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A Wireless Sensor Network for Electrical Distribution

Substations

Ricardo André Pinto Faria

Dissertation submitted to obtain the Master (Msc) degree in

Electrical and Computer Engineering

Jury

President: Marcelino Bicho dos Santos

Supervisor: Maria Helena da Costa Matos Sarmento

Member: José Luis Costa Pinto de Sá

Member: Mário Serafim dos Santos Nunes

October 2011

iii

ACKNOWLEDGMENTS

A major research project like this is never the work of a lone individual. The contributions of

many different people, in their different ways, have made this possible. I would like to extend my

personal gratitude to the following.

To Prof. Helena Sarmento, my thanks for making this research possible. Her support,

guidance and advice throughout the research project, as well as her pain-staking efforts in proof

reading the reports, are greatly appreciated. Indeed, without her guidance, I would not be able

to put the topic together.

Prof. Pinto Sá for his availability and for sharing knowledge and experience gained over

several years in the field of Energy. His sagacity has often been a light in dark places.

The opportunity of integrating my research into the Kic InnoEnergy innovations project, in the

area “Intelligent, Energy-Efficient Buildings and Cities” on the project “Active Substations” is one

that I am most proud in having taken part. My special thanks to Dr. Tom de Rybel, for the

suggestions and doubts clarified during the project.

My deepest thanks to the research institution INESC-ID, for graciously allowing me the use

of the space and equipment needed for the development of the prototype.

My heartfelt appreciation goes to all my friends and colleagues, who helped me through

these difficult years, always ready to offer support in good moments and bad.

Last but not least, I would like to thank my parents for their unconditional support, both

financially and emotionally throughout my degree, in particular, the patience and understanding

shown by my mother, father and sister throughout these years of college.

It's very simple -

you read the protocol and write the code.

Bill Joy

http://en.wikipedia.org/wiki/Bill_Joy

iv

ABSTRACT

Accidents occurring in large-investment electrical substations for transmission and

distribution of electric energy may cause great financial damage due to a lack of control and

monitoring of devices. The automation of devices in the substations has thus become a priority,

in order to prevent such accidents. The IEC 61850 is a new automation protocol, designed for

the purpose of establishing a common standard for all substations, one that all manufacturers of

all different devices must comply with.

In the present day, not all distribution substations are automatized, as is the case with

secondary substations, due to the fact that the automation costs are higher than the cost of the

substation itself. However, these substations are the only places from where important data may

be drawn about the distribution network, to be transmitted to substations at a higher hierarchic

level.

This thesis advocates the projection, implementation and evaluation of a wireless sensor

network (WSN) for the control and acquiring of non-critical data. The sensor network is

constituted by wireless modules, using ZigBee technology. The structure of communication in

WSN and the format of the messages are based on IEC 61850.

The tests, performed in different environments, have shown that it is possible to adopt

ZigBee technology while complying with the main reporting requirements demanded by IEC

61850, such as those pertaining to lack of data loss and meet some of data transmission time.

However, some parameters of the format communications and some data transmission times do

not meet the requirements. The noise in the substation does not influence the performance of

the WSN and communication between modules is guaranteed, allowing a cheap, simple and

reliable automation system for secondary substations.

Keywords:

IEC 61850, Secondary Substations, ZigBee, WSN, Smart Grid

v

RESUMO

As subestações eléctricas de transmissão e distribuição de energia eléctrica representam

um grande investimento. A ocorrência de acidentes devido a falta de controlo e monitorização

dos equipamentos podem gerar prejuízos económicos sérios. A automatização dos

equipamentos nas subestações é, portanto, uma prioridade. Para que o modelo de

comunicação fosse comum em todas as subestações e que os diversos fabricantes de

equipamentos cumprissem todos os requisitos, foi desenvolvido um novo protocolo de

comunicação para subestações eléctricas, definido como IEC 61850.

Actualmente, nem todas as subestações de distribuição se encontram automatizadas, como

é o caso dos postos de transformação, porque o custo do equipamento é inferior ao custo do

sistema de automação. Contudo, estas subestações são os únicos locais de onde se pode

retirar dados importantes acerca da rede de distribuição para serem transmitidos para

subestações de nível hierárquico superior.

Esta tese descreve a concepção, implementação e a avaliação de uma rede de sensores

sem fios (WSN – Wireless Sensor Network) para controlar e adquirir dados de equipamentos

não críticos do posto de transformação. A rede de sensores é constituída por módulos sem fios,

usando a tecnologia ZigBee. A estrutura de comunicação e o formato das mensagens é

baseado no IEC 61850.

Os testes realizados em diferentes ambientes mostram que os principais requisitos exigidos

pelo IEC 61850 ao adoptar a WSN utilizando a tecnologia ZigBee são cumpridos, tais como os

requisitos sobre a perda de pacotes de dados e os principais tempos de transmissão de dados.

Contudo, os requisitos de alguns parâmetros do formato de comunicação e alguns tempos de

transmissão de dados não são cumpridos. O ruido electromagnético existente nas subestações

não influencia a performance da WSN, garantido uma comunicação segura entre os módulos.

Tudo isto possibilita uma solução barata, simples e fiável de sistema de automatização para os

postos de transformação.

Palavras-Chave:

IEC 61850, Posto de Transformação, ZigBee, WSN, Rede Inteligente

vi

CONTENTS

ACKNOWLEDGMENTS ......................................................................................................... III

ABSTRACT ......................................................................................................................... IV

RESUMO ............................................................................................................................ IV

CONTENTS ......................................................................................................................... VI

LIST OF FIGURES .............................................................................................................. VIII

LIST OF TABLES ................................................................................................................... X

1. INTRODUCTION ............................................................................................................... 1

1.1. POWER SYSTEM GRID ............................................................................................................... 1

1.2. OBJECTIVES ............................................................................................................................. 4

1.3. DISSERTATION LAYOUT ............................................................................................................. 4

2. IEC 61850 ..................................................................................................................... 5

2.1. HISTORY .................................................................................................................................. 5

2.2. ANALYSIS OF IEC 61850 ........................................................................................................... 6

2.2.1. IEC 61850 CONCEPTS ...................................................................................................... 6

2.2.2. DATA STRUCTURE IN IEC 61850 ....................................................................................... 8

2.2.3. COMMUNICATION SYSTEM ...............................................................................................11

2.3. WIRELESS COMMUNICATION IN IEC 61850 ..............................................................................15

2.3.1. IEEE 802.15.4 AND ZIGBEE ............................................................................................16

2.3.2. ANALYSIS OF IEC61850 OVER ZIGBEE AND IEEE 802.15.4 .............................................18

3. WIRELESS SENSOR NETWORK FOR MEDIUM-LOW VOLTAGE SUBSTATION........................ 20

3.1. CHARACTERIZATION OF THE SS ...............................................................................................21

3.2. ARCHITECTURE .......................................................................................................................23

3.3. WSN COMMUNICATION ...........................................................................................................24

3.4. PROTOTYPE IMPLEMENTATION .................................................................................................25

3.4.1. CREATION OF THE WSN ..................................................................................................26

vii

3.4.2. DEFINITION OF COMMUNICATION MESSAGES IN WSN .......................................................30

4. EVALUATION TESTS ...................................................................................................... 37

4.1. TYPES OF TESTS .....................................................................................................................37

4.2. LABORATORY TESTS ................................................................................................................39

4.2.1. SV TESTS .......................................................................................................................40

4.2.2. MMS TESTS ....................................................................................................................41

4.3. SS#1 TESTS ...........................................................................................................................42

4.4. SS#2 TESTS ...........................................................................................................................44

4.5. CONCLUSIONS OF THE RESULTS ..............................................................................................47

5. CONCLUSION AND FUTURE WORK ................................................................................. 48

5.1. CONCLUSION ..........................................................................................................................48

5.2. FUTURE WORK .......................................................................................................................49

BIBLIOGRAPHY .................................................................................................................. 50

ANNEX A. ......................................................................................................................... 54

A.1 EXAMPLE OF A LOGICAL NODE AND DATA CLASS .....................................................................54

A.2 TABLE OF PICOM TYPES ......................................................................................................56

ANNEX B. ......................................................................................................................... 58

B.1 THE LN AND LD PROGRAM FLOWCHART .................................................................................58

B.2 DATA FORMAT ADOPTED FOR COMMUNICATIONS USED IN WSN ...............................................60

ANNEX C. ......................................................................................................................... 65

C.1 PREPARATION OF WIRELESS MODULES...................................................................................65

C.2 TEST SITES AND THE DISPOSITION OF WIRELESS MODULES ......................................................66

C.3 RESULTS OF THE TESTS ........................................................................................................70

viii

LIST OF FIGURES

Figure 1: Basic structure of the electric system [4]. ............................................................................................. 1

Figure 2: IEC 61850 Interface Model and levels of SAS [17], [15] ...................................................................... 7

Figure 3: Data structure in IEC 61850 [18]. ......................................................................................................... 8

Figure 4: Hierarchical structure of a logical node XCBR ..................................................................................... 9

Figure 5: Example of a protection function [15] ................................................................................................. 10

Figure 6: Communication modes on SAS levels [18] ........................................................................................ 15

Figure 7: Performance of the wireless technologies [27] .................................................................................. 16

Figure 8: Communication system in a SS [10] .................................................................................................. 21

Figure 9: Architecture of the WSN on SS .......................................................................................................... 23

Figure 10: WSN communication ....................................................................................................................... 25

Figure 11: ZigBee Model ................................................................................................................................... 26

Figure 12: The principal flowchart of the WSN .................................................................................................. 29

Figure 13: Contents of the universal data frame [40] ........................................................................................ 30

Figure 14: Model communication to monitor the devices [41] ........................................................................... 32

Figure 15: Example for ASN.1 coded frame structured [23] .............................................................................. 33

Figure 16: Data frame of synchronization message [25] ................................................................................... 34

Figure 17: SV message Transfer time .............................................................................................................. 38

Figure 18: MMS message transfer time ............................................................................................................ 38

Figure 19: Laboratory environment ................................................................................................................... 40

Figure 20: SV message performance in Laboratory.......................................................................................... 40

Figure 21: MMS message performance in Laboratory ...................................................................................... 41

Figure 22: SS#1 environment ........................................................................................................................... 42

Figure 23: SV performance in SS#1 ................................................................................................................. 43

Figure 24: MMS performance in SS#1 .............................................................................................................. 44

Figure 25: SS#2 environment ........................................................................................................................... 45



Figure 28: Substation Controller program flowchart .......................................................................................... 58

Figure 29: Module LN program flowchart .......................................................................................................... 59

Figure 30: Portable Wireless Module ................................................................................................................ 65

Figure 31: Laboratory and disposition of the wireless modules ........................................................................ 66

Figure 32: Format of SS#1 ................................................................................................................................ 66

Figure 33: Disposition of the LN SIML and ZMDS ............................................................................................ 67

Figure 34: Disposition the LN ADEC and the ZigBee Coordinator .................................................................... 67

ix

Figure 35: Format of SS#2 ................................................................................................................................ 68

Figure 36: Disposition of the LN ADEC ............................................................................................................. 68

Figure 37: Disposition of the wireless modules ................................................................................................. 69

Figure 38: New disposition for the LN SIML and ZMDS .................................................................................... 69

Figure 39: MMS performance in Laboratory ..................................................................................................... 70

Figure 40: MMS performance in Laboratory ..................................................................................................... 71





x

LIST OF TABLES

Table 1: Message Types and the corresponding requirements [15], [7] ........................................................... 11

Table 2: Communication stacks and the OSI-7 layer model [24] ...................................................................... 12

Table 3: Data classes for the LN circuit breaker [28] ........................................................................................ 54

Table 4: The "DPC" common data class for "POS" [28] .................................................................................... 55

Table 5: PICOM types [15] ................................................................................................................................ 57

Table 6: Encoding for the basic data types [23] ................................................................................................ 60

Table 7: Value and data class of the LN SIML [20] ........................................................................................... 61

Table 8: Value and data class of the LN ZMDS ................................................................................................ 61

Table 9: Value and data class of the LN ADEC ................................................................................................ 61

Table 10: Common Data Class SPS [19] .......................................................................................................... 62

Table 11: Common Data Class MV [19] ............................................................................................................ 62

Table 12: MMS type description definition for USVCB MMS structure [23] ....................................................... 63

Table 13: Parameter name for USVCB services [41] ........................................................................................ 63

Table 14: SNTP client operations [25] .............................................................................................................. 64

Table 15: SNTP server operations [25] ............................................................................................................. 64

xi

LIST OF ACRONYMS

Acronyms

APL

ASN.1

ACSI

ADEC

CSMA/CA

CSMA/CD

CDC

CIGRE

Definition

Application Layer

Abstract Syntax Notation One

Abstract Communication Service Interface

Automatic Door Entrance Control

Carrier Sense Multiple Access with Collision Avoidance

Carrier Sense Multiple Access with Collision Detection

Common Data Class

Conseil International des Grands Réseaux Électriques

DAS

DCP

DS

EDP

EPRI

FFD

FC

GOOSE

GSSE

GPS

HMI

IEC

IP

IED

IEEE

IUC

ISO

LD

LN

LV

MAC

MMS

MV

MSVC

NTP

OSI

PAN

PICOM

PYL

PCB

Data Acquisition System

Controllable Double Point

Distribution Substation

Energias de Portugal

Electric Power Research Institute

Full Function Device

Functional Constraints

Generic Object Oriented Substation Events

Generic Substation Status Event

Global Position System

Human Machine Interface

International Electrotechnical Commission

Internet Protocol

Intelligent Electronic Device

Institute of Electric and Electronic Engineers

Integrated Utility Communication

International Organization for Standardization

Logical Device

Logical Nodes

Low Voltage

Media Access Control

Manufacturing Message Specification

Medium Voltage

Multicast Sampled Value Control

Network Time Protocol

Open Systems Interconnection

Personal Area Network Coordinator

Piece of Information for Communication

Physical Layer

Printed Circuit Board

xii

RFD

SV

SVC

SS

SIMG

SIML

SNTP

SAS

SCADA

SCSM

SCL

SPS

TCP

TS

USVC

UCA

UDP

USVCB

WSN

ZMDS

Reduced Function Device

Sampled Value Measurement

Sampled Value Control

Secondary Substation

Sensor Insulation Medium Gas

Sensor Insulation Medium Liquid

Simple Network Time Protocol

Substation Automation System

Supervisory Control and Data Acquisition

Specific Communication Service Mapping

Substation Configuration Language

Single Point Status

Transmission Control Protocol

Transmission Substation

Unicast Sampled Value Control

Utility Communications Architecture

User Datagram Protocol

Unicast Sampled Value Control Block

Wireless Sensor Network

Further Meteorologist Device Substation

1

1. INTRODUCTION

1.1. POWER SYSTEM GRID

Currently, electricity is a basic need in any household and a basic human right. It is the

dominant form of energy for communication, technology and the production of assets. Before

reaching the consumer, electric energy is dependent on an elaborate and complex power

system which enables its generation, transport and distribution, making the aforementioned

ease a deceptive way of describing this form of energy [1].

In order to facilitate the delivery of electric power to the consumer, a system was deemed

necessary. Therefore, what we now know as the electric grid was created, so that the need for

distribution could be satisfied. The Figure 1 illustrates the electrical grid of the United Kingdom,

called “National Grid”, but this network is very similar in all European Union countries [2]. This

network generally supports one of the following functions [3]:

Power generation: Power generation includes the facilities for generating power in

central as well as in distributed locations.

Electricity transmission: Electricity transmission refers to the very high, high and

medium voltage network of electric cables used to take bulk power from generation facilities to

power distributions facilities near populated areas.

Electricity distribution: Electricity distribution is the process in which the high-voltage

power is down converted and disseminated to the consumers through a mesh network of cables

reaching all the way to consumer premises.

Figure 1: Basic structure of the electric system [4].

2

The electric grid depends on electrical subsystems, where voltage is converted from high to

low or the reverse using transformers [3], as shown on Figure 1.

The electric subsystems are classified as generation, transmission and distribution. The

generation subsystem has the capability of connecting the generation of electricity to the

electricity grid. The voltage levels are: 25kV and 33kV [4]. The transmission subsystem (TS) is

used to transmit high and medium voltages [2]. The voltage levels are 400kV, 275kV and 132kV

[4].

Distribution subsystem (DS) is used to lower transmission voltages of distribution process to

end users [2]. They normally serve a single small area, being located near end users. The

voltage levels are: 240V, 11kV and 33kV [4]. The DS can be classified according to metering

arrangements and type of supply (overhead line or underground). This is the specific case of the

substation which converts 11kV to 240 V, which is the last step of the electric grid before energy

reaches individual consumers [5], designated by Medium to Low Voltage Distribution Substation

(MLVDS). The word substation comes from the time where only one power generation existed

and the other substations were subsidiaries of the power generation [1].

The substations generally consist of devices for switching, protection, control and

transforming voltage. [6]. The transmission substations have circuit breakers to interrupt

any short circuits or overload currents that may occur on the network [6]. Distribution stations

may use recloser circuit breakers or fuses for the protection of the distribution circuit. The

equipment here described possesses some types of setting, monitoring and/or control

parameters. The managing of these devices and the enabling of the various devices for

intercommunication in a fast and efficient manner is performed by the substation automation

system (SAS) [7].

The order control process involves the exchange of information between the devices and the

device that coordinates all substation devices. An example of this is the acquiring of empty or

load voltage values, in order to assess whether they are within the limits.

In the beginning, the telephone was used to communicate line loadings back to the control

center and if problems on the substation occurred, orders were given to the operators for

performing switching operations at substations. In the mid-1930s [8], the telephone line was

used to remotely control switching-based remote control units and to provide status and control

about the position of the contacts. When digital communications became a viable option in the

1960s [8], data acquisition systems (DAS) were installed to automatically collect measurement

data from the substations. As we move into the digital age [8], literally thousands of analog and

digital data points become available in a single Intelligent Electronic Device (IED) and

communication bandwidth is no longer a limiting factor. It thus became a priority to define the

formats of messages, transmission times and other criteria, with the purpose of connecting IED

from different vendors and the devices into the substation. Given this, a new protocol was

designed to standardize communication between substations and IED devices, known as

International Electrotechnical Commission (IEC) 61850.

3

The international standard IEC 61850 – Communication Networks and Systems in

Substations is the standard which defines the communication between intelligent electronic

devices (IED) in the substation and the related system requirements. This standard is a suite of

multiple communication protocols, defining data models for electrical applications, data types

and communication services, modeling devices, functions, process and architectures. In

addition to standardize communications within the substation, it sets standards for control and

monitor the automation system starting from the outside specifically with the supervisory control

and data acquisition (SCADA) using the remote terminal unit [9].

The standard only covers distribution and transmission substations. Given its necessity, only

the transmission and some distribution substations are automated in an efficient form, fulfilling

requirements set by the IEC 61850. However, the Secondary Substation (SS) are not

automatized, the main reason being that the value of the investment is greater than the value of

the DS itself.

The SS automation can greatly improve the operation of the distribution network operation.

The main reason to automate the SS is that it is the only spot from which short-circuit voltage

values may be acquired, as well as the load impedance at the extremities of distribution lines of

medium voltage. This data may be transmitted to automated substations of higher hierarchy or

the SCADA, which is crucial for understanding the state of the line of medium voltage

distribution [10]. Additional information about the substation, such as door position, transformer

temperature, position of the switchgear and values of voltage and current can also be

transmitted.

The SS has no critical devices such as substations of the upper hierarchy. Given this, the

requirements for IEC 61850 as applied to some transmission substations and distribution

substations should not be applicable to SS, because the critical levels are distinct. However, for

the implementation of communication between substations of different hierarchies and SCADA,

the standard may be the base. Using an IEC 61850 based protocol will reduce complexity of

communication in the entirety of the electrical grid, being therefore a good option as even in

communication between SS devices, the IEC 61850 must be complied with so that any operator

may understand the existing automation system [11].

The SS built recently are substations with broad space, which allows one to easily insert and

handle new cables. The same does not apply to older SS, due to not being as broad and to

sometimes having concrete walls inside, making it difficult to install new cables. This leads to a

higher cost of installation.

A solution for these problems is the adoption of wireless communication. Wireless

communication is flexible, inexpensive (equipment and installation) and allows the operator

access to all devices within the substation. This solution applies to all types of SS, making it a

more advantageous solution than cable.

For wireless communication, the ZigBee technology, based on the IEEE 802.15.4, is well

suited for this application because it is very cheap and the sizes of the modules are small,

facilitating the integration of devices in the substation [12]. The IEC 61850 does not establish

4

special requirements for these types of communications to SS; therefore it applies the global

specifications to ZigBee technology. However, there is a mishap; the standard defines critical

time for data transmission of critical devices, which cannot be supported on a WSN. For non-

critical devices, defined transmission times can be met by the ZigBee technology. In either case,

it must be confirmed that the devices of the WSN will comply or not the specifications required

by IEC 61850.

This work was developed in the scope of the Kic InnoEnergy project “Active Substations”.

The “Active Substations” project has, as the main objective, the creation of an automation

system for efficient and inexpensive SS, using advanced technology and introducing the

concept of smart grid distribution networks of low voltage [10] and [13].

1.2. OBJECTIVES

The main objective of this dissertation is to develop a system for the monitoring of the non-

critical equipment available in a SS using a wireless sensor network. The underlying wireless

technology is ZigBee and the IEC 61850 protocol is used for the communication between

devices. To achieve the main objective, specific goals are outlined:

- To analyze the IEC 61850 standard.

- To analyze the ZigBee technology.

- To choose the IEC 61850 types of message suitable each specific function.

- To implement a prototype by using the IEC 61850 on top of ZigBee.

- To adapt existing modules ZigBee at system.

- To test the prototype in different Distribution Substations.

1.3. DISSERTATION LAYOUT

This thesis is organized in five Chapters. The present chapter introduces the concept and

presents the objectives and motivations. Chapter 2 describes analysis of the IEC 61850

standard and different wireless technologies to be used for communication. The architecture of

the developed prototype and its functionalities, as well as the implementation of the prototype is

described on Chapter 3. Results are discussed in Chapter 4. Chapter 5 summarizes some

conclusions and suggests improvements and future work.

l%20

5

2. IEC 61850

IEC 61850 is an international standard designed to provide interoperability and fast

communications among field devices, guaranteeing data delivery time and supporting field

devices configuration, also defines the semantics and syntax of data being communicated.

Its primary objective is to define the communication between IED in the substation [9]. The

communication architecture is composed of abstract definitions of classes and services which

are independent of underlying concrete protocol stacks and deployment platforms. In addition to

intra-substation communications, IEC 61850 information models are applicable to inter-

substation communication, control center communication, metering, electrical equipment

condition monitoring and diagnosis, and define a communication method between IED to

engineering systems communications. This chapter concerns itself with two main subjects - the

analysis and description of the IEC 61850 and the analysis of a possible communication

technology capable of implementing this standard.

The structure of the IEC 61850 is described in a set of documents of over 1,400 pages.

These documents are divided into 10 parts and consist of the following [9].

2.1. HISTORY

In 1988 EPRI and IEEE initiated the Utility Communications Architecture (UCA) project under

the Integrated Utility Communication (IUC) program. The objective of the UCA project was to

make provision for interoperability between control systems employed to monitor and control the

electric power utilities. Initially the UCA project focused on communications between control

centers, and communications between substations and control centers. EPRI and IEEE carried

out the UCA project in collaboration with the Pacific Gas and Electric Company and Houston

Light and Power Company. The result of this collaboration was a standard communications

architecture referred to as UCA version 1.0 [9].

UCA version 1.0 did not provide a detailed description of how the UCA communication

architecture was to be practically implemented and used in field devices, therefore the adoption

of UCA architecture in the electrical power industry was limited. EPRI and IEEE continued with

their efforts to improve the UCA architecture by sponsoring a number of research projects such

as the substation integrated protection, control, and data acquisition, and the MMS Forum

Working Groups. These efforts lead to thorough specifications of object models of field devices,

i.e. definitions of data and control functions provided by these field devices, in what became

UCA version 2.0 [9].

In 1997 EPRI and IEEE joined efforts with Working Group 10 (WG10) of the IEC Technical

Committee 57 (TC57) to build a common international standard for electrical utility

communications. These efforts were based on concepts and definitions of the UCA architecture

and lead to the creation of a standard named IEC 61850 [9].

6

2.2. ANALYSIS OF IEC 61850

This section provides an overview of the standard. Firstly, basic concepts are explained.

Then a standard content is briefly presented. Finally, the different parts of the standard are

analyzed in more detail.

2.2.1. IEC 61850 CONCEPTS

There are some concepts adopted by the IEC 61850 standard that are not defined by it, yet

they are essential to understand the whole automation system. The main concepts which are

necessary for the development and realization of the automation system are the IED, which are

the main devices of the system that takes all the protection functions of control and monitoring

of all devices in the substation; SCADA, which allows controlling the operation of various

electrical substations directly from a single place; finally, the SAS, which defines the structure of

substation automation.

IED are microprocessor-based controllers of power system equipment’s such as sensors

and actuators, which are capable of receiving or sending data/control from human interface

machines or external sources [14]. IED can be classified by their functions. Common types of

IED include relay devices, circuit breaker controllers, recloser controllers and voltage regulators.

IEC 61850 defines client-server communication, where the server provides services for a

client. For instance, logging, reporting and settings control are services that are offered by a

substation controller to automation operators or SCADA. These services are defined by the

standard and are available through the ACSI (Abstract Communication Service Interface) [15].

SCADA is a control system that centrally controls and monitors substation devices, acquiring

data from them, analyzing that data and saving it onto servers. This data is available to all

automated substations, including substations of power generation or other SCADA, so that all

information circulates throughout the electricity grid. It may be located within a TS or installed

close to other substations [16].

The SAS is only used for the medium and high voltage substation [9]. It divides the

structure of automation in three different hierarchical levels, defining the location of the devices

and their features, from data acquisition to the analysis of data by operators or even the sending

of data to the outside. These three levels are station, bay and process.

Figure 2 shows the three levels and the interfaces originally identified to be within scope of

the standard, specifically the measurement process from devices (e.g.-voltages, currents,

status) to IED, IED to station level, IED to IED, and IED to Technical Services. Each interface

brings with it different requirements for performance, quality of service and reliability. Identified

but not yet implemented interfaces are the station level to SCADA and local device to other,

more remote electric substations existing in the grid [15].

7

Figure 2: IEC 61850 Interface Model and levels of SAS [17], [15]

At the process level, the devices are switchyard equipment’s, remote I/O devices which send

process values via digital link (e.g. remote terminal unit), actuators (e.g. automatic reclosure,

voltage control) and sensors (arc detection, measuring current and voltage transformer). The

bay level includes the IED, a device that has the ability to control and protect the devices of the

process level. The operator has access to all data and control devices at the station level. The

bus station and the bus processes both hold different requirements for each type of

communication. The bus process facilitates critical communication between protection and

control IED to the primary equipment, while the station bus facilitates communication between

station level and bay level, as shown on Figure 2.

The IEC 61850 is characterized by interoperability between devices and covers three levels

of the SAS [17]. However, the standard defines the communication between all three levels.

Communications themselves are mapped by other communication protocols. The mapping of

IEC 61850 to concrete protocols is carried out by the Specific Communication Service Mapping

(SCSM). SCSM associates abstract communication services, data objects and parameters with

proper elements of communication protocols.

8

2.2.2. DATA STRUCTURE IN IEC 61850

The standard models numerous devices, defines functions to be implemented on

substations, the semantics (meaning and behavior) and the syntax of the data being

communicated. Models and functions (e.g. measure, status, description and control) are

organized into virtual Logical Nodes (LN), localized on the process level (Figure 3).

Figure 3: Data structure in IEC 61850 [18].

Figure 3 describes the data model hierarchy defined by the standard. The LN, represented in

blue on Figure 3 (measurement unit#2 and circuit breaker#1) has several data objects, each

belonging to a defined data class, like current measurements or breaker position control

represented in green and yellow on the Figure 3. A Logical Device (LD), that is a collection of

LN, is implemented in a Physical Device. A physical device can host one or more LD. The

Physical Device has a unique Internet protocol address. Each physical device is associated to

only one Logical Device.

2.2.2.1. LOGICAL NODES

The various operations within a substation, such as control, protection, monitoring and

measurement functions are modeled using LN. A LN is a collection of data objects, log-control

objects and a list of list of data elements and attributes. The types of data contained in a LN can

be operational data (e.g. measurement values, position status) or configuration data (e.g.

configure the acquiring values sample rate).

It is identified by the initials of its name. For instance, the standard LN name for a

measurement unit for 3-phase power is MMXU or for a circuit breaker XCBR, shown in blue in

Figure 3. The prefix M or X identifies the instance of a logical node name. The ‘Metering and

Measurement’ represents the logical node group M, while the ‘Switching Devices’ represents

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the logical node group X. The suffix shall be defined in relation to the English name of the LN

class name [15]. For example, the suffix MXU indicates measuring for operative purpose and

the CBR circuit breaker [15]. The number allows us to distinguish the three phase’s feeders [15].

Each LN is composed of several data objects and each data object in turn has several data

attributes associated with it [8]. The type and structure of the data within the LN, conforms to the

specification of a Common Data Class (CDC), per IEC 61850-7-3 [19], that serves as the

predefined building blocks for creating larger data objects. Each CDC has a defined name and a

set of CDC attributes each with a defined name, defined type, and specific purpose. Each

individual attribute of a CDC belongs to a set of functional constraints (FC) that groups the

attributes into categories [19].

Figure 4: Hierarchical structure of a logical node XCBR

Figure 4 illustrates in greater detail the hierarchical structure and data attributes associated

to the specific data object “Pos” of the XCBR. The complete data objects and the respective

data attributes of the XCBR are described on annex A.1.

The data object “Pos” contains the common data information (which the mode of operation),

settings, status info and controls of the switch position [18] and [20]. The Controllable Double

Point (DPC) is the CDC defines for these data object.

The DPC defines data attributes such as control, status, and substitute values in different

categories, as shown on Figure 4. Each category of attributes corresponds to the functional

constraint. A complete description of the CDC can be found on annex A.1.

10

2.2.2.2. FUNCTION AND PICOM DEFINITION

Functions necessary for substation automation and application (e.g. protection, control,

measurement, monitoring) are classified in IEC 61850 in the following categories: system

support, system configuration or maintenance, operational or control, local process automation,

distributed automatic support and distributed process automation [15].

Figure 5: Example of a protection function [15]

Figure 5 illustrates an example of a protection function, where there are two LN, a circuit

breaker (XCBR), a current transformer (TCTR), one substation computer running the human

machine interfaces (HMI) and one LD acting as the protective relay (P). These virtual nodes

reside in three distinct physical devices.

The logical communication between devices, such as when the HMI must communicate with

the devices of the Remote Process Interface, has to be transmitted via the protective relay,

which retransmits this data to the devices. The same applies in vice-versa.

The IEC 61850 adopts, for data transfers, the Piece of Information for Communication

(PICOM) structure. PICOM describes the information transfer on a given logical connection with

given communication attributes between two LN [15]. The components of PICOM types are:

Data, meaning the content of the information and its identification as needed by the

functions (semantics).

Type, describing the structure of the data, i.e. if it is an analog or a binary value, if it is a

single value or a set of data, etc.

Performance, meaning the permissible transmission time (defined by performance

class), the data integrity and the method or cause of transmission (for example periodic, event

driven, on request).

Logical connection, containing the logical source (sending LN) and the logical sink

(destination or receiving LN).

PICOM does not represent the actual structure and format for data exchanged over the

communication network. However, it defines the type of communication for each existing

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function in SAS. A description of data to be exchanged over, described on [21], [22] and [23], is

briefly detailed in the next subsection.

2.2.3. COMMUNICATION SYSTEM

According to the IEC 61850, transmission time requirements for SAS network messages

must be ensured under any operating conditions and contingencies inside the substation.

The message types being carried will vary from moment to moment, depending on the

activity both in the substation and on the system. These messages are classified into 7

categories, as follows [15]:

Type 1 – Fast messages, used when a response is needed, for example “Stop”, “Start”

and some functions.

Type 2 – Medium speed messages, used to transfer status and measurement

information.

Type 3 – Low speed messages, used for slow speed auto-control functions and

transmission of event records.

Type 4 – Raw data messages, used to transfer output data from digitizing transducers

and digital instrument transformers.

Type 5 – File transfer messages, used to transfer big files of data for recording.

Type 6 – Time synchronization messages, used to synchronize devices internal clocks.

Type 7 – Command messages with access control, used to transfer control information,

issued from local or remote HMI functions, where a higher degree of security is required.

All messages types and the corresponding transmission time requirements for the

distribution and transmission bays are sorted on Table 1.

Message Type Message Level

Requirements (Transmission Times)

Distribution

Bay

Transmission

Bay

1A Fast messages (Trip/Block) Extremely Fast 10ms 3ms

1B Fast messages (Others) Fast 100ms 20ms

2 Medium Speed Medium Speed ≤100ms

3 Low Speed Slow Speed ≤500ms

4 Raw Data (Protection and Control) Extremely Fast 10ms 3ms

5 File Transfer Slow Not Request ≥1000ms

6A Time Synchronization (Control and Protection)

The accuracies are

functional requirements

±1ms ±0.1ms

6B Time Synchronization (For Measurement)

±25µs ±4/±1µs

7 Command messages with access control

Low/Fast 1

Table 1: Message Types and the corresponding requirements [15], [7]

1 If it is used to operate switchgear, transmission time should meet requirement for fast message.

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Table 2 presents the IEC 61850 protocol stack layers of functionality, which provide

flexibility in advancement of communication technology, its relationship with the Open Systems

Interconnection (OSI) model and the respective protocol specification for each type of

communication stack.

7 Application Layer

Time Sync

SNTP

SV

Sampled Values

GOOSE Generic

Object Oriented Substation Event

GSSE Generic Substation

Status Event MMS IS9506 Connectionless ACSE ISO/IEC 8649,10035

MMS ISO 9506 Core

ACSI Services Connection oriented ACSE ISO/ IEC 8649,8650

6 Presentation Layer

n/a n/a ASN.1, BER ISO/IEC 8824.1

Connectionless presentation ISO/IEC 8649, 10035 ASN.1, BER ISO/IEC 8824.1

Connection-oriented presentation protocol ISO/IEC 882, 8823 ASN.1, BER ISO/IEC 8824.1

5 Session Layer

n/a n/a n/a Connectionless session ISO/ IEC 9548

Connection-oriented session ISO/IEC 8326,8327

4 Transport Layer

UDP /IP

n/a n/a GSSE T-Profile ISO/IEC 8602 ISO COT-Profile ISO/IEC 8073

TCP/IP T- Profile ISO Transport on top of TCP (RFC 1006)

3 Network Layer

IP( RFC 791)

n/a n/a ISO/IEC 9542 ISO/IEC 8473

IP (RFC 791)

2 Link Layer RFC 894

Priority Tagging/Vlan ( IEEE 802.1Q) CSMA/CD ( ISO/IEC 8802.3)

ISO/IEC 8802-2 LLC RFC 894

1 Physical Layer

ISO/IEC 8802.3 Etherype ISO/IEC 8802.3 ISO/IEC 8802.3 Ethertype

Table 2: Communication stacks and the OSI-7 layer model [24]

The information in Table 1 and Table 2 relates to the fact that seven types of messages are

mapped to different communication stacks. These communication stacks are used on the

application layer of the OSI model. For each communication stack are defined the profiles of the

seven layers of the OSI model, as shown on Table 2.

Generic Object Oriented Substation Events (GOOSE) is implemented using type 1A and 1B

messages, while Sampled Measurement (SV) uses the type 4 message. Therefore, the time of

data transmission for GOOSE and SV is critical, being directly embedded to a low-level Ethernet

link layer [7]. This gives the advantage of improved performance for real time messages, by

shortening the Ethernet frame (no upper layer protocol overhead) and reducing the processing

time. MMS (Manufacturing Message Specification) is an application protocol for real devices

and functions, employed for exchanging information about the real device. The exchanging

process data are referred to as client-server communication messages [8]. It uses the medium

speed message (type 2), the low speed message (type 3), the file transfer message (type 5)

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and the command message with access control (type 7). MMS uses TCP/IP stacks above the

Ethernet layer [7]. The time synchronization messages (type 6) are broadcasted to all IED and

the corresponding devices in the substation by using User Datagram Protocol/Internet Protocol

(UDP/IP) [7]. Finally, the message type 1A and 1B can also be mapped to Generic Substation

Status Event (GSSE), which is identical in implementation as the GOOSE but used only for

state change information, operating over connectionless OSI services.

The message types are based on a grouping of the performance related PICOM attributes

and, therefore, define the performance requirements to be supported. Since the performance

requirements are defined per message, they are independent of the size of the substation. On

annex A.2 is described the types of PICOM.

There are two independent groups of performance classes, one for control and protection,

another one for metering and power quality applications. Since the performance classes are

defined according to the functionality needed, they are independent from the size of the

substation [15].

Within a specific substation, communication links do not necessarily need to support the

same performance class. Station level and process level communications may be selected

independently of each other and within the process level, while different performance classes

can be used for communications in different bays, depending on the number and rating of

equipment located in each bay [15].

The distinction of the types of messages depends on the role each device has to play in the

substation, implying that, for each communication stack, specific functions in each level of the

SAS are performed.

The GOOSE messages exchange information from one output IED to either the input of

several other IED or to station level. They are used to model the transmission of time-sensitive

information like commands or interlocking. This data is directly embedded into Ethernet data

packets and works on publisher-subscriber mechanisms on multicast or broadcast MAC

address. The same GOOSE message is retransmitted with varying and increasing re-

transmission intervals. A new event occurring within any GOOSE dataset element will result in

the existing GOOSE retransmission message being stopped. A state number within the GOOSE

protocol identifies whether a GOOSE message is a new or a retransmitted message. This

method of media access control is called Carrier Sense Multiple Access with Collision Detection

(CSMA/CD).

The GSSE message is identical to GOOSE. However, it deals with information about status

devices of the substation. There is a difference in the transport layer and the network in relation

to GOSSE [15]. The GSSE has a specific ISO protocol on its.

The SV message contains the information of the source to be transmitted to all levels of

SAS. This data is drawn from a modern device, which has current or voltage measurement

sensors as well as action sensors (among many other types), which gather information from the

primary power system. Like with GOOSE, the data is directly embedded into Ethernet data

packets and the method of access to transmit data is the CSMA/CD. The exchange is based on

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a subscriber/publisher mechanism and takes into account time constraints and sampling rates.

Two methods are provided, namely MULTICAST-APPLICATION-ASSOCIATION (using

Multicast Sampled Value Control, MSVCB) and TWO-PARTY-APPLICATION-ASSOCIATION

(using Unicast Sampled Value Control, USVCB) [15].

GOOSE and SV use VLAN and priority tagging, as per IEEE 802.1Q, to have a separate

virtual network within the same physical network and sets appropriate message priority levels.

The MMS can be used from the process level to the station level, as shown in Figure 6.

MMS models the behavior of two devices in the form of a Client/Server model. The client can,

for example, operate and monitor the system from a control center or IED. The server can

represent one real device, several real devices or whole systems. MMS uses an object-oriented

modeling method with object classes (Named Variable, Domain, Program Invocation, and Event

Condition etc.), as well as instances from the object classes and methods (services like read,

write, store, start, stop etc.), as defined by IEC 61850-8-1 [21]. In certain situations, the packet

size of data to be transmitted can be great. Therefore, it is necessary to adopt a system of

encoding messages so that the message becomes small and easy to decode. The MMS

adopted the standard Abstract Syntax Notation One (ASN.1) to describe the data structure,

encoding, transmitting and decoding. The method of sending data is done via unicast, in TCP/IP

and over the IEC/ISO protocols.

The message TimeSync defines the time and time synchronization model which shall

provide the UCT2 synchronized times to applications in the server and client substation IED.

The model of synchronization is based on Simple Network Time Protocol (SNTP) version 4,

which is an adaptation of the Network Time Protocol (NTP) message format. This version can

operate over IP version 6 and OSI addressing. The time value is the count of seconds relative to

0h on 1 January 1900 [25].

Figure 6 illustrates the communications modes on SAS levels in order to summarize and

combine all information described in this subsection. The MMS communication stack is used

between all devices in the substation, while GOOSE and GSSE are used only between IED and

TimeSync between IED and the computer station. The SV message is only used in messages

from sensor to IED because the reverse communication is via MMS.

2 Is the primary time standard by which the world regulates clocks and time

http://en.wikipedia.org/wiki/Time_standard

15

Figure 6: Communication modes on SAS levels [18]

2.3. WIRELESS COMMUNICATION IN IEC 61850

The IEC 61850 standard requires that communication between the devices must be wired

via fiber optic or cable. This is so for they are mature technologies that ensure compliance with

the requirements of the standard [15]. However, great strides have been made in new types of

wireless systems in the non-industrial arena, in particular, data over cellular phone systems and

the IEEE 802 series. Personal and commercial wireless data communications systems are

becoming widespread, with increasingly mature technologies and standards, as well as

decreasing costs. As such, they offer the benefits of inexpensive products, rapid development,

low cost installations widespread access, and mobile communications which wired technologies

and even the older wireless technologies often cannot provide. In addition, with cyber security

becoming of greater importance to the power industry, the newer wireless systems are including

improved security technologies such as in the IEEE 802.11i standards.

New wireless standards are being developed in the IEEE 802.11, 802.15 and 802.16 series

which could become very useful in certain electrical substations applications. Wireless

technologies have a number of vulnerabilities related to the impact of noisy electrical

environments on wireless media, the reliability of the commercial wireless equipment, the

consequences of many users in the unlicensed frequencies and influences the performance for

time-sensitive data, as well as the security of communications [26].

Currently there are five different types of wireless technologies considered more mature and

capable of meeting the requirements of the standard. These technologies are: WiFi – IEEE

802.15.1; Bluetooh – IEEE 802.15.7; ZigBee – IEEE 802.15.4; Wimax – IEEE 802.16 and

Cellphone – Group Speciale Mobile (GSM) [26]. As illustrated on Figure 7, these technologies

16

are the most common for wireless communication, which are compared in terms of

cost/complexity, power consumption and data rates.

Figure 7: Performance of the wireless technologies [27]

The sizes of data packets to be transmitted between devices on the electrical substation are

low size, from 80 bytes to 1000 bytes depending on the number of devices to control and

monitor [28]. Therefore, is not necessary to have a technology that has a high data rate but one

that can guarantee that the requirements for communications between devices are met as well

as low cost. Verifying the Figure 7, the technology that possesses all these characteristics (low

cost, low complexity and enough data rate) is the IEEE 802.15.4 with ZigBee.

The IEEE 802.15.4 defines the Physical Layer (PHY) and MAC layer, while the ZigBee

covered the higher layers. This technology was designed to address the need for a low-cost and

low-power wireless solution and has become a solid foundation for monitoring and controlling

the network. However, before implementing the IEC 61850 on this technology, it shall be

necessary to analyze the ability of this technology to meet the requirements of IEC 61850

standard. The following subsections will describe the IEEE 802.15.4/Zigbee and offer an

analysis of this technology when applied with the IEC 61850 standard.

2.3.1. IEEE 802.15.4 AND ZIGBEE

The IEEE 802.15.4 is used to investigate a low data rate solution with multi-month to multi-

year battery life and very low complexity. The specification for PHY defines a low-power spread

spectrum radio operating at 16 channels in the 2.4 GHz with a basic bit rate of 250, 40 and 20

Kilobits per second [26]. Alternately, PHY specifications have 10 channels for 915 MHz and one

channel to 868 MHz that operate ate even lower data rates, but they are not as popular [29].

The IEEE 802.15.4 defines 2 types of devices. A device can be a full-function device (FFD)

or reduced-function device (RFD). A network shall include at least one FFD, operating as the

17

PAN coordinator. The FFD can operate in three modes: a personal area network coordinator

(PAN), a coordinator or a device. An RFD is intended for applications that are extremely simple

and do not need to send large amounts of data. An FFD can talk to RFD or FFD while an RFD

can only talk to an FFD [29].

The 3 types of network topologies that ZigBee supports are: star, peer-to-peer and cluster

tree topologies [30]. In star topology, the communication is established between devices and a

single central controller, called the PAN coordinator. The peer-to-peer topology is also a PAN

coordinator. In contrast to star topology, any device can communicate with any other device as

long as they are in range of one another. Cluster-tree network is a special case of a peer-to-

peer network in which most devices are FFD and an RFD may connect to a cluster-tree network

as a leave node at the end of a branch.

The communication between network nodes is done according to either the beacon3 mode

or the non-beacon mode, using the Carrier Sense Multiple Access with Collision Avoidance

(CSMA/CA) method for MAC. The main distinction between both modes is the existence of a

mechanism that allows for a synchronized/organized communication between the network

nodes [29].

ZigBee builds upon this IEEE 802.15.4 standard to define the network, security and

application framework layers, profiles that can be shared among different manufactures to

provide system-to-system interoperability. It specifies the management of multiple IEEE

802.15.4 nodes, and theoretically can manage up to 64,000 ZigBee devices. Distances between

devices range from 10 to 100 meters, depending on power output and environmental

characteristics.

ZigBee Network Layer extends the IEEE 802.15.4 MAC layer by defining an addressing

scheme and a routing mechanism. It also provides authentication and encryption. ZigBee

defines types of nodes based on their role rather than on hardware capabilities (FFD and RFD).

A ZigBee network has a coordinator, routers and/or end devices. The coordinator is the

responsible for starting a network and for choosing/setting some key network parameters. The

router is a device capable of extending the network coverage. It routes messages between

other devices and enables new devices to associate with it. Finally, the end device only

associates with one device at a time, cannot directly exchange messages, and can sleep in

order to save power.

The network topologies adopted by the ZigBee are star and cluster tree topology, already

defined by IEEE 802.15.4 and mesh [29]. The mesh topology defines any node which can also

serve as a router for other devices on the network. The security includes the spread spectrum

security characteristics at the physical layer, and provides key establishment, key transport,

encryption of messages using AES, and network management. This level is quite strong, but

cannot provide total end-to-end communications. It only covers the media and a part of the

transport.

3 Is a frame periodically sent by the coordinator, without CSMA/CA, and retransmitted by the routers

18

Recently, the 6LoWPAN (IPv6 for Low power Wireless Personal Area Networks) Working

Group (WG) has provided a new adaptation layer between the IPv6 network layer and the IEEE

802.15.4 layer allowing direct communications between devices (e.g., 6LoWPAN) and an

external IP-based network (e.g., Internet). In fact, researchers and Engineers have provided uIP

(micro IP), which is an implementation of the TCP/IP protocol stack dedicated for small 8-bit and

16-bit microcontrollers [31].

2.3.2. ANALYSIS OF IEC61850 OVER ZIGBEE AND IEEE 802.15.4

The standard specifies that communication should be done via cable or fiber optics [28].

However, the standard does not exclude the entry of new communication technologies,

requiring only that they meet the requirements [15].

At present, positive results on the performance of WSN in electrical substations by several

research projects have been reported. However, these projects did not opt for ZigBee

technology integrating IEC 61850, [32] and [33]. The development of WSN projects using

ZigBee technology and integrating the IEC 61850 are at an early stage of defining the structure

of the WSN, how to integrate the IEC 61850, and evaluate the performance of the WSN through

simulations. This research on the performance of IEEE 802.15.4 based WSN was recently done

by a team of engineers from the University of Sannio, Italy [34]. The network performances have

been assessed by simulating the WSN behavior in supporting data exchange based on the

standard IEC 61850. However, it is not specified what kind of communication stacks are used or

what equipment was tested.

Based on these simulation results, the ZigBee technology cannot guarantee certain

transmission times for the communication stack, SV and GOOSE/GSSE defined by IEC 61850

[34]. There is a range between 11 ms to 2500 ms for the same test situation, which requires that

this technology can only be used for MMS communication [34].

The time transmission values of this technology depend on many factors, such as the

distance between devices from other devices, competition from other devices that communicate

with the 2.4 GHz, electromagnetic noise, the adoption of this method to access the channel

(CSMA/CA) and the number of devices on network [35], [34] and [36]. Of the many

investigations carried on this problem, the values of transmission times in normal situations are

around 10 ms to 120 ms, but in the worst cases can get to one second, becoming more limited

in the integration of IEC 61850 in ZigBee [35].

The GOOSE, GSSE and SV, the maximum value of transmission is either 3 ms or 10 ms.

ZigBee currently does not warrant or keep these values [9]. In addition to this factor, which

prevents ZigBee from integrating with the GOOSE and SV, both have priority tags and the

channel access method used is a CSMA/CD method which does not adopt by IEEE 802.15.4.

The TimeSync message adopts as its transport layer the UDP/IP, which can also be

implemented by 6LoWPAN, but have accuracy in the order of microseconds. The only

communication stack that has a transmission time for which ZigBee is able to meet, even in the

19

worst cases, is MMS. However, the transport layer of MMS uses TCP/IP, which is not supported

by ZigBee. Yet, the adoption of 6LoWPAN makes it so that MMS becomes the only possible

communication system that can run over ZigBee.

In the near future, it is foreseeable that the necessity to create special functional

requirements for the wireless equipment and technologies to provide the necessary reliability,

availability, and security of communications for different power system functions in the electrical

environments becomes evident. In addition, guidelines for using wireless need to be developed

so that users can better understand the issues, pros, and cons of implementing wireless

communications for different applications.

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3. WIRELESS SENSOR NETWORK FOR MEDIUM-

LOW VOLTAGE SUBSTATION

The IEC 61850 standard covers all types of TS or DS. Currently, almost every electrical

substation of High Voltage and Medium Voltage is automated, but some specific DS are not

automated. The main reason is the cost is higher than the substation itself. This is the case of

distribution substations which convert medium voltage to low voltage (SS), in which the cost of

one IED is more than one low voltage substation.

In the current system from the electricity grid, automated substations and SCADA cannot get

information about electrical conductors that connect the distribution substations of MV with SS.

The values of short-circuit voltages and load impedance at the ends of the conductors can only

be acquired in SS. In addition to this acquisition of values, it is necessary to control and monitor

substation devices, such as to check the status of the fuses or to control the contact of different

SS from a single location. Once the communications between SS, SAS and other substations

are established, information can be completed by means of auxiliary data about the operation of

SS. This data can be, for instance, positioning of contacts, values concerning the ambient and

transformer temperature, as well as substation door positioning [10]. If an alarm indicates that

the substation door is open the operator in the control center, can investigate the alarm reason,

in order to determine if there are intruders or maintenance workers.

The automation of SS and their integration into the smart grid of electrical substations

permits the detection of technical failures or other problems and the ability to solve them without

human interference. The automation system applicable to this substation must be cheaper than

the investment cost of itself. The system must have simple and reliable operations for any

specialized operator to understand its functioning, as well as cheap sensors and low cost

installation.

In some substations, is difficult to install new cables due to the existence of walls inside,

increasing the cost of installation [10]. To reduce the number of cables and the cost of

installation in the SS, wireless communication can be a good approach. Besides these

advantages, it allows the operator to get access to devices anywhere in the SS.

ZigBee technology has many advantages of relevance, such as the relatively cheap price

and the size of its modules, small enough that they permit adaptation or embedding in devices

of low implementation complexity [37]. However, ZigBee does not meet all IEC 61850

requirements for timing and format communication. As such, this technology can only be

adopted in the transmission of non-critical data, while using fixed wiring for critical data [10].

Despite these limitations, this study intends to use the IEC 61850 over ZigBee to standardize

communication throughout the smart grid, reducing compatibility problems of communication

between substations and SCADA. Even within the substation, communications must be based

on IEC 61850 so that any operator may understand the automation system.

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3.1. CHARACTERIZATION OF THE SS

The SS may be installed either indoors, in rooms specially built for the purpose, within a

building or at an outdoor installation, which could be:

Installed in a dedicated enclosure, prefabricated or not, with indoor equipment

(closed cabin) and can be found at a location near a village.

Pole mounted with dedicated outdoor equipment, located on the top of the

distribution poles.

The indoors and closed cabin substations are essentially constituted by three blocks:

medium-voltage switchgear, distribution transformer and low-voltage switchboard as shown on

Figure 8. The medium-voltage switchgear controls the contact break of the MV conductors. The

distribution transformer convert medium to low voltage. The third consists of low-voltage

switchgear used to control the contact break of the conductor’s LV [38]. The pole mounted

substation is composed for one transformer and uses fuses for their protection.

Closed cabin or indoor substations occupy the area of a small room or even a container,

which cost around 10 thousand euros, and can reach up to 50 thousand euros [39]. The pole

mounted is very small, its volume is similar in size to a washing machine and the cost is around

400 to 1500 euros [37]. The SS that have higher priority to be automated are closed cabin and

indoor substations because they have more equipment, demand more investment, receive and

control the energy generated by environmentally friendly power generation and have a door that

allows any human being to enter.

Figure 8: Communication system in a SS [10] Figure 8 illustrates the constitution of a substation, the process of automation and

communications with the outside. The automation process consists of the control and

monitoring of devices, so that they are observed and acted, as described on Figure 8.

22

Observation consists of the following; acquiring current and voltage values as well as short-

circuit voltage values and load impedance in MV or LV conductors; current and voltage values

in the transformer; auxiliary values, such as temperature, atmospheric pressure, door

positioning. Actions consist in the altering of operations in devices, such as transformer control,

breaker or separator switches of LV/MV conductors [38].

Actions and observations cover all operations of SS. These operations range from the

electric conductors that enter and leave the substation to the control and protection of MV

conductors in the connection to the transformer, to the control and protection of the transformer

itself and the control and protection for the LV conductors. However, it is necessary to have

access to operations that manage auxiliary data outside the operation of devices such as

temperature or humidity, door position control, all of which are outside factors that may influence

the operations of the SS. The smart grid, which involves SCADA and all other automated

substations, allows faults to be detected in a substation, making it so these flaws do not affect

the operations of other substations and having the ability to resolve these failures without

human interference.

The actions and observations can be classified in two classes, critical and non-critical. The

distinction of both depends of the influence directly or indirectly exerted on the operation,

protection and monitoring devices in the substation. Critical data and commands directly affect

substation operation, such as control of devices such as act as the smart breaker separator and

the controllable transformer and observe the values such as current, voltage and short circuit.

The acquisition of auxiliary values to the operation of the substation such as environmental or

transformer temperature, door position and others not directly associated with the operation of

the substation are considered non-critical data.

The IEC 61850 does not define special requirements for these SS. Therefore, the same

requirements applied to high and medium voltage substations are used for the SS. The critical

data and commands, in accordance with IEC 61850-5 [15], requires transmission times below

10 ms to ensure the protection and proper functioning of the substation devices. For non-critical

data, transmission times range from 50 ms, 100 ms, 500 ms, 1000 ms and 5000 ms, depending

on the functionality of each data [15]. Considering these transmission times, the wireless

communications almost guarantee compliance with the non-critical data transmission time

requirements. For critical data and commands wired communication is adopted.

Critical data, commands and non-critical data are transmitted to a control device capable of

evaluating and acting in the operation of devices. This control device is known as substation

controller, which permits communication with the outside, as shown on Figure 8.

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3.2. ARCHITECTURE

The communication architecture of SS is supported by a wired and a wireless network.

Wired communication is used for critical data and commands, while a wireless sensor network

(WSN) is used for non-critical data. For the WSN, the requirements and structure of

communication between the devices is defined based on “Active Substation” project [10], which

are the scope of this thesis.

The architecture of the automation system for the SS is composed by a gateway, a

substation controller and the sensors, outlined on Figure 9. The gateway allows receiving and

sending of data to and from other automated substations and SCADA. The substation controller

sends and receives data through the gateway and the sensors, assessing and managing the

obtained information. The sensors allow acquiring data information of the operations of the SS.

Assignment of the LD and LN to devices of the SS is based on IEC 61850. The sensors

correspond to the LN and the collections of various sensors are the LD, as all sensor are each

independent and each LN corresponds to a different LD. The substation controller corresponds

to an IED, because the operating principles of both are very similar.

Figure 9: Architecture of the WSN on SS

Figure 9 illustrates the architecture of the WSN on the SS automation, based on the “Active

Substation” project [10]. The WSN is constituted by three sensors and the substation controller.

Communication between the other devices of the automation system is effected via cable.

Throughout the automation framework all communications are bidirectional.

Devices concerned with door positioning ensure that the life of an ordinary citizen is not in

danger. Another deals with temperature and pressure values of the transformer, which, while

not directly influencing the functioning transformer, can prevent it from being damaged when

there is an oil leak. Yet another deals with meteorological conditions within the substation, in

order to prevent possible accidents that affect the functioning of substation devices (e.g. excess

heat and humidity).

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The definition of the categories of LN for different sensors and the data that is to be

transmitted by them are based on IEC 61850, described on chapter 2 and [20].

The data acquired from the transformer is housed in the group of sensors and monitoring.

Within this group, there are two types of LN, where the difference lies in the characteristics of

the transformers. The SIMG (Sensor Insulation Medium Gas) is assigned to transformers with

gas insulation and the insulation liquid is defined by SIML (Sensor Insulation Medium Liquid)

[20].

The device that controls the position of the door and the device that tends to acquisition of

values about the environmental temperature of the substation are not covered by the IEC

61850, yet the standard permits the addition of new LN which may tend to these functions.

The standard suggests in [20] insert de sensor of position door in the "Automatic Control"

group and device that acquires meteorological data inserts in the group "Further Equipment".

The naming of the LN may be assigned by the operator, but in a way that is both within reason

and compliant with the rules established by the standard. The name of LN specified for the

control of the position door adopted are "ADEC" (Automatic Door Entrance Control), while the

other LN is named "ZMDS" Further Meteorologist Device Substation equipment.

Integration of LN and the substation controller on WSN consists in assigning each LN and

substation controller to the corresponding node of the WSN. Each LN corresponds to a router of

the WSN, while the substation controller that coordinates and controls a set of LN is assigned

the task of coordinator of the WSN, as shown on Figure 9. The LN is a router and not an end-

device because only the router has the capability of re-transmitting the messages, therefore

ensuring that all messages reach their destination.

3.3. WSN COMMUNICATION

For the WSN communication, it was decided to use the semantics of the IEC 61850

communication stacks, as well as, the communication structure, in order to maintain

compatibility with other automation devices in the SS, with other automated substations and

with SCADA. The MMS, SV and TimeSync communication stacks are used.

As the WSN only manages non-critical data, MMS communication stack can be used.

Coding and decoding of MMS messages are more complex, requiring more processor capacity.

Also, MMS use client/server communication, making communications slower. This

communication stack is used for transporting large data packets, monitoring devices, requests

and for the transmitting of non-critical data. The MMS will be used in this WSN to monitor the

sensors, so as to order the stopping of data sending or to change the period of data acquisition.

Although it is suspected that the performance of ZigBee does not meet SV timing

requirements, so the SV communication message is implemented, in order to evaluate it [34].

The SV message type is easy to encode / decode has small size, and its communication mode

can be unicast and or multicast. The SV format is ideal for transmitting data acquired from

sensors to the substation controller (critical data) via a quick message, but not adequated due to

25

the timing requirements. This type of message is used developed with the sole purpose of

transmitting data between sensors or actuators and the IED.

The TimeSync message is used to synchronize all devices. It has an accuracy of

microseconds, being, essential in a network to avoid timing differences between devices. The

synchronization system for IEC 61850 is the same used to stncronize the computer clocks in the

global internet.

As we are considering only one secondary substation, where only exist one substation

controller, it does not make sense to use the GOOSE/GSSE communication stack.

Figure 10: WSN communication

Figure 10 illustrates the complete WSN communication system, the constitution and the

definition of message formats between the LN and the substation controller. The WSN consists

of only four nodes, one of which corresponds to a substation controller, and three LN.

Temperature values, pressure of the transformer, environmental temperature and door

positioning data are sent from LN to the substation controller using the SV message. The

substation controller’s monitoring of the LN and the requests to update date and times are

defined by the MMS format. For the synchronization of all devices, the TimeSync message is

used. The substation controller is only synchronized by external sources such as the Global

Position System (GPS) [28].

The organization and structure of data communications between the modules shall be

covered in detail in the next subsection.

3.4. PROTOTYPE IMPLEMENTATION

A prototype was developed to analyze the implementation of the IEC 61850 on the WSN.

The ZigBee model used, (Figure 11) in addition to performing the duties, for which it was

developed, has the ability to emulate sensor data. These modules have already been

assembled and tested for this WSN, based on PCBs (Print Circuit Board) previously

manufactured [29].

26

Figure 11: ZigBee Model

Figure 11 illustrates the ZigBee model available which is used on this WSN. These modules

include an integrated circuit, an external antenna and a power supply. The ZigBee integrated

circuit is a CC2430 from Texas Instruments, consist of: a 8051 microcontroller, operating at

maximal frequency of 32MHz; up to 256 kB of flash; 8 kB of RAM; 3 general purpose timers (2

with 8 bits and 1 with 16 bits) and a 1 timer for the MAC layer; a DMA controller with 5 channels;

a 12 bits ADC, 3 test-points and a AES co-processor and 2 UART [29]. They also provide two

serial interfaces (UART and RS232).

At the beginning of the projections on this project, there were insufficient ZigBee modules to

implement WSN. However there are six PCBs in the laboratory, ready for mounting

components. During the welding process, several errors occurred, the result being that all

devices became inoperable. The problems consisted of components with the terminals short-

circuited and grinding welds components. A model available after the repair process is shown

on Figure 11.

The ZigBee module was programmed in C language using the IAR Embedded Workbench

v7.30b. The setback of this module is that the development platform does not support 6LowPan

technology, so is not possible to transmit data over TCP/IP as required by the MMS

communication stack.

3.4.1. CREATION OF THE WSN

The main application for the creation of WSN using ZigBee technology is already designed

and tested in previous projects. However, to make the integration of the application into each LN

and substation controller it was necessary to restructure the main application yet keep the basic

structure of the main application. This application defines the creation of a ZigBee network, the

process of transmitting or re-transmission of data and hardware configuration [29].

27

The process of creation of the WSN consists in the definition of the router and the

coordinator. In case a network does not exist, the WSN coordinator waits for a node to be

connected to it in order to create the WSN. If the WSN network exists, it adheres to the

coordinator and works as a secondary coordinator. The router’s function is to check if there is a

WSN network compatible. If it exists, the router adheres to or creates a WSN. The main

application defines, for each message, a specific format which allocates 85 bytes of message

space as a packet for the transmission of data. In this package of 85 bytes, the semantics of the

communication stacks defined by IEC 61850 will be transmitted. The mechanism of

transmission/re-transmission used by the main application ensures that all messages reach

their destination. In order to ensure no data loss in the transmission process, the adopted

system sends a specific flag to confirm that the message arrived. This flag is called ACK and

the transmission system operates as follows; if the router module sends a message to the

coordinator, it shall constantly continue to send that message until it receives a positive ACK

from the coordinator [29]. Finally, the main application configures hardware such as RS-232

interface, test points and others.

For each application of the LN or substation controller, a unique application was developed

that integrates the specific application of each LN or substation controller within one main

application. This structure is based on the creation and management of events1. As previously

mentioned, the IEEE 802.15.4 with ZigBee technology does not allow priority tags as required

by IEC 61850 for some communications stacks. However, this events system, once adopted,

allows certain features to have higher priority than others. For both applications is to create a

closed cycle of events that is constantly being performed, until the activation of each event for

the performance of its designated functions. The priority assigned to each event depends on a

sequential hierarchy, in which the function first chosen by the operator will be performed before

any other is set in motion. In addition to this similarity between the applications to the LN and

substation controller, the main distinction between them is mode of operation on network and is

characterized by:

LN

o The WSN contains three routers and each corresponds to a specific LN,

(SIML, ZMDS and ADEC). The structure of the application for both LN is

very similar, the difference residing the size and content of the messages.

The main function of these LN nodes is to transmit values and evaluate data

by emulating data acquisition. The transmissions of this data will be in

accordance with the semantics based on the SV communication stack

established by IEC 61850.

In each LN all emulated data is evaluated and if such data exceeds the

limits set, it triggers an alarm and then sends a message to the substation

1 Technical terms, that represents an occurrence that performs one or more functions

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controller. In cases that there are no alarm situations, the LN sends data

periodically to update the substation controller database.

To update date and time, each LN makes a request to the substation

controller when joining the WSN and then makes it periodically while the

network remains in LN.

Substation Controller

o The substation controller shall act as the coordinator of the WSN. It shall

create, manage, monitor and control the WSN. This application allows

controlling and monitoring of LN parameters, creating and managing a

database of WSN. It is the only device that can communicate with the

outside. The database created then stores the received data and the

identifiers/addresses of each LN. The external communication is performed

by the RS-232 interface, which allows updating date and timing via GPS,

acquiring data from LN and sending commands to the LN in more detail in

the Annex B.1.

All features described above for each LN and substation controller can be found in more

detail in the Annex B.1. The orders of execution of the functions in the automation system at

MLVDS, illustrated on Figure 12 are:

1. Establish communication of the substation controller with at least one LN router

(SIML, or ZMDS and ADEC) in order to create the WSN.

2. Each LN router makes a request to the substation controller for the update of the

date and time.

3. Each LN router analyzes the simulated data while the substation controller is waiting

for new instructions.

4. The LN router sends data and continues to analyze the simulated data.

5. The substation controller receives data and updates the database.

6. The substation controller sends a message to get or change parameter settings for a

specific LN router.

7. The LN router receives the request and either sends its outcome or confirms the

change of parameters to the substation controller. It continues to analyze data

afterward.

8. The substation controller receives data and updates the database or communicates

with the outside, resting in standby mode afterward.

The structure of communication between the modules of the WSN in the automation system

is via unicast. This mode only allows communication between two specific modules. However,

when it is necessary to send the same message to several modules from the substation

controller using the multicast method, this WSN cannot support it. In order to send the same

29

message for all devices of the WSN from the substation controller, the messages will be sent

sequentially via unicast.

Figure 12: The principal flowchart of the WSN

30

3.4.2. DEFINITION OF COMMUNICATION MESSAGES IN WSN

This topic shall seek to describe all the options taken and the description of the format of the

messages. The contents of the messages are based on the semantics of communications

stacks set by IEC 61850. All technical details are described in the Annex B.2.

3.4.2.1. DATA TRANSMISSION THE LN TO LD USING SV MESSAGE

This message is used to communicate the LN to the substation controller and its format is

based on the SV communication stack to transmit data acquired by the sensors (these being

emulated), via unicast which is described in the part 9-1 of the IEC 61850 [22]. Each LN

contains different data and the resulting messages of each LN are different. The IEC 61850

defines, for each LN, a data set to include in the message. All data types used in each LN of the

WSN are described in Annex B.2.1.

The data frame which contains the data, according to IEC 61850, is defined by IEC 60044-8

[22]. Figure 13 describes the universal data frame used in both LN.

Figure 13: Contents of the universal data frame [40]

This standard defines a data frame universal with data set already defined. However, this

data set does not include some data which are defined for these specific LN [20] and [40]. To

bypass this problem, the IEC 60044-8 establishes two different data sets, one in which data is

31

already defined and the other in which the discretion of each operator defines the content of the

data set. In conjunction with IEC 61850, a data set is established that meets all the

requirements of both standards. These data sets are described in more detail, for each LN, in

Annex B.2.1.

The data set is the type of specification that distinguishes the messages of each LN. The

other specifications of the data frame are the same for both LN. The contents of each data

type’s specification of the data frame are defined as follows [40] and [22]:

o Length of Data Set – Number of bytes of data set

o LNName – Identifier of the logical node name not defined by the IEC 61850

and thus awarded according to the order described by the IEC 61850. For

the specific cases of SIML, ADEC and ZMDS, the identifiers are: 67, 92 and

93 respectively.

o DataSetName - The unique number to identify the structure of the dataset,

i.e. the data channel assignment. The number that corresponds to a new

data set is 254 [40].

o LDName – Unique identifier of the logical device name in the WSN used to

identify the source of the data. The identifier is assigned according to the

example of the IEC 61850 part 9-1 and is 0xFF01 [22].

o Rated phase current, neutral current, phase voltage and delay time - In

WSN there are no LN that have sensors which acquire the data, so it was

decided to exclude this data and occupy the space they would usually take

with data from the data set.

o Data Set - The data set that has data acquired by the sensors, in which

each value corresponds to a “datachannel” [40]. The number of

“datachannels” corresponds to the number of data required by the IEC

61850 of each LN.

o Status Word 1 and 2 – Data that describe the status of the LN, such as

alarm indication or if the data acquired are valid.

o SmpCnt - Indicator of how many messages have been sent.

The last two bytes of the data frame are reserved for the IEC 61850-9-1. These fields were

used as indicators of transmission times during the tests. Each data frame is inserted in the

space reserved by the IEEE 802.15.4 for data transfer. The final data frame for the LN; SIML;

ZMDS and ADEC are 48, 24 and 22 bytes respectively. Any of the data frames exceeds the

space limits, which means that only one message is needed to transmit all data. That allows

messages to be transmitted with the lowest transmission time.

Although this communication only uses the semantics of the SV communication stack and

does not meet certain requirements set by the IEC 61850, but the transmission times in a real

environment do not exceed the 10 ms required by IEC 61850.

32

3.4.2.2. MONITORING OF LN, USING MMS MESSAGE

For the monitoring of the parameters of the LN from the ZigBee Coordinator, such as

controlling the sample rate of data sending and data acquiring, as well as for starting or

stopping of message sending, the IEC 61850 defines a model of transmission which is shown

on the figure [41].

The information exchanged shall be based on client/server mechanisms and the method

used to exchange sampled values between a client and a server is the unicast sampled value

control, as described on Figure 14.

The client sends requests to set or get parameter values defined by the ZigBee Coordinator;

the server receives requests and sends responses to the client, to confirm the change or relay

informative data [41].

Figure 14: Model communication to monitor the devices [41]

Figure 14 illustrates the model for control and monitoring of the parameters of LN consists of

two blocks; Sampled Value Control (SVC) and DATE:SAV [41]. The SVC in the LN, shall be

used to control the communication procedure and receives requests to change the parameter

settings; the DATE:SAV has information about each data value of data set and receives

requests from the substation controller, requesting information from them [41].

The unicast sampled value and the services are mapped to a MMS unicast sampled value

control block as define. As described above, the MMS at the transport layer requires a means of

data transmission using TCP/IP and some tags notation harmonized with the MMS grammar

used in IEC 61850-8-1 [21].

For the transmission of sampled monitor values, the ASN.1 basic encoding rules will be

used, to encode and decode the MMS messages. However, the entire model of message

33

encoding is too heavy to implement in models of low processing [42]. As such, only parts of the

system shall be used. The example of frame structures for the monitoring cases of LN,

illustrated on IEC 61850 described on the Figure 15 is used [23].

Figure 15: Example for ASN.1 coded frame structured [23]

Figure 15 illustrates the sequence of data in the construction of the message. The sequence

begins with indicating the total size of the message and ends with the values of the data-set.

Each data has its value tag as attributed by the ASN.1, the total size of its data and its

respective value, all in hexadecimal. The field of data parameters and data set of LN is defined

by various types of data as described in Part 9-2 of IEC 61850 and are characterized as follows

[23].

o Sequence of Data Message - the total size of the sequence data

o svID – A unique identification of the sampled value buffer related to the

sampled values.

o Datset – Specifies the reference of the data-set whose values of members

are to be transmitted in the Unicast Sampled Value Control Block (USVCB)

message.

o confRev – Contain a count of the number of times that the configuration with

the regard to the USVCB has been changed.

o smpRate – Sample rate from the instance of USVCB.

o Sequence of data - Corresponds to the data set of the LN.

For each field of data the IEC 61850 defines a specific type of data format, which can be

viewed in more detail in Annex B.2.2, as well as the entire contents of each message request or

response.

34

Despite this, the size of these messages is over 85 bytes, which is the space reserved for

data transmission by IEEE 802.15.4. The size of each message is less than 190 bytes, which

implies that it has to be divided into two, a process that was not developed by the main

application. Due to this division, each complete message is sent in two bursts, which implies

that the transmission time is greater.

The transmission times defined by IEC 61850 for these cases of monitoring must be less

than 1000 ms, to which the ZigBee modules may comply even by transmitting the message in

two bursts. The total transmission time is not set by IEC 61850 in regards to situations when the

substation controller sends the same message via unicast for all devices of the WSN. The

general consensus is that the total time of data sending for all devices must be less than 5000

ms maximum value for MMS.

3.4.2.3. SYNCHRONIZATION ALL DEVICES USING TIMESYNC MESSAGE

In a WSN, it is necessary that all modules are synchronized with the precise same time and

date. The communication mode for this synchronization function in the WSN is the unicast mode

and the request is used MMS format. The LN of the WSN sends a request to the substation

controller and awaits a reply from which it. Then transmit a synchronization message with the

exact time, roundtrip delay and local clock. The requests are performed the moment one is

joined to the WSN, being conducted periodically afterward [25].

Like MMS message, this message uses semantics of the SNTP protocol because it has a

transport layer via IP and UDP, not supported by these modules. The protocol defines how to

format date and time values, the count being in seconds relative to 0h of the 1rst of January

1900 [25]. The data frame adopted is composed by 13 different types of data and the total

message size is 48 bytes adopted by the SNTP protocol, as shown on Figure 16 [25].

Figure 16: Data frame of synchronization message [25]

35

The data frame illustrated on Figure 16, the function of each field described is briefly

summarized below [11];

o Leap Indicator – This is a two-bit code warning of an impending leap second

to be inserted and deleted in the last minute of the current day.

o Version Number – Indicator of NTP/SNTP version number

o Mode – Indicator of the type of mode. In unicast modes, the client sets this

field to 3 and the server to 4.

o Stratum – Type of stratum level of the local clock.

o Poll Interval – The maximum interval between successive messages in

seconds. The example of the IEC 61850-9-1, set in its annex suggests the

value of the range of 1024 seconds [22].

o Precision – Indicator of the precision of the local clock.

o Root Delay – Represents the total roundtrip delay to the primary reference

source in seconds.

o Root Dispersion – Represents the nominal error relative to the primary

reference source in seconds.

o Reference Identifier – Indicator of the external reference, the most common

and adopted, is the GPS.

o Reference Timestamp – This is the time at which the local clock was last set

or corrected.

o Originate Timestamp – This is the time at which the request departed the

client for the server.

o Receive Timestamp – This is the time at which the request arrived at the

server.

o Transmit Timestamp – This is the time at which the reply departed the

server for the client

The contents of the fields of each component in the SNTP client and server, the requests

and response operations are pre-defined. The data assigned for each field and its components

are detailed in the Annex B.2.3.

The SNTP protocol does not establish requirements for transmission times, yet the IEC

61850 requires accuracy in the order of ‘µs’ for the SV messages. The hardware of the ZigBee

modules does not have timers with this kind of precision; they only have timers in the order of

milliseconds, which makes it impossible that they comply with the requirements set by the

standard.

The transmission times defined by IEC 61850 for date and time update requests from LN

to substation controller must be less than 1000 ms.

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3.4.2.4. EMULATION

As described above, due to hardware limitations, it is not possible to acquire data from

sensors. Taking this into account, data emulation was developed, in order to test the

performance of WSN. Each LN has an emulator in which data is generated according to pre-

established times, transmitted with differing sample rates.

The SIML sensor is considered the most critical module in WSN. This is so due to the fact

that when there is a short circuit in the transformer, in 2 seconds it reaches high temperatures,

and at 4 seconds it reaches temperatures that damage the equipment of the substation

[43].Taking account of the these time values, the emulator of the SIML is defined to generate a

new value every 500 ms, emulating the acquisition of data from the sensor. The normal

temperature of a transformer when in operation ranges between 50 and 60 Celsius degrees

[43]. If the emulated value exceeds 60 Celsius degrees, an alarm is activated, post which a

message is sent every 500 ms to the substation controller until the temperature returns to

normal. If the emulated data does not exceed the maximum limits of temperature, it sends the

last simulated value after ten “acquisitions” to update the database of the substation controller.

The ZMDS sensor has as emulation model very similar to the SIML, except that the values

of sample rate differ. The temperature of the substation is influenced by the energy released

during the operations of equipment or external factors such as fires outside of the substation.

The variations of environmental temperature take more time to reach higher temperatures, with

high levels of temperature taking up to 10 seconds to settle [44]. The conditions of temperature

required for the transformer’s correct functioning depends on the manufacturer yet, to ensure a

smooth operation, the temperature should not be lower than -25 or higher than 40 Celsius

degrees respectively [44]. These values are simulated every 2 seconds, with the aim of

ensuring that possible variations are transmitted. If the temperatures exceed the limits, the LN

sends an alarm message every 2 seconds until it returns to the normal temperature. If it does

not, it sends data after 10 “acquisitions”, to update the database of the substation controller.

The substation controller does not emulate any value; it is the device that allows evaluating

the state of the WSN and acting on it based on those evaluations made. Communication is

performed by the RS-232 interface and allows the coordinator to send commands to the WSN,

update date and time values and draw data from the database.

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4. EVALUATION TESTS

The tests were performed in laboratory and in live environments. The WSN was installed in

two different SS in order to verify how electromagnetic noise affects its operation. The two SS

are closed cabin-type substations. The tests consist of transmission time measurements of SV

and MMS messages and confirming the messages’ arrival to their destination.

In these tests, the substation controller and the LN were named as ZigBee coordinator and

ZigBee router respectively, because the modules are not connected directly to the substation

devices. At the end of the chapter, a complete analysis of the influence of different

environments on the WSN is presented, as well as a description of WSN deployment in

substations.

4.1. TYPES OF TESTS

Communications between substation devices, using ZigBee, need to guarantee the

IEC61850 timing requirements. In addition to these requirements, it is necessary to ensure that

there are no communication errors and lost data packets in the WSN, regardless of the

environment where it is located [45]. If these requirements are not met, the WSN cannot be

seen as a solution for automation systems for electricity substations based on IEC 61850.

Testing the performance in different substations permits the analysis of the influence of the

environment on ZigBee communications. Different substations can be affected by different

levels of interference caused by electromagnetic noise and type of construction.

The tests performed can be classified into two different types: measurement of message

transmission times, in verification that there are no communication errors and lost data packets

in the WSN, [12] [46]. Analysis of transmission times is only important for SV and MMS

messages. TimeSync messages are not included in these tests because, as previously

mentioned, it is not possible to guarantee microsecond precision using these ZigBee modules,

as referred on chapter 3. Nevertheless, transmission times for the date and time update

requests are included in the tests of the MMS messages.

The tool used to measure transmission times, detect communication problems and count

data packets lost in the network was the packet sniffer. The packet sniffer is a PC software

application used to display and store packets captured with a listening Device, allowing the

tester to get transmission times in the order of microseconds. However, this method to

determine transmission times has a setback; the accuracy of the values obtained depends on

the location of the platform of the packet sniffer in regards to the ZigBee module, that is, the

closer the packet sniffer to the ZigBee module, the lower the error.

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Figure 17: SV message Transfer time

Transmission times are calculated differently for SV and MMS messages. Figure 17

illustrates this process for SV messages. The transmission time is measured from the ZigBee

Router (LN) until arrival at the ZigBee Coordinator (Substation Controller) [15]. The time is

determined from the beginning of the process of sending the message ZigBee Router until the

ZigBee Coordinator gets it properly. As illustrated on Figure 17, transmission time is calculated,

adding the time to process the message (T1), communication time (T2) and time to process the

ACK (T3). The communication time of the ACK is not included. Times T1 and T3 were acquired

by the timers in each module. We obtain T2 by subtracting the two times acquired by the packet

sniffer, which is placed close to each module.

Figure 18: MMS message transfer time

The calculation of the transmission time of a MMS message via unicast is done from the

start of when the request is made by the ZigBee Coordinator until it receives the response, sent

from the ZigBee Router [15]. This process is illustrated on Figure 18.

The total transmission time includes the processing time of sending and receiving of data

in the ZigBee Coordinator, as well as the time of sending, processing and receiving of data in

the ZigBee Router. This calculation process is described on Figure 18. The time of

communication is also a factor in the calculation of the total transmission time. The values T2

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and T6 are determined by the packet sniffer, T1 and T7 by the ZigBee Coordinator and finally

the ZigBee Router calculates the T3, T4 and T5. However, the total transmission time is

acquired by the timer in the ZigBee Coordinator, starting when sends the data and stopping

when it receives the response from their router.

In addition to using the packet sniffer to analyze the number/percentage of successful

delivery of messages, the number of messages sent by the ZigBee Router and the number of

messages received by the ZigBee Coordinator are counted.

In order to see if the number of modules of WSN influences the value of transmission

times, the tests were run for a WSN with one, two and three ZigBee Routers. Due to a lack of

more ZigBee modules, it was not possible to insert more nodes in WSN. This makes these

results inconclusive for WSN with multiple nodes, given the fact that, in the application of WSN

in live situations, the SS is likely to have, in its WSN, more than three nodes.

These tests permit analysis of the transmission times for a WSN with one, two or three

ZigBee Routers in function on the order of sequence of sending messages in the WSN. They

also permit the analyzing of possible retransmissions between ZigBee modules, send attempt

failures and communication errors.

The retransmission of messages does not occur due to the small size of closed-cabin

substations. Electromagnetic noise is the main factor which may bear influence in the

occurrence of this phenomenon. In both substation and laboratory environments, distances

between ZigBee Routers (LN) and the ZigBee Coordinator (substation controller) are similar.

This removes distance between the ZigBee Coordinator and the ZigBee Router as a factor in

the arising of small differences in test results, focusing attention on studying the environment

[35]. For testing purposes, the three ZigBee Router were set towards actively sending

messages at the same time. The modules were placed in areas with a lot of noise such as

those close to transformer and LV switchboard and MV switchgear [45].

The testing equipment was prepared so that the electromagnetic noise does not interfere

with the operation of the PCB, described on Annex C.1.

4.2. LABORATORY TESTS

The dimensions of the laboratory room, the location of each module in the room and the

distances between ZigBee Router and ZigBee Coordinator are illustrated on Figure 19. Annex

C.2 presents illustrative photographs.

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Figure 19: Laboratory environment

4.2.1. SV TESTS

Figure 20 illustrates transmission times for SV messages, depending on message order,

for one, two and three ZigBee Router.

Figure 20: SV message performance in Laboratory

Excluding the 17th message, when three ZigBee Routers are used, and the 24

th message,

for which two ZigBee Routers were used, transmission times range from 7.164 to 12.05 ms.

The difference in values for one, two and three ZigBee Router is minimal, meaning that WSN

behavior is not influenced by the number of ZigBee Routers. The average transmission times

for WSN with one, two and three ZigBee Router are 10 ms, 10.2 ms and 10.3 ms respectively.

The values exceed the 10 ms specified by IEC 61850. Retransmission of messages in WSN is

not verified.

Upon analysis of the higher transmission time of the 17th

and 24th messages, it was

discovered that the ZigBee Router attempted to send these messages when the communication

channel was occupied. For the 17th message, only at the third attempt was the message sent.

Therefore, when the communication channel is busy, the transmission times may rise from 15 to

20 ms, which, for this specific type of data, does not influence the operation of devices in the

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substation. However, for process values (instantaneous values) like current and voltage, it may

compromise protection of the devices. For the 24th, a similar situation occurred. However, the

channel was only occupied during the first attempt.

When the channel is occupied, the ZigBee router enters on standby mode for some time,

increasing transmission time. In WSN with more nodes, these cases occur more frequently and

with possible times greater than 20 ms.

A total of 500 SV messages were exchanged in the WSN. There were no communication

errors or lost data in the WSN. The graph with the results of the tests can be consulted, with

better resolution, in Annex C.3.

4.2.2. MMS TESTS

Figure 21 presents results for tests performed to evaluate MMS messages performance,

including the results for when the substation controller sends the same message to all the WSN

devices via unicast.

Figure 21: MMS message performance in Laboratory

Upon analysis of Figure 21, a large discrepancy in transmission times can be noted for

WSN with one, two or three ZigBee Routers. In cases in which these discrepancies occur, the

ZigBee Coordinator sends the same monitor message via unicast to all ZigBee Routers of the

WSN. The total transfer time for these cases is determined by the sum of time starting from the

ZigBee Coordinator’s first request until its reception is confirmed by the ZigBee router to which it

was destined. Concerning the WSN with three ZigBee Routers, considering the analysis of

messages four, five and six of the Figure 21 as an example, the transmission time for the first

LN that is reported is 39 ms, 82 ms for the second LN and the third is 138 ms. But the total

MMS transmission time is 138 ms. For the WSN with two LN, the transmission time for the first

LN is of 38 ms and 81 ms for the second, data taken from Figure 21 in regards to messages 28

and 29. When these cases do not occur, the transmission times do not depend on the number

of ZigBee Routers.

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As can be seen on Figure 21, for the 16th and 27

th messages, transmission times range

between 29 and 33 ms. With these results, it is concluded once again that the number of

ZigBee Routers that constitute the WSN does not influence the results.

The transmission times for MMS messages are below the 1000 ms required by the IEC

61850. When sending the same message for all devices in WSN, the total time is far below the

5000 ms mark due to the fact that the WSN is composed of few modules and the size of the

data packets transmitted is small. In a SS, it shall not be necessary to have more than ten

nodes in the WSN. Considering that each MMS message takes an average of 35 ms, the total

of 350 ms still does not exceed 5000 ms.

A total of 500 MMS messages were exchanged in the WSN. No communication errors or

lost data in WSN were detected. The graph with the results of the tests is available, with better

resolution, in Annex C.3.

4.3. SS#1 TESTS

This SS, built in 2010 and upgraded with electrical equipment installed in March 20111,

includes new electronic power equipment’s, such as a ring main unit (MV switchgear) and a

low-voltage switchboard.

Figure 22: SS#1 environment

Figure 22 presents a block diagram of the SS, its dimensions and the arrangement of the

ZigBee modules for testing purposes. Annex C.2 presents photographs of the substation and

the location of the ZigBee modules. Due to the greater distance between the devices in the

substation and the existence of protective railings, it was not possible to create an arrangement

of modules as in the laboratory. Greater priority was given to placing the modules closer to the

substation devices. The method of analysis for these tests is similar to the tests in the

laboratory.

1 Located in the village Cova da Iria, near Fátima on São João de Deus Street

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Figure 23: SV performance in SS#1

Figure 23 illustrates SV message transmission times in function of the order of messages

transmitted in the WSN, with one, two and three ZigBee Routers. Unlike the situation which

occurred in the laboratory, there was no occurrence of situations where a ZigBee Router tries to

send data and the communication channel is busy. Transmission time values range from 7.5 to

12.6 ms. Average transmission times for WSN with one, two and three ZigBee Routers are 10,

10.2 and 10.3 ms respectively. These results are very similar to those obtained in the

laboratory; therefore the analysis of the results is the same in both tests. The similarity proves

that the performance of WSN is not influenced by the electromagnetic noise that exists in this

substation. The values exceed the requirements set by the IEC 61850 for 10 ms.

Retransmission of messages in the WSN is not verified.

The WSN with three LN has the highest transmission time and does not have transmission

times below 8ms. The differences between transmission times are minimal in relation to the

different configurations of the other WSN. These small differences may be due to the distance

of a LN that is not part of the WSN with one and two LN, which may be situated at a greater

distance to the ZigBee Coordinator.

A total of 350 SV messages were exchanged in the WSN. No communication errors or lost

data in WSN were detected. The graph with the results of the tests is available, with better


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Figure 24: MMS performance in SS#1

Figure 24 illustrates MMS message transmission times in function of the order of

messages transmitted in the WSN, with one, two and three ZigBee Routers. Comparisons

drawn between Figure 24 and Figure 21 show that results drawn in both tests are similar.

For situations when the ZigBee coordinator sends the same message to all the WSN, the

average of the total transmission are 133 ms. In WSN with three ZigBee Routers that send 1rst

,

2nd

and 3rd

messages, the first ZigBee Router that receives the request has a transmission time

of 39 ms, the second has 85 ms and the third has 132 ms. In cases where these situations do

not occur, the transmission times range between 30 ms and 35 ms for the three different

constitutions of the WSN.

The test results drawn from the SS are very similar to results obtained in the laboratory.

The analysis of the results follows the same pattern as the laboratory tests. This similarity of

results serves as more proof that the performance of WSN is not influenced by the noise

produced by the devices.

A total of 500 MMS messages were exchanged in the WSN. No communication errors or

lost data in WSN were detected. The graph with the results of the tests is available, with better


4.4. SS#2 TESTS

This SS, built in 1992 and refurbished in 2007

2, has obsolete but still functional electronic

equipment. It has concrete walls inside.

SS details and dimensions, as well as arrangement of the wireless modules, are described

on Figure 25. In Annex C.2, one may find photographs of the modules available in the SS.

2 Located in the village of Moita near Fatima, on São João Eudes Street

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Figure 25: SS#2 environment

In this SS, it was possible to place the ZigBee modules close to the substation devices and

the arrangement of wireless modules is very similar to that used in the laboratory. Nevertheless,

there are concrete walls on the alignment of communication between the modules, which may

minimally influence the results.

Figure 26 illustrates the results of this test, which represents the transmission time of SV

messages in function of the order of messages transmitted in the WSN, with one, two and three

ZigBee Router.


Figure 23 illustrates SV message transmission times in function of the order of messages

transmitted in the WSN, with one, two and three ZigBee Routers. This test has not detected

situations in which the ZigBee Router tried to send data and the channel rejected the request,

nor has it detected retransmission of messages. The remaining values of transmission times

vary between 8 ms and 12 ms for the three types of WSN constitutions and they have very

similar results to tests carried out previously. However, as is shown in Figure 26, there are three

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cases with transmission times above 13 ms. After a brief review, it was found that these

transmission times correspond to the ZigBee Router which has a wall between it and the

ZigBee coordinator. These results prove that the existence of obstacles between the wireless

modules influences the performance of communication, as explained in [35].

The average transmission times for WSN with one, two and three ZigBee Routers are

10.2, 10.3 and 10.4 ms respectively. These values are significantly higher than those obtained

previously. This difference is mainly due to the existence of obstacles between the wireless

modules on the SS. The values prove that the ZigBee technology does not meet the time of 10

ms required by IEC 61850.

A total of 500 SV messages were exchanged in the WSN. No communication errors or lost

data in WSN were detected. The graph with the results of the tests is available, with better



Figure 27 illustrates MMS message transmission times in function of the order of

messages transmitted in the WSN, with one, two and three ZigBee Routers. Comparisons

drawn between Figure 24, Figure 21 and Figure 27 show that results drawn in all tests are

similar.

As can be seen in Figure 27, cases where the same message is sent to all devices occur

again. The transmission times for these cases continues to be similar to those reported

previously. The averages of total transmission are 134 ms. For example, for a WSN with three

ZigBee Routers, the first ZigBee Router that receives the request has a transmission time of 37

ms, the second has 89 ms, the third has 135 ms. In the absence of these situations, values are

similar to those previously obtained, ranging between 29 and 36 ms. Due to the similarity of the

results, conclusions drawn from the tests for MMS messages also apply to this test. The

similarity of these results to previous ones shows that the performance of the WSN is not

influenced by the type of environment or format SS building.

These results were not affected by the existence of obstacles between the wireless

modules, because the calculation for the transmission times of this test lacks the precision used

for testing the SV message. Of 500 MMS messages exchanged in each type of constitution of

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the WSN, all reached their destination, with no communication errors to report and no data lost

in WSN. The graph with the results of the tests is available, with better resolution, in Annex C.3.

4.5. CONCLUSIONS OF THE RESULTS

To analyze the effect of retransmission on transmission times, several locations for

wireless modules were tested. To force retransmissions, the ZigBee Router must be outside of

the SS and distanced from the substation controller by 5 meters. The tests show that there is no

retransmission of messages in the WSN of the SS caused by electromagnetic noise or the

architecture of the substation.

When ZigBee modules try to send data, the communication channel cannot be accessed if

it is occupied. This situation is difficult to occur with three or fewer modules, as already reported

in [47]. In fact, in the full breadth of hundreds of tests performed only for SV messages, only two

situations of this sort were detected. However, for situations with more nodes, these cases

occur more frequently. These tests are inconclusive for WSN with more than three nodes [47].

Results obtained in the laboratory and in an SS environment show similar values for SV

transmission times, yet, in all tests, the values exceed the 10 ms requirement defined by IEC

61850. Small differences detected can be due to "packet sniffer" errors, different distances

between the platform and ZigBee modules and the obstacles between the modules. It was not

possible to place the packet sniffer closer to the modules. Even in test situations where the

channel was occupied, values of 15 ms and 20 ms were obtained. No problems should come

from transmitting specific data in this WSN [9]. However, the IEC 61850 defines that the SV

communication stack must only transmit critical data, such as samples of voltage and current

values [10]. These results exclude the use of the SV message in a WSN, for failing to meet this

requirement; the protection and control of devices is put in question.

Transmission times for MMS messages are lower than the 1000 ms required by IEC

61850, even if the message must be fragmented into two messages. It is also important to note

that the IEC 61850 defines the size requirement of the data packet as being between 100 bytes

and 1000 bytes. For cases when the ZigBee Coordinator sends the same message to all

ZigBee Routers via unicast the IEC 61850 does not specify requirements. The total

transmission time of this situations observed on the tests is less than 5000 ms, which complies

with the requirements [15]. For WSN with more than three nodes it is possible that the results

suffer small changes. However, for WSN with ten nodes in the SS, and considering that the

message transmission time on average is 35 ms, the requirements are still met.

In 2400 messages were transmitted between WSN nodes. No messages were lost, which

makes this system compliant with the no-failure requirements set by the IEC 61850.

During these tests, no problems or accidents were detected in the substation equipment.

These error situations are those that can generate the most electromagnetic noise, situations

which can affect test results. Therefore, this analysis of the results is only valid for all

substations in normal operation.

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5. CONCLUSION AND FUTURE WORK

5.1. CONCLUSION

This dissertation proposes the use of a wireless sensor network, using ZigBee technology

and the IEC 61850 standard, to monitor non-critical devices of an electrical distribution

substation of small size.

Currently, researchers and experts from the electric power are not confident about the use

of low power wireless technology. They argue that this technology does not guarantee reliable

communication in noisy environments. However, results obtained in the scope of this

dissertation demonstrate the contrary, with tests conducted in small distribution substations.

WSN, with the use of ZigBee technology, can thus become a valid solution.

Accomplished tests did not include operation in critical situations. Therefore, WSN

compliance with all IEC 61850 requirements is not confirmed. However, results show that

wireless technology can be taken into account. However, SV messages in WSN cannot be

adopted.

As non-critical data can be sent in the MMS format, all communication can be done in the

WSN using the MMS format, in order to meet the requirements set by the IEC 61850 and to

perform the same functions in the same level of performance.

Furthermore, results show that more LN can be inserted into the WSN, as defined by the

IEC 61850, for such functions as fire detection, fuse status, determination of root mean square

voltages and current values. These LN have functions with timing requirements between 50,

100 and 500 ms [10]. ZigBee technology can thus be used to create a WSN to be successfully

used in the the "Active Substations" project.

The system defended in this dissertation was developed for a specific application.

However, results of the tests proves it can be used for other projects such as equipment control

in home and building automation, personal health and fitness and less harmful defense

systems. In a more advanced stage of the “Active Substations” project, it may be used for the

developing of a smart-grid for efficient management and monitoring of electrical power

generated by individuals [48].

49

5.2. FUTURE WORK

The developed WSN prototype presents some limitations that may be overcome in the

future, in the implementation of the final prototype on the "Active Substations" project.

Improvements and exploration of certain details may still be researched in order to refine the

quality of the adaptation techniques developed and the overall performance of the prototype.

The WSN here discussed has some limitations that can be upgraded for an insertion into

an automation system of SS using IEC 61850, such as:

The limitation of current hardware and software to implement the modules in

wireless 6LoWPAN, which would use as a medium of data transport the TCP/IP.

The applications of modules are inefficient for data processing. In making the

applications more efficient, better transmission times are possible.

Use of emulated values, such as values acquired from sensors. The next step is to

develop or change the hardware of the modules to handle the data coming from

sensors.

Dependence on batteries as a power source circuit. In making hardware changes, it

adapts the power circuit modules, for they can be fed through the existing electrical

power substation.

The current application of the ZigBee coordinator has certain limitations for

communication with the outside. It will be necessary to develop a new interface so

communication with the substation controller is made more feasible.

Once all of the modifications mentioned above are made, the performance of the WSN

should see good improvements, as well as guaranteeing that the system is reliable and credible

in real situations.

The next step of the project concerns itself with the WSN’s implementation using only the

MMS communication stack. Therefore, it is necessary to restructure the WSN, implementing the

complete format of MMS communication and develop a decoder/encoder for ASN.1 messages.

In the near future, with new changes introduced into the IEC 61850 for the covering of all

special cases regarding the automation in electrical substations, ZigBee technology will be fully

capable of complying with all requirements.

50

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53

ANNEXES

54

ANNEX A.

This section explains in greater detail some general requirements defined by IEC 61850.

More specifically, it exemplifies the constitution of the data structure of an LN, the definition of a

communication format and the measuring of messages’ transmission times for each function to

be performed on the system.

A.1 EXAMPLE OF A LOGICAL NODE AND DATA CLASS

Table 3 shows an example of a list of data classes for a circuit breaker (XCBR). The data

classes that make up the circuit breaker are grouped into three categories (basic LN

information, controllable data, and status information). Each category comprises some data

classes, for example, “Mode” and “Switch position”. These data classes are referenced by their

DataName: “Mode” and “Pos” [28]. To be more precise, each data class also has CDC, defining

the details, i.e, and the attributes of the data class. The last column specifies whether this data

class is mandatory (M) or optional (O).

Logical Node: Circuit Break Name: XCBR

Data-Class DataName Common Data Class (CDC) M/O

Basic Logical Node Information

Mode Mod INC - Controllable Integer Status M

Behavior Beh INS – Integer Status M

Health Health INS – Integer Status M

Name Plate NamPlt LPL – Logical Node name plate M

Local operation (local means without substation automation communication, hardwired direct control)

Loc SPS – Single point Status

External equipment health EEHealth INS – Integer Status

External equipment name plate

EEName DPL – Device name plate

Operation counter OpCnt INS – Integer Status

Controllable Data

Switch position Pos DPC – Controllable Double Point M

Block opening BlkOpn SPC – Controllable Single Point M

Block closing BlkCls SPC – Controllable Single Point M

Charger motor enabled ChMotEna SPC – Controllable Single Point O

Metered Values

Sum of Switched Amperes, resetable

SumSwARs

BCR – Binary Counter reading O

Status Information

Circuit breaker operating capability

CBOpCap INS – Integer Status M

Point On Wave switching capability

POWCap INS – Integer Status O

Circuit breaker operating capability when fully charged

MaxOpCap INS – Integer Status O

Table 3: Data classes for the LN circuit breaker [28]

55

Since many data classes use the same details (attributes), these details are therefore collected for re-use in common CDC (common to many data classes) defined in part 7-3 of the IEC 61850 standard [19]. As an example, the “Controllable double point” (DPC), CDC for “Pos” is shown in Table 4.

DPC class

Attribute Name

Attribute type FC TrgOp Value/ value range M/O/C

DataName Inherited from data Class (see IEC 618507-2)

DataAttribute

Control and status

ctlVal BOOLEAN CO Off (False) | on (TRUE) AC_CO_M

operTim TimeStamp CO AC_CO_M

origin Originator CO,ST AC_CO_M

ctlNum INT8U CO,ST 0…255 AC_CO_M

stVal Coded Enum ST dchg Intermediate-state | off | on | bad-state

M

q Quality ST qchg M

t TimeStamp ST M

stSeld Boolean St dchg AC_CO_M

substation

subEna BOOLEAN SV PICS_SUBST

subVal Coded Enum SV Intermediate-state | off | on | bad-state

PICS_SUBST

subQ Quality SV PICS_SUBST

subID VisibleString64 SV PICS_SUBST

Configuration, description and extension

pulseConfig PulseConfig CF AC_CO_O

ctlModel CtlModels CF M

sboTimeout INT32U CF AC_CO_O

sboClass SboClasses CF AC_CO_O

d VisibleString255 DC Text O

cdcNs VisibleString255 EX AC_DLNDA_M

cdcName VisibleString255 EX AC_DLNDA_M

dataNs VisibleString255 EX AC_DLN_M

Services

…

Table 4: The "DPC" common data class for "POS" [28]

The “DPC” CDC is composed of a list of 20 data attributes. Each attribute has a name, type,

functional constraint, trigger option, value/value range, and an indication of whether the attribute is

mandatory or optional [28].

At least all the mandatory attributes of all mandatory data classes of the “XCBR” on Table 3,

make up the attributes of the “XCBR”. Optional data classes (for example, Point On Wave

switching capability – POWCap and optional data attributes (for example, origin - Originator) shall

be used if required by an application [28].

56

A.2 TABLE OF PICOM TYPES

The PICOM types defined by IEC 61850 for each type of situation automation are

described in Table 5. Each PICOM type is assigned a specific type of mode, size of the value,

the value of number attributes combined, transfer time and the message type.

PICOM TYPE ID

Meaning of PICOM and its value attribute

Type Mode

Number of value attributes combined

Size of value attribute in bits

Transfer Time (response/ cycle) ms

Message Type

1 Process Values (Sample)

Value Cyclic

1 to 8 1,2,3,5

16 . 10 .0,1;0,5;1;2;5;10

4a

2 Process Value (r.m.s)

Value Cyclic

1 to 8 1,2,3,5

16 . 1000 . 50;100;500

2b

3 Measured Value (calculated) such as energy

Value Cyclic Reqst

1 to 64 4,6,64

16 . 1000 . 100;500;1000

3

4 Metered Value (calculated) such as energy

Value Cyclic Reqst

1 to 512 1, 512

16 . 1000 to 5000 . 1000; 5000

3

5 Process value (non electrical such as temperature

Value Cyclic

1 to 8 1

16 .1000 to 5000 .1000; 5000

3c

6 Report (calculated) Such as energy list

File Reqst

1 1024 .1000 to 5000 .1000; 5000

5

7 Fault value (calculated) such as fault distance

Value Reqst

1 to 2 1

16 .1000 to 5000 .1000; 5000

3

8 Mixed fault info (calculated) such as extensive

File Reqst

1 512 .1000 to 5000 5

9 Mixed fault data (calculated) such as disturbance recording

File Reqst

1

20 000 20 0000

.5000 5

10 Event/alarm Event Spont- neous

1 to 16 1

1 .100 to 1000 .100;500;1000

3d

11 Event/alarm List/group

File Spont-neous reqst

1 128 1024

.100 to 1000

.100;500;1000 5

12 Trigger (calculated) for example for start of another function

Event Spont- neous

1 1 .10 to 1000 .10;50;100;1000

1

13 Complex block or Release (calculated)

Event Spont- neous

1 16 .10 to 100 .10; 100

1

14 Request (calculated) for Synrocheck interlocking, etc.

File Spont-neous reqst

1 1 .10 to 100 .10;100

2

15 Fast broadcast message, for example for block/release

Event Spont- neous

1 1 .1 .1

1

See notes on the next page

57

PICOM TYPE ID

Meaning of PICOM and its value attribute

Type Mode

Number of value attributes combined

Size of value attribute in bits

Transfer Time (response/ cycle) ms

Message Type

16 Process state Status Rqst Cyclic

1 1 .1 to 100 .1;10;20;50;100

2e

17 Calculated state Status Rqst

1 1 .1 to 100 .1;10;20;50;100

2e

18 External condition Status Rqst Cyclic

1 1 .1 to 100 .10;100

2e

19 Mode of operation Status Rqst Cyclic

1 1 16

.10 to 100

.10; 100 3

20 Process state changed

Event Spont- neous

1 1 .1 to 10 .1, 10

1

21 Command Cmd. Spont- neous

1,5 1 .1 to 1000 .1;2;5;10;50;100 1000

7f

22 Trip Cmd. Spont- neous

1 1 .1 1

23 Set point Value Spont- neous

1 16 .100 to 1000 .100; 1000

3

24 ID data, setting File Spont- neous

1 1024 .1000 to 5000 .1000; 5000

5

25 Diagnostic Data File Spont- neous

1 1024 .5000 5

26 Acknowledge by Operator or auto.

Cmd. Spont- neous

1 1 .10 to 1000 .10;100;1000

3

27 Date and Time Value Cyclic Reqst

1 32 .100 to 1000 .100; 1000

3

28 Synchronization “pulse”

Cmd. Cycl.

1 1 .0.1 to 10 .0.1;0.5;1;2;5;10

6

1 By basic definition, a PICOM consists of one data element (value only). Some of these basic data

elements may combine if this makes sense from the application point of view. 2 Without a tima tag; not a requirement but some idea about the net data and input for data flow

calculations are necessary. 3 See 12.2 of IEC 61850-5 for a definition

4 According to 12.5 of IEC 61850-5

a Accuracy 25µs or less.

b In future, some values regarding power quality may be of message type 1a.

c Special values such as pressure may need message type 2

d Alarms and events as seen from the alarm and event handling, automatics may need message

class 2. e For some fast functions, message type 1 may be requested.

f The command message created as type 7 by the operator may be propagate at lower levels faster, for example according to type 1 on the process bus such as a trip.

Table 5: PICOM types [15]

58

ANNEX B.

This annex describes in more detail some aspects which require special attention. This annex is

divided into two parts, the individual description of the communication architecture of both the LN

and the LD, the description of the WSN and the data content of each communication.

B.1 THE LN AND LD PROGRAM FLOWCHART

As described in Chapter 3, the substation controller and LN modules have a structure of similar

application between them. However, they perform completely different roles, as the LN

corresponds to the WSN router while the substation controller is the coordinator of the WSN. In

Figure 28 and Figure 29, the program flowchart for the substation controller and LN are

represented respectively.

The program flowchart is the same for the entire LN, because, while the operation is the same,

what differ are the content and parameter settings.

Figure 28: Substation Controller program flowchart

59

Figure 29: Module LN program flowchart

60

B.2 DATA FORMAT ADOPTED FOR COMMUNICATIONS USED IN WSN

WSN communications between the modules adopt three types of message formats; SV

message, MMS message and TimeSync message. All messages have different data types, as

defined in Part 7-2 and 8-1 of IEC 61850, [41] and [21].

Table shows the encoding for the basic data types used for data values referenced by the data

set members.

Data types according to IEC 61850-7-2

Encoding in data set

BOOLEAN 8 Bit set to 0 False; anything else = True

INT8 8 Bit Big Endian (signed)




INT8U 8 Bit Big Endian (unsigned)



FLOAT32 32 Bit IEEE Floating Point (IEEE 754)

FLOAT64 64 Bit IEEE Floating Point (IEEE 754)

ENUMERATED 32 Bit Big Endian

CODED ENUM 32 Bit Big Endian

OCTEC STRING 20 Bytes ASCII Text, Null terminated

VISIBLE STRING 35 Bytes ASCII Text, Null terminated

UNICODE STRING 20 Bytes ASCII Text, Null terminated

ObjectName 20 Bytes ASCII Text, Null terminated

ObjectReference 20 Bytes ASCII Text, Null terminated

TimeStamp 64 Bit Timestamp as defined in IEC 61850-8-1

Entrytime 48 Bit Timestamp as defined in IEC 61850-8-1

Data types according to IEC 61850-8-1

Encoding in data set

BITSTRING 32 Bit Big Endian

Table 6: Encoding for the basic data types [23]

B.2.1 TYPE OF DATA FOR EACH LN, USED IN THE SEMANTICS OF THE SV MESSAGE

Of the three LN that constitute the WSN, only one has a defined standard. The other two were

developed in compliance with the rules established by the IEC 61850. The IEC 61850 defines

several data for each LN, but only a few are required, classified as mandatory (M) or optional (O)

[20].

Table 7, Table 8 and Table 9 describe which values and class data are adopted for the LN,

SIML, ZMDS and ADEC respectively.

61

SIML Class

Attribute Name Data Class Explanation M/O

LNName Shall be inherited from Logical-Node Class [41]

Data

LN shall inherit all Mandatory Data from Common LN M

Measured Values

Tmp MV Insulation liquid temperature O

Pres MV Insulation liquid pressure O

H20 MV Relative Saturation of moisture in insulating liquid O

Status Information

InsAlm SPS Insulation liquid Critical M

TmpAlm SPS Insulation liquid temperature Alarm O

PressAlm SPS Insulation liquid pressure Alarm O

Table 7: Value and data class of the LN SIML [20]

ZMDS Class



Data


Measured Values

Amtmp MV Ambient temperature O

Hum MV Humidity Level O

Status Information

AmbAlm SPS Ambient temperature Critical M

Table 8: Value and data class of the LN ZMDS

ADEC Class



Data


Measured Values

PosDor MV Ambient temperature O

Status Information

DrOpen SPS Door Open Critical M

Table 9: Value and data class of the LN ADEC

The data class defines the size and type of representation for each value. All three LN uses two

common types of class, which are designated as Measured Value (MV) and Status Point Single

(SPS). The SPS is used for the status indication group, while MV is used to indicate the values of

the sensors. On the Table 10 and Table 11 the encoding of common data class SPS and MV is

described.

62

Common data class SPS (IEC 61850-7-3) Comment

Attribute name Attribute type

stVal BOOLEAN

grpVal BIT STRING 16 individual status values

q Quality 1 Byte

validity CODED ENUM

detailQual PACKED LIST

t TimeStamp

SecondsSinceEpoch INT32

FractionOfSecond INT24

TimeQuality TimeQuality

LeapSecondsKnown BOOLEAN SecondsSinceEpoch includes leap seconds

ClockFailure BOOLEAN Time function is unreliable

ClockNotSynchonized BOOLEAN Clock is not synchronized to the reference source

TimeAccuracy CODED ENUM Reserved

Table 10: Common Data Class SPS [19]

Common data class SPS (IEC 61850-7-3)

Comment Attribute name Attribute type

mag AnalogueValue UI16

Instmag Analoque Not mapped

q Quality 1 Byte

validity CODED ENUM

detailQual PACKED LIST Not mapped, see Note1

t TimeStamp

SecondsSinceEpoch INT32

FractionOfSecond INT24

TimeQuality TimeQuality

LeapSecondsKnown BOOLEAN Not mapped

ClockFailure BOOLEAN Not mapped

ClockNotSynchonized BOOLEAN Not mapped

TimeAccuracy CODED ENUM Not mapped, see Note1

Note 1: According to IEC 61850-7-3, t and detailQual is a mandatory attribute and not require in the sample value buffer. So in this WSN only uses mag and the validity attribute.

Table 11: Common Data Class MV [19]

B.2.2 THE TYPES OF DATA USED ON THE MODEL CLIENT/SERVER OF THE

COMMUNICATION STACK.

The control and monitoring of LN depends on the semantics of the MMS communication stack.

The transmission system is based on the client/server method, which defines the client responsible

for sending the request to the server and the receiving of an answer. The IEC 61850 defines two

types of services, set or get data. The format of each service has different request and response

data.

In the Table 12, all types of MMS description used in both services are described. In Table 13,

the description of the requests and responses for each service is made.

63

MMS component

name

MMS type description

Comments

UsvCBNam ObjectName MMS object name: the value of this component shall be of the format of the ObjectReference.

UsvCBRef ObjectReference MMS object name: the value of this component shall be of the format of the ObjectReference.

SvEna Boolean True = transmission of sampled value buffer is activated False= transmission of sampled value buffer is deactivated.

Resv Boolean True = USVCB is exclusively reserved for the client that has set this value to true.

UsvID Visible-String System-wide unique identification.

Datset ObjectReference MMS object name: the value of this component shall be of the format of the ObjectReference.

ConfRev Integer Count of configuration changes regard to USVCB.

SmpRate Integer Amont of samples per nominal periods

SmpCnt Integer Count of a new sample of the analogue value is taken

FuncionalConstr Visible-String Indicates which services can be used to access the values of the data attributes. For this case is “US”

Table 12: MMS type description definition for USVCB MMS structure [23]

Parameter name for get USVCB values service

Parameter name for set USVCB values service

Request Request

UsvCBRef UsvCBRef

FuncionalConstr FuncionalConstr

Response+ SvEna

SvEna Resv

Resv UsvID

UsvID Datset

Datset ConfRev

ConfRev SmpRate

SmpRate Response+

Response- Response-

Service error Service error

Table 13: Parameter name for USVCB services [41]

B.2.3 THE TYPES OF DATA USED ON THE MODEL CLIENT/SERVER OF THE SNTP

PROTOCOL

The SNTP protocol pre-defines the content of the fields of each type of addressing. The

communication model is based on client/server for each request and responses are defined or

ignore values for the respective field. In the table, indicates the data set by SNTP and adopted for

the date and time synchronization in WSN.

64

Field Name Request Reply

Leap Indicator 0 0-2

Version Number 4 Copied from request

Mode 3 4

Stratum 0 1-14

Poll ignored ignored

Precision 0 ignored

Root Delay 0 ignored

Root Dispersion 0 ignored

Reference Identifier 0 ignored

Reference Timestamp 0 ignored

Originate Timestamp 0 Time request sent by client

Receive Timestamp 0 Time request received by server

Transmit Timestamp Time reply sent by server nonzero

Table 14: SNTP client operations [25]

Field Name Request Reply

Leap Indicator ignore 0 or 3

Version Number 4 Copied from request

Mode 3 4

Stratum ignored 1

Poll ignored Copied from request

Precision ignored -log2 server significant bits

Root Delay ignored 0

Root Dispersion ignored 0

Reference Identifier ignored Source indent (GPS)

Reference Timestamp ignored Time of the last radio update

Originate Timestamp ignored Copied from transmit timestamp

Receive Timestamp ignored Time of day

Transmit Timestamp Time reply sent by server Time of day

Table 15: SNTP server operations [25]

65

ANNEX C.

In this annex are shown photographs of the test sites and the disposition of the wireless

modules. For security reasons, it was not possible in certain cases to place the wireless

modules at desired locations, such as putting them on top of the transformer. Even with these

impediments, they were placed closer to the desired locations and in places with more intense

electromagnetic noise.

C.1 PREPARATION OF WIRELESS MODULES

The wireless modules were prepared so that the electromagnetic noise did not interfere in

the functioning of the PCB. The PCB is placed inside a plastic box, thus avoiding direct contact

with the devices of the substation and preventing noise influences to the electrical circuit. The

power supply of the wireless modules is a battery, to make them portable and easy to carry for

the tests. Figure 30 illustrates a wireless module with these modifications.

Figure 30: Portable Wireless Module

66

C.2 TEST SITES AND THE DISPOSITION OF WIRELESS MODULES

Laboratory

Figure 31: Laboratory and disposition of the wireless modules

SS#1

Figure 32: Format of SS#1

67

Figure 33: Disposition of the LN SIML and ZMDS

Figure 34: Disposition the LN ADEC and the ZigBee Coordinator

68

SS#2

Figure 35: Format of SS#2

Figure 36: Disposition of the LN ADEC

69

Figure 37: Disposition of the wireless modules

Figure 38: New disposition for the LN SIML and ZMDS

70

C.3 RESULTS OF THE TESTS

C.3.1 LABORATORY TESTS

Figure 39: MMS performance in Laboratory

71

Figure 40: MMS performance in Laboratory

72

C.3.2 SS#1 TESTS


73


74

C.3.3 SS#2 TESTS


75


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