PLC Based Communication System for Distribution

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2nd IEEE International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor Baharu, Malaysia

Power Line Carrier (PLC) Based

Communication System for Distribution

Automation System

* M. M. Ahmed, Member, IEEE and ** W. L. Soo

* Universiti Teknikal Malaysia Melaka (UTeM), Melaka, Malaysia. Email: [email protected]** Universiti Teknikal Malaysia Melaka (UTeM), Melaka, Malaysia. [email protected]

Abstract—This paper presents the development of a power

line carrier in low distribution automation system (DAS) for

operating and controlling low voltage (LV) down stream of

415/240V. Supervisory Control and Data Acquisition

(SCADA) based Remote Terminal Unit (RTU) along power

line communication (PLC) system are used for DAS

development that practically simulates the down stream

distribution system functions in an automated manner. It is

the first DAS research work done on customer side

substation for operating and controlling between the

consumer side system and the substation using PLC. Most

of the work in this paper is focused on PLC that provides an

effective communication system for both RTU and SCADA

systems. The Human Machine Interface for SCADA system

is developed using customized software and an RTU

microprocessor and its software implements.

Index Terms — SCADA system, RTU, RS-232, RS-485,

Serial Modules, Modbus, TCP/IP, Master-Slave.

I. INTRODUCTION

The main advantage with PLC is the use of anexisting infrastructure. A recent study was published inMetering International Magazine recent edition Issue 2,2002 mentioned the communication media used in theautomatic meter reading (AMR). It appears that the mostcommon AMR communication technology worldwide isradio (RF) and the second most common communicationmedia is the PLC [1].

The PLC communications take place over thesame lines that deliver electricity. This techniqueinvolves injecting a high frequency AC carrier onto the power line and modulating this carrier with dataoriginating from the remote meter or central station.Power line carrier techniques are used successfully andcost effectively for short distances.

In this paper, PLC types, the basic technique inPLC, PLC modem, system architecture using PLC for DAS are presented.

II. PLC IN POWER SYSTEM

The first carrier frequency systems began tooperate around the year 1922 which operated in thefrequency spectrum from 10KHz to 490kHz [2]. The PLCwas applied for telemetry purposes. In the 1930s, ripplecarrier signaling was introduced on the medium (10-20kV) and low voltage (240/415V) distribution systems [3].In the mid 1980s, with the introduction to SCADA, someof the utility companies have made further developementof PLC communication [4]. PLC is used in control andtelemetry applications for both utility side applicationsand consumer-side applications [5]. The challenges faced by the PLC communication are noise, disturbances, power line channel impedance variations, and signalattenuation.

Fig. 1. Simple PLC Communication System [2]

Fig. 1 shows a typical PLC system for runningcommunication signals in the distribution network. Thesignal is generated in the data centre and is sent throughthe power line to the receiver which passes the signal tothe coupler. The low voltage concentrator picks up andenhances the signal which is passes to the other coupler

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and sends to the building. The desired signal finallyreaches the users’ premises. In the data transmission, thedistribution transformer is completely bypassed to avoidthe undesired influence of the device.

III. BASIC TECHNIQUES OF PLC

Fig. 2. Basic Power Line Carrier Terminal

The major components of a PLC channel areshown in Fig. 2. The problem associated with the PLCchannel is the requirement to put the carrier signal onto

the high voltage line without damaging the carrierequipment. Once the signal is on the power line it mustbe directed in the proper direction in order for it to bereceived at the remote terminal line.

A. Transmittesr and Receivers

The carrier transmitters and receivers are usuallymounted in a rack or cabinet in the control house. Theline tuner is out in the switchyard. This means that thereis a large distance between the equipment and the tuner.The connection between the two is made using a coaxialcable. The coaxial cable provides shielding so that thenoise cannot get into the cable and cause interference.

The coaxial cable is connected to the line tuner whichmust be mounted at the base of the coupling capacitor. If there is more than one transmitter involved per terminal,the signal must go through isolation circuits beforeconnecting to the line tuner.

B. Hybrids and Filters

The purpose of the hybrid circuits is to enablethe connection of two or more transmitters together onone coaxial cable. The hybrid circuits preventintermodulation distortion due to the signal from onetransmitter affecting the output stages of the other transmitter. There are many forms of hybrids, such as

resistive hybrids, reactance hybrids, and skewed hybridsto name the most popular types.Fig.3 shows the resistive hybrid. By assuming

that the secondary of the transformer is terminated with a

50 Ω resistor and a voltage (V) is applied to the input port

#1. The 50 Ω load will be reflected in the primary of the

transformer as a 25 Ω quantity from point (a) to thecenter tap (ct). As shown in Fig. 2, the voltage across theinput port #2 is 0 volts, and the hybrid isolates the voltageat one input port from the other input port.

Fig. 3. Resistive Hybrid

Fig. 4. Reactance Hybrid

When a hybrid is connected to the power line

through a line tuner and coupling capacitor, the

termination impedance may not always be a 50

resistive. This type of hybrid is called a reactance hybridand is shown in Fig. 4. Resistive Hybrid and ReactanceHybrid are balanced hybrids which have equal lossesfrom each input port to the output port.

The success of a PLC channel will depend onthe received signal to noise ratio (SNR), and this can beobtained by maximizing the amount of transmitter signalthat is coupled to the phase wire. For long distancetransmission, losses may be high if using balancedhybrids in the application. Another type of hybrid can beused in an application of this type. It is called a skewedhybrid.

Fig. 5 shows the skewed hybrid. In skewedhybrid, the losses from input port #1 to the output are notthe same as the losses from port #2 to the output. TheSNR is better if it is compared with balanced hybrid. Thehigh losses in the receive path do not affect the SNRsince the noise is attenuated by the same amount as thesignal.

Fig. 5. Skewed Hybrid

C. Line Tuners

Line tuning units (LTUs) are used to tune to thecarrier frequency and provide impedance matching

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between the power line and the transmitter/receiver. TheLTU includes an impedance-matching transformer, aseries-resonant L-C circuit tuned to the carrier frequency,and also a protective device [6].

D. Line Traps

Line traps provide blocking of the carrier signal, preventing it from continuing into other transmission linesections. Single and two-frequency line traps are parallelL-C circuits with parameters of variable inductances andcapacitances selected so as to resonate at a specificfrequency (or at two frequencies) thus blocking thecarrier frequency (Fig. 6). Line traps are available invarious inductance ratings and continuous power-frequency current ranges [4].

Fig. 6. Equivalent Circuit Diagrams of Line Traps

IV. PLC MODEM

Fig. 7. Single Phase PLC Modem

Fig. 8. Three Phase PLC Modem

The PLC Modem is a dedicated device for transferring data over low voltage power line. Fig. 7 andFig.8 show the PLC modems used in the power line cableas communication medium. It is convenient as iteliminates the need to lay additional cables. The modemat the transmission end modulates the signal from dataterminal through RS-232 interface onto the carrier signal

in the power line. At the receiving end, the modemrecovers the data from the power line carrier signal bydemodulation and sends the data to data terminalsthrough RS-232 interface.

Fig.9. Remote Automatic Meter Reading System Using PLC Modem

Fig. 9 shows an example in a remote automaticmeter reading system. It illustrates the schematic diagramof the connection between power line modem and thedata terminals using power line cables.

V. SYSTEM ARCHITECTURE USING PLC

Fig. 10 shows the system architecture of SCADA for DAS that is used PLC modem as thecommunication media. The Embedded Ethernet RTU andcomputer are connected directly to the PLC modems. ThePLC modems use spread spectrum (SS) technology -CEbus. The SS technology is referred as the method of signal modulation where the transmitted signal occupies a bandwidth which is greater than the minimum necessary

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to send the information. The spread spectrum techniquesthat have been applied to the PLC systems use chirpingmethod. Fig.11 shows the carrier chirp waveformmeasured from the PLC systems. In the CEbus standard,the frequency range of the chirping method is from 100kHz to 400 kHz over a 100s meaning a Unity SymbolTime (UST). In CEbus protocol, the data is transmitted in

short frame as shown in Fig. 12. The PLC target modemwill answer back to the source PLC modem when itreceives a request.

Fig. 10. System Architecture

Fig. 11 CEBus Carrier Chirp

Fig. 12. CEBus Frame Packet.

VI. EXPERIMENTS AND R ESULTS

The fault isolation operation is built in thisresearch to locate fault point and isolate the fault point.There are two modes of operation which are the manualmode and automatic mode. In manual mode, the operator will manually switch on or off the loads. In automaticmode, the system will detect automatically the fault loadand isolate the fault load from other healthy loads. Allthese functions are done by PLC system to communicatefrom section of the system to another.

Fig.13 shows the experiment for manual mode atcustomer service substation panel. By using HMIapplication developed in this research, each load which

consists of 40W bulb lamp is switched on in sequence bythe user. The experiment starts by clicking on reset buttonto reset the system. This experiment is repeated four times and the result is shown in Fig. 17.

Fig. 13. Manual Mode Experiment Procedure-1

Fig. 14. Manual Mode Experiment Procedure-2

Fig. 15. Auto Mode Experiment Procedure-1

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Fig.14 shows the experiment for manual mode atthe service substation panel. This experiment is repeatedfour times and the result is shown in Fig. 17. Zone 1,Zone 2, Zone 3 and Zone 4 represent the MCCB1,MCCB2, MCCB3 and MCCB4 respectively.

Fig.15 shows the experiment for automatic modeat the service substation panel. In automatic mode, the

fault point is manually triggered by using the “TestButton” of Mikro 2200. Mikro 2200 is an over-currentand earth fault relay. This experiment is repeated threetimes and the result is shown in Fig. 17.

Fig. 16. Auto Mode Experiment Procedure-2

Fig.16 shows the experiment for automatic modeat customer service substation panel. In automatic mode,the fault point is manually triggered by using the “TestButton” of earth leakage circuit breaker (ELCB). Thisexperiment is repeated three times and the result is shownin Fig. 17.

The percentages failure of the system for notable to operate correctly based on the experimental procedures are described in Fig.17. The failure operationswere during the manual mode of service substation toswitch on Zone 2 and Zone 3. The reason of this failure is

due to loose wiring at the digital output module. Onefailure was recorded during automatic mode of customer service substation when fault was simply triggered atZone 2. The system was not able to switch on the other healthy loads during the operation because of the loosewiring at the second feeder. After the causes of failureshave been fixed, the experiments were continued and nooperation failure was recorded. The percentages of failureoperation in Fig.17 are contributed by human factors.

Fig. 17. Percentage of Failure Operation

VII. CONCLUSIONS

Based on the results from the experiments, thePLC modem is reliable. The application of PLC can bevery simple or can be quite challenging depends on the

desired use. The PLC communication technology is stillrapidly evolving. However it has limited to narrow bandapplication such as telemetry and SCADA for remotemetering. The PLC communication technology hasstrengths and also weaknesses. There is a lack of standardization and interoperability of PLCcommunication relevant products. The bandwidth in thePLC communication is constrained by regulatoryauthorities to prevent the interference with other networks.The standards for limiting the bandwidth depend on thecountries of use. The PLC communication technology hasthe potential to further extension in the future.

R EFERENCES

[1] CIRED, “Distribution Utility TelecommunicationInterfaces, Protocols and Architectures”, Final Report of the CIRED Working Group WG06, September 2003. pp.52

[2] K.W. Louie, A. Wang, P.Wilson, and P.Buchanan, “Discussion onPower Line Carrier Applications”, Manitoba HVDC ResearchCentre, IEEE, 2006, pp.655

[3] K.W. Louie, A. Wang, P.Wilson, and P.Buchanan, “Discussion onPower Line Carrier Applications”, Manitoba HVDC ResearchCentre, IEEE, 2006, pp.656

[4] K Dostert, 1997, Telecommunications over the Power DistributionGrid- Possibilities and Limitations Proc 1997 Internat. Symp. onPower Line Comms and its Applications pp1-9

[5] T J Sheppard 'Mains Communications- a practical meteringsystem' 7th International Conference on Metering Applications

and Tariffs for Electricity Supply pp 223-227 17-19 Nov 1992(London UK: IEE 1992)

[6] Miriam P. Sanders and Roger E. Ray, “Power Line Carrier Channel & Application Condiserations For Transmission LineRelaying”, Pulsar Document Number C045-P05967, 2000.

[7] Archnet, “Embedded Power Line Carrier Modem”, availableonline: http://www.archnetco.com/english/product/ATL90.htm

[8] Customized Non-interruptible Distribution Automation System,Short Term Project No. PJP/2006/FKE (1) , UTeM, 2005-2006

[9] Intelligent Distribution Automation System: Customized SCADABased Rtu For Distribution Automation System, M.Sc. ResearchProject, UTeM, 2005-2007.

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BIOGRAPHIES

Dr. Musse Mohamud Ahmed is anassociate Prof. at Faculty of electricalEngineering, UTeM. He graduatedfrom UTM and got his Ph.D in theyear 2000. He worked Multimedia

University (MMU), as lecturer at theFaculty of Engineering & Technologyin Malacca campus from 2000 to 2002.He joined UTeM in March 2002 as alecturer. In October 2002, he wasappointed as deputy dean,

postgraduate, research & development at the Faculty of ElectricalEngineering, UTeM, a position he held till March 2007. Since then hehas been with UTeM.Dr. Musse has been IEEE-PES member for eleven years and PESExecutive Committee in Malaysia chapter for the last five years. Hisresearch interests include: Distribution Automation System, Power System Operation and Control Simulation & Modeling of Large Power Systems, Intelligent Power Systems, Energy & Renewable Energy andRisk Assessment of Electricity Supply

Soo Wai Lian was born in Malacca, Malaysia, onJune 3, 1978. She received her B.S degree in electrical engineering fromthe University Technology Malaysia and Master of Science inUniversity Technical Malaysia Melaka (UTeM). She is studying in theUniversiti Teknikal Malaysia Melaka pursuing her PHD degree. She isspecializing in electrical power distribution system.

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PLC Based Communication System for Distribution

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