Introduction to Transmission

Substation & Protection of

Transmission Line

SIEMENS Ltd.

KOLKATA (INDIA)

2016

SUBMITTED BY MANISH KR SINHA

B.E (ELECTRONICS & TELECOMMUNICATION)

6/15/2016

Training Report

ACKNOWLEDGEMENT

I would like to express my gratitude to Mr. Saikat Ganguly who gave me the

opportunity to work on this project. This project could not have been written without

Mr. Aukhoya Chatterjee who not only served as my project guide but also

encouraged and challenged me throughout y training program. I would also like to

acknowledge and extend my heartfelt gratitude to all those who have directly or

indirectly helped in making my training a success and making it a great educational

experience.

MANISH KUMAR SINHA

Pre-final year B.E in Electronics & Telecommunication,

Indian Institute of Engineering Science & Technology, Shibpur (IIESTS).

COMPLETION CERTIFICATE

This is to certify that Mr. MANISH KUMAR SINHA pursuing his B.E in

Electronics & Telecommunication from Indian Institute of Engineering Science

and Technology, Shibpur (IIESTS) has successfully completed his training on

“Transmission Substation and Protection of Transmission Line” under “Energy

Automation Business Unit” at Siemens Ltd., Kolkata (WB) as a summer trainee

during the period 16th May 2016 to 15th June 2016.

The training has been completed under my supervision. He has done excellent job and

was sincere during his training period.

DATE: 15th June 2016

SUPERVISOR

Mr. AUKHOYA CHATTERJEE

Group Leader-Engineering,

Energy Automation,

Siemens Ltd,

Kolkata (WB)

Table of Contents

Company Profile...................................................................................................................................................... 6

Energy Management Division ............................................................................................................................. 6

Business Units of Energy Management Division ............................................................................................. 7

Overview of Electrical Energy Systems ................................................................................................................... 8

Switchyard .............................................................................................................................................................. 8

Typical Bus Configurations .................................................................................................................................. 8

Single Bus ........................................................................................................................................................ 9

Sectionalized Bus ............................................................................................................................................ 9

Main and Transfer Bus .................................................................................................................................. 10

Ring Bus ........................................................................................................................................................ 10

Breaker-and-a-Half ....................................................................................................................................... 11

Double Breaker-Double Bus .......................................................................................................................... 11

Surge Arrester ................................................................................................................................................... 12

Line/Wave Traps ............................................................................................................................................... 13

Power Line Carrier Communication System ...................................................................................................... 13

Equipments used in PLCC system .................................................................................................................. 13

Circuit Breaker .................................................................................................................................................. 14

Isolator .............................................................................................................................................................. 14

Current & Voltage Transformers....................................................................................................................... 15

Saturation of CT ............................................................................................................................................ 16

Classification of CTs ...................................................................................................................................... 16

Voltage Transformers ....................................................................................................................................... 16

CCVT in Power Line Communication ................................................................................................................. 16

Ferro Resonance Problem in CCVT ................................................................................................................ 16

Need of Protection ............................................................................................................................................... 17

Types of Protection ............................................................................................................................................... 17

Apparatus Protection ........................................................................................................................................ 17

System Protection ............................................................................................................................................. 17

Relay ..................................................................................................................................................................... 17

Numerical Relays .................................................................................................................................................. 18

Introduction to transmission line .......................................................................................................................... 19

Factors Influencing line Protection ....................................................................................................................... 19

Line Differential Protection ................................................................................................................................... 19

Basic Concept .................................................................................................................................................... 19

Numerical Current Differential Protection System............................................................................................ 20

Differential Protection Using Siemens 7SD5 Relay ........................................................................................... 21

Distance Protection ............................................................................................................................................... 23

Zone 1 of Protection .......................................................................................................................................... 24

Zone 2 and Zone 3 for Protection...................................................................................................................... 25

Problem of Load Encroachment ........................................................................................................................ 26

Distance Protection Using Siemens 7SD5 Relay ................................................................................................ 27

Earth Fault Detection .................................................................................................................................... 27

Power Swing Detection ................................................................................................................................. 31

AUTO-RECLOSING ................................................................................................................................................. 32

Overvoltage Protection ......................................................................................................................................... 34

Voltage Surge ................................................................................................................................................... 34

Switching Impulse or Switching Surge .......................................................................................................... 35

Methods of Protection Against Lightning ..................................................................................................... 35

Bibliography .......................................................................................................................................................... 36

Company Profile

Siemens was founded in Berlin by Werner von Siemens in 1847. As an extraordinary

inventor, engineer and entrepreneur, Werner von Siemens made the world's first pointer telegraph

and electric dynamo; inventions that helped put the spin in the industrial revolution. He was the man

behind one of the most fascinating success stories of all time - by turning a humble little workshop into

one of the world's largest enterprises.

As Werner had envisioned, the company he started grew from strength to strength in

every field of electrical engineering. From constructing the world's first electric railway to laying the

first telegraph line linking Britain and India, Siemens was responsible for building much of the modern

world's infrastructure.

Siemens is today a technology giant in more than 190 countries, employing some

440,000 people worldwide. Our work in the fields of energy, industry, communications, information,

transportation, healthcare, components and lighting has become essential parts of everyday life.

While Werner was a tireless inventor during his days, Siemens today remains a

relentless innovator. With innovations averaging 18 a day, it seems like the revolution Werner started

is still going strong.

Siemens was established in India in 1922. However, the story of Siemens’ association

with India began in 1867 when Werner Von Siemens personally supervised the laying of the first

telegraph line between Calcutta and London. Making the country’s priorities its own, Siemens has put

its experience in the major core sectors namely Power, Industry, Telecommunications, Transportation

and Healthcare. In the last three decades, Siemens has played an active role in India’s technological

progress and has 12 offices and 5 manufacturing units in the country.

The company has around 400,000 employees (in continuing operations) working to

develop and manufacture products ,design and install complex system and projects ,and tailor a wide

range of solution for individual requirements .

Various divisions of Siemens-

Power and Gas

Power Generation Services

Building Technologies

Digital Factory

Healthcare

Wind Power and Renewables

Energy Management

Mobility

Process Industries and Drives

Financial Services

Energy Management Division

The Energy Management Division is one of the leading global suppliers of products, systems,

solutions, and services for the economical, reliable, and intelligent transmission and distribution of

electrical power.

As the trusted partner for the development and extension of an efficient and reliable power

infrastructure, the Energy Management Division provides utility companies and industries with the

portfolio that meets their needs. This includes facilities and systems for the low-voltage and

distribution power grid level, smart grid and energy automation solutions, power supply for industrial

plants, and high-voltage transmission systems.

The Division develops innovative solutions which have the potential to cope with the new challenges

our energy systems worldwide are facing. This includes a growing range such as the efficient

transmission of bulk volumes of green power over long distances, enabling dedicated power

exchange between power grids, connecting micro grids with the main grids.

Siemens offers an open and flexible architecture of solutions and services with the industry’s most

comprehensive energy management portfolio: The Siemens Smart Grid Suite. The suite enables a

multitude of customized solutions for smarter infrastructure grids and introduces unforeseen

opportunities to further stabilize systems, develop new business models and optimize energy trade.

The solution concept totally Integrated Power (TIP) for electrification and especially power

transmission and distribution completes the Smart Grid Suite. TIP is based on our comprehensive

range of products, systems, and solutions for low, medium and high voltage, rounded out by our

support throughout the entire lifecycle – from planning with our own software tools to installation,

operation, and services.

Business Units of Energy Management Division

High Voltage Products

Low-Voltage Power Distribution

Power Transmission

Smart Grid Solutions & Services

Transformers

Medium Voltage switchgear and devices

Energy Automation

Energy Automation

This Business unit provides solutions for -

Control, protection & Automation of power transmission & generation systems

Control Centres / Load Dispatch Centres

Manufacturing of Numerical & Auxiliary Relays, Control & Relays Panels

In India, Manufacturing facilities of this business unit are at Goa Works & Kalwa Works of Siemens

while engineering & Project management functions are based at Gurgaon.

Sales functions is distributed region-wise & has its offices at Gurgaon, Mumbai, Vadodara, Chennai &

Kolkata.

Protection Relays Product Lines

SIPROTEC 5 SIPROTEC 4 SIPROTEC Compact Reyrolle

Overview of Electrical Energy Systems

The purpose of an electrical Energy system is to generate and supply electrical energy to consumers. The system should be designed to deliver this energy both reliably and economically.

An electrical energy system consists of various equipments connected together. Typically,

power is generated at lower voltages (a few kV) (3-phase ac voltage source) which is stepped up by a transformer and fed into a transmission grid. Thermal power should be generated at pit heads and hydro power at reservoirs. A transmission grid is a meshed network of high voltage lines and transformers. It can have multiple voltage levels like 400 kV, 220 kV, etc. The power is delivered to load centers which may be far off (even housands of km's apart).

It can be seen that large amount of generation is concentrated in the eastern end while large

load centers are concentrated in the western end. The power is transferred through the ac network and HVDC lines. At load centers, voltage levels are stepped down by step down transformers in multiple stages and finally power is delivered to the end user by a distribution system which is mostly radial (no loops) in nature. A unique feature of electrical energy systems is its natural mode of synchronous operation. It implies that during steady state the electrical frequency is same all through the system irrespective of the geographical location. This closely knits the system together. We can perceive all generators acting in tandem like the ballet dancers in a dance. They may occupy different angular positions, but all machines rotate at the same electrical speed. This close knitting implies an embedded interaction of generators through the transmission network which is governed by the differential and algebraic equations of the apparatus and interconnects. This aspect is referred to as the system behaviour. This system has to be protected from abnormalities which is the task of protection system.

Switchyard

Place where switching operation of power transmission is performed.It provides the facilities for switching, protection and control of electric power

It handle high voltage power with proper safety measures. To isolate the noises coming from the grid with true 50hz power

Switchyard is the place where switching operation of power transmission is promoted. o Busbar o Lighting arrestor o Circuit breaker o Isolator o Current transformer o Capacitive voltage transformer o Wave trapper

Typical Bus Configurations

Busbar configuration or Bus switching scheme is the circuit adopted for substation based on following:

• System reliability • Operational flexibility • Ease of maintenance • Limitation of fault level • Simplicity of Protection system • Ease of extension • Availability of Land • Cost

Single Bus

Below figure shows the single-line diagram of a single bus substation configuration. This is

the simplest of the configurations, but is also the least reliable. It can be constructed in either of low profile or high-profile arrangement depending on the amount of space available. In the arrangement shown, the circuit must be de-energized to perform breaker maintenance, which can be overcome by the addition of breaker bypass switches, but this may then disable protection systems.

Single Bus Advantages:

Lowest cost

Small land area

Easily expandable

Simple in concept and operation

Relatively simple for the application of protective relaying Single Bus Disadvantages:

Single bus arrangement has the lowest reliability

Failure of a circuit breaker or a bus fault causes loss of entire substation

Maintenance switching can complicate and disable some of the protection schemes and overall relay coordination

Sectionalized Bus

Below figure shows the layout of a sectionalized bus, which is merely an extension of the single bus layout. The single bus arrangements are now connected together with a centre circuit breaker that may be normally open or closed. Now, in the event of a breaker failure or bus bar fault, the entire station is not shut down. Breaker bypass operation can also be included in the sectionalized bus configuration

Sectionalized Bus Advantages:

Flexible operation

Isolation of bus sections for maintenance

Loss of only part of the substation for a breaker failure or bus fault Sectionalized Bus Disadvantages:

Additional circuit breakers needed for sectionalizing, thus higher cost

Sectionalizing may cause interruption of non-faulted circuits

Main and Transfer Bus

A main and transfer bus configuration is shown in below figure. There are two separate and independent buses; a main and a transfer. Normally, all circuits, incoming and outgoing, are connecting the main bus. If maintenance or repair is required on a circuit breaker, the associated circuit can be then fed and protected from the transfer bus, while the original breaker is isolated from

the system.

Main and Transfer Bus Advantages:

Maintain service and protection during circuit breaker maintenance

Reasonable in cost

Fairly small land area

Easily expandable Main and Transfer Bus Disadvantages:

Additional circuit breaker needed for bus tie

Protection and relaying may become complicated

Bus fault causes loss of the entire substation

Ring Bus

Below figure depicts the layout of a ring bus configuration, which is an extension of the sectionalized bus. In the ring bus a sectionalizing breaker has been added between the two open bus ends. Now there is a closed loop on the bus with each section separated by a circuit breaker. This provides greater reliability and allows for flexible operation. The ring bus can be easily adopted to a breaker-and-a-half scheme, which will be looked at next.

Ring Bus Advantages:

Flexible operation

High reliability

Double feed to each circuit

No main buses

Expandable to breaker-and-a-half configuration

Isolation of bus sections and circuit breakers for maintenance without circuit disruption

Ring Bus Disadvantages:

During fault, splitting of the ring may leave undesirable circuit combinations

Each circuit has to have its own potential source for relaying

Usually limited to 4 circuit positions, although larger sizes up to 10 are in service. 6 is usually the maximum terminals for a ring bus

Breaker-and-a-Half

A breaker-and-a-half configuration has two buses but unlike the main and transfer scheme, both busses are energized during normal operation. This configuration is shown in below figure. For every two circuits there are three circuit breakers with each circuit sharing a common centre breaker. Any breaker can be removed for maintenance without affecting the service on the corresponding exiting feeder, and a fault on either bus can be isolated without interrupting service to the outgoing lines. If a centre breaker should fail, this will cause the loss of two circuits, while the loss of an outside breaker would disrupt only one. The breaker-and-a-half scheme is a popular choice when upgrading a ring bus to provide more terminals.

Breaker-and-a-Half Advantages:

Flexible operation and high reliability

Isolation of either bus without service disruption

Isolation of any breaker for maintenance without service disruption

Double feed to each circuit

Bus fault does not interrupt service to any circuits

All switching is done with circuit breakers

Breaker-and-a-Half Disadvantages:

One-and-a-half breakers needed for each circuit

More complicated relaying as the centre breaker has to act on faults for either of the two circuits it is associated with Each circuit should have its own potential source for relaying

Double Breaker-Double Bus

The final configuration shown is the double breaker – double bus scheme in below figure like the breaker-and-a-half, the double breaker-double bus configuration has two main buses that are both normally energized. Here though, each circuit requires two breakers, not one-and-a-half. With the addition of the extra breaker per circuit, any of the breakers can fail and only affect one circuit. This added reliability comes at the cost of additional breakers, and thus is usually only used at large generating stations.

Double Breaker-Double Bus Advantages:

Flexible operation and very high reliability

Isolation of either bus, or any breaker without disrupting service

Double feed to each circuit

No interruption of service to any circuit from a bus fault

Loss of one circuit per breaker failure

All switching with circuit breakers Double Breaker-Double Bus Disadvantages:

Very high cost – 2 breakers per circuit Out of the above bus configuration different types of substation use a specific configuration like- Power evacuation Substations and Transmission Substations uses double bus, three bus, Two Main & one Transfer, One & half breaker scheme Distribution Substations uses Single Bus with/without sectionaliser, Double bus scheme, three bus scheme (Two Main & one Transfer, One Main & Main cum Transfer)

Surge Arrester

A surge arrestor is installed between communication equipment and coaxial cable connector or between two communication equipments to protect communication equipment from damage caused by transient state voltage formed by lightning induction. It adopts quarter-wave technology, is designed according to VSW theory and frequency spectrum of lightning wave. It has features of quick reaction, big current passing capacity, wide frequency band, low VSWR, low insertion loss, easy installation and free maintenance. It can be used to meet protection requirements of various communication equipments and lightning sensitivity.

Line/Wave Traps

Line Traps are connected in series with HV transmission lines. The main function of the Line Trap is to present high impedance at the carrier frequency band while introducing negligible impedance at the power frequency. The high impedance limits the attenuation of the carrier signal within the power system by preventing the carrier signal from being:

dissipated in the substation

grounded in the event of a fault outside the carrier transmission path

dissipated in a tap line or a branch of the main transmission path.

Power Line Carrier Communication System

Power line carrier communication (PLC) is a system for carrying data on a conductor that is also used for electric power transmission. It is also known as power line carrier communication, power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), and broadband over power lines (BPL) Power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of power line communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60Hz, power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications.

Equipments used in PLCC system

LINE TRAPS

High voltage transmission lines are used for the transmission of carrier signals between 30KHz to 50KHz for Speech, Data, and Protection signals. Line traps are used to prevent the loss of these high frequency signals to the sub-station side. The Line Traps are connected in series to the Transmission line and are designed to carry the rated power frequency current, as well as to withstand the substation fault currents.

COUPLING DEVICE

The 650W Coupling Device comprising of coupling filters and protective devices is used to couple the PLCC terminals onto the high voltage transmission line. The device is purely passive and

is inserted between the low voltage terminals of the coupling capacitor and the co-axial connection to the carrier terminal. The coupling filter together with the coupling capacitor forms T section band pass

filter with multi band selections.

TELE-PROTECTION EQUIPMENT

Tele-protection equipment transmits the trip signal from the protection equipment in power station and substations. Tele-protection links using power line carrier channels are the most economical way of performing all the tasks associated with transmitting tripping signals. Security, dependability and transmission time are the important characteristics of any Tele-protection equipment. Interface on the communication channels should neither be interpreted as a trip command (security) nor in admissibly delay the transmission of genuine trip commands.

Circuit Breaker

A Circuit Breaker (CB) is basically a switch used to interrupt the flow of current. It opens on relay command. The relay command initiates mechanical separation of the contacts. It is a complex element because it has to handle large voltages (few to hundreds of kV's) and currents (in kA's). Interrupting capacity of the circuit breaker is therefore expressed in MVA. Power systems under fault behave more like inductive circuits. X/R ratio of lines is usually much greater than unity. For 400 kV lines, it can be higher than 10 and it increases with voltage rating. From the fundamentals of circuit analysis, we know that current in an inductive circuit (with finite resistance) cannot change instantaneously. The abrupt change in current, if it happens due to switch opening, will result in infinite di/dt and hence will induce infinite voltage. Even with finite di/dt, the induced voltages will be quite high. The high induced voltage developed across the CB will ionize the dielectric between its terminals. This results in arcing. When the current in CB goes through the natural zero, the arc can be extinguished (quenched). However, if the interrupting medium has not regained its dielectric properties then the arc can be restruck. The arcing currents reduce with passage of time and after a few cycles the current is finally interrupted. Usually CB opening time lies in the 2-6 cycles range. CBs are categorized by the interrupting medium used. Minimum oil, air blast, vacuum arc and SF6

CBs are some of the common examples. CB opening mechanism requires much larger power input than what logical element relay can provide. Hence, when relay issues a trip command, it closes a switch that energizes the CB opening mechanism powered by a separate dc source (station battery). The arc struck in a CB produces large amount of heat which also has to be dissipated.

Isolator

In electrical engineering, a disconnector or isolator switch is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance. Often the isolation switch is not intended for normal control of the circuit and is used only for isolation

In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems where both end of the circuit need to be isolated.

Usually isolators used in a substation are of two types. 1. three post double break centre post rotating type 2. two post single break isolating switch The major difference between an isolator and a circuit breaker is that an isolator is an off-load

device intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the isolator while it supplies a circuit

Current & Voltage Transformers

Practically all electrical measurements and relaying decisions are derived from current and voltage signals. Since relaying hardware works with smaller range of current (in amperes and not kA) and voltage (volts and not kV), real life signals (feeder or transmission line currents) and bus voltages have to be scaled to lower levels and then fed to the relays. This job is done by current and voltage transformers (CTs and VTs). CTs and VTs also electrically isolate the relaying system from the actual power apparatus. The electrical isolation from the primary voltage also provides safety of both human personnel and the equipment. Thus,CT and VTs are the sensors for the relay. CT and VT function like ‘ears' and the ‘eyes' of the protection system. They listen to and observe all happening in the external world. Relay itself is the brain which processes these signals and issues decision commands implemented by circuit breakers, alarms etc Clearly, quality of the relaying decision depends upon ‘faithful' reproduction on the secondary side of the transformer.

A fundamental difference between power transformer & current transformer is that while regular power transformers are excited by a voltage source, a current transformer has current source excitation. Primary winding of the CT is connected in series with the transmission line. The load on the secondary side is the relaying burden and the lead wire resistance. Total load in ohms that is introduced by CT in series with the transmission line is insignificant and hence, the connection of the CT does not alter current in the feeder or the power apparatus at all. Hence it is reasonable to assume that CT primary is connected to a current source.

As impedance in series with the current source can be neglected, we can neglect the primary

winding resistance and leakage reactance in CT.For the convenience in analysis, we can shift the magnetizing impedance from the primary side to the secondary side of the ideal transformer. Note that the secondary winding resistance and leakage reactance is not neglected as it will affect the performance of CT. The total impedance on the secondary side is the sum of relay burden, lead wire resistance and leakage impedance of secondary winding. Therefore, the voltage developed in the secondary winding depends upon these parameters directly. The secondary voltage developed by the CT has to be monitored because as per the transformer emf equation, the flux level in the core depends upon it. The transformer emf equation is given by E2=4.44fN2ᶲm, where ᶲm is the peak sinusoidal flux developed in the core. If Bm corresponding to this flux is above the knee point, it is more or less obvious that the CT will saturate. During saturation, CT secondary winding cannot replicate the primary current accurately and hence, the performance of the CT deteriorates. Thus, we conclude that in practice, while selecting a CT we should ascertain that it should not saturate on the sinusoidal currents that it would be subjected to. Use of numerical relays due to their very small burden vis-a-vis solid state and electromechanical relays, improves the CT performance. CT is to be operated always in closed condition. If the CT is open circuited, all the current Ip/N, would flow through Xm. This will lead to the development of dangerously high level of voltage in secondary winding which can even burn out the CT.

Saturation of CT

One of the major problems faced by the protection systems engineer is the saturation of CT on large ac currents and dc offset current present during the transient. When the CT is saturated, primary current source cannot be faithfully reflected to the secondary side. Also, the magnetizing impedance falls down during saturation. Then the transformer behaves more like an air core device, with negligible coupling between the primary and secondary winding. The high reluctance due to the air path implies that the magnetizing impedance (inductance) falls down.

Classification of CTs

The CTs can be classified into following types:

Measurement CTs

Protection CTs A measurement grade CT has much lower VA capacity than a protection grade CT. A measurement

CT has to be accurate over its complete range e.g. from 5% to 125% of normal current. In other words, its magnetizing impedance at low current levels. (and hence low flux levels) should be very high. Note that due to non-linear nature of B-H curve, magnetizing impedance is not constant but varies over the CT's operating range. It is not expected to give linear response (secondary current a scaled replica of the primary current) during large fault currents. In contrast, for a protection grade CT, linear response is expected up to 20 times the rated current. Its performance has to be accurate in the range of normal currents and upto fault currents. Specifically, for protection grade CT's magnetizing impedance should be maintained to a large value in the range of the currents of the order of fault currents. When a CT is used for both the purposes, it has to be of required accuracy class to satisfy both accuracy conditions of measurement CTs and protection CTs. In other words, it has to be accurate for both very small and very large values of current. Typically, CT secondary rated current is standardized to 1A or 5A (more common). However, it would be unreasonable to assume that the linear response will be independent of the net burden on the CT secondary.

Voltage Transformers

Many relaying applications like distance relays, directional overcurrent relays require measurement of voltages at a bus.This task is done by a voltage transformer (VT). The principle of a voltage transformer is identical to the conventional transformer. Typically, the secondary voltage of the VT is standardized to 110 V (ac).Hence, as the primary voltage increases, the turns ratio N1:N2 increases and transformer becomes bulky.To cut down the VT size and cost, a capacitance potential divider is used. Thus, a reduced voltage is fed to primary of the transformer. This reduces the size of VT. This leads to development of coupling capacitor voltage transformers (CCVT).

CCVT in Power Line Communication

The capacitance potential divider also serves the dual purpose of providing a shunt path to high frequency signal used in power line carrier communication. Normally, CCVT is used in HV/EHV systems where carrier line communication is used. High frequency i.e. Radio Frequency (RF) signals (50 - 400 kHz) can be coupled to power line for communication. At high frequency,the capacitive shunt impedance is very small and hence these signals can be tapped by the potential divider. To block the path to ground for the RF signal, a small drainage reactor is connected in series with the capacitance divider.At power frequencies, it has a very small impedance. Thus, the role of capacitance potential divider at power frequency is not compromised. On the other hand, at RF, the impedance of drainage reactor is large and it blocks the RF signal.Also, compensating reactor and transformer leakage reactance by their inductive nature, block the path of RF signal. This signal is then tapped by a tuning pack which provides low impedance to the RF signal.

Ferro Resonance Problem in CCVT

The iron cores of the reactor and transformer will not only introduce copper and core losses but it can also produce ferroresonance caused by the nonlinearity of the iron cores. Hence a ferroresonance suppression

circuit is also included in the secondary of the transformer. The dangerous overvoltages caused by ferroresonance are eliminated by this circuit. Unfortunately, it can aggravate CCVT transients.

Need of Protection

Electrical power system operates at various voltage levels from 415 V to 400 kV or even more. Electrical apparatus used may be enclosed (e.g., motors) or placed in open (e.g., transmission lines). All such equipments undergo abnormalities in their life time due to various reasons. For example, a worn out bearing may cause overloading of a motor. A tree falling or touching an overhead line may cause a fault. A lightning strike can cause insulation failure. Pollution may result in degradation in performance of insulators which may lead to breakdown. Under frequency or over-frequency of a generator may result in mechanical damage to its turbine requiring tripping of an alternator. Even otherwise, low frequency operation will reduce the life of a turbine and hence it should be avoided. It is necessary to avoid these abnormal operating regions for safety of the equipment. Even more important is safety of the human personnel which may be endangered due to exposure to live parts under fault or abnormal operating conditions. Small current of the order of 50 mA is sufficient to be fatal! Whenever human security is sacrificed or there exists possibility of equipment damage, it is necessary to isolate and de-energize the equipment. Designing electrical equipment from safety perspective is also a crucial design issue which will not be addressed here. To conclude, every electrical equipment has to be monitored to protect it and provide human safety under abnormal operating conditions. This job is assigned to electrical protection systems. It encompasses apparatus protection and system protection.

Types of Protection

Protection systems can be classified into apparatus protection and system protection.

Apparatus Protection

Apparatus protection deals with detection of a fault in the apparatus and consequent protection. Apparatus protection can be further classified into following:

Transmission Line Protection and feeder protection

Transformer Protection

Generator Protection

Motor Protection

Busbar Protection

System Protection

System protection deals with detection of proximity of system to unstable operating region and consequent control actions to restore stable operating point and/or prevent damage to equipments. Loss of system stability can lead to partial or complete system blackouts. Under-frequency relays, out-of-step protection, islanding systems, rate of change of frequency relays, reverse power flow relays, voltage surge relays etc are used for system protection. Wide Area Measurement (WAM) systems are also being deployed for system protection. Control actions associated with system protection may be classified into preventive or emergency control actions. .

Relay

Formally, a relay is a logical element which processes the inputs (mostly voltages and currents) from the system/apparatus and issues a trip decision if a fault within the relay's jurisdiction is detected.

In below figure, a relay R1 is used to protect the transmission line under fault F1. An identical

system is connected at the other end of the transmission line relay R3 to open circuit from the other ends as well. To monitor the health of the apparatus, relay senses current through a current transformer (CT), voltage through a voltage transformer (VT). VT is also known as Potential Transformer (PT). The relay element analyzes these inputs and decides whether (a) there is a abnormality or a fault and (b) if yes, whether it is within jurisdiction of the relay. The jurisdiction of relay R1 is restricted to bus B where the transmission line terminates. If the fault is in it's jurisdiction, relay sends a tripping signal to circuit breaker(CB) which opens the circuit. A real life analogy of the jurisdiction of the relay can be thought by considering transmission lines as highways on which traffic (current/power) flows. If there is an obstruction to the regular flow due to fault F1 or F2, the traffic police (relay R1) can sense both F1 and F2 obstructions because of resulting abnormality in traffic (power flow). If the obstruction is on road AB, it is in the jurisdiction of traffic police at R1; else if it is at F2, it is in the jurisdiction of R2. R1 should act for fault F2, if and only if, R2 fails to act. We say that relay R1 backs up relay R2. Standard way to obtain backup action is to use time discrimination i.e., delay operation of relay R1 in case of doubt to provide R2 first chance to clear the fault.

Numerical Relays

The hardware comprising of numerical relay can be made scalable i.e., the maximum number of v and i input signals can be scaled up easily. A generic hardware board can be developed to provide multiple functionality. Changing the relaying functionality is achieved by simply changing the relaying program or software. Also, various relaying functionalities can be multiplexed in a single relay. It has all the advantages of solid state relays like self checking etc. Enabled with communication facility, it can be treated as an Intelligent Electronic Device (IED) which can perform both control and protection functionality. Also, a relay which can communicate can be made adaptive i.e. it can adjust to changing apparatus or system conditions. For example, a differential protection scheme can adapt to transformer tap changes. An overcurrent relay can adapt to different loading conditions. Numerical relays are both "the present and the future". Hence, in this course, our presentation is biased towards numerical relaying. This also gives an algorithmic flavour to the course.

Introduction to transmission line

Transmission lines are a vital part of the electrical distribution system, as they provide the path to transfer power between generation and load. Transmission lines operate at voltage levels from 69kV to 765kV, and are ideally tightly interconnected for reliable operation. Any fault, if not detected and isolated quickly will cascade into a system wide disturbance causing widespread outages for a tightly interconnected system operating close to its limits. Transmission protection systems are designed to identify the location of faults and isolate only the faulted section . The key challenge to the transmission line protection lies in reliably detecting and isolating faults compromising the security of the system.

Factors Influencing line Protection

The high level factors influencing line protection include the criticality of the line (in terms of load transfer and system stability),fault clearing time requirements for system stability, line length, the system feeding the line, the configuration of the line (the number of terminals, the physical construction of the line, the presence of parallel lines), the line loading, the types of communications available, and failure modes of various protection equipment. The physical construction of the transmission line is also a factor in protection system application. The type of conductor, the size of conductor, and spacing of conductors determines the impedance of the line, and the physical response to short circuit conditions, as well as line charging current. In addition, the number of line terminals determines load and fault current flow, which must be accounted for by the protection system. Parallel lines also impact relaying, as mutual coupling influences the ground current measured by protective relays. The presence of tapped transformers on a line, or reactive compensation devices such as series capacitor banks or shunt reactors, also influences the choice of protection system, and the actual protection device settings.

The graded overcurrent systems, though attractively simple in principle, do not meet all the protection requirements of a power system. Application difficulties are encountered for two reasons: firstly, satisfactory grading cannot always be arranged for a complex network, and secondly, the settings may lead to maximum tripping times at points in the system that are too long to prevent excessive disturbances occurring. These problems led to the concept of 'Unit Protection', whereby sections of the power system are protected individually as a complete unit without reference to other sections. One form of ‘Unit Protection’ is also known as ‘Differential Protection’, as the principle is to sense the difference in currents between the incoming and outgoing terminals of the unit being protected. Other forms can be based on directional comparison, or distance tele-protection schemes.

Line Differential Protection

Basic Concept

Differential protection is based on the fact that any fault within electrical equipment would cause the current entering it, to be different, from the current leaving it. Thus by comparing the two currents either in magnitude or in phase or both we can determine a fault and issue a trip decision if the difference exceeds a predetermined set value .An auxiliary ‘pilot’ circuit interconnects similar current transformers at each end of the protected zone, as shown in below figures. Current transmitted through the zone causes secondary current to circulate round the pilot circuit without producing any current in the relay. For a fault within the protected zone the CT secondary currents will not balance, compared with the through-fault condition, and the difference between the currents will flow in the relay. An alternative arrangement the CT secondary windings are opposed for through-fault conditions so that no current flows in the series connected relays. The former system is known as a ‘Circulating Current’ system, while the latter is known as a ‘Balanced Voltage’ system.

Circulating Current System

Balanced Voltage System

Numerical Current Differential Protection System

A digital or numerical unit protection relay may typically provide phase-segregated current differential protection. This means that the comparison of the currents at each relay is done on a per phase basis. For digital data communication between relays, it is usual that a direct optical connection is used (for short distances) or a multiplexed link. Link speeds of 64kbit/s are normal, and up to 2 Mbit/s in some cases. Through current bias is typically applied to provide through fault stability in the event of CT saturation.

Once the relay at one end of the protected section has determined that a trip condition exists, an intertrip signal is transmitted to the relay at the other end. Relays that are supplied with information on line currents at all ends of the line may not need to implement intertripping facilities. However, it is usual to provide intertripping in any case to ensure the protection operates in the event of any of the relays detecting a fault. The problem remains of compensating for the time difference between the current measurements made at the ends of the feeder, since small differences can upset the stability of the scheme, even when using fast direct fibre-optic links. The problem is overcome by either time

synchronisation of the measurements taken by the relays, or calculation of the propagation delay of the link continuously.

Differential Protection Using Siemens 7SD5 Relay

The differential protection is the first main protection function of the device. It is based on current comparison. For this, one device must be installed at each end of the zone to be protected. The devices exchange their measured quantities via communication links and compare the received currents with their own values. In case of an internal fault the allocated circuit breaker is tripped. Depending on the version ordered, 7SD5 is designed for protected objects with up to 6 ends. Thus, with exception of normal lines, three and multi-branch lines can also be protected with or without connected transformers in block as well as small busbars. The protected zone is selectively limited by the current transformers at its ends. Transmission of measured values If the entire protected object is located in one place — as is the case with generators, transformers, busbars — the measured quantities can be processed immediately. This is different for lines where the protected zone spans a certain distance from one substation to the other. To be able to process the measured quantities of all line ends at each line end, these have to be transmitted in a suitable form. In this way, the tripping condition at each line end can be checked and the respective local circuit breaker can be operated if necessary. 7SD5 transmits the measured quantities as digital telegrams via communication channels. For this, each device is equipped with at least one protection data interface. Each device measures the local current and sends the information on its intensity and phase relation to the opposite end. The interface for this communication between protection devices is called protection data interface. As a result, the currents can be added up and processed in each device.

Restraint The precondition for the basic principle of the differential protection is that the total sum of all currents flowing into the protected object is zero in healthy operation. This precondition is only valid for the primary system and even there only if shunt currents of a kind produced by line capacitances or magnetizing currents of transformers and parallel reactors can be neglected. The secondary currents which are applied to the devices via the current transformers, are subject to measuring errors caused by the response characteristic of the current transformers and the input circuits of the devices. Transmission errors such as signal jitters can also cause deviations of the measured quantities. As a result of all these influences, the total sum of all currents processed in the devices in healthy operation is not exactly zero. Therefore, the differential protection is restrained against these influences. Charging current compensation

Charging current compensation is an additional function for the differential protection. It allows to achieve a higher sensitivity by compensating the charging currents that flow through the capacitance of the line and that are caused by the capacitances of the overhead line or the cable. Due to the phase-to-earth and phase-to-phase capacitances, charging currents are flowing even in healthy operation and cause a difference of currents at the ends of the protected zone. Especially

when cables and long lines have to be protected, the capacitive charging currents can reach considerable magnitude. If the feeder-side transformer voltages are connected to the devices, the influence of the capacitive charging currents can be compensated to a large extent arithmetically. It is possible to activate a charging current compensation which determines the actual charging current. In healthy operation charging currents can be considered as being almost constant under steady-state conditions, since they are only determined by the voltage and the capacitances of the lines. Without charging current compensation, they must therefore be taken into account when setting the sensitivity of the differential protection. With charging current compensation, no charging currents need to be taken into account here. With charging current compensation, the steadystate magnetizing currents across shunt reactances are taken into account as well. The devices have a separate inrush restraint feature for transient inrush currents. Further influences Further measuring errors which may arise in the actual device by hardware tolerances, calculation tolerances, deviations in time or due to the „quality“ of the measured quantities such as harmonics and deviations in frequency are also estimated by the device and automatically increase the local self-restraining quantity. Here, the permissible variations in the data transmission and processing periods are also considered. Deviations in time are caused by residual errors during the synchronization of measured quantities, data transmission and operating time variations, and similar events. When GPS synchronization is used, these influences re eliminated and do not increase the self-restraining quantity. It is due to the self-restraint that the differential protection always operates with the maximum possible sensitivity since the restraining quantities automatically adapt to the maximum possible errors. In this way, also highresistance faults, with high load currents at the same time, can be detected effectively. Using GPS synchronisation, the self-restraint when using communication networks is once more minimised since differences in the transmission times are compensated by the precise calculation of the two-way transmission times. A maximum sensitivity of the differential protection consists of an optical-fiber connection. Breaker Intertrip and Remote Tripping 7SD5 allows to transmit a trip command created by the local differential protection to the other end or ends of the protected object (intertripping). Likewise, any desired command of another internal protection function or of an external protection, monitoring or control equipment can be transmitted for remote tripping. The reaction when such a command is received can be set individually for each device. Thus, selection can be made for which end(s) the intertrip command should be effective. Commands are transmitted separately for each phase, so that a simultaneous single-pole auto-reclosure is always possible, provided that devices and circuit breakers are designed for single-pole tripping. Blocking/interblocking The distance protection, provided that it is available and configured, automatically takes over as protection function if the differential protection is blocked by a binary input signal. The blocking at one end of a protected object affects all ends via the communications link (interblocking). If the distance protection is not available or ineffective, and if overcurrent protection has been configured as emergency function, all devices automatically switch to emergency mode. Please keep in mind that the differential protection is phase-selectively blocked at all ends when a wire break is detected at one end of the protected object. The message „Wire break“ is only generated at the device in which the wire break has been detected. All other devices show the phase-selective blocking of the differential protection by displaying dashes instead of the differential and restraint current for the failed phase. In the case of a phase-selective blocking of the differential protection, the distance protection, even if it is available and configured, does not take over the protection function for the failed phase.

Distance Protection

Overcurrent protection scheme is essentially a simple protection scheme. Consequently, its accuracy is not very high. It is comparatively cheap as non-directional protection does not require VT. However, it is not suitable for protection of meshed transmission systems where selectivity and sensitivity requirements are more stringent. Overcurrent protection is also not a feasible option, if fault current and load currents are comparable.

Distance protection provides the following features:

More accurate as more information is used for taking decision.

Directional, i.e. it responds to the phase angle of current with respect to voltage phasor.

Fast and accurate.

Back-up protection.

Primarily used in transmission line protection. Also it can be applied to generator backup, loss of field and transformer backup protection.

In a simple radial system fed from a single source, apparent impedance will be (V/I) at the sending end, for the unloaded system, I = 0, and the apparent impedance seen by the relay is infinite. As the system is loaded, the apparent impedance reduces to some finite value (ZL+Zline) where ZL is the load impedance and Zline is the line impedance. In presence of a fault at a per-unit distance ‘m', the impedance seen by the relay drops to a m*Zline as shown in below figure.

The basic principle of distance relay is that the apparent impedance seen by the relay, which

is defined as the ratio of phase voltage to line current of a transmission line (Zapp), reduces drastically in the presence of a line fault. A distance relay compares this ratio with the positive sequence impedance (Z1) of the transmission line. If the fraction Zapp/Z1 is less than unity, it indicates a fault. This ratio also indicates the distance of the fault from the relay. Because, impedance is a complex number, the distance protection is inherently directional. The first quadrant is the forward direction i.e. impedance of the transmission line to be protected lies in this quadrant. However, if only magnitude information is used, non- directional impedance relay results. Below figures show a characteristic of an impedance relay and ‘mho relay' both belonging to this class. The impedance relay trips if the magnitude of the impedance is within the circular region. Since, the circle spans all the quadrants, it leads to non-directional protection scheme. In contrast, the mho relay which covers primarily the first quadrant is directional in nature.

Impedance Relay (Non Dir.) MHO Relay (Directional) As we have considered bolted and unloaded faults. Therefore, there would be errors introduced when the fault has some impedance. Hence, the apparent impedance seen by the relay will not exactly lie on transmission line impedance AB. Rather it would lie in a region shown by trapezoid in below figure.

Arcing faults are primarily resistive in nature. Usually, distance relay characteristics are visualized by drawing the relay characteristics in R-X plane. If the apparent impedance seen by the relay falls inside the trip region (enclosed region), then relay declares a fault and issues a trip decision. This decision making can be done in about 1/2 - 1 cycle time, if no intentional time delays are introduced, e.g, for backup protection. While trapezoid or quadrilateral characteristics are quite popular with the numerical relays, previous generation of electromechanical and solid state relays used other characteristics like 'mho' characteristics, which were easier to derive. Mho relay circles usually enclosed a larger area than the quadrilateral characteristics for identical line impedance and arcing impedance parameters. Thus, they are more susceptible to nuisance tripping. Hence, these characteristics have been superceded by the trapezoidal characteristics.

Zone 1 of Protection

The distance relay R1 has to provide primary protection to line AB and back up protection to lines BC, BD and BE.

The primary protection should be fast and hence preferably it should be done without any intentional time delay, while back up protection should operate if and only if corresponding primary relay fails. In above figure, R1 backs operation of relays R3, R5 and R7. Typically, distance relays are provided with multiple zones of protection to meet the stringent selectivity and sensitivity requirements. At least three zones of protection are provided for distance relays. Zone 1 is designated by Z1 and zones 2 and 3 by Z2 and Z3 respectively. Zone 1 is meant for protection of the primary line. Typically, it is set to cover 80% of the line length. Zone 1 provides fastest protection because there is no intentional time delay associated with it. Operating time of Z1 can be of the order of 1 cycle. Zone 1 does not cover the entire length of the primary line because it is difficult to distinguish between faults at F1 and F2/F3/F4 all of which are close to bus B. In other words, if a fault is close to bus, one cannot ascertain if it is on the primary line, bus or on back up line. This is because of the following reasons:

1. CTs and PTs have limited accuracy. During fault, a CT may undergo partial or complete saturation. The resulting errors in measurement of apparent impedance seen by relay, makes it difficult to determine fault location at the boundary of lines very accurately.

2. Derivations for equations of distance relays made some assumptions like neglecting capacitance of line, unloaded system transposed lines and bolted faults. In practice none of these assumptions are valid. Fault on a line will also destroy effect of transposing. Such factors affect accuracy of distance relaying. Further, algorithms for numerical relays may use a specific transmission line model. For example, a transmission line may be modeledas a series R – L circuit and the contribution of distributed shunt capacitance may be neglected. Due to model limitation and because of transients accompanied with the fault, working of numerical algorithm is prone to errors.

3. With only local measurements, and a small time window, it is difficult to determine fault impedance accurately.

4. There are infeed and outfeed effects associated with working of distance relays. Recall that a distance relaying scheme uses only local voltage and current measurements for a bus and transmission line. Hence, it cannot model infeed or outfeed properly.

Zone 2 and Zone 3 for Protection

Usually zone 2 is set to 120% of primary line impedance Z1. This provides sufficient margin to account for non-zero fault impedance and other errors in relaying. Also one should note that Z2 also provides back up protection to a part of the adjacent line. Therefore, one would desire that Z2 should be extended to cover as large a portion of adjacent line as possible. Typically, Z2 is set to reach 50% of the shortest back up line.If the shortest back up line is too short then, it is likely that ZP + 1.5ZB will be less than 1.2ZP. In such a case, Z2 is set to 1.2ZP. Since, back up protection has to be provided for entire length of remote line, a third zone of protection, Z3 is used.

Zone-3 is set to cover the farthest (longest) remote lines (BD in above figure-a for relay R1 acting as a backup relay). Since its operation should not interfere with Z2 operation of relays , it is set up to operate with a time delay of 2 CTI where CTI is the coordination time interval. The settings of relay R1 on an R-X plane is visualized in fig(b). timing diagrams are shown in fig (c).

Problem of Load Encroachment

Consider the steady state positive sequence model of a transmission line shown below-

Then, it can be shown that apparent impedance seen by relay R is given by,

From this equation, We can derive following conclusions

1. Quadrant of ZR in the R - X plane correspond to the quadrant of apparent power (Sij) in (Pij - Qij) plane.

2. The apparent impedance seen by the relay is proportional to square of the magnitude of bus voltage. If the bus voltage drops say to 0.9 pu from 1 pu, then ZR reduces to 81% of its value with nominal voltage. Further, if the bus voltage drops to say 0.8 pu, then the apparent impedance seen by the relay will drop to 64% of its value at 1 pu.

3. The apparent impedance seen by the relay is inversely proportional to the apparent power flowing on the line. If the apparent power doubles up, the impedance seen by relay will reduce by 50%. During peak load conditions, it is quite likely that combined effect of (2) and (3) may reduce the apparent impedance seen by the relay to sufficiently small value so as to fall in Z2 or Z3 characteristic. This is quite likely in case of a relay backing up a very long line. In such a case, Z3 impedance setting can be quite large. If the impedance seen by relay due to large loads falls within the zone, then it will pick up and trip the circuit after its time dial setting requirement are met. Under such circumstances, the relay is said to trip on load encroachment. Tripping on load encroachment compromises security and it can even initiate cascade tripping which in turn can lead to black outs.

Thus, safeguards have to be provided to prevent tripping on load encroachment. A distinguishing feature of load from faults is that typically, loads have large power factor and this leads to Zapp with large R/X ratio. In contrast, faults are more or less reactive in nature and the ratio X/R is quite high. Thus, to prevent tripping on load encroachment, the relay characteristic are modified by excluding an area in R – X plane, which corresponds to high power factor. A typical modified characteristic to account for load encroachment is shown in below figure.

Distance relay characteristics modified for load Enchroachment

Distance Protection Using Siemens 7SD5 Relay

Distance protection is the second main function of the device. It can operate as a fully-fledged redundant second protection function (Main2) in parallel to differential protection, or be configured as the only main protection function of the device (Main only). The distance protection distinguishes itself by high measuring accuracy and the ability to adapt to the given system conditions

Earth Fault Detection

Functional Description

Recognition of an earth fault is an important element in identifying the type of fault, as the determination of the valid loops for measurement of the fault distance and the shape of the distance zone characteristics substantially depend on whether the fault at hand is an earth fault or not. The 7SD5 has a stabilized earth current measurement, a zero sequence current/negative sequence current comparison as well as a displacement voltage measurement. Furthermore, special measures are taken to avoid a pickup for single earth faults in an isolated or resonant earthed system.

Earth Current 3I0

For earth current measurement, the fundamental component of the sum of the numerically filtered phase currents is supervised to detect if it exceeds the set value (parameter 3I0> Threshold). It is stabilized against spurious operation resulting from unsymmetrical operating currents and error currents in the secondary circuits of the current transformer due to different degrees of current transformer saturation during short-circuits without earth: the actual pick-up threshold automatically increases as the phase current increases (Figure 2-35). The dropout threshold is approximately 95 % of the pickup threshold.

Negative Sequence Current 3I2>

On long, heavily loaded lines, large currents could cause excessive restraint of the earth current measurement (ref. Figure 2-35). To ensure secure detection of earth faults in this case, a negative sequence comparison stage is additionally provided. In the event of a single-phase fault, the negative sequence current I2 has approximately the same magnitude as the zero sequence current I0. When the ratio zero sequence current / negative sequence current exceeds a preset ratio, this stage picks up. For this stage a parabolic characteristic provides restraint in the event of large negative sequence currents. Figure 2-36 illustrates this relationship. A release by means of the negative sequence current comparison stage requires currents of at least 0.2·IN for 3I0 and 3I2.

Displacement Voltage 3U0

For the neutral displacement voltage recognition the displacement voltage (3·U0) is numerically filtered and the fundamental frequency is monitored to recognize whether it exceeds the set threshold. The dropout threshold is approximately 95 % of the pickup threshold. In earthed systems (3U0> Threshold) it can be used as an additional criterion for earth faults. For earthed systems, the U0–criterion may be disabled by applying the ∞ setting.

Logical Combination for Earthed Systems

The current and voltage criteria supplement each other, as the displacement voltage increases when the zero sequence to positive sequence impedance ratio is large, whereas the earth current increases when the zero sequence to positive sequence impedance ratio is smaller. Therefore, the current and voltage criteria for earthed systems are normally OR-ed. However, the two criteria may also be AND-ed (settable, see Figure 2-37). Setting 3U0> Threshold to infinite makes this criterion ineffective. If the device detects current transformer saturation in any phase current, the voltage criterion is indeed crucial to the detection of an earth fault since irregular current transformer saturation can cause a faulty secondary zero-sequence current although no primary zero-sequence current is present. If displacement voltage detection has been made ineffective by setting 3U0> Threshold to infinite, earth fault detection with the current criterion is possible even if the current transformers are saturated. The earth fault detection alone does not cause a general fault detection of the distance protection, but merely controls the further fault detection modules. It is only alarmed in case of general fault detection.

Distance Protection with Quadrilateral Characteristic Operating polygons In total, there are six independent zones and one additional controlled zone for each fault impedance loop. Figure 2-52 shows the shape of the polygons as example. Zone Z6 is not shown in Figure 2-52. The first zone is shaded and forward directional. The third zone is reverse directional. In general, the polygon is defined by means of a parallelogram which intersects the axes with the values R and X as well as the tilt ϕDist. A load trapezoid with the setting RLoad and ϕLoad may be used to cut the area of the load impedance out of the polygon. The axial coordinates can be set individually for each zone; ϕDist, RLoad and ϕLoad are common for all zones. The parallelogram is symmetrical with respect to the origin of the R-X-coordinate system; the directional characteristic however limits the tripping range to the desired quadrants (refer to „Direction determination“ below). The R-reach may be set separately for the phase-to-phase faults and the phase-to-earth faults to achieve a larger fault resistance coverage for earth faults if this is desired.

For the first zone Z1, an additional settable tilt α exists, which may be used to prevent overreach resulting from angle variance and/or two ended infeed to short-circuits with fault resistance. For Z1B and the higher zones, this tilt does not exist.

Pickup and assignment to the polygons Using the fault detection modes I, U/I or U/I/ϕ, the impedances that were calculated from the valid loops, are assigned, after the pick-up, to the zone characteristics set for the distance protection. The loop information is also converted to phase-segregated information. Using the impedance pickup, the calculated loop impedances are also assigned to the zone characteristics set for the distance protection, but without consideration of an explicit fault detection scheme. The pickup range of the distance protection is determined from the thresholds of the largest-set polygon taking into consideration the respective direction. Here the loop information is also converted into phase-segregated indications. For each zone „pickup“signals are generated and converted to phase information, e.g. „Dis Z1 L1“ (internal message) for zone Z1 and phase L1; this means that each phase and each zone is provided with separate pickup information; the information is then processed in the zone logic and by additional functions (e.g. teleprotection logic, Section 2.7). The loop information is also converted to phase-segregated information. Another condition for „pickup“ of a zone is that the direction matches the direction configured for this zone (refer also to Section 2.6). Furthermore the distance protection may not be blocked or switched off completely. Figure 2-58 shows these conditions.

Controlled zone Z1B The overreaching zone Z1B is a controlled zone. The normal zones Z1 to Z6 are not influenced by Z1B. Zone Z1B is often used in combination with automatic reclosure and/or teleprotection schemes. It can be activated internally by the teleprotection functions or the integrated automatic reclosure, or externally by a binary input. It is generally set to at least 120 % of the line length.

Power Swing Detection

The 7SD5 has an integrated power swing supplement which allows both the blocking of trips by the distance protection during power swings (power swing blocking) and the tripping during unstable power swings (out-ofstep tripping). To avoid uncontrolled tripping, the distance protection devices are supplemented with power swing blocking functions. At particular locations in the system, out-of-step tripping devices are also applied to split the system into islanded networks at selected locations, when system stability (synchronism) is lost due to severe (unstable) power swings. Following dynamic events such as load jumps, faults, reclose dead times or switching actions it is possible that the generators must realign themselves, in an oscillatory manner, with the new load balance of the system. The distance protection registers large transient currents during the power swing and, especially at the electrical centre, Small voltages with simultaneous large currents apparently imply small impedances, which again could lead to tripping by the distance protection. In expansive networks with large transferred power, even the stability of the energy transfer could be endangered by such power swings. System power swings are three-phase symmetrical processes. Therefore a certain degree of measured value symmetry may be assumed in general. System power swings may, however, also occur during asymmetrical processes, e.g. after faults or during a single-pole dead time. Thus the power swing detection in the 7SD5 is based on three measuring systems. For each phase, there is a measuring system that ensures phase-selective power swing detection. In case of faults, the detected power swing is terminated in the corresponding phases, which enables selective tripping of the distance protection. Use of the Ohm Characteristic During severe power swing conditions from which a system is unlikely to recover, stability might only be regained if the swinging sources are separated. Where such scenarios are identified, power swing, or out-of-step, tripping protection can be deployed, to strategically split a power system at a preferred location. Ideally, the split should be made so that the plant capacity and connected loads on either side of the split are matched. This type of disturbance cannot normally be correctly identified by an ordinary distance protection. As previously mentioned, it is often necessary to prevent distance protection schemes from operating during stable or unstable power swings, to avoid cascade tripping. To initiate system separation for a prospective unstable power swing, an out-of-step tripping scheme employing ohm impedance measuring elements can be deployed.

Power swings can be classified as either stable or unstable. Basically, a relay which is expected to issue trip decision on a fault should not pick up on a swing (either stable or unstable). When a power swing is a consequence of stable disturbance, unwanted line tripping can aggravate disturbance and lead to instability. On the other hand, when the power swing is a on sequence of disturbance, classified as unstable, then interconnected operation of the system is simply not possible. This implies that the system has to be split into multiple islands each of which can have independent existence i.e. each island can maintain synchronism of generators. Now to achieve stable operation in each island, generator load balance has to be ascertained. If an island has excess generation, it should be shelved and similarly if an island has excess load then load shedding is required. Load shedding is usually initiated by under-frequency relays, as excess load tends to pull the frequency down. However, to minimize the loss of service to consumers, the boundary of islands has to be selected carefully. To illustrate this point, consider a simple two area system as shown in fig 26.1

consequent to a disturbance, let the system be unstable and let the location of electrical center be on line AB. Recall that at the electrical center, voltage zero point is created when the two generators are out of step. Alternatively, electrical center appears when the power swing intersects the transmission line characteristics. This implies that relays located at the two ends of the transmission line, perceive the out of step condition as a bolted three phase fault on the transmission line. Consequently, relays R1 and R2 will issue a trip decision, thereby islanding the system. Now, the generator at A (PG = 0.666pu) islands with a load of 0.333pu and generator at B (PG= 0.333pu) islands with a load of 0.666pu. The resulting loss of load is 0.333pu in island B and loss of generation in island A is 0.333pu.However, if we had islanded the system by tripping line BC then an ideal solution of zero load or generation shedding would have been achieved. This suggests that during unstable swings, we should block the relays from operation. Consequently, more selective tripping can be initiated to achieve the desirable islands. We now, arrive at a thumb rule that under out of step condition, distance relays should be blocked from operation on swings. In case of a stable power swing. The resulting movement of apparent impedance seen by relay on the R-X plane may encroach Z2 or Z3 of a relay. If the swing stays inside the zone for long enough time, then the relay will issue a trip command. This is also not desirable. Hence, even under stable swings, the distance relays have to be blocked from tripping.

it is not desirable for distance relay to trip on power swing whether the swing is stable or not. This implies that distance relay should be equipped with swing detection and blocking mechanism.

The basic idea in detecting a power swing is that change in apparent impedance seen by relay ΔZ due to fault occurrence is instantaneous. In contrast, the change in ΔZ due to power swing is a slow process limited by inertia of generators. Thus, this time discrimination can be used to distinguish swings from faults.

AUTO-RECLOSING

Experience shows that about 85% of the arc faults on overhead lines are extinguished automatically after being tripped by the protection. The line can therefore be re-energised. Reclosure is performed by an automatic reclose function (AR). Automatic reclosure function is only permitted on overhead lines because the possibility of extinguishing a fault arc automatically only exists there. It must not be used in any other case. If the protected object consists of a mixture of overhead lines and other equipment (e.g. overhead line in block with a transformer or overhead line/cable), it must be ensured that reclosure can only be performed in the event of a fault on the overhead line.

If the circuit breaker poles can be operated individually, a 1-pole automatic reclosure is usually initiated in the case of 1-phase faults and a 3-pole automatic reclosure in the case of multi-phase faults in the network with earthed system star point. If the fault still exists after reclosure (arc not extinguished or metallic short-circuit), the protection issues a final trip. In some systems several reclosing attempts are performed. In the model with 1-pole tripping the 7SD5 allows phase-selective 1-pole tripping. A 1- and 3-pole, one- and multi-shot automatic reclosure is integrated depending on the order variant. The 7SD5 can also operate in conjunction with an external automatic reclosure device. In this case, the signal exchange between 7SD5 and the external reclosure device must be effected via binary inputs and outputs. It is also possible to initiate the integrated auto reclose function by an external protection device (e.g. a backup protection). The use of two 7SD5 with automatic reclosure function or the use of one 7SD5 with an automatic reclosure function and a second protection with its own automatic reclosure function is also possible. Reclosure is performed by an automatic reclosure circuit (ARC). An example of the normal time sequence of a double reclosure is shown in the figure below.

Initiation Initiation of the automatic reclosure function means storing the first trip signal of a power system fault that was generated by a protection function which operates with the automatic reclosure function. In case of multiple reclosure, initiation therefore only takes place once, with the first trip command. This storing of the first trip signal is the prerequisite for all subsequent activities of the automatic reclosure function. The starting is important when the first trip command has not appeared before expiry of an action time (see below under „Action times“). Automatic reclosure function is not started if the circuit breaker has not been ready for at least one OPEN-CLOSE- OPEN–cycle at the instant of the first trip command. This can be achieved by setting parameters. For further information, please refer to „Interrogation of Circuit Breaker Ready State“. Each short-circuit protection function can be parameterized as to whether it should operate with the automatic reclose function or not, i.e. whether it should start the reclose function or not. The same goes for external trip commands applied via binary input and/or the trip commands generated by the teleprotection via permissive or intertrip signals. Those protection and monitoring functions in the device which do not respond to short-circuits or similar conditions (e.g. an overload protection) do not initiate the automatic reclosure function because a reclosure will be of no use here. The circuit breaker failure protection must not start the automatic reclosure function either.

The most important parameters of an auto-reclose scheme are: o Dead time o Reclaim time o Single or multi-shot

These parameters are influenced by: o Type of protection o Type of switchgear o Possible stability problems o Effects on the various types of consumer loads

Protection against over voltage .

Overvoltage Protection

There are several instances when the elements of power system (generators,transformers,transmission line ,insulators etc.) are subjected to overvoltage i.e. voltage greater then the normal value. These overvoltages in the power system may be caused due to many reasons such as Lightening, opening of a circuit breaker ,the grounding of a conductor etc

Under Electrical Protection

Voltage Surge

Switching Impulse or Switching Surge

Methods of Protection Against Lightning

Earthing Screen

Overhead Earth Wire

Lightning Arrester

There are always a chance of suffering an electrical power system from abnormal over

voltages. These abnormal over voltages may be caused due to various reasons such as, sudden

interruption of heavy load, lightening impulses, switching impulses etc. These over voltage stresses

may damage insulation of various equipments and insulators of the power system. Although, all the

over voltage stresses are not strong enough to damage insulation of system, but still these over

voltages also to be avoided to ensure the smooth operation of electrical power system.

These all types of destructive and non destructive abnormal over voltages are eliminated from the system by means of overvoltage protection.

Voltage Surge

The over voltage stresses applied upon the power system, are generally transient in nature.

Transient voltage or voltage surge is defined as sudden sizing of voltage to a high peak in very short

duration. The voltage surges are transient in nature, that means they exist for very short duration. The

main cause of these voltage surges in power system are due to lightning impulses and switching

impulses of the system. But over voltage in the power system may also be caused by, insulation

failure, arcing ground and resonance etc.

The voltage surges appear in the electrical power system due to switching surge, insulation

failure, arcing ground and resonance are not very large in magnitude. These over voltages hardly

cross the twice of the normal voltage level. Generally, proper insulation to the different equipment of

power system is sufficient to prevent any damage due to these over voltages. But over voltages occur

in the power system due to lightning is very high. If over voltage protection is not provided to the

power system, there may be high chance of severe damage. Hence all over voltage protection

devices used in power system mainly due to lightning surges.

Switching Impulse or Switching Surge

When a no load transmission line is suddenly switched on, the voltage on the line becomes

twice of normal system voltage. This voltage is transient in nature. When a loaded line is suddenly

switched off or interrupted, voltage across the line also becomes high enough current chopping in the

system mainly during opening operation of air blast circuit breaker, causes over voltage in the system.

During insulation failure, a live conductor is suddenly earthed. This may also caused sudden over

voltage in the system. If emf wave produced by alternator is distorted, the trouble of resonance may

occur due to 5th or higher harmonics. Actually for frequencies of 5th or higher harmonics, a critical

situation in the system so appears, that inductive reactance of the system becomes just equal to

capacitive reactance of the system. As these both reactance cancel each other the system becomes

purely resistive. This phenomenon is called resonance and at resonance the system voltage may be

increased enough.

But all these above mentioned reasons create over voltages in the system which are not very high in magnitude.

But over voltage surges appear in the system due to lightning impulses are very high in amplitude and highly destructive. The affect of lightning impulse hence must be avoided for over voltage protection of power system.

Methods of Protection Against Lightning

These are mainly three main methods generally used for protection against lightning. They are

1. Earthing screen. 2. Overhead earth wire.

3. Lighning arrester or surge dividers.

Earthing Screen

1. Earthing screen is generally used over electrical sub-station. In this arrangement a net of GI wire is mounted over the sub-station. The GI wires, used for earthing screen are properly grounded through different sub-station structures. This network of grounded GI wire over

2. This method of high voltage protection is very simple and economic but the main drawback is, it can not protect the system from travelling wave which may reach to the sub-station via different feeders.

Overhead Earth Wire

This method of over voltage protection is similar as earthing screen. The only difference is, an

earthing screen is placed over an electrical sub-station, whereas, overhead earthwire is placed over

electrical transmission network. One or two stranded GI wires of suitable cross-section are placed

over the transmission conductors. These GI wires are properly grounded at each transmission tower.

These overhead ground wires or earthwire divert all the lightning strokes to the ground instead of

allowing them to strike directly on the transmission conductors.

Lightning Arrester

The previously discussed two methods, i.e. earthing screen and over-head earth wire are very

suitable for protecting an electrical power system from directed lightning strokes but system from

directed lightning strokes but these methods can not provide any protection against high voltage

travelling wave which may propagate through the line to the equipment of the sub-station.

The lightning arrester is a devices which provides very low impedance path to the ground for high voltage travelling waves.

The concept of a lightning arrester is very simple. This device behaves like a nonlinear electrical resistance. The resistance decreases as voltage increases and vice-versa, after a certain level of voltage.

The functions of a lightning arrester or surge dividers can be listed as below.

1. Under normal voltage level, these devices withstand easily the system voltage as electrical insulator and provide no conducting path to the system current.

2. On occurrence of voltage surge in the system, these devices provide very low impedance path for the excess charge of the surge to the ground.

3. After conducting the charges of surge, to the ground, the voltage becomes to its normal level. Then lightning arrester regains its insulation properly and prevents regains its insulation property and prevents further conduction of current, to the ground.

There are different types of lightning arresters used in power system, such as rod gap arrester, horn gap arrester, multi-gap arrester, expulsion type LA, value type LA.

In addition to these the most commonly used lightning arrester for over voltage protection now-a-days gapless ZnO lightning arrester is also used.

Bibliography

1. NPTEL 2. 7SD5 Manual (Siemens) 3. SIPROTEC-4 Catalogue (Siemens) 4. NPAG (Alstom)