psg notes_opt.pdf

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Department Of Electrical And Electronics Engineering

EE2402 PROTECTION AND SWITCHGEAR

Question fand answers

For

Two mark and 16,8,4 mark questions

UNIT I INTRODUCTION

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

UNIT III APPARATUS PROTECTION

UNIT IV THEORY OF CIRCUIT INTERRUPTION

UNIT V CIRCUIT BREAKERS

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EE2402 PROTECTION AND SWITCHGEAR L T P C 3 0 0 3

AIM:

To introduce the students to the various abnormal operating conditions in power system anddescribe the apparatus and system protection schemes. Also to describe the phenomena of current interruption to study the

various switchgears.

OBJECTIVES:

i. To discuss the causes of abnormal operating conditions (faults, lightning and switchingsurges) of the apparatus and system.ii. To understand the characteristics and functions of relays and protection schemes.

iii. To understand the problems associated with circuit interruption by a circuit breaker.

UNIT I INTRODUCTION 9

Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and faultcurrents - relay terminologydefinitions - and essential qualities of protection. Protection against over voltages due to

lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber and divertersPower System earthingneutral Earthing - basic ideas of insulation coordination.

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9

Electromagnetic relaysover current, directional and non-directional, distance, negative sequence, differential and underfrequency relaysIntroduction to static relays.

UNIT III APPARATUS PROTECTION 9Main considerations in apparatus protection - transformer, generator and motor protection -

protection of bus bars. Transmission line protection - zones of protection. CTs and PTs and their applications in protectionschemes.

UNIT IV THEORY OF CIRCUIT INTERRUPTION 9

Physics of arc phenomena and arc interruption. DC and AC circuit breaking - restriking voltage and recovery voltage -rate of rise of recovery voltage - resistance switching - current chopping - interruption of capacitive current.

UNIT V CIRCUIT BREAKERS 9

Types of circuit breakersair blast, air break, oil, SF6 and vacuum circuit breakerscomparative merits of differentcircuit breakerstesting of circuit breakers.

TOTAL : 45 PERIODS

TEXT BOOKS:

1. M.L. Soni, P.V. Gupta, V.S. Bhatnagar, A. Chakrabarti, A Text Book on Power SystemEngineering, Dhanpat Rai & Co., 1998. (For All Chapters 1, 2, 3, 4 and 5).2. R.K.Rajput, A Tex book of Power System Engineering. Laxmi Publications, First

Edition Reprint 2007.

REFERENCES:1. Sunil S. Rao, Switchgear and Protection, Khanna publishers, New Delhi, 1986.

2. C.L. Wadhwa, Electrical Power Systems, Newage International (P) Ltd., 2000.

3. B. Ravindranath, and N. Chander, Power System Protection & Switchgear, Wiley Eastern Ltd.,1977.4. Badri Ram, Vishwakarma, Power System Protection and Switchgear, Tata McGraw Hill, 2001. 5. Y.G. Paithankar and S.R. Bhide, Fundamentals of Power System Protection, Prentice Hall of India Pvt. Ltd., New

Delhi110001, 2003.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EE2402 PROTECTION AND SWITCHGEAR

UNIT I INTRODUCTION 9

Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and

fault currents - relay terminologydefinitions - and essential qualities of protection. Protection against over

voltages due to lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber anddiverters

Power System earthingneutral Earthing - basic ideas of insulation coordination.

Short Answers 2 Marks

1] List the types of faults in power system

Active Faults

Passive Faults

Transient & Permanent Faults

Symmetrical & Asymmetrical Faults

A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents asteady state condition.

An asymmetrical fault displays a d.c. offset, transient in nature and decaying to the steady state of thesymmetrical fault after a period of time:

Faults on a Three Phase SystemTypes of Faults on a Three Phase System.

(A) Phase-to-earth fault(B) Phase-to-phase fault

(C) Phase-to-phase-to-earth fault

(D) Three phase fault(E) Three phase-to-earth fault

(F) Phase-to-pilot fault *(G) Pilot-to-earth fault *

2] What is the need for protection zones in the system?

Any fault occurring within the given zone will provide necessary tripping of relays or disconnecting or opening

of circuit breakers and thus the healthy section is safe guarded.

If a fault occurs in the overlapping zone in a proper protected scheme, more circuit breakers than the minimum

necessary to isolate the faulty part of the system would trip.

3] what is surge absorber? How do they differ from surge diverter?

Surge Absorber: it is a protective device used to reduce the steepness of the wave front of a surge and

absorbs energy contained in the travelling wave.

Surge Diverter

It will divert excess voltages from an electrical surge to earth. It measures the volts coming in and once

it gets above a set amount (normally 260 volts), will divert the excess volts to earth.

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An Electrical Surge Diverter is a great way to ensure adequate lightning protection for your valuable electronicequipment. Unlike the more common Surge Protector Powerboards that simply switch off if there is spike involts, a Surge Diverter will just divert the excess volts away. It is also installed on your main switchboard,

thereby protecting all powerpoints.

4] Define the term Insulation Coordination.

Insulation Coordination is the process of determining the proper insulation levels of various components

in a power system as well as their arrangements. It is the selection of an insulation structure that will withstandvoltage stresses to which the system, or equipment will be subjected to, together with the proper surge arrester.

The process is determined from the known characteristics of voltage surges and the characteristics of surgearresters.

5] Write any two functions of protective relaying?

(i) The function of a protective relay is to detect and locate a fault and issue a command to the circuit

breaker to disconnect the faulty element.

(ii)It is a device which senses abnormal conditions on a power system by constantly monitoring

electrical quantities of the system which differ under normal and abnormal conditions.

6] What are the desirable qualities of protective relaying? Or Mention the essential features of the power

system protection. Or List the essential features of switchgear.

1. Selectivity 2. Speed & time 3. Sensitivity

4. Reliability 5. Simplicity 6. Economy

7] What is meant by switchgear?

The apparatus used for switching, controlling and protecting the electrical circuits and equipment isknown as switchgear.

8] What are the functions of protection relaying?

The principal function of protective relaying is to cause the prompt removal from service of any elementof the power system when it starts to operate in an abnormal manner or interface with the operation of rest of

the system.

9] What are the causes of faults in power system?

(i) Internal causes.

(ii) Heavy short circuit current may cause damage to damage equipment or other element of the systemof the system due to overheating and high mechanical forces set up due to heavy current.

(iii) Arc associated with short circuits may cause fire hazards. Such fires resulting from arcing may

destroy the fault element of the system. There is also possibility of firing spreading to the other devices if thefault is not isolated quickly.

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10] What are the different types of fault in power system transmission lines?

1. Symmetrical faults 3 phase faults

2. Unsymmetrical faults Single phase to ground, single phase to open circuit, Two phase to groundfault; phase to phase short circuit.

11] List out the types of faults in power system

A].Single phase to ground B] phase to phase faults C] Two phase to ground fault and D] Three phase

short circuit faults.

12] Explain the need for overlapping the zones of protection.

1. The circuit breakers are located in the connection to each power element.

2.

This provision makes it possible to disconnect only the faulty element from the system.

13] Differentiate between primary and backup protection.

No Primary protection BackUp Protection

1It is designed to protect the components

of the power system. [main protection]

It is second line of protection in case main

protection fails.

2It is for instantaneous protection It is designed to operate with enough time

delay

3 Only faulty element will be removed. Larger part of the power system is removed.

14] What are the causes of faults in power system?

1. Internal causes of the equipment.

2. Heavy short circuit current may cause s damage the equipment or other element of the system due to

overheating and high mechanical forces set up due to heavy current.

3. Deterioration of insulation.

15] What are the functions of protective relays

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from

the rest of the system, thereby protecting the system from damages consequent to the fault.

16. Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If

protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing

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its contacts. The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the

system.Also, a low voltage may be created which may damage systems connected to the supply.

17. Define protected zone.

Are those which are directly protected by a protective system such as relays, fuses or switchgears. If a fault

occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that

particular zone.

18. What are unit system and non-unit system?

A unit protective system is one in which only faults occurring within its protected zone are isolated.

Faults occurring elsewhere in the system have no influence on the operation of a unit system.

A non-unit system is a protective system which is activated even when the faults are external to itsprotected zone.

19.What is primary protection?

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit

breaker. It serves as the first line of defence.

20. What is back up protection?

Is the second line of defence, which operates if the primary protection fails to activate within a definite

time delay.

21. Name the different kinds of over current relays.

Induction type non-directional over current relay, Induction type directional over current relay & current

differential relay.

22. Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

23. Define operating time of a relay.

It is defined as the time period extendind from the occurrence of the fault through the relay detecting thefault to the operation of the relay.

24. Define resetting time of a relay.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the

fault is removed and the relay can be reset.

25. What are over and under current relays?

Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg:

Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent

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3. Resistance Neutral Earthing System.

1. Low Resistance Earthing.2. High Resistance Earthing.

4. Resonant Neutral Earthing System.

5. Earthing Transformer Earthing.

(1) Ungrounded Neutral Systems:

In ungrounded system there is no internal connection between the conductors and earth. However, as

system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces.Consequently, the ungrounded system is, in reality, a capacitive grounded system by virtue of the

distributed capacitance.

Under normal operating conditions, this distributed capacitance causes no problems. In fact, it isbeneficial because it establishes, in effect, a neutral point for the system; As a result, the phase

conductors are stressed at only line-to-neutral voltage above ground.

But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-line

voltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on allinsulation in the system. This situation can often cause failures in older motors and transformers, due to

insulation breakdown.

Advantage:

1. After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation,

permitting continued production until a convenient shut down for maintenance can be scheduled.

Disadvantages:1. The interaction between the faulted system and its distributed capacitance may cause transient over-

voltages (several times normal) to appear from line to ground during normal switching of a circuit

having a line-to ground fault (short). These over voltages may cause insulation failures at points other

than the original fault.2. A second fault on another phase may occur before the first fault can be cleared. This can result in very

high line-to-line fault currents, equipment damage and disruption of both circuits.

3. The cost of equipment damage.4. Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct

feeder, then the branch, and finally, the equipment at fault. The result is unnecessarily lengthy and

expensive down downtime.

(2) Solidly Neutral Grounded Systems:

Solidly grounded systems are usually used in low voltage applications at 600 volts or less.

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In solidly grounded system, the neutral point is connected to earth.

Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on theungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the

system three phase faul t cur rent.However, if the reactance of the generator or transformer is too great,

the problem of transient over voltages will not be solved.

While solidly grounded systems are an improvement over ungrounded systems, and speed up thelocation of faults, they lack the current limiting ability of resistance grounding and the extra protection

this provides.

To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must beextend from the source to the furthest point of the system within the same raceway or conduit. Its

purpose is to maintain very low impedance to ground faults so that a relatively high fault current will

flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimizedamage. It also greatly reduces the shock hazard to personnel

If the system is not solidly grounded, the neutral point of the system would float with respect toground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability.

The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current.

The magnitude of the current depends on the fault location and the fault resistance. One way to reduce

the earth fault current is to leave some of the transformer neutrals unearthed.

Advantage:1. The main advantage of solidly earthed systems is low over voltages, which makes the earthing design

common at high voltage levels (HV).

Disadvantage:

1.

This system involves all the drawbacks and hazards of high earth fault current: maximum damage anddisturbances.

2. There is no service continuity on the faulty feeder.3. The danger for personnel is high during the fault since the touch voltages created are high.

Applications:

1. Distributed neutral conductor.

2. 3-phase + neutral distribution.

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3. Use of the neutral conductor as a protective conductor with systematic earthing at each transmission

pole.4. Used when the short-circuit power of the source is low.

(3) Resistance earthed systems:

Resistance grounding has been used in three-phase industrial applications for many years and it resolvesmany of the problems associated with solidly grounded and ungrounded systems.

Resistance Grounding Systems limits the phase-to-ground fault currents. The reasons for limiting the

Phase to ground Fault current by resistance grounding are:

1. To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers,

cables, and rotating machines.

2. To reduce mechanical stresses in circuits/Equipments carrying fault currents.3. To reduce electrical-shock hazards to personnel caused by stray ground fault.

4. To reduce the arc blast or flash hazard.

5. To reduce the momentary line-voltage dip.

6.

To secure control of the transient over-voltages while at the same time.7. To improve the detection of the earth fault in a power system.

Grounding Resistors are generally connected between ground and neutral of transformers, generators

and grounding transformersto limit maximum faul t curr ent as per Ohms Law to a value which wil l no

damage the equipmentin the power system and allow sufficient flow of fault current to detect and

operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with highresistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced. As a result

of this fact, protection devices may not sense the fault.

Therefore, it is the most common application to limit single phase fault currents with low resistanceNeutral Grounding Resistors to approximately rated current of transformer and / or generator.

In addition, limiting fault currents to predetermined maximum values permits the designer to selectivelycoordinate the operation of protective devices, which minimizes system disruption and allows for quicklocation of the fault.

There are two categories of resistance grounding:(1) Low resistance Grounding.

(2) High resistance Grounding.

Ground fault current flowing through either type of resistor when a single phase faults to ground will

increase the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation andsur ge arrestor r atings must be based on li ne-to-l ine voltage. This temporary increase in phase-to-ground voltage should also be considered when selecting two and three pole breakers installed on

resistance grounded low voltage systems.

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Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with

phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazards

associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses andreduce thermal damage to electrical equipment, circuits, and apparatus carrying faulted current.

The difference between Low Resistance Grounding and High Resistance Grounding is a matter ofperception and, therefore, is not well defined. Generall y speaking hi gh-resistance grounding refers to a

system in which the NGR let-thr ough cur rent i s less than 50 to 100 A. Low resistance grounding

indicates that NGR cur rent would be above 100 A. A better distinction between the two levels might be alarm only and tripping. An alarm-only system

continues to operate with a single ground fault on the system for an unspecified amount of time. In a

tripping system a ground fault is automatically removed by protective relaying and circuit interrupting

devices. Alarm-only systems usually limit NGR current to 10 A or less.

Rating of The Neutral grounding resistor:

1. Voltage: Line-to-neutral voltage of the system to which it is connected.

2. Initial Current: The initial current which will flow through the resistor with rated voltage applied.

3. Time: The on time for which the resistor can operate without exceeding the allowable temperaturerise.

(A).Low Resistance Grounded:

Low Resistance Grounding is used for large electrical systems where there is a high investment in

capital equipment or prolonged loss of service of equipment has a significant economic impact and it isnot commonly used in low voltage systems because the limited ground fault current is too low to

reliably operate breaker trip units or fuses. This makes system selectivity hard to achieve. Moreover, low

resistance grounded systems are not suitable for 4-wire loads and hence have not been used incommercial market applications

A resistor is connected from the system neutral point to ground and generally sized to permit only 200A

to 1200 ampsof ground fault current to flow. Enough current must flow such that protective devices can

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detect the faulted circuit and trip it off-line but not so much current as to create major damage at the

fault point.

Since the grounding impedance is in the form of resistance, any transient over voltages are quickly

damped out and the whole transient overvoltage phenomena is no longer applicable. Althoughtheoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the system

voltage dropped across the grounding resistor, there is not enough voltage across the arc forcing current

to flow, for the fault to be reliably detected. For this reason, low resistance grounding is not used for

low voltage systems(under 1000 volts line to-line).

Advantages:

1. Limits phase-to-ground currents to 200-400A.

2. Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-groundarcing current conditions only.

3. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery

windings.

Disadvantages:

1. Does not prevent operation of over current devices.

2. Does not require a ground fault detection system.

3. May be utilized on medium or high voltage systems.

4. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutralloads must be served through an isolation transformer.

Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.

(B).High Resistance Grounded:

High resistance grounding is almost identical to low resistance groundingexcept that the ground fault

current magnitude is typically limited to10 amperes or less. High resistance grounding accomplishestwo things.

The first is that the ground f ault cur rent magnitude is suf fi ciently low enough suchthat no appreciable

damage is done at the fault point. This means that the faulted circuit need not be tripped off-line when

the fault first occurs. Means that once a fault does occur, we do not know where the fault is located. Inthis respect, it performs just like an ungrounded system.

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The second point is it cancontrolthe transient overvoltage phenomenonpresent on ungrounded

systems if engineered properly.

Under earth fault conditions, the resistance must dominate over the system charging capacitance but not

to the point of permitting excessive current to flow and thereby excluding continuous operation

High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shorts

or arcs to ground, but at lower levels than low resistance systems.

In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A.

HRGs are continuous current rated, so the description of a particular unit does not include a time rating.

Unlike NGRs, ground fault current flowing through a HRG is usually not of significant magnitude to

result in the operation of an over current device. Since the ground fault current is not interrupted, aground fault detection system must be installed.

These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically

opens and closes). When the contactor is open, ground fault current flows through the entire resistor.

When the contactor is closed a portion of the resistor is bypassed resulting in slightly lower resistance

and slightly higher ground fault current.

To avoid transient over-voltages, an HRG resistor must be sized so that the amount of groundfault currentthe unit will allow to flow exceeds the electrical systems charging current. As a rule of

thumb, charging current is estimated at 1A per 2000KVA of system capacity for low voltage systemsand 2A per 2000KVA of system capacity at 4.16kV.

These estimated charging currents increase if surge suppressors are present. Each set of suppressors

installed on a low voltage system results in approximately 0.5A of additional charging current and eachset of suppressors installed on a 4.16kV system adds 1.5A of additional charging current.

A system with 3000KVA of capacity at 480 volts would have an estimated charging current of

1.5A.Add one set of surge suppressors and the total charging current increases by 0.5A to 2.0A. A

standard 5A resistor could be used on this system. Most resistor manufacturers publish detailedestimation tables that can be used to more closely estimate an electrical systems charging current.

Advantages:

1. Enables high impedance fault detection in systems with weak capacitive connection to earth2. Some phase-to-earth faults are self-cleared.

3.

The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times thefundamental frequency maximum voltage.

4. Limits phase-to-ground currents to 5-10A.5. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground

arcing current conditions only.

6. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer androtating machinery windings.

7. Prevents operation of over current devices until the fault can be located (when only one phase faults to

ground).

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8. May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993

states that high resistance grounding should be restricted to 5kV class or lower systems with chargingcurrents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding

relaying is employed.

9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral

loads must be served through an isolation transformer.

Disadvantages:

1. Generates extensive earth fault currents when combined with strong or moderate capacitive connection

to earth Cost involved.2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has

occurred.

(4) Resonant earthed system:

Adding inductive reactance from the system neutral point to ground is an easy method of limiting theavailable ground fault from something near the maximum 3 phase short circuit capacity (thousands of

amperes) to a relatively low value (200 to 800 amperes).

To limit the reactive part of the earth fault current in a power system a neutral point reactor can beconnected between the transformer neutral and the station earthing system.

A system in which at least one of the neutrals is connected to earth through an

1. Inductive reactance.

2. Petersen coil / Arc Suppression Coil / Earth Fault Neutralizer.

The current generated by the reactance during an earth fault approximately compensates the capacitive

component of the single phase earth fault current, is called a resonant earthed system.

The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitiveearth fault current of the system.

A system in which the inductive current is slightly larger than the capacitive earth fault current is overcompensated. A system in which the induced earth fault current is slightly smaller than the

capacitiveearth fault current is under compensated

However, experience indicated that this inductive reactance to ground resonates with the system shunt

capacitance to ground under arcing ground fault conditions and creates very high transient over voltageson the system.

To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuit

current to flow underground fault conditions.

Example. A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacityavailable. Due to the high magnitude of ground fault current required to control transient over voltages,

inductance grounding is rarely used within industry.

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Petersen Coils:

A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the

capacitive current in the earth faul t i s compensated by an inductive current passed by the PetersenCoil.A small residual current will remain, but this is so small that any arc between the faulted phase andearth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pin

insulator, could be held on the system without the supply being interrupted. Transient faults would not

result in supply interruptions.

Although the standard Peterson coil does not compensate the entire earth fault current in a network due

to the presence of resistive losses in the lines and coil, it is now possible to apply residual current

compensation by injecting an additional 180 out of phase current into the neutral via the Peterson coil.The fault current is thereby reduced to practically zero. Such systems are known as Resonant earthing

with residual compensation, and can be considered as a special case of reactive earthing.

Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively

act as a high impedance NER, which will substantially reduce any earth fault currents, and hence alsoany corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, including

any EPR hazards impressed onto nearby telecommunication networks).

Advantages:

1. Small reactive earth fault current independent of the phase to earth capacitance of the system.

2.

Enables high impedance fault detection.

Disadvantages:

1. Risk of extensive active earth fault losses.2. High costs associated.

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(5) Earthing Transformers:

For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an

earthing transformer may be used to provide a return path for single phase fault currents

In such cases the impedance of the earthing transformer may be sufficient to act as effective earthing

impedance. Additional impedance can be added in series if required. A special zig-zag transformer issometimes used for earthing delta windings to provide a low zero-sequence impedance and high positive

and negative sequence impedance to fault currents.

Conclusion:

Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazard

reduction, limiting mechanical and thermal damage associated with faults, and controlling transient overvoltages.

High resistance grounding systems may also be employed to maintain service continuity and assist withlocating the source of a fault.

When designing a system with resistors, the design/consulting engineer must consider the specific

requirements for conductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings,and method of serving phase-to-neutral loads.

2] Discuss the essential qualities of protective relaying. 16

Protective Relaying (Part11)

A protective relaying scheme should have certain important qualities. Such an essential qualities ofprotective relaying are,1. Reliability

2. Selectivity and Discrimination

3. Speed and Time

4. Sensitivity5. Stability

6. Adequateness

7. Simplicity and Economy

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1.1 Reliability

Aprotective relaying should be reliable is its basic quality. It indicates the ability of the relay system tooperate under the predetermined conditions. There are various components which go into the operation before a

relay operates. Therefore every component and circuit which is involved in the operation of a relay plays an

important role. The reliability of a protection system depends on the reliability of various components like

circuit breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables, trip circuits etc. Theproper maintenance also plays an important role in improving the reliable operation of the system. The

reliability can not be expressed in the mathematical expressions but can be adjusted from the statistical data

The statistical survey and records give good idea about the reliability of the protective system. The inherentreliability is based on the design which is based on the long experience. This can be achieved by the factors like

i) Simplicity ii) Robustness

iii) High contact pressure iv) Dust free enclosureiv) Good contact material vi) Good workmanship and

vii) Careful Maintenance

1.2 Selectivity and Discrimination

The selectivity id the ability of the protective system to identify the faulty part correctly and disconnect that

part without affecting the rest of the healthy part of system. The discrimination means to distinguish between.

The discrimination quality of the protective system is the ability to distinguish between normal condition and

abnormal condition and also between abnormal condition within protective zone and elsewhere. The protectivesystem should operate only at the time of abnormal condition and not at the time of normal condition. Hence i

must clearly discriminate between normal and abnormal condition. Thus the protective system should select thefault part and disconnect only the faulty part without disturbing the healthy part of the system.

The protective system should not operate for the faults beyond its protective zone. For example, consider

the portion of a typical power system shown in the Fig. 1.

Fig. 1

It is clear from the Fig. 1 that if fault F 2 occurs on transmission line then the circuit breakers 2 and 3 should

operate and disconnect the line from the remaining system. The protective system should be selective in

selecting faulty transmission line only for the fault and it should isolate it without tripping the adjacenttransmission line breakers or the transformer.

If the protective system is not selective then it operates for the fault beyond its protective zones and

unnecessary the large part of the system gets isolated. This causes a lot of inconvenience to the supplier andusers.1.3 Speed and Time

a protective system must disconnect the faulty system as fast as possible. If the faulty system is not

disconnect for a long time then,1. The devices carrying fault currents may get damaged.

2. The failure leads to the reduction in system voltage. Such low voltage may affect the motors and generators

running on the consumer sude.

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3. If fault persists for long time, then subsequently other faults may get generated.

The high speed protective system avoids the possibility of such undesirable effects.The total time required between the instant of fault and the instant of final arc interruption in the circuit

breaker is called fault clearing time. It is the sum of relay time and circuit breaker time. The relay time is the

time between the instant of fault occurrence and the instant of closure of relay contacts. The circuit breaker

times is the time taken by the circuit breaker to operate to open the contacts and to extinguish the arccompletely. The fault clearing time should be as small as possible to have high speed operation of the protective

system.

Though the small fault clearing time is preferred, in practice certain time lag is provided. This is because,1. To have clear discrimination between primary and backup protection

2. To prevent unnecessary operation of relay under the conditions such as transient, starting inrush of current

etc.Thus fast protective system is an important quality which minimises the damage and it improves the overall

stability of the power system.

1.4 Sensitivity

The protective system should be sufficiently sensitive so that it can operate reliably when required. Thesensitivity of the system is the ability of the relay system to operate with low value of actuating quantity.

It indicates the smallest value of the actuating quantity at which the protection starts operating in relation

with the minimum value of the fault current in the protected zone.

The relay sensitivity is the function of the volt-amperes input to the relay coil necessary to cause itsoperation. Smaller the value of volt-ampere input, more sensitive is the relay. Thus 1 VA input relay is more

sensitive than the 5VA input relay.

Mathematically the sensitivity is expressed by a factor called sensitivity factor . It is the ratio of minimumshort circuit current in the protected zone to the minimum operating current required for the protection to start.

Ks= Is/Io

where Ks = sensitivity factor

Is = minimum short circuit current in the zoneIo= minimum operating current for the protection

1.5 Stability

The stability is the quality of the protective system due to which the system remains inoperative and stableunder certain specified conditions such as transients, disturbance, through faults etc. For providing the stability,certain modifications are required in the system design. In most of the cases time delays, filter circuits

mechanical and electrical bias are provided to achieve stable operation during the disturbances.

1.6 Adequateness

There are variety of faults and disturbance those may practically exists in a power system. It is impossibleto provide protection against each and every abnormal condition which may exist in practice, due to economical

reasons. But the protective system must provide adequate protection for any element of the system. The

adequateness of the system can be assessed by considering following factors,

1. Ratings of various equipments2. Cost of the equipments

3. Locations of the equipments4. Probability of abnormal condition due to internal and external causes.5. Discontinuity of supply due to the failure of the equipment

1.7 Simplicity and Economy

In addition to all the important qualities, it is necessary that the cost of the system should be well within

limits. In practice sometimes it is not necessary to use ideal protection scheme which is economically

unjustified. In such cases compromise is done. As a rule, the protection cost should not be more than 5% of thetotal cost. But if the equipments to be protected are very important, the economic constrains can be relaxed.

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The protective system should be as simple as possible so that it can be easily maintained. The complex

system are difficult from the maintenance point of view. The simplicity and reliability are closely related toeach other. The simpler system are always more reliable.

3] Discuss the nature and causes of different faults in a power system. (16)

Nature and causes of Faults:Any faults in electrical apparatus are nothing but the defect in its electricalcircuit which makes current path directed from its intended path. Normally due to breaking of conductors orfailure of insulation, these faults occur. The other reasons for occurrence of fault include mechanical failure,

accidents. Excessive internal and external stresses. The impedance of the path in the fault is low and the fault

currents are comparatively large. The induction of insulation is not considered as a fault until it shows some

effect sucj as excessive current flow or reduction of impedance between conductors or between conductors andearth.

When a fault occurs on a system, the voltage of the three phases become unbalanced. As the fault currents arelarge, the apparatus may get damaged. The flow of power is diverted towards the fault which affects the supply

to the neighboring zone.

A power system consists of generators, transformers, switchgear, transmission and distribution circuits. There

is always a possibility in such a large network that some fault will occur in some part of the system. The

maximum possibility of fault occurrence is on the transmission lines due to their greater lengths and exposure toatmospheric conditions.

The faults cannot be classified according to the causes of their incidence. The breakdown may occur at normalvoltage due to deterioration of insulation. The breakdown may also occur due to damage on account of

unpredictable causes which include perching of birds, accidental short circuiting by snakes, kite strings, three

branches etc. The breakdown may occur at abnormal voltages due to switching surges or surges caused bylighting.sss

4] List the types of faults in power system

Active Faul ts

The Activefault is when actual current flows from one phase conductor to another

(phase-to-phase) or alternatively from one phase conductor to earth (phase-to-earth).This type of fault can also be further classified into two areas, namely the solid

fault and the incipient fault.

Passive Faul ts

Passive faults are not real faults in the true sense of the word but are rather conditionsthat are stressing the system beyond its design capacity, so that ultimately active

faults will occur.

Typical examples are:

Overloading - leading to overheating of insulation (deteriorating quality,

reduced life and ultimate failure).

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Overvoltage - stressing the insulation beyond its limits.

under frequency - causing plant to behave incorrectly.

Power swings - generators going out-of-step or synchronism with each other

Transient & Permanent Faults

Transient faults are faults which do not damage the insulation permanently and allow

the circuit to be safely re-energized after a short period of time. A typical example would be an insulator

flashover following a lightning strike, which would be successfully cleared on opening of the circuit breaker,

which could then be automatically reclosed.Transient faults occur mainly on outdoor equipment where air is the main insulating medium.

Permanent faults, as the name implies, are the result of permanent damage to the insulation. In this case, theequipment has to be repaired and reclosing must not be entertained.

Symmetri cal & Asymmetr ical Faul ts

A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a

steady state condition.An asymmetrical fault displays a d.c. offset, transient in nature and decaying to the steady state of the

symmetrical fault after a period of time:

Faults on a Three Phase System

The types of faults that can occur on a three phase A.C. system are as follows:Types of Faul ts on a Three Phase System.

(A) Phase-to-earth fault(B) Phase-to-phase fault

(C) Phase-to-phase-to-earth fault

(D) Three phase fault

(E) Three phase-to-earth fault(F) Phase-to-pilot fault *

(G) Pilot-to-earth fault *

5] What is a surge absorber? Write a short note on Ferranti surge absorber. [8]

Surge absorbers are protective devices used to absorb the complete surge i,e. due to lightening surge or anytransient surge in the system..........unlike the

lightening arrestor in which a non-linear resistor is provided which provides a low resistance path to the

dangerously high voltages on the system to the earth...

Ferranti surge absorber

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6. What are the causes of over voltage on a power system? (8)

Overvoltage - Causes and Protection

Over voltages occur in a system when the system voltage rises over 110% of the nominal rated voltage.

Overvoltage can be caused by a number of reasons, sudden reduction in loads, switching of transient loads,

lightning strikes, failure of control equipment such as voltage regulators, neutral displacement,. Overvoltage can

cause damage to components connected to the power supply and lead to insulation failure, damage to electroniccomponents, heating, flashovers, etc.

Overvoltage relays can be used to identify overvoltages and isolate equipment. These relays operate when the

measured voltage exceeds a predetermined set-point. The voltage is usually measured using a PotentialTransformers. The details of the ratio of the potential transformer are also entered into the relay. These relays

are usually provided with a time delay. The time delay can be either instantaneous, fixed time or for IDMT(inverse definite minimum time) curves.

Generally, overvoltage relays are provided with sufficient time delay in order to avoid unwanted trippings due

to transients (See article on Transients).

These relays can be used to isolate feeders and other equipment connected to the network. In the case of

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generators, these relay also switch off the excitation system to the generators thereby preventing voltage build-

up.

7. Describe the phenomenon of lightning. (16)

Lightning, an awesome and terrifying natural phenomenon, is really nothing more than an electrical

discharge that happens to be at an enormous voltage. Lightning equalizes a difference of electrical potential,

whether it is between a cloud and the ground, between two clouds, or between two different areas of the same

cloud. Due to various mechanisms in storm cloud formation, such as the rising and falling of air currentscarrying moisture and ice particles, the storm cloud generates an electrical charge at its base. Charged regions in

a thundercloud create conductive ionization channels through the atmosphere called stepped leaders, while anopposite electrical charge or shadow accumulates on the ground below.

These leaders jump and branch ever downward in a quest to connect with their oppositely charged image

or shadow on the earths surface. As the fiercely charged electrical field builds, streamers rise from objects on

the ground. When a stepped leader from the sky chooses to meet the strongest rising streamer from below, a

lightning circuit is completed. The main lightning stroke erupts up the ionized path. Numerous additionalstrokes along the same channel are common, creating a mesmerizing strobe light effect. We are most interested

in protecting our clients from the shadow and its resulting electrical dischargea lightning strike.

Whatever lightning protection method is implemented, the importance of thegrounding system

supporting it cannot be overemphasized. A well designed, correctly installed, low impedance and low resistanceconnection from the earth to the components of the lightning protection system is essential. With the addition oftransient voltage surge suppression,a comprehensive facility protection approach addressing all concerns of

safety and power quality finally is made possible. Only then may we watch in unflinching wonder as the sky

explodes in its timeless electrical display.

8. (a) What is necessity of protecting electrical equipment against traveling waves? (6)

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9. Describe the construction and principle of operation of

(i) expulsion type lightning arrester, (8) (ii) Value type lightning arrester. (8)

(i) expulsion type lightning arrester, (8)

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10. Write short notes on the following.

(i) klydonograph and magnetic link (4)

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(ii) Rod gap (4)It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in

between as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth

The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so

that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdownshould occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the

insulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap

across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, thegap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and

the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly

conducted to earthLimitations:(i)After the surge is over, the arc n the gap is maintained by the normal supply voltage, leading to short-circuit

on the system.

(ii) The rods may melt or get damaged due to excessive heat produced by the arc.

(iii) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.

(iv) The polarity of the f the surge also affects the performance of this arrester.

Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main

arresters.

It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap inbetween as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth

The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so

that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown

should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across theinsulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap

across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the

gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and

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the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly

conducted to earth

(iii) Arcing horns (4)

Transmission and other electrical equipment can be exposed to overvoltages. Overvoltages can be

caused by a number of reasons such as lightning strikes, transient surges, sudden load fluctuation, etc. In theevent of an overvoltage, the insulating equipment such as the insulators on a transmission line or bushings in a

transformer can be exposed to high voltages which may lead to their failure.

Arcing horns are protective devices that are constructed in the form of projections in the conductingmaterials on both sides of an insulator. Arcing horns are fitted in pairs. Thus in transmission lines they are

found on the conducting line and the transmission tower across the insulators. In transmission lines, in theevent of a lightning strike on the tower, the tower potential rises to dangerous levels and can result in flashovers

across the insulators causing their failure. Arcing horns prevent this by conducting the arc across the air gap

across them.

Arcing horns function by bypassing the high voltage across the insulator using air as a conductive

medium. The small gap between the horns ensures that the air between them breaks down resulting in a

flashover and conducts the voltage surge rather than cause damage to the insulator.The horns are constructed inpair so that one horn is on the line side and the other is on the ground side.

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Arcing Horns are also used along with air insulated switchgear equipment. Air insulated switchgear are

vulnerable to damage due to arcing. Arcing horns serve to divert the arc towards themselves thus protecting the

switching equipment. The arcing horns serve to move the arc away from the bushings or the insulators.Figure shows the horn gap arrester. It consists of a horn shaped metal rods A and B separated by a small air

gap. The horns are so constructed that distance between them gradually increases towards the top as shown. The

horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and

choke coil L while the other end is effectively grounded. The resistance R helps in limiting the follow current to

a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very

high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be

protected. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc

across the gap.

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Under normal conditions, the gap is non-conducting i.e. normal supply voltage is insufficient to initiate the arc

between the gap. On the occurrence of an over voltage, spark-over takes place across the small gap G. The

heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves

progressively into positions 1,2 and 3. At some position of the arc (position 3), the distance may be too great for

the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus

conducted through the arrester to the ground.

(iv) Basic impulse insulation level (4)

BIL or basic impulse insulation level is the dielectric insulation gradient of a material tested to withstand the

voltage stress at a voltage impressed between the material and a conductive surface beyond the BIL rating, anelectric tracking starts to occur which will then result into an arcing flashover to the conductive surface. In

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addition it is the capacity of an equipment to withstand mechanical stress like lightning strike without causing

any damage to the equipment...

11. What is Peterson coil? What protective functions are performed by this device? (16)

[Ref Q.No 1 IN 16 MARKS]

ASSIGNMENT

1. What are the requirements of a ground wire for protecting power conductors against direct lightning

stroke? Explain how they are achieved in practice. (16)

2. Determine the inductance of Peterson coil to be connected between the neutral and ground to

neutralize the charging current of overhead line having the line to ground capacitive of 0.15f. If thesupply frequency is 50Hz and the operating voltage is 132 KV, find the KVA rating of the coil. (16)

3. (a) Explain the term insulation coordination. (8)

(b) Describe the constn of volt-time curve and the terminology associated with impulse-testing. (8)

4. Explain the operation of various types of surge absorbers (16)

5. Why is it necessary to protect the lines and other equipment of the power system against over voltages?REF Q.No

6.Describe in brief the protective devices used for protection of equipment against such waves? (10)

7. What protective measures are taken against lightning over voltages? (16)

8. (a) What is tower-footing resistance? (4)

(b) Why is it required to have this resistance as low as economically possible? (4)

(c) What are the methods to reduce this resistance? (8)

9.Describe the protection of stations and sub-stations against direct lightning stroke. (16)

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UNIT 2

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9

Electromagnetic relaysover current, directional and non-directional, distance, negative sequence, differential and under

frequency relaysIntroduction to static relays.

1] Draw the block diagram of a static differential relay.

2] What are the advantages of over current relays over electromagnetic relays?o Low power consumption as low as 1mW

o No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement ofcontacts

o No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc.

o A single relay can perform several functions like over current, under voltage, single phasing protection

by incorporating respective functional blocks. This is not possible in electromagnetic relays

o Static relay is compact

o Superior operating characteristics and accuracy

o Static relay can think , programmable operation is possible with static relay

o Effect of vibration is nil, hence can be used in earthquake-prone areas

o Simplified testing and servicing. Can convert even non-electrical quantities to electrical

in conjunction with transducers.

3] Compare static and electromagnetic relay.

Electromagnetic relaysare1st generation relays they use principle of electromagnetic principle. They

depend upon gravitation also and the value changes to the surrounding magnetic fields also.

Static relaysare 2nd generation relays. In this relays transistors and IC's r been used. There value mayvary with respect to temperature also.

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Numerical relaysare present generation relays. They use Microprocessor. Within built software and

predefined values if any values extend the predefined values then relay gets on by this relay we can finddistance of fault also.

4] A relay is connected to 400/5 ratio current transformer with current setting of 150%. Calculate the

Plug Setting Multiplier when circuit carries a fault current of 4000A.

5] What are the features of directional relay?

Dual polarized directional element capability

Zero sequence (Residual) voltage (3V0)

Zero sequence (Neutral) current (I0) Directional element is insensitive to third or higherharmonics.

12 Field selectable, inverse, definite time, and BritishStandard (BS142) time over current curves.

Independent time and instantaneous overcurrent elements. Optional independent directional and non-directional instantaneous over current functions.

Wide continuous setting ranges:

Current pickup accuracy +2% Timing accuracy +5%

Time Over current ResetInstantaneous

Optional Auxiliary Output Contact follows operation ofuser defined function (TOC, Dir INST, Non

Dir INST) Drawout construction, testable-in-case

Provision for trip circuit testing

Less than 0.1 Ohmburden for all sensing inputs.

Standard magnetically latched targets for each tripfunction.

6] What are the advantages and disadvantages of static relays?

a. Low burden on CTs & PTs.

b. No moving contactsc. Fast operation and long life.;

d. Sensitivity is more.

e. Fast reset & no overshoot.

f. Size of the relay is small since measuring circuit needs very low current.

7] Classify the different types of over current relays based on inverse time characteristics?

a. Static instantaneous over current relay

b. Directional static over current relayc. Inverse time over current relay

8] Mention any two applications of differential relay.

It is comparing miniature to heavy loads.

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Fault current can be easily detected.

Used for protecting generator, generator unit transformer unit, large motors and bus bars

9] Derive and draw the characteristics of an impedance relay.

Operating time Z distance

Time of operation of relay

distance

where V- voltage in volts Zimpedance in ohmsIcurrent in amperes

10] For what purpose distance relays used?

It is used in the main transmission lines or sub transmission lines of 33KV,66KV,& 132KV.

It measures in terms of impedance, reactance etc., from relay to the point of fault.

11] Define the terms a] pick up value b]plug setting multiplier.

a] pick up value

As the current or voltage on an un operated relay is increased, the value at or below which all contacts function

b]Plug Setting Multiplier

The actual r.m.s current flowing in the relay expressed as a multiple of the setting current (pick upcurrent) is known as plug setting multiplier.

mathematically ,

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PSM= SECONDARY CURRENT/RELAY CURRENT SETTI NG

or

PSM= (primary curr ent during fault)/(relay current setting *C.T ratio)

PSM =current through operating coil

plug setting current =

CT secondary current

plug setting current

PSM =

line current CT ratio

plug setting current =

line current

CT ratio plug setting current

12] What type of relay is best suited for long distance , very high voltage transmissions?

Impedance relay

13] What type of relay is best suited for generator protection?

Differential relay

14] Give any two applications of electromagnetic relays.

To protect the ac & dc equipments.

Over/under current and over/under voltage protection of various ac & dc equipments.

15. What is the need of relay coordination?The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest

possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be

sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults canalways be isolated quickly without serious disturbance to the rest of the system.

16. Mention the short comings of Merz Price scheme of protection applied to a power transformer.In a power transformer, currents in the primary and secondary are to be compared. As these two currents

are usually different, the use of identical transformers will give differential current, and operate the relay under

no-load condition. Also, there is usually a phase difference between the primary and secondary currents of threephase transformers. Even CTs of proper turn-ratio are used, the differential current may flow through the relay

under normal condition

.

17. What are the various faults to which a turbo alternator is likely to be subjected?Failure of steam supply; failure of speed; overcurrent; over voltage; unbalanced loading; stator winding

fault .

18. What is an under frequency relay?

An under frequency relay is one which operates when the frequency of the system (usually an alternatoror transformer) falls below a certain value.

19. Define the term pilot with reference to power line protection.Pilot wires refers to the wires that connect the CTs placed at the ends of a power transmission line as

part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

20. Mention any two disadvantage of carrier current scheme for transmission line only.

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The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response

time ofband pass filter; capacitance phase-shift of the transmission line

21. What are the features of directional relay?

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal

ratio; burden must not be excessive.

22. What are the causes of over speed and how alternators are protected from it?Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are

provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the primemover when a dangerous over-speed occurs.

23. What are the main types of stator winding faults?

Fault between phase and ground; fault between phases and inter-turn fault involving turns of the samephase winding.

23. Give the limitations of Merz Price protection.

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomesimpossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the

voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CTs. In

addition to this, there always an inherent phase difference between the primary and the secondary quantities anda possibility of current through the relay even when there is no fault.

24. What are the uses of Buchholzs relay?Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer

and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil

immersion transformers with a rating over 750KVA.

16 Mark questions

1] What are the different types of electromagnetic relays? Discuss their field of applications. [16]

Type of protection Over current. Directional over current.

Distance.

Over voltage.

Differential.

Reverse power.

Electromagnetic relays

Electromagnetic relays are constructed with electrical, magnetic and mechanical components, havean operating coil and various contacts and are very robust and reliable. The construction characteristics

can be classified in three groups, as detailed below.

Attraction relays

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Attraction relays can be supplied by AC or DC, and operate by the movement of a piece of metal

when it is attracted by the magnetic field produced by a coil. There are two main types of relay in this class.The attracted armature relay, which is shown in figure 1, consists of a bar or plate of metal which

pivots when it is att racted towards the coil.

The armature carries the moving part of the contact, which is closed or opened according to the

design when the armature is attracted to the coil. The other type is the piston or solenoid relay, illustrated inFigure 2, in which bar or piston is attracted axially within the field of the solenoid. In this case, the piston also

carries the operating contacts.

It can be shown that the force of attraction is equal to K1I2- K2, where 1depends upon the number of turns onthe operating solenoid, the air gap, the effective area and the reluctance of the magnetic circuit, among other

factors. K2 is the restraining force, usually produced by a spring. When the relay is balanced, the resultant force is

zero and therefore 112= K2,

So that

In order to control the value at which the relay starts to operate, the restraining tension of the spring or

the resistance of the solenoid circuit can be varied, thus modifying the restricting force. Attraction relayseffectively have no time delay and, for that reason, are widely used when instantaneous operations are

required.

Relays with moveable coilsThis type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to

rotate between the poles of a permanent magnet. The coil is restrained by two springs which also serve asconnections to carry the current to the coil.

The torque produced in the coil is given by:

T = B.l.a.N.i

Where:

T= torque

B = flux densityL=length of the coil

a= diameter of the coil

N = number of turns on the coil

i= current flowing through the coil

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Figure 2 Solenoid-type relay

Figure 3 Inverse time characteri sticFrom the above equation it will be noted that the torque developed is proportional to the current. The speed of

movement is controlled by the damping action, which is proportional to the torque. It thus follows that the relay

has an inverse time characteristic similar to that illustrated in Figure 3. The relay can be designed so that thecoil makes a large angular movement, for example 80.

Induction relaysAn induction relay works only with alternating current. It consists of an electromagnetic system which operates ona moving conductor, generally in the form of a disc or cup, and functions through the interaction of

electromagnetic fluxes with the parasitic Fault currents which are induced in the rotor by these fluxes. These

two fluxes, which are mutually displaced both in angle and in position, produce a torque that can be expressed

by

T= 1.1.2.sin ,

Where 1 and 2are the interacting fluxes and is the phase angle between 1 and 2. It should be noted that

the torque is a maximum when the fluxes are out of phase by 90, and zero when they are in phase.

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Figure 4 Electromagnetic forces in induction r elays

It can be shown that 1= 1sin t, and 2= 2sin (t+ ) , where is the angle by which 2 leads 1.Then:

And

Figure 4 shows the interrelationship between the currents and the opposing forces. Thus:

F = ( F 1 - F 2 ) (2i1+1i2 )

F 21sin T

Induction relays can be grouped into three classes as set out below.

Shaded-pole relay

In this case a portion of the electromagnetic section is short-circuited by means of a copper ring or coil.

This creates a flux in the area influenced by the short circuited section (the so-called shaded section) whichlags the flux in the nonshaded section, see Figure 5.

Figure 5 Shaded-pole relay

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Figure 6 Wattmetric-type relay

In its more common form, this type of relay uses an arrangement of coils above and below the disc with the upper andlower coils fed by different values or, in some cases, with just one supply for the top coil, which induces an out-of-

phase f lux in the lower coil because of the air gap. Figure 6 illust r ates a typical arrangement.

Cup-type relay

This type of relay has a cylinder similar to a cu which can rotate in the annular air gap between the poles of the coils,and has a fixed central core, see Figure 7. The operation of this relay is very similar to that

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Figure 7Cup-type relay

Of an induction motor with salient poles for the windings of the stator. Configurations with four or eight polesspaced symmetrically around the circumference of the cup are often used. The movement of the cylinder is

limited to a small amount by the contact and the stops. special spring provides the restraining torque.

The torque is a function of the product of the two currents through the coils and the cosine of the angle

between them. The torque equation is

T= ( KI1I2cos (12) Ks),

Where K, .s and are design constants, 1and I2are the currents through the two coils and 12is the anglebetween I1and I2.

In the first two types of relay mentioned above, which are provided with a disc, the inertia of the disc

provides the time-delay characteristic. The time delay can be increased by the addition of a permanent magnet.

The cup-type relay has a small inertia and is therefore principally used when high speed operation is required,

for example in instantaneous units.

2.What are the various types of over current relays? Discuss their area of application. (16)

STATIC INSTANTANEOUS OVER-CURRENT RELAYThe block diagram of an instantaneous over-current relay is shown in fig 21. The same construction may

be used for under-voltage, over-voltage and earth fault relays too.

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The secondaries of the line CTs are connected to a summation circuit (not shown in the fig). The output of this

summation CT is fed to an auxiliary CT, whose output is rectified smoothened and supplied to the measuring

unit (level detector). The measuring unit determines whether the quantity has attained the threshold value (setvalue) or not. When the input to measuring unit is less than the threshold value, the output of the level detector

is zero.

For an over-current relay

For I input< I threshold; Ioutput= 0For I input> I threshold; Ioutput= Present

In an actual relay I thresholdcan be adjusted.

After operation of the measuring unit, the amplifier amplifies the output. Amplified output is given to the output

circuit to cause trip/alarm. If time-delay is desired, a timing circuit is introduced before the level detector.Smoothing circuit and filters are introduced in the output of the bridge rectifier. Static over-current relay is

made in the form of a single unit in which diodes, transistors, resistors, capacitors etc., are arranged on printed

board and are bolted with epoxy resin.

STATIC OVER-CURRENT TIME RELAY

The block diagram of static over current time relay is shown in fig

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The current from the line CT is reduced to 1/1000 th by an auxiliary CT, the auxiliary has taps on the primary

for selecting the desired pick-up and current range and its rectified output is supplied to level detector I (over-

load level detector) and an R-C timing circuit. When the voltage on the timing capacitor Vc attains the threshold

value of the level detector II, tripping occurs. Time delay given by the timing circuit shown in fig 22 b is given

as Tc = RC log eE/(EVT) .

Where VTis the threshold value of the level detector II. By varying values of R and C the time can be varied

without difficulties

3.Describe the operating principle, constructional features and area of applications of reverse power or

directional relay. (16)Directionalized relays are relays that use a polarizing circuit to determine which "direction" (in the zone of

protection, or out of the zone protection) a fault is. There are many different types and different polarizing

methods - ground polarizing, voltage polarizing, zero sequence voltage polarizing, negative sequencepolarizing, etc.

The basic operation of this relay is just like any nondirectional relay, but with an added torque control - the

directionalizing element. This element allows the relay to operate when it is satisfied that the fault is within thezone of protection (ie not behind where the relay is looking).

A reverse power relay is a directional power relay that is used to monitor the power from a generator running inparallel with another generator or the utility. The function of the reverse power relay is to prevent a reverse

power condition in which power flows from the bus bar into the generator. This condition can occur when there

is a failure in the prime mover such as an engine or a turbine which drives the generator.

Causes of Reverse Power

The failure can be caused to a starvation of fuel in the prime mover, a problem with the speed controller or an

other breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a

condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives theprime mover. This happens as in a synchronized condition all the generators will have the same frequency. Anydrop in frequency in one generator will cause the other power sources to pump power into the generator. The

flow of power in the reverse direction is known as the reverse power relay.

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Another cause of reverse power can occur duringsynchronization.If the frequency of the machine to be

synchronized is slightly lesser than the bus bar frequency and the breaker is closed, power will flow from thebus bar to the machine. Hence, during synchroniza

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