EE6702 - PROTECTION AND SWITCHGEAR

UNIT 3

Part – A

1. What are the types of graded used in line of radial relay feeder?

Definite time relay and inverse-definite time relay.

2. What are the various faults that would affect an alternator?

(a) Stator faults

1. Phase to phase faults 2. Phase to earth faults 3. Inter turn faults

(b) Earth faults

1. Fault between turns 2. Loss of excitation due to fuel failure

(c) Over speed

1. Loss of drive 2. Vacuum failure resulting in condenser pressure rise, resulting in shattering of the

turbine low pressure casing

(d) Fault on lines

1. Fault on busbars

3. Why neutral resistor is added between neutral and earth of an alternator?

In order to limit the flow of current through neutral and earth a resistor is introduced between them.

4. What is the backup protection available for an alternator?

Overcurrent and earth fault protection is the backup protections

5. What are faults associated with an alternator?

(a) External fault or through fault

(b) Internal fault

1. Short circuit in transformer winding and connection 2. Incipient or slow developing faults

6. What are the main safety devices available with transformer?

Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator .

7. What are the problems arising in differential protection in power transformer and how are they overcome?

a. Difference in lengths of pilot wires on either sides of the relay. This is overcome by

connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.

b. Difference in CT ratio error difference at high values of short circuit currents that makes

the relay to operate even for external or through faults. This is overcome by introducing

bias coil.

c. Tap changing alters the ratio of voltage and currents between HV and LV sides and the

relay will sense this and act. Bias coil will solve this.

d. Magnetizing inrush current appears wherever a transformer is energized on its primary side

producing harmonics. No current will be seen by the secondary. CT’s as there is no load in

the circuit. This difference in current will actuate the differential relay. A harmonic

restraining unit is added to the relay which will block it when the transformer is energized.

8. What is REF relay?

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the

transformer, the voltage available to drive the earth circuit is very small, which may not be

sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of

protection in the winding of the transformer is restricted to cover only around 85%. Hence the

relay is called REF relay.

9. What is over fluxing protection in transformer?

If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability

of the transformer to withstand this is limited to a few minutes only. This phenomenon is called

over fluxing.

10. Why busbar protection is needed?

a) Fault level at busbar is high

b) The stability of the system is affected by the faults in the bus zone.

c) A fault in the bus bar causes interruption of supply to a large portion of the system network.

11. What are the merits of carrier current protection?

Fast operation, auto re-closing possible, easy discrimination of simultaneous faults .

12. What are the errors in CT?

a) Ratio error Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100

b) The value of transformation ratio is not equal to the turns ratio.

c) Phase angle error:

Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]

13. What is field suppression?

When a fault occurs in an alternator winding even though the generator circuit breaker is tripped,

the fault continues to fed because EMF is induced in the generator itself.

Hence the field circuit breaker is opened and stored energy in the field winding is discharged

through another resistor. This method is known as field suppression.

14. What are the causes of bus zone faults?

a) Failure of support insulator resulting in earth fault

b) Flashover across support insulator during over voltage

c) Heavily polluted insulator causing flashover

d) Earthquake, mechanical damage etc.

15. What are the problems in bus zone differential protection?

a) Large number of circuits, different current levels for different circuits for external faults.

b) Saturation of CT cores due to dc component and ac component in short circuit currents.

c) The saturation introduces ratio error.

d) Sectionalizing of the bus makes circuit complicated.

e) Setting of relays need a change with large load changes

16. What is static relay?

It is a relay in which measurement or comparison of electrical quantities is made in a static network

which is designed to give an output signal when a threshold condition is passed which operates a

tripping device.

17. What are the advantages of static relay over electromagnetic relay?

a) Low power consumption as low as 1mW

b) No moving contacts; hence associated problems of arcing, contact bounce, erosion,

replacement of contacts

c) No gravity effect on operation of static relays. Hence can be used in vessels ie, ships,

aircrafts etc.

d) A single relay can perform several functions like over current, under voltage, single

phasing protection by incorporating respective functional

DHANALAKSHMI COLLEGE OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EE 6702 – PROTECTION AND SWITCHGEAR

UNIVERSITY QUESTIONS AND ANSWERS

UNIT 3

1. Explain protection scheme of an AC induction motor (N/D-16)

Three-phase or poly-phase motors are of mainly two types: induction or asynchronous motors and synchronous

motors. Synchronous motors are special types of motors used in constant speed applications, whereas most of the

motors used in the industrial applications are of induction type. This article concentrates only on three-

phase induction motor and its protection.

These motors are squirrel and slip-ring type induction motors. Three-phase induction motor consists of a stator and a

rotor, and there is no electrical connection between these two. These stator and rotors are made up of high-magnetic

core materials with less hysteresis and eddy current losses. The Stator consists of three phase windings overlapped

with one another at 120 degree phase shift. These windings are excited by three-phase main supply.

This three-phase AC motor rotor is different for the slip ring and squirrel cage induction motors. In a squirrel-cage

motor, the rotor consists of heavy aluminum or copper bars that are shorted on both ends of the cylindrical rotor. In a

slip-ring-type induction motor, the rotor consists of three phase windings that are internally starred at one end, and

the other ends are brought outside and connected to the slip rings mounted on the rotor shaft, as shown in the figure.

With the help of carbon brushes, rheostat is connected to these windings for developing a high starting torque.

Principle of Operation: Whenever three-phase supply is given to the three-phase stator winding, a rotating

magnetic field with 120 displacements at constant magnitude and rotating at a synchronous speed is produced in it.

This changing magnetic field travels over to the rotor conductor causing to induce a current in the rotor conductors

according to the Faradays laws of electromagnetic induction. As the rotor conductors are shorted, the current starts

to flow through these conductors.

According to the Lenz’s law, these induced currents oppose the cause for its production, i.e., rotating magnetic field.

As a result, the rotor starts rotating in the same direction of the rotating magnetic field. However, the rotor speed

must be less than the stator speed – otherwise no currents are induced in the rotor because the relative speed of the

magnetic fields of the rotor and the stator is the reason for rotor motion. This difference between the stator and the

rotor fields is called slip. Due to this relative speed difference between the stator and the rotors, this 3-phase motor is

called asynchronous machine.

Types of Protections Needed for Induction Motor

Three-phase induction motors are accountable for 85 percent of the installed capacity of the industrial driving

systems. Therefore, the protection of these motors is necessary for reliable operation of loads. Motor failures are

mainly divided into three groups: electrical, mechanical and environmental. Mechanical stresses cause overheating

resulting in the rotor bearings’ wear and tear, whereas the over mechanical load causes heavy currents to draw, and

thus results in increasing temperatures. Electrical failures are caused by various faults like Phase-to-phase and

phase-to-ground faults, single phasing, over and under voltage, voltage and current unbalance, under frequency, etc.

Starting of Current of Induction motor

In addition to the motor protection systems for the above mentioned faults, it is also necessary to use three-phase

motor starter to limit the staring current of the induction motor. As we know – in every electrical machine, when

supply is provided, there is opposition to this supply by an induced EMF – which is called back EMF. This limits the

current drawing by the machine, but at the beginning, the EMF is zero because it is directly proportional to the speed

of the motor. And therefore, the zero back EMF’s huge current will be drawn by the motor at the start, and this will be

8-12 times the full-load current as shown in the figure.

To protect the motor from the high-staring current, there are different staring methods available like reduced voltage,

rotor resistance, DOL, star-delta starter, auto transformer, soft starter, etc. And, for protecting the motor from the

above discussed faults; various protection equipments like relays, circuit breakers, contractors and various drives are

implemented.

These are some of the protection systems for three-phase induction motors against starting inrush currents,

overheating and single phasing faults with the use of microcontroller for low-level applications for better

understanding of the students.

Electronic Soft Start for 3-Phase Induction Motor

This soft start of induction motor is the modern method of starting that reduces the mechanical and electrical stresses

caused in the DOL and star delta starters. This limits the starting current to the induction motor by using thyristors.

This 3-phase motor starter consists of two major units: one is power unit and the other control unit. Power unit

consists of back to back SCRs for each phase, and these are controlled by the logic implemented in the control

circuit. This control unit consists of zero voltage crossing circuit with capacitors for producing delay time.

Electronic Soft Start for 3-Phase Induction Motor

In the above block diagram, when a three-phase supply is given to the system, the control circuit rectifies each

phase supply, regulates it and compares for zero crossing voltage by operational amplifier. This Op-Amp output

drives the transistor, which is responsible for producing time delay with the use of capacitor. This capacitor

discharging enables another Op-Amp output for certain time so that Opto-isolators are driven for this elapsed time.

During this time, the opto-isolator output triggers back-to-back thyristors; and, the output applied to the motor is

reduced during this time. After this starting time, a full voltage is applied to the induction motor, and hence, the motor

runs at full speed. In this way, zero voltage triggering for certain time period at the starting of an induction motor

deliberately reduces the starting inrush current of the induction motor.

Induction Motor Protection System

This system protects the 3-phase AC motor from single phasing and overheating. When any of the phases is out,

then this system recognizes it and immediately turns off the motor, which is powered by the mains.

Induction Motor Protection System

All the three phases are rectified, filtered and regulated and given to operational amplifier where this supply voltage is

compared with certain voltage. If any of the phases is missed, then it gives zero voltage at the Op-amp input, and

therefore, it gives low logic to the transistor which further de-energizes the relay. Hence, the main relay gets turned

off and the power to the motor is interrupted.

Similarly, when the temperature of the motor exceeds certain limit, the operational amplifier output de-energizes the

appropriate relay; even then also the main relay gets turned off. In this way, the single phasing faults and over-

temperature conditions can be overcome in the induction motor.

2.Explain protection scheme for protection of transformer against incipient fault (N/D-16)

Transformer Protection

There are different kinds of transformers such as two winding or three winding electrical power transformers, auto

transformer, regulating transformers, earthing transformers, rectifier transformers etc. Different transformers demand

different schemes of transformer protection depending upon their importance, winding connections, earthing

methods and mode of operation etc.

It is common practice to provide Buchholz relay protection to all 0.5 MVA and above transformers. While for all small

size distribution transformers, only high voltage fuses are used as main protective device. For all larger rated and

important distribution transformers, over current protection along with restricted earth fault protection is applied.

Differential protection should be provided in the transformers rated above 5 MVA.

Depending upon the normal service condition, nature of transformer faults, degree of sustained over load, scheme of

tap changing, and many other factors, the suitable transformer protection schemes are chosen.

Nature of Transformer Faults

Although an electrical power transformer is a static device, but internal stresses arising from abnormal system

conditions, must be taken into consideration.

A transformer generally suffers from following types of transformer fault-

1. Over current due to overloads and external short circuits,

2. Terminal faults,

3. Winding faults,

4. Incipient faults.

All the above mentioned transformer faults cause mechanical and thermal stresses inside the transformer winding

and its connecting terminals. Thermal stresses lead to overheating which ultimately affect the insulation system of

transformer. Deterioration of insulation leads to winding faults. Some time failure of transformer cooling system, leads

to overheating of transformer. So the transformer protection schemes are very much required.

The general winding faults in transformer are either earth faults or inter-turns faults. Phase to phase winding faults in

a transformer is rare. The phase faults in an electrical transformer may be occurred due to bushing flash over and

faults in tap changer equipment. Whatever may be the faults, the transformer must be isolated instantly during fault

otherwise major breakdown may occur in the electrical power system.

Incipient faults are internal faults which constitute no immediate hazard. But it these faults are over looked and not

taken care of, these may lead to major faults. The faults in this group are mainly inter-lamination short circuit due to

insulation failure between core lamination, lowering the oil level due to oil leakage, blockage of oil flow paths. All

these faults lead to overheating. So transformer protection scheme is required for incipient transformer faults also.

The earth fault, very nearer to neutral point of transformer star winding may also be considered as an incipient fault.

Influence of winding connections and earthing on earth fault current magnitude.

There are mainly two conditions for earth fault current to flow during winding to earth faults,

1. A current exists for the current to flow into and out of the winding.

2. Ampere-turns balance is maintained between the windings.

The value of winding earth fault current depends upon position of the fault on the winding, method of winding

connection and method of earthing. The star point of the windings may be earthed either solidly or via a resistor. On

delta side of the transformer the system is earthed through an earthing transformer. Grounding or earthing

transformer provides low impedance path to the zero sequence current and high impedance to the positive and

negative sequence currents.

Star Winding with Neutral Resistance Earthed

In this case the neutral point of the transformer is earthed via a resistor and the value of impedance of it, is much

higher than that of winding impedance of the transformer. That means the value of transformer winding impedance is

negligible compared to impedance of earthing resistor. The value of earth current is, therefore, proportional to the

position of the fault in the winding. As the fault current in the primary winding of the transformer is proportional to the

ratio of the short circuited secondary turns to the total turns on the primary winding, the primary fault current will be

proportional to the square of the percentage of winding short circuited. The variation of fault current both in the

primary and secondary winding is shown below.

Star Winding with Neutral Solidly Earthed

In this case the earth fault current magnitude is limited solely by the winding impedance and the fault is no longer

proportional to the position of the fault. The reason for this non linearity is unbalanced flux linkage.

Percentage differential protection of transformer

The Differential Protection of Transformer has many advantages over other schemes of protection.

1. The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any

fault occurs in the transformer but not in oil then it can not be detected by Buchholz relay. Any flash over at

the bushings are not adequately covered by Buchholz relay. Differential relays can detect such type of

faults. Moreover Buchholz relay is provided in transformer for detecting any internal fault in the transformer

but Differential Protection scheme detects the same in more faster way.

2. The differential relays normally response to those faults which occur in side the differential protection zone

of transformer.

Principle of Differential Protection

Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually

compares between primary current and secondary current of power transformer, if any unbalance found in between

primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of

the transformer.

Suppose you have one transformer which has primary rated current Ip and secondary current Is. If you install CT of ratio Ip/1A at the primary side and similarly, CT of ratio Is/1A at the secondary side of the transformer. The secondaries of these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other. In other words, the secondaries of both CTs should be connected to the same current coil of a differential relay in such an opposite manner that there will be no resultant current in that coil in a normal working

condition of the transformer. But if any major fault occurs inside the transformer due to which the normal ratio of the transformer disturbed then the secondary current of both transformers will not remain the same and one resultant current will flow through the current coil of the differential relay, which will actuate the relay and inter trip both the primary and secondary circuit breakers. To correct phase shift of current because of star-delta connection of transformer winding in the case of three-phase transformer, the current transformer secondaries should be connected in delta and star as shown here.

At maximum through fault current, the spill output produced by the small percentage unbalance may be substantial. Therefore, differential protection of transformer should be provided with a proportional bias of an amount which exceeds in effect the maximum ratio deviation.

3. Explain carrier current protection scheme in detail. Explain carrier phase comparision relay.(N/D-15)

Carrier current protection scheme is mainly used for the protection of the long transmission line. In the carrier, current

protection schemes, the phase angle of the current at the two phases of the line are compared instead of the actual

current. And then the phase angle of the line decides whether the fault is internal and external. The main elements of

the carrier channel are a transmitter, receiver, coupling equipment, and line trap.

The carrier current receiver receives the carrier current from the transmitter at the distant end of the line. The receiver

converts the received carrier current into a DC voltage that can be used in a relay or other circuit that performs any

desired function. The voltage is zero when the carrier current, is not being received.

Line trap is inserted between the bus-bar and connection of coupling capacitor to the line. It is a parallel LC network

tuned to resonance at the high frequency. The traps restrict the carrier current to the unprotected section so as to

avoid interference from the with or the other adjacent carrier current channels. It also avoids the loss of the carrier

current signal to the adjoining power circuit.

The

coupling capacitor connects the high-frequency equipment to one of the line conductors and simultaneously separate

the power equipment from the high power line voltage. The normal current will be able to flow only through the line

conductor, while the high current carrier current will circulate over the line conductor fitted with the high-frequency

traps, through the trap capacitor and the ground.

Methods of Carrier Current Protection

The different methods of current carrier protection and the basic form of the carrier current protection are

1. Directional Comparison protection

2. Phase Comparison Protection

These types are explained below in details

1. Directional Comparison Protection

In this protection schemes, the protection can be done by the comparison of a fault of the power flow direction at the

two ends of the line. The operation takes place only when the power at both the end of the line is on the bus to a line

direction. After the direction comparison, the carrier pilot relay informs the equipment how a directional relay

behaves at the other end to a short circuit.

The relay at both the end removes the fault from the bus. If the fault is in protection section the power flows in the

protective direction and for the external fault power will flow in the opposite direction. During the fault, a simple signal

through carrier pilot is transmitted from one end to the other. The pilot protection relaying schemes used for the

protection of transmission are mainly classified into two types. They are

Carrier Blocking Protection Scheme – The carrier blocking protection scheme restricts the operation of

the relay. It blocks the fault before entering into the protected section of the system. It is one of the most

reliable protecting schemes because it protects the system equipment from damage.

Carrier Permitting Blocking Scheme – The carrier, protective schemes allows the fault current to enter

into the protected section of the system.

2. Phase Comparison Carrier Protection

This system compares the phase relation between the current enter into the pilot zone and the current leaving the

protected zone. The current magnitudes are not compared. It provided only main or primary protection and backup

protection must be provided also. The circuit diagram of the phase comparison carrier protection scheme is shown in

the figure below.

The

transmission line CTs feeds a network that transforms the CTs output current into a single phase sinusoidal output

voltage. This voltage is applied to the carrier current transmitter and the comparer. The output of the carrier current

receiver is also applied to the comparer. The comparer regulates the working of an auxiliary relay for tripping the

transmission line circuit breaker.

Advantage of Carrier Current Protection

The following are the advantage of the carrier current protection schemes. These advantages are

1. It has a fast and simultaneous operation of circuit breakers at both the ends.

2. It has a fast, clearing process and prevents shock to the system.

3. No separate wires are required for signalling because the power line themselves carry the power as well as

communication signalling.

4. It’s simultaneously tripping of circuit breakers at both the end of the line in one to three cycles.

5. This system is best suited for fast relaying also with modern fast circuit breakers.

The main operation of power line carrier has been for the purpose of supervisory control, telephone communication,

telemeter and relaying.

CARRIER PHASE COMPARISION RELAY

There are different methods of carrier current protection such as :

(1) Directional comparison method.

(2) Phase comparison method.

RELAY UNIT

Phase comparison method compares the phase relation between current entering in the protected zone and

current leaving the protected zone. The magnitudes of current are not compared. Phase comparison provides only

main protection. Back up protection should be provided in addition. In one of the phase comparison methods signals

are sent from each end of the line and received at the other end. The signals are related to the current flow in the

main line, as they are divided from CT secondary current. Where there is no fault, the signal is sent for alternate 1/2

cycles from each end which result in continuous signal over the line half the cycle from one end, remaining half from

the other. The same condition holds good from an external fault. During internal fault the current in one of the line

reverses in phase or differs in phase and remains below the fault detector setting, so that carrier is sent only for half

the time. The relay is arranged to sense the absence of signal in the line. Depending upon the setting, the tripping

occurs when the phase angle between the two signals reaches a certain value.

Block diagram of phase comparison circuit

Referring to Fig, for internal fault condition shown on right hand side, the transmitted signals and receiver signals

are almost in phase. The comparator compares these signals. Due to absence of signals of alternate half cycles, the

comparator gives output causing operation of trip relay.

Carrier signals are transmitted to the line from both ends. For external faults the effect produced by the sum of

these two signals is similar to that obtained when a continuous high frequency carrier is available on the line, and the

protection is designed to remain stable under the condition. The sum of these two signals on all internal faults

produces produces and effect similar to the periodic suppression of such a continuous carrier, the duration of each

suppression being proportional to the phase displacements greater than a normal angle 30o. Thus for displacements

less than 30o the protection will stabilise. This angle is usually referred to as the stabilising angle of the protection

(angle X in Fig).

Fig. below illustrates the two extreme cases with symmetrical fault conditions. The external fault condition is

implied by the fact that the primary current at both ends is in phase and the internal fault condition by the fact that the

two primary currents are 180oout of phase.

As a first step to produce the required carrier signals the secondary current at one end only (end B) is made

180o out of phase with the primary current by the reversal of the current transformer connections. Thus for external

faults the secondary currents at the two ends are 180oout of phase with each other .

It will be seen that the carrier-signal produced at both ends takes the form of a continuous carrier which is

periodically suppressed. In other words, a high frequency signal is only transmitted on alternative half-cycles of the

power-frequency corresponding say to the period when the secondary current is positive. The type of high frequency

signals is achieved by a process of modulation, whereby the normally consistent magnitude of a high frequency

carrier is made to vary in accordance with a square wave-shaped derived from the power current and having the

same period .

Diagram illustrating the working principle phase comparison method

3. Explain stepped time-distance characteristic of three distance relaying units used for 1, II, and III

zone of protection.(N/D-14)

Distance relying is considered for protection of transmission lines where the time-lag can not be permitted

and selectivity can not be obtained by overcurrent relying. Distance protection is used for secondary lines

and main lines.

A distance relay measures the ratio V/I at relay location which gives the measure of distance between

the relay and fault location. The impedance (resistance/reactance/admittance) of a fault loop is proportional

to the distance between the relay location and the fault point. For a given setting, the distance relay picks up

when impedance measured by it is less than the set value. Hence it protects a certain length of line. Hence

it is called distance relay.

Measurement: Considering zero fault impedance the voltage at fault point will be zero. The voltage at

relay location O will be equal to the voltage drop along the path OF, whereas same current I is flowing in the

line at O upto F. If fault had occurred near O, the voltage at O would be different. Current would be more

because of the reduction in line impedance. If fault occurred away from O, the voltage at O would be lesser

and current would also be lesser. In distance relays the ratio V/I is measured. The current gives operating

torque and voltage gives restraining torque. Hence for values of Z above certain setting the relay does not

operate. Hence it protects only a certain length of line equivalent to its impedance setting.

Stepped Characteristic of distance relay:

The distance relays early day used to have Inverse characteristic vs. Time. Now such characteristic is no

more preferred and distance relays have stepped characteristic.

The stepped characteristic may be either single stepped (Fig. 13) or three stepped, (Fig. 14).

Single Stepped Distance-Time Characteristic

Single step distance relay can be used where high set instantaneous overcurrent relays can not be

used. The typical applications of single-step distance protection are protection for transformer feeder,

protection of single section transmission lines, protection of bus bars etc. The conventional distance

measuring element has instantaneous time-distance characteristics. The operating time becomes infinite at

relay reach point. The distance relay is set for a value say Z corresponding to length of line L. Then if a fault

occurs within length L, the ideal distance relay operate instantaneously.

However the d.c. component of wave. fault resistance, influence the relay measurement and cause

over-reach or under reach.

Over-reach. When short circuit occurs, the current wave has d.c. component which causes a distance

relay to over-reach, i.e. to operate for a large impedance than desired. The tendency to over-reach is

minimized by adjusting the voltage to 90 to 80% of its normal value.

Effect of arc resistance, under-reach. The arc resistance is approximately given by

Rarc = 2.9 x104 L /I1.4

where Rarc = Resistance of arc, ohm

L = Length of arc in m. in open standstill air

I = Fault current Amperes.

Due to the extra arc resistance the distance measured by impedance relays inaccurate. The distance

relay will measure an impedance Zf + Rarc where Zf is the impedance of line.

By adding Zf and Rarc the measured point on R-X diagram goes out of impedance circle and relay does

not operate even though the fault in within the protection zone. This is called Under reach.

Single-step characteristic of high speed impedance relay : above certain Z, the relay is

inoperative. For smallest Z the relay operates in time.

Three Step Distance-Time Characteristic

The transmission lines having successive line section can be protected by means of three-zone

distance protection schemes. By such schemes, quick protection can be obtained and back up of the

sections as well as adjoining lines/bus bars can also be provided.

Referring to Fig. 14, the distance relay RA , located at section A has a 3-step characteristic given by

dashed into marked tA1, tA2, tA3, tA1, is called the first step and covers about 80 per cent of the first line

section AB, and gives instantaneous protection. tA2 is the second step of relay at station A(RA) and covers

the remaining portion of section AB and about 20 to 50 per cent of the next section (BC). The third step

having timing TA3 covers the entire remaining line. The steps are obtained by one of the following methods :

- Changing taps on auxiliary voltage transformer,

- Switching resistance in relay restraint circuit at per-set time intervals by means of time element.

- Separate measuring element for zone 2 and zone 3.

Three step characteristic of distance relay

4.Explain harmonic restrained differential relay used for transformer protection(N/D-15)

POWER transformer is important part of every power system and its protection is important to ensure the

reliability of power supply. Differential Protection is used to protect the transformer from internal faults.

Differential Protection is a unit protection as it only operates for faults on the unit it is protecting. It is based

on the fact that it only operates when differential current (difference between primary and secondary current)

exceeds a predetermined value. However, the differential current can sometimes be significant even without

an internal fault.

The reason is some of the characteristics of current transformers (different saturation levels, nonlinearities)

measuring the input and output currents, and of the power transformer being protected. With the exception

of the inrush and over excitation currents, most of the other problems, can be solved by means of the

percentage biased transformer differential relay, which adds to the normal differential relay two restraining

coils fed by the zone-through current, by proper choice of the resulting percent differential characteristic,

and by proper connection of the current transformers on each side of the power transformer.

INRUSH CURRENT:

Magnetizing inrush current in transformers results from a fault, voltage recovery after clearing an external

fault, change of the character of a fault and out-of-phase synchronizing of a connected generator. This

magnetizing current disturbs the currents at the transformer terminals, and is therefore detected by the

differential relay as a “false” differential current and operates. This leads to a very undesirable situation

because relay must not operate on this situation and should remain stable in the presence of inrush

currents. This inrush current contains dc offset, odd harmonics, and even harmonics (The content of 2nd

harmonic being the largest).

2ND HARMONIC RESTRAINT CIRCUITRY:

2ND Harmonic restrain works on the fact that the second harmonic content in the differential current is much

larger in the case of inrush current as compared to fault condition so this large content of second harmonic

is used to restrain the relay from operation in the presence of inrush current

A microprocessor based transformer relay using 2nd Harmonic restraint scheme consists of numerous

Fourier transforms, relational and logical operators, flip-flops and data type converters. The scheme also

uses an AC source, two three phase breaker and a three phase power transformer to implement this

scheme. Input currents from the primary and secondary of a power transformer are fed to the differential

relay.

This scheme receives low level currents from either side of the transformer through CTs. The relay logic is

designed in a way that the Fourier transform is used to extract the second harmonic from the differential

current which is then used to differentiate between inrush current and fault current and make a trip decision.

The relational operator is then used to compare the 2nd harmonic content with a specified value and if this

value exceeds a predetermined value which is an indication of inrush current then an input signal is fed to

the relay to restrain from operating. However if the 2 harmonic content is less than the specified value then

a signal is sent to the operating coil of relay causing it to operate. Flip flops and logical operators are also

used in this scheme to obtain the desired relay logic.

A differential relay works on the principle of difference of currents on primary side and secondary side of the

equipment under consideration. The equipment under consideration is power transformer. Tripping signal is

given to the circuit breaker (CB) when the difference in currents on primary side and secondary side of the

power transformer is greater than the prescribed limit. Relational operators, flip flops and logical operators

are used to fulfill transformer differential scheme.

But there is a practical problem in the simple differential relay. Practically, when a power transformer is

energized at no load, a transient current, 6 to 10 times of the rated current occurs only at the primary side of

the power transformer. This transient current is known as the inrush current . As this current occurs only at

the primary side of the transformer and there is no current on the secondary side of the transformer as

transformer is energized at no load. So the simple differential relay treats the inrush current as a fault

current and generates a tripping signal and sends to the CB. The circuit breaker receives the tripping signal

and isolates the power transformer without any fault

Now days, this problem is tackled by desensitizing the differential relay for that specific time period in which

inrush occurs.

5. Write brief notes on generator protection and bus bar protection.(M/J-14)

Generator Protection:

A generator is subjected to electrical traces imposed on the insulation of the machine, mechanical forces

acting on the various parts of the machine, and temperature rises. These are the main factors which make

protection necessary for the generator or alternator. Even when properly used, a machine in its perfect

running condition does not only maintain its specified rated performance for many years, but it does also

repeatedly withstand certain excess of over load.

Hence, preventive measures must be taken against overloads and abnormal conditions of the machine so

that it can serve safely. Despite of sound, efficient design, construction, operation, and preventive means of

protection, the risk of that fault cannot be completely eliminated from any machine.The devices used

in generator protection, ensure the fault, made dead as quickly as possible.

An electrical generator can be subjected to either internal fault or external fault or both. The generators are

normally connected to an electrical power system, hence any fault occurred in the power system should also

be cleared from the generator as soon as possible otherwise it may create permanent damage in the

generator. The number and variety of faults occur in generator, are huge. That is why generator or alternator

is protected with several protective schemes. The generator protection is of both discriminative and non-

discriminative type. Great care is to be taken in coordinating the systems used and the settings adopted, so

that the sensitive, selective and discriminative generator protection scheme is achieved.

Types of Generator Protection

The various forms of protection applied to the generator can be categorized into two manners,

1. Protective relays to detect faults occurring outside the generator.

2. Protective relays to detect faults occurring inside the generator.

Other than protective relays, associated directly with the generator and its associated transformer, there

are lightning arrestors, over speed safe guards, oil flow devises and temperature measuring devises for

shaft bearing, stator winding, transformer winding and transformer oil etc. Some of these protective

arrangement are of non-trip type i.e. they only generate alarm during abnormalities. But the other protective

schemes ultimately operate master tripping relay of the generator. This should be noted that no protective

relay can prevent fault, it only indicates and minimises the duration of the fault to prevent high temperature

rise in the generator otherwise there may be permanent damage in it. It is desirable to avoid any undue

tresses in the generator, and for that it is usual practice to install surge capacitor or surge divertor or both to

reduce the effects of lightning and other voltage surges on the machine.

The protection schemes usually applied to the generator are discussed here below in brief.

Protection against Insulation Failure

The main protection provided in the stator winding against phase to phase or phase to earth fault, is

longitudinal differential protection of generator. Second most important protection scheme for stator winding

is inter turn fault protection. This type of protection was considered unnecessary in previous days because

breakdown of insulation between points in the same phase winding, contained in the same slot, and

between which a potential difference exists, very rapidly changes into an earth fault, and then it is detected

by either the stator differential protection or the stator earth fault protection. A generator is designed to

produce relatively high voltage in comparison to its output and which therefore contains a large number

of conductors per slot. With increasing size and voltage of the generator, this form of protection is becoming

essential for all large generating unit.

Stator Earth Fault Protection

When the stator neutral is earthed through a resistor, a current transformer is mounted in the neutral to

earth connection. Inverse Time Relay is used across the CT secondary when the generator is connected

directly to the bus bar. In case of generator feeds power via a delta star transformer, an Instantaneous

Relay is used for the same purpose. In the former case, the earth faults relay is required to be graded with

other fault relays in the system. This is the reason why Inverse Time Relay is used in this case. But in the

letter case, the earth fault loop is restricted to the stator winding and primary winding of the transformer,

hence, there is no need of grading or discrimination with other earth fault relays in the system. That is why

Instantaneous Relay is preferable in the case.

Rotor Earth Fault Protection

A single earth fault does not create any major problem in the generator but if the second earth fault is

occurred, however, part of the field winding will become short-circuited and resulting and

unbalanced magnetic field in the system and consequently there may be major mechanical damage to the

bearings of the generated. There are three methods available to detect the types of fault in the rotor. The

methods are

1. Potentiometer method

2. AC injection method

3. DC injection method

Unbalanced Stator Loading Protection

Unbalancing in loading produces negative sequence currents in the stator circuit. This negative sequence

current produces a reaction field rotating at twice of synchronous speed with respect to the rotor and hence

induce double frequency current in the rotor. This current is quite large and causes overheating in the rotor

circuit, especially in the alternator. If any unbalancing occurred due to fault in the stator winding itself, that

would be cleared instantaneously by the differential protection provided in the generator. If the unbalancing

is occurred due to any external fault or unbalanced loading in the system, it may remain undetected or may

persist for a significant period of time depending on the protection coordination of the system. These faults

then be cleared by installing a negative phase sequence relay with the characteristics to match the

withstand curve of the machine.

Protection against Stator Overheating

Overloading, causes overheating in the stator winding of the generator. Not only overloading, failure of

cooling systems and insulation failure of stator laminations also cause overheating of the stator winding. The

overheating is detected by embedded temperature detectors at various points in the stator winding. The

temperature detector coils are normally resistance elements which form one arm of the Wheatstone bridge

circuit. In the case of smaller generator normally below 30 MW, the generators are not equipped with

embedded temperature coil but are usually fitted with thermal relay and they are arranged to measure the

current flowing in the stator winding. This arrangement only detects overheating caused by overloading and

does not provide any protection against overheating due to failure of cooling systems or short circuited

stator laminations. Although over current relays, negative phase sequence relays, and devises for

monitoring constant flow are also used to provide a certain degree of thermal overload protection.

Low Vacuum Protection

This protection, usually in the form of a regulator which compares the vacuum against atmospheric

pressure, it is normally fitted to the generator set above 30 MW. The modern practice is for the regulator to

unload the set via the secondary governor until normal vacuum conditions are restored. If the vacuum

conditions do not improve below 21 inch the stop valves are closed and the main circuit breaker is tripped.

Protection against Lubrication Oil Failure

This protection is not considered essential since the lubrication oil is normally obtained from the same pump

as governor oil and a failure of the governor oil will automatically make stop valve to close.

Protection against Loss of Boiler Firing

Two methods are available for detecting the loss of boiler firing. In the first methods, normally opened (NO)

contacts are provided with the fan motors which may trip the generator if more than two motors fail. The

second methods uses a boiler pressure contacts which unload the generator in boiler pressure falls below

approximately 90%.

Protection against Prime Mover Failure

If the prime mover fails to supply mechanical energy to the generator, the generator will continue to rotates

in motoring mode that means it takes electrical energy from the system instead of supplying it to the system.

In steam turbine set the steam acts as a coolant maintaining the turbine blades at a constant temperature.

Failure of the supply will therefore result in overheating due to friction, with subsequent distortion of turbine

blades. The, failure of steam supply can cause severe mechanical damage in addition of imposing a heavy

motoring load on the generator. Reverse power relay is used for this purpose. As soon as the generator

starts rotating in motoring mode, the reverse power relay will trip the generator set.

Over Speed Protection

While it is the general practice to provide mechanical over speed devises on both steam and hydro turbine,

which operate directly on the steam throttle valve or main step valve, it is not usual to backup this devises

by an over speed relay on steam driven sets. It is, however, considered good practice on hydroelectric units,

as the response of the governor is comparatively slow and the set is more prone to over-speed. The relay

when fitted is usually supplied from the permanent magnet generator used for the control of governor.

Protection against Rotor Distortion

The cooling rates following shutdown, at the top and bottom of the turbine casing, are different and this

uneven temperature distribution tends to cause destruction of the rotor. To minimize the disruption, it is

common practice to turn the rotor at low speed during the cooling down period. In the view of the forces

involved with large modern rotor, it is now standard practice to fit shaft eccentricity detectors.

Protection against Difference in Expansion between Rotating and Stationary parts

During the running up period, the rate of heating of the rotor differs from that of the casing, due to the

difference in mass. As a result, the rotor expands at a different rate to the casing and it is necessary to

overcome this unequal expansion. To this end, proposition is made on the larger machine for independent

supplies of steam to be set to certain joints on the casing. It is desirable therefore to provide a means of

measuring the axial expansion to assist the operator to feed the steam to the correct points and also to

provide indication of any dangerous expansion. The shaft axial expansion detector is basically similar to the

equipment described for rotor distortion equipment, except that the detector magnets are fixed to the turbine

casing.

Protection against Vibration

Vibration detectors are usually mounted on the bearing pedestals. The detector consists of a coil mounted

on springs between U shaped permanent magnets. The voltage output from the coil, which is proportional to

the degree of vibration, is passed from the coil into integrating circuits and then into interval indicating

instrument.

Back up Protection of Generator

Back up protection should always be given in highly rated machine like synchronous generator or alternator.

If faults occurred had not been cleared by the appropriate protection scheme then back up protection

relays should be operated to clear the fault. Over current relays are generally used for this purpose.

Because the synchronous reactance of modern machine is often greater than hundred percent, the

sustained fault current fed from the machine into an external fault is invariably below the normal full load

current. The normal IDMTL relays would not prove satisfactory because they are current settings must be

close to the full load and their time sitting short if operation is to be obtained, resulting in probable lack of

discrimination with other over current relays in the system. Father, the over current relay would most

probably operate for loss of field on the machine, disconnecting it prematurely. To overcome this problem is

it has become customary to apply an over current relay in combination with under voltage relay, the letter

relay controlling the fault settings of the former as shown in the figure below.

Busbar protection:

Differential Busbar Protection

Current Differential Protection

The scheme of busbar protection, involves, Kirchoff’s current law, which states that, total current entering

an electrical node is exactly equal to total current leaving the node.

Hence, total current entering into a bus section is equal to total current leaving the bus section.

The principle of differential busbar protection is very simple. Here, secondaries of CTs are connected

parallel. That means, S1 terminals of all CTs connected together and forms a bus wire. Similarly S2 terminals

of all CTs connected together to form another bus wire.

A tripping relay is connected across these two bus wires.

Here, in the figure above we assume that at normal condition feed, A, B, C, D, E and F carries current IA, IB,

IC, ID, IE and IF.

Now, according to Kirchoff’s current law, Essentially

all the CTs used for differential busbar protection are of same current ratio. Hence, the summation of all

secondary currents must also be equal to zero. Now, say current through the relay connected in parallel with

all CT secondaries, is iR, and iA, iB, iC, iD, iE and iF are secondary currents.

Now, let us apply KCL at node X. As per KCL at node X,

So, it is clear that

under normal condition there is no current flows through the busbar protection tripping relay. This relay is

generally referred as Relay 87. Now, say fault is occurred at any of the feeders, outside the protected zone.

In that case, the faulty current will pass through primary of the CT of that feeder. This fault current is

contributed by all other feeders connected to the bus. So, contributed part of fault current flows through the

corresponding CT of respective feeder. Hence at that faulty condition, if we apply KCL at node K, we will still

get, iR = 0.

That means, at external faulty condition, there is no current flows through relay 87. Now consider a situation

when fault is occurred on the bus itself.

At this condition, also the faulty current is contributed by all feeders connected to the bus. Hence, at this

condition, sum of all contributed fault current is equal to total faulty current.

Now, at faulty path there is no CT. (in external fault, both fault current and contributed current to the fault by

different feeder get CT in their path of flowing).

The sum of all secondary currents is no longer zero. It is equal to secondary equivalent of faulty current.

Now, if we apply KCL at the nodes, we will get a non zero value of iR.

So at this condition current starts flowing through 87 relay and it makes trip the circuit

breaker corresponding to all the feeders connected to this section of the busbar.

As all the incoming and outgoing feeders, connected to this section of bus are tripped, the bus becomes

dead.

This differential busbar protection scheme is also referred as current differential protection of busbar.

Differential Protection of Sectionalized Bus

During explaining working principle of current differential protection of busbar, we have shown a simple non

sectionalized busbar. But in moderate high voltage system electrical bus sectionalized in than one sections

to increase stability of the system. It is done because, fault in one section of bus should not disturb other

section of the system. Hence during bus fault, total bus would be interrupted.

Let us draw and discuss about protection of busbar with two sections.

Here, bus section A or zone A is bounded by CT1, CT2 and CT3 where CT1 and CT2 are feeder CTs and

CT3 is bus CT.

Similarly bus section B or zone B is bounded by CT4, CT5 and CT6 where CT4 is bus CT, CT5and CT6 are

feeder CT.

Therefore, zone A and B are overlapped to ensure that, there is no zone left behind this busbar

protection scheme.

ASI terminals of CT1, 2 and 3 are connected together to form secondary bus ASI;

BSI terminals of CT4, 5 and 6 are connected together to form secondary bus BSI.

S2 terminals of all CTs are connected together to form a common bus S2.

Now, busbar protection relay 87A for zone A is connected across bus ASI and S2.

Relay 87B for zone B is connected across bus BSI and S2.

This section busbar differential protection scheme operates in some manner simple current differential

protection of busbar.

That is, any fault in zone A, with trip only CB1, CB2 and bus CB.

Any fault in zone B, will trip only CB5, CB6 and bus CB.

Hence, fault in any section of bus will isolate only that portion from live system.

In current differential protection of busbar, if CT secondary circuits, or bus wires is open the relay may be

operated to isolate the bus from live system. But this is not desirable.

DC Circuit of Differential Busbar Protection

A typical DC circuit for busbar differential protection scheme is given below.

Here, CSSA and CSSB are two selector switch which are used to put into service, the busbar

protection system for zone A and zone B respectively.

If CSSA is in “IN” position, protection scheme for zone A is in service.

If CSSB is in “IN” position, protection for zone B is in service.

Generally both of the switches are in “IN’ position in normal operating condition. Here, relay coil of 96A and

96B are in series with differential busbar protection relay contact 87A-1 and 87B-1 respectively.

96A relay is multi contacts relay. Each circuit breaker in zone A is connected with individual contact of 96A.

Similarly, 96B is multi contacts relay and each circuit breaker in zone-B is connected with individual contacts

of 96B.

Although here we use only one tripping relay per protected zone, but this is better to use one individual

tripping relay per feeder. In this scheme one protective relay is provided per feeder circuit breaker, whereas

two tripping relays one for zone A and other for zone B are provided to bus section or bus coupler circuit

breaker.

On an interval fault in zone A or bus section A, the respective bus protection relay 87A, be energized

whereas during internal fault in zone B, the respective relay 87B will be energized.

As soon as relay coil of 87A or 87B is energized respective no. contact 87A-1 or 87B-1 is closed.Hence, the

tripping relay 96 will trip the breakers connected to the faulty zone. To indicate whether zone A or B busbar

protection operated, relay 30 is used.

For example, if relay 87A is operated, corresponding “No” contact 87A-2 is closed which energized relay

30A. Then the No contact 30A-1 of relay 30A is closed to energized alarm relay 74. Supervision relay 95 of

respective zone is also energized during internal fault, but it has a time delay of 3 second. So, it reset as

soon as the fault is cleared and therefore does not pick up zone bus wire shorting relay 95x which in turn

shorts out the bus wires. An alarm contact is also given to this auxiliary 95x relay to indicate which CT is

open circuited. No volt relay 80 is provided in both trip and non-trip section of the DC circuit of differential

busbar protection system to indicate any discontinuity of D. C. supply.

Voltage Differential Protection of Busbar

The current differential scheme is sensitive only when the CTs do not get saturated and maintain same

current ratio, phase angle error under maximum faulty condition. This is usually not 80, particularly, in the

case of an external fault on one of the feeders. The CT on the faulty feeder may be saturated by total

current and consequently it will have very large errors. Due to this large error, the summation of secondary

current of all CTs in a particular zone may not be zero. So there may be a high chance of tripping of all

circuit breakers associated with this protection zone even in the case of an external large fault. To prevent

this maloperation of current differential busbar protection, the 87 relays are provided with high pick up

current and enough time delay.

The greatest troublesome cause of current transformer saturation is the transient dc component of the short

circuit current.

This difficulties can be overcome by using air core CTs. This current transformer is also called linear

coupler. As the core of the CT does not use iron the secondary characteristic of these CTs, is straight line.

In voltage differential busbar protection the CTs of all incoming and outgoing feeders are connected in

series instead of connecting them in parallel.

The secondries of all CTs and differential relay form a closed loop. If polarity of all CTs are properly

matched, the sum of voltage across all CT secondries is zero. Hence there would be no resultant voltage

appears across the differential relay. When a buss fault occurs, sum of the all CT secondary voltage is no

longer zero. Hence, there would be current circulate in the loop due to the resultant voltage. As this loop

current also flows through the differential relay, the relay is operated to trip all the circuit breaker associated

with protected bus zone. Except when ground fault current is severally limited by neutral impedance there is

usually no selectivity problem when such a problem exists, it is solved by use of an additional more sensitive

relaying equipment including a supervising protective relay.

6.Explain detail about impedance relay with R-X diagram(M/J-14)

Impedance Type Distance Relay

An impedance relay is a voltage controlled overcurrent relay. The relay measures impedance up to the point

of fault and gives tripping command to the circuit breaker if the impedance is less than the relay setting Z.

Relay setting Z is the is known as the replica impedance, and it is proportional to the set impedance, i.e., the

impedance up to the reach of the relay. The relay monitors continuously the line current I through CT and

the bus voltage V through PT and operates when the V/I ratio decline below the set value.

Principle of Operation of Impedance Relay

The principle of operation of an impedance relay is illustrated in the figure shown below. The voltage

element of the relay is excited through a potential transformer (PT) from the line under protection, and

current element of the relay is excited from a current transformer (CT) in series with the line.

The portion AB of the line is the protected zone. Under normal operating conditions, the impedance of the

protected line is Z. The relay is so designed it closes its contact whenever impedance of the protected

sections falls below the set value, i.e., Z in this case.

Now assume that the fault occurs at a point F1 in the protected zone. The impedance, the ratio of the bus

voltage and fault current (V/I), between the point where the relay is located and point of fault will become

less than Z and hence the relay operates. When the fault occurs beyond the protected zone (at point F2) the

impedance will become more than Z, and relay contacts do not close.

Operating Characteristic of an Impedance Relay

The impedance relay is a double actuating relay and essentially consist of two elements – operated voltage

element and the current operated element. The current operated elements produce a positive (pick up

torque) while the voltage elements develop a negative or reset torque. Taking spring control effects as –K3,

the torque equation of the relay is

where V and I are the RMS value of voltage and current

respectively. At balance point, when the relay is on the extreme of operating the net torque is zero, and

The effect of control spring magnitude is negligible. Its effect is

noticeable only at current, magnitude well below those normally encountered. Hence, taking K3 = 0 the relay

torque equation becomes

The operating characteristic regarding voltages V and current I is

shown in the figure, causing a notable bend in the characteristic only at the current low end. The dashed line

represents an operating characteristic which represents a constant value of Z, may be considered as

operating characteristic.

The relay will

pick up for any combination of V, and I represented a point above the characteristic in the positive torque

region, or in other words, for any value of impedance less than the constant value represented by the

operating characteristic.

The operating characteristic of a

distance relay shown by impedance diagram or R-X diagram, as illustrated in the figure below. The

numerical value of the ratio of V and I is shown as the length of the radius vector, such as Z, and the phase

angle between V and I persuade the position of the vector, as shown below.

Any value of impedance less

than the radius of the circle will result in positive torque and any value of impedance greater than this radius

will result in negative torque. The impedance relays normally used are high-speed relays. These relays may

be of electromagnetic or induction type described below.

Electromagnetic Type Induction Relay

In this relay two torques are produced by the electromagnetic action on the voltage and the current and

these torque are compared. The solenoid B is voltage excited from the secondary of the PT and develop a

clockwise torque pulling a plunger P2 downwards and tends to rotate the balance arm in the clock direction.

The spring act as a restraining force and set up a mechanical torque in the clockwise direction.

Another torque is developed by the solenoid in the A in the clockwise direction which tends to pull the

plunger P1 downwards. The solenoid is current excited from the secondary of CT, which is connected to the

line under protection. This torque is called deflecting or pick-up torque.

In the normal operating condition when

there is no fault and equilibrium prevails, the balance arm is horizontal, and the relay contacts are open.

When a fault occurs in the protected zone, the current in these CT primary goes up and so in the relay. Thus

the torque developed by solenoid A is increased. Also, due to the voltage drop with the occurrence of fault

the magnitude of restraining torque developed by solenoid decrease. Thus the balance arm rotates in a

counterclockwise direction, and the relay contacts are closed.

The pull of the solenoid A, i.e., (current element) is proportional to I2 and that due to solenoid B (voltage

element) to V2. Consequently, the relay will operate when

The value of the constants k1 and k2 depend on the ampere-turns of the two

solenoids, and the ratios of the instrument transformers. By providing tappings on the coil, the setting of the

relay can be changed.

The time impedance characteristic of the relay is shown in the figure. The ordinate represents the time of

operations for the relay, and the abscissa represents impedance which is proportional to distance. The

operating time for almost the entire length is constant. Towards the ends of the predetermined length, the

curve rises gradually because the pull of the voltage and current elements becomes nearly equal and after

the point, A has been passing the operating time rapidly become infinite.

Induction Type Impedance Relay

Induction type impedance relay consists of a combination of an overcurrent element with a voltage restraint

element. Their circuit diagram is shown in the figure below.

It consists of two metallic disc, usually made up of copper or aluminum which is capable of rotating between

two electromagnets. The upper electromagnet has two two separate windings similar to that of overcurrent

relay. The primary winding is connected to the secondary of the CT connected in the line to be protected.

The winding has some tappings so as to vary the current settings; the tapping is connected to the plug

bridge.

The secondary windings on the upper electromagnet are connected in series with windings on the lower

electromagnet. By this arrangement leakage fluxes of upper and lower electromagnets are sufficiently

displaced in space and phase to set up a rotational torque on the induction disc, as in the shaded pole

induction disc motor. The controlling or braking torque caused by the permanent magnet varies directly as

the driving torque.

In normal operating conditions the pull exerted by armature is more than that of the induction element and

thus the trip circuit contacts remain open. When the fault occurs, the induction disc starts to rotate with

a speed approximately proportional to the operating currents. Hence the time taken by the disc to turn

through a given angle varies inversely as the current.

Also, as the disc rotates the spring is wound. The disc continuously rotating till the tension of the spring is

sufficient.to overcome the pull of the voltage restraint the voltage restrained electromagnet over its armature

and as soon as this armature is released the trips contacts are closed.

Thus the angle through which the induction disc is to rotate for the operation of the relay depends on the

value of the pull on the restraint armature. The greater this pull, the more significant would be the travel of

the disc. This pull is also approximately proportional to the voltage, therefore, the angle through which the

disc is to rotate for the operation of the relay is direct to voltage V.

Thus, in this type of the relay, the time required is directly proportional to the line voltage V and inversely

proportional to current I i.e., the time of operation is proportional to V/I or the impedance of the line or

section.

Time-Characteristic of High-Speed Type Impedance Relay

The operating time characteristic of a high-speed type impedance relays is shown in the figure below. The

curve shown in the figure is a particular value of current magnitude. For other current values, a similar

characteristic is obtained. The curve for higher currents will lie above it. It is observed that for impedance

values above 100 % pick-up impedance the relay does not operate. The curve represents the actual

characteristic while curve II is a simplified representation of the same curve.

Drawbacks of Plan Impedance Relay

It can responds on both side of CT, PT location so that it can discriminate between externals and internal

fault. It is also affected by arc resistance of line fault and result under reach. Such type of relay is sensible to

power swing as a large area is covered by the circle on each on R-X plane. During power swing which is

caused by severe faults, the relays see a fictitious impedance, and if this impedance is less than the relay

setting, the relay may operate.