DHANALAKSHMI COLLEGE OF ENGINEERING

DEPARTMENT OF ELECTRICAL ENGINEERING

EE 6702 – PROTECTION AND SWITCHGEAR

UNIVERSITY QUESTIONS AND ANSWERS

UNIT – 2

PART B

1. Working principle of directional overcurrent relay:(N/D-16)

The over-current protection can be given directional feature by adding directional element in the protection

system. Directional over-current protection responds to over-currents for a particular direction flow. If power

flow is in the opposite direction, the directional over-current protection remains un-operative.

Directional over-current protection comprises over-current relay and power directional relay- in a single

relay casing. The power directional relay does not measure the power but is arranged to respond to the

direction of power flow.

Directional operation of relay is used where the selectivity can be achieved by directional relaying. The

directional relay recognizes the direction in which fault occurs, relative to the location of the relay. It is

set such that it actuates for faults occurring in one direction only. It does not act for faults occurring in the

other direction. Consider a feeder AC passing through sub-section B. The circuit breaker CB3 is

provided with a directional Relay `R' which will trip the breaker CB3 if fault power flow in direction C alone.

Therefore for faults in feeder AB, the circuit breaker CB3 does not trip unnecessarily.

However for faults in feeder BC the circuit-breaker CB3 trips. Because it's protective relaying is set with a

directional feature to act in direction AC.

The classic electromechanical and solid state relay, as well as some common numeric relays, determines

the direction to fault by comparing the phase angle relationship of phase currents to phase voltages. If only

per phase watt flow (32 element) is to be considered, the basic concept would be that if IPh is in phase with

VPh-N (0°, ±90°), then power flow on that phase is indicated as forward (or reverse, depending on one’s

perspective). However, for a phase to ground fault, the VPh-N may collapse to 0, and I may be highly

lagging, so that VPh-N x IPh may be mostly VAR flow, and thus prevent the relay from making a correct

directional decision. To resolve the low voltage issue, quadrature voltages (i.e., VBC vs. IA) are commonly

used. To resolve the issue that fault current is typically highly lagging, the relay current vs. voltage

detection algorithm is skewed so that the relay is optimized to detect lagging current conditions rather then

1.0 power factor conditions. One approach, seen in Fig, is to phase shift the voltage signal so that the

relay’s internal voltage signal (VPolarity, abbreviated as VPol) is in phase with current when current lags

the 1.0 power factor condition by some setting, typically between 300 and 900 . The angle setting is

commonly referred to as the maximum torque angle, MTA. In some designs of this concept, the current

signal is skewed rather than the voltage signal. In some designs, other phase voltages are used. For

instance, IA could be compared to VAB, VCA, VBN, or VCN, and the detection algorithm would work,

though the quadrature voltage VBC gives the most independence of the voltage signal from the effects of

an A-N, A-B, or A-C fault.

Directional power protection operates in accordance with the direction of power flow. Reverse power

protection operates when the power direction is reversed in relation to the normal working direction.

Reverse power relay is different in construction than directional over-current relay.

In directional over-current relay, the directional element does not measure the magnitude of power. It

senses only direction of power flow. However, in Reverse Power Relays, the directional element measures

magnitude and direction of power flow.

Torque Equation:

The universal relay torque equation can be given as

where I = RMS value of current in current coil

V = RMS value of voltage fed to the voltage coil

ϕ = Electrical angle between V and I

T = The maximum torque angle K1, K2

and K3 = Relay constant

K = Mechanical restraining torque

Differential Protection:

It is used for transformer and generator protection. It simultaneously compares the phaser difference &

magnitude of the current entering & leaving the protected zone. Differential protection is a unit protection,

relay works on the principle of Kirchoff's current Law. Measuring element is Current Transformer. The

differential current measured between the incoming current and Outgoing current must be negligible current

during stable and through fault condition. In case of in zone fault or unstable condition (due to CT saturate)

relay will sense the differential current and issue the trip signal.

Applications of Differential protection: Transformer, Generator and Cable Protection.

Connection of directional relay:

30° relay connection (0° MTA)

60° No. 1 connection (0° MTA)

2.Operation of impedance,reactance and admittance relay.(N/D-16)

Impedance relay:

The working principle of distance relay or impedance relay is very simple. There is one voltage element

from potential transformer and an current element fed from current transformer of the system. The

deflecting torque is produced by secondary current of CT and restoring torque is produced by voltage of

potential transformer.

In normal operating condition, restoring torque is more than deflecting torque. Hence relay will not operate.

But in faulty condition, the current becomes quite large whereas voltage becomes less. Consequently,

deflecting torque becomes more than restoring torque and dynamic parts of the relay starts moving which

ultimately close the No contact of relay. Hence clearly operation or working principle of distance relay,

depends upon the ratio of system voltage and current. As the ratio of voltage to current is nothing but

impedance a distance relay is also known as impedance relay.

The operation of such relay depends upon the predetermined value of voltage to current ratio. This ratio is

nothing but impedance. The relay will only operate when this voltage to current ratio becomes less than its

predetermined value. Hence, it can be said that the relay will only operate when the impedance of the line

becomes less than predetermined impedance (voltage / current). As the impedance of a transmission

line is directly proportional to its length, it can easily be concluded that a distance relay can only operate if

Reactance Relay

The reactance relay is a high-speed relay. This relay consists of two elements an overcurrent element and

a current-voltage directional element. The current element developed positive torque and a current-voltage

developed directional element which opposes the current element depending on the phase angle between

current and voltage.

Reactance relay is an overcurrent relay with directional limitation. The directional element is arranged to

develop maximum negative torque when its current lag behinds its voltage by 90°. The induction cup or

double induction loop structures are best suited for actuating reactance type distance relays.

Construction of Reactance Relay

A typical reactance relay using the induction cup structure is shown in the figure below. It has a four-pole

structure carrying operating, polarizing, and restraining coils, as shown in the figure below. The operating

torque is developed by the interaction of fluxes due to current carrying coils, i.e., the interaction of fluxes of

2, 3 and 4 and the restraining torque is produced by the interaction of fluxes due to poles 1, 2 and 4.

The operating torque will be proportional to the square of

the current while the restraining torque will be proportional to VI cos (Θ – 90°). The desired maximum

torque angle is obtained with the help of resistance-capacitance circuits, as illustrated in the figure. If the

control effect is indicated by –k3, the torque equation becomes

where Θ, is defined as positive when I lag behind V. At the balance point net torque is zero, and hence

The spring control effect is neglected in the above equation, i.e., K3 = 0

Admittance Relay:

A mho Relay is a high-speed relay and is also known as the admittance relay. In this relay operating torque

is obtained by the volt-amperes element and the controlling element is developed due to the voltage

element. It means a mho relay is a voltage controlled directional relay.

A mho relay using the induction cup structure is shown in the figure below. The operating torque is

developed by the interaction of fluxes due to pole 2, 3, and 4 and the controlling torque is developed due to

poles 1, 2 and 4.

If the spring controlling effect is indicated by –K3, the torque equation becomes,

Where Θ and τ are defined as positive when I lag behind V. At balance point, the net torque is zero, and

hence the equation becomes

3.Working Principle of Inductional type overcurrent relay(N/D-15)

Induction type relay: These are classified as Non-directional and directional over current relays

INDUCTION TYPE DIRECTIONAL OVER CURRENT RELAY The directional power relay is not suitable under short circuit conditions because as short circuitoccurs the system voltage falls to a low value resulting in insufficient torque to cause relayoperations. This difficulty is overcome in the directional over current relay, which is independentof system voltage and power factor. Constructional details: Figure shows the constructional details of a typical induction typedirectional over current relay. It consists of two relay elements mounted on a common case viz.(i) directional element and (ii) non-directional element.(i) Directional element: It is essentially a directional power relay, which operates when power flows in a specific direction. The potential of this element is connected through a potentialtransformer (PT.) to the system voltage. The current coil of the element is energized through aCT by the circuit current. This winding is carried over the upper magnet of the non-directionalelement. The trip contacts (1 and 2) of the directional element are connected in series withsecondary circuit of the over current element. The latter element cannot start to operate until itssecondary circuit is completed. In other words, the directional element must first operate (ie.contacts 1 and 2 should close) in order to operate the over current element. (ii) Non-directional element: It is an over current element similar in all respects to a non-directional over current relay. The spindle of the disc of this element carries a moving contactwhich closes the fixed contact after the operation of directional element. Plug setting bridge is provided for current setting. The tappings are provided on the upper magnet of over currentelement and are connected to the bridge. Operation:- Under normal operating conditions, power flows in the normal direction in thecircuit operated by the relay. Therefore, directional power relay does not operate, therebykeeping the (lower element) un-energized. However, when a short circuit occurs, there is atendency for the current or power to flow in the reverse direction. The disc of the upper elementrotates to bridge the fixed contacts 1 and 2. This completes the circuit for over current element.

The disc of this element rotates and the moving contact attached to closes the trip circuit. This operates the circuit breaker which isolates the faulty section Non-directional: This relay is also called earth leakage induction type relay .The overcurrent relay operates when the current in the circuit exceeds a certain preset value. The induction type non directional overcurrent relay has a construction similar to a watt-hour meter, with slight modification .The fig shows the constructional details of non directional induction type over current relay.

It consists of two electromagnets. The upper is E shaped while the lower is shaped the aluminium disc is

free to rotate between the two magnets. The spindle of the disc carries moving contacts and when the disc

rotates the moving contacts come in contact with fixed contacts which are the terminals of a trip circuit. The

upper magnet has two windings. Primary and secondary. The primary connected to the secondary of C.T

should be protected. This winding is tapped at intervals. The tapping’s are connected to plug setting. With

the help of this bridge, number of turns of primary winching can be adjusted. Thus the desired current

setting for the relay can be obtained. There are usually seven sections of tapping’s to have the overcurrent

range from 50% to 20%, in steps of 25%. These values are percentages of the current rating of the relay.

Thus a relay current may be i.e it can be connected to C.T. with secondary current rating of WA but with

50% setting the relay will start operating at SA. So adjustment of the current setting is made by inserting a

pin between spring loaded jaw of the bridge socket at the proper tap value required.

When the pin is withdrawn for the purpose of changing the setting while relay is in service then relay

automatically adopts a higher current setting thus secondary of C.T. is not open circuited. So relay remains

operative for the fault occurring during the pant of changing the setting. The secondary winding on the

central limb of upper magnet is connected in series with winching on the lower magnet. This winding is

energised by the induction from primary By this arrangement: of secondary winding, the leakage duxes of

upper and lower magnets are sufficiently displaced in space and time to produce a rotational torque on the

aluminium disc. The control torque is provided by the spiral spring. When current exceeds its preset value,

disc rotates and moving contacts on spindle make connection with trip circuit terminals. Angle through

which the disc rotates is between 0' to 360". The travel of the moving contacts can be adjusted by adjusting

angle of rotation of disc. This gives the relay any desired setting which o indicated by a pointer on a time

setting dial. The dial is calibrated from 0 to I. This does not give direct operating time but it gives multiplier

which can be used along with the time-plug setting multiplier curve to obtain actual operating time of the

relay.

4. Techniques used to realize various time-current characteristics using electromechanical relays.

Also, compare the time-current characteristics of inverse, very inverse and extremely inverse over

current relays. Discuss their applications.(N/D-14)

(A) Instantaneous Over Current Relay (Define Current):

Definite current relay operate instantaneously when the current reaches a predetermined value.

Operates in a definite time when current exceeds its Pick-up value.

Its operation criterion is only current magnitude (without time delay).

Operating time is constant.

There is no intentional time delay.

Coordination of definite-current relays is based on the fact that the fault current varies with the

position of the fault because of the difference in the impedance between the fault and the source

The relay located furthest from the source operate for a low current value

The operating currents are progressively increased for the other relays when moving towards the

source.

It operates in 0.1s or less

Application: This type is applied to the outgoing feeders

(B) Definite Time Over current Relays:

In this type, two conditions must be satisfied for operation (tripping), current must exceed the

setting value and the fault must be continuous at least a time equal to time setting of the relay.

Modern relays may contain more than one stage of protection each stage includes each own

current and time setting.

For Operation of Definite Time Over Current Relay operating time is constant

Its operation is independent of the magnitude of current above the pick-up value.

It has pick-up and time dial settings, desired time delay can be set with the help of an intentional

time delay mechanism.

Easy to coordinate.

Constant tripping time independent of in feed variation and fault location.

Drawback of Relay:

The continuity in the supply cannot be maintained at the load end in the event of fault.

Time lag is provided which is not desirable in on short circuits.

It is difficult to co-ordinate and requires changes with the addition of load.

It is not suitable for long distance transmission lines where rapid fault clearance is necessary for

stability.

Relay have difficulties in distinguishing between Fault currents at one point or another when fault

impedances between these points are small, thus poor discrimination.

Application: Definite time over current relay is used as:

Back up protection of distance relay of transmission line with time delay.

Back up protection to differential relay of power transformer with time delay.

Main protection to outgoing feeders and bus couplers with adjustable time delay setting.

(C) Inverse Time Over current Relays (IDMT Relay):

In this type of relays, operating time is inversely changed with current. So, high current will operate

over current relay faster than lower ones. There are standard inverse, very inverse and extremely

inverse types.

Discrimination by both ‘Time’ and ‘Current’. The relay operation time is inversely proportional to the

fault current.

Inverse Time relays are also referred to as Inverse Definite Minimum Time (IDMT) relay

The operating time of an over current relay can be moved up (made slower) by adjusting the ‘time

dial setting’. The lowest time dial setting (fastest operating time) is generally 0.5 and the slowest is

10.

Operates when current exceeds its pick-up value.

Operating time depends on the magnitude of current.

It gives inverse time current characteristics at lower values of fault current and definite time

characteristics at higher values

An inverse characteristic is obtained if the value of plug setting multiplier is below 10, for values

between 10 and 20 characteristics tend towards definite time characteristics.

Widely used for the protection of distribution lines.

Based on the inverseness it has three different types.

(1) Normal Inverse Time Over current Relay:

The accuracy of the operating time may range from 5 to 7.5% of the nominal operating time as

specified in the relevant norms.

The uncertainty of the operating time and the necessary operating time may require a grading

margin of 0.4 to 0.5 seconds.

used when Fault Current is dependent on generation of Fault not fault location

Relatively small change in time per unit of change of current.

Application:

Most frequently used in utility and industrial circuits. especially applicable where the fault

magnitude is mainly dependent on the system generating capacity at the time of fault

(2) Very Inverse Time Over current Relay:

Gives more inverse characteristics than that of IDMT.

Used where there is a reduction in fault current, as the distance from source increases.

Particularly effective with ground faults because of their steep characteristics.

Suitable if there is a substantial reduction of fault current as the fault distance from the power

source increases.

Very inverse over current relays are particularly suitable if the short-circuit current drops rapidly

with the distance from the substation.

The grading margin may be reduced to a value in the range from 0.3 to 0.4 seconds when over

current relays with very inverse characteristics are used.

Used when Fault Current is dependent on fault location.

Used when Fault Current independent of normal changes in generating capacity.

(3) Extremely Inverse Time Over current Relay:

It has more inverse characteristics than that of IDMT and very inverse over current relay.

Suitable for the protection of machines against overheating.

The operating time of a time over current relay with an extremely inverse time-current characteristic

is approximately inversely proportional to the square of the current

The use of extremely inverse over current relays makes it possible to use a short time delay in

spite of high switching-in currents.

Used when Fault current is dependent on fault location

Used when Fault current independent of normal changes in generating capacity.

Application:

Suitable for protection of distribution feeders with peak currents on switching in (refrigerators,

pumps, water heaters and so on).

Particular suitable for grading and coordinates with fuses and re closes

For the protection of alternators, transformers. Expensive cables, etc.

(4) Long Time Inverse over current Relay:

The main application of long time over current relays is as backup earth fault protection.

(D) Directional Over current Relays

When the power system is not radial (source on one side of the line), an over current relay may not

be able to provide adequate protection. This type of relay operates in on direction of current flow

and blocks in the opposite direction.

Three conditions must be satisfied for its operation: current magnitude, time delay and

directionality. The directionality of current flow can be identified using voltage as a reference of

direction.

Application of Over Current Relay:

Motor Protection:

Used against overloads and short-circuits in stator windings of motor.

Inverse time and instantaneous over current phase and ground

Over current relays used for motors above 1000kW.

Transformer Protection:

used only when the cost of over current relays are not justified

Extensively also at power-transformer locations for external-fault back-up protection.

Line Protection:

On some sub transmission lines where the cost of distance relaying cannot be justified.

primary ground-fault protection on most transmission lines where distance relays are used for

phase faults

For ground back-up protection on most lines having pilot relaying for primary protection.

Distribution Protection:

Over Current relaying is very well suited to distribution system protection for the following reasons:

It is basically simple and inexpensive

Very often the relays do not need to be directional and hence no PT supply is required.

It is possible to use a set of two O/C relays for protection against inter-phase faults and a separate

Over Current relay for ground faults.

5. Explain in what way distance protection is superior to over current protection for the protection

of transmission lines. (N/D-14)

The key advantage of distance protection is that its fault coverage of the protected circuit is independent of

source impedance variations. Considering an example to understand how distance protection is

independent of source impedance. Consider the figure below.

In the figure above, R1 is an over current relay which is used for the protection of Transmission Line. If

there is a fault at F1,

Equivalent source impedance Zs = 10×10/20 = 5 Ω

Impedance up to the point of Fault = 5+4 = 9 Ω

Fault current IF1= 220×103/1.732*9 = 220×103/15.588 = 14113.5 A

Therefore the setting of over current Relay should be more than 14113.5 A.

Now consider the case,

Here fault is not on the Transmission Line but it is assumed to be inside Switchyard and only one source is

feeding the power to the network. Proceeding in the similar manner,

Fault Current IF2= 220×103/1.732*10 = 12702A

Therefore for the protection of Transmission Line, the setting of Relay shall be kept less than 12702 A. But

for earlier case we saw that setting of Relay R1 shall be more than 14113.5 A thus overall the setting shall

be > 14113.5 but <12702 A which is impractical. Therefore over current Relay is not suitable here and

it depends on the source impedance.

Distance protection is therefore used for the protection of Transmission Line. It is simple to apply and fast in

isolating the faulty section from the healthy network. Distance Protection provides primary as well as back-

up protection to the protected line.

6.Write short notes on i)Under-frequency relays ii)Static relays (N/D-14)

i) Under Frequency relays:

Frequency relays are used whenever deviations from nominal system frequency need to be detected.

Frequency deviations can be harmful to connected objects, such as generators and motors, or when

abnormal frequency creates inconvenience for power consumers and may cause failures of electrical

apparatuses. Frequency relays are also used where detection of high or low frequency indicates system

abnormalities, such as faults in speed regulation units or system overload.

Underfrequency relays should be considered for applications where the detection of underspeed conditions

for synchronous motors and condensers is required. On lines where reclosing of the source breaker is

utilized, damage to large synchronous motors can be avoided by disconnecting the motors from the

system. Likewise, disconnection of synchronous condensers can be initiated upon loss of power supply.

The under frequency relay is a solidstate device that functions to protect the load in the event

generator frequency decreases below preset limits. It actuates when the frequency decreases to 55 hertz

for 60-hertz operation and 46 hertz for 50-hertz operation. Upon actuation, contacts within the relay close to

signal the annunicator and open to de-energize the generator breaker (contactor), resulting in a display of

the fault condition and removal of the load from the generator.

Frequency sensing is accomplished by a tuned circuit consisting of capacitors C1 and C2 and components

in the encapsulated base. Zener diodes CR1, CR2, and CR3 limit the peak voltage to the tuned circuit. The

ac output of the tuned circuit is rectified by diode CR4 and applied to a voltage divider consisting of

resistors R1, R2, R3, and R4. Transistor Q1 compares the voltage at the wiper of potentiometer R3 with the

reference voltage established by zener diode CR7.

When transistor Q1 conducts, transistor 42 operates as a switch to control the coil voltage on a relay

contained in the encapsulated base. Both transistors Q1 and Q2 and the relay in the encapsulated base

are energized when the frequency of the input voltage to terminals 1 and 2 is normal frequency (50 to 60

hertz). When an underfrequency condition occurs, the voltage at the base of transistor Q1 is not sufficient

for conduction. This causes the relay to be de-energized and its contacts to switch. The underfrequency trip

point is adjusted by potentiometer R3.

ii)Static relays:

The static relay is the combination of both the static and the electromagnetic relay. In this relay, there is no

armature and moving contacts and response is developed by the components without mechanical motion.

The solid state components used are transistors, diodes, resistors, capacitor and so on. In the static relay,

the measurement is performed by electronic, magnetic, optical or another component without mechanical

motion.

The static components of a static relay are shown in the figure below. Here the relaying quantity, i.e., the

output of a CT or PT of a transducer is rectified by the rectifier. The rectified output is given to a measuring

unit constitute of comparators, level detectors, and logic circuits. The output is actuated when the dynamic

input attains the thereshold value.

The output of the measuring unit is fed to the output unit devices after it is amplified by the amplifiers. The output unit activates the trip coil only when the relay operates. The relaying quantity such as the voltage and current is rectified and measured. When the quantity under measurement attains certain well-defined value, the output device is energized and hence, the circuit breaker trip is triggered.

The static relay can be arranged to respond to electrical inputs. The other types of input such as heat, light, magnetic field, traveling waves, etc., can be suitably converted into equivalent analog and digital signal and then supplied to the static relay

7.Describe the principle of negative sequence relay.(M/J-14) A relay which protects the electrical system from negative sequence component is called a negative sequence relay or unbalance phase relay. A negative phase sequence or unbalance relay is essentially provided for the protection of generators and motors against unbalanced loading that may arise due to phase-to-phasefaults. Negative phase sequence relay has a filter circuit which is responsible only for the negative sequence components. Since small magnitude over current can cause a dangerous condition, it becomes necessary to have low settings of such relays. Negative sequence relays are mainly required for the phase to phase faultprotection. The figure shown below illustrates the scheme used for negative phase sequence relay. A network consists of four impedance Z1, Z2, Z3 and Z4 of equal magnitude connected in a bridge formation is energized from three CTs. A single pole relay having an inverse-time characteristic is connected to the circuit shown in the figure.

Z1 and Z2 are non-inductive resistors while Z2 and Z4 are composed of both resistance and inductance. The value of Z2 and Z1 are so adjusted that the current flows through them lag behind those in impedance Z3 and Z1 by 60º. The relay is assumed to have negative impedance. The current from phase R at junction

A is equally divided into two branches as I1 and I4, but, I4 will lag behind I1 by 60º.

Similarly, current from phase B split at junction C into two equal components I3 and I2, I2 lagging behind I3 by 60º.

I1 leads IR by 30º while I4 lags behind IR by 30º. Similarly, I2 lags behind IB by 30º,whereas I3 leads IB by 30º. The current through the relay operating coil at junction B will be equal to phasor sum of I1, I2 and IY.

The flow of Positive Sequence Current – The phasor diagram of positive sequence components is shown in the figure below. When the load is in balanced conditions, then there is no negative sequence current. The current flow through the relay is given by the equation

So the relay remains operative for a balanced system. The flow of Negative Sequence Current – In the bridge circuit it is shown that the current I1 and I2 are equal but opposite to each other, so they cancel each other and IY current flow through the relay operating coil. Thus the relay operates due to the flow of the IY. A low setting value well below the normal full load rating of the machine is provided with comparatively small values of unbalanced current produces a great danger.

The flow of Zero Sequence Current – The current at junction B of the relay is represented by the phasor diagram from which it is observed that the current I1 and I2 are displaced from each other by 60º, so the resultant of these current is in phase with current IY. Thus the relay would operate by the twice of the total current flow through it. For making the current inoperative, the CTs are connected in delta as shown in the figure and then no zero sequence current can flow in the network circuit.

Induction type Negative Sequence Relay

The construction of induction type negative phase sequence relay is similar as that of an induction type over current relay. This relay consists of a metallic disc usually made up of an aluminum coil, and this is rotating between two electromagnets the upper and the lower electromagnets. The upper electromagnet has two winding, the primary winding of the upper electromagnet is connected to the secondary of the CT connected in the line to be protected. The secondary winding of the upper electromagnet is connected in series with the windings on the lower electromagnet The primary windings provided on the central limb of the upper electromagnet which is provided by the central tap resulting into three terminals 1, 2, and 3 of these windings. The upper half is energized from phase R through CT and an auxiliary transformer while the lower half is energized from phase Y through CT. The auxiliary transformer has a special construction such that the output of this transformer lag by 120º instead of 180º. The operation for Positive Sequence Currents – The current IR and IY flowing through the primary of the relay are in oppositions, the auxiliary transformer is so arranged that I’R and I’y are of equal magnitude. Thus the relays remain inoperative for a balanced system. The operation for Negative Sequence currents – When there is a fault on the system, resulting in phase sequence currents, there is a flow of current I through the primary windings of the relay.

8. With a neat schematic diagram, explain the protection of transformer with differential protection scheme.(M/J-14) The transformer is one of the major equipment in power system. It is a static device, totally enclosed and usually oil immersed, and therefore the fault occurs on them are usually rare. But the effect of even a rare fault may be very serious for a power transformer. Hence the protection of power transformer is very important. The fault occurs on the transformer is mainly divided into two type external faults and internal fault. External fault is cleared by the relay system outside the transformer within the shortest possible time in order to avoid any danger to the transformer due to these faults. The protection for internal fault in such type of transformer is to be provided by using differential protection system.

Differential protection schemes are mainly used for protection against phase-to-phase fault and phase to earth faults.The differential protection used for power transformers is based on Merz-Prize circulating current principle. Such types of protection are generally used for transformers of rating exceeding 2 MVA.

Connection for Differential Protection for Transformer The power transformer is star connected on one side and delta connected on the other side. The CTs on the star connected side are delta-connected and those on delta-connected side are star-connected. The neutral of the current transformer star connection and power transformer star connections are grounded. The restraining coil is connected between the secondary winding of the current transformers. Restraining coils controls the sensitive activity occurs on the system. The operating coil is placed between the tapping point of the restraining coil and the star point of the current transformer secondary windings.

Working of Differential Protection System Normally, the operating coil carries no current as the current are balanced on both the side of the power transformers. When the internal fault occurs in the power transformer windings the balanced is disturbed

and the operating coils of the differential relay carry current corresponding to the difference of the current among the two sides of the transformers.Thus, the relay trip the main circuit breakers on both sides of the power transformers. Problem Associated with Differenctial Protection System When the transformer is energizing the transient inrush of magnetizing current is flows in the transformer. This current is as large as 10 times full load current and its decay respectively.This magnetizing current is flows in the primary winding of the power transformers due to which it causes a difference in current transformer output and it makes the differential protection of the transformer to operate falsely. To overcome this problem the kick fuse is placed across the relay coil. These fuses are of the time-limit type with an inverse characteristic and do not operate in short duration of the switch in the surge. When the fault occurs the fuses blow out and the fault current flows through the relay coils and operate the protection system. This problem can also be overcome by using a relay with an inverse and definite minimum type characteristic instead of an instantaneous type. 9. Draw and explain about differential protection of transmission lines.(M/J-14) As the length of electrical power transmission line is generally long enough and it runs through open atmosphere, the probability of occurring fault in electrical power transmission line is much higher than that of electrical power transformers and alternators. That is why a transmission line requires much more protective schemes than a transformer and an alternator. Protection of line should have some special features, such as-

1. During fault, the only circuit breaker closest to the fault point should be tripped. 2. If the circuit breaker closest the faulty point, fails to trip the circuit breaker just next to this breaker

will trip as back up. 3. The operating time of relay associated with protection of line should be as minimum as possible in

order to prevent unnecessary tripping of circuit breakers associated with other healthy parts of power system.

These above mentioned requirements cause protection of transmission line much different from protection of transformer and other equipment of power systems. The main three methods of transmission line protection are -

1. Time graded over current protection. 2. Differential protection. 3. Distance protection.

Time Graded Over Current Protection

This may also be referred simply as over-current protection of electrical power transmission line. Let'

discuss different schemes of time graded over current protection.

Protection of Radial Feeder

In radial feeder, the power flows in one direction only, that is from source to load. This type of feeders can

easily protected by using either definite time relays or inverse time relays.

Line Protection by Definite Time Relay

This protection scheme is very simple. Here total line is divided into different sections and each section is

provided with definite time relay. The relay nearest to the end of the line has minimum time setting while time setting of other relays successively increased, towards the source.

For example, suppose there is a source at point A, in the figure below

At point D the circuit breaker CB-3 is installed with definite time of relay operation 0.5 sec. Successively, at

point C an other circuit breaker CB-2 is installed with definite time of relay operation 1 sec. The next circuit

breaker CB-1 is installed at point B which is nearest of the point A. At point B, the relay is set at time of

operation 1.5 sec.

Now, assume a fault occurs at point F. Due to this fault, the faulty current flow through all the current

transformers or CTs connected in the line. But as the time of operation of relay at point D is minimum the

CB-3, associated with this relay will trip first to isolate the faulty zone from rest part of the line. In case due

to any reason, CB-3 fails to trip, then next higher timed relay will operate the associated CB to trip. In this

case, CB-2 will trip. If CB-2 also fails to trip, then next circuit breaker i.e. CB-1 will trip to isolate major

portion of the line.

Advantages of Definite Time Line Protection

The main advantage of this scheme is simplicity. The second major advantage is, during fault, only nearest

CB towards the source from fault point will operate to isolate the specific position of the line.

Disadvantage of Definite Time Line Protection

If the number of sections in the line is quite large, the time setting of relay nearest to the source, would be

very long. So during any fault nearer to the source will take much time to be isolated. This may cause

severe destructive effect on the system.

Over Current Line Protection by Inverse Relay

The drawback as we discussed just in definite time over current protection of transmission line, can easily

be overcome by using inverse time relays. In inverse relay the time of operation is inversely proportional to

fault current.

In the above figure, overall time setting of relay at point D is minimum and successively this time setting is

increased for the relays associated with the points towards the point A. In case of any fault at point F will

obviously trip CB-3 at point D. In failure of opening CB-3, CB-2 will be operated as overall time setting is

higher in relay at point C. Although, the time setting of relay nearest to the source is maximum but still it will

trip in shorter period, if major fault occurs near the source, as the time of operation of relay is inversely

proportional to faulty current.

Over Current Protection of Parallel Feeders

For maintaining stability of the system it is required to feed a load from source by two or more than two

feeders in parallel. If fault occurs in any of the feeders, only that faulty feeder should be isolated from the

system in order to maintain continuity of supply from source to load. This requirement makes the protection

of parallel feeders little bit more complex than simple non direction over current protection of line as in the

case of radial feeders. The protection of parallel feeder requires to use directional relays and to grade the

time setting of relay for selective tripping.

There are two feeders connected in parallel from source to load. Both of the feeders have non-

directional over current relay at source end. These relays should be inverse time relay. Also both of the

feeders have directional relay or reverse power relay at their load end. The reverse power relays used here

should be instantaneous type. That means these relays should be operated as soon as flow of power in the

feeder is reversed. The normal direction of power from source to load.

Now, suppose a fault occurs at point F, say the fault current is If. This fault will get two parallel paths from

source, one through circuit breaker A only and other via CB-B, feeder-2, CB-Q, load bus and CB-P. This is

clearly shown in figure below, where IA and IB are current of fault shared by feeder-1 and feeder-2

respectively.

As per Kirchoff's current law, IA + IB = If.

Now, IA is flowing through CB-A, IB is flowing through CB-P. As the direction of flow of CB-P is reversed it

will trip instantly. But CB-Q will not trip as flow of current (power) in this circuit breaker is not reversed. As

soon as CB-P is tripped, the fault current IB stops flowing through feeder and hence there is no question of

further operating of inverse time over current relay. IA still continues to flow even CB-P is tripped. Then

because of over current IA, CB-A will trip. In this way the faulty feeder is isolated from system.

Differential Pilot Wire Protection

This is simply a differential protection scheme applied to feeders. Several differential schemes are applied

for protection of line but Mess Price Voltage balance system and Translay Scheme are most popularly

used.

Merz Price Balance System

The working principle of Merz Price Balance system is quite simple. In this scheme of line protection, identical CT is connected to each of the both ends of the line. The polarity of the CTs are same. The secondary of these current transformer and operating coil of two instantaneous relays are formed a closed loop as shown in the figure below. In the loop pilot wire is used to connect both CT secondary and both relay coil as shown.

Now, from the figure it is quite clear that when the system is under normal condition, there would not be any current flowing through the loop. As the secondary current of one CT will cancel out secondary current of other CT. Now, if any fault occurs in the portion of the line between these two CTs, the secondary current of one CT will no longer equal and opposite of secondary current of other CT. Hence there would be a resultant circulating current in the loop. Due this circulating current, the coil of both relays will close the trip circuit of associate circuit breaker. Hence, the faulty line will be isolated from both ends.