Protective Relays

7/13/2019 Protective Relays

1/331

PROTECTIVE SYSTEM-GENRAL ASPECTS:-

In an electrical power system, as and when a fault occurs or any parameter becomes abnormal, it

becomes necessary to isolate the faulty section instantaneously from the rest of the system. The isolation

should be automatic, such that the supply must get disconnected from the faulty section leaving healthy

remainder in service. This can be achieved by using the protective devices like relays or fuses in the circuit to

isolate the circuit from damage to occur due to short circuit, overhead, earth fault etc.

The use of fuse for isolation or breaking the circuit for protection of the system is limited to low voltagecircuits i.e., upto 11kV. 11kV onwards, the circuit is protected by providing the protective relays.

Main features of good protective system/gear.The main features of a good protective system/gear are:

1. Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity. The

protective system should be so sensitive that it operates even at low values of fault current.

2. Selectivity:It is the ability of the protective system to select correctly that part of the system in trouble

and disconnect the faulty part without disturbing the rest of the system. The protective system should

select correctly the faulty part of the power system and disconnect the same without disturbing the rest

of the system. Thus it is absolutely necessary for a protective system to have the property of

discrimination.3. Reliability: The reliability is one of the most important factors in the system. The component installed

as a part of the protective system should operate definitely under pre-determined condition.

4. Simplicity: The relaying system should be simple so that it can be easily maintained. The simpler the

protection system, the greater will be its reliability.

5. Quickness:The protect system should respond as quickly as possible in order to improve the quality of

service, increase safety of life and equipment and increase stability of operation.

6. Non-interference with future extension: The installation of protective system should be carried out in

such a way that future extension is possible without any interference with the original one.

7. Economy: While selecting a protective gear, due to consideration should be given to its cost. However,

when utmost important and sensitive apparatus (like alternator, transformer etc.) is to be protected thecost factor or economic consideration are ignored.

Types of protection:

The protectionis of the following two types:

1. Main or primary protection: This type of protection is the first line of defence and ensures quick

acting and selective clearing of faults within the boundary of the circuit section or element it produces.

Main protection as a rule, is provided for each section of an electrical installation.

2. Back up or secondary protection: the backup protection is quite important for the proper functioning

of the good system of electrical protection since percent reliability of the of protective system as well as

associated CTs, PTs and circuit breakers cannot be guaranteed. It is the second line of the defence

which function to isolate a faulty section of the system when the main protection fails to function

properly. Back up protection is only provided where the main protection of the adjacent is unable to

backup the main protection of the given circuit.

PROTECTIVE RELAYS-INTRODUCTION

Relay is a device by means of whi ch an electric cir cuit (tr ip circuit of alarm cir cui t), is controlled by change

in the other cir cui t. Relays are automati c. There are several types and appli cations of r elays. Relays are

essential components of protective systems.

Protective relay is an electr ical relay used for protective of electr ical devices. I t is a device whi ch closed its

contacts, when operating quanti ty reaches certain predetermined magni tude/phase. Closing of relay contacts

in iti ates an alarm cir cuit or tri p cir cuit.

Primary Relays. These are the relays which are connected directly in the circuit to be protected.


2/332

Secondary Relays. These are the relays which are connected in the circuit through current and potential

transformers.

Auxiliary Relays. These are the relays which operate in response to the opening or closing to its operating

circuit to assist another relay in the performance of its function. This relay maybe instantaneous or may have a

time delay.

(example of such a relay is a television power button. When a viewer presses the power button to turn the

television ON, he is opening the relay. A signal is then sent to the main power to turn the television ON. When

he presses the power button again, he is closing the relay and the power will shut off)

Whenever fault occurs on the power system then the relay

detect that fault and closes the tri p coil circuit. This results in

opening of circuit breaker which disconnect the faulty circuit.

Thus this ensures the safety of the circuit equipment being

damaged.

A typical relay circuit is shown in above figure on occurrence

of short circuit at appoint F on the transmission line. There is

enormous increase in the value of current flowing in the line.

This causes a heavy flow of current through the relay coil(R.C), making the relay to operate by closing it contacts. This

in turn close the trip circuit of the breaker, consequently the

circuit breaker opens and the faulty section is isolated from the rest of the system.

Trip circuit: The circuit comprising trip coil, relay contacts, auxiliary switch, battery supply etc., which

controls the circuit breaker for opening operation.

Auxiliary switch: A multipoint switch which operates in conjunction with circuit breaker and

connects/disconnects certain protective, indicating and control circuits in each position.

Auto reclosure: this is the process of automatic re-closing of circuit breaker after opening. When thedisturbance occurs for a short time the relay operates; but it re-closes after sensing it as a temporary fault.

Temporary faults are due to lightning and trees falling on lines.

For EHV line one reclosure in 12 cycles is sufficient to clear the fault.

For lines upto 33kV three re-closures are necessary to clear the fault due to tree falling (with interval of 15-

120secs)

Based on experience it is found that about 80% of faults are cleared after first re closure, 10% after second re-

closure, 2% after third re-closure and 8% not cleared after third re-closure is permanent fault.

Rapid auto reclosure of CBs at both ends of transmission lines is advantageous, because

1. It improves transient stability of the system

2. It improves the power transfer of the system

Essential fundamental elements of the relays: All the relays have the following three essential fundamental

elements:

1. Sensing element: it is also called measuring element .it is the element which responds to change in theactuating quantity, the current in a protected system, in case of over-current relay.

2. Comparing elements: This element compare the action of the actuating quantity on the relay with apredesign relay setting. The relay picks up only if the actuating (operating) quantity is more than the

relay setting.


3/333

3. Control elements: This element accomplishes a sudden change in control quantity such as closing of

the operative current circuit.

Operation of relay: It means either closing or opening of its contacts. Most of the relays are restrained by

spring so that they are at a pre-determined position when correctly de-energized. At this position if the status of

the contact is closed it is said 'NC' (normally closed) contact and if it is open it is said 'NO' (normally open)

contact.

-If a relay operates to open a NC contact or close a NO contact it is said that a relay 'picks up' and the

smallest value of this actuating quantity is called the pick-up value.

-If a relay operates to return back to its original position it is said that the relay rests and the largest value of

operating quantity at which it this occurs is called reset value. Reset value is less than the pick-up value.

Terms Connected with Relays:

Some commonly used terms relating to relays are discussed below:

1. Operating force or torque: It is torque (or force) which tends to close the contacts of the relay.2. Restraining force or torque: It is a torque (or force) which opposes the operating torque and tends to

prevent the closer of relay contacts.

3. Operating or pick up level: It is defined as the threshold value of current, voltage etc., above whichthe relay will close its contacts.

4. Drop out or reset (level): It is defined as the value of the current, voltage etc., below which the relaywill open its contacts and return to normal position.

The ratio of drop out (or reset) value to the operating (or pick up) value is called drop out (or reset)

ratio.

5. Flag or target: It is a device (usually spring or gravity operated) which indicates the operation of arelay.

6. Power consumption (burden): It is power absorbed by circuits of the relay expressed in VA (volt-amperes) in A.C. and in W (watts) in D.C. circuits at the rated current or voltage.

7. Operating time: It is defined as the time which elapses form the moment when the actuating quantityattains a value equal to the pickup value until the relay operates its contacts.

8. Reset time: The time which elapses between the instant when the actuating quantity becomes less thanthe reset value to the instant when the relay contact returns to its normal position.

9. Seal-in-coil: It is the coil which does not permit the relay contacts to open when the current is flowingthrough them.

10.Maximum torque angle: It is defined as the design angle of the relay that will yield maximum torque.It is also known as characteristics angle of relay.

11.Over-reach: A relay is said to over-reach when it operates at a current which is lower than its setting.'Under-reach' is just reverse of over-reach.

12.Pick-Up Current. If the minimum current in relay coil at which the relay starts to operate.13.Current Setting/Plug setting. Often it is desirable to adjust the pickup current to any required value.

This is known as current setting and is usually achieved by the use of tapings on the relay operating

coil. The value assigned to each tap are expressed in terms of percentage full load rating of C.T.


4/334

with which the relay is associated and represents the value above which the disc commences to rotate

and finally closes the trip circuit.

Therefore, Pick-up current (relay pick up current) =Rated secondary current (C.T) x Current setting.

The current setting is sometimes referred as current plug setting. But Plug Setting Multiplier is different

from plug setting.

Plug setting/Current settingbridge is provided with induction disc relays and it provides a wide range

of current settings. The plug setting refers to the magnitude of current at which the relay starts to

operate. The plug setting bridge comprises connections tapped from relay coil. By inserting the plug ina particular gap in the bridge, a certain no of turns of the relay coil are brought into circuit (as we know,

minimum pick up value of the deflecting forceof an electrical relay is constant & the deflecting force

of the coil is proportional to its number of turns and electric current flowing through the coil. Now, if

we can change the number of active turns of any coil, the required current to reach at minimum pick

value of the deflecting force in the coil also changes. That means if active turns of the relay coil is

reduced, then proportionately more current is required to produce desired relay actuating force.

Similarly if active turns of the relay coil is increased, then proportionately reduced current is required to

produce same desired deflecting force)

Suppose, you want that, an over current relay should operate when the system current just crosses 125%

of rated current. If the relay is rated at 2A, the normal pick up current of the relay is 2A and it should be

equal to secondary rated current of current transformer connected to the relay.

That means, the relay should be operated when the electric current of CT secondary becomes more than

or equal 2.5A (which is 125% of rated current).

As per definition,

Current setting = 2.5/2 x 100% = 125%

14.Plug Setting Multiplier (P.S.M).It is the ratio of fault current in relay coil to the pick-up current, thatis

P.S.M=

Pk up urret=

Rated edary urret f C.T x Curret ettg.

PSM will give the severity of the fault, i.e., if PSM


5/335

Time setting.

Fault current.

Current transformer ratio,

The following is the procedure to calculate the actual relay operating time:

By the use of current transformer ratio, convert the fault current into the relay coil current.

Express the relay current as a multiplier of current setting i.e., calculate P.S.M.

Using Time/P.S.M. curve of the relay, read-off the time of the operation for the calculated P.S.M. Determine the actual time of operation by multiplying the above time of the relay by the time setting

multiplier in use.

Classification of Relays:

There are many kind of relays used in the power systems. The relays can be designed and constructed to

operate in response to one or more electrical quantity such as voltage, current, phase angle etc.

The type of relays may be classified as under:

1. According to their construction and principle of operation : Electromagnetic attraction type relays:

o Attracted armature type relaysOperation depends on movement of an armature under the

influence of attractive force due to magnetic field setup by the current flowing through the relay

winding.

o Solenoid type relaysOperation depends on the movement of an iron plunger core along the

solenoid axis.

o Balanced beam type relays

Electromagnetic induction or simply induction relaysOperation depends on movement of a metallic

disc or cylinder free to rotate by the interaction of induced eddy currents and the alternating magnetic

field producing them.o Shaded pole type

o Watt-hour meter type

o Induction cup type

Electro-dynamic relaysIn such a relay, moving member consists of a coil free to rotate in the air gap

of a permanent magnet.

Moving coil type relaysIn such a relay, moving member consists of a coil free to rotate in air gap of a

permanent magnet.

Thermal relaysIn this type of relay, movement depends upon the action of the heat produced by the

current flowing through the element of relay.

Physio-electric relaysBuchholzs relay is an example of this type of relay. Static relaysThis relays employ thermionic valves, transistors or magnetic amplifiers to obtain the

operating characteristics.

2. According to their applications : Under-voltage, Under-current and under-power relaysOperation occurs when the voltage, current or

power falls below a specified value (mostly instantaneous or induction relays).

Over-voltage, over-current and over-power relaysOperation takes place when the voltage, current or

the power rises above a specified value (mostly instantaneous or induction relays).

Directional or reverse current relaysOperation occurs when the applied current assumes a specific

phase displacement with the respect to the applied voltage and the relay is compensated for fall in

voltage (induction current relays). Directional or reverse power relays. This relay is able to sense whether the fault lied in the forward

direction or reverse direction w.r.t the relay location. It can also sense the direction of power flow i.e.,

in the normal direction or in the opposite direction


6/336

Differential relays Operation occurs at some specific phase or magnitude difference between two or

more electrical quantities.

Distance relaysoperation depends upon the ratio of voltage to current.

Polarised relay:the operation of this relay depends on the direction of the current.

Slave relay:it is an output device and does not perform the function of comparison or measurement. It

simply closes the contacts, when an output is received from the static rely. Most of the modern static

relays are using D.C. polarized relay as slave relay because of its low cost and simpler electromagnetic

multi-contact tripping arrangement

3. According to their time of operation or timing characteristics:

1)Instantaneous relaysOperation occurs after a negligibly small interval of time from the incidence of the

current or other quantity which causes operation.

An instantaneous relay is one in which no intentional time delay is provided. In this case, the relay contacts are

closed immediately after current in the relay coil exceeds the minimum calibrated value.

This relay is effective only where the impedance between the relay and source is small compared to the

protected section impedance

Its operating time is sometimes expressed in cycle based on the power system frequency.

e.g., In a 50 cycle system one cycle would be 1/50 sec.

2) Inverse time-lag relayA time of operation is approximately inversely proportional to the magnitude off the

current or other quantity causing operation.

The inverse time delay can be achieved by associating mechanical accessories (e.g., Drag magnet (for

induction type relays), Oil dash pot or time limit fuse).

At values of current less than pick up, the relay never operates.

At higher values the time of operation of relays decreases steadily with the increase of current.

3)Definite time-lag relays...In this type of relay there is a definite time elapse between the instant of pick up

and the closing of relay contacts. This particular time setting is independent of the amount of current through

the relay coil being the same for all values of current in excess of the pickup value.

It may be mentioned here that all inverse time relays are also provided with definite minimum time feature in

order that the relay may never become instantaneous in its action for very long overloads.

4)Inverse-Definite minimum time lag(I.D.M.T) relaysThe time of operation is approximately inversely

proportional to the smaller values of current or other quantities causing operation and tends to a definite

minimum time as the value increases without limit.

The time lag in induction type relays may be achieved by using a permanent magnet which is so

arranged that the relay rotor cuts the flux between the poles of this

magnet. This type of magnet is called Drag magnet.

In other relays it may be achieved in the following 2 ways:

1) When a series connected overload trip coil is used (without any

CT), an oil dash pot can be attached in which a piston is connected

to the lower end of the solenoid plunger of the relay as shown. On

the occurrence of a fault, when the plunger is pulled, the piston,

immersed in the oil, retards plunger motion and thus provides the

necessary drag or delay in the operation of the relays.


7/337

2)If the trip coil is transformer (CT)operated, time delay is

achieved by connecting a time- limit fuse across the trip coil

terminal(as shown).This fuse should have inverse time

current characteristic and will normally carry the C.T

secondary current to by-pass the trip coil. When sufficient

current flows to blow this fuse, the whole current is then

transferred to the trip coil which operates to trip the circuitbreaker.

Most of the relays in service on electric power system today are of electro-mechanical type. They work on the

following two main operating principles:

Electromagnetic attraction

Electromagnetic induction

Electromagnetic Attraction Relays

Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an

electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by d.c. or a.c. quantities.

The force of attraction F=ki2

The important types of electromagnetic attraction relays are:

Attracted armature type relay

Solenoid type relay

Balanced beam type relay

Attracted armature type relay:Figure shows the schematic arrangementof an attracted armature type relay. It consists of a laminated

electromagnet M carrying a coil C and a pivoted laminated armature. The

armature is balanced by a counterweight and carries a pair of spring

contact fingers at its free end. Under normal operating conditions, the

current through the relay coil C is such that counterweight holds the

armature in the position shown. However, when a short-circuit occurs, the

current through the relay coil increases sufficiently and the relay armature

is attracted upwards. The contacts on the relay armature bridge a pair of

stationary contacts attached to the relay frame. This completes the trip circuit which results in the opening of

the circuit breaker and, therefore, in the disconnection of the faulty circuit.

It is a usual practice to provide a number of tappings on the relay coil so that the number of turns in use and

hence the setting value at which the relay operates can be varied

Solenoid type relay: Figure shows the schematic arrangement of a solenoid type

relay. It consists of a solenoid and movable iron plunger arranged as shown. Under

normal operating conditions, the current through the relay coil C is such that it holds

the plunger by gravity or spring in the position shown. However, on the occurrence of

a fault, the current through the relay coil becomes more than the

pickup value, causing the plunger to be attracted to the solenoid. The

upward movement of the plunger closes the trip circuit, thus openingthe circuit breaker and disconnecting the faulty circuit.

Balanced beam type relay:Figure shows the schematic arrangement

of a balanced beam type relay. It consists of an iron armature fastened


8/338

to a balance beam. Under normal operating conditions, the current through the relay coil is such that the beam

is held in the horizontal position by the spring. However, when a fault occurs, the current through the relay coil

becomes greater than the pickup value and the beam is attracted to close the trip circuit. This causes the

opening of the circuit breaker to isolate the faulty circuit.

Induction Relays

Electromagnetic induction relays operate on the principle of induction motor and are widely used for protective

relaying purposes involving a.c. quantities. They are not used with d.c. quantities owing to the principle of

operation.

An induction relay essentially consists of a pivoted aluminium disc placed in two alternating magnetic fields of

the same frequency but displaced in time and space. The torque is produced in the disc by the interaction of one

of the magnetic fields with the currents induced in the disc by the other.

2 sin

The following points may be noted from above expression

The greater the phase angle between the fluxes, the greater is the netforce applied to the disc.

The following three types of structures are commonly used for obtaining the phase difference in the fluxes and

hence the operating torque in induction relays:

Shaded-pole structure

Watt-hour-meter or double winding structure

Induction cup structure

Plug setting and time setting in induction disc relays:In these relays, there is a facility for selecting the plug

setting and time setting such that the same relay can be used for a wide range of current, time characteristics.

Shaded-pole structure:The general arrangement of shaded-pole structure is shown in Figure. It consists of a

pivoted aluminium disc free to rotate in the air-gap of an electromagnet. One half of each pole of the magnet issurrounded by a copper band known as shading ring.

The alternating flux s in the shaded protion of the

poles will, owing to the reaction of the current

induced in thering, lag behind the flux u in the

unshaded portion by an angle . These two a.c. fluxes

differing in phase will produce the necessary torque

to rotate the disc. As proved earlier, the driving

torque T is

sinAssuming the fluxes s and u to be proportional to

the current I in the relay coil then

2 sin

This shows that driving torque is proportional to the square of current in the relay coil.


9/339

Watt-hour meter structure: This structure gets its name from the fact that it is used in watt-hour meters. The

general arrangement of this type of relay is shown in Fig. It

consists of a pivoted aluminium disc arranged to rotate freely

between the poles of two electromagnets. The upper

electromagnet carries two windings; the primary and the

secondary. The primary winding carries the relay current I1

while the secondary winding is connected to the winding of the

lower magnet. The primary current induces e.m.f. in the

secondary and so circulates a current I2 in it. The flux 2

induced in the lower magnet by the current in the secondary

winding of the upper magnet will lag behind 1by

an angle . The two fluxes 1 and 2 differing in phase by will

produce a driving torque on the disc proportional to

2 sin.

An important feature of this type of relay is that its operation can be controlled (ON/OFF of relay) by opening

or closing the secondary winding circuit. That means, If secondary circuit is opened, no flux can be set by the

lower magnet however great the value of current in the primary winding but no torque will be produced (since

from the equation). Therefore, the relay can be made inoperative by opening its secondary winding circuit.

Induction cup structure: Fig shows the general arrangement of an

induction cup structure. It most closely resembles an induction motor,

except that the rotor iron is stationary, only the rotor conductor portion

being free to rotate.

The moving element is a hollow cylindrical rotor which turns on its axis.

The rotating field is produced by two pairs of coils wound on four poles as

shown. The rotating field induces currents in the cup to provide the

necessary driving torque. If 1and 2represent the fluxesproduced by the

respective pairs of poles, then torque produced is proportional to

2 sinwhere is the phasedifference between the two fluxes.

A control spring and the back stopfor closing of the contacts carried on an

arm are attached to the spindle of the cup to prevent the continuous rotation.

Induction cup structures are more efficient torque producers than either the shaded-pole or the watt-hour meter

structures. Therefore, this type of relay has very high speed and may have an operating time less than 01

second. Induction cup relay is very suitable for directional or phase comparison units. This is because, besides

the sensitivity, induction cup relay have steady non vibrating torque and parasitic torques due to current or

voltage alone are small.

Reactance and MHO type Induction Cup Relay

By manipulating the current voltage coil arrangements and the relative phase displacement angles between the

various fluxes, induction cup relay can be made to measure either pure reactance or admittance. Such

characteristics are discussed in greater detail in a session on electromagnetic distance relay.

Induction Type Overcurrent Relay (non-directional):

This type of relay works on the induction principle and initiates corrective measures when current in the circuit

exceeds the predetermined value. The actuating source is a current in the circuit supplied to the relay from acurrent transformer. These relays are used on a.c. circuits only and can operate for fault current flow in either

direction.

Constructional details: Figure shows the important constructional details of a typical non-directional

induction type overcurrent relay. It consists of a metallic (aluminium) disc which is free to rotate in between


10/3310

the poles of two electromagnets. The upper electromagnet has a primary and a secondary winding. The primary

is connected to the secondary of a C.T. in the line to be protected and is tapped at intervals. The tappings are

connected to a plug-setting bridge by which the number of active turns on the relay operating coil can be

varied, thereby giving the desired current setting.

The secondary winding is energised by induction from primary and is

connected in series with the winding on the lower magnet. The controlling

torque is provided by a spiral spring. The spindle of the disc carries a moving

contact which bridges two fixed contacts (connected to trip circuit) when thedisc rotates through a pre-set angle. This angle can be adjusted to any value

between 0 and 360. By adjusting this angle, the travel of the moving contact

can be adjusted and hence the relay can be given any desired time setting.

Operation: The driving torque on the aluminium disc is set up due to the

induction principle. This torque is opposed by the restraining torque provided

by the spring. Under normal operating conditions, restraining torque is greater

than the driving torque produced by the relay coil current. Therefore, the

aluminium disc remains stationary. However, if the current in the protected circuit exceeds the pre-set value,

the driving torque becomes greater than the restraining torque. Consequently, the disc rotates and the moving

contact bridges the fixed contacts when the disc has rotated through a pre-set angle. The trip circuit operates the

circuit breaker which isolates the faulty section.

Induction Type Directional Power Relay: This type of relay operates when power in the circuit flows in a

specific direction. Unlike a non-directional overcurrent relay, a directional power relay is so designed that it

obtains its operating torque by the interaction of magnetic fields derived from both voltage and current source

of the circuit it protects. Thus this type of relay is essentially a wattmeter and the direction of the torque set up

in the relay depends upon the direction of the current relative to the voltage with which it is associated.

Constructional details: Fig shows the essential parts of a typical induction type directional power relay. It

consists of an aluminum disc which is free to rotate in between the

poles of two electromagnets. The upper electromagnet carries a

winding (called potential coil) on the central limb which is connected

through a potential transformer (P.T.) to the circuit voltage source. The

lower electromagnet has a separate winding (called current coil)

connected to the secondary of C.T. in the line to be protected. The

current coil is provided with a number of tappings connected to the

plug setting bridge (not shown for clarity, which would be same as

non-directional type). This permits to have any desired current setting.

The restraining torque is provided by a spiral spring.

Operation: The flux 1due to current in the potential coil will benearly 90 lagging behind the applied voltage V. The flux 2due to

current coil will be nearly in phase with the operating current I. The

interaction of fluxes 1 and 2 with the eddy currents induced in the

disc produces a driving torque(energy meter principle) given by:

2 sin

&2 = (90 )

Therefore, 2 sin(90 )

2 cos


11/3311

It is clear that the direction of driving torque on the disc depends upon the direction of power flow in

the circuit to which the relay is associated. When the power in the circuit flows in the normal direction, the

driving torque and the restraining torque (due to spring) help each other to turn away the moving contact from

the fixed contacts. Consequently, the relay remains inoperative. However, the reversal of cur rent in the circui t

reverses the direction of dr iving torque on the disc. When the reversed dri ving torque is large enough, the

disc rotates in the reverse dir ection and the moving contact closes the trip cir cui t. Th is causes the operation

of the cir cui t breaker which disconnects the faul ty section.

Induction Type Directional Overcurrent Relay:

The directional power relay discussed above is unsuitable for use as a directional protective relay under

short-circuit conditions. When a short-circuit occurs, the system voltage falls to a low value and there may be

insufficient torque developed in the relay to cause its operation. This difficulty is overcome in the directional

overcurrent relay which is designed to be almost independent of system voltage and power factor.

Constructional details:

Directional element: It is essentially a directional

power relay which operates when power flows in a

specific direction. The potential coil of this element is

connected through a potential transformer (P.T.) to

the system voltage. The current coil of the element is

energised through a C.T. by the circuit current. This

winding is carried over the upper magnet of the non-

directional element. The trip contacts (1 and 2) of the

directional element are connected in series with the

secondary circuit of the overcurrent element.

Therefore, the latter element cannot start to operate

until its secondary circuit is completed. In other

words, the directional element must operate first (i.e.,

contacts 1 and 2 should close) in order to operate the

overcurrent element.

Non-di rectional element:It is an overcurrent element

similar in all respects to a non-directional overcurrent

relay as shown.

It may be noted that plug-setting bridge is also

provided in the relay for current setting but has been

omitted in the figure for clarity and simplicity. The

tappings are provided on the upper magnet of overcurrent element and are connected to the bridge.Operation:Under normal operating conditions, power flows in the normal direction in the circuit protected by

the relay. Therefore, directional power relay (upper element) does not operate, thereby keeping the overcurrent

element (lower element) unenergised. However, when a short-circuit occurs, there is a tendency for the current

or power to flow in the reverse direction. Should this happen, the disc of the upper element rotates to bridge the

fixed contacts 1 and 2. This completes the circuit for overcurrent element. The disc of this element rotates and

the moving contact attached to it closes the trip circuit. This operates the circuit breaker which isolates the

faulty section. The two relay elements are so arranged that final tripping of the current controlled by them is

not made till the following conditions are satisfied:

Current flows in a direction such as to operate the directional element. Current in the reverse direction exceeds the pre-set value.

Excessive current persists for a period corresponding to the time setting of overcurrent element.

Distance or Impedance Relays:


12/3312

The operation of the relays discussed so far depended upon the magnitude of current or power in the

protected circuit. However, there is another group of relays in which the operation is governed by the ratio of

applied voltage to current in the protected circuit. Such relays are called distance or impedance relays (since,

Impedance is an electrical measure of distance along a transmission line).

Distance relays are double actuating relays, one coil is energized by voltage and other by current,

voltage coil gives restraining torque and current coil gives operating torque i.e., the torque produced by a

current element is opposed by the torque produced by a voltage element. The relay will operate when the ratio

V/I is less than a predetermined value.


13/3313

Differential protection/Pilot Wire Protection

Circulating current differential protection (Merz-Price protection):

Figure shows an arrangement of an overcurrent relay connected to operate as a differential relay. A pair of

identical current transformers are fitted on either end of the section to be protected (alternator winding in this

case). The secondaries of CTs are connected in series in such a way that they carry the induced currents in the

same direction. The operating coil of the overcurrent relay is connected across the CT secondary circuit. This

differential relay compares the current at the two ends of thealternator winding.

Under normal operating conditions, suppose the alternator winding

carries a normal current of1000 A. Then the currents in the two

secondaries of CTs are equal. These currents will merely circulate

between the two CTs and no current will flow through the

differential relay. Therefore, the relay remains inoperative. If a

ground fault occurs on the alternator winding as shown in Figure the

two secondary currents will not be equal and the current flows

through the operating coil of the relay, causing the relay to operate.

Disadvantages:

The impedance of the pilot cables (The two CTs are connected through conductors called pilot cable)

generally causes a slight difference between the currents at the two ends of the section to be protected. If

the relay is very sensitive, then the small differential current flowing through the relay may cause it to

operate even under no fault conditions.

Pilot cable capacitance causes incorrect operation of the relay when a large through-current flows.

Accurate matching of current transformers cannot be achieved due to pilot circuit impedance.

The above disadvantages are overcome to a great extent in biased beam relay.

Biased or Percent differential relay:

The reason for this modification over the circulating current differential relay is to overcome the trouble arising

out of differences in CT ratios for high values of external short circuit currents. The percentage differential

relay has an additional restraining coil connected in the pilot wire as shown.

In this relay the operating coil is connected to the mid-point of the restraining coil. The total no of ampere turns

in the restraining coil becomes the sum of ampere turns in its two halves, i.e.,

2

2which gives the average

restraining current of+

2in N turns. For external faults both I1& I2increase and thereby the restraining torque


14/3314

increases which prevents the mal-operation.

The operating characteristic of such a relay is given figure

The ratio of differential operating current to average restraining

current is fixed percentage. Hence the relay is called percentage

differential relay.

The relay is also called biased differential relay because the

restraining coil is also called a biased coil as it provides additional

flux.

Opposed voltage or Balanced voltage differential protection:

Figure shows the arrangement of voltage balance protection. In this scheme of protection, two similar current

transformers are connected at either end of the element to be protected (e.g.an alternator winding) by means of

pilot wires. The secondaries of current transformers are connected in series with a relay in such a way that

under normal conditions, their induced e.m.f.s are in opposition hence the name has come.

Note: The CTs used in such protection are with air gap core so that the core does not get saturated and over-

voltages are not produced during zero secondary current under normal working conditions.

Disadvantages:

As discussed above a multi-gap current transformer construction is required to achieve the accurate

balance between current transformer pairs.

The system is not suitable for protection of long cables, the charging current may be sufficient to operate

the relay even if a perfect balance of current transformers is attained. (It is suitable for cables of relatively

short lengths due to the capacitance of pilot wires)

The above disadvantages have been overcome in Translay (modified balanced voltage) system

Translay System:

This system is the modified form of opposition voltage-balance system. Principle is same but, here opposition

is between voltages induced in the secondary coils wound on the relay magnets and not between the secondary

voltages of the line current transformers. Hence secondary current never be zero so, no problem due to over-

voltages during zero secondary current under normal working conditions. So the current transformers used can

be made of normal design without any air gaps. This permits the scheme to be used for feeders of any voltage.

Constructional details: These relays cover the function of transformer as well as relay. Hence the name

Translay.

Fig shows the simplified diagram illustrating the principle of Translay scheme. It consists of two identical

double winding induction type relays fitted at either end of the feeder to be protected. The primary circuits of

these relays are supplied through a pair of current transformers. The secondary windings of the two relays are


15/3315

connected in series by pilot wires in such a way that voltages induced in the former opposes the other.The

compensating devices (cu rings) neutralize the effects of pilot-wire capacitance currents and of inherent lack of

balance between the two current transformers.

Operation: Under healthy conditions, current at the

two ends of the protected feeder is the same and the

primary windings of the relays carry the same

current. These windings induce equal e.m.f.s in the

secondary windings. As these windings are soconnected that their induced voltages are in

opposition, no current will flow through the pilots or

operating coils and hence no torque will be exerted

on the disc of either relay. In the event of fault on

the protected feeder, current leaving the feeder will

differ from the current entering the feeder.

Consequently, unequal voltages will be induced in

the secondary windings of the relays and current

will circulate between the two windings, causing the torque to be exerted on the disc of each relay. As the

direction of secondary current will be opposite in the two relays, therefore, the torque in one relay will tend toclose the trip circuit while in the other relay, the torque will hold the movement in the normal un-operated

position. It may be noted that resulting operating torque depends upon the position and nature of the fault in the

protected zone and at least one element of either relay will operate under any fault condition.


16/3316

Split core/Split Conductor protection:

The probability of faults occurrence on the overhead lines is much more due to their greater lengths

and atmospheric conditions. The protective schemes employed for alternators and transformers, with slight

modifications, may also be employed for protection of feeders.

The split-conductor is another method of securing the benefits of a balanced method of protections

without the necessity of using pilot wires. The principle of operation depends on the fact that two conductors of

equal length and impedance, when connected in parallel will share the load equally, provided that insulation of

the system is sound. When a fault develops on any conductor it will carry current than the other, and this

inequality of currents is arranged to operate a relay and thus isolate the faulty line.

In this system of protection each phase of the line is split into two sections having equal impedances. The

two sections are lightly insulated from each other .In this system, as shown in figure a single-turn currenttransformer is inserted at each end of the split conductor. The current transformer consists of laminated iron

rings on which a secondary winding is wound all around the periphery.

Under healthy conditions the current flowing along the two splits are equal and since these are threaded

through the current transformers CT1 and CT2 in the opposite directions hence the voltage across the terminals

of the evenly spread secondary winding is zero. In fault conditions one of the split takes more current than the

other one, thereby giving rise to an unbalance of the primary side of the current transformers .Due to

unbalancing of currents on the primary side of current transformers resultant flux will be set up in the core of

the one of the current coil R will be emergised. The relay contacts will be closed and trip coil will trip the

circuit breaker and isolate the fault

In the best arrangement the splits are carried into the circuit breakers on both sides of the feeders so that

the splits are opened by the breakers. This is explained as follows-

Let the splits be not carried into the circuit breakers and a fault develop at the receiving end of a long

line. Under these conditions, the impedance of the differential current transformer at this end maybe

insufficient to cause unbalance between the currents carried by each split conductor. Hence such a fault will

not will not be cleared by circuit breakers since the relay will not operate. But when the splits are carried into

the circuit breakers the fault current is confined to the faulty split after the sending end circuit breaker has

tripped. In the former case, although the sending end current breaker trips, the fault current is not confined to

the faulty split but it would divide practically equal between the two splits being solidly connected ,so the

receiving end current breaker will not trip. But in the latter case, the fault current is confined to the faulty split

the opening of the receiving end circuit breaker takes place.

From above discussion it is obvious that the split current breakers impart a high sensitivity of operation to the

protection and switchgear.


17/3317

DISADVANTAGES:-

The disadvantages of this system is that we have to make use of a special type of cable with the lower

limits for the voltages .In case of overhead feeders, for each phase ,a duplicate set of conductors , insulators,

etc, have to be employed .The lines having step-up or step-down transformers or voltage regulators cannot be

protected by this method.


18/3318

Alternator Protection

Different types of fault in generator

External faults.

Thermal overloading.

Unbalanced Loading.

Stator Winding faults .

Field Winding Faults.

Over-voltages.

External Faults:-

During external faults with large short circuit currents, severe mechanical stress will be imposed on the stator

windings. If any mechanical defect already exists in winding, these may be further aggravated. The temperature

rise is however, relatively slow and a dangerous temperature level may be obtained after about 10 seconds with

asymmetrical faults, severe vibration and overheating of the rotor may occur.

The external faults such as fault on bus bar are not covered by generator protection zone. Hence differential

protection of generator does not respond to external faults.

The over current and earth fault protection of generator provides back-up protection to external fault, while the

primary protection as provided by the protective system of respective equipment (e.g. bus-bars, transmission

lines).

Thermal Overloading:-

Continued overloading may increase the winding temperature to such extent that the insulation will be

damaged and its useful life reduced. Temperature rise can also be caused by failure of cooling system. In largemachines thermal element (thermo-couples or resistance thermometers) are embedded in the stator slots and

cooling system. The electrical over current protection (not thermal protection) is generally set at higher value

for responding the excessive overloads. Hence it cannot sense the continuous overloads of less value. Neither

can it sense the failure of cooling system.

Unbalanced loading:-

Continued unbalanced loads, equal to or more than 10% of the rated current cause dangerous heating of the

cylindrical rotor in turbo generator. Salient pole rotor in hydro generators often include damper winding and

are, therefore, much less effected by unbalance loading (i.e., for negative phase sequence current).

Unbalanced loading on generator can be due to

- Unsymmetrical fault in the system near the generating station

- Mal-operation of a circuit breaker near generating station, the 3-phase not being cleared.

Stator Winding Faults:-

Stator winding faults involve armature winding and must therefore be cleared quickly by complete shutdown of

the generator. Only opening the circuit does not help since the e.m.f is induced in the stator winding itself. The

field is opened and de-energized by field suppression.

The stator faults include.1. Stator inter-turn faults.

2. Phase-to-phase faults.

3. Phase-to-earth faults.


19/3319

Phase to phase faults and phase inter-turn faults are less common. Inter turn faults are more difficult to detect.

Stator Inter-turn faults:-

Short circuit between the turns of one coil may occur if the stator winding is made up of multi turn coils. Such

faults may develop owing to incoming current surges with a steep wave front which may cause a high voltage

across the turn at the entrance of the stator winding. If however the stator winding is made up of single

turn coil, with only one coil per slot, it is, of course, impossible to have an inter-turn fault. If there are two

coils per slot the insulation between the coil is in such a way, dimensions that an inter-turn fault is not likely tooccur.

Differential protection and over current protection does not sense inter-turn faults. Stator inter turn fault

protection detect the inter turn faults.

Phase to Earth faults:-

These faults normally occur in the armature slots. The damage at the point of fault is directly related to the

selected neutral earthing resistor. With fault current less than 20 A, if the machine is tripped within some

seconds then a negligible burning of the iron core will result. A repair work then amounts to changing the

damaged coil without restacking of core laminations. If, however, the earthing resistor is selected to pass a

much larger earth fault current, severe burning of the stator core will take place, necessitating restacking of

laminations. Even when a high speed earth fault differential protection is used, severe damaged may be caused

owing to the large time constant of the field current and the relatively long time required to completely

suppress the field flux. In the case of high earth fault currents it is therefore normal practice to install a circuit

breaker in the neutral of the circuit breaker in order to reduce the total fault clearance time.

Circulating current biased differential protection provides the earth fault protection. However the sensitivity of

such a protection for earth fault depends upon the resistance in neutral to earth connection and the position of

earth fault in the winding. A separate and sensitive earth fault protection is generally necessary for generator

with resistance earthing.

Phase to Phase Fault:-

Short circuit between the stator windings is very rare because the insulation in a slot between coils of different

phases is at least twice as large as the insulation between one coil and the iron core. However a phase to earth

fault may cause a phase to phase faults within the slots. A phase to phase fault most likely to be located at the

end connection of the armature windings i.e. in the overhanging parts outside the slots. A fault of this nature

causes severe arcing with high temperature, melting of copper and risk of fire if the insulation is not made of

fire resistance, non-flammable material. Since the short circuit current in this case do not pass via the stator

core, the laminations will not be particularly damaged. The repair work may therefore be limited to replacing

the affected coil and mechanical part of the end structure.

Field Winding Faults:-

Rotor faults include rotor inter turn fault and conductor to earth faults these are called by mechanical and

temperature stresses.

The field system is normally not connected to the earth so that a single earth fault does not give rise to any

fault current. A second earth fault will short circuit part of the winding and may thereby produce an

unsymmetrical field system, giving unbalanced force on the rotor. Such a force will cause excessive pressure

on bearing and shaft distortion, if not cleared quickly.

The unbalanced loading on generator gives rise to negative sequence current which cause negative sequence

component of magnetic field. The negative sequence field to rotate in opposite direction of the main field and

induces e.m.f in rotor winding. Thus the unbalanced loading causes rotor heating. Rotor earth fault protection

is provided for large generators.

Over-voltage:-


20/3320

Atmospheric surge voltage is caused by direct lighting stroke to the aerial line in the H.V system

induced and capacitive transferred voltage surges can, however, reach the generator via the unit

transformer. To protect generator from severe voltages surges, surge arresters and surge capacitors

and often used. In the case of smaller machines directly connected to a distribution network comprising

overhead line, such protective devices are of prime importance.

Switching Surges: Switching operation may cause relatively high transient voltage if re-striking occurs

across the contacts of the circuit breakers. This transient is similar to those obtained during intermittent

earth fault (arcing grounds) and may be limited by using modern circuit breakers. Arcing Ground:The amplitude of the transient voltage during arcing grounds may theoretically, under

the most unfavorable conditions of arc re-striking, reach a value of five times normal line to neutral

peak voltage by means of the resistance earthing of the generator neutral these over voltages will be

reduced to a maximum value of about 2.5 times of the rated peak voltage.

Biased differential protection:

In this method, the currents at the two ends of the protected section are sensed using current transformers. The

wires connecting relay coils to the current transformer secondaries are called pilot wires.

Under normal conditions, when there is no fault in the windings, the currents in the pilot wires fed fromC.T. secondaries are equal. The differential current i1- i2through the operating coils of the relay is zero. Hence

the relay is inoperative and system is said to be balanced.

When fault occurs inside the protected section of the stator windings, the differential current i1- i2flows

through the operating coils of the relay. Due to this current, the relay operates. This trips the generators circuit

breaker to isolate the faulty section.

Fig. 1 Biased differential protection for star connected alternator Fig. 2 For delta connected alternator

The C.T.s on the delta connected machine winding side are connected in delta while the C.T.s at

outgoing ends are connected in star. The restraining coils are placed in each phase, energized by the secondary

connections of C.T.s while the operating coils are energized from the restraining coil tappings and the C.T.

neutral earthing.

If there is a fault due to a short circuit in the protected zone of the windings, it produces a difference

between the currents in the primary windings of C.T.s on both sides of the generator winding of the same

phase. This results in a difference between the secondary currents of the two currents transformers. Thus, under

fault conditions, a differential current flows through the operating coils which is responsible to trip the relay

and open the circuit breaker. The differential relay operation depends on the relation between the current in the

operating coil and that in the restraining coil.

In addition to the tripping of circuit breaker, the percentage differential relay trip a hand reset multi-contact

auxiliary relay. This auxiliary relay simultaneously initiates the following operations,

1. Tripping of the main circuit breaker of generator


21/3321

2. Tripping of the field circuit breaker

3. Tripping of the neutral circuit breaker

4. Shut down of the prime mover

5. Turn on of CO2 gas if provided for safety of generator under faulty conditions.

6. Operation of alarm and / or annunciator to indicate the occurrence of the fault and the operation of

the relay the field must be opened immediately otherwise it starts feeding the fault.

When differential relaying is used for the protection, C.T.s at both the ends of generator must be of equal

accuracy otherwise if the error is excessive, wrong operation of the relay may result. The cause of unequalcurrents on both the sides of C.T.s without any fault are ratio errors, unequal lengths of the leads, unequal

secondary burdens etc.

This scheme provides very fast protection to the stator winding against phase to phase faults and phase to

ground faults. If the neutral is not grounded or grounded through resistance then additional sensitive earth fault

relay should be provided.

The advantages of this scheme are,

1. Very high speed operation with operating time of about 15 msec.

2. It allows low fault setting which ensures maximum protection of machine windings.

3. It ensures complete stability under most severe through and external faults.

4. It does not require current transformers with air gaps or special balancing features.

Restricted earth fault protection

Generally Merz-Price protection based on circulating current principle provides the protection against internal

earth faults. But for large generators, as there are costly, an additional protection scheme called restricted earth

fault protection is provided.

When the neutral is solidly grounded then the generator gets completely protected against earth faults. Butwhen neutral is grounded through earth resistance, then the stator windings gets partly protected against earth

faults. The percentage of windings protected depends on the value of earthing resistance and the relay setting.

In this scheme, the value of earth resistance, relay setting, current rating of earth resistance must be carefully

selected. The earth faults are rare near the neutral point as the voltage of neutral point with respect to earth is

very less. But when earth fault occurs near the neutral point then the insufficient voltage across the fault drivers

very low fault current than the pickup current of relay coil. Hence the relay coil remains unprotected in this

scheme. Hence it is called restricted earth fault protection. It is usual practice to protect 85% of the winding.

Consider that earth fault occurs on phase B due to breakdown of its insulation to earth, as shown in the

Fig. 3. The fault current If will flow through the core, frame of machine to earth and complete the path throughthe earthing resistance. The C.T. secondary current Is flows through the operating coil and the restricted earth

fault relay coil of the differential protection. The setting of restricted earth fault relay and setting of overcurrent

relay are independent of each other. Under this secondary current Is, the relay operates to trip the circuit

breaker. The voltage Vbxis sufficient to drive the enough fault current If when the fault point x is away from the

neutral point.


22/3322

Fig. 3 Restricted earth fault protection

If the fault point x is nearer to the neutral point then the voltage Vbxis small and not sufficient to drive

enough fault current If. And for this If, relay cannot operate. Thus part of the winding from the neutral point

remains unprotected. To overcome this, if relay setting is chosen very low to make it sensitive to low faultcurrents, then wrong operation of relay may result. The relay can operate under the conditions of heavy through

faults, inaccurate C.T.s, saturation of C.T.s etc. Hence practically 15% of winding from the neutral point is kept

unprotected, protecting the remaining 85% of the winding against phase to earth faults.

Automatic field suppression and neutral circuit breakers:

When a fault

develops in an alternator

winding even though the

generator circuit breaker is

tripped, the fault continuesto be fed because e.m.f is

induced in the generator

itself. Hence the field circuit

breaker is opened and the

stored energy in the field

winding is discharged

through another resistor.

This method is known as field suppression.

The earthing resistor is selected to pass a much larger earth fault current severe burning of the statorcore will take place, necessitating restacking of laminations. Even when a high speed earth fault differential

protection is used, severe damaged may be caused owing to the large time constant of the field current and the

relatively long time required to completely suppress the field flux. In the case of high earth fault currents it is

therefore normal practice to install a circuit breaker in the neutral of the circuit breaker in order to reduce the

total fault clearance time.

Negative Phase Sequence Protection:

Negative phase sequence relays are used in protection against unbalanced loads. The unbalanced 3-

phase stator currents cause double frequency currents to be induced in rotor. They cause heating of rotor and

damage the rotor. Unbalanced stator currents also cause severe vibrations and heating of stator. From thetheory of symmetrical components, we know that unbalance three-phase currents have a negative sequence

component. This component rotates at synchronous speed in a direction opposite to the direction of rotation of

rotor. Therefore double frequency currents are induced in the rotor.


23/3323

Negative sequence current filter with over-current relay provides protection against unbalanced loads

(as shown in figure).

The relative asymmetry of a three-phase generator is defined as the ratio of negative sequence current

(I2to rated current In), i.e.,% =

100

In case of loss of one phase, the

relative asymmetry %S=58%. The time for

which the machine can be allowed to operate

for various amounts of relative asymmetriesdepends on type of machine. The additional

heat caused by negative sequence currents in

rotor is proportional to I22t. The product I22tis

a machine characteristic.

I22t=30 is a generally accepted figure

as per ASA, (I2 in per unit, t in sec.) For

wound rotor machines and 40 for salient pole

machine.

It is generally necessary to install

negative sequence relays that match with theI22t characteristic of the machine.

Fig 1. Protection against unbalanced load using negative sequence filter

Negative sequence filter circuit comprises resistors and inductors connected in the secondary circuit in such a

way that negative sequence component flows through the relay coil, ZL as shown in below figure.

The over-current relay (ZL) of negative phase sequence protection is with inverse characteristics

matching with the I22trating curve of the machine and is arranged to trip the unit.

Negative Phase Sequence Circuit:-

The given figure illustrates the principle of the negative phase sequence circuit. The twin windings of

the two auxiliary current-transformers are so connected to the line current-transformers that under normalbalanced-load condition, currents Ia, Ib and Ic

flow in the direction shown. Impedance Z1

and Z2 are connected across auxiliary current-

transformers T1and T2, and a load impedance

ZLis connected across the terminals XX.

When primary load current flows, the

current through T1 will be (Ib-Ic) and that

through T2will be (Ia-Ib). For a given value of

load impedance ZL, (over-current relay) the

impedance Z1 and Z2 are chosen such thatpoints P and R remain at the same potential,

i.e., the voltages across QR and QP are equal

and opposite. Under balanced conditions,

these voltages differ, and an output is

produced proportional to the negative phase

sequence across XX (voltage E) so as to

operate the relay. The protection remains

stable on symmetrical overloads upto about 3

times rated full load.

As the output is instantaneous inoperation, it is necessary to operate the

equipment in conjunction with a time-lag

relay. These relays are used for protection

against unbalanced loads.


24/3324

Transformer Protection

Buchholz relay

Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil immersed

transformers having rating more than 500 KVA. Buchholz relay is not provided in relays having rating below

500 KVA from the point of view of economic considerations.

Buchholz Relay in transformeris an oil container housed the connecting pipe from main tank to conservator

tank. It has mainly two elements (for both the alarm and trip circuits) along with hinged float and mercury

switch assembly. The entire assembly is in an oil proof case which has two glass windows.

The upper element consists of a float. The float is attached to a hinge in such a way that it can move up and

down depending upon the oil level in the Buchholz RelayContainer. One mercury switch is fixed on the float.

The alignment of mercury switch hence depends upon the position of the float.

The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just in front of

the inlet (main tank side) of Buchholz Relay in transformerin such a way that when oil enters in the relay

from that inlet in high pressure the alignment of the baffle plate along with the mercury switch attached to it,

will change.

Operation:

The operation of Buchholz relay is as follows:

In case of incipient faults within the transformer, the heat

due to fault causes the decomposition of some transformer

oil in the main tank. The products of decomposition contain

more than 70% of hydrogen gas. The hydrogen gas being

light tries to go into the conservator and in the process gets

entrapped in the upper part of relay chamber. When a

predetermined amount of gas gets accumulated, it exerts

sufficient pressure on the float to cause it to tilt and close

the contacts of mercury switch attached to it. This

completes the alarm circuit to sound an alarm (because, the

conditions described do not call for the immediate removal

of the faulty transformer. It is because sometimes the air bubbles in the oil circulation system of a healthy

transformer may operate the float. For this reason, float is arranged to sound an alarm upon which steps can

be taken to verify the gas and its composition)

If a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The

oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to

close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling

the transformer.

Advantages

It is the simplest form of transformer protection.

It detects the incipient faults at a stage much earlier than is possible with other forms of protection.


25/3325

Disadvantages

It can only be used with oil immersed transformers equipped with conservator tanks.

The device can detect only faults below oil level in the transformer. Therefore, separate protection is

needed for connecting cables.

Mercury Switch Description:

Mercury switches have one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. Theenvelope may also contain air, an inert gas, or a vacuum. Gravity is constantly pulling the drop of mercury to the lowest point in the

envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing the electrical

circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts,

thus breaking that circuit. The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for

example, single-pole, double-throw (SPDT) operation.

Advantages

Advantages of the mercury switch over other types are that the contacts are enclosed, so oxidation of the contact points is unlikely. In

hazardous locations, interrupting the circuit will not emit a spark that can ignite flammable gases. Contacts stay clean, and even if an

internal arc is produced, the contact surfaces are renewed on every operation, so the contacts don't wear out. The sensitivity of the

drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic

operation. The switches are quiet, as there are no contacts that abruptly snap together. The mass of the moving mercury drop can

provide an "over center" effect to avoid chattering as the switch is tilted. Multiple contacts can be included in the envelope for two or

more circuits.

Disadvantages

Mercury switches have disadvantages when compared with other types in certain applications. Mercury switches have a relatively

slow operating rate due to the inertia of the mercury drop, so they are not used when many operating cycles are required per second.

Mercury switches are sensitive to gravity so may be unsuitable in portable or mobile devices that can change orientation or that

vibrate. Mercury compounds are highly toxic and accumulate in any food chain, so mercury is not permitted in many new designs.

Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damaging the envelope. The mercury drop

forms a common electrode, so circuits are not reliably isolated from each other if a multi-pole switch is used.

Restricted Earth Fault Protection:

Earth fault relay connected in residual circuit of

line CTs(as shown) give protection against earth

faults on the delta or unearthed star connected

windings of transformer. Earth fault on secondary

side are not reflected on primary side, when the

primary winding is delta connected or has

unearthed star point. In such cases, an earth fault

relay connected in residual circuit of 3 CTs on

primary side operates on internal earth faults inprimary windings only because earth faults on

secondary side do not produce zero sequence

currents on primary side. Restricted earth fault

protection may then be used for high speed

tripping for faults on star connected earthed

secondary winding transformer.


26/3326

In this fig the star connected side is protected by Restricted Earth Fault Protection. An earth fault (F1)

beyond the transformer

causes the currents I2 and I1

in CT secondaries as shown

in Fig. Therefore, the

resultant current in earth fault

relay is negligible. For earth

fault within the transformer

star connected winding (F2),only I2 flows and I1 is

negligible. Hence I2 flows

through the earth fault relay.

Thus restricted earth fault-

relay does not operate for

earth fault beyond the

protected zone of the

transformer.

When fault occurs very near to the neutral point of the transformer, the voltage available for driving earth fault

current is small. Hence fault current would be low. If the relay is to sense such faults, it has to be too sensitive

and would therefore operate for spurious signals, external faults and switching surges. Hence the practice is to

set the relay such that it operates for earth fault current of the order of 15 % of rated winding current. Such

setting protects restricted portion of the winding. Hence the name restricted earth fault protection.

Harmonic Restraint Differential Relay

When an unloaded transformer is switched on, it draws a large initial magnetizing current which may be

several times the rated current of the transformer. This initial magnetizing current is called the magnetizing

inrush current.

As the current flows only through the primary windings, the differential protection will see this inrush

current as an internal fault. The harmonic contents in the inrush current are different than those in usual faultcurrent. The dc component varies from 40 to 60%, the second harmonic 30 to 70% and the third harmonic 10 to

30 %. The other harmonics are progressively less. The third harmonic and its multiples do not appear in C.T.

terminals as these harmonics circulate in the delta winding of the transformer and the delta connected C.T.s on

the Y side of the transformer. As the second harmonic is more in the inrush current than in the fault current,

this feature can be utilized to distinguish between a fault and magnetizing inrush current.

The operation of the relays because of the magnetizing inrush current can be avoided by using relays

with inverse and definite time (IDMT) characteristics. However for EHV transformers, the relay current and

time ratings necessary to ensure stability on the magnetizing inrush current caused by switching in the

transformer are not adequate for providing high speed operation. So, a high speed biased differential relay

incorporating a harmonic restraint feature is immune to the magnetizing inrush current. As the magnetizing

inrush currents have a high component of even and odd harmonics (about 63% of second harmonics and 26.8%

of third harmonics) while harmonic components of short-circuit current is negligible. The use of these facts is

made for restraining the relay from operation during initial current inrush.

The harmonic restraint differential relay is sensitive to fault currents but is immune to the magnetizing

current. The operating coil of the relay carries only the fundamental component of current only while the

restraining coil carries the sum of the fundamental and harmonic components.

Basic circuit of a harmonic restraint differential relay is illustrated in figure below. The restraining coil

is energized by a direct-current proportional to bias winding current as well as the direct current due to

harmonics. Harmonic restraint is had from the tuned circuit (XC XL) that allows only the fundamental

component of current to enter the operating circuit. The DC and higher harmonics (mostly second harmonics)

are diverted into the rectifier bridge feeding the restraining coil. The relay is adjusted so that it will not operate


27/3327

when the harmonic current exceeds 15% of the fundamental current. Both the DC and higher harmonics are of

large magnitude during magnetizing inrush.

Fig:- Basic circuit of Harmonic Restraint Differential Relay

The relay may fail to operate due to harmonic restraint feature if an external fault has considerable harmonics

that may be present in the fault current itself due to an arc or due to saturation of CT.

Also, if a fault exists at the instant of energization of transformer harmonics present in the magnetizing current

may prevent the operation of the relay. This problem can be overcome by providing instantaneous overcurrent

relay in the differential circuit which is set above the maximum inrush current but will operate in less than one

cycle on internal faults. Thus fast tripping is ensured for all ensured faults.

Fig :- Conceptual representation of Harmonic Restraint Differential Relay


28/3328

Generator-Transformer Unit Protection

In a high voltage transmission systems, the bus bars are at very high voltages than the generators. The

generators are directly connected to step up transformer to which it is connected, together from a generator

transformer unit. The protection of such a unit is achieved by differential protection scheme using circulating

current principle. While providing protection to such a unit, it is necessary to consider the phase shift and

current transformation in the step up

transformer.

The figure shows a biased differential

protection scheme used for generator

transformer unit. The zone of such a

scheme includes the stator windings, the

step up transformer and the intervening

connections.

The transformer is delta-star hence the

current transformers on high voltage side

are delta connected while those on

generator side are star connected. Thiscancels the displacement between line

currents introduced by the delta connected primary of the transformer. Where there is no fault, the secondary

currents of the current transformer connected on generator side are equal to the currents in the pilot wires from

the secondaries of the delta connected current transformers on the secondary of main transformer. When a fault

occurs, the pilot wires carry the differential current to operate the percentage differential relay.

For the protection against the earth faults, an earth fault relays is put in the secondary winding of the main

step up transformers as shown. In such a case, differential protection acts as a backup protection to the

restricted earth fault protection. This overall differential protection scheme does not include unit transformer as

a separate differential scheme is provided to it.


29/3329

Protection against Over-voltages

Causes of Over-voltages

1. Internal causes

Switching surges

Insulation failure

Arcing ground (The phenomenon of intermittent arc taking place in line-to-ground fault of a 3-

system with consequent production of transients is known as arcing ground, these may exist onungrounded systems)

Resonance

2. External causes (i.e. lightning)

Internal causes do not produce surges of large magnitude. Experience shows that surges due to internal

causes hardly increase the system voltage to twice the normal value. Generally, surges due toi nternal causes

are taken care of by providing proper insulation to the equipment in the power system. However, surges due to

lightning are very severe and may increase the system voltage to several times the normal value. If the

equipment in the power system is not protected against lightning surges, these surges may cause considerable

damage. In fact, in a power system, the protective devices provided against over-voltages mainly take care of

lightning surges.

An electric discharge between cloud and earth, between clouds or between the charge centres of the same

cloud is known as lightning

Introduction: Benjamin Franklin (1706-90) performed his famous experiment (1745) of flying kite in thundercloud. Before his discovery the lighting was considered to be Act of God. Frankling proved that the lighting

stroke is due to the discharge of electricity. Franklin also invented lighting rods to be fixed on tall buildings and

earthed to protect them from lighting strokes.

Static induced charges: An overhead conductor accumulate statically induced charge when a charged

clouds come above the conductor. If the cloud swept away from its place, the charges on conductor is released.The charge travels on either sides giving rise to two travelling waves. The earth wire does not prevent such

surges.

Types of Lightning Strokes

Direct stroke

Indirect stroke

Direct stroke:In the direct stroke, the lightning discharge (i.e., current path) is directly from the cloud to the

subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the

pole to the ground. The over-voltages set up due to the stroke may be large enough to flashover this path

directly to the ground. The direct strokes can be of two types viz. (i) Stroke A ii) stroke B.

Two points are worth noting about direct strokes. Firstly, direct strokes on the power system are very rare.

Secondly, stroke A will always occur on tall objects and hence protection can be provided against it. However,

stroke B completely ignores the height of the object and can even strike the ground. Therefore, it is not possible

to provide protection against stroke B.

Indirect stroke: Indirect strokes result

from the electrostatically induced charges

on the conductors due to the presence of

charged clouds. This is illustrated in Fig.

24.6. A positively charged cloud is above

the line and induces a negative charge on

the line by electrostatic induction. This


30/3330

negative charge, however, will be only on that portion of the line right under the cloud and the portions of the

line away from it will be positively charged as shown in Fig. The induced positive charge leaks slowly to earth

via the insulators. When the cloud discharges to earth or to another cloud, the negative charge on the wire is

isolated as it cannot flow quickly to earth over the insulators. The result is that negative charge rushes along the

line is both directions in the form of travelling waves. It may be worthwhile to mention here that majority of

the surges in a transmission line are caused by indirect lightning strokes.

Protection against Lightning

The most commonly used devices for protection against lightning surges are:

Earthing screen

Overhead ground wires

Lightning arresters or surge diverters

Earthing screen provides protection to power stations and sub-stations against direct strokes whereas overhead

ground wires protect the transmission lines against direct lightning strokes. However, lightning arresters or

surge diverters protect the station apparatus against both direct strokes and the strokes that come into the

apparatus as travelling waves. We shall briefly discuss these methods of protection.

Ground wire/Earth wire:

A ground wire is a form of lighting protection using a conductor or conductors, well-grounded at a regular

intervals, preferably at each support, and attached from support to support above the transmission line. And it is

provided above the overhead transmission lines for protection from lightning strokes. The ground wire shields

the phase or line conductors by attracting itself the lightning strokes which, in its absence would strike the

phase conductors. Besides it, the ground wire reduces the voltage electrostatically or electromagnetically

induced in the conductors by the discharge of a neighboring cloud. It also provides additional protective effect

by causing attenuation of travelling waves set in lines by acting as a short-circuited secondary of the lineconductors.

Limitations:

Earth wire do not provide 100% protection. Weak strokes are not attracted by earth wire. However for the most

dangerous direct strokes earth wire has proved to be a very good solution.

Zone of protection:

Practical experience has shown that

earth wire has a shielding angle. The

conductors coming in the shielded zone are

protected against direct strokes. The shieldingangle () define as follows: A vertical line is

drawn from the earth wire. Angle () is plotted

on each side of this vertical line. The envelope

within angle 2 is called zone of protection.

Shielding angle is between 30 deg. to 40 deg.

An angle of 35 deg. is supposed to be

satisfactory and economical for overhead lines.

Surge absorber

The device, which reduces the steepness of the wave front of a particular surge and thus minimizes the

danger due to over-voltage is called the surge absorber. A condenser when placed between the line and earth

reduces the steepness of the wave front to a considerable extent and hence protects the other apparatus from

damage due to overvoltage. The condenser also provides protection against comparatively low-voltage, high-

Zone of protection


31/3331

frequency waves. Since the impedance of a condenser is inversely proportional to the frequency, it is low at

high frequencies and large at low frequencies. The normal frequency voltage produces only a small current in

the condenser, so that negligible loss is caused during normal operation. A condenser is

Protective Relays

Documents

Transcript of Protective Relays