Unit:4 (Equipment Protection)

Alternator protection: Some important faults which may occur on an alternator:

(i) Failure of prime-mover

(ii) Failure of field

(iii) Over current

(iv) Over speed

(v) Over voltage

(vi) Unbalanced loading

(vii) Stator winding fault

As the alternator is exposed to more harmful operating condition than any other power system

element, more sophisticated and innovative protection schemes are required. These are:

(i) Percentage differential protection

(ii) Rotor protection due to loss of excitation

(iii) Rotor over heating protection due to unbalanced loading

(iv) Over speed protection

(v) Protection against motoring and field suppression

Differential protection of alternator:

Operation: Under normal condition the current at both ends of each winding will be equal

and hence the current in the secondary of two CTs will also be equal. Therefore, there is

balanced circulating current in the pilot wire and no current flows through the operating coil

(R1, R2 and R3) of the relays. When earth fault or phase to phase fault occurs, the current which

is flowing in the secondary of two CTs through pilot wire becomes unequal and the difference

of the two current will flow through the relay coil and returning via neutral wire. And the relay

then closes its contact to trip the circuit breaker.

Rotor protection due to loss of excitation: When the alternator losses its field

excitation, it speeds up slightly and continues to run as an induction generator deriving

excitation from the system and supplying power at a leading power factor. A fall in voltage will

also occur due to loss-of-excitation which may result in loss of synchronism and system

stability. There is a possibility of over-heating of the rotor due to induced current in the rotor

and damper winding. This can be avoided by using a tripping scheme which is so arranged that

opening of field circuit breaker causes the tripping of generator unit breaker.

Rotor over heating protection: Unbalanced loading means there are different phase

current in the alternator. Unbalanced loading arises from phase to earth fault or fault between

phases. Due to unbalanced loading burns the mechanical fixing of the rotor core or damage the

field winding.

Under normal condition equal current flows through the different phases of the alternator and

their algebraic sum is zero. Therefore, the sum of the current flowing in the secondary is zero.

And no current flows through the relay coil. However, if unbalanced loading occurs the current

induced in the secondary will be different and resultant of these current will flow through the

relay coil. Then relay will trip the circuit breaker to disconnect the alternator from the system.

Over speed protection: The main cause of over speed is the sudden loss of all or the

major part of load on the alternator. Modern alternators are usually provided with mechanical

centrifugal devices mounted on their driving shafts to trip the main valve of the prime-mover

when a dangerous over speed occurs.

Protection against motoring: In case of prime-mover failure the alternator continues

to rotate as a synchronous motor drawing electrical power from the system. This motoring

action of the alternator is known as “inverted running”. In case of turbo-alternator sets, failure

of steam supply may cause inverted running. This operation is prevented by reverse power

relay.

Field suppression: when a fault develops in an alternator winding even though the

alternator circuit breaker is tripped, the fault continues to be fed because e.m.f. is induced in

the alternator 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”.

Transformer faults: The most common faults which may occur in transformer

(i) Open circuit fault

(ii) Overheating fault

(iii) Winding short circuit fault: These are phase to earth, phase to phase and inter turn

fault.

Protection system of transformer:

(i) Buchholz Relay: This providing the protection against all kind of incipient faults i.e.

slow developing faults such as insulation failure of winding, core-heating, fall of oil

level due to leaky joints.

(ii) Earth fault relay: This providing the protection against earth fault only.

(iii) Over current relay: This providing the protection against phase to phase fault and

over loading.

(iv) Differential relay: This providing the protection against both earth and phase fault.

Buchholz relay: It is a gas actuated relay used in oil immersed transformer (above 500kVA)

for protection against all kinds of faults. It is used to give an alarm in case of slow developing

faults of transformer and to disconnect the transformer from the supply in case of heavy faults.

Construction: It is placed in the pipe connecting between main tank and conservator. The

device has two elements. The upper element consists of a mercury type switch attached to a

float. The lower element contains a mercury switch mounted on a hinged type flap.

Operation: In case of slow developing fault in the transformer, the heat due to fault causes

the decomposition of oil. The products of decomposition contain more than 70% of hydrogen

gas. Due to light in weight of hydrogen it tries to go into the conservator and is trapped in the

upper part of relay chamber. When the large amount of gas is accumulated, it exerts sufficient

pressure on the float to activate the mercury switch to give an alarm. In case of heavy fault a

large amount of gas is generated and it rushes towards the conservator via the Buchhloz relay.

And it tilts the flap to close the mercury switch. This completes the trip circuit to open the

circuit breaker.

Earth fault or leakage protection/ Core balanced leakage protection:

Construction: Three leads of primary winding of power transformer are taken through the

core of a CT which carries a single secondary winding. The operating coil of a relay is connected

to this secondary.

Operation: Under normal condition the phasor sum of the three phase current is zero and

there is no resultant flux in the core of CT. So, no current flows through the relay coil and it

remain inoperative. However, on the occurrence of an earth fault the resultant flux is produced

in the core of CT. And current is induced in the secondary of CT, which energizes the relay to

trip the circuit breaker.

Differential protection of transformer OR Merz-Price current

circulating protection:

Construction: This system is used for protection of a 3-phase delta/delta power

transformer against phase to phase fault and phase to ground fault. The CTs on the both side of

the transformer are connected in star. This compensates for the phase difference between

primary and secondary of the transformer, the CTs on the two sides are connected by pilot wire

and one relay is used for each pair of CTs.

Operation: During normal condition the secondary of CTs carry identical currents.

Therefore, the current entering and leaving the pilot wire at both ends are same, and no

current flows through the relay coil. If ground fault or phase to phase fault occurs a differential

current flows through pilot wire, which energizes the relay coil to trip the circuit breaker.

Primary protection: It is the protection system which is designed to protect the

equipments of the power system. Each line has an over current relay that protects the line. If a

fault occurs on any line it will be cleared by its relay and circuit breaker. This forms the primary

or main protection and serves as the first line of defense. If some time fault is not cleared by

primary relay system because of trouble within the relay wiring system or breaker. Under such

condition back-up protection does the required job.

Back-up protection: It is the second line of defense in case of failure of the primary

protection. It is designed to operate with sufficient time delay so that primary relaying will be

given enough time function. Relay A provides back-up protection for each of the four lines. If a

line fault is not cleared by it relay and breaker the relay A will operate after a definite time

delay to protect the line.

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