Protection of alternator

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PROTECTION OF ALTERNATER

Prepared By: Manavar Milan

INDEX:

o Introductiono Abnormalities and fault in alternatoro Differential protection o Differential Protection for Alternator whose neutral

earthed through high resistance o Negative phase sequence protectiono Balanced Earth fault protectiono Over current protectiono % Bias differential protectiono Application

INTRODUCTION:

o The generating units, especially the larger ones are relatively few in number and higher in individual cost then most other equipments, therefore it is desirable and necessary to provide protection to the alternators.

o In generating station ,as a continuous operation of generators is much more necessary so the faulty part has to be cleared very quickly for uninterruptable power supply.

o Unlike other apparatus, opening a breaker to isolate the faulty

generator is not sufficient to prevent further damage.

ABNORMALITIES AND FAULT IN ALTERNATOR:

(A) Abnormalities: 1. Failure of prime mover

2. Failure of exciter

3. Overloading

4. Unbalanced loading

5. Over voltage

6. Over speed

(B) Faults:

(b1) Stator faults: 1. Phase to phase faults

2. Phase to earth fault

3. Inter turn fault

(b2)Rotor faults: 1. Earth fault

2. Inter turn fault

Differential Protection:

o It provide protection against phase to phase and phase to ground

o When the stator winding (Armature winding) is healthy and when there is no internal fault, equal currents flow the through the secondary windings of CTs.

o So no currents flows through any winding of the relay.

o But the current balance is distributed when there is earth fault in one phase or when there is phase to phase fault, current flows through one or winding of the relay and the circuit breaker is tripped.

Differential Protection for Alternator whose neutral earthed

through high resistance:

o Many times the neutral of alternator is earthed through high resistance.

o Value of the earth fault current becomes less if the value of earthling resistance is high.

o This means that some portion of the alternator winding will not get protection.

o To overcome this difficulty, some change is made in the above arrangement.

o In this two relay coils are used for the phase fault and the third relay is used for earth fault.

o In the healthy condition the currents on the two sides of the stator winding will be equal.

o So the currents balance is obtained as the secondary currents of CT are equal no current will flow through the relay winding.

o Current flow through the earth relay when there is earth fault in any phase.

o If there is fault between two phases, the fault current will circulate in the secondary winding of the two transformer through two coils and no current will flow through the earth relay.

o Thus the sensitive earth fault relay can be used so no difficulty is encountered due to high earthing resistance.

NEGATIVE PHASE SEQUENCE PROTECTION:

NEGATIVE PHASE SEQUENCE PROTECTION:

o The negative sequence component is produced when the load is become unbalanced.

o This component rotates with synchronous speed in the direction opposite to that of the rotor.

o So, negative phase sequence filter is used to detect the unbalanced loading.

BALANCED EARTH FAULT PROTECTION:

BALANCED EARTH FAULT PROTECTION:

o The value of the pickup current for the over current relay is more so the relay cannot operate with low value of the earth fault current.

o So, to give earth fault protection, an earth fault relay is connected in the neutral circuit through CT as shown in fig.

OVER CURRENT PROTECTION:

OVER CURRENT PROTECTION:

o The time setting of the over current relay is kept higher then that of differential protection.

o Because the first differential protection should work and if it fails to operate, after some time this over current protection system will work.

% BIAS DIFFERENTIAL PROTECTION:

% BIAS DIFFERENTIAL PROTECTION:

o When the alternator feeds the external fault, the currents through the three windings increase to high value so the CT saturate.

o Now the characteristics of the CTs on the two sides may not be identical so some out of balance current flows through the relay coil and the relay operate.

o To overcome this difficulty percentage biased differential relay is used.

Application

To detect the abnormality and fault condition.

The protection system of alternator is provide protection.

THANK YOU