Generator Protection Part 10
The Merz-Price protection system gives protection against phase to phase faults and earth faults. It does not give protection against interturn faults. The interturn fault is a short circuit between the turns of the same phase winding. Thus the current produced due to such fault is a local circuit current and it does not affect the currents entering and leaving the winding at the two ends, where C.T.s are located. Hence Merz-Price protection can not give protection against interturn faults.
In single turn generator, there is no question of interturn faults but in multimum generators, the interturn fault protection is necessary. So such interturn protection is provided for multiturn generators such as hydroelectric generators. These generator have double winding armatures. This means, each phase winding is divided into two halves, due to the very heavy currents which they have to carry. This splitting of single phase winding into two is advantageous in providing interturn fault protection to such hydroelectric generators.
The Fig. 1 shows the interturn fault protection scheme used for the generator with double winding armatures.
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| Fig. 1 Interturn fault protection |
The schemes uses cross differential principle. Each phase of the generator is doubly wound and split into two parts S1 and S2 as shown in the Fig.1. The current transformers are connected in the two parallel paths of the each phase winding. The secondaries of the current transformers are cross connected. The current transformers work on circulating current principle. The relay is connected across the cross connected secondaries of the current transformers.
Under normal operation conditions, when the two paths are sound then currents in the two parallel paths S1 and S2 are equal. Hence currents in the secondaries of the current transformers are also equal. The secondary current flows round the loop and is same at all the points. Hence no current flows through the relay and the relay is inoperative.
If the short circuit is developed between the adjacent turns of the part S1 of the winding say then currents through S1 and S2 no longer remain same. Thus unequal currents will be induced in the secondaries of the current transformers. The difference of these currents flows through the relay R. Relay then closes its contacts to trip the circuit breaker which isolates the generator from the system.
Such an interturn fault protection system is extremely sensitive but it can be applied to the generators having doubly wound armatures.
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