S6 Alternator Protection.pdf

Under normal operating conditions the currents

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Under normal operating conditions, the currents flowing in the alternator leads and hence the currents flowing in secondaries of the line current transformers add to zero and no current flows through the relay. Also under these conditions, the current in the neutral wire is zero and the secondary of neutral current transformer supplies no current to the relay. If an earth-fault develops at F 2 external to the protected zone, the sum of the currents at the terminals of the alternator is exactly equal to the current in the neutral connection and hence no current flows through the relay. When an earth-fault occurs at F 1 or within the protected zone, these currents are no longer equal and the differential current flows through the operating coil of the relay. The relay then closes its contacts to disconnect the alternator from the system. 6.1.5 Stator Inter-turn Protection: Merz-price circulating-current system protects against phase-to-ground and phase- to phase faults. It does not protect against turn-to-turn fault on the same phase winding of the stator. It is because the current that this type of fault produces flows in a local circuit between the turns involved and does not create a difference between the currents entering and leaving the winding at its two ends where current transformers are applied. However, it is usually considered unnecessary to provide protection for inter- turn faults because they invariably develop into earth-faults. In single turn generator ( e.g. large steam-turbine generators), there is no necessity of protection against inter- turn faults. However, inter turn protection is provided for multi-turn generators such as hydro-electric generators. These generators have double-winding armatures ( i.e. each phase wind- ing is divided into two halves) owing to the very heavy currents which they have to carry. Advantage may be taken of this necessity to protect inter-turn faults on the same winding. Fig. 22.7 shows the schematic arrangement of circulating-current and inter-turn protection of a 3-phase
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double wound generator. The relays RC provide protection against phase-to-ground and phase-to-phase faults whereas relays R 1 provide protection against inter-turn faults. Fig. 22.8 shows the duplicate stator windings S 1 and S 2 of one phase onlywith a provision against inter-turn faults. Two current transformers are connected on the circulating-current principle. Under normal conditions, the currents in the stator windings S 1 and S 2 are equal and so will be the currents in the secondaries of the two CTs. The secondary current round the loop then is the same at all points and no current flows through the relay R 1. If a short- circuit develops between adjacent turns, say on S 1, the currents in the stator windings S 1 and S 2 will no longer be equal. Therefore, unequal currents will be induced in the secondaries of CTs and the difference of these two currents flows through the relay R 1. The relay then closes its contacts to clear the generator from the system.
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