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. It is usually unnecessary to provide inter turn faults because they invariably develop into earth faults. In single turn generator there is no necessity of protection against inter turn faults. However it is provided for multi turn generators such as hydroelectric generators. These generators have double winding armatures owing to the very heavy read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

The over current and earth fault protection is same as the over current and earth fault protection of transformers which will be discussed later. Fig 6 shows the over current and earth fault protection scheme for generators. read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

This is very sensitive type of earth fault detection scheme, which necessarily consists of two connecting cables connected to the two ends of each phase and the cables are passed through the circular aperture of the ring type C.T as shown in fig 5. Under normal working conditions the current carried by two cables in the central aperture of the C.T will be in opposite direction resulting in no magnetization of the C.T. If there is an earth fault on any one of the phases. The fault current will pass only once through the circular ring of the C.T of that particular phase, this results into production of flux and thereby an e.m.f is induced in the relay circuit, which will trip the circuit breaker. The relay will also trip for any fault between two phases. It does not provide protection read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

The biased circulating relays are useful because they allow non-similarity between two sets of C.T.s. the restraining coil is inserted in series with one of the phases, while the operating coil is connected to the central tapping and the neutral pilot wire as in fig 4. Only one biased relay is shown for simplicity. The advantage of biased relay is that when generator carries through heavy fault current the current transformers will fail to balance each other and if ordinary relay is used the relay will trip: but biased relay does not operate at one definite current setting, its operation increases with the through current passing into operating coil read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

This type of protection provides a continuous check on faults within the points, where the C.T.s are used as in shown in fig 2. There are two sets of C.T.s; each set is mounted on either end of the stator phase. The secondaries of these current transformer sets are connected in star and their ends are connected together through pilot wires. The over current inverse time relay is connected across the equipotential points in the pilot wires, which will naturally be in center of these wires as shown in fig 2. But it’s convenient to locate these relays adjacent to the main circuit breaker; this can be achieved by inserting balancing resistance in series with the pilot wires to achieve equipotential point near the main circuit breaker as shown in fig 3. If there is no fault in the read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »

The major faults in case of generator can be classified as: (a)Failure of prime movers: Whenever there is a fault on prime mover side the conversion of mechanical power to electrical power stops. If this generator works in parallel with other generator sets it will start working as a synchronous motor running at synchronous speed and the prime mover will act as load on it. When generator starts running as a motor it is called as inverted operation. If the fault is cleared the generator will automatically pick up generation. (b)Failure of excitation: The turbo generator set under consideration works in parallel with other sets. If the field of the generator is interrupted it will continue to operate as an induction generator-delivering load at very low power factor. So the other read more

If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »