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. 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 currents, which they have to carry. Fig 7 shows the arrangement of a 3-phase double wound generator. The relays Rc provide protection against phase to ground and phase-to-phase faults whereas relays R1 provide protection against inter turn faults. S1 and S2 are the stator windings of one phase. Two current transformers are connected on the circulating current principle. Under normal conditions, the current in the stator windings are equal and so will be the currents in secondaries of the two C.T.s. the secondary current round the loop then is
same at all points and no current flows through the relay R1. If a short circuit develops between the adjacent turns on S1 the currents in stator windings S1 and S2 will no longer be equal. Therefore unequal currents will be induced in the secondaries of C.T.s and the difference of these two current flows through the relay R1. The relay then closes its contacts to clear the generator from the system