Generator Rotor Field Suppression and Neutral Circuit Breaker:

In the event of a fault on a generator winding even though the generator circuit breaker is tripped, the fault continues to be fed as long as the excitation will exist because emf is induced in the generator itself. For the quick removal of the fault during emergency, it is necessary to disconnect the field simultaneously with the disconnection of the generator. Thus it is absolutely necessary to discharge its magnetic field in the shortest possible interval of time. Hence it is to be ensured that all the protection system not only trip the generator circuit breaker but also trip the automatic field discharge switch. The field discharge switch is an automatic control unit designed to remove the voltage from the generator after its isolation from the system.

The schematic diagram for field suppression and opening of the neutral circuit breaker is illustrated in Fig. 11.21 (a). In the event of fault the circulating relay contact is closed and so the trip coils TC₁, TC₂ and TC₃ are energized. The trip coil TC₁ opens the main circuit breaker while the trip coil TC₃ opens the neutral circuit breaker. The trip coil TC₂ opens the upper contacts, shorts the lower contacts so as to short-circuit the field winding through resistor R. Thus the energy of the generator is dissipated in the resistor R.

An alternative arrangement of field suppression and dissipation of energy of the generator is illustrated in Fig. 11.21 (b).

Alternative Arrangement of Field Suppression:

This arrangement is similar to that described above except that alternator field winding is also discharged through resistor R₂ by using trip coil TC₄, as illustrated in Fig. 11.22. This process of discharging consists of the isolation of the exciter from the generator rotor field winding and involves the dissipation of magnetic energy stored in the inductive reactance of the rotor and the main exciter windings. This is achieved by switching-y in the rotor and the dc exciter field windings across the field discharge resistors, as illustrated in Fig. 11.22. In order to avoid any overvoltages, the switching-over is performed immediately and without a break in the excitation system.

The quality of operation of an automatic field discharge system depends upon the rate of decrease of field, which in turn depends upon the time constant of the field winding. This term is also known as the field discharge time constant T_d. Normally the value of field discharge time constant lies between 3 and 8 seconds.

The discharge time T_d is defined as the time required for the drop in voltage at the generator stator terminals to a minimum value (usually 500 V).

The value of resistance R₂ in the rotor circuit is determined on the basis of a fall in generator voltage to 30% of normal voltage within 3 to 4 seconds. Resistance R₂ comes out to be roughly 4 to 5 times rotor winding hot resistance.

The resistance R₁ should be roughly equal to 10 times the exciter field winding resistance.