Underexcitation Protection, Field Failure Protection

In case of a loss of exciting current of a generator, the stator current increases and the damper winding is growing warm. To protect in such a case, generators of up to 50 MVA have been equipped with undercurrent relays in the excitation circuit. Since the excitation controller can also define small currents without the danger of underexcitation, the magnitude of excitation current could not be the only criteria.

BBC realized a solution against the unintentional interruption of the excitation circuit of a synchronous machine in 1958  (Figure 3). This protection device supervises not only the inductor current, but even the field circuit current of the main exciter and the voltages of all exciters. Different phenomena, all of which could cause an interruption, required a certain amount of protection elements. The supervision element consists of two over-resistance relays type TH, an under-voltage relay type RGBmin (Figure 2) and a timing element MLT (Figure 1). The first relay supervises the main exciter, the second - the excitation circuit of the main exciter and the under-voltage relay - the voltage of the auxiliary exciter.

To detect the loss of excitation it was possible to supervise inductor and stator.

A circuit to detect the interruption of an excitation circuit in the rotor developed by OERLIKON in 1964 is shown in Figure 4.

Main- and auxiliary circuit have been supervised separately, in front and behind the voltage controller. The main excitation circuit was supervised with the resistance 9. To use an undercurrent relay behind the voltage controller 7 was not possible, since the excitation current in the salient-pole machine could become negative or zero. The over-resistance relay was supplied with voltage and current. The voltage knits in the tripping direction of the relay, the current in the opposite direction. Due to the huge amplitude of the current (several hundred Amperes) the current was measured with a shunt 12.The tripping element 14 was connected with a timing element 14. The delay was necessary due to transients and the time constant of the machine. In the auxiliary excitation circuit an over-resistance relay 10 is used. Differing from relay 9 it was connected to the exciting current directly. Since the relay 10 could not detect the loss of the auxiliary excitation voltage or an interruption in front of controller 7, an under-voltage relay (11) measured the voltage (more or less constant) across terminals of the auxiliary exciter. To avoid false tripping during run-up and synchronization, especially of the under-voltage relay 11, the control voltage was connected via an auxiliary contact to the circuit breaker 3. Inactivation during the synchronization was not a problem, since the synchronizer could detect an interruption in the excitation circuit (and the loss of voltage) already on its own.

To detect loss of excitation on stators, OERLIKON developed the device shown in Figure 5.

In case of lost excitation the generator obtains the magnetizing current needed from the grid. Measuring this reactive current allows detection of this faulty state. Since this current was proportional with the voltage, the current was not measured directly but compared with an operating current proportional to the voltage. The interposing transformer 7 was connected to the phase current and a current proportional to the phase-to-phase voltage (resistance 9).

The difference, provided on the secondary side of the transformer was provided to a Ferraris relay 8. Choosing the polarization voltage of the relay and tuning the polarization current with resistance 10 is done in such a manner, that in case of an improper magnitude of the magnetizing current the relay trips. The tripping value was chosen with the resistance 9. The tripping was delayed to avoid false tripping in case of transient phenomena during switching (depending from the time constant of the generator).