Exciter Discharge Protection
An exciter in a synchronous generator is used to provide the DC supply to the electromagnetic field winding which is mounted on the rotor of the generator. In order to prevent damage being sustained to excitation systems during shut down, a suitable means to discharge the energy stored in the field coil must be available.
For many years, Metrosil silicon carbide varistors have provided a reliable solution for exciter discharge applications, being used by many leading OEMs in world flagship power projects.
Depending on the discharge system configuration, Metrosil units may be switched or permanently connected across the electrical supply:
- For low discharge currents (less than 600 Amps) and suitable field and discharge voltages, it is possible to supply a Metrosil which can be connected directly across the field at all times. In these configurations, the Metrosil discs have to possess a high degree of non-linearity to prevent excess conduction at operating voltages, whilst being able to clamp transients to a prescribed voltage
- For large static exciter discharge applications, it is common to switch in the exciter discharge system at the same time as switching out the supply voltage. This may be achieved via a thyristor crowbar control system or a field breaker. In these applications, the Metrosil is not connected across the electrical supply under normal operating conditions and as such considerations towards continuous power dissipation in the discs does not have to be made. For these types of applications, discs may be selected with higher current ratings, albeit with higher beta values and a lower degree of non-linearity.
Both types are supplied as assemblies of 150mm diameter discs, electrically connected in parallel. The number of discs and their thickness depends on the application details.
In the design of a Metrosil unit for an exciter discharge application, the following range of parameters are taken into account:
- Required protection voltage
- Maximum discharge current from the field coil
- Energy stored in the field coil
The required protection voltage is the voltage which the Metrosil discs limit the system to when the maximum discharge current flows from the coil into the Metrosil unit. In the discharge current - time characteristics, this maximum current occurs as soon as the discharge resistor is switched in. Under normal discharge conditions, the magnitude of this current is equal to the magnetising current used in the excitation of the field winding.
The discharge energy is the energy stored in the field winding, which is to be dissipated in the Metrosil unit.
Considerations towards the protection voltage and energy to be dissipated in should also be made with regards to three phase short circuit currents, which may be up to three times the magnitude of the maximum discharge current.