Generator de‑excitation

Exciter discharge protection

Metrosil delivers reliable protection to exciter discharge applications and are used by many leading OEMs in flagship power projects across the world.


Supplied in a wide range of assembly configurations, Metrosil units provide excellent protection for low and high discharge currents. Depending on the discharge system configuration, Metrosil units may be switched-in or permanently connected across the electrical supply:

  • Low discharge currents (600 amps) – Metrosil varistors can be connected directly across the supply at all times. The discs possess a high degree of non-linearity, to prevent excess conduction at operating voltages, while being able to clamp transients to a prescribed voltage
  • Large discharge currents (>600 amps) – Metrosil varistors are not connected across the electrical supply under normal operating conditions. In larger static exciter discharge units, it is common to switch in the exciter discharge system at the same time as switching out the supply voltage. For these applications, varistors can have higher current ratings, higher beta values and a lower degree of non-linearity

The design of a Metrosil unit for an exciter discharge application depends on the following parameters:

  • Required protection voltage – the voltage that the Metrosil products limit the system to, when the maximum discharge current flows from the coil into the Metrosil unit
  • Maximum discharge current from the field coil – the maximum current that occurs as soon as the discharge varistor is switched in
  • Energy stored in the field coil – the energy stored in the field winding, which is to be dissipated in the Metrosil unit

Consideration of the protection voltage and dissipated energy should also be made in relation to the three phase short circuit currents. This may be up to three times the magnitude of the maximum discharge current.

With our High Energy Test Laboratory, which is literally our very own on-site power station, we are able to expose our units to the highest level of energy they are ever likely to encounter. All units are tested prior to despatch, enabling us to instil risk reduction, safety assurance and confidence within flagship power projects on a global basis.

Excitation applications data sheet

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