Excitation protection

Effectively safeguarding electrical generators.

Synchronous generators are a major source of commercial electrical energy. Metrosil exciter discharge varistors are specified in flagship power projects worldwide to prevent damage to excitation systems during shutdown.

Protecting Synchronous Power Generators

6000 Series

The 6000 Series is our classic high energy varistor design. For many decades, these varistors have protected valuable assets in power generation stations. The 6000 Series is superseded by the 8000 Series.

Metrosil 6000 Series De-Excitation Protection Unit

8000 Series

The 8000 Series was created as a direct response to both client and industry needs, and consequently was developed directly with leading OEMs and industry experts. The design has been optimised for ease of installation and future-proofed to be compatible with industry developments. The 8000 Series, with its modern features, is suited to large applications.

Metrosil 8000 Series De-Excitation Protection Unit

9000 Series

The 9000 Series provides the highest energy density of any power varistor system available today. Delivering double the energy density of the popular 6000 and 8000 Series of power varistors, the 9000 Series delivers a much more compact, space-saving installation – assisted by unique, patented varistor tiles. For extra high energy systems, multiple assemblies can be connected in parallel.

Metrosil 9000 Series De-Excitation Protection Unit


Rated up to 2,000 Volts, 8,000 Amps and 2 Megajoules, the unique High Energy Test Laboratory (HETL) is literally our very own on-site power station. With such extreme power, HETL is able to put our exciter discharge resistors to the ultimate test – hitting them with the highest level of energy they are ever likely to face.

Close up of Metrosil's High Energy Test Laboratory Testing an 8000 Series Assembly

Design Specification

Supplied in a wide range of configurations, Metrosil excitation protection units provide excellent protection for low and high discharge currents. Depending on the discharge system configuration, Metrosil assemblies 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). To select the most appropriate configuration for your excitation system, please contact our customer services team with the above parameters.

Need advice surrounding excitation protection? contact us