Excitation systems

Metrosil delivers reliable protection to exciter discharge applications. Our non-linear resistors are used by many leading OEMs in flagship power projects across the world – including Grand Coulee, La Grande, Mica, Revelstoke, Itaipu, Xiangjiaba, Xiluodu and Three Gorges.

Excitation System Protection

A power station’s synchronous generator consists of a rotor spinning in a magnetic field. This magnetic field is produced by the rotor’s field coils, and in order for electricity to be produced, a current must flow in the coils to generate the field. Coupling an excitation system to the same shaft as the rotor provides this field current, and enables the rotor to force the movement of electric charges.

Transients may occur in excitation systems when the voltage to the field coil is suddenly removed, causing a rapid decrease in current with time. This is because the energy stored in the coil tries to maintain the magnitude of the current by creating a back electromotive force (EMF), which can be considerably larger than the supplied voltage and, if uncontrolled, can be sufficient enough to damage components in the system. Therefore, plant shutdowns – whether planned or not, must be accounted for. To prevent damage being sustained to an excitation system during shutdown, a generator surge protector must be in place to quickly absorb the high energy levels stored in the rotor’s field coils. IEEE 421.6-2017 states that silicon carbide is the preferred technology for high energy discharge.

Metrosil varistors control the magnitude of a back EMF by dissipating stored energy in the coil to an appropriate load. The degree of non-linearity between the voltage and current characteristics of our varistor discs, and our ability to match discs according to their energy absorption properties, means that superfast discharge of excitation systems is possible. We can assure Protection Engineers of this via our High Energy Test Laboratory (HETL). HETL enables us to test resistor assemblies to their maximum rating prior to despatch, providing our customers with complete peace of mind that their valuable assets are fully protected against energy transients.


Metrosil Silicon Carbide varistors were borne out of the high-voltage department of Metropolitan-Vickers in 1936 and mass produced in 1937. As a major power house in the 20th century, Metrovicks was renowned for its industrial electrical equipment, which included generators, steam turbines, switchgear, transformers, electronics and railway traction equipment. Consequently, Metrosil resistors were specified into major flagship projects that paved the way for the efficient distribution of electricity. To this day our resistors remain in place within established power networks, which has instilled trust in major OEMs and utilities alike. As modern power infrastructures develop, we continue to innovate and be specified on a global basis.

Discharging excitation systems

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Three Gorges Dam in China

Metrosil excitation protection units

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Four Disc Metrosil Excitation Protection Unit

High energy testing

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Why Metrosil non-linear resistors?

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