Relay protection

Protecting High Impedance Relays

The stability of a high impedance differential scheme that contains CTs depends upon the relay circuit setting voltage being greater than the maximum voltage which appears across the relay circuit under a given through fault condition.

In some applications of high impedance relays for protection and control duties, the maximum internal fault current can develop high voltages that can damage the relay insulation. A range of Metrosil varistors, primarily based on 150mm diameter discs, has been developed to limit this voltage to a safe level. Selecting the most suitable Metrosil component is vital to ensure that sufficient protection is provided. Depending on the relay setting voltage and the maximum secondary internal fault current, single discs are available for secondary internal fault currents up to 50A rms, while multiple discs connected in parallel are used for larger fault currents.

Choosing the correct relay Metrosil varistor will ensure that the system is held to a safe clamping voltage and will have negligible effects on the measurement accuracy of the relay.

Why Metrosil?

Metrosil Silicon Carbide varistors were borne out of the high-voltage department of Metrovicks Research in 1936 and mass produced in 1937. As a major power house in the 20^th 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 into substations on a global basis.