Harmonic Filters

Harmonic Filter Resistors, Discharge Resistors and Damping Resistors

transmission network resistor

600kW naturally cooled resistor assemblies being manufactured in the UK at Cressall. These resistors, which are virtually maintenance free, will be supplied as part of a new series of compensation project being installed in the UK. The project is designed to improve the efficiency and power transfer capability of the transmission network.

pre insertion resistor

Cressall Pre-Insertion Resistor being prepared for RIV (Radio Interference Voltage) and through impulse testing. The resistor passed the RIV test at 275kV and the through impulse tests at 475kV.


The twin demands for higher power quality and better network utilisation are leading to the increasing use of static VAR compensators (SVC’s) and manually switched capacitor damping networks (MSCDN’s) in high-voltage three phase distribution networks throughout the world.

Good harmonic filter design requires that the capacitors and inductors in these installations allow through the fundamental frequency and divert the harmonics into the damping resistor where the harmonic currents are dissipated as heat in a safe, controlled and reliable way and so removed from the system.

For the same reasons, discharge resistors are used as safety devices to reduce the risks of sudden, unexpected and dangerous discharges from capacitors and inductors which have become isolated from their networks. The discharge resistor does this by dissipating the stored energy in a controlled fashion.

Cressall builds such resistors to meet the stringent operating conditions of customers such as Siemens, Areva and the National Grid Company in the UK and its counterparts overseas.

Our design expertise in this field is well known.

Cressall offers compact, type-tested designs for all HV filter applications, using any of several different resistor technologies and configurations.

We use low-inductance expanded mesh resistor elements for easy matching with capacitors and inductors.

Filter resistors are also an essential component in HVDC converter stations, where they act as part of the harmonic filters used to dissipate unwanted harmonics produced by the solid-state converters.

Hydro Quebec Harmonic Filter

Our harmonic filters resistors are built to withstand extremes of weather from desert heat to the worst Canadian winters.