Many circuit-breakers operating in the EHV/UHV network are equipped with closing resistors in order to reduce switching transients. Such closing resistors are switched temporarily in the circuit, just before closing the main circuit-breaker and before closing the main contacts of the circuit-breaker. See the schematic in Figure 1.
This implies that the full pre-strike and its prearc upon closing have to be absorbed by the contact system of the closing resistor switch. Though electrically a simple circuit, the physical construction of the closing resistor assembly and its operating mechanism are quite complicated.
Several utilities have faced problems with the operation of the closing resistor switch. Therefore verification of the closing resistor switch operation under relevant operating conditions (voltages and currents) becomes very important.
Appropriate test-circuits for testing this function of the closing resistor switch have been developed at the KEMA Laboratories and successfully applied during testing of a gas-insulated 800 kV switchgear.
The main issue here is the very high AC voltage that has to be applied across the resistor switch just before making, followed by the full resistor inrush current after pre-strike. Figure 3 shows a typical test result, where an AC voltage of 1375 kV peak was applied across the switch prior to pre-strike.
This voltage is generated by superposition of three synthetic installations. Figure 2 shows a principle scheme of the used test-circuit. After pre-strike, a current of 3000 A is supplied, first feeding the pre-arc in the switch which will last until galvanic touch of the switch contacts.
The thermal capability of the resistors may be verified separately in a direct test circuit.
It is common practice to employ closing resistor switch system for damping transients associated with the closing operations. However, it is important to verify their functioning in appropriate test circuits that reproduces the correct stresses, in a laboratory environment.