scope:

This annex is applicable to resistor fitted disconnectors in metal-enclosed gas insulated switchgear (GIS). The maximum VFTO (very fast transient overvoltage) in GIS systems with a rated voltage of 800 kV and above may reach the insulation level of lightning impulse withstand voltage in certain cases during switching of disconnectors. The purpose of fitting a resistor is to mitigate the VFTO in such cases. Three different types of resistor fitted disconnectors are given as examples in Figure H.1. The resistor can be positioned in parallel or in series to the switching gap. In the arc-commutation method the current is switched to the parallel resistor during an opening operation by commutation of the arc from the main contact to the resistor contact. NOTE Typically resistors of values 200 O to 1 000 O are used. The overvoltage damping factor depends on the ratio of the resistance of the resistor and the system impedance.