Optimum sizing of neutral resistor in a high inductance-earthed network utilizing vacuum interrupters

• A new method for optimum sizing of neutral grounding resistor (NGR) is proposed.
• IEEE C62.92.5 criteria relating to NGR sizing are scrutinized.
• An innovative method for VCB modeling in PSCAD environment is presented.
• VCB performance in interruption of SLF current is analytically discussed.
• There are some safe and dangerous points classified wrongly by IEEE C62.92.5.

Vacuum interrupters have been recognized as one of the most important sources of switching over-voltages in MV networks, leading to voltage escalation. These over-voltages are usually observed as a result of inductive current interruption which may accompany with multiple re-ignitions. Single phase fault current in high inductance-earthed networks is inherently inductive and therefore prone to cause dangerous over-voltages during interruption. To mitigate this problem, a resistor is usually inserted between ground and inductor neutral. In this paper, sizing of this resistor is investigated for an industrial plant. Also, prominent conditions affected by resistance value, leading to re-ignitions are studied. Firstly, resistor is sized through relevant IEEE recommendation. Then vacuum circuit breaker is efficiently modeled and using a new simulation-based method in PSCAD/EMTDC platform, the maximum over voltage figures are obtained for variety of operating points. Finally, simulation results are compared with those given by IEEE criterion and appropriate value for neutral resistance will be effectively determined.