Coupling to control cables in HV substations

PLCs and microprocessor relays located in control buildings inside the HV substations were subject to electrical interference in the substation yard during high voltage air disconnect switching. The interference was due to modifications to a circuit breaker that produced common mode voltages on control circuit wiring in the substation control building and led to equipment damage and misoperation. This paper describes the unusual coupling mechanism that created transient voltages, a comparison between predicted and measured transients, and a description of use of shielded cable to minimize interference coupling.