Simultaneous pole and neutral faults on an HVDC line with a dedicated metallic return

The dedicated metallic return conductor (DMR) of an HVDC transmission line may flashover as a consequence of a pole insulation failure or backflash over simultaneously with a pole during a lightning-induced event. The transient voltage surges induced on the DMR of the Quebec-New England HVDC intertie by pole faults are analyzed, and the sensitivity of simultaneous pole and DMR backflashover probability to variations in line insulating and footing impedance, is evaluated. It is found that for any reasonable DMR insulator string length, any lighting backflashover of a pole will be accompanied by a DMR flashover. Because coincident flashovers from lighting cannot be practically eliminated, the only practical solution for pole plus DMR faults on the New England portion of the Quebec-New England HVDC intertie is to install a high-speed ground switch at Sandy Pond, MA. The DMR insulator string length was increased to four units on the Sandy Pond-Comerford (NH) section, and to five units on the Comerford-Quebec order section in order to minimize the frequency for which this ground switch must operate