Neutral voltage displacement (NVD) protection applied to condenser bushings (capacitor cones)

On distribution systems similar to that shown in Figure 1, in the event of a single phase to earth fault, the source protection will trip on earth fault. After the source has cleared, an unearthed network is left, backfed via the primary substation. Other than a very low level charging current, no fault current will flow. UK Power Networks detect this by neutral voltage displacement (NVD). Where a suitable VT is available, this can be used to drive NVD protection. These VTs are expensive. Where foils are included in the transformer bushings, as part of the bushing, we can use these capacitive couplings to the three lines to detect NVD, without the cost implications of a VT, or separate capacitor cones. This paper examines how NVD from capacitor cones can be applied using the options available on modern numerical relays, hence reducing cost and space.