Development and site trial of a numerical pilot-wire differential protection

A new numerical current differential relay that operates by digital communication over metallic pilot circuits has been developed, allowing utilities to re-use their existing pilot-wire protection signalling infrastructure while introducing the benefits of numerical relay technology. Sophisticated techniques were applied to achieve digital communication over metallic pilots without the use of modems. The new relay has been successfully applied in a site trial installation for a period of one year, the trial circuit being a 33 kV cable feeder within the network of the UK utility SP power systems. During the course of the trial a number of faults were experienced on the primary system (both internal and through-faults) for which the protection responded correctly.