A NEW APPROACH TO DIGITAL CURRENT DIFFERENTIAL PROTECTION FOR LOW AND MEDIUM VOLTAGE FEEDER CIRCUITS USING A DIGITAL VOICE-FREQUENCY GRADE COMMUNICATIONS CHANNEL

A new approach to digital current differential protection is described which provides the capabilities of conventional pilot wire differential protection while using the limited data transfer capabilities of a digital data voice-frequency grade communications channel. This approach compares polyphase polarised current measurements taken at the feeder's terminals to detect faults and can trip in under two power system cycles. The algorithms decouple the measurements from the power system frequency and its waveform, and thereby have a high inherent tolerance to the complications introduced by communication channel delays and their fluctuations. For this project, the communications requirements have been limited to a full duplex 2400 bits/sec system suitable for operation over a voice-frequency grade channel. Naturally a higher data rate would have provided less problems, but the object of the exercise was to tackle these problems.