Optimum data speed for numerical current differential protection operating over public switched telephone networks

This paper considers the issues of data reliability and data transfer time delay in a public switched telephone network (PSTN) channel. It provides analysis and evidence of the reasoning necessary to determine the optimum data speed for a numerical current differential relay for distribution feeder protection. A communication channel testing system was constructed to measure the data transfer reliability and delay over the PSTN channel under different data rates. The results show that for a given current differential message frame, optimum data speed selection can both maximise the data channel reliability and minimise the channel delay. The paper also presents and discusses the results obtained when testing the protection by transmitting protection messages over PTSN channels