Optimum message transfer rate for distribution feeder protection operating over switched telephone networks

Distribution feeder protection relays require reliable and low cost communication channels. The reliability in sending protection messages over a private or a public switched telephone network (STN) and the associated communication time delay are considered in this paper. The reasoning used to determine the optimum message transfer rate for a numerical current differential relay used for distribution feeder protection is described. A communication channel testing facility was developed and used to measure the time delay necessary to reliably send a protection message over an STN channel operating at different modem data rates. This paper presents and discusses the results obtained and shows how they can be validated using an analytical expression for data transfer reliability. For the defined current differential message frame, test results show that the selected data speed maximizes channel reliability and minimizes channel delay.