Development of an improved multifunction high speed operating current differential relay for transmission line protection

Toshiba developed and launched the first fully numerical phase-segregated current differential relay as a transmission line unit protection in 1980. High-speed operation was achieved by communicating the instantaneous values of the three-phase currents, sampled at 600 Hz in a 50 Hz system, to all terminals. Japanese utilities have constructed wide band communication networks, such as G-band microwave communication networks and dedicated optical fibres (OPGWs), which make high-speed and large bandwidth data communication feasible. Therefore, this simple, highly selective, high-speed protection system has been very popular as a unit-protection for HV and EHV circuits ranging from 66 kV to 500 kV. To date over 3,500 sets of this current differential relay have been supplied to domestic utilities over the last 20 years. Recently, Toshiba have developed a new relay utilising Multi-MPU, i.e. three, 60-MIPS, 32-bit RISC microprocessors. High-speed operation is achieved, (typically 16 ms), in a multi-function current differential relay benefiting from 20 years experience in the field of phase-segregated current differential protection. Out-of-step protection, high accuracy fault location and autoreclosing are also discussed.