Microcontroller based overcurrent relay and directional overcurrent relay with ground fault protection

This paper presents the design and construction of overcurrent and directional overcurrent relays with ground fault protection for the protection of three-phase subtransmission and distribution systems, using a 16-bit microprocessor, the Intel 8096BH. The relay obtains the system currents at the rate of 12 samples per cycle and estimates the fundamental-frequency components of the current signals using discrete Fourier transform techniques. In the case of the directional overcurrent relay, the direction of the current flow is identified to determine whether the fault current is flowing into its protected zone. For this purpose, several internally stored voltage vectors, corresponding to the different directional element settings, are synchronized accurately with the system voltage and used to determine the direction of the power flow. Facilities to change relay characteristics, the time dial and plug settings are provided. The user can also set the relay as an instantaneous overcurrent relay. The desired operating characteristics are achieved by direct curve data storage in the memory.