A microprocessor-based measuring unit for high-speed distance protection

This paper describes a microprocessor-based measuring unit which is suitable for use in high-speed digital distance relays. The unit uses an algorithm that provides accurate estimates of apparent resistance and inductance of the line in relatively short times. The algorithm deals effectively with the presence of nonfundamental frequency components in signals without making assumptions about their compositions. The hardware and software of the unit are described in the paper. Test results indicate that the estimates converge to their true values within about 10 ms after fault inception. Also, the ability of the unit to provide accurate estimates is not adversely affected by the system frequency's drifting from its nominal value and by the distortion of the input signals.