Research of implementing least squares in digital distance relaying for AC electrified railway

In AC electrified railway feeder, distance relays are the most popular used protector. Sensed impedance of distance relay may be distorted by the nonperiodic and harmonic values, which are involved in the sensed voltage and current, especially in AC electrified railway circuit. To insure the operating correctness of distance relay, the fundamental voltage and current for calculating impedance must be separated from the sensed voltage and current. The popular principle of parametrical estimation is the least squares algorithm that is applied to the distance relay in this paper. The system model based on the differential equation is reconstructed. Since the error of estimation resistance based on the model of differential equation is unacceptable, a new model based on the instant power consumption for estimating resistance is suggested. Both models based on the differential equation and the instant power consumption are validated by simulation. The results of analysis and simulation experiment show that the sensed impedance for distance relay can be obtained by individually estimating inductance and resistance.