A Differential Protection Scheme based on Pi-Model for Bipolar HVDC Transmission Lines

The distributed capacitive current and harmonics are some of the factors that adversely impact operation of the conventional differential current schemes protecting HVDC transmission lines. To overcome these problems, a more effective differential current scheme using low frequency components of voltage and current signals is proposed in this paper. In this way, Pi-model can be used more accurately for long transmission lines and the distributed capacitive current at each pole can be calculated based on Pi-model by considering the mutual parameters between the two poles. Moreover, using the low frequency voltage and current signals solves the problems associated with harmonics generated under external faults. The proposed scheme is very simple and has a low computational burden, and simultaneously detects internal faults and faulty pole. To evaluate the proposed scheme, $a \pm 500 kV$ bipolar CSC-HVDC system is modeled in PSCAD/EMTDC. The simulation results, confirm the fast and selective operation of the suggested method in detection of faults up to a considerably high fault resistance. Moreover, the method has a low sensitivity to noise, sampling frequency change, and power flow changes.