A Novel Pilot Protection Scheme Based on Differential Voltage for Hybrid LCC/MMC HVDC Transmission Lines

The wide application of hybrid LCC/MMC HVDC system will be a trend in the future, and the performance of DC overhead line protection directly affects the safety of the system. The differential protection is used as backup protection to deal with high fault resistance conditions. However, it needs a long time delay to avoid mal-operation because of the distributed capacitance current. A new pilot protection scheme based on differential voltage is proposed in this paper to enhance the backup protection performance. In the proposed scheme, the compensated voltages at the setting points will be obtained from the measured voltage and current of the relay devices. The differential voltage at a setting point on the line is defined as the difference between the compensated voltages obtained by the rectifier-side relay devices and the inverter-side relay devices. The differential voltages are very tiny in external fault scenarios. Nevertheless, the differential voltages are large in internal fault scenarios. To improve the sensitivity and enhance protection performance, two setting points are set on the line. To simplify the calculation and ensure the calculation accuracy, the measured voltage and current are band-pass filtered. The proposed method has fast operation speed, simple calculation process, and can accurately estimate the fault distance. A +/- 800 kV hybrid LCC/MMC HVDC system is modelled in PSCAD/EMTDC to test the performance of the proposed scheme, and the effectiveness and reliability of it is verified.