A Pilot Protection for HVDC Transmission Lines Based on Transient Energy Ratio of DC Filter Link

To overcome shortages in the traditional current differential protection for HVDC transmission lines, a pilot protection based on transient energy ratio is proposed. The fault identification criterion is put forward based on the impedance-frequency characteristic of the line boundary, and the fault analysis which shows that under internal faults, with regard to either terminal of the dc line, the transient energy within a specific frequency band on the line side of the boundary is much greater than that on the valve side of the boundary, whereas after rectifier-terminal (inverter-terminal) external faults, the transient energy on the line side of the rectifier-terminal (inverter-terminal) boundary is much less than that on the valve side of the boundary. Moreover, the lightning disturbance identification criterion is implemented based on the magnitude ratio of fault components of high- and low-frequency band currents. Fault-pole selection is constructed by using the fault components of pole voltages. Simulation results demonstrate that the proposed scheme, with low computational complexity, can distinguish internal faults from external ones and protect the entire line reliably. It can also identify high-impedance faults and select the faulted pole correctly. Besides, it is not subject to lightning interferences and the dc line distributed capacitor.

I. Introduction

Due TO remarkable advantages such as asynchronous interconnection of AC power networks and long-distance bulk power transmission, high voltage direct current (HVDC) system has been widely used [1]–[3]. Due to lengthy transmission lines, as well as complex terrain and weather conditions of transmission corridors, the fault probability of HVDC lines can be high [4]. Thus, it is critical to investigate HVDC line protection with satisfactory performance. At present, in most practical HVDC projects, the main protection for HVDC lines is traveling-wave based protection and differential under-voltage protection, while the backup protection principally includes under-voltage protection and current differential protection [5], [6]. Therein, the traveling wave front detection is difficult for high-impedance grounded faults. Besides, issues such as high sampling rate and complexity of setting the threshold still need to be addressed [7]. For differential under-voltage protection, the adaptability to fault resistance is poor [8]. Current differential protection is mainly used to identify high-impedance faults, yet a reasonable time delay needs to be set to avoid the influence of capacitive current during the fault transient, which results in its slow operating speed even up to 1.1 s [9].