Response Of The DC Protection To AC Faults And Its Output Timing Sequence Characteristics

In AC / DC hybrid system, when faults occur in HVDC transmission system, DC protection should rapidly respond to protect DC equipment from threats and faults. When faults occur in AC system, the DC protection should not act instead of AC protection to cut off the faults. However, the DC protection does not act strictly following the principles above. There has been several accidents that caused by DC protection’s malfunction under AC faults and resulted in the block of HVDC system. It shows that the challenges AC / DC interaction brings to the safe operation of DC protection should not to be ignored anymore. In order to solve this problem, this paper analyzes and accesses the operation risk of DC protection in AC / DC hybrid system, which provides guidance for the safe and reliable operation of HVDC transmission system.This paper firstly analyzes the formation and development of transient process of DC system under AC faults, and sums up its characteristics which including low voltage, current imbalance, and 100 Hz component.The response mechanism of DC protection to AC faults is revealed by two aspects, namely the similarity between AC faults and DC faults as well as the imperfection of both protection principle and setting method of DC protection. Based on this response mechanism, the types of DC protections, which can reflect AC faults in principle, are summarized.Combining with PSCAD/EMTDC experimental data of a certain UHVDC transmission system, the malfunction of DC protection due to harmonic characteristics, low-voltage characteristics and current unbalance characteristics under AC faults at both sides are analyzed in detail. The malfunction mechanism of 100 Hz protection, DC low-voltage protection, valve group differential protection and bridge differential protection under AC faults are analyzed in depth, and corresponding improvement suggestions are put forward.Finally, this paper analyzes the timing principle of DC protection on the problem of action uncertainty of DC protection under the AC faults. This paper points out that the internal timer of DC protection counts accumulatively, which will lead to the uncertainty of DC protection trip time. DC protection trip time depends on the dynamic process of protection characteristic during the period of faults, and it relates to fault type and fault occurrence time. The trip time of the DC protections that response to AC faults is close, so the trip timing sequence of the protections determines the final result of protections action. When AC faults occur at different time, DC protection trip time and trip timing sequence will be different too, which will result in the action uncertainty of DC protection.