Abstract
Commutation failure, primarily caused by voltage drops and distortion, may lead to high-voltage direct current (HVDC) power interruption and AC system oscillation. To date, there have been few studies on the mechanism and prevention of commutation failure based on voltage distortion caused by harmonics. This study analyses the influence of harmonic voltage on commutation failure based on voltage time area and proposes a method for quantitative analysis of harmonic effects. The analysis shows that limiting the DC current is an effective method to mitigate commutation failure caused by harmonics. A voltage distortion-dependent commutation failure prevention strategy is proposed, and the controller is applied on the inverter side. The selection of parameters is based on the influence of various harmonic orders. The validity and effectiveness of the proposed methods are verified by simulations, which show that voltage distortion-dependent commutation failure prevention strategy can improve the DC system recovery characteristics and suppress subsequent commutation failure caused by harmonics.
1 Introduction
With the advantage of economical operation and high controllability, high-voltage direct current (HVDC) has been widely used in the field of long-distance transmission [1, 2]. Currently, hundreds of HVDC have been put to use in the world, and China is expected to have 38 HVDC by 2020. Consequently, many heavy load areas will form a pattern of multi-infeed HVDC, such as eastern and southern China.
As the most common faults in HVDC systems, commutation failures (CFs) can lead to temporary interruption of transmitted power and stressing the converter equipment [3]. Short-time CFs can produce large power and voltage fluctuations, while long-time and continuous CF may cause HVDC blocking [4]. There have been many studies related to the factors influencing CF and preventive measures.
5 Conclusion
In this paper, the mechanism of commutation failure caused by harmonics is analysed. A quantitative method for determining the influence of various harmonic orders on commutation failure is proposed, and the analysis shows that the harmonic amplitude, phase, and harmonic commutation coefficient are important factors leading to commutation failure. During the recovery period of an HVDC after commutation failure, the harmonics of the inverter bus may lead to subsequent commutation failure, and low-order harmonics play a major role. A VDDCFP strategy is proposed in this paper to prevent CFs caused by harmonics. Simulation results show that the proposed method and controller are useful for analysing and preventing CFs caused by harmonics. Moreover, the recovery characteristics are improved with the proposed VDDCFP controller.
