A Study On Fault Analysis And Protection Of The Hvdc Transmission Line

A large number of unnecessary HVDC outages are induced by the HVDC line faults because of the low level of sensitivity of HVDC line protections, leading to a significant negative influence on the safety of the power system. On the one hand, there is a lack of effective analytical theory and setting calculation method due to the complexity of HVDC line faults. It is difficult to reveal the underlying principle and the border of influencing factors just by the numerical simulation of the HVDC system. On the other hand, the transient characteristics of the HVDC line fault and thus the line protections are affected by the HVDC control system, which is very complicated and still a hard nut to crush. In this paper, the analytical theory of the HVDC line fault, dynamic characteristics of the protections and the control system and their coordination are studied and an improved protection scheme is proposed to meet the significant demand of the HVDC system, which is not only valuable in the academic area but also utility in the engineer field.The work is summarized as following:(1) Analytical treatment is applied to indicate the reflection principles at the fault position, between the modal quantities and at the terminals of the HVDC line. Based on the distortionless line model, the analytical solutions of the traveling waves on both the monopole system and the bipolar system are developed to reveal effects of the smoothing reactor, DC filters, fault resistance on the traveling waves caused by the line fault. It is shown that the wave-front of the voltage at the terminal is consist of a exponential decay component and a sinusoidal component and thus the amplitude and the derivative of the wave-front both attenuate, although the initial incident wave has a step waveform. The initial amplitude mainly depends on the fault resistance. Furthermore, the change rate of the voltage which is the dominating discriminant of the existing traveling wave protection can be obtained for setting calculation of the traveling wave protections of the HVDC line. The simulation results obtained from the EMTDC model of the real HVDC system shows the effectiveness and credibility of the setting method.(2) A novel protection principle based on the polarity comparison of modal back waves is proposed in the paper, which is effective to improve the capability of detecting the high resistance line faults. The threshold of the change rate of the traveling waves which is the dominating discriminant of the existing traveling wave protection must be greater than the ones caused by the external faults. Thus, the response of the existing traveling wave protections to the high resistance and the far distance line faults. The polarity comparison instead of amplitude comparison is used to improve sensitivity of the traveling wave protection. The simulation results based on the EMTDC show the effectiveness of the proposed protection principle.(3) The operation time of the existing DC Line Differential Protection (DLDP) is too long as the blocking criterion can't distinguish the DC line fault from the AC system failure. A novel blocking criteria, according to different spectrum of the differential currents between the DC line fault and the AC system failure, is presented. Simulation results obtained by the EMTDC show that the proposed criteria can discriminate the line fault and the faults in the AC system availably and rarely affected by the fault impedance and the fault distance, while it does shorten the operation time of the DLDP.(4) The effect of the control system on the HVDC line faults is studied using the EMTDC model of real HVDC system. It is shown that the continuous fluctuation of HVDC line current may be induced by voltage adjusting function and transient current control function of the Voltage Dependent Current Limit (VDCL) at the rectifier, which leads to miss-trip of DLDP. For this issue, a criterion using the voltage of the commutation bus at the rectifier is proposed to avoid the unfavorable influence of the control system on the DLDP.(5) The dynamic characteristics of the Derivative and Under Voltage Protection (DUVP) is presented by means of the analysis of field data and the simulation results. The DUVP may mal-operate because of the harmonic distortion of DC voltage and in-coordination between the AC main protection and the DUVP when the cascading failure happens in the AC system. To address the problem, the DC current criteria is added to the DUVP.(6) A novel HVDC line protection scheme, based on the sampling frequency and channels of the existing HVDC line protection. A simulation system including both the existing scheme and the proposed one is developed based on the EMTDC models of an real HVDC system. The comparison of the two schemes shows the validity of this paper's effort.This work was supported by the National Natural Science Foundation of China (50337010) and the co-operative researches with China Southern Power Grid(CSG). Some of the results has been applied in the modification of the HVDC projects in CSG, which shows effectiveness and credibility of this paper's efforts.