交流系统接地故障对HVDC的影响分析

以最常见的交流系统单相接地故障为例,详细分析了 HVDC换流器的动态过程,包括换流母线电压特性、锁相环输出特性和阀换相特性。提出改进的开关函数法以求解故障期间的直流电压,该方法将直流侧电压表示为3部分,分别对应交流侧正序电压、负序电压以及导通时间变化量。建立了直流侧谐波计算的等值电路,该电路中故障侧换流器以谐波电压源表示,其幅值由开关函数计算得到,对侧换流器计及了换相过程的影响并以换相电抗的平均值表示。算例以及与 PSCAD／EMTDC的结果对照表明对故障期间换流器的动态过程分析正确,直流电压和谐波计算准确。该文所提出的改进的开关函数法解决了开关函数法不能应用于换流器不对称运行的问题,并可用于交流系统单相接地故障及阀不对称运行时的谐波分析。该谐波计算方法可用于HVDC控制保护中谐波保护定值的整定计算与校核。

Analysis of the Impact of AC System Single-Phase Earth Fault on HVDC

The paper analyses the dynamic process of HVDC converter, including voltage characteristic of converter AC bus, phase-locked loop (PLL) and the commutation characteristic of valve, when single-phase earth fault occurs in AC system, which is the most typical fault among all AC system faults. An improved method based on switching function is presented to determine voltage harmonic component in dc side. In the method, voltage in dc side is divided into three groups, corresponding to positive sequence of AC side voltage, negative sequence of AC side voltage and variation of valve conduction time respectively. An equivalent circuit for calculating dc side harmonics is set up, in which the converter at fault side is represented by an equivalent harmonic voltage source while the converter at the other side is represented by average commutation impedance accounting for the commutation courses. A test system is established in PSCAD/EMTDC to verify the analysis. Results show that the analysis of converter's dynamic process is correct and high accuracy is gained in harmonics calculation. The improved switching function method solved the problem that switching function cannot be used to unsymmetrical operation of converter, and thus can be used in harmonic analysis in single-phase earth fault and unsymmetrical operation of converter. The presented method can be applied to the value determination and verification of harmonic protection in HVDC control and protection system.