换流变压器和应涌流的正序二次谐波特性分析

近几年,南方电网发生了多起因换流变压器空载合闸而导致运行极直流50 Hz保护误动事故。事故主要原因是运行换流变压器的和应涌流含有大量正序二次谐波,流经换流器转化为50Hz分量。文中分析了交流系统电压以及直流输送功率等运行工作点改变引起换流变压器分接头挡位、滤波器投切组数的变化,进而影响和应涌流正序二次谐波的规律;从同塔双回直流线路的极性布置、电流极性等耦合影响因素分析换流变压器和应涌流的正序二次分量在运行线路上的不平衡性。为防止直流50Hz保护误动,建议提高直流接入交流系统的强度,适当抬高换流站交流母线电压,限制运行极直流的输送功率。同时,应重点关注同塔双回直流另一回同一极性的换流变压器的和应涌流。

Analysis on Positive-sequence Second Harmonic Characteristic of Sympathetic Inrush Current for Converter Transformer

A number of DC 50 Hz protection malfunctions have occurred in recent years in China Southern power grid due to no-load closing of the converter transformers. The main cause is that the sympathetic inrush current of the operating converter transformer contains abundant positive-sequence second harmonic, which has been transformed to 50 Hz harmonic by converter. This paper analyzes the impact of the DC operation conditions, such as the AC system voltage and DC transmission power, on the tap of converter transformer and AC filters, which changes the positive-sequence second harmonic of sympathetic current finally. It also analyzes the imbalances of positive-sequence second harmonic of the double circuit DC transmission lines from the view of the positions of poles, current interconnection and so on. To avoid the malfunction of 50 Hz protection, some suggestions are proposed, such as decreasing the impedance of AC system, increasing the voltage of AC bus, limiting the transporting power of running pole, etc. The sympathetic inrush current of the other DC transmission line on the same pole as no-load closing pole should be concerned seriously.