换流器并联运行的直流配电系统振荡分析

Oscillation analysis of DC distribution system with parallel converters

目前直流配电系统的稳定性分析大多采用单一换流器建模研究,而当直流配电系统采用多换流器并联运行模式时容易发生交互耦合现象,导致系统出现振荡,甚至引发失稳。因此,找出多换流器并联时的失稳机理以及影响系统稳定性的因素至关重要。首先推导了基于P-Udc下垂控制的换流器并联系统模型,建立了两换流器并联系统的简化电路,并以其中一个换流器为激励源,将系统等效为RLC谐振电路并求得振荡频率的解析表达式,准确诠释了多换流器并联时的振荡机理;然后由频域分析所得的奈奎斯特图与时域仿真曲线也可以得到多换流器系统较单一换流器系统更容易出现低频振荡的结论,并且仿真所得的振荡频率与解析表达式所求的结果一致。此外,下垂系数以及交流线路电感等参数的变化均会对系统的振荡稳定性产生影响,并有一定的规律,当下垂系数增加或交流线路电感减小时,有利于提高系统的稳定性。

At present,the stability analysis of DC distribution systems is modeled mostly based on single converter.However,when the multi-converter parallel operation mode is adopted for the DC distribution system,interactive coupling is prone to occurrence,which results in system oscillation,even system instability. Therefore,it is very important to find out the instability mechanism and the factors which affect the system stability. The model of parallel converter system based on P-Udc droop control is derived. The simplified circuit of the system with two parallel converters is established. With one of the converters as the excitation source,the system is equivalent to RLC(radio link control) resonant circuit,and the analytical expression of oscillation frequency is obtained. The oscillation mechanism of multiple converters in parallel is explained accurately. And then,from the Nyquist diagram obtained with frequency domain analysis and the time domain simulation curves,it can be concluded that the multi-converter system is more prone to low frequency oscillation than the single converter system,and the oscillation frequency obtained by simulation is consistent with that obtained by the analytical expression. In addition,the variation of droop coefficient and inductance of AC line will affect the system stability,and the variation has a certain rules. When the droop coefficient increases or the inductance of AC line decreases,the system stability will be improved.