基于平均模型的高压链式STATCOM功率模块直流电压失控机理

Mechanism of Incontrollable DC Voltage of Sub Modules for Cascaded STATCOM Under Higher Voltage Based on Average Modeling

高压链式STATCOM在重载时功率模块的直流电容电压容易控制,在空载时却可能发生失控。空载失控机理分析是解决该问题的关键。针对该机理问题,主要开展了如下研究:分别将串联阻抗、并联阻抗和控制电源能耗化简并绘制出等效电路图。基于平均模型将直流电容电压分解为直流分量和交流分量,交流电流分解为基波和谐波分量。并利用调制度等效开关函数,得到单个功率模块直流电容电压的数学方程。根据稳定性分析的小信号法,分别计算功率模块直流电容电压稳态分量和扰动分量。针对未采取均压控制和采取均压控制两种情况,推导并计算任意两个功率模块在重载和空载下的直流电压平衡方程,证明了换流链的直流电压平衡与调制度、开关频率等因素有关。提出一种改进均压控制策略,在空载时增加开关频率来改善均压性能。最后,经过35 kV装置的仿真和试验,结果表明:在空载时所提均压控制策略能够实现功率模块直流电容电压均衡,从而提高了装置的可靠性。

Cascaded STATCOM under high voltage is easily balanced during heavy loads but probably out of control at empty loads. Research on the mechanism of out-of-control at empty loads is the key to solution of this problem. Consequently, we emphatically research the main contents include as follows:①Equivalent circuit diagram of serial impedance, parallel impedance and control power consumption are simplified and drawn, respectively.②According to the average modeling theory, the DC capacitor voltage is decomposed into the direct current component and the alternating current component, the alternating current is decomposed into the fundamental wave and the harmonic component, and the modulation degree equivalent switch function is used to list the mathematical expression of the DC voltage of the single power module.③According to the small signal method of stability analysis, the steady-state component and the disturbance component of the DC capacitor voltage of a power module are calculated.④In the two cases whether the DC voltage balance control is adopted or not, the DC voltage balance equation of any two power modules of the convert chain at the heavy load and the light load is deduced and calculated, and the DC voltage balance is related to the modulation degree, switching frequency, and other factors.(5)An improved DC voltage balance control strategy is proposed. The switching frequency is increased to improve the voltage balancing performance at empty load. In the end, simulations and tests are used to compare the results under four typical operating conditions. The results show that the voltage-sharing control strategy proposed under empty-load conditions can achieve DC capacitor voltage equalization of the power module to increase reliability of the cascaded STATCOM.