新型高压直流输电系统接入三相不平衡电网分析

新型高压直流输电系统接入三相电压不平衡电网时会遇到直流电容电压出现2倍工频纹波的问题,从而导致换流器性能下降,影响整个系统的正常运行。若只考虑基波电动势,使用对称分量法可以将不平衡电压分解成对称的正序和负序2部分,而这2部分平衡且相互独立。换流器d、q轴分量是直流正序分量与2次交流谐波的负序分量之和,参考在电压平衡情况下的数学模型,可以得到三相电压不平衡情况下电压源型换流器在两相同步旋转坐标系下的数学模型,这个数学模型包含正序和负序2个部分。由三相输入的复功率可知,有功功率包含2次余弦和正弦部分,通过正序电流和负序电流分别独立控制的双电流闭环控制方法使其为零,从理论上就可以消除直流电容电压的2次纹波。利用电力系统仿真软件Matlab/Simulink对系统进行仿真,结果表明,双电流闭环的控制方法可消除直流电压的纹波,保证了新型高压直流输电系统在电压不平衡的情况下仍然具有良好的性能。

Analysis of HYDC system connecting to three-phase unbalanced grid

When connect HVDC system to 3-phase unbalanced grid,100 Hz / 120 Hz ripples may occur in DC capacitor voltage,which weakens converter performance and breaks formal system operation. If only the fundamental is considered,the unbalanced voltages can be decomposed by the symmetrical components method into positive and negative sequence components,which are balanced and independent of each other. The d and q axis components of the converter are the sum of DC positive sequence component and 100 Hz / 120 Hz AC harmonic negative sequence component. According to the mathematic model of balanced voltages,the mathematic model of unbalanced 3-phase voltages for voltage source converter in two phase synchronous rotary frame is obtained,which contains both the positive and negative sequence components. According to the complex power of three-phase inputs,the active power includes 100 Hz / 120 Hz sine and cosine components,which can be theoretically reduced to zero to eliminate the DC capacitor voltage ripples by controlling both positive and negative sequence currents with dual current closed loops. Simulation with Matlab / Simulink shows that the dual current closed loop control method eliminates the DC voltage ripples and ensures the excellent performance of HVDC system under unbalanced voltage condition.