希尔排序算法优化偏差控制的MMC电容均压模型

Capacitor Voltage Balancing Model for Modular Multilevel Converter Using Shell Sorting Algorithm and Bias Control

模块化多电平换流器MMC(modular multilevel converter)子模块均压控制问题一直是高压直流输电系统能否稳定运行的关键。当子模块数目较为庞大时,传统基于排序算法的均压模型会使得电容电压排序计算量加大,占用大量资源,为硬件的设计带来巨大困难。为此,设计了一种希尔排序算法优化偏差控制的MMC电容均压模型。通过对当前与上一个周期中所投入子模块的数量及各子模块电容电压与其平均值的偏差是否超过界限,判断是否再次计算触发脉冲,偏差的大小决定子模块是否投入,利用希尔排序算法对桥臂处于不同状态时满足相应判定条件的子模块进行排序,按照排列后的顺序依次投入,保证了系统的高效可靠运行。在PSCAD/EMTDC上搭建模块化多电平换流器的仿真模型,验证所提出的子模块均压控制模型的正确性。仿真结果验证了该控制模型在低频下能够很好地实现子模块电容电压的均衡控制。

The voltage balancing problem of sub-modules(SMs)in a modular multilevel converter(MMC)is the key to the stable operation of high voltage direct current(HVDC)transmission system.However,when the number of SMs is ex-cessive,the voltage balancing model based on the traditional sorting algorithm will lead to a large amount of computation load when sorting the capacitor voltages and thus occupies numerous resources,which poses a challenge to the design of hardware.To this end,an improved capacitor voltage balancing model for MMC is designed based on Shell sorting algorithm and bias control.To determine whether to recalculate the trigger pulse or not,the numbers of on-state SMs in the previous and present control cycles are compared,together with the detection of deviation between capacitor voltages of SMs and their average value.In addition,the value of deviation determines whether the SMs should be put into operation or not.The Shell sorting algorithm is used to sort the SMs that satisfy the corresponding decision criteria when the bridge arm is in different states,and the SMs are input according to the sequence after arrangement,thus ensuring the efficient and reliable operation of the system.An MMC model was set up in PSCAD/EMTDC,and the validity of the voltage balancing control model for SMs was verified.Simulation results showed that this control model can effectively realize the balancing control of capacitor voltage of SMs at a low frequency.