基于离散模型的MMC环流抑制控制器的设计

Control of Circulating Current Suppressing Based on Discrete Model for Modular Multilevel Converter

模块化多电平换流器内部三相环流直接影响MMC的运行特性和系统损耗,为了抑制MMC的内部三相环流,本设计在MMC内部连续数学模型的基础上,采用二倍频负序坐标变换将换流器内部的三相环流分解为d-q旋转坐标系下的直流分量,利用欧拉近似公式建立了MMC内部离散数学模型;在此基础上设计了由坐标变换、离散调节器及反坐标变换环节构成的环流抑制控制器,得到内部环流压降,协同载波移相调制策略,有效抑制了MMC的二倍频环流,减少了桥臂电流的畸变程度,使其逼近正弦波,同时还降低了子模块电容电压的波动幅度;在PSCAD/EMTDC环境下搭建了21电平MMC-HVDC仿真模型,仿真结果表明:本设计中的控制器在不改变MMC外部特性的前提下使二倍频环流的幅值降低了84.27%,能有效抑制MMC内部的二倍频环流,有利于MMC的稳定运行,便于工程实际应用。

The performance and power loss of modular multilevel converters(MMC) was directly affected by inner circulating current. The internal circulation three- phase of MMC is decomposed into the dc component of d- q rotating coordinate system by using double frequency negative sequence coordinate transformation,based on MMC internal continuous mathematical model to restrain it. By using the Euler approximation formula, an inner discrete mathematical model of MMC is established. Based on this, the circulation inhibition of controller is consisted of rotational frame, discrete adjustor and inverse rotational frame is designed. Meanwhile, the carrier phase shifted- pulse width modulation control is adopted. The circulating current suppressing controller(CCSC) can minimize the peak value and the distortion degree of the arm current significantly,and reduce the degree of sub- module capacitor voltage unbalance. Finally,the 21- level MMC- HVDC model is built under the PSCAD/EMTDC.The simulation results show that the CCSC decrease the inner circulating current of MMC about 84.27% without affecting the AC side dynamics of MMC, which is benefit for improving the stability and reliability of MMC.