交流背景谐波对MMC多端直流的影响分析及抑制策略

连接弱交流系统是基于模块化多电平换流器(MMC)的多端直流输电系统的重要应用场景之一。然而,弱交流系统更易受到谐波扰动的影响,该交流系统中的非线性设备可能形成总谐波畸变率偏高的背景谐波电压,这些背景谐波电压将在MMC的三相桥臂中产生正序、负序及零序电压分量及相应的电流分量。在一些极端情形下,某些频率的谐波将会引起直流网络的谐振,导致直流网络的谐波含量显著增大,并通过直流网络传播到其他互联的交流系统中。因此,本文首先从理论上分析了交流系统电压背景谐波对MMC桥臂电压的影响,获得了谐波传递规律;在此基础上,设计了利用MMC子模块储能能力的抑制直流侧谐波的附加控制策略。在PSCAD/EMTDC中的仿真表明,本文所提出的控制策略能有效阻断背景谐波向直流网络的传播,避免直流网络由于背景谐波产生谐振现象。

Impact of AC background harmonics on MMC multi-terminal HVDC system and mitigation strategy

One of the possible applications of modular multilevel converter ( MMC ) based multi-terminal HVDC ( MTDC) is to interconnect weak AC systems which may be more prone to harmonic disturbance. This paper inves-tigates the potential resonance of the MMC-based MTDC excited by the AC background harmonics and proposes a control strategy to eliminate the resonance. Firstly, the harmonic transfer rule across one MMC is revealed based on the phase-leg voltage analysis;then the equivalent circuit of the MMC-based MTDC deriving from the transfer rule is studied to illustrate how the resonance is excited by the AC background harmonics; lastly, a control strategy u-sing the MMC' s energy storage capability is proposed in this paper to avoid the resonance of the MMC-based MTDC from being excited by AC background harmonics. To verify the analytical results and the effectiveness of the pro-posed control strategy, a 5-terminal MMC-based MTDC system is modeled and simulated in PSCAD/EMTDC. The simulation results accord with the analytical results precisely and the proposed control strategy functions effectively.