基于电容能量的模块化多电平换流器次同步振荡抑制及其阻抗分析

针对模块化多电平换流器(modular multilevel converter,MMC)易与风机和弱电网相互作用导致次同步振荡的情况,文中提出一种基于MMC电容能量控制的次同步振荡抑制方法。该方法通过控制环流的零序分量来调制MMC中储存的电容能量,使得电容能量根据公共连接点频率对次同步振荡进行阻尼。基于谐波状态空间理论建立考虑能量控制的MMC序阻抗模型,从交流侧阻抗角度分析能量控制对MMC动态的影响,并研究MMC与风机和弱电网相互作用导致次同步振荡的机理。分析结果表明,能量阻尼控制抑制了MMC阻抗中次同步频段的谐振峰;风电出力过大或电网短路比过小时,MMC易出现次同步振荡;在MMC采取定交流电压与定功率控制两种情形下,文中提出的能量阻尼控制都能有效阻尼次同步振荡。此外,理论阻抗建模及控制器性能都在电磁暂态仿真中得到验证。

Sub-synchronous Oscillation Suppression of Modular Multilevel Converter Based on Capacitance Energy and Its Impedance Analysis

Aiming at the situation that modular multilevel converter(MMC) easily interacts with wind turbines and weak grids to cause sub-synchronous oscillation, a sub-synchronous oscillation suppressing method of MMC based on capacitance energy control was proposed. This method modulates the capacitor energy stored in MMC by controlling the zero-sequence component of the circulating current, so that the capacitor energy damps the sub-synchronous oscillation according to the frequency of the common connection point. An MMC sequence impedance model considering energy control was established based on the theory of harmonic state-space. The influence of energy control on MMC dynamic characteristics was analyzed from the perspective of AC-side impedance, and the mechanism of sub-synchronous oscillation caused by the interaction of MMC with wind turbine and weak power grid was studied. The analysis results show that energy damping control suppresses the resonance peaks around the sub-synchronous band of MMC impedance. When the output power of wind is too large or the grid short circuit ratio is too small, the MMC is prone to sub-synchronous oscillation. In the case of MMC adopting constant AC voltage and constant power control, the energy damping control method can effectively damp the sub-synchronous oscillation. In addition, theoretical impedance modeling and controller performance were verified in electromagnetic transient simulations.