基于并网电流谐波微分的有源阻尼策略

Research on Active Damping Strategy Based on the Differentiation of Injected Grid Current Harmonics

采用有源阻尼抑制弱电网下并网变流器系统失稳振荡的方法近年来受到广泛关注。已有的直接以公共耦合点(PCC)电压为反馈量的有源阻尼方法可有效抑制系统的失稳振荡,但会削弱系统对电网背景谐波电压扰动的抑制能力,进而影响到并网电流质量。为此,该文提出一种基于并网电流谐波微分的有源阻尼策略,通过微分运算将系统失稳振荡引发的谐振电流转换成PCC处的谐振电压,避免了将电网背景谐波电压扰动引入到并网电流参考值,可兼顾弱电网下并网变流器系统稳定控制和高质量并网电流两方面需要。仿真和实验结果验证了该文中理论分析的正确性和所提控制策略的有效性。

The active damping strategy that simulates a virtual resistor at the point of common coupling(PCC)is effective to improve the converter stability under weak grid.However,in the traditional strategy,the PCC voltage is generally fed to the current reference by multiplying a gain.Naturally,the disturbances of grid background harmonic voltages will be also introduced to the current reference,and the unexpected harmonics will arise in the current.Thus,the traditional strategy will weaken the rejection abilities of the converter on the background harmonic voltages and reduce the current quality.To solve these issues,this paper proposes an active damping strategy based on the differentiation of injected grid current harmonics,which can ensure the system stability and the current quality simultaneously.First,the control structure of the active damping strategy based on the differentiation of injected grid current harmonics is given,in which the differential operation is used to transfer the harmonic currents to the harmonic voltages.This way,only the harmonic voltages caused by the unstable resonance will be fed to the output current reference,and the grid background harmonic voltages can be excluded.Therefore,the disturbances in the output current reference from grid background harmonic voltages can be eliminated and the output current quality will not be influenced by the active damping control.In addition,the rejection abilities of the converter on the background harmonic voltage disturbances before and after applying the proposed active damping strategy are compared.Simulation and experimental results show that,with the proposed active damping strategy,the minimum short circuit ratio(SCR)for the grid-connected converter system to operate stably is reduced from 20 to 2.9,which indicates that the proposed method can markedly improve the adaptability of the converter to weak grid condition.Moreover,under a distorted grid voltage condition,the comparison of current total harmonic distortion(THD)with the different active damping methods shows that,the current THDs are 2.18%(simulation result)and 2.11%(experimental result)with the proposed method,while the current THDs are 3.3%(simulation result)and 3.0(experimental result)with the conventional method.This demonstrates that the proposed active damping method can obtain higher current quality compared to the conventional method.This is because the rejection abilities of the converter on the background harmonic voltage disturbances can remain unchanged when the proposed active damping method is used.The following conclusions can be drawn from the simulation and experimental analysis:(1)If the grid impedance is not considered,the larger grid inductance of weak grid may affect the converter stability and even to destabilize the system.(2)The active damping strategy based on PCC voltage feedback can improve the stability of the converter under weak grid.However,it will introduce the disturbances of background harmonic voltages to the current reference and weaken the rejection abilities of the converter on these disturbances,thereby reducing the current quality.(3)For the active damping strategy based on the differentiation of injected grid current harmonics,the differential operation is used to transfer the resonant current caused by the instability to the resonant voltage at PCC.Therefore,the disturbances in the current reference from background harmonic voltages can be avoided.The proposed method can realize the active damping performance,and meanwhile,the rejection abilities of the converter on the background harmonic voltages will not be weakened.Both the stability control of the converter system under weak grid and the high quality of grid-injected current can be satisfied.