提高LCL型并网逆变器鲁棒性的电容电流即时反馈有源阻尼方法

A Capacitor-current Real-time Feedback Active Damping Method for Improving Robustness of the LCL-type Grid-connected Inverter

LCL型并网逆变器对高频谐波的衰减效果显著,但是存在谐振问题。电容电流反馈有源阻尼可有效阻尼LCL滤波器的谐振峰,在模拟控制下,它等效为在滤波电容上并联一个电阻。然而,采用数字控制时,控制延时使其不再等效为一个电阻,而是一个与频率相关的阻抗。并且在高于1/6的采样频率时,该等效阻抗表现出负阻特性,当谐振频率高于1/6的采样频率时,负阻会降低系统对电网阻抗的鲁棒性。特别地,当谐振频率等于1/6的采样频率时,系统无法稳定。为此,该文提出电容电流即时采样方法,以减小电容电流反馈有源阻尼的控制延时,使其更接近于滤波电容并联电阻的特性。这样,不仅提高了系统的鲁棒性,而且,即使谐振频率等于1/6的采样频率,系统也具有很好的稳定性。以单相LCL型并网逆变器为例,进行实验验证,实验结果证明了所提出的电容电流即时采样方法是有效的。

The grid-connected inverter with an LCL filter has the ability of attenuating the switching frequency current harmonics.However,the resonance hazard of the LCL filter requires proper damping solutions.As an effective method to damp the LCL-filter resonance,capacitor-current-feedback active damping is equivalent to a resistor in parallel with the filter capacitor in analog control.However,due to the control delay introduced by digital control,capacitor-current-feedback active damping exhibits the characteristic of negative resistor at the frequency range higher than one sixth of sampling frequency.As a consequence,the current loop yields a poor robustness against grid-impedance variation if the resonance frequency is higher than one sixth of sampling frequency.Especially,if the resonance frequency is equal to one sixth of sampling frequency,system will be unstable all the time.To overcome these issues,capacitor-current real-time feedback with reduced control delay is proposed in this paper.In doing so,the characteristic of negative resistor is eliminated,and the current loop has satisfactory performance while dealing with the wide range grid-impedance variation.Moreover,a stable operation is obtained even if the resonance frequency is equal to one sixth of sampling frequency.Experimental results from an LCL-type single-phase grid-connected inverter confirm the theoretical expectations.