虚拟惯性控制的负荷变换器接入弱电网的序阻抗建模与稳定性分析

Sequence Impedance Modeling and Stability Analysis of Load Converter with Virtual Inertia Control Connected to Weak Grid

负荷虚拟同步机(LVSM)与基于直流侧电容虚拟惯性控制的负荷变换器(DLCVIC-LC)都能增强系统惯性,但也可能在弱电网下发生谐波振荡等交互稳定性问题。该文考虑直流电压控制以及频率耦合的影响,推导三相负荷变换器的序阻抗计算通式,并据此建立LVSM以及DLCVIC-LC的精确序阻抗模型,进而对比分析两者的序阻抗特性。分析发现LVSM的正序阻抗基本呈感性,与电网的阻抗特性基本一致,不易发生谐波振荡问题;而DLCVIC-LC的正序阻抗在中频段呈负阻容性(相位在−90°~−180°),容易在弱电网中引发谐波振荡问题。此外,该文基于所建序阻抗模型和Nyquist稳定判据分析电网阻抗及控制参数对两种负荷变换器并网系统稳定性的影响。最后,通过实验验证了该文分析的正确性。

Both load virtual synchronous machine(LVSM)and load converter based on DC-link capacitor virtual inertia control(DLCVIC-LC)can enhance the power system inertia.However,inevitably,there are some interaction problems like harmonic resonance in weak grids.In this paper,considering the DC-link voltage dynamics and frequency-coupling effects,the general sequence impedance formulas for three-phase load converters were derived.Then,the precise sequence impedance models were built for impedance characteristics comparison analysis of LVSM and DLCVIC-LC.The analysis shows that the positive-sequence impedance of LVSM is generally inductive,which is basically consistent with the grid impedance characteristics and hardly causes harmonic resonance.In contrast,the positive-sequence impedance of LCVIC is negative-resistive-capacitive(i.e.,phase angle is between−180°and−90°)in the middle-frequency band,which is likely to cause harmonic oscillation in weak grids.Furthermore,based on the derived model and Nyquist stability criterion,the effects of grid impedance and other control parameters on the stability of the two load converters connected to the grid were analyzed.Finally,experiments were performed to validate the analysis.