Abstract
Load converters with inertial support are used to enhance the system inertia. However, these converters may fail to operate stably in the weak grid. In this article, sequence impedance modeling and stability comparison analysis are proposed for the load virtual synchronous machine (LVSM) and the load converter with virtual inertia control (LCVIC). First, the universal sequence impedance formulas are derived by considering dc-link voltage dynamics and frequency-coupling effects. Then, the precise sequence impedance models are built for impedance characteristics analysis of LVSM and LCVIC. The analysis shows that the positive-sequence impedance of LVSM is generally inductive in the middle-frequency band, which is basically consistent with the grid impedance. 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 may lead to harmonic oscillation in the weak grid. Furthermore, the effects of grid impedance, load power, and other control parameters on the stability are analyzed for the two load converters based on the derived model and Nyquist stability criterion. The analysis results reveal that LVSM has better stability than LCVIC in the weak grid. Finally, the experimental results validate the correctness of the theoretical analyses.
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