Harmonic Stability Analysis and Harmonic Suppression Strategy for Grid-Forming Converter with Dual-Loop Control

With the increasing proportion of renewable energy in the power grid, the stability of the power system is facing challenges. Compared to grid-following converters, grid-forming (GFM) converters have voltage source characteristics, which can improve the support capability of power electronic equipment and enhance the security level of the power grid. When grid-forming converter adopts vector voltage and current dual closed-loop control, due to the delay effect of digital control, the output impedance of the converter presents negative damping characteristics at high frequencies, which may lead to harmonic oscillation after being connected to the power grid. In response to this issue, this paper studies the harmonic stability of grid-forming converters. Firstly, the control methodology of grid-forming converter is studied. Secondly, impedance model of grid-forming converter is built, and the impedance analysis method for harmonic stability analysis is studied. Then, a harmonic suppression strategy based on capacitive current feedback is proposed. Finally, the effectiveness of the harmonic suppression strategy based on capacitor current feedback is simulated and verified.

I. Introduction

In recent years, with the rapid development of new energy generation such as photovoltaic and wind power, the proportion of new energy in the power grid has gradually increased. New energy units, represented by wind power and photovoltaic, are mostly connected to the grid through converters^[1], which have problems such as high power fluctuations, high reactive power compensation, and instability when connected to weak power grid. This has led to a continuous decline in power grid strength, posing a huge challenge to the stable operation of traditional grid following converters.