A stabilizer for suppressing harmonic resonance in multi-parallel inverter system

In the paper a simple stability-equivalent single- inverter system is proposed to analyze the stability of the complex multi-inverter system. Investigation shows that the harmonic resonance even instabilities will be triggered with the increase of the paralleled inverter quantities or the grid inductance due to the interaction between the inverter admittance and the grid admittance. From the perspective of reshaping the admittance, a stabilizer based on the rectifier is proposed to improve the system stability. To both offset the capacitive characteristics of the inverter admittance and the inductive characteristics of the grid admittance the rectifier connected to the point of the common coupling (PCC) is controlled as a resistive-inductive admittance within the current controller bandwidth. With the equivalent shunt admittance, the phase of the admittance seen by the power grid is much lower than 90° in a wide frequency range, and the phase of the admittance seen by the inverters is much higher than −90° in the vicinity of their intersection frequency which is constrained at the high frequency. After the stabilizer is added the system can keep sufficient phase margins as the grid impedance varies in a wide frequency range. Simulation results have verified the performance of the proposed stabilizer for suppressing the harmonic resonance.