Sideband-Harmonic Instability of Paralleled Inverters with Asynchronous Carriers

Paralleled inverters with asynchronous carriers are increasingly emerging into renewable power plants and other power-electronic-based power systems. The sideband harmonics resulting from the pulse width modulation tend to be different from one inverter to another, which may trigger sideband-harmonic resonances among the paralleled inverters. To address the sideband-harmonic instability, the nonlinearity of the digital pulse width modulator is analyzed first in this paper based on the harmonic balance principle. It is revealed that the dynamic sideband components corresponding to a given small-signal perturbation can be far below the Nyquist frequency, and they may not be effectively attenuated by anti-aliasing filters. A multi-frequency impedance model is then developed for the inverter, which characterizes the dynamic coupling between the perturbed frequency component and the sideband frequency component. The impact of asynchronous carries on the sideband-harmonic instability of paralleled inverters can thus be analyzed, and the frequency and phase sequence of the resulting oscillation components are predicted. Experimental results of two paralleled inverters confirm the effectiveness of the developed model and the stability analysis.