Near-Synchornous Resonance Interaction of Paralleled Grid-forming Converters in Islanded Operation

The natural synchronous resonance issue is one of the key features of grid-forming converters. It is caused by the frequency-shifting nature of power calculation when adopting power based synchronization mechanism. This paper is a report of a 33kW power-hardware-in-the-loop test, whose results show that not only synchronous resonance but also near-synchronous resonance phenomenon can occur due to interaction between the active power control loops of grid-forming converters when operating in islanded mode. In addition to that, the test results also show that the different resonance modes are highly related to the cutting frequency design of the low-pass filter used in the power calculation process. In this paper, analytical model is established to figure out the root cause of such interaction. A preliminary design guideline for the filter design is also proposed to avoid such unstable interaction.