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A Universal Controller for Grid-Forming Inverters in Microgrid during Islanding for Low Transient Current

A universal controller to reduce the transient current during islanding of microgrid is presented in this paper. In the proposed universal controller, two sets of semi-parallel paths are formed. One set produces the PWM references for grid-following (GFL) mode, and the other produces the PWM references for the grid-forming (GFM) mode for the inverters. Herein, the controller enables an inverter to universally operate in GFL and GFM mode, whereas only one set of voltage and phase-angle references is engaged for PWM signal generation. The universal controller switches the voltage and phase-angle references from GFL to GFM mode during islanding of the microgrid. The proposed controller ensures lower transient current by keeping the two sets of semi-parallel paths synchronized throughout the operation of the inverter and enabling a seamless transition between the modes. The proposed controller is verified through hardware experiments under different scenarios in a laboratory-scale microgrid for inverters transitioning from GFL to GFM mode after islanding.