Simultaneous Voltage and Current Harmonic Control of Grid-Forming Converters Considering Both Islanded and Grid-Connected Modes

As a promising technology, grid-forming (GFM) converters can provide voltage and frequency support in grid-connected mode, and power local loads in islanded mode. Multiple GFM converters connected to the same point of common coupling (PCC) should not only ensure harmonic current sharing, but also reduce the PCC voltage harmonics. Most of the existing harmonic control of GFM converters do not consider both islanded and grid-connected modes. Moreover, these methods need to extract and control each harmonic sequence separately, resulting in enormous calculation. To solve this problem, a harmonic control method for GFM converters based on unified harmonic voltage-current droop is proposed. By establishing a unified droop relation between all harmonic components of PCC voltage and output current, we manage to share harmonic current while reducing PCC voltage harmonics. The proposed method is applicable in both grid-connected and islanded modes, without extracting individual harmonic sequences. Compared with existing control methods, it requires less computation burden. The design of harmonic control parameters based on closed-loop pole analysis is discussed in detail. Experimental results show that with limited computation resources, the proposed control achieves superior harmonic control effects in both grid-connected and islanded modes than existing methods.