Controllable Harmonic Current Sharing in Islanded Microgrids: DG Units With Programmable Resistive Behavior Toward Harmonics

In order to obtain a coordinated approach to integrate the increasing presence of distributed generation (DG) units in the electric power system, the microgrid concept has been introduced. Since most DG units use a converter as an interface with the grid, new control strategies for these converters are being developed. For the islanded mode of the microgrid, with the voltage-based droop control strategy, active and reactive power balancing and sharing between multiple DG units are achieved. This method also makes it easy to delay changing the output power of the renewable DG units compared to that of the dispatchable DG units without communication. However, to deal with harmonic and nonlinear loads, the power control strategies in general, with the voltage-based control strategy as a specific example, need to be modified. Otherwise, the grid-forming DG units form short circuits for harmonic currents. Therefore, in this paper, the shunt harmonic impedance method for harmonic damping in the grid-connected mode is adapted for application in islanded microgrids. The voltage-based droop control strategy of the DG units is extended with programmable resistive behavior toward harmonics. In this way, harmonic current sharing between DG units can be achieved in a controllable manner (e.g., according to the ratings of the units).