Power Sharing in Three-Level NPC Inverter Based Three-Phase Four-Wire Islanding Microgrids With Unbalanced Loads.

The droop-based control schemes are widely used for power-sharing and control of the isolated microgrids. The conventional droop control scheme with inner current and voltage controllers can work effectively under ideal voltage. However, the existence of unbalance tends its performance to worsen. This paper aims to propose an enhanced droop control method to improve the power-sharing, and maintain the distributed generators' (DGs) terminal voltages and the system frequency under the balanced as well as unbalanced loads. The enhancements of the proposed control scheme include the droop controller, inner current and voltage controller, and virtual impedance loop. The proposed virtual impedance deals with the accurate sharing of powers in the case of unequal line impedance and the control of zero sequence current. Moreover, to enable the proposed control to compensate for the unbalance, three-level neutral point clamped (NPC) inverters are used to form a three-phase four-wire microgrid. With this control scheme, the voltage unbalance factors (VUFs) of the DGs' terminal voltages are reduced from 4% to less than 1%, negative sequence reactive powers, which are also an indication of unbalance, have been reduced significantly, and the system frequency is maintained within the standard permissible limit of 2.5%. Furthermore, a local load controller is also proposed to control the reactive loading of the DGs when they have local loads supplied from their terminals. The investigations have been carried out under simulation as well as lab-based environments to validate the efficacy of the proposed control method.