Voltage Control of Microgrid Systems Based on 3L-NPC Inverters with LCL-Filter in Islanding Operation

In microgrid systems one or more inverters should be grid-forming in order to control the voltage during island mode operation. Usually the voltage regulation is operated controlling the capacitor voltage of the LCL filter interconnected between the converter and the load. In the present paper a cascade control is proposed for a Three-Level Neutral Point Clamped (3L-NPC) inverter where the voltage and the current at the output of the LCL filter are used as state variables instead of the capacitor voltage and the converter-side current. Instability issues inherent in the control of the filter output voltage are solved using a Virtual Resistor (VR) active damping method. The proposed control strategy allows loads voltage control eliminating the error due to the voltage drop on load-side inductance of the LCL filter. Since the filter output voltage and current measurements are required anyway, in order to perform the active and reactive power control based on droop control techniques, the proposed control scheme provides also a number reduction of the measurements sensors compared with the classical implementation of the hierarchical control. Simulation results, considering two different operating scenarios, validate the proposed control strategy.