Design and implementation of a virtual capacitor based DC current suppression method for grid-connected inverters

Integration of renewable energy (RE) sources, such as wind energy and photo voltaic (PV) energy, to a power network (grid) is usually achieved through an intermediate power electronic inverter. Ideally, the inverter is not expected to inject any form of DC component into the grid. However, this is not the case in practice as DC component are invariably generated and, subsequently, injected into the grid by the power converter, impacting on the overall power quality. To mitigate this problem, the International Grid Certification stipulates the maximum extent of DC component that can be injected into the grid current. Thus, various techniques have been proposed to maintain the DC component within the stipulated limit, however these techniques have the drawbacks of complicated control algorithm, extra power losses, and increased high costs. To solve these problems, this paper proposes a virtual-capacitor based DC current suppression control technique for grid-connected inverters, which has the advantages of fast implementation and good DC component suppression performance in utility. An LCL filter interfaced 3-phase inverter is used and virtual capacitors are incorporated into the inverter which connects the inverter with the grid. The allowable region and design methods of the inverter controller parameters which are combined with the active damping and grid voltage feedforward strategies are described in detail. Furthermore, both simulation and experimental results show that the DC component can be successfully mitigated and could satisfy the International Grid Certification standards.