Multiloop current control for an inductive–capacitive–inductive-filtered grid-connected inverter with frequency-adaptive capability under distorted grid environment

To eliminate the adverse effects of grid voltages such as the harmonic distortion and frequency variation, this study presents a multiloop current control scheme for an inductive-capacitive-inductive-filtered grid-connected inverter. The proposed scheme is achieved by multiloop control structure which is composed of the grid-side current control loop, capacitor voltage control loop, and inverter-side current control loop. This structure has an advantage of seamless mode transfer between the grid-connected mode and islanding mode. In addition, the proposed scheme offers a frequency adaptation capability even under distorted grid environment, providing a high quality of injected current in grid-connected mode. Whereas the conventional schemes require lots of state measurements, the proposed scheme eliminates the need of additional sensing devices by employing a discrete-time state observer. The resonant controllers are implemented in outer grid-side current controller to deal with harmonically distorted grid voltage at the 5th, 7th, 11th, and 13th orders. The proposed scheme ensures the quality of grid currents even in the presence of both the harmonic distortion and frequency change in grid voltages by using only the measurement of the grid-side currents and grid voltages. Simulation and experimental results are given to validate the usefulness of the proposed scheme.