H∞ repetitive voltage control of gridconnected inverters with a frequency adaptive mechanism

A voltage controller is proposed and implemented for grid-connected inverters based on H^∞ and repetitive control techniques. A frequency adaptive mechanism is introduced to improve system performance and to cope with grid frequency variations. The repetitive control, based on the internal model principle, offers excellent performance for voltage tracking, as it can deal with a very large number of harmonics simultaneously. This leads to a very low total harmonic distortion and improved tracking performance. It turns out that the controller can be reduced to a proportional gain cascaded with the internal model (in a re-arranged form), which can be easily implemented in real applications. The proposed controller is experimentally tested to validate its performance, focusing on reducing tracking error and total harmonic distortion, under different scenarios (e.g. in the stand-alone mode or in the grid-connected mode, with or without the frequency adaptive mechanism, with linear or non-linear loads etc).