Mixed-frame and stationary-frame repetitive control schemes for compensating typical load and grid harmonics

In this study, repetitive current controllers operating in either the mixed or stationary frame are proposed for high-precision reference tracking and disturbance rejection of power converters. Both controllers use a proportional-resonant regulator in the stationary frame for regulating the positive-and negative-sequence fundamental currents, which are known to directly influence the flow of active and reactive power in most energy conversion systems. Moreover, for the tracking or compensation of harmonics, the controllers include a repetitive control path in either the synchronous or stationary frame, whose inherent feedback and feedforward structure is proven to resemble a bank of resonant filters in either reference frames. Unlike other existing controllers, the proposed repetitive controllers function by introducing multiple resonant peaks at only those harmonic frequencies typically produced by most non-linear loads, with no high-resonant gain introduced at all other non-characteristic components. Hence, they are expected to be more robust and less sensitive to implementation noises. By performing synchronous transformation using an unfiltered angle or by using its equivalent frequency-shifted variant, the tracking abilities of the controllers are also shown to remain undisturbed even if a distorted grid or back-emf source is intentionally connected to the system. These described performance features, together with the controller simple implementations, were physically tested with a digitally controlled commercial converter.