This paper presents a multirate fractional-order repetitive control (MRFORC) of three-phase shunt active power filter (APF) suitable for microgrid applications. The proposed APF control scheme includes an inner proportional-integral control loop with a sampling rate identical to switching frequency and an external plug-in repetitive control (RC) loop with a reduced sampling rate. The MRFORC loop is implemented in the dq-frame with interleaving for the reduction of computational burden in each control cycle. Moreover, in order to deal with the harmonics in the presence of wide grid frequency variations, FORC, which replaces the fractional-order elements with the Lagrange-interpolation-based finite impulse response filter, is adopted instead of the conventional RC. The synthesis, stability analysis, and parameter tuning criteria of the MRFORC system are derived in detail. A step-by-step design example of the proposed controller is also given in this paper. Finally, experiments are performed to validate the feasibility and effectiveness of the proposed scheme.

Resonant controllers (RSCs) are widely adopted for controlling power converters since they can track AC signals of both positive and negative sequences without steady-state error. However, the performance of RSCs has not been fully exploited due to the improper phase compensation angle and insufficient controller gain. In this paper, a novel controller parameters design method, which is based on the system error transfer function, is proposed to further explore the potential high performance of RSCs. Comparing with the conventional methods, more appropriate phase compensation angles can be obtained, which means that the stable region of the controller gain is extended. Since the proposed RSCs design method is able to maximize the controller gain, the controller sensitivity to system frequency variations can be decreased and the system response speed can be improved. The RSCs tuning procedure is given in detail, and comparative experiments between the proposed method and the conventional method are performed to validate the superiority of the proposed method.

Resonant controller (RSC) is one of the most popular approaches for ac current/voltage control due to the high performance of zero steady-state tracking error. However, it suffers from heavy computational burden issue when multiple RSCs are demanded to control multiple harmonics. To alleviate this issue, a down-sampled multirate RSCs (MRRSCs) scheme is proposed for controlling power inverters, e.g., grid-connected inverters and active power filters. The proposed control scheme is composed of an inner control loop with a fast sampling rate, which is identical to the switching frequency, and an array of paralleled MRRSCs-based external control loop with a reduced sampling rate. With the reduced sample rate, the MRRSCs can be executed alternatively in the dq -frame, such that the computational burden can be significantly reduced per cycle. Moreover, to reinforce the wide frequency adaptability of the MRRSC, its central resonant frequency is online updated according to the output of the phase-locked loop. In the paper, the equivalent single-rate closed-loop model of the overall system is developed, based on which the controller parameters are designed employing the Nyquist diagram and root locus. Finally, experiments are performed on a grid-connected inverter to validate the superiority of the proposed scheme.

Harmonics have been considered as one of the major issues in modern power grids. Considering the high penetration level of power electronic converter interfaced distributed generation (DG) units, it is of interest to provide ancillary services through DG interfacing converters, such as harmonic compensation. In that case, multiple DG interfacing converters are utilized to compensate harmonics, and the compensation effort should be properly shared among these converters. However, it is rarely considered in existing literatures that converters operating in different modes, such as voltage-controlled mode (VCM) and current-controlled mode (CCM), need to cooperatively provide the compensation function. Aiming at this objective, this paper proposes a unified voltage harmonic mitigation strategy for VCM and CCM converters with high harmonic current sharing accuracy. Another advantage of the proposal is that the grid-side and load-side current measurements are avoided, which reduces the implementation cost. Experimental results are presented to demonstrate the effectiveness of the method.

Harmonics have been considered as one of the major issues in future power grids. With the increasing demand in advanced control functions, power electronic converter interfaced distributed generators (DGs) are expected to perform harmonic compensation when necessary. It has been demonstrated in a number of studies that DG converters operating in voltage-controlled mode can be easily configured to realize voltage harmonic compensation utilizing naturally embedded voltage control loop. While for DGs operating in current-controlled mode (CCM), such function was rarely studied. Considering that CCM is commonly used in renewable energy based generators and energy storage systems, it has certain significance to achieve the same function with CCM converters. Aiming at such objectives, this paper proposes a voltage detection based harmonic compensator (HC) for CCM converters. The novelty and main advantages of the proposed method include the following. It realizes seamless interface of HC with inner fundamental current control loop. Compared with a conventional active power filtering method, it does not require remote load harmonic current measurement since it is local voltage detection based. Compared with a conventional voltage detection based method, it offers better performance because of directly harmonic voltage regulation. Experimental results are presented to demonstrate the effectiveness of the method.

Current control is a crucial issue of the operation of grid-connected voltage source converters (VSCs). The challenge of the current control lies in how to track the current reference in a fast and accurate way. A second-order generalized integrators (SOGIs)-based current controller is one of the highly popular ways to achieve these objectives since they can track ac signals of both positive and negative frequencies/sequences without steady-state error. However, this may lead to extra unnecessary computational burden when SOGIs are applied in three-phase systems. Therefore, a reduced order generalized integrators (ROGIs)-based current controller is proposed to reduce the computational burden. In this paper, a phase compensation embedded ROGI is proposed, which can not only retain the advantage of a less computational burden of the conventional ROGI compared with the SOGI, but also more importantly, further improve the harmonic control bandwidth and frequency adaptivity in comparison with the SOGI or conventional ROGI. In addition, the derivation of the phase compensated ROGI is provided in the paper along with its parameters tuning strategy analysis. A case study regarding an LCL -filtered three-phase grid-connected VSC with auxiliary harmonic compensation capability is given in detail. Finally, the experimental results are presented to validate the superiority of the proposed method.