For LCL-type grid-connected inverters, inverter-side current, grid-side current, capacitor current and voltage are four commonly used variables in the control serving as different roles, such as control objective, active damping, synchronization, etc. This article builds a general admittance model, in which the total output admittance is decomposed into four paralleled subadmittance corresponding to the above four variables in the control law, to facilitate the passivity-based stability assessment and controller parameter optimization for different control objectives and schemes. Based on the passivity analysis, an appropriate combination scheme of subadmittance and a simple unified analytical controller parameter design method are proposed to obtain a passive output admittance from 0 Hz up to Nyquist frequency for either inverter-side or grid-side current control. Therefore, it can not only ensure the stable operation of LCL-type inverters regardless of grid impedance (either inductive or capacitive) but also avoid the conventional design constraints of an LCL filter, i.e., f _r <; or > f _s /6 is required for the single-loop inverter-side or the grid-side current control. Finally, experiments verify the theoretical results.

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.

This paper proposes a triangle carrier based discontinuous PWM (TCB-DPWM) implement method for three-level neutral-point-clamped (3L-NPC) inverters. The function equivalent relationship of proposed TCB-DPWM and space vector based discontinuous PWM (SVB-DPWM) is mathematically analyzed in this paper. The proposed TCB-DPWM method effectively simplified the implementation of the SVB-DPWM by injecting twice common mode voltages (CMVs) to original modulation signals, where different DPWM methods can be easily obtained in a unified scheme by setting different values of proportional allocation factor for small vectors in different sectors. In addition, a TCB-DPWM based neutral point voltage balancing method is investigated to demonstrate its application. The polarity of the CMV injection is regulated according to the polarity of the proportional allocation factor to realize the neutral point voltage balancing. Finally, the proposed TCB-DPWM and neutral point voltage balancing method are both verified by simulation and experimental results. The results show that the proposed TCB-DPWM can effectively simplify the implementation of modulation and improve system efficiency.

The frequency-domain passivity theory offers an effective way to assess the stability of inverters connecting to complex grid conditions, e.g., inductive grid, capacitive grid, and system comprising multiple inverters. In this article, an admittance model for the grid-side current-controlled LCL-type inverter with capacitor voltage feedforward active damping (CVF-AD) is built to facilitate the passivity-based stability assessment and controller parameter design. Especially, it is found that if the antiresonance frequency of the inverter-side inductor and the filter capacitor is designed in a certain region, i.e., (0.66f _c + 0.034f ₈ , 0.22f ₈ ), all frequencies' passive output admittance of the inverter can be achieved via a modified CVF-AD scheme with simple digital implementation. Thanks to the proposed methods, the inverter can be connected to the grid regardless of the grid impedance, which means a robust plug-and-play functionality suitable for microgrid applications. In addition, the controller parameters are analytically designed; time-consuming iterations are, hence, avoided. The allowable tolerance of power circuit parameters for a valid controller parameter design is as high as 64% or more. The validity of the theoretical analysis and effectiveness of the proposed method are verified by using experimental results on a laboratory prototype.

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.