An ac/dc/ac power converter is an important device used to extract power from variable speed permanent magnet wind generators and feed it into the grid. This paper describes how these converters incorporate maximum power point tracking based on its power feed to the grid at different wind speeds. Using the permanent magnet generator voltage, grid current, and grid voltage samples, the proposed system achieves an enhanced dynamic behavior. This feature effectively prevents the grid from "boost" charging the dc side of the H-bridge inverter at the start of operation. Since small wind turbines normally do not have expensive pitch control mechanisms, a thyristor-based "dump-load circuit" is employed to protect the turbine from high wind speed operation when disconnected from the grid. The thyristor controller also protects the inverter from high dc voltage input from the wind generator at high wind speed. Preliminary results are included using a laboratory 2-kW prototype converter.

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This paper proposes a passive islanding detection method based on computing a set of frequency dependent reference impedance magnitudes and comparing them with the measured impedance magnitudes at the point of common coupling (PCC). The reference impedance may be identified online over a selected frequency band from the measured voltage and current at PCC. Islanding is detected when the magnitude differences between reference impedance and measured impedance over the chosen frequency band are smaller than a preset threshold. Robustness of the proposed method is verified in a distribution network, the method avoids misdiagnosing islanding condition as normal condition. Performance analyses are discussed from two perspectives of non-detection zone (NDZ) and fault detection zone (FDZ). By combining the under voltage/over voltage (UV/OV) and under frequency/over frequency (UF/OF) methods with the proposed method, the NDZs of UV/OV and UF/OF methods can be reduced.

Grid harmonic interactions due to aggregated flyback micro-inverters are investigated in this study. An impedancebased model based on the Norton model of flyback micro-inverters, which are with quasi-resonant peak-current control, is obtained by adopting the small-signal modelling approach. As a supplement to the existing output impedance modelling of string inverters with linear controllers, further study on micro-inverters under non-linear control is addressed in this study. Based on the derived impedance model, the admittance matrix of aggregated micro-inverters connected to the grid is formulated and the impedance-based analysis of the system stability is also presented. Consequently, by modelling the output admittance of the quasi-resonant peak-current controlled flyback micro-inverters, harmonic interactions with a distorted grid can be effectively forecasted. Results obtained from modelling, analysis and verifications have shown that the proposed method is a simple and valid way of dealing with the harmonic quasi-resonance problems.

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Energy storage as an alternative solution for integrating renewable energy into grid has been studied recently. Vanadium Redox Battery (VRB) has been received much attention for its excellent characteristics, especially for large capacity energy storage. This paper focuses on the structure, modeling and control of VRB energy storage system. To cooperate with large scale wind farm /PV station, the structure for large capacity battery energy storage system (BESS), including configuration site, battery system and power condition system (PCS), is discussed. The equivalent circuit model of VRB, concerning of electrical, physical and chemical factors, is established, and also the PCS. The PQ decoupled scheme is adopted to control active power and reactive power independently, and the zero-sequence circulating current control scheme is employed for parallel converters .To coordinate the operation of battery system with PCS, the battery-based restriction control and multiple operation modes are proposed. The simulation results with MATLAB/Simulink are presented to verify the model and control schemes.

Inverters with distributed storage(DS) play a very important role in a microgrid. A new kind of inverter called virtual synchronous generator(VSG) is proposed in this paper to emulate the behavior of synchronous generator(SG) in the utility. The core algorithm of VSG is to employ an electromechanical transient mathematic model of round SG. There are two important operation modes in microgrid: active power/reactive power control (PQ control) and voltage/frequency control(Vf control). So a governor and an excitation controller are developed to fulfill these two functions. If Vf control is selected in islanded operation, the objective of governor is to keep frequency in an acceptable range by adjusting the input active power to match the output power. The excitation controller is to maintain the output voltage of VSG by controlling the excitation current; If PQ mode is selected in grid-connected mode, active power could follow the dispatch command very well by the governor. But a Q controller should be developed to control the reactive power by adjusting the voltage reference of excitation controller. Additionally, two kinds of PQ control are discussed here: (1) unit output power flow control (UPC) and (2) feeder power flow control (FFC). Finally, the proposed algorithm and control methods are verified by a single VSG experiment.

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