A novel repetitive controller directly combined with an open loop SPWM inverter is presented in this paper. To cope with the high-resonant peak of the open loop inverter that may cause instability, a zero-phase-shift notch filter other than the inverse transfer function of the inverter or a conventional second-order filter is incorporated in the controller. The proposed method has good harmonic rejection and large tolerance to parameter variations. To further reduce the steady-state error, low-pass-filter ( ) algorithm is applied. DC bias problem is also taken into consideration and solved with the repetitive controller itself. The method is implemented with a digital signal processor and achieves low THD% (1.4%-1.7%) with nonlinear loads and fast error convergence (3-5 fundamental periods). It proves to be a cost-effective solution for common UPS products where high-quality output voltage is more stressed than fast dynamic response.

This paper presents a control strategy to improve the low-voltage ride-through capability of a doubly fed induction generator (DFIG); since the stator of a DFIG is directly connected to a grid, this sort of machine is very sensitive to grid disturbance. Grid voltage sag causes overcurrents and overvoltages in rotor windings, which can damage the rotor-side converter (RSC). In order to protect the RSC, a classical solution based on installation of the so-called crowbar is adopted; however, as the DFIG absorbs reactive power from the grid, this type of solution deteriorates grid voltage sags and cannot meet the requirements of a new grid code. An improved control strategy which uses virtual resistance to limit rotor side overcurrents is proposed in this paper, which can make a crowbar inactive and supply reactive power to fulfill the latest grid code requirement during voltage sags. In order to validate the proposed strategy, simulations and experiments have been carried out, and the results demonstrate the effectiveness of the proposed strategy.

Abstract-Maximum power point tracking (MPPT) techniques are employed in photovoltaic (PV) systems to make full utilization of PV array output power which depends on solar irradiation and ambient temperature. Among all the MPPT strategies, the incremental conductance (INC) algorithm is widely used due to the high tracking accuracy at steady state and good adaptability to the rapidly changing atmospheric conditions. In this paper, a modified variable step size INC MPPT algorithm is proposed, which automatically adjusts the step size to track the PV array maximum power point. Compared with the conventional fixed step size method, the proposed approach can effectively improve the MPPT speed and accuracy simultaneously. Furthermore, it is simple and can be easily implemented in digital signal processors. A theoretical analysis and the design principle of the proposed method are provided and its feasibility is also verified by simulation and experimental results.Index Terms-Incremental conductance (INC), maximum power point tracking (MPPT), variable step size.

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