Research On Grid-connected Inverter For Distributed Power System

The looming environmental deterioration and energy crisis encourage all the nations to better utilize the abundant renewable energy sources. Distributed generators improve the efficient of photovoltaic power and wind turbines by local consumer satisfaction, which increases economic benefits and received a great popularity. Grid-connected inverters are the essential interfaces between distribu ted renewable resources and utilities. As the penetration of distributed generators increasing, the control strategy of grid-connected inverters plays a more vital role in utilities. In this thesis, resonances among inverters, power control strategy and fault protection measures are discussed, and then modified methods in current loop and power loop are proposed. The specific contents are as follows:According to the resonance peak of LCL filter, six different kinds of passive damping and three kinds of active damping are comparatively studied. The frequency responses of damping methods illustrate that the state-feedback based active damping strategy has advantages of strong stability-robustness and excellent harmonic-rejection-ability. Based on analysis the state-feedback based active damping strategy, a practical design method is presented to calculate feedback coefficients and parameters of current regulator.The impedance reshaping control strategy for multiple grid-connected inverter system is studied. To maximize the magnitude of output impedance, a parallel virtual impedance in Norton equivalent circuit for grid-connected inverter is presented. Therefore, the total harmonic distortion of the injected grid currents can be declined, as well as avoiding possible low frequency resonancesThe power allocation control strategy for multiple grid-connected inverter system is studied. In order to improve the dynamic frequency response to load verification, a novel droop control based on the virtual synchronous generator technique is proposed. The virtual torque for synchronous generator is applied to increase inertia and damping ratio, while droop control discipline enables inverters to participate the secondary frequency regulation. Then a small-signal model of virtual synchronous generator and droop control is presented on the basis of the proposed droop control strategy. Therefore, the relationship between frequency fluctuation and load verification can be calculated, and the influence of control parameters is analyzed.The low voltage ride through control for grid-connected inverter under the power grid fault condition is studied. To solve injected grid currents distortions with unbalanced voltage sag, a modified balance positive sequence control is proposed. Injecting reactive power to utilities enables a voltage rise, which accelerates fault recovery. In addition, passive islanding detection and active islanding detection methods are analyzed and comparatively studied.