| As an effective way to organize renewable sources and distributed generation, microgrid can be connected to large power system or run as an independent power system. While operating independently, microgrid should keep power balance so as to ensure voltage and frequency stability, resulting in high technical requirements for micro sources and energy storage devices connected to the microgrid.Unlike traditional large generators, micro sources have various output characteristics and present typical intermittence and randomness. The majority of micro sources are connected to microgrid through power electronic inverters. Due to the coupling effects between the inverter topology control and the parameters of microgrid, new issues are raised with grid-connection of the micro sources, such as phase detection method of the grid voltage as the CCVSI (Current Controlled Voltage Source Inverter) is connected to microgrid, as well as transient protection technology for three-phase inverter bridge. Preferable solutions to these problems can improve the reliability of grid-connection of the micro sources. Meanwhile, in order to realize power balance as well as voltage and frequency stability, each micro source and energy storage device should generate electricity according to designated quota by the power dispatcher. Therefore, corresponding designated power output control strategy is indispensable and should be analyzed based on the output characteristics of the micro sources and energy storage devices. Exploration of possible solutions to these crucial technical problems forms the basis of this thesis.Most micro sources are connected to microgrid through current controlled type voltage source inverters (CCVSI). Due to the interaction of the line inductances and the power device switching process, switched voltage ripples will superimpose onto the voltage waveform at the grid-connection point, leading to the appearance of multiple zero-crossing phenomenon at the actual zero-crossing moment, resulting in large deviation in phase detection of the grid voltage. Based on theoretical analysis and experiment study of the dynamic behaviors, the impact of line inductances on the voltage waveform at the grid-connection point is analyzed, which proves the multiple zero-crossings area of the sampled voltage presents strict symmetrical feature regarding to the actual zero-crossing of the microgrid voltage waveform. Based on the above analysis, an improved zero-crossing based phase detection method is proposed, which can be applied to the situation with multiple zero crossing points in a single switching cycle. To deal with large switching voltage ripples, a novel method that combines PLL technology and Fourier series analysis is further developed, so as to accurately track the phase of the grid voltage at the connection site. Based on the hysteresis current control, the calculation accuracy of the voltage amplitude and phase by Fourier series is validated, which is not influenced by the voltage on DC side of the inverter, the grid inductance and the switching voltage. In order to improve the real-time performance of phase tracking and restrain the extraneous interference as well as the influence of switching ripple voltage, a method based on improved three-phase software PLL algorithm is brought forward, which adopts 8k decomposition and weighted mean technique in a primitive period, and thereby can restrain negative effects while especially applicable for three-phase three-wire system. Both simulation analysis and experimental results have verified the method.To solve the transient protection problem of typical CCVSI bridge, an equivalent circuit model is established based on stray parameters of the devices. Short through phenomenon of CCVSI bridge arm is thoroughly studied and the mechanism for surge current generation is elucidated, in addition, quantitative formula to determine the current amplitude and rate of change is also deduced. Through theoretical analysis and experimental research, a novel transient protection topology for CCVSI bridge arm is presented based on a LCRD combinational network. The operating principle of the protection topology is analyzed in details and the effective criterion to choose the typology parameters is also given. Experiments indicate that the new protection typology can effectively limit amplitude of the surge current and rate of change of the instantaneous current and voltage during power device switching process, hence, false trigging and damage of power devices can be avoided effectively, and normal operation of the three-phase inverter and reliable connection of micro sources to microgrid can be achieved.The balance of both active and reactive power with a microgrid among the generating devices and the utilization equipment is the precondition for reliable operation of the microgrid. In order to maintain voltage and frequency stability of the microgrid, designated power control over micro sources and energy storage devices is indispensable. Based on theoretical analysis and experimental study, designated power output control strategies for micro sources and energy storage devices are investigated in the thesis, specifically with photovoltaic array, battery energy storage device and wind power generation device as the controllable objects.Based on the fundamental relationship between parameters of a photovoltaic battery cell, the monotony characteristics of theâ… -â…¤curve and dP/dV curve of the photovoltaic arrays are deduced, which lays theoretical foundation for further development of tracking control method at designated power output. A new method based a high order constructed function to implement designated power tracking control is presented, where the first derivative of the operation voltage-based function at the designated reference point is zero while the second derivative is less than zero. This method guarantees that the photovoltaic battery array can self-adaptively approach the designated power point from the present status, and the response speed of the designated power tracking is significantly raised as well. Based on short-circuit current method and open-circuit voltage method, calculation formulas for key parameters in the constructed function are deduced. A nonlinear control system model based on the above proposed method is established and key factors influencing the system stability are analyzed based on small voltage perturbation and linear smoothing, then selection criterion and reliable range of the control parameters are also given. Simulation and experimental results validate effectiveness and speediness of the control method.For designated power control strategy of battery storage devices within a independent microgrid, the calculation method for instruction current on the three-phase AC side is deduced based on coordinates transformation, while slide-mode variable structure control theory is used in the output current tracking and control. Based on adaptive changes of the parameters in the discrete reaching law function, an adaptive discrete slide-mode variable structure current control method is brought forward, which can effectively achieve real-time tracking and flexible control of both charging and discharging currents of the batteries. Critical condition and quantitative formula are deduced, which can be used in the optimization design of the control parameters. Simulation model and experimental prototype of the battery storage device for designated power charging/discharging are established. Simulation and experimental results demonstrates effectiveness of the proposed method, with preferable robustness to exterior interference and parameter perturbation.With regard to the double-feed wind power generation unit in the microgrid, an effective method to control the output power by changing the tip speed ratio of the wind turbine is proposed. Further studies are carried out specifically on power fluctuation phenomenon while a sudden step change of the wind speed happens, and a real-time compensation method based on controllable supercapacitor buffer is presented. Simulations are also given to verify effectiveness of the proposed method.Based on photovoltaic arrays unit, wind power generation unit, battery storage unit and central control unit, an experimental platform for microgrid study is constructed, and multipurpose experiments for physical simulation are implemented to verify the theoretical analysis and the proposed control methodologies.This proposed research enriches the theoretical foundation and analytical methods for systematic study on grid-connection and designated power output control of micro sources, which provides referential basis for further development of the microgrid technologies.
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