| The research contains Distributed generation (DG) microgrids connected into the main electric power grid possesses strong practical significances. Microgrid is a small electric power system includes distributed generations, energy storage devices, power conversion devices, loads, and the protection devices. This thesis introduces the characteristics of the microgrid technology, and analyzes the randomness of the distributed generation which could lead to difficulties in control the microgrid and adverse impacts on the power quality. The power quality of the grid could be affected from several factors such as the operation characteristics and control method, connection point and capacity, operation mode and control method of the microgrid; as well as power electronics, energy storage devices, and load characteristics will all affect the power quality. They result in difficulties and complexities in detecting, analyzing, evaluating, and improving the power quality of the microgrid in comparison with the main grid.The power quality of the microgrid possesses the particularity is determined by the micro power sources, loads and the microgrid’s operation and control methods.The power quality control of the microgrid has the following requirements:(1) Reactive voltage control; (2) Frequency stability; (3) Unbalance control; (4) Harmonic suppression. Therefore, researching on the improvement of the power quality of the microgrid based on the operational control of the hierarchical control has important practical significances.The manifestations of the above power quality issues of the microgrid are different, but the solutions depend upon the energy balance control. The hierarchical control strategy is an important means to achieve energy balance and to ensure the normal operation of the microgrid. Hierarchical control applied in the microgrid needs to solve some key technical issues, such as the relationship to the micro grid economic operation and optimal operation, hierarchical control strategy of controlling power fluctuations. The research contents mainly involve the following four aspects:the model and control method for each distributed generation, the voltage and frequency deviation of the secondary-level control, unbalance voltage in the secondary-level control and harmonic suppression in the microgrid. The main research contents of the thesis are as follows:(1) Starting from the microgrid’s architecture and a variety of mathematical models of distributed generation including wind power, solar photovoltaic battery, and battery energy storage system. According to the basic state-space averaging method, the distributed generations often use power electronic conversion devices to connect to the microgrid. The grid connected control models of the battery energy storage system are divided into battery system, two-way half bridge DC/DC converter system, and inverter’s control system. The grid connected control models of the solar photovoltaic battery generation system are divided into solar photovoltaic battery system, two-way half bridge DC/DC converter system, and inverter’s control system. Wind power adopts direct-drive permanent magnet synchronous generator mainly composed of generator side rectifier and the network side inverter for connection. The simulation results verified the influence of the distributed generation on the power quality of the microgrid. From that, a hierarchical control strategy has been proposed to improve the power quality.(2) The voltage and frequency deviations are two indices of the power quality of the microgrid, which could be caused when the inverter of the traditional distributed generation adopts the droop control method. In primary control of the microgrid. most of the distributed generation adopts the power electronic interface, loads, and changeable network structure, leading to increase the difficulty in control voltage and frequency stably. Therefore, how to ensure the microgrid to satisfy the stability indices of voltage and frequency in the islanding operation mode is the key to reliable operation of the grid. The microgrid’s control strategy based on the secondary control was established to compensate the effects of voltage and frequency. This control method uses the primary control signal to control the output voltage amplitude and frequency of the inverter again to make the voltage and frequency deviation at the point of common coupling (PCC) within a certain range. Additionally, the smooth and stable voltages and frequency synchronization of the microgrid and the main grid could be achieved by adopting the grid connected control strategy of the microgrid based on the secondary control, so as to avoid the impact of impulse current occurring in grid-connection process, and finally, to achieve smooth switching from the islanding mode to the grid mode.(3) The microgrid at the situation of containing single phase distributed generations and load characteristic will cause the voltage at PCC point unbalanced. Under this condition, the voltage of the primary control of the microgrid composed of positive and negative sequence components which are respectively converted into the direct current component and the double frequency component. To compensate the negative sequence component of the voltage, it must adopt a suitable phase-locked loop under the unbalanced situation of the micro grid. The micro grid control strategy based on secondary control was proposed, in combination with the using of the improved decoupled double synchronous reference frame phase-locked loop (IDDSRF-PLL) control, to ensure that the grid has undistorted voltage and symmetrically connected. Consequently, ensuring the grid-connected power quality, and obtaining the balanced voltage at the PCC point of the microgrid.(4) In the microgrid, the distributed the generations and energy storage devices could generate harmonics at their power electronic converter output, and the nonlinear loads could cause current harmonics, leading to the voltage at the PCC point of the microgrid distorted seriously. The primary control of the microgrid control adopts multiple PR proportional resonance controllers of voltage and current loop controls in combination with the virtual impedance droop control strategy. This can eliminate the 5th,7th etc voltage and current harmonic distortions at the output of the microgrid inverter, satisfying the grid-connected standard of distributed generations.In addition, the nonlinear loads will cause harmonic currents, making the voltage at the PCC point to be distorted. In order to compensate this voltage distortion, the microgrid control strategy was proposed based on the secondary control. This control method can offer harmonic suppression effect in the grid by means of a harmonic compensator. |
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