Research On Power Quality Control And Correction Of DC Micro-grid

With the rapid development of modern economy, environment pollution and energy crisis have become more severe. And, distributed power and micro-grid is an important method to solve environmental and energy problems. In micro-grids, various types of distributed power generation systems all need power electronic devices to access to grid. DC type can reduce the degree of coupling between micro-network and the main network and improve the utilization of equipment. In addition, DC micro-grids, which do not have frequency and phase, just need to maintain the stability of bus voltage. Therefore, it can provide good power quality.This thesis is focused on studying DC micro-grid system topology, voltage level and internal micro-source model. Currently, there are several typical power quality problems in DC micro-grid, such as circulation, islands conversion, and distributed generation, etc. So, this thesis based on the Shanghai20kW DC micro-grid demonstration project, studies to optimize the power quality from the control strategy and system architectures. A two-stage inverter is chosen as the main controller; which can reduce the impact from grid and achieve a smooth handover between grid-connected mode and island mode. The photovoltaic system is in PQ control to maximize the use of solar energy. Because of the different roles in grid-connected mode and islanding mode, the battery is respectively in PQ control or constant voltage control. And, the double-loop control is used to maintain the load-side voltage stability. Meanwhile, according to theoretical calculations and simulation analysis, this thesis determines the optimal values of each converter’s inductors, capacitors and PWM frequency.Based on the above researches, this thesis sets up a DC micro-grid model in PSCAD simulation environment to study the DC bus situation in grid-connected mode and island mode. The simulation cases include three-phase unbalance of AC system voltage, photovoltaic output changing suddenly, the input and excision of load and the conversion process of the two modes. The simulation results shows that, the proposed control method is effective and can provide good power quality. Last, this thesis complete the secondary circuit’s design of AC/DC converter and DC/DC converter, including the TMS320F28335control circuit, the HCPL316J driver circuit and the Blue Ocean microchip HMI. These two devices are the core control of the DC micro-grid. Through demonstration projects testing, these two devices have good transient response in grid-connected mode and the island mode. The results show that, in a variety of harsh conditions the, the bus voltage can be maintained stable and meet the ripple does not exceed5%.