Research On Bus Voltage Control And Energy Coordination Strategy For DC Microgrid

With the development of energy technology, the application of DG (Distributed Generation) in power system is imperative. As a controllable system, microgrid may operate in either grid-connected mode or islanding mode. When output abrupt change of DG, plenty of loads switching or operation mode switching happen, it would significant impact on the bus voltage, or even cause protection system malfunction. So it is important to study on the DC microgrid operation control technology, and maintain stability of the bus voltage, thus realizing the energy optimal coordination control for the theoretical significance and application value. In this dissertation, each unit converter control strategy and the system control strategy based on hierarchical coordination control are analyzed and designed in the DC microgrid system.Firstly, the related issues and the future trend in city distribution system of DC microgrid are analyzed. The research statuses of DC microgrid are studied, including DC bus voltage control and optimal energy coordination control. The DC microgrid system model is built, which could meet the requirements of DC microgrid for operation stability and reliability. The model can provide some basis for the research on unit converter autonomous control strategies and system control strategies.Secondly, the DC microgrid which comprise photovoltaic cell, grid-connect converter, battery unit, fuel cell unit and load is taken as the research objects. Conveter autonomous control strategies for each unit are established. It is in order to achieve quickly and smoothly switch between different working statuses.Thirdly, the control strategy for DC microgrid optimal energy utilization is built. This strategy can be used when DC microgrid is under grid-connect state, or under the islanding status which has a power balancing condition within DC microgrid. The bus voltage stability can be maintained. The optimal utilization of renewable energy and batteris can be accomplished at the same time.Fourthly, in order to solve the problem of bus voltage fluchtuation due to power unbalancing under islanding operation. The self-regulating control strategy of energy storage charge and discharge is designed. When the bus voltage value enters the preset voltage range of the strategy, the power buffering ability of battery can be utilized. As the slack terminal, battery converters can autonomously switch to droop charging control or droop discharging control based on the voltage change. The power within DC microgrid can reach a new equilibrium. Then, it is able to maintain the bus voltage reliability.Finally, the extreme operation control strategy is built, which considering serious power imbalance within DC microgrid. To ensure the operation reliability under extreme operation status for DC microgrid, the wide range fluctuation of bus voltage can be effectively inhibited by autonomous output control of photovoltaic converter and utilization of auxiliary power supply. Hence, it is able to avoid the possibility of system collapse.