Research On Autonomous Control And Stability Analysis Of Ring-Bus DC Microgrids

Since the control of DC micro grid doesn't need to consider problems about synchronization and reactive power,with a relatively simple coordinated control in different distributed power generation units,and its efficiency and reliability are better than that of AC micro grid,increasing attention has been paid to the study in recent years.DC micro grid with the ring network structure is suitable for places with high reliability of power supply because of its more power supply path.However,the complexity of the network structure brings new problems to its operation control and stability analysis,preventing its development.Therefore,with the four-terminal ring-bus DC micro grid as the research object,this paper studies precise distribution control of the load using droop control converter,compensation control of load node voltage,coordination control between units of a system and the stability of the DC micro grid.The research results have theoretical significance and practical application value for the operation control and stability design of ring-bus DC micro grid.The specific contents of this paper are as follows:Firstly,based on the traditional DC grid flow calculation method,an improved algorithm of power flow calculation is proposed for the ring-bus DC micro grid using voltage droop control.Based on this,two methods for precise distribution of dynamic load are studied,respectively using the vertical coefficient ratio and the reference voltage difference as the controlled variable.Considering the voltage fluctuation of the load node cansed.by the droop control and the network impedance,two methods for comprehensive implementation of dynamic load precise distribution control and voltage compensation control are studied,respectively using the droop voltage reference and the dropping coefficient ratio as the controlled variable,and using two droop voltage reference as the controlled variable.In the latter method,studies show that when keeping the load distribution ratio and the ioad node voltage is the controlled objective,either variation of the two droop voltage reference presents linear relationship of different slopes with load power and photovoltaic power.On this basis,the linear function models of the two droop voltage variable and load power and photovoltaic power are respectively established.When the system operates in real time,with the help of the linear function models,we can easily calculate the size of each droop voltage reference in accordance with the precise distribution of load and the constant control of load node voltage,according to the size of the load power and photovoltaic power.A coordination control objective function based on economic operation is established for the 4-port ring-bus DC micro grid,on the basis of which the eleven working modes of DC micro grid are analyzed.In order to implement the economical and reliable operation of DC micro grid,seamless switching between different modes and constant control of load node voltage,an improvement hierarchical coordination control strategy is proposed.The upper layer control achieves mode selection based on the economic operation,the power flow calculation,the droop control parameter calculation,the load node voltage compensation control,photovoltaic(PV)inverter working mode switching in extreme circumstances and load lightening and recovery;Local control not only needs to receive the upper control instructions and parameters and operates in the designated mode.Meanwhile,in order to maximize the reliability of the DC micro grid power supply,the automatic switching of the local control mode is also needed in case of communication system failure.The switching includes the local controller charging,discharging and standby mode switching of the energy storage node,the MPPT and CV control mode switching of the local node of the PV node,and the automatic loading and unloading functions of the load node local controller.The simulation results and experimental results in different operating conditions verify the feasibility and effectiveness of the proposed layered coordination control strategy in the normal and faulty communication system.Finally,based on the Lyapunov stability theory,the author establishes a 23-order full-order small-signal linearized model of the 4-port ring-bus DC micro-grid,analyses the influence of the capacitance value of the load node on the stability of the system,simplifies the model to get a reduced 5-order model regardless of the dynamic characteristics of the converters,analyzes the influence of the circuit equivalent capacitance,equivalent resistance,equivalent reactance,droop coefficient and load power on the stability of the system based on this,analyzes the influence of impedance and voltage regulator on the stability of bidirectional AC/DC converters.