Research On Modeling And Coordination Control Strategy Of DC Micro-grid

The widely use of renewable energy promotes rapid development of Micro-grid. AC micro-grid is the mainstream form of Micro-grid, but if DC load and DC distributed sources, such as photovoltaic, battery and super conductor are directly combined as AC micro-grid, it will lead to many problems such as high cost, low efficiency. Compared with AC micro-grid, in the DC micro-grid, energy conversion is less, system control is easier, system loss is lower and it is much easier for DC element to connect into system. So the DC micro-grid is classed as the important development direction of the micro-grid.Firstly, this paper analyses the working principle and the mathematical model of photovoltaic cells, battery and direct-driven permanent magnet synchronous generator, this paper continues to study the model of grid converter, researches the network topology of converters and decoupling control strategy. This above research work lays the foundation for coordinated control strategy of DC micro-grid.Whether in AC or DC micro-grid, three-phase two-level voltage source converter is an important interface of distributed generation and power grid. Under the influence of various unbalanced factors, active power and reactive power in power grid and DC voltage of the converter all can result in the fluctuation at double frequency, which seriously influences power quality of the system. To solve above problem, this paper has used two kinds of compensation control strategies which are power resonance and current resonance to eliminate the double frequency fluctuation of power and DC voltage under unbalanced grid voltage, as the simulation result, this control strategies improve power quality of system.In DC micro-grid, the DC bus voltage and active power are two most important control variables. To balance the active power in multiple types of distributed generators, this paper divides the operation state of DC micro-grid into four modes, and designs different coordinated control methods for balancing active power in different modes, then puts forward to a switching operation coordinated control strategy. In transient process of switching operation, the current injects into DC Bus mutates fiercely, and then will cause DC voltage fluctuate a lot. To eliminate severely fluctuation of DC voltage, this paper analyzes the cause of the fluctuation and its influence factors, treats the instantaneous injection current in DC side of grid converter as a disturbance factor, adds the nonlinear disturbance observer into originally DC voltage control circuit, proposes the DC bus voltage control strategy based on a nonlinear disturbance observer(NDO).Finally, a DC micro-grid simulation case is constructed with PSCAD/EMTDC, the above power resonance and current resonance control?switching operation coordinated control?DC bus voltage control based on nonlinear disturbance observer are successively adopted into different simulation cases. And from simulation results, the effectiveness of these control strategies is verified.