| Nowadays,with the problems of energy shortage and environmental pollution highlighted increasingly and the scale of new energy resources incorporated into the grid increasing,the application of microgrid is more and more wide.While microgrid guarantee the reliability of power supply,it is more complicated compared with the traditional power grid in the matter of voltage regulation and power control.However,due to the big line impedance ratio in the microgrid,the control of line power flow meets the strong coupling nonlinear problems,and the characteristics of its have also been analyzed so far.Therefore,in order to enhance the economy and efficiency of microgrid in grid connected mode and to improve its energy management system,unified power flow controller(UPFC)is applied to power flow control of the point of common coupling(PCC).In view of the nonlinear and external disturbance on the control performance in power system,a new and suitable power flow control strategy must be proposed,and then the robust controller is designed.Firstly,on the basis of introducing the basic structure and power flow control methods of microgrid briefly,setting forth photovoltaic,fuel cells and supercapacitor in detail,from many aspects expounding the basic structure,principle and operation modes,the model of microgrid in grid connected mode including multiple micro sources and energy storage devices has been built.Considering the influence of the big line impedance ratio and change of the node voltage on performance,the characteristics of power flow control has been obtained by means of comparing the theory calculation and simulation results.Meanwhile,the UPFC power flow calculation has also been finished,and so validated the correctness of the simulation model.The above work lays a foundation for the design of the power flow controller and the example analysis.Then,in view of the nonlinear of UPFC,its nonlinear mathematical model has been established and four control targets have been also given.Aimed at the strong coupling nonlinear problems,the paper has proposed a new nonlinear power control strategy based on Backstepping method and sliding mode control for the series-side controller of UPFC,and decoupling control strategy which is based on the reactive power control mode for the shunt-side controller of UPFC,and then the nonlinear power flow controller is designed.It has been verified the power flow control function and characteristics of the controller are valid.And the paper obtains the power regulation range,and also realizes the power balance of the whole system.Finally,in the actual engineering application,in order to avoid many adverse factors including signal delay and noise interference,and so on,which caused by the design of Backstepping sliding mode controller,the paper has proposed a solution that an additional series impedance is set at the end of the UPFC series-side and determined the selection principle of the additional impedance.For the microgrid with the improved UPFC set at the PCC,the paper has designed the nonlinear power flow controller.Simulation results presented that the proposed controller can not only effectively flexibly control the line power flow and realize the global regulation,but also have advantages over cross decoupling controller and verify the good robust performance of nonlinear controller. |
