Study On Time-varying Tracking Optimal Strategy Of Grid-connected Wind Power System

With the increasing penetration of wind power,photovoltaics and other renewable energy power generation,the power grid structure is changing,and the traditional power grid operation regulation solutions need to be adjusted.With a high proportion of wind power connected to the grid,the fluctuation and intermittency of wind power bring huge pressure to the peak regulation,voltage regulation and frequency regulation of the power grid.It is difficult to ensure the safe and economic operation of the power grid by relying on thermal power units and traditional regulatory equipment.Therefore,it is necessary to curb the adverse effects of wind power uncertainty from the source,that is,the self-regulation ability of the grid-connected wind power system should be deeply explored,and the wind power system should have the ability to be scheduled and controlled by the power grid.The uncertainty of wind power determines that set points regulation of every generation unit in the wind power system should be quick,online,and time-varying.The maturity of power electronics technology,and the development of measurement communication technology,make online time-varying optimization of the wind power system can be realized.At the same time,the wind power generators represented by doubly fed induction generators have the ability to regulate active and reactive power flexibly,which make the grid-connected wind power system have certain self-governance ability and scheduled,controllable potential.Under the above background,and based on doubly-fed wind power system,this paper is from the perspective of wind power system managers,and study on the time-varying optimal autonomy potential of doubly-fed wind turbines and time-varying coordinated optimization method of the wind power system when tracking scheduling commands.The main contents are as follows.1)Based on the steady state mathematical model of doubly fed induction generator(DFIG),the active and reactive power regulation range is deduced,and the output characteristics of DFIG when the active and reactive power is mutually restricted is analyzed.From the point of steady state,the active and reactive power regulation principle of DFIG is analyzed.Taking the grid-connected wind power system with one generator as an example,the voltage regulation mechanism of DFIG is explored.Results show that the voltage fluctuation caused by wind power can be stabilized by regulating the reactive power of DFIG,and when the capacity of reactive power regulation is insufficient,additional reactive power regulation range can be obtained by reducing active power,and the voltage problem can be solved.2)In order to solve the voltage problems of internal nodes(especially generators terminal nodes)and point of common coupling(PCC)of the wind power system caused by uncertain wind power,and to explore the time-varying optimal autonomy potential of the wind power system which consists of doubly fed induction generators,this paper proposes a time-varying optimal method for tracking grid-connected maximum active power of doubly fed wind turbines.On the premise of ensuring voltage quality of internal nodes and PCC,the nonlinear optimization model is established,which ensures the maximum active power output of the PCC.With the fast development of online measurement and communication technologies,this optimization model is linearized using Taylor expansion.After this simplification,the model can be solved in milliseconds,which makes set points of each wind turbine can be adjusted online and quickly.Thus,the maximum active power output of the PCC can be obtained,and time-varying optimal power flow of the power system can be tracked.3)On the basis of 2),this paper proposes a solution idea to calculate the aggregated active and reactive power regulation range of the wind power system which consists of doubly fed induction generators.The steady-state aggregated output characteristics of the doubly-fed wind power are analyzed,and this can provide some reference for the higher power grid dispatching departments.Due to the existence of internal components,such as electric lines and transformers,the aggregated P-Q regulation range of the doubly-fed wind turbines is not equivalent to the range which is obtained by adding the P-Q regulation range of each DFIG In fact,the aggregated P-Q regulation range is restricted by the voltage safety of generators,so the actual aggregated P-Q regulation range of the wind power system will be significantly smaller than the range calculated by adding the P-Q regulation range of each DFIG.4)In order to make the grid-connected wind power system participate in the regulation of the power grid,a time-varying optimal scheduling method is proposed,which make the wind power storage system track the active and reactive power scheduling commands online.This method includes two layers,namely,day-ahead wind storage scheduling layer and online time-varying optimal tracking layer.The purpose of these two layers are as follows.The first layer fully considers the uncertainty of wind power,and calculates the total active power plan of the wind power storage system which is reported to the higher power grid dispatching center the next day.The second layer aims at smoothing the wind power fluctuation in the actual operation,and tracking active and reactive power dispatch commands online.In the first layer,scenario set is used to describe the uncertainty of wind power,and the day-ahead wind storage scheduling model is established.The model is solved by scenario reduction method,so the day-ahead active power output plan is obtained.The second layer is based on the online measurement and communication,and uses the power electronic quick regulation ability of DFIG and the storage to adjust the active and reactive power set points of each control unit online and quickly,so the wind power fluctuation can be smoothed,and the scheduling commands can be tracked online.This method can track active and reactive power scheduling commands synchronously,and can provide certain auxiliary services for the power grid on the basis of ensuring its own operation safety,which is of great significance for the operation and regulation of the power grid.