Mid-long Term Voltage Emergency Coordinated Control Of Power System

With the growing transmission power of power grid,the system is operating more and more frequently near its limit point in recent years,voltage instability is an increasingly serious threat to the safe operation of power grid.As the power system is a large-scale and complex system with very high nonlinearity,it is always a challenging task to find a voltage coordinated control strategy which can guarantee the safety of the system and minimize the control cost at the same time.This thesis mainly studies the voltage emergency coordinated control in mid-long time frame of the power system.The main contributions are as follows:Aiming at problems,such like the online computation burden is heavy and the forecasting model is difficult to balance the accuracy and fastness,encountered in the application of model predictive control to mid-long term voltage emergency coordinated control in power systems,an online rolling single-step voltage control optimization model is proposed.The proposed model based on trajectory sensitivity only predicts and calculates controls at current sampling instant during each optimization,it can predict the future output trajectory of the system accurately and rapidly,and greatly enhance the speed of the online rolling optimization of the voltage control optimization model.In order to improve the practicability and efficiency of the model,the following work is done: Firstly,a desired trajectory adjusted online along with rolling optimization is proposed.The desired trajectory is adjusted according to the actual sampling value in each rolling optimization process,which makes the proposed model more adaptable and makes sure existence of optimization solution;Secondly,trajectory sensitivity is calculated with explicit Euler integral method to relieve computational burden;Thirdly,a control screening index based on trajectory sensitivity is proposed,selecting effective controls which helps most of the system recovery before rolling optimization,and determining the optimal load shedding point and the minimum load shedding amount of the system in each rolling optimization;Lastly,tracing low voltage bus along with rolling optimization to make the proposed model well-targeted and reduce constraint number at the same time.Simulation results demonstrate that the optimization problem can be solved rather fast with the proposed model.With optimal solutions,system trajectory can trace desired value precisely.The proposed model has good robustness even at presence of load model parameter perturbations.As a kind of clean energy,the installed capacity of wind power is growing in our country.Considering the large-scale access of wind farms will have a significant impact on the mid-long term voltage stability of power system,this thesis analyzes the mid-long dynamic process of power system with constant speed and constant frequency wind farms.The operating characteristic of the asynchronous generator is analyzed,on the basis of the mathematical model of asynchronous generator,a quasi-steady-state model of constant-speed constant-frequency wind turbine is established for mid-long term dynamic simulation of power system.The simulation results of a system with constant-speed and constant-frequency wind turbine demonstrate the validity of the proposed model.