Research On Coordinated Optimization Of Preventive Control And Emergency Control In Power Grid Based Upon Risk

Modern electric power system has become interconnected system with large capacity and high voltage,which promotes the development of national economy and improves the level of social life.However,the complexity and the uncertainty of large-scale power system also affect the security of the system.Therefore,applying effective methods to evaluate the security of the system and formulating corresponding control strategies are of great significance to the safe and economic operation of the system.The security assessment method and the control strategy,which are based on the risk theory,consider the effect of fault probability and fault severity on power system.Compared with traditional method,risk method is more conducive to achieve the balance between security and economy.Based on the risk theory,this paper constructs the power system risk index,and establishes the optimal control model based on the risk constraints.The main works of this paper are as follows:(1)At first,the fault model of transmission line under windy weather is established,considering the impact of accidental and aging failures.The model is solved by Monte Carlo method,and the fault probability of transmission line under windy weather is calculated.Then,according to the probability and the severity of transmission line fault,the risk index of branch overload is constructed in DC power flow.And the preventive control model based on the branch overload risk constraints is established.In the preventive control model,the line outage distribution factor is used to reduce the number of optimization variables and constraints of the model,and the solving speed of the model is improved.(2)The steady-state risk index of power system is constructed and the coordination coefficient is introduced.Based on the steady-state risk constraints,the coordinated control model between preventive control and emergency control is established.By introducing the Tanh-type continuous and derivable function to approximate the continuous but non-derivable function in the model,the coordinated optimization model is transformed into two-layer optimization model,which could be sovled by gradient method.In the first layer of optimization,adjusting the value of the coordination coefficient can change the limits of steady-state risk constraints in the second-level optimization,and achieve the purpose of adjusting the total control cost.In the second layer of optimization,under the satisfiction of the steady-state risk constraints,the aim of this layer is to minimize the sum of preventive control cost and emergency control expected cost.According to the characteristics of coordinated optimization model,this paper proposes a solution method combining golden section method and interior point method.(3)The transient risk index of power system is constructed and the coordination coefficient is introduced.Based on the transient risk constraints,the coordinated optimization model of preventive control and emergency control is established,this model is two-layer optimization model.In the first layer of optimization,adjusting the value of the coordination coefficient can change the limits of transient risk constraints in the second-level optimization,and achieve the purpose of adjusting the total control cost.In the second layer of optimization,under the satisfiction of the transient risk constraints,the aim of this layer is to minimize the sum of preventive control cost and emergency control expected cost.