The Start-up And Control Strategy Of HVDC Connected To AC System With Weak Strength

The modern power network has been developed in the direction of AC-DC hybrid transmission with high voltage.The operating conditions of the HVDC system connected to AC system with weak strength include the HVDC island operation and the operation of the HVDC during the black start.HVDC island operation can effectively reduce the effect of the fault in DC system on the main network and improve the safety and stability of the power grid.While during black start,the DC transmission system can speed up the recovery and interconnection of power grid.However,due to the connection to weak AC system,the start-up of HVDC and the increase of load are likely to lead to over-voltage and frequency stability problems of AC system.Therefore,it is of great significance to improve the stability of system by studying the start-up and control strategy of HVDC connected to AC System with weak strength.In this paper,we mainly study the start-up and control strategy of HVDC during black start.Aiming at the problem of HVDC start-up,we study the start mode of single circuit HVDC whose impact to AC power grid is the least and the minimum starting conditions to meet the AC system voltage and frequency requirements when DC is started.Put forward the start strategy of multi-circuit HVDC.In order to speed up the recovery of load during the black start,this paper proposes two frequency control strategies to achieve the maximum load under the frequency constraint.(1)Frequency modulation strategy based on variable parameter FLC.The mathematic model of AC/DC system is established.FLC parameters are optimized by particle swarm optimization(PSO).The optimal FLC parameters with the lowest frequency offset of both sengding and receiving system are obtained.According to the influence of parameters of the generator and the system capacity on optimal FLC parameters,the frequency control strategy based on variable parameter FLC is proposed.(2)An AC/DC system coordinated control strategy based on linear quadratic optimal control and adaptive control is proposed for the AC/DC systems where the units are continuously put and the generator parameters are difficult to be identified.The optimal prime mover and governor parameters are selected within a reasonable range of parameters.Based on the linear quadratic optimal control,the DC transmission power control law is obtained after the load is put.The frequency response of the optimal model under the quadratic optimal control is used as the reference signal.The Narendra adaptive tracking control method with correction value is used to obtain the control law of the target value of the speed control system,so that the actual system tracks the ideal reference model.The frequency modulation strategy can reduce the computational complexity in the FLC parameter optimization process and avoid the difficulty of parameter identification of generator unit.The simulation results verify the effectiveness of the proposed control strategys.