Research On Improving The Transient Stability Of Energy Storage System After DC Locking Of AC/DC Power Grid

Due to the imbalance of energy distribution and load distribution in China,HVDC transmission technology with the characteristics of large transmission capacity,long transmission distance and small footprint is more and more widely used in China.With the gradual operation of large-capacity high-voltage direct current transmission systems,the characteristics of "strong direct and weak alternating" of the power grid have become increasingly obvious.However,the introduction of large-scale power electronics in new power systems has made the dynamic characteristics of the grid more complex and changeable.Transient energy shocks caused by faults exceed the limits of power electronics,eventually leading to DC lockout.When the DC line is blocked,the large amount of unbalanced power accumulated at the sender is transferred to the adjacent AC system through large-scale power transfer.When the transmission power of the AC channel reaches its transmission limit,the relative power angle of the sending generator and the receiving generator will continue to swing,and the power angle instability will inevitably be accompanied by voltage instability,which will eventually lead to the loss of transient stability of the power system.Therefore,it is of great significance to study the emergency control and stability strategy of the DC blocking and sending end system of AC and DC power grid.In this paper,the composition of HVDC transmission system is analyzed,and the AC/DC hybrid system is established according to the structure of HVDC system and the equivalent model of traditional AC system.Based on the equivalent model of AC and DC system,the transient process after DC blocking is analyzed theoretically,which provides a theoretical basis for the emergency control of energy storage system after participating in DC blocking.By further connecting the energy storage system to the AC/DC system,the equivalent model of the AC/DC system including the energy storage system is proposed,and the adjustment mechanism of the transient power angle and transient voltage of the AC/DC system after the energy storage system is put into operation is analyzed.The energy storage system has the technical characteristics of flexible installation,fast response speed,and can effectively smooth the fluctuation of new energy power,which can improve the stability of the AC and DC system under the large-capacity active and reactive impact after DC blocking.A electromechanical transient model of battery energy storage applied in AC and DC systems is proposed.The model regulates the power angle and voltage fluctuation of the AC and DC system by controlling the current injected into the grid through the decoupling control of phase angle difference/active,voltage difference / reactive power.Based on the equal area rule of including energy storage system,a joint control strategy combining the energy storage system after DC blocking with the emergency measures of cutting machine control and stability is obtained.The AC/DC system is built in the power system comprehensive simulation program PSASP,which consists of two improved CEPRI-36 systems,and the simulation analysis of the joint control strategy after DC blocking is carried out in this system.