Study On Evaluation Method Of Transient Stability Of AC-DC Hybrid Power System Based On Direct Method

With the development of HVDC transmission technology and the gradual formation of AC-DC interconnection,accurate transient stability assessment of power systems provides a powerful guarantee for the safe operation of large-scale AC-DC systems.As one of the main methods of transient stability analysis,the direct method is applied to the transient stability analysis of large-scale AC-DC hybrid power systems.It can not only quickly and quantitatively determine the system transient stability,but also provide stability margin information of the system,which is convenient for the system to give control decisions on the ranking of accident stability.However,the huge scale of the multi-zone interconnected AC-DC system increases the complexity of system modeling and energy function calculation.In addition,the dependence of the DC system transient energy on the system response trajectory also brings challenges to the calculation of DC transient energy.To this end,this article carried out the following research work:(1)The energy function of the AC system under the lossless structure-preserving models is derived.Based on the second-order model of generators,the models of constant power load and AC system network,the construction mechanism of the AC system energy function under the lossless structure-preserving models is derived and combed.Provide a theoretical basis for the construction of the energy function of the AC-DC systems later.In this paper,the non-positive definite nature of the energy function of the AC system along the postfault trajectory is numerically studied and analyzed.A numerical example verifies that the energy function satisfies the condition of becoming an energy function and can effectively evaluate the transient stability of the AC system.(2)A fast evaluation method for transient stability of multi-zone interconnection ACDC system based on DC port energy is proposed.Based on the port energy,the transient energy of the DC system is expressed by the transient energy injected into the DC system port,and the port energy function of the DC system is established.Further,for a decoupling multi-zone interconnected system,the port energy of DC system is used to represent the transient energy accumulated at the port of the DC line connecting the regional systems.The transient energy function of each regional system with DC port energy is constructed.Numerical simulations show that,compared with the time-domain simulation method,the accuracy of the proposed method on the stability of the entire interconnected system is more than 90%,which has a higher evaluation accuracy.At the same time,the proposed method can provide information reflecting the stability of each regional system.(3)A numerical algorithm for transient energy of the AC-DC power grid considering the transient response trajectory of the system is proposed.By simulating the equivalent model of AC-DC hybrid power system,the approximate transient response trajectory of the system is obtained.The power injected into the DC system port is determined based on the DC port voltage,phase angle,and DC current transient response data.Furthermore,the DC port energy is integrated to calculate the DC system energy function value,which improves the accuracy of the AC and DC system transient energy calculation.The analysis of the calculation example shows that,in the case of system stability and instability,the method in this paper can obtain more accurate calculation results than the trapezoidal approximation method and the linear path method.It can be used to evaluate the transient stability of the actual AC-DC hybrid power system.The direct method of AC-DC hybrid power system transient stability evaluation method proposed in this paper can provide a theoretical reference for the large-scale ACDC system transient stability research.