Research On Modeling Method For Dynamic Equivalent Of AC-DC Power Systems Considering Security Control Strategy

Due to the unbalanced distribution of energy resources and economic development,the situation of UHV(Ultra High Voltage)AC-DC transmission and interconnection is developing rapidly.The multi-DC pattern has greatly changed the power grid architecture,the degree of mutual coupling between AC and DC has been enhanced,and the impact of fault in the grid on the system changes from local to global.Traditional security control measures have been unable to adapt to the new power grid needs.In order to prevent the risks caused by cascading failures,it is necessary to conduct a transient safety assessment of the multi-resource security control strategy adopted after the system fault,and verify its effectiveness in restoring the stable operation of the system.To reduce the computational burden of a large number of simulation scenarios,the dynamic equivalent for the AC-DC power grids is necessary to reduce the scale of the system.But at the same time,the reduced model also needs to ensure the accuracy of the transient security assessment in the verification.Under the new trend mentioned above,whether the dynamic equivalent model can accurately simulate the dynamic characteristics and response of the original system after the fault is the key challenge for the dynamic equivalent of AC-DC power systems.In this paper,combined with the demand of improving the security risk prevention and control of the power grid,the dynamic equivalent is studied in terms of the influence of the equivalent boundary and the method of determining the equivalent boundary,which makes up for the lack of the existing systematic methods in this link,thus forming a complete dynamic equivalent modeling method.The main work of this article is as follows:Firstly,the principle and realization process of each link of the coherency-based equivalent method in dynamic equivalent are studied.To improve the accuracy of coherent bus simplification and the complexity of the process,a practical improvement measure for bus simplification is proposed based on the phase shifting transformer.The example analysis shows that the improved dynamic equivalent method can basically maintain the AC and DC dynamic characteristics of the original system,and lays the foundation for the later study and the equivalent calculation after the boundary is determined.Secondly,the influence of different equivalent boundaries on the dynamic characteristics of the AC and DC systems after the fault is analyzed,and the principle of determining equivalent boundaries is obtained.First,based on the coupling relationship between AC and DC,determine the electrical distance between the buses,select the boundary based on the bus voltage after the fault,and then compare the simulation results of the dynamic characteristics of different boundaries,and the degree to which the DC response characteristics are affected by faults at different buses.The basic principles guiding the division of the equivalent boundary are obtained,which provides a basis for the determination of the initial equivalent boundary.Thirdly,a method for determining the dynamic equivalent boundary of AC-DC power systems considering security control measures is proposed.In this paper,aiming at specific fault,based on the idea of graph theory,Dijkstra algorithm is used to search for paths in the system network to obtain equivalent boundaries,and the supplementary boundary is added according to the principle of the previous research to merge into the comprehensive boundary.According to different researches on the different requirements of error accuracy,the equivalent boundary is dynamically adjusted to provide a referable visual equivalent boundary for the establishment of different dynamic equivalent models under different expected faults,avoiding the uncertainty of the artificially specified boundary.The analysis of the calculation example shows that the equivalent system with the method not only meets the equivalent error requirements of the original system,but also retains the main dynamic characteristics of the original system,and at the same time reduces the system size,which verifies the effectiveness of the method.Finally,the dynamic equivalent program and the co-simulation with PSS/E are designed and developed.The design framework of program co-simulation is established,each sub-module is described in detail,and the influence of DC system and security control strategy is considered in it.The design framework of program co-simulation is established,each sub-module is described in detail,and the influence of DC system and security control strategy is considered,which satisfies the need of considering the cascading failures of the current AC-DC coupling gradually increasing.