Research On Overvoltage Simulation Models Of Power Electronic Equipment In VSC-HVDC Grids And Their Applications

Voltage Source Converter based High Voltage Direct Current(VSC-HVDC) grid which using Modular Multilevel Converter(MMC) is an important development direction of VSC-HVDC transmission technology in the future.At present,China is building the world’s first VSC-HVDC grid-Zhangbei VSC-HVDC grid.The project adopts the transmission mode of half bridge sub-module combined with DC circuit breaker(DCCB),and the four-terminal converter station is connected into a network through overhead lines erected on the same tower.Many new technologies and equipment have been applied to the project,which also brings more technical challenges.Due to the application of overhead lines,short-circuit faults are more likely to occur in this project.The calculation of overvoltage during short-circuit is a very important part of the engineering design stage.However,there are still few studies on the calculation of over-voltage in VSC-HVDC grid,and there are still some deficiencies in the study of power grid simulation model and over-voltage characteristics.In order to solve the above problems,this paper focuses on the simulation model of power electronic equipment(MMC and DCCB) in VSC-HVDC grid and the over-voltage characteristics of the grid.According to the requirement of accuracy and efficiency of MMC simulation model in over-voltage simulation,the bridge average value model(AVM) and port AVM of MMC are proposed in this paper.The bridge AVM is based on the Thevenin equivalent circuit of the sub-module,and the equivalent arm model of MMC bridge is realized by means of energy sharing method.The simulation accuracy of the average model of the bridge arm is similar to that of the Thevenin model.The simulation speed has been significantly improved and the improvement effect is positively correlated with the number of sub-modules.The prot AVM of MMC is an equivalent model for port of MMC DC side.An port AVM is proposed in this paper.By adding controlled current source on AC side and charging module on bridge arm,this model solves the defect that the existing models can not accurately simulate AC and DC components of bridge arm current and capacitance charging process of sub-module after blocking.The simulation accuracy of the enhanced port AVM is equivalent to that of the Thevenin model,and the simulation efficiency is improved by an order of magnitude.The application scope,simulation efficiency and modeling difficulty of the bridge AVM and port AVM are also compared.Compared with the bridge AVM,the port AVM can not simulate AC side faults,but the simulation speed of the port AVM is better than that of the bridge AVM.Moreover,the modeling difficulty of port AVM is less than that of bridge AVM.So the appropriate model can be selected according to the need in the over-voltage simulation.In this paper,the breaking characteristics of capacitance forced commutation based DCCBs are analyzed.It is found that the electromagnetic transient characteristics of the ports of hybrid DCCBs with different topologies are identical.Based on the requirements and characteristics of over-voltage simulation in engineering design stage,the equivalent port model of capacitance forced commutation based DCCBs is proposed.The topology and parameter calculation method of the model are introduced in detail.A remarkable advantage of the proposed model is that the parameters needed in modeling are simple and the internal parameters of DCCBs are non-essential,which provides great convenience for the modeling of DCCBs in engineering design stage.In this paper,the breaking test of 500 kV diode full bridge DCCB is carried out in the laboratory,and the correctness of the model is verified by the test data.The applicability of the model in the simulation of over-voltage in VSC-HVDC grid is further analyzed by simulations.Aiming at Zhangbei VSC-HVDC grid,this paper establishes the equivalent circuit model of Zhangbei VSC-HVDC grid in short-circuit fault,deduces the mathematical expression of bus overvoltage of converter station fault pole under the most serious condition,and analyses the change process of bus voltage during the development of short-circuit fault.Though the analysis the causes of bus overvoltage are reavled.The development process of non-fault pole bus’s voltage in unipolar grounding fault is analyzed by simulations,and the cause of non-fault pole bus voltage is revealed.Based on the enhanced port AVM of MMCs and port equivalent model of DCCBs proposed in the previous paper,a complete simulation model of Zhangbei VSC-HVDC grid is established in this paper.Through a lot of simulations,the influence of system operation scheme,current limiting equipment parameters and guarantee strategy on the over-voltage of VSC-HVDC grid is studied,and these influence laws are explained theoretically.Based on the above simulation and analysis,this paper proposes the overvoltage suppression measures for Zhangbei VSC-HVDC grid.The proposed suppression measures are based on the existing design of Zhangbei Project,and can significantly reduce the overvoltage level of the grid without increasing construction investment.