The Voltage Interactioninmulti-Infeed HVDC Systems And Its Influence

With the increasing penetration of the HVDC transmission projects in China, the grid in South China and East China have formed complex multi-infeed ac/dc power systems. According to the plan, new HVDC lines will be built to transmit the power into Central China, where multiple rectifier and inverter stations will feed into the same ac network. This leads to another type multi-infeed system. The voltage interaction between two converter ac buses will affect the relative strength of the ac system, commutation failure of the inverters and transient overvoltage in the multi-infeed system. In order to analysis the influence of the voltage interaction in different types of multi-infeed system, this thesis carries out the study from the following aspects:1) The comparative study among the three voltage interaction evaluation factors are conducted, the corresponding factors for the same multi-infeed system at different voltage interaction level arc calculated. Base on the calculation results, the indicators for the system strength for are acquired and compared to the real system strength. The multi-infeed interaction factor (MIIF) is selected for its accurate evaluation. An analytical method for MIIF is proposed and the examples are given, the results show that the interaction voltage level between the converters in the multi-infeed system is decided by the value of mutual impedance, as well as the converter station power characteristics. The power characteristics of the converter are different under different control modes, the impact of the control system on the voltage interaction and rectifier-inverter voltage interaction are also analyzed.2) Based on the maximum available power (MAP) method, the system strength of the rectifier/inverter mixed multi-infeed system is studied. The existing factor-multi-infeed effective short circuit ratio (MIESCR) fails on predicting the system strength of the rectifier/inverter mixed multi-infeed system. In order to describe the influence of the rectifier-inverter voltage interaction on the system strength, a test dual-infeed system is established. A new MIESCR definition is proposed after conducting a range of parametric studies based on the simulation results. This new definition is verified in the triple-infeed systems with different structures. Meanwhile, the transient overvoltage in the rectifier/inverter mixed multi-infeed system is analyzed. The result also shows the accuracy of the new MIESCR definition. The new MIESCR definition can be used for the ac system strength evaluation in two type multi-feed systems. The impact of the control modes on the ac strength is calculated and analyzed. 3) The study methods on commutation failure (CF) are analyzed. The linear relationship between the commutation failure immunity index (CFII) and the ESCR is acquired from the electromagnetic transient (EMT) simulation results of the CIGRE HVDC benchmark model, which implies the voltage drop at the inverter ac bus corresponding to the CFII index is fixed. The EMT models with different voltage interaction levels are established to study the CF in the rectifier/inverter mixed multi-infeed system. The simulation results show that the voltage interaction between the rectifier and inverter can increase the immunity of the inverter station to CF. The equation for calculating the index CFII is also derived and verified.4) The indicators discussed in the thesis are used to analyze the practical multi-infeed systems. The voltage interaction between two converter ac bus and the MIESCR are calculated. The method EMT modeling of the large-scale multi-infeed system is disscussed. The corresponding conclusions are given after the study.