Influence And Countermeasures Of UHVDC Transmission On Power System Stability

Ultra-High Voltage Direct Current(UHVDC) is an efficient way of "point-to-point" power transmission, which is an important part of China UHV power grid planning and also the only way for Chinese power industry development. UHVDC with huge transmission power would transport a lot of power to the receiving end, while has a major impact in the security and stability of power system. And there is still a risk in the current control measures taken after UHVDC line being with short-circuit fault. This thesis discusses about the UHVDC power transmission system modeling, the UHVDC's affection on security and stability of AC transmission system, and the corresponding countermeasures. Research on UHVDC line short-circuit fault backup protection after DC converter case of failure, etc., is also carried out.As the main model for the current HVDC power system analysis software used in different models and different precisions resulting in some disadvantages,this thesis establishes a quasi-steady-state model of UHVDC, including the modeling of DC converter and DC control system,based on the customized environment (UD) of China EPRI PSASP simulation system. To make out the characteristics of UHVDC transmission system, simulation is made in various working conditions. The simulation results verify the rationality of the established model, which laid a foundation for follow-up study.This thesis summarizes the definition, classification of power system voltage stability and the stability control method, and analyzes the strength of the end of the power grid and voltage stability under various faults with examples of simulation UHVDC accessed. With respect to the serious fault under the system of voltage instability problems, it puts forward to use STATCOM dynamic reactive power compensation device in the relevant site, and the simulation verify the effectiveness of its effect on improving system voltage stability level.The simulation analyzes the effects of UHVDC on small signal stability of AC system. Through the analysis of various oscillation modes under different operation modes, it is concluded that AC-DC system damping will be changed when the UHVDC project delivers different power. The way of enhancing system damping presented in this thesis is to optimize PSS parameters. The simulated results show that the low frequency oscillation can be restrained by optimizing PSS parameters.Because of the deficiency of STATCOM in restraining power oscillation, and the adaptability of optimizing PSS parameters is not strong, the shunt STATCOM with energy storage is presented in this thesis to solve both problems of transient voltage and power oscillation in AC-DC hybrid system. The simulation results show that shunt STATCOM with energy storage can not only effectively support the system transient voltage and promote voltage of DC inverter station to avoid DC block caused by commutation failure, but also can significantly enhance the system damping and restrain regional power oscillation in case of system failure.To avoid the problems and risks of the strategy for isolating the short-circuit fault, under the fault of DC converter, a new backup protection scheme for short-circuit fault based on the new DC circuit breaker is presented. A novel current-source hybrid electro-mechanical DC circuit breaker is designed. The simulated results validate that the proposed new DC circuit breaker can interrupt the fault of DC transmission line. The new DC breaker and converter blocking have the same effect. The effectiveness of the proposed backup protection based on the new DC circuit breaker is verified.