Stability Analysis For VSC-HVDC Feeding Into Weak AC Systems

Global energy and environmental issues are becoming increasingly serious,and it is imperative to construct a clean,intelligent,efficient and reliable transmission network.Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)transmission technology will have been being widely employed in various application areas.Especially in power supply for weak AC systems,VSC-HVDC has an incomparable superiority to conventional HVDC.However,stability problem is found when VSC-HVDC is used to power weak systems.Based on the above,a series of studies are carried out in this dissertation,focusing on stability analysis for VSC-HVDC feeding into weak AC systems.Firstly,basic principle of VSC-HVDC and vector current control strategy,which is widely employed in VSC-HVDC,is introduced in this paper.In order to improve the dynamic response characteristics of the whole VSC-HVDC system,PI parameters of the control system are optimized with Optimum Run in PSCAD/EMTDC.Secondly,according to reasonable simplification,the reduced order small signal model of VSC-HVDC feeding into low short circuit ratio(SCR)AC systems is deduced.The small signal model of VSC-HVDC system is reduced from 16 th order to 7th order.Modular modeling method is adopted,and the system can be divided into three parts: AC system,control system and converter.To validate the correctness of the proposed small signal model,the corresponding detailed electromagnetic transient model is also established based on PSCAD/EMTDC.Then two models are compared under the small disturbances in time domain in details.The simulation results show that the output waveform of the two models is almost completely overlap,which proves the small signal model is correct.Then,based on the proposed small signal model,the influence of control parameters on the stability of VSC-HVDC is analyzed by using the root locus method.It is found that the parameters of phase locked loop(PLL)and outer loop controller reactive component loop have a very important influence on the stability of the system.Finally,using the same method,small signal modeling and stability analysis for VSC-HVDC feeding into low inertia AC systems is carried out.And it is concluded that the PLL parameters have important effect on the stability of low inertia systems.