Research On The Interactions Between Voltage And Current Source Converters In Dual-infeed HVDC System

With the development of VSC-HVDC and the widely application of LCC-HVDC,hybrid multi-infeed HVDC system has attracted a lot of attentions in recent years.It has theoretical and engineering significance to have a research on the interactions between the VSC-HVDC and LCC-HVDC in multi-infeed HVDC system.In this thesis,the static and dynamic performance of a dual-infeed HVDC system is studied to investigate the mutual impacts between the LCC and VDC converters.To reveal the interactions between LCC-HVDC and VSC-HVDC,three main issues are analyzed,including the small signal model analysis,the quantitative evaluation of equivalent short circuit ratio and the commutation failure and power recovery performance.(1)The mathematical model and control strategies of the dual-infeed HVDC systemIn order to have a research on the interactions between the LCC-HVDC and the VSC-HVDC in dual-infeed HVDC system,the basic structures and control strategies of LCC and VSC are analyzed.The mathematical models are derived,including the ac and dc voltage transformation model and the power transmission model.Then the topology and control strategy of the multi-infeed HVDC system are proposed.Based on the dual-infeed HVDC system,the impacts of different control methods of LCC-HVDC and VSC-HVDC on the commutation failure and power recovery performance of LCC-HVDC are studied.It is demonstrated that the control method of LCC-HVDC will have obvious impact on the commutation failure and power recovery performance of LCC-HVDC,while control methods of VSC-HVDC will make little difference on these performance.Finally,the control strategies of the dual-infeed HVDC system are selected,which is the basic of the following research.In LCC-HVDC,the constant current control method and the constant extinguish angle control method are adopted in the rectifier and inverter,respectively.While the constant dc voltage and constant dc power control strategies are used in the rectifier and inverter of VSC-HVDC,respectively.(2)The stability analysis of the dual-infeed HVDC system based on small signal modelThe small signal analysis is an important way the reveal the interactions between the VSC-HVDC and LCC-HVDC in dual-infeed HVDC system.Firstly,based on the power conservation,the linear models of LCC and VSC in dq frame are derived.Combing the mathematical model of the basic unit in the converter station,including LCC,control system and filter,the small signal models of the LCC and VSC station are derived.Then with each station regarded as a subsystem,the interconnection method of the converters and the transmission model between the variables in different coordinates are built,and then the method to establish the small single model of multi-infeed HVDC system is proposed.Utilizing the dual-infeed HVDC system,the small signal model and the simulation mode in PSCAD/EMTDC are compared to validate the established small signal model.Based on the small signal model,the stability issues with the variation the ac system strength,the power rating of VSC-HVDC and the power rating of the LCC-HVDC are analyzed.The main unstable oscillatory modes and the main participating variables of the system are investigated by eigenvalue and participating factor analysis.It is demonstrated that in the above conditions the variables in LCC and VSC have the similar participation in the main modes.It can be concluded that the LCC and VSC has a strong interaction when the system become unstable due to the decreasing of the system strength.Finally,the effective short circuit ratio of the dual-infeed HVDC system is calculated,it is demonstrated that ESCR can indicate the system strength qualitatively.But for quantitative evaluation,the ESCR will not work and need to be improved.(3)The impedance based effective short circuit ratio in dual-infeed HVDC systemIn order to extend the ESCR index into the multi-infeed HVDC system and reveal the interactions between the LCC-HVDC and VSC-HVDC,the concept of operating impedance of the converter and the operating impedance based equivalent effective short circuit ratio are proposed.Firstly,based on the established small signal model,the port characteristics of the converter are analyzed by applying a small disturbance at the operating point.The relationship between the current and voltage at the converter ac port is analyzed,and then the calculation method of the fundamental frequency equivalent impedance of converter is derived.This impedance is determined by the operating point of the system,thus it is referred as operating impedance in this thesis.At the specific operating point,the ac network is equalized with converter regarded as impedance,and then the impedance based the effective short circuit ratio is proposed.The IESCR method is validated by simulation.Based on the IESCR,the system strength interactions between the LCC-HVDC and VSC-HVDC in dual-infeed HVDC system are analyzed.It is found that the system strength of LCC-HVDC is influenced by the VSC control method,VSC power rating and the distance between VSC and LCC.The impact of LCC-HVDC on the system strength of VSC-HVDC is similar.Finally,based on the small signal model,the smallest IESCR under which the HVDC system can transmit the rated power is analyzed.It is found that the constant extinguish angle controller and the PLL have significant impacts on the smallest IESCR.The system is much easy to operate under the critical ESCR when the control parameters are small.(4)The commutation failure and power recovery performance of the dual-infeed HVDC systemTo reveal the impact of the VSC-HVDC on transient characteristics of LCC-HVDC,the VSC control method,VSC power rating and the distance between VSC and LCC are varied to compare the commutation failure and power recovery performance of LCC-HVDC.It is found that the LCC-HVDC can achieve a better commutation performance when the constant ac voltage control rather than constant reactive power control method is adopted in VSC-HVDC,especially when the distance between VSC and LCC is small and the response of the controller is fast.Then the reactive power of VSC and the commutation voltage and current of LCC are analyzed to reveal the principle of utilizing VSC-HVDC to improve the commutation performance of LCC-HVDC.It is found that the way to improve the commutation voltage of LCC by a paralleled VSC is constrained by the electrical distance and the speed of the controller.To make full use of the voltage source characteristic of VSC,a new LCC topology is proposed,in which the arm comprises of the thyristor valves and sub-modules.It is demonstrated that the proposed topology can further improve the commutation failure and power recovery performance of LCC-HVDC.