Research On Topology And Stability Of Hybrid High Voltage Direct Current System

The traditional energy and load center in China is in reverse distribution,Wind energy,solar energy and other renewable energy are concentrated in the central and western regions.At the same time,the environmental pollution in the eastern region is severe with frequent bad weather.Increasing the amount of clean energy power generation and using large-scale and long-distance power transmission have become the inevitable choice for solving the energy and environmental problems in China.Compared to AC transmission,DC transmission is a more ideal way of transmission.HVDC is divided into conventional HVDC and flexible HVDC transmission.Conventional HVDC transmission has the advantages of large capacity,mature technology and good economic benefit,but the problem of commutation failure threatens the safe operation of power grid.VSC-HVDC transmission is more advantageous in the fields of new energy grid connection and large city power supply,but its running loss is large and it is not economical.Combining conventional HVDC and flexible HCDC system into hybrid HVDC transmission system,to some extent,can make use of their advantages to make up for the shortcomings.In some applications,it can improve the overall transmission benefit,and has good prospects for development.In the process of power system operation,any disturbances are likely to make the system in a non-normal state.The research on the stability of hybrid HVDC transmission system will help to deepen the understanding of this nonlinear complex transmission system and improve the comprehensive application of the system.The main contents of this paper are as follows:(1)On the basis of summarizing the characteristics of LCC and VSC,The advantages and disadvantages of the hybrid HVDC system topologies with engineering prospects and the basic control strategies and applications are analyzed.Hybrid HVDC transmission technology has a very broad development prospect in the new energy grid,large city power supply and other occasions.(2)Using appropriate assumptions,the state space equation of each module of the conventional HVDC transmission system and the flexible HVDC transmission system is established,and the small signal model of each module is obtained by linearizing the state space equation.(3)A small signal model of a hybrid dual infeed HVDC system is established in Matlab.In order to verify the correctness of the model,the small signal model is compared with the electromagnetic transient model in time domain.When the system instruction changes,the dynamic response of the two model is almost identical.Study on the stability of hybrid doubly fed DC transmission system using small signal model.In addition to the system stability affected by the strength of the AC system,the LCC and VSC control system parameters will affect the system stability under weak systems.The influence of VSC outer loop reactive power control has a great influence on the stability of the system.As the parameter increases,the system stability decreases or even instability occurs.