Research On Analysis And Control Strategy Of VSC For Improving Stability Of Power System

In order to absorb the renewable energy reliably and promote the energy revolution,voltage source converter(VSC)has drawn attention from both industry and academia.VSC has many technical advantages,such as immunity against commutation failure,low harmonic levels and ac voltage support capability,etc.Therefore,VSC can be widely used in different scenarios,such as power transmission between cities,supplying power to passive systems and wind power integration,etc.Howerver,in some special scenarios,problems on VSC analysis and control strategies still need further research.In this paper,the AC/DC power system with VSCs is taken as the research object and the issues on analysis and control stratigies under special scenarios are studied.The main works are organized as follows(1)A VSC control strategy to enhance the fault ride-through capability of the passive industrial load is designed.Since industrial loads are relative sensitive to voltage drops,it's essential to guarantee the stability of voltage during severe faults.First,the main factor that affects the ac voltage in the passive industrial system is analyzed.Secondly,according to the analytical results,a modified current limit strategy is proposed and a frequency hysteresis control is added to the VSC controller.The simulation tests under different faults are done in PSCAD/EMTDC,and the results verify the validity of the control methods.(2)A VSC-HVDC control strategy for improving frequency stability of segmented power grid is proposed.In the multi-infeed HVDC system,since the inverters are located in close proximity,concurrent commutation failures of the HVDC system may happen during faults.In order to alleviate the problem,the dc-segmentation technique based on VSC-HVDC transmission technology is planned to be used in Guangdong power grid of China.The Guangdong power grid is planned to be split into two asynchronously operated sectors,i.e.,the eastern and western sectors.However,after segmentation,the frequency regulation capacity of generators is small and the frequency stability of the system is poor.In this case,it is necessary for the VSC-HVDC system to participate in controlling the frequency of the segmented power grid.This paper proposes a frequency control strategy for the VSC-HVDC transmission system.The development of the control method includes two steps.First,the d-axis control scheme of the VSC station is proposed.The VSC stations can adjust the active power automatically.Secondly,the q-axis auxiliary control scheme is added to the VSC controller,which can further enhance the frequency stability of the system.The method is carried out in Guangdong power system by use of PSS/E.The results show that under the proposed frequency control strategy,the frequency stability of the segmented power grid can be improved efficiently.(3)A method of selecting the optimal access point for the offshore wind farm transmitted by the VSC-HVDC system is proposed.The optimal access point is chosen from the alternative access points.The alternative access schemes are made by system planners considering limitation factors such as the effective transmission of the wind power.This paper focuses on the voltage characteristic of the receiving power grid under different access schemes The development of the selecting method includes three steps.First,three major factors,which have significant impacts on deciding the optimal location of the onshore VSC,are introduced,i.e.,voltage support from onshore VSCs,voltage fluctuation caused by the change of wind power and construction costs of the VSC-HVDC transmission system.Secondly,three evaluation indexes that are corresponding to the three factors are put forward.Thirdly,in the face of this multi-objective programming problem,a linear weighted model is proposed in this paper.The comprehensive weight method based on the theory of information entropy and the analytic hierarchy process method is adopted to get the weight coefficients.The method is carried out in Zhejiang power system of China.The results demonstrate that the proposed methodology is an effective way for selecting the optimal access point of the offshore wind farm.(4)The concept of constant voltage operation realized by the MMC-STATCOM is proposed.Firstly,the benefits of constant voltage operation are discussed,i.e,resisting disturbances,eliminate the voltage fluctuation problem involved with the integration of large scale renewable energy and minimzing the grid power loss.This paper proposes a method for evaluating the capacity of the MMC-STATCOM needed for achieving constant voltage operation.The simulation is carried out in Guangdong power grid.The results demonstrate the effectiveness and feasibility of constant voltage operation.