Research On Control Strategy Of VSC-MTDC System In Urban Power Grid

With the development of society and the continuous advancement of urbanization,the urban power grid is facing some operating problems such as lack of transmission corridors,insufficient internal power source,and over-capability of the short-circuit current at key buses,which cause the urban power grid to meet many power supply security challenges and has an urgent need for new technology to meet the safe operation requirement of the urban power system.Voltage source converter-based high voltage direct current（VSC-HVDC）transmission is considered as an effective solution for urban power system upgrading.In recent years,there are many practices and academic research on the application of the VSC-HVDC system into the urban power grid.With the further wide application and continuous mature of the VSC technology,it could be seen that the multiple terminal VSC-HVDC（VSC-MTDC）will be a developing trend in the urban power grid.However,the effective operation of the VSC-MTDC system will depend on the safe,reliable,and reasonable operation control strategy,which should be paid more attention by academics and industry.In this thesis,the VSC-MTDC system that is applied in the urban power grid is taken as the research object,in order to investigate the control strategy of the VSC-MTDC system in the urban power grid that focused on the safety of the AC/DC hybrid operation,the rapidity of the VSC-MTDC system on the power regulation and operation mode switch and the flexibility of the coordination control strategy on the VSCMTDC system.The main work is summarized as follows:1)Deadbeat predictive control method for converters in VSC-HVDC system.Due to the complex operating environment,multiple operation conditions and higher power supply quality requirements compared to the transmission system,the control strategy of the converter has higher requirements on rapidity,reliability and robustness.This thesis proposes a deadbeat predictive control method for MMCs to control the inner loop current in order to improve the fast response capability of the converter.With the reasonable control of the upper/lower arm voltage,the proposed control strategy realizes the fast and accurate control for multiple control objective in MMC.Due to the complex optimization calculation process,such as the cost function and so on,is not required,the calculation amount of this method can be effectively reduced.Then,two important factors of SM capacitor voltage fluctuation and circuit parameter mismatch of prediction model that may influence the control performance are investigated in detail and the specific control methods for solving these problems are developed to improve the operation performance of the system.2)The fast steady-state operation point adjustment strategy of the VSC-MTDC system.In the high power demand period of the urban power grid,the improper power distribution results of VSC-MTDC system after the DC accident will lead to a large load rate deviation of the key transmission corridors.The VSC-MTDC system can be used to adjust the steady-state operation point of the DC system rapidly to balance the line load rate before ac system is involved and improve the safe operation level of urban power grid.Conventional steady-state operation point adjustment method cannot be applied timely due to the complex coordination between the AC system and VSC-MTDC system.In this thesis,considering the power supply security of the key transmission corridors,a fast steady-state operation point adjustment strategy of the VSCMTDC system is proposed.By putting forward the AC/DC dynamic regulation capacity,the originally fixed rated active power capacity can be changed to flexible capacity,which is related with the active power capability of the key transmission corridors.In this way.after a DC accident,the calculation and decision center can be moved down from the AC/DC system control layer to the DC system control layer,so as to reduce the data exchange between the AC and DC system,reduce the execution link of steady-state operation point adjustment,and improve the response speed of the VSC-MTDC system on the steady-state operation point adjustment after the DC accident.3)VSC-MTDC coordination control strategy.With the application of the VSC-MTDC system into the urban power grid,the design of the coordination control strategy receives more attention.In order to improve the security and flexibility of the urban AC/DC system,a VSCMTDC coordination control strategy is proposed in this thesis.Considering the safety operation requirements of different power supply regions in urban power grid,seven coordination control modes as well as the selection and switching strategy among the coordination control modes are developed based on the regulation capacity of the DC unbalance power in the most serious VSC-MTDC system event.The three coordination control modes under the basic operation mode can effectively realize the accurate and rapid adjustment of DC system under different security operation requirements of urban power grid.The four coordination control modes under the piecewise operation mode can enlarge the active power regulation capacity of VSCMTDC system and reduce the risk of DC voltage out of control when the unbalanced power regulation capacity is insufficient.The selection and switching strategy can realize the orderly selection and switch among the coordination control modes of the VSC-MTDC system according to different operating conditions of each region of the urban power grid.