Research On Control Strategy Of VSC-HVDC Inverter Supplying Power To The Isolated Grid

With the development of power electronics and power system,high voltage direct current transmission system based on voltage source converter(VSC-HVDC)has been researched and applied widely.With the expansion of the offshore drilling platform and the development of the island,the power consumption of the offshore drilling platform has also increased.More and more attention has been paid to the mode of power transmission from the VSC-HVDC to the island grid when the installed power supply in the island is unable to satisfy the demand for the increased load.In order to ensure the reliability of VSC-HVDC power supply to the isolated island grid,it is necessary to study the control strategy of VSC-HVDC according to the characteristics of the isolated island grid.Therefore,it is of great significance to study the VSC-HVDC control strategy with the different running state of the isolated island grid to improve the reliability of the power supply.In this context,the following research is carried out on the control strategy of the inverter station for VSC-HVDC supplying to the isolated island grid.Firstly,according to the situation that the isolated island grid has changed from an active power grid to a passive power grid,a control switching mode of VSC inverter is designed,and control strategies of phase-following and active power priority are proposed.The electromagnetic transient simulation model of VSC-HVDC supplying power to the isolated island grid is built on PSCAD.The correctness of phase-following and active power priority control strategy is proved by simulation.The switching scheme can effectively ensure the reliability and continuity of the power supply from VSC-HVDC to the isolated island when the isolated island grid is transformed from the active power grid to the passive power grid.Secondly,considering the uncertainty of the fault and the inaccuracy of the switching time,an additional control strategy for VSC inverter station is proposed.The reliable and stable power supply to the isolated island power grid can be guaranteed by reasonably designing the control of VSC inverter station without switching the control mode when the isolated island grid will change from an active network to a passive network.Based on the mathematical model of induction motor,the transfer function model of the island grid can describe the relationship between the frequency and the active power of the passive network,and the VSC active power instruction value is regulated by the frequency deviation of the isolated island grid.The additional frequency control strategy of VSC inverter is proposed,and the controller is designed based on the pole placement method.The simulation model is established in PSCAD/EMTDC,simulation results verify that the additional frequency controller not only can maintain the stability of the isolated grid when the grid is changed from the active network into passive network,but also can suppress the disturbance of active grid or passive grid and achieve effective control for the active grid or passive grid.Finally,considering the diversity of island power grid running states,the transfer function model of the island power grid is uncertain.Based on the robust control theory,a robust additional controller for VSC inverter station is designed,which is effective not only for the two running conditions of isolated island,but also has good controller performance under different operation modes of the island,and the controller is robust.The PSCAD simulation proves that the proposed robust additional controller is effective and robust,and can achieve continuous power supply from VSC-HVDC to the active system and passive system under multiple operation conditions...