Research On Relative Issues Of VSC-HVDC Links On AC Power Grid

DC grid has become the prevailing evolution of VSC-HVDC development for its technical advantages in flexibility,reliability and economy.Because of the defects of traditional master-slave control strategy and voltage droop control,it is in great urgent to develop a novel control strategy in order to fill the gap.With the great VSC-HVDC project planning to be built,the dispatching and operating schedule of AC/DC hybrid system should be solved.Meanwhile,on account of the fast and flexible control of VSC-HVDC,it can be used as auxiliary controls for AC/DC hybrid system in addition to power transmission.Based on these,this paper launched a study on the control strategy of DC grid and the operation and control of AC/DC hybrid system.In this paper,firstly,the basic principle of VSC-HVDC system,the type and mathematical model of voltage source converter are introduced.On this basis,the control strategies of DC grid are studied and the merits and demerits of these strategies are analyzed.A hierarchical and reginal power flow strategy is designed which composed of error-accumulation control,regional power flow control and emergent power support control.This control strategy is simulated on a seven-terminal two-region DC Grid.The simulating result proves that the control strategy is able to eliminate the active power deviation in steady state and achieve the regional management of a DC grid.Secondly,the operating and dispatching schedule of AC/DC hybrid system is studied in this paper.The optimal power flow problem of AC/DC hybrid system is researched and the power flow equations of AC/DC hybrid system are analyzed.An AC/DC power flow algorithm is proposed focused on MTDC and AC hybrid system,taking master-slave control and voltage droop control strategy into consideration.Based on a 30 nodes AC/DC hybrid system,the optimal power flow in steady state and the adjustment strategy of variables after fault are calculated considering the optimal of generator cost.Finally,based on the fact that damping controllers can be added to either the active or reactive control loop of VSC-HVDC to improve the damping of the power oscillation,these two ways of damping control design were studied and validated.The simulation is carry on a back-to-back VSC-HVDC and AC hybrid system.Results show that combination of damping controls based on active control and reactive control could fast decrease power oscillation as well as stabilize voltage variation.