Research On The Effect Of VSC-HVDC Power Modulation On Power Grid Stability

In recent years,under the trend of protecting environment and building the efficient intelligent transmission network,the large regional power grids are interconnected and a large number of new energy sources are connected to the power system.As the scale of the power grid expands and the operation mode becomes more complex,the power network stability problem,including sub-synchronous oscillation(SSO)and voltage sag fault,is becoming more and more important.With the development of power electronics and control technology,the proportion of voltage source converter based high voltage direct current(VSC-HVDC)is increasing gradually and the effect of VSC-HVDC on power grid is increasingly important.VSC-HVDC is composed of full-controlled power electronic devices and adopts PWM control technology.It can realize fast and independent control of active and reactive power with good control performance.Making full use of VSC-HVDC power modulation can quickly adjust output power of the converter,which can provide emergency power support for AC system during fault period and enhance the stability of power grid.Combined with this problem,this paper first introduces the structure of VSC-HVDC system,analyses its operation principle,deduces the mathematical model of the system in different coordinates and studies the converter vector control strategy with active and reactive power decoupling.On this basis,the simulation model is built and the results show that VSC-HVDC has fast active and reactive power control response characteristics.For the sub-synchronous oscillation problem,this paper studies how to use VSC-HVDC power modulation to improve system damping to suppress power oscillation and ensure stable operation of the power grid.In this study,the equivalent mathematical model and transfer function of the system is obtained by Prony identification method.Then the Supplementary Sub-synchronous oscillation Damping Controller(SSDC)in VSC-HVDC control system is designed based on pole configuration method.A sub-synchronous oscillating system model with parallel VSC-HVDC is built and the simulation results verify the effectiveness of the SSDC designed in this paper.When AC-side failure leads to power grid voltage sag,the control strategy of Multi-Terminal VSC-HVDC(VSC-MTDC)system is designed in this paper.It provides emergency power support for the power grid by quickly adjusting the output power of the converter station,and finally realizes the voltage stability.On the basis of providing reactive power support to maintain the voltage level,this control strategy takes full account of the power limiting level of converter and the stability of the DC system.Combined with the droop control strategy,the method of reducing active power of fault-side converter is proposed to avoid overcurrent.The effectiveness of this control strategy is verified by the simulation of a three-terminal VSC-MTDC model.