Study On Control Strategy Of Additional Frequency Regulation And Asymmetric Fault At Receiving End For Flexible HVDC Transmission

With the development of flexible HVDC transmission technology,more and more flexible HVDC transmission systems are put into operation.Compared with AC transmission technology,HVDC transmission has many advantages,but the traditional control strategy of flexible HVDC transmission system can not respond to the disturbance of AC system.Therefore,when the proportion of DC transmission system connected to power grid is more and more,the inertia and frequency modulation ability of the system are relatively insufficient.The HVDC system can provide inertia for AC system and participate in AC side frequency modulation,which can provide strong frequency support for AC power grid and is conducive to maintaining the stability of AC power grid.When an asymmetric fault occurs in the flexible HVDC system based on MMC,negative sequence current and circulating current will be generated under the traditional control strategy,which will bring additional power loss and threaten the safe operation of MMC converter.In the case of asymmetric fault,the improved control strategy can protect the converter,reduce the loss,and make the converter continue to transmit power without quitting operation,which is conducive to improving the reliability of HVDC system operation.Firstly,aiming at the problem of participating in AC system frequency regulation,a frequency regulation control strategy for flexible HVDC system is proposed.Retaining the advantages of droop control,the system can automatically distribute power without communication on DC side.The AC side frequency is adjusted without error by frequency deviation feedback,and the virtual inertia control strategy is used to provide inertia for the system.It can stabilize frequency and reduce Transient fluctuation and overshoot in the process of frequency modulation.In addition,aiming at the contradiction between frequency fluctuation suppression and DC side voltage deviation in the selection of inertia coefficient,an adaptive virtual inertia coefficient control method is further proposed.It can take into account both the safety requirements of DC voltage and frequency modulation,so that the inertia can be adjusted dynamically according to the DC voltage state.When the DC voltage deviation is large,the inertia can be reduced to prevent voltage exceeding the limit,otherwise the inertia can be improved to obtain better suppression effect of frequency fluctuation.Secondly,aiming at the problem of MMC-HVDC system control strategy under asymmetric fault,the traditional control strategy of MMC-HVDC system is improved.Considering the the coordination of all aspects,a joint control strategy is proposed to improve the performance of the controller under fault.by combining phase sequence decomposition algorithm,improved inner loop controller and improved circulating current suppression controller,the strategy can reduce the negative sequence current and circulating current,and realize the safe and reliable operation of MMC-HVDC system under asymmetric fault.The work can also provide reference for further research on fault characteristics and quantitative evaluation under asymmetric fault.Finally,simulation models of five terminal VSC-HVDC and two terminal MMC-HVDC are established based on PSCAD / EMTDC.The simulation analysis of the proposed frequency modulation control strategy is carried out in five terminal VSC-HVDC system,and the simulation analysis of the control effect under asymmetric fault condition is carried out in two terminal MMC-HVDC system,which verifies the feasibility and effectiveness of the proposed control strategy.