Research On Coordinate Control For The VSC-HVDC Grid Based On Modular Multilevel Converter

The voltage-source-converter-based high-voltage direct-current(VSC-HVDC)system has already achieved independent active-and reactive-power control.The advent of modular multilevel converter(MMC)enables a VSC station with high voltage and capacity levels and excellent output voltage quantity,while reducing switching frequency and losses and operating consistency of switch devices.Therefore,VSC-HVDC is commercially used to deal with the tough situations,such as large scale power transmission and integration of renewable power sources.So far in China,it has Nanao and Zhoushan multi-terminal VSC-HVDC systems for integrating offshore wind farms.Hence,HVDC grid,which contains converter stations,renewable power sources and energy storage devices,is not a concept but a reality for the future's transmission system.Coordinate-control achieves power balance between the VSC-HVDC grid and its host AC systems and maintaines a stable DC voltage for the DC system.This paper proposes an enhanced DC voltage droop-control for the VSC-HVDC grid as a coordinate-control strategy.The proposed droop-control divides the VSC stations into 4 groups,and then voltage margins and dead-bands are assigned for each converter station group to determine their priorities in enabling the droop-control.Thus,the pre-specified voltage margins and dead-bands are not affected by the number of converter stations in the HVDC grid.Therefore,the feasibility of the proposed droop-control is better than that of the exsiting droop-control.This paper also provides an optimal solution for voltage margins and dead-bands,considering over-correction and adjustment-time of an electric variable in transient states and new steady-state DC volage.An improved power-voltage characteristic is presented to obtain droop rates more convenient based on the practical situations.Discussion on cooperations between the proposed droop-control and additional DC voltage control,and between the proposed droop-control and smooth start strategy are made in this paper to show the compatibility of the proposed droop-control.The aforementioned cooperation ways are good indicators for other control strategies which need to cooperate with the droop-control.The EMTDC-based,MMC control system models are established in this paper for large scale simulations of the HVDC grids.Equivalent functions are established and the oringial control components are unnecessary.Multi-demention signal interfaces are designed for the difined control models so that the convertion of signal dementions is eliminated.Therefore,the simulation platform using the defined control models reduces the labor and complexity of modeling the control system of an MMC.The effectiveness and feasibility of the proposed control strategies in this paper is tested by a large scale of AC-DC hybrid system.The achievements of this paper can be good indicators for the practical applications.