Control And Protection Strategies For MMC-HVDC Grid

With the development of HVDC technology and engineering based on IGBT,VSC-HVDC grid has drawn intensive attention.Compared with traditional two-level or three-level VSC topology,modular multilevel converter(MMC)is one of the most promising converters for future VSC-HVDC grid because MMC exhibits attractive advantages for high-voltage levels of power transmission,such as lower switching frequency and energy loss.MMC-HVDC grid is a DC network with a plurality of MMCs,partially meshed and partially radial.MMC-HVDC grid is able to supply different power to users in multiple space span.However,the MMC-HVDC grid still faces many challenges,including establishment of protection mechanism like that in AC grid and coordinated control strategies between converters.Given this background,some topics of control and protection strategy for MMC-HVDC grid is preliminarily studied.The main focuses of this dissertation can be summarized as follows:1)The basic principle of MMC-HVDC and the DC-side fault characteristics of pole-to-pole short-circuit fault in MMC-HVDC are analyzed.Then the fault current before the MMC blocking of DC pole-to-pole short-circuit fault is calculated.By comparing three isolation techniques of DC fault including AC circuit breaker,MMCs with fault blocking capability and DC circuit breaker(DCCB),DCCB is considered to be the most promising because it can interrupt DC fault both selectively and quickly.2)A mature hybrid DCCB topology proposed by ABB is introduced.Then based on a differential-current fault detection technology,a DC grid protection scheme is proposed,which includes a DC transmission line protection,a DC busbar protection that can consider the distributed line capacitive current.The hybrid DCCB is used to clear DC fault selectively and quickly.A scheme of failure protection in MMC-HVDC grid is proposed,which can send command to adjacent DCCBs to interrupt DC fault completely if any DCCB fails.3)The control principles between DC and AC grid are compared.Three main coordination control strategies for multi-terminal HVDC grid(MTDC)are analyzed.Based on the MMC-HVDC grid model proposed by CIGRE,a coordinated control strategy of DC voltage for complex MMC-HVDC grid with a plurality of MMCs partially meshed and partially radial is proposed.