Design And Control For AC Voltage Boosted FBSM-MMC

Modular multilevel converter(MMC)is a promising candidate for VSC-HVDC systems owing to its advantages of modular design,low switching frequency,high efficiency,and excellent output voltage waveforms.The half-bridge sub-module(HBSM)based MMC(HBSM-MMC)is the main topology in the existing VSC-HVDC projects.The drawback of HBSM-MMC is that it lacks DC fault handling capability.It has to rely on an AC or DC breaker to clear the DC fault current.The AC breaker handling process is rather slow and complicated,and the DC breaker is expensive and has not been thoroughly studied.Therefore,converters with DC fault handling capability,such as full-bridge sub-module(FBSM)based MMC(FBSM-MMC)have attracted considerable attention in recent years.However,the addition of power devices in FBSMs results in higher converter losses and costs,which are the main obstacles in the application of FBSM-MMC to practical projectsReducing the SM capacitance,converter loss are crucial for economical and reliable operation of FBSM-MMC.To decrease the SM capacitance,this paper proposes an SM capacitance reduction method for FBSM-MMC,which combines AC voltage boosting and circulating current injection.The negative voltage state of FBSMs is fully utilized to boost the AC voltage under fixed DC voltage.And by increasing the AC voltage,the SM capacitor voltage ripple can be slightly decreased.To further decrease the SM capacitor voltage ripples,the second-harmonic voltage is added to the reference arm voltage.The capacitor voltage ripple can be controlled to be a targeted value under all operating conditions.Consequently,the capacitance requirement for AC voltage boosting FBSM-MMC can be significantly reduced as compared to conventional FBSM-MMC.Experimental results confirm the feasibility and validity of the proposed SM capacitance reduction method.To decrease the converter loss,this paper proposes a coordinated control method of power loss reduction and capacitor voltage balancing for AC voltage boosted FBSM-MMC by controlling the second harmonic component in the circulating current.The analytic expressions of converter loss and SM capacitor voltage ripple are derived.Based on the analysis,the optimal second-harmonic circulating current is injected into the arm current to optimize the converter loss and ripple.Simulation results of an 400MW/±200kV AC voltage boosted FBSM-MMC using PSCAD/EMTDC confirm the feasibility and validity of the proposed loss and ripple reduction method.