Modulation Method And Fault Isolation Of Hybrid Modular Multilevel Converter In Flexible DC System

How to efficiently isolate short-circuit fault at DC side is one of the problems of flexible DC transmission and distribution system. Hybrid modular multilevel converter(MMC) is a newly proposed converter topology applied in flexible DC grid, whose arms contain both half-bridge sub-modules(HBSM) and full-bridge sub-modules(FBSM). This type of converter not only has the FBSM's ability of isolating two-pole short-circuit fault in DC side, but also costs less power devices for HBSMs' adopting, which is a practical converter topology. Hybrid MMC' control strategy is studied in this paper, and a phase shifted carrier modulation method with sub-module's(SM) s capacitor voltage balance strategy is proposed for hybrid MMC. Based on this, under two-pole short-circuit fault at DC side, hybrid MMC with different proportions of FBSM's fault isolating capability is further analyzed. The main content of this paper is as follows:Based on the different operation principles of HBSM and FBSM of hybrid MMC, a phase shifted carrier modulation method with SM capacitor voltage balance strategy is proposed. Through AC side output voltage control and inner circulating current suppressing control, each arm's reference voltage can be acquired. The drive signal of FBSMs and HBSMs can be then respectively produced by using this proposed modulation method. Under the control of drive signals, FBSMs and HBSMs work collaboratively, and each arm of hybrid MMC output corresponding reference voltage. Use this modulation method to design the drive circuit, hybrid MMC can output high quality AC waveform at a low switching frequency, in addition, the switching frequency of FBSMs and HBSMs' IGBTs are respectively equal. This proposed modulation method and SM capacitor voltage balance strategy is validated by simulation.The mechanism of hybrid MMC with different proportions of FBSMs' isolating a two-pole short circuit fault at DC side is studied. The equivalent circuit diagram under the fault is proposed and the expression of isolating voltage of FBSM capacitor are derived. The fault process can be derived into three phases: short circuit fault phase, SM capacitor charging phase and fault isolated phase. During SM capacitor charging phase, the process of FBSM capacitor charging is the key to isolate a two-pole short-circuit fault at DC side. The isolating voltage of FBSM capacitors among hybrid MMCs with different proportions of FBSMs is derived, and the proportion of FBSMs is also derived to ensure FBSM's isolating voltage not higher than normal operation voltage, the conclusion is drawn: the cost of power devices can be reduced by lowering the proportion of FBSMs, at the price of raising the withstand voltage of the FBSM capacitor. Thus, the proportion of FBSMs should be determined after considering all relevant factors. The derivation is validated by results of simulations. Meanwhile, the expression of isolating voltage of FBSM capacitor can be used to guide the design of withstand voltage of hybrid MMC's FBSM capacitors.