The Research of SM Topology With DC Fault Tolerance in MMC-HVDC

Abstract

The high voltage direct current transmission system based on modular multilevel converter (MMC-HVDC) technology is now studied and used widely. But traditional half bridge topology of sub module (HBSM) of MMC can't block dc link fault current because of the freewheeling effect of diodes. In order to solve this problem, firstly, this paper improves HBSM topology, a series connected double sub module (SDSM) is designed and dc link fault current blocking capability is realized without tripping ac circuit breaker, so MMC-HVDC with this improved topology can immediately rebuild dc link voltage and resume power transmission. Secondly, the self starting and fault disposing procedure are also designed. At last, simulations in the software PSCAD/EMTDC are carried out to verify its effectiveness and feasibility.

STATCOM Operation Scheme of the CDSM-MMC During a Pole-to-Pole DC Fault

“…An activating threshold current is set as thrice the size of the rated dc-link current [5]. If the dc-link current exceeds the activating threshold and at the meantime the dc-link voltage drops to almost zero, then a pole-to-pole dc fault is detected [22]. If the dc-link current drops to zero, then the fault type is a broken line fault.…”

“…To ensure that the diodes in the blocked arms are reverse-biased, the following equations should be followed by substituting (22) to (5)(6):…”

Simplified Analytic Approach of Pole-to-Pole Faults in MMC-HVDC for AC System Backup Protection Setting Calculation

“…Under pole-to-pole faults, the AC system feeds the fault current through the anti-parallel diodes of MMC arms even when the MMC is blocked [14][15][16]. AC circuit breakers are employed to clear conduct periodically, consequently, the variation of fundamental frequency current shows periodicity.…”

“…In the conducting set {D 1 ,D 2 ,D 3 ,D 4 }, current i 2 has reached its peak value. Thus, the peak value and phase of i ac is obtained, and is rewritten to form Equation (15). …”

Full bridge MMC converter optimal design to HVDC operational requirements

“…Several topologies of DC fault tolerant MMC have been reported in the literature [9] - [18], such as the MMC using the clamp double submodules [9], the hybrid cascaded multilevel converter (HCMC) [10], the alternate arm converter [11], a series connected double sub-module [12], the cross-connected half-bridge submodules [13], the hybrid MMC with HB submodules at the DC side and FB submodules at the AC side [14]- [15], the MMC based on unipolar voltage full-bridge SM and three-level cross-connected SM [16], the MMC based on FB (full bridge) submodules [9], [17]- [18] and the MMC with mixed HB submodules and FB submodules [19]- [21]. Among all these DC fault tolerant MMCs, the MMC based on FB submodules (including the mixed submodules MMC) is the only commercially available fault tolerant technology for HVDC application.…”