Design and Operation of a Hybrid Modular Multilevel Converter

Abstract

This paper presents a hybrid Modular Multilevel Converter (MMC), which combines full-bridge sub-modules (FBSM) and half-bridge sub-modules (HBSM). Compared with the FBSM based MMC, the proposed topology has the same dc fault blocking capability but uses fewer power devices hence has lower power losses. To increase power transmission capability of the proposed hybrid MMC, negative voltage states of the FBSMs are adopted to extend the output voltage range. The optimal ratio of FBSMs and HBSMs, and the number of FBSMs generating a negative voltage state are calculated to ensure successful dc fault blocking and capacitor voltage balancing. Equivalent circuits of each arm consisting of two individual voltage sources are proposed and two-stage selecting and sorting algorithms for ensuring capacitor voltage balancing are developed. Comparative studies for different circuit configurations show excellent performance balance for the proposed hybrid MMC, when considering dc fault blocking capability, power losses, and device utilization. Experimental results during normal operation and dc fault conditions demonstrate feasibility and validity the proposed hybrid MMC.

Full bridge MMC converter optimal design to HVDC operational requirements

“…Each HB submodule employs 2 IGBTs, bypass switch and bypass thyristors which have lower costs and will be neglected in this study. According to (19) and (20), the relationship between the total number of IGBTs per arm and the k MMC is:…”

“…Successful HB submodule balancing demands that the arm current has positive and negative segments in each cycle. This implies that the peak of AC component in (34) must be larger than DC component: In steady state, the instantaneous dc power equals the AC power, and therefore using (8), (23) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Therefore, for a FB-MMC with design requirement V minpu ≥0.5k MMC , the N FB and N HB can be dimensioned according to (19) and (20). For the other design requirement -0.5k MMC <V minpu <0.5k MMC , to enable successful voltage balancing of the submodule voltages all the inserted submodules should be of FB type.…”

“…Combining (19), (20), (39) and (40), generic method of calcuating the number of FB submodules and HB submodules in a FB-MMC under any specified design condition is: Fig. 4 shows the number of required FB submodules (in pu relative to the total number of submodules in an arm N sm ) as the function of V minpu (in pu relative to V dcn ) and k MMC .…”

Fault Handling Methods and Comparison for Different DC Breaker Topologies and MMC Topologies of the HVDC System

“…Hence, many MMC submodule topologies are developed based on the half bridge MMC, such as full bridge SMs [6] and clampdouble SMs [7]. Because the modified SMs have more losses during normal operation, the hybrid MMC is considered [8]. This second method has two types.…”

Protection of large partitioned MTDC Networks Using DC-DC converters and circuit breakers

“…Thus, fast and reliable protection is mandatory for fast fault clearance to avoid complete shutdown of the entire network so as to minimize the disturbances to the connected AC networks. Some MMC topologies [15][16][17] can block or control DC fault current but resulting additional capital cost and power loss. In addition, such converter topologies cannot isolate fault from the MTDC network apart from protecting themselves from over-current so additional DC protection equipment is still required.…”