Hybrid Design of Modular Multilevel Converters for HVDC Systems Based on Various Submodule Circuits

Abstract

The modular multilevel converter (MMC) has become the most promising converter technology for high-voltage direct current (HVDC) transmission systems. However, similar to any other voltage-sourced converter-based HVDC system, MMC-HVDC systems with the half-bridge submodules (SMs) lack the capability of handling dc-side short-circuit faults, which are of severe concern for overhead transmission lines. In this paper, two new SM circuit configurations as well as a hybrid design methodology to embed the dc-fault-handling capability in the MMC-HVDC systems are proposed. By combining the features of various SM configurations, the dc-fault current path through the freewheeling diodes is eliminated and the dc-fault current is enforced to zero. Several MMC configurations based on the proposed hybrid design method and various SM circuits, that is, the half-bridge, the full-bridge, the clamp-double, and the five-level cross-connected SMs, as well as the newly proposed unipolar-voltage full-bridge and three-level cross-connected SMs, are investigated and compared in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements. The studies are carried out based on time-domain simulation in the PSCAD/EMTDC software environment for various SM configurations and dc-fault conditions. The reported study results demonstrate the proposed hybrid-designed MMC-HVDC system based on the combination of the half-bridge and the proposed SM circuits is the optimal design among all evaluated systems in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements.Index Terms-DC-side short-circuit fault, fault clearance, modular multilevel converter (MMC).

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

“…It is well known that half bridge MMC has many advantages, but one main disadvantage is that it has no ability to cut of the fault current. 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. The first type is the topologies in [6][7][8]. These topologies can store rest energy in the capacitors in SMs and the arms build up the counter-emf; then the current is cut off.…”

Protection of the MMCs of HVDC Transmission Systems against DC Short-Circuit Faults

“…One of the significant drawbacks of a MMC-HVDC using an HBSM is its lack of DC-fault current blocking capability, where the freewheeling diode operates as a rectifier and produces an excessively high DC fault current from the AC grid through the short-circuit point in the DC link as illustrated in Fig. 2 [6]. So far, there have been three kinds of approaches that theoretically and practically deal with DC-cable faults in HVDC systems [7]- [14].…”

Neutral-point-clamped hybrid multilevel converter with DC fault blocking capability for medium-voltage DC transmission

“…Compared with the HCMC, the proposed NHMC replaces the TLC stage with a three-level NPC stage and replaces the FBSMs with two types of unipolar full-bridge SMs (UFBSMs) [21], thereby reducing the number of IGBTs and the cost of the converter. The special hybrid design offers the proposed NHMC the capability to control the DSs to switch the fundamental frequency while working in a wide modulation range.…”