The Coordinated Fault Current Limiting Strategy for a Hybrid Multilevel Modular Converter (MMC) based VSC-HVDC Applications

Background: High-voltage direct current (HVDC) is suitable for high capacity and longdistance power transmission, thus becoming ideal for connecting renewable energies such as solar power and wind power to grids. Objective: Overhead lines in HVDC are vulnerable to short-circuit faults. Non-permanent DC short circuit faults are the most common in HVDC transmission, which can lead to pause in power transmission and interruption in large grids. Thereby, it is crucial to deploy techniques to suppress fault current. Method: To lower the fault current economically, a coordinated fault current limiting strategy based on a hybrid multilevel modular converter (MMC) is proposed in this paper. Results: Combining hybrid MMC and small-capacity DC circuit breaker reduces total IGBTs required and avoids MMC blocking during pole-to-ground short-circuit fault. This approach is verified using a two-terminal MMC-based system in PSCAD/EMTDC simulation environment. Conclusion: By implementing the introduced scheme, the peak fault current can be lowered by 33.0% using hybrid-MMC with 80% of FBSMs. Economic efficiency can be improved by adopting proposed scheme.

A Minimal Capacitor Modular Converter for Ultra-Dense Power Conversion

<div>Modular multilevel power electronic converters are considered an increasingly critical family of converters for myriad high voltage high power applications. With the ever-growing emphasis on electrification of the economy, they play a crucial role in serving energy sources and loads whose electrical ratings go beyond the ratings of the conventional power electronic building blocks. In particular, modular multilevel converter (MMC) topology enjoy its dominance in such applications due to modularity, scalability, performance and fault-tolerance capability. However, the MMC topology design imposes low-frequency ac components on the module capacitors and thus is inhibited by the capacitor size. Capacitor sizing plays a significant role in the overall system’s size, cost and reliability. This paper introduces a minimal capacitor module based topology for DC to three-phase AC conversion. The unique design feature of the module includes minimal capacitor requirement due to elimination of single-phase ac power processing requirements. Together with improved power density, reduction of capacitor size permits the use of only film capacitors thus eliminating the weakest link of the overall system. Along with the step-by-step analytical derivation of the proposed approach, the paper presents detailed simulation studies, comparative analysis and experimental results from a proof-of-concept laboratory-scale prototype.</div>

A Minimal Capacitor Modular Converter for Ultra-Dense Power Conversion

<div>Modular multilevel power electronic converters are considered an increasingly critical family of converters for myriad high voltage high power applications. With the ever-growing emphasis on electrification of the economy, they play a crucial role in serving energy sources and loads whose electrical ratings go beyond the ratings of the conventional power electronic building blocks. In particular, modular multilevel converter (MMC) topology enjoy its dominance in such applications due to modularity, scalability, performance and fault-tolerance capability. However, the MMC topology design imposes low-frequency ac components on the module capacitors and thus is inhibited by the capacitor size. Capacitor sizing plays a significant role in the overall system’s size, cost and reliability. This paper introduces a minimal capacitor module based topology for DC to three-phase AC conversion. The unique design feature of the module includes minimal capacitor requirement due to elimination of single-phase ac power processing requirements. Together with improved power density, reduction of capacitor size permits the use of only film capacitors thus eliminating the weakest link of the overall system. Along with the step-by-step analytical derivation of the proposed approach, the paper presents detailed simulation studies, comparative analysis and experimental results from a proof-of-concept laboratory-scale prototype.</div>

Multi-Modular Converters with Automatic Interleaving for Synchronous Generator Based Wind Energy System

Among different options available for wind energy system, this research is focused on direct driven Synchronous generator based variable speed wind turbines that are connected to power grid via modular converter units. Compared to single full size power converter, modular design has higher reliability/redundancy, better harmonic performance, lower developmental cost and higher efficiency. Better harmonic performance of modular structure is possible through interleaving which effectively reduces ripple in the output current, enabling use of smaller sized filter components. Focus of this research is to design a controller that can perform automatic interleaving of modular three-phase converters used in above cited wind energy system. Developed control algorithm will have critical decisions carried out by local controllers. With minimum communication overhead the controller will ensure interleaved operation of parallel modules under all conditions. Developed control algorithm is verified through simulation and laboratory testing. Results prove effectiveness of the designed controller.