Search

Medium‐/High Voltage DC grids are interesting for the integration of renewable energy sources. DC‐DC conversion systems are highly needed for the development of DC grids. Recently, Modular Multilevel Converter (MMC) is the most promising technology for medium‐/high‐voltage applications, but employing the conventional MMC with DC output voltage leads to diversion in the Submodule (SM) capacitor voltages, that is energy drift. This paper proposes a modified modular multilevel DC‐DC converter with self‐energy equalization, which ensures successful DC‐DC conversion with balanced capacitors voltages. In the modified topology, clamping Insulated Gate Bipolar Transistors (IGBTs) are employed in each arm to enable parallel‐connection of the capacitors in the same arm. During the operation (equalization period), the parallel‐connected upper capacitors are connected to the parallel‐connected lower capacitors in each leg through a small limiting inductor to transfer energy between the arms to ensure balanced capacitor voltages. The proposed configuration, along with the operational concepts, mathematical analysis, and design, are presented. Finally, simulation and experimental results are presented for validation. [ABSTRACT FROM AUTHOR]

Conventional modular multilevel converters (MMCs) are promising converters for high-voltage high-power applications. Unlike DC-AC conversion, DC-DC conversion of MMCs suffers from unbalanced capacitor voltages due to the energy drift problem. In this study, a new modular DC-DC converter is proposed, which consists of front-end half-bridge MMC with half-bridge submodules followed by full-bridge MMC with full-bridge submodules. The proposed modular multilevel DC-DC converter can be used effectively for connecting two different DC voltage levels in high-/medium-voltage DC grids. The proposed architecture provides successful DC-DC conversion with balanced capacitor voltages throughout the arm interchange concept. It also provides a high conversion ratio, bidirectional power flow, low losses, low current stresses, reduced number of submodules, and DC fault blocking capability. Detailed illustration, operational concept, and design of the proposed architecture with the arm interchange concept are presented. Simulation results for a high-voltage high-power DC-DC converter with the suggested approach are presented to demonstrate the system performance during different normal as well as abnormal operating conditions. Finally, a scaled-down prototype has been implemented for validation. The extracted simulation and experimental results elucidate the presented claims. [ABSTRACT FROM AUTHOR]

This paper proposes a new dual modular multilevel converter (MMC) topology as a medium-voltage drive for adjustable-speed applications incorporating open-end stator winding machines. A novel concept of sharing one capacitor between each two adjacent-arm sub-modules (SMs) of MMC phase-legs, operating with out-of-phase modulation, is realised through new SM arrangement. This concept allows the MMC to utilise half the number of the SM capacitors, compared to a traditional MMC topology. Additionally, the sizing requirement of the shared capacitor is diminished, which significantly reduces the volume of the drive system and its stored energy. The switching scheme of the shared capacitor between two oppositely modulated SMs eliminates the problem of capacitor wide-voltage fluctuations, independent of the operating frequency. Further, the proposed MMC can efficiently operate at near zero frequency; therefore, a machine speed-range from zero speed to the rated speed is possible under rated torque operating condition. The proposed MMC topology is elucidated in detail, and its effective performance is verified through simulation. [ABSTRACT FROM AUTHOR]

This paper presents a modular multilevel converter (MMC) configuration that utilises energy exchange between sub-modules (SMs) of upper and lower arms, for energy rebalancing. The configuration is applicable to medium-voltage high-power variable-speed drives with any number of motor phases, where the traditional MMC topology experiences challenging shortcomings. With the out-of-phase alternation of the fundamental ripple power in upper and lower arms, the proposed MMC configuration decouples this ripple power by employing dual half-bridge modules linking opposite SMs in upper and lower arms of the same MMC-leg. This counter-balances arm ripple-power through bidirectional power transfer between opposite SMs, resulting in a reduction in the SM capacitance and the MMC system stored energy. The proposed MMC configuration solves the problem of wide SM capacitor voltage fluctuation, especially at low operating frequencies, where the SM capacitor voltage ripple profile is almost constant, independent of the operating frequency. Therefore, the configuration is able to drive multi-megawatt machines from stand-still to the rated speed, at rated torque. The operation of the proposed converter topology is elucidated in detail, and its effectiveness is verified through simulation and experimentation. [ABSTRACT FROM AUTHOR]

This paper proposes a new high-voltage pulse generator (PG) fed from a low-voltage DC supply Vs, which charges one arm of N series-connected full-bridge (FB) modular multilevel converter (MMC) sub-module (SM) capacitors sequentially, through a resistive-inductive branch. By utilising FB-SMs, the proposed PG is able to generate bipolar rectangular pulses of peak NVs and unipolar rectangular pulses of either polarity, at high repetition rates. Asymmetrical pulses are also possible. The proposed topology is assessed via simulation and scaled-down experimentation, which establish the viability of the topology for water treatment applications. [ABSTRACT FROM AUTHOR]

In this study, a cost‐effective HVDC shunt tap with low‐number of relatively low‐voltage semiconductor devices and small‐sized passive components is proposed. The proposed architecture is based on employing three single‐phase half‐bridge Modular Multilevel Converters (MMCs), where the DC sides of MMCs are connected in series across the total DC‐link voltage. The MMCs are adopted instead of conventional Two‐Level Voltage Source Converters (2L‐VSCs) to avoid the complications of series connection of Insulated Gate Bipolar Transistors (IGBTs). The involved MMCs in the suggested architecture are operated with the conventional 2L modulation, which results in insignificant arm inductors and Sub‐Modules (SMs) capacitances (in range of µH and µF, respectively). This, in turn, affects positively the converter cost and footprint. Each arm of the involved MMCs can be considered as a high‐voltage valve of a 2L‐VSC. To maintain the balance of SMs capacitors, each arm of the involved (N + 1)‐level MMC has an extra SM (a balancing SM) to select N out of N + 1 SMs during the turn‐off condition. The operational concept, design, and assessment of the proposed architecture are presented in this study. Simulation results are provided for substantiation of the proposed concept. Finally, a scaled down single‐phase prototype is used for experimental validation. [ABSTRACT FROM AUTHOR]

Pulse forming networks and Marx generators are the classical rectangular waveform pulse generators (PGs). They are inflexible and their capacitors must be fully charged to the required voltage from 0 V before delivering each high-voltage (HV) pulse. They are only able to generate unipolar pulses; if bipolar pulses are sought another generator fed from a negative supply voltage is added. Recently, several power electronics based PGs have been proposed. This study presents an HV power electronics based PG, which is based on half-bridge modular multilevel converter (MMC) sub-modules (SMs) charged sequentially in a voltage-boost mode. Each SM capacitor and main switch form a boost converter with the charging input supply and inductor. As a result, all SM capacitors are charged to a voltage greater than the input. During the discharging process, the SM capacitors are connected in series, producing a rectangular HV pulse across the load. The proposed charging method allows a reduction in the converter footprint in comparison with recently proposed MMC sequentially charged PG topologies. Although only rectangular pulse waveforms are sought in this study, an SM-capacitor voltage balance method allows multilevel pulse generation. The viability of the proposed converter is confirmed by MATLAB/Simulink simulation and scaled-down experimentation. [ABSTRACT FROM AUTHOR]