Abstract
This paper presents a modular multilevel converter (MMC) configuration that utilises energy exchange between submodules (SMs) of upper and lower arms, for energy rebalancing. The configuration is applicable to mediumvoltage 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.
Highlights
With its superior features, the modular multilevel converter (MMC) is considered a competitor for high-voltage highpower applications, outperforming its counterparts [1]
The motor currents are near dc at zero motor speed, the capacitor voltage ripple is totally controllable
This paper extended a previously proposed ripple power decoupling approach for MMC-fed adjustable-speed drives by introducing a new placement scheme for the power decoupling channels
Summary
The modular multilevel converter (MMC) is considered a competitor for high-voltage highpower applications, outperforming its counterparts [1]. Due to its unique operating principles, the MMC performance is highly depending on the operating frequency, where pulsating ripple power at both the fundamental and secondorder harmonic are experienced by the MMC arms This ripple power is eventually manifested in the MMC submodule (SM) capacitor in a voltage ripple profile that has the first and second harmonics as the most dominant components. The power decoupling approach has been applied to three-phase machines with both open-end stator windings [13] and star and delta connections [14], in addition to symmetrical six-phase machines [15] In these three applications, the power decoupling is implemented in a horizontal scheme, where the excess of the capacitive energy at each SM is transferred to the adjacent-arm SM in another phase-leg, to cater for the lack of energy. In the two sections, the pulsating ripple power problem and its effects on the SM capacitor voltage ripple are briefly discussed, the proposed solution is presented
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