Donoso Merlet, Felipe Octavio
(2022)
Hybrid modular multilevel converter (MMC) applications under over-modulation.
PhD thesis, University of Nottingham.
Abstract
The Modular Multilevel Converter (MMC) has become a prominent converter topology due to its several advantages: high modularity, high scalability, low harmonic distortion, and high efficiency. In particular, due to its modular structure, it is possible to increase its voltage rating by stacking extra cells. The MMC has been successfully applied to high voltage direct current (HVDC) transmission systems and drive applications.
Most MMC-based projects use the half-bridge sub-module (HBSM) as a building block to reduce semiconductor losses. Despite the MMC advantages, there are still open challenges regarding its control and operation. For instance, in the HBSM-based MMC, the minimum dc-port voltage cannot be lower than two times the AC output voltage. The over-modulation operation of the MMC, i.e. operation with reduced dc-link voltage, has shown some benefits. For instance, the MMC can operate with a reduced dc-port voltage in HVDC applications to avoid flashovers under extreme atmospheric conditions. In addition, for back-to-back MMC-based drive applications, it is possible to reduce the energy arm oscillations by controlling the dc-port voltage.
The operation with a reduced dc-port voltage can be accomplished by using full-bridge sub-modules (FBSM) instead of the HBSMs. However, this solution has higher semiconductor losses. A possible alternative is to use a hybrid MMC. In this case, each arm is composed of HBSMs and FBSMs. However, in the hybrid MMC, the capacitor voltages of the HBSMs and FBSMs may drift apart if the converter operates with reduced dc-port voltage because the arm current becomes unipolar, i.e. the arm currents do not have zero-crossing angles.
This thesis presents two control strategies to ensure the cell capacitor voltage balance of the hybrid MMC operating in over-modulation. A decoupled control is developed and shown to regulate the inner and outer converter variables independently. An optimisation problem is proposed to ensure the local balance between HBSMs and FBSMs. In addition, a closed-loop controller is considered to correct any mismatch between the control and actual system parameters. The proposed controller is validated through simulation and experimental results. In particular, a 5 kW hybrid MMC of 18 cells has been built to validate the proposed strategies.
Finally, this thesis presents the control systems and experimental evaluation of a hybrid back-to-back (BTB) modular multilevel converter (MMC) for drive applications. The grid-side converter is a hybrid MMC composed of half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs), while the drive-side converter is an HBSM-based MMC. The proposed topology can operate with a variable dc-port voltage. By controlling the dc-port voltage as a function of the machine operational point, it is possible to reduce the high sub-module capacitor voltage oscillations in the machine-side MMC during low machine speed. An experimental rig composed of 36 cells was built and tested to validate the proposed control.
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