Abstract
The modular multilevel matrix converter (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> C) is an ac to ac power converter composed of 9 arms and is proposed for high power applications such as motor drive and wind energy conversion systems. Energy Control of the M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> C is achieved using four circulating currents, and is frequently divided into the different frequency mode (DFM) and equal frequency mode (EFM). EFM is more challenging, because of the larger capacitor voltage oscillations that can be produced. The control schemes are typically different for EFM/DFM operation and this further increases the complexity. In this article, a continuous-control-set model predictive control for energy management of the M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> C is proposed. The control scheme is based on solving an equality constrained quadratic programming problem, where the optimal solution is analytically obtained. The result is a single and simple control law to obtain the circulating current references, where good performance is achieved for both EFM and DFM. The proposed strategy is experimentally validated using a scaled-down M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> C prototype composed of 27 power cells.
Highlights
Modular Multilevel Cascaded Converters (MMCC) were initially proposed for AC to DC applications, and are being widely used in High Voltage Direct Current (HVDC) transmission systems [1], [2]
The main novelty of the work reported here is the introduction of a balancing and mitigation scheme for the squares of the cluster capacitor voltages (SSCV) terms, based on a Continuous-Control-Set Model Predictive Control (CCS-MPC)
If the cost weight of the corresponding affected pair is increased during Equal Frequency Mode (EFM), the circulating currents will adopt the shape of the Common-mode Voltage (CMV) waveform to mitigate the low-frequency oscillations
Summary
Modular Multilevel Cascaded Converters (MMCC) were initially proposed for AC to DC applications, and are being widely used in High Voltage Direct Current (HVDC) transmission systems [1], [2]. In feedforward EFM control, the average values of the CCVs are regulated using nested controllers, similar to DFM control, but an offline-obtained feedforward component is introduced into the control to mitigate the voltage oscillations [24] These approaches have some limitations, mainly based on the use of off-line calculated mitigation signals that cannot compensate non-linearities, uncertainties and changes in the operating conditions [19]. The strategy uses a three-stage optimisation scheme, where the common-mode voltage and the circulating current references are obtained for DFM and EFM operation. The CCS-MPC enables both DFM and EFM operation, using a simple control structure with a straightforward formulation based on a M 3C state-space model Using this methodology, optimal circulating current references are obtained from the modelling presented, allowing the fast dynamic response that is typical of MPC, with a fixed and pre-determined computational burden. An appraisal of the proposed control methodology is presented in the Conclusions
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