چكيده به لاتين
In this thesis, an improved control scheme that is based on the Predictive Torque Control (PTC) approach, is proposed, and experimentally implemented for three-phase induction motors fed by Modular Multilevel Converters (MMC). The PTC scheme exhibits several benefits like easy procedure of controller design, and excellent dynamic response. Unlike two-level inverters, the MMC has also internal dynamics and therefore, implementing the predictive-based approaches for controlling three-phase motors (Predictive Current Control (PCC) and PCC) is not a straightforward task for this topology. In fact, the complicated system model employed in the predictive scheme, and huge number of MMC switching states are the main reasons for the aforementioned issue. On the other hand, any control scheme designed for MMC-fed induction motors, should consider the well-known problem of these drives in low speeds, i.e. the large magnitude of the cell capacitor voltages.
In order to fulfil these control goals by the proposed strategy, an initial output voltage vector reference is firstly determined based on deadbeat approach principles; in order to control the flux and torque of the motor. Next, the admissible candidates that should be evaluated in the predictive scheme cost function, are restricted to the vicinity of this initial voltage vector. In this way, the computational load is significantly and effectively reduced. In the next stage, besides simultaneously controlling the flux and torque by the set of restricted admissible switching states, the circulating current control is also accomplished by the redundant states. It should be noted that considering the control of flux and torque in the first stage, and then adding the circulating current control capability in the second stage, would also simplify the required modelling. On the other hand, restricting the set of admissible vectors is independent of the number of cells, which is an important benefit. Another important feature of the proposed strategy is the capability of injecting high frequency circulating current, in order to effectively reduce the capacitor voltage ripple components at the lower speeds. Finally, the proposed method is experimentally implemented on a three-phase MMC, and its various performance aspects and features are evaluated.
