| Energy has become a major global problem nowadays, thus energy saving is a major direction for development in current stage. And medium-voltage inverter industry is an important means for energy saving. The multi-level technology it takes has become the mainstream in medium-voltage power transmission field, mainly in the following three topologies: diode clamp, flying capacitor and cascaded H-bridge power structure. Currently, these three types of multi-level structure are relatively mature, but there are still many problems that restrict their applications. For example, the first two structures can hardly meet the demands in 6k V voltage level or above, and cascaded H-bridge medium voltage inverter although can achieve a higher level of voltage by cascading a series of power units together, but it's restricted when the high performance system needs four quadrant operation. Therefore, this paper proposes a modular multi-level converter(MMC) topology application in motor drive fields, and constitute a new type of high-performance medium-voltage inverter system.Firstly, The working principle of the topology and the modular multi-level converter are introduced in this paper, also the analysis of its basic characteristics in motor drive system. And the main topology is given as a whole, including the MMC rectifier connected to the AC grid, the DC bus and the MMC inverter connected with AC induction motor. Besides, the half-bridge-type sub-module structure is focused for its work states and the operating mode, on the basis of which a comparative analysis of its several electrical models is taken. Meanwhile, the used modulation strategies are introduced, and the pre-charge side of the rectifier and inverter system when you start making elaborate.The control of the whole article is divided into two parts: the control side of the rectifier and inverter control. The grid-connected rectifier, is to achieve unity input power factor control, reduce the harmonic pollution and stabilize the DC bus voltage. The paper begins with MMC rectifier, and the mathematical models in abc coordinate system and dq coordinate system are build based on the electrical model for the further studies of MMC rectifier such as the cause and existing pattern of interphase circulation, and also its mathematical relationship. From the analysis we know that the existence of interphase circulation does not affect of MMC external characteristics, but will increase the leg loss. Thus circulation suppression measures should be taken. This paper focuses on the analysis of voltage balancing, and the voltage balance control process based on carrier phase-shifted modulation strategy is elaborated. Its feasibility is proved through simulation, and therefore a steady DC voltage is achieved.Finally, the MMC inverter and asynchronous motors are combined together according to the rotor flux oriented vector control for motor speed regulation. Taking into account that most of the motor in drive field run in low frequency situation, the paper discusses the case of MMC model in low frequency, and a scheme for low-frequency control is proposed. And the correctness of theoretical analysis and the feasibility of the methods are validated. |
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