Research On Modulation Strategies And Power Balance Control Of Asymmetric Hybrid Multilevel Inverter

With the high voltage and high power fields ceaseless requirements to the functions and performance of the inverter, and the multilevel inverter could output multilevel high voltage, which has lower harmonics and electromagnetic interference with lower voltage power switches, therefore it has been to the research hotspot of high voltage and high power conversion technology. While the present traditional multilevel inverters applied in engineering need more power switches, and their structures are more complicated, so they have the sensitive problems which include that the inversion system has huge volume, more expensive price and lower efficiency. And the actual application range of the multilevel inverter is extremely limited by these problems. But, the asymmetric and hybrid multilevel inverter (AHMI) with its control technology could output more numbers of voltage levels with less power switches and DC supplies. It extremely simplifies the multilevel topology, improves the power density and reduces the cost, thus it will become the developing trend of multilevel inverter. At the present time, the research of AHMI is at the beginning step, there is no unified and recognized topology in nation or abroad yet, thereby, its practicality may only be estimated by cost and performance. While the operating performance of the AHMI, is up to the proper hybrid multilevel modulation strategy and its correlative control strategy. Thereby, the AHMIs are taken as the research objects, then their respective modulation strategies and control strategies are investigated, and it both has the definite theory significance and the engineering practical value.The topology characteristics of the asymmetric H bridges which include in AHMIs are analysed deeply, then a novel carriers-staggered modulation strategy is proposed, which is proper to the asymmetric H bridges. According to three kinds of five-level asymmetric H bridges which have higher performance, the bidirection switch asymmetric H bridge is taken as the example. The behavior rules of each power switch during the fundamental period are deeply analysed to form the unified modulation modes, and the carriers which are distributed as the modalities of positive and negative staggered cascades are confirmed, then the carriers- staggered SPWM (CS-SPWM) modulation strategy is obtained with the sinusoidal wave and the carriers which are layered and divisional. This modulation is also proper to the diode-clamped asymmetric H bridge. Finally, the three-phase natural sampling method of the CS-SPWM is implemented by FPGA, and the CS-SPWM with phase distribution has better performance by contrasting analysis of simulation and experimental results.The mathematics models of the capacitor-clamped asymmetric H bridge are established, and the CS modulation strategy with phase distribution impact on the capacitor voltage is analysed deeply, it is pointed that the clamped capacitor voltage needs a fundamental period to be balanced, and there is a problem that the fluctuating amplitude of the capacitor voltage is uncontrollable. A novel improved control strategy is proposed by means of the staggered carriers'freedoms, and it not only controls the fluctuating amplitude of the capacitor voltage effectively, but also balances the utilization of the homogeneous power switches in the topology. On the base of it, the mathematics relationship of capacitor capacitance and switching frequency versus load current and voltage ripples is established, to ensure the choice between the capacitance and the switching frequency better. By means of the principle of the generated line to line voltage, it is found that the output voltage space vectors couldn't conform to the principle of the nearest three vectors (NTV), and the line to line voltage exists the problem of overlapped level layers, then the optimization control method is proposed to enhance the performance of the line to line voltage.In order to increase the level of output voltage, the multilevel inverter with hybrid cascaded H bridges is formed by the capacitor-clamped asymmetric H bridge and traditional H bridges, and its switching function models are established. With the deep analysis of the conventional basic ideas of hybrid modulation strategy, the CS hybrid modulation strategy is proposed with the respective modulation of each power cell, and it increases the equivalent carrier frequency of the output voltage. On the base of it, the hybrid modulation strategy is extended to more asymmetric cascaded H bridges, and it has the better control performance to the inverter with hybrid asymmetric H bridges. In addition, aiming at the efficiency problem of the topology, the optimization control method injecting hybrid components to the modulation waves is proposed, it not only improves the DC voltage utilization, but also reduces the loss of high frequency power switches.Another problem from the multilevel inverter with hybrid cascaded H bridges is how to balance the output power of each cell. Thus, the function between the active power and the fundamental voltage amplitude is derived from instantaneous power, then the function between the fundamental voltage amplitude and modulation depths is analysed deeply, and it's found that the output assignment of each power cell has the extremity problem during the lower modulation depths. Therefore, the strategy of linearization control of power cells outputs is proposed to settle the problem completely. In order to make the outputs of the power cells be balanced, the power balance curves are rebuilt, then the restriction functions of power balance is established to propose the power balance control strategies including both higher and lower modulation depths. Finally, the outputs of power cells are balanced during the whole modulation depths.