Controllability Analysis And M-mode Control Of Power Inverters Based On Switched System Models

A power converter is a switched system which featured with typical characteristic of nonlinearity. However, it lacks of a systematic way to investigate due to the limitations of linear circuit theory. Power inverters are widely used in modern industrial production and daily life, it is necessary to analysis performance of inverters so that the inverters can operate stably and reliably. Therefore, the switched system theory was applied in power inverters to study the controllability-related problems, then new proposals and mythologies for designing circuit topologies and control strategies are presented based on the proposed method.The main contributions of the thesis are listed as follows.1. Three novel models, i.e. switched system models of three-phase four-wire inverter, three-phase three-wire inverter and nine-switch inverter, are built. In the switched system models, operation modes and control strategies of the inverters are described as state equations and switching sequences respectively, compared with the former models, switched system models can be used to study the controllability property of the inverter since the model retained all the dynamic characteristics.2. The existing methods for studying structural controllability were developed from linear systems, where the structure of the system is invariant. However, these methods cannot be applied in switched system model of inverters, because the inverter model switch among different system structures periodically. Hence, a novel directed graph method based on the convex combinations of operation modes, was proposed to study the controllability of the three-phase four-wire inverter, three-phase three-wire inverter and nine-switch inverter. Simulation and experiment results showed that, the three-phase four-wire inverter, three-phase three-wire inverter and nine-switch inverter are structural controllable with the three-phase balanced loads. However, when the loads are unbalanced, the three-phase three-wire inverter is no longer controllable, which means that it cannot be used in unbalance situations.3. Based on the controllability criterion of switched systems, state controllability of three-phase four-wire inverter, three-phase three-wire inverter and nine-switch inverter were studied. The research showed that three-phase four-wire inverter and nine-switch inverter were completely state controllable but not the three-phase three-wire one, which further explained why the three-phase three-wire one cannot operate stably with unbalanced loads.4. Theoretically, for a inverter with k operation modes, only m(m<k) operation modes are needed to make the inverters state controllable. Hence, minimum-mode controllability of the inverters were studied based on the controllability theory, consequently, m-mode controllability and m-mode modulation strategies were proposed, and it is proved that m-mode modulation strategies covered all types of PWM and SVPWM strategies.5. According to the m-mode controllability theory, 3-mode modulation, 4-mode modulation of three-phase four-wire inverter and 6-mode modulation of nine-switch inverter were proposed. Simulation and experiment results have proved that, the switching frequency of the 3-mode modulation and 6-mode modulation strategies were significantly reduced by two-thirds compared with the traditional SVPWM method, hence, the switching losses of the power semiconductors can be reduced and the efficiency of the inverter can be improved.6. Based on the analysis results of m-mode controllability of MMC full bridge sub-module, a topological optimization method was presented to simplify the structure of complex inverters. With the proposed optimization method, a new MMC sub-module circuit was designed. Compared with the traditional full bridge sub-modules, the new sub-module circuit spared one semi-conductor which reduced the cost and simplified the structure, meanwhile, the voltage utilization ratio of MMC inverter improved and the harmonic distortion is reduced with the new sub-modules. In addition, it has been proved that the proposed optimization method can be extended to be applied in other power electronic converters.