Research On Analvsis Methods,Carrier PWM Technologies And Fault Diagnosis For Hybrid Clamped Multilevel Inverters

Hybrid clamped multilevel inverters have the advantages of low THD, low voltage stress on power switches and low system EMI, and they can also implement the functions of reliable clamping on power devices and self-voltage balancing on DC link capacitors. Two or more kinds of clamping components are used together in hybrid clamped multilevel inverters, thus leading to more complicated topology structure and control method, so topology analyses become more difficult and system reliability are decreased, especially for n (n>3) level inverters. The analysis and testing method, the carriers PWM control techniques and the devices fault diagnosis methods for hybrid clamped multilevel inverters are studied in this dissertation.Firstly, the ideas of topology decomposition and switching modes combination are proposed in this dissertation, which offer a new way of constructing new multilevel inverters topologies. Based on the idea of topology decomposition, a new topology combining a switched-capacitor circuit with a flying-capacitor four-level inverter is constructed. It can balance the capacitor voltages inherently without designing the redundant switching states, and can be applied to real power or reactive power conversions regardless of load characteristics. Preliminary simulation result shows the validity and rationality of this new topology. Integrating the basic constructing units of multilevel inverters with switched-capacitor circuits, a new method of constructing new multilevel inverter topology is proposed, and its rationality is also discussed. By altering the structure of switched-capacitor circuit of the hybrid clamped multilevel inverter with self-voltage balancing, an improved topology named MCT-BSD (Multilevel Circuit Topologies Based on the Switched-capacitor Converter and Diode-clamped Converter) is attained, and its generality and feasibility under three-phase operation are discussed. Simulated and experimental results show its validity and rationality. The MCT-BSD topology remains the advantage of self-voltage balancing, and the number of power switches and DC link capacitors are decreased. Moreover, stepping up output voltage can be realized in MCT-BSD, which can reduced the step-up ratio of the input transformer, thus reducing the cost of transformer.Then, the analysis methods of switching modes transitions for multilevel inverters are proposed in this dissertation. The analyzing and modeling of hybrid clamped multilevel inverters with self-voltage balancing are completed based on analysis methods of switching modes transitions. On the other hand, the concept of quasi on-line testing is proposed in this dissertation. Based on the experimental prototype of hybrid clamped multilevel inverters with self-voltage balancing, a quasi on-line testing platform is built in order to verify the analysis on the switching modes transitions.Furthermore, the concepts of carrier cells and carrier sub cells are proposed in this dissertation. By constructing some kinds of new carrier wave arrangements with carrier cells and sub cells, some new kinds of carrier PWM methods are put forward so as to improve the performance of multilevel inverters from the different aspects. The carrier waveforms of each switching device in hybrid clamped multilevel inverters with self-voltage balancing were rearranged with carrier cells, and then a new carrier PWM control method--HLCCAPO PWM (Higher and Lower Carrier Cells Alternative Phase Opposition PWM) is attained. Simulated and experimental results show that comparing with the carrier PD PWM method, HLCCAPO PWM method can effectively reduce the number of devices switching on or off within broad modulation index range, consequently reducing switching losses. Also, the energy of lower harmonics transfers to higher harmonics band with the HLCCAPO PWM method. The reduction of the energy of lower harmonics can simplify the design of output filter and reduce its size. Setting the reasonable dead time between the complementary switching devices of hybrid clamped multilevel inverters is necessary in order to ensure the reliable operation of multilevel inverters. However, the existing of dead time brings about the unexpected level on the output voltage waveform. Based on the carrier cells, a method of effectively eliminating the unexpected output levels by improving the carrier waveform is proposed in this dissertation. Simulations and experiments show the unexpected output levels, which are caused by the dead time, can be effectively eliminated by applying the improved PWM method to the hybrid clamped multilevel inverter within broad modulation index range.Last, the devices fault diagnosis method for hybrid clamped multilevel inverters based on the analysis and testing on switching modes transitions is proposed in this dissertation. The fault features are extracted from the output voltage waveform of inverters, and then devices diagnosis can be preliminarily classified and sifted according to the fault features. Devices fault diagnosis and fault location are finally completed by measuring the specific voltages or currents under the certain switching modes. Taking a hybrid clamped five-level inverter with self-voltage balancing as instance, devices fault features under all kinds of fault conditions are analyzed and generalized in detail, and the rules of devices fault diagnosis are given, which has practicality.