Translated Abstract
High-voltage and high-power frequency conversion technology is of great significance for energy saving. Cascaded H-bridge inverter topology is the prevailing topology of high-voltage inverters for motor drives. It has the advantages of lower harmonics，fewer switching devices required to output equal number of levels and good fault-tolerance ability. Although cascaded H-bridge inverters have been used in the field of high-voltage motor drives for years, there are still a lot to be improved whether in terms of circuit analysis or in terms of operation control of the inverters.
Firstly, existing methods of analyzing and calculating the voltages and currents inside power cells are not accurate enough. This makes it difficult for engineers to design inverters with high cost performance. Secondly, traditional fault-tolerant strategy for cascaded H-bridge inverters usually needs injection of a zero-sequence component into output voltages. Inappropriate injection of zero-sequence components may lead to backflow of real power, which threatens the safe operation of those inverters which cannot return the real power to the grid. Thirdly, most of exisitng motor speed identification methods need the knowledge of accurate mathematical model of motors. If the motor parameters are unknown or vary, the stabiliy and the accuracy of the identification system may be deteriorated. This undoubtly affects the practical application of speed sensorless tecknology.
To solve the problems faced by traditional high-voltage inverters, this paper has done researches into three aspects: analysis and calculation for power cells, fault-tolerant control for cascaded H-bridge inverters and parameter sensitivity analysis for speed identification.
In terms of analysis and calculation for power cells, this paper calculates harmonics in the input current of the H bridge at first. Then the possible types of harmonics in the output current of the rectifier are analyzed. On this basis, an approximately linear equivalent circuit of the rectifying circuit is constructed by methods such as linear approximation and harmonic balance. Accroding to the equivalent circuit, the output current of the rectifier, the capacitor current and the input current of the rectifier are successively calculated analytically. According to the calculation results, this paper has investigated the relationships between the capacitance of the DC-bus capacitor and the quantities of the root-mean-square(RMS) value of the capacitor current, the fluctuation amplitude of the capacitor voltage and the RMS value of the input current of the rectifier.
In terms of the fault-tolerant control of cascaded H-bridge inverters, this paper proposes new methods of generating zero-sequence voltages both for linear modulation and overmodulation. The proposed fault-tolerant control method in linear modulation region reduces the fundamental component of the zero-sequence voltage by limiting and reshaping the waveform of it. Thus the backflow of real power is suppressed effectively without sacrificing the voltage output ability of the inverter. As to the fault-tolerant control in the overmodulation region, the feasible region of the drifting neutral point is analyzed and the way of drifting to maximize the safe interval of load power factor angle is acquired through calculation. Compared with traditional methods, the proposed fault-tolerant strategy enables the inverter to work safely within a wider load power factor so that the fault-tolerant operating range of the inverter is extended.
In terms of speed identification technology, parameter sensitivity is analyzed for an adaptive full-order observer with both speed and stator resistance estimation. The analytical relationship between the stable-state estimation errors and parameter errors is acquired. The stability judgement method for the observer is proposed in cases that parameter errors are taken into consideration. This paper also improves the parameter design method for the traditional feedback gain matrix and the adaptive rules, which reduces the influences of the parameter errors on the stability of the observer.
In conclusion, in this paper, a more accurate analysis and calculation method for the power cell circuit and concerned voltages and currents of cascaded H-bridge inverters is proposed; The fault-tolerant control stategy is optimized and the reliability of the fault-tolerant operation is enhanced; The impacts of the parameter accuracy on the adaptive full-order observer simultaneously estimating speed and stator resistance are analyzed and the robustness of the observer is improved. The research results of this paper provide a more accurate theoretical basis for the selection of devices for cascaded H-bridge inverters. They also provide a safer and more reliable control strategy for the fault-tolerant operation of the inverter. Moreover, they may help to analyze and solve the parameter sensitivity problem of speed identification systems. Therefore, these results are expected to improve the overall performance of cascaded H-bridge inverters and promote technological progress in the field of inverters for high-voltage drives.
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