| With the rapid development of green intelligent rail transits, it puts forward higher requirements on power density, output performance, conversion efficiency and reliability of auxiliary inverters, which ensure stable and comfortable running of rail trains. Based on the key problems of auxiliary inverters, high-frequency link topology, hybrid modulation algorithm, output optimization and efficiency improvement strategies are studied and discussed. Furtherly, the correctness and feasibility of the research results are verified with co-simulations and prototype experiments.From the perspective of high-frequency link topology, the intermediate DC-link LC filter of conventional high-frequency link auxiliary inverter is eliminated, and a three-level Boost-half bridge converter is adopted to restrict the rectified voltage spike when the range of input voltage is wide and there exists secondary parasitic ringing, which improve the system power density, reliability and lifetime. Focusing on the three-level Boost converter, this dissertation adopts a pulse phase delay control strategy to adjust the deviation voltage, solving the coupling problems of independent duty control. By revealing the limitations of state space average model, an inductor current ripple-based modeling approach has been adopted to accurately model and analyze the converter. Then the piecewise expressions of deviation voltage gains are derived, and a piecewise fitting method for maximum deviation voltage conditions is proposed to determine the boundaries of pulse phase delays in different duty ratios. Finally, an optimized double-loop controller with different bandwidths is proposed to achieve decoupling control of output voltage and deviation voltage.From the perspective of output optimization, the different zero-portion effects on DC links with two hybrid modulation algorithms are investigated based on the coupling characteristics. By establishing average model of output voltages in a carrier cycle and Fourier analytical model of deviation voltages in a modulation wave cycle, the harmonic distortion, efficiency of DC voltage and DC offset of output voltages are explored with different carrier frequency differences, zero-portion widths and modulation indexes. Subsequently, a piecewise compensation strategy is proposed based on the average model, improving the output performance in limited modulation ranges. Based on the zero portion effects, a carrier-phase-shift hybrid modulation algorithm is developed in certain conditions, which improves the quality of output waveform and contributes to realize soft switching. By revealing the changing rules of harmonic characteristic, the design principle of carrier frequencies is determined. And based on the impedance matching principle, the influence of intermediate DC-link LC filter on the system output filter performance is studied.From the perspective of efficiency improvement, a loss model of three-level Boost converter is established, and the converter’s voltage gain is optimized considering the rectified voltage spike. Subsequently, an input voltage feed-forward compensation control strategy is proposed to improve the conversion efficiency during the whole operation period. Based on the pulsating DC-link current, zero-vector synchronized soft-switching control and zero current soft switching method with active clamp are proposed to decrease the switching losses of half bridge converter and clamp the voltage spike. In order to realize relative minimum loss of three-phase voltage source inverter under all load conditions, a modified generalized discontinuous pulse width modulation algorithm based on continuously adjustable zero-sequence component injection is proposed. And the boundary conditions of modulation index with superior performance are obtained with the constraints of voltage modulation gain and harmonic characteristic. |
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