Diode bypassed transistor-voltage-source units and harmonic control in multilevel inverters
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2012
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Abstract
Multilevel inverters make use of a large number of switching components to synthesize a staircase output voltage waveform that resembles a sine wave. A present research trend is to reduce switch count while maintaining or lowering output voltage harmonic distortion. In this disse11ation a switching unit is proposed that can bypass a voltage source in series with a transistor using a diode instead of the conventional approach that uses a bypass transistor. This diode bypass unit is employed with the deĀlink and the neutral point inverters. The major part of this dissertation focuses on harmonic optimization in multilevel inverters. The objective is to find the switching angle values that can eliminate specific low order harmonics in the output voltage waveform while keeping control of the value of the fundamental voltage. The conventional approach is to equate the undesired harmonic voltages to zero and solve the resulting equations using Newton's method. The approach used in this dissertation employs the harmonic power quantities to eliminate the undesired harmonics. The powers of these harmonics are added together to constitute a cost function that can be minimized using a gradient search technique. The proposed algorithm uses the sign of the gradient instead of the exact value of the gradient, thus reducing analytical complexity of the approach. The gradient sign approach is shown to have very close results in the search path and outcomes of the optimization process compared with the exact gradient approach. Two more cost functions are also considered based on the least squares approach. Weighted versions of the present and past total power and the squared total power of the undesired harmonics are taken as two cost functions and minimized using the sign of the gradient. The algorithms proposed can take into account the cases of unequal voltage sources and a wide range of the modulation index. Simulation and experimental results support the proposed theory.
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Bibliography: p. 115-122
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Al-Judi, A. (2012). Diode bypassed transistor-voltage-source units and harmonic control in multilevel inverters (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/4778