Harmonic Optimization For High-Power Three-Phase PWM Converters

Three phase high power pulse width modulation (PWM) converters are widely used in industry, commercial, residential, aerospace and military applications with the emphasis for energy saving, increasing productivity and improving product quality. The modulation technique for high power PWM converters is a key issue for high power PWM converter control. The traditional carrier based and space vector based PWM strategies can achieve basic control for high power PWM converters. However, these control methods do not completely eliminate specified harmonics. Selective harmonic elimination (SHE) modulation technique can eliminate the selected harmonics while generate desired fundamental, but it also results in an increase in residual harmonics. In addition, due to the limited switching frequency, the device switching frequency is normally below 1 kHz to reduce switching losses. The harmonic distortions of the high power PWM converters increase more with low switching frequency, which may destroy the devices used in the system and result in electromagnetic interference (EMI). It is possible to reduce harmonics by additional large LC filters, but this may increase the cost of the high power devices. In this case, it is necessary to optimize switching time of the high power PWM converters to obtain better electric energy.Converters that change energy to or from alternating current form involve much more complex processes than those only involve direct current. So the purpose of this thesis is to explore the converter modulation issue in detail as it relates to three phase high power DC-AC and AC-DC converters, including voltage source two level, three level, multilevel converters and current source two level converter. In this thesis, the harmonic components of the output voltage/current of these converters are analyzed. And a new modulation control method is developed to conquer some disadvantages for the existing methods. This modulation control method applies modern heuristic optimization technique to generate PWM signals which can achieve harmonic optimization of three phase high power PWM converters.Heuristic optimization methods, such as genetic algorithm (GA), clonal selection algorithm (CSA), and particle swarm optimization (PSO), are proposed to achieve certain optimization, such as selective harmonic elimination (SHE) and total harmonic distortion (THD) minimization, for the output of the high power PWM converters. Compared with traditional PWM technique, the THDs obtained by heuristic methods base modulation technique are the lowest. And this modulation method can more efficiently reduce the output harmonics of high power PWM converters without the restriction of the number of variables in the harmonic equations. Heuristic methods base PWM control methods can solve the harmonic optimization problem with a simpler formulation and are more universal with any kind level of converters without extensive derivation of differential expressions.The simulation and experimental studies have also verified that the heuristic optimization based modulation technique can reduce the harmonic distortion more effectively and obtain higher quality waveforms.