Multilevel inverters are gaining interest in industry, especially for renewable energy sources, as a means to reduce output distortion and also reduce electromagnetic interference effects. However, multilevel inverters have a high-transistor component count. In order to reduce the number of power transistors within a multilevel inverter, a bypass diode technique is suggested for two multilevel inverter topologies. In the first inverter configuration, referred to here as the diode-bypassed multilevel dc-link inverter, one H-bridge is required for the entire single-phase system, with one transistor and one bypass diode needed for each voltage source used in the inverter. Each voltage source contributes in supplying power to the load in both the positive and negative half cycles of the output waveform. In the second configuration, referred to here as the diodebypassed neutral point inverter, one transistor and one bypass diode is required for each voltage source, plus two additional transistors for the entire single-phase inverter system. For this configuration, each dc source contributes only to one half-cycle of the output waveform, either positive or negative. For both configurations, it is possible to ensure even power distribution among all the voltage sources (or compensate for imbalance in the state of charge of the dc sources). Experimental results suggest the feasibility of constructing the proposed inverter topologies and simulations provide preliminary power-efficiency data.
A technique for reducing the total harmonic distortion in the voltage output of naturally sampled pulse width modulated inverters is investigated. This technique is based on synchronization of parallel-connected inverters and makes use of a phase-shift in transistor gating between these inverters. Often, in practice parallel inverters are operated with no synchronization, resulting in far less than optimal harmonic distortion reduction. The output waveform of a single inverter, two parallel inverters, and three parallel inverters is analyzed and compared in simulation and experimentally. This is done for two and three level pulse width modulation, for several values of modulation index, and different carrier to fundamental frequency ratios. Both simulation and experimental results indicate that a significant reduction in total harmonic distortion occurs using the synchronization technique, and observations are provided on the ability to reduce harmonic distortion in multiple inverter systems.
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