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Tuesday, March 25, 2008

cascade multilevel inverter

cascade multilevel inverter
Abstract
A method is presented showing that a cascade multilevel inverter can be implemented using only a single DC power source and capacitors. A standard cascade multilevel inverter requires n DC sources for 2n + 1 levels. Without requiring transformers, the scheme proposed here allows the use of a single DC power source (e.g., a battery or a fuel cell stack) with the remaining n-1 DC sources being capacitors. It is shown that one can simultaneously maintain the DC voltage level of the capacitors and choose a fundamental frequency-switching pattern to produce a nearly sinusoidal output.
Basics behind this project:
Power-electronic inverters are becoming popular for various industrial drives applications. In recent years also high-power and medium-voltage drive applications have been installed. To overcome the limited semiconductor voltage and current ratings, some kind of series and/or parallel connection will be necessary. Due to their ability to synthesize waveforms with a better harmonic spectrum and attain higher voltages, multi-level inverters are receiving increasing attention in the past few years. The multilevel inverter was introduced as a solution to increase the converter operating voltage above the voltage limits of classical semiconductors. The multilevel voltage source inverter is recently applied in many industrial applications such as ac power supplies, static VAR compensators, drive systems, etc. One of the significant advantages of multilevel configuration is the harmonic reduction in the output waveform without increasing switching frequency or decreasing the inverter power output. The output voltage waveform of a multilevel inverter is composed of the number of levels of voltages, typically obtained from capacitor voltage sources. The so-called multilevel starts from three levels. As the number of levels reach infinity, the output THD (Total Harmonic Distortion) approaches zero. The number of the achievable voltage levels, however, is limited by voltage unbalance problems, voltage clamping requirement, circuit layout, and packaging constraints. Multilevel inverters synthesizing a large number of levels have a lot of merits such as improved output waveform, a smaller filter size, a lower EMI (Electro Magnetic Interference), and other advantages. The principle advantage of using multilevel inverters is the low harmonic distortion obtained due to the multiple voltage levels at the output and reduced stresses on the switching devices used. Improvements in fast switching power devices have led to an increased interest in voltage source inverters (VSI) with pulse width modulation control (PWM). It is generally accepted that the performance of an inverter, with any switching strategies, can be related to the harmonic contents of its output voltage. Power electronics researchers have always studied many novel control techniques to reduce harmonics in such waveforms. Up-to-date, there are many techniques, which are applied to multilevel inverter topologies. Pulse Width Modulation (PWM) is widely employed to control the output of static power inverters. The reason for using PWM is that they provide voltage and/or current wave shaping customized to the specific needs of the application under consideration. It is lastly performance and cost criteria, which determines the choice of a PWM method in a specific application. PWM inverters can control their output voltage and frequency simultaneously. And also they can reduce the harmonic components in load currents. These features have made them power candidate in many industrial applications such as variable speed drives, uninterruptible power supplies, and other power conversion systems. However, the reduction of harmonic components in output currents is still the focus of major interest to alleviate the influences of electromagnetic interferences or noise and vibrations.
In general, neutral point clamped PWM three-phase inverter which uses four switching elements in each arm has the five- level voltage waveforms that results in considerable suppression of the harmonic currents comparing with the conventional full-bridge type three-level PWM inverters. However, this is not the case of single-phase PWM inverter. In these days, the popular single-phase inverters adopt the full-bridge type using approximate sinusoidal modulation technique as the power circuits. The output voltage of them has three values: zero, positive and negative supply dc voltage levels. Therefore, the harmonic components of their output voltage are determined by the carrier frequency and switching functions. Moreover, the harmonic reduction of them is limited to a certain degree. Under these technical backgrounds, a single-phase three-level PWM (Pulse Width Modulation) is presented in the project.

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