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Optimal PWM control of a multilevel inverter through Curve Fitting Technique (CFT)

Ahmad Azli, Naziha and Mohd. Yatim, Abdul Halim (2008) Optimal PWM control of a multilevel inverter through Curve Fitting Technique (CFT). In: Recent Advances In Power Inverter. Penerbit UTM , Johor, pp. 281-293. ISBN 978-983-52-0647-4

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Abstract

The advantages of the optimal PWM switching strategies over the carrier modulated sinusoidal PWM switching strategies in traditional two-level output inverters have been noted. In particular, an inverter employing optimal PWM switching strategies is found to achieve great reduction in its effective switching frequency. This contributes to the decrease in switching losses for the same amount of reduction in low order harmonics when compared to the one using the carrier modulated sinusoidal PWM switching strategies [1]. The typical practical implementation of the optimal PWM switching strategies has shown good timing precision using external hardware timers and microprocessors. However the utilization of the microprocessors in such implementation is essentially memory-based. It involves solving of the related non-linear equations offline for either variable amplitude of the fundamental of the inverter output phase voltage in per unit value (ap1), or for both variable ap1 and number of switching angles per quarter cycle (N) to control the inverter output frequency as well. The former is necessary for regulating the inverter output voltage in AC power supply applications while the latter are required in Adjustable Speed Drive (ASD) applications. This is followed by pre-programming of the various sets of the optimal PWM switching angles obtained from the numerical method into the microprocessor’s memory in the form of look-up tables, which are then used to generate the PWM waveforms online in real time. Since in general, the relationship between the switching angles and ap1 are non-linear, a large number of look-up tables corresponding to each discrete ap1 or both ap1 and N are required [2]. Alternatively, some form of interpolation between the tables can also be made, which involves fewer look-up tables. However, the switching angles calculated by the microprocessor for the values of ap1 or N that are not available in the look-up tables are merely the approximate switching angles, whose accuracy depends greatly on the interpolation technique used. Recent studies on optimal PWM switching strategies have shown great effort in developing techniques that fully utilizes the computational ability of the microprocessor rather than its memory space [3][4][5] in generating the optimal PWM switching angles online in real time. Most of these techniques are developed for the conventional single-phase or three-phase two-level output inverter configuration, particularly for Adjustable Speed Drive (ASD) application except in [5] that implements its version of online optimal PWM switching strategies on a bridge rectifier. The multilevel inverter topology has gained increasing attention in recent years particularly in applications involving high voltage and power. This is mostly due to the limitations of the conventional two-level output inverters in handling high power conversion. The main feature of a multilevel inverter is its ability to reduce the voltage stress on each power device due to the utilization of multiple levels on the DC bus [6]. This is especially important when a high DC side voltage is imposed by an application. There are several types of multilevel inverters but the one considered in this work is the cascaded H-bridge type. The features particularly in terms of its structure that is not only simple and modular but also requires the least number of components compared to other types of multilevel inverters. This in turn, provides the flexibility in extending the CHMI to higher number of levels without undue increase in circuit complexity as well as facilitates packaging.

Item Type:Book Section
Uncontrolled Keywords:optimal PWM switching, Curve Fitting Technique (CFT), inverter
Subjects:T Technology > TK Electrical engineering. Electronics Nuclear engineering
Divisions:Electrical Engineering
ID Code:24957
Deposited By: Ms Zalinda Shuratman
Deposited On:24 Apr 2012 08:52
Last Modified:24 Apr 2012 08:52

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