14-05-2012, 03:01 PM
A General Space Vector PWM Control Algorithm for Multilevel Inverters
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INTRODUCTION
The research on the multilevel inverter has been receiving
wide attention mainly due to its capability of high voltage
operation without switching devices connected in series. In
addition, with the increase of voltage levels, the inverter output
contains less harmonics and will eventually approach a desired
sinusoidal waveform [1]. Therefore, the multilevel inverters
have been selected as a preferred power converter topology for
high voltage and high power applications [2], [3].
SELECTION OF SWITCHING STATES
In the multilevel inverter, the redundant switching states
increase with the voltage level. For example, the 5, 7, and
9-level inverters have the redundant states of 4, 6 and 8,
respectively for zero voltage vector. In the diode clamped
multilevel inverters, the redundant switching states can be
utilized to balance the voltage of capacitors in the dc link. The
cascaded H-bridge inverter inherently does not have this
problem. In this paper, the redundancy is employed to minimize
the voltage harmonic distortion.
EXPERIMENTS
A three-phase DSP (TMS320F240) controlled 7-level
cascaded H-bridge multilevel field-oriented induction motor
drive system prototype was built and tested. The induction
machine was rated at 3KW, 380V, 6.8A and 1420rpm. The
control cycle is 200us and the flowchart of the program
CONCLUSIONS
A simple and general space vector PWM algorithm has been
proposed in this paper. To facilitate the design and digital
implementation of the space vector algorithm, all the space
vectors are transformed from the commonly used Cartesian
coordinate system to a 60° coordinate system. The proposed
algorithm substantially simplifies the calculation of space
vectors and their corresponding dwell times. Based on the
proposed algorithm, a new switching pattern, referred to as
Large-Small Alternation (LSA), for the cascaded H-bridge
multilevel inverters has been developed.