21-08-2012, 03:23 PM
Multilevel Selective Harmonic Elimination PWM Technique in Series-Connected Voltage Inverters
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Abstract
Selective harmonic elimination pulsewidth modulation
(SHEPWM) method is systematically applied for the first
time to multilevel series-connected voltage-source PWM inverters.
The method is implemented based on optimization techniques.
The optimization starting point is obtained using a phase-shift
harmonic suppression approach. Another less computationally
demanding harmonic suppression technique, called a mirror
surplus harmonic method, is proposed for double-cell (five-level)
inverters. Theoretical results of both methods are verified by
experiments and simulations for a double-cell inverter. Simulation
results for a five-cell (11-level) inverter are also presented for the
multilevel SHEPWM method.
INTRODUCTION
MEDIUM/LARGE motor drives, uninterruptible power
supply (UPS) systems, and high-power inverters in
flexible alternate current transmission systems (FACTS) need
switching elements which can bear high voltages and currents.
To overcome the limitations of semiconductor switches, several
new techniques and topologies have been developed [1]–[4],
such as multiple switching elements in one leg of an inverter
[5], [6], series-connected inverters [7]–[15], parallel-connected
inverters [16], [17], multilevel reactive power compensators
[18]–[21], multiple rectifiers for unity power factor correction
[22], optimization of motor performance indexes (such as
harmonic current, torque ripple, common mode voltage, and
bearing currents) [23], [24], and neutral-point-clamped (NPC)
inverters [25]–[27].
MULTILEVEL SHEPWM TECHNIQUE
The multilevel SHEPWM technique has a theoretical potential
to achieve the highest output power quality at low switching
frequencies in comparison to other methods. However, because
of its mathematical complexity, no significant results have been
reported thus far. One of the main challenges is to obtain a good
starting point when the solution of a nonlinear system of equations
of the (3) type is attempted. This paper presents a concept
of obtaining the starting point by means of SHEPWM with a
phase shift. Then, a nonconstraint optimization [35] is used to
calculate the final solution of the multilevelSHEPWM problem.
CONCLUSIONS
A multilevel selected harmonic elimination PWM method
has been proposed. The computational difficulties of multilevel
SHEPWM methods are overcome by development of an inverter
model for nonconstraint optimization. The optimization starting
point is obtained using a phase-shift surplus harmonic suppression
technique. Simulation and experimental results are presented
for a double-cell series-connected voltage source PWM
inverter in single-phase and three-phase configuration. Simulation
results for a three-phase five-cell inverter are also given.
The multilevel SHEPWM method is capable of providing veryhigh-
quality output waveforms.
A new reduced-order method of mirror harmonic suppression
in a double-cell series-connectedPWMinverter is also suggested.
Instead of using a difficult-to-solve system of nonlinear
equations, the two inverter cells are considered separately.
low-order harmonics in the first cell are eliminated with
a standard SHEPWM harmonic elimination scheme. An additional
switching angle is allowed in the second cell to shape its
frequency spectrum in such a way that it mirrors the spectrum of
the first cell.