Abstract—
In this paper, the method of triangular carrier
switching control of two-level inverters is extended to cascaded
multilevel inverters using phase-shifted multicarrier unipolar
pulsewidth modulation (PWM). The condition for smooth modulation
is obtained using the Bessel’s function representation of
the PWM output and the switching condition of the multilevelinverter-
controlled system. A method is proposed for the determination
of the minimum amplitude of the triangular carrier
for smooth modulation at fixed switching frequency. It is shown
that the multilevel modulation based on the phase-shifted carriers
significantly reduces the ripple magnitude in the switching
function and allows the use of a smaller carrier amplitude under
closed loop. This increases the forward gain and, hence, improves
the tracking characteristics. The proposed cascaded multilevel
inverter control is implemented for the operation of a distribution
static compensator (DSTATCOM) in the voltage control mode.
The experimental verification of the theoretical and simulation
results is provided through a field-programmable gate array
(FPGA) based control of a laboratory model of a single-phase
DSTATCOM.
Index Terms—Bessel’s function, carrier amplitude, cascaded
multilevel inverter, distribution static compensator
(DSTATCOM), field-programmable gate array (FPGA), multilevel
modulation, switching control.
I. INTRODUCTION
THE BASIC motivation for the use of multilevel inverters
is the reduction of voltage stress on the switching devices
[1]. A cascaded multilevel inverter is a cost-effective option due
to the series connections of identical H-bridge modules. The use
of phase-shifted multicarrier unipolar pulsewidth modulation
(PWM) reduces the harmonic contents of the voltage delivered
by the cascaded multilevel inverter [2], operating at a lower
switching frequency because of low switching loss. Applications
such as medium-voltage distribution system compensation
use cascaded multilevel inverters as the voltage source
inverters (VSIs) for compensating devices [3], [4].
The hysteresis-based instantaneous method of control has
been a preferred method for accurate and fast dynamic performance
of two-level inverter-controlled systems [5]–[8].
Recently, a hysteresis-based control has been used for the
cascaded multilevel topology [4]. However, the method suf-
A. Ghosh is with School of Engineering Systems, Queensland University of
Technology, Brisbane, Qld. 4001, Australia (e-mail: a.ghosh[at]qut.edu.au).
A. Joshi is with the Department of Electrical Engineering, Indian Institute of
Technology, Kanpur 208 016, India (e-mail: ajoshi[at]iitk.ac.in).
Digital Object Identifier 10.1109/TIE.2007.896274
fers from the disadvantages of variable switching frequency
and unequal switching stress among various switches. The
satisfactory performance is obtained on an average basis only
after a sequential change in the switching order. The method
of triangular carrier switching control is well known for the
closed-loop control of two-level inverters [5]–[9]. The method
yields dynamic performance close to the hysteresis control
and, additionally, provides a constant switching frequency. The
important limitation of this method is that it leads to a tracking
error in the steady state for low-frequency references that is
proportional to the carrier amplitude [6], whereas the value
of the carrier amplitude cannot be arbitrarily reduced and is
limited by the ripple magnitude of the switching function. The
carrier amplitude forms an important design parameter and is
difficult to a priori estimate for smooth modulation at the fixed
switching frequency. There has been a recent trend for the
generalization of the two-level inverter control techniques to the
multilevel inverters [10]–[12].
In this paper, the method of triangular carrier switching
control of two-level inverters is extended for the cascaded
multilevel-inverter-controlled systems. A multilevel output
voltage is generated following the phase-shifted multicarrier
unipolar PWM for the cascaded inverters. The method leads to
a fixed switching frequency operation with uniform distribution
of the switching stress among all the switches. The condition
for smooth modulation is obtained using the Bessel’s function
representation of the PWM output [2] and the switching
condition of multilevel inverters. A method is proposed for the
determination of the minimum amplitude of a triangular carrier
for the smooth modulation at a fixed switching frequency
of the multilevel control. The proposed cascaded multilevel
control is implemented for the operation of a distribution static
compensator (DSTATCOM) in the voltage control mode [6]
and verified through an experimental model of a low-voltage
distribution system.


Download full report
http://ieeexplore.ieeeiel5/41/4459824/04...er=4401134