04-12-2012, 05:47 PM
A Novel Overmodulation Technique for Space-Vector PWM Inverters
A Novel Overmodulation.pdf (Size: 228.96 KB / Downloads: 214)
Abstract
In this paper, a novel overmodulation technique
for space-vector pulsewidth modulation (PWM) inverters is proposed.
The overmodulation range is divided into two modes
depending on the modulation index (MI). In mode I, the reference
angles are derived from the Fourier series expansion of the
reference voltage which corresponds to the MI. In mode II, the
holding angles are also derived in the same way. The strategy,
which is easier to understand graphically, produces a linear
relationship between the output voltage and the MI up to sixstep
operation. The relationship between those angles and the MI
can be written in lookup tables or, for real-time implementation,
can be piecewise linearized. In addition, harmonic components
and total harmonic distortion (THD) of the output voltage are
analyzed. When the method is applied to the V/f control of the
induction motor, a smooth operation during transition from the
linear control range to the six-step mode is demonstrated through
experimental results.
INTRODUCTION
THREE-PHASE voltage-source pulsewidth modulation
(PWM) inverters have been widely used for dc/ac power
conversion since they can produce a variable voltage and
variable frequency power. However, they require a dead time
to avoid the arm-short and snubber circuits to suppress the
switching spike. Apart from these ancillary aspects, the PWM
inverters have an essential problem that they cannot produce
voltages as large as the six-step inverters can. That is, the dc
bus voltage cannot be utilized to the maximum.
To increase the voltage utilization of the sinusoidal PWM
inverter, a method of the addition of the third harmonics to
the reference voltage was proposed by which the fundamental
component can be increased by 15.5% [1]. In a space-vector
PWM inverter, which is widely used, the voltage utilization
factor can be increased to 0.906, normalized to that of the sixstep
operation [2]. On the other hand, different discontinuous
PWM strategies were analyzed in [3], where the modulation
waveform of a phase has at least one segment of 60 which
is clamped to the positive and/or negative dc bus for, at most,
a total of 120 in a fundamental period during which no
switching in either inverter arm occurs.
A NOVEL OVERMODULATION STRATEGY
In this section, a novel overmodulation strategy for the
space-vector PWM is derived from developing Fourier series
expansion of the waveform of the phase voltage reference
which gives the desired fundamental component. For simple
analysis, a dead-time effect is neglected.
EXPERIMENTS AND DISCUSSIONS
To confirm the validity of the proposed scheme, experiments
were performed for the V/f control of induction motor drive
fed by an insulated gate bipolar transistor (IGBT) PWM
inverter. Fig. 10 shows the experiment system with a digital
signal processor (DSP) board. In practice, such a highperformance
DSP is not required for implementation of the
V/f control of the induction motor. Also, the dc-link voltage
is usually measured for the space-vector modulation and
overvoltage protection, and the current is measured only for
monitoring.
CONCLUSIONS
In the space-vector modulation, a linear control of the
inverter output voltage was obtained up to MI by
a novel overmodulation strategy. The method is based on
Fourier series representation of the reference voltage, where
a graphical transformation between complex voltage vectors
and the phase voltage in time domain is used implicitly.
The reference angles in mode I and the holding angles in
mode II were derived as a function of the modulation index
by numerical analysis. These data can be written in
lookup tables or, for real-time implementation, piecewise
linearized