Download High Power Machine Drive, Based on Three

High Power Machine Drive, Based on Three-Stage Connection of “H” Converters,
and Active Front End Rectifiers.
Juan Dixon, Alberto Bretón, Felipe Ríos
Luis Morán
Department of Electrical Engineering
Pontificia Universidad Católica de Chile
Casilla 306, Correo 22, Santiago, Chile
fax 56-2-552-2563
e-mail [email protected]
Department of Electrical Engineering
Universidad de Concepción
Casilla 53-C, Concepción, Chile
fax 56-41-246-999
e-mail [email protected]
Abstract. A three-stage inverter using “H” converters is being
analyzed for high power machine drive applications. The great
advantage of this kind of converter is the minimum harmonic
distortion obtained at the machine side. The drawbacks are the
isolated power supplies required for each one of the three stages
of the multiconverter. In this paper this problem has been
overcome in two ways: 1) by using independent windings for
each phase of the motor, or 2) by using independent input
transformers. Special configurations and combinations of
passive rectifiers and active front end rectifiers for one of the
stages of the drive are used to eliminate all input harmonics.
The topology can also keep unity power factor at the input
terminals. Simulation results are shown and some experiments
with small four-stage prototypes are displayed. The control of
this multi-converter is being implemented using DSP
controllers, which give flexibility to the system.
I. INTRODUCTION
Power Electronics devices contribute with important part
of harmonics in all kind of applications, such as power
rectifiers, thyristor converters, and static var compensators
(SVC). On the other hand, the PWM techniques used today
to control modern static converters such as high power
machine drives, strongly depend on switching frequency of
the power semiconductors. Normally, voltage (or current in
dual devices) moves to discrete values, forcing the design of
machines with good isolation, and sometimes loads with
inductances in excess of the required value. In other words,
neither voltage nor current are as expected. This also means
harmonic contamination, additional power losses, and high
frequency noise that can affect the controllers. All these
reasons have generated many research works on the topic of
PWM modulation [1-4]. More recently, multilevel converters
[5-7] have permitted to have many levels or steps of voltage
to reduce the THD levels.
Multi-stage converters [8, 9] work more like amplitude
modulation rather than pulse modulation, and this fact makes
the outputs of the converter very much cleaner. This way of
operation allows having almost perfect currents, and very
good voltage waveforms, eliminating most of the undesirable
harmonics. And even better, the bridges of each converter
work at a very low switching frequency, which gives the
possibility to work with low speed semiconductors, and to
generate low switching frequency losses. The objective of
this paper is to show the advantages of multi-stage converters
for high power machine drive applications. The drawbacks of
requiring isolated power supplies is solved using different
0-7803-7906-3/03/$17.00 ©2003 IEEE.
techniques, based on the fact that the first converter, called
Master, takes more than 80% of the total power delivered to
the machine. A three-stage converter using “H” power
modules, which gives 27 different levels of voltage
amplitude is studied. The current and voltage waveforms for
a standard 4 kV, 2 MW induction machine is simulated.
There are also some experiments with a small laboratory
prototype, using a four-stage three-phase converter.
II. BASICS OF MULTI-STAGE CONVERTERS
A. Basic Principle
The circuit of fig.1 shows the basic topology of one
converter used for the implementation of multi-stage
converters. It is based on the simple, four switches converter,
used for single phase inverters or for dual converters. These
converters are able to produce three levels of voltage in the
load: +Vdc, -Vdc, and Zero.
Driver
+
_
Vdc
LOAD
Fig. 1. Three-level module for building multiconverters
The Fig. 2 displays the main components of a three-stage
converter which is being analysed in this work. The figure
only shows one of the three phases of the complete system.
As can be seen, the dc power supplies of the four converters
are isolated, and the dc supplies are scaled with levels of
voltage in power of three. The scaling of voltages in power
of three allows having, with only three converters, 27 (33)
different levels of voltage: 13 levels of positive values, 13
levels of negative values, and zero. The converter located at
the top of the figure has the biggest voltage, and will be
called Master. The other two modules will be the Slaves. The
Master works at a lower switching frequency and carries
more than 80% of the total power, which is an additional
advantage of this topology for high power machine drives
applications.
226
With 27 levels of voltage, a three-stage converter can
follow a sinusoidal waveform in a very precise way. It can
control the load voltage as an AM device (Amplitude
Modulation). The Fig. 3 shows the voltage modulation of
each one of the Three “H” converters, for 100% amplitude
modulation.
Driver
+
9xVdc
Master
_
B. Power Distribution
One of the good advantages of the strategy described here
for multiconverters is that most of the power delivered to the
machine comes from the Master. The example of Fig. 4
shows the power distribution in one phase of the three-stage
converter, feeding a pure resistive load with sinusoidal
voltage. A little more than 80% of the real power is delivered
by the Master converter, and only 20% for the Slaves. Even
more, the last slave only delivers 5% of the total power. That
means, the dc power sources needed by the second Slave is
small.
5 % P O W E R IN 2 N D S L A V E
Machine
phase
+
Driver
_
3xVdc
0
1 5 % P O W E R IN 1 ST S L A V E
1st Slave
0
8 0 % P O W E R IN M A S T E R
+
Driver
Vdc
_
0
2nd Slave
Fig. 4. Active power distribution in a four-stage converter.
Fig. 2. Main components of the three-stage converter.
The Fig. 3 shows the voltage modulation of each one of
the three converters of the chain of Fig. 2.
2ND SLAVE
0
1ST SLAVE
0
MASTER
0
Fig. 3. Voltage modulation in each converter
This characteristic makes possible to feed the second Slave
with low power dc supplies. However, as in some levels of
low voltage regulation the power goes through the system,
the sources need to be bi-directional. There are three
solutions for this problem: 1) active front-end rectifiers, 2)
bi-directional dc-dc power supplies, or 3) passive rectifiers
with dissipative resistors. In the last case it should be
required to evaluate the power losses.
Another attribute of this configuration, which is possible
to see in Figs. 3 and 4, is the very low switching frequency of
each converter, specially the Master, which carries most of
the power. Then, the larger the power of the unit, the lower
its switching frequency. In the case analized here, the Master
has been implemented with GTOs, and the Slaves with
IGBTs.
III. INPUT POWER TOPOLOGY
As it was already mentioned, isolated power supplies for
each converter are required. In Fig. 5 the electrical schematic
of the complete power part including rectifiers and inverters
is displayed. The three Masters are fed with standard
rectifiers, each one in 6-pulse configuration. These rectifiers
are isolated from the supply by a four winding transformer,
to create three secondary voltage systems, one for each of the
three Masters, and shifted in +20°, 0° and –20°. With this
configuration, a very low harmonic distortion from the mains
point of view is obtained [10].
Each one of the first Slaves (“Slaves 1” A, B and C in
Fig. 5), which carries 15% of the total power, needs
227
bidirectional power supplies because at some low voltage
operation the power goes from the machine to the mains. To
solve this problem, three PWM active rectifiers are used. The
advantage of using this type of rectifier at this stage is that
they work as power factor compensator and active power
filters from the mains point of view, allowing to have almost
perfect current waveforms at the supply side.
Finally, the second Slaves (“Slaves 2” A, B and C in Fig.
5), are fed with simple Graetz bridges with a dissipative
resistors, which are necessary when the machine operates
with very low voltage (less than 15%) during starting.
However, they can also be implemented with PWM rectifiers
like “Slaves 1”.
“H” bridges
Transformers
“H” bridges
Transformers
6-pulse conv
+20
°
0°
Master
A
Master
B
-20°
Transformers
PWM conv
Slave-1
A
N
M
Slave-1
B
6-pulse conv
Master
A
+20
°
0°
Slave-1
C
Master
B
80%
Power
Transformers
6-pulse rect
Slave-2
A
Master
C
-20°
Transformers
Slave-2
B
PWM conv
Slave-1
A
Slave-2
C
Slave-1
B
15%
Power
Fig. 6. Another topology using independent motor windings
Slave-1
C
Transformers
Master
C
III. SIMULATED WAVEFORMS
6-pulse rect
Slave-2
A
5%
Power
Slave-2
B
Slave-2
C
The following simulations were performed using PSIM, a
special simulator for power electronics circuits [11]. The Fig.
7 shows the output voltages and the motor current produced
by the three-stage converter. The converter voltage, the
phase-to-phase voltage, the phase-to-neutral voltage, and the
motor current are displayed. In the case of Fig. 6 topology,
the output voltage of the three-stage converter is the same as
the phase-voltage of the machine, because the windings are
independent and isolated. The machine is a 2MW, 4kV
induction motor.
M
a)
Fig. 5. One of the proposed topologies for high power drives
The drawback of the configuration of Fig. 5 is that the
power rectifiers of the Masters need a good filter at the dc
link, because each Master represents a single-phase load. To
avoid this problem, the three Masters can be fed in parallel,
keeping the transformer configuration with the rectifiers
connected in series as shown in Fig. 6. However, the three
windings of the machine have to be fed independently (no
electrical connection between them).
228
b)
c)
Fig. 7. a) converter voltage, b) phase-to-phase voltaje, c)
phase-to-motor neutral voltage and current.
The Fig. 8 shows the harmonic spectrum of machine
voltages for the case of Fig. 5 and Fig. 6. It can be noticed
that the spectrum is cleaner for the case of Fig. 5 (machine
with neutral connection), but in both cases the amplitude of
the higher harmonics is less than 1%, and hence the
amplitude of current harmonics are absolutely negligible.
1.2
1.0
a)
%
0.8
a)
0.6
0.4
0.2
b)
0.0
1.2
1.0
0.8
0.6
%
b)
0.4
0.2
c)
0.0
Fig. 8. Voltage harmonics spectrum at the motor windings
a) Fig. 6 topology, b) Fig. 5 topology
The Fig. 9 shows the three phase-voltages generated by the
thre-stage converters, and the machine current, which looks
perfectly sinusoidal. On the other hand, the Fig. 10 shows the
current distortion when the voltage of the machine varies
from 100% to 10%, for the case of Fig. 6 topology
(independent no neutral connection windings), which is the
worst of the two systems from the machine point of view. It
can be noticed that the current remains almost sinusoidal
even with 25% voltage amplitude, without the need of PWM
modulation. For this simulation the frequency and the slip of
the machine have been kept constant.
d)
e)
a)
Fig. 10 current waveform distortion at the machine
a) 100% voltage, b) 75% voltage, c) 50% voltage, d)
25% voltage, and e) 10% voltage
b)
Fig. 9. a) phase voltajes at the three phases of the converter,
and b) winding voltage and current for Fig. 6 topology
It is also important to show the power distribution in each
stage of the power converter, particularly in some cases
where the power is reversed with the voltage variations. The
Fig.11 shows the particular case when the Slave 1 is
returning power from the motor to the system, and this
situation happens because the system is trying to keep the
current sinusoidal. This reason justifies the fact of using
active front end rectifiers at the first Slave level. Otherwise,
the power could not be returned to the mains. As it was
229
mentioned before, these rectifiers also allow to keep the input
currents of the system free of harmonics.
a)
b)
c)
Fig. 14. Single-phase current and three-phase voltages
Fig. 11 Power distribution in the three stages of the
converter. a) Master, b) Slave 1, c) Slave 2
IV. EXPERIMENTAL RESULTS
The Fig. 12 shows the voltage steps waveforms obtained
with a 3 kW four-stage prototype. The figure shows only half
wave. On the other hand, the Fig. 13 shows the phase voltage
and currents in one of the three phases of the multiconverter
when it feds an induction machine. Finally, in Fig. 14, the
voltages of the three phases, and the current in one of them
are observed. It is noticed again that the voltages are very
good.
The prototype used for the experiments is shown in
Fig.14. It can be observed that the voltages are quite
sinusoidal and the resultant current is also very clean. These
results justify the research developed with this kind of
converter because, as was shown in figures 5 and 6, they are
specially suited for very large machine drives, which can be
implemented with GTOs at the Master level, and with IGBTs
at the Slaves levels.
Fig. 12. Voltage steps waveforms in a four-stage converter
Fig. 15. Four-stage multiconverter prototype
V. CONCLUSIONS
Fig. 13. Voltage and current waveforms in a four-stage
converter
A three-stage inverter using “H” converters has been analyzed for
high power machine drive applications. The great advantage of this
kind of converter is the minimum harmonic distortion obtained at
the machine side. The need of isolated power supplies required for
each one of the three stages of the multiconverter has been solved in
three ways: passive rectifiers at the Master level (80% of the
power), active front-end PWM rectifiers (which act as a power
filters and var compensators) at the Slave 1 level (15% of the
power), and passive rectifiers with dissipative power resistors
during very low voltage operation at the Slave 2 (only 5% of the
total power). Simulation results were shown and some experiments
with a small four-stage prototype was displayed. The control of this
multi-converter is being implemented using DSP controllers, which
will give flexibility to the system.
230
with Floating Capacitor Technology”, European
Power Electronics Conference, EPE 2001.
VI. ACKOWLEDGEMENTS
The authors want to thank Conicyt through Projects
Fondecyt 1020460 and 1020982, for the support given to this
work.
[11]
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Powersim Technologies, Vancouver, Canada, Web
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