high power machine drive, based on three-stage...
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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 Department of Electrical Engineering Pontificia Universidad Católica de Chile Universidad de Concepción Casilla 306, Correo 22, Santiago, Chile
fax 56-2-552-2563 e-mail [email protected]
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
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.
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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.
9xVdc + _
Vdc + _
Driver
Driver
Driver
Machinephase
3xVdc + _
2nd Slave
1st Slave
Master
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.
MASTER
1ST SLAVE
2ND SLAVE
0
0
0
Fig. 3. Voltage modulation in each converter
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.
8 0 % P O W E R IN M A S T E R
1 5 % P O W E R IN 1 S T S L A V E
5 % P O W E R IN 2 N D S L A V E
0
0
0
Fig. 4. Active power distribution in a four-stage 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
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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”.
M
Transformers
Transformers 6-pulse conv
0°
-20°
+20°
Master A
Master B
MasterC
Slave-1 A
Slave-1 B
Transformers 6-pulse rect
Slave-2 A
Slave-2 B
Slave-2C
Slave-1C
N
PWM conv
“H” bridges
80% Power
15% Power
5% Power
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).
Transformers
Transformers 6-pulse conv
0°
-20°
+20°
Master A
MasterB
MasterC
Slave-1 A
Slave-1B
Transformers 6-pulse rect
Slave-2 A
Slave-2B
Slave-2C
Slave-1C
PWM conv
“H” bridges
M
Fig. 6. Another topology using independent motor windings
III. SIMULATED WAVEFORMS
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.
a) b) c)
Fig. 7. a) converter voltage, b) phase-to-phase voltaje, c) phase-to-motor neutral voltage and current.
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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.
%
%
a)
b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
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.
a) b)
Fig. 9. a) phase voltajes at the three phases of the converter,
and b) winding voltage and current for Fig. 6 topology
a)
b )
c )
d )
e )
Fig. 10 current waveform distortion at the machine a) 100% voltage, b) 75% voltage, c) 50% voltage, d)
25% voltage, and e) 10% voltage
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
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mentioned before, these rectifiers also allow to keep the input currents of the system free of harmonics.
a) b) c)
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.
Fig. 12. Voltage steps waveforms in a four-stage converter
Fig. 13. Voltage and current waveforms in a four-stage
converter
Fig. 14. Single-phase current and three-phase voltages
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. 15. Four-stage multiconverter prototype
V. CONCLUSIONS
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.
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VI. ACKOWLEDGEMENTS
The authors want to thank Conicyt through Projects Fondecyt 1020460 and 1020982, for the support given to this work.
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