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DonNTU | Master's portal |
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Khanin AlexisFaculty: "Electrotechnical" Department: "Electric drive and automation of industrial-scale plants" Speciality:
Research of system control asynchronous motor with asymmetrical cascade multilevel inverter.Scientific adviser: Ph.D. Shavelkin Alexander |
Abstract
Table of contents
1. Introduction
2. Asymmetrical multilevel inverter structures and modulation сontrol schemes
3. Induction motor control system with an asymmetric multilevel inverter
4. Conclusions and future work
5. References
Numerous industrial applications have begun to require higher power apparatus in recent years. Some medium voltage motor drives and utility applications require medium voltage and megawatt power level. For a medium voltage grid, it is troublesome to connect only one power semiconductor switch directly [1]. As a result, an asymmetrical multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations. An asymmetrical multilevel converter not only achieves high power ratings, but also enables the use of renewable energy sources. Renewable energy sources such as photovoltaic, wind, and fuel cells can be easily interfaced to a multilevel converter system for a high power application
Concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc voltage sources. The commutation of the power switches aggregate these multiple dc sources in order to achieve high voltage at the output; however, the rated voltage of the power semiconductor switches depends only upon the rating of the dc voltage sources to which they are connected [4].
An asymmetrical multilevel converter has several advantages over a conventional two-level converter that uses high switching frequency pulse width modulation (PWM). The attractive features of a multilevel converter can be briefly summarized as follows [3,5].
- Input current: Asymmetrical multilevel converters can draw input current with low distortion.
- Staircase waveform quality:Asymmetrical multilevel converters not only can generate the output voltages with very low distortion, but also can reduce the dv/dt stresses; therefore electromagnetic compatibility (EMC) problems can be reduced.
- Switching frequency: Asymmetrical multilevel converters can operate at both fundamental switching frequency and high switching frequency PWM. It should be noted that lower switching frequency usually means lower switching loss and higher efficiency.
Asymmetrical multilevel converters do have some disadvantages. One particular disadvantage is the greater number of power semiconductor switches needed. Also, circulation of energy between different single-phase inverters [5,7].
That why induction motor control system must be considered in the complex problems associated with asymmetric topology
2. Asymmetrical multilevel inverter structures and modulation сontrol schemes
Simplified block diagram of an asymmetric multilevel inverter in Figure 1
Figure 1 – Simplified block diagram with three inverters per phase
The hybrid cascaded H-bridge inverter power circuit is shown in Fig. 1. The inverter is composed of three legs, in each one is a series connection of three H-bridge inverters fed by independent dc sources that are not equal (V1≠V2≠V3). Asymmetric multilevel inverters use different voltage levels among the cascaded inverter cells. By addition and subtraction of these voltages, more unique output-voltage levels can be generated with the same number of components, compared to a symmetric multilevel inverter [1,2].
The control schemes for the multilevel inverter can be divided into three categories [4,6]:
1. Fundamental switching frequency (Fig. 2). A multilevel converter can produce a quarter-wave symmetric stepped voltage waveform synthesized by several dc voltages.
2. Sinusoidal PWM (SPWM). There may be two options: multiplexer SPWM (Fig 3), and multilevel SPWM (Fig 4).
3. Space vector PWM. High complexity in the implementation of such a PWM. The number of possible switch combinations is equal to the cube of the level (m³). For this six-level inverter, there are 216 possible switching states.
Figure 2 – Output phase voltage waveform of an 7-level cascade inverter (Animation: 10 shots, 3 seconds, 113 KB, the number of repetitions: 7)
Figure 3 – Multiplexer sinusoidal PWM
Figure 4 – Multilevel sinusoidal PWM
Figure 5 – Space vector PWM
Are possible a combinations of these methods.
3. Induction motor control system with an asymmetric multilevel inverter
In this paper we have considered scalar control and vector control system of AC Induction Motor
Model softstarter induction motor is presented in Figure 6. And the simulation results are shown in Figures 7. To assess the results in Figure 8 shows the transient torque induction motor with "Classical" inverter.
Figure 6 –Model softstarter of AC induction motor.
Figure 7 – Transient processes induction motor with asymmetrical multilevel inverter
Figure 8 – Transient processes induction motor with "Classical" inverter
As you can see from the figures 7,8, a decrease torque ripple
Model of vector control induction motor is shown in Figure 9. This has two inner current control loops, one for isx control and one for isy control. The two outer loops namely, the speed control loop and the flux control loop generates reference values for isy and isx control respectively. Complete model of the drive is shown in Figure 10
The simulation results are shown in Figures 11,12,13.
Figure 9 – Model of vector control
Figure 10 – Complete model of the drive with vector control
Figure 11 – Transient processes in motor torque with asymmetrical multilevel inverter
Figure 12 – Transient processes in motor torque with "Classical" inverter
Figure 13 – Rotor speed the induction motor
4. Conclusions and future work
Asymmetrical multilevel inverters are attracting an increasing interest in power conversion field because they can offer high power possibility with low output harmonics. They can be used in a variety of areas such as traction motor drive or renewable energy utility interface. In this work,
• Were considered the possibility of using an asymmetric multilevel inverter in scalar control and vector control system
• Were carried out research ripple torque
• Were developed universal models of control systems drive
Future plans
• Research of other types of asymmetrical multilevel inverters
• Research the possibility of replacing in the asymmetrical multilevel inverters DC with a minimum voltage to the capacitor
Important!
At this time Master's work was not completed. Final completion of the work planned for December 2011. Full text of the work and materials on the subject can be obtained from the author or his manager after that date.
1.Haiwen Liu, Leon M. Tolbert, Burak Ozpineci, Zhong Du, "Hybrid multilevel Inverter with single DC source," IEEE International Midwest Symposium on Circuits and Systems, Knoxville, TN, Page 538-541, August 10-13, 2008.
2. J. Rodriguez, J. S. Lai and F. Z. Peng, "Multilevel Inverters: Survey of Topologies, Controls, and Applications," IEEE Transactions on Industry Applications, vol. 49, no. 4, Aug. 2002, pp. 724-738.
3. Haiwen Liu, Leon M. Tolbert, Surin Khomfoi, Burak Ozpineci, Zhong Du, "Hybrid cascaded multilevel inverter with PWM method," IEEE Power Electronics Specialist Conference, Island of Rhodes, Greece, Page 162-166, June 15-19, 2008.
4. Haiwen Liu, "Design and Application of Hybrid Multilevel Inverter for Voltage Boost," a dissertation, The University of Tennessee, Knoxville, August 2009.
5. Шавьолкін О.О. Перетворювальна техніка: навчальний посібник/ О.О. Шавьолкін, О.М. Наливайко; за загальною ред. канд. техн. наук, доц. О.О. Шавьолкіна.- Краматорськ: ДДМА, 2008. - 328с.
6. M. F. Aiello, P. W. Hammond, and M. Rastogi, "Modular Multi-Level Adjustable Supply with Parallel Connected Active Inputs," U.S. Patent 6 301 130,Oct. 2001.
7. Шавёлкин А.А. Несимметричный гибридный многоуровневый преобразователь частоты на базе трехуровневого инвертора напряжения/А.А. Шавёлкин // Наукові праці ДонНТУ. Серія: “Електротехніка та енергетика”.- Донецьк: ДВНЗ «ДонНТУ», 2009.-вип. 9(158)- С.242-249.
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