Aside 9. Example of a PI Voltage Controller Design for a UPS Application
5.5 OTHER APPLICATIONS OF THE CURRENT CONTROLLED VSI
5.5.2 The Active Power Filter
The active power filter application can be represented by Fig. 5.18 as well. In this case, the ILOAD generator is considered and used to represent the distorting or reactive loads the filter
EXTERNAL CONTROL LOOPS 145
has to compensate, while the dc load,RDC, may not be present. If there is no dc load, the active power filter is not required to process any active power, with the exception of that due to its losses, and can thus be sized to sustain only the reactive and harmonic load currents. A typical control objective for this application is to compensate the harmonic and reactive load currents, so as to make the ac source current proportional to the source voltage. This implies that, from the source standpoint, the load will be seen as an equivalent resistor, absorbing only the active power required by the distorting loads. The achievement of this objective requires again two different control loops: (i) a current control loop, used to impose the desired ac currentIACto the source, and (ii) a dc voltage control loop, used to regulate the load voltage,VDC, keeping it equal to a given reference value.
Apparently, this situation seems identical to that of the rectifier discussed in the previous section. This is actually the case for the voltage loop, which can be designed exactly as that of the rectifier. It is not at all the case for the current loop: the compensation of high-order harmonic currents normally requires some high-performance current control loop. Indeed, the implementation of a simple PI current controller is normally able to offer only a limited harmonic compensation capability, which is very often quite far from being satisfactory.
Therefore, more complex solutions have to be taken into account. As we have illustrated for the UPS voltage loop, in this case it is as well possible to follow two different design philosophies:
(i) implementing a large bandwidth current controller or (ii) implementing a narrow bandwidth current controller. The former solution is aimed at the instantaneous compensation of any deviation of the current injected into the line from its reference waveform. The latter is instead aimed at the slow compensation of the same deviation, typically requiring several fundamental frequency periods to be accomplished.
The large bandwidth controllers that, in the digital domain, are exactly of the predictive type we have discussed in Chapter 3 are normally suited to all those situations where the distorting and harmonic load currents are characterized by unpredictable and frequent variations.
The narrow bandwidth controllers can be based on the resonant filters or, equivalently, on the rotating reference frame regulators seen in Chapter 4. In the active filter application, several parallel regulators will be implemented to take care of the different harmonic frequencies to be compensated. Repetitive or DFT filter based controllers, of the type seen in Section 5.4, are also viable solutions. Of course, since the dynamic response of these regulators normally extends to some fundamental frequency periods, their adoption should be limited to those cases where the distorting and reactive load currents are not subject to frequent variations and therefore the controller steady state is not too frequently perturbed. The design of the narrow bandwidth regulators exactly follows the principles we have illustrated for the UPS voltage control case.
The last issue we need to examine to complete this brief description of active power filter control is related to the generation of the inverter reference current signal. From Fig. 5.18 we
can see that in order to achieve the desired compensation and inject a voltage proportional current into the ac source, the inverter simply needs to generate a current equal to the algebraic sum of the desired source current and the load current. Therefore, in the most simple approach the inverter current reference can be built as
IOREF= −IAC∗ +ILOAD= −GEQEAC+ILOAD, (5.25) whereGEQ, as in the rectifier case, represents the output of the dc link voltage regulator. Of course, the implementation of (5.25) is straightforward only if the measurement of the distorting and harmonic loads’ currentILOADis possible. If this is the case, the result of its application will be the cancelation of the reactive current component from the ac source current. In addition, any harmonic current not present in the ac source voltage will also be canceled. The quality of the cancelation is, of course, limited only by the chosen current controller reference tracking capabilities [15].
If current ILOAD cannot be measured, or if the active power filter is designed for more complex tasks, like the partial, controlled compensation of some selected harmonics and/or the compensation of the load reactive power only, different approaches for the computation of the converter current reference can be employed, the illustration of which, however, goes beyond the scope of this book.
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[5] S. Buso, S. Fasolo and P. Mattavelli, “Uninterruptible power supply multi-loop control employing digital predictive voltage and current regulators,”IEEE Trans. Indust. Appl., Vol. 37, No. 6, pp. 1846–1854, Nov. /Dec. 2001.doi.org/10.1109/28.968200
[6] O. K¨ukrer, “Deadbeat control of a three-phase inverter with an output LC filter,”IEEE Trans. Power Electron., Vol. 11, No. 1, pp. 16–23, Jan. 1996.doi.org/10.1109/63.484412 [7] O. K¨ukrer and H. Komurcugil “Deadbeat control method for single-phase UPS inverters with compensation of computational delay,” IEE Proc., Electr. Power Appl., Vol. 146, No. 1, pp. 123–128, Jan. 1999.dx.doi.org/10.1049/ip-epa:19990215
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[9] T. Yokoyama and A. Kawamura, “Disturbance observer based fully digital controlled PWM inverter for CVCF operation,”IEEE Trans. Power Electron., Vol. 9, No. 5, pp. 473–
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[13] K. Zhang, Y. Kang, J. Xiong and J. Chen, “Direct repetitive control of SPWM inverters for UPS purpose,” IEEE Trans. Power Electron., Vol. 18, No. 3, pp. 784–792, May 2003.doi.org/10.1109/TPEL.2003.810846
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[15] S. Buso, L. Malesani and P. Mattavelli, “Comparison of current control techniques for active filter applications,” IEEE Trans. Indust. Electron., Vol. 45, No. 5, pp. 722–729, Oct. 1998.doi.org/10.1109/41.720328
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C H A P T E R 6
Conclusions
This book has been conceived to give to the reader a basic and introductory knowledge of some typical power converter control problems and of their digital solutions. Although the presented material has been focused on a single converter topology, i.e., the half-bridge voltage source inverter, the control topics we have been dealing with represent, in our opinion, a significant spectrum of the more frequently encountered digital control applications in power electronics.
Moving from the pulse width modulation modeling, we have described the fundamental types of digital current control loop implementation, i.e., the PI controller and the predictive controller. These basic techniques have subsequently allowed us to present the fundamental issues related to three phase current control, with particular consideration for the concepts of rotating reference frame and the controllers that can be based on it.
In the last part of our discussion, we have approached some more advanced control orga- nizations, essentially based on multiloop strategies. We have consequently presented the typical case of the voltage controller for a single-phase uninterruptible power supply. We have seen how both large bandwidth and narrow bandwidth control strategies can be digitally implemented, and analyzed their merits and limitations. In addition, we have seen how the controllers we have analyzed can allow the implementation of other applications of voltage source inverters, like the controlled rectifier of the active power filter.
Of course, we are aware that a lot of other extremely interesting applications could have been dealt with, and also that the more advanced research topics could have been taken into account and presented. We hope the choice we have made, for the sake of conciseness, and the method we have chosen to present the selected material, starting from the very basic issues, will be good enough to give to the readers that we have not been able to completely satisfy the motivation for further autonomous study.
On the other hand, we hope that what has been presented will allow inexperienced readers to successfully experiment with digital control techniques in power electronics.
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About the authors
Simone Busograduated in electronic engineering at the University of Padova in 1992. He received the Ph.D. degree in industrial electron- ics and informatics from the same university in 1997. Since 1993, he has been cooperating with the power electronics research group of the University of Padova. Currently he is a member of the staff of Depart- ment of Information Engineering (DEI) of the University of Padova, where he is holding the position of associate professor. His main re- search interests are in the industrial and power electronics fields and are specifically related to dc/dc and ac/dc converters, smart power in- tegrated circuits, digital control and robust control of power converters, solid state lighting, electromagnetic compatibility applied to integrated circuits, and switch mode power supplies.
Simone Buso is a member of the IEEE.
Paolo Mattavelli graduated (with honors) and received the Ph.D.
degree, both in electrical engineering, from the University of Padova (Italy) in 1992 and in 1995, respectively. From 1995 to 2001, he was a researcher at the University of Padova. In 2001 he joined the Department of Electrical, Mechanical and Management Engineering (DIEGM) of the University of Udine, where he was an associate professor of Electronics from 2002 to 2005. From 2001 to 2005 he was leading the Power Electronics Laboratory of the DIEGM at the University of Udine, which he founded in 2001. Since 2005 he has been with the Department of Technology and Management of Industrial System at the University of Padova in Vicenza. He was also Visiting Researcher at the Massachusetts Institute of Technology in 1995 and in 1997.
His major fields of interest include analysis, modeling, and control of power converters, digital control techniques for power electronic circuits, active power filters and power quality issues.
Paolo Mattavelli is a member of IEEE Power Electronics, IEEE Industry Applications, IEEE Industrial Electronics Societies, and the Italian Association of Electrical and Electronic Engineers (AEI). He currently (2006) serves as an associate editor for IEEE Transactions on Power Electronics, IPCC (Industrial Power Converter Committee) Review Chairman for the IEEE Transactions on Industry Applications, and Member-at-Large of PELS Adcom.