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Voltage Sag Ride-Through of AC Drives: Control and Analysis Kai Pietil¨ ainen
Voltage Sag Ride-Through of AC Drives:
Control and Analysis
Kai Pietil¨
ainen
Stockholm 2005
Electrical Machines and Power Electronics
Department of Electrical Systems
Royal Institute of Technology
Submitted to the School of Electrical Engineering, KTH,
in partial fulllment of the requirements for the degree of
Doctor of Technology. TRITA-ETS-2005-12
ISSN 1650674X
ISRN KTH/R-0504-SE
ISBN 91-7178-165-X
KTH, Royal Institute of Technology
Department of Electrical Systems
Division of Electrical Machines and Power Electronics
KTH/ETS/EME
Teknikringen 33
SE-10044 Stockholm
Sweden
c Kai Pietil¨ainen, 2005 Abstract
This thesis focuses on controller design and analysis for induction motor (IM)
drives, ux control for electrically excited synchronous motors with damper
windings (EESMs), and to enhance voltage sag ride-through ability and analy-
sis for a wind turbine application with a full-power grid-connected active recti-
er. The goal is to be able to use the existing equipment, without altering the
hardware. Further, design and analysis of the stabilization of DC-link voltage
oscillations for DC systems and inverter drives is studied, for example traction
drives with voltage sags in focus.
The proposed IM controller is based on the eld-weakening controller of Kim
and Sul [31], which is further developed. Applying the proposed controller to
voltage sag ride-through gives a cheap and simple ride-through system.
The EESM controller is based on setpoint adjustment for the eld current
controller. The analysis also concerns stability for the proposed ux controller.
The DC-link stabilization algorithm is designed following Mosskull [38],
where a component is added to the current controller. The algorithm is fur-
ther developed.
Analysis is the main focus, and concerns the impact of the dierent parame-
ters involved. Proper parameter selection for the controller, switching frequency,
and DC-link capacitor is given.
The impact of voltage sags is investigated for a power-grid-connected recti-
er. Here, we analyze the impact of dierent types of voltage sags and phase-
angle jumps. The analysis gives design rules for the DC-link capacitor and the
switching frequency.
Experimental results and simulations verify the theoretical results.
Acknowledgments
I would like to express my gratitude to my research supervisors Profs. Lennart
Harnefors and Hans Peter Nee as well as Dr. Rolf Ottersten for proof reading
the thesis and helping me with technical diculties.
Further, I would thank the members of my steering committee, Prof. Lars
Gertmar, Sven Ivner, Dr. Magnus Olofsson, and Prof. Lennart S¨oder, who have
helped me with their industrial experience and expertise at several interesting
meetings.
For contributing with many stimulating discussions giving progress I thank
my fellow PhD student Magnus Jansson.
I would also like to thank Mats Leksell at KTH, division EME, for helping
me with the experimental setup. Without his guidance I would have had large
problems with experimental results. With the experimental results at Chalmers
for Publications V and VI, I would like to thank Dr. Andreas Petersson for his
valuable help.
Last I would like to thank Holger Persson and my family for giving me
motivation and help with the closure of this work.
This work was supported nancially by Elforsk/Elektra and my parents.
A sunny day in August 2005
Kai Pietil¨ainen
Contents
1 Introduction
1
1.1 Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
1.2.1
Publications not Included in this Thesis . . . . . . . . . .
3
1.3 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2 Electric Power Quality
7
2.1 Voltage Sags . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.2 Impact of Voltage Sags . . . . . . . . . . . . . . . . . . . . . . . .
8
2.3 Voltage Sag Ride-Through Methods . . . . . . . . . . . . . . . .
11
2.3.1
Voltage Sag Ride-Through for IMs . . . . . . . . . . . . .
11
2.3.2
Voltage Sag Ride-Through for Line-Connected EESMs . .
11
2.3.3
Stabilization of the DC-Link Voltage and Voltage Sag
Ride-Through for Inverter Dives . . . . . . . . . . . . . .
12
2.3.4
Voltage Sag Response for Active Rectiers in Wind Tur-
bine Applications . . . . . . . . . . . . . . . . . . . . . . .
12
3 Electrical Models & Vector Control
15
3.1 Space Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3.2 Electrical Models . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
3.2.1
Induction Machine . . . . . . . . . . . . . . . . . . . . . .
16
3.2.2
Permanent Magnet Synchronous Motor (PMSM)
Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
3.2.3
Grid Model . . . . . . . . . . . . . . . . . . . . . . . . . .
19
3.2.4
A Generalized PMSM, IM, and Grid Model . . . . . . . .
20
3.2.5
EESM Model (With Damper Windings) . . . . . . . . . .
21
3.3 Review of Vector Control . . . . . . . . . . . . . . . . . . . . . .
23
3.3.1
Voltage Model . . . . . . . . . . . . . . . . . . . . . . . .
24
3.3.2
Statically Compensated Voltage Model . . . . . . . . . . .
26
3.3.3
Current Model . . . . . . . . . . . . . . . . . . . . . . . .
27
4 Current, Voltage, and Speed Control
29
4.1 Internal Model Control (IMC) . . . . . . . . . . . . . . . . . . . .
29
4.1.1
Active Damping . . . . . . . . . . . . . . . . . . . . . . .
31
4.1.2
Back Calculation/Realizable Reference . . . . . . . . . . .
32
4.2 Current Control . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
4.2.1
Voltage Limitation and Back Calculation . . . . . . . . .
34
4.3 Speed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
i ii
CONTENTS
4.3.1
DC-Link Voltage Controller . . . . . . . . . . . . . . . . .
38
5 Conclusions
41
5.1 Further Research . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
A Nomenclature
43 Chapter 1
Introduction
This thesis deals with voltage sag ride-through. First, we will cover variable-
speed drives (VSDs), which consist of a converter, a pulse-width modulator
(PWM), and an electrical machine. For VSDs we will concentrate on induction
machines (IMs). In the industry today, the IM is the machine most commonly
used, since it is robust, cheap, and needs no maintenance. We will investigate
the stability aspects and parameter selection for induction motor drives fed by
a diode rectier or a DC grid.
Second we will for voltage sag ride-through for synchronous machines in-
vestigate the possibility of ride-through through ux control for the electrically
excited synchronous machine with damper windings (EESM).
Third we will for wind-turbine applications cover the behavior of the con-
trolled rectier and voltage sag ride-through. Design rules for the controllers
and selection rules for the switching frequency and DC-link capacitor will be
given.
1.1
Outline of the Thesis
The thesis consists of the following chapters:
Chapter 1 introduces the research in the thesis, gives a background, and
summarizes the publications and contributions.
Chapter 2 describes power quality problem in the industry. We con-
centrate on voltage sags, as they are the main cause for trouble in the
industry. We introduce the concept of voltage sag ride-through by intelli-
gent control for variable-speed drives and electrically excited synchronous
machines.
Chapter 3 is an introduction to vector control for AC machines. Here we
describe models for the induction machine and the synchronous machine
with damper windings, as well as three-phase grids.
Chapter 4 explains the design of current, voltage, and speed controllers
by internal model control (IMC) design. We explain the concepts of active
damping and back calculation.
Chapter 5 gives conclusions and suggestions for future work.
1 2
CHAPTER 1. INTRODUCTION
1.2
Publications
The thesis is based on the following publications. A short introduction to each
publication is made in the list below.
Publication I: Unied Sensorless Vector Control of Synchronous and
Induction Motors, L. Harnefors, K. Pietil¨ainen, and M. Jansson, IEEE
Transactions on Industrial Electronics, Vol. 50, No. 1, Feb. 2003, pp. 153
160.
In this publication we develop a unied method for sensorless vector
control for induction and synchronous machines. A unied model for
both synchronous and induction machines is developed, which serves
as base for the ux estimator design. This publication also serves as
an introduction to the concept of eld orientation, which is built on
in the following publications.
Publication II: Torque-Maximizing Field-Weakening Control: Design,
Analysis, and Parameter Selection, L. Harnefors, K. Pietil¨ainen, and L.
Gertmar, IEEE Transactions on Industrial Electronics, Vol. 48, No. 1, Feb.
2001, pp. 161168.
In this publication we design an algorithm for maximum torque in
the eld-weakening region. We analyze the algorithm and give sim-
ple design and parameter selection rules. The theory is veried by
simulations and experiments.
Publication III: Voltage Sag Ride-Through of Induction Motor Drives,
K. Pietil¨ainen, M. Jansson and L. Harnefors, NORPIE, Stockholm, Au-
gust, 2002