NORPIE 2004 Trondheim, Norway
1
Voltage Regulator for Reactive Power Control
on Synchronous Generators in Wind Energy
Power Plants
Balduino Rabelo 1) , Wilfried Hofmann, Member, IEEE
Abstract—This paper presents a voltage regulator for the field
circuit of a separately excited synchronous generator in wind
power plants. A hydrodynamic variable speed planetary gear is
used to maintain constant speed on the generator shaft. The
generator is directly connected to the net without power
electronics . This new concept promises better power quality and
do not need bulky output filters. In order to fulfil new
requirements on wind generation units the field voltage
controller must present some features like reactive power and
dynamical stability control. The used model and the design
procedures, as well as some simulation results are shown
2)
, Martin Tilscher 3) , Andreas Basteck
4)
1. Support net voltage during fault conditions;
2. Control reactive power in a desired range;
3. Limit maximum generated power;
4. Limit start -up current transients.
Index Terms—Renewable energy systems, control of electrical
drives, powerquality.
I. INTRODUCTION
Recent studies have shown that the most used electrical
generators in wind power plants are the doubly-fed induction
generator, the gear-less synchronous generator and the
squirrel-cage induction generator, respectively [1]. Although
separately excited synchronous generators with small number
of poles are not so popular in this field, a new horizon is
opened for them when the shaft speed is maintained constant
mechanically. This goal is achieved with a hydrodynamic
controlled variable speed gear (Vorecon) from the Voith
Company. The Vorecon keeps the output shaft speed to the
generator constant while the input shaft speed from the turbine
is variable. This enables the direct connection of the
synchronous generator stator circuit to the mains supply
without needing a power electronics converter to correct
voltage and frequency. This improves the power quality and
does not require the bulky output LC-filter, reducing losses
and weight. Further advantage of this topology is the high
efficiency of the system, since the Vorecon presents losses
compared to a controlled electrical machine over the whole
speed range. The system overview with the planetary gear and
the hydraulic torque converter connecting the turbine and
generator is shown in figure 1.
In order to fulfil the new guidelines from the power
companies throughout Europe for connection of wind energy
conversion systems to the high-voltage net, depicted in [2],
an induced voltage controller is designed and presented in
this study, as well as some simulation results. For this new
generation of wind power stations there are four basic
requirements from the power company:
Fig.1 – System overview.
The first, second and fourth part of the 4. requirements must
be fulfilled through the excitation controller. The first item is
described in the norms as the voltage drop profile of figure 2
on the high- voltage net connection point (NCP). The
generator must not be turned off if the voltage level is above
it. This point has been discussed by wind mill operators and
machine constructors. For a single generator support the
voltage loss depicted in figure 2 the value of the required
internal induced voltage would make the generator production
costs impracticable. Furthermore the field circuit and exciter
had to be over-dimensioned increasing the machine size.
Fig.2 – Voltage Drop Profile.
NORPIE 2004 Trondheim, Norway
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Taking into account the short -circuit impedances of the
transmission and distribution lines and of the transformer
between the machine terminals and the faulty point the
voltage level would increase by the voltage drop over the
equivalent impedance. Normally more generating units are
connected to the same NCP, so that supporting the voltage or
feeding the fault would be shared through these plants. To
accomplish this task the excitation regulators must be
interconnected by a master monitoring system that coordinates the control actions of the sole generators.
Controlling the reactive power and synchronising with the
mains supply are normally built-in functions of the field exciter
for commercial synchronous generators over a few hundred
kilowatts. For wind generation applications one must have in
mind that the prime mover is depending on the stochastic
wind speed variations. The pitch control is too slow to avoid
completely the torque disturbances caused by wind gusts.
The guide vane actuates faster in a limited operating range.
The combination of these two controllers enables a good
operation of the wind station in an appropriate range.
Nevertheless, the field controller must guarantee dynamical
stability of the system under extreme conditions.
Fig.3 – Block diagram of the synchronous machine model.
The differential equation of the field circuit is given by the
following expression
u f = Rf if +
d
ψ .
dt f
(2)
In order to consider the effects of the damping circuit the
following flux equations in frequency domain are introduced.
ψ sd ( s ) = GL ( s )isd ( s ) + G f ( s )u f ( s )

ψ sq ( s ) = GLq ( s )isq( s )

d
A 2.5 MW separately excited synchronous generator
connected to the wind turbine by a variable speed planetary
gear was modelled and the voltage regulator was designed.
These were then simulated under some normal condition
operations, like synchronisation, input power step and
reactive power step. Detailed explanation about the modelling
and design of the machine and controller are given on the
further topics.
where the equations of the transfer functions Gf (s) , GLd (s) ,
and G Lq (s) , as functions of the generator inductances and
time constants, are derived in the literature [1,2] and shown by
expressions (4), (5) and (6), respectively.
GLd ( s ) = Lsd
II. M ACHINE MODEL AND CONTROLLER
A. Machine Modelling
A classical model of a separately excited synchronous
machine with damping circuit in a synchronous rotating dqcoordinate was utilised [3,4]. The basic stator equations in time
domain are shown below.
d

u sd = R sisd + dt ψ sd − ω rψ sq

usq = R sisq + d ψ sq + ω rψ sd

dt
generator shaft.
GLq ( s ) = Lsq
Gf ( s ) =
(1)
where ψ are the fluxes and ωr is the angular speed of the
(3)
s 2Td' Td'' + s (Td' + Td'' ) + 1
s 2Td' 0Td''0 + s (Td' 0 + Td''0 ) + 1
1 + sTq''
1 + sTq''0
(4)
(5)
Lsd − L' sd Td' 0
L f 1+ sTd' 0
(6)
These expressions model all the interactions between the
different circuits on a synchronous machine.
The electromagnetic torque expression as a function of the
stator currents and fluxes is done by (7).
Te =
(
3
Z p ψ sd isq − ψ sqi sd
2
)
(7)
where Zp is the number of pole pairs. Figure 3 shows the block
diagram of the machine model.
NORPIE 2004 Trondheim, Norway
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Further modifications have to be carried out in the model
structure from figure 2 in order to represent a non-connected
machine before synchronisation. This situation is described by
the equations (8) and (9) where the machine with no currents
and induced voltages at the terminals is modelled.
e = d ψ − ωψ
r sq
 sd dt sd

d
esq = ψsq + ωrψsd

dt
applied to the main generator field circuit. This is the preferred
type for high-powered machines.
The proposed controller structure can be applied to any
generator type and do not present an inner current controller.
This can be further implemented and improves the dynamical
response of the system. The structure is depicted in figure 4.
(8)
e is the induced voltage. If the field flux is constant and
different from zero and regarding equations (3) and (8) one can
derive the following expression.
ψf = ψsd = cte. ⇒ ψ& sd = 0  esd = 0
⇒
 ψ = 0 ⇒ ψ& = 0
sq
sq

esq = ωrψf
Fig.4 – Reactive power and synchronisation controllers.
(9)
which depicts the real situation before synchronisation. The
voltage at the machine terminals equals the internal induced
voltage on the quadrature axis.
Both non-connected and connected model were built in the
same simulation block and interchanged at the synchronisation
moment.
B. Controller Design
In order to accomplish the electrical requirements, like
reactive power control for example, a control algorithm for
regulating the field voltage and so the induced voltage on the
stator windings was developed and simulated. This controller
has to carry out the following tasks: regulate the induced
voltage at the machine terminals during synchronisation while
the prime mover controls the speed and position, control the
power factor or the reactive power flow during normal
operation and guarantee the dynamical stability of the machine
during undesired transient conditions, like voltage drops and
input or output power steps.
Here some comments must be done about the field excitation
circuit type. The basic configurations use slip-rings or are
brushless types. The first one can be self-excited, where the
terminal voltage supplies the field controller on the machine
stator side. After rectification and signal processing the output
excitation voltage is passed to the field circuit through the sliprings. Another variant posses a rotating rectifier bridge in the
rotor feeding the field circuit directly after signal processing on
the stator side.
The brushless types carry out the control tasks on the
stator side and the output current feeds the field circuit of an
auxiliary machine mounted on the same shaft of the main
generator. This exciter induces voltages in the rotating
armature coils that are rectified by a diode bridge converter
also mounted on the rotor side. The rectified voltage is then
This controller delivers the reference value of the excitation
voltage of the field circuit Uf by controlling the induced
voltage on the stator circuit UPol. The reference value for this
latter is the mains voltage un before synchronisation. After the
machine is connected, the power factor controller is activated
and generates a ∆un value that is subtracted (or added) to the
original reference supply voltage. This enables the generator
to work under- or over-excited, controlling the reactive power
flow.
Under faulty conditions the controller is blocked and the
excitation voltage chosen is proportional to the voltage drop
and to the active and reactive powers flowing on the moment
of the fault. A stability controller working in parallel with the
original structure was implemented later and improved the
performance of the machine during non-synchronous
operation. Further computations of the static and dynamic
stability under faulty conditions will give precise information
for an optimal control.
The induced voltage regulator was designed using linear
control techniques and posses 2 different controllers for before
and after synchronisation. The plant structure on the closedcontrol loop can be seen on figure 5.
Fig.5 – Induced voltage closed loop control.
The 2 regulators are projected based on optimal module (BO)
and symmetrical optimum (SO) design techniques. The first
one has the net voltage as reference value before
synchronisation. The second regulator is put in operation after
synchronisation and receives some variation of these reference
value from the power factor controller, as seen before in fig. 4.
NORPIE 2004 Trondheim, Norway
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The frequency response of the open and closed-loop control
using both regulators is shown in figure 6.
00..55
00..44
II
aa bb cc
00..33
Bode Diagram
//II
nn
150
150
Magnitude (dB)
00..22
BO
BO open
open
B
BO
O cclloosseedd
SO
SO open
open
S
SO
O cclloosseedd
100
100
5500
00..11
00
00
--00..11
--5500
--00..22
-100
-100
00
--00..33
--00..44
Phase (deg)
--4455
--00..55
11..22
--9900
11..2255
11..33
11..3355
tt (( ss ))
11..44
11..4455
Fig.7 – Start-up currents during synchronisation.
-135
-135
-180
-180
1100
--22
00
1100
22
1100
Frequency (Hz)
44
xx 1100
00
Fig.6 – Bode diagram of the open and closed loop induced
voltage control.
--00..55
--11
The SO designed regulator was made faster in order to catch
up stepwise changes of active and reactive power without
losing synchronism. This is observed by the higher corner
frequencies on the dashed and dotted-dashed curves.
Furthermore, stability is guaranteed for both control loops, as
shown on the open-loop phase margin curves.
The power factor controller was calculate by the optimum
damping criteria (DO) after linearizing the plant over the
operation point. The power angle instead of the power factor
was used as controlled variable as it presents a more linear
characteristic over the desired range and provides the
information of under- or over-excited operation by its sign.
--11..55
m
m
--22
aa
(( N
Nm
m ))
--22..55
--33
--33..55
m
m
--44
ee
(( N
Nm
m ))
--44..55
55
55..55
66
66..55
tt (( ss ))
77
77..55
88
Fig.8 – Input torque step
33
III. SIMULATION RESULTS
Some simulation results of synchronisation, active and
reactive power steps are shown. Connecting the machine to
the mains with an angle error from less than 10 degrees gives
the transient currents observed in figure 7. The peak values lie
under 40 % of rated value and the transients vanish in less
than one second. Further low frequency oscillations of the
speed are also observed.
Figures 8 and 9 show an input torque step from 0 to rated
torque and the respective current overshoot due to the
increase on the power input.
Even with the extreme power step the generator is kept in
synchronism and the currents reach the rated value after the
transient period.
Figures 10 and 11 bring an active power step input and the
effect on the power factor control
II
22
//II
aa bb cc
nn
11
00
-- 11
-- 22
-- 33
55
55..55
66
66..55
77
77..55
88
tt (( ss ))
Fig.9 –Phase currents overshoot during an input power step.
.
NORPIE 2004 Trondheim, Norway
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44
--11..22
44
xx 1100
-0.8
-0.8
xx 1100
--11
--11..33
-1.2
-1.2
m
m
ee
((N
Nm
m))
--11..44
m
m
ee
(( N
Nm
m ))
-1.4
-1.4
--11..55
-1.6
-1.6
--11..66
-1.8
-1.8
m
m
--11..77
aa
(( N
Nm
m ))
m
m
aa
((N
Nm
m))
--22
-2.2
-2.2
--11..88
-2.4
-2.4
--11..99
-2.6
-2.6
--22
-2.8
-2.8
--22..11
2266
2277
2288
2299
3300
3311
3322
3333
tt (( ss ))
0.4
0.4
S
Siinnuuss P
Phhii
00..3355
C
Cooss P
Phhii 00..9922 iinndd
0.3
0.3
00..2255
0.2
0.2
C
Cooss P
Phhii 00..9988 iinndd
2277
2288
2299
3300
3311
3322
3333
t(s)
Fig.11 –Cross-coupling with the reactive power control.
Once again over-shoots on the electromechanical torque
occur during the transient period. These could be better
damped with a proper selection of the controller gains or with
an active damping procedure. Of course a compromise must be
met concerning the stability. The coupling with the reactive
power control can be observed on the power factor in figure
11. The well-damped controller reduced the over-shoots and let
the actual value reach the reference smoothly after some
seconds.
0.4
0.4
S
Siinnuuss P
Phhii
0.3
0.3
Cos
Cos Phi
Phi 0.92
0.92 ind
ind
0.2
0.2
0.1
0.1
00
-0.1
-0.1
-0.2
-0.2
-0.3
-0.3
-0.4
-0.4
3333
C
Cooss P
Phhii 00..9922 kkaapp
3333..55
3344
t(s)
3344..55
Fig.12 – Reactive power step.
3333..55
3344
tt((ss))
3344..55
3355
Fig.13 –Cross-coupling with the active power.
Fig.10 – Negative input torque step.
00..1155
2266
--33
3333
3355
Figures 12 and 13 show the effect of a reactive power step
on the active power or electromagnetic torque.
It is remarkable here that the coupling from the reactive
power canal with the active power. The increase on the torque
caused by the reference power factor step is less damped than
the reaction caused again on the powe r factor. This extreme
reactive power step must be avoided in the normal operation of
the generator. Faulty conditions reproducing the voltage
profile from figure 2 on the machine terminals were also
simulated but not shown here. In order to keep the machine in
stable operation without loss of synchronism the field circuit
must over-excite the machine and so experience higher
voltages and currents. With a non-saturated model the voltage
on the field circuit had to be increased until six times the rated
value, which would be impracticable. These results will point
out in a further work the stability margin that could be reached
by controlling the field voltage.
IV. CONCLUSIONS
The machine classical model has to be adapted to simulate
situations before synchronisation. A voltage regulator for the
field excitation of a synchronous machine is proposed. This
controller has to guarantee stable operation of the generator
under various conditions including faults.
Simulation results show the good performance of the
controller. With the already existing controller the machine is
kept stable during extreme conditions like torque steps and
reactive power variations. Faulty conditions were also
simulated.
Further studies will investigate the effects of faulty
conditions on the mechanical drive train caused by high
electromechanical torque and its harmonics and of the
distribution line and the transformer on the performance of the
machine under voltage drops.
NORPIE 2004 Trondheim, Norway
A CKNOWLEDGMENT
The authors would like to thank the Voith company, the
Chemnitz University of Technology and the company AEM for
the financial and technical support of this work.
REFERENCES
[1] Rabelo, B., and Hofmann, W. “Optimal Reactive Power Splitting
with the Doubly Fed Induction Generators for Wind-Turbine”,
Proceedings of DEWEK’2002, CD. Wilhemshaven, Germany,
October 2002.
[2] Rabelo, B., Hofmann, W. “Wind Generator Control in Compliance
with New Norms”, Proceedings of ISIE’2003, CD. Rio de Janeiro,
Brazil, July 2003. H. Poor, An Introduction to Signal Detection
and Estimation. New York: Springer-Verlag, 1985, ch. 4.
[3] Kovács, K.P., Rácz, I. “Transiente Vorgänge in Wechsltrommaschinen”, Verlag der Ung. Akademie der Wisss. Budapest,
Hungary, 1959.
[4] Müller, G., “Betriebsverhalten rotierender elektrischer Maschinen”,
VEB Verlag Technik, Berlin, 1990. J. Wang, “Fundamentals of
erbium-doped fiber amplifiers arrays (Periodical style—Submitted
for publication),” IEEE J. Quantum Electron., submitted for
publication.
1)
Dipl.-Ing. Balduino Rabelo is with the Chemnitz University of
Technology, Dept. of Electrical Machines and Drives, 09127
Chemnitz, Germany (corresponding author to provide phone:
+49371-531-3586; fax: +49371-531-3324; e-mail: barab@hrz.tuchemnitz.de).
2)
Prof. Dr.. Wilfried Hofmann is the chair of the Dept. of
Electrical Machines and Drives at the Chemnitz University of
Technology
phone:
+49371-531-3323;
(e-mail:
wilfried.hofmann@e-technik.tu-chemnitz.de).
3)
Dr. Martin Tilscher is with the Voith Turbo Company,
Controlled Drives Department, 74564 Crailsheim, Germany (email: martin.tilscher@voith.de).
4)
Dr.Andreas Basteck is with the Voith Turbo Company,
Controlled Drives Department, 74564 Crailsheim, Germany (e mail: andreas.basteck@voith.de)
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