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NORPIE 2004 Trondheim, Norway 1

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

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 thesquirrel-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 lossesand 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 basicrequirements from the power company:

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.

Fig.1 – System overview.

The first, second and fourth part of the 4. requirements mustbe 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.

Balduino Rabelo1)

, Wilfried Hofmann,Member, IEEE 2)

, Martin Tilscher3)

, Andreas Basteck 4)

Voltage Regulator for Reactive Power Control

on Synchronous Generators in Wind EnergyPower Plants

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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 co-

ordinates 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.

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.

II. MACHINE MODEL AND CONTROLLER

A. Machine Modelling

A classical model of a separately excited synchronous

machine with damping circuit in a synchronous rotating dq-

coordinate was utilised [3,4]. The basic stator equations in time

domain are shown below.

++=

−+=

sd r sq sq s sq

sqr sd sd s sd

dt

d i Ru

dt

d

i Ru

ψ ωψ

ψ ωψ (1)

whereψ are the fluxes andr

ω is the angular speed of the

generator shaft.

Fig.3 – Block diagram of the synchronous machine model.

The differential equation of the field circuit is given by the

following expression

f f f f dt d i Ru ψ += . (2)

In order to consider the effects of the damping circuit the

following flux equations in frequency domain are introduced.

=

+=

) s( i ) s( G ) s(

) s( u ) s( G ) s( i ) s( G ) s(

sq Lq sq

f f sd L sd d

ψ

ψ (3)

where the equations of the transfer functions G f (s) , G Ld (s) ,

and G Lq (s) , as functions of the generator inductances and

time constants, are derived in the literature [1,2] and shown byexpressions (4), (5) and (6), respectively.

( )( ) 1

1

0000

2

2

+++

+++=

''

d

'

d

''

d

'

d

''

d

'

d

''

d

'

d

sd Ld T T sT T s

T T sT T s L ) s( G (4)

''

q

''

q

sq Lq sT

sT L ) s( G

01

1

+

+= (5)

'

d

'

d

f

'

sd sd

f sT

T

L

L L ) s( G

0

0

1+

−=

(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).

( ) sd sq sq sd pe ii Z T ψ ψ −=2

3(7)

where Zp is the number of pole pairs. Figure 3 shows the block

diagram of the machine model.

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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.

+=

−=

sd r sq sq

sqr sd sd

dt

d e

dt

d e

ψ ωψ

ψ ωψ (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.

=

=

⇒

=⇒=

=⇒==

f r sq

sd

sq sq

sd sd f

e

e.cte

ψ ωψ ψ

ψ ψ ψ 0

00

0

&

&(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 slip-

rings. 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

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.

Fig.4 – Reactive power and synchronisation controllers.

This controller delivers the reference value of the excitation

voltage of the field circuit U f by controlling the induced

voltage on the stator circuit U Pol . The reference value for this

latter is the mains voltage u n before synchronisation. After the

machine is connected, the power factor controller is activated

and generates a ∆u n 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 momentof 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 closed-

control 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 referencevalue from the power factor controller, as seen before in fig. 4.

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The frequency response of the open and closed-loop control

using both regulators is shown in figure 6.

-100-100

-50-5 0

00

5 050

100100

150150

M a g n i t u d e

( d B )

1010-2-2

101 000

1 01022

-180-180

-135-135

-90-9 0

-45-4 5

00

P h a s e

( d e g )

BO openBO open

BO closedBO closedSO openSO open

SO closedSO closed

Bode D iagram

Frequency (Hz)

Fig.6 – Bode diagram of the open and closed loop induced

voltage control.

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 factorwas 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.

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

1 . 21 . 2 1 . 251 . 2 5 1 . 31 . 3 1 . 351 . 35 1 . 41 . 4 1 . 451 . 4 5-0 .5-0 .5

-0 .4-0 .4

-0 .3-0 .3

-0 .2-0 .2

-0 .1-0 .1

00

0 . 10. 1

0 . 20. 2

0 . 30. 3

0 . 40. 4

0 . 50. 5

I I a a b b c c

/ I / I n n

t ( s ) t ( s )

Fig.7 – Start-up currents during synchronisation.

55 5 . 55 . 5 66 6 . 56 . 5 77 7 . 57 . 5 88

-4 .5-4 .5

-4-4

-3 .5-3 .5

-3-3

-2 .5-2 .5

-2-2

-1 .5-1 .5

-1-1

-0 .5-0 .5

00

x 10x 1044

t ( s ) t ( s )

m m e e

( N m ) ( N m )

m m a a

( N m ) ( N m )

Fig.8 – Input torque step

55 5 . 55 . 5 66 6 . 56 . 5 77 7 . 57 . 5 88- 3- 3

- 2- 2

- 1- 1

00

11

22

33

I I a a b b c c

/ I / I n n

t ( s ) t ( s )

Fig.9 –Phase cu rrents ove rshoot during an input power step.

.

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2 626 2727 282 8 292 9 3 030 3131 323 2 333 3-2 .1-2 .1

-2-2

-1 .9-1 .9

-1 .8-1 .8

-1 .7-1 .7

-1 .6-1 .6

-1 .5-1 .5

-1 .4-1 .4

-1 .3-1 .3

-1 .2-1 .2x 1 0x 1 0

44

t ( s ) t ( s )

m m e e

( N m ) ( N m )

m m a a

( N m ) ( N m )

Fig.10 – Negative input torque step.

2626 272 7 2 828 2929 3 030 313 1 3 232 333 30.150.15

0.20.2

0.250.25

0.30.3

0.350.35

0.40.4

t(s)

Sinus Phi Sinus Phi

Cos Phi 0.98 ind Cos Phi 0.98 ind


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.

3 333 33.533.5 3 434 34.534.5 3 535-0.4-0.4

-0.3-0.3

-0.2-0.2

-0.1-0.1

00

0.10.1

0.20.2

0.30.3

0.40.4

t(s)

Sinus Phi Sinus Phi

Cos Phi 0.92 kap Cos Phi 0.92 kap


Fig.12 – Reactive power step.

3333 33.533.5 3434 34.534.5 353 5-3-3

-2.8-2.8

-2.6-2.6

-2.4-2.4

-2.2-2.2

-2-2

-1.8-1.8

-1.6-1.6

-1.4-1.4

-1.2-1.2

-1-1

-0.8-0.8x 1 0x 1 0

44

t(s) t(s)

m m e e

(Nm) (Nm)

m m a a

(Nm) (Nm)

Fig.13 –Cross-coupling wi th the active power.

Figures 12 and 13 show the effect of a reactive power stepon 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 voltageon 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.

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ACKNOWLEDGMENT

The authors would like to thank the Voith company, the

Chemnitz University of Technology and the company AEM forthe 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 Wechsltromm-

aschinen”, 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: [email protected]

chemnitz.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:

[email protected]).

3) Dr. Martin Tilscher is with the Voith Turbo Company,

Controlled Drives Department, 74564 Crailsheim, Germany (e-

mail: [email protected]).

4) Dr.Andreas Basteck is with the Voith Turbo Company,

Controlled Drives Department, 74564 Crailsheim, Germany (e-

mail: [email protected])

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