Kein Folientitel

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NORPIE04 - Nordic Workshop on Power and
Industrial Electronics 14-16 June, 2004
Trondheim, Norway
Voltage Regulator for Reactive Power Control on
Synchronous Generators in Wind Energy Power
Plants Balduino Rabelo Wilfried Hofmann
Andreas Basteck Martin Tilscher
VOITH TURBO CONTROLABLE DRIVES
FACULTY OF ELECTRICAL ENGINEERING DEPARTMENT OF
ELECTRICAL MACHINES AND DRIVES
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Topics
1. Introduction 2. E-On Guidelines 3. Machine
Modelling 4. Controller Design 5. Simulation
Results 6. Conclusion
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Motivation

• In a project with the Voith Co. the AEM Co. and
  the TU Dresden preliminary
• studies of a windmill using a hydrodynamic torque
  converter (VORECON)
• were carried out.
• The TU Chemnitz accomplished the following tasks
• Modelling and simulation of a synchronous
  generator
• Emulation of the E-On cases for power plant
  connection to the electrical grid
• 1. Synchronisation and connection with the mains
  supply
• 2. Reactive power exchange with the net
• 3. Load drop
• 4. Active power limiting with frequency variation
• 5. Short-circuit behaviour

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Guidelines for net connection from E-On
Ergänzende Netzanschlussregein für
Windenergieanlagen, 1.8.2003, E-on Netz Ltd.
Why?

• Further expansion of wind energy. 2010 - 20,000
  MW 2020 40,000 MW

• Wind generators will have to support the grid in
  case of faults

• European interconnected system can bare a maximum
  drop of 3,000MW

Who?

• Wind farms with more than 100 MW connected to the
  high voltage and extra-high voltage grids.

How?
1 - Support the grid in case of 15 to 60
voltage drops for not more than 3s
2 - Reduce the active power and frequency
fluctuation. 10 Pc/min
3 - Limit the cut-in in 1.2 of the rated power Pc
at the connection point
4 - Control the reactive power in a desired range
Verification of compliance with the new norms

• Guideline from manufacturers and measuring
  institutes

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Active Power Limiting Curve
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Reactive Power Control Range
t lt 30min
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Voltage Drop Profile
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Hydrodynamic Torque Converter VORECON
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Dynamical Model of the VORECON
Torque
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Basic Equations
The voltage equation of a synchronous
machine where the underline defines a complex
vector
Description in a rotating reference frame
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Torque and Flux Linkage Equations
The electromechanical torque expression
The dynamical interactions between the stator,
damping and field circuits are given the
following operators
The fluxes can be obtained by the expressions
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Block Diagram of the Synchronous Generator
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Control Variables

• The system has 3 control variables
• pitch angle and the guide vane position control
  the main power flow
• field voltage controls the magnetising of the
  synchronous generator and the reactive power
  flow.
• This latter can also influence the stability of
  the system.

Wind Turbine
Guide Vane Position H
Pitch Angle b
Synchronous Generator
Vorecon
Field Voltage uf
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Induced Voltage Controller
The induced voltage controller UPol uses the
field voltage uf to regulate the flux In
high-powered generators these self-excited
control schemes present a faster inner field
current control not shown here Considering the
speed constant the induced voltage dynamics
depends only on the flux
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Controller Tasks and Structure

• The field controller has to accomplish the
  following tasks
• regulate the induced voltage at the machine
  terminals during synchronisation
• control the power factor or the reactive power
  flow during normal operation
• guarantee the dynamical stability during
  undesired transient conditions
• An outer power factor control loop influences the
  induced voltage in normal operating conditions,
  as well as compensates the voltage drop over the
  stator windings during loading

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Controller Optimisation
The induced voltage controller posses 2 basic PI
structures for synchronisation and for normal
operation that were optimised by module (BO) and
by symmetrical optimum (SO) rules,
respectively.
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Synchronisation Starting Currents
Connecting the machine to the mains with an angle
error from less than 10 degrees gives transient
currents which peak values lie under 40 of
rated value vanishing in less than one second, as
shown in the left figure. With a neglectiable
angle error the start-up currents are much
smaller, as can be observed in the right figure.
stator phase currents
stator phase currents
t(s)
t(s)
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Torque Step from No-load to Rated Value
An extreme power step is simulated where the
input torque is increased from no-load condition
to rated torque. The generator is kept in
synchronism and the currents reach the rated
value after the transient period.
stator phase currents
generator torque
t(s)
t(s)
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Coupling with the Reactive Power Control
After a negative mechanical torque step
over-shoots on the electromechanical torque occur
during the transient period. The well-damped
power factor controller reduced the over-shoots
due to the coupling with the active power canal
and let the actual value reach the reference
smoothly after some seconds.
power factor
generator torque
t(s)
t(s)
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Power Factor Control
The power factor controller presents good
responses over the desired range. Higher damping
is observed on the capacitive range, as expected.
power factor
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Power Factor Step
The power factor step presents a retarded
response due to the coupling with the active
power channel. The increase on the torque
caused by the reference power factor step is less
damped than the reaction caused again on the
power factor. Such extremes reactive power steps
must be avoided in the normal operation of the
generator in order to avoid the observed torque
steps.
generator torque
power factor
t(s)
t(s)
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Voltage Drop
net voltages
The 3-phase voltage drop profile from E-ON have a
similar effect of a 3-phase short-circuit on the
machine. After the well-known transient periods
further oscillations appear on the torque and on
the currents due to the slow increase of the
mains voltage.
t(s)
generator torque
stator phase currents
t(s)
t(s)
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Power Angle and Speed
The oscillations observed previously on the
torque and on the currents are also observed on
the speed. The power angle crosses the stability
limit for a short period of time. The machine is
kept in synchronism due to the reached dynamical
stability enabled by the power factor
controller. However, the required field voltages
to magnetise the machine in such cases are higher
than the maximum allowed.
pole pitch angle
rotor speed
d break down
d ()
n (rpm)
d Rated
t(s)
t(s)
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Load Drop
A 100 load drop was simulated in order to
verify the possible overvoltage effects on the
machine terminals. The induced voltage increases
after at the load drop moment but the field
controller actuates faster and limits the
increase ratio letting the intern voltage in
acceptable levels until switching off or
reloading.
stator phase currents
induced voltage
t(s)
t(s)
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Summary and Future Works
A synchronous machine classical model was used to
simulate different situations before and after
synchronisation with the electrical grid. A
voltage regulator for the field excitation of the
synchronous generator was designed. 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.