Note: Descriptions are shown in the official language in which they were submitted.
CA 02875840 2014-12-19
PORTABLE ELECTRIC BRAKING SYSTEM FOR WIND TURBINES WITH
INDUCTION GENERATORS
Technical Field
[0001] The present disclosure relates to wind turbines.
Background
[0002] Most wind turbines have braking mechanisms that can be used to stop
the
rotor from spinning in the event of a malfunction, to permit maintenance or
repairs, or
other situations. Some wind turbines have secondary braking mechanisms to stop
or
slow the rotor if the primary braking mechanism fails. However, if there is no
secondary
braking mechanism, or if the secondary braking mechanism is not capable of
stopping a
malfunctioning turbine, the turbine could spin for an extended period of time
before the
rotor stops and the turbine may be safely accessed. For example, if a
turbine's primary
brakes fail, secondary braking mechanisms may be insufficient to stop the
turbine or
may cause excessive wear or damage to the turbine if engaged.
[0003] The inventors have identified a need for improved apparatus and
methods
for stopping wind turbine rotors.
Summary
[0004] The present disclosure provides systems and methods for electrically
braking an induction generator-based wind turbine. Some embodiments provide
portable apparatus that may be moved to the site of a wind turbine to stop the
turbine,
for example in the case of a malfunction in the turbine's primary brakes.
[0005] One aspect provides a process for stopping a wind turbine having an
induction generator with a generator rotor connected to be driven by the wind,
and a
generator stator having a plurality of generator leads for connecting the
generator to a
power grid. The process comprises connecting outputs of a portable voltage
sourced
converter to the generator leads, applying a probing waveform from the
portable voltage
sourced converter to the generator leads, the probing waveform comprising an
AC
electrical signal having a first amplitude and a probing frequency configured
to induce a
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magnetic field in the stator that rotates at a probing rotational velocity,
measuring
current and voltage on the generator leads to determine a back EMF from the
generator
in response to the probing waveform, adjusting the probing frequency until the
back
EMF is substantially zero to determine an initial generator rotor rotational
velocity,
applying a braking waveform having a second amplitude initially higher than
the first
amplitude and having a braking frequency configured to induce a magnetic field
in the
stator that rotates at a braking rotational velocity slightly lower than the
initial generator
rotor velocity, and, adjusting the braking waveform to ramp down the braking
rotational
velocity to a target rotational velocity.
[0006] Another aspect provides a portable electric braking system for a
wind
turbine having an induction generator with a generator rotor connected to be
driven by
the wind, and a generator stator having a plurality of generator leads for
connecting the
generator to a power grid. The system comprises a voltage sourced converter
having a
power unit with input connectable to a power source and an output connectable
to stator
leads of the wind turbine generator, a controller connected to control
operation of the
power unit to cause the power unit to apply a probing waveform from the
portable
voltage sourced converter to the generator leads, the probing waveform
comprising an
AC electrical signal having a first amplitude and a probing frequency
configured to
induce a magnetic field in the stator that rotates at a probing rotational
velocity,
measure current and voltage on the generator leads to determine a back EMF
from the
generator in response to the probing waveform, adjust the probing frequency
until the
back EMF is substantially zero to determine an initial generator rotor
rotational velocity,
apply a braking waveform having a second amplitude higher than the first
amplitude and
having a braking frequency configured to induce a magnetic field in the stator
that
rotates at a braking rotational velocity slightly lower than the initial
generator rotor
velocity, and, adjust the braking waveform to ramp down the braking rotational
velocity
to a target rotational velocity.
[0007] Another aspect provides an electric braking system for a wind
turbine
having an induction generator with a generator rotor connected to be driven by
the wind,
and a generator stator having a plurality of generator leads for connecting
the generator
to a power grid. The system comprises a voltage sourced converter having a
power
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unit with input connectable to the power grid and an output connected to
stator leads of
the wind turbine generator, a controller connected to control operation of the
power unit
to cause the power unit to apply a probing waveform from the portable voltage
sourced
converter to the generator leads, the probing waveform comprising an AC
electrical
signal having a first amplitude and a probing frequency configured to induce a
magnetic
field in the stator that rotates at a probing rotational velocity, measure
current and
voltage on the generator leads to determine a back EMF from the generator in
response
to the probing waveform, adjust the probing frequency until the back EMF is
substantially zero to determine an initial generator rotor rotational
velocity, apply a
braking waveform having a second amplitude higher than the first amplitude and
having
a braking frequency configured to induce a magnetic field in the stator that
rotates at a
braking rotational velocity slightly lower than the initial generator rotor
velocity, and,
adjust the braking waveform to ramp down the braking rotational velocity to a
target
rotational velocity.
[0008] Other aspects and features of the present disclosure will become
apparent
to those ordinarily skilled in the art upon review of the following
description of specific
embodiments in conjunction with the accompanying figures.
Drawings
[0009] The following figures set forth embodiments in which like reference
numerals denote like parts. Embodiments are illustrated by way of example and
not by
way of limitation in the accompanying figures.
[0010] FIG. 1 shows an example wind power installation wherein a portable
braking system is being used to stop a wind turbine.
[0011] FIG. 2 schematically illustrates circuit elements of an example
portable
electric braking system for stopping a wind turbine.
[0012] FIG. 3 is a flowchart showing steps of an example process for
stopping a
wind turbine.
[0013] FIG. 4A is a graph showing example rotational velocities over time
for a
generator rotor and an induced magnetic field in a generator stator of a wind
turbine
being braked by an electric braking system according to one embodiment.
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[0014] FIG. 4B is a graph showing an example voltage over time for a
generator
stator of a wind turbine being braked by an electric braking system according
to one
embodiment.
[0015] FIG. 5A is a graph showing example rotational velocities over time
for a
generator rotor and an induced magnetic field in a generator stator of a wind
turbine
being braked by an electric braking system according to one embodiment.
[0016] FIG. 5B is a graph showing example power over rotational velocity
of the
wind turbine of the FIG. 5A example.
[0017] FIG. 6A schematically illustrates an example portable electric
braking
system according to one embodiment.
[0018] FIG. 6B schematically illustrates an example portable electric
braking
system according to another embodiment.
Detailed Description
[0019] The following describes a portable electric braking system for a
wind
turbine. The example portable electric braking systems disclosed herein may be
utilized
in situations in which a primary and/or secondary braking system of the wind
turbine
have failed, are malfunctioning, or are otherwise unable to stop the turbine.
[0020] For simplicity and clarity of illustration, reference numerals may
be
repeated among the figures to indicate corresponding or analogous elements.
Numerous details are set forth to provide an understanding of the examples
described
herein. The examples may be practiced without these details. In other
instances, well-
known methods, procedures, and components are not described in detail to avoid
obscuring the examples described. The description is not to be considered as
limited to
the scope of the examples described herein.
[0021] FIG. 1 shows a wind turbine 10 coupled to an electric braking
system 100.
The wind turbine 10 includes a tower 12 that supports a nacelle 14, which
houses an
AC generator (not shown) comprising a generator rotor and a generator stator.
The
nacelle 14 supports a wind rotor 16 having a plurality of blades 18. Wind
blowing
across the blades 18 generates a rotational force or "torque" dependent on
wind velocity
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which spins the wind rotor 16. The wind rotor 16 is coupled to a generator
rotor, for
example by a drive train (not shown) such that rotation of the wind rotor 16
rotates the
generator rotor to convert the mechanical energy of the rotation of the wind
rotor 16 to
electrical energy, as is known in the art. The generator is connected to
provide
electrical power to an electrical grid (not shown) through a master control
cabinet 20.
[0022] The generators of some wind turbines 10 are asynchronous/induction
generators. In an asynchronous/induction generator, the generator stator has a
plurality
of coils configured such that when the stator leads are energized with AC
electric power
(e.g. from the grid) the stator produces an induced magnetic field that
rotates at a
"synchronous" speed. The synchronous speed depends on the number of poles of
the
generator and the frequency of the grid that the generator is connected to.
For
example, a 4 pole AC generator on a 50 Hz grid has a synchronous speed of 1500
rpm,
whereas the same generator on a 60 Hz grid has a synchronous speed of 1800
rpm. In
order to produce electrical power, the generator rotor of an asynchronous
generator is
driven to rotate at a rotational speed slightly greater than the synchronous
speed, which
causes current to flow from the generator stator. The difference between the
generator
rotor speed and the synchronous speed is known as the "slip," which is often
expressed
as a percentage of the synchronous speed. In some generators, power is
typically
generated with a slip of about 0.5 to 3% of the synchronous speed (e.g. about
1508-
1545 rpm for a synchronous speed of 1500 rpm). If the generator rotor speed
exceeds
a safe slip threshold, the turbine can exceed recommended operating
conditions, which
can lead to increased risk of mechanical or electrical failures. For example,
some
turbines have a maximum generator speed of 112% of the synchronous speed (e.g.
1680 rpm where the synchronous speed is 1500 rpm), and an overspeed shutdown
may
triggered if the generator speed exceeds the maximum, wherein the generator is
disconnected from the electrical grid and brakes are applied to slow or stop
the wind
rotor. A generator may also be shut down for other reasons, such as for
example to
perform maintenance or repairs, or if an operator presses an emergency stop
button.
During a shutdown, a main contactor connecting the generator to the grid opens
(thus
de-energizing the stator and removing the stator magnetic field) and a braking
mechanism is engaged (e.g. by clamping brake calipers on a braking disc, or
other
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suitable mechanism). However, if the brakes fail to stop the turbine, for
example due to
worn brake pads or another malfunction, the wind rotor can continue to rotate,
and may
continue to increase its speed if the wind is still blowing.
[0023] Some wind turbines 10 may include a rotor 16 having an automatic
mechanical blade pitching functionality that rotates the blades 18 to depower
the wind
turbine 10 when the rotational velocity of the wind rotor exceeds a threshold.
Automatic
mechanical blade pitching can advantageously depower the turbine 10 without
the need
for grid power or an active control system. Automatic mechanical blade
pitching may
be provided, for example by coupling each of the blades 18 to the wind rotor
16 (or in
some implementations, coupling a tip portion of the blades to a base portion
of the
blades) with a coupling comprising a helical track and providing spring
tension toward
the center of the wind rotor 16. When the rotational velocity of the wind
rotor 16
increases to the point that the centrifugal force on the blades 18 (or tips
thereof)
overcomes the spring tension (which may be referred to as the "blade-pitching"
threshold speed), each blade 18 (or tip thereof) moves outward from the wind
rotor 16
(or base of the blade) in the helical track causing the blade 18 (or tip
thereof) to rotate
about an axis along the length of the blade 18 such that the wind exerts less
torque on
the blade and the turbine 10 is depowered. Once the velocity of rotation of
the wind
rotor 16 is sufficiently reduced, the spring tension may pull the blades 18
(or tips
thereof) back to the default position. Alternatively, the blades 18 (or tips
thereof) may
lock in place once they have been pitched.
[0024] The electric braking system 100 is electrically coupled to the
generator of
the wind turbine 10, for example through an access panel or the like on the
master
control cabinet 20. The electric braking system 100 is preferably located at
some safe
distance from the base of the tower 12, as it can be dangerous to approach a
spinning
wind turbine. In installations where the master control cabinet 20 is located
at the base
of the tower 12, the system 100 can be located at a point further from the
tower 12 than
the master control cabinet 20 and an electrical connection to the generator
leads may
be established at such further point. As discussed further below, the
electrical braking
system 100 is typically connected between the generator stator leads and the
electrical
grid. In some embodiments the electrical braking system 100 may be operable
without
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a grid connection, and could be powered by an alternate power supply such as a
generator or the like, provided that the system 100 is capable of dissipating
the
electrical power generated by the braking, as discussed below.
[0025] The electric braking system 100 applies a braking waveform to the
leads
of the generator of the wind turbine 10. As described in more detail below,
the braking
waveform induces a rotating magnetic field that has a rotational velocity
slightly lower
(e.g., 0.5-1% lower) than an initial rotational velocity of the generator
rotor, to induce a
braking force on the generator rotor. The rotational velocity of the braking
waveform is
then ramped down (e.g., by following a control setpoint or braking profile
determined
based on characteristics of the generator) to slow the wind rotor 16 utilizing
the electric
torque generated by the generator.
[0026] The electric braking system 100 shown in FIG. 1 is a self-contained
portable unit that may be brought to the site of a malfunctioning wind turbine
10. The
portable electric braking system 100 may be connected to the generator without
an
operator having to climb the tower 12 or approach the immediate area of the
tower 12,
which may be unsafe when the wind rotor 16 is spinning. By being provided as a
portable unit, a single electric braking system 100 can be used to service
multiple
turbines and the additional expense of providing built-in electric braking
systems in each
turbine can be avoided.
[0027] As discussed in more detail below, the electric braking system 100
may be
controlled remotely through, for example, connectivity over a local area
network, the
internet, or a cellular network. Controlling the electric braking system 100
from a
remote location may be desirable when the rotor 16 is rotating at a
sufficiently high
velocity that approaching the wind turbine 10 would be dangerous due to, for
example,
the risk of mechanical failure of the rotor 16, or so that, for example,
personnel may
control the braking process from a remote location, without having to
physically be
onsite near the turbine 10.
[0028] FIG. 2 shows a schematic view of an electric braking system 100
connected to a generator 200 and to an electrical grid 300. As shown in FIG. 1
the
electric braking system 100 is connected between the generator 200 and the
electrical
grid 300.
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[0029] The electric braking system 100 includes a power module 102 for
providing the braking voltage waveform to the generator, a controller 104 for
controlling
the power module 102, an interface 106 for the controller 104, a transformer
108 for
powering the electric braking system 100 utilizing energy from the electrical
grid 300.
Alternatively, the electric braking system 100 may receive power from another
power
source (not shown) in the event the grid power is down. In the illustrated
example, the
electric braking system 100 also includes a network interface 110 for enabling
the
control of the electric braking system 100 remotely. The network interface 110
may be
configured to connect the electric braking system 100 to a local area network
(LAN), the
internet, or a cellular network, to facilitate controlling and monitoring the
electric braking
system 100 remotely.
[0030] The power module 102 utilized in the electric braking system 100
may
comprise, for example, a voltage sourced converter. An example of a voltage
sourced
converter that is suitable for use in an electric braking system 100 as
described herein
is the Sinamics G120 manufactured by Siemens . The capabilities of the voltage
sourced converter may be selected based on the characteristics of the
generator 200.
[0031] The electrical grid 300 shown in FIG. 2 is a three-phase electrical
grid
having three phase lines 302, 304, 306 and a ground line 308. The three phase
lines
302, 304, 306 of the electrical grid 300 are coupled to inputs 103 of the
power module
102. The phase lines 302, 304, 306 may be coupled to the power module 102
through
a circuit protection element 112 (e.g. a circuit breaker, or three fuses with
one fuse in
each line) which protects the power module 102 from over current conditions.
The
circuit protection element 112 may, for example, be configured to disconnect
the power
module 102 from the electrical grid 300 within a certain period of time from
when the
current through the circuit protection element 112 exceeds a predetermined
threshold,
depending on the time-current curve of the circuit protection element 112. In
some
embodiments, the predetermined threshold may be about 150A. The illustrated
example contemplates a three-phase grid, and a correspondingly configured
generator,
but it is to be understood that the system 100 could be adapted for use with
grids/generators with different numbers of phases/poles, different voltages,
different grid
frequencies, etc.
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[0032] Outputs 105 of the power module 102 are electrically connected to
the
leads of the generator 200 (e.g., through the master control cabinet 20,
possibly via a
more distant connection point) such that the electric braking system 100 is in
series
between the generator 200 and the electrical grid 300. In some embodiments,
the
electric braking system 100 may take the place of the grid 300, and may
provide an
alternate power source and/or current sink, as discussed further below with
respect to
FIGS 6A and 6B.
[0033] The controller 104 is coupled to the power module 102 and controls
the
power module 102 by setting different operational parameters and options of
the power
module 102. The controller 104 may, for example, comprise a PID controller.
The
interface 106 is utilized to facilitate input and output of the controller
104. The interface
106 may also be utilized to set and read operational parameters and
characteristics of
the power module 102, the controller 104 and the generator 200. For example,
the
interface 106 may comprise an input device for setting parameters of the power
module
102 and a display showing data associated with the electric braking system
100. In an
embodiment in which the power module 102 is a Sinamics G120 unit manufactured
by
Siemens, the interface may utilize Siemens Total Integration Automation (TIA)
software
for setting the parameters through the controller 104.
[0034] Although the power module 102, the controller 104, and the interface
106
are shown in FIG. 2 as separate elements, any or all of the features of the
power
module 102, the controller 104, and the interface 106 may be combined as a
single
element.
[0035] The transformer 108 may be coupled to one or more phase lines of the
electrical grid. In the FIG. 2 example, the transformer 108 is coupled to two
of the
phase lines 302, 304 of the electrical grid 300 and utilized for stepping down
the voltage
from the electrical grid 300 to a voltage that may be utilized by the power
module 102,
as well as other components of the electric braking system 100, in order to
power the
components with power from the electrical grid 300. For example, the
transformer may
step down the voltage of the electrical grid 300, and may include an AC/DC
power
adaptor to provide a suitable form of electrical power to the controller 104
and/or
interface 106 (e.g., in some embodiments, the controller 104 and interface 106
are
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provided with 24V DC power). The transformer 108 may be adaptable such that
the
electric braking system 100 can be adapted to utilize the voltage and
frequency of the
particular electrical grid 300 at the site of the wind turbine 10. An
adaptable transformer
108 enables the electric braking system 100 to be employed in various
different
countries, each having electrical grids operating at different voltages and
frequencies.
[0036] Power the electric braking system 100 utilizing the transformer 108
as
shown in FIG. 2 requires that the electrical grid 300 be live. Alternatively
or additionally
to the transformer 108, the electric braking system 100 may include a power
source (not
shown) such as for example a battery, other electrical power storage device,
or portable
generator, that may be utilized to power the components of the electric
braking system
100 when the electrical grid is not live and no other power is available for
the
transformer 108. In implementations wherein the grid is not live or the grid
is not used
to power the electric braking system 100, the inputs 103 would be connected to
the
alternative power supply rather than the grid lines. Example portable braking
systems
adapted to operate without a grid connection are discussed below with
reference to
FIGS 6A and 6B.
[0037] Referring now to FIG. 3, a flow chart of a method 400 of stopping a
wind
turbine 10 utilizing an electric braking system 100 is shown. Other than the
connecting
step at 402, the method shown in FIG. 3 may be carried out by, for example, a
processor of the controller 104, the interface 106 or remotely connected
through
interface 110.
[0038] At 402, the outputs 105 of the power module 102 are connected to
the
leads of the generator 200. The inputs 103 are connected to the grid or an
alternate
power source. Connecting the power module 102 to the generator 200 may include
positioning a portable electric braking system 100 in the vicinity of the
generator 200 at
a safe distance from the turbine. The controller 104, or other processor
connected
locally or remotely to control the power module 102, may be programmed with
parameters of the turbine, or may have access to a memory with parameters of
the
turbine stored therein. The parameters of the turbine may, for example,
include
characteristics of the generator, the wind rotor and the drive train, as
discussed further
below.
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[0039] At 404, a probing waveform is applied by the power module 102 to
the
leads of the generator 200. The probing waveform has a probing frequency that
is
selected to induce a magnetic field in the generator stator that rotates at a
probing
rotational velocity. The initial probing rotational velocity is selected to be
significantly
lower than the expected rotational velocity of the generator rotor. In some
embodiments, the initial probing rotational velocity is selected as 50% of the
generator's
synchronous speed.
[0040] The probing waveform has a relatively small amplitude compared to
the
waveforms utilized when electrically braking the wind rotor 16, as discussed
below. For
example, in some embodiments the probing waveform may have a voltage amplitude
of
less than 50% of the grid voltage. In some embodiments, the probing waveform
may
have a voltage amplitude of about 10-30% of the grid voltage. For example, in
some
embodiments the probing waveform may comprise an AC voltage with an initial
frequency of about 25Hz and a voltage amplitude of less than 50V, for example
in the
range of about 10-30V.
[0041] The initial probing waveform may be applied for a time period
sufficient for
several (e.g. 5-10) rotations of the induced magnetic field in the stator to
occur, then the
back electromotive force (EMF) is determined, for example by measuring the
voltage
and current from the generator. The back EMF may be determined periodically or
continuously. For example, in some embodiments, the probing waveform may be
initially applied for approximately 0.2 seconds, following which the back EMF
is
determined.
[0042] At 406, a determination is made whether the measured back EMF is
substantially zero. A back EMF of substantially zero indicates that the
probing rotational
velocity of the magnetic field induced by the probing frequency is
substantially equal to
the rotational velocity of the generator rotor. In an embodiment, the back EMF
may be
determined to be substantially zero when the measured back EMF is less than a
predetermined threshold, such as for example a threshold voltage amplitude.
[0043] If the back EMF is determined not to be substantially zero at 406,
the
method proceeds to 408 where the probing frequency is adjusted. The probing
frequency may be adjusted by stepping the probing frequency upward (or
downward) by
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a predetermined step size. After the probing frequency is adjusted, the
process returns
to 404 where the probing waveform is applied at the adjusted probing frequency
and a
back EMF is determined.
[0044] In an example, the initial probing frequency is set to induce a
rotating
magnetic field at 50% of the nominal rotational velocity of the wind turbine
10 which the
electric braking system 100 is connected to. Probing waveforms may adjusted by
a
predetermined increment such as, for example, 0.5Hz, and a "factor" parameter
may be
utilized to specify how long the probing waveform is applied at each
incrementally
adjusted frequency. In some embodiments, each frequency of the probing
waveform is
applied for at least 5-10 cycles in order to measure the back EMF for that
frequency.
[0045] In some embodiments, the probing frequency may be adjusted to
induce a
rotating magnetic field at up to a maximum velocity that is higher than the
maximum
estimated rotor velocity in the present conditions. When the maximum velocity
is
reached and the measured back EMF has not been substantially zero, then the
rotational rate of the generator rotor is determined to likely be lower than
the range of
probing frequencies utilized. In this case, the probing frequency may be reset
to nearer
zero and the probing waveform is applied and adjusted as described above.
[0046] When the back EMF is determined to be substantially zero at 406,
the
method proceeds to 410, where a generator rotor velocity is determined to be
the
probing velocity of the probing waveform at which the measured back EMF was
measured as substantially zero.
[0047] At 412, a braking waveform having a braking rotational velocity
slightly
lower than the determined generator rotor velocity is applied by the power
module 102
to the generator 200. For example, in some embodiments the initial braking
rotational
velocity may have a magnitude of about 0.5-1% lower than the magnitude of the
determined generator rotor velocity. The braking waveform exerts a negative
torque
(i.e. a torque that opposes the normal spinning direction of the wind rotor
16) on the
generator rotor, which acts to slow down the wind rotor 16 through the
drivetrain. The
braking waveform has a voltage amplitude higher than the voltage amplitude of
the
probing waveform. The braking waveform may, for example, have a voltage
amplitude
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similar to the grid voltage, but which is higher at braking frequencies
greater than the
grid frequency and lower at braking frequencies less than the grid frequency.
[0048] As discussed below, the braking waveform is adjusted to lower the
frequency thereof at 414 to slow the rotor. Adjustment of the braking waveform
depends on a number of factors, including how quickly rotation of the rotor 16
is to be
reduced, the power limits of the power module 102 utilized in the electric
braking system
100, and mechanical characteristics of the turbine (e.g. the drivetrain, the
wind rotor,
etc.). The power module 102 may be sized to have power limits suitable for the
expected braking power generated when stopping the type of turbine that the
power
module 102 is connected to. For example, faster braking of the rotor 16
requires more
power to be dissipated through the power module 102 (e.g. to the grid) and
transferring
more power than the power rating of the power module 102 could damage the
power
module 102. In some embodiments, the power module 102 is controlled to
discontinue
application of the braking waveform in the event a power dissipation threshold
is
exceeded. In some embodiments, the rate of adjustment of the braking waveform
may
be controlled to avoid extended periods of high braking power output.
[0049] At 414, the frequency of the braking waveform (and thus the
rotational
velocity of the braking magnetic field) is ramped down in order to slow the
rotation of the
rotor. In some embodiments, the frequency of the braking waveform is ramped
down at
a rate determined by a braking profile. Examples of how the braking waveform
may be
adjusted relative to the rotor velocity are discussed further below with
reference to
FIGS. 4A and 5A.
[0050] The braking profile may, for example, be determined based on
generator
parameters stored in a memory accessible to the controller. For example, the
controller
104 may utilize a "turbine model" that is based on turbine parameters. The
turbine
model may, for example be based on customized variables determined by turbine
construction and testing of the turbine, such as for example, wind rotor mass,
inertia,
drivetrain construction, generator configuration, etc. The turbine model may
be used
(e.g. by the controller) to determine the rotor speed based on the frequency
of the
braking waveform and the slip. When the turbine parameters are known, the slip
can be
determined by measurement of the current output from the generator at a given
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frequency of the braking waveform. Thus, the system can determine the rotor
speed
when applying a given braking waveform based on current measurements. In some
embodiments, the frequency of the braking waveform may be ramped down
according
to a predetermined braking profile. The predetermined braking profile for
ramping down
the braking frequency (and thus rotor velocity) may be determined by the
controller 104
based on turbine parameters, or alternatively entered into the controller 104
by the
interface 106 of 110. The "shape" of the braking profile may be controlled by
the
controller 104. In an embodiment, the shape of the braking profile is
configured to
provide underdamped braking (i.e. to reduce or eliminate oscillations). In
some
embodiments, the power module 102 may be configured to "release" the rotor
(e.g. stop
applying the braking waveform) in the event of a wind gust, blade tip rotation
or other
event that causes a generator output power spike and corresponding power
module
power spike, in order to reduce the risk of damage to the power module 102.
[0051] In some situations, it is desired that the braking profile provides
a smooth
slowing of the rotor 16 to a safe frequency of rotation within 10-15 seconds.
In some
embodiments, the rotor velocity may be ramped down over a longer period of
time, for
example up to about one minute, in order to avoid exceeding the safe operating
limits of
the power module 102, and/or to reduce mechanical loads on the drivetrain and
wind
rotor. In general, it is not desired to have the wind rotor 16 rotating for
extended periods
of time at higher speeds that produce large torques or energy output as this
may stress
the electrical and mechanical systems of the wind turbine 10 and/or electric
braking
system 100.
[0052] In some embodiments, the portable electric braking system 100 may
be
provided with a simplified user interface, such that an on-site operator can
brake a
turbine with only minimal user input. For example, in some embodiments, the
system
100 can be configured to require only a target speed input (e.g. 0 rpm, or
some other
target selected based on the situation). In some embodiments, a local or
remote
operator may input, such as through the interface 106 of 110, a target speed
of rotation
of the generator rotor and desired time to reach the target speed. The
controller 104
determines the "shape" of the braking profile that will safely brake the
generator rotor,
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CA 02875840 2014-12-19
based on the parameters and characteristics of the generator 200 and the power
module 102.
[0053] In the case of a rotor 16 that includes an automatic mechanical
blade
pitching functionality, as described above, a two-stage ramping of the braking
frequency
may be desired to compensate for a power surge that may occur when the rotor
blades
18 return to the normal position. Two-stage (or more than two-stage) ramping
could
also be used in other situations to address power surges due to the power
response
characteristics of the turbine. A power surge can cause the power module 102
to
exceed its power limits, causing damage to the power module 102. An example of
two-
stage braking is discussed below with reference to FIG. 5A.
[0054] Rather than have the controller 104 automatically ramp down the
braking
frequency, either directly to a single target or as part of a multi-stage
braking profile, as
described above, the braking frequency may optionally be ramped down manually
by an
operator during some or part of the braking operation. The user may optionally
manually
ramp down the frequency of the braking waveform at the site of the electric
braking
system 100, or from a remote location utilizing the interface 110.
[0055] At 416, a determination is made whether the generator rotor rpm is
substantially zero (e.g. based on the braking waveform applied, the turbine
model and
current measurements), which indicates that the wind rotor 16 has stopped
spinning. If
the wind rotor 16 is still spinning, the process may continue to cycle through
414 and
416 the wind rotor 16 has stopped.
[0056] Once the wind rotor 16 has stopped, at 418 the braking waveform is
maintained. In a typical emergency braking situation, there will still be a
wind force
causing a positive torque on the wind rotor, and in some embodiments, the
ramping
setpoint may be controlled to ramp down to produce a negative rotational
velocity for
the magnetic field induced by the braking waveform maintained at 418. This
negative
rotational velocity produces a negative torque which opposes the positive
torque from
the wind, to hold the wind rotor 16 still until on-site technicians can lock
the wind rotor
16 or otherwise stop the blades 18 from spinning to secure the turbine 10. The
power
output is periodically or continuously monitored, and the braking waveform is
adjusted
at 414 as necessary to account for any changes in the wind force. In some
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CA 02875840 2014-12-19
embodiments, the system 100 may include a power supply sufficient to maintain
the
braking waveform at 418 without grid power, so that the system may be
decoupled from
the grid once the wind rotor has stopped, as discussed below. Once the wind
rotor 16
and/or blades 18 have been secured, the system 100 may be turned off and
disconnected.
[0057] FIG. 4A is a graph 500 showing example rotational velocities for a
generator rotor (represented by solid trace 502) and the magnetic fields
induced in a
generator stator (represented by broken traces 504, 506, 508) during an
example
braking operation. From the start of the graph to time to the generator is
functioning
normally, with the generator rotor velocity 502 holding steady, and the
operating
magnetic field 504 (dashed trace) at the stator is rotating at the synchronous
speed. At
time to a shutdown is initiated, at which point the stator field is turned off
such that the
rotor starts accelerating due to positive wind torque on the blades 18 that is
not
countered by back EMF induced by the stator magnetic field , and the primary
brakes
are applied. However, the primary brakes are malfunctioning, such that the
rotor
velocity increases rapidly in the presence of continued wind torque. At time
ti the
rotational velocity of the rotor exceeds a secondary braking threshold Rs and
a
secondary braking mechanism (e.g., an automatic blade pitching mechanism) is
engaged, such that the rotational velocity of the rotor levels off.
Alternatively of
additionally, the rotational velocity of the rotor may level off due to the
aerodynamic
properties of the wind rotor in its nominal position.
[0058] At time t2 the stator leads are connected to a portable electric
braking
system according to the present disclosure, and a probing waveform is applied
to
generate a probing magnetic field 506 (dash-dotted trace) that initially
rotates relatively
slowly, the frequency of which is ramped up until the rotational velocity of
the probing
waveform induced magnetic field 506 substantially matches the rotational
velocity of the
rotor 502 at time t3. When the rotational velocity of the probing waveform
induced
magnetic field 506 matches the rotational velocity of the rotor 502 at time
t3, a braking
waveform is applied to generate a braking magnetic field 508 (dash-dot-dotted
trade)
that initially rotates at a slightly lower velocity than the rotational
velocity of the rotor
502. The rotational velocity of the braking waveform induced magnetic field
508 is
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CA 02875840 2014-12-19
ramped down until the rotor stops at time t4. At time t4 and thereafter until
the turbine
can be secured, the rotational velocity of the braking waveform induced
magnetic field
508 is maintained at a negative value Ro (and adjusted as necessary based on
monitoring of the power output from the generator, as discussed above) to
produce a
negative torque sufficient to counteract a continuing positive torque from the
wind.
[0059] FIG. 4B is a graph 510 showing example respective voltages used to
generate the rotating magnetic fields 504, 506, 508 of FIG. 4A. The grid
voltage 514
that generates the operating magnetic field 504 may, for example, be about
400V in
some embodiments. The probing voltage 516 that generates the probing magnetic
field
506 may, for example, be less than 50V, and about 10-30 V in some embodiments.
The braking voltage 518 that generates the braking magnetic field 508 may, for
example, be ramped down from an initial value which may be higher than the
grid
voltage to a final voltage Vo that may depend on the amount of negative torque
necessary to keep the rotor stopped in the presence of ongoing wind. In some
embodiments, the voltage amplitude of the braking waveform may range from
about
400V or more initially at higher frequencies (e.g. around 50 Hz) down to about
20V at
lower frequencies (e.g. about 0-2 Hz).
[0060] FIG. 5A is a graph 520 similar to FIG. 4A showing example
rotational
velocities for a generator rotor (represented by solid trace 502) and the
magnetic fields
induced in a generator stator (represented by broken traces 504, 506, 508-1,
508-2)
during an example braking operation. Graph 520 is similar to graph 500 except
that
graph 520 illustrates an example two stage braking operation, illustrated by
the two
portions of the braking waveform indicated by traces 508-1 and 508-2. As
discussed
above, two stage braking may be useful when braking a turbine having an
automatic
mechanical blade pitching mechanism or other characteristics likely to cause a
power
surge as the rotor is electrically braked.
[0061] In a first stage of the two-stage ramping process, the braking
frequency is
ramped down at a first rate until time t3.1, as indicated by trace 508-1.
After time t3.1, in a
second stage the frequency of the braking waveform is further ramped down at a
second rate to a target speed (e.g. to a stop), as indicated by trace 508-2.
For example,
in some embodiments the braking frequency may be ramped down at the first rate
until
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CA 02875840 2014-12-19
the speed is just below a speed R1 at which the blades 18 will return to their
normal
position, which will cause a temporary increase in the power output from the
generator
200.
[0062] FIG. 5B is a graph 530 illustrating an example power response of an
example wind turbine, where the power output from the generator increases as
the
rotational velocity drops from the velocity at time t3, then the power
decreases again as
the rotational velocity reaches the velocity at time t3.1. The braking speed
may be
ramped down more quickly through this transitional region between time t3and
time t3.1,
to avoid exceeding the power limitations of the power module 102.
[0063] The present disclosure describes a portable electric braking system
for
use in braking a wind turbine. The portable electric braking system may be
brought to
the site of a wind turbine, rather than building wind turbines with expensive,
built-in
electric braking systems. The disclosed portable electric braking system can
slow down
or completely stop a wind turbine without utilizing friction based brakes.
Certain
embodiments of the disclosed portable electric braking system enable fully
controllable
deceleration periods, enabling fast or slow braking.
[0064] The disclosed portable electric braking system may convert the
kinetic
energy of the turbine that is removed during braking into electric energy,
which may be
transferred to the electrical grid. In particular, electric energy generated
during braking
may be provided from the power module 102 to the grid, or to another power
dump if
the grid is down. As discussed above, the rate of braking may be controlled to
ensure
that power dissipation from the power module 102 remains within safe
operational
limits.
[0065] FIGS. 6A and 6B respectively show example portable braking
apparatus
600A and 600B that may be configured to operate even in the absence of grid
power.
Each of the apparatus 600A of FIG. 6A and the apparatus 600B of FIG. 6B may be
installed on a mobile platform 610 (e.g. a truck, van, trailer, etc.) to
facilitate transport to
a wind turbine needing braking.
[0066] The apparatus 600A of FIG 6A comprises a portable power unit such
as,
for example a battery/UPS module 620, that is connected to the inputs of the
power
module of a braking system 100 as disclosed above. In operation, the inputs of
the
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CA 02875840 2014-12-19
system 100 of FIG. 6A would be connected to the grid during braking, then may
be
disconnected from the grid once braking is complete, and rely on the
battery/UPS
module 620 to maintain the braking waveform (and absorb any additional braking
power
produced) once the turbine is stopped. The battery/UPM module 620 may, for
example,
include inverters or other elements for providing the AC output used for the
braking
waveform. The apparatus 600A may also include a suitable switching mechanism
(not
shown) for breaking the connection to the grid and establishing the connection
to the
battery/UPS module. The apparatus 600A may safely maintain the turbine
stationary so
that service personnel can secure the turbine, rather than relying on
continued grid
power, which may be unreliable in the event of a storm or other disruption.
[0067] The apparatus 600B of FIG 6B comprises a portable AC generator 630
and a dump load 640 that are connected to the inputs of the power module of a
braking
system 100 as disclosed above. The AC generator 630 is configured to provide
reactive power to energize the generator stator during braking operations, and
the dump
load 640 (e.g. a power dissipating resistor or other element for dissipating
or absorbing
braking power) is configured to handle the power generated during braking
operations.
The AC generator may, for example be configured to produce a power output of
about
30% of the rating power of the turbine generator.
[0068] The examples discussed above relate to portable electric braking
systems, but the teachings of the present disclosure may also be applied to
built-in
braking systems. Such systems may advantageously be used to provide auxiliary
electric braking for wind turbines with induction generators. In some
embodiments, a
built-in electric braking system may include an auxiliary power supply such as
a
battery/UPS module similar to the portable apparatus 600A of FIG. 6A, such
that the
built-in electric braking system can hold the turbine stationary for a time
without relying
on grid power. For example, in some embodiments an electric braking system of
the
type shown in FIG. 2 may be incorporated into the master control cabinet 20 of
the wind
turbine 10, or may be incorporated into a base portion of the tower 12
supporting the
wind turbine 10, with the power module 102 connected between the master
control
cabinet 20 and the generator 200.
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CA 02875840 2014-12-19
[0069] In the preceding description, for purposes of explanation, numerous
details are set forth in order to provide a thorough understanding of the
embodiments.
However, it will be apparent to one skilled in the art that these specific
details are not
required. In other instances, well-known electrical structures and circuits
are shown in
block diagram form in order not to obscure the understanding. For example,
specific
details are not provided as to whether the embodiments described herein are
implemented as a software routine, hardware circuit, firmware, or a
combination thereof.
[0070] Embodiments of the disclosure can be represented as a computer
program product stored in a machine-readable medium (also referred to as a
computer-
readable medium, a processor-readable medium, or a computer usable medium
having
a computer-readable program code embodied therein). The machine-readable
medium
can be any suitable tangible, non-transitory medium, including magnetic,
optical, or
electrical storage medium including a diskette, compact disk read only memory
(CD-
ROM), memory device (volatile or non-volatile), or similar storage mechanism.
The
machine-readable medium can contain various sets of instructions, code
sequences,
configuration information, or other data, which, when executed, cause a
processor to
perform steps in a method according to an embodiment of the disclosure. Those
of
ordinary skill in the art will appreciate that other instructions and
operations necessary
to implement the described implementations can also be stored on the machine-
readable medium. The instructions stored on the machine-readable medium can be
executed by a processor or other suitable processing device, and can interface
with
circuitry to perform the described tasks.
[0071] The present disclosure may be embodied in other specific forms
without
departing from its spirit or essential characteristics. The described
embodiments are to
be considered in all respects only as illustrative and not restrictive.
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