BACKGROUND
The field of the disclosure relates generally to methods and systems for use
in controlling operation of a wind turbine, and more specifically, to controlling the
operation of a wind turbine using dynamic braking in response to an islanding event.
Generally, wind turbine systems regulate a positive sequence voltage with a
closed-loop current regulation scheme that minimizes negative sequence current.
Such systems work well and are known to be reliable at constant output power levels.
However, as the length of transmission line feeder to the DFIG wind turbine system is
increased, response to grid transients and grid disturbances causes oscillations of
power into and out of the converter which create disturbances on the DC bus voltage
in the converter. Such power oscillations may, over time, lead to degradation of
system controllability andlor equipment malfunctions. In some grid faults, upon
clearing, the wind plant is left with no remaining connection to the grid, but still with
the wind turbines connected to the cables and lines of the wind plant and at least a
portion of a I.ong transmission grid. This can be considered an "islanded" condition
for the wind park that is characterized by potentially significant deviations in voltage
and frequency. This condition is not to be confused with other usages of the term
"islanding," where the intent is to ensure safety of maintenance personnel.
The above-described events pose a potential for damage to. the wind turbine
electrical system due to high voltages within that system that exceed equipment
capability. It is desirable for the wind turbine to ride through the grid events, both
low-voltage and high-voltage, when the grid remains partially intact after clearing the
grid fault. However, when the grid becomes open-circuited after clearing the fault,
then it is desirable that the wind turbines continue operating without damage and
eventually shut down based on inability to transfer power.
One control method for regulating the power flow during some grid
disturbances involves operating a "rotor crowbar," which is used as a last resort to
limit power flow into the DC bus of the converter to keep the converter from being
damaged. Generally; however, such a system does not allow the wind turbine system
to recover fast enough to meet the some grid code standards and/or regulations. With
existing control methods, as longer transmission line lengths are desired, possibly
coupled with larger grid voltage transients, the voltage overshoots on the DC bus
voltage in the converter may reach a level to damage the components in the converter.
Accordingly, a need exists to more effectively control wind turbine systems
using dynamic braking to protect electrical equipment from disturbances caused by
the power grid.
BRIEF DESCRIPTION
In one aspect, a method of dissipating energy in a direct current (dc) bus of a
doubly-fed induction generator (DFIG) converter during a grid event is provided.
One embodiment of the method comprises monitoring operating conditions of an
electrical system, the electrical system comprising at least a DFIG generator and a line
side converter and a rotor side converter connected by a dc bus having a dynamic
brake connected thereto; detecting an overvoltage on the dc bus or a condition
indicative of an overvoltage on the dc link is detected, the overvoltage on the dc bus
or condition indicative of the overvoltage caused by a grid event; and causing energy
in the dc link to be dissipated using the dynamic brake.
In another aspect, a method of dissipating energy in a direct current (dc) bus
of a doubly-fed induction generator (DFIG) converter during an islanding event is
provided. One embodiment of the method comprises detecting an indicator of
islanding of a DFIG generator and a DFlG converter, the DFIG converter comprising
a line side converter and a rotor side converter connected by a dc bus having a
dynamic brake connected thereto where the DFIG generator and line side converter
connected to at least a portion of an electrical grid subsequent to the islanding; and
causing energy in the dc bus of the DFIG converter to be dissipated based on the
detected indicator of islanding condition, the energy dissipated using the dynamic
brake.
In another embodiment, a system for dissipating energy in a direct current
(dc) bus of a doubly-fed induction generator (DFIG) converter during a grid event is
provided. One embodiment of the system comprises a DFIG generator coupled to a
utility grid via a dual path, the dual path defined by a stator bus and a rotor bus; a
DFIG converter coupled to the generator via rotor bus and coupled to the utility grid
via a line bus, the DFIG converter comprising a line side converter and a rotor side
converter connected by a dc bus having a dynamic brake connected thereto; and a
controller, wherein the controller is configured to receive and transmit signals to and
from an electrical system comprising at least the utility grid, DFIG generator and the
DFIG converter, the controller further configured to: monitor operating conditions of
the utility grid, DFIG generator, line side converter, rotor side converter, dc bus and
dynamic brake; detect an overvoltage on the dc bus or a condition indicative of an
overvoltage on the dc link, the overvoltage on the dc bus or condition indicative of the
overvoltage caused by a grid event; and cause energy in the dc link to be dissipated
using the dynamic brake.
BRIEF DESCRIPTION OF THE DRAWINGS
A full and enabling disclosure of embodiments of the present invention,
including the best mode thereof, directed to one of ordinary skill in the art, is set forth
in the specification, which makes reference to the appended figures, in which:
FIG. 1 is a perspective view of a portion of an exemplary wind turbine;
FIG. 2 is a schematic of an exemplary generator control system including a
doubly fed induction generator (DFIG) that may be used with the wind turbine shown
in FIG. 1 ;
FIG. 3A illustrates the normal operating condition of a wind turbine in a wind
park;
FIG. 3B illustrates the conditions when a remote breaker opens leaving the
wind park in an islanded condition and the power flow to the grid is suddenly
interrupted for a case where the rotor torque and speed remain substantially the same
as pre-islanding condition;
FIG. 4 illustrates a block diagram of one embodiment of suitable components
that may be included within an embodiment of a controller, or any other computing
device that receives signals indicating a high-voltage grid event in accordance with
aspects of the present subject matter;
FIG. 5A is a flow chart of an exemplary method of controlling dc bus voltages
of a DFIG converter during a grid event such as islanding; and
FIG. 5B is a flow chart of an exemplary method of dissipating energy in a
direct current (dc) bus of a doubly-fed induction generator (DFIG) converter during
an islanding event.
DETAILED DESCRlPTIOW
Before the present methods and systems are disclosed and described, it is to
be understood that the methods and systems are not limited to specific synthetic
methods, specific components, or to particular compositions. It is also to be
understood that the terminology used herein is for describing particular embodiments
only and is not intended to be limiting.
As used in the specification and the appended claims, the singular forms
-'a," "an" and "the" include plural referents unless the context clearly dictates
otherwise. Ranges may be expressed herein as from "about" one particular value,
and/or to .'aboutw another particular value. When such a range is expressed, another
embodiment includes from the one particular value andfor to the other particular
value. Similarly, when values are expressed as approximations, by use of the
antecedent "about," it wilI be understood that the particular value forms another
embodiment. It wilI be further understood that the endpoints of each of the ranges are
significant both in relation to the other endpoint, and independentIy of the other
endpoint.
"Optional" or "optionally" means that the subsequently described event or
circumstance may or may not occur, and that the description includes instances where
said event or circumstance occurs and instances where it does not.
Throughout the description and claims of this specification, the word
"comprise" and variations of the word, such as "comprising" and "comprises," means
"including but not limited to," and is not intended to exclude, for example, other
additives, components, integers or steps. "Exemplary" means "an example of' and is
not intended to convey an indication of a preferred or ideal embodiment. "Such as" is
not used in a restrictive sense, but for explanatory purposes.
Disclosed are components that can be used to perform the disclosed
methods and systems. These and other components are disclosed herein, and it is
understood that when combinations, subsets, interactions, groups, etc. of these
components are disclosed that while specific reference of each various individual and
collective combinations and permutation of these may not be explicitly disclosed,
each is specifically contemplated and described herein, for all methods and systems.
This applies to all aspects of this application including, but not limited to, steps in
disclosed methods. Thus, if there are a variety of additional steps that can be
performed it is understood that each of these additional steps can be performed with
any specific embodiment or combination of embodiments of the disclosed methods.
As used herein, the term "wind turbine" refers to any device that generates
rotational energy from wind energy, and more specifically, converts the kinetic
energy of wind into mechanical energy. As used herein, the term "wind turbine
generator" refers to any wind turbine that generates electrical power from rotational
energy generated from wind energy, and more specifically, converts mechanical
energy converted from kineticenergy of wind to electrical power.
As used herein, the terms "disturbance," "grid disturbance," "fault,"
''system fault," "transient" and other similar terms generally refer to any event that
causes perturbations in the input signal from the electric/power grid. For example,
such disturbances can include impulses, notches, glitches, momentary interruptions,
vo It age sag/swelIs, harmonic distortions, flickers, and islanding where the wind
turbine generator is left with no remaining connection to the grid, but still with the
wind turbine generator connected to the cables and lines of the wind plant and at least
a portion of a long transmission grid. Generally, the grid signal is a three-phase signal
that includes sequence components having particular frequencies. The three-phase
signal includes positive sequence components, negative sequence components, and
zero or neutral sequence components. Each of the components includes frequency
information. phase information, and magnitude information. Typically, wind turbine
systems regulate a positive sequence voltage with a closed-loop current regulation
scheme that minimizes negative sequence current. Such systems work well and are
known to be reliable at constant output power levels. However, as the length of
transmission line feeder to a wind turbine generator is increased, response to grid
transients and grid disturbances may cause oscillations of power into and out of the
converter, which can create disturbances on the DC bus voltage in the converter. Such
power oscilIations may, over time, lead to degradation of system controllability andlor
equipment malfunctions. The present methods and systems may be understood more
readily by reference to the following detailed description of preferred embodiments
and the Examples included therein and to the Figures and their previous and following
description.
Generally disclosed herein are systems and methods of responding to a
high-voltage grid event on an electrical system connected with one or more DFIGs.
FIG. 1 is a perspective view of an exemplary wind turbine 10. In the
exemplary embodiment, wind turbine 10 is a wind turbine generator that generates
electrical power from wind energy. Wind turbine 10 may have a horizontal-axis
configuration, however, in an alternative embodiment wind turbine 10 includes, in
addition to, or in the alternative to, the horizontal-axis configuration, a vertical-axis
configuration. In the exemplary embodiment, wind turbine 10 is coupled to an
electrical load, such as, but not limited to, an electric/power grid, for receiving
electrical power therefrom to drive operation of wind turbine 10 and/or its associated
components and/or for supplying electricaI power generated by wind turbine 10
thereto. Although only one wind turbine 10 is shown in FIG. 1, a plurality of wind
turbines 10 may be grouped together, sometimes referred to as a !'wind farm" or a
"wind park."
Wind turbine 10 includes a body or nacelle 12 and a rotor 14 coupled to
nacelle 12 for rotation with respect to nacefle 12 about an axis of rotation 20. In the
exemplary embodiment, nacelle 12 is mounted on a tower 16. In an alterative
embodiment, nacelle 12 may be positioned adjacent to the ground andlor adjacent to a
surface of water. A height of tower 16 may be selected to be any suitable height that
enables wind turbine 10 to function as described herein. Rotor 14 includes a hub 22
and a plurality of blades 24 that extend radially outwardly from hub 22 for converting
wind energy into rotational energy. Although rotor I4 is shown as having three
blades 24, rotor 14 may have any number of blades 24.
FIG. 2 is a schematic of an exemplary generator control system 100 that
incIudes a doubly fed induction generator (DFIG) 102 that may be used with wind
turbine 10, shown in FIG. 1. In the exemplary embodiment, generator control system
100 includes a plurality of rotor blades 104 coupled to a rotating hub 106, which
together generally define a rotor 108. Rotor 108 is coupled to a gear box I 10, which
is coupled to a generator 102. In the exemplary embodiment; generator 102 is a DFIG
or a wound rotor generator.
In the exemplary embodiment, generator 102 is coupled to a statorsynchronizing
switch 120 via a stator bus 122. Stator-synchronizing switch 120 is
coupled to power grid 124. Generator 102 is also .coupled to a power conversion
component 130 via a rotor bus 132. In the exemplary embodiment, power conversion
component 130 is coupled to a conversion circuit breaker 133 via a line bus 135, and
conversion circuit breaker 150 is coupled to grid 124. In the exemplary embodiment,
stator bus 122 outputs three-phase power from a stator of generator 102 and rotor bus
132 outputs three-phase power from a rotor of generator 102, though any numkr of
phases are contemplated within the scope of embodiments of the present invention.
Power conversion component 130 includes a rotor-side converter 134 and
a line-side converter 136. in one aspect, rotor-side converter 134 and line-side
converter 136 can be configured for a normal operating mode in a three-phase, two
level, Pulse Width Modulation (PWM) arrangement. Rotor-side converter 134 and
line-side converter 136 are coupled together via a direct current (DC) bus 137 having
a positive link 138 and a negative link 140. In the exemplary embodiment, a dynamic
brake I42 and a DC bus capacitor 144 are coupled to DC bus 137, between rotor-side
converter 134 and line-side converter 136.
In one exemplary embodiment, dynamic brake 142 includes a fully
controllable switch 146 placed in series with a resistor 148. In one exemplary
embodiment, switch 146 is a semiconductor such as an Insulated Gate Bipolar
Transistor (IGBT) or any other electronic gated switch. In one embodiment, a diode
is coupled in parallel with switch 146. Jn an alternative embodiment, a diode is
coupled in parallel with the resistor 148. In another embodiment, a diode is coupled
in parallel with both switch 146 and resistor 148. Alternatively, diode placement
within dynamic brake I42 is not limited to a single diode, but rather any number of
diodes can be coupled in any parallel combination of switch 146 and resistor 148 that
enables a wind turbine 10 to operate as described herein.
In operation, power generated at generator 102 is provided via a dual path
to grid 124. The dual paths are defined via stator bus 122 and rotor bus 132. In the
exemp1ar.y embodiment, sinusoidal three-phase alternating current (AC) power is
converted to direct current (DC) power on rotor bus 132 via power conversion
component 130. Converted power supplied from power conversion component 130 is
combined with the power supplied from generator 102 to provide three-phase power
at a frequency that is maintained substantially constant. In one embodiment, the
frequency is maintained at about 60 Hertz AC, 50 Hertz AC, and the like.
Alternatively, the frequency can be maintained at any Ievel that enables operation of
wind turbine 10 as described herein.
The normal operating condition of a wind turbine in a wind park is
illustrated in FIG. 3A. This figure shows power flows within the converter 130 and
the wind park electrical system 300 during super-synchronous operation typical of
moderate to high wind conditions. The power from the DFIG 102 splits in two paths,
one power flow (PStator) 302 flows directly from the stator of the generator 102
through a wind turbine circuit breaker 214, wind turbine transformer 234 and local
grid breaker 238 into the grid connection 124. The other power flow (PRotor) 304
from DFIG 102 flows via the rotor of the generator 102, which passes though the
rotor converter 134, to the dc Iink 137, to the line converter 136, through a line reactor
312 (not required) and then on to the grid connection point 124 via the wind turbine
circuit breaker 214, wind turbine transformer 234 and the local grid breaker 238. The
sum (Pgrid) 306 of these two power flows 302,304 is the net output of the generator
102. Note that the split of power between the generator 102 rotor and stator is a
function of rotor speed relative to synchronous. Similarly, power from other wind
turbines 3 I4 in the wind park can flows from the local grids of each additional wind
turbine 314 to the grid connection point 124. At super-synchronous operation the
rotor speed is higher than synchronous and the power splits as shown. At subsynchronous
operation the rotor speed is less than synchronous and the rotor winding
draws power from the rotor canverter 134, i.e. the power flows through the converters
136, 134 into the rotor of the generator 102.
FIG. 3B illustrates the conditions when a remote breaker opens leaving the
wind park in an islanded condition and the power flow to the grid 124 is suddenly
interrupted for a case where the rotor torque and speed remain substantially the same
as pre-islanding conditions. The power (PLine) 308 on the line converter 136 is
suddenly forced to reverse, since the power that was flowing from the DFIG 102
stator to the grid 124 now has only the line converter 136 as a path. This causes the
voltage on the dc Iink 137 to rise very rapidly, When an islanded condition occurs, it
is desirable to disconnect the wind turbine system 300 from the grid 124 in a manner
that does not cause damage to components of the electrical system 300. However,
damage to components can happen in a few milliseconds, which is typically faster
than circuit breakers can operate. Also, the remote breaker opening may Ieave some
portion of the local grid connected to the generator 102, e.g. cables that make up the
wind park collector system, a portion of the transmission grid 124, and the like. The
capacitance 310 of the remaining connected grid can be a source of ac voltage
amplification on the remaining network. Control action is needed quickly to prevent
damaging voltage IeveIs and there is a need to dissipate energy in the system 300
during the control process. In some aspects, a controller or other computing device
can be used to detect conditions on the electrical system 300 or grid 124 indicative of
an overvoltage on the dc link 137 or that would cause an overvoltage on the dc link
137 and cause the switch 146 of the dynamic brake 142 to allow current to pass
through the switch 146 and energy be at least partially dissipated by the resistor of the
dynamic brake 142.
In one aspect, the dynamic brake I42 can be selectively activated on the
level of the DC bus voltage in converter 130. For example, voltage on the dc link 137
can be monitored by a controller or other computing device and if it reaches or
exceeds a threshold the switch 146 of the dynamic brake 142 can be caused by the
controller or other computing device to allow current to pass through the switch 146
and be at least partially dissipated by the resistor of the dynamic brake 142. In one
aspect, the switch 146 stays in a conducting state until the voltage on the dc bus 137
drops below the threshoId, the resistor approaches a thermal limit, or other monitored
operational conditions cause the switch 146 to be placed in a non-conducting state. In
another aspect, the switch 146 acts as a chopper and switches between a conducting
and non-conducting state. An alternate control scheme includes monitoring AC
voltage and/or current on the remaining connected grid portion of the electrical
system in a feedback loop using the controller or other computing device and timing
the operation of the switch 146 in the dynamic brake circuit such that oscillations
between the grid and the dc link 137 are reduced. Furthermore, embodiments of the
dynamic brake 142 as described herein can be used to dissipate ovewoltages or
energy on the dc link 137, which allows the usable speed range of the wind turbine
generator 102 to be expanded as the response time of the pitch control system, when
responding to grid fluctuations, can be reduced by allowing the dynamic brake I42 to
dissipate energy caused by the fluctuations.
Referring now to FIG. 4, some embodiments of systems for dissipating
energy in a dc link of a DFIG converter caused by an islanding condition can include
a control system or controller 202. in general, the controller 202 may comprise a
computer or other suitable processing unit. Thus, in several embodiments, the
controller 202 may include suitable computer-readable instructions that, when
implemented, configure the controller 202 to perform various different functions, such
as receiving, transmitting and/or executing control signals. As such, the controller
202 may generally be configured to control the various operating modes (e.g.,
conducting or non-conducting states) of the one or more switches 146 andlor
components of embodiments of the electrical system 300. For example, the controller
202 may be configured to implement methods of dissipating energy in a dc link of a
DFIG converter caused by an islanding condition.
FIG. 4 illustrates a block diagram of one embodiment of suitable
components that may be included within an embodiment of a controller 202, or any
other computing device that receives signals indicating grid .conditions in accordance
with aspects of the present subject matter. In various aspects, such signals can be
received from one or more sensors or transducers 58, 60, or may be received from
other computing devices (not shown) such as a supervisory control and data
acquisition (SCADA) system, a turbine protection system, PLL regulator, and the
like. Received signals can include, for example, voltage signals such as DC bus 137
voltage and AC grid voltage, corresponding phase angles for each phase of the AC
grid, current signals, power flow (direction) signals, power output from the converter
system 130, total power flow into (or out of) the grid, and the like. In some instances,
signals received can be used by the controller 202 to calculate other variables such as
changes in voltage phase angles over time, and the like. As shown, the controller 202
may include one or more processor(s) 62 and associated memory device(s) 64
configured to perform a variety of computer-implemented functions (e.g., performing
the methods, steps, calculations and the like disclosed herein). As used herein, the
term "processor" refers not only to integrated circuits referred to in the art as being
included in a computer, but also refers to a controller, a microcontroller, a
tnicrocomputer, a programmable logic controller (PLC), an application specific
integrated circuit, and other programmable circuits. Additionally, the memory
device(s) 64 may generally comprise memory element(s) including, but not limited to,
computer readable medium (e.g., random access memory (RAM)), computer readable
non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only
memory (CD-ROM),a magneto-opticaI disk (MOD),a digital versatile disc (DVD)
andlor other suitable memory elements. Such memory devicels) 64 may generally be
configured to store suitable computer-readable instructions that, when implemented
by the processor(s) 62, configure the controller 202 to perform various functions
including, but not limited to, directly or indirectly transmitting suitable control signals
to one or more switches that comprise the bi-directional power conversion assembly
130, switches of the dynamic brake 142, monitoring operating conditions of the
electrical system 300, and various other suitable computer-implemented functions.
Additionally, the controller 202 may also include a ~ornmu~licationmso dule
66 to facilitate communications between the controller 202 and the various
components of the electrical system 390 including the one or more sources of
electrical generation 102. For instance, the communications module 66 may serve as
an interface to permit the controller 202 to transmit control signals to the bidirectional
power conversion assembly 130, dynamic brake 142, and/or other
components of the wind turbine and electrica1 system. Moreover, the
communications module 66 may include a sensor interface 68 (e.g., one or more
analog-to-digital converters) to permit signals transmitted from the sensors (e-g., 58,
60) to be converted into signals that can be understood and processed by the
processors 62. Alternatively, the controller 202 may be provided with suitable
computer readable instructions that, when implemented by its- processor(s) 62,
configure the controller 202 to take various actions depending upon the control mode
of the wind turbine. For example, in normal operation (i.e., rotor control), the rotor
converter has dominant control over the flow of real and reactive power from the
generator. The line converter acts primarily to regulate dc link voltage by adjusting
the real power exchange to the grid connection point. The line converter can also
draw reactive current from the grid in case of high ac voltage. If a grid event is
detected by the controller 202, then the operation mode can change such that the
switch 146 of the dynamic brake 142 is caused to conduct in order to protect the
converters 134, 136 and other electrical components.
FIG. 5A is a flow chart of an exemplary method of dissipating energy in a
dc bus of a DFlG converter during a grid event such as islanding. Steps of the
flowchart may be implemented by the controller 202 or other suitable computing
devices. In the exemplary embodiment, method includes step 502, monitoring
electrical system operating conditions. In one aspect, monitoring electrical system
operating conditions comprises monitoring voltage on the dc link 137 by a controller
or other computing device. In another aspect, AC voltage andlor current on the
remaining connected grid portion of the electrical system can be monitored in a
feedback loop using the controller or other computing device. At step 504, an
overvoltage on the dc link 137 is detected or a condition indicative of an overvoltage
on the dc link is detected such as, for example, high ac voltage, high ac current and
the like. For example, the overvoltage, high ac voltage or high ac current may be 196,
5% lo%, 50%, 150% or greater, and any values therebetween, of the measured
voltage or current over the nominal voltage or current. At step 506, protective action
is taken using the dynamic brake to dissipate energy in the dc link of the DFIG
converter. For example, if the monitored voltage on the dc link reaches or exceeds a
threshold, the switch 146 of the dynamic brake 142 can be caused by the controller or
other computing device to allow current to pass through the switch 146 and energy be
at least partially dissipated by the resistor of the dynamic brake 142. In one aspect,
the switch 146 stays iil a conducting state until the voltage on the dc bus 137 drops
below the threshold, the resistor approaches a thermal limit, or other monitored
operational conditions cause the switch 146 to be placed in a non-conducting state. In
another aspect, the switch 146 acts as a chopper and switches between a conducting
and non-conducting state. In another aspect, AC voltage andlor current on the
remaining connected grid portion of the electrical system is monitored in a feedback
loop using the controller or other computing device and timing the operation of the
switch 146 in the dynamic brake circuit such that oscillations between the grid and the
dc link I37 are reduced. Though not shown in FIG. 5A, in one aspect, the abovedescribed
process is carried out upon detecting islanding or upon receiving, by the
controller, one or more early indicators of islanding of the wind turbine generator
such its high ac voltage, high dc link voltage, frequency and/or phase shifts, and the
like. Such a process may be carried out concurrently while the contrailer or other
computing device is making a determination whether the grid event comprises an
islanding event. In one aspect, islanding can be detected by detecting that the grid
connections have all been lost, leaving the wind plant in an islanded condition. There
are several ways to do this including, for example, standard wind turbine monitor and
protection functions, including grid frequency deviation, grid voltage deviation,
measured torque not following commanded torque for a predetermined time, turbine
overspeed, tower vibration, etc. Another way of detecting that grid connections have
been lost includes special monitoring functions enacted by the high-voltage condition,
e.g, increasing the sensitivity of existing functions such as frequency and voltage
deviations. And yet another way of detecting loss of grid connections includes
receiving a signal from an external device that knows status of the grid connections.
FIG. 5B is a flow chart of an exemplary method of dissipating energy in a
direct current (dc) bus of a doubly-fed induction generator (DFIG) converter during
an islanding event. Aspects of the steps of the process can be performed by a
computing device such as controller 202 as described herein. One embodiment of the
method comprises step 508, detecting an indicator of islanding of a DFIG generator
and a DFIG converter where the DFIG converter comprises at least a line side
converter and a rotor side converter connected by a dc bus having a dynamic brake
connected thereto. The DFIG generator and line side converter are connected to at
least a portion of an electrical grid subsequent to the islanding. In one aspect,
detecting an indicator of islanding of the DFIG generator and DFIG converter
comprises detecting an indicator of islanding by one or more of grid frequency
deviation, grid voltage deviation, measured torque not following commanded torque
for a predetermined time, turbine overspeed, tower vibration, receiving a signal from
an external device indicating status of grid connections, and the like. At step 510,
energy in the dc bus of the DFIG converter is caused to be dissipated based on the
detected indicator of islanding condition, the energy dissipated using the dynamic
brake. In one aspect, causing energy in the dc link to be dissipated using the dynamic
brake comprises determining that a monitored voltage on the dc link reaches or
exceeds a threshold and causing a switch of the dynamic brake to allow current to
pass through the switch and energy be at least partially dissipated by a resistor of the
dynamic brake. In another aspect, causing energy in the dc link to be dissipated using
the dynamic brake comprises monitoring alternating current (AC) voltage or current
on the portion of the electrical grid connected to the DFIG generator and the line side
converter and timing operation of a switch in the dynamic brake such that osciIlations
between the portion of the electrical grid and the dc link are reduced.
It is to be noted that the above embodiments of systems and methods can be
used to dissipate energy in the dc link of a DFIG converter regardless of whether the
converter, generator or other components of the electrical system are energized and/or
operating or whether they are not.
As described above and as will be appreciated by one skilled in the art,
embodiments of the present invention may be configured as a system, method, or a
computer program product. Accordingly, embodiments of the present invention may
be comprised of various means including entirely of hardware, entirely of software, or
any combination of software and hardware. Furthermore, embodiments of the present
invention may take the form of a computer program product on a computer-readable
storage medium having computer-readable program instructions (e.g., computer
software) embodied in the storage medium. Any suitable non-transitory computerreadable
storage medium may be utilized including hard disks, CD-ROMs, optical
storage devices, or magnetic storage devices.
Embodiments of the present invention have been described above with
reference to block diagrams and flowchart illustrations of methods, apparatuses (i-e.,
systems) and computer program products. It will be understood that each block of the
block diagrams and flowchart illustrations, and combinations of blocks in the block
diagrams and flowchart illustrations, respectively, can be implemented by various
means including computer program instructions. These computer program
instructions may be loaded onto a general purpose computer, special purpose
computer, or other programmable data processing apparatus, such as the processor(s)
62 discussed above with reference to FIG. 4, to produce a machine, such that the
instructions which execute on the computer or other programmable data processing
apparatus create a means for implementing the functions specified in the flowchart
block or blocks.
These computer program instructions may also be stored in a non-transitory
computer-readable memory that can direct a computer or other programmable data
processing apparatus (e.g., processor(s) 62 of FIG. 4) to function in a particular
manner, such that the instructions stored in the computer-readable memory produce
an article of manufacture including computer-readable instructions for implementing
the function specified in the flowchart block or blocks. The computer program
instructions may also be Ioaded onto a computer or other programmable data
processing apparatus to cause a series of operational steps to be performed on the
computer or other programmable apparatus to produce a computer-implemented
process such that the instructions that execute on the computer or other programmable
apparatus provide steps for implementing the functions specified in the flowchart
block or blocks.
Accordingly, blocks of the block diagrams and flowchart illustrations
support combinations of means for performing the specified functions, combinations
of steps for performing the specified functions and program instruction means for
performing the specified functions. It will also be understood that each block of the
block diagrams and flowchart illustrations, and combinations of blocks in the block
diagrams and flowchart illustrations, can be implemented by special purpose
hardware-based computer systems that perform the specified functions or steps, or
combinations of special purpose hardware and computer instructions.
Unless otherwise expressly stated, it is in no way intended that any method
set forth herein be construed as requiring that its steps be performed in a specific
order. Accordingly, where a method claim does not actually recite an order to be
followed by its steps or it is not otherwise specifically stated in the claims or
descriptions that the steps are to be limited to a specific order, it is no way intended
that an order be inferred, in any respect. This holds for any possible non-express
basis for interpretation, including: matters of logic with respect to arrangement of
steps or operational flow; plain meaning derived from grammatical organization or
punctuation; the number or type of embodiments described in the specification.
Throughout this application, various publications may be referenced. The
disclosures of these publications in their entireties are hereby incorporated by
reference into this application in order to more fully describe the state of the art to
which the methods and systems pertain.
Many modifications and other embodiments of the inventions set forth
herein will come to mind to one skilled in the art to which these embodiments of the.
invention pertain having the benefit of the teachings presented in the foregoing
descriptions and the associated drawings. Therefore, it is to be understood that the
embodiments of the invention are not to be limited to the specific embodiments
disclosed and that modifications and other embodiments are intended to be included
within the scope of the appended claims. Moreover, although the foregoing
descriptions and the associated drawings describe exemplary embodiments in the
context of certain exemplary combinations of elements andlor Functions, it should be
appreciated that different combinations of elements and/or functions may be provided
by alternative embodiments without departing from the scope of the appended claims.
In this regard. for example, different combinations of elements andlor functions than
those explicitly desci-ibed above are also contemplated as may be set forth in some of
the .appended claims. Although specific terms are employed herein, they are used in a
generic and descriptive sense only and not for purposes of limitation.
259393-1
Parts List
234
23 8
3 00
3 02
3 04
3 06
308
3 10
312
314
wind turbine transformer
local grid breaker
wind park electrical system
one power flow (PStator)
other power flow (PRotor)
sum (Pgrid)
power (PLine)
Capacitance
line reactor
other wind turbines

We claim:
1. A method of dissipating energy in a direct current (dc) bus of a doublyfed
induction generator (DFIG) converter during a grid event, said method
comprising:
monitoring, by a controller, operating conditions of an electrical system,
said electrical system comprising at least a DFIG generator and a line side converter
and a rotor side converter connected by a dc bus having a dynamic brake connected
thereto;
detecting, by the controller, an overvoltage on the dc bus or a condition
indicative of an overvoltage on the dc link is detected, said overvoltage on the dc bus
or condition indicative of the overvoltage caused by a grid event; and
dissipating, by the con@olIer, the energy in the dc link using the
dynamic brake.
2. The method of Claim 1, wherein monitoring, by the controller,
operating conditions of the electrical system comprises monitoring voltage on the dc
link.
3. The method of Claim 1, wherein monitoring, by the controller,
operating conditions of the electrical system comprises monitoring alternating
current (AC) voltage or current on at least a portion of an electrical grid connected to
the DFIG generator the line side converter.
4. The method of Claim 1, wherein detecting, by the controller, a
condition indicative of an overvoltage on the dc link comprises detecting a high
alternating current (AC) voltage or current on at least a portion of an electrical grid
connected to the DFIG generator the line side converter.
5. The method of Claim 1, wherein dissipating, by the controller, energy in
the dc link using the dynamic brake comprises determining that the monitored
voltage on the dc link reaches or exceeds a threshold and causing a switch of the
dynamic brake to allow current to pass through the switch and energy be dissipated
by a resistor of the dynamic brake.
6. The method of Claim 1, wherein dissipating, by the controller, energy in
the dc link to be dissipated using the dynamic brake comprises monitoring
alternating current (AC) voltage or current on at least a portion of an electrical grid
connected to the DFIG generator and the line side converter and timing operation of
a switch in the dynamic brake to reduce oscillations between the portion of the
electrical grid and the dc link.
7. The method of Claim 1, wherein detecting, by the controller, the
overvoltage on the dc bus or condition indicative of the overvoltage caused by a grid
event comprises detecting the overvoltage on the dc bus or condition indicative of the
overvoltage caused by islanding.
8. A method of dissipating energy in a direct current (dc) bus of a doublyfed
induction generator (DFIG) converter during an islanding event, said method
comprising:
detecting, by a controller, an indicator of islanding of a DFIG generator
and a DFIG converter, said DFIG converter comprising a line side converter and a
rotor side converter connected by a dc bus having a dynamic brake connected
thereto, said DFIG generator and line side converter connected to at least a portion of
an electrical grid subsequent to the islanding; and
dissipating, by the controller, energy in the dc bus of the DFIG
converter based on the detected indicator of islanding condition, said energy
dissipated using the dynamic brake.
9. The method of Claim 8, wherein detecting, by the controller, the
indicator of islanding of a DFIG generator and a DFIG converter comprises detecting
the indicator of islanding by one or more of grid frequency deviation, grid voltage
deviation, measured torque not following commanded torque for a predetermined
time, turbine overspeed, tower vibration, or receiving a signal from an external
device indicating status of grid connections.
10. The method of Claim 8, wherein dissipating, by the controller, energy in
the dc link using the dynamic brake comprises determining that a monitored voltage
on the dc link reaches or exceeds a threshold and causing a switch of the dynamic
brake to allow cumnt to pass through the switch and energy be at least partially
dissipated by a resistor of the dynamic brake.
11. The method of Claim 8, wherein dissipating, by the controller, energy in
the dc link using the dynamic brake comprises monitoring alternating current (AC)
voltage or current on the portion of the electrical grid connected to the DFIG
generator and the line side converter and timing operation of a switch in the dynamic
brake to reduce oscillations between the portion of the electrical grid and the dc link.
12. A system for dissipating energy in a direct current (dc) bus of a doublyfed
induction generator (DFIG) converter connected to a wind turbine during a grid
event, the system comprising:
a DFIG generator coupled to a utility grid via a dual path, said dual
path defined by a stator bus and a rotor bus;
a DFIG converter coupled to said generator via a rotor bus and coupled
to the utility grid via a line bus, said DFIG converter comprising a line side converter
and a rotor side converter connected by a dc bus having a dynamic brake connected
thereto; and
a controller, wherein the controller is configured to receive and
transmit signals to and from an electrical system comprising at least the utility grid,
DFIG generator and the DFIG converter, said controller further configured to:
monitor operating conditions of the utility grid, DFIG
generator, line side converter, rotor side converter, dc bus and dynamic
brake;
detect an overvoltage on the dc bus or a condition indicative of
an overvoltage- on the dc link, said overvoltage on the dc bus or
condition indicative of the overvoltage caused by a grid event; and
dissipating energy in the dc link using the dynamic brake.
13. The system of Claim 12, wherein monitoring, by the controller,
operating conditions of the electrical system comprises monitoring voltage on the dc
link.
14. The system of Claim 12, wherein monitoring, by the controlIer,
operating conditions of the electrical system comprises monitoring alternating
current (AC) .voltage or current on at least a portion of the utility grid connected to
the DFlG generator and the line side converter.
15. The system of Claim 12, wherein detecting, by the controller, a
condition indicative of an overvoltage on the dc link comprises detecting a high
alternating current (AC) voltage or current on at least a portion of the utility grid
connected to the DFIG generator and the line side converter.
16. The system of Claim 12, wherein dissipating, by.the controller, energy
in the dc link using the dynamic brake comprises determining that the monitored
voltage on the dc link reaches or exceeds a threshold and causing a switch of the
dynamic brake to allow current to pass through the switch and energy be at least
partially dissipated by a resistor of the dynamic brake.
17. The system of Claim 12, wherein causing, by the controller, energy in
the dc link to be dissipated using the dynamic brake comprises monitoring
alternating current (AC) voltage or current on at least a portion of the utility grid
connected to the DFIG generator and the line side converter and timing operation of
a switch in the dynamic brake to reduce oscillations between the portion of the utility
grid and the dc link.
18. The system of CIaim 12, wherein detecting, by the controller, the
overvoltage on the dc bus or condition indicative of the overvoltage caused by a grid
event comprises detecting the overvoltage on the dc bus or condition indicative of the
overvoftage caused by islanding.
19. The system of Claim 12, wherein the controller is further configured to
detect islanding of the DFIG generator and the DFIG converter and cause energy in
the dc bus of the DFIG converter to be dissipated based on the detected islanding
condition, said energy dissipated using the dynamic brake, wherein detecting, by the
controller, islanding of a DFIG generator and a DFIG converter comprises detecting
islanding by one or more of grid frequency deviation, grid voltage deviation,.
measured torque not folIowing commanded torque for a predetermined time, turbine
overspeed, tower vibration, or receiving a signal from an external device indicating
status of grid connections.
20. The system of Claim 12, wherein dissipating energy using the dynamic
brake allows the usable speed range of the wind turbine to be expanded and
dissipating energy using the dynamic brake aids in the response time of the wind
turbine's pitch control system.