Note: Descriptions are shown in the official language in which they were submitted.
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LOW VOLTAGE RIDE THROUGH
BACKGROUND
[001] This application relates to wind turbine generators (WTGs).
[002] Wind energy has emerged as the fastest growing source of energy,
offering a
clean, renewable, and ecological-friendly alternative to fossil-based energy
supplies.
At the present growth rate, wind energy conversion is projected to produce
more than
117,000 MW by the year of 2009, claiming about 1.25% of the global electricity
generation. In addition to their traditional role in servicing rural
residences in grid-
isolated areas, wind turbine generators are now increasingly installed in
large-scale
(e.g., multi-megawatt) wind farms and integrated into power grids that can
deliver
electricity to consumers nationwide.
[003] The performance of a grid-connected WTG can be influenced by many
factors, such as voltage fluctuations on the grid. For example, a short
circuit on the
grid may result in a sudden voltage drop, which reduces the effective drag on
the
WTG and may cause both the turbine and the generator to accelerate rapidly. To
ensure safe operation, some WTGs have been designed to trip off-line (i.e.,
disconnect
from the grid and shut down) as soon as grid voltage drops below a prescribed
level
(e.g., 85% of nominal voltage). After fault clearance, these WTGs enter a
restart
cycle that can last several minutes before resuming power transmission to the
grid.
[004] During this off-line period, the loss of power generation may impact the
stability of utility grids to which WTGs are connected. As the number of grid-
integrated wind plants/farms continues to grow, regulatory agencies in many
countries
have started to adopt strict interconnection standards that require large WTGs
to
remain online during disturbances and continue to operate for an extended
period - a
process called "low-voltage ride through" (LVRT).
[005] Among various interconnection standards, the Spanish Grid Code, for
example, requires WTGs to be able to sustain ("ride-through") line voltage at
20% of
rated level for at least 500 ms. FIG. 1A shows an example of voltage
transients when
a low-voltage event occurs. In this case, after an initial dip of 500ms, line
voltage
starts to recover and within 15 seconds has returned to 95% of nominal. During
the
entire low-voltage period (-15s), the Spanish Grid Code requires WTGs to
continue
to operate and supply current in controlled amounts to help stabilize the
grid. FIG. lB
shows the required current behavior, measured by the ratio of the magnitude of
reactive current to total current (IYea~~~ ~l~o~) as a function of line
voltage. Note that
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other countries may have different regulations on grid-connected WTGs' current
and
voltage behaviors in response to low voltage disturbances.
SUMMARY
[006] In one general aspect of the invention, a system is provided for
connecting a
wind turbine generator to a utility power network. A first power converter
converts
an AC signal from the wind turbine generator to a DC signal and supplies a
controlled
amount of reactive current to the wind turbine generator. A second power
converter,
connected in series with the first converter, converts the DC signal from the
first
power converter to a line-side AC signal and supplies a controlled amount of
current
to the utility power network. A power dissipation element is coupled to the
first and
second power converters for dissipating power from the first power converter.
[007] Embodiments of this aspect of the invention may include one or more of
the
following features.
[008] The amount of current supplied to the utility power network satisfies a
predetermined criterion associated with a voltage condition of the utility
power
network. The predetermined criterion may include that when a voltage of the
utility
power network falls below a predetermined threshold, the magnitude of reactive
current supplied to the utility power network is at least twice as much as the
magnitude of real current supplied to the utility power network.
[009] The first and second power converters are connected via a DC bus. A
capacitor is coupled to the DC bus. A first and second AC filter reactor may
be
coupled to the first and second power converter, respectively. The power
dissipation
element may include a resistor. The resistor may include a dynamic braking
resistor.
A controllable switching device may be coupled to the resistor for regulating
a current
passing through the resistor. A power factor correction unit may be provided
for
adjusting a power factor of the electric power supplied to the utility power
network.
The power factor correction unit may include a controllable capacitor that can
be
switched on and off by electrical signals.
[010] In another general aspect of the invention, a control system is provided
for
controlling an interconnection between a wind turbine generator and a utility
power
network. Upon an occurrence of a low voltage event, the control system
electrically
opens a first path of the interconnection. A second path of the
interconnection is
controlled during the low voltage event to provide a first current suitable
for
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maintaining an operation of the wind turbine generator and a second current
having a
predetermined characteristic associated with an operation of the utility power
network.
[011] Embodiments of this aspect of the invention may include one or more of
the
following features.
[012] The control system may determine the occurrence of a low voltage event
based on a voltage condition associated with the utility power network, or
alternatively, on a current condition associated with the wind turbine
generator, or a
combination of both of these methods.
[013] The first current includes a reactive current component sufficient for
maintaining an excitation of the wind turbine generator. The second current
includes
a real current component and a reactive current component. During the low
voltage
event, the second current is controlled so that the magnitude of the reactive
current
component is at least twice the magnitude of the real current component.
[014] The first path includes a switch unit controllable by external signals,
and may
further include a forced commutation circuit configured to provide a
commutation
signal to the switch unit. The second path includes a first power converter
for
converting AC power from the wind turbine generator to DC power and for
providing
the first current. A second power converter is connected in series with the
first
converter for converting the DC power from the first power converter to line-
side AC
power and for providing the second current. A power dissipation element is
coupled
to the first and second power converter for dissipating power from the first
power
converter. The power dissipation element may include a resistor and a
controllable
switching device coupled to the resistor configured for regulating a current
passing
through the resistor. A capacitor is coupled to the first and second power
converter.
[015] The control system may further control a power factor correction unit to
adjust
a power factor of the electric power supplied to the utility power network.
The power
factor correction unit may include a controllable capacitor that can be
switched on and
off by electrical signals.
[016] Among other advantages and features, a system for connecting a wind
turbine
generator to a utility power network is provided. During normal operations,
electric
power generated by the WTG can be delivered to the utility power network with
near
unity power factor and negligible power loss in the LVRT system (e.g., less
than
0.3%). When faults on the network cause line voltage to drop, the system
maintains
near nominal voltages at generator terminals and provides sufficient impedance
to the
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generator. As a result, the WTG continues to operate without experiencing low-
voltage impacts (e.g., over-speeding). The amounts of real and reactive power
delivered to the network can also be controlled based on voltage conditions.
For
example, when desired, reactive power can be injected to the grid in
sufficient
amounts (e.g., at least twice the amount of real power) to help stabilize the
utility
network in a major low voltage event. In some cases, proper selection of power
electronics and circuit design can also reduce system response time to faults.
[017] Other features and advantages of the invention are apparent from the
following description, and from the claims.
BRIEF DESCRIPTIONS OF DRAWINGS
[018] FIGs. 1A and lB illustrate some aspects of LVRT requirements in the
Spanish
Grid Code.
[019] FIGs. 2A and 2B provide an overview and an exemplary implementation of a
wind power generation system with LVRT capability, respectively.
[020] FIG. 3 is a flow chart illustrating a control scheme of the wind power
generation system.
[021] FIGs. 4A to 4D are examples of steady-state and transient operations of
one
implementation of the wind power generation system.
DETAILED DESCRIPTIONS
1 System Overview
[022] Referring to FIG. 2A, a wind power generation system 200 with LVRT
capability includes a rotor 202 (e.g., a low speed propeller) which drives a
wind
turbine generator 204 for converting wind power to electric power in the form
of
alternating current (AC). Through an interconnection system 208, AC power is
provided to a transformer 242, which by stepping up the AC voltage, transmits
the
power to a local grid 244.
[023] The interconnection system 208 includes a switch unit 210 and a back-to-
back
conversion unit 220, which provide a first and second paths 211 and 221
respectively,
between the generator 204 and the transformer 242. Generally, switch unit 210
can be
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electrically turned "ON" (closed) or "OFF" (open) by external signals (e.g.,
control
signals) to allow or block current passage in first path 211. Switch unit 210
can be a
single power electronic switch (e.g., a thyristor), or a circuit that
functions essentially
as an electric switch having at least two states of distinct impedance.
Preferably,
when gated "ON," switch unit 210 presents negligible impedance to the current
generated by the generator 204, thereby minimizing potential power loss during
transmission.
[024] When the grid is operating under normal conditions (e.g., voltage
fluctuation
remains within 10% of nominal), switch unit 210 is closed, allowing power
from the
generator to be transmitted via first path 211 to transformer 236 in full
capacity.
When a low voltage event occurs (e.g., grid voltage drops below 90% of
nominal),
switch unit 210 is quickly opened to block first path 211. Subsequently, the
full
output of the generator is delivered through second path 221 to back-to-back
conversion unit 220. When the grid voltage drops significantly to, for
example, one-
fifth its nominal value (i.e. 20%), five times nominal current will flow for
the grid to
absorb the pre-sag power generated by the WTG. To prevent components in the
WTG system from being overloaded during the low voltage event, back-to-back
conversion unit 220 provides power in controlled amounts based on voltage
conditions. Preferably, back-to-back conversion unit 220 also provides
reactive
current necessary to excite generator 204 so that the generator continues to
operate
and generate power without being affected by the voltage drop. Other functions
of
the back-to-back conversion unit include a means to absorb or dissipate the
excess
power from the WTG that cannot be absorbed by the grid and, optionally,
provide
reactive current to the grid to aid in post-fault voltage recovery, which is
described in
greater detail below.
[025] In some examples, a master controller 270 is provided in interconnection
system 208 to control power transmission between the generator and the grid.
Preferably, master controller 270 is able to detect low voltage faults (as
will be
described in greater detail below) and act upon these faults to coordinate and
control
the operations of the switch and conversion units 210 and 220 to provide LVRT
features of this power generation system. The implementation and logic of
master
controller 270 will be described in greater detail in the context of an
exemplary
interconnection system provided below.
2 An Example of an Interconnection System
[026] Referring to FIG. 2B, an exemplary implementation of the interconnection
system 208 shown in FIG. 2A is provided. Each of switch unit 210, back-to-back
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conversion unit 220, master controller 270, and an optional power factor
correction
unit 234 is described in the following sections.
2.1 Switch Unit
[027] Switch unit 210 includes a static switch 212 consisting of two
controllable
semiconductor switching devices, here, thyristors 212a and 212b. When closed,
the
pair of thyristors conducts AC current in alternative half-cycles, allowing
the full
output of the generator through the first path 211 with near zero voltage
drop.
Preferably, thyristors 212a and 212b are selected to be "over-sized" (i.e.,
current
ratings higher than required) to minimize on-state power consumption.
[028] Under low voltage conditions, to open the static switch 212, normally
waiting
for the current to be zero ("zero cross over") is needed to set thyristors in
their off-
state. If static switch 212 were to naturally commutate off due to an AC line
current
(e.g., a 50Hz output of the generator), a time delay of up to lOms may occur
before
the 50Hz current reaches a zero crossover. This time delay can be
disadvantageous
for a wind power generation system that is designed to adjust quickly to fault
conditions (e.g., on the order of milliseconds). Therefore, a means of forced
commutation of the thyristors is provided. In one method, switch unit 210 has
a built-
in forced commutation circuit 214 to which the thyristors are connected. When
a
control signal is received by the forced commutation circuit 214, the
commutation
circuit generates a current pulse of sufficient magnitude with a polarity that
generates
a zero crossover of current with the thyristors. By forced commutation, the
static
switch 212 can be quickly turned off to help reduce system response time and
improve transient performance.
[029] Alternatively, the back-to-back converters 222 and 224 can be controlled
to
also generate a commutation current pulse in the static switch thyristors 212.
2.2 Back-to-back Conversion Unit
[030] Back-to-back conversion unit 220 includes a generator-side AC/DC
converter
222 and a line-side DC/AC converter 224 connected in series via a DC bus 225.
Also
coupled to DC bus 225 are one or multiple DC bus capacitors 226 which supports
a
DC bus voltage Vdc, and a power dissipation device 228 capable of dissipating
real
power. Power dissipation device 228 can include for example, a resistor (e.g.,
a
dynamic braking resistor) to dissipate real power, and a controllable
switching device
that controls the amount of current passing through the resistor. In some
examples,
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AC filter reactors (not shown) for reducing undesired harmonics and distortion
in AC
signals are also provided on both the generator and line sides.
[031] When the grid voltage drops, the amount of real power that can be safely
transferred to the grid without overloading WTG components decreases. With
generator 204 continuing to operate, back-to-back conversion unit 220 receives
the
full output power of the generator while passing only a safe amount of power
onto the
grid. Real power in excess of the safe amount is dissipated by power
dissipation
device 228. As a result, power generation system 200 can ride through severe
voltage
drops without either 1) sending large amounts of current through transformer
242
(which may potentially damage the transformer and trip the turbine generator
on over-
current); or 2) increasing the speed of the turbine generator (which may
potentially
trip the generator on over-speed).
[032] In some situations where it is desired to feed reactive power to utility
grid 244
to help stabilize the grid at fault, line-side converter 224 is configured to
provide not
only real power but also reactive power in controlled amounts (e.g., reactive
power at
least twice as much as real power) to grid 244. The exact ratio of reactive to
real
power may be arbitrarily set or imposed by applicable grid interconnection
requirements (e.g. the Spanish Grid Code). For some wind turbine generators
(e.g.,
induction generators) that require reactive power to establish and sustain
their electric
and magnetic fields, generator-side converter 222 also provides reactive
current
necessary to keep the generators excited and operating at constant speed
during low
voltage events while simultaneously absorbing the real power output of the
generator.
[033] For some wind turbine generators (e.g., induction generators) that
require
reactive power to establish and sustain their electric and magnetic fields,
generator-
side converter 222 also provides reactive current necessary to keep the
generators
excited and operating at constant speed during low voltage events while
simultaneously absorbing the real power output of the generator. In these type
of
generators, without the reactive current being applied under low voltage
conditions,
the generator sees reduced torque and begins to accelerate rapidly, which can
damage
the WTG.
2.3 Power Factor Correction Unit
[034] In some wind power generation systems, a power factor correction unit
234 is
optionally coupled to line-side terminal 232 for improving the power factor
(PF) of
the electricity delivered to utility grids. Generally, in an AC system where
both real
power and reactive power are present, PF is a dimensionless number between 0
and 1,
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representing the ratio of real power to total power (also referred to as
apparent
power). A power factor of zero indicates that energy flow in the circuit is
entirely
reactive and stored energy in the load returns to the source on each cycle,
whereas a
power factor of unity indicates that energy flow is entirely real and thus uni-
directional from source to load. Under normal conditions, it is generally
desirable to
operate power generation systems at near unity power factor to provide high
efficiency power to utility grids.
[035] In wind power generation system 200, power factor correction unit 232
includes a group of capacitors that can be individually switched on and off by
means
of contactors (e.g., electrically controlled switches). During normal
operation, these
capacitors provide reactive power in adjustable amounts (e.g., depending on
the
number of capacitors switched on) to help achieve near unity power factor
(e.g.,
above 0.9) at grid connection points. This power factor correction unit 232
may be
provided as part of an existing wind turbine system, the interconnection
system 208,
or a combination of both.
2.4 Master Controller
[036] Master controller 270 is coupled to each of static switch 212, forced
commutation circuit 214, back-to-back conversion unit 220, and possibly other
components in interconnection system 208. Master controller 270 oversees
system
operation and controls power transmission between the generator and the grid
based
on various grid conditions.
[037] Referring to FIG. 3, the logic and functions of master controller 270
are
briefly illustrated in a flow chart 300. Generally, the master controller uses
feedback
signals from multiple sensors (e.g., line-side and generator-side
current/voltage
sensors) to monitor current/voltage dynamics for determining system states. If
the
system is operating in steady state (step 310) - that is, grid voltage appears
within
10% of nominal, the master controller functions to maintain the on-state of
the
switch unit 210 and disable/disconnect the back-to-back conversion unit 220.
As a
result, power is transmitted to the transformer only through first path 211.
[038] Line faults, including both unbalanced faults and sudden voltage drops,
can be
detected by the master controller upon sensing voltage or current anomalies.
In many
systems, a line fault is often followed by generator current exceeding a
preset
instantaneous level (e.g., 120% of nominal depending on system configuration),
or
line voltage falling below a preset threshold (e.g., 90% of nominal). In
detecting
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either event, the master controller immediately turns off the switch unit
(step 330) and
initiates a low voltage ride through (step 340).
[039] One way to turn off switch unit 210 is to command forced commutation
circuit 214 to generate a defined width commutation current pulse to commutate
off
the thyristor (212a or 212b) that is in its conducting state. Pulse polarity
can be
determined as a function of generator current polarity. An alternative way to
turn off
the switch unit uses the generator-side and/or line-side converter. Current is
injected
by the converter in reverse direction to the existing current in thyristors,
thereby
creating zero current crossover that biases the thyristors off-state. In some
systems,
having a converter on each side of the switch unit helps offset source
impedance
effects that often contribute to the delay in thyristors' response time (i.e.
line
impedance limiting the rate of change in the commutation current). This
commutation process can occur simultaneously on all three phases of the LVRT
system regardless of how many line phases are faulted.
[040] Once switch unit 210 is off, master controller 270 controls the
operation of
back-to-back conversion unit 220 to provide LVRT capability. Here, the desired
output of conversion unit 220 may vary depending on system design in
compliance
with specific grid connection standards. For example, to meet the requirements
in the
Spanish Grid Code, master controller 270 regulates conversion unit 220 so that
1)
generator-side converter 222 receives generator power and provides reactive
power to
keep the generator excited and rotating at constant speed; and 2) line-side
converter
224 supplies a safe amount of real power to grid 244 and injects sufficient
reactive
power to help stabilize the grid. Generator power in excess of the amount that
can be
safely absorbed by grid 244 is dissipated by power dissipation device 228,
which
consumes power in response to a regulated DC bus voltage, or can be controlled
directly by matching the power dissipated to the excess generator power.
Optionally,
master controller 270 also controls power factor correction unit 234 to
provide
reactive power in suitable amounts for improving power factor at gird
connection
points.
[041] After fault clearance, grid voltage begins to recover. When detected
line
voltage has restored nearly to its pre-fault level (e.g., above 90%),
generator side
converter 222 synchronizes the generator voltage and phase to that of the grid
(step
360) and switch unit 210 is quickly turned on (step 370). With back-to-back
conversion unit 220 disconnected again from the generator, the system returns
to
steady state operation (step 310), feeding generator power through the first
path 211
to the transformer 242.
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3 Examples of Steady-state and Transient Operations
[042] FIGs. 4A to 4D further illustrate how an interconnection system operates
to
provide satisfactory electric power to utility grids in ways that conform to
the Spanish
Grid Code. Circuit performance during each of several stages, including a
steady
state and multiple transient states following a low voltage event, is
described in detail
below.
[043] Referring to FIG. 4A, wind power generation system 200 is operating in
steady state with line-side voltage at nearly 100% of rated level. In this
case, 706 kW
of real power produced by turbine generator 204 is delivered entirely through
switch
unit 212 to transformer 242, with less than 0.3% of energy loss. No power
passes
through generator-side converter 222, line-side converter 224, or the power
dissipation device (e.g., a resistor 227). The power factor correction unit
(e.g., a set of
capacitors 233) provides about 250 kVAR of reactive power to excite the wind
turbine generator 204. With zero net reactive output at terminal 236,
electricity is
being provided to the grid at a power factor of unity.
[044] Referring now to FIG. 4B, when a grid failure causes line-side voltage
to drop
to 20% of rated levels, the switch unit 212 is quickly turned off (e.g., by
forced
commutation) to disconnect generator 204 from AC line 232. Subsequently,
generator-side converter 222 is controlled to absorb real power from generator
204 to
prevent the turbine from storing power and overspeeding. As converter 222 now
provides the reactive exciting current (which was formerly supplied by the
power
correction unit 233, the utility, or a combination of both), the generator
continues to
be excited. At the same time, generator-side voltage is maintained by the
converter
222 at near rated level (although line voltage has fell to 20%).
[045] Once real power starts flowing into generator-side converter 222 and
onto DC
bus 225, the DC bus voltage begins to rise. In response, line-side converter
224 starts
to operate to supply both real and reactive current to AC line 232. In this
example,
the amount of real and reactive current transferred by line-side converter 224
is
controlled such that the reactive power is twice the real power (e.g., 134kVAR
and
67kW, respectively) and the total current does not significantly exceed the
current
rating of the turbine transformer 242. Since only a small portion of the
generated
power (67kW out of 706kW) is transferred to the AC line 232, energy builds up
on
DC bus 225. This excess power (about 639kW) is dissipated in resistor 227, for
example, by modulating the duty cycle of a switching device 229 to which the
resistor
227 is coupled.
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[046] At AC line 232, the net output of line-side converter 224 includes 280A
of real
current and 560A of reactive current. Together with the diminished reactive
current
provided by the power factor correction unit 233 (at 20% of line voltage, the
correction unit provides 20% of rated current), the total current supplied to
the
transformer 242 is 663A. This amount of total current represents only 106% of
transformer rating (well within transformer capability), with an
Ireactive/Itotal ratio of
0.907.
[047] Referring now to FIG. 4C, as line voltage starts to recover from the
fault, the
amount of real power that can be transferred to the grid increases, and the
amount of
power to be dissipated in resistor 227 drops. For example, with line voltage
at 60% of
pre-fault level, line-side converter 224 transfers about 201 kW of real power
to AC
line 232, and the power dissipated by resistor 227 is reduced to 505kW.
Including the
contribution of the power factor correction unit 233, the net current provided
to the
transformer now increases to 740A (i.e., 118% of transformer current rating).
The
IreaetivelItotal ratio of the net current is 0.926.
[048] Referring now to FIG. 4D, once the line-side voltage recovers to near
rated
level (e.g., above 95%), the switch unit 212 is turned on after the generator
side
converter synchronizes the generator voltage and phase to that of the grid. As
real
power from the turbine resumes flowing through switch unit 212 to AC line 232,
both
generator-side converter 222 and line-side converter 224 cease to transfer
real power.
Resistor 227 no longer dissipates power. Subsequently, interconnection system
208
returns to operate in steady state (as previously shown in FIG. 4A). In some
cases,
before returning to steady-state, line-side converter 224 may continue to
supply
reactive current for an extended period (e.g., 150ms) unless line voltage
exceeds a
predetermined level (e.g., 110% of nominal). Preferably, this additional
supply of
reactive current provides post-fault voltage support that may be desired in
some
systems following a major low voltage event.
[049] In this application, although some examples are provided primarily in
the
context of a system designed to satisfy the Spanish Grid Code, the approach
described
above can be generally applied in many power generation systems to provide
steady-
state and transient fault behaviors that satisfy the requirements of one or
multiple grid
interconnection standards. In addition to providing low-voltage ride through,
the
interconnection systems described in FIGs. 2A and 2B may also be modified to
allow
wind turbine generators to continue to operate and supply electricity to grid
under
other fault conditions. Moreover, the power electronics used in these systems
can be
conveniently coupled to a wide variety of wind turbine generators (e.g.,
Squirrel Cage
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Induction Generators, Doubly Fed Induction Generators, and Synchronous
Generators) operating in either constant speed or variable speed modes.
[050] There can be many alternatives to the thyristors used in the static
switch. For
instance, thyristors capable of switching off by gate control (instead of zero
current)
can be coupled in use with the master controller that is configured to provide
such
gate control signals. Examples of gate control thyristors include Gate Turn-
Off
thyristors (GTOs) and Integrated Gate-Commutated Thyristors (IGCTs). There are
also non-thyristor solid-state devices (e.g., transistors) that could be used
for the static
switch.
[051] Line faults may be detected by the master controller upon sensing
generator
current exceeding a preset instantaneous level, or line voltage falling below
a preset
threshold. Alternatively, the master control may monitor a rate of change of
line
voltage and/or current together with absolute thresholds as a means of
detecting a sag
event.
[052] In the event of small voltage sags, it is also possible to leave the
static switch
closed while commanding one or both of the converters to output capacitive
reactive
power. This capacitive reactance may interact with existing transformer and
source
impedance to help achieve a voltage boost.
[053] In some applications, during low voltage events, the line-side converter
is
controlled to provide power compensation by outputting reactive current that
is twice
the amplitude of real current. In some other applications, line-side converter
may
instead output zero real current while providing capacitive reactive current
of an
arbitrary amount (up to the overload limit of the converter). In addition,
both line-
side and generator-side converters may operate in an "overload" mode to reduce
cost.
Operating converters in so-called "overload" mode is described in U.S.
6,577,108,
which issued on June 10, 2003 and whose disclosure is incorporated herein by
reference.
[054] During normal WTG operations, one or both of the converters may be
turned
on to provide additional power-factor correction. For example, in cases where
wind
turbine power factor correction units are only capable of improving PF to 0.95
lagging
(inductive), this additional PF correction from converter(s) can potentially
boost the
PF to 1.0, or even to a leading (capacitive) PF when desired.
[055] It is to be understood that the foregoing description is intended to
illustrate and
not to limit the scope of the invention, which is defined by the scope of the
appended
claims. Other embodiments are within the scope of the following claims.
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