The subject matter described herein relates generally to
controlling operation of power generation and delivery systems, and more
specifically, to stabilizing a power converter after an electrical grid contingency
event.
Wind turbine generators utilize wind energy to produce
electrical power. Wind tiu-bine generators typically include a rotor having multiple
blades that transform wind energy into rotational motion of a drive shaft, which in
turn is utilized to drive an electrical generator to produce electrical power. Each of
the multiple blades may be pitched to increase or decrease the rotational speed of the
rotor. A power output of a wind turbine generator increases with wind speed imtil the
wind speed reaches a rated wind speed for the turbine. At and above the rated wind
speed, the wind turbine generator operates at a rated power.
Variable speed operation of the wind turbine generator
facilitates enhanced capture of energy by the wind turbine generator when compared
to a constant speed operation of the wind turbine generator. However, variable speed
operation of the wind turbine generator produces electricity having varying voltage
and/or firequency. More specifically, the frequency of the electricity generated by the
variable speed wind turbine generator is proportional to the speed of rotation of the
rotor. A power converter may be coupled between the electric generator and an
electrical grid. The power converter outputs electricity having a fixed voltage and
firequency for delivery on the electrical grid.
Power generated by an electric utility, using renewable
sources of energy or fossil fuel based sources of energy, is typically delivered to a
customer over an electrical grid. Electricity applied to the electrical grid is required to
meet grid connectivity expectations. These requirements address safety issues as well
as power quality concerns. For example, the grid connectivity expectations include
operating the power generation system during a transient event, also referred to herein
as a grid fault event and/or a grid contingency event. This capability may be referred
to as low voltage ride through (LVRT) or zero voltage ride through (ZVRT). An
LVRT/ZVRT event is a condition where the alternating current (AC) utility voltage is
low on either one phase of the electrical grid or miUtiple phases of the electrical grid.
During an LVRT/ZVRT event, the capacity of the electrical grid to accept power firom
the power generation system is low. Following switching actions in the external grid,
the impedance of the grid may increase substantially leading to a condition referred to
herein as a "weak grid".
Operation of the power converter is controlled to facilitate
LVRT/ZVRT. Once the LVRT/ZVRT event dissipates, the power converter is
controlled to facilitate recovery fi-om the event and return the power generation
system to steady-state operation. During the recovery, system oscillations may cause
instability, for example, instability in a power output by the power converter.
BRIEF DESCRIPTION OF THE INVENTION
In one aspect, a stabilizer system associated with a power
converter controller is provided. The stabilizer system includes a regulator stabilizer
configured to receive a phase locked loop (PLL) error signal and to generate a
regulator stabilization signal based at least partially on the PLL error signal. The
stabilizer system also includes a regulator coupled to the regulator stabilizer and a
converter interface controller. The regulator is configured to receive the regulator
stabilization signal, generate a first command signal, based at least partially on the
regulator stabilization signal, that reduces system oscillations, and transmit the first
command signal to the converter interface controller.
In another aspect, a converter controller for controlling
operation of a power converter is provided. The converter controller includes a
stabilizer system configured to receive a phase locked loop (PLL) error signal and
generate a first command signal, based at least partially on the PLL error signal, that
reduces system oscillations. The converter controller also includes a converter
interface controller communicatively coupled to the stabilizer system and configured
to generate control signals based at least partially on the first command signal and
transmit the control signals to a power conversion assembly.
In yet another aspect, a method for controlling a power
generation and delivery system that includes an electrical generator, a power
converter, and a controller is provided. The method includes monitoring an output
parameter of the power generation and delivery system indicative of system
oscillations. The method also includes generating, using the controller, a command
signal based at least partially on the output parameter. The method also includes
controlling operation of the power converter based at least partially on the command
signal to reduce system oscillations.
BRffiF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of an exemplary power generation
system.
Figure 2 is a perspective view of a portion of an exemplary
wind turbine that may be used in the power generation system shown in Figure 1.
Figure 3 is a partially cut-away view of a portion of the wind
turbine shown in Figure 2.
Figure 4 is a block diagram of the wind turbine shown in
Figure 2.
Figure 5 is a block diagram of an exemplary power generation
and delivery system that may include the Avind turbine shown in Figure 2.
Figure 6 is a block diagram of an exemplary converter control
system that may be included within the power generation and delivery system shown
in Figure 5.
Figure 7 is a block diagram of an exemplary stabilizer system
that may be included within the converter control system shown in Figure 6.
Figure 8 is a block diagram of an alternative converter control
system that may be included within the power generation and delivery system shown
in Figure 5.
Figures 9-18 are graphical views illustrating operation of a
power generation and delivery system after a grid contingency event.
Figure 19 is a flow chart of an exemplary method for
controlling the power generation and delivery system shown in Figure 5.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term "blade" is intended to be
representative of any device that provides reactive force when in motion relative to a
surrounding fluid. As used herein, the term "wind turbine" is intended to be
representative of any device that generates rotational energy from wind energy, and
more specifically, converts kinetic energy of wind into mechanical energy. As used
herein, the term "wind turbine generator" is intended to be representative of any wind
turbine that generates electrical power from rotational energy generated from wind
energy, and more specifically, converts mechanical energy converted from kinetic
energy of wind to electrical power.
Technical effects of the methods, systems, and computerreadable
media described herein include at least one of: (a) monitoring an output
parameter of the power generation and delivery system, wherein oscillations within
the output parameter correspond to system oscillations; (b) generating a command
signal based at least partially on the output parameter; and, (c) controlling operation
of the power converter based at least partially on the command signal to reduce
system oscillations.
The methods, systems, and computer readable media
described herein facilitate reducing system oscillations that may occur during
recovery from a grid contingency event. As described herein, a voltage regulator
stabilizer generates a voltage regulator stabilization signal based at least partially on a
measured PLL error. The voltage regulator stabilization signal is provided to a
voltage regulator that determines a reactive current command based at least partially
on the voltage regulator stabilization signal. Furthermore, a power regulator stabilizer
may generate a power regulator stabilization signal based at least partially on the
measured PLL error. The power regulator stabilization signal is provided to a power
regulator that determines a real current command based at least partially on the power
regulator stabilization signal. Controlling the reactive current output and/or the real
current output of the power converter as a function of the PLL error facilitates
reducing system oscillations that may occur during recovery from a grid contingency
event. Furthermore, reducing system oscillations stabilizes the power generation
system and utility grid. Although generally described herein with respect to a wind
turbine, the methods and systems described herein are applicable to any type of
electric generation system including, for example, solar power generation systems,
fiiel cells, geothermal generators, hydropower generators, and/or other devices that
generate power from renewable and/or non-renewable energy sources.
Figure 1 is a block diagram of an exemplary power generation
system 10 that includes a power generator 12. Power generator 12 includes one or
more power generation units 14. Power generation units 14 may include, for
example, wind turbines, solar cells, fiiel cells, geothermal generators, hydropower
generators, and/or other devices that generate power from renewable and/or nonrenewable
energy sources. Although three power generation units 14 are shown in the
exemplary embodiment, in other embodiments, power generator 12 may include any
suitable number of power generation units 14, including only one power generation
unit 14.
In the exemplary embodiment, power generator 12 is coupled
to a power converter 16, or a power converter system 16, that converts a substantially
direct current (DC) power output from power generator 12 to alternating current (AC)
power. The AC power is transmitted to an electrical distribution network 18, or
"grid." Power converter 16, in the exemplary embodiment, adjusts an amplitude of
the voltage and/or current of the converted AC power to an amplitude suitable for
electrical distribution network 18, and provides AC power at a frequency and a phase
that are substantially equal to the frequency and phase of electrical distribution
network 18. Moreover, in the exemplary embodiment, power converter 16 provides
three phase AC power to electrical distribution network 18. Alternatively, power
converter 16 provides single phase AC power or any other number of phases of AC
power to electrical distribution network 18. Furthermore, in some embodiments,
power generation system 10 may include more than one power converter 16. For
example, in some embodiments, each power generation unit may be coupled to a
separate power converter 16.
In an exemplary embodiment, power generation units 14
include solar panels coupled to form one or more solar arrays to facilitate operating
power generation system 10 at a desired power output. Each power generation unit
14 may be an individual solar panel or an array of solar panels. In one embodiment,
power generation system 10 includes a plurality of solar panels and/or solar arrays
coupled together in a series-parallel configuration to facilitate generating a desired
current and/or voltage output from power generation system 10. Solar panels include,
in one embodiment, one or more of a photovoltaic panel, a solar thermal collector, or
any other device that converts solar energy to electrical energy. In the exemplary
embodiment, each solar panel is a photovoltaic panel that generates a substantially
direct current power as a result of solar energy striking solar panels. In the exemplary
embodiment, the solar array is coupled to power converter 16, or power converter
system 16, that converts the DC power to alternating current power that is transmitted
to electrical distribution network 18.
In other embodiments, power generation units 14 include one
or more wind turbines coupled to facilitate operating power generation system 10 at a
desired power output. Each wind turbine generates substantially direct current power.
The wind turbines are coupled to power converter 16, or power converter system 16,
that converts the DC power to AC power that is transmitted to an electrical
distribution network 18, or "grid." Methods and systems will be further described
herein with reference to such a wind turbine based power generation system.
However, the methods and systems described herein are applicable to any type of
electric generation system including, for example, fuel cells, geothermal generators,
hydropower generators, and/or other devices that generate power from renewable
and/or non-renewable energy sources.
Figure 2 is a perspective view of an exemplary wind turbine
20 that may be used in power generation system 10. Figure 3 is a partially cut-away
perspective view of a portion of wind turbine 20. Wind turbine 20 described and
shown herein is a wind turbine generator for generating electrical power fi"om wind
energy. Moreover, wind turbine 20 described and illustrated herein includes a
horizontal-axis configuration. However, in some embodiments, wind turbine 20 may
include, in addition or alternative to the horizontal-axis configuration, a vertical-axis
configuration (not shown). Wind turbine 20 may be coupled to an electrical load (not
shown in Figure 2), such as, but not limited to, a power grid, for receiving electrical
power therefrom to drive operation of wind turbine 20 and/or its associated
components and/or for supplying electrical power generated by wind turbine 20
thereto. Although only one wind turbine 20 is shown in Figures 2 and 3, in some
embodiments, a plurality of wind turbines 20 may be grouped together, sometimes
referred to as a "wind farm."
Wind turbine 20 includes a body or nacelle 22 and a rotor
(generally designated by 24) coupled to nacelle 22 for rotation with respect to nacelle
22 about an axis of rotation 26. In the exemplary embodiment, nacelle 22 is mounted
on a tower 28. However, in some embodiments, in addition or alternative to towermounted
nacelle 22, nacelle 22 may be positioned adjacent the ground and/or a
surface of water. The height of tower 28 may be any suitable height enabling wind
turbine 20 to function as described herein. Rotor 24 includes a hub 30 and a plurality
of blades 32 (sometimes referred to as "airfoils") extending radially outwardly firom
hub 30 for converting wind energy into rotational energy. Although rotor 24 is
described and illustrated herein as having three blades 32, rotor 24 may have any
number of blades 32. Blades 32 may each have any length that allows wind turbine
20 to fimction as described herein. For example, in some embodiments, one or more
rotor blades 32 are about one-half meter long, while in some embodiments one or
more rotor blades 32 are about fifty meters long. Other examples of blade 32 lengths
include ten meters or less, about twenty meters, about thirty-seven meters, and about
forty meters. Still other examples include rotor blades between about fifty and about
one-hundred meters long, and rotor blades greater than one-hundred meters long.
Despite how rotor blades 32 are illustrated in Figure 2, rotor
24 may have blades 32 of any shape, and may have blades 32 of any type and/or any
configuration, whether such shape, type, and/or configuration is described and/or
illustrated herein. One example of another type, shape, and/or configuration of blades
32 is a Darrieus wind turbine, sometimes referred to as an "eggbeater" turbine. Yet
another example of another type, shape, and/or configuration of blades 32 is a
Savonious wind turbine. Moreover, wind turbine 20 may, in some embodiments, be a
wind turbine wherein rotor 24 generally faces upwind to harness wind energy, and/or
may be a wind turbine wherein rotor 24 generally faces downwind to harness energy.
Of course, in any of the embodiments, rotor 24 may not face exactly upwind and/or
downwind, but may face generally at any angle (which may be variable) with respect
to a direction of the wind to harness energy therefirom.
Referring now to Figure 3, wind turbine 20 includes an
electrical generator 34 coupled to rotor 24 for generating electrical power from the
rotational energy generated by rotor 24. Generator 34 may be any suitable type of
electrical generator, such as, but not limited to, a wound rotor induction generator, a
double-fed induction generator (DFIG, also known as dual-fed asynchronous
generators), a permanent magnet (PM) synchronous generator, an electrically-excited
synchronous generator, and a switched reluctance generator. Generator 34 includes a
stator (not shown) and a rotor (not shown) with an air gap included therebetween.
Rotor 24 includes a rotor shaft 36 coupled to rotor hub 30 for rotation therewith.
Generator 34 is coupled to rotor shaft 36 such that rotation of rotor shaft 36 drives
rotation of the generator rotor, and therefore operation of generator 34. In the
exemplary embodiment, the generator rotor has a generator shaft 38 coupled thereto
and coupled to rotor shaft 36 such that rotation of rotor shaft 36 drives rotation of the
generator rotor. In other embodiments, the generator rotor is directly coupled to rotor
shaft 36, sometimes referred to as a "direct-drive wind turbine." In the exemplary
embodiment, generator shaft 38 is coupled to rotor shaft 36 through a gearbox 40,
although in other embodiments generator shaft 38 is coupled directly to rotor shaft 36.
The torque of rotor 24 drives the generator rotor to thereby
generate variable frequency AC electrical power from rotation of rotor 24. Generator
34 has an air gap torque between the generator rotor and stator that opposes the torque
of rotor 24. A power conversion assembly 42 is coupled to generator 34 for
converting the variable frequency AC to a fixed frequency AC for delivery to an
electrical load (not shown in Figure 3), such as, but not limited to an electrical grid
(not shown in Figure 3), coupled to generator 34. Power conversion assembly 42 may
include a single frequency converter or a plurality of frequency converters configured
to convert electricity generated by generator 34 to electricity suitable for delivery over
the power grid. Power conversion assembly 42 may also be referred to herein as a
power converter. Power conversion assembly 42 may be located anywhere within or
remote to wind turbine 20. For example, power conversion assembly 42 may be
located within a base (not shown) of tower 28.
In the exemplary embodiment, wind turbine 20 includes at
least one system controller 44 coupled to at least one component of wind turbine 20
for generally controlling operation of wind turbine 20 and/or controlling operation of
the components thereof. For example, system controller 44 may be configured to
control operation of power conversion assembly 42, a disk brake 46, a yaw system 48,
and/or a variable blade pitch system 50. Disk brake 46 brakes rotation of rotor 24 to,
for example, slow rotation of rotor 24, brake rotor 24 against fiiU wind torque, and/or
reduce the generation of electrical power from electrical generator 34. Yaw system
48 for rotating nacelle 22 about an axis of rotation 52 for changing a yaw of rotor 24,
10
and more specifically for changing a direction faced by rotor 24 to, for example,
adjust an angle between the direction faced by rotor 24 and a direction of wind.
Furthermore, variable blade pitch system 50 controls,
including but not limited to changing, a pitch angle of blades 32 (shown in Figures 2-
3) with respect to a wind direction. Pitch system 50 may be coupled to system
controller 44 for control thereby. Pitch system 50 is coupled to hub 30 and blades 32
for changing the pitch angle of blades 32 by rotating blades 32 with respect to hub 30.
The pitch actuators may include any suitable structure, configuration, arrangement,
means, and/or components, whether described and/or shown herein, such as, but not
limited to, electrical motors, hydraulic cylinders, springs, and/or servomechanisms.
Moreover, the pitch actuators may be driven by any suitable means, whether
described and/or shown herein, such as, but not limited to, hydraulic fiuid, electrical
power, electro-chemical power, and/or mechanical power, such as, but not limited to,
spring force.
Figure 4 is a block diagram of an exemplary embodiment of
wind turbine 20. In the exemplary embodiment, wind turbine 20 includes one or more
system controller 44 coupled to at least one component of wind turbine 20 for
generally controlling operation of wind turbine 20 and/or controlling operation of the
components thereof, regardless of whether such components are described and/or
shown herein. For example, in the exemplary embodiment system controller 44 is
coupled to pitch system 50 for generally controlling rotor 24. In the exemplary
embodiment, system controller 44 is mounted within nacelle 22 (shown in Figure 3),
however, additionally or alternatively, one or more system controller 44 may be
remote from nacelle 22 and/or other components of wind turbine 20. System
controllers 44 may be used for overall system monitoring and control including,
without limitation, pitch and speed regulation, high-speed shaft and yaw brake
application, yaw and pump motor application, and/or fault monitoring. Alternative
distributed or centralized control architectures may be used in some embodiments.
11
In an exemplary embodiment, wind turbine 20 includes a
plurality of sensors, for example, sensors 54, 56, and 58. Sensors 54, 56, and 58
measure a variety of parameters including, without limitation, operating conditions
and atmospheric conditions. Each sensor 54, 56, and 58 may be an individual sensor
or may include a plurality of sensors. Sensors 54, 56, and 58 may be any suitable
sensor having any suitable location within or remote to wind turbine 20 that allows
wind turbine 20 to function as described herein. In some embodiments, sensors 54,
56, and 58 are coupled to system controller 44 for transmitting measurements to
system controller 44 for processing thereof
In some embodiments, system controller 44 includes a bus 62
or other communications device to communicate information. One or more
processor(s) 64 are coupled to bus 62 to process information, including information
from sensors 54, 56, 58 and/or other sensor(s). Processor(s) 64 may include at least
one computer. As used herein, the term computer is not limited to integrated circuits
referred to in the art as a computer, but broadly refers to a processor, a
microcontroller, a microcomputer, a programmable logic controller (PLC), an
application specific integrated circuit, and other programmable circuits, and these
terms are used interchangeably herein.
System controller 44 may also include one or more random
access memories (RAM) 66 and/or other storage device(s) 68. RAM(s) 66 and
storage device(s) 68 are coupled to bus 62 to store and transfer information and
instructions to be executed by processor(s) 64. RAM(s) 66 (and/or storage device(s)
68, if included) can also be used to store temporary variables or other intermediate
information during execution of instructions by processor(s) 64. System controller 44
may also include one or more read only memories (ROM) 70 and/or other static
storage devices coupled to bus 62 to store and provide static (i.e., non-changing)
information and instructions to processor(s) 64. Processor(s) 64 process information
transmitted from a plurality of electrical and electronic devices that may include,
without limitation, speed and power transducers. Instructions that are executed
include, without limitation, resident conversion and/or comparator algorithms. The
12
execution of sequences of instructions is not limited to any specific combination of
hardware circuitry and software instructions.
System controller 44 may also include, or may be coupled to,
input/output device(s) 72. Input/output device(s) 72 may include any device known
in the art to provide input data to system controller 44 and/or to provide outputs, such
as, but not limited to, yaw control and/or pitch control outputs. Instructions may be
provided to RAM 66 from storage device 68 including, for example, a magnetic disk,
a read-only memory (ROM) integrated circuit, CD-ROM, and/or DVD, via a remote
connection that is either wired or wireless providing access to one or more
electronically-accessible media. In some embodiments, hard-wired circuitry can be
used in place of or in combination with software instructions. Thus, execution of
sequences of instructions is not limited to any specific combination of hardware
circuitry and software instructions, whether described and/or shown herein. Also, in
the exemplary embodiment, input/ou^ut device(s) 72 may include, without limitation,
computer peripherals associated with an operator interface such as a mouse and a
keyboard (neither shown in Figure 4). Alternatively, other computer peripherals may
also be used that may include, for example, a scanner (not shown in Figure 4).
Furthermore, in the exemplary embodiment, additional output channels may include,
for example, an operator interface monitor (not shown in Figure 4). System controller
44 may also include a sensor interface 74 that allows system controller 44 to
communicate with sensors 54, 56, 58 and/or other sensor(s). Sensor interface 74 may
include one or more analog-to-digital converters that convert analog signals into
digital signals that can be used by processor(s) 64.
In an exemplary embodiment, wind turbine 20 includes a
phase locked loop (PLL) regulator 80. PLL regulator 80 is coupled to sensor 56. In
the exemplary embodiment, sensor 56 is a voltage transducer configured to measure a
terminal grid voltage ou^ut by power conversion assembly 42. Alternatively, PLL
regulator 80 is configured to receive a plurality of voltage measurement signals from
a plurality of voltage transducers. In an example of a three-phase generator, each of
three voltage transducers is electrically coupled to each one of three phases of a grid
13
bus. PLL regulator 80 may be configured to receive any number of voltage
measurement signals from any number of voltage transducers that allow PLL
regulator 80 to function as described herein.
Figure 5 is a block diagram of an exemplary power generation
and delivery system 150. Power generation and delivery system 150 may be used
with, or included within, wind turbine 20 (shown in Figures 2 and 3). System 150
includes an energy source, for example, generator 34. Although described herein as
wind turbine generator 34, the energy source may include any type of electrical
generator that allows system 150 to fimction as described herein, e.g. a solar power
generation system. System 150 also includes a power converter, such as, power
conversion assembly 42. Power conversion assembly 42 receives electrical power
(Pv) 132 generated by generator 34 and converts electrical power 132 to an electrical
power (R) 134 (referred to herein as terminal power 134) suitable for transmission
over an electric power transmission and distribution grid 136 (referred to herein as
utility grid 136). A terminal voltage (Vt) 138 is defined at a node between power
conversion assembly 42 and utility grid 136. A bulk power system 140 is coupled to
utility grid 136. Bulk power system 140 includes a plurality of loads and/or power
sources.
In the exemplary embodiment, system 150 includes a griddependent
power limiter system 152. In the exemplary embodiment, a controller, for
example, but not limited to, system controller 44 (shown in Figure 4), is programmed
to perform the functions of grid-dependent power limiter system 152. However, in
alternative embodiments, the functions of grid-dependent power limiter system 152
may be performed by any circuitry configured to allow system 150 to function as
described herein. Power limiter system 152 is configured to identify the occurrence
of a grid contingency event, and provide power conversion assembly 42 with control
signals that facilitate providing a stable recovery from the grid event. Generally,
upon detection of a grid contingency event, power limiter system 152 provides signals
to reduce the power output of power conversion assembly 42. During recovery from
the grid contingency event, power limiter system 152 provides signals to increase the
14
active power output of power conversion assembly 42. In some embodiments, power
limiter system 152 provides a signal, or signals, to increase the active power output of
power conversion assembly 42 gradually imtil the output power of power conversion
assembly 42 is returned to its pre-fault level.
In the exemplary embodiment, system 150 also includes a
stabilizer system 182 (shown in Figure 6) configured to output a command signal that
is provided to converter interface controller 156 and used to control operation of
power converter 42. In the exemplary embodiment, a controller, for example, but not
limited to, system controller 44 (shown in Figure 4), is programmed to perform the
functions of stabilizer system 182. However, in alternative embodiments, the
functions of stabilizer system 182 may be performed by any circuitry configured to
allow system 150 to function as described herein. Oscillations within an output of
power converter 42 are reduced when power converter 42 is operated in accordance
with control signals from converter interface controller 156 that are based at least
partially on the command signal, for example, a reactive current command signal 192
and/or a real current command signal 166.
In the exemplary embodiment, stabilizer system 182 includes
a regulator 184 and a regulator stabilizer 186. In the exemplary embodiment,
regulator 184 is a voltage regulator configured to generate a reactive power command,
for example, reactive current command signal 192. Regulator stabilizer 186 is
configured to generate a regulator stabilization signal 188 that stabilizes regulator 184
as system 150 recovers from a grid contingency event. For example, regulator
stabilizer 186 may generate a voltage regulator stabilization signal and/or a power
regulator stabilization signal. In certain embodiments, power conversion assembly 42
responds according to the signals provided by stabilizer system 182 and reduces
system oscillations that may occur during recovery from the grid event.
A grid event, also referred to herein as a grid contingency
event, may leave utility grid 136 in a degraded mode where the grid impedance is
high. An example of a grid event includes a short-circuit fault on one of the
15
transmission lines within utility grid 136. Electrical transmission protection actions
remove the faulted portion of utility grid 136 to permit operation of the remaining
unfaulted portion of utility grid 136. A transmission path remains that is degraded in
its ability to transmit power from system 150 to bulk power system 140. Such grid
events cause a brief period of low voltage on utility grid 136 prior to clearing the
faulted portion of the utility grid 136. Typically, terminal voltage 138 will be
significantly degraded at the time of the grid event. The high grid impedance after the
fault clearing can result in an oscillatory response of the regulators within the
generator (e.g., power regulator 204 and/or voltage regulator 184). These oscillations
are typically in a frequency range of approximately 10 hertz (Hz) to 30 Hz, and in
some instances, can become imstable if not properly accounted for by system 150.
As shown in Figure 5, in the exemplary embodiment, power
conversion assembly 42 is configured to receive control signals 154 from a converter
interface controller 156. Control signals 154 are based on sensed operating conditions
or operating characteristics of wind turbine 20 as described herein and used to control
the operation of power conversion assembly 42. Examples of measured operating
conditions may include, but are not limited to, a terminal grid voltage, a PLL error, a
stator bus voltage, a rotor bus voltage, and/or a current. For example, sensor 56
(shown in Figure 4) measures terminal grid voltage 138 and transmits a terminal
voltage feedback signal 160 to a voltage regulator 184 and power limiter system 152.
Furthermore, PLL regulator 80 (shown in Figure 4) may generate a PLL error signal
190 and transmit signal 190 to stabilizer system 182 and power limiter system 152.
In the exemplary embodiment, voltage regulator stabilizer
186 generates, based at least partially on PLL error signal 190, voltage regulator
stabilization signal 188 and transmits voltage regulator stabilization signal 188 to
voltage regulator 184. Voltage regulator 184 generates reactive ciurent command
signal 192, based at least partially on voltage regulator stabilization signal 188 and
transmits reactive current command signal 192 to converter interface controller 156.
In some embodiments power limiter system 152 also receives terminal voltage
feedback signal 160 and generates a power command signal, for example, real current
16
command signal 166 based at least partially on PLL error signal 190 and terminal
voltage feedback signal 160. After a grid contingency event, PLL error signal 190
may oscillate as system 150 gradually increases an active power output of power
conversion assembly 42. In other words, oscillations within PLL error signal 190 are
indicative of system oscillations. Voltage regulator stabilizer 186 applies a transfer
function to the oscillating PLL error signal 190, which outputs voltage regulator
stabilization signal 188. This feedback loop is configured to reduce system
oscillations.
More specifically, system oscillations occurring after a grid
contingency event are identified by oscillations in PLL error signal 190, PLL error
signal 190 is provided to voltage regulator stabilizer 186, voltage regulator stabilizer
186 generates voltage regulator stabilization signal 188, and voltage regulator
stabilization signal 188 is provided to voltage regulator 184. Voltage regulator
stabilization signal 188 causes reactive current command signal 192 to oscillate in a
manner that reduces and/or cancels system oscillations. Voltage regulator 184
transmits reactive current command signal 192 to converter interface controller 156.
In an alternative embodiment, converter interface controller 156 is included within
system controller 44. Other operating condition feedback from other sensors also
may be used by controller 44 and/or converter interface controller 156 to control
power conversion assembly 42.
Figure 6 is a block diagram of an exemplary converter control
system 200 configured to generate control signals provided to a power converter, for
example, power conversion assembly 42 (shown in Figure 5), for control of power
conversion assembly 42. In the exemplary embodiment, converter control system 200
includes power limiter system 152, stabilizer system 182, and converter interface
controller 156. In the exemplary embodiment, power limiter system 152 includes a
power limiter 202 and a power regulator 204 and outputs a power command signal,
for example, real current command signal 166. In the exemplary embodiment, power
limiter 180 receives at least one measured operating condition of system 150. The at
least one measured operating condition may include, but is not limited to, a PLL error
17
signal 190 from PLL regulator 80 and terminal grid voltage feedback signal 160 from
sensor 54. Power limiter 180 also receives a stored reference power control signal
194 from, for example, system controller 44 (shown in Figure 3). In some
embodiments, power limiter 180 receives terminal grid voltage feedback signal 160
and stored reference power control signal 194, In other embodiments, power limiter
180 receives PLL error signal 190 and stored reference power control signal 194. In
other embodiments, power limiter 180 receives both PLL error signal 190 and
terminal grid voltage feedback signal 160, as well as stored reference power control
signal 194. In the exemplary embodiment, power limiter 180 generates a power
command signal 206 and transmits power command signal 206 to power regulator
204. Power regulator 204 generates real current command signal 166 and transmits
real current command signal 166 to converter interface controller 156. Converter
interface controller 156 may also be referred to herein as a converter firing control.
As described above, PLL regulator 80 may be included within system controller 44,
or may be coupled to, but separate from, system controller 44.
In the exemplary embodiment, PLL regulator 80 receives
terminal voltage feedback signal 160. For example, PLL regulator 80 may receive
terminal voltage feedback signal 160 (shown in Figure 3 as Vt) provided by sensor 54
(shown in Figure 3). As described above, PLL regulator 80 generates PLL error
signal 190 and a PLL phase angle signal 208. PLL phase angle signal 208 is
transmitted to converter interface controller 156 for control of power conversion
assembly 42 and for subsequent control of electrical currents injected onto utility grid
136 (shown in Figure 4).
In the exemplary embodiment, voltage regulator stabilizer
186 also receives PLL error signal 190. Furthermore, in the exemplary embodiment,
voltage regulator stabilizer 186 applies a predefined fransfer function to PLL error
signal 190 to generate voltage regulator stabilization signal 188. Voltage regulator
stabilization signal 188 is applied to voltage regulator 184, which combines signal
188 with voltage feedback signal 160 to generate reactive current command signal
192. Oscillations within PLL error signal 190 provide an indication of system
18
oscillations that may occur after a grid contingency event. More specifically,
oscillations within PLL error signal 190 correspond to system oscillations, for
example, oscillations within terminal voltage 138 (shown in Figure 5) and/or
oscillations within output power 134 (shown in Figure 5). Determining reactive
current command signal 192 based partially on PLL error signal 190 facilitates
reducing the system oscillations.
An example of the transfer ftinction applied by voltage
regulator stabilizer 186 isolates a frequency range within PLL error signal 190 that
includes an indication of regulator oscillations (e.g., a band pass filter between 10 Hz
and 30 Hz), and applies a gain selected to cause the regulator oscillations to be
positively damped. The transfer function may be determined based on, for example,
calculations, simulations, and/or testing where voltage regulator stabilizer 186 applies
various voltage regulator stabilization signals 188 to voltage regulator 184. The
transfer function may include linear components, for example, band pass filtering and
gain, and may also include any nonlinear components, for example, but not limited to,
limiters and dead bands, that allow system 150, in conjunction with electrical grid
136, to function as described herein. More specifically, voltage regulator stabilizer
186 may apply linear and/or nonlinear transfer functions to PLL error signal 190 to
generate a voltage regulator stabilization signal 188 that dampens system oscillations.
Voltage regulator 184 receives voltage regulator stabilization
signal 188 and generates reactive current command signal 192. Reactive current
command signal 192 is provided to converter interface controller 156, which controls
operation of power conversion assembly 42 in accordance with reactive current
command signal 192.
Figure 7 is a block diagram of an exemplary voltage
regulator, for example, voltage regulator 184 (shown in Figure 6) and an exemplary
voltage regulator stabilizer, for example voltage regulator stabilizer 186 (shown in
Figure 6). As described above with respect to Figure 6, in the event of a grid
contingency such as a weak grid, power output of conversion assembly 42 may
19
oscillate. Voltage regulator stabilizer 186 receives PLL error signal 190 and
generates voltage regulator stabilization signal 188. Voltage regulator 184 generates
reactive current command signal 192 based on voltage regulator stabilization signal
188 and voltage feedback signal 160 and sends reactive current command signal 192
to converter interface controller 156. Reactive current command signal 192 instructs
converter interface controller 156 to inject current onto utility grid 136 that includes a
reactive component configiu-ed to dampen power output oscillations. Dampening
power output oscillations increases the stability of grid 136 and power generation and
delivery system 150.
In the exemplary embodiment, voltage regulator 184 receives
voltage stabilizer signal 188 from voltage regulator stabilizer 186, receives terminal
voltage feedback signal 160, and receives a reference voltage command signal
(VREF) 240 from at least one volt-ampere reactive (VAR) regulator 242. VREF 240
is also referred to herein as a reference voltage. Upon detection of a grid contingency
event, power limiter system 152 transmits real current command signal 166 (shown in
Figure 6) to converter interface controller 156 to reduce the output power of power
conversion assembly 42. After the grid contingency event is completed, power limiter
system 152 generates signals, for example real current command signal 166, that
command a gradual increase in the power ou^ut of power conversion assembly 42.
During the grid contingency event, e.g., terminal voltage 138 indicates occurrence of
a grid contingency event, voltage regulator 184 generates a reactive current command
signal 192 that increases the reactive current output by power conversion assembly 42
to support terminal grid voltage 138 imtil the grid contingency event is resolved. At
the resolution of the grid contingency event, reactive current command signal 192
returns to a lower level, causing reactive current output by power conversion
assembly 42 to decrease to approximately the level of reactive current output by
power conversion assembly 42 prior to the grid contingency event. As the output
power of power conversion assembly 42 increases during recovery from the grid
contingency event, additional reactive current may be needed to maintain terminal
voltage 138 and avoid voltage collapse of utility grid 136.
20
To facilitate reducing oscillations in the output power of
conversion assembly 42, voltage regulator stabilizer 186 generates voltage regulator
stabilization signal 188 and transmits voltage regulator stabilization signal 188 to
voltage regulator 184. Voltage regulator stabilization signal 188 is added to reference
voltage command signal 240. Hence, voltage regulator 184 generates a reactive
current command signal 192 that includes a reactive current component configured to
cancel oscillations in the power output by power conversion assembly 42. Voltage
regulator 184 sums voltage regulator stabilization signal 188 and reference voltage
command signal 240 and subtracts terminal voltage feedback signal 160 to produce an
error signal. A control block 246 receives the error signal and generates reactive
current command signal 192.
Figure 8 is a block diagram of an alternative embodiment of
converter control system 200 (shown in Figure 6) and identified herein as converter
control system 220. Converter control system 220 is configured to generate control
signals provided to a power converter, for example, power conversion assembly 42
(shown in Figure 5), for control of power conversion assembly 42. In the alternative
embodiment, stabilizer system 182 includes power regulator 204 that is configured to
generate a real power command, for example, real current command signal 166. In
the alternative embodiment, regulator stabilizer 186 is a power regulator stabilizer
configured to generate stabilization signal 188, which is, more specifically, a power
stabilization signal. Power stabilization signal 188 is provided to power regulator
204, which generates control signals based at least partially on signal 188. The
control signals, for example, real current command signal 166, are provided to
converter interface controller 156. In the alternative embodiment, converter control
system 220 includes power limiter system 152, stabilizer system 182, and converter
interface controller 156.
Figures 9-18 are graphical views illustrating operation of a
power generation and delivery system after a grid contingency event. More
specifically. Figures 9-13 illustrate operation of a power generation and delivery
system that does not include a regulator stabilizer, for example, regulator stabilizer
21
186 (shown in Figure 6). In contrast, Figures 14-18 illustrate operation of a power
generation and delivery system, for example, power generation and delivery system
150 (shown in Figure 5), that includes regulator stabilizer 186. The exemplary
measurements illustrated in Figures 9-18 were obtained through experimentation
and/or calculation and are included to illustrate the effect of operation of regulator
stabilizer 186 on power generation and delivery system 150.
Figures 9 and 14 are graphical views of PLL error signal 190
versus time. As described above, after a grid contingency event, system oscillations
arising from operation of voltage regulator 184 (shown in Figure 6) are measured and
apparent in PLL error signal 190 (see Figure 9). Figure 14 illustrates the reduction in
system oscillations, as shown by the reduction in PLL error signal 190 oscillations.
Figures 10 and 15 are graphical views of a sum 250 of
reference voltage command signal 240 and voltage regulator stabilization signal 188
(both shown in Figure 7) versus time. As illustrated in Figure 10, without voltage
regulator stabilizer 186, no voltage regulator stabilization signal 188 is provided to
voltage regulator 184. Therefore, sxmi 250 of reference voltage command signal 240
and voltage regulator stabilization signal 188 equals reference voltage command
signal 240, which in the illustrated example, is a constant over time.
As shown in Figure 15, sxmi 250 of reference voltage
command signal 240 and voltage regulator stabilization signal 188 varies over time.
Reference voltage command signal 240 remains a constant, however, voltage
regulator stabilization signal 188 varies over time.
Figures 11 and 16 are graphical views of terminal voltage
feedback signal 160 (shown in Figure 7) versus time. In the illustrated example, the
oscillation of terminal voltage feedback signal 160 is an example of a system
oscillation that occurs, for example, while system 150 is recovering from a grid
contingency event. Figure 11 illustrates a system oscillation (e.g., oscillations of
terminal voltage feedback signal 160) increasing over time. Figure 16 illustrates the
reduction in system oscillations (e.g., reduction in terminal voltage feedback signal
22
160 oscillations), caused by operation of voltage regulator stabilizer 186. More
specifically, Figure 16 illustrates how application of sum 250 (shown in Figure 15) to
operation of voltage regulator 184 dampens oscillations of terminal voltage feedback
signal 160.
Figures 12 and 13 are graphical views of electrical power 134
(shown in Figure 5) versus time in a power generation and delivery system that does
not include voltage regulator stabilizer 186. More specifically. Figure 12 illustrates a
reactive power component of electrical power 134 and Figure 13 illustrates a real
power component of electrical power 134. The oscillations of electrical power 134
illustrated in Figures 12 and 13 are another example of system oscillations that may
occur while system 150 is recovering fi-om a grid contingency event.
Figures 17 and 18 are graphical views of electrical power 134
(shown in Figure 5) versus time in a power generation system that includes a voltage
regulator stabilizer, for example, power generation system 150 that includes voltage
regulator stabilizer 186. Figures 17 and 18 illustrate the reduction in system
oscillations, more specifically, the reduction in the oscillations of electrical power
134, caused by operation of voltage regulator stabilizer 186.
Figure 19 is a flow chart 260 of an exemplary method 270 for
controlling a power generation and delivery system, for example, power generation
and delivery system 150 (shown in Figure 5). In the exemplary embodiment, power
generation and delivery system 150 includes an electrical generator, for example,
electrical generator 34 (shown in Figure 5), a power converter, for example power
conversion assembly 42 (shown in Figure 5), and a system controller, for example,
system controller 44 (shown in Figure 4).
In the exemplary embodiment, method 270 includes
monitoring 272 an output parameter of power generation and delivery system 150 that
is indicative of system oscillations. For example, a PLL regulator, for example, PLL
regulator 80 (shown in Figure 4), may monitor 272 a PLL error, and generate a PLL
error signal, for example, PLL error signal 190. The output parameter may also
23
include, but is not limited to, a voltage feedback signal, for example, voltage feedback
signal 160 (shown in Figure 5). As described above, oscillations within PLL error
signal 190 are indicative of system oscillations.
In the exemplary embodiment, method 270 also includes
generating 276 a command signal, for example, reactive current command signal 192
(shown in Figure 5) and/or real current command signal 166 (shown in Figure 5),
based at least partially on the output parameter. For example, system controller 44
may generate 276 the command signal by applying a transfer function to PLL error
signal 190 to generate a voltage regulator stabilization signal, for example, voltage
regulator stabilization signal 188 (shown in Figure 6). A voltage regulator, for
example, voltage regulator 184 (shown in Figure 6) is configiured to generate
command signal 192 based at least partially on voltage regulator stabilization signal
188. In an alternative embodiment, system controller 44 may generate 276 the
command signal by applying a transfer function to PLL error signal 190 to generate a
power regulator stabilization signal, for example, power regulator stabilization signal
188 (shown in Figure 8). A power regulator, for example, power regulator 204
(shown in Figxire 8) is configured to generate command signal 166 based at least
partially on power regulator stabilization signal 188.
More specifically, generating 276 command signal 192 may
include summing voltage regulator stabilization signal 188, a reference voltage
command signal, for example, reference voltage command signal 240 (shown in
Figure 7), and an inverse of terminal voltage feedback signal 160 (shown in Figure 7).
Moreover, applying the transfer function may include applying a predefined transfer
function to PLL error signal 190 that isolates a frequency range within PLL error
signal 190 that includes an indication of system oscillations. Applying the transfer
function may also include applying a predefined gain to PLL error signal 190 to
positively dampen system oscillations.
In the exemplary embodiment, method 270 also includes
controlling 278 operation of power converter 42 based at least partially on reactive
24
current command signal 192 and/or the real current command signal 166 to reduce
system oscillations.
The above-described embodiments facilitate efficient and
cost-effective operation of a wind turbine. The wind turbine includes a voltage
regulator stabilizer system that generates a voltage regulator stabilization signal based
at least partially on a measured PLL error. The voltage regulator stabilization signal
is provided to a voltage regulator that determines a reactive current command based at
least partially on the voltage regulator stabilization signal. Controlling the reactive
current output as a fimction of the PLL error facilitates reducing system oscillations
that may occur during recovery from a grid contingency event. The method and
systems described herein facilitate increasing the stability of the voltage regulator, and
furthermore, the stability voltage and/or power output by the wind turbine following a
grid contingency event.
Exemplary embodiments of a wind turbine, voltage regulator
stabilizer system, and methods for operating a wind turbine in response to an
occurrence of a grid contingency event are described above in detail. The methods,
wind turbine, and voltage regulator stabilizer system are not limited to the specific
embodiments described herein, but rather, components of the wind turbine,
components of the voltage regulator stabilizer system, and/or steps of the methods
may be utilized independently and separately from other components and/or steps
described herein. For example, the voltage regulator stabilizer system and methods
may also be used in combination with other wind turbine power systems and methods,
and are not limited to practice with only the power system as described herein.
Rather, the exemplary embodiment can be implemented and utilized in connection
with many other wind turbine or power system applications.
Although specific features of various embodiments of the
invention may be shown in some drawings and not in others, this is for convenience
only. In accordance with the principles of the invention, any feature of a drawing
25
may be referenced and/or claimed in combination with any feature of any other
drawing.
This written description uses examples to disclose the
invention, including the best mode, and also to enable any person skilled in the art to
practice the invention, including making and using any devices or systems and
performing any incorporated methods. The patentable scope of the invention is
defined by the claims, and may include other examples that occur to those skilled in
the art. Such other examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal language of the claims, or if
they include equivalent structural elements with insubstantial differences from the
literal language of the claims.
26
METHODS AND SYSTEMS FOR CONTROLLING A
POWER CONVERTER
PARTS LIST
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
62
power generation system
power generator
power generation units
power converter
electrical distribution network
wind turbine
nacelle
rotor
axis of rotation
tower
rotor hub
blades
generator
rotor shaft
generator shaft
gearbox
power conversion assembly
system controller
disk brake
yaw system
pitch system
axis of rotation
sensor
sensor
sensor
bus
27
64
66
68
70
72
74
80
132
134
136
138
140
150
152
154
156
160
166
180
182
184
186
188
190
192
194
200
202
204
206
208
processor(s)
RAM
storage device
ROM
input/output device
sensor interface
phase locked loop (PLL) regulator
electrical power
electrical power
electrical grid
terminal grid voltage
bulk power system
power generation and delivery system
power limiter system
control signal
converter interface controller
terminal voltage feedback signal
real current command signal
power limiter
stabilizer system
regulator
regulator stabilizer
regulator stabilization signal
PLL error signal
reactive current command signal
reference power control signal
converter control system
power limiter
power regulator
power command signal
PLL phase angle signal
28
240
242
246
250
260
270
272
276
278
reference voltage command signal
volt-ampere reactive (VAR) regulator
control block
sum
flow chart
method for controlling a power generation and delivery system
monitoring an output parameter of the power generation and delivery
system
generating a command signal
controlling operation of the power converter
29

We claim:
1. A stabilizer system (182) associated with a power converter
controller (44), the stabilizer system comprising:
a regulator stabilizer (186) configured to receive a phase locked loop
(PLL) error signal (190) and to generate a regulator stabilization signal (188) based at
least partially on the PLL error signal; and,
a regulator (184, 204) coupled to said regulator stabilizer and a
^ converter interface controller (156), said regulator configured to:
receive the regulator stabilization signal;
generate a first command signal (192, 166), based at least
partially on the regulator stabilization signal, that reduces system oscillations;
and,
transmit the first command signal to the converter interface
controller.
2. A system (182) in accordance with claim 1, wherein said
regulator stabilizer (186) is fiirther configwed to apply a predefined transfer fimction
to the PLL error signal (190), wherein the regulator stabilization signal (188) is an
^ output of the predefined transfer fimction, and wherein the predefined transfer
fimction isolates a frequency range within the PLL error signal (190) that includes an
indication of system oscillations and applies a gain to positively dampen the system
oscillations.
3. A system (182) in accordance with claim 1, wherein
oscillations within the PLL error signal (190) correspond to system oscillations
including at least one of oscillations of a terminal voltage (138) at an output of a
power converter (42) associated with power converter controller (44) and electrical
30
power (134) output by the power converter, and wherein the system oscillations
correspond to system instability.
4. A system (182) in accordance with claim 3, wherein the first
command signal (192,166), when provided to the converter interface controller (156)
and used to control operation of the power converter (42), dampens the system
oscillations.
5. A system (182) in accordance with claim 1, wherein said
regulator (184, 204) comprises at least one of a voltage regulator (184) and a power
^ regulator (204), and wherein the first command signal (192, 166) comprises at least
one of a reactive current command signal (192) generated by said voltage regulator
and a real ciurent command signal (166) generated by said power regulator.
6. A converter controller (200, 220) for controlling operation of a
power conversion assembly (42), said converter controller comprising:
a stabilizer system (182) configured to:
receive a phase locked loop (PLL) error signal (190); and,
generate a first command signal (192, 166), based at least
partially on the PLL error signal, that reduces system oscillations; and,
#
a converter interface controller (156) commxmicatively coupled to said
stabilizer system and configured to generate control signals based at least partially on
the first command signal and transmit the control signals to the power conversion
assembly.
7. A converter controller (200, 220) in accordance with claim 6,
wherein the PLL error signal (190) is indicative of system oscillations including at
least one of oscillations of terminal voltage (138) and/or electrical power (134) output
by the power conversion assembly (42), and wherein the system oscillations
correspond to system instability.
31
8. A converter controller (200, 220) in accordance with claim 6,
wherein said stabilizer system (182) comprises:
a regulator stabilizer (186) configured to receive the PLL error signal
(190) and to generate a regulator stabilization signal (188); and,
a regulator (184, 204) coupled to said regulator stabilizer and
configured to receive the regulator stabilization signal, generate the first command
signal (192, 166), based at least partially on the regulator stabilization signal, and
provide the first command signal to said converter interface controller (156).
9. A converter controller (200, 220) in accordance with claim 8,
wherein said regulator (184, 204) comprises at least one of a voltage regulator (184)
and a power regulator (204), and wherein the first command signal comprises at least
one of a reactive current command signal (192) generated by said voltage regulator
and a real current command signal (166) generated by said power regulator (204).
10. A converter controller (200, 220) in accordance with claim 8,
wherein said regulator stabilizer (186) is configured to apply a predefined transfer
function to the PLL error signal (190), wherein the regulator stabilization signal (188)
is an output of the predefined transfer function, wherein the predefined transfer
function isolates a frequency range within the PLL error signal that includes an
g^ indication of system oscillations and applies a gain to positively dampen the system
oscillations.