Claims:1. A method for operating a power generation system, comprising:
generating a first electrical power and a second electrical power by a doubly-fed induction generator (DFIG), wherein the DFIG is mechanically coupled to a wind turbine and comprises a generator comprising a rotor winding and a stator winding, a rotor side converter electrically coupled to the rotor winding, a line side converter electrically coupled to the stator winding at a point of turbine coupling (PTC), and a direct current (DC) link disposed between the rotor side converter and the line side converter, wherein the first electrical power is generated at the stator winding and the second electrical power is generated at the rotor winding;
supplying a third electrical power to the DC-link of the DFIG from an auxiliary power source;
determining whether to limit the second electrical power, the third electrical power, or both the second electrical power and the third electrical power if an input power to the line side converter is higher than a rated power of the line side converter, wherein the input power to the line side converter comprises the second electrical power and the third electrical power; and
limiting the determined second electrical power, third electrical power, or both second electrical power and third electrical power.
2. The method of claim 1, wherein the determined second electrical power, third electrical power, or both second electrical power and third electrical power are limited based on at least one of a wind power tariff and an auxiliary power tariff such that at least one of a revenue benefit metric and a power production metric is maximized.
3. The method of claim 1, further comprising, in response to determining to limit the second electrical power:
determining a level of the third electrical power received at the DC-link from the auxiliary power source; and
determining a desired level of the second electrical power based on the level of the third electrical power and the rated power of the line side converter.
4. The method of claim 3, further comprising:
determining a desired slip value of the generator corresponding to the desired level of the second electrical power; and
operating the generator at the desired slip value.
5. The method of claim 4, further comprising determining a generator rotor speed corresponding to the desired slip value, and operating a generator rotor at the determined generator rotor speed.
6. The method of claim 5, wherein operating the generator rotor at the determined generator rotor speed comprises:
determining at least one of a pitch value of rotor blades and a torque value of a wind rotor corresponding to the desired slip value; and
adjusting at least one of a pitch and a torque of the wind rotor at the determined pitch value and the torque value.
7. The method of claim 6, wherein at least one of the pitch of the rotor blades and the torque of the wind rotor is adjusted such that the determined torque value is lower than a rated torque of the wind turbine.
8. The method of claim 6, wherein at least one of the pitch of the rotor blades and the torque of the wind rotor is adjusted such that a level of thrust is maintained lower than a rated thrust of the wind turbine.
9. The method of claim 6, wherein the determined pitch value is such that the rotor blades are not operated in a stall condition.
10. The method of claim 1, further comprising determining a level of the second electrical power received at the DC-link from the rotor winding of the generator in response to determining to limit the third electrical power.

11. The method of claim 10, further comprising determining a desired level of the third electrical power based on the level of the second electrical power and the rated power of the line side converter.
12. The method of claim 11, further comprising limiting the third electrical power to the desired level of the third electrical power via a DC-DC converter.
13. A power generation system, comprising:
a wind turbine;
a doubly-fed induction generator (DFIG) mechanically coupled to the wind turbine and comprising: a generator comprising a rotor winding and a stator winding, a rotor side converter electrically coupled to the rotor winding, a line side converter electrically coupled to the stator winding at a point of turbine coupling (PTC), and a direct current (DC) link disposed between the rotor side converter and the line side converter, wherein the generator is configured to generate a first electrical power at the stator winding and generate or absorb a second electrical power at the rotor winding;
an auxiliary power source electrically coupled to the DC-link and configured to supply a third electrical power to the DC-link; and
a controller operably coupled to the wind turbine, the rotor side converter, and the line side converter, wherein, if an input power to the line side converter is higher than a rated power of the line side converter, the controller is configured to:
determine whether to limit the second electrical power, the third electrical power, or both the second electrical power and the third electrical power; and
limit the determined second electrical power, third electrical power, or both second electrical power and the third electrical power,
wherein the input power to the line side converter comprises the second electrical power and the third electrical power.

14. The power generation system of claim 13, wherein auxiliary power source comprises a photo-voltaic (PV) power source.
15. The power generation system of claim 13, wherein the controller is configured to determine whether to limit the second electrical power, the third electrical power, or both the second electrical power and the third electrical power based on at least one of a wind power tariff and an auxiliary power tariff such that at least one of a revenue benefit metric and a power production metric is maximized.
16. The power generation system of claim 15, wherein, in response to determining to limit the second electrical power, the controller is configured to:
determine a level of the third electrical power received at the DC-link from the auxiliary power source; and
determine a desired level of the second electrical power based on the level of the third electrical power and the rated power of the line side converter.
17. The power generation system of claim 16, wherein the controller is further configured to:
determine a desired slip value of the generator corresponding to the desired level of the second electrical power; and
operate the generator at a generator rotor speed corresponding to the desired slip value.
18. The power generation system of claim 17, wherein the wind turbine comprises a wind rotor comprising a plurality of rotor blades rotatable via wind, and wherein, to operate the generator rotor at the determined generator rotor speed, the controller is configured to:
determine at least one of a pitch value of the rotor blades and a torque value of the wind rotor corresponding to the desired slip value; and
adjust at least one of a pitch and a torque of the wind turbine at the determined pitch value and the torque value.
19. The power generation system of claim 13, wherein, in response to determining to limit the third electrical power, the controller is configured to determine a level of the second electrical power received at the DC-link from the rotor winding of the generator.
20. The power generation system of claim 19, further comprising a DC-DC converter electrically coupled between the auxiliary power source and the DC-link, wherein controller is configured to:
determine a desired level of the third electrical power based on the level of the second electrical power and the rated power of the line side converter; and
limit the third electrical power to the desired level of the third electrical power.
, Description:BACKGROUND
[0001] The present application relates generally to generation of electrical
power and more particularly relates to a power generation system employing a
wind turbine and an auxiliary power source.
[0002] Typically, in hybrid power generation systems that employ a doublyfed
induction generator (DFIG), power sources such as a primary power source
(e.g., wind turbine) and an auxiliary power source (e.g., solar/photo-voltaic power
source) may be coupled to the DFIG. The DFIG includes a generator having a
stator winding, a rotor winding, a rotor side converter coupled to the rotor
winding, and a line side converter coupled to the stator winding. The rotor side
converter and the line side converter are coupled to each other via a direct-current
(DC) link.
[0003] When the generator is operated in a super-synchronous mode, in
addition to the power generated at the stator winding, additional power is also
generated at the rotor winding which may be supplied to the DC-link. Moreover,
auxiliary power may also be supplied to the DC-link from the auxiliary power
source. In such a power generation system, the line side converter is preconfigured
and an amount of power transferred therethrough is restricted by
corresponding rated power. There are situations when both the primary power
source and the auxiliary power source generate power such that an effective
amount of power transferred via the line side converter is limited to the rated
power of the line side converter. Accordingly, the hybrid power generated by the
power generation system may be limited. Additionally, supplying higher than
rated power to the line side converter may cause damage thereto and result in
reduced reliability of the power generation system.
287294-1
3
BRIEF DESCRIPTION
[0004] In accordance with aspects of the present specification, a method for
operating a power generation system is disclosed. The method includes
generating a first electrical power and a second electrical power by a doubly-fed
induction generator (DFIG), wherein the DFIG is mechanically coupled to a
wind turbine and comprises a generator comprising a rotor winding and a stator
winding, a rotor side converter electrically coupled to the rotor winding, a line
side converter electrically coupled to the stator winding at a point of turbine
coupling (PTC), and a direct current (DC) link disposed between the rotor side
converter and the line side converter, wherein the first electrical power is
generated at the stator winding and the second electrical power is generated at the
rotor winding. The method further includes supplying a third electrical power to
the DC-link of the DFIG from an auxiliary power source. Furthermore, the
method includes determining whether to limit the second electrical power, the
third electrical power, or both the second electrical power and the third electrical
power if an input power to the line side converter is higher than a rated power of
the line side converter, wherein the input power to the line side converter
comprises the second electrical power and the third electrical power. Moreover,
the method includes limiting the determined second electrical power, third
electrical power, or both second electrical power and third electrical power.
[0005] In accordance with aspects of the present specification, a power
generation system is disclosed. The power generation system includes a wind
turbine. The power generation system further includes a doubly-fed induction
generator (DFIG) mechanically coupled to the wind turbine. The DFIG includes
a generator having a rotor winding and a stator winding, a rotor side converter
electrically coupled to the rotor winding, a line side converter electrically coupled
to the stator winding at a point of turbine coupling (PTC), and a DC-link disposed
between the rotor side converter and the line side converter, wherein the generator
is configured to generate a first electrical power at the stator winding and generate
or absorb a second electrical power at the rotor winding. Moreover, the power
287294-1
4
generation system also includes an auxiliary power source electrically coupled to
the DC-link and configured to supply a third electrical power to the DC-link.
Furthermore, the power generation system includes controller operably coupled to
the wind turbine, the rotor side converter, and the line side converter, wherein, if
an input power to the line side converter is higher than a rated power of the line
side converter, the controller is configured to determine whether to limit the
second electrical power, the third electrical power, or both the second electrical
power and the third electrical power, wherein the input power to the line side
converter includes the second electrical power and the third electrical power. The
controller is further configured to limit the determined second electrical power,
third electrical power, or both second electrical power and the third electrical
power.
DRAWINGS
[0006] These and other features, aspects, and advantages of the present
invention will become better understood when the following detailed description
is read with reference to the accompanying drawings in which like characters
represent like parts throughout the drawings, wherein:
[0007] FIG. 1 is a block diagram of an electrical power distribution system, in
accordance with aspects of the present specification;
[0008] FIG. 2 is a flowchart of an example method for operating a power
generation system, in accordance with aspects of the present specification;
[0009] FIG. 3 is a flowchart of an example method for determining whether
to limit a second electrical power, a third electrical power, or both the second
electrical power and the third electrical power and subsequently limiting the
determined electrical power, in accordance with aspects of the present
specification;
287294-1
5
[0010] FIG. 4 is a flowchart of an example method for limiting a second
electrical power, in accordance with aspects of the present specification; and
[0011] FIG. 5 is a flowchart of an example method for limiting a third
electrical power, in accordance with aspects of the present specification.
DETAILED DESCRIPTION
[0012] The specification may be best understood with reference to the
detailed figures and description set forth herein. Various embodiments are
described hereinafter with reference to the figures. However, those skilled in the
art will readily appreciate that the detailed description given herein with respect to
these figures is for explanatory purposes as the method and the system may
extend beyond the described embodiments.
[0013] In the following specification, the singular forms “a”, “an” and “the”
include plural referents unless the context clearly dictates otherwise. As used
herein, the term “or” is not meant to be exclusive and refers to at least one of the
referenced components being present and includes instances in which a
combination of the referenced components may be present, unless the context
clearly dictates otherwise.
[0014] As used herein, the terms “may” and “may be” indicate a possibility of
an occurrence within a set of circumstances; a possession of a specified property,
characteristic or function; and/or qualify another verb by expressing one or more
of an ability, capability, or possibility associated with the qualified verb.
Accordingly, usage of “may” and “may be” indicates that a modified term is
apparently appropriate, capable, or suitable for an indicated capacity, function, or
usage, while taking into account that in some circumstances, the modified term
may sometimes not be appropriate, capable, or suitable.
[0015] In accordance with some aspects of the present specification, a power
generation system is disclosed. The power generation system includes a wind
287294-1
6
turbine. The power generation system further includes a doubly-fed induction
generator (DFIG) mechanically coupled to the wind turbine. The DFIG includes
a generator having a rotor winding and a stator winding, a rotor side converter
electrically coupled to the rotor winding, a line side converter electrically coupled
to the stator winding at a point of PTC, and a DC-link disposed between the rotor
side converter and the line side converter, where the generator is configured to
generate a first electrical power at the stator winding and generate or absorb a
second electrical power at the rotor winding. Moreover, the power generation
system also includes an auxiliary power source electrically coupled to the DClink
and configured to supply a third electrical power to the DC-link.
Furthermore, the power generation system includes a controller operably coupled
to the wind turbine, the rotor side converter, and the line side converter, wherein
the controller is configured to limit at least one of the second electrical power and
the third electrical power if an input power to the line side converter is higher
than a rated power of the line side converter, where the input power to the line
side converter includes the second electrical power and the third electrical power.
A method of operating the power generation system is also disclosed.
[0016] FIG. 1 is a block diagram of an electrical power distribution system
100, in accordance with aspects of the present specification. The electrical power
distribution system 100 may include a power generation system 101 coupled to an
electric grid 102 at a point of common coupling (PCC) 117. In some
embodiments, the power generation system 101 may be coupled to the PCC 117
via a point of turbine coupling (PTC) 103. In some embodiments, the PCC 117
may be coupled to a local electrical load 105 to enable supply of an AC power to
the local electrical load 105. In some embodiments, the PTC 103 and the PCC
117 may include a transformer.
[0017] The electric grid 102 may be representative of an interconnected
network for delivering a grid power (e.g., electricity) from one or more power
generation stations (different from the power generation system 101) to
consumers (e.g., the local electrical load 105) through high/medium voltage
287294-1
7
transmission lines. The grid power may be received at the PCC 117 from the
electric grid 102. The local electrical load 105 coupled to the PCC 117 may
include electrical devices that are operable using the electric power received from
the electric grid 102 or from the power generation system 101.
[0018] In some embodiments, the power generation system 101 may include a
wind turbine 106, a doubly-fed induction generator (DFIG) 108, and an auxiliary
power source 110. In some embodiments, the power generation system 101 may
also include a controller 124 operatively coupled to at least one of the wind
turbine 106 and the DFIG 108. The controller 124 may be configured to control
the operations of the wind turbine 106 and the DFIG 108. In some embodiments,
the DFIG 108 may include one or more of a generator 112, a rotor side converter
114, and a line side converter 116.
[0019] In one embodiment, the controller 124 may include one or more of a
specially programmed general purpose computer, a microprocessor, a digital
signal processor, and/or a microcontroller. The controller 124 may also include
input/output ports, and a storage medium, such as, an electronic memory.
Various examples of the microprocessor include, but are not limited to, a reduced
instruction set computing (RISC) architecture type microprocessor or a complex
instruction set computing (CISC) architecture type microprocessor. Further, the
microprocessor may be a single-core type or multi-core type. Alternatively, the
controller 124 may be implemented as hardware elements such as circuit boards
with processors or as software running on a processor such as a commercial, offthe-
shelf personal computer (PC), or a microcontroller. In certain embodiments,
the wind turbine 106, the rotor side converter 114 and the line side converter 116
may include controllers / control units / electronics to control their respective
operations. These controllers / control units / electronics may be controlled by the
controller 124. The controller 124 may be capable of executing program
instructions for controlling operations of the power generation system 101, the
electrical devices constituting the local electrical load 105. In some
287294-1
8
embodiments, the controller 124 may aid in executing a method for operating the
power generation system 101 (see FIG. 2).
[0020] The wind turbine 106 may include a rotor 107 which is typically
mounted on a tower 109, and a shaft 111 coupled to the rotor 107. The rotor 107
may include a plurality of rotor blades 113 coupled to the shaft 111. During
operation, based on a speed and direction of wind, the rotor blades 113 rotate
causing rotations of the shaft 111. The rotational speed of the rotor 107 may be
based on various parameters including, but not limited to, a pitch of the rotor
blades and/or a torque on the rotor 107. The term “pitch” as used herein refers to
an angle of a given rotor blade 113.
[0021] The DFIG 108 may include the generator 112. In a non-limiting
example, the generator 112 may be a wound rotor induction generator. The
generator 112 may include a stator 126, a rotor 128, a stator winding 130 disposed
on the stator 126, and a rotor winding 132 disposed on the rotor 128. More
particularly, the stator winding 130 may be coupled (directly or indirectly) to the
PTC 103. The DFIG 108 may be mechanically coupled to the wind turbine 106.
In some embodiments, the rotor 128 of the generator 112 may be mechanically
coupled to the shaft 111 of the wind turbine 106, such that, during operation,
rotations of the shaft 111 may cause a rotary motion of the rotor 128 of the
generator 112. In some embodiments, the shaft 111 of the wind turbine 106 may
be coupled to the rotor 128 of the generator 112 through a gear box (not shown in
FIG. 1).
[0022] During operation, when the rotor 128 is rotated, the generator 112 may
be configured to generate a first electrical power (voltage and current) at the
stator winding 130 and generate or absorb a second electrical power (voltage and
current) at the rotor winding 132 based at least on an operating speed (rpm) of the
rotor 128. Typically, a slip of the generator 112 may be defined as represented
by Equation (1):
287294-1
9
s
s r
N
S N N
-
= ... Equation (1)
where, r N represents operating speed of the rotor 128 in revolution per
minute (rpm) and s N represents a synchronous speed of the generator 112.
Further, s N is represented by Equation (2):
p
N f s
= 120* ... Equation (2)
where, f represents frequency of current flowing through the stator
winding 130, and p represents number of stator poles.
[0023] The generator 112 may operate in different modes depending on the
operating speed (rpm) of the rotor 128. For example, the generator 112 may
operate in sub-synchronous mode if r N is lower than s N . The generator 112 may
operate in synchronous mode if r N is same as s N . The generator 112 may
operate in super-synchronous mode if r N is greater than s N . In some
embodiments, when the generator 112 operates in sub-synchronous mode, the
generator 112 may be configured to absorb the second electrical power at the
rotor winding 132. In some embodiments, when the generator 112 operates in
super-synchronous mode, the generator 112 may be configured to generate
second electrical power at the rotor winding 132.
[0024] In some embodiments, the DFIG 108 may further include the rotor
side converter 114 and the line side converter 116. Each of the rotor side
converter 114 and the line side converter 116 may act as an AC-DC converter or a
DC-AC converter, and may be controlled by the controller 124. The rotor side
converter 114 may be electrically coupled to the rotor winding 132. Further, the
line side converter 116 may be electrically coupled to the stator winding 130
directly or via a transformer (not shown in FIG. 1). In one embodiment, the rotor
side converter 114 and line side converter 116 are also coupled to each other. For
287294-1
10
example, the rotor side converter 114 and the line side converter 116 are
electrically coupled to each other via a DC-link 118. The DC-link 118 may
include two electrically conductive lines (not shown in FIG. 1) – one at a positive
potential and other at a negative or zero potential. In some embodiments, a
capacitor (not shown in FIG. 1) may be coupled to the DC-link 118. For
example, one terminal of the capacitor may be coupled to the electrically
conductive line of positive potential and other terminal of the capacitor may be
coupled to the electrically conductive line of negative potential.
[0025] Further, the power generation system 101 may include the auxiliary
power source 110 electrically coupled to the DFIG 108 at the DC-link 118. The
auxiliary power source 110 may be configured to supply a third electrical power
to the DC-link 118. In a non-limiting example, the auxiliary power source 110
may include a PV power source 115. The PV power source 115 may include one
or more PV arrays (not shown in FIG. 1), where each PV array may include at
least one PV module (not shown in FIG. 1). A PV module may include a suitable
arrangement of a plurality of PV cells (diodes and/or transistors). The PV power
source 115 may generate a DC voltage contributing to the third electrical power
that depends on solar insolation, weather conditions, and/or time of day.
[0026] In some embodiments, the auxiliary power source 110 may be
electrically coupled to the DFIG 108 at the DC-link 118 via a first DC-DC
converter 134. The first DC-DC converter 134 may be electrically coupled
between the auxiliary power source 110 and the DC-link 118. In such an
instance, the third electrical power may be supplied from the auxiliary power
source 110 to the DC-link 118 via the first DC-DC converter 134. The first DCDC
converter 134 may be operated as a buck converter, a boost converter, or
buck-boost converter, and may be controlled by the controller 124.
[0027] In some embodiments, the auxiliary power source 110 may optionally
also include an energy storage device 122. The energy storage device 122 may
employ one or more batteries, capacitors, and the like. In some embodiments, the
287294-1
11
energy storage device 122 may be electrically coupled to the DFIG 108 at the
DC-link 118 to supply DC electrical power to the DC-link 118 that may also
contribute the third electrical power. The energy storage device 122 may be
electrically coupled to the DFIG 108 at the DC-link 118 via a second DC-DC
converter 136. The second DC-DC converter 136 may be electrically coupled
between the energy storage device 122 and the DC-link 118. In such an instance,
the DC electrical power from the energy storage device 122 may be supplied to
the DC-link 118 via the second DC-DC converter 136. The second DC-DC
converter 136 may be operated as a buck converter, a boost converter, or buckboost
converter, and may be controlled by the controller 124. In some
embodiments, the energy storage device 122 may be charged via the second DCDC
converter 136 using a DC power from the DC-link 118.
[0028] In some embodiments, the power generation system 101 may also
include a third DC-DC converter 138. The third DC-DC converter 138 may be
electrically coupled between the energy storage device 122 and the PV power
source 115. In some embodiments, the third DC-DC converter 138 may be
configured to charge the energy storage device 122 via the PV power source 115.
For example, in some embodiments, the energy storage device 122 may receive a
charging current via the third DC-DC converter 138 from the PV power source
115. The third DC-DC converter 138 may be operated as a buck converter, a
boost converter, or buck-boost converter, and may be controlled by the controller
124.
[0029] As previously noted, the controller 124 may be operatively coupled to
the wind turbine 106, the generator 112, the rotor side converter 114, and the line
side converter 116. Moreover, in some embodiments, the controller 124 may also
be operatively coupled (as shown using dashed connectors) to at least one of the
first DC-DC converter 134, the second DC-DC converter 136, and the third DCDC
converter 138 to control their respective operations. Furthermore, in some
embodiments, the controller 124 may be operatively coupled (as shown using
287294-1
12
dashed connector) to the local electrical load 105 to selectively connect and
disconnect the respective electrical device to manage load.
[0030] As previously noted, the controller 124 may aid in executing a method
for operating a power generation system 101 (see FIG. 2). In some embodiments,
the controller 124 may aid in executing steps 202-216 of FIG. 2. In order to
execute the steps 202-216 of FIG. 2, the controller 124 may be configured to
control the operation of one or more of the wind turbine 106, the generator 112,
the rotor side converter 114, the line side converter 116, the first DC-DC
converter 134, and/or the second DC-DC converter 136. By way of example, the
controller 124 may be programmed to control operation of the rotor side converter
114, the line side converter 116, the first DC-DC converter 134, and/or the second
DC-DC converter 136 to allow or disallow a flow of the power therethrough in a
determined direction to aid in operating a power generation system 101.
Moreover, the controller 124 may be configured to control the operation of the
wind turbine 106 by adjusting one or more operating parameters, such as, the
pitch and the torque of the rotor blades 113.
[0031] FIG. 2 is a flowchart 200 of an example method for operating the
power generation system 101, in accordance with aspects of the present
specification. More particularly, the flowchart 200 represents an example method
of generating maximum possible hybrid power at the PTC 103 by selectively
limiting at least one of the second electrical power or the third electrical power.
[0032] At step 202, the first electrical power and the second electrical power
may be generated by the DFIG 108. As previously noted, the generator 112 of
the DFIG 108 may be configured to generate the first electrical power at the stator
winding 130 and generate the second electrical power at the rotor winding 132
based at least on the operating speed (rpm) of the rotor 128. More particularly,
the second electrical power may be generated at the rotor winding 132 when the
generator 112 is operated in the super-synchronous mode (i.e., when r N > s N ).
The first electrical power and the second electrical power are hereinafter
287294-1
13
collectively referred to as wind power. Further, at step 204, a third electrical
power may be supplied to the DC-link 118 of the DFIG 108 from the auxiliary
power source 110.
[0033] Furthermore, at step 206, a level (e.g., a magnitude) of an input power
to the line side converter 116 may be determined by the controller 124. In certain
embodiments, to determine the level of the input power to the line side converter
116, the controller 124 may be configured to determine magnitude of a DC-link
power, for example, using one or more sensors disposed along the DC-link 118.
[0034] The input power to the line side converter includes the second
electrical power and the third electrical power. Accordingly, in some
embodiments, to determine the input power to the line side converter 116, the
controller may be configured to determine both a level of the second electrical
power and a level of the third electrical power.
[0035] In some embodiments, the controller 124 may determine the level of
the second electrical power by using voltage and current sensor measurements of
an input or an output from rotor side converter 112. In other embodiments, the
controller 124 may determine the level of the second electrical power based on a
level of the first electrical power. In some embodiments, the controller 124 may
determine the level of the second electrical power based on current operating
speed (rpm) of the rotor 128 and a current value of the slip of the generator 112.
In some embodiments, the controller 124 may either determine the third power
directly via current and voltage measurements from an output of the auxiliary
power source 110 or may be configured to estimate the level of the third power
generated based on parameters including, but not limited to, solar insolation,
weather conditions, and/or time of day. Subsequently, to determine the input
power to the line side converter 116, the controller 124 may be configured to sum
the determined level of the second electrical power and the estimated level of the
third electrical power.
287294-1
14
[0036] Moreover, at step 208, the controller 124 may be configured to
perform a check to determine whether the level of the input power (e.g., the DClink
voltage / power) to the line side converter 116 is higher than the rated power
of the line side converter 116. A value of the rated power 210 of the line side
converter 116 may be received by the controller 124 or known to the controller
124. Accordingly, the controller 124 may be configured to compare the level of
the input power with the value of the rated power 210 to determine whether the
level of the input power to the line side converter 116 is higher than the value of
the rated power 210 of the line side converter 116.
[0037] At step 208, if it is determined that the level of the input power to the
line side converter 116 is lower than the value of the rated power 210 of the line
side converter 116, the controller 124, at step 212, may determine not to limit the
second electrical power and the third electrical power. However, at step 208, if it
is determined that the level of the input power to the line side converter 116 is
higher than the value of the rated power 210 of the line side converter 116, the
controller 124, at step 214, may determine whether to limit the second electrical
power, the third electrical power, or both the second electrical power and the third
electrical power. Moreover, at step 216, the controller 124 may be configured to
limit the determined second electrical power, third electrical power, or both
second electrical power and third electrical power. A method of determining
whether to limit the second electrical power, the third electrical power, or both the
second electrical power and the third electrical power and subsequently limiting
the determined electrical power is described in conjunction with FIG. 3
[0038] FIG. 3 is a flowchart 300 of an example method for determining
whether to limit the second electrical power, the third electrical power, or both the
second electrical power and the third electrical power and subsequently limiting
the determined electrical power, in accordance with aspects of the present
specification. The flowchart 300 represents sub-steps of the steps 214 and 216 of
the flowchart 200. More particularly, steps 302, 310, 314 represent sub-steps of
287294-1
15
the step 214 of the flowchart 200. Further, steps 308, 316, 308 represent substeps
of the step 216 of the flowchart 200.
[0039] To determine whether to limit the second electrical power or the third
electrical power, inputs to the controller 124 may include a wind power tariff and
an auxiliary power tariff. The term “auxiliary power tariff” refers to a payment
that a buyer of an auxiliary power (i.e., the third electrical power) is required to
make per unit of energy (e.g., rupees/kW-hour). The term “wind power tariff”
refers to a payment that a buyer of the wind power is required to make per unit of
energy (e.g., rupees/kW-hour). Typically the buyer is a utility company. In some
embodiments, one objective of selectively limiting one of the second electrical
power or the third electrical power is to maximize at least one of a revenue
benefit metric and a power production metric.
[0040] The term “revenue benefit metric” as used herein refers to a gain in
revenue (e.g., a profit) that may be achieved by limiting one of the second
electrical power and the third electrical power over another for a determined
amount of hybrid power at the PTC 103 that is to be supplied from the power
generation system 101 when the wind power tariff is different from the auxiliary
power tariff. In a non-limiting example for purposes of illustration, the
determined amount of hybrid power to be supplied via the power generation
system is 250 kW, the rated power of the line side converter is 100 kW, the value
of the first electrical power generated at the stator winding 130 is 150 kW, the
value of the second electrical power generated at the rotor winding 130 is 75 kW,
the value of the third electrical power that can be supplied from the auxiliary
power source 110 is 100 kW, the wind power tariff is INR 4/unit, the auxiliary
power tariff is INR 7/unit. As the rated power of the line side converter 116 is
100 kW and a maximum possible input power to the line side converter 116 is
175 kW (i.e., 75 kW + 100 kW), it may be desirable to limit one of the second
electrical power or the third electrical power so as not to exceed the rated power
of 100 kW. In such a situation, the controller 124 may be configured to
determine whether it is beneficial to limit the second electrical power or it is
287294-1
16
beneficial to limit the third electrical power. Accordingly, the controller 124 may
be configured to limit one of the second electrical power or the third electrical
power for which the revenue benefit metric is maximized based on the given wind
power tariff (INR 4/unit) and the auxiliary power tariff (INR 7/unit).
[0041] The term “power production metric” may refer to a maximum amount
of the hybrid power that can be supplied by the power generation system 101 at
the PTC 103. For example, the power production metric may be represented by
following Equation (3) as below:
T p s T fP s
? + ,? , max ... Equation (3)
where, T represents a torque demanded by rotor 128, ? represents the
operating speed of the rotor 128, s P represents demanded third electrical power,
f represents ratio of auxiliary power tariff and wind power tariff.
[0042] In some embodiments, in situations when the wind power tariff is the
same as the auxiliary power tariff, the controller 124 may determine to maximize
the power production metric. When the wind power tariff is same as the auxiliary
power tariff, value of f equals unity.
[0043] Accordingly, at step 302, the controller 124 may be configured to
perform a check to determine if the wind power tariff 304 is same as the auxiliary
power tariff 306. At step 302, if it is determined that the wind power tariff 304
same as the auxiliary power tariff 306, the controller 124, at step 308, may limit at
least one of the second electrical power or the third electrical power such that the
power production metric is maximized (i.e., the sum of the third electrical power
and the wind power is maximized). In some embodiments, the second electrical
power may be limited (e.g., reduced) by reducing a slip of the generator 112,
thereby creating a slip margin. Further details of limiting the second electrical
power are described in conjunction with FIG. 4. In some embodiments, the slip
margin may be created such that higher amount of the third electrical power (i.e.,
287294-1
17
the auxiliary power) is extracted via the line side converter 116. Typically, an
aero power (i.e., the wind power) may be based on a torque applied on the rotor
blades 113 and the operating speed (rpm) of the rotor 128 of the generator 112.
Therefore, at a given wind speed, while limiting the second electrical power by
way of creating the slip margin, the rotor blades 113 are configured to operate at a
maximum torque ( max T ). The controller 124 may determine the maximum torque
( max T ) for a given wind speed based on a performance characteristics of the wind
turbine 106. In one example, the performance characteristics may be provided as
a look-up table. Accordingly, while reducing the operating speed (rpm) of the
rotor 128, the controller 124 may operate the rotor blades 113 such that a torque
applied on the rotor blades 113 may be maintained at about the maximum torque (
max T ). In a non-limiting example, the maximum torque ( max T ) for a determined
generator rotor speed ( r N ) (determined at step 410 of FIG. 4) may be represented
by Equation (4) as represented below:
( )
Desired level of the second electrical power
max
r s N N
T
-
= ... Equation (4)
where, the desired level of the second electrical power may be
determined at step 406 of FIG. 4.
[0044] Accordingly, in some embodiments, while creating the slip margin, the
generator rotor speed ( r N ) may be reduced to an extent the torque may be
maintained at about the maximum torque (Tmax). A minimum value of the
generator rotor speed ( r N ) for which the torque may be maintained at about the
maximum torque ( max T ) is hereinafter referred to as a minimum generator rotor
speed ( r-min N ).
[0045] In some embodiments, while limiting the second electrical power, if
the generator rotor speed ( r N ) reaches the minimum generator rotor speed (
r-min N ), it may not be desirable to further reduce the slip as the torque may not be
287294-1
18
maintained at about the maximum torque ( max T ). In such an instance, while
maintaining the generator rotor speed ( r N ) at the minimum generator rotor speed
( r-min N ), the controller 124 may be configured to additionally limit the third
electrical power. In some embodiments, the third electrical power may be limited
(e.g., reduced) to allow for a higher amount of the second electrical power to pass
through the line side converter 116. Further details of limiting the third electrical
power is described in conjunction with FIG. 5.
[0046] However, at step 302, if it is determined that the wind power tariff 304
is different from the auxiliary power tariff 306, the controller 124, at step 310,
may determine revenue benefit metrics (e.g., a first revenue benefit metric and a
second revenue benefit metric) corresponding to limiting the second electric
power and corresponding to limiting the third electric power. By way of
example, the revenue benefit metric in the case of limiting the second electric
power is referred to as a first revenue benefit metric and the revenue benefit
metric in the case of limiting the third electric power is referred to as a second
revenue benefit metric. Subsequently, the controller 124 may be configured to
determine maximum of the two revenue benefit metrics. Accordingly, the
controller 124, at step 314, may carry out a check to determine if the first revenue
benefit metric is maximum. At step 314, if it is determined that the first revenue
benefit metric is maximum, the controller 124, at step 318, may be configured to
limit the second electrical power. Further details of limiting the second electrical
power is described in conjunction with FIG. 4. However, at step 314, if it is
determined that the second revenue benefit metric is maximum, the controller 124
may limit the third electrical power at step 316. Further details of limiting the
third electrical power is described in conjunction with FIG. 5.
[0047] FIG. 4 is a flowchart 400 of an example method for limiting the
second electrical power, in accordance with aspects of the present specification.
In some embodiments, the method of limiting the second electrical power may be
performed, at step 308, in response to determining (at step 302) that the wind
287294-1
19
power tariff 304 is same as the auxiliary power tariff 306. In some embodiments,
the method of limiting the second electrical power may be performed, at step 318,
in response to determining (at step 314) that the first revenue benefit metric is
maximum. The controller 124 may aid in executing steps 402-412 of FIG. 4.
[0048] At step 402, the controller 124 may be configured to determine a level
of the third electrical power received at the DC-link 118 from the auxiliary power
source 110 using one of the techniques discussed above with respect to step 206
of FIG. 2, for example.
[0049] Furthermore, at step 404, the controller 124 may be configured to
determine a level of the first electrical power generated by the generator 112. In
some embodiments, the power generation system 101 may include one or more
voltage or current sensors (not shown in FIG. 1) disposed at the stator winding
130. The controller 124 may determine the level of the first electrical power
based on signals received from the voltage or current sensors disposed at the
stator winding 130.
[0050] Further, at step 406, the controller 124 may be configured to determine
a desired level of the second electrical power based on the level of the third
electrical power (determined at step 402) and the value of the rated power 210
(received at step 208 in flowchart 200 of FIG. 2) of the line side converter 116.
In a non-limiting example, to determine the desired level of the second electrical
power, the controller 124 may be configured to subtract the level of the third
electrical power from the value of the rated power 210 of the line side converter
116. The difference between the values of the third electrical power and the rated
power 210 of the line side converter 116 may represent the desired level of the
second electrical power.
[0051] Moreover, at step 408, the controller 124 may be configured to
determine a desired slip value of the generator 112 based on the level of the first
electrical power and the desired level of the second electrical power. In some
embodiments, the desire slip value may be lower than a current slip value of the
287294-1
20
generator 112. In some embodiments, a relationship between the first electrical
power and the second electrical power may be represented by Equation (5) as
represented below:
2 1 P = S *P ... Equation (5)
where, 1 P represents the level of the first electrical power (determined
at step 404), 2 P represents the level of the second electrical power (determined at
step 406), and S represents the desired slip value.
[0052] Accordingly, the desired slip value ( S ) may be determined based on
Equation (6) as represented below:
1
2
P
S = P ... Equation (6)
[0053] Once the desired slip value ( S ) is determined, the controller 124 may
be configured to determine a generator rotor speed (i.e., the operating speed of the
rotor 128 r N ) corresponding to the desired slip value ( S ) at step 410. In a nonlimiting
example, the generator rotor speed ( r N ) may be determined using
following Equation (7) as represented below:
s r
s
r N N
N S N
-
= * ... Equation (7)
[0054] Subsequently, at step 412, the rotor 128 of the generator 112 may be
operated at the determined generator rotor speed ( r N ). In one example, the rotor
128 of the generator 112 may be operated at the determined generator rotor speed
( r N ) by operating the rotor 107 of the wind turbine 106 at the corresponding
speed, depending a gear ratio (if the wind turbine 106 is coupled to the rotor 128
via a gear box).
[0055] Accordingly, to operate the rotor 128 at the determined generator rotor
speed ( r N ), the controller 124 may be configured to determine at least one of a
287294-1
21
pitch value of the rotor blades 113 and a torque value of a wind rotor 107
corresponding to the desired slip value ( S ). In some embodiments, the controller
124 may determine the pitch value of the rotor blades 113 and the torque value of
a wind rotor 107 based on one or more look-up tables. The one or more look-up
tables may contain different values of the pitch and torque corresponding to
different values of the generator rotor speed ( r N ). Subsequently, at least one of
the pitch and the torque of the wind rotor 107 may be adjusted by the controller
124 such that the rotor 128 may be operated at the determined generator rotor
speed ( r N ).
[0056] In some embodiments, it may not be desirable to operate the wind
turbine 106 such that corresponding rated specification values (e.g., rated torque,
rated thrust) are exceeded. For example, it may not be desirable to operate the
wind turbine 106 in a torque limited regime. The term “torque limited regime”
refers to operation of the wind turbine 106 at a torque such that the torque on the
rotor 107 exceeds the rated torque of the wind turbine 106. Accordingly, in some
embodiments, at least one of the pitch of the rotor blades 113 and the torque of
the wind rotor 107 may be adjusted such that the determined torque value of the
wind rotor is maintained lower than or equal to the rated torque of the wind
turbine 106.
[0057] Furthermore, in some embodiments, it may not be desirable to operate
the wind turbine 106 in a thrust limited regime. The term “thrust limited regime”
refers to operation of the wind turbine 106 at such that the thrust on the rotor 107
exceeds the rated thrust of the wind turbine 106. Accordingly, in some
embodiments, at least one of the pitch of the rotor blades 113 and the torque of
the wind rotor 107 may be adjusted such that the determined thrust on the rotor
107 is maintained lower than or equal to the rated thrust of the wind turbine 106.
[0058] Moreover, in certain embodiments, the determined pitch value is such
that the rotor blades 113 are not operated in a stall condition. For example, a
collective pitch angle of the rotor blades 113 may not adjusted at values that can
287294-1
22
induce the stall condition. The term “stall condition” as used herein refers to a
situation when there exists a loss of lift on the rotor blades 113. Such loss of lift
on the rotor blades 113 may substantially reduce the torque applied on the rotor
blades 113. The stall condition may happen if the pitch angle of the rotor blades
113 is set to be below a minimum pitch setting for a given blade 113. The stall
condition may be defined in terms of a range of tip speed ratio (TSR) and pitch
values which result in stalling of the rotor blades 113. Accordingly, the pitch
value may be determined such that the determined pitch value is higher than the
“minimum pitch setting” based on current operating TSR. In embodiments where
the wind power tariff is similar to the auxiliary power tariff, and it is a challenge
to operate within the above discussed limits by solely limiting the second
electrical power, it may be preferable to limit the third electrical power instead of
or in addition to the second electrical power.
[0059] FIG. 5 is a flowchart 500 of an example method for limiting a third
electrical power, in accordance with aspects of the present specification. In some
embodiments, the method of limiting the third electrical power may be
performed, at step 308, in response to determining (at step 302) that the wind
power tariff 304 is the same as the auxiliary power tariff 306. In some
embodiments, the method of limiting the third electrical power may be
performed, at step 316, in response to determining (at step 314) that the first
revenue benefit metric is not maximum (i.e., the second revenue benefit metric is
maximum). The controller 124 may aid in executing steps 502-506 of FIG. 5.
[0060] At step 502, the controller 124 may be configured to determine a level
of the second electrical power received at the DC-link 118 from the rotor winding
132 using one of the techniques discussed above with respect to step 206 of FIG.
2, for example.
[0061] Further, at step 504, the controller 124 may be configured to determine
a desired level of the third electrical power based on the level of the second
electrical power (determined at step 502) and the value of the rated power 210
287294-1
23
(received at step 208 in flowchart 200 of FIG. 2) of the line side converter 116.
In a non-limiting example, to determine the desired level of the third electrical
power, the controller 124 may be configured to subtract the level of the second
electrical power from the value of the rated power 210 of the line side converter
116. The difference between the values of the second electrical power and the
rated power 210 of the line side converter 116 may represent the desired level of
the third electrical power.
[0062] Furthermore, at step 506, the controller 24 may be configured to limit
the third electrical power to the desired level of the third electrical power that was
determined at step 504. In some embodiments, to limit the third electrical power,
the controller 124 may operate one or more of the first DC-DC converter 134 and
the second DC-DC converter 136 as the buck converter.
EXAMPLE:
[0063] The value of the rated power 210 of the line side converter 116 is 350
kW, the level of first power generated by at the stator winding is 1000 kW, a
current level of the second power generated at the rotor winding 132 is 250 kW,
the level of third electrical power supplied from the auxiliary power source 110 is
150 kW, a current slip value is 0.25. Input power to the line side converter 116 is
400 kW (250 kW+150 kW). In a given situation, the power supplied via the line
side converter is restricted to the rated power (350 kW). Consequently, the
hybrid power supplied at the PTC 103 is 1350 kW (1000 kW + 350 kW).
[0064] In the given situation, the input power (determined at step 206) to the
line side converter 116, absent control measures for limiting, may be 400 kW.
Accordingly, it may be determined (at step 208) that the input power is higher
than the rated power 210 of the line side converter 116 which is 350 kW.
Therefore, it may be determined (at step 214) to limit one of the second electrical
power or the third electrical power.
287294-1
24
[0065] Assuming f = 1 (i.e., the wind power tariff 304 is same as the
auxiliary power tariff 306), the controller 124 may determine (at step 308)
whether to limit the second electrical power. The controller 124 may determine
(at step 402) the level of third electrical power supplied from the auxiliary power
source 110 as 150 kW. The controller 124 may determine (at step 404) the level
of first electrical power supplied from the stator winding 130 as 1000 kW. The
controller 124 may further determine (at step 406) a desired level of the second
electrical power as being 200 kW (350 kW-150 kW). Accordingly, the controller
124 may determine (at step 408) the desired slip value ( S ) as 0.225 based on the
feasible region of operation (i.e., without exceeding the rated torque and thrust
values) of the turbine and the optimal operating point in terms of total hybrid
power. If the generator 112 is operated at this slip (steps 410-412) by adjusting
the generator rotor by way of adjusting the pitch of the rotor blades 113 and the
torque of the rotor 107, the generator 112 may generate 990 kw (i.e., first
electrical power) at the stator winding 130 and 222.75 kW (i.e., second electrical
power) at the rotor winding 132. Consequently, the hybrid power supplied at the
PTC 103 is 1362 kW (990 kW+222.75kW+150 kW) which is 12 kW higher than
1350 kW generated at the slip od 0.25. Clearly, adjusting the slip of the generator
112 in accordance with embodiments of the present specification maximizes the
generation of hybrid power.
[0066] Any of the foregoing steps and/or system elements may be suitably
replaced, reordered, or removed, and additional steps and/or system elements may
be inserted, depending on the needs of a particular application, and that the
systems of the foregoing embodiments may be implemented using a wide variety
of suitable processes and system elements and are not limited to any particular
computer hardware, software, middleware, firmware, microcode, etc.
[0067] Furthermore, the foregoing examples, demonstrations, and method
steps such as those that may be performed by the controller 124 may be
implemented by suitable code on a processor-based system, such as a generalpurpose
or special-purpose computer. Different implementations of the systems
287294-1
25
and methods may perform some or all of the steps described herein in different
orders, parallel, or substantially concurrently. Furthermore, the functions may be
implemented in a variety of programming languages, including but not limited to
C++ or Java. Such code may be stored or adapted for storage on one or more
tangible or non-transitory computer readable media, such as on data repository
chips, local or remote hard disks, optical disks (that is, CDs or DVDs), memory or
other media, which may be accessed by a processor-based system to execute the
stored code. Note that the tangible media may include paper or another suitable
medium upon which the instructions are printed. For instance, the instructions
may be electronically captured via optical scanning of the paper or other medium,
then compiled, interpreted or otherwise processed in a suitable manner if
necessary, and then stored in the data repository or memory.
[0068] In accordance with some embodiments of present specification, the
power generation system 101 may be operated such that power generation is
maximized. This is achieved, in part, via selectively limiting power supplied from
one of the wind turbine (via the rotor winding 132) and the auxiliary power
source 110. Additionally, when wind power tariff and the auxiliary power tariff
are different, the power generation is maximized such that cost per unit of power
is minimized. Moreover, useful life of the line side converter may also be
increased by limiting input power to the line side converter. Consequently,
reliability of the power generation system 101 may also be improved.
[0069] The present invention has been described in terms of some specific
embodiments. They are intended for illustration only, and should not be
construed as being limiting in any way. Thus, it should be understood that
modifications can be made thereto, which are within the scope of the invention
and the appended claims.
[0070] It will be appreciated that variants of the above disclosed and other
features and functions, or alternatives thereof, may be combined to create many
other different systems or applications. Various unanticipated alternatives,
287294-1
26
modifications, variations, or improvements therein may be subsequently made by
those skilled in the art and are also intended to be encompassed by the following
claims.