U.S. patent application number 13/593841 was filed with the patent office on 2013-03-07 for transformer chamber for a wind turbine, wind turbine structure component, wind turbine, and method for assembling a wind turbine.
The applicant listed for this patent is Jesper Gaard, Thorkil Munk-Hansen, Jacob Blach Nielsen, Alex Yundong Wang. Invention is credited to Jesper Gaard, Thorkil Munk-Hansen, Jacob Blach Nielsen, Alex Yundong Wang.
Application Number | 20130058070 13/593841 |
Document ID | / |
Family ID | 44674419 |
Filed Date | 2013-03-07 |
United States Patent
Application |
20130058070 |
Kind Code |
A1 |
Gaard; Jesper ; et
al. |
March 7, 2013 |
Transformer chamber for a wind turbine, wind turbine structure
component, wind turbine, and method for assembling a wind
turbine
Abstract
A transformer chamber for a wind turbine is described. The
transformer chamber includes a liquid-tight tank for receiving a
liquid-filled, in particular oil-filled, transformer, a wind
turbine structure component includes a component bedframe adapted
for receiving such a transformer chamber and a wind turbine
includes such a transformer chamber and such a wind turbine
structure component. Furthermore, a method for assembling a wind
turbine is described.
Inventors: |
Gaard; Jesper; (Odense S,
DK) ; Munk-Hansen; Thorkil; (Give, DK) ;
Nielsen; Jacob Blach; (Engesvang, DK) ; Wang; Alex
Yundong; (Herning, DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Gaard; Jesper
Munk-Hansen; Thorkil
Nielsen; Jacob Blach
Wang; Alex Yundong |
Odense S
Give
Engesvang
Herning |
|
DK
DK
DK
DK |
|
|
Family ID: |
44674419 |
Appl. No.: |
13/593841 |
Filed: |
August 24, 2012 |
Current U.S.
Class: |
361/836 |
Current CPC
Class: |
Y02E 10/72 20130101;
H01F 27/321 20130101; F03D 80/82 20160501; H01F 27/06 20130101;
H01F 27/12 20130101; H01F 27/025 20130101; H01F 27/085 20130101;
F05B 2240/14 20130101 |
Class at
Publication: |
361/836 |
International
Class: |
H05K 7/14 20060101
H05K007/14 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2011 |
EP |
EP11179874 |
Claims
1. A wind turbine, comprising: a transformer chamber for a wind
turbine, comprising a liquid-tight tank for receiving an oil-filled
transformer, a nacelle comprising a component bedframe adapted for
receiving a transformer chamber; and a rotor, wherein the
transformer chamber is detachably connected to the component
bedframe, and wherein the transformer chamber is placed on the
opposite side of the rotor.
2. The wind turbine according to claim 1, further comprising: low
voltage terminals; and high voltage terminals, wherein the low
voltage terminals and high voltage terminals being accessible from
a top surface of the transformer chamber.
3. The wind turbine according to claim 1, further comprising: a
damper including hydraulic cylinders.
4. The a wind turbine according to claim 1, wherein the oil-tight
tank is an explosion safe tank and comprises an explosion
plate.
5. The a wind turbine according to claim 4, wherein the explosion
plate is made from aluminum.
6. The wind turbine according to claim 1, further comprising:
comprising guiding elements.
7. wind turbine according to claim 6, wherein the guiding elements
are wheels.
8. The wind turbine according to claim 1, further comprising: a
cooling channel system.
9. The wind turbine according to claim 1, wherein the component
bedframe comprises a damper.
10. The wind turbine according to claim 1, wherein the damper is a
hydraulic cylinder damper.
11. The wind turbine according to claim 1, further comprising: a
winch and/or a hatch.
12. The wind turbine structure component according to claim 1,
further comprising: a cooling air inlet and a cooling air outlet
adapted to provide a cooling air path in the transformer
chamber.
13. The wind turbine structure component according to claim 12,
further comprising: a fan.
14. The wind turbine according to claim 1, wherein the transformer
chamber is movable relative to the wind turbine structure
component.
15. The wind turbine according to claim 1, wherein the transformer
chamber is movable the direction of gravity.
16. A method for assembling a wind turbine according to claim 1.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of European Patent Office
application No. 11179874.0 EP filed Sep. 2, 2011. All of the
applications are incorporated by reference herein in their
entirety.
FIELD OF INVENTION
[0002] The present disclosure relates to the field of wind
turbines. In particular, the disclosure relates to a transformer
chamber, a wind turbine structure component, a wind turbine, and a
method for assembling a wind turbine.
BACKGROUND OF INVENTION
[0003] Wind turbines becoming more and more popular for ecological
power generation.
[0004] Power generated by wind turbines has to be transformed to be
transportable via high voltage transmission lines to the
consumers.
[0005] Liquid leaking from a liquid-filed transformer in case of a
transformer failure may harm, in particular electrical and
controlling, components near the liquid-filed transformer.
SUMMARY OF INVENTION
[0006] There may be a need for a transformer chamber, a wind
turbine structure component, a wind turbine, and a method for
manufacturing a wind turbine reducing the risk associated with
leaking liquid.
[0007] This need may be met by the subject matter according to the
independent claims Embodiments are described by the dependent
claims.
[0008] According to a first aspect, there is provided a transformer
chamber for a wind turbine comprising a liquid-tight tank for
receiving a liquid-filled, in particular oil-filled, transformer.
This aspect is based on the idea that a second shell may limit the
effect of transformer failures, as liquid originating from the
transformer may be kept contained within the liquid-tight tank.
[0009] Liquid-filled transformers are used as they may allow for
better heat dissipation from coils and transformer sheets during
operation of the transformer. In particular, insulating liquids may
be used for liquid-filled transformers. These insulating liquids
may help to avoid corona formation and arcing within the
transformer.
[0010] Some liquids like polychlorinated biphenyls may combine good
insulating properties, i.e. a high dielectric strength, with good
cooling properties. However, these liquids may be toxic. A
transformer chamber with a liquid-tight tank may thus prevent that
persons are exposed to these toxic substances.
[0011] Transformer oil has also been found to combine good cooling
properties and insulating properties. However, transformer oil may
be less fire-resistant. The liquid-tight tank may prevent that
leaking transformer oil reaches surfaces having a temperature above
a flame point of the transformer oil. Hence, a transformer chamber
with a liquid-tight tank may prevent inflammation of the
transformer oil.
[0012] The transformer chamber may comprise a pit, in particular an
oil pit. The pit may allow collecting liquid, in particular
transformer oil, during transformer chamber maintenance and reduce
the amount of, e.g. toxic or burnable, substances within the
transformer chamber.
[0013] According to an embodiment of the transformer chamber for a
wind turbine, the transformer chamber comprises a liquid-filled
transformer with low voltage terminals and high voltage terminals,
wherein the low voltage terminals and high voltage terminals are
accessible from a top surface of the transformer chamber.
[0014] This may reduce the time necessary for electrical
installation of the transformer while at the same time maintaining
the structural integrity of the liquid-tight tank. In particular,
the transformer chamber may remain an essentially sealed tank such
that no liquids may leak through the transformer chamber.
[0015] According to another embodiment of the transformer chamber
for a wind turbine, the transformer chamber comprises a damper, in
particular a damper including hydraulic cylinders. Typically,
transformers have a quite high mass, because they are essentially
composed of huge amounts of copper and iron. Providing the
transformer chamber for a wind turbine with a damper may reduce the
oscillations associated with the transformer being arranged in at
the top of a wind turbine. Thus, the load on the wind turbine tower
may be reduced.
[0016] According to yet another embodiment of the transformer
chamber for a wind turbine, the liquid-tight tank is an explosion
safe tank comprising an explosion plate, in particular an explosion
plate made from aluminum.
[0017] Gas may be produced during operation of the transformer,
which gas may explode. If the liquid-tight tank is constructed so
as to be an explosion safe tank comprising an explosion plate the
risk to damage elements near the transformer in case of a
transformer explosion may be limited.
[0018] The explosion plate of the explosion tank may work as a
predetermined breaking point. Thus, pressure due to the explosion
may be released in a determined way.
[0019] Aluminum may be in particular suitable because of its lower
strength compared to steel, which may be the preferred material for
the transformer chamber.
[0020] According to a still further embodiment of the transformer
chamber for a wind turbine, the transformer chamber comprises
guiding elements, in particular wheels.
[0021] These guiding elements may facilitate installation of the
transformer chamber. Typically, the transformer chamber is
installed at the top of the wind turbine, e.g. in the wind turbine
nacelle or the wind turbine tower. The guiding elements, in
particular wheels, may allow supporting the transformer chamber
against the wind turbine tower either from the outside or form the
inside when it is winched to the top. The exchangeability of the
transformer chamber may likewise be improved.
[0022] According to a second aspect there is provided a wind
turbine structure component comprising a component bedframe adapted
for receiving a transformer chamber. The component bedframe may
allow a particularly easy fixing of the transformer chamber to the
wind turbine structure component.
[0023] According to a first embodiment of the wind turbine
structure component, the wind turbine structure component comprises
a cooling channel system.
[0024] During operation heat has to be dissipated from the
liquid-filled transformer. Providing the transformer chamber with a
cooling channel system may allow for heat dissipation by convection
that may be more efficient than heat dissipation by radiation
and/or conduction. In particular, the cooling channels may be
construed to guide air through fins of the liquid-filled
transformer. Fins may augment the effective surface for heat
dissipation.
[0025] According to second embodiment the wind turbine structure
component comprises a damper, in particular a hydraulic cylinder
damper.
[0026] Typically, transformers have a quite high mass, because they
are essentially composed of huge amounts of copper and iron.
Providing the wind turbine structure component with a damper may
reduce the oscillations associated with the transformer being
provided at the top of a wind turbine. Thus, the load on the wind
turbine tower may be reduced.
[0027] According to another embodiment of the wind turbine
structure component is a wind turbine tower segment.
[0028] If the wind turbine structure component is a wind turbine
tower segment, the transformer chamber may be guided within the
wind turbine tower during installation. Thus, installation of the
transformer chamber may be performed in an environment protected
from wind and rain. Furthermore, the transformer chamber may not be
subjected to rotational forces when the wind turbine rotor is moved
into the wind direction.
[0029] According to a yet another embodiment of the wind turbine
structure component is a wind turbine nacelle.
[0030] The weight of the transformer may be beneficial when the
wind turbine nacelle is adapted to receive the transformer chamber.
The transformer may in this way be a counterweight to the wind
turbine rotor.
[0031] Placing the transformer chamber at the top of the wind
turbine may help to reduce the cable length from the generator or
converter to the transformer, in particular when the transformer
chamber is provided within the wind turbine nacelle.
[0032] According to a further embodiment the wind turbine structure
component comprises a winch.
[0033] The winch may render allow for installation of the wind
turbine chamber without providing a crane. Thus, installation of
the wind turbine chamber may be cheaper, in particular for offshore
wind turbines. All means to install the transformer chamber may be
provided on site.
[0034] According to a still further embodiment the wind turbine
structure component comprises a hatch. The hatch may allow moving
the transformer chamber into the wind turbine structure
[0035] According to another embodiment the wind turbine structure
component comprises at least one cooling air inlet and at least one
cooling air outlet adapted to provide the transformer chamber with
cooling air.
[0036] During operation heat has to be dissipated from the
liquid-filled transformer and the transformer chamber. Providing
the transformer chamber with at least one cooling air inlet and at
least one cooling air outlet adapted to provide the transformer
chamber with cooling air may allow using the wind driving the wind
turbine rotor to be used for cooling.
[0037] According to yet another embodiment the wind turbine
structure component comprises a fan. A fan may augment the amount
of air provided to the transformer chamber and/or other components
of the wind turbine. Thus, even when there is a minimal wind,
overheating of wind turbine components may be avoided.
[0038] According to a third aspect there is provided a wind turbine
comprising a transformer chamber as has been described hereinbefore
and a wind turbine structure component, wherein the transformer
chamber is detachably connected to the component bedframe.
[0039] Such a wind turbine allows easy exchangeability of the
transformer chamber and the transformer in case of a transformer
failure. Furthermore, installation of the wind turbine, in
particular at offshore location, may be simplified.
[0040] According to an embodiment of the wind turbine the
transformer chamber is movable, in particular in the direction of
gravity, relative to the wind turbine structure component.
[0041] Such a wind turbine may further facilitate the installation
of the transformer chamber within the wind turbine structure
component. A wind turbine nacelle may, for example, be provided
with a hatch at the bottom such that the transformer chamber may be
winched through the opened hatched.
[0042] According to a forth aspect there is provided a method for
assembling a wind turbine as has been described hereinbefore.
[0043] The method may in particular be useful to erect and to
overhaul a wind turbine.
[0044] It has to be noted that embodiments have been described with
reference to different subject matters. In particular, some
embodiments have been described with reference to method type
claims whereas other embodiments have been described with reference
to apparatus type claims. However, a person skilled in the art will
gather from the above and the following description that, unless
other notified, in addition to any combination of features
belonging to one type of subject matter also any combination
between features relating to different subject matters, in
particular between features of the method type claims and features
of the apparatus type claims is considered as to be disclosed with
this document.
[0045] The aspects defined above and further aspects are apparent
from the examples of embodiment to be described hereinafter and are
explained with reference to the examples of embodiment. The
embodiments will be described are in more detail hereinafter and
are meant to be illustrative and not limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 shows a transformer chamber in a perspective
view.
[0047] FIG. 2 shows a cutaway view of a transformer chamber.
[0048] FIG. 3 shows another cutaway view of a transformer
chamber.
[0049] FIG. 4 shows yet another cutaway view of a transformer
chamber.
[0050] FIG. 5 shows a further cutaway view of a transformer
chamber.
[0051] FIG. 6 shows a still further cutaway view of a transformer
chamber.
[0052] FIG. 7 shows a cooling channel system of a transformer
chamber.
[0053] FIG. 8 shows a section of a wind turbine.
[0054] FIG. 9 depicts a wind turbine nacelle.
[0055] FIG. 10 shows a section of a wind turbine nacelle.
[0056] FIG. 11 depicts a section of a wind turbine nacelle.
[0057] FIG. 12 shows a transformer chamber
[0058] FIG. 13 shows a transformer chamber
[0059] FIG. 14 shows a section of a wind turbine nacelle.
DETAILED DESCRIPTION OF INVENTION
[0060] The illustration in the drawing is schematically.
[0061] FIG. 1 shows a transformer chamber 1 with an explosion safe
tank 2 in a perspective view. The transformer chamber 1 surrounds
an oil-filled transformer 3 only the top of which is visible. Air
may flow through a first opening 4 into a cooling channel system of
the transformer chamber 1, where it may absorb heat generated by
the oil-filled transformer 3. The hot air may then leave the
cooling channel system of the transformer chamber 1 through a
second opening 5.
[0062] The bottom section of the transformer chamber 1 up to the
height of the first opening 4 may be formed as an oil pit 6.
Leaking oil from a damaged oil-filled transformer, e.g. after an
explosion of the oil-filled transformer, may be collected within
the oil pit 6. An explosion plate 7 made of aluminum may work as a
predetermined breaking point and may reduce the damages in case of
an explosion of the oil-filled transformer 3.
[0063] An upper side of the transformer chamber 1 is open and
allows easy access to a high voltage terminal 8 and a low voltage
terminal 9 of the oil-filled transformer 3. This may allow a quick
and easy replacement and reconnection of the oil-filled transformer
3 to electric wires. The top of the oil-filled transformer 3
further comprises transport rings 10, 11, to which a hook or chain
can be attached when an exchange of the oil-filled transformer 3
becomes necessary and the transformer chamber 1 with the oil-filled
transformer therein needs to be lowered to or picked from surface
level with a winch.
[0064] The transformer chamber 1 further comprises two support
rails 12, 13, which may be used to attach the transformer chamber 1
to a bedframe of, e.g. a wind turbine nacelle. Two support beams 14
and 15 may enhance the rigidity of the transformer chamber 1.
[0065] FIG. 2 shows a cutaway view of a transformer chamber 16 with
an explosion safe tank 17. Like the transformer chamber 1 shown in
FIG. 1 the transformer chamber 16 comprises an oil-filled
transformer 18, a first opening 19, a second opening 20, an oil pit
21, an explosion plate 22, a high voltage terminal 23, a low
voltage terminal 24, transport rings 25, 26, support rails 27, 28,
and support beams 29, 30. The features of this embodiment have been
described with reference to FIG. 1. The oil-filled transformer 18
is placed in the transformer chamber 16 such that the distance
between cooling fins 31 of the oil-filled transformer 18 and the
wall of the transformer chamber 16 is kept very narrow. The narrow
arrangement forces the air coming from the first opening along the
cooling fins 31 for better cooling of the surfaces of the
oil-filled transformer 18.
[0066] FIGS. 3, 4, 5 show further cutaway views of transformer
chambers 32, 45, 58 with an explosion safe tank 33, 46, 59. These
transformer chambers 32, 45, 58 also include an oil-filled
transformer 34, 47 a first opening, a second opening 35, 48 an oil
pit, an explosion plate, a high voltage terminal 36, 49, 60 a low
voltage terminal 37, 50, 61 transport rings 38, 39, 51, 52, 62, 63
support rails 40, 41, 53, 54, 64, 65, support beams 42, 43, 55, 56,
66, 67, and cooling fins 44, 57 comparable to those as have been
described hereinbefore.
[0067] FIG. 6 shows a still further cutaway view of a transformer
chamber 68 focusing on its explosion plate 69 above its first
opening 70. The explosion plate 69 is made from aluminum and
represents a predetermined breaking point, which bursts in case of
an explosion and allows release of pressure at a secure location
avoiding unpredictable and harmful damage to the transformer
chamber 68 and its features. The explosion plate is fixed to the
transformer chamber 68 with seven bolts 71, 72, 73, 74, 75, 76, 77
of which five bolts 71, 72, 73, 74, 75 are located along a bottom
portion parallel to the first opening 70. A bending section of the
explosion plate is created with slits 78 running parallel to the
first opening 70. Two deformation bolts 76, 77 hold the explosion
plate 69 to the transformer chamber 68 on an upper section of the
explosion plate 69. In case of an explosion these two bolts 76, 77
and the explosion plate 69 bends outwardly along the bending
section. The bending section works similar to a hinge.
[0068] FIG. 7 shows a cooling channel system 79 for a transformer
chamber. Two inlet interfaces 80, 81 may be connected to
transformer chamber's first openings. One outlet interface 82 may
be joinable to a second opening of a transformer chamber. A fan may
be located behind the outlet interface and create a cooling airflow
by sucking in ambient air. This air may then be fed into the
transformer chamber through the two inlet interfaces 80, 81 and be
received by the outlet interface 82 before it is blown out through
the channel outlet 83 of the cooling channel system 79.
[0069] FIG. 8 shows a section of a wind turbine 84 comprising a
wind turbine tower 85. A person 86 is depicted to give an
indication of the size of the wind turbine 84. A wind turbine
nacelle 87 is mounted to the top of the wind turbine tower 85. The
outer walls of the wind turbine nacelle 87 are not shown. The wind
turbine nacelle 87 comprises a support structure and a component
bedframe 88, which is bolted to the support structure. However, the
component bedframe 88 and the support structure may in an
alternative embodiment also be made in one piece.
[0070] The support structure is rotatable connected to the wind
turbine tower 85 and supports a generator 89 with a wind turbine
rotor 90 connected thereto. The wind turbine rotor 90 comprises a
hub 91 and blades 92.
[0071] The component bedframe 88 and the support structure carry
inter alia electrical components 93 such as a converter,
communication units and control units. A transformer chamber 94
holding an oil-filled transformer is mounted to the bottom side of
the component bedframe 88 in close vicinity to the wind turbine
tower 85. The center of mass is located close to the wind turbine
tower 85 by positioning the heavy oil-filled transformer close to
the wind turbine tower 85. Thus, structural loads on the wind
turbine 84, in particular on the wind turbine tower 85, the support
structure and the component bedframe 88, may be minimized.
[0072] Furthermore, by placing the transformer chamber 94 on the
opposite side of the wind turbine rotor 90 the weight of the
transformer chamber 94 comprising the oil-filled transformer may
counteract the weight of the wind turbine rotor 90. This may
additionally reduce structural loads.
[0073] The transformer chamber 94 comprises guiding in elements in
form of wheels. These wheels may allow an easy movement of the
transformer chamber 94 relative to the wind turbine tower 85, which
may be used as guidance when the transformer chamber 94 is lowered
to the surface or raised therefrom. Guiding the transformer chamber
94 may reduce the security risks associated with swinging
masses.
[0074] The wind turbine nacelle 87 may additionally comprise a
winch with a chain or rope, which is attachable to the transformer
chamber 94. This may allow exchanging the transformer chamber 94
without the need of additional cranes.
[0075] FIG. 9 depicts a wind turbine nacelle 95 mounted to the top
of a wind turbine tower 96. A transformer chamber 97 comprising an
explosion safe tank 98 is bolted to a component bedframe 99. A
ladder 100 provides ease of access to the transformer chamber 97
and to electrical components 101 from the support structure. The
ladder 100 and the placement of the transformer chamber 97 with the
transformer therein inside the canopy of the wind turbine nacelle
95 may allow repair work and service to be done even under adverse
weather conditions.
[0076] FIG. 10 shows a section of a wind turbine nacelle 102 in a
cutaway view. The wind turbine nacelle 102 houses a transformer
chamber 103 with an explosion safe tank. The transformer chamber
103 comprises an oil-filled transformer 104 and is attached to a
component bedframe of the wind turbine nacelle 102. A hatch 105 is
provided such that the transformer chamber 103 may easily be
lowered from the wind turbine nacelle 102 to the surface. The wind
turbine nacelle 102 further houses electric components 106, e.g. a
converter, communication units and control units, which may be
arranged in cabinets on a support structure of the wind turbine
nacelle 102. The access to the transformer chamber 103 and to the
electric components 106 may be facilitated by means of a ladder
107. The wind turbine nacelle 102 further comprises a cooling
channel system 108 for the transformer chamber 103. Fans 109 blow
ambient air into the cooling channel system 108 and the transformer
chamber 103, where it serves to dissipate from the oil-filled
transformer 104. The hot air then is guided by the cooling channel
system 108 to the channel outlet 110 where it is released to the
outside of the wind turbine nacelle 102.
[0077] FIG. 11 depicts a section of a wind turbine nacelle 111
housing a transformer chamber 112. Only parts of the transformer
chamber 112 are shown. A transformer (not shown) rests on two pads
113, 114 of the transformer chamber 112. The pads 113, 114 may be
made of a resilient material and absorb the vibrations associated
with the frequency of the power grid. The material may in
particular be adapted to absorb vibrations with a frequency of 50
Hertz or 60 Hertz. A support beam 115 is provided to augment the
rigidity of the transformer chamber 112. The transformer chamber
112 is attached to the component bedframe of the wind turbine
nacelle 111 with two support rails 116, 117 such that in case of an
explosion of the transformer an explosion plate 118 of the
transformer chamber 112 may work as a predetermined breaking point
and the pressure may be released in a direction laterally of the
wind turbine nacelle 111. Thus, harmful damages to other components
within the wind turbine nacelle 111 may be avoided or at least
reduced. A wind turbine tower 119 supports the wind turbine nacelle
111 and a cable 120 transmits the power generated from a high
voltage terminal of the oil-filled transformer along the inside of
the wind turbine tower 119 to the main power grid.
[0078] FIGS. 12 and 13 show a transformer chamber 120. The
transformer chamber 120, 128 comprises L-shaped support rails 121,
122, 129. With these L-shaped support rails 121, 122, 129 the
transformer chamber 120, 128 is bolted at four corners 123, 124,
125 to a component bedframe 126 of a wind turbine nacelle 127. The
transformer 130 is bolted to the top 131 and the bottom 132 of the
transformer chamber 128.
[0079] FIG. 14 shows a section of a wind turbine nacelle 133 at the
top of a wind turbine tower 134. The wind turbine nacelle 133
comprises an inlet 135 for sucking in ambient air for cooling of
components within the wind turbine nacelle 133 and an outlet 136
for blowing out hot air.
[0080] While specific embodiments have been described in detail,
those with ordinary skill in the art will appreciate that various
modifications and alternative to those details could be developed
in light of the overall teachings of the disclosure. For example,
elements described in association with different embodiments may be
combined. Accordingly, the particular arrangements disclosed are
meant to be illustrative only and should not be construed as
limiting the scope of the claims or disclosure, which are to be
given the full breadth of the appended claims, and any and all
equivalents thereof. It should be noted that the term "comprising"
does not exclude other elements or steps and the use of articles
"a" or "an" does not exclude a plurality.
* * * * *