U.S. patent application number 11/159337 was filed with the patent office on 2006-05-18 for electroplating method for sealing liquid-cooled generator stator bar structures.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Yu Wang.
Application Number | 20060103261 11/159337 |
Document ID | / |
Family ID | 36313996 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060103261 |
Kind Code |
A1 |
Wang; Yu |
May 18, 2006 |
Electroplating method for sealing liquid-cooled generator stator
bar structures
Abstract
The interior of a stator bar clip that is joined to the end of a
stator bar is electroplated so that a metallic barrier coating
overlies the braze joint between the stator bar and the clip to
define a seal at and around the joint that is substantially
impermeable to liquid.
Inventors: |
Wang; Yu; (Clifton Park,
NY) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
36313996 |
Appl. No.: |
11/159337 |
Filed: |
June 23, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10990958 |
Nov 18, 2004 |
|
|
|
11159337 |
Jun 23, 2005 |
|
|
|
Current U.S.
Class: |
310/254.1 ;
205/122 |
Current CPC
Class: |
H02K 15/12 20130101;
C25D 21/10 20130101; C25D 7/04 20130101; H02K 15/0093 20130101 |
Class at
Publication: |
310/254 ;
205/122 |
International
Class: |
H02K 1/12 20060101
H02K001/12; C25D 5/02 20060101 C25D005/02 |
Claims
1-11. (canceled)
12. A method of sealing interior surfaces of a fitting receiving a
stator bar end, to define a seal substantially impermeable to
liquid, the fitting having a chamber for receiving a liquid through
an opening in the fitting and said chamber being in communication
with hollow strands forming at least a portion of said stator bar,
said method comprising the steps of: disposing an electroplating
solution in said chamber; disposing an anode of the metal or metal
alloy to be deposited in said electroplating solution; connecting
said anode to the positive terminal of a current source as a
cathode; electrically connecting said stator bar to the negative
terminal of said current source; and establishing an electrical
potential between said anode and said cathode to initiate migration
of metal ions from the anode to the electrically conductive inner
surface of the component.
13. A method as in claim 12, wherein said anode is formed from at
least one of nickel, chromium, zinc and copper.
14. A method as in claim 12, wherein said anode comprises a
consumable metal source.
15. A method as in claim 12, further comprising pumping
electroplating solution into said chamber and drawing
electroplating solution out of said chamber, to thereby circulate
electroplating solution through said chamber.
16. A method as in claim 15, wherein said electroplating solution
is pumped into said chamber from a reservoir of electroplating
solution.
17. A method as in claim 12, wherein said disposing an anode
comprises disposing an anode in said chamber.
18. A method as in claim 12, further comprising coupling an access
tubing to an inlet end of said component for defining an
electroplating bath with said interior of said component.
19. A method as in claim 18, wherein said disposing an anode
comprises disposing a wire formed from the metal or metal alloy to
be deposited through the access tubing into the electroplating
bath.
20. A method of sealing a stator bar with an interior liquid pass
for flow of coolant forming at least a portion of said stator bar,
comprising electroplating at least a portion of an interior wetted
surface of said stator bar.
Description
BACKGROUND OF THE INVENTION
[0001] Water-cooled stator bars for electrical generators are
comprised of a plurality of small rectangular solid and hollow
copper strands brazed to one another to form a bar. The ends of the
strands are brazed to an end fitting, typically referred to as
stator bar clip. A cover is brazed to the clip window. The end
fitting serves as both an electrical and a cooling flow connection
for the stator bar.
[0002] The hollow end fitting typically includes an enclosed
chamber for ingress or egress of stator bar cooling liquid,
typically deionized water. At one end, the end fitting receives the
ends of the strands of the stator bar. The fitting and the
peripherally outermost copper strands of the stator bar are brazed
to one another. The opposite end of the fitting is connected to a
stator cooling conduit.
[0003] Liquid cooled stator bar clips have gone through design
changes over the years. However, they typically contain mixed solid
and hollow strands brazed to one another, and a cover brazed to a
clip window. During operation, the hollow strands carry water to
cool off the bar. Over time, leaks can develop about the connection
between the stator bar ends and the stator bar fitting, between
cover and clip as well as between adjacent strands. Leaks may also
occur at various plumbing connections. It is believed that the
major leak mechanism is a crevice corrosion process which initiates
in the braze alloy at the interior surface of the braze joint.
Crevice corrosion is a localized form of corrosion usually
associated with a stagnant solution on the micro-environmental
level. Such stagnant microenvironments tend to occur in crevices
such as micro surface voids formed during brazing, especially at
the boundary of strands and braze alloy. Crevice corrosion is
initiated by changes in local chemistry within the crevice, such as
shift to phosphorous acid conditions in the crevice. Stagnant water
in the chamber of the fitting is in contact with the braze alloy
and the copper strands. This coolant contact with the braze joint
and cooper strands is believed to cause corrosion and consequent
leakage.
[0004] Field repair of coolant leaks through the stator bar end
connections has been successful. A leak site is identified by
several different tests, such as vacuum decay and traceable Helium
test.
[0005] An epoxy barrier coating method has been used as a leak
repair and prevention method. An example of an epoxy barrier
coating method is disclosed in U.S. Pat. No. 5,605,590, the
disclosure of which is incorporated herein by this reference. This
epoxy barrier coating has been applied to provide protection
against water initiated corrosion mechanisms along the brazed
length of the strand package. Epoxy coating is manually injected.
The voids and air pockets are possible during injection. Thus, the
process is labor intensive and requires 100% inspection. As a
result, the process can be labor intensive, takes a long time to
complete, and can produce defects.
[0006] There are also other leak issues in liquid cooled
generators. Indeed, there are many types of leaks associated with
water-cooled generators. Stator bar end crevice corrosion is the
major one. But other leaks are caused by porosity, cracking and
localized damage during manufacturing rather than as a result of
corrosion.
[0007] There is a need for an improved method for leak repair and
prevention in liquid-cooled generators. In particular, there is a
need for corrosion protection at the junction between the stator
bars and their clips. The corrosion protection should be robust and
be applicable to various stator bar clip designs, including clips
for recessed braze, flush braze and raised hollow strand braze
designs. There is also a need for an improved method to repair and
prevent leaks within the generator water pass that arise from other
causes.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The invention proposes an electroplating method to deposit a
thin layer of metallic barrier coating on a part of or the entire
interior surface of a liquid-cooled stator bar clip. The metallic
layer provides a corrosion resistant barrier coating to prevent
water access to corrosion susceptible region(s) such as
clip-to-strand braze joints. Different materials may be deposited,
such as copper or nickel as the metallic barrier coating.
[0009] Thus, the invention may be embodied in an apparatus for
electroplating an interior of a component, comprising: an inflow
tube for flowing electroplating solution to an interior of said
component; an outflow tube for flowing electroplating solution out
of an interior of said component; a pump for conducting
electroplating solution through said inflow tube and for drawing
said electroplating solution through said outflow tube; an anode
disposed in said electroplating solution and electrically connected
to a power supply; and said component being electrically connected
to said power supply as a cathode.
[0010] The invention may also be embodied in a method of sealing a
stator bar with an interior liquid pass for flow of coolant forming
at least a portion of said stator bar, comprising electroplating at
least a portion of an interior wetted surface of said stator
bar.
[0011] The invention may also be embodied in a method of sealing
interior surfaces of a fitting receiving a stator bar end, to
define a seal substantially impermeable to liquid, the fitting
having a chamber for receiving a liquid through an opening in the
fitting and said chamber being in communication with hollow strands
forming at least a portion of said stator bar, said method
comprising the steps of: disposing an electroplating solution in
said chamber; disposing an anode of the metal or metal alloy to be
deposited in said electroplating solution; connecting said anode to
the positive terminal of a current source as a cathode;
electrically connecting said stator bar to the negative terminal of
said current source; and establishing an electrical potential
between said anode and said cathode to initiates electrophoretic
migration of metal ions from the anode to the electrically
conductive inner surface of the component.
[0012] The invention is also embodied in stator bar end and a
fitting receiving the end to define a seal substantially
impermeable to liquid, the fitting having a chamber for receiving a
liquid through an opening in the fitting and in communication with
hollow strands forming at least a portion of said stator bar and
for flow of the liquid through the hollow strands, wherein at least
a portion of an interior surface of said fitting has a metallic
layer disposed thereon, wherein said metallic layer is deposited by
electroplating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic illustration of a liquid-cooled stator
winding arrangement illustrating the stator bars and end fittings
coupled to inlet and outlet coolant headers;
[0014] FIG. 2 is a schematic cross-section of a plating set-up for
plating the interior of a component, such as a stator bar clip;
and
[0015] FIG. 3 is a schematic illustration of a plating set-up for
simultaneous plating wherein the bar is flooded with circulating
solution.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Water-cooled stator bars for electrical generators are
comprised of a plurality of small rectangular solid and hollow
copper strands which are brazed to one another and brazed to an end
fitting. The end fitting serves as both an electrical and a
hydraulic connection for the stator bar. The end fitting typically
includes an enclosed chamber for ingress or egress of stator bar
cooling liquid, typically deionized water. Another opening of the
end fitting receives the ends of the strands of the stator bar, the
fitting and peripherally outermost copper strands of the stator bar
being brazed to one another. Over time, leaks have variously
developed about the connection between the stator bar ends and the
stator bar fitting as well as between adjacent strands. It is
believed, based on leak analysis results, that the leak mechanism
is due to a crevice corrosion process which initiates in the braze
alloy at the interior surface of the braze joint. Crevice corrosion
is initiated by changes in local chemistry within the crevice, such
as shift to phosphorous acid conditions in the crevice.
[0017] There are also other leak issues in liquid cooled
generators. Examples of other leaks in addition to the clip crevice
corrosion leaks are clip crack leaks, clip window leaks, plumbing
and fitting leaks, and connection ring leaks.
[0018] Field repair of leaks through the stator bar end connections
has only been moderately successful.
[0019] In an embodiment of the invention, an electroplating method
is proposed to deposit a thin layer of metallic barrier coating on
the entire interior surface of liquid-cooled stator bars' water
pass, or selectively on target areas, e.g., with high corrosion
risks or otherwise having a high leak potential. The metallic layer
provides a corrosion resistant barrier coating to prevent water
access to corrosion susceptible region(s) such as clip-to-strand
braze joints. Concurrently the coating has the capability to repair
and prevent substantially all other types of leaks.
[0020] Referring now to the drawings, particularly to FIG. 1, there
is illustrated a liquid-cooled stator winding arrangement used in a
typical liquid-cooled generator. A stator core having stator core
flanges 12 and core ribs 14 is illustrated, with stator bars 16
passing through radially extending slots and terminating at
opposite ends in end fittings 18 and 20. Inlet hose 22 connects
inlet fitting 18 to an inlet coolant header 24 and outlet hose 26
connects the outlet fitting 20 to an outlet coolant header 28. Each
stator bar 16 includes a plurality of hollow and solid copper
strands respectively disposed in side-by-side and superposed
relation one to the other. The fitting, for example, fitting 20,
includes a clip 30 formed of an electrically conductive material,
such as copper. The clip comprises a body having a rectilinear
opening at one end for receiving the strands of the stator bar 16.
At the opposite end, there is provided an opening which in use is
normally engaged with a copper tube 32 which serves as both an
electrical connection and a hydraulic connection for flowing liquid
coolant, e.g., deionized water, into or from the chamber 34 defined
by the stator bar clip 30 and the exposed ends of the hollow and
solid copper strands. The liquid in the chamber either flows into
the fitting and through the hollow strands for cooling purposes
when the fitting comprises an inlet fitting or receives the liquid
coolant from the hollow strands for egress when the fitting is
employed as an outlet fitting. As mentioned above, the solid and
hollow strands are brazed to one another, and a cover is brazed to
clip window. These junctions are among the potential sites for
leaks over time.
[0021] In an embodiment of the invention, the interior of the
stator bar clip 30, including the corrosion susceptible braze joint
between the clip 30 and the stator bar 16, and the cover to clip
braze seal, are electroplated so that a thin metallic layer is
deposited on the entire interior surface. The metallic layer
provides a corrosion resistant barrier coating to prevent water
access to corrosion and/or leak susceptible region(s) such as the
clip-to-strand braze joints and the cover-to-clip braze seal.
[0022] Electroplating is facilitated by isolating the target stator
bar clip, including the braze joint between it and the stator bar,
such as by partial disassembly to provide access to the end of the
clip remote from the stator bar. Referring to FIG. 2, a suitable
assembly for electroplating the stator bar clip is schematically
illustrated by way of example. In the illustrated embodiment,
plastic tubing 40 is securely coupled to the clip and extends up
and away from the clip to define a plating solution bath upper
surface 42 whereas the plating solution 44 will fill the chamber 34
defined by the stator clip. A pump 46 is provided to circulate
plating solution 44 through the bath defined by the interior of the
stator clip 30 and the interior of the tubing 40. The plating
solution may be initially loaded to the clip interior through the
tubing 40, or the pump can load the bath through inflow tube 48. In
the latter case, a reservoir of plating solution (not shown) is
provided and operatively coupled to the pump. In either case,
recirculating flow tube 50 recirculates fluid to the pump (and
reservoir, if any) and removes any gas, such as hydrogen bubbles,
trapped against the uppermost portion of the clip interior. The
resultant circulating flow advantageously promotes agitation.
[0023] In the embodiment illustrated in FIG. 2, the anode 52 is a
length of 10 gauge insulated copper wire stripped for several
inches and rolled into a coil to reduce its overall dimensions. Of
course, a copper wire is just one example of a consumable metal
source that may be provided as the anode. Other alternatives
include a metal mesh structure. Electrical contact between the
anode and the component being plated should of course not occur.
Therefore, in an exemplary embodiment the anode is inserted into,
e.g., a double walled porous bag 54 of non-conductive material to
prevent electrical contact but to allow solution access. The anode
is connected to a suitable poser supply (a 2-3 V DC power supply
was used in a test apparatus). The device itself is electrically
coupled to the power supply as a cathode, e.g., via contact 56.
[0024] In the assembly illustrated in FIG. 2, the reservoir may be
closed remote from the access tube. In the alternative, a
recirculating system may be provided wherein the remote end is not
closed, but instead the flow is through the clips 30 at each end of
the stator bar(s), and the interior of both clips and the
respective braze joints are plated simultaneously, as schematically
illustrated in FIG. 3. In the illustrated embodiment, plating
solution reservoirs 60, 62 are disposed in flow communication with
each clip, and a pump 64 is provided to circulate the plating
solution through the clips and bar 16.
[0025] Although not illustrated in detail, in practice the device
is heated, e.g. with heating tape, to between 40 and 50.degree. C.
Heating may also be accomplished with an in-line heater in the
electrolyte return tube.
[0026] An exemplary acid copper electroplating solution is a
mixture of e.g., water, sulfuric acid, copper sulfate, and a trace
of hydrochloric acid. To this mixture a number of organic
constituents are added that serve to regulate and distribute the
delivery of, e.g., copper to the surface being plated. The two
basic organic additives are commonly referred to as the
"brightener/leveler" and the "carrier".
[0027] An electroplating cell is typically comprised of a
(non-metallic) container full of the electroplating solution in
flow communication with the area or component surface to be treated
and a source of plating metal ions, as the anode. In the
illustrated embodiment, the electroplating cell is comprised of the
component (clip 30) interior 34, the access tubing 40 interior
volume, the inflow and outflow tubes 48,50, and the remote
reservoir for the solution (if provided) and, as the anode, the
coiled copper wire 52, or other consumable metal source, disposed
within the access tubing 40 and/or clip 30. This ion supply must be
capable of continuous sourcing into a near short circuit load. A
typical copper electroplating bath has an effective full load
operating "impedance" that ranges from 0.025 Ohms and 0.015 Ohms.
The surface for receiving the electroplated coating, referred to as
the cathode, in this case the clip, the stator bar, and the braze
joint therebetween, is connected to the negative terminal of the
current source. Deposition of, e.g., copper in this example, on the
target component occurs when an electrical potential is established
between the anode and the cathode (target component). The resulting
electrical field initiates electrophoretic migration of copper ions
from the anode to the electrically conductive surface of the
cathode, where the ionic charge is neutralized as the metal ions
plate out of solution. As a result, a uniform, thin coating of
smooth, bright copper is deposited on the target surface(s).
[0028] It should be noted that if it is anticipated that air or
evolved gas may become trapped, e.g., in a crevice or a blind
recessed area, it will prevent plating in such areas and pressure
treatment may be necessary. Pressure treatment may be applied by
defining the water pass system as a closed loop that can be sealed
and pressurized. Additionally, locating the return flow tube to
draw fluid from the vertically upper portion of the stator clip, as
mentioned above, facilitates the removal of trapped air or evolved
gas to facilitate uniform plating.
[0029] The electroplating process is continued until a deposit
layer of about 1 to 3 mm is achieved. The process can be ceased at
any time simply by disengaging the power source.
[0030] As an alternative to electroplating a single metal, the
plating of the braze joint may be accomplished as a double-layered
plating with a first plating followed by a second plating applied
over the first plating layer. As will appreciated, a double layered
plating provides enhanced durability as compared to a single
layered plating particularly where the plating coatings have
selective properties; for example, an underlying plated film having
strong corrosion resistance and a second plated film applied for
air tightness, durability and corrosion resistance.
[0031] As a further alternative, the water pass of the stator bars
and/or the clips, for example, may be plated before assembly and
then plating can again be carried out after assembly to plate the
braze joints, to effectively provide a double layered plating in
certain areas or on certain parts.
[0032] In accordance with this invention, all of the joints which
have the potential for forming a leakage path are electroplated. In
this way, the existing leak or leaks at the stator bar end
connections of a generator in the field are repaired. Additionally,
by providing a protective coating to all potential leakage paths of
the joints, a seal is provided which will ensure against the
formation of leakage paths in the future. That is, the
electroplating not only isolates the liquid coolant from the
brazing material and seals between the joints of adjacent strands
and the outermost strands, but can be used to deposit a layer of
material over, to seal, the entire interior surface of the coolant
flow path, whereby all potential leakage paths are sealed to
prevent future leaks. While the present invention is particularly
applicable to field repairs of existing generators, it may also be
applied to generators during initial manufacture to provide
protection against future leakage.
[0033] As noted above, since the method disclosed herein is capable
of applying a coating over the entire wetted surface of the flow
system, the method can address the clip crevice corrosion leak
issue and also other leak issues in liquid cooled generators. For
example, the method embodying the invention may used to address
clip crack leaks, clip window leaks, plumbing and fitting leaks,
and connection ring leaks. Thus, while the invention has been
described in connection with what is presently considered to be the
most practical and preferred embodiment, it is to be understood
that the invention is not to be limited to particulars of the
disclosed embodiment, but on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims.
* * * * *