Measuring and testing – With fluid pressure – Leakage
Reexamination Certificate
1999-09-30
2002-04-09
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Leakage
C073S049100, C073S040000
Reexamination Certificate
active
06367311
ABSTRACT:
BACKGROUND OF THE INVENTION
The present application is directed to techniques for testing a water-cooled generator for leaks, and more particularly to improvements in the process for drying water channels in the generator's stator to prepare the stator for pressure-decay testing and vacuum-decay testing.
FIG. 1A
schematically illustrates a stator
10
of a water-cooled generator. The stator
10
comprises of stator bars
14
, as shown in
FIG. 1B
, which are formed by uniting a number of individual conductors
16
in a meandering pattern. The meandering pattern (transposition) helps minimizing electrical losses. A cross-sectional view of one of the conductors
16
is shown in FIG.
1
C. As will be seen, the conductor
16
has a channel
18
for passage of water to cool the stator. Although not shown, the stator bars
14
are covered with electrical insulation.
A metal fixture called a water box (not illustrated) is connected to each end of every stator bar
14
. Further metal fixtures (not illustrated) are then used to hydraulically connect adjacent water boxes and to also electrically connect the stator bars
14
so as to form stator windings. These further metal fixtures are hydraulically connected by electrically insulating fixtures to an inlet header
20
at one end of the stator
10
and to an outlet header
22
at the other end. Thus, by way of the various fixtures, one end of each stator bar
14
is in hydraulic communication with the inlet header
20
, and the other end of the stator bar
14
is in hydraulic communication with the outlet header
22
.
FIG. 1A
shows the stator bars
14
in one stator winding loop of the stator
10
.
Coupling members
24
and
26
are provided outside the stator
10
to provide fluid communication with the inlet header
20
. Similarly, coupling members
28
and
30
are provided outside the stator
10
to provide fluid communication with the outlet header
22
. The coupling members
29
-
30
are accessible from outside the generator itself.
It will be apparent that a number of components are connected together to form the stator
10
. Many of these connections are brazed connections, which are susceptible to failure. Water leaks may also form in regions other than the connections. Although the water used for cooling the stator bars
14
is pure, and thus has a very low conductivity, any water that leaks out of joints of the fixtures connected to a stator bar
14
may soak its insulation and thereby degrade the insulation's ability to withstand high voltage. In particularly severe cases, a short circuit due to water leakage may cause catastrophic failure of the stator
10
.
Due to the risk of generator failure, or the risk of an unscheduled shut-down for less severe leakage, it has become common practice in the power generation industry to periodically test water-cooled generators for leaks. The General Electric Company, a major manufacturer of generators, has published information about leakage testing in
Technical Information Letter Number
1098, one version of which was published on Jan. 24, 1995 and was updated by Alan M. Iversen in November of 1996. Further information about testing is provided in a paper by Bruce Faulk et al., entitled “Diagnosing and Repairing Water Leaks in Stator Windings,” that was presented at an EPRI (Electric Power Research Institute) conference in 1995.
The periodic testing typically begins by draining the stator and then drying it. After the stator has been thoroughly dried, pressure decay and vacuum decay tests are conducted to confirm that the stator is able to hold pressure and vacuum. If the pressure within the stator falls too rapidly after the stator has been pressurized with compressed air, or if the pressure rises too rapidly after air has been evacuated, then a leak that requires further attention has been detected. The generator is then opened so that further testing can be conducted to identify the site of the leak so that it can be repaired.
Equipment for drying a stator and for conducting the pressure and vacuum decay testing may be collected together on a chassis or housing to provide an arrangement called a test skid. A conventional test skid typically includes an air tank or air receiver for holding compressed air. A compressor is not needed, since most power plants have piping systems for delivering compressed “instrument air” (cleaned and dried air for instrumentation) and compressed “service air” (utility compressed air for pneumatic tools and other applications where instrument air is not necessary). A conventional test skid also includes a vacuum pump. An arrangement of conduits, valves, and sensors for conducting pressure and vacuum drying and for performing the pressure and vacuum decay tests themselves is also present. Auxillary equipment, such as conduits for connecting the skid to the generator, may also be housed on the skid.
The drying procedure using a conventional skid is typically conducted in two stages—a pressure drying stage and a vacuum drying stage. After the stator has been drained, the air receiver is charged with air to a predetermined pressure; the pressurized air is introduced to the stator via one of the upper coupling members
24
or
28
; and then the pressurized air is discharged from the stator via a diagonally disposed bottom coupling member
26
or
30
. After the pressure falls to a predetermined value, the air receiver is again charged with pressurized air, and this pressurized air is discharged through the stator. A number of such pressure drying cycles are conducted. Next, the stator is closed and air is evacuated. Water that remains trapped in tiny nooks or crevices after completion of the pressure drying stage evaporates into the vacuum and is removed. When the stator is finally dry enough, as indicated by vacuum and dewpoint sensors, it is charged with compressed air to a predetermined pressure, and the pressure is measured periodically during a monitoring interval to determine whether the pressure decays or falls at an acceptably slow rate. During the vacuum decay test, the stator is evacuated and measurements are made during a monitoring interval to determine whether the vacuum decays (in this case, meaning that the sub-atmospheric pressure rises) at an acceptably slow rate.
The drying procedure in preparation for the pressure and vacuum decay testing can be conducted fairly expeditiously if it is begun while the generator remains hot, soon after it has been taken off line. If there is a delay, however, the drying procedure may take a week or more, particularly if the generator is housed in an unheated building. One reason for this may be that ice crystals form due to cooling because of rapid evaporation of water droplets during the vacuum drying stage. The ice crystals sublimate slowly into the vacuum, and moreover may clog crevices which contain residual water.
More than a year before the present application was filed, the inventor observed a test skid that was being operated by employees by a company then named MDA, or Mechanical Dynamic Analysis. The skid included a small heater which slightly raised the temperature of compressed air entering the air receiver. The purpose of this may have been to avoid water condensation inside the air receiver.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved method for testing water-cooled generators and a test skid for use in the method.
Another object is to provide a method to accelerate drying-out of a stator before pressure and/or vacuum decay testing is conducted.
Another object is to provide a test skid in which the same dewpoint sensor may be used during both pressure decay and vacuum decay testing.
These and other objects which will become apparent in the ensuing detailed description can be attained by providing a method for testing a water-cooled generator having a stator with water channels, which method includes the steps of drying the water channels and then conducting at least one of a pressure decay test and a vacuum decay test. The step of drying
Garg Trilok C.
Kunitz Norman N.
Venable
Wiggins David J.
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