Note: Descriptions are shown in the official language in which they were submitted.
1 5D-5602
LEAK DETECTOR FOR VAPORIZATION COOLED TRANSFORMERS
Canadian patent application Serial No.
313,355 filed October 13, 1978l discloses a
vaporization cooled transformer wherein a vaporizable
, fluid is used for providing both cooling facility and
dielectric capability to transformer cores and
windings. An effective liquid level gage for sensing
the quantity of coolant is known. The liquid
, level gage adequately provides shutoff facility
when a vaporizable fluid such as trichlorotrifluoroethane
leaks out of the transformer tank. The aforementioned
vaporization cooled transformer utilizes a quantity
of molecular sieve material in the vapor path
between the transformer tank and the heat exchanger
as a water scavenger to remove any moisture that
may be released from the cellulosic insulation
materials during transformer operation. Since the
insulating material continuously release water
vapor to the transformer interior over the operating
life of the transformer, a sufficient quantity of
the molecular sieve material is employed to provide
for adequate water adsorption throughout the
; operating life of the transformer. In the event that
the sieve material becomes saturated, excess moisture
can occur within the transformer and behave as an ideal
gas under certain temperature conditions. The excess
moisture under these conditions can cause corrosion
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of ihe heat exchanger.
When leaks develop ~hrough openings occurring
within Ihe heat exchanger or tank assemly a negative
internal pressure within the heat exchanger or tank
5 assembly allows a substantial quantity of ambient air
to enter through the lea~ openings. The presence of a
quantity of atmospheric air within the heat exchanger
can be detrimental to the transformer operation. One
; long-term deleterious effect is the premature saturation
10 of the molecular sieve material due to the presence of
substantial quantities of water vapor present within
the admitted air.
Short-term deleterious effects which can
occur due to the presence of the admitted air include
15 both an overpressure condition caused by reduced heat
,~: exchanger efficiency as well as coolant loss by the
;,; excape of the vaporizable coolant out through the leak
openings. The heat exchanger efficiency is decreased
because the presence of the trapped air within the heat
20 exchanger headers and cooling tubes prevents the
, vaporized coolant from entering into these areas
during the condensation periods of the vaporization-
condensation cycle. The loss in cooling efficiency in
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iurn causes the transformer to operate at a higher
25 temperature causing further increases in pressure until
; an overpressure mechanism becomes energized and the
transformer becomes automatically disconnected.
The purpose of this invention is to provide
, means for sensing the differences in temperature that
30 exist between the heat exchanger and the transformer
tank to determine the presence of admitted air within
the heat exchanger assembly as well as the presence of
excess moisture.
Temperature sensing means are installed
35 both in the heat exchanger ~nd the transformer tank in
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vaporization cooled transformers. The temperature
differential between the heat exchanger and the tank
is sensed to determine when a predetermined temperature
differential is exceeded. The excess temperature
~; 5 differential indicates the presence of ambient air
or excess moisture within the transformer. Alarm
indicating means ana transformer disconnect relays are
-~ actuated when the temperature differential exceeds the
predetermined range.
Figure 1 is a graphic representation of the
coolant vapor pressure of a vaporization cooled trans-
, former as a function of transformer loading for contours
P of ambient air temperature;
Figure 2 is a front section view of a
15 vaporization cooled transformer containing the leak
detection means according to the invention;
Figure 3 is a top section view of the heat
-~ exchanger of Figure 2 through the plane 3-3;
Figure 4 is an enlarg~d section view of the
'~ 20 lower header within the embodiment of Figure 1 with the
temperature sensor connected to the end of the header;
" Figure 5 is a side section view of a vertically
,~l arranged heat exchanger as an alternative to the hori-
~, zontally arranged heat exchanger of Figure 2;
Y 25 Figure 6 is a graphic representation of the
¢,~ relation between temperature and time for the embodiment
of Figure 2 in the absence of a leak;
Figure 7 is a graphic representation of the
relation between temperature and time for the embodiment
30 of Figure 2 in the presence of a leak;
Figure 8 is a graphic representation af the
temperature differential between the temperatures
depicted in Figures 6 and 7 as a function of time;
Figures 9A - 9C are diagrammatic representa-
35 tions of the temperature separation which occurs within
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the side, front and bottom section views of a portion
of a heat exchanger at 25% transformer loading;
, Figures lOA - lOC are diagrammatic representa-
tions of the temperature separation which occurs within
.- 5 the side, front and bottom section views of a portion of
: a heat exchanger at 75~ transformer loading;
.- Figures llA - llC are diagrammatic representa-
.- tions of the temperature separation which occurs within
the side, front and bottom sections of a portion of
z 10 a heat exchanger at 100% transformer loading;
.~ Figure 12 is a front section view of a vapori-
zation cooled transformer containing a further arrange-
ment of the leak detection means of the invention;
. Figure 13 is a graphic representation of the
. 15 relation between temperature and time for the embodiment
of Figure 12 in the absence of a leak;
Figure 14 is a graphic representation of the
relation between temperature and time for the embodiment
of Figure 12 in the presence of a leak;
~,;, 20 Figure 15 is a graphic representation of the
temperature differential between the temperatures
: depicted in Figures 13 and 14 as a function of time; and
, Figure 16 is a diagrammatic representation of
, the temperature separation which occurs within a
vaporization cooled transformer depicted in a partial
section front view having vertical cooling tubes and
containing the leak detection means according to the
invention.
Figure 1 shows the relationship between
vaporized coolant pressure and transformer loading
existing within a vaporization cooled transformer for
various ambient temperatures. It can be seen that
the coolant vapor pressure within the transformer at
low conditions of loading and low ambient temperatures
can be quite low. A standard atmospheric pressure
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line P is included at 15 pounds per square inch
absolute (PSIA) for comparison purposes. Pressures
; beneath the standard atmospheric pressure line P are
considered as negative pressures for the purposes of
this disclosure.
Figure 2 contains a vaporizati~n cooled
transformer 10 as described within the aforementioned
Canadian Patent application. The vaporization
; cooled transformer 10 comprises a transformer tank
11 and a heat exchanger 12 wherein the transformer
windings 13, core 14, and bushing 15 are cooled by
means of vaporizable coolant 16. The coolant
becomes vaporized by the heat generated by the trans-
former core and the windings and the vaporized coolant
16' enters up into the intake manifold 17 within the
heat exchanger by means of intake pipe 18 in the
direction indicated by arrow B. A plurality of headers
19 are connected to the intake manifold and in turn are
connected with a plurality of inclined cooling tubes 20
' 20 connecting between the intake manifold 17 and return
- manifold 21. The condensed coolant droplets 16" return
to the transformer tank from the return manifold by
means of return pipe 22. An expansion tank 23 is
provided for the expansion of the vaporized coolant and
is connected with the intake manifold by means of
connector pipe 24. Any coolant vaporizing within the
expansion tank returns to the transformer tank by
means of expansion return pipe 25. Located between the
transformer tank 11 and intake pipe 18 is a molecular
sieve container 26 consisting of a quantity of molecular
sieve material 27 within housing 28 which contains a
plurality of apertures 29 for retaining the molecular
sieve material while permitting the transport of the
coolant. Upon becoming vaporized, coolant 16' transmits
out from the transformer tank through opening 30 in the
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direction indicated by arrow A.
As previously discussed, the presence of a
; leak in any portion of the heat exchanger 12 or tank 11
causes an influx of ambient air into the heat exchanger
assembly. During transformer operation the admitted
air siabilizes within heat exchanger headers 19 after a
period of extended use and settles in lowest header 19'
within the intake manifold. The presence of a quantity
of admitted air within any of the headers 19 prevents
vaporized coolant 16' from en~ering those headers and
their associated cooling tubes 20 so that the headers
and cooling tubes remain at a somewhat lower temperature
than the remaining cooling tubes and headers. This is -~
i~ because the vaporized coolant is prevented from con-
densing and transmitting its heat of vaporization
within the "air locked" headers and cooling tubes. The
' air locked condition also occurs when fans are employed
to increase the cooling efficiency of the heat exchanger.
The insertion of a temperature sensor 31 such as a
~hermocouple, within the lowest header 19' and a
sensor 32 within the transformer tank provides one
means for sensing the temperature of the lowest header
19' and of the coolant within the transformer tank at
any given time. Connecting the thermocouples 31, 32
, 25 by means of electrical connectors 33, 34 to a differential
- temperature gage 35 allows the temperature difference
between the header and the transformer tank to be
- rapidly determined. Once the temperature differential
; is determined a direct readout is provided by indicator
36 in order for an operator to monitor the temperature
differential while the transformer is operating. A
terminal 37 is provided on the differential temperature
gage in order io provide an output signal for connecting
to an audio-visual alarm device and to a circuit breaker
relay for interrupting input power to the transformer
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when a predetermined temperature differential is
; exceeded.
The temperature sensor 31 can be connected to
- heai exchanger 12, which is horizontally arranged, by
connection through the side of the lowest header l9' as
indicated in Figure 3 or by connection through the end
of the lowesc header l9' as shown in Figure 4.
The-temperature sensor 31 can be located at
the botiom of heat exchanger 12 of Figure 5 by connecting
' 10 through the side of lowest header l9' which is higher
than enirance pipe 18 as indicated.
For normal operating conditions and in the
absence of any leaks the temperature of the interior
surface of the header or tube being moniiored is
' 15 approximately the same temperature as the temperature
of the vaporized coolant 16' within the associated header
or tube. A series of tests were performed on a 1000
Kva transformer within a 31C ambient. Figure 6
shows the temperatures from time of startup at a
relatively high load value of 15380 watts. As shown,
the temperatures of the coolant in the tank A and the
coolant vapor in the lowes~ header B, are approximately
equal. The temperature of the lowest header is about
3C less due to thermocouple errors and conduction
, 25 heat loss through the header. It had previously been
concluded that upon transformer startup, coolant in
the tank would heat up in the vicinity of sensor 32
(Figure 2) before hot coolant vapor could move through
the heat exchanger to sensor 31. However, the coolant
vapor rapidly reaches sensor 32 so that sensor 32
stays approximately at the same temperature as sensor
31 during the startup cycle. This is indicated by the
near equal temperature conditions for both A and B.
A quantity of air admitted to the transformer
resulted in the temperatures A and B shown in Figure 7.
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When the air was admitted into the transformer, the
load had to be reduced to 9770 watts in order not to
damage ihe windings. For this load value the steady
state pressure measured 8.6 psig. For the iemperature
values shown in Figure 6 the pressure measured 5.3
-psig at the higher load value. As the transformer
containing the admitted air is further loaded, the
temperature of sensor 32 begins to rise. However, due
to the air leak, the temperature of sensor 31 remains
:10 constant for the first 30 minutes. The temperature
difference ~ To is 6.7C after 15 minutes. The
temperature difference ~ Tl is 18.8C after one hour.
The leak detector can therefore give an early indication
of the presence of a leak before a dangerous over
pressure develops. The temperature of sensor 31 begins
to heat up after one hour, possibly due to the hot
vapor in the heat exchanger intermixing with the admitted
air. However, the lowest header remains at a lower
temperature due to the air leak. At steady state
conditions the temperature difference ~ T2 was still
approximately 13.4C.
Figure 8 is a graphic representation of the
temperature differential occurring between sensors
31 and 32 with and without a leak. An alarm can be
connected to terminal 37 on the temperature gage
35 in Figure 2 and preset above approximately 6C to
allow for sensor errors and load deviation. Temperature
differentials above preset values of from 0 to 10C would
operate the alarm to give an audible and visual indica-
tion of the presence of a leak.
The leak detector which comprises the combinationof temperature gage 35 and sensors 31 and 32 can have
a direct reading visual scale, an audible alarm, or a
ivisual and audible signaling mechanism, depending on
transformer location and operator preference.
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The leak detector can also be electrically coupled
with a transformer disconnector relay.
Figures 9 through 11 depict the location of
ihe hot vapor and cooler air in a 750 Kva transformer
for loads of 3600, 9000, and 12000 watts. The 12000 watts
value represents the approximate full load rating and
the 9000 watt and 3600 watt values represent 75%
and 25% load values respectively. As shown in Figures
9A - 9C for 25% loading and Figures lOA - lOC for 75%
loading, the lowest row of cooling tuhes 20' are
partially cold at 25% loading and entirely hot at the
higher 75% rating. When the loading is increased to
100% the lowest row of cooling tubes has a well defined
cool region as shown in Figures llA - llC.
When loads are less than 100%, the increase
in coolant vapor pressure, which occurs in the presence
of a leak, is not as detrimental as full load conditions
and can be withstood until the load conditions increase
; further to 100%.
As shown in Figure 12, sensor 31 may be located
in the highest header 19"~ An additional sensor 31'
can be located within expansion tank 23 and connected
to gage 35 by conductor 38 for use in combination with
sensor 31 so that an indication can be received from
25 either sensor 31 or sensor 31' relative to sensor 32.
Under leak conditions at full load, temperature
increases were sensed at sensor 32 while the tempera-
tures at 31 and 31' decreased to lower levels. Tem-
perature differentials in excess of the 6C setting are
' 30 sufficient to cause standard overpressure gages to
become energized at high loads and to cause the
transformers to be automatically shut down by standard
auxiliary relay equipment. This occurs since pressures
generated in excess of the standard overpressure gage
setting of 15 psig can cause damage to the heat
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exchanger assembly. It is to be noted that pressure
increases within the heat exchanger assembly and the
transformer tank during operation cause a corresponding
increase in the boiling temperature of the coolant so
; 5 that the transformer operates at a higher temperature.
The temperatures monitered at sensor 32
within the transformer tank and sensor 31' within the ;~
expansion tank, are shown at C and D respectively in
~ Figure 13 for transformer operation in the absence of a
- 10 leak. The same temperatures monitered, when air was
admitted are shown in Figure 14. Ii is to be noted
that a differential occurs similar to that described
earlier for the header mounted sensor 31 of Figure 2.
This differential is shown in Figure 15 under both regular
and leak conditions over a period of time.
Intentionally forming leaks of various sizes
at different locations within the horizontal heat
exchanger assemblies of Figures 2 and 12 confirmed the
fact that the admitted air generally settles in the
lowest header on the intake manifold side of the heat
exchanger assembly. The reason for the settling of
the admitted air in the lowest header within the intake
manifold is not at this time well understood. It
is noted however, that regardless of where the leak
occurs most of the admitted air does in fact locate
within the lowest header within the intake manifold.
There is also a temperature differential occurring
within the lowest cooling tubes 20' associated with
, the lowest header 19' relative to the temperature of
the coolant within the transformer tank.
The leak detector of this invention can be
used with other heat exchanger designs. Figure 16
depicts a vertical type heat exchanger 12 described in
afore-mentioned Canadian patent application Serial No.
313,355 filed October 13, 1978. Air leaking into this
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heat exchanger 12 consisting of vertical tubes 20-
segregates in upper header 40' as well as within the
upper region of expansion chamber 41 and cooling tubes
20. Sensor 31 may op~ionally be located within upper
header 40' or wiihin expansion iank 41.
Since the remainder of the cooling tubes and
headers are hot relative to the top end portions of
the tubes and headers it is to be clearly understood
that sensor 32 can be connected to the hot portions
~ 10 of the headers and cooling tubes as well as to the
-~ transformer tank. An auxiliary sensor 32' connected
to the hot portion of one of the headers 40 can provide
. a temperature differential relative to sensor 31 in
the colder region of the heat exchanger.
Sensor 32 may be mounted within coolant 16 in
tank 11, in the hot vapor space above the coolant
as shown in Figures 2 and 12, or within the main vapor
; supply pipe 18 to the heat exchanger. Sensor 32 can
also be located within the hot portion of one of the
; 20 headers 40 or cooling tubes 20 in the heat exchanger
for providing a temperature differential relative to
sensor 31.
The leak detector of the invention also
provides effective high moisture indication. When the
molecular sieve material ~ in Figure 2 becomes inopera-
tive due to excessive moisture within the transformer,
the excess water vapor behaves as a noncondensable gas
- and segregates in a manner similar to the admitted air
~i in the presence of a leak and provides the same hot
and cold regions indicated earlier in Figures 9 - 11.
Although thermocouples are used as the tem-
perature sensors within the transformer tank and the
heat exchanger assembly, other temperature sensing
means such as thermistors, resistive elements and direct
reading thermometers can also be employed.
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It is to be understood that the pressure
within both the heat exchanger and ~he transformer tank
can increase for a variety of reasons. This invention
:` is directed to means for determining the presence of a
leak either within the heat exchanger assembly or
the transformer tank above the coolant liquid level
that is not readily detectable until the transformer
becomes energized. The presence of admitted air within
`:~ the transformer tank, for example, can cause serious
dielectric problems immediately upon transformer startup.
The admitied air is not determinable with standard
type pressure sensing means either within the tank or
heat exchanger assembly. A sufficient quantity of
admitted air will immediately indicate a temperature
lS differential between sensor 31 located within the heat
exchanger and sensor 32 within the transformer tank
before any dielectric breakdown problems can ocaur
within the transformer windings during startup conditions.
Although the leak detector of the invention is
directed to vaporization cooled transformers, this
. is by way of example only. The invention finds appli-
y~ cation wherever heat exchangers and vaporization
~;~ cooled e1ectrical1y heated e1ements may be employed.
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