Note: Descriptions are shown in the official language in which they were submitted.
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VAPORIZATION COOLED TRANSFORMER
'HAVING'~'HI'GH:VOLTA'GE'RATING
Canadian patent application SN 313,355 filed ~.October 13, 1978, discloses a vaporization cooled
transformer design for transformers rated at 3750 kVA :
and having a basic insulation level t~IL) of 95 KV. The
coolant employed within the described vaporization cooled
transformer comprises a chlorinated fluorocarbon having
a dielectric strength in its liquid state approximately
equivalent to transformer mineral oil, When the
transformer is operated at rated loading, the vaporized
coolant exhibits a sufficiently high pressure to prevent
dielectric breakdown from occurring within the winding
~ and between the winding and the transformer wall. When
`~ the~transformer is operated under conditions of low
~ ~ ambient temperatures,:the pressure exhibited by the
: : ~15 ~vaporized coolant is not sufficient to provide the
: necessary dielectric strength. In order to prevent
breakdown from occurring at low ambient conditions, a
: :large amount of soIid insulation must be employed around
the winding and the spacing between the internal
20. electrical conductors must be large enough to ensure that
: breakdown does not occur.
When basic~insulation levels higher than 95 kV
are desired the added insulation material required to -
co.mpensate for the decreased dielectric strength occurring
at low coolant pressures makes the overall transformer
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transformer size economically infeasible. The larger
transformer tank needed to provide the necessary
separation distances between the windings and the tank
wall, requires a larger quantity of liquid coolant
within the tank. The design of a vaporization cooled
transformer for operating at higher ratings than 3750
kVA requires a large he`at exchanger to cool the coolant
in order to prevent the transformer core and windings
from becoming overheated at the higher operating
temperatures. Since the cooling facility of the heat
exchanger depends upon the surface area exposed to the
ambient air, a larger number of condenser tubes and
auxiliary cooling fans must be employed.
The purpose of this invention is to provide a
vaporization cooled transformer having increased voltage
ratings without requiring a substantial increase in the
overall size of the transformer tank and the heat
exchanger.
A vaporization cooled transformer provides
cooling and heating acility to the transformer to
operate the condensable coolant at a predetermined
pressure over a wide range of ambient temperatures. The
predetermined pressure provides a dielectric strength to
the coolant equivalent to transformer mineral oil. One
embodiment provides a microprocessor in combination with
a series of heat exchangers and an auxiliary heater to
control the rates of heating and cooling. The micro-
processor is programmed to control the number of heat
exchangers required and the operating cycle of the
auxiliary heater to adjust the coolant vapor pressure
under varying climatic conditions.
FIGURE 1 is a side sectional view of the
vaporization cooled transformer according to the
invention .
FIGURE 2 is a front view of the transformer of
FIGURE 1.
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FIGURE 3 is a graphic representation of the
coolant vapor pressure of a vaporization cooled
transformer as a function of transformer loading for
contours of ambient air temperatures.
EIGURE 4 is a graphic representation of the
dielectric strength'of the coolant vapor as a function
of coolant vapor pres'sure.
FIGU~E 5 is a graphic representation of the
vapor pressure of the liquid coolant as a function of
coolant temperature.
FIGURE 6 is a graphic representa~ion of the
relation between the vapor pressure and time for the
vaporization cooled transformer of the invention.
FIGURE 7 is a schematic diagram showing the
interconnection between thè elements of FIGURES 1 and
2 implemented by means of a microprocessor.
FIGURE 1 shows a vaporization cooled transformer
10 similar in operation to that described within the
aforementioned Canadian patent application and containing
a transformer tank 11 in combination with a heat
exchanger 12 for cooling and electrically insulating
winding 13'and core 14 within the tank. High voltage
bushing 15 situated at the top o the tank and low
voltage bushing 16 on the side of the tank wall provide
eIectrical connection with the winding by means of
cables 17. The tank contains a ~uantity of'coolant 18
which is a chIorinated fluorocarbon such as
trichIorotrif~uoroethane which when heated vaporizes and
enters intake manifold 19 in the direction indicated by
Arrow A. m e vaporized coolant then enters a plurality
of cooLiny tubes 20 wherein the coolant condenses and
returns through the exit manifold 2I and return pipe 22
back to the transformer tank. A quantity of molecular
sieve material 23'is located within the'intake manifold
for removing any moisture releàsed by the'cellulosic
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insulation materials in the winding during transformer
operation. The cooling tubes also contain a plurality
of ~ins 24 to increase the effective cooling area of the
tubes when used in combination with one or more fans 25
connected together by means of shroud 26. An auxiliary
heater 27 is employed in combination with a layer of gas
filled foam insulation material 28 on the inner surface
of the tank to heat the coolant and keep the coolant at
a sufficiently high`temperature in cold weather
operation. The`insulating foam can also be arranged on
the outside`of the tank but is-situated internal to the
tank in this embbdiment for the purpose of displacing
some o tlle expensive coolant as a cost savings feature.
FIGURE 2 is a front view of the transformer 10
depicted in FIGURE 1 wherein like reference numbers are
used to designate corresponding eIements such as tank
11 and bushing lS. Three heat exchangers 12a, 12b, and
12c are arranged along the wall o~ the tank and each
hèat exchanger contains a pair of ~ans 25a, 25b, and
25c. The`he`at exchàngers are connected to the tank by
means of intake manifolds l9a, 19b and l9c located at
thè top of the heat exchangers and return pipes` 22a,
22b, and 22c which connect witn- exit manifolds 21a,
21b, and 21c located at the bottom of the heat exchangers.
The requirement of morè than one heat exchanger and more
than one fan within each heat exchanger will be discussed
in detail beIo~.
The operation of a vaporization cooled
transformer with a single large heat exchanger revealed
that the` vapor pressure of the coolant above the liquid
in the transformer tank was a sensitive function of the
transformer loading as well as the ambient air
temperature in the` vicinity of the heat exchanger. The
vapor pressure of the coolant for purposes of this
disclosure means the pressùre`exerted by the vaporized
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coolant in equilibrium with'the liquid coolant at a
given temperature. The'equilibrium pressure will also
be referred to as-the "saturation pressure" since a
change in the temperature of the liquid coolant
immediately causes a corresponding increase in the
vapor pressure exerted by the vaporized coolant.
FIGURE 3, which'shows the coolant vapor
pressure as a function of loading'and ambient
temperature, indicates that the vapor pressure of the
coolant increases with'transformer loading due'to the
heat generated within the transformer core and the
winding and that the limit in the pressure is governed
by the ambient temperature conditions for a given heat
exchanger design. It can be seen from comparing the
steady state vapor pressllre with the trans~ormer
operating at 100 per cent loading in an ambient
temperat,ure'o~ 10C to that in an ambient 50C that
the vapor pressure is nearly doubled at the higher
ambient temperature.
The operating design point for 30C ambient
operating conditions indicated at C results in a coolant
vapor pressure of approximately 27 pounds per square
inch absolute (PSIA). Since the coolant vapor pressure
for a ~aporiæation cooled transformer varies from less
than 5 PSIA at startup to approximately 27 PSIA at 100
per cent loading the effect of the low- and high-coolant
vapor pres'sure'upon the dielectric properties of the
vaporized coolant were in~,estigated in oxder to determine
whéther more insulation should be applied to the winding
to protect the winding ~rom arc-over at low ambient
~emperatures during startup when the transformer rating
is increased.
FIGURE 4 shows the dielectric strength in
kilovolts as measured between a 1/4" diameter rounded
electrode and a flat plate with a 3" separation distance
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between the electrode and Lhe plate, tes'ted for impulse
and 60 Hertz breakdown conditions in an atmosphere of
saturated coolant vapor. The` impulse'~oltage dielectric
strength D is approxi~ateIy 200 kV when the coolant
vapor pressure exceeds 20 PSIA. The 60 Hertz dielectric
strength E for the same coolant vapor pressure is
approximately 160 kilovolts. Since the dielectric
strength in kilovolts for the vaporized coolant at 20
PSIA is approximately equivalent to the dielectric
lQ strength for transformer mineral oil it was then first
realized that maintaining the coolant vapor pressure
above 20 PSIA would result in a vaporization cooled
transformer having the same eIectrical properties as a
mineral oil cooled transformer at equivalen-t ratings.
Since the dieIectric constant of trichlorotri~luoroethane
as a liquid is approximateIy equal to the dielectric
constant of standard transformer mineral oil~ it wa5
expected that the dieIectric strength properties would
only be equivalent while the coolant remained in its
liquid phàse. The dielectric strength of the liquid
coolant at 21.5C for both impulse and 60 Hz voltages
preincluded in FIGURE 4 for comparison purposes. A
serious dielectric problem was anticipated when the
coolant vaporized` during its ~apor transport cycle and
the winding has to reIy in part upon the dielectric
properties o~ the coolant vapor. It was heretofore
anticipated that t~e`dielectric strength o~ the vaporized
coolant would ~e no greater than air. FIGURE 4 shows,
however, that maintaining the'coolant vapor pressure at
30- or in exces's of 20 PSI~ results in a dielectric strength
approx`imately equivalent to that of the liquid coolant
at 21.5C.
The vapor pressure o~ trichlorotrifluoroethane
coolant as-a function of temperature is shown at F in
35 FIGURE 5. In order to maintain a vapor pressure equal
to or greater than 20 PSIA the temperature of the
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coolant must be maintained in excess of approximately
57C. Since temperatures less than 50C, for
example, would result in a coolant vapor pressure
having reduced dielectric strength, and temperatures
greater than 100C would result in coolant vapor
pressures in excess of the strength properties of the
heat exchanger assembly, some means must be employed
for keeping the coolant temperature within the 50 to
100C range. As shown earlier in FIGURE 3, a
design point of 25 PSIA could be employed and the
pressure could vary from 20 to 30 PSIA in an ambient
temperature range of 30 to 40C.
The vaporization-cooled transformer described
in the aforementioned Canadian patent application
utilizes a large heat exchanger for the purpose of
insuring that the vapor pressure of the coolant
remained within reasonable values over wide ranges of
ambient temperature. The heat exchangers of the
instant invention depicted in FIGURES l and 2 are
substantially smaller than the aforementioned heat
exchanger in total surface area exposed and are
operated sequentially in a controlled manner for
closely regulating the coolant temperature. Auxiliary
heater 27 connected to the side of the transformer tank
2~ by means of feed throughs 29 and electrically connected
with a voltage source by means of electrical conductors
; ~ 30 and a switch, is used to heat the liquid coolant
within the tank up to 50C before the transformer is
energized. This assures that the vapor pressure of the
vaporized coolant above the liquid, which can be
determined for example by pressure or temperature
sensor 9, is in excess of 20 PSIA and that the
dielectric strength of the vaporized coolant is
sufficient for protecting the internal components of
the transformer. When the transformer becomes fully
energized the heater is shut off and at least one of
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the heat exchangers becomes operatively connected with
the tank by means of solenoid valve 31~for example.
The first heat exchanger 12a of FIGURE 2 could be
connected to the tank by means of an electrically
operated solenoid valve within intake manifold l9a for
example valve 31 or by means of a pressure actuated
valve designed to operate when the coolant vapor
pressure exceeds the design operating pressure of ~5
PSIA as an alternative to solenoid valve 31. With the
first heat exchanger 12a in operation and with the
transformer at rated power, fans 25a would become
actuated in the event that the ambient conditions were
such that the cooling tubes alone were incapable of
reducing the coolant temperature and the resulting
vapor pressure increased above the 30 PSIA upper
limit. Either one or both fan~ 25a could become
activated depending upon the amount of cooling
required. When ambient temperatures are high, the
first heat exchanger is insufficient to cool the
vaporized coolant and to cause the coolant vapor
pressure to remain within the ~0 PSIA upper limit. A
second solenoid valve or pressure-actuated valve could
become actuated connecting second heat exchanger 12b
and allowing the vaporized coolant to enter by means of
second intake manifold l9b and return to the tank by
means of second exit manifold 21b and second return
pipe 22b. Fans 25b are employed within the second
manifold to provide added cooling facility as described
earlier for the first manifold. Third heat exchanger
12c is provided in the event that the ambient
temperature conditions are such that further cooling is
required. It is to be understood that a single heat
exchanger having a plurality of fans located along the
extent of the cooling tubes could be employed and that
the fans could be connected to a control system for
automatically starting and stopping the fans depending
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upon the degree of cooling or heating required. When
more than one heat exchanger is employed the heat
exchangers can be operatively connected with the tank
in a parallel arrangement, or the heat exchangers can
be serially connected with each other depending upon
the particular transformer design. The heat exchangers
can also be directly connected to the tank without
valves. In this case merely turning on the fans would
increase the cooling within each separate heat
exchanger.
Operating a 200 kVA transformer from 0 to 100
rating over an ambient temperature range of from 17
to 23C resulted in the relatively constant vapor
pressure G shown in FIGURE 6. The load was increased
in 33% increments over a 3 hour period and the fans
(FIGURES 1 and 2) were cycled on and off to keep the
pressure at the 27 to 28 PSIA design point. The number
of fans to be operated in accordance with transformer
load and ambient temperature can be determined and a
~0 program designed for each transformer rating. A
smaller number of fans can be employed and the
operating cycle of the fans can be programmed to switch
on and off as an alternative to the sequential use of a
larger number of fans. For the constant vapor pressure
G of FIGURE 6 a direct reading pressure gage 9 was
included in the transformer tank, as shown in FIGURE 1,
and the heater and fans were manually switched to
maintain the pressure at a constant value as the
transformer loading and ambient varied. For long term
operation, a pressure control device can be employed to
sense the pressure and automatically switch on the fàns
and the heater as required.
Although pressure-sensing mechanisms are
employed it is to be well understood that temperature-
sensing mechanisms such as thermocouples; thermistors,and direct reading thermometers can also be employed to
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determine the coolant vapor pressure. This can be seen
by the relationship indicated between coolant vapor
pressure and temperature shown earlier in FIGURE 5.
This is true as long as there remains some coolant in
liquid form and the vapor exhibits a vapor pressure and
does not behave as an ideal gas. When temperature-
sensing devices are employed within the transformer
tank or heat exchanger the fans and the heater can be
operatively connected in a manner similar to that for
the pressure sensing embodiment. Temperature sensors
can be used for determining the ambient temperature
condition and electrical meters can be connected within
the transformer control circuitry to determine the
transformer load. These parameters in combination with
the coolant vapor temperature or pressure are
sufficient to control the operating coolant vapor
pressures within the transformer tank over the full
operating range of the transformer over a wide range of
ambient temperatures.
FIGURE 7 shows one arrangement for operatively
connecting heat exchanger 12 and heat source 27 to
transformer tank 11. This arrangement employs a
microprocessor 8 electrically connected to a sensor 9
in the transformer tank for sensing the temperature of
the liquid coolant or the vapor pressure of the
vaporized coolant and, in turn, activating either the
heat exchanger valve 31 or the heat source switch
depending upon whether the coolant temperature and
vapor pressure are too high or too low, respectively.
Alternatively as indicated by broken lines, the
transformer sensor 9 can be directly connected with the
heat exchanger wherein a temperature or pressure sensor
9 within the transformer tank directly actuates the
heat exchanger valves or the fan controls without the
microprocessor control unit 8. The heat source can
also be electrically connected with a temperature or
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pressure sensor 9 within the transformer tank for
directly causing the heat source to become energized
upon sensing low coolant temperatures and coolant vapor
pressures and de-energized when the coolant temperature
and vapor pressure reach a predetermined amount. The
microprocessor can be programmed to sense an electrical
output signal of a particular maynitude which is
generated by a thermocouple pressure gauge or a
thermistor temperature-sensing device 9 projecting
within the tank to provide output control facility to
both the heat exchanger and the heat source.
It is to be well understood that the
properties of the vaporizable coolant employed
determine the corresponding vapor pressure and electric
strength so that the operating range of coolant
temperatures and coolant vapor pressures may vary when
different coolants are employed. The microprocessor
would have to be individually programmed for operating
with a prescribed coolant. The nature and design of
the heat exchangers and the heat source as well as the
transformer operating characteristics would have to be
carefully determined for each microprocessor program.
Although the controlled pressure arrangement
of the invention is described for operation with a
transformer, this is by way of example only. The
controlled coolant vapor pressure arrangement of the
invention finds application wherever any electrical
device requiring both cooling and electrical insulation
is to be employed.
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