Hot spot thermometer

A thermometer unit for monitoring the temperature of a fluid cooled electric transformer, the thermometer unit having a ceramic body insertable between the coils of the transformer winding immersed in cooling fluid in the transformer case. A nylon rod responsive to heat changes is confined in the interior of the body and is adapted to expand and contract according to the heat changes to vary volume flow of a gas circulated by a pump in a closed circuit connected with the body. Variations in the gas pressure are indicated on a sight gage externally of the transformer.

BACKGROUND OF THE INVENTION 
This invention relates to thermometers and it is particularly concerned 
with improvements in hot spot thermometers. 
While a hot spot thermometer may have other applications, it is especially 
suited for use in sensing temperature changes in a fluid cooled electric 
transformer. 
A hot spot thermometer is so named because of its structural 
characteristics whereby it is enabled to be located for temperature 
monitoring purposes at the spot of hottest temperature development in a 
transformer. This spot is between the coils of the transformer winding. 
This type of thermometer has various advantages. Because it is applied to 
the hottest area of the transformer, it obtains not only a more accurate 
reading but also a more rapid indication of temperature changes than would 
be otherwise provided. 
A hot spot thermometer is known from U.S. Pat. No. 3,960,017. It utilizes a 
high heat resistant ceramic body insertable between the coils of a 
transformer winding and containing a heat sensitive dielectric rod adapted 
to vary passage of light through fibre optic elements to produce in an 
external read-out instrument electrical signals indicative of temperature 
changes developing in the transformer. 
An object of the present invention is to provide a hot spot thermometer 
which utilizes a high heat resistant ceramic body insertable between the 
coils of a transformer winding and containing a heat sensitive rod which 
is cooperable with a closed tube containing a circulating gas to produce, 
according to variations in temperature in the transformer winding, 
pressure changes in the circulating gas, the pressure changes being 
readable on an associated pressure gage. 
In accordance with the invention, there is provided the combination 
comprising an electrical transformer apparatus having a tank containing 
cooling fluid in which the coils of a transformer winding are immersed, a 
thermometer unit having a hollow open-ended body of ceramic dielectric 
material inserted into a space between the coils, the body being connected 
in a closed circuit filled with gas, a pump connected in the circuit for 
circulating the gas about the circuit, a heat sensing element of 
dielectric material arranged in the hollow of the body having response to 
temperature changes developing in the transformer winding to cause 
corresponding variations in the pressure of the circulating gas, and a 
sight gas pressure gage connected in the circuit adapted to indicate said 
pressure variations.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
In the accompanying drawing is disclosed a conventional fluid cooled high 
voltage transformer apparatus 10 to which a thermometer unit 11 embodying 
the invention has been applied. Only as much of the transformer apparatus 
has been shown as deemed needed to provide a proper understanding of the 
invention and its mode of application. 
The transformer apparatus includes the usual tank 12 containing cooling oil 
in which the transformer winding 13 is immersed. The thermometer unit 11 
has been applied to the transformer by inserting a heat sensor member 14 
thereof into one of the usual spaces 15 separating the coils 16 of the 
winding. 
The sensor member 14 includes a hollow open-ended body 17 formed of 
dielectric material, such as ceramics, which is capable of withstanding 
the high temperature that can develop in a transformer. The body is 
dimensioned in its thickness so as to obtain a friction fit when inserted 
between the coils of the winding and thereby insure its being retained in 
place. 
The hollow of the body is defined by a major bore portion 18 extending for 
substantially the length of the body and opening out of a rear end of the 
latter, and by a short counterbore 19 of reduced diameter extending 
through a forward stem portion 21 of the body. A heat sensing non-metallic 
rod 22 formed of suitable dielectric material (here, nylon) is disposed in 
the major bore. An enlarged diameter rear portion 23 of the rod is fixedly 
fitted to the surrounding wall of the body and projects in part out of the 
latter. A forwardly extending elongated portion 24 of the rod is of 
reduced diameter so as to be free, of and to provide an annular clearance 
between it and, the surrounding wall of the body. A tip of the rod is 
disposed in close spaced relation to a seat 26 defined by a shoulder at 
the juncture of the major bore with the counterbore 19. 
A passage 27 through the rod and body is connected by tubing 28, 30 in a 
closed fluid circuit with a motorized gas pump 29 and a sight pressure 
gage 31. 
The tubing is of a dielectric non-metallic flexible material (here, silicon 
rubber). Section 28 has one end fitted over the projecting portion of the 
rod at 32, and its other end connected to a outlet port 33 from the pump. 
The second section 30 has one end fitted over the stem of the body and its 
other end connected to an inlet port 34 to the pump. A branch 35 off the 
first tubing section connects with the sight gage. 
The tubing extends from the body 17 through a wall 36 of the tank to the 
outside. The sight gage and pump are located externally of the tank. The 
circuit is filled with a gas. Here, a gas known as sulfur hexafluoride is 
used. 
In the use of the thermometer unit in association with the transformer, the 
pump is set in continuous operation causing the gas (indicated by the 
arrows) to be circulated around the closed circuit. The nylon rod 22 is 
adapted in response to change in temperature within the coils of the 
transformer winding to correspondingly expand and contract linearly 
relative to the seat so as to vary the volume of gas flow through the seat 
26. Variations in gas pressure developing in the circuit because of 
variations in flow through the seats are indicated by the pressure 
responsive sight gage.