Temperature controls
are required in order to turn on and off the cooling equipment. These
controls are usually combined with a visible dial-type gauge that
measures ‘winding temperature’.
This is really a misnomer because the gauge does not actually contact the winding.
Instead, it measures the top oil temperature
plus a temperature gradient produced by a small heater surrounding the
thermometer bulb. This heater is connected to a current transformer on
one of the phases of the secondary leads, so as the secondary load
increases, so does the current through the heater resistance.
The
heater then becomes a mimic of the actual transformer winding with the
winding temperature gauge measuring a temperature that is roughly
equivalent to the true winding temperature.
In addition to (or instead of
) the winding temperature gauge, most transformers have an oil
temperature gauge that measures the actual top oil temperature. The
difference in temperature between the winding temperature gauge and the
top oil temperature gauge is one indication of how heavily the
transformer is loaded.
Winding and/or top oil temperatures should be routinely observed to see whether the transformer is operating within normal temperature limits. There have been many cases where an abnormal temperature indication has uncovered serious problems with transformers.
One example of this was a case of an OA cooling class transformer
operating with a very high oil temperature. Further investigation
revealed that a leak in the radiator had caused the oil level to drop
below the radiator inlet, resulting in a virtual total loss of cooling.
In other cases, investigations of elevated transformer temperatures have
revealed blocked coolers and malfunctioning cooler controls.
An oil level gauge
is required so that the correct oil level can be maintained. There is
usually a mark on the gauge that indicates the 25°C level, which is the
proper oil level at that temperature. Maintaining the proper oil level
is extremely important because if the oil level falls below the level of
the radiator inlet, flow through the radiator will cease and the
transformer will overheat.
A very low oil level can expose
energized and current-carrying components that are designed to operate
in oil and could result in overheating or an electrical flashover. If
the oil level is too high, it could cause over pressurization when the
oil expands.
If the transformer is equipped with a constant-pressure nitrogen oil preservation system, then pressure gauges are included to measure the pressure of the gas blanket and the nitrogen in the make-up bottle.
Transformers
that have a gas blanket over the oil in the main tank usually come
equipped with pressure gauges. On random inspections, a sealed-tank unit
should have a slight positive or a slight negative pressure. If the
pressure gauge consistently records zero pressure under all loading and
temperature conditions, this is an indication that the transformer tank
has a leak, allowing it to ‘breathe‘.
This condition must be taken seriously and fixed, because a transformer that breathes is probably contaminated with moisture.
Reference: Power Transformers Principles and Applications – John J. Winders, Jr.
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