Technical
-> Insulation Systems
The insulation materials within a transformer are designed
to separate the various electrically conductive parts preventing
shorting or arcing between these parts. The different materials
available for insulation have varying resilience to the temperature
at which they can safely be operated, beyond these limits
the materials de-grade both chemically and mechanically until
they cease to function as an insulator. For this reason UL
(Underwriters Laboratories) an independent testing organisation
operating in the United States developed a system of classifying
insulation systems to give a reliable life expectancy for
the transformers at a specific maximum operating temperature.
This system has been adopted internationally, within Europe
it is covered by IEC 85
Having specified the maximum operating temperature for the
system, a defined maximum temperature rise can be specified
for the expected maximum ambient temperature in the operational
area.
These classes are listed as follows.
Class |
Max Temperature |
Max Temp Rise |
A
|
105°C |
65°C |
E
|
120°C |
80°C |
B
|
130°C |
90°C |
F
|
155°C |
115°C |
H
|
180°C |
140°C |
N
|
200°C |
160°C |
220 |
220°C |
180°C |
These figures are based on European climatic conditions in
Tropical regions the allowed temperature rise is reduced by
10°C.
It is generally the case that transformers will be constructed
with a small margin of temperature within these limits
Note transformers to be housed in
sealed enclosures fully protected against the ingress of solid
and liquid contaminants are subject to elevated ambient temperatures
within the enclosure and we have to reduce the maximum temperature
rise of the transformer. When ordering a transformer to be
installed in a high IP rated enclosure customers must specify
this requirement
It should be noted that a higher temperature system is not
by definition a better system, all of the systems have equivalent
life expectancy and reliability characteristics. The selection
of system is often determined by the combination of price,
size and application.
Higher temperature rated insulation materials are generally
more expensive however a transformer of a higher thermal rating
will be physically smaller using less materials to offset
some of these costs.
Also components surrounding a transformer e.g. capacitors
and semi-conductors may have low temperature ratings so there
may be a restriction against using high thermal class transformers
in their vicinity.
Generally Carroll & Meynell transformers comply with
the following system ratings and designed temperature rise
limits
Rating
|
Insul Class |
Typical Temp Rise
|
0 to 10KVA |
E |
75-80°C |
10 to 49KVA |
F |
85-90°C |
50 to 99KVA |
F |
95-100°C |
100 to 150KVA |
H |
105-115°C |
150KVA+ |
H |
125°C |
|
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