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
|
B
|
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
|
