Handbook of Electrical Installation Practice - BeKnowledge

356 Handbook of Electrical Installation Practice
Table 13.10 Recommended maintenance tests for high fire-point fluids.
Acceptable values
Synthetic ester Unacceptable values
Test (Midel) Silicone fluid indicate
Minimum
Visual Clear straw- Crystal clear, Particulates, free water
coloured liquid, free of particles colour change
free of particles
Odour Faintly sweet Odourless Arced or burned
insulation, volatile
contamination (solvent
pcb)
Dielectric 50kV new 50kV new (first test) Particulates,
breakdown contamination, water
BS 148: test cell 25kV in 25kV in
transformer transformer
Additional
Water content 200ppm maximum 100ppm maximum Excessive water
Viscosity 95cSt at 20°C 50cSt at 25°C Fluid degradation,
contamination
Fire-point 310°C minimum 350°C minimum Contamination of fluid by
volatile materials
Permittivity 3.2 at 20°C 2.7 at 25°C Polar/ionic contamination
significantly reduce the fire-point. It is preferable that separate pumping and handling
equipment is used for different fluids. Silicone fluids have minimal lubricating
properties and care must be taken in selecting the appropriate pumping equipment
to handle these materials. Solid contaminants may be removed from these fluids by
cartridge filters or by filter presses, moisture being extracted by conventional
degassing equipment.
Table 13.10 details physical and electrical properties of silicone and synthetic ester
fluids that can be checked as part of maintenance operations. Transformers containing
these fluids are normally of hermetically sealed construction, thus contamination
by air-borne pollutants is prevented and the amount of maintenance required
is reduced. As with mineral oil-filled transformers, synthetic liquid cooled types
should be checked for leaks and to ensure that deterioration of the paint finish has
not occurred.
Dry-type transformers
The most important aspect of maintenance is that of ensuring that all cooling air
ducts are kept free of foreign objects and contaminating dirt. A vacuum cleaner
can be used for this purpose. Any blockage can cause the winding temperature to