dissipated; the sludge forms a blanket barrier to the fl ow of heat from the oil
to the coolant and from the core and coils to the cool oil. If allowed to continue
long enough, the sludge may even block off the fl ow of oil through the
cooling ducts. As a result, the transformer insulation gets too hot and is damaged,
particularly between turns of the windings. Deterioration of the turn
insulation may eventually lead to short circuits between turns and the breakdown
of the transformer. When oxidation progresses to the point where
sludge is being precipitated, the fi rst step should be to remove the sludge
from the transformer by a high-pressure stream of oil or hot oil circulation to
dissolve the sludge, or to either replace the sludged oil or treat it with activated
clay to remove the acid. Under favorable conditions, complete treatment
of the oil is less costly than replacing it with new oil.
TABLE 4.1
Maximum Allowable Moisture in Oil
Voltage Level (kV) Maximum Moisture (ppm)
5 30
15 30
35 25
69 20
138 and up 15
240
200
160
120
80
40
0 10 20
Temperature (