Page 83 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

4.12 CHAPTER FOUR

FIGURE 4.7 (a) Examples of damage to top and bottom coils in a transformer; (b) bottom view of coil; (c)
top view of coil.

and a hoop force (horizontal) of 3,000,000 lb. Therefore, the windings must be braced to
withstand these forces. If the windings are not properly braced, physical movement occurs.
Short circuit is developed, leading to transformer failure. Therefore, the root cause of elec-
trical faults is mechanical in nature.
When the Kraft paper is impregnable with a good, clean, dry oil, it becomes one of the
best dielectrics known in industry. Water has devastating effects on Kraft paper. Most
transformers fail due to the presence of water. The water that weakens the Kraft paper is
the microscopic droplets formed by paper degradation and oil oxidation (Fig. 4.8). The
water droplets are produced by the inner layers of paper and oil that is trapped between
the coil (copper or aluminum) and the paper. The water acts as a solvent to dissolve and
weaken the paper by destroying the fiber of the Kraft paper. This results in loosening the
windings. The paper insulation will get abraded by the constant moving of the windings.
A total failure is created by having a failure in an extremely small amount of the paper in
the transformer. Adequate measures must be taken early, and promptly to protect the
transformer.
The water generated in the “innards of the transformer” that causes the destruction of
the unit is significantly below the level of detection through oil test procedures and/or elec-

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