Page 82 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.11

Kraft paper
providing turn to End rings using dense
turn insulation of pressboard to aid in
winding coil. mechanical strength.
Spaced to also provide
for cooling of the inner
winding(s).
Conductor tap
leads wrapped
in paper Dense pressboard
insulation. used for support.

FIGURE 4.6 Three-phase power transformer (75/125 MVA).

(about 10 times more than cellulose insulation). In general, Nomex is not cost-effective for
use as a major insulation in a power transformer.

Reasons for Deterioration
Insulation deteriorates due to the following:

1. Heat
2. Contamination such as dirt, moisture, or oxygen
3. Electrical stress
4. Mechanical stress and strain
Insulation deterioration normally results in the loss of its mechanical properties. As the
insulation weakens, it loses flexibility and becomes brittle. It would not be able to resist the
mechanical stresses resulting from the magnetic forces, differential temperature expansion,
and vibration. The insulation disintegrates, leading to electrical faults.

FORCES

During normal operation, the axial and radial forces between the windings are moderate.
These forces become severe during a short circuit. The transformer must be able to with-
stand fault conditions. This includes forces 10 times higher than normal. Figure 4.7 illus-
trates damaged top and bottom coils in a transformer.

CAUSE OF TRANSFORMER FAILURES

Most transformers fail due to mechanical reasons. The windings are subjected to physical
forces that operate in all directions. These forces can become astronomical under short-circuit
conditions. For example, a 16-MVA transformer will develop a 500,000-lb vertical force

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