72A-2243-01 Rev. B 9/04 2-1
September 17, 2004
2. Understanding Leakage Reactance Testing
Introduction
Winding deformation which leads to an immediate transformer failure may be
the result of several overcurrent events. The probability of overcurrent
conditions is not very high and, as a result, a transformer can remain in service
with partially deformed windings, although the reliability of such a
transformer is reduced. Many transformer failures begin with mechanical
deformation but eventually occur for electrical reasons. Consequently,
determining mechanical deformation should be given very serious
consideration. Even small changes in measured parameters should be treated
with the utmost respect.
Several methods have been used to detect winding deformation. They are:
• frequency response analysis
• Iow-voltage impulse test
• capacitance measurement
• leakage reactance measurement
Methods one and two have inherently very promising searching capabilities.
The relative sophistication of the instruments and the expertise required for
these measurements has yet to allow them to become “household tools” at
many utilities.
Capacitance measurements are performed as a part of the routine
AC-insulation tests and normally include all three phases. Capacitance
between the windings, and between each winding and the core/tank, is a
function of their geometric relationships, as well as the dielectric constants of
the intervening insulation. It is known that capacitance may exhibit minor
variations due to temperature changes or serious contamination.
Leakage reactance measurements are performed by short-circuiting the low
voltage winding. During that test, the reluctance encountered by the magnetic
flux is determined predominantly by the leakage channel (Figure 2.1).
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