Page 58 - Energize August 2021

TECHNICAL

Extreme overloads can result in
catastrophic transformer failure. The
temperature of the top oil should
never exceed 100°C for power
transformers with 55°C insulation or
110°C for those with 65°C insulation.
The consequence of exceeding these
limits could be oil overflow, excessive
pressure, or tank rupture.
If winding hot spot temperatures
exceed 140°C when moisture is
present, free bubbles may form and
result in internal faults. Due to these
considerations, the peak short duration
loading of power transformers less than
100 MVA should never exceed 200% of
the nameplate rating.
The resulting fault current passes
Figure 1: Distribution transformer at substation
through the transformer and shakes
the windings with a mechanical force
proportional to the square of the fault
current magnitude. If thermal aging
has caused the insulation to become
sufficiently brittle, a crack will form and an
internal transformer fault will result.
Low impedance transformers
tend to fail more often than high
impedance transformers since they will
experience more severe fault currents.
Autotransformers generally have very low
impedances and tend to fail more often
than multiple-winding transformers.
Extensive research has been done
in the area of transformer condition
assessment. The goal is to determine
the health of the transformer and
identify incipient problems before they
lead to catastrophic failure. Simple
Figure 2: Expected transformer life vs. hot spot temperature
methods include visual and audio
inspection. More complex methods
include thermal load tests, power factor pumps, fans and blocked radiators reduce the ability of transformers to dissipate heat but
tests, high potential tests and dissolved do not cause outages directly. Other failures, like cracked insulators and broken seals, may
gas analysis, and other electrical, result in outages but can be fixed quickly and inexpensively. Oil-filled load tap changers have
mechanical, and chemical techniques. historically been prone to failure and can substantially reduce the reliability of a transformer.
Some of these tests can be performed Manufacturers have addressed this problem so that new devices using vacuum
continuously, referred to as condition technology have succeeded in reducing load tap changer failure rates. n
monitoring and can automatically notify
operators if monitored values exceed Acknowledgement
warning thresholds. This article was first published by Electrical Engineering Portal
Transformers have many accessories
that can also fail. Failures associated with Send your comments to rogerl@nowmedia.co.za

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