[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

3.12.3.2.2.4 Resistance — Such measurements must be made using suitable high-accuracy instruments;
these measurements are not easy to make under service conditions.
3.12.3.2.2.5 Insulation Power Factor, Resistance, and Capacitance — A polarization index can be determined
in the same manner. Polarization index results that are less than 2.0 indicate deterioration of the
insulation.
3.12.3.2.2.6 Low-Voltage Exciting Current (if Previous Data Are Available) — This information is an
excellent indication of winding movement, but the test must be performed under circumstances similar
to the benchmark test to be of value.
3.12.3.2.2.7 Short-Circuit Impedance (if Faults Have Been Involved) — This test is an indicator of winding
movement that may also give indication of shorted turns.
3.12.3.2.3 Internal Inspection
If the problem cannot he identified from the test data and behavior analyses, an internal inspection may
be necessary. In general, internal inspections should be avoided because the probability of failure increases
after persons have entered transformers. The following items should be checked when the transformer
is inspected.
• Is there evidence of carbon tracks indicating flashovers or severe partial discharges?
• Check leads for evidence of overheating. The insulation will be tan, brown, or black in extreme
cases.
• Check for evidence of partial-discharge trees or failure paths. Do odors indicate burned insulation
or oil?
• Check bolted connections for proper tightness.
• Are leads and bushing lower shields in position? Is the lead insulation tight?
• Inspect the windings for evidence of distortion or movement.
• Check the end insulation on core-form designs for evidence of movement or looseness.
• Are the support members at the ends of the phases tight in shell-form designs?
• Are coil clamping devices tight and in position?
• Check tap changers for contact deterioration. Is there evidence of problems in the operating
mechanisms?
• In core-form units, check visible parts of the core for evidence of heating.
After the inspection has been completed, look over everything again for evidence of “does anything
look abnormal.” Such final inspections frequently reveal something of value.
3.12.4 Failure Investigations
The same processes are required in failure investigations as for problem analyses. In fact, it is recommended
that the approaches described for problem investigations be performed before dismantling of
the transformer to determine possible causes for the failure. Performing this work in advance usually
results in hypotheses for the failure and frequently indicates directions for the dismantling. Thus, the
only additional steps to be performed following a failure are to dismantle the transformer and make
a determination whether it should be repaired. There is never a 100% certainty that the cause of failure
can be determined. All theories should be tested against the facts available, and when assumptions
come into doubt, the information must be replaced with data that is confirmed by the other facts
available.
As a general rule, all steps of the process should be documented with photographs. All too often it
happens that something that was not considered relevant in the early stages of the investigation is later
determined to be important. A photographic record will ensure that evidence that has been destroyed
in the dismantling process is available for later evaluation.
3.12.4.1 Dismantling Process
Complete dismantling is usually performed when a failure has occurred. In a few instances, such operations
are performed to determine the cause for a problem, such as excessive gassing that could not be
explained by other investigations.
The following steps are recommended for this process.
3.12.4.1.1 Transformer Expert
The process should be directed by a person or persons having knowledge of transformer design. If it is
done in the original manufacturer’s plant, the manufacturer’s experts will usually be available. However,
it is recommended that the user have experts available if the failure mechanism is in doubt. It is good
to have two experts available because they may look into the failure from different perspectives and will
provide the opportunity to discuss various aspects of the investigation from differing viewpoints.
3.12.4.1.2 Inspection before Untanking
Inspect the tank for distortion resulting from high internal pressures that sometimes result from failures.
Check the position of leads and connections. Determine if there has been movement of bushings.
3.12.4.1.3 Inspection after Removal of Core and Windings
• Make a detailed inspection of the top ends of windings and cores.
• Inspect the mechanical and electrical condition of the interphase insulation and the insulation
between the windings and the tank.
• Check the core ground.
• Check lead entrances to windings for mechanical, thermal, and electrical condition.
• Check the general condition of leads, connections, and tap changers.
• Check the leakage flux shields on tank walls or on frames for heating or arcing.
• Examine the wedging between phases and from the windings to the core on shell-form units and
the winding clamping structures on core-form units to determine if there is still pressure on the
windings.
3.12.4.1.4 Detailed Inspection of the Windings
• Is there evidence of electrical failure paths from the windings over the major insulation to ground?
• Is there evidence of distortion of the coils or windings resulting from short-circuit failures?
• Has electrical failure occurred between turns, and has there been mechanical distortion of the
turns?
• Have failures occurred between windings or between coils? Are there weaknesses in nonfailed
portions that could affect the electrical strength? Such instances include damage to turn insulation,
distorted disks or coils, or insulation pieces not properly assembled.
• Is there evidence of metallic contamination on the insulation and windings?
• Is there evidence of hot spots at the ends of windings or in cap leads inside the disks or coils?
• Is there any evidence of partial discharges or overheating in the leads or connections?
• Were the windings properly supported for short circuit?
• Are spacers in alignment?
• Is insulation between windings and between the inner winding and the core tight?
3.12.4.1.5 Examination of the Magnetic Circuit
• Have electrical arcs occurred to the core? If so, determine if the fault current flowed from the
failure point to ground, resulting in damage to core laminations in this path. Note that any such
damage will usually require scrapping of the laminations.
• Is there evidence of leakage flux heating in the outer laminations?
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC