[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

3.12.2 Background Investigation
The background investigation usually involves exploring the following.
• Identification of failure. How did the transformer problem manifest itself or how did the failure
occur? This starting point assumes that some event has occurred, such as a trip, malfunction, or
abnormal diagnostic test results.
• The original transformer design
• A history of similar transformer designs
• The original manufacturing and testing of the transformer
• System application and operational data
3.12.2.1 Transformer Records
3.12.2.1.1 Transformer Application
Cooperation between parties involved will speed the investigation and lead more easily to a correct
conclusion. The manufacturer should be contacted even when the transformer is many years old. Most
equipment manufacturers track problems with their designs, and awareness of problems can help them
improve designs and manufacturing processes. The manufacturer may also be able to provide information
leading to a solution.
Application of the transformer in the power system should be determined. A party with expertise in
transformer application should confirm that the transformer was properly rated for the purpose for
which it was intended and that subsequent operation has been consistent with that intent.
Transformer problems can lead to service outages. Careful analysis of the situation before restoring
service can avoid a repetition of the outage and avoid increasing damage to the transformer.
The performance of transformers made by the manufacturer should be studied. Industry records
of transformers of similar rating and voltage class may be helpful in establishing base data for the
investigation. For example, there have been transformers made by certain manufacturers that have a
history of short-circuit failures in service. At one time, the failure record of extra high voltage (EHV)
transformers designed with three steps of reduced basic impulse insulation level (BIL) had a higher
failure rate than those with higher BILs.
3.12.2.1.2 Transformer Design
The specifications for the transformer, instruction books and literature, nameplate, and drawings such
as the outline and internal assembly should be examined. If failures involving the core and windings have
occurred, the investigative process is much like a design review in which the details of the insulation
design, winding configuration, lead configurations and clearances, short-circuit capability, and the core
construction and normal operating induction must be studied. If components are involved, bushings
and bushing clearances, tap changers, heat exchanger equipment, and control equipment should be
investigated.
3.12.2.1.2.1 Design Review Process — An explanation of the design review process would require a separate
chapter and is beyond the scope of this subject. The process involves a detailed study of the winding
and insulation designs, short-circuit capability, thermal design, magnetic-circuit characteristics, leakage
flux losses and heating analysis, materials used, oil preservation systems, etc. In some cases, it may be
desirable to conduct part or all of a design review as a part of a problem investigation.
3.12.2.1.2.2 Determine if Similar Designs Experienced Problems or Failures — This can be difficult, since
there is no agency that collects and distributes data on all industry problems. It has been made even
more difficult by the closing of several major transformer manufacturing plants. However, some
information can usually be obtained by discussions with users having transformers made by the same
manufacturer.
3.12.2.1.3 Manufacturing and Testing of the Transformer
The manufacturing and test records should be studied to determine if discrepancies occurred. Of
particular importance are deviations from normal manufacturing specifications or practices. Such
deviations could be involved in the problem or failure. All parts of the test data must be studied to
determine if discrepancies or deviations existed. Partial-discharge and impulse test data that have any
deviation from good practices should be recorded. Approvals of deviations made during testing,
especially relating to dielectric and other test standards, are sometimes indications of difficulty during
testing.
3.12.2.1.4 Transformer Installation
The records of installation and initial field tests should be used as a benchmark for any future test results.
These initial field tests are more easily compared with subsequent field tests than with factory tests.
Factory tests are performed at full load current or full voltage, and adjustments must be made to field
test results to account for differences in losses at reduced load and differences in excitation at the operating
voltage.
3.12.2.2 Transformer Protection
The protection of the transformer is as important a part of the application as the rating values on the
transformer. Entire texts, as well as Chapter 3.8 in this text, are devoted to the subject of transformer
protection. When investigating a failure, one should collect all the protection-scheme application and
confirm that the operation of any tripping function was correct.
3.12.2.2.1 Surge Arresters
Surge arrester protective level must be coordinated with the BIL of the transformer. Their purpose, to
state what may seem obvious, is to protect the transformer from impulse voltages and high-frequency
transients. Surge arresters do not eliminate voltage transients. They clip the voltages to a level that the
transformer insulation system is designed to tolerate. However, repeated impulse voltages can have a
harmful effect on the transformer insulation.
3.12.2.2.2 Overcurrent Protection
Overcurrent devices must adequately protect the transformer from short circuits. Properly applied, the
time–current characteristic of the device should coordinate with that of the transformer. These characteristics
are described in IEEE C57.109-1993, Guide for Liquid-Immersed Transformer Through-Fault
Duration. Overcurrent devices may be as simple as power fuses or more complex overcurrent relays.
Modern overcurrent relays contain recording capability that may contain valuable information on the
fault being investigated.
3.12.2.2.3 Differential Protection
Differential relays, if applied, should be coordinated with the short-circuit current available, the transformer
turns ratio and connection, and the current transformers employed in the differential scheme. If
differential relays have operated correctly, a fault occurred within the protected zone. One must determine
if the protected zone includes only the transformer, or if other devices, such as buswork or circuit breakers,
might have faulted.
3.12.2.3 Recording Devices
The occurrence of unusual trends or events that indicated a possible problem should be recorded.
Operation of relays or protective equipment that indicated a failure should be studied. Copies of oscillographic
or computer records of the events surrounding the problem or failure should be obtained.
Records of events immediately prior to the observation of a problem or immediately prior to a trip
are often only the final events in a series that led to the failure. Investigations at the transformer location
should concentrate on collecting such data as relay targets, event recordings, and on-line monitoring
records as well as making observations of the condition of the transformer and associated equipment.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC