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