The protection of the transformer is as important a part of
the application as the rating values on the transformer. Entire texts 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.
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.
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.
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.
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