There is no universal practice with respect to reenergizing
a transformer that has been disconnected from the system by relay action that
may have been caused by a transformer fault. Since no one would intentionally
energize an internally faulted transformer, the differences in practice seem to
be based on the lack of knowledge of where the fault was or whether there was a
fault.
Consider a transformer differential arrangement that
includes external leads. A fault within the differential zone may not be an
internal fault. If the transformer has a pressure relay, this may give
indication of an internal fault. If not, one has to rely on the presence or
lack of evidence indicating an external fault. In the absence of definite
information that a fault was external, most operating companies will not
reenergize the power transformer without a complete check.
Now consider a form of transformer protection that includes
just the transformer. This may be a differential relay (operating from
transformer bushing CTs) or a pressure relay. The one reason to reenergize a
transformer so protected is the lack of confidence in the relays. While a few
may reenergize a transformer so protected, it may be argued that such a
practice does not appear to be warranted with modern relays.
The use and location of the transformer will affect the
decision whether or not to reenergize. One is less likely to reenergize a
generator step-up transformer or a large system tie transformer than a small
substation transformer. The presence of a spare transformer would lessen the
necessity to reenergize right away. A history of failures of a certain type
transformer may affect the decision by operating companies to reenergize that
type of transformer.
If a user’s practice is not to reenergize after a protective
relay has disconnected the transformer from the system, a real and continuing
problem is how to proceed after such a relay operation; that is, if no fault is
evident on visual inspection, what should be done to determine whether or not
an actual fault exists?
Several tests are available to check a transformer prior to
reenergizing. Turns ratio tests, resistance tests, and low-voltage impulse
tests are available, but gas analysis is now the most used test. Gas analysis
has become increasingly popular and found to be quite reliable when properly
performed. See IEEE Committee Report [B51] and Pugh and Wagner [B76].
Normally, power transformers are not reenergized by
automatic reclosing schemes except where the transformer may be connected to a
line or bus that may be reenergized after a relay trip by the line or
bus-protective relays. The transformer protective relays usually operate a
lockout relay that trips the local interrupting devices (power circuit breaker,
circuit switcher, or disconnect switch) and prevents the devices from closing.
Where a local interrupting device is not present, transfer
trip may be used to operate a remote interrupting device. The transfer trip may
also be used to lock out the remote interrupting device, thus preventing
reenergizing the transformer.
If an automatic grounding switch is used on the high side of
a transformer and high-speed reclosing is used on the line, the transformer
will probably be reenergized before a high-side motor-operated disconnect
switch (MODS) can open. However, if
delayed reclosing is used on the line, the MODS will have time to open and the
transformer will not be reenergized.
Usually, high-speed reclosing would not be used on lines
with automatic grounding switches. If a transformer tapped on a line is fused
on the high side, there is no way to prevent its reenergizing if the line
relays detect the fault and trip, unless all three fuses blow.
Philosophies have changed somewhat in recent years, in that
operating companies seem to have an increasing reluctance to reenergize
transformers after a protective relay operation where the transformer might be
subjected to a second fault. This reluctance is partly because of recent
transformer failure rates and partly because of increased cost and time to
repair internal failures.
Also, operating companies are gaining more confidence
in protective relays, particularly pressure relays.
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