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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