Transformer Load Transfer

Synchronizing module ensures that voltage, frequency and phase angle difference between the two systems is within acceptable limits relative to one another. Since the upstream source of two identical transformers is a common bus, so synchronizing module is not required (provided tap position of transformer is same). As load transfer is to be done without any interruption, so momentary paralleling is required. When planned outage of one of the transformer is required, the respective incomers should trip automatically after the bus section breaker is closed manually. For this purpose, a selection should be available in plant control room or a trip selector switch on switchboard for selecting the incomer to be tripped. Depending on the selection made, incomer to bus section – A or to bus section – B will be tripped once bus section (tie) breaker is closed manually, maintaining continuity of supply. A timer with a time delay of 0.5 – 1 sec. for annunciation of the selected running breaker failing to trip within a pre-set time, (i.e. warning that the two sources remain paralleled for more than a pre-set time) is provided and last closed breaker that is bus section circuit breaker should trip automatically, to prevent momentary paralleling for more than pre-set time.

As commented before, the point is system impedance during momentary paralleling of two sources will be halved. This will cause double the short circuit current, so do we need to have incomer and bus coupler circuit breaker as well as busbar of switchboard of higher short circuit capacity? Please give opinion in this regard.

Transformer 1 feeding to bus section A will have fault level of 22.5kA and voltage transformer -2 feeding to bus section-B will have fault level of 25.31kA. So selecting higher of the two 25.31kA and considering tie circuit breaker closed and incomer -1 circuit breaker out as well as downstream contributions then 31.5kA should be rating of each incomer and bus coupler circuit breaker as well as busbar of switchboard. However during momentary paralleling same equivalent impedance will be parallel combination of 5.89 and 5.94 which will be 3. Hence fault level will be approximately 50kA. So what SC capacity should be selected for the entire switch board.

Keep in mind the two transformers will be paralleled for very short duration (less than a second) and the possibility of fault occurring exactly when the two sources are momentarily paralleled is very rare.