Recently, a Generation Station called my company out to their site to investigate and unexpected outage that occurred from a transmission fault. The transmission transformer had a fault occur at the end of a bushing that was outside of the Zone 1 CT's, thus, the Zone 2 protection had to cleared the fault. During the fault, the Beckwith Syncrotran Transfer Unit sensed a reduced voltage on the Main Bus, Tie Bus, and Load Bus. Due to the timing delay with the Zone 2 fault, it fell into the timing range for a transfer and upon restoration of nominal voltage as the fault cleared and the generator continued to generate; an unexpected trip of the main breaker was sent out from the Transfer unit without a close of the tie resulting in loss of the feed motors on the load bus.
To remedy this problem an engineering change was made to install a variable voltage contactor (we called it a 27 device) to the tie bus where when an acceptable voltage was on the tie bus, if the Transfer unit fell into the timing for a transfer to occur and tie voltage was available above the pickup point of the 27 device; an "a" contact from the 27 device in series with the "Auto Transfer Enabled" (vice before where a jumper was in place to allow for a transfer at any time) would allow for the transfer process to start. Further retesting verified this engineering change. When contacting Beckwith about this issue they stated that they no longer support the Syncrotran units.
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