The following one-line is relevant for this story:
SLLA ==== CNTC —- PALM —- FATC
After an all-weekend line reconductoring outage on one of the two parallel 115 KV lines between the SLLA and CNTC substations everything seemed to be smooth sailing that afternoon. For the duration of the outage, all the load in what constitutes a single 115 KV loop around the region as shown on the one-line above, is being fed radially from the SLLA substation through the remaining parallel line by opening the breaker of the CNTC-PALM 115 KV line at CNTC and dividing the load just as simulated in the load flow runs. The 38 KV lines between the same substations have N.O. ACB’s in them to avoid parallel flows over their ratings.
The time comes to energize the line, the breaker at SLLA was closed and the line was energized, the open breaker at the other end at CNTC failed to operate close through SCADA multiple times. The breaker at SLLA is opened and the breaker at CNTC closes on by SCADA. When the breaker at SLLA was commanded to close, it fails to close but luckily a substation maintenance crew wrapping up their work right at the substation was called to verify the “inoperative” GCB, they found nothing wrong with it other than the closing coil isn’t getting energized whenever the breaker control handle in the control house is rotated to the close position.
Easy-peazy, the substation maintenance supervisor resolves to energize the GCB’s closing coil directly, the operator agrees on this and the breaker at SLLA did close, coincidentally the breaker on the parallel line to SLLA at CNTC trips.
The CNTC load was left radially fed by the freshly reconductored SLLA – CNTC line while the parallel line was also radially energized with no load from SLLA.
The operator still unaware of the overall picture issues a close command on the open CNTC breaker of the CNTC -SLLA line, it closes! The GCB of the reconductored parallel line at SLLA trips too!
The substation crew notices a led flag on one of the line protection relays at SLLA. After some measurements, the relay techs determine the breaker isn’t closing because the line is not synchronizing to the system.
The operators got lucky because when their phases got swapped – by pure chance – the rotation remained the same as the original system’s.
Other utilities have not been so lucky and they’ve left a couple of towns and industries with their phase rotation in the opposite direction…for days!