A station-service tap is easy to identify on a single-line diagram and easy to misunderstand in an operating review. Its usefulness depends on the breakers between it and an available source. Naming it a “unit supply” does not establish what powers it before the generator starts. Use IEC 60076-1 for general transformer specification [1].
Locate the tap electrically, then mark its physical isolation points. These two views may differ: a bus conductor can remain energized from the GSU while a nearby generator compartment is isolated. The drawing should identify the whole conducting section, including voltage transformers and surge arresters connected to it.
Compare the source through the unit cycle
| Supply location | Normal source | Startup question | Maintenance question |
|---|---|---|---|
| Generator-terminal tee | Generator through its terminal bus | Can the tee be backfed without energizing the machine? | Does generator-breaker work remove the auxiliary source? |
| GSU-side tee behind generator breaker | Generator or grid through GSU | Is the GSU reverse-feed state permitted? | Which GSU and bus isolation removes supply? |
| Separate grid-fed startup transformer | Independent grid connection | Is supply available before unit synchronization? | Is the connection independent of the equipment under repair? |
“Independent” needs a named failure boundary. Two transformers fed from the same bus section may have separate breakers yet lose supply in the same bus fault. The table should be expanded with actual bus sections and common upstream equipment.
Follow the transfer, not just the steady supplies
For each normal-to-startup transfer, identify the loads that can coast, the loads that must remain energized and the loads that restart automatically. Rotating motors can sustain residual bus voltage after disconnection. That affects transfer timing and the phasor mismatch seen by the incoming source.
Do not solve this by assuming all transfers are open-transition or that a brief interruption is acceptable. Use the plant’s transfer philosophy and study the defined equipment. An approved parallel transfer also imposes a temporary circulating-current duty that a steady-load diagram omits.
Remove the shared failed source before counting backups
Assume two 5 MVA auxiliary transformers and essential load 4 MVA. Either carries 0.80 pu if its source survives. If both share the failed upstream bus, available main-source paths are zero. Identify the actual emergency path before describing the arrangement as redundant.
Turn the drawing into an availability test
Select one state at a time: cold startup, synchronized operation, turbine trip, generator-breaker maintenance and grid loss. For each state, trace a continuous path from a live source to every essential auxiliary bus. Cross out unavailable equipment before counting a backup.
Where no path remains, record the intended emergency source or the required shutdown consequence. This method reveals a missing supply without inventing a new transformer rating. It also makes tap selection meaningful: the auxiliary transformer must meet its voltage duty for each surviving source, including the source impedance during motor starting.
Apply the plant isolation procedure to every identified energy source [2].
References
- IEC, IEC 60076-1:2011, Power transformers – Part 1: General.
- U.S. Occupational Safety and Health Administration, 29 CFR 1910.147, The control of hazardous energy (lockout/tagout), official text accessed 2026-10-24.