When several transformers must be energized, sequence changes the source condition seen by each subsequent unit. An already energized transformer adds load and can participate in sympathetic inrush; generator or weak-grid voltage may also recover differently between steps. The energization sequence and minimum recovery time should therefore be explicit study inputs rather than left as an operational afterthought.
The second operation does not start from the first operation’s baseline
After one transformer is energized, its magnetizing branch, any connected load and the source-control state remain part of the system. The next transformer therefore sees a different network than the first. In a weak grid or generator island, voltage and frequency may still be recovering. A study that runs each transformer independently from the same ideal source state can miss the cumulative interaction and the sympathetic response of the units already connected.
Treat the interval as a study variable, not a guessed delay
Compare the credible sequence and intervals using defined voltage, frequency and protection criteria. A suitable interval depends on source recovery, transformer response and the existing loads; it is not determined simply by dividing total transformer MVA by generator MVA. Where recovery is conditional, specify the engineering condition that must be satisfied rather than publishing an unsupported universal number of seconds. The operating procedure must implement those conditions through the approved control and authorization process.
Include unsuccessful and alternative sequences
A transformer may fail to energize, a source may become unavailable, or the planned next unit may be under maintenance. Identify which alternative states remain permitted and which require a new assessment. Evaluate the applicable source and relay behavior after an interrupted sequence rather than assuming the system returns automatically to its original condition. Retain the approved sequence matrix as an engineering input to restoration planning, not as a stand-alone set of breaker commands.
Technical reference framework
[1] CIGRE Technical Brochure 568 — Transformer energization in power systems: A study guide.
[2] IEC 60076-8:1997 — Application guide.
[3] IEEE C37.91-2021 — Protecting power transformers.