Faced with a nameplate crowded with numbers, beginners get lost. The solution is a fixed methodology: five steps for reading any transformer nameplate in the world in a logical order, extracting what you need for any operating or testing decision.
The methodology: five steps in order
- 1. Power: Read the kVA for each cooling mode (for example, 1000/1250 kVA for ONAN/ONAF). This is the transformer's ceiling.
- 2. Voltages: The HV/LV side voltages at the nominal tap position, from which the turns ratio is derived.
- 3. Connection group: Such as Dyn11 — the connection type on each side and the angular displacement, which are critical for paralleling and protection.
- 4. Impedance Z%: The key to short-circuit current calculations: the smaller it is, the larger the potential short-circuit current.
- 5. Tap changer table: The taps and the voltage at each tap — your reference when adjusting voltage and performing the TTR test.
A nameplate reads: 1500 kVA — 13800/400 V — Dyn11 — Z=6% — ONAN.
- Full secondary current ≈ 1,500,000 ÷ (1.732×400) ≈ 2165 A.
- Primary current ≈ 1,500,000 ÷ (1.732×13,800) ≈ 62.8 A.
- Approximate maximum short-circuit current at the secondary terminals ≈ 2165 ÷ 0.06 ≈ 36 kA (neglecting the network's impedance).
- Delta on the high-voltage side, star with neutral on the low-voltage side, a 30-degree displacement (clock position 11).
Field tips
- Do not confuse the no-load voltage written on the nameplate with the actual operating voltage under load.
- When two cooling ratings exist, verify which capacity applies to your current situation.
- Always compare test results (TTR, winding resistance) against the nameplate and catalog values, not against a similar transformer.
Sample answer: First, the rated currents: the primary ≈ 2,000,000÷(1.732×33,000) ≈ 35 A and the secondary ≈ 105 A, which are the basis for protection and cabling. Second, the connection group Dyn11: delta/star with neutral and a 30° displacement, so it can only be paralleled with a transformer of the same group. Third, the impedance of 7%: the short-circuit current at the secondary is approximately the rated current ÷ 0.07 ≈ 1.5 kA on the 11 kV side, used for protection coordination.
Reading the rated voltage and assuming it is fixed across all tap positions. The written voltage corresponds to a specific tap changer position; changing the tap changes the ratio, and every test comparison must record the tap position at the time.
Want to master electrical transformers step by step?
Follow trainer Fahad Refai's Electrical Transformers course — a practical walkthrough from the fundamentals to testing and reading catalogs.
Browse Fahad Refai's Courses