In this guide 6 sections
A transformer routine test record holds the serial number, rating and tap position, then the readings: turns ratio, winding resistance, insulation resistance, no load loss and load loss. Each reading carries its instrument, temperature and tester. Keep one signed sheet per unit, filed under the serial number.
Start with a header that identifies the unit
The top block ties the sheet to one transformer. Put the serial number, customer, order number, kVA rating, primary and secondary voltage, vector group, frequency and cooling type. Add the tap changer type and the tap position under test.
Include the date, ambient temperature and, for a liquid filled unit, the oil temperature. Resistance and loss readings shift with temperature. A sheet without it cannot be compared later.
Name the standard or customer specification the test follows.
- Serial number and order number
- Rating, voltages and vector group
- Tap changer type and position
- Ambient and winding temperature
- Specification followed
Turns ratio and vector group check
The ratio test confirms the winding turns match the nameplate on each tap. Record the expected ratio, the measured ratio and the difference for every tap position, with phase displacement or polarity beside it.
A ratio sheet with only a pass tick hides trouble. Keep the numbers. A figure that drifts from unit to unit on the same design points to a winding problem long before anything fails.
Polarity and vector group get their own line. A wrong connection group is a costly surprise once the transformer is on a plinth at site, with the crane already gone.
Winding resistance and insulation resistance
Winding resistance is measured phase by phase on the high voltage and low voltage sides. Note the terminals, the reading, the temperature and the unbalance between phases. A loose joint or a broken strand shows here.
Insulation resistance is measured between windings and from each winding to earth. Record the test voltage, the duration and the megohmmeter used. Many sheets also log the one minute and ten minute readings so a polarisation index can be worked out.
Leave room for the instrument serial number and its calibration due date.
- Resistance per phase, HV and LV
- Phase unbalance
- Test voltage and duration
- One minute and ten minute readings
- Instrument and calibration due date
Loss measurement and impedance
No load loss and load loss are measured on a calibrated power analyser, then corrected to the reference temperature. Record the applied voltage, the current, the measured watts and the corrected value.
Next to them, note the committed value from the order and the margin left. Impedance voltage belongs in the same block.
Customers compare these figures with tender values and evaluate your unit against the committed numbers. They must be legible and traceable. Write the power analyser model and the current transformer ratio beside every reading.
- No load loss and excitation current
- Load loss at reference temperature
- Impedance voltage
- Committed value from the order
Sign off and filing
The foot of the sheet carries the tester’s name, the checker, the date and a space for the customer’s inspector. A pass or fail line follows, with any retest noted rather than overwritten.
File the sheet under the serial number, scanned if it is paper. If the record lives in software, link it to the dispatch entry so an untested unit cannot leave the yard.
Print this layout, run it on the next unit through your test bay and strike every box your testers call awkward.
Transformer routine test sheet
A two page sheet. Page one has the unit header and tables for turns ratio per tap, winding resistance per phase and insulation resistance with one minute and ten minute readings. Page two covers no load loss, load loss, impedance, an instrument calibration list, a retest log and signature blocks for tester, checker and customer inspector.
Open a blank worksheet to printQuestions owners ask
Which tests count as routine for a transformer?
Routine tests normally cover ratio and vector group, winding resistance, insulation resistance, no load loss, load loss and impedance, plus dielectric checks. Your customer’s specification and the applicable standard define the exact list.
Why record temperature on the sheet?
Resistance and loss values change with temperature. Without it you cannot correct readings to a reference temperature or compare one unit with another. Note the winding temperature at the start of each measurement, because a unit that has just come off the oven reads very differently from a cold one.
Should failed readings be corrected on the sheet?
No. Strike through with a date and initials, or open a retest entry. Overwriting a failed number makes the record unreliable and weak in any later query.
Can the test record live in a software system?
Yes. Store it against the serial number, keep the instrument and calibration fields, and block dispatch until the sheet shows a pass. Export a copy for the customer’s inspection.