What a transformer routine test report actually tells you

"Tested unit by unit" is easy to write and hard to verify – unless you know how to read the document that proves it. IEC 60076-1 defines which tests must be performed on every transformer (routine tests), on one unit of a design (type tests) and on request (special tests). This article walks through the routine test report of a distribution transformer.

Routine tests – performed on every unit

Test	What it checks	Typical result, 630 kVA oil unit
Winding resistance	Correct copper/aluminium cross-section, sound joints	HV ≈ 5–6 Ω per phase, LV ≈ 1.5 mΩ, corrected to 75 °C
Voltage ratio and phase displacement	Right number of turns, vector group, all tap positions	Ratio within ±0.5 % of nominal on every tap; Dyn11 confirmed
Short-circuit impedance and load losses	uk and Pk at rated current, referred to 75 °C	uk 4.0 % (tolerance ±10 %); Pk ≤ 4 600 W (tolerance +15 %)
No-load losses and current	P0 and I0 at rated voltage and frequency	P0 ≤ 540 W (tolerance +15 %); I0 ≈ 0.3–0.8 % of In
Separate-source AC withstand (applied voltage)	Insulation between windings and to earth	50 kV (Um 24 kV) for 60 s on HV, 3 kV on LV – no breakdown
Induced AC withstand	Turn-to-turn and layer insulation	2 × rated voltage at 100–200 Hz for 40–60 s – no breakdown
Tank tightness (oil units)	Hermetic seal	30 kPa for 12 h, no leak
Partial discharge (cast resin)	Voids in the resin	≤ 10 pC at 1.1 × Um

How to read the loss figures

The report gives the measured values corrected to the reference temperature of 75 °C. IEC 60076-1 allows a tolerance of +15 % on each of P0 and Pk and +10 % on their sum – but Ecodesign Regulation (EU) 2019/1783 sets absolute maximum values that must not be exceeded, tolerance or not. A compliant report therefore shows both figures below the Tier 2 limit. If the guaranteed value is lower than the limit (a low-loss design), the tolerance applies to the guaranteed value.

Type tests and special tests

• Temperature rise test – verifies the cooling design; performed on the first unit of a design.
• Lightning impulse test – 125 kV (Um 24 kV) 1.2/50 µs full and chopped waves.
• Sound level measurement (IEC 60076-10).
• Short-circuit withstand test (IEC 60076-5) – performed by an independent laboratory on a representative unit; expensive, so it is done per design family, not per order.

Type test reports are available on request for the PNACHO design family; they are not part of the standard delivery because they refer to the design, not to your unit.

What arrives with a PNACHO transformer

1. Routine test report of the individual unit (serial number matches the nameplate).
2. EU declaration of conformity (LVD, EMC, RoHS, Ecodesign).
3. Operating and installation manual.
4. Dimension drawing; DWG on request.

These are the four documents distribution network operators in most EU countries ask for before approving a connection. They are delivered with every unit from the PNACHO-O and PNACHO-D ranges.