Transformer routine tests and type tests explained for buyers

Every transformer that leaves a factory has passed routine tests, and every design has passed type tests before the first unit was sold. Knowing what transformer routine tests and type tests actually measure lets an engineer or procurement specialist in Uzbekistan read a test report critically, ask for the right certificates and organise a useful factory acceptance test, whether the unit is free-standing or destined for a box-type substation (KTP). The scope here is 10 kV class oil-immersed and cast-resin distribution transformers tested to GB/T 1094 (harmonised with IEC 60076).

Transformer routine tests: what is measured on every unit

Routine tests are carried out on every transformer after final assembly and form the routine test report that travels with the unit.

• Winding resistance. DC resistance of each winding per phase, recorded together with the winding temperature. It confirms joints and tap-changer contacts; the three phases should agree closely, and a spread of more than about 2 % (a common acceptance practice, not a standard limit) points to a poor joint or tap contact.
• Voltage ratio and vector group. Ratio on every tap, tolerance ±0.5 % at the principal tap (IEC 60076-1), plus a check that phase displacement matches the nameplate (Yyn0 or Dyn11 for a 10/0.4 kV unit).
• No-load loss and no-load current. Rated voltage on the LV winding with the HV open. No-load loss is present for all 8 760 hours of the year regardless of load and is the figure most sensitive to core steel and workmanship.
• Load loss and short-circuit impedance. Rated current with one winding short-circuited. Load loss is corrected to the reference temperature (75 °C for oil-immersed, 120 °C for class F cast-resin); impedance must sit within ±10 % of the guaranteed value (IEC 60076-1, for impedance below 10 %) because it fixes fault levels and parallel operation.
• Applied voltage. Power-frequency voltage between each winding and earth for 60 s: 35 kV for a 10 kV class winding under GB/T 1094.3 (IEC 60076-3 lists 28 kV as the minimum for Um = 12 kV).
• Induced voltage. Twice rated voltage at raised frequency so that the core does not saturate; the duration follows from the test frequency (120 × 50 Hz / test frequency, so 40 s at 150 Hz, never less than 15 s). It stresses the turn-to-turn insulation that the applied test cannot reach.
• Partial discharge (cast-resin). For cast-resin units such as the SC(B)13 model of the SG(B)13 / SC(B)13 series, GB/T 1094.11 / IEC 60076-11 makes partial discharge (PD) a routine test: pre-stress at 1.8 × rated voltage for 30 s, then a reading at 1.3 × rated voltage over 3 min against a 10 pC limit. A high reading means voids in the resin.
• Tightness (oil-immersed). The sealed corrugated tank is held under pressure for a set time and inspected for leaks; the report records pressure and holding time.

At the Jiangsu Xinhong plant these tests run in a shielded partial-discharge test room on computer-controlled benches that log each result against the serial number. Reports are issued in Chinese and English, with a Russian translation on request.

Type tests: what the design has already proved

Type tests are performed once on a representative unit of each design (capacity, voltage class, performance level), normally at an independent national laboratory, and the certificate covers later units of that design. IEC 60076-1 separates type tests from "special tests"; Chinese type-test certificates usually group both.

• Temperature rise (IEC 60076-2 / 60076-11): the unit runs at rated losses until temperatures stabilise. Limits are 60 K top oil and 65 K average winding for oil-immersed units, 100 K (class F) or 125 K (class H) for dry-type windings.
• Lightning impulse: 1.2/50 µs impulses, 75 kV peak for the 10 kV class in GB/T 1094.3; the pass criterion is an unchanged oscillogram between reduced-level and full-level shots.
• Short-circuit withstand (IEC 60076-5): full asymmetrical fault current, several shots per phase; pass criteria are a limited change in short-circuit reactance, repeated routine tests and inspection of the active part. It is the only test that directly proves the mechanical strength of the windings and their bracing.
• Sound level (IEC 60076-10): sound pressure measured around the unit and converted to sound power in dB(A). The S13 design, for example, is specified on average 20 % below the JB/T 10088-2016 noise limits.
• Energy-efficiency verification: measured losses compared with the GB 20052 grade limits. Where the test basis on the certificate includes GB 20052, as on the SCB18-100/10-NX1 certificate for the SC(B)-NX1 grade 1 cast-resin transformer, the laboratory has checked the measured losses against that standard, not only the manufacturer.

The Xinhong certificate set includes S13-M-500/10 and S13-M-1000/10, SCB13 at 500 and 1000 kVA, and SCB18 at 100, 500 and 2000 kVA, alongside S11 and S22 ratings.

At a glance

How to read a test report

1. Identify the unit. Serial number, model designation (for example S13-M-1000/10), capacity, voltages, vector group and standard must match the order and the approved drawing.
2. Check the reference temperature. Load loss belongs at 75 °C for oil-immersed and 120 °C (class F) or 145 °C (class H) for dry-type. The resistive part of copper losses rises about 22 % from 20 °C to 75 °C, so losses quoted at ambient temperature are not comparable with guaranteed values.
3. Compare with guaranteed values, not only with the standard. A report for an S13-M-1000/10 should show no-load loss against the catalogue 830 W, load loss against 10 300 W and impedance against 4.5 %. IEC 60076-1 allows +15 % on each loss component and +10 % on total losses, but when an energy-efficiency grade is contractual many buyers specify no positive tolerance.
4. Read the dielectric section for values and durations, not just "passed": applied voltage and 60 s, induced voltage with frequency and time, PD voltages with the reading in pC.
5. Check traceability: test engineer, inspector and calibration status of the loss-measuring system.

Certificates to ask for

• Type-test certificate for the exact model, capacity and voltage class, issued by a national laboratory, with the tests listed. A certificate for 1000 kVA does not cover 2500 kVA.
• CQC China Energy-Saving Product certificate for the transformer family (oil-immersed, dry-type or amorphous-alloy), confirming GB 20052 conformity.
• ISO 9001, 14001 and 45001 of the manufacturer: evidence that routine testing is part of a controlled process.
• Routine test report for each serial number, with passport and outline drawing.

Scanned originals of every certificate on the certificates page are available on request.

How a factory acceptance test is organised

A factory acceptance test (FAT) is the witnessing of routine tests by the buyer plus a visual and dimensional inspection. A typical sequence:

1. Inspection and test plan agreed at order: witnessed tests, units covered (all, or a sample from the batch), acceptance criteria, documentation language.
2. Notification before the test date, with results of units already tested. Ten to fourteen days is a planning assumption; the contract fixes the notice period.
3. Visual and dimensional check: nameplate, dimensions and terminals against the approved drawing, paint, accessories (thermometer, pressure-relief device, tap changer, fan controller on dry-type units), no leaks.
4. Witnessed routine tests on the bench; the inspector signs the bench printouts.
5. Additional checks where specified: oil breakdown-voltage sample and PD measurement on oil-immersed units, or an indicative sound-level measurement.
6. FAT report and release for packing: punch list closed, test report and packing list agreed before dispatch to Uzbekistan.

Remote witnessing by video link is a workable alternative for small batches.

Summary / checklist

• Routine tests on every unit: resistance, ratio, losses and impedance, dielectric tests, PD (cast-resin), tightness (oil-immersed); one report per serial number.
• Type tests per design: temperature rise, impulse, short-circuit, sound level, energy-efficiency grade; one certificate for the exact design.
• Read losses at the correct reference temperature and against guaranteed values; state your tolerance policy in the enquiry.
• Ask for the type-test certificate, CQC energy-saving certificate, ISO 9001/14001/45001 and the routine test report.
• Agree FAT scope, sample size and notice period in the contract.

For S13 oil-immersed, SC(B)13 and SC(B)-NX1 (SCB18) cast-resin transformers we can attach the relevant type-test certificates and a sample routine test report to the quotation. Send your specification through the contact page and say whether you would like to attend the FAT in person or by video.