The answer first: commission with 8 tests — insulation resistance above the IEEE 43 minimum, a turns ratio within ±0.5% of nameplate, winding resistance balanced within 2%, oil breakdown above 30 kV, and a confirmed vector group — never energize on “it looks fine”
A transformer is a precision machine that has just survived a sea voyage or a long road journey, and shipping is exactly when most latent faults are introduced. Oil sloshes and picks up moisture, a tap-changer contact can shift, a bolted connection can loosen, and a phase wired wrongly on site can destroy the unit the moment the breaker closes. Commissioning is the disciplined sequence of electrical tests that catches these faults before first energization. The pass/fail numbers are published in standards, and they are what separate a professional installation from a gamble:
- Insulation resistance (IR) at or above the IEEE 43 / IEC 60076-1 minimum, with a Polarization Index (PI) above 1.5 for oil-immersed units.
- Turns ratio (TTR) within ±0.5% of the nameplate ratio on every tap.
- Vector group confirmed to match the nameplate (Dyn11, Yyn0, etc.) before any parallel or phasing work.
- Winding resistance balanced within 2% between phases and matching the factory test report.
- Oil dielectric strength above 30 kV breakdown with moisture below 20 ppm for a ≤72.5 kV unit (IEC 60422).
- Protection and alarms functionally tested: Buchholz, pressure-relief device, winding-temperature indicator, oil-level gauge.
- Earthing confirmed at ≤10 Ω, with the neutral earthed exactly as specified.
- An energization sequence with the first close on no-load, then a monitored soak, before any load is applied.
Each test has a specific job and a specific number to hit. Here is exactly what to measure, what it should read, and what it tells you.
Why you cannot skip commissioning
Every transformer leaves a good factory with a full routine test certificate — but that certificate describes the unit at the factory gate. Between the gate and your substation, the unit was lifted, trucked, shipped, unloaded and stored. Mechanical shock can crack a bushing or a CT, vibration can loosen a bolted tap connection, and even a small ingress of moisture through a breather can pull the insulation resistance down by an order of magnitude. Commissioning is not a repeat of the factory test — it is a verification that transport did no harm and that site wiring is correct. For the tests you should demand at the factory before the unit ships, see our Pre-Shipment Inspection (PSI) checklist — commissioning and PSI are two halves of the same quality chain.
The 8 tests: what to measure and what it should read
1. Insulation resistance (IR) and Polarization Index (PI)
Apply a DC megger (typically 2.5 kV or 5 kV for a distribution unit) between each winding and earth, and between HV and LV, and read the resistance after 1 minute. The IEEE 43 rule of thumb is a minimum of 1 MΩ per kV of rated voltage plus 1 MΩ, temperature-corrected to 20°C — a 12 kV winding should read at least ~13 MΩ. The Polarization Index is the 10-minute reading divided by the 1-minute reading: above 1.5 is acceptable for a clean oil-immersed unit, above 2.0 is good, and below 1.0 means moisture or contamination that must be dried out before energization. A low PI is the classic signature of a unit that absorbed moisture in transit.
2. Transformer turns ratio (TTR)
A TTR set applies a low voltage to the HV winding and measures the induced voltage on the LV side on every tap position. The measured ratio must be within ±0.5% of the nameplate ratio (IEEE C57.12.90). A deviation larger than that points to a shorted or open turn, or a tap-changer that landed between positions — the tap-changer is by far the most common culprit, and the fix is often as simple as operating the handle through its full range and re-seating the contact.
3. Vector group / phase-relationship check
Confirm the winding configuration and phase displacement actually match the nameplate — a Dyn11 transformer must show a 30° lead of the LV relative to the HV, and a Yyn0 must show 0°. Getting this wrong is the single most dangerous site error, because a mismatched vector group will either refuse to parallel, reverse the phase rotation, or let a circulating current flow on the first close. The symbol, the displacement angles and how to specify them correctly are covered in our Vector Group guide.
4. Winding resistance
Measure the DC resistance of each winding on every tap with a winding-resistance meter (Kelvin four-wire method). The three phases must balance within 2% of each other, and the absolute values should match the factory test report within its tolerance. A higher-than-expected phase points to a loose bolted connection, a bad tap-changer contact, or a partially open conductor — all of which will overheat in service. Because the test current can be large, record the winding temperature at the same time and correct the reading.
5. Insulating oil tests and Dissolved Gas Analysis (DGA)
Draw an oil sample and run the two checks that matter for a new unit. First, dielectric breakdown voltage (BDV): for equipment up to 72.5 kV the oil must withstand ≥30 kV (IEC 60422), and the moisture content must be below 20 ppm; at higher voltage classes the bar rises to ~50–60 kV and <10 ppm. Second, a Dissolved Gas Analysis (DGA) baseline: a new, healthy unit should show negligible dissolved gases. That baseline is gold later — you trend future DGA samples against it to catch incipient faults years before they fail. For the oil-immersed vs dry-type choice and what each means for maintenance, see our Oil-Immersed vs Dry-Type guide.
6. Tan delta / insulation power factor
For distribution units this is optional, but for anything from ~5 MVA up (and always for higher voltage classes) run a tan-delta test on the HV and LV windings. The dielectric loss angle (tan δ) of a new unit should be below 0.5% at 20°C (IEC 60076-3 / IEEE C57.12.90); a rising tan δ flags aged or moist insulation that the simple IR test can miss. It is the cheapest way to buy confidence on an expensive unit.
7. Protection, alarm and control function checks
Do not rely on the factory — prove each protective device works on site: the Buchholz relay (gas and oil-surge elements), the pressure-relief device, the winding-temperature indicator, the oil-level gauge, and the trip/alarm wiring back to the breaker. Inject or simulate each signal and confirm the correct alarm or trip lands. A transformer whose protection is never functionally tested is uninsurable in most markets.
8. Earthing and neutral connection check
Measure the earth resistance of the transformer earthing point and confirm it meets ≤10 Ω (IEEE 142 / local codes), and verify the neutral is earthed exactly as the specification says — solidly, through an impedance, or left isolated. A high-resistance or wrongly-configured earth is what lets a surge or a fault destroy an otherwise healthy unit; the earthing and bonding rules are spelled out in our Lightning Protection guide.
The pass/fail numbers in one table
| Test | Pass criterion | Standard |
|---|---|---|
| Insulation resistance | ≥1 MΩ per kV + 1 MΩ (20°C corrected) | IEEE 43 / IEC 60076-1 |
| Polarization Index | >1.5 (oil-immersed), >2.0 good | IEEE 43 |
| Turns ratio (TTR) | ±0.5% of nameplate, all taps | IEEE C57.12.90 |
| Winding resistance | ≤2% phase imbalance, matches factory report | IEC 60076-1 |
| Oil breakdown voltage | ≥30 kV (≤72.5 kV), moisture <20 ppm | IEC 60422 |
| Tan delta | <0.5% at 20°C | IEC 60076-3 |
| Earthing resistance | ≤10 Ω | IEEE 142 |
The test sequence: order matters
Run the tests in a logical order so that each result is trustworthy and nothing is damaged. The safe sequence is:
- Visual and mechanical inspection — check for transport damage, oil leaks, loose fixings, and confirm the nameplate data before touching anything electrical.
- Earthing check — establish the earth reference first, so every later measurement has a valid ground.
- Insulation resistance and PI — the quickest health scan; if it fails, nothing downstream is worth doing until the unit is dried or cleaned.
- Winding resistance and turns ratio — confirm the magnetic circuit and tap-changer are sound.
- Oil tests and DGA baseline — confirm the insulating liquid is dry and clean.
- Vector group and phase relationship — verify the connections before you ever consider closing the breaker.
- Protection and control function checks — prove the trip and alarm paths end-to-end.
- No-load energization — close the breaker with the secondary open, watch for abnormal inrush, noise or gas, and let the unit soak under monitoring (typically 24 hours) before loading.
The no-load first close is deliberate: an inrush current of several times rated current is normal and harmless, but if there is a fault the no-load condition limits the energy released. For how the impedance voltage (Uk%) limits short-circuit and inrush behaviour on a real fault, see our Short-Circuit Current calculation guide.
How to put commissioning into your RFQ and project plan
- State it in the contract. “Supplier shall provide the factory routine test report; commissioning shall be performed per IEC 60076-1 and IEEE 43 before energization.”
- Ask for the PSI report. Demand the factory pre-shipment inspection report and the factory test data so the site team has a reference to compare against.
- Budget the instruments. A megger, a TTR set, a winding-resistance meter and an oil test kit are a small fraction of the transformer cost and pay for themselves on the first fault they catch.
- Fix responsibility. Name who runs each test, who signs off, and what happens if a value is out of tolerance — re-test after drying, or reject.
- Keep the records. File the commissioning results with the nameplate photo and the factory report; they are the baseline for every future maintenance and warranty claim.
QDTB supplies oil-immersed and dry-type transformers with a complete factory test report and commissioning support, and we size and specify the unit so it passes every one of these tests first time. Use the QDTB Engineering Toolbox to size the kVA and voltage class, browse our oil-immersed and dry-type ranges, or contact us with your nameplate data for a commissioning-ready quotation.