A Power Transformer should pass routine tests before it leaves the factory, but the more practical question for a buyer is: which results do I actually need to see, and what do I do if something looks wrong? The test report is the only objective record of what was built. Reading it properly is part of the purchasing job, not an afterthought.
Winding Resistance, Ratio & Vector Group
Winding resistance measurement catches loose connections and broken strands. What matters is the phase-to-phase deviation - if one phase reads noticeably higher than the others, stop and ask for the winding record before accepting the unit. Ratio testing confirms the actual ratio against the design value, typically within ±0.5%. Vector group verification confirms the phase displacement. This last one is often skipped by buyers ordering a single unit, but if the transformer will run in parallel with existing equipment, a wrong vector group means it cannot be paralleled at all. For a 250 kVA 3 Phase Transformer, these three tests together decide whether the unit can be safely connected and paralleled.
Insulation Performance
Insulation resistance and power-frequency withstand tests are standard. Partial discharge is where buyers with sensitive loads should pay closer attention - a low PD level (typically ≤10 pC) indicates no internal voids. But here is a detail worth checking: is the PD value a routine test result on your actual unit, or a type test result from a similar design? The two are not the same. For oil-immersed units, oil breakdown voltage above 60 kV is a common acceptance threshold, and the oil test report should be dated close to the shipment date, not months earlier.
No-Load and Load Loss
Loss measurements are where quotations and test reports most often diverge. Compare the measured values against the guaranteed values written in the contract, not the nameplate and not the quotation. Check the reference conditions: load loss is temperature-dependent, so a figure without a stated reference temperature cannot be compared with another supplier's number. If the measured loss exceeds the guaranteed maximum, the supplier is typically obligated to correct or replace the unit - which is exactly why the guaranteed values belong in the purchase order.
Checking the Report
Ask for the routine test report before shipment. Check that the serial number matches your order, that the test date and instrument calibration status are recorded, and that numerical values are shown rather than just "pass". A report listing only "qualified" without data cannot be used to verify anything. On larger orders, a third-party inspection before dispatch is a reasonable clause to add to the contract.
About Hebei Hozee Electric Co., Ltd.
Hebei Hozee Electric Co., Ltd. manufactures oil-immersed transformers, dry-type transformers, single-phase transformers, and transformer cores including laminated cores and single-phase wound cores. Routine tests on every Power Transformer cover winding resistance, ratio, vector group, insulation, and losses, and the factory test report ships with the unit. If you need the test protocol or the guaranteed loss values confirmed in writing before you place an order, our engineering team can provide them.