What should accompany transformer percent impedance?
Keep the percent value beside its winding pair, power base, tap position, frequency, reference temperature and stated tolerance. For a multi-winding transformer, request the applicable pairwise values; one impedance number does not describe every possible winding duty.
| Record | Question to resolve |
|---|---|
| Winding pair | Which two windings does the value describe? |
| Power base | On which kVA or MVA rating is the percentage stated? |
| Tap and voltage basis | Which tap and rated voltages were used? |
| Test conditions | What frequency and reference temperature apply? |
| Requirement versus result | Is this a specified target, design value or final measured result, and what tolerance applies? |
Why does impedance matter when buying a replacement transformer?
A different impedance can change how the candidate behaves in the system even when voltage ratio and MVA match. Have the responsible engineer assess the candidate against the applicable fault, protection, voltage and operating studies rather than accepting the closest-looking percentage.
- Keep the existing unit's measured data and the project's allowed deviations in the comparison.
- Identify whether the offered data uses the same winding and power base before comparing percentages.
- Record any tertiary duty and the corresponding data needed for the proposed application.
- Do not use an isolated impedance value to select switchgear or set protective devices.
Does matching impedance prove transformers can operate in parallel?
No. Parallel operation also requires review of ratios, taps, phase displacement, connections, grounding, ratings, load sharing and protection. Equal percent values on different bases are not enough to establish equivalent behavior. Provide both units' full technical records for the project engineer's review.
What should I send to compare a transformer offer?
Email Tommy the existing nameplate or project specification, required voltage ratio and MVA, impedance requirement and basis, connection and grounding, any parallel-operation requirement, quantity and needed date. Ask for the candidate's final test data and flag missing values before purchase.
Common questions
Written quotes and supporting records control unit-specific ratings, availability, condition, price, and delivery.
Can I compare two transformers using only their percent impedance?
No. Confirm the winding pair, kVA/MVA base, tap, frequency, temperature and tolerance, then review the full project requirement.
Is lower transformer impedance always better?
No. The appropriate value depends on the complete system design and operating requirements. The project engineer evaluates the candidate's effects on the applicable studies.
Does a test report replace the project specification?
No. The report describes the tested equipment. Compare it with the controlling specification and have any deviations approved by the responsible project team.