Transformer Impedance and Parallel Operation: A Buyer’s Checklist
Reviewed by the Fengguang engineering team · Last updated 7 Nov 2026
Impedance controls fault current, voltage regulation and load sharing. Use this checklist before paralleling a new transformer with an existing unit.
What impedance does
Transformer impedance is the percentage voltage drop caused by the transformer’s own leakage reactance and resistance at rated current. It limits short-circuit current and affects voltage regulation. A low impedance gives better voltage control but higher fault current; a high impedance reduces fault current but increases voltage drop and losses.
Parallel operation conditions
Two transformers can be paralleled safely only when their voltage ratio, tap position, vector group and phase sequence match, and their impedances are close enough to share load predictably. A small impedance difference is acceptable; a large difference can overload one unit before the other reaches rating.
Confirm with the protection study
Do not select impedance from a catalogue page alone. The value must coordinate with the upstream protection, the LV breaker rating and the parallel-operation plan. Ask for the guaranteed impedance and tolerance on the datasheet.
| Parallel condition | Why it matters |
|---|---|
| Voltage ratio and tap | Prevents circulating current |
| Vector group | Phase angle must match |
| Impedance | Controls load sharing |
| Phase sequence | Prevents reverse rotation / fault |
| Neutral / earthing | Affects earth-fault protection |
FAQ
Can transformers with different kVA be paralleled?
Yes, if the impedance and ratio are compatible and the protection study confirms load sharing. The smaller unit must not be overloaded at the combined peak.
What is a typical impedance?
Distribution transformers commonly use values in the 4–6% range, but the correct value is project-specific and must be confirmed against the fault level and protection settings.
Does impedance affect losses?
Yes. Impedance is linked to the winding and leakage design, and load loss changes with the design. Compare the guaranteed load loss, not just the impedance percentage.
Related pages
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