Transformers
Load sharing between two transformers in parallel
- where
- rating divided by per-unit impedance (kVA/%)
- transformer ratings (kVA)
- transformer impedances (%)
- load carried by each transformer (kVA)
- total load (kVA)
Explain this equation
Shows how a load divides between two transformers in parallel and the largest load they can carry before either is overloaded. Use it before paralleling units of different size or impedance.
Y is a transformer’s rating divided by its percentage impedance, which is proportional to its admittance on a common base. A larger, lower-impedance unit has a larger Y and takes more of the load.
The two units are in parallel between the same busbars, so they share the same voltage drop. Current then divides in inverse proportion to ohmic impedance, which is the same as in proportion to S/z.
Sense check: 1000 kVA at 5 % with 1500 kVA at 6 % carrying 2000 kVA puts about 889 kVA (89 %) on the small unit and 1111 kVA (74 %) on the large one. The pair tops out at 2250 kVA, not 2500 kVA.
Watch out: unequal ratios or tap positions drive a circulating current not included here, and very different X/R ratios make the magnitude-only method approximate.
How a load divides between two transformers in parallel with different ratings and impedances, and the largest load they can carry without either being overloaded.
Enter your values to see the result.
Good to know
Each unit takes load in proportion to its rating divided by its per-unit impedance. With equal per-unit impedances, load shares in proportion to rating. IEC 60076-8 recommends impedances within about ±10 % of each other and a rating ratio not above about 3:1. Transformer 2 loading is L2/S2 × 100.
Common mistake
Assuming the combined capacity is S1 + S2. The lower-impedance unit reaches its rating first and caps the total.
Limits
Assumes identical voltage ratios and tap positions (no circulating current), the same vector group, and similar X/R ratios so that impedances can be added as magnitudes. Mismatched taps cause a circulating current that this calculator ignores.
Sources
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.