Transformers
Transformer full-load currents and voltage ratio
- where
- HV full-load current (A)
- rated power (kVA)
- HV rated voltage (kV)
- LV full-load current (A)
- LV rated no-load voltage (kV)
- voltage ratio
Explain this equation
Gives the rated line currents on each side of a three-phase transformer and its voltage ratio. Use it for protection settings, CT ratio selection and cable sizing.
S is the nameplate rating, which by definition is the same on both windings. U1 and U2 are the rated line-to-line voltages, the LV one being the no-load value such as 433 V. The currents are in inverse ratio to the voltages.
Each side is a balanced three-phase circuit, so I = S/(√3·U). With S in kVA and U in kV the answer is in amperes.
Sense check: an 11/0.433 kV 1 MVA unit carries about 52.5 A on HV and 1333 A on LV; the ratio is about 25.4.
Watch out: inside a delta winding the current is line current divided by √3, and for Dy or Yd units the turns ratio differs from the voltage ratio by √3.
Rated line currents on both sides of a three-phase transformer, and its rated voltage ratio.
Enter your values to see the result.
Good to know
IEC 60076-1 defines rated current from rated power and rated voltage, the LV rated voltage being the no-load voltage (e.g. 433 V for a 400 V system). For a delta winding the current inside the winding is the line current / √3. The turns ratio equals the voltage ratio for Yy and Dd; for Dy or Yd it is the phase-voltage ratio, which differs by √3.
Common mistake
Using 400 V instead of the 433 V (or 420 V) rated no-load voltage, which overstates the LV current by about 8 %.
Limits
Principal tapping. On other tappings the rated current changes inversely with the tapping voltage.
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.