Fault level
MVA method: source plus transformer plus local infeed
- where
- transformer short-circuit MVA (MVA)
- transformer rating (MVA)
- transformer impedance (%)
- fault level through the transformer (MVA)
- upstream fault level (MVA)
- total busbar fault level (MVA)
- local motor or generator infeed (MVA)
- busbar fault current (kA)
- busbar nominal voltage (kV)
Explain this equation
A quick estimate of busbar fault level using the MVA method. Handy for sanity-checking a study, a quotation or a switchgear rating before doing a full impedance calculation.
Each element is turned into its own short-circuit MVA. A transformer can pass its rating divided by its per-unit impedance, so ST = SrT/(uk/100). S″kQ is the upstream fault level, Sm any motor or generator infeed connected at the busbar, and Un the busbar voltage.
Short-circuit MVA is the inverse of per-unit impedance on a common base. So elements in series combine like resistors in parallel, which is the product-over-sum Sbus, while infeeds in parallel simply add. The current then follows from I = S/(√3·Un).
Sense check: a 20 MVA, 10% transformer can pass 200 MVA on its own. Behind a 500 MVA source it gives about 143 MVA, or 7.5 kA at 11 kV. An infinitely stiff source would leave just ST.
Watch out: the method ignores angles, the voltage factor c and KT. Treat it as an estimate, and use the IEC 60909 impedance method for equipment ratings.
Quick fault level estimate: series elements combine like resistors in parallel (reciprocals), parallel infeeds simply add.
Enter your values to see the result.
Good to know
Each element’s short-circuit MVA is its rating divided by its per-unit impedance (ST = SrT/uk, motor ≈ rating × LRC ratio). Good for checking a study or switchgear rating at a glance. A 20 MVA, 10% transformer on a 500 MVA source gives about 143 MVA at 11 kV.
Common mistake
Combining MVAs of elements with very different X/R as if in phase, then treating the result as an IEC 60909 value. The MVA method ignores angle and c, so it is an estimate, not a rating calculation.
Limits
Scalar (magnitude-only) method with no voltage factor c and no KT. Typically within a few per cent of an impedance calculation when X/R ratios are similar; use the impedance method (IEC 60909) for equipment ratings.
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.