Electrical
Transformer Protection
Generates a starting set of transformer protection settings — overcurrent, earth fault, differential, REF, thermal and over-fluxing — from the transformer nameplate and system fault level.
Standard method — stated scope limits apply
Enter transformer data and generate.
About This Calculation
A power transformer's differential protection compares current entering one winding against current leaving the other — under normal conditions and external faults these are equal (after CT ratio and vector-group correction), but an internal fault creates a genuine mismatch the relay can detect almost instantly. The hard part is that transformer inrush current (a large, temporary magnetizing surge on energization) also looks like a differential mismatch to a naive relay — which is why real transformer differential protection includes harmonic restraint, specifically checking for the second-harmonic content that distinguishes inrush from a genuine internal fault.
Frequently Asked Questions
Why does transformer inrush current look like a fault to a differential relay?
On energization, the transformer core briefly draws a large magnetizing current from one side only (to build up flux) — the secondary sees essentially no current during this transient, creating a large apparent current imbalance that a simple differential comparison would read as an internal fault.
What is second-harmonic restraint and why 2nd harmonic specifically?
Inrush current is rich in even harmonics, especially 2nd harmonic, because of the asymmetric, one-sided way the core saturates during energization — a genuine internal fault current does not have this signature. Relays measure 2nd harmonic content in the differential current and restrain (block) tripping when it exceeds a set threshold, typically 15-20%.
Why does the CT ratio matching matter so much for differential protection?
The relay is comparing scaled-down secondary currents from CTs on both sides of the transformer, which normally have different turns ratios (since primary and secondary transformer currents differ by the transformer’s own turns ratio). Any mismatch between the actual CT ratios and what the relay compensates for shows up as a false, permanent differential current even with no fault at all.
What does percentage (biased) differential protection add over simple differential?
It scales the tripping threshold up as through-current rises, since CT ratio errors and mismatches genuinely get larger in absolute terms at higher current — a fixed differential threshold would either be too sensitive at low load or too insensitive at high load; percentage bias keeps sensitivity appropriate across the whole load range.