Non-linear loads—VFDs, LED/SMPS lighting, servers—pull peaky current rich in 3rd, 5th, 7th… harmonics. Those harmonics raise copper I²R and stray losses (especially in windings and structural parts), pushing temperature rise beyond what a “standard” transformer was designed for. K-Factor is a simple way to specify transformers that survive this extra heating and preserve insulation life.
What K-Factor means (in practice)
K-Factor quantifies how “hard” a harmonic spectrum will heat a transformer compared to pure sine. A K-rated transformer is built with:
- Lower stray losses (winding geometry, conductor placement)
- Heavier conductors / cooler hot-spots
- Higher thermal margins (often Class F/H)
- Sometimes electrostatic shields to reduce capacitive coupling to ground
Result: for the same kVA, a K-rated unit runs cooler on harmonic loads—slowing insulation aging and extending life.
Quick chooser/ K-Factor Selection Guide (K-rating vs typical load mix)
| Typical load mix | Example applications | Recommended K |
| Light non-linear (office IT, POS, LED) | Retail, small offices | K-4 |
| Mixed non-linear (IT + drives, welders) | Plants, hospitals, campuses | K-13 |
| Heavy non-linear (dense VFDs/rectifiers) | Process lines, data centers | K-20 |
| Extreme spectra / specialty | Large rectifiers, UPS front-ends | K-30 to K-50 |
Rule of thumb: If you don’t have a measured spectrum, K-13 is a safe baseline for most mixed panels. Go K-20 when VFD/rectifier density is high.
Selection in 5 steps
- Identify non-linear loads (kW/kVA and quantity): VFDs, UPS/SMPS, welders.
- Check data: THDi at feeders or a recent PQ study. If missing, use conservative assumptions.
- Pick K-rating from the table (or computed K from spectrum if available).
- Size kVA for continuous load + diversity + growth (avoid overloading to “fake” K).
- Decide options: electrostatic shield (for noise/CM current), temperature sensors, enclosure/IP, noise limits.
Why K-rating protects life
Insulation life halves for roughly every 10 °C extra hot-spot temperature. K-rated windings and layouts reduce stray hot-spot formation under harmonics, keeping the hot-spot cooler, so you retain years of service life instead of burning it in months.
When K-Factor isn’t enough
- Severe PCC limits / utility penalties → you still need harmonic mitigation (line reactors, passive/active filters).
- Motor dv/dt/bearing issues → that’s an output-side problem (dv/dt or sine filters), not solved by the transformer alone.
RFQ checklist (paste into your enquiry)
- Duty: “K-rated transformer for non-linear loads; target K-__.”
- Rating: __ kVA, __ V (Pri) / __ V (Sec), 50 Hz, Vector group __.
- Thermal: Class __; temp rise ≤ __ K at K-duty; hot-spot sensors (PT100/NTC).
- Losses & test: No-load/load losses at K-duty; routine and type tests per IEC/IEEE.
- Shield/EMC: Electrostatic screen (Y/N), ground lug ≤ __ mΩ.
- Noise & enclosure: dB(A) max ; IP / indoor/outdoor.
- Docs: Outline & terminal drawings, weight, lifting/clearances, warranty.
Bottom line: K-Factor doesn’t “fix” harmonics—but it armors the transformer against them. Specify the right K, and you’ll keep winding temperatures—and insulation aging—under control for a longer, quieter, more reliable life.