Mission Brief

Prove the Machine Before You Load It

A new 20 MVA 69/13.8 kV Dyn11 transformer sits on its pad, oil filled and ready. Before it's switched on it must be proven as a machine — a full commissioning suite: the turns ratio true at every tap, the OLTC stepping cleanly, the windings balanced, the vector group right, the insulation dry, the core sound (open-circuit / no-load test), the impedance and copper loss right (short-circuit test), and the oil dielectrically strong. A wrong ratio, a shifted impedance, or wet oil only shows up on the test set — not after energizing.

• Measure turns ratio (TTR) at every tap; confirm the OLTC steps cleanly
• Check winding-resistance phase balance and the vector group
• Open-circuit (no-load) test — exciting current and core loss
• Short-circuit (impedance) test — %Z and load (copper) loss
• Insulation resistance / PI, and the insulating-oil dielectric strength (BDV)