Calculates steady-state top-oil temperature rise and winding hot-spot rise at rated and overloaded conditions across ONAN, ONAF, and OFAF cooling regimes using the non-linear exponent thermal model in IEC 60076-2.
Governing Formulas & Standards
Standards Basis: IEC 60076-2 / IEC 60076-7 / IEEE Std C57.91
\Delta\theta_{to} = \Delta\theta_{to,r} \cdot \left[\frac{1 + R_L \cdot K^2}{1 + R_L}\right]^x \quad ; \quad \theta_H = \theta_{amb} + \Delta\theta_{to} + H \cdot g_r \cdot K^y
Applies cooling exponents x and y to total loss ratio RL and load factor K to establish hot-spot temperature θH.
Worked Engineering Example: ONAN Transformer at 120% Overload in 40°C Ambient
- Top-Oil Temperature: Top Oil Rise = 55.0 × (1.20)^1.6 = 73.8 K ; θ_to = 40 + 73.8 = 113.8°C → 113.8°C
- Hot-Spot Temperature: Winding Gradient = (68.0 - 55.0) × (1.20)^1.6 = 17.4 K ; θ_H = 113.8 + 17.4 → 131.2°C (Accelerated Aging)
Final Solution: Hot-Spot Temperature: 131.2°C (Relative Aging Rate: ~11x normal)
Frequently Asked Questions
- What is the consequence of operating above 110°C hot-spot temperature?
- Operating above 110°C accelerates paper depolymerization, releasing moisture and CO/CO2, reducing paper tensile strength, and causing cumulative irreversible loss of transformer operating life.
Interactive calculation engine and real-time CAD solver available online at https://amithvijayan.in/tools/transformer-temperature-rise.