Substation power transformers rely on cellulose insulation and mineral or synthetic oil. Both age primarily due to temperature. Excess heat accelerates polymer degradation, reduces dielectric strength, and shortens service life. Grid operators therefore impose strict thermal limits based on hot-spot temperature, top-oil temperature, and ambient conditions. Cooling systems (radiators, pumps, fans, OLTC oil circulation) maintain thermal margins during peak loading. Real-time thermal models and DGA monitoring help prevent overload, thermal runaway, or insulation failure.