Forced-air-cooled transformers have fan-cooled radiators through which the transformer oil circulates by gravity, as shown in figure 22a. Fans force air through radiators, cooling the oil.
Forced-air/oil/water-cooled transformers have a self-cooled (kVA or MVA) rating and one or more forced cooling ratings (higher kVA or MVA). Higher ratings are due to forced cooling in increasing amounts. As temperature increases, more fans or more oil pumps are turned on automatically.
Figure 22 – Forced-Air/Oil/Forced-Oil-Cooled Transformers
The forced-cooling principle is based on a tradeoff between extra cooling and manufacturing costs. Transformers with forced-cooling have less weight and bulk than self-cooled transformers with the same ratings. In larger-sized transformers, it is more economical to add forced cooling, even though the electricity needed to operate fans and pumps increases the operating cost.