Cooling Classes of Transformers

As a point of clarification, the cooling classes of transformers
have changed in recent years and are explained in
the following information. The IEEE transformer cooling
designations were changed to become consistent with the
IEC (IEC 60076-2: 1998). The new classifications are
detailed in IEEE C57.12.00-2000.
The new cooling designations have four-letter descriptions
that indicate specific criteria relative to 1) the type of
oil, 2) how the oil is internally circulated, 3) what is used to
cool the oil, and 4) how the oil is externally cooled.
As an example:

Figure 1 — Cooling Designations

The cooling class is identified by the following
methodology:

First Letter

Internal Cooling Medium in Contact with the Windings
Letter Definition
O Mineral oil or synthetic insulating liquid with fire point ≤ 300°C
K Insulating liquid with fire point > 300°C
L Insulating liquid with no measurable fire point

Second Letter

Circulation Mechanism for Internal Cooling Medium
Letter Definition
N Natural convection flow through cooling equipment
and in windings
F Forced circulation through cooling equipment (i.e.,
coolant pumps) and natural convection flow in
windings (also called nondirected flow)
D Forced circulation through cooling equipment, directed
from the cooling equipment into at least the
main windings

Third Letter

External Cooling Medium
Letter Definition
A Air
W Water

Fourth Letter

Circulation Mechanism for External Cooling Medium
Letter Definition
N Natural convection
F Forced circulation [fans (air cooling) or pumps
(water cooling)]

Comparison of past transformer cooling designations
versus present-day transformer cooling designations is
detailed in the following table: