Page 54 - Practical Power System and Protective Relays Commissioning

Power Transformers Theory Testing and Commissioning Chapter | 5 51

FIGURE 5.2 Principle of the Buchholz relay.

FIGURE 5.3 OFAF transformer cooling system. OFAF, Oil forced air forced.

Refer to Fig. 5.3 for a diagram of oil forced air forced (OFAF) cooling
and Fig. 5.4 for oil forced water forced (OFWF) cooling. Table 5.1 shows
transformer cooling codings and an explanation of their methods.

5.4 TRANSFORMER THEORY AND CIRCUIT ANALYSIS
A power transformer transfers energy from one voltage level to another volt-
age level, consisting of two or more coils of wires wound around a magnetic
core (see Fig. 5.5).
Transformer impedance can be represented in percentage impedance or
per unit value. The impedance represents the losses in the transformers due
to leakage flux (defined as magnetic flux that does not follow the particularly
intended path in a magnetic circuit).