TYPES AND CONSTRUCTION OF TRANSFORMERS
The function of a transformer is to convert ac power from a voltage level to another voltage level at the same frequency. The core of a transformer is constructed from thin laminations electrically isolated from each other to reduce eddy current losses (Fig. 3.1).
The primary and secondary windings are wrapped one on top of the other around the core with the low-voltage winding innermost. This arrangement serves two purposes:
1. The problem of insulating the high-voltage winding from the core is simplified.
2. It reduces the leakage flux compared to if the windings were separated by a distance on the core.
The transformer that steps up the output of a generator to transmission levels (110 kV) is called the unit transformer. The transformer that steps the voltage down from transmis- sion levels to distribution levels (2.3–34.5 kV) is called a substation transformer. The transformer that steps down the distribution voltage to the final voltage at which the power is used (110, 208, 220 V, etc.) is called a distribution transformer.
There are also two special-purpose transformers used with electric machinery and power systems. The first is used to sample a high voltage and produce a low secondary volt- age proportional to it (potential transformers). The potential transformer is designed to handle only a very small current. A current transformer is designed to give a secondary cur- rent much smaller than its primary current.