medium-voltage transformer
A medium-voltage distribution transformer or service transformer is a transformer type that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer.
Practical levels of voltage are often referred to as medium voltage, meaning the incoming voltage to the transformer is on the order of 5 kV to 35 kV. Some distribution voltages may exceed 35 kV and would be considered high voltage, but most of the distribution system is within the medium-voltage range. Modern distribution transformers are manufactured in accordance with many standards, most notably by IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission).
In the United States, the features and functionality of most distribution transformers fall under IEEE standard C57.12.00 (Standard General Requirements for Liquid-Immersed Distribution Power and Regulating Transformers), however, there are a multitude of standards that apply to specific types and applications of transformers including substation type, compartmental padmount type, single-phase pole mounted transformers, generator duty transformers, high-temperature transformers, and many more.
medium-voltage distribution transformers
Substation transformer
The substation transformer is the heart of the electrical substation. This transformer changes the relationship between the incoming voltage and current and the outgoing voltage and current. Substation transformers are rated by their primary and secondary voltage relationship and their power carrying capability. For example, a typical substation transformer would be rated 15 kV, 25 kV, 35 kV or 46 kV on the primary at a power rating of about 5-20 MVA. The secondary or low voltage can be 15 kV down to 5 kV or even less than 600 V. Substation-style transformer design and functionality is dictated by IEEE standards C57.12.00 and C57.12.36. These type transformers consist of a core and coils immersed in oil or dielectric fluid in a steel tank. The oil or fluid serves both as an insulator and as a coolant to keep the core at reliable operating temperatures. Substation units are easily identified by their exposed bushings, gauges, panels or monitoring equipment and are typically located behind a fence or with a restricted area.
