Medium Voltage Transformer: What Is It? What Are Its Types?

9 May 2023

A medium voltage transformer is a type of transformer used in the electrical distribution system to provide the final voltage transformation, reducing the voltage from distribution lines to a level that can be used by customers.

The voltage typically falls within the medium range, meaning the voltage entering the transformer is between 5 kV and 35 kV. Some distribution voltages may exceed 35 kV and are considered high voltage, but most distribution systems operate within the medium voltage range.

Medium voltage transformers are designed in compliance with international standards, primarily by organizations such as IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission).

What Are the Types of Medium Voltage Transformers?

Substation Transformers:

Substation transformers are the heart of an electrical substation. These transformers change the voltage and current relationships between incoming and outgoing power. Substation transformers are rated based on their primary and secondary voltage relationships and power-carrying capacities.

For instance, a typical substation transformer might be rated at approximately 5-20 MVA with primary voltages of 15 kV, 25 kV, 35 kV, or 46 kV. Secondary or low voltage can range from 15 kV down to 5 kV.

These medium voltage transformers consist of a core and coils immersed in oil or dielectric fluid contained within a steel tank. The oil or fluid serves as an insulator and coolant to maintain the core at a reliable operating temperature.

Transformer design and functionality are determined by IEEE standards C57.12.00 and C57.12.36, among others.

Three-Phase Pad-Mounted Transformers:

Three-phase pad-mounted transformers are medium voltage distribution transformers installed on a concrete pad and enclosed in a locked steel cabinet. These transformers can be smaller (45-5000 kVA) or larger in station sizes (up to 10 MVA) when compared to other types of medium voltage transformers.

They are suitable for installation near or within public areas where public safety is a concern. The compartmentalized, tamper-resistant, pad-mounted design makes them ideal for use in areas where public safety is a critical consideration.

Design and functionality are defined by IEEE standard C57.12.34, with other specifications governed by IEEE standards C57.12.28 or C57.12.29, among others.

Single-Phase Pad-Mounted Transformers:

Single-phase pad-mounted transformers are designed for use in residential areas and to facilitate power distribution through underground systems. These transformers are typically rated between 10-167 kVA with primary voltages of up to 35 kV.

Specifications and installations are defined by IEEE standard C57.12.38.

Single-Phase Pole-Mounted Transformers:

Single-phase pole-mounted transformers are often used in residential areas. They come in various sizes, ranging from small 5 kVA units to larger 500 kVA units, with primary voltages ranging up to 35 kV. These transformers are mounted on utility poles and are commonly used for overhead distribution systems.

Design, installation, and functionality adhere to IEEE standard C57.12.20.

Air-Cooled Dry-Type Transformers:

Air-cooled dry-type transformers are insulation devices that utilize air circulation for cooling, rather than using oil or other liquids. The transformer casing is ventilated to allow air circulation and cooling of the coils. Additional ventilation louvers may be added for outdoor applications.

Dry-type transformers can be rated from 15 kVA up to 30 MVA. Primary voltages exceeding 601 V must conform to IEEE standards C57.12.01 and C57.12.91.

These are the various types of medium voltage transformers used in electrical distribution systems, each serving specific purposes based on their design and application requirements.

Nano Medya