Standard Sizes of 3-Phase Power Transformers

Three-phase power generation is standard in the United States and Canada, since 3-phase power transmission is very efficient in carrying electricity over great distances without significant power loss. Transformers are used to step up (increase) and step down (decrease) the voltage. Transformers are defined on their type of wiring, whether or not they are immersed in oil, and the voltage level they produce. The standard secondary or stepped down voltages produced by transformers in North America are 125 and 277 volts. Power transformers are measured in thousands of volt-amperes (kVA) or millions of volt-amperes (MVA).

Types of Transformers

Transformers convert high voltage power to a lower voltage suitable for homes and offices.

A three-phase, four-wire system within a three-phase power transformer uses the fourth wire as grounding. A delta-wye or delta-Y transformer uses three wires without a fourth ground wire, giving it the shape of a Y. Transformers can be dry-type or filled with oil. Transformers sitting in oil are better protected from oxidation and corrosion than dry-type transformers. Oil-filed transformers up to 2,500 kilo-volt-amperes (kVA) are either in a hermetically sealed tank or in a tank with a conservator. Network transformers are distribution transformers used to step down power to local networks. Network transformers are divided into vault-type transformers and subway-type transformers.

Standard Industrial Sizes

Power transformer sizes can be grouped based on their voltage-amperes. Industrial power transformers are measured in thousands of volt-amperes or kVA. Small power transformers range from 500kVA to 7,500 kVA. Medium power transformers range from 7,500 kVA to 100 MVA. Large power transformers supply 100 MVA and higher. Standard power transformers are 5 MVA, 10 MVA, 15 MVA and 22 MVA. When the calculated power transformer requirements are different than a standard transformer size, it is recommended that two standard size transformers be installed to handle peak loads and overloads. Three-phase power transformers can also be custom built to any size.

Submersible Transformers

Submersible transformers are used in subway lines due to frequent tunnel flooding.

Submersible power transformers are used in water pumps, boats, and in areas where the power transformer may be flooded periodically. Submersible power transformers are smaller than standard industrial sizes. According to "Electric Power Transformer Engineering", "standard three-phase kVA ratings from 75 to 1000kVA are available with secondary voltage of 208Y/120V. With a 480Y/277-V secondary, the available sizes are 75 to 2,500 kVA". Subway-type network transformers can be frequently or continuously submerged. Vault-type network transformers can be submerged occasionally.

Service Transformers

Service transformers step down local power line voltage for homes and small businesses. They convert the distribution voltage from 20,000 volts to 100 to 600 volts. Service transformers are available in 10 kVA, 25kVA, 50kVA, 75kVA, 100 kVA and 150 kVA sizes.

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