6 Transformer Types You Can See in Commercial Installations _ EEP

8/20/2019 6 Transformer Types You Can See in Commercial Installations _ EEP

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6 Transformer Types You Can See In CommercialInstallations

6 Transformer Types You Can See In Commercial Buildings (photo credit: iml.bg)

Transformer Types and their Characteristics

Transformers in commercial installations are normally used to change a voltage level

from a utility distribution voltage to a voltage that is usable within the building, and are

also used to reduce building distribution voltage to a level that can be utilized by specific

equipment. Applicable standards are the ANSI C57 Series and NEMA TR and ST

Series.

The following six types of transformers are normally used in commercial buildings:

1. Substation2. Primary-unit substation

3. Secondary-unit substation (power center)

4. Network

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5. Pad-mounted

6. Indoor distribution

Many other types of transformers are manufactured for special applications, such as

welding, constant voltage supply, and high-impedance requirements. Discussion of the

special transformers and their uses is beyond the scope of this recommended practice.

1. Substation Transformers

Used with outdoor substations, they are rated 750-5000 kVA for single-phase

units and 750-25 000 kVA for three-phase units.

High voltage transformer 40MVA (Steps down 150kv to 10kV in a substation in Belgium. Photo taken 1983.)

The primary voltage range is 2400 V and up. Taps are usually manually operated while

de-energized; but automatic load tap changing may be obtained. The secondary voltage

range is 480-13 800 V. Primaries are usually delta connected, and secondaries are

usually wye connected because of the ease of grounding the secondary neutral.

The insulation and cooling medium is usually liquid. High-voltage connections are on

cover-mounted bushings. Low-voltage connections may be cover-mounted bushings or an

air terminal chamber.

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2. Primary-Unit Substation Transformers

Used with their secondaries connected to medium-voltage switchgear, they are

rated 1000-10 000 kVA and are three-phase units. The primary voltage range is 6900-

138 000 V. The secondary voltage range is 2400-34 500 V.

Primary-Unit Substation Transformer (photo credit: actom.co.za)

Taps are usually manually changed while de- energized; but automatic load tap changingmay be obtained. Primaries are usually delta connected. The type may be oil, less-

flammable liquid, air, dry, cast-coil, or gas. The high-voltage connections may be cover

bushings, an air terminal chamber, or throat. The low-voltage connection is a throat.


3. Secondary-Unit Substation Transformers

Used with their secondaries connected to low-voltage switchgear or switchboards, they

are rated 112.5-2500 kVA and are three-phase units. The primary voltage range

is 2400-34 500 V. The taps are manually changed while de-energized. The secondary

voltage range is 120-480 V.




Trihal – Dry-type transformer 1600 kVA 10/0,42kV

connected to busbar system Canalis KTA 2500A

(Schneider Electric)

The primaries are usually delta-connected, and secondaries are usually wye

connected. The type may be oil, less-flammable liquid, air, dry, cast-coil, or gas. The

high-voltage connections may be cover bushings, an air terminal chamber, or throat. The

low-voltage connection is a throat but it may also be by bus duct.


4. Network Transformers

Used with secondary-network systems, they are rated 300-2500 kVA. The primary

voltage range is 4160-34 500 V. The taps are manually operated while de-energized. The

secondary voltages are 208Y/120 V and 480Y/277 V.




Network transformer – Subway type (photo credit: pioneertransformers.com)

The type may be oil, less-flammable liquid, air, dry, cast-coil, or gas. The primary is delta

connected, and the secondary is wye-connected. The high-voltage connection is

generally a network switch (on-off-ground) or an interrupter-type switch with or without a

ground position. The secondary connection is generally an appropriate network protector,

or a low-voltage power air circuit breaker designed to provide the functional equivalent of

a network protector.

A subway-type unit is suitable for frequent or continuous operation while submerged in

water; a vault-type unit is suitable for occasional submerged operation.


5. Pad-Mounted Transformers

Used outside buildings where conventional unit substations might not be appropriate, and

are either single-phase or three-phase units. Because they are of tamper-resistant

construction, they do not require fencing.




Pad-mount outdoor transformer

Primary and secondary connections are made in compartments that are adjacent to each

other but separated by barriers from the transformer and each other. Access is through

padlocked hinged doors designed so that unauthorized personnel cannot enter either

compartment.

Where ventilating openings are provided, tamper-resistant grills are used. Gauges and

accessories are in the low- voltage compartment.

These units are rated 75-2500 kVA.

The primary voltage range is 2400-34 500 V.

Taps are manually changed while de-energized.

The secondary voltage range is 120-480 V.

Primaries are almost always delta connected or special construction wye connected,

and secondaries are usually wye connected. A delta-connected tertiary is not acceptable

with a three-legged core unless an upstream device opens all three phases for a single-

phase fault.

The type may be oil, less-flammable liquid, air, dry, cast-coil, or gas. The high-voltage

connection is in an air terminal chamber that may contain just pressure- or disconnecting-

type connectors or may have a disconnecting device, either fused or unfused. The

connections may be for either single or loop feed. The low-voltage connection is usually

by cable at the bottom; but it may also be by bus duct.

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The dry-type, pad-mounted transformer does not have the inherent fire hazards of the oil

filled, pad-mounted transformer and frequently the dry-type, pad-mounted transformer

is mounted on the roofs of buildings so that it will be as near to the load center as

possible.

ANSI C57.12.22-1989 [5] applies to oil immersed units with primary voltages of 16

340 V and below.


6. Indoor Distribution Transformers

Used with panelboards and separately mounted, they are rated 1-333 kVA for single-

phase units and 3-500 kVA for three-phase units. Both primaries and secondaries

are 600 V and below (the most common ratio is 480-208Y/120V).


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Indoor substation transformer

The cooling medium is air (ventilated or nonventilated). Smaller units have been

furnished in encapsulated form. High- and low-voltage connections are pressure-type

connections for cables. Impedances of distribution transformers are usually lower than

those of substation or secondary-unit substation transformers.

Indoor and outdoor distribution transformers are also available at primary

voltages of up to 34 500 V and 150 kV basic impulse insulation level (BIL).

The majority of transformers for distributing power at 480 V in a commercial building are

usually referred to as “general-purpose transformers” and secondaries are typically

rated at 208Y/120 V. These transformers are mostly of the dry-type, and some of the

smaller sized ones are encapsulated. General-purpose transformers are used for serving

120 V lighting, appliances, and receptacles.


Other Transformer Types //

Virtually all power transformers used in commercial buildings are of the two-winding type,

which may be referred to as isolating or insulating transformers, and are distinct from the

one-winding type known as the autotransformer . The two-winding-type

transformer provides a positive isolation between the primary and secondary circuits;

which is desirable for safety, circuit isolation, reduction of fault levels, coordination, andreduction of electrical interference.




There are also a number of “specialty transformers”used for applications,

such as x-ray machines, laboratories, electronic equipment, and special

machinery applications.

Specialty transformers used in applications where the least amount of leakage current

could cause an arc and ignite the atmosphere (such as in an oxygenated environment) or cause personal injury (such as in open heart surgery) will require an ungrounded

secondary.

Figure 2 – Direct-Current Electric Arc Furnace (DC EAF) Transformer

In the most sensitive applications, the leakage current may be monitored and is

controlled by introducing a grounded shield between the primary and secondary coils.

Such a shield also reduces electromagnetic interference (EMI), which may be present in

the primary.

Reference // IEEE Recommended Practice for Electric Power Systems in Commercial

Buildings

6 Transformer Types You Can See in Commercial Installations _ EEP

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