View source for Pad-mounted transformer

{{short description|Ground mounted transformer}}
[[File:CERN Computer Centre for LHC - Transformers.jpg|thumb|right|Large pad-mount transformers supplying power to a computer data center. No live wires are exposed.]]
[[File:Prolec GE Pad Mounted.jpg|thumb|right|Padmounted transformers in Caguas, Puerto Rico.]]
A '''padmount''' or '''pad-mounted transformer'''  is a ground-mounted electric power [[distribution transformer]] in a locked steel cabinet mounted on a concrete pad. Since all energized connection points are securely enclosed in a grounded metal housing, a padmount transformer can be installed in places that do not have room for a fenced enclosure. Padmount transformers are used with underground [[electric power distribution]] lines at [[service drop]]s to step down the primary [[voltage]] on the line to the lower secondary voltage supplied to utility customers. A single transformer may serve one large building or many homes.

Pad-mounted transformers are made in power ratings from around 15 to around 5000 [[kilovolt-ampere|kVA]]  and often include built-in fuses and switches. Primary power cables may be connected with elbow connectors, which can be operated when energized using a [[hot stick]] and allows for flexibility in repair and maintenance.

== Design ==
Pad-mount transformers are available in various electrical and mechanical configurations. Pad-mount transformers operate on medium-voltage distribution systems, up to about 35 kV. The low-voltage winding matches the customer requirement and may be single-phase or three-phase.

Pad-mount transformers are (nearly always) [[transformer oil|oil-filled]] units and so must be mounted outdoors only. The core and coils are enclosed in a steel oil-filled tank, with terminals for the transformer accessible in an adjacent lockable wiring cabinet. The wiring cabinet has high and low-voltage wiring compartments. High and low-voltage underground cables from below enter the terminal compartments directly. The top of the tank has a cover secured with carriage bolt-nut assemblies. The wiring cabinet has sidewalls on two ends with doors that open sideways to expose the high and low voltage wiring compartments.<ref name="Padmount Transformer Design">{{cite web|url=https://www.electricityforum.com/td/utility-transformers/padmount-transformer|title=Padmount Transformer |publisher=The Electricity Forum |access-date=2022-02-02}}</ref>

Pad-mount transformers have self-protecting [[fuse (electrical)|fuses]] consisting of a [[bayonet mount]] fuse placed in a high voltage compartment, with a backup high energy [[current limiting]] fuse in series to protect against secondary faults and transformer overload. The bayonet mount fuse protects against secondary faults and transformer overload and is a [[Field Replaceable Unit|field replaceable device]]. The backup current-limiting fuse operates only during transformer failure; therefore, it is not field replaceable. These transformers also serve the conventional low voltage fusing requirements.

The use of a polymeric cable and load break elbows enable switching and isolation to be carried out in the HV chamber in what is known as a "''dead front''" environment, i.e., all terminations are fully screened and watertight.<ref name="Transformer Voltages">{{cite web |author=Langley Engineering. The information contained within this site is copyrighted to Langley Engineering unless otherwise stated. |url=http://www.langley-eng.co.uk/langley_products/pad_mount_transformers_substations.html |title=Pad Mount Transformers |publisher=Langley Engineering |access-date=2011-01-02 |archive-url=https://web.archive.org/web/20101203125127/http://www.langley-eng.co.uk/langley_products/pad_mount_transformers_substations.html |archive-date=2010-12-03 |url-status=dead }}</ref>

Single- and three-phase pad-mounted transformers are used in underground industrial and residential [[power distribution]] systems, where there is a need for safe, reliable, and aesthetically appealing transformer design. Their enclosed construction allows the installation of pad-mount transformers in public areas without protective fencing. Pad-mount transformers are usually located on the street [[easement]]s and supply multiple households in residential areas.

While most traditional pad-mount transformers are fixed on a concrete 'pad,' today, small single-phase designs are also available with the transformer already mounted on a 'polypad' base to be mounted on hard ground, connected, and switched on.

== Standards ==
[[ANSI|American National Standards Institute]] /[[Institute of Electrical and Electronics Engineers]] (ANSI/IEEE)
* ANSI C57.12.00 – Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
* ANSI C57.12.22 – Standard for Transformers - Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers with High-Voltage Bushings, 2500 kVA and Smaller: High Voltage, 34,500Grd/19,920 Volts, and Below; Low-Voltage, 480 Volts, and Below - Requirements
* ANSI C57.12.26 (Withdrawn) – Standard for Transformers-Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase Distribution Transformers for Use with Separable Insulated High-Voltage Connectors, 34,500 Grd/19,920 Volts and Below; 2500 kVA and Smaller
* ANSI C57.12.28 – Standard for Switchgear and Transformers, Pad-Mounted Equipment – Enclosure Integrity
[[National Electrical Manufacturers Association]] (NEMA) Standards
* NEMA TR 1-1993 (R2000) – Transformers, Regulators and Reactors, Table 0-2 Audible Sound Levels for Liquid-Immersed Power Transformers.
* NEMA 260-1996 (2004) – Safety Labels for Pad-Mounted Switchgear and Transformers Sited in Public Areas

== References ==
{{reflist}}

{{Electric transformers}}

[[Category:Electric transformers]]