Loop Feed, Single Phase, Primary Deadfront, Distribution, Padmount, Transformer, Structure

 Introduction

A Loop Feed, Single Phase, Primary Deadfront, Distribution, Padmount, Transformer, Structure is typically installed along feeders underground residential developments (URD's). Padmounted transformers step down the voltage to feed one or multiple service drops of the secondary bushings. Many padmounted transformers may serve a single large building, or it may serve many residences.

They housed in a locked metal enclosure that give the transformer a tamper-resistant rating. They typically rest on a concrete pad with conduit openings so that incoming and outgoing cables can be brought into the termination compartment from underground.  Almost all padmounted transformers are oil filled type.

On the primary (high) side of the transformer they will have two medium voltage bushings (H1A and H1B),  one set for the incoming feed and one for the outgoing feed. This means there are two bushings for one phase to allow the customer allow the customer to connect all of their transformers in a loop configuration.

A typical enclosure includes a tank for holding the core/coil assembly of the padmounted transformer immersed in oil or the like, and a wiring cabinet having high and low voltage wiring compartments for enclosing the high and low voltage bushings, respectively, of the transformer, etc.

Equipment

Typical equipment used to create a Loop Feed, Single Phase, Primary Deadfront, Distribution, Padmount, Transformer, Structure is:

  • Padmount, Low Profile, Single Phase, Distribution, Transformer
  • Bay-O-Net Fuse
  • Current Limiting Fuse
  • Primary Side Elbow Connectors (Elbow, 200A, Loadbreak)
  • Secondary Connections
  • Neutral Ground Strap
  • Service Post Connector
  • Hold Down Device
  • Pad or Pre-Cast Base, Transformer, Single Phase
  • Connector, Compression
  • Wire, Ground
  • Sealing Compound
  • Tap Changer
  • Split Bolt, Copper
  • Clamp, Ground Rod
  • Padlock
  • Secondary Terminal, Transformer, Assembly
  • Wire, Stranded, Copper, Soft Drawn
  • Optional
    • Fault Indicator, Underground

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Padmount, Low Profile, Single Phase, Distribution, Transformer

Pad-mount transformers are (nearly always) oil-filled units. Oil filled transformers can operate in typical indoor or outdoor environments. The core and coils are enclosed in a steel oil-filled tank, with terminals for the transformer accessible in a lockable compartment. The lockable compartment primary and secondary bushings. Primary and Secondary voltage underground cables enter the lockable compartment from from below. The lockable compartment is typically secured with a carriage bolt which needs a special tool to open it and/or a padlock.

The major manufacturers of these transformers in North America are:

  • ABB
  • Cooper Power Systems (Eaton)

Current Limiting Fuse

The current limiting fuse is a backup fuse to a Bayonet because it is extremely fast acting and can break high fault current  allowing it to minimize the effects of high fault current stresses on equipment and the distribution system. It will prevent catastrophic failure of an oil-filled padmount transformer in the event of a internal low impedance fault. A catastrophic failure of a transformer within a residential neighborhood is a safety concern as their would be pressure build up in the oil tank, resulting in a oil and fire explosion. Even with a pressure relief valve, pressure can build up faster then the valve can release it.  The current limiting fuse will limit the energy to fault, thus preventing a catastrophic failure. An excellent program developed by Hi-Tech (ABB) to coordinate bay-o-net and current limiting fuses for a padmount transformer is http://www.tnb.com/Hi-TechFACT

The major manufacturers of current limiting fuses in North America are:

  • Cooper Power Systems (Eaton)
  • Hi-Tech (ABB)
  • Ferraz Shawmut (Mersen)

Bay-O-Net Fuse

Padmount transformers for safety will have two types a fuses, a bay-o-net fuse and a current limiting fuse. The Bay-o-net fuse is an expulsion fuse which is used to open on faults downstream of the transformer. Bay-o-net fuses are rest in the transformer oil and are externally removable with a hotstick and the fuse cartridges are replaceable.  Bayonet are the least expensive fuse for a transformer. The largest manufacturer of Bay-O-Net fuses in North America is Cooper Power Systems (Eaton).

It is important when removing the Bay-O-Net fuse because it is immersed in oil to not drip oil onto the primary molded rubber elbow connectors as oil can be absorbed into the rubber and effect the dielectric properties of the rubber.

Primary Side 200A Bushing and Elbow Connectors (Elbow, 200A, Loadbreak)

A 200A Transformer bushing assembly with 200A bushing and 200A loadbreak elbow. This assembly requires grounding with a minimum of #14 solid copper or equivalent wire. It will bond the shield of the elbow and insert and bushing mounting plate.  A separable connector (elbow) is a fully insulated and shielded system for terminating and electrically connecting an insulated power cable to electrical apparatus, other power cables, or both, so designed that the electrical connection can be readily established or broken by engaging or separating the connector at the operating interface. Click here to learn more about 200A Loadbreak Elbow Connectors.

The major manufacturers of current limiting fuses in North America are:

  • Elastimold (ABB)
  • Cooper Power Systems (Eaton)
  • Richards Manufacturing

Transformer Secondary Terminal Assembly

Padmount transformers always have a low side and high side. In a typical distribution system you will have the high side as the source where the low side is the load side, where there might be many secondary lines coming from the secondary bushings of the transformer to feed multiple different services near the transformer.

Description Drawing/Picture
Transformer Low Voltage Spade Terminal with Lugs, Aluminum, Secondary Assembly
Transformer Low Voltage Spade with Lay-In Terminal, Secondary Assembly
Transformer Low Voltage Stud with Slip-Fit Terminal, Secondary Assembly
Transformer Low Voltage Stud with Submersible Multi-Port Connector and Lugs, Secondary Assembly
Transformer Low Voltage Spade Terminal with Submersible Multi-Port Connector and Lugs, Secondary Assembly

The major manufacturers of secondary transformer connectors in North America are:

  • Burndy (Hubbell)
  • Homac (ABB)

Split Bolt, Copper

A split bolt connector is a mechanical connector which holds two electrical wires in close contact with a simple assembly. Wires are fed through the device and held in place by a nut tightened on a threaded shaft. In this application they are used to connect grounding connectors together whenever needed.

 

The major manufacturers of copper split bolt connectors in North America are:

  • Burndy (Hubbell)
  • Homac/Blackburn (ABB)
  • Ilsco (CDI Technoology)

Clamp, Ground Rod

A ground rod clamp is used to connect the ground wire to a ground rod. They are typically manufactured from cast bronze alloy, and are supplied with a bolt of non-ferrous metal.

The major manufacturers of grounding connectors in North America are:

  • Burndy (Hubbell)
  • Homac/Blackburn (ABB)
  • Ilsco (CDI Technoology)

Wire, Stranded, Copper, Soft Drawn

Bare copper conductor, soft-drawn (annealed), concentric-lay stranded it to create a ground grid around the padmount transformer. Should meet ASTM B3: Soft or Annealed Copper wire.

 

The major manufacturers of secondary transformer connectors in North America are:

  • Southwire

 Ground Rod

Ground Rods are typically solid copper, however there are many other different ground rods such copper-bonded ground, stainless steel rods, or galvanized rods.

The major manufacturers of secondary transformer connectors in North America are:

  • Erico
  • Homac/Blackburn (ABB)

Ground Grid

Padmount Transformers must have ground grids around the pre-cast base. The ground grid typical consists of four ground rods interconnected by
#4/0 bare copper grounding conductor and an additional two #4/0 bare copper grounding conductor tails, connected to the ground bus of the equipment.  Ground rod resistance must measure 6 ohms or less. A back-fill such as fine-fill clay is required around the ground grid.

Pad or Pre-Cast Base

Precast reinforced concrete transformer pads and pre-cast bases are used as a platform for transformers. The pre-cast base has a vault under it to allow for cable to be coiled so that if needed extra cable is available without splices being used.

Description Drawing/Picture
Transformer Pre-Cast Base
Transformer Pre-Cast Pad

Conduit

Conduits shall be sealed with duct seal or foam.

Underground Faulted Circuit Indicators

Underground Fault Indicators are utilized on the underground distribution cables to detect and assist in locating the short circuit fault. A fault indicator provides visual or remote indication of a fault on the electric power system. Underground Fout Circuit indicators are typically mounted inside a padmount equipment and are equipped with LED visual indication which can be seen on the outside of the enclosure. Also called a faulted circuit indicator (FCI) in North America FCI's are governed by IEEE 495 - Guide for Testing Faulted Circuit Indicators.

The major manufacturers of secondary transformer connectors in North America are:

  • Horstmann GmbH
  • Schweitzer Engineering Laboratories (SEL)
  • Cooper Power Solutions (Eaton)
  • Fisher Pierce (ABB)

 References

  1. Underground Construction Standards, San Diego Gas & Electric, 2016