Padmount transformer

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Large pad-mount transformers supplying power to a computer data center. No live wires are exposed. CERN Computer Centre for LHC - Transformers.jpg
Large pad-mount transformers supplying power to a computer data center. No live wires are exposed.
Padmounted transformers in Taiwan. Taiwan Power Company Grid.jpg
Padmounted transformers in Taiwan.

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 drops, 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.

Distribution transformer transformer that provides the final voltage transformation in an electric power distribution system

A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882.

Electric power distribution Final stage of electricity delivery to individual consumers in a power grid

Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment or household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.

Service drop

In electric power distribution, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers. The customer connection to an underground distribution system is usually called a "service lateral". Conductors of a service drop or lateral are usually owned and maintained by the utility company, but some industrial drops are installed and owned by the customer.

Contents

Pad-mounted transformers are made in power ratings from around 75 to around 5000 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.

Hot stick

In the electric power distribution industry, a hot stick is an insulated pole, usually made of fiberglass, used by electric utility workers when engaged on live-line working on energized high-voltage electric power lines, to protect them from electric shock. Depending on the tool attached to the end of the hot stick, it is possible to test for voltage, tighten nuts and bolts, apply tie wires, open and close switches, replace fuses, lay insulating sleeves on wires, and perform various other tasks while not exposing the crew to a large risk of electric shock.

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) 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. [1]

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

Pad-mount transformers have self protecting fuses consisting of a bayonet mount fuse placed in a high voltage compartment, with a back-up 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 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.

Fuse (electrical) type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, and it must be replaced or rewired, depending on type.

Bayonet mount

A bayonet mount or bayonet connector is a fastening mechanism consisting of a cylindrical male side with one or more radial pins, and a female receptor with matching L-shaped slot(s) and with spring(s) to keep the two parts locked together. The slots are shaped like a capital letter L with serif ; the pin slides into the vertical arm of the L, rotates across the horizontal arm, then is pushed slightly upwards into the short vertical "serif" by the spring; the connector is no longer free to rotate unless pushed down against the spring until the pin is out of the "serif".

Current limiting is the practice in electrical or electronic circuits of imposing an upper limit on the current that may be delivered to a load with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load.

The use of 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. [2]

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 the need of protective fencing. In residential areas, pad-mount transformers are usually located on street easements and supply multiple households.

An easement is a nonpossessory right to use and/or enter onto the real property of another without possessing it. It is "best typified in the right of way which one landowner, A, may enjoy over the land of another, B". It is similar to real covenants and equitable servitudes; in the United States, the Restatement (Third) of Property takes steps to merge these concepts as servitudes.

Three-phase pad-mounted transformers range in sizes from 75 kVA up to 5000 kVA with voltages ranging from 2,400 up to 34,500 delta or wye. Low-voltage pad-mounted range in size from 208y/120 through 24,940y/14,000.

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

Standards

American National Standards Institute /Institute of Electrical and Electronic Engineers (ANSI/IEEE)

National Electrical Manufacturers Association (NEMA) Standards

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In an electrical power distribution system, a ring main unit(RMU) is a factory assembled, metal enclosed set of switchgear used at the load connection points of a ring-type distribution network. It includes in one unit two switches that can connect the load to either or both main conductors, and a fusible switch or circuit breaker and switch that feed a distribution transformer. The metal enclosed unit connects to the transformer either through a bus throat of standardized dimensions, or else through cables and is usually installed outdoors. Ring main cables enter and leave the cabinet. This type of switchgear is used for medium-voltage power distribution, from 7200 volts to about 36000 volts.

References

  1. ↑ "Padmount Transformer". The Electricity Forum. Retrieved 2011-01-02.
  2. ↑ Langley Engineering. The information contained within this site is copyrighted to Langley Engineering unless otherwise stated. "Pad Mount Transformers". Langley Engineering. Retrieved 2011-01-02.
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