Distribution Transformers transformer is a static electrical device, which transfers energy from one electrical circuit to another by magnetic coupling. It can be used to convert from high to low voltages and between low and high currents (“Transformer”).
Transformers consist of “two or more insulated windings, to carry current” and “a core, in which the mutual magnetic field couples the windings” (“Transformer”).
In a transformer, the primary coil is fed by an alternating electric current, which creates a magnetic field around the conductor. The secondary coil, which is placed in the varying magnetic field, develops a potential difference that is referred to as an electromotive force (EMF). EMF refers to the force, measured in volts, produced by the interaction between a current and a magnetic field. EMF can also be referred to as electrical potential or potential difference. When the ends of the secondary are connected to form a circuit, the EMF causes a current to flow in the secondary coil. By this method, electrical power that was fed into the primary is delivered to the secondary (“Transformer”). The EMF developed in the secondary coil is “proportional to the ration of the number of turns in the secondary coil to the number of turns in the primary coil” (“Transformer”). The electrical power in the secondary can then be distributed outside of the transformer.
There are various types of transformers: step-up, step-down, isolating, and variable. The classification is dependent upon the ratio fo the number of turns in the coils and whether or not the coils are isolated. In a step-up transformer, the secondary coil has more turns than the primary coil. In a step-down transformer, the primary coil has more turns that the secondary coil. In isolating transformers, each coil has approximately the same number of turns, because the transformers are intended to transform from one voltage to another. The difference in turns on each coil is simply to account for loss. In variable transformers, the primary and secondary coils each have an adjustable number of coils, which can be reselected without reconnecting the transformer. “In all cases the primary winding, or the secondary winding, or both, may have taps that allow selection of one of several different ratios of primary to secondary turns” (“Transformer”).
In addition, there are two main groupings of transformer types: core and shell. In core-type transformers, the core has to be designed so that it does not reach magnetic saturation (“Transformer”). Cores can be made of steel, powdered iron cores, non-conductive magnetic materials such as ferrites, and even air. Core-type transformers are also divided into toroidal cores and EI cores. In an EI transformer, the laminated core is made from E-shaped and I-shaped pieces of steel. In contrast, toroidal transformers are built around a ring-shaped core, with the primary and secondary coils wound concentrically to cover the entire surface of the core (“Transformer”). In shell-type transformers, the core surrounds the windings. In addition, in a shell-type transformer, “all primary and secondary coils are assembled insulated from each other after which the entire coil assembly is dipped in an insulating varnish and baked” (“What is a Transformer?”).
The history of the transformer begins on August 29, 1831, when Michael Farady invented the induction ring. In 1881, Lucien Gaulard and John Dixon Gibbs exhibited a device they referred to a secondary generator in London. Later the men sold the device to Westinghouse. William Stanley, a Westinghouse engineer, invented the first commercially used transformer in 1885. In 1885, Otto Blathy, Miksa Deri, and Karoly Zipernowsky created the ZBD model of transformer. In 1891, Nikola Tesla invented the Tesla transformer or Tesla coil, a high-voltage, air-core, dual-tuned resonant transformer, which is used to generate very high voltages at high frequency (“Transformer”).
Distribution transformers are transformers that distribute power to various loads. Distribution transformers are used to distribute electrical power to homes from power lines. Except for a few notable exceptions, distribution transformers are designed like other transformers. The primary coil has two windings with an equal number of turns. If the windings are connected in series, the transformer has a higher voltage rating. If the windings are connected in parallel, the transformer has a lower voltage rating. The secondary coil has a variety of windings, which are each designated for the different loads. The role of the secondary winding is to simulate a separate transformer. In order to protect machinery from voltage surges, the primary and secondary coils are isolated from each other.
Distribution transformers consist of five types: conventional, types, completely self-protected, pad-mounted, and submersible. “The most common type of distribution transformer used in North America is the single-phase oil-filled, pole-mounted transformer, normally referred to as a “pole-top” or “overhead” transformer” (“Overhead Transformers”). There are also transformers that serve customers from underground distribution lines.
Most of those transformers are pad-mount transformers. Pad-mount transformers are:
electrically the same as pole-top units, but are packaged in a box-like, oil-filled metal enclosure and installed on a ground-level concrete foundation, or “pad.” Underground cable enters and exits the unit from under the ground and out of sight. There are no exposed live parts outside a pad mount transformer, so no fence or further confinement is required. The case or box is designed to be resistant to unauthorized or accidental entry. It is very safe even though it sits essentially on the ground (“Underground Transformers”).
Submersible transformers are transformers that are designed for installation underground, either in a manhole or underground vault. Submersible transformers are fully insulated and waterproof. Completely self-protected transformers contain lightning arresters, fuses, and thermal overload breakers within them.
Distribution transformers are protected by a variety of devices, including: lightning arresters, fuses, and thermal overload breakers. These safety devices can be contained in the transformer or be externally located. Lightning arresters protect the transformers from lightning surge voltages and prevent those surges from damaging other protective equipment. Fuses remove the damaged transformer from the primary line. “Thermal overload protection for the transformer is designed to trip in the event of a short circuit on the secondary side, or in the event of ‘thermal overload’ due to sustained high currents” (“Protective Equipment”).
The kVA rating of a transformer defines “the load carrying or power capability and stands for kilovolt-amperes” (“kVA Rating”). A volt is the potential difference across a conductor when a current of one ampere dissipates one watt of power, or one joule of energy per coulomb of charge. An ampere is the base unit used to measure electrical currents.
Polarity refers to the electric charge, whether positive or negative. Transformer polarity refers to the direction of the induced voltages between the terminals on a transformer. However, in the field, polarity refers to the way that the leads come out of the transformer. The industry standard is that distribution transformers are additive polarity. Additive polarity and subtractive polarity also applies to the arrangement of insulators on the transformers. In addition, polarity can refer to the materials used in the windings of the transformers; if opposite polarities are used, the windings can be forced apart, much like magnets of opposite polarity can be forced apart. One of the requirements for paralleling a transformer is that the polarities must be the same.
Three-phase transformers are used throughout the industry because three phase power is the most common way in which power is used and produced. There are only four possible three-phase transformer connections. Delta to delta is used in industrial applications. Delta to wye is the most common connection, and is used in commercial and industrial applications. Wye to delta is used for high-voltage transmissions. Wye to wye is the least frequent connection. A wye-delta transformer’s primary winding is connected in a wye and its secondary winding connected in a delta. A delta-wye transformer’s primary winding is connected in a delta and its secondary winding is connected in a wye. Delta systems are used for short-distance distribution, such as neighborhoods and small commercial loads. Delta systems are limited because only one voltage is available between any two wires in a delta system. In a wye system, the voltage between any two wires is constant throughout a three-phase system, but the voltage between any one of the conductors and the neutral is less than the voltage between the power conductors.
There are two ways of creating electrical circuits: in series and in parallel. The names refer to the method of attaching the components. If components are placed in a continuous loop, the circuit is said to be a series circuit. In a series circuit, the same current passes through every component in the series. Amps are consistent in a series circuit. If the components are attached to the circuit independently of each other, the circuit is said to be a parallel circuit. Voltage is consistent across the components in a parallel circuit.
Because of the large current loads carried by transformers, they can be extremely dangerous. In fact, line workers are frequently injured while repairing or maintaining transformers. The reason for these injuries can be attributed to a small bunch of hazards presented by distribution transformers. The first hazard is that the primary and secondary windings are close together, which creates the potential for a primary to secondary short in the event of insulation failure. In addition, there is a possibility that the transformer will explode because of the hermetically sealed tank of flammable oil.
There are several steps that line workers can take in order to prevent injuries from distribution transformers. First and foremost, line workers should wear the following protective equipment: hard hats, rubber gloves, safety glasses, and fire retardant clothing. A line worker should always connect the neutral first before connecting the transformer and remove it last before disconnecting the transformer. To avoid the possibility of backfeed, there should not be any paralleling of transformers across any point that could be used to isolate a line section. A line worker should always wear rubber gloves if working on the secondary coil of a faulty transformer. Transformers should be de-energized before servicing or repairing. Finally, before a transformer is re-fused, a line worker should thoroughly inspect the transformer and test it with an approved instrument. When refusing transformers, line workers should also disconnect the customer’s load, and use an extendable live line tool. In fact, it is recommended that line workers maintain a distance of at least 10 feet from the transformer prior to re-fusing. Prior to refusing, transformers should be tested with a smaller test fuse. Finally, transformers should be replaced rather than re-fused if the pressure relief valve has operated or an auxiliary current limiting fuse has blown.
Works Cited kVA Rating.” Fundamentals of Electricity. 2005. El Paso Electric Company. 21 Mar. 2005 http://elpaso.apogee.net/foe/ftdtk.asp.
Overhead Transformers.” Fundamentals of Electricity. 2005. El Paso Electric
Company. 21 Mar. 2005 http://elpaso.apogee.net/foe/ftdto.asp.
Protective Equipment.” Fundamentals of Electricity. 2005. El Paso Electric
Company. 21 Mar. 2005 http://elpaso.apogee.net/foe/ftdtp.asp.
Transformer.” Wikipedia.org. 2005. Wikimedia. 21 Mar. 2005 http://en.wikipedia.org/wiki/Transformer.
Underground Transformers.” Fundamentals of Electricity. 2005. El Paso Electric Company. 21
Mar. 2005 http://elpaso.apogee.net/foe/ftdtu.asp.
What is a Transformer?” Cutler-Hammer Training: Module 4. 2004. Eaton Electrical, Inc. 21
Mar. 2005 http://www.eatonelectrical.com/unsecure/html/101basics/Module04/Output/TypesTransformers.html
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