The distribution transformer or service transformer is a transformer that provides the final voltage transformation in the power distribution system, shrinking the voltage used in the distribution line to the level used by the customer. The invention of efficient transformers practically makes the AC power distribution feasible; systems using distribution transformers were demonstrated in early 1882.
When mounted on power lines, they are called pole-mounted transformers . If the distribution line is located on the ground or underground, the distribution transformer is mounted on a concrete bearing and locked in a steel box, known as a pad-mount transformer.
Distribution transformers typically have ratings of less than 200 kVA, although some national standards may allow for units up to 5000 kVA to be described as distribution transformers. Because distribution transformers are energized for 24 hours a day (even when they do not carry any load), reducing iron loss has an important role in their design. Since they usually do not operate at full load, they are designed to have maximum efficiency at lower loads. To have better efficiency, the voltage setting on this transformer must be kept to a minimum. Therefore they are designed to have small leak reactance.
Video Distribution transformer
Classification
Distribution transformers are classified into different categories based on certain factors such as:
- Installation location - pole, pad, underground vault
- Type of insulation - liquid-drown or dry-type
- Number of single or three phase phases
- Voltage class
- Basic impulse insulation level (BIL).
Maps Distribution transformer
Use
Distribution transformers are typically located in service drops, where cables run from power lines or underground power lines to customer premises. They are often used for power supply facilities outside of settlements, such as isolated homes, agricultural fields or pumping stations at voltages below 30 kV. Another application is the power supply from the wire over an electrically powered train with AC. In this case a single phase distribution transformer is used.
The number of subscribers given a distribution transformer varies depending on the number of subscribers in an area. Some homes may be fed single transformers in urban areas; rural distribution may require one transformer per customer. Large commercial or industrial complexes will have several distribution transformers. In urban areas and environments where major distribution lines run underground, padmount transformers, transformers in locked metal enclosures mounted on a concrete pad, are used. Many large buildings have electrical services provided at primary distribution voltages. These buildings have customer-owned transformers in the basement for the purpose of stepping down.
Distribution transformers are also found in electricity collecting networks of wind farms, where they increase the power of each wind turbine to connect to a substation that may be several miles (kilometers) away.
Connection
The mount and pad-mount transformer converts the high 'primary' voltage from the upper or lower distribution channel to the lower 'secondary' voltage of the inner distribution cable of the building. Its predecessors used a three-phase system. The main distribution line always has three wires, while the smaller "lateral" (close to the customer) may include one or two phases, which are used to serve all customers with single phase power. If a three-phase service is desirable, a person must have a three-phase inventory. Primary provides power to the standard distribution voltages used in the area; this ranges from 2300 volts to about 35,000 volts depending on local and standard distribution practices; often 11,000 V (50 Hz system) and 13,800 V (60 Hz system) are used but many other voltages are standard.
Primary
The main coil of high voltage is brought out to the busing at the top of the casing.
- Single-phase transformers, commonly used in US systems, are attached to overhead cables with two different connection types:
- Wye - On the wye distribution circuit, 'wye' or 'phase to neutral' transformers are used. Single phase wye transformers usually only have one busing on top, connected to one of the three main phases. The other end of the primary winding is connected to the case of the transformer, which is connected to the neutral wire of the wye system, and is also earthed. The wye distribution system is preferred because the transformer presents an unbalanced load on the line causing the current in the neutral wire and then earthed. But with a delta distribution system, unbalanced loads can cause variations in voltage across 3 phase cables.
- Delta - In the delta distribution circuit, 'delta' or 'phase to phase' transformers are used. A single phase delta transformer has two buses on which are connected to two of the three primary cables, so the voltage across the primary winding is a phase-to-phase voltage. The disadvantage of a delta transformer is that if only one of the two primary phases are deenergized, the remaining phase will feed back through the transformer winding into the deenergized phase, which can be a hazard to line workers.
- Transformers that provide three-phase electrical power, used for home services in European systems, have three main windings and are attached to the three primary phase cables. The reels are almost always connected in a 'wye' configuration, with the ends of all three reels connected and grounded.
The transformer is always connected to the main distribution line through the protective fuse and the breaker switch. For transformers installed at this pole usually take the form of 'fused fractions'. An electrical failure causes the fuse to melt, and the device will open to provide a visual indication of the problem. It can also be opened manually when the channel is energized by line workers using an isolated hot stick. In some cases, a fully protected transformer is used, which has a circuit breaker embedded in it, so a unified piece is not required.
Secondary
Low voltage secondary winding is attached to three or four terminals on the side of the transformer.
- In the United States and countries using the system, the secondary system is most often split-phase 120/240 volts. The 240 V secondary winding is centered on the center and the central earthed wire is earthed, making the two end conductors "hot" with respect to the central faucet. All three of these cables reduce service to the power meter and service panel inside the building. Connecting the load between hot and neutral wire gives 120 volts. Connecting between the two hot wires gives 240 volts.
- In Europe and countries using the system, the secondary system is often a three-phase 400Y/230 system. There are three secondary turns of 230 V, each receiving power from the primary windings attached to one of the main phases. One end of each secondary winding is connected to a 'neutral' wire, which is earthed. The other end of the 3 secondary windings, together with the neutral, is dropped from service to the service panel. The 230 V load is connected between one of three phase and neutral cables.
Higher secondary voltage, such as 480 volts, is sometimes required for commercial and industrial use. Some industrial customers require three phase power at secondary voltage. To provide this, a three-phase transformer can be used. In the US, most of which use single-phase transformers, three identical single-phase transformers are often transferred to the transformer bank in a wye or delta connection, to create a three phase transformer.
Construction
The distribution transformers are made using a core made of laminated stacked sheet steel and bonded with resin or joined with a steel rope. Where a large number of transformers are made for standard design, the C-shaped core is economical to produce. A streaked steel strip, pressed into shape and then cut into two C-shaped parts, which are reassembled on a copper reel.
The primary winding is the wound of enameled and high-wire copper or aluminum wires, low-wound low-voltage tapes using thick bands of aluminum or copper. The coils are insulated with resin-impregnated paper. The whole assembly is roasted to cure the resin and then soaked in a powder-coated steel tank which is then filled with transformer oil (or other insulating fluid), which is inert and non-conductive. The transformer oil cools and isolates the winding, and protects the transformer winding from water vapor, which will float on the oil surface. The tank is temporarily distressed to remove any residual moisture that will cause the curve and closed against the weather with a gasket at the top.
Previously, distribution transformers for indoor use would be filled with polychlorinated biphenyl (PCB) liquids. Because these liquids survive in the environment and have adverse effects on animals, they have been banned. Other flame-retardant liquids such as silicon are used where liquid-filled transformers should be used indoors. Certain vegetable oils have been used as transformer oils; this has the advantage of a high fire point and is completely biodegradable in the environment.
Pile-mounted transformers often include accessories such as surge arresters or fuse protective links. Self-protected transformers include internal fuses and flow controls; Other transformers have components that are installed separately outside the tank. Pile-mounted transformers may have lugs that allow direct mounting of poles, or they can be mounted on cross-poles that are bolted to the poles. Air transformers, larger than about 75 kVA, can be mounted on platforms supported by one or more poles. The three phase service can use three identical transformers, one per phase.
Transformers designed for sub-level installations can be designed for periodic immersion in water.
The distribution transformer may include an off-load tap changer to allow for a slight adjustment of the ratio between the primary and secondary voltages, to bring the customer voltage within the desired range on a long or heavy channel.
The peg-mounted transformer has a locked and securely locked metal enclosure to prevent unauthorized access to live internal components. This enclosure may also include fuses, isolation switches, load-breaking buses, and other accessories as described in technical standards. Transplant-mounted transformers typically range from about 100 to 2000 kVA, although some larger units are also used.
References
Bibliography
- Bakshi, V.B.U.A. (2009). Transformer & amp; Induction Machine . Technical Publications. ISBN: 9788184313802 . Retrieved 2014-01-14 .
- Harlow, James H. (2012). Power Transformer Technique, Third Edition, Volume 2 . Press CRC. ISBN: 143985629X.
- Pansini, Anthony J. (2005). Guide to Power Distribution Systems . The Fairmont Press, Inc. ISBNÃ, 088173506X. Source of the article : Wikipedia
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