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The Background of three phase transformers

Three-phase electric poweris a common method ofalternating-currentelectric powergeneration,transmission, and distribution. It is a type ofpolyphase systemand is the most common method used byelectrical gridsworldwide to transfer power. It is also used to power largemotorsand other heavy loads. Athree-phasesystem is usually more economical than an equivalentsingle-phaseortwo-phasesystem at the same line to groundvoltagebecause it uses less conductor material to transmit electrical power.[2]The three-phase system was independently invented byGalileo Ferraris,Mikhail Dolivo-Dobrovolsky,Jonas WenstrmandNikola Teslain the late 1880s.

Types of three phase balanced transformersA three phase transformer having a pair of additional coupled windings on the secondary side of each phase, with these coupled windings properly connected in series to develop a voltage in phase with a particular secondary voltage but driven from alternate phase primaries. The primary and secondary windings are connected in a Y or configuration based on the coupled windings.

There are two type of 3 phase:

1. Core type

2. Shell type

Termination marking of three phase transformers

Transformer manufactures have agreed to standardize the marking of terminals to indicate their priority.

Transformer can be either subtractive or additive priority

The connections of three phase balanced transformers: Wye-Wye/ Wye-Delta/Wye-Delta/Delta-Delta.

The balanced three-phase wye-wye connection is shown below. Note that the line impedance for each of the individual phases in included in the circuit. The line impedances are assumed to be equal for all three phases. The line currents (IaA, IbB and IcC) are designated according to the source/load node naming convention. The source current, line current, and load current are all one in the same current for a given phase in a wye-wye connection. Wye source Wye load Assuming a positive phase sequence, the application of Kirchoffs voltage law around each phase gives where Ztotal is the total impedance in each phase and 2Z is the phase angle associated with the total phase impedance. The preceding equations can be solved for the line currents.

Wye-Delta

TheY- transform, also writtenwye-deltaand also known by many other names, is a mathematical technique to simplify the analysis of an electrical network. The name derives from the shapes of the circuit diagrams, which look respectively like the letter Y and the Greek capital letter. This circuit transformation theory was published byArthur Edwin Kennellyin 1899.[1]It is widely used in analysis ofthree-phase electric powercircuits.

The transformation is used to establish equivalence for networks with three terminals. Where three elements terminate at a common node and none are sources, the node is eliminated by transforming the impedances. For equivalence, the impedance between any pair of terminals must be the same for both networks. The equations given here are valid for complex as well as real impedances.

Delta-Delta Connection

In this type of connection, both the three phase primary and secondary windings areconnected in deltaas shown in theFigure 1:

Figure 1 - Delta-delta transformer connection

The voltages on primary and secondary sides can be shown on thephasor diagram(Figure 2):

Figure 2 - Phasor diagram of delta delta transformer connection

This connection proves to be economical for large low voltage transformers as it increases number of turns per phase.

Regulations/Efficiency of balanced three phase transformers Three-phase circuits are the most economical for a.c. power transmission and distribution. As a consequence, three-phase transformers are the most widely used in power systems. A three phase transformer may be a single unit (all windings wound around the same core, immersed in one tank) or it may be made up of three single-phase units. In practice the choice between one or another type is governed mainly by economical reasons, transportation, future expansion, reliability, etc.

Transformer taps changing Atap changeris a connection point selection mechanism along a powertransformerwinding that allows a variable number of turns to be selected in discrete steps. A transformer with a variable turns ratio is produced, enabling steppedvoltage regulation of the output. The tap selection may be made via an automatic or manualtap changermechanism

Determine the total cost of the transformers and compare three different manufacturers' costs

1. Autotransformer: Anautotransformerhas one winding which is tapped at some point along the winding. Voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The equivalent power rating of the autotransfomer is lower than the actual load power rating. It is calculated by: load VA(|VinVout|)/Vin.[1]For example, an auto transformer used to adapt a 1000VA load rated at 120Volts to a 240Volt supply has an equivalent rating of at least: 1,000VA(240V120V)/240V=500VA. However, the actual rating (which is what is shown on the tally plate) would have to be at least 1000VA.

For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller, and more efficient than an isolating (two-winding) transformer of the same rating.[2]Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33kV and 66kV sub-transmission networks.

Grounding transformerMain article:Zigzag transformerGrounding transformersare used to allow three wire (delta)polyphase systemsupplies to accommodate phase to neutral loads by providing a return path for current to a neutral. Grounding transformers most commonly incorporate a single winding transformer with a zigzag winding configuration but may also be created with a wye-delta isolated winding transformer connection.

Variable autotransformer[edit]

Variable autotransformer

Main article:Autotransformer Variable autotransformersBy exposing part of the winding coils of an autotransformer, and making the secondary connection through a sliding carbonbrush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for wide voltage adjustment in very small increments.