Electrical Engineering Portal.com ScottT Transformer Connection Overview

7/27/2019 Electrical Engineering Portal.com ScottT Transformer Connection Overview

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Largest furnace transformer with 105-MVA capacity manufactured at Siemens transformer factory in Dresden

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Scott-T Transformer Connection Overview

Transforming 3 Phase to 2 Phase

There are two main reasons f or the need to t ransform fro m three phases to two phases,

1. To give a supply to an exist ing two phase system f rom a three phase supply.

2. To supply two phase f urnace transf ormers f rom a three phase source.

Two-phase syst ems can have 3-wire, 4-wire, or 5-wire circuits . It is needed to be considering that a two-phassystem is not 2/3 of a three-phase system. Balanced three-wire, two-phase circuits have two phase wires,bot h carrying approximately the same amount o f current, with a neutral wire carrying 1.414 times t he currents the phase wires. The phase-t o- neutral voltages are 90 out o f phase with each ot her.

Two phase 4- wire circuits are essent ially just two ungrounded single-phase circuits that are electrically 90 ouof phase with each other. Two phase 5-wire circuits have fo ur phase wires plus a neutral; the four phase wireare 90 out o f phase with each other.
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Scott-T Connec tion - 2 and 3 phase wire co nnection

The easiest way to transf orm three-phasevoltages into two-phase voltages is with twoconventional single-phase transformers. Thefirst transformer is connected phase-to-neutralon the primary (three-phase) side and thesecond transf ormer is connected between theother two phases on the primary side.

The secondary windings of the two transf ormersare then connected to the two-phase circuit. Thephase-to -neutral primary voltage is 90 out o fphase with the phase-to-phase primary voltage,producing a two-phase voltage across thesecondary windings. This s imple connection,called the T connection, is shown in Figure

The main advantage of the T connection is t hatit uses transformers with standard primary andsecondary voltages. The disadvantage of the T

connection is that a balanced two-phase loadst ill produces unbalanced three-phase currents; i.e., the phase currents in the three-phase system do not havequal magnitudes, their phase angles are not 120 apart, and there is a cons iderable amount of neutral currenthat must be returned to t he source.

The Scott Connection of Transformer

A Scot t-T trans f ormer (also called a Scott connect ion) is a type o f circuit used to derive t wo-phase powerf rom a three-phase source or vice-versa. The Scot t connection evenly dist ributes a balanced load between th

phases of the source.

Scot t T Transf ormers require a three phase power input and provide two equal single phase outputs calledMain and Teaser. The Main and Teaser outputs are 90 degrees out o f phase. The MAIN and the Teaserout puts must not be connected in parallel or in series as it creates a vecto r current imbalance on the primaryside.

MAIN and Teaser outputs are o n separate cores. An external jumper is also required to connect the primaryside of the MAIN and Teaser sect ions.

The schematic of a typical Scott T Transformer is shown below:
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Typic al Scott T Transforme r sche matic diagram

Scott-T co nnection co nvert 3-phase to 2-phase

Scot t T Transformer is built with two s inglephase transf ormers o f equal power rating. TheMAIN and Teaser sections can be enclosed in af loor mount enclosure with MAIN on the bott omand Teaser o n to p with a connecting jumpercable. They can also be placed side by side inseparate enclosures.

Assuming the des ired vo ltage is the same onthe two and three phase sides, the Scot t- Ttransf ormer connection consists o f a center-tapped 1:1 ratio main transf ormer, T1, and an86.6% (0.53) ratio teaser transformer, T2. Thecenter- tapped side of T1 is connected betweentwo o f the phases on the three-phase side. Its center tap then connects to one end of the lower turn countside of T2, the ot her end connects to the remaining phase.

The other side of the transformers then connects directly to the two pairs o f a two-phase fo ur-wire system.

The Scott -T transf ormer connection may be also used ina back to back T to T arrangement f or a three-phase to 3phase connection. This is a cost saving in the smaller kVAtransf ormers due to the 2 coil T connected to asecondary 2 coil T in-lieu of the t raditional three-coilprimary to three-coil secondary transf ormer. In thisarrangement the Neutral tap is part way up on thesecondary teaser transf ormer .

The voltage stability of this T to T arrangement ascompared to the t raditional 3 coil primary to t hree-coil

secondary transf ormer is questioned.

Key Points

If the main transformer has a t urns rat io o f 1: 1, then the teaser t ransformer requires a turns ratio of 0.866: f or balanced operation. The principle of operation o f the Scot t connection can be most easily seen by firstapplying a current to the t easer secondary windings, and then applying a current to the main secondarywinding, calculating the primary currents separately and superimpos ing the results .

Load connect ed bet ween phaseY1 and phase Y2 of the secondary:

Secondary current f rom the teaser winding into phase X1 =1.0


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The reason that the primary current f rom H3 phase into t he teaser winding is 1.1547 due to 0.866: 1turns rat io o f the teaser, transf orming 1/0.866= 1.1547 times the secondary current. This current mustsplit in half at the center tap o f the main primary winding because both halves o f the main primarywinding are wound on the same core and the tot al ampere-turns o f the main winding must equal zero.

Load connect ed between phase X2 and phase X1 of the secondary:

Secondary current from the main winding into phase X2 =1.0< 0

Secondary current f rom the main winding into phase X4= -1.0


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The transformation ratio of the coils and the voltage obtained may be slightly unbalanced due tomanufacturing variances of the interconnected coils.

This designs neutral has to be solidly grounded. If it is not grounded so lidly, the secondary voltages couldbecome unstable. Since this design will have a low impedance, special care will have to be taken on the primaryprotection fault current capacity. This could be an issue if the system was designed for a Delta-Starconnection.

The inherent single phase cons truction and characteristics of this connection produces a comparatively bulkyand heavier transf ormer when compared with a normal three phase transf ormer of the same rating.

Application

Main application is f or For Indust rial Furnace Transf ormer.

For Traction Purpose: The power is obtained f rom the 220 kV or 132 kV or 110 kV or 66 kV, three-phase,ef f ectively earthed transmission network o f the State Electricity Board, through single-phase t ransformers orScott connected transformer installed at the Traction Substation. The primary winding of the single-phase

transf ormer is connected to two phases of the transmission network or Where Scott -connected transf ormeris used, the primary windings are connected to the three phases of the transmission network.

The single-phase transf ormers at a Traction Substation are connected to the same two phases o f thetransmission network (ref erred as single-phase connection), or alternatively to dif f erent pairs o f phases- t hethree single phase transformers forming a delta-connection on the primary side. Out of three single-phasetransf ormers, one transf ormer feeds t he overhead equipment (OHE) on one s ide of the Traction Substation,another feeds the OHE on the other side of the Traction Substation, and the third remains as standby. Thusthe two single-phase transformers which feed the OHE constitute an open-delta connection (alternatively,referred as V-connection) on the three-phase transformers network.

The Scot t- connected t ransformer and V-connected single-phase t ransformers are ef f ective in reducingvoltage imbalance on the transmission network. The spacing between adjacent substat ions is no rmallybetween 70 and 100 km.

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Electrical Engineering Portal.com ScottT Transformer Connection Overview