Transforming 3 Phase to 2 Phase

There are two main reasons for the need to transform from three phases to two phases,

1. To give a supply to an existing two phase system from a three phase supply.

2. To supply two phase furnace transformers from a three phase source.

Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed to be considering that a two-

phase system is not 2/3 of a three-phase system. Balanced three-wire, two-phase circuits have two phase

wires, both carrying approximately the same amount of current, with a neutral wire carrying 1.414 times

the currents in the phase wires. The phase-to-neutral voltages are 90° out of phase with each other.

Two phase 4-wire circuits are essentially just two ungrounded single-phase circuits that are electrically

90° out of phase with each other. Two phase 5-wire circuits have four phase wires plus a neutral; the four

phase wires are 90° out of phase with each other.

Scott-T Connection - 2 and 3 phase wire connection

The easiest way to transform three-phase voltages into two-phase voltages is with two conventional

single-phase transformers. The first transformer is connected phase-to-neutral on the primary (three-

phase) side and the second transformer is connected between the other two phases on the primary side.

The secondary windings  of the two transformers are then connected to the two-phase circuit. The

phase-to-neutral primary voltage is 90° out of phase with the phase-to-phase primary voltage, producing a

two-phase voltage across the secondary windings. This simple connection, called the T connection, is

shown in Figure

The main advantage of the T connection is that it uses transformers with standard primary and secondary

voltages. The disadvantage of the T connection is that a balanced two-phase load still produces

unbalanced three-phase currents; i.e., the phase currents in the three-phase system  do not have equal

magnitudes, their phase angles are not 120° apart, and there is a considerable amount of neutral current

that must be returned to the source.

The Scott Connection of Transformer

A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power

from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load

between the phases of the source.

Scott T Transformers require a three phase power input and provide two equal single phase outputs

called Main and Teaser. The Main and Teaser outputs are 90 degrees out of phase. The MAIN and the

Teaser outputs must not be connected in parallel or in series as it creates a vector current imbalance on

the primary side.

MAIN and Teaser outputs are on separate cores. An external jumper is also required to connect the

primary side of the MAIN and Teaser sections.

The schematic of a typical Scott T Transformer is shown below:

Typical Scott T Transformer schematic diagram

Scott T Transformer is built with two single phase transformers of equal power rating. The MAIN and

Teaser sections can be enclosed in a floor mount enclosure with MAIN on the bottom and Teaser on top

with a connecting jumper cable. They can also be placed side by side in separate enclosures.

Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer

connection consists of a center-tapped 1:1 ratio main transformer, T1, and an 86.6% (0.5√3) ratio teaser

transformer, T2. The center-tapped side of T1 is connected between two of the phases on the three-

phase side. Its center tap then connects to one end of the lower turn count side of T2, the other end

connects to the remaining phase.

The other side of the transformers  then connects directly to the two pairs of a two-phase four-wire

system.

Scott-T connection convert 3-phase to 2-phase

The Scott-T transformer connection may be also used in a back to back T to T arrangement for a three-

phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T

connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to three-coil secondary

transformer. In this arrangement the Neutral tap is part way up on the secondary teaser transformer .

The voltage stability of this T to T arrangement as compared to the traditional 3 coil primary to three-coil

secondary transformer is questioned.

Key Points

If the main transformer has a turn’s ratio of 1: 1, then the teaser transformer requires a turn’s ratio of

0.866: 1 for balanced operation. The principle of operation of the Scott connection can be most easily

seen by first applying a current to the teaser secondary windings, and then applying a current to the main

secondary winding, calculating the primary currents separately and superimposing the results.

Load connected between phaseY1 and phase Y2 of the secondary:

Secondary current from the teaser winding into phase X1 =1.0 <90°

Secondary current from the teaser winding into phase X2 =-1.0< 90°

Primary current from H3 phase into the teaser winding= 1.1547< 90°

Primary current from H2  phase into the main winding= 0.5774 <90°

Primary current from H1 phase into the main winding= -0.5774< 90°

The reason that the primary current from H3 phase into the teaser winding is 1.1547 due to 0.866:

1 turn’s ratio of the teaser, transforming 1/0.866= 1.1547 times the secondary current. This current

must split in half at the center tap of the main primary winding because both halves of the main

primary winding are wound on the same core and the total ampere-turns of the main winding must

equal zero.

Load connected between phase X2 and phase X1 of the secondary:

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

Secondary current from the main winding into phase X4= -1.0 <0°

Primary current from H2 phase into the main winding =1.0 <0°

Primary current from H1 phase into the main winding=- 1.0 <0°

Primary current from H3 phase into the teaser winding= 0

Superimpose the two sets of primary currents:

I H3= 1.1547 <90° +0= 1.1547 <90°

I H2 =0.5774 <90° +1.0< 0°= 1.1547 < 30°

I H1 =0.5774 <90°+ 1.0 <0°=1.1547 <210°

Notice that the primary three-phase currents are balanced; i.e., the phase currents have the same

magnitude and their phase angles are 120° apart. The apparent power supplied by the main

transformer is greater than the apparent power supplied by the teaser transformer. This is easily

verified by observing that the primary currents in both transformers have the same magnitude;

however, the primary voltage of the teaser transformer is only 86.6% as great as the primary

voltage of the main transformer.Therefore, the teaser transforms only 86.6% of the apparent

power transformed by the main.

We also observe that while the total real power delivered to the two phase load is equal to the total

real power supplied from the three-phase system, the total apparent power transformed by both

transformers is greater than the total apparent power delivered to the two-phase load.

The apparent power transformed by the teaser is 0.866 X IH1= 1.0 and the apparent power

transformed by the main is 1.0X IH2 =1.1547 for a total of 2.1547 of apparent power transformed.

The additional 0.1547 per unit of apparent power is due to parasitic reactive power owing between

the two halves of the primary winding in the main transformer.

Single-phase transformers used in the Scott connection are specialty items that are virtually

impossible to buy ‘‘off the shelf ’’ nowadays. In an emergency, standard distribution transformers

can be used.

Advantages of the Scott T Connection

If desired, a three phase, two phase, or single phase load may be supplied simultaneously.  The neutral

points can be available for grounding or loading purposes

Disadvantages when used for 3 Phase Loading

This type of asymmetrical connection (3 phases, 2 coils), reconstructs three phases from 2 windings. This

can cause unequal voltage drops in the windings, resulting in potentially unbalanced voltages to be

applied to the load.

The transformation ratio of the coils and the voltage obtained may be slightly unbalanced due to

manufacturing variances of the interconnected coils.

This design’s neutral has to be solidly grounded. If it is not grounded solidly, the secondary voltages could

become unstable. Since this design will have a low impedance, special care will have to be taken on the

primary protection fault current capacity. This could be an issue if the system was designed for a Delta-

Star connection.

The inherent single phase construction and characteristics of this connection produces a comparatively

bulky and heavier transformer when compared with a normal three phase transformer of the same rating.

Application

Main application is for For Industrial Furnace Transformer.

For Traction Purpose: The power is obtained from the 220 kV or 132 kV or 110 kV or 66 kV, three-

phase, effectively earthed transmission network of the State Electricity Board, through single-phase

transformers or Scott connected transformer installed at the Traction Substation. The primary winding of

the single-phase transformer is connected to two phases of the transmission network or Where Scott-

connected transformer is used, the primary windings are connected to the three phases of the

transmission network.

The single-phase transformers at a Traction Substation are connected to the same two phases of the

transmission network (referred as single-phase connection), or alternatively to different pairs of phases-

the three single phase transformers forming a delta-connection on the primary side. Out of three single-

phase transformers, one transformer feeds the overhead equipment (OHE) on one side of the Traction

Substation, another feeds the OHE on the other side of the Traction Substation, and the third remains as

standby. Thus the 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 Scott-connected transformer and V-connected single-phase transformers are effective in reducing

voltage imbalance on the transmission network. The spacing between adjacent substations is normally

between 70 and 100 km.

Recommended EE articles //

Configurations and Characteristics Of Distribution Substations APRIL 13, 2015

Maintenance of Load Tap Changers (LTCs) MARCH 25, 2015

Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks! FEBRUARY 23, 2015

Basic Transformer Routine Test – Measurement of Winding Resistances FEBRUARY 20, 2015

Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks! (on photo 3-core type oil-immersed rectifier transformer 380kV; via electric-power-transformers.com)

Obligatory Checklist

The following tests and checks should be performed at a minimum to ensure that  the transformer is

ready to be energized. Do not energize the transformer without performing these tests and checks.

If any of below explained tests fail and transformer is energized anyway, it may lead to the serious hazard,

and that’s not good   That’s not what we want.

Let’s see these 15 tests…

1. Insulating Fluid Test

Draw a fluid sample and test its dielectric strength . Dielectric strength of new fluid should be 26 kv or

greater.

The breakdown voltage indicates how well insulating oil can withstand an electrical load and is therefore decisive for the operational efficiency of a transformer. The breakdown voltage is measured according to VDE 0370 part 5 (IEC 60156); photo credit: SIEMENS

2. Pressure Test

Check the transformer tank for leaks by pressurizing the tank with dry air or dry nitrogen through the

pressure test fitting to a pressure of 3 to 4 PSIG. let the tank stand under pressure for one to two hours,

then examine the tank and fittings for leaks. leaks above the fluid level can be detected by applying soap

solution to all welds, joints, pipe fittings, and cable connections.

Upon completion of this test, reduce the internal pressure to 1 or 2 PSIG.

Repairing transformer oil-tank for leakage (photo credit: khia.belzona.com)

3. Insulation Megger Test

Perform a 1000-volt Megger test and a power factor test to ensure that none of the windings is

grounded. See the complete procedure here .

Regular insulation testing is one of the most cost effective methods of identifying aging of transformer

4. Ratio Test

Perform a ratio test at each tap position to ensure that transformer coil ratios and tap changer connections

are correct. Read the complete procedure of transformer turns ratio test step by step .

5. Continuity Test and Resistance Test

Perform a continuity check on each winding. Measure the winding resistance of each winding and

compare results to factory test values.

An increase of more than 10% could indicate a loose internal connection.

6. Line Connections

In preparation for making line connections, check to make sure that all mating connector surfaces

are clean and smooth. Connections must be tightened appropriately to prevent overheating and possible

subsequent failure of the connection.

Connections to should be made with care to avoid placing undue stress on the bushings.

Oil-filled transformer bushings (photo credit: khia.belzona.com)

7. Tap Changer Setting

Check the tap changer setting to ensure it is set to the proper position for the required voltage. See  an

example of transformer tap-changer correct adjustment .

On-Line Oil Filtration for Load Tap Changer Compartment (photo credit: velcon.com)

8. Delta/Wye and Series/Multiple Switch Settings

Check delta/wye and series/multiple switch settings to make sure they are set correctly.

If these connections are made using an internal terminal board, check to ensure that these connections

are made properly according to the chart on the transformer nameplate .

GE distribution transformer nameplate (photo credit: tucsontransformer.com)

9. Grounding

Check to ensure that the transformer tank is permanently and effectively grounded. The transformer

tank ground pad is located near the bottom of the tank.

10. Wiring

Check wiring of control and alarm circuits (if provided) to make sure there are no loose

connections and no damage to insulation.

Oil-filled transformer terminal box

11. Fluid level

Figure 1 – Liquid Level Gauge

Check to make sure the fluid level as indicated by the fluid level gauge is as follows:

The fluid level gauge indicates the fluid level inside the tank. The transformer tank is filled with fluid at the

factory to the normal level at a reference temperature of 25°C.

12. Tank Finish

Check all painted surfaces to make sure that there is no damage or corrosion.

13. Bolted Connections

Check all bolted connections for tightness.

Oil transformer bolted connections (photo credit: polywater.com)

14. Tools

Check to make sure that all tools and equipment are accounted for and have been removed from the

transformer.

15. Fluid Temperature

Read the fluid temperature gauge and make sure the temperature is no lower than minus 20°C before

the unit is energized.

Figure 2 – Liquid Temperature gauge

Reference: Installation, Operation, and Maintenance of Medium Power Substation Transformers –

Howard Industries

Recommended EE articles //

Configurations and Characteristics Of Distribution Substations APRIL 13, 2015

Maintenance of Load Tap Changers (LTCs) MARCH 25, 2015

Basic Transformer Routine Test – Measurement of Winding Resistances FEBRUARY 20, 2015

4 Essential Features of Transformer On-Load Tap Changer (OLTC) JANUARY 28, 2015

Share with engineers //Article Tags //

fluid level, insulating fluid test, oil filled transformer, tap changer, transformer, transformer oil, transformer

tank,

Filed Under Category //

Testing and Commissioning Transformers

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high

power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of

LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+

 RSS Feed for Comments

15 Comments

1.

NikolayAPR 25, 2015

Also need to check if:

- Radiator valves are open

- Cooling fan motors rotate in the correct direction

- There are no visible oil leaks

- Off-load tap changer handle has been locked

(reply)

2.

DeepAPR 14, 2015

I believe that there are other important test like core balance, short circuit etc

(reply)

3.

Mhmoud RefaatAPR 14, 2015

Many thanks

(reply)

4.

ASIMAPR 13, 2015

thank you Mr.Edvard ,, very nice one but i think there is other tests to be added here like the polarization

index and tan delta test.

(reply)

« Older Comments RSS Feed for Comments

Leave a CommentTell us what you're thinking... we care about your opinion!

and oh, not to forget - if you want a picture to show with your comment, go get a freeGravatar !

NAME *

EMAIL *

WEBSITE

WE BELIEVE YOU'RE AN ENGINEER, BUT NEVERTHELESS ... *

six −   = 5

 SIGN ME UP FOR THE NEWSLETTER

Submit Comment