elect rical-engineering-po rt al.co mhttp://electrical-engineering-portal.com/power-transformer-turns-ratio -test?utm_source=Technical+Articles&utm_campaign=0db9f82282-RSS_EMAIL_CAMPAIGN&utm_medium=email&utm_term=0_ae069f0d91-0db9f82282-315654417

Po we r Transfo rme r Turns Ratio Te st

Edvard

Power Transformer Turns Ratio Test

Turns ratio between the windings

The turns ratio test is an AC low voltage test which determines the ratio of the high voltage winding to allother windings at no- load. The turns ratio test is perf ormed on all taps of every winding.

The Transformer Turns Ratio tester (TTR) is device used to measure the turns ratio between the windings(example shown below).

Voltage is applied on the H marked leads and measured of the X marked lead by the test set.

Ratio measurements are conducted on all tap posit ions and calculated by dividing the induced voltagereading into the applied voltage value. When ratio tests are being made on three-phase transf ormers, theratio is taken on one phase at a t ime with a three-phase TTR until the ratio measurements of all threephases are completed.

Fig ure 1 – Thre e -p hase Transfo rme r Turns Rati (TTR) Co nne ctio n d iag ram.

Examp le o f s ing le p hase , hand -cranke d TTR – Transfo rme rTurns Ratio Te st Se t (Me asure s the turns ratio and e xciting

curre nt o f wind ing s in p o we r, p o te ntial and curre nttransfo rme rs.)

Measuredratiovariationsshould bewithin 0.5%of thenameplatemarkings.

Some TTRcan perf ormtransf ormerratio

measurement and also assess if on- load tap changer contacts are making satisf actorily during itstransit ion f rom one tap posit ion to the next posit ion.

Turns Ratio Test Procedure, Step byStep

Step 1.

Isolate the equipment, apply working grounds to allincoming and outgoing cables and disconnect allincoming and outgoing cables f rom the transf ormerbushing terminals connections.

Disconnected cables should have sufficientclearance f rom the switchgear terminals greater thatthe phase spacing distance. Use nylon rope to holdcable away f rom incoming and outgoing terminals asrequired.

Step 2.

Connect the H designated three-phase test lead with the military style connector at one end to the matingconnection on the test set marked with an H. Ensure that the connector ’s index notch lines up properly.

Step 3.

Connect the X designated three-phase test of lead military style connector at one end to the matingconnection on the test set marked with an X. Ensure that the connector ’s index notch lines up properly.

Step 4.

Connect the H1, H2, H3 designated test lead to the corresponding H1, H2, H3 transformer terminal /bushing. Connect the H0 test lead if H0 terminal/bushing is present.

Ref er to Figure 1.

Step 5.

Connect the X1, X2, X3 designated test leads to the corresponding X1, X2,X3 transf ormer terminals /bushings. Connect the X0 test lead if X0 terminal/bushing is present.

Step 6.

Perf orm turns ratio measurements f or all tap posit ions.

Step 7.

Confirm that the measured ratios is within 0.5% of the calculated ratios.

Important Note:

Transf ormers that have wye connections but do not have the neutral of the wye brought out shall betested f or ratio with three-phase power supply.

Any inequality in the magnetizing characteristics of the three phases will then result in a shif t of theneutral and thereby cause unequal phase voltages. When such inequality is f ound, the connection shouldbe changed, either to a delta or to a wye connection, and the line voltages measured.

When these are f ound to be equal to each other and the proper values (1.732 times the phase voltageswhen connected in wye), the ratio is correct.

Power Transformer Test ing – Automatically measuring rat io and winding resistance of alltaps/phases

Cant see this video? Click here to watch it on Youtube.

Reference: Substation Commissioning Course – Raymond Lee, Technical Trainer