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BEST Transf ormers - OSB Power Transf ormers T estLaboratory

http://electr ical-engineering-portal.com/transformer-routine-test-measurement-of-winding-resistance January 6, 2013

Transformer Routine Test Measurement of WindingResistance

Calculating Load Loss Values

Measurement is made to checktransformer windings and terminalconnections and also bot h to use asreference for future measurements andto calculate t he load loss values atreference (e.g. 75C) temperature.

Measuring the winding resistance isdone by using DC currentand is verymuch dependent on temperature.

Temperature correction is madeaccording to the equations below:

R2 winding resistance at t emperature t2,

R1 winding resistance at temperature t1

Because of this, temperatures must be measured when measuring the winding resistances andtemperature during measurement should be recorded as well.

Winding resistances are measured between all connection terminals of windings and at alltap positions. During this, winding temperature should also be appropriately measured andrecorded.

The measuring current can be obtained either f rom a batteryor from a constant (stable)current source. The measuring current value should be high enough to obt ain a correct andprecise measurement and small enough not to change the winding temperature.

In practice, this value should be larger than 1,2 x I0and smaller than 0,1 x IN, if possible.
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Figure 1 - Measuring the resist ance by Current-Voltagemethod

A transformer consists of a resistance Rand an inductance L connected in serial. If a voltageUis applied to this circuit ;

The value of current measurement will be :

Here, the time coef f icient depends on L/R ratio.

As the measurement current increases, the core will be saturated and inductance will decrease.In this way, the current will reach the saturation value in a shorter t ime.

Af ter the current is applied to the circuit, it should be waited until the current becomesstat ionary (complete saturation) before taking measurements, otherwise, there will bemeasurement errors.

Measuring circuit and performing the measurement

The transformer winding resistances can be measured either by current-voltage method orbridge method. If digital measuring instruments are used, the measurement accuracy will behigher.

Measuring by the current-voltage method is shown in figure 1 below:

In the current voltage method, themeasuring current passing through thewinding also passes through a standardresistor with a known value and thevoltage drop values on both resistors(winding resistance and standardresistance) are compared to f ind theunknown resistance (windingresistance).

One should be careful not to keep the

voltage measuring voltmeterconnected to t he circuit t o protect itf rom high voltages which may occurduring switching the current circuit onand off .

The bridge method is based on comparing an unknown (being measured) resistor with a knownvalue resistor.

When the currents f lowing in the arms are balanced, the current through the galvanometer willbe zero. In general, if the small value resistors (e.g. less than 1 ohm) are measured with a Kelvin

bridge and higher value resistors are measured with a Wheatstone bridge, measurement errorswill be minimised.


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Figure 2 (left ) - Kelvin bridge; Figure 3 (right) - Wheatstonebridge

The resistance measured with theKelvin Bridge:

The resistance measured with the Wheatstone Bridge:

BEST Transformers laboratory


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BEST Transf ormers - OSBLaboratory

BEST Test laboratory is equipped with the most advancedtesting facilit ies and is capable of conduct ing all testsrequired by IEC standards except short circuit mechanicalwithstand test, conducted in an independent internationallaboratory, CESI-Italy.

Tests performed on the transformers can be classified as

follows:

Tests during manufacturing, rout ine tests, type tests, specialtests, acceptance tests, site tests, defect analysis /identificat ion and tests before maintenance.

Resource: BEST Transformer Tests (BALIKESRELEKTROMEKANK SANAY TESSLER A..)

electrical-engineering-portal.com-transformer routine test measurement of winding resistance

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