Transformer routine test – The measurement of winding resistance

Calculating Load Loss Values

Measurement is made to check transformer windings and terminal connections and also both to use as reference for future measurements and to calculate the load loss values at reference (e.g. 75ºC) temperature.

Measuring the winding resistance is done by using DC current and is very much dependent on temperature.

Temperature correction is made according to the equations below:

R2 – winding resistance at temperature t₂,
R1 – winding resistance at temperature t₁

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

The measuring current can be obtained either from a battery or from a constant (stable) current source. The measuring current value should be high enough to obtain a correct and precise measurement and small enough not to change the winding temperature.

In practice, this value should be larger than 1,2 x I₀ and smaller than 0,1 x I_N, if possible.

A transformer consists of a resistance R and an inductance L connected in serial. If a voltage U is applied to this circuit;

The value of current measurement will be :

Here, the time coefficient 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 time.

After the current is applied to the circuit, it should be waited until the current becomes stationary (complete saturation) before taking measurements, otherwise, there will be measurement errors.

Measuring circuit and performing the measurement

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

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

In the current – voltage method, the measuring current passing through the winding also passes through a standard resistor with a known value and the voltage drop values on both resistors (winding resistance and standard resistance) are compared to find the unknown resistance (winding resistance).

One should be careful not to keep the voltage measuring voltmeter connected to the circuit to protect it from high voltages which may occur during switching the current circuit on and off.

When the currents flowing in the arms are balanced, the current through the galvanometer will be 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 errors will be minimised.

The resistance measured with the Kelvin Bridge:

The resistance measured with the Wheatstone Bridge:

BEST Transformers laboratory

BEST Test laboratory is equipped with the most advanced testing facilities and is capable of conducting all tests required by IEC standards except short circuit mechanical withstand test, conducted in an independent international laboratory, CESI-Italy.

Tests performed on the transformers can be classified as follows:

Tests during manufacturing, routine tests, type tests, special tests, acceptance tests, site tests, defect analysis / identification and tests before maintenance.

Resource: BEST Transformer – Tests (BALIKESİR ELEKTROMEKANİK SANAYİ TESİSLERİ A.Ş.)