Routine Transformer Test – Measuring Winding Resistance

Calculate the pressure drop values

Measurements are made to check the Routine transformer windings and terminal connections and both to use as a reference for future measurements and to calculate the pressure drop values at the reference temperature (eg 75 ° C).

Winding resistance measurement is done using direct current and is very temperature-dependent.

Routine Transformer Test – Measuring Winding Resistance

The temperature correction is made according to the equations below:

R2 – winding resistance at temperature 2,
R1 – winding resistance at temperature 1

Therefore, temperatures should be measured when measuring winding resistances, and temperature during measurement should also be recorded.

Winding resistances are measured between all winding connection terminals and at all tap positions. During this time, the winding temperature should also be measured and recorded appropriately.

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

In practice, this value should be greater than 1.2 x I 0 and less than 0.1 x I N, if possible.

A transformer consists of a resistor R and an inductor L connected in series. If a voltage U is applied to this circuit;

The current measurement value will be:

Here, the time coefficient depends on L / R ratio.

 Measuring Winding Resistance
Measuring Winding Resistance

As the measurement current increases, the core will be saturated and the inductance will decrease. In this way, the current will reach the saturation value faster.

After current is applied to the circuit, wait until the current is off (full saturation) before taking any measurements, otherwise, measurement errors will occur.

Measuring circuit and carrying out the measurement

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

The measurement by the current voltage method is illustrated.

In the current-voltage method, the measuring current through the winding also passes through a standard resistor of known value and the voltage drop values ​​on the two resistors (winding resistance and standard resistance) are compared to find the unknown resistance (winding resistance).

Be careful not to leave the voltage measuring voltmeter Routine Transformer connected to the circuit to protect it from high voltages that may occur when switching the circuit on / off.

The bridge method is based on the comparison of an unknown resistance (being measured) with a resistance of known value.

When the currents flowing in the arms are balanced, the current through the galvanometer will be zero. In general, if resistors of a small value (eg less than ≤ 1 ohm) are measured with a Kelvin bridge and if resistors of a higher value are measured with a Wheatstone bridge, measurement errors will be minimized.

The resistance measured with the Kelvin bridge:

The resistance measured with the Wheatstone bridge:

BEST Transformers Laboratory

The BEST test laboratory is equipped with the most advanced testing facilities and capable of performing all tests required by IEC standards with the exception of the mechanical resistance to short circuit test, carried out in an independent international laboratory, CESI -Italy.

Tests performed on transformers can be classified as follows:

In-process testing, routine testing, type testing, special testing, acceptance testing, on-site testing, fault analysis / identification, and pre-maintenance testing.

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