Measurement cycle and method of DC resistance of transformer winding

Purpose of measuring DC resistance of winding
Check the quality of the winding welding joints and whether there is a short circuit between the turns of the winding;
Check if there are any broken wires or open circuits in the winding conductors or lead wires;
Check whether the contact of each position of the tap changer is good, and whether the actual position of the tap changer matches the indicated position;
Are there any broken strands or other issues with windings wound in parallel.
Test cycle
1) During handover
2) After major repairs
3) 1-3 years
4) Off load tap changer transformer for changing tap positions
5) After the maintenance of the tap changer of the on load tap changer transformer (at all taps)
6) When necessary
DC resistance tester
Experimental principle
When time t is zero, I=0, and when t is ∞, I=En/R, reaching stability.
Due to the high inductance and low resistance of transformer windings, the inductance can reach several hundred henries and the time constant is relatively large. For high-voltage and large capacity transformers, measuring the stability time of a resistor takes several minutes, tens of minutes, or even hours. Choosing appropriate measurement methods and equipment is the key to ensuring measurement accuracy.
Shortening the measurement time (i.e. reducing the T-value) is very effective in improving the experiment. To achieve a decrease, the method of reducing L or increasing R (i.e. increasing additional resistance) can be used. Reducing L can increase the measurement current and improve the saturation degree of the iron core, that is, reducing the magnetic permeability coefficient of the iron core and increasing R. It can be achieved by adding an appropriate additional resistor in series in the circuit. Generally, the additional resistor is 4-6 times the measured resistor. At this time, the measurement voltage should also be increased to avoid the current being too small and affecting the sensitivity of the measurement.
Analysis and judgment of experimental results
1) For transformers above 1.6MVA, the difference in resistance between the windings of each phase should not exceed 2% of the average value of the three phases; The difference between the wires of a winding without a neutral point should not exceed 1% of the average value of the three phases (unbalance rate=(measured maximum value in the three phases - minimum value)/arithmetic mean of the three phases).
2) For transformers of 1.6MVA and below, the phase to phase difference should generally not exceed 4% of the average value of the three phases; The difference between lines should generally not exceed 2% of the average value of the three phases.
3) There should be no significant difference in the resistance of each phase winding compared to previous results at the same location and temperature. The difference should not exceed 2%, and attention should be paid when it exceeds 1%.
4) Reactor reference execution