Practice Ohmic Resistance of Windings
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With the ohmic resistance test of transformer coils, it is more common to find problems in the mechanical connections between the bushings and the windings (coils on the primary or secondary side). A false connection can cause high temperatures that “burn” the surrounding insulation and contaminate the dielectric oil with combustible gases. We also perform the ohmic resistance test of the coils to detect possible damage between the insulation of the turns (in this sense it is a complementary test to the transformation ratio tests). This is because when there are turns in short circuit, the ohmic resistance of the winding will be very different with respect to another winding of the same transformer. Although the ohmic resistance values of the windings are very different from the ohmic resistance values of contacts (such as those of the blades, disconnectors or switches), the measurement principle is the same and the interpretation of the results is similar. If you do not yet have the theoretical basis of the ohmic resistance test, I recommend you watch the following video: • Ohmic Resistance Test of Conta... Regarding the reference standard used, you can consult the following standards: • SOM-3531 Reference Standard of the Federal Electricity Commission (CFE: Mexican electric company) Item 3.7 OHM RESISTANCE TEST FOR WINDINGS. “The current used in the measurement must not exceed 15% of the nominal value of the winding, since higher values may obtain inaccurate results caused by variations in resistance due to heating of the winding” • SOM-3531 Item 3.8.5 “To date, variations in the plate impedance of the transformer are evaluated; • IEEE Std 62. Item 6.1.1 Winding resistance “The interpretation of the results is generally based on a comparison of the measurements made separately in each phase in the case of a star-connected winding or between pairs of terminals in a delta-connected winding. A comparison with the original data measured at the factory can also be made. Agreement within 5% for any of the above comparisons is generally considered satisfactory.” • IEEE Std 62. Item 6.1.1.1.1 Voltmeter-ammeter method “The currents used for these measurements normally do not exceed 15% of the nominal current. This avoids heating the winding and therefore changing its resistance. The resistance variation shall not exceed 5% from phase to phase.” • NETA – MTS Item 7.2.2.7 Test values: electrical “7. Temperature-corrected winding resistance values shall be compared within one percent of previously obtained results.”
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