Práctica Resistencia Óhmica de Devanados

When testing the ohmic resistance of transformer coils, it's most common to find problems in the mechanical connections between the bushings and the windings (coils on the primary or secondary side). A poor connection can generate high temperatures that burn the surrounding insulation and contaminate the dielectric oil with flammable gases. We also perform the ohmic resistance test on coils to detect potential damage between the insulation of the turns (in this sense, it's a complementary test to the turns ratio test). This is because, when turns are short-circuited, the ohmic resistance of the winding will be very different from that of another winding in the same transformer. Although the ohmic resistance values ​​of the windings are very different from the ohmic resistance of contacts (such as those of switches, disconnectors, or circuit breakers), the measurement principle is the same, and the interpretation of the results is similar. If you don't yet have a theoretical understanding of ohmic resistance testing, I recommend watching the following video:    • Prueba de Resistencia Óhmica de Contactos ...   Regarding the reference standard used, you can consult the following: • SOM-3531 Reference Standard of the Federal Electricity Commission (CFE: Mexican Electric Company) Item 3.7 OHMIC RESISTANCE TEST OF WINDINGS. “The current used in the measurement should not exceed 15% of the winding's nominal value, as higher values ​​may produce inaccurate results due to variations in resistance caused by winding heating.” • SOM-3531 Item 3.8.5 “Currently, variations in the transformer's nameplate impedance are evaluated; deviations between 3% and 5% are considered permissible, within which the equipment is considered to be in good condition (values ​​that correspond to American and European criteria, respectively).” Note: SOM-3531 is a Reference Standard of the Federal Electricity Commission (CFE: Mexico's electric company). • IEEE Std 62. Item 6.1.1 Winding Resistance “The interpretation of the results is generally based on a comparison of measurements taken separately on each phase in the case of a wye-connected winding or between pairs of terminals in a delta-connected winding. Alternatively, Compare the results with the original data measured at the factory. 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 rated current. This prevents heating the winding and, therefore, changing its resistance. The resistance variation should not exceed 5% from phase to phase.” • NETA – MTS Item 7.2.2.7 Test Values: Electrical “7. Temperature-corrected winding resistance values ​​should be compared to within one percent of previously obtained results.”