IEEE C57.12.91-2001 pdf free.IEEE Standard Test Code for Dry-Type Distribution and Power Transformers.
Resistance measurements are of fundamental importance for the calculation of the I’R component ofconductor losses , for the calculation of winding temperatures at the end of a temperature rise test, and as abase for assessing possible damage in the field.
Cold-resistance measurements shall be taken on all phases of each primary and secondary winding on therated tap connection.If a temperature rise test is to be performed,cold resistance measurements shall also betaken on all phases of each primary and secondary winding on the combination of connections and taps to beused for the temperature rise test. When transferring leads from one winding to another, the same relativepolarity should be maintained with regard to the measuring leads and the transformer terminals.
The induction time for the measuring current to become stable should be noted during the cold-resistancemeasurements in order to ensure that sufficient time elapses for the induction effect to disappear before hotresistance readings are taken during the temperature rise tests. Cold-resistance measurements shall not bemade on a transformer when it is located in drafts or when it is located in a room in which the temperature isfluctuating rapidly.
The cold temperature of the winding shall be determined as accurately as possible when measuring the coldresistance.The following precautions shall be observed.The temperature of the windings for ventilated unitsshall be recorded as the average readings of several thermometers or thermocouples inserted between thecoils.Care shall be taken to see that the measuring points of the thermocouples or thermometers are as nearlyas possible in actual contact with the winding conductors. The temperature of the windings for sealed unitsshall be recorded as the average readings of several temperature sensors in contact with the tank and cover(see 11.8.6 and Figure 30).
lt should not be assumed that the windings are at the same temperature as the surrounding air. To ensure thatthe windings are at ambient temperature, the following conditions shall be met immediately before takingcold-resistance measurements:
a)All internal temperatures measured by the internal temperature sensors shall not differ from ambient temperature by more than 2 °C.
b) Tank surface temperatures for sealed units shall not differ from ambient temperature by more than 2 C.IEEE C57.12.91 pdf download.