IEEE C57.12.91-2020 pdf free.IEEE Standard Test Code for Dry-Type Distribution and Power T ransformers.
When making resistance measurements:
a) Measurement time intervals of5 s to 10 s are recommended. Longer intervals may be needed for the current and voltage readings to reach stable steady-state values. Readings shall not be taken before the current and voltage measurement values reach steady state.
b) The measuring instruments shall have ranges that give a reasonably large deflection for a needle type of instrument and a high percentage of scale range for a digital instrument to reduce errors of observation.
c) Current shunts and digital voltmeters, or high-accuracy digital ammeters, or other high-accuracy instrumentation should be used to meet the requirements of IEEE Std C57.12.01 due to inaccuracy of deflecting ammeters and voltmeters. This equipment should have a resolution of equal to or better than 0. 1% of the measured value.
d) Passing de current of the same polarity through other windings in the same phase or current of the opposite polarity through windings in other phases during resistance measurements can help reduce charging time. Charging time can also be reduced by opening the delta connection for delta conected windings.
The cold termperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The following precautions shall be observed. The temperature of the windings for ventilated units shall be recorded as the average readings of several thermometers or thermocouples inserted between the coils. Care shall be taken to see that the measuring points of the thermocouples or thermometers are as nearly as possible in actual contact with the winding conductors. The temperature of the windings for sealed units shall be recorded as the average readings of several temperature sensors in contact with the sides and cover (see 11.8.7 and Figure 31).
It should not be assumed that the windings are at the same temperature as the surrounding air. To help ensure that the windings are at ambient temperature, the following conditions shall be met immediately before taking cold-resistance measurements:
1) All internal temperatures measured by the internal temperature sensors shall not differ from ambient temperature by more than 2 °C.
b) Enclosure surface temperatures for sealed units shall not differ from ambient temperature by more than2 °C.
c) Ambient temperature shall not have changed by more than 3 °C for at least 3 h.
d) The transformer has been in a draft. free area for 24 h, and neither voltage nor current has been applied to it for 24 h to 72 h, depending on transformer size.IEEE C57.12.91 pdf download.