IEEE Std C57.12.91 pdf free download

IEEE Std C57.12.91 pdf free download.Dry-Type Distribution and Power Transformers.
5.2.4 Making measurements
When making resistance measurements:
a) Measurement time intervals of 5 s to lOs 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 Sid 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 dc 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 connected windings.
5.3 Determination of cold temperature
5.3.1 Cold temperature value
The cold temperature 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 1 1.8.7 and Figure 31).
5.3.2 AmbIent temperature
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:
a) 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 than 2 °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.
The cold temperature shall be the average of the internal temperature sensors or, in the case of a sealed unit where the internal sensors are not available, the average of the surface temperature sensors.
5.3.3 Measurement time
When measuring the cold resistance preparatory to making a temperature rise test, the time required for the readings to become constant should be noted. The period thereby determined should be allowed to elapse before taking the first reading when final winding hot-resistance measurements are being made.
5.3.4 Residual flux
The residual flux in the core should be same for both the cold- and hot-resistance measurements by saturating the core with dc current prior to the measurement.IEEE Std C57.12.91 pdf download.