IEEE Std C57.12.91:2001 pdf free download

IEEE Std C57.12.91:2001 pdf free download.IEEE Standard Test Code for Dry-Type Distribution and Power Transformers.
NOTE—A reversal in magnctl!ation of the core can change thc time constant and result in crroncous readings.
5.4.2.3 Independent voltmeter and current leads
The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to avoid inciLiding in (he reading the resistances of current-carrying leads and their contacts and of extra lengths of leads. To protect the voltmeter from injury by off-scale dellecfions. the voltmeter should be disconnected from the circuit before the current is switched on or otT. To protect test personnel from induclive kick, the current should be switched off by a suitably insulated switch.
If the drop of voltage is less than I V. a potentiometer or millivoltmeter shall be used.
5.4.2.4 The effect of winding dc time constant
Readings shall not be taken until after the current and voltage have reached steady-state values.
When measuring the cold-resistance preparatory to making a heat run, the time required for the readings to become constant should be noed. The period thereby determined should be allowed to elapse before taking the first reading when final winding hot-resistance measurements are being made.
In general. the winding will exhibit a long dc time constant. To reduce the time required for the current to reach its steadY-state value, a noninductive external resistor should be added in series with the dc source. The resistance should he large compared to the inductance of the winding. It will then be necessary to increase the source voltage to compensate for the voltage drop in the series resistor. The time will also be reduced by operating all other transformer windings open-circuited during the tests.
5.4.2.5 Maximum dc current in measuring circuit
Rcadings shall be taken with fbi less than four values of current when deflecting instruments are used. The average of the resistances calculated from these measurements shall be considered to be the resistance of the circuit.
The current used shall not exceed l5 of the rated current of the winding’s resistance tobe measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.
When the current is too low to be read on a deflecting ammeter, a shunt and digital millivolimeter or potentiometer shall be used.
6. Polarity and phase-relation tests
Polarity and phase-relation tests are of interest primarily because o their bearing on paralleling or banking two or more transformers. Phase-relation tests are made to determine angular displacement and relative phase sequence.