IEEE C57.12.91-2020 pdf download

IEEE C57.12.91-2020 pdf download.IEEE Standard Test Code for Dry-Type Distribution and Power Transformers.
b) The voltage leads shall be independent of the current leads and shall be connected to the transformer at the point of measurement as specified in 5.2.1. This is to avoid including into the readings the resistances of leads and their contact resistances.
c) Transformer winding resistance measurements may exhibit a long dc time constant to reach stable steady-state values. To reduce the time required for the current to reach its steady-state value, a noninductive external resistor may be added in series with the dc source. The external resistance should be large compared to the impedance of the winding. It is then necessary to increase the source voltage to compensate for the voltage drop in the external resistor.
d) To protect test personnel from inductive kickback, the measuring circuit current source should be connected and disconnected by a suitably insulated switch with a protective circuit to safely discharge the energy. The voltmeter should be disconnected from the circuit before switching the current on or off to protect the voltmeter from overvoltage injury from inductive kickback.
5.5.2 Bridge method
Bridge methods or other high-accuracy digital instruments may be used in cases where the rated current of the transformer winding requires the measuring current to be less than 1 A. Wheatstone bridges (or equivalent) are commonly used for resistances greater than 1 : Kelvin bridges (or equivalent) are commonly used for resistances less than 1 2; and some of the newer, digital resistance bridges have capabilities in both ranges.
5.6 Resistance measurement connections and reporting
5.6.1 Resistances to be reported
The total winding resistances, defined in 5.6.2 through 5.6.5, shall be reported. The individual terminal-to- terminal resistances should also be reported.
5.6.2 Single-phase windings
For single-phase windings. the reported resistance measurements are from terminal to terminal. The reported total winding resistance is the terminal-to-terminal measurement. For series-parallel windings, the reported total winding resistance is the sum of the series terminal-to-terminal section measurements.
5.6.3 Wye windings
For wye windings, the reported resistance measurement may be from terminal to terminal or from terminal to neutral. The reported total winding resistance is the sum of the three terminal-to-terminal measurements divided by 2. The neutral connection resistance, which is a portion of each terminal-to-neutral resistance measurement. may be excluded.
5.6.4 Delta windings
For delta windings, the reported resistance measurement may be from terminal to terminal with the delta closed or from terminal to terminal with the delta open to obtain the individual phase resistances. If the delta is open, the reported total winding resistance is the sum of the three terminal-to-terminal measurements. If the delta is closed, the reported total winding resistance is the sum of the three terminal-to-terminal measurements multiplied by 1 .5.
5.6.5 Autotransformer windings
For autotransformer windings, the reported resistance measurements are from terminal to terminal. The reported total winding resistance is the terminal-to-terminal measurement.
For autotransformer series winding resistance, the test current shall be circulated between the high-voltage series terminal and neutral terminal, and the test voltage measured between the high-voltage series terminal and the low-voltage common terminal. For the common winding resistance, the test current shall be circulated between the high-voltage series terminal and neutral terminal, and the test voltage measured between the low-voltage common terminal and the neutral terminal. If the resistance of the common terminal connection is needed, the test current shall be applied between the high-voltage series terminal and the low-voltage common terminal, and the test voltage measured between the low-voltage common terminal and the neutral terminal.
6. Polarity and phase-relation tests
6.1 General
Polarity and phase-relation tests are of interest primarily because of their bearing on paralleling or banking two or more transformers. Phase-relation tests are made to determine angular displacement and relative phase sequence.
6.2 Subtractive and additive polarity
Windings arranged for subtractive and additive polarity are shown in Figure 2 and Figure 3. Leads and polarity marks arranged for subtractive polarity and additive polarity are shown in Figure 4 and Figure 5.