Power and Current Transformers Sample Clauses
Power and Current Transformers. 9 questions
a. Transformer theory and operation
b. Transformer operating losses (hysteresis, eddy currents, and copper losses)
c. Single-line symbols for three-phase transformer banks
d. Three-phase transformer bank power, primary current, and secondary current calculations using nameplate values
e. Transformer primary and secondary connections (internal and external)
f. Transformer polarity and insulation tests
g. Current transformer (CT) theory and operation
h. Multi-ratio (tapped) CT’s
i. Safety precautions working with CT’s
j. Wye system operation with connected meters and relays k. CT ratings and classifications l. CT burden m. Polarity markings
Power and Current Transformers. IEEE Standard C57.91-1995 (R2004), “Guide for Loading Mineral-Oil-Immersed Transformers” [Reference 3] recommends using “average daily temperatures” for the month involved in determining normal ratings and “average of maximum daily temperatures” for the month involved for emergency ratings. The Guide also recommends the use of a 5°C adder to be conservative. The ambient temperatures indicated in Table A1 are consistent with the recommendations for determining ambient temperatures set forth in the C57.91 including the recommended 5°C adder as based on the data of Table A2.
1) Normal ambient temperatures were derived from the Daily Mean temperatures of Table A2; Column 3 and emergency ambient temperatures were derived from the Average of the Daily Maximum temperatures of Table A2, Column 1.
2) However, weighted averages of temperatures appropriate to the summer and winter periods were used instead of monthly temperatures as suggested by the Guide. Winter temperatures were equally weighted over the 5-month period. Summer temperatures were determined by equal weighting of the temperatures for the months of June through September. A recalculation of the ambient temperature values using the data of Table A3 compares favorably with the recommendations of Table A1. Only the Summer Emergency ambient temperature differed, being lower by less than 2ºC. The criteria to be used for developing ambient temperature for current transformers will be the same as power transformers.