Handbook of Electric Power Calculations, Third Edition
A powerful calculations tool for electric power engineers and technicians, this book offers essential, step-by-step procedures and examples for solving a wide array of electric power problems.
TABLE OF CONTENTS Handbook of Electric Power Calculations, Third Edition
Section 13: SHORT-CIRCUIT COMPUTATIONS
Lawrence J.Hollander, P.E.
Dean of Engineering Emeritus
Union College
TRANSFORMER REGULATION DETERMINED FROM SHORT-CIRCUIT TEST
A single-phase transformer has the following nameplate data: 2300/220 V, 60 Hz, 5 kVA. A short-circuit test (low-voltage winding short-circuited) requires 66 V on the high-voltage winding to produce rated full-load current; 90 W is measured on the input. Determine the transformer s percent regulation for a load of rated current and a power factor of 0.80, lagging.
Calculation Procedure
1. Compute the Rated Full-Load Current (High-Voltage Side)
For a single-phase ac circuit use the relation kVA= VI/1000, where kVA=apparent power in kilovolt-amperes, V=potential in volts, and I=current in amperes. Rearranging the equation yields I=1000 kVA/V=(1000)(5)/2300=2.17 A.
2. Compute the Circuit Power Factor for the Short-Circuit Test
Use the relation for power factor: pf=W/VA, where W is real power in watts and VA is apparent power in volt-amperes (volts amperes); pf=90/(66)(2.17)=0.628. The power factor is the cosine of the angle between voltage and current; cos 1 0.628= ?=51.1 lagging. This is the pf angle of the transformer s internal impedance.
3. Compute the Circuit Power-Factor Angle for the Operating Condition
As in Step 2, use the relation cos 1 0.80= ?=36.9 lagging. This is the pf angle of the load served by the transformer.
4. Compute the Transformer Output Voltage for Serving an 0.80-pf Load at Rated Full-Load Current
Use the relation for IR drop in the transformer: V IR= V sc
Copyright The McGraw-Hill Companies, Inc. 1984 under license agreement with Books24x7
