EE306 Power System Analysis: Standard Transformer


Design and specify the ratings of a single-phase transformer to supply a number of loads from 400-V source. The design must include the determination of kVA rating, and specifications of primary and secondary currents. It also attempts to determine the efficiency of the transformer under different loading conditions.Starting a transformer generally requires more current than running it. To account for this additional current requirement, it’s often helpful to put a start factor into your calculations.A good rule of thumb is to multiply the (VA) by an additional start factor of 125%.
The building has the following loads:
Type of Loads Rating
Load Voltage Power Factor
Lighting load 5000 220 1.0
Laundry equipment 1000 220 0.85
Kitchen equipment 1000 220 0.9
Dishwasher 1000 220 0.80
Disposal 500 220 0.9
Air-conditioning 5000 220 0.85
Design steps
a. Consider additional future loads of 10% of current total load capacity).
b. Calculate the total kVA required for the transformer (including the additional future loads and the starting factor).
c. Use the typical “Standard Transformer Sizes”; see table below.
d. Determine the primary and secondary currents of the transformer.
e. Determine the full load capacity in kW.
f. Calculate the full load efficiency using the following assumption:
1. The core losses are equal to 300-W + XX-W. XX is your last two digits serial number as posted in BB under EE306 common sections (e.g., 01,04, 10, 14, 21 …etc.)
2. The full load copper losses are equal to 400-W + Y-W. Y is your one digit section number as posted in BB under EE306 common sections (e.g., 1, 2,6 …etc.)
g. Calculate the efficiency of at least 3 different loading conditions (LDs) of your own choice (e.g., LDs1: Air Conditions & Lighting; LDs2: Laundry & Kitchen equipment; …etc.)
h. Calculate the maximum efficiency of the transformer when operating at a power factor of 0.9
Standard Transformer Sizes (KVA)
3 30 112.5
6 37.5 150
9 45 225
15 75 300
Submit a formal report indicating all detailed design and calculation aspects.

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