Medium Voltage Application Guide

SWITCHGEAR
CPT selection - Inrush VA at selected voltage drop
Inrush VA
Nominal VA rating
85% voltage drop 90% voltage drop 95% voltage drop
347 289 216 63
338 290 229 80
907 745 541 130
1267 1039 754 200
1394 1116 781 250
2870 2298 1584 350
3786 3013 2065 500
7360 5763 3786 750
7360 5763 3786 800
8837 6785 4329 1000
14921 11328 7070 1600
20500 14850 9100 2000
CPT selection - power factor adjustment factor
Power factor 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Adjustment factor 1.5 1.29 1.11 1.00 0.91 0.82 0.78 0.71 0.67 0.64
Exercise
A transformer with its primary connected to 7.2 kV, has a 110 VAC secondary with a total inrush loading of
850 VA and a resistive load of 100 VA. The total sealed VA of the load is 200 VA.
A voltage drop of 85% is acceptable during the inrush stage. The power factor of the inrush current is 0.3.
Calculate the necessary power rating of the control supply power transformer.
1. The total inrush loading is 950 VA (850 VA + 100 VA). The next highest Inrush VA figure in the 85% volt
drop column is 1267 VA.
2. An Inrush VA of 1267 VA equates to a Nominal VA rating of 200 VA.
3. 200 VA seems acceptable as this is equivalent to the total sealed load VA.
4. The inrush current has a power factor of 0.3. Using the power factor adjustment factor, the transformer has
a revised Nominal VA rating of 222 VA (200 x 1.11). The next highest standard size would be 250 VA.
Use a 7.2 kV/110 VAC, 250 VA single phase control supply power transformer.
Transformer primary fuses must withstand the inrush magnetising current which flows when a
transformer primary is switched on. With a medium voltage primary, E-rated fuses are used and often
selected to withstand 25 times the nominal primary current for 0.01 second and 12 times the nominal
primary current for 0.1 second.
710-12280-00A Medium Voltage Application Guide Page 129