Question 2.7: Avoiding a New Transformer by Improving the Power Factor. A ......
Avoiding a New Transformer by Improving the Power Factor. A factory with a nearly fully loaded transformer delivers 600 kVA at a power factor of 0.75. Anticipated growth in power demand in the near future is 20%. How many kVAR of capacitance should be added to accommodate this growth so they don’t have to purchase a larger transformer?
At PF = 0.75, the real power delivered at present is 0.75 × 600 kVA = 450 kW. And the phase angle is \theta \ = \ \cos ^{-1}\left(0.75\right) \ = \ 41.4º. If demand grows by 20%, then an additional 90 kW of real power will need to be supplied. At that point, if nothing is done, the new power triangle would show
For this transformer to still supply only 600 kVA, the power factor will have to be improved to at least
PF \ = \ {540 \ kW}/{600 \ kVA} \ = \ 0.90The phase angle now will be \theta \ = \ \cos ^{-1} \left(0.90\right) \ = \ 25.8º. The reactive power will need to be reduced to
Q \ = \ 600 \ kVA \ \sin 25.8º \ = \ 261 \ kVARThe difference in reactive power between the 476 kVAR needed without power factor correction and the desired 261 kVAR must be provided by the capacitor. Hence
PF \ \text{correcting capacitor} \ = \ 476 \ − \ 261 \ = \ 215 \ kVARThe power triangles before and after PF correction are shown below:
