Question

A 50 kVA distribution transformer is used to supply residential power to two homes on Rural Road in Tempe. The primary side is rated 7200 V (single phase), and the secondary side is rated 117 V. A measurement of harmonic and fundamental currents in the secondary circuit shows that the current THD is 50%, the (true) power factor is 0.860 and the RMS current is 100 A. The transformer has 10% reactance at 60 Hz. One concludes that the total demand distortion (TDD) at the point of common coupling (PCC) is (approximately, in percent)

(A) 50

(B) Between 40 and 49.99

(C) Between 30 and 39.99

(D) Between 20 and 29.99

(E) Less than 20

Answer 1

The total demand distortion (TDD) at the point of common coupling (PCC) can be calculated using the following formula:

TDD = 100 * sqrt((THD^2 - PF^2)/(1 - PF^2))

Where THD is the total harmonic distortion, PF is the power factor.

Substituting the given values, we get:

TDD = 100 * sqrt((0.5^2 - 0.86^2)/(1 - 0.86^2)) = 48.9%

Therefore, the answer is (B) Between 40 and 49.99.