Question

Can you help me solve this problem and show the steps?

The local transformer shown below provides the last voltage stepdown in a power distribution system. A common sight on utility poles in residential areas, it is a single-phase transformer that typically has a 13.8-kV rms line to neutral on its primary coil, and a center tap secondary coil provides both 120 V rms and 240 V rms to service several residences. A typical example of this transformer is shown below. Find the turns ratio necessary to produce the 240-V rms secondary voltage. Assuming that the transformer provides 200-A rms service to each of 10 13.8 kV houses, let us determine the minimum power rating for the transformer and the maximum current in the primary. 1:n a Substation 13.8 kV rms + 120 Vrms 240 Vrms + 120 Vrms n Local step-down transformer transformer 240/120 V output

Answer 1

Turns ratio = N2/N1= V2/V1= 240/13800= 1/57.5

n=57.5

1:n=1:57.5

Total secondary current= I2 = 10*200 = 2000A

(Since there are 10 houses which takes 200A each)

Power rating of transformer= V2*I2

= 240*2000

=480 KVA

Primary current I1= 480kva/13.8kv

Therefore , I1 = 34.782A