Question

A three-phase transformer bank consists of 3 single-phase transformers to handle 400 kVA witha 34.5kV/13.8kV voltage ratio. Find the rating of each individual single-phase transformer in the bank (high voltage, low voltage, turns ratio, and apparent power) for the following transformer connections:

Answer 1

Answer:

(a) high voltage = 19.9 kV, low voltage = 7.97 kV, turns ratio = 2.50:1, apparent power = 133 kVA

(b) high voltage = 19.9 kV, low voltage = 13.8 kV, turns ratio = 1.44:1, apparent power = 133 kVA

(c) high voltage = 34.5 kV, low voltage = 7.97 kV, turns ratio = 4.33:1, apparent power = 133 kVA

(d) high voltage = 34.5 kV, low voltage = 13.8 kV, turns ratio = 2.50:1, apparent power = 133 kVA

Explanation:

The turn ratio can be estimated by taking the ratio of the values of the high voltage and the low voltage. For example, the turn ratio for (a) is 19.9 kV/7.97 kV = 2.50:1. Similarly, the apparent power for each transformer is calculated as 400 kVA/3 = 133 kVA. Furthermore, the high and low voltages for the Δ are the same for the three-phase transformer i.e. 34.5 kv/13.8 kV. The high and high voltages for Y connection is calculated as 34.5 kV/1.73 = 19.9 kV and 13.8 kV/1.73 = 7.97 kV.