Problems #4

1. Two single-phase transformers work in parallel to supply a load of 500 kVA at 0.8 power factor lagging. Determine the secondary voltage, output (in kVA) and power factor of each transformer. Test data are:

Open-circuit: 11 kV/ 3.3 kV for each transformer

Short-circuit: with HV winding short-circuited

Transformer A: 200 V, 400 A, 15 kW

Transformer B: 100 V, 400 A, 20 kW

2. Two single-phase transformers, rated 1000 kVA and 500 kVA respectively, are connected in parallel to supply a load of 1000 kVA at 0.8 power factor lagging. They have equal voltage ratings of 11 kV/ 400 V and their per-unit impedances are (0.02 + j0.07) and (0.03 + j0.08) respectively. What is the largest value of the unity power factor load that can be delivered by the parallel combination at the rated voltage?

3. Two single-phase transformers (say Transformer A and Transformer B) with the rating of 3000 kVA and 2000 kVA respectively are to operate in parallel to supply a load having a resistance of 25 ohms and reactance of 15 ohms. The no-load ratio of each transformer is 33 kV/ 11 kV, but the tap-changers are set incorrectly and the no-load secondary voltage with 33 kV on the primary are 11.2 kV on Transformer A and 10 kV on Transformer B. Calculate the load in kVA on transformer A, assuming the percentage equivalent voltage drops at rated load and nominal ratio to be:

Transformer A; resistance 1.5%, leakage reactance 6%

Transformer B: resistance 1%, leakage reactance 5%