QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 State TWO functions of the oil used in oil-filled transformers - NSC Electrical Technology Power Systems - Question 3 - 2016 - Paper 1

To determine the secondary phase voltage ($V_{PH}$), use the relationship between line voltage ($V_{L_{S}}$) and phase voltage:

$V_{PH} = \frac{V_{L_{S}}}{\sqrt{3}}$

Substituting the values:

• $V_{L_{S}} = 380$ V

$V_{PH} = \frac{380}{\sqrt{3}} \approx 219.39 V$

The transformer ratio ($TR$) can be calculated using the primary and secondary phase voltages:

$TR = \frac{V_{P_{H}}}{V_{S_{H}}}$

Where:

• $V_{P_{H}} = 6600$ V
• $V_{S_{H}} = 380$ V

Thus,

$TR = \frac{6600}{380} \approx 30:1$