QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 State TWO functions of the oil used in oil-filled transformers. 3.2 Name TWO losses that occur in transformers. 3.3 State TWO advantages of a three-phase transformer over a single-phase transformer. 3.4 Explain why a transformer cannot step up power. 3.5 Describe what would happen to the primary current of a step-down transformer if the load of the transformer were increased. 3.6 A 20 kVA three-phase transformer has a delta-connected primary and a star-connected secondary. The primary line voltage is 6.6 kV and the secondary line voltage is 380 V. Calculate the: 3.6.1 Primary line current 3.6.2 Secondary phase voltage 3.6.3 Transformer ratio FIGURE 3.1: THREE-PHASE TRANSFORMER

NSC Electrical Technology Electronics Calculations: QUESTION 3: THREE-PHASE TRANSFO

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

Question 3

QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 State TWO functions of the oil used in oil-filled transformers. 3.2 Name TWO losses that occur in transformers. 3.3 Stat... show full transcript

Worked Solution & Example Answer:QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 State TWO functions of the oil used in oil-filled transformers - NSC Electrical Technology Electronics - Question 3 - 2016 - Paper 1

Step 1

3.6.1 Primary line current

96%

114 rated

Answer

To calculate the primary line current ( $I_{PL}$ ), we use the formula:

$I_{PL} = \frac{S}{\sqrt{3} V_{LP}}$

Substituting the values:

S = 20000 VA (20 kVA)
$V_{LP} = 6.6 kV = 6600 V$

We have: $I_{PL} = \frac{20000}{\sqrt{3} \times 6600} = \frac{20000}{11425.51} \approx 1.75 A$

Step 2

3.6.2 Secondary phase voltage

99%

104 rated

Answer

To calculate the secondary phase voltage ( $V_{PH}$ ), we use the relationship between line voltage ( $V_{L}$ ) and phase voltage in a star-connected system:

$V_{PH} = \frac{V_{L}}{\sqrt{3}}$

Given:

$V_{L} = 380 V$ , we calculate:

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

Step 3

3.6.3 Transformer ratio

96%

101 rated

Answer

The transformer ratio ( $TR$ ) can be calculated using the formula:

$TR = \frac{V_{PH (primary)}}{V_{PH (secondary)}}$

We know:

$V_{PH (primary)} = 6.6 kV = 6600 V$
$V_{PH (secondary)} \approx 219.39 V$

Thus: $TR = \frac{6600}{219.39} \approx 30 : 1$

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

Worked Solution & Example Answer:QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 State TWO functions of the oil used in oil-filled transformers - NSC Electrical Technology Electronics - Question 3 - 2016 - Paper 1

3.6.1 Primary line current

3.6.2 Secondary phase voltage

3.6.3 Transformer ratio

Join the NSC students using SimpleStudy...