5.1 Name TWO types of transformer core constructions used in three-phase transformers. 5.2 Explain why dielectric oil is used inside a transformer. 5.3 State where the Buchholz relay is situated in an oil-cooled transformer. 5.4 Draw a three-phase delta-step-down transformer unit by using three identical single-phase transformers. 5.5 A 10 kVA three-phase transformer has a copper loss of 300 W and a core loss of 50 W. The system operates at a power factor (p.f.) of 0.8. Calculate the following: 5.5.1 Output power 5.5.2 Efficiency 5.6 FIGURE 5.6 below shows a three-phase delta-star transformer. Given: $V_L1 = 6 kV$ $I_L1 = 2 A$ $V_{ph2} = 240 V$ $P = 18 kW$ Calculate the following: 5.6.1 Rating of the transformer (apparent kV) 5.6.2 Power factor of the load 5.6.3 Primary phase voltage 5.6.4 Turns ratio

NSC Electrical Technology Power Systems Motor Principles: 5.1 Name TWO types of transforme

5.1 Name TWO types of transformer core constructions used in three-phase transformers - NSC Electrical Technology Power Systems - Question 5 - 2022 - Paper 1

Question 5

5.1 Name TWO types of transformer core constructions used in three-phase transformers. 5.2 Explain why dielectric oil is used inside a transformer. 5.3 State where... show full transcript

Worked Solution & Example Answer:5.1 Name TWO types of transformer core constructions used in three-phase transformers - NSC Electrical Technology Power Systems - Question 5 - 2022 - Paper 1

Step 1

5.1 Name TWO types of transformer core constructions used in three-phase transformers.

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Answer

The two types of transformer core constructions used in three-phase transformers are:

Core-type: This construction features a core that encircles the windings of the transformer, providing efficient magnetic coupling.
Shell type: In this design, the windings are located inside a shell-like core, which offers better leakage flux control.

Step 2

5.2 Explain why dielectric oil is used inside a transformer.

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Answer

Dielectric oil is used in transformers primarily for its electric insulating properties. It ensures that the high-voltage parts of the transformer are insulated from each other, preventing electrical discharge. Additionally, the oil acts as a coolant by transferring heat away from the components, maintaining operational efficiency and safety.

Step 3

5.3 State where the Buchholz relay is situated in an oil-cooled transformer.

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The Buchholz relay is typically positioned in-line between the conservator and the transformer housing. Its location allows it to monitor gas accumulation and oil flow, providing critical protection against faults.

Step 4

5.4 Draw a three-phase delta-step-down transformer unit by using three identical single-phase transformers.

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The drawing should depict three single-phase transformers connected in a delta configuration on the primary side, with appropriate connections indicating that the secondary side is also set to deliver a lower voltage. The arrangement should represent the step-down functionality clearly, showing primary and secondary line voltages and load connections.

Step 5

5.5.1 Output power

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To calculate the output power, use the formula:

$P_{out} = S imes ext{p.f.}$

Substituting the values:

$P_{out} = 10,000 imes 0.8 = 8,000 ext{ W}$

Step 6

5.5.2 Efficiency

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The efficiency can be calculated using the formula:

$ext{Efficiency} ( ext{n}) = \frac{P_{out}}{P_{out} + ext{losses}} \times 100$

Where the losses are the sum of copper loss and core loss:

$\text{losses} = 300 + 50 = 350 ext{ W}$

Thus:

$n = \frac{8,000}{8,000 + 350} \times 100 = \frac{8,000}{8,350} \times 100 \approx 95.8\%$

Step 7

5.6.1 Rating of the transformer (apparent kV)

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To find the apparent power rating, use:

$S(P_{app}) = \sqrt{3} \times V_L \times I_L$

Substituting the values:

$S(P_{app}) = \sqrt{3} \times 6,000 \times 2 \approx 20,784.61 ext{ VA}$

Step 8

5.6.2 Power factor of the load

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The power factor can be calculated using:

$ext{Cos } \theta = \frac{P}{S}$

Substituting the values:

$\cos \theta = \frac{18,000}{20,784.61} \approx 0.87$

Step 9

5.6.3 Primary phase voltage

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For a delta connection, the phase voltage ( $V_{ph}$ ) is given by:

$V_{ph} = V_L$ For the primary side:

$V_L = 6,000 ext{ V}$

Step 10

5.6.4 Turns ratio

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The turns ratio (TR) can be determined by the relation:

$TR = \frac{V_{ph(1)}}{V_{ph(2)}}$

Where:

$TR = \frac{6,000}{240} = 25$ Thus, the turns ratio is 25:1.

5.1 Name TWO types of transformer core constructions used in three-phase transformers - NSC Electrical Technology Power Systems - Question 5 - 2022 - Paper 1

Worked Solution & Example Answer:5.1 Name TWO types of transformer core constructions used in three-phase transformers - NSC Electrical Technology Power Systems - Question 5 - 2022 - Paper 1

5.1 Name TWO types of transformer core constructions used in three-phase transformers.

5.2 Explain why dielectric oil is used inside a transformer.

5.3 State where the Buchholz relay is situated in an oil-cooled transformer.

5.4 Draw a three-phase delta-step-down transformer unit by using three identical single-phase transformers.

5.5.1 Output power

5.5.2 Efficiency

5.6.1 Rating of the transformer (apparent kV)

5.6.2 Power factor of the load

5.6.3 Primary phase voltage

5.6.4 Turns ratio

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