QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding. 5.2 Explain how an alternating magnetic field is created in the primary winding of a transformer. 5.3 List THREE properties that must be identical in single-phase transformers so that they can be used as a three-phase transformer unit. 5.4 Explain where delta-delta transformers are mainly used. Give a reason for the answer. 5.5 A three-phase transformer unit can be obtained by two methods: either by using single-phase transformers and connecting them as a three-phase unit or by a pre-manufactured three-phase transformer. Compare the two methods and state which method: 5.5.1 Is more expensive. 5.5.2 Has a higher efficiency. 5.5.3 Uses thicker-sized conductors. 5.6 Transformers generate a lot of heat during normal operation and therefore need cooling systems. Answer the following questions: 5.6.1 With reference to cooling, transformers are divided into two categories. Name the TWO categories. 5.6.2 Name TWO cooling methods used in transformers. 5.6.3 Name the protective device that monitors gas formation in high-power transformers. 5.7 FIGURE 5.7 below shows a three-phase delta-star transformer. Given: $V_{L1} = 6 kV$ $TR = 25 : 1$ $P_{out} = 50 kW$ $pf = 0.9$ Use the data in FIGURE 5.7 to calculate the following: 5.7.1 Secondary phase voltage. 5.7.2 Secondary line voltage. 5.7.3 Apparent power. 5.7.4 Primary line current.

NSC Electrical Technology Power Systems Transformer Losses: QUESTION 5: THREE-PHASE TRANSFOR

QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding - NSC Electrical Technology Power Systems - Question 5 - 2023 - Paper 1

Question 5

QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding. 5.2 Explain ho... show full transcript

Worked Solution & Example Answer:QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding - NSC Electrical Technology Power Systems - Question 5 - 2023 - Paper 1

Step 1

Name the process used by transformers to transfer energy from the primary winding to the secondary winding.

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Answer

The process used by transformers to transfer energy is called mutual induction.

Step 2

Explain how an alternating magnetic field is created in the primary winding of a transformer.

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Answer

In a transformer, the AC supply voltage connected to the primary winding creates an alternating current that flows through the primary coils. This alternating current generates a magnetic field around the coils, which expands and collapses as the supply voltage increases and decreases, thereby inducing voltage in the secondary winding.

Step 3

List THREE properties that must be identical in single-phase transformers so that they can be used as a three-phase transformer unit.

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Answer

The three properties that must be identical in single-phase transformers include:

Size – to ensure physical compatibility.
Voltage rating – to maintain consistent voltage levels across all transformers.
Efficiency – to ensure balanced performance in the three-phase formation.

Step 4

Explain where delta-delta transformers are mainly used. Give a reason for the answer.

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Answer

Delta-delta transformers are mainly utilized in industries where high power transfer is essential. This is due to their capability to handle larger loads and provide a more robust connection for high voltage applications.

Step 5

Is more expensive.

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Using single-phase transformers connected as a three-phase unit is generally more expensive due to additional installation and material costs.

Step 6

Has a higher efficiency.

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Answer

The pre-manufactured three-phase transformer typically has a higher efficiency compared to using individual single-phase transformers.

Step 7

Uses thicker-sized conductors.

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Answer

The method using single-phase transformers uses thicker-sized conductors to handle the higher currents arising from multiple units.

Step 8

With reference to cooling, transformers are divided into two categories. Name the TWO categories.

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Answer

Transformers are divided into two cooling categories:

Dry type transformers
Oil immersed transformers

Step 9

Name TWO cooling methods used in transformers.

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Answer

The two cooling methods used in transformers include:

Air natural cooling
Oil forced cooling

Step 10

Name the protective device that monitors gas formation in high-power transformers.

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Answer

The protective device that monitors gas formation in high-power transformers is known as the Buchholz relay.

Step 11

Secondary phase voltage.

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Answer

To calculate the secondary phase voltage, we use the turns ratio:

V_{ph2} = \frac{N_2}{N_1} \times V_{ph1} = \frac{1}{25} \times 6000 = 240 V

Step 12

Secondary line voltage.

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Answer

The secondary line voltage can be calculated using:

V_{L2} = \sqrt{3} \times V_{ph2} = \sqrt{3} \times 240 \approx 415.69 V

Step 13

Apparent power.

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Answer

Apparent power (S) is calculated using:

S = \frac{P}{pf} = \frac{50000}{0.9} \approx 55555.56 VA

Step 14

Primary line current.

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Answer

The primary line current (I1) can be calculated by:

I_{L1} = \frac{S}{\sqrt{3} \times V_{L1} \times cos \theta} = \frac{50000}{\sqrt{3} \times 6000 \times 0.9} \approx 5.35 A

QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding - NSC Electrical Technology Power Systems - Question 5 - 2023 - Paper 1

Worked Solution & Example Answer:QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Name the process used by transformers to transfer energy from the primary winding to the secondary winding - NSC Electrical Technology Power Systems - Question 5 - 2023 - Paper 1

Name the process used by transformers to transfer energy from the primary winding to the secondary winding.

Explain how an alternating magnetic field is created in the primary winding of a transformer.

List THREE properties that must be identical in single-phase transformers so that they can be used as a three-phase transformer unit.

Explain where delta-delta transformers are mainly used. Give a reason for the answer.

Is more expensive.

Has a higher efficiency.

Uses thicker-sized conductors.

With reference to cooling, transformers are divided into two categories. Name the TWO categories.

Name TWO cooling methods used in transformers.

Name the protective device that monitors gas formation in high-power transformers.

Secondary phase voltage.

Secondary line voltage.

Apparent power.

Primary line current.

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