QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 List THREE external conditions that may cause transformer failure. 4.2 Explain what would happen if an earth fault occurs in one of the three phases of a protected transformer. 4.3 Describe how an increase in the load would affect the magneto motive force in the primary windings. 4.4 Name TWO types of cooling methods for a dry transformer. 4.5 State why the output power of a transformer is slightly less than the input power. 4.6 Describe the construction of a three-phase core-type transformer. 4.7 A star-delta connected transformer has 600 turns in the primary windings and 80 turns in the secondary windings. The transformer is connected to a 6 kV supply. Given: N_p = 600 turns N_s = 80 turns V_{L(P)} = 6 kV Calculate the following: 4.7.1 Turns ratio 4.7.2 Primary phase voltage 4.7.3 Secondary phase voltage 4.7.4 Secondary line voltage 4.8 With reference to Faraday's law, describe how mutual induction occurs in a transformer.

NSC Electrical Technology Power Systems Transformer Applications: QUESTION 4: THREE-PHASE TRANSFOR

QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 List THREE external conditions that may cause transformer failure - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

Question 4

QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 List THREE external conditions that may cause transformer failure. 4.2 Explain what would happen if an earth fault occurs ... show full transcript

Worked Solution & Example Answer:QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 List THREE external conditions that may cause transformer failure - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

Step 1

4.1 List THREE external conditions that may cause transformer failure.

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Answer

Overload: Operating a transformer above its rated capacity can lead to overheating and eventual failure.
Lightning: Electrical surges caused by lightning can damage transformer insulation and components.
Poor ventilation: Insufficient airflow can prevent adequate cooling, leading to overheating and failure.

Step 2

4.2 Explain what would happen if an earth fault occurs in one of the three phases of a protected transformer.

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In the event of an earth fault, the phases may become unbalanced, leading to differences in voltage between the phases. This imbalance would activate the protective relay, isolating the transformer from the power supply to prevent damage.

Step 3

4.3 Describe how an increase in the load would affect the magneto motive force in the primary windings.

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As the load increases, the current drawn from the secondary windings will also increase, which causes the magneto motive force in the secondary windings to rise. Consequently, the primary magneto motive force will need to adjust to balance the load, leading to an increase in the magneto motive force in the primary windings.

Step 4

4.4 Name TWO types of cooling methods for a dry transformer.

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Air natural cooling (AN): Uses natural airflow to cool the transformer.
Air forced cooling (AF): Utilizes fans to increase the airflow over the transformer for more effective cooling.

Step 5

4.5 State why the output power of a transformer is slightly less than the input power.

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During the transformation process, some energy is lost as heat due to resistance in the windings and other losses in the core. Therefore, the output power is always slightly less than the input power.

Step 6

4.6 Describe the construction of a three-phase core-type transformer.

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A three-phase core-type transformer consists of three sets of windings wound around a common iron core. The core forms a closed magnetic circuit that improves efficiency and reduces losses. The winding arrangement is typically designed to maximize magnetic coupling and minimize leakage inductance.

Step 7

4.7.1 Turns ratio

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

$TR = \frac{N_p}{N_s} = \frac{600}{80} = 7.5 : 1$

Step 8

4.7.2 Primary phase voltage

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The primary phase voltage can be calculated as:

$V_{ph} = \frac{V_{L(P)}}{\sqrt{3}} = \frac{6000}{\sqrt{3}} \approx 3464 V$

Step 9

4.7.3 Secondary phase voltage

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The secondary phase voltage can be calculated using the turns ratio:

$V_{ph(s)} = V_{ph} \times \frac{N_s}{N_p} = 3464 \times \frac{80}{600} \approx 461.88 V$

Step 10

4.7.4 Secondary line voltage

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The secondary line voltage is given by:

$V_L = V_{ph} \times \sqrt{3} \approx 461.88 \times \sqrt{3} \approx 800 V$

Step 11

4.8 With reference to Faraday's law, describe how mutual induction occurs in a transformer.

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Mutual induction occurs when the magnetic field created in the primary windings induces a voltage in the secondary windings. According to Faraday's law, the induced voltage is proportional to the rate of change of magnetic flux. Thus, when current flows in the primary winding, it creates a changing magnetic field that couples with the secondary winding, inducing a voltage according to the equation:

$V_L = N \frac{d\Phi}{dt}$

QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 List THREE external conditions that may cause transformer failure - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

Worked Solution & Example Answer:QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 List THREE external conditions that may cause transformer failure - NSC Electrical Technology Power Systems - Question 4 - 2019 - Paper 1

4.1 List THREE external conditions that may cause transformer failure.

4.2 Explain what would happen if an earth fault occurs in one of the three phases of a protected transformer.

4.3 Describe how an increase in the load would affect the magneto motive force in the primary windings.

4.4 Name TWO types of cooling methods for a dry transformer.

4.5 State why the output power of a transformer is slightly less than the input power.

4.6 Describe the construction of a three-phase core-type transformer.

4.7.1 Turns ratio

4.7.2 Primary phase voltage

4.7.3 Secondary phase voltage

4.7.4 Secondary line voltage

4.8 With reference to Faraday's law, describe how mutual induction occurs in a transformer.

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