Refer to FIGURE 5.1 below and answer the questions that follow. 5.1.1 State whether this is a core-type or shell-type three-phase transformer. 5.1.2 State ONE advantage of a shell-type transformer over a core-type transformer. 5.1.3 State the configuration of the primary and secondary windings in FIGURE 5.1 above. 5.1.4 Briefly describe the principle of operation of the three-phase transformer in FIGURE 5.1 above. 5.2 With reference to transformers, answer the following questions. 5.2.1 Describe the effect that an increase in the load will have on the primary current of a transformer. 5.2.2 Name ONE function of the dielectric oil used in transformers. 5.2.3 Name TWO types of losses other than copper losses that occur in transformers. 5.2.4 Name and explain TWO cooling methods used for dry transformers. 5.2.5 Compare conductor sizes between a single-phase transformer and a three-phase transformer for the same power. 5.3 Given: A 300 kW delta-connected load with a power factor of 0.87 that is connected to a delta-star transformer. The primary and secondary line voltages of the transformer are 8 kV and 600 V respectively. Assume that the transformer is 100% efficient. Answer the questions that follow. 5.3.1 Calculate the following: 5.3.1 Line current of the load 5.3.2 Phase current of the load 5.3.3 Primary line current 5.4 Protection devices used with three-phase transformers are categorised in two categories, relay and switches. Name TWO protection relays used in three-phase transformers.

NSC Electrical Technology Power Systems Transformer Applications: Refer to FIGURE 5.1 below and an

Refer to FIGURE 5.1 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 5 - 2024 - Paper 1

Question 5

Refer to FIGURE 5.1 below and answer the questions that follow. 5.1.1 State whether this is a core-type or shell-type three-phase transformer. 5.1.2 State ONE adva... show full transcript

Worked Solution & Example Answer:Refer to FIGURE 5.1 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 5 - 2024 - Paper 1

Step 1

5.1.1 State whether this is a core-type or shell-type three-phase transformer.

96%

114 rated

Answer

This is a core-type three-phase transformer.

Step 2

5.1.2 State ONE advantage of a shell-type transformer over a core-type transformer.

99%

104 rated

Answer

One advantage of a shell-type transformer is that it is less prone to magnetic stray loss compared to a core-type transformer.

Step 3

5.1.3 State the configuration of the primary and secondary windings in FIGURE 5.1 above.

96%

101 rated

Answer

The configuration of the primary and secondary windings in the transformer is star-delta (or delta-star) configuration.

Step 4

5.1.4 Briefly describe the principle of operation of the three-phase transformer in FIGURE 5.1 above.

98%

120 rated

Answer

When an alternating voltage is applied to the primary windings of the transformer, an alternating magnetic flux is established in the core. This flux links with the secondary windings, inducing an electromotive force (e.m.f.) of the same frequency. When the load is connected to the secondary windings, a current flows through it. Power is thus transferred magnetically from the primary to the secondary windings.

Step 5

5.2.1 Describe the effect that an increase in the load will have on the primary current of a transformer.

97%

117 rated

Answer

An increase in the load will increase the secondary current, resulting in a more prominent primary current drawn from the supply.

Step 6

5.2.2 Name ONE function of the dielectric oil used in transformers.

97%

121 rated

Answer

The dielectric oil provides electrical insulation between the windings and the case, helping to provide cooling and preventing breakdown of the windings.

Step 7

5.2.3 Name TWO types of losses other than copper losses that occur in transformers.

96%

114 rated

Answer

The two types of losses include hysteresis losses and eddy current losses.

Step 8

5.2.4 Name and explain TWO cooling methods used for dry transformers.

99%

104 rated

Answer

Two cooling methods include:

Air Natural: Transformers are cooled by natural air flow around the transformer windings.
Air Forced: Transformers are cooled by air being forced through a fan above the windings.

Step 9

5.2.5 Compare conductor sizes between a single-phase transformer and a three-phase transformer for the same power.

96%

101 rated

Answer

The conductor size in a single-phase transformer is larger than that required for a three-phase transformer. For the same power, the conductor size for a three-phase transformer is 75% of what is needed for a single-phase transformer.

Step 10

5.3.1 Calculate the following: Line current of the load

98%

120 rated

Answer

To calculate the line current, we use the formula: $I_L = \frac{P_{tr}}{\sqrt{3} \times V_{L1} \times \cos \phi}$ By substituting the known values: $I_L = \frac{300 \times 10^3}{\sqrt{3} \times 600 \times 0.87} = 331.82 A$

Step 11

5.3.2 Phase current of the load

97%

117 rated

Answer

The phase current can be calculated using the formula: $I_{ph} = \frac{I_L}{\sqrt{3}}$ Substituting the values gives: $I_{ph} = \frac{331.83}{\sqrt{3}} = 191.58 A$

Step 12

5.3.3 Primary line current

97%

121 rated

Answer

To find the primary line current, we use the formula: $I_{L1} = \frac{P}{\sqrt{3} \times V_{L1} \times \cos \phi}$ Substituting in the values: $I_{L1} = \frac{300 \times 10^3}{\sqrt{3} \times 8 \times 10^3 \times 0.87} = 24.89 A$

Step 13

5.4 Protection devices used with three-phase transformers are categorised in two categories, relay and switches. Name TWO protection relays used in three-phase transformers.

96%

114 rated

Answer

Two protection relays used in three-phase transformers are:

Buchholz relay (B)
Restricted fault relay (REF)

Refer to FIGURE 5.1 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 5 - 2024 - Paper 1

Worked Solution & Example Answer:Refer to FIGURE 5.1 below and answer the questions that follow - NSC Electrical Technology Power Systems - Question 5 - 2024 - Paper 1

5.1.1 State whether this is a core-type or shell-type three-phase transformer.

5.1.2 State ONE advantage of a shell-type transformer over a core-type transformer.

5.1.3 State the configuration of the primary and secondary windings in FIGURE 5.1 above.

5.1.4 Briefly describe the principle of operation of the three-phase transformer in FIGURE 5.1 above.

5.2.1 Describe the effect that an increase in the load will have on the primary current of a transformer.

5.2.2 Name ONE function of the dielectric oil used in transformers.

5.2.3 Name TWO types of losses other than copper losses that occur in transformers.

5.2.4 Name and explain TWO cooling methods used for dry transformers.

5.2.5 Compare conductor sizes between a single-phase transformer and a three-phase transformer for the same power.

5.3.1 Calculate the following: Line current of the load

5.3.2 Phase current of the load

5.3.3 Primary line current

5.4 Protection devices used with three-phase transformers are categorised in two categories, relay and switches. Name TWO protection relays used in three-phase transformers.

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