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

Dielectric oil serves two primary functions in transformers:

• It acts as a non-conductor of electricity, providing electrical insulation between the internal components of the transformer.
• It helps in cooling the transformer by facilitating the dissipation of heat generated during operation, thus preventing overheating.

The Buchholz relay is located in-line between the conservator and the transformer housing. This placement allows it to detect gas and oil movement, signaling potential faults within the transformer.

The three-phase delta-star step-down transformer can be depicted as follows:

• In the delta configuration, the three primary windings are connected in a loop.
• The secondary windings are connected in a star configuration with a common neutral point. This arrangement provides the necessary voltage transformation from high to low.

To calculate the output power (P_out) of the transformer, use the formula:

$P_{out} = S imes ext{p.f.} = 10,000 imes 0.8 = 8,000 ext{ W}$

Thus, the output power is 8,000 W.

To find the efficiency (η) of the transformer:

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

Substituting the values:

• Losses = Copper loss + Core loss = 300 W + 50 W = 350 W

Therefore,

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

The efficiency of the transformer is 95.8%.

To calculate the apparent power (S) of the transformer:

$S(P_{app}) = \sqrt{3} \times V_{L} \times I_{L} = \sqrt{3} \times 6,000 \times 2$

= 20,784.61 VA or approximately 20.78 kVA.

To find the power factor (cos φ) of the load:

$cos φ = \frac{P}{S} = \frac{18,000}{20,784.61} = 0.87$

Therefore, the power factor of the load is 0.87.

For a delta connected transformer:

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

Thus, primary phase voltage:

$V_{PH} = \frac{V_{L}}{\sqrt{3}} = \frac{6,000}{\sqrt{3}} = 3,464.1 V$

To calculate the turns ratio (TR) between primary and secondary:

$TR = \frac{V_{PH(1)}}{V_{PH(2)}} = \frac{6,000}{240} = 25$

Hence, the turns ratio is 25.