5.1 Name TWO parts of the cooling system in three-phase transformers - NSC Electrical Technology Power Systems - Question 5 - 2023 - Paper 1

The type of loss that contributes most to heat in three-phase transformers is Copper losses, also known as I²R losses.

A star-delta transformer is commonly used in high voltage supplies to reduce voltage levels in transmission networks.

Eddy current losses are reduced by constructing the core out of thin laminations, which reduces the loops that allow eddy currents to form.

Heat in transformers should be dissipated to maintain the temperature within safe limits, preventing degradation of the insulation material. Excessive heat can lead to operational inefficiencies and failures.

The preferred cooling method for very large transformers is Oil forced; Water forced (OFWF).

The Buchholz relay monitors the gas formation inside the oil of a transformer. It sounds an alarm if gas forms due to internal faults and isolates the transformer from the supply if the gas formation exceeds a certain level.

The primary line current is calculated using the formula:

$I_{L1} = \frac{S}{\sqrt{3} V_{L1}}$

Substituting the values:

$I_{L1} = \frac{100000 \text{ VA}}{\sqrt{3} \times 11000 \text{ V}} = 5.25 \text{ A}$

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

$\frac{N_1}{N_2} = \frac{V_{PH1}}{V_{PH2}}$

Therefore:

$V_{PH2} = \frac{V_{L1}}{\sqrt{3} \times TR} = \frac{11000}{48} = 229.17 \text{ V}$

The active (true) power can be calculated by:

$P = S \times pf = 100000 \text{ VA} \times 0.9 = 90000 \text{ W} = 90 \text{ kW}$

The diagram should represent a three-phase delta connection for the primary side and a star connection for the secondary side, annotated appropriately with labels.

This is a step-down transformer since the transformer ratio of 48:1 indicates that the primary voltage is reduced to a lower secondary voltage.

In a step-down transformer, the primary voltage is reduced while the current increases, maintaining power conservation as per the formula:

$P_{primary} = P_{secondary}$

Thus, as voltage decreases, current increases to ensure that power remains constant across the transformer.