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 the most to heat in three-phase transformers is copper losses, which arise due to the resistance in the windings.

A star-delta transformer is commonly used in high voltage supplies to step down voltage levels, ensuring safe distribution to industrial and residential loads.

Eddy current losses are reduced in transformers by constructing the core out of thin laminations of silicon steel, which minimizes the circulating currents.

Heat should be dissipated in transformers to maintain the operational efficiency and reliability of the equipment. Excessive heat can degrade insulation materials, leading to failures, reduced lifespan, and potential safety hazards.

The preferred cooling method for very large transformers is oil forced, water forced (OFWF) cooling, where oil circulates through radiators to enhance heat dissipation.

The Buchholz relay monitors the gas formation inside the oil of a transformer. It serves to sound an alarm when gas is formed, isolate the transformer when gas exceeds a certain level, and protect the transformer from damage.

To calculate the primary line current ($I_{L1}$):

$I_{L1} = \frac{S}{\sqrt{3} V_{L1}} = \frac{100\, \text{kVA}}{\sqrt{3} \cdot 11\, \text{kV}} \approx 5.25\, \text{A}$

To determine the secondary phase voltage ($V_{PH2}$):

Using the transformer ratio: $\frac{N_{1}}{N_{2}} = \frac{V_{PH1}}{V_{PH2}}$

Calculating: $V_{PH2} = \frac{V_{L1}}{N_{R}} = \frac{11\, \text{kV}}{48} \approx 229.17\, \text{V}$

The active (true) power ($P$) can be calculated as:

$P = S \cdot \text{pf} = 100000 \cdot 0.9 = 90\, \text{kW}$

Here is a simple diagrammatic representation of the transformer coils:

L1    L1    L2    L2    L3    L3
  |------|    |------|    |------|
  |      |    |      |    |      |
  N      N    N      N    N      N

In this representation, the lines indicate connections and the primary coils are denoted accordingly.

This is a step-down transformer because the winding ratio is 48:1, meaning the primary voltage is higher than the secondary voltage.

In a step-down transformer, the voltage is reduced while the current is increased to maintain power conservation. This is a consequence of the transformer equation, where the power input equals the power output, leading to the relationship: $P_{in} = P_{out} \Rightarrow V_{in}I_{in} = V_{out}I_{out}$