To solve this problem, let's first understand the situation:

• The transformer is a three-phase transformer with a primary voltage (High Tension, HT) of 33 kV and a secondary voltage (Low Tension, LT) of 6.6 kV.
• The transformer is connected in a star/delta configuration.
• The CTs on the LT side have a ratio of 300/5 A.

Step 1: Calculate the turns ratio of the transformer

The voltage ratio (turns ratio) between the primary and secondary side of the transformer is given by: $\text{Turns ratio} = \frac{V_{HT}}{V_{LT}} = \frac{33\text{ kV}}{6.6\text{ kV}} = 5$

Step 2: Calculate the current ratio on the HT side

The turns ratio of the transformer affects the current in such a way that the current on the high tension side will be lower by the same ratio. Therefore: $\text{Current ratio on HT side} = \frac{I_{LT}}{I_{HT}} = 5$

Step 3: Determine the CT ratio on the HT side

Given the CT ratio on the LT side is 300/5 A, and the transformer current ratio is 5, we calculate the corresponding CT ratio on the HT side: $\text{CT ratio on HT side} = \frac{300 \text{ A}}{5} = 60 \text{ A}$

Step 4: Account for the star/delta connection

Because the transformer is star/delta connected, the current on the delta (LT) side must be multiplied by $\sqrt{3}$ to convert it into the equivalent star side (HT) current for proper differential protection.

Thus, the correct CT ratio on the HT side should be: $\frac{60}{\sqrt{3}} : 5 \text{ A}$

But in practice, the CT ratio would typically be given as 60:5√3 A to account for the star/delta transformation.

Final Answer:

The correct CT ratio on the HT side should be 60:5√3 A.

Would you like more details or have any questions?

Here are some related questions to explore further:

1. How does the star/delta connection affect the phase currents?
2. What is the significance of the differential protection scheme in transformers?
3. How is the CT ratio selected for protection schemes?
4. Why is it necessary to consider the $\sqrt{3}$ factor in the CT ratio for a star/delta transformer?
5. What are the typical causes of differential protection tripping in transformers?

Tip: When working with three-phase transformers, always consider the effect of the connection type (star, delta) on the current and voltage ratios.