Question

A three-phase, 66/11KV, star-delta connected transformer is protected by the Merz-Price system. The CT’s on the LV side have a ratio of 400/5. Find the ratio of CT’s on the HV side.

Answer 1

To find the ratio of the Current Transformers (CTs) on the High Voltage (HV) side of a three-phase, 66/11 kV transformer protected by the Merz-Price system, follow these steps:

### 1. **Understand the Transformer and Protection System**

- **Transformer Ratings:** 66 kV (HV side) / 11 kV (LV side)
- **CT Ratio on LV Side:** 400/5

The Merz-Price protection system is used for the protection of transformers against internal faults. It involves the use of CTs on both sides of the transformer to measure the currents. The CTs on the HV side and the LV side should be chosen such that their ratios are compatible with the system’s protection and measurement requirements.

### 2. **Calculate the Transformation Ratio of the Transformer**

The transformer has a voltage ratio of:
\[ \text{Voltage Ratio} = \frac{\text{HV Voltage}}{\text{LV Voltage}} = \frac{66 \text{kV}}{11 \text{kV}} = 6 \]

This means that the HV side voltage is 6 times the LV side voltage.

### 3. **Relate the CT Ratios**

The CTs on both sides should have a ratio that reflects the transformation ratio of the transformer. This is to ensure that the current values measured on the HV side and LV side are compatible for protection calculations.

The CT ratio on the LV side is 400/5, which implies:
\[ \text{CT Ratio}_{\text{LV}} = \frac{400}{5} = 80 \]

The CTs on the HV side must be chosen such that their ratio is adjusted by the same factor as the voltage transformation ratio. Therefore, the ratio of the CTs on the HV side (\( \text{CT Ratio}_{\text{HV}} \)) should be:
\[ \text{CT Ratio}_{\text{HV}} = \text{CT Ratio}_{\text{LV}} \times \text{Voltage Ratio} \]
\[ \text{CT Ratio}_{\text{HV}} = 80 \times 6 = 480 \]

### 4. **Verify the CT Ratio**

To ensure consistency, we can double-check by converting the CT ratios for different sides into a common scale.

On the LV side, if the CT ratio is 400/5, the primary current to secondary current ratio is 80.

On the HV side, the CT ratio should be 480 to reflect the same primary to secondary current ratio, keeping in line with the transformer’s voltage ratio.

### **Conclusion**

The CT ratio on the HV side of the transformer should be 480/5 to match the LV side CT ratio, in line with the transformer's voltage ratio.

Answer 2

To determine the ratio of the Current Transformers (CTs) on the High Voltage (HV) side of a Three-Phase, 66/11 kV, Star-Delta connected transformer protected by the Merz-Price system, we need to understand how the CT ratios on both sides of the transformer relate to each other.

### Background

The Merz-Price system is a type of differential protection used for transformers. For this system to work correctly, the CT ratios on the High Voltage (HV) side and the Low Voltage (LV) side must be coordinated. In general, the CT ratio on the HV side is selected so that the currents measured by the CTs on both sides of the transformer are comparable, considering the transformer’s turns ratio.

### Given Data
- Transformer ratings: 66 kV (HV) / 11 kV (LV)
- LV CT ratio: 400/5

### Steps to Find the HV CT Ratio

1. **Determine the Transformer Turns Ratio:**

   The turns ratio of the transformer can be calculated using the rated voltages of the HV and LV sides. The turns ratio is given by:

   \[
   \text{Turns Ratio} = \frac{\text{HV Voltage}}{\text{LV Voltage}} = \frac{66\, \text{kV}}{11\, \text{kV}} = 6
   \]

2. **Relate the CT Ratios:**

   In a transformer protected by the Merz-Price system, the ratio of the CTs on the HV side to those on the LV side should be the same as the turns ratio of the transformer. This ensures that the differential protection operates correctly. Therefore, the CT ratio on the HV side (\( \text{HV CT Ratio} \)) can be found by multiplying the LV CT ratio by the turns ratio of the transformer:

   \[
   \text{HV CT Ratio} = \text{LV CT Ratio} \times \text{Turns Ratio}
   \]

   The given LV CT ratio is 400/5. We need to find the ratio on the HV side:

   \[
   \text{HV CT Ratio} = \left(\frac{400}{5}\right) \times 6
   \]

   Simplify the calculation:

   \[
   \text{HV CT Ratio} = 80 \times 6 = 480
   \]

   Therefore, the HV CT ratio should be 480/5.

### Summary

The ratio of the Current Transformers on the High Voltage (HV) side should be 480/5 to ensure proper differential protection coordination with the given LV CT ratio and the transformer’s turns ratio.