Question

A three-phase 66/11 kV, star-delta connected transformer is protected by Merz-Price protection. The CTs on the LT side have a ratio of 420/5. Find the ratios of the CTs on the HT side. Draw the neat labeled connection diagram of the complete scheme.

Answer 1

To find the ratios of the current transformers (CTs) on the high tension (HT) side of a three-phase star-delta connected transformer protected by Merz-Price protection, we'll first need to understand the relationship between the primary and secondary voltages and currents in this transformer configuration.

### Given Information:
- Transformer ratings: 66 kV / 11 kV
- CT ratio on the LT side: \( 420/5 \)

### Steps to Determine CT Ratios on the HT Side:

1. **Transformer Voltage Ratio:**
   The voltage ratio of the transformer can be calculated using:
   \[
   \text{Voltage Ratio} = \frac{V_{HT}}{V_{LT}} = \frac{66 \text{ kV}}{11 \text{ kV}} = 6
   \]

2. **Current Ratio:**
   The current in the secondary (LT side) will be inversely proportional to the voltage in the primary (HT side). Therefore, the CT ratio on the HT side can be calculated as follows:
   \[
   \text{CT Ratio on HT side} = \text{CT Ratio on LT side} \times \text{Voltage Ratio}
   \]
   Where:
   - CT Ratio on LT side = \( \frac{420}{5} \)
   - Voltage Ratio = 6

3. **Calculate HT CT Ratio:**
   \[
   \text{CT Ratio on HT side} = \frac{420}{5} \times 6 = \frac{420 \times 6}{5} = \frac{2520}{5} = 504
   \]

### CT Ratios:
- **CT Ratio on HT side**: \( 504/5 \)

### Connection Diagram:
In a Merz-Price protection scheme, two sets of CTs are typically used to measure the line currents. Here is a simplified explanation of the connection:

1. **HT Side:**
   - Connect the HT CTs (504/5) in each phase (R, Y, B).
   - These CTs are connected to a relay for measuring the phase currents.

2. **LT Side:**
   - Connect the LT CTs (420/5) in each phase (R, Y, B).
   - These CTs are also connected to a relay for monitoring the phase currents.

3. **Relay Configuration:**
   - The outputs of the CTs from both sides feed into a Merz-Price relay which calculates the difference and provides protection against earth faults.

### Neat Labeled Connection Diagram
I'll provide a description of the connection diagram as I can't draw here directly. You can create a diagram based on this description.

#### Diagram Description:
- Draw three horizontal lines to represent the three phases: R (Red), Y (Yellow), and B (Blue).
- On the left, label the lines as HT side (66 kV).
- On the right, label the lines as LT side (11 kV).
- Place the CTs on both sides:
  - For HT side, label each CT as \( CT_{HT} (504/5) \) in series with each phase.
  - For LT side, label each CT as \( CT_{LT} (420/5) \) in series with each phase.
- Connect the output of each CT to a relay labeled as "Merz-Price Relay".
- Draw the connections between the CTs to the respective relays ensuring the connections represent the star-delta transformation.

This layout captures the fundamental aspects of the Merz-Price protection scheme for a star-delta connected transformer. If you need an actual diagram, I can guide you through drawing one.

Answer 2

To determine the ratios of the CTs on the HT side and draw the connection diagram for the Merz-Price protection scheme, follow these steps:

### 1. **Calculating the CT Ratios on the HT Side**

The Merz-Price protection scheme is used for transformer protection and involves CTs on both HT and LT sides. The protection system generally requires that the CT ratios on the HT and LT sides maintain the same ratio as the voltage ratio of the transformer.

**Given Data:**
- Transformer Voltage Ratio: 66 kV (HT) / 11 kV (LT)
- CT Ratio on LT Side: 420/5

**Steps:**

1. **Determine the Voltage Ratio:**
   \[
   \text{Voltage Ratio} = \frac{\text{HT Voltage}}{\text{LT Voltage}} = \frac{66\, \text{kV}}{11\, \text{kV}} = 6
   \]

2. **Find the HT CT Ratio:**
   The CT ratio on the HT side should be such that the ratio of HT CT to LT CT is the same as the voltage ratio.

   Given:
   \[
   \text{CT Ratio on LT Side} = \frac{420}{5} = 84
   \]
   Therefore, the CT ratio on HT side should be:
   \[
   \text{HT CT Ratio} = \text{LT CT Ratio} \times \text{Voltage Ratio} = 84 \times 6 = 504
   \]

   So, the CT ratio on the HT side should be 504/5.

### 2. **Connection Diagram of the Merz-Price Protection Scheme**

Here is a simplified description of the connection diagram:

**Diagram Components:**
- **Transformer:** Star-Delta Connected
- **CTs:** Current Transformers on HT and LT sides
- **Relay:** Differential Relay for Merz-Price Protection
- **Busbars:** For connecting CTs and the relay

**Connection Diagram Description:**

1. **Transformer:**
   - HT Side: Star (Y) connection
   - LT Side: Delta (Δ) connection

2. **CTs:**
   - **HT Side CTs:** Connected in a Star configuration (one CT per phase)
   - **LT Side CTs:** Connected in a Delta configuration (one CT per phase)

3. **Relay:**
   - The relay is connected to the secondary terminals of the HT side CTs and LT side CTs. The differential relay compares the currents from both sides. If there’s a significant difference (indicative of a fault), the relay will trip the transformer circuit.

**Diagram:**

```
HT Side                           LT Side
   |                                |
   |                                |
   |                                |
   +----[CT1]----[Relay]----[CT4]----+
   |                                |
   |                                |
   |                                |
   +----[CT2]----[Relay]----[CT5]----+
   |                                |
   |                                |
   |                                |
   +----[CT3]----[Relay]----[CT6]----+
   |                                |
   |                                |
   +-------------------[Transformer]--+
   |                                |
   |                                |
   +---[HT Busbar]      [LT Busbar]---+
```

**Key Points:**
- **CT1, CT2, CT3:** HT side CTs connected in Star configuration.
- **CT4, CT5, CT6:** LT side CTs connected in Delta configuration.
- **Relay:** Differential relay connected to the CTs' secondary windings.

This diagram helps in ensuring that the currents are measured accurately and compared for any discrepancies which might indicate a fault condition.