To calculate the Current Transformer (CT) ratio on the 22kV side of a three-phase transformer, you need to consider the relationship between the line currents on both sides of the transformer and the connection types (Star/Delta in this case).
### Step 1: Understand the Transformer Connections
- **Primary Side (220V):** Star (Y) connection.
- **Secondary Side (22kV):** Delta (Δ) connection.
### Step 2: Determine the Line Current Ratio
The line current ratio between the primary and secondary sides of the transformer can be found using the voltage transformation ratio. For a star-delta connected transformer, the line voltage ratio is:
\[
\text{Voltage Ratio} = \frac{V_{\text{secondary}}}{V_{\text{primary}}} = \frac{22\,\text{kV}}{220\,\text{V}} = \frac{22000\,\text{V}}{220\,\text{V}} = 100
\]
For a star-delta transformer, the line current ratio is the inverse of the voltage ratio:
\[
\text{Current Ratio} = \frac{1}{\text{Voltage Ratio}} = \frac{1}{100} = 0.01
\]
### Step 3: Apply the Current Ratio to the CT Ratio
On the 220V side, the CT ratio is given as 400/5.
This means the actual line current on the 220V side is \(I_{L1} = 400 \times \frac{5}{400} = 5\,\text{A}\).
The corresponding line current on the 22kV side is:
\[
I_{L2} = I_{L1} \times \frac{1}{\text{Current Ratio}} = 5\,\text{A} \times 100 = 500\,\text{A}
\]
### Step 4: Determine the CT Ratio on the 22kV Side
To have the same secondary current (5 A) on the 22kV side, the CT ratio should be:
\[
\text{CT Ratio on 22kV Side} = \frac{500\,\text{A}}{5\,\text{A}} = 100/1
\]
Therefore, the CT ratio on the 22kV side should be **100/1**.