Question

How is CT size calculated?

Answer 1

The size of a Current Transformer (CT) is calculated based on the primary current rating, the desired accuracy class, and the required secondary current. Here's how it's typically calculated:

### 1. **Primary Current Rating (Iₚ):**
   - The primary current is the current flowing through the conductor to which the CT is connected. It's the full-scale current that the CT is meant to measure.
   - CTs are rated for specific current values (e.g., 100A, 200A, 400A, etc.).

### 2. **Secondary Current Rating (Iₛ):**
   - The standard secondary current for most CTs is 5A or 1A.
   - The ratio between the primary and secondary currents is what defines the CT's **turns ratio**.

### 3. **CT Ratio (Turns Ratio):**
   - The CT ratio is the ratio of the primary current (Iₚ) to the secondary current (Iₛ).
   - **CT Ratio = Primary Current (Iₚ) / Secondary Current (Iₛ)**

   For example:
   - If the primary current is 100A and the secondary current is 5A, the CT ratio is **100A/5A = 20**. This means that for every 20A of current flowing in the primary, 1A flows in the secondary.

### 4. **Burden (Secondary Load):**
   - The burden is the load connected to the secondary side of the CT (e.g., meters, relays). It is typically expressed in VA (Volt-Amps).
   - The burden, combined with the CT's accuracy class, determines the sizing of the CT to ensure proper operation.

### 5. **Accuracy Class:**
   - Accuracy class defines how much error is allowed in the measurement. For example, a CT might have an accuracy class of 0.5, 1.0, or 3.0, which means the maximum permissible error is 0.5%, 1%, or 3% at full-scale current.
   - The accuracy class, along with the current rating and burden, influences the CT’s size and selection.

### 6. **Core Size:**
   - The physical size of the CT core is chosen based on the current rating and the required accuracy. Higher primary current ratings require larger cores to maintain accuracy, as larger cores allow for a higher number of turns in the winding.
   
### 7. **Design Considerations:**
   - CTs are also designed to handle specific short-circuit and fault currents. The core and winding need to be sized to handle peak currents without saturating.
   
### Example:
   If you want to size a CT for a 200A primary current with a 5A secondary and a burden of 10VA, you would:
   - Select a CT with a 200:5 ratio.
   - Check that the burden does not exceed the CT's rated burden.
   - Ensure that the accuracy class (e.g., 1.0) is met for your measurement requirements.

To summarize, the size of the CT is based on the primary current, secondary current, burden, accuracy class, and design limits. All these factors need to be considered to ensure proper operation and measurement accuracy.