1 Answer
The size of a **Current Transformer (CT)** is generally determined based on a few key factors related to the application and the system it is monitoring. Here’s how you can calculate or choose the correct size:
### 1. **Nominal Current Rating (Primary Side)**
The primary current rating is the amount of current that the CT will measure in the system. This is typically based on the maximum current expected in the circuit, and CTs are usually rated in multiples of 5A or 1A on the secondary side.
- **Primary Current (I₁):** This is the current on the high-voltage (primary) side of the system, which the CT will step down to a manageable level on the secondary side.
- **Secondary Current (I₂):** This is typically 5A or 1A (standard CT ratings), which is what will be fed to the measurement or protection device.
### 2. **Burden (Load on the Secondary Side)**
The burden is the impedance (in ohms) connected to the secondary side of the CT. It includes the measurement or protection device and any wiring. The burden limits the CT’s accuracy and performance, and it needs to be matched with the CT’s rated secondary current.
**Formula for Burden**:
\[ \text{Burden} (Z) = \frac{V_{\text{secondary}}}{I_{\text{secondary}}} \]
Where:
- \( V_{\text{secondary}} \) is the voltage developed across the secondary winding of the CT.
- \( I_{\text{secondary}} \) is the rated current of the secondary side (usually 5A or 1A).
### 3. **CT Rating (Rated Current and Accuracy Class)**
When selecting a CT, it’s important to choose one with an appropriate **accuracy class** and **rated primary current** based on the application. For example:
- **Accuracy Class**: Defines how much error you can tolerate in the measurements. For example, a CT with a 0.2% accuracy class means the current measurement will have less than 0.2% error.
- **Rated Current**: This should match the system's expected primary current.
### 4. **CT Ratio (Primary Current to Secondary Current)**
The CT ratio determines how the CT reduces the primary current to the secondary current. For example, a CT with a 100:5 ratio will step down 100A on the primary side to 5A on the secondary side.
**CT Ratio = Primary Current / Secondary Current**
### 5. **Core Size (Physical Size of the CT)**
The core size is typically determined by the amount of current the CT is rated to handle. Larger currents require larger cores to ensure the CT operates without saturation.
- **Saturation** occurs when the core of the CT is unable to handle the high current, resulting in inaccurate measurements.
- A **larger core** allows the CT to handle higher currents without saturating.
### Example Calculation:
Suppose you need a CT for a system with a maximum current of **500A** on the primary side, and you want a **5A secondary side**:
- **Primary Current (I₁)** = 500A
- **Secondary Current (I₂)** = 5A
- **CT Ratio** = 500A / 5A = 100:5
Now, you also need to ensure that the CT has an adequate **accuracy class**, **burden**, and core size for the system’s needs.
So, to summarize:
- Determine the **primary current**.
- Choose a **CT ratio** based on the expected load and secondary current standard (5A or 1A).
- Ensure the **accuracy class** meets your measurement needs.
- Select the **core size** to avoid saturation at the highest expected current.
Does that help clarify how to calculate or choose the right CT size?
### 1. **Nominal Current Rating (Primary Side)**
The primary current rating is the amount of current that the CT will measure in the system. This is typically based on the maximum current expected in the circuit, and CTs are usually rated in multiples of 5A or 1A on the secondary side.
- **Primary Current (I₁):** This is the current on the high-voltage (primary) side of the system, which the CT will step down to a manageable level on the secondary side.
- **Secondary Current (I₂):** This is typically 5A or 1A (standard CT ratings), which is what will be fed to the measurement or protection device.
### 2. **Burden (Load on the Secondary Side)**
The burden is the impedance (in ohms) connected to the secondary side of the CT. It includes the measurement or protection device and any wiring. The burden limits the CT’s accuracy and performance, and it needs to be matched with the CT’s rated secondary current.
**Formula for Burden**:
\[ \text{Burden} (Z) = \frac{V_{\text{secondary}}}{I_{\text{secondary}}} \]
Where:
- \( V_{\text{secondary}} \) is the voltage developed across the secondary winding of the CT.
- \( I_{\text{secondary}} \) is the rated current of the secondary side (usually 5A or 1A).
### 3. **CT Rating (Rated Current and Accuracy Class)**
When selecting a CT, it’s important to choose one with an appropriate **accuracy class** and **rated primary current** based on the application. For example:
- **Accuracy Class**: Defines how much error you can tolerate in the measurements. For example, a CT with a 0.2% accuracy class means the current measurement will have less than 0.2% error.
- **Rated Current**: This should match the system's expected primary current.
### 4. **CT Ratio (Primary Current to Secondary Current)**
The CT ratio determines how the CT reduces the primary current to the secondary current. For example, a CT with a 100:5 ratio will step down 100A on the primary side to 5A on the secondary side.
**CT Ratio = Primary Current / Secondary Current**
### 5. **Core Size (Physical Size of the CT)**
The core size is typically determined by the amount of current the CT is rated to handle. Larger currents require larger cores to ensure the CT operates without saturation.
- **Saturation** occurs when the core of the CT is unable to handle the high current, resulting in inaccurate measurements.
- A **larger core** allows the CT to handle higher currents without saturating.
### Example Calculation:
Suppose you need a CT for a system with a maximum current of **500A** on the primary side, and you want a **5A secondary side**:
- **Primary Current (I₁)** = 500A
- **Secondary Current (I₂)** = 5A
- **CT Ratio** = 500A / 5A = 100:5
Now, you also need to ensure that the CT has an adequate **accuracy class**, **burden**, and core size for the system’s needs.
So, to summarize:
- Determine the **primary current**.
- Choose a **CT ratio** based on the expected load and secondary current standard (5A or 1A).
- Ensure the **accuracy class** meets your measurement needs.
- Select the **core size** to avoid saturation at the highest expected current.
Does that help clarify how to calculate or choose the right CT size?