In a current transformer (CT), the burden refers to the total impedance (resistance and reactance) presented by the connected measuring devices, such as ammeters, relays, or meters, to the secondary winding of the CT. It is usually expressed in volt-amperes (VA) at a specific secondary current (typically 1A or 5A) and can include the impedance of the connecting wires and instruments.
Key Points:
- Burden Components: The burden consists of the impedance of the connected devices (like meters, protection relays, and wires) which draw power from the CT’s secondary winding.
- Burden Rating:
- CTs are rated for a maximum burden in VA. For example, a CT rated for 10 VA means that the connected load should not exceed 10 VA.
- If the burden exceeds the CT’s rated value, it can cause errors in current measurement or protection malfunctions because the CT may not be able to maintain its accuracy or properly step down the current.
- Importance of Proper Burden:
- The burden affects the accuracy of the CT. If the burden is too high (i.e., the impedance of the connected devices is too large), the CT will not provide an accurate current transformation, leading to incorrect measurements or protection relay malfunctions.
- On the other hand, if the burden is too low, the CT may operate closer to saturation, which also leads to inaccuracies.
- Practical Consideration:
- When installing a CT, it is crucial to calculate the burden of all connected devices and ensure it falls within the CT’s rated burden for proper performance.
In summary, the burden is the load imposed on the secondary winding of the current transformer, and ensuring it is within the CT’s rated capacity is essential for accurate current measurement and safe operation.
