The concept of burden in relation to current transformers and its impact on the overall measurement accuracy

In the context of current transformers (CTs), the concept of "burden" refers to the electrical load connected to the secondary winding of the CT. The burden is the combination of the impedance of the connected circuit or equipment, including the wires, resistors, and other components that form the secondary circuit of the CT. Understanding the burden is essential for accurate current measurement, as it can significantly impact the overall measurement accuracy. Here's how it works:

1. Measurement Principle:

CTs are used to measure the current in a primary circuit by transforming it into a proportional current in the secondary circuit. The secondary winding of the CT is connected to measuring instruments, protective relays, or other equipment.

2. Burden Rating:

CTs are typically rated for a specific burden, which is expressed in ohms (Ω) and is the maximum load that the CT can accommodate while maintaining accurate current measurement. The burden rating is often specified on the CT's nameplate.

3. Impact on Accuracy:

If the burden connected to the CT exceeds its rated value, it can lead to measurement inaccuracies. This is because the additional impedance of the burden affects the CT's ability to accurately reproduce the primary current in the secondary circuit.

The burden introduces voltage drops, phase shifts, and other electrical effects that can distort the measurement.

4. Voltage Output:

The burden's impedance affects the voltage across the CT's secondary winding. The higher the burden, the greater the voltage drop, which can lead to measurement errors if not taken into account.

5. Ratio Correction:

Some CTs have multiple taps or are adjustable to allow for different burden values. This feature enables ratio correction, where the CT's turns ratio can be adjusted to compensate for the burden's impact on accuracy.

6. Careful Selection:

When selecting a CT for a specific application, it's crucial to consider the burden requirements of the connected equipment. Using a CT with the appropriate burden rating is essential to maintain measurement accuracy.

Oversized CTs with lower burden ratings than required for the application can result in voltage saturation and may lead to accuracy issues.

7. Impedance Matching:

Proper impedance matching between the Current Transformers and the connected equipment helps maintain accurate current measurements. Using matching resistors or other components can achieve this.

8. Testing and Calibration:

Periodic testing and calibration of CTs with their connected burdens are essential to verify their accuracy. Calibration ensures that the CT accurately reproduces the primary current in the secondary circuit, accounting for the burden's impact.