What is the significance of burden in a CT?

Short Answer:

The burden in a Current Transformer (CT) refers to the total load connected to its secondary winding, expressed in volt-amperes (VA). It includes the impedance of connected meters, relays, and the connecting wires. The burden is important because it affects the accuracy and performance of the CT.

If the burden exceeds the rated value, it may cause errors in current measurement, overheating, or even failure of the CT. Maintaining the burden within its specified limit ensures that the CT delivers accurate, safe, and reliable readings.

Detailed Explanation:

Significance of burden in a CT

A Current Transformer (CT) is designed to reduce high currents to a lower, measurable value for instruments and protective devices. The burden is a critical factor in its proper functioning. It affects both the efficiency and accuracy of the CT. Understanding the concept and significance of burden helps ensure that the CT operates within safe and precise limits, especially in power monitoring and protection systems.

Definition of Burden:

  • The burden is defined as the total impedance (resistance + reactance) connected across the secondary winding of the CT.
  • It is usually expressed in volt-amperes (VA) at a given power factor, often 0.8 lagging.
  • It includes:
    • The connected measuring device (meter or relay)
    • The length and resistance of secondary leads
    • Terminal connections

Why Burden is Important:

  1. Accuracy of Measurement:
    • CTs are calibrated for a certain burden.
    • If the burden is too high, the CT may not provide an accurate secondary current, causing ratio error and phase error.
  2. Safety:
    • High burden can lead to voltage build-up on the secondary side if the circuit opens.
    • This may result in electric shock, equipment damage, or insulation failure.
  3. CT Saturation:
    • Exceeding the burden causes magnetic core saturation.
    • In saturated conditions, the CT cannot reproduce the input current properly, especially during fault conditions, which affects protective relay operation.
  4. Relay Operation:
    • Protective relays depend on accurate secondary current for detecting faults.
    • A burden beyond rated value may lead to delayed or failed tripping, compromising system protection.

How to Manage Burden:

  1. Use Shorter and Thicker Cables:
    • Reduces wire resistance, keeping the burden within limit.
  2. Proper CT Selection:
    • Choose a CT with a burden rating higher than the total connected load.
  3. Avoid Unused Devices:
    • Only connect necessary instruments to minimize unnecessary burden.
  4. Regular Testing and Maintenance:
    • Periodically check the CT secondary loop for resistance increase due to ageing or corrosion.
  5. Use of Burden Calculations:
    • Before installation, calculate the total VA of all secondary devices and wiring to match CT specifications.
Conclusion:

The burden in a CT plays a major role in ensuring accurate measurement, system safety, and correct relay operation. If the total burden connected to the CT secondary exceeds its rated limit, it can cause errors, failures, and unsafe conditions. Therefore, managing and maintaining the correct burden is essential for reliable and safe operation of power system monitoring and protection devices.