What is the knee point voltage in a CT?

Short Answer:

The knee point voltage in a Current Transformer (CT) is the voltage at which a small increase in the secondary voltage causes a large increase in the magnetizing current. It marks the point beyond which the CT core begins to saturate, and the transformer can no longer operate accurately.

This voltage is especially important in protection class CTs, where accurate performance under high fault currents is critical. Operating beyond the knee point can cause errors in current measurement and lead to malfunction of protective relays.

Detailed Explanation:

Knee point voltage in a CT

The knee point voltage is a key specification in protection-grade current transformers, particularly those used in high-voltage and power system applications. It defines the limit of linearity of the CT’s core magnetic response. Understanding this voltage helps in selecting the right CT for fault detection and ensuring accurate relay operation during abnormal conditions like short circuits.

Definition and Behavior:

  • The knee point is the voltage at which the CT core starts to saturate significantly.
  • Beyond this point, a small rise in voltage leads to a sharp increase in magnetizing current with little or no increase in secondary current.
  • The CT no longer maintains a proportional relationship between the primary and secondary current, causing measurement errors.

Example Behavior:

  • Below the knee point: Linear increase of voltage and magnetizing current.
  • At knee point: Magnetizing current starts increasing sharply.
  • Beyond knee point: CT saturates and becomes inaccurate.

Why Knee Point Voltage is Important:

  1. Relay Protection Accuracy:
    • Protection relays rely on accurate current inputs.
    • If the CT saturates during a fault, the relay may misjudge the current and either fail to trip or trip incorrectly.
  2. System Fault Detection:
    • During high fault currents, CTs must remain linear to correctly pass secondary current to differential or overcurrent relays.
    • The knee point defines the maximum voltage CT can handle without losing accuracy.
  3. Selection Criteria for Protection CTs:
    • The knee point voltage is specified as per standards like IEC 60044-1.
    • Protection CTs are often marked with classes like 5P10, 10P20, where the number indicates how many times the rated current the CT can handle before saturation, and P stands for protection.

How Knee Point Voltage is Determined:

  • It is obtained from the magnetization curve of the CT.
  • The knee point is defined as the point on the curve where:

An increase of 10% in voltage causes more than 50% increase in magnetizing current.

  • This helps manufacturers and engineers know the exact voltage limit for linear operation.

Applications Where It Matters:

  • Differential protection schemes
  • Distance protection relays
  • Backup overcurrent protection
  • High-voltage transmission systems
  • Generator and transformer protection
Conclusion:

The knee point voltage of a CT is the voltage limit beyond which the CT core begins to saturate, and it can no longer accurately replicate the primary current. This value is crucial in protective CTs, where precision and quick response are needed during faults. Selecting CTs with an appropriate knee point voltage ensures that relays function correctly, and the system remains protected during abnormal events.