Knee of the Curve

Current transformers are used to measure alternating current levels. They produce a current in their secondary which is accurately proportional to but smaller than the current flowing through its primary. The current transformer presents a negligible load to the primary circuit, while providing a secondary current small enough for safety within instruments and protective relays.

Current transformers are subject to knee-point core-saturation voltage effects; these must be allowed for as they limit the range for the transformers’ accuracy. The knee-point voltage is the magnitude of the CT’s secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy limits.

In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the knee point is reached. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals.

The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much higher but constrained within a very small range of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times rated current.