Knee Point Voltage of Current Transformer

Knee Point Voltage of Current Transformer

This is the significance of saturation level of a CT core mainly used for protection purposes. The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%. The CT core is made of CRGO steel. It has its won saturation level. The EMF induced in the CT secondary windings is
E₂ = 4.44φfT₂ Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T₂ is the number of turns of the secondary winding. The flux in the core, is produced by excitation current I_e. We have a non-liner relationship between excitation current and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation current. This non-liner relation curve is also called B - H curve. Again from the equation above, it is found that, secondary voltage of a current transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage and excitation current as shown below. It is clear from the curve that, linear relation between V and I_e is maintained from point A and K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′.