Current Error
The error which a transformer introduces into the measurement of a current and which arises from the effect that the actual transformation ratio is not equal to the rated transformation ratio.
The current error expressed as a percentage is given by
current error %= (Kn*Is-Ip)x100/Ip
where,
Kn=rated transformation ratio.
Ip=actual primary current
Is= actual secondary current when Ip is flowing under the conditions of measurement.
Example,
Ip=100
100/5=20
(20x4.98-100)x100/100=current error= 4%
Phase Displacement
The displacement in phase between the primary and secondary current vectors, the direction of vectors being so chosen that the angle is zero for a perfect transformer.
The phase displacement is said to be positive when the secondary current vector leads the primary current vector and negative when it lags behind the primary current vector. It is usually expressed in minutes or in cent radians.
