The actual transformation ratio is affected by errors from three sources:
a) the difference between the inverse of the turns ratio and the rated transformation ratio;
b) the core exciting current (Ie);
c) the currents which flow in the stray capacitances associated with the windings.
In most cases, it is reasonable to assume that for a given secondary winding induced e.m.f. (Es), the error currents due to stray capacitances and core magnetization will maintain a constant value irrespective of the value of the primary energizing current. Es can theoretically be maintained at a constant value for a range of energizing currents, provided that the secondary loop impedance can be appropriately adjusted. For current transformers designed to be of the low-leakage reactance type, the secondary leakage reactance can be ignored and only the secondary winding resistance has to be considered. Thus, for any two currents l's and I"s the basic equation defining the test requirement is
given by
where R is the actual resistance of the secondary winding.
The respective error currents will be given by:
whence:
If I’s = 2I’’s, the turns ratio error is given by that.
A test at rated current with minimum secondary connected burden, followed by a test at half rated current and suitable increase in secondary loop resistance, will usually give satisfactory results.
