Current Transformer Saturation

The secondary current is proportional to the primary current in the linear zone, and no longer so once saturaion happens. (see knee-point voltage)

Study more here

Any CT will maintain its accurcay upto a level of primary current, generally a pre-determined multiple of primary current. Beyond this Accuracy Limit Primary Current, because of the very high flux density, the core is driven to saturation and it no more does its duty of linear conversion properly. The errors of the CT would mulitply then.

As described by posts #1 & #2.

It is useful to note that with high saturation the secondary current becomes "spikes" about zero primary current time.

Also all the effects of "flux doubling" and "flux trebling" which are featured for voltage transformers apply.