Ct Saturation Detection - GE T60 Instruction Manual

T60 Transformer Protection System
Chapter 9: Theory of operation
Theory of operation
This chapter outlines advanced information on operation.
9.1 Saturation detector

9.1.1 CT saturation detection

The saturation detector of the T60 takes advantage of the fact that any CT operates correctly for a short period of time,
even under very large primary currents that would subsequently cause a very deep saturation. As a result, in the case of
an external fault, the differential current stays very low during the initial period of linear operation of the CTs while the
restraining signal develops rapidly. Once one or more CTs saturate, the differential current increases. The restraining signal,
however, yields by at least a few milliseconds. During internal faults, both the differential and restraining currents develop
simultaneously. This creates characteristic patterns for the differential - restraining trajectory, as shown.
Figure 9-1: CT saturation detection internal and external fault patterns
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 9-1