Internal CT secondary monitoring

The CT/VT input modules with enhanced diagnostics are monitoring internal CT circuitry integrity.

This is done to eliminate relay erroneous operation in case of CT secondary winding failure.

The secondary winding of CT is divided into two equal parts with a central tap in the middle.

CT current values from both halves of the CT secondary windings are run through

a differential amplifier to get a true current sample for the phasor calculations.

It also compares between each other by simply summing two samples together.

(Note that due to CT central tap, samples have different polarity.)

In case that values from different halves of the same CT accumulated over

one protection pass per adaptable per signal level do exceed the fixed error boundary,

failure is declared and protection is blocked.

CT secondary monitoring is performed at all four CT channels (A, B, C, and N) individually at each CT bank.