After the multiplexer samples analog channels via the sample and hold (S/H) mechanism, samples from all four channels are summed up digitally, using the same equation to get a digital value Sd. The “Mean Square” of the difference between the channels is summed up in the hardware and an equivalent, computed digitally over one protection pass is supplied to the comparator. If the summed squared difference Σ(Sa – Sd)2 is greater than the adaptable per signal level error boundary, then “invalid data” is declared and protection functions using the measured data are momentarily blocked. In addition, if the error keeps repeating, a major self-test diagnostics error message is displayed on the UR. As a result of such failure, the relay’s self-test alarm contact operates and the relay is taken out of service. The relay can be rebooted and if no more failures are detected, it continues to be functional unless a new failure is detected and the relay is again taken out of service. The above analog data integrity check is performed on both analog banks (that is, four currents or four voltages) within each CT/VT input module.

In addition, these modules with enhanced diagnostics include power supply voltage rail monitoring to further monitor hardware health. This is achieved by continuous check of the power supply voltage Vdc against safe operate reference voltage Vref, which ensures that all electronic components of the CT/VT module operate in the safe operating conditions.

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.