Decision Making in Protective Relays - Digital Signal Processing in Power System Protection and Control - page 212

Checking of
trip conditions
on the Diff-
Bias plane

Calculation of
diff and bias
currents

Trip
decision

Excluding
inrush
conditions

AND

Excluding
stationary
overexcitation

OR

Blocking
signal

Excluding
CT saturation
for ext. faults

Fig. 10.11 Asserting the tripping decision in transformer differential protection

• current transformer errors due to saturation in transients and in steady sates
during internal short circuits at the transformer terminals may greatly distort
waveshapes of the differential currents.

Whatever the reason of an inadvertent tripping, it brings the following
consequences. First, the power system stability margin may be drastically reduced,
what may lead to the developing disturbance. Second, the operating personnel
becomes confused, suspecting an internal fault in the transformer, what may cause
hesitation whether to reclose the transformer or not.
To prevent malfunction of the differential relays the standard ways of per-
centage and harmonic restraint are adopted [ 14 ]. In most of the relays, if the
second harmonic ratio measured in the differential current exceeds 15-20% of the
fundamental harmonic level, the relay is blocked. It is sufficient in preventing
maloperation during external short circuits, and in the majority of inrush cases.
The situations of stationary overexcitation can be excluded through analysis of the
fifth harmonic content in the differential current. If the ratio of fifth harmonic to the
first harmonic is higher than 30%, the relay should remain blocked. Checking, if
there was no CT saturation during external fault, can also be done by applying the
so-called

''add-on stabilization'' [ 6 ], where the course of trajectory on the

Diff-Bias plane is observed to discriminate the event.
Whatever specific criteria and algorithms are adopted, the blocking signal of the
relay results from aggregation of sub-decisions for protection stabilization against
above-mentioned inrush, external faults with CT saturation and overexcitation
conditions. Simple logic with OR, AND blocks, as presented in Fig. 10.11 ,is
sufficient to assert the final protection decision. Transformer tripping is not per-
mitted, if the blocking signal coming from any of the three stabilization units is
issued.