ABB Relion Technical Manual page 142

Section 6
Differential protection
136
IEC06000430-2-en.vsd
IEC06000430 V2 EN
Figure 50: Position of current transformers; the recommended (default) orientation
If the fault is internal, the faulty generator must be quickly tripped, that is, disconnected
from the network, the field breaker tripped and the agent to the prime mover interrupted.
GENPDIF (87G) function always uses reference (default) directions of CTs towards the
protected generator as shown in figure 50. Thus, it always measures the currents on the
two sides of the generator with the same reference direction towards the generator
windings. With the orientation of CTs as in figure 50, the difference of currents flowing
in, and out, of a separate stator winding phase is simply obtained by summation of the two
currents fed to the differential protection function.
Numerical IEDs have brought a large number of advantages and new functionality to the
protective relaying. One of the benefits is the simplicity and accuracy of calculating
symmetrical components from individual phase quantities. Within the firmware of a
numerical IED, it is no more difficult to calculate negative-sequence components than it
is to calculate zero-sequence components. Diversity of operation principles integrated in
the same protection function enhances the overall performance without a significant
increase in cost.
A novelty in GENPDIF (87G), namely the negative-sequence-current-based internal-
external fault discriminator, is used advantageously in order to determine whether a fault
is internal or external. Indeed, the internal-external fault discriminator not only positively
discriminates between internal and external faults, but can independently detect minor
faults which may not be ascertained (until they develop into more serious faults) by the
"usual" differential protection based only on operate-restrain characteristic.
GENPDIF (87G) is using fundamental frequency phase current phasors and negative
sequence current phasors. These quantities are derived outside the differential protection
function block, in the general pre-processing blocks. GENPDIF (87G) is also using with
advantage the DC component of the instantaneous differential current and the 2
harmonic components of the instantaneous differential currents. The instantaneous
differential currents are calculated from the input samples of the instantaneous values of
the currents measured at both ends of the stator winding. The DC and the 2nd and 5th
harmonic components of each separate instantaneous differential current are extracted
inside the differential protection.
1MRK 502 048-UUS A
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