ABB REG650 Technical Manual page 124

Section 6
Differential protection
118
IEC06000430-2-en.vsd
IEC06000430 V2 EN
Figure 46: 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 power to the prime
mover interrupted.
GENPDIF (87G) function always uses reference (default) directions of CTs towards
the protected generator as shown in figure 46. 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 46, 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 with advantage 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 felt (until they develop into more serious faults)
by the "usual" differential protection based 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
nd th
and the 2 and 5 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 502 043-UUS B
Technical Manual