6.3.6 Monitored data
PID-3551-MONITOREDDATA v6
Table 69: GENPDIF (87G) Monitored data
Name Type Values (Range) Unit Description
IDMAG_A REAL - A Fund. freq. differential current, phase A; in
primary A
IDMAG_B REAL - A Fund. freq. differential current, phase B; in
primary A
IDMAG_C REAL - A Fund. freq. differential current, phase C; in
primary A
IDMAG_NS REAL - A Negative Sequence Differential current; in
primary Amperes
IBIAS REAL - A Magnitude of the common Bias current; in
primary Amperes
6.3.7 Operation principle
SEMOD156685-1 v3
SEMOD155649-4 v7
The task of Generator differential protection GENPDIF (87G) is to determine whether a fault is
within the protected zone, or outside the protected zone. The protected zone is delimited by the
position of current transformers, as shown in figure
51.
IEC06000430 V2 EN-US
Figure 51: 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
51. 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 51, 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
1MRK 502 066-UUS B Section 6
Differential protection
153
Technical manual
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