ABB Relion 670 series, Technical Manual

Page: 160 / 1432

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
nd
and 5
th
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.
6.3.7.1 Function calculation principles
SEMOD155649-14 v2
To make a differential protection as sensitive and stable as possible, the restrained differential
characteristic is used. The protection must be provided with a proportional bias, which makes the
protection operate for a certain percentage differential current related to the current through the
generator stator winding. This stabilizes the protection under through fault conditions while still
permitting the system to have good basic sensitivity. The following chapters explain how these
quantities are calculated.
The fundamental frequency phasors of the phase currents from both sides of the generator (the
neutral side and the terminal side) are delivered to the differential protection function by the pre-
processing module of the IED.
6.3.7.2 Fundamental frequency differential currents
SEMOD155649-18 v7
The fundamental frequency RMS differential current is a vectorial sum (that is, sum of
fundamental frequency phasors) of the individual phase currents from the two sides of the
protected generator. The magnitude of the fundamental frequency RMS differential current, in
phase A, is as calculated in equation 25 :
2 2
_ [(Re( )) (Im( )) ] Idiff A IAn IAt IAn IAt
= + + +
ANSIEQUATION2316 V2 EN-US
(Equation 25)
One common fundamental frequency bias current is used. The bias current is the magnitude of the
highest measured current in the protected circuit. The bias current is not allowed to drop
instantaneously, instead, it decays exponentially with a predefined time constant. These principles
make the differential IED more secure, with less risk to operate for external faults. The “maximum”
principle brings as well more meaning to the breakpoint settings of the operate-restrain
characteristic.
max( , , , , , ) Ibias IAn IBn ICn IAt IBt ICt
=
EQUATION2049-ANSI V2 EN-US
(Equation 26)
Section 6 1MRK 502 066-UUS B
Differential protection
154
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