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REG 316*4 1MRB520049-Uen / Rev. F
ABB Switzerland Ltd
All three of these influences cause a differential current, which
flows during normal system conditions. The setting for "g" must
be chosen above the level of this differential current. A typical
setting is g = 0.3 I
(i.e. 30 % I
).
N
N
Pick-up ratio v
The pick-up ratio "v" is decisive for the stability of the protection
during external phase and earth faults, i.e. in the presence of
high through-fault currents.
The value of "v" defines the ratio of the operating current to re-
straint current. The setting should be such that when operating
under load conditions, weak faults causing only a low differential
current can still be detected, but at the same time there is no risk
of false tripping during through-faults. A typical setting is v = 0.5.
Restraint current b
The restraint current b defines the point at which the character-
istic is switched.
The sloped section of the characteristic ensures that the relay
remains stable during through-faults with c.t. saturation.
The ability to switch between two different slopes enables the
characteristic to adapt to different conditions.
The recommended setting for "b" is 1.5. This provides high sta-
bility during high through-fault currents and sufficient sensitivity
to detect fault currents in the region of the operating current.
Factors a
and a
2
3
The full setting range for the factors a
and a
for compensating
2
3
current amplitude only applies, if the reference value for the c.t.
input channels is set to 1.000. At all other settings, the per-
missible upper limit reduces in proportion to the ratio of the c.t.
input channel reference values (transformer winding 1/winding 2,
respectively winding 3/winding 1).
3-33
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