ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-148
E. Setting instructions
Settings:
Impedance Z-Setting
Delay Delay
The underimpedance function serves as back-up protection for
phase faults on the generator/transformer unit. It is faster and
more sensitive than the overcurrent protection. Its disadvantage
is that the zone of protection is shorter than the differential pro-
tection, which serves as the main protection.
The underimpedance scheme is connected to the c.t’s at the
generator star-point and to the v.t’s at the generator terminals.
The underimpedance operating characteristic is a circle in the
R/X plane, whereby the origin represents the location of the v.t’s.
The zone of protection covers the generator windings the cables
and the step-up transformer.
The setting of the underimpedance function is determined by the
short-circuit reactance of the step-up transformer. Otherwise, the
distance between the step-up transformer and the HV circuit-
breaker is mostly too short to be able to discriminate reliably with
the impedance setting between faults in the generator/trans-
former unit zone and faults on the other side of the HV circuit-
breaker. The impedance is thus set to 70 % of the transformer
impedance which at least includes the transformer winding on
the generator side in the zone of protection.
The setting of the underimpedance function is referred to rated
voltage and current.
The impedance of the protected zone is determined by the short-
circuit reactance of the step-up transformer and is given by:
z
1
= 0.7 x
T
[p.u.]
The impedance seen by the underimpedance function depends
on the c.t and v.t. ratios K
i
and K
u
and the rated data of step-up
transformer and protection:
K
I
I
i
N
N2
=
1
K
U
U
u
N
N2
=
1
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