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Pickup
The maximum load current during operation is the most important criterion for setting
overcurrent pickup. A pickup by an overload must be excluded! For this reason, the
3302 IMP I> pickup value must be set above the maximum (over) load current to be
expected. Recommended setting: 1.2 to 1.5 times the nominal machine current. The
pickup logic corresponds to the logic of the definite time-overcurrent protection.
If the excitation is derived from the generator terminals with the short circuit current
possibly falling below the pickup value (address 3302) due to the collapsing voltage,
the undervoltage seal-in feature of the pickup is used, i.e. address 3303 U< SEAL-
IN is switched to ON.
The undervoltage seal-in setting U< (address 3304) is set to a value just below the
lowest phase-to-phase voltage occurring during operation, e.g. to U< = 75 % to 80 %
of the nominal voltage. The seal-in time (address 3305 T-SEAL-IN) must exceed the
maximum fault clearance time in a back-up case (recommended setting: Address
3312 T END + 1 s).
Impedance Stages
The protection has the following characteristics which may be set independently:
1. Zone (fast tripping zone Z1 ) with parameters
ZONE Z1
T-Z1
Overreach zone Z1B, externally controlled via binary input, with parameters
ZONE Z1B
T-Z1B
2. Zone (zone Z2) with parameters
ZONE Z2
ZONE2 T2
Non-directional final stage with parameter
T END
As the user may assume that impedance protection measurement extends into the
unit transformer, parametrization selection must sufficiently consider the transformer
control range.
Therefore ZONE Z1 is normally set to a reach of approx. 70 % of the protected zone
(i.e. about 70 % of the transformer reactance), with no or only a small delay (i.e. T-Z1
= 0.00 s to 0.50 s). Protection then switches off faults on this distance after its operat-
ing time or with a slight time delay (high speed tripping). A time delay of 0.1 s is pre-
ferred.
For ZONE Z2 the reach could be set to about 100 % of the transformer reactance, or
in addition to a network impedance. The corresponding ZONE2 T2 time stage is to be
set so that it overreaches the power system protective equipment of the following
lines. The T END time is the last back-up time.
7UM61 Manual
C53000-G1176-C127-3
2.14 Impedance Protection (ANSI 21)
Reactance = reach,
= 0 or short delay, if required.
Reactance = reach,
T1B = 0 or short delay, if required.
Reactance = reach,
The user must select a value for T2 above the grading
time of the network protection.
The user must select T END so that the 2nd or 3rd
stage of the series-connected power system distance
protection is overreached.
97
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