Delay
The delay is used to achieve discrimination of the overcurrent function. It is set according to
the grading table for all the over-current units on the power system. The zone of protection of
the overcurrent function under consideration extends to the location of the next downstream
overcurrent relay.
Should the downstream relay fail to clear a fault, the overcurrent function trips slightly later in
a backup role.
Setting MaxMin
This parameter enables the following operating modes to be selected:
•
MIN (3ph): Pick-up when the highest phase current also falls below the setting. This
setting is not permitted for single-phase meas.
•
MIN (1ph): Pick-up when the lowest phase current falls be-low the setting.
•
MAX (3ph): Pick-up when the lowest phase current also exceeds the setting. This setting is
not permitted for single-phase measurement.
•
MAX (1ph): Pick-up when the highest phase current exceeds the setting.
•
MAX-Inrush: Blocking of inrush currents when a phase current exceeds the setting.
Operation of the inrush blocking feature (parameter
MaxMin set to MAX-Inrush)
The inrush detector picks up and blocks operation of the function when the amplitude of the
fundamental component of the current exceeds the current function setting.
The inrush detector is based on the evaluation of the second harmonic component of the
current I
2h
in relation to the fundamental frequency component I
1h
(evaluation of the
amplitudes).
The output of the function is disabled when the ratio I
2h
/I
1h
exceeds 10% and enabled again
when it falls below 8%.
There is no setting for the peak value of I
2h
/I
1h
.
The function can operate with inrush blocking in both the single and three-phase mode
(parameter
NrOfPhase).
In the three-phase mode, the phase used for evaluation is the one with the highest amplitude
at rated frequency (pick-up and inrush detection).
5.3
Inverse time overcurrent protection 51 (OC)
5.3.1
Mode of operation
GUID-FF80694B-B995-460F-9783-D6B0A852D201 v1
Overcurrent function with time delay inversely proportional to the current and definite
minimum tripping time.
5.3.2
Features
GUID-4BAE5289-4B64-4659-94DF-02DEB395E65A v1
•
Operating characteristics according to British Standard 142:
•
c = 0.02: normal inverse
•
c = 1 : very inverse and long time earth fault
•
c = 2 : extremely inverse
•
Insensitive to DC component
•
Insensitive to harmonics
Section 5
1MRK 505 406-UEN B
Bay protection functions
94
Bay protection functions REB500
Technical manual
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