ABB RED615 Technical Manual page 154

Section 4
Protection functions
148
A070980 V2 EN
Figure 73: Numerical overcurrent protection functionality for a typical sub-
transmission/distribution substation (feeder protection not shown).
Blocking output = digital output signal from the start of a protection
stage, Blocking in = digital input signal to block the operation of a
protection stage
The operating times of the time selective stages are very short, because the grading
margins between successive protection stages can be kept short. This is mainly due
to the advanced measuring principle allowing a certain degree of CT saturation,
good operating accuracy and short retardation times of the numerical units. So, for
example, a grading margin of 150 ms in the DT mode of operation can be used,
provided that the circuit breaker interrupting time is shorter than 60 ms.
The sensitivity and speed of the current-selective stages become as good as
possible due to the fact that the transient overreach is practically zero. Also, the
effects of switching inrush currents on the setting values can be reduced by using
the IED logic, which recognizes the transformer energizing inrush current and
blocks the operation or multiplies the current start value setting of the selected
overcurrent stage with a predefined multiplier setting.
Finally, a dependable trip of the overcurrent protection is secured by both a proper
selection of the settings and an adequate ability of the measuring transformers to
reproduce the fault current. This is important in order to maintain selectivity and
also for the protection to operate without additional time delays. For additional
information about available measuring modes and current transformer
requirements, see the Measurement modes
Radial outgoing feeder overcurrent protection
The basic requirements for feeder overcurrent protection are adequate sensitivity
and operation speed taking into account the minimum and maximum fault current
chapter in this manual.
1YHT530004D05 D
615 series
Technical Manual