ABB RELION Series Technical Manual page 217

1MRS758755 A
REC615 and RER615
Technical Manual
A070980 V2 EN
Figure 117: 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 very low. Also, the effects of switching
inrush currents on the setting values can be reduced by using the protection relay's
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 chapter in this manual.
Section 4
Protection functions
211