ABB 615 series Technical Manual page 227

1MAC059074-MB A
615 series ANSI
Technical Manual
A070980-ANSI V1 EN
Figure 108: Numerical overcurrent protection functionality for a typical sub-
transmission/distribution substation (feeder protection not
shown).Blocking output = digital output signal from the pickup 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 pickup 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
modes chapter in this manual.
Section 4
Protection functions
Measurement
221