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Section 4
1MAC050144-MB C
Protection functions
116
615 series ANSI
Technical Manual
Figure 47:
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 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 General function
block features section in this manual.
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