Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
68
Figure 2-13 Logic diagram of the 51N ground overcurrent relay element
If an ANSI Curve is configured, parameter 1309 51N TIME DIAL is used instead of parameter 1308 51N
TIME DIAL.
If parameter 1313 MANUAL CLOSE is set to 51N instant. and manual close detection is used, a pickup
causes instantaneous tripping even if theElement is blocked via a binary input.
Dropout Behavior
For the ANSI or IEC characteristics, you can select whether an Element drops out instantaneously after a
threshold has been undershot or whether dropout is performed by means of disk emulation. "Instantaneous"
means that the picked-up element drops out when 95 % of the pickup value is undershot. For a new pickup,
the time delay starts at zero.
The disk emulation evokes a dropout process (timer counter is decrementing) which begins after de-energiza-
tion. This process corresponds to the reset of a Ferraris disk (explaining its denomination "disk emulation"). In
case several faults occur in succession, the "history" is taken into consideration due to the inertia of the Ferraris
disk and the time response is adapted. Reset begins as soon as 90 % of the setting value is undershot, in ac-
cordance to the dropout curve of the selected characteristic. In the range between the dropout value (95 % of
the pickup value) and 90 % of the setting value, the incrementing and the decrementing processes are in idle
state.
Disk emulation offers advantages when the overcurrent relay elements must be coordinated with conventional
electromechanical overcurrent relays located towards the source.
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