Siemens SIPROTEC 7SK80 Manual page 63

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
When using an ANSI or IEC curve, it can be selected whether the dropout of an Element is to occur instanta-
neously or whether dropout is to be performed by means of the disk emulation mechanism. "Instantaneously"
means that the pickup will drop out when the value falls below approx. 95 % of the pickup value. For a new
pickup the timer is restarted.
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 Fer-
raris-disk and the time response is adapted. Reset begins as soon as 90 % of the setting value is undershot,
in accordance with 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.
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
2.2 Overcurrent Protection 50, 51, 50N, 51N
Functions
63