Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
72
Figure 2-11
Logic diagram of the inverse-time overcurrent protection element for ground
If parameter
MANUAL CLOSE
is set to
51N instant.
and manual close detection applies, the trip is initiated
as soon as the pickup conditions arrive, even if the element is blocked via binary input. The same applies to
79AR 51N instantaneous.
Dropout Behavior
When using an ANSI curve, it can be selected whether the dropout of an element is to occur instantaneously
after the threshold has been undershot or whether dropout is to be performed by means of disk emulation. "In-
stantaneous" means that pickup drops off 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 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.
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