Functions
2.6 Negative Sequence Protection 46
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
113
Dropout for ANSI Curves
When using an ANSI curve it can be selected whether the dropout of the element is to occur instantaneously
or whether dropout is to be performed by means of the disk emulation mechanism. „Instantaneous“ means that
the drop out will occur when a 95 % of the pickup value is reached. For a new pickup the time counter 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. This ensures a proper simulation of the temperature rise of the
protected object even for extremely fluctuating unbalanced load values. Reset begins as soon as 90 % of the
setting value is reached, 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 dec-
rementing process is in idle state.
Disk emulation offers advantages when the behavior of the negative sequence protection must be coordinated
with other relays in the system based on electromagnetic measuring principles.
Logic
The following figure shows the logic diagram for the negative sequence protection function. The protection may
be blocked via a binary input. This resets pickup and time elements and clears measured values.
When the negative sequence protection criteria are no longer satisfied (i.e. all phase currents below 0.1 x I
Nom
or at least one phase current is greater than 10 x I
Nom
) all pickups issued by the negative sequence protection
function are reset.
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