Functions
2.7 Negative Sequence Protection 46
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
153
Dropout for IEC Curves
The element drops out when the negative sequence current decreases to approx. 95% of the pickup setting.
The time delay resets immediately in anticipation of another pickup.
Dropout for ANSI Curves
When using an ANSI curve, select if dropout after pickup is instantaneous or with disk emulation. "Instanta-
neous" means that pickup drops out when the pickup value of approx. 95 % 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. 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 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 decre-
menting 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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