297
FUNCTION CHARACTERISTICS
To avoid heavy switch operations, the open command related to 78 protection trip com-
mand is delayed as long as following conditions are not present:
• δ lower δ
TRIP
, the value is defined by user and typically lower than 90°
• Negative value of Z measured real component
Figures E1 and E2 show 78 function state related to Z trajectory in R-X plane in case of out
of step with sliding in External (Fig. E2) and Internal (Fig. E3) area.
Out of step protection is unaffected by electrical system failure events and is able to
reliably recognize out of step events from stable system swings, avoiding unwanted trips.
In order to assure protection stability, a functional block for 78 protection is foreseen in
case of:
• Inverse sequence current larger than a settable threshold value (protection stability
must be unaffected by asymmetrical phenomena eg single-phase faults, two-phase
faults)
• Absence of direct cyclic sense current
• Voltage close to nominal values (out of step phenomena are characterized by voltage
levels lower than nominal value)
• δ angle frequency oscillation out of range 0.2Hz and 8Hz (typical oscillation frequency
for out of step phenomena is included in that range).
XMR-D EQUIPMENT MANUAL
Ed. 2.9 - 02/2021
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