2 Functions
158
7SA522 Manual
C53000-G1176-C155-3
The power-time characteristic can be displaced in power direction via a reference
value S
ref
(= basic value for the inverse characteristic for ϕ = ϕ
comp
) and in time direc-
tion by a factor k.
Figure 2-67 shows the logic diagram. The tripping time depends on the level of the
compensated zero sequence power S
r
as defined above. For meshed earthed
systems the zero sequence voltage and the zero sequence current increase towards
the earth fault location. The inverse characteristic results in the shortest command
time for the relay closest to the fault. The other relays then reset.
Figure 2-67 Zero-sequence power protection
Phase Current
Stabilization
Non-symmetrical load conditions in multiple-earthed systems or different current
transformer errors can result in a zero sequence current. This zero sequence current
could cause faulty pickup of the earth current stages if low pick-up thresholds are set.
To avoid this, the earth current stages are stabilized by the phase current: As the
phase currents increase, the pick up thresholds are increased (Figure 2-68). The sta-
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