Siemens siprotec 7SA6 User Manual page 288

Functions
6-106
Figure 6-64 illustrates the functional principle. The tripping time depends on the level
of the zero sequence voltage U
voltage increases 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.
A further time stage 7 UHY 8LQY provokes non-directional tripping with a
voltage-independent delay. This stage can be set above the directional stage. When
tripping with this stage it is, however, a prerequisite that the time of the voltage-
controlled stage has already expired (without directional check). In case the zero
voltage is too low or the voltage transformer circuit-breaker is tripped, this stage is also
disabled.
The function of the zero sequence voltage time protection can be blocked by the
distance protection. This gives the selective fault detection, carried out by the distance
protection with preference over the tripping by the zero sequence voltage time
protection.
The zero sequence voltage time protection can also be blocked during a single-pole
auto-reclosure cycle. This prevents from false measurement caused by the zero-
sequence values then appearing.
If the device operates with external automatic reclosure or if single-pole tripping is
possible via a different protection (operating in parallel), the zero sequence voltage
time protection must be blocked during the single-pole open condition via a binary
input.
. For meshed earthed systems the zero sequence
0
7SA6 Manual
C53000-G1176-C133-1