Functions
2.6 Voltage Protection 27, 59
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
143
2.6.3
Undervoltage Protection 27
Function
Undervoltage protection consists of two definite time elements (
27-1 PICKUP
and
27-2 PICKUP
). Therefore,
tripping can be time-graded depending on how severe voltage collapses are. Voltage thresholds and time
delays can be set individually for both elements.
The dropout ratio for the two undervoltage elements (= V
dropout value
/V
pickup value
) can be set.
The undervoltage protection works in an additional frequency range. This ensures that the protection function
is preserved even when it is applied e.g. as motor protection within the context of decelerating motors. Howev-
er, the true r.m.s. value of the positive-sequence voltage component is considered too small when severe fre-
quency deviations exist. This function therefore exhibits an overfunction. If applications are anticipated in which
the frequency range of f
Nom
±
10 %, will be exceeded, the current criterion will not give a correct result and must
be switched off.
Figure 2-46 shows a typical voltage profile during a fault for source side connection of the voltage transformers.
Because full voltage is present after the circuit breaker has been opened, current supervision CS described
above is not necessary in this case. After the voltage has dropped below the pickup setting, tripping is initiated
after time delay
27-1 DELAY
. As long as the voltage remains below the dropout setting, reclosing is blocked.
Only after the fault has been cleared, i.e. when the voltage increases above the dropout level, the element
drops out and allows reclosing of the circuit breaker.
Figure 2-46
Typical fault profile for source side connection of the voltage transformer (without current su-
pervision)
Figure 2-47 shows a fault profile for a load side connection of the voltage transformers. When the circuit breaker
is open, the voltage disappears (the voltage remains below the pickup setting), and current supervision is used
to ensure that pickup drops out after the circuit breaker has opened (
BkrClosed I MIN
).
After the voltage has dropped below the pickup setting, tripping is initiated after time delay
27-1 DELAY
. When
the circuit breaker opens, voltage decreases to zero and undervoltage pickup is maintained. The current value
also decreases to zero so that current criterion is reset as soon as the release threshold (
BkrClosed I MIN
)
is exceeded. Thanks to the AND-combination of voltage and current criteria, pickup of the protection function
is also reset. As a consequence, energization is admitted anew when the minimum command time has elapsed.
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