Undervoltage Protection 27 - Siemens SIPROTEC 7SK80 Manual

2.5.3

Undervoltage Protection 27

Function
Undervoltage protection consists of two definite time elements (27-1 PICKUP and 27-2 PICKUP). Therefore,
tripping can be time-coordinated 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
Like the other protection functions, the undervoltage protection operates in an extended frequency range. This
ensures that the protection function is ensured even for the protection of decelerating motors, for instance.
However, the r.m.s. value of the positive voltage component is considered too small when the frequency devi-
ates considerably so that the device will tend to overfunction.
Figure 2-27 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. After the fault has been cleared, i.e. when the voltage increases above the
dropout threshold, the element will drop out and allow closing of the circuit breaker.
Figure 2-27
Figure 2-28 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. Pickup of the protection function is also reset by the action of the AND-combination of voltage and
current. As a consequence, energization is admitted anew when the minimum command time elapsed.
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
Typical fault profile for supply-side connection of the voltage transformers (without current su-
pervision)
/V ) can be set.
dropout value pickup value
Functions
2.5 Voltage Protection 27, 59
103