Undervoltage Protection - Siemens Siprotec 7VK61 Manual

Functions
2.5 Under and over-voltage protection (optional)
2.5.2

Undervoltage protection

Undervoltage Phase–Earth
Figure 2-25 depicts the logic diagram of the phase voltage stages. The fundamental frequency is numerically
filtered from each of the three measuring voltages so that harmonics or transient voltage peaks are largely
harmless. Two threshold stages Uph-e< and Uph-e<< are compared with the voltages. If phase voltage falls
below a threshold it is indicated phase-segregated. Furthermore, a general pickup indication „Uph-e<
Pickup" „Uph-e<< Pickup" is given. The drop-out to pickup ratio can be set (Uph-e<(<) RESET).
Every stage starts a time delay which is common to all phases. Expiry of the respective time delay T Uph-e<
or T Uph-e<< is signalled and results in the trip command „Uph-e<(<) TRIP".
Depending on the configuration of the substations, the voltage transformers are located on the busbar side or
on the outgoing feeder side. This results in a different behaviour of the undervoltage protection when the line
is de-energised. While the voltage usually remains present or reappears on the busbar side after a trip
command and opening of the circuit breaker, it becomes zero on the outgoing side. For the undervoltage pro-
tection this results in a pickup state being present if the voltage transformers are on the outgoing side. If this
pickup must be reset, the current can be used as an additional criterion (current supervision CURR.SUP.
Uphe<) to achieve this result. Undervoltage will then only be detected if, together with the undervoltage condi-
tion, the minimum current PoleOpenCurrent of the corresponding phase is also exceeded. This condition is
communicated by the central function control of the device.
The undervoltage protection phase–earth can be blocked via a binary input „Uph-e<(<) BLK". The stages
of the undervoltage protection are then automatically blocked if a voltage failure is detected („Fuse–Failure–
Monitor", also see Section 2.7.1) or if the trip of the mcb of the voltage transformers is indicated (internal block-
ing) via the binary input „>FAIL:Feeder VT".
Also during a single-pole automatic reclose dead time the stages of the undervoltage protection are automati-
cally blocked in the pole open state. If necessary, the current criterion will be considered so that they do not
respond to the undervoltage of the disconnected phase when voltage transformers are located on the outgoing
side. Only such stages are blocked during the single-pole dead time that can actually generate a trip command
according to their setting.
This automatic blocking function requires the condition of the system to be detected without doubt. This means
that it is only active if all three current and voltage transformers are connected.
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SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009