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2 Functions
2.11.2 Undervoltage protection
Undervoltage
Figure 2-58 depicts the logic diagram of the phase voltage stages. The fundamental
Phase–Earth
frequency is numerically filtered from each of the three measuring voltages so that har-
monics 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 und-
ervoltage protection this results in a pickup state being present if the voltage trans-
formers 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 con-
dition, 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
2.15.1) 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 automatically 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 dis-
connected 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.
158
7SD610 Manual
C53000-G1176-C145-4
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