Functions
2.8 Undervoltage and Overvoltage Protection 27/59 (Optional)
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
121
2.8.2 Undervoltage Protection (ANSI 27)
Undervoltage Phase–Ground
Figure 2-50 depicts the logic diagram of the phase voltage elements. 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 elements 27-1-Vph PICKUP (address 3752) and 27-2-Vph PICKUP (address
3754) are compared with the voltages. If phase voltage falls below a threshold it is indicated phase-segregat-
ed. Furthermore, a general pickup indication „27-1-Vpg Pickup“ „27-2-Vpg Pickup“ exists for each
element. The dropout ratio can be set (27-Vph RESET, address 3759).
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 behavior of the undervoltage protection when the line is
de-energized. 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 protection 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 criterion CURR.SUP 27-Vph, address
3758) 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-to-ground can be blocked via a binary input „27-Vphg BLK“. The ele-
ments of the undervoltage protection are then automatically blocked if a voltage failure is detected („fuse failure
monitor“, also see Section 2.14.1) or if the trip of the mcb of the voltage transformers is indicated (internal block-
ing) via the binary input „>FAIL:Feeder VT“.
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