Functions
2.19 Undervoltage and Overvoltage Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
368
2.19.2
Undervoltage protection
Undervoltage Phase-to-earth
Figure 2-177 depicts the logic diagram of the phase voltage stages. The fundamental component is numerically
filtered from each of the three measuring voltages so that harmonics or transient voltage peaks are largely elim-
inated. Two threshold stages
Uph-e<
(address
3752
) and
Uph-e<<
(address
3754
) are compared with the
voltages. If the phase voltage falls below a threshold it is indicated in a phase-segregated way. Furthermore, a
general pickup indication
„Uph-e< Pickup“
and
„Uph-e<< Pickup“
is given. The drop-out to pickup ratio
can be set (
Uph-e<(<) RESET
, address
3759
).
Every stage starts a time delay which is common to all phases. The expiry of the respective time delay
T Uph-
e<
(address
3753
) or
T Uph-e<<
(address
3755
) is signalled and usually 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<
, address
3758
) to achieve this result. Undervoltage will then only be detected if, together with the un-
dervoltage condition, 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-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 Mon-
itor“, also see Section 2.24.1) or if the trip of the mcb of the voltage transformers is indicated (internal blocking)
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 the stages do
not respond to the undervoltage of the disconnected phase when voltage transformers are located on the out-
going side. Only such stages are blocked during the single-pole dead time that can actually generate a trip
command according to their setting.
