Functions
2.21 Undervoltage Protection (ANSI 27)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
176
Figure 2-73 Logic diagram of the undervoltage protection
2.21.2 Setting Notes
General
The undervoltage protection is only effective and available if this function was set during protective function
configuration (Section 2.4, address 140, UNDERVOLTAGE is set to Enabled). If the function is not required
Disabled is set. Address 4001 UNDERVOLTAGE serves to switch the function ON or OFF or to block only the
trip command (Block relay).
Settings
It must be noted that the positive phase-sequence voltages and thus also the pickup thresholds are evaluated
as phase-to-phase quantities (terminal voltage ·√3
). The first undervoltage protection stage is typically set to
about 75% of the nominal machine voltage, i.e. address 4002 U< is set to 75 V. The user must select a value
for the 4003 T U< time setting that ensures that voltage dips which would affect operating stability are discon-
nected. On the other hand, the time delay must be large enough to avoid disconnections during admissible
short-time voltage dips.
For the second stage, a lower pickup threshold 4004 U<< e.g. = 65 V should be combined with a shorter trip
time 4005 T U<< e.g. = 0.5 s to achieve an approximate adaptation to the stability behaviour of the consumers.
All setting times are additional time delays which do not include the operating times (measuring time, dropout
time) of the protective function.
The drop-out ratio can be adapted in small steps to the operating conditions at address 4006 DOUT RATIO.
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