Functions
2.25 Inverse-Time Undervoltage Protection (ANSI 27)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
191
Figure 2-79 Logic Diagram of the Inverse-Time Undervoltage Protection
2.25.2 Setting Notes
General
The inverse-time undervoltage protection is only effective and available if this function was set during protective
function configuration (Section 2.4, address 144, INV.UNDERVOLT. is set to Enabled. If the function is not
required Disabled is set. Address 4401 INV. UNDERVOLT. serves to switch the function ON or OFF or to
block only the trip command (Block relay).
Settings
It must be considered that the positive phase-sequence voltages and thus also the pickup thresholds are eval-
uated as phase-to-phase quantities (terminal voltage · √3).
There are no clear cut procedures on how to set the pickup values. But since the protection is used mainly to
protect consumers (induction machines) from the consequences of voltage drops, and to prevent loss of sta-
bility, the value is normally set to approx. 75 % of the nominal machine voltage, i.e. address 4402 Up< PICKUP
is set to 75 V. In exceptional cases, where the voltage drop during startup is too large, it may be necessary to
set the protection to lower values. The time multiplier 4403 T MUL must be selected such that voltage drops
which would cause unstable operation are reliably disconnected. On the other hand, the time delay must be
large enough to avoid disconnections during permissible short-time voltage dips.
If required, the tripping time can also be extended by an additional time stage 4404 T Up<.
All setting times are additional time delays which do not include the operating times (measuring time, drop-out
time) of the protective function.
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