Functions
2.25 Inverse-Time Undervoltage Protection (ANSI 27)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
190
2.25 Inverse-Time Undervoltage Protection (ANSI 27)
The inverse undervoltage protection mainly protects consumers (induction machines) from the consequences
of dangerous voltage drops in island networks thus avoiding inadmissible operating conditions and possible
loss of stability. It can also be used as a criterion for load shedding in interconnected networks. Two-pole short
circuits or earth faults cause asymmetrical voltage collapse. Compared with single phase measuring systems,
the detection of the positive phase-sequence system is not influenced by these procedures and is therefore
especially useful for assessing stability problems.
2.25.1 Functional Description
Measured Quantity
For the above reasons, the positive sequence system is calculated from the fundamental waves of the three
phase-earth voltages, and fed to the protection function. After numerical filtering only the fundamental wave is
evaluated.
If voltage transformers in open delta connection (V connection) are available on the plant side, protection is
applied to the phase-to-phase voltages and the internal starpoint is left empty. A virtual starpoint is thus formed
so that the (virtual) phase-to-earth voltages can still be detected (see connection example in Appendix A.3).
Tripping characteristic
By means of a voltage-time dependent tripping characteristic protection can be matched exactly to the stability
characteristic of motors. If the motor falls below the stability characteristic, it will stall or run at substantially
reduced speed, even if full voltage is restored after a short time. Only squirrel-cage machines for which the
torque characteristic of the driven machine lies below the motor characteristic at all speeds will regain their
rated speed. All other machines will be thermally and perhaps mechanically overstressed on running down,
after return of voltage.
Undervoltage protection consists of one inverse time element. In order to avoid malfunction of the protection in
the event of secondary voltage failure, it can be blocked via a binary input, e.g. by a voltage transformer min-
iature circuit breaker or th auxilary contact of main circuit breaker when the machine is at stand-still. Also the
integrated Fuse-Failure Monitor will block the two stage (see section 2.42.2).
If no measured values are available at the device (operation condition 0), no trip signal is issued if there was
no pickup. This ensures that the undervoltage protection does not pick up immediately when it is switched on
if no measured value is available. Once the protection has been activated, it can only be deactivated again only
by blocking.
If a pickup signal is present when the device enters operating condition 0 (i.e. no measured values, or frequen-
cy outside the permissible range), it is maintained. The delay time until tripping is calculated in the same way
as for a drop to 0V. The sealed-in pickup or tripping signal can only be reset on voltage restoration, or if the
blocking input is activated.
The pickup/dropout ratio is 101 % or 0.5 V absolute of the threshold set at address 4402 Up< PICKUP. The
integral action of the tripping time determination is „frozen“ between the pickup and the dropout value.
The following figure shows the logic diagram of the inverse undervoltage protection.
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