Application - ABB 615 series Technical Manual

Section 4
Protection functions
4.7.3.5
710
reset timer reaches the value set by Reset delay time, the operating timer resets and the
PICKUP output is deactivated.
The Timer calculates the pickup duration value PICKUP_DUR, which indicates the
percentage ratio of the start situation and the set operating time (DT). The value is
available in the Monitored data view.
Blocking logic
There are three operation modes in the blocking functionality. The operation modes are
controlled by the BLOCK input and the global setting Configuration/System/Blocking
mode which selects the blocking mode. The BLOCK input can be controlled by a binary
input, a horizontal communication input or an internal signal of the protection relay's
program. The influence of the BLOCK signal activation is preselected with the global
setting Blocking mode.
The Blocking mode setting has three blocking methods. In the "Freeze timers" mode, the
operate timer is frozen to the prevailing value. In the "Block all" mode, the whole function
is blocked and the timers are reset. In the "Block TRIP output" mode, the function operates
normally but the TRIP output is not activated.

Application

The three-phase underimpedance protection is used as a backup protection against short
circuit faults at the generator terminals or on the high-voltage side of a step-up
transformer. This function can be used either instead of the definite time voltage-
dependent overcurrent protection or to obtain a limited protection zone and the optimum
operating time instead of the non-directional overcurrent protection.
Comparison between overcurrent and underimpedance protection
Phase current for three-phase short circuit is shown in
ordinary over current relay having the current setting as 1.2 · I
less than 0.2 seconds, since with a higher value the short-circuit current decays below the
set value and the relay drops off. The current setting can also be reduced to 1.1 · I
although this provides no substantial rise in the time setting. In some situations, either of
the above current settings is appropriate, but if longer tripping times are required to
maintain the time selectivity, the underimpedance protection is needed.
1MAC059074-MB A
Figure 353. In this case, with an
, the time setting should be
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615 series ANSI
Technical Manual
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