ABB RELION REX640 Technical Manual page 1031

1MRS759142 F
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
The
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 OPERATE output"
mode, the function operates normally but the OPERATE output is not activated.
4.7.4.6 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
an ordinary over current relay having the current setting as 1.2 · I
should be 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
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.
REX640
Technical Manual
Blocking mode setting has three blocking methods. In the "Freeze timers"
, although this provides no substantial rise in the time setting. In
n
Blocking mode .
Figure 573
Protection functions
. In this case, with
, the time setting
n
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