Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.7.4.6
946
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 OPERATE output" mode,
the function operates normally but the OPERATE 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
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 , although this provides no substantial rise in the time setting. In some
n
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.
1MRS759142 C
Figure 512. In this case, with an
, the time setting should
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Technical Manual
REX640