Section 5
1MAC050144-MB C
Protection related functions
388
615 series ANSI
Technical Manual
The function contains a blocking functionality. It is possible to block the function outputs,
if desired.
5.2.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are Enable and Disable.
The operation of breaker failure protection can be described by using a module diagram.
All the blocks in the diagram are explained in the next sections. Also further information
on retrip and back-up trip logics is given in sub-module diagrams.
Figure 188:
Functional module diagram. I_A, I_B and I_C represent phase currents
and I
0
residual current.
Level detector 1
The measured phase currents are compared phase-wise with the set
Current value
. If the
measured value exceeds the set
Current value
, the level detector reports the exceeding of
the value to the retrip and back-up trip logics. The parameter should be set low enough so
that situations with small fault current or high load current can be detected. The setting can
be chosen in accordance with the most sensitive protection function to start the breaker
failure protection.
Level detector 2
The measured ground current is compared with the set
Current value Res
. If the measured
value exceeds the set
Current value Res
, the level detector reports the exceeding of the
value to the back-up trip logic. In high impedance grounded systems, the ground current
at phase to ground faults are normally much smaller than the short circuit currents. To
detect a breaker failure at single-phase ground faults in these systems, it is necessary to
measure the ground current separately. In effectively grounded systems, also the setting of
the ground-fault current protection can be chosen at a relatively low current level. The
CB
failure trip mode
is set "1 out of 4". The current setting should be chosen in accordance
with the setting of the sensitive ground-fault protection.
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