ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-134
Typical settings:
V-Setting 0.05 U
N
Delay 0.5 s
Note:
If a generator circuit-breaker is installed between the generator
and the step-up transformer, a second E/F protection scheme is
required for the zone between the step-up transformer and the
unit transformer. The second scheme is connected to the broken
delta secondary windings of three v.t’s. This scheme must also
remain stable for E/F’s on the HV system and during ferroreso-
nance phenomena and for this reason there is a resistor across
the broken delta as well. Frequently, the E/F protection is only
required to protect the cables and bar conductors, because the
transformers are protected by differential schemes and Buchholz
relays. In this case, the E/F protection setting is determined by
the voltage offset for an E/F at the lowest load voltage. A typical
setting for reliable E/F detection in an ungrounded system is
60 % U
N
, i.e.
V-Setting = 0.6 U
N
The delay can remain the same at 0.5 s. The second E/F
scheme usually gives only an alarm. Since the plant remains in
operation for an E/F on the cables, the resistor across the bro-
ken delta must be continuously rated.
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