4.6.1
Systems with out rectifiers or frequency
converters.
If a fault between the primary and secondary
winding in a transformer occurs. the voltage
level of the secondary network can be exposed
to the voltage level of the primary
winding. This can cause extensive damage to
the low voltage network.
AfIashover protection consisting of an
instantaneous voltage relay type RXEG 21 or
RXEL 24 connected to a voltage transformer
between the neutral point of the low voltage
system and ground can be used to trip the
circuit breakers. The voltage relay is normally
set at 1 .5-2 times the phase voltage and no
delay of the relay is allowed.
No rectifiers or frequency converters are
allowed to be connected to the low voltage
system. The voltage transformer will get saturated
by dc from the rectifiers or converters
and damaged if a ground-fault occurs on the
dc-system.
This type of flashover protection can only be
used if the ground-fault current on the
primary side of the power transformer is
limited to about 25 A. The condition has to be
fulfilled to be able to use a voltagedependent
resistor connected in parailei with
the primary side of the voltage transformer in
the grounded neutral. The resistor is used to
limit the voltage across the relay to max 2 kV
in order to protect the low voltage system and
the relay against excessive overvoltage. See
tig 16.
A delayed residual voltage relay with RXEG
21 or RXEL 24 can be connected in paralIei
with the flashover relay The relay is normally
set for operation at 20-40% of the phase