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Check Using
Network Earth Fault
7UM62 Manual
C53000-G1176-C149-3
With the earth fault bridge in place, the resistance stages of the 100-% protection
(warning and trip stage) must pick up immediately on switching in the supply voltage
of the 20 Hz generator.
To check the pickup behaviour of the current stage SEF100 I>>, read out the mea-
sured value I SEF from the operational measured values at approx. 10 % to 20 % of
the displacement voltage. The value thus obtained should be about the same as the
pickup value SEF I>> selected at address 5306. This ensures that the current stage
of the 100-% stator earth fault protection covers a protection zone of about 80 % to
90 % of the winding in addition to the 100 % resistance calculation.
With the primary plant voltage-free and earthed, install a single-pole earth fault bridge
on the primary side of the unit transformer.
DANGER!
Primary measurements may only be carried out with the generator at stand–still
on disconnected and grounded equipment of the power system.
Caution!
The starpoints of the unit transformer must not be connected to earth during this test!
Start up the generator and slowly excite to 30 % of rated machine voltage (max. 60 %).
The 100-% and the 90-% stator earth fault protection do not pick up.
The checks to be performed for the 90-% stator earth fault protection are described in
Section 3.4.6.1, side title "Checking for Generator Earth Fault".
For the 100-% stator earth fault protection, read out the operational measured value
I SEF. This value is extrapolated to approx. 1.3 times the rated machine voltage. The
current thus extrapolated should not exceed half the pick-up value SEF I>> (address
5306) in order to achieve the desired safety margin of the current stage of the 100-%
stator earth fault protection.
Shut down and de-excite the generator. Remove earth fault bridge.
If the starpoint of the high-voltage side of the unit transformer is to be earthed
during normal operation, re-establish starpoint earthing.
If the 20 Hz generator is to be fed by the voltage transformers of the machine termi-
nals, the appropriate connections, or a different type of supply connection (e.g. DC
voltage supply from a battery) must now be permanently made.
If no more special tests are to be made, switch the 100-% stator earth fault protection
to be operative: address 5301 100% SEF-PROT. = ON.
3.4 Primary Commissioning Tests with the Generator
383
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