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This formula applies only for almost ideal earthing or neutral transformers. If necessary, the measuring result
from the primary tests must be set as FACTOR R SEF. For this the inserted fault resistance (trip stage) is related
to the measured secondary fault resistance.
The primary fault resistance should be set between 1 and 2 kΩ for the trip stage and to between approx. 3 and
8 kΩ for the alarm stage. The default delay times have proven to be practical.
Example:
Loading resistance
Voltage divider
Miniature CT
The transformation ratio of the miniature CT 400 A:5 A has been halved to 200A:5 A by passing the primary
conductor twice through the transformer window.
For FACTOR R SEF the following value results:
If a generator-side fault resistance of 1000 Ω is selected for the trip stage R<<, a resistance value of R<< SEF
TRIP = 1000 Ω/8.33 = 120 Ω is set in address 5303. For the warning stage, a primary resistance of 3 kΩ yields
a setting value of R< SEF ALARM = 360 Ω.
Earth Current Stage
The earth current stage has a backup protection function. It is set to a protected zone of approx. 80 %. Refer-
enced to the maximum secondary fault current, the pickup threshold is at approx. 20 %, and the setting value
is calculated as follows:
The delay time T SEF TRIP (address 5305), which is also relevant for the earth current stage, must be less
than the tolerable time of the loading transformer (in this example 50 A for 20 s). The overload capability of the
earthing or neutral transformer must also be considered if it lies below that of the loading resistor.
Monitoring
At addresses 5307 and 5308 the monitoring thresholds are set with U20 MIN and I20 MIN. If the 20 Hz
voltage drops below the pickup value without the 20 Hz current rising, there must be a problem with the 20 Hz
connection. The default settings will be adequate for most applications. In applications where the loading re-
sistor is less than 1 Ω, the threshold U20 MIN must be reduced to 0.5 V. The current threshold I20 MIN may
be left at 10 mA.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
2.31 100-%-Stator Earth Fault Protection with 20 Hz Voltage Injection (ANSI 64G - 100%)
R
L
VD
Ratio
CT
Ratio
10 Ω
(10 A continuously, 50 A for 20 s)
500 V/200 V
200 A/5 A
Functions
227
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