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Functions
2.15 Earth Current Differential Protection (ANSI 87GN,TN)
Figure 2-51
• Phase current monitoring
To exclude spurious tripping due to CT saturation in the presence of external faults, the protection function
is blocked as soon as a maximum phase current is reached. For this purpose, the phase currents of side 1
are monitored. As soon as one phase current exceeds the threshold, blocking takes effect. This blocking is
not a drawback, since high-current faults are sufficiently managed by other protection functions such as dif-
ferential protection, impedance protection and overcurrent protection.
• Zero sequence voltage monitoring
Where the phase current transformers model zero sequence currents on the secondary side after load ad-
dition, and where there is no direct evaluation of the starpoint current, zero sequence voltage monitoring
should be used. It also provides additional restraint in the presence of external faults without earth involve-
ment. The zero sequence voltage is calculated from the phase-to-earth voltages. On detection of a zero se-
quence voltage, an enable signal is issued.
Logic
The logic interconnection of all signals and the most important settings, as well as the indications output, are
shown in the following logic diagram (Figure ). The function can be blocked with the input „>BLOCK REF".
Using the CFC, this input also allows blocking of other features such as if the measured zero sequence voltage
is to be injected via the U
in V connection (open delta connection).
The following figure shows the blocking of the phase currents and their release on the basis of the calculated
zero voltage. This is followed by the monitoring of the operating characteristic with possibly an additional query
of the starpoint current, and the angle enabling. When all conditions are met, the earth current differential pro-
tection picks up. The subsequent timer T I-REF> is usually set to zero.
132
Operating ranges of the direction criterion
input. This is necessary if the voltage inputs are connected to a voltage transformer
E
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
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