Siemens 7SC80 Manual page 174

Functions
2.5 Single-Phase Overcurrent Protection
[fohochimpp-1a-20120514, 1, en_US]
Please bear in mind that when choosing a higher pickup value Ι
doing so, power loss will increase significantly.
The varistor B (see following figure) must be dimensioned such that it remains high-resistive until reaching
knee-point voltage, e.g.
approx. 100 V for 5 A CT,
approx. 500 V for 1 A CT.
[sj6x-ueb-einph-hochimpedanz3-141103, 1, en_US]
Figure 2-47
Even with an unfavorable external circuit, the maximum voltage peaks should not exceed 2 kV for safety
reasons.
If several varistors are switched in parallel for performance reasons, preference should be given to types with a
flat characteristic to avoid unbalanced load. We therefore recommend the following types from METROSIL:
600A/S1/S256 (k = 450, β = 0.25)
600A/S1/S1088 (k = 900, β = 0.25)
The pickup value (0.1 A or 0.05 A in the example) is set in address 2706 50 1Ph-1 PICKUP in the device.
The 50-2 element is not required (address 2703 50 1Ph-2 PICKUP = ∞ ).
The trip command of the protection can be delayed via address 2707 50 1Ph-1 DELAY. Normally, such
delay is set to 0.
If a higher number of CTs is connected in parallel, e.g. as busbar protection with several feeders, the magnet-
izing currents of the transformers connected in parallel cannot be neglected anymore. In this case, the
magnetizing currents at half the knee-point voltage (corresponds to the setting value) have to be summed up.
These magnetizing currents reduce the current through the resistor R. Therefore the actual pickup value will
be correspondingly higher.
Application as Tank leakage Protection
The use as tank protection requires a sensitive input transformer to be available at device input Ι
case, only the pickup value for 1-phase overcurrent protection is set at the 7SC80 device for the current at
input Ι .
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The tank leakage protection is a sensitive overcurrent protection which detects the leakage current between
the isolated transformer tank and ground. Its sensitivity is set in address 2706 50 1Ph-1 PICKUP. The 50-2
element is not required (address 2703 50 1Ph-2 PICKUP = ∞).
The trip command of the element can be delayed in address 2707 50 1Ph-1 DELAY. It is normally set to 0.
174
Connection diagram of the ground fault differential protection according to the high-impe-
dance principle
, the resistance must be decreased and, in
pu
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
. In this
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