Functions
2.5 Single-Phase Overcurrent Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
137
Calculation Example:
For the 5 A CT as above
desired pickup value I
pu
= 0.1 A (equivalent to 16 A primary)
For the 1 A CT as above
desired pickup value I
pu
= 0.05 A (equivalent to 40 A primary)
The required short-term power of the resistor is derived from the knee-point voltage and the resistance:
As this power only appears during ground faults for a short period of time, the rated power can be smaller by
approx. factor 5.
Please bear in mind that when choosing a higher pickup value I
pu
, the resistance must be decreased and, in
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.
Figure 2-44 Connection diagram of the ground fault differential protection according to the high-impedance
principle
Even with an unfavorable external circuit, the maximum voltage peaks should not exceed 2 kV for safety rea-
sons.
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