Siemens siprotec SJ62 User Manual page 133

2 Functions
131
Calculation Example:
For 5-A CT as above
desired pickup value I
pu
For 1-A CT as above
desired pickup value I
pu
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
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 2-42 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 reasons.
= 0.1 A (equivalent to 16 A primary)
= 0.05 A (equivalent to 40 A primary)
, the resistance must
pu
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015