Siemens 7SR18 Solkor Manual page 397

7SR18 Applications Guide
As the system is radial the phase comparator will generally not operate as the fault current and load is often of a
very similar power factor. Therefore it is essential the relay is set to ensure this minimum earth fault level is
detected by the magnitude comparator.
Secondary Currents under Fault conditions:
For a minimum earth fault of 400 A, the currents measured by the relays at either end of the feeder will be: -
Local End = 600 A + 400 A = 1000 A / 1200 = 0.833 A
Remote End = 600 A / 1200= 0.500 A
Restraint or Bias Current = (0.833 + 0.500) / 2 = 0.666A
Differential Current = 0.833 - 0.500 = 0.333 A
The measured relay point is therefore 0.333 A differential current at bias current 0.666 A.
This can be compared graphically with the relay default P/F Diff. Setting of 0.3xIn and 30% Slope.
Setting of P/F Diff. Setting for Load Bias
Figure 7.5.2-2
It is clear that the relay will not operate for this fault. To detect this level of fault the relay P/F Differential and/or
Slope 1 setting must be reduced. This setting must be set in excess of the multiple of steady state charging
current required by the relay to ensure stable operation. For relays with shaped current magnitude comparators
the required minimum limit is 4 x Ic, where Ic is the steady state cable charging current of the feeder.
Cable Charging Current:
The P/F Differential Setting > 4 x charging current [Resistance Earthed System]
The secondary charging current can be estimated to be: 7 km x 4.5 A x 1/1200 = 0.0263 A
P/F Differential Setting > 4 x 0.0263 A / rated current = 0.106 x I
N
The relay should be set to the next highest Differential Setting of 0.15 x I
N
Setting of Bias Slope for Load Bias
Figure 7.5.2-3
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