GE Multilin 850 Instruction Manual page 294

PROTECTION
4–162
Example: Transformer: 5MVA, 13.8kV/4.16kV, D/Yg1 type
Rg = 10 ohms
Phase CTs: 800:5
Ground CT: 300:5
Ifgnd (max) = 4.16kV/(10 ohms *v3) = 240A - maximum ground fault current
To detect a ground fault on the Wye winding at 15% distance from the neutral point, the
relay shall detect ground differential current of:
Igd = (15x240)/100 = 36 A/800 = 0.045 x CT – PKP setting for the RGF protection
The transformer loading is almost unaffected for ground faults close to the winding
neutral, so that the winding rated load can be used as a reference in defining the RGF 1
slope setting. The rated load for 4.16kV Wye winding is:
Irated = 5MVA/(4.16kV*v3) = 693 A
Slope setting, % = (36 A/693A)*100 = 5.1 %. Select 5% if the transformer is expected to run
most of the time at full load. If most of the time the transformer runs at lower than the
nominal loads, the slope setting can be set to higher value:
@ 70% load the slope setting would be: (36 / 485 A)*100 = 7%
@ 50% load the slope setting would be: (36 / 346 A)*100 = 10%
@ 30% load the slope setting would be: (36 / 208 A)*100 = 17%
The ground current supervision feature and/or the RGF 1 Pickup time delay can be used if
CT saturation due to heavy external fault, is a concern.
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS