GE Multilin 850, Instruction Manual

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6–52 850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CURRENT ELEMENTS CHAPTER 6: PROTECTION SETPOINTS
Restricted Ground Fault (87G)
The 850 relay is equipped with the Restricted Ground Fault (RGF) element, sometimes
referred to as the Restricted Earth Fault (REF) element.
The Restricted Ground Fault (RGF) protection provides ground fault detection for low-
magnitude ground fault currents primarily for ground faults close to the neutral point of
the wye connected winding. An internal ground fault on an impedance grounded wye
winding produces a low magnitude ground fault current depending on the position of the
fault with respect to the winding neutral point.
The diagram below shows the dependence of the fault current on the fault distance from
the neutral point.
Figure 6-19: Fault current with respect to distance from neutral
The resultant primary current can be negligible for ground winding faults within 35% of the
distance from the neutral point since the fault voltage is not the system voltage, but rather
the result of the transformation ratio between the primary windings and the percentage of
shorted turns.
Application of Restricted Ground Fault protection extends the fault coverage towards the
neutral point for low-impedance grounded winding.
Figure 6-20: Restricted Ground Fault zone of protection