GE 845 Instruction Manual page 296

PROTECTION
4–192
The Restricted Ground (Earth) 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 transformer 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 4-74: 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.
Figure 4-75: Restricted Ground (Earth) Fault zone of protection
845 implementation of the Restricted Ground (Earth) Fault protection is a low impedance
current differential scheme. The 845 calculates the magnitude of the ground differential
current as an absolute value from the vector summation of the computed residual current,
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS