GE T60 Instruction Manual page 505

CHAPTER 5: SETTINGS GROUPED ELEMENTS
Restricted ground fault (RGF) protection provides sensitive ground fault detection for low-magnitude fault currents,
primarily faults close to the neutral point of a wye-connected winding. An internal ground fault on an impedance grounded
wye winding produces a fault current dependent on the ground impedance value and the fault position on the winding
with respect to the neutral point. The resultant primary current is negligible for faults on the lower 30% of the winding
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 on the secondary. Therefore, the resultant differential currents can be below
the slope threshold of the main differential element and the fault can go undetected. Application of the restricted ground
fault protection extends the coverage towards the neutral point (see the following figure).
Figure 5-162: RGF and percent differential zones of protection
This protection is often applied to transformers having impedance-grounded wye windings. The element can also be
applied to the stator winding of a generator having the neutral point grounded with a CT installed in the grounding path, or
the ground current obtained by external summation of the neutral-side stator CTs. The following figure shows the basic
application and wiring rules.
5
Figure 5-163: Typical applications of RGF protection
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 5-287