GE L90 Instruction Manual page 448

GROUPED ELEMENTS
These elements measure the current that is connected to the ground channel of a CT/VT module. The conversion
range of a standard channel is from 0.02 to 46 times the CT rating.
5.7.11.4 Restricted ground fault (ANSI 87RGF, IEC PNDF)
SETTINGS  GROUPED ELEMENTS  SETTING GROUP 1(6)  GROUND CURRENT  RESTRICTED GROUND FAULT
1(4)
 RESTRICTED GROUND
 FAULT 1
5
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).
5-262
Figure 5-141: Ground IOC1 logic
RESTD GND FT1

FUNCTION: Disabled
RESTD GND FT1

SOURCE: SRC 1
RESTD GND FT1

PICKUP: 0.080 pu
RESTD GND FT1

SLOPE: 40%
RESTD GND FT1 PICKUP

DELAY: 0.10 s
RESTD GND FT1 RESET

DELAY: 0.00 s
RESTD GND FT1 BLOCK:

Off
RESTD GND FT1

TARGET: Self-reset
Range: Disabled, Enabled
Range: SRC 1, SRC 2, SRC 3, SRC 4
Range: 0.005 to 30.000 pu in steps of 0.001
Range: 0 to 100% in steps of 1
Range: 0.00 to 600.00 s in steps of 0.01
Range: 0.00 to 600.00 s in steps of 0.01
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS