GE T60 Instruction Manual page 223

5 SETTINGS
d) RESTRICTED GROUND FAULT (ANSI 87G)
PATH: SETTINGS GROUPED ELEMENTS
RESTRICTED GROUND
FAULT 1
MESSAGE
MESSAGE
MESSAGE
MESSAGE
MESSAGE
MESSAGE
MESSAGE
MESSAGE
As of T60 firmware revision 3.20, the definition of the restraining signal has been significantly changed
compared to previous versions. The restraint during external faults is generally not lower, and often much
NOTE
higher, compared to the previous definition of the restraining signal (enhanced security). The restraint on
internal faults has been greatly reduced compared to previous versions (enhanced sensitivity), particularly
during low-current internal faults. Using time delay as a means of dealing with CT saturation is no longer
obligatory; pickup and slope are the primary means of addressing CT saturation. Increasing the slope set-
ting is recommended when migrating from the 3.1x or earlier firmware revisions. The default value for the
slope has been changed from 10% to 40%.
Restricted ground fault (RGF) protection provides sensitive ground fault detection for low-magnitude fault currents, prima-
rily faults close to the neutral point of a wye-connected winding. An internal ground fault on an impedance grounded wye
winding will produce 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 will be 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 may be below the slope
threshold of the main differential element and the fault could go undetected. Application of the restricted ground fault pro-
tection extends the coverage towards the neutral point (see the RGF and Percent Differential Zones of Protection diagram).
Figure 5–60: RGF AND PERCENT DIFFERENTIAL ZONES OF PROTECTION
GE Multilin
Courtesy of NationalSwitchgear.com
SETTING GROUP 1(6)
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
RESTD GND FT1
EVENTS: Disabled
WINDING
35%
RGF
Rg
ZONE
T60 Transformer Management Relay
5.5 GROUPED ELEMENTS
GROUND CURRENT RESTRICTED GROUND FAULT 1(4)
Range: Disabled, Enabled
Range: SRC 1, SRC 2, SRC 3, SRC 4
Range: 0.000 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
Range: Disabled, Enabled
DIFFERENTIAL ZONE
842731A1.CDR
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