GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual page 113

5 SETPOINTS
mary (I ) and fault (I ) currents as a function of the distance of the fault point from the neutral and Figure 5–24: RGF and
P F
Percent Differential Zones of Protection outlines the zones of effective protection along the winding for an impedance
grounded wye.
Figure 5–24: RGF AND PERCENT DIFFERENTIAL ZONES OF PROTECTION
The 745 implementation of RGF (shown below) is a low impedance current differential scheme where "spill" current due to
CT tolerances is handled via load bias similar to the percent differential. The 745 calculates the vectorial difference of the
residual and ground currents (i.e. 3I
value. The slope setting allows the user to determine the sensitivity of the element based on the class and quality of the
CTs used. Typically no more than 4% overall error due to CT "spill" is assumed for protection class CTs at nominal load.
Ia
Ib
Ic
Ig
GE Multilin
Courtesy of NationalSwitchgear.com
100
90
80
70
60
50
40
30
20
Ip(x)
10
Ifault(x)
0
0 10
x = distance of fault from neutral
Figure 5–23: FAULT CURRENTS VS. POINTS FROM NEUTRAL
WINDING
35%
RGF
ZONE
Rg
- I ) and divides this by the maximum line current (I
0 g
Imax
Calculate
Maximum Phase
Slope = Igd/Imax
Current
Slope > Setpoint
Calculate
3I
0
Igd
Calculate
|3I – Ig|
0
Measure
Ig
Figure 5–25: RESTRICTED GROUND FAULT IMPLEMENTATION
745 Transformer Management Relay
Ifault
Ip
20 30 40 50 60 70 80
DIFFERENTIAL
ZONE
745 RELAY
AND
Igd > Setpoint
5.6 S4 ELEMENTS
90 100
) to produce a percent slope
max
Timer
OUTPUT
0 to 0.5 s
5
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