GE Multilin
745 Transformer Management Relay
8-25
8 COMMISSIONING
8.5 PROTECTION SCHEMES
8
8.5.10 RESTRICTED GROUND FAULT
a) RGF POLARITY TEST
This procedure verifies the correct wiring of field CTs (phase and ground) to the corresponding phase and ground CT termi-
nals on the relay for the purposes of the Restricted Ground Fault protection. The correct wiring is determined by the distri-
bution of fault current during external Phase A to ground faults on Wye-connected windings with grounded neutral.
From the figure below, the
I
f
fault current travels through the Wye grounded neutral as
I
g
and can be simulated by injecting
a single current into the Phase A (W2) and G1/2 terminals.
Figure 8–10: FAULT CURRENT DISTRIBUTION DUE TO AN EXTERNAL PHASE TO GROUND FAULT ON W2
The procedure for this test is shown below:
1.
Select the D/Y30° transformer type into the relay and set the same CT ratio for both phase and ground winding inputs.
The selected transformer type assures the G1/2 ground input is associated to Winding 2 (Wye).
2.
Enable the RGF protection only and monitor the ground differential current under the relay actual values.
3.
Connect the single current source to inject current into G10 of the ground G1/2 terminal and to the connected in-series
Winding 2 Phase A terminals as shown on the figure above.
4.
Verify the ground current and Phase A current are in phase
5.
Verify the ground differential current is zero
The polarities and wirings of the CTs for the RGF protection are correct if the external phase-to-ground fault current
is seen on both relay terminals (phase and ground) in the same direction. The response of the RGF protection is
based on the magnitude of the ground differential current resulting from the vector difference of the neutral and
ground currents; that is,
.
b) RGF ELEMENT TEST
This procedure verifies that the Restricted Ground Fault performance matches the in-service settings. The ground element
measures the current signal connected to the ground current input CT, H10 and G10 or F12 and E12. The neutral (3
I
o
) cur-
rent is calculated from the vector sum of the three phase currents. Injecting current into one phase automatically produces
a neutral current (i.e. 3
I
o
=
I
A
). Settings for these elements are found in the
S4 ELEMENTS
ÖØ
RESTRICTED GROUND
ÖØ
W1(3) RESTD GND FAULT
setpoints menu.
WINDING 1 ELEMENT:
To ensure that only the Restricted Ground Fault element operates the trip relays (and any other output relays selected by
the logic) disable all protection features except Restricted Ground Fault.
H4 *
G4
H10 *
G10
AC source
50/60 Hz signal
Ig
3I0
I3Io – Ig I = 0
RGF polarity wiring test
Ground CT
*
*
*
*
Phase A CT(W2)
745 relay
H4 *
G4
G10
H10 *
G1/2 input
Phase A (W2)
Ig
If
Ig = I2
If = Ig = I1 = I2
If = I1
I1
I2
NOTE
I
gd
3
I
0
I
g
–
=
