Protection Schemes
745
Transformer Management Relay
Commission
ing
http://www.GEindustrial.com/multilin
7–26
GE Multilin
Ground Instantaneous
Overcurrent 2
The Ground Instantaneous Overcurrent 2 elements are identical to the Ground
Instantaneous Overcurrent 1 elements. Consequently, the same test procedure may
be used to verify their correct operation. Disable all protection features except
Ground Instantaneous Overcurrent 2. Make the appropriate changes for the display
indications and output relays operated by the Ground Instantaneous Overcurrent 2
elements.
Restricted Ground Fault
Polarity Test
This procedure verifies the correct wiring of field CTs (phase and ground) to the
corresponding phase and ground CT terminals on the relay for the purposes of the
Restricted Ground Fault protection. The correct wiring is determined by the
distribution 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 7–10: Fault Current Distribution due to an External Phase to
Ground Fault on Winding 2
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,
.
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
–
=
Summary of Contents for Multilin 745
Page 2: ......
