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GE Multilin 489 Generator Management Relay A-3
APPENDIX A A.1 STATOR GROUND FAULT
A
Again the time delay on this element must be coordinated with protection elements downstream, if the generator is
grounded. Refer to Section 4.6.7: Ground Overcurrent on page 4–29 for the range of settings of the pickup levels and the
time delays. The time delay on this element should always be longer than the longest delay on line protection downstream.
Figure A–3: GROUND OVERCURRENT ELEMENT WITH DIFFERENT CURRENT SOURCE SIGNALS
A.1.4 GROUND DIRECTIONAL ELEMENT
The 489 can detect internal stator ground faults using a Ground Directional element implemented using the V
neutral
and the
ground current inputs. The voltage signal is obtained across the grounding impedance of the generator. The ground, or
zero sequence, current is obtained from a core balance CT, as shown below (due to CT inaccuracies, it is generally not
possible to sum the outputs of the conventional phase CTs to derive the generator high-side zero sequence current, for an
impedance-grounded generator).
If correct polarities are observed in the connection of all signals to the relay, the V
neutral
signal will be in phase with the
ground current signal. The element has been provided with a setting allowing the user to change the plane of operation to
cater to reactive grounding impedances or to polarity inversions.
This element’s normal "plane of operation" for a resistor-grounded generator is the 180° plane, as shown in Figure A–4:
Ground Directional Element Polarities and Plane of Operation, for an internal ground fault. That is, for an internal stator-to-
ground fault, the V
o
signal is 180° away from the I
o
signal, if the polarity convention is observed. If the grounding imped-
ance is inductive, the plane of operation will be the 270° plane, again, with the polarity convention shown below. If the polar-
ity convention is reversed on one input, the user will need to change the plane of operation by 180°.
Figure A–4: GROUND DIRECTIONAL ELEMENT POLARITIES AND PLANE OF OPERATION
808736A1.CDR
489
GENERATOR
CORE
BALANCE
CT
CORE
BALANCE
CT
Phase CTs
BREAKER
Breaker
Aux.
Option 2
Option 1
Option 5
(similar to
Option 4)
Option 3
Option 4
Ground
Overcurrent
Element
Ground current input
from one of the five
options
808735A1.CDR
GENERATOR
I
o
CORE
BALANCE
CT
Plane of operation
for resistive
grounding impedance
180°
270°
0°
90°
Grounding
Resistor
Isolating
Transformer
489
Relay
V
o
F10
E10
H10
G10
I
o
I
o
I
o
±
±