A.1.4 Ground Directional Element - GE 489 Instruction Manual

APPENDIX 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.
CORE
BALANCE
CT
Option 2
Option 1
Figure A–3: GROUND OVERCURRENT ELEMENT WITH DIFFERENT CURRENT SOURCE SIGNALS
The 489 can detect internal stator ground faults using a Ground Directional element implemented using the V
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
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 signal is 180° away from the I
o
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°.
Grounding
Resistor
Isolating
Transformer
Figure A–4: GROUND DIRECTIONAL ELEMENT POLARITIES AND PLANE OF OPERATION
GE Multilin
GENERATOR
Option 5
(similar to
Option 4)
Ground
Ground current input
from one of the five Overcurrent
Element
options
signal, if the polarity convention is observed. If the grounding imped-
o
GENERATOR
CORE
BALANCE
CT
I
o
I
o
±
F10 H10
489
I
o
Relay
E10 G10
±
489 Generator Management Relay
A.1 STATOR GROUND FAULT
Phase CTs
CORE
BALANCE
CT
Option 3
489
Option 4

A.1.4 GROUND DIRECTIONAL ELEMENT

signal will be in phase with the
neutral
Plane of operation
for resistive
grounding impedance
180°
V
o
BREAKER
Breaker
Aux.
808736A1.CDR
and the
neutral
90°
I
o
0°
270°
808735A1.CDR
A-3
A