B.1.4 Ground Directional Element - GE 489 Instruction Manual

B.1 STATOR GROUND FAULT PROTECTION
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).
B
Aux.
Contact
To Relay
Grounding
Impedance
(Trans. &
Resistor)
Figure B–4: GROUND DIRECTIONAL ELEMENT CONCEPTUAL ARRANGEMENT
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 B–5:
GROUND DIRECTIONAL ELEMENT POLARITIES AND PLANE OF OPERATION, for an internal ground fault. That is, for
an internal stator-to-ground fault, the V
grounding impedance is inductive, the plane of operation will be the 270° plane, again, with the polarity convention shown
below. If the polarity convention is reversed on one input, the user will need to change the plane of operation by 180°.
The operating principle of this element is quite simple: for internal ground faults the two signals will be 180° out of phase
and for external ground faults, the two signals will be in phase. This simple principle allows the element to be set with a high
sensitivity, not normally possible with an overcurrent element.
The current pickup level of the element can be adjusted down to 0.05 × CT primary, allowing an operate level of 0.25 A pri-
mary if the 50:0.025 ground CT is used for the core balance. The minimum level of V
is determined by hardware limitations and is internally set at 2.0 V.
Because this element is directional, it does not need to be coordinated with downstream protections and a short operating
time can be used. Definite time delays are suitable for this element.
B-4
GENERATOR
Grounding
Switch
Ground
Directional
V
neutral
Element
Input
(or O/C)
Isolating
Transformer
Neutral
O/V
Element
Grounding
G.S.
Switch
Status
Aux. Cont.
signal is 180° away from the I
o
489 Generator Management Relay

B.1.4 GROUND DIRECTIONAL ELEMENT

CORE
BALANCE
CT
489
Ground
Current
Input
Ground
O/C
Element
Breaker
Status
signal will be in phase with the
neutral
signal, if the polarity convention is observed. If the
o
at which the element will operate
neutral
APPENDIX B
and the
neutral
BREAKER
Aux.
Breaker
GDECON2.CDR
GE Power Management