Ground Directional Element - GE 489 Instruction Manual

A.1 STATOR GROUND FAULT
A
Aux.
Contact
To Relay
Grounding
Impedance
(Trans. &
Resistor)
Figure A–5: GROUND DIRECTIONAL ELEMENT CONCEPTUAL ARRANGEMENT
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.
Applications with generators operated in parallel and grounded through a common impedance require special consider-
ations. If only one generator is grounded and the other ones left floating, the directional element for the floating generators
does not receive a correct V
use of auxiliary contacts off the grounding switch and the unit breaker to turn the element into a simple overcurrent element,
with the pickup level set for the directional element (note that the ground directional element and the ground overcurrent
elements are totally separate elements). In this mode, the element can retain a high sensitivity and fast operate time since
it will only respond to internal stator ground faults. The table below illustrates the status of different elements under various
operating conditions.
Table A–1: DETECTION ELEMENT STATUS
GENERATOR UNIT
CONDITION BREAKER
Shutdown Open
Open Circuit and Open
grounded
Loaded and Closed
Grounded
Loaded and Not Closed
Grounded
A-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 and therefore cannot operate correctly. In those applications, the element makes
neutral
GROUNDING
SWITCH
DIRECTIONAL
Open
Closed In-service (but will not
operate due to lack of LO)
Closed
Open In service as a simple
overcurrent element
489 Generator Management Relay
CORE
BALANCE
489
Ground
Current
Input
Ground
O/C
Element
Breaker
Status
neutral
ELEMENT
GROUND NEUTRAL
OVERVOLTAGE
Out-of-service Out-of-service
In-service
In-service In-service
Out-of-service
APPENDIX A
BREAKER
CT
Aux.
Breaker
808734A1.CDR
at which the element will operate
GROUND
OVERCURRENT
In-service
In-service
In-service
In-service
GE Multilin