GE 489 - Manual (Page 232)

A-4 489 Generator Management Relay GE Multilin
A.1 STATOR GROUND FAULT APPENDIX A
A
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
neutral
at which the element will operate
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
neutral
signal and therefore cannot operate correctly. In those applications, the element makes
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
CONDITION
UNIT
BREAKER
GROUNDING
SWITCH
ELEMENT
GROUND
DIRECTIONAL
NEUTRAL
OVERVOLTAGE
GROUND
OVERCURRENT
Shutdown Open Open Out-of-service Out-of-service In-service
Open Circuit and
grounded
Open Closed In-service (but will not
operate due to lack of LO)
In-service In-service
Loaded and
Grounded
Closed Closed In-service In-service In-service
Loaded and Not
Grounded
Closed Open In service as a simple
overcurrent element
Out-of-service In-service
808734A1.CDR
GENERATOR
CORE
BALANCE
CT
BREAKER
Aux.
Contact
Grounding
Switch
To Relay
Grounding
Impedance
(Trans. &
Resistor)
Grounding
Switch
Aux. Cont.
G.S.
Status
Breaker
Status
Ground
O/C
Element
Isolating
Transformer
V
Input
neutral
Ground
Directional
Element
(or O/C)
489
Neutral
O/V
Element
Ground
Current
Input
Aux.
Breaker