Ground Overcurrent Element - GE Multilin 489 Instruction Manual

CHAPTER A: APPENDIX
A.1.3

Ground Overcurrent Element

489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
circulating third harmonic current will be quite small). With a common ground point, the
same V signal is brought to all the relays but only the one which is grounded should
neutral
have the neutral overvoltage element in service.
For these cases, the neutral overvoltage element has been provided with a supervising
signal obtained from an auxiliary contact off the grounding switch. When the grounding
switch is opened, the element is disabled. The grounding switch auxiliary contact is also
used in the ground directional element, as is the breaker auxiliary contact, as discussed
later.
If all the generators are left grounded through the same impedance, the neutral
overvoltage element in each relay will respond to a ground fault in any of the generators.
For this reason, the ground directional element should be used in each relay, in addition to
the neutral overvoltage element.
Common
Grounding
Impedance
Trans. & R
Isolating
Trans.
FIGURE A–2: Parallel Generators with Common Grounding Impedance
The ground overcurrent element can be used as a direct replacement or a backup for the
neutral overvoltage element, with the appropriate current signal from the generator
neutral point, for grounded generators. This element can also be used with a Core Balance
CT, either in the neutral end or the output end of the generator, as shown below. The use of
the special CT, with its dedicated input to the relay, offers very sensitive current detection,
but still does not offer protection for the full stator. The setting of this element must be
above the maximum unbalance current that normally flows in the neutral circuit. Having
the element respond only to the fundamental frequency component allows an increase in
sensitivity.
G1
Grounding
Switch
Aux.
Contact
489
Relay
V
neutral
G2
Grounding
Switch
Aux.
Contact
489
Relay
V
neutral
Other Generators,
as the case may be
Breaker
Aux.
Contact
Breaker
Aux.
Contact
808737A1.CDR
A–3