Stator Ground Fault; Description - GE Multilin 489 Instruction Manual

Digital Energy
Multilin
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Stator Ground Fault

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Description

489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
489 Generator Management Relay
Appendix
Appendix
This application note describes general protection concepts and provides guidelines on
the use of the 489 to protect a generator stator against ground faults. Detailed
connections for specific features must be obtained from the relay manual. Users are
also urged to review the material contained in the 489 manual on each specific
protection feature discussed here.
The 489 Generator Management Relay offers a number of elements to protect a generator
against stator ground faults. Inputs are provided for a neutral-point voltage signal and for
a zero-sequence current signal. The zero-sequence current input can be into a nominal 1 A
secondary circuit or an input reserved for a special GE Multilin type HGF ground CT for very
sensitive ground current detection. Using the HGF CT allows measurement of ground
current values as low as 0.25 A primary. With impedance-grounded generators, a single
ground fault on the stator does not require that the unit be quickly removed from service.
The grounding impedance limits the fault current to a few amperes. A second ground fault
can, however, result in significant damage to the unit. Thus the importance of detecting all
ground faults, even those in the bottom 5% of the stator. The fault detection methods
depend on the grounding arrangement, the availability of core balance CT, and the size of
the unit. With modern full-featured digital generator protection relays such as the 489,
users do not incur additional costs for extra protection elements as they are all part of the
same device. This application note provides general descriptions of each of the elements in
the 489 suitable for stator ground protection, and discusses some special applications.
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