GE 489 Instruction Manual page 91

4 SETPOINTS
The neutral undervoltage function responds to 3rd harmonic voltage measured at the generator neutral and output termi-
nals. When used in conjunction with the Neutral Overvoltage (fundamental frequency) function, it provides 100% ground
fault protection of the stator windings.
WYE CONNECTED VTS:
Since the amount of third harmonic voltage that appears in the neutral is both load and machine dependent, the protection
method of choice is an adaptive method. If the phase VT connection is wye, the following formula is used to create an
adaptive neutral undervoltage pickup level based on the amount of third harmonic that appears at the generator terminals.
-------------------------------------- -
(
The 489 tests the following permissives prior to testing the basic operating equation to ensure that V
surable magnitude for an unfaulted generator:
' 0.25 volts >
V
P3
where: V = the magnitude of the third harmonic voltage at generator neutral
N3
V = the magnitude of the third harmonic voltage at the generator terminals
P3
V ´ = the VT secondary magnitude of the third harmonic voltage measured at the generator terminals
P3
V ´ = the VT secondary magnitude of the third harmonic voltage at generator neutral
N3
Permissive Threshold = 0.15 volts for the alarm element and 0.1875 volts for the trip element
Refer to Appendix B for Application Notes.
OPEN DELTA CONNECTED VTS:
If the phase VT connection is open delta, it is not possible to measure the third harmonic voltages at the generator termi-
nals and a simple third harmonic neutral undervoltage element is used. The level is programmable in terms of Neutral VT
secondary voltage. In order to prevent nuisance tripping at low load or low generator voltages, two blocking functions are
provided. They apply to both the alarm and trip functions. When used as a simple undervoltage element, settings should be
based on measured 3rd harmonic neutral voltage of the healthy machine.
This method of using 3rd harmonic voltages to detect stator ground faults near the generator neutral has proved
feasible on generators with unit transformers. Its usefulness in other generator applications is unknown.
NOTE
GE Multilin
V
N3
≤
0.15 which simplifies to V
3 ⁄ )
V V
+
P3 N3
' Permissive Threshold ≥
and V
P3
489 Generator Management Relay
4.7 S6 VOLTAGE ELEMENTS
≥
17V
P3 N3
N3
Neutral VT Ratio
× × ------------------------------------------- -
17
Phase VT Ratio
(EQ 4.21)
' should be of a mea-
(EQ 4.22)
4
4-41