Ge 489 User Manual download pdf (Page 91)

GE Multilin 489 Generator Management Relay 4-41

4 SETPOINTS 4.7 S6 VOLTAGE ELEMENTS

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.

(EQ 4.21)

The 489 tests the following permissives prior to testing the basic operating equation to ensure that V

’ should be of a mea-

surable magnitude for an unfaulted generator:

(EQ 4.22)

where: V

= the magnitude of the third harmonic voltage at generator neutral

= the magnitude of the third harmonic voltage at the generator terminals

´ = the VT secondary magnitude of the third harmonic voltage measured at the generator terminals

´ = the VT secondary magnitude of the third harmonic voltage at generator neutral

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.

3⁄()V

-------------------------------------- -

0.15 which simplifies to V

17V

≥≤

′ 0.25 volts and V

′ Permissive Threshold 17

Neutral VT Ratio

Phase VT Ratio

--------------------------------------------

××≥>

NOTE

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Ge 489 User Manual Page 91