5–54
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
(EQ 5.25)
where:
V
N3
= magnitude of the third harmonic voltage at generator neutral;
V
P3
= magnitude of the third harmonic voltage at the generator terminals
V
P3
´ = VT secondary magnitude of the third harmonic voltage measured at the
generator terminals;
V
N3
´ = VT sec. magnitude of 3
rd
harmonic voltage at generator neutral;
Threshold = 0.15 V for the alarm element and 0.1875 V for the trip element
For 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 terminals 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.
Note
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.
V
P
3
′
0.25 V and
V
P
3
′
Threshold 17 Neutral VT Ratio
Phase VT Ratio
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×
×
≥
>
