GE 489 - Manual (Page 233)

GE Multilin 489 Generator Management Relay A-5
APPENDIX A A.1 STATOR GROUND FAULT
A
A.1.5 THIRD HARMONIC VOLTAGE ELEMENT
The conventional neutral overvoltage element or the ground overcurrent element are not capable of reliably detecting stator
ground faults in the bottom 5% of the stator, due to lack of sensitivity. In order to provide reliable coverage for the bottom
part of the stator, protective elements, utilizing the third harmonic voltage signals in the neutral and at the generator output
terminals, have been developed (see Reference 4).
In the 489 relay, the third-harmonic voltage element, Neutral Undervoltage (3rd Harmonic) derives the third harmonic com-
ponent of the neutral-point voltage signal from the V
neutral
signal as one signal, called V
N3
. The third harmonic component
of the internally summed phase-voltage signals is derived as the second signal, called V
P3
. For this element to perform as
originally intended, it is necessary to use 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. The following formula is used to create an adaptive third-harmonic scheme:
(EQ A.1)
The 489 tests the following conditions prior to testing the basic operating equation to ensure that V
N3
is of a measurable
magnitude:
(EQ A.2)
where: V
N3
is the magnitude of third harmonic voltage at the generator neutral
V
P3
is the magnitude of third harmonic voltage at the generator terminals
V
P3
' and V
N3
' are the corresponding voltage transformer secondary values
Permissive_Threshold is 0.15 V for the alarm element and 0.1875 V for the trip element.
In addition, the logic for this element verifies that the generator positive sequence terminal voltage is at least 30% of nomi-
nal, to ensure that the generator is actually excited.
This method of using 3rd harmonic voltages to detect stator ground faults near the generator neutral has proved
feasible on larger generators with unit transformers. Its usefulness in other generator applications is unknown.
If the phase VT connection is "open delta", it is not possible to measure the third harmonic voltage at the generator termi-
nals and a simple third harmonic neutral undervoltage element is used. In this case, the element is supervised by both a
terminal voltage level and by a power level. When used as a simple undervoltage element, settings should be based on
measured 3rd harmonic neutral voltage of the healthy machine. It is recommended that the element only be used for alarm
purposes with open delta VT connections.
A.1.6 REFERENCES
1. C. R. Mason, "The Art & Science of Protective Relaying", John Wiley & Sons, Inc., 1956, Chapter 10.
2. J. Lewis Blackburn, "Protective Relaying: Principles and Applications", Marcel Dekker, Inc., New York, 1987, chapter 8.
3. GE Multilin, "Instruction Manual for the 489 Generator Management Relay".
4. R. J. Marttila, "Design Principles of a New Generator Stator Ground Relay for 100% Coverage of the Stator Winding",
IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 4, October 1986.
V
N3
V
P3
3 ⁄ V
N3
+
---------------------------------- 0.15 which simplifies to V
P3
17V
N3
≥ ≤
V
P3 ′
0.25 V and V
P3 ′
Permissive_Threshold 17
Neutral CT Ratio
Phase CT Ratio
-------------------------------------------- Ч Ч ≥ >
NOTE