CHAPTER A: APPENDIX
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
A–7
(EQ 1.2)
where:
V
N
3
is the magnitude of third harmonic voltage at the generator neutral
V
P
3
is the magnitude of third harmonic voltage at the generator terminals
V
P
3
'
and
V
N
3
'
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 nominal, to ensure that the generator is actually excited.
Note
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 terminals 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
P
3
′
0.25 V and
V
P
3
′
Permissive_Threshold 17 Neutral CT Ratio
Phase CT Ratio
----------------------------------------
×
×
≥
>
