Third Harmonic Voltage Element - GE Multilin 489 Instruction Manual

A.1.5

Third Harmonic Voltage Element

A–6
Courtesy of NationalSwitchgear.com
Applications with generators operated in parallel and grounded through a common
impedance require special considerations. If only one generator is grounded and the other
ones left floating, the directional element for the floating generators does not receive a
correct V signal and therefore cannot operate correctly. In those applications, the
neutral
element makes use of auxiliary contacts off the grounding switch and the unit breaker to
turn the element into a simple overcurrent element, with the pickup level set for the
directional element (note that the ground directional element and the ground overcurrent
elements are totally separate elements). In this mode, the element can retain a high
sensitivity and fast operate time since it will only respond to internal stator ground faults.
The table below illustrates the status of different elements under various operating
conditions.
Table A–1: Detection Element Status
Generator Unit Ground
Condition Breaker Switch
Shutdown Open Open
Open Circuit
and Open Closed
grounded
Loaded and
Closed Closed
Grounded
Loaded and
Not Closed Open
Grounded
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 component of the neutral-point voltage signal from the V
signal as one signal, called V
phase-voltage signals is derived as the second signal, called
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:
V
N3
------------------------------- ≤
V 3 ⁄ V
+
P3 N3
The 489 tests the following conditions prior to testing the basic operating equation to
ensure that V is of a measurable magnitude:
N3
Ground
Directional
Out-of-service Out-of-service
In-service (but will
not operate due to
In-service
lack of I )
0
In-service In-service
In service as a
simple overcurrent Out-of-service
element
. The third harmonic component of the internally summed
N3
0.15 which simplifies to V
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER A: APPENDIX
Element
Neutral Ground
Overvoltage Overcurrent
In-service
In-service
In-service
In-service
neutral
V . For this element to
P3
≥ 17V
(EQ 1.1)
P3 N3