Field Ground Fault Protection - GE G60 Instruction Manual

5 SETTINGS
e) APPLICATION CONSIDERATIONS FOR SUB-HARMONIC STATOR GROUND FAULT DETECTOR
The following special application considerations must be followed when employing sub-harmonic injection based 100% sta-
tor ground protection.
1. A generator with wye-grounded primary VT will provide a path for the injected 20 Hz signal to flow through the ground.
Thus the voltage measured using this VT will have both the fundamental and 20 Hz components superimposed at all
times.
2. Special algorithms address impact of this 20 Hz component on the aux over voltage element, which is normally used
for 95% ground fault protection by measuring NGT secondary voltage.
3. There are applications where a wye-grounded primary VT is normally used on the bus side of a generator breaker and
a neutral over voltage is used to detect ground faults on the bus duct or cable, etc. In such applications, after careful
analysis on the sensitivity of setting levels, this function may be blocked (if required) when the generator is online,
since the sub-harmonic injection based protection covers this zone.
4. The third harmonic based stator ground fault protection elements should not be used while using the sub-harmonic
injection based stator ground protection.
5. It is recommended that phase-phase voltages be used in G60 for the synchrocheck function rather than phase-to-
ground voltages under applications where a single voltage is being connected to one of the auxiliary VT channels of
G60 device.
6. The current from the CT in the secondary of the NGT fed to the sensitive ground input of G60 shall only be used for
sub-harmonic based stator ground protection and not for any other protection element such as ground over current or
neutral directional overcurrent.
a) MAIN MENU

PATH: SETTINGS GROUPED ELEMENTS
 FIELD GROUND
 PROTECTION
MESSAGE
A block diagram of the field ground detection scheme using the G60 and the GPM-F is shown below. The field winding of a
synchronous generator is represented electrically by the impedance Z
cuit is ungrounded. The capacitance C
itance represents the only path for current to flow to ground under normal conditions.
The resistance R represents a breakdown of the field winding insulation providing a resistive path to ground through the
G
field grounding brush. The insulation failure can occur anywhere, so the impedances Z
impedance R is also unknown. The purpose of the field ground detector is to measure this resistance.
G
GE Multilin

SETTING GROUP 1(6)
 FIELD
 GROUND
 FIELD
 CURRENT
is the stray capacitance of the field, distributed along the field winding. This capac-
F
Figure 5–133: FIELD GROUND FAULT DETECTION
G60 Generator Protection System
5.6 GROUPED ELEMENTS

5.6.18 FIELD GROUND FAULT PROTECTION


FIELD GROUND PROTECTION
See page 5–251.
See page 5–253.
= Z + Z . Under normal conditions the field cir-
F F1 F2
and Z
F1
are unknown. The fault
F2
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