GE Multilin g60, Instruction Manual

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GE Multilin G60 Generator Protection System 5-249
5 SETTINGS 5.6 GROUPED ELEMENTS
5
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.
5.6.18 FIELD GROUND FAULT PROTECTION
a) MAIN MENU
PATH: SETTINGS

GROUPED ELEMENTS

SETTING GROUP 1(6)

FIELD GROUND PROTECTION
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
F
= Z
F 1
+ Z
F 2
. Under normal conditions the field cir-
cuit is ungrounded. The capacitance C
F
is the stray capacitance of the field, distributed along the field winding. This capac-
itance represents the only path for current to flow to ground under normal conditions.
Figure 5–133: FIELD GROUND FAULT DETECTION
The resistance R
G
represents a breakdown of the field winding insulation providing a resistive path to ground through the
field grounding brush. The insulation failure can occur anywhere, so the impedances Z
F 1
and Z
F 2
are unknown. The fault
impedance R
G
is also unknown. The purpose of the field ground detector is to measure this resistance.

FIELD GROUND

PROTECTION

FIELD

GROUND
See page 5–251.
MESSAGE

FIELD

CURRENT
See page 5–253.
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