GE Multilin g60 Instruction Manual page 370

5.6 GROUPED ELEMENTS
• V is the measured sub-harmonic voltage.
X
• I is the measured sub-harmonic current.
G
The ground fault resistance can be easily calculated by measuring the 20 Hz voltage and current phasors at the secondary
of the grounding transformer. This is done by the G60. An overcurrent element responding to the sub-harmonic provides
backup protection.
For machines that may operate at sub-synchronous frequencies (for instance a gas turbine that employs static starting), the
function is blocked at frequencies between 15 and 25 Hz. This ensures that the voltage and current produced by the gener-
ator does not leak into the sub-harmonic ground fault measurements.
Figure 5–115: STATOR GROUND FAULT DETECTION BY SUB-HARMONIC INJECTION
In addition, a check for minimum values of injected voltage and current guards against a failure of the injection unit or a
5
short or open circuit in the external circuit. In addition, the critical-fail relay contact of the 20 Hz generator can also be con-
nected to one of the contact inputs of G60 and the sub-harmonic stator ground element may be blocked.
The stator ground source settings determine the signals that are applied for V
reported in primary ohms. Therefore the VT ratio setting of the auxiliary VT input must match the turns ratio of the neutral
grounding transformer and the CT primary setting of the ground CT input must match that of the CT used to measure
ground current.
When the magnitude of R
should be used to make this measurement. Thus a CT/VT module with sensitive ground current input should be present in
a G60 if it were to be used for sub-harmonic stator ground protection.
Accessory modules required for sub-harmonic injection based stator ground protection:
• 20 Hz injection module (GE Multilin order code): GPM-S-G.
• Coupling filter (GE Multilin order code): GPM-S-B.
• Current transformer (GE order code): 204-SD-43737.
The following settings are available for the sub-harmonic stator ground fault feature.
• SH STATOR GND STG1 PICKUP: If the measured stator ground primary resistance is less than the value specified by
this setting, the stage 1 element will pickup. Normally stage 1 is used to raise alarms and typical settings may fall in the
range of 5 to 10 kΩ. This setting is given in primary ohms.
• SH STATOR GND STG1 DELAY: This setting specifies a time delay for stage 1. Typical settings are in the range of 5
to 10 seconds. This delay will have to be added to the operating time of the element to obtain the overall delay.
• SH STATOR GND STG2 PICKUP: If the measured stator ground primary resistance is less than the value specified
this setting, stage 2 element will pickup. Normally stage 2 is used to raise trip signals and typical settings may fall in the
range of 1 to 5 kΩ. This setting is given in primary ohms.
• SH STATOR GND STG2 DELAY: This setting specifies a time delay for stage 2. Typical settings are in the range of 1
to 2 seconds. This delay will have to be added to the operating time of the element to obtain the overall delay.
• SH CT ANGLE COMPEN: The CT used may introduce phase shifts and this should be compensated for accurate fault
resistance calculations. Perform a test during commissioning at no fault condition by measuring the
reported by the G60. In a healthy machine, the SH impedance is purely capacitive and the angle should be 90°.
ANGLE
5-210
is large the resulting current I
G
G60 Generator Protection System
, I , and frequency. The resistance is
X G
will be very small therefore the G60 sensitive ground input
G
5 SETTINGS
SH CURRENT
GE Multilin