GE G60 Instruction Manual page 522

GROUPED ELEMENTS
A voltage source placed at the neutral of the generator produces a current for ground faults anywhere on the stator
winding. This source is coupled to the primary circuit using the existing neutral grounding transformer. The following figure
shows a typical high impedance grounded generator that is protected against ground faults using the sub-harmonic
injection method. The neutral resistor is chosen to limit the ground fault current to a low value (less than 25 A) in order to
minimize damage. A sub-harmonic frequency voltage signal is injected into the neutral of the generator. Under normal
conditions a resulting current flows through the surge capacitors (C
step-up transformer windings. When a ground fault develops on the stator winding, an additional current flows through
the resistance R . The value of R
G
An AC signal is injected so that it can be coupled through an injection transformer to the primary circuit. The signal is of a
low frequency in order to minimize the effects of the capacitance of the primary circuit. A frequency of 20 Hz is chosen.
Referring to the following figure, the admittance seen looking into the grounding transformer is
where
R is the ground fault resistance
G
C is the total capacitance to ground
T
N is the neutral grounding transformer ratio
V is the measured sub-harmonic voltage
X
I is the measured sub-harmonic current
G
The ground fault resistance can be 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.
5
For machines that can 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
generator does not leak into the sub-harmonic ground fault measurements.
In addition, a check for minimum values of injected voltage and current guards against a failure of the injection unit or a
short or open circuit in the external circuit. In addition, the critical-fail relay contact of the 20 Hz generator can also be
connected to one of the contact inputs of G60 and the sub-harmonic stator ground element can 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
this measurement. Thus, a CT/VT module with sensitive ground current input needs to be present in a G60 when used for
sub-harmonic stator ground protection.
The following accessory modules are required for sub-harmonic injection based stator ground protection:
5-308
is derived from the measurement of the injected voltage and the resulting current.
G
Figure 5-178: Stator ground fault detection by sub-harmonic injection
is large, the resulting current I
G
) and through the stray capacitance of the stator and
s
, I , and frequency. The resistance is
X G
is very small and the G60 sensitive ground input is used to make
G
G60 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Eq. 5-45