Function Range or value Accuracy
Reset ratio, undercurrent < 105% -
Reset ratio, overvoltage > 95% -
Reset ratio, undervoltage < 105% -
Overcurrent:
Critical impulse time 10 ms typically at 0 to 2 x I
set
-
Impulse margin time 15 ms typically -
Undercurrent:
Critical impulse time 10 ms typically at 2 to 0 x I
set
-
Impulse margin time 15 ms typically -
Overvoltage:
Critical impulse time 10 ms typically at 0.8 to 1.2 x V
set
-
Impulse margin time 15 ms typically -
Undervoltage:
Critical impulse time 10 ms typically at 1.2 to 0.8 x V
set
-
Impulse margin time 15 ms typically -
11.2 Rotor ground fault protection (64R)
SEMOD172064-1 v3
11.2.1 Functionality
SEMOD151286-4 v8
The field winding, including the rotor winding and the non-rotating excitation equipment, is
always insulated from the metallic parts of the rotor. The insulation resistance is high if the rotor
is cooled by air or by hydrogen. The insulation resistance is much lower if the rotor winding is
cooled by water. This is true even if the insulation is intact. A fault in the insulation of the field
circuit will result in a conducting path from the field winding to ground. This means that the fault
has caused a field ground fault.
The field circuit of a synchronous generator is normally ungrounded. Therefore, a single ground
fault on the field winding will cause only a very small fault current. Thus the ground fault does not
produce any damage in the generator. Furthermore, it will not affect the operation of a generating
unit in any way. However, the existence of a single ground fault increases the electric stress at
other points in the field circuit. This means that the risk for a second ground fault at another point
on the field winding has increased considerably. A second ground fault will cause a field short-
circuit with severe consequences.
The rotor ground fault protection is based on injection of an AC voltage to the isolated field
circuit. In non-faulted conditions there will be no current flow associated to this injected voltage. If
a rotor ground fault occurs, this condition will be detected by the rotor ground fault protection.
Depending on the generator owner philosophy this operational state will be alarmed and/or the
generator will be tripped. An injection unit RXTTE4 and an optional protective resistor on plate are
required for correct rotor ground fault protection operation.
Section 11 1MRK 502 066-UUS B
Multipurpose protection
714
Technical manual
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