Rotor Earth Fault Protection R, fn (ANSI 64R)
201
7UM62 Manual
C53000-G1176-C149-3
2.30
Rotor Earth Fault Protection R, fn (ANSI 64R)
General
Rotor earth fault protection is used to detect earth faults in the excitation circuit of
synchronous machines. One earth fault in the rotor winding does not cause immediate
damage; however, if a second earth fault occurs, then this represents a winding short-
circuit of the excitation circuit. Magnetic unbalances can occur resulting in extreme
mechanical forces which can lead to the destruction of the machine.
2.30.1 Functional Description
Measuring Proce-
dure
The rotor earth fault protection in the 7UM62 uses an external auxiliary voltage of ap-
proximately 36 to 45 V AC, which can be taken from the voltage transformers via a
coupling unit 7XR6100-0*A00. This voltage is symmetrically coupled to the excitation
circuit via the capacitors of the coupling unit and simultaneously connected to the
measurement input of the 7UM62. The capacitors C
K
of the 7XR6100 coupling unit are
protected by series resistors R
series
and - in case high harmonics content is expected
in the excitation circuit (e.g. excitation by thyristor circuits) - by an additional filter
choke (for a connection example with terminal assignment see Figure A-31 in Appen-
dix A.4).
The auxiliary AC voltage drives a small charging current through the coupling unit,
brush resistance and capacitance to earth of the excitation circuit. This current
I
RE
amounts to only a few mA during normal operation and is measured by the device
(Figure 2-95).
Figure 2-95
Determination of the Rotor Earth Resistance R
E
(7XR61 – series device for the rotor short circuit to earth protection;
3PP13 – from UPU > 150 V, resistors in the 7XR61 must be shorted!)
7UM62
7XR61
R
E
C
E
C
K
C
K
R
series
R
series
3PP13
Rotor earth
fault protec-
tion
I
RE
U
RE
I
EE1
U
E
R
series
R
series
