ABB RELION REX640 Technical Manual page 490

Section 4
Protection functions
484
GUID-392EB0A0-2BB1-4FD4-913A-7D2686BEEC6B V1 EN
Figure 265: Principle of the rotor earth-fault protection with the current injection
device
The auxiliary AC voltage forms a small charging current I
capacitors, resistances of the brushes and the leakage capacitance between the field
circuit and earth. The field-to-earth capacitance C
current to an extent which is a few milliamperes during normal no-fault operating
conditions.
If an earth fault arises in the rotor field circuit, this current increases and can reach a
level of 130 mA at a fully developed earth fault (fault resistance R
capacitor C = 2μF is used). The integrated current transformer of the injection device
1
REK 510 then amplifies this current with the ratio of 1:10 to a measurable level.
MREFPTOC is used to measure this current.
An example of the measured curves with various field-to-earth leakage capacitance
values is given in Figure
It is recommended that the alarm and operation stages of MREFPTOC are both used.
The alarm stage for giving an indication for weakly developed earth faults with a start
value setting corresponds to a 10 kΩ fault resistance with a 10-second delay. The
operation stage for a protection against fully developed earth faults with a start value
setting corresponds to a 1...2 kΩ fault resistance with a 0.5-second delay.
The current setting values corresponding to the required operating
fault resistances can be tested by connecting an adjustable fault-
simulating resistor between the excitation winding poles and the
earth. Whether only one of the coupling capacitors or both should be
used in a parallel connection should be determined on a case-by-case
basis, taking into consideration the consequences of a possibly
excessive current at a direct earth fault.
266.
1MRS759142 C
to flow via the coupling
1
affects the level of the resulting
E
= 0, one coupling
E
Technical Manual
REX640