ABB REG670 - Rotor Earth Fault Protection Function

7.8.9.2 Rotor Earth Fault Protection function
The injection to the rotor is schematically shown in figure 127.
DC
AC
REX
061
C
rot
R
f
+
U
inj
-
I
inj
C
rot
Rotor Reference
Impedance
R
f
+
U
inj
-
I
inj
IEC11000065-1-en.vsd
IEC11000065 V1 EN
Figure 127: Equivalent diagram for Sensitive rotor earth fault protection principle
The impedance Z
Measured
is equal to the capacitive reactance between the rotor
winding and earth (1/ωC
rot
) and the earth fault resistance (R
f
). The series resistance in
the injection circuit is eliminated. R
f
is very large in the non-faulted case and the
measured impedance, called the rotor reference impedance and can be calculated as :
1
ref
rot
Z j
C
w
= -
EQUATION2510 V1 EN
alternative
1
rot
ref
j C
Z
w =
EQUATION2511 V1 EN
Where
2
inj
f w p = ×
EQUATION2512 V1 EN
The injected frequency f
inj
of the square wave, is a set value, deviating from the
fundamental frequency (50 or 60 Hz). The injected frequency can be set within the
range 75 – 250 Hz with the recommended value 113 Hz in 50 Hz systems and 137 Hz
in 60 Hz systems.
R
series
is a resistance in the REX061 unit used to protect against overvoltage to the
injection unit. Such overvoltages can occur if the unit is fed from static excitation
system.
1MRK502052-UEN B Section 7
Impedance protection
287
Technical manual