Rotor Earth Fault Protection Function - ABB REG670 Applications Manual

1MRK 502 071-UEN -
8.7.2.1
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
signal with a frequency different from the generator rated frequency is injected into
the rotor circuit.
To implement the above concept, a separate injection device is required. The injection
device generates a square wave voltage signal which is fed into the rotor winding of
the generator.
The magnitude of the injected voltage signal is measured at the injection point; the
magnitude of the injected current is measured through a resistive shunt located within
REX060. These two measurements are fed to the IED. Based on these two measured
quantities, the IED determines the rotor winding resistance to earth. The resistance
value is then compared to the preset fault resistance alarm and trip levels.
The protection function can detect earth faults in the entire rotor winding and
associated connections. The function can also detect excitation system earth faults on
the AC side of the excitation rectifier. The measuring principle used is not influenced
by the generator operating mode and is fully functional even with the generator at
standstill.

Rotor earth fault protection function

The injection to the rotor is schematically shown in figure 154.
C
rot
R
f
IEC11000065 V1 EN
Figure 154: Equivalent diagram for Sensitive rotor earth fault protection principle
The impedance Z
Measured
winding and earth (1/ωC
the injection circuit is eliminated. R
measured impedance, called the rotor reference impedance and can be calculated as :
1
= -
Z j
w
ref C
rot
EQUATION2510 V1 EN
alternative
DC
AC
REX 061
I
inj
is equal to the capacitive reactance between the rotor
) and the earth fault resistance (R
rot
is very large in the non-faulted case and the
f
Section 8
Impedance protection
R
C
f
rot
+
Rotor Reference
U
inj
Im pedance
-
IEC11000065-1-en.vsd
). The series resistance in
f
I
inj
+
U
inj
-
291