ABB Relion 670 series - Page 409

DC
AC
REX 061
R
f
+
V
inj
-
I
inj
C
rot
Rotor Reference
Impedance
R
f
+
V
inj
-
I
inj
C
rot
ANSI11000065_1_en.vsd
ANSI11000065 V1 EN-US
Figure 216: Equivalent diagram for Sensitive rotor earth fault protection principle
The impedance Z
Measured
is equal to the capacitive reactance between the rotor winding and
ground (1/ωC
rot
) and the ground 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-US
alternative
1
rot
ref
j C
Z
w =
EQUATION2511 V1 EN-US
Where
2
inj
f w p = ×
EQUATION2512 V1 EN-US
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.
If the machine (generator) is stationary running (a few seconds or more) on 1/3 of the injection
frequency, the injection function will start to send errors. Therefore, 1/3 of the injection frequency
will not be used for spinning reserve, either the machine frequency or the injection frequency has
to be changed. The error occurs because the 3
rd
harmonic of the machine is on top of the injection
frequency and causes interference.
For example, if the injection frequency is 87Hz and the fundamental frequency of machine is on
87/3 = 29Hz, then the 3
rd
harmonic of machine is 87Hz, which can cause injection interference
errors.
The solution is to either change machine frequency or injection frequency.
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.
1MRK 502 066-UUS B Section 7
Impedance protection
403
Technical manual