ABB Relion 670 series, Technical Manual

Page: 435 / 1432

to use RMS to determine a change of machine condition because the RMS makes a distinction
between the measured values and the total amplitude of the signal. The standstill condition only
contains the injected frequency, while the full load condition and full speed condition contains
other frequencies, which amplitudes may change under varying machine conditions.
In case of a stator ground fault with fault impedance Z
f
the measured admittance will be:
1 1 1
ref f
Z Z Z
= +
EQUATION2513 V1 EN-US
and this gives:
1 1 1
ref
f
Z Z Z
= -
EQUATION2514 V1 EN-US
and the real part gives the fault conductance:
1 1 1
Re
f measured ref
R Z Z
 
= −
 
 
 
EQUATION2421 V2 EN-US
In the settings there are given two resistance levels:
• RAlarm given in Ω. If
f Alarm
R R
<
EQUATION2524 V1 EN-US
an alarm signal ALARM is given after a set delay
tAlarm.
•
RTrip given in Ω. If
f Trip
R R
<
EQUATION2523 V1 EN-US
a start signal BFI is given.
If the fault resistance is slightly below the set value RTrip the trip time will be about 10 s with
default filter length of 1 s. If the fault resistance is estimated to 0 Ω the trip delay will be 2 s with
default filter length of 1 s. For values in between the delay will follow linear interpolation
describing the fault resistance time relation, as shown in figure 231 .
Note that actual tripping time is dependent on the set parameter
FilterLength which has default
value of 1s.
1MRK 502 066-UUS B Section 7
Impedance protection
429
Technical manual