Rockwell Automation 865 Differential Protection Relay - Manual (Page 62)

3-18 Protection Functions
865-UM001A-EN-P – July 2009
Figure 3.12 shows a functional block diagram of the I
0
> earth
overcurrent stage with definite time and inverse time operation time.
Figure 3.13 shows a functional block diagram of the I
0
>>, I
0
>>> and
I
0
>>>> earth fault stages with definite time operation delay.
Input Signal Selection
Each stage can be connected to supervise any of the following inputs
and signals:
•
Input I
01
for all networks other than rigidly earthed
•
Input I
02
for all networks other than rigidly earthed
•
Calculated signal I
0Calc
for rigidly and low impedance earthed
networks. I
0Calc
= I
L1
+ I
L2
+ I
L3
•
Calculated signal I’
0Calc
for rigidly and low impedance earthed
networks. I’
0Calc
= I’
L1
+ I’
L2
+ I’
L3
Additionally the stage I
0
> have two more input signal alternatives to
measure current peaks to detect a restriking intermittent earth fault:
•
I
01Peak
to measure the peak value of input I
01
•
I
02Peak
to measure the peak value of input I
02
Intermittent Earth Fault Detection
Short earth faults make the protection to start (pick up), but will not
cause trip. When starting happens often enough, such intermittent
faults can be cleared using the intermittent time setting.
When a new start happens within the set intermittent time, the
operation delay counter is not cleared between adjacent faults and
finally the stage will trip. By using input signals I
01Peak
or I
02Peak
a
single one-millisecond current peak is enough to start the stage and
increase the delay counter by 20 ms. For example if the operating
time is 120 ms, and the time between two peaks does not exceed the
intermittent time setting, the sixth peak will cause a trip.
Four Independent Undirectional Earth Fault Overcurrent Stages
There are four separately adjustable earth fault stages: I
0
>, I
0
>>,
I
0
>>>, and I
0
>>>>. The first stage I
0
> can be configured for definite
time (DT) or inverse time operation characteristic (IDMT). The other
stages have definite time operation characteristic. By using the definite
delay type and setting the delay to its minimum, an instantaneous
(ANSI 50N) operation is obtained.