TB XRI1-IE 11.01 E
21
6.2
Setting value calculation
6.2.1
Definite time overcurrent element
Low set element I>
The pickup current setting is determined by the load
capacity of the protected object and by the smallest
fault current within the operating range. The pickup
current is usually selected about 20% for power lines,
about 50% for transformers and motors above the
maximum expected load currents.
The delay of the trip signal is selected with considera-
tion to the demand on the selectivity according to sys-
tem time grading and overload capacity of the pro-
tected object.
High set element I>>
The high set element is normally set to act for near-by
faults. A very good protective reach can be achieved
if the impedance of the protected object results in a
well-defined fault current. In case of a line-transformer
combination the setting values of the high set element
can even be set for the fault inside the transformer.
The time delay for high set element is always inde-
pendent to the fault current.
6.2.2
Inverse time overcurrent
element
Beside the selection of the time current characteristic
one set value each for the phase current path and
earth current path is adjusted.
Low set element I>
The pickup current is determined according to the
maximum expected load current. For example:
Current transformer ratio: 400/5A
Maximum expected load current: 300A
Overload coefficient: 1.2 (assumed)
Starting current setting:
Is = (300/400) x 1.2 = 0.9 x I
N
Time multiplier setting
The time multiplier setting for inverse time overcurrent is
a scale factor for the selected characteristics. The
characteristics for two adjacent relays should have a
time interval of about 0.3 - 0.4 s.
High set element I>>
The high set current setting is set as a multiplier of the
nominal current. The time delay tI>> is always inde-
pendent to the fault current.
6.3
Indication of measuring and fault
values
6.3.1
Measuring values
The following measuring quantities can be indicated
on the display during normal service:
•
Apparent current in phase 1 (LED L1 green)
•
Apparent current in phase 2 (LED L2 green)
•
Apparent current in phase 3 (LED L3 green)
•
Apparent earth current (LED E green)
6.3.2
Indication of fault data
All faults detected by the relay are indicated on the
front plate optically. For this purpose, the four LEDs (L1,
L2, L3, E) and the four function LEDs (I>, I>>, IE> and
IE>>) are equipped at
XRI1-IE
. If, for example an
overcurrent occurs, first the LEDs of the corresponding
phases will light up. LED I> lights up at the same time.
After tripping the LEDs are lit permanently.