Startco FPU-16 - Mode Descriptions; General; MODE 0 - I T LIMIT

Startco Engineering Ltd. Page 7
FPU-16 Feeder Protection Unit Rev. 1
Pub. FPU-16-M, November, 1997 Downloaded from the Internet, see Disclaimer.
3. MODE DESCRIPTIONS
3.1 GENERAL
This section contains a description of each mode. It explains operational
characteristics, describes special functions, and lists set-point ranges, display
codes, and default values.
3.2 MODE 0 — I
2
t LIMIT
This mode is used to enter long-time overcurrent set points and to annunciate
long-time overcurrent alarms and trips.
Thermal protection provided by the FPU-16 is based upon a constant I
2
t
characteristic that is defined by the Mode-0 set point and the rated current (I
n
)
programmed in Mode 8. The Mode-0 set point is the time, in seconds, required
for the FPU-16 to trip at 6 x I
n
. The Mode-0 set point is determined by equating
I
2
t at 6 x I
n
to the allowable thermal rating. For example, if the allowable thermal
rating is 2 x I
n
for 3 minutes, the I
2
t limit is:
6
2
x t = 2
2
x 180, and
t = 20 seconds—this is the Mode-0 set point.
One of the 45 available protection curves is shown in Figure 2. The largest of
the three phase currents is used for the I
2
t thermal model and the 0%-I
2
t curve is
the maximum-time-to-trip curve for current above I
n
. Current above I
n
causes I
2
t
to tend toward a limit above 100%, and current below I
n
causes I
2
t to tend toward
a limit that is less than 100% as shown in the inset graph in Figure 2. In all
cases, I
2
t responds exponentially toward its final value with a time constant
derived from the Mode-0 set point so that I
2
t tracks used thermal capacity.
Used thermal capacity determines the time-to-trip for excursions above I
n
. For
the Class-20 example shown in Figure 2, sustained operation at 0.87 x I
n
will
ultimately result in a used thermal capacity of 75% and the time to trip for
current above I
n
will be reduced as shown by the 75%- I
2
t curve.
The FPU-16 trips in Mode 0 when I
2
t reaches 100%. A Mode-0 trip cannot be
reset until I
2
t has decreased to 30% unless Mode 5 has been programmed to
allow an immediate reset. See Section 3.7.3 and Table 2. If Mode 5 has not
been programmed to allow an immediate reset, the time required for I
2
t to
decrease to 30% is a function of the derived time constant. Percent I
2
t and
minutes-to-reset are displayed in Mode d. The FPU-16 can be programmed in
Mode 5 to automatically reset when I
2
t decreases to 30%. See Section 3.7.1 and
Table 2.