Parameter Description
Explanation
C
Coefficient specific
to selected curve
Affects the multiple of PU where the curve would approach infinity if
allowed to continue below pickup. Has greatest effect on curve
shape near pickup.
N
Exponent specific
to selected curve
Affects how inverse the characteristics are. Has greatest effect on
curve shape at low to medium multiples of tap.
K
Constant
Characteristic minimum delay term.
T
R
Time to reset
Relevant if 51 function is set for integrating reset.
R
Coefficient specific
to selected curve
Affects the speed of reset when integrating reset is selected.
Setting Programmable Curves
Curve coefficients are entered using BESTCOMS. Alternately, curve coefficients can be entered using the
SP-CURVE ASCII (Settings Protection-programmable curve) command. Table 4-17 lists the
programmable curve settings.
Table 4-17. Programmable Time Current Characteristic Curve Coefficients
Setting Range
Increment
Default
A Coefficient
0 to 600
0.0001
0.2663
B Coefficient
0 to 25
0.0001
0.0339
C Coefficient
0.0 to 1.0
0.0001
1.0000
N Coefficient
0.5 to 2.5
0.0001
1.2969
R Coefficient
0 to 30
0.0001
0.5000
Curve coefficients are entered by selecting the
Curve
Coefficients
button on the
51
tab in the
Time Overcurrent
screen. (Refer to Figure 4-25.) The Curve Coefficients screen
will appear (see Figure 4-27). Enter the calculated values for
each constant and select
Done
.
9365200990 Rev F
BE1-CDS240 Protection and Control
4-33
Programmable curve coefficients can be entered regardless of
the curve chosen for the protection element. However, the
programmable curve will not be enabled until
P
is selected as
the curve for the protective element.
Figure 4-27. Curve Coefficients
46 Curve
The 46 curve is a special curve designed to emulate the I
2
t withstand ratings of generators using what is
frequently referred to as the generator’s K factor. Do not confuse the 46 curve with the 46 element. The
46 curve was designed for use with the 46 function. But, in actuality, the 46 curve may be selected for use
with the 51P, 51N, and 51Q protection functions as well (though in actual practice, it is doubted that this
will be done very often).
To use the 46 curve, the user should determine the K factor of the generator and the continuous (I
2
)
2
t
rating of the generator (supplied by the manufacturer) and use this to set the time dial and pickup for the
46 curve by the process described in Appendix A,
Time Overcurrent Characteristic Curves
. The K factor
is the time the generator can withstand 1 per unit I
2
where 1 pu is the relay setting for nominal current.
Summary of Contents for BE1-CDS240
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