Basler BE1-CDS220, Instruction Manual

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BE1-CDS220 Settings Calculations D-13
To compare the two expressions, we take the ratio SF’/SF:
) (
) ( '
Rs Zc
Rs ZB
ZB
Zc
SF
SF
+
+
⋅ =
Since this expression varies with the ratio of the actual relay circuit burden (ZB) to the accuracy class
burden (Zc) and the CT internal resistance (Rs), it is best visualized with a surface plot (Figure D-5)
showing simultaneous variations of the parameters. The following example is based on a C200 (Zc=2)
with Rs varying from 0.1 to 0.8 ohms and ZB varying from 0.1 to 2 times ZC ohms. (load angles are
neglected).
Figure D-5. SF’/SF Ratio
The 0 to 20 axis represents the variations X10 of ZB (20 is 2xZC). The 0 to 8 axis represents the
variations x10 of Rs in ohms. The vertical axis (0 to 4) shows that for ZB values equal to or greater than
the burden value Zc, the two saturation factor equations are nearly identical. The ANSI Accuracy Class
method yields the larger, more conservative result. For low values of ZB and large values of Rs, the
Excitation Curve method yields a larger saturation factor. Since the Excitation Curve method is closely
following the CT characteristics, it may be said that the ANSI Class method which neglects the CT
internal resistance, is too optimistic in this range and should be discarded in favor of the Excitation Curve
method. The absolute values of SF and SF’ are compared in Figure D-6 for the particular case where
Zc=2, ZB=0.5, and Rs=0.8 when IF varies from 0 to 100A.
Figure D-6. Comparing SF and SF’
0
2
4
6
8
0
2
4
6
8
10
12
14
16
18
20
0
1
2
3
SF
IF
SF'
IF
IF
0 20 40 60 80 100
0
0.2
0.4
SF and SF` for Zc=2, ZB=0.5, Rs=0.2