2. APPLICATION
2-8
DTP-B Digital Transformer Protection
GEK-106243E
2.3.1.2 Calculation of CT Ratio Error
1. CT Burdens
Assumptions: One-way cable resistance
R= 0.25 ohms
CT resistance per turn
e= 4.1 milliohms (100/5 CT)
e=
2.6
milliohms
(200/5
CT)
CT resistance per lead
f= 25 milliohms
Winding A:
(20 x 4.1 + 2.5 x 25)
Z = 0.04 +
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+ 2.27 x 0.25 = 0.75
1000
Winding B:
(20 x 4.1 + 2.5 x 25)
Z = 0.04 +
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+ 2.27 x 0.25 = 0.75
1000
Winding C:
(40 x 2.6 + 2.5 x 25)
Z = 0.04 +
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
+ 2.27 x 0.25 = 0.77
1000
2.
Impedance
(ohms)
0.75
0.75
0.77
3. Current I at 8 times the tap
value
(A)
20.0
40.0
40.0
4. Required Es (Is xZ) for the CT
15.0
30.0
30.0
5. Required exciting current, Ie
(from the excitation curve) 0.87A
>100A
0.26A
6.
Ration
error
in
%
4.35%
>100%
1.08%
The excitation current in winding B is excessive; therefore a higher CT ratio must be selected. The CT ratio for
winding B will be increased and the matching error and CT Ratio error will be checked again.
Summary of Contents for DTP-B
Page 23: ...3 OPERATING PRINCIPLES 3 6 DTP B Digital Transformer Protection GEK 106243E ...
Page 53: ...10 INSTALLATION AND MAINTENANCE 10 2 DTP B Digital Transformer Protection GEK 106243E ...
Page 68: ...12 FIGURES GEK 106243E DTP Digital Transformer Protection 12 3 Figure 3 RS 232 Connection ...
Page 69: ...12 FIGURES 12 4 DTP B Digital Transformer Protection GEK 106243E Figure 4 Dimensions Diagram ...
Page 70: ...12 FIGURES GEK 106243E DTP Digital Transformer Protection 12 5 Figure 5 Front View ...