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6 F 2 S 0 8 5 7
Calculation steps Primary Secondary Tertiary
(1) Transformer capacity (kVA)
40 × 10
3
(2) Voltage(kV) 154 66 11
(3) Rated line current(A) 150 350 2100
=(1)/(
3 × (2))
(4) CT ratio 60 120 240
(5) Secondary rated line current(A) =(3)/(4)
2.50 2.92 8.75
(6) CT secondary rating(A) 5 5 5
(7) Setting =(6)/(5) Kct1=2.00 Kct2=1.71 Kct3=0.57
Note: kct1 to kct3 should be set to 2.00 or less. If more, the CT ratio matching of relay input current
may be not stable.
Secondary
40MVA
66kV
Primary
40MVA
154kV
CT1
300/5
CT2
600/5
CT3
1200/5
GRT100
A
B
C
kct1 kct3
kct2
Tertiary
12MVA
11kV
Figure 2.2.5.1 CT Ratio Matching
As explained in Section 2.2.3 for Mathcing of CT Secondary Currents, examples of setting for
both α-method and β-method are described as follows: