Phase k1 = 30%
Phase k2 = 100%
Therefore, settings in primary/secondary values for each device can be calculated as follows:
Setting End A End B End C
Phase CT Primary 4000A 4000A 1200A
Phase CT Sec’y 5A 5A 5A
Phase Is1 [p.u.] 0.2 0.2 0.2
Phase Is1 (Primary) 0.2*4000 = 800A 0.2*4000 = 800A 0.2*1200 = 240A
Phase Is1 (Secondary) 0.2*5 = 1A 0.2*5 = 1A 0.2*5 = 1A
Phase Is2 [p.u.] 2 2 2
Phase Is2 (Primary) 2*4000 = 8000A 2*4000 = 8000A 2*1200 = 2400A
Phase Is2 (Secondary) 2*5 = 10A 2*5 = 10A 2*5 = 10A
Note:
Settings shown in primary values at ends A and B appear different compared with end C. This is not a problem as the currents
at ends A and B will be multiplied by the Correction Factor, when the differential calculation is done.
Susceptance settings for Ends A and B
With a VT ratio 275 kV/110 V and CT ratio 4000/5:
R
CT
= 800
R
VT
= 2500
B = 310
m
S
Secondary susceptance = 310
m
S(R
VT
/ R
CT
)= 968
m
S
Susceptance settings for End C
With a VT ratio 275 kV/110 V and CT ratio 1200/5:
B = 310
m
S
Secondary susceptance = 310
m
S(R
VT
/ R
CT
)= 3.22 mS
P543i/P545i Chapter 6 - Current Differential Protection
P54x1i-TM-EN-1 131
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