Basic criterion for transformer neutral point grounding is magnitude of capacitive earth fault current. Boundary values of earth fault currents [according to R. Detjen] are (until quoted values arc fault can extinguish without relay tripping of faulted feeder):
Ic = 20 A (Un = 10 kV) Ic = 15 A (Un = 20 kV) Ic = 10 A (Un = 35 kV)
R. Detjen, “Grounding Transformer Applications and Associated Protection Schemes”, IEEE Transactions on industry applications, Vol. 28, N0.4, 199.
If a grounding resistance is intended to be employed then
Ig>=3*Ico Ig=3*20=60 A
Rg=VLG/sqrt(3)/Ig=12.47*1000/3/60=69.3 ohm
Pnresg=Vo*Ig/sqrt(3)=12.47/sqrt(3)*60/sqrt(3)=249.4 kVA
If Isc[done] it is three-phase short-circuit current then Z1=12.47/SQRT(3)/11.5=0.626 ohm
If Z1=0.626;Z2=0.626 and Zo=0 [unknown actually] then the Isczigzag=
Ik1=sqrt(3)*c*Un/(Z1+Z2+Zo+3*Rg)=sqrt(3)*1.05*12.47/sqrt((3*69.2)^2+(0.626*3)^2)=0.1092 kA
If no grounding resistance and considering Zo=Z1 then[IEC 60909-0]:
Ik1=sqrt(3)*c*Un/(Z1+Z2+Zo)=sqrt(3)*1.1*12.47/(0.626*3)=12.65 kA
ANSI/IEEE Standard 32-1972 requires a continuous rating of 3% for a 10 second rated unit.
Then Irat for 10 sec will be=0.03*12.65 =0.38 kA
Sxfr=SQRT(3)*0.38*12.47=8.2 MVA for 10 sec,
