standards / manuals / guidelines for small hydro development - AHEC

Zero sequence impedance would be that of transformers above as 12 % on 100 MVA.Impedance X of diagram is shown in figure.X 0 = 0.04 0.24 x0.120.04 0.240. 12=0.28x0.120.28 0.12= 0.0840.240.12X 0 = 0.040.24Since X 0 is greater than X 1 , single line to ground fault need not be considered.4. Selection of Breaker rating as per IEEE std. 37010-1999.Load current =437 x 2 = 874ARated normal continuous current =As per table Xc of IS 131181250ARated short breaking current = 20,000A at 145 kVThree phase short circuit current is 7056A is less than 80% of symmetrical interruptingcapability ( 16,000A) and large growth margin exists; the 132 kV breakers at step upsub station are rated 1250A; with rated short circuit breaking current of 20,000A.Rating as per earlier AIEEE Committee methodMaximum initial symmetrical current =7050ARating for 5 cycle breakers & factor = 1.1and mandatory duty factor = 1.6short circuit rating = 7050 x 1.1 x 1.6= 12419Aand for 3 cycle breakers = 134547.5According 20,000 kA; 1250 A breakers as O. K.4-25