Cell construction for current limiting reactors

FOR MANY YEARS, current-limiting reactor manufacturers have been showing on outline drawings dimensions for distance to magnetic materials. Instruction leaflets define magnetic materials as “… I-beams, channels, plates, and other steel structures hidden or exposed. Steel reinforcing rods not over 3/4 inch in diameter which do not form a complete electric circuit are not considered magnetic material.” Warning is given that conducting loops which enclose flux act as the short-circuited secondary winding of a transformer. The clearances to magnetic materials shown in Fig. 1 are sufficient to keep magnetic saturation losses low enough so parts will not exceed 30 C rise. However, clearances do not permit constructing the cell to form a closed circuit about the reactor; nor are they great enough to permit use of high conductivity metals. The construction in Fig. 1 of an all-metal cell is not recommended unless the coils are magnetically shielded. Consider a 5%, 400 ampere feeder reactor having 40 turns of conductor placed in such a cell. From Fig. 2, we can approximate wall currents in the cell. For 20 times normal current, the coil would develop 400 × 40 × 20 = 320,000 ampere turns plus the possibility that the first half cycle would be fully offset to increase the ampere turns to 512,000 rms. For W/d = 2 we find the current in the cell wall will be 6.4% for aluminum (32,700 amperes); 2.4% for 11 microhm magnetic steel, 3,200 amperes per sq. in. (12,300 amperes); 0.66% for 70 microhm nonmagnetic metal (3,400 amperes). The aluminum cell would probably be blown open. The magnetic and nonmagnetic steel cells might also be opened, depending on how many and how tightly the bolts were drawn up that fasten the side sheets to the framing. At normal reactor current, the aluminum cell wall would have 6.4% of normal ampere turns (1,024 amperes); the current density in the magnetic steel would make its ampere turns the same as the 70 microhm nonmagnetic 0.66% (106 amperes). None of the foregoing cells would have excessive heating at normal current but, under fault conditions, they might prove to be dangerous. Insulated joint or magnetic shielded construction should be used.