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BELL SYSTEM TECHNICAL JOURNAL 



voltages, and the impedances and currents are the negative and zero 

 sequence quantities. 



The reduction of a network to an equivalent star is usually a tedious 

 and sometimes a difficult process especially in large interconnected 

 systems. Methods of accomplishing the reduction, such as delta-star 

 transformations, simultaneous equations or direct measurements on 

 calculating boards can be found in the literature.^- ^ 



Having reduced the three sequence networks to equivalent stars, 

 the equations are developed as shown in the Appendix. 



The following set of equations (1) is the general solution for fault 

 currents during simultaneous three-phase faults to ground at two 

 different locations in a power network. The set applies directly to 

 the calculation of ground fault currents on a system having finite 



FAULT A FAULT B 



Fig. 2 — Reduced positive sequence diagram. 



neutral impedances or an isolated system in which the zero sequence 

 capacitance is taken into account. For other types of faults, such as 

 faults to ground in isolated systems in which zero sequence capacitance 

 has been neglected, or for phase to phase faults, set (1) is not directly 

 applicable since some of the constants become infinitely large. How- 

 ever, by certain transformations of set (1), more convenient sets (2) 

 and (3) are obtained, directly applicable for solution of these latter 



cases. 



Neutral Grounded System 



(1) 



