Consideration of fault current projection on account of future growth for earthing design – sharing an experience

Design of earthing system of AC substation is largely depend upon the Grid current. The grid current Ig is derived from formula given in IEEE -80 where fault circuit current is multiplied with two other factors Df (decrement factor due to DC component of AC waveform) and Sf (current division factor). Decrement factor and Current Division factor can be considered from well established formulae and table in IEEE 80. Further the increase in fault level due to future development in the power system is really most critical issue and shall be considered with due care to take the final grid current.

It is humble effort to provoke the thoughts on the issue at large level amongst various utilities so that some reference value can be judged while designing the earthing grid. Even though it is purely customized value for particular system where the design is going to be adopted, sharing experience of various utilities may help the design fraternity in assuming the correct factor.

This paper describes practical trend of increase in fault level of a substation based on the author's experience and database collected. IEEE 80–2013 suggests that due care should be taken in consideration of increase in fault current as future development will reduce the current division factor as some new lines and/or neutrals will come into circuit and hence it may compensate the increase in current thus any more factor if considered for increase in current will cause the over design the grid. Example is quoted in the paper to illustrate the effect of increase in fault current with the decrease in division factor.