My assignment:
A simple power system consists of Generator of reactance j0.26 p.u., transformer of reactance j0.16 p.u. and a circuit breaker connected to a system of reactance j0.19 p.u. A three-phase fault occurs between transformer and circuit breaker. What is the fault current in per unit?
My initial solution considers only the impedances of the generator and transformer, so total fault current would be 1/(0.16 + 0.26) = 2.38 pu.
Another interpretation of the problem argues that the j0.19 pu system reactance pertains to the grid, so then, it should be 1/[(0.16 + 0.26)^-1 + 0.19^-1]^-1 = 7.64 pu fault current.
But I still maintain my 2.38 pu answer for three reasons:
- There is no other fault current contributor after the breaker.
- Realistically, the breaker should open, so the fault loop only includes Generator -> X-MER -> Reference Bus.
- Problem states a simple power system, so a radial system that terminates at a load (perhaps static since only generator is mentioned which therefore acts as sole contributor to fault current) seems a more fitting interpretation than an interconnected or network system.
