Power Systems X/R Ratio

When people mention System X/R ratio, it indicates - during a fault condition - the impedance between the generating units and the fault point. The fault current can be represented by a sinusoidal current (AC component) and a DC offset that decays with a time constant t = L/R. The power systems usually have a X/R value of 10 to 20. The higher value of X/R, the higher voltage the power systems turns to be. Figure below shows the relationship between the X/R and the system time constant.

We know that low voltage system has a smaller value of X/R. For instance, a system with X/R = 10 takes time constant of 0.03 seconds to have the DC component to decay to 36.8% (1/e) of its initial value. On the other hand, a X/R = 20 system takes 0.05 seconds (three cycles) to reduce to 36.8%.