3.2. Circuit Simulation 43
current, 28.9 pu on the 5-kVA base. At the 50-kVA transformer, this
fault will result in 1/0.346 pu = 2.89 pu current on a 50 kVA base, and
at the 500-kVA transformer the fault is 1/3.46 = 0.289 pu on a
500-kVA base. At any point in the system, one can define a base
impedance as
Zbase
=
V
LL
2
/
VA
or
Zbase
=
V
LL
2
/(1000
×
kVA) where
VA or kVA is a three-phase rating. Then, in ohms, Zohms = Zbase Zpu
at that base. The base impedance is the impedance that, when con-
nected to each line of a three-phase system at rated voltage, will draw
rated load current and develop rated voltamperes.
It is worth the effort to develop a familiarity with the per unit sys-
tem, because it greatly eases conversations with utility engineers, ...