I read a book about power system faults - they all use per unit. They show examples of generators supplying motors through a transmission lines. They convert real voltages, powers and resistances into per unit quantities and then use ohms laws to calaculte internal per unit voltages in the motors and generators to find short circuit fault currents. Why convert into per unit? Why not just use resistances ect to calaculte fault values?
Good ol' Wikipedia to the rescue. The Per-unit system article states:
In the power systems analysis field of electrical engineering, a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of a transformer to the other. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. Moreover, similar types of apparatus will have the impedances lying within a narrow numerical range when expressed as a per-unit fraction of the equipment rating, even if the unit size varies widely. Conversion of per-unit quantities to volts, ohms, or amperes requires a knowledge of the base that the per-unit quantities were referenced to. The per-unit system is used in power flow, short circuit evaluation, motor starting studies etc.
The main idea of a per unit system is to absorb large differences in absolute values into base relationships. Thus, representations of elements in the system with per unit values become more uniform. [Emphasis mine.]
So back to your questions:
Why convert into per unit? Why not just use resistances ect to calaculte fault values?
While the resistance may help solve a calculation at one particular voltage level - on a 10 kV feed, for example - you then need to scale it to see its effect on the other side of the transformer feeding it - on a 38 kV feed, for example. This gets tedious and confusing. The per-unit system solves this. A per unit measurement on one side of the transformer is the same on the other.
Disclaimer: I don't work in this end of the business and never heard of per-unit measures until it came up in previous questions on EE.SE.