Per Unit

Per Unit Calculation -  this allows systems to be analyzed on all voltages levels at the same base. Transformers are represented by an impedance and since everything is on the same base(voltage, impedance, current, and power) the voltage step up and step down aspect is ignored. Equipment like transformers has % PU impedance at transformer rated base that can be converted to the system base, usually 100 MVA, and be compared to other items in the system. Some items like transformers have very small %PU ranges (remember it is in the transformers base, not in the common system base of 100 MVA). Transformers regardless of size will have a %PU between 6-12%. This allows for you to estimate the ohmic impedance of a transformer based on its MVA rating.

One thing that will look odd when reviewing the per unit calculations you will see the PU voltage value being used interchangeably in non-PU calculations you would use either VLN or VLL. PU voltage is the same for line-to-line as line-to-neutral for balance aka "normal" conditions for a balanced system, The triangle showing all the phasors L-L, and L-N vectors scales equally with different PU values.

The health of the system voltage is often represented in PU due to there are several voltage classes and standards reference %PU and not specific voltages.

Online Calculator
NEPSI simple calculator to change between ohmic and PU values

Michigan Tech Per Unity System - Lecture and Notes
https://huskycast.mtu.edu/Panopto/Pages/Viewer.aspx?id=94834965-9a26-4ef5-addd-04293e665779

http://www.ece.mtu.edu/faculty/bamork/ee5200/PreReq_L03.pdf

https://huskycast.mtu.edu/Panopto/Pages/Viewer.aspx?id=cc2c304a-52c0-425e-99f3-916986a674b3

http://www.ece.mtu.edu/faculty/bamork/ee5200/PreReq_L04.pdf

Books that make good reference of this:
Power System Analysis by Grainger and Stevenson

Power Systems Analysis and Design by Glover and Sarma

Presentation to IAS "Power System Calculations - Part 2" by Kurt Ederhoff