The Per-Unit System

Charles A. Gross

Auburn University

1.1    Impact on Transformers

1.2    Per-Unit Scaling Extended to Three-Phase Systems

1.3    Per-Unit Scaling Extended to a General Three-Phase System

In many engineering situations, it is useful to scale or normalize quantities. This is commonly done in power system analysis, and the standard method used is referred to as the per-unit system. Historically, this was done to simplify numerical calculations that were made by hand. Although this advantage has been eliminated by using the computer, other advantages remain:

•  Device parameters tend to fall into a relatively narrow range, making erroneous values conspicuous.

•  The method is defined in order to eliminate ideal transformers ...

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