The Per- Unit System, abbreviated PU, is in the electric power industry an auxiliary unit of measurement relative to a reference value; they do serves to electrical data as relative and dimensionless pu - value express and to allow easier comparisons of relevant electrical parameters in power grids. In contrast to the also dimensionless and logarithmic level statements, which find application in the field of communications and can be expressed for example as Bel, the pu - specification is linear.

The Per- Unit System is used in performance specifications as the indication of active and apparent powers, as well as the details of electric voltages and impedances. In the pu - specifying a value, the reference variable and its dimension is essential.

Applications lie in the load flow calculation or Einliniendiagrammen. It will be made ​​in the key parameters such as load flows or impedances of power transformers and rotating electrical machines as relative pu specification.


There are several reasons for using the per- unit system:

  • In the power range used equipment such as generators, transformers or high voltage lines have similar parameters such as impedances, performance data or loss of information. Due to the relative reference in the pu system, the absolute size does not matter.
  • In energy networks, which are operated with three-phase alternating current, the chaining factor is added to the reference value, which can be information such as performance data of single-phase and three-phase systems compared directly without conversion factor.
  • Information in the per- unit system are independent of specific operating voltages and impedances or directly comparable on both sides of a power transformer with an appropriate choice.
  • The terms of the variables to a common base simplifies in particular the manual envelope calculation.


The underlying assets are arbitrarily chosen as the variables to set in relation in the same order of magnitude. Are usual round values ​​. For performance information, as the basis of the apparent power SB, reference values ​​from 1 MVA to 10 MVA, 100 MVA or 1 GVA be chosen depending on the application. When the reference voltage UB usually the rated voltage as 110 kV or 400 kV is selected. The reference value for the current IB is then obtained:

And for the reference value of the impedance Z:

In three-phase systems, the values ​​of the individual strands as phase voltage and phase current are generally used in making three-phase systems, the reference values ​​should be divided by the factor of concatenation. Performance specifications should be divided by a factor of 3.

The usual in the complex AC circuit analysis complex representation is also possible in the Per- Unit System: The amounts of the complex variables are divided into the polar form by the reference values ​​, the angles remain unchanged.


An overhead line 400 kV for the three- phase system is designed for the transfer of 1,200 MVA. The two reference values ​​are chosen to be:

In a short-term power boost to 2,100 MVA, the voltage drops to 390 kV. This corresponds in the pu system:


  • Jr William D. Stevenson: Elements of Power System Analysis Third Edition. McGraw- Hill, New York, 1975, ISBN 0-07-061285-4.
  • BM Weedy: Electric Power Systems Second Edition. John Wiley and Sons, London 1972, ISBN 0-471-92445-8.
  • Electrical Distribution
  • Auxiliary unit