Choose 100MVA as the S_{base} (apparent power.) For calculation purpose, this value is reasonable and is widely used.

You may alternatively choose the MVA rating of the largest generator or the transformer in the power system. Equipment manufacturers who build these machines, list its MVA rating on the name plate.

Whichever MVA you choose as the base, remember that in per unit system, this value will never change. Be it at generation, transmission, or distribution.

Sample per unit problem solved:

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5 Responses to Choosing Sbase in the power system

  1. Lawal says:

    I use an MVA base of 100 for a Newton Raphson solution load flow, the solution did not converge until I chose a base MVA of 200. Can you please explain why?

  2. Admin says:

    -Sbase is not specified in per units. Voltages, currents, and impedances are.
    -yes, symmetrical components are used to analyse unbalanced faults.

  3. Antony Alapatt says:


    Is there a standard value you assume for Sbase like 1 p.u. or do I give any random value like 1.5 p.u. or 2 p.u.

    Also in your 4th Note of reply, you meant that Unbalanced 3-phase faults can be calculated, am I right?


  4. Admin says:

    -You don’t really calculate Sbase. You assume a value for it. It is like a fundamental quantity using which you calculate your Zbase and currents in the system.
    -Same goes for the Vbase. If the taps on the transformer are at the nominal position then you can assume the nominal voltage of the transformer as the base voltage (for each side).
    – Using Sbase and Vbase, you can proceed on to calculate other parameters in the system.
    -Note that when you are using per unit systems, you are assuming the system is balanced. Balanced 3-phase faults (not involving ground) can be analyzed using per unit systems too. You will need to use symmetrical components to deal with an unbalanced system.

    I will provide examples at a later time when I have availability. Hope this helps a bit.

  5. sam says:

    Hey Guru,
    Calculating the System Sbase
    I need some examples on how to perform system calculations using per unit system where you have 2 generators and transfomers connected on either side feeamdinding into lines with a combination of reactances in some sort of delta forming. Please help.


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