Forums » Electrical Engineering

Transformer impedance

    • 156 posts
    January 14, 2020 12:39 PM PST


    based on which criteria manufacturer determines the impedance for transformers? In Europe, less than 630kVA, the transformer impedance will be 4% and greater than 630kA, the impedance of transformer will be 6%. The higher the percentage, lesser will be short circuit and the lesser cost of switchgear.

    I am looking to know the reason and why they choose 4% for less than 630kVA ?

    can it be based on core losses, stray losses or any other thing?

    Thanks in advance.

    with regards


    • 1 posts
    January 19, 2020 1:41 AM PST
    The impedance is determined by the specifications, the customer and the design. The design and size normally dictates the standard impedance, as you explained but it is possible to change the impedance by changing the core steel , number of turns etc.
    An auto-transformer has a very low impedance for example (normally under 1%) because of the design and will have very high short circuit currents as you explained. Not much can be done to increase the impedance on this transformer.
    This post was edited by Floris Erasmus at January 19, 2020 1:43 AM PST
    • 42 posts
    January 19, 2020 2:38 AM PST

    Due to full load there is always voltage drop to the winding resistance and leakage reactance. This is called Z%. This impedance represents the amount of normal primary voltage that should be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by multiplying the reciprocal of the impedance times the full load current.

    This post was edited by Ronnie Rana at January 19, 2020 2:38 AM PST