Transformer Protection

inter-teknik Back-Up fuse-links for transformer protection

inter-teknik MV Back-Up fuse-links are primarily used to protect distribution type transformers from over¬heating in case of internal and external short-circuit faults.

Accordingly, fuse-link selection is based on two criteria (see fig. 1):

  • The fuse-links shall interrupt a three-phase bolted short-circuit current (Isc) on the l.v. termi¬nals within 2 s corresponding to the transformer withstand time. Isc is calculated as follows;

    Isc = InTr / uk

  • The fuse-links must not operate nor single fuse-elements be damaged by transformer inrush currents (magnetizing currents). A representative value for distribution type transformer inrush currents has been found 12 times transformer rated current for a duration of 100 ms.

Selection: All you need to do is to mark Isc and transformer inrush current; shown with red dots on Fig 1. Then, you can check which fuses’ time-current curves lie in between these values. Note that there may be more than one applicable fuse for a given application.

For ease of application, you may refer to the FUSE SELECTION TABLE FOR TRANSFORMER PROTECTION.

IMPORTANT: Selection is as simple as this! However, when fuses are combined with on-load switches, additional requirements come into play. Please visit Switch-Fuse combinations for more information.

Related Links:

Electrical Utilities
Medium Voltage Switchgears
Distribution Transformers