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Utility line crews frequently perform manual re-energizing of distribution transformers. Industry studies indicate on average between 3% and 5% of the in-service population of pole top transformers are re-energized annually.

[1] Because of the potential hazards caused by the possibility of reclosing on a faulty transformer, all reclosing should follow strict safety procedures. These procedures are prone to human error and crews are often under pressure to restore power rapidly, putting themselves at risk. These risks can be eliminated when transformers are equipped with a reliable indicator that will signal the presence of an internal fault. The Internal Fault Detector (IFD) described in this report is such an indicator.

The IFD is designed for installation in the air space of oil filled electrical distribution equipment and is activated by the unique, rapid pressure rise that accompanies an internal arcing fault.

Once activated it displays a highly visible signal,
identifiable day or night to a ground based observer.

The feasibility of the concept was proved and patented in 1993/4. Design criteria satisfying side mounting, inclusion of the PRV and other requirements arising from discussions with Utilities and Transformer Manufacturers are detailed and information provided as to how these requirements were met. The operating conditions reflected in the section on design criteria include:

  • A required minimum operating life of thirty years
  • Be maintenance free over the life of the transformer
  • Be able to reliably detect internal faults at current levels as low as 500 A.

To satisfy these requirements a detailed set of design criteria was established and explanations given of how these criteria were met. As an example, to meet the design criteria of responding reliably to a wide range of pressure transients that might occur in the transformer air space, starting from peak pressure values as low as 0.5 psig (3.5 kPa) within five to seven milliseconds. To meet this requirement a pressure sensing membrane was developed.

On-going development projects are led by IFD Corporation with utility and manufacturer technical reviews held formally and informally through each stage of the process. This includes each iteration of design, prototyping, testing, IFD production and installation into transformers in manufacturing plants.

Certification tests are performed on completed units to ensure they meet the specified requirements. Powertech Labs performs a wide range of high power equipment testing including internal arcing fault testing on randomly selected units installed in various designs of transformers and with various input currents ranging from 500 A through 8000 A. These internal fault tests are performed using the withstand test protocol outlined in the IEEE C57.12.20 transformer standard as a guideline.

Read the IFD Test Summaries document