Transformer Failures

Forensic Services Newsletter

Multiple points of failure of HT winding stack of dry-type transformer.
Severe damage to paper insulation on coil windings of oil-filled transformer.

This month's newsletter on transformer failures is prepared by Tony Jarratt, an electrical engineer engaged in failure investigation and restoration.

Arcing damage to high tension conductors in oil-filled transformer.

We are encountering an increasing number of losses involving dry-type transformers. Although there are some suggestions that these might have a higher failure rate than their oil-filled counterparts, the increasing frequency of failures that we see is more likely due to the dry-type transformer being increasingly used throughout the world.

The following table is presented listing the pros and cons of each type.

Fire Significant risk of fire in event of failure. Low risk of fire.
/ Monitoring
Monitoring required, with analysis of oil. Low.
Cost Lower than dry type. Higher than oil-type.
Warning Probable indication of failure if monitoring carried out correctly. No warning of failure.
Repairability Can be repaired. Unlikely to be cost effective.
Soot from breakdown of HT winding of dry-type transformer.

Up until recently, there was a widespread usage of oil-filled transformers for almost all locations in the distribution of electrical energy. While oil-filled transformers are still very popular and are used to almost the exclusion of anything else in the distribution networks of most national and urban power companies, there is a growing trend for the use of dry-type transformers in other situations. This is because there is an increasing requirement for indoor locations of distribution transformers.

Arcing damage between core lamination and clamping bolt of oil-filled transformer.

From the point of view of insurers, there is less chance of a fire loss with dry-type transformers. However it remains that whether oil-filled or dry, the transformers are usually the only providers of electrical energy for the buildings in which they are located, and their failure without prior warning usually means a general power outage of noticeable proportions, with considerable associated financial losses.

Damage to inside of coil winding stack of oil-filled transformer.

It also remains that in the event of a loss, insurers might require certain questions to be answered concerning the failure. Examples follow, in no particular order.

  1. What is the proximate cause of the failure?
  2. Does the loss involve fire?
  3. Was maintenance carried out correctly?
  4. Was there a manufacturing defect?
  5. Is the failure electrical in nature?

Forensic Services is skilled in investigating transformer failures and answering these questions. Insurers and adjusters should direct inquiries to :

Tony Jarratt (H/P : +60 12 376 7730)
Khairul Za'im (H/P : +60 12 205 8967)


Barry Dillon