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Distribution Transformers what purchasers should consider.

By Ian W. Robertson Managing Director of Tanelec Limited(Tanzania)
There are many questions asked by customers about distribution transformers. There are many people, including utility engineers, who have preconceptions and misconceptions about certain aspects of features, options and what type or design of transformer is optimal for its intended purpose. Whilst many of these people are well qualified and very knowledgeable there can be no doubt that recognized top transformer manufacturers in the world know best – that is their core business and is what they do every day of their lives.

The top transformer producers in the world spend vast amounts of funds on R&D, some as much as 5% of revenues. These levels of R&D mean that, despite transformers technology being extremely mature, there is a gradual improvement in technology and design expertise. The main progress in recent years comes from improvements in the materials used in the manufacture of transformers.There are different ways how these improvements can be used, whether it is to enable cost, and thus price improvements, or alternatively used to design transformers capable of withstanding extreme climatic or operational conditions.


The key for customers is therefore to ensure that they specify and purchase the distribution transformers most suited to the required service application. The rewards for doing that will be optimal lifetime cost. Not understanding what the best option is, can result in buying equipment that is not best designed for its purpose and with the risk that the expected lifetime will not be attained. Involving manufacturers at an early stage to assist in identifying the best specifications can be very worthwhile. One common mistake in some African countries is for “first time cost” to be the major purchasing criteria and that is often achieved through buying cheap and poor quality transformers.
The answer to that is “you get what you pay for”.


It has generally been accepted that the design life of a distribution transformer is 25 years whilst, in reality, it can be 40 years or more if it is looked after properly. In some African countries the life expectancy can be as little as 5 years – why is that? It can be attributed to a number of the following reasons:
Poor maintenance(oil sampling etc)
Inadequate or no protection being installed in the lines against voltage and current surges.



Every surge weakens a transformer. Overloading, resulting in higher operating temperatures, deterioration of internal insulation materials resulting in sludge formation which reduces circulation and further accelerates temperature increases and so the cycle continues.
Is there too much pressure on utilities to create connections for customers at the expense of installing a quality distribution network? Again the lesson can be that higher first time costs will very probably result in lower lifetime costs.


Some utilities are now buying, or considering buying, dry type transformers because they like the idea that there is no transformer oil for stealing – a common problem in some African countries. The same utilities even refer to these as being “new technology”. The truth of the matter is that dry types have been around for many decades and have been designed for specific applications where they can minimize fire hazards, reduce the need for costly fire precautions/installations and especially for reducing overall installation costs in high rise buildings.
Substituting a dry type for a standard oil filled distribution transformer does not always make economic sense. They are much larger and heavier and thus ground mounting instead of pole mounting may be required. They are also considerably more expensive. Oil filled units need minimal maintenance but dry types need much more as, depending on location the windings will attract dust, insects etc and will need cleaned at regular intervals.
The final reminder is that the manufacturers know best and can advise what is best.


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