Analysing Transformer Oil - How Often ?

We have ten 11kV to 415V transformers at my place of work.
Can anyone suggest how often we should have the transformer oil analysed ?
We have had various suggestions from contractors, from anually to once every eight years.
TIA,
woodglass
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every
As this is a relatively low cost operation, it should be done annually, by drawing a sample in clean bottles from near the bottom of the tank (Drain Cock). When I was involved in this operation, as a maintenance engineer in the 70's, the company who we allocated the refiltering and replacement of the oil, supplied the bottles and carried out the analysis free of charge. We drew the samples. Jaymack
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woodglass wrote:

If the transformer is in good condition, then annual testing is recommended. Once any increase in gases is found, then more frequent testing is required to stay ahead of a potential failure.
We recently had a customer with a 3750 KVA transformer that tested within limits for five consecutive years. Then all of a sudden there was a spike that continued to rise over monthly tests. The odd thing is that it was a problem created by Square D during the assembly of the transformer. For five years, a very poor weld in the low voltage leads was fine but suddenly became a hot spot. The gases helped us to pinpoint the problem and repair it before there was a significant failure.
Steve Lockridge Manager Alfa Transformer www.alfatransformer.com
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