Electrical

Hi there,

A Furan test is used to detect the breakdown of the paper insulation. Furans are usually reported in ppb (parts per billion). The Furan results are used to calculate an average expected DP for the paper in the equipment. The calculation is dependent on whether the transformer insulation is made up of thermally upgraded paper or not.

The calculated DP is used to estimate the percentage of the solid insulation life that remains.

The solid insulation in a transformer is made up of paper. Paper is made up of cellulose fibers. Cellulose is a polymer formed from glucose molecules. When the paper is brand new, before it has been installed in a transformer and factory dried, the average cellulose polymer chain is 1000 to 1200 glucose molecules long. Installation and drying breaks down the cellulose a little bit, so that new paper in a new transformer has slightly shorter polymer chains – about 800 to 1000 glucose molecules long. We call the average cellulose chain length the "Degree of Polymerization" (DP) of the paper. As the paper ages, there is a natural and gradual breakdown of the polymer chains. As the chains get shorter, the mechanical strength of the paper is also reduced.

DP of the paper has a direct relationship to the mechanical strength (tensile strength) of the paper. When DP has been reduced by aging to 200, the paper is so weak that any stress will disrupt the paper and lead to failure. This is the practical definition of the end of reliable life for the solid insulation and therefore the end of life for the equipment.

When the cellulose chain breaks and two shorter chains are formed, the breakdown process "kicks out" one or more of the glucose molecules and also creates some water, carbon monoxide, and carbon dioxide. The glucose molecule changes chemically during this process and forms a compound containing a furan ring.

The change in furan content – the amount of furanic compounds generated during the testing interval – is the most important parameter for determining whether there is an active fault in the equipment that needs to be addressed. For a first analysis, where there is no past history – or where the past history is so old as to be virtually meaningless – we use the following standards for interpreting results:

In addition to our AC, QU, and UN ranges, we consider very high levels to be of more immediate concern. Levels over 1000 ppb indicate severe, irreversible damage to the solid insulation. We consider this to be the start of the "danger zone" because we start to see transformer failures in the range of 1000 to 1500 ppb total furans. We typically do not recommend reclaiming or other oil maintenance procedures for transformers where the total furan content is in this range. End of reliable life for a transformer with thermally upgraded insulation that has aged gradually, without hot spots, is approximately 2800 ppb total furans.

Regards,

Craig