Furans in oil
When cellulose insulation decomposes due to overheating, organic compounds, in addition to CO2 and CO, are released and dissolved in the oil. These chemical compounds are known as furanic
compounds or furans. The most important one, for our purposes, is 2-furfuraldehyde. When DGAs are required, always request that furans testing be completed by the laboratory to check for paper deterioration. In healthy transformers, there are no detectable furans in the oil, or they are less than 100 parts per billion (ppb). In cases where significant damage to paper insulation from heat has occurred, furan levels may be at least 100 ppb and up to 70,000 ppb. Furanic content in the oil is especially helpful in estimating remaining life in the paper insulation, particularly if several prior tests can be compared and trends established.
Use the furan numbers in table 18 for assessment; do not base any evaluation on only one test; use several DGAs over a period of time to develop trending. See An Introduction to the Half-Century
Transformer by the Transformer Maintenance Institute, S.D. Myers Co., 2002 . The first column in table 18 is used for transformers with non-thermally upgraded paper, and the second column is for transformers with thermally upgraded paper. Testing is completed for five different furans which are caused by different problems. The five furans and their most common causes are
♦ 5H2F (5-hydroxymethyl-2-furaldehyde) caused by oxidation
(aging and heating) of the paper
♦ 2FOL (2-furfurol) caused by high moisture in the paper
♦ 2FAL (2-furaldehyde) caused by overheating
♦ 2ACF (2-acetylfuran) caused by lightning (rarely found in DGA)
♦ 5M2F (5-methyl-2-furaldehyde) caused by local severe
Doble inservice limits are reproduced below to support the above recommended guidelines.
Table 19 is excerpted from Doble Engineering Company’s Reference Book on Insulating Liquids and Gases . These Doble Oil Limit tables support information given in prior sections and are shown here as summary tables.
Additional guidelines given in table 20 are useful.
1 D 877 test is not as sensitive to dissolved water as the D 1816 test and should not be used with oils for extra high-voltage (EHV) equipment. Dielectric breakdown tests do not replace specific tests for water content..
2 The use of absolute values of water-in-oil (ppm) do not always guarantee safe conditions in electrical apparatus. The percent by dry weight should be determined from the curves provided. See the information in section 126.96.36.199.
3 ND = None detectable.
These recommended limits for inservice oils are not intended to be used as absolute requirements for removing oil from service but to provide guidelines to aid in determining when remedial action is most beneficial. Remedial action will vary depending upon the test results. Reconditioning of oil, that is, particulate removal (filtration) and drying, may be required if the dielectric breakdown voltage or water content do not meet these limits. Reclamation (clay filtration) or replacement of the oil may be required if test values for power factor, interfacial tension, neutralization number, or soluble sludge do not meet recommended limits.
1 When an oil is allowed to sludge in service, special treatment may be required to clean the core, coil, and tank.