Estimate of Paper Deterioration
9.9.1 Degree of Polymerization (DP)
Do not open a transformer for the sole purpose of doing this test. Perform this test only if the unit is being opened for other reasons. See the reasons for an internal inspection in section 9.9.2.
220.127.116.11 Background – One of the most dependable means of determining paper deterioration and remaining life is the DP test of the cellulose. The cellulose molecule is made up of a long chain of glucose rings which form the mechanical strength of the molecule and the paper. DP is the average number of these rings in the molecule.
As paper ages or deteriorates from heat, acids, oxygen, and water, the number of these rings decreases. When the insulation is new, the DP is typically between 1,000 and 1,400. As paper deteriorates, bonds between the rings begin to break. When the DP reaches around 200, the insulation has reached the end of life. All mechanical strength of the insulation has been lost; the transformer must be replaced.
18.104.22.168 Process – When doing an internal inspection, or if the transformer is opened and oil is fully or partially drained for any reason on a service-aged transformer, perform a DP test. Remove a
sample of the paper insulation about 1 centimeter square from a convenient location near the top of the center phase with a pair of tweezers. In general, in a three-phase transformer, the hottest most thermally aged paper will be at the top of the center phase. If it is not possible to take a sample from the center phase, take a sample from the top of one of the other phases. Send this sample to an oil testing laboratory for the DP test. Analyze results of the DP test with table 25 taken from EPRI’s Guidelines for the Life Extension of Substations, 2002 Update, chapter 3 . Table 25 has been developed by EPRI to estimate remaining life.
9.9.2 Internal Inspection
22.214.171.124 Background – If an internal inspection is absolutely necessary, it must be completed by an experienced person who knows exactly what to look for and where to look. Many times, more damage is done by opening a transformer and doing an internal inspection than what is gained.
There are very few reasons for an internal inspection; some are shown below:
♦ Extensive testing shows serious problems.
♦ Unexplained relay operation takes the transformer offline, and testing is inconclusive.
♦ Acetylene is being generated in the DGA (indicates active internal arcing).
♦ Ethylene and ethane are being generated in sufficient quantities to cause grave concern. This generally indicates a bad connection on a bushing bottom or tap changer, circulating currents, additional core ground, or static discharges.
♦ A core ground must be repaired, or an additional core ground has developed which must be removed.
♦ Vibration and ultrasonic analysis indicate loose windings that are generating gases from heat caused by friction of the vibrating coils; loose wedges must be located and replaced.
♦ CO2/CO ratio are very low (around 2 or 3), indicating severe paper deterioration from overheating. Cooling must be checked carefully before opening the transformer.
♦ Furans are high (see section 7.6), indicating excessive aging rate; a DP test must be completed.
♦ The metal particle count is above 5,000 in 10 milliliters of oil taken specifically for detecting metal particle count. (See EPRI’s Guidelines for the Life Extension of Substations, 2002 Update  for exact procedures for detecting and correcting metal particle contamination).
If a service-aged transformer is opened for any reason, a sample of the paper should be taken for DP analysis. If possible, take the paper sample from the top of the center phase winding because this will be near the hot spot. If it is not possible to get a paper sample from the top of B phase, take a sample from the top of one of the other windings.
126.96.36.199 Transformer Borescope – A new technology has been developed for internal transformer inspections using a specifically designed borescope. The borescope can be used with oil inside the transformer; core, windings, connections, etc., can be examined and photographed. If it is necessary to go inside the transformer for repairs, workers will possibly know exactly what is defective and exactly what must be done. This technology, used properly, can save generating time and repair dollars.
Currently, this technology is available only as a contracting service. However, plans are being made by the developing company to make the instrument available for sale.