Degree of Polymerization (DP) Test to Estimate Deterioration of Paper (Offline During Internal Inspection)
One of the most effective tests to determine 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 decrease. 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.
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 center phase with a pair of tweezers. In general, in a threephase
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 the table below taken from EPRI’s Guidelines for the Life Extension of Substations.
Commentary Note:
The transformer shall not be opened to perform the DP tests only.
Internal Inspection
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 whichmust 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 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. Refer to EPRI’s Guidelines for the Life Extension of Substations for exact procedures for detecting and correcting metal particle contamination.
Commentary Note:
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.
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 costs.
Transformer Operating History – Power Transformer Diagnostic
One of the most important transformer diagnostic tools is the operating history including temperatures, overload history (especially through faults), relay operations, and nearby lightning strikes. Many times, a though fault or nearby lightning strike will generate acetylene or ethylene and other gases, and these will remain in future DGAs.
Through faults (i.e., where fault current passes through the windings) subject a transformer to severe electrical and mechanical stresses which can degrade insulation and mechanical strength. Likewise, close-in lightning strikes subject the transformer to stresses. Excessive heating caused by sustained overloads or loss of cooling will degrade insulation. The damage may not be immediately evident but can show up years later. Through faults, overload history, and temperatures should be in operations records, and a review of these is required when diagnosing a transformer problem.