Estimate of Paper Deterioration

Estimate of Paper Deterioration (Online)

The two methods below should be used together:

CO2 and CO Accumulated Total Gas Values

IEEE Standard C57.104™ Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers, gives status conditions, based on accumulated values of CO2 and CO. Accumulated dissolved gas levels provide four status conditions: normal operation, modest concern (investigate), major concern (more investigation), and imminent risk (nearing failure). The CO2 and CO levels in ppm for each status are given in table shown on Picture 1.

CO2 /CO Ratio

The CO2/CO ratio should also be analyzed in conjunction with the CO2 and CO accumulated values.

Picture 1: Paper Status Conditions Using CO2 and CO


CAUTION: The status from table on Picture 1 should be at least in Condition 2 or 3 from one or both gases before a detailed investigation is begun. There is no need to look at the ratios, unless a substantial amount of these gases have already been generated. If the transformer is relatively new, CO2 and other atmospheric gases (N2, O2, and even some CO) may be migrating out of the paper into the oil because the paper was stored in air prior to transformer assembly. If the paper was stored in a polluted city atmosphere, a considerable amount of CO may show up in the DGA. This may look like the transformer has a problem and is generating a lot of CO. However, if the transformer has a real problem, H2 and perhaps other heat gases (CH4, C2H6, C2H4) should also be increasing.

Estimate of Paper Deterioration (Offline During Internal Inspection)

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. 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.


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 shown on Picture 2 taken from EPRI’s Guidelines for the Life Extension of Substations (2002 Update, chapter 3). Table on Picture 2 has been developed by EPRI to estimate remaining life.

Picture 2: DP Values for Estimating Remaining Paper Life

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 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, 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.

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 dollars.