Sandra Rintoul, Wilks Enterprise Inc.
The electrical transformer is an essential link in the power distribution grid used to step down the high voltage electricity transmission for use in businesses and homes. More than 70 percent of large power transformers are 25 years old or older with the average age approaching the expected 40-year design life. Because of the large capital investment, utilities would rather extend the life of a transformer than replace it. Coupled with this, however, are risks and consequences of transformer failure. A severe failure can come with a high cost, therefore, avoiding a failure and extending the life of a transformer has obvious financial benefits.
Many utilities have instituted life cycle transformer management programs in an attempt to extend the life of this major asset. About 3 percent of transformer failures are due to deteriorated liquid insulation. Maintaining the proper antioxidant levels to reduce degradation caused by oxidation of the mineral oil insulation is one aspect of transformer management. Two common inhibitors that prevent oxidation of the transformer’s insulation oil are 2,6 ditertiary-butyl para cresol (DBPC) and 2,6-di-tert-butyl phenol (DBP). Periodic testing typically is not done as often as it should be because utilities want to avoid the time and expense required to transport samples to a laboratory. Easy-to-use, portable filter-based infrared analyzers are relatively inexpensive and ideal for on-site testing of DBPC and DBP. On-site measurements not only encourages more frequent testing but save money by reducing expenses associated with transporting samples to an off-site laboratory for analysis. Technicians also are able to make an immediate correction while still at the location.
Antioxidant Analysis: Infrared vs. Gas Chromatography
During the life of the transformer, mineral oil and paper are subjected to three main factors—heat, moisture and oxygen—that can cause it to break down. To minimize the detrimental actions of oxygen during operation, the oxidation inhibitors DBPC or DBP are added to the mineral oil at about 0.3 weight percent. This amount of inhibitor is considered the optimum concentration for prolonging transformer life. As the inhibitor is depleted over time, it becomes necessary to analyze the oil to determine the remaining antioxidant level and add an additional amount, bringing it back to the optimum concentration. This analysis can be accomplished quantitatively with infrared spectroscopy.
ASTM D2668 calls for mid-infrared (mid-IR) spectroscopy for the measurement of antioxidant levels in transformer oil. The mid-IR range is an ideal choice for the analysis due to the excellent absorbance of organic chemicals in this spectral region. DBPC and DBP have characteristic infrared absorption at 3,650 cm-1 [2.7 micrometers (àŽ¼m)] with no interference from the mineral oil. As the inhibitor concentration decreases, the infrared absorbance at that wavelength also decreases. The infrared absorbance can be directly calibrated to display in weight percent.
ASTM D4768 provides another method for DBPC or DBP inhibitor determination. This method is performed by gas chromatography (GC) using a solvent solution and flame ionization detector (FID). Typical analysis time for each sample, including standard preparation, is one to two days. Not only does the GC method require the use of flammable solvents, but five standards must be run prior to sample analysis. This method cannot be performed on-site and requires a skilled technician.
Infrared analysis, using a portable filter-based analyzer, requires less than 7 milliliters of sample for analysis or calibration verification. A pre-prepared standard can be easily stored for more than six months under refrigeration, eliminating the need to make a standard prior to calibration verification or sample analysis. In addition, this infrared method eliminates the disposal of flammable solvent and only a small volume of waste transformer oil is left after testing. The complete analysis takes less than one minute and can be performed at the transformer location by a non-technical operator. This analysis method is also useful for facilities that recondition transformers.
Filter-based Infrared Analyzers and Procedure
A simplified filter-based portable infrared analyzer can be used, especially for on-site measurements, instead of a FTIR (Fourier Transform Infrared) spectrometer. This system has a filter mounted on a detector that is specific to the analysis, in this case 3,650 cm-1 or 2.7 àŽ¼m. Infrared light is passed through a sample cell containing the transformer oil and is then focused onto the filter/detector as shown in Figure 1. The detector measures how much energy was absorbed by the sample due to the antioxidant. The signal is then converted to so it displays in weight percent.
Many advantages of a filter-based infrared analyzer exist: convenient size, portability, lower cost, ruggedness, decreased power usage and operation by non-technical personnel. A rugged six-inch square box with no moving parts that will easily fit in a truck, operate off a 12-volt car battery, and function in a wide temperature and humidity range is ideal for on-site transformer oil analysis in remote locations. This portable instrument allows technicians to test antioxidant levels and immediately adjust them to ensure optimum liquid insulation.
The analyzer is zeroed on uninhibited oil using a syringe to fill the 1 millimeter path-length sample cell. It must be zeroed only every few hours. The operator transfers the sample into the cell with the syringe, allowing several milliliters to flush the previous sample or zeroing oil through. Once the sample cell is filled, the operator presses the “run” button, and 30 seconds later the analyzer displays the result.
Personnel can create a calibration verification sample by measuring a specific weight of the antioxidant into the oil and heating it until it dissolves. As mention earlier, a large volume can be prepared in advance and used for future calibration checks if desired. While the calibration will remain constant for several years, periodic calibration checks ensure that the system is working properly. The graph in Figure 2 shows that the calibration standards line up on the calibration curve up to the highest prepared standard at 0.60 weight percent.
To validate the fixed filter infrared method against the ASTM method, samples for this study were obtained by the S. D. Myers Inc. laboratory in Tallmadge, Ohio. The dielectric fluids were drawn from transformers sent to their facility for service. Lab personnel analyzed the customers’ samples for inhibitor content with a laboratory bench top FTIR using ASTM method D2668. They tested Split samples of the oil on the Wilks InfraCal Filtometer. The table in Figure 3 shows that the two methods are comparable.
As utilities juggle the cost of replacing aging transformers with the cost of failures, they should consider every aspect of transformer management that will allow them to maintain the health of the system. A quick, inexpensive on-site test that reduces the deterioration of the liquid insulation and prolongs transformer life is a valuable addition to both power utilities and the companies who manufacture, rebuild and recondition transformers.
Sandra Rintoul is president of Wilks Enterprise Inc. She wishes to acknowledge and thank Ron Hontert and the S. D. Myers team for the FTIR test results they provided on the inhibitor content from various transformers. Their efforts helped her and Wilks validate that the fixed filter infrared method was comparable to ASTM D2668.
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