Jocelyn Jalbert

Methanol: A Novel Approach to Power Transformer Asset Management

All electrical utilities deal with the task of determining the residual life of their in-service power transformers. Given the difficulties experienced with the use of first and second generations of markers (carbon oxides and 2-furfuraldehyde), several organizations are now considering the use of methanol for this purpose. Hydro-Québec, which discovered this approach, uses this molecule on a regular basis to evaluate the state of the cellulose insulation of in-service power transformers and applies it to Électricité de Frances nuclear power plant transformers. In this paper, some examples of the application of methanol in the field are presented against the information received from the early marker generations.

Robust and sensitive analysis of methanol and ethanol from cellulose degradation in mineral oils.

Methanol and ethanol have been identified as oil-soluble by-products generated by the aging of oil-impregnated cellulosic insulation materials of power transformers. Their presence provides useful information for diagnostics and end-of-life transformer estimation. Despite their value as cellulose degradation indicators, their sensitive and accurate determination is challenged by the complex oil matrix. To overcome this constraint, we present a simple, fast and direct procedure for their simultaneous determination in mineral insulating oil samples. The procedure uses a static headspace sampler coupled with a gas chromatograph equipped with a mass spectrometer. The selected method parameters permitted adequate separation of these two compounds from the complex oil matrix and quantification at ng g(-1) concentrations. An original internal standard procedure was developed, in which ethanol-d6 was added to all studied samples and blanks, with adequate resolution between the internal standard and its isotopomer ethanol. The method was validated in terms of accuracy and reproducibility for both analytes. The method detection limit, 4 ng g(-1) for methanol and ethanol, is well below the value (μg g(-1)) achieved by a standardized method for methanol determination in crude oil. During method validation studies, a relative error of approximately 6% was obtained for both methanol and ethanol with excellent reproducibility, average %RSD, below 2%. An experiment control chart, constructed to evaluate long-term reproducibility, indicate an overall good reproducibility (%RSD<3%) for 1000 ng g(-1) control solutions. The applicability of the method to the direct analysis of trace methanol and ethanol in oil from field transformer samples was successfully demonstrated. This analytical method is of high relevance to the electrical utilities as it allows indirectly assessment of the level of deterioration of the critical cellulose, an inaccessible part of a power transformer.

Cellulose chemical markers in transformer oil insulation Part 1: Temperature correction factors

The concentrations of cellulose chemical markers, in oil, are influenced by various parameters due to the partition between the oil and the cellulose insulation. One major parameter is the oil temperature which is a function of the transformer load, ambient temperature and the type of cooling. To accurately follow the chemical markers concentration trends during all the transformer life, it is crucial to normalize the concentrations at a specific temperature. In this paper, we propose equations for the normalization of methanol, ethanol and 2-furfural at 20 °C. The proposed equations have been validated on some real power transformers.

Relationships between methanol marker and mechanical performance of electrical insulation papers for power transformers under accelerated thermal aging

The mechanical performance of two commercial papers used as solid insulation in power transformers, namely standard Kraft and a thermally upgraded Kraft paper, were studied during accelerated aging in Luminol oil at 170 °C. The results show a relationship between the degree of polymerization and the mechanical properties measured by tensile testing. A linear relationship was found between the mechanical properties of paper, the tensile index (Tidx), and the concentration of methanol present in the oil. The methanol chemical marker has been proven to be an accurate assessment tool for the aging of cellulosic paper. The results show a promising tool for correlating the methanol concentration in oil, as an indirect indicator, with the mechanical performance of the paper. This approach can be used to monitor the real state of the cellulose chains in the power transformer insulation paper.

Determination of dissolved gases and furan-related compounds in transformer insulating oils in a single chromatographic run by headspace/ capillary gas chromatography

Abstract A static headspace/ capillary gas chromatographic technique has been developed allowing dissolved gases and furan-related compounds in power transformer oils to be determined in a single chromatographic run. The simultaneous determination of these two classes of compounds from a single sample injection represents a definite advantage over current procedures for the detection of impending performance failure of the unit. The method uses gas chromatography with porous-layer open tubular (PLOT) columns and a valve operated in sequence for flowpath selection. Also, comparison with the current method using packed columns showed that dissolved-gas analysis with PLOT columns provides better peak shapes and lower detection limits ( S/N b = 3): 5, 3 and 1 ppm (v/v) for hydrogen, carbon oxides and light hydrocarbons respectively. Detection of 2-furaldehyde is possible down to 0.5 ppm (w/w).

Relationship between some chemical markers and the mechanical properties of the solid insulation used in power transformers

Solely some degradation by-product of cellulose paper, detectable in oil may provide information on the condition of the paper without requiring invasive measures or de-energizing the transformer. In this contribution, oil impregnated Kraft paper samples were submitted to thermal aging with oxygen inlet in laboratory conditions. Many key parameters providing a “picture” of the paper condition have been determined. From the obtained results, some correlations were found between the amounts of alcoholic chemical markers (methanol, ethanol) dissolved in oil and the degree of polymerization (DPv) together with the mechanical properties of Kraft papers. These correlations can be used as tools to assess the degradation of the solid insulation and ultimately to estimate the lifespan of power transformers.

Unusual ethylene production of in-service transformer oil at low temperature

The study sponsored by the CEATI Life Cycle Management of Station Equipment & Apparatus Interest Group deals with an unusual gassing trend of dissolved gas-in-oil in power transformers. The history of gas generation development in these free-breathing transformers shows that ethylene and carbon dioxide gases were produced progressively under normal operating conditions without the simultaneous generation of hydrogen and other hydrocarbon gases. Thermal aging experiments performed in the laboratory on inservice oil in closed air-saturated glass ampoules revealed a dominant influence of oxygen and temperature on the oils chemical stability. The results suggest that the degradation of the in-service oil is governed by an oxidation process in an oxygen-rich environment at unexpectedly low temperatures. The possible raisons explaining the unusual gassing trend of dissolved gas-in-oil in power transformers were not elucidated in this study.

Influence of transformer oil acidity on insulating paper degradation

The transformer oil on Hydro-Québecs grid is a major investment. Therefore, it is essential to preserve the oils quality thus the value of the asset is maintained and, more important, a high level of reliability on the equipment is ensured. In order to preserve this non-renewable resource, Hydro-Québec decided in the 1990s to reclaim the oil of its power transformers. The quality of the oil has an impact on its own properties (dielectric, physico-chemical), and eventually on the solid insulation and its degradation rate. In order to optimize the reclamation activities and establish criteria to initiate the reclaiming operation, this manuscript presents the preliminary results on the influence of the initial oil acidity on the paper degradation rate. The results show that the degradation rates for Kraft paper and thermally upgraded paper at 110°C increase when using oil with a relative low acidity. Moreover, the degradation rate of the thermally upgraded paper seems to be relatively more affected by oil acidity than for Kraft paper.

Insulating oil decaying assessment by FTIR and UV-Vis spectrophotometry measurements

The most easily accessible and efficient way to determine the transformer condition is to use the insulating oil as diagnostic medium. Practical experiences have shown that more than 70% of incipient failures can be detected through oil tests. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. In this contribution, oil samples aged in laboratory condition were monitored with traditional diagnostic methods - i.e., interfacial tension (IFT) and total acid number (TAN) - and alternative techniques based on spectral analysis, using Fourier Transform Infrared (FTIR) and Dissolved decay products (DDP) assessed by spectrophotometer. From the obtained results, the sensitivity and reliability of each method in monitoring step-by-step the decay products formation are analyzed and discussed. It is shown that ratio of DDP to FTIR transmittance measurements at 1710 cm-1 might be possibly be used as index for assessing the degradation of transformer oil.

Determination of the analytical performance of a headspace capillary gas chromatographic technique and karl Fischer coulometric titration by system calibration using oil samples containing known amounts of moisture.

Over the past few years, concerns have been raised in the literature about the accuracy of the Karl Fischer (KF) method for assessing moisture in transformer mineral oils. To better understand this issue, the performance of a static headspace capillary gas chromatographic (HS-CGC) technique was compared to that of KF coulometric titration by analyzing moisture in samples containing known amounts of water and various samples obtained from the National Institute of Standards and Technology (NIST). Two modes of adding samples into the KF vessel were used:  direct injection and indirect injection via an azeotropic distillation of the moisture with toluene. Under the conditions used for direct injection, the oil matrix was totally dissolved in the anolyte, which allowed the moisture to be titrated in a single-phase solution rather than in a suspension. The results have shown that when HS-CGC and combined azeotropic distillation/KF titration are calibrated with moisture-in-oil standards, a linear relation is observed over 0-60 ppm H(2)O with a correlation coefficient better than 0.9994 (95% confidence), with the regression line crossing through zero. A similar relation can also be observed when calibration is achieved by direct KF addition of standards prepared with octanol-1, but in this case an intercept of 4-5 ppm is noted. The amount of moisture determined by curve interpolation in NIST reference materials by the three calibrated systems ranges from 13.0 to 14.8 ppm for RM 8506 and 42.5 to 46.4 ppm for RM 8507, and in any case, the results were as high as those reported in the literature with volumetric KF titration. However, titration of various dehydrated oil and solvent samples showed that direct KF titration is affected by a small bias when samples contain very little moisture. The source of error after correction for the large sample volume used for the determination (8 mL) is about 6 ppm for Voltesso naphthenic oil and 4 ppm for toluene, revealing a matrix effect on the measurement. Finally, the results revealed that HS-CGC is a good technique for measuring moisture in oil samples and that the use of azeotropic vapors for introducing moisture into the titrator almost completely eliminates the matrix effect observed with the oil components. Direct KF injection could also be used provided the system is calibrated with moisture-in-oil standards prepared in the same matrix which is to be used for the determination.