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Rapid analytical method for elemental sulphur detection in power transformers insulation
Abstract — Over recent decades there have been reported failures of oil/paper-based transformers caused by corrosion of copper conductor surfaces and breakdown of insulation. In a small number of failure was caused by silver corrosion in tap-changers, promoted by the presence of elemental sulphur at ppm levels. The electrical power infrastructure and the entire transformer fleet can be at risk from unexpected failures, so preventive measures are necessary. To improve this, a rapid analytical technique to monitor elemental sulphur levels (< 1 ppm) in insulating mineral oil has been developed. This method can be used as a routine test or to confirm the presence of elemental sulphur in samples where corrosion has occurred. The potential of the approach adopted is demonstrated through its application to some real mineral oil samples from transformers.
Keywords—transformers, mineral oil, silver corrosion, elemental sulphur, analytical method.
Transformers are key assets in any ac network, and one of the most expensive, with high voltage (HV) super grid transformers costing millions of pounds. Since the beginning of the new millennium, after decades with relatively few sporadic failures of transformers being attributed to corrosive sulphur, the recurrence of this type of failure has underwent a marked increase.
The promotion of corrosion, mainly promoted by the reaction of sulphur species within the oil and the naked copper or silver surfaces, is of critical concern. It has been reported that levels of elemental sulphur (S8) as low as 1 ppm may be sufficient to induce silver corrosion within tap changers .
Consequently, a fleet of transformers contaminated with S8 presents a high financial and operational risk. Standard corrosion tests are time consuming, requiring 18 h for completion, followed by a subjective visual rating process . Hence, the development of rapid and reliable analytical methods to quantify and thereby monitor the presence of S8 is required.
Several analytical techniques have been applied to determine elemental sulphur in hydrocarbons [3,4]. A number of variants of GC methods using selective detection for sulphur have been published . In this paper, we describe a GC-MS method that does not require the use of an element selective detector, which is highly advantageous to laboratories that do not have this specialised equipment.
Elemental sulphur exists predominantly at room temperature as S8 ring structures, but under catalytic conditions or at elevated temperatures (as found in a standard Gas Chromatography (GC) injector) an allotropic equilibrium (… S6 ↔ S7 ↔ S8 …) ensures that sulphur does not elute from gas-chromatographic columns as a single peak. To overcome this a derivatisation method using triphenylphosphine (TPP) was employed . This results in the formation of a single compound, namely triphenylphosphine sulphide (TPPS, Fig. 1).
The use of this derivatisation technique allows the detection of sub ppm levels of elemental sulphur in transformer mineral oil without the need for element selective detection. This is possible as the derivatisation process results in a 9.2-fold increase in the mass concentration, i.e., one mole of elemental sulphur produces 8 moles of the derivatised product TPPS, in combination with improved chromatography due to the analysis of a single sulphur-containing species.
Fig. 3. Derivatisation reaction.
A. Materials and chemicals
All solvents and chemicals were used as supplied and without any further purification, drying or treatment. Toluene (high-performance liquid chromatography (HPLC) grade) was purchased from ThermoFisher Scientific (Loughborough, UK). Elemental sulphur (S8) (99.998%), triphenylphosphine (TPP) and triphenylphosphine sulphide (TPPS) were purchased from Sigma−Aldrich (Gillingham, U.K.). Insulating mineral oil samples were kindly supplied by National Grid plc (London, UK).
B. General Experimental
A number of samples of mineral oil samples, some of which are believed to cause corrosion of the transformers and all with varying compositions and chemistries were studied. The detection of S8 was conducted with the GC-MS method (described below).
All the samples were prepared for analysis by diluting 5 mL of the oil sample in 5 mL of toluene (solvent mixture optimum), this is followed by addition of 25 mg of TPP. This will cover an oil sample with a concentration of S8 up to ~300 ppm. The TPPS formation is based on the selective reaction of triphenylphosphine with elemental sulphur; It was reported that the reaction was instantaneous in toluene and that the solutions were stable for at least 24 h . In this work, the samples were allowed to react for 30 min before being loaded into autosampler vials. All samples were analysed immediately after preparation. The autosampler vials were prepared at a concentration of 10% of the sample and 90% of toluene.
Fig. 4. EI GC-MS of new mineral oil spiked with 1500 ppm of TPPS
D. Instrumentation and GC-MS Method
Samples were analysed using a Thermo (Hemel Hempstead, UK) Trace GC-MS single quadrupole mass spectrometer. Gas chromatography was performed using a Phenomenex ZB5-MS 30 m x 0.25 mm 0.25 μm thickness non-polar column using helium as a carrier gas at 1.2 mL/min. The injector temperature was set at 220 °C and 1 μL of sample was injected in splitless mode. The temperature program used was 60 °C for 3 min which then increased at 10 °C/min to 320 °C and then held for 10 min. Positive EI mass spectrometry with a Selected Ion recording (SIR) method was also created for the ions of interest m/z 183, 262 and 294 which corresponds to TPPS. Low resolution positive ion electron ionisation mass spectra were recorded over a mass range of m/z 40-500 at 70 eV. The ion source temperature was 200 °C and the detector voltage of 350 V. Xcalibur software was used for GC-MS data acquisition and processing.
III. Results and Discussion
A. Development of the GC-MS Method
When detecting S8, the elevated temperatures (found in a standard GC injector) form an allotropic equilibrium that leads to the complication that elemental sulphur does not elute from gas-chromatographic columns as a single peak. In addition to this, S8 elutes in a region in the chromatogram where other mineral oil compounds elute. To overcome the derivatisation method is used (Fig.1).
The chemical basis of the derivatisation method is the reaction of S8 with TPP that results in the formation of the correspondent sulphide (TPPS). TPP is converted to TPPS by one eight equivalent of S8. There are several advantages arising from this reaction . Firstly, as a result of the derivatisation a 9.2 fold increase in the mass occurs, so a sample containing 1 ppm of elemental sulphur will contain 9 ppm of the derivative. Secondly, all the cyclic sulphur species (S6, S7, S8, etc.) react to form a single product. In addition, the derivative has a high boiling point and elutes in a region of the chromatogram where no mineral oil components should elute. As a result of these factors, is it possible to detect elemental sulphur without the use of an element-selective detector as we can observe in Fig.2. The amount of S8 in the sample is therefore proportional to the amount of TPPS that is formed. The selectivity of the
TPP for the elemental sulphur and their reaction to form TPPS is the basis of this analytical method. Side reactions of the TPP with the other sulphur compounds present in the mineral oil would present us with false positives/increased concentrations.
The TPP reactivity with other sulphur compounds was studied and the side reactions of the TPP with other major sulphur compounds present in the oil are neglectable. These results ensure that TPPS formed is from the presence of S8 and not by side reactions. The choice of the solvent is of most importance. After many considerations, we settle with toluene. There are three main reasons for this choice. Firstly, the use of toluene as almost no interference with the analyte determination. The S8 content is very small, such concentration is not likely to have a significant impact on the total concentration. To avoid this, high quality sulphur free toluene was used. Secondly, it dissolves mineral oils readily even when they are severely aged. Thirdly and most importantly, dissolves S8 and TPPS in very high concentrations. This allows to cover a very wide range of S8 content that might be present in the oil samples.
As low levels as 1 ppm of S8 are believed to cause silver corrosion, the analytical method must be able to detect the TPPS to the level of < 9 ppm. To study the limit of detection (LOD) our GC-MS method standard solutions of TPPS were prepared by volumetric dilutions of the working solution with a concentration of 100 ppm TPPS that was prepared in a solution of 10% transformer mineral oil and 90% toluene. The LOD of the method is 4 ppm of TPPS, which corresponds to less than 0.5 ppm of elemental sulphur. The chromatogram of the standard solution with 4ppm of TPPS is shown in Fig.3. To test the validity of this method, different oil samples, with varying compositions and chemistries were analysed (see below).
B. Analyses of real mineral transformer insulating oils
A variety of mineral insulating oils were provided by National Grid UK plc, some of which were believed to present corrosive properties were analysed. At the beginning of each analysis, three blanks of toluene are conducted and one blank of toluene between samples to prevent carryover issues. Two of the samples analysed, oil sample A and oil sample B are shown in Fig. 4 and Fig. 5, respectively. The result of the analysis of the mineral oil A and B indicated that elemental sulphur is present in the oil samples. The formation of TPPS is demonstrated clearly in both figures by the presence of the peak at ~25 min. The MS spectrum of both peaks matches with that reported in the literature for TPPS. The peak of TPPS in the oil sample B, elutes with other compounds of the mineral oil, so quantification of the TPPS and consequently the concentration of elemental sulphur present in the sample cannot be calculated accurately. In this cases where mineral oil components interfere with the TPPS peaks, the use of a different GC method or a selective detector for sulphur or phosphorous is required for accurate quantification.
The new GC-MS method for S8 detection in transformer mineral oil has proven to be a quick and reliable tool to identify oil samples that might be a risk for the transformer. This method is capable to detect S8 in samples of mineral insulating oils with varying compositions and chemistries, including in service oils. This method uses standard GC-MS equipment that can be found in most analytical laboratories. Our technique allows the detection of trace levels of elemental sulphur levels in mineral oil down to 0.5 ppm. Due to the properties of some mineral oils, quantification might not be possible, because of the overlap of the elution of some compounds present in the mineral oil with TPPS, and future work will seek to address this potential limitation.
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Figure 4 – EICC of the oil sample A provided by National Grid plc..
Figure 5 – EICC of the oil sample B provided by National Grid plc..
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