Use of Headspace-Gas Chromatography for Transformer Oil Gas Analysis

Topics Covered

Introduction

Insulating fluids, generally mineral oils, are used in transformers. Under normal, mild conditions there is very little decomposition.

Occasionally however (localized or general) overheating of the oil occurs and decomposition products are formed. If the concentration of these gases reaches a critical point, the chances of catastrophic transformer failure are high.

The ASTM D 3612 method describes in detail three different routes:

A. Vacuum Extraction

The gases are extracted from the oil via a vacuum extraction device and analyzed via gas chromatography.

B. Stripper Column Extraction

Dissolved gases are extracted from a sample of oil by sparging the oil with the carrier gas on a stripper column containing a high surface area bead. The gases are then flushed from the stripper column into a gas chromatograph for analysis.

C. Headspace Sampling

An oil sample is brought into contact with a gas phase (headspace) in a closed vessel purged with Argon. As a result, a portion of a gas dissolved in the oil is transferred to the Headspace.

This article describes analysis of a transformer oil by method C - Headspace-Gas Chromatography

Instrumentation Used

• Gas Chromatograph - Bruker TOGA Analyzer based on 450-GC
• Headspace Sampler - SHS-40 Headspace Analyzer
• GC control and data handling - Compass CDS software

Figure 1. TOGA Analysis, TCD Channel

Materials and Reagents

“True North” DGA Oil Standard by Morgan Schaffer:

Sample Preparation

Figure 2. TOGA analysis FID channel

Figure 3. Schematic overview hardware.

The Morgan Schaffer Calibration Standard is carefully transferred into the headspace vial. The gases are extracted from the oil by means of a headspace sampler and injected onto a short Hayesep P precolumn and then to a micro packed Carboxen-1000 column.

The fraction containing Hydrogen, Oxygen, Nitrogen, Carbon Monoxide, and Methane will elute direct from the Carboxen-1000 column to the micro packed Molsieve column. Hydrogen, Oxygen and Nitrogen are detected by the TCD. Carbon Monoxide, and Methane are detected by the FID, after passing the Methanizer. When the Molsieve column is bypassed, Carbon Dioxide and the C₂-C₃ isomers are eluting from the Carboxen-1000 column and detected by the FID after passing the Methanizer. The back flush time is set to completely elute the C₃ isomers. C₄ and higher are back flushed.

Conditions

Table 2. TCD, FID, Methanizer settings

Table 3. Valves

Results and Discussion

Chromatograms of both TCD and FID channels are shown in Figure 1 and Figure 2. The complete hardware configuration (SHS-40/TOGA) is shown schematically in Figure 3.

Repeatability is tested by analyzing multiple samples from the same source. Data can be found in Table 4.

Table 4. Repeatability Data

A graphic representation of the data is shown in Figure 4 and Figure 5.

Besides the analytical result also the window specified in the ASTM D 3612 method is presented. From the data presented in Table 4, Figure 4 and Figure 5 it is clear that the repeatability of the system is well within the window specified by the ASTM D 3612.

Figure 4. Repeatability results of a Transformer Oil.

Conclusion

Full separation of all components of interest, easy and reliable quantification results with very good repeatability was achieved.

The analysis of dissolved gases in transformer oil according to ASTM D 3612, Method C, can be performed perfectly with the Bruker Transformer Oil Gas Analyzer (TOGA Analyzer) in conjunction with the Bruker SHS-40 Automated Headspace Sampler.

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This information has been sourced, reviewed and adapted from materials provided by Bruker - Chemical and Applied Markets (CAM).

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