Is SF6 Dissolved in Your Transformer Oil?
Gas insulated substations (GIS) are a preferred design choice when space is limited or when the operating environment is challenging (due to risk of sandstorms or salt-spray for instance). GIS achieve high reliability by putting many high-voltage components inside chambers filled with pressurized high-dielectric-strength sulfur hexafluoride (SF6) gas. Over time, the seals that keep the gas inside can develop leaks, leading to loss of this strong green-house gas and potential air ingress. A gas detector can be used to locate outboard SF6 leaks, but how can one know if SF6 gas is leaking through a bushing seal into the main transformer tank?
Records from our dissolved gas analysis (DGA) laboratories indicate that approximately 0.3% of oil samples in North America contain traces of dissolved SF6 gas. We expect this percentage is much higher in regions where GIS facilities are more common, including the Middle East, South America, and Japan, for example.
Obviously, for the GIS operator, it is useful to detect SF6 leaking into the oil tank as a first step toward localizing and repairing the leak. Also, SF6 in the oil can complicate the Dissolved Gas Analysis (DGA) processes that are so important to transformer health assessment and maintenance decisions. For example, dissolved SF6 will cause some DGA monitors and portable analysers that use IR measurement methods (e.g. Photo Acoustic Spectroscopy, Non-Dispersive IR) to report false high readings for one or more key fault gases. So, unknown SF6 in the oil may potentially lead to poor transformer maintenance decisions.
Morgan Schaffer Calisto5 and Calisto9 products are designed to provide accurate measurements of DGA fault gases even with SF6 in the oil. These products contain a rugged industrial gas chromatograph optimized for reliability, high sensitivity, and lifetime accuracy. The chromatography column in these products separates the SF6 signal so that it does not interfere with the measurement of other gases. Also, unique in the industry, the Calisto C901S model specifically measures and reports the dissolved SF6 concentration. The C901S monitor can detect the first appearance of an SF6 leak into the oil and quantify changes in leak severity over time.
Calisto DGA monitors are ideal for assessing the health of transformers in GIS facilities. They provide accurate DGA data no matter if SF6 leaks into the oil. And the C901S model specifically enables timely maintenance decisions if ever such a leak arises.
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