Preventive Maintenance of Transformer Health Index Through Stray Gassing: A Case Study

Preventive maintenance endeavors in transformers aim to appraise the risk by means of the computation and surveillance of the health index of transformers in service. A prevalent method employed in the preventive maintenance and assessment of the health index of transformers is the Dissolved Gas Analysis (DGA). A contemporary trend is to utilize an online DGA monitoring instrument while carrying on to conduct investigations in the test laboratory. Most recently, the testing of stray gassing has only been conducted by oil suppliers. With the sharp increase in reports of peculiar stray gassing phenomena in relatively new transformers, typically not more than ten years in-service, transformer manufactures and utility owners began conducting this test in-house. Though an established testing procedure for stray gassing exists, there is still lack of coherent criterion on the stray gassing limits and appropriate period when stray gassing patterns should be a concern. In this paper, numerous DGA methods have been adopted to analyze the oil sample results of a 2000kVA transformer in-service that has been exhibiting stray gassing patterns. The objective of this paper is to distinguish the stray gassing phenomena of uninhibited mineral oil so that they are unambiguous from other occurring fault types. A wide-scale study on transformers filled with uninhibited mineral oils has been carried out to develop local stray gassing limits for transformers in the South African power grid, specifically in solar photovoltaic (PV) plants. Premier findings indicate that the Hydrogen gas constituted by stray gassing is often misinterpreted to corona partial discharges in the transformers.