Abstract
Dissolved gas in oil analysis (DGA) is one of the most reliable condition monitoring techniques, which is currently used by the industry to detect incipient faults within the power transformers. While the technique is well matured since the development of various offline and online measurement techniques along with various interpretation methods, no much attention was given so far to the oil sampling time and its correlation with the transformer loading. A power transformer loading is subject to continuous daily and seasonal variations, which is expected to increase with the increased penetration level of renewable energy sources of intermittent characteristics, such as photovoltaic (PV) and wind energy into the current electricity grids. Generating unit transformers also undergoes similar loading variations to follow the demand, particularly in the new electricity market. As such, the insulation system within the power transformers is expected to exhibit operating temperature variations due to the continuous ramping up and down of the generation and load. If the oil is sampled for the DGA measurement during such ramping cycles, results will not be accurate, and a fault may be reported due to a gas evolution resulting from such temporarily loading variation. This paper is aimed at correlating the generation and load ramping with the DGA measurements through extensive experimental analyses. The results reveal a strong correlation between the sampling time and the generation/load ramping. The experimental results show the effect of load variations on the gas generation and demonstrate the vulnerabilities of misinterpretation of transformer faults resulting from temporary gas evolution. To achieve accurate DGA, transformer loading profile during oil sampling for the DGA measurement should be available. Based on the initial investigation in this paper, the more accurate DGA results can be achieved after a ramping down cycle of the load. This sampling time could be defined as an optimum oil sampling time for transformer DGA.
Highlights
Since the Kyoto agreement was ratified by most of the nations, there has been a global common goal for reducing greenhouse gas emissions through adopting more renewable energy sources such as photovoltaic (PV) and wind power generation [1]–[3]
Results show that all characteristic gases are impacted by the momentary thermal change in the entire thermal cycles and generation and load ramping in real transformer operation
CH4 concentration is increasing from the reference level (0 ppm) to 10 ppm when the temperature increases from 25◦ to 30◦ after which it slightly reduces and stabilizes at a level of 6 ppm when the temperature is maintained at 60◦
Summary
Since the Kyoto agreement was ratified by most of the nations, there has been a global common goal for reducing greenhouse gas emissions through adopting more renewable energy sources such as photovoltaic (PV) and wind power generation [1]–[3]. Due to the intermittent characteristics of these sources such as solar irradiations and wind speed fluctuations and impact of diffused solar irradiations resulting from passing clouds, power transformers are expected to exhibit. There are some instantaneous pulsations in the generated power that may be attributed to a passing cloud during the entire generation period.
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