42 / 2018-08-16 11:05:36

Investigation on Lightning Impulse Breakdown and Dissolved Gas Characteristics of The New Three-Element Mixed Insulation Oil

the mixed inulation oil,mineral oil,dissolved gas,breakdown voltage,lightning impluse

With the development of the power industry, mineral oil is gradually unsatisfied with the requirements of oil-immersed transformer. Although mineral oil has excellent dielectric and thermal characteristics, it is non-biodegradable and could cause serious environmental problems if leakages or spills occurred. Besides, low flash and fire points are also serious disadvantages for mineral oil. In recent years, more and more attentions are focused on finding alternative dielectric liquids. Comparing with mineral oil, natural esters not only have better anti-aging performance and fire safety characteristics, but also be more environmental friendly. However, natural esters has been proved less desirable than mineral oil due to its inferior oxygen stability, higher pour point and viscosity. Taking into account the advantageous or disadvantageous properties of mineral oil and natural esters, some researches on mixed insulating oils carry out gradually. But some properties, such as viscosity, are still not meet the requirements of IEC 60296:2012. A new type of three-element mixed insulation oil which is prepared by mixing mineral oil, palm oil and soybean oil in a volume ratio at 76:19:5 can be a proper alternative with good physical and electrical performance. Based on basic performance testing, physico-chemical and electrical properties of the three-element mixed insulation oil basically meets the requirements of IEC 60296:2012.
Lightning impulse breakdown of insulation oil is an important factor restricting the safe operation of the oil-immersed transformers. So it is necessary to obtain the breakdown strength of the mixed oil under lightning impulse voltage which is an indispensable electrical characteristic for transformer insulation system design. This paper presents the experimental results of lightning impulse breakdown properties of mineral oil and mixed insulation oil. The main equipment consists of a Marx generator, a test cell containing a needle-plane electrode arrangement and a signal acquisition system. The mixed insulation oil was treated at 90℃, vacuum 50Pa, to dry for 48h. In order to keep the same moisture content between mineral oil and the mixed insulation oil, the mineral oil only be degassed for 48h. The moisture content measured by Karl Fisher titration (IEC 60814) and the moisture content of two kinds of insulating oil was kept near 28 ppm after the treatments. The insulation oils were withstood non-uniform lightning impulses(1.2/50µs), and the gap between needle-plane electrode varied from 5 mm up to 25 mm. The voltage was increased in steps starting from 80% of expected withstand voltage up to the breakdown. In order to reduce the contingency of the experiment, the breakdown voltage(BDV) value only after at least three times of lightning impulse without breakdown will be meaningful. Breakdown voltage and the velocity of streamer were measured. The dissolved gasses in oil was also measured after 20, 40, 60 times of breakdown.
The BDV result shows that the mixed insulation oil has lower breakdown strength in every oil gap under lightning impulse voltage when the needle is negative. As the increase of the oil gap, the BDV of mineral oil is far greater than the mixed insulation oil. When the needle is positive, the BDV of mixed insulation oil is slightly better than mineral oil. For the velocities of streamers, mineral oil has faster velocity because of its low viscosity. After the several times breakdown of two kinds of insulation oils in same voltage, dissolved hydrocarbon gases of mineral oil is much more than the mixed insulation oil, which means lightning impulse has a greater pollution of mineral oil.