Early Deterioration and Its Detection of Natural Monoester Type Insulating Oil Used for Transformer 変圧器用天然モノエステル絶縁油の早期劣化とその検出

1. Introduction

2. Literature Review

3. Gas Generation of Monoesters under Low Temperature Overheating

4. Gas Generation of PFAE under Partial Discharge

5. PD Behavior of PFAE under Different Moisture Content

6. Conclusions

Oil-filled transformers have been widely used for many decades, and proven to be more efficient than transformers of dry type. Oil is used as an insulation, in conjunction with solid insulation, in addition to its other function as a cooling medium. Mineral oil is still the most popular oil, but there has been an emerging interest in using natural ester （vegetable） oil during the last two decades. The main reasons are that natural esters are readily biodegradable, non-toxic, and possess low fire risk due to their high flash and fire points, hence, the oils are environmentally friendly. Moreover, the depletion of the resource is not the case since vegetable oils are plentiful available in nature. In Japan, the Lion Corporation developed a natural ester of monoester type insulating oil called palm fatty acid ester （PFAE）. The viscosity of PFAE is 0.6 times less than mineral oil makes the cooling efficiency of the PFAE-immersed transformer better than that of mineral oil. The oil can be expected to have better oxidation stability than that of the triglyceride type since the oil contains only saturated fatty acid chains. However, the flash point of the PFAE is lower than that of the triglyceride type. This thesis deals with the investigation research on the early deterioration and its detection methods of PFAE and other monoesters. The gas generations of the oils under low temperature overheating and under partial discharge （PD） are investigated. The effect of moisture content on the gas generation of PFAE and PD behavior of the oils are also explored. The low temperature overheating is conducted by heating the oils locally at temperatures ranging from 200 to 300 ℃. Local heating is realized using two ceramic heaters immersed in the oil sample, fed by two DC power supplies. It is shown that natural monoesters, in general, produce a higher amount of carbon monoxide （CO）, but less amount of methane （CH4） than the mineral oil does. It is also revealed that natural monoesters having C=C bond in their hydrocarbon chain produce remarkable amount of ethane, whereas those without C=C bond, and mineral oil of naphthenic type do not. It is then disclosed that ethane generation, which is previously regarded as the key gas to distinguish natural esters from mineral oil under thermal fault, is now clear as the difference between oil having C=C bond in their hydrocarbon chains and those without C=C bond, irrespective of the oil types. None of the existing DGA fault interpretation methods can be used to interpret the low temperature overheating fault. Instead, the ratio of CO/CH4 is proposed. The gas generation under PD is investigated by applying an AC high voltage of 20 kV on a needle-plane electrode configuration immersed in the oil sample. PD pulses are detected using RC detector, and the detector is connected to an oscilloscope for measurement. It is found that PFAE does not produce hydrogen, or produce hydrogen in not significant amount. This differs from that in mineral oil stressed with PD at which hydrogen is usually found, as mentioned in the literature. It is also obtained that the change in relative moisture content up to 17 % does not cause a significant difference in PD behavior of the oil, but the change in PD number does. Among existing DGA fault interpretation methods, only Duval Triangle method can be used, resulting mainly in the overestimated interpretation. PD faults mainly considered as the discharge of high energy. The effect of moisture content on PD behavior of PFAE is studied using two PFAE samples of different conditions, dry and moist samples. The needle-plane electrode configuration is still used, but the PD detector used to detect PD pulses is a combination of R detector, detecting impedance （DI） and a PD monitoring device （CD6）. It is observed that the presence of moisture intensifies PD activities of PFAE oil. The PD number increases drastically, and occurs mainly at the negative polarity of the AC applied voltage. The positive PDs still occur, but with a drastic decrease in PD number and with a slight decrease in PD charge. It is proposed that the change in PD behavior is due to the electronic affinity of water molecules. A mechanism is then proposed, and the schematic representation is presented.

九州工業大学博士学位論文 学位記番号:工博甲第429号 学位授与年月日:平成29年3月24日

平成28年度

1. Introduction||2. Literature Review||3. Gas Generation of Monoesters under Low Temperature Overheating||4. Gas Generation of PFAE under Partial Discharge||5. PD Behavior of PFAE under Different Moisture Content||6. Conclusions

九州工業大学博士学位論文（要旨）学位記番号：工博甲第429号 学位授与年月日：平成29年3月24日