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On-Ioad tap changer (OLTC), extinguishing arc with oil, is one of the important components of transformer. Combined many years of field experience, physicochemical properties and electrical properties of oil deteriorate under the combined action of electric, thermal and mechanical agitation. Taking the oil for OLTC in 110kV transformers as research object, ageing characteristics of oil in different operation periods, such as water content, breakdown voltage and oil group composition, were studied comparatively. The results show that the decrease of breakdown voltage is determined by the water content in oil, operation time and operation times of OLTC. Furthermore, the low content of aromatic carbon in the oil is also cause the breakdown voltage to decrease obviously, but the change of water content is small. The water content undulates during operation. The leak of sealed for OLTC is the main factor that makes the breakdown voltage and the water content greatly change.

As an important component of power transformer, transformer bushing cause thirty percent of transformer accidents. It is difficult to find defects in transformer bushing in advance because of the lack of on-site inspection methods. Frequency domain dielectric spectroscopy is widely used in the field of insulation evaluation of electrical equipment because of its frequency bandwidth and rich information. However, the frequency domain dielectric spectroscopy in the inspection of oil-paper insulated transformer bushing is affected by lacking substation test case and diagnostic criteria. In this paper, a dielectric response model of the transformer bushing is established to reveal the different defect types corresponding to the test curves in different frequency bands. In the low frequency band which is less than 0.0lHz, it is reflecting the water content of bushing. In the middle frequency band which is between 0.0lHz to 100Hz, it is reflecting the aging of bushing. Several cases of substation bushing defects detected by frequency domain dielectric spectroscopy have verified the above results. It is difficult to propose uniform fault diagnosis criteria for the frequency domain dielectric spectrum of transformer bushing which is affected by the structure and type. Based on a large number of substation case studies, a defect diagnosis criterion based on Frechet similarity method is proposed in this paper, which is universal for different structures and different types of bushing.

As an essential component of power transformer, transformer bushing’s reliable operation does not only impact the safety of transformer itself but also other equipment located nearby. To prevent bushing failures, it is therefore important to routinely assess bushing insulation and pin-point the potential problem as early as possible. In this paper, fault diagnosis and tear-down analysis of five 500kV oil-impregnated paper (OIP) transformer bushings are presented. Results of different field and laboratory diagnostic tests are discussed and compared. Key features of partial discharge-led defect with x-wax deposits are analysed. The effectiveness of adopting frequency domain spectroscopy (FDS) in combination with dissolved gas analysis (DGA) as supplementary field diagnostic test is then demonstrated.

The misoperation of gas protection in transformer out-of-zone faults will seriously affect the security and stability level of power system. In this paper, the oil flow characteristics in transformer under different operating conditions are analyzed by multi-physical field coupling simulation. The principle of common gas protection relays and their causes of misoperation are analyzed. A numerical simulation model of multi-physical field inside transformer is established. The inner core and winding of transformer are modeled and simulated by ANSYS Maxwell 3D and ANSYS Fluent software. The variation of multiphysical field in normal operation, out-of-zone fault and in-zone fault of transformer is obtained. The changes of magnetic field and electromagnetic stress, oil flow and outlet pressure of inner winding and core of transformer are analyzed. This study provides a new basis for formulating measures to improve the reliability of gas protection operation.