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Operating experience has shown that characteristic gases can be found in oil‐immersed power transformers and high‐voltage reactors under strong vibration without discharge or thermal faults occurring. This will cause misdiagnosis of Dissolved Gases Analysis (DGA). In this study, generation characteristics and mechanism of characteristic gases in transformer oil dependent on vibration are investigated. The results show that there is a vibration pressure threshold that makes characteristic gases such as hydrogen (H2), methane (CH4), ethylene (C2H4), ethane (C2H6), and acetylene (C2H2) appear in transformer oil. The concentration of each characteristic gas increases with the increase of vibration pressure. Furthermore, misdiagnosis of insulation status which may be caused by vibration is illustrated based on Duval triangles method. Besides, vibration bubbling phenomenon in the experiment is observed. The generation mechanism of characteristic gases is analyzed with bubble dynamics under vibration. Higher vibration pressure induces cavitation processes and creates local thermal effects, which is the main reason for the generation of characteristic gases. A method based on combination of Duval triangle 1 and assistant triangle is proposed to distinguish characteristic gases generated by vibration from those generated by discharge and thermal faults. The results may provide theoretical support for fault identification and insulation state assessment of oil‐immersed power transformers and high‐voltage reactors. [ABSTRACT FROM AUTHOR]

The temperature gradient and mismatching between the thermal expansion of the core and flange readily lead to cracks and discharges on the core surface of the dry-type valve-side bushing, which severely impact the safety of power systems. It is vital to clarify the cracking risk of bushing cores under temperature gradients and establish corresponding control methods. The mechanical properties of epoxy resin impregnated paper (ERIP) material were measured in this study at different temperatures, and a thermal–mechanical coupling simulation model was established. The thermal and stress distributions of the core were obtained and the cracking risk was defined accordingly. The crack development mode was explored as it relates to the phase-field mode. Various elastic cushion materials affecting the stress distribution of the core were investigated. The results show that the mechanical properties of the ERIP material decrease rapidly as the temperature increases. When under severe working conditions, the maximum first principal stress of the core may be significantly higher than the tensile strength of the ERIP material resulting in significant axial cracks. Adding an elastic cushion layer made of polyurethane rubber can effectively relax the interface stress and reduce the cracking risk. [ABSTRACT FROM AUTHOR]

Voltage–time (V –t) characteristics of the SF6 long gap under very fast transient overvoltage (VFTO) draw wide attention at the high‐voltage transmission systems. In this research, a generating system of VFTO and lightning impulse (LI) up to 2.5 MV was set up. The V –t characteristics for SF6 gaps with hundreds of millimetres length under negative LI and VFTO were researched. Experimental results indicated the rate of rising of V –t curve for the sphere‐plane gap under VFTO was moderate, and V –t curve under LI was rapid, resulting in the intersection of the two types of V –t curves. However, for the rod‐plane gap, the V –t curve under VFTO was below that under LI at 0.5 MPa. The shape of V –t curve had many relationships with discharge propagation velocity in the SF6 gap, which was determined by the gap distance, field non‐uniformity, gas pressure and applied voltages. Based on analysis of discharge process and physical background of V –t curve, the semi‐empirical formulae were derived and the prediction model was established. It was demonstrated the predicted V –t curves could accurately predict breakdown characteristics of long SF6 gas gaps [ABSTRACT FROM AUTHOR]