Investigating Aging Characteristics of Oil-Immersed Power Transformers' Insulation in Electrical-Thermal-Mechanical Combined Conditions.

The condition and health of large oil-immersed power transformers' insulation have a direct impact on the safety and stability of the power grid. Therefore, it is crucial to investigate the aging characteristics of oil-paper insulation in power transformers. In this study, we developed a computational model for reclosing current calculation and multiphysics coupling models for magnetic-circuit-force, electrostatic field, and temperature field simulations. The calculated aging resulted in a mechanical stress of 8.71 MPa, an electric field strength of 2.26 × 106 V/m, and a temperature of 113.7 °C. We conducted combined electrical-thermal-mechanical aging tests on the oil-paper insulation and measured various insulating paper performance parameters at different aging stages. Our study revealed that both the mechanical and electrical properties of the insulating paper deteriorated in both aging groups. However, the changes were more pronounced in the electrical-thermal-mechanical aging group compared to the electrical-thermal aging group, indicating that mechanical stress accelerated the aging process of the insulating paper. In the early stages of aging, the rate of performance changes in the electrical-thermal aging group was similar to that in the electrical-thermal-mechanical aging group. However, as the aging time increased, the degradation of performance induced by mechanical aging became more significant. This suggests that the insulating paper's resistance to mechanical damage, specifically short-circuit resistance, noticeably decreased after prolonged aging.

Diagnosis Method of Decay-like Composite Insulators in a High-Humidity Environment Based on Characteristic Coefficient of Temperature Rise Gradient.

At present, the temperature rise in insulators is observed using infrared thermometry as a common method of diagnosing decay-like insulators. However, the original characteristic data obtained by infrared thermometry cannot effectively distinguish some of the decay-like insulators from those with ageing sheaths. Therefore, it is imperative to find a new diagnostic characteristic quantity. Based on statistical data, this article first explains that existing diagnostic methods have limited diagnostic effectiveness and a high false detection rate for insulators in a slightly heated state. A full-scale temperature rise test is carried out on a batch of composite insulators returned from the field under high-humidity conditions. Two different defective insulators with similar temperature rise profiles are identified, and an electro-thermal coupling simulation model is developed based on the dielectric characteristic parameters of the above insulators for both core rod defects and sheath ageing. A new infrared diagnostic feature, the temperature rise gradient coefficient, is then obtained to identify the source of abnormal heat in insulators using statistical analysis of an infrared image gallery of abnormally hot composite insulators obtained from field inspections and laboratory tests.

Defect Identification of Composite Insulator Based on Infrared Image.

The deterioration of a composite insulator's mandrel will cause a serious power-grid accident, so it is necessary to find the defect as soon as possible. Infrared imaging is an effective means to detect mandrel-deterioration defects, but sheath-aging defects will cause trouble to screen mandrel deterioration. Therefore, it is urgent to distinguish the two defects. In this paper, two composite insulators that are out of service with different heating characteristics are tested and analyzed, and the temperature-rise characteristics are analyzed by building a simulation model. The results show that the temperature rise of composite insulators with oxidative decomposition of epoxy resin is large, and the range usually extends from the hardware of the high-voltage side to several umbrella skirts. The temperature rise caused by the composite insulator with the fiber completely wrapped by epoxy resin is small, which mainly occurs from the hardware to the first umbrella skirt. The simulation model constructed in this paper is consistent with the experimental results, which verifies the accuracy of the model. The model can distinguish the mandrel-deterioration defect and sheath-aging defect, and has guiding significance and important reference value for the detection rate of the mandrel-deterioration composite insulator and ensuring the safe and stable operation of the power grid.

Use of ultrasonic array method for positioning multiple partial discharge sources in transformer oil.

Fast and accurate positioning of partial discharge (PD) sources in transformer oil is very important for the safe, stable operation of power systems because it allows timely elimination of insulation faults. There is usually more than one PD source once an insulation fault occurs in the transformer oil. This study, which has both theoretical and practical significance, proposes a method of identifying multiple PD sources in the transformer oil. The method combines the two-sided correlation transformation algorithm in the broadband signal focusing and the modified Gerschgorin disk estimator. The method of classification of multiple signals is used to determine the directions of arrival of signals from multiple PD sources. The ultrasonic array positioning method is based on the multi-platform direction finding and the global optimization searching. Both the 4 × 4 square planar ultrasonic sensor array and the ultrasonic array detection platform are built to test the method of identifying and positioning multiple PD sources. The obtained results verify the validity and the engineering practicability of this method.