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| DC Breakdown Characteristics of Flowing Transformer Oil Contaminated by Metallic Particles |
| Yao Yuhang1, Tao Xiantao2, Yu Mengyue1, Pan Cheng1, Tang Ju1 |
1. State Key Laboratory of Power Grid Environmental Protection School of Electrical Engineering and Automation Wuhan University Wuhan 430072 China;
2. China Electric Power Research Institute Wuhan 430074 China |
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Abstract The presence of free metallic particles in power transformer oil poses a significant threat to its insulation due to their conductivity and their ability to distort the electric field. With the increasing rated voltage levels, the negative effects resulting from particle motion and accumulation have gained considerable attention. However, previous investigations have primarily focused on static liquid conditions, overlooking the fact that transformer oil in operational equipment keeps consistentlyflowing due to circulation systems and temperature gradients. The motion behaviors of metallic particles are altered by the flowing of oil, consequently affecting the breakdown characteristics of contaminated oil. The breakdown characteristics and mechanisms of transformer oil containing metallic particles under flowing conditions are not well understood.
Therefore, this study aims to address this knowledge gap. Before the test, 6.2 g of metallic particles with a diameter of ~150 μmare injected into the circulating system of transformer oil, with a particle concentration of 3 500 pieces/100 mL. The flow velocity of the transformer oil is controlled by adjusting the oil pump speed. Experimental tests on the breakdown characteristics of flowing transformer oil are conducted, using a custom-built experimental platform. And two types of breakdown tests, without preload and with preload, are performed. During the experiments, a synchronized data acquisition system comprising a high-speed camera and oscilloscope is employed to record the distribution of metallic particles before breakdown and the discharge signals at the moment of breakdown.
The results indicate that the presence of metallic particles significantly reduces the insulation withstand capability of flowing transformer oil. Compared to pure transformer oil, the DC breakdown voltage of oil decreases by more than 30%. Furthermore, the characteristic breakdown voltage of contaminated oil initially exhibits a rapid increase with flow velocity and then gradually stabilizes. When the oil flow velocity is below 0.25 m/s, the breakdown voltage after preload is lower than that without preload. However, as the flow velocity continues to increase, the two values gradually converge.
Finally, the influence mechanism of oil flow velocity on the breakdown characteristics of transformer oil is discussed in conjunction with the variation in particle distribution in the oil gap. The images captured before breakdown indicate that the breakdown of transformer oil containing metallic particles is primarily caused by individual metallic particles in close proximity to the energized electrode surface. When these particles come into contact with the electrode, it results in severe distortion of the local electric field, leading to partial discharge (PD). Following discharge, the metallic particles can be regarded as metallic protrusions on the high-voltage electrode, further contributing to gap breakdown. The oil flow velocity affects the breakdown characteristics by modifying the distribution of metallic particles during the pre-breakdown stage. As the flow velocity increases, the number of particles in the oil gap gradually decreases, reducing the possibility of breakdown. After preload for 20 minutes at 10 kV, metallic particles tend to accumulate in stagnant and low-velocityflow. Thereby, the particle concentrationprior to breakdownincreases and the electric field in the contaminated oilis distorted, resulting in a further reduction of the breakdown voltage.
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Received: 24 February 2023
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2023, Vol. 38 
