Insulation Characteristics and Influencing Factors of Transformer Oil Containing Cellulose Particles under Low-Frequency Voltage
Liu Yunpeng1,2, Zhao Jiaying1, Liu Hechen1,2, Zhao Tao1, Yin Ziao1
1. Hebei Key Laboratory of Green and Efficient New Electrical Materials and Equipment North China Electric Power University Baoding 071003 China; 2. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China
Abstract：Low-frequency transformer is the key equipment in low-frequency transmission systems, and its insulation characteristics under low-frequency voltage are important for its design and operation. In this paper conducted transformer oil breakdown tests at different cellulose particle size (low, medium, and high) levels to study the insulation characteristics of transformer oil containing cellulose particles under low-frequency voltage. The test frequency was selected as 20, 30, 40, and 50 Hz, and the content of cellulose particles in oil samples was selected as low, medium, and high particle size levels. According to the IEC 60156 standard, the pressure was increased step by step at a constant rate of 2 kV/s to the oil sample breakdown, and the oil sample in the oil cup was replaced every 6 times after the breakdown. In order to ensure the consistency of the test, 30 times of breakdown tests were repeated respectively at each frequency and particle size level, and the effective value of breakdown voltage was recorded. The Weibull distribution method was used to analyze the test results. The test results show that at the low particle size level, the breakdown voltage at 50 Hz is taken as the reference, and the breakdown voltage at 20, 30, and 40 Hz is increased by 12.25%, 25.62%, and 39.14%, respectively. At the high particle level, the breakdown voltage increases by 21.45%, 83.69%, and 97.30% at 20, 30, and 40 Hz, respectively. At the same time, the accumulation process of cellulose particles at the electrode was observed by experiments, and the movement of cellulose particles in transformer oil at different frequencies was analyzed by multi-physical field simulation. In this paper, an observation test was carried out on the motion characteristics of cellulose particles for transformer oil with high particle size levels. 18 kV was selected as the test voltage and carried out at room temperature. The distribution state of cellulose particles between electrodes was observed with a camera. Finally, COMSOL finite element software “AC-DC module” and “Particle tracking module” are used to simulate the motion trajectory of particles in the slightly uneven field of transformer oil under four frequencies. The results show that the agglomeration rate of particles towards the electrode at a low frequency 20 Hz was significantly lower than that at a power frequency 50 Hz. More impurities would accumulate near the electrode at 50 Hz, and the bridge of impurities was broader than that at 20 Hz. The results show that the AC voltage frequency significantly affects the bridge formation characteristics and breakdown voltage of cellulose particles in transformer oil. When the transformer is running, the breakdown voltage of transformer oil increases with the decrease of AC voltage frequency, and the higher the granularity level, the more significant the difference in breakdown voltage between frequencies. The frequency of AC voltage will affect the moving speed of impurity particles and the formation process of impurity Bridges in transformer oil. The research results of this paper provide essential data support for the design, operation, and maintenance of low-frequency transformers.
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2024, Vol. 39 
