Evaluation Method of Oil-Paper Insulation Damp State Based on the Frequency-Dependent Characteristics of Dielectric Response Current
Xia Yuan1, Yang Lijun1, Lü Xiaolu2, Wang Ke2, Li Jinzhong2
1. State Key Laboratory of Power Transmission Equipment Technology Chongqing University Chongqing 400044 China; 2. China Electric Power Research Institute Beijing 100192 China
Abstract Moisture in the oil-paper insulation is an important cause of performance degradation in power transformers, and accurate assessment of the moisture state of the insulation inside the equipments is important to ensure its safe operation. At present, the method of projecting the moisture content of insulating paper according to the moisture content in oil has led to great errors in field application due to the complicated influencing factors. In recent years, the insulation condition diagnosis technology based on dielectric response test method has received wide attention, and some characteristic parameters and evaluation methods have been proposed, but the accuracy of the evaluation results is greatly influenced by the differences in transformer insulation structures and cannot yet meet the needs of practical engineering applications. Based on the previous research results, this paper further explores the influence of moisture on the response current characteristics of oil-paper insulation, and aims to propose an efficient evaluation and diagnosis method of insulation moisture state that is less affected by the difference of insulation structure and easy to implement in the field. First, oil-paper insulation samples with different moisture contents and different polymerization degrees were prepared. In order to measure the response current signals of the oil-paper insulation under voltage excitation of different frequencies and amplitudes, a response current test system for oil-paper insulation was built. The response currents of the samples were tested in the frequency range of 0.01 Hz to 50 Hz. The response current amplitude at 0.01 Hz was used as the reference value, and the response current amplitude at different frequencies was normalized to obtain the f-I* curve. The f-I* curve of oil-paper insulation will change from a straight line when dry to a concave curve as the moisture content in the paper increases, while the f-I* curves of oil-paper insulation with different polymerization degrees basically overlap. When the moisture content is low, the f-I* curve basically does not change with the voltage amplitude, but when the moisture content is large the voltage increase will cause the right side of the f-I* curve to shift down slightly. Oil-paper composite insulation systems with different structures were made using circular cardboard and ring cardboard, and the test results showed that the FDS method is significantly affected by the insulation structure, while the difference in insulation structure has minimal effect on the f-I* curve when the moisture content in the insulation paper is the same. Finally, I*@50Hz (i.e., the I* value at 50 Hz) was proposed as a characteristic parameter to quantify the moisture content of the transformer oil-paper insulation, and a fitting equation between it and the moisture content of oil-paper insulation was established. For oil-impregnated insulated cardboard with different times of natural moisture absorption, the absolute error between the evaluation results and the actual measurement results of the Karl Fischer instrument was less than 0.09%. For actual oil-paper insulated bushings and transformers, the evaluation results were reasonable and consistent with the overall pattern of the results obtained by commercial insulation diagnostic analyzers. The following conclusions can be drawn from the experimental results: (1) The f-I* curve is closely related to the damp state of the oil-paper insulation. When the oil-paper insulation is dry, its f-I* curve is a straight line. However, the rise law of the f-I* curve in the low-frequency band will significantly change after the insulation is damp, and the whole curve will turn into a concave curve. (2) The test results of the f-I* curves are basically not affected by the insulation structure and insulation aging. This mechanism has more advantages compared with the FDS test method, which is greatly affected by the insulation structure. (3) This study proposed to take I*@50Hz, wherein the I* value at 50 Hz in the f-I* curve is used as the characteristic parameter to evaluate the dampness of the oil-paper insulation, and the test time is only approximately 100 s. The effectiveness of this method to evaluate the moisture content of oil-immersed equipment is verified by testing the actual oil-paper insulation bushings and transformers.
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2024, Vol. 39 
