Abstract
With the wide application of vegetable insulating oil in power transformers, it is essential to investigate the influence of moisture on the frequency domain dielectric spectrum (FDS) analysis in order to propose an accurate moisture assessment method for vegetable oil-paper insulation system. In this regard, various vegetable oil-paper samples are prepared in a laboratory environment. FDS curves for all samples are obtained at different moisture contents and operating temperatures. A new method to estimate the moisture content in insulation paper is proposed based on the low-frequency characteristic quantity. Also, the effect of operating temperature and water migration between the oil and paper insulation on the FDS analysis is investigated. Based on this analysis, a compensation factor is introduced to eliminate the influence of temperature on paper moisture assessment. Furthermore, an improved normalization method for the temperature effect on the FDS of vegetable oil-paper insulation with different moisture contents is proposed. The proposed moisture assessment and temperature normalization methods are validated thorough their application on an operating power transformer.
Highlights
Moisture is the main factor affecting the insulation dielectric strength of oil-immersed power transformers [1]
In [14], a frequency-temperature normalization curve for mineral oil under different moisture contents is proposed, but the effect of water migration between solid and liquid insulation is not taken into account in the process of normalization, which affects the results of this method
With the decrease of the test frequency, the rapid polarization process of water can be established completely, so the permittivity of the insulation system in the low frequency band increases with the increase of water content
Summary
Moisture is the main factor affecting the insulation dielectric strength of oil-immersed power transformers [1]. Results presented in [10] reveal that the transformer operating temperature affects the insulation dielectric loss curve of water content and a method to eliminate the influence of temperature is proposed. This method is effective for mineral insulating oil only. In [14], a frequency-temperature normalization curve for mineral oil under different moisture contents is proposed, but the effect of water migration between solid and liquid insulation is not taken into account in the process of normalization, which affects the results of this method. Dielectric response analyzer IDAX300 of frequency range 10−4∼103Hz as shown in Fig. 1 is used for FDS measurements
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