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Porcelain insulators are widely used in overhead high-voltage power transmission lines in order to provide adequate insulation to withstand switching and lightning over voltages. Since the useful serviceable life of the individual insulator element and insulator strings is hard to predict, they must be verified periodically to insure that adequate reliability is maintained at all times. Over the years, many testing methods have been used for this purpose, each one with its own advantages and disadvantages. A novel insulator inspection method using low-frequency harmonics is proposed for human safety. Using this inspection device with a mobile robot can keep humans far away from power transmission lines. The working principle of this new method is based on the relationship between low-frequency harmonic components and the defective characteristic of the insulators. In this paper, the retrieval of low-frequency harmonics from complex noise is solved by using extended high-order cumulants (HOCs). The concept of cross mixing is newly proposed and the harmonic retrieval method using the slice of the sixth-order time-averaged moment poly-spectra is introduced based on this condition. It can be successfully used in the real environment where a full spectrum of multiplicative and colored noises exists. In the experiment, as one of our dedicated contributions, low-frequency harmonics and the defective characteristics of popular porcelain insulators are illustrated.