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Online monitoring of the contamination condition of power transmission line is important to prevent insulation from flashover. The conventional online monitoring methods are easily disturbed by the strong electromagnetic interference around the high voltage line. In this letter, Bragg wavelength of polyimide coated fiber Bragg grating (FBG) is found to be slightly restrained by the salt deposited on the polyimide surface. Based on the effect, a special designed FBG sensor is proposed for detecting equivalent salt deposit density (ESDD) on insulators of power transmission line. To improve the sensor sensitivity, multiple polyimide layers are coated on the naked FBG, and in the curing process a 0.15-N force is applied on the free end of FBG to form uniform layers. Experiment results show the restrain wavelength varies with the ESDD. The resolution can be improved with multiple polyimide layers.

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