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.

To cope with the large state estimation error due to sensor delay, a novel flexible model is explored to describe a linear dynamic system with multiple-step random delays in this paper. Compared with existing models, this model is more consistent with the actual situation. Based on the new model, the main difficulty, which is to determine the probability of any number of steps delayed, is overcome by applying techniques of high-order Markov chain. Then, the Kalman filtering problem with measurement delays is converted to random parameter matrices Kalman filtering(RKF), the new approximate state estimators are proposed. For a n-step random delay model, we prove that it can be treated as a (2n –1)th-order Markov chain, making it theoretically feasible to apply the method in this paper to deal with any multiple-step delay model. Some illustrative numerical examples are presented to demonstrate the efficiency of the new model and superiority over existing algorithms.