**1. Introduction**

Insulators are key components on transmission lines because they provide sufficient insulation space between the conductor line and the ground. Usually, insulators are made of glass, ceramics, and polymers [1]. During prolonged outdoor service, contamination or pollution is inevitably deposited on insulator surfaces and consists of dust, particles, and other airborne substances. This contamination cannot be avoided because transmission lines are the main energy transport channel for every grid. Under dry conditions, this contamination is relatively safe. Still, in heavy fog or rainy weather, the soluble components in the contamination can dissolve in the water and form conductive paths on the insulator surfaces, thus reducing the flashover voltage and causing discharge and even accidental flashover. In China, there were several power outage incidents in the 1990s due to transmission flashover caused by contamination deposited on insulators in heavy fog weather [2]. This phenomenon was deemed surface flashover in high-voltage engineering and has become a topic of interest in this field. Thus, the detection of contamination composition has become an important task in the regular operation of the State Grid.

The main factors affecting the surface discharge activity and flashover process of pollution insulators are the equivalent salt deposit density (ESDD) level and relativity air humidity (RH). Higher ESDD or higher RH will lead to more intense discharges and lower flashover voltage [3,4]. Contaminant composition, especially soluble salts (including common salts such as NaCl, NaNO3, Na2SO4, MgC12, Mg(NO3)2, MgSO4, CaCl2, Ca(NO3)2, and CaSO4.), and material properties can affect the flashover process, which may result in an excess or lack of insulation during insulation design. A previous study showed that the tendency for an insulator to flashover is dependent on the type of contaminant, as well as on the equivalent salt density [5]. At present, the only way to analyze these salt compositions is by collecting the contamination during a power outage and taking the sample back to the lab for analysis with chemistry equipment. It needs a new method that can be used for on-site and on-line detection of contamination in insulators.

In recent years, laser-induced breakdown spectroscopy (LIBS) has developed rapidly in power engineering because of its advantages, such as no sample preparation, harmless sampling, and fast detection speed. LIBS has been widely applied as a tool for mineral analysis, archaeology, biomedical analysis, aerospace exploration, etc. [6–8]. The basic experimental process involves exciting plasma with a high-energy laser and collecting its characteristic spectral information to obtain the elemental composition and surface condition of the target material.

In recent years, LIBS has also played an important role in the online detection of power equipment status in high-voltage engineering. Huan et al. [9] applied LIBS technology to detect the vacuum degree of vacuum breakers. Based on the fact that the intensity of characteristic spectral lines of different elements, including Cu, O, N, and H, will change with the vacuum degree of the vacuum breaker, the vacuum degree can be predicted. In their study, principal component analysis (PCA) and the artificial neural network (ANN) model were used to optimize the spectral line selection, and the accuracy of the final model reached 96.67% [10,11]. Our previous work proved that the number of laser pulses has a linear relationship with the depth of ablation. Thus, the distribution of elements along with depth on one test point of silicon rubber was obtained, and the thickness of the aging layer could be calculated accordingly [12–14].

A remote laser-induced breakdown spectroscopy technique combined with a photometric device was proposed and demonstrated at the laboratory scale. It was used for the remote sensing and quantification of surface pollutants such as salt deposits on wind turbine blades from different standoff distances [15]. Nearly every element was studied with LIBS to obtain calibration curves for quantitative analysis of soils, rocks, compounds, and other materials [16–18]. However, on the surface of transmission line insulators, the contamination consists of various compounds that may have the same cations. It is necessary to determine the influence of various compound ratios and system parameters on the calibration results to improve the detection accuracy of LIBS for the online monitoring of insulator surface pollution. Calibration curves are crucial in determining the sample contents with LIBS. There have been many studies on the composition analysis of contamination, and we used artificial pollution and compressed it into pellets. In this work, compounds containing various ratios of Na and Ca were designed and analyzed to determine the effects of these mixture ratios on the LIBS calibration results.
