**1. Introduction**

Outdoor insulators are essential hardware of the power delivery system. They are a basic requirement of open-air outdoor switchgears, and a failure of them means a failure in the system. These essential hardware are found in the transmission and distribution of electricity from power stations to substations, where the voltage is stepped down and distributed to commercial and residential consumers. Due to the remoteness of power plants, the energy is generally transported over long distances using high voltage (HV) overhead transmission lines supported by pylons. Insulators provide the mechanical means by which high voltage transmission lines (composed of bare conductors) are suspended from transmission structures (pylons), while also providing the required electrical insulation [1,2]. These important hardware are exposed to various electrical and environmental stresses that a ffect their performance and increase their premature aging and degradation. When insulators fail, either in their mechanical or electrical role, the consequences are power outages and, in some cases, additional equipment and structure damages. Reducing the risk of failure through proper installation, inspection, and assessment practices is the focus of ongoing research and utilities engineer training. The reduction in the performance of outdoor insulators occurs mainly by the pollution accumulation at the insulating surfaces. Surface discharges are precursors of flashover. Outdoor insulators are exposed to various pollution sources (sea salts, domestic pollution, dirt, and chemical residues in the industrial areas) [3,4]. Insulator pollution can lead to flashover. In polluted areas, overhead lines may see their reliability and performance decline due to pollution insulators. When the contaminated insulating surface is wet, it becomes a conductive electrolyte. The leakage currents then increase on the insulating surface with potential flashover [5]. Identification of the factors causing insulation surface pollution, pollution severity assessment, and tackling its unfavorable e ffects have an important role in increasing the grid reliability. This is highly remarkable, particularly in environments with high dynamics (wind, pollution, humidity, etc.). Significant di fferences within the seriousness of pollution between sites can be assessed through equivalent salt deposit density (ESDD), non-soluble deposit density (NSDD), and the leakage current measurements. One of the major problems faced by this factory is the pollution of ceramic insulators installed on the 13.2 kV distribution grid. The interaction between the air transporting dust and the insulators creates a pollution layer on the insulator's surface. Once this layer is moistened, the withstand voltage drops considerably, causing the insulator to flashover. The concomitant power outages are not acceptable because they lead to huge financial losses and loss of control of the production process cycle. Since the insulators were initially designed considering a light pollution level, it is suspected that the environmental parameters dynamics resulting from the factory's production have a ffected the type and amount of particle accumulation. In this paper, seven service-aged 13.2 kV ceramic insulators, surrounding a Canadian aluminum plant, are collected to examine the severity of the pollution. The assessment was conducted through the leakage current and the ESDD and NSDD (non-soluble deposit density) measurements. The results are useful for assessing the insulation performance in the di fferent areas and to propose an updated pollution severity map.

#### **2. Background on Insulators**

Generally, insulators are made based on porcelain or tempered glass. They are composed of a wet mixture of four primary materials: feldspar, flint, ball clay, and talc [6]. The mixture is molded and heated at a temperature of 1200 ◦C–1400 ◦C, and then glazed. The excellent dielectric strength of porcelain, which is around 1.574–11.02 kV/mm, and its relative permittivity (between 5.1 and 5.9) allow improving the resistance to materials' aging due to electrical and environmental stresses [7]. The Achilles heel of porcelain insulators is, essentially, their hydrophobic surfaces [8]. Later on, polymer insulators were introduced because of their excellent performance against pollution. Polymer insulators are of two types: in resin or composite.

Composite insulators are used extensively for the levels of voltage distribution and high voltage transmission lines [1,9]. In contaminated environments, the leakage current at their level is much lower than that of ceramic insulators [10]. However, a composite insulator has some disadvantages: brittle fractures, erosion, tracking, and chemical changes on the surface due to weathering are the main reasons for failures [1].

A significant cause of both service interruptions and flashovers is due to polluted insulators. Their surface is mostly responsible. There are two types: hydrophilic (ceramic insulators) or hydrophobic (polymer insulators) [3,4]. The pollution flashover process for ceramic insulators can be seen in [5]. Pollution sources can be found in Table 1 [6].

In non-ceramic and porcelain insulators, the contamination process is the same, but non-ceramic insulators collect less pollution than ceramic insulators [6,11]. Recently, with nanotechnology and nanoscience, new materials with innovative properties have been proposed for several applications [12–14]. A wide range of such monitoring devices and techniques has been developed over the years. The most widely used ones are [3]: directional dust deposit gauge, NaCl, ESDD, environmental monitoring (air sampling, climate measurements), NSDD, surface conductance, insulator flashover stress, surge counting, and leakage current measurement. New diagnostic tools have emerged [1].


