energies Article Optical Radiation from an Electric Arc at Different Frequencies Łukasz Nagi * , Michał Kozioł and Jarosław Zygarlicki Institute of Electrical Power Engineering and Renewable Energy, Opole University of Technology, Prószkowska 76 Street, 45-758 Opole, Poland; [email protected] (M.K.); [email protected] (J.Z.) * Correspondence: [email protected] Received: 22 January 2020; Accepted: 25 March 2020; Published: 3 April 2020 Abstract: The article presents research on the electric arc generated by AC current at different frequencies. The measurement procedure and system are described. Optical spectra of the generated arc in the air were recorded using a spectrophotometer. Optical spectra for five frequencies were obtained. The article also presents the energy balance of the components of the registered spectrum. The visible changes in the spectra that depend on the frequency of the AC current generating the electric arc can be significant to the diagnostics of gas insulators. The research presented in the article can be used in multiple areas of technology where an electric arc is used. The influence of the frequency of the current supplying the electric arc on the electromagnetic radiation spectrum in the area of light radiation emitted by the electric arc allows for the construction of systems that can shape the desired characteristics of the electric arc. Keywords: optical method; electric arc; spectrophotometer; electromagnetic radiation; gas insulator; aeronautics; arc lamps; welding; arc frequencies 1. Introduction The electric arc generated in gas insulators is the penultimate phase of the electrical discharge and leads to a breakdown. The order in which this happens is corona extinction voltage (CEV); partial discharge initiative voltage (PDIV); partial discharge extinction voltage (PDEV); arcing voltage; breakdown voltage. There are many mechanisms to stop the formation of an arc [1,2] and prevent a breakdown. Just as important as the breakdown is the diagnostics of the gas isolation state in the direction of partial discharges (PDs) and corona discharges, which are the phenomena preceding the formation of an arc. Many diagnostic methods are used for this purpose: acoustic emission (AE), electromagnetic wave detection in the UHF range, high energy ionizing and UV or visible light (with a spectrophotometer). The main aim of the research presented in this article was to determine and analyze the spectrum of optical radiation in the ultraviolet, visible and near-infrared range (UV–NIS–NIR) emitted by a generated electric arc in air at atmospheric pressure for different AC voltage frequencies. The recording of the arc spectrum is one of the elements of high-voltage diagnostics. It is possible to define some descriptors for a certain phenomenon by their shape or intensity. From the scientific point of view, for different frequencies of alternating voltage generating an electric arc, the spectrum of the arc differs depending on the preset frequency. Characterization of parameters for different frequencies of currents may help us to identify risks depending on the frequency of current used. Previous research on electric arcs was more focused on phenomena related to direct current. Guan et al., in the article “DC arc self-extinction and dynamic arc model in open-space condition using a Yacob Ladder” [3], focused on the study of the arc generated at the initial current from 50 A to 200 A, while the voltage of the generating system was 560 V from a three-phase system. The entire process of arc evolution, from its production to development to self-extinguishing, was studied. The phenomenon Energies 2020, 13, 1676; doi:10.3390/en13071676 www.mdpi.com/journal/energies Energies 2020, 13, 1676 2 of 9 was recorded with a fast camera in the visible light range. A Yacob Ladder was used in the study. Another study, in which a Yacob Ladder was used to generate the arc, is called “Application of Optical Spectrophotometry for Analysis of Radiation Spectrum Emitted by Electric Arc in the Air” [4]. The authors also used DC current. However, the arc measurements were performed with an optical method, using a spectrophotometer in the visible and near UV light range. Other studies aimed at characterization of the electric arc are presented in a paper by Martins et al. [5]. The authors used a high-speed camera to record the optical signal. In this work, different levels of current peaks are used, from 10 kA to 100 kA, with a short peak duration of about 15 µs. The camera itself is synchronized with the trigger of the arc generator. The ionic lines of nitrogen and oxygen are used to determine the radial temperature profiles and electron density in the arc channel over a period spanning from 2 µs to 36 µs. Tests in contaminated isolators using the optical method were described in a paper titled “Study of the AC arc discharge characteristics over polluted insulation surface using optical emission spectroscopy” [6]. In other tests, the arc propagation growth and its shape and leakage current at various air pressures were checked. The length of the discharge path is related to air pressure and it is always shorter with decreasing pressure [7]. A description of arc formation and propagation is also included in the study “Performance and Characteristics of a Small-Current DC Arc in a Short Air Gap” [8]. This paper shows that the color of the DC arc changes from blue to purple and then yellow, forming a flame in the visible light range as the current increases. Research on the electric arc with diagnostic methods for PD detection has been described in an article by Chen et al. [9]. The authors simulated the wave of acoustic pressure and then carried out experiments demonstrating the convergence of theory and practice. It was found that the spatial distribution of power density in the arc is highly heterogeneous and the power density in the area close to the electrodes is much higher. The acoustic method is widely used for PD detection in both gaseous and electrical insulating liquids [10,11]. The phenomenon of acoustic wave formation itself, at the moment of arc generation, can be very dangerous. High pressure and high energy can cause a lot of damage and can be dangerous for life [12]. As mentioned in the article by Martins et al. [5], the electric arc excites atoms of ambient elements. The optical spectrum recorded in this study was based on the nitrogen and the oxygen spectra. With direct current, the excitations change only depending on the energy supplied to generate the arc. Extended studies, with currents from 100 kA to 250 kA at the peak, were presented in [13]. In the case of sine wave currents, the excitations can look different. For different frequencies, different elements may be induced into resonances at the nuclear level at different instances of the phenomenon. The excitation energy concerns mainly electrons, but it may also affect the nuclear spin of the elements. Non-zero nuclear spin includes almost all atoms with an odd number of nucleons (e.g., hydrogen 1H, carbon 13C, nitrogen 15N, oxygen 17O, fluorine 19F, sodium 23Na and phosphorus 31P). Put simply, a nuclear spin can be imagined as the rotation of the nucleus around its axis. It is related to the internal momentum of the nucleus. The main purpose of the research presented in this article was to determine the spectrum of light in the visible range coming from a generated electric arc in air at atmospheric pressure, depending on the frequency of alternating voltage. In the study, an optical method was used to detect and identify partial discharges in insulators of equipment and power cables [14]. This method is often combined with others using different ranges for the detection of electromagnetic radiation emitted from PDs [15,16]. Research on arcs and preceding corona discharges (CDs) is also important in aeronautics. The electric arc is a major risk to aircraft systems. Professor Riba’s team was engaged in diagnostics of corona discharges under aeronautical conditions. In the article [17], they described an experiment using a low-cost camera for early detection of UV radiation from corona discharges. They also compared the corona discharge measurements for positive and negative DC and AC for 50 Hz. Experimental results presented in [18] clearly show that the sphere–plane gap follows a correlation similar to Peek’s law for cylindrical conductors. This conclusion is true for 50 Hz AC, positive DC and negative DC supply. 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