energies Article Energy Distribution of Optical Radiation Emitted by Electrical Discharges in Insulating Liquids Michał Kozioł Faculty of Electrical Engineering, Automatic Control and Informatics, Opole University of Technology, Proszkowska 76, 45-758 Opole, Poland; [email protected] Received: 26 March 2020; Accepted: 29 April 2020; Published: 1 May 2020 Abstract: This article presents the results of the analysis of energy distribution of optical radiation emitted by electrical discharges in insulating liquids, such as synthetic ester, natural ester, and mineral oil. The measurements of optical radiation were carried out on a system of needle–needle type electrodes and on a system for surface discharges, which were immersed in brand new insulating liquids. Optical radiation was recorded using optical spectrophotometry method. On the basis of the obtained results, potential possibilities of using the analysis of the energy distribution of optical radiation as an additional descriptor for the recognition of individual sources of electric discharges were indicated. The results can also be used in the design of various types of detectors, as well as high-voltage diagnostic systems and arc protection systems. Keywords: optical radiation; electrical discharges; insulating liquids; energy distribution 1. Introduction One of the characteristic features of electrical discharges is the emission to the space in which they occur, an electromagnetic wave with a very wide range. Such typical ranges of emitted radiation include ionizing radiation, such as X-rays, optical radiation, acoustic emission, and radio wave emission. Based on most of these emitted ranges, diagnostic methods were developed, which enables the detection and location of the source of electrical discharges, which is a great achievement in the diagnostics of high-voltage electrical insulating devices [1–4]. These methods are constantly being improved and modified in terms of increasing their effectiveness and speed of operation. Parallel to these activities, research was also carried out in the field of basic studies aimed at learning new possibilities of using the physicochemical properties of electrical discharge forms [5–10]. Examples of not fully understood areas are X-ray radiation and optical radiation emitted by electrical discharges [11–14]. The research topic discussed in this article is focused in particular on the analysis of optical radiation emitted by electrical discharges, which is usually interpreted using a designated spectrum. For this study, the optical radiation range from 200 nm to 1100 nm was assumed. The radiation spectrum represents the visual form of electromagnetic radiation distributed over the individual components of the wavelengths. Using the radiation spectrum, information about the range of waves that are involved in the analyzed radiation is presented, but their quantitative values were not determined. The dependence of the quantitative size on the occurring wavelength component was represented by the spectral distribution. Spectral distribution, in addition to the range of wavelengths of occurring radiation, most often shows the intensity value of individual components of wavelengths. Registration of optical radiation is a particularly difficult task in the case of emissions in insulating liquids where there is a large attenuation of the optical signal [15–17]. In addition, there was also an environment with high electric field strength. Therefore, to record radiation in such conditions, it required the use of advanced measuring devices that enabled transmission and processing of optical signals without interference. An additional problem was the correct positioning of the measuring probe Energies 2020, 13, 2172; doi:10.3390/en13092172 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 10 conditions, it required the use of advanced measuring devices that enabled transmission and processing of optical signals without interference. An additional problem was the correct positioning ofEnergies the measuring2020, 13, 2172 probe (optical fiber) in the expected location of the electrical discharge, so that2 the of 9 emitted optical radiation can be introduced and transmitted by means of an optical fiber. Currently, effective(optical fiber) measuring in the expected probes have location not ofyet the been electrical developed discharge,, and so all that measuremen the emittedts optical carried radiation out in this can areabe introduced are of an experimental and transmitted nature. by means of an optical fiber. Currently, effective measuring probes have not yetConducted been developed, and published and all measurements studies were carried mainly out focused in this areaon arethe ofpossibility an experimental of recording nature. dischargesConducted and determining and published spectral studies distributions were mainly on focused their basis on the [18 possibility–21]. However, of recording there is dischargesmuch less workand determining devoted to spectralthe development distributions of useful on their descriptors basis [18 –which,21]. However, determined there on is the much basis less of work the obtaineddevoted spectral to the development distributions, of could useful be descriptors used to identify which, the determined forms of electrical on the discharges basis of the in obtained various insulationspectral distributions, systems (both could gas beand used liquid). to identify Such an the approach forms of electricalwas presented discharges in the in work various [22] insulation, where a groupsystems of (bothdescriptors gas and for liquid). identifying Such forms an approach of electrical was discharges presented in insulating the work [ 22oil], w whereere developed. a group of descriptorsWith regard for identifying to the already forms conducted of electrical research discharges related in insulatingto the registration oil were and developed. analysis of optical radiationWith emitted regard toby the electric already discharges conducted, in research terms of related the possibility to the registration of using andtheir analysis results ofin optical high- voltageradiation diagnostics, emitted by the electric author discharges, proposed in a terms new ofapproach the possibility to the ofinterpre usingtation their results of recorded in high-voltage spectral distributions.diagnostics, the This author solution proposed is based a new on approach the analysis to the of interpretation the optical spectrum of recorded in terms spectral of distributions.the share of individualThis solution ranges is based of optical on the analysisradiation of and the opticaltheir use spectrum as a descriptor in terms ofto therecognize share of single individual-source ranges forms of ofoptical electrical radiation discharge and theirs. use as a descriptor to recognize single-source forms of electrical discharges. 2.2. Method Method of of M Measuringeasuring O Opticalptical S Spectrapectra TheThe tests tests were were carried carried out out on two electrode systems systems,, on which single single-source-source forms of of electrical dischargesdischarges were were generated. The The f firstirst system system consisted consisted of of needle needle–needle–needle electrodes, electrodes, where where a a high high voltagevoltage was was applied applied to one of of the electrodes electrodes and the other was earthed. The The second second system system consisted consisted ofof a a needle needle electrode electrode,, and and a a solid solid dielectric dielectric was was used used to to generate generate surface surface discharges. discharges. Both Both systems systems can can bebe used used as as models models of of pot potentialential damage damage in in the the high high power power insulating insulating liquid liquid filled filled transformers transformers,, where where thethe needle needle–needle–needle electrode electrode system system wa wass a a model model of of damage damage to to neighboring neighboring transformer transformer windings, windings, while the the surface surface discharge discharge system system wa wass a model a model of damage of damage at the at contact the contact between between the solid the and solid liquid and dielectric.liquid dielectric. The electrode The electrode systems systems were weresubsequently subsequently immersed immersed in ininsulating insulating liquids liquids,, and and the the measurementsmeasurements were were carried carried out in identical metrological conditions for each variant. Figure Figure 11 showsshows thethe general general scheme scheme of of the the measuring measuring system. system. HR4000 USB POF HV Ro S1 S1 kV ~U L1 L2 FigureFigure 1. DiagramDiagram of of the the measuring system system:: Ro Ro—protective—protective water water resistor resistor (1 (1.1.1 M Ω)W);; POF POF—Polymer—Polymer OpticalOptical F Fiber;iber; HR4000 HR4000—optical—optical spectrophotometer spectrophotometer;; L1 L1 and and L2 L2—control—control signalling signalling;; S1 S1—voltage—voltage switchswitch;; kV kV—voltmeter;—voltmeter; and U U—mains—mains voltage 230 V.V. TheThe schematic schematic diagram diagram and and general general view view of of the the spark spark gap gap for for generating generating electric electric discharges discharges in in a a needleneedle–needle–needle system system is is shown shown in in Fig Figureure 22.. Two identicalidentical electrodeselectrodes withwith thethe followingfollowing dimensionsdimensions were used: used: total total length length,, 35 35 mm mm;; base base