Energy Distribution of Optical Radiation Emitted by Electrical Discharges in Insulating Liquids

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 area on are the of possibility 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 spectral the 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 to by the electric already discharges conducted, in research terms of related the possibility to the registration of using and their analysis results of in 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 were subsequently subsequently immersed immersed in in insulating 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)Ω);; POF POF—Polymer—Polymer OpticalOptical FFiber;iber; HR4000HR4000—optical—optical spectrophotometer spectrophotometer;; L1 L1 and and L2 L2—control—control signallingsignalling;; 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 diameter diameter,, 20 20mm mm;; apex apex ang angle,le, 32°; 32 and◦; and needle needle head head diameter diameter,, 0.8 0.8 mm. Distance between the electrodes in the needle–needle system was constant for all cases and was 2 cm. The electrodes were made of copper, and their surfaces were electroplated with nickel. The Energies 2020, 13, x FOR PEER REVIEW 3 of 10 Energies 2020, 13, x FOR PEER REVIEW 3 of 10 mm. Distance between the electrodes in the needle–needle system was constant for all cases and was mm. Distance between the electrodes in the needle–needle system was constant for all cases and was Energies2 cm. 2020The, 13electrodes, 2172 were made of copper, and their surfaces were electroplated with nickel. The3 of 9 2 cm. The electrodes were made of copper, and their surfaces were electroplated with nickel. The galvanic coating of the copper electrode with nickel improves its mechanical resistance and thermal galvanic coating of the copper electrode with nickel improves its mechanical resistance and thermal strength, which also allows multiple uses of the same electrode. galvanicstrength, coating which also of the allows copper multiple electrode uses with of the nickel same improves electrode. its mechanical resistance and thermal strength, which also allows multiple uses of the same electrode.

(a) (b) (a) (b) Figure 2. Needle–needle electrode system: schematic diagram side view (a) and view from above (b). FigureFigure 2. Needle–needle2. Needle–needle electrode electrode system: system: schematic diagram diagram side side view view (a ()a and) and view view from from above above (b). ( b). A system of two metal electrodes with a solid dielectric between them was used to generate AA systemsystem ofof twotwo metalmetal electrodeselectrodes withwith a solid dielectric between them them was was used used to to generate generate electrical discharges in the surface system. A schematic diagram and general view of the spark gap electricalelectrical dischargesdischarges inin thethe surfacesurface system.system. A s schematicchematic diagram diagram and and general general view view of of the the spark spark gap gap in the surface system is shown in Figure 3. The supplying electrode was a needle electrode, and the inin the the surface surface systemsystem isis shownshown inin FigureFigure3 3.. TheThe supplyingsupplying electrodeelectrode was a needle electrode electrode,, and and the the grounded electrode was a flat cylindrical plate with a base diameter of 69 mm and thickness of 9 mm, grounded electrode was a flat cylindrical plate with a base diameter of 69 mm and thickness of 9 mm, groundedwhich was electrode made of wasmetal. a ﬂat The cylindrical spark gap plateelectrodes with awere base separated diameter ofby 69a solid mm anddielectric, thickness which of 9was mm, which was made of metal. The spark gap electrodes were separated by a solid dielectric, which was whicha plate was made made of sodium of metal. glass, The with spark external gap electrodes dimensions were of separated90 mm × 90 by mm a solid and dielectric,a thickness which of 10 mm. was a platea plate made made of of sodium sodium glass, glass, with with external external dimensions dimensions of of 90 90 mm mm ×90 90 mmmm andand aa thickness of 10 mm. ×

(a) (b) (a) (b) FigureFigure 3.3. Surface discharge system: schematic schematic diagram diagram side side view view (a (a) )and and view view from from above above (b (b). ). Figure 3. Surface discharge system: schematic diagram side view (a) and view from above (b). TheThe threethree mostmost frequentlyfrequently used electroinsulating liquids liquids in in power power engineering engineering were were used used for for The three most frequently used electroinsulating liquids in power engineering were used for testingtesting naturalnatural ester ester Midel Midel 1204, 1204, synthetic synthetic ester ester Midel Midel 7131, 7131 and, and mineral mineral oil Orlen oil Orlen Trafo Trafo EN. All EN. liquids All testing natural ester Midel 1204, synthetic ester Midel 7131, and mineral oil Orlen Trafo EN. All wereliquids brand were new brand and new free and of any free contamination. of any contamination. The temperature The temperature of the insulating of the insulating liquids wereliquids the liquids were brand new and free of any contamination. The temperature of the insulating liquids samewere the in all same examined in all examined cases and cases was and 20 was◦C. 20 Due °C. to Due the to experimental the experimental nature nature of basic of basic research, research, the were the same in all examined cases and was 20 °C. Due to the experimental nature of basic research, inﬂuencethe influence of liquid of liquid temperature temperature on the on obtainedthe obtained measurement measurement results results was was not not analyzed. analyzed. the influence of liquid temperature on the obtained measurement results was not analyzed. TheThe opticaloptical spectrophotometer,spectrophotometer, HR4000 fro fromm Ocean Ocean Optics Optics ( (Dunedin,Dunedin, FL FL,, USA USA)) was was used used to to The optical spectrophotometer, HR4000 from Ocean Optics (Dunedin, FL, USA) was used to recordrecord thethe opticaloptical radiationradiation emitted by electrical electrical discharges. discharges. The The applied applied spectrophotometer spectrophotometer record recordeded record the optical radiation emitted by electrical discharges. The applied spectrophotometer recorded opticaloptical radiationradiation inin the ultraviolet, visible visible,, and and near near-infrared-infrared range range (UV (UV–VIS–NIR–VIS–NIR spectral spectral range range from from optical radiation in the ultraviolet, visible, and near-infrared range (UV–VIS–NIR spectral range from 200200 nmnm toto 11001100 nm).nm). The device is equipped with with a a 3648 3648-element-element linear linear silicon silicon CCD CCD array array and and an an 200 nm to 1100 nm). The device is equipped with a 3648-element linear silicon CCD array and an opticaloptical resolutionresolution ofof 0.470.47 nmnm FWHM (Full Width at Half Maximum). This This enable enabledd the the detection detection of of optical resolution of 0.47 nm FWHM (Full Width at Half Maximum). This enabled the detection of 36483648 componentscomponents ofof thethe recordedrecorded optical spectrum in the range of of 200 200 nm nm to to 1100 1100 nm. nm. 3648 components of the recorded optical spectrum in the range of 200 nm to 1100 nm. Polymer optical ﬁber 600SR (POF) manufactured by Ocean Optics was used as the measuring head, and one of its endpoints was placed near the electrode system. The basic parameters of the optical

ﬁber were presented in Table1. During the emission of optical radiation by electrical discharge, the light beam was introduced into the optical ﬁber and sent to spectrophotometer. The spectrophotometer Energies 2020, 13, x FOR PEER REVIEW 4 of 10

Polymer optical fiber 600SR (POF) manufactured by Ocean Optics was used as the measuring head, and one of its endpoints was placed near the electrode system. The basic parameters of the optical fiber were presented in Table 1. During the emission of optical radiation by electrical Energiesdischarge,2020, 13the, 2172 light beam was introduced into the optical fiber and sent to spectrophotometer. 4The of 9 spectrophotometer converts the light beam into a parallel stream with a spectral range of 200 nm to converts1100 nm theand light counts beam the intonumber a parallel of emitted stream photons with a for spectral each wavelength. range of 200 The nm integration to 1100 nm time and countsof the thespectrophotometer number of emitted (matrix photons exposure for each time) wavelength. was the same The in integration all cases and time was of set the to spectrophotometer 1 s. Obtained data (matrixwere presented exposure in time) the was form the of same spectral in all characteristics, cases and was where set to the1 s. intensity Obtained correspond data wereed presented to the innumber the form of counts of spectral for wavelength characteristics,s in the where analyzed the intensity range. corresponded to the number of counts for wavelengths in the analyzed range. Table 1. Basic parameters of the optical fiber.

TableParameter 1. Basic parameters of the opticalValue fiber. spectralParameter range 200 nm Value–1100 nm fiber core type polymer spectral range 200 nm–1100 nm fibercore core diameter type 600 polymer ± 10 μm operatingcore temperature diameter range −65 600 °C…+30010 µm °C ± operating temperature range 65 C ... +300 C fiber bend radius − ◦ 12 cm ◦ fiberacceptance bend radius angle 1225 cm° acceptance angle 25◦ Figure 1 shows how the fiber was placed in the electrode system area. The optical fiber head was placedFigure at a1 distance shows how of 2. the5 cm fiber from was the placed expected in the source electrode of optical system radiation area. The emission. optical fiber This head distance was placedwas determined at a distance on ofthe 2.5 basis cm fromof the the fiber expected acceptance source cone of opticalparameter radiation and due emission. to the metal This distanceelements was of determinedthe fiber head. on the basis of the fiber acceptance cone parameter and due to the metal elements of the fiber head.All measurements were made in a darkened laboratory room, separated from external sources of opticalAll measurements radiation. Before were made each in measurement a darkened laboratory series, the room, spectrophotometer separated from wasexternal calibrated sources to of opticaldetermine radiation. the minimum Before each background measurement level. series,This operation the spectrophotometer aimed to eliminate was calibrated interference to determineresulting thefrom minimum the process background of converting level. the This optical operation signal to aimed digital to form. eliminate The background interference calibration resulting function from the processis available of converting in the device the optical software. signal Spectral to digital calibration form. The of background the spectrophotometer calibration function with a isdedicated available inPOF the was device performed software. by the Spectral manufacturer. calibration The of supply the spectrophotometer voltage of electrode with systems a dedicated was from POF 25 to was 50 performedkV (RMS v byoltage) themanufacturer. of 50 Hz alternating The supply current voltage (AC), ofwith electrode gradation systems every was 5 kV from. In order 25 to 50to kVlimit (RMS the voltage)discharge of current, 50 Hz alternating a water resistor current (Ro) (AC), of 1 with.1 MΩ gradation was used, every which 5 kV. limited In order the tocurrent limit theto mA discharge range current,(about 100 a water mA for resistor this system). (Ro) of For 1.1 each MΩ supplywas used, voltage, which 10 limitedmeasurement the current tests were to mA performed. range (about 100 mA for this system). For each supply voltage, 10 measurement tests were performed. 3. Measurement Results 3. Measurement Results Figure 4 presents examples of registered optical spectra emitted by electrical discharges generatedFigure on4 presents the system examples of needle of registered–needle electrodes optical spectra for each emitted of the by tested electrical insulating discharges liquids. generated on the system of needle–needle electrodes for each of the tested insulating liquids.

Energies 2020, 13, x FOR PEER REVIEW 5 of 10 (a) (b)

(c)

FigureFigure 4.4. Example results of measurements generated on a needle needle–needle–needle spark spark gap gap at at 35 35 kV kV supply supply voltagevoltage forfor thethe followingfollowing insulatinginsulating liquids:liquids: Midel 1204 ( a); Midel 7131 ( b); and Mineral Mineral oil (cc).).

Obtained optical spectra in the needle–electrode system shows some similarity in the shape of the spectral characteristics in all three analyzed liquids. The spectral range of the characteristics mainly includes visible light and, to a small extent, near infrared and ultraviolet. Figure 5 presents examples of registered optical spectra emitted by electrical discharges generated on the surface discharge system for each of the tested insulating liquids.

(a) (b)

(c)

Figure 5. Example results of measurements generated on a surface discharge system at 35 kV supply voltage for the following insulating liquids: Midel 1204 (a); Midel 7131 (b); and Mineral oil (c).

By comparing the obtained spectral characteristics in both analyzed electrode systems and for the same supply voltage level, significant differences in their shapes and spectral ranges can be observed. For the needle–needle electrode system, the spectral range is mainly in visible light, and a small extent in the near-infrared and ultraviolet range. In turn, the spectra obtained for the surface discharge system contained a higher proportion of ultraviolet radiation. This showed the potential possibility of using optical spectra analysis for the recognition of single-source forms of electrical discharges.

4. Optical Radiation Energy Based on the obtained characteristics of spectral distributions and using the quantum description where optical radiation is described as a photon stream, the share of emitted energy can

Energies 2020, 13, x FOR PEER REVIEW 5 of 10

(c)

Figure 4. Example results of measurements generated on a needle–needle spark gap at 35 kV supply Energiesvoltage2020, 13 for, 2172 the following insulating liquids: Midel 1204 (a); Midel 7131 (b); and Mineral oil (c). 5 of 9

Obtained optical spectra in the needle–electrode system shows some similarity in the shape of the spectralObtained characteristics optical spectra in in all the three needle–electrode analyzed liquids. system The shows spectral some rangesimilarity of the in the characteristics shape of the mainlyspectral includes characteristics visible inlight all threeand, to analyzed a small liquids.extent, near The spectralinfrared rangeand ultraviolet. of the characteristics Figure 5 presents mainly examplesincludes visible of registered light and, optical to a small spectra extent, emitted near infrared by electrical and ultraviolet. discharges Figure generated5 presents on the examples surface dischargeof registered system optical for spectra each of emitted the tested by electricalinsulating discharges liquids. generated on the surface discharge system for each of the tested insulating liquids.

(a) (b)

(c)

FigureFigure 5. ExampleExample results results of of measurements measurements generated generated on on a a surface surface discharge discharge system system at at 35 35 kV kV supply supply voltage for the following insulating liquids: Midel Midel 1204 1204 ( a);); Midel Midel 7131 7131 ( b); and Mineral oil ( c).

ByBy c comparingomparing thethe obtainedobtained spectralspectral characteristics characteristics in in both both analyzed analyzed electrode electrode systems systems and and for for the thesame same supply supply voltage voltage level, level significant, significant differences differences in their in shapes their and shapes spectral and spectralranges can ranges be observed. can be observed.For the needle–needle For the needle electrode–needle system, electrode the system, spectral the range spectral is mainly range in is visible mainly light, in visible and a smalllight, extentand a smallin the extent near-infrared in the near and ultraviolet-infrared and range. ultraviolet In turn, range. the spectra In turn, obtained the spectra for the obtain surfaceed discharge for the surface system dischargecontained system a higher contain proportioned a higher of ultraviolet proportion radiation. of ultraviolet This showed radiation. the potential This show possibilityed the potential of using possibilityoptical spectra of using analysis optical for thespectra recognition analysis of for single-source the recognition forms of of single electrical-source discharges. forms of electrical discharges. 4. Optical Radiation Energy 4. OpticalBased R onadiation the obtained Energy characteristics of spectral distributions and using the quantum description whereBased optical on radiation the obtained is described characteristics as a photon of stream, spectral the distributionsshare of emitted and energy using can the be estimated. quantum descriptionEach wavelength where of optical emitted radiation radiation is corresponds described as to a aphoton specific stream, photon the energy share which of emitted can be determinedenergy can from the relation: E = n h υ (1) · · where: E—total energy of the photon stream (J), n—number of photons counted (-), h—Planck constant 6.626 10 34 (J s), and υ—wave frequency (1/s). × − · The frequency of the waveform is expressed by the formula: c υ = (2) λ where: υ—wave frequency (1/s); c—phase speed of the wave, speed of light in vacuum 2.998 108 × (m/s); and λ—wavelength (nm). Energies 2020, 13, 2172 6 of 9

Table2 presents examples of the calculated energy values of optical radiation emitted by electric discharges generated in the tested electrode systems. In order to better present the determined total energy values, the physical unit of energy description in the form of electronvolts (eV) was used. They were calculated from a simple relationship resulting from the deﬁnition of eV, where 1 J 6.241509126(38) 1018 eV. The calculated energy was not the total energy radiated by electrical ≈ × discharges. The presented values were estimated based on the registered optical radiation by the spectrophotometer. This stage of research does not include an attempt to prepare energy balance, but only presents the possibility of applying energy distribution analysis to recognize the form of electrical discharges.

Table 2. Examples of optical radiation energy values.

Energy in UV Energy in VIS Energy in NIR Total Energy (E = Range (E ) 200 Range (E ) 380 Range (E ) 780 c UV VIS NIR E + E + E ) Type of Liquid nm–380 nm nm–780 nm nm–1100 nm UV VIS NIR (J) (J) (J) (J) GeV needle–needle system natural ester 6.56 10 13 9.07 10 11 2.21 10 12 9.35 10 11 0.58 × − × − × − × − synthetic ester 2.39 10 12 1.96 10 10 4.50 10 12 2.03 10 10 1.27 × − × − × − × − mineral oil 1.08 10 12 3.07 10 10 1.02 10 11 3.17 10 10 1.98 × − × − × − × − surface discharge system natural ester 9.04 10 12 8.15 10 11 3.34 10 12 9.38 10 11 0.59 × − × − × − × − synthetic ester 1.07 10 11 1.57 10 10 1.63 10 12 1.69 10 10 1.05 × − × − × − × − mineral oil 3.09 10 11 2.05 10 10 1.74 10 12 2.37 10 10 1.48 × − × − × − × −

Analysis of the Variability of Optical Energy Shares in the UV–VIS–NIR Range The analysis of optical radiation variability in the UV–VIS–NIR range was carried out using traditional statistical methods. The main aim of the variability analysis was to determine the share of the energy of optical radiation, in particular spectral ranges for recorded optical spectra emitted by electric discharges generated in the analyzed electrode conﬁgurations. The results of the variability analysis are presented in Table3. The results presented in Table3 were determined on the basis of 10 registered characteristics for each variant of the measuring system.

Table 3. Comparison of the share of optical radiation emitted by electric discharges.

Average Standard Coeﬃcient Range of Typical Values of Range of Variance Type of Energy (%Ec) Deviation of Variation Radiation Energy Share Optical Liquid ¯ 2 Radiation x s s Vs Xtyp needle–needle system

UV 0.70 0.01 0.06 8.57 0.64 < Xtyp < 0.76 Natural VIS 97.60 0.62 0.79 0.81 96.81 < Xtyp < 98.39 ester NIR 1.70 0.08 0.29 17.06 1.41 < Xtyp < 1.99

UV 0.50 0.02 0.15 30.00 0.35 < Xtyp < 0.65 Synthetic VIS 97.90 1.13 1.06 1.08 96.84 < Xtyp < 98.96 ester NIR 1.60 0.13 0.36 22.50 1.24 < Xtyp < 1.96

UV 0.40 0.07 0.26 65.0 0.64 < Xtyp < 0.76 Mineral VIS 97.80 0.51 0.71 0.73 97.09 < Xtyp < 98.51 oil NIR 1.80 0.03 0.16 8.89 1.64 < Xtyp < 1.96 Energies 2020, 13, x FOR PEER REVIEW 7 of 10

Table 3. Comparison of the share of optical radiation emitted by electric discharges.

Range of Typical Average Coefficient Range of Standard Values of Type of Energy Variance of Optical Deviation Radiation Energy Liquid (%Ec) Variation Radiation Share s2 s Vs Xtyp needle–needle system Energies 2020, 13, 2172 7 of 9 UV 0.70 0.01 0.06 8.57 0.64 < Xtyp < 0.76 Natural ester VIS 97.60 0.62 0.79 0.81 96.81 < Xtyp < 98.39 NIR 1.70 Table0.08 3. Cont. 0.29 17.06 1.41 < Xtyp < 1.99 UV 0.50 0.02 0.15 30.00 0.35 < Xtyp < 0.65 Synthetic Average Standard Coeﬃcient Range of Typical Values of Range of VIS 97.90 Variance1.13 1.06 1.08 96.84 < Xtyp < 98.96 Typeester of Energy (%Ec) Deviation of Variation Radiation Energy Share Optical NIR 1.60 0.13 0.36 22.50 1.24 < Xtyp < 1.96 Liquid ¯ 2 Radiation UV x 0.40 s 0.07 s0.26 Vs 65.0 0.64 X

Figures 6–8 present the percentage share of energy in optical radiation ranges emitted by electricFiguresal discharges6–8 present generated the percentage on the share system of energy of needle in optical–needle radiation electrodes ranges and emitted surface by discharge electrical dischargessystems in generatedthe insulating on the liquids system adopted of needle–needle for testing. electrodesThe UV radiation and surface was less discharge than 1% systems of the intotal the insulatingenergy radiation liquids adoptedand poorly for detectable testing. The in UVall tested radiation insulating was less liqu thanids. 1% However, of the total it wa energys different radiation in andthe poorlycase of detectableelectrical discharges in all tested generated insulating in liquids. a surface However, discharge it was system, diﬀerent where in the the case proportion of electrical of dischargesultraviolet generatedradiation was in a much surface higher discharge. system, where the proportion of ultraviolet radiation was much higher.

(a) (b)

EnergiesFigureFigure 2020,6. 16.3 ,Percentage Percentagex FOR PEER ofREVIEW optical radiation energy energy for for electrical electrical discharges discharges generated generated in in natural natural ester, ester,8 of 10 MidelMidel 1204,1204, onon electrodeelectrode systems:systems: needle–needleneedle–needle ( a)) and and for for surface surface discharges discharges ( (bb).).

(a) (b)

FigureFigure 7.7. PercentagePercentage of optical radiation energy energy for for electrical electrical discharges discharges generated generated in in natural natural ester, ester, MidelMidel 7131,7131, onon electrodeelectrode systems:systems: needle–needleneedle–needle ( a)) and and for for surface surface discharges discharges ( (bb).).

(a) (b)

Figure 8. Percentage of optical radiation energy for electrical discharges generated in mineral oil, Orlen Trafo EN, on electrode systems: needle–needle (a) and for surface discharges (b).

This is probably due to various energy transformations accompanying the phenomenon of electrical discharges, which occurs in the used electrode systems. In the electrode system of the needle–needle type, electric discharges mostly occur in the form of a spark, while in the system for surface discharges, the phenomena occur continuously. Radiation in the ultraviolet range, in the case of discharges in the needle–needle system, is short-lived and effectively suppressed.

5. Conclusions The spectral distribution of the optical radiation emitted by electrical discharges in insulating liquids differed according to the electrode geometry. Needle–needle electrodes had < 1% UV radiation in all analyzed cases. In contrast, surface discharges had 7% or more UV radiation, depending on the type of electrical insulating liquid. This result might allow identification of the discharge type from the radiation spectrum and might be incorporated in expert diagnostic systems used in various technical areas. The results also justify further research, in terms of the applicability of the proposed indicator, for recognizing forms of electrical discharges in high-voltage insulation systems.

Funding: This research was co-funded by the National Science Centre, Poland (NCN) as a part of the Preludium Research Project No. 2017/25/N/ST8/00590.

Conflicts of Interest: The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Energies 2020, 13, x FOR PEER REVIEW 8 of 10

(a) (b)

Figure 7. Percentage of optical radiation energy for electrical discharges generated in natural ester, EnergiesMidel2020 7131,, 13, 2172 on electrode systems: needle–needle (a) and for surface discharges (b). 8 of 9

(a) (b)

FigureFigure 8. Percentage of of optical optical radiation radiation energy energy for for electrical electrical discharges discharges generated generated in mineral in mineral oil, Orlen oil, OrlenTrafo EN,Trafo on EN, electrode on electrode systems: systems: needle–needle needle–needle (a) and (a for) and surface for surface discharges discharges (b). (b).

ThisThis is is probably probably due due to various energy transformations accompanying accompanying the the phenomenon phenomenon of of electricalelectrical discharges discharges,, which which occur occurss in in the the used electrode systems. In In the the electrode electrode system system of of the the needleneedle–needle–needle type, type, electric electric discharges discharges mostly mostly occur occur in in the the form form of of a a spark, spark, while while in in the the system system for for surfacesurface discharges, discharges, the the phenomena phenomena occur occur continuously. continuously. R Radiationadiation in in the the ultraviolet ultraviolet range range,, in in the the case case ofof discharges discharges in in the the needle needle–needle–needle system system,, is short short-lived-lived and effectively eﬀectively suppressed.

5.5. Conclusions Conclusions TheThe spec spectraltral distribution distribution of of the the optical optical radiation radiation emitted emitted by by electrical electrical discharges discharges in in insulating insulating liquidsliquids didifferedffered according according to to the the electrode electrode geometry. geometry. Needle–needle Needle–needle electrodes electrodes had < 1% had UV < radiation1% UV radiationin all analyzed in all cases. analyzed In contrast, cases. surface In contrast, discharges surface had discharges 7% or more had UV 7% radiation, or more depending UV radiation on the, dependingtype of electrical on the insulating type of electrical liquid. This insulating result might liquid allow. This identification result might of allow the discharge identification type from of the the dischargeradiation spectrumtype from and the mightradiation be incorporatedspectrum and in might expert be diagnostic incorporated systems in expert used indiagnostic various technicalsystems usedareas. in The various results technical also justify areas further. The results research, also in justify terms further of the applicability research, in terms of the of proposed the applicability indicator, offor the recognizing proposed formsindicator of electrical, for recognizing discharges forms in high-voltage of electrical insulation discharges systems. in high-voltage insulation systems. Funding: This research was co-funded by the National Science Centre, Poland (NCN) as a part of the Preludium Research Project No. 2017/25/N/ST8/00590. Funding: This research was co-funded by the National Science Centre, Poland (NCN) as a part of the Preludium Conflicts of Interest: The author declares no conflict of interest. The funders had no role in the design of the Research Project No. 2017/25/N/ST8/00590. study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to Conflictspublish the of results.Interest: The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publishReferences the results. 1. Kunicki, M. Variability of the UHF signals generated by partial discharges in mineral oil. Sensors 2019, 19, 1392. [CrossRef][PubMed]

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