Research on the optical signal characteristics at different discharge stages in transformer oil based on fluorescent fiber detection

Research on the optical signal characteristics at different discharge stages in transformer oil based on fluorescent fiber detection | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Research on the optical signal characteristics at different discharge stages in transformer oil based on fluorescent fiber detection Wei Wang, Qilin Wang, XiaoHui Wang, ZhiFei Yang, ShengHui Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6429164/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The optical signal emitted by the discharge in oil is an important symptom of the operating status of transformers. A transformer oil discharge optical signal detection system based on fluorescent optical fibers and silicon photomultiplier are designed in this paper. The characteristics of optical signals, ultrasonic signals, ultra-high frequency signals, and pulse current signals of corona discharge, spark discharge, and arc discharge in a transformer model are compared and studied. The experimental results show that as the discharge increases, the amplitude of the above signals also increases accordingly, the amplitude of optical signals has a certain degree of randomnesst, but there is an approximate linear relationship with the pulse current signal. Optical signals have higher detection sensitivity for hazardous high-energy arc discharges, which can be used as a signal to reflect the severity of discharge failure, and can be used to provide relevant trip signals for relaying protection devices to reduce the probability of transformer explosion accidents. Oil immersed transformer Optical signal Fluorescent fiber Discharge characteristic Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Oil immersed large power transformers are one of the key equipment in substations, their faults can affect the stable operation of the transformer itself and even the power grid, in severe cases, they can also lead to huge economic losses. In a statistical report provided by the CIGRE Transformer Working Group in 2013, it was pointed out that after an arc short circuit fault occurred inside an oil immersed transformer, about 54% of the transformers experienced oil tank cracking, explosions, and other situations due to the untimely action of relay protection devices. Among them, 10% caused serious explosion accidents [1]. Numerous studies have shown that the main cause of transformer power outages is insulation discharge in transformers [2-4]. Therefore, it is necessary to detect the internal discharge in transformers to provide early warning, cut off fault current timely, and reduce the probability of explosion accident. Traditional discharge detection methods include pulse current method [5], ultra-high frequency method [6] ,ultrasonic detection method [7], gas chromatography method [8] and so on. However, the traditional methods of measuring electrical and acoustic signals are susceptible to on-site electromagnetic signal interference, and gas chromatography has a significant delay in the detection process, and it is difficult to detect sudden faults in time. Discharge in oil is accompanied by optical signal radiation, in recent years, optical detection method has become a new non electrical detection method, and its fast response speed and high sensitivity have been paid more and more attention in engineering. A fiber optic sensing system for detecting partial discharge signals was proposed, and found that its detection results were highly consistent with the ultra-high frequency method [9]; Reference [10] utilized the principle of photoacoustic spectroscopy to achieve rapid detection of dissolved fault gases in transformer oil in the laboratory; The principle of optical fiber sensor monitoring partial discharge has been described theoretically[11], and a design scheme of partial discharge on-line monitoring system is proposed; The simulation test of partial discharge was carried out, and good photoelectric response characteristics were obtained[12]; Reference [13] established a corona discharge model in air and conducted experimental research on the performance influencing factors of fluorescent fiber probes, such as fiber coupling length and its position. However, the current focus of research on optical measurement methods is mostly on theoretical discussions and sensor arrangement methods, and the main research is focused on discharge in gas environments, such as discharges in air and SF 6 . Oil, as a liquid insulation medium, has significantly different optical signal generation and radiation characteristics compared to gas environments, but there is limited research on the characteristics of discharge optical signals in transformer oil. Based on the above research status, this paper simulates the internal structure of oil immersed transformers, establishes a typical discharge model in oil, and designs an optical signal detection system based on fluorescent fiber and silicon photomultiplier. Experimental research is conducted on the optical signal characteristics during the corona discharge stage, spark discharge stage, and arc discharge stage. The research results have certain reference value for the internal discharge detection and the identification of discharge development stages in oil immersed transformers, as well as the explosion warning of transformers and proposing new relay protection control strategies. 2. Spectral distribution of discharge in transformer oil 2.1 Discharge spectral distribution measurement system In order to study the optical signal characteristics of discharge in transformer oil, it is necessary to measure the spectral distribution of discharge in oil, a typical needle-plate discharge model and spectral measurement platform are shown in Figure 1. The test chamber is made of acrylic material, with the thickness of 15 mm, length, width and height of 500 mm×500 mm×400 mm respectively, and it is filled with 25 # Karamay naphthenic transformer oil. The length of needle electrode is 10 mm, the electrode angle is 15 degree, the thickness of plate electrode is 10 mm with a diameter of 100 mm. In oil-immersed transformers, the insulation paper can be roughly divided into two categories: one is close to the surface of the winding, and the other is the thick insulation paperboard between the windings [14-15]. In the test, 2 mm thick insulation paperboard is used in the test. The insulation paperboard has been vacuum dried and immersed in oil before the test [16], and its side length is 10mm. A fiber optic connector is installed on the side of the experimental chamber, a high-purity quartz fiber is used to transmit the discharge light signal to the spectral analyzer, and the distance between the end of the fiber optic probe and the needle-plate electrode is about 5cm. The model of the spectral analyzer is QEPRO, its spectral detection wavelength is 200-950 nm, and the peak detection efficiency is 90%, the detection efficiency at 250 nm is 65%, and the spectral resolution is 1.7 nm FWHM. During the experiment, adjust the distance between the needle-electrode and the cardboard to about 5 mm, apply AC voltage to the discharge model, and gradually increase the voltage, and the spectral distribution characteristics are measured under different discharge intensities. 2.2 Spectral distribution of discharge When discharge occurs in transformer oil, the average free path in oil molecules is relatively short and the molecular weight is high, so there may be significant differences in the spectral distribution to the air. Optical detection systems have different requirements for the excitation wavelength, emission wavelength, and sensitive area of the photoelectric converter, which need to be adjusted according to the situation. To compare and analyze the differences between discharge in oil and in air, the spectral distribution of discharge in air and transformer oil are measured, and the results are shown in the Figure 2. From Figure 2, it can be seen that the spectral distribution of corona discharge in the air is a mixed spectrum mainly composed of line spectra and partially continuous spectra. The spectral energy is mainly concentrated in the near ultraviolet band of 270-430 nm. The spectral distribution range of discharge in transformer oil is relatively wide, covering the near ultraviolet, visible, and infrared regions. The main energy distribution is in the wavelength range of 320-950 nm. The discharge spectrum in transformer oil is a combination of line spectrum and band spectrum, and the wavelength is continuous over a wide range. The spectral energy is relatively high in the range of 320-950 nm, with a rapid increase in spectral energy at 320 nm and a maximum peak at 650 nm. In summary, for discharge in oil, in order to ensure high detection efficiency, the sensitive area of fluorescent fiber detection needs to be selected within the range of 320-950 nm. 3. Optical Signal Detection System Based on Fluorescent Fiber The schematic diagram of the optical signal detection system based on fluorescent fiber is shown in Figure 3. After the fluorescent fiber senses the discharge optical signal, it is transmitted to the low-loss quartz fiber through a fiber coupler, and then projected to the photoelectric conversion device through the collimator. To ensure accurate perception of optical signals and high transmission efficiency, the device size, main wavelength range, and transmission loss in fluorescent fiber must meet certain conditions [17]. 1. Fluorescent fibers should have good toughness and ductility, making them easy to install in transformers. At the same time, the fiber cladding should be transparent, the core diameter should be large, and it should be easy to receive optical signals over a large angle range. 2. The excitation wavelength (detection wavelength) of the fluorescent fiber material matches the distribution range of the discharge spectrum in the transformer oil, and the emission wavelength matches the photosensitive unit. 3. The transmission wavelength range of quartz fiber is wide, the low loss area matches the emission wavelength of fluorescent fiber, and it is easy to connect. 4. The signal loss of fiber coupler and collimator is small, and the rated wavelength range matches the transmission wavelength of the optical signal. 5. The photosensitive unit has sufficient detection sensitivity and magnifying ability, which can detect weak optical signals and convert them into easily processed electrical signals. 3.1 Fluorescent fiber selection At present, there are four kinds of fluorescent optical fibers commonly used, which are plastic optical fiber, glass optical fiber, quartz optical fiber and liquid core optical fiber, the corresponding characteristics are shown in Table 1. Table 1. Comparison of Four Types of Optical Fiber Performance Parameters Plastic Optical Fiber Glass Optical Fiber Quartz Optical Fiber Liquid Core Optical Fiber Extensibility Good Poor Poor Good Bending Radius D×8 D×15 D×20 D×15 N. A. Aperture 0.5~3.0 0.63 0.2~0.37 0.6 Cost Low Cost Medium High High From Table 1, it can be seen that plastic optical fibers have the following advantages: good toughness, small bending radius, strong ductility, and low cost, it is convenient to install in narrow areas inside the transformer. Therefore, this paper chooses plastic fluorescent fiber as the detection fiber, with an excitation spectrum of 350-850 nm, which can cover the high-energy region of discharge spectra in oil. The emission spectrum of the fluorescent fiber is shown in Figure 4, mainly in the range of 500-700 nm, which is consistent with the detection sensitive area of mainstream photoelectric conversion devices. 3.2 Selection of photoelectric conversion equipment Usually, the discharge light radiation in transformer oil is weak pulse signal, and the duration is in the ns range, so the output signal of the fluorescent fiber is also low. Therefore, a photodetector with high sensitivity and fast response speed is needed to measure the discharge light signal in transformer oil. At present, the commonly used optoelectronic devices for single photon detection are: vacuum photomultiplier (PMT), avalanche diode (APD), silicon photomultiplier (SIPM) and pin-photodiode (PIN-PD). Their working principle and performance characteristics are shown in Table 2. Table 2. Performance characteristics of typical photoelectric converters Performance Parameters PMT APD SIPM PIN-PD Basic Principles External Effects Avalanche Effect Geiger Mode junction photodiode Luminous Efficiency 80% + 80% + 50% + 70%+ Spectral Range 100 nm~1 μm 200 nm～1μm 200 nm~1 μm 200 nm~2.6 μm Driving Voltage ~1 kV ~00 V ~10 V 5～30 V Size ~cm μm~mm ~mm ~mm Photon Counting Supported Not Supported Supported Not Supported Considering the cost and performance, the DET10A2 silicon photodiode of Soreibo was used in this detection system. Based on the above device selection, the specific parameters of the transformer oil discharge light signal detection system designed in this paper are shown in Table 3. Table 3. Parameters of Optical Signal Measurement system Device name Model number Parameters Fluorescent fiber Custom plastic fiber Core diameter：1.5 mm Excitation wavelength：350~850 nm Emission wavelength：500~700 nm Quartz fiber VIS-UV Core diameter：1.5 mm Transmission wavelength：200~1200 nm Optical coupler Customizable Applicable wavelength：200~2500 nm Fiber collimator Customizable Core diameter：1.5 mm Applicable wavelength：200~2500 nm Silicon photodiode DET10A2 Light-sensitive area：0.8 mm 2 Wavelength Range:200-1100nm Peak Response:0.44 A/W (Typ.) Output current：0~10 mA rising time：1ns 4. Discharge Optical Signal Characteristics in Transformer Oil 4.1 Transformer model and signal acquisition system At a transformer production plant, relevant tests were conducted using a simulated transformers tank, and the wiring is shown in Figure 5. The AC high voltage generated by the resonant high voltage generator is introduced into the test chamber through a bushing and connected to the needle-plate discharge model. The discharge optical signal is sensed with a fluorescent fiber, its length is about 200cm, in order to increase the reception of more discharge light signals, the fluorescent fiber is wound into a ring shape with a total of 2 turns and a radius of 10cm.The needle-plate electrode is placed at the center of the fluorescent fiber ring, the fluorescent fiber is connected to the low loss quartz fiber through the fiber connector on the oil tank flange. The quartz fiber is about 3m long and connected to the silicon photomultiplier. Its output electrical signal is connected to the oscilloscope through a shielded cable with a length of about 15m. To compare and analyze the relationship between optical signals and other traditional discharge signals, the discharge ultrasonic signals, ultra-high frequency signals, and pulse current signals are also measured simultaneously. The ultrasonic sensor model is G&TGT30PA, which has the gains of 20dB, 40dB, and 60dB; The model of ultra-high frequency(UHF) sensor is UHF0315, which covers a wide frequency range of 300 MHz~1.5 GHz and has a detection sensitivity of -75 dBm~-35 dBm; The current sensor is BCT-5, its response frequency can reach 30 kHz. During the experiment, the distance between the needle-electrode and the insulating cardboard is 1mm. The characteristics of the discharge optical signal in the transformer oil are related to the discharge stage, in the first stage, the voltage was first increased at a speed of 5 kV/min to form the corona discharge, in the second stage, the voltage was slowly increased at a speed of 50 V/s to form the intermittent sparks discharge, and in the third stage, the voltage was increased 100 V each time and held for 1 minute until a breakdown arc discharge occurred. 4.2 Typical Signal Wave forms at Different Discharge Stages During the experiment, when the voltage increases to about 18.5 kV, there is a significant corona discharge sound, indicating that the corona discharge in the oil has reached a relatively stable partial discharge stage. The typical power frequency voltage signal, optical signal, ultrasonic signal, ultra-high frequency signal, and pulse current curve signals are shown in Figure 6. It can be seen from the Figure 6 that during corona discharge stage, the discharge phases of the positive and negative half cycles are distributed around 45° and 215°respectively. At this time, the discharge energy is small, so the optical signal is relatively weak, and the signal amplitude is about 1-2 mV. Continue to increase the voltage, spark discharge occurs at about 20 kV. When slowly increase the voltage to 21 kV, the frequency of sparks discharge increases significantly, and the discharge signals at this time are shown in Figure 7. From Figure 7, it can be seen that compared to the corona discharge, the discharge phase of spark discharge has changed, with the positive half cycle around 90° and the negative half cycle around 315°. The optical signal is stronger, with an amplitude of about 10-13 mV, and its energy is about 10 times that of corona discharge. The amplitude of the pulse current signal increases about 10-20%, the amplitude of the UHF signal increases about 1 times, and the amplitude of the ultrasound signal increases about 50-80%. The faster increase in amplitude of the optical signal compared to other signals indicates that during the weak discharge stage of corona discharge, the portion of energy converted into optical signal is relatively small. However, during the spark discharge, the portion of energy converted into optical signal increases sharply. Continue to increase the voltage, the typical signals during the arc discharge stage are shown in Figure 8, the discharge phase is concentrated at 90° and 270°, and the amplitude of the optical signal is extremely high, rising from the mV level to the V level. The significant increase in the amplitude of the optical signal indicates that it has a "threshold" characteristic in the light signal. When the amplitude is less than a certain value, the energy converted into the optical signal during discharge is relatively less, while when the discharge energy is greater than a certain "threshold value", the energy converted into the optical signal increases sharply. It means that fluorescent optical fiber have high sensitivity for detecting arc discharges, and large arc discharges in engineering are the main cause of transformer explosions accident. Due to the excellent insulation performance of fluorescent optical fibers, they can be directly embedded near the bushing elevation base inside transformers, effectively avoiding external interference signal. At the same time, using optical fibers to transmit optical signals can further reduce external interference, Therefore, the detected optical signal will have a higher signal-to-noise ratio, and its signal will be purer. If a transformer explosion warning system based on optical signals is adopted, it can help solve the problem of false alarms or misoperations in relay protection devices and effectively improve the reliability of transformer operation. 4.3. Relationship between Optical Signal and Electric Pulse Signal Figure 9 shows The amplitude distribution between the optical and electrical signals. It can be seen that there is a good linear relationship between optical signal and the electrical signal, but the data still has a certain degree of discreteness, this may be because the discharge phenomenon in transformer oil is a random process of energy conversion, release, and transfer. The wavelength distribution of the light signal exhibits a certain degree of randomness, and the position, angle, and distance of the fluorescent fiber have different collection and transmission efficiencies for photons [18]. Considering the randomness of optical signals, it is difficult to establish an accurate functional relationship between optical signals and electrical signals. However, in the overall trend, the optical signal increases with the increase of pulse current, which can to some extent reflect the severity of discharge phenomena. As a non electrical signal detection method, optical signals have higher resistance to electromagnetic interference than traditional electrical signals, which can provide new ideas and methods for online monitoring of the operating status of oil immersed equipment. Based on the above experimental data, the histogram of the average value and dispersion of the optical signal amplitude under the three discharge types is shown in Figure 10. The average discharge optical signal value during corona discharge is 1.3 mV and the dispersion is 22.7%, the average and dispersion of spark discharge and arc discharge are 14.0 mV / 23.5% and 2.1 V / 24.7%, respectively. The amplitude of the discharge optical signal in the three stages shows a significant upward trend, in the arc discharge stage, the amplitude of the optical signal increases from mV level to V level. That is to say, the optical signal has higher sensitivity to the serious high-energy discharge. This characteristic is conducive to the timely detection of arc discharge by fluorescence optical fiber method, and provides relevant signals for preventing misoperation and and refusal operation of oil-immersed transformer protection equipment. 5. Conclusions In this paper, a set of optical signal detection system for discharge in transformer oil is established based on the fluorescent fiber and silicon photomultiplier. The optical signals and electrical signals of corona discharge, spark discharge and arc discharge are compared and analyzed. The main conclusions are as follows: 1. The spectral distribution range of discharge in transformer oil is relatively wide, covering the near ultraviolet, visible, and infrared regions. The main energy distribution of the spectrum is in the wavelength range of 320-950 nm. For discharges in oil, to ensure detection sensitivity, the detection sensitive area of fluorescent fibers should be in the range of 320-950 nm. 2. The average value of the discharge optical signal during corona discharge is 1.3 mV, and the average and dispersion of spark discharge and arc discharge are 14.0 mV and 2.1 V, respectively. There is a good correspondence between the light signal and the pulse current signal, with similar changing trends, but also showing strong random fluctuations. The light signal can effectively reflect the severity of the discharge phenomenon. 3. Optical signal has good anti-electromagnetic interference ability. It can not only effectively detect weak discharge in oil-immersed equipment, but also has higher sensitivity to high-energy discharge with serious harm. Optical signal can be used as a parameter to reflect the severity of discharge fault in oil-immersed equipment. It can be used for on-line monitoring of transformer discharge, and can also provide relevant control signals for relay protection devices based on optical signal to reduce the probability of transformer explosion accidents. Declarations Funding . State Grid Henan Electric Power Company; Research on Optical Detection and Early Warning Method for Discharge in Transformer Oil Based on Multichannel Fiber Optic Multiplication(52170223000C).. Acknowledgment. This research was funded by State Grid Henan Electric Power Company Technology Project: Research on Optical Detection and Early Warning Method for Discharge in Transformer Oil Based on Multichannel Fiber Optic Multiplication Conflicts of Interest. The authors declare no conflicts of interest. Data availability . The data supporting the findings of this study are available from the corresponding author upon reasonable request.. Author Contribution Wei Wang and QiLin Wang: Methodology; XiaoHui Wang: conceptualization; QiLin Wang, XiaoHui Wang, ZhiFei Yang: software; Wei Wang, ZhiFei Yang, ShengHui Wang: resources;ShengHui Wang: data curation; QiLin Wang and XiaoHui Wang: writing-review and editing; Wei Wang, QiLin Wang, ZhiFei Yang, XiaoHui Wang: supervision; Wei Wang, QiLin Wang, ZhiFei Yang, XiaoHui Wang: project administration;XiaoHui Wang: funding acquisitionAll authors have read and agreed to the published version of the manuscript. References Working Group A2.33. Guide for transformer fire safety practices. Paris, France: CIGRE, 2013. Available online:https://static.mimaterials.com/midel/documents/sales/Guide_for_Transformer_Fire_Safety_Practices.pdf (accessed on 11 November 2023). 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Development process of discharge defects in transformer oil-paper insulation needle plate. High. Volt. 2011, 37, 1362-1370. DOI: 10.13336/j.1003-6520.hve.2011.06.028. Chi M.; Chen Q.; Chi M.; Wang X.; Wei X. Effect of temperature on electric field distribution of oil-paper insulation under combined voltage. Chin. J. Electr. Eng. 2015, 35, 1524-1532. DOI: 10.13334/j.0258-8013.pcsee.2015.06.029. Yang H.; Luo H. Research on PD monitoring system based on optical fiber sensor. Transformer, 2017, 54, 39-42. DOI: 10.19487/j.cnki.1001-8425.2017.09.008. Qin S.; Qin S.; Ke Y.; et al. Influence of fluorescent fibers layout methods on detection of partial discharge signals. Insulating Materials, 2023, 56, 64-72. DOI:10.16790/j.cnki.1009-9239.im.2023.08.010. Additional Declarations No competing interests reported. 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(a) Discharge Spectral Distribution in air; (b) Discharge Spectral Distribution in transformer oil.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/1d3d9d0c46fc286eb09adf9f.png"},{"id":84474832,"identity":"f9a004be-7aec-4685-9dca-cb026ba64969","added_by":"auto","created_at":"2025-06-12 11:08:43","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":15362,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of fluorescence fiber optic detection system.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/4b8bd04236b11fcbbe2b0cf5.png"},{"id":84474123,"identity":"939998de-f23d-4a6e-924c-e20588fd4674","added_by":"auto","created_at":"2025-06-12 11:00:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":58038,"visible":true,"origin":"","legend":"\u003cp\u003eEmission spectrum of plastic fluorescence optical fiber.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/23c126d3bc45bcea71f4c176.png"},{"id":84475303,"identity":"da01c67b-324a-4eae-8d58-4d6f947110ed","added_by":"auto","created_at":"2025-06-12 11:16:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":202054,"visible":true,"origin":"","legend":"\u003cp\u003eWiring diagram for discharge test in oil.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/20eb07cc7bd100a0f36803d8.png"},{"id":84474126,"identity":"b7e24d33-d30d-4f2c-a2db-6bada9d51677","added_by":"auto","created_at":"2025-06-12 11:00:43","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":92857,"visible":true,"origin":"","legend":"\u003cp\u003eCorona discharge signal curves.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/e50f224be0b4a9b0dd0e1905.png"},{"id":84475304,"identity":"bfa9af89-8554-456a-9151-eb518d26ecb9","added_by":"auto","created_at":"2025-06-12 11:16:43","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":86555,"visible":true,"origin":"","legend":"\u003cp\u003eSpark discharge signal curves.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/48478e0e0e324c3566c2a63d.png"},{"id":84472964,"identity":"839f37bf-b5ad-411c-b4dd-476358be89d3","added_by":"auto","created_at":"2025-06-12 10:52:43","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":81169,"visible":true,"origin":"","legend":"\u003cp\u003eArc discharge signal curves.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/075ece0b67de811f70abc656.png"},{"id":84474851,"identity":"6a6b56b9-9b0e-490a-8e94-da1989f604f8","added_by":"auto","created_at":"2025-06-12 11:08:44","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":38272,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship of discharge optical and the electrical pulse signal.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/3f81c3ae188497496ce02387.png"},{"id":84474129,"identity":"0151d68f-483c-4ff5-8bb9-fc398c6ca07f","added_by":"auto","created_at":"2025-06-12 11:00:43","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":27014,"visible":true,"origin":"","legend":"\u003cp\u003eDischarge optical signal amplitude distribution.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/a376a349f8409ddcf2ad5465.png"},{"id":96403999,"identity":"fe77202b-ee4d-41d5-ac6b-4161cc6699ef","added_by":"auto","created_at":"2025-11-20 16:38:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1125051,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6429164/v1/641297d5-4a21-4e76-89a4-da96192eea12.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Research on the optical signal characteristics at different discharge stages in transformer oil based on fluorescent fiber detection","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOil immersed large power transformers are one of the key equipment in substations, their faults can affect the stable operation of the transformer itself and even the power grid, in severe cases, they can also lead to huge economic losses. In a statistical report provided by the CIGRE Transformer Working Group in 2013, it was pointed out that after an arc short circuit fault occurred inside an oil immersed transformer, about 54% of the transformers experienced oil tank cracking, explosions, and other situations due to the untimely action of relay protection devices. Among them, 10% caused serious explosion accidents [1]. Numerous studies have shown that the main cause of transformer power outages is insulation discharge in transformers [2-4]. Therefore, it is necessary to detect the internal discharge in transformers to provide early warning, cut off fault current timely, and reduce the probability of explosion accident.\u003c/p\u003e\n\u003cp\u003eTraditional discharge detection methods include pulse current method [5], ultra-high frequency method [6] ,ultrasonic detection method [7], gas chromatography method [8] and so on. However, the traditional methods of measuring electrical and acoustic signals are susceptible to on-site electromagnetic signal interference, and gas chromatography has a significant delay in the detection process, and it is difficult to detect sudden faults in time. Discharge in oil is accompanied by optical signal radiation, in recent years, optical detection method has become a new non electrical detection method, and its fast response speed and high sensitivity have been paid more and more attention in engineering. A fiber optic sensing system for detecting partial discharge signals was proposed, and found that its detection results were highly consistent with the ultra-high frequency method [9]; Reference [10] utilized the principle of photoacoustic spectroscopy to achieve rapid detection of dissolved fault gases in transformer oil in the laboratory; The principle of optical fiber sensor monitoring partial discharge has been described theoretically[11], and a design scheme of partial discharge on-line monitoring system is proposed; The simulation test of partial discharge was carried out, and good photoelectric response characteristics were obtained[12]; Reference [13] established a corona discharge model in air and conducted experimental research on the performance influencing factors of fluorescent fiber probes, such as fiber coupling length and its position. However, the current focus of research on optical measurement methods is mostly on theoretical discussions and sensor arrangement methods, and the main research is focused on discharge in gas environments, such as discharges in air and SF\u003csub\u003e6\u003c/sub\u003e. Oil, as a liquid insulation medium, has significantly different optical signal generation and radiation characteristics compared to gas environments, but there is limited research on the characteristics of discharge optical signals in transformer oil.\u003c/p\u003e\n\u003cp\u003eBased on the above research status, this paper simulates the internal structure of oil immersed transformers, establishes a typical discharge model in oil, and designs an optical signal detection system based on fluorescent fiber and silicon photomultiplier. Experimental research is conducted on the optical signal characteristics during the corona discharge stage, spark discharge stage, and arc discharge stage. The research results have certain reference value for the internal discharge detection and the identification of discharge development stages in oil immersed transformers, as well as the explosion warning of transformers and proposing new relay protection control strategies.\u003c/p\u003e"},{"header":"2.\tSpectral distribution of discharge in transformer oil","content":"\u003cp\u003e2.1 Discharge spectral distribution measurement system\u003c/p\u003e\n\u003cp\u003eIn order to study the optical signal characteristics of discharge in transformer oil, it is necessary to measure the spectral distribution of discharge in oil, a typical needle-plate discharge model and spectral measurement platform are shown in Figure 1.\u003c/p\u003e\n\u003cp\u003eThe test chamber is made of acrylic material, with the thickness of 15 mm, length, width and height of 500 mm\u0026times;500 mm\u0026times;400 mm respectively, and it is filled with 25 # Karamay naphthenic transformer oil. The length of needle electrode is 10 mm, the electrode angle is 15 degree, the thickness of plate electrode is 10 mm with a diameter of 100 mm. In oil-immersed transformers, the insulation paper can be roughly divided into two categories: one is close to the surface of the winding, and the other is the thick insulation paperboard between the windings [14-15]. In the test, 2 mm thick insulation paperboard is used in the test. The insulation paperboard has been vacuum dried and immersed in oil before the test [16], and its side length is 10mm. A fiber optic connector is installed on the side of the experimental chamber, a high-purity quartz fiber is used to transmit the discharge light signal to the spectral analyzer, and the distance between the end of the fiber optic probe and the needle-plate electrode is about 5cm. The model of the spectral analyzer is QEPRO, its spectral detection wavelength is 200-950 nm, and the peak detection efficiency is 90%, the detection efficiency at 250 nm is 65%, and the spectral resolution is 1.7 nm FWHM. During the experiment, adjust the distance between the needle-electrode and the cardboard to about 5 mm, apply AC voltage to the discharge model, and gradually increase the voltage, and the spectral distribution characteristics are measured under different discharge intensities.\u003c/p\u003e\n\u003cp\u003e2.2 Spectral distribution of discharge\u003c/p\u003e\n\u003cp\u003eWhen discharge occurs in transformer oil, the average free path in oil molecules is relatively short and the molecular weight is high, so there may be significant differences in the spectral distribution to the air. Optical detection systems have different requirements for the excitation wavelength, emission wavelength, and sensitive area of the photoelectric converter, which need to be adjusted according to the situation. To compare and analyze the differences between discharge in oil and in air, the spectral distribution of discharge in air and transformer oil are measured, and the results are shown in the Figure 2.\u003c/p\u003e\n\u003cp\u003eFrom Figure 2, it can be seen that the spectral distribution of corona discharge in the air is a mixed spectrum mainly composed of line spectra and partially continuous spectra. The spectral energy is mainly concentrated in the near ultraviolet band of 270-430 nm. The spectral distribution range of discharge in transformer oil is relatively wide, covering the near ultraviolet, visible, and infrared regions. The main energy distribution is in the wavelength range of 320-950 nm. The discharge spectrum in transformer oil is a combination of line spectrum and band spectrum, and the wavelength is continuous over a wide range. The spectral energy is relatively high in the range of 320-950 nm, with a rapid increase in spectral energy at 320 nm and a maximum peak at 650 nm. In summary, for discharge in oil, in order to ensure high detection efficiency, the sensitive area of fluorescent fiber detection needs to be selected within the range of 320-950 nm.\u003c/p\u003e"},{"header":"3. Optical Signal Detection System Based on Fluorescent Fiber","content":"\u003cp\u003eThe schematic diagram of the optical signal detection system based on fluorescent fiber is shown in Figure 3. After the fluorescent fiber senses the discharge optical signal, it is transmitted to the low-loss quartz fiber through a fiber coupler, and then projected to the photoelectric conversion device through the collimator.\u003c/p\u003e\n\u003cp\u003eTo ensure accurate perception of optical signals and high transmission efficiency, the device size, main wavelength range, and transmission loss in fluorescent fiber must meet certain conditions [17].\u003c/p\u003e\n\u003cp\u003e1.\u0026nbsp; \u0026nbsp;\u0026nbsp;Fluorescent fibers should have good toughness and ductility, making them easy to install in transformers. At the same time, the fiber cladding should be transparent, the core diameter should be large, and it should be easy to receive optical signals over a large angle range.\u003c/p\u003e\n\u003cp\u003e2.\u0026nbsp; \u0026nbsp;\u0026nbsp;The excitation wavelength (detection wavelength) of the fluorescent fiber material matches the distribution range of the discharge spectrum in the transformer oil, and the emission wavelength matches the photosensitive unit.\u003c/p\u003e\n\u003cp\u003e3.\u0026nbsp; \u0026nbsp;\u0026nbsp;The transmission wavelength range of quartz fiber is wide, the low loss area matches the emission wavelength of fluorescent fiber, and it is easy to connect.\u003c/p\u003e\n\u003cp\u003e4.\u0026nbsp; \u0026nbsp;\u0026nbsp;The signal loss of fiber coupler and collimator is small, and the rated wavelength range matches the transmission wavelength of the optical signal.\u003c/p\u003e\n\u003cp\u003e5. \u0026nbsp; \u0026nbsp;The photosensitive unit has sufficient detection sensitivity and magnifying ability, which can detect weak optical signals and convert them into easily processed electrical signals.\u003c/p\u003e\n\u003cp\u003e3.1 Fluorescent fiber selection\u003c/p\u003e\n\u003cp\u003eAt present, there are four kinds of fluorescent optical fibers commonly used, which are plastic optical fiber, glass optical fiber, quartz optical fiber and liquid core optical fiber, the corresponding characteristics are shown in Table 1.\u003c/p\u003e\n\u003cp\u003eTable 1. Comparison of Four Types of Optical Fiber\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"509\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003ePerformance Parameters\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003ePlastic Optical Fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003eGlass Optical Fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 98px;\"\u003e\n \u003cp\u003eQuartz Optical Fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 115px;\"\u003e\n \u003cp\u003eLiquid Core Optical Fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eExtensibility\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003eGood\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003ePoor\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 98px;\"\u003e\n \u003cp\u003ePoor\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 115px;\"\u003e\n \u003cp\u003eGood\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eBending Radius\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003eD\u0026times;8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003eD\u0026times;15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 98px;\"\u003e\n \u003cp\u003eD\u0026times;20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 115px;\"\u003e\n \u003cp\u003eD\u0026times;15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eN. A. Aperture\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003e0.5~3.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003e0.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 98px;\"\u003e\n \u003cp\u003e0.2~0.37\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 115px;\"\u003e\n \u003cp\u003e0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 113px;\"\u003e\n \u003cp\u003eCost\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 100px;\"\u003e\n \u003cp\u003eLow Cost\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 83px;\"\u003e\n \u003cp\u003eMedium\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 98px;\"\u003e\n \u003cp\u003eHigh\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 115px;\"\u003e\n \u003cp\u003eHigh\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eFrom Table 1, it can be seen that plastic optical fibers have the following advantages: good toughness, small bending radius, strong ductility, and low cost, it is convenient to install in narrow areas inside the transformer. Therefore, this paper chooses plastic fluorescent fiber as the detection fiber, with an excitation spectrum of 350-850 nm, which can cover the high-energy region of discharge spectra in oil. The emission spectrum of the fluorescent fiber is shown in Figure 4, mainly in the range of 500-700 nm, which is consistent with the detection sensitive area of mainstream photoelectric conversion devices.\u003c/p\u003e\n\u003cp\u003e3.2 Selection of photoelectric conversion equipment\u003c/p\u003e\n\u003cp\u003eUsually, the discharge light radiation in transformer oil is weak pulse signal, and the duration is in the ns range, so the output signal of the fluorescent fiber is also low. Therefore, a photodetector with high sensitivity and fast response speed is needed to measure the discharge light signal in transformer oil.\u003c/p\u003e\n\u003cp\u003eAt present, the commonly used optoelectronic devices for single photon detection are: vacuum photomultiplier (PMT), avalanche diode (APD), silicon photomultiplier (SIPM) and pin-photodiode (PIN-PD). Their working principle and performance characteristics are shown in Table 2.\u003c/p\u003e\n\u003cp\u003eTable 2. Performance characteristics of typical photoelectric converters\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePerformance Parameters\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePMT\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eAPD\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eSIPM\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePIN-PD\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003eBasic Principles\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eExternal Effects\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eAvalanche Effect\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eGeiger Mode\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003ejunction photodiode\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003eLuminous Efficiency\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc23156\"\u003e80% +\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc20168\"\u003e80% +\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc4829\"\u003e50% +\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e70%+\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003eSpectral Range\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc30220\"\u003e100 nm~1 \u0026mu;m\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc21044\"\u003e200 nm～1\u0026mu;m\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc13725\"\u003e200 nm~1 \u0026mu;m\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e200 nm~2.6 \u0026mu;m\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003eDriving Voltage\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc9258\"\u003e~1 kV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc6511\"\u003e~00\u0026nbsp;V\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc13973\"\u003e~10\u0026nbsp;V\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e5～30 V\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003eSize\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc22706\"\u003e~cm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc22053\"\u003e\u0026mu;m~mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp id=\"_Toc21043\"\u003e~mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003e~mm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 19px;\"\u003e\n \u003cp\u003ePhoton Counting\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eSupported\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eNot Supported\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eSupported\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 20px;\"\u003e\n \u003cp\u003eNot Supported\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eConsidering the cost and performance, the DET10A2 silicon photodiode of Soreibo was used in this detection system.\u003c/p\u003e\n\u003cp\u003eBased on the above device selection, the specific parameters of the transformer oil discharge light signal detection system designed in this paper are shown in Table 3.\u003c/p\u003e\n\u003cp\u003eTable 3. Parameters of Optical Signal Measurement system\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"485\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 117px;\"\u003e\n \u003cp\u003eDevice name\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 130px;\"\u003e\n \u003cp\u003eModel number\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 238px;\"\u003e\n \u003cp\u003eParameters\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 117px;\"\u003e\n \u003cp\u003eFluorescent fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 130px;\"\u003e\n \u003cp\u003eCustom plastic fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 238px;\"\u003e\n \u003cp\u003eCore diameter：1.5 mm\u003c/p\u003e\n \u003cp\u003eExcitation wavelength：350~850 nm\u003c/p\u003e\n \u003cp\u003eEmission wavelength：500~700 nm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 117px;\"\u003e\n \u003cp\u003eQuartz fiber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 130px;\"\u003e\n \u003cp\u003eVIS-UV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 238px;\"\u003e\n \u003cp\u003eCore diameter：1.5 mm\u003c/p\u003e\n \u003cp\u003eTransmission wavelength：200~1200 nm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 117px;\"\u003e\n \u003cp\u003eOptical coupler\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 130px;\"\u003e\n \u003cp\u003eCustomizable\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 238px;\"\u003e\n \u003cp\u003eApplicable wavelength：200~2500 nm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 117px;\"\u003e\n \u003cp\u003eFiber collimator\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 130px;\"\u003e\n \u003cp\u003eCustomizable\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 238px;\"\u003e\n \u003cp\u003eCore diameter：1.5 mm\u003c/p\u003e\n \u003cp\u003eApplicable wavelength：200~2500 nm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 117px;\"\u003e\n \u003cp\u003eSilicon photodiode\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 130px;\"\u003e\n \u003cp\u003eDET10A2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 238px;\"\u003e\n \u003cp\u003eLight-sensitive area：0.8 mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eWavelength Range:200-1100nm\u003c/p\u003e\n \u003cp\u003ePeak Response:0.44 A/W (Typ.)\u003c/p\u003e\n \u003cp\u003eOutput current：0~10 mA\u003c/p\u003e\n \u003cp\u003erising time：1ns\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"4.\tDischarge Optical Signal Characteristics in Transformer Oil","content":"\u003cp\u003e4.1 Transformer model and signal acquisition system\u003c/p\u003e\n\u003cp\u003eAt a transformer production plant, relevant tests were conducted using a simulated \u0026nbsp;transformers tank, and the wiring is shown in Figure 5. The AC high voltage generated by the resonant high voltage generator is introduced into the test chamber through a bushing and connected to the needle-plate discharge model. The discharge optical signal is sensed with a fluorescent fiber, its length is about 200cm, in order to increase the reception of more discharge light signals, the fluorescent fiber is wound into a ring shape with a total of 2 turns and a radius of 10cm.The needle-plate electrode is placed at the center of the fluorescent fiber ring, the fluorescent fiber is connected to the low loss quartz fiber through the fiber connector on the oil tank flange. The quartz fiber is about 3m long and connected to the silicon photomultiplier. Its output electrical signal is connected to the oscilloscope through a shielded cable with a length of about 15m.\u003c/p\u003e\n\u003cp\u003eTo compare and analyze the relationship between optical signals and other traditional discharge signals, the discharge ultrasonic signals, ultra-high frequency signals, and pulse current signals are also measured simultaneously. The ultrasonic sensor model is G\u0026amp;TGT30PA, which has the gains of 20dB, 40dB, and 60dB; The model of ultra-high frequency(UHF) sensor is UHF0315, which covers a wide frequency range of 300 MHz~1.5 GHz and has a detection sensitivity of -75 dBm~-35 dBm; The current sensor is BCT-5, its response frequency can reach 30 kHz.\u003c/p\u003e\n\u003cp\u003eDuring the experiment, the distance between the needle-electrode and the insulating cardboard is 1mm. The characteristics of the discharge optical signal in the transformer oil are related to the discharge stage, in the first stage, the voltage was first increased at a speed of 5 kV/min to form the corona discharge, in the second stage, the voltage was slowly increased at a speed of 50 V/s to form the intermittent sparks discharge, and in the third stage, the voltage was increased 100 V each time and held for 1 minute until a breakdown arc discharge occurred.\u003c/p\u003e\n\u003cp\u003e4.2 Typical Signal Wave forms at Different Discharge Stages\u003c/p\u003e\n\u003cp\u003eDuring the experiment, when the voltage increases to about 18.5 kV, there is a significant corona discharge sound, indicating that the corona discharge in the oil has reached a relatively stable partial discharge stage. The typical power frequency voltage signal, optical signal, ultrasonic signal, ultra-high frequency signal, and pulse current curve signals are shown in Figure 6.\u003c/p\u003e\n\u003cp\u003eIt can be seen from the Figure 6 that during corona discharge stage, the discharge phases of the positive and negative half cycles are distributed around 45\u0026deg;\u0026nbsp;and 215\u0026deg;respectively. At this time, the discharge energy is small,\u0026nbsp;so the optical signal is relatively weak, and the signal amplitude is about 1-2\u0026nbsp;mV.\u003c/p\u003e\n\u003cp\u003eContinue to increase the voltage, spark discharge occurs at about 20 kV. When slowly increase the voltage to 21 kV, the frequency of sparks discharge increases significantly, and the discharge signals at this time are shown in Figure 7.\u003c/p\u003e\n\u003cp\u003eFrom Figure 7, it can be seen that compared to the corona discharge, the discharge phase of spark discharge has changed, with the positive half cycle around 90\u0026deg; and the negative half cycle around 315\u0026deg;. The optical signal is stronger, with an amplitude of about 10-13 mV, and its energy is about 10 times that of corona discharge. The amplitude of the pulse current signal increases about 10-20%, the amplitude of the UHF signal increases about 1 times, and the amplitude of the ultrasound signal increases about 50-80%. The faster increase in amplitude of the optical signal compared to other signals indicates that during the weak discharge stage of corona discharge, the portion of energy converted into optical signal is relatively small. However, during the spark discharge, the portion of energy converted into optical signal increases sharply.\u003c/p\u003e\n\u003cp\u003eContinue to increase the voltage, the typical signals during the arc discharge stage are shown in Figure 8, the discharge phase is concentrated at 90\u0026deg; and 270\u0026deg;, and the amplitude of the optical signal is extremely high, rising from the mV level to the V level.\u003c/p\u003e\n\u003cp\u003eThe significant increase in the amplitude of the optical signal indicates that it has a \u0026quot;threshold\u0026quot; characteristic in the light signal. When the amplitude is less than a certain value, the energy converted into the optical signal during discharge is relatively less, while when the discharge energy is greater than a certain \u0026quot;threshold value\u0026quot;, the energy converted into the optical signal increases sharply. It means that fluorescent optical fiber have high sensitivity for detecting arc discharges, and large arc discharges in engineering are the main cause of transformer explosions accident. Due to the excellent insulation performance of fluorescent optical fibers, they can be directly embedded near the bushing elevation base inside transformers, effectively avoiding external interference signal. At the same time, using optical fibers to transmit optical signals can further reduce external interference, Therefore, the detected optical signal will have a higher signal-to-noise ratio, and its signal will be purer.\u003c/p\u003e\n\u003cp\u003eIf a transformer explosion warning system based on optical signals is adopted, it can help solve the problem of false alarms or misoperations in relay protection devices and effectively improve the reliability of transformer operation.\u003c/p\u003e\n\u003cp\u003e4.3. Relationship between Optical Signal and Electric Pulse Signal\u003c/p\u003e\n\u003cp\u003eFigure 9 shows The amplitude distribution between the optical and electrical signals. It can be seen that there is a good linear relationship between optical signal and the electrical signal, but the data still has a certain degree of discreteness, this may be because the discharge phenomenon in transformer oil is a random process of energy conversion, release, and transfer. The wavelength distribution of the light signal exhibits a certain degree of randomness, and the position, angle, and distance of the fluorescent fiber have different collection and transmission efficiencies for photons [18].\u003c/p\u003e\n\u003cp\u003eConsidering the randomness of optical signals, it is difficult to establish an accurate functional relationship between optical signals and electrical signals. However, in the overall trend, the optical signal increases with the increase of pulse current, which can to some extent reflect the severity of discharge phenomena. As a non electrical signal detection method, optical signals have higher resistance to electromagnetic interference than traditional electrical signals, which can provide new ideas and methods for online monitoring of the operating status of oil immersed equipment.\u003c/p\u003e\n\u003cp\u003eBased on the above experimental data, the histogram of the average value and dispersion of the optical signal amplitude under the three discharge types is shown in Figure 10. The average discharge optical signal value during corona discharge is 1.3 mV and the dispersion is 22.7%, the average and dispersion of spark discharge and arc discharge are 14.0 mV / 23.5% and 2.1 V / 24.7%, respectively. The amplitude of the discharge optical signal in the three stages shows a significant upward trend, in the arc discharge stage, the amplitude of the optical signal increases from mV level to V level. That is to say, the optical signal has higher sensitivity to the serious high-energy discharge. This characteristic is conducive to the timely detection of arc discharge by fluorescence optical fiber method, and provides relevant signals for preventing misoperation and and refusal operation of oil-immersed transformer protection equipment.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eIn this paper, a set of optical signal detection system for discharge in transformer oil is established based on the fluorescent fiber and silicon photomultiplier. The optical signals and electrical signals of corona discharge, spark discharge and arc discharge are compared and analyzed. The main conclusions are as follows:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e1.\u0026nbsp; \u0026nbsp;\u0026nbsp;The spectral distribution range of discharge in transformer oil is relatively wide, covering the near ultraviolet, visible, and infrared regions. The main energy distribution of the spectrum is in the wavelength range of 320-950 nm. For discharges in oil, to ensure detection sensitivity, the detection sensitive area of fluorescent fibers should be in the range of 320-950 nm.\u003c/p\u003e\n\u003cp\u003e2.\u0026nbsp; \u0026nbsp;\u0026nbsp;The average value of the discharge optical signal during corona discharge is 1.3 mV, and the average and dispersion of spark discharge and arc discharge are 14.0 mV and 2.1 V, respectively. There is a good correspondence between the light signal and the pulse current signal, with similar changing trends, but also showing strong random fluctuations. The light signal can effectively reflect the severity of the discharge phenomenon.\u003c/p\u003e\n\u003cp\u003e3. \u0026nbsp; \u0026nbsp;Optical signal has good anti-electromagnetic interference ability. It can not only effectively detect weak discharge in oil-immersed equipment, but also has higher sensitivity to high-energy discharge with serious harm. Optical signal can be used as a parameter to reflect the severity of discharge fault in oil-immersed equipment. It can be used for on-line monitoring of transformer discharge, and can also provide relevant control signals for relay protection devices based on optical signal to reduce the probability of transformer explosion accidents.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e State Grid Henan Electric Power Company; Research on Optical Detection and Early Warning Method for Discharge in Transformer Oil Based on Multichannel Fiber Optic Multiplication(52170223000C)..\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgment.\u003c/strong\u003e This research was funded by State Grid Henan Electric Power Company Technology Project: Research on Optical Detection and Early Warning Method for Discharge in Transformer Oil Based on Multichannel Fiber Optic Multiplication\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest.\u0026nbsp;\u003c/strong\u003eThe authors declare no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e The data supporting the findings of this study are available from the corresponding author upon reasonable request..\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWei Wang and QiLin Wang: Methodology; XiaoHui Wang: conceptualization; QiLin Wang, XiaoHui Wang, ZhiFei Yang: software; Wei Wang, ZhiFei Yang, ShengHui Wang: resources;ShengHui Wang: data curation; QiLin Wang and XiaoHui Wang: writing-review and editing; Wei Wang, QiLin Wang, ZhiFei Yang, XiaoHui Wang: supervision; Wei Wang, QiLin Wang, ZhiFei Yang, XiaoHui Wang: project administration;XiaoHui Wang: funding acquisitionAll authors have read and agreed to the published version of the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eWorking Group A2.33. Guide for transformer fire safety practices. Paris, France: CIGRE, 2013. Available online:https://static.mimaterials.com/midel/documents/sales/Guide_for_Transformer_Fire_Safety_Practices.pdf (accessed on 11 November 2023).\u003c/li\u003e\n \u003cli\u003eMohammad Akbari Azirani.; Mohamadreza Ariannik.; Peter Werle.; Asghar Akbari. Optimal frequency selection for detection of partial discharges in power transformers using the UHF measurement technique. Measurement, 2021, 172, 108895. DOI: https://doi.org/10.1016/j.measurement.2020.108895.\u003c/li\u003e\n \u003cli\u003eChina Electric Power Enterprise Federation. China Electric Power Statistics Yearbook 2021. Beijing: China Statistics Publishing House, 2021.\u003c/li\u003e\n \u003cli\u003eLiao R.; Yang L.; Zheng H.; Wang K.; Ma Z. Review of thermal aging research on oil-paper insulation of power transformer. Journal of Electrical Technology, 2012, 27, 1-12. DOI: 10.19595/j.cnki.1000-6753.tces.2012.05.001.\u003c/li\u003e\n \u003cli\u003eZhao X.; Yang J.; Lu X.; Yuan P.; Wang S.; Li Y. Comparison of pulse current method and ultra-high frequency method for partial discharge detection in oil. High. Volt. 2008, 34, 1401-1404. DOI: 10.13336/j.1003-6520.hve.2008.07.021.\u003c/li\u003e\n \u003cli\u003eChang W.; Tang Z.; Li C.; Wang C. Simulation analysis of UHF signal propagation characteristics of transformer partial discharge. High. Volt. 2009, 35, 1629-1634. DOI: 10.13336/j.1003-6520.hve.2009.07.034.\u003c/li\u003e\n \u003cli\u003eZhou L.; He L.; Li W. Ultrasonic signal characteristics of transformer partial discharge and discharge source location. High. Volt. 2003, 11-13+16. DOI: 10.13336/j.1003-6520.hve.2003.05.0051.\u003c/li\u003e\n \u003cli\u003eXu K.; Zhou J.; Ru Q.; Zhou Z. Development and Prospect of On-line Monitoring Technology for Dissolved Gases in Transformer Oil. High. Volt. 2005, 30-32+35. DOI: 10.13336/j.1003-6520.hve.2005.08.011.\u003c/li\u003e\n \u003cli\u003eTang J.; Ouyang Y.; Fan M.; Zhang X.; Liu Y. Development of fluorescence optical fiber sensing system for detecting partial discharge in transformers. High. Volt. 2011, 37, 1129-1135. DOI: 10.13336/j.1003-6520.hve.2011.05.015.\u003c/li\u003e\n \u003cli\u003eWen q.; Zhou L.; Wang D.; Wang D.; Jiang Y.; Ma F.; Liu X. Research on on-line monitoring technology of transformer based on photoacoustic spectroscopy. Electrical Measurement and Instrumentation, 2020, 57, 23-27+125. DOI: 10.19753/j.issn1001-1390.2020.13.005.\u003c/li\u003e\n \u003cli\u003eYang H.; Luo H. Research on PD monitoring system based on optical fiber sensor. Transformer, 2017, 54, 39-42. DOI: 10.19487/j.cnki.1001-8425.2017.09.008.\u003c/li\u003e\n \u003cli\u003eZhang C.; Xie F.; Huang X.; et al. Ground simulation experiment of partial discharge fault of near space vehicle based on fluorescent fiber. J. Opt. 2021, 41, 56-61. 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DOI:10.16790/j.cnki.1009-9239.im.2023.08.010.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Oil immersed transformer, Optical signal, Fluorescent fiber, Discharge characteristic","lastPublishedDoi":"10.21203/rs.3.rs-6429164/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6429164/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe optical signal emitted by the discharge in oil is an important symptom of the operating status of transformers. A transformer oil discharge optical signal detection system based on fluorescent optical fibers and silicon photomultiplier are designed in this paper. The characteristics of optical signals, ultrasonic signals, ultra-high frequency signals, and pulse current signals of corona discharge, spark discharge, and arc discharge in a transformer model are compared and studied. The experimental results show that as the discharge increases, the amplitude of the above signals also increases accordingly, the amplitude of optical signals has a certain degree of randomnesst, but there is an approximate linear relationship with the pulse current signal. Optical signals have higher detection sensitivity for hazardous high-energy arc discharges, which can be used as a signal to reflect the severity of discharge failure, and can be used to provide relevant trip signals for relaying protection devices to reduce the probability of transformer explosion accidents.\u003c/p\u003e","manuscriptTitle":"Research on the optical signal characteristics at different discharge stages in transformer oil based on fluorescent fiber detection","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-12 10:52:39","doi":"10.21203/rs.3.rs-6429164/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7e07ba8e-fb1e-4a6f-82af-388dda48307a","owner":[],"postedDate":"June 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-20T16:38:26+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-12 10:52:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6429164","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6429164","identity":"rs-6429164","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}