A method for coupling partial discharge electromagnetic wave signals of tank joint type transformer

Abstract

The application discloses to the technical field of power equipment fault diagnosis, and particularly relates to a kind of oil tank joint type transformer partial discharge electromagnetic wave signal coupling method, comprising: according to the polarization characteristics of electromagnetic wave line in oil tank joint, the main gain direction of line polarization coupling device is determined;According to the main gain direction, select the conical antenna as the conical coupling structure of line polarization coupling device;Coaxial output port of line polarization coupling device is constructed;Line polarization coupling device is installed in the outermost limit of oil tank joint type transformer square steel fracture and realizes coaxial feed, and the coupling of oil tank joint type transformer partial discharge electromagnetic wave signal is completed.The actual measurement results show that the sensitivity is 2-3 times of the traditional external sensor, the high sensitivity detection of transformer insulation state is realized, the basic structure of transformer does not need to be changed on a large scale, and it is suitable for transformers of various voltage levels that have been put into operation, and has important engineering application value for partial discharge detection of in-station transformer.

Description

Technical Field

[0001] This invention belongs to the field of power equipment fault diagnosis technology, and specifically relates to a method for coupling electromagnetic wave signals for partial discharge in a tank-joint transformer. Background Technology

[0002] As one of the core pieces of equipment in a power system, the operating condition of a power transformer directly affects the safety and reliability of the entire power system, while the insulation condition of the transformer directly determines whether the transformer can operate stably.

[0003] According to a statistical analysis by CIGRE WG A2.37 of the operating status of 167,459 transformers in 21 countries worldwide between 1996 and 2010, the failure rate of 110kV and above transformers reached 0.48%, with the main causes of failure being insulation failure (38%), mechanical failure (22%), and electrical failure (18%). A statistical analysis by D. Martin et al. of 6,637 power transformers in Australia and New Zealand between 2000 and 2016 showed that the failure rate of 110kV and above transformers reached 0.32%, with the majority of failures caused by insulation aging and deterioration. The transformer tank is filled with insulating oil; when an insulation failure occurs inside the transformer, it can lead to more serious safety accidents such as transformer fires and explosions. Statistics show that approximately 10% of transformers in Australia and New Zealand catch fire and explode after an insulation failure, comparable to the 12% reported by CIGRE WG A2.37, posing a serious threat to substation operation and maintenance personnel.

[0004] Partial discharge (PD) is generally considered a significant factor contributing to the aging and deterioration of transformer insulation, and is also a crucial indicator and manifestation of insulation failure. Detecting partial discharge within a transformer allows for the early detection of internal insulation defects, thus preventing greater damage from sudden and latent accidents. Therefore, partial discharge detection technology is of great importance for ensuring the safe and reliable operation of transformers.

[0005] Common methods for detecting partial discharge in transformers include pulsed current method, dissolved gas in oil method, ultrasonic method, and ultra-high frequency (UHF) method. Among these, the UHF method has been widely used in actual field testing. UHF sensors are mainly installed in two ways: internal and external. Internal sensors are placed inside the transformer tank via a drain valve, manhole, or pre-installed during transformer manufacturing. However, installing internal sensors carries the risk of disrupting the transformer's internal electromagnetic balance. External sensors, on the other hand, are installed outside the transformer tank by creating a dielectric window. Because this requires pre-planning for the dielectric window during the transformer design phase, it cannot be used on operating power transformers.

[0006] Since the oil tank of a power transformer is a metal-enclosed structure, existing built-in and external ultra-high frequency sensors have the aforementioned defects. Therefore, there is an urgent need for a method for coupling electromagnetic wave signals of partial discharge in transformers with oil tank seams. Based on the propagation characteristics of partial discharge electromagnetic waves at the seams, this method can be applied to operating power transformers to achieve high-sensitivity detection of partial discharge electromagnetic wave signals in transformers. Summary of the Invention

[0007] The purpose of this invention is to provide a method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer, characterized by the following steps:

[0008] S1: Based on the electromagnetic wave linear polarization characteristics in the tank seam, determine the main gain direction of the linear polarization coupling device, wherein the main gain direction is perpendicular to the tank seam plane;

[0009] S2: Based on the main gain direction, a conical antenna is selected as the conical coupling structure of the linear polarization coupling device to increase the equivalent height and effective receiving area of ​​the linear polarization coupling device;

[0010] S3: Construct the coaxial output port of the linear polarization coupling device;

[0011] S4: Install the linear polarization coupling device at the outermost limit square steel break of the oil tank joint transformer to achieve coaxial power supply and complete the partial discharge electromagnetic wave signal coupling of the oil tank joint transformer.

[0012] The step of determining the main gain direction of the linearly polarized coupling device in S1 includes:

[0013] S11: Establish a scaled-down model of the transformer tank using finite difference simulation software;

[0014] S12: Set a short dipole antenna at the center of the transformer tank scaled-down model and apply a steep pulse current to the short dipole antenna to simulate a partial discharge source;

[0015] S13: The short dipole antenna is set to the X direction, Y direction and Z direction respectively, and the Z direction is perpendicular to the fuel tank joint plane;

[0016] S14: Set up a potential observation point inside the gap of the fuel tank;

[0017] S15: Based on the observation results, the Z direction is determined to be the main gain direction of the linearly polarized coupling device.

[0018] The step of installing the linear polarization coupling device at the outermost limiting square steel break of the oil tank joint transformer in S4 includes:

[0019] S41: Determine the opening position, the opening is located on the upper surface of the fuel tank body; the orthographic projection of the opening on the lower surface of the fuel tank body falls within the orthographic projection of the outermost limiting square steel cut on the lower surface of the fuel tank body;

[0020] S42: Determine the aperture diameter D and the inner conductor diameter d based on the impedance matching constraint conditions;

[0021] S43: Based on the opening position and opening diameter determined by S41 and S42, an opening is made on the upper surface of the tank body so that the inner conductor and the opening form a coaxial structure, thereby achieving impedance matching of the power supply port.

[0022] The impedance matching constraint condition in S42 is:

[0023]

[0024] In the formula, Z0 is the transmission impedance of the coaxial structure, and ε r Let ε be the relative permittivity of the medium, where the medium is air. r Let 1 be the diameter of the inner conductor and D be the diameter of the opening.

[0025] Furthermore, the aperture diameter D is 6.9 mm, and the inner conductor diameter d is 3 mm.

[0026] The following are included after S3 and before S4:

[0027] A conical antenna is placed inside a metal cavity to form a cavity antenna, which is equivalent to a binary antenna array.

[0028] The width and height of the metal back cavity are determined based on the dimensions of the outermost limiting square steel section of the oil tank joint transformer.

[0029] The length of the metal back cavity is determined based on array antenna theory.

[0030] The length of the metal back cavity is determined according to Equation 2:

[0031]

[0032] Where: h is the length of the metal back cavity, E1 is the electric field intensity emitted by the antenna itself in space, λ0 is the wavelength of the center frequency of the electromagnetic wave, and E is the electric field intensity.

[0033] Furthermore, the width of the metal back cavity is selected to be 33mm, the height of the metal back cavity is 17mm, and the length of the metal back cavity is 150mm.

[0034] This invention discloses a linear polarization coupling device for partial discharge electromagnetic wave signals of a tank-joint transformer. The device is characterized in that it realizes the coupling of electromagnetic wave signals for partial discharge of the tank-joint transformer based on the linear polarization characteristics of electromagnetic waves in the tank joint.

[0035] The linear polarization coupling device includes: a tapered coupling structure and a coaxial output port;

[0036] The tapered coupling structure is connected to the coaxial output port to achieve coaxial power supply.

[0037] The conical coupling structure is a conical antenna, which can increase the equivalent height and effective receiving area of ​​the linearly polarized coupling device;

[0038] The coaxial output port includes a coaxial line and a power supply port connected in sequence, and an inner conductor is provided inside the coaxial line;

[0039] An opening is provided on the upper surface of the fuel tank body, and the orthographic projection of the opening on the lower surface of the fuel tank body falls within the orthographic projection of the outermost limiting square steel cut on the lower surface of the fuel tank body.

[0040] The inner conductor and the opening form a coaxial structure to achieve impedance matching at the power supply port.

[0041] The partial discharge electromagnetic wave signal line polarization coupling device of the oil tank joint transformer also includes a back cavity surrounding the conical antenna to form a back cavity antenna, which is equivalent to a binary antenna array.

[0042] The beneficial effects of this invention are as follows:

[0043] To address the challenge of existing external sensors failing to detect partial discharge in the metal-enclosed structure of power transformer tanks, this invention proposes a method for coupling electromagnetic wave signals of partial discharge in transformer tank joints, based on the propagation characteristics of electromagnetic waves during partial discharge in the tank joints. This method offers the following advantages:

[0044] First, based on the electromagnetic wave linear polarization characteristics in the tank joint, a linear polarization coupling method is proposed, and a cone-shaped partial discharge signal coupling device suitable for installation in the transformer tank joint is developed.

[0045] Second, a conical coupling structure is proposed to increase the equivalent height and effective receiving area;

[0046] Third, optimize the coaxial output port structure to improve impedance matching characteristics;

[0047] Fourth, an electromagnetic wave reflection multiplier cavity structure is added to improve detection sensitivity and accuracy.

[0048] Experimental results show that the designed seam-type coupling device has a sensitivity 2-3 times that of traditional external sensors, and can be applied to operating power transformers to achieve high-sensitivity detection of transformer insulation status. The method for coupling electromagnetic wave signals of partial discharge in a tank-type transformer using the seam-type coupling method disclosed in this invention does not require large-scale modifications to the basic structure of the transformer and is applicable to transformers of various voltage levels already in operation. It has significant engineering application value for partial discharge detection in substation transformers. Attached Figure Description

[0049] Figure 1 This is a flowchart of a partial discharge electromagnetic wave signal coupling method for a tank joint transformer according to the present invention.

[0050] Figure 2 This is a scaled-down model diagram of a transformer tank.

[0051] Figure 3 A schematic diagram of the transformer tank joint structure and the layout of potential observation points;

[0052] Figure 4 A basic structural model diagram of a linearly polarized coupling device;

[0053] Figure 5 This is a schematic diagram of the axial ratio direction of the XOZ plane of a conical antenna;

[0054] Figure 6 This is a schematic diagram showing the axial ratio direction of the YOZ plane of a conical antenna;

[0055] Figure 7 A schematic diagram showing the installation location of the linear polarization coupling device;

[0056] Figure 8 Enlarged view of the cut of the limiting square steel at the installation position of the linear polarization coupling device;

[0057] Figure 9 This is a schematic diagram showing the mirror effect at the bottom of the dorsal cavity;

[0058] Figure 10 This is a schematic diagram of the structure of a typical defective fuel tank model detection platform;

[0059] Figure 11The following are comparison diagrams of the signal intensity of the tip defect discharge sensor in oil, where (a) is the time domain waveform of the conical antenna, (b) is the time domain waveform of the bowtie antenna, (c) is the frequency domain waveform of the conical antenna, and (d) is the frequency domain waveform of the bowtie antenna.

[0060] Figure 12 The following are comparison diagrams of the signal intensity of the suspended defect discharge sensor in oil, where (a) is the time domain waveform of the conical antenna, (b) is the time domain waveform of the bowtie antenna, (c) is the frequency domain waveform of the conical antenna, and (d) is the frequency domain waveform of the bowtie antenna.

[0061] Figure 13 The following are comparison diagrams of the signal intensity of surface defect discharge sensors in oil, where (a) is the time-domain waveform of the conical antenna, (b) is the time-domain waveform of the bowtie antenna, (c) is the frequency-domain waveform of the conical antenna, and (d) is the frequency-domain waveform of the bowtie antenna.

[0062] Figure 14 The following are comparison diagrams of the signal intensity of the gas bubble defect discharge sensor in oil: (a) is the time domain waveform of the conical antenna, (b) is the time domain waveform of the bowtie antenna, (c) is the frequency domain waveform of the conical antenna, and (d) is the frequency domain waveform of the bowtie antenna.

[0063] Figure 15 The following are comparison diagrams of test results with and without a reflective back cavity, where (a) is the test result without a reflective back cavity, (b) is the test result with a reflective back cavity added, and (c) is the test result with an external bowtie antenna. Detailed Implementation

[0064] Most large oil-immersed transformer tanks operating in substations adopt bell-shaped and barrel-shaped structures. Limiting square steel bars are welded between the tank body and the upper end cover or between the tank body and the base. Rubber sealing rings confine the tank body between these limiting square steel bars, forming a non-metallic insulating gap around the transformer. Partial discharge electromagnetic wave signals can propagate to the outside through this gap. Existing technologies lack research on the propagation characteristics of partial discharge electromagnetic waves at this gap. Based on extensive research and experiments, this invention discovers the linear polarization characteristics of discharge electromagnetic wave signals in the tank gap. Based on these characteristics, a partial discharge electromagnetic wave signal coupling method for transformers with tank joints is designed and developed. A conical partial discharge signal coupling device suitable for installation in transformer tank joints is also developed. The coaxial output port structure is optimized for matching impedance, and a reflection multiplier cavity structure is added, effectively improving detection sensitivity and accuracy. The invention will be further described in detail below with reference to the accompanying drawings.

[0065] like Figure 1 The embodiment of the present invention shown provides a method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer, comprising the following steps:

[0066] S1: Based on the electromagnetic wave linear polarization characteristics in the tank seam, determine the main gain direction of the linear polarization coupling device, wherein the main gain direction is perpendicular to the tank seam plane;

[0067] First, determine the electromagnetic wave polarization direction and polarization matching coupling method in the fuel tank joint.

[0068] To clarify the polarization direction of the electromagnetic wave signal in the gap, a system was established in the finite difference simulation software as follows: Figure 2 The transformer tank scale model shown has a short dipole antenna at its center, and a steep pulse current is applied to it to simulate a partial discharge source. The directions of the short dipole antenna are set to X, Y, and Z (where the Z direction is perpendicular to the tank gap plane). A transformer tank joint structure is created, and potential observation points are set inside the tank gap.

[0069] like Figure 3 As shown, the transformer tank joint structure includes: an upper cover 1 and a tank body 2; between the upper cover 1 and the tank body 2, a first limiting square steel 31, a second limiting square steel 32, and a third limiting square steel 33 are sequentially arranged from the center of the tank outwards; a rubber sealing ring 4 is provided between the first limiting square steel 31 and the second limiting square steel 32, the height of the rubber sealing ring 4 being greater than the height of the first limiting square steel 31 and the second limiting square steel 32, thus forming a gap 5 between the upper cover 1 and the tank body 2. The tank joint plane is the horizontal plane where the gap 5 is located.

[0070] The locations of the potential observation points are arranged as follows: Figure 3 The five gaps shown are as follows.

[0071] Based on the electromagnetic wave linear polarization characteristics in the fuel tank joint, the main gain direction of the linear polarization coupling device is determined. The specific steps include:

[0072] S11: Establish a scaled-down model of the transformer tank using finite difference simulation software;

[0073] S12: Set a short dipole antenna at the center of the transformer tank scaled-down model and apply a steep pulse current to the short dipole antenna to simulate a partial discharge source;

[0074] S13: The short dipole antenna is set to the X direction, Y direction and Z direction respectively, and the Z direction is perpendicular to the fuel tank joint plane;

[0075] S14: Set up a potential observation point inside the gap of the fuel tank;

[0076] S15: Based on the observation results, the Z direction is determined to be the main gain direction of the linearly polarized coupling device.

[0077] The observation results are shown in Table 1. Regardless of the direction of the discharge pulse, the component of the electric field intensity in the gap in the Z direction is always higher, averaging 64.59 times higher than the component in the X direction and 3.65 × 10⁻⁶ higher than the component in the Y direction. 5 Therefore, a linearly polarized coupling device with the main gain direction in the Z direction needs to be designed. Thus, the main gain direction is determined to be perpendicular to the tank joint plane.

[0078] Table 1 Maximum Electric Field Strength

[0079]

[0080] S2: Based on the main gain direction, a conical antenna is selected as the conical coupling structure of the linear polarization coupling device to increase the equivalent height and effective receiving area of ​​the linear polarization coupling device.

[0081] A broadband, high-sensitivity conical coupling component is constructed. Existing UHF antennas used for partial discharge detection mainly include: linear antennas primarily based on monopole antennas, surface antennas primarily based on microstrip antennas, aperture antennas primarily based on horn antennas, and non-frequency-varying antennas primarily based on helical antennas and conical antennas. Conical antennas possess broadband and linear polarization characteristics, and have a compact structure. Therefore, considering the required polarization characteristics and the limitation on sensor size imposed by the fuel tank gap, a conical antenna is selected as the electromagnetic signal coupling device for partial discharge at the fuel tank seam.

[0082] like Figure 4 As shown, the linear polarization coupling device includes a tapered coupling structure and a coaxial output port; the tapered coupling structure is a tapered antenna 61, which can increase the equivalent height and effective receiving area of ​​the linear polarization coupling device; the coaxial output port includes a coaxial line 62 and a feed port 63 connected in sequence, and an inner conductor (not shown in the figure) is provided inside the coaxial line 62;

[0083] In this embodiment, to accommodate a slot size sensor with a top diameter of 32mm and a height of 16mm, the bottom is fed via a coaxial line. In the three-dimensional radiation direction of the conical antenna, the gain in the main radiation direction is -1.5dB. The axial ratio direction of the XOZ plane of the conical antenna is as follows... Figure 5 As shown, the axial ratio direction of the YOZ plane of the conical antenna is as follows: Figure 6 As shown, the axial ratios in the main radiation direction are all greater than 70dB, which indicates that the conical antenna has good linear polarization characteristics.

[0084] S3: Construct the coaxial output port of the linear polarization coupling device;

[0085] like Figure 8 As shown, the coaxial output port includes a coaxial line 62 and a power supply port 63 connected in sequence, and an inner conductor 64 is provided inside the coaxial line 62.

[0086] The coaxial output port needs to achieve impedance matching. As known from antenna reception theory, in order for an antenna to reliably and distortion-free sense transient electric field signals and to achieve a response consistent with theoretical calculations, in addition to the antenna's manufacturing process meeting requirements, the coaxially fed conical antenna must ensure impedance matching at the feed port.

[0087] S4: Install the linear polarization coupling device at the outermost limit square steel break of the oil tank joint transformer to achieve coaxial power supply and complete the partial discharge electromagnetic wave signal coupling of the oil tank joint transformer.

[0088] The step of installing the linear polarization coupling device at the outermost limiting square steel break of the oil tank joint transformer in S4 includes:

[0089] S41: Determine the opening position, the opening is located on the upper surface of the fuel tank body; the orthographic projection of the opening on the lower surface of the fuel tank body falls within the orthographic projection of the outermost limiting square steel cut on the lower surface of the fuel tank body;

[0090] like Figure 3 As shown, the transformer used in this embodiment has three limiting square steel structures. Between the upper end cover 1 and the tank body 2, from the center of the tank outwards, there are three limiting square steel structures: a first limiting square steel 31, a second limiting square steel 32, and a third limiting square steel 33. To prevent rainwater from seeping into the tank through the sealing ring in the tank flange gap, a break is provided on the outermost third limiting square steel 33. Experimental verification shows that changing the width of the break in the outermost limiting square steel has little effect on the electric field strength at the two inner limiting square steels; its effect on the electric field strength inside the gap is mainly reflected at the outermost limiting square steel. Therefore, as... Figure 7 As shown, the linear polarization coupling device is installed at the break 9 of the third limiting square steel 33 on the outermost side of the oil tank joint transformer. The partial discharge electromagnetic wave signal coupling method for oil tank joint transformer disclosed in this invention can not only achieve better detection results, but also does not require large-scale modification of the basic structure of the transformer. It is applicable to transformers of various voltage levels that have been put into operation, and has important engineering application value for partial discharge detection of transformers in the station.

[0091] S42: Determine the aperture diameter D and the inner conductor diameter d based on the impedance matching constraint conditions;

[0092] like Figure 8 As shown, the conical antenna 61 is installed at the break 9 of the outermost limiting square steel 33. The inner conductor 64 and the opening of the box form a coaxial structure. In the S42 impedance matching constraint condition, the diameter of the inner conductor and the diameter of the opening are determined according to formula (1) to meet the 50Ω matching.

[0093]

[0094] In the formula, Z0 is the transmission impedance of the coaxial structure, and ε r Let ε be the relative permittivity of the medium, where the medium is air. r Let 1 be the diameter of the inner conductor and D be the diameter of the opening.

[0095] In this embodiment, combined with the actual structure and the constraints of equation (1), the inner conductor diameter d is 3mm and the opening diameter D is 6.9mm.

[0096] S43: Based on the opening position and opening diameter determined by S41 and S42, an opening is made on the upper surface of the tank body so that the inner conductor and the opening form a coaxial structure, thereby achieving impedance matching of the power supply port.

[0097] When a partial discharge occurs inside the transformer tank, the electromagnetic wave signal is transmitted to the outside of the transformer tank through a linear polarization coupling device. The linear polarization coupling device is installed at the outermost limit square steel break of the tank joint transformer to achieve coaxial power supply and complete the coupling of the electromagnetic wave signal of the partial discharge of the tank joint transformer.

[0098] In this embodiment, a reflective back cavity enhancement component is introduced to improve detection sensitivity.

[0099] A conical antenna receives signals from both the front and back sides simultaneously. This not only reduces the antenna's reception efficiency in the primary detection direction but also allows it to pick up ambient clutter, leading to signal distortion and a lower signal-to-noise ratio. Therefore, it is necessary to place the antenna within a metal cavity to create a cavity-backed antenna.

[0100] The specific steps, following S3 and before S4, include:

[0101] A conical antenna is placed inside a metal cavity to form a cavity antenna, which is equivalent to a binary antenna array.

[0102] The width and height of the metal back cavity are determined based on the dimensions of the outermost limiting square steel section of the oil tank joint transformer.

[0103] The length of the metal back cavity is determined based on array antenna theory.

[0104] In this embodiment, the length of the back cavity in the antenna plane normal has a major impact on the antenna performance, while the width and height of the back cavity can be determined according to the application requirements of the antenna. Based on the size requirements, the width of the back cavity is initially selected as 33mm and the height as 17mm. The length h of the back cavity is calculated below.

[0105] like Figure 9As shown, the bottom surface of the back cavity has a mirror effect on the antenna, thus making the antenna equivalent to a binary antenna array. The transmitted field strength of the antenna can then be analyzed according to the theory of array antennas. The length h of the metal back cavity is determined according to equation (2):

[0106]

[0107] Where: h is the length of the metal back cavity, E1 is the electric field strength emitted by the antenna itself in space, λ0 is the wavelength of the electromagnetic wave center frequency, and E is the electric field strength. The emitted electric field strength E is strongest when the length of the metal back cavity is h = λ0 / 4. Based on calculations, h is set to 150 mm.

[0108] like Figure 15 As shown, the test results with and without a reflective back cavity are compared. Figure 15 In the diagram, (a) shows the test results without a reflective back cavity, (b) shows the test results with a reflective back cavity added, and (c) shows the test results with an external bowtie antenna. The test results show that the added reflective back cavity increased the signal of the cone sensor by 78.87%, which is 3.2 times that of the bowtie antenna.

[0109] In summary, the partial discharge electromagnetic wave signal coupling method for a tank-joint transformer disclosed in this invention has the following technical effects:

[0110] First, based on the electromagnetic wave linear polarization characteristics in the fuel tank joint, a linear polarization coupling method is proposed;

[0111] Second, a conical coupling structure is proposed to increase the equivalent height and effective receiving area;

[0112] Third, a coaxial output port structure was proposed to improve impedance matching characteristics;

[0113] Fourth, an electromagnetic wave reflection multiplier cavity structure is proposed to improve detection sensitivity.

[0114] In this embodiment, in order to apply the partial discharge electromagnetic wave signal coupling method of the oil tank joint transformer, a linear polarization coupling device for the partial discharge electromagnetic wave signal of the oil tank joint transformer is disclosed. The linear polarization coupling device realizes the coupling of the partial discharge electromagnetic wave signal of the oil tank joint transformer based on the electromagnetic wave linear polarization characteristics in the oil tank joint.

[0115] The linear polarization coupling device includes: a tapered coupling structure and a coaxial output port;

[0116] The tapered coupling structure is connected to the coaxial output port to achieve coaxial power supply.

[0117] The conical coupling structure is a conical antenna 61, which can increase the equivalent height and effective receiving area of ​​the linearly polarized coupling device;

[0118] The coaxial output port includes a coaxial cable 62 and a power supply port 63 connected in sequence, and an inner conductor 64 is provided inside the coaxial cable 62;

[0119] An opening is provided on the upper surface of the fuel tank body, and the orthographic projection of the opening on the lower surface of the fuel tank body falls within the orthographic projection of the outermost limiting square steel cut on the lower surface of the fuel tank body.

[0120] The inner conductor 64 and the opening form a coaxial structure to achieve impedance matching of the power supply port 63.

[0121] The partial discharge electromagnetic wave signal line polarization coupling device of the oil tank joint transformer also includes a back cavity surrounding the conical antenna to form a back cavity antenna, which is equivalent to a binary antenna array.

[0122] To verify the practical effect of the partial discharge electromagnetic wave signal coupling method for a tank seam transformer disclosed in this invention, a sensitivity comparison test of the tank seam sensor was conducted.

[0123] Build such a custom-designed physical model of the transformer tank. Figure 10 The testing platform shown places the discharge model in transformer oil. The testing platform includes a corona-free experimental transformer 81, a protective resistor 82, a voltage divider 83, a bushing 84, a discharge model 85, an oscilloscope 86, sensor signals 87, and a discharge current pulse signal 88. Figure 7 As shown, the conical antenna is installed at the break 9 of the outermost third limiting square steel 33. An existing bowtie antenna in the laboratory is used as a control; during the test, the polarization direction of the bowtie antenna is kept consistent with the polarization direction of the electromagnetic wave in the gap.

[0124] To address common insulation defects in transformers, a comparative experiment was conducted on slotted conical antennas and external bowtie antennas.

[0125] Figure 11 The results of test observations on tip discharge for slotted conical antennas and external bowtie antennas are presented. Figure 11 In the diagram, (a) shows the time-domain waveform of the conical antenna, (b) shows the time-domain waveform of the bow-tie antenna, (c) shows the frequency-domain waveform of the conical antenna, and (d) shows the frequency-domain waveform of the bow-tie antenna. The signal amplitude of the conical antenna is 23.8 mV, and the signal amplitude of the bow-tie antenna is 10.7 mV. The energy of both sensor signals is mainly concentrated in the range of 400 MHz to 800 MHz. In comparison, the signal energy of the seam-type sensor is more concentrated.

[0126] Figure 12 The paper presents test results for levitation discharge using a slotted conical antenna and an external bowtie antenna. Figure 12In the diagram, (a) shows the time-domain waveform of the conical antenna, (b) shows the time-domain waveform of the bow-tie antenna, (c) shows the frequency-domain waveform of the conical antenna, and (d) shows the frequency-domain waveform of the bow-tie antenna. The signal amplitude of the conical antenna is 1980mV, and the signal amplitude of the bow-tie antenna is 990mV. The energy of both sensor signals is mainly concentrated in the range of 300MHz to 900MHz.

[0127] Figure 13 The results of test observations on surface discharge using a slotted conical antenna and an external bowtie antenna are presented. Figure 13 In the diagram, (a) shows the time-domain waveform of the conical antenna, (b) shows the time-domain waveform of the bow-tie antenna, (c) shows the frequency-domain waveform of the conical antenna, and (d) shows the frequency-domain waveform of the bow-tie antenna. The signal amplitude of the conical antenna is 19.4 mV, and the signal amplitude of the bow-tie antenna is 8.4 mV. The energy of both sensor signals is mainly concentrated in the range of 300 MHz to 800 MHz.

[0128] Figure 14 The results of test observations on tip discharge for slotted conical antennas and external bowtie antennas are presented. Figure 14 In the diagram, (a) shows the time-domain waveform of the conical antenna, (b) shows the time-domain waveform of the bow-tie antenna, (c) shows the frequency-domain waveform of the conical antenna, and (d) shows the frequency-domain waveform of the bow-tie antenna. The signal amplitude of the conical antenna is 26.8mV, and the signal amplitude of the bow-tie antenna is 7.4mV. The energy of both sensor signals is mainly concentrated in the range of 400MHz to 700MHz. In comparison, the signal energy of the seam-type sensor is more concentrated. The experimental results show that the sensitivity of the designed seam-type coupling device is 2-3 times that of the traditional external sensor, achieving high-sensitivity detection of transformer insulation status. The method for coupling electromagnetic wave signals of partial discharge in a transformer using a seam-type coupling device for a tank, as disclosed in this invention, does not require large-scale modification of the basic structure of the transformer and is applicable to transformers of various voltage levels already in operation. It has significant engineering application value for partial discharge detection of transformers in substations.

Claims

1. A method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer, characterized in that, Includes the following steps: S1: Based on the electromagnetic wave linear polarization characteristics in the tank seam, determine the main gain direction of the linear polarization coupling device, wherein the main gain direction is perpendicular to the tank seam plane; S2: Based on the main gain direction, a conical antenna is selected as the conical coupling structure of the linear polarization coupling device to increase the equivalent height and effective receiving area of ​​the linear polarization coupling device; S3: Construct the coaxial output port of the linear polarization coupling device; S4: Install the linear polarization coupling device at the outermost limit square steel break of the oil tank joint transformer to achieve coaxial power supply and complete the coupling of partial discharge electromagnetic wave signals of the oil tank joint transformer. The step of determining the main gain direction of the linearly polarized coupling device in S1 includes: S11: Establish a scaled-down model of the transformer tank using finite difference simulation software; S12: Set a short dipole antenna at the center of the transformer tank scaled-down model and apply a steep pulse current to the short dipole antenna to simulate a partial discharge source; S13: The short dipole antenna is set to the X direction, Y direction and Z direction respectively, and the Z direction is perpendicular to the fuel tank joint plane; S14: Set up a potential observation point inside the gap of the fuel tank; S15: Based on the observation results, the Z direction is determined to be the main gain direction of the linearly polarized coupling device; The step of installing the linear polarization coupling device at the outermost limiting square steel break of the oil tank joint transformer in S4 includes: S41: Determine the opening position, the opening is located on the upper surface of the fuel tank body; the orthographic projection of the opening on the lower surface of the fuel tank body falls within the orthographic projection of the outermost limiting square steel cut on the lower surface of the fuel tank body; S42: Determine the aperture diameter D and the inner conductor diameter d based on the impedance matching constraint conditions; S43: Based on the opening position and opening diameter determined by S41 and S42, an opening is made on the upper surface of the tank body so that the inner conductor and the opening form a coaxial structure, thereby achieving impedance matching of the power supply port. The following are included after S3 and before S4: A conical antenna is placed inside a metal cavity to form a cavity antenna, which is equivalent to a binary antenna array. The width and height of the metal back cavity are determined based on the dimensions of the outermost limiting square steel section of the oil tank joint transformer. The length of the metal back cavity is determined based on array antenna theory.

2. The method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer according to claim 1, characterized in that, The impedance matching constraint condition in S42 is: ， In the formula, Z0 is the transmission impedance of the coaxial structure, and ε r Let ε be the relative permittivity of the medium, where the medium is air. r Let 1 be the diameter of the inner conductor and D be the diameter of the opening.

3. The method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer according to claim 1, characterized in that, The aperture diameter D is 6.9 mm, and the inner conductor diameter d is 3 mm.

4. The method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer according to claim 1, characterized in that, The length of the metal back cavity is determined according to Equation 2: ， Where: h is the length of the metal back cavity, E1 is the electric field intensity emitted by the antenna itself in space, λ0 is the wavelength of the center frequency of the electromagnetic wave, and E is the electric field intensity.

5. The method for coupling electromagnetic wave signals of partial discharge in a tank-joint transformer according to claim 4, characterized in that, The width of the metal back cavity is selected as 33mm, the height of the metal back cavity is 17mm, and the length of the metal back cavity is 150mm.

6. A partial discharge electromagnetic wave signal linear polarization coupling device for a tank-joint type transformer, characterized in that, The linear polarization coupling device realizes the coupling of electromagnetic wave signals for partial discharge in the oil tank joint transformer based on the linear polarization characteristics of electromagnetic waves in the oil tank joint. The linear polarization coupling device includes: a tapered coupling structure and a coaxial output port; The tapered coupling structure is connected to the coaxial output port to achieve coaxial power supply. The conical coupling structure is a conical antenna (61), which can increase the equivalent height and effective receiving area of ​​the linearly polarized coupling device; The coaxial output port includes a coaxial line (62) and a power supply port (63) connected in sequence, and the coaxial line (62) is provided with an inner conductor (64); An opening is provided on the upper surface of the fuel tank body, and the orthographic projection of the opening on the lower surface of the fuel tank body falls within the orthographic projection of the outermost limiting square steel cut on the lower surface of the fuel tank body. The inner conductor (64) and the opening form a coaxial structure to achieve impedance matching of the power supply port (63).

7. The partial discharge electromagnetic wave signal linear polarization coupling device for a tank joint type transformer according to claim 6, characterized in that, It also includes forming a cavity antenna by enclosing a metal cavity around the conical antenna, the cavity antenna being equivalent to a binary antenna array.

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