Method for determining remaining life of electric power device due to electrical tree deterioration

Abstract

To provide a technique for determining a remaining life of an epoxy mold electric power device through simple operations without causing a power outage.SOLUTION: A method comprises the steps of: (a) detecting an electrical signal related to partial discharge generated from an epoxy mold electric power device to obtain a φ-q distribution; (b) determining whether the shape of the φ-q distribution is linear or planar; (c) determining, when the shape of the φ-q distribution is determined to be planar, whether a virtual figure formed by a closed curve smoothly connecting outer edges of an area showing the planar shape has a first shape in which there are substantially no notches, or a second shape in which there are notches in the virtual figure; (d) calculating an estimation value of an electrical tree length on the basis of a variance value Var related to a discharge charge q when the area showing the planar shape is the first shape; (e) calculating an estimation value of a propagation speed of an electrical tree; and (f) calculating an estimation value of a remaining life of the epoxy mold electric power device on the basis of the estimation value of the propagation speed of the electrical tree.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for determining the remaining life of electric power equipment associated with electrical tree degradation, and more particularly to a method for determining the remaining life of epoxy molded electric power equipment. [Background technology]

[0002] For substation equipment such as transformers and high-voltage panels, it is important to predict the lifespan and prevent accidents before they occur. For this reason, predictive maintenance has been proposed, which focuses on precursor phenomena that lead to deterioration and accidents in substation equipment and takes measures in advance. One such precursor phenomenon that occurs in substation equipment is partial discharge, which occurs when the insulation function in the equipment deteriorates.

[0003] Partial discharge occurs when there is a defect such as a void in the insulation of electric power equipment. More specifically, partial discharge occurs when the voltage applied to the defect exceeds a critical voltage, i.e., a spark voltage, due to partial insulation breakdown at the defect. If this partial discharge occurs repeatedly, it may eventually lead to insulation breakdown in the entire insulator, causing a power outage or other disruption to power transmission and distribution. However, it is extremely cumbersome to stop power transmission and distribution in order to detect this partial discharge.

[0004] Therefore, in order to detect partial discharges in insulators without stopping power transmission and distribution, i.e., in a live line state, a method has been proposed for detecting partial discharges by detecting ultrasonic waves generated during partial discharges (see, for example, Patent Document 1 below). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2004-335953 A Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, in electric power equipment such as transformers, in addition to those that use insulating oil as an insulating material, there are also those that use solid insulating materials such as epoxy resin. In the case of electric power equipment that uses such solid insulating materials, if a partial discharge continues to occur, the insulating material is eroded in a dendritic form, and an electrical tree is generated. As a result, the dielectric strength decreases, and finally, it leads to a total breakdown.

[0007] Once an electrical tree occurs in an electric power device, it will not disappear but will progress over time. If the electrical tree progresses sufficiently, the electric power device may experience a total circuit breakdown in the near future. If a total circuit breakdown occurs in an electric power device while it is energized, it will cause a power outage due to a ground fault, so a total circuit breakdown is an event that should be avoided.

[0008] On the other hand, even if an electrical tree has occurred in electric power equipment, depending on the extent of the development, some electric power equipment requires urgent replacement and some does not. Replacement of electric power equipment requires a power outage, and the work inevitably incurs costs. Furthermore, since there are a huge number of electric power equipment, it is not realistic to replace all of the electric power equipment. For this reason, it is preferable to give priority to replacing electric power equipment that is highly in need of replacement.

[0009] In view of the above problems, the present invention aims to provide a technique for determining the remaining life of epoxy-molded electric power equipment through simple operations without causing a power outage. [Means for solving the problem]

[0010] The method for determining the remaining life of an electric power device associated with electrical tree degradation according to the present invention comprises the steps of: A step (a) of detecting an electrical signal related to partial discharge generated in an epoxy-molded electric power device and obtaining a φ-q distribution, which is a relationship between an applied voltage phase angle φ and a discharge charge amount q; a step (b) of determining whether the shape of the φ-q distribution obtained in the step (a) is a linear shape in which the spread in the q-axis direction is dominant compared to the spread in the φ-axis direction, or a planar shape that shows spread in both the φ-axis direction and the q-axis direction; a step (c) of determining whether the shape of the φ-q distribution is the planar shape, and the shape of the region representing the planar shape is a first shape in which a virtual figure formed by a closed curve smoothly connecting the outer edges of the region representing the planar shape is substantially free of a notch, or a second shape in which a notch is present in the virtual figure; (d) calculating a variance value Var relating to the discharge charge amount q from the φ-q distribution when the region showing the surface shape is the first shape, and calculating an estimated value of an electrical tree length occurring in the epoxy molded electric power device based on the variance value Var; (e) calculating an estimate of the propagation speed of the electrical tree in the epoxy molded electric power device based on a combination of the length of the predetermined period and the estimated value of the electrical tree length obtained by performing the step (d) multiple times with an interval of one day or more; The method further comprises a step (f) of calculating an estimated remaining life of the epoxy-molded power equipment based on the estimated rate of propagation of the electrical tree calculated in the step (e).

[0011] When an electrical tree occurs in an epoxy resin, it usually branches into multiple branches, resulting in multiple electrical trees with different tree lengths. In this case, the voltage phase characteristics when partial discharge occurs vary, so the φ-q distribution, which shows the relationship between the voltage phase and the discharge charge amount, varies in phase angle φ. In other words, when an electrical tree occurs, there is no phase tendency to show a high discharge charge amount at a specific phase angle φ. In this case, the φ-q distribution shows a surface shape that shows expansion in both the φ axis direction and the q axis direction.

[0012] On the other hand, partial discharges occur from the presence of gaps such as voids and cracks that occur in epoxy resin. In the initial stage, partial discharges erode the walls of the gaps, and as this erosion progresses, tiny depressions called pits are formed. After that, as partial discharges continue, the pits grow, and eventually electrical trees develop from specific pits.

[0013] In other words, at the stage where the electrical tree has not yet progressed, in other words, at the pit stage, there is no significant difference in the length of erosion of the epoxy resin compared to the progress of the electrical tree. As a result, there is a tendency for a high discharge charge to be shown at a specific phase angle φ. In this case, the φ-q distribution shows a line shape in which the spread in the q-axis direction is dominant compared to the spread in the φ-axis direction. Here, "the spread in the q-axis direction is dominant compared to the spread in the φ-axis direction" means that it can be clearly recognized that the discharge charge q shows a high value under a specific phase angle φ. More specifically, it means that the region in which the discharge charge q shows a high value is within a range of 30° with respect to the φ-axis direction.

[0014] In other words, if the shape of the φ-q distribution is determined to be linear in the above step (b), it can be estimated that the partial discharge in the epoxy molded power equipment is in the initial stage (pit stage), and if the shape of the φ-q distribution is determined to be planar, it can be estimated that the partial discharge in the epoxy molded power equipment has progressed from the pit stage and is at the stage where an electrical tree is formed.

[0015] Incidentally, there are two types of electrical trees that can occur in epoxy-molded power equipment: non-conductive trees and conductive trees. A non-conductive tree is an electrical tree that occurs in a state where the root and tip are insulated, and in this case, a potential difference occurs between the tree root and the tree tip. On the other hand, a conductive tree is an electrical tree that occurs in a conductive state, and in this case, the tree root and tree tip are at approximately the same potential.

[0016] In a non-conductive tree, when a high voltage exceeding the withstand voltage occurs between the root and tip of the tree, a discharge occurs throughout the entire tree. On the other hand, in a conductive tree, the root and tip of the tree are almost at the same potential, so when an electric field concentrates at the tip of the tree and exceeds the withstand voltage, a discharge occurs at the tip of the tree.

[0017] According to the inventor's intensive research, it has been confirmed that the shape of the φ-q distribution shows further different tendencies depending on whether the electrical tree is non-conductive or conductive.

[0018] More specifically, when the electrical tree is non-conductive, the virtual figure formed by the closed curve that smoothly connects the outer edges of the region where the φ-q distribution shows a planar shape does not substantially have a notch. On the other hand, when the electrical tree is conductive, the virtual figure formed by the closed curve that smoothly connects the outer edges of the region where the φ-q distribution shows a planar shape does have a notch. Note that "substantially no notch" here means that the figure can be distinguished visually or by AI when compared with the case where a notch is present. As a typical example, when there is no notch, a mountain-shaped shape is shown, and when there is a notch, a wing-shaped shape is shown.

[0019] As described above, when the electrical tree is non-conductive, discharge occurs throughout the entire inside of the tree, so that the region in which the φ-q distribution shows a planar shape generally shows a smooth shape, and no peculiar region is generated. In other words, when a virtual figure is formed by a closed curve that smoothly connects the outer edges of the region showing the planar shape, this virtual figure also shows a smooth shape. In this way, when the virtual figure created based on the region showing the planar shape shows a smooth shape, the shape of the region showing the planar shape is referred to as the "first shape" for convenience.

[0020] On the other hand, when the electrical tree is conductive, the discharge develops near the tip of the tree, and depending on the extent of the development, a peculiar region is likely to be created in the region where the φ-q distribution shows a planar shape. More specifically, in a virtual figure formed by a closed curve smoothly connecting the outer edges of the region where the φ-q distribution shows a planar shape, a notch is created that cannot be confirmed in the case of a non-conductive electrical tree. In this way, when a notch is created in the virtual figure created based on the region showing the planar shape, the shape of the region showing the planar shape is referred to as the "second shape" for convenience.

[0021] In step (c), if the region in which the φ-q distribution shows a planar shape is determined to be the "first shape", in other words, if it is estimated that a non-conductive tree has occurred in the epoxy molded power device, an estimation process of the electrical tree length is performed in the above step (d).

[0022] If the electric tree is non-conductive, internal discharge occurs throughout the entire electric tree, so when the non-conductive electric tree develops, partial discharges are likely to occur at the root and intermediate points of the electric tree, in addition to the tip. As a result, the distance between the high voltage side and the ground side changes, the intensity of the discharge varies, and the value of the variance value Var related to the discharge charge q becomes significantly large. This means that the extent of the development of the electric tree, in other words, the length of the electric tree, can be estimated according to the value of the variance value Var.

[0023] By performing step (d) multiple times at intervals, the propagation speed of the electrical tree can be estimated in step (e). Since the initial dimensions (thickness) of the epoxy resin contained in the epoxy-molded power device are usually known, it is possible to predict how many days it will take for the epoxy-molded power device to reach insulation breakdown, in other words, to predict the remaining life, based on the propagation speed of the electrical tree estimated in step (e).

[0024] The method for determining the remaining life of electric power equipment due to electrical tree deterioration may include a step (g) of determining that the epoxy molded electric power equipment has reached the end of its life when the area showing the surface shape in step (c) is the second shape.

[0025] When the electrical tree is conductive, the discharge develops near the tip of the electrical tree, and therefore, unlike the case of a non-conductive electrical tree, there is little correlation between the electrical tree length and the variance value Var related to the discharge charge q. In other words, it is not possible to determine how far the electrical tree has developed based on the variance value Var related to the discharge charge q.

[0026] On the other hand, when it is determined in step (c) that the region showing the surface shape is the second shape, it is confirmed that a conductive electrical tree has been generated in the epoxy molded power equipment and is currently in a developing state. Therefore, according to the above method, when the region showing the surface shape is the second shape, it is determined in step (g) that the life of the epoxy molded power equipment has arrived, in consideration of safety. It is recommended to replace the epoxy molded power equipment judged in this way promptly.

[0027] If it is determined in step (b) that the shape of the φ-q distribution is linear, the epoxy molded power equipment may be determined to be subject to follow-up observation, and step (a) may be performed again after a specified waiting period has elapsed.

[0028] When the shape of the φ-q distribution is determined to be linear, it is estimated that although the electrical tree has not yet been formed in the epoxy resin, pits, which are the initial stage of the electrical tree, have been formed. In other words, in epoxy-molded power equipment determined in this way, if the pits grow over time, there is a high probability that electrical trees will occur. Therefore, by listing such equipment as equipment to be subjected to the determination process again at a later time, it is possible to estimate the remaining life in advance and encourage equipment replacement at an appropriate time before an accident such as a ground fault occurs.

[0029] The method for determining remaining life of electric power equipment associated with electrical tree degradation includes the steps of: a step (h) of determining whether the epoxy-molded electric power device is a remaining life judgment target before the step (a); The step (h) is a step of receiving an electromagnetic wave signal emitted from the epoxy-molded electric power equipment, breaking it down into intensities according to frequency, and determining that the epoxy-molded electric power equipment is a target for remaining life determination when the signal intensity of a reference frequency band that is within a range of 100 MHz or more and less than 200 MHz and includes at least 150 MHz exceeds a predetermined threshold value that exceeds a noise level; The step (a) may be performed on the epoxy-molded electric power equipment determined in the step (h) to be a remaining life judgment target.

[0030] Through intensive research by the present inventors, it has been confirmed that when partial discharge occurs in epoxy molded power equipment, an electromagnetic wave signal containing frequency components around 150 MHz is transmitted from the epoxy molded power equipment. In other words, according to the above method, it is possible to determine whether or not the epoxy molded power equipment is a target for remaining life judgment simply by receiving the electromagnetic wave signal from the epoxy molded power equipment in advance and analyzing the frequency. The process required for receiving the electromagnetic wave signal and analyzing the frequency is completed in a short time compared to each of the above steps (a) to (f). Therefore, according to the above method, by performing step (h) in advance, it is possible to screen in advance the equipment for which remaining life judgment is to be performed.

[0031] More specifically, the step (a) can be carried out in several ways.

[0032] For example, the step (a) may include a step of installing an antenna near the epoxy-molded power equipment and a step of receiving an electromagnetic wave signal with the antenna, and may be a step of obtaining the φ-q distribution based on the intensity of the received electromagnetic wave signal.

[0033] Also, for example, the step (a) may include a step of measuring the current flowing through the ground wire of the epoxy-molded power equipment with a current sensor, and obtaining the φ-q distribution based on the intensity of the measured current.

[0034] Also, for example, the method may include a step of installing a transient ground voltage sensor on the surface of the housing of the epoxy molded power equipment and a step of measuring the transient ground voltage of the epoxy molded power equipment with the transient ground voltage sensor, and a step of obtaining the φ-q distribution based on the magnitude of the measured transient ground voltage. Effect of the Invention

[0035] According to the present invention, the remaining life of epoxy-molded electric power equipment can be determined by a simple operation, which makes it possible to identify, for example, epoxy-molded electric power equipment that is particularly in need of replacement. [Brief description of the drawings]

[0036] [Figure 1] 2 is a flowchart showing an example of a procedure of a remaining life determination method according to the present invention. [Diagram 2] 1 is a diagram illustrating an example of a method for acquiring an electromagnetic wave signal emitted from an epoxy molded power device. [Figure 3A] 1 is a diagram showing a schematic configuration of an experimental system. [Figure 3B] 1 is a schematic diagram showing a configuration of a test piece for generating creeping discharge. [Figure 3C] 1 is a schematic diagram showing a configuration of a test piece for generating partial discharge. [Figure 4A] 1 is a diagram showing an example of a spectrum decomposition by frequency of an electromagnetic wave signal received by an antenna in the case of Example 1-1. [Figure 4B] 1 is an example of a spectrum decomposition by frequency of an electromagnetic wave signal received by an antenna in the case of Example 1-2. [Figure 4C]1 is an example of a spectrum decomposition by frequency of an electromagnetic wave signal received by an antenna in the case of Example 1-3. [Figure 4D] 1 is an example of a spectrum decomposition of an electromagnetic wave signal received by an antenna according to frequency in the case of Example 1-4. [Figure 5A] 13 is an example of a display screen when an electromagnetic wave signal received by an antenna is displayed on an oscilloscope in the case of Example 2-1. [Figure 5B] 13 is an example of a display screen when an electromagnetic wave signal received by an antenna is displayed on an oscilloscope in the case of Example 2-2. [Figure 5C] 13 is an example of a display screen when an electromagnetic wave signal received by an antenna is displayed on an oscilloscope in the case of Example 2-3. [Figure 5D] 13 is an example of a display screen when an electromagnetic wave signal received by an antenna is displayed on an oscilloscope in the case of Example 2-4. [Figure 6A] 1 is a diagram illustrating an example of a method for measuring a current flowing through a ground wire of an epoxy-molded power device. [Figure 6B] 1 is a diagram illustrating an example of a method for measuring a transient ground voltage of an epoxy-molded power device. [Figure 7A] 1 is an example of a graph showing a φ-q distribution. [Figure 7B] 1 is an example of a graph showing a φ-q distribution. [Figure 8A] 1 is a diagram showing a schematic configuration of an experimental system. [Figure 8B] 1 is a schematic diagram showing a configuration of a test piece for generating partial discharge. [Figure 9A] This is an image of a test piece taken with a macroscope. [Figure 9B] This is an image of another test piece taken with a macroscope. [Figure 10A] 9B is a graph showing a φ-q distribution based on a current signal detected by a partial discharge analyzer at the time of imaging in FIG. 9A. [Figure 10B]9C is a graph showing a φ-q distribution based on a current signal detected by a partial discharge analyzer at the time of imaging in FIG. 9B. [Figure 11A] 10B is a diagram that illustrates a virtual figure obtained by smoothly connecting the outer edges of the region showing the surface shape of the φ-q distribution obtained in FIG. 10A. [Figure 11B] 10C is a diagram that illustrates a virtual figure obtained by smoothly connecting the outer edges of the region showing the surface shape of the φ-q distribution obtained in FIG. 10B. [Figure 12] 11 is a graph showing an example of the relationship between the variance value Var related to the discharge charge amount q obtained from the result of the φ-q distribution and the electrical tree length. [Figure 13] 1 is a diagram for explaining an example of a method for estimating an electrical tree growth rate based on electrical tree lengths derived at different timings; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] An embodiment of a method for determining remaining life of an electric power device associated with electrical tree degradation (hereinafter, abbreviated as "remaining life determination method") according to the present invention will be described with reference to the drawings as appropriate. Note that the dimensional ratios in the drawings do not necessarily match the actual dimensional ratios in each drawing.

[0038] 1 is a flow chart showing an example of a procedure of a remaining life determination method according to the present invention. In the following description, reference will be made to the symbols in the flow chart shown in FIG.

[0039] The target equipment for the remaining life judgment method according to the present invention is epoxy-molded electric power equipment. The epoxy-molded electric power equipment refers to electric power equipment including an insulator made of an insulating material containing epoxy resin as an insulating material. Examples of such electric power equipment include transformers, switches, circuit breakers, bushings, etc.

[0040] (Step #1: Pre-screening) Epoxy molded electric power equipment is used for the above-mentioned transformers, switches, circuit breakers, bushings, etc. These electric power equipment are installed at many sites, and in many cases, multiple pieces of equipment are installed at one site. For this reason, if all electric power equipment is subject to remaining life assessment, the number of target equipment may become enormous.

[0041] Therefore, in the flowchart shown in FIG. 1, screening is performed in advance to identify epoxy-molded electric power equipment for which the remaining life span is to be judged.

[0042] Specifically, the electromagnetic wave signal emitted from the epoxy molded power equipment is received, broken down into intensities according to frequency, and the signal intensities are analyzed. Figure 2 is a diagram showing a schematic diagram of an example of a method for performing step #1. In this example, a case is shown in which the epoxy molded power equipment to be pre-screened is a transformer 5 in a cubicle 3.

[0043] As shown in Fig. 2, antenna 11 is placed near the epoxy molded electric power device (here, transformer 5) to be screened. As will be described later, if transformer 5 is deteriorated, a predetermined electromagnetic wave signal is transmitted from transformer 5, and antenna 11 is placed at a position where this electromagnetic wave signal can be received. Specifically, antenna 11 is preferably placed within 3 m, and more preferably within 2 m, of the outer surface of transformer 5. In this case, antenna 11 may be fixed by a magnet or the like.

[0044] 2, when the target electric power device (transformer 5) is placed in a housing such as a cubicle 3, it is preferable to place the antenna 11 in the housing and then close the door 4. This reduces the reception sensitivity of the antenna 11 to the environmental radio waves present outside the cubicle 3, thereby suppressing erroneous diagnosis due to disturbances.

[0045] 2, the antenna 11 is connected to a waveform analysis unit 12, and a signal from the waveform analysis unit 12 is output to a computer 13. The electromagnetic wave signal received by the antenna 11 is input to the waveform analysis unit 12, and the intensity of the electromagnetic wave signal is resolved by frequency. As the waveform analysis unit 12, for example, an oscilloscope can be used.

[0046] Information regarding the strength of the electromagnetic signal by frequency (frequency spectrum) is output to computer 13. Computer 13 is provided with a judgment unit as a functional means. When the signal strength of a reference frequency band that is in the range of 100 MHz or more and less than 200 MHz and includes at least 150 MHz exceeds a predetermined threshold that exceeds the noise level, the judgment unit judges that there is a high possibility that an internal discharge is occurring in the target electric power equipment (transformer 5). In this case, the screened transformer 5 is determined to be a target for remaining life judgment (1A in FIG. 1). On the other hand, when the signal strength of the reference frequency band is equal to or less than the noise level, the transformer 5 is not determined to be a target for remaining life judgment (1B in FIG. 1).

[0047] Fig. 3A is a schematic diagram showing the configuration of the experimental system. A test piece 33 and an antenna 32 were placed on a test stand 31 placed in a test space 30, and an electromagnetic wave signal received by the antenna 32 was input to an oscilloscope 34 via a cable 36. The test piece 33 was grounded via a resistor of 1.1 kΩ. The antenna 32 was placed 1 m away from the test piece 33. A small mobile antenna was used as the antenna 32.

[0048] 3B is a schematic diagram showing the configuration of a test piece 33 for generating creeping discharge. The test piece 33 used was a 6 kV support insulator 61 with an AC voltage of 1.5 kV applied to the high-voltage pleat surface via a needle electrode 62.

[0049] FIG. 3C is a schematic diagram showing the configuration of a test piece 33 for generating partial discharge. A small piece 51 of epoxy resin (bisphenol A type, no filler) was used as the test piece 33, to which an AC voltage of 13 kV was applied via a needle electrode 53. A gap (void) of about 0.5 mm was provided at the tip of the needle electrode 53, and an aluminum electrode was placed on the ground side of the small piece 51 via a conductive paste 54, so that a voltage was applied uniformly in the planar direction to the test piece 33. The test piece 33 used had dimensions of 30 mm x 20 mm x 5 mm in length x width x depth. The distance from the tip of the needle electrode 53 to the ground side surface (bottom surface) of the test piece 33 was about 3 mm.

[0050] Figures 4A to 4C are graphs showing the results of receiving an electromagnetic wave signal transmitted from test piece 33 shown in Figure 3C by antenna 32 and spectrally decomposing it by frequency on oscilloscope 34. Also, Figure 4D is a graph showing the results of receiving an electromagnetic wave signal transmitted from test piece 33 shown in Figure 3B by antenna 32 and spectrally decomposing it by frequency on oscilloscope 34. When test piece 33 shown in Figure 3C was used, the spectrum at the beginning of voltage application corresponds to Figure 4C, and as the application time became longer, the waveform changed in the order of Figure 4B and Figure 4A.

[0051] According to Fig. 4C and Fig. 4D, the strength of the electromagnetic wave signal is not confirmed near 150 MHz. According to Fig. 4D, when a partial discharge does not occur but a creeping discharge occurs, a high strength of the electromagnetic wave signal is confirmed in a frequency band of less than 100 MHz. In contrast, in Fig. 4A and Fig. 4B, where a partial discharge occurs, a high strength of the electromagnetic wave signal is confirmed near 150 MHz, particularly in a frequency band of 100 MHz or more and less than 200 MHz (hereinafter referred to as the "reference frequency band"). Thus, when the computer 13 confirms that the electromagnetic wave signal includes at least 150 MHz and that the signal in the reference frequency band within the range of 100 MHz to less than 200 MHz exceeds a predetermined threshold exceeding the noise level, it can determine that there is a high possibility that an internal discharge is occurring in the transformer 5.

[0052] As a more detailed example, the signal in the reference frequency band may be determined to exceed a predetermined threshold value when the integrated intensity of the signal in the reference frequency band relative to the integrated intensity of the signal in the range of 0 MHz or more and 500 MHz or less exceeds 30%. As another example, the signal in the reference frequency band may be determined to exceed a predetermined threshold value when the peak intensity of the signal in the reference frequency band is 0.2 or more when normalized by the peak intensity of the signal in a frequency band lower than the reference frequency band (e.g., 0 MHz or more and less than 100 MHz).

[0053] 5A to 5D are examples of screens when electromagnetic wave signals transmitted from transformers 5 installed in different cubicles 3 are received by antenna 11 and displayed on the display screen of an oscilloscope serving as waveform analysis unit 12. Each figure also shows a spectrum waveform for each frequency. In each of Figs. 5A to 5D, the waveform displayed on the upper side is a voltage waveform, the horizontal axis corresponds to time, and the vertical axis corresponds to intensity. The waveform displayed on the lower side is a spectrum for each frequency, the horizontal axis corresponds to frequency, and the vertical axis corresponds to intensity. The horizontal axis corresponding to the voltage waveform has a scale of 100 ns, and the horizontal axis corresponding to the spectrum has a scale of 50 MHz.

[0054] Fig. 5A is a waveform of an electromagnetic signal transmitted from a transformer 5 (Example 2-1) in which an electrical tree is developing. Fig. 5B is a waveform of an electromagnetic signal transmitted from a transformer 5 (Example 2-2) in which an electrical tree is not developing but partial discharge is occurring. Fig. 5C is a waveform of an electromagnetic signal transmitted from a transformer 5 (Example 2-3) in which neither an electrical tree nor partial discharge is occurring. Fig. 5D is a waveform of an electromagnetic signal transmitted from a transformer 5 (Example 2-4) in which neither an electrical tree nor partial discharge is occurring but creeping discharge is occurring.

[0055] 5A and 5B, it is confirmed that, for the electromagnetic wave signal from a transformer in which partial discharge is occurring, the signal in the reference frequency band in the range of 100 MHz to less than 200 MHz, including at least 150 MHz, exceeds the noise level not only in the experimental system but also in the waveforms obtained from the actual transformer 5. On the other hand, for Figures 5C and 5D, which correspond to the waveforms of the electromagnetic wave signal from a transformer in which partial discharge is not occurring, it is confirmed that the intensity of the signal in the reference frequency band is below the noise level.

[0056] As an alternative screening method, a method of analyzing the waveform of the electromagnetic wave signal obtained by the waveform analysis unit 12 can be adopted. According to Figures 5A and 5B, it can be seen that the transformer 5 in which partial discharge occurs emits an electromagnetic wave signal showing a triangular voltage waveform whose rise time is less than 100 ns and which converges within 1 μs. In contrast, according to Figures 5C and 5D, the transformer 5 in which partial discharge does not occur does not emit an electromagnetic wave signal showing such a voltage waveform.

[0057] If the timing of the rising edge of the voltage waveform is synchronized with the phase of the voltage applied to the transformer 5, there is a high possibility that the electromagnetic wave signal received by the antenna 11 is a signal derived from a partial discharge occurring within the transformer 5. On the other hand, if the timing of the rising edge of the voltage waveform is unrelated to the phase of the voltage, it is considered to be due to a noise signal such as inverter noise.

[0058] Therefore, if the rise time of the voltage waveform of the electromagnetic wave signal obtained by the waveform analysis unit 12 is less than 100 nsec, and shows a shape (triangular shape) that converges within 1 μsec, and the voltage waveform of the electromagnetic wave signal is synchronized with the phase of the voltage applied to the transformer 5, it may be determined that there is a high possibility that partial discharge has occurred in the transformer 5, and this transformer 5 may be determined to be the target of determination (1A in FIG. 1). This waveform analysis process may be performed on the waveform analysis unit 12 side or on the computer 13 side.

[0059] The reason why the electromagnetic wave signal received by antenna 11 exhibits the triangular shape described above when a partial discharge occurs in transformer 5 is thought to be that a partial discharge is a phenomenon that lasts for several n seconds per discharge, and therefore the received electromagnetic wave signal also converges within a short period of time of less than 1 μsecond.

[0060] On the other hand, if the voltage waveform does not satisfy the above condition, the transformer 5 may be determined to be outside the scope of the judgment (1B in FIG. 1). However, even in this case, for safety reasons, it is acceptable to perform a strength analysis for each frequency of the electromagnetic wave signal and further judge whether or not the signal in the reference frequency band exceeds the noise level.

[0061] This step #1 corresponds to process (h).

[0062] However, in the present invention, step #1 is not an essential step. For example, it may be decided in advance to judge the remaining life of a certain epoxy-molded power device, in which case step #1 is unnecessary. Also, step #1 is unnecessary when the total number of epoxy-molded power devices that can be the subject of the test is small.

[0063] (Step #2: Obtaining the φ-q distribution) In step #1, if it is determined that the epoxy molded electric power equipment is an equipment for which the remaining life is to be determined (1A in FIG. 1), the received electromagnetic wave signal is analyzed into the intensity for each phase angle φ, and the discharge charge amount q according to the phase angle φ is derived to calculate the φ-q distribution (also called "q-φ distribution"). As mentioned above, step #1 can be omitted, and the process may start from step #2.

[0064] Specifically, the phase analysis is performed by recognizing the change over time in the voltage value indicated by the electromagnetic wave signal received by the antenna 11. Here, the frequency band to be subjected to the phase analysis may be a predetermined frequency band that is highly likely to be caused by partial discharge. As an example, as described above in step #1, the frequency band may be within the range of 100 MHz to 200 MHz. As another example, the frequency band may be within the range of several MHz to several tens of MHz (for example, 4 MHz to 40 MHz). According to Figs. 5A to 5D, in the case of Fig. 5C where no discharge occurs, no signal in the frequency band of several MHz to several tens of MHz is observed, whereas in Figs. 5A, 5B, and 5D where partial discharge or creeping discharge occurs, a signal in the frequency band of several MHz to several tens of MHz is observed. This point can also be understood from the verification results of Figs. 4A to 4D.

[0065] In step #2, instead of receiving the electromagnetic wave signal by the antenna 11, a current sensor 11a may be used to measure the ground current flowing through the ground wire 10a, as shown in FIG. 6A. A high-frequency CT may be used as the current sensor 11a. In this case, information about the ground current detected by the current sensor 11a is input to the waveform analysis unit 12. The information input to the waveform analysis unit 12 is processed in the same manner as the electromagnetic wave signal received by the antenna 11.

[0066] Furthermore, in step #2, instead of receiving the electromagnetic wave signal by the antenna 11, as shown in Fig. 6B, a transient ground voltage sensor 11b may be used to measure the transient ground voltage of the epoxy molded power equipment (here, the transformer 5) by using the transient ground voltage sensor 11b attached to the surface of the housing of the epoxy molded power equipment. In this case, information on the transient ground voltage detected by the transient ground voltage sensor 11b is input to the waveform analysis unit 12. The processing is similar to that for the electromagnetic wave signal received by the antenna 11.

[0067] It should be noted that the current sensor 11a or the transient ground voltage sensor 11b can also be used in step #1.

[0068] 7A and 7B are examples of diagrams showing φ-q distributions derived based on electromagnetic wave signals received from different power devices. A similar φ-q distribution can also be derived based on information about the ground current or transient ground voltage.

[0069] This step #2 corresponds to process (a).

[0070] (Step #3: Identifying the φ-q distribution shape) Next, the shape of the φ-q distribution obtained in step #2 is identified. Specifically, it is determined whether the shape of the φ-q distribution is a linear shape in which the spread in the q axis direction is dominant compared to the spread in the φ axis direction, or a planar shape that shows spread in both the φ axis direction and the q axis direction.

[0071] In the case of the example shown in Fig. 7A (instrument X1), the shape of the φ-q distribution is a planar shape that shows expansion in both the φ-axis direction and the q-axis direction, whereas in the case of the example shown in Fig. 7B (instrument X2), the shape of the φ-q distribution is a linear shape that is dominated by expansion in the q-axis direction compared to the expansion in the φ-axis direction.

[0072] As a method for determining whether the shape of the φ-q distribution is linear or planar, an operator may visually determine it, or an AI may automatically determine it. In the latter case, step #3 may be executed by the computer 13 shown in FIG. 2, FIG. 6A, or FIG. 6B, or may be executed by a server (not shown).

[0073] This step #3 corresponds to process (b).

[0074] (Step #4: Wait) If it is determined in step #3 that the shape of the φ-q distribution is linear (3A in step #3), then after waiting for a predetermined period of time, step #2 is executed again.

[0075] When the φ-q distribution is linear, it means that there is a tendency for a particularly high discharge charge q to be shown at a specific phase angle φ. In this case, it can be determined that the partial discharge has not yet reached the stage where an electrical tree has developed, but has only just formed pits, and it is not yet time to assess the remaining life and recommend replacing the equipment. On the other hand, because there is a high possibility that pits have formed, if the partial discharge continues to develop, it may lead to the formation and development of an electrical tree.

[0076] Therefore, the electric power device whose φ-q distribution shape is determined to be linear in step #3 can be determined to be a target device for follow-up observation. From this viewpoint, step #2 can be performed again after a predetermined period (for example, one or several years) has passed.

[0077] (Step #5: Further identify surface shape) On the other hand, if it is determined in step #3 that the shape of the φ-q distribution is planar (3B in step #3), the shape of the φ-q distribution is further identified.

[0078] Specifically, a virtual figure formed by a closed curve that smoothly connects the outer edges of an area representing a surface shape is determined to be a first shape in which a cutout is substantially absent, or a second shape in which a cutout is present.

[0079] This shape will be explained below based on data obtained using an experimental system.

[0080] 8A is a schematic diagram showing the configuration of an experimental system. A container 42 containing a plate-shaped test piece 33 was placed on a test stand 31 placed in a test space 40. Note that the test piece 33 was contained in the container 42 with insulating oil filled in the container 42 to prevent creeping discharge from occurring in the test piece 33.

[0081] The container 42 was made of transparent acrylic. The test piece 33 in the container 42 was observed and photographed using a macroscope 48 (Leica Z16APO, zoom range 0.57x to 9.2x) installed outside the container 42. The test piece 33 was placed on an insulating sheet 41.

[0082] A transformer 47 connected to a power supply Vcc was installed in the test space 40. A high voltage of 14 kV to 17 kV at 60 Hz was applied to the test piece 33 via the transformer 47. A ground wire 33a was connected to the test piece 33, and the current flowing through this ground wire 33a was detected by a high-frequency CT 49 (PRODYN I-125-1 HF, 120 kHz to 600 MHz).

[0083] The current signal detected by the high frequency CT 49 was detected by a partial discharge analyzer 44 (DAC-PD-9, 40 kHz to 40 MHz, manufactured by Soken Denki Co., Ltd.). The phase of the current signal detected by the partial discharge analyzer 44 was detected by a coupling capacitor 45 connected to the same node as the test piece 33 and a phase synchronizer 46 connected to this coupling capacitor 45. In other words, the waveform analysis unit 12 is simulated by the partial discharge analyzer 44, the coupling capacitor 45 and the phase synchronizer 46.

[0084] FIG. 8B is a diagram showing the configuration of a test piece 33 for generating partial discharge, and is substantially the same as FIG. 3C. In this verification, a small piece 51 of epoxy resin (bisphenol A type, no filler) was used as the test piece 33, to which an AC voltage of 14 to 17 kV was applied via a needle electrode 53. A gap (void) of about 0.5 mm was provided at the tip of the needle electrode 53, and an aluminum electrode was placed on the ground side of the small piece 51 via a conductive paste 54, so that a voltage was applied uniformly in the planar direction to the test piece 33. The test piece 33 used had dimensions of 30 mm x 20 mm x 5 mm in length x width x depth. The distance from the tip of the needle electrode 53 to the ground side surface (bottom surface) of the test piece 33 was about 3 mm.

[0085] 9A and 9B are photographs showing the results of an experiment conducted using the above-mentioned experimental system, in which a high voltage was continuously applied to different test pieces 33 to intentionally generate partial discharges, and the photographs were taken with a macroscope 38. An electrical tree Tre1 was confirmed in state α shown in FIG. 9A, and an electrical tree Tre2 was confirmed in state β shown in FIG. 9B.

[0086] Figures 10A and 10B are graphs showing the φ-q distribution based on the current signal detected by the partial discharge analyzer 44 at the time when the photographs of Figures 9A and 9B were taken, respectively. Also, Figures 11A and 11B are schematic drawings of imaginary figures smoothly connecting the outer edges of the regions showing the surface shapes of the φ-q distributions obtained in Figures 10A and 10B.

[0087] 10A and 10B, it is confirmed that the φ-q distributions both show a planar shape, as in Fig. 7A. Furthermore, when comparing the two, it is confirmed that the shapes of the regions showing the planar shape are clearly different.

[0088] In the case of the shape shown in Fig. 10A, the virtual figure Y1 that smoothly connects the outer edges of the area showing the plane shape is substantially free of a notch. This is also clear from the schematic drawing shown in Fig. 11A. On the other hand, in the case of the shape shown in Fig. 10B, the virtual figure Y2 that smoothly connects the outer edges of the area showing the plane shape has a notch Z2 that is clearly visible. This is also clear from the schematic drawing shown in Fig. 11B.

[0089] The shape of the region showing the surface shape when there is no substantial notch in the virtual figure (the shape shown in Fig. 10A and Fig. 11A) is referred to as the "first shape" for convenience. Also, the shape of the region showing the surface shape when there is a notch in the virtual figure (the shape shown in Fig. 10B and Fig. 11B) is referred to as the "second shape" for convenience.

[0090] As a method of determining whether the area showing the surface shape is the first shape or the second shape, an operator may visually determine it, or an AI may automatically determine it. The first shape is a mountain-like shape as shown in Figures 10A and 11A, while the second shape is a feather-like shape as shown in Figures 10B and 11B, so that automatic determination by an AI is possible by having the AI ​​learn such shapes. In the latter case, step #3 may be executed by the computer 13 shown in Figures 2, 6A, or 6B, or may be executed by a server not shown.

[0091] This step #5 corresponds to process (c).

[0092] (Step #6: Estimating Electrical Tree Length) If it is determined in step #5 that the region of the φ-q distribution indicating the surface shape is the first shape (5A in step #5), an estimation process of the electrical tree length is performed.

[0093] Specifically, a variance value Var related to the discharge charge q is calculated from the φ-q distribution obtained in step #2. Then, an estimated value of the electrical tree length occurring in the epoxy molded power device (here, the transformer 5) is derived based on this variance value Var.

[0094] Using the same experimental system as in Figures 8A and 8B, a high voltage was continuously applied to the same test piece 33 to intentionally generate partial discharge, and the state of change over time of the test piece 33 was photographed by a macroscope 38, and the electrical tree length was measured from the photograph. In addition, the variance value Var related to the discharge charge amount q was calculated from the result of the φ-q distribution at the time of photographing. The correspondence relationship between the electrical tree length and the variance value Var related to the discharge charge amount q is shown in Table 1 and Figure 12.

[0095] [Table 1]

[0096] According to the results of Table 1 and FIG. 12, it can be seen that as the tree length of the electrical tree grows, the value of the variance value Var related to the discharge charge amount q increases.

[0097] There are two types of electrical trees that can occur in epoxy-molded power equipment: non-conductive trees and conductive trees. A non-conductive tree is an electrical tree that occurs with insulation between its root and tip, in which case a potential difference occurs between its root and tip. On the other hand, a conductive tree is an electrical tree that occurs with conductivity, in which case the root and tip are at approximately the same potential.

[0098] In other words, in a non-conductive tree, when a high voltage exceeding the withstand voltage occurs between the root and tip of the tree, a discharge occurs throughout the inside of the tree, so the region in which the φ-q distribution shows a planar shape generally shows a smooth shape, and no peculiar region is created. In other words, a virtual figure formed by a closed curve that smoothly connects the outer edges of the region showing the planar shape tends to be a smooth figure (first shape).

[0099] On the other hand, in a conductive tree, the root of the tree and the tip of the tree are almost at the same potential, so that the electric field concentrates at the tip of the tree and exceeds the withstand voltage, causing a discharge to occur at the tip of the tree and propagating. Therefore, depending on the extent of the tree propagation, a specific region is likely to be created in the region where the φ-q distribution shows a planar shape. In other words, a notch is likely to be created in a virtual figure formed by a closed curve that smoothly connects the outer edges of the region showing the planar shape (second shape).

[0100] In other words, if the region of the φ-q distribution showing the surface shape is determined to be the first shape in step #5, it can be estimated that a non-conductive electrical tree has occurred in the target epoxy molded electric power equipment (transformer 5). When a non-conductive electrical tree develops, partial discharges tend to occur not only at the tip of the electrical tree but also at the root and intermediate points, and the number of types of distance between the high voltage side and the ground side increases. As a result, the intensity of the discharge varies, and the value of the variance value Var related to the discharge charge amount q becomes significantly large.

[0101] In other words, when a non-conductive electrical tree occurs, there is a correlation between the variance value Var related to the discharge charge q and the electrical tree length, so the electrical tree length can be estimated based on the variance value Var. This is also shown in the results of Table 1 and Figure 12.

[0102] For example, by measuring the relationship between the variance value Var related to the discharge charge amount q and the electrical tree length in advance in an experimental system and recording information on the calibration curves of both in computer 13, it is possible to obtain an estimate of the tree length of the electrical tree occurring in transformer 5 based on the variance value Var actually derived from the φ-q distribution.

[0103] This step #6 corresponds to process (d).

[0104] (Step #7: Calculate the electrical tree growth rate) Next, by repeating step #7 multiple times at predetermined intervals, the state of change over time of the estimated value of the electrical tree length can be obtained. Based on this data, the propagation speed of the electrical tree is calculated.

[0105] 13, when an estimated value of the electrical tree length derived on a certain date D1 is L1 and an estimated value of the electrical tree length derived on a date D2 D days after D1 is L2, the electrical tree propagation speed v can be calculated as v=(L2-L1) / D. However, the electrical tree propagation speed v may be derived based on estimated values ​​of the electrical tree length at three different points in time.

[0106] This step #7 corresponds to process (e).

[0107] (Steps #8 and #9: Estimating remaining life) Based on the estimated value of the propagation speed of the electrical tree calculated in step #7, the remaining life of the transformer 5 is estimated (step #8). Since the initial dimensions (thickness) of the epoxy resin contained in the epoxy-molded power equipment (transformer 5) are usually known, it is possible to predict how many days it will take for the epoxy-molded power equipment to reach insulation breakdown based on the propagation speed of the electrical tree calculated in step #7, in other words, to predict the remaining life.

[0108] The standard for determining the remaining life to be zero does not necessarily have to be the time when dielectric breakdown occurs, but may be, for example, the time when the thickness of the epoxy resin is equal to or less than a predetermined ratio to the initial thickness of the epoxy resin. The predetermined ratio here may be, for example, a value within the range of 5% to 25%.

[0109] On the other hand, if the region of the φ-q distribution showing the planar shape is determined to be the second shape in step #5 (5B in step #5), in other words, if it is estimated that a conductive electrical tree has occurred in the target epoxy molded power equipment (transformer 5), it is deemed that the life of transformer 5 has arrived (step #9).

[0110] As described above, when the electrical tree is conductive, the discharge progresses near the tip of the electrical tree, and therefore, unlike the case of a non-conductive electrical tree, no significant correlation is found between the electrical tree length and the variance value Var related to the discharge charge q. Therefore, the life of the transformer 5 cannot be estimated from information on the variance value Var related to the discharge charge q.

[0111] From this perspective, and taking safety into consideration, if there is a high possibility that conductive electrical trees have occurred in the transformer 5, it is assumed that the transformer has reached the end of its life, and prompt replacement of the equipment is encouraged.

[0112] Step #8 corresponds to process (f), and step #9 corresponds to process (h).

[0113] The present invention is not limited to the above-described embodiment, and includes various modifications. For example, the above-described embodiment has been described in detail for a better understanding of the present invention, and is not necessarily limited to the embodiment having all the configurations described. The scope of the present invention is indicated by the claims, and it is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0114] 3: Cubicle 4: Opening and closing door 5: Transformer 10a: Ground wire 11: Antenna 11a: Current sensor 11b: Transient ground voltage sensor 12: Waveform analysis section 13: Computer 30: Test space 31: Test stand 32: Antenna 33: Test piece 33a: Ground wire 34: Oscilloscope 36: Cable 38: Macroscope 40: Test space 41: Insulation sheet 42: Container 44: Partial Discharge Analyzer 45: Coupling capacitor 46 :Phase synchronizer 47: Transformer 48: Macroscope 51 :Small piece 53: Needle electrode 54: Conductive paste 61: Support insulator 62: Needle electrode Y1: Virtual shape Y2: Virtual shape Z2: Notch

Claims

1. A step (a) of detecting an electrical signal related to partial discharge generated in an epoxy-molded electric power device to obtain a φ-q distribution, which is a relationship between an applied voltage phase angle φ and a discharge charge amount q; a step (b) of determining whether the shape of the φ-q distribution obtained in the step (a) is a linear shape in which the spread in the q-axis direction is dominant compared to the spread in the φ-axis direction, or a planar shape showing spread in both the φ-axis direction and the q-axis direction; a step (c) of determining whether the shape of the region representing the surface shape is a first shape in which a notch is substantially not present in a virtual figure formed by a closed curve smoothly connecting an outer edge of the region representing the surface shape, or a second shape in which a notch is present; (d) calculating a variance value Var relating to the discharge charge amount q from the φ-q distribution when the region showing the surface shape is the first shape, and calculating an estimated value of an electrical tree length occurring in the epoxy molded electric power device based on the variance value Var; (e) calculating an estimate of the propagation speed of the electrical tree in the epoxy molded electric power device based on a combination of the length of the predetermined period and the estimated value of the electrical tree length obtained by performing the step (d) multiple times with an interval of one day or more; A method for determining the remaining life of electric power equipment due to electrical tree degradation, comprising: a step (f) of calculating an estimated value of the remaining life of the epoxy molded electric power equipment based on the estimated value of the propagation rate of the electrical tree calculated in the step (e).

2. 2. The method for determining the remaining life of an electric power device due to electrical tree degradation as described in claim 1, further comprising a step (g) of determining that the life of the epoxy molded electric power device has arrived when the area showing the surface shape in step (c) is the second shape.

3. 3. A method for determining the remaining life of electric power equipment due to electrical tree degradation as described in claim 1 or 2, characterized in that if it is determined in step (b) that the shape of the φ-q distribution is linear, it is determined that the epoxy molded electric power equipment is subject to follow-up observation, and step (a) is performed again after a predetermined waiting period has elapsed.

4. a step (h) of determining whether the epoxy-molded electric power equipment is a remaining life judgment target before the step (a); The step (h) is a step of receiving an electromagnetic wave signal emitted from the epoxy-molded electric power equipment, breaking it down into intensities according to frequency, and determining that the epoxy-molded electric power equipment is to be subjected to remaining life determination when the signal intensity of a reference frequency band within a range of 100 MHz or more and less than 200 MHz and including at least 150 MHz exceeds a predetermined threshold value exceeding a noise level; 3. A method for determining remaining life of electric power equipment due to electrical tree degradation as described in claim 1 or 2, characterized in that the step (a) is performed on the epoxy-molded electric power equipment determined to be subject to remaining life determination in the step (h).

5. The method for determining the remaining life of electric power equipment due to electrical tree degradation as described in claim 1 or 2, characterized in that the step (a) includes a step of installing an antenna in the vicinity of the epoxy molded electric power equipment and a step of receiving an electromagnetic wave signal with the antenna, and is a step of obtaining the φ-q distribution based on the intensity of the received electromagnetic wave signal.

6. 3. A method for determining the remaining life of electric power equipment due to electrical tree degradation as described in claim 1 or 2, characterized in that the step (a) includes a step of measuring the current flowing through the ground wire of the epoxy-molded electric power equipment with a current sensor, and obtaining the φ-q distribution based on the intensity of the measured current.

7. A method for determining the remaining life of electric power equipment due to electrical tree degradation as described in claim 1 or 2, characterized in that step (a) includes a step of installing a transient ground voltage sensor on the surface of the housing of the epoxy molded electric power equipment and a step of receiving an electromagnetic wave signal with the transient ground voltage sensor, and a step of obtaining the φ-q distribution based on the intensity of the received electromagnetic wave signal.

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