Black box model-based medium-voltage distribution cable latent fault modeling method and system

Abstract

The invention belongs to the field of power distribution networks, and provides a medium-voltage distribution cable latent fault modeling method and system based on a black box model. Wherein it is assumed that the arc surface temperature is constant and the temperature change amplitude of the cable insulation layer in the cable latent fault duration is smaller than a set threshold value, and based on the initial temperature distribution of the cable insulation layer and arc parameters, an expression of initial arc outward conduction heat flow is obtained; according to the lumped steady-state thermal circuit model of the medium-voltage distribution cable, correcting the expression of the initial arc external conduction heat flow as the arc external solid conduction power, taking the arc external solid conduction heat flow as the arc black box model loss power, and calculating the arc dynamic resistance based on the arc black box model; the arc dynamic resistor and the constant resistor are connected in series, and a medium-voltage distribution cable latent fault model is constructed; the constant resistance represents the part without arc discharge in the water tree channel and the electric tree channel; the arc dynamic resistance explains the zero break phenomenon occurring in the latent fault of the cable.

Description

Technical Field

[0001] The present invention belongs to the field of power distribution networks, and in particular relates to a method and system for modeling latent faults of medium-voltage distribution cables based on a black box model. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] As a key equipment for power distribution, power cables are widely used in urban power distribution networks. At present, under the combined effects of many factors such as cable design and production, actual operation and maintenance, and operating environment, cable failures occur from time to time. When a cable fails, it is often accompanied by explosions, burning, and thick smoke. Once such a failure occurs, it will cause irreversible and serious damage to the main insulation of the cable, and will seriously affect the safe operation of the power grid, causing major economic losses and adverse social impacts. It can be said that cables have become one of the safety hazards in the actual operation of the power grid. If external force damage factors are ignored, more than 97% of cable failures are insulation failures, and the locations of occurrence are mainly concentrated in cable accessories such as connectors and terminals in the cable and the cable body. Before a serious cable failure occurs, due to the decline in insulation capacity, some latent faults are inevitable in the operation of the cable line. The cable latent fault is an instantaneous, self-recovering grounding fault that occurs at the same position of the cable before a permanent fault. This type of fault has a short duration (1 / 4 to 4 power frequency cycles) and a small fault current. Traditional protection devices cannot be activated. These latent faults have little effect on power supply, but can play a certain role in predicting serious cable failures. In recent years, there have been many studies on the identification, location and prediction of cable faults based on latent fault characteristics. However, since latent faults are difficult to detect and their on-site samples are extremely limited, many studies based on latent cable faults need to be carried out with the help of simulation, and there is an urgent need to fit the cable latent fault model of the real fault situation.

[0004] The mainstream view today is that latent faults are mainly arc faults caused by the aging of the insulation materials of the cable body insulation layer and the cable connector. The aging of the cable insulation material generally has a development process from quantitative change to qualitative change. Due to the influence of various factors in the manufacturing process, handling, laying and other processes, there are insulation defects such as bubbles, micropores, and cracks in the insulation materials of the cables and connectors. In the process of production and operation, the insulation defects of the cables are affected by electromagnetic, mechanical external forces, operating heat and chemical corrosion, and the insulation strength of the insulation materials continues to decline. And in this decline process, electrical trees, water trees and partial discharge phenomena are very likely to occur at the defects. When electrical trees, water trees and partial discharge occur in the insulation, the physical conditions such as light and heat generated at the defects will further accelerate the aging of the insulation. When aging to a certain extent, an electrical channel will be formed in the cable insulation layer, resulting in latent faults. At present, in the identification and location algorithms of latent faults in medium-voltage distribution cables, the latent fault models of medium-voltage distribution cables used are mostly Mayr models based on energy balance and their improved models, including cybernetics models, Kizilcay models, etc. The above models do not focus on the complex physical process inside the arc, but regard the arc as a "black box". After assuming or speculating some internal characteristic parameters of the arc, they start from studying the external characteristics of the arc and give the dynamic conductivity of the arc. Therefore, they are also called "black box" models. These models are mainly derived from the characteristics of long arcs of high-resistance faults in open environments such as air, which are quite different from the environment where latent faults in medium-voltage distribution cables occur. Summary of the invention

[0005] In view of the problem that the latent fault mechanism of medium-voltage distribution cables is missing in the application of black box models, the present invention provides a method and system for modeling latent faults of medium-voltage distribution cables based on a black box model, which can fit real fault situations.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A first aspect of the present invention provides a method for modeling latent faults of medium voltage distribution cables based on a black box model.

[0008] In one or more embodiments, a method for modeling latent faults of medium voltage distribution cables based on a black box model is provided, comprising:

[0009] Assuming that the arc surface temperature is constant and the temperature change of the cable insulation layer during the duration of the cable latent fault is less than the set threshold, the expression of the initial arc heat flow to the outside is obtained based on the initial temperature distribution of the cable insulation layer and the arc parameters;

[0010] According to the lumped steady-state thermal circuit model of medium-voltage distribution cables, the expression of the initial arc conduction heat flux is corrected to the arc's external solid conduction power, and the arc's external solid conduction heat flux is used as the arc black box model loss power, and then the arc dynamic resistance is calculated based on the arc black box model.

[0011] The arc dynamic resistance and constant resistance are connected in series to construct a latent fault model of medium-voltage distribution cables. The constant resistance is used to characterize the part of the water tree and electric tree channels where no arc discharge occurs. The arc dynamic resistance is used to explain the zero-break phenomenon that occurs in latent cable faults.

[0012] As an implementation method, the latent fault model of medium voltage distribution cable is expressed as:

[0013]

[0014] Where R is the latent fault model of medium voltage distribution cable; R s is a constant resistance; is the dynamic resistance of the arc; u is the voltage at both ends of the arc; i is the current flowing through the arc channel; φ is the heat flow conducted by the arc to the outside; τ is the arc time constant.

[0015] As an implementation method, based on the cable core loss φ1, insulation layer loss φ2, outer shielding layer and outer sheath loss φ3, and external environment temperature T e , the thermal resistances R1 and R2 of the insulating layer, the outer semiconductor layer and the external environment, and the corrected arc heat flux φ: Among them, T arc is the arc surface temperature; r is the arc axis radius; λ is the thermal conductivity; the arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L.

[0016] As an implementation method, the expression of the initial arc heat flux conducted to the outside is:

[0017]

[0018] The arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. After the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at T arc ; λ is the thermal conductivity.

[0019] A second aspect of the present invention provides a medium voltage distribution cable latent fault modeling system based on a black box model.

[0020] In one or more embodiments, a medium voltage distribution cable latent fault modeling system based on a black box model includes:

[0021] The module for characterizing the heat flux conducted externally by the initial arc is used to obtain the expression of the heat flux conducted externally by the initial arc based on the initial temperature distribution of the cable insulation layer and the arc parameters, assuming that the arc surface temperature is constant and the temperature change amplitude of the cable insulation layer during the duration of the cable latent fault is less than the set threshold.

[0022] The arc dynamic resistance calculation module is used to correct the expression of the initial arc external conduction heat flow to the solid conduction power of the arc according to the lumped steady-state thermal circuit model of the medium-voltage distribution cable, and use the solid conduction heat flow of the arc to the outside as the loss power of the arc black box model, and then calculate the arc dynamic resistance based on the arc black box model;

[0023] The cable latent fault model construction module is used to connect the arc dynamic resistance and the constant resistance in series to construct a latent fault model of the medium-voltage distribution cable; among them, the constant resistance is used to characterize the part of the water tree and electric tree channel where no arc discharge occurs; the arc dynamic resistance is used to explain the zero-break phenomenon that occurs in the cable latent fault.

[0024] As an implementation mode, in the cable latent fault model construction module, the expression of the medium voltage distribution cable latent fault model is:

[0025]

[0026] Where R is the latent fault model of medium voltage distribution cable; R s is a constant resistance; is the dynamic resistance of the arc; u is the voltage at both ends of the arc; i is the current flowing through the arc channel; φ is the heat flow conducted by the arc to the outside; τ is the arc time constant.

[0027] As an implementation mode, in the arc dynamic resistance calculation module, based on the cable core loss φ1, insulation layer loss φ2, outer shielding layer and outer sheath loss φ3, external environment temperature T e , the thermal resistances R1 and R2 of the insulating layer, the outer semiconductor layer and the external environment, and the corrected arc heat flux φ: Among them, T arc is the arc surface temperature; r is the arc axis radius; λ is the thermal conductivity; the arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L.

[0028] As an implementation manner, in the initial arc external conduction heat flux characterization module, the expression of the initial arc external conduction heat flux is:

[0029]

[0030] The arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. After the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at T arc ; λ is the thermal conductivity.

[0031] A third aspect of the present invention provides a computer-readable storage medium.

[0032] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps in the method for modeling latent faults of medium-voltage distribution cables based on a black box model as described above.

[0033] A fourth aspect of the present invention provides an electronic device.

[0034] An electronic device comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, the steps in the method for modeling latent faults of medium-voltage distribution cables based on a black box model as described above are implemented.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention is based on the evolution mechanism of cable latent faults and decomposes the cable latent fault model into constant resistance and arc dynamic resistance. Considering that the cable latent fault is first characterized as a high-resistance fault, a constant resistance is used to characterize the part of the water tree and electric tree channels where no arc discharge occurs, so that the volt-ampere characteristic curve is linear as a whole, which better describes the breakdown process of the solid dielectric of the cable insulation layer. The arc dynamic resistance is used to explain the zero-break phenomenon occurring in the cable latent fault. The constructed medium-voltage distribution cable latent fault model can fit the real fault situation and is used for the subsequent identification and positioning of the medium-voltage distribution cable latent fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0038] Figure 1 It is a flow chart of a method for modeling latent faults of medium-voltage distribution cables based on a black box model according to an embodiment of the present invention;

[0039] Figure 2 Schematic diagram of arc black box model principle in an embodiment of the present invention;

[0040] Figure 3 is a schematic diagram of the structure of a medium voltage distribution cable used in an embodiment of the present invention;

[0041] Figure 4 It is a lumped steady-state thermal circuit model of a medium-voltage distribution cable proposed in an embodiment of the present invention;

[0042] Figure 5 is a volt-ampere characteristic curve diagram of arc dynamic conductance according to an embodiment of the present invention;

[0043] Figure 6 It is a medium voltage distribution network model containing latent cable faults constructed by simulation in an embodiment of the present invention;

[0044] FIG. 7( a ) is a waveform of a potential fault voltage of an under-cycle cable obtained by simulation according to an embodiment of the present invention;

[0045] FIG. 7( b ) is a waveform of a latent under-cycle cable fault current obtained by simulation according to an embodiment of the present invention;

[0046] FIG. 7( c ) is a waveform of the latent dynamic resistance of an under-cycle cable obtained by simulation according to an embodiment of the present invention;

[0047] FIG8( a ) is a multi-cycle cable latent fault voltage waveform obtained by simulation according to an embodiment of the present invention;

[0048] FIG8( b ) is a multi-cycle cable latent fault current waveform obtained by simulation according to an embodiment of the present invention;

[0049] FIG8( c ) is a waveform of latent dynamic resistance of a multi-cycle cable obtained by simulation according to an embodiment of the present invention;

[0050] FIG9( a ) is a diagram of cable latent fault current under different arc time constants obtained by simulation according to an embodiment of the present invention;

[0051] FIG9( b ) is a partial enlarged diagram of the cable latent fault current under different arc time constants obtained by simulation according to an embodiment of the present invention;

[0052] Fig.10 A latent fault current diagram of a cable under different externally conducted heat flows obtained by simulation in an embodiment of the present invention.

[0053] Fig.11 It is a schematic diagram of the structure of a medium-voltage distribution cable latent fault modeling system based on a black box model according to an embodiment of the present invention. DETAILED DESCRIPTION

[0054] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0055] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0056] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0057] Figure 1 is a flow chart of a method for modeling latent faults of medium voltage distribution cables based on a black box model in an embodiment of the present invention, such as Figure 1 The method for modeling latent faults of medium voltage distribution cables based on a black box model in the embodiment shown may include:

[0058] S101, assuming that the arc surface temperature is constant and the temperature change amplitude of the cable insulation layer during the duration of the cable latent fault is less than a set threshold, based on the initial temperature distribution of the cable insulation layer and the arc parameters, an expression for the initial arc heat flow conducted to the outside is obtained.

[0059] Consider the arc to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. Considering that there are energy losses in the conductor, insulation layer, and semiconductor layer of the medium-voltage distribution cable, and there are certain differences in the temperature at different positions of the cable, the penetrating arc conducts heat flux φ0 to the outside as shown in the following formula:

[0060]

[0061] Since the duration of the cable latent fault is short, only 1 / 4 to 4 power frequency cycles, it can be considered that the temperature change of the cable insulation layer is weak during the duration of the cable latent fault. Assume that after the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at T arc , then the expression of the initial arc heat flux conducted to the outside is:

[0062]

[0063] The arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. After the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at Tarc ; λ is the thermal conductivity.

[0064] Arc surface temperature T arc Ratio to radius It can be given in advance through experiments and other means, and the heat flux conducted by the arc to the outside can be obtained by calculating the initial temperature distribution T0(l) of the cable insulation layer.

[0065] S102, based on the lumped steady-state thermal circuit model of the medium-voltage distribution cable, the expression of the initial arc external conduction heat flux is corrected to the arc external solid conduction power, and the arc external solid conduction heat flux is used as the arc black box model loss power, and then the arc dynamic resistance is calculated based on the arc black box model.

[0066] In the embodiment of the present invention, the black box model regards the arc as a cylindrical gas channel based on energy balance. The arc is ignited when the input energy of the gas channel is greater than the output energy, and the arc is extinguished when the input energy is less than the output energy. Figure 2 The arc model shown, its energy changes are as follows:

[0067]

[0068] In the formula, Q is the internal energy of the arc, u is the voltage at both ends of the arc, i is the current flowing through the arc channel, and p is the power loss, that is, the power released by the arc through conduction, convection, and radiation. The energy change of the arc maps the current carrying capacity of the arc gap, and the arc conductance g can be expressed as a function of the internal energy Q of the arc, that is, g = F (Q), and the two ends are differentiated to obtain:

[0069]

[0070] Combining formula (3) and formula (4), we can get:

[0071]

[0072] By introducing the arc time constant The above formula can be further simplified as:

[0073]

[0074] Formula (6) is the original formula of the black box model. Arc models such as the Mayr model and the cybernetics model are all based on this formula. By introducing restrictions on the arc environment, the arc time constant τ and the loss power p are explained and corrected.

[0075] In this embodiment, considering that many black box models such as Mayr model, Cassie model, and cybernetics model are all proposed for long gap and large arc scenarios in gas circuit breaker environments, the arc occurrence environment is quite different from the latent fault of medium voltage distribution cables. In a gas environment, the arc power loss mainly comes from the convection between the arc and the arc blowing gas, while the cable latent fault occurs in the cable insulation layer, and the power loss is mainly due to the heat conduction of the solid medium. The heat conduction of the solid medium is different from the gas convection, and the arc power loss is related to the temperature gradient.

[0076] For Figure 3 The lumped steady-state thermal circuit model of the medium voltage distribution cable model shown in Figure 4 As shown in the figure, φ1 represents the cable core loss, φ2 represents the insulation layer loss, φ3 represents the outer shielding layer and outer sheath loss, T e represents the external environment temperature, T1 represents the surface temperature of the cable core conductor, T2 represents the surface temperature of the insulation layer, R1 and R2 represent the thermal resistance of the insulation layer, the outer semiconductor layer and the external environment respectively.

[0077] Thermal circuit calculations are very similar to circuit calculations. Figure 4 Based on Kirchhoff's law, the following equation can be obtained:

[0078]

[0079] By solving the above, the cable core conductor surface temperature T1 and the insulation layer surface temperature T2 are calculated, and the expression of T0(l) can be obtained as follows:

[0080] Substituting formula (8) into formula (2), we can obtain the corrected arc external heat flux φ:

[0081]

[0082] The power loss of latent fault in medium-voltage distribution cable is mainly composed of the solid conduction power of the arc to the outside, which can be expressed by the corrected heat flow of the arc to the outside.

[0083] At this point, the power loss of the improved and corrected black box model based on the cable thermal circuit model is completed.

[0084] S103, the arc dynamic resistance and the constant resistance are connected in series to construct a latent fault model of medium-voltage distribution cables; wherein the constant resistance is used to characterize the part of the water tree and electric tree channel where no arc discharge occurs; and the arc dynamic resistance is used to explain the zero-break phenomenon occurring in the latent fault of the cable.

[0085] One of the difficulties in applying the black box model is that it is difficult to determine the fault parameters of the arc, especially for the explanation of the mechanism. The improvement of the power loss in the arc black box model disclosed in this disclosure is essentially an explanation and prior confirmation of the arc fault parameters. The arc waveform established by formula (9) and formula (6) is still consistent with the Mayr model in form and has a similar volt-ampere characteristic curve. The volt-ampere characteristic curve of the arc model is as follows: Figure 5 As shown, it can be seen that the change of fault current in the black box model based on energy balance theory lags far behind the change of fault voltage, and has the dynamic conductivity characteristics of a typical arc fault.

[0086] Considering that the latent cable fault is first characterized by a high-resistance fault, the volt-ampere characteristic curve is linear as a whole. By introducing a constant resistance R s Characterize the part of the water tree and electric tree channel where arc discharge does not occur, and better describe the breakdown process of the solid dielectric of the cable insulation layer. The arc resistance is obtained by solving equation (6) and a constant resistance R is connected in series. s , the latent fault model of medium voltage distribution cable established by the present invention is shown in formula (10):

[0087]

[0088] Where R is the latent fault model of medium voltage distribution cable; R s is a constant resistance; is the arc dynamic resistance; u is the voltage across the arc; i is the current flowing through the arc channel; φ is the heat flow conducted by the arc to the outside, determined by formula (9); τ is the arc time constant.

[0089] Adjust the constant resistance R by formula (9) s The latent fault waveform of the medium voltage distribution cable is fitted with the arc time constant τ. The arc time constant τ can be calculated at a given constant resistance value R s After that, the rate of change of dynamic arc conductivity over time is solved. Thus, a latent fault model of medium voltage distribution cable based on the improved black box model is established.

[0090] The cable latent fault model disclosed in this paper is simulated in the Matlab / Simulink environment, and the following Figure 6 The neutral point is grounded by a small resistance distribution network model shown in the figure. The positive sequence impedance Z1 of the 10km cable line is 0.27+j0.08Ω / km, the zero sequence impedance Z0 is 2.7+j0.35Ω / km, the positive sequence admittance b1 is j118.12μS / km, and the zero sequence admittance b0 is j86.71μS / km. The fault parameters of the latent fault model are set, and the constant resistance R sThe arc time constant τ is 500Ω, the arc time constant τ is 0.6ms, the external conduction heat flux φ is 500W, and the cable latent fault is set to occur at 4km of the cable line. The fault occurs at the peak of the operating voltage. The fault lasts for 1 / 4 power frequency cycle (under-cycle latent fault) and 4 power frequency cycles (multi-cycle latent fault). In the simulation, the latent fault is set to occur at 0.02s, the under-cycle latent fault is restored at 0.025s, and the multi-cycle latent fault is restored at 0.10s. The fault waveforms are as follows: Figure 7(a)-Figure 7(c) and Figure 8(a)-Figure 8(c) As shown. It can be seen that this model conforms to the characteristics of the cable latent fault. First, it characterizes the characteristics of the cable high-resistance fault. The fault voltage changes slightly. The fault voltage and fault current both present a sinusoidal waveform. The dynamic resistance characteristics are good. When the current is near zero, the arc gap current capacity is weakened and the resistance increases. When the current increases, the arc gap current capacity is weakened and the resistance decreases.

[0091] By adjusting the latent fault parameters, the fault waveform differences are analyzed, and the arc time constant τ and the external conduction heat flow φ are adjusted respectively. The fault currents are as follows: Figure 9(a)-Figure 9(b) and Fig.10 As shown. Figure 9(a)-Figure 9(b) It can be seen that the arc time constant τ has a very weak effect on the fault current waveform. When the arc time constant τ is small, the zero-break phenomenon of the fault current waveform is aggravated. Fig.10 It can be seen that with the increase of the external conduction heat flux φ, the arc zero-break phenomenon becomes more obvious, and the fault current amplitude has a slight downward trend. For the latent fault model of medium-voltage distribution cables based on the improved black box model proposed in this disclosure, the fault current amplitude is mainly affected by the constant resistance R s The influence is determined. The arc time constant τ and the external conduction heat flux φ have a certain influence on the zero-break phenomenon of the fault current.

[0092] The present invention uses arc dynamic resistance to explain the zero-break phenomenon occurring in latent cable faults, and derives a "black box" model by analyzing the energy balance principle; based on the cable steady-state thermal circuit model, the "black box" model is corrected to obtain the power loss, and then the arc dynamic resistance is calculated; based on the evolution mechanism of latent cable faults, the cable latent fault model is decomposed into a constant resistance and an arc dynamic resistance, and the two resistances are connected in series to form a latent fault model of a medium-voltage distribution cable of the improved black box model disclosed in the present invention, and the latent fault model of a medium-voltage distribution cable of the improved black box model is simulated based on matlab / Simulink simulation.

[0093] Fig.11 Schematic diagram of the structure of a medium voltage distribution cable latent fault modeling system based on a black box model in an embodiment of the present invention. Figure 1 This corresponds to the modeling method of latent faults of medium voltage distribution cables based on black box model, such as Fig.11As shown, the medium voltage distribution cable latent fault modeling system based on the black box model in this embodiment may include:

[0094] An initial arc external conduction heat flux characterization module 111 is used to obtain an expression for the initial arc external conduction heat flux based on the initial temperature distribution of the cable insulation layer and arc parameters, assuming that the arc surface temperature is constant and the temperature change amplitude of the cable insulation layer during the duration of the cable latent fault is less than a set threshold;

[0095] The arc dynamic resistance calculation module 112 is used to correct the expression of the initial arc external conduction heat flow to the solid conduction power of the arc according to the lumped steady-state thermal circuit model of the medium-voltage distribution cable, and use the solid conduction heat flow of the arc to the outside as the loss power of the arc black box model, and then calculate the arc dynamic resistance based on the arc black box model;

[0096] The cable latent fault model construction module 113 is used to connect the arc dynamic resistance and the constant resistance in series to construct a medium-voltage distribution cable latent fault model; wherein the constant resistance is used to characterize the part of the water tree and electric tree channel where no arc discharge occurs; the arc dynamic resistance is used to explain the zero-break phenomenon occurring in the cable latent fault.

[0097] Specifically, in the cable latent fault model building module 113, the expression of the medium voltage distribution cable latent fault model is:

[0098]

[0099] Where R is the latent fault model of medium voltage distribution cable; R s is a constant resistance; is the dynamic resistance of the arc; u is the voltage at both ends of the arc; i is the current flowing through the arc channel; φ is the heat flow conducted by the arc to the outside; τ is the arc time constant.

[0100] In the arc dynamic resistance calculation module 112, based on the cable core loss φ1, insulation layer loss φ2, outer shielding layer and outer sheath loss φ3, and external environment temperature T e , the thermal resistances R1 and R2 of the insulating layer, the outer semiconductor layer and the external environment, and the corrected arc heat flux φ: Among them, T arc is the arc surface temperature; r is the arc axis radius; λ is the thermal conductivity; the arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L.

[0101] In the initial arc external conduction heat flow characterization module 111, the expression of the initial arc external conduction heat flow is:

[0102]

[0103] The arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. After the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at T arc ; λ is the thermal conductivity.

[0104] It should be noted here that Fig.11 The various modules in the medium voltage distribution cable latent fault modeling system based on black box model are Figure 1 The various steps in the medium-voltage distribution cable latent fault modeling method based on the black box model correspond one to one, and the specific implementation process is the same, which will not be repeated here.

[0105] In one or more embodiments, an electronic device is provided, and when a central processing unit in the electronic device executes the program, the following is achieved: Figure 1 The steps in the black box model-based latent fault modeling method for medium voltage distribution cables are shown.

[0106] In particular, according to an embodiment of the present application, the process described above with reference to the flowchart can be implemented as a computer software program. For example, an embodiment of the present application includes a computer program product, which includes a computer program carried on a computer readable medium, the computer program including a computer program for executing Figure 1 In such an embodiment, the computer program can be downloaded and installed from a network through a communication part, and / or installed from a removable medium. When the computer program is executed by a central processing unit, various functions defined in the device of the present application are executed.

[0107] in, Figure 1 The computer program instructions corresponding to the method shown may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce a product including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0108] A person skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for modeling latent faults of medium voltage distribution cables based on a black box model, characterized in that: include: Assuming that the arc surface temperature is constant and the temperature change of the cable insulation layer during the duration of the cable latent fault is less than the set threshold, the expression of the initial arc heat flow to the outside is obtained based on the initial temperature distribution of the cable insulation layer and the arc parameters; According to the lumped steady-state thermal circuit model of medium-voltage distribution cables, the expression of the initial arc conduction heat flux is corrected to the arc's external solid conduction power, and the arc's external solid conduction heat flux is used as the arc black box model loss power, and then the arc dynamic resistance is calculated based on the arc black box model. The arc dynamic resistance and constant resistance are connected in series to construct a latent fault model of medium-voltage distribution cables. The constant resistance is used to characterize the part of the water tree and electric tree channel where no arc discharge occurs. Arc dynamic resistance is used to explain the zero-break phenomenon that occurs in latent cable faults.

2. The method for modeling latent faults of medium voltage distribution cables based on a black box model according to claim 1, characterized in that: The expression of the latent fault model of medium voltage distribution cable is: Where R is the latent fault model of medium voltage distribution cable; R s is a constant resistance; is the dynamic resistance of the arc; u is the voltage at both ends of the arc; i is the current flowing through the arc channel; φ is the heat flow conducted by the arc to the outside; τ is the arc time constant.

3. The method for modeling latent faults of medium voltage distribution cables based on a black box model as claimed in claim 1, characterized in that: Based on the cable core loss φ1, insulation layer loss φ2, outer shielding layer and outer sheath loss φ3, external ambient temperature T e , the thermal resistances R1 and R2 of the insulating layer, the outer semiconductor layer and the external environment, and the corrected arc heat flux φ: Among them, T arc is the arc surface temperature; r is the arc axis radius; λ is the thermal conductivity; the arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L.

4. The method for modeling latent faults of medium voltage distribution cables based on a black box model as claimed in claim 1, characterized in that: The expression of the initial arc heat flow to the outside is: The arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. After the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at T arc ; λ is the thermal conductivity.

5. A latent fault modeling system for medium voltage distribution cables based on a black box model, characterized in that: include: The module for characterizing the heat flux conducted externally by the initial arc is used to obtain the expression of the heat flux conducted externally by the initial arc based on the initial temperature distribution of the cable insulation layer and the arc parameters, assuming that the arc surface temperature is constant and the temperature change amplitude of the cable insulation layer during the duration of the cable latent fault is less than the set threshold. The arc dynamic resistance calculation module is used to correct the expression of the initial arc external conduction heat flow to the solid conduction power of the arc according to the lumped steady-state thermal circuit model of the medium-voltage distribution cable, and use the solid conduction heat flow of the arc to the outside as the loss power of the arc black box model, and then calculate the arc dynamic resistance based on the arc black box model; The cable latent fault model construction module is used to connect the arc dynamic resistance and the constant resistance in series to construct a latent fault model of the medium-voltage distribution cable; wherein the constant resistance is used to characterize the part of the water tree and electric tree channel where no arc discharge occurs; Arc dynamic resistance is used to explain the zero-break phenomenon that occurs in latent cable faults.

6. The method for modeling latent faults of medium voltage distribution cables based on a black box model as claimed in claim 5, characterized in that: In the cable latent fault model construction module, the expression of the medium voltage distribution cable latent fault model is: Where R is the latent fault model of medium voltage distribution cable; R s is a constant resistance; is the dynamic resistance of the arc; u is the voltage at both ends of the arc; i is the current flowing through the arc channel; φ is the heat flow conducted by the arc to the outside; τ is the arc time constant.

7. The method for modeling latent faults of medium voltage distribution cables based on a black box model as claimed in claim 5, characterized in that: In the arc dynamic resistance calculation module, based on the cable core loss φ1, insulation layer loss φ2, outer shielding layer and outer sheath loss φ3, external environment temperature T e , the thermal resistances R1 and R2 of the insulating layer, the outer semiconductor layer and the external environment, and the corrected arc heat flux φ: Among them, T arc is the arc surface temperature; r is the arc axis radius; λ is the thermal conductivity; the arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L.

8. The method for modeling latent faults of medium voltage distribution cables based on a black box model as claimed in claim 5, characterized in that: In the initial arc external conduction heat flux characterization module, the expression of the initial arc external conduction heat flux is: The arc is considered to have a radius of R arc , an ideal cylindrical gas channel with a length of L radially penetrates the cable insulation layer. After the fault occurs, the solid medium at the arc axis radius r maintains the initial temperature T0(l) before the fault occurs, and the arc surface temperature is constant at T arc ; λ is thermal conductivity.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps in the method for modeling latent faults of medium-voltage distribution cables based on a black box model as described in any one of claims 1 to 4 are implemented.

10. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps in the method for modeling latent faults of medium-voltage distribution cables based on a black box model as described in any one of claims 1 to 4 are implemented.

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