A method for constructing a multi-arc fault model inside a cable

Abstract

The application discloses a cable internal multi-form arc fault model. Experiments show that the cable internal arc generated between the core and the metal shielding layer presents multiple forms. Based on experimental data analysis, the cable internal arc is attributed to three forms of oscillation form, steady state form and semi-steady state form, and a cable internal multi-form arc fault model is proposed accordingly. The core part of the model includes an oscillation arc module constructed by a triangular wave controlled Gaussian random pulse function, a steady state arc module constructed by a 1 / 2 cosine-1 / 4 sine function, and a semi-steady state arc module constructed by a proportion control circuit. Then, a random switching module is used to select the arc form, and a fault timing module is used to control the arc starting time point, and finally a dynamic time-varying arc conductance is output to control the multi-form arc fault process. The application firstly proposes to construct a cable arc fault model in a multi-form manner, discovers the unique waveform characteristics of the cable internal arc fault, enriches the random diversity of the cable arc fault, and provides a new model basis for cable arc fault analysis.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power cables, and particularly relates to a construction method of a cable internal multi-form arc fault model. BACKGROUND

[0002] The underground power cable widely used in the 10kV distribution network system has a multi-layer insulation structure with a metal sheath, and the metal sheath is usually directly grounded through a grounding electrode. When a local insulation defect occurs in the cable, the path of line core→main insulation layer→metal sheath (ground)→filling layer→armoring layer→outer insulation sheath→laying environment (air / soil)→ground is gradually broken down, accompanied by arc discharge. However, the multi-layer insulation medium of the cable is not degraded at one time, but is a natural process of gradual evolution. In addition, the distribution network in China adopts a small current grounding system, and the fault current is generally only a few amperes, so the above-mentioned multi-layer insulation medium is not easy to be completely broken down to ground at one time, and therefore the cable is often prone to form an internal intermittent arc fault through the shortest grounding path, i.e. the line core to the metal sheath (ground), before being broken down to ground.

[0003] As a common initial fault form between partial discharge and permanent fault, the cable internal intermittent arc has the characteristics of relatively high fault electrical quantity amplitude and obvious fault waveform distortion characteristics, and is a kind of early warning fault category with great research value and as the focus of attention.

[0004] At present, the unique arc voltage fault waveform generated by means of the arc model is the key to the design of most arc fault detection methods. The arc waveform characteristics simulated by the existing model are that the arc voltage lasts for several power frequency cycles in the form of an approximate square wave, and the arc current has obvious periodic zero pause phenomenon. This typical characteristic waveform is derived from the arc generated in the air, however, in fact, the cable internal arc is different from the arc caused by the arc light breaking down to ground in many ways: the cable internal arc is earlier than the arc light breaking down to ground, the arc path is wrapped by the cable insulation medium and is not affected by the external environment, and the arc space is closed and the arc path is short. These different factors determine that the cable internal arc should have different characteristics from the arc caused by the arc light breaking down to ground, and also affect the waveform shape of the arc fault voltage and the construction of the corresponding model. SUMMARY

[0005] In view of the above problems, the present application takes the 10kV cross-linked polyethylene power cable widely used in the distribution network in China as the object, simulates the internal arc fault between the cable core and the metal sheath, confirms the essential difference between the waveform shape of the internal arc and the arc caused by the arc light breaking down to ground, and the fact that there are many forms. Based on the main waveform forms of the measured cable internal arc, the main characteristics are analyzed and summarized, and a cable internal multi-form arc fault model is proposed.

[0006] The application is realized by the following technical solutions:

[0007] A cable internal multi-form arc fault model, which comprises three core sub-modules of typical cable internal arc forms, a random switching module and a fault timing module; the core sub-modules randomly output any one of the three cable internal arc forms under the control of the random switching module and the fault timing module arc to control the cable multi-form arc fault process; the three core sub-modules are an oscillating arc module, a steady-state arc module and a semi-stable arc module.

[0008] The oscillating arc module is internally controlled by a triangular wave controlled Gaussian random pulse function to generate an oscillating arc conductance G osci (t); the specific control function is:

[0009]

[0010] In the formula, is the average peak value of the arc conductance during oscillation; t0 is the pulse starting time point; sigma is a time constant for controlling the pulse width; the oscillation control function is a triangular wave function oscillating n times with an average frequency f osci , an amplitude [-1 / (2f osci ), 1 / (2f osci )].

[0011] The steady-state arc module is internally controlled by a 1 / 2 cosine-1 / 4 sine function to generate a steady-state arc conductance G stab (t); the specific control function is:

[0012]

[0013] In the formula, A gc is the triangular function amplitude of the first derivative of the arc conductance in the drop zone; A gs is the triangular function amplitude of the first derivative of the arc conductance in the holding zone; T s is the steady-state arc burning duration; D c is the percentage of the drop zone relative to T s ; t0 is the pulse starting time point.

[0014] The semi-stable arc module is internally controlled by the outputs of the oscillating arc module and the steady-state arc module to obtain a semi-stable arc conductance G semi (t) in a proportion control manner; the specific control function is:

[0015]

[0016] In the formula, T semi is the arc burning duration of the semi-stable arc; D semi is the percentage of the oscillation phase of the arc relative to T semi ; t0 is the pulse starting time point.

[0017] The random switching module is composed of a time element, a logic switch element and a flip-flop.

[0018] The fault timing module is composed of a derivative element, a time element and a flip-flop.

[0019] Compared with the prior art, the present application firstly proves the multi-form feature of the arc inside the arc through experimental tests, innovatively proposes to construct a cable arc fault model in a multi-form mode, explores the unique waveform feature of the cable internal arc fault, enriches the random diversity of the cable arc fault, provides a new model basis and data support for cable arc fault analysis, and effectively promotes the new development of cable fault prediction, detection and positioning methods. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The proposed cable internal multi-form arc fault model structure diagram.

[0021] Figure 2 The cable internal arc generation site.

[0022] Figure 3 The specific implementation of the simulation circuit of the present application.

[0023] Figure 4 The cable internal multi-form arc fault voltage waveform simulated by the present application. DETAILED DESCRIPTION

[0024] The present application will be further described below in combination with the drawings and examples, but the protection scope of the present application is not limited by the examples.

[0025] Example 1

[0026] This example is to build a multi-form arc model by using the present application, and the model structure diagram is as shown in Figure 1 The simulated effect is to generate a multi-form arc fault inside the cross-linked polyethylene cable, as shown in Figure 2 .

[0027] The cable internal multi-form arc fault model construction of this example includes the following steps:

[0028] As shown in Figure 3As shown, firstly, according to the calculation principle of the internal control function of each module, three arc morphology sub-module internal control circuits are constructed by using the CSMF element library in the PSCAD software. Among them, the oscillating arc module is constructed by triangular wave element, square element, basic operation element and power function element; the steady-state arc module is constructed by time element, basic operation element, trigonometric function element, integral element and logic switch element; and the semi-stable arc module is constructed by logic switch element and flip-flop. Then, the random switching module is constructed by using time element, logic switch element and flip-flop; and the fault timing module is constructed by using derivative element, time element and flip-flop, so as to control the selective output of the random arc morphology and the arc starting time point.

[0029] Then, based on the experimental analysis and fitting data, the input parameters of each sub-module in the model are set as follows:

[0030] 1) The input parameter setting of the oscillating arc module: t0 (arc starting point): 0.245-0.255 power frequency period, (oscillation depth): 2.2-12.6 mS, σ (oscillation pulse width): 0.005 ms, f osci-n (oscillation frequency / time): 1.58 kHz-4.22 kHz / 1-12 times.

[0031] 2) The input parameter setting of the steady-state arc module: t0 (arc starting point): 0.245-0.255 power frequency period, T s (duration): 3.1-6.7 ms, D c (fall region width): 23%-54%, A gc (fall region arc): 0.0042-0.0082 mS / ms, A gs (maintaining region stability): 0.0036-0.0077 mS / ms.

[0032] 3) The input parameter setting of the semi-stable arc module: T semi (duration): 4.1-6.2 ms, D semi (control ratio): 12%-52%.

[0033] Finally, the model is run, and the parameters of the random switching and fault timing modules are set to control the cable internal multi-morphology arc fault voltage waveform generated by the model as Figure 4 shown. Figure 4The model can randomly output any one of the three internal arc conductances at any specified time in an intermittent manner. The dotted line in the figure represents that the oscillating arc voltage waveform is generated, the waveform repeatedly reignites and extinguishes in the form of oscillation, and lasts for about 1 / 4 of the power frequency period. The long dashed line in the figure represents that the semi-stable arc voltage waveform is generated, the waveform presents an oscillating form in the first half, and presents a stable form in the second half, the arc voltage is close to a straight line, and lasts for about 1 / 4 of the power frequency period. The short dashed line in the figure represents that the stable arc voltage waveform is generated, the waveform falls in the form of a quadratic curve, and then remains a straight line, and lasts for about 1 / 4 of the power frequency period.

[0034] The simulation analog waveform fully characterizes the randomness and diversity of the internal arc of the cable, can reflect the key characteristic quantity of the multi-form arc voltage of the cable, indicates the effectiveness of the random simulation of the multi-form arc fault in the internal arc of the cable, and the model can be used for further analysis of the internal arc fault of the cable.

[0035] Those skilled in the art should understand that the present application is not limited to the embodiment, the cable internal arc fault model following the basic principles and features of the present application falls within the scope of the present application, and is defined by the claims and equivalents thereof.

Claims

1. A method for constructing a multi-morphology arc fault model inside a cable, characterized in that: The model comprises three core sub-modules of typical cable internal arc forms, a random switching module and a fault timing module; the core sub-modules randomly output any one of the three cable internal arc forms under the control of the random switching module and the fault timing module to control the cable multi-form arc fault process; the three core sub-modules are an oscillating arc module, a steady-state arc module and a semi-steady-state arc module. The oscillating arc module is internally controlled by a triangular wave controlled Gaussian random pulse function to generate an oscillating arc conductance G osci (t), and the specific control function is: wherein is the average peak value of the arc conductance during the oscillation; t0is the time point of the pulse start; σ is the time constant for controlling the pulse width; the oscillation control function is the triangular wave function oscillating n times with the average frequency f osci , the amplitude [-1 / (2f osci ), 1 / (2f osci )]; the steady state arc module, which is internally controlled by the 1 / 2 cosine - 1 / 4 sine function to generate the steady state arc conductance G stab (t), the specific control function is: wherein A gc is the amplitude of the trigonometric function of the first derivative of the arc conductance in the drop region; A gs is the amplitude of the trigonometric function of the first derivative of the arc conductance in the sustain region; T s is the duration of the steady state arc burning period; D c The percentage of the falling zone relative to T s is the percentage of the falling zone relative to T The semi-stable arc module, the inside is obtained by the semi-stable arc conductance G with the output of the oscillation arc module and the stable arc module in the proportion control semi (t), the specific control function is: In the formula, T semi is the half-steady arc burning time; D semi The percentage of the oscillation phase of the arc relative to T semi ; t0 is the starting point of the pulse.

2. The method of claim 1, wherein: The random switching module is composed of a time element, a logic switch element and a flip-flop.

3. The method of claim 1, wherein: The fault timing module is composed of a derivative element, a time element and a flip-flop.

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