Online diagnosis method of sheath protector fault based on sheath current difference amplitude-frequency characteristic

Abstract

The present invention provides an online diagnosis method for sheath protector faults based on the amplitude-frequency characteristics of sheath current difference, the method comprising the following steps: judging whether the current difference between the end and the beginning of the cable is equal to the sum of the leakage currents flowing into each segmented cable inside the cable, if it is equal, no ground short circuit fault occurs in the cable loop; otherwise, judging whether the abnormal current difference contains harmonics, if not, no other type of ground short circuit fault occurs in the cable loop; if it contains, a ground short circuit fault occurs in the cable loop. The present invention provides a calculation method for the current difference under power frequency and the leakage current of the faulty sheath protector and establishes an equivalent circuit model of the harmonic component after the sheath protector fault, designs a sheath protector fault diagnosis method based on the amplitude-frequency characteristics of the cable sheath current difference, and improves the efficiency of sheath protector maintenance at this stage.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage cable lines, and more specifically, to an online fault diagnosis method for a sheath protector based on sheath current difference amplitude-frequency characteristics. Background Art

[0002] The existing sheath protector fault monitoring methods mainly rely on manual inspections, and the power frequency sheath current changes caused by sheath protector faults and cable outer sheath damage are similar, and it is difficult to distinguish the two by relying on sheath current amplitude, sheath current relative value, and sheath current and sheath current difference phase angle. Aiming at the nonlinear resistance characteristics of the zinc oxide valve plate inside the sheath protector, the present invention improves the accuracy of sheath protector fault diagnosis by using the amplitude-frequency characteristics of the sheath current difference. Summary of the invention

[0003] In view of the technical problems existing in the prior art, the present invention provides a method for calculating the current difference under power frequency and the leakage current of the faulty sheath protector, establishes an equivalent circuit model of the harmonic components after the sheath protector fault, and designs a sheath protector fault diagnosis method based on the amplitude-frequency characteristics of the cable sheath current difference, thereby improving the efficiency of the current sheath protector maintenance.

[0004] The present invention provides a method for online diagnosis of sheath protector faults based on sheath current difference amplitude-frequency characteristics, comprising:

[0005] Obtain the current difference between the end and the beginning of the cable, and determine whether the current difference is equal to the sum of the leakage currents flowing into each segment of the cable. If so, there is no ground short circuit fault in the cable loop; otherwise, there is a ground short circuit fault in the line.

[0006] Based on the above technical solution, the present invention can also make the following improvements.

[0007] Preferably, after a ground short circuit fault occurs in the line, the method further includes: determining whether the current difference contains harmonics; if not, no other type of ground short circuit fault occurs in the cable loop; if so, performing sheath protector fault diagnosis and positioning according to preset criteria and diagnosis results.

[0008] Preferably, the sheath protector fault diagnosis and location according to preset criteria and diagnostic results includes, by establishing an equivalent circuit model of the current difference of the sheath circuit after the sheath protector fault under the power frequency and the harmonic component after the sheath protector fault, formulating a fault diagnosis criterion according to the amplitude-frequency characteristics of the sheath current caused by the sheath protector fault, and determining the circuit where the sheath protector fault is located.

[0009] Preferably, formulating a fault diagnosis criterion based on the amplitude-frequency characteristics of the sheath current caused by the sheath protector fault includes: treating each component of the leakage current in the faulty sheath protector as equivalent to a harmonic source, injecting it into the metal sheath loop from the position of the faulty sheath protector, and through the filtering effect of the cable sheath, the harmonic content in the sheath current at both ends of the cable changes due to the fault position of the sheath protector, and formulating a graph of the harmonic content of the leakage current in the faulty sheath protector as a fault diagnosis criterion.

[0010] Preferably, obtaining the current difference between the end of the cable and the head end comprises the following steps:

[0011] Install 6 power frequency current transformers in the grounding boxes at both ends of the cross-connected high-voltage cable lines to synchronously obtain the sheath current signals at both ends of the cable;

[0012] The collected sheath current signals are grouped according to sheath loops, and the sheath current differences of the corresponding groups are calculated respectively;

[0013] Perform Fourier transform on the sheath current difference waveform to obtain the harmonic content in the leakage current.

[0014] Preferably, the determining whether the current difference is equal to the sum of the leakage currents flowing into each segmented cable inside the cable comprises:

[0015] Obtain the equivalent impedance of the sheath protector;

[0016] The equivalent circuits of the capacitive coupling component and the inductive coupling component are established respectively, and the sheath current vector difference caused by the sheath fault protector fault is obtained according to the equivalent circuits;

[0017] The leakage current containing harmonic components of the faulty sheath protector is calculated according to the sheath current vector difference, and the current difference containing harmonic components of the fundamental current difference phase angle is obtained according to the leakage current.

[0018] Preferably, obtaining the equivalent impedance of the sheath protector comprises the following steps:

[0019] Obtain the parallel equivalent resistance and the parallel equivalent capacitance of the sheath protector;

[0020] Establish an equivalent impedance calculation model;

[0021] Substitute the parallel equivalent resistance and the parallel equivalent capacitance into the equivalent impedance calculation model to obtain the equivalent impedance.

[0022] Preferably, the equivalent impedance calculation model is expressed as:

[0023]

[0024] In the formula, Z Lrepresents equivalent impedance, j represents imaginary factor, ω represents power frequency angular velocity; Rsv l represents parallel equivalent resistance, Csv l represents parallel equivalent capacitance.

[0025] Preferably, the current difference including the harmonic component of the fundamental current difference phase angle obtained according to the leakage current comprises:

[0026] In the simulation of the cable line after the sheath protector fails, the sheath circuit models at the power frequency and harmonic frequency are established respectively. In the equivalent circuit model of the harmonic component after the sheath protector fails, the faulty sheath protector is regarded as a harmonic source, the cable sheath circuit is equivalent to a series RL circuit, and it is assumed that the ground impedance at the beginning and end of the cable is equal. In the line with a total loop length of l, the transfer function of the sheath current difference to the harmonic component of the leakage current of the faulty sheath protector is defined as the ratio of the two.

[0027] Technical effects and advantages of the present invention:

[0028] The present invention monitors the fault of the sheath protector based on the nonlinear resistance characteristics of the zinc oxide valve plate in the internal structure of the sheath protector and the harmonic content of the current difference; establishes an equivalent circuit model of a cross-connected high-voltage cable, proposes a detection method using the harmonic content of the current difference as the characteristic quantity of the sheath protector fault, and provides a method for calculating the current difference during a fault, and formulates a fault diagnosis process for the sheath protector in a three-phase nine-section cross-connected high-voltage cable system, thereby improving the efficiency of the current maintenance of the sheath protector.

[0029] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flow chart of the fault diagnosis method of the present invention;

[0031] Figure 2 Formulate a flow chart for the fault criterion of the sheath protector of the present invention;

[0032] Figure 3 is the harmonic content of the leakage current in the fault sheath protector of the present invention;

[0033] Figure 4 This is a structural diagram of a three-phase nine-section cross-interconnected high-voltage sheath protector according to the present invention;

[0034] Figure 5 This is a schematic diagram of the sheath current of loop 1 of the present invention;

[0035] Figure 6It is the equivalent circuit diagram of the capacitive coupling component of the sheath circuit of loop 1 of the present invention;

[0036] Figure 7 It is an equivalent circuit diagram of the inductive coupling component of the sheath circuit of loop 1 of the present invention;

[0037] Figure 8 It is the equivalent circuit model of the harmonic component after the sheath protector of loop 1 of the present invention fails;

[0038] Fig. 9 This is a diagram of the current content of harmonic components after the sheath protector of loop 1 of the present invention fails. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] When the protective layer protector is broken down, a carbonization channel will appear from the upper electrode through the zinc oxide valve plate to the lower electrode. At this time, the equivalent impedance and other physical properties of the protective layer protector will change. The present invention is based on the characteristics of the zinc oxide valve plate, which is the main component of the protective layer protector. When the protective layer protector fails, the differential amplitude-frequency characteristic change of the protective layer current is used to realize the online diagnosis and positioning of the protective layer protector failure.

[0041] In order to solve the shortcomings of the prior art, the present invention proposes an online fault diagnosis method for a sheath protector based on the sheath current difference amplitude-frequency characteristic. Figure 1 As shown, the method comprises the following steps:

[0042] Install 6 power frequency current transformers in the grounding boxes at both ends of the cross-connected high-voltage cable lines to synchronously obtain the sheath current signals at both ends of the cable;

[0043] Specifically, after obtaining the current difference between the end and the beginning of the cable, it is determined whether the current difference between the end and the beginning of the cable is equal to the sum of the leakage currents flowing into the three small sections of the cable. If so, no ground short circuit fault occurs in the cable loop, otherwise a ground short circuit fault occurs in the line.

[0044] The collected current signals are grouped according to the sheath loops, and the corresponding sheath current differences are calculated respectively;

[0045] Specifically, the steps of calculating the corresponding sheath current difference include measuring the sheath protector parameters, establishing the cable line model, sheath protector fault diagnosis, calculation of the fault sheath protector leakage current and current difference, and fault classification and detection. The steps of calculating the corresponding sheath current difference will be described in detail later and will not be repeated here. Figure 2 The figure shows a flow chart for formulating fault judgment criteria for the sheath protector. In the figure, after determining that a ground short circuit fault has occurred in the line, it also includes determining whether the current difference contains harmonics. If not, no other type of ground short circuit fault has occurred in the cable loop; if contained, a ground short circuit fault has occurred in the cable loop.

[0046] Perform fast Fourier transform on the sheath current difference waveform to obtain the contents of the 3rd, 5th, 7th and 9th harmonics.

[0047] Specifically, the obtained contents of the 3rd, 5th, 7th and 9th harmonics are used as the diagnosis result of the fault diagnosis.

[0048] The sheath protector fault diagnosis and location are carried out according to the preset criteria and diagnostic results.

[0049] Specifically, the sheath protector fault diagnosis and location are performed according to preset criteria and diagnostic results by establishing an equivalent circuit model of the current difference of the sheath circuit after the sheath protector fault under the power frequency and the harmonic component after the sheath protector fault, and formulating a fault diagnosis criterion based on the amplitude-frequency characteristics of the sheath current caused by the sheath protector fault, and determining the circuit where the sheath protector fault is located.

[0050] According to the amplitude-frequency characteristics of the sheath current caused by the sheath protector fault, the fault diagnosis criterion is formulated as follows: each component of the leakage current in the faulty sheath protector is equivalent to a harmonic source, which is injected into the metal sheath loop from the position of the faulty sheath protector. After filtering by the cable sheath, the harmonic content in the sheath current at both ends of the cable changes due to the fault position of the sheath protector. Based on this, a diagram of the harmonic content of the leakage current in the faulty sheath protector is formulated as a preset criterion for fault diagnosis.

[0051] In this embodiment, the harmonic content of the leakage current in the faulty sheath protector is tested according to the experiment. In the case of a ground short circuit fault in the line, the zinc oxide valve plate inside the sheath protector is under the power frequency 100V voltage, and the harmonics in the leakage current are mainly the 3rd, 5th, 7th, and 9th harmonics. Figure 3 The figure shows the harmonic contents of the leakage current in the faulty sheath protector. Taking the fundamental current as a reference, the harmonic contents are 65.4%, 27.5%, 5.9% and 3.4% respectively. The present invention has formulated the standard for sheath protector fault diagnosis in three circuits. The amplitude-frequency characteristics of the fault current difference of the sheath protector are shown in Table 1 below:

[0052]

[0053]

[0054] It should be noted that the present invention diagnoses and locates the sheath protector fault by detecting the sheath current of the high-voltage cable and calculating the current phase difference for diagnosis. By applying the technical solution of the present invention, the detection current data can be effectively used in the high-voltage cable sheath current inspection to timely discover and locate the sheath protector fault, so as to facilitate the subsequent replacement of the faulty protector, thereby effectively protecting the safety of the high-voltage cable sheath and insulation, and ensuring the safe operation of the cable line.

[0055] At the same time, the above calculation of the corresponding sheath current difference includes the following parts:

[0056] 01)Measure the parameters of the sheath protector

[0057] In this embodiment, the cable includes an outer sheath, a metal sheath, an insulating layer and a cable core from the outside to the inside, wherein the sheath protector is installed in a cross-connected grounding box between two metal sheath segments in a cross-connected cable line. Usually, 35kV large-section power cables and power cables of 66kV, 110kV and above voltage levels are single-core cables. The three ends of the cable metal sheath are interconnected and grounded, and the other end is not grounded. When lightning waves or internal overvoltages flow along the cable core, a high impulse overvoltage will appear at the ungrounded end of the cable metal sheath, or when the system short-circuit fault current flows through the cable core, a high power frequency induced overvoltage will also appear at the ungrounded end of the sheath. The above-mentioned overvoltage may break through the insulation of the cable outer sheath, causing a multi-point grounding fault of the cable metal sheath, seriously affecting the normal operation of the power cable and even greatly reducing the service life of the cable. Therefore, in order to limit the induced voltage and fault overvoltage on the metal sheath of the power cable and avoid the formation of a loop in the sheath, one end of the cable sheath is directly grounded, and the other end must be grounded through the sheath protector. If the line is long, the cable sheath can be divided into three sections or multiples of three to insulate each other. The sheaths at the sections are cross-connected and grounded through the sheath protector. The sheath protector uses ZnO varistors or ZnO valve plates as protection elements. The ZnO valve plates have no series gaps and have excellent voltage-current characteristic curves. The sheath protector uses insulating materials as external insulation. Compared with traditional discharge gap protection, gapped silicon carbide resistor protection and other methods, its volt-ampere characteristics are more superior.

[0058] In practical applications, if the protective layer protector is broken down, a carbonization channel will appear from the upper electrode through the zinc oxide valve plate to the lower electrode. At this time, the physical properties of the protective layer protector such as the equivalent impedance will change. According to the characteristics of the zinc oxide valve plate, the main component of the protective layer protector, an equivalent circuit model of the protective layer protector is established. The establishment of the equivalent circuit model of the protective layer protector includes the following steps: obtaining the parallel equivalent resistance R of the faulty protective layer protector svlAnd the parallel equivalent capacitance C svl ; Establish an equivalent impedance calculation model, and connect the equivalent resistance R svl And the parallel equivalent capacitance C svl Substitute into the equivalent impedance calculation model to obtain the equivalent impedance Z L In this embodiment, the parallel equivalent resistance R of the faulty sheath protector can be measured by using an AC bridge. svl And the parallel equivalent capacitance C svl , equivalent resistance R svl and equivalent capacitance C svl Connect in parallel and calculate the equivalent impedance Z L Including the parallel equivalent resistance R svl And the parallel equivalent capacitance C svl Substitute into the equivalent impedance calculation model of the following formula 1 to obtain the equivalent impedance Z L ,

[0059]

[0060] In the formula, j represents the imaginary factor, ω represents the power frequency angular velocity; R svl Represents the parallel equivalent resistance, C svl Represents the parallel equivalent capacitance.

[0061] 02) Build a cable line model

[0062] The present invention adopts a three-phase nine-segment cross-connected high-voltage sheath protector, specifically Figure 4 As shown, the cable sheath is divided into three sections and insulated from each other. Figure 4 The 9 metal sheath sections of the cable sheath will consist of the first metal sheath S1, the second metal sheath S2, the third metal sheath S3, the fourth metal sheath S4, the fifth metal sheath S5, the sixth metal sheath S6, the seventh metal sheath S7, the eighth metal sheath S8 and the ninth metal sheath S9. Among them, the first metal sheath S1, the fifth metal sheath S5 and the ninth metal sheath S9 form the sheath loop 1; the fourth metal sheath S4, the eighth metal sheath S8 and the third metal sheath S3 form the sheath loop 2; the seventh metal sheath S7, the second metal sheath S2 and the sixth metal sheath S6 form the sheath loop 3. The first sheath protector Z SVL1 , Second sheath protector Z SVL2 、Third sheath protector Z SVL3 、The fourth sheath protector Z SVL4 、Fifth sheath protector Z SVL5 、The sixth sheath protector Z SVL6The first current sensor CS1, the second current sensor CS2 and the third current sensor CS3 are installed at the beginning of the cable line, and the fourth current sensor CS4, the fifth current sensor CS5 and the sixth current sensor CS6 are installed at the end of the cable line. A 、i B 、i C is the core current of the three-phase cable.

[0063] 3) Sheath protector fault diagnosis

[0064] The present invention is mainly explained by taking loop 1 as an example, and loops 2 and 3 are similar to loop 1 and will not be described in detail here. Figure 5 The figure shows the schematic diagram of the sheath current of loop 1. The first metal sheath S1, the fifth metal sheath S5 and the ninth metal sheath S9 are connected in series. cs1 ,I cs5 The sheath-to-ground currents measured at current sensors cs1 and cs5 are respectively. Under normal fault-free conditions, the current difference in the metal sheath is equal to the leakage current flowing into the first metal sheath S1 according to Kirchhoff's current law:

[0065] I cs5 -I cs1 =I LA1 +I LB2 +I LC3 (2)

[0066] In the formula, I LA1 ,I LB2 and I LC3 They respectively represent the leakage current flowing from the cable core into the first metal sheath S1, the fifth metal sheath S5 and the ninth metal sheath S9.

[0067] In the event of a fault in the protective layer protector, the physical properties of the protective layer protector, such as the equivalent impedance, will change, based on the characteristics of the zinc oxide valve plate, which is the main component of the oxygen protective layer protector. When the protective layer protector fails, the differential amplitude-frequency characteristics of the protective layer current change to achieve online diagnosis and positioning of the protective layer protector failure. The present invention takes into account the nonlinear resistance characteristics of the faulty protective layer protector, and distinguishes the protective layer protector failure from other types of ground short circuit failures based on whether the abnormal current difference contains harmonics; by establishing an equivalent circuit model of the current difference of the protective layer circuit after the protective layer protector fails at the power frequency and the harmonic components after the protective layer protector fails, a fault diagnosis criterion is formulated based on the amplitude-frequency characteristics of the protective layer current caused by the protective layer protector failure, and the circuit where the protective layer protector failure is located is determined.

[0068] 3) Calculation of leakage current and current difference of fault sheath protector

[0069] In order to analyze the relationship between fault impedance and detection sheath current, the present invention provides a method for calculating the current difference under power frequency and the leakage current of the fault sheath protector. Compared with the insulation impedance, the impedance of the sheath protector to the ground can be ignored. Therefore, it is assumed that when the sheath protector is grounded, the leakage current is equal to that in normal conditions. Compared with the fault point, the impedance of another normal sheath protector in the same circuit is very large and is regarded as an open circuit to the ground. For the convenience of calculation, it is assumed that the ground impedance at the end of the cable line is approximately equal to that at the beginning, that is, R e1 =R e2 =R.

[0070] Judging whether the current difference is equal to the sum of the leakage currents flowing into each segmented cable inside the cable includes: calculating the leakage current and current difference of the sheath protector, which specifically establishes equivalent circuits of capacitive coupling components and inductive coupling components respectively, and obtains the sheath current vector difference caused by the sheath fault protector fault according to the equivalent circuit; calculating the leakage current containing harmonic components of the faulty sheath protector according to the sheath current vector difference, and obtaining the current difference containing harmonic components of the fundamental current difference phase angle according to the leakage current. In this embodiment, the sheath current of the cross-connected high-voltage cable is the superposition of the capacitive coupling current component and the inductive coupling current component, so it is necessary to establish equivalent circuit models of the capacitive coupling component and the inductive coupling component respectively.

[0071] ① Calculation of capacitive coupling current component

[0072] Figure 6 This is the equivalent circuit diagram of the capacitive coupling component of the sheath circuit of loop 1. In the figure, E S1 is the induced voltage at the first metal sheath S1, E S5 is the induced voltage at the fifth metal sheath S5, E S9 is the induced voltage at the ninth metal sheath S9; Z S1 is the sheath impedance of the first metal sheath S1, Z S2 is the sheath impedance of the fifth metal sheath S5, Z S3 is the sheath impedance of the ninth metal sheath S9. cs1m and I cs5m They are the inductively coupled currents measured at the first current sensor cs1 and the fifth current sensor cs5 respectively.

[0073] Cable metal sheath impedance Z S The calculation process is shown in formula (3).

[0074]

[0075] Where l is the cable length; R S is the resistance per unit length of the metal sheath; D S is the average diameter of the sheath; f is the operating frequency of the system, and ω represents the angular velocity of the operating frequency.

[0076] M X,Sn The mutual inductance between the x-phase cable core and the cable Sn section sheath:

[0077]

[0078] Where, d x,Sy is the average geometric distance between the x-phase cable core and the Sn-segment cable sheath, μ0 is the vacuum magnetic permeability, l n is the length of each small cable segment.

[0079] The calculation formula for the sheath induced voltage on the first section metal sheath S1, the fifth section metal sheath S5 and the ninth section metal sheath S9 is as follows:

[0080]

[0081] In the formula, E S1 、E S5 、E S9 are the sheath induced voltages generated on sheath segments S1, S5 and S9 respectively; M A,S1 、M B,S1 、M C,S1 are the mutual inductances of the three-phase cables A, B, and C to the first metal sheath S1; M A,S5 、M B,S5 、M C,S5 are the mutual inductances of the three-phase cables A, B, and C to the first metal sheath S5; M A,S9 、M B,S9 、M C,S9 I is the mutual inductance of the three-phase cables A, B and C to the first metal sheath S9; A ,I B ,I C They are the load currents of the three phases of cables A, B and C respectively.

[0082] ②Calculation of inductively coupled current component

[0083] Figure 7 This is the equivalent circuit diagram of the inductive coupling component of loop 1 sheath circuit. In the figure, U A is the induced voltage at the first metal sheath S1, U B is the induced voltage at the fifth metal sheath S5, U C is the induced voltage at the ninth metal sheath S9; Z iS1 is the sheath impedance of the first metal sheath S1, Z iS5 is the sheath impedance of the fifth metal sheath S5, Z iS9 is the sheath impedance of the ninth metal sheath S9. cs1L and I cs5LThey are the inductively coupled currents measured at the first current sensor cs1 and the fifth current sensor cs5 respectively.

[0084] ③Calculation of sheath current vector difference

[0085] Specifically, obtaining the sheath current vector difference caused by the sheath fault protector fault according to the equivalent circuit includes: analyzing the circuit S1-S2-S3 of the sheath protector fault of loop 1 at the power frequency f1=50Hz;

[0086] Calculate the cable insulation capacitance C i :

[0087]

[0088] Among them, ε is the relative dielectric constant of the insulating layer; ε0 is the dielectric constant in vacuum; D C D is the core conductor diameter; S is the average geometric diameter of the cable metal sheath.

[0089] In the first circuit, I LS1 is the capacitive coupling component current at the first metal sheath S1, I LS5 is the capacitive coupling component current at the fifth metal sheath S5, I LS9 is the capacitive coupling component current at the ninth metal sheath S9, where ω1 is the angular velocity corresponding to the power frequency.

[0090]

[0091] The working voltage vector in the first circuit is U loop1 , where U A , U B , U C are the operating voltages of phases A, B, and C respectively; the load current vector is I loop1 , where I A ,I B ,I C are the load currents of phases A, B, and C respectively; the equivalent capacitance vector of the cable insulation layer is C i , where C S1 , C S5 , C S9 are the equivalent capacitances of the first metal sheath S1, the fifth metal sheath S5 and the ninth metal sheath S9 respectively; M loop1 is the mutual inductance matrix of the cable sheath and the core in loop 1, where the mutual impedance between the x-phase core and the sheath Sn segment is defined as Z x,Sn :

[0092] U loop1 =[U A U B UC ] (8)

[0093] I loop1 =[I A I B I C ] T (9)

[0094] C i =[C S1 C S5 C S9 ] (10)

[0095]

[0096] Sheath current vector difference I caused by sheath protector SV11 fault in loop 1 loop1,svl1 for:

[0097]

[0098] Similarly, the sheath current vector difference I caused by the sheath protector SV l5 fault can be obtained loop1,svl5 for:

[0099]

[0100] ④Calculation of leakage current containing harmonic components of fault sheath protector

[0101] According to the sheath current vector difference I loop1,svl5 Calculation of the leakage current containing harmonic components of the faulty sheath protector includes:

[0102] The calculation formula for the leakage current through the faulty sheath protector after the sheath protector SV L1 and SV L5 fail under power frequency is as follows:

[0103]

[0104]

[0105] Due to the randomness of harmonic phase angle, the phase angle of each harmonic current is assumed to be equal to the phase angle of the fundamental current φ svl(1) , the harmonic order is h. Taking the sheath protector SV L 1 fault as an example, the leakage current containing harmonic components of the faulty sheath protector is:

[0106]

[0107] In the simulation of the cable line after the sheath protector fails, the sheath circuit model at the power frequency and harmonic frequency will be established respectively. The faulty sheath protector is regarded as a harmonic source, and the cable sheath circuit is equivalent to a series RL circuit. The equivalent circuit model of the harmonic component after the sheath protector fails is as follows: Figure 8 As shown in the figure, R S1 , L s1 , R S5 , L s5 , R S9 , L s9 In series, I cs1m and I cs5m are the inductive coupling currents measured at the first current sensor cs1 and the fifth current sensor cs2 respectively; in the series RL circuit, the unit length inductance L S (Unit: H / m), resistance per unit length R S (Unit: Ω / m), and it is assumed that the ground impedance at the beginning and end of the cable is equal. In a line with a total length of loop 1 of l, the transfer function of the sheath current difference to the harmonic component of the fault sheath protector leakage current is defined as the ratio of the two G(ω).

[0108]

[0109] ⑤ Calculation of the fundamental current difference phase angle including the current difference of harmonic components;

[0110] The fundamental current difference phase angle obtained according to the leakage current contains the current difference of harmonic components, including:

[0111] When the sheath protector SV L 1 fails, the fundamental current difference phase angle φ (1) The current difference calculation formula including harmonic components is:

[0112]

[0113] 5) Fault classification and detection

[0114] Take the collected fault sheath protector as an example, the equivalent resistance R svl =216Ω, Fig. 9 The amplitude-frequency characteristics of the current difference in the corresponding sheath circuit when the sheath protectors SV 1 1 to SV 1 6 fail respectively. In this embodiment, the cable parameters are shown in Table 2 below:

[0115] Parameter / Unit Numeric Parameter / Unit Numeric AB phase distance / m 0.27 Working voltage / kV 110 BC phase spacing / m 0.27 <![CDATA[Resistance per unit length of sheath / Ω·m -1 > <![CDATA[4.26×10 -5 ]]> AC phase spacing / m 0.54 Insulation layer volume resistivity / Ω·m <![CDATA[10 15 ]]> Ground resistance / Ω 0.50 Length of the first section / m 425 Metal conductor outer diameter / mm 35.90 Length of the second section / m 477 Insulation layer thickness / mm 17.30 Length of the third section / m 536 Metal sheath thickness / mm 2 Free space dielectric constant <![CDATA[8.85×10 -12 ]]> Frequency / Hz 50 Relative dielectric constant 2.30

[0116] In the case of known line parameter table 2 and sheath protector fault impedance, the current difference of the sheath loop where the fault is located can be calculated by the above method. And determine whether the current difference is equal to the sum of the leakage currents flowing into each segment cable inside the cable. If it is equal, there is no ground short circuit fault in the cable loop; otherwise, there is a ground short circuit fault in the line. When a ground short circuit fault occurs in the line, it is also necessary to determine whether the current difference contains harmonics. If not, there is no other type of ground short circuit fault in the cable loop; if it is, there is a ground short circuit fault in the cable loop, and the harmonics are compared with Fig. 9 Compare the harmonics in the leakage current in the circuit to determine where the sheath protector fault is located.

[0117] In this embodiment, the influence of factors such as field cable parameters, cable core current, lengths of three small cable sections, ground impedance, and core voltage need to be considered when formulating the judgment criteria.

[0118] In summary, the present invention establishes an equivalent model of a three-phase nine-section cross-connected high-voltage cable, and proposes a method for calculating the leakage current and current difference in the sheath protector at the power frequency when the sheath protector fails. Each harmonic current equivalently generated by the faulty sheath protector is injected into the cable sheath circuit as a harmonic source, and a corresponding harmonic circuit model is established. The present invention uses the harmonic content of the current difference as a characteristic quantity to identify whether the fault in the line is a sheath protector fault and determine the circuit where the sheath protector fault is located.

[0119] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristics, characterized in that: The method comprises the following steps: Obtain the current difference between the end and the beginning of the cable, and determine whether the current difference is equal to the sum of the leakage currents flowing into each segment of the cable, including: Establishing an equivalent circuit model of the sheath protector, including: obtaining an equivalent impedance of the sheath protector; establishing equivalent circuits of capacitive coupling components and inductive coupling components respectively, and obtaining a sheath current vector difference caused by a sheath fault protector fault according to the equivalent circuits; The leakage current of the faulty sheath protector containing harmonic components is calculated according to the sheath current vector difference, the transfer function of the sheath current difference to the harmonic components of the leakage current of the faulty sheath protector is determined, and the current difference containing harmonic components of the fundamental current difference phase angle is obtained according to the leakage current; wherein, the transfer function of the sheath current difference to the harmonic components of the leakage current of the faulty sheath protector is defined as the ratio of the two G ( ω ), expressed as: When the sheath protector fails, the fundamental current difference phase angle including the harmonic component current difference calculation formula is: In the formula, L S is the inductance per unit length, R S is the resistance per unit length of the metal sheath, j represents the imaginary factor, ω represents the angular velocity, l is the cable length, φ （1） is the fundamental current difference phase angle, I loop1,svl1 It represents the sheath current vector difference caused by the sheath protector SVL1 fault, h is the harmonic order, ω 1 Angular velocity corresponding to the power frequency; If they are equal, then there is no ground short circuit fault in the cable loop; otherwise, there is a ground short circuit fault in the line.

2. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 1 is characterized in that: After a ground short circuit fault occurs in the line, the method further includes: determining whether the current difference contains harmonics. If not, no other type of ground short circuit fault occurs in the cable loop; if so, performing sheath protector fault diagnosis and positioning according to preset criteria and diagnosis results.

3. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 2 is characterized in that: The fault diagnosis and location of the sheath protector according to the preset criterion and the diagnosis result includes: By establishing the equivalent circuit model of the current difference of the sheath circuit after the sheath protector fault under power frequency and the harmonic component after the sheath protector fault, the fault diagnosis criterion is formulated according to the amplitude-frequency characteristics of the sheath current caused by the sheath protector fault, and the circuit where the sheath protector fault is located is determined.

4. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 3 is characterized in that: The fault diagnosis criteria are formulated based on the amplitude-frequency characteristics of the sheath current caused by the sheath protector fault, including: The various components of the leakage current in the faulty sheath protector are equivalent to harmonic sources and injected into the metal sheath loop from the position of the faulty sheath protector. After filtering by the cable sheath, the harmonic content in the sheath current at both ends of the cable changes due to the fault position of the sheath protector, and the harmonic content of the leakage current in the faulty sheath protector is used as the fault diagnosis criterion.

5. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 1 is characterized in that: The step of obtaining the current difference between the end of the cable and the head end includes the following steps: Install 6 power frequency current transformers in the grounding boxes at both ends of the cross-connected high-voltage cable lines to synchronously obtain the sheath current signals at both ends of the cable; The collected sheath current signals are grouped according to sheath loops, and the sheath current differences of the corresponding groups are calculated respectively; Perform Fourier transform on the sheath current difference waveform to obtain the harmonic content in the leakage current.

6. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 1 is characterized in that: The method of obtaining the equivalent impedance of the sheath protector comprises the following steps: Obtain the parallel equivalent resistance and the parallel equivalent capacitance of the sheath protector; Establish an equivalent impedance calculation model; Substitute the parallel equivalent resistance and the parallel equivalent capacitance into the equivalent impedance calculation model to obtain the equivalent impedance.

7. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 6 is characterized in that: The equivalent impedance calculation model is expressed as: In the formula, Z L represents equivalent impedance, j represents imaginary factor, ω represents power frequency angular velocity; Rsvl represents parallel equivalent resistance, and Csvl represents parallel equivalent capacitance.

8. The method for online diagnosis of sheath protector fault based on sheath current difference amplitude-frequency characteristic according to claim 1 is characterized in that: The current difference including harmonic components of the fundamental current difference phase angle obtained according to the leakage current comprises: In the simulation of the cable line after the sheath protector fails, the sheath circuit models at the power frequency and harmonic frequency are established respectively. In the equivalent circuit model of the harmonic component after the sheath protector fails, the faulty sheath protector is regarded as a harmonic source, the cable sheath circuit is equivalent to a series RL circuit, and it is assumed that the ground impedance at the beginning and end of the cable is equal. In the line with a total loop length of l, the transfer function of the sheath current difference to the harmonic component of the leakage current of the faulty sheath protector is defined as the ratio of the two.

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