Online diagnosis method for grounding fault of metal sheath of power transmission cable

Abstract

The invention discloses a power transmission cable metal sheath grounding fault on-line diagnosis method, which comprises the steps of S1, dividing a to-be-detected high-voltage power transmission cable into a plurality of sections, defining one end of the to-be-detected section of the high-voltage power transmission cable as a head end, and defining the other end of the to-be-detected section of the high-voltage power transmission cable as a tail end; s2, a distributed circulating current detection device is installed on the high-voltage power transmission cable to be detected; s3, collecting a current signal on the to-be-detected section of the high-voltage power transmission cable by adopting a distributed circulating current detection device, and estimating the amplitude and phase of the collected current signal by adopting a Prony method; s4, synchronizing current phases of all the distributed circulating current detection devices mounted on the high-voltage power transmission cable to be detected; and S5, calculating characteristic currents, comparing the amplitudes, phases or vectors of the calculated three groups of characteristic currents, and judging that the cable sheath has a grounding fault when data are inconsistent in comparison. When the method is applied, accurate diagnosis of the ground fault of the metal sheath of the high-voltage power transmission cable on site can be realized.

Description

Technical Field

[0001] This invention relates to the field of fault detection in power transmission cables, specifically an online diagnostic method for grounding faults in the metallic sheath of power transmission cables, belonging to the field of inspection and maintenance technology for power transmission cables. Background Technology

[0002] Against the backdrop of urbanization, cross-linked polyethylene (XLPE) power cables, with their excellent electrical insulation and mechanical properties, are increasingly being used in power transmission systems. For high-voltage transmission cables, single-core XLPE cables with a metallic sheath are typically used to ensure reliable power supply. During operation, induced voltages can appear on the metallic sheath of single-core XLPE cables. To prevent excessive grounding currents caused by these induced voltages, a grounding method is usually employed, involving direct grounding at one end, protective grounding at the other, or cross-connection grounding. However, during cable service, damage to the outer sheath due to environmental corrosion, external forces, or inherent quality issues can lead to grounding faults in the metallic sheath. This alters the normal grounding method of the metallic sheath, forcing the generation of large grounding currents. Under the thermal effect of these grounding currents, critical structural components such as the cable's insulation layer may be damaged, leading to insulation breakdown and affecting the stability of the transmission system. Therefore, in order to improve the operational reliability of power transmission cables, it is necessary to quickly and accurately diagnose grounding faults in the metallic sheath of power transmission cables.

[0003] Existing methods for diagnosing grounding faults in the metallic sheath of power transmission cables mainly include power outage testing and online testing. Power outage testing primarily includes the DC withstand voltage method and the high-voltage pulse method. The DC withstand voltage method applies a DC voltage to the metallic sheath of the test section of the cable and then reads the amplitude of the leakage current to diagnose grounding faults in the metallic sheath. The high-voltage pulse method applies a high-voltage pulse electrical signal to the metallic sheath of the test section of the cable and then analyzes the characteristics of the discharge signal to diagnose grounding faults in the metallic sheath. Although power outage testing methods can diagnose grounding faults in the cable sheath, they suffer from problems such as long power outage times and untimely diagnosis.

[0004] Online testing methods primarily analyze the circulating current or related characteristic quantities (such as temperature) of the cable sheath during operation to diagnose cable sheath grounding faults. This approach not only eliminates the need for power outages but also allows for timely detection of grounding faults in the cable sheath. Existing online testing methods mainly include the following approaches:

[0005] The invention patent published by Chinese Patent Publication No. CN111123041A on May 8, 2020, entitled "A Method for Locating Cable Sheath Faults Based on Temperature Characteristics," provides a method for diagnosing cable sheath grounding faults by measuring the temperature difference before and after the sheath. However, since the circulating current after a sheath grounding fault may not necessarily cause a significant temperature rise in the cable sheath, and the temperature measurement results are greatly affected by the ambient temperature, the versatility of this method needs to be improved.

[0006] The invention patent published by Chinese Patent Publication No. CN115201715A on October 18, 2022, entitled "Identification Method and System for Grounding Fault Mode of Cross-Interconnected High-Voltage Cable Sheath", provides a method for diagnosing cable sheath grounding faults by constructing the clustering results of fault feature quantities using the sheath equivalent circuit model and comparing them with the sheath circulating current value of the target high-voltage cable. However, this method requires the prior establishment of an accurate sheath equivalent circuit model, which makes the application of this method in engineering quite difficult.

[0007] The invention patent published by China Patent Publication No. 31 on January 31, 2023, entitled "Online Diagnosis Method for High Voltage Cable Sheath Defects Based on Sheath Current Anomalies," provides a method for recording changes in sheath current to diagnose cable sheath grounding faults. However, since normal fluctuations in the load current in the cable can also cause changes in the sheath current, this method may lead to misjudgments of cable sheath grounding faults.

[0008] The invention patent published by Chinese Patent Publication No. CN112881863A on June 1, 2021, entitled "Online Monitoring Method for High Voltage Cable Faults Based on a Novel Criterion Constructed by Sheath Current," provides a method for diagnosing cable sheath grounding faults by comparing the amplitude and phase of the sheath grounding current at the beginning and end of the main cross-interconnection section of the cable. However, this method is only applicable to the single case where the intermediate joints at the beginning and end of the main cross-interconnection section are all insulated joints, and it does not have universality.

[0009] The invention patent published by Chinese Patent Publication No. CN116087678A on May 9, 2023, entitled "An Online Location Method and System for Grounding Faults in the Sheath of High-Voltage Transmission Cables," provides a method for diagnosing grounding faults in the cable sheath by monitoring the sudden change in circulating current at the beginning and end of a cable section. However, this method does not consider the time error of the devices at the beginning and end, which may lead to deviations in the vector calculation of the circulating current, making it difficult to accurately diagnose grounding faults in the cable sheath.

[0010] In summary, existing online diagnostic methods for grounding faults in the metallic sheath of power transmission cables suffer from problems such as difficulty in time synchronization, poor versatility, and high misjudgment rate. Therefore, researching new diagnostic methods for grounding faults in the metallic sheath of power transmission cables is of great significance. Summary of the Invention

[0011] The purpose of this invention is to solve the problems of existing online diagnostic methods for grounding faults in the metallic sheath of power transmission cables, such as difficulty in time synchronization, poor versatility, and high misjudgment rate. This invention provides an online diagnostic method for grounding faults in the metallic sheath of power transmission cables. It proposes a segmented diagnostic scheme for the sheath of high-voltage power transmission cables and an arrangement scheme for a distributed circulating current detection device. Furthermore, it proposes a current acquisition method and a phase synchronization method for the distributed circulating current detection device. Finally, it determines the grounding fault diagnosis method for the cable sheath. When applied, this method can accurately diagnose grounding faults in the metallic sheath of high-voltage power transmission cables in the field.

[0012] The objective of this invention is mainly achieved through the following technical solutions:

[0013] An online diagnostic method for grounding faults in the metallic sheath of power transmission cables includes the following steps:

[0014] Step S1: Divide the high-voltage transmission cable to be tested into multiple segments, and define one end of the high-voltage transmission cable to be tested as the beginning end and the other end as the end end;

[0015] Step S2: Install a distributed circulating current detection device on the high-voltage transmission cable section to be tested; wherein, if the high-voltage transmission cable section to be tested is a single-section cable sheath, install one set of distributed circulating current detection devices at the sheath grounding box at the beginning and end of the cable; if the high-voltage transmission cable section to be tested is a double-section cable sheath, install one set of distributed circulating current detection devices at the sheath grounding box at the beginning, middle and end of the cable.

[0016] Step S3: Use a distributed circulating current detection device to collect the current signal on the high-voltage transmission cable under test, and use the Prony method to estimate the amplitude and phase of the collected current signal.

[0017] Step S4: Synchronize the current phase of all distributed circulating current detection devices installed on the high-voltage transmission cable section to be tested;

[0018] Step S5: Calculate the characteristic currents of phases A, B, and C, or calculate the characteristic currents of phases AB, BC, and CA. Compare the amplitude, phase, or vector of the three sets of characteristic currents. If the data is inconsistent, it is determined that there is a grounding fault in the cable sheath.

[0019] Furthermore, the single-segment cable sheath is the sheath at both ends of the high-voltage transmission cable sheath of the section under test, which is an insulating joint, including:

[0020] The first sheath grounding box is an independent direct grounding box, and the last sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box.

[0021] The first sheath grounding box is a cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a cross-interconnected ABC grounding box.

[0022] The first sheath grounding box is a cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a cross-interconnected ACB grounding box.

[0023] The grounding box for the first sheath layer is a protective grounding box, and the grounding box for the last sheath layer is an independent direct grounding box.

[0024] The dual-section cable sheath is the sheath of the high-voltage transmission cable section under test, with insulated joints at both ends and a straight-through joint in the middle, including:

[0025] The first sheath grounding box is a protective grounding box, the middle sheath grounding box is a common direct grounding box, and the last sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box.

[0026] The first sheath grounding box is a cross-interconnected ABC grounding box, the middle sheath grounding box is a common direct grounding box, and the end sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box.

[0027] The first sheath grounding box is a cross-interconnected ACB grounding box, the middle sheath grounding box is a common direct grounding box, and the last sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box.

[0028] Furthermore, the distributed circulating current detection device includes six current transformers, namely, the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers.

[0029] The installation method for the A, B, and C phase main core current transformers is as follows: For independent direct grounding boxes, protective grounding boxes, cross-interconnected ABC grounding boxes, and cross-interconnected ACB grounding boxes, when it is the grounding box at the beginning of the sheath section of the high-voltage transmission cable under test, the A, B, and C phase main core current transformers are respectively installed on the cable body of phases A, B, and C at the end of the insulating joint or cable terminal; when it is the grounding box at the end of the sheath section of the high-voltage transmission cable under test, the A, B, and C phase main core current transformers are respectively installed on the cable body of phases A, B, and C at the beginning of the insulating joint or cable terminal; for shared direct grounding boxes, the A, B, and C phase main core current transformers should be respectively installed on the cable body of phases A, B, and C at the end of the straight-through joint; the direction of all main core current transformers is from the beginning to the end.

[0030] The installation methods for the sheath current transformers of phases A, B, and C are as follows: For independent direct grounding boxes, protective grounding boxes, and shared direct grounding boxes, the sheath current transformers of phases A, B, and C are respectively installed on the grounding wires of the single-core cables of phases A, B, and C of the grounding box, with the connectors facing the grounding box. For cross-interconnected ABC grounding boxes and cross-interconnected ACB grounding boxes, the sheath current transformers are installed using either coaxial measurement mode or continuous measurement mode. In coaxial measurement mode, the sheath current transformers of phases A, B, and C are respectively installed on the coaxial cable grounding wires of phases A, B, and C of the grounding box, with the connectors facing the grounding box. In continuous measurement mode, the sheath current transformers of phases A, B, and C are respectively installed on the continuous sections A to B, B to C, and C to A inside the grounding box, with the directions of the sheath current transformers of phases A, B, and C being A to B, B to C, and C to A, respectively.

[0031] 4. The online diagnostic method for grounding faults in the metallic sheath of transmission cables according to claim 3, characterized in that, the estimation of the amplitude and phase of the acquired current signal using the Prony method in step S3 includes the following steps:

[0032] The Prony method uses a linear combination of exponential functions to fit periodically sampled signals. Setting the damping factor in the exponential function to 0, the current signal i(n) is then expressed as:

[0033] i(n) = Abz n +A(bz n ) -1 +w(n)

[0034] In the formula, n is the counting variable, w(n) is white noise, and Abz n with A(bz n ) -1 These are complex sinusoidal signals at 50Hz and -50Hz, respectively. z = exp(j2πf)g T c ), where j is the imaginary unit, and 2A is the magnitude of i(n). It is the phase of i(n), f g It is the frequency of the power system, T c It is the sampling interval period; where i(n) is a real sine wave signal, which consists of a positive frequency complex sine wave signal and a corresponding negative frequency complex sine wave signal, let

[0035]

[0036] In the formula, δ, Q, and I are all defined computational matrices, and N is the data length of i(n). We can obtain...

[0037] I = δQ

[0038] Using the least squares method, we can obtain

[0039] Q=(δ H δ) -1 δ H I

[0040] In the formula, H It is about finding the transpose of a matrix;

[0041] After obtaining A and b, the amplitude F and phase P of the current signal are further obtained as follows:

[0042] F = 2A

[0043] P = angle(b)

[0044] In the formula, angle is used to calculate the phase angle.

[0045] Furthermore, step S4 includes current phase synchronization in single-segment cable sheath test mode and current phase synchronization in double-segment cable sheath test mode.

[0046] Current phase synchronization in the single-segment cable sheath test mode includes the following steps:

[0047] Define the current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the head-end sheath grounding box as F, respectively. A1 F B1 F C1 F a1 F b1 F c1 The phases are P A1 P B1 P C1 P a1 P b1 P c1The vectors formed by their amplitude and phase are respectively I A1 I B1 I C1 I a1 I b1 I c1 The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the end sheath grounding box are defined as F, respectively. A2 F B2 F C2 F a2 F b2 F c2 The phases are P A2 P B2 P C2 P a2 P b2 P c2 ;

[0048] The amplitude deviations of the three-phase main core cables of the two sets of distributed circulating current detection devices are calculated as follows:

[0049] ΔF A12 =|F A1 -F A2 |

[0050] ΔF B12 =|F B1 -F B2 |

[0051] ΔF C12 =|F C1 -F C2 |

[0052] Compare ΔF A12 ΔF B12 ΔF C12 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation value of the main core cable of phase X of the two sets of distributed circulating current detection devices is calculated as follows:

[0053] ΔP X12 =P X1 -P X2

[0054] In the formula, P X1 It refers to P A1 P B1 P C1 The corresponding value of the X phase; P X2 It refers to P A2 P B2 P C2 The corresponding value of phase X in the middle;

[0055] Based on the phase deviation value ΔP X12 The phase of the current data in the distributed circulating current detection device at the end sheath grounding box is corrected to obtain the corrected phase. They are respectively:

[0056]

[0057]

[0058]

[0059]

[0060]

[0061]

[0062] Define the original amplitude F of the distributed circulating current detection device at the end sheath grounding box. A2 F B2 F C2 F a2 F b2 F c2 With the corrected phase The current vectors formed are I A2 I B2 I C2 I a2 I b2 I c2 ;

[0063] Current phase synchronization in the dual-segment cable sheath test mode includes the following steps:

[0064] Define the current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the head-end sheath grounding box as F, respectively. A1 F B1 F C1 F a1 F b1 F c1 The phases are P A1 P B1 P C1 P a1 P b1 P c1 The vectors formed by their amplitude and phase are respectively I A1 I B1 I C1 I a1 I b1I c1 ;

[0065] The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the end sheath grounding box are defined as F. A2 F B2 F C2 F a2 F b2 F c2 The phases are P A2 P B2 P C2 P a2 P b2 P c2 ;

[0066] The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers of the distributed circulating current detection device at the intermediate sheath grounding box are defined as F. A3 F B3 F C3 F a3 F b3 F c3 The phases are P A3 P B3 P C3 P a3 P b3 P c3 The vectors formed by their amplitude and phase are respectively Meanwhile, the intermediate variable vector of this distributed circulation detection device is calculated as follows:

[0067]

[0068]

[0069]

[0070] The amplitude deviations of the main core cable of the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the middle sheath grounding box are calculated as follows:

[0071] ΔF A13 =|F A1 -E(I AM3 )|

[0072] ΔF B13 =|F B1 -E(I BM3 )|

[0073] ΔF C13 =|FC1 -E(I CM3 )|

[0074] In the formula, E() is used to calculate the magnitude of the vector;

[0075] Compare ΔF A13 ΔF B13 ΔF C13 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation of the X-phase main core cable between the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the middle sheath grounding box is calculated as follows:

[0076] ΔP X13 =P X1 -G(I XM3 )

[0077] In the formula, P X1 It refers to P A1 P B1 P C1 The corresponding value of the X phase; I XM3 It refers to I AM3 I BM3 I CM3 The corresponding value of phase X in the middle; G() is used to find the phase of the vector;

[0078] Based on the phase deviation value ΔP X13 The phase of the current data in the distributed circulating current detection device at the intermediate sheath grounding box is corrected to obtain the corrected phase. They are respectively:

[0079]

[0080]

[0081]

[0082]

[0083]

[0084]

[0085] Define the original amplitude F of the distributed circulating current detection device at the intermediate sheath grounding box. A3 F B3 F C3 F a3 F b3 F c3 With the corrected phase The vectors formed are I A3I B3 I C3 I a3 I b3 I c3 ;

[0086] The amplitude deviations of the main core cables of the distributed circulating current detection device at the intermediate sheath grounding box and the distributed circulating current detection device at the end sheath grounding box are calculated as follows:

[0087] ΔF A32 =|E(I A3 )-F A2 |

[0088] ΔF B32 =|E(I B3 )-F B2 |

[0089] ΔF C32 =|E(I C3 )-F C2 |

[0090] Compare ΔF A32 ΔF B32 ΔF C32 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation between the distributed circulating current detection device at the intermediate sheath grounding box and the phase X main core cable installed at the end sheath grounding box is calculated as follows:

[0091] ΔP X32 =G(I X3 )-P X2

[0092] Based on the phase deviation value ΔP X32 The phase of the current data in the distributed circulating current detection device at the end sheath grounding box is corrected to obtain the corrected phase. They are respectively:

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099] Define the original amplitude F of the distributed circulating current detection device at the end sheath grounding box.A2 F B2 F C2 F a2 F b2 F c2 With the corrected phase The vectors formed are I A2 I B2 I C2 I a2 I b2 I c2 .

[0100] Furthermore, when the ABC grounding box in the continuous cross-interconnection mode is the first-end sheath grounding box in step S5, its current vector is corrected as follows:

[0101]

[0102]

[0103]

[0104]

[0105]

[0106]

[0107] In the formula: It is the intermediate variable of the current vector of the device at the grounding box of the first end sheath;

[0108] When the interconnected ACB grounding box in the continuous pattern is the end sheath grounding box, its current vector is corrected as follows:

[0109]

[0110]

[0111]

[0112]

[0113]

[0114]

[0115] In the formula: It is an intermediate variable of the current vector of the device at the end sheath grounding box;

[0116] When testing a single section of cable sheath, the characteristic current is calculated as follows:

[0117] When the first sheath grounding box is an independent direct grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box; when the first sheath grounding box is a connected cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a protective grounding box, and the last sheath grounding box is an independent direct grounding box; these 8 grounding box layout methods adopt I 合a =I a1 +I a2 Calculate the characteristic current of phase A using I 合b =I b1 +I b2 Calculate the characteristic current of phase B using I 合c =I c1 +I c2 Calculate the characteristic current of phase C;

[0118] When the first sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box, and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; these two grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 -I a2 Calculate the characteristic current of phase CA;

[0119] When the first sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; these two grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1+I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c2 Calculate the characteristic current of phase CA;

[0120] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box, these two grounding box layout methods adopt I. 合a -I 合b =I a1 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0121] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box, this grounding box layout adopts I. 合a -I 合b =I a1 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a2 Calculate the characteristic current of phase CA;

[0122] When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box, these two grounding box layout methods adopt I 合a -I 合b =-I b1 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +Ib2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0123] When both the first and last sheath grounding boxes are coaxial cross-interconnected ACB grounding boxes, this type of grounding box layout adopts I. 合a -I 合b =-I b1 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c2 Calculate the characteristic current of phase CA;

[0124] The characteristic current in step S5, which tests the sheath of a double-section cable, is calculated as follows:

[0125] When the first sheath grounding box is a protective grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ABC grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ACB grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; these 9 grounding box layout methods adopt I 合a =I a1 +I a3 +I a2 Calculate the characteristic current of phase A using I 合b =I b1 +I b3 +I b2 Calculate the characteristic current of phase B using I 合c =I c1 +I c3 +I c2 Calculate the characteristic current of phase C;

[0126] When the first sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; these three grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA;

[0127] When the first sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; these three grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA;

[0128] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box, these three grounding box layout methods adopt I 合a -I 合b =I a1 +I a3 -I b3 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0129] When both the first and last sheath grounding boxes are coaxial cross-interconnected ABC grounding boxes, this type of grounding box layout adopts I. 合a -I 合b =I a1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA;

[0130] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; this grounding box layout method adopts I. 合a -I 合b =I a1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I合b -I 合c =I b1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA;

[0131] When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; this grounding box layout method adopts I. 合a -I 合b =-I b1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA;

[0132] When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box, and the last sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, these three grounding box layout methods adopt I. 合a -I 合b =-I b1 +I a3 -I b3 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -Ia3 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0133] When both the first and last sheath grounding boxes are coaxial cross-interconnected ACB grounding boxes, this type of grounding box layout adopts I 合a -I 合b =-I b1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA.

[0134] Furthermore, step S5 also includes comparing the calculated characteristic current with a set amplitude threshold. When the amplitude of a certain characteristic current is greater than the set amplitude threshold, it is determined that a single-phase grounding fault has occurred in the test sheath section corresponding to the characteristic current.

[0135] Current online diagnostic methods for grounding faults in the metallic sheath of power cables do not consider issues such as load current fluctuations, straight-through joint installations, and device time synchronization. This makes these methods unsuitable for cable sheaths with various connection methods used in engineering projects, and may even lead to incorrect diagnoses of grounding faults in the cable sheath. This invention provides an online diagnostic method for grounding faults in the metallic sheath of high-voltage transmission cables, applicable to engineering sites, enabling accurate diagnosis of such faults in the field.

[0136] In summary, the present invention has the following advantages compared with the prior art: (1) The present invention adopts a segmented diagnostic scheme for the sheath of high-voltage transmission cables, and determines the single-segment cable sheath test method and the double-segment cable sheath test method. It is applicable to common cable sheath connection methods, cable joint types and sheath grounding box types, thus ensuring the universality of the method of the present invention.

[0137] (2) This invention designs an installation method for a distributed circulating current detection device. This method can detect all relevant current data of the cable sheath in real time, and the test current data is independent of the load fluctuation of the cable under test and its co-channel cables, which can ensure the accuracy of the diagnostic results. At the same time, two methods, coaxial measurement and continuous measurement, are considered for the cross-connection (ABC) grounding box and the cross-connection (ACB) grounding box, which can facilitate on-site testing.

[0138] (3) The present invention proposes a current acquisition scheme for a distributed circulating current detection device. The amplitude and phase of the current signal acquired in the distributed circulating current detection device are estimated by using the Prony method, which can avoid problems such as spectrum leakage and picket fence effect in the traditional Fourier transform algorithm, so as to accurately estimate the amplitude and phase of the signal.

[0139] (4) The present invention has developed a phase synchronization scheme for distributed circulating current detection devices. By collecting current data from different distributed circulating current detection devices, phase synchronization of each device can be achieved. This not only has low implementation difficulty and low device cost, but also high accuracy of phase synchronization, which can ensure the accuracy of vector operation between circulating current data of each device.

[0140] (5) Based on the current data of each distributed circulating current detection device, the present invention defines a characteristic current, and by comparing the amplitude, phase and vector of the characteristic current, the grounding fault of the cable sheath can be quickly and accurately diagnosed. Attached Figure Description

[0141] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0142] Figure 1 A flowchart of a specific embodiment of the present invention;

[0143] Figure 2 Schematic diagram of the installation of a distributed circulating current detection device at an independent direct grounding box;

[0144] Figure 3 A schematic diagram of the installation of the distributed circulating current detection device at the protective grounding box;

[0145] Figure 4 A schematic diagram of the installation of a distributed circulating current detection device at the cross-connection (ABC) grounding box (coaxial measurement mode);

[0146] Figure 5 A schematic diagram of the installation of a distributed circulating current detection device at the cross-connection (ABC) grounding box (connected measurement mode);

[0147] Figure 6Schematic diagram of the installation of a distributed circulating current detection device at the cross-connection (ACB) grounding box (coaxial measurement mode);

[0148] Figure 7 Schematic diagram of the installation of a distributed circulating current detection device at the cross-connection (ACB) grounding box (connected measurement mode);

[0149] Figure 8 Schematic diagram of the installation of a distributed circulating current detection device at a shared direct grounding box;

[0150] Figure 9 This is a schematic diagram showing the flow of current in the cable sheath when there is no grounding fault point.

[0151] Figure 10 This is a schematic diagram showing the current flow direction on the cable sheath when a grounding fault occurs.

[0152] The corresponding names of the reference numerals in the attached diagram are as follows: 1. Phase A cable body, 2. Phase B cable body, 3. Phase C cable body, 4. Intermediate joint, 5. Sheath grounding box, 6. Phase A main core current transformer, 7. Phase B main core current transformer, 8. Phase C main core current transformer, 9. Phase A sheath current transformer, 10. Phase B sheath current transformer, 11. Phase C sheath current transformer, 12. Protector. Detailed Implementation

[0153] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0154] Example 1:

[0155] like Figure 1As shown, an online diagnostic method for grounding faults in the metallic sheath of a power transmission cable includes the following steps: Step S1: Divide the high-voltage power transmission cable under test into multiple segments, defining one end of the segment as the starting end and the other end as the ending end; Step S2: Install a distributed circulating current detection device on the segment of the high-voltage power transmission cable under test; wherein, if the segment of the high-voltage power transmission cable under test is a single-segment cable sheath, a distributed circulating current detection device is installed at the sheath grounding box at the starting and ending ends respectively; if the segment of the high-voltage power transmission cable under test is a double-segment cable sheath, a distributed circulating current detection device is installed at the sheath grounding box at the starting end, the middle, and the ending ends respectively. A distributed circulating current detection device is installed; Step S3: The distributed circulating current detection device is used to collect the current signal on the high-voltage transmission cable under test, and the Prony method is used to estimate the amplitude and phase of the collected current signal; Step S4: The current phases of all distributed circulating current detection devices installed on the high-voltage transmission cable under test are synchronized; Step S5: The characteristic currents of phases A, B, and C, or the characteristic currents of phases AB, BC, and CA are calculated, and the amplitude, phase, or vector of the three sets of characteristic currents are compared. If the data is inconsistent, it is determined that there is a grounding fault in the cable sheath. In this embodiment, when the high-voltage transmission cable under test is divided into multiple segments in step S1, it should be ensured that grounding boxes are distributed at both ends of each segment of the high-voltage transmission cable under test.

[0156] Common cable sheath grounding methods can be divided into three types: single-end protective grounding, ABC (overlapping) grounding, and ACB (overlapping) grounding. Single-end protective grounding refers to one end of the metallic sheath being directly grounded, and the other end being protectively grounded. ABC grounding refers to both ends of the metallic sheath being directly grounded, where the three sheath segments are defined from the beginning to the end as the first segment, second segment, and third segment. The A, B, and C phase sheaths of the first segment are connected to the B, C, and A phase sheaths of the second segment, respectively, and the A, B, and C phase sheaths of the second segment are connected to the B, C, and A phase sheaths of the third segment. ACB grounding refers to both ends of the metallic sheath being directly grounded, where the three sheath segments are defined from the beginning to the end as the first segment, second segment, and third segment. The A, B, and C phase sheaths of the first segment are connected to the C, A, and B phase sheaths of the second segment, respectively, and the A, B, and C phase sheaths of the second segment are connected to the C, A, and B phase sheaths of the third segment.

[0157] For the common cable sheath grounding methods mentioned above, the intermediate joints used can be divided into two types: insulated joints and straight-through joints. The insulation body of insulated joints and straight-through joints is the same. The difference is that the shielding layer and metal sheath of the cable at both ends inside the insulated joint are disconnected, while the shielding layer and metal sheath of the cable at both ends inside the straight-through joint are continuous. Therefore, the cable sheaths at both ends of the insulated joint are connected to the sheath grounding box by two single-core cables or one coaxial cable (the cable sheath at one end is connected to the shield of the coaxial cable, and the cable sheath at the other end is connected to the core of the coaxial cable). The cable sheaths at both ends of the straight-through joint are connected to the sheath grounding box by one single-core cable.

[0158] Cable sheath grounding boxes can be classified into five types: independent direct grounding boxes, protective grounding boxes, cross-connection (ABC) grounding boxes, cross-connection (ACB) grounding boxes, and shared direct grounding boxes. Among them, an independent direct grounding box refers to a grounding box where the sheaths of the three phases of the cable under test at the insulation joint or cable terminal are connected to the grounding box directly via three single-core cables; a protective grounding box refers to a grounding box where the sheaths of the three phases of the cable under test at the insulation joint or cable terminal are connected to the grounding box via three single-core cables and then grounded via a sheath protector; a cross-connection (ABC) grounding box refers to a grounding box where the sheaths of the three phases of the cable on both sides of the insulation joint are connected to the grounding box via three coaxial cables, and then grounded via a sheath protector after an ABC cross-connection. At the joint, the sheaths of phases A, B, and C on the first end side are connected to the sheaths of phases B, C, and A on the last end side, respectively. A cross-connection (ACB) grounding box refers to a grounding box where the sheaths of the three-phase cables on both sides of the insulation joint enter the grounding box via three coaxial cables, and after cross-connection in an ACB manner, are grounded via a sheath protector. The sheaths of phases A, B, and C on the first end side of the joint are connected to the sheaths of phases C, A, and B on the last end side, respectively. A shared direct grounding box refers to a grounding box where the sheaths of the three-phase cables at the straight-through joint enter the grounding box via three single-core cables and are directly grounded.

[0159] Based on the distinctions regarding cable sheath connection methods, cable joint types, and sheath grounding box types, this embodiment conducts segment-by-segment testing of the transmission cable sheath, setting up single-segment cable sheath testing and double-segment cable sheath testing methods. The single-segment cable sheath testing method is suitable for cases where both ends of the cable sheath under test are insulated joints, while the double-segment cable sheath testing method is suitable for cases where both ends of the cable sheath under test are insulated joints, and the middle is a straight-through joint. The single-segment cable sheath testing method requires the installation of two sets of distributed circulating current detection devices at the sheath grounding boxes at the beginning and end points, while the double-segment cable sheath testing method requires the installation of three sets of distributed circulating current detection devices at the sheath grounding boxes at the beginning, middle, and end points. The segment types for the single-segment and double-segment cable sheath testing methods are shown in Tables 1 and 2.

[0160] Table 1. Applicable Grounding Box Layout Methods for Single-Segment Cable Sheath Testing Methods

[0161] Serial Number Type of grounding box for first end sheath End sheath grounding box type 1 Independent direct grounding box Protective grounding box 2 Independent direct grounding box Cross-connection (ABC) grounding box 3 Independent direct grounding box Cross-connection (ACB) grounding box 4 Cross-connection (ABC) grounding box Independent direct grounding box 5 Cross-connection (ABC) grounding box Cross-connection (ABC) grounding box 6 Cross-connection (ACB) grounding box Independent direct grounding box 7 Cross-connection (ACB) grounding box Cross-connection (ACB) grounding box 8 Protective grounding box Independent direct grounding box

[0162] As shown in Table 1, in this embodiment, the sheath of a single cable segment is the sheath of the high-voltage transmission cable to be tested. The sheaths at both ends are insulated joints. The grounding box layout includes: the grounding box of the first sheath is an independent direct grounding box, and the grounding box of the last sheath is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box; the grounding box of the first sheath is a cross-interconnected ABC grounding box, and the grounding box of the last sheath is an independent direct grounding box or a cross-interconnected ABC grounding box; the grounding box of the first sheath is a cross-interconnected ACB grounding box, and the grounding box of the last sheath is an independent direct grounding box or a cross-interconnected ACB grounding box; the grounding box of the first sheath is a protective grounding box, and the grounding box of the last sheath is an independent direct grounding box.

[0163] Table 2 Applicable Grounding Box Layout Methods for Two-Section Cable Sheath Testing Methods

[0164] Serial Number Type of grounding box for first end sheath Intermediate sheath grounding box type End sheath grounding box type 1 Protective grounding box Shared direct grounding box Protective grounding box 2 Protective grounding box Shared direct grounding box Cross-connection (ABC) grounding box 3 Protective grounding box Shared direct grounding box Cross-connection (ACB) grounding box 4 Cross-connection (ABC) grounding box Shared direct grounding box Protective grounding box 5 Cross-connection (ABC) grounding box Shared direct grounding box Cross-connection (ABC) grounding box 6 Cross-connection (ABC) grounding box Shared direct grounding box Cross-connection (ACB) grounding box 7 Cross-connection (ACB) grounding box Shared direct grounding box Protective grounding box 8 Cross-connection (ACB) grounding box Shared direct grounding box Cross-connection (ABC) grounding box 9 Cross-connection (ACB) grounding box Shared direct grounding box Cross-connection (ACB) grounding box

[0165] In this embodiment, the double-segment cable sheath is the sheath of the high-voltage transmission cable to be tested. The sheaths at both ends are insulated joints, and the middle one is a straight joint. The grounding box layout includes: the grounding box at the first end of the sheath is a protective grounding box, the grounding box at the middle sheath is a common direct grounding box, and the grounding box at the end of the sheath is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box; the grounding box at the first end of the sheath is a cross-interconnected ABC grounding box, the grounding box at the middle sheath is a common direct grounding box, and the grounding box at the end of the sheath is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box; the grounding box at the first end of the sheath is a cross-interconnected ACB grounding box, the grounding box at the middle sheath is a common direct grounding box, and the grounding box at the end of the sheath is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box.

[0166] The distributed circulating current detection device in this embodiment includes six current transformers, namely, the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers. It should be noted that the transfer functions of the six cable current transformers need to have the same phase shift characteristics. If their phase shift characteristics are different, their phase shift characteristics can be measured in advance and digital compensation can be performed.

[0167] For the five types of cable sheath grounding boxes, the installation diagram of the distributed circulating current detection device is as follows: Figures 2 to 8 As shown, the cross-connection (ABC) grounding box and the cross-connection (ACB) grounding box can be set to two modes: coaxial measurement and continuous measurement. Figures 2 to 8 In the attached diagram, the corresponding names of the reference numerals are as follows: 1. A-phase cable body, 2. B-phase cable body, 3. C-phase cable body, 4. Intermediate joint, 5. Sheath grounding box, 6. A-phase main core current transformer, 7. B-phase main core current transformer, 8. C-phase main core current transformer, 9. A-phase sheath current transformer, 10. B-phase sheath current transformer, 11. C-phase sheath current transformer, 12. Protector.

[0168] like Figures 2 to 8 As shown, the installation method for the A, B, and C phase main core current transformers is as follows: For independent direct grounding boxes, protective grounding boxes, cross-interconnected ABC grounding boxes, and cross-interconnected ACB grounding boxes, when it is the grounding box at the beginning of the sheath section of the high-voltage transmission cable under test, the A, B, and C phase main core current transformers are respectively installed on the cable body of phases A, B, and C at the end of the insulation joint or cable terminal; when it is the grounding box at the end of the sheath section of the high-voltage transmission cable under test, the A, B, and C phase main core current transformers are respectively installed on the cable body of phases A, B, and C at the beginning of the insulation joint or cable terminal; for shared direct grounding boxes, the A, B, and C phase main core current transformers should be installed on the cable body of phases A, B, and C at the end of the straight-through joint. All main core current transformers are oriented from the beginning to the end. It should be noted that... Figures 2 to 8 Only the case where the main core current transformer is installed at the end of an insulated joint or cable terminal is shown.

[0169] The installation methods for the sheath current transformers of phases A, B, and C are as follows: For independent direct grounding boxes, protective grounding boxes, and shared direct grounding boxes, the sheath current transformers of phases A, B, and C are respectively installed on the grounding wires of the single-core cables of phases A, B, and C in the grounding box, with the joints facing the box body. For cross-interconnected ABC grounding boxes and cross-interconnected ACB grounding boxes, the sheath current transformers are installed using either coaxial measurement mode or continuous measurement mode. In coaxial measurement mode, the sheath current transformers of phases A, B, and C are respectively installed on the coaxial cable grounding wires of phases A, B, and C in the grounding box, with the joints facing the box body. In continuous measurement mode, the sheath current transformers of phases A, B, and C are respectively installed on the continuous sections A to B, B to C, and C to A inside the grounding box, with the directions of the sheath current transformers of phases A, B, and C being A to B, B to C, and C to A, respectively.

[0170] China uses a power system with a nominal frequency of 50Hz. Therefore, in China's high-voltage transmission cables, the main frequency component of the core current is 50Hz. According to the law of electromagnetic induction, the grounding loop of the metal sheath will generate an induced current with a main frequency component of 50Hz. Distributed circulating current detection devices collect current signals from the high-voltage transmission cables. In this embodiment, the amplitude and phase of this current signal are estimated using the Prony method. The Prony method uses a linear combination of exponential functions to fit the periodically sampled signals. Setting the damping factor in the exponential function to 0, the current signal i(n) is then expressed as:

[0171] i(n) = Abz n +A(bz n ) -1 +w(n)

[0172] In the formula, n is the counting variable, w(n) is white noise, and Abz n with A(bz n ) -1 These are complex sinusoidal signals at 50Hz and -50Hz, respectively. z = exp(j2πf) g T c ), where j is the imaginary unit, and 2A is the magnitude of i(n). It is the phase of i(n), f g It is the frequency of the power system, T c It is the sampling interval period; where i(n) is a real sine wave signal, which consists of a positive frequency complex sine wave signal and a corresponding negative frequency complex sine wave signal, let

[0173]

[0174] In the formula, δ, Q, and I are all defined computational matrices, and N is the data length of i(n). We can obtain...

[0175] I = δQ

[0176] Using the least squares method, we can obtain

[0177] Q=(δ H δ) -1 δ H I

[0178] In the formula, H It is about finding the transpose of a matrix;

[0179] After obtaining A and b, the amplitude F and phase P of the current signal are further obtained as follows:

[0180] F = 2A

[0181] P = angle(b)

[0182] In the formula, angle is used to calculate the phase angle.

[0183] Because this embodiment uses a linear combination of exponential functions to fit the periodically sampled signal in order to estimate the amplitude and phase of the periodically sampled signal, which is different from the principle of the traditional Fourier transform algorithm, this embodiment does not have the inherent problems of spectrum leakage and picket fence effect of the traditional Fourier transform algorithm. Even under non-integer period sampling conditions, the method of this embodiment can still accurately estimate the amplitude and phase of the current signal.

[0184] Assuming the current signal has an amplitude of 40A, a phase of 45°, a sampling interval of 2.5ms, and a data length of 85, the amplitude and phase are estimated using the method of this embodiment and the traditional Fourier transform algorithm, respectively. The amplitude and phase estimated by the traditional Fourier transform algorithm are 31.3754A and -22.8135°, respectively, while the amplitude and phase estimated by the method of this embodiment are 40.0000A and 45.0000°, respectively.

[0185] The results show that, under non-integer period sampling conditions, the method in this embodiment can accurately estimate the amplitude and phase of the current signal, while the estimation results of the Fourier transform algorithm have a larger deviation. Analysis of this phenomenon reveals that the frequency resolution of the traditional Fourier transform algorithm is 4.7059Hz. Since 50Hz is not an integer multiple of 4.7059Hz, the traditional Fourier transform algorithm struggles to accurately estimate the amplitude and phase of the current signal due to issues such as spectral leakage and the picket fence effect.

[0186] Vector calculation of sheath circulating current is a crucial step in diagnosing sheath grounding faults. Phase synchronization of each distributed circulating current detection device is a key prerequisite for this vector calculation. In traditional sheath circulating current analysis schemes, GPS synchronization clocks are typically used to synchronize the phase of the distributed circulating current detection devices. This method is not only costly to implement but also difficult to apply to underground cable tunnels without signals. Chinese Patent Publication No. CN114859104A, published on August 5, 2022, discloses an invention patent entitled "Method and System for Offline Detection Time Synchronization of Sheath Current in Cross-Interconnected High-Voltage Cables." This invention provides a method for synchronizing each circulating current detection device by calculating the optimal time offset of the capacitive current residual. However, an intact three-phase cable sheath is a prerequisite for accurately calculating the capacitive current residual, making this method unsuitable for cables with existing sheath faults. Chinese Patent Publication No. CN112067946A published on December 11, 2020, an invention patent entitled "Cable Sheath Fault Detection Device and Method with Dorothecus Coil Broadcast Synchronization Signal," which proposes using a broadcast signal as the synchronization signal for each circulating current detection device. However, this method has a large synchronization error, and the transmission and acquisition results of the broadcast signal are affected by strong electromagnetic interference in the cable channel. This embodiment provides a novel phase synchronization scheme for a distributed circulating current detection device to correct the time error of each device and ensure the accuracy of vector calculations between the circulating current data of each device.

[0187] Step S4 in this embodiment includes current phase synchronization in single-segment cable sheath testing mode and current phase synchronization in double-segment cable sheath testing mode. The current phase synchronization in single-segment cable sheath testing mode includes the following steps: defining the current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers of the distributed circulating current detection device at the first-end sheath grounding box as F0. A1 F B1 F C1 F a1 F b1 F c1 The phases are P A1 P B1 P C1 P a1 P b1 P c1 The vectors formed by their amplitude and phase are respectively I A1 I B1 I C1 I a1 I b1 I c1 The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the end sheath grounding box are defined as F, respectively. A2F B2 F C2 F a2 F b2 F c2 The phases are P A2 P B2 P C2 P a2 P b2 P c2 ;

[0188] The amplitude deviations of the three-phase main core cables of the two sets of distributed circulating current detection devices are calculated as follows:

[0189] ΔF A12 =|F A1 -F A2 |

[0190] ΔF B12 =|F B1 -F B2 |

[0191] ΔF C12 =|F C1 -F C2 |

[0192] Compare ΔF A12 ΔF B12 ΔF C12 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation value of the main core cable of phase X of the two sets of distributed circulating current detection devices is calculated as follows:

[0193] ΔP X12 =P X1 -P X2

[0194] In the formula, P X1 It refers to P A1 P B1 P C1 The corresponding value of the X phase; P X2 It refers to P A2 P B2 P C2 The corresponding value of phase X in the middle;

[0195] Based on the phase deviation value ΔP X12 The phase of the current data in the distributed circulating current detection device at the end sheath grounding box is corrected to obtain the corrected phase. They are respectively:

[0196]

[0197]

[0198]

[0199]

[0200]

[0201]

[0202] Define the original amplitude F of the distributed circulating current detection device at the end sheath grounding box. A2 F B2 F C2 F a2 F b2 F c2 With the corrected phase The current vectors formed are I A2 I B2 I C2 I a2 I b2 I c2 At this point, the current data from the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the last sheath grounding box have achieved phase synchronization, correcting the time error between the two sets of distributed circulating current detection devices.

[0203] In this embodiment, the current phase synchronization under the dual-section cable sheath testing mode includes the following steps: The current amplitudes collected by the A, B, and C phase main core current transformers of the distributed circulating current detection device at the first-end sheath grounding box and the A, B, and C phase sheath current transformers are defined as F... A1 F B1 F C1 F a1 F b1 F c1 The phases are P A1 P B1 P C1 P a1 P b1 P c1 The vectors formed by their amplitude and phase are respectively I A1 I B1 I C1 I a1 I b1 I c1 ;

[0204] The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the end sheath grounding box are defined as F. A2 F B2 FC2 F a2 F b2 F c2 The phases are P A2 P B2 P C2 P a2 P b2 P c2 ;

[0205] The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers of the distributed circulating current detection device at the intermediate sheath grounding box are defined as F. A3 F B3 F C3 F a3 F b3 F c3 The phases are P A3 P B3 P C3 P a3 P b3 P c3 The vectors formed by their amplitude and phase are respectively Meanwhile, the intermediate variable vector of this distributed circulation detection device is calculated as follows:

[0206]

[0207]

[0208]

[0209] The amplitude deviations of the main core cable of the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the middle sheath grounding box are calculated as follows:

[0210] ΔF A13 =|F A1 -E(I AM3 )|

[0211] ΔF B13 =|F B1 -E(I BM3 )|

[0212] ΔF C13 =|F C1 -E(I CM3 )|

[0213] In the formula, E() is used to calculate the magnitude of the vector;

[0214] Compare ΔF A13 ΔF B13 ΔFC13 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation of the X-phase main core cable between the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the middle sheath grounding box is calculated as follows:

[0215] ΔP X13 =P X1 -G(I XM3 )

[0216] In the formula, P X1 It refers to P A1 P B1 P C1 The corresponding value of the X phase; I XM3 It refers to I AM3 I BM3 I CM3 The corresponding value of phase X in the middle; G() is used to find the phase of the vector;

[0217] Based on the phase deviation value ΔP X13 The phase of the current data in the distributed circulating current detection device at the intermediate sheath grounding box is corrected to obtain the corrected phase. They are respectively:

[0218]

[0219]

[0220]

[0221]

[0222]

[0223]

[0224] Define the original amplitude F of the distributed circulating current detection device at the intermediate sheath grounding box. A3 F B3 F C3 F a3 F b3 F c3 With the corrected phase The vectors formed are I A3 I B3 I C3 I a3 I b3 I c3 ;

[0225] The amplitude deviations of the main core cables of the distributed circulating current detection device at the intermediate sheath grounding box and the distributed circulating current detection device at the end sheath grounding box are calculated as follows:

[0226] ΔF A32 =|E(I A3 )-F A2 |

[0227] ΔF B32 =|E(I B3 )-F B2 |

[0228] ΔF C32 =|E(I C3 )-F C2 |

[0229] Compare ΔF A32 ΔF B32 ΔF C32 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation between the distributed circulating current detection device at the intermediate sheath grounding box and the phase X main core cable installed at the end sheath grounding box is calculated as follows:

[0230] ΔP X32 =G(I X3 )-P X2

[0231] Based on the phase deviation value ΔP X32 The phase of the current data in the distributed circulating current detection device at the end sheath grounding box is corrected to obtain the corrected phase. They are respectively:

[0232]

[0233]

[0234]

[0235]

[0236]

[0237]

[0238] Define the original amplitude F of the distributed circulating current detection device at the end sheath grounding box. A2 F B2 F C2 F a2 F b2 F c2 With the corrected phase The vectors formed are I A2 I B2 I C2 I a2 I b2 I c2 At this point, the current data from the distributed circulating current detection devices at the first sheath grounding box, the middle sheath grounding box, and the end sheath grounding box have achieved phase synchronization, correcting the time error of the three sets of distributed circulating current devices.

[0239] An analysis of a certain section of the cable sheath is conducted. When there is no grounding fault point in the cable sheath, the schematic diagram of the current flow direction on the sheath is as follows: Figure 9 As shown in the figure, I s It refers to the current flowing from the cable sheath to the beginning of the cable, I. e It refers to the current flowing from the cable sheath to the end, I. c This is the leakage current in the main insulation of the cable, primarily manifested as a capacitive current component. According to Kirchhoff's law (KCL), we obtain...

[0240] I 合 =I s +I e =I c

[0241] When a grounding fault occurs in the cable sheath, the current flow direction on the sheath is shown in the diagram below. Figure 10 As shown in the figure, I g This refers to the current flowing from the cable sheath to the point of grounding fault. According to Kirchhoff's law (KCL), we obtain...

[0242] I 合 =I s +I e =I c -I g .

[0243] Comparing the two equations above, it can be seen that through the characteristic current I 合 This can determine whether there is a grounding fault in the cable sheath. When there is no single-phase sheath grounding fault in the cable, the characteristic current I... 合 It will manifest as capacitive current, at which point the I of the three phases 合 The amplitudes are consistent, but the phase difference is 120°. When a single-phase ground fault occurs in the sheath of a certain phase of the cable, the characteristic current I of that phase... 合 It will manifest as the difference between the capacitive current and the ground fault current. In this case, the capacitive current can be ignored, and the I of this phase... 合 The amplitude will be relatively large, and the I of the three phases will also be relatively large. 合 The amplitudes are no longer consistent, and the phases are no longer 120° apart, which can be used to diagnose grounding faults in the cable sheath.

[0244] For ease of calculation, when the cross-connected (ABC) grounding box (continuous measurement mode) is the first-end sheath grounding box, its current vector is corrected as follows:

[0245]

[0246]

[0247]

[0248]

[0249]

[0250]

[0251] In the formula: It is the intermediate variable of the current vector of the device at the grounding box of the first end sheath.

[0252] When the cross-connection (ACB) grounding box (continuous measurement mode) is the end sheath grounding box, its current vector is corrected as follows:

[0253]

[0254]

[0255]

[0256]

[0257]

[0258]

[0259] In the formula: It is an intermediate variable of the current vector of the device at the end sheath grounding box.

[0260] In this embodiment, when diagnosing sheath grounding faults under the single-segment cable sheath test method, the characteristic current calculation formula is defined as shown in Table 3 for different combinations of the first-end sheath grounding box type and the last-end sheath grounding box type in Table 1.

[0261] Table 3. Formulas for calculating characteristic current under single-segment cable sheath testing method

[0262]

[0263]

[0264] Based on the characteristic current calculation formula defined in Table 3, the characteristic current calculation results under the single-segment cable sheath test method are shown in Table 4.

[0265] Table 4. Calculation results of characteristic current under single-segment cable sheath testing method

[0266]

[0267]

[0268] As shown in Tables 3 and 4, in this embodiment, when the first sheath grounding box is an independent direct grounding box and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ABC grounding box and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box; when the first sheath grounding box is a connected cross-interconnected ACB grounding box and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a protective grounding box and the last sheath grounding box is an independent direct grounding box; these eight grounding box layout methods adopt I. 合a =I a1 +I a2 Calculate the characteristic current of phase A using I 合b =I b1 +I b2 Calculate the characteristic current of phase B using I 合c =I c1 +I c2 Calculate the characteristic current of phase C;

[0269] When the first sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box, and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; these two grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 -I a2 Calculate the characteristic current of phase CA;

[0270] When the first sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; these two grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c2 Calculate the characteristic current of phase CA;

[0271] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box, these two grounding box layout methods adopt I. 合a -I 合b =I a1 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0272] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box, this grounding box layout adopts I. 合a -I 合b =I a1 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a2 Calculate the characteristic current of phase CA;

[0273] When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box, these two grounding box layout methods adopt I 合a -I 合b =-I b1 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0274] When both the first and last sheath grounding boxes are coaxial cross-interconnected ACB grounding boxes, this type of grounding box layout adopts I. 合a -I 合b =-I b1 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c2 Calculate the characteristic current of phase CA.

[0275] In this embodiment, when diagnosing sheath defects under the double-segment cable sheath testing method, the characteristic current calculation formula is defined as shown in Table 5 for different combinations of the first-end sheath grounding box type, the middle sheath grounding box type and the last sheath grounding box type in Table 2.

[0276] Table 5. Calculation formulas for characteristic current under the test method of double-section cable sheath.

[0277]

[0278]

[0279] Based on the characteristic current calculation formula defined in Table 5, the characteristic current calculation results under the double-section cable sheath test method are shown in Table 6.

[0280] Table 6. Calculation results of characteristic current under the test method of double-section cable sheath.

[0281]

[0282] As shown in Tables 5 and 6, in this embodiment, when the first sheath grounding box is a protective grounding box and the last sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box; when the first sheath grounding box is a continuous cross-interconnected ABC grounding box and the last sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box; when the first sheath grounding box is a continuous cross-interconnected ACB grounding box and the last sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box; these nine grounding box layout methods adopt I. 合a =I a1 +I a3 +I a2 Calculate the characteristic current of phase A using I 合b =I b1 +I b3 +I b2 Calculate the characteristic current of phase B using I 合c =I c1 +I c3 +I c2 Calculate the characteristic current of phase C;

[0283] When the first sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; these three grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA;

[0284] When the first sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; these three grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA;

[0285] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box, these three grounding box layout methods adopt I 合a -I 合b =I a1 +I a3 -I b3 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0286] When both the first and last sheath grounding boxes are coaxial cross-interconnected ABC grounding boxes, this type of grounding box layout adopts I. 合a -I 合b =Ia1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA;

[0287] When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; this grounding box layout method adopts I. 合a -I 合b =I a1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA;

[0288] When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; this grounding box layout method adopts I. 合a -I 合b =-I b1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +Ic3 -I a3 -I a2 Calculate the characteristic current of phase CA;

[0289] When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box, and the last sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, these three grounding box layout methods adopt I. 合a -I 合b =-I b1 +I a3 -I b3 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 +I c2 -I a2 Calculate the characteristic current of phase CA;

[0290] When both the first and last sheath grounding boxes are coaxial cross-interconnected ACB grounding boxes, this type of grounding box layout adopts I 合a -I 合b =-I b1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA.

[0291] When the circulating current at the cross-connection (ABC) grounding box and the cross-connection (ACB) grounding box is not measured using the coaxial measurement mode, the three sets of characteristic currents calculated are I 合a I 合b I合c When there is no grounding fault in the sheath of the three-phase cable, I 合a I 合b I 合c These are the leakage currents (mainly capacitive currents) of the main insulation of phases A, B, and C of the test cable section, respectively. Therefore, I 合a I 合b I 合c The amplitudes are consistent, and the phase difference is 120°. When a ground fault occurs in the cable sheath of a certain phase, the characteristic current of that phase is the difference between the leakage current of the main insulation of that phase and the current at the ground fault point. Since the leakage current of the main insulation is smaller, the current at the ground fault point is dominant. Therefore, the amplitude of the characteristic current of that phase will be significantly larger than that of the other two phases, and the phase difference between the characteristic current of that phase and the characteristic current of the other two phases will no longer be 120°.

[0292] When the circulating current at the cross-connection (ABC) grounding box and the cross-connection (ACB) grounding box is measured in coaxial mode, the three sets of characteristic currents calculated are I 合a -I 合b I 合b -I 合c I 合c -I 合a When there is no grounding fault in the sheath of the three-phase cable, I 合a -I 合b I 合b -I 合c I 合c -I 合a These are the interphase leakage currents of the main insulation of phases AB, BC, and CA of the test cable section, respectively. Therefore, I 合a -I 合b I 合b -I 合c I 合c -I 合a The amplitudes are consistent, and the phase difference is 120°. When a ground fault occurs in the cable sheath of a certain phase, the corresponding two sets of characteristic currents are the difference between the interphase main insulation leakage current of the test cable section and the ground fault point current. Since the interphase main insulation leakage current is small, the ground fault point current dominates. Therefore, the amplitudes of the corresponding two sets of characteristic currents will be significantly larger than the other one set, and the phase difference of the three sets of characteristic currents will no longer be 120°.

[0293] In summary, the presence of a grounding fault in the cable sheath can be determined by comparing the amplitude, phase, or vector of three sets of characteristic currents. Step S5 of this embodiment further includes comparing the calculated characteristic current with a set amplitude threshold. When the amplitude of a certain characteristic current exceeds the set amplitude threshold, it is determined that a single-phase grounding fault has occurred in the test sheath segment corresponding to that characteristic current.

[0294] It should be noted that since the induced circulating current of the cable sheath is proportional to the cable load, while the leakage current of the main insulation is independent of the cable load, it is necessary to select a high-load period to carry out the induced circulating current detection of the cable. At this time, the circulating current value at the sheath fault grounding point is larger, which makes it easier to distinguish from the leakage current of the main insulation and increases the accuracy of fault diagnosis.

[0295] Considering measurement and calculation errors, when the coaxial test mode of the cross-connected (ABC) grounding box or cross-connected (ACB) grounding box is not used, the amplitude threshold T of the three characteristic currents is... F Defined as

[0296] T F =k F i d

[0297] When using a coaxial test mode with either an ABC or ACB grounding box, the amplitude threshold T of the three characteristic currents... F Defined as

[0298]

[0299] In the formula: k F This is the amplitude threshold coefficient, preferably set to 1.2; d The capacitive current of the cable section to be tested can be obtained using theoretical values ​​or no-load current. Its theoretical value is...

[0300]

[0301] In the formula: U is the system voltage of the cable; C is the capacitance of the section of cable under test; ω=2πf g ω is the angular frequency of the fundamental component, i.e., ω = 100π; ε is the dielectric constant of the dielectric; r c and r s These are the radii of the cable core and the shielding layer, respectively; l is the length of the section of cable to be tested.

[0302] The no-load state refers to the cable line being in a hot standby state, meaning the load current in the cable line is the capacitive current of the entire line. From this, the i-value of the cable section under test can be obtained. d for

[0303]

[0304] i L The load current measured at the power supply side under no-load conditions is L, where L is the total length of the cable line.

[0305] The following is a specific example of this implementation:

[0306] Taking a 220kV transmission cable in a certain urban area as an example, its core cross-section is 2000mm². 2 Each cable segment is 500m long. Grounding fault diagnosis and analysis were carried out on two cable segments. It was determined that both cable segments were tested using a single-segment cable sheath method. The corresponding grounding box types for the first and last cable segments are shown in the table below.

[0307] Table 7 shows the types of grounding boxes for the first and last sheaths in the embodiments.

[0308]

[0309]

[0310] Table 8 shows the circulating current test data for the first and last sheath grounding boxes in serial number 1.

[0311]

[0312] Table 9 shows the circulating current test data for the first and last sheath grounding boxes in serial number 2.

[0313]

[0314] Table 10 shows the calculation results of the characteristic current in the examples.

[0315]

[0316] The amplitude threshold T in sequence 1 and sequence 2 is obtained by theoretical calculation. F The current is 8.5A. Based on the threshold test, it can be seen that the characteristic currents of all three phases in sequence 1 are less than the threshold, therefore it can be determined that there is no grounding fault in the sheath in Example 1. In sequence 2, the characteristic currents of phases AB and CA are both greater than the threshold, therefore it can be determined that there is a grounding fault in the sheath of phase A. Through cable inspection, it was found that the cable support arm damaged the outer sheath of the cable, leading to a grounding fault in the cable sheath.

[0317] It should be noted that, by comparing the characteristic currents in examples 1 and 2, it can be seen that when there is no grounding fault in the cable sheath, the amplitudes of the three sets of characteristic currents are almost identical, and the phases differ by about 120°. When there is a grounding fault in the cable sheath, the amplitudes of the three sets of characteristic currents are inconsistent, and the phases no longer satisfy the 120° difference relationship. Therefore, other amplitude, phase, and vector analysis methods can also be used to determine whether the three sets of characteristic currents are normal, thereby determining whether there is a grounding fault in the cable sheath. The relevant technologies will not be elaborated in this invention.

[0318] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for online diagnosis of grounding faults in the metallic sheath of power transmission cables, characterized in that, Includes the following steps: Step S1: Divide the high-voltage transmission cable to be tested into multiple segments, and define one end of the high-voltage transmission cable to be tested as the beginning end and the other end as the end end. Step S2: Install a distributed circulating current detection device on the high-voltage transmission cable section to be tested; wherein, if the high-voltage transmission cable section to be tested is a single-section cable sheath, install one set of distributed circulating current detection devices at the sheath grounding box at the beginning and end of the cable; if the high-voltage transmission cable section to be tested is a double-section cable sheath, install one set of distributed circulating current detection devices at the sheath grounding box at the beginning, middle and end of the cable. Step S3: Use a distributed circulating current detection device to collect the current signal on the high-voltage transmission cable under test, and use the Prony method to estimate the amplitude and phase of the collected current signal. Step S4: Synchronize the current phase of all distributed circulating current detection devices installed on the high-voltage transmission cable section to be tested; Step S5: Calculate the characteristic currents of phases A, B, and C, or calculate the characteristic currents of phases AB, BC, and CA. Compare the amplitude, phase, or vector of the three sets of characteristic currents. If the data is inconsistent, it is determined that there is a grounding fault in the cable sheath.

2. The online diagnostic method for grounding faults in the metallic sheath of power transmission cables according to claim 1, characterized in that, The single-segment cable sheath is the sheath of the high-voltage transmission cable section under test, and the sheaths at both ends are insulating joints, including: The first sheath grounding box is an independent direct grounding box, and the last sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box. The first sheath grounding box is a cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a cross-interconnected ABC grounding box. The first sheath grounding box is a cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a cross-interconnected ACB grounding box. The grounding box for the first sheath layer is a protective grounding box, and the grounding box for the last sheath layer is an independent direct grounding box. The dual-section cable sheath is the sheath of the high-voltage transmission cable section under test, with insulated joints at both ends and a straight-through joint in the middle, including: The first sheath grounding box is a protective grounding box, the middle sheath grounding box is a common direct grounding box, and the last sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box. The first sheath grounding box is a cross-interconnected ABC grounding box, the middle sheath grounding box is a common direct grounding box, and the end sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box. The first sheath grounding box is a cross-interconnected ACB grounding box, the middle sheath grounding box is a common direct grounding box, and the last sheath grounding box is a protective grounding box, a cross-interconnected ABC grounding box, or a cross-interconnected ACB grounding box.

3. The online diagnostic method for grounding faults in the metallic sheath of power transmission cables according to claim 2, characterized in that, The distributed circulating current detection device includes six current transformers, namely, the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers. The installation method for the A, B, and C phase main core current transformers is as follows: For independent direct grounding boxes, protective grounding boxes, cross-interconnected ABC grounding boxes, and cross-interconnected ACB grounding boxes, when it is the grounding box at the beginning of the sheath section of the high-voltage transmission cable under test, the A, B, and C phase main core current transformers are respectively installed on the cable body of phases A, B, and C at the end of the insulating joint or cable terminal; when it is the grounding box at the end of the sheath section of the high-voltage transmission cable under test, the A, B, and C phase main core current transformers are respectively installed on the cable body of phases A, B, and C at the beginning of the insulating joint or cable terminal; for shared direct grounding boxes, the A, B, and C phase main core current transformers should be respectively installed on the cable body of phases A, B, and C at the end of the straight-through joint; the direction of all main core current transformers is from the beginning to the end. The installation methods for the sheath current transformers of phases A, B, and C are as follows: For independent direct grounding boxes, protective grounding boxes, and shared direct grounding boxes, the sheath current transformers of phases A, B, and C are respectively installed on the grounding wires of the single-core cables of phases A, B, and C of the grounding box, with the connectors facing the grounding box. For cross-interconnected ABC grounding boxes and cross-interconnected ACB grounding boxes, the sheath current transformers are installed using either coaxial measurement mode or continuous measurement mode. In coaxial measurement mode, the sheath current transformers of phases A, B, and C are respectively installed on the coaxial cable grounding wires of phases A, B, and C of the grounding box, with the connectors facing the grounding box. In continuous measurement mode, the sheath current transformers of phases A, B, and C are respectively installed on the continuous sections A to B, B to C, and C to A inside the grounding box, with the directions of the sheath current transformers of phases A, B, and C being A to B, B to C, and C to A, respectively.

4. The online diagnostic method for grounding faults in the metallic sheath of power transmission cables according to claim 3, characterized in that, The step S3, which uses the Prony method to estimate the amplitude and phase of the acquired current signal, includes the following steps: The Prony method uses a linear combination of exponential functions to fit periodically sampled signals. Setting the damping factor in the exponential function to 0, the current signal i(n) is then expressed as: i(n)=Abz n +A(bz n ) -1 +w(n) In the formula, n is the counting variable, w(n) is white noise, and Abz n with A(bz n ) -1 These are complex sinusoidal signals at 50Hz and -50Hz, respectively. z = exp(j 2πf g T c ), where j is the imaginary unit, and 2A is the magnitude of i(n). It is the phase of i(n), f g It is the frequency of the power system, T c It is the sampling interval period; where i(n) is a real sine wave signal, which consists of a positive frequency complex sine wave signal and a corresponding negative frequency complex sine wave signal, let In the formula, δ, Q, and I are all defined computational matrices, and N is the data length of i(n). We can obtain... I = δQ Using the least squares method, we can obtain Q=(δ H d) -1 d H I In the formula, H It is about finding the transpose of a matrix; After obtaining A and b, the amplitude F and phase P of the current signal are further obtained as follows: F = 2A P = angle(b) In the formula, angle is used to calculate the phase angle.

5. The online diagnostic method for grounding faults in the metallic sheath of power transmission cables according to claim 3, characterized in that, Step S4 includes current phase synchronization in single-segment cable sheath test mode and current phase synchronization in double-segment cable sheath test mode. Current phase synchronization in the single-segment cable sheath test mode includes the following steps: Define the current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the head-end sheath grounding box as F, respectively. A1 F B1 F C1 F a1 F b1 F c1 The phases are P A1 P B1 P C1 P a1 P b1 P c1 The vectors formed by their amplitude and phase are I, respectively. A1 I B1 I C1 I a1 I b1 I c1 The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the end sheath grounding box are defined as F, respectively. A2 F B2 F C2 F a2 F b2 F c2 The phases are P A2 P B2 P C2 P a2 P b2 P c2 ; The amplitude deviations of the three-phase main core cables of the two sets of distributed circulating current detection devices are calculated as follows: ΔF A12 =|F A1 -F A2 | ΔF B12 =|F B1 -F B2 | ΔF C12 =|F C1 -F C2 | Compare ΔF A12 ΔF B12 ΔF C12 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation value of the main core cable of phase X of the two sets of distributed circulating current detection devices is calculated as follows: ΔP X12 =P X1 -P X2 In the formula, P X1 It refers to P A1 P B1 P C1 The corresponding value of the X phase; P X2 It refers to P A2 P B2 P C2 The corresponding value of the X phase; Based on the phase deviation value ΔP X12 The phase of the current data in the distributed circulating current detection device at the end sheath grounding box is corrected to obtain the corrected phase. They are respectively: Define the original amplitude F of the distributed circulating current detection device at the end sheath grounding box. A2 F B2 F C2 F a2 F b2 F c2 With the corrected phase The current vectors formed are I A2 I B2 I C2 I a2 I b2 I c2 ; Current phase synchronization in the dual-segment cable sheath test mode includes the following steps: Define the current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the head-end sheath grounding box as F, respectively. A1 F B1 F C1 F a1 F b1 F c1 The phases are P A1 P B1 P C1 P a1 P b1 P c1 The vectors formed by their amplitude and phase are I, respectively. A1 I B1 I C1 I a1 I b1 I c1 ; The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers at the distributed circulating current detection device at the end sheath grounding box are defined as F. A2 F B2 F C2 F a2 F b2 F c2 The phases are P A2 P B2 P C2 P a2 P b2 P c2 ; The current amplitudes collected by the A, B, and C phase main core current transformers and the A, B, and C phase sheath current transformers of the distributed circulating current detection device at the intermediate sheath grounding box are defined as F. A3 F B3 F C3 F a3 F b3 F c3 The phases are P A3 P B3 P C3 P a3 P b3 P c3 The vectors formed by their amplitude and phase are respectively Meanwhile, the intermediate variable vector of this distributed circulation detection device is calculated as follows: The amplitude deviations of the main core cable at the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the middle sheath grounding box are calculated as follows: ΔF A13 =|F A1 -E(I AM3 )| ΔF B13 =|F B1 -E(I BM3 )| ΔF C13 =|F C1 -E(I CM3 )| In the formula, E() represents the magnitude of the vector; Compare ΔF A13 ΔF B13 ΔF C13 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation of the X-phase main core cable between the distributed circulating current detection device at the first sheath grounding box and the distributed circulating current detection device at the middle sheath grounding box is calculated as follows: ΔP X13 =P X1 -G(I XM3 ) In the formula, P X1 It refers to P A1 P B1 P C1 The corresponding value of the X phase; I XM3 It refers to I AM3 I BM3 I CM3 The corresponding value of phase X in the middle; G() is used to find the phase of the vector; Based on the phase deviation value ΔP X13 The phase of the current data in the distributed circulating current detection device at the intermediate sheath grounding box is corrected to obtain the corrected phase. They are respectively: Define the original amplitude F of the distributed circulating current detection device at the intermediate sheath grounding box. A3 F B3 F C3 F a3 F b3 F c3 With the corrected phase The vectors formed are I A3 I B3 I C3 I a3 I b3 I c3 ; The amplitude deviations of the main core cables of the distributed circulating current detection device at the intermediate sheath grounding box and the distributed circulating current detection device at the end sheath grounding box are calculated as follows: ΔF A32 =|E(I A3 )-F A2 | ΔF B32 =|E(I B3 )-F B2 | ΔF C32 =|E(I C3 )-F C2 | Compare ΔF A32 ΔF B32 ΔF C32 The minimum value is determined to correspond to phase X, where X is one of phases A, B, and C. Phase X of the cable is considered the normal phase. The phase deviation between the distributed circulating current detection device at the intermediate sheath grounding box and the phase X main core cable installed at the end sheath grounding box is calculated as follows: ΔP X32 =G(I X3 )-P X2 Based on the phase deviation value ΔP X32 The phase of the current data in the distributed circulating current detection device at the end sheath grounding box is corrected to obtain the corrected phase. They are respectively: Define the original amplitude F of the distributed circulating current detection device at the end sheath grounding box. A2 F B2 F C2 F a2 F b2 F c2 With the corrected phase The vectors formed are I A2 I B2 I C2 I a2 I b2 I c2 .

6. The online diagnostic method for grounding faults in the metallic sheath of power transmission cables according to claim 3, characterized in that, In step S5, when the interconnected ABC grounding box in the continuous mode is the first-end sheath grounding box, its current vector is corrected as follows: In the formula: It is the intermediate variable of the current vector of the device at the grounding box of the first end sheath; When the interconnected ACB grounding box in the continuous pattern is the end sheath grounding box, its current vector is corrected as follows: In the formula: It is an intermediate variable of the current vector of the device at the end sheath grounding box; For single-segment cable sheath testing, Its characteristic current is calculated as follows: When the first sheath grounding box is an independent direct grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box; when the first sheath grounding box is a connected cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a protective grounding box, and the last sheath grounding box is an independent direct grounding box; these 8 grounding box layout methods adopt I 合a =I a1 +I a2 Calculate the characteristic current of phase A using I 合b =I b1 +I b2 Calculate the characteristic current of phase B using I 合c =I c1 +I c2 Calculate the characteristic current of phase C; When the first sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box, and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; these two grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 -I a2 Calculate the characteristic current of phase CA; When the first sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; these two grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ABC grounding box, these two grounding box layout methods adopt I. 合a -I 合b =I a1 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c2 -I a2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box, this grounding box layout adopts I. 合a -I 合b =I a1 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box, and the last sheath grounding box is an independent direct grounding box or a connected cross-interconnected ACB grounding box, these two grounding box layout methods adopt I 合a -I 合b =-I b1 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c2 -I a2 Calculate the characteristic current of phase CA; When both the first and last sheath grounding boxes are coaxial cross-interconnected ACB grounding boxes, this type of grounding box layout adopts I. 合a -I 合b =-I b1 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c2 Calculate the characteristic current of phase CA; When testing the sheath of a double-section cable in step S5, the characteristic current is calculated as follows: When the first sheath grounding box is a protective grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ABC grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; when the first sheath grounding box is a connected cross-interconnected ACB grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box; these 9 grounding box layout methods adopt I 合a =I a1 +I a3 +I a2 Calculate the characteristic current of phase A using I 合b =I b1 +I b3 +I b2 Calculate the characteristic current of phase B using I 合c =I c1 +I c3 +I c2 Calculate the characteristic current of phase C; When the first sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; these three grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; these three grounding box layout methods adopt I 合a -I 合b =I a1 -I b1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 -I c1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 -I a1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box, and the last sheath grounding box is a protective grounding box, a connected cross-interconnected ABC grounding box, or a connected cross-interconnected ACB grounding box, these three grounding box layout methods adopt I 合a -I 合b =I a1 +I a3 -I b3 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 +I c2 -I a2 Calculate the characteristic current of phase CA; When both the first and last sheath grounding boxes are coaxial cross-interconnected ABC grounding boxes, this type of grounding box layout adopts I. 合a -I 合b =I a1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ABC grounding box and the last sheath grounding box is a coaxial cross-interconnected ACB grounding box; this grounding box layout method adopts I. 合a -I 合b =I a1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =I b1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =I c1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box and the last sheath grounding box is a coaxial cross-interconnected ABC grounding box; this grounding box layout method adopts I. 合a -I 合b =-I b1 +I a3 -I b3 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 -I a2 Calculate the characteristic current of phase CA; When the first sheath grounding box is a coaxial cross-interconnected ACB grounding box, and the last sheath grounding box is a protective grounding box, a continuous cross-interconnected ABC grounding box, or a continuous cross-interconnected ACB grounding box, these three grounding box layout methods adopt I. 合a -I 合b =-I b1 +I a3 -I b3 +I a2 -I b2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 +I b2 -I c2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 +I c2 -I a2 Calculate the characteristic current of phase CA; When both the first and last sheath grounding boxes are coaxial cross-interconnected ACB grounding boxes, this type of grounding box layout adopts I 合a -I 合b =-I b1 +I a3 -I b3 +I a2 Calculate the characteristic current of phase AB using I 合b -I 合c =-I c1 +I b3 -I c3 +I b2 Calculate the characteristic current of phase BC using I 合c -I 合a =-I a1 +I c3 -I a3 +I c2 Calculate the characteristic current of phase CA.

7. A method for online diagnosis of grounding faults in the metallic sheath of transmission cables according to any one of claims 1 to 6, characterized in that, Step S5 further includes comparing the calculated characteristic current with a set amplitude threshold. When the amplitude of a certain characteristic current is greater than the set amplitude threshold, it is determined that a single-phase grounding fault has occurred in the test sheath section corresponding to the characteristic current.

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