A high-voltage cable temporary grounding device and a temporary grounding method

Abstract

The application provides a high-voltage cable temporary grounding device and a temporary grounding method, which comprises a front window opening detection grounding assembly, a main temporary grounding assembly and a sheath current limiting resistance assembly; the front window opening detection grounding assembly is pre-grounded and electrically connected with a metal sheath of the high-voltage cable, and the front window opening detection grounding assembly comprises a front grounding lead which is directly connected with an earth grounding electrode; the main temporary grounding assembly is installed on the metal sheath of the high-voltage cable, and the main temporary grounding assembly comprises a sheath connecting end and a grounding connecting end; wherein the sheath connecting end is electrically connected with the metal sheath; and the grounding connecting end is connected with the earth grounding electrode after being connected in series with the sheath current limiting resistance assembly. The front circuit of the application can not only fix the loose sheath structure but also instantaneously release the suspended induced charge, so as to prevent the electric shock danger in the opening stage; the main grounding circuit is connected in series with the current limiting resistance to build a closed loop with impedance, so as to inhibit the large current formed by the mutual inductance circulating current and the capacitive current after the double-end grounding.

Description

Technical Field

[0001] This invention belongs to the field of high-voltage cable operation and maintenance technology, specifically a temporary grounding device and method for high-voltage cables. Background Technology

[0002] High-voltage cables are core infrastructure of urban power grids. my country has nearly 69,000 kilometers of 66 kV and above high-voltage cables, with cable coverage exceeding 70% in first-tier cities. The metal sheath of high-voltage single-core power cables serves to shield electric fields, provide grounding protection, and offer mechanical protection. Its grounding reliability is directly related to operational safety.

[0003] In existing sheath grounding methods, single-end grounding permanently grounds only one end of the cable's aluminum sheath while leaving the other end suspended, in order to break the closed loop of the sheath and prevent induced circulating current. However, two prominent problems exist during long-term operation and maintenance:

[0004] First, grounding failure can cause floating overvoltage in the sheath. Cables operating in damp tunnels or buried environments are highly susceptible to corrosion of the permanent grounding wire in the aluminum sheath, leading to a significant increase in grounding circuit resistance or even complete open circuit, resulting in a floating potential in the aluminum sheath. The core's power frequency voltage continuously charges the cable through the distributed capacitance between the core and sheath. Simultaneously, the load current's mutual inductance induces a longitudinal power frequency electromotive force on the sheath, causing a floating induced overvoltage of several thousand volts in the aluminum sheath. Maintenance personnel face a high risk of electric shock when touching the cable or opening holes for repairs. To address this, the industry typically installs a temporary grounding device before maintenance to temporarily connect the floating aluminum sheath to the earth.

[0005] Secondly, the installation of a temporary grounding device introduces a potential hazard of double-ended closed-loop current. Once the temporary grounding device is installed, the aluminum sheath forms a double-ended closed loop between the permanent grounding terminal and the temporary grounding terminal. Within this loop, capacitive current and mutual inductance current are simultaneously superimposed. When the permanent grounding electrode is in good working condition and the circuit resistance is low, the mutual inductance current can reach tens of amperes. This, combined with the capacitive current, forms a large circulating current in the sheath at tens of amperes, leading to localized overheating and accelerated corrosion of the aluminum sheath, as well as overheating and burning of the grounding copper stranded wire. Simultaneously, the large current generates a power frequency voltage drop across the grounding lead resistance, raising the potential to ground at the work point and posing a risk of electric shock.

[0006] Traditional temporary grounding devices use only pure metal conductors for direct grounding, lacking a current-limiting voltage regulation structure, and cannot suppress large circulating currents in the sheath. If the loop resistance is directly increased to suppress the circulating current, the sheath's potential to ground will rise accordingly, resulting in a loss of safety protection. Some simplified current-limiting schemes do not quantitatively match the resistance parameters; if the resistance value is too small, the current limiting will be insufficient and the heat generation will be severe; if the resistance value is too large, the residual potential of the sheath to ground will exceed the standard, posing a safety hazard. Summary of the Invention

[0007] The main objective of this invention is to provide a temporary grounding device and method for high-voltage cables that can solve the problems of overheating and potential rise caused by large circulating currents in double-ended grounding.

[0008] The high-voltage cable temporary grounding device provided by this invention includes a pre-opening detection grounding component, a main temporary grounding component, and a sheath current-limiting resistor component. The pre-opening detection grounding component is electrically connected to the metal sheath of the high-voltage cable for pre-grounding. The pre-opening detection grounding component includes a pre-grounding lead, which is directly connected to the earth grounding electrode. The main temporary grounding component is installed on the metal sheath of the high-voltage cable and includes a sheath connection end and a grounding connection end. The sheath connection end is electrically connected to the metal sheath. The grounding connection end is connected to the earth grounding electrode after being connected in series with the sheath current-limiting resistor component.

[0009] In one embodiment of the above-mentioned temporary grounding device, the main temporary grounding assembly includes a main grounding fixing base, and a sheath contact bolt and a grounding lead fixing bolt disposed on the main grounding fixing base. The sheath contact bolt is used for electrical connection with the metal sheath; the grounding lead fixing bolt is used to connect to the earth grounding electrode after being connected in series with the sheath current limiting resistor assembly.

[0010] In one embodiment of the aforementioned temporary grounding device, the rated resistance of the current-limiting resistor assembly of the sheath is 200mΩ.

[0011] In one embodiment of the aforementioned temporary grounding device, the sheath current-limiting resistor assembly includes a current-limiting resistor body, a ceramic insulating package covering the current-limiting resistor body, heat-dissipating aluminum fins disposed on the outer surface of the ceramic insulating package, and quick-plug terminals disposed at both ends and with tin-plated surfaces.

[0012] In one embodiment of the aforementioned temporary grounding device, a grounding reliability detection component is included. The grounding reliability detection component is electrically connected to the front grounding lead and the sheath connection terminal and grounding connection terminal of the main temporary grounding component, respectively, and is used to detect the grounding loop impedance, induced current, and power frequency voltage drop across the sheath current limiting resistor component in real time.

[0013] In one embodiment of the aforementioned temporary grounding device, a sealing and repair component is further included, comprising room-temperature curing silicone rubber and waterproof insulating tape; the room-temperature curing silicone rubber fills the opening formed on the outer sheath of the cable by the pre-window detection grounding component, and the waterproof insulating tape is used to wrap and reinforce the opening for sealing.

[0014] In one embodiment of the aforementioned temporary grounding device, the pre-opening detection grounding assembly includes an opening fixing base, a blunt-tipped grounding pin, a sealing gasket, and the pre-grounding lead; the inner side of the opening fixing base is provided with a buffer insulating pad, the arc-shaped inner wall of which is used to fit the outer sheath surface of the high-voltage cable; the blunt-tipped grounding pin adopts a blunt-tipped structure, which can penetrate through the micro pre-installed hole on the high-voltage cable and make flexible contact with the cable's metal sheath.

[0015] A temporary grounding method utilizing the aforementioned temporary grounding device includes the following steps:

[0016] Install the pre-opening detection grounding component on the high-voltage cable and make it electrically connected to the pre-grounding of the cable metal sheath. Connect the pre-grounding lead directly to the earth grounding electrode without connecting the current limiting resistor in series to form an impedance-free pre-grounding path to release the residual induced charge on the metal sheath.

[0017] The main temporary grounding assembly is installed on the high-voltage cable. After being electrically connected to the cable metal sheath through the sheath connection end of the main temporary grounding assembly, it is directly connected to the earth grounding electrode. Then, after being connected to the earth grounding electrode through the sheath current-limiting resistor assembly in series through the grounding connection end, it forms a grounding path with no impedance and a grounding path with current-limiting impedance. This ensures reliable grounding of the metal sheath and prevents the metal sheath potential from floating, while suppressing the sheath circulating current generated when the metal sheath is grounded at both ends.

[0018] The grounding status of the main temporary grounding components is quantitatively tested to confirm that the grounding condition is qualified.

[0019] After completing the cable maintenance work, remove the front-end window detection grounding component and seal and repair the opening formed by the front-end window detection grounding component on the cable outer sheath; the main temporary grounding component is left on the cable after installation.

[0020] The grounding status of the main temporary grounding component is quantitatively detected, specifically including:

[0021] After installing the front-opening grounding detection component, use the grounding reliability detection component to detect the grounding loop resistance R from the metal sheath to the direct grounding box at the location of the front-opening grounding detection component;

[0022] After installing the main temporary grounding assembly and the sheath current-limiting resistor assembly, the parallel resistance R / / (Rj+Rx) of the metal sheath grounding circuit is detected at the same location of the front-opening grounding assembly using the grounding reliability detection assembly, where / / indicates parallel connection, Rx=200mΩ, and the three-phase contact resistance Rj between the main temporary grounding assembly and the metal sheath is calculated using the parallel formula.

[0023] The three-phase contact resistance Rj not exceeding 15mΩ and the voltage drop across the three-phase current-limiting resistor Rx not exceeding 1.2V are used as the criteria for determining whether the grounding condition is qualified.

[0024] The openings on the outer sheath of the cable are sealed and repaired. Specifically, for the micro pre-installed holes, after cleaning the debris inside the holes, room temperature curing silicone rubber is injected as filler. After being left to cure at room temperature, and after initial shaping, multiple layers of waterproof insulating tape are wrapped around the holes for reinforcement and sealing.

[0025] The beneficial effects of this invention are as follows:

[0026] 1. An independent pre-grounding circuit and a main grounding current-limiting circuit are set up. The pre-grounding circuit flexibly conducts aluminum sheath in advance through a miniature pre-installed hole, which can fix the corroded and loosened sheath structure, buffer the mechanical vibration of the opening, and avoid the risk of aluminum sheath breaking during operation from the root. It can also release the suspended induced charge and residual voltage instantly, eliminating the danger of high voltage electric shock during the opening stage. The main grounding circuit is connected in series with a current-limiting resistor to form an impedance closed circuit, which effectively suppresses the large current formed by the superposition of mutual inductance circulating current and capacitive current after double-end grounding, avoids overheating and burning of aluminum sheath and grounding lead, and prevents abnormal rise of ground potential at the maintenance point, so as to achieve inherent safety throughout the entire operation process.

[0027] 2. Based on the dual constraints of the upper limit of the current carrying capacity of the resistance heating power and the limit of the safe potential of the aluminum sheath to ground, the standard current limiting resistance value is quantified. On the one hand, the total current of the sheath is stably suppressed to within 6A, and the matching heat dissipation structure can achieve long-term current carrying without overheating risk. On the other hand, the residual potential of the aluminum sheath to ground is clamped within the safe range of 1.2V, without weakening the core protection functions of temporary grounding to discharge induced charge and clamping the sheath potential.

[0028] 3. The grounding reliability test adopts the parallel resistance calculation method of two measurements. By opening the window to detect the same measuring point at the location of the grounding component, the loop resistance and parallel resistance are obtained, and then the three-phase contact resistance is calculated. This enables a quantitative judgment of the grounding installation quality, avoids the difficulty of directly measuring the contact resistance in the traditional method, and improves the operability and accuracy of the test. Attached Figure Description

[0029] Figure 1 This is an installation diagram of one embodiment of the present invention.

[0030] Figure 2 for Figure 1 Installation diagram of the front-mounted window detection grounding component and the main temporary grounding component.

[0031] Figure 3 for Figure 2 A schematic diagram of the structure of the front-mounted window detection grounding component.

[0032] Figure 4 for Figure 2 A schematic diagram of the structure of the main temporary grounding assembly. Detailed Implementation

[0033] The relevant technical solutions will now be clearly and completely described with reference to the accompanying drawings of the embodiments of the present invention. The described embodiments are only a part of the embodiments, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figure 1 and Figure 2 As shown, the high-voltage cable temporary grounding device disclosed in this embodiment includes a grounding reliability detection component 1, a pre-opening detection grounding component 2, a main temporary grounding component 3, a sheath current limiting resistor component 4, and a sealing and repair component.

[0035] The pre-opening detection grounding assembly 2 and the main temporary grounding assembly 3 are arranged sequentially along the axial direction of the high-voltage cable to avoid localized stress concentration. In the operation process, the pre-opening detection grounding assembly is installed before the main temporary grounding assembly. It is used to connect testing instruments for grounding loop parameter testing and to perform preliminary discharge on the aluminum sheath. After the main temporary grounding assembly is installed and grounding reliability is confirmed, the pre-opening detection grounding assembly is removed. The main temporary grounding assembly remains on the cable after installation and does not need to be removed.

[0036] like Figure 3 As shown, the front-mounted window opening detection grounding assembly 2 includes a window opening fixing base 201, a blunt-tipped grounding pin 202, a strap 203, a screw handle 204, a ratchet tightening handle 205, a safety grounding terminal 206, a sealing gasket, and a front-mounted grounding lead.

[0037] When installing the front-mounted window detection grounding component 2, a miniature pre-installation hole with a diameter of 5mm is opened on the corresponding side wall of the high-voltage cable. The inner wall of the pre-installation hole is polished and smooth without burrs.

[0038] The window-opening fixing base 201 adopts an aluminum alloy arc-shaped structure with a silicone buffer insulation pad on its inner side. The arc-shaped inner wall can tightly fit the outer sheath surface of the high-voltage cable. The binding strap 203 is inserted on both sides of the window-opening fixing base to secure it to the outer wall of the cable. The ratchet tightening handle 205 is linked to the binding strap, and the ratchet mechanism tightens the binding strap in one direction, making the window-opening fixing base firmly attached to the cable surface.

[0039] The blunt-tipped grounding pin 202 is aligned with the miniature pre-installed hole. It features a copper blunt-tipped structure with no sharp edges and a smooth, rounded end. The lead screw handle 204 is connected to the tip of the blunt-tipped grounding pin, which is threaded into the miniature pre-installed hole. Rotating the lead screw handle drives the blunt-tipped grounding pin axially along the miniature pre-installed hole, allowing its end to pierce the cable's plastic outer sheath and gently reach the aluminum sheath surface, achieving electrical continuity without causing hard compression damage to the aluminum sheath.

[0040] A sealing gasket is fitted between the blunt-tipped grounding pin 202 and the miniature pre-installed hole to maintain the seal at the pre-installed hole after the pin passes through, preventing moisture from entering the cable along the hole wall.

[0041] The safety grounding terminal 206 is located on the side of the window mounting base 201 and is electrically connected to the blunt-tipped grounding pin 202. One end of the pre-grounding lead is connected to the safety grounding terminal, and the other end is directly connected to the earth grounding electrode. The pre-grounding lead uses multi-strand high-voltage resistant soft copper stranded wire with a cross-sectional area of ​​16mm². No current-limiting resistor is connected in series in this pre-grounding branch to ensure that the residual induced charge on the aluminum sheath can be discharged instantaneously and quickly.

[0042] After the pre-discharge and grounding parameter testing tasks are completed and the main temporary grounding component 3 is installed, the pre-opening detection grounding component 2 is completely removed. The 5mm micro pre-installed hole formed after removal is filled and repaired by the sealing and repair component in subsequent processes.

[0043] like Figure 4 As shown, the main temporary grounding assembly 3 includes a main grounding fixing base 301, an aluminum sheath contact bolt 302, a grounding lead fixing bolt 303, a fixing steel strip 304, a sealing waterproof gasket 305, a first main grounding lead, and a second main grounding lead.

[0044] The main grounding fixing seat 301 adopts an arc-shaped insulation structure and is in the form of a semi-enclosed card seat. A buffer pad is pasted on its inner side. During installation, it is locked to the outer wall of the high-voltage cable by the fixing steel strip 304. Its installation position is staggered with the front window detection grounding component 2 along the cable axis to avoid stress concentration caused by two openings at the same cross section of the cable.

[0045] The fixing steel strip 304 is inserted on both sides of the main grounding fixing seat 301, wraps around the outer circumference of the cable and then tightens and locks it, so that the inner buffer pad of the fixing seat is tightly attached to the surface of the cable outer sheath, buffering the mechanical vibration of subsequent drilling and grinding operations.

[0046] The main grounding mounting base 301 has two spaced-apart installation channels, which are used to pass through the aluminum sheath contact bolt 302 and the grounding lead fixing bolt 303, respectively.

[0047] The aluminum sheath contact bolt 302 is inserted into one of the mounting channels on the main grounding mounting base 301. Its top features an arc-shaped end face design, the arc profile matching the trough curve of the cable's aluminum sheath. After tightening, the top of the bolt passes through the cable's plastic outer sheath and achieves large-area contact with the aluminum sheath surface, effectively reducing contact resistance. A first main grounding lead is crimped to the tail terminal of the aluminum sheath contact bolt 302. The other end of the first main grounding lead is directly connected to the earth grounding electrode, forming a direct grounding path. No current-limiting resistor is connected in series in this direct grounding path, providing a continuous and rapid discharge channel for residual induced charge on the aluminum sheath during maintenance work, while simultaneously clamping the aluminum sheath's potential to earth directly to the earth potential.

[0048] The grounding lead fixing bolt 303 passes through another mounting channel on the main grounding fixing base 301. Its top also passes through the cable's plastic outer sheath and contacts the surface of the aluminum sheath, achieving electrical continuity with the aluminum sheath. A second main grounding lead is crimped to the terminal at the tail of the grounding lead fixing bolt 303. The other end of the second main grounding lead is first connected in series with the sheath current-limiting resistor assembly 4 before being connected to the earth grounding electrode, forming a current-limiting grounding path. This current-limiting grounding path contains a current-limiting resistor of rated resistance, used to increase the total impedance of the circuit when the aluminum sheath forms a double-ended closed loop, suppressing the large current formed by the superposition of mutual inductance circulating current and capacitive current.

[0049] The sealing and waterproof gasket 305 is made of silicone and is 10mm thick. It is located at the bottom of the main grounding fixing base 301 and serves as a waterproof seal. Both the first and second main grounding leads are made of multi-strand high-voltage resistant soft copper stranded wire.

[0050] The sheath current-limiting resistor assembly 4 is arranged in series in the current-limiting grounding path, that is, between the second main grounding lead and the earth grounding electrode. The sheath current-limiting resistor assembly 4 includes a current-limiting resistor body, a ceramic insulating package, heat dissipation aluminum fins, and two quick-plug copper terminals.

[0051] The rated resistance of the current-limiting resistor body is 200mΩ. A ceramic insulating enclosure completely covers the current-limiting resistor body, and heat-dissipating aluminum fins are fixed to the outer surface of the ceramic insulating enclosure to increase the heat dissipation area. Two quick-plug copper terminals are located at both ends of the ceramic insulating enclosure, and their surfaces are tin-plated to reduce contact heat generation. One quick-plug copper terminal connects to the second main grounding lead on the aluminum sheath side, and the other quick-plug copper terminal connects to the grounding lead on the earth side. This quick-plug structure allows the sheath current-limiting resistor assembly 4 to be quickly replaced according to different cable voltage levels and line lengths.

[0052] The grounding reliability testing component 1 employs a live-line diagnostic instrument for high-voltage cable grounding status, with a detection resolution of 0.001mΩ. During the operation of the pre-opening grounding testing component 2, this diagnostic instrument is electrically connected to the pre-grounding lead to preliminarily detect the grounding loop resistance and residual voltage data of the aluminum sheath, determining whether the pre-grounding status of the aluminum sheath is qualified. After the main temporary grounding component 3 is installed, the diagnostic instrument then becomes electrically connected to the first and second main grounding leads, acquiring and displaying in real time the grounding loop resistance, three-phase induced current, and power frequency voltage drop data across the current-limiting resistor. When the voltage drop across the current-limiting resistor exceeds 1.2V or the loop current exceeds a preset threshold, the diagnostic instrument automatically triggers an audible and visual alarm.

[0053] The sealing and repair components consist of room-temperature curing silicone rubber and butyl waterproof insulating tape. The room-temperature curing silicone rubber fully cures after 12 hours of standing at room temperature, achieving an insulation strength of no less than 20kV / mm. The 5mm micro-pre-installed hole left after removing the pre-installed window detection grounding component 2 is filled and repaired by injecting room-temperature curing silicone rubber after maintenance, and then reinforced and sealed with multiple layers of butyl waterproof insulating tape in conjunction with the silicone rubber.

[0054] The overall function and principle of this temporary grounding device for high-voltage cables are as follows:

[0055] Under the original operating mode where the aluminum sheath of the high-voltage cable was only permanently grounded at one end, safe grounding was achieved through phased operations. First, a no-impedance pre-grounding path was established by the pre-opening detection grounding component 2 to complete the rapid release of residual charge in the aluminum sheath, safe pre-clamping of the sheath potential, and preliminary detection of grounding circuit parameters. After the main temporary grounding component 3 was installed in place, the pre-opening detection grounding component 2 was removed, and the main temporary grounding component assumed all grounding protection functions and was retained on the cable for a long period of time.

[0056] In the double-bolt grounding structure of the main temporary grounding component 3, the sheath contact bolt 302 forms a direct grounding path through the first main grounding lead, providing an impedance-free charge discharge channel to ensure that the aluminum sheath's potential to ground is reliably clamped within a safe range; the grounding lead fixing bolt 303 forms a current-limiting grounding path through the second main grounding lead in series with the 200mΩ sheath current-limiting resistor component 4, increasing the total impedance of the closed loop, suppressing the total current of the sheath formed by the superposition of mutual inductance circulating current and capacitive current after double-end grounding within a safe amplitude, avoiding overheating and burning of the aluminum sheath and grounding lead due to large current, and at the same time, the voltage drop to ground generated by the 200mΩ resistor under extreme working conditions is only about 1V, and the residual potential of the aluminum sheath to ground is reliably clamped within the safe voltage range for the human body, without weakening the core protective functions of the temporary grounding device in releasing induced charge and eliminating floating overvoltage.

[0057] The grounding reliability detection component 1 quantifies the three-phase contact resistance by performing two measurements and using the parallel resistance calculation method, and simultaneously monitors parameters such as induced current and current-limiting resistor voltage drop, thereby achieving standardized and data-driven judgment of grounding installation quality.

[0058] After the maintenance work was completed, the sealing and repair component used a silicone rubber filling and waterproof insulating tape multi-layer wrapping method to repair the 5mm micro pre-installed hole left after the removal of the front window detection grounding component 2, restoring the insulation and waterproof performance of the hole to the same level as the original cable.

[0059] Regarding the selection of the 200mΩ current-limiting resistor value in the above-mentioned sheath current-limiting resistor component 4, the design concept of this invention was not obtained through simple experimental attempts, but rather it was a creative selection result determined after quantitative analysis based on a bidirectional constraint model. The following detailed demonstration is provided in conjunction with specific comparative examples.

[0060] In the case of temporary grounding of high-voltage cables, the selection of the current-limiting resistor value faces two sets of contradictory technical constraints.

[0061] The first constraint is the resistance's heating power and current-carrying capacity. This constraint sets a lower limit on the resistance value, meaning the resistance cannot be too small. If the resistance value is too small, the increase in the total impedance of the closed loop will be limited, resulting in insufficient suppression of the circulating current in the sheath. Consequently, the heating power of the resistor body will increase dramatically and exceed the continuous load-carrying capacity of conventionally packaged resistors.

[0062] Taking a typical operating condition as an example, with a cable load current of 500A and a total cable length of 500m:

[0063] When no current-limiting resistor is connected in series, the total resistance of the sheath closed loop includes only the resistance of the aluminum sheath body and the resistance of the copper stranded wire of the grounding lead, which is less than 0.005Ω. The theoretical value of the mutual inductance circulating current can reach more than 60A. After adding the capacitive current generated by the distributed capacitance of the entire line, the total grounding current of the 110kV cable will exceed 30A, and the total current of the 220kV cable will approach 40A. The grounding lead will be softened by high temperature or even burned out due to the continuous flow of large current.

[0064] When a 50mΩ resistor is connected in series, the total circuit resistance is approximately 0.0547Ω, and the circulating current of the mutual inductance is still close to 6A. After adding the capacitor current, the total current of the 110kV cable is approximately 7.9A. The heating power of the resistor is P=I²R=7.9²×0.05≈3.12W. Although the surface temperature rise has decreased somewhat, it is still at a high level, and the circulating current suppression effect is insufficient. The aluminum sheath still has the risk of local overheating.

[0065] When a 200mΩ resistor is connected in series, the total circuit resistance is approximately 0.2047Ω, and the circulating current of the mutual inductance is effectively limited to approximately 1.57A. After adding the capacitor current, the total current of the 110kV cable is approximately 2.95A, and the total current of the 220kV cable under extreme operating conditions is approximately 5.24A. At this time, the heating power of the resistor is P = 5.24² × 0.2 ≈ 5.49W. With the heat dissipation aluminum fins and ceramic encapsulation structure, it can stably dissipate heat for a long time, and the surface temperature rise of the resistor can be controlled within 30K. There is no risk of thermal breakdown or high-temperature aging, which meets the safety requirements of continuous current-carrying conditions.

[0066] The second constraint is the safety potential constraint of the aluminum sheath to ground. This constraint sets an upper limit on the resistance value, meaning the resistance cannot be too high. A current-limiting resistor is connected in series in the current-limiting grounding path. All capacitive current and induced circulating current of the sheath flow through this resistor, generating a power frequency voltage drop across it. This voltage drop directly manifests as the residual potential of the aluminum sheath to ground. According to power operation and maintenance safety regulations, the potential of the aluminum sheath to ground during maintenance must be controlled within the safe voltage range for humans. If the resistance value is too high, the residual potential of the aluminum sheath to ground will significantly increase, weakening or even eliminating the core protective function of the temporary grounding device in clamping the sheath potential.

[0067] The comparison will still be based on the above typical working conditions:

[0068] When a 200mΩ resistor is connected in series, based on the total current of 5.24A under extreme operating conditions of a 220kV cable, the residual potential of the aluminum sheath to ground is U=I×R=5.24×0.2=1.048V, which is an extremely low safe voltage, and maintenance personnel have no risk of electric shock.

[0069] When a 1Ω resistor is connected in series, the residual potential can reach over 5.24V, which exceeds the recommended limit for contact voltage during maintenance operations in some safety regulations.

[0070] If the cable length is further increased, the distributed capacitive current along the entire line will rise accordingly, and the sheath-to-ground potential will rise further. The core functions of the temporary grounding device in releasing induced charges and eliminating floating overvoltages will be severely weakened, rendering the pre-grounding safety protection function ineffective.

[0071] The above bidirectional constraint analysis shows that 200mΩ is the optimal balance point that takes into account both the lower limit of the safety limit for resistive heating and the upper limit of the safety limit for the sheath to ground potential. Furthermore, the range of the series resistance can be 100mΩ to 500mΩ; if the resistance is below 100mΩ, although the residual potential of the sheath to ground is lower, the circulating current suppression effect is insufficient and the resistive heating power is too high, failing to guarantee long-term current carrying reliability; if the resistance is above 500mΩ, although the circulating current suppression effect is further enhanced and the heating power is reduced, the residual potential of the sheath to ground is significantly increased, exceeding the safe voltage range and violating the basic protective purpose of the temporary grounding device.

[0072] The 200mΩ current-limiting resistor achieves optimal bidirectional balance. On one hand, it increases the total impedance of the sheath closed loop to approximately 0.2Ω, suppressing the combined current of mutual inductance and capacitance to a safe range of 5-6A. The resistor generates approximately 5.5W of heat, and the accompanying heat dissipation structure can stably withstand this load over a long period. On the other hand, the power frequency voltage drop across the resistor is only about 1V, reliably clamping the residual potential of the aluminum sheath to ground within a safe range, without affecting the core functions of the temporary grounding device in discharging induced charges and eliminating floating overvoltages. Furthermore, the 200mΩ resistor is a standard low-value resistor, compact in size, easy to modularly integrate, and suitable for installation requirements in confined cable working spaces.

[0073] The determination of this resistance value is not a simple result of conventional selection or limited testing in this field, but a creative selection conclusion obtained by establishing a two-way constraint model and comparing a large number of operating condition calculations after quantitatively analyzing two sets of contradictory constraints: the upper limit of the heating power current carrying capacity and the limit of the sheath to ground safety potential.

[0074] This embodiment also provides a temporary grounding method for the aforementioned high-voltage cable temporary grounding device. Taking the temporary grounding maintenance of a 500m long 110kV high-voltage cable in a tunnel as an example, the cable's aluminum sheath suffers from electrochemical corrosion and potential localized loosening. The normal load current is 500A, and the original operating mode was single-end permanent grounding of the aluminum sheath, with the permanent grounding electrode in good condition. The specific steps for carrying out temporary grounding work using this device are as follows:

[0075] Step 1: Job Pre-processing

[0076] Clean the surface of the high-voltage cable's pre-designated working area, removing asphalt, graphite layers, dust, and impurities. Use a fine grinding head to lightly polish the working contact surface, ensuring it is smooth and clean. Inspect the appearance of each of the pre-opening grounding detection component 2, the main temporary grounding component 3, and the sheath current-limiting resistor component 4, confirming that the 200mΩ current-limiting resistor body is undamaged, the heat dissipation aluminum fins are not deformed, and the tin plating of the quick-connect terminals is intact. At the same time, confirm that the grounding reliability detection component 1 has sufficient power and is functioning normally.

[0077] Step 2: Install the pre-grounding detection component to establish a pre-grounding path:

[0078] The window-opening fixing bracket 201 is attached to the outer wall of the straight section of the cable at the pre-set working position. The ratchet tightening handle 205 tightens the binding strap 203 in one direction, making the window-opening fixing bracket securely hug the cable. The rotating screw handle 204 drives the blunt-tipped grounding pin 202 to be axially fed along the 5mm micro pre-installed hole. The end of the blunt-tipped pin pierces the plastic outer sheath of the cable and abuts against the surface of the aluminum sheath, realizing electrical conduction. One end of the pre-grounding lead is connected to the safety grounding terminal 206, and the other end is directly connected to the tunnel-specific grounding electrode, forming an impedance-free pre-grounding path. No current-limiting resistor is connected in series in the pre-grounding branch to ensure that the floating induced charge and residual voltage accumulated on the aluminum sheath can be instantly and quickly discharged to the ground.

[0079] After pre-grounding is completed, connect grounding reliability detection component 1 to the front grounding lead, detect the grounding loop resistance R from the metal sheath to the direct grounding box, and judge the grounding loop status of the single-ended grounding system according to the diagnostic criteria in Table 1 below:

[0080]

[0081] If the diagnosis result is "attention or abnormal", it indicates that there is a potential for deterioration in the original permanent grounding circuit. Continue to perform the subsequent steps to install the main temporary grounding assembly to build a reliable double-bolt grounding protection.

[0082] Step 3: Install the main temporary grounding assembly to construct dual-path grounding protection:

[0083] Install the main temporary grounding component 3 at a position offset from the pre-opening detection grounding component 2 along the cable axis to avoid stress concentration caused by two openings at the same cross section of the cable. Secure the main grounding fixing seat 301 to the outer wall of the cable by wrapping it around the cable with the fixing steel strap 304, so that the inner buffer pad of the fixing seat is tightly attached to the surface of the cable outer sheath.

[0084] Using a specialized hole saw, two standard 14mm mounting holes are made at a uniform speed at the location of the cable outer sheath corresponding to the main grounding mounting bracket 301. The hole depth is limited to penetrating the plastic outer sheath and the asphalt protective layer. The hole saw is stopped immediately after the tip touches the surface of the aluminum sheath to avoid scratching the aluminum sheath substrate. After the holes are made, a specialized grinding tool is used to finely grind the surface of the aluminum sheath to remove the surface oxide layer.

[0085] Insert the aluminum sheath contact bolt 302 and the grounding lead fixing bolt 303 into the corresponding two mounting channels on the main grounding fixing seat 301 and tighten them so that the top arc end faces of the two bolts pass through the cable plastic outer sheath and make large-area contact with the surface of the aluminum sheath.

[0086] One end of the first main grounding lead is pressed and connected to the tail terminal of the sheath contact bolt 302, and the other end is directly connected to the earth grounding electrode to form a direct grounding path. This path is a direct grounding without impedance, which is used to continuously provide a fast discharge channel for the residual induced charge of the aluminum sheath during maintenance and subsequent operation, and to clamp the aluminum sheath to the earth potential.

[0087] One end of the second main grounding lead is pressed tightly and connected to the tail terminal of the grounding lead fixing bolt 303. The other end is first connected in series with the sheath current-limiting resistor assembly 4 and then connected to the earth grounding electrode to form a current-limiting grounding path. This path is connected in series with a 200mΩ rated current-limiting resistor to increase the total impedance of the double-ended closed loop of the aluminum sheath, suppressing the superposition amplitude of the inductive circulating current generated by the mutual inductance of the conductor load current and the capacitive current generated by the distributed capacitance of the entire cable within a safe range. The quick-plug tinned terminals at both ends of the sheath current-limiting resistor assembly 4 are respectively plugged and locked with the second main grounding lead on the aluminum sheath side and the earth grounding lead to ensure reliable electrical connection.

[0088] Step 4: Grounding reliability test;

[0089] After the main temporary grounding component 3 is installed in place, the grounding reliability detection component 1 is connected to the first main grounding lead and the second main grounding lead respectively. The detection program is started, and the following core parameters are collected in real time and judged one by one:

[0090] 1. After installing the main temporary grounding component 3 and the sheath current limiting resistor component 4, use the grounding reliability detection component 1 to detect the parallel resistance R / / (Rj+Rx) of the metal sheath grounding circuit at the same location as the front-opening detection grounding component 2, where / / indicates the parallel relationship; Rx=200mΩ, which is the rated resistance value of the sheath current limiting resistor component 4.

[0091] 2. Based on the measured grounding loop resistance R from the metal sheath to the direct grounding box, calculate the three-phase contact resistance Rj between the main temporary grounding component 3 and the metal sheath using the parallel formula; substitute the known quantities R, Rx and the measured parallel resistance value to calculate Rj in reverse.

[0092] 3. The standard for judging whether the grounding condition is qualified is that the three-phase contact resistance Rj does not exceed 15mΩ and the power frequency voltage drop across the three-phase current-limiting resistor Rx does not exceed 1.2V. If the voltage drop exceeds the limit, it indicates that the combined current of the circuit is too large or the current-limiting resistor is abnormal. The contact condition or the wiring of the current-limiting resistor needs to be checked.

[0093] 4. Confirmation of no abnormal high-current surges in the three-phase induced current;

[0094] Once all the above parameters meet the standards, the grounding installation quality is deemed qualified, and the current-limiting resistor is considered to be operating normally. If any parameter fails to meet the standards, the tightening of the aluminum sheath contact bolt 302 or the grounding lead fixing bolt 303 should be readjusted, or the locking of the plug-in terminals at both ends of the sheath current-limiting resistor assembly 4 should be checked until the test is qualified. After the test is qualified, the pre-opening detection grounding assembly 2 should be completely removed, and the main temporary grounding assembly 3 should independently assume all grounding protection functions and remain on the cable permanently.

[0095] Step 5: Conduct cable maintenance work.

[0096] After confirming the reliability of both the direct grounding path and the current-limiting grounding path, the normal operation of the current-limiting resistor, and that all electrical parameters meet the standards, maintenance and repair work was carried out, including corrosion detection of the aluminum sheath of the high-voltage cable and investigation of defects in the outer sheath. Throughout the operation, the potential of the aluminum sheath to ground remained stable below 1V, with no risk of overheating or electric shock.

[0097] Step Six: Device Removal and Differentiated Sealing Repair:

[0098] After the maintenance work is completed, remove each component in the following order: first disconnect the grounding reliability detection component 1 from each grounding lead, then remove the front window detection grounding component 2 as a whole and clean up its installation traces;

[0099] The main temporary grounding component 3 remains on the cable and does not need to be removed.

[0100] Repair the 5mm micro pre-installed hole left after the removal of the front window detection grounding component 2. Clean the debris and residual debris in the hole, inject room temperature curing silicone rubber into the hole until it is completely filled, let it stand at room temperature for 12 hours to cure naturally, and after initial shaping, wrap and seal it with multiple layers of butyl waterproof insulating tape, with no less than 3 layers.

[0101] Step 7: On-site re-inspection and acceptance:

[0102] After the sealing repair is completed, visually inspect the repair location of the 5mm micro pre-installed hole to confirm that the silicone rubber is fully filled, without gaps or cracks, and that the waterproof insulating tape is tightly wrapped without loosening. Confirm that the main temporary grounding component 3 is in good condition, and that its sealing waterproof gasket 305 is not misaligned or shows signs of aging. After confirming that all openings and component installation locations are properly insulated and waterproofed, with no risk of water leakage, clean the work site and complete the entire work process.

[0103] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although detailed descriptions have been provided with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A temporary grounding device for high-voltage cables, characterized in that: This includes a front-mounted window detection grounding assembly, a main temporary grounding assembly, and a sheath current-limiting resistor assembly; The pre-grounding connection between the pre-window detection grounding component and the metal sheath of the high-voltage cable is achieved. The pre-grounding connection grounding component includes a pre-grounding lead, which is directly connected to the earth grounding electrode. The main temporary grounding assembly is installed on the metal sheath of the high-voltage cable. The main temporary grounding assembly includes a sheath connection end and a grounding connection end. The sheath connection end is electrically connected to the metal sheath. The grounding connection end is connected to the earth grounding electrode after being connected in series with the sheath current-limiting resistor assembly.

2. The temporary grounding device for high-voltage cables as described in claim 1, characterized in that: The main temporary grounding assembly includes a main grounding mounting base, and a sheath contact bolt and a grounding lead fixing bolt disposed on the main grounding mounting base. The sheath contact bolt is used for electrical connection with the metal sheath; the grounding lead fixing bolt is used to connect to the earth grounding electrode after being connected in series with the sheath current limiting resistor assembly.

3. The temporary grounding device for high-voltage cables as described in claim 1, characterized in that: The rated resistance of the current-limiting resistor assembly of the sheath is 200mΩ.

4. The temporary grounding device for high-voltage cables as described in claim 1, characterized in that: The current-limiting resistor assembly includes a current-limiting resistor body, a ceramic insulating package covering the current-limiting resistor body, heat-dissipating aluminum fins disposed on the outer surface of the ceramic insulating package, and quick-plug terminals disposed at both ends and with tin-plated surfaces.

5. The temporary grounding device for high-voltage cables as described in claim 1, characterized in that: It also includes a grounding reliability detection component, which is electrically connected to the sheath connection end and grounding connection end of the front grounding lead and the main temporary grounding component, respectively, and is used to detect the grounding loop impedance, induced current and power frequency voltage drop across the sheath current limiting resistor component in real time.

6. The temporary grounding device for high-voltage cables as described in claim 1, characterized in that: It also includes a sealing and repair component, which comprises room temperature curing silicone rubber and waterproof insulating tape; the room temperature curing silicone rubber fills the opening formed on the outer sheath of the cable by the pre-opening detection and grounding component, and the waterproof insulating tape wraps around the opening to reinforce and seal it.

7. The temporary grounding device for high-voltage cables as described in claim 1, characterized in that: The pre-opening detection grounding assembly includes a window fixing base, a blunt-tipped grounding pin, a sealing gasket, and the pre-grounding lead; the inner side of the window fixing base is provided with a buffer insulating pad, and its arc-shaped inner wall is used to fit the outer sheath surface of the high-voltage cable; the blunt-tipped grounding pin adopts a blunt-tipped structure, which can penetrate through the micro pre-installed hole on the high-voltage cable and make flexible contact with the cable's metal sheath.

8. A temporary grounding method using the temporary grounding device according to any one of claims 1-7, characterized in that, Includes the following steps: Install the pre-opening detection grounding component on the high-voltage cable and make it electrically connected to the pre-grounded metal sheath of the cable. Connect the pre-grounding lead directly to the earth grounding electrode. Install the main temporary grounding assembly on the high-voltage cable; after being electrically connected to the cable's metal sheath through the sheath connection end of the main temporary grounding assembly, directly connect it to the earth grounding electrode, and then connect it to the earth grounding electrode through the sheath current-limiting resistor assembly connected in series through the grounding connection end.

9. The method for temporary grounding of high-voltage cables as described in claim 8, characterized in that, Also includes: After installing the front-opening grounding detection component, use the grounding reliability detection component to detect the grounding loop resistance R from the metal sheath to the direct grounding box at the location of the front-opening grounding detection component; After installing the main temporary grounding assembly and the sheath current-limiting resistor assembly, the parallel resistance R / / (Rj+Rx) of the metal sheath grounding circuit is detected at the same location of the front-opening grounding assembly using the grounding reliability detection assembly, where / / indicates parallel connection, and Rx=100-500mΩ. The three-phase contact resistance Rj between the main temporary grounding assembly and the metal sheath is calculated using the parallel formula. When the three-phase contact resistance Rj does not exceed 15mΩ and the voltage drop across the three-phase current-limiting resistor Rx does not exceed 1.2V, the grounding condition is deemed qualified.

10. The method for temporary grounding of high-voltage cables as described in claim 8, characterized in that, The openings on the outer sheath of the cable are sealed and repaired. Specifically, for the micro pre-installed holes, after cleaning the debris inside the holes, room temperature curing silicone rubber is injected as filler. After being left to cure at room temperature, and after initial shaping, multiple layers of waterproof insulating tape are wrapped around the holes for reinforcement and sealing.

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