Coal mine ground fracturing equipment power supply system and method based on underground power grid

Abstract

The invention relates to the technical field of coal mining, and discloses a coal mine ground fracturing equipment power supply system and method based on an underground power grid, and the system uses an underground power supply module to evaluate the residual capacity of the power grid, and leads a power supply to the ground through a cable laying module in a drill hole communication module; the ground power transformation module performs voltage transformation and dynamic reactive compensation; the intelligent control module collects wellhead pressure and real-time displacement of fracturing operation, and a theoretical electric power demand is predicted through feedforward of an electro-mechanical transformation model; when the demand is close to the capacity limit value, the system reversely determines the maximum displacement set value based on the energy inverse operation, and forcibly adjusts the motor frequency of the fracturing pump truck to limit the power; and the grading tripping logic of ground priority action is matched to ensure quick isolation when a fault occurs. According to the invention, the redundant capacity of the mine power grid is effectively utilized to replace diesel oil for power generation, and low-carbon and high-efficiency operation of fracturing operation is realized while the underground override trip risk is solved.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, specifically to a power supply system and method for coal mine surface fracturing equipment based on an underground power grid. Background Technology

[0002] Most coal seams are characterized by extremely low permeability. Traditional underground borehole gas drainage suffers from problems such as long pre-drainage cycles and low efficiency, and is usually limited to point or linear treatment, making it difficult to achieve regional coverage of the entire working face or mining area, which seriously restricts the continuity of coal mining. In addition, for dynamic disasters such as rockbursts induced by hard roofs, underground blasting or hydraulic fracturing methods are limited by construction space and equipment capabilities, making it difficult to effectively disturb the high-level key strata above the coal seam. Surface fracturing technology, with its advantages of large displacement, high pump pressure, and wide coverage, can achieve regional permeability enhancement of deep coal seams and strategic weakening of roof strata, which is of great significance for ensuring safe and efficient coal mining.

[0003] However, surface fracturing operations rely on high-power electric drive equipment such as fracturing pump trucks and mixing skids, placing extremely high demands on the capacity and stability of the power supply system. Currently, such projects use either newly built surface power lines connected to the public power grid or leased high-power diesel generator sets for power supply. For mining areas located in remote mountainous regions or with complex terrain, where the public power grid is not covered or has insufficient end-point capacity, constructing new long-distance high-voltage lines not only faces difficulties in land acquisition, huge investments, and lengthy approval cycles, but is also easily restricted by geographical conditions, making it difficult to implement and unable to meet the temporary and urgent power needs of fracturing. If diesel generator sets are used for power supply, although they offer greater mobility, their operating costs are high, and they are accompanied by significant fuel consumption, exhaust emissions, and high-decibel noise, which deviates from the trend of green mining and low-carbon development in the coal industry.

[0004] Meanwhile, coal mines have highly reliable central substations with grid capacity configured for maximum production load, providing power redundancy during maintenance shifts or off-peak periods. However, current technology lacks a secure physical connection between the underground power grid and the surface work site, and corresponding collaborative control mechanisms are also lacking. Simply drawing underground power to supply high-power equipment on the surface could easily disrupt the stability of the underground power grid due to drastic fluctuations in fracturing load, potentially causing cascading trips or even mine-wide power outages, thus threatening safe production underground. Therefore, how to safely and economically utilize idle underground power resources to solve the power supply problem for fracturing operations on the surface is an urgent technical issue that needs to be addressed. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a power supply system and method for coal mine surface fracturing equipment based on underground power grids. It solves the problems of high construction costs, high pollution emissions, and long deployment cycles caused by geographical constraints on the construction of new power lines or reliance on diesel generators in existing surface fracturing operations, as well as the problems of cascading trips and threats to underground safe production caused by the lack of load coordination control when directly using underground power grids for power supply.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The first aspect of the present invention provides a power supply system for coal mine surface fracturing equipment based on an underground power grid. The system includes an underground power supply module, a borehole connection module, a cable laying module, a surface substation module, and an intelligent control module.

[0008] The underground power supply module is deployed in the underground substation of the coal mine, serving as the energy starting point of the system. The underground power supply module does not directly overdraw the full capacity of the underground power grid; instead, it is equipped with a relay protection unit to perform capacity assessment. Specifically, the underground power supply module collects and analyzes historical load data during mine maintenance shifts to construct a background load prediction model, thereby determining the apparent power of the base load. The system defines the difference between the rated apparent power of the underground transformer and the apparent power of the base load as the available redundant capacity, utilizing only this redundant capacity to supply power to the surface, thus ensuring that the normal power consumption for underground coal mine production is not affected.

[0009] The borehole connection module and the cable laying module form a physical transmission channel connecting the downhole and the surface. The borehole connection module is constructed using directional drilling technology and contains a tiered casing assembly including surface casing and technical casing. A sealing cementing layer is filled between the casing and the borehole wall to prevent downhole gas escape. The cable laying module is located inside the casing and uses a seamless, armored high-voltage mining cable. A distributed fixing device establishes frictional contact with the inner wall of the casing to limit the cable's sway during vertical laying, ensuring the mechanical stability of high-voltage power transmission.

[0010] The surface substation module serves as an energy receiving and regulating terminal, converting the high-voltage power transmitted underground into a voltage level suitable for fracturing equipment. This module integrates a dynamic reactive power compensation unit, utilizing static var generator (SVM) technology to acquire voltage and current signals, separating the fundamental reactive current and higher harmonic components. It then outputs a reverse cancelling current to correct the power factor and filter out higher harmonics generated by the fracturing frequency converter, preventing grid pollution from affecting the underground operation.

[0011] The intelligent control module is the core logic unit of the system, used to realize feedforward prediction and closed-loop control of power supply load. Its working principle lies in establishing a mapping relationship between fracturing hydraulic parameters and electrical load. This intelligent control module includes a fracturing load feedforward prediction unit, used to read the wellhead pressure, real-time discharge rate, and fluid density data of the fracturing equipment in real time. Based on a pre-set electromechanical conversion model, this fracturing load feedforward prediction unit uses the product of wellhead pressure and real-time discharge rate, combined with the electro-hydraulic integrated energy conversion efficiency coefficient, to calculate in advance the theoretical power demand required to maintain the current operation before the actual electrical load changes abruptly. Furthermore, this intelligent control module also monitors the discharge rate growth rate and dynamically compensates for the theoretical power demand to eliminate the impact of transient response lag.

[0012] Furthermore, the intelligent control module is equipped with a closed-loop feedback protection unit to execute an active current-limiting strategy based on energy inverse calculation. When the calculated theoretical power demand approaches the real-time capacity limit assessed by the downhole power supply module, the system no longer passively responds to the displacement command. Instead, it subtracts the power consumption of auxiliary equipment from the current real-time capacity limit to obtain the net available power, and, combined with the current wellhead construction pressure, calculates the maximum allowable displacement setting value for the fracturing pump set. This maximum displacement setting value is sent as a mandatory command to the fracturing equipment, locking the total system power by limiting the operating frequency of the variable frequency motor, ensuring that it always operates within the safety margin of the downhole power grid.

[0013] Regarding fault protection mechanisms, this system employs a tiered tripping strategy. The protection action delay setting value of the circuit breaker in the surface substation module is set to be less than that of the circuit breaker in the underground power supply module, and the difference between the two is greater than the preset tiered coordination margin time. This time tier setting ensures that when an overload or short-circuit fault occurs in the system, the surface circuit breaker can operate first to isolate the fault, achieving physical isolation between the fault point and the underground power grid, and avoiding large-scale power outages underground due to surface equipment failures.

[0014] A second aspect of the present invention provides a power supply method for coal mine surface fracturing equipment based on an underground power grid.

[0015] This method relies on the aforementioned power supply system and includes the following steps: First, the power supply channel was constructed by using directional drilling and cementing technology to establish a physical channel with electrical insulation and airtightness between the underground substation and the surface, and to complete the vertical laying and connection of high-voltage cables.

[0016] Secondly, the system is powered on and its status is monitored. After the power is supplied underground, the power quality is automatically adjusted using the ground reactive power compensation equipment, and the maximum active power limit allowed by the underground power supply module is periodically obtained to provide dynamic boundary conditions for subsequent control.

[0017] Subsequently, the system enters the fracturing load feedforward prediction stage. Instead of relying on the hysteretic feedback of the current transformer, it directly collects the hydraulic parameters at the source of the fracturing operation. Based on the electromechanical conversion model, it calculates the total active power demand corresponding to the current operating condition in real time, achieving millisecond-level prediction of load changes.

[0018] Based on this, load margin analysis and closed-loop feedback control are performed. The system continuously compares the theoretical power demand with the maximum power limit. Once it detects that the power demand is approaching the limit, it immediately triggers the energy conversion inverse operation, calculates the maximum allowable displacement based on the current remaining power margin, and forcibly adjusts the variable frequency operating frequency of the fracturing pump truck. This step dynamically switches the traditional constant displacement, variable power operation mode to a constant power, variable displacement mode, realizing continuous operation under limited capacity.

[0019] Finally, implement graded protection and fault isolation, and in the event of extreme faults, strictly follow the tripping logic of ground priority and underground delay to ensure the absolute safety of the underground power supply system in coal mines.

[0020] This invention provides a power supply system and method for coal mine surface fracturing equipment based on an underground power grid. It has the following beneficial effects: 1. This invention establishes a vertical power supply channel through a drilling connection module and a cable laying module, utilizing the surplus capacity of the underground substation to power surface equipment. This avoids the construction investment and approval process required for building new long-distance high-voltage lines on the ground, while replacing diesel generator sets, reducing fuel consumption, exhaust emissions and noise interference during on-site operations. This method improves the utilization rate of existing power supply facilities in the mine and realizes the electrification and low-carbon operation of fracturing operations.

[0021] 2. This invention utilizes an intelligent control module to execute feedforward prediction and closed-loop feedback control based on an electromechanical conversion model. According to the real-time capacity limit assessed by the underground power supply module, the maximum displacement of the fracturing equipment is adjusted in reverse to dynamically adapt the working load to the remaining capacity of the underground power grid. Combined with the tiered tripping logic where the action time of the surface circuit breaker is better than that of the underground circuit breaker, this system reduces the risk of tiered tripping in the underground due to overload or failure of surface equipment while utilizing the mine power supply, thus ensuring the continuity of power supply for underground production.

[0022] 3. This invention uses directional drilling technology and staged casing process to construct energy transmission channels. Its engineering implementation is less affected by ground topography and vegetation distribution. The construction and deployment cycle is usually shorter than that of traditional ground power transmission and transformation projects. The system can respond to the power demand of fracturing in remote mountainous areas or areas not covered by the ground power grid, and provides a feasible power supply solution for coal mine surface treatment operations in geographically limited areas. It has good engineering adaptability. Attached Figure Description

[0023] Figure 1 This is an overall structural block diagram of a power supply system for coal mine surface fracturing equipment based on an underground power grid, provided in an embodiment of the present invention. Figure 2 This is a flowchart illustrating the capacity assessment and background load prediction of the downhole power supply module in this embodiment of the invention. Figure 3 This is a detailed flowchart of the intelligent control module performing fracturing load feedforward prediction and closed-loop feedback control in an embodiment of the present invention; Figure 4 This is a flowchart of the hierarchical tripping protection and fault isolation logic in an embodiment of the present invention; Figure 5 This is a main flowchart of a power supply method for coal mine surface fracturing equipment based on an underground power grid, provided in an embodiment of the present invention. Figure 6 This is a dynamic comparison curve of electrical load demand and actual output power in an embodiment of the present invention; Figure 7 This is the adaptive adjustment response curve of hydraulic parameters for fracturing operations in an embodiment of the present invention.

[0024] Among them, 10 is the downhole power supply module; 20 is the borehole connection module; 30 is the cable laying module; 40 is the surface substation module; and 50 is the intelligent control module. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all 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.

[0026] See Figure 1 The present invention provides a power supply system for coal mine surface fracturing equipment based on underground power grid. The system mainly includes an underground power supply module 10, a borehole connection module 20, a cable laying module 30, a surface substation module 40, and an intelligent control module 50.

[0027] The underground power supply module 10 is installed in the central substation or mining area substation of the coal mine, serving as the power source for the system. This underground power supply module 10 includes a mine-use explosion-proof high-voltage vacuum power distribution device, a relay protection unit, and a safety isolation device. The mine-use explosion-proof high-voltage vacuum power distribution device is physically connected to the high-voltage busbar of the underground power grid, and draws power through a built-in circuit breaker assembly. The relay protection unit monitors the electrical status of the power supply circuit and performs instantaneous overcurrent and overcurrent protection. The safety isolation device includes a mechanical interlocking mechanism and an electrical interlocking mechanism, used to lock the circuit breaker in the open position during system maintenance or fault conditions.

[0028] The borehole connection module 20 is constructed between the underground coal mine and the surface, serving as a physical passage connecting the underground and surface spaces. This module 20 consists of a directional borehole, a staged casing assembly, and a sealing cementing layer. The trajectory of the directional borehole extends from the underground inlet coordinates to the surface outlet coordinates. The staged casing assembly is coaxially arranged within the directional borehole, comprising an outer surface casing and an inner technical casing. The sealing cementing layer, formed by a solidified material, fills the annular space between the staged casing assembly and the formation rock mass, preventing underground gas from escaping to the surface and providing an electrically insulating environment.

[0029] The cable laying module 30 is installed inside the technical casing of the borehole connection module 20 and serves as a power transmission carrier. The cable laying module 30 includes a mining armored high-voltage cable, distributed fixing clamps, and a grounding device. The mining armored high-voltage cable is laid vertically along the axial direction of the borehole connection module 20 and mechanically anchored to the inner wall of the technical casing at predetermined intervals using distributed fixing clamps to counteract the axial tension generated by the cable's own weight. The bottom end of the cable laying module 30 is connected to the output end of the underground power supply module 10, and the top end is connected to the input end of the surface substation module 40.

[0030] The surface substation module 40 is installed at the surface fracturing operation site, serving as the power receiving and energy management end of the system. This surface substation module 40 includes an explosion-proof prefabricated substation and a dynamic reactive power compensation unit. The explosion-proof prefabricated substation integrates a dry-type transformer to convert the input downhole power voltage to a voltage suitable for the fracturing equipment. The dynamic reactive power compensation unit is connected in parallel to the low-voltage side bus of the surface substation module 40, and outputs inductive or capacitive reactive power according to the real-time load status to adjust the system power factor and bus voltage.

[0031] The intelligent control module 50 establishes data connections with the downhole power supply module 10, the surface substation module 40, and the main control unit of the fracturing equipment via an industrial communication network. The intelligent control module 50 mainly includes a hardware communication network, a fracturing load feedforward prediction unit, and a closed-loop feedback protection unit. The hardware communication network is used to transmit electrical parameters of the downhole power grid, surface electrical parameters, and hydraulic parameters of the fracturing equipment. The fracturing load feedforward prediction unit is configured to calculate future electrical load requirements in advance based on the hydraulic parameter variation characteristics of the fracturing equipment. The closed-loop feedback protection unit is configured to send adjustment commands to the surface substation module 40 or power limitation and cut-off commands to the fracturing equipment based on the load prediction results and real-time electrical parameters, thereby achieving coordinated control of the system.

[0032] See Figure 2The underground power supply module 10 is installed in the central substation or mining area substation of the coal mine, serving as the power source for the system. The underground power supply module 10 includes a mine-use explosion-proof high-voltage vacuum power distribution device, a relay protection unit, and a safety isolation device.

[0033] The mine-use explosion-proof high-voltage vacuum power distribution device is physically connected to the high-voltage busbar of the underground power grid, and draws power through a built-in circuit breaker assembly. This power distribution device adopts an explosion-proof structure with a shell resistant to gas explosion pressure. The internal circuit breaker assembly uses a high-voltage vacuum circuit breaker. The incoming line is connected to the substation busbar via a copper busbar, and the outgoing line is connected to the system's cable laying module 30 via a cable sealing introduction device. A high-voltage isolating switch is also connected in series between the circuit breaker assembly and the busbar to create a visible electrical break after the circuit breaker is tripped.

[0034] The relay protection unit is integrated into the intelligent control room of the mine explosion-proof high-voltage vacuum power distribution device. It monitors the electrical status of the power supply circuit and performs instantaneous overcurrent and overcurrent protection, as well as zero-sequence protection (leakage protection). It also performs power supply capacity assessment. Specifically, it assesses the apparent power of the underground foundation load. Based on this determination, the relay protection unit can also be configured to focus on analyzing load characteristic curves during maintenance shifts and establishing background load prediction models to more accurately calculate available redundant capacity. This relay protection unit is connected to voltage transformers and current transformers for real-time acquisition of underground power grid operation data.

[0035] Regarding power supply capacity assessment, the relay protection unit has pre-set redundancy capacity calculation logic. First, the calculation time is based on the transformer's rated parameters. Theoretical redundancy apparent power The calculation expression is: ; in, This indicates the rated apparent power of the underground transformer; This represents the apparent power of the downhole foundation load, which is collected and calculated in real time by the current transformer. This represents the preset safety operating coefficient, with a value ranging from 0.8 to 0.9.

[0036] Furthermore, the relay protection unit, combining the system power factor and load fluctuation allowance coefficient, calculates the maximum active power limit that the ground fracturing equipment can obtain. : ; in, This indicates the current system power factor of the underground power grid; This represents the load fluctuation reserve factor used to cope with the impact of downhole motor startup. The calculated value is... The data is sent to the intelligent control module 50 via the communication interface.

[0037] In terms of fault protection, the relay protection unit performs time-delayed overcurrent protection and instantaneous overcurrent protection. To prevent cascading tripping of the underground power supply due to surface equipment failure, the instantaneous overcurrent protection setting value of the relay protection unit is... The following constraints must be met: ; In the formula, This represents the two-phase short-circuit current value at the power take-off point under minimum operating conditions; This indicates the sensitivity coefficient of the protection device. When the monitored line current exceeds... At that time, the relay protection unit directly drives the trip coil of the vacuum circuit breaker assembly to operate.

[0038] The safety isolation device includes a mechanical interlocking mechanism and an electrical interlocking mechanism, used to lock the circuit breaker in the open position during system maintenance or fault conditions. The mechanical interlocking mechanism is constructed as a linkage interlock between the isolating switch operating handle and the circuit breaker main shaft, and is configured to enforce the operating sequence of opening the circuit breaker first, then opening the isolating switch. The electrical interlocking mechanism connects the circuit breaker's auxiliary contacts in series to the closing control circuit, and is configured to physically cut off the power supply to the closing circuit when the grounding switch is detected to be closed or the insulation resistance of the downstream line is lower than a threshold, preventing accidental energization.

[0039] The borehole connection module 20 is constructed between the roof of an underground roadway and the surface of a coal mine, traversing the overburden strata, and serves as a physical passage connecting the underground space and the surface space. The borehole connection module 20 mainly consists of directional boreholes, staged casing assemblies, and sealing cementing layers.

[0040] Directional drilling is the drilling trajectory formed using directional drilling equipment. The design of the borehole trajectory follows the principle of avoiding goaf areas, fault fracture zones, and aquifers. The geometric path of the trajectory is determined by the coordinates of the downhole entrance. Extend to ground exit coordinates To accurately determine the required cable length and the basis for electrical parameter verification, the actual trajectory length of directional drilling was determined. Determined based on the following integral relationship: ; in, This indicates the actual trajectory length of the directional drilling; Represents the vertical depth coordinates of the well entrance point; Indicates the vertical elevation coordinates of the ground exit point; and These represent the abscissa and ordinate of the borehole trajectory projected onto the horizontal plane, respectively. The vertical coordinates representing the borehole trajectory; This represents the rate of change of the horizontal coordinate of the trajectory relative to the vertical coordinate. This represents the rate of change of the longitudinal axis relative to the vertical axis. To ensure sufficient space within the borehole to accommodate the high-voltage cable and meet heat dissipation requirements, the diameter of the directional drilling is generally above 200mm. During construction, the verticality deviation of the directional drilling is controlled within 1% and the azimuth deviation within 2° to reduce frictional resistance during subsequent cable lowering.

[0041] The staged casing assembly is coaxially installed within the directional borehole to support the wellbore and create a smooth cable channel. The staged casing assembly employs a double-layer nested structure, consisting of an outer surface casing and an inner technical casing.

[0042] Surface casing is installed in the near-surface section of the borehole, penetrating through the loose surface layer and reaching a certain depth (e.g., 30 meters) into the bedrock. It is used to isolate surface water and loose strata. Surface casing is made of large-diameter seamless steel pipe (e.g., with specifications of...). ).

[0043] The technical casing coaxially passes through the interior of the surface casing and extends to the downhole borehole inlet, achieving full borehole coverage. The technical casing is made of high-strength oil casing (e.g., with specifications of...). Its inner wall is smooth and burr-free, and it is set as the direct carrier of the cable laying module 30.

[0044] The sealing cement layer fills the annular space between the staged casing assembly and the formation rock mass. It is formed by grouting and solidifying oil well cement slurry throughout the entire borehole section. The grouting process employs either upward grouting from downhole or downward grouting from the surface to ensure the cement slurry completely fills the annulus between the casing and the borehole wall. The solidified sealing cement layer forms a dense annular barrier, which both electrically isolates the casing from the formation and tightly seals formation fractures, preventing downhole gas from escaping to the surface along the borehole annulus.

[0045] At the surface outlet of the borehole connection module 20, a wellhead sealing device is also installed. This wellhead sealing device includes a blowout preventer flange and a waterproof sealing ring to prevent rainwater from flowing back into the well along the inner wall of the casing and to seal the wellhead in an emergency. At the downhole inlet of the borehole connection module 20, a wellhead guide casing is installed to guide the cable in and protect the cable from damage caused by the shear force of the rock at the wellhead.

[0046] The cable laying module 30 is installed inside the technical casing of the borehole connection module 20 and serves as the power transmission carrier connecting the downhole power supply module 10 and the surface substation module 40. The cable laying module 30 mainly consists of a mining armored high-voltage cable, distributed fixing devices, surface extension protection components, and an electrical grounding system.

[0047] The armored high-voltage cable for mining uses cross-linked polyethylene insulated, coarse steel wire armored power cable, with a voltage rating not lower than the rated voltage of the underground power grid. The cable is laid continuously and perpendicularly along the axial direction of the borehole connection module 20 without intermediate joints to ensure insulation strength. The coarse steel wire armor layer of the cable is designed to bear the axial tensile load generated by the cable's own weight, preventing the conductor from deforming due to tensile stress.

[0048] The distributed fixing device includes multiple cable clamps spaced at preset intervals (e.g., every 3 to 5 meters). These cable clamps are fastened to the outer sheath of the mining armored high-voltage cable, and their outer diameter is slightly smaller than the inner diameter of the technical sleeve. The distributed fixing device is designed to limit the lateral swing of the cable within the technical sleeve and to provide auxiliary support through frictional contact with the inner wall of the technical sleeve, preventing slapping or torsional damage to the cable during vertical transmission.

[0049] The ground extension protection assembly is installed between the ground outlet of the borehole connection module 20 and the ground substation module 40. This assembly uses hot-dip galvanized steel pipe as a sheath, through which the ground section of the mining armored high-voltage cable is laid and buried underground at a depth not less than 0.8 meters below the frost line. At the connection point between the cable and the ground substation module 40, an explosion-proof junction box is installed. The core connection area is treated with heat-shrink double insulation, and the box is filled with explosion-proof sealant to ensure the airtightness of the electrical connection.

[0050] The electrical grounding system is used to ensure transmission safety and electromagnetic compatibility. The metal shield and steel wire armor of the mining armored high-voltage cable are connected to the local grounding electrode via copper braided wire at both the underground and surface ends. The grounding resistance at the surface end is no greater than 4Ω to ensure rapid current discharge in the event of a single-phase ground fault.

[0051] To ensure that the terminal voltage quality after long-distance vertical transmission meets the start-up requirements of the fracturing equipment, the selection and length configuration of the cable laying module 30 must be verified by voltage drop. (Line voltage loss) Determined based on the following formula: ; in, This indicates line voltage loss, measured in volts (V). Represents the line voltage coefficient of a three-phase AC system; This indicates the maximum operating current of the transmission circuit, measured in amperes (A). This indicates the total length of the cable laid, in kilometers. This indicates the AC resistance value per unit length of the selected cable, in ohms per kilometer (Ω). ); The power factor represents the load current; This indicates the inductive reactance per unit length of the selected cable, expressed in ohms per kilometer. ); This represents the sine value of the load current impedance angle. During system design, it is necessary to ensure that the power supply voltage minus... The remaining voltage value after the test shall not be lower than the lower limit of the allowable deviation of the rated voltage of the ground equipment.

[0052] The ground substation module 40 is installed in the pre-designated power distribution area at the ground fracturing operation site. It receives electrical energy transmitted from the cable laying module 30 and converts it into the rated operating voltage required by the fracturing equipment. The core of the ground substation module 40 adopts an explosion-proof mobile box-type substation structure, with a skid-mounted base at the bottom to meet the transportation and rapid deployment needs of complex terrain in the field.

[0053] The explosion-proof mobile prefabricated substation adopts a compartmentalized isolation structure, with the internal space physically divided into independent high-voltage, transformer, and low-voltage compartments by steel partitions. Electrical interlocking devices are installed between each compartment, designed to enforce a preset operating sequence and prevent accidental entry into energized areas.

[0054] The transformer room houses a mine-use explosion-proof dry-type transformer. This transformer employs an epoxy resin cast coil structure, with the winding connection group set to Dyn11. This uses a delta connection to suppress high-order harmonic components and reduce harmonic pollution to the underground power grid. To address bus voltage deviations caused by severe load fluctuations during fracturing operations, the dry-type transformer integrates an on-load tap changer (OLTC). This OLTC is configured to dynamically switch tap positions based on adjustment commands sent by the intelligent control module 50 without interrupting the load current, thereby maintaining the stability of the low-voltage side output voltage. A PT100 temperature sensor is pre-embedded inside the transformer coil, connected to a temperature controller to control the start and stop of the air-cooling unit.

[0055] The high-voltage compartment is configured as both the power supply and protection terminal, with its incoming cabinet connected to the top interface of the cable laying module 30 via a high-voltage cable connector. The high-voltage compartment is equipped with a high-voltage vacuum load switch and fuse combination appliance, or a high-voltage vacuum circuit breaker, serving as the main protection element for the transformer. A high-voltage live display device is installed on the high-voltage compartment panel to visually indicate the live status of the three-phase voltage.

[0056] The low-pressure compartment is configured as the power distribution and output terminal, housing a low-voltage busbar and multiple outgoing circuit breakers. Considering the multi-unit operation characteristics of fracturing trucks, the output terminal of the low-pressure compartment is equipped with multiple high-power quick-connect interfaces. These interfaces directly connect to the power input terminals of each fracturing pump truck, enabling plug-and-play electrical connections. The low-pressure compartment also integrates multi-functional power meters for real-time measurement of three-phase voltage, current, active power, and power factor on the ground side, uploading the data to the intelligent control module 50. Furthermore, a dedicated interface is reserved on the low-voltage busbar for connecting a dynamic reactive power compensation unit to achieve local reactive power balancing.

[0057] The ground substation module 40 provided by the present invention further includes a dynamic reactive power compensation unit connected in parallel on the low-voltage side bus.

[0058] This dynamic reactive power compensation unit is built based on Static Var Generator (SVG) technology using a self-commutated bridge circuit. Its physical structure mainly includes IGBT power switching modules, DC-side energy storage capacitor banks, connecting reactors, and core control circuitry. The AC output of the dynamic reactive power compensation unit is connected to the low-voltage bus of the prefabricated substation via a dedicated circuit breaker, providing continuous, bidirectional adjustable inductive or capacitive reactive power to the system.

[0059] The dynamic reactive power compensation unit includes a high-frequency sampling circuit that synchronously acquires three-phase voltage and current signals through precision current transformers and voltage transformers installed at the bus. The core control circuit has a built-in instantaneous reactive power theoretical algorithm, which is used to separate the fundamental active current, fundamental reactive current, and higher harmonic current components from the acquired non-sinusoidal waveform.

[0060] To maintain stable voltage on the ground power supply bus and reduce line losses, the dynamic reactive power compensation unit executes closed-loop control logic based on the target power factor. This unit calculates the required reactive power compensation at the current moment based on real-time monitored load conditions. Its calculation expression is: ; in, This indicates that the dynamic reactive power compensation unit is at time [time]. The amount of reactive power compensation that should be output is expressed in kilovars (kvar), and the sign of the sign indicates whether the output is inductive or capacitive reactive power. Indicates time The actual active power consumed by the ground fracturing equipment, in kilowatts; Represents the tangent function in trigonometric functions; Represents the inverse cosine function in the inverse trigonometric functions; This represents the system's natural power factor detected without compensation, and is a dimensionless value. This represents the system's preset control target power factor constant, which is a dimensionless value.

[0061] The drive circuit of the dynamic reactive power compensation unit is based on the calculated... A pulse width modulation (PWM) control signal is generated. This PWM signal directly drives the gate of the IGBT power switching module, and by controlling the on and off of the inverter bridge arm, it adjusts the amplitude and phase of the inverter output voltage, forcing the connected reactor to generate a compensation current that matches its command value and injects it into the grid.

[0062] Furthermore, to address the high-order harmonics generated by the frequency converter drive system of the fracturing pump, the dynamic reactive power compensation unit is also configured to perform active filtering. The core control circuit calculates specific harmonic current components (such as the 5th, 7th, and 11th) in the load current and superimposes them in reverse phase onto the... The instruction controls the IGBT module to output a reverse canceling current. This configuration is used to prevent harmonic currents from flowing back into the cable laying module 30 and the underground power grid, avoiding overheating of long-distance vertical cables due to the skin effect caused by high-frequency harmonics.

[0063] The intelligent control module 50 serves as the logical core of the entire power supply system, and at the physical level, it constructs an industrial-grade communication network covering the downhole power supply end, the surface substation end, and the fracturing operation end. The hardware core of this module includes a main controller located in the surface main control room, a human-machine interface terminal (HMI), and network switching equipment distributed at various functional nodes.

[0064] The main controller employs an industrial-grade programmable logic controller (PLC) or industrial control computer (IPC) with dual-machine hot standby capability. This main controller, including a high-speed processing unit and multiple communication interface modules, serves as the system's computational hub. The main controller exchanges data with remote I / O modules located in each subsystem via an Ethernet bus. To ensure real-time data transmission and interference immunity, the main controller and the human-machine interface terminal are connected via an independent, isolated control network.

[0065] In terms of the vertical communication link, i.e., the connection between the surface and the well, the intelligent control module 50 adopts a gigabit industrial Ethernet architecture based on fiber optic transmission. Fiber optic composite cables or independent optical cables are laid along the borehole connection module 20, with the bottom end connected to the explosion-proof fiber optic switch of the downhole power supply module 10 and the top end connected to the core switch in the surface control room. This link is used to transmit bus voltage, current, switch status, and fault recording data collected by the downhole relay protection unit. Due to the use of optical signal transmission, this link effectively eliminates electromagnetic interference generated by the high-voltage cables inside the borehole.

[0066] In terms of horizontal communication links, i.e., connections between ground-based devices, the intelligent control module 50 adopts a star network topology. The integrated protection and control devices and dynamic reactive power compensation unit controllers within the ground substation module 40 all function as slave nodes, connected to the core switch via shielded twisted-pair cables. The communication protocol uses standard industrial automation protocols (such as Modbus TCP / IP or IEC 61850) to ensure interoperability between devices from different manufacturers.

[0067] For cross-system communication with fracturing equipment, the intelligent control module 50 is equipped with a dedicated communication gateway. The gateway's input connects to the fracturing truck's instrument data center via a CAN bus or J1939 protocol interface to acquire real-time data on fracturing pump displacement, wellhead pressure, and fracturing fluid density. The gateway internally runs a protocol conversion program to convert fracturing condition data into unified format Ethernet data packets, which are then sent to the main controller as input variables for the feedforward prediction logic.

[0068] In addition, the intelligent control module 50 integrates a GPS / BeiDou dual-mode time synchronization device. This device receives satellite clock signals via an antenna and synchronizes the time at the microsecond level with the downhole power supply module 10, the ground substation module 40, and the main controller via the NTP network time synchronization protocol. This configuration ensures that all electrical and hydraulic parameters within the system have a unified timestamp, providing an accurate timing reference for subsequent fault tracing and load correlation analysis.

[0069] See Figure 3 The intelligent control module 50 provided by this invention further includes a fracturing load feedforward prediction unit. This unit is used to establish a mapping relationship between the hydraulic operation parameters of the fracturing equipment and the electrical load parameters of the power supply system. It reads the operation data sent by the fracturing instrument vehicle in real time through a communication interface, including wellhead pressure, real-time discharge rate (flow rate), and fracturing fluid density. The fracturing load feedforward prediction unit has a pre-stored electromechanical conversion model of the fracturing pump group, which is used to calculate the corresponding theoretical electrical power demand using the instantaneous values ​​of the hydraulic parameters before a sudden change in the actual electrical load occurs.

[0070] To achieve accurate load forecasting, the fracturing load feedforward prediction unit executes a power conversion algorithm based on energy conservation. This algorithm converts water power into shaft power, then into electrical power, and takes into account overall system efficiency losses. The predicted total active power demand for fracturing operations is then calculated. Determined based on the following formula: ; in, Indicates time The predicted total active power demand for fracturing operations, in kilowatts; This indicates the real-time wellhead construction pressure, measured in megapascals (MPa). This indicates the total displacement of the fracturing pump unit collected in real time, in liters per minute; This represents a physical constant used to convert the product of pressure and flow rate into kilowatts of power. This represents the electro-hydraulic integrated energy conversion efficiency coefficient of the fracturing unit. The coefficient ranges from 0.85 to 0.92, and the specific value is set according to the motor nameplate and transmission chain parameters of the fracturing equipment. This represents the sum of the fixed base power of auxiliary equipment such as sand mixing trucks and instrument trucks at the fracturing site, expressed in kilowatts.

[0071] The fracturing load feedforward prediction unit is further equipped with trend analysis logic. This trend analysis logic calculates the total displacement of the fracturing pump unit. The time derivative is used to identify the proppant addition or displacement increase phase of the fracturing operation. When the displacement increase rate is detected to exceed a preset threshold, the fracturing load feedforward prediction unit automatically adjusts the parameters accordingly. Multiply by a dynamic response coefficient greater than 1 to compensate for the lag error in power prediction caused by the starting current surge during motor acceleration.

[0072] The fracturing load feedforward prediction unit will calculate the obtained With the maximum allowable power from the downhole power supply module 10 Real-time comparisons are performed. This fracturing load feedforward prediction unit generates the remaining power supply capacity. Its definition is: ; in, This indicates the system's remaining power supply capacity at the current moment. If If the current value is lower than the preset alarm threshold, the fracturing load feedforward prediction unit immediately generates a power warning signal and sends it to the closed-loop feedback protection unit first, without waiting for the actual current to exceed the protection setpoint. This mechanism ensures that the control system can proactively identify overload risks before electrical faults occur.

[0073] See Figure 4 The intelligent control module 50 provided by the present invention further includes a closed-loop feedback protection unit for performing bidirectional linkage control between power supply safety and fracturing operation.

[0074] The closed-loop feedback protection unit mainly consists of a comparison logic circuit, a flow limit calculation module, and a graded trip control module. This unit receives load margin index data from the fracturing load feedforward prediction unit and real-time capacity limit data from the downhole power supply module 10. Based on the electrical constraints on the power supply side, the closed-loop feedback protection unit reverse-regulates the hydraulic operation parameters on the fracturing side, forming an electro-hydraulic closed-loop control circuit.

[0075] The flow limit calculation module is used to automatically calculate the maximum allowable discharge setting of the fracturing pump unit when the system load is detected to be close to the critical value. To ensure that the electrical load does not exceed the allowable output power of the underground substation, the flow limit calculation module determines the upper limit of flow based on the inverse energy conversion operation. The calculation expression is as follows: ; In the formula, This indicates the maximum allowable output displacement setting of the fracturing pump unit under the current operating conditions, in liters per minute. This indicates the maximum allowable active power output of the downhole power supply module 10 in real time, in kilowatts. This indicates the fixed base power of the fracturing field auxiliary equipment, in kilowatts. The physical conversion constant representing power versus flow rate and pressure; This represents the electro-hydraulic integrated energy conversion efficiency coefficient of the fracturing unit; This indicates the real-time collected wellhead construction pressure value, in megapascals (MPA).

[0076] The closed-loop feedback protection unit transmits the calculated data through a communication gateway. The command is sent in real time to the control center (instrument vehicle) of the fracturing pump truck. This command is used to forcibly rewrite the frequency setpoint or torque limiting logic of the fracturing pump truck's variable frequency drive (VFD) system when the set displacement by the driver or automatic control system exceeds... At this time, the fracturing truck's control system automatically limits the output speed or output power of the high-power variable frequency motor, forcing the actual displacement to remain within the calculated safety threshold, thereby suppressing the generation of overload current on the grid side at the source.

[0077] The tiered tripping control module serves as the system's last line of defense, handling extreme situations such as communication delays or feedback control failures. This module executes longitudinal selective protection logic based on action time tiers. To ensure that surface faults do not affect the underground power grid, the tiered tripping control module sets the protection action delay setting value for the 40 incoming circuit breaker of the surface substation module. Strict constraints were set: ; in, This indicates the protection action delay setting value of the 40-type circuit breaker in the ground substation module, in seconds; This indicates the protection action delay setting value of the feeder circuit breaker of the downhole power supply module 10, in seconds; This represents the differential coordination margin time, in seconds. Its value depends on the inherent tripping time of the circuit breaker and the operation time of the protection device, and is usually set to 0.2 to 0.3 seconds.

[0078] When the closed-loop feedback protection unit detects that the input current of the ground substation module 40 continuously exceeds the protection setting and does not decrease after a preset feedback adjustment period, or when the system detects serious fault characteristics such as short circuit, the graded trip control module is used to directly drive the ground circuit breaker to perform a power cut-off operation within 100 milliseconds, and send a blocking signal to the main controller after tripping to prevent reclosing before the fault is cleared.

[0079] See Figure 5 The power supply method provided by this invention, through the above-described system architecture, executes the entire process from downhole power acquisition, vertical transmission, surface transformation to intelligent load control.

[0080] First, step S1 is executed: power supply channel construction. This step S1 is completed before the fracturing operation begins. A directional drilling hole is constructed using directional drilling technology to connect the nearby roadway of the underground substation to the surface fracturing operation area. After installing the staged casing assembly and completing grouting and cementing within the borehole, the mining armored high-voltage cable is vertically lowered along the internal space of the casing. The bottom end of the cable is connected to the feeder switch of the underground power supply module 10, and the top end is connected to the high-voltage incoming cabinet of the surface substation module 40, thus establishing a physical power transmission link through the rock strata. Simultaneously, the fiber optic communication network is connected to complete the data handshake between the surface intelligent control module 50, the underground power supply module 10, and the fracturing instrument vehicle. A GPS / BeiDou dual-mode device is used to synchronize the clock across the entire system.

[0081] Then, proceed to step S2: system power-on and status monitoring. The underground power supply module 10 is switched on and powered on, and high-voltage power is transmitted to the surface via vertically laid cables. The integrated protection device in the surface substation module 40 monitors the voltage amplitude and phase sequence at the receiving end in real time. At this time, the dynamic reactive power compensation unit automatically starts, calculates and outputs inductive or capacitive reactive power by collecting bus voltage and current signals, corrects the power factor on the surface side to a preset target value (e.g., above 0.95), and stabilizes the bus voltage under no-load or light-load conditions. The intelligent control module 50 periodically reads the current maximum active power limit from the underground power supply module 10. This value is dynamically determined by the rated capacity of the underground transformer and the current occupancy of other underground loads.

[0082] During fracturing operations, step S3 is executed: fracturing load feedforward prediction. The intelligent control module 50 collects the hydraulic parameters of the fracturing unit in real time via the communication gateway, including the wellhead construction pressure. Total displacement of fracturing pump unit The system uses a pre-set electromechanical conversion model and formulas... Real-time calculation of the total active power requirement for fracturing operations under the current working conditions. This prediction process is completed before the actual step change in electrical load occurs, providing the control system with a millisecond-level look-ahead time window.

[0083] Next, step S4 is executed: load margin analysis and closed-loop feedback control. The system will predict power... With the upper limit of maximum active power The system compares the predicted power output to calculate the real-time load margin. When the predicted power approaches or exceeds the allowable upper limit, the intelligent control module 50 immediately activates the flow restriction logic, reversing the calculation to determine the maximum allowable displacement setpoint under the current pressure. The flow restriction command is sent first to the data center of the fracturing instrument vehicle, forcing the fracturing vehicle control system to adjust the operating frequency of the variable frequency motor and limit the pump speed, thereby locking the actual power consumption within the safe capacity range of the downhole power supply system.

[0084] Finally, step S5 is executed: graded protection and fault isolation. In extreme cases where the aforementioned feedforward control and feedback regulation fail to prevent overload, or a short-circuit fault occurs in the system, the surface substation module 40 and the underground power supply module 10 perform longitudinal coordinated protection based on time-level differences. The circuit breaker of the surface substation module 40 is set to first-level protection, and its protection action delay setting value... Strictly less than the protection action delay setting value of the feeder switch of the downhole power supply module 10 When the fault current persists, the surface-side circuit breaker will trip first to clear the fault, ensuring that the underground power supply backbone is not affected and guaranteeing the continuous operation of other mining equipment. After the fault is cleared, the system must receive a manual reset signal before it can resume the power-on process.

[0085] See Figure 6 and Figure 7 To verify the control stability of this system under conditions of severe fluctuations in downhole background load, this embodiment constructs a typical fracturing operation simulation scenario lasting 60 minutes.

[0086] Figure 6 The dynamic response of electrical parameters is shown, with the vertical axis representing power (unit: kW). Figure 7 The corresponding hydraulic parameter response is displayed, with the left vertical axis representing the real-time discharge (unit: m³). 3 / min), the right vertical axis is the wellhead construction pressure (unit: MPa).

[0087] The specific control process and technical details are analyzed as follows: Initial power capping and displacement adaptation (0-20 minutes): In the initial stage of this simulation scenario (0-20 minutes), the substation transformer connected to the underground power supply module 10 is under light load.

[0088] observe Figure 6 The real-time capacity limit (red dashed line, which coincides with the blue solid line in the figure) serves as the maximum allowable power boundary of the system and is stably set at 2000kW. After the operation begins, as the theoretical power demand (black dotted line) attempts to climb and exceed the 2000kW limit in a very short time (about 3 minutes), the intelligent control module 50 immediately intervenes.

[0089] Within this range, the actual active power (solid blue line) is forcibly reduced and overlaps with the real-time capacity limit (dashed red line), forming a horizontal straight line of 2000kW. This indicates that the system entered power-limited mode from the initial stage of operation, precisely capping the output power at the boundary allowed by the downhole power grid.

[0090] To achieve this constant power control, the system must actively adjust the displacement. (Observation) Figure 7 The wellhead pressure (magenta dashed line) gradually increased from approximately 30 MPa to nearly 50 MPa. To maintain a constant power output of 2000 kW, the real-time displacement (blue solid line) was used as the controlled variable, starting from an initial 2.5 m³ / h. 3 The speed was automatically adjusted down to approximately 1.7m. 3 / min.

[0091] Increased downhole background load triggers secondary power limiting in the system (20-45 minutes): At the 20-minute mark of the simulation, high-power equipment is started at the simulated underground fully mechanized mining face, increasing the underground background load. The underground power supply module 10 detects this change and adjusts the remaining available capacity reserved for the surface fracturing equipment from 2000kW to 1600kW.

[0092] Reflected in Figure 6 In the diagram, the red dashed line (real-time capacity limit) experiences a downward step change at the 20-minute mark, dropping to 1600kW. The actual active power (blue solid line) immediately follows suit and decreases, overlapping with the new capacity limit to form a horizontal straight line of 1600kW. This area, marked in the diagram, represents the intelligent peak-shaving protection, demonstrating the system's rapid response and precise control to dynamic changes in the underground power grid.

[0093] Accordingly, in Figure 7 At the 20-minute mark, the real-time displacement (solid blue line) also experienced a step drop, from approximately 1.8 m³ / s. 3 / min suddenly dropped to 1.5m 3 / min. Subsequently, as the wellhead pressure continued to fluctuate, the discharge rate remained at 1.5m³ / min. 3 / min to 1.2m 3The power is dynamically adjusted between / min to ensure that the actual power is always locked at 1600kW.

[0094] Downhole load restored, system automatically resets (45-60 minutes): When the time reaches 45 minutes, the simulated downhole high-power load stops running and the background load decreases. Figure 6 The real-time capacity limit (red dashed line) quickly rebounded to 2000kW.

[0095] The system detected a recovery in power margin, and the actual active power (solid blue line) immediately jumped accordingly, once again overlapping with the 2000kW capacity limit. Meanwhile, Figure 7 The real-time displacement (blue solid line) also rebounded and was dynamically adjusted according to pressure changes to maintain a power output of 2000kW.

[0096] In summary, Figure 6 and Figure 7 Simulation results demonstrate that the present invention can utilize the dynamic remaining capacity of the downhole power grid to provide power for fracturing. Throughout the entire operation, regardless of changes in theoretical demand or fluctuations in downhole background load, the system consistently and precisely limits the actual output power within the dynamic safety boundary of the downhole power grid through active displacement adjustment based on energy inverse calculation, thus achieving an effective combination of electrical safety and the continuity of fracturing operations.

[0097] Specific embodiments have been used to illustrate the principles and implementation methods of this invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of this invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A power supply system for coal mine surface fracturing equipment based on underground power grid, characterized in that, include: The underground power supply module is installed in the underground substation of the coal mine. It is used to draw out the power supply from the high-voltage busbar of the underground power grid and perform electrical protection and capacity assessment on the output underground power supply. The output end of the underground power supply module is connected to the bottom end of the cable laying module. The borehole connection module is constructed with a directional borehole extending from the downhole inlet to the surface outlet. The directional borehole is equipped with a staged casing assembly, which is used to construct a physical channel connecting the downhole space and the surface space. A cable laying module is installed inside the graded casing group of the borehole communication module. The top of the cable laying module is connected to the input end of the surface substation module, which is used to vertically transmit the downhole power output by the downhole power supply module to the surface substation module. The surface power conversion module is installed at the surface fracturing operation site to convert the received downhole power into rated voltage and supply it to the fracturing equipment, and at the same time to adjust the power factor of the power supply circuit. The intelligent control module establishes communication connections with the downhole power supply module, the surface substation module, and the fracturing equipment, respectively. It is used to collect the hydraulic parameters of the fracturing equipment to calculate the electrical load demand, and when the electrical load demand approaches the system capacity limit, it sends control commands to the fracturing equipment to limit the operating frequency of the fracturing pump truck's variable frequency motor.

2. The power supply system for coal mine surface fracturing equipment based on underground power grid according to claim 1, characterized in that, The underground power supply module includes a mine explosion-proof high-voltage vacuum power distribution device and a relay protection unit. The high-voltage vacuum circuit breaker inside the mine explosion-proof high-voltage vacuum power distribution device is used to control the power supply on and off. The relay protection unit is used to perform instantaneous overcurrent protection, overcurrent protection and leakage protection, and is used to establish a background load prediction model by analyzing the load characteristic curve during the maintenance shift, thereby calculating the available redundancy capacity of the underground power supply module by subtracting the base load apparent power determined by the background load prediction model from the rated apparent power of the underground transformer. The downhole power supply module also includes a safety isolation device, which has a mechanical interlocking mechanism and an electrical interlocking mechanism, used to forcibly lock the high-voltage vacuum circuit breaker in the open position during system maintenance.

3. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 1, characterized in that, The verticality deviation and azimuth deviation of the directional drilling trajectory of the drilling communication module are controlled within the range that meets the requirements for smooth installation of the graded sleeve group and vertical laying of the cable, and the diameter of the directional drilling is designed to meet the requirements for high-voltage cable accommodation and heat dissipation. The graded casing assembly includes a surface casing located near the surface for sealing loose layers, and a full-hole technical casing that passes through the surface casing and extends to the downhole inlet. The borehole connection module also includes a sealing cementing layer, which fills the annular space between the technical casing and the borehole wall to provide an airtight barrier for electrical insulation and to prevent gas escape.

4. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 1, characterized in that, The cable laying module includes a mining armored high-voltage cable that is continuously laid along the axial direction of the drilling and connecting module without any joints in the middle. The cable laying module also includes a distributed fixing device, which is fastened to the outer sheath of the mining armored high-voltage cable at a preset interval, and is used to limit the cable swing by frictional contact with the inner wall of the graded bushing group. The cable laying module also includes a grounding device for connecting the metal shielding layer and steel wire armor layer of the mining armored high-voltage cable to a local grounding electrode, and the grounding resistance value of the grounding device is limited to below a preset safety threshold.

5. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 1, characterized in that, The ground substation module includes an explosion-proof box-type substation, which integrates a dry-type transformer with a voltage regulating switch to dynamically switch the tap position according to the adjustment command sent by the intelligent control module, thereby maintaining stable output voltage. The low-voltage room of the explosion-proof box-type substation is equipped with a high-power quick-connect interface that can be directly connected to the power input terminal of the fracturing pump truck, and a dedicated interface is reserved for connecting the dynamic reactive power compensation unit.

6. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 5, characterized in that, The ground substation module also includes a dynamic reactive power compensation unit connected in parallel to the low-voltage side bus, which is constructed based on static var generator technology. The dynamic reactive power compensation unit is used to separate the fundamental active current, fundamental reactive current and higher harmonic current components by collecting three-phase voltage and current signals. The dynamic reactive power compensation unit is used to output inductive or capacitive reactive power to correct the power factor, and to generate a reverse canceling current to filter out high-order harmonics generated by the frequency conversion drive system of the fracturing pump truck.

7. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 1, characterized in that, The intelligent control module includes a fracturing load feedforward prediction unit; The fracturing load feedforward prediction unit is used to read the wellhead pressure, real-time discharge rate and fracturing fluid density data sent by the fracturing equipment in real time through the communication interface. The fracturing load feedforward prediction unit has a pre-stored electro-hydraulic conversion model, which is used to calculate the corresponding theoretical electrical power demand before the actual electrical load changes abruptly by using the product of the wellhead pressure and the real-time discharge rate, combined with the electro-hydraulic integrated energy conversion efficiency coefficient. The fracturing load feedforward prediction unit is also used to monitor the growth rate of the real-time discharge and to dynamically compensate the theoretical power demand when the growth rate exceeds a threshold.

8. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 7, characterized in that, The intelligent control module also includes a closed-loop feedback protection unit; The closed-loop feedback protection unit is used to receive the theoretical power demand calculated by the fracturing load feedforward prediction unit and the real-time capacity limit data of the downhole power supply module. The closed-loop feedback protection unit includes a flow limit calculation module, which is used to determine the maximum allowable discharge setting value of the fracturing pump group based on energy conversion inverse operation when the theoretical power demand is close to the real-time capacity limit. The closed-loop feedback protection unit is used to send the maximum displacement setting value as a control command to the fracturing equipment, forcibly rewriting the control logic of the fracturing pump truck frequency conversion drive system.

9. A power supply system for coal mine surface fracturing equipment based on an underground power grid according to claim 8, characterized in that, The closed-loop feedback protection unit also includes a graded tripping control module; The graded trip control module is used to directly drive the circuit breaker in the ground substation to perform a disconnection operation when the input current of the ground substation module continuously exceeds the protection setting value and the feedback adjustment is ineffective, or when short-circuit fault characteristics are detected. The protection action delay setting value of the circuit breaker in the ground substation module of the graded tripping control module is less than the protection action delay setting value of the high-voltage vacuum circuit breaker in the underground power supply module, and the difference between the protection action delay setting value of the high-voltage vacuum circuit breaker in the underground power supply module and the protection action delay setting value of the circuit breaker in the ground substation module is greater than the preset graded coordination margin time.

10. A power supply method for coal mine surface fracturing equipment based on underground power grid, characterized in that, The power supply system for coal mine surface fracturing equipment based on an underground power grid, as described in any one of claims 1-9, comprises the following steps: S1. Power supply channel construction: directional drilling technology is used to construct a directional borehole connecting the underground substation and the surface fracturing operation area. After installing a graded casing group in the directional borehole and completing grouting and cementing, the mining armored high-voltage cable is vertically lowered along the inside of the graded casing group, and the two ends of the mining armored high-voltage cable are physically connected to the underground power supply module and the surface substation module, respectively. S2. System power-on and status monitoring: Control the underground power supply module to close, so that the underground power supply is transmitted to the ground substation module through the mine armored high-voltage cable. The dynamic reactive power compensation unit in the ground substation module automatically adjusts the bus voltage and power factor, and the intelligent control module periodically reads the maximum allowable active power limit of the underground power supply module. S3. Fracturing load feedforward prediction: The intelligent control module collects hydraulic parameters of fracturing operations in real time, and calculates the total active power demand of fracturing operations corresponding to the current working condition based on the preset electromechanical conversion model, using the wellhead construction pressure and the total discharge of the fracturing pump group. S4. Load margin analysis and closed-loop feedback control: The total active power demand for fracturing operations obtained in step S3 is compared with the maximum active power limit obtained in step S2. When the total active power demand for fracturing operations is close to the maximum active power limit, the net power obtained by subtracting the basic power of auxiliary equipment from the maximum active power limit is used. Combined with the wellhead construction pressure and the electro-hydraulic integrated energy conversion efficiency coefficient, the energy conversion inverse operation is performed to calculate the maximum allowable discharge setting value. The fracturing equipment is forced to adjust the operating frequency of the variable frequency motor to limit the actual discharge. S5. Graded protection and fault isolation: When the control in step S4 fails to prevent overload or short circuit fault in the system, the circuit breaker of the ground substation module is controlled to perform a tripping operation before the circuit breaker of the underground power supply module, thereby cutting off the power supply circuit.

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