A three-core power cable jacketed series impedance suppression jacketed circulating current method and system

Abstract

The application discloses a kind of three-core power cable sheath string impedance suppression sheath circulating current method and system, belong to power engineering transmission and distribution technical field.The method includes: based on three-phase sheath current loop, the calculation equation of sheath circulating current is established;Establish the power system model of three-core power cable as transmission line, calculate the circulating current size and sheath loss when sheath is not stringed into impedance and sheath is stringed into different impedance;Under the constraint of sheath circulating current, the optimal solution of sheath string impedance is searched by disturbance observation method with the minimum sheath loss as target.The application suppresses sheath circulating current by the method of series impedance in the sheath loop of three-core power cable, and then reduces the heating capacity and temperature rise of power cable.The method has the advantages of large impedance adjustment range, good circulating current suppression effect and other advantages, and has important significance for improving the safety and stability of power engineering transmission and distribution system operation.

Description

Technical Field

[0001] This invention belongs to the field of power transmission and distribution technology, and relates to a method and system for suppressing circulating current in the sheath of a three-core power cable by series impedance. Background Technology

[0002] Three-core power cables are mainly used for three-phase AC power transmission and are a widely used cable type in power systems, suitable for industrial power supply and household appliance connections. Excessive circulating current in the sheath of a three-core power cable can cause it to overheat. Because the structure of a three-core power cable is tightly secured within the inner and outer insulation layers and armor layer, its heat dissipation is not as good as that of a single-core power cable. For the same amount of heat generated, a three-core power cable is more likely to reduce its current carrying capacity than a single-core power cable, and may even damage the cable and cause accidents.

[0003] Suppressing circulating current in the sheath by series impedance in the power cable sheath circuit can reduce the heat generation and temperature rise of the power cable, which is of great significance for improving the safety and stability of the power transmission and distribution system.

[0004] Suppression of circulating current in power cable sheaths mainly focuses on single-core power cables. For single-core power cables with cross-connected sheaths, due to the presence of the cross-connection box, it is usually necessary to monitor and regulate the voltage of the cross-connection box while suppressing the circulating current in the sheath to prevent high-voltage hazards to instruments, equipment and operators. This has attracted great attention from domestic scholars. For example, the literature "Research on the Optimization of Grounding Circulation Current Suppression Mechanism and Method and Grounding Defect Monitoring of High Voltage Cables" (Zong Hongbao, Tianjin University, 2020) compares and analyzes different circulation current suppression methods based on the ATP / EMTP simulation model, and concludes that resistors, inductors, and resistive-inductive elements can all effectively suppress sheath circulation current, and that the circulation current suppression effect is better when 4 to 5 times the sheath impedance is connected in series; the literature "Calculation and Analysis of Circulation Current in Metal Sheath of Single-Core Power Cable" (Qin Xi, Liu Yanping, Zhou Wenjun, et al. Electrical Engineering Technology, 2021, (19): 134-137) establishes an equivalent circuit model of sheath grounding circulation current and analyzes the sheath circulation current under different operating conditions of single-core cables. The results show that the greater the asymmetry of the three-phase system and load, the greater the sheath circulation current; the literature "High Voltage Single-Core Power Cables" compares and analyzes different operating conditions of single-core cables, and that the literature "Calculation and Analysis of Circulation Current in Metal Sheath of Single-Core Power Cables" compares and analyzes different operating conditions of single-core cables, and that the literature "Calculation and Analysis of Circulation Current in Metal Sheath of Single-Core Power Cables" compares and analyzes different operating conditions of single-core power cables, and that the greater the asymmetry of the three-phase system and load, the greater the sheath circulation current; the literature "Calculation and Analysis of Circulation Current in Metal Sheath of Single-Core Power Cables" compares and analyzes different operating conditions of single-core power cables, and that ... The paper "Research on Circulating Current Analysis and Suppression Method of Metal Sheath of High-Voltage Single-Core Cable" (Guo Chunming, Shenyang University of Technology, 2022) derives the theoretical calculation formulas for the induced voltage and circulating current of the sheath of high-voltage single-core cable based on electromagnetic induction, and studies the method of suppressing sheath circulating current by connecting different attribute impedances in series with the sheath based on the influencing factors of sheath circulating current. The paper "Parameter Design of Cable Circulating Current Suppression Device Based on Impedance Optimization" (Chen Xiaoru, Huang Longyi, Yang Mingjia, et al. Electrical Transmission, 2024, 54(04):89-96) addresses the lack of flexibility in the input and output of existing high-voltage cable sheath circulating current suppression devices, and the failure to consider the excessive sheath voltage and excessive loss when selecting the suppression impedance. It designs a circulating current suppression device that can dynamically input or postpone the sheath circulating current suppression impedance according to the field conditions.

[0005] Existing literature primarily focuses on suppressing sheath circulating current in single-core high-voltage cables. These cables suffer from significant three-phase asymmetry due to the large spacing between the three-phase conductors, resulting in a high induced electromotive force (EMF) in the sheath and consequently, a large circulating current. Therefore, single-core high-voltage cables often employ cross-interconnection and uniform transposition of the sheaths. In each unit, the sheath undergoes three transpositions via two grounding boxes, ensuring that each transposition segment is of equal length and placed outside the three-phase conductors to reduce induced EMF and circulating current. Long-term research has often assumed that three-core power cables have low asymmetry and low induced EMF in the sheath, leading to low circulating current and the absence of sheath transposition, neglecting the study and suppression of sheath current in three-core power cables. In reality, with longer cable lines, the asymmetry of the cable accumulates. Furthermore, leakage current between sheaths causes nonlinear changes in the induced EMF, self-impedance, and mutual impedance per unit length of the sheath, with the nonlinearity increasing with cable length. Both of these factors contribute to an increase in sheath circulating current in three-core power cables as the cable length increases. Summary of the Invention

[0006] This invention provides a method and system for suppressing circulating current in the sheath of a three-core power cable by introducing series impedance, which solves the problems of large circulating current, high loss and heat generation, and high temperature rise in the cable system of existing three-core power cables.

[0007] The technical solution of this invention is as follows:

[0008] A method for suppressing circulating current in the sheath of a three-core power cable by series impedance is provided, comprising the following steps:

[0009] S1. Based on the three-phase sheath current loop, establish the calculation equation for the sheath circulating current;

[0010] Ignoring leakage current between sheaths and nonlinearity of the cable system, the calculation equation for sheath circulating current is theoretically analyzed, and the influence of sheath series impedance on sheath circulating current is derived.

[0011] S2. Establish a power system model with a three-core power cable as the transmission line, and calculate the circulating current and sheath loss when the sheath is not connected in series with impedance and when the sheath is connected in series with different impedances.

[0012] S21. Based on the PSCAD simulation platform, establish a PSCAD simulation model of the power system with buried three-core power cables as transmission lines, taking into account the burial depth, internal actual structure, arrangement, sheath leakage current and nonlinear characteristics of the three-core power cables.

[0013] S22. Based on the PSCAD simulation model, accurately calculate the circulating current of the sheath of a three-core power cable, the circulating current and sheath loss when different impedances are introduced into the sheath, and obtain the relationship between the sheath circulating current, loss and the sheath impedance.

[0014] S3. The optimal solution for the sheath series impedance is searched using the perturbation observation method to minimize sheath loss under constraints, based on the sheath circulating current.

[0015] In step S1, the three-phase sheath current is calculated based on the electromagnetic induction principle of the power cable electromagnetic system.

[0016] In step S3, the sheath circulating current is used as a constraint and the minimum sheath loss is used as the objective function. The perturbation and observation method is used to search for the optimal solution of the sheath series impedance. The optimization is divided into two aspects: the impedance of the series sheath loop when the sheath loss is minimized is greater than the impedance corresponding to the critical value of the sheath circulating current.

[0017] In step S3, a digital potentiometer based on the STM32F103C8T6 embedded processing chip is used to implement the optimal series impedance system for the sheath.

[0018] The optimal serial impedance system for the sheath includes: a minimum system based on an STM32F103C8T6, a single-pole single-throw electronic switch MOSFET driver, and a digital potentiometer.

[0019] The optimal series impedance system of the sheath is powered by a 5V DC power supply. The 3.3V and 24V power supply voltages are obtained through a DC-DC converter to power the minimum system with STM32F103C8T6 as the core and the single-pole single-throw electronic switch MOS transistor driver, respectively.

[0020] The 24V power supply voltage drives the MOSFET power ground and is connected to the source of the single-pole single-throw electronic switch MOSFET. Each single-pole single-throw electronic switch MOSFET is configured with a 24V power supply.

[0021] The digital potentiometer is implemented by independently controlling the series impedance through the GPIO port pins of the STM32F103C8T6 embedded processing chip.

[0022] The single-pole single-throw electronic switch MOSFET uses an optocoupler (optical coupler) and an STM32F103C8T6 embedded processing chip for isolation and amplification, realizing strong and weak current isolation of the sheath series impedance suppression sheath circulating current system to ensure system safety;

[0023] The 24V power supply uses a 9.1V Zener diode to generate two voltage sources: 14.9V and 9.1V. These are used as the driving voltage when the MOSFET is turned on and the voltage to accelerate the discharge of charge when the MOSFET is turned off.

[0024] The sheath circulating current detection is performed by the analog-to-digital converter (ADC) module, which integrates the STM32F103C8T6 embedded processing chip;

[0025] The implementation of the perturbation-observation method for searching the optimal solution of the sheath serial impedance in the STM32F103C8T6 embedded processing chip is programmed in C language.

[0026] A digital potentiometer based on the STM32F103C8T6 embedded processing chip is used to implement a system for finding the optimal serial impedance of the sheath. By utilizing the resource modules of the STM32F103C8T6 embedded processing chip, hardware is designed to detect the sheath current, calculate the sheath loss, and programmatically implement the perturbation and observation method to search for the optimal serial impedance of the sheath.

[0027] A three-core power cable sheath series impedance suppression sheath circulating current suppression system includes:

[0028] The sheath circulating current module is used to establish the calculation equation for the sheath circulating current based on the three-phase sheath current loop;

[0029] The calculation module is used to establish a power system model of a three-core power cable as a transmission line, and to calculate the circulating current and sheath loss when the sheath is not connected in series with impedance and when the sheath is connected in series with different impedances.

[0030] The impedance optimization module is used to search for the optimal solution of the sheath series impedance under the constraint of sheath circulating current, with the goal of minimizing sheath loss, by using the perturbation and observation method.

[0031] The advantages of this invention are:

[0032] (1) Based on the electromagnetic theory of the power system of three-core power cable as a transmission line, establish the theoretical calculation equation of sheath circulating current, solve the sheath circulating current related to the core (load) current and cable geometric parameters, and analyze the variation characteristics of sheath circulating current with sheath series impedance.

[0033] (2) This invention introduces the PSCAD platform for simulation modeling analysis of the circulating current and loss of the sheath of three-core power cables. It fully considers the laying method of three-core power cables, the internal structure of the cables, and electrical parameters (such as relative permittivity, relative permeability, etc.), and can accurately calculate the leakage current between sheaths and the nonlinear characteristics of the cable system.

[0034] (3) Through theoretical analysis of sheath circulating current and simulation modeling on PSCAD platform, this invention obtains the relationship between the series impedance of sheath circuit and circulating current and sheath loss, optimizes the design of series impedance of sheath circuit, so that the sheath loss is minimized under the constraint that the sheath circulating current meets the requirements.

[0035] (4) The present invention designs a digital potentiometer based on the STM32F103C8T6 embedded processing chip. The objective function is to minimize the sheath loss under the constraint of sheath circulating current, and the series impedance of the sheath is optimized by the perturbation observation method.

[0036] This invention elucidates a method for suppressing circulating current in the sheath of three-core power cables by introducing series impedance from electromagnetic theory and simulation modeling. It has important reference value for the study of heating, temperature rise, and insulation aging of three-core power cables as transmission lines, and is an important component of power cable circulating current suppression. Attached Figure Description

[0037] Figure 1 This is a flowchart of the method of the present invention;

[0038] Figure 2 This is the three-phase sheath current loop diagram of the present invention;

[0039] Figure 3 This is the equivalent circuit diagram of the three-phase sheath of the cable of the present invention grounded by impedance;

[0040] Figure 4 This is a graph showing the variation of cable sheath circulating current with series impedance obtained by the theoretical method of this invention;

[0041] Figure 5 This is a configuration diagram of a three-core power cable conduit under the PSCAD simulation platform of this invention;

[0042] Figure 6 This is a diagram showing the outer radius, relative permittivity, and permeability of the inner / outer insulation / armor layers of a three-core power cable conduit under the PSCAD simulation platform of this invention.

[0043] Figure 7 This is a PSCAD simulation model diagram of the three-core power cable transmission system established in the implementation of this invention;

[0044] Figure 8 This is a waveform diagram of the three-phase normal operating conductor (load) current according to an embodiment of the present invention;

[0045] Figure 9 This is a diagram showing the effective values ​​of the three-phase normal operating conductor (load) current according to an embodiment of the present invention;

[0046] Figure 10 This is a waveform diagram of the sheath current when the three-phase system is operating normally without series impedance according to an embodiment of the present invention.

[0047] Figure 11 This is a diagram showing the effective value of the sheath current when the sheath is in normal operation without an impedance connected in series according to an embodiment of the present invention.

[0048] Figure 12 The three-phase normal operation sheath of this invention is inserted in series. Waveform of sheath current under impedance;

[0049] Figure 13 The three-phase normal operation sheath of this invention is inserted in series. Plot of RMS value of sheath current under impedance;

[0050] Figure 14 This is a graph showing the variation of sheath circulating current with the impedance of the inserted sheath in an embodiment of the present invention;

[0051] Figure 15 This is a graph showing the variation of sheath loss with the impedance inserted into the sheath according to an embodiment of the present invention;

[0052] Figure 16 In this embodiment of the invention, the circulating current of the sheath is less than the minimum loss after the sheath is connected in series with impedance. A schematic diagram;

[0053] Figure 17 In this embodiment of the invention, the circulating current of the sheath is greater than the minimum loss after the sheath is connected in series with impedance. A schematic diagram;

[0054] Figure 18 This is a diagram of a digital potentiometer controlled by a digital signal according to an embodiment of the present invention;

[0055] Figure 19 This is the minimum system schematic diagram of the STM32F103C8T6 embedded processing chip as the control core in this embodiment of the invention;

[0056] Figure 20 This is a schematic diagram of the first 5 impedances connected in series in a digital potentiometer controlled by the GPIOA0~4 pins of this invention.

[0057] Figure 21 This is a schematic diagram of the series connection of the last five impedances of the digital potentiometer controlled by pins GPIOA5~9 in an embodiment of the present invention.

[0058] Figure 22 This is a flowchart of a digital potentiometer system based on the STM32F103C8T6 embedded processing chip according to an embodiment of the present invention. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0060] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0061] like Figure 1 As shown, a method for suppressing circulating current in the sheath of a three-core power cable by series impedance is disclosed, including the following steps:

[0062] S1. Based on the three-phase sheath current loop, establish the calculation equation for the sheath circulating current;

[0063] Ignoring leakage current between the sheaths of the three-core power cable and the nonlinear characteristics of the cable system, the calculation equations for the sheath circulating current are theoretically analyzed.

[0064] like Figure 2 As shown, each phase of the three-core power cable (A, B, and C) consists of a conductor and a sheath. The three-phase conductors, along with the three-phase power supply and the three-phase load, constitute a three-phase circuit. To introduce the total impedance into the sheath, the first and last ends are connected in series. . It is the equivalent total impedance of the grounding impedance and earth resistance at both ends of the sheath throughout the entire cable line.

[0065] like Figure 3 As shown, the induced voltage in the sheath forms a circulating current in the sheath circuit. In the equivalent circuit where the three-phase sheath is grounded via impedance, the three-phase sheath terminals directly form a loop with the ground, or through an impedance connected in series at both ends to limit the circulating current. Forming a loop with the earth, The self-impedance per unit length of the sheath, This refers to the length of the cable line. Based on electromagnetic theory, the three-phase load current (passing through the conductors to form a circuit)... , , The electromotive force induced per unit length of the sheath , , They are respectively:

[0066] (1)

[0067] In the formula: The system angular frequency; Let be the mutual inductance per unit length between the conductor and its sheath, which is:

[0068] (2)

[0069] In the formula: The vacuum permeability; This represents the equivalent depth of the earth when the earth is the loop. The outer radius of the sheath; The radius of the wire core; Let be the mutual inductance per unit length between a phase conductor and a non-phase sheath, which is:

[0070] (3)

[0071] In the formula: The distance between the centers of any two phase cables.

[0072] Electromotive force induced in the third phase sheath by the current in any two phase sheaths , , They are respectively:

[0073] (4)

[0074] In the formula: Let be the mutual inductance per unit length between any two phase sheaths, which is:

[0075] (5)

[0076] Self-impedance per unit length of sheath for:

[0077] (6)

[0078] In the formula: Resistance per unit length of the sheath; The self-inductance per unit length of a circuit formed by a single-phase sheath and ground is:

[0079] (7)

[0080] The three-phase voltage equations for the sheath circuit are as follows:

[0081] (8)

[0082] From equation (8), we can obtain:

[0083] (9)

[0084] Currently, single-core cables, due to their high voltage levels, experience large leakage currents between the core and sheath, requiring consideration of leakage current in sheath circulating current analysis. Three-core cables, however, are typically used for medium- and low-voltage applications, such as 35kV and below, where leakage currents between the core and sheath are small. Ignoring leakage current allows for easy derivation of the relationship between sheath circulating current and sheath series impedance, providing a theoretical basis for implementing sheath series impedance suppression. Here, only the trend of sheath circulating current variation with series impedance is needed; high-precision sheath circulating current calculation is not required. In practice, the sheath circulating current is measured in real-time, and sheath losses are calculated. Step S1's calculation method is simple, and the physical concepts of the loop voltage equation are clear.

[0085] like Figure 4 The figure shows the variation of the circulating current in a power system using a 35kV three-core power cable as a transmission line, with different impedances introduced into the cable sheath. The horizontal axis of the figure is... The ordinate represents the resistance of the sheath itself as an integer multiple of the resistance of the sheath itself; the vertical axis max( This represents the maximum effective value of the sheath circulating current. This indicates that the circulating current of the sheath with series impedance is a multiple of the circulating current without series impedance. It can be seen that as the series sheath impedance increases, the sheath circulating current decreases. In this embodiment, the power system parameters of the 35kV three-core power cable used as the transmission line are shown in Table 1:

[0086]

[0087] Table 1

[0088] S2. Based on the PSCAD simulation platform, establish a power system model with buried three-core power cables as transmission lines, taking into account the burial depth, actual internal structure, arrangement, sheath leakage current and nonlinear characteristics.

[0089] For cable model selection, the frequency-dependent Bergeron model is adopted. Its simple structure and high computational efficiency allow for electromagnetic system modeling based on geometric parameters, describing the frequency relationship between loss and propagation characteristics. It facilitates simultaneous solutions for three-phase loads, three-phase sheath series impedance, loop grounding, three-phase asymmetrical laying, and sheath cross-interconnection. The time-domain PSCAD simulation platform for power system electromagnetic transient analysis can handle distributed parameter lines, grounding networks, and multi-loop coupling problems. For high-voltage cable systems, its modeling mechanism facilitates the introduction of electromagnetic coupling and grounding conditions of the conductor, sheath, and ground loop, enabling simulation of the time-domain response of sheath circulating currents.

[0090] like Figure 5 As shown, the pipeline configuration of the PSCAD simulation model of the three-core power cable of the present invention can set the cable placement method (surface / buried), the number of cable cores, burial depth, etc.

[0091] like Figure 6 As shown, the parameters for configuring a three-core power cable conduit can be set, including the outer radius of the inner / outer insulation / armor layer, and the relative permittivity / permeability.

[0092] Figures 5-6In this context, Inner Insulator, Outer radius, Relative permittivity, Relative permeability, Pipe Conductor, Resistivity, and Relative permeability all represent the same properties of pipes.

[0093] like Figure 7 As shown, based on the PSCAD simulation platform, a power system is established consisting of a three-core power cable used as a transmission line, adjustable impedances inserted at both ends of the three-core power cable, a three-phase power supply, and a three-phase symmetrical load. It can simulate power systems with different voltage levels, different transmission capacities, and different sheath series impedances. It can also simulate fault conditions such as symmetrical short circuits, asymmetrical short circuits, open circuits, sheath grounding, and sheath open circuits in the power system.

[0094] Since apparent power and line voltage at the end of the transmission line determine the current in the conductor, and the transmission system usually only provides apparent power and line voltage, and does not provide conductor current separately, it can be solved by the following formula.

[0095] When the three-phase circuit is operating normally, the relationship between the power, voltage, and current at the end of the line is as follows:

[0096] (10)

[0097] In the formula: , , These are respectively three-phase apparent power, three-phase active power, and three-phase reactive power; , These are the line voltage and line current at the end of the cable transmission line, respectively.

[0098] The relationship between the active and reactive power of a three-phase load and voltage and frequency is as follows:

[0099] (11)

[0100] In the formula: , These are the three-phase active power and three-phase reactive power of the load, respectively. , These are the rated values ​​of three-phase active power and three-phase reactive power (values ​​at rated voltage and rated frequency). It is the load voltage. This is the rated value of the load voltage. , These are the voltage coefficients for active power and reactive power, respectively. , These are the frequency coefficients for active power and reactive power, respectively.

[0101] When current flows through the sheath, the sheath loss will cause the cable to heat up. After introducing impedance, this loss is:

[0102] (12)

[0103] In the formula: The sheath loss is the loss when the series impedance is applied. RMS value of sheath current; For calculation The real part of impedance, which is a function of resistance.

[0104] Figure 8 The figure shows the current waveform of the conductor (load) when the power system transmitting the rated capacity load through the three-core power cable is in operation. Its frequency, amplitude, and three-phase phase are completely consistent with the actual system.

[0105] Figure 8 In the figure, Ia-A phase conductor current instantaneous value; Ib-B phase conductor current instantaneous value; Ic-C phase current instantaneous value; sec-second; kA-kiloampere.

[0106] Figure 9 The effective value of the conductor (load) current when the power system transmitting power through the three-core power cable is carrying a rated capacity load is calculated based on the 50Hz fundamental frequency. Therefore, the effective value stabilizes after approximately one power frequency cycle, and the effective values ​​of the three phases are equal. It is close to the rated current of 190A for the wire core system.

[0107] Figure 9 In the figure, IaRMS-A ​​phase conductor current RMS value; IbRMS-B phase conductor current RMS value; IcRMS-C phase conductor current RMS value; sec - second; kA - kiloampere.

[0108] Figure 10 This is the sheath current waveform when no impedance is connected in series at both ends of the sheath. Its frequency and three-phase phase are consistent with the frequency and phase of the conductor system.

[0109] Figure 10 In the figure, the instantaneous value of the sheath current for phase Isa-A; the instantaneous value of the sheath current for phase Isb-B; the instantaneous value of the sheath current for phase Isc-C; sec; kA.

[0110] Figure 11 This is the effective value of the sheath current when no impedance is connected in series at both ends of the sheath, which is also a stable calculated value obtained after about one power frequency cycle. From equation (12), the sheath loss is obtained. .

[0111] Figure 11 In the figure, the effective value of the sheath current for phase A of IsaRMS; the effective value of the sheath current for phase B of IsbRMS; the effective value of the sheath current for phase C of IscRMS; sec; kA.

[0112] Figure 12 Impedance is inserted into both ends of the sheath The sheath current waveform at that time has the same frequency and three-phase phase as the core system frequency and phase.

[0113] Figure 12 In the figure, the instantaneous value of the sheath current for phase Isa-A; the instantaneous value of the sheath current for phase Isb-B; the instantaneous value of the sheath current for phase Isc-C; sec; kA.

[0114] Figure 13 Impedance is inserted into both ends of the sheath The effective value of the sheath current at that time is also a stable calculated value obtained after about one power frequency cycle. From equation (12), the sheath loss is obtained. .

[0115] Figure 13 In the figure, IsaRMS - A phase sheath current RMS value; IsbRMS - B phase sheath current RMS value; IscRMS - C phase sheath current RMS value; sec - second; kA - kiloampere.

[0116] It is evident that introducing impedance into the sheath can reduce the circulating current in the sheath, but it may increase the sheath loss, which will cause the cable to heat up and cause the temperature to rise.

[0117] like Figure 14 As shown, an impedance is inserted in series at both ends of the sheath. Changes in the sheath circulation over time.

[0118] Figure 14 middle, - Impedance of the sheath ; - Effective value of sheath current (A).

[0119] like Figure 15 As shown, an impedance is inserted in series at both ends of the sheath. Changes in sheath wear over time.

[0120] Figure 15 middle, - Impedance of the sheath ; -Sheath loss (W).

[0121] It can be seen that as the series impedance increases, the sheath circulating current decreases, and the sheath loss first decreases and then increases.

[0122] The magnitude of the sheath circulating current is an important reference for selecting sheath circuit current measuring instruments and setting protection devices; it is generally necessary to suppress the sheath circulating current within a certain range. Furthermore, the magnitude of the sheath circulating current is closely related to sheath losses and directly affects the temperature rise of the cable system.

[0123] S3. Based on the variation of sheath circulating current and loss with series impedance, this invention proposes a sheath series impedance optimization method that minimizes sheath loss under sheath circulating current constraints, expressed as follows:

[0124] (13)

[0125] In the formula: The sheath loss represents its series impedance. The function; To find the minimum value function; This is the critical value for sheath current.

[0126] The sheath circulating current is reduced by increasing the series impedance of the sheath circuit. When the sheath circulating current is below a critical value, the impedance of the circuit connected in series with the sheath is stored. The perturbation and observation method is used to search for the impedance of the circuit connected in series with the sheath that minimizes the sheath loss. If this impedance is greater than the stored impedance value, it is selected as the optimal impedance for the final series connection to the sheath circuit. If this impedance is less than the stored impedance value, it is selected as the optimal impedance for the final series connection to the sheath circuit.

[0127] Among the conventional methods for suppressing circulating current in the sheath by introducing impedance into the sheath, the most basic is to introduce a fixed impedance value (too mechanical and not well-matched to load changes), while the most comprehensive is to use a genetic algorithm to obtain the impedance value to be introduced into the sheath (the algorithm logic is too complex). Both of these methods are either too simple or too complex.

[0128] The method proposed in this invention is simple to implement and has a good matching degree for suppressing circulating currents under different loads.

[0129] like Figure 16 As shown, when the sheath circulates Greater than Increase the series impedance of the sheath circuit until... Store the impedance value of the series connection. (i.e., the initial value of the sheath series impedance), in The sheath loss was minimized by using the perturbation observation method in the vicinity. series impedance ,when At that time, the circulating current in the sheath is at its minimum after the impedance is introduced into the sheath. Less than ,choose The optimal series impedance for the sheath circuit.

[0130] like Figure 17 As shown, in The sheath loss was minimized by using the perturbation observation method in the vicinity. series impedance ,when When, choose The optimal series impedance for the sheath circuit, i.e., the sheath circulating current with the minimum loss after the impedance is introduced into the sheath. Greater than .

[0131] This invention also provides a digital potentiometer-based system for achieving optimal serial impedance of the sheath based on the STM32F103C8T6 embedded processing chip, comprising: a minimum system with STM32F103C8T6 as the core, a single-pole single-throw electronic switch MOS transistor driver, and a digital potentiometer;

[0132] like Figure 18 As shown, the digital implementation method for suppressing circulating current in the sheath by incorporating optimized impedance is a digital potentiometer controlled by a digital signal, which internally consists of... Equal impedances In series, each impedance is connected to an electronic switch across its terminals. In parallel connection, the electronic switch is typically a power MOSFET, used as a tap of a digital potentiometer. This analog switch is equivalent to a single-pole single-throw switch, where a digital signal controls the state of each impedance connected to the sheath circuit, thereby controlling the total impedance of the sheath circuit.

[0133] like Figure 19 As shown, the STM32F103C8T6 embedded processing chip based on the Cortex-M3 core is selected as the control core. Its general purpose input / output ports GPIOA, B, and C have 14 pins (0-12, 15), GPIOB has 16 pins (0-15), and GPIOC has 3 pins (13-15), totaling 33 pins for outputting digital control signals. The schematic diagram shows the minimum system with the STM32F103C8T6 embedded processing chip as the control core. This system is easy to power, requiring only a 5V input voltage, which is managed by a power management chip. Convert to The voltage supplies power to the chip core; it uses an external high-speed clock with a crystal oscillator frequency of 8MHz and an external low-speed clock with a crystal oscillator frequency of 32.768KHz; the debugging adopts a serial line mode (Wire Debug), which only requires two signal lines, resulting in a small system size and low cost.

[0134] like Figure 20 As shown in the schematic diagram, the first five impedances of the digital potentiometer controlled by pins GPIOA0-4 of the STM32F103C8T6 embedded processing chip are connected in series. ZxR and HalfCon are the series impedance connection points. Since the maximum output capability of the GPIO pin is only 20mA, the optocoupler chip TLP250, which acts as an isolation amplifier as shown in the diagram, is used to isolate and drive the MOSFETs. After isolation, the sources of the MOSFETs simulating single-pole single-throw switches are in an isolated state, allowing for independent control of the series impedance. This schematic uses a DC-DC boost power management chip of model B0524-2WRS to generate 10 auxiliary 24V voltage sources {(24V1-AGND1), (24V2-AGND2), ..., (24V10-AGND10)} from a single 5V voltage source, which are used as the driving power supply for the MOSFETs.

[0135] The power of the current-limiting resistor of the input diode of the optocoupler The calculation is as follows:

[0136]

[0137] In the formula: This indicates a high level on the GPIO pin of the STM32F103C8T6 embedded processing chip. The voltage drop across the input diode conductor of the optocoupler. The input current-limiting resistor is used for the optocoupler.

[0138] The auxiliary drive 24V voltage is divided into 14.9V and 9.1V by a 9.1V Zener diode 1N4937A and a 2.2kΩ 1206 surface mount resistor (power 1 / 4W). The power of the 2.2kΩ resistor is calculated as follows:

[0139] (14)

[0140] The common point voltage of the Zener diode and the 2.2kΩ resistor is 9.1V relative to the AGND voltage, connected to the source of the MOSFET. A 47uF electrolytic capacitor and a 1uF ceramic capacitor are connected in parallel with the Zener diode and the 2.2kΩ resistor respectively to stabilize the voltage. The high and low levels of the signals from pins GPIOA0~9 of the STM32F103C8T6 embedded processing chip are 3.3V and 0V respectively. The forward voltage drop of the input diode of the TLP250 optocoupler is... A 1.2V, 330Ω, 0603 surface-mount resistor (power 1 / 16W) is inserted in series to limit the on-state current of the input diode. Its power is calculated as follows:

[0141] (15)

[0142] In the optocoupler driving circuit, when the GPIO pin outputs a high level, the input diode of the TLP250 optocoupler conducts, and the emitted optical signal causes the upper transistor of the output push-pull transistor to saturate and conduct. Since pins 6 and 7 of the TLP250 optocoupler are internally connected to pin 8, the voltage is 24V, and the voltage between the gate and source of the MOSFET is 14.9V, driving the MOSFET to conduct. When the GPIO pin outputs a low level, the input diode of the TLP250 optocoupler is cut off. Without an optical signal, the lower transistor of the output push-pull transistor saturates and conducts. Since pins 6 and 7 of the TLP250 optocoupler are internally connected to pin 6, the voltage is 0V, and the voltage between the gate and source of the MOSFET is -9.1V. This negative voltage will cause the gate charge of the MOSFET to discharge rapidly, accelerating the MOSFET turn-off. The 22Ω 1206 resistor in the diagram limits the gate turn-on current and suppresses gate voltage oscillation. The 1N4148 fast recovery diode further accelerates the gate charge discharge during turn-off. A 16V Zener diode 1N4745 connected in parallel to the gate and source of the MOSFET ensures that the gate turn-on voltage does not exceed 16V, in order to prevent excessive voltage from breaking down the gate mask.

[0143] like Figure 21 As shown in the schematic diagram of the last five impedances connected in series in the digital potentiometer controlled by GPIOA5~9 pins, ZxL and HalfCon are the connection points for the series impedances.

[0144] like Figure 22 As shown in the flowchart of the digital potentiometer system based on the STM32F103C8T6 embedded processing chip, the detection of sheath circulating current is completed by the integrated analog-to-digital converter (ADC) unit inside the embedded processing chip, which can realize the detection of sheath circulating current without the need for additional hardware resources.

[0145] First, check the circulating current of the sheath. If the circulating current is greater than the critical value, increase the series impedance of the sheath until the circulating current is reduced. Less than the critical value And store the impedance value inserted in series. ; Calculate sheath loss The sheath's series impedance generates an increment. , Detect the sheath wear at this time ;like The series impedance of the sheath continues to increase. , Detect the sheath wear at this time This cycle continues until... ;if ,but ,otherwise .

[0146] A three-core power cable sheath series impedance suppression sheath circulating current suppression system includes:

[0147] The sheath circulating current module is used to establish the calculation equation for the sheath circulating current based on the three-phase sheath current loop;

[0148] The calculation module is used to establish a power system model of a three-core power cable as a transmission line, and to calculate the circulating current and sheath loss when the sheath is not connected in series with impedance and when the sheath is connected in series with different impedances.

[0149] The impedance optimization module is used to search for the optimal solution of the sheath series impedance under the constraint of sheath circulating current, with the goal of minimizing sheath loss, by using the perturbation and observation method.

[0150] The advantages of this invention are:

[0151] (1) Based on the electromagnetic theory of the power system of three-core power cable as a transmission line, establish the theoretical calculation equation of sheath circulating current, solve the relationship between sheath circulating current and core (load) current and cable geometric parameters, and theoretically analyze the characteristics of sheath circulating current change with sheath series impedance.

[0152] (2) This invention introduces the PSCAD platform for simulation modeling analysis of the circulating current and loss of the sheath of three-core power cables. It fully considers the laying method of three-core power cables, the internal geometry of the cables, and electrical parameters (such as relative permittivity, relative permeability, etc.), and can accurately calculate the leakage current between sheaths and the nonlinear characteristics of the cable system.

[0153] (3) The present invention designs a digital potentiometer based on the STM32F103C8T6 embedded processing chip. The objective function is to minimize the sheath loss under the constraint of sheath circulating current, and the series impedance of the sheath is optimized by the perturbation observation method.

[0154] Based on the unique electromagnetic relationship of a three-core power cable system, this invention analyzes the theoretical equation of sheath circulating current, establishes a power system simulation model of a three-core power cable as a transmission line under the PSCAD simulation platform, obtains the relationship between sheath circulating current, sheath loss and sheath series impedance, and establishes a method for optimizing the sheath series impedance using a digital potentiometer based on the perturbation and observation method with the STM32F103C8T6 embedded processor chip as the core.

[0155] This invention elucidates the theory, method, and implementation of suppressing sheath circulating current in three-core power cables by introducing series impedance from electromagnetic theory, simulation modeling, embedded processor hardware schematic design, and software program flow design. It has important reference value for the study of heating, temperature rise, and insulation aging of three-core power cables as transmission lines and is an important component of power cable circulating current suppression.

[0156] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for suppressing circulating current in the sheath of a three-core power cable by introducing series impedance, characterized in that, Includes the following steps: S1. Based on the three-phase sheath current loop, establish the calculation equation for the sheath circulating current; S2. Establish a power system model with a three-core power cable as the transmission line, and calculate the circulating current and sheath loss when the sheath is not connected in series with impedance and when the sheath is connected in series with different impedances. S3. Under the constraint of sheath circulating current, with the goal of minimizing sheath loss, the optimal solution of sheath series impedance is searched by perturbation and observation method.

2. The method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 1, characterized in that, In step S1, the three-phase sheath current circuit includes: Each phase of a three-core power cable (A, B, and C) consists of a conductor and a sheath. The three-phase conductors, along with the three-phase power supply and the three-phase load, constitute a three-phase circuit. The first and last ends are connected in series. The total impedance is the series impedance of the sheath; the equivalent total impedance of the grounding impedance and earth resistance at both ends of the sheath throughout the entire cable line is... .

3. The method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 2, characterized in that, In step S1, the specific expression for the sheath circulation is: In the formula: , , These are the sheath current phasors for phases A, B, and C, respectively. , , These represent the electromotive forces induced per unit length of the sheath by the three-phase load current, where j is an imaginary number. The system angular frequency, Let be the mutual inductance per unit length between any phase conductor and its sheath. The self-impedance per unit length of the sheath, This refers to the length of the cable line.

4. The method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 1, characterized in that, Step S2 includes: S21. Based on the PSCAD simulation platform, establish a PSCAD simulation model of the power system with buried three-core power cables as transmission lines, taking into account the burial depth, internal actual structure, arrangement, sheath leakage current and nonlinear characteristics of the three-core power cables. S22. Based on the PSCAD simulation model, accurately calculate the circulating current of the sheath of a three-core power cable, the circulating current when different impedances are inserted into the sheath, and the sheath loss.

5. A method for suppressing circulating current in the sheath of a three-core power cable by introducing series impedance according to claim 4, characterized in that, In step S22, When current flows through the sheath, the sheath loss will cause the cable to heat up. After introducing impedance, this loss is: In the formula: The sheath loss is the loss when the series impedance is applied. RMS value of sheath current; For calculation Real part of impedance; The resistance per unit length of the sheath.

6. The method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 1, characterized in that, In step S3, the method for optimizing the sheath series impedance to minimize sheath loss under the sheath circulating current constraint is expressed as follows: In the formula: The sheath loss represents its series impedance. The function; To find the minimum value function; This is the critical value for sheath current; For sheath circulation.

7. A method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 6, characterized in that, In step S3, the optimal impedance for suppressing the circulating current in the sheath is selected using the perturbation and observation method; specifically: When the sheath circulates Greater than the critical value Increase the series impedance of the sheath circuit until... Store the impedance value of the series connection. ,exist The sheath loss was minimized by using the perturbation observation method in the vicinity. series impedance ; when At that time, the circulating current in the sheath is at its minimum after the impedance is introduced into the sheath. Less than ,choose The optimal series impedance for the sheath circuit; when When, choose The optimal series impedance for the sheath circuit, i.e., the sheath circulating current with the minimum loss after the impedance is introduced into the sheath. Greater than .

8. A method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 1, characterized in that, In step S3, a digital potentiometer based on the STM32F103C8T6 embedded processing chip is used to implement the optimal series impedance system for the sheath. The optimal series impedance system for the sheath includes: a minimum system based on an STM32F103C8T6, a single-pole single-throw electronic switch MOSFET driver, and a digital potentiometer. A 5V DC power supply is used, and a DC-DC converter is used to obtain 3.3V and 24V power supply voltages to power the minimum system with STM32F103C8T6 as the core and the single-pole single-throw electronic switch MOSFET driver, respectively. The digital potentiometer is implemented by independently controlling the series impedance through the GPIO port pins of the STM32F103C8T6 embedded processing chip. The single-pole single-throw electronic switch MOSFET is isolated and amplified by an optocoupler and an STM32F103C8T6 embedded processing chip. The sheath circulating current detection is performed by the analog-to-digital converter module with an integrated STM32F103C8T6 embedded processing chip; The implementation of the perturbation-observation method for searching the optimal solution of the sheath serial impedance in the STM32F103C8T6 embedded processing chip is programmed in C language.

9. A method for suppressing circulating current in the sheath of a three-core power cable by series impedance according to claim 8, characterized in that, The power of the current-limiting resistor of the input diode of the optocoupler The calculation is as follows: In the formula: This indicates a high level on the GPIO pin of the STM32F103C8T6 embedded processing chip. The voltage drop across the input diode conductor of the optocoupler. The input current-limiting resistor is used for the optocoupler.

10. A three-core power cable sheath series impedance suppression system for suppressing sheath circulating current, characterized in that, include: The sheath circulating current module is used to establish the calculation equation for the sheath circulating current based on the three-phase sheath current loop; The calculation module is used to establish a power system model of a three-core power cable as a transmission line, and to calculate the circulating current and sheath loss when the sheath is not connected in series with impedance and when the sheath is connected in series with different impedances. The impedance optimization module is used to search for the optimal solution of the sheath series impedance under the constraint of sheath circulating current, with the goal of minimizing sheath loss, by using the perturbation and observation method.

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