A low-voltage cable branch box load distribution dynamic adjustment method

Abstract

The application provides a low-voltage cable branch box load distribution dynamic adjustment method, comprising: monitoring each-phase load current and zero-line zero-sequence harmonic content, and analyzing zero-discharge side impedance distribution before and after auxiliary branch on-off; according to the zero-discharge side impedance distribution and the zero-line zero-sequence harmonic content, using a Fourier transform algorithm to analyze harmonic component proportion, and generating an identification mark of an amplification circulating current condition; according to the identification mark of the amplification circulating current condition, evaluating whether the harmonic component proportion exceeds a critical range of the amplification circulating current condition, after confirming that the amplification circulating current condition is established through analysis, determining a conversion condition of a dredging shunt condition; according to the conversion condition of the dredging shunt condition and the harmonic component proportion, analyzing the influence of the auxiliary branch low-resistance passage on the zero-sequence harmonic, and determining a switching time sequence of zero-line thermal stress reduction.

Description

Technical Field

[0001] This invention relates to the field of information technology, and in particular to a method for dynamic adjustment of load distribution in low-voltage cable branch boxes. Background Technology

[0002] Low-voltage cable distribution boxes, as crucial nodes in power distribution networks, undertake load distribution and current transmission tasks, and their operational stability directly impacts the safety and efficiency of the power supply system. In modern power systems, low-voltage cable distribution boxes face increasingly complex operating conditions during dynamic load distribution adjustments. Nonlinear loads generate distorted currents containing multiple harmonics. Odd-order zero-sequence harmonics, such as the 3rd, 9th, and 15th harmonics, superimpose in phase within a three-phase system, failing to cancel each other out and instead accumulating directly at the neutral and neutral busbar sides, significantly increasing the neutral current, sometimes exceeding that of a single-phase current. Currently, common solutions to excessive neutral current or three-phase imbalance in low-voltage cable distribution boxes include thickening the neutral wire, reducing its impedance, or adding auxiliary current-carrying branches to divert excessive neutral current, attempting to stabilize the neutral point potential and alleviate neutral wire heating. However, these methods often rely on fixed hardware structures and struggle to adapt promptly to load fluctuations, changes in harmonic content, and variations in impedance distribution at the neutral busbar side. While auxiliary current-carrying branches can provide a shunt path for unbalanced currents, when the impedance of this branch is too low and dynamic switching control is lacking, zero-sequence harmonics will preferentially flow back along the low-resistance branch, forming a recurring circulating current channel between the neutral bus, neutral wire, and auxiliary current-carrying branch. In this case, the intended shunt path, meant to divert excessive neutral wire current, may instead attract more zero-sequence harmonic currents, repeatedly superimposing and generating a circulating current effect similar to eddy currents. This not only exacerbates the neutral wire current burden but also leads to an abnormally high residual current detection value, interfering with the normal judgment of leakage protection devices. If multiple low-voltage cable branch boxes simultaneously carry a large number of charging devices, the zero-sequence harmonic circulating current may cause the protection device to misjudge a ground fault and frequently trip, resulting in regional power outages and severely reducing power supply continuity. Furthermore, circulating current amplification also enhances electromagnetic interference around outdoor power distribution facilities, affecting the operating accuracy of traffic signal control boxes, communication base station equipment, outdoor monitoring terminals, or charging station metering and acquisition devices, further amplifying system-level risks. Summary of the Invention

[0003] This invention provides a method for dynamically adjusting the load distribution of a low-voltage cable branch box, mainly comprising: Monitor the load current of each phase and the zero-sequence harmonic content of the neutral line, and analyze the impedance distribution of the neutral line side before and after the auxiliary branch is switched on and off. Based on the impedance distribution on the zero-outlet side and the zero-sequence harmonic content of the zero line, the proportion of harmonic components is analyzed using the Fourier transform algorithm to generate an identification mark for the amplified circulating current condition. Based on the identification criteria of the amplified circulation condition, assess whether the proportion of harmonic components exceeds the critical range of the amplified circulation condition. After confirming the establishment of the amplified circulation condition through analysis, determine the switching conditions for the diversion condition. Based on the switching conditions and harmonic component ratio of the diversion operation, the influence of the low-resistivity path of the auxiliary branch on the zero-sequence harmonic is analyzed, and the switching sequence for the decrease of the zero line thermal stress is determined. By controlling the on / off state of the auxiliary current-conducting branch through the switching timing, the changes in neutral current, neutral outlet temperature and zero-sequence harmonic content before and after switching are collected to assess the risk of increased neutral temperature rise and obtain the updated zero-sequence harmonic content. Based on the updated zero-sequence harmonic content and the switching timing, the Kalman filter algorithm is used to predict the risk of leakage protection malfunction, evaluate the three-phase imbalance conduction effect, and quantify the circulating current amplification suppression level of the dynamic adjustment of load distribution in low-voltage cable branch boxes.

[0004] Furthermore, monitor the load current of each phase and the zero-sequence harmonic content of the neutral line, and analyze the impedance distribution on the neutral side before and after the auxiliary branch is switched on and off, including: The instantaneous values ​​of the load currents of phases A, B, and C are collected by current transformers deployed at the incoming end, and the amplitude of the odd-order zero-sequence harmonic components flowing through the neutral line is extracted to obtain a current sampling set. For the current sampling set, the loop voltage drop and return current values ​​from the neutral pin to the neutral point are measured under two on / off states: the auxiliary current guide branch is open and the return current value is connected. The impedance distribution spectrum of the neutral pin side is plotted, and the proportion of zero-sequence harmonic return current borne by the low-resistance path in the spectrum is compared with the decrease in neutral current to obtain the conflict state identification mark.

[0005] Furthermore, the step of analyzing the proportion of harmonic components using a Fourier transform algorithm based on the zero-line impedance distribution and the zero-sequence harmonic content to generate an identification marker for amplified circulating current conditions includes: The zero-sequence harmonic amplitude waveform data of the zero-line is extracted and analyzed in the frequency domain. The amplitudes and phase angles of the 3rd, 9th, and 15th odd-order zero-sequence harmonics are extracted to form an odd-order zero-sequence harmonic amplitude array. The component proportion is calculated according to the ratio of each harmonic amplitude to the fundamental amplitude. The component proportions of each order are paired with the low-impedance path impedance values ​​on a frequency band basis to obtain a harmonic impedance pairing set. The harmonic impedance pairing set is compared with the circulating current amplification critical threshold and the low-impedance judgment threshold. The harmonic order exceeding the threshold and the corresponding proportion value and pairing impedance value are marked to obtain the identification mark of the amplified circulating current condition.

[0006] Furthermore, based on the identification criteria of the amplified circulation condition, the proportion of harmonic components is assessed to determine whether it exceeds the critical range of the amplified circulation condition. After confirming the establishment of the amplified circulation condition through analysis, the switching conditions for the diversion and diversion condition are determined, including: The marked harmonic order, corresponding proportion value, and paired impedance value are read from the identification mark of the amplified circulating current condition. The proportion value is compared with the upper limit threshold to obtain the proportion evaluation result. After the proportion value is continuously higher than the upper limit threshold and the paired impedance value is continuously lower than the low resistance judgment threshold within the continuous observation window, the confirmed result of the circulating current condition is obtained. The timing of the auxiliary current guiding branch is adjusted and the change trend of the neutral current drop rate and the zero-sequence harmonic return direction after the branch is put into operation are checked to obtain the conversion condition of the diversion current condition.

[0007] Furthermore, the transition conditions for the diversion of the diversion condition are determined, including: analyzing the amplified circulating current channel formed by the convergence of harmonic currents on the zero-sequence side, identifying the backflow carrying capacity of the low-resistance path constructed by the switchable auxiliary diversion branch for zero-sequence harmonics, and obtaining the transition conditions for the diversion of the diversion condition when the amplified circulating current condition is established.

[0008] Furthermore, the step of analyzing the impact of the low-resistivity path of the auxiliary branch on the zero-sequence harmonics based on the switching conditions and harmonic component ratio of the diversion operation, and determining the switching sequence for the decrease in neutral wire thermal stress, includes: For the low-resistance path of the auxiliary guide branch, the loop impedance value is collected at each order, and the impact assessment result is obtained by comparing the change of the zero-sequence harmonic amplitude on the neutral line side before and after the low-resistance path is put into operation. The moving average of the zero-sequence harmonic amplitude and the difference of the three-phase phase current are tracked synchronously. When the convergence enhancement period and the unbalance rise period coincide, the operation command is triggered to obtain the operation start time point. When the neutral line current continues to decrease and remains stable and the neutral line temperature stops rising, the exit command is triggered. The operation start time point and the exit time point together constitute the operation and exit sequence of the decrease of the neutral line thermal stress.

[0009] Furthermore, the process of controlling the on / off state of the auxiliary current-conducting branch through the switching timing, collecting changes in neutral current, neutral outlet temperature, and zero-sequence harmonic content before and after switching, assessing the risk of increased neutral temperature rise, and obtaining updated zero-sequence harmonic content includes: The power electronic switch or mechanical contactor is driven to form an auxiliary current-conducting branch on / off state sequence according to the start and end times of the switching sequence. For the on / off state sequence, the difference between the neutral current amplitude, neutral outlet temperature and the content of each zero-sequence harmonic is collected before and after each switching. The rising rate of the neutral outlet temperature and the synchronous change trend of the neutral current amplitude are compared to obtain the risk assessment result of the increased neutral temperature rise. Based on the risk assessment result of the increased neutral temperature rise, the frequency domain is re-decomposed using the Fourier transform algorithm for the neutral current sampling sequence after each switching and exit time. The proportion of each zero-sequence harmonic component is calculated according to the ratio of each order amplitude to the fundamental frequency amplitude, and arranged in the order of harmonic order to form a new round of zero-sequence harmonic content array, thus obtaining the updated zero-sequence harmonic content.

[0010] Furthermore, the step of using a Kalman filter algorithm to predict the risk of leakage protection maloperation based on the updated zero-sequence harmonic content and the switching timing, evaluating the three-phase imbalance mitigation effect, and quantifying the circulating current amplification suppression level of the dynamic adjustment of load distribution in low-voltage cable branch boxes includes: For each switching sequence number, the residual current amplitude on the neutral side and the content of each zero-sequence harmonic are arranged in time sequence to construct a leakage current protection maloperation observation sequence. The Kalman filter algorithm is used to perform state estimation and one-step prediction on the leakage current protection maloperation observation sequence, and the predicted mean and corresponding covariance of the residual current sampling value in the next switching cycle are output. The ratio of the difference between the predicted mean and the setting value of the leakage current protection device to the setting value is used as the predicted value of leakage current protection maloperation risk. The starting point for the activation of the auxiliary current-conducting branch is delayed until the predicted average value falls back below the setting value of the leakage current protection device, based on the predicted value of the leakage current protection maloperation risk exceeding the maloperation risk threshold. This results in a refined sequence of activation times for the auxiliary current-conducting branch. Based on the refined auxiliary current guiding branch commissioning timing sequence, the decrease in the difference between the maximum and minimum values ​​of the three-phase current before and after each commissioning is collected to obtain the three-phase imbalance guiding effect index. The arithmetic mean of the zero-sequence harmonic amplitude of the neutral line and the convergence intensity of the main path of the amplified circulating current channel is collected according to the decrease ratio to obtain the circulating current amplification suppression index. The average value of the circulating current amplification suppression index under all commissioning sequences is taken to quantify the circulating current amplification suppression level.

[0011] The technical solutions provided by the embodiments of the present invention may include the following beneficial effects: This invention discloses a dynamic adjustment method for load distribution in low-voltage cable branch boxes. Addressing the conflict between the convergence and amplification of zero-sequence harmonics and the diversion and mitigation of three-phase unbalanced current in low-voltage cable branch boxes, the method monitors the load current of each phase and the zero-sequence harmonic content of the neutral wire. It analyzes the proportion of harmonic components using a Fourier transform algorithm to identify amplified circulating current conditions and predicts the risk of leakage protection maloperation based on a Kalman filter algorithm. This invention constructs switchable auxiliary current-diverting branches and dynamically adjusts their activation timing. The auxiliary branches are activated when zero-sequence harmonics increase and three-phase unbalanced demand rises, achieving diversion and mitigation. They are deactivated after the neutral wire current stabilizes, reducing thermal stress on the neutral wire. This invention effectively suppresses circulating current amplification, optimizes load distribution, reduces the risk of neutral wire temperature rise, and improves system operational stability. Attached Figure Description

[0012] Fig. 1 This is a flowchart of a method for dynamically adjusting the load distribution of a low-voltage cable branch box according to the present invention.

[0013] Fig. 2 This is a schematic diagram of a dynamic adjustment method for load distribution in a low-voltage cable branch box according to the present invention.

[0014] Fig. 3This is another schematic diagram of a dynamic adjustment method for load distribution in a low-voltage cable branch box according to the present invention. Detailed Implementation

[0015] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. 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.

[0016] like Figs. 1-3 This embodiment of a method for dynamically adjusting the load distribution of a low-voltage cable branch box may specifically include: Step S101: Monitor the load current of each phase and the zero-sequence harmonic content of the neutral line, and analyze the impedance distribution of the neutral side before and after the auxiliary branch is switched on and off.

[0017] The instantaneous values ​​of the load currents of phases A, B, and C are collected by current transformers deployed at the inlet of the low-voltage cable branch box. A dedicated zero-sequence current sampling element is installed on the neutral side to extract the amplitude of the odd-order zero-sequence harmonic components flowing through the neutral wire. The waveform data of the phase current amplitude and the zero-sequence harmonic amplitude of the neutral wire changing with time are recorded simultaneously to obtain a current sampling set reflecting the current load distribution status of the low-voltage cable branch box. For the current sampling set, under the two on / off states of the auxiliary current-conducting branch being open and closed, the loop voltage drop and corresponding return current value from the neutral terminal to the neutral point are measured respectively. The loop impedance value of each return path on the neutral terminal side in the odd-order zero-sequence harmonic frequency band is calculated one by one according to Ohm's law. The impedance distribution spectrum of the neutral terminal side before and after the switching of the auxiliary branch is plotted, and the proportion of zero-sequence harmonic return current borne by the low-resistance path in the spectrum is compared. Based on the impedance distribution spectrum of the zero line side and the amplitude of the zero-sequence harmonic, compare the decrease in the zero line current after the auxiliary branch is put into operation with the increase in the proportion of the zero-sequence harmonic return current. If the amplitude of the zero line current shows a continuous downward trend but the proportion of the zero-sequence harmonic return current increases synchronously, it is determined that the circulating current convergence and amplification and the three-phase unbalanced current diversion and diversion are in conflict, and the conflict state identification mark of the current low-voltage cable branch box zero line side is obtained.

[0018] When a low-voltage cable branch box carries a nonlinear load, odd-order zero-sequence harmonics on the neutral side tend to flow back along low-resistance paths and form circulating currents. Therefore, it is necessary to simultaneously monitor the phase current, neutral zero-sequence harmonics, and impedance distribution on the neutral busbar side. The neutral busbar side refers to the side of the neutral busbar in the low-voltage cable branch box, i.e., the return path on the neutral grounding side, to identify the conflict between circulating current amplification and unbalanced current shunting. In one embodiment, at the incoming busbar of the low-voltage cable branch box, through-type current transformers with the same transformation ratio are installed for phases A, B, and C respectively; a broadband zero-sequence current sampling element is added on the neutral side, covering the frequency band from power frequency to 2000Hz. The secondary side of the current transformer is connected to the sampling channel after anti-aliasing filtering, and the instantaneous values ​​of the phase current and neutral current are simultaneously collected at a sampling rate of 10kHz.

[0019] Specifically, the 3rd, 9th, and 15th odd harmonics generated by the nonlinear load in the three-phase system are 360 ​​degrees out of phase in phases A, B, and C, exhibiting in-phase characteristics. Therefore, they cannot cancel each other out on the neutral line, but instead superimpose to form zero-sequence harmonic components.

[0020] In one embodiment, the amplitudes of each harmonic are extracted from the neutral current sampling sequence using a Fast Fourier Transform (FFT). The root mean square (RMS) values ​​of the 3rd, 9th, and 15th harmonics are synthesized to obtain the zero-sequence harmonic amplitude, denoted as Iz. Simultaneously, the fundamental effective values ​​Ia, Ib, and Ic of the three-phase currents are calculated, and the difference between the maximum and minimum phase currents is used as a reference for unbalance. Ia, Ib, Ic, and Iz, along with their corresponding timestamps, are written into the current sampling set to form time-series data reflecting the current load distribution status.

[0021] It should be noted that the sampling window length is set to four power frequency cycles to ensure the spectral resolution of odd harmonic components. Furthermore, an on / off control signal is applied to the switchable auxiliary current-carrying branch. Under the same load condition, current sampling sets are recorded for a steady-state period for both the off and on states of the auxiliary branch to avoid interference from switching transients in the impedance calculation results.

[0022] For example, the process of obtaining the zero-row side loop impedance is as follows: With the auxiliary branch disconnected, a low-range voltage sampling module is connected between the zero-row terminal and the neutral point to record the power frequency voltage drop U0 and the corresponding power frequency return current I0 along this path. The power frequency loop impedance Z0 = U0 / I0 is obtained according to Ohm's law. When switching to the 3rd and 9th harmonic frequency bands, the voltage drop component and current component of the same frequency band are extracted respectively to obtain the loop impedances Z3 and Z9 for each frequency band. With the auxiliary branch connected, the above measurements are repeated along the auxiliary branch path to obtain the loop impedances Zf0, Zf3, and Zf9 of the auxiliary branch at the power frequency and each odd harmonic frequency band. The impedance values ​​of each return path at the zero-row terminal in different frequency bands are arranged according to the path number and frequency band number to construct the zero-row side impedance distribution map.

[0023] Specifically, the horizontal axis of the spectrum represents the harmonic frequency band number, the vertical axis represents the impedance value of each return path, and the impedance ratio of the low-impedance path of the auxiliary branch in the 3rd and 9th frequency bands is marked.

[0024] It is understandable that after the auxiliary branch is put into operation, if its loop impedance in the odd-order zero-sequence harmonic frequency band is significantly lower than that of the original neutral line path, the zero-sequence harmonics tend to flow back along the low-resistance path of the auxiliary branch; this return flow ratio is the ratio of the current component corresponding to the frequency band of the auxiliary branch impedance to the total amplitude of the zero-sequence harmonics in the spectrum. In one embodiment, the change in the neutral line current amplitude ΔIn before and after the auxiliary branch is put into operation is synchronously compared with the change in the zero-sequence harmonic return flow ratio ΔK. If ΔIn is negative and its absolute value exceeds the preset current fall threshold, and ΔK is positive and exceeds the preset return flow ratio increase threshold, it is determined that the zero-sequence harmonics are repeatedly converging along the low-resistance auxiliary branch, rather than being effectively diverted, and a conflict state identification mark is output at this time. Through the above implementation method, the low-voltage cable branch box can complete the joint observation of phase current, zero-sequence harmonic amplitude and neutral line impedance distribution within the same acquisition cycle.

[0025] Step S102: Based on the impedance distribution of the zero-line side and the zero-sequence harmonic content of the zero line, the Fourier transform algorithm is used to analyze the proportion of harmonic components and generate an identification mark for the amplified circulating current condition.

[0026] The impedance distribution spectrum of the zero line side and the waveform data of the zero-sequence harmonic amplitude changing with time are obtained. A preset number of power frequency cycles are extracted from the waveform data of the zero-sequence harmonic amplitude changing with time as an analysis window. The Fourier transform algorithm is used to perform frequency domain decomposition on the time-domain current signal within the analysis window to obtain the fundamental component and each order harmonic component. For each order harmonic component, the amplitude and phase angle of the 3rd, 9th, and 15th odd-order zero-sequence harmonics are extracted and arranged in order of harmonic order to form an array of odd-order zero-sequence harmonic amplitudes, thus obtaining the spectrum decomposition result reflecting the distribution pattern of the zero-sequence harmonics on the zero line side. Based on the spectral decomposition results, the component proportions of each odd-order zero-sequence harmonic are calculated item by item according to the ratio of the amplitude of each odd-order zero-sequence harmonic to the amplitude of the fundamental component. For the loop impedance values ​​of the low-resistance path of the auxiliary branch in the zero-row impedance distribution spectrum at the corresponding odd-order frequency band, the component proportions of each order are paired with the impedance values ​​of the low-resistance path band by band. The phase angle deviation range of the harmonic in-phase superposition in each pairing item is marked according to the phase angle, so as to obtain the harmonic impedance pairing set that characterizes the mapping relationship between the degree of zero-sequence harmonic superposition and the return current carrying capacity of the low-resistance path. Based on the harmonic impedance pairing set, the proportion of each order component is compared with the preset circulating current amplification critical threshold, and the impedance value of the low-impedance path in the corresponding frequency band is checked to see if it is lower than the preset low-impedance judgment threshold. If the proportion of any order component exceeds the circulating current amplification critical threshold, and the corresponding low-impedance path impedance value is lower than the low-impedance judgment threshold, it is determined that the zero-sequence harmonic is repeatedly flowing back along the low-impedance path of the auxiliary branch and forming an amplifying circulating current channel. The order of harmonics, the corresponding proportion value and the pairing impedance value are marked to obtain the identification mark of the amplifying circulating current condition.

[0027] Identifying amplified circulating current conditions relies on a fine frequency domain characterization of the odd-order zero-sequence harmonic components in the neutral current, and matching the results of this frequency domain characterization with the impedance characteristics of the low-resistance path of the auxiliary branch in the impedance distribution spectrum of the neutral side. In one embodiment, the instantaneous current value sequence collected from the neutral side of the low-voltage cable branch box is continuously truncated, and a preset number of power frequency cycles are used as the analysis window. The analysis window length is 4 power frequency cycles, corresponding to 80 milliseconds at a power frequency of 50 Hz, the sampling rate is 12.8 kHz, and a single window contains 1024 sampling points to ensure that the 3rd (150 Hz), 9th (450 Hz), and 15th (750 Hz) odd-order zero-sequence harmonics have sufficient resolution in the spectrum.

[0028] Specifically, before entering the Fourier transform algorithm, a Hanning window is added to the sequence of instantaneous zero-line current values ​​within the analysis window to suppress spectral leakage caused by non-integer period truncation.

[0029] It should be noted that the Hanning window is one of the commonly used cosine window functions, which can concentrate the main lobe and attenuate the side lobes faster, thereby improving the stability of odd harmonic amplitude extraction.

[0030] For example, the Fourier transform algorithm transforms the windowed time-domain sequence into a frequency-domain complex sequence. Let Xn be the complex component corresponding to the nth harmonic. Then, the amplitude An = |Xn| of this harmonic, and the phase angle θn = arctan(Im(Xn) / Re(Xn)), where Im and Re represent the imaginary and real parts of the complex number, respectively, and n takes orders of 1, 3, 9, 15, etc. The fundamental component amplitude A1 is obtained from the complex modulus when n = 1, and the odd-order zero-sequence harmonic amplitudes A3, A9, and A15 are obtained from the complex modulus when n = 3, 9, and 15, respectively. The corresponding phase angles θ3, θ9, and θ15 are obtained from their arguments. A3, A9, and A15 are arranged in harmonic order to form an odd-order zero-sequence harmonic amplitude array, and θ3, θ9, and θ15 are arranged in the same order to form a phase angle sequence. Both together constitute the spectral decomposition result.

[0031] Understandably, the above spectral decomposition results directly reflect the distribution pattern of the zero-sequence harmonics on the zero-line side. The amplitude array is used for subsequent component proportion calculation, and the phase angle sequence is used for subsequent in-phase superposition discrimination. Furthermore, based on the ratio of the amplitudes An, n=3, 9, 15 of each odd-order zero-sequence harmonic to the amplitude A1 of the fundamental component, the component proportion Kn=An / A1 of each odd-order zero-sequence harmonic is calculated item by item.

[0032] In one embodiment, the proportion K3 corresponding to the 3rd order represents the most significant odd-order component contribution of the three-phase nonlinear load concentrated on the neutral line of the low-voltage cable branch box, while the proportions K9 and K15 corresponding to the 9th and 15th orders represent the secondary dominant odd-order component contributions. K3, K9, and K15 together constitute the component proportion vectors of each order.

[0033] Specifically, the loop impedance values ​​of the low-resistance paths of the auxiliary branches in the zero-order impedance distribution spectrum are extracted at the corresponding odd-order frequency bands. The loop impedance of the auxiliary branches at the 3rd order is denoted as Zf3, at the 9th order as Zf9, and at the 15th order as Zf15. The proportion Kn is paired with Zfn of the same order band by band to obtain pairings of the form (K3, Zf3), (K9, Zf9), and (K15, Zf15). In one embodiment, the degree of in-phase superposition of the three-phase zero-sequence harmonic phase angles in each pairing is evaluated. The deviation between the phase angles of the three harmonics of the same order and the zero-sequence reference phase angle is examined. If the phase angle deviations of all three phases fall within the range of 0 to 30 degrees, then the harmonics are determined to be in-phase superposition. The above pairings and the corresponding phase angle deviation ranges are recorded together to obtain the harmonic impedance pairing set. The harmonic impedance pairing set aligns the "degree of concentration of harmonics on the neutral line" and "whether the auxiliary branch provides a low-resistance return path" on the same order, thereby avoiding misjudgments that may occur when looking at the proportion or impedance alone.

[0034] In one embodiment, the proportion of each of the components is compared with a preset critical threshold Kth for circulation amplification.

[0035] Preferably, Kth is set to 0.15, meaning that if any of the K3, K9, and K15 values ​​exceeds 15%, it is considered an abnormal increase in the proportion of that harmonic; at the same time, it is checked whether the impedance value of the low-impedance path in the corresponding frequency band is lower than the preset low-impedance judgment threshold Zth.

[0036] Preferably, Zth is taken as 50% of the loop impedance of the neutral path in the same frequency band, that is, the low impedance is considered to be established when the impedance of the auxiliary branch is less than half of the neutral path.

[0037] For example, if K3=0.22 and Zf3 is less than 0.4 times the corresponding neutral line impedance under actual operating conditions, and the phase angle deviation of the three-phase third harmonic falls within 0 to 30 degrees, then it is determined that the third zero-sequence harmonic is repeatedly flowing back along the low-resistance path of the auxiliary branch and forming an amplified circulating current channel. The harmonic order is marked as 3, the corresponding proportion value is 0.22, and the paired impedance value is Zf3, outputting an identification mark for the amplified circulating current operating condition. This identification mark consists of three fields: harmonic order number, proportion value, and paired impedance value, clearly indicating the order and intensity of the amplified circulating current. Low-voltage cable branch boxes can output an amplified circulating current operating condition identification mark in similar operating conditions, including centralized access for charging equipment, centralized access for energy-saving lighting, and centralized access for variable frequency air conditioners.

[0038] Step S103: Based on the identification mark of the amplified circulation condition, assess whether the proportion of harmonic components exceeds the critical range of the amplified circulation condition. After confirming the establishment of the amplified circulation condition through analysis, determine the switching conditions for the diversion condition.

[0039] The marked harmonic order, corresponding percentage value, and paired impedance value are read from the identification flag of the amplified circulating current condition. The critical range of the amplified circulating current condition is determined by a preset percentage upper limit threshold. The corresponding percentage value is compared with the percentage upper limit threshold item by item, and the offset of the corresponding percentage value from the percentage upper limit threshold is recorded to obtain the percentage evaluation result reflecting the degree to which the percentage value of each marked order deviates from the critical range. Based on the percentage evaluation result and the paired impedance value, it is checked whether the percentage value of the marked harmonic order is continuously higher than the percentage upper limit threshold, and whether the paired impedance value of the same order is continuously lower than the preset low impedance judgment threshold. If the percentage value exceeds the percentage upper limit threshold within a continuous observation window, and the paired impedance value is simultaneously lower than the low impedance judgment threshold, then the amplified circulating current condition is confirmed to be established, and a circulating current condition establishment confirmation result is obtained. Based on the confirmation results of the circulating current condition, the current on / off state of the switchable auxiliary diversion branch is adjusted. During the period when the zero-sequence harmonic convergence is enhanced and the three-phase imbalance is increased, the timing of the auxiliary branch is adjusted, and the trend of the neutral current drop rate and the zero-sequence harmonic return direction after the branch is put into operation is checked. If the neutral current shows a downward trend and the zero-sequence harmonic no longer repeatedly returns along the low-resistance path, the switching conditions of the diversion condition are determined.

[0040] In one implementation, the marked harmonic order n, the corresponding proportion value Kn, and the paired impedance value Zfn are read sequentially from the identification flag of the amplifying circulating current condition, where n is 3, 9, or 15. The critical range of the amplifying circulating current condition is determined by a preset upper limit threshold Kmax, i.e., when the proportion Kn of a certain order is greater than Kmax, it is considered that the proportion of that harmonic order has entered the critical range of the amplifying circulating current condition. Preferably, Kmax is 0.15.

[0041] Specifically, the corresponding proportion value Kn is compared item by item with the proportion upper limit threshold Kmax, and the offset ΔKn = Kn - Kmax is recorded and arranged in harmonic order to form an offset sequence. The offset sequence and the corresponding paired impedance value Zfn together constitute the proportion evaluation result. The proportion evaluation result is not equivalent to the conclusion that the operating condition is valid. A mere instantaneous exceedance of the threshold is insufficient to determine that the amplified circulating current operating condition is stable, and there is still a possibility of misjudgment due to transient load fluctuations or occasional harmonic rises. Furthermore, a continuous observation window is introduced to continuously check the offset sequence.

[0042] In one embodiment, the observation window length is 10 power frequency cycles, i.e., 200 milliseconds at a power frequency of 50 Hz. Within this window, Kn and Zfn are collected cycle by cycle. For the marked harmonic order, it is checked whether Kn is higher than Kmax throughout the entire observation window, and simultaneously checked whether Zfn of the same order is lower than a preset low-impedance threshold Zth within the window. Preferably, Zth is taken as 50% of the loop impedance of the neutral path at the same order.

[0043] It is understood that the above dual threshold determination must be satisfied simultaneously. If the proportion value exceeds the upper limit threshold of the proportion in the continuous observation window, and the paired impedance value is synchronously lower than the low impedance determination threshold, then it is determined that the zero-sequence harmonic is repeatedly flowing back along the low impedance auxiliary branch and forming a stable amplified circulating channel, confirming that the amplified circulating condition is established, and outputting the confirmation result of the established circulating condition. This result includes the set of established order and the corresponding threshold deviation.

[0044] Specifically, adjustments are made to the current on / off state of the switchable auxiliary diversion branch.

[0045] Preferably, the auxiliary branch is connected to the parallel circuit between the zero-output terminal and the neutral point via a power electronic switch or a mechanical contactor. In one embodiment, based on the confirmation result of the circulating current condition, the periods of increased zero-sequence harmonic convergence and the periods of increased three-phase imbalance are identified. The former is marked by the rise of the moving average of Kn within the observation window, and the latter by the rise of the moving average of the maximum difference in the three-phase current amplitude. The time interval in which the two overlap is taken as the timing for the auxiliary branch to be put into operation. The degree of three-phase imbalance can be calculated using the voltage imbalance factor (VUF). Input the three-phase voltages Va, Vb, and Vc, and calculate the average voltage Vavg = (Va + Vb + Vc) / 3. The maximum deviation Dmax = max(|Va - Vavg|, |Vb - Vavg|, |Vc - Vavg|), and VUF = (Dmax / Vavg) × 100%. If VUF > 5%, it is considered an increase. For example, when Va = 230V, Vb = 220V, and Vc = 210V, Vavg = 220V, Dmax = 10V, and VUF = 4.55%, there is no increase. When Va = 235V, Vb = 220V, and Vc = 205V, Vavg = 220V, Dmax = 15V, and VUF = 6.82%, there is an increase.

[0046] For example, after the auxiliary branch is closed at the activation time, the rate of return of the neutral current and the trend of the zero-sequence harmonic return direction are continuously checked. Taking the moment of activation as the starting point, the amplitude of the neutral current In(t) is sampled at fixed time intervals, and the difference between adjacent sampling points is calculated as the rate of return dIn / dt. If dIn / dt is continuously negative, the neutral current shows a decreasing trend. At the same time, the amplitudes of the zero-sequence harmonic currents of the same order on both the auxiliary branch and the original neutral path are collected, and the relative changes of the zero-sequence harmonic amplitudes on the two paths are compared. If the zero-sequence harmonic amplitude on the auxiliary branch path no longer continues to rise while the zero-sequence harmonic amplitude on the original neutral path side decreases synchronously, it is determined that the zero-sequence harmonics no longer repeatedly return along the low-resistance path. When both of the above checks pass simultaneously, the neutral current decreases and the zero-sequence harmonic return direction changes from "repeated convergence" to "unidirectional diversion", then the conversion condition for the diversion diversion working condition is determined by the following three factors: the auxiliary branch is in a closed on / off state, the neutral current drop rate is continuously negative, and the zero-sequence harmonic amplitude of the auxiliary branch path no longer increases.

[0047] Another way to determine the switching conditions of the diversion condition is to extract the in-phase superposition phenomenon of the 3rd, 6th, and 9th zero-sequence harmonics in the three-phase system on the neutral line, analyze the amplified circulating current channel formed by the convergence of harmonic currents on the zero-discharge side, identify the backflow carrying capacity of the low-resistance path constructed by the switchable auxiliary current guiding branch for the zero-sequence harmonics, and obtain the switching conditions of the diversion condition when the amplified circulating current condition is established.

[0048] The instantaneous current sequences of phases A, B, and C of the three-phase system and the instantaneous neutral current sequence are obtained. The amplitudes and phase angles of the 3rd, 6th, and 9th zero-sequence harmonic components are extracted from the instantaneous neutral current sequence. The phase angles of the zero-sequence harmonic components of the same order in phases A, B, and C are extracted respectively. The maximum difference between the three-phase phase angles of each order is compared with a preset in-phase determination threshold. The order in which the difference between the three-phase phase angles is less than the in-phase determination threshold is marked. The corresponding zero-sequence harmonic amplitude of the neutral line is output according to the order, thus obtaining the set of in-phase superimposed orders and the corresponding superimposed amplitudes on the neutral line. For the set of in-phase superposition orders and their corresponding superposition amplitudes, for each in-phase superposition order, the current component distribution and flow direction of the zero-sequence harmonic on each return path on the zero-row side are tracked, and the vector sum of the current components of each path at the zero-row terminal node is compared with the corresponding superposition amplitude. The combination of zero-sequence harmonic paths that repeatedly enter and exit the zero-row terminal node without leaking out along the outgoing line side is identified, and the return path with the lowest impedance in the path combination is marked as the main path of the amplifying circulation channel. The convergence intensity is recorded according to the amplitude of the zero-sequence harmonic of the same order on the main path to obtain the structural description of the amplifying circulation channel. According to the description of the amplified circulating channel structure, the loop impedance value and the corresponding zero-sequence harmonic return current value of the low-resistance path constructed by the switchable auxiliary guiding branch are collected at each in-phase superposition order. The ratio of the return current value to the convergence intensity is used as the return current carrying capacity of the low-resistance path for the zero-sequence harmonic, and the return current carrying capacity sequence is obtained by arranging them in order of order. If the values ​​of each item in the return current carrying capacity sequence are continuously higher than the preset return current carrying capacity threshold, and the loop impedance value of the low-resistance path at each order is lower than the preset low-resistance judgment threshold, then the switching condition of the diversion and diversion working condition is determined.

[0049] The determination of the transition conditions for the diversion of the current under the amplified circulating current condition depends on the stepwise identification of the in-phase superposition of three-phase zero-sequence harmonics, the convergence path on the neutral side, and the return current carrying capacity of the low-resistance path of the auxiliary diversion branch. In one embodiment, current transformers are installed for phases A, B, and C at the incoming busbar of the low-voltage cable branch box, and a broadband zero-sequence current sampling element is installed on the neutral side with a sampling rate of 12.8kHz to synchronously acquire the instantaneous current sequence of each phase and the instantaneous current sequence of the neutral line.

[0050] Specifically, the analysis window length is set to 4 power frequency cycles. The instantaneous value sequence of the neutral current is decomposed in the frequency domain, and the amplitudes and phase angles of the 3rd, 6th, and 9th zero-sequence harmonic components are output. Further, the phase angles of the same-order zero-sequence harmonic components in phases A, B, and C are extracted, and the phase angles of phases A, B, and C at the nth order are denoted as θa,n, θb,n, and θc,n, where n is 3, 6, or 9. The maximum difference between the three-phase phase angles of this order, Δθn = max(|θa,n-θb,n|,|θb,n-θc,n|,|θa,n-θc,n|), is calculated and compared with the preset in-phase determination threshold θth.

[0051] Preferably, θth is set to 15 degrees, meaning that the phase difference between the three identical orders is within 15 degrees, which is considered as in-phase superposition. All orders satisfying Δθn less than θth are marked, and the corresponding zero-sequence harmonic amplitude In is output according to the order, thus obtaining the set M of in-phase superposition orders on the neutral line and the corresponding superposition amplitude sequence. Each element in the set M corresponds to a set (n, In), representing the harmonic order of that order and the zero-sequence harmonic amplitude flowing through the neutral line after superposition.

[0052] It should be noted that the 6th harmonic usually exhibits positive sequence characteristics in a strictly balanced three-phase system. However, under conditions of asymmetrical nonlinear load access, such as inconsistent numbers of three-phase charging devices or concentrated access of single-phase energy-saving lighting, the 6th harmonic can also have in-phase components on the neutral line, and therefore it is included in the extraction scope.

[0053] Specifically, for each order n in the set M, node current analysis is performed on the zero-row terminal node. The zero-row terminal of a low-voltage cable branch box typically connects to multiple return paths, including the original neutral line path, the PE protective grounding path, and switchable auxiliary current-carrying branches. The zero-sequence harmonic current components Ik,n of the same order n on each return path are collected, where k is the path number.

[0054] Understandably, according to Kirchhoff's current law, the vector sum of all zero-sequence harmonic current components of the same order flowing into and out of the zero-row node should be equal to the inlet amplitude of that node, i.e., ΣIk,n should be approximately equal to In. If the difference between the vector sum and the corresponding superimposed amplitude is less than the unbalance margin threshold, then the node current conservation condition is considered to be met, and the path combination identification can proceed.

[0055] For example, the direction of the current component of the same order on each return path is continuously observed, and the direction of current inflow and outflow on each path is counted within an observation window. If the zero-sequence harmonic current of the same order in a certain path combination exhibits a repeated inflow and outflow characteristic of "inflow and outflow, outflow and inflow," and no current of the same order with the same amplitude is leaked on the outgoing line, then the path combination is determined to form a local circulating current at the zero-output end. In the path combination, the loop impedance of each path at this order is compared, and the return path with the lowest impedance value is marked as the main path of the amplifying circulating current channel, denoted as Pn. Further, according to the zero-sequence harmonic amplitude IPn of the same order on the main path Pn, it is recorded as the convergence intensity Sn=IPn of this order, and a convergence intensity sequence is formed according to the order. The amplifying circulating current channel structure description consists of a triplet consisting of the main path identifier Pn, the convergence intensity Sn, and the corresponding order n.

[0056] In one embodiment, for the low-resistance path constructed by the switchable auxiliary current-conducting branch, its loop impedance value Zf,n is measured at the 3rd, 6th, and 9th orders, respectively. The method is to apply a small-signal disturbance of the same frequency band to both ends of the auxiliary branch, record the voltage drop across the auxiliary branch and the current value of the same order flowing through the auxiliary branch, and calculate Zf,n according to Ohm's law. At the same time, the return current value If,n of the same order zero-sequence harmonic on the auxiliary branch is collected.

[0057] Specifically, the return current carrying capacity Rn of the low-resistance path for zero-sequence harmonics is obtained by the ratio of the return current value If,n to the convergence intensity Sn, i.e., Rn = If,n / Sn. Physically, this represents the proportion of zero-sequence harmonics of that order that return along the low-resistance path of the auxiliary branch. The Rn values ​​for each in-phase superposition order are arranged in order to form a return current carrying capacity sequence. In one embodiment, a dual-threshold determination is performed using a return current carrying capacity threshold Rth and a low-resistance determination threshold Zth.

[0058] Preferably, Rth is 0.6, meaning the auxiliary branch circuit carries at least 60% of the zero-sequence harmonic return current; Zth is 50% of the original neutral line circuit impedance at the same order. If Rn in the return current carrying capacity sequence is consistently higher than Rth, and Zf,n of the low-resistance circuit at each order is lower than Zth, then the switching condition for the diversion of the diversion condition is determined by a combination of three factors: the auxiliary branch circuit is in a closed-on / off state, Rn at each order is consistently higher than Rth, and Zf,n at each order is lower than Zth. Through the above implementation process, the low-voltage cable branch box can output clear diversion conditions for the diversion of the diversion condition in nonlinear load scenarios such as centralized access of charging equipment, centralized access of energy-saving lighting, and centralized access of variable frequency air conditioners, based on the phenomenon of in-phase superposition of zero-sequence harmonics and the convergence structure of the zero-row side, providing a switching basis for real-time control of the switching sequence of the auxiliary branch circuit.

[0059] Step S104: Based on the switching conditions and harmonic component ratio of the diversion operation, analyze the influence of the low-resistance path of the auxiliary branch on the zero-sequence harmonics, and determine the switching sequence for the decrease of the thermal stress of the neutral line.

[0060] The conversion conditions of the diversion operation and the proportion of harmonic components are obtained. The loop impedance values ​​of the low-resistance path constructed by the auxiliary diversion branch are collected under each order of zero-sequence harmonics. The changes in the amplitude of zero-sequence harmonics of each order on the neutral line side before and after the low-resistance path is put into operation are compared. The decrease in the amplitude of zero-sequence harmonics on the neutral line and the decrease in the proportion of the corresponding components are recorded in order of harmonic order. The impact assessment results of the low-resistance path on the distribution of zero-sequence harmonic return path are obtained. Based on the decrease in amplitude of each order in the impact assessment results, the moving average tracking of the amplitude of zero-sequence harmonics of each order on the neutral line side is performed. The difference between the maximum and minimum values ​​of the three-phase current is tracked synchronously. The period when the moving average of the zero-sequence harmonic amplitude shows a continuous increase is marked as the convergence enhancement period, and the period when the difference between the three-phase currents shows a continuous increase is marked as the imbalance rise period. When the convergence enhancement period and the imbalance rise period coincide, the auxiliary diversion branch is put into operation command, and the starting time of the auxiliary diversion branch is obtained. Based on the starting point of the auxiliary current guiding branch, the amplitude of the neutral current and the temperature of the neutral outlet are continuously observed, and the change curve of the neutral current amplitude is recorded at a preset sampling interval. When the amplitude of the neutral current continues to decrease and remains within a stable range within a preset observation window, and the temperature of the neutral outlet stops rising synchronously, the auxiliary current guiding branch exit command is triggered. The starting point of the input and the corresponding exit point together constitute a complete switching sequence, and the switching sequence of the decrease of the neutral thermal stress is obtained.

[0061] Determining the switching sequence for the decrease in thermal stress on the neutral wire requires precise judgment of the timing for the activation and deactivation of the auxiliary current-guiding branch, based on the identified switching conditions for the diversion of the current-guiding operation and the proportion of harmonic components. In one embodiment, the loop impedance values ​​of the low-resistance path constructed by the auxiliary current-guiding branch are collected at each order of the 3rd, 9th, and 15th zero-sequence harmonics. The collection method is to record the voltage drop and the current value of the same order at the moment of switching the auxiliary branch on and off, and obtain the loop impedance Zf,n of that order according to Ohm's law. Subsequently, the change in the amplitude of the zero-sequence harmonics on the neutral wire side before and after the activation of the low-resistance path, ΔIn=In,front-In,back, is compared, and the decrease in the amplitude of the zero-sequence harmonics on the neutral wire at each order, ΔIn, and the decrease in the proportion of the corresponding component, ΔKn, are recorded in order of harmonic order to obtain the evaluation result of the impact of the low-resistance path on the distribution of the zero-sequence harmonic return path. The magnitude of the decline in the impact assessment results reflects the auxiliary branch's ability to guide the zero-sequence harmonics of the neutral line during historical switching processes, and serves as a reference benchmark for determining the current timing of its commissioning.

[0062] Specifically, a moving average tracking method is used for the amplitude In of each order of zero-sequence harmonics on the neutral side, with a moving window length of 5 power frequency cycles, corresponding to 100 milliseconds. Simultaneously, a synchronous moving average tracking method is used for the difference between the maximum and minimum values ​​Imax and Imin of the phase currents in phases A, B, and C, ΔIabc = Imax - Imin, with a window length consistent with the zero-sequence harmonic tracking window. Furthermore, the slopes of the In curve and the ΔIabc curve after the moving average are checked respectively.

[0063] Preferably, the slope increase determination threshold is 0.5 amperes per second. That is, when the moving average slope of In is continuously greater than this threshold for more than 10 power frequency cycles, the current period is marked as a convergence enhancement period; when the moving average slope of ΔIabc is continuously greater than the same slope increase determination threshold for more than 10 power frequency cycles, the current period is marked as an imbalance increase period. When the convergence enhancement period and the imbalance increase period coincide on the time axis, an auxiliary diversion branch activation command is triggered, and the start time of this coincidence is recorded as the auxiliary diversion branch activation start time t0.

[0064] For example, the auxiliary branch circuit is connected using a power electronic switch or a mechanical contactor, which is driven to close by an activation command. After activation, the neutral current amplitude I... n_total Continuous monitoring of the zero-bus terminal temperature Tb was conducted. The zero-line current amplitude was collected by a sampling element on the zero-line side at 50-millisecond intervals, while the zero-bus terminal temperature was collected by a thermocouple or negative temperature coefficient thermistor attached to the surface of the zero-bus copper busbar at 500-millisecond intervals. The zero-line thermal stress is mainly caused by the continuous load of the zero-line current and the accumulation of the zero-bus terminal temperature rise. Simply relying on the current amplitude decline is insufficient to determine whether the thermal stress release is complete. Therefore, the zero-bus terminal temperature was introduced simultaneously as an auxiliary criterion for the exit condition.

[0065] Specifically, the change curve of the neutral current amplitude is recorded according to a preset sampling interval. n_total(t) The first-order difference of the curve is used to obtain the fall rate dI. n_total / dt.

[0066] Preferably, the stability range threshold ε is set to 5% of the zero-line current amplitude before energization, i.e., I n_total(t) The absolute value of the difference between it and its moving average is less than ε, and the preset observation window length is 200 milliseconds. When I n_total(t)When the temperature continues to decrease within the observation window and remains stable within the stable range threshold, and the sliding average slope of the zero-discharge temperature Tb is no longer positive (i.e., the temperature synchronously stops rising), an auxiliary current-conducting branch exit command is triggered, and this moment is recorded as the exit time point t1. Further, a switching sequence pair (t0, t1) is formed by the input start time point t0 and the corresponding exit time point t1, and arranged chronologically to obtain the complete switching sequence {(t0, k, t1, k)}, where k is the switching sequence number. This switching sequence is the switching sequence for the decrease in zero-line thermal stress. In nonlinear load conditions such as centralized access to charging equipment, centralized access to energy-saving lighting, and centralized access to variable frequency air conditioners, the low-voltage cable branch box can output a clear auxiliary current-conducting branch switching sequence, allowing the zero line to be guided during high-current impact periods and promptly disconnecting the auxiliary path during recovery periods, avoiding the auxiliary branch from carrying zero-sequence harmonic backflow for extended periods and causing secondary heat source accumulation.

[0067] Step S105: By controlling the on / off state of the auxiliary current guiding branch through the switching timing, collect the changes in neutral current, neutral outlet temperature and zero-sequence harmonic content before and after switching, assess the risk of increased neutral temperature rise, and obtain the updated zero-sequence harmonic content.

[0068] According to the switching sequence of neutral wire thermal stress reduction, the power electronic switch or mechanical contactor of the auxiliary current guiding branch is driven at each start and corresponding exit point of the switching sequence, so that the auxiliary current guiding branch is closed at the start of the switching and open at the exit point. The current on / off state of the auxiliary current guiding branch is recorded in real time and arranged in the switching sequence to obtain the on / off state sequence of the auxiliary current guiding branch synchronized with the switching sequence. According to the on / off state sequence of the auxiliary current guiding branch, the difference between the neutral wire current amplitude, the neutral outlet temperature and the content of each zero-sequence harmonic is collected within a preset time window before and after each switching. The synchronous change trend of the neutral wire current amplitude and the neutral outlet temperature is compared. If the rate of increase of the neutral outlet temperature exceeds the preset temperature rise rate threshold and the neutral wire current amplitude does not decrease synchronously, it is determined that there is a risk of increased neutral wire temperature rise, and the risk assessment result of increased neutral wire temperature rise is obtained. Based on the risk assessment results of the increased temperature rise of the neutral wire, the frequency domain is re-decomposed using the Fourier transform algorithm for the neutral wire current sampling sequence after each switching and exit point. The amplitudes of the 3rd, 9th, and 15th zero-sequence harmonic components and the fundamental component are extracted. The proportion of each zero-sequence harmonic component is calculated according to the ratio of each order amplitude to the fundamental amplitude, and arranged in order of harmonic order to form a new round of zero-sequence harmonic content array, thus obtaining the updated zero-sequence harmonic content.

[0069] In one embodiment, the auxiliary current-guiding branch is connected to a parallel circuit between the zero-displacement terminal and the neutral point. The switching device is either a power electronic switch or a mechanical contactor. The power electronic switch has a response time in the millisecond range and is suitable for high-frequency switching scenarios; the mechanical contactor has a response time in the tens of millisecond range and is suitable for medium- and low-frequency switching scenarios such as thermal stress control. The switching device is activated by the switching control unit according to the activation start time t in the switching sequence. 0,k With the corresponding exit time point t 1,k The switching is driven sequentially, where k is the switching sequence number. The switching control unit operates at t. 0,k At any time, a closed level is sent to the switching device, at t 1,k A disconnection level is sent continuously, and the current on / off state is synchronously written to the status register. The on / off state is encoded as "0" representing open and "1" representing closed, and is recorded sequentially to form the on / off state sequence of the auxiliary guide branch {(t 0,k ,1,t 1,k ,0)}.

[0070] Specifically, for each switching action in the on / off state sequence, at t 0,k Previously with t 1,k Data was then collected during preset time windows. Both the initial and subsequent time windows were set to 2 seconds in length to cover the transient and steady-state transition processes during the switching.

[0071] For example, the zero-line current amplitude I is collected in the preceding time window and the following time window, respectively. n_total Zero-end temperature Tb and zero-sequence harmonic content K n,前 K n,后 Where n takes values ​​of 3, 9, or 15. Calculate the difference between the three types of observed quantities before and after each switching operation, including the zero-line current difference ΔI. n_total =I n_total,后 -I n_total,前 Zero-end temperature difference ΔT b =T b,后 -T b,前 The difference in the content of zero-sequence harmonics of each order is ΔKn=K n,后 -K n,前 The differences are stored using a joint index of the switching sequence number k and the harmonic order n, forming a dataset of changes before and after the switching. Further, the synchronous change trends of the neutral current amplitude and the neutral outlet temperature are compared to calculate the rate of temperature rise vT = ΔT. b / Δt, where Δt is the time interval between points in the preceding window and points in the following window.

[0072] Preferably, the preset temperature rise rate threshold vTh is set to 2 degrees Celsius per minute.

[0073] It is understandable that when the rate of increase of the zero-displacement temperature vT exceeds vTh, and the amplitude of the zero-line current does not show a decreasing trend after switching, i.e., ΔI... n_total When the value is greater than or equal to 0, it is determined that the auxiliary current-conducting branch failed to divert the neutral wire current after being put into operation. Instead, due to the resistive loss of the auxiliary branch itself or the superposition of secondary heat sources caused by its parallel connection with the original neutral wire path, there is a risk of increased neutral wire temperature rise. Otherwise, it is determined that this switching action did not cause any additional temperature rise risk. The determination results are sequentially written into the neutral wire temperature rise aggravation risk assessment results according to the switching sequence number k.

[0074] Specifically, for each dropout point t 1,k The subsequent neutral current sampling sequence is truncated, with four consecutive power frequency cycles serving as the update analysis window. The Fourier transform algorithm is used to perform frequency domain decomposition on the time-domain current signal within the update analysis window to obtain the fundamental component A. 1,k The amplitudes A of the 3rd, 9th, and 15th zero-sequence harmonic components 3,k A 9,k A 15,k Furthermore, the proportion K of each zero-sequence harmonic component is calculated based on the ratio of each order amplitude to the amplitude of the fundamental component. 3,k =A 3,k / A 1,k K 9,k =A 9,k / A 1,k K 15,k =A 15,k / A 1,k Arrange the three percentage values ​​in order of harmonic order to form a new array of zero-sequence harmonic content (K). 3,k ,K 9,k ,K 15,k ), which is the updated zero-sequence harmonic content.

[0075] Step S106: Based on the updated zero-sequence harmonic content and the switching timing, the Kalman filter algorithm is used to predict the risk of leakage protection malfunction, evaluate the three-phase imbalance conduction effect, and quantify the circulating current amplification suppression level of the dynamic adjustment of load distribution in the low-voltage cable branch box.

[0076] The updated zero-sequence harmonic content and the switching sequence are obtained. For each switching sequence number, the residual current amplitude on the neutral side and the zero-sequence harmonic content of each order are arranged in time sequence to construct a leakage current protection maloperation observation sequence. The Kalman filter algorithm is used to perform state estimation and one-step prediction on the leakage current protection maloperation observation sequence. The predicted mean Ipred (unit mA) of the residual current sampling value in the next switching cycle and the corresponding covariance are output. The predicted mean Ipred is compared with the leakage current protection device setting value Ith (unit mA). (Ipred-Ith) / Ith is used as the leakage current protection maloperation risk prediction value, where Ipred is estimated from the observation sequence by Kalman filtering. Based on the predicted risk value of leakage current protection malfunction, the starting time of each switching sequence is slightly adjusted forward or delayed backward, and the delay time is calculated according to the trajectory of the predicted average value changing over time. If the predicted risk value of leakage current protection malfunction exceeds the preset malfunction risk threshold at a certain switching starting time, the corresponding switching starting time is delayed until the predicted average value falls back below the setting value of the leakage current protection device. If the predicted risk value of leakage current protection malfunction is lower than the preset malfunction risk threshold, the original switching starting time remains unchanged, and the switching timing sequence of the auxiliary current guiding branch is obtained by rearranging the adjusted switching starting times. Based on the refined auxiliary current guiding branch commissioning sequence, the difference between the maximum and minimum values ​​of the three-phase current before and after each commissioning is collected, and the three-phase imbalance guiding effect index is obtained according to the decrease in the difference. The zero-sequence harmonic amplitude of the neutral line and the convergence intensity of the main path of the amplified circulating current channel are collected before and after each commissioning, and the circulating current amplification suppression index is obtained according to the arithmetic mean of the decrease ratio of the zero-sequence harmonic amplitude of the neutral line and the decrease ratio of the convergence intensity. The circulating current amplification suppression index under all commissioning sequences is averaged to quantify the circulating current amplification suppression level of the dynamic adjustment of the load distribution of the low-voltage cable branch box.

[0077] During the dynamic adjustment of load distribution in low-voltage cable branch boxes, leakage current protection devices are prone to malfunction due to deviations in residual current sampling values ​​from the normal range during periods of increased zero-sequence harmonic circulating current. In one implementation, the residual current amplitude I on the neutral side is set to k for each switching sequence number. r,k With the content of zero-sequence harmonics of each order K 3,k ,K 9,k ,K 15,k Arranged sequentially, they are combined into a state observation vector. The yk values ​​of multiple consecutive switching cycles are connected in series in chronological order to form a leakage current protection malfunction observation sequence. N represents the cumulative number of cuts / switches within the observation window.

[0078] Specifically, establishing a discrete linear state-space model , , where x kThe implicit state vector contains the slowly varying components of the residual current and the zero-sequence harmonics. A is the state transition matrix, H is the observation matrix, and w k-1 For process noise, v k To observe the noise, both the process noise covariance Q and the observed noise covariance R are pre-calibrated based on historical load data from low-voltage cable branch boxes. The Kalman filter algorithm performs a two-step iteration on the leakage current protection malfunction observation sequence, with the prediction step obtaining a priori state estimate. Covariance of prior error A T The transpose of A; update step by Kalman gain. The prior estimate is corrected to obtain the posterior state estimate. With posterior covariance Based on x k|k Using the state transition matrix A, we can further calculate the predicted mean μ of the residual current samples in the next switching cycle. k+1 With the corresponding covariance σ k+1 2 .

[0079] It should be noted that the predicted mean μ k+1 The covariance σ represents the expected amplitude of the residual current on the neutral side during the next switching cycle. k+1 2 This represents the uncertainty of the prediction.

[0080] Specifically, for the predicted mean μ k+1 With the setting value I of the leakage protection device th A comparison is performed, and the ratio ρ of the difference between the two values ​​to the setpoint is used. k+1 =(μ k+1 -I th ) / I th As a predictive value for the risk of malfunction of leakage current protection.

[0081] Preferably, the setting value I of the leakage current protection device th A current of 30 mA corresponds to Level I personal safety protection. Furthermore, regarding the starting time t of each input in the aforementioned switching sequence... 0,k Make forward adjustments or delay backwards.

[0082] Preferably, the preset malfunction risk threshold ρ th We take 0.2, which is the risk prediction value ρ. k+1 A rate exceeding 20% ​​is considered to indicate a significant risk of malfunction.

[0083] It is understandable that if the predicted risk value ρ of the leakage protection malfunction... k+1 At a certain initial investment time point t 0,k The risk of malfunction exceeds the preset threshold ρ thIf the starting time of the current protection device is delayed, the delay time is calculated based on the trajectory of the predicted mean over time, until the predicted mean falls back below the setting value of the current protection device, which is then taken as the new starting time t. 0,k' If the predicted value of the leakage current protection malfunction risk is lower than the preset malfunction risk threshold, the original start time of the protection will remain unchanged.

[0084] For example, based on the adjusted input start time t 0,k' With the corresponding exit time point t 1,k Reorganize the switching pairs and arrange them in chronological order to obtain the refined auxiliary guide branch deployment timing sequence {(t 0,k' ,t 1,k Furthermore, for each activation of the refined auxiliary current-guiding branch in the activation sequence, the instantaneous values ​​of the three-phase current are collected before and after each activation, and the difference ΔI between the maximum and minimum values ​​of the three-phase current is calculated. abc Let the difference before investment be ΔI. abc,前 The difference after input is ΔI abc,后 According to the decrease in difference η abc =(ΔI abc,前 -ΔI abc,后 ) / ΔIabc, the three-phase imbalance conduction effect index η is obtained before abc Simultaneously, the zero-sequence harmonic amplitude I before and after each input is... n_zero The convergence intensity Sn along the main path of the amplified circulation channel was collected separately, and the amplitude of the zero-sequence harmonic was decreased according to the ratio ηI=(I n_zero,前 -I n_zero,后 ) / I n_zero,前 Compared with the decrease in convergence intensity, ηS=(S n,前 -S n,后 The arithmetic mean of the values ​​before ηI and ηS is used to obtain the circulating current amplification suppression index γk = (ηI + ηS) / 2.

[0085] Specifically, the arithmetic mean of the circulating current amplification suppression index γk for all switching sequence numbers k=1,2,...,N is taken to obtain the circulating current amplification suppression level Γ=(1 / N)·Σγk for dynamic adjustment of load distribution in low-voltage cable branch boxes.

[0086] Preferably, the value of Γ ranges from 0 to 1, with the value closer to 1 indicating a more significant effect in suppressing amplified circulating current. Through the above implementation process, in nonlinear load scenarios such as centralized access of charging equipment, centralized access of energy-saving lighting, and centralized access of variable frequency air conditioners, the low-voltage cable branch box can dynamically refine the timing of auxiliary current guiding branch activation based on the predicted risk value of leakage protection maloperation, and simultaneously output the three-phase imbalance conduction effect index ηabc and the circulating current amplification suppression level Γ, so that the leakage protection device is free from maloperation during the period of aggravated zero-sequence harmonic circulating current, and provides a quantitative evaluation basis for the degree of reduction of neutral wire thermal stress.

[0087] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately. Furthermore, various different embodiments of the present invention can also be arbitrarily combined, as long as they do not violate the spirit of the present invention, and should also be regarded as the content disclosed by the present invention.

Claims

1. A method for dynamically adjusting the load distribution of a low-voltage cable branch box, characterized in that, The method includes: Monitor the load current of each phase and the zero-sequence harmonic content of the neutral line, and analyze the impedance distribution of the neutral line side before and after the auxiliary branch is switched on and off. Based on the impedance distribution on the zero-outlet side and the zero-sequence harmonic content of the zero line, the proportion of harmonic components is analyzed using the Fourier transform algorithm to generate an identification mark for the amplified circulating current condition. Based on the identification criteria of the amplified circulation condition, assess whether the proportion of harmonic components exceeds the critical range of the amplified circulation condition. After confirming the establishment of the amplified circulation condition through analysis, determine the switching conditions for the diversion condition. Based on the switching conditions and harmonic component ratio of the diversion operation, the influence of the low-resistivity path of the auxiliary branch on the zero-sequence harmonic is analyzed, and the switching sequence for the decrease of the zero line thermal stress is determined. By controlling the on / off state of the auxiliary current-conducting branch through the switching timing, the changes in neutral current, neutral outlet temperature and zero-sequence harmonic content before and after switching are collected to assess the risk of increased neutral temperature rise and obtain the updated zero-sequence harmonic content. Based on the updated zero-sequence harmonic content and the switching timing, the Kalman filter algorithm is used to predict the risk of leakage protection malfunction, evaluate the three-phase imbalance conduction effect, and quantify the circulating current amplification suppression level of the dynamic adjustment of load distribution in low-voltage cable branch boxes.

2. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, The monitoring of load current in each phase and zero-sequence harmonic content of the neutral line, and the analysis of the impedance distribution on the neutral side before and after the auxiliary branch is switched on and off, include: The instantaneous values ​​of the load currents of phases A, B, and C are collected by current transformers deployed at the incoming end, and the amplitude of the odd-order zero-sequence harmonic components flowing through the neutral line is extracted to obtain a current sampling set. For the current sampling set, the loop voltage drop and return current values ​​from the neutral pin to the neutral point are measured under two on / off states: the auxiliary current guide branch is open and the return current value is connected. The impedance distribution spectrum of the neutral pin side is plotted, and the proportion of zero-sequence harmonic return current borne by the low-resistance path in the spectrum is compared with the decrease in neutral current to obtain the conflict state identification mark.

3. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, The method involves analyzing the harmonic component ratio using a Fourier transform algorithm based on the zero-line impedance distribution and the zero-sequence harmonic content, and generating identification markers for amplified circulating current conditions, including: The zero-sequence harmonic amplitude waveform data of the zero-line is extracted and analyzed in the frequency domain. The amplitudes and phase angles of the 3rd, 9th, and 15th odd-order zero-sequence harmonics are extracted to form an odd-order zero-sequence harmonic amplitude array. The component proportion is calculated according to the ratio of each harmonic amplitude to the fundamental amplitude. The component proportions of each order are paired with the low-impedance path impedance values ​​on a frequency band basis to obtain a harmonic impedance pairing set. The harmonic impedance pairing set is compared with the circulating current amplification critical threshold and the low-impedance judgment threshold. The harmonic order exceeding the threshold and the corresponding proportion value and pairing impedance value are marked to obtain the identification mark of the amplified circulating current condition.

4. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, Based on the identification criteria of the amplified circulation condition, the proportion of harmonic components is assessed to determine whether it exceeds the critical range of the amplified circulation condition. After confirming the establishment of the amplified circulation condition through analysis, the switching conditions for the diversion and diversion condition are determined, including: The marked harmonic order, corresponding proportion value, and paired impedance value are read from the identification mark of the amplified circulating current condition. The proportion value is compared with the upper limit threshold to obtain the proportion evaluation result. After the proportion value is continuously higher than the upper limit threshold and the paired impedance value is continuously lower than the low resistance judgment threshold within the continuous observation window, the confirmed result of the circulating current condition is obtained. The timing of the auxiliary current guiding branch is adjusted and the change trend of the neutral current drop rate and the zero-sequence harmonic return direction after the branch is put into operation are checked to obtain the conversion condition of the diversion current condition.

5. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, The transition conditions for the diversion of the diversion current are determined by: analyzing the amplified circulating current channel formed by the convergence of harmonic currents on the zero-sequence side, identifying the backflow carrying capacity of the low-resistance path constructed by the switchable auxiliary diversion branch for zero-sequence harmonics, and obtaining the transition conditions for the diversion of the diversion current when the amplified circulating current condition is established.

6. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, The analysis of the influence of the low-resistivity path of the auxiliary branch on the zero-sequence harmonics based on the switching conditions and harmonic component ratio under the diversion condition, and the determination of the switching sequence for the decrease of neutral wire thermal stress, includes: For the low-resistance path of the auxiliary guide branch, the loop impedance value is collected at each order, and the impact assessment result is obtained by comparing the change of the zero-sequence harmonic amplitude on the neutral line side before and after the low-resistance path is put into operation. The moving average of the zero-sequence harmonic amplitude and the difference of the three-phase phase current are tracked synchronously. When the convergence enhancement period and the unbalance rise period coincide, the operation command is triggered to obtain the operation start time point. When the neutral line current continues to decrease and remains stable and the neutral line temperature stops rising, the exit command is triggered. The operation start time point and the exit time point together constitute the operation and exit sequence of the decrease of the neutral line thermal stress.

7. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, The process involves controlling the on / off state of the auxiliary current-conducting branch through the switching timing, collecting changes in neutral current, neutral outlet temperature, and zero-sequence harmonic content before and after switching, assessing the risk of increased neutral temperature rise, and obtaining updated zero-sequence harmonic content, including: The power electronic switch or mechanical contactor is driven to form an auxiliary current-conducting branch on / off state sequence according to the start and end times of the switching sequence. For the on / off state sequence, the difference between the neutral current amplitude, neutral outlet temperature and the content of each zero-sequence harmonic is collected before and after each switching. The rising rate of the neutral outlet temperature and the synchronous change trend of the neutral current amplitude are compared to obtain the risk assessment result of the increased neutral temperature rise. Based on the risk assessment result of the increased neutral temperature rise, the frequency domain is re-decomposed using the Fourier transform algorithm for the neutral current sampling sequence after each switching and exit time. The proportion of each zero-sequence harmonic component is calculated according to the ratio of each order amplitude to the fundamental frequency amplitude, and arranged in the order of harmonic order to form a new round of zero-sequence harmonic content array, thus obtaining the updated zero-sequence harmonic content.

8. The method for dynamic adjustment of load distribution in a low-voltage cable branch box according to claim 1, characterized in that, The method of predicting the risk of leakage protection malfunction using a Kalman filter algorithm based on the updated zero-sequence harmonic content and the switching timing, evaluating the three-phase imbalance conduction effect, and quantifying the circulating current amplification suppression level of the dynamic adjustment of load distribution in low-voltage cable branch boxes includes: For each switching sequence number, the residual current amplitude on the neutral side and the content of each zero-sequence harmonic are arranged in time sequence to construct a leakage current protection maloperation observation sequence. The Kalman filter algorithm is used to perform state estimation and one-step prediction on the leakage current protection maloperation observation sequence, and the predicted mean and corresponding covariance of the residual current sampling value in the next switching cycle are output. The ratio of the difference between the predicted mean and the setting value of the leakage current protection device to the setting value is used as the predicted value of leakage current protection maloperation risk. The starting point for the activation of the auxiliary current-conducting branch is delayed until the predicted average value falls back below the setting value of the leakage current protection device, based on the predicted value of the leakage current protection maloperation risk exceeding the maloperation risk threshold. This results in a refined sequence of activation times for the auxiliary current-conducting branch. Based on the refined auxiliary current guiding branch commissioning timing sequence, the decrease in the difference between the maximum and minimum values ​​of the three-phase current before and after each commissioning is collected to obtain the three-phase imbalance guiding effect index. The arithmetic mean of the zero-sequence harmonic amplitude of the neutral line and the convergence intensity of the main path of the amplified circulating current channel is collected according to the decrease ratio to obtain the circulating current amplification suppression index. The average value of the circulating current amplification suppression index under all commissioning sequences is taken to quantify the circulating current amplification suppression level.

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