Control methods for composite grounding devices, composite grounding systems and electrical equipment

Abstract

This application relates to a control method for a composite grounding device, a composite grounding system, and electrical equipment. The method includes: upon detecting a low-resistance grounding fault, controlling the operation of a low-resistance grounding device to ground the neutral point through the low-resistance grounding device; upon detecting a high-resistance grounding fault, controlling the operation of a fully controlled flexible device, and selecting a voltage reference value based on the sampled zero-sequence current value and a zero-sequence current threshold value determined by whether to consider the influence of line impedance and load; and controlling the fully controlled flexible device to adjust the neutral point voltage to the voltage reference value. Through the efficient coordination of the low-resistance grounding device and the fully controlled flexible device, more reliable grounding can be achieved to cope with various types of grounding faults. Automatically selecting the neutral point voltage reference value based on the zero-sequence current value can eliminate the influence of line impedance and load factors on the arc-suppression effect, improving the stability and reliability of the distribution network operation.

Description

Technical Field

[0001] This application relates to the field of power grid technology, and in particular to a control method for a composite grounding device, a composite grounding system, and electrical equipment. Background Technology

[0002] With the increasing number of nonlinear loads in power systems, traditional arc suppression coils can no longer meet the requirements for arc extinguishing. To achieve reliable arc extinguishing of single-phase ground faults, traditional techniques have explored active arc extinguishing methods, primarily using power electronic converters to regulate the fault point current or voltage. These methods can be categorized into current-based arc extinguishing and voltage-based arc extinguishing. Current-based arc extinguishing compensates for the fault point current by injecting zero-sequence current into the distribution network, while voltage-based arc extinguishing suppresses the fault point voltage by adjusting the neutral point voltage, thus achieving reliable arc extinguishing.

[0003] Current active voltage arc suppression control strategies typically target the bus voltage of the faulted phase. However, in practice, due to voltage drops in the lines, the residual fault current remains significant, even reaching tens of amperes, after active compensation, especially under conditions of high line impedance or heavy load, making effective arc suppression impossible. Furthermore, as traditional overhead lines are gradually replaced by cables, low-resistance grounding systems are becoming more common. However, in distribution networks using low-resistance grounding, high-resistance grounding faults may occur, with fault characteristic quantities potentially falling below the zero-sequence protection threshold, preventing the faulty line from being isolated. Therefore, more efficient and reliable grounding methods and more precise and effective control strategies are needed. Summary of the Invention

[0004] Based on this, it is necessary to provide a control method, a composite grounding system, and electrical equipment for the composite grounding device to address the above-mentioned technical problems. This method can eliminate the influence of line impedance and load factors on the arc suppression effect, and effectively suppress fault residual current and reliably suppress arc under different line impedance and load conditions.

[0005] In a first aspect, this application provides a control method for a composite grounding device, the composite grounding device including a fully controlled flexible device and a low-resistance grounding device, both of which are connected to a neutral point; the method includes:

[0006] In the event of a low-resistance grounding fault, the small-resistance grounding device is activated to ground the neutral point through the small-resistance grounding device.

[0007] When a high-resistance grounding fault is detected, the fully controlled flexible device is put into operation, and a voltage reference value is selected based on the zero-sequence current value obtained from sampling and the voltage correction threshold; the voltage correction threshold is the critical value of zero-sequence current for determining whether to consider the influence of line impedance and load.

[0008] The fully controlled flexible device adjusts the neutral point voltage to the voltage reference value.

[0009] In one embodiment, a voltage reference value is selected based on the sampled zero-sequence current value and a voltage correction threshold, including:

[0010] If the zero-sequence current value obtained by sampling is less than the voltage correction threshold, it is determined that there is no need to consider the influence of line impedance and load, and the first voltage reference value is selected.

[0011] If the zero-sequence current value obtained by sampling is greater than or equal to the voltage correction threshold, it is determined that the influence of line impedance and load needs to be considered, and a second voltage reference value is selected; the second voltage reference value is a parameter obtained by correcting the first voltage reference value.

[0012] In one embodiment, the method further includes:

[0013] The transition resistance threshold is determined based on the line voltage drop and fault residual current limit;

[0014] The zero-sequence current corresponding to the transition resistance threshold is used as the voltage correction threshold.

[0015] In one embodiment, the method further includes: determining a correction factor based on the load impedance of the faulted line, the positive sequence impedance per unit length of the faulted line, and the fault distance; and determining a second voltage reference value based on a first voltage reference value and the correction factor.

[0016] In one embodiment, the first voltage reference value is the opposite of the fault phase power supply voltage.

[0017] In one embodiment, the method further includes: comparing the sampled zero-sequence current value with a ground fault threshold and a zero-sequence current protection threshold, respectively; if the zero-sequence current value is greater than the ground fault threshold and less than the zero-sequence current protection threshold, then a high-resistance ground fault is determined to have occurred; if the zero-sequence current value is greater than the ground fault threshold and not less than the zero-sequence current protection threshold, then a low-resistance ground fault is determined to have occurred.

[0018] In one embodiment, the method further includes: after the low-resistance grounding device or the fully controlled flexible device is put into operation, determining whether the fault has been cleared; if the fault has been cleared, maintaining the operation of the low-resistance grounding device, or disconnecting the fully controlled flexible device and restoring the operation of the low-resistance grounding device; if a permanent fault is identified, performing subsequent relay protection.

[0019] Secondly, this application also provides a composite grounding system, which includes a composite grounding device and a controller. The composite grounding device includes a fully controllable flexible device and a low-resistance grounding device, and the controller is used to perform the steps of the method described above.

[0020] In one embodiment, the controller includes a coordination controller and a voltage regulation controller;

[0021] The coordinated controller is used to control the small-resistance grounding device to be put into operation when a low-resistance grounding fault is detected, so that the neutral point is grounded through the small-resistance grounding device; when a high-resistance grounding fault is detected, it controls the fully controlled flexible device to be put into operation, and selects a voltage reference value based on the sampled zero-sequence current value and the voltage correction threshold; the voltage correction threshold is the zero-sequence current critical value for determining whether to consider the influence of line impedance and load.

[0022] A voltage regulator controller is used to control the neutral point voltage of a fully controlled flexible device to adjust it to the voltage reference value.

[0023] Thirdly, this application also provides an electrical device that includes the composite grounding system described above.

[0024] The control method, system, and electrical equipment described above for the composite grounding device, when a low-resistance grounding fault is detected, control the small-resistance grounding device to operate, ensuring the neutral point is grounded through it. When a high-resistance grounding fault is detected, control the fully controlled flexible device to operate, and select a voltage reference value based on the sampled zero-sequence current value and a voltage correction threshold. The voltage correction threshold is the critical value for zero-sequence current used to determine whether to consider the influence of line impedance and load. The fully controlled flexible device adjusts the neutral point voltage to the voltage reference value. Through this method, the efficient coordination between the small-resistance grounding device and the fully controlled flexible device achieves more reliable grounding to cope with various types of grounding faults. Automatically selecting the neutral point voltage reference value based on the zero-sequence current value, while considering the influence of line impedance and load, eliminates the impact of line impedance and load factors on the arc-suppression effect. Under different line impedance and load conditions, it effectively suppresses residual fault current, reliably extinguishes arcs, and improves the stability and reliability of the distribution network. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the drawings used in the description of the embodiments of this application or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is an application environment diagram of the control method for the composite grounding device in one embodiment;

[0027] Figure 2This is a flowchart illustrating the control method of a composite grounding device in one embodiment;

[0028] Figure 3 This is a flowchart illustrating the step of selecting a voltage reference value in one embodiment;

[0029] Figure 4 This is a surface plot showing the effect of line impedance and load on fault residual current in one embodiment;

[0030] Figure 5 This is a composite sequence net and Thevenin equivalent circuit diagram in one embodiment;

[0031] Figure 6 This is a block diagram of the control strategy of the voltage regulator controller in one embodiment;

[0032] Figure 7 This is a control flowchart of a composite grounding device in one embodiment;

[0033] Figure 8 This is a simulation result diagram verifying the arc suppression effect in one embodiment;

[0034] Figure 9 This is a simulation result diagram of collaborative control in one embodiment;

[0035] Figure 10 This is a structural block diagram of a composite grounding system in one embodiment. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0037] The control method for the composite grounding device provided in this application embodiment can be applied to, for example... Figure 1 The application environment shown is described. The composite grounding system includes a composite grounding device 10 and a controller 20. The composite grounding device 10 includes a fully controllable flexible device 102 and a low-resistance grounding device 104. The controller 20 is used in the steps of the control method for the composite grounding device described in the embodiments of this application.

[0038] In one exemplary embodiment, such as Figure 2 As shown, a control method for a composite grounding device is provided. The composite grounding device includes a fully controllable flexible device and a low-resistance grounding device, both of which are connected to the neutral point. This method can be applied to applications such as... Figure 1 Taking the controller 20 shown as an example, the method includes:

[0039] Step 202: In the event of a low-resistance grounding fault, control the small-resistance grounding device to be put into operation so that the neutral point is grounded through the small-resistance grounding device.

[0040] In the case of a low-resistance grounding fault, the neutral point is grounded through a low-resistance grounding device by control, thus completing the relay protection action.

[0041] Step 204: In the event of a high-resistance grounding fault, the fully controlled flexible device is put into operation, and a voltage reference value is selected based on the zero-sequence current value obtained from sampling and the voltage correction threshold. The voltage correction threshold is the critical value of the zero-sequence current to determine whether to consider the influence of line impedance and load.

[0042] In this embodiment, under high-resistance grounding fault conditions, the fully controlled flexible device can inject compensation voltage into the neutral point through control, thereby suppressing the fault phase voltage. Under different grounding fault conditions, the low-resistance grounding device or the fully controlled flexible device is switched into operation to achieve more reliable grounding and cope with various types of grounding faults.

[0043] Optionally, the grounding resistance can be calculated through parameter estimation, and the calculated grounding resistance can be used to determine whether a low-resistance grounding fault or a high-resistance grounding fault has occurred. If the grounding resistance is lower than a set resistance threshold, a low-resistance grounding fault is determined to have occurred; if the grounding resistance is not lower than the set resistance threshold, a high-resistance grounding fault is determined to have occurred.

[0044] Optionally, the occurrence of a low-resistance ground fault or a high-resistance ground fault can be determined by collecting the zero-sequence current value. If the zero-sequence current value is greater than the ground fault threshold but less than the zero-sequence current protection threshold, a high-resistance ground fault is determined to have occurred; if the zero-sequence current value is greater than the ground fault threshold but not less than the zero-sequence current protection threshold, a low-resistance ground fault is determined to have occurred.

[0045] The process involves sampling and acquiring the zero-sequence current value, comparing it with a voltage correction threshold, and selecting a voltage reference value based on the comparison result. This method allows for the selection of different voltage reference values ​​according to actual conditions, making the voltage compensation process more reliable. Optionally, if it is determined that the influence of line impedance and load does not need to be considered, the neutral point voltage reference value before correction is selected; if it is determined that the influence of line impedance and load needs to be considered, the neutral point voltage reference value after correction is selected.

[0046] The voltage correction threshold is a zero-sequence current threshold used to determine whether to consider the effects of line impedance and load. This zero-sequence current threshold is a parameter that is set in advance based on the transition resistance threshold determined by the line voltage drop and fault residual current limit.

[0047] Step 206: Control the fully controlled flexible device to adjust the neutral point voltage to the voltage reference value.

[0048] By controlling the neutral point voltage to be adjusted to the voltage reference value, the fault phase voltage and fault residual current can be suppressed, thus achieving reliable arc suppression.

[0049] In the control method of the aforementioned composite grounding device, when a low-resistance grounding fault is detected, the small-resistance grounding device is activated to ground the neutral point through it. When a high-resistance grounding fault is detected, the fully controlled flexible device is activated, and a voltage reference value is selected based on the sampled zero-sequence current value and the voltage correction threshold. The voltage correction threshold is the critical value of the zero-sequence current used to determine whether to consider the influence of line impedance and load. The fully controlled flexible device is then used to adjust the neutral point voltage to the voltage reference value. Through this method, the efficient coordination between the small-resistance grounding device and the fully controlled flexible device achieves more reliable grounding to cope with various types of grounding faults. Automatically selecting the neutral point voltage reference value based on the zero-sequence current value, and considering the influence of line impedance and load, eliminates the impact of line impedance and load factors on the arc suppression effect. Under different line impedance and load conditions, it can effectively suppress fault residual current, reliably extinguish arcs, and improve the stability and reliability of the distribution network operation.

[0050] In one exemplary embodiment, refer to Figure 3 Based on the zero-sequence current value obtained from sampling and the voltage correction threshold, a voltage reference value is selected, including:

[0051] Step 302: If the zero-sequence current value obtained by sampling is less than the voltage correction threshold, it is determined that there is no need to consider the influence of line impedance and load, and the first voltage reference value is selected.

[0052] Among them, the zero-sequence current value is less than the voltage correction threshold, indicating that the actual transition resistance is large enough. At this time, there is no need to consider the influence of line impedance and load. The first voltage reference value is selected, that is, the neutral point voltage reference value before correction.

[0053] Step 304: If the zero-sequence current value obtained by sampling is greater than or equal to the voltage correction threshold, it is determined that the influence of line impedance and load needs to be considered, and a second voltage reference value is selected; the second voltage reference value is a parameter obtained by correcting the first voltage reference value.

[0054] Among them, if the zero-sequence current value is greater than or equal to the voltage correction threshold, it indicates that the actual transition resistance is small and the influence of line impedance and load is large. It is necessary to consider the influence of line impedance and load and select a second voltage reference value, that is, the corrected neutral point voltage reference value.

[0055] In this embodiment, when it is determined that there is no need to consider the influence of line impedance and load, the neutral point voltage reference value before correction is selected; when it is determined that the influence of line impedance and load needs to be considered, the neutral point voltage reference value after correction is selected. Different voltage reference values ​​can be selected according to the actual situation, making the voltage compensation process more reliable.

[0056] In an exemplary embodiment, the method further includes: determining a transition resistance threshold based on the line voltage drop and the fault residual current limit; and using the zero-sequence current corresponding to the transition resistance threshold as a voltage correction threshold.

[0057] To keep the fault residual current below the fault residual current limit, a corresponding transition resistance threshold needs to be set. When the actual transition resistance is lower than the threshold, the impact of line impedance and load on the arc suppression effect needs to be considered; when the actual transition resistance is not lower than the threshold, the impact of line impedance and load does not need to be considered. The zero-sequence current corresponding to the transition resistance threshold is used as the voltage correction threshold so that the current value can be used to determine whether the impact of line impedance and load needs to be considered.

[0058] The following example illustrates this: For instance, the voltage drop in a 10kV distribution network is generally not allowed to exceed 10%, which is 600V. If the requirement is to control the fault residual current to below 1A, the transition resistance threshold is set to 600Ω. The zero-sequence current of 10A corresponding to 600Ω is the voltage correction threshold.

[0059] Reference Figure 4 , Figure 4 This is a surface plot showing the effect of line impedance and load on fault residual current in one embodiment. It reflects the effect under different load currents (reference). Figure 4 I in LC 200A and I LC Under the condition of 100A, fault residual current (reference) Figure 4 The trend of fault current (in the figure) with fault distance and transition resistance shows that the larger the load current, the larger the residual fault current, and the residual fault current increases with the increase of fault distance. When the transition resistance is large enough, the residual fault current can be ignored, that is, there is no need to consider the influence of line impedance and load, and the arc will extinguish naturally. When the transition resistance is small, the arc is difficult to extinguish due to the large residual fault current generated by the line voltage drop. Therefore, when selecting the neutral point reference voltage value, the influence of line impedance and load needs to be considered.

[0060] In an exemplary embodiment, the method further includes: determining a correction factor based on the load impedance of the faulted line, the positive sequence impedance per unit length of the faulted line, and the fault distance; and determining a second voltage reference value based on a first voltage reference value and the correction factor.

[0061] The second voltage reference value is calculated according to the following formula:

[0062]

[0063] In the formula, U2 represents the second voltage reference value, k represents the correction coefficient, U1 represents the first voltage reference value, and Z... load Z represents the load impedance of the faulty line. l1 α represents the positive sequence impedance per unit length of the faulty line, and α represents the fault distance.

[0064] Reference Figure 5 , Figure 5 for Figure 1 Based on the composite sequence network and Thevenin equivalent circuit diagram corresponding to the structure shown, it can be deduced that, considering the influence of line impedance and load, to suppress the fault residual current to zero, the corrected neutral point voltage reference value should be U2 = eq Thevenin equivalent power supply E eq The calculation is the negative of the fault phase power supply voltage multiplied by a coefficient k, where the coefficient k is calculated as follows:

[0065]

[0066] By using the above method, the influence of line impedance on arc suppression is reflected by the load impedance of the faulted line, the positive sequence impedance per unit length of the faulted line, and the fault distance. The first voltage reference value is corrected by a correction factor to obtain the second voltage reference value, so as to take into account the influence of line impedance and load, and to compensate the neutral point voltage, thereby achieving reliable arc suppression and improving the stability and reliability of the distribution network operation.

[0067] In one exemplary embodiment, the first voltage reference value is the opposite of the fault phase power supply voltage.

[0068] The first voltage reference value, i.e. the neutral point voltage reference value before correction, ignores the influence of line impedance and load and is calculated as the negative of the power supply voltage of the fault phase.

[0069] In one exemplary embodiment, a voltage regulation controller controls a fully controllable flexible device to adjust the neutral point voltage to a voltage reference value. (Refer to...) Figure 6 , Figure 6 The diagram shows the control strategy block diagram of the voltage regulator controller. The voltage regulator controller adopts a dual closed-loop control strategy, including an inner current loop and an outer voltage loop. The inner current loop is based on a proportional-integral controller, and the outer voltage loop is based on a proportional-resonant controller. A lead compensation element is connected in series to compensate for the phase margin and improve the stability of the control system.

[0070] In an exemplary embodiment, the method further includes: comparing the sampled zero-sequence current value with a ground fault threshold and a zero-sequence current protection threshold, respectively; if the zero-sequence current value is greater than the ground fault threshold and less than the zero-sequence current protection threshold, then a high-resistance ground fault is determined to have occurred; if the zero-sequence current value is greater than the ground fault threshold and not less than the zero-sequence current protection threshold, then a low-resistance ground fault is determined to have occurred.

[0071] The system sets three threshold levels for zero-sequence current: a ground fault threshold, a voltage correction threshold, and a zero-sequence current protection threshold. The sampled zero-sequence current value is compared with these three threshold levels: if the zero-sequence current value is higher than the ground fault threshold, a ground fault is determined to have occurred; if a ground fault has occurred and the zero-sequence current value is lower than the zero-sequence current protection threshold, a high-resistance ground fault is determined to have occurred, and the fully controlled flexible device is put into operation; if a high-resistance ground fault has occurred and the zero-sequence current value is lower than the voltage correction threshold, the neutral point voltage reference value before correction is selected, i.e., the first voltage reference value; if a high-resistance ground fault has occurred and the zero-sequence current value is not lower than the voltage correction threshold, the corrected neutral point voltage reference value is selected, i.e., the second voltage reference value; if a ground fault has occurred and the zero-sequence current value is not lower than the zero-sequence current protection threshold, a low-resistance ground fault is determined to have occurred, the low-resistance grounding device is put into operation, and the zero-sequence protection is activated.

[0072] In this embodiment, by setting a three-level threshold for zero-sequence current, the fault type is automatically determined. During a low-resistance grounding fault, the small-resistance grounding device is activated, triggering the zero-sequence protection. During a high-resistance grounding fault, the small-resistance grounding device is disconnected, and the fully controlled flexible device is put into operation. The reference value for the neutral point voltage before or after correction is selected to eliminate the influence of line impedance and load factors on the arc-suppression effect, achieving reliable arc suppression and improving the stability and reliability of the distribution network operation.

[0073] In one embodiment, the method further includes: after the low-resistance grounding device or the fully controlled flexible device is put into operation, determining whether the fault has been cleared; if the fault has been cleared, maintaining the operation of the low-resistance grounding device, or disconnecting the fully controlled flexible device and restoring the operation of the low-resistance grounding device; if a permanent fault is identified, performing subsequent relay protection.

[0074] If the low-resistance grounding fault has been cleared, the low-resistance grounding device should be kept in operation so that the neutral point is grounded through the low-resistance grounding device. If the high-resistance grounding fault has been cleared, the fully controlled flexible device should be disconnected from operation, and the low-resistance grounding device should be restored to operation so that the neutral point is grounded through the low-resistance grounding device.

[0075] If the fault has been cleared, disconnecting the grounding device can quickly restore the normal operation of the distribution network system. If a permanent fault is detected, subsequent relay protection can isolate the faulty area and protect other non-faulty parts, thus improving the stability of the distribution network operation.

[0076] In one exemplary embodiment, refer to Figure 7 , Figure 7 This is a control flowchart of a composite grounding device in one embodiment. Three threshold levels for zero-sequence current are set, namely, ground fault threshold I... set1 Voltage correction threshold I set2 and zero-sequence current protection threshold I set3 The zero-sequence current value I is obtained by sampling. N Determine I N >I set1 Whether it is true or not, and sample the voltage and current of the non-faulty phases.

[0077] If I N >I set1 If it is true, then a ground fault has occurred. If I N ≤I set1 If no fault has occurred, the zero-sequence current sampling will be performed again.

[0078] In the event of a ground fault, determine I N set3 Is it true? If I... N set3 If this condition is met, a high-resistance grounding fault is confirmed, and the fully controlled flexible grounding device is activated, ensuring the neutral point is grounded through the device. If I... N ≥I set3 If a low-resistance grounding fault is detected, the small-resistance grounding device will be put into operation, so that the neutral point is grounded through the small-resistance grounding device.

[0079] In the event of a high-resistance grounding fault, determine I N set2 Is it true? If I... N set2 If true, then the control center point voltage is adjusted to the reference voltage value U1 before correction. If I N ≥I set2 Then, the coefficient k is calculated, and the neutral point voltage is controlled to the corrected voltage reference value kU1. Subsequently, the system checks whether the fault has been cleared and detects permanent faults. If the fault is cleared, the fully controlled flexible device is disconnected. If a permanent fault is detected, the circuit breaker is tripped.

[0080] ​​​​In the event of a low-resistance grounding fault, the low-resistance grounding device is activated, executing zero-sequence protection to attempt reclosing. Subsequent checks are performed to determine if the fault has been cleared and to detect permanent faults. If the fault is cleared, the low-resistance grounding device is disconnected. If a permanent fault is detected, the circuit breaker is tripped.

[0081] In one exemplary embodiment, refer to Figure 8 , Figure 8 This is a simulation result diagram verifying the arc suppression effect in one embodiment. (Refer to...) Figure 1 Of the four lines (L1, L2, L3, L4), L1 and L2 are cables with lengths of 20km and 10km respectively, while L3 and L4 are overhead lines, each 10km long. The loads of L1 and L2 are 0.5MV·A, L3 is 0.5MV·A, and L4 is 0.8MV·A; the transition resistance of all lines is 100Ω. It can be seen that under different grounding fault conditions, regardless of whether it is a cable or an overhead line, and regardless of the line length and load capacity, a large residual current exists at the fault point after arc suppression using the original voltage reference value (i.e., the first voltage reference value), failing to meet the arc suppression requirements. However, using the corrected voltage reference value (i.e., the second voltage reference value) can effectively suppress the voltage and current at the fault point, reliably suppressing the arc.

[0082] In one exemplary embodiment, refer to Figure 9 , Figure 9 The diagram shows the simulation results of coordinated control in one embodiment. It can be seen that in the case of a low-resistance ground fault, the neutral point current reaches the zero-sequence current protection threshold, the zero-sequence protection operates, and the faulty line is disconnected, maintaining grounding via the small-resistance grounding device throughout the process. In the case of a high-resistance ground fault, the small-resistance grounding device is disconnected, and the fully controlled flexible device is put into operation. When the zero-sequence current is lower than the voltage correction threshold, the original neutral point voltage reference value (i.e., the first voltage reference value) is used to suppress the fault residual current to below 1A; when the zero-sequence current is higher than the voltage correction threshold, the corrected neutral point voltage reference value (i.e., the second voltage reference value) is used to suppress the fault phase voltage and current to zero.

[0083] It should be understood that, although the various steps in the flowcharts involved in the various embodiments described above are displayed in sequence according to the instructions of the arrows, these steps are not necessarily executed in sequence in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order restriction on the execution of these steps, and these steps can be executed in other orders. Moreover, at least a portion of the steps in the flowcharts involved in the various embodiments described above can include multiple steps or multiple stages, and these steps or stages are not necessarily executed and completed at the same time, but can be executed at different times, and the execution order of these steps or stages is not necessarily to be carried out in sequence, but can be executed in turn or alternately with other steps or at least a portion of steps or stages in other steps.

[0084] Based on the same inventive concept, this application also provides a composite grounding system for implementing the control method of the composite grounding device described above. The solution provided by this device is similar to the implementation described in the above method; therefore, the specific limitations in one or more composite grounding system embodiments provided below can be found in the limitations of the control method for the composite grounding device described above, and will not be repeated here.

[0085] In one exemplary embodiment, such as Figure 10 As shown, a composite grounding system is provided, which includes a composite grounding device 10 and a controller 20. The composite grounding device 10 includes a fully controllable flexible device 102 and a low-resistance grounding device 104. The controller 20 is used to perform the steps of the method described above.

[0086] In one exemplary embodiment, controller 20 includes a co-controller 1002 and a voltage regulation controller 1004;

[0087] The coordinating controller 1002 is used to control the small-resistance grounding device 104 to operate when a low-resistance grounding fault is detected, so that the neutral point is grounded through the small-resistance grounding device 104; and to control the fully controlled flexible device 102 to operate when a high-resistance grounding fault is detected, and to select a voltage reference value based on the sampled zero-sequence current value and the voltage correction threshold; the voltage correction threshold is the zero-sequence current critical value for determining whether to consider the influence of line impedance and load.

[0088] The voltage regulation controller 1004 is used to control the fully controlled flexible device 102 to adjust the neutral point voltage to the voltage reference value.

[0089] In an exemplary embodiment, the cooperative controller 1002 is further configured to: if the sampled zero-sequence current value is less than the voltage correction threshold, determine that the influence of line impedance and load does not need to be considered and select a first voltage reference value; if the sampled zero-sequence current value is greater than or equal to the voltage correction threshold, determine that the influence of line impedance and load needs to be considered and select a second voltage reference value; the second voltage reference value is a parameter obtained by correcting the first voltage reference value.

[0090] In an exemplary embodiment, the cooperative controller 1002 is further configured to determine a transition resistance threshold based on the line voltage drop and the fault residual current limit; and use the zero-sequence current corresponding to the transition resistance threshold as a voltage correction threshold.

[0091] In an exemplary embodiment, the cooperative controller 1002 is further configured to determine a correction factor based on the load impedance of the faulted line, the positive sequence impedance per unit length of the faulted line, and the fault distance; and to determine a second voltage reference value based on a first voltage reference value and the correction factor.

[0092] In one exemplary embodiment, the first voltage reference value is the opposite of the fault phase power supply voltage.

[0093] In an exemplary embodiment, the cooperative controller 1002 is further configured to compare the sampled zero-sequence current value with a ground fault threshold and a zero-sequence current protection threshold, respectively; if the zero-sequence current value is greater than the ground fault threshold and less than the zero-sequence current protection threshold, then a high-resistance ground fault is determined to have occurred; if the zero-sequence current value is greater than the ground fault threshold and not less than the zero-sequence current protection threshold, then a low-resistance ground fault is determined to have occurred.

[0094] In an exemplary embodiment, the coordination controller 1002 is further configured to determine whether the fault has been cleared after the low-resistance grounding device 104 or the fully controlled flexible device 102 is put into operation; if the fault has been cleared, the operation of the low-resistance grounding device 104 is maintained, or the fully controlled flexible device 102 is disconnected and the operation of the low-resistance grounding device 104 is restored; if a permanent fault is identified, subsequent relay protection is performed.

[0095] In an exemplary embodiment, the voltage regulation controller 1004 includes an inner current loop and an outer voltage loop; the inner current loop is constructed based on a proportional-integral controller; and the outer voltage loop is constructed based on a proportional-resonant controller.

[0096] In one exemplary embodiment, the voltage outer loop is connected in series with a lead correction circuit.

[0097] Each module in the aforementioned composite grounding system can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in the processor of a computer device in hardware form or independent of it, or stored in the memory of the computer device in software form, so that the processor can call and execute the operations corresponding to each module.

[0098] Based on the same inventive concept, embodiments of this application also provide an electrical device, which includes the composite grounding system described in any embodiment of this application.

[0099] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of the relevant data must comply with relevant regulations.

[0100] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile memory and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, artificial intelligence (AI) processors, etc., and are not limited to these.

[0101] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0102] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be determined by the appended claims.

Claims

1. A control method for a composite grounding device, characterized in that, The composite grounding device includes a fully controlled flexible device and a low-resistance grounding device, both of which are connected to the neutral point; the method includes: In the event of a low-resistance grounding fault, the low-resistance grounding device is activated to ground the neutral point through the low-resistance grounding device. In the event of a high-resistance grounding fault, the fully controllable flexible device is put into operation, and a voltage reference value is selected based on the sampled zero-sequence current value and the voltage correction threshold; the voltage correction threshold is the zero-sequence current critical value used to determine whether to consider the influence of line impedance and load. The fully controllable flexible device is controlled to adjust the neutral point voltage to the voltage reference value; The step of selecting a voltage reference value based on the zero-sequence current value obtained from sampling and the voltage correction threshold includes: If the zero-sequence current value obtained by sampling is less than the voltage correction threshold, it is determined that there is no need to consider the influence of line impedance and load, and the first voltage reference value is selected. If the zero-sequence current value obtained by sampling is greater than or equal to the voltage correction threshold, it is determined that the influence of line impedance and load needs to be considered, and a second voltage reference value is selected; the second voltage reference value is a parameter obtained by correcting the first voltage reference value. The method further includes: The transition resistance threshold is determined based on the line voltage drop and fault residual current limit; Use the zero-sequence current corresponding to the transition resistance threshold as the voltage correction threshold; The method further includes: The correction factor is determined based on the load impedance of the faulty line, the positive sequence impedance per unit length of the faulty line, and the fault distance. The second voltage reference value is determined based on the first voltage reference value and the correction factor; The first voltage reference value is the opposite of the power supply voltage of the faulty phase.

2. The method according to claim 1, characterized in that, The method further includes: The sampled zero-sequence current value is compared with the ground fault threshold and the zero-sequence current protection threshold, respectively. If the zero-sequence current value is greater than the ground fault threshold and the zero-sequence current value is less than the zero-sequence current protection threshold, then a high-resistance ground fault is determined to have occurred. If the zero-sequence current value is greater than the ground fault threshold and the zero-sequence current value is not less than the zero-sequence current protection threshold, then a low-resistance ground fault is determined to have occurred.

3. The method according to claim 1, characterized in that, The method further includes: After the low-resistance grounding device or the fully controlled flexible device is put into operation, determine whether the fault has been cleared; If the fault has been cleared, continue the operation of the small resistance grounding device, or disconnect the fully controlled flexible device that has been put into operation and restore the operation of the small resistance grounding device; If a permanent fault is identified, subsequent relay protection will be implemented.

4. A composite grounding system, characterized in that, The composite grounding system includes a composite grounding device and a controller. The composite grounding device includes a fully controllable flexible device and a low-resistance grounding device. The controller is used to perform the steps of the method as described in any one of claims 1 to 3.

5. The system according to claim 4, characterized in that, The controller includes a coordination controller and a voltage regulation controller; The collaborative controller is used to control the small resistance grounding device to operate when a low resistance grounding fault is detected, so that the neutral point is grounded through the small resistance grounding device; and to control the fully controlled flexible device to operate when a high resistance grounding fault is detected, and to select a voltage reference value based on the zero-sequence current value and voltage correction threshold obtained by sampling. The voltage correction threshold is the critical value of zero-sequence current used to determine whether to consider the effects of line impedance and load. The voltage regulation controller is used to control the fully controlled flexible device to adjust the neutral point voltage to the voltage reference value; The co-controller is further configured to: if the sampled zero-sequence current value is less than the voltage correction threshold, determine that the influence of line impedance and load does not need to be considered and select a first voltage reference value; if the sampled zero-sequence current value is greater than or equal to the voltage correction threshold, determine that the influence of line impedance and load needs to be considered and select a second voltage reference value; the second voltage reference value is a parameter obtained by correcting the first voltage reference value. The co-controller is also used to determine the transition resistance threshold based on the line voltage drop and the fault residual current limit; and to use the zero-sequence current corresponding to the transition resistance threshold as the voltage correction threshold. The collaborative controller is further configured to determine a correction coefficient based on the load impedance of the faulted line, the positive sequence impedance per unit length of the faulted line, and the fault distance; and to determine a second voltage reference value based on the first voltage reference value and the correction coefficient. The first voltage reference value is the opposite of the power supply voltage of the faulty phase.

6. The system according to claim 5, characterized in that, The collaborative controller is also used to compare the sampled zero-sequence current value with a ground fault threshold and a zero-sequence current protection threshold, respectively; if the zero-sequence current value is greater than the ground fault threshold and the zero-sequence current value is less than the zero-sequence current protection threshold, then a high-resistance ground fault is determined to have occurred; if the zero-sequence current value is greater than the ground fault threshold and the zero-sequence current value is not less than the zero-sequence current protection threshold, then a low-resistance ground fault is determined to have occurred.

7. The system according to claim 5, characterized in that, The collaborative controller is also used to determine whether the fault has been cleared after the low-resistance grounding device or the fully controlled flexible device is put into operation; if the fault has been cleared, the operation of the low-resistance grounding device is maintained, or the fully controlled flexible device is disconnected from operation and the operation of the low-resistance grounding device is restored; if a permanent fault is identified, subsequent relay protection is performed.

8. An electrical device, characterized in that, The electrical equipment includes the composite grounding system as described in any one of claims 4 to 7.

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