An intelligent switching device for parallel resistors and its control method

Abstract

The present invention relates to a parallel medium resistor intelligent switching device and its control method. The parallel medium resistor intelligent switching device includes an intelligent controller, a medium resistor input circuit, a medium resistor self-checking circuit, a transfer switch SW, a potential transformer (PT), and a current transformer (CT). The parallel medium resistor intelligent switching device and its control method have the capabilities of online self-diagnosis of the parallel medium resistor, fault warning, and adaptive ability of the resistance value of the inserted medium resistor, while reducing the operation and maintenance workload of the medium resistor, improving the operation reliability of the medium resistor, the anti-transition resistance ability and accuracy of the medium resistor grounding line selection method.

Description

Technical Field

[0001] The present invention relates to the technical field of shunt resistors, in particular to an intelligent switching device for shunt resistors and its control method. Background Art

[0002] In China's medium and low voltage power grids, the grounding method of the neutral point is usually ungrounded or grounded through an arc suppression coil. The selection of the neutral point grounding method is related to many aspects such as power supply reliability, equipment safety, and personal safety.

[0003] When the neutral point is grounded through an arc suppression coil, when a single-phase grounding fault occurs in the system, the inductive current of the arc suppression coil compensates for the capacitive current at the fault point, which can not only make the grounding arc at the fault point easy to extinguish, but also avoid the reignition of the grounding arc and make it completely extinguish, limiting the damage of the grounding current and the electrodynamic force and thermal effect of the arc to equipment and systems, and ensuring the power supply reliability of the system, equipment and personal safety. At the same time, due to the compensation effect of the arc suppression coil, the fault current and fault characteristics of the faulty line are greatly reduced, further reducing the line selection accuracy of the small current grounding line selection device.

[0004] When a single-phase grounding fault occurs in the system with the neutral point grounded through an arc suppression coil, the voltage of the non-faulty phase to the ground will rise from the phase voltage to the line voltage, bringing great pressure to the insulation of cables and equipment. It is very easy for the power grid to have problems such as single-phase grounding of the system, the voltage of the non-faulty phase to the ground rising, the fault point not being isolated in time, and the cable insulation being broken down after long-term operation, resulting in cable trench fires and further causing large-scale power outages and greater equipment losses.

[0005] Currently, the shunt resistor in the arc suppression coil is mainly used to cooperate with the medium resistor line selection method of the small current grounding line selection device. However, the existing medium resistor usually only has a fixed resistance value. When the transition resistance at the fault point is too large, even if the medium resistor is put into operation, the fault current of the faulty line is still too small, and the fault characteristics are not obvious, still reducing the line selection accuracy of the small current grounding line selection device.

[0006] The existing medium resistor switching device has no online self-check function and still requires manual maintenance, which is time-consuming and laborious and requires the arc suppression coil to be powered off, affecting the power supply reliability of the power grid. In addition, most of the existing medium resistors use mechanical switches for switching, and the switching time is difficult to accurately control. It is very easy for the medium resistor to be burned out due to the fault current flowing through it for a long time without being cut off in time. If it is not discovered in time after being burned out, it will affect the line selection accuracy next time.

[0007] In summary, in a system with a neutral point grounded through an arc suppression coil, when a single-phase ground fault occurs in the system, issues such as whether the resistance value of the shunt resistor is appropriate, the magnitude of the transition resistance at the fault point, the performance of the shunt resistor, and the switching time of the shunt resistor will all affect the line selection accuracy of the small current grounding line selection device, further affecting the fault isolation time and the power supply quality and reliability of the power grid. Summary of the Invention

[0008] The purpose of the present invention is to provide a shunt resistor intelligent switching device and its control method, which have the capabilities of on-line self-diagnosis of the shunt resistor, fault warning, and adaptive ability of the resistance value when the shunt resistor is put into operation. While reducing the operation and maintenance workload of the shunt resistor, it improves the reliability of the shunt resistor operation, the ability to resist transition resistance, and the accuracy of the shunt resistor grounding line selection method.

[0009] To achieve the above purpose, the technical solution of the present invention is: a shunt resistor intelligent switching device, including an intelligent controller and a shunt resistor input circuit, a shunt resistor self-check circuit, a changeover switch SW, a voltage transformer for collecting the zero-sequence voltage component of the bus voltage, and a current transformer for collecting the zero-sequence current flowing through the shunt resistor, which are connected to the intelligent controller.

[0010] In an embodiment of the present invention, the intelligent controller includes a controller DC power supply contact for accessing a 220V DC power supply as the DC power supply of the intelligent controller, a line selection signal output contact for starting the line selection function of the small current grounding line selection device, a DC power supply output contact for outputting a 24V DC power supply as the power supply of the shunt resistor self-check circuit, a zero-sequence voltage sampling contact connected to the voltage transformer for collecting the zero-sequence voltage component of the bus voltage, a zero-sequence current sampling contact connected to the current transformer for collecting the zero-sequence current flowing through the shunt resistor, a changeover switch SW control contact connected to the changeover switch SW for controlling the closing of different contacts of the changeover switch, and an electronic switch control contact for controlling the on and off of the IGBT-based electronic switch.

[0011] In an embodiment of the present invention, the medium resistance input circuit includes six electronic switches and six identical resistors with a resistance value of R each. Among them, R1, R2, and R3 are connected in series to form resistor string 1, and R4, R5, and R6 are connected in series to form resistor string 2. Resistor string 1 and resistor string 2 are connected in parallel to form the medium resistance; electronic switch 1 controls the access of R1, R2, and R3 to the primary circuit; electronic switch 2 controls the access of R2 and R3 to the primary circuit; electronic switch 3 controls the access of R3 to the primary circuit; electronic switch 4 controls the access of R4, R5, and R6 to the primary circuit; electronic switch 5 controls the access of R5 and R6 to the primary circuit; electronic switch 6 controls the access of R6 to the primary circuit; by controlling the on / off of the six electronic switches, the resistance value of the medium resistance connected to the primary circuit is controlled, and the adjustable resistance values are 3R, 2R, 1.5R, 1.2R, R, 0.75R, 0.67R, and 0.5R respectively; electronic switch 1, electronic switch 2, electronic switch 3, electronic switch 4, electronic switch 5, and electronic switch 6 are connected to the electronic switch control contacts.

[0012] In an embodiment of the present invention, the controller DC power supply contact, the 2-3 contact of the changeover switch SW, the medium resistance, the current transformer, and the zero-sequence current sampling contact constitute the medium resistance self-checking circuit.

[0013] In an embodiment of the present invention, the medium resistance has three states during operation, namely the medium resistance input state, the medium resistance self-checking state, and the medium resistance cut-off state; by controlling the conduction of different contacts of the changeover switch SW, the medium resistance is in different operating states; when the 1-3 contact of the changeover switch SW is switched to be conductive, the medium resistance is connected to the primary circuit, and at this time the medium resistance is in the input state to bypass the arc suppression coil; when the 2-3 contact of the changeover switch SW is switched to be conductive, the medium resistance self-checking circuit is connected at this time and is in the self-checking state for detecting the performance of the medium resistance; when the 4-3 contact of the changeover switch SW is switched to be conductive, the medium resistance is in the cut-off state at this time.

[0014] In an embodiment of the present invention, the voltage transformer is connected to the zero-sequence voltage sampling contact.

[0015] In an embodiment of the present invention, the current transformer is connected to the zero-sequence current sampling contact.

[0016] The present invention also provides a control method for a parallel medium resistance intelligent switching device. Using the above-described device, the self-checking logic of the medium resistance self-checking circuit is as follows:

[0017] Step S1: When the medium resistance self-checking circuit is conductive, the medium resistance is in the self-checking state; first, close electronic switch 1 to connect R1, R2, and R3 to the medium resistance self-checking circuit; measure whether the loop current is equal to U dc / 3R through the current transformer; if it is equal, it is considered that the three resistors R1, R2, and R3 are all in good condition; disconnect electronic switch 1, skip steps S2 - S4, and go to step S5;

[0018] Step S2. If the current measured by the current transformer in Step S1 is not equal to U dc / 3R, it indicates that there is a damage in one of the three resistors R1, R2, and R3. At this time, disconnect the electronic switch 1 and close the electronic switch 2 to connect R2 and R3 to the medium resistor self-checking circuit. Measure whether the loop current is equal to U dc / 2R through the current transformer. If it is equal, it is considered that R1 is damaged and the two resistors R2 and R3 are intact. In the future, when the medium resistor is put into operation, the electronic switch 1 will be locked and the resistor R1 will no longer be put in. Disconnect the electronic switch 2, skip Steps S3 - S4, and go to Step S5;

[0019] Step S3. If the current measured by the zero-sequence CT in Step S2 is not equal to U dc / 2R, it indicates that there is a damage in one of the two resistors R2 and R3. At this time, disconnect the electronic switch 2 and close the electronic switch 3 to connect R3 to the medium resistor self-checking circuit. Measure whether the loop current is equal to U dc / R through the current transformer. If it is equal, it is considered that R2 is damaged and the resistor R3 is intact. In the future, when the medium resistor is put into operation, the electronic switches 1 and 2 will be locked and the resistors R1 and R2 will no longer be put in. Disconnect the electronic switch 3, skip Step S4, and go to Step S5.

[0020] Step S4. If the current measured by the zero-sequence CT in Step S3 is not equal to U dc / R, it indicates that the resistor R3 is damaged. At this time, disconnect the electronic switch 3. In the future, when the medium resistor is put into operation, the electronic switches 1, 2, and 3 will be locked and the resistor string 1 will no longer be put in;

[0021] Step S5. Similar to Steps S1 - S4, check the quality of the three resistors R4, R5, and R6 in the resistor string 2 and lock the corresponding electronic switches in the switching logic, and no longer put in the damaged resistors.

[0022] In an embodiment of the present invention, the intelligent controller collects the bus 3U0 voltage through the voltage transformer. When the collected zero-sequence voltage is greater than the threshold value Um, the medium resistor input logic will be started.

[0023] In an embodiment of the present invention, the medium resistor input logic is specifically as follows:

[0024] (1). First, input the maximum resistance value Rmax that can be input by the medium resistor currently. The maximum resistance value Rmax has automatically recorded the resistance value that can be input by the medium resistor currently through the self-checking logic of the medium resistor self-checking circuit and sorted it, and measure the zero-sequence current passing through the medium resistor through the current transformer. When the zero-sequence current is greater than the set threshold value Im, close the line selection signal output contact to start the line selection function of the small current grounding line selection device;

[0025] (2) If the zero-sequence current measured by the current transformer in step (1) is less than the set threshold value Im, the intelligent controller will automatically select the optimal resistance value from the currently available resistance values of the medium resistance according to the maximum resistance value Rmax input, the zero-sequence current measured after the maximum resistance value is input, and the set threshold value Im, and close the line selection signal output contact to start the line selection function of the small current grounding line selection device.

[0026] (3) If, through calculation, the calculated optimal resistance value exceeds the range of the currently available resistance values of the medium resistance, the minimum value of the currently available resistance values will be automatically selected for input, and the line selection signal output contact will be closed to start the line selection function of the small current grounding line selection device.

[0027] Compared with the prior art, the present invention has the following beneficial effects: The present invention has the capabilities of online self-diagnosis of the shunt medium resistance, fault warning, and adaptability of the input resistance value of the medium resistance. While reducing the operation and maintenance workload of the medium resistance, it improves the reliability of the medium resistance operation, the ability to resist transition resistance, and the accuracy of the medium resistance grounding line selection method. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic block diagram of an intelligent switching device for a shunt medium resistance according to the present invention.

[0029] Figure 2 It is a schematic diagram of an intelligent switching device for a shunt medium resistance according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The technical solution of the present invention will be specifically described below with reference to the drawings.

[0031] As Figure 1 shown, an intelligent switching device for a shunt medium resistance according to the present invention includes an intelligent controller, a medium resistance input circuit connected to the intelligent controller, a medium resistance self-checking circuit, a change-over switch SW, a voltage transformer for collecting the zero-sequence voltage component of the bus voltage, and a current transformer for collecting the zero-sequence current flowing through the medium resistance.

[0032] As Figure 2 shown, this embodiment provides an intelligent switching device for a shunt medium resistance, which includes an intelligent controller, a medium resistance input circuit, a medium resistance self-checking circuit, a change-over switch SW, a voltage transformer (PT), and a current transformer (CT).

[0033] Furthermore, the intelligent controller includes a controller DC power supply contact point (for connecting to a 220V DC power supply as the power supply for the intelligent controller device), a line selection signal output contact point (for starting the line selection function of the small current grounding line selection device), a DC power supply output contact point (for outputting a 24V DC power supply as the power supply for the medium resistance self-check loop), a zero-sequence voltage sampling contact point (for collecting the zero-sequence voltage component of the bus voltage), a zero-sequence current sampling contact point (for collecting the zero-sequence current flowing through the medium resistance), a change-over switch SW control contact point (for controlling the closing of different contacts of the change-over switch), and an electronic switch control contact point (for controlling the opening and closing of the IGBT-based electronic switch).

[0034] Furthermore, when the medium resistance is working, it has three states, namely the medium resistance input state, the medium resistance self-check state, and the medium resistance cut-off state. By controlling the conduction of different contacts of the change-over switch SW, the medium resistance is in different working states. When the change-over switch contacts 1-3 are conducting, the medium resistance is connected to the primary circuit, and at this time the medium resistance is in the input state and bypasses the arc suppression coil; when the change-over switch contacts 2-3 are conducting, the medium resistance self-check loop is connected and is in the self-check state, which is used to detect the performance of the medium resistance; when the change-over switch contacts 4-3 are conducting, the medium resistance is in the cut-off state.

[0035] Furthermore, the medium resistance input loop includes 6 IGBT-based electronic switches and 6 resistors with the same resistance value (resistance value is R). R1, R2, and R3 are connected in series to form resistor string 1, and R4, R5, and R6 are connected in series to form resistor string 2. Resistor string 1 and resistor string 2 are connected in parallel to form the medium resistance. The electronic switch T1 controls R1, R2, and R3 to be connected to the primary circuit. The electronic switch T2 controls R2 and R3 to be connected to the primary circuit. The electronic switch T3 controls R3 to be connected to the primary circuit. The electronic switch T4 controls R4, R5, and R6 to be connected to the primary circuit. The electronic switch T5 controls R5 and R6 to be connected to the primary circuit. The electronic switch T6 controls R6 to be connected to the primary circuit. By controlling the on-off of the 6 electronic switches, the resistance value of the medium resistance connected to the primary circuit can be controlled, and the changeable resistance values are 3R, 2R, 1.5R, 1.2R, R, 0.75R, 0.67R, and 0.5R respectively.

[0036] Furthermore, the medium resistance self-check loop includes a DC power supply output contact point (output voltage is 24V), the 2-3 contact of the change-over switch SW, the medium resistance, the zero-sequence CT, and the zero-sequence current sampling contact point of the intelligent controller.

[0037] Furthermore, the medium resistance self-check logic is as follows:

[0038] Step 1: When the medium resistance self-check loop is conducting, the medium resistance is in the self-check state. First, close the electronic switch T1 to connect R1, R2, and R3 to the self-check loop. Measure whether the loop current is equal to U through the zero-sequence CT dc / 3R. If they are equal, it is considered that all three resistors R1, R2, and R3 are in good condition. Disconnect the electronic switch T1, skip steps 2 - 4, and go to step 5.

[0039] Step 2: If the current measured by the zero-sequence CT in step 1 is not equal to U dc / 3R, it indicates that there is a damage in the three resistors R1, R2, and R3. At this time, disconnect the electronic switch T1, close the electronic switch T2, and connect R2 and R3 into the self-checking circuit. Measure whether the current in the circuit is equal to U through the zero-sequence CT dc / 2R. If they are equal, it is considered that R1 is damaged and the two resistors R2 and R3 are in good condition. In the future, when the medium resistor is put into operation, the electronic switch T1 will be locked and the resistor R1 will no longer be put into operation. Disconnect the electronic switch T2, skip steps 3 - 4, and go to step 5.

[0040] Step 3: If the current measured by the zero-sequence CT in step 2 is not equal to U dc / 2R, it indicates that there is a damage in the two resistors R2 and R3. At this time, disconnect the electronic switch T2, close the electronic switch T3, and connect R3 into the self-checking circuit. Measure whether the current in the circuit is equal to U through the zero-sequence CT dc / R. If they are equal, it is considered that R2 is damaged and the resistor R3 is in good condition. In the future, when the medium resistor is put into operation, the electronic switches T1 and T2 will be locked and the resistors R1 and R2 will no longer be put into operation. Disconnect the electronic switch T3, skip step 4, and go to step 5.

[0041] Step 4: If the current measured by the zero-sequence CT in step 3 is not equal to U dc / R, it indicates that the resistor R3 is damaged. At this time, disconnect the electronic switch T3. In the future, when the medium resistor is put into operation, the electronic switches T1, T2, and T3 will be locked and the resistor string 1 will no longer be put into operation.

[0042] Step 5: Similar to steps 1 - 4, check the quality of the three resistors R4, R5, and R6 in the resistor string 2 and lock the corresponding electronic switches in the switching logic, and no longer put into the damaged resistors.

[0043] Furthermore, the intelligent controller collects the bus 3U0 voltage through the PT. When the collected zero-sequence voltage is greater than the threshold value Um, the medium resistor input logic will be started.

[0044] The medium resistor input logic is as follows

[0045] Step 1: First, put into the maximum resistance value R that can be currently put into the medium resistor max(During daily self-check of the loop by the intelligent controller, the resistance values that can be currently inputted by the neutral resistor have been automatically recorded and sorted in the logic), and the zero-sequence current passing through the neutral resistor is measured by the zero-sequence CT. When the zero-sequence current is greater than the set threshold value Im, the line selection signal output contact is closed to start the line selection function of the small-current grounding line selection device.

[0046] Step 2: If the zero-sequence current measured by the zero-sequence CT in Step 1 is less than the set threshold value Im. Then the intelligent controller will, according to the maximum resistance value R max inputted, the zero-sequence current measured after the maximum resistance value is inputted, and the set threshold value Im, automatically select the optimal resistance value from the resistance values that can be currently inputted by the neutral resistor for input, and close the line selection signal output contact to start the line selection function of the small-current grounding line selection device.

[0047] Step 3: If, through calculation, the calculated optimal resistance value exceeds the range of the resistance values that can be currently inputted by the neutral resistor, then automatically select the minimum value of the resistance values that can be currently inputted for input, and close the line selection signal output contact to start the line selection function of the small-current grounding line selection device.

[0048] In the above preferred embodiments, the purpose, technical solution and advantages of the present invention have been further described in detail. It should be understood that the above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0049] The above are the preferred embodiments of the present invention. Whenever changes are made according to the technical solution of the present invention and the functions and effects generated do not exceed the scope of the technical solution of the present invention, they shall fall within the protection scope of the present invention.

Claims

1. A parallel medium resistor intelligent switching device, characterized in that, it includes an intelligent controller and a medium resistor input circuit, a medium resistor self-checking circuit, a change-over switch SW, a voltage transformer for collecting the zero-sequence voltage component of the bus voltage, and a current transformer for collecting the zero-sequence current flowing through the medium resistor, which are connected to the intelligent controller; the intelligent controller includes a controller DC power supply contact point for accessing a 220V DC power supply as the DC power supply of the intelligent controller, a line selection signal output contact point for starting the line selection function of the small current grounding line selection device, a DC power supply output contact point for outputting a 24V DC power supply as the power supply of the medium resistor self-checking circuit, a zero-sequence voltage sampling contact point connected to the voltage transformer for collecting the zero-sequence voltage component of the bus voltage, a zero-sequence current sampling contact point connected to the current transformer for collecting the zero-sequence current flowing through the medium resistor, a change-over switch SW control contact point connected to the change-over switch SW for controlling the closing of different contacts of the change-over switch, and an electronic switch control contact point for controlling the opening and closing of the electronic switch based on IGBT; the medium resistor input circuit includes 6 electronic switches and 6 identical resistors with a resistance value of R each. Among them, R1, R2, and R3 are connected in series to form resistor string 1, and R4, R5, and R6 are connected in series to form resistor string 2. Resistor string 1 and resistor string 2 are connected in parallel to form the medium resistor; electronic switch 1 controls R1, R2, and R3 to be connected to the primary circuit; electronic switch 2 controls R2 and R3 to be connected to the primary circuit; electronic switch 3 controls R3 to be connected to the primary circuit; electronic switch 4 controls R4, R5, and R6 to be connected to the primary circuit; electronic switch 5 controls R5 and R6 to be connected to the primary circuit; electronic switch 6 controls R6 to be connected to the primary circuit; by controlling the on-off of the 6 electronic switches, the resistance value of the medium resistor connected to the primary circuit is controlled to be changed. The changeable resistance values are 3R, 2R, 1.5R, 1.2R, R, 0.75R, 0.67R, and 0.5R respectively; electronic switch 1, electronic switch 2, electronic switch 3, electronic switch 4, electronic switch 5, and electronic switch 6 are connected to the electronic switch control contact point; the controller DC power supply contact point, the 2-3 contact point of the change-over switch SW, the medium resistor, the current transformer, and the zero-sequence current sampling contact point constitute the medium resistor self-checking circuit; the medium resistor has three working states, namely the medium resistor input state, the medium resistor self-checking state, and the medium resistor cut-off state; by controlling the conduction of different contacts of the change-over switch SW, the medium resistor is in different working states; when the 1-3 contact of the change-over switch SW is switched to conduct, the medium resistor is connected to the primary circuit. At this time, the medium resistor is in the input state and bypasses the arc suppression coil; when the 2-3 contact of the change-over switch SW is switched to conduct, the medium resistor self-checking circuit is connected at this time and is in the self-checking state for detecting the performance of the medium resistor; when the 4-3 contact of the change-over switch SW is switched to conduct, the medium resistor is in the cut-off state at this time.

2. The parallel medium resistor intelligent switching device according to claim 1, characterized in that, the voltage transformer is connected to the zero-sequence voltage sampling contact point.

3. The parallel medium resistor intelligent switching device according to claim 1, characterized in that, The current transformer is connected to the zero-sequence current sampling contact point.

4. A control method for an intelligent switching device of a shunt medium resistor, characterized in that the device described in claim 1 is adopted, and the self-checking logic of the medium resistor self-checking circuit is as follows: Step S1: When the medium resistance self-checking loop is conducting, the medium resistance is in the self-checking state; First, close the electronic switch 1 to connect R1, R2, and R3 to the medium resistance self-checking loop; Measure whether the loop current is equal to U dc / 3R through the current transformer; If it is equal, it is considered that the three resistors R1, R2, and R3 are all intact; Open the electronic switch 1, skip steps S2 - S4, and go to step S5; Step S2: If the current measured by the current transformer in Step S1 is not equal to U dc / 3R, it indicates that there is a damage in one of the three resistors R1, R2, and R3. At this time, disconnect the electronic switch 1 and close the electronic switch 2 to connect R2 and R3 to the medium resistor self-checking circuit. Measure whether the loop current is equal to U dc / 2R through the current transformer. If it is equal, it is considered that R1 is damaged and the two resistors R2 and R3 are intact. In the future, when the medium resistor is put into operation, lock the electronic switch 1 and no longer put in the resistor R1. Disconnect the electronic switch 2, skip Steps S3 - S4, and go to Step S5; Step S3. If the current measured by the zero-sequence CT in Step S2 is not equal to U dc / 2R, it indicates that there is damage in the two resistors R2 and R3. At this time, disconnect the electronic switch 2 and close the electronic switch 3 to connect R3 to the medium resistor self-checking circuit. Measure whether the loop current is equal to U dc / R through the current transformer. If it is equal, it is considered that R2 is damaged and R3 is intact. In the future, when the medium resistor is put into operation, lock the electronic switch 1 and the electronic switch 2, and no longer put the resistors R1 and R2 into operation. Disconnect the electronic switch 3, skip Step S4, and go to Step S5; Step S4: If the current measured by the zero-sequence CT in Step S3 is not equal to U dc / R, it indicates that the R3 resistor is damaged; At this time, the electronic switch 3 is disconnected; in the future, when the medium resistor is put into operation, the electronic switches 1, 2 and 3 will be blocked, and the resistor string 1 will not be put into operation anymore; Step S5: Adopt steps S1 - S4 to check the quality of the three resistors R4, R5, and R6 in the resistor string 2 and block the corresponding electronic switches in the switching logic, and no longer put into the damaged resistor.

5. A control method for an intelligent switching device of a shunt medium resistor according to claim 4, characterized in that the intelligent controller collects the bus 3U0 voltage through the voltage transformer. When the collected zero-sequence voltage is greater than the threshold value Um, the medium resistor input logic will be started.

6. A control method for an intelligent switching device of a shunt medium resistor according to claim 5, characterized in that the medium resistor input logic is specifically as follows: (1). First, input the maximum resistance value Rmax that can be input by the medium resistor currently. The maximum resistance value Rmax has automatically recorded the resistance value that can be input by the medium resistor currently through the self-checking logic of the medium resistor self-checking circuit and sorted it, and measures the zero-sequence current passing through the medium resistor through the current transformer; when the zero-sequence current is greater than the set threshold value Im, close the line selection signal output contact point and start the line selection function of the small current grounding line selection device; (2). If the zero-sequence current measured by the current transformer in step (1) is less than the set threshold value Im; then the intelligent controller will automatically select the optimal resistance value from the resistance values that can be input by the medium resistor currently according to the maximum resistance value Rmax input, the zero-sequence current measured after the maximum resistance value is input, and the set threshold value Im, and close the line selection signal output contact point and start the line selection function of the small current grounding line selection device; (3). If through calculation, the calculated optimal resistance value exceeds the range of the resistance values that can be input by the medium resistor currently, then automatically select the minimum value of the resistance values that can be input currently and close the line selection signal output contact point and start the line selection function of the small current grounding line selection device.

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