Device and method for suppressing overvoltage caused by shell-contact grounding transfer on the high-voltage side of a distribution transformer

Abstract

The present invention belongs to the technical field of distribution networks, and provides a device and method for suppressing transfer overvoltage caused by shell-touching and grounding on the high-voltage side of a distribution transformer. The device adds a current-limiting resistor between the housing of the distribution transformer and the grounding grid. The current-limiting resistor and the current-limiting resistor switching contactor are connected in series and in parallel with the single-phase voltage transformer. The control logic circuit is used to compare the collected actual secondary voltage of the single-phase voltage transformer with the grounding fault alarm setting value. If it is greater than the setting value, a shell-touching and grounding fault occurs, a grounding fault alarm signal is sent, and the current-limiting resistor switching contactor is simultaneously activated to control the switching on and off of the current-limiting resistor and cut off the fault current; otherwise, the current-limiting resistor is controlled to exit. By adding a current-limiting resistor between the housing of the distribution transformer and the grounding grid, the present invention can effectively reduce transfer overvoltage and effectively prevent personal injury and equipment damage caused by transfer overvoltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution networks, and in particular relates to a device and method for suppressing overvoltage caused by shell grounding transfer on the high-voltage side of a distribution transformer. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Transfer overvoltage occurs when a grounding fault occurs on the high-voltage (10 kV) side of a distribution transformer. This creates a direct electrical connection between the high-voltage and low-voltage sides of the transformer through the grounding electrode. The fault current flowing through the grounding electrode significantly raises the neutral point potential on the low-voltage side of the distribution transformer. This voltage is then transferred through the neutral line to the outer casing of electrical equipment, causing a transfer overvoltage. This transfer overvoltage can cause personal injury, equipment damage, fire, and other accidents, posing a serious threat to the power supply security of the distribution system.

[0004] At present, the neutral point of the 10kV distribution system widely adopts an ungrounded or arc-suppression coil grounding system. If the capacitive current of a single-phase grounding fault is 10A or less, it is advisable to adopt an ungrounded neutral point method. If it exceeds 10A and is less than 100A~150A, it is advisable to adopt a neutral point grounding method through an arc-suppression coil. The residual current of the grounding fault after compensation should generally be controlled within 10A. Therefore, when a single-phase grounding occurs in the system, the current at the fault point is relatively small.

[0005] However, with improvements in distribution network technology and increasing demands for power supply reliability, more and more regions are piloting the use of low-resistance grounding of the neutral point, or through arc suppression coils and low-resistance grounding. When a single-phase ground fault occurs, the current flowing through the grounded point is much greater than that in an ungrounded system. With a neutral point resistance of 10Ω, the theoretical maximum fault current can reach 600A. This fault current flowing through the grounding resistor significantly raises the potential at the fault point. When a ground fault occurs in the form of a side-to-shell contact, the overvoltage is transmitted through the neutral line on the low-voltage side of the distribution transformer to the outer casing of the user's electrical equipment, generating a transferred overvoltage. Furthermore, the distribution transformer outer casing is directly connected to the grounding electrode (grid), sharing the same grounding electrode (grid) as the 400-volt working ground. This makes it impossible to suppress the transferred overvoltage, potentially causing personal injury and equipment damage. Summary of the Invention

[0006] In order to solve at least one technical problem existing in the above-mentioned background technology, the present invention provides a device for suppressing transfer overvoltage caused by shell grounding on the high-voltage side of a distribution transformer, which adds a current-limiting resistor between the distribution transformer casing and the grounding grid, thereby effectively reducing the transfer overvoltage.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A first aspect of the present invention provides a device for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer, comprising a current-limiting resistor, a current-limiting resistor switching contactor, a single-phase voltage transformer, and a control logic circuit;

[0009] Add a current limiting resistor between the distribution transformer casing and the grounding grid, the current limiting resistor and the current limiting resistor switching contactor are connected in series and in parallel with the single-phase voltage transformer;

[0010] The control logic circuit is used to compare the collected actual secondary voltage of the single-phase voltage transformer with the ground fault alarm setting value. If it is greater than the setting value, a shell ground fault occurs, and a ground fault alarm signal is sent. At the same time, the current limiting resistor switching contactor is activated to control the switching on of the current limiting resistor and cut off the fault current; otherwise, the current limiting resistor is controlled to be disconnected.

[0011] As an implementation method, the resistance of the current limiting resistor satisfies the calculation formula:

[0012]

[0013] Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R Tequ is the equivalent grounding resistance of the distribution transformer, U max is the transfer overvoltage allowable value, R0 is the small resistor value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

[0014] As an implementation manner, the ground fault alarm setting value is calculated based on the voltage drop across the current limiting resistor when a shell-touching ground fault occurs.

[0015] As an implementation manner, the calculation formula for the voltage drop across the current-limiting resistor when a shell-to-ground fault occurs is:

[0016]

[0017] Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R x is the resistance of the current limiting resistor, R Tequ is the equivalent grounding resistance of the distribution transformer, R0 is the small resistance value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

[0018] As an implementation method, the operating time of the current limiting resistor switching contactor satisfies the following conditions:

[0019] If it is necessary to cooperate with the upper protection of the distribution transformer, the action time should be longer than the action time of the upper protection by Δt;

[0020] If there is no need to coordinate with the upper level protection, the contactor will trip instantly without delay.

[0021] A second aspect of the present invention provides a method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer, comprising the following steps:

[0022] Get the secondary voltage across the current limiting resistor;

[0023] Compare the secondary voltage with the ground fault alarm setting value. If it is greater than the setting value, a shell ground fault occurs and a ground fault alarm signal is sent. At the same time, the current limiting resistor switching contactor is activated to control the current limiting resistor to cut off the fault current; otherwise, the current limiting resistor is controlled to be disconnected.

[0024] As an implementation method, the resistance of the current limiting resistor satisfies the calculation formula:

[0025]

[0026] Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R Tequ is the equivalent grounding resistance of the distribution transformer, U max is the transfer overvoltage allowable value, R0 is the small resistor value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

[0027] As an implementation manner, the ground fault alarm setting value is calculated based on the voltage drop across the current limiting resistor when a shell-touching ground fault occurs.

[0028] As an implementation manner, the calculation formula for the voltage drop across the current-limiting resistor when a shell-to-ground fault occurs is:

[0029]

[0030] Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R x is the resistance of the current limiting resistor, R Tequ is the equivalent grounding resistance of the distribution transformer, R0 is the small resistance value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

[0031] As an implementation method, the operating time of the current limiting resistor switching contactor satisfies the following conditions:

[0032] If it is necessary to cooperate with the upper protection of the distribution transformer, the action time should be longer than the action time of the upper protection by Δt;

[0033] If there is no need to coordinate with the upper level protection, the contactor will trip instantly without delay.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention adds a current-limiting resistor between the distribution transformer casing and the grounding grid, and provides a method for suppressing the overvoltage transferred to the low-voltage side when a single-phase grounding fault occurs on the high-voltage side of the distribution transformer through the casing. This method can effectively reduce the transferred overvoltage and solve the problem that the distribution transformer casing is directly connected to the grounding electrode (grid) and shares the grounding electrode (grid) with the 400-volt side working ground, making it impossible to suppress the transferred overvoltage.

[0036] This invention accurately calculates the fault current when a single-phase ground fault occurs on the high-voltage side of a 10kV distribution transformer, as well as the transferred overvoltage transmitted to the enclosure of electrical equipment via the neutral line on the 400V side. Furthermore, the alarm and control circuit detects the enclosure ground fault, sends an alarm signal, and activates its protective function to disconnect the ground fault, effectively preventing personal injury and equipment damage caused by transferred overvoltage.

[0037] Advantages of additional aspects of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0039] Figure 1 This is a schematic diagram of a transfer overvoltage suppression device according to an embodiment of the present invention;

[0040] Figure 2 This is the installation position and primary wiring diagram of the transfer overvoltage suppression device in an embodiment of the present invention. DETAILED DESCRIPTION

[0041] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0042] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0043] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0044] In the present invention, terms such as "connected" and "connection" should be interpreted broadly to mean a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediary. Relevant researchers or technicians in this field may determine the specific meanings of these terms in the present invention based on specific circumstances, and they should not be construed as limitations on the present invention.

[0045] Explanation of terms

[0046] The 10kV distribution transformer adopts a connection method in which the safety ground and the working ground share a common grounding electrode (grounding grid). Safety grounding means that the outer casing of the distribution transformer is directly connected to the grounding electrode without passing through any device or equipment. Working grounding means that the low-voltage (400V) side of the distribution transformer is directly connected to the grounding electrode.

[0047] Example 1

[0048] Figure 1 This is a schematic diagram of a transfer overvoltage suppression device. This embodiment provides a transfer overvoltage suppression device for a high-voltage side shell grounding of a distribution transformer, including a current limiting resistor, a current limiting resistor switching contactor, a unidirectional voltage transformer, and a control logic circuit.

[0049] One end of the current limiting resistor is connected to the housing of the distribution transformer, and the other end is connected to the neutral point of the low-voltage side of the distribution transformer;

[0050] A unidirectional voltage transformer is connected in parallel at both ends of the current-limiting resistor, and the current-limiting resistor and the current-limiting resistor switching contactor are connected in series;

[0051] The control logic circuit is used to compare the collected actual secondary voltage of the single-phase voltage transformer with the ground fault alarm setting value. If it is greater than the setting value, a shell ground fault occurs, and a ground fault alarm signal is sent. At the same time, the current limiting resistor switching contactor is activated to control the switching on of the current limiting resistor and cut off the fault current; otherwise, the current limiting resistor is controlled to be disconnected.

[0052] Figure 2 To transfer the overvoltage suppression device installation location and primary wiring diagram, such as Figure 2As shown, when installing the transfer overvoltage suppression device, first remove all the connections between the original distribution transformer casing and the grounding grid, and connect the transfer overvoltage suppression device in series between the distribution transformer casing and the grounding grid through cables.

[0053] The advantage of the above scheme is that by adding a current-limiting resistor between the distribution transformer casing and the grounding grid, the transfer overvoltage can be effectively reduced, solving the problem that the distribution transformer casing is directly connected to the grounding electrode (grid) and shares the grounding electrode (grid) with the 400-volt side working ground, which makes it impossible to suppress the transfer overvoltage.

[0054] As one or more embodiments, the invention is described with specific implementations.

[0055] Among them, the current limiting resistor R x Its main functions are: first, to suppress the fault current when a ground short circuit fault occurs; second, to share the fault voltage so that the voltage borne by the grounding electrode is as small as possible to reduce the transferred overvoltage.

[0056] The calculation formula satisfied by the resistance of the current limiting resistor is:

[0057]

[0058] Among them, U N is the rated voltage of the high voltage system of the distribution transformer, which is 10 kV in this embodiment, R Tequ is the equivalent grounding resistance of the distribution transformer, U max is the transfer overvoltage allowable value, R0 is the small resistor value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

[0059] For example, if U N Take 6000 volts and ignore the ground impedance Z T0 , line impedance Z l According to GB / T 16895.10-2010 "Electrical Installations Part 4-44: Safety Protection against Voltage Disturbance and Electromagnetic Disturbance", the transfer overvoltage U max The long-term (10s) allowable value is approximately no more than 80V. The calculated minimum value of the current-limiting resistor for different distribution transformer equivalent grounding resistance values ​​is shown in Table 1 below. Therefore, a 290 ohm resistor can meet all operating conditions.

[0060] Table 1 Minimum current limiting resistance under different equivalent grounding resistance values ​​of distribution transformers

[0061] Serial number <![CDATA[R Tequ (Regulations stipulate that the resistance should not exceed 4 ohms)]]> <![CDATA[R x (Ohm)]]> 1 0.5 27 2 1 65 3 1.5 102 4 2 140 5 2.5 177 6 3 215 7 3.5 252 8 4 290

[0062] A single-phase voltage transformer (PT) converts the voltage across a current-limiting resistor into a low-voltage secondary voltage for use in measurement and control circuits. The PT's rated primary voltage matches the phase voltage on the high-voltage side of the distribution transformer (e.g., a 10 kV system can use a PT with a rated primary voltage of 6 kV), and its rated secondary voltage is 100 V.

[0063] Current limiting resistor switching contactor K x It is used to control the switching on and off of the current limiting resistor and cut off the fault current. After the current limiting resistor is connected, the fault current is limited to a smaller range, and the contactor can meet the switching function. The rated voltage of the contactor is not less than the rated phase voltage of the system.

[0064] The local alarm light is used to indicate a fault when the shell is grounded.

[0065] The control logic circuit mainly includes the following functions:

[0066] (1) Calculate the ground fault current I f , as shown below:

[0067]

[0068] Among them, U2 is the PT secondary side voltage collected by the control circuit, n PT is the PT ratio.

[0069] (2) The ground fault alarm setting value is calculated based on the voltage drop across the current limiting resistor when a shell-touching ground fault occurs.

[0070]

[0071] In this embodiment, considering the influence of excessive resistance, the alarm setting value should be set to 0.83U based on the calculation of 1.2 to 2 times sensitivity when a ground fault occurs. 2C ~0.5U 2C .

[0072] (3) Ground fault alarm logic: When the actual PT secondary voltage U2 collected by the control circuit is greater than the set value, it proves that a shell ground fault has occurred. The control circuit instantly sends a ground alarm signal and uploads it wirelessly to the substation or dispatching control center to which the distribution transformer belongs.

[0073] (4) Ground fault protection logic: When the actual PT secondary voltage U2 collected by the control circuit is greater than the set value, it proves that a ground fault has occurred, and the protection logic operates to trip the contactor K x , the action time should meet the following requirements: if it needs to be coordinated with the upper protection of the distribution transformer, the action time should be longer than the action time of the upper protection by Δt; if it does not need to be coordinated with the upper protection, the contactor will trip instantly without delay.

[0074] It should be noted that in this embodiment, Δt is set to 0.2s. Those skilled in the art can set it according to specific working conditions, which is not described in detail here.

[0075] The advantages of this solution are that it can accurately calculate the fault current when a single-phase ground fault occurs through the casing on the high-voltage side of a 10kV distribution transformer, as well as the transferred overvoltage transmitted to the casing of the electrical equipment through the neutral conductor on the 400V side. Furthermore, the alarm and control circuit can detect the casing ground fault, send an alarm signal, and activate its protective function to disconnect the ground fault, effectively preventing personal injury and equipment damage caused by transferred overvoltage.

[0076] Example 2

[0077] This embodiment provides a method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer, including the following steps:

[0078] Get the secondary voltage across the current limiting resistor;

[0079] Compare the secondary voltage with the ground fault alarm setting value. If it is greater than the setting value, a shell ground fault occurs and a ground fault alarm signal is sent. At the same time, the current limiting resistor switching contactor is activated to control the current limiting resistor to cut off the fault current; otherwise, the current limiting resistor is controlled to be disconnected.

[0080] As one or more embodiments, specific implementation methods are described below.

[0081] Step 1: Determine the neutral point grounding method of the substation's 10kV busbar, obtain the substation's neutral point grounding device and 10kV line parameters;

[0082] Specifically include:

[0083] (1) Neutral point grounding system with small resistance: Get the small resistance value R0, grounding impedance Z T0 .

[0084] (2) Obtain the line impedance Z between the 10 kV outgoing line start and the distribution transformer l .

[0085] Step 2: Measure the equivalent grounding resistance of the distribution transformer, including:

[0086] (1) Confirm the grounding method of the distribution transformer safety ground and the working ground, and clarify the grounding method of the distribution transformer low-voltage system.

[0087] (2) Find the grounding electrode of the distribution transformer and process the above-ground part to expose the metal part. The exposed area should be large enough to be clamped by the grounding electrode clamp of the ground resistance tester.

[0088] (3) Use a ground resistance tester to measure the equivalent ground resistance R of the distribution transformer Tequ .

[0089] Step 3: Design of transfer overvoltage suppression device. The principle of the device is as follows: Figure 1 As shown, it mainly includes the following parts:

[0090] (1) Current limiting resistor R x Its main functions are: first, to suppress the fault current when a ground short circuit fault occurs; second, to distribute the fault voltage so that the voltage on the grounding electrode is as small as possible to reduce the transferred overvoltage. The selection of the current limiting resistor value should meet the following requirements:

[0091]

[0092] Among them, U N is the rated voltage of the high voltage system of the distribution transformer, which is 10 kV in this embodiment, and U max It is the allowable value of transfer overvoltage.

[0093] (2) Single-phase voltage transformer (PT), whose main function is to convert the voltage across the current-limiting resistor into a secondary low voltage for use in measurement and control circuits. The rated primary voltage of the PT is consistent with the phase voltage on the high-voltage side of the distribution transformer (for example, a PT with a rated primary voltage of 6 kV can be selected for a 10 kV system), and the rated secondary voltage is 100 V.

[0094] (3) Current limiting resistor switching contactor K x Its main function is to control the switching on and off of the current limiting resistor and cut off the fault current. After the current limiting resistor is connected, the fault current is limited to a smaller range, and the contactor can meet the switching function. The rated voltage of the contactor is not less than the rated phase voltage of the system.

[0095] (4) Local alarm light, used to indicate fault when the shell touches the ground.

[0096] (5) Control logic circuit, mainly including the following functions:

[0097] 1) Calculate the ground fault current as shown below:

[0098]

[0099] Among them, U2 is the PT secondary side voltage collected by the control circuit, n PT is the PT ratio.

[0100] 2) Setting of ground fault alarm value: Calculate the voltage drop U on the current limiting resistor when a ground fault occurs. 2C .

[0101]

[0102] Taking into account the influence of excessive resistance, the alarm setting value should be set to 0.83U based on the 1.2 to 2 times sensitivity when a ground fault occurs. 2C ~0.5U 2C .

[0103] 3) Ground fault alarm logic: When the actual PT secondary voltage U2 collected by the control circuit is greater than the set value, it proves that a shell ground fault has occurred. The control circuit instantly sends a ground alarm signal and uploads it wirelessly to the substation or dispatching control center to which the distribution transformer belongs.

[0104] 4) Ground fault protection logic: When the actual PT secondary voltage U2 collected by the control circuit is greater than the set value, it proves that a ground fault has occurred, and the protection logic acts to trip the contactor K x The action time should meet the following requirements: If it is necessary to coordinate with the upper protection of the distribution transformer, the action time should be longer than the action time of the upper protection by Δt. It is recommended that Δt be 0.2s. If it is not necessary to coordinate with the upper protection, the contactor will trip instantly without delay.

[0105] The advantages of this solution are that it can accurately calculate the fault current when a single-phase ground fault occurs through the casing on the high-voltage side of a 10kV distribution transformer, as well as the transferred overvoltage transmitted to the casing of the electrical equipment through the neutral conductor on the 400V side. Furthermore, the alarm and control circuit can detect the casing ground fault, send an alarm signal, and activate its protective function to disconnect the ground fault, effectively preventing personal injury and equipment damage caused by transferred overvoltage.

[0106] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. The high-voltage side shell grounding transfer overvoltage suppression device of the distribution transformer is characterized by: Including current limiting resistor, current limiting resistor switching contactor, single-phase voltage transformer and control logic circuit; Add a current limiting resistor between the distribution transformer casing and the grounding grid, the current limiting resistor and the current limiting resistor switching contactor are connected in series and in parallel with the single-phase voltage transformer; The control logic circuit is used to compare the collected actual secondary voltage of the single-phase voltage transformer with the ground fault alarm setting value. If it is greater than the setting value, a shell ground fault occurs, and a ground fault alarm signal is sent. At the same time, the current limiting resistor switching contactor is activated to control the withdrawal of the current limiting resistor and cut off the fault current; otherwise, the current limiting resistor is controlled to be switched on.

2. The device for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 1, characterized in that: The calculation formula satisfied by the resistance of the current limiting resistor is: Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R Tequ is the equivalent grounding resistance of the distribution transformer, U max is the transfer overvoltage allowable value, R0 is the small resistor value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

3. The device for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 1, characterized in that: The ground fault alarm setting value is calculated based on the voltage drop across the current limiting resistor when a shell-touching ground fault occurs.

4. The device for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 3, characterized in that: The calculation formula for the voltage drop across the current limiting resistor when a shell-to-ground fault occurs is: Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R x is the resistance of the current limiting resistor, R Tequ is the equivalent grounding resistance of the distribution transformer, R0 is the small resistance value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

5. The device for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 1, characterized in that: The conditions for the action time of the current limiting resistor switching contactor to be satisfied are: If it is necessary to cooperate with the upper protection of the distribution transformer, the action time should be longer than the action time of the upper protection by Δt; If there is no need to coordinate with the upper level protection, the contactor will trip instantly without delay.

6. A method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer, characterized in that: The device for suppressing overvoltage transfer caused by shell grounding on the high-voltage side of a distribution transformer as claimed in any one of claims 1 to 5 comprises the following steps: Get the secondary voltage across the current limiting resistor; Compare the secondary voltage with the ground fault alarm setting value. If it is greater than the setting value, a shell ground fault occurs and a ground fault alarm signal is sent. At the same time, the current limiting resistor switching contactor is activated to control the withdrawal of the current limiting resistor and cut off the fault current; otherwise, the current limiting resistor is controlled to be switched on.

7. The method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 6, characterized in that: The calculation formula satisfied by the resistance of the current limiting resistor is: Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R Tequ is the equivalent grounding resistance of the distribution transformer, U max is the transfer overvoltage allowable value, R0 is the small resistor value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

8. The method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 6, wherein: The ground fault alarm setting value is calculated based on the voltage drop across the current limiting resistor when a shell-touching ground fault occurs.

9. The method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 8, characterized in that: The calculation formula for the voltage drop across the current limiting resistor when a shell-to-ground fault occurs is: Among them, U N is the rated voltage of the high voltage system of the distribution transformer, R x is the resistance of the current limiting resistor, R Tequ is the equivalent grounding resistance of the distribution transformer, R0 is the small resistance value, Z T0 is the grounding impedance, Z l It is the line impedance between the starting point of the high-voltage outgoing line of the distribution transformer and the distribution transformer.

10. The method for suppressing overvoltage caused by shell grounding on the high-voltage side of a distribution transformer according to claim 6, characterized in that: The conditions for the action time of the current limiting resistor switching contactor to be satisfied are: If it is necessary to cooperate with the upper protection of the distribution transformer, the action time should be longer than the action time of the upper protection by Δt; If there is no need to coordinate with the upper level protection, the contactor will trip instantly without delay.

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