A microgrid current and voltage protection method and system employing grid-side short-circuit current constraint.

Abstract

This invention discloses a microgrid current and voltage protection method and system employing grid-side short-circuit current constraints. This method is applicable to microgrids containing a high proportion of renewable energy sources. By equipping the connection point (PCC point) between the microgrid and the main grid with a current-limiting power electronic switch, the method ensures that the main grid provides a certain short-circuit current to the microgrid while also protecting the operational safety of the power electronic equipment within the microgrid. This microgrid current and voltage protection method, through the use of a current-limiting power electronic switch at the PCC point, can constrain the short-circuit current from the main grid, meeting the requirements of relay protection characteristics while also ensuring the safety of the power electronic devices within the microgrid.

Description

Technical Field

[0001] This invention relates to a microgrid current and voltage protection method and system that employs grid-side short-circuit current constraints, belonging to the field of microgrid relay protection. Background Technology

[0002] Microgrids differ significantly from large power grids in network structure, output characteristics, and spatial features, making it difficult to directly apply traditional protection methods used in large power grids. When a short-circuit fault occurs within a microgrid, existing protection schemes can be broadly categorized into two types. One is to maintain the microgrid in grid-connected mode, i.e., keeping the microgrid connected to the large power grid. This involves selectively identifying faults within the microgrid by utilizing short-circuit currents from the large power grid that exhibit clear fault characteristics. However, sufficiently high short-circuit currents from the large power grid can pose a significant threat to the operational safety of power electronic equipment within the microgrid.

[0003] Another approach is to keep it in islanded mode, i.e., disconnect the microgrid from the main grid. Because the large number of renewable energy sources contained within a microgrid usually maintain limited output after a fault, typically 1.2-1.5 times the rated current, the limited short-circuit current can ensure the safe operation of the power electronic equipment within the microgrid, but it cannot provide sufficiently obvious fault characteristic quantities for the operation of relay protection.

[0004] In addition, the small size of microgrids results in shorter electrical distances between electrical devices, leading to smaller variations in short-circuit currents under fault conditions at different points on the line. Therefore, current protection methods widely used in large power grids will be difficult to apply to microgrids.

[0005] Therefore, the significant differences between microgrids and large power grids in terms of network morphology, operation mode, and spatial characteristics make it difficult to directly apply traditional protection methods used in large power grids in microgrids. Designing a relay protection method specifically for microgrids that takes into account the fault characteristics of microgrid relay protection, the operational safety of internal power electronic equipment, and the unique features of microgrids is a pressing problem that needs to be solved in the field of microgrid relay protection. Summary of the Invention

[0006] The purpose of this invention is to provide a microgrid current and voltage protection method and system that adopts grid-side short-circuit current constraint. By configuring a current-limiting power electronic switch at the connection between the large power grid and the microgrid, and referring to the traditional three-stage overcurrent protection method, a three-stage undervoltage protection method with current-driven activation is equipped inside the microgrid. This can achieve both selective fault clearing inside the microgrid and safe operation of the microgrid, effectively improving the reliability of regional power supply.

[0007] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0008] In a first aspect, the present invention provides a microgrid current and voltage protection method employing grid-side short-circuit current constraint, based on a current-limiting power electronic switch equipped at the connection point between the microgrid and the main grid, comprising:

[0009] Collect the current at the connection point between the microgrid and the main power grid;

[0010] If the current at the connection point between the microgrid and the main grid is less than the limit value, the current-limiting power electronic switch is in a zero-impedance state and does not operate.

[0011] If the current at the connection point between the microgrid and the main grid exceeds the limit, the current-limiting power electronic switch will be in a high-impedance state and enter the current-limiting state to constrain the short-circuit current from the grid side.

[0012] Furthermore, the limiting value is the maximum short-circuit current value restricted by the current-limiting power electronic switch set at the PCC point, which is taken as 3-5 times the line rated current.

[0013] Furthermore, the method also includes:

[0014] The microgrid is equipped with three-stage undervoltage protection.

[0015] Furthermore, the microgrid is equipped with three-stage undervoltage protection, including:

[0016] Perform three-stage undervoltage protection setting;

[0017] Based on the adjusted three-stage undervoltage protection settings, three-stage undervoltage protection is implemented.

[0018] Furthermore, the method for setting the three-stage undervoltage protection settings is as follows:

[0019] The operating voltage value of the undervoltage protection stage I of protection 1 must be less than the residual voltage at bus A when a short circuit occurs on bus B. Therefore, the operating setting value of the undervoltage protection stage I of protection 1 is... :

[0020]

[0021] In the formula, This represents the normal current supplied from the main power grid to the microgrid before the fault occurred; This indicates the normal current flowing through protection 1 before the fault; This indicates the limiting factor of the current-limiting power electronic switch set at the PCC point; This indicates the limiting factor of the new energy power supply connected to bus A after the fault; The impedance of the line between busbars A and B is the total length of the line. To determine the reliability coordination factor for the low voltage stage I protection;

[0022] If the undervoltage protection stage II of protection 1 is coordinated with the undervoltage protection stage I of the next line, then the operating setting value of the undervoltage protection stage II of protection 1 is... for:

[0023]

[0024] In the formula, To determine the reliability coordination factor for the protection of the low voltage stage II; This is the setting value for the low voltage protection stage I of the next line protection 2.

[0025] The undervoltage stage III protection setting for protection 1, for overload conditions, must be less than the residual voltage at bus A under maximum load. Therefore, the undervoltage stage III protection setting value for protection 1 is... for:

[0026]

[0027] In the formula, To determine the reliability coordination factor for the low voltage stage III protection; This refers to the potential overload current on the line. The impedance of the line between busbars B and C is the total length of the line. The impedance is the total length of the line between busbars C and D.

[0028] Furthermore,

[0029] Take 3-5;

[0030] Take 1.2-1.5;

[0031] Take 0.8-0.9;

[0032] Take 0.8-0.9;

[0033] Take 0.8-0.9.

[0034] Furthermore, based on the adjusted three-stage undervoltage protection settings, three-stage undervoltage protection is implemented, including:

[0035] Step A: Measure the current at the protection installation point and obtain the current measurement value;

[0036] Step B: If the measured current value is greater than the protection activation value, measure the residual voltage of the bus; otherwise, proceed to step A.

[0037] Step C: If the residual voltage of the bus is less than the operating setting value of the low voltage I stage protection of this line, then the short circuit fault is located within the low voltage I stage protection range of this line, and the corresponding protection will operate; otherwise, the low voltage I stage protection of this line will not operate, and the process will proceed to step D.

[0038] Step D: If the residual voltage of the bus is less than the setting value of the low voltage stage II protection of this line, the low voltage stage II protection of this line will start timing; if the residual voltage of the bus rises to near the rated value before the timing time arrives, jump to step A; if the residual voltage of the bus remains less than the setting value of the low voltage stage II protection of this line after the timing time arrives, the fault will be cleared by the low voltage stage II protection of this line.

[0039] Step E: If the residual voltage of the busbar is less than the setting value of the low voltage stage III protection, start timing; if the residual voltage of the busbar is still low after the timing time is reached, trip; if the residual voltage of the busbar is greater than the setting value of the low voltage stage III protection before the timing time is reached, it means that the fault or overload has disappeared, then return to step A.

[0040] Furthermore, the protection activation value is set to 1.2-1.5 times the line rated current.

[0041] Furthermore, the delay time of stage III is shorter than that of stage II, and the delay time of stage II is shorter than that of stage I, thus enabling sequential selective operation of undervoltage protection for different lines.

[0042] In a second aspect, the present invention provides a microgrid current and voltage protection system employing grid-side short-circuit current constraint, comprising:

[0043] Current-limiting power electronic switches are equipped at the junction of microgrids and main grids;

[0044] The current-limiting power electronic switch is used to collect the current at the connection between the microgrid and the main grid. If the current at the connection between the microgrid and the main grid is less than the limit value, the current-limiting power electronic switch is in a zero-impedance state and does not operate. If the current at the connection between the microgrid and the main grid is greater than the limit value, the current-limiting power electronic switch is in a high-impedance state and enters the current-limiting state to constrain the short-circuit current from the grid side.

[0045] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0046] 1. The present invention proposes a microgrid current and voltage protection method and system that uses grid-side short-circuit current constraint. By equipping the connection between the microgrid and the main grid with a current-limiting power electronic switch, the current-limiting power electronic switch is in a zero-impedance state under normal conditions so as not to affect the normal power exchange. Under fault conditions, it is in a high-impedance state to achieve the purpose of constraining the short-circuit current from the grid side. It can effectively take into account both the operational safety of power electronic equipment inside the microgrid and the relay protection characteristic quantity requirements of the microgrid.

[0047] 2. This invention addresses microgrids containing a large number of renewable energy sources by proposing a microgrid current and voltage protection method that employs grid-side short-circuit current constraints. By referencing the traditional three-stage overcurrent protection method, a three-stage undervoltage protection method is incorporated within the microgrid. This method effectively improves protection performance by utilizing the relatively short electrical distances between electrical devices within the microgrid and employing significantly varying voltage values ​​as relay protection characteristic quantities.

[0048] 3. The microgrid current and voltage protection method with grid-side short-circuit current constraint proposed in this invention is designed based on the inverter component contained in all new energy power sources. It can be widely used in microgrids containing different types of new energy sources and has high versatility.

[0049] 4. The microgrid current and voltage protection method with grid-side short-circuit current constraint proposed in this invention is applicable to microgrids with a high proportion of new energy sources. It has important economic and social significance for ensuring the reliability of power supply in remote areas (such as islands and remote mountainous areas) that rely on microgrids for power supply. Attached Figure Description

[0050] Figure 1 This is a schematic diagram illustrating the principle of a microgrid current and voltage protection method and system that employs grid-side short-circuit current constraints.

[0051] Figure 2 This is a flowchart illustrating the operation of a microgrid current and voltage protection method and system employing grid-side short-circuit current constraints. Detailed Implementation

[0052] The present invention will be further explained in detail below with reference to the accompanying drawings and specific embodiments, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0053] Example 1:

[0054] This embodiment provides a microgrid current and voltage protection method using grid-side short-circuit current constraints, the principle of which is illustrated in the figure below. Figure 1As shown, microgrids contain a high proportion of renewable energy sources. Due to their internal self-control characteristics, their output exhibits significant nonlinear limiting characteristics after a fault. Typically, microgrids operate in two modes: grid-connected and islanded. When a line short-circuit fault occurs within a microgrid, if the microgrid is in grid-connected mode, the high-amplitude short-circuit current from the main grid can meet the relay protection requirements, but it can easily damage internal power electronic devices. However, when the microgrid is in islanded mode, the short-circuit current is provided by internal renewable energy sources, with a smaller amplitude (typically only 1.2-1.5 times the rated current). This ensures the safety of power electronic devices, but cannot provide sufficient fault characteristic quantities to meet relay protection requirements.

[0055] This invention selects a current-limiting power electronic switch at the connection point (PCC point) between the microgrid and the main grid. By limiting the short-circuit current from the main grid, it achieves a balance between the relay protection characteristic requirements of the microgrid and the operational safety of the internal power electronic equipment. The current-limiting power electronic switch is in a zero-impedance state under normal conditions (so as not to affect the normal power exchange), and in a high-impedance state during a fault (so as to constrain the short-circuit current from the grid side).

[0056] Microgrids are generally small in scale, resulting in short electrical distances between internal devices. Short-circuit current variations under line faults at different points are small, making it difficult to ensure selective protection operation. However, the residual bus voltage varies significantly under line faults at different points, effectively distinguishing fault locations. This invention, referencing the traditional three-stage overcurrent protection method, incorporates a three-stage undervoltage protection method within the microgrid. Simultaneously, an increased current is used as the protection activation value; for reliable activation, the activation value can be set to 1.2-1.5 times the line rated current.

[0057] This invention provides a microgrid current and voltage protection method and system that uses grid-side short-circuit current constraint. The feature is that a current-limiting power electronic switch is equipped at the connection point (PCC point) between the microgrid and the main grid to constrain the short-circuit current from the main grid.

[0058] Specifically, considering the needs of microgrid relay protection characteristics and the safety of power electronic equipment inside the microgrid, the limiting factor of the current-limiting power electronic switch is set to 3-5 times the rated line current.

[0059] Specifically, referring to the three-stage overcurrent protection, a three-stage undervoltage protection is equipped inside the microgrid.

[0060] Specifically, the increased short-circuit current after a short circuit is used as the starting value for the three-stage undervoltage protection.

[0061] Specifically, to ensure reliable protection activation, the activation value can be set to 1.2-1.5 times the line rated current.

[0062] Specifically, the setting method for the three-stage undervoltage protection of microgrids is as follows:

[0063] As attached Figure 1 As shown, taking protection 1 as an example, the operating voltage value of the low voltage protection stage I of protection 1 must be less than the residual voltage at bus A when a short circuit occurs on bus B. Therefore, the operating setting value of the low voltage protection stage I of protection 1 is... :

[0064]

[0065] In the formula, This represents the normal current supplied from the main power grid to the microgrid before the fault occurred; This indicates the normal current flowing through protection 1 before the fault; This indicates the limiting factor of the current-limiting power electronic switch set at the PCC point, which can be 3-5 (recommended value, dynamically adjusted as needed in engineering applications). This indicates the limiting factor for new energy power supplies connected to bus A after a fault, which can generally be taken as 1.2-1.5; The impedance of the line between busbars A and B is the total length of the line. To ensure the reliability coordination factor of the low voltage stage I protection, a value of 0.8-0.9 can be used.

[0066] If the undervoltage protection stage II of protection 1 is coordinated with the undervoltage protection stage I of the next line, then the operating setting value of the undervoltage protection stage II of protection 1 is... for:

[0067]

[0068] In the formula, To ensure the reliability coordination factor of the protection for the low voltage stage II, a value of 0.8-0.9 can be used. This is the setting value for the low voltage protection stage I of the next line protection 2.

[0069] The undervoltage stage III protection setting for protection 1, for overload conditions, must be less than the residual voltage at bus A under maximum load. Therefore, the undervoltage stage III protection setting value for protection 1 is... for:

[0070]

[0071] In the formula, To ensure the reliability coordination factor of the low voltage stage III protection, a value of 0.8-0.9 can be used. This refers to the potential overload current on the line. The impedance of the line between busbars B and C is the total length of the line. The impedance is the total length of the line between busbars C and D.

[0072] Specifically, after the protection is activated, the bus voltage is detected. When the residual bus voltage is less than the setting value of the undervoltage protection, each protection section will selectively operate after a corresponding delay.

[0073] Specifically, the three-stage undervoltage protection operation includes the following process:

[0074] If the residual voltage of the busbar is less than the operating setting value of the low voltage I stage protection of this line, then the short circuit fault is within the low voltage I stage protection range of this line, and the corresponding protection will operate; otherwise, the low voltage I stage protection of this line will not operate.

[0075] If the residual voltage of the busbar is less than the setting value of the low voltage stage II protection of this line, the low voltage stage II protection of this line will start timing. If the residual voltage of the busbar rises to near the rated value before the timing time arrives, it indicates that the fault occurred on the next line and has been cleared by the low voltage stage I protection of the next line. If the residual voltage of the busbar remains less than the setting value of the low voltage stage II protection of this line after the timing time arrives, it indicates that the fault occurred on this line or the protection of the next line failed to operate. Therefore, the fault is cleared by the low voltage stage II protection of this line, realizing the full line protection of this line and the backup protection of the next line outlet.

[0076] The low voltage stage I protection and low voltage stage II protection of all the above lines constitute a complete protection range in sequence.

[0077] In addition, low-voltage stage III protection needs to be set. If the residual voltage of the busbar is less than the low-voltage stage III protection setting, the timer starts; if the residual voltage of the busbar is still low after the timer expires, it indicates that the fault or overload still exists, so the circuit breaker trips; if the residual voltage of the busbar is greater than the low-voltage stage III protection setting before the timer expires, it indicates that the fault or overload has disappeared, so the circuit breaker returns to normal. Furthermore, due to the strong nonlinearity of the power supply, the low-voltage stage III protection setting values ​​of each line are not significantly different. In this case, different delay times can be adopted, that is, the delay time of the next line is less than the delay time of the previous line by one time interval. This enables sequential selective operation of the three-stage protection for low voltage on different lines.

[0078] Referring to three-stage overcurrent protection, a three-stage undervoltage protection system is installed within the microgrid. An increased short-circuit current is used as the activation value for the microgrid's undervoltage protection. After activation, the three stages of undervoltage protection work in sequence with appropriate time delays to selectively clear faults. This three-stage undervoltage protection system, referencing three-stage overcurrent protection, effectively addresses the short electrical distances between devices within the microgrid, ensuring the safe operation of the microgrid and reliable power supply to the region.

[0079] Example 2:

[0080] This embodiment provides a microgrid current and voltage protection system employing grid-side short-circuit current constraints, including:

[0081] A current-limiting power electronic switch is installed at the connection between the microgrid and the main grid. The current-limiting power electronic switch is in a zero-impedance state under normal conditions (so as not to affect the normal exchange of power) and in a high-impedance state under fault conditions (so as to constrain the short-circuit current from the grid side).

[0082] The current-limiting power electronic switch is used to collect the current at the connection between the microgrid and the main grid. If the current at the connection between the microgrid and the main grid is less than the limit value, the current-limiting power electronic switch is in a zero-impedance state and does not operate. If the current at the connection between the microgrid and the main grid is greater than the limit value, the current-limiting power electronic switch is in a high-impedance state and enters the current-limiting state to constrain the short-circuit current from the grid side.

[0083] This invention selects to configure a current-limiting power electronic switch at the connection point (PCC point) between the microgrid and the main grid. By limiting the short-circuit current from the main grid, it can achieve a balance between the relay protection characteristic requirements of the microgrid and the operational safety of the internal power electronic equipment.

[0084] The system also includes a three-stage undervoltage protection system within the microgrid, referencing a three-stage overcurrent protection system.

[0085] Specifically, the three-stage undervoltage protection operation includes the following process:

[0086] If the residual voltage of the busbar is less than the operating setting value of the low voltage I stage protection of this line, then the short circuit fault is within the low voltage I stage protection range of this line, and the corresponding protection will operate; otherwise, the low voltage I stage protection of this line will not operate.

[0087] If the residual voltage of the busbar is less than the setting value of the low voltage stage II protection of this line, the low voltage stage II protection of this line will start timing. If the residual voltage of the busbar rises to near the rated value before the timing time arrives, it indicates that the fault occurred on the next line and has been cleared by the low voltage stage I protection of the next line. If the residual voltage of the busbar remains less than the setting value of the low voltage stage II protection of this line after the timing time arrives, it indicates that the fault occurred on this line or the protection of the next line failed to operate. Therefore, the fault is cleared by the low voltage stage II protection of this line, realizing the full line protection of this line and the backup protection of the next line outlet.

[0088] The low voltage stage I protection and low voltage stage II protection of all the above lines constitute a complete protection range in sequence.

[0089] In addition, low-voltage stage III protection needs to be set. If the residual voltage of the busbar is less than the low-voltage stage III protection setting, the timer starts; if the residual voltage of the busbar is still low after the timer expires, it indicates that the fault or overload still exists, so the circuit breaker trips; if the residual voltage of the busbar is greater than the low-voltage stage III protection setting before the timer expires, it indicates that the fault or overload has disappeared, so the circuit breaker returns to normal. Furthermore, due to the strong nonlinearity of the power supply, the low-voltage stage III protection setting values ​​of each line are not significantly different. In this case, different delay times can be adopted, that is, the delay time of the next line is less than the delay time of the previous line by one time interval. This enables sequential selective operation of the three-stage protection for low voltage on different lines.

[0090] Microgrids are generally small in scale, resulting in short electrical distances between internal devices. Short-circuit current variations under line faults at different points are small, making it difficult to ensure selective protection operation. However, the residual bus voltage varies significantly under line faults at different points, effectively distinguishing fault locations. This invention, referencing the traditional three-stage overcurrent protection method, incorporates a three-stage undervoltage protection method within the microgrid. Simultaneously, an increased current is used as the protection activation value; for reliable activation, the activation value can be set to 1.2-1.5 times the line rated current.

[0091] This invention provides a microgrid current and voltage protection method and system that uses grid-side short-circuit current constraint. The feature is that a current-limiting power electronic switch is equipped at the connection point (PCC point) between the microgrid and the main grid to constrain the short-circuit current from the main grid.

[0092] Specifically, considering the needs of microgrid relay protection characteristics and the safety of power electronic equipment inside the microgrid, the limiting factor of the current-limiting power electronic switch is set to 3-5 times the rated line current.

[0093] Specifically, referring to the three-stage overcurrent protection, a three-stage undervoltage protection is equipped inside the microgrid.

[0094] Specifically, the increased short-circuit current after a short circuit is used as the starting value for the three-stage undervoltage protection.

[0095] Specifically, to ensure reliable protection activation, the activation value can be set to 1.2-1.5 times the line rated current.

[0096] Specifically, the setting method for the three-stage undervoltage protection of microgrids is as follows:

[0097] As attached Figure 1 As shown, taking protection 1 as an example, the operating voltage value of the low voltage protection stage I of protection 1 must be less than the residual voltage at bus A when a short circuit occurs on bus B. Therefore, the operating setting value of the low voltage protection stage I of protection 1 is... :

[0098]

[0099] In the formula, This represents the normal current supplied from the main power grid to the microgrid before the fault occurred; This indicates the normal current flowing through protection 1 before the fault; This indicates the limiting factor of the current-limiting power electronic switch set at the PCC point, which can be 3-5 (recommended value, dynamically adjusted as needed in engineering applications). This indicates the limiting factor for new energy power supplies connected to bus A after a fault, which can generally be taken as 1.2-1.5; The impedance of the line between busbars A and B is the total length of the line. To ensure the reliability coordination factor of the low voltage stage I protection, a value of 0.8-0.9 can be used.

[0100] If the undervoltage protection stage II of protection 1 is coordinated with the undervoltage protection stage I of the next line, then the operating setting value of the undervoltage protection stage II of protection 1 is... for:

[0101]

[0102] In the formula, To ensure the reliability coordination factor of the protection for the low voltage stage II, a value of 0.8-0.9 can be used. This is the setting value for the low voltage protection stage I of the next line protection 2.

[0103] The undervoltage stage III protection setting for protection 1, for overload conditions, must be less than the residual voltage at bus A under maximum load. Therefore, the undervoltage stage III protection setting value for protection 1 is... for:

[0104]

[0105] In the formula, To ensure the reliability coordination factor of the low voltage stage III protection, a value of 0.8-0.9 can be used. This refers to the potential overload current on the line. The impedance of the line between busbars B and C is the total length of the line. The impedance is the total length of the line between busbars C and D.

[0106] Specifically, after the protection is activated, the bus voltage is detected. When the residual bus voltage is less than the setting value of the undervoltage protection, each protection section will selectively operate after a corresponding delay.

[0107] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only used to facilitate the description of the present invention and to simplify the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the scope of protection of the present invention.

[0108] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0109] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0110] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A microgrid current and voltage protection method employing grid-side short-circuit current constraint, characterized in that, Based on current-limiting power electronic switches equipped at the junction of microgrids and mains grids, including: Collect the current at the connection point between the microgrid and the main power grid; If the current at the connection point between the microgrid and the main grid is less than the limit value, the current-limiting power electronic switch is in a zero-impedance state and does not operate. If the current at the connection between the microgrid and the main grid exceeds the limit, the current-limiting power electronic switch is in a high-impedance state and enters the current-limiting state to constrain the short-circuit current from the grid side. The method further includes: The microgrid is equipped with three-stage undervoltage protection. The microgrid is equipped with three-stage undervoltage protection, including: Perform three-stage undervoltage protection setting; Based on the adjusted three-stage undervoltage protection settings, three-stage undervoltage protection is implemented. The method for setting the three-stage undervoltage protection settings is as follows: Protection 1 is installed between busbars A and B; Protection 2 is installed between busbars B and C. The operating voltage value of the undervoltage protection stage I of protection 1 must be less than the residual voltage at bus A when a short circuit occurs on bus B. Therefore, the operating setting value of the undervoltage protection stage I of protection 1 is... : ； In the formula, This represents the normal current supplied from the main power grid to the microgrid before the fault occurred; This indicates the normal current flowing through protection 1 before the fault; This indicates the limiting factor of the current-limiting power electronic switch installed at the PCC point where the microgrid and the main grid connect; This indicates the limiting factor of the new energy power supply connected to bus A after the fault; The impedance of the line between busbars A and B is the total length of the line. To determine the reliability coordination factor for the low voltage stage I protection; If the undervoltage protection stage II of protection 1 is coordinated with the undervoltage protection stage I of the next line, then the operating setting value of the undervoltage protection stage II of protection 1 is... for: ； In the formula, To determine the reliability coordination factor for the protection of the low voltage stage II; This is the setting value for the low voltage protection stage I operation of the next line protection 2; The undervoltage stage III protection setting for protection 1, for overload conditions, must be less than the residual voltage at bus A under maximum load. Therefore, the undervoltage stage III protection setting value for protection 1 is... for: ； In the formula, To determine the reliability coordination factor for the low voltage stage III protection; This refers to the potential overload current on the line. The impedance of the line between busbars B and C is the total length of the line. The impedance is the total length of the line between busbars C and D.

2. The microgrid current and voltage protection method using grid-side short-circuit current constraint according to claim 1, characterized in that, The specified limit value is the maximum short-circuit current value that is restricted by the current-limiting power electronic switch installed at the PCC point at the connection between the microgrid and the main grid, and is taken as 3-5 times the rated current of the line.

3. The microgrid current and voltage protection method using grid-side short-circuit current constraint according to claim 1, characterized in that, Take 3-5; Take 1.2-1.5; Take 0.8-0.9; Take 0.8-0.9; Take 0.8-0.

9.

4. The microgrid current and voltage protection method using grid-side short-circuit current constraint according to claim 1, characterized in that, Based on the adjusted three-stage undervoltage protection settings, three-stage undervoltage protection is implemented, including: Step A: Measure the current at the protection installation point and obtain the current measurement value; Step B: If the measured current value is greater than the protection activation value, measure the residual voltage of the bus; otherwise, proceed to step A. Step C: If the residual voltage of the bus is less than the operating setting value of the low voltage I stage protection of this line, the short circuit fault is located within the low voltage I stage protection range of this line, and the corresponding protection will operate; otherwise, the low voltage I stage protection of this line will not operate, and the process will proceed to step D. Step D: If the residual voltage of the bus is less than the setting value of the low voltage stage II protection of this line, the low voltage stage II protection of this line will start timing; if the residual voltage of the bus rises to near the rated value before the timing time arrives, jump to step A; if the residual voltage of the bus remains less than the setting value of the low voltage stage II protection of this line after the timing time arrives, the fault will be cleared by the low voltage stage II protection of this line. Step E: If the residual voltage of the busbar is less than the setting value of the low voltage stage III protection, start timing; if the residual voltage of the busbar is still low after the timing time is reached, trip; if the residual voltage of the busbar is greater than the setting value of the low voltage stage III protection before the timing time is reached, it means that the fault or overload has disappeared, then return to step A.

5. The microgrid current and voltage protection method using grid-side short-circuit current constraint according to claim 4, characterized in that, The protection activation value is set to 1.2-1.5 times the line rated current.

6. The microgrid current and voltage protection method using grid-side short-circuit current constraint according to claim 5, characterized in that, The delay time of stage III is less than the delay time of stage II, and the delay time of stage II is less than the delay time of stage I, so as to achieve sequential selective operation of low voltage protection for different lines.

7. A microgrid current and voltage protection system employing grid-side short-circuit current constraint, characterized in that, include: Current-limiting power electronic switches are equipped at the junction of microgrids and main grids; The current-limiting power electronic switch is used to collect the current at the connection between the microgrid and the main grid. If the current at the connection between the microgrid and the main grid is less than the limit value, the current-limiting power electronic switch is in a zero-impedance state and does not operate. If the current at the connection between the microgrid and the main grid is greater than the limit value, the current-limiting power electronic switch is in a high-impedance state and enters the current-limiting state to constrain the short-circuit current from the grid side. The microgrid is equipped with three-stage undervoltage protection. The microgrid is equipped with three-stage undervoltage protection, including: Perform three-stage undervoltage protection setting; Based on the adjusted three-stage undervoltage protection settings, three-stage undervoltage protection is implemented. The method for setting the three-stage undervoltage protection settings is as follows: Protection 1 is installed between busbars A and B; Protection 2 is installed between busbars B and C. The operating voltage value of the undervoltage protection stage I of protection 1 must be less than the residual voltage at bus A when a short circuit occurs on bus B. Therefore, the operating setting value of the undervoltage protection stage I of protection 1 is... : ； In the formula, This represents the normal current supplied from the main power grid to the microgrid before the fault occurred; This indicates the normal current flowing through protection 1 before the fault; This indicates the limiting factor of the current-limiting power electronic switch installed at the PCC point where the microgrid and the main grid connect; This indicates the limiting factor of the new energy power supply connected to bus A after the fault; The impedance of the line between busbars A and B is the total length of the line. To determine the reliability coordination factor for the low voltage stage I protection; If the undervoltage protection stage II of protection 1 is coordinated with the undervoltage protection stage I of the next line, then the operating setting value of the undervoltage protection stage II of protection 1 is... for: ； In the formula, To determine the reliability coordination factor for the protection of the low voltage stage II; This is the setting value for the low voltage protection stage I operation of the next line protection 2; The undervoltage stage III protection setting for protection 1, for overload conditions, must be less than the residual voltage at bus A under maximum load. Therefore, the undervoltage stage III protection setting value for protection 1 is... for: ； In the formula, To determine the reliability coordination factor for the low voltage stage III protection; This refers to the potential overload current on the line. The impedance of the line between busbars B and C is the total length of the line. The impedance is the total length of the line between busbars C and D.

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