Adaptive distance protection method for distribution network with inverter power supply access

Abstract

The present invention discloses an adaptive distance protection method for a distribution network under inverter power access, and relates to the field of distribution networks under inverter power access. The specific steps of the present invention are as follows: first, read the line voltage and current data, solve the fault current phase angle, determine whether there is a fault according to the starting criterion, and if so, solve the fault additional impedance angle under inverter power access according to the fault type identification result: then calculate the real impedance conversion coefficient and the real impedance, distinguish the fault area, and send a fault signal. The present invention can accurately calculate the short-circuit impedance corresponding to the line fault position, has a strong transition resistance tolerance, eliminates the adverse effects of the output current being controlled after the inverter power fault, and all calculations use the inverter power side to protect the single-ended electrical quantity, which effectively reduces the communication cost of the distribution network while being unaffected by the change of the fault position, and embodies good selectivity and reliability.

Description

Technical Field

[0001] The present invention relates to the field of relay protection of power distribution systems under inverter power access, and in particular to an adaptive distance protection method for power distribution networks under inverter power access. Background Art

[0002] With the increasing global energy shortage and environmental pollution, distributed generation (DG) driven by renewable energy is becoming increasingly widespread in the power industry. Distributed power sources, such as photovoltaic power generation and energy storage power plants, are connected to the power system via converters, known as inverter-type power sources. Their high penetration and large capacity have profoundly changed the topology and fault characteristics of traditional distribution networks, posing even greater challenges to the design of distribution network protection schemes. Therefore, studying novel protection principles for distribution networks connected to inverter-type power sources to improve the stability and reliability of distribution systems is of great theoretical and engineering significance.

[0003] In response to the current complexities and challenges, experts and scholars both domestically and internationally have conducted extensive research. In the early days of inverter-powered power distribution networks, line faults generally involved first disconnecting distributed devices to ensure reliable protection. However, according to my country's current DG grid-connected technical standards, distributed power sources connected to medium and low voltage distribution networks should possess strong fault ride-through capabilities in the event of a fault. Some researchers have proposed algorithms for calculating the DG access capacity of distribution networks, but these algorithms are inconsistent with the current development prospects for open DG access. A protection scheme that uses real-time IIDG operating parameters to adjust the set value of three-stage current protection requires real-time reporting of system operating parameters, placing high demands on distribution network communication capabilities. With the increasing proportion of IIDGs in distribution networks, researchers have begun to introduce distance protection into distribution networks at voltage levels of 35 kV and lower. Some researchers have proposed a distance protection scheme based on adaptive impedance circle adjustment. By deriving an adaptive adjustment algorithm for the set impedance on a complex impedance plane, they achieve adaptive changes in the operating range and improve the reliability of traditional distance protection. Some scholars have also proposed an improvement scheme for adaptive distance protection. Based on the existing curved quadrilateral action area, the concept of line short-circuit impedance is introduced to improve the action performance of the existing adaptive distance protection. However, this type of distance protection with adaptive action area adjustment will have the problem of excessive action area changes and protection malfunction under small and medium interference. Regarding the proposed protection scheme that combines distance protection action area adjustment with fault load reduction algorithm, obtaining impedance measurement values at different locations in the system requires large-scale instant communication. The phasor method is used to calculate the actual fault impedance using the voltage, current and complex impedance components at the protection installation, but the influence of the fault output controlled characteristics of the IIDG in the event of a grid-side fault is not considered. The negative sequence voltage on the IIDG side is used to calculate the fault point current and then the fault impedance, but this method only considers asymmetric faults.

[0004] Therefore, it is clear that the existing traditional distance protection improvement scheme is difficult to meet the distribution network protection requirements after large-scale IIDG access. It is of great significance to design a new reliable and fast distance protection method suitable for inverter power access. Summary of the Invention

[0005] In order to solve the problem that the existing traditional distance protection improvement scheme is difficult to meet the distribution network protection requirements after large-scale IIDG access, the present invention provides a distribution network adaptive distance protection method suitable for inverter power supply access.

[0006] The present invention is achieved through the following technical solutions: a method for adaptive distance protection of distribution networks suitable for inverter-type power supply access, which analyzes the fault output characteristics after IIDG is connected to the grid and obtains its impact on traditional distance protection. On this basis, an adaptive distance protection scheme based on the calculation of real short-circuit impedance is proposed. This scheme calculates the additional impedance angle of the fault after the distribution network is connected to the IIDG according to the fault type. Under different fault forms, the product of the additional impedance angle, the sine ratio of the set impedance angle and the measured impedance is used as the real short-circuit impedance, and the entire solution process uses single-ended electrical quantities on the inverter power supply side. Specifically, it includes the following steps:

[0007] S1: Calculate the total current I at the fault point f And the protection measurement current I k The phase angle is calculated as follows:

[0008] Three-phase short circuit fault:

[0009]

[0010] Single-phase ground fault:

[0011]

[0012] Two-phase short circuit fault:

[0013]

[0014] Two-phase short circuit grounding fault:

[0015]

[0016] Where: Solve I f Used It is called the line virtual positive sequence voltage drop, is the positive sequence voltage drop of the busbar on the inverter power supply side. Both are special phase measurement values used in composite sequence network analysis under different line faults (due to the fault phase in single-phase grounding and the non-fault phase in two-phase fault), and their magnitudes can only be obtained from the electrical quantities on the IIDG side; is the line impedance angle; I θ represents the phase current, θ = a, b, c, and the superscripts 1 and 0 represent the positive-sequence and zero-sequence components, respectively;

[0017] S2: Calculate the additional fault impedance angle α when the inverter power supply is connected:

[0018] When different types of faults occur in the distribution network containing IIDG, only the IIDG side protection measurement value is used to add impedance Z after the inverter power supply is connected. ad The angle with the positive direction of the negative impedance plane R axis, namely the additional impedance angle α, is solved. The specific calculation formula is as follows:

[0019]

[0020] I f with I k Measure the current for the fault current and protection installation location respectively;

[0021] From formula (5), we can know that by calculating the phase difference between the fault current and the measured current, we can get the size of the additional impedance angle.

[0022] S3: True impedance conversion coefficient K real and the real impedance Z real The calculation formula is as follows:

[0023]

[0024] Set the impedance angle for distance protection, Indicates the measured impedance Z k The angle with the positive direction of the R axis, α is the additional fault impedance angle;

[0025] When different types of faults occur, the real impedance conversion coefficient K real and the real impedance Z real The performance is: when the fault type is an internal fault, Z real falls within the protection setting circle; when the fault type is a fault outside the positive zone, Z real Falling outside the protection setting circle, K real >0; when the fault type is a reverse zone fault, Z real Falling outside the protection setting circle, K real <0;

[0026] S4: Start criterion design:

[0027] During normal system operation, the bus voltage fluctuation detected by the IIDG protection is close to zero. After a fault occurs, the voltage at the grid connection point fluctuates violently. Based on this voltage fluctuation characteristic, the protection activation criterion is constructed. When the following formula is established, the line protection is activated:

[0028]

[0029] They are the line and phase voltage fluctuation measurement values at the protection installation location, The line and phase voltages during normal system operation; to ensure the reliability of protection when the system is subject to various noises and disturbances, 0.3 times the normal voltage is taken as the starting threshold;

[0030] S5: Adaptive distance protection criterion:

[0031] Based on the protection criterion of traditional directional distance protection, the solution method of the real short-circuit impedance is improved. By solving the real short-circuit impedance conversion coefficient K real , can accurately distinguish between faults within the zone, faults outside the forward zone and faults in the reverse direction; using Z real Replace the Z in the traditional directional distance protection action criterion k , which can effectively improve the operation reliability of traditional directional distance protection in distribution networks containing IIDGs; the designed adaptive distance protection criterion can be expressed as:

[0032]

[0033] Preferably, in the design of the start criterion of step S4, the design of the start threshold also takes into account: different distributed power supply capacities and their control strategies in actual applications, different voltage levels and the error influence of the measuring device, which can be adjusted according to the sensitivity principle of protection.

[0034] Compared with the prior art, the present invention has the following beneficial effects: the present invention provides an adaptive distance protection method for distribution networks under inverter power access, and obtains its influence on traditional distance protection by analyzing the fault output characteristics after IIDG is connected to the grid. On this basis, an adaptive distance protection scheme based on the calculation of real short-circuit impedance is proposed. The scheme calculates the additional impedance angle of the fault after the distribution network is connected to the IIDG according to the fault type. Under different fault forms, the product of the additional impedance angle and the sine ratio of the set impedance angle and the measured impedance is used as the real short-circuit impedance. The entire solution process uses single-ended electrical quantities on the inverter power side, which effectively reduces the communication cost of the distribution network while being unaffected by changes in the fault location. In principle, it eliminates the influence of transition resistance, solves the problem of reduced performance of distance protection action when the inverter power supply is connected to the grid, and can operate correctly under different fault forms; it meets the protection requirements of the distribution network after large-scale IIDG access. The present invention can accurately calculate the short-circuit impedance corresponding to the line fault location, has a strong transition resistance tolerance capability, eliminates the adverse effect of output current control after the inverter power supply fails, and all calculations use the inverter power supply side to protect single-ended electrical quantities. While effectively reducing the distribution network communication cost, it is not affected by changes in the fault location, and embodies good selectivity and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The figure is a schematic diagram of the topological structure of a distribution network including an IIDG according to a specific embodiment of the present invention.

[0036] Figure 2 This is an impedance phasor relationship diagram when a three-phase short circuit fault occurs in the line area involved in the specific embodiment of the present invention.

[0037] Figure 3 This is a fault equivalent circuit diagram when a three-phase short circuit fault occurs in the line involved in the specific embodiment of the present invention.

[0038] Figure 4 This is a circuit diagram of normal operation of the circuit involved in a specific embodiment of the present invention.

[0039] Figure 5 This is a breakdown diagram of an additional equivalent circuit when a three-phase short circuit fault occurs in a specific embodiment of the present invention.

[0040] Figure 6 This is a composite sequence network diagram of a circuit in which a single-phase grounding fault occurs in a line according to a specific embodiment of the present invention.

[0041] Figure 7 This is a composite sequence network diagram of a circuit in which a two-phase short circuit fault occurs in a line according to a specific embodiment of the present invention.

[0042] Figure 8This is a composite sequence network diagram of a circuit in which a two-phase short circuit and grounding fault occurs in a line involved in a specific embodiment of the present invention.

[0043] Figure 9 This is a diagram showing the relationship between various impedances under fault conditions within the line area involved in a specific embodiment of the present invention.

[0044] Figure 10 This is a diagram showing the relationship between impedances under forward out-of-zone faults and reverse faults of a line according to a specific embodiment of the present invention.

[0045] Figure 11 It is a flow chart of the adaptive distance protection scheme involved in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be further described below with reference to specific embodiments.

[0047] Taking 35kV distribution network as an example, the impact of IIDG access on traditional line protection is studied. A simple equivalent model of IIDG distribution network is shown as follows: Figure 1 As shown in the figure, the IIDG is connected to the distribution network via busbar B. Side A is the system power supply, and side C is the load connected to the distribution network. Distance protection devices R1-R3 are configured on lines AB and BC. To study the performance of distance protection during faults in the distribution network containing the IIDG, different faults are set at different locations f1 and f2 for analysis.

[0048] Take the three-phase short circuit fault at point f1 on line AB as an example. At this time, the measured impedance Z at the protection installation is k It can be expressed as:

[0049]

[0050] Where: U k , I k Respectively represent the measured voltage and measured current at the protection installation; Z real Indicates the actual short-circuit impedance from the protection installation to the fault point when the line fails; Z ad =(I f / I k )R g It represents the additional impedance caused by the transition resistance and the auxiliary current at the opposite end, I f is the total current at the fault point, R g is the transition resistance.

[0051] According to formula (1), for protection elements at different positions on the line, the measured value I k With fault current I f It will be affected by the fault current I provided by the system power supply and IIDG at the same time. G and I IIDGThe influence of Z under different fault conditions k The inability to accurately reflect the short-circuit impedance of the line causes traditional distance protection to have problems of refusal to operate and false operation;

[0052] Figure 2 Given Z k 、Z real and Z ad The geometric relationship on the complex impedance plane is shown in the figure: To set the impedance angle for distance protection, there are usually is the line impedance angle; Indicates the measured impedance Z k The angle with the positive direction of the R axis is called the measurement impedance angle; α represents the additional impedance Z ad The angle with the positive direction of the R axis is called the additional impedance angle, and α=arg(Z ad );

[0053] The specific verification and implementation process of this embodiment includes the following steps:

[0054] S1: Determination of additional impedance angle:

[0055] The analysis shows that the expression of the fault additional impedance angle α is:

[0056]

[0057] It can be seen from formula (2) that to solve α, it is only necessary to calculate the fault current I f With the measured current I k Phase difference.

[0058] The measured value U in traditional line distance protection k , I k Determined by the wiring method of the protection, considering the current various types of distance protection wiring forms at home and abroad, U under different fault types k , I k The expression is shown in Table 1, where is the zero-sequence compensation coefficient, and the 1, 2, and 0 in the superscript represent the positive, negative, and zero-sequence components, respectively.

[0059] Table 1 U under different types of faults k , I k

[0060]

[0061] I f The corresponding α under different fault types needs further analysis. The specific solution is taken as an example of a three-phase short circuit fault. The solution process is as follows:

[0062] There is only positive sequence component in the system before and after the three-phase short circuit fault, so Figure 3 All physical quantities in are expressed using positive sequence components. and Respectively represent the fault current input to the fault point from both sides A and B, and the subscript represents the input side bus; I 1f Indicates the total fault current; Indicates the equivalent impedance of the line; Indicates the equivalent impedance on the system side; It represents the equivalent impedance on the IIDG side, and its magnitude changes in real time due to the different working status and control strategies of the IIDG. x represents the ratio of the line length from the fault point to the protection installation to the total line length. and Indicates the equivalent voltage of the system and IIDG.

[0063] The fault equivalent circuit diagram can be decomposed into two parts: normal operation circuit and fault additional circuit by using superposition principle. Figure 4 and Figure 5 As shown:

[0064] Depend on Figure 4 It can be seen that before the fault, the currents provided by A and B are equal in magnitude and opposite in direction, so I 1f It is determined only by the additional state of the fault, and its specific expression is as follows:

[0065]

[0066] in Corresponding to the current after the fault occurs and increment;

[0067] Compared with IIDG, the system power supply equivalent impedance is extremely small. Combine Figure 5 We can get:

[0068]

[0069] According to formula (4), we can get I 1f The phase angle is:

[0070]

[0071] Generally speaking Available The specific formula is as follows: G is a real number:

[0072]

[0073] Depend on Figure 5 Available for:

[0074]

[0075] Substituting equations (6) and (7) into equation (5), we get:

[0076]

[0077] In the formula It is called the line virtual positive sequence voltage drop, is the positive sequence voltage drop of bus B. Both can be obtained only through the electrical quantities on the IIDG side during line faults; is the line impedance angle.

[0078] When the three-phase short circuit occurs, the fault current is equal to the positive sequence fault current. Then α can be expressed as:

[0079]

[0080] The composite sequence network diagram of single-phase grounding fault, two-phase interphase fault and two-phase grounding fault is as follows Figures 6 to 8 As shown, similar to the analysis of three-phase short-circuit fault, the obtained α can be expressed as:

[0081] Single-phase grounding fault (taking phase A grounding fault as an example):

[0082]

[0083] Two-phase short circuit fault (taking BC phase short circuit fault as an example):

[0084]

[0085] Two-phase short circuit grounding fault (taking BC grounding short circuit fault as an example):

[0086]

[0087] From the above analysis, it can be concluded that by establishing a fault composite sequence network to analyze the relationship between each sequence current, the fault current phase under asymmetric fault can be expressed by the positive sequence component. Using the phase analysis conclusion of the positive sequence fault current under three-phase fault, the formula for calculating the additional impedance angle α corresponding to different fault types is obtained, thereby realizing the K real , Z real The solution.

[0088] S2: Determination of actual short-circuit impedance:

[0089] (1) Analysis of faults within the area:

[0090] The fault analysis within the line occurrence area of this embodiment is as follows:

[0091] When a fault occurs in the protection R2 area on the IIDG side, it is affected by R g Influence, as α changes, Z k It may fall outside the operating area of any quadrant of the impedance complex plane, causing the protection to refuse to operate. At this time, Z k , Z real and Z ad The geometric relationship is as follows Figure 9 The circle in the figure is the operating area of the directional impedance relay, and its setting impedance Z set and setting impedance angle is certain, and by using the relationship between the sides and angles of the circle and the triangle, the corresponding relationship between the phasor and the impedance can be obtained as shown in formula (13):

[0092]

[0093] OM is the diameter of the action circle, and Z is made through point M. ad The parallel line corresponding to the phase quantity NF intersects the impedance characteristic circle at point P, and the angle relationship in each right triangle satisfies formula (14):

[0094]

[0095] According to formula (14), the relationship between the actual short-circuit impedance and the measured impedance amplitude is as shown in formula (15):

[0096] |Z real |×sin(∠ONQ)=|Z k |×sin(∠OFQ) (15)

[0097] Transforming Equation (15) to obtain the amplitude of the real short-circuit impedance is:

[0098]

[0099] Combine Figure 9 From formula (16), we can get that when α takes different values between (-π, π), Z k Falling into four different quadrants of the complex impedance plane, ∠OFQ, ∠ONQ and The relationship between α and its sine ratio is shown in Table 2.

[0100] Table 2 Relationship between various angles under fault conditions within the zone

[0101]

[0102] The actual short-circuit impedance Z of the fault in the actual operation zone of the system real ≥0 is always true, and from Table 2, it is easy to see that the ratio of the sine values of ∠OFQ and ∠ONQ as the interior angles of a triangle is always greater than 0 and satisfies formula (17):

[0103]

[0104] Therefore, substituting equation (17) into equation (16) yields the fault time Z real The solution is as follows:

[0105]

[0106] Where, the real impedance conversion coefficient is defined as

[0107] (2) Analysis of forward out-of-zone faults and reverse faults:

[0108] When the system has a forward fault or a reverse fault, Z k 、Z real and Z ad The geometric relationship in the complex impedance plane is as follows Figure 10 As shown. Figure 10 It can be inferred that the relationship between the actual short-circuit impedance and the measured impedance in equation (15) still holds. k By analyzing the geometric relationship between them, it can be found that if the traditional distance protection for faults outside the forward zone malfunctions, Z ad Should fall into the area below the tangent of the action circle passing through point M; and if the protection malfunctions during reverse fault, Z ad Should fall into the area above the tangent of the action circle passing through point O; at this time, α is in a certain special interval. Reverse fault

[0109] Then when the fault is outside the forward zone or the reverse fault occurs, ∠OFQ, ∠ONQ and The relationship between α and the ratio of their sine values is shown in Table 3.

[0110] Table 3 Relationship between angles under forward out-of-zone fault and reverse fault

[0111]

[0112] As shown in Table 3, the calculation formula of the real short-circuit impedance for the forward out-of-zone fault is the same as that for the in-zone fault; while for the reverse fault Satisfy Z real <0.

[0113] At this time, generalizing formula (18) yields: For the line protected by R2, by calculating K real The fault direction can be determined. real The solution method is exactly the same when an in-zone fault, a forward out-zone fault, or a reverse fault occurs, and can be expressed by formula (18).

[0114] In summary, in a distribution network with IIDG equipped with traditional directional distance protection, whether a fault occurs inside or outside the area, the real short-circuit impedance Z real They can all be expressed by formula (19):

[0115]

[0116] Among them U k , I k 、 and α is the influence of Z real The main parameters of U k , I k and Can be measured directly after a fault occurs, It is certain in the actual operation line. Therefore, it is only necessary to calculate the fault additional impedance angle α according to the steps described in the previous section to calculate the real short-circuit impedance Z real .

[0117] In this embodiment, a method for adaptive distance protection of a distribution network applicable to inverter-type power supply access includes the following steps:

[0118] S1: Start criteria:

[0119] When the system is operating normally, the bus voltage fluctuation detected by the IIDG side protection is almost zero. After the fault occurs, the grid voltage fluctuates violently. The protection start criterion is constructed based on this voltage fluctuation characteristic. When the following equation (20) is established, the line protection is started.

[0120]

[0121] Where, They are the line and phase voltage fluctuation measurement values at the protection installation location, This is the line and phase voltage during normal system operation. To ensure protection reliability under various noise and disturbance conditions, this embodiment uses 0.3 times the normal voltage as the startup threshold. In practical applications, the influence of different distributed power generation capacities and control strategies, different voltage levels, and measurement device errors must be considered, and adjustments can be made based on protection sensitivity principles.

[0122] S2: Adaptive distance protection criterion:

[0123] This embodiment improves the method for calculating the real short-circuit impedance based on the protection criterion of traditional directional distance protection. real , can accurately distinguish between faults within the zone, faults outside the forward zone and faults in the reverse direction. real Replace the Z in the traditional directional distance protection action criterion k, which can effectively improve the reliability of the traditional directional distance protection in the distribution network containing IIDG. The adaptive distance protection criterion designed in this embodiment can be expressed as:

[0124]

[0125] According to the principle of this embodiment, the flow chart of the adaptive distance protection scheme based on inverter power access is as follows: Figure 11 After determining the fault type, the additional impedance angle α can be obtained according to the solution formula under the corresponding fault type, and then K can be calculated. real , Z real To identify the fault area. To avoid the drastic fluctuation of the measurement signal after the fault occurs, Z real The protection action will be activated when the judgment result meets the protection action criteria for two consecutive cycles.

[0126] The scope of protection claimed by the present invention is not limited to the above specific embodiments. For those skilled in the art, the present invention may have various variations and modifications. Any modifications, improvements and equivalent substitutions made within the concept and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for adaptive distance protection of a distribution network suitable for inverter-type power supply access, characterized by: The steps include: S1: Calculate the total current I at the fault point f And the protection measurement current I k The phase angle is calculated as follows: Three-phase short circuit fault: Single-phase ground fault: Two-phase short circuit fault: Two-phase short circuit grounding fault: Where: Solve I f Used It is called the line virtual positive sequence voltage drop, is the positive sequence voltage drop of the busbar on the inverter power supply side. Both are special phase measurements used in composite sequence network analysis under different line faults, and their magnitudes are obtained only through the electrical quantities on the IIDG side. is the line impedance angle; I θ represents the phase current, θ = a, b, c, and the superscripts 1 and 0 represent the positive-sequence and zero-sequence components, respectively; S2: Calculate the additional fault impedance angle α when the inverter power supply is connected: When different types of faults occur in the distribution network containing IIDG, only the IIDG side protection measurement value is used to add impedance Z after the inverter power supply is connected. ad The angle with the positive direction of the negative impedance plane R axis, namely the additional impedance angle α, is solved. The specific calculation formula is as follows: I f with I k Measure the current for the fault current and protection installation location respectively; From formula (5), we can know that by calculating the phase difference between the fault current and the measured current, we can get the size of the additional impedance angle. S3: True impedance conversion coefficient K real and the real impedance Z real The calculation formula is as follows: Set the impedance angle for distance protection, Indicates the measured impedance Z k The angle with the positive direction of the R axis, α is the additional fault impedance angle; When different types of faults occur, the real impedance conversion coefficient K real and the real impedance Z real The performance is: when the fault type is an internal fault, Z real falls within the protection setting circle; when the fault type is a fault outside the positive zone, Z real Falling outside the protection setting circle, K real >0; when the fault type is a reverse zone fault, Z real Falling outside the protection setting circle, K real <0; S4: Start criterion design: During normal system operation, the bus voltage fluctuation detected by the IIDG protection is close to zero. After a fault occurs, the voltage at the grid connection point fluctuates violently. Based on this voltage fluctuation characteristic, the protection activation criterion is constructed. When the following formula is established, the line protection is activated: They are the line and phase voltage fluctuation measurement values at the protection installation location, The line and phase voltages during normal system operation; to ensure the reliability of protection when the system is subject to various noises and disturbances, 0.3 times the normal voltage is taken as the starting threshold; S5: Adaptive distance protection criterion: Based on the protection criterion of traditional directional distance protection, the solution method of the real short-circuit impedance is improved. By solving the real short-circuit impedance conversion coefficient K real , can accurately distinguish between faults within the zone, faults outside the forward zone and faults in the reverse direction; using Z real Replace the Z in the traditional directional distance protection action criterion k , which can effectively improve the operation reliability of traditional directional distance protection in distribution networks containing IIDGs; the designed adaptive distance protection criterion can be expressed as:

2. The method for adaptive distance protection of a distribution network suitable for inverter-type power supply access according to claim 1, characterized in that: In the design of the start-up criterion in step S4, the design of the start-up threshold also takes into account: different distributed power supply capacities and their control strategies in actual applications, different voltage levels and the error influence of the measurement device, which can be adjusted according to the sensitivity principle of protection.

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