A single-phase grounding protection method and system based on a small-resistance grounding system

Abstract

The present invention belongs to the technical field of relay protection for distribution networks, and particularly relates to a single-phase grounding protection method and system for a small-resistance grounding system. This method adjusts the operating threshold of zero-sequence overcurrent protection according to the secondary harmonic content of the distribution line and the change of zero-sequence voltage, which not only improves the sensitivity of the protection for high-resistance grounding faults, but also considers the zero-sequence unbalanced current generated by the self-starting of large motors and the no-load closing of transformers, including: when the collected zero-sequence voltage U0 is greater than the starting value U set or the sudden change of zero-sequence voltage ΔU is greater than the starting threshold ΔU set or the zero-sequence current I0 is greater than the starting threshold I 0set , calculate the maximum value K of the secondary harmonic content of the three-phase current of the distribution line r , compare K r with K set and make corresponding processing; when the grounding resistance in the small-resistance grounding system changes from small to large, compare the relationship between the zero-sequence current zero I0 and the operating threshold of overcurrent protection, and determine whether it is a faulty line.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay protection for distribution networks, and in particular relates to a single-phase grounding protection method and system for a low-resistance grounding system. Background Art

[0002] Low-resistance grounding systems are widely used in distribution networks in large and medium-sized cities. They effectively reduce overvoltage levels and minimize equipment damage during single-phase ground faults. By connecting a small resistor in series with the neutral point, they limit fault current and ensure more stable system operation. However, in high-resistance grounding, the fault current is smaller, making detection more difficult. This can lead to long-term faults, compromising equipment insulation, and even causing more serious accidents. Although high-resistance grounding faults only account for 5% to 10% of all faults, their hidden nature makes detection and prevention particularly important.

[0003] Zero-sequence overcurrent protection is a common protection method for low-resistance grounding systems. It relies on detecting zero-sequence current to identify ground faults. When a three-phase system is unbalanced, zero-sequence current is generated, and the protection device uses this to detect faults. Traditional zero-sequence overcurrent protection uses a fixed setting value and is suitable for low-resistance ground faults, as shown in the invention patents CN114884033A and CN105787819A.

[0004] However, in high-resistance ground faults, the fault current is low and may not trigger the protection, resulting in a false trip. In addition, the setting value is usually high to avoid false trips, which further limits the detection sensitivity of high-resistance ground faults.

[0005] For example, the invention patent with authorization announcement number CN105140897B discloses a protection method for single-phase grounding faults in low-resistance grounding systems. The method uses the zero-sequence voltage as the braking amount to determine the zero-sequence overcurrent protection setting value based on the relationship between the transition resistance of the fault point and the zero-sequence voltage and the zero-sequence current of each outgoing line during a grounding fault. That is, if the zero-sequence voltage amplitude is greater than the inflection point voltage, the zero-sequence overcurrent protection setting value is increased by a certain proportion; otherwise, the zero-sequence overcurrent protection setting value remains unchanged and is the set value for normal operation. However, the patent does not consider the influence of the magnetizing inrush current when the transformer is closed at no load and the unbalanced zero-sequence current that may be generated during the self-start of large motors. In the face of these situations, false operation may occur, affecting the stability of the power system operation. Summary of the invention

[0006] Objective of the Invention: Aiming at the problems of poor detection sensitivity of high-resistance grounding faults in the prior art and poor system stability caused by unbalanced zero-sequence current, the present invention provides a single-phase grounding protection method for a small-resistance grounding system. The present invention also discloses a single-phase grounding protection system for a small-resistance grounding system. The present invention adjusts the operating threshold of zero-sequence over-current protection according to the secondary harmonic content of the distribution line and the change of zero-sequence voltage, which not only improves the sensitivity of the protection for diagnosing high-resistance grounding faults, but also takes into account the zero-sequence unbalanced current generated by the self-starting of large motors and the no-load closing of transformers.

[0007] Technical Solution: In the first aspect, the present invention discloses a single-phase grounding protection method based on a small-resistance grounding system, and the method includes the following steps:

[0008] S1 Collect and calculate the zero-sequence voltage, zero-sequence current, and three-phase current of the faulty line and the normal line;

[0009] S2 When the collected zero-sequence voltage U0 is greater than the starting value U set or the sudden change in zero-sequence voltage ΔU is greater than the starting threshold ΔU set or the zero-sequence current I0 is greater than the starting threshold I 0set the protection starts and enters step S3; otherwise, it is determined that the current distribution line is a non-faulty line;

[0010] S3 Calculate the maximum value K r of the secondary harmonic content of the three-phase current of the distribution line, and compare K r with K set When K r ≥ K set the operating threshold of zero-sequence over-current protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, where K set is a fixed threshold;

[0011] When K r < K set compare the zero-sequence voltage U0 with the inflection point voltage U 0.g When U0 < U 0.g the operating threshold of zero-sequence over-current protection is the minimum operating setting value plus the threshold part increased to avoid the unbalanced current generated by the self-starting of large motors;

[0012] When U0 ≥ U 0.g the operating threshold of zero-sequence over-current protection is a threshold that floats according to the amplitude of the zero-sequence voltage;

[0013] S4 When the grounding resistance in the small-resistance grounding system changes from small to large, compare the relationship between the zero-sequence current zero I0 and the operating threshold of over-current protection, and determine whether it is a faulty line.

[0014] Furthermore, it includes:

[0015] In step S3, when K r ≥K set the operating threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line during a solid single-phase ground fault on the distribution line, that is, the setting value of the zero-sequence overcurrent protection, which is expressed as:

[0016]

[0017] where I set1 is the setting value of the zero-sequence overcurrent protection; E A is the power supply voltage of the faulty phase; R N is the neutral grounding resistance.

[0018] Furthermore, it includes:

[0019] In step S3, when K r <K set and U0 < U 0.g the operating threshold of the zero-sequence overcurrent protection is expressed as:

[0020]

[0021] where I set.min is the minimum operating current, K0 is the motor braking coefficient, I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

[0022] Furthermore, it includes:

[0023] When K r <K set and U0 ≥ U 0.g the operating threshold of the zero-sequence overcurrent protection is expressed as:

[0024] I set3 =K1(U0 - U 0.g ) + I set.min (3)

[0025] where K1 is the braking coefficient.

[0026] Furthermore, it includes:

[0027] The inflection point voltage U 0.g is expressed as:

[0028]

[0029] Furthermore, it includes:

[0030] In step S4, comparing the relationship between the zero-sequence current I0 and the operating threshold of the over-current protection includes:

[0031] When I0 < I set2 or I0 < I set3 , the current distribution line is a non-faulty line; otherwise, the current distribution line is a faulty line.

[0032] Further, it includes:

[0033] When K r ≥ K set , the faulty line with a metallic single-phase ground fault occurs in the distribution line, thereby enabling protection blocking.

[0034] In the second aspect, the present invention provides a single-phase ground protection system based on a small-resistance grounding system, and this system includes:

[0035] An acquisition module, used for acquiring and calculating the zero-sequence voltage, zero-sequence current, and three-phase current of the faulty line and the normal line;

[0036] A line start determination module, used for when the acquired zero-sequence voltage U0 is greater than the start setting value U set or the sudden change in zero-sequence voltage ΔU is greater than the start threshold ΔU set or the zero-sequence current I0 is greater than the start threshold I 0set , the protection starts and enters the comparison module; otherwise, it is determined that the current line is a non-faulty line;

[0037] A comparison module, used for calculating the maximum value K r of the second harmonic content of the three-phase current of the distribution line, comparing the magnitudes of K r and K set , when K r ≥ K set , the operating threshold of the zero-sequence over-current protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, where K set is a fixed threshold;

[0038] When K r < K set , comparing the magnitudes of the zero-sequence voltage U0 and the inflection point voltage U 0.g , when U0 < U 0.g , the operating threshold of the zero-sequence over-current protection is the minimum operating setting value plus the threshold part increased to avoid the unbalanced current generated by the self-starting of large motors;

[0039] When U0 ≥ U 0.g , the operating threshold of the zero-sequence over-current protection is a threshold that floats according to the magnitude of the zero-sequence voltage;

[0040] A line fault determination module, configured to compare the relationship between the zero-sequence current I0 and the over-current protection action threshold when the grounding resistance in the small-resistance grounding system changes from small to large, and determine whether it is a faulty line.

[0041] Further, it includes:

[0042] In the comparison module, when K r ≥K set The zero-sequence over-current protection action threshold is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, that is, the zero-sequence over-current protection setting value, which is expressed as:

[0043]

[0044] Where, I set1 is the zero-sequence over-current protection setting value; E A is the power supply voltage of the faulty phase; R N is the neutral point grounding resistance.

[0045] Further, it includes:

[0046] In the step comparison module, when K r <K set and U0<U 0.g The zero-sequence over-current protection action threshold is expressed as:

[0047]

[0048] Where, I set.min is the minimum operating current, K0 is the motor braking coefficient, I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

[0049] Further, it includes:

[0050] When K r <K set and U0≥U 0.g The zero-sequence over-current protection action threshold is expressed as:

[0051] I set3 =K1(U0 - U 0.g ) + I set.min (7)

[0052] Where, K1 is the braking coefficient.

[0053] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0054] The present invention attempts to adjust the operating threshold of zero-sequence over-current protection according to the secondary harmonic content of the distribution line and based on the change of zero-sequence voltage. This not only improves the sensitivity of the protection for diagnosing high-resistance grounding faults but also takes into account the zero-sequence unbalanced current generated by the self-starting of large motors and the no-load closing of transformers. This method is simple and easy to implement, significantly improving the sensitivity of detecting high-resistance grounding faults in a small-resistance grounding system and effectively preventing the misoperation of the protection device caused by the zero-sequence unbalanced current generated by the self-starting of large motors and the no-load closing of transformers. Description of the Drawings

[0055] Figure 1 It is a flow chart of the single-phase grounding protection method based on a small-resistance grounding system described in an embodiment of the present invention;

[0056] Figure 2 It is a MATLAB model diagram of a single-phase grounding protection method for a small-resistance grounding system described in an embodiment of the present invention.

[0057] Figure 3 It is a waveform diagram of the three-phase exciting inrush current of the system under the condition of no-load closing of the transformer described in an embodiment of the present invention.

[0058] Figure 4 It is a diagram of the change of the zero-sequence current and the operating threshold of the normal line and the fault line of the system under the condition of grounding resistances of 0, 100, 500, and 1000 Ω respectively in an embodiment of the present invention. Detailed Embodiment

[0059] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0060] The present invention discloses a single-phase grounding protection method for a small-resistance grounding system. It includes:

[0061] Step 1: Collect and calculate the zero-sequence voltage, zero-sequence current, and three-phase current of the distribution line;

[0062] Step 2: When the zero-sequence voltage is greater than the starting value, or the sudden change in zero-sequence voltage is greater than the starting value, or the zero-sequence current is greater than the starting value, the protection starts;

[0063] Step 3: If the maximum value of the secondary harmonic content of the three-phase current of the distribution line exceeds the threshold, the operating threshold of the zero-sequence over-current protection is the value of the zero-sequence current flowing through the fault line when a metallic single-phase ground fault occurs in the distribution line.

[0064] Step 4: When the maximum value of the second harmonic content of the three-phase current in the distribution line is less than the starting threshold, select the zero-sequence voltage, zero-sequence current, and three-phase current data 0.02 s after the fault occurs. When the zero-sequence voltage is less than the inflection point voltage, the operating threshold of the zero-sequence overcurrent protection is the minimum operating setting value plus the increased threshold part to avoid the unbalanced current generated by the self-starting of large motors. Its value is equal to the ratio of the difference between the maximum effective value of the phase current and the rated value of the phase current to the rated value of the phase current, multiplied by the product of the motor braking coefficient K0 and the minimum operating setting value, and then added to the minimum operating setting value. When the zero-sequence voltage is greater than the inflection point voltage, the operating threshold of the zero-sequence overcurrent protection is a floating threshold set according to the amplitude of the zero-sequence voltage. Its value is equal to the braking coefficient K1 multiplied by the difference between the zero-sequence voltage and the inflection point voltage, plus the minimum operating setting value. This method is simple and easy to implement, significantly improving the sensitivity of high-resistance grounding fault detection in a small-resistance grounding system and effectively preventing the misoperation of protection devices caused by the zero-sequence unbalanced current generated by the self-starting of large motors and the no-load closing of transformers.

[0065] Specifically, as Figure 1 shown, the method includes:

[0066] Step (1): Collect and calculate the zero-sequence voltage, zero-sequence current, and three-phase current of the line.

[0067] The zero-sequence current, zero-sequence voltage, and three-phase current collected after the fault are the basis for setting the operating threshold of the zero-sequence overcurrent protection in the algorithms of Step 2, Step 3, and Step 4.

[0068] The specific steps for deriving the single-phase grounding zero-sequence current and voltage in Step (1) are as follows:

[0069] First, calculate the zero-sequence voltage formula as follows:

[0070]

[0071] The zero-sequence current of the sound line is:

[0072]

[0073] The zero-sequence current of the fault outgoing line is:

[0074]

[0075] Among them, is the pre-fault phase voltage of the fault phase; R f is the short-circuit grounding resistance; R N is the grounding resistance; C 0Σ is the sum of the zero-sequence capacitances of all lines to the ground for one phase; C 0∑H is the zero-sequence capacitance of all sound lines to the ground.

[0076] From the above analysis, it can be concluded that the zero-sequence current of the sound line and the zero-sequence current of the faulty outgoing line are both proportional to the zero-sequence voltage at the fault point.

[0077] Step (2): When U0 > U set or ΔU > ΔU set or I0 > I 0set the protection is activated.

[0078] Step (3): When the second-harmonic content K r in the three-phase current of the line is greater than or equal to the threshold value K set the operating threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line during a metallic single-phase ground fault on the distribution line, and its value is:

[0079]

[0080] where I set is the setting value of the zero-sequence overcurrent protection; E A is the power supply voltage of the faulty phase; R N is the neutral grounding resistance, generally taken as 10 Ω.

[0081] Step (4): When K r < K set compare the magnitudes of the zero-sequence voltage U0 and the inflection-point voltage U 0.g When U0 < U 0.g the operating threshold of the zero-sequence overcurrent protection can be expressed as

[0082]

[0083] where I set.min is the minimum operating current, generally taken as about 1.33 A; K0 is the motor braking coefficient, generally taken as 2 - 5; I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

[0084] When U0 ≥ U 0.g the operating threshold of the zero-sequence overcurrent protection can be expressed as

[0085] I set = K1(U0 - U 0.g ) + I set.min (6)

[0086] where K1 is the braking coefficient, and its value range is 0.0038 - 0.0256. In this method, it is taken as 0.01

[0087] In step (2), the starting setting value U set of the zero-sequence voltage is generally taken as 5 - 10 V, and the starting setting value ΔU of the voltage mutationset Generally, it is taken as 0.5 - 1V, and the starting setting value I of the zero-sequence current 0set is generally taken as 0.5 - 1A.

[0088] In step (3), the value of K set is generally taken as 0.15 - 0.2.

[0089] For a single-phase grounding protection method of a small-resistance grounding system proposed by the present invention, the action threshold of the zero-sequence overcurrent protection can be expressed by the following formula:

[0090]

[0091] To illustrate the effectiveness of this method, the following tests are conducted, including:

[0092] See Figure 2 , a system model is established, and a single-phase grounding protection method for a small-resistance grounding system is proposed by the present invention.

[0093] Step (1): Collect and calculate the zero-sequence voltage, zero-sequence current, and three-phase current of the line.

[0094] The zero-sequence current, zero-sequence voltage, and three-phase current collected after the fault are the basis for setting the action threshold of the zero-sequence overcurrent protection in steps 2, 3, and 4 algorithms.

[0095] Step (2): When U0 > U set or ΔU0 > ΔU set or I0 > I 0set , the protection starts.

[0096] A Step (3): When the content of the second harmonic K r in the three-phase current of the line is greater than or equal to the threshold K set , the action threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, and its value is:

[0097]

[0098] Among them, I set is the setting value of the zero-sequence overcurrent protection; E A is the power supply voltage of the faulty phase; R N is the neutral point grounding resistance, generally taken as 10Ω.

[0099] Step (4): When K r < K set , compare the magnitudes of the zero-sequence voltage U0 and the inflection point voltage U 0.g . When U0 < U 0.g , the action threshold of the zero-sequence overcurrent protection can be expressed as

[0100]

[0101] Among them, I set.min is the minimum operating current, generally about 1.33 A; K0 is the motor braking coefficient, generally taken as 2 - 5; I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

[0102] When U0 ≥ U 0.g , the operating threshold of the zero-sequence overcurrent protection can be expressed as

[0103] I set = K1(U0 - U 0.g ) + I set.min (3)

[0104] Among them, K1 is the braking coefficient, and its value range is 0.0038 - 0.0256. In this method, 0.01 is taken.

[0105] In step (2), the starting setting value U set of the zero-sequence voltage is generally taken as 5 - 10 V, the starting setting value ΔU set of the voltage mutation is generally taken as 0.5 - 1 V, and the starting setting value I 0set of the zero-sequence current is generally taken as 0.5 - 1 A.

[0106] In step (3), the value of K set is generally taken as 0.15 - 0.2.

[0107] For a single-phase grounding protection method of a small-resistance grounding system proposed by the present invention, the operating threshold of the zero-sequence overcurrent protection can be expressed by the following formula:

[0108]

[0109] This description simulates the system through MATLAB / Simulink, with a frequency of 50 Hz, and the lengths of the three feeders are 5 km, 8 km, and 10 km respectively. First, the situation of transformer no-load closing is simulated, as Figure 3 shown, and it is found that the ratio of the second harmonic to the first harmonic of the three-phase current reaches 0.56, and the operating threshold is set to E A / 3R N . Then, the situation of single-phase grounding in the small-resistance grounding system is simulated. The grounding fault is a single-phase grounding fault of phase A, and the grounding resistances at the short-circuit point are 0, 100, 500, and 1000 Ω respectively. The calculation results of the algorithm are shown in the following table:

[0110] Table 1 Fault conditions with different grounding resistances

[0111]

[0112] As can be seen from Table 1 and Figure 4 it can be seen that when the grounding resistance in the low-resistance grounding system ranges from 0 to 1000 Ω, the operating threshold of the zero-sequence over-current protection calculated by this algorithm is between the effective value of the zero-sequence current of the normal line and the effective value of the zero-sequence current of the faulty line, and the faulty grounding line can be reliably identified.

[0113] In summary, based on the relationship between the zero-sequence voltage, zero-sequence current and three-phase current, the new protection criterion proposed by the present invention can sensitively detect the high-resistance grounding fault of 1 kΩ in the low-resistance grounding system, and has a small amount of calculation and is easy to be implemented on the microcomputer protection device.

[0114] On the other hand, the present invention provides a single-phase grounding protection system based on a low-resistance grounding system, and the system includes:

[0115] An acquisition module, configured to acquire and calculate the zero-sequence voltage, zero-sequence current and three-phase current of the faulty line and the normal line;

[0116] A line start determination module, configured to, when the acquired zero-sequence voltage U0 is greater than the start setting value U set or the sudden change amount ΔU of the zero-sequence voltage is greater than the start threshold ΔU set or the zero-sequence current I0 is greater than the start threshold I 0set the protection starts and enters the comparison module; otherwise, it is determined that the current line is a non-faulty line;

[0117] A comparison module, configured to calculate the maximum value K of the second harmonic content of the three-phase current of the distribution line r , compare K r with K set When K r ≥K set the operating threshold of the zero-sequence over-current protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase grounding occurs in the distribution line, where K set is a fixed threshold;

[0118] When K r <K set compare the zero-sequence voltage U0 with the inflection point voltage U 0.g When U0<U 0.g the operating threshold of the zero-sequence over-current protection is the minimum operating setting value plus the threshold part increased to avoid the unbalanced current generated by the self-starting of large motors;

[0119] When U0≥U 0.g the operating threshold of the zero-sequence over-current protection is a threshold that floats according to the amplitude of the zero-sequence voltage;

[0120] A line fault determination module, configured to compare the relationship between the zero-sequence current I0 and the over-current protection operation threshold when the grounding resistance in a small-resistance grounding system changes from small to large, and determine whether it is a faulty line.

[0121] Further, it includes:

[0122] In the comparison module, when K r ≥K set The zero-sequence over-current protection operation threshold is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, that is, the zero-sequence over-current protection setting value, expressed as:

[0123]

[0124] Where, I set1 is the zero-sequence over-current protection setting value; E A is the power supply voltage of the faulty phase; R N is the neutral point grounding resistance.

[0125] Further, it includes:

[0126] In the step comparison module, when K r <K set and U0<U 0.g The zero-sequence over-current protection operation threshold is expressed as:

[0127]

[0128] Where, I set.min is the minimum operating current, K0 is the motor braking coefficient, I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

[0129] Further, it includes:

[0130] When K r <K set and U0≥U 0.g The zero-sequence over-current protection operation threshold is expressed as:

[0131] I set3 =K1(U0-U 0.g )+I set.min (7)

[0132] Where, K1 is the braking coefficient.

[0133] Other technical features of the single-phase ground protection system based on the small-resistance grounding system provided by the present invention are similar to those of the corresponding single-phase ground protection method based on the small-resistance grounding system, and will not be elaborated here.

[0134] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memory, CD-ROM, optical memory, etc.) that contain computer-usable program code.

[0135] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications that fall within the scope of the present invention.

[0136] Obviously, those skilled in the art can make various changes and modifications to the embodiments of the present invention without departing from the spirit and scope of the embodiments of the present invention. Thus, if these modifications and variations of the embodiments of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these changes and modifications.

Claims

1. A single-phase grounding protection method based on a small-resistance grounding system, characterized in that, The method includes the following steps: S1 Collect and calculate the zero-sequence voltage, zero-sequence current, and three-phase current of the faulty line and the normal line; S2 When the zero-sequence voltage U0 collected is greater than the starting value U set or the sudden change in zero-sequence voltage ΔU is greater than the starting threshold ΔU set or the zero-sequence current I0 is greater than the starting threshold I 0set then the protection is started and step S3 is entered; otherwise, it is determined that the current distribution line is a non-faulty line. S3 calculates the maximum value K of the second harmonic content of the three-phase current of the distribution line r , and compares K r with K set . When K r ≥ K set , the operating threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs on the distribution line. Among them, K set is a fixed threshold; When K r <K set When it is, compare the zero-sequence voltage U0 with the knee voltage U 0.g . When U0 < U 0.g , the operating threshold of the zero-sequence overcurrent protection is the minimum operating setting value plus the threshold part increased to avoid the unbalanced current generated by the self-starting of large motors; When U0≥U 0.g the operating threshold of the zero-sequence over-current protection is a floating threshold set according to the zero-sequence voltage amplitude; S4 When the grounding resistance in the small-resistance grounding system changes from small to large, compare the relationship between the zero-sequence current I0 and the operating threshold of the overcurrent protection, and determine whether it is a faulty line.

2. The single-phase grounding protection method based on a small-resistance grounding system according to claim 1, characterized in that, In the step S3, when K r ≥K set is satisfied, the operating threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, that is, the setting value of the zero-sequence overcurrent protection, which is expressed as: Among them, I set1 is the setting value of zero-sequence over-current protection; E A is the power supply voltage of the fault phase; R N is the neutral point grounding resistance.

3. The single-phase grounding protection method based on a small-resistance grounding system according to claim 1, characterized in that In the said step S3, when K r <K set and U0<U 0.g the operating threshold of zero-sequence overcurrent protection is expressed as: Among them, I set.min is the minimum operating current, K0 is the motor braking coefficient, and I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

4. The single-phase grounding protection method based on a small-resistance grounding system according to claim 3, characterized in that, When K r <K set and U0≥U 0.g the operating threshold of zero-sequence overcurrent protection is expressed as: I set3 = K1(U0 - U 0.g ) + I set.min (3) Wherein, K1 is the braking coefficient.

5. The single-phase grounding protection method based on a small-resistance grounding system according to claim 4, characterized in that The inflection point voltage U 0.g is expressed as:

6. The single-phase grounding protection method based on a small-resistance grounding system according to claim 5, characterized in that In the step S4, comparing the relationship between the zero-sequence current I0 and the operating threshold of the overcurrent protection includes: When I0 < I set2 or I0 < I set3 , the current distribution line is a non-faulty line; otherwise, the current distribution line is a faulty line.

7. The single-phase grounding protection method based on a small-resistance grounding system according to claim 6, characterized in that, When K r ≥K set , the faulty line with a metallic single-phase ground fault occurs in the distribution line, thus enabling protection blocking.

8. A single-phase grounding protection system based on a small-resistance grounding system, characterized in that, The system includes: A collection module for collecting and calculating the zero-sequence voltage, zero-sequence current, and three-phase current of the faulty line and the normal line; Line startup determination module, which is used to determine that when the zero-sequence voltage U0 collected is greater than the startup setting value U set or the sudden change in zero-sequence voltage ΔU is greater than the startup threshold ΔU set or the zero-sequence current I0 is greater than the startup threshold I 0set the protection starts and enters the comparison module; otherwise, it is determined that the current line is a non-faulty line; A comparison module for calculating the maximum value K of the second harmonic content of the three-phase current of a distribution line r , comparing K r with K set in magnitude. When K r ≥K set , the operating threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line when a metallic single-phase ground fault occurs in the distribution line, where K set is a fixed threshold value; When K r <K set , compare the zero-sequence voltage U0 with the knee-point voltage U 0.g . When U0 < U 0.g , the operating threshold of the zero-sequence overcurrent protection is the minimum operating setting value plus the additional threshold part for avoiding the unbalanced current generated by the self-starting of large motors; When U0≥U 0.g the operating threshold of the zero-sequence overcurrent protection is a floating threshold set according to the magnitude of the zero-sequence voltage; A line fault determination module for comparing the relationship between the zero-sequence current I0 and the operating threshold of the overcurrent protection when the grounding resistance in the small-resistance grounding system changes from small to large, and determining whether it is a faulty line.

9. The single-phase grounding protection system based on a small-resistance grounding system according to claim 8, wherein In the comparison module, when K r ≥K set is satisfied, the operating threshold of the zero-sequence overcurrent protection is the value of the zero-sequence current flowing through the faulty line during a solid single-phase ground fault on the distribution line, that is, the setting value of the zero-sequence overcurrent protection, which is expressed as: Among them, I set1 is the setting value of zero-sequence over-current protection; E A is the power supply voltage of the fault phase; R N is the neutral grounding resistance.

10. The single-phase grounding protection system based on a small-resistance grounding system according to claim 9, wherein In the described step comparison module, when K r <K set and U0 < U 0.g the operating threshold of the zero-sequence overcurrent protection is expressed as: Among them, I set.min is the minimum operating current, K0 is the motor braking coefficient, I max is the maximum effective value of the phase current; I n is the rated value of the phase current.

11. The single-phase grounding protection system based on a small-resistance grounding system according to claim 10, characterized in that, When K r <K set and U0≥U 0.g the operating threshold of zero-sequence over-current protection is expressed as: I set3 = K1(U0 - U 0.g ) + I set.min (7) Wherein, K1 is the braking coefficient.

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