Low-voltage protection method suitable for asymmetric fault of AC power grid containing new energy

Abstract

The invention discloses a low-voltage protection method suitable for an asymmetric fault of an AC power grid containing new energy, and relates to the technical field of power system relay protection. Firstly, according to the relative magnitude of the amplitude of each phase voltage, which type the fault condition belongs to can be judged, that is, the fault type and the fault phase are determined. Secondly, according to which effective interval the phase difference of each phase of voltage belongs to and the phase sequence is ABC or ACB in the fault type, it can be determined that at the moment of the fault, compared with a positive sequence component, the output influence of all new energy in the system is generally in the forward direction or the reverse direction. Then, according to the direction of the new energy influence component at the moment, a setting constant value used under the working condition is selected from the double constant values; and finally, by taking the selected constant value as a criterion of current protection, judging whether the fault is located in the line or not and whether protection acts or not. According to the invention, the protection range of low-voltage protection can be greatly improved, no extra required measurement equipment is added, and the method has economical efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of electric power systems, and in particular to a low voltage protection method suitable for asymmetric faults of an AC power grid containing new energy. Background Art

[0002] When a short-circuit fault occurs in the power system, the new energy has two working conditions: emitting inductive current and absorbing inductive current, and it has the characteristics of a current source. This will cause the power system containing new energy to have different phase current distribution characteristics from the conventional power system. Since overcurrent protection is related to the fault current amplitude, this characteristic of new energy may affect the calculation of overcurrent protection setting. Overcurrent protection requires starting with the most severe working condition of the protected line, taking the most serious short-circuit fault in the maximum mode as the corresponding working condition for setting calculation, and using this setting value to apply to all fault types of the protected line. However, after taking into account the impact of new energy, the fault current amplitude of the most severe working condition will increase, that is, at this time, the impact component of new energy is in the same direction as the positive sequence component. However, there will be working conditions in the line where the impact component of new energy is in the opposite direction to the positive sequence component, and the line current amplitude will decrease at this time. Due to the influence of new energy, the setting value will increase, and the fault working condition will have more current amplitude reduction. Therefore, the protection range of the overcurrent protection will be reduced, that is, the sensitivity will be reduced.

[0003] To this end, a low voltage protection method suitable for asymmetric faults in AC power grids containing new energy is provided to solve the above problems. Summary of the invention

[0004] The present invention discloses a low voltage protection method for asymmetric faults in an AC power grid containing renewable energy. The method sets multiple fixed values ​​to respectively apply different directions of the renewable energy impact component, thereby improving the sensitivity of overcurrent protection and solving the problems raised in the above-mentioned background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A low voltage protection method applicable to asymmetric faults in an AC power grid containing new energy sources comprises the following steps:

[0007] S1: Determine the fault type and fault phase according to the relative magnitude of the voltage amplitudes of each phase, wherein the fault types include single-phase grounding fault, two-phase interphase fault and two-phase grounding fault;

[0008] S2: Determine the direction of the new energy impact component based on the voltage phase difference and the phase sequence of the three-phase voltage;

[0009] S3: According to the direction of the new energy impact component, select the set value applicable to this fault condition from the double set values;

[0010] S4: According to the selected constant value, determine whether the fault is within the protection range at this time. If it is determined that the fault is within the protection range, the protection will be activated; otherwise, the protection will not be activated.

[0011] Preferably, the calculation process of the voltage amplitude of each phase is the same, wherein the calculation process of the voltage amplitude of phase A in the B phase ground short circuit fault is as follows:

[0012] According to the boundary conditions of the single-phase short-circuit grounding fault, the positive sequence current I at the fault point f is calculated. fa(1) :

[0013]

[0014] In the formula, I fa(1) is the positive sequence current at the fault point f; is the voltage at point f before the fault, taking 1.0pu; Z ff(1) , Z ff(2) and Z ff(0) are the positive-sequence, negative-sequence and zero-sequence self-impedance of the fault node f, respectively;

[0015] Thus, according to the obtained positive sequence current, each sequence voltage at the fault point f can be obtained;

[0016]

[0017] Where U fa(1) , U fa(2) and U fa(0) are the positive sequence, negative sequence and zero sequence voltages of phase A at the fault node f respectively;

[0018] Similarly, using the superposition theorem, the positive sequence, negative sequence and zero sequence voltages of phase A at node s where the protection is installed can be obtained;

[0019]

[0020] Where U sa(1) , U sa(2) and U sa(0) are the positive sequence, negative sequence and zero sequence voltages of phase A at node s where the protection is installed, respectively; is the voltage at point s before the fault, which is taken as 1.0pu.

[0021] Preferably, the process of determining the fault type according to the voltage amplitude is as follows:

[0022] The voltage amplitude characteristics of the single-phase short-circuit grounding fault TP1 are: two phases have the same voltage amplitude, and the voltage amplitude of another phase is greater than the same voltage amplitude of the two phases;

[0023] The voltage amplitude characteristics of the two-phase short-circuit fault TP2 are: the voltage amplitudes of two phases are the same, and the voltage amplitude of another phase is smaller than the same voltage amplitudes of the two phases, and there is no zero-sequence component in the system;

[0024] The voltage amplitude characteristics of the two-phase short-circuit grounding fault TP3 are: two phases have the same voltage amplitude, and the voltage amplitude of another phase is smaller than the same voltage amplitude of the two phases, and there is a zero-sequence component in the system.

[0025] Preferably, step S2 is specifically as follows:

[0026] The three types of faults in the three asymmetric faults, a total of 9 asymmetric fault conditions, are concentrated, and their phase angle differences and phase sequences are sorted and summarized in the following table:

[0027]

[0028] Combining the phase angle difference of each phase voltage of the fault and the phase sequence of the three-phase voltage, the direction of the new energy impact component under the fault condition is uniquely determined.

[0029] Preferably, in step S3, two types of constants A\B are set, and the type A constant is larger than the type B constant, and is suitable for the working condition that the direction of the new energy impact component is in the same direction as the positive sequence component; the type B constant is smaller than the type A constant, and is suitable for the working condition that the direction of the new energy impact component is opposite to the positive sequence component.

[0030] Preferably, the step S3 is specifically as follows:

[0031] Set different Class A and Class B settings for three types of faults respectively;

[0032] According to the direction of the new energy impact component, if it is in the same direction, select the Class A constant under the corresponding fault type; if it is in the opposite direction, select the Class B constant under the corresponding fault type.

[0033] A computer-readable storage medium includes a stored program, wherein when the program is running, the device where the computer-readable storage medium is located is controlled to execute the low-voltage protection method applicable to asymmetric faults of an AC power grid containing new energy as described above.

[0034] A processor is used to run a program, wherein when the program is run, the low voltage protection method applicable to asymmetric faults of an AC power grid containing new energy as described above is executed.

[0035] The beneficial effects of the present invention are:

[0036] The dual-setting low voltage protection method applicable to renewable energy sources described in the present invention analyzes the change of fault voltage amplitude caused by renewable energy influence components in different directions, i.e., the reduction of the protection range of low voltage protection, and proposes to give different applicable setting values ​​to the fault conditions in different directions of renewable energy influence components. The phase angle difference of phase voltage and phase sequence are used to comprehensively judge the direction of renewable energy influence components. This method can greatly improve the protection range of low voltage protection, and will not increase the required measurement equipment, so it is economical. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0038] Figure 1 It is a schematic diagram of a dual-setting low voltage protection method applicable to new energy sources according to the present invention;

[0039] Figure 2 It is a flow chart of a dual-setting low voltage protection method applicable to new energy sources according to the present invention;

[0040] Figure 3 This is a system structure diagram for simulating the method described in the present invention in an IEEE39 node. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0043] It should also be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0044] It should be further understood that the term "and / or" used in the present description and the appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0045] The present invention provides a dual-setting low voltage protection method suitable for new energy sources, combined with Figure 1 To illustrate, the following steps are included:

[0046] (1) Due to different asymmetric fault types, the three-phase voltage difference amplitudes measured at the protection installation are quite different. Therefore, according to the characteristics of the voltage difference amplitude, the fault type (TP1, TP2, TP3) can be first determined, and the selection range of the setting value can be narrowed down to one of the two.

[0047] (2) Subsequently, since the impact component of new energy has two directions: in the same direction as the positive sequence component; in the opposite direction to the positive sequence component. When the impact component of new energy is in the same direction as the positive sequence component, it has an increasing effect on the voltage; when the impact component of new energy is in the opposite direction to the positive sequence component, it has a weakening effect on the voltage. Therefore, the direction of the impact component of new energy is determined based on which valid interval the phase difference of each phase voltage belongs to within the fault type, and whether the phase sequence is ABC or ACB.

[0048] (3) According to the direction of the new energy impact component, the setting value suitable for the current fault condition is selected from the two setting values.

[0049] (4) Finally, the selected setting value is compared with the voltage amplitude measured at the protection installation location. If it is determined that the fault is within the protection range, the protection is activated; otherwise, the protection is not activated.

[0050] Among them, the voltage sequence component analysis applicable to asymmetric faults is as follows:

[0051] The symmetrical sequence component method is used for the asymmetrical faults in the system. Taking the B-phase grounding short-circuit fault as an example, the positive sequence current I at the fault point f can be calculated based on the boundary conditions of the single-phase short-circuit grounding fault. fa(1) .

[0052]

[0053] In the formula, I fa(1) is the positive sequence current at the fault point f; is the voltage at point f before the fault, usually 1.0pu; Z ff(1) , Z ff(2) and Z ff(0) are the positive-sequence, negative-sequence and zero-sequence self-impedances of the fault node f respectively.

[0054] Therefore, according to the positive sequence current obtained above, the sequence voltages at the fault point f can be obtained.

[0055]

[0056] Where U fa(1) , U fa(2) and U fa(0) are the positive-sequence, negative-sequence and zero-sequence voltages of phase A at the fault node f respectively.

[0057] Similarly, using the superposition theorem, the positive-sequence, negative-sequence and zero-sequence voltages of phase A at node s where the protection is installed can be obtained.

[0058]

[0059] Where U sa(1) , U sa(2) and U sa(0) are the positive sequence, negative sequence and zero sequence voltages of phase A at node s where the protection is installed, respectively; It is the voltage at point s before the fault, usually 1.0pu.

[0060] (1.2) Method for determining the direction of the new energy impact component based on phase angle difference and phase sequence for low voltage protection

[0061] The three types of faults in the three asymmetric faults, a total of 9 asymmetric fault conditions, are concentrated, and their phase angle differences and phase sequences are sorted and summarized in Appendix Table 1. According to the data in Table 1, the direction of the new energy impact component under the fault condition can be uniquely determined by combining the phase angle difference of the fault phase voltage and the phase sequence of the three-phase voltage.

[0062] For a single-phase short-circuit grounding fault, the direction of the new energy impact component can be determined only by the phase sequence: the fault condition with phase sequence ABC corresponds to a new energy impact component in the same direction as the positive sequence component; the fault condition with phase sequence ACB corresponds to a new energy impact component in the opposite direction to the positive sequence component. It can be seen from Appendix Table 1 that whether it is a phase A short-circuit grounding fault, a phase B short-circuit grounding fault, or a phase C short-circuit grounding fault, the above judgment conditions are met, regardless of the fault phase.

[0063] For a two-phase short-circuit fault, the direction of the new energy impact component can also be determined only by the phase sequence: the fault condition with phase sequence ABC corresponds to a new energy impact component in the same direction as the positive sequence component; the fault condition with phase sequence ACB corresponds to a new energy impact component in the opposite direction to the positive sequence component. It can be seen from Appendix Table 1 that whether it is an AB phase short-circuit fault, a BC phase short-circuit fault, or a CA phase short-circuit fault, the above judgment conditions are met, regardless of the fault phase.

[0064] For a two-phase grounding short-circuit fault, the direction of the new energy impact component cannot be determined solely by the phase sequence, and the phase sequence needs to be combined with the phase voltage phase angle difference for judgment. It can be seen from Appendix Table 1 that due to the two-phase short-circuit grounding fault, there are two fault conditions with significant differences. This is because when the new energy impact component is reversed, if the new energy impact component is small, the phase voltage of the non-fault phase cannot be reversed to the positive sequence voltage, which is condition 1; if the new energy impact component is large, the phase voltage of the non-fault phase is reversed to the positive sequence voltage, which is condition 2. It is easy to see that the above two conditions have completely different phase angle difference characteristics, which need to be discussed and analyzed separately. Among them, taking the BC two-phase ground short circuit fault as an example, the θ0 in the table is the phase angle difference between the voltages of phase A and phase B when there is no new energy impact component; the θ1 in the table is the phase angle difference between the phase voltage of phase B and the negative sequence voltage of phase A when the new energy impact component increases to just make the voltage amplitude of phase A equal to 0; the θ2 in the table is the phase angle difference between the phase voltage of phase A and the phase voltage of phase B when the new energy impact component increases to make the voltage amplitude of phase A, phase voltage amplitude of phase B, and phase voltage amplitude of phase C equal. It can be seen from Appendix Table 1 that the direction of the new energy impact component can be determined by combining the phase sequence and the voltage phase angle difference. Whether it is the AB phase ground short circuit fault, the BC phase ground short circuit fault, or the CA phase ground short circuit fault, the above judgment conditions are met and it has nothing to do with the fault phase.

[0065] The fixed value calculation taking into account the different directions of the impact of new energy components is as follows:

[0066] As Appendix Figure 2 As shown in the flow chart, for the three types of faults, namely, single-phase short-circuit grounding fault, two-phase phase-to-phase fault and two-phase grounding fault, different setting values ​​are set for the new energy impact component in the same direction and the opposite direction of the positive sequence component in this type.

[0067] Taking the A-phase grounding fault in a single-phase grounding fault as an example, the constant A1 corresponds to the fault type in which the new energy impact component and the positive sequence component are in the same direction. The constant is calculated as follows:

[0068] I stage low voltage protection setting

[0069] In order to avoid the most severe fault of the largest mode at the end of this line, the voltage amplitude measured at the protection installation is used for setting calculation.

[0070] Stage II low voltage protection setting

[0071] The II-stage protection needs to be coordinated with the I-stage protection of the downstream line, and needs to be smaller than the voltage amplitude measured at the installation location of the protection when the downstream line fails. In order to ensure the selectivity of the protection, so that the protection does not operate, that is, when a fault occurs within the protection range of the I-stage of the downstream line, the II-stage protection of the line does not operate, it is necessary to add a delay to the II-stage of the line protection to ensure that there will be no false operation for the downstream line fault.

[0072] Stage III low voltage protection setting

[0073] The calculation method for selecting the setting value of the stage III low voltage protection setting is basically the same as that of the stage II low voltage protection setting.

[0074] In summary, the dual-setting low voltage protection method applicable to new energy sources is as follows:

[0075] The flowchart of the dual-setting low voltage protection method applicable to new energy sources is shown in the appendix. Figure 2 As shown, first, according to the characteristics of the relative magnitude of the voltage amplitude of each phase, the fault condition type (TP1, TP2, TP3) can be judged, and the fault type and fault phase can be determined. Secondly, according to which valid interval the phase difference of each phase voltage belongs to within the fault type, and whether the phase sequence is ABC or ACB, it can be determined that at this time of the fault, compared with the positive sequence component, the output impact of all new energy in the system is generally positive or reverse. Then, according to the direction of the new energy impact component at this time, the setting value used in this condition is selected from the dual setting. Finally, the selected setting is used as the criterion for current protection to determine whether the fault is located in this line and whether the protection is activated.

[0076] The following takes the A-phase short-circuit grounding fault of a single-phase short-circuit grounding fault as an example to further illustrate this solution (other fault types are similar):

[0077] For a single-phase short-circuit grounding fault, taking the A-phase short-circuit grounding fault as an example, due to the fact that the new energy impact component and the positive sequence component are in the same direction, as the new energy impact component gradually increases, the phase angle difference between the phase voltages changes monotonically. And the phase sequence of the three-phase voltage is always ABC. When the new energy impact component and the positive sequence component are in the opposite direction, when the absorption component amplitude increases from 0, the A-phase voltage amplitude increases, and the B and C-phase voltage amplitudes decrease accordingly. The phase angle difference between the A-phase voltage and the B-phase voltage decreases monotonically; the phase angle difference between the A-phase voltage and the C-phase voltage decreases monotonically; the phase angle difference between the B-phase voltage and the C-phase voltage first increases monotonically to 180 degrees, and then decreases monotonically. And the phase sequence of the three-phase voltage is always ACB. Based on the above analysis, it can be easily known that the direction of the new energy impact component in a single-phase short-circuit grounding fault can be determined according to the phase sequence.

[0078] For a two-phase short-circuit fault, taking the BC phase short-circuit grounding fault as an example, due to the same direction of the new energy impact component and the positive sequence component, as the new energy impact component gradually increases, the phase angle difference between the phase voltages changes monotonically. And the phase sequence of the three-phase voltage is always ABC. When the new energy impact component and the positive sequence component are in the opposite direction, when the absorption component amplitude increases from 0, the A phase voltage amplitude increases, and the B and C phase voltage amplitudes decrease accordingly. The phase angle difference between the A phase voltage and the B phase voltage increases monotonically; the phase angle difference between the A phase voltage and the C phase voltage increases monotonically; the phase angle difference between the B phase voltage and the C phase voltage decreases monotonically from 180 degrees. And the phase sequence of the three-phase voltage is always ACB. Based on the above analysis, it can be easily known that the direction of the new energy impact component in a two-phase short-circuit fault can be determined according to the phase sequence.

[0079] For a two-phase short-circuit grounding fault, taking the BC phase short-circuit grounding fault as an example, due to the same direction of the new energy impact component and the positive sequence component, as the new energy impact component gradually increases, the phase angle difference between the phase voltages changes monotonically. And the phase sequence of the three-phase voltage is always ABC. When the new energy impact component and the positive sequence component are in the opposite direction, when the absorption component amplitude increases from 0, the A phase current amplitude first decreases monotonically to 0, and then increases monotonically; the B and C phase voltage amplitudes have been monotonically increasing, and the phase angle difference between the A phase voltage and the B phase voltage and the phase angle difference between the A phase voltage and the C phase voltage first increase monotonically, and the phase angle difference between the B phase voltage and the C phase voltage decreases monotonically. And at this time, the three-phase voltage sequence is ABC. When the new energy impact component increases to make the A phase voltage amplitude 0, the electrical quantity characteristics will change due to the subsequent reverse phase voltage of the A phase. When the impact of new energy continues to increase, the phase angle difference between the voltage of phase A and the voltage of phase B and the phase angle difference between the voltage of phase A and the voltage of phase C decreases monotonically, and the phase angle difference between the voltage of phase B and the voltage of phase C increases monotonically. And at this time, the phase sequence of the three-phase voltage is ACB. Based on the above analysis, it can be known that the direction of the impact of new energy in a two-phase short-circuit grounding fault cannot be determined based on the phase sequence alone. As shown in Appendix Table 1, the phase sequence needs to be combined with the phase angle difference of each phase voltage, and the characteristics of two electrical quantities can be combined to determine the direction of the impact of new energy.

[0080] Table 1 Summary of effective range of phase difference of fault current

[0081]

[0082] That is, the protection scheme of the multiple fixed values ​​is: after the effective interval of the current amplitude and phase difference of each phase measured at the protection installation, first determine whether the maximum value of the three-phase current amplitude is greater than the setting value. If the condition is not met, the protection will not operate; if the condition is met, the relative size of the three-phase current amplitude is used to determine which type of working condition (TP1, TP2, TP3) the working condition belongs to. Then determine whether the phase difference of the three-phase current falls within a certain effective setting value interval corresponding to the type of fault. If the condition is not met, the protection will not operate; if the condition is met, it can be obtained according to Appendix Table 1 which type of the 9 asymmetric faults the fault working condition belongs to, so that the protection can operate accurately.

[0083] In order to further illustrate the dual-setting low voltage protection method applicable to new energy provided by the embodiment of the present invention, the present invention is further described below in conjunction with the embodiment.

[0084] Taking the IEEE39 node system as an example, node 30 is set to be connected to a new energy source, and the new energy impact component is set to increase gradually from 0 to 1.0 pu in the same / reverse direction to simulate the network with different new energy penetration rates in the actual system, making the simulation test universal. It can be seen from the results that the protection range of the low voltage protection method with multiple fixed values ​​is greatly improved compared to the low voltage protection range with only one fixed value.

[0085] In summary, the present invention provides a dual-setting low-voltage protection method suitable for new energy sources. By analyzing the two-directional influence (increase or decrease) of the new energy power supply on the fault voltage amplitude, the phase angle difference and phase sequence between the phases of the fault voltage are comprehensively analyzed to uniquely screen out the direction of the new energy impact component under the fault condition at this time. And according to the direction, select the applicable setting value for protection setting comparison. In this way, different new energy impact components correspond to different setting values, which can increase the protection range of low voltage protection and improve protection sensitivity.

[0086] Those of ordinary skill in the art will appreciate that the units of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the composition of each example has been generally described in terms of function in the above description. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0087] In the embodiments provided by the present invention, it should be understood that the division of units is only a logical function division, and there may be other division methods in actual implementation, for example, multiple units can be combined into one unit, one unit can be split into multiple units, or some features can be ignored, etc.

[0088] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0089] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a computer device (which can be a personal computer, a server or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, read-only memory (ROM, Read-0nlyMemory), random access memory (RAM, RandomAccessMemory), mobile hard disk, magnetic disk or optical disk, etc., which can store program code.

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims

1. A low voltage protection method suitable for asymmetric faults in an AC power grid containing new energy, characterized in that: The following steps are involved: S1: Determine the fault type and fault phase according to the relative magnitude of the voltage amplitudes of each phase, wherein the fault types include single-phase grounding fault, two-phase interphase fault and two-phase grounding fault; S2: Determine the direction of the new energy impact component based on the voltage phase difference and the phase sequence of the three-phase voltage; S3: According to the direction of the new energy impact component, select the set value applicable to this fault condition from the double set values; S4: According to the selected constant value, determine whether the fault is within the protection range at this time. If it is determined that the fault is within the protection range, the protection will be activated; otherwise, the protection will not be activated.

2. A low voltage protection method applicable to asymmetric faults of AC power grid containing new energy according to claim 1, characterized in that: The calculation process of the voltage amplitude of each phase is the same, and the calculation process of the voltage amplitude of phase A in the B phase ground short circuit fault is as follows: According to the boundary conditions of the single-phase short-circuit grounding fault, the positive sequence current I at the fault point f is calculated. fa(1) : In the formula, I fa(1) is the positive sequence current at the fault point f; is the voltage at point f before the fault, taking 1.0pu; Z ff(1) , Z ff(2) and Z ff(0) are the positive-sequence, negative-sequence and zero-sequence self-impedance of the fault node f, respectively; Thus, according to the obtained positive sequence current, each sequence voltage at the fault point f can be obtained; Where U fa(1) , U fa(2) and U fa(0) are the positive sequence, negative sequence and zero sequence voltages of phase A at the fault node f respectively; Similarly, using the superposition theorem, the positive sequence, negative sequence and zero sequence voltages of phase A at node s where the protection is installed can be obtained; Where U sa(1) , U sa(2) and U sa(0) are the positive sequence, negative sequence and zero sequence voltages of phase A at node s where the protection is installed, respectively; is the voltage at point s before the fault, which is taken as 1.0pu.

3. A low voltage protection method applicable to asymmetric faults of AC power grid containing new energy according to claim 2, characterized in that: The process of judging the fault type based on the voltage amplitude is as follows: The voltage amplitude characteristics of the single-phase short-circuit grounding fault TP1 are: two phases have the same voltage amplitude, and the voltage amplitude of another phase is greater than the same voltage amplitude of the two phases; The voltage amplitude characteristics of the two-phase short-circuit fault TP2 are: the voltage amplitudes of two phases are the same, and the voltage amplitude of another phase is smaller than the same voltage amplitudes of the two phases, and there is no zero-sequence component in the system; The voltage amplitude characteristics of the two-phase short-circuit grounding fault TP3 are: two phases have the same voltage amplitude, and the voltage amplitude of another phase is smaller than the same voltage amplitude of the two phases, and there is a zero-sequence component in the system.

4. A low voltage protection method applicable to asymmetric faults of AC power grid containing new energy according to claim 1, characterized in that: Step S2 is specifically as follows: The three types of faults in the three asymmetric faults, a total of 9 asymmetric fault conditions, are concentrated, and their phase angle differences and phase sequences are sorted and summarized in the following table: Combining the phase angle difference of each phase voltage of the fault and the phase sequence of the three-phase voltage, the direction of the new energy impact component under the fault condition is uniquely determined.

5. A low voltage protection method applicable to asymmetric faults of AC power grid containing new energy according to claim 4, characterized in that: In step S3, two types of constants A\B are set. The type A constant is larger than the type B constant, and is suitable for the working condition where the direction of the new energy impact component is in the same direction as the positive sequence component; the type B constant is smaller than the type A constant, and is suitable for the working condition where the direction of the new energy impact component is in the opposite direction to the positive sequence component.

6. A low voltage protection method applicable to asymmetric faults of AC power grid containing new energy according to claim 5, characterized in that: The step S3 is specifically as follows: Different A-type and B-type settings are set for three types of faults respectively; According to the direction of the new energy impact component, if it is in the same direction, select the Class A constant under the corresponding fault type; if it is in the opposite direction, select the Class B constant under the corresponding fault type.

7. A computer-readable storage medium, characterized in that: The computer-readable storage medium includes a stored program, wherein when the program is running, the device where the computer-readable storage medium is located is controlled to execute the low voltage protection method applicable to asymmetric faults of an AC power grid containing new energy as described in any one of claims 1 to 6.

8. A processor, characterized in that: The processor is used to run a program, wherein when the program is run, the low voltage protection method applicable to asymmetric faults of an AC power grid containing new energy sources as described in any one of claims 1 to 6 is executed.

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