Direct-driven wind power plant differential protection discrimination method based on harmonic trend difference comparison

Abstract

The invention discloses a direct-drive wind power plant differential protection judgment method based on harmonic trend difference comparison, relates to the technical field of power system relay protection, and solves the problem of delay action of transformer differential protection in a new energy alternating current grid-connected system. The method comprises the following steps: acquiring current instantaneous values at protection mounting positions of high and low voltage sides of the boost transformer of the direct-driven wind power plant in real time, calculating the differential current of the boost transformer, and discriminating that the differential current is located in an action area of steady-state ratio differential protection, and when the differential current is located in an action area of steady-state high-value ratio differential protection, judging that the differential current is in the action area of steady-state ratio differential protection. Whether the protection action is performed or not is judged by judging whether the excitation surge current criterion meets the open condition or not. And when the differential current is in a steady-state low-value ratio differential protection action area, taking the condition that the excitation surge current criterion and the TA transformer saturation criterion both meet the open condition as the condition for carrying out the protection action. On the basis, differential protection can be carried out more accurately, and safe and stable operation of a power system is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of power systems, and in particular to a direct-drive wind farm differential protection identification method based on harmonic trend difference comparison. Background Art

[0002] As an important part of new energy, wind farms are welcoming opportunities for rapid development. By converting wind energy into electrical energy, wind farms can not only reduce dependence on fossil fuels, but also effectively reduce greenhouse gas emissions. The step-up transformer raises the output voltage of the doubly-fed wind farm and then centrally integrates it into the high-voltage AC power grid. Therefore, in order to ensure the safe and stable operation of the direct-drive wind farm, the step-up transformer needs to quickly and reliably cut off the fault when a fault occurs. At present, longitudinal current differential protection is the main protection method for step-up transformers in direct-drive wind farms. In theory, current differential protection can provide absolute selectivity when a fault occurs inside the step-up transformer, ensuring rapid and reliable operation; in the event of an external fault, it can maintain the reliability of non-action. However, when a direct-drive wind farm is connected to an AC grid-connected system, a low-resistance fault may occur inside the step-up transformer. At this time, the short-circuit current amplitude provided by the direct-drive wind farm is limited and contains harmonic components, which will cause the transformer current transformer (TA) saturation discrimination element and the excitation inrush current discrimination element to malfunction and cause the transformer differential protection to be mistakenly locked, thereby causing the differential protection to have a delayed action, which is not conducive to the safe and stable operation of the power system.

[0003] In view of this, a direct-drive wind farm differential protection identification method based on harmonic trend difference comparison is needed. Summary of the invention

[0004] In view of the problem of delayed action of transformer differential protection in the new energy AC grid-connected system in the prior art, the present invention provides a direct-drive wind farm differential protection discrimination method based on harmonic trend difference comparison, which can perform differential protection more accurately and ensure the safe and stable operation of the power system. The specific technical solution is as follows:

[0005] A direct-drive wind farm differential protection identification method based on harmonic trend difference comparison includes the following steps:

[0006] S1: Real-time acquisition of the instantaneous current value at the high-voltage and low-voltage side protection installation of the direct-drive wind farm, phase compensation of the high-voltage and low-voltage side electrical quantities after filtering, and conversion of the low-voltage side current to the high-voltage side of the transformer;

[0007] S2: Perform Fourier decomposition on the currents on the high and low voltage sides of the boost converter to obtain the amplitude and phase of the fundamental wave, second harmonic and third harmonic on the high and low voltage sides;

[0008] S3: Calculate the currents on each side to obtain the proportions of the second and third harmonics, and at the same time compare the fundamental wave, second harmonic, and third harmonic trends of the currents on the high-voltage and low-voltage sides;

[0009] S4: Calculate the boosting differential current, enter the steady-state ratio differential protection discrimination element, and identify whether the differential current is in the action area of the steady-state ratio differential protection;

[0010] S5: If the differential current is in the action area of the steady-state high-value ratio differential protection, if the inrush current criterion meets the opening condition, the protection operates; if the inrush current criterion does not meet the opening condition, and at the same time the fundamental wave trends of the high-voltage and low-voltage side currents are quite different and the proportion trends of the second and third harmonics are quite similar, the protection operates; otherwise, the protection is blocked;

[0011] S6: If the differential current is in the action area of the steady-state low-value ratio differential protection, if both the inrush current criterion and the TA transformer saturation criterion meet the opening conditions, the protection operates; if the inrush current criterion and the TA transformer saturation criterion cannot be satisfied simultaneously, and at the same time the fundamental wave trends of the high-voltage and low-voltage side currents are quite different and the proportion trends of the second and third harmonics are quite similar, the protection operates; otherwise, the protection is blocked.

[0012] Preferably, the S1 includes:

[0013] Obtain the instantaneous values of the currents at the protection installation points on the high-voltage and low-voltage sides of the step-up transformer in the direct-drive wind farm in real time. After filtering, phase angle compensation is performed on the electrical quantities on the high-voltage and low-voltage sides. Taking Y0 / Δ-11 as an example for phase angle compensation, for the current on the Y0 side, the following formula is used for phase angle compensation:

[0014]

[0015] For the current on the Δ side, the following formula is used for phase angle compensation:

[0016]

[0017] After phase angle compensation for other wiring methods, the following formula is used to convert the low-voltage side current to the high-voltage side of the transformer by amplitude conversion:

[0018]

[0019] In the formula: is the current on the Δ side, is the current of each phase after phase angle compensation on the Δ side; is the current on the Y0 side, is the current of each phase after phase angle compensation on the Y0 side, is the low-voltage side current after amplitude conversion, and k is the transformer ratio.

[0020] Preferably, the S2 includes:

[0021] The Fourier decomposition of the current on the high-voltage side of the step-up transformer is carried out using the following formula to obtain the amplitudes and phases of the fundamental wave, second harmonic, and third harmonic of the high-voltage side current:

[0022]

[0023] The Fourier decomposition of the current on the low-voltage side of the step-up transformer is carried out using the following formula to obtain the amplitudes and phases of the fundamental wave, second harmonic, and third harmonic of the low-voltage side current:

[0024]

[0025] Where: i h 、I h1 、I h2 、I h3 are the high-voltage side current, high-voltage side fundamental wave current, high-voltage side second harmonic current, and high-voltage side third harmonic current respectively; i l 、I l1 、I l2 、I l3 are the low-voltage side current, low-voltage side fundamental wave current, low-voltage side second harmonic current, and low-voltage side third harmonic current respectively, and T is the time of one cycle of the power frequency.

[0026] Preferably, the S3 includes:

[0027] The ratios of the second and third harmonics of the high-voltage side current are obtained using the following formula:

[0028]

[0029] The ratios of the second and third harmonics of the low-voltage side current are obtained using the following formula:

[0030]

[0031] The trends of the fundamental wave, second harmonic, and third harmonic of the high and low voltage sides are compared using the following formula:

[0032]

[0033] Where: n is the number of sampling points in half a cycle, PLV1 represents the trend difference of the fundamental wave component of the current, PLV2 represents the trend difference of the second harmonic ratio, and PLV3 represents the trend difference of the third harmonic ratio.

[0034] Preferably, the S4 includes:

[0035] The three-phase differential current of the transformer is calculated using the following formula:

[0036]

[0037] Wherein, φ represents three phases A, B, and C;

[0038] Compare the three-phase differential currents to obtain their maximum value |I dφmax |. Compare the maximum value |I dφmax | of the three-phase differential currents with the differential current starting setting value I cdqd . If |I dφmax | > I cdqd , the protection is started;

[0039] Use the following formula to judge the steady-state ratio differential protection:

[0040]

[0041] Where: I 1φ......mφ are the currents on each side of the transformer respectively, I dφ is the single-phase differential current, |I dφmax | is the maximum value of the three-phase differential currents, I cdqd is the differential current starting setting value. I e is the rated current of the transformer, I 1φ......mφ are the currents on each side of the transformer respectively, I dφ is the single-phase differential current, I rφ is the single-phase braking current, φ represents three phases A, B, and C, I cdqd is the steady-state ratio differential starting setting value, K b1 is the ratio braking coefficient setting value (0.2 ≤ K b1 ≤ 0.75).

[0042] Preferably, the S5 includes:

[0043] If the differential current is in the steady-state high-value ratio differential protection action area, use the following formula to judge the inrush current:

[0044]

[0045] Where: I 2nd , I 3nd are the second harmonic and the third harmonic in each phase differential current respectively. I 1st is the fundamental wave of the differential current of the corresponding phase. k 2xb , k 3xb are the second harmonic and the third harmonic braking coefficient setting values respectively. In the device, k 2xb is fixed at 0.15. If the equation is satisfied, the inrush current discrimination element satisfies the opening condition. If t set consecutive sampling points satisfy the opening condition, the protection acts;

[0046] If the above equation is not satisfied, i.e., the inrush current criterion is blocked, then the following formulas are used to compare the trends of the fundamental waves of the high- and low-side currents and the ratios of the second and third harmonics on the high- and low-sides:

[0047]

[0048] If the trend comparison formula is satisfied, then the protection action is taken; otherwise, the protection is blocked.

[0049] Preferably, the S6 includes:

[0050] If the differential current is in the steady-state low-value ratio differential action area, then the following formulas are respectively used to discriminate the inrush current:

[0051]

[0052] At the same time, the following formulas are used to discriminate the TA saturation:

[0053]

[0054] If all the above equations are satisfied, i.e., the inrush current discrimination element satisfies the opening condition, and at the same time the TA saturation discrimination element satisfies the opening condition, and continuous t set sampling points satisfy the opening condition, then the protection action is taken; if the above equations are not completely satisfied, then the inrush current criterion and the TA saturation criterion are blocked, and the following formulas are used to compare the trends of the fundamental waves of the high- and low-side currents and the ratios of the second and third harmonics on the high- and low-sides:

[0055]

[0056] If the trend comparison formula is satisfied, then the protection action is taken; otherwise, the protection is blocked.

[0057] A computer-readable storage medium, the computer-readable storage medium includes a stored program, wherein when the program runs, it controls the device where the computer-readable storage medium is located to execute the above-mentioned direct-drive wind farm differential protection discrimination method based on harmonic trend difference comparison.

[0058] A processor, the processor is used to run a program, wherein when the program runs, it executes the above-mentioned direct-drive wind farm differential protection discrimination method based on harmonic trend difference comparison.

[0059] Compared with the prior art, the beneficial effects of the present invention are:

[0060] The present invention obtains the instantaneous current values at the protection installation locations of the high and low voltage sides of the step-up transformer in a direct-drive wind farm in real time. After filtering, the electrical quantities on the low voltage side are phase-compensated and uniformly converted to the high voltage side of the transformer. The differential current of the step-up transformer is calculated and fed into the steady-state ratio differential protection discrimination element, and then it is determined whether the differential current is in the action area of the steady-state ratio differential protection. The currents on the high and low voltage sides of the step-up transformer and the differential current are subjected to Fourier decomposition to obtain the amplitudes and phases of the fundamental, second harmonic, and third harmonic of the high and low voltage sides and the differential current. The ratios of the second and third harmonics of the differential current and the currents on each side are calculated respectively, and at the same time, the trends of the fundamental, second harmonic, and third harmonic of the currents on the high and low voltage sides are compared. If the differential current is in the action area of the steady-state high-value ratio differential protection and the magnetizing inrush current criterion meets the opening condition, the protection operates; if the magnetizing inrush current criterion does not meet the opening condition, and at the same time, the difference in the fundamental wave trends of the high and low voltage side currents is large, and the difference in the ratios of the second and third harmonics is small, the protection operates; otherwise, the protection is blocked. If the differential current is in the action area of the steady-state low-value ratio differential protection and both the magnetizing inrush current criterion and the TA transformer saturation criterion meet the opening conditions, the protection operates; if the magnetizing inrush current criterion and the TA transformer saturation criterion cannot be simultaneously satisfied, and at the same time, the difference in the fundamental wave trends of the high and low voltage side currents is large, and the difference in the ratios of the second and third harmonics is small, the protection operates; otherwise, the protection is blocked. Based on this, the present invention can perform differential protection more accurately and ensure the safe and stable operation of the power system. Description of the Drawings

[0061] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or 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 do not necessarily draw to actual scale.

[0062] Figure 1 It is a flowchart of the technical solution provided by the present invention in an embodiment of the present invention;

[0063] Figure 2 It is a schematic diagram of the structure of a typical new energy power station transmission system provided in an embodiment of the present invention;

[0064] Figure 3 It is the situation of the ratios of the second and third harmonics of the fault-phase current flowing through the protection installation location on the low voltage side during a low-resistance fault in the low voltage side area of the step-up transformer of a new energy power station in an embodiment of the present invention;

[0065] Figure 4 It is the curve of the fundamental wave trend, the second harmonic ratio trend, and the third harmonic ratio trend in the present invention during a fault in the high voltage side area of the step-up transformer of a new energy power station in an embodiment of the present invention

[0066] Figure 5In an embodiment of the present invention, the curves of the fundamental wave trend, the second harmonic ratio trend, and the third harmonic ratio trend in the present invention are adopted for the inrush current on the high-voltage side of the booster transformer in the new energy power station. Detailed implementation manners

[0067] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. 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.

[0068] It should be understood that when used in this specification and the appended claims, the terms "comprises" and "comprising" indicate the presence of the described features, wholes, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or their combinations.

[0069] It should also be understood that the terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the specification of the present invention and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an", and "the" are intended to include the plural forms.

[0070] It should be further understood that the term " / and" as used in the specification of the present invention and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0071] It should be noted that the following embodiments are described by taking a typical direct-drive wind farm transmission system as an example, as Figure 2 shown.

[0072] The wind farm mainly includes direct-drive wind turbines, box transformers, collector lines, and a main transformer in the power station. The wind farm is connected to an external system through a transmission line. u s is the equivalent voltage source of the external system; N and M are the installation locations of the high- and low-voltage side protections of the booster transformer.

[0073] A differential protection discrimination scheme for the booster transformer in a direct-drive wind farm based on the comparison of harmonic trend differences is provided, including:

[0074] S1: Obtain the instantaneous current values at the installation locations of the high- and low-voltage side protections of the booster transformer in the direct-drive wind farm in real time. After filtering, phase angle compensation is performed on the electrical quantities on the high- and low-voltage sides, and the low-voltage side current is converted to the high-voltage side of the transformer;

[0075] S2: Perform Fourier decomposition on the currents of the high and low voltage sides of the step-up transformer to obtain the amplitudes and phases of the fundamental wave, second harmonic, and third harmonic on the high and low voltage sides;

[0076] S3: Calculate the ratios of the second and third harmonics in the currents on each side, and at the same time compare the trends of the fundamental wave, second harmonic, and third harmonic of the currents on the high and low voltage sides;

[0077] S4: Calculate the differential current of the step-up transformer, enter the steady-state ratio differential protection discrimination element, and identify whether the differential current is in the action area of the steady-state ratio differential protection;

[0078] S5: If the differential current is in the action area of the steady-state high-value ratio differential protection, and if the inrush current criterion meets the opening condition, the protection operates; if the inrush current criterion does not meet the opening condition, and at the same time the difference in the fundamental wave trends of the high and low voltage side currents is large, and the difference in the ratios of the second and third harmonics trends is small, then the protection operates; otherwise, the protection is blocked; (where "large" and "small" are judged by those skilled in the art according to the actual situation by setting corresponding comparison thresholds).

[0079] S6: If the differential current is in the action area of the steady-state low-value ratio differential protection, and if both the inrush current criterion and the TA transformer saturation criterion meet the opening conditions, the protection operates; if the inrush current criterion and the TA transformer saturation criterion cannot be satisfied simultaneously, and at the same time the difference in the fundamental wave trends of the high and low voltage side currents is large, and the difference in the ratios of the second and third harmonics trends is small, then the protection operates; otherwise, the protection is blocked. (where "large" and "small" are judged by those skilled in the art according to the actual situation by setting corresponding comparison thresholds).

[0080] In one embodiment, the S1 includes:

[0081] Obtain the instantaneous values of the currents at the protection installation locations on the high and low voltage sides of the step-up transformer in the direct-drive wind farm in real time. After filtering, perform phase angle compensation on the electrical quantities on the high and low voltage sides. Taking Y0 / Δ-11 as an example for phase angle compensation, for the current on the Y0 side, use the formula for phase angle compensation, and for the current on the Δ side, use the formula for phase angle compensation. Other wiring methods can be analogized. After phase angle compensation, then use the formula to convert the current on the low voltage side to the high voltage side of the transformer through amplitude conversion.

[0082] In the formula: is the current on the Δ side, is the current of each phase after phase angle compensation on the Δ side; is the current on the Y0 side, is the current of each phase after phase angle compensation on the Y0 side, is the current on the low voltage side after amplitude conversion, and k is the transformer ratio.

[0083] In one embodiment, S2 includes:

[0084] Using the formula to perform Fourier decomposition on the current on the high-voltage side of the step-up transformer to obtain the amplitudes and phases of the fundamental wave, second harmonic, and third harmonic of the high-voltage side current;

[0085] Using the formula to perform Fourier decomposition on the current on the low-voltage side of the step-up transformer to obtain the amplitudes and phases of the fundamental wave, second harmonic, and third harmonic of the low-voltage side current.

[0086] Where: i h , I h1 , I h2 , I h3 are respectively the high-voltage side current, high-voltage side fundamental wave current, high-voltage side second harmonic current, and high-voltage side third harmonic current; i l , I l1 , I l2 , I l3 are respectively the low-voltage side current, low-voltage side fundamental wave current, low-voltage side second harmonic current, and high-voltage side third harmonic current, and T is the time of one cycle of the power frequency.

[0087] In one embodiment, S3 includes:

[0088] Using the formula to obtain the ratios of the second and third harmonics of the high-voltage side current, and using the formula to obtain the ratios of the second and third harmonics of the low-voltage side current. Using the formula to compare the trends of the fundamental wave, second harmonic, and third harmonic of the high and low voltage sides. Where: n is the number of sampling points in half a cycle, PLV1 represents the trend difference of the fundamental wave component of the current, PLV2 represents the trend difference of the second harmonic ratio, and PLV3 represents the trend difference of the third harmonic ratio. In this example, n = 40.

[0089] In one embodiment, S4 includes:

[0090] Using the formula to calculate the three-phase differential current of the transformer, where φ is the three phases of A, B, and C. Then compare the three-phase differential current to obtain its maximum value |I dφmax |, and compare the maximum value of the three-phase differential current |I dφmax | with the differential current starting setting value I cdqd . If |I dφmax | > I cdqd is satisfied, the protection is started. Using the formula

[0091] to perform steady-state ratio differential protection discrimination. Where: I1φ......mφ are the currents on each side of the transformer, I dφ is the single-phase differential current, |I dφmax | is the maximum value of the three-phase differential current, I cdqd is the setting value for starting differential current. I e is the rated current of the transformer, I 1φ......mφ are the currents on each side of the transformer, I dφ is the single-phase differential current, I rφ is the single-phase braking current, φ represents three phases A, B, and C, I cdqd is the setting value for steady-state ratio differential starting, K b1 is the setting value for ratio braking coefficient (0.2 ≤ K b1 ≤ 0.75), and it can be fixedly set to 0.5.

[0092] In one embodiment, the S5 includes:

[0093] If the differential current is in the steady-state high-value ratio differential protection action area, use the formula to judge the inrush current. If the above equation is satisfied, that is, the inrush current discrimination element satisfies the opening condition, and if t set successive sampling points satisfy the opening condition, the protection will act; if the above equation is not satisfied, that is, the inrush current criterion is blocked, then use the formula to compare the trends of the fundamental waves of the high- and low-voltage side currents and the ratios of the second and third harmonics of the high- and low-voltage sides. The fault characteristics of restorative inrush current and sympathetic inrush current are that the difference in the fundamental waves of the high- and low-voltage side currents is small, the harmonic of the high-voltage side current is large, and the harmonic of the low-voltage side current is small. If the above trend comparison formula is satisfied, it indicates that the difference in the fundamental waves of the high- and low-voltage side currents is large, at the same time, the second and third harmonics of the high-voltage side current are small, and the second and third harmonics of the low-voltage side current are large, and the fault characteristics are greatly affected by the new energy power source, and the protection will act; otherwise, the protection will be blocked. Where: I 2nd 、I 3nd are the second and third harmonics in each phase of the differential current respectively. I 1st is the fundamental wave of the differential current of the corresponding phase. k 2xb 、k 3xb are the setting values for the second and third harmonic braking coefficients respectively. In the device, k 2xb is fixed at 0.15, and k 3xb is fixed at 0.2.

[0094] In one embodiment, the S6 includes:

[0095] If the differential current is in the steady-state low-value ratio differential action area, then use the formula to judge the inrush current and the formula Perform TA saturation discrimination. If the above equations are all satisfied, that is, the inrush current discrimination element satisfies the opening condition, and at the same time the TA saturation discrimination element satisfies the opening condition, and continuous t set sampling points satisfy the opening condition, then the protection operates; if the above equations are not fully satisfied, the inrush current criterion and the TA saturation criterion are blocked, and the formula is used to compare the trends of the fundamental waves of the high- and low-side currents and the ratios of the second and third harmonics of the high- and low-side currents. The fault characteristics of restorative inrush current and sympathetic inrush current are that the difference in the fundamental waves of the high- and low-side currents is small, the harmonics of the high-side current are large, and the harmonics of the low-side current are small. If the above trend comparison formula is satisfied, it indicates that the difference in the fundamental waves of the high- and low-side currents is large, at the same time the second and third harmonics of the high-side current are small, and the second and third harmonics of the low-side current are large, and the fault characteristics are greatly affected by the new energy power source, and the protection operates; otherwise the protection is blocked.

[0096] Using the comparison of the fundamental wave trend, the second harmonic ratio trend and the third harmonic ratio trend of the high and low sides in the present invention, the curve of its calculation result with time is as Figure 4 、 Figure 5 shown. At this time, when a fault occurs in the zone, the difference in the fundamental wave trends of the high and low sides is large, and the difference in the ratios of the second and third harmonics is small. When there is a sympathetic inrush current, the difference in the fundamental wave trends of the high and low sides is small, and the difference in the ratios of the second and third harmonics is large.

[0097] In summary, the present invention obtains the instantaneous current values at the protection installation locations of the high and low sides of the step-up transformer in the direct-drive wind farm in real time, compensates the phase angle of the electrical quantities on the low side after filtering processing, and uniformly converts them to the high side of the transformer. Calculate the differential current of the step-up transformer and enter the steady-state ratio differential protection discrimination element, and then identify that the differential current is in the action area of the steady-state ratio differential protection. Perform Fourier decomposition on the high and low side currents and the differential current of the step-up transformer to obtain the amplitudes and phases of the fundamental waves, second harmonics and third harmonics of the high and low side currents and the differential current. Calculate the ratios of the second and third harmonics of the differential current and the currents on each side respectively, and at the same time compare the trends of the fundamental waves, second harmonics and third harmonics of the high and low side currents. If the differential current is in the action area of the steady-state high-value ratio differential protection, and if the inrush current criterion satisfies the opening condition, the protection operates; if the inrush current criterion does not satisfy the opening condition, and at the same time the difference in the fundamental wave trends of the high and low side currents is large, and the difference in the ratios of the second and third harmonics is small, the protection operates; otherwise the protection is blocked. If the differential current is in the action area of the steady-state low-value ratio differential protection, and if both the inrush current criterion and the TA transformer saturation criterion satisfy the opening condition, the protection operates; if the inrush current criterion and the TA transformer saturation criterion cannot be satisfied simultaneously, and at the same time the difference in the fundamental wave trends of the high and low side currents is large, and the difference in the ratios of the second and third harmonics is small, the protection operates; otherwise the protection is blocked. Based on this, the present invention can perform differential protection more precisely and ensure the safe and stable operation of the power system.

[0098] Those of ordinary skill in the art can realize that the units of each example described in combination with the embodiments disclosed herein can be implemented by electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the composition of each example has been generally described according to functions in the above description. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present invention.

[0099] 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.

[0100] In addition, the functional units in each embodiment of the present invention can be integrated into one processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above-mentioned integrated units can be implemented in the form of hardware or in the form of software functional units.

[0101] If the above-mentioned 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 this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to enable a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: various media such as USB flash drives, read-only memories (ROMs), random access memories (RAMs), mobile hard disks, magnetic disks, or optical discs that can store program codes.

[0102] 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 them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of each embodiment of the present invention, and they should all be covered by the scope of the claims and the description of the present invention.

Claims

1. A differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison, characterized in that, It includes the following steps: S1: Obtain the instantaneous current values at the installation locations of the protection devices on the high-voltage and low-voltage sides of the step-up transformer in the direct-drive wind farm in real time. After filtering, perform phase angle compensation on the electrical quantities on the high- and low-voltage sides, and convert the low-voltage side current to the high-voltage side of the transformer; S2: Perform Fourier decomposition on the currents on the high- and low-voltage sides of the step-up transformer to obtain the amplitudes and phases of the fundamental, second harmonic, and third harmonic on the high- and low-voltage sides; S3: Calculate the ratios of the second and third harmonics in the currents on each side, and at the same time compare the trends of the fundamental, second harmonic, and third harmonic currents on the high- and low-voltage sides; S4: Calculate the differential current of the step-up transformer, and enter the steady-state ratio differential protection discrimination element to identify whether the differential current is in the action area of the steady-state ratio differential protection; S5: If the differential current is in the action area of the steady-state high-value ratio differential protection, if the inrush current discrimination criterion meets the opening condition, the protection operates; if the inrush current discrimination criterion does not meet the opening condition, and at the same time the difference in the trends of the fundamental currents on the high- and low-voltage sides is greater than the comparison threshold, and the difference in the trends of the ratios of the second and third harmonics is less than the comparison threshold, then the protection operates; otherwise, the protection is blocked; S6: If the differential current is in the action area of the steady-state low-value ratio differential protection, if both the inrush current discrimination criterion and the TA transformer saturation discrimination criterion meet the opening conditions, the protection operates; if the inrush current discrimination criterion and the TA transformer saturation discrimination criterion cannot be simultaneously opened, and at the same time the difference in the trends of the fundamental currents on the high- and low-voltage sides is greater than the comparison threshold, and the difference in the trends of the ratios of the second and third harmonics is less than the comparison threshold, then the protection operates; otherwise, the protection is blocked.

2. The differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to claim 1, wherein The S1 includes: Obtain the instantaneous current values at the installation locations of the protection devices on the high- and low-voltage sides of the step-up transformer in the direct-drive wind farm in real time. After filtering, perform phase angle compensation on the electrical quantities on the high- and low-voltage sides. Taking Y0 / Δ-11 as an example for phase angle compensation, for the current on the Y0 side, use the following formula for phase angle compensation: For the current on the Δ side, use the following formula for phase angle compensation: After phase angle compensation for other wiring methods, then use the following formula to convert the low-voltage side current to the high-voltage side of the transformer through amplitude conversion: Wherein: is the current on the Δ side, is the current of each phase after phase angle compensation on the Δ side; is the current on the Y0 side, is the current of each phase after phase angle compensation on the Y0 side, is the current on the low-voltage side after amplitude conversion, and k is the transformer ratio.

3. A differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to claim 1, characterized in that, The S2 includes: Use the following formula to perform Fourier decomposition on the current on the high-voltage side of the step-up transformer to obtain the amplitudes and phases of the fundamental, second harmonic, and third harmonic of the high-voltage side current: Use the following formula to perform Fourier decomposition on the current on the low-voltage side of the step-up transformer to obtain the amplitudes and phases of the fundamental, second harmonic, and third harmonic of the low-voltage side current: Where: i h , I h1 , I h2 , I h3 are the high-voltage side current, fundamental wave current of the high-voltage side, second harmonic current of the high-voltage side, and third harmonic current of the high-voltage side, respectively; i l , I l1 , I l2 , I l3 are the low-voltage side current, fundamental wave current of the low-voltage side, second harmonic current of the low-voltage side, and third harmonic current of the high-voltage side, respectively, and T is the time of one cycle of the power frequency.

4. A differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to claim 1, characterized in that, The S3 includes: Use the following formula to obtain the ratios of the second and third harmonics in the high-voltage side current: Use the following formula to obtain the ratios of the second and third harmonics in the low-voltage side current: Use the following formula to compare the trends of the fundamental, second harmonic, and third harmonic on the high- and low-voltage sides In the formula: n is the number of sampling points in half a cycle, PLV1 represents the difference in the trends of the fundamental current components, PLV2 represents the difference in the trends of the ratios of the second harmonics, and PLV3 represents the difference in the trends of the ratios of the third harmonics.

5. A differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to claim 1, characterized in that, The S4 includes: Use the following formula to calculate the three-phase differential current of the transformer: In the formula, φ represents the three phases of A, B, and C; Compare the three-phase differential currents to obtain their maximum value |I dφmax |, and compare the maximum value |I dφmax | of the three-phase differential currents with the differential current starting setting value I cdqd . If |I dφmax | > I cdqd is satisfied, the protection is started; Use the following formula for steady-state ratio differential protection discrimination: Where: I 1φ......mφ are the currents on each side of the transformer respectively, I dφ is the single-phase differential current, |I dφmax | is the maximum value of the three-phase differential current, I cdqd is the setting value of the differential current starting, I e is the rated current of the transformer, I 1φ......mφ are the currents on each side of the transformer respectively, I dφ is the single-phase differential current, I rφ is the single-phase braking current, φ represents the three phases A, B, and C, I cdqd is the setting value of the steady-state ratio differential starting, K b1 is the setting value of the ratio braking coefficient (0.2 ≤ K b1 ≤ 0.75).

6. The differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to claim 1, wherein The S5 includes: If the differential current is in the action area of the steady-state high-value ratio differential protection, use the following inrush current discrimination formula for inrush current discrimination: Where: I 2nd and I 3nd are the second harmonic and the third harmonic in the differential current of each phase respectively, and I 1st is the fundamental wave of the differential current of the corresponding phase, k 2xb and k 3xb are the setting values of the braking coefficients of the second harmonic and the third harmonic respectively. In the device, k 2xb is fixed at 0.

15. If the equation is satisfied, that is, the inrush current discrimination element satisfies the opening condition, and if the continuous t set sampling points satisfy the opening condition, the protection will operate; If the magnetizing inrush current discrimination equation is not satisfied, i.e., the magnetizing inrush current criterion is blocked, the following formula is used to compare the trends of the fundamental waves of the high- and low-side currents and the ratios of the second and third harmonics on the high- and low-sides: If the trend comparison formula is satisfied, the protection action is performed; otherwise, the protection is blocked.

7. A differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to claim 1, characterized in that The S6 includes: If the differential current is in the steady-state low-value ratio differential action area, the following magnetizing inrush current discrimination formulas are respectively used for magnetizing inrush current discrimination: At the same time, the following formula is used for TA saturation discrimination: If all the inrush current discrimination equations are satisfied, that is, the inrush current discrimination element satisfies the opening condition, and at the same time the TA saturation discrimination element satisfies the opening condition, and the continuous t set sampling points satisfy the opening condition, the protection will operate; if the inrush current discrimination equation is not fully satisfied, the inrush current criterion and the TA saturation criterion will be blocked, and the following formula will be used to compare the trends of the fundamental waves of the high- and low-side currents and the ratios of the second and third harmonics on the high- and low-sides: If the trend comparison formula is satisfied, the protection action is performed; otherwise, the protection is blocked.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored program, wherein, when the program runs, it controls the device where the computer-readable storage medium is located to execute the differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to any one of claims 1 to 7.

9. A processor, characterized in that, The processor is used to run the program, wherein, when the program runs, it executes the differential protection discrimination method for a direct-drive wind farm based on harmonic trend difference comparison according to any one of claims 1 to 7.

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