Wind farm transmission line protection method considering similarity of current variation

Abstract

The present invention belongs to the technical field of power system relay protection and provides a wind farm transmission line protection method that considers the similarity of current variation. The method comprises: installing relay protection devices at both ends of a large-scale offshore wind farm AC transmission line, collecting the instantaneous current value of each phase at both ends of the line; determining the fault moment and determining whether protection should be activated based on a phase current mutation detection method; when the activation conditions are met, subtracting the instantaneous current value of the current cycle before the fault from the collected instantaneous current value of the cycle after the fault to obtain the current variation on both sides of the line, and transmitting the obtained current variation to the opposite side of the line; based on the fault moment, using the Kendall algorithm to calculate the similarity of current variation at both ends of the line at the same fault moment and within the same data window for a long time; and controlling the protection action based on the current variation similarity and the protection setting value. The present invention is not affected by the data window length, noise, or the control strategy on both sides of the line.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system relay protection, and in particular relates to a wind farm transmission line protection method considering the similarity of current variation. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the increasing depletion of fossil fuels worldwide, renewable energy generation technologies, such as wind power and photovoltaics, have rapidly developed. Offshore wind power, in particular, has attracted attention from researchers worldwide due to its advantages such as high utilization hours and minimal land occupation. Because offshore wind farms are often located far from users, flexible high-voltage direct current (HVDC) transmission (VSC-HVDC) technology is often used to connect the electricity generated by offshore wind farms to the grid. As the installed capacity of offshore wind farms increases, a certain level of fault ride-through capability is required to ensure the normal operation of the grid.

[0004] For AC transmission lines with power electronic devices at both ends, the different low-voltage ride-through methods and control strategies on both sides of the line result in the current after the fault being limited in amplitude and having a phase angle difference between the currents on both sides, which are different from the fault characteristics of traditional power grids. This makes the traditional current longitudinal protection face adaptability issues. Therefore, it is necessary to study new protection principles from other perspectives of fault characteristics. Summary of the Invention

[0005] In order to solve the technical problems existing in the above-mentioned background technology, the present invention provides a wind farm transmission line protection method that considers the similarity of current variation. The present invention forms a protection principle by utilizing the characteristics that the current variation of the AC transmission line of an offshore wind farm is basically zero when the system is operating normally, the current variation on both sides of the line changes in the same direction when a fault occurs within the area, has the same variation trend, and is highly similar; when a fault occurs outside the area, the current variation on both sides of the AC transmission line changes in the opposite direction, has the opposite variation trend, and is relatively weak in similarity. The Kendall correlation coefficient that is not affected by the amplitude is selected to reflect the above characteristics to form a protection criterion. The method has the advantages of not being affected by the data window length, noise, and control strategies on both sides of the line, and having a strong ability to withstand transition resistance. It overcomes the problems faced by traditional longitudinal protection in offshore wind farm transmission lines connected to the flexible direct current grid.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a wind farm transmission line protection method taking into account the similarity of current variation.

[0008] A wind farm transmission line protection method considering similarity of current variation comprises:

[0009] Install relay protection devices at both ends of the AC transmission line of large-scale offshore wind farms and collect the instantaneous current value of each phase at both ends of the line;

[0010] Determine the fault moment and judge whether the protection is activated based on the phase current sudden change detection method;

[0011] When the starting conditions are met, the current change on both sides of the line is obtained by subtracting the instantaneous current value of the cycle before the fault from the instantaneous current value of the cycle after the fault, and the obtained current change is transmitted to the opposite side of the line;

[0012] According to the fault time, the Kendall algorithm is used to calculate the similarity of current changes at the same fault time and in the same data window for a long time at both ends of the line;

[0013] The protection action is controlled according to the similarity of the current variation and the protection setting value.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] The present invention first uses relay protection devices installed at both ends of the line to collect instantaneous current values ​​and uses the phase current mutation detection method as a method to determine whether the protection is activated. The protection is activated when three consecutive current mutations are greater than the set value. The value of the current cycle after the fault minus the cycle before the fault is selected as the current change on both sides of the line. The amplified Kendall correlation coefficient, which is not affected by the waveform amplitude, is selected to measure the similarity of the current change at both ends of the line within the same time window length. When the calculated result is greater than the protection setting value, the protection is activated; otherwise, the protection is not activated. The method of the present invention is not affected by the system's weak feedback, control strategy, and data window length, has a strong ability to withstand transition resistance, and can adapt to AC transmission lines with power electronic devices at both ends. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0017] Figure 1 A schematic flow chart of a wind farm transmission line protection method considering the similarity of current variation provided by an embodiment of the present invention;

[0018] Figure 2 The offshore wind farm system structure provided by the embodiment of the present invention is connected to the grid through flexible direct current;

[0019] Figure 3 This is the waveform of the current change on both sides of the line after a phase A grounding fault occurs in the area (point K2);

[0020] Figure 4 This is the waveform of the current change on both sides of the line after a phase A grounding fault occurs outside the area (point K3);

[0021] Figure 5 The changing trend of τ'2 value within 20ms after the phase A ground fault occurs in the area (point K2);

[0022] Figure 6 This is the changing trend of the τ'2 value within 20ms after a phase A grounding fault occurs outside the zone (point K3). DETAILED DESCRIPTION

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0025] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0026] It should be noted that the flowcharts and block diagrams in the accompanying drawings illustrate the possible implementation architecture, functions and operations of the methods and systems according to the various embodiments of the present disclosure. It should be noted that each box in the flowchart or block diagram can represent a module, program segment, or a part of code, and the module, program segment, or a part of code can include one or more executable instructions for implementing the logical functions specified in the various embodiments. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in an order different from that marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, or they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the flowchart and / or block diagram, and the combination of the boxes in the flowchart and / or block diagram, can be implemented using a dedicated hardware-based system that performs the specified function or operation, or can be implemented using a combination of dedicated hardware and computer instructions.

[0027] Example

[0028] like Figure 1 As shown, this embodiment provides a wind farm transmission line protection method considering the similarity of current variation, including the following steps:

[0029] Step 1: Install relay protection devices at both ends of the AC transmission line of a large-scale offshore wind farm and collect the instantaneous value of each phase current.

[0030] Step 2: Use the phase current mutation detection method as the basis for judging whether the protection is activated and determining the time when the fault occurs. When any phase among the three phases A, B, and C meets the condition that the instantaneous current mutation is greater than the set value for three consecutive times, the protection is activated and the first point greater than the set value is selected as the time when the fault occurs. The judgment criterion of the phase current mutation detection method is shown in formula (1):

[0031]

[0032] Where: is the current phase difference, represents the current mutation at the kth sampling point, represents the kth current sampling point, and N represents the sampling points of one current cycle.

[0033] Step 3: When the starting conditions are met, the instantaneous value of the current for one cycle before the fault is subtracted from the instantaneous value of the current for one cycle after the fault to obtain the current change on both sides of the line, and the obtained current change is transmitted to the opposite side of the line. The calculation formula of the current change is shown in formula (2):

[0034]

[0035] Where: represents the kth sampling value of the current; Indicates the current change of the kth sampling value; N indicates the number of sampling points in one cycle of current. Indicates separation.

[0036] Step 4: Based on the fault time determined in step 2, the Kendall algorithm, which is not affected by amplitude, is used to calculate the similarity of the current changes at the same fault time and in the same data window over a long period of time at both ends of the line. The current changes calculated on the wind farm side of the line are placed in set A, as shown in formula (3):

[0037]

[0038] Where: Indicates the phase difference of the current change, F indicates the wind farm side, and n indicates the nth current change calculated in chronological order.

[0039] The current changes calculated on the flexible DC side of the line are put into set B respectively as shown in formula (4):

[0040]

[0041] Where: Indicates the phase difference of the current change, M indicates the flexible DC side, and n indicates the nth current change calculated in chronological order.

[0042] Take the corresponding elements from the two sets to form the set C, as shown in formula (5):

[0043]

[0044] For a more intuitive observation, the calculated Kendall coefficient is multiplied by 10 and the protection setting value is determined. The amplified Kendall correlation coefficient is denoted by τ'2 in the following text. Its calculation formula is shown in formula (6):

[0045]

[0046] Where τ'2 represents the Kendall correlation coefficient after amplification by 10 times, n is the number of sampling points, n c Represents the number of logarithms of elements in set C that satisfy consistency, n d represents the number of logarithms of inconsistency elements in set C, Where f represents the number of small sets of the same elements on the wind farm side of the transmission line, t z The number of elements in the zth small set is represented by n2. Similarly, the same is true for n2 relative to the flexible direct side of the line.

[0047] Take any two elements from the set C and Among them (1≤i≤n), (1≤j≤n), when and or and When , the two waveforms are said to have a positive correlation. If all elements in the set C are consistent, the value of τ'2 is 10, that is, the current changes on both sides should increase or decrease at the same time. and or and The two are said to be inconsistent. When all elements in set C are inconsistent, the value of τ'2 is -10, which means that the current changes on both sides of the line are completely opposite. When τ'2 is 0, it means that the number of consistent elements and inconsistent elements in set C is equal or all elements are the same size. Other cases are between (-10, 0) or (0, 10).

[0048] Step 5: Compare the value of τ'2 of each phase with the set protection action setting value. When the calculated result is greater than the setting value, the protection is activated. When the calculated result is less than the setting value, the protection is not activated.

[0049] When the current flowing from the busbar to the line is specified as the positive direction of the current, ideally, the current change during normal system operation is zero, so the protection setting value can be considered to be set to 0. Considering that during actual normal system operation, the current change of the previous and next cycle may not be exactly the same, but the current change on both sides of the line must be opposite. According to the selected similarity algorithm, the similarity calculation result of the current change on both sides of the line must be less than 0. Therefore, it is reasonable to set the protection setting value to 0.

[0050] According to the comparison between the value of the current variation τ'2 of each phase of the line and the set value, there are the following situations:

[0051] When the value of the current change τ'2 on both sides of the three phases A, B, and C of the AC transmission line is greater than the protection action setting value for any phase, it is judged as a single-phase grounding fault, the corresponding fault phase circuit breaker trips, and the non-fault phase protection does not operate.

[0052] When the value of the current change τ'2 on both sides of the three phases A, B, and C of the AC transmission line is greater than the protection setting value for any two phases, it is judged as a two-phase-to-phase or two-phase-to-ground short circuit fault. At this time, the circuit breaker of the corresponding fault phase trips, and the protection of the non-fault phase does not operate.

[0053] When the values ​​of the current variation τ'2 on both sides of the three phases A, B, and C of the AC transmission line are all greater than the protection setting value, it is judged as a three-phase fault. At this time, the faulty phase circuit breaker trips and the protection is activated.

[0054] When the values ​​of the current variation τ'2 on both sides of the three phases A, B, and C of the AC transmission line are all less than the protection setting value, it is judged that there is no fault inside the line. At this time, the protection does not operate and the circuit breaker does not trip.

[0055] In this embodiment, a grid-connected model of an offshore wind farm connected to the grid via flexible direct current (FDC) is constructed using EMTP-RV simulation software, and a wind farm transmission line protection method considering the similarity of current variation proposed in the embodiment is simulated and verified.

[0056] 1) Model establishment

[0057] The grid connection model of offshore wind farms connected via flexible direct current is as follows: Figure 2As shown in the figure, the total capacity of the wind farm is 400MW, the voltage output by the wind farm through the transformer is 35kV, which is stepped up to 220kV by the main transformer and sent to the flexible DC terminal through the 220kV AC transmission line. The generated electricity is transmitted to the onshore power grid through the MMC-HVDC. The rated voltage of the MMC-HVDC is ±400kV. The length of the 220kV AC transmission line of the wind farm is 10km. The positive sequence resistance and inductance of the line are 0.0529Ω / km and 0.45mH / km respectively, the zero sequence resistance and inductance of the line are 0.0530Ω / km and 0.45mH / km respectively, the positive sequence capacitance and zero sequence capacitance of the line are c1=0.155μF / km and c0=0.155μF / km respectively. The simulation adopts a data window length of 20ms and a sampling frequency of 4kHz.

[0058] 2) Simulation analysis

[0059] In order to verify the feasibility of the wind farm transmission line protection method proposed in this embodiment considering the similarity of current variation, faults with different conditions are set at points K2 and K3, such as Figure 3 and Figure 4 The following are the waveforms of the current changes on both sides of the line when the A-phase metallic grounding fault occurs inside and outside the area. The trend of the current changes at both ends of the line is consistent with the previous analysis. Figure 5 and Figure 6 are the changes of τ'2 when the metallic ground fault of phase A occurs inside and outside the area, respectively. Figure 5 It can be seen that when a fault occurs in the area, the value of τ'2 increases rapidly, which can accurately and quickly identify the fault. Figure 6 It can be seen that when an out-of-zone fault occurs, the value of τ'2 always remains below the set value, and the protection is reliable and does not operate. At the same time, to verify the impact of different fault types on the protection method, the results corresponding to different fault types when metallic faults occur are recorded in Table 1.

[0060] Table 1 Impact of different fault types on protection

[0061]

[0062] From the data analysis in Table 1, it can be seen that the protection has good adaptability to different fault types and can accurately identify the location of the fault.

[0063] Considering that when a high-resistance single-phase grounding fault occurs, there is a large phase difference between the currents on both sides of the AC transmission line, which makes the traditional longitudinal protection composed of current phasors have the risk of failure to operate. Therefore, this paper analyzes the Figure 2 The adaptability of protection when phase A grounding fault occurs at points K2 and K3 through different transition resistances is calculated, and the calculation results are recorded in Table 2 and Table 3 respectively.

[0064] Table 2 τ'2 values ​​when K2 point passes through different transition resistance faults

[0065]

[0066] Table 3 τ'2 values ​​of K3 point when it passes through different transition resistance faults

[0067]

[0068] As shown in Table 2 and Table 3, the data show that the method of the invention has good resistance to transition resistance, can overcome the phase angle difference caused by the control strategy and transition resistance on both sides of the line, and has good adaptability to the AC transmission line of the offshore wind farm connected to the flexible direct current grid.

[0069] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A wind farm transmission line protection method considering the similarity of current variation is characterized by: include: Install relay protection devices at both ends of the AC transmission line of large-scale offshore wind farms and collect the instantaneous current value of each phase at both ends of the line; Determine the fault moment and judge whether the protection is activated based on the phase current sudden change detection method; The specific process of determining the fault moment and judging whether the protection is activated based on the phase current sudden change detection method includes: when any phase of the three phases A, B, and C meets the condition that the instantaneous current sudden change is greater than the set value for three consecutive times, the protection is activated, and the first point greater than the set value is selected as the fault occurrence moment; The criterion of the phase current sudden change detection method is: Where: is the current phase difference, represents the current mutation at the kth sampling point, represents the kth current sampling point, and N represents the number of sampling points in one current cycle; When the startup conditions are met, the current variation on both sides of the line is obtained by subtracting the instantaneous current value of the cycle before the fault from the instantaneous current value of the cycle after the fault, and the obtained current variation is transmitted to the opposite side of the line; the two ends of the line include the wind farm side of the line and the flexible DC side of the line; According to the fault time, the Kendall algorithm is used to calculate the similarity of the current change at the same fault time and in the same data window for a long time at both ends of the line. The amplified Kendall correlation coefficient in the Kendall algorithm is: Where, It represents the Kendall correlation coefficient after 10 times magnification. , is the number of sampling points, represents the number of logarithms of elements in set C that satisfy consistency, represents the number of logarithms of inconsistency elements in set C, ,in Indicates the number of small sets composed of the same elements on the wind farm side of the line, Indicates the The number of elements contained in a small set, similarly The same is true on the flexible straight side of the line; The protection action is controlled according to the similarity of the current variation and the protection setting value.

2. The wind farm transmission line protection method considering the similarity of current variation according to claim 1 is characterized in that: Construct the current variation set A on the wind farm side of the transmission line: Construct the current variation set B on the flexible direct current side of the line: Based on the current variation set A and the current variation set B, the current variation set C is constructed: Where: Indicates the phase difference of current change, Indicates the wind farm side, Indicates the soft straight side, Indicates the first A current change.

3. The wind farm transmission line protection method considering the similarity of current variation according to claim 1 is characterized in that: Take any two elements from the set C and ,in ; when and or and If all elements in set C are consistent, then The value of is 10; when and or and If the elements in set C are inconsistent, then The value of is -10, when When it is 0, it means that the number of consistent elements and inconsistent elements in set C is equal or all elements are of the same size; The rest of the cases are between (-10,0) or (0,10).

4. The wind farm transmission line protection method considering the similarity of current variation according to claim 1 is characterized in that: The controlling of the protection action according to the similarity of the current variation and the protection setting value specifically includes: comparing the current variation of each phase of the line with the protection setting value, and when the calculation result is greater than the protection setting value, controlling the protection action; when the calculation result is less than the protection setting value, the protection does not act.

5. The wind farm transmission line protection method considering the similarity of current variation according to claim 4 is characterized in that: According to the change of each phase current of the line The numerical value of is compared with the set value, and there are the following situations: When the current changes on both sides of the three-phase A, B, and C of the AC transmission line When the value of any phase is greater than the protection action setting value, it is judged as a single-phase grounding fault, the corresponding fault phase circuit breaker trips, and the non-fault phase protection does not operate; When the current changes on both sides of the three-phase A, B, and C of the AC transmission line When the values ​​of any two phases are greater than the protection setting value, it is judged as a two-phase phase-to-phase or two-phase grounding short circuit fault. At this time, the corresponding fault phase circuit breaker trips and the non-fault phase protection does not operate; When the current changes on both sides of the three-phase A, B, and C of the AC transmission line When the values ​​of the three phases are all greater than the protection setting value, it is judged as a three-phase fault. At this time, the circuit breaker of the faulty phase trips and the protection is activated; When the current changes on both sides of the three-phase A, B, and C of the AC transmission line When the values ​​of the three phases are all less than the protection setting value, it is judged that there is no fault inside the line. At this time, the protection does not operate and the circuit breaker does not trip.

6. The wind farm transmission line protection method considering the similarity of current variation according to any one of claims 1 to 5, characterized in that: The protection setting value is 0.

Images