Pseudo-power sum-difference waveform comparison method for offshore wind power transmission line longitudinal protection

Abstract

The present invention belongs to the technical field of power system relay protection and provides a pseudo-power sum and difference waveform comparison-type longitudinal protection method for offshore wind power transmission lines. The method comprises installing relay protection devices at both ends of an AC transmission line and collecting the instantaneous voltage and current values ​​on both sides of the line. When the sudden changes in three consecutive instantaneous current values ​​on either side meet the starting conditions, the first sudden change point is used as the fault occurrence time, the data of one cycle before the first current sudden change point is selected as the voltage and current operating data under normal conditions, and the data of one cycle after the first sudden change point is selected as the fault value of the instantaneous voltage and current values. The pseudo-power theorem is used to calculate the pseudo-power sum and pseudo-power difference. The Kendall correlation coefficient is used to measure the degree of difference between the pseudo-power sum and pseudo-power difference waveforms, and the protection action is controlled in combination with the setting value of the protection action. This method has the advantages of being unaffected by the fault type and having a strong ability to withstand transition resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system relay protection, and in particular relates to a pseudo-power and difference waveform comparison type offshore wind power transmission line longitudinal protection method. 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 advent of the "dual carbon" goals, new energy power generation technologies have experienced unprecedented development. Offshore wind power, with its significant advantages such as its lack of land-based resources and high utilization hours, has attracted widespread attention from researchers. Given the large capacity and long-distance transmission requirements of offshore wind power, it is often connected to the grid via flexible high-voltage direct current (VSC-HVDC) transmission. Furthermore, direct-drive permanent magnet synchronous motors (PMSGs) are often chosen as the primary generator set for large-scale offshore wind farms due to their high efficiency, compact size, wide air flow adjustment range, and excellent low-voltage ride-through performance.

[0004] However, since power electronic devices are used on both sides of the offshore wind farm connected to the grid via flexible direct current, the two sides of the line have limited amplitude, phase difference between the currents on both sides, and frequency offset is prone to occur, which makes the traditional longitudinal protection unable to operate correctly.

[0005] At the same time, with the increase in the capacity of offshore wind farms, fault ride-through capability is often required for a period of time after a fault occurs. The different control strategies on both sides of the line also make the AC transmission line present fault characteristics different from those of traditional power sources. Summary of the Invention

[0006] To address the inapplicability of traditional pilot protection for AC transmission lines from offshore wind farms, this invention proposes a pseudo-power sum-difference waveform comparison method for offshore wind power transmission line pilot protection. This method utilizes the fundamental pseudo-power theorem in circuits. By collecting the instantaneous values ​​of voltage and current at both ends of the line, mathematical operations are performed on the expressions that satisfy the pseudo-power theorem before and after the fault. By measuring the difference between the pseudo-power sum and pseudo-power difference waveforms after the operations, internal and external faults are determined. This method is unaffected by the control strategy or data window length on either side of the line, exhibits strong resistance to transition resistance, and requires no line parameters.

[0007] According to some embodiments, the present invention adopts the following technical solutions:

[0008] A pseudo-power sum-difference waveform comparison method for offshore wind power transmission line longitudinal protection, comprising:

[0009] Install relay protection devices at both ends of the AC transmission line and collect the instantaneous voltage and current values ​​on both sides of the line;

[0010] When the mutation amount of three consecutive instantaneous current values ​​on either side meets the starting condition, the first mutation point is taken as the fault occurrence time, the data of one cycle before the first current mutation point is selected as the voltage operation data and current operation data under normal conditions, and the data of one cycle after the first mutation point is selected as the fault value of the instantaneous voltage value and the instantaneous current value. Combined with the pseudo-power theorem, the pseudo-power sum and pseudo-power difference are calculated;

[0011] The Kendall correlation coefficient is used to measure the difference between the pseudo-power sum and the pseudo-power difference waveforms, and the protection action is controlled in combination with the setting value of the protection action.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] (1) It is not affected by the weak feedback of offshore wind farms and the phase angle difference of currents on both sides, and can solve the difficulties faced by the phase ratio braking protection method.

[0014] (2) It has strong adaptability to offshore wind farms connected to the flexible direct current grid with power electronic devices on both sides, is less affected by distributed capacitance and control strategy, and has strong resistance to transition resistance.

[0015] (3) Compared with the protection method using phasor values, it can effectively prevent inaccuracies and frequency leakage in Fourier transform, which is of great significance to offshore wind farms containing a large number of power electronic devices. 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 pseudo-power sum difference waveform comparison method for offshore wind power transmission line longitudinal protection provided by an embodiment of the present invention;

[0018] Figure 2 This is a single-line schematic diagram of the equivalent system of AC transmission lines for offshore wind farms connected to the grid via flexible direct current.

[0019] Figure 3 Schematic diagram of the system structure and fault location of an offshore wind farm connected to the grid via flexible direct current provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0021] 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.

[0022] 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.

[0023] like Figure 1 As shown, this embodiment provides a flowchart of a pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection, the specific steps of which include:

[0024] Step 1: Install identical relay protection devices on both sides of the wind farm AC transmission lines M and W. The relay protection devices on both sides independently collect the instantaneous values ​​of voltage and current on both sides of the line and determine the time when the fault occurs using the following formula:

[0025]

[0026] Where: is the current phase difference, Indicates the current mutation amount at the kth sampling point, represents the kth current sampling point, and N represents the sampling value of one current cycle.

[0027] If three consecutive points on any side satisfy When the fault is detected, the fault diagnosis begins.

[0028] Step 2: When the starting conditions are met, select the moment when the first mutation point occurs as the fault start time, select the instantaneous value of the voltage and current of one cycle before the first mutation point as the value during normal operation, select the instantaneous value of the voltage and current of one cycle after the first mutation point as the value after the fault, and combine Figure 2 The line can be written as shown in formula (1) with the pseudo-power theorem:

[0029]

[0030] Where d represents the total number of branches, u m ,i m , They are the instantaneous values ​​of voltage and current on the flexible DC side during normal operation and after a fault, respectively. w ,i w , are the instantaneous values ​​of voltage and current on the wind farm side during normal operation and after a fault, respectively. c ,i c , are the instantaneous values ​​of voltage and current of each branch of the line during normal operation and after fault respectively, u f ,i f , They are the voltage and current before the fault and the voltage and current after the fault of the fault branch respectively. Under normal circumstances, when the system is operating normally and an out-of-zone fault occurs, the fault branch inside the line can be regarded as having infinite impedance. At this time, the current before the fault can be considered to be i f =0.

[0031] Step 3: Perform sum and difference operations on the written Tellegen pseudo-power theorem to obtain the pseudo-power sum and pseudo-power difference. When the system is operating normally, the fault branch current i f =0, and then we can get the formula (1) At this time, the pseudo power sum and pseudo power difference can be expressed as shown in formula (2) and formula (3) respectively:

[0032]

[0033]

[0034] Step 4: Express the instantaneous value of voltage with the instantaneous value of current, such as Figure 2 The instantaneous voltage values ​​of each branch in the equivalent model of the AC transmission line shown in FIG1 during normal operation and after a fault can be expressed as Equation (4) and Equation (5) respectively:

[0035]

[0036]

[0037] Where: R c ,C c ,L c They are Figure 2 The equivalent impedance of each branch includes resistance, capacitance, and inductance, i c , are the instantaneous current values ​​before and after the c-th branch fault respectively.

[0038] When a fault occurs in the AC transmission line, the current in each branch contains not only the fundamental frequency component but also the DC component and the high-frequency component. Therefore, the current after the fault can be written as shown in formula (6):

[0039]

[0040] Where: represents the current after the cth branch fault, represents the DC component, represents the amplitude of the nth high-frequency component of the cth branch after the fault occurs, ω represents the angular frequency, It represents the initial phase angle of the nth harmonic of the cth branch after the fault.

[0041] During normal operation, the branch current can be expressed as shown in formula (7):

[0042]

[0043] Where: i c (t) represents the instantaneous current value of the cth branch during normal operation, I c Indicates the magnitude of the current.

[0044] In practical applications, the fundamental frequency component and the second frequency component in the post-fault current are greater than the other high-frequency components, and the cable line resistance is much smaller than the line inductance. Therefore, for simplicity, only the fundamental frequency component and the second frequency component in the fault current are considered in the formula derivation process, and the influence of the line resistance is ignored.

[0045] Combining the above formulas, substituting equations (4) to (7) into equation (1) and simplifying them, we can obtain the pseudo-power sum as shown in equation (8):

[0046]

[0047] The pseudo power difference is shown in formula (9):

[0048]

[0049] In formula (8) and formula (9): R c ,C c ,L c are the resistance, capacitance and inductance of each branch respectively, I c Indicates the amplitude of the current during normal operation, They represent the amplitudes of the DC component, fundamental frequency component and double frequency component in the fault current of the cth branch respectively. represents the initial phase angle of the current in the cth branch during normal operation, They represent the initial phase angles of the fundamental frequency component and the double frequency component after the fault respectively.

[0050] From the analysis of formula (8) and formula (9), it can be seen that when an out-of-area fault occurs, there is no fault branch inside the line. In the branch capacitor C c Among the related components, the first item is exactly the same, and the third and fourth items are the same except for the amplitude. In addition, there is a difference between the two in terms of the double frequency component and the DC component.

[0051] And with the inductor Lc In the relevant part, the first term in the two formulas is exactly the same. At this time, the difference between the two formulas in this term is small, and they have a high degree of similarity. c The third item in the correlation component has opposite signs and a 3-fold difference in amplitude, and the fourth item still has opposite signs and unequal amplitudes. In addition, the pseudo-power sum and the pseudo-power difference still differ by a double frequency component and a DC component. Therefore, with the inductance L c Correlated components contain both identical components and components with opposite signs.

[0052] At the same time, for large-scale offshore wind power AC transmission lines, which are usually submarine cables, the distributed capacitance is larger than that of traditional overhead lines. According to the above analysis, when an out-of-area fault occurs, there are differences in the components contained in the pseudo-power sum and pseudo-power difference waveforms, but the content of similar components is relatively high. At the same time, the amount of pseudo-power and mid-double frequency content will also have a great influence on the similarity between the two.

[0053] Therefore, when an out-of-area fault occurs on the AC transmission line, there is no fault branch inside the line, that is, the difference between the pseudo power sum and the pseudo power difference is is 0, so and Fully reflecting the shape and changing characteristics of the waveform, according to the aforementioned differences in the components contained in the two, will result in the two waveforms not being able to completely overlap, and there must be certain differences.

[0054] When a fault occurs within the area, u f is the line voltage before the fault, is the current flowing to the fault branch after the fault, and Far greater than and At this time, the pseudo power sum and pseudo power difference waveforms Both of them have a dominant position, so it can be considered that the two formulas only contain Ignoring the other components, the waveforms of the pseudo-power sum and the pseudo-power difference should be almost completely consistent.

[0055] Moreover, according to the previous derivation, the pseudo-power sum and the pseudo-power difference also contain different DC components. In order to reduce the influence of the DC component in the waveform, the present invention uses the Kendall correlation coefficient, which is not affected by the waveform amplitude, to measure the degree of difference between the two waveforms, and then determine the location of the fault.

[0056] Step 5: Use the Kendall correlation coefficient, which can reflect the similarity of waveforms, to measure the difference between the two waveforms. When the sampling points of the two waveforms increase or decrease at the same time over time, it indicates consistency. When the sampling points of the two waveforms change in opposite directions over time, it indicates inconsistency. The Kendall correlation coefficient formula is shown in formula (10):

[0057]

[0058] Where τ2 represents the calculated Kendall correlation coefficient, N is the number of sampling points, N c Indicates the number of sampling points that meet consistency, N d represents the number of pairs of inconsistent sampling points, Where r represents the number of small sets composed of the same elements on the wind farm side, t z Indicates the number of elements contained in the zth small set. Similarly, N B The same is true for the soft straight side.

[0059] The Kendall correlation coefficient is a statistical method for measuring the degree of similarity. Its value varies between [-1, 1]. When the Kendall correlation coefficient is close to 1, it means that the two waveforms being compared have a high degree of similarity. When the Kendall correlation coefficient is close to -1, it means that the two waveforms being compared have a low degree of similarity. The Kendall coefficient has no relationship with the amplitude of the compared objects.

[0060] When the system is in normal operation, the protection will not start if the starting conditions are not met.

[0061] When an out-of-area fault occurs on the AC transmission line, the components contained in the pseudo-power sum and pseudo-power difference waveforms are significantly different. According to the Kendall correlation coefficient calculation formula, the two waveforms are significantly different and have poor similarity.

[0062] When an AC transmission line fault occurs in the area, the pseudo power sum and pseudo power difference waveforms The proportion is much greater than that of the other components, so the two waveforms can be considered to be exactly the same. According to the Kendall correlation coefficient calculation formula, there is almost no difference between the two waveforms at this time, and the similarity is strong and close to 1.

[0063] Since the calculated result of the Kendall coefficient is between [-1,1], it is greatly affected by uncertain factors and the observation is not intuitive enough. Therefore, the result of the Kendall calculation can be multiplied by a coefficient of 10 to amplify it. The amplified Kendall correlation coefficient will be represented by τ′2 in the following text.

[0064] Step 6: Determine the protection action threshold value based on the protection principle described above. From the above analysis, it can be seen that although the two waveforms of the pseudo-power sum and the pseudo-power difference contain different components, the components with stronger similarity dominate, making the waveforms show similar characteristics to a certain extent. Therefore, based on experience, it can be considered to set the threshold τ′ of whether the amplified Kendall correlation coefficient is activated. set Set it to 6. To ensure a certain margin, multiply the result by the margin factor 1.4, and the set value is 8.4.

[0065] Step 7: Calculate the relationship between τ′2 and the set protection setting value for each phase to determine the location and type of fault.

[0066] In the specific implementation process, by comparing with the setting results, there will be the following situations:

[0067] If the value of τ′2 of one phase of the AC transmission lines A, B, and C of the offshore wind farm is greater than the set value, it means that a single-phase grounding fault has occurred, and the faulty phase circuit breaker will trip.

[0068] If the values ​​of τ′2 of two or three phases of the AC transmission lines A, B, and C of the offshore wind farm are greater than the set value, it means that a two-phase short-circuit fault or a three-phase short-circuit fault has occurred. At this time, the relay protection device issues a two-phase or three-phase tripping command.

[0069] If the values ​​of τ′2 of the three phases A, B, and C of the AC transmission lines of the offshore wind farm are all less than the set values, it means that the line has an out-of-zone fault. At this time, the relay protection device does not issue a tripping command and the circuit breaker does not trip.

[0070] Example

[0071] An offshore wind farm grid-connected model via flexible direct current (FDC) was built using the EMTP-RV simulation software, and a longitudinal protection method for offshore wind farm transmission lines based on pseudo-power theorem waveform differences proposed in the embodiment was simulated and verified.

[0072] 1) Model establishment

[0073] The grid connection model of offshore wind farms connected via flexible direct current is as follows: Figure 3As shown in the figure, there are four wind farms, each with a capacity of 100 MW. The rated output voltage of the direct-drive wind turbine in each wind farm is 0.69 kV, which is stepped up to 35 kV by the box transformer, connected through a 35 kV medium-voltage collector, stepped up to 220 kV by the main transformer, and sent to the flexible direct current terminal through the 220 kV 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 ±400 kV. The length of the 220 kV AC transmission line of the wind farm is 10 km. The positive-sequence resistance and inductance of the line are 0.0529 Ω / km and 0.45 mH / km respectively, the zero-sequence resistance and inductance of the line are 0.0530 Ω / km and 0.45 mH / km respectively, the positive-sequence capacitance and zero-sequence capacitance of the line are c1 = 0.155 μF / km and c0 = 0.155 μF / km respectively. The simulation adopts a data window length of 20 ms and a sampling frequency of 4 kHz.

[0074] 2) Simulation analysis

[0075] To verify the feasibility of the offshore wind power transmission line longitudinal protection method based on pseudo-power and differential waveform comparison proposed in this embodiment, faults with different conditions were set at points K2 and K3, and the results are recorded in Tables 1, 2, and 3. Table 1 shows the results corresponding to different fault types when a metallic fault occurs. Table 2 shows the results corresponding to different fault types when an in-zone short-circuit fault occurs through different transition resistances. Table 3 shows the results corresponding to different fault types when an out-of-zone short-circuit fault occurs through different transition resistances.

[0076] Table 1 Impact of different fault types on protection

[0077]

[0078] Table 2 Effects of different transition resistances on protection when K2 point fault occurs

[0079]

[0080]

[0081] Table 3 Effects of different transition resistances on protection when K3 point fault occurs

[0082]

[0083]

[0084] Table 1 shows that the offshore wind power transmission line longitudinal protection method based on pseudo-power and differential waveform comparison proposed in this embodiment can accurately determine the fault location under different fault types. Tables 2 and 3 show that as the transition resistance gradually increases, the value of τ'2 remains far from the protection setting value. The protection can reliably operate for in-zone faults and reliably fail to operate for out-of-zone faults.

[0085] 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 are intended to be within the scope of protection of the present invention.

Claims

1. A pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection, characterized in that: include: Install relay protection devices at both ends of the AC transmission line and collect the instantaneous voltage and current values ​​on both sides of the line; When the mutation amount of three consecutive instantaneous current values ​​on either side meets the starting condition, the first mutation point is taken as the fault occurrence time, the data of one cycle before the first current mutation point is selected as the voltage operation data and current operation data under normal conditions, and the data of one cycle after the first mutation point is selected as the fault value of the instantaneous voltage value and the instantaneous current value. Combined with the pseudo-power theorem, the pseudo-power sum and pseudo-power difference are calculated; The Kendall correlation coefficient is used to measure the difference between the pseudo-power sum and the pseudo-power difference waveforms, and the protection action is controlled in combination with the setting value of the protection action. The pseudo power sum is: Where d represents the total number of branches, They are the instantaneous values ​​of voltage and current on the flexible DC side during normal operation and after a fault, respectively. are the instantaneous values ​​of voltage and current on the wind farm side during normal operation and after a fault, respectively. are the instantaneous values ​​of voltage and current of each branch of the line during normal operation and after fault respectively. They are the instantaneous values ​​of voltage and current before and after the fault of the faulty branch of the line; The pseudo power difference is: Where d represents the total number of branches, They are the instantaneous values ​​of voltage and current on the flexible DC side during normal operation and after a fault, respectively. are the instantaneous values ​​of voltage and current on the wind farm side during normal operation and after a fault, respectively. are the instantaneous values ​​of voltage and current of each branch of the line during normal operation and after fault respectively. They are the instantaneous values ​​of voltage and current before and after the fault of the faulty branch of the line; The Kendall correlation coefficient is an amplified Kendall correlation coefficient. The amplified Kendall correlation coefficient is: Where, It represents the Kendall correlation coefficient after 10 times magnification. , is the number of sampling points, represents the logarithm of sampling points that meet consistency, represents the logarithm of sampling points that satisfy inconsistency, ,in Indicates the number of small sets of points with the same value on the wind farm side, Indicates the The number of elements contained in a small set, similarly The same is true on the flexible DC side of the line.

2. The pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection according to claim 1 is characterized in that: The starting conditions are: in, , Indicates the current phase difference, Indicates the current mutation amount at the kth sampling point, represents the kth current sampling point, N represents the sampling value of one cycle of current, It is the protection start setting value.

3. The pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection according to claim 1, characterized in that: Before calculating the pseudo-power sum and pseudo-power difference, it is necessary to first obtain the pseudo-power theorem expressions at both ends of the line: Where d represents the total number of branches, They are the instantaneous values ​​of voltage and current on the flexible DC side during normal operation and after a fault, respectively. are the instantaneous values ​​of voltage and current on the wind farm side during normal operation and after a fault, respectively. are the instantaneous values ​​of voltage and current of each branch of the line during normal operation and after fault respectively. They are the instantaneous values ​​of voltage and current before and after the fault of the faulty branch of the line.

4. The pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection according to claim 1, characterized in that: The two sides of the line include a flexible straight line side and a wind farm side.

5. The pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection according to claim 1, characterized in that: Before using the Kendall correlation coefficient, the method includes: expressing the instantaneous value of voltage with the instantaneous value of current, and analyzing the difference degree of the meaning components in the expanded pseudo-power sum and pseudo-power difference expressions.

6. The pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection according to claim 1, characterized in that: The setting value of the protection action is determined according to the characteristics of the components contained in the internal and external faults when the fault occurs and combined with the amplified Kendall correlation coefficient.

7. The pseudo-power and difference waveform comparison method for offshore wind power transmission line longitudinal protection according to claim 1, characterized in that: Calculate the relationship between the Kendall correlation coefficient and the set protection setting value for each phase to determine the location and type of fault, including: If the Kendall correlation coefficient of one phase of the AC transmission line A, B, and C of the offshore wind farm is greater than the set value, it means that a single-phase grounding fault has occurred, and the circuit breaker of the faulty phase will trip. If the Kendall correlation coefficient of two or three phases of the AC transmission lines A, B, and C of the offshore wind farm is greater than the set value, it means that a two-phase short circuit fault or a three-phase short circuit fault has occurred. At this time, the relay protection device issues a two-phase or three-phase trip command; If the three-phase Kendall correlation coefficients of the AC transmission lines A, B, and C of the offshore wind farm are all less than the set values, it means that the fault on the line is out of range. At this time, the relay protection device does not issue a tripping command and the circuit breaker does not trip.

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