A method and system for longitudinal protection of offshore wind power AC submarine cable lines

Abstract

The present invention discloses a pilot protection method and system for an AC submarine cable line of an offshore wind farm, which is characterized by comprising: determining the sampling values of three-phase currents on both sides of the line; after normalizing the sampling values of the three-phase currents on both sides, determining the sampling values of the three-phase currents on both sides after compensating for the capacitive current and the sampling values of the zero-sequence currents on both sides after compensation; constructing a first discrete signal x(k) and a second discrete signal y(k) according to the sampling values of the three-phase currents on both sides after compensating for the capacitive current and the sampling values of the zero-sequence currents on both sides after compensation; determining the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) xy ; according to the correlation degree ρ xy , determining a pilot protection criterion based on the correlation degree. Thus, internal and external faults can be quickly and correctly distinguished, and it has a strong ability to withstand high resistance, and can also weaken the influence of the distributed capacitance of the submarine cable and the weak feed characteristics of the wind power

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and more specifically, to an AC submarine cable longitudinal protection method and system. Background Art

[0002] Wind power systems contain a large number of power electronic components, and the large-scale grid connection of wind farms will bring many challenges to the safe and stable operation of the power grid. Relay protection is the first line of defense for the safe operation of the power grid. Traditional protection based on power frequency can protect the safety of the power grid in most cases, but wind power systems have different fault characteristics from traditional synchronous generators (such as weak feed, high harmonics, frequency offset, etc.). These differences reduce the performance of traditional AC line protection based on power frequency, and even cause false operation and refusal to operate. Some experts have tried to improve the traditional AC line protection after the wind power system is connected to the grid. Although traditional protection can be improved in some aspects, it is difficult to fully adapt to the wind power system.

[0003] With the increase in offshore wind power transmission capacity and transmission distance, the use of flexible direct current transmission to connect it to the grid has become an important way to connect offshore wind power to the grid. Compared with wind power systems, offshore wind power transmission systems through flexible direct current transmission not only need to consider the current extraction problem, but also need to consider the temporary locking of offshore flexible direct current converter equipment, resulting in the loss of fault characteristics. Traditional AC submarine cable protection will face more severe challenges. Therefore, it is urgent to study new protection principles that are suitable for offshore wind power transmission systems through flexible direct current transmission.

[0004] Correlation is the result of two sets of discrete data processed by correlation coefficient, which is mainly used to measure the degree of association between them. It is widely used in engineering fields such as physics, biology and medicine. In recent years, some scholars have applied relevant technologies to the principle of relay protection and conducted meaningful research. The application of correlation in traditional protection and wind farm transmission line protection has achieved good results. However, when the AC submarine cable line of the offshore wind power flexible direct current transmission system is taken as the protection object, the distributed capacitance current of the line and the control characteristics of the flexible direct current side will affect the performance of the correlation protection, and the ability of the correlation protection to tolerate transition resistance is generally poor. Summary of the invention

[0005] According to the present invention, a longitudinal protection method and system for an offshore wind power AC submarine cable line are provided to solve the technical problems in the prior art that when the AC submarine cable line of an offshore wind power flexible direct current transmission system is the protection object, the line distributed capacitance current and the control characteristics of the flexible direct current side will affect the correlation protection performance, and the correlation protection is generally poor in its ability to tolerate transition resistance.

[0006] According to a first aspect of the present invention, there is provided a method for longitudinal protection of an offshore wind power AC submarine cable line, comprising:

[0007] Determine the three-phase current sampling values ​​on both sides of the line;

[0008] After normalizing the three-phase current sampling values ​​on both sides, the three-phase current sampling values ​​on both sides after compensating the capacitor current and the zero-sequence current sampling values ​​on both sides after compensation are determined;

[0009] Constructing a first discrete signal x(k) and a second discrete signal y(k) according to the three-phase current sampling values ​​on both sides after the capacitor current is compensated and the zero-sequence current sampling values ​​on both sides after the compensation;

[0010] Determine the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) xy ;

[0011] According to the correlation degree ρ xy , determine the correlation longitudinal protection criterion.

[0012] Optionally, the three-phase current sampling values ​​on both sides include A-phase current sampling values ​​on both sides, B-phase current sampling values ​​on both sides, and C-phase current sampling values ​​on both sides.

[0013] Optionally, the first discrete signal x(k) includes a first discrete signal of the A-phase current sampling values ​​on both sides, a first discrete signal of the B-phase current sampling values ​​on both sides, a first discrete signal of the C-phase current sampling values ​​on both sides, and a first discrete signal of the zero-sequence current sampling value;

[0014] The second discrete signal y(k) includes a second discrete signal of the A-phase current sampling values ​​on both sides, a second discrete signal of the B-phase current sampling values ​​on both sides, a second discrete signal of the C-phase current sampling values ​​on both sides, and a second discrete signal of the zero-sequence current sampling values.

[0015] Optionally, the correlation degree ρ xy It includes: the correlation between the first discrete signal of the A-phase current sampling values ​​on both sides and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal of the B-phase current sampling values ​​on both sides and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal of the C-phase current sampling values ​​on both sides and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal of the zero-sequence current sampling value and the second discrete signal of the zero-sequence current sampling value.

[0016] Optionally, according to the correlation degree ρ xy , determine the relevant longitudinal protection criteria, including:

[0017] When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling value adopt an OR logic, a protection action signal is determined;

[0018] When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​are respectively logically negated and then logically ANDed to determine the protection non-action signal.

[0019] Optionally, after normalizing the three-phase current sampling values ​​on both sides, determining the three-phase current sampling values ​​on both sides after compensating the capacitor current includes:

[0020] According to the relationship between the current and voltage generated by the distributed capacitance, it is determined that the capacitor current is equal to the capacitance value multiplied by the derivative of the capacitor voltage with respect to time;

[0021] Based on the fact that the capacitor current is equal to the capacitor value multiplied by the derivative of the capacitor voltage with respect to time, according to the total ground capacitance C of the line pg , total phase-to-phase capacitance C pp , the derivative of the capacitor voltage on the three-phase side of both sides with respect to time, and determine the three-phase capacitor current on both sides that needs to be compensated represented by the derivative;

[0022] The calculation formula of the three-phase capacitor current on both sides that needs to be compensated represented by the derivative is converted into the calculation formula of the three-phase capacitor current on both sides that needs to be compensated represented by the numerical differential;

[0023] Calculate the sampling values ​​of the capacitor currents of the three phases on both sides that need to be compensated according to the calculation formula of the three-phase capacitor currents on both sides that need to be compensated represented by the numerical differential;

[0024] The three-phase current sampling values ​​on both sides after compensating the capacitor current are determined by subtracting the capacitor current sampling values ​​on both sides that need to be compensated for the three-phase current sampling values ​​on both sides.

[0025] Optionally, according to the three-phase current sampling values ​​on both sides after the capacitor current is compensated, a first discrete signal x(k) and a second discrete signal y(k) are constructed, including:

[0026]

[0027] Among them, i' m (k) is the three-phase current sampling value i' on the m side after compensating the capacitor current n(k) is the sampling value of the three-phase current on the n side after compensating the capacitor current.

[0028] Optionally, constructing a first discrete signal and a second discrete signal according to the zero-sequence current sampling values ​​on both sides after the capacitor current is compensated includes:

[0029] According to the following formula, the first discrete signal and the second discrete signal are constructed:

[0030]

[0031] Where: i′ m0 (k) is the zero-sequence current sampling value on the m side after the capacitor current is compensated, i′ n0 (k) is the zero-sequence current sampling value on the n side after compensating the capacitor current, i′ m0 (k)=(i′ mA0 (k)+i′ mB0 (k)+i′ mC0 (k)) / 3, i′ mA0 (k) is the sampling value of phase A zero-sequence current on side m after compensation of capacitor current, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the m side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the C phase zero-sequence current on the m side after the capacitor current is compensated, i′ n0 (k)=(i′ nA0 (k)+i′ nB0 (k)+i′ nC0 (k)) / 3, i′ nA0 (k) is the sampling value of the zero-sequence current of phase A on the n side after the capacitor current is compensated, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the n side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the zero-sequence current of phase C on the n side after compensating the capacitor current.

[0032] Optionally, according to the first discrete signal x(k) and the second discrete signal y(k), a correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment is determined: xy ,include:

[0033] According to the following formula, the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment is determined xy :

[0034]

[0035] Where N is the number of sampling points in a certain period of time.

[0036] Optionally, according to the correlation degree ρxy , determine the improved correlation longitudinal protection criterion, including:

[0037] According to the correlation degree ρ xy , determine the correlation of zero-sequence current on both sides ρ0 and the correlation of three-phase current on both sides p xy , the improved correlation longitudinal protection criterion is determined as:

[0038] p xy >ρ set Or ρ0>ρ set

[0039] Among them, ρ set To protect the fixed value.

[0040] According to another aspect of the present invention, there is also provided an offshore wind power AC submarine cable line longitudinal protection system, comprising:

[0041] A sampling value determination module is used to determine the three-phase current sampling values ​​on both sides of the line;

[0042] A compensation current determination module is used to determine the three-phase current sampling values ​​on both sides after compensation and the zero-sequence current sampling values ​​on both sides after compensation after normalizing the three-phase current sampling values ​​on both sides;

[0043] A signal construction module is used to construct a first discrete signal x(k) and a second discrete signal x(k) according to the three-phase current sampling values ​​on both sides after the capacitor current is compensated and the zero-sequence current sampling values ​​on both sides after the compensation;

[0044] A correlation determination module, configured to determine a correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment according to the first discrete signal x(k) and the second discrete signal y(k) xy ;

[0045] Determine a protection criterion module for determining the protection criterion module according to the correlation degree ρ xy , determine the correlation longitudinal protection criterion.

[0046] According to another aspect of the present invention, a computer device is also provided, which includes: a processor and a memory; wherein the memory is used to store executable instructions of the processor; the processor is used to read the executable instructions from the memory and execute the instructions to implement a method for longitudinal protection of an offshore wind power AC submarine cable line.

[0047] According to another aspect of the present invention, there is further provided a computer-readable storage medium storing a computer program for executing a method for longitudinal protection of an offshore wind power AC submarine cable line.

[0048] Thus, after normalizing the current sampling values ​​on both sides of the line, the first discrete signal and the second discrete signal are constructed, and the correlation between the first discrete signal and the second discrete signal is determined, and finally the correlation longitudinal protection criterion is determined. The AC submarine cable longitudinal protection of the offshore wind power flexible direct current transmission system based on improved correlation can not only quickly and correctly distinguish the faults inside and outside the zone and has a strong high resistance ability, but also can weaken the influence of the distributed capacitance of the submarine cable and the weak feedback characteristics of wind power. It can correctly operate when a fault occurs in the long-distance (no more than 300km) uncompensated offshore wind power AC submarine cable line, and has strong adaptability to the AC submarine cable line of offshore wind power transmitted by flexible direct current. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] A more complete understanding of exemplary embodiments of the present invention may be obtained by referring to the following drawings:

[0050] Figure 1 It is a schematic diagram of a flow chart of a method for longitudinal protection of an offshore wind power AC submarine cable line according to this embodiment;

[0051] Figure 2 Schematic diagram of a Π-type equivalent circuit according to the time domain method described in this embodiment;

[0052] Figure 3 Schematic diagram of the variation trend of the correlation calculation result according to the present embodiment with the current amplitude ratio K on both sides;

[0053] Figure 4 It is a logic diagram of the improved correlation longitudinal joint protection action according to this embodiment;

[0054] Figure 5 A topological diagram of an offshore wind power flexible direct current transmission system according to this embodiment;

[0055] Figure 6 is a schematic diagram of correlation under different transition resistances according to this embodiment;

[0056] Figure 7 is a schematic diagram of correlation under different submarine cable lengths according to this embodiment;

[0057] Figure 8 This is a schematic diagram of longitudinal protection for an offshore wind power AC submarine cable line according to the present embodiment. DETAILED DESCRIPTION

[0058] Now, exemplary embodiments of the present invention are described with reference to the accompanying drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to disclose the present invention in detail and completely and to fully convey the scope of the present invention to those skilled in the art. The terms used in the exemplary embodiments shown in the accompanying drawings are not intended to limit the present invention. In the accompanying drawings, the same units / elements are marked with the same reference numerals.

[0059] Unless otherwise specified, the terms (including technical terms) used herein have the commonly understood meanings to those skilled in the art. In addition, it is understood that the terms defined in commonly used dictionaries should be understood to have the same meanings as those in the context of the relevant fields, and should not be understood as idealized or overly formal meanings.

[0060] According to this embodiment, a method 100 for protecting an offshore wind power AC submarine cable line is provided. Figure 1 As shown, the method 100 includes:

[0061] S101: Determine the sampling values ​​of the currents on both sides of the line;

[0062] S102: after normalizing the sampled values ​​of the currents on both sides, determine the sampled values ​​of the three-phase currents on both sides after compensating the capacitor current and the sampled values ​​of the zero-sequence currents on both sides after compensation;

[0063] S103: constructing a first discrete signal x(k) and a second discrete signal y(k) according to the three-phase current sampling values ​​on both sides after compensation and the zero-sequence current sampling values ​​on both sides after compensation;

[0064] S104: Determine the correlation degree p between the first discrete signal x(k) and the second discrete signal y(k) xy ;

[0065] S105: According to the correlation degree p xy , determine the correlation longitudinal protection criterion.

[0066] The specific implementation of the method is as follows:

[0067] 1. Constructing related longitudinal protection:

[0068] Assume that at a certain sampling time, two discrete signals are x(k) and y(k), and the number of sampling points in a certain period is N. To study the similarity or correlation between them, define ρ xy is the correlation between x(k) and y(k). According to the Schwartz correlation calculation formula:

[0069]

[0070] The following two discrete signals are constructed using the sampled values ​​of the current on both sides of the line:

[0071]

[0072] In formula 2, i m (k) and i n (k) is the kth sampling value of the current on both sides of the line.

[0073] Substituting x(k) and y(k) into equation (1), we can get the correlation between the current waveforms on both sides of the line. During normal operation (or fault outside the line area), the current waveforms on both sides are equal in magnitude and opposite in direction, and the correlation calculation result is -1; when there is a fault inside the line area, the currents on both sides are generally different in magnitude, but opposite in phase, and the correlation calculation result is in [0, 1]. Reasonable selection of protection settings based on the difference in correlation calculation results can effectively distinguish between faults inside and outside the area.

[0074] 2. Amplitude normalization processing:

[0075] When a fault occurs in the line area, assuming that the currents on both sides are in opposite phases and the amplitude ratio is K, the correlation calculation result ρ changes with the change of K, as shown in the attached figure. Figure 2 The weak feedback characteristics of the current on the wind farm side make the calculation result of the current correlation on both sides biased towards 0 when there is a fault in the area, which loses some margin for the selection of protection settings and has low protection sensitivity.

[0076] Normalize the current on both sides to get:

[0077]

[0078] where i' m (k), i' n (k) is the sampling value of the current on both sides after time domain capacitance current compensation.

[0079] After normalization, the calculated similarity value is no longer affected by the current amplitude on both sides. It is close to 1 when there is a fault within the zone and closer to -1 when there is a fault outside the zone or in normal operation. The protection sensitivity is greatly increased when there is a fault within the zone.

[0080] 3. Time domain capacitance current compensation:

[0081] The relationship between the current generated by the distributed capacitance and the voltage on it is shown in the following equation (4):

[0082]

[0083] Time domain capacitor current compensation is to list the equations satisfied by the capacitor current to be compensated by the transmission line equivalent circuit in the time domain to achieve effective compensation of the capacitor current. The equivalent circuit of the π-type network in the time domain method is shown in the attached figure. Figure 3 As shown in FIG. 1 , according to the relationship between the current and voltage generated by the distributed capacitance, the capacitance current is equal to the capacitance value multiplied by the derivative of the capacitance voltage with respect to time. Combined with the Π-type equivalent circuit of the transmission line, the calculation formula of the capacitance current that needs to be compensated for the line a phase represented by the derivative can be listed. Based on the fact that the capacitance current is equal to the capacitance value multiplied by the derivative of the capacitance voltage with respect to time, according to the total line capacitance to ground C pg , total phase-to-phase capacitance C pp , the derivative of the capacitor voltage on the m side of phase A with respect to time The derivative of the capacitor voltage on the n-side of phase A with respect to time The derivative of the capacitor voltage on the m side of the AB phase with respect to time The derivative of the capacitor voltage on the n side of the AB phase with respect to time The derivative of the capacitor voltage on the m side of the AC phase with respect to time And the derivative of the capacitor voltage on the AC phase n side with respect to time Determine the instantaneous value of the capacitive current that needs to be compensated on both sides of the line of phase A represented by the derivative, wherein the instantaneous value of the capacitive current that needs to be compensated on both sides of the line of phase A includes the instantaneous value of the capacitive current that needs to be compensated on the m side of the line of phase A And the instantaneous value of the capacitive current that needs to be compensated on the n-side of the A-phase line

[0084] Taking the semi-compensation method based on the π-type network as an example, the instantaneous value of the capacitor current that needs to be compensated for phase A on both sides of the line can be obtained according to formula (4):

[0085]

[0086] Where C pg is the total capacitance of the line to ground, C pp , the total phase-to-phase capacitance.

[0087] Similarly, the capacitor current sampling values ​​that need to be compensated for phase B and phase C are:

[0088]

[0089]

[0090] C pg , C pp The positive-sequence and zero-sequence capacitances of the entire line length have the following relationship:

[0091]

[0092] When solving engineering problems, the derivative is approximated by differentials. Therefore, the calculation formula of the capacitive current that needs to be compensated for phase a of line represented by the derivative is transformed into the calculation formula of the capacitive current that needs to be compensated for phase a of line represented by numerical differentials, and the sampling value of the capacitive current that needs to be compensated for phase a of line is calculated according to the formula.

[0093] Right now:

[0094]

[0095] Combining equations (5)-(7), it can be known that the instantaneous value of the capacitor current that needs to be compensated for phase a is (similar to phases b and c):

[0096]

[0097]

[0098]

[0099] At this time, the currents on both sides after compensation are:

[0100]

[0101] The x(k) and y(k) formed by the currents on both sides after compensation become:

[0102]

[0103] In formula 10, i' m (k), i' n (k) is the current sampling value after the currents on both sides are compensated by the time domain capacitor current.

[0104] The correlation pilot protection after time-domain capacitance current compensation can greatly weaken the influence of capacitance current distributed on the submarine cable line on the correlation calculation results.

[0105] 4. Zero-sequence current correlation protection:

[0106] Aiming at the problem that the correlation algorithm is not able to withstand transition resistance, a method of using zero-sequence currents on both sides for correlation calculation is proposed. The zero-sequence current is improved accordingly by combining amplitude normalization and time-domain capacitance current compensation.

[0107] The discrete sets x(k) and y(k) formed by the improved zero-sequence currents on both sides are shown in Equation 9:

[0108]

[0109] Where: i′ m0 (k)=(i′ mA0 (k)+i′ mB0 (k)+i′mC0 (k)) / 3, i′ n0 (k)=(i′ nA0 (k)+i′ nB0 (k)+i′ nC0 (k)) / 3 is the zero-sequence current sampling value on both sides of the line after capacitor current compensation.

[0110] After a fault occurs in the line area, although the improved zero-sequence current will also decrease with the increase of transition resistance, since the zero-sequence current is almost 0 during normal operation of the line, even in the case of a high-resistance fault under a transition resistance of 1000Ω, the correlation after the fault can increase rapidly, and the protection can quickly enter the action area. Therefore, the zero-sequence current correlation protection has a higher tolerance to transition resistance than the phase current correlation protection.

[0111] 5. Composition of protection criteria:

[0112] The final improved x(k) and y(k) are selected as shown in formula (12):

[0113]

[0114] In formula 12, i′ m (k), i′ n (k) is the sampling value of the current on both sides after time domain capacitance current compensation.

[0115] Substituting equations (11) and (12) into equation (1) respectively, we can obtain the correlation of zero-sequence current on both sides ρ0 and the correlation of current on both sides p xy The improved correlation longitudinal protection criterion thus formed is shown in formula (13):

[0116] p xy >ρ set orρ0>ρ set (13)

[0117] In formula 13: set To protect the fixed value.

[0118] The protection setting is set according to the maximum transmission error during normal operation. Considering that the maximum transmission error of CT is ±10%, and considering a certain margin, the reliability coefficient is selected as 0.9, so ρ is selected. set =-0.9 is the setting value of the main criterion.

[0119] The protection action logic mentioned in this article is as shown in the attached Figure 4As shown, when the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​adopt an OR logic, a protection action signal is determined;

[0120] When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​are respectively logically negated and then logically ANDed to determine the protection non-action signal.

[0121] In order to verify the performance of the offshore wind power AC submarine cable longitudinal protection based on improved correlation proposed in this paper, an offshore wind power flexible direct current transmission system is selected, and its system topology is as follows: Figure 5 shown. Figure 5 The medium wind farm contains 90 5MW direct-drive wind turbines. The wind farm is connected to the external system through a 35kV / 220kV transformer. The submarine cable line is 30km long. The wind turbine adopts the negative sequence current suppression control strategy, the offshore converter station adopts the fixed U / F control strategy, and the onshore converter station adopts the fixed active power and fixed reactive power control strategy. F1 and F5 are fault points outside the AC submarine cable area, and F2, F3, and F4 are the fault points in the area close to the offshore converter station side, the midpoint of the submarine cable, and the fault points in the area close to the wind farm side. The system parameters are shown in Table 1.

[0122] Table 1 System parameters

[0123]

[0124]

[0125] Various types of faults inside and outside the line area are simulated, and the correlation results at 5ms after the fault occurs are shown in Table 2 below.

[0126] Table 2 Correlation of various faults occurring inside and outside the transmission line area

[0127]

[0128]

[0129] The improved correlation longitudinal protection can operate correctly within 5ms when a metallic fault occurs in the zone, and can reliably not operate when a fault occurs outside the zone.

[0130] Taking the A phase grounding fault at F3 at 0.2s through 100Ω, 200Ω, 500Ω, and 1000Ω transition resistance as an example, the tolerance of transition resistance of improved correlation protection is analyzed. The correlation of zero-sequence current on both sides of the line is as follows: Figure 6 shown.

[0131] Figure 6 Medium AR100 , AR200 , AR500 , AR1000 They are the correlation of zero-sequence current on both sides when the transition resistance is 100Ω, 200Ω, 500Ω, and 1000Ω respectively.

[0132] from Figure 6 It can be seen that when the transition resistance is 100Ω, 200Ω, 500Ω, and 1000Ω, the zero-sequence current correlation protection can still quickly enter the action area and the protection can operate correctly. In addition, Figure 2 It can be seen from the small window that with the increase of transition resistance, the correlation after the fault in the zone gradually decreases, but even in the case of 1000Ω transition resistance, it can still reach around 0.9985. The improved correlation protection has a high sensitivity.

[0133] Taking the AN fault at F3 at 0.2s as an example, the improved correlation longitudinal protection performance under different submarine cable line lengths is studied. The correlation results of phase A current on both sides are as follows: Figure 7 shown.

[0134] Figure 7 Medium AL30 , AL50 , AL100 , AL150 , AR200 , AL300 They represent the correlation of phase A current on both sides when the length of the submarine cable line is 30, 50, 100, 150, 200 and 300 km respectively.

[0135] from Figure 7 It can be seen that the longer the length of the submarine cable, the longer it takes for the protection to enter the action area, and the lower the correlation at 5ms after the fault. When the length of the submarine cable is 300km, the correlation at 5ms after the fault is -0.789, and the protection can be operated. Therefore, the improved correlation longitudinal protection can at least ensure correct operation on a 300km uncompensated submarine cable line.

[0136] Therefore, the AC submarine cable longitudinal protection of the offshore wind power flexible direct current transmission system based on improved correlation can not only quickly and correctly zone internal and external faults and has strong high resistance resistance, but also weaken the influence of the distributed capacitance of the submarine cable and the weak feedback characteristics of wind power. It can correctly operate when an internal fault occurs in the long-distance (no more than 300km) uncompensated offshore wind power AC submarine cable line, and has strong adaptability to the AC submarine cable lines for offshore wind power transmission via flexible direct current.

[0137] Optionally, the three-phase current sampling values ​​on both sides include A-phase current sampling values ​​on both sides, B-phase current sampling values ​​on both sides, and C-phase current sampling values ​​on both sides.

[0138] Optionally, the first discrete signal x(k) includes a first discrete signal of the A-phase current sampling values ​​on both sides, a first discrete signal of the B-phase current sampling values ​​on both sides, a first discrete signal of the C-phase current sampling values ​​on both sides, and a first discrete signal of the zero-sequence current sampling value;

[0139] The second discrete signal y(k) includes a second discrete signal of the A-phase current sampling values ​​on both sides, a second discrete signal of the B-phase current sampling values ​​on both sides, a second discrete signal of the C-phase current sampling values ​​on both sides, and a second discrete signal of the zero-sequence current sampling values.

[0140] Optionally, the correlation degree ρ xy It includes: the correlation between the first discrete signal of the A-phase current sampling values ​​on both sides and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal of the B-phase current sampling values ​​on both sides and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal of the C-phase current sampling values ​​on both sides and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal of the zero-sequence current sampling value and the second discrete signal of the zero-sequence current sampling value.

[0141] Optionally, according to the correlation degree ρ xy , determine the relevant longitudinal protection criteria, including:

[0142] When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling value adopt an OR logic, a protection action signal is determined;

[0143] When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​are respectively logically negated and then logically ANDed to determine the protection non-action signal.

[0144] Optionally, after normalizing the three-phase current sampling values ​​on both sides, determining the three-phase current sampling values ​​on both sides after compensating the capacitor current includes:

[0145] According to the relationship between the current and voltage generated by the distributed capacitance, it is determined that the capacitor current is equal to the capacitance value multiplied by the derivative of the capacitor voltage with respect to time;

[0146] Based on the fact that the capacitor current is equal to the capacitor value multiplied by the derivative of the capacitor voltage with respect to time, according to the total ground capacitance C of the line pg , total phase-to-phase capacitance C pp , the derivative of the capacitor voltage on the three-phase side of both sides with respect to time, and determine the three-phase capacitor current on both sides that needs to be compensated represented by the derivative;

[0147] The calculation formula of the three-phase capacitor current on both sides that needs to be compensated represented by the derivative is converted into the calculation formula of the three-phase capacitor current on both sides that needs to be compensated represented by the numerical differential;

[0148] Calculate the sampling values ​​of the capacitor currents of the three phases on both sides that need to be compensated according to the calculation formula of the three-phase capacitor currents on both sides that need to be compensated represented by the numerical differential;

[0149] The three-phase current sampling values ​​on both sides after compensating the capacitor current are determined by subtracting the capacitor current sampling values ​​on both sides that need to be compensated for the three-phase current sampling values ​​on both sides.

[0150] Optionally, according to the three-phase current sampling values ​​on both sides after the capacitor current is compensated, a first discrete signal x(k) and a second discrete signal y(k) are constructed, including:

[0151]

[0152] Among them, i' m (k) is the three-phase current sampling value i' on the m side after compensating the capacitor current n (k) is the sampling value of the three-phase current on the n side after compensating the capacitor current.

[0153] Optionally, constructing a first discrete signal and a second discrete signal according to the zero-sequence current sampling values ​​on both sides after the capacitor current is compensated includes:

[0154] According to the following formula, the first discrete signal and the second discrete signal are constructed:

[0155]

[0156] Where: i′ m0 (k) is the zero-sequence current sampling value on the m side after the capacitor current is compensated, i′ n0 (k) is the zero-sequence current sampling value on the n side after compensating the capacitor current, i′ m0 (k)=(i′ mA0 (k)+i′ mB0 (k)+i′ mC0 (k)) / 3, i′ mA0 (k) is the sampling value of phase A zero-sequence current on side m after compensation of capacitor current, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the m side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the C phase zero-sequence current on the m side after the capacitor current is compensated, i′ n0 (k)=(i′ nA0 (k)+i′ nB0 (k)+i′ nC0 (k)) / 3, i′ nA0 (k) is the sampling value of the zero-sequence current of phase A on the n side after the capacitor current is compensated, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the n side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the zero-sequence current of phase C on the n side after compensating the capacitor current.

[0157] Optionally, according to the first discrete signal x(k) and the second discrete signal y(k), a correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment is determined: xy ,include:

[0158] According to the following formula, the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment is determined xy :

[0159]

[0160] Where N is the number of sampling points in a certain period of time.

[0161] Optionally, according to the correlation degree ρ xy , determine the improved correlation longitudinal protection criterion, including:

[0162] According to the correlation degree ρ xy , determine the correlation of zero-sequence current on both sides ρ0 and the correlation of three-phase current on both sides p xy , the improved correlation longitudinal protection criterion is determined as:

[0163] p xy >ρ set Or ρ0>ρ set

[0164] Among them, ρ set To protect the fixed value.

[0165] According to another aspect of the present invention, there is also provided an offshore wind power AC submarine cable line longitudinal protection system 800, comprising:

[0166] The sampling value determination module 810 is used to determine the sampling values ​​of the currents on both sides of the line;

[0167] A compensation current determination module 820 is used to determine the three-phase current sampling values ​​on both sides after compensation and the zero-sequence current sampling values ​​on both sides after compensation after normalizing the three-phase current sampling values ​​on both sides;

[0168] A signal construction module 830 is used to construct a first discrete signal x(k) and a second discrete signal x(k) according to the three-phase current sampling values ​​on both sides after the capacitor current is compensated and the zero-sequence current sampling values ​​on both sides after the compensation;

[0169] The correlation determination module 840 is used to determine the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment according to the first discrete signal x(k) and the second discrete signal y(k). xy ;

[0170] Determine the protection criterion module 850, for determining the protection criterion module 850 according to the correlation degree ρ xy , determine the correlation longitudinal protection criterion.

[0171] An offshore wind power AC submarine cable line longitudinal protection system 800 of an embodiment of the present invention corresponds to an offshore wind power AC submarine cable line longitudinal protection method 100 of another embodiment of the present invention, which will not be repeated here.

[0172] According to another aspect of the present invention, a computer device is also provided, which includes: a processor and a memory; wherein the memory is used to store executable instructions of the processor; the processor is used to read the executable instructions from the memory and execute the instructions to implement a method for longitudinal protection of an offshore wind power AC submarine cable line.

[0173] According to another aspect of the present invention, there is further provided a computer-readable storage medium storing a computer program for executing a method for longitudinal protection of an offshore wind power AC submarine cable line.

[0174] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0175] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0176] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0177] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0178] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0179] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A method for longitudinal protection of an offshore wind power AC submarine cable line, characterized in that: include: Determine the three-phase current sampling values ​​on both sides of the line; After normalizing the three-phase current sampling values ​​on both sides, the three-phase current sampling values ​​on both sides after compensating the capacitor current and the zero-sequence current sampling values ​​on both sides after compensation are determined; According to the three-phase current sampling values ​​on both sides after the capacitor current is compensated and the zero-sequence current sampling values ​​on both sides after the compensation, a first discrete signal x(k) and a second discrete signal y(k) are constructed, and according to the zero-sequence current sampling values ​​on both sides after the capacitor current is compensated, a first discrete signal and a second discrete signal are constructed, including: According to the following formula, the first discrete signal and the second discrete signal are constructed: Where: i′ m0 (k) is the zero-sequence current sampling value on the m side after the capacitor current is compensated, i n ′0(k) is the zero-sequence current sampling value on the n side after the capacitor current is compensated, i′ m0 (k)=(i′ mA0 (k)+i′ mB0 (k)+i′ mC0 (k)) / 3, i′ mA0 (k) is the sampling value of phase A zero-sequence current on side m after compensation of capacitor current, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the m side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the C phase zero-sequence current on the m side after the capacitor current is compensated, i n ′0(k)=(i n ' A0 (k)+i n ' B0 (k)+i n ' C0 (k)) / 3,i n ' A0 (k) is the sampling value of the zero-sequence current of phase A on the n side after the capacitor current is compensated, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the n side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the zero-sequence current of the C phase on the n side after the capacitor current is compensated; determine the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) xy ; According to the correlation degree ρ xy , determine the relevant longitudinal protection criteria, including: When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​adopt an OR logic, a protection action signal is determined; When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​are respectively logically negated and then logically ANDed to determine the protection non-action signal.

2. The method according to claim 1, characterized in that The three-phase current sampling values ​​on both sides include A-phase current sampling values ​​on both sides, B-phase current sampling values ​​on both sides, and C-phase current sampling values ​​on both sides.

3. The method according to claim 2, characterized in that The first discrete signal x(k) includes a first discrete signal of the A-phase current sampling values ​​on both sides, a first discrete signal of the B-phase current sampling values ​​on both sides, a first discrete signal of the C-phase current sampling values ​​on both sides, and a first discrete signal of the zero-sequence current sampling value; The second discrete signal y(k) includes a second discrete signal of the A-phase current sampling values ​​on both sides, a second discrete signal of the B-phase current sampling values ​​on both sides, a second discrete signal of the C-phase current sampling values ​​on both sides, and a second discrete signal of the zero-sequence current sampling values.

4. The method according to claim 3, characterized in that The correlation degree ρ xy It includes: the correlation between the first discrete signal of the A-phase current sampling values ​​on both sides and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal of the B-phase current sampling values ​​on both sides and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal of the C-phase current sampling values ​​on both sides and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal of the zero-sequence current sampling value and the second discrete signal of the zero-sequence current sampling value.

5. The method according to claim 2, characterized in that: After normalizing the three-phase current sampling values ​​on both sides, determining the three-phase current sampling values ​​on both sides after compensating the capacitor current includes: According to the relationship between the current and voltage generated by the distributed capacitance, it is determined that the capacitor current is equal to the capacitance value multiplied by the derivative of the capacitor voltage with respect to time; Based on the fact that the capacitor current is equal to the capacitor value multiplied by the derivative of the capacitor voltage with respect to time, according to the total ground capacitance C of the line pg , total phase-to-phase capacitance C pp , the derivative of the capacitor voltage on the three-phase side of both sides with respect to time, and determine the three-phase capacitor current on both sides that needs to be compensated represented by the derivative; The calculation formula of the three-phase capacitor current on both sides that needs to be compensated represented by the derivative is converted into the calculation formula of the three-phase capacitor current on both sides that needs to be compensated represented by the numerical differential; Calculate the sampling values ​​of the capacitor currents of the three phases on both sides that need to be compensated according to the calculation formula of the three-phase capacitor currents on both sides that need to be compensated represented by the numerical differential; The three-phase current sampling values ​​on both sides after compensating the capacitor current are determined by subtracting the capacitor current sampling values ​​on both sides that need to be compensated for the three-phase current sampling values ​​on both sides.

6. The method according to claim 5, characterized in that According to the three-phase current sampling values ​​on both sides after the capacitor current is compensated, a first discrete signal x(k) and a second discrete signal y(k) are constructed, including: Among them, i' m (k) is the three-phase current sampling value i' on the m side after compensating the capacitor current n (k) is the sampling value of the three-phase current on the n side after compensating the capacitor current.

7. The method according to claim 1, characterized in that Determining the correlation degree ρxy between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment according to the first discrete signal x(k) and the second discrete signal y(k) comprises: According to the following formula, the correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment is determined xy : Where N is the number of sampling points in a certain period of time.

8. The method according to claim 7, characterized in that According to the correlation degree ρ xy , determine the improved correlation longitudinal protection criterion, including: According to the correlation degree ρ xy , determine the correlation of zero-sequence current on both sides ρ0 and the correlation of three-phase current on both sides p xy , the improved correlation longitudinal protection criterion is determined as: p xy >r set or p0>p set Among them, ρ set To protect the fixed value.

9. An offshore wind power AC submarine cable line longitudinal protection system, characterized in that: include: A sampling value determination module is used to determine the three-phase current sampling values ​​on both sides of the line; A compensation current determination module is used to determine the three-phase current sampling values ​​on both sides after compensation and the zero-sequence current sampling values ​​on both sides after compensation after normalizing the three-phase current sampling values ​​on both sides; A signal construction module is used to construct a first discrete signal x(k) and a second discrete signal y(k) according to the three-phase current sampling values ​​on both sides after the capacitor current is compensated and the zero-sequence current sampling values ​​on both sides after the compensation, and to construct the first discrete signal and the second discrete signal according to the zero-sequence current sampling values ​​on both sides after the capacitor current is compensated, including: According to the following formula, the first discrete signal and the second discrete signal are constructed: Where: i′ m0 (k) is the zero-sequence current sampling value on the m side after the capacitor current is compensated, i n ′0(k) is the zero-sequence current sampling value on the n side after the capacitor current is compensated, i′ m0 (k)=(i′ mA0 (k)+i′ mB0 (k)+i′ mC0 (k)) / 3, i′ mA0 (k) is the sampling value of phase A zero-sequence current on side m after compensation of capacitor current, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the m side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the C phase zero-sequence current on the m side after the capacitor current is compensated, i n ′0(k)=(i n ' A0 (k)+i n ' B0 (k)+i n ' C0 (k)) / 3,i n ' A0 (k) is the sampling value of the zero-sequence current of phase A on the n side after the capacitor current is compensated, i′ mB0 (k) is the sampling value of the zero-sequence current of phase B on the n side after the capacitor current is compensated, i′ mC0 (k) is the sampling value of the zero-sequence current of phase C on the n side after the capacitor current is compensated; A correlation determination module, configured to determine a correlation degree ρ between the first discrete signal x(k) and the second discrete signal y(k) at a certain moment according to the first discrete signal x(k) and the second discrete signal y(k) xy ; Determine a protection criterion module for determining the protection criterion module according to the correlation degree ρ xy , determine the correlation longitudinal protection criterion; According to the correlation degree ρ xy , determine the relevant longitudinal protection criteria, including: When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​adopt an OR logic, a protection action signal is determined; When the correlation between the first discrete signal and the second discrete signal of the A-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the B-phase current sampling values ​​on both sides, the correlation between the first discrete signal and the second discrete signal of the C-phase current sampling values ​​on both sides, and the correlation between the first discrete signal and the second discrete signal of the zero-sequence current sampling values ​​are respectively logically negated and then logically ANDed to determine the protection non-action signal.

10. A computer device, characterized in that: The computer device comprises: a processor and a memory; wherein, The memory is used to store the processor executable instructions; The processor is used to read the executable instructions from the memory and execute the instructions to implement the method according to any one of claims 1 to 8.

11. A computer-readable storage medium, characterized in that: The storage medium stores a computer program, and the computer program is used to execute the method according to any one of claims 1 to 8.

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