Transient negative sequence current energy protection method and its application for offshore AC collection lines

Abstract

The transient negative sequence current energy protection method and application of the offshore AC collection line detects the transient negative sequence current energy at the moment n of each feeder protection installation of the bus; selects the minimum transient negative sequence current energy at the moment n of each feeder protection installation in the same bus as the reference value; compares the transient negative sequence current energy at each feeder protection installation of the same bus with the reference value to obtain a ratio; identifies the fault direction according to the relationship between the transient negative sequence current energy ratio and the set threshold; the two-end protection of the AC collection line interacts with the locking signal to distinguish between the zone and the outer fault. The invention constructs negative sequence current energy, amplifies the difference characteristics of the negative sequence current amplitude between the fault line and the non-fault line, and realizes the rapid identification of the fault line. It can effectively solve the problem that the AC collection line adopts a negative sequence suppression strategy at both ends of the converter and the fault characteristics are weak, resulting in the degradation of the performance of the traditional power frequency quantity protection, and ensures the safe and stable operation of the offshore wind power transmission system through flexible direct current.

Description

Technical Field

[0001] The invention belongs to the technical field of relay protection of electric power systems, and in particular relates to a transient negative sequence current energy protection method and application of an offshore AC collection line. Background Art

[0002] The flexible direct current transmission system based on modular multilevel converter (MMC) has the advantages of low harmonic content, flexible operation and regulation, and support for passive networks. Offshore wind power has become the preferred solution for offshore wind power transmission through the flexible direct current transmission system. Since the offshore AC collection line is in a harsh marine environment with high salinity and strong corrosion for a long time, its failure rate is greatly increased. When an AC collection line fails, since the wind turbines and offshore converter stations connected at both ends are power electronic equipment, their output characteristics are limited by the control strategy. The fault line current will show a weak fault feature with limited amplitude and controlled phase angle, and the difficulty of fault identification is significantly increased.

[0003] At present, domestic and foreign research on offshore wind power flexible direct current transmission systems pays little attention to the fault characteristics and protection of AC collection lines. In the study of the fault characteristics of offshore AC collection lines, Chen Helin et al. (Chen Helin et al. Analysis of transient behavior of MMC-HVDC grid-connected system of offshore wind farm [J]. Automation of Electric Power Systems, 2014, 38(12): 112-118.) analyzed the transient fault characteristics of AC collection lines, but did not consider the short-circuit current impact of wind farms. Wang Xiaohe et al. (Wang Xiaohe et al. Analysis of overvoltage mechanism and suppression strategy of power grid fault at the sending end of offshore wind power flexible direct current system [J]. High Voltage Technology, 2021, 47(08): 2688-2699.) analyzed the fault characteristics of AC collection lines under the negative sequence suppression strategy adopted by wind farms and offshore converter stations, and concluded that the system fault current is zero under single-phase grounding faults and phase-to-phase faults. Application No. 202111249880.2, entitled "An offshore wind power flexible DC transmission system and control method thereof", proposed an offshore wind power flexible DC transmission system and control method, but did not carry out the fault characteristics analysis of the collection line. Application No. 202210126332.9, entitled "Low voltage ride-through control method and application suitable for offshore wind power collection line faults", proposed a low voltage ride-through strategy based on the active voltage reduction method, but did not solve the problem of difficulty in identifying fault lines in the collection line.

[0004] In the research on relay protection of offshore AC collection lines, Zheng Liming et al. (Zheng Liming et al. Analysis of fault characteristics on the AC side of offshore wind power access to flexible DC system and its impact on protection [J]. Power System Protection and Control, 2021, 49(20): 20-32.) pointed out through theoretical analysis that due to the limited amplitude and controlled phase angle of short-circuit current of power electronic equipment, conventional power frequency line protection such as distance protection and current differential protection has the risk of refusal to operate. In this regard, Zheng Liming et al. (Zheng Liming, et al. AC line protection of offshore wind power access to flexible DC system based on singular value decomposition [J]. Proceedings of the CSEE, 2020, 40(S1): 75-83.) proposed a fast protection scheme based on transient current singular value decomposition, which uses the fault mutation characteristics of short-circuit current on both sides to realize the identification of internal and external faults. This scheme is not affected by the control strategy of power electronic equipment, but the algorithm is relatively complex and highly dependent on communication. From the above analysis, it can be seen that there is no good solution for the protection of offshore AC collection lines, and it is urgent to carry out research on relay protection methods suitable for offshore wind power AC collection lines.

[0005] In the research of transient quantity protection, the application number is 201310107108.6 and the name is "A method for selecting fault lines in a resonant grounding system using a combination of complex wavelets and ANN". By using complex wavelets to calculate the characteristic frequency band of transient zero-sequence current, the energy and center frequency corresponding phase of each feeder in the characteristic frequency band are trained using an artificial intelligence algorithm ANN to select the fault line, but the method requires a large amount of sample data, the algorithm is complex, and the amount of calculation is large. The application number is 201410107301.4 and the name is "A method for detecting single-phase grounding faults in distribution networks based on transient components". According to the transient zero-sequence voltage and transient zero-sequence current, the magnitude of the fifth harmonic component of the line and the positive and negative characteristics of the capacitance parameters are calculated to identify single-phase grounding faults, but the method introduces voltage and can only identify single-phase grounding faults. The application number is 201910909701.X and the name is "A distribution network single-phase grounding protection method based on active and passive joint detection of weak faults". It uses the busbar zero-sequence voltage and zero-sequence current energy to identify single-phase grounding faults in the distribution network, but it is only applicable to single-phase grounding faults and the algorithm is complex. The application number is 201810122090.X and the name is "A current sequence component comparison fault section location method containing DG feeders". It uses the phase relationship of negative sequence current and the amplitude relationship of positive sequence current to locate the fault section, which is mainly applicable to distribution networks connected to DG. Application number 201510566344.3 is titled "A method for longitudinal protection of high-voltage DC transmission lines based on S transformation", application number 201210295511.1 is titled "A method for protecting high-voltage DC transmission lines based on the direction of voltage and current mutation", and application number 201811098051.7 is titled "A method for rapid protection of DC lines in multi-terminal flexible DC grids based on single-end voltage". These applications propose protection methods that comprehensively utilize voltage and current information for DC lines, but the sampling rate is relatively high. Application No. 201810426285.3 is named "A flexible DC distribution line protection method based on transient current similarity", Application No. 201810402951.X is named "A transient energy protection method for DC transmission line fault current", Application No. 202111124892.2 is named "A single-pole grounding line selection method based on zero-mode current correlation", Application No. 201910670794.5 is named "Flexible DC power grid short-circuit noble fast detection method based on MMC". The transient current characteristics of the DC line are used to construct the protection method. However, the sampling rate is high, and because the voltage and current of the AC line are sinusoidal, the fault characteristics are more complex, so whether the above DC protection scheme can be directly applied to the AC line needs further study.The application number is 201910531594.1 and the name is "Directional longitudinal transient quantity protection method applicable to UPFC transmission lines". It uses the high-frequency information difference of the protection connected to the same bus to identify internal and external faults. Considering that the busbars at both ends of the offshore AC collection line are connected to multiple feeders, it provides ideas for constructing a protection method for offshore AC collection lines based on the protection difference characteristics of different feeders connected to the busbars. Therefore, for offshore AC collection lines with double-ended power electronic devices, the present invention proposes a transient negative sequence current energy protection method for offshore AC collection lines, which can effectively identify internal and external faults. Summary of the invention

[0006] In order to solve the problem that the offshore AC collection line presents weak fault characteristics due to the influence of the control strategy of the power electronic equipment on both sides, resulting in the AC protection scheme of the conventional power frequency quantity having a reduced action performance, the present invention proposes a transient negative sequence current energy protection method and application of the offshore AC collection line, and its technical solution is as follows:

[0007] A transient negative sequence current energy protection method for an offshore AC collection line is used for rapid and accurate identification of a faulty line; the method is implemented based on an offshore wind power flexible direct current transmission system, the offshore wind power flexible direct current transmission system comprising an offshore wind farm, a wind farm collection line, an offshore booster station, an AC collection line, an offshore converter station, a DC submarine cable, an onshore converter station, and an AC system; the transient negative sequence current energy is obtained by accumulating the negative sequence current within a certain time window, and is characterized in that:

[0008] Step 1: Calculate the negative sequence current energy S at each feeder protection installation on the same bus j (n);

[0009] Step 2: Select the minimum negative sequence current energy S at each feeder protection installation location of the same busbar r (n) as a benchmark value;

[0010] Step 3: Calculate the ratio k of the negative sequence current energy to the reference value at each feeder protection installation on the same busbar j ;

[0011] Step 4: Compare the ratio k j With the threshold k set Determine the fault direction by the size relationship of

[0012] Step 5: Identify the internal and external faults according to the fault direction determination results of the protection at both ends of the AC collection line.

[0013] Preferably, the step 1 calculates the negative sequence current energy S at the time n of each feeder protection installation of the same busbar. j (n):

[0014]

[0015] Where: n is the nth sampling time, m is the mth sampling time, Δt is the sampling period, T is the data window length, To protect the negative sequence current at point j at time m, S j (n) is the negative sequence current energy at protection point j at time n, which is the accumulated sum of all negative sequence currents in the T data window before time n.

[0016] Preferably, step 2 further includes the following contents: selecting the minimum negative sequence current energy S at the time n of each feeder protection installation of the same busbar r (n) As a reference value:

[0017] S r (n) = min{S j (n)}.

[0018] Where: S r (n) is the minimum negative sequence current energy in each feeder protection installation of the bus connected to protection j at time n, S j (n) is the negative sequence current energy at protection point j at time n.

[0019] Preferably, step 3 calculates the ratio k of the negative sequence current energy to the reference value at each feeder protection installation of the same busbar. j :

[0020]

[0021] Where: k j is the ratio of the negative sequence current energy at protection j to the reference value, S r (n) is the minimum negative sequence current energy among all protections of the busbar connected to protection j at time n, S j (n) is the negative sequence current energy at protection point j at time n.

[0022] Preferably: the step 4 compares the ratio k j With the threshold k set The fault direction can be determined by the size relationship of

[0023] When a fault occurs in the positive direction of the protection, its transient negative sequence current energy is greater than the transient negative sequence current energy of other lines connected to the same bus; when a fault occurs in the reverse direction of the protection, its transient negative sequence current energy is less than the transient negative sequence current energy of the fault line connected to the same bus; the criterion for identifying the fault direction at protection j is as follows:

[0024] a) If k j >k set , then protection j determines that the fault direction is a positive direction fault;

[0025] b) If k j <k set , then protection j determines that the fault direction is a reverse direction fault;

[0026] Where: k j is the ratio of the negative sequence current energy at protection j to the reference value, k set To set the threshold, considering that the non-fault line has basically the same line model and voltage level, the negative sequence current energy at the non-fault line protection is basically the same as the reference value, and the ratio is close to 1, so the threshold k is selected set =1.2.

[0027] Preferably, step 5 further includes the following contents:

[0028] When the protection at both ends of the AC collection line identifies a positive direction fault, the fault is identified as an internal fault and the protection operates reliably;

[0029] When any of the protections at both ends of the AC collection line identifies a reverse direction fault, a blocking signal is sent to the opposite side, identifying the fault as an out-of-zone fault, and the protection is reliable and does not operate.

[0030] The transient negative sequence current energy protection method for offshore AC collection lines is applied to the relay protection system of offshore wind power AC collection lines.

[0031] Beneficial effects:

[0032] According to the difference in negative-sequence current characteristics between the fault line and the non-fault line connected to the same bus, the present invention constructs a protection method based on transient negative-sequence current energy, further amplifies the difference in negative-sequence current characteristics between the fault line and the non-fault line, realizes rapid identification of the fault line, and the protection criterion action constant is easy to set. It has a certain ability to withstand transition resistance, and can effectively solve the problem of decreased action performance of traditional power frequency quantity protection due to weak fault characteristics caused by the negative-sequence suppression strategy adopted by the converters at both ends of the AC collection line. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0034] Figure 1 This is a schematic diagram of the simulation model of the offshore wind power transmission system via flexible direct current;

[0035] Figure 2 This is the negative sequence network diagram of the fault point f1;

[0036] Figure 3 This is the negative sequence network diagram for the fault at point f2;

[0037] Figure 4 is the negative sequence current energy at each feeder of the offshore converter station (busbar N);

[0038] Figure 5 It is the negative sequence current energy at each feeder of the offshore booster station (busbar M);

[0039] Figure 6 is the negative sequence current energy ratio at each feeder of the offshore converter station (busbar N);

[0040] Figure 7 It is the negative sequence current energy ratio at each feeder of the offshore converter station (busbar M). DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. The present invention proposes a transient negative sequence current energy protection method and application for offshore AC collection lines. The present invention is further explained below in conjunction with the drawings and embodiments.

[0042] The invention discloses a transient negative sequence current energy protection method for an offshore AC collection line and an application thereof. Figure 1 The offshore wind power transmission system shown in the figure is composed of an offshore wind farm, a wind farm collection line, an offshore booster station, an AC collection line, an offshore converter station, a DC submarine cable, an onshore converter station, and an AC system. The transient negative sequence current energy is obtained by accumulating the negative sequence current within a certain time window. The protection method based on the transient negative sequence current energy is used to accurately identify the fault line.

[0043] Firstly, taking the faults inside and outside the AC collection line in the offshore wind power transmission system through flexible DC system as an example, the distribution of negative sequence current is explained, laying the foundation for constructing a protection method based on transient negative sequence current energy.

[0044] Figure 2The offshore wind farm in China increases the outlet voltage of 35kV wind turbines to 220kV through the offshore booster station, and transmits the electric energy to the offshore converter station through the AC collection line. The offshore converter station side protection of the AC collection line L1 is protection 2, and the offshore booster station side protection of the AC collection line L1 is protection 1; the offshore converter station side protection of the AC collection line L2 is protection 4, and the offshore booster station side protection of the AC collection line L2 is protection 3; the offshore converter station side protection of the AC collection line L3 is protection 6, and the offshore booster station side protection of the AC collection line L1 is protection 5; the offshore booster station side protection of the wind farm collection line L4 is protection 7; the offshore booster station side protection of the wind farm collection line L5 is protection 8; the offshore booster station side protection of the wind farm collection line L6 is protection 9.

[0045] When an asymmetric fault occurs at the midpoint f1 of the AC collection line L1, the negative sequence current flow path is as follows: Figure 2 As shown by the black dotted line in the middle, the negative sequence current starts from the fault point and forms a loop with distributed capacitance. For the protection on the offshore converter station (busbar N) side, the negative sequence current is shunted to the non-fault lines L2 and L3 and the AC side of the offshore converter station after protection 2. The relationship between protection 2 of the fault line L1, protection 4 of the non-fault line L2, protection 6 of the non-fault line L3, and the negative sequence current measured at the outlet of the AC side of the offshore converter station satisfies the following relationship:

[0046]

[0047] Where: Respectively Figure 2 Negative sequence current component at middle protection 2, 4, 6, express Figure 2 Negative sequence current component at the AC side outlet of the offshore converter station.

[0048] For the protection on the offshore booster station (busbar M) side, the negative sequence current is shunted to the non-fault lines L4, L5, and L6 after protection 1. The negative sequence current measured at each protection installation location satisfies the following relationship:

[0049]

[0050] Where: Respectively Figure 2 Negative sequence current components at middle protection points 1, 7, 8, and 9.

[0051] Therefore, when a fault occurs in the collecting line, the fault point is located in the positive direction of the protection at both ends of the collecting line. The current at the fault point flows to the end of the fault line through the distributed capacitance and then is diverted to other non-fault lines. That is, the negative sequence current measured at the protection at both ends of the fault line is greater than the negative sequence current of other non-fault lines connected to the same bus.

[0052] Figure 3The offshore wind farm in China increases the outlet voltage of 35kV wind turbines to 220kV through the offshore booster station, and transmits the electric energy to the offshore converter station through the AC collection line. The offshore converter station side protection of the AC collection line L1 is protection 2, and the offshore booster station side protection of the AC collection line L1 is protection 1; the offshore converter station side protection of the AC collection line L2 is protection 4, and the offshore booster station side protection of the AC collection line L2 is protection 3; the offshore converter station side protection of the AC collection line L3 is protection 6, and the offshore booster station side protection of the AC collection line L1 is protection 5; the offshore booster station side protection of the wind farm collection line L4 is protection 7; the offshore booster station side protection of the wind farm collection line L5 is protection 8; the offshore booster station side protection of the wind farm collection line L6 is protection 9.

[0053] When an asymmetric fault occurs at point f2 outside the AC collection line L1, the negative sequence current flow path is as follows: Figure 3 As shown by the black dotted line in the middle, for the protection on the offshore converter station (busbar N) side, the negative sequence current flows into the non-fault lines L1, L2, and L3 connected to the busbar N. The negative sequence current measured at the protection near the fault side of the non-fault line is expressed as follows:

[0054]

[0055] Where: express Figure 3 Negative sequence current component at the AC side outlet of the offshore converter station, Respectively Figure 3 Negative sequence current component at middle protection 2, 4, and 6.

[0056] The negative sequence current analysis of non-fault lines L1, L2, and L3 is similar. Taking line L1 as an example, the fault current flows into the near-fault side protection 2 of the non-fault line L1 and flows out from the far-fault side protection 1. For the protection on the offshore substation (busbar M) side, the negative sequence current relationship measured at the far-fault side protection 1 of the non-fault line is still similar to that at the time of the f1 point fault, which is the following relationship:

[0057]

[0058] Where: Respectively Figure 3 Negative sequence current components at middle protection points 1, 7, 8, and 9.

[0059] Therefore, when an out-of-zone fault occurs on the collecting line, the fault point is located in the opposite direction of the protection on the near-fault side of the collecting line and in the positive direction of the protection on the far-fault side. The negative-sequence current measured at the protection on the near-fault side of the collecting line is smaller than the negative-sequence current of the fault line connected to the same bus.

[0060] From the above analysis, it can be seen that the distribution of negative sequence current of offshore AC collection line is obviously different when there is a fault inside and outside the area. Therefore, a transient negative sequence current energy protection method for offshore wind power AC collection line is constructed. The method of identifying the fault line by the transient negative sequence current energy method of offshore AC collection line includes the following steps:

[0061] Step 1: Calculate the negative sequence current energy S at each feeder protection installation on the same bus j (n);

[0062] Step 2: Select the minimum negative sequence current energy S at each feeder protection installation location of the same busbar r (n) as a benchmark value;

[0063] Step 3: Calculate the ratio k of the negative sequence current energy to the reference value at each feeder protection installation on the same busbar j ;

[0064] Step 4: Compare the ratio k j With the threshold k set Determine the fault direction by the size relationship of

[0065] Step 5: Identify the internal and external faults according to the fault direction determination results of the protection at both ends of the AC collection line.

[0066] Step 1 calculates the negative sequence current energy S of each feeder of the same bus j (n):

[0067] Figure 1 The offshore wind farm in China uses an offshore substation to increase the outlet voltage of 35kV wind turbines to 220kV, and the AC collection line transmits the electric energy to the offshore converter station, and then connects to the onshore converter station via submarine cables, and finally integrates into the 500kV AC system. Figure 1 The two ends of the offshore AC collection line are connected to the offshore converter station and the offshore booster station respectively. The negative sequence current measured at the protection 2, 4, 6 of the line connected to the offshore converter station (busbar N) side and the protection 7, 8, 9 of the line connected to the offshore booster station (busbar M) side are detected respectively. The negative sequence current measured at the protection j at the sampling time m is defined as The negative sequence current measured at the protection j at the sampling time m is accumulated within a certain time window to obtain the negative sequence current energy S at the time n. j (n), then the negative sequence current energy S at protection point j at time n is j (n) is:

[0068]

[0069] Where: n is the nth sampling time, Δt is the sampling period, T is the data window length 5ms, the sampling frequency is 5kHz, To protect the negative sequence current at point j at time m, S j (n) is the negative sequence current energy at protection point j at time n, which is the accumulated sum of all negative sequence currents in the T data window before time n.

[0070] Step 2, select the minimum value S of the negative sequence current energy of each feeder of the same bus r (n) as a benchmark value;

[0071] According to the step 1, the negative sequence current energy S2(n), S4(n), S6(n) of the protections 2, 4, 6 connected to the offshore converter station (busbar N) at the time n and the negative sequence current energy S7(n), S8(n), S9(n) of the protections 7, 8, 9 connected to the offshore booster station (busbar M) at the time n are obtained. The negative sequence current energy S2(n), S4(n), S6(n) of the feeder protections 2, 4, 6 connected to the offshore converter station (busbar N) at the time n are compared; the negative sequence current energy S7(n), S8(n), S9(n) of the feeder protections 7, 8, 9 connected to the offshore booster station (busbar M) at the time n are compared. For the offshore converter station (busbar N) and the offshore booster station (busbar M), the minimum negative sequence current energy S of the protections connected to the busbars at the time n is selected respectively. r (n) As a reference value, the negative sequence current energy reference value S r (n) is:

[0072] S r (n) = min{S j (n)}

[0073] Where: S r (n) is the minimum negative sequence current energy among all protections of the busbar connected to protection j at time n, S j (n) is the negative sequence current energy at protection point j at time n.

[0074] Step 3, calculate the negative sequence current energy S of each feeder of the same bus j (n) and the reference value S r The ratio k of (n) j :

[0075] The negative sequence current energy S2(n), S4(n), S6(n) of the feeder protection 2, 4, 6 connected to the offshore converter station (busbar N) at time n and the negative sequence current reference value S in each feeder of the offshore converter station (busbar N) selected in step 2 are used. r(n) is used to perform ratio operation, and the ratio of the negative sequence current energy at protection j to the reference value is calculated. Similarly, the negative sequence current energy S7(n), S8(n), S9(n) of the feeder protections 7, 8, and 9 connected to the offshore booster station (busbar M) at time n is used together with the negative sequence current reference value S in each feeder of the offshore booster station (busbar M) selected in step 2. r (n) Perform ratio operation to calculate the ratio of the negative sequence current energy at protection point j to the reference value.

[0076]

[0077] Where: k j is the ratio of the negative sequence current energy at protection j to the reference value, S r (n) is the minimum negative sequence current energy among all protections of the busbar connected to protection j at time n, S j (n) is the negative sequence current energy at protection point j at time n.

[0078] The step 4 compares the ratio k j With the threshold k set Determine the fault direction by the size relationship:

[0079] When a fault occurs in the AC collection line area, the fault point is located in the positive direction of the protection at both ends of the fault line. The negative sequence current measured at the protection at both ends of the fault line is greater than the negative sequence current of other non-fault lines connected to the same bus. The negative sequence current shunting of other non-fault lines connected to the same bus is basically the same due to the same voltage level and line model. Therefore, the negative sequence current energy ratio of the fault line will be greater than the energy ratio of the non-fault line connected to the same bus.

[0080] When a fault occurs outside the AC collecting line area, the fault point is located in the opposite direction of the protection on the near-fault side of the collecting line and in the positive direction of the protection on the far-fault side of the collecting line. For the protection on the near-fault side of the collecting line, its negative-sequence current will be smaller than the negative-sequence current at the fault line protection of the bus connected to the protection on the near-fault side of the collecting line. Therefore, the negative-sequence current energy ratio at the protection on the near-fault side of the collecting line will be smaller than the negative-sequence current energy ratio at the fault line protection connected to the same bus.

[0081] From the above analysis, it can be seen that the fault direction can be determined according to the difference in the transient negative sequence current energy ratio between the fault line and the non-fault line connected to the same busbar. Therefore, the criterion for determining the fault direction is as follows:

[0082] a) If k j >k set , then protection j determines that the fault direction is a positive direction fault;

[0083] b) If k j <k set, then protection j determines that the fault direction is a reverse direction fault.

[0084] Where: k j is the ratio of the negative sequence current energy at protection j to the reference value, k set To set the threshold, considering that the non-fault line has basically the same line model and voltage level, the negative sequence current energy at the non-fault line protection is basically the same as the reference value, and the ratio is close to 1, so the threshold k is selected set =1.2.

[0085] The step 5 identifies the fault direction information based on the protection at both ends of the AC collection line to determine whether the fault is inside or outside the area.

[0086] When the protections at both ends of the AC collection line (taking the offshore converter station side protection 2 and the offshore booster station side protection 1 as examples) are both identified as positive direction faults, the fault is identified as an intra-area fault and the protection operates reliably; among them, the offshore booster station side protection 1 of the collection line needs to rely on the judgment results of the offshore booster station side wind farm collection line protections 7, 8, and 9, which are located on both sides of the busbar M. When the offshore booster station side wind farm collection line protections 7, 8, and 9 are all judged as reverse direction faults, the offshore booster station side protection 1 of the collection line is judged as a positive direction fault.

[0087] When any of the protections at both ends of the AC collection line (taking the offshore converter station side protection 2 and the offshore booster station side protection 1 as examples) identifies a reverse fault, a blocking signal is sent to the opposite side to identify the fault as an out-of-zone fault, and the protection is reliable and does not operate. Among them, the offshore booster station side protection 1 of the collection line needs to rely on the judgment results of the offshore booster station side wind farm collection line protection 7, 8, and 9, which are located on both sides of the busbar M. When any of the offshore booster station side wind farm collection line protections 7, 8, and 9 are judged as a positive fault, the offshore booster station side protection of the collection line is judged as a reverse fault.

[0088] Example

[0089] Figure 1 In the transmission line, the voltage level of the AC collection line is 220kV, the positive-sequence resistance per unit length is 0.0472Ω / km, the positive-sequence inductance per unit length is 0.3955mH / km, the positive-sequence capacitance per unit length is 0.1673μF / km, the zero-sequence resistance per unit length is 0.1186Ω / km, the zero-sequence inductance per unit length is 0.1042mH / km, and the zero-sequence capacitance per unit length is 0.1673μF / km.

[0090] When a phase A ground fault occurs at the midpoint f1 of the AC collection line L1 (such as Figure 2 As shown in Figure 2, the simulated waveforms of negative sequence current energy and negative sequence current energy ratio at each feeder of the offshore converter station (busbar N) and the offshore booster station (busbar M) are as follows: Figure 4-7 shown.

[0091] According to step 1, the negative sequence current energy measured at the protections 2, 4, 6 of the lines connected to the offshore converter station (busbar N) side and the protections 7, 8, 9 of the lines connected to the offshore booster station (busbar M) side are calculated respectively. The negative sequence current energies S2(n), S4(n), and S6(n) at the protections 2, 4, and 6 are respectively as follows: Figure 4 As shown by the solid, dotted and dashed lines, the negative sequence current energies S7(n), S8(n) and S9(n) at protections 7, 8 and 9 are respectively Figure 5 Indicated by the solid line, dotted line and dashed line. Figure 4-5 It can be seen that for the protection on the offshore converter station (busbar N) side, the negative sequence current energy at protection 2 is significantly greater than the negative sequence current energy at protection 4 and protection 6; for the protection on the offshore booster station (busbar M) side, the negative sequence current energy at protection 7, protection 8 and protection 9 is basically the same.

[0092] According to step 2, for the offshore converter station side, the minimum negative sequence current energy S4(n) of protection 6 in each feeder protection of the same bus is selected as the reference value, and for the offshore converter station side, the minimum negative sequence current energy S7(n) of protection 7 in each feeder protection of the same bus is selected as the reference value.

[0093] According to step 3, for the offshore converter station side, the negative sequence current energies S2(n), S4(n), and S6(n) at protections 2, 4, and 6 are respectively ratioed to the reference value S4(n), and the negative sequence current energy ratios k2, k4, and k6 at protections 2, 4, and 6 are obtained as follows: Figure 6 For the offshore booster station side, the negative sequence current energies S7(n), S8(n), and S9(n) at protections 7, 8, and 9 are respectively compared with the reference value S7(n) to obtain the negative sequence current energy ratios k7, k8, and k9 at protections 7, 8, and 9, respectively. Figure 7 Shown in solid line, dotted line and dashed line.

[0094] According to step 4, for the offshore converter station side, the negative sequence current energy ratio k2 at protection 2 is 2.067, and the negative sequence current energy ratios k4 and k6 at protection 4 and 6 are both 1. set =1.2, we can see that: k2>k set 、k4<k set 、k6<k set , then protection 2 is judged as a positive direction fault, and protections 4 and 6 are judged as reverse direction faults. For the offshore substation side, the negative sequence current energy ratios k7, k8, and k9 of protections 7, 8, and 9 are all 1, and each ratio is compared with the set threshold k set =1.2, we can see that: k7<k set 、k8<k set、k9<k set , then protection 7, 8, 9 are judged as reverse direction fault.

[0095] According to step 5, the offshore booster station side protection 1 of the collection line needs to rely on the judgment results of the offshore booster station side wind farm collection line protection 7, 8, 9, which are respectively located on both sides of the busbar M. When the offshore booster station side wind farm collection line protection 7, 8, 9 are all judged as reverse direction faults, that is, the offshore booster station side protection 1 of the collection line is judged as positive direction fault. Therefore, the protections at both ends of the AC collection line L1 (protection 2, protection 1) are both identified as positive direction faults, then the fault occurs in the area of ​​the AC collection line L1, and the protection is reliably operated, verifying the correctness of the present invention.

[0096] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions only describe the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A transient negative sequence current energy protection method for an offshore AC collection line, used for rapid and accurate identification of faulty lines; the method is implemented based on an offshore wind power flexible direct current transmission system, the offshore wind power flexible direct current transmission system comprising an offshore wind farm, a wind farm collection line, an offshore booster station, an AC collection line, an offshore converter station, a DC submarine cable, an onshore converter station, and an AC system; the transient negative sequence current energy is obtained by accumulating the negative sequence current within a certain time window, and is characterized in that: Step 1: Calculate the negative sequence current energy S at each feeder protection installation on the same bus j (n); Step 2: Select the minimum negative sequence current energy S at each feeder protection installation point of the same busbar r (n) as a benchmark value; Step 3: Calculate the ratio k of the negative sequence current energy to the reference value at each feeder protection installation point of the same busbar j ; Step 4: Compare the ratio k j With the threshold k set set The fault direction is determined by the magnitude relationship of the protection; when a fault occurs in the positive direction, its transient negative sequence current energy is greater than the transient negative sequence current energy of other lines connected to the same bus; when a fault occurs in the reverse direction, its transient negative sequence current energy is less than the transient negative sequence current energy of the fault line connected to the same bus; the criterion for identifying the fault direction at protection j is as follows: a) If k j >k set , then protection j determines that the fault direction is a positive direction fault; b) If k j <k set , then protection j determines that the fault direction is a reverse direction fault; Where: k j is the ratio of the negative sequence current energy at protection j to the reference value, k set To set the threshold, considering that the non-fault line has the same line model and voltage level, the negative sequence current energy at the non-fault line protection is the same as the reference value, and the ratio is close to 1, so the threshold k is selected set =1.2; Step 5: Identify internal and external faults according to the fault direction discrimination results of the protection at both ends of the AC collection line: When the protection at both ends of the AC collection line identifies a positive direction fault, the fault is identified as an internal fault and the protection is reliably actuated; When any of the protections at both ends of the AC collection line identifies a reverse direction fault, a blocking signal is sent to the opposite side, identifying the fault as an out-of-zone fault, and the protection is reliable and does not operate.

2. The transient negative sequence current energy protection method for offshore AC collection lines according to claim 1 is characterized in that: The step 1 calculates the negative sequence current energy S at the time n of each feeder protection installation of the same busbar j (n): Where: n is the nth sampling time, m is the mth sampling time, Δt is the sampling period, T is the data window length, To protect the negative sequence current at point j at time m, S j (n) is the negative sequence current energy at protection point j at time n, which is the accumulated sum of all negative sequence currents in the T data window before time n.

3. The transient negative sequence current energy protection method for offshore AC collection lines according to claim 2 is characterized in that: The step 2 further includes the following contents: selecting the minimum negative sequence current energy S at the time n of each feeder protection installation of the same busbar r (n) As a reference value: S r (n)=min{S j {noun} Where: S r (n) is the minimum negative sequence current energy in each feeder protection installation of the bus connected to protection j at time n, S j (n) is the negative sequence current energy at protection point j at time n.

4. The transient negative sequence current energy protection method for offshore AC collection lines according to claim 3 is characterized in that: Step 3 calculates the ratio k of the negative sequence current energy to the reference value at each feeder protection installation of the same busbar j : Where: k j is the ratio of the negative sequence current energy at protection j to the reference value, S r (n) is the minimum negative sequence current energy among all protections of the busbar connected to protection j at time n, S j (n) is the negative sequence current energy at protection point j at time n.

5. The relay protection system of the offshore wind power AC collection line is characterized by: The system adopts the transient negative sequence current energy protection method of the offshore AC collection line described in any one of claims 1-4.

Images