Single-phase earth fault improved differential protection method for collecting line of offshore wind plant

Abstract

An improved differential protection method for a single-phase earth fault of an offshore wind plant collection line comprises the following steps: continuously reading voltage and current data at a protection installation position, calculating a fault distance and a fault identification factor corresponding to the fault distance when the fault type is judged to be a single-phase earth fault after the start of protection is confirmed; when fault identification factors calculated according to the voltage and current data of five continuous sampling points at the protection installation position all exceed a threshold value, it is judged that an internal fault occurs and a tripping signal is sent out, and otherwise, it is judged that an external fault occurs when at least one fault factor value is lower than the threshold value. According to the method, enough sensitivity and correct action can be kept when single-phase grounding and two-phase short-circuit faults happen to the collection line, different reactive power support strategies and operation modes of an offshore wind power plant can be adapted, and the problem that traditional current differential protection fails to act under low fault current scenes such as double-end inversion type power supply transmission lines is solved.

Description

Technical Field

[0001] The present invention relates to a technology in the field of offshore wind farms, specifically an improved differential protection method for single-phase grounding faults of offshore wind farm collection lines. Background Art

[0002] Distance protection reflects the fault distance and is not affected by the system operation mode. It is widely used as the main protection and backup protection for lines. Under the background of carbon peaking and carbon neutrality, the new energy industry has developed rapidly, and offshore wind power has received extensive attention in recent years. Offshore wind farm clusters are connected to the grid through submarine cables. Human or natural factors such as external force machinery and chemical corrosion may cause damage to the outer insulation of submarine cables, resulting in short-circuit faults in the collection lines. With the rapid development of the offshore wind power industry, the protection problem of offshore wind farm collection lines has become increasingly prominent. Summary of the Invention

[0003] In view of the problems of insufficient sensitivity and high requirements for protection hardware existing in the existing differential or pilot protection, the present invention proposes an improved differential protection method for single-phase grounding faults of offshore wind farm collection lines, which can maintain sufficient sensitivity and operate correctly when single-phase grounding and two-phase short-circuit faults occur in the collection lines, can adapt to different reactive power support strategies and operation modes of offshore wind farms, and solves the problem of refusal to operate of traditional current differential protection in low-fault-current scenarios such as transmission lines of double-ended inverter-type power supplies.

[0004] The present invention is realized through the following technical solutions:

[0005] The present invention relates to an improved differential protection method for single-phase grounding faults of offshore wind farm collection lines. By continuously reading voltage and current data at the protection installation location, after confirming the activation of the protection, when the fault type is determined to be a single-phase grounding fault, the fault distance and its corresponding fault identification factor are calculated. When the fault identification factors calculated from the voltage and current data of five consecutive sampling points at the protection installation location all exceed the threshold, it is determined as an in-zone fault and a tripping signal is issued; otherwise, when at least one fault factor value is lower than the threshold, it is determined as an out-of-zone fault.

[0006] The calculation of the fault distance includes: calculating the fault distances α and 1 - α on both sides, specifically: Where: is the positive-sequence voltage of phase A on the wind farm side, is the negative-sequence voltage of phase A on the wind farm side, is the positive-sequence current of phase A on the wind farm side, is the positive-sequence impedance of the collection line, Imag is to take the imaginary part of the phasor, and the rotation factor F = 1∠(-I MMC-A + -I Wind-A + ), is the positive sequence current of phase A on the flexible DC side, and ∠ is the angle of the phasor.

[0007] The described fault identification factor is the reciprocal sum of the fault distances on both sides, specifically:

[0008] The determination of an in-zone fault means: Where: M set is the setting value; during an in-zone fault, M is greater than 4; during normal operation and out-of-zone faults, M is less than 2.

[0009] Preferably, the setting value M set = 3. Technical effects

[0010] The present invention indirectly obtains the negative sequence voltage at the fault point through the negative sequence voltage at the protection installation location and calculates the fault location. At the same time, the influence of the transition resistance is eliminated by using the rotation factor. The present invention can maintain sufficient sensitivity and operate correctly when a single-phase ground fault occurs on the collection line, can adapt to different reactive power support strategies and operation modes of the offshore wind farm, and solves the problem of refusal to operate of traditional current differential protection in low fault current scenarios such as transmission lines with double-ended inverter power supplies. Description of the drawings

[0011] Figure 1 is the flow chart of the present invention;

[0012] Figure 2 is the test system diagram of the collection line of the offshore wind farm;

[0013] Figure 3 is the identification result of a metallic single-phase ground fault inside and outside the collection line;

[0014] Figure 4 is the identification result of a single-phase ground fault inside and outside the collection line with a 100Ω transition resistance;

[0015] Figure 5 is the influence of the operation mode of the wind farm on the protection. Specific implementation manners

[0016] To verify the rationality of the high-sensitivity differential protection for single-phase ground faults on the collection line of the offshore wind farm, this embodiment uses the Jiangsu Rudong Offshore Wind Power Flexible DC Transmission Demonstration Project as a verification example, and the topology diagram is as Figure 2 . The offshore wind farm groups H6, H10, and H8 are connected to the 220kV offshore booster station (transformer on the fan side) through their respective on-site 35kV collection systems, and are then connected to the offshore flexible DC converter station through the 220kV collection line. The capacities of the offshore wind farm groups H6, H10, and H8 are 400MW, 400MW, and 300MW respectively. This embodiment takes the collection line of the offshore wind farm group H6 as an example for verification.

[0017] As shown Figure 1 in the figure, this embodiment relates to an improved differential protection method for single-phase grounding faults in the collection line of an offshore wind farm. By continuously reading voltage and current data at the protection installation location, when it is confirmed to start the protection and the fault type is determined to be a single-phase grounding fault, the fault distance and its corresponding fault identification factor are calculated. When the fault identification factors calculated from the voltage and current data of five consecutive sampling points at the protection installation location all exceed the threshold, it is determined as an in-zone fault and a tripping signal is issued; otherwise, when at least one fault factor value is lower than the threshold, it is determined as an out-of-zone fault.

[0018] Through specific experiments, the results of identifying metallic single-phase grounding faults at the 80%, 50%, 20% of the collection line, the outlet of the 220 kV side of the flexible DC transformer, the 50% of the adjacent collection line, and the 220 kV collection bus are as Figure 3 shown in the figure; the results of identifying single-phase grounding faults through a 100 Ω transition resistance at the 80%, 50%, 20% of the collection line, the outlet of the 220 kV side of the flexible DC transformer, the 50% of the adjacent collection line, and the 220 kV collection bus are as Figure 4 shown in the figure.

[0019] In the above two cases, this embodiment can detect in-zone faults within 30 ms and can maintain sufficient sensitivity; in the case of out-of-zone faults, this embodiment can reliably not operate, and as the transition resistance increases, this embodiment can still maintain sufficient sensitivity.

[0020] Affected by the intermittent and fluctuating characteristics of wind power, the operation mode of the wind farm changes greatly. The above-mentioned simulations are all results when the output power of the offshore wind farm is 1 pu. When the output power of the wind farm is 0.5 pu, the results of identifying single-phase grounding faults through a 10 Ω transition resistance at the 80%, 50%, 20% of the collection line, the outlet of the 220 kV side of the flexible DC transformer, the 50% of the adjacent collection line, and the 220 kV collection bus are as Figure 4 shown in the figure. It can be seen that when the output power of the offshore wind farm is 0.5 pu, this embodiment can still detect in-zone faults within 30 ms and can reliably not operate in the case of out-of-zone faults.

[0021] Compared with the prior art, the simulation results show that applying the present invention in the collection line of an offshore wind farm can effectively improve the ability of the offshore wind farm to identify single-phase grounding faults and enhance its safe and stable operation ability.

[0022] The above specific embodiments can be locally adjusted in different ways by those skilled in the art without departing from the principles and purposes of the present invention. The protection scope of the present invention is subject to the claims and is not limited by the above specific embodiments. All implementation solutions within its scope are subject to the present invention.

Claims

1. An improved differential protection method for single-phase grounding faults of a collection line in an offshore wind farm, characterized in that, By continuously reading voltage and current data at the protection installation location, when it is confirmed that the protection is activated and the fault type is determined to be a single-phase grounding fault, calculate the fault distance and its corresponding fault identification factor. When the fault identification factors calculated from the voltage and current data of five consecutive sampling points at the protection installation location all exceed the threshold, it is determined as an in-zone fault and a trip signal is issued; otherwise, when at least once the fault factor value is lower than the threshold, it is determined as an out-of-zone fault. The calculation of the fault distance includes: calculating the fault distances α and 1 - α on both sides, specifically: Where: U Wind-A + is the positive - sequence voltage of phase A on the wind - farm side, U Wind-A - is the negative - sequence voltage of phase A on the wind - farm side, I Wind-A + is the positive - sequence current of phase A on the wind - farm side, Z L + is the positive - sequence impedance of the collection line, Imag is the imaginary part of the phasor, the rotation factor F = 1∠(-I MMC-A + -I Wind-A + ), I MMC-A + is the positive - sequence current of phase A on the VSC - HVDC side, ∠ is the angle of the phasor.

2. The improved differential protection method for single-phase grounding faults of a collection line in an offshore wind farm according to claim 1, characterized in that, The described fault identification factor is the sum of the reciprocals of the fault distances on both sides, specifically:

3. The improved differential protection method for single-phase grounding faults of a collection line in an offshore wind farm according to claim 1, characterized in that, The determination of a fault within the zone means that: Where: M set is the setting value; during a fault within the zone, M is greater than 4; during normal operation and a fault outside the zone, M is less than 2.

4. The improved differential protection method for single-phase grounding faults of a collection line in an offshore wind farm according to claim 3, characterized in that, The setting value M set = 3

Images