CN-122026343-A - Low-frequency alternating current transmission system and fault suppression method, system, equipment and medium thereof

CN122026343ACN 122026343 ACN122026343 ACN 122026343ACN-122026343-A

Abstract

The invention provides a low-frequency alternating current sending-out system and a fault suppression method, a system, equipment and a medium thereof, wherein the system comprises an offshore wind farm, an offshore transformer, an alternating current converter, an onshore transformer, an onshore power grid and an angle joint reactive generator; the system comprises an AC/DC converter, an on-shore power grid, an angle joint reactive power generator, a coordination control strategy and a control system, wherein the AC/DC converter is connected to the on-shore wind power plant through an on-shore transformer at a low frequency side, the AC/DC converter is connected to the on-shore power grid through the on-shore transformer at a power frequency side, the power frequency side is connected with the angle joint reactive power generator in parallel, and the AC/DC converter and the angle joint reactive power generator are used for inhibiting unbalanced faults at the power frequency side through the coordination control strategy. In the system, the reactive generator is connected in parallel at the power frequency side, so that the defect of current regulation capability of a star connection structure of the AC-AC converter can be overcome, and the unbalance condition caused by the fault at the AC network side of the AC-AC converter can be regulated.

Inventors

Guo gaopeng
SHE DONGJIN
LI LANFANG
ZHANG FAN
LU HONGGANG
JIN XUEFEN
HAO JING
WANG XIAOFEI
CAO TIANZHI

Assignees

中电普瑞电力工程有限公司
国网电力科学研究院有限公司
国网冀北电力有限公司电力科学研究院
国家电网有限公司

Dates

Publication Date: 20260512
Application Date: 20251224

Claims (20)

1. The low-frequency alternating current sending-out system is characterized by comprising an offshore wind farm, an offshore transformer, an alternating current converter, an onshore transformer, an onshore power grid and an angle joint reactive generator; the ac converter is coupled to the offshore wind farm at a low frequency side by the offshore transformer; the ac-dc converter is coupled to the onshore grid at a power frequency side via the onshore transformer, and the corner-junction reactive generator is connected in parallel to the power frequency side; The AC-DC converter and the angle joint reactive generator are used for restraining unbalanced faults at the power frequency side through a coordination control strategy.
2. The system of claim 1, wherein the corner joint reactive generator comprises a corner joint static reactive generator SVG.
3. A system according to claim 1 or 2, characterized in that the corner joint var generator comprises a delta connection topology of three commutation chains; each converter chain is of an H-bridge cascade multi-level structure.
4. A fault suppression method for a low frequency ac delivery system, comprising: When unbalance faults occur on the power frequency side of the low-frequency alternating current sending-out system, acquiring the negative sequence current of the grid-connected point; Based on the negative sequence current and the bearing capacity difference of the AC-DC converter and the corner-junction reactive generator in the low-frequency AC sending system to the negative sequence current, generating a negative sequence current control instruction for the AC-DC converter and the corner-junction reactive generator by utilizing a coordination control strategy, wherein the corner-junction reactive generator is connected in parallel with the power frequency side of the AC-DC converter; And controlling the AC-DC converter and the corner junction reactive power generator to perform current compensation based on the negative sequence current control instruction so as to inhibit the unbalanced fault and realize system fault ride-through.
5. The method of claim 4, wherein generating negative sequence current control commands for the ac converter and the corner-to-corner reactive generator using a coordinated control strategy based on the negative sequence current and a difference in load carrying capacity of the ac converter and the corner-to-corner reactive generator in the low frequency ac delivery system comprises: extracting d-axis components and q-axis components of the negative sequence current by adopting a symmetrical component method; Calculating a first distribution weight of the AC-DC converter and a second distribution weight of the corner-junction reactive generator by using a capacity ratio example normalization distribution method in a coordination control strategy based on the bearing capacity difference of the AC-DC converter and the corner-junction reactive generator on negative-sequence current; And according to the first distribution weight and the second distribution weight, combining the d-axis component and the q-axis component, and utilizing a weighted distribution algorithm in a coordination control strategy to generate negative sequence current control instructions for the AC-DC converter and the corner joint reactive power generator.
6. The method of claim 5, wherein calculating the first distribution weight of the ac-dc converter and the second distribution weight of the corner-junction reactive generator using a capacity ratio normalization distribution method in a coordinated control strategy based on the difference in the load capacity of the ac-dc converter and the corner-junction reactive generator for negative-sequence currents comprises: Determining the rated current and the maximum negative sequence current proportion of the AC-DC converter and the rated current and the maximum negative sequence current proportion of the corner-junction reactive generator based on the bearing capacity difference of the AC-DC converter and the corner-junction reactive generator on the negative sequence current; Calculating a negative sequence current capacity ratio coefficient by using a capacity ratio example normalization distribution method in a coordination control strategy based on the rated current and the maximum negative sequence current proportion of the AC-DC converter and the rated current and the maximum negative sequence current proportion of the corner joint reactive power generator; and calculating a first distribution weight of the alternating current converter and a second distribution weight of the corner joint reactive generator based on the negative sequence current capacity ratio coefficient.
7. The method of claim 6, wherein the negative sequence current capability ratio coefficient satisfies the following formula: where k is the negative sequence current capability ratio coefficient, For the rated current of the ac-to-ac converter, For the maximum negative sequence current ratio of the ac-to-dc converter, For the rated current of the corner joint reactive generator, For the maximum negative sequence current ratio of the corner junction reactive generator, Is the number of phases of the ac-to-dc converter.
8. The method of claim 6 or 7, wherein after determining the current rating and the maximum negative sequence current ratio of the ac-to-ac converter and the current rating and the maximum negative sequence current ratio of the corner-to-corner reactive generator based on the difference in the load carrying capacity of the ac-to-corner reactive generator for negative sequence currents, further comprising: Acquiring the current bridge arm current of the AC-DC converter, the unbalance degree of capacitance and voltage of each sub-module and the junction temperature state; And correcting the maximum negative sequence current proportion corresponding to the AC-DC converter and the corner junction reactive power generator respectively by taking the thermal stability and overmodulation inhibition during the fault period as optimization targets based on the bridge arm current, the unbalance degree of the capacitance voltage of each sub-module and the junction temperature state.
9. The method of claim 6, wherein said generating negative sequence current control instructions for said ac-to-ac converter and said corner-to-corner reactive generator using a weighted allocation algorithm in a coordinated control strategy in accordance with said first allocation weight and said second allocation weight in combination with said d-axis component and q-axis component comprises: Based on the d-axis component and the q-axis component, respectively carrying out linear weighting according to the first distribution weight and the second distribution weight to generate initial negative sequence current instructions for the AC-DC converter and the corner joint reactive power generator; If the initial negative sequence current instruction causes that the bridge arm current of the AC-DC converter and/or the corner junction reactive power generator exceeds the upper limit of the corresponding negative sequence current, correcting the initial negative sequence current instruction by adopting a scaling and rebalancing algorithm based on the current safety constraint of the AC-DC converter and the corner junction reactive power generator to obtain a final negative sequence current control instruction; Otherwise, the initial negative sequence current instruction is used as a final negative sequence current control instruction.
10. The method of claim 4, wherein generating negative sequence current control commands for the ac converter and the corner-to-corner reactive generator using a coordinated control strategy based on the negative sequence current and a difference in load carrying capacity of the ac converter and the corner-to-corner reactive generator in the low frequency ac delivery system comprises: Determining current safety constraints of an AC-AC converter and an angular joint reactive generator based on the difference of carrying capacity of the AC-AC converter and the angular joint reactive generator on negative sequence current in the low-frequency AC delivery system; constructing a multi-optimization objective function of a coordination control strategy by taking the voltage unbalance degree of the minimum grid-connected point and the total active loss of the system as optimization targets; And solving the multi-optimization objective function based on the negative sequence current by taking the current safety constraint as a constraint condition to obtain a negative sequence current control instruction for the AC-DC converter and the corner joint reactive power generator.
11. The method of claim 4, wherein controlling the ac-to-ac converter and the corner-to-corner reactive generator for current compensation based on the negative sequence current control command comprises: Adopting a three-phase independent synchronous rotation coordinate system current control strategy to track a fundamental wave reactive power instruction under a positive sequence dq coordinate system, and controlling the actual output current of the angle joint reactive power generator to track the first negative sequence d-axis current and the first negative sequence q-axis current respectively under the negative sequence dq coordinate system; and generating a low-frequency side alternating current voltage reference signal through a bidirectional modulation strategy of an internal matrix converter of the alternating current converter, and controlling actual output current output from a power frequency side to track the second negative sequence d-axis current and the second negative sequence q-axis current respectively.
12. A fault suppression system for a low frequency ac delivery system, comprising: the acquisition module is used for acquiring the negative sequence current of the grid-connected point when the power frequency side of the low-frequency alternating current sending-out system has an unbalanced fault; The coordination control module is used for generating negative sequence current control instructions for the AC-DC converter and the corner joint reactive power generator by utilizing a coordination control strategy based on the negative sequence current and the bearing capacity difference of the AC-DC converter and the corner joint reactive power generator in the low-frequency AC transmission system, wherein the corner joint reactive power generator is connected in parallel with the power frequency side of the AC-DC converter; And the fault suppression module is used for controlling the AC-AC converter and the corner joint reactive generator to perform current compensation based on the negative sequence current control instruction so as to suppress the unbalanced fault and realize system fault ride-through.
13. The system of claim 12, wherein the coordination control module is specifically configured to: extracting d-axis components and q-axis components of the negative sequence current by adopting a symmetrical component method; Calculating a first distribution weight of the AC-DC converter and a second distribution weight of the corner-junction reactive generator by using a capacity ratio example normalization distribution method in a coordination control strategy based on the bearing capacity difference of the AC-DC converter and the corner-junction reactive generator on negative-sequence current; And according to the first distribution weight and the second distribution weight, combining the d-axis component and the q-axis component, and utilizing a weighted distribution algorithm in a coordination control strategy to generate negative sequence current control instructions for the AC-DC converter and the corner joint reactive power generator.
14. The system of claim 13, wherein the coordination control module is specifically configured to: Determining the rated current and the maximum negative sequence current proportion of the AC-DC converter and the rated current and the maximum negative sequence current proportion of the corner-junction reactive generator based on the bearing capacity difference of the AC-DC converter and the corner-junction reactive generator on the negative sequence current; Calculating a negative sequence current capacity ratio coefficient by using a capacity ratio example normalization distribution method in a coordination control strategy based on the rated current and the maximum negative sequence current proportion of the AC-DC converter and the rated current and the maximum negative sequence current proportion of the corner joint reactive power generator; and calculating a first distribution weight of the alternating current converter and a second distribution weight of the corner joint reactive generator based on the negative sequence current capacity ratio coefficient.
15. The system of claim 14, wherein the negative sequence current capability ratio coefficient satisfies the following formula: where k is the negative sequence current capability ratio coefficient, For the rated current of the ac-to-ac converter, For the maximum negative sequence current ratio of the ac-to-dc converter, For the rated current of the corner joint reactive generator, For the maximum negative sequence current ratio of the corner junction reactive generator, Is the number of phases of the ac-to-dc converter.
16. The system of claim 14 or 15, wherein the coordination control module is further configured to: Acquiring the current bridge arm current of the AC-DC converter, the unbalance degree of capacitance and voltage of each sub-module and the junction temperature state; And correcting the maximum negative sequence current proportion corresponding to the AC-DC converter and the corner junction reactive power generator respectively by taking the thermal stability and overmodulation inhibition during the fault period as optimization targets based on the bridge arm current, the unbalance degree of the capacitance voltage of each sub-module and the junction temperature state.
17. The system of claim 14, wherein the coordination control module is specifically configured to: Based on the d-axis component and the q-axis component, respectively carrying out linear weighting according to the first distribution weight and the second distribution weight to generate initial negative sequence current instructions for the AC-DC converter and the corner joint reactive power generator; If the initial negative sequence current instruction causes that the bridge arm current of the AC-DC converter and/or the corner junction reactive power generator exceeds the upper limit of the corresponding negative sequence current, correcting the initial negative sequence current instruction by adopting a scaling and rebalancing algorithm based on the current safety constraint of the AC-DC converter and the corner junction reactive power generator to obtain a final negative sequence current control instruction; Otherwise, the initial negative sequence current instruction is used as a final negative sequence current control instruction.
18. The system of claim 12, wherein the coordination control module is specifically configured to: Determining current safety constraints of an AC-AC converter and an angular joint reactive generator based on the difference of carrying capacity of the AC-AC converter and the angular joint reactive generator on negative sequence current in the low-frequency AC delivery system; constructing a multi-optimization objective function of a coordination control strategy by taking the voltage unbalance degree of the minimum grid-connected point and the total active loss of the system as optimization targets; And solving the multi-optimization objective function based on the negative sequence current by taking the current safety constraint as a constraint condition to obtain a negative sequence current control instruction for the AC-DC converter and the corner joint reactive power generator.
19. The system of claim 12, wherein the fault suppression module comprises: The angle joint reactive power generator control unit is used for decomposing the negative sequence current control instruction to obtain a first negative sequence d-axis current and a first negative sequence q-axis current of the angle joint reactive power generator; tracking a fundamental wave reactive power instruction under a positive sequence dq coordinate system by adopting a three-phase split-phase independent synchronous rotation coordinate system current control strategy, and controlling the actual output current of the corner joint reactive power generator under a negative sequence dq coordinate system to track the first negative sequence d-axis current and the first negative sequence q-axis current respectively; The AC-to-AC converter control unit is used for decomposing the negative sequence current control instruction to obtain a second negative sequence d-axis current and a second negative sequence q-axis current of the AC-to-AC converter, generating a low-frequency side alternating voltage reference signal through a bidirectional modulation strategy of an internal matrix converter of the AC-to-AC converter, and controlling the actual output current output by a power frequency side to track the second negative sequence d-axis current and the second negative sequence q-axis current respectively.
20. The electronic equipment is characterized by comprising at least one processor and a memory, wherein the memory and the processor are connected through a bus; the memory is used for storing one or more programs; The fault suppression method of a low frequency ac delivery system as claimed in any one of claims 4 to 11 is implemented when the one or more programs are executed by the at least one processor.

Description

Low-frequency alternating current transmission system and fault suppression method, system, equipment and medium thereof Technical Field The invention relates to the technical field of low-frequency alternating current transmission of power systems, in particular to a low-frequency alternating current transmission system and a fault suppression method, system, equipment and medium thereof. Background Compared with the traditional flexible direct current transmission system based on MMC (Modular Multilevel Converter, modularized multi-level converter), the wind power plant transmits electric energy through a low-frequency alternating current transmission system, saves offshore platforms, greatly reduces the volume and the cost within the economic range of 70 km-300 km, and remarkably improves the economical efficiency. However, when the system unbalance fault of the new energy wide area networking is caused by the system unbalance fault of the power frequency alternating current side, the flexible low frequency sending system can cause overvoltage and overcurrent of the alternating current converter, and cause locking or system tripping, so that the stable operation of the alternating current converter is affected. Therefore, corresponding measures are needed to restrain overvoltage and overcurrent when the power frequency alternating current side system fails, and the grid-connected requirement of the low-frequency power transmission system is met. The current inner loop of the M3C adopts a controller based on ADRC (Active Disturbance Rejection Control, self-disturbance rejection control) to adapt to single-phase grounding, two-phase short-circuit and two-phase short-circuit grounding faults, and the active self-adaptability and the capability of coping with the asymmetrical faults of the system are improved. However, the control strategy of the M3C converter valve is only improved, and although asymmetric current can be restrained to a certain extent, the serious unbalance fault condition of the AC-AC converter network side is limited by insufficient negative sequence current restraining capability of the M3C star connection topology on faults and limited capacity of the M3C, and the serious unbalance condition cannot be regulated. Disclosure of Invention In order to overcome the defect that the star connection topological structure limited by M3C in the traditional scheme is insufficient in negative sequence current regulation capability under the unbalanced fault condition, the invention provides a low-frequency alternating current sending system which comprises an offshore wind farm, an offshore transformer, an alternating current converter, an onshore transformer, an onshore power grid and an angle joint reactive generator; the ac converter is coupled to the offshore wind farm at a low frequency side by the offshore transformer; the ac-dc converter is coupled to the onshore grid at a power frequency side via the onshore transformer, and the corner-junction reactive generator is connected in parallel to the power frequency side; The AC-DC converter and the angle joint reactive generator are used for restraining unbalanced faults at the power frequency side through a coordination control strategy. Optionally, the corner joint reactive generator comprises a corner joint static reactive generator SVG; optionally, the corner joint reactive power generator comprises a triangle connection topological structure formed by three converter chains; each converter chain is of an H-bridge cascade multi-level structure. In another aspect, the present invention provides a fault suppression method for a low frequency ac transmission system, including: When unbalance faults occur on the power frequency side of the low-frequency alternating current sending-out system, acquiring the negative sequence current of the grid-connected point; Based on the negative sequence current and the bearing capacity difference of the AC-DC converter and the corner-junction reactive generator in the low-frequency AC sending system to the negative sequence current, generating a negative sequence current control instruction for the AC-DC converter and the corner-junction reactive generator by utilizing a coordination control strategy, wherein the corner-junction reactive generator is connected in parallel with the power frequency side of the AC-DC converter; And controlling the AC-DC converter and the corner junction reactive power generator to perform current compensation based on the negative sequence current control instruction so as to inhibit the unbalanced fault and realize system fault ride-through. Optionally, the generating, based on the negative sequence current and the difference in carrying capacity of the ac-dc converter and the corner-junction reactive generator in the low-frequency ac delivery system to the negative sequence current, a negative sequence current control instruction for the ac-dc converter and the corner-junctio