A multi-voltage-level offshore wind power DC networking system control method
By establishing a multi-voltage-level offshore wind power DC grid system topology and MMC interconnection, combined with the constant active power control of the mid-section converter station, the interconnection problem of multi-voltage-level offshore wind power DC systems was solved, realizing low-cost and efficient wind power collection and transmission, and improving the steady-state operating margin and reliability of the system.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ZHEJIANG UNIV
- Filing Date
- 2026-03-03
- Publication Date
- 2026-06-05
AI Technical Summary
Existing technologies are insufficient to effectively solve the interconnection problem of offshore wind power DC systems with multiple voltage levels. In particular, due to high hardware costs, limited control dimensions, and difficulties in power allocation, it is difficult to achieve efficient collection and flexible transmission of offshore wind power, and it cannot alleviate the pressure on coastal power consumption.
By establishing a multi-voltage-level offshore wind power DC grid system topology, adopting MMC interconnected DC lines and combining constant active power control of the mid-section converter station, system-level optimized control is achieved, reducing hardware complexity and improving steady-state operating margin.
It reduced system construction costs, improved the system's steady-state operating margin and reliability, and enabled optimized power allocation of DC grids at multiple voltage levels and stable transmission of wind power.
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Figure CN122159333A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of power system technology, specifically relating to a control method for a multi-voltage-level offshore wind power DC grid system. Background Technology
[0002] With the continued growth in global demand for clean energy, new energy technologies such as offshore wind power have developed rapidly. Deep-sea areas, characterized by high wind speeds, stable wind conditions, and vast geographical areas, have become an important direction for offshore wind power development. Against this backdrop, high-voltage direct current (HVDC) transmission technology based on modular multilevel converters (MMCs) has become one of the preferred solutions for long-distance, large-capacity offshore wind power transmission due to its advantages such as high waveform quality and low transmission loss.
[0003] However, offshore wind farms vary in size, installed capacity, and distance from shore. To optimize the economics of project construction, the DC transmission voltage level needs to be adapted to the actual power transmission requirements. Furthermore, with technological advancements, the capacity, transmission distance, and DC voltage level of offshore wind farms will gradually increase, and in the future, offshore wind power will be connected to the grid via DC lines of different voltage levels. Meanwhile, although coastal areas possess abundant offshore wind power and tidal flat photovoltaic resources, their local absorption capacity is limited, necessitating the transmission of electricity to inland load centers. With the large-scale development of new energy sources along the coast, existing AC transmission channels are facing increasingly severe congestion problems.
[0004] Currently, existing technologies for interconnecting DC systems of different voltage levels mainly fall into two categories: one is the DC interconnection scheme based on modular multilevel dual active bridge (DAB-MMC), as seen in the literature [S. Kenzelmann, et al. "Isolated DC / DC structure based on modular multilevel converter" IEEE Trans. Power Electron. 2015]. This scheme uses two full-scale MMC converter stations and an intermediate high-frequency / medium-frequency isolation transformer to form a DC-DC converter, achieving voltage transformation and electrical isolation. However, its hardware cost is extremely high, requiring two complete converter stations, and the size and insulation design of the isolation transformer are extremely difficult in ultra-high voltage and high-capacity scenarios. In addition, this scheme can only achieve point-to-point DC voltage conversion, lacking the ability to interact with local AC systems, making it difficult to alleviate the pressure on coastal power consumption.
[0005] Another type is the DC interconnection scheme based on high-voltage, high-capacity resonant DC-DC converter, see reference [D. Jovcic, "Bidirectional, high-power DC transformer" IEEE Trans. Power Deliv. 2009]. This scheme uses an LC resonant circuit to achieve DC voltage boosting and downscaling, reducing the number of converter devices. However, in multi-voltage level network scenarios, its control dimension is singular, making it difficult to achieve fine-grained coordinated power allocation between multiple voltage level systems. At the same time, the resonant components experience significant voltage / current stress under steady-state operation and lack the ability to flexibly support the power of the receiving-end AC grid. Summary of the Invention
[0006] In view of the above, the present invention provides a control method for a multi-voltage-level offshore wind power DC grid system, which interconnects DC transmission lines of different voltage levels in coastal areas through MMC to form a DC grid architecture, realizes efficient collection and flexible transmission of offshore wind power, and enhances the reliability and redundancy of the system, providing technical support for future large-scale offshore wind power grid connection and long-distance power transmission.
[0007] A control method for a multi-voltage-level offshore wind power DC grid system includes the following steps: (1) Establish a topology for a multi-voltage-level offshore wind power DC grid system; (2) Based on the above topology, establish a mathematical model of the multi-voltage level offshore wind power DC grid system and analyze the system operation characteristics; (3) Based on the above mathematical model, establish a power optimization control method for offshore wind power DC grid system with multiple voltage levels.
[0008] Furthermore, in step (1), for the offshore wind power DC grid system with N voltage levels, its topology includes N offshore wind farms, N sending-end converter stations, N intermediate converter stations, N DC transmission lines, and N receiving-end converter stations, wherein the active power of the nth offshore wind farm is transmitted through the sending-end converter station S-MMC. n The rectified DC power is then transmitted to the S-MMC converter station at the sending end. n The DC side is connected to the corresponding receiving-end converter station R-MMC via the nth DC line. n All receiving-end converter stations have their AC sides connected to the same receiving-end AC power grid; N DC transmission lines correspond to N different DC voltage levels, and the nth DC line is connected to the (n-1)th DC line via the intermediate converter station M-MMC. nThe DC side of each converter station is connected to the first DC line at one end and grounded at the other end. The AC side of each converter station is connected to its own independent AC system, where n is a natural number and 1≤n≤N.
[0009] Furthermore, the sending-end converter station, the intermediate converter station, and the receiving-end converter station all adopt MMC.
[0010] Furthermore, the mathematical model expression for the multi-voltage-level offshore wind power DC grid system in step (2) is as follows: , , , in: P w_n This represents the measured output active power of the nth offshore wind farm. I s_n Indicates the sending-end converter station S-MMC n The output DC current, U dc_n Indicates the intermediate converter station M-MMC n The positive terminal potential on the DC side, P n Indicates the receiving-end converter station R-MMC n Active power transmitted to the receiving-end AC power grid. I r_n Indicates the receiving-end converter station R-MMC n Inflowing DC current, R r_n Indicates the receiving-end converter station R-MMC n To the intermediate converter station M-MMC n The resistance of the DC line between them, P M_n Indicates the intermediate converter station M-MMC n Active power absorbed from the AC system, I M_n Indicates the intermediate converter station M-MMC n DC current.
[0011] Furthermore, the power optimization control method in step (3) includes: under steady-state conditions, all sending-end converter stations in the system adopt constant voltage-frequency ratio control, each intermediate converter station adopts constant active power control, and each receiving-end converter station adopts constant DC voltage control; the active power of each intermediate converter station is selected as the control variable to adjust the active power of each receiving-end converter station; based on the output power value of the offshore wind farm detected at a certain time step and the DC positive terminal potential of each intermediate converter station, the active power command value is calculated and issued to each intermediate converter station through system-level optimization control, so that the active power command values of each intermediate converter station cooperate with each other to achieve optimized control of the active power of each receiving-end converter station.
[0012] Furthermore, the system-level optimization control obtains the active power command value of each intermediate converter station by optimizing and solving the following objective function; in: F ( P M () is the active power vector P M The relaxation penalty function, P M =[ P M_1 , P M_2 , P M_3 ,…, P M_N ], P Mt It is the active power command value vector and P Mt =[ P M_1t , P M_2t , P M_3t ,…, P M_Nt ], P M_nt For the mid-section converter station M-MMC n The active power command value, s n For the mid-section converter station M-MMC n Slack variables.
[0013] A computer device includes a memory and a processor, wherein the memory stores a computer program and the processor executes the computer program to implement the above-described control method for a multi-voltage-level offshore wind power DC grid system.
[0014] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described control method for a multi-voltage-level offshore wind power DC grid system.
[0015] Based on the above technical solution, the present invention has the following beneficial technical effects: 1. Reduced system construction costs and hardware complexity. This invention directly connects the intermediate converter station to DC lines of adjacent voltage levels, replacing the traditional isolated two-stage DC-DC structure. Since the intermediate converter station serves as both a DC voltage connection point and connects to an independent AC system through its AC side, it achieves multi-functionality, reducing the total installed capacity of the converter station and the demand for transformer equipment, thus significantly reducing the initial investment in offshore wind power interconnection systems.
[0016] 2. Significantly improves the steady-state operating margin and reliability of the system. Through a multi-variable collaborative control strategy, this invention solves the problem of deep power coupling between adjacent voltage levels in multi-voltage DC grids. Under wind speed fluctuation conditions, converter stations at each level can cooperate to achieve power compensation, effectively avoiding voltage instability caused by power exceeding limits at the receiving-end converter station. Calculation results show that this invention can suppress DC voltage fluctuations within a small range, providing a solid technical guarantee for the stable transmission of large-scale offshore wind power. Attached Figure Description
[0017] Figure 1 This is a schematic diagram of the topology of the N-voltage level offshore wind power DC grid system of the present invention.
[0018] Figure 2 This is a schematic diagram of the topology of a three-voltage-level offshore wind power DC grid system.
[0019] Figure 3 This is a schematic diagram of the wind speed variation waveforms for each wind farm.
[0020] Figure 4 This is a schematic diagram of the wind power waveform detected at each level of the middle section.
[0021] Figure 5 This is a schematic diagram of the DC voltage waveforms at each intermediate stage.
[0022] Figure 6 This is a schematic diagram of the output active power waveform of each intermediate MMC.
[0023] Figure 7 This is a schematic diagram of the active power waveform transmitted by each receiving end MMC. Detailed Implementation
[0024] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
[0025] This embodiment provides a control method for a multi-voltage-level offshore wind power DC grid system, including the following steps: (1) Establish the topology of the offshore wind power DC grid system with multiple voltage levels.
[0026] This embodiment takes a system with N DC voltage levels as an example for analysis. The topology of an N-voltage-level DC grid is as follows: Figure 1 As shown, the active power of each offshore wind farm is rectified and transmitted as DC power through S-MMC1, S-MMC2, S-MMC3, ..., S-MMCN at the sending end, and R-MMC1, R-MMC2, R-MMC3, ..., R-MMCN at the receiving end are connected to the AC grid of the same zone. The N DC voltage levels are U... dc_1 U dc_2 U dc_3 ,…,U dc_N DC lines of different voltage levels are connected by the DC sides of the intermediate sections M-MMC1, M-MMC2, M-MMC3, ..., M-MMCN to form a multi-voltage level DC power grid. The AC sides of M-MMC1, M-MMC2, M-MMC3, ..., M-MMCN are respectively connected to the AC system. Wherein P w_1 ,P w_2 ,P w_3 ,…,P w_N To determine the active power output of each offshore wind farm measured at the mid-section of the line, P M1 ,P M2 ,P M3 ,…,P MN P1, P2, P3, ..., PMMC represent the active power absorbed from the AC system by each MMC in the middle section. N R represents the active power transmitted by each MMC at the receiving end to the AC system. r_1 ,R r_2 ,R r_3 ,…,R r_N This represents the resistance of the DC line from each receiving-end MMC to the middle section MMC; since the DC line distance between each middle section MMC is relatively short, its line resistance is ignored in the steady-state operation characteristic analysis.
[0027] (2) Construct a mathematical model of a multi-voltage-level offshore wind power DC grid system and analyze the system's operating characteristics.
[0028] for Figure 1The DC grid topology shown has 3N MMCs. The N sending-end S-MMCs need to use constant V / f control to provide grid connection voltage for offshore wind farms. The N DC voltage levels in the DC grid require N MMCs using constant DC voltage control, while the remaining N MMCs use constant active power control. To reduce the difficulty of constructing the communication and control systems, the N MMCs in the same area use the same active power control strategy. Therefore, in this embodiment, the multi-voltage level DC grid can use constant active power control for the N M-MMCs in the middle section and constant DC voltage control for the N receiving-end R-MMCs, defining U... dc_n The voltage value is the potential at the positive terminal of each M-MMC.
[0029] make U dc_0 =0, P M_N+1 =0, the active power of the sending-end MMC, intermediate-end MMC and receiving-end MMC at each voltage level can be expressed as: Where: n represents the number of DC voltage levels, n=1,2,3,…,N I s_n This represents the DC current output from the terminal of the nth voltage level. I r_n This is the DC current flowing into the receiving end of the nth voltage level.
[0030] The current relationship between each sending end, receiving end, and intermediate MMC can be expressed as: in: I M_n This is the DC current output by the MMC in the middle of the nth stage.
[0031] Combining the above formulas, we can obtain the active power expression for the nth-stage receiving-end MMC: In the formula, I r_n It can be represented as: Combining the above formulas, we can also obtain the expression for the active power of the mid-stage MMC in the nth stage: (3) Establish a power optimization control method for offshore wind power DC grid system with multiple voltage levels.
[0032] Under steady-state conditions, the control strategy for each converter station in a DC grid is as follows: constant V / f control for each MMC at the sending end, constant active power control for each MMC in the intermediate section, and constant DC voltage control for each MMC at the receiving end. In the DC grid, each MMC at the receiving end acts as an active power balance node. The active power and DC voltage of the nth-level receiving-end MMC, along with the active power output of the wind farm at the same level, are considered. P w_n Active power of MMC in the middle section of the same level P M_n And the active power of the MMC in the middle section of the next higher level. P M_n+1 It is not possible to adjust the active power of the receiving-end MMC at a corresponding level on a one-to-one basis by changing the active power reference value of the MMC at the intermediate stage. For DC systems, the DC voltage needs to remain stable under steady-state conditions and cannot be used as a control variable for power regulation.
[0033] To provide power support after a sending-end fault and power absorption after a receiving-end fault, and to prevent the receiving-end MMC from exceeding its capacity limit during power control, the active power command values of each intermediate-level MMC need to be coordinated to ensure stable system operation. Therefore, the active power of each intermediate-level MMC is selected as the control variable to adjust the active power of each receiving-end MMC.
[0034] control variables P M control P The sensitivity matrix can be expressed as: This leads to the derivation of the M-MMCj active power at level j. P M_j For the active power of R-MMCi at level i P i Sensitivity It can be represented as: As can be seen from the above formula, the power command value of the mid-section MMC can adjust the active power of the receiving-end MMC at a DC voltage level that is equal to or one level lower than its DC voltage level, but cannot adjust the active power of the receiving-end MMC at a higher DC voltage level. The non-zero sensitivity is related to the value of the voltage level and the active power of the receiving-end converter.
[0035] Based on a reasonable relationship between DC voltage and capacity, and the configuration of line impedance, the following can be analyzed: Sensitivity matrix can be inferred S Maintaining full rank within the operating range, all states can be controlled independently, while SThe diagonal line and the adjacent element of the previous parallel diagonal line have similar magnitudes, exhibiting significant adjacency coupling, making one-to-one control difficult and requiring multi-variable collaborative optimization. Therefore, the active power of each mid-section MMC is selected as the control variable, and the active power of each receiving-end MMC is the controlled variable.
[0036] To achieve multivariable nonlinear optimization control, this implementation proposes a power control strategy. Based on the output power value of the offshore wind farm detected at a certain time step and the potential at the positive terminal of each mid-section MMC, the system-level optimization control calculates and issues power command values to the mid-section converter station, so that the active power command values of each mid-section MMC cooperate with each other to achieve optimized control of the power of each MMC at the receiving end.
[0037] The output active power of each MMC in the middle section under steady state is expressed as: P M =[ P M_1 , P M_2 , P M_3 ,…, P M_N It can track its power command value well, and the control variable is defined as the output active power vector of each MMC in the middle section. P M The state variable is the active power vector output by each MMC at the receiving end to the AC grid at the receiving end. P =[ P 1, P 2, P 3,…, P N [The active power target value vector given by each MMC in the middle section] P Mt =[ P M_1t , P M_2t , P M_3t ,…, P M_Nt ].
[0038] Let s be the slack variable, and let minimizing the vector sum s be the slack penalty. Establish the objective function as follows: In the power optimization control problem, the control variable P M The converter capacity limit constraint must be met: in: P M_n_max and PM_n_min These are the upper and lower limits of active power for the nth mid-segment MMC, respectively.
[0039] To mitigate long-term power losses caused by the disconnection of DC lines due to faults, a minimum constraint is set on the sum of the receiving-end power: in: P min_set This is the minimum setpoint for the sum of the MMC power at each receiving end. This value is a lower limit set to avoid the total active power sent to the receiving end system being too low.
[0040] The state variable P needs to satisfy the receiving-end converter capacity limit constraint, and the non-reverse power of the receiving-end converter station should be considered: in: P n_max This represents the upper limit of active power for the nth receiving-end MMC.
[0041] The mid-section MMC needs a certain power output under steady-state conditions. Therefore, relaxation constraints are set for the control variables under steady-state conditions. Based on the above objective function and constraints, the optimization problem is solved. Under this power optimization control strategy, the objective function is the sum of squares, and least squares optimization is selected for the solution.
[0042] (4) Verification by example.
[0043] To verify the effectiveness of the control strategy of this invention, we built a system in PSCAD / EMTDC as follows: Figure 2 The simulation model of the three-voltage-level DC grid test system is shown in Table 1. The main parameters of the system are shown in Table 1. Each S-MMC at the sending end is connected to an offshore wind farm that is not connected to the AC synchronous grid. Each M-MMC in the middle section is connected to a new energy base that is connected to the AC synchronous grid. Each R-MMC at the receiving end is connected to the same AC grid to transmit power to the load center. The delay of the power optimization control strategy is set to 50ms.
[0044] Table 1 At 3 seconds, the system had entered steady-state operation. From 4 seconds onwards, the wind speed changes at the three wind farms are as follows: Figure 3 As shown, the intermediate converter stations M-MMC1, M-MMC2, and M-MMC3 employ power optimization control to uniformly regulate the active power reference value, while the receiving-end converter stations R-MMC1, R-MMC2, and R-MMC3 employ DC-DC voltage control. An invented control strategy is used for power optimization allocation at the receiving-end converter stations.P Mt =[600,300,300]MW.
[0045] Wind power detected at each level of the mid-section, such as Figure 4 As shown, the DC voltage of each intermediate stage is as follows: Figure 5 As shown, the output active power of each intermediate MMC is as follows: Figure 6 As shown, the active power transmitted by each receiving end MMC is as follows: Figure 7 As shown. By Figure 3 It can be seen that the wind speed fluctuations are inconsistent across different wind farms, therefore Figure 4 The changes in active power output from each converter station at the sending end of the transmission line are different. From Figure 5 It is evident that the DC voltage at each level remains stable during power fluctuations at the sending end, indicating that the active power at each end of the DC grid can be balanced under power optimization control. Figure 6 and Figure 7 It can be seen that when the active power of the receiving-end converter station does not reach the upper limit, the power optimization control can maintain the active power transmitted by the mid-section MMC at a given target value. However, when the wind power changes and causes the active power of the receiving-end converter to reach the upper limit, the active power of the mid-section MMC is no longer maintained at the target value. Under the condition of rapid fluctuation in wind power, due to the millisecond-level delay in power optimization control, the active power of the receiving-end MMC will fluctuate to a certain extent before quickly stabilizing.
[0046] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.
Claims
1. A control method for a multi-voltage-level offshore wind power DC grid system, characterized in that, Includes the following steps: (1) Establish a topology for a multi-voltage-level offshore wind power DC grid system; (2) Based on the above topology, establish a mathematical model of the multi-voltage level offshore wind power DC grid system and analyze the system operation characteristics; (3) Based on the above mathematical model, establish a power optimization control method for offshore wind power DC grid system with multiple voltage levels.
2. The control method for a multi-voltage level offshore wind power DC grid system according to claim 1, characterized in that: In step (1), the topology of the offshore wind power DC grid system with N voltage levels includes N offshore wind farms, N sending-end converter stations, N intermediate converter stations, N DC transmission lines, and N receiving-end converter stations. The active power of the nth offshore wind farm is transmitted through the sending-end converter station S-MMC. n The rectified DC power is then transmitted to the S-MMC converter station at the sending end. n The DC side is connected to the corresponding receiving-end converter station R-MMC via the nth DC line. n All receiving-end converter stations have their AC sides connected to the same receiving-end AC power grid; N DC transmission lines correspond to N different DC voltage levels, and the nth DC line is connected to the (n-1)th DC line via the intermediate converter station M-MMC. n The DC side of each converter station is connected to the first DC line at one end and grounded at the other end. The AC side of each converter station is connected to its own independent AC system, where n is a natural number and 1≤n≤N.
3. The control method for a multi-voltage level offshore wind power DC grid system according to claim 2, characterized in that: The sending-end converter station, the intermediate converter station, and the receiving-end converter station all adopt MMC.
4. The control method for a multi-voltage level offshore wind power DC grid system according to claim 2, characterized in that: The mathematical model expression for the multi-voltage-level offshore wind power DC grid system in step (2) is as follows: ， ， ， in: P w_n This represents the measured output active power of the nth offshore wind farm. I s_n Indicates the sending-end converter station S-MMC n The output DC current, U dc_n Indicates the intermediate converter station M-MMC n The positive terminal potential on the DC side, P n Indicates the receiving-end converter station R-MMC n Active power transmitted to the receiving-end AC power grid. I r_n Indicates the receiving-end converter station R-MMC n Inflowing DC current, R r_n Indicates the receiving-end converter station R-MMC n To the intermediate converter station M-MMC n The resistance of the DC line between them, P M_n Indicates the intermediate converter station M-MMC n Active power absorbed from the AC system, I M_n Indicates the intermediate converter station M-MMC n DC current.
5. The control method for a multi-voltage level offshore wind power DC grid system according to claim 2, characterized in that: The power optimization control method in step (3) includes: under steady-state conditions, all sending-end converter stations in the system adopt constant voltage-frequency ratio control, each intermediate converter station adopts constant active power control, and each receiving-end converter station adopts constant DC voltage control; the active power of each intermediate converter station is selected as the control variable to adjust the active power of each receiving-end converter station; based on the output power value of the offshore wind farm detected at a certain time step and the DC positive terminal potential of each intermediate converter station, the active power command value is calculated and issued to each intermediate converter station through system-level optimization control, so that the active power command values of each intermediate converter station cooperate with each other to achieve optimized control of the active power of each receiving-end converter station.
6. The control method for a multi-voltage level offshore wind power DC grid system according to claim 5, characterized in that: The system-level optimization control obtains the active power command value of each intermediate converter station by optimizing and solving the following objective function; in: F ( P M () is the active power vector P M The relaxation penalty function, P M =[ P M_1 , P M_2 , P M_3 ,…, P M_N ], P Mt It is the active power command value vector and P Mt =[ P M_1t , P M_2t , P M_3t ,…, P M_Nt ], P M_nt For the mid-section converter station M-MMC n The active power command value, P M_n Indicates the intermediate converter station M-MMC n Active power absorbed from the AC system, s n For the mid-section converter station M-MMC n Slack variables.
7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the control method for a multi-voltage level offshore wind power DC grid system as described in any one of claims 1 to 6.
8. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the control method for a multi-voltage level offshore wind power DC grid system as described in any one of claims 1 to 6.