Offshore wind power flexible low-frequency sending-out system through parallel DRU-MMC and control method

Abstract

The invention relates to the technical field of power grid control and offshore wind power sending-out topology, in particular to a flexible low-frequency sending-out system for offshore wind power through parallel DRU-MMC and a control method. The land frequency conversion station provided by the invention comprises a low-frequency side transformation module, a land rectifier station, a land inverter station and a power frequency side transformer, the land rectification station comprises an MMC rectifier and a DRU rectification module formed by connecting a plurality of DRU rectifiers in series. According to the invention, the DRU rectifier and the MMC are integrated on the land frequency conversion station, and land AC-AC frequency conversion is realized, so that the land frequency conversion station can be directly connected with the offshore wind plant through the low-frequency AC submarine cable, and black-start power is provided for the offshore wind plant. Moreover, the characteristics of higher reliability, lower investment cost and lower power loss of the DRU rectifier are fully utilized, the defects of high offshore low-frequency alternating-current power transmission cost and poor electric energy quality are overcome, and bidirectional optimization of power transmission quality and construction cost management and control is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of power grid control and offshore wind power transmission topologies, and in particular to an offshore wind power flexible low-frequency transmission system and control method via a parallel DRU-MMC. Background Art

[0002] Offshore wind power resources are rich and stable, especially in the deep and far sea areas, where the technically exploitable amount of wind energy resources is huge. With the advancement of offshore wind power development towards the deep and far sea, the traditional power frequency AC power transmission technology can no longer meet the requirements due to transmission distance and capacity limitations. Although the high-voltage flexible DC power transmission technology based on the modular multilevel converter (MMC) has advantages in long-distance and large-capacity power transmission, its high cost and complex construction of the offshore converter platform limit its wide application. Therefore, exploring a more economical and reliable power transmission technology has become an urgent task.

[0003] In recent years, offshore low-frequency AC power transmission technology has received attention. By reducing the AC power transmission frequency, the problem of short power transmission distance of the power frequency AC technology can be overcome, and only an AC-AC frequency converter needs to be installed on shore without an offshore converter platform, which can save costs and operation and maintenance expenses. Therefore, the low-frequency AC power transmission technology is considered a beneficial supplement to the flexible DC power transmission technology. Since the offshore wind power transmission mainly features unidirectional power transmission, the bidirectional power conversion capabilities of the traditional modular multilevel matrix converter (M3C) and back-to-back modular multilevel converter (BTB-MMC) do not have advantages in this scenario. Based on this unidirectional power transmission characteristic, in the field of DC power transmission, some studies have proposed using a diode rectifier unit (DRU) instead of the MMC offshore. In the low-frequency AC power transmission scenario, some studies have proposed constructing an AC-AC frequency converter by using a diode rectifier on the low-frequency side and an MMC on the power frequency side, which can significantly reduce the requirements for switching devices and capacitors. However, due to the uncontrolled rectification characteristic of the DRU, it cannot provide a synchronous grid-connected AC voltage for offshore wind turbines, which requires the wind turbine units to operate in a grid-forming control mode. However, currently, grid-following wind turbines are still the mainstream, and in actual projects, there is a lack of specific application practices for grid-forming wind turbines. In addition, when the DRU is used on the low-frequency side, the onshore AC power grid cannot provide a black start power supply for the offshore wind farm, and an additional black start power supply needs to be provided, which undoubtedly increases the construction cost of the system. At the same time, the DRU consumes a certain amount of reactive power and generates harmonic currents during normal operation, further bringing reactive power compensation and filtering problems. Summary of the Invention

[0004] In order to overcome the defects of high cost and poor power quality in the existing offshore low-frequency AC power transmission technology, the present invention proposes a flexible low-frequency transmission system for offshore wind power through a parallel DRU-MMC. By taking advantage of the low cost and compactness of the DRU rectifier, the DRU rectifier is combined with the MMC to achieve flexible control of the AC and DC side voltages and reliable transmission of black-start power. This solution has the characteristics of low reactive power and low harmonics, and can realize stable transmission of high-quality electric energy.

[0005] A flexible low-frequency transmission system for offshore wind power through a parallel DRU-MMC proposed by the present invention sets up an onshore frequency converter station connecting the onshore AC power grid and the offshore AC power grid. The onshore frequency converter station includes: a low-frequency side voltage conversion module, an onshore rectifier station, an onshore inverter station, and a power frequency side transformer; the onshore rectifier station includes an MMC rectifier and a DRU rectifier module formed by a plurality of DRU rectifiers connected in series. The low-frequency side voltage conversion module includes transformers corresponding one by one to the DRU rectifiers in the DRU rectifier module and a transformer corresponding to the MMC rectifier; the low-frequency sides of each DRU rectifier module and the low-frequency side of the MMC rectifier are both connected to the offshore booster station through the corresponding transformers. The onshore inverter station adopts an MMC inverter. The onshore inverter station and the MMC rectifier are connected back-to-back through a DC transmission line to form a DC loop; the DRU rectifier module and the MMC rectifier are connected in parallel on the same side of the onshore inverter station; the power frequency side of the onshore inverter station is connected to the onshore AC power grid through the power frequency side transformer.

[0006] Preferably, the transformer corresponding to the low-frequency side of a single DRU rectifier adopts a Y / Y wiring structure or a Y / △ wiring structure, and when an even number of DRU rectifiers are connected in series, the transformers corresponding to the low-frequency side of the DRU rectifier module adopt a configuration in which the Y / Y wiring structure and the Y / △ wiring structure each account for half.

[0007] Preferably, the DRU rectifier module is composed of 2 or 4 six-pulse DRU rectifiers.

[0008] Preferably, the active power capacity of the DRU rectifier module is greater than the active power capacity of the MMC rectifier, and the active power capacity of the onshore inverter station is equal to the sum of the active power capacities of the DRU rectifier module and the MMC rectifier.

[0009] Preferably, the power capacity of the MMC rectifier is: ; where P MMC is the active power required in the black-start stage of the offshore wind farm; Q MMC is the sum of the inductive reactive power absorbed by the DRU rectifier module and the capacitive reactive power absorbed by the low-frequency AC submarine cable when the low-frequency AC bus of the MMC rectifier is used as the common connection point; HMMC The power capacity required for harmonic compensation of the MMC rectifier.

[0010] Preferably, it includes an offshore wind farm, an offshore booster station, a low-frequency AC submarine cable, an onshore frequency converter station, and an onshore AC power grid connected in sequence; The onshore AC power grid is connected to the onshore power frequency terminal of the onshore frequency converter station through a current-limiting resistor; the offshore wind farm includes multiple groups of wind turbine generators; the offshore low-frequency terminal of the onshore frequency converter station is connected to the offshore booster station through a low-frequency AC submarine cable.

[0011] A control method applied to the offshore wind power flexible low-frequency transmission system with parallel DRU-MMC proposed by the present invention first controls the black start of the system. After all the wind turbine generators in the offshore wind farm are grid-connected and operating, the d-axis outer-loop voltage on the low-frequency side of the MMC rectifier is dynamically adjusted to achieve the command value U ld_ref : ; where K P and K I are the proportional coefficient and integral coefficient of the controller respectively; U ref_0 is the set threshold voltage, P r is the measured active power value of the DRU rectifier module; s is the complex frequency variable of the Laplace transform; P W is the measured active power value of the offshore wind farm; P r_ref is the reference active power value of the MMC rectifier.

[0012] Preferably, the calculation method of the reference active power value of the MMC rectifier is: ; ; where S1 is the switching function; P rN is the rated transmission power of the DRU rectifier module; P w is the measured active power value of the offshore wind farm.

[0013] Preferably, its black start method is: First, the onshore AC power grid charges the onshore inverter station through a current-limiting resistor, and the DC bus voltage of the onshore inverter station is stabilized at the rated value U dN ; Pre-charge the MMC rectifier, and use the V / f control mode to control the PCC voltage at the low-frequency side AC bus of the MMC rectifier to raise the low-frequency side AC output voltage to the set steady-state target value to establish the AC voltage of the offshore wind farm; Start the grid connection operation of the wind turbines in the offshore wind farm one by one. When the power of the offshore wind farm is transmitted to the onshore AC grid, start the grid connection of the remaining wind turbines in the offshore wind farm, and at the same time, the DRU rectifier module transmits the active power of the offshore wind farm.

[0014] An apparatus for implementing the control method of the offshore wind power parallel DRU-MMC flexible low-frequency transmission system proposed by the present invention includes a black start module, a power monitoring and control module, a voltage / frequency coordinated control module, and a harmonic suppression and reactive power compensation module; The black start module is used to perform the black start of the system; the power monitoring and control module monitors the output power of the offshore wind farm and the actual output power of the DRU rectifier module in real time. When the output power of the offshore wind farm is greater than the rated power of the DRU rectifier module, the power monitoring and control module controls the DRU to transmit the rated active power, and the MMC rectifier transmits the remaining power generated by the wind farm; The voltage / frequency coordinated control module communicates with the MMC rectifier and adjusts the voltage amplitude and frequency of the point of common coupling through the MMC rectifier; The harmonic suppression and reactive power compensation module is built into the MMC rectifier and is used to inject reverse harmonic current into the MMC rectifier and compensate for reactive power.

[0015] The advantages of the present invention are as follows: The offshore wind power parallel DRU-MMC flexible low-frequency transmission system proposed by the present invention uses an onshore frequency converter station to achieve AC-AC frequency conversion. Among them, the onshore rectifier station is designed with a large-capacity DRU rectifier module and a small-capacity MMC rectifier on the low-frequency side. Both are connected in parallel on the AC and DC sides. In this way, the MMC rectifier M1 can draw power from the onshore inverter station through the DC transmission line and provide black start power for the offshore wind farm.

[0016] The onshore inverter station uses the MMC inverter M2. In this way, M1 and M2 are connected back-to-back DC, which makes it more convenient for M1 to draw power from M2 and provide black start power for the offshore wind farm, ensuring the stable access of conventional grid-following wind turbines and improving the compatibility and reliability of the system.

[0017] In the present invention, the DRU rectifier and the MMC are integrated on the onshore frequency converter station to achieve onshore AC-AC frequency conversion, so that the offshore wind farm can be directly connected to the onshore through a low-frequency AC submarine cable to provide black start power for the offshore wind farm. Moreover, the present invention makes full use of the characteristics of the DRU rectifier, such as higher reliability, smaller investment cost, and smaller power loss, to achieve a two-way optimization of power transmission quality and construction cost control.

[0018] Traditional flexible DC and AC power transmission requires two converter stations, one onshore and one offshore; the proposed parallel DRU-MMC flexible low-frequency power transmission system for offshore wind power only needs to set up a cycloconverter station onshore, without an offshore converter platform, reducing the use of a high proportion of power electronic devices and lowering the construction cost.

[0019] An active power controller is added to the MMC rectifier M1 on the rectifier side of the present invention, which can dynamically adjust the amplitude of the AC bus voltage of the onshore rectifier station according to the change of the active power output of the offshore wind farm, ensure that the DRU module transmits all the active power of the offshore wind power, and at the same time automatically provide reactive power compensation for the DRU module to maintain the reactive power balance of the offshore AC system, improving the stability and economy of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 FIG. is the topology structure of a parallel DRU-MMC flexible low-frequency power transmission system for offshore wind power proposed by the present invention; Figure 2 FIG. is the topology structure of an onshore frequency conversion station proposed by the present invention; Figure 3 FIG. is Figure 1 the flowchart of the black start method of the system shown; Figure 4 FIG. is the V / f control block diagram of the active power control added to the MMC rectifier; Figure 5 FIG. is the schematic diagram of the DRU rectifier circuit proposed by the present invention; Figure 6 FIG. is Figure 1 the module diagram of the control device of the system shown; Figure 7 FIG. is the module diagram of the computer device; Figure 8 FIG. is the measured value of the active power absorbed by the DRU rectifier module and the MMC rectifier M1 on the rectifier side; Figure 9 FIG. is the measured value of the reactive power absorbed by the DRU rectifier module and the MMC rectifier M1 on the rectifier side; Figure 10 FIG. is the measured value of the AC bus voltage on the low-frequency side of the onshore rectifier station; Figure 11 FIG. is the measured value of the AC current on the low-frequency side of the onshore rectifier station; Figure 12 FIG. is the measured value of the active power and the measured value of the reactive power sent out by the offshore wind farm. DETAILED DESCRIPTION OF THE INVENTION

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] As Figure 1 shown, the offshore wind power transmitted through the parallel DRU-MMC flexible low-frequency transmission system proposed in this embodiment includes an offshore wind farm, an offshore booster station, a low-frequency AC submarine cable, an onshore frequency converter station, and an onshore AC power grid connected in sequence; The offshore wind farm is connected to the offshore booster station, the offshore booster station is connected to the onshore frequency converter station through a low-frequency AC submarine cable, and the onshore frequency converter station is finally connected to the onshore AC power grid.

[0023] The onshore frequency converter station includes a low-frequency side voltage conversion module, an onshore rectifier station, an onshore inverter station, and a power frequency side transformer connected in sequence.

[0024] The onshore rectifier station includes an MMC (modular multilevel converter) rectifier M1 and a first DRU (diode rectifier unit) rectifier D1, a second DRU rectifier D2; the first DRU rectifier D1 and the second DRU rectifier D2 are connected in series to form a DRU rectification module.

[0025] The low-frequency side of the first DRU rectifier D1 is connected to the low-frequency AC submarine cable through a first transformer K1, and the low-frequency side of the second DRU rectifier D2 is connected to the low-frequency AC submarine cable through a second transformer K2. The wiring structures between the two DRU rectifiers and their low-frequency side transformers respectively adopt Y / Y wiring structure and Y / Δ wiring structure; for example, if the first DRU rectifier D1 and the first transformer K1 adopt Y / Y wiring structure, then the second DRU rectifier D2 and the second transformer K2 adopt Y / Δ wiring structure. The low-frequency side of the MMC rectifier M1 in the onshore rectifier station is connected to the low-frequency AC submarine cable through a corresponding third transformer K3, and the MMC rectifier M1 and the third transformer K3 adopt Y / Δ wiring structure. The first transformer K1, the second transformer K2, and the third transformer K3 constitute the low-frequency side voltage conversion module.

[0026] The onshore inverter station adopts the MMC inverter M2. The MMC inverter M2 and the MMC rectifier M1 are connected back-to-back through a DC transmission line. A DC loop is formed between the MMC rectifier M1 and the MMC inverter M2 to provide voltage support for the MMC rectifier M1 and the MMC inverter M2. The left side of the MMC rectifier M1 is the low-frequency side, and the right side of the MMC inverter M2 is the power-frequency side, so that the entire system forms a functional structure of "low-frequency - commutation - power-frequency". In this way, when arranging the offshore wind power transmission, the offshore rectifier station can be omitted, and it can provide DC voltage support for the MMC rectifier M1 during the black start process of the offshore wind farm, enabling the MMC rectifier M1 to output AC voltage on the low-frequency side. In addition, the MMC rectifier M1 can compensate for the reactive power and harmonics generated by the DRU rectifier module to improve the power quality of the low-frequency AC side.

[0027] The DRU rectifier module composed of the first DRU rectifier D1 and the second DRU rectifier D2 in series is connected in parallel with the MMC rectifier M1 on the same side of the MMC inverter M2.

[0028] In this way, the DRU rectifier module composed of the first DRU rectifier D1 and the second DRU rectifier D2 and the MMC rectifier M1 form a parallel structure on the low-frequency side, that is, the AC side, and also form a parallel structure on the DC side.

[0029] In specific implementation, both the first DRU rectifier D1 and the second DRU rectifier D2 in the onshore rectifier station adopt 6-pulse rectifier units based on phase-shifting transformers. The first DRU rectifier D1 and the second DRU rectifier D2 are connected in series to form a DRU rectifier module of a 12-pulse rectifier unit. This DRU rectifier module is used to undertake the power output of the MMC rectifier M1. Therefore, the sum of the power capacities of the first DRU rectifier D1 and the second DRU rectifier D2, that is, the power capacity of the DRU rectifier module, is greater than the power capacity of the MMC rectifier M1; the MMC inverter M2 is used to undertake the power output of the first DRU rectifier D1, the second DRU rectifier D2, and the MMC rectifier M1. Therefore, the power capacity of the MMC inverter M2 is equal to the sum of the power capacities of the first DRU rectifier D1, the second DRU rectifier D2, and the MMC rectifier M1.

[0030] The power capacity of the MMC rectifier M1 is relatively small. Specifically, the power capacity S of the MMC rectifier M1 can be calculated according to the following formula MMC : (1); where P MMC is the active power required during the black start stage of the offshore wind farm; Q MMCIt is the sum of the inductive reactive power absorbed by the first DRU rectifier D1 and the second DRU rectifier D2 and the capacitive reactive power absorbed by the low-frequency AC submarine cable when the low-frequency side AC bus of the MMC rectifier M1 is used as the point of common coupling (PCC); H MMC It is the power capacity that the MMC rectifier M1 needs to provide for harmonic compensation; Taking the low-frequency side AC bus of the MMC rectifier M1 as the point of common coupling, that is, the connection between the DRU rectifier module and the low-frequency side of the MMC rectifier M1 and the low-frequency AC submarine cable is used as the point of common coupling PCC.

[0031] When the first DRU rectifier D1 and the second DRU rectifier D2 form a 12-pulse rectifier unit based on a phase-shifting transformer, H is calculated based on the 11th, 13th, 23rd, and 25th harmonics in the AC current of the DRU rectifier module MMC 。

[0032] It should be noted that the DRU rectifier modules in the onshore frequency conversion station are generally formed by connecting an even number of DRU rectifiers in series. A transformer is set on the low-frequency side of each DRU rectifier, and the low-voltage side transformers are all connected to the offshore booster station through low-frequency AC submarine cables, as specifically shown in Figure 2 shown. Considering that the number of DRU rectifiers in the DRU rectifier module increases, while the rectification effect is improved, the harmonic intensity is enhanced synchronously, thus affecting the current quality. Therefore, in specific implementation, the DRU rectifiers in the DRU rectifier module can specifically adopt 6-pulse DRU rectifiers, and the series connection number can be specifically set to 2 or 4. In addition, the low-frequency side transformers of the DRU rectifiers can specifically adopt the Y / Y wiring structure or the Y / Δ wiring structure. In order to ensure the system stability, multiple low-frequency side transformers of the DRU rectifiers need to include both the Y / Y wiring structure and the Y / Δ wiring structure at the same time, and the two wiring structures can be evenly distributed.

[0033] Refer to Figure 3 The black start method of the offshore wind power transmitted through the parallel DRU-MMC flexible low-frequency system includes the following steps.

[0034] St1. Pre-charge the MMC inverter M2. Specifically, the onshore AC grid can charge the onshore frequency conversion station through a current-limiting resistor, that is, first charge the MMC inverter M2; St2. Unlock the pulses of the MMC inverter M2 and set it to the constant DC voltage control mode, and control the DC bus voltage of the MMC inverter M2 to the rated value U dN ; St3. Wait for the DC bus voltage to stabilize at the rated value After that, pre-charge the MMC rectifier M1 in the onshore rectification station, then unlock the pulses of the MMC rectifier M1, and control the voltage at the point of common coupling (PCC) of the low-frequency AC bus of the MMC rectifier M1 in the AC voltage / frequency (V / f) control mode, so that the AC output voltage on the low-frequency side of the flexible low-frequency transmission system slowly rises from zero to the set steady-state target value, and establish the AC voltage of the offshore wind farm to provide power for the initial startup of the wind turbines in the offshore wind farm. The PCC is the connection node between the low-frequency AC submarine cable and the onshore frequency conversion station.

[0035] St4. As the AC voltage of the offshore wind farm is established, start the wind turbines in the offshore wind farm one by one and connect them to the grid for operation; St5. When the first DRU rectifier D1 and the second DRU rectifier D2 start to transmit active power, the remaining all wind turbines are connected to the grid, and the electric energy is transmitted from the entire offshore wind farm to the onshore AC grid.

[0036] Establish the AC voltage of the offshore wind farm through the onshore frequency conversion station to provide black start energy for some wind turbines in the offshore wind farm. As the number of started wind turbines increases, the electric energy output of the offshore wind farm gradually increases, and then the electric energy is output to the outside. In this step, when the DRU rectifier module composed of the first DRU rectifier D1 and the second DRU rectifier D2 outputs active power, it means that the started wind turbines in the offshore wind farm are sufficient to meet the local load and there is a surplus, that is, the power generated by the offshore wind farm is transmitted from the offshore wind farm to the onshore AC grid. At this time, it is sufficient to meet the startup of all wind turbines, so as to realize the power supply output of the offshore wind farm.

[0037] In the system proposed in this application, the MMC rectifier M1 adopts the AC voltage amplitude / frequency control (V / f control) with additional active power control. Thus, its control mode specifically refers to Figure 4 、 Figure 5 As shown, the steps are as follows: Step 1, the active power controller is used as the starting link, receives the active power P w of the offshore wind farm and the rated transmission powers P rN of the DRU rectifiers D1 and D2, and calculates the reference value P r_ref of the active power of the MMC rectifier M1; when the power of the offshore wind farm exceeds the rated value of the DRU rectifier module, make the active power of the MMC rectifier M1 switch from the rated value to the reference value P r_ref of the active power to avoid overload; otherwise, the active power of the MMC rectifier M1 maintains the rated value.

[0038] P r_ref The specific calculation is as follows: (2); (3); Among them, S1 is a switching function; P rN is the rated transmission power of the first DRU rectifier D1 and the second DRU rectifier D2; P w is the measured active power value output by the offshore wind farm.

[0039] Step 2: Then, the power error P r_ref -P r is adjusted by a PI controller (the additional active power controller of M1) to generate a low-frequency side d-axis voltage reference command value U ld_ref and execute it to achieve the conversion from power to voltage reference; (4); Among them, 1 is the per-unit value reference, corresponding to the rated voltage; K P and K I are the proportional coefficient and integral coefficient of the PI controller respectively, used to adjust the dynamic response; U ref_0 is the set threshold voltage, set to 0.3 per-unit value in a specific embodiment; P r_ref is the active power reference value of the MMC rectifier M1; P r is the measured active power value of the DRU rectifier module; s is the complex frequency variable of the Laplace transform; P w is the measured active power value output by the offshore wind farm. Thus, when the offshore wind farm generates electricity, the output voltage reference command value U ld_ref after passing through the PI regulator is output. On the contrary, a fixed threshold U ref_0 is output to maintain the lowest voltage of the system.

[0040] The purpose of executing U ld_ref is to perform steady-state regulation of the voltage through low-frequency side outer-loop voltage control. When U ld_ref is executed, the d-axis control of the MMC rectifier M1 receives the U ld_ref output by the PI controller and the actually measured low-frequency side d-axis voltage U ld , and outputs a steady-state correction amount of the low-frequency side d-axis voltage through PI regulation; The q-axis control targets the low-frequency side q-axis reactive power to be 0, compares the low-frequency side q-axis voltage reference value U lq_ref =0 with the actual low-frequency side q-axis voltage value U lq , and outputs a steady-state correction amount of the q-axis voltage through PI regulation to track the voltage reference value and maintain voltage stability.

[0041] After calculating U ld_ref , through the following Steps 3 and 4, the low-frequency side d-axis outer loop of the MMC rectifier M1 executes U ld_ref in real time, and the q-axis outer loop executes U lq_ref in real time.

[0042] Step 3: The inner-loop current control on the low-frequency side is responsible for dynamic tracking and solving the problem of inductance cross-coupling in the dq coordinate system. First, the d-axis decoupling term of the MMC rectifier M1 is introduced - ω l L 0 i lq and the q-axis decoupling term ω l L 0 i ld to cancel the cross-coupling term, enabling independent control of the d- and q-axis currents of the MMC rectifier M1; the outer-loop voltage control output, decoupling term, and actual current generate the dq-axis voltage dynamic control quantity through PI regulation, quickly responding to current changes and improving the control accuracy.

[0043] ω l represents the angular frequency of the low-frequency side AC voltage of the MMC rectifier M1, L 0 represents the equivalent inductance of the low-frequency side AC circuit of the MMC rectifier M1, i ld represents the measured value of the d-axis current on the low-frequency side of the MMC rectifier M1, i lq represents the measured value of the q-axis current on the low-frequency side of the MMC rectifier M1; The outer-loop voltage control output includes the d-axis voltage U ld and the q-axis voltage U lq on the low-frequency side of the MMC rectifier M1; the decoupling term includes the d-axis decoupling term - ω l L 0 i lq and the q-axis decoupling term ω l L 0 i ld ; the actual current includes the measured value of the d-axis current on the low-frequency side of the MMC rectifier M1 i ld and the measured value of the q-axis current on the low-frequency side i lq .

[0044] Step 4: The dq / abc coordinate transformation and PWM modulation complete the physical layer implementation. First, the dq-axis voltage dynamic control quantity is converted into the modulation waves in the abc three-phase stationary coordinate system through coordinate transformation u com,u , u com,v and ucom,w , and then through PWM modulation, the modulation wave u com,u , u com,v and u com,w are converted into switching signals. The switching signals are used to drive the MMC rectifier M1 to output the required low-frequency AC voltage, realizing V / f control and ensuring the stable operation of the system. The entire structure achieves flexible regulation of active power and maintenance of power quality through hierarchical control, adapting to the grid connection requirements of offshore wind power.

[0045] In this embodiment, the first DRU rectifier D1 and the second DRU rectifier D2 specifically adopt 6-pulse rectification units.

[0046] The circuit structure of the 6-pulse DRU rectifier is shown in Figure 5 . The active power and reactive power generated by the offshore wind farm are injected into the low-frequency side AC bus of the onshore rectification station (i.e., at the point of common coupling PCC). The active power and reactive power absorbed by the DRU rectification module are respectively and . The MMC rectifier M1 on the rectifier side is connected in parallel with the DRU rectifier at the PCC. The reactive power output by the MMC rectifier M1 is , providing reactive power compensation for the DRU rectifier.

[0047] Among them, the DC voltage of the 6-pulse DRU rectifier with load can be expressed as: (5); In formula (5), is the effective value of the line voltage at the PCC, is the turns ratio of the DRU rectifier transformer, is the leakage reactance of the DRU rectifier transformer referred to the valve side, is the DC current of the DRU rectification station, refers to the rated value of the DRU rectification station under steady state; the DRU rectifier transformer is the transformer on the low-frequency side of the DRU rectifier.

[0048] According to formula (6), the DC current can be expressed as: (6); From formulas (5) and (6), the active power absorbed by the DRU rectification station is: (7); In formula (7), is the number of series-connected 6-pulse rectifiers.

[0049] Refer to Figure 6 , the control device of the offshore wind power transmitted through the parallel DRU-MMC flexible low-frequency transmission system, includes a black start module, a power monitoring and control module, a voltage / frequency coordinated control module, and a harmonic suppression and reactive power compensation module.

[0050] The black start module is used to execute the black start method during the system startup phase. The black start module first starts the MMC inverter M2, enabling the onshore AC grid to gradually increase the voltage at the PCC through the onshore frequency converter to provide the initial power supply for the wind turbines and complete the system black start; then, through the coordinated operation of the multi-level converters and the phased voltage construction strategy, it completes the full-process startup from the DC bus pre-charging to the grid connection of the offshore wind farm.

[0051] The power monitoring and control module obtains the output power of the offshore wind farm and the actual output power of the DRU rectifier module in real time, calculates the power deviation value, and dynamically adjusts the power distribution between the DRU rectifier module and the MMC rectifier M1 based on the power deviation value.

[0052] In order to make the best use of the reactive power capacity of the small-capacity MMC rectifier M1 on the rectifier side, under normal conditions, the MMC rectifier M1 tries not to transmit active power as much as possible, and the active power output of the offshore wind power needs to be sent out through the DRU rectifier module; if the total active power generated by the wind farm is less than the rated power of the DRU rectifier module, the DRU rectifier module transmits all the active power; if the total active power generated by the offshore wind farm is greater than the rated power of the DRU rectifier module, the DRU transmits the rated active power, and the MMC rectifier M1 transmits the remaining power generated by the offshore wind farm.

[0053] The voltage / frequency coordinated control module adjusts the voltage amplitude of the point of common coupling (PCC) through the MMC rectifier M1, indirectly controls the output power of the DRU rectifier module, and is used to maintain the voltage and frequency stability of the low-frequency AC grid, thereby maintaining the stability of the power system and the power quality.

[0054] Specifically, the point of common coupling can be regarded as the AC bus on the low-frequency side of the DRU rectifier module, that is, the connection node between the low-frequency AC submarine cable and the onshore frequency converter.

[0055] The harmonic suppression and reactive power compensation module is built into the MMC rectifier M1. By injecting reverse harmonic currents, it cancels the characteristic harmonics such as the 11th, 13th, 23rd, and 25th generated by the DRU rectifier module, reducing the harmonic distortion rate of the grid current to within the national wind power grid connection standard range; at the same time, the harmonic suppression and reactive power compensation module provides dynamic reactive power support for the MMC rectifier M1 to maintain the voltage stability of the offshore AC grid, especially suppressing voltage fluctuations through reactive power control during offshore AC system faults.

[0056] The above-mentioned offshore wind power parallel DRU-MMC flexible low-frequency transmission system and control method are verified below in conjunction with specific embodiments.

[0057] In the offshore wind power parallel DRU-MMC flexible low-frequency transmission system constructed in this embodiment, the main circuit parameters are shown in Tables 1-5.

[0058] Table 1 Grid-connected system parameters of direct-drive permanent magnet synchronous wind turbines ; Table 2 Offshore substation parameters ; Table 3 DRU rectifier module parameters ; The rated AC frequency in Table 1 is applicable to the DRU rectifier module and transformers K1 and K2.

[0059] Table 4 MMC rectifier M1 parameters ; The rated AC frequency in Table 1 is applicable to the MMC rectifier M1 and transformer K3.

[0060] Table 5 MMC inverter M2 parameters ; In this embodiment, first, the black start method is used to start the offshore wind farm, and then the above control method is used to control the power distribution of the DRU rectifier module and the MMC rectifier M1, so that the d-axis outer loop and q-axis outer loop on the low-frequency side of the MMC rectifier M1 respectively execute U ld_ref and U lq_ref , thereby controlling the voltage and frequency of the offshore AC system, and then observing the offshore power output situation. The measured active power and reactive power absorbed by the rectifier side are respectively as shown in Figure 8 and Figure 9 ; the measured AC bus voltage and AC current on the low-frequency side of the onshore rectifier station are as shown in Figure 10 , Figure 11 ; the measured active power and reactive power output from the offshore wind farm are as shown in Figure 12 . Combining Figures 8 - 12 it can be seen that after the offshore wind farm is started, the stable output of the offshore wind farm is achieved by using the control method of this application.

[0061] Figure 7It is a schematic structural diagram of a computer device provided by an embodiment of the present application. A computer device provided by an embodiment of the present application includes a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, it implements a black start method and a control method for a parallel-connected DRU-MMC flexible low-frequency transmission system for offshore wind power.

[0062] An embodiment of the present application also provides a storage medium on which a computer program is stored. When the computer program is run by a processor, it executes the above-mentioned black start method and control method for a parallel-connected DRU-MMC flexible low-frequency transmission system for offshore wind power.

[0063] Among them, the storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random access memory (abbreviated as SRAM), electrically erasable programmable read-only memory (abbreviated as EEPROM), erasable programmable read-only memory (abbreviated as EPROM), programmable read-only memory (abbreviated as PROM), read-only memory (abbreviated as ROM), magnetic memory, flash memory, magnetic disk or optical disk.

[0064] Certainly, for those skilled in the art, the present invention is not limited to the details of the above exemplary embodiments, but also includes the same or similar structures that can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0065] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0066] The technologies, shapes, and structures not described in detail in the present invention are all well-known technologies.

Claims

1. A parallel-connected DRU-MMC flexible low-frequency transmission system for offshore wind power, characterized in that, An onshore frequency converter station is set up to connect the onshore AC grid and the offshore AC grid. The onshore frequency converter station includes: a low-frequency side voltage transformation module, an onshore rectifier station, an onshore inverter station, and a power frequency side transformer; the onshore rectifier station includes an MMC rectifier and a DRU rectification module formed by connecting multiple DRU rectifiers in series; The low-frequency side voltage transformation module includes transformers corresponding one-to-one to the DRU rectifiers in the DRU rectification module and a transformer corresponding to the MMC rectifier; the low-frequency sides of each DRU rectification module and the low-frequency side of the MMC rectifier are both connected to the offshore booster station through the corresponding transformers; The onshore inverter station adopts an MMC inverter. The onshore inverter station and the MMC rectifier are connected back-to-back through a DC transmission line to form a DC loop; the DRU rectification module and the MMC rectifier are connected in parallel on the same side of the onshore inverter station; the power frequency side of the onshore inverter station is connected to the onshore AC grid through the power frequency side transformer.

2. The offshore wind power integrated with a parallel DRU-MMC flexible low-frequency transmission system according to claim 1, characterized in that The transformer corresponding to the low-frequency side of a single DRU rectifier adopts a Y / Y wiring structure or a Y / Δ wiring structure. And when an even number of DRU rectifiers are connected in series, the transformers corresponding to the low-frequency side of the DRU rectification module adopt a configuration method in which the Y / Y wiring structure and the Y / Δ wiring structure each account for half.

3. The offshore wind power parallel DRU-MMC flexible low-frequency transmission system according to claim 2, wherein The DRU rectification module is composed of 2 or 4 six-pulse DRU rectifiers.

4. The offshore wind power parallel DRU-MMC flexible low-frequency transmission system according to claim 1, characterized in that, The active power capacity of the DRU rectification module is greater than the active power capacity of the MMC rectifier. The active power capacity of the onshore inverter station is equal to the sum of the active power capacities of the DRU rectification module and the MMC rectifier.

5. The offshore wind power parallel DRU-MMC flexible low-frequency transmission system according to claim 4, wherein The power capacity of the MMC rectifier is: ； Among them, P MMC is the active power required during the black start phase of the offshore wind farm; Q MMC is the sum of the inductive reactive power absorbed by the DRU rectifier module and the capacitive reactive power absorbed by the low-frequency AC submarine cable when the low-frequency side AC bus of the MMC rectifier is used as the point of common coupling; H MMC is the power capacity required for the MMC rectifier to compensate for harmonics.

6. The offshore wind power integrated with a parallel DRU-MMC flexible low-frequency power transmission system according to any one of claims 1-5, characterized in that, It includes an offshore wind farm, an offshore booster station, a low-frequency AC submarine cable, an onshore frequency converter station, and an onshore AC grid connected in sequence; The onshore AC grid is connected to the onshore power frequency end of the onshore frequency converter station through a current-limiting resistor; the offshore wind farm includes multiple groups of wind turbine generators; the offshore low-frequency end of the onshore frequency converter station is connected to the offshore booster station through a low-frequency AC submarine cable.

7. A control method applied to the offshore wind power parallel DRU-MMC flexible low-frequency transmission system as described in claim 6, characterized in that, First, control the black start of the system. After all the wind turbines in the offshore wind farm are connected to the grid and operating, dynamically adjust the d-axis outer-loop voltage on the low-frequency side of the MMC to achieve the command value U ld_ref : ； Among them, K P and K I are the proportional coefficient and integral coefficient of the controller respectively; U ref_0 is the set threshold voltage, P r is the measured active power value of the DRU rectifier module; s is the complex frequency variable of the Laplace transform; P W is the measured active power value of the offshore wind farm; P r_ref is the reference active power value of the MMC rectifier.

8. The control method of the offshore wind power parallel DRU-MMC flexible low-frequency transmission system according to claim 7, characterized in that, The calculation method of the reference value of the active power of the MMC rectifier is: ； ； Among them, S1 is the switching function; P rN is the rated transmission power of the DRU rectification module; P w is the measured active power value of the offshore wind farm.

9. The control method of the offshore wind power flexible low-frequency transmission system via parallel DRU-MMC as claimed in claim 7, wherein Its black start method is: First, the onshore AC power grid charges the onshore inverter station through a current-limiting resistor, and the DC bus voltage of the onshore inverter station is stabilized at the rated value U by adopting a constant DC voltage control mode dN ; Pre-charge the MMC rectifier, and use the V / f control mode to control the PCC voltage at the low-frequency side AC bus of the MMC rectifier to raise the low-frequency side AC output voltage to the set steady-state target value to establish the AC voltage of the offshore wind farm; Start the wind turbine generators in the offshore wind farm to be connected to the grid one by one. When the power of the offshore wind farm is transmitted to the onshore AC grid, start the remaining wind turbine generators in the offshore wind farm to be connected to the grid, and at the same time, the DRU rectification module transmits the active power of the offshore wind farm.

10. An apparatus for implementing a control method of an offshore wind power parallel DRU-MMC flexible low-frequency transmission system according to any one of claims 1-5, characterized in that, It includes a black start module, a power monitoring and control module, a voltage / frequency coordinated control module, and a harmonic suppression and reactive power compensation module; The black start module is used to execute the black start of the system; the power monitoring and control module monitors the output power of the offshore wind farm and the actual output power of the DRU rectification module in real time. When the output power of the offshore wind farm is greater than the rated power of the DRU rectification module, the power monitoring and control module controls the DRU to transmit the rated active power, and the MMC rectifier transmits the remaining power generated by the wind farm; The voltage / frequency coordinated control module communicates with the MMC rectifier and adjusts the voltage amplitude and frequency of the point of common connection through the MMC rectifier; The harmonic suppression and reactive power compensation module is built into the MMC rectifier, which is used to inject reverse harmonic current into the MMC rectifier and compensate for reactive power.

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