Offshore wind power parallel drus-mmc flexible low-frequency transmission system and control method

Abstract

The present application relates to the technical field of power grid control and offshore wind power transmission topology, and particularly relates to a parallel DRU-MMC flexible low-frequency transmission system for offshore wind power and a control method thereof. The onshore frequency conversion station comprises a low-frequency side transformer module, an onshore rectifier station, an onshore inverter station and a power frequency side transformer. The onshore rectifier station comprises an MMC rectifier and a DRU rectifier module formed by a plurality of DRU rectifiers connected in series. In the present application, the DRU rectifier and the MMC are integrated on the onshore frequency conversion station, the onshore AC-AC frequency conversion is realized, and thus the offshore wind farm can be directly connected to the onshore through the low-frequency AC submarine cable to provide black start power for the offshore wind farm. Moreover, the present application makes full use of the characteristics of the DRU rectifier, such as higher reliability, smaller investment cost and smaller power loss, overcomes the defects of high cost and poor power quality of offshore low-frequency AC power transmission, and realizes bidirectional optimization of power transmission quality and construction cost control.

Description

Technical Field

[0001] This invention relates to the fields of power grid control and offshore wind power transmission topology, and in particular to a flexible low-frequency transmission system and control method for offshore wind power via parallel DRU-MMC. Background Technology

[0002] Offshore wind power resources are abundant and stable, especially in deep-sea areas where the exploitable amount of wind energy is enormous. As offshore wind power development extends into deeper waters, traditional power frequency AC transmission technology is no longer sufficient due to limitations in transmission distance and capacity. While high-voltage flexible DC transmission technology based on modular multilevel converters (MMCs) has advantages in long-distance, high-capacity transmission, its high cost and complex offshore converter platform construction limit its widespread application. Therefore, exploring more economical and reliable transmission technologies has become an urgent priority.

[0003] In recent years, offshore low-frequency AC power transmission technology has attracted attention. By reducing the AC transmission frequency, the short transmission distance problem of power frequency AC technology can be overcome, and only AC-AC converters need to be installed on shore, eliminating the need for offshore converter platforms, thus saving costs and maintenance expenses. Therefore, low-frequency AC power transmission technology is considered a beneficial supplement to flexible DC power transmission technology. Since offshore wind power transmission is mainly unidirectional power transmission, the bidirectional power conversion capabilities of traditional modular multilevel matrix converters (M3C) and back-to-back modular multilevel converters (BTB-MMC) are not advantageous in this scenario. Based on this unidirectional power transmission characteristic, in the field of DC power transmission, research has proposed using diode rectifiers (DRUs) to replace MMCs at sea. Applying this to the low-frequency AC power transmission scenario, research has proposed constructing back-to-back AC-AC converters by using diode rectifiers on the low-frequency side and MMCs on the power frequency side, which can significantly reduce the demand for switching devices and capacitors. However, due to the uncontrolled rectification characteristics of the DRU, it cannot provide synchronous grid-connected AC voltage for offshore wind turbines, requiring the wind turbines to operate in a grid-connected control mode. However, grid-connected wind turbines remain the mainstream, and their practical application is limited in engineering projects. Furthermore, when using a DRU on the low-frequency side, it cannot provide black-start power to the offshore wind farm through the onshore AC grid, requiring an additional black-start power source, which undoubtedly increases the system's construction cost. Simultaneously, the DRU consumes reactive power and generates harmonic currents during normal operation, further introducing reactive power compensation and filtering issues. Summary of the Invention

[0004] To overcome the shortcomings of high cost and poor power quality in existing offshore low-frequency AC power transmission technologies, this invention proposes a flexible low-frequency power transmission system for offshore wind power via parallel DRU-MMC. Utilizing the low cost and compactness of the DRU rectifier, the system combines the DRU rectifier with the MMC to achieve flexible control of AC and DC side voltages and reliable black-start power transmission. This solution features low reactive power and low harmonics, enabling stable transmission of high-quality power.

[0005] This invention proposes a flexible low-frequency transmission system for offshore wind power via parallel DRU-MMC, which sets up an onshore frequency converter station to connect the onshore AC power grid and the offshore AC power grid. The onshore frequency converter station includes: a low-frequency side transformer 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 multiple DRU rectifiers connected in series.

[0006] The low-frequency side transformer module includes a transformer corresponding to the DRU rectifier in the DRU rectifier module and a transformer corresponding to the MMC rectifier; the low-frequency side of each DRU rectifier module and the low-frequency side of the MMC rectifier are connected to the offshore substation through the corresponding transformer.

[0007] The onshore inverter station uses MMC inverters. The onshore inverter station and the MMC rectifier are connected back-to-back through DC transmission lines to form a DC loop. The DRU rectifier module is connected in parallel with the MMC rectifier 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 a power frequency side transformer.

[0008] Preferably, the transformer corresponding to the low-frequency side of a single DRU rectifier adopts a Y / Y connection structure or a Y / Δ connection structure. When an even number of DRU rectifiers are connected in series, the transformer corresponding to the low-frequency side of the DRU rectifier module adopts a configuration in which the Y / Y connection structure and the Y / Δ connection structure each account for half.

[0009] Preferably, the DRU rectifier module consists of two or four 6-pulse DRU rectifiers.

[0010] 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 capacity of the DRU rectifier module and the active power capacity of the MMC rectifier.

[0011] Preferably, the power capacity of the MMC rectifier is:

[0012] ;

[0013] Among them, P MMC Q represents the active power required for the black start phase of an offshore wind farm. MMCThis 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; H MMC The power capacity required to compensate for harmonics in the MMC rectifier.

[0014] 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;

[0015] 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 wind turbine units; the offshore low-frequency terminal of the onshore frequency converter station is connected to the offshore booster station via a low-frequency AC submarine cable.

[0016] This invention proposes a control method for the aforementioned offshore wind power transmission system via a parallel DRU-MMC flexible low-frequency power transmission line. First, the system undergoes a black start control. After all wind turbines in the offshore wind farm are connected to the grid, the low-frequency side d-axis outer loop voltage of the MMC rectifier is dynamically adjusted to achieve the command value U. ld_ref :

[0017] ;

[0018] Among them, K P and K I These are the proportional and integral coefficients of the controller, respectively; U ref_0 For the set threshold voltage, P r is the measured active power of the DRU rectifier module; s is the complex frequency variable of the Laplace transform; P W P represents the measured active power of an offshore wind farm. r_ref This is the active power reference value for the MMC rectifier.

[0019] Preferably, the active power reference value of the MMC rectifier is calculated as follows:

[0020] ;

[0021] ;

[0022] Where S1 is the switching function; P rN P represents the rated transmission power of the DRU rectifier module. w This represents the measured active power of an offshore wind farm.

[0023] Preferably, the black boot method is as follows:

[0024] First, the onshore AC grid is used to charge the onshore inverter station through a current-limiting resistor. Then, a constant DC voltage control mode is used to stabilize the DC bus voltage of the onshore inverter station at the rated value U. dN ;

[0025] The MMC rectifier is pre-charged, and the PCC voltage at the low-frequency AC bus of the MMC rectifier is controlled by the V / f control mode, so that the low-frequency AC output voltage is raised to the set steady-state target value and the AC voltage of the offshore wind farm is established.

[0026] One by one, the wind turbines in the offshore wind farm are started and connected to the grid. When the power of the offshore wind farm is transmitted to the onshore AC grid, the remaining wind turbines in the offshore wind farm are started and connected to the grid. At the same time, the DRU rectifier module transmits the active power of the offshore wind farm.

[0027] The present invention proposes an apparatus for implementing the control method of the offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission, comprising 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;

[0028] 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 to transmit the remaining power generated by the wind farm.

[0029] The voltage / frequency co-control module communicates with the MMC rectifier and adjusts the voltage amplitude and frequency at the common coupling point through the MMC rectifier.

[0030] 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 to compensate for reactive power.

[0031] The advantages of this invention are:

[0032] The offshore wind power transmission system proposed in this invention uses an onshore frequency converter station to achieve AC-AC conversion. 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 to provide black start power to the offshore wind farm.

[0033] The onshore inverter station uses MMC inverter M2. In this way, M1 and M2 are connected back-to-back by DC, which makes it easier for M1 to draw power from M2 and provide black start power to the offshore wind farm. This ensures the stable connection of conventional grid-connected wind turbines and improves the system's compatibility and reliability.

[0034] In this invention, a DRU rectifier and MMC are integrated into an onshore frequency converter station, realizing onshore AC-AC frequency conversion. This allows for direct connection from the onshore station to the offshore wind farm via a low-frequency AC submarine cable, providing black start power to the offshore wind farm. Moreover, this invention fully utilizes the characteristics of the DRU rectifier, such as higher reliability, lower investment cost, and lower power loss, to achieve bidirectional optimization of power transmission quality and construction cost control.

[0035] Traditional flexible DC and AC transmission require two converter stations, one offshore and one on land. The offshore wind power transmission system proposed in this invention, which uses a parallel DRU-MMC flexible low-frequency transmission system, only requires an AC-AC converter station on land, eliminating the need for an offshore converter platform. This reduces the use of a high proportion of power electronic devices and lowers construction costs.

[0036] This invention adds an active power controller to the MMC rectifier M1 on the rectifier side, which can dynamically adjust the AC bus voltage amplitude of the onshore rectifier station according to the changes in the active power output of the offshore wind farm, ensuring that the DRU module transmits all the active power of the offshore wind power, and automatically provides reactive power compensation for the DRU module to maintain the reactive power balance of the offshore AC system, thereby improving the stability and economy of the system. Attached Figure Description

[0037] Figure 1 This invention proposes a topology for a flexible low-frequency transmission system for offshore wind power via parallel DRU-MMC.

[0038] Figure 2 This invention proposes a topology for an onshore frequency converter station.

[0039] Figure 3 for Figure 1 The flowchart of the black boot method of the system shown is as follows;

[0040] Figure 4 Block diagram of V / f control structure for adding active power control to MMC rectifier;

[0041] Figure 5 This is a schematic diagram of the DRU rectifier circuit proposed in this invention;

[0042] Figure 6 for Figure 1 The control device module diagram of the system shown is as follows;

[0043] Figure 7 A block diagram of computer equipment;

[0044] Figure 8 The measured value of active power absorbed by the DRU rectifier module and MMC rectifier M1 on the rectifier side;

[0045] Figure 9Measured values ​​of reactive power absorbed by the DRU rectifier module and MMC rectifier M1 on the rectifier side;

[0046] Figure 10 This is the measured value of the AC bus voltage on the low-frequency side of the onshore rectifier station;

[0047] Figure 11 This is the measured value of the AC current on the low-frequency side of the onshore rectifier station;

[0048] Figure 12 Measured values ​​of active power and reactive power delivered to offshore wind farms. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0050] like Figure 1 As shown, the offshore wind power transmission system proposed in this embodiment via parallel DRU-MMC flexible low-frequency transmission 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.

[0051] Offshore wind farms are connected to offshore booster stations, which are then connected to onshore frequency converter stations via low-frequency AC submarine cables. Finally, the onshore frequency converter stations are connected to the onshore AC power grid.

[0052] The onshore frequency converter station includes a low-frequency side transformer module, an onshore rectifier station, an onshore inverter station, and a power frequency side transformer connected in sequence.

[0053] The onshore rectifier station includes an MMC (Modular Multilevel Converter) rectifier M1, a first DRU (Diode Rectifier Unit) rectifier D1, and a second DRU rectifier D2; the first DRU rectifier D1 and the second DRU rectifier D2 are connected in series to form a DRU rectifier module.

[0054] The low-frequency side of the first DRU rectifier D1 is connected to a low-frequency AC submarine cable via the first transformer K1, and the low-frequency side of the second DRU rectifier D2 is connected to the low-frequency AC submarine cable via the second transformer K2. The wiring structures between the two DRU rectifiers and their low-frequency side transformers adopt Y / Y and Y / Δ wiring structures, respectively; for example, if the first DRU rectifier D1 and the first transformer K1 adopt a Y / Y wiring structure, then the second DRU rectifier D2 and the second transformer K2 adopt a 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 via the corresponding third transformer K3, and the MMC rectifier M1 and the third transformer K3 adopt a Y / Δ wiring structure. The first transformer K1, the second transformer K2, and the third transformer K3 constitute the low-frequency side transformer module.

[0055] The onshore inverter station uses MMC inverter M2. MMC inverter M2 and MMC rectifier M1 are connected back-to-back via a DC transmission line, forming a DC loop that provides voltage support for both. The left side of MMC rectifier M1 is the low-frequency side, and the right side of MMC inverter M2 is the power frequency side, creating a "low-frequency-converter-power frequency" functional structure for the entire system. This eliminates the need for an offshore rectifier station when deploying offshore wind power transmission. It provides DC voltage support for MMC rectifier M1 during the black start process of the offshore wind farm, allowing MMC rectifier M1 to output AC voltage on the low-frequency side. Furthermore, MMC rectifier M1 can compensate for reactive power and harmonics generated by the DRU rectifier module, improving the power quality on the low-frequency AC side.

[0056] The DRU rectifier module, consisting of the first DRU rectifier D1 and the second DRU rectifier D2 connected in series, is connected in parallel with the MMC rectifier M1 on the same side of the MMC inverter M2.

[0057] Thus, the DRU rectifier module, consisting of the first DRU rectifier D1 and the second DRU rectifier D2, forms a parallel structure with the MMC rectifier M1 on the low-frequency side (AC side) and also on the DC side.

[0058] In practical implementation, both the first DRU rectifier D1 and the second DRU rectifier D2 in the onshore rectifier station adopt a 6-pulse rectifier unit based on a phase-shifting transformer. The first DRU rectifier D1 and the second DRU rectifier D2 are connected in series to form a 12-pulse rectifier unit DRU rectifier module. This DRU rectifier module is used to receive 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, i.e., 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 receive 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 power capacity of the first DRU rectifier D1, the second DRU rectifier D2, and the MMC rectifier M1.

[0059] The power capacity of the MMC rectifier M1 is relatively small. Specifically, the power capacity S of the MMC rectifier M1 can be calculated using the following formula. MMC :

[0060] (1);

[0061] Among them, P MMC Q represents the active power required for the black start phase of an offshore wind farm. MMC H 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 AC bus of MMC rectifier M1 is used as the point of common coupling (PCC). MMC The power capacity required to compensate harmonics for the MMC rectifier M1;

[0062] The low-frequency AC bus of MMC rectifier M1 is used as the common connection point, that is, the connection point between the DRU rectifier module and the low-frequency side of MMC rectifier M1 and the low-frequency AC submarine cable is used as the common connection point PCC.

[0063] When the first DRU rectifier D1 and the second DRU rectifier D2 constitute a 12-pulse rectifier unit based on a phase-shifting transformer, the H is calculated based on the 11th, 13th, 23rd, and 25th harmonics in the AC current of the DRU rectifier module. MMC .

[0064] It is worth noting that the DRU rectifier modules in onshore frequency converter stations are generally formed by an even number of DRU rectifiers connected in series. Each DRU rectifier has a transformer on its low-frequency side, and the low-voltage side transformers are all connected to the offshore booster station via low-frequency AC submarine cables, as detailed below. Figure 2As shown. Considering that increasing the number of DRU rectifiers in the DRU rectifier module improves the rectification effect but also simultaneously increases harmonic intensity, thus affecting current quality, in specific implementations, the DRU rectifiers in the DRU rectifier module can specifically adopt a 6-pulse DRU rectifier, and the number in series can be set to 2 or 4. In addition, the low-frequency side transformer of the DRU rectifier can specifically adopt a Y / Y connection structure or a Y / Δ connection structure. To ensure system stability, the multiple low-frequency side transformers of the DRU rectifier must simultaneously include both Y / Y connection structures and Y / Δ connection structures, and the two connection structures can be evenly distributed.

[0065] Reference Figure 3 The black-start method for offshore wind power via a parallel DRU-MMC flexible low-frequency transmission system includes the following steps.

[0066] St1, pre-charge the MMC inverter M2. Specifically, the onshore AC grid can be used to charge the onshore frequency converter station through a current-limiting resistor, that is, charge the MMC inverter M2 first.

[0067] St2, unlock the pulse of MMC inverter M2 and set it to constant DC voltage control mode, controlling the DC bus voltage of MMC inverter M2 to the rated value U. dN ;

[0068] St3. Wait for the DC bus voltage to stabilize at the rated value. Subsequently, the MMC rectifier M1 in the onshore rectifier station is pre-charged, and then the pulse of the MMC rectifier M1 is unlocked. The low-frequency AC bus common coupling point (PCC) voltage of the MMC rectifier M1 is controlled using an AC voltage / frequency (V / f) control mode. This causes the AC output voltage on the low-frequency side of the flexible low-frequency transmission system to slowly rise from zero to the set steady-state target value, establishing the AC voltage for the offshore wind farm to provide power for the initial startup of the wind turbines. The PCC is the connection node between the low-frequency AC submarine cable and the onshore frequency converter station.

[0069] St4. Once the AC voltage of the offshore wind farm is established, start up and connect the wind turbine units in the offshore wind farm to the grid one by one.

[0070] When St5, the first DRU rectifier D1, and the second DRU rectifier D2 begin transmitting active power, all remaining wind turbine units are connected to the grid, realizing the transmission of electricity from the entire offshore wind farm to the onshore AC power grid.

[0071] An AC voltage is established at the offshore wind farm via an onshore frequency converter station, providing black-start energy for some of the wind turbines. As the number of wind turbines started increases, the offshore wind farm's power output gradually increases, thus supplying electricity to the outside world. 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 indicates that the started wind turbines at the offshore wind farm are sufficient to meet their own load with a surplus. That is, the power generated by the offshore wind farm is transmitted from the offshore wind farm to the onshore AC grid, which is sufficient to start all wind turbines, thereby realizing the offshore wind farm's power output.

[0072] In the system proposed in this application, the MMC rectifier M1 employs AC voltage amplitude / frequency control (V / f control) with additional active power control. The specific control mode is detailed in [reference needed]. Figure 4 , Figure 5 As shown, the steps are as follows:

[0073] Step 1: The active power controller acts as the starting point, receiving the active power P from the offshore wind farm. w The rated transmission power P of DRU rectifiers D1 and D2 rN The active power reference value P of the MMC rectifier M1 was calculated. r_ref When the power output of the offshore wind farm exceeds the rated value of the DRU rectifier module, the active power of the MMC rectifier M1 is switched from the rated value to the active power reference value P. r_ref To avoid overload; otherwise, the active power of the MMC rectifier M1 will remain at the rated value.

[0074] P r_ref The specific calculations are as follows:

[0075] (2);

[0076] (3);

[0077] Where S1 is the switching function; P rN P represents the rated transmission power of the first DRU rectifier D1 and the second DRU rectifier D2; w This represents the measured value of the active power output of an offshore wind farm.

[0078] Step two, the power error P is then adjusted by the PI controller (the active power controller attached to M1). r_ref -P r Generate the low-frequency side d-axis voltage reference command value U ld_ref And execute it to achieve the conversion from power to voltage reference;

[0079] (4);

[0080] Where 1 is the per-unit value reference, corresponding to the rated voltage; K P and K I These are the proportional and integral coefficients of the PI controller, used to adjust the dynamic response; U ref_0 The threshold voltage is set, specifically 0.3 per unit in this embodiment; P r_ref P is the reference value for the active power of the MMC rectifier M1; r is the measured active power of the DRU rectifier module; s is the complex frequency variable of the Laplace transform; P w This represents the measured active power output of the offshore wind farm. Thus, when the offshore wind farm generates electricity, the output voltage reference command value U, after passing through the PI regulator, is... ld_ref Conversely, the output is a fixed threshold U. ref_0 To maintain the system at its lowest voltage.

[0081] U ld_ref The objective is to achieve steady-state voltage regulation through low-frequency outer loop voltage control. ld_ref During execution, the d-axis control of the MMC rectifier M1 receives the U output from the PI controller. ld_ref The actual measured low-frequency side d-axis voltage U ld The steady-state correction of the low-frequency side d-axis voltage is regulated by PI.

[0082] q-axis control aims to achieve zero reactive power on the low-frequency side q-axis, and sets the low-frequency side q-axis voltage reference value U... lq_ref =0 and the actual q-axis voltage value U on the low-frequency side lq In contrast, the PI-regulated output q-axis voltage steady-state correction is used to track the voltage reference value and maintain voltage steady state.

[0083] Calculate U ld_ref Then, through steps three and four, the low-frequency side d-axis outer ring of the MMC rectifier M1 is made to execute U in real time. ld_ref The outer ring of the q-axis executes U in real time. lq_ref .

[0084] Step three: The low-frequency side inner loop current control is responsible for dynamic tracking and resolving the inductor cross-coupling problem 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 q-axis decoupling term ω l L 0 i ldThe cross-coupling terms are canceled, allowing the d-axis and q-axis currents of the MMC rectifier M1 to be controlled independently. The outer loop voltage control output, decoupling terms, and actual current are PI-regulated to generate dynamic control quantities for the d-q axis voltage, which quickly respond to current changes and improve control accuracy.

[0085] ω l This 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 AC circuit of the MMC rectifier M1. i ld This represents the measured value of the low-frequency side d-axis current of the MMC rectifier M1. i lq This represents the measured value of the low-frequency side q-axis current of the MMC rectifier M1;

[0086] The outer loop voltage control output includes the low-frequency side d-axis voltage U of the MMC rectifier M1. ld and low-frequency side q-axis voltage U lq The decoupling terms include the d-axis decoupling terms of the MMC rectifier M1. ω l L 0 i lq and q-axis decoupling term ω l L 0 i ld The actual current includes the measured d-axis current on the low-frequency side of the MMC rectifier M1. i ld and low-frequency side q-axis current measurement value i lq .

[0087] Step four: Physical layer implementation of dq / abc coordinate transformation and PWM modulation. First, the dynamic control quantity of the dq axis voltage is converted into a modulated wave in the abc three-phase stationary coordinate system through coordinate transformation. u com,u , u com,v and u com,w Then, the modulated wave is modulated by PWM. u com,u , u com,v and u com,w The signal is converted into a switching signal, which drives the MMC rectifier M1 to output the required low-frequency AC voltage, achieving V / f control and ensuring stable system operation. The entire structure, through layered control, achieves flexible active power regulation and power quality maintenance, adapting to the grid connection requirements of offshore wind power.

[0088] In this embodiment, the first DRU rectifier D1 and the second DRU rectifier D2 specifically adopt a 6-pulse rectifier unit.

[0089] The circuit structure of the 6-pulse DRU rectifier is shown below. Figure 5 Active power generated by offshore wind farms and reactive power Injected into the low-frequency side AC bus of the onshore rectifier station (i.e., at the point of common coupling PCC), the active power and reactive power absorbed by the DRU rectifier 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 of the MMC rectifier M1 is... This provides reactive power compensation for the DRU rectifier.

[0090] The DC voltage of the 6-pulse DRU rectifier under load It can be represented as:

[0091] (5);

[0092] In equation (5), This is the effective value of the line voltage at PCC. The turns ratio of the DRU rectifier transformer. To calculate the leakage reactance of the DRU rectifier transformer referred to the valve side, This refers to the DC current of the DRU rectifier station. This refers to the rated value of the DRU rectifier station under steady state; the DRU rectifier transformer is the transformer on the low-frequency side of the DRU rectifier.

[0093] According to equation (6), the direct current It can be represented as:

[0094] (6);

[0095] From equations (5) and (6), we can obtain the active power absorbed by the DRU rectifier station. for:

[0096] (7);

[0097] In equation (7), This refers to the number of 6-pulse rectifiers connected in series.

[0098] Reference Figure 6 The control device for the offshore wind power transmission system via 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.

[0099] 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, which causes the onshore AC grid to gradually increase the voltage at the PCC through the onshore frequency converter station, providing initial power to the wind turbine and completing the system black start; then, through multi-stage converter coordination and a phased voltage construction strategy, it completes the entire startup process from DC bus pre-charging to offshore wind farm grid connection.

[0100] The power monitoring and control module acquires the real-time output power of the offshore wind farm and the actual output power of the DRU rectifier module, 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.

[0101] To maximize the reactive power capacity of the small-capacity MMC rectifier M1 on the rectifier side, under normal conditions, the MMC rectifier M1 should not transmit active power as much as possible, and all active power output from offshore wind power should 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, then the DRU rectifier module will transmit all active power. If the total active power generated by the offshore wind farm is greater than the rated power of the DRU rectifier module, then the DRU will transmit the rated active power, and the MMC rectifier M1 will transmit the remaining power generated by the offshore wind farm.

[0102] The voltage / frequency coordinated control module adjusts the voltage amplitude of the common coupling point PCC through the MMC rectifier M1, indirectly controlling the output power of the DRU rectifier module to maintain the voltage and frequency stability of the low-frequency AC power grid, thereby maintaining the stability and power quality of the power system.

[0103] The common connection point can be specifically 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 station.

[0104] The harmonic suppression and reactive power compensation module is built into the MMC rectifier M1. By injecting reverse harmonic current, it cancels the characteristic harmonics of the 11th, 13th, 23rd and 25th orders generated by the DRU rectifier module, reducing the grid current harmonic distortion rate 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, maintaining the voltage stability of the offshore AC grid, especially suppressing voltage fluctuations through reactive power control during offshore AC system faults.

[0105] The following specific embodiments verify the above-mentioned offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission and control method.

[0106] The main circuit parameters of the offshore wind power transmission system constructed in this embodiment via the parallel DRU-MMC flexible low-frequency transmission system are shown in Table 1-5.

[0107] Table 1 Parameters of Grid-Connected System for Direct-Drive Permanent Magnet Synchronous Wind Turbine Generators

[0108] ;

[0109] Table 2 Parameters of the offshore substation

[0110] ;

[0111] Table 3 DRU Rectifier Module Parameters

[0112] ;

[0113] The rated AC frequencies in Table 1 apply to DRU rectifier modules and transformers K1 and K2.

[0114] Table 4 Parameters of MMC Rectifier M1

[0115] ;

[0116] The rated AC frequencies in Table 1 apply to MMC rectifier M1 and transformer K3.

[0117] Table 5 Parameters of MMC Inverter M2

[0118] ;

[0119] In this embodiment, the offshore wind farm is first started using a black start method. Then, the power distribution between the DRU rectifier module and the MMC rectifier M1 is controlled using the aforementioned control method, so that the low-frequency side d-axis outer loop and q-axis outer loop of the MMC rectifier M1 execute U in real time. ld_ref and U lq_ref This allows for the control of the voltage and frequency of the marine AC system, followed by observation of the marine power output. The measured values ​​of active power and reactive power absorbed on the rectifier side are shown below. Figure 8 and Figure 9 As shown; the measured values ​​of AC bus voltage and AC current on the low-frequency side of the onshore rectifier station are as follows. Figure 10 , Figure 11 As shown; the measured values ​​of active power and reactive power delivered by the offshore wind farm are as follows. Figure 12 As shown. Combined with Figures 8-12 It can be seen that after the offshore wind farm is started up, the control method of this application has achieved stable output of the offshore wind farm.

[0120] Figure 7This is a schematic diagram of the structure of a computer device provided in an embodiment of this application. The computer device provided in this application includes a memory, a processor, and a computer program stored in 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 an offshore wind power transmission system via a parallel DRU-MMC flexible low-frequency transmission system.

[0121] This application embodiment also provides a storage medium storing a computer program, which, when run by a processor, executes the aforementioned black start method and control method for the offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission.

[0122] 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 (SRAM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Erasable Programmable Read Only Memory (EPROM), Programmable Red-Only Memory (PROM), Read-Only Memory (ROM), magnetic storage, flash memory, magnetic disk, or optical disk.

[0123] Of course, those skilled in the art will recognize that the present invention is not limited to the details of the exemplary embodiments described above, but also includes the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0124] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider 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.

[0125] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A flexible low-frequency transmission system for offshore wind power via parallel DRU-MMC, characterized in that, An onshore frequency converter station is set up to connect the onshore AC power grid and the offshore AC power grid. The onshore frequency converter station includes: low-frequency side transformer module, onshore rectifier station, onshore inverter station and power frequency side transformer; the onshore rectifier station includes MMC rectifier and DRU rectifier module formed by multiple DRU rectifiers connected in series; The low-frequency side transformer module includes a transformer corresponding to the DRU rectifier in the DRU rectifier module and a transformer corresponding to the MMC rectifier; the low-frequency side of each DRU rectifier module and the low-frequency side of the MMC rectifier are connected to the offshore substation through the corresponding transformer. The onshore inverter station uses MMC inverters. The onshore inverter station and the MMC rectifier are connected back-to-back through DC transmission lines to form a DC loop. The DRU rectifier module is connected in parallel with the MMC rectifier 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 a power frequency side transformer. Its black boot method is as follows: First, the onshore AC grid is used to charge the onshore inverter station through a current-limiting resistor. Then, a constant DC voltage control mode is used to stabilize the DC bus voltage of the onshore inverter station at the rated value U. dN ; The MMC rectifier is pre-charged, and the PCC voltage at the low-frequency AC bus of the MMC rectifier is controlled by the V / f control mode, so that the low-frequency AC output voltage is raised to the set steady-state target value and the AC voltage of the offshore wind farm is established. One by one, the wind turbines in the offshore wind farm are started and connected to the grid. When the power of the offshore wind farm is transmitted to the onshore AC grid, the remaining wind turbines in the offshore wind farm are started and connected to the grid. At the same time, the DRU rectifier module transmits the active power of the offshore wind farm. The system control method is as follows: First, control the system to start in black mode. After all the wind turbines in the offshore wind farm are connected to the grid, dynamically adjust the low-frequency side d-axis outer loop voltage of the MMC rectifier to achieve the command value U. ld_ref ; Among them, K P and K I These are the proportional and integral coefficients of the controller, respectively; U ref_0 For the set threshold voltage, P r is the measured active power of the DRU rectifier module; s is the complex frequency variable of the Laplace transform; P W P represents the measured active power of an offshore wind farm. r_ref This is the active power reference value for the MMC rectifier.

2. The offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission as described in claim 1, characterized in that, The transformer corresponding to the low-frequency side of a single DRU rectifier adopts a Y / Y connection structure or a Y / Δ connection structure. When an even number of DRU rectifiers are connected in series, the transformer corresponding to the low-frequency side of the DRU rectifier module adopts a configuration in which the Y / Y connection structure and the Y / Δ connection structure each account for half.

3. The offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission as described in claim 2, characterized in that, The DRU rectifier module consists of two or four 6-pulse DRU rectifiers.

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

5. The offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission as described in claim 4, characterized in that, The power capacity of the MMC rectifier is: Among them, P MMC Q represents the active power required for the black start phase of an offshore wind farm. MMC This 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; H MMC The power capacity required to compensate for harmonics in the MMC rectifier.

6. The offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission as described in 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 power grid that are 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 wind turbine units; the offshore low-frequency terminal of the onshore frequency converter station is connected to the offshore booster station via a low-frequency AC submarine cable.

7. The offshore wind power transmission system via parallel DRU-MMC flexible low-frequency transmission as described in claim 1, characterized in that, The calculation method for the active power reference value of the MMC rectifier is as follows: Where S1 is the switching function; P rN P represents the rated transmission power of the DRU rectifier module. w This represents the measured active power of an offshore wind farm.

8. An apparatus for performing the control method of the offshore wind power transmission system via a parallel DRU-MMC flexible low-frequency transmission system as described in any one of claims 1-5, characterized in that, 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 to transmit the remaining power generated by the wind farm. The voltage / frequency co-control module communicates with the MMC rectifier and adjusts the voltage amplitude and frequency at the common coupling point 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 to compensate for reactive power.

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