Method and device for controlling auxiliary commutation equipment in offshore wind power direct current system

Abstract

The invention provides an auxiliary converter equipment control method and device in an offshore wind power DC system, and the method comprises the steps: carrying out the proportional-integral control of an offshore AC voltage frequency deviation through a frequency control module, generating a DC voltage reference value, assigning the DC voltage reference value to a DC internal potential of auxiliary converter equipment, and achieving the indirect control of frequency; meanwhile, a d / q-axis controller is used for controlling capacitor voltage and PCC point voltage deviation, a d / q-axis current reference value is generated, a harmonic suppression module is combined to compensate harmonic current, and finally a voltage reference value calculation module is used for generating a three-phase bridge arm voltage reference value by using a proportional resonance controller and a linear transformation matrix; and active / reactive decoupling control and frequency stability control are realized. The invention also provides a corresponding control device, electronic equipment, a storage medium and a program product, and can be applied to an open sea high-capacity wind power delivery system, so that the system oscillation risk is effectively reduced, the capacity demand of auxiliary commutation equipment is reduced, and the system stability and the electric energy quality are improved.

Description

A control method and device for auxiliary converter equipment in an offshore wind power DC system Technical Field

[0001] This invention relates to the field of converters, and more particularly to a control method and apparatus for auxiliary converter equipment in an offshore wind power DC system. Background Technology

[0002] Renewable energy is key to achieving low-carbon and green development, and wind power is currently the most widely used and has the greatest potential for large-scale development. Offshore wind resources are abundant, wind speeds are relatively stable, space is vast, and larger wind turbines are permitted, making it a future trend in wind power development. In recent years, the focus of wind power development has shifted from onshore to offshore, and offshore wind power has become a key technology for achieving climate goals. According to data from the World Offshore Wind Forum (WFO) and the Global Wind Energy Council (GWEC), the total installed capacity of offshore wind power globally has reached 57.6 GW, with 9.4 GW added in 2022, and it is projected that the annual new installed capacity of offshore wind power globally will reach 45.7 GW by 2030. my country's eastern coast possesses the resource conditions for large-scale offshore wind power development, and its electricity demand is enormous, allowing for local absorption of electricity. China has become the biggest driving force for the development of offshore wind power in the world. By the end of 2022, my country's installed offshore wind power capacity reached 25.6GW, accounting for 44% of the global total installed capacity. In 2022, my country's newly added offshore wind power capacity was 6.8GW, accounting for 70% of the global newly added capacity that year.

[0003] With technological advancements, offshore wind power development is showing a trend towards larger capacities and farther offshore distances. According to existing research, when the transmission distance exceeds the equivalent distance of AC / DC transmission, DC transmission of offshore wind power is more cost-effective. Therefore, for large-capacity offshore wind power transmission scenarios, the adoption of DC has become an inevitable trend. Currently, several offshore wind power DC transmission projects have been put into operation both domestically and internationally. Most of the offshore wind power projects already in operation both domestically and internationally are based on flexible DC transmission using Modular Multilevel Converters (MMCs). Although MMCs have many advantages, their large number of sub-modules, capacitors, and semiconductor switching devices results in extremely large size, weight, and high cost for the offshore converter platform. While some scholars have studied methods for compacting MMCs, the actual degree of compactness is still relatively limited.

[0004] Given that offshore wind power is primarily transmitted unidirectionally from the sea to the land, DC transmission technology based on diode rectifiers (DRs) has become a research hotspot in recent years. Since DRs lack control capabilities, conventional grid-connected control is difficult to apply, necessitating grid-based control to establish an offshore AC grid. Assuming a constant onshore DC voltage, the DC transmission power is related to the DR's DC voltage, which in turn is proportional to the AC voltage amplitude. Therefore, current research typically couples active power and voltage amplitude, employing active / voltage amplitude and reactive / frequency grid control, i.e., "P / V, Q / f" grid construction. However, this "active / voltage amplitude, reactive / frequency" grid construction method contradicts the typical AC system characteristics of "active / frequency, reactive / voltage amplitude," leading to active / reactive coupling issues and potentially causing system oscillations.

[0005] To address the challenges of conventional "P / V, Q / f" grid configurations, some research has proposed a wind turbine grid control method based on "active power / frequency, reactive power / voltage amplitude," or "P / f, Q / V." In this method, the active power of the wind turbine is coupled with frequency, and reactive power with voltage amplitude, exhibiting characteristics similar to typical AC systems, resulting in better system stability. However, current methods rely on transmitting frequency information from the sea to onshore inverters via communication, and then using the inverters to dynamically adjust DC voltage and power to maintain frequency stability at sea. On one hand, this method depends on the onshore inverters having a certain DC voltage regulation capability; on the other hand, the continuously dynamically adjusting DC voltage operation is difficult to adapt to future multi-terminal DC application scenarios.

[0006] Other studies have proposed a hybrid approach using a DR (Diverter) and a small-capacity auxiliary MMC (Multi-Module Controlling Unit), including series and parallel configurations. The auxiliary MMC is primarily used to control the AC voltage at sea. In the parallel configuration, the auxiliary MMC is directly connected in parallel to the DC line; therefore, its rated DC voltage must be designed to match the system voltage, preventing a significant reduction in the number of submodules and thus hindering size and cost reduction. In the series configuration, although the auxiliary MMC and DR work together to support the DC voltage, effectively reducing the DC voltage and number of submodules, current research indicates that if the auxiliary MMC is used to control the AC voltage at sea, its capacity must be at least 30% of the system capacity, resulting in limited reductions in size and cost for the offshore converter platform, as shown in Figure 1. Summary of the Invention

[0007] The present invention aims to at least partially solve one of the technical problems in the related art.

[0008] This invention proposes a control method for auxiliary converter equipment in an offshore wind power DC system, aiming to solve the active / reactive coupling problem of grid-connected wind turbines in existing offshore wind power DC transmission systems, improve system stability, reduce the capacity requirements and system costs of auxiliary converter equipment, and at the same time reduce dependence on communication systems, thereby improving the engineering feasibility and economy of the system.

[0009] Another objective of this invention is to provide a control system for auxiliary converter equipment in an offshore wind power DC system.

[0010] To achieve the above objectives, this invention proposes a control method for auxiliary converter equipment in an offshore wind power DC system, comprising: generating a DC voltage reference value via a proportional-integral controller in response to the deviation between the offshore AC voltage frequency and the rated frequency; setting the DC voltage reference value as the DC internal potential of the auxiliary converter equipment; generating a d-axis current reference value via proportional-integral control in response to the deviation between the actual value and the reference value of the capacitor voltage; generating a q-axis current reference value via proportional control in response to the deviation between the actual value and the reference value of the PCC point voltage of the offshore AC grid; generating a new dq-axis current reference value in response to the dq component of the harmonic current and adding the component to the d-axis and q-axis current reference values; calculating a three-phase bridge arm voltage reference value based on the new dq-axis current reference value via a proportional resonant controller and a linear transformation matrix; and outputting the three-phase bridge arm voltage reference value to the auxiliary converter equipment to achieve active / reactive power decoupling control and frequency stabilization control of the offshore AC grid.

[0011] The auxiliary converter equipment control method in the offshore wind power DC system of the present invention may also have the following additional technical features: In one embodiment of the present invention, the frequency control module includes a proportional-integral controller, the output of which is used to generate a DC voltage reference value to adjust the DC internal potential of the auxiliary converter equipment, thereby indirectly controlling the offshore AC voltage frequency.

[0012] In one embodiment of the present invention, the d-axis controller module and the q-axis controller module are used to control active power and reactive power, respectively. The d-axis controller module controls the capacitor voltage deviation through proportional-integral control, and the q-axis controller module controls the PCC point voltage deviation through proportional control, thereby achieving decoupled control of active and reactive power.

[0013] In one embodiment of the present invention, the harmonic suppression module includes a harmonic current detection unit and an addition unit. The harmonic current detection unit is used to extract the dq component of the harmonic current, and the addition unit is used to add the dq component to the d-axis and q-axis current reference values ​​to generate new dq-axis current reference values.

[0014] In one embodiment of the present invention, the voltage reference value calculation module includes a proportional resonant controller and a linear transformation matrix. The proportional resonant controller is used to perform harmonic compensation on the new dq-axis current reference value, and the linear transformation matrix is ​​used to convert the dq-axis voltage reference value into a three-phase bridge arm voltage reference value.

[0015] To achieve the above objectives, another aspect of the present invention proposes a control device for auxiliary converter equipment in an offshore wind power DC system, comprising: a frequency control module for generating a DC voltage reference value via a proportional-integral controller in response to the deviation between the offshore AC voltage frequency and the rated frequency; a DC internal potential setting module for setting the DC voltage reference value as the DC internal potential of the auxiliary converter equipment; a d-axis controller module for generating a d-axis current reference value via proportional-integral control in response to the deviation between the actual value and the reference value of the capacitor voltage; and a q-axis controller module for responding to the deviation between the actual value and the reference value of the offshore AC grid PCC point voltage. The deviation between the actual voltage value and the reference value is used to generate a q-axis current reference value through proportional control; the harmonic suppression module is used to respond to the dq component of the harmonic current and add the component to the d-axis and q-axis current reference values ​​to generate a new dq-axis current reference value; the voltage reference value calculation module is used to calculate the three-phase bridge arm voltage reference value based on the new dq-axis current reference value, through a proportional resonant controller and a linear transformation matrix; the control output module is used to output the three-phase bridge arm voltage reference value to the auxiliary converter equipment to achieve active / reactive power decoupling control and frequency stabilization control of the offshore AC power grid.

[0016] The control method and apparatus for auxiliary converter equipment in offshore wind power DC systems according to embodiments of the present invention significantly reduce the capacity requirements of the auxiliary converter equipment through a series hybrid topology and optimized control strategy, which helps to reduce the size and cost of the converter platform. Furthermore, the auxiliary converter equipment also has harmonic suppression capabilities, improving the power quality of the system. The control method of the present invention eliminates the need for frequent communication with the shore inverter to adjust the DC voltage, reducing the system's dependence on communication and improving operational reliability.

[0017] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0018] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of embodiments in conjunction with the accompanying drawings, in which: Figure 1 is a prior art structural diagram based on DR, DR+auxiliary MMC in parallel, and DR+auxiliary MMC in series according to an embodiment of the present invention; Figure 2 is a flowchart of a control method for auxiliary converter equipment in an offshore wind power DC system according to an embodiment of the present invention; Figure 3 is a control diagram of auxiliary converter equipment in an offshore wind power DC system according to an embodiment of the present invention; Figure 4 is a structural diagram of a control device for auxiliary converter equipment in an offshore wind power DC system according to an embodiment of the present invention. Detailed Implementation

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0020] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0021] The following description, with reference to the accompanying drawings, illustrates a control method and apparatus for auxiliary converter equipment in an offshore wind power DC system according to an embodiment of the present invention.

[0022] Figure 2 is a flowchart of a control method for an auxiliary converter device in an offshore wind power DC system according to an embodiment of the present invention. As shown in Figure 2, the method includes: S1, generating a DC voltage reference value through a proportional-integral controller in response to the deviation between the offshore AC voltage frequency and the rated frequency; S2, setting the DC voltage reference value as the DC internal potential of the auxiliary converter device; S3, generating a d-axis current reference value through proportional-integral control in response to the deviation between the actual value and the reference value of the capacitor voltage; S4, generating a q-axis current reference value through proportional control in response to the deviation between the actual value and the reference value of the PCC point voltage of the offshore AC grid; S5, generating a new dq-axis current reference value by adding the dq component of the harmonic current to the d-axis and q-axis current reference values ​​in response to the new dq-axis current reference value; S6, calculating a three-phase bridge arm voltage reference value through a proportional resonant controller and a linear transformation matrix based on the new dq-axis current reference value; S7, outputting the three-phase bridge arm voltage reference value to the auxiliary converter device to achieve active / reactive power decoupling control and frequency stabilization control of the offshore AC grid.

[0023] The frequency control module further includes a proportional-integral controller, the output of which is used to generate a DC voltage reference value to adjust the DC internal potential of the auxiliary converter equipment, thereby indirectly controlling the frequency of the marine AC voltage.

[0024] Furthermore, the d-axis controller module and the q-axis controller module are used to control active power and reactive power, respectively. The d-axis controller module controls the capacitor voltage deviation through proportional-integral control, and the q-axis controller module controls the PCC point voltage deviation through proportional control, thereby achieving decoupled control of active and reactive power.

[0025] Furthermore, the harmonic suppression module includes a harmonic current detection unit and an addition unit. The harmonic current detection unit is used to extract the dq component of the harmonic current, and the addition unit is used to add the dq component to the d-axis and q-axis current reference values ​​to generate new dq-axis current reference values.

[0026] Furthermore, the voltage reference value calculation module includes a proportional resonant controller and a linear transformation matrix. The proportional resonant controller is used to perform harmonic compensation on the new dq-axis current reference value, and the linear transformation matrix is ​​used to convert the dq-axis voltage reference value into a three-phase bridge arm voltage reference value.

[0027] Specifically, this invention proposes a control method for the auxiliary MMC based on the existing DR+auxiliary MMC series hybrid method. This provides more control dimensions for offshore rectifiers, enabling offshore wind turbines to utilize P / f and Q / V grids while maintaining the onshore voltage at the rated voltage, thus achieving both low cost and good control performance. The proposed control block diagram is shown in Figure 3.

[0028] For frequency control, the difference between the frequency ωpcc of the marine AC voltage and its rated value ω0 is calculated and passed through a proportional-integral (PI) controller to obtain the DC voltage reference value u*dcm. Then, the value of u*dcm is directly assigned to the DC internal potential edc.

[0029] For AC voltage / current controllers, there are d-axis and q-axis controllers. In the outer loop of the d-axis controller, the reference value uc0 of the capacitor voltage is subtracted from the actual value uc, and this difference is processed by a PI controller to obtain the d-axis current reference value i*d. In the outer loop of the q-axis controller, the voltage reference value Upcc0 at the PCC point is subtracted from the actual value Upcc, and this difference is processed by a proportional controller to obtain the q-axis current reference value i*q. Simultaneously, the auxiliary MMC also has harmonic suppression functionality. The dq component of the harmonic current, ihdr(dq), is also added to the d and q-axis current reference values ​​to become new d and q-axis current reference values, and the a, b, and c-phase voltages are calculated using the PR controller. Finally, the bridge arm voltage reference values ​​are calculated using the following linear transformation matrix: In summary, by adopting the control method proposed in this invention, the auxiliary MMC can achieve "P / f, Q / V" grid configuration of the wind turbine without frequent changes in DC voltage, avoiding active and reactive power coupling and oscillation risks. Simultaneously, the auxiliary MMC can also perform filtering functions, filtering out harmonic currents to a certain extent.

[0030] The control method for auxiliary converter equipment in offshore wind power DC systems according to embodiments of the present invention can be applied to large-capacity offshore wind power transmission systems, effectively reducing system oscillation risks, reducing the capacity requirements of auxiliary converter equipment, and improving system stability and power quality.

[0031] To implement the above embodiments, as shown in Figure 4, this embodiment also provides an auxiliary converter equipment control device 10 for an offshore wind power DC system, including: a frequency control module 100, used to generate a DC voltage reference value through a proportional-integral controller in response to the deviation between the offshore AC voltage frequency and the rated frequency; a DC internal potential setting module 200, used to set the DC voltage reference value as the DC internal potential of the auxiliary converter equipment; a d-axis controller module 300, used to generate a d-axis current reference value through proportional-integral control in response to the deviation between the actual value and the reference value of the capacitor voltage; and a q-axis controller module 400, used to respond to the deviation between the offshore AC voltage and the reference value. The deviation between the actual value and the reference value of the voltage at the PCC point is used to generate a q-axis current reference value through proportional control. The harmonic suppression module 500 is used to respond to the dq component of the harmonic current and add the component to the d-axis and q-axis current reference values ​​to generate a new dq-axis current reference value. The voltage reference value calculation module 600 is used to calculate the three-phase bridge arm voltage reference value based on the new dq-axis current reference value through a proportional resonant controller and a linear transformation matrix. The control output module 700 is used to output the three-phase bridge arm voltage reference value to the auxiliary converter equipment to achieve active / reactive power decoupling control and frequency stabilization control of the offshore AC power grid.

[0032] Furthermore, the frequency control module 100 includes a frequency deviation detection unit and a PI controller unit, wherein the frequency deviation detection unit is used to acquire the frequency of the marine AC voltage and calculate its deviation from the rated frequency; the PI controller unit is used to perform proportional-integral control on the deviation to generate a DC voltage reference value.

[0033] Furthermore, the d-axis controller module 300 and the q-axis controller module 400 respectively include a current deviation detection unit and a controller unit. The current deviation detection unit is used to collect the actual values ​​of the capacitor voltage and the PCC point voltage and calculate their deviation from the reference value. The controller unit performs proportional-integral control and proportional control on the deviation to generate d-axis and q-axis current reference values.

[0034] Furthermore, the harmonic suppression module 500 also includes a harmonic extraction unit, which is used to perform dq transformation on the harmonic current in the offshore AC power grid, extract its dq components, and feed the components back to the d-axis and q-axis controller module.

[0035] Furthermore, the voltage reference value calculation module 600 also includes a PR controller unit and a linear transformation matrix unit, wherein the PR controller unit is used to perform harmonic compensation on the new dq axis current reference value; and the linear transformation matrix unit is used to convert the dq axis voltage reference value into a three-phase bridge arm voltage reference value.

[0036] The auxiliary converter equipment control device in the offshore wind power DC system according to the present invention can be applied to the offshore large-capacity wind power transmission system, effectively reducing the risk of system oscillation, reducing the capacity requirement of auxiliary converter equipment, and improving system stability and power quality.

[0037] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0038] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

Claims

1. A control method for auxiliary converter equipment in an offshore wind power DC system, characterized in that, include: In response to the deviation between the offshore AC voltage frequency and the rated frequency, the frequency control module performs proportional-integral control on the deviation to generate a DC voltage reference value. This DC voltage reference value is then assigned to the DC internal potential of the auxiliary converter equipment to achieve indirect control of the offshore AC voltage frequency. In response to the deviation between the actual and reference values ​​of the capacitor voltage, the d-axis controller module performs proportional-integral control on the deviation to generate a d-axis current reference value. In response to the deviation between the actual and reference values ​​of the PCC point voltage in the offshore AC power grid, the q-axis controller module performs proportional control on the deviation to generate a q-axis current reference value. In response to the dq component of the harmonic current, the harmonic suppression module adds the harmonic current component to the d-axis and q-axis current reference values ​​to generate a new dq-axis current reference value. Based on the new dq-axis current reference value, the voltage reference value calculation module calculates the three-phase bridge arm voltage reference value using a proportional resonant controller and a linear transformation matrix. The three-phase bridge arm voltage reference value is then output to the auxiliary converter equipment to achieve active / reactive decoupling control and frequency stabilization control of the offshore AC power grid.

2. The control method as described in claim 1, characterized in that, The frequency control module includes a proportional-integral controller, the output of which is used to generate a DC voltage reference value to adjust the DC internal potential of the auxiliary converter equipment, thereby indirectly controlling the frequency of the marine AC voltage.

3. The control method as described in claim 2, characterized in that, The d-axis controller module and the q-axis controller module are used to control active power and reactive power, respectively. The d-axis controller module controls the capacitor voltage deviation through proportional-integral control, and the q-axis controller module controls the PCC point voltage deviation through proportional control, thereby achieving decoupled control of active and reactive power.

4. The control method as described in claim 3, characterized in that, The harmonic suppression module includes a harmonic current detection unit and an addition unit. The harmonic current detection unit is used to extract the dq component of the harmonic current, and the addition unit is used to add the dq component to the d-axis and q-axis current reference values ​​to generate new dq-axis current reference values.

5. The control method as described in claim 4, characterized in that, The voltage reference value calculation module includes a proportional resonant controller and a linear transformation matrix. The proportional resonant controller is used to perform harmonic compensation on the new dq-axis current reference value, and the linear transformation matrix is ​​used to convert the dq-axis voltage reference value into a three-phase bridge arm voltage reference value.

6. A control device for auxiliary converter equipment in an offshore wind power DC system, characterized in that, include: The frequency control module is used to generate a DC voltage reference value through a proportional-integral controller in response to the deviation between the marine AC voltage frequency and the rated frequency. The DC internal potential setting module is used to set the DC voltage reference value as the DC internal potential of the auxiliary converter equipment; the d-axis controller module is used to generate a d-axis current reference value through proportional-integral control in response to the deviation between the actual value and the reference value of the capacitor voltage; the q-axis controller module is used to generate a q-axis current reference value through proportional control in response to the deviation between the actual value and the reference value of the PCC point voltage of the offshore AC power grid. The harmonic suppression module is used to respond to the dq component of the harmonic current and add the component to the d-axis and q-axis current reference values ​​to generate a new dq-axis current reference value; the voltage reference value calculation module is used to calculate the three-phase bridge arm voltage reference value based on the new dq-axis current reference value, through a proportional resonant controller and a linear transformation matrix. The control output module is used to output the reference value of the three-phase bridge arm voltage to the auxiliary converter equipment to realize active / reactive decoupling control and frequency stabilization control of the offshore AC power grid.

7. The control device as described in claim 6, characterized in that, The frequency control module includes a frequency deviation detection unit and a PI controller unit. The frequency deviation detection unit is used to collect the frequency of the marine AC voltage and calculate its deviation from the rated frequency. The PI controller unit is used to perform proportional-integral control on the deviation to generate a DC voltage reference value.

8. The control device as described in claim 7, characterized in that, The d-axis controller module and the q-axis controller module each include a current deviation detection unit and a controller unit. The current deviation detection unit is used to collect the actual values ​​of the capacitor voltage and the PCC point voltage and calculate their deviation from the reference value. The controller unit performs proportional-integral control and proportional control on the deviation to generate d-axis and q-axis current reference values.

9. The control device as described in claim 8, characterized in that, The harmonic suppression module also includes a harmonic extraction unit, which is used to perform dq transformation on the harmonic current in the offshore AC power grid, extract its dq components, and feed the components back to the d-axis and q-axis controller module.

10. The control device as claimed in claim 9, characterized in that, The voltage reference value calculation module further includes a PR controller unit and a linear transformation matrix unit. The PR controller unit is used to perform harmonic compensation on the new dq-axis current reference value. The linear transformation matrix unit is used to convert the dq-axis voltage reference value into a three-phase bridge arm voltage reference value.

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