An offshore wind power flexible direct current transmission system and control method thereof

Abstract

The present invention relates to an offshore wind power flexible direct current transmission system and a control method thereof. The system is composed of a current source converter device composed of two fully controlled current source converter valve components and a modular multi-level integrated compensation component, which respectively constitute an offshore converter station and an onshore converter station, and can reduce the harmonic problem of the current source converter of the base frequency modulation without the help of a filter group. The technical solution provided by the present invention can realize the power transmission of the offshore wind power system at a lower switching frequency (for example, 50Hz) with less loss; it has a harmonic suppression function, does not need to configure a large filter group, and has volume and cost advantages; at the same time, it can realize independent smooth regulation of reactive power, realize the decoupling of active power and reactive power, so as to better maintain the voltage stability of the offshore AC system.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission, and in particular to an offshore wind power flexible direct current transmission system and a control method thereof. Background Art

[0002] Offshore wind power has the advantages of abundant resources, stable wind speed, higher available hours, and closer to the load, which is conducive to the use of centralized large-scale transmission, so as to give full play to the scale effect and cluster effect of offshore wind power. In order to smoothly transmit offshore wind power resources to the land power grid, DC transmission is currently mainly used. However, due to the special application environment of the offshore, it is necessary to build supporting facilities such as offshore converter stations and offshore booster stations, which makes the current system construction cost very high, and the overall cost is highly sensitive to factors such as the volume and weight of the DC transmission system topology. Therefore, it is urgent to propose an offshore wind power DC transmission system topology with good economic efficiency, and at the same time have the functions of offshore AC voltage control and DC transmission. Summary of the invention

[0003] Based on the above situation of the prior art, the purpose of the present invention is to provide an offshore wind power flexible DC transmission system and a control method thereof, which can reduce the harmonic problem of the base frequency modulated current source converter without the help of a filter group, and realize independent control of the active and reactive power of the current source converter, thereby avoiding severe voltage fluctuations in the offshore AC system.

[0004] To achieve the above object, according to one aspect of the present invention, a current source converter device is provided, comprising a modular multi-level integrated compensation component, a first fully controlled current source converter valve component and a second fully controlled current source converter valve component; wherein:

[0005] The first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly are connected in parallel with the modular multi-level integrated compensation assembly through a Y / Y type converter transformer and a Y / △ type converter transformer respectively;

[0006] Each of the fully controlled current source converter valve components includes a three-phase H-bridge structure;

[0007] The modular multi-level integrated compensation component includes three switch bridge arms, and the three switch bridge arms are connected to each other in a triangle or star manner.

[0008] Furthermore, each of the switch bridge arms includes a plurality of switch sub-modules, and the switch sub-module includes a full-bridge structure having four fully-controlled switch devices and a capacitor.

[0009] According to a second aspect of the present invention, there is provided an offshore wind power flexible direct current transmission system, comprising an offshore wind farm, an offshore converter station, an onshore converter station and an alternating current power grid; wherein:

[0010] The AC side of the onshore converter station is connected to the AC power grid, and the DC side is connected to the DC side of the offshore converter station via a DC submarine cable, so as to maintain power balance in the DC system;

[0011] The AC side of the offshore converter station is connected to the offshore wind farm via an AC submarine cable, and is used to provide AC system voltage and frequency support for the offshore wind farm;

[0012] The offshore converter station and the onshore converter station both include the current source type converter device as described in the first aspect of the present invention.

[0013] According to a third aspect of the present invention, there is provided a control method for an offshore wind power flexible DC transmission system as described in the second aspect of the present invention, comprising:

[0014] The first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly of the offshore converter station and the onshore converter station are subjected to base frequency modulation and PI control;

[0015] The modular multi-level integrated compensation components of the offshore converter station and the onshore converter station are used to perform harmonic compensation and reactive power compensation.

[0016] Furthermore, the first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly of the offshore converter station adopt fundamental frequency modulation and PI control, including:

[0017] The voltage amplitude of the AC side of the offshore converter station With AC voltage amplitude command value The error value generated after the comparison is passed through the PI controller to generate the first trigger angle of the first fully controlled current source converter valve component. ;

[0018] The trigger angle of the first bridge arm component After the first phase is shifted, a first trigger angle of the second fully controlled current source converter valve assembly is generated. ;

[0019] The frequency command value After integration, the offshore AC voltage reference phase is generated .

[0020] Furthermore, it also includes:

[0021] The first firing angle and the first firing angle Triggering occurs after limiting.

[0022] Furthermore, the first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly of the onshore converter station adopt fundamental frequency modulation and PI control, including:

[0023] The DC side current value of the onshore converter station With DC current command value The error value generated is passed through a PI controller to generate a trigger angle of the first fully controlled current source converter valve component and the second fully controlled current source converter valve component. ;

[0024] The trigger angle Triggering occurs after limiting.

[0025] Furthermore, the modular multi-level integrated compensation components of the offshore converter station and the onshore converter station are used to perform harmonic compensation and reactive power compensation, including:

[0026] The average voltage of each switch submodule in the modular multi-level integrated compensation component With the submodule voltage command value The error value generated after comparison is passed through the PI controller to generate the fundamental current Axis reference value ;

[0027] Reactive power Reactive power command value The error value generated after comparison is passed through the PI controller to generate the fundamental current Axis reference value ;

[0028] The fundamental current Axis reference value and fundamental current Axis reference value Output after passing through fundamental current controller;

[0029] The current harmonics on the AC side of the offshore converter station The error value generated after comparison with the current harmonic command value is passed through multiple proportional resonant controllers to perform phase compensation on different harmonics, and the compensation results are superimposed;

[0030] The output result of the fundamental current controller and the superimposed compensation result are respectively After transformation and superposition, the output voltage reference value of the modular multi-level comprehensive compensation component is output .

[0031] Furthermore, the current harmonic command value is 0.

[0032] Furthermore, the phase-splitting compensation of different harmonics by multiple proportional resonant controllers includes:

[0033] Four proportional resonant controllers are used to perform phase compensation for the 11th, 13th, 23rd and 25th harmonics.

[0034] In summary, the present invention provides an offshore wind power flexible direct current transmission system and a control method thereof. The system is composed of a current source converter device composed of two fully controlled current source converter valve components and a modular multi-level integrated compensation component, which respectively constitute an offshore converter station and an onshore converter station, and can reduce the harmonic problem of the current source converter of the base frequency modulation without the help of a filter group; in the control method of the present invention, the fully controlled current source converter valve components of the offshore converter station and the onshore converter station are subjected to base frequency modulation and PI control; and the modular multi-level integrated compensation components of the offshore converter station and the onshore converter station are used for harmonic compensation and reactive power compensation. The technical solution provided by the present invention can realize the power transmission of the offshore wind power system at a lower switching frequency (for example, 50Hz) with less loss; it has a harmonic suppression function, does not need to configure a large filter group, and has volume and cost advantages; at the same time, it can realize independent smooth regulation of reactive power, and realize the decoupling of active power and reactive power, so as to better maintain the voltage stability of the offshore AC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the circuit structure of a hybrid converter topology in the prior art;

[0036] Figure 2 Schematic diagram of the circuit structure of the current source type commutation device of the present invention;

[0037] Figure 3 It is a schematic diagram of the topological structure of the offshore wind power flexible DC transmission system of the present invention;

[0038] Figure 4 It is a schematic diagram of the control strategy of the fully controlled current source converter valve assembly of the offshore converter station;

[0039] Figure 5 It is a schematic diagram of the control strategy of the fully controlled current source converter valve assembly of the onshore converter station;

[0040] Figure 6 It is a schematic diagram of the control strategy of the modular multi-level integrated compensation component for harmonic compensation and reactive power compensation. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0042] The existing offshore wind power DC transmission system mainly includes voltage source converter and current source converter. The voltage source converter is more mature in technology and mainly adopts modular multi-level topology (MMC). Each converter consists of thousands of sub-modules and is flexible to control. However, the converter needs to use a large number of IGBT devices, and the one-time construction cost is high. Another DC transmission technology based on current source converter was proposed later because it requires higher frequency PWM modulation, which greatly increases system losses, and the driving technology required for a large number of switching devices in series at high frequencies is difficult to achieve.

[0043] In recent years, some scholars have proposed using base frequency modulation (50Hz) in current source converters, combining the advantages of grid-commutated converters (LCCs) to reduce system losses. This solution can use IGCT switching devices to achieve higher voltage levels through series connection, realize high-power energy transmission, and is suitable for switching frequencies of around 50Hz. It has certain cost advantages and realizes offshore AC system voltage control through the switchable characteristics of IGCT devices. Figure 1 The circuit structure diagram of the hybrid converter topology in the prior art is shown in FIG. However, the current source converter using fundamental frequency modulation has large harmonics when used. Similar to the LCC converter station, a large number of filter groups need to be configured, such as Figure 1 As shown in the dotted box in the upper left center, for offshore wind power transmission, the size and weight of the offshore converter station will be greatly increased, thereby increasing the investment cost of the project. On the other hand, there is a strong coupling between the active power and reactive power of the current source converter using base frequency modulation, which cannot be controlled independently. The change of active power will cause a large fluctuation in the voltage of the offshore AC system, affecting the safety of wind farm operation.

[0044] In view of the above technical problems, according to one embodiment of the present invention, a current source type commutation device is provided. The circuit structure diagram of the current source type commutation device is shown in FIG. Figure 2 As shown, the device includes a modular multi-level comprehensive compensation component, a first fully-controlled current source converter valve component and a second fully-controlled current source converter valve component; wherein the first fully-controlled current source converter valve component and the second fully-controlled current source converter valve component are respectively connected in parallel with the modular multi-level comprehensive compensation component through a Y / Y type converter transformer and a Y / △ type converter transformer; each of the fully-controlled current source converter valve components includes a three-phase H-bridge structure; each fully-controlled current source converter valve component includes 6 bridge arms, and the bridge arms can be formed by a plurality of fully-controlled switch devices (such as IGCT) connected in series. The modular multi-level comprehensive compensation component includes three switch bridge arms, and the three switch bridge arms are interconnected in a triangle or star manner. Each of the switch bridge arms includes a plurality of switch submodules, and the switch submodule includes a full-bridge structure having four fully-controlled switch devices and a capacitor.

[0045] According to a second embodiment of the present invention, a flexible DC transmission system for offshore wind power is provided. The topological structure diagram of the flexible DC transmission system for offshore wind power is shown in FIG. Figure 3 As shown, the system includes an offshore wind farm, an offshore converter station, an onshore converter station and an AC power grid. The AC side of the onshore converter station is connected to the AC power grid, and the DC side is connected to the DC side of the offshore converter station through a DC submarine cable, so as to maintain the power balance in the DC system; the AC side of the offshore converter station is connected to the offshore wind farm through an AC submarine cable, so as to provide AC system voltage and frequency support for the offshore wind farm; the offshore converter station and the onshore converter station both include a current source converter device as provided in the first embodiment of the present invention. The onshore converter station and the offshore converter station can adopt the same topology, including two fully controlled current source converter valve assemblies and a modular multi-level integrated compensation assembly. Considering that the onshore converter station does not need to consider the construction of offshore platforms like the offshore converter station, the modular multi-level integrated compensation assembly can be replaced with the AC filter and reactive power compensation device in the traditional LCC DC converter station.

[0046] According to a third embodiment of the present invention, a control method for an offshore wind power flexible DC transmission system as provided in the second embodiment of the present invention is provided, comprising:

[0047] Base frequency modulation and PI control are adopted for the first fully controlled current source converter valve assembly and the second fully controlled current source converter valve assembly of the offshore converter station and the onshore converter station; and the modular multi-level integrated compensation assembly of the offshore converter station and the onshore converter station is adopted to perform harmonic compensation and reactive power compensation. In the control method provided in the embodiment of the present invention, both the offshore converter station and the onshore converter station adopt base frequency modulation and PI control, but the instructions and feedback of PI control are different (that is, the control targets are different), the control target of the offshore converter station is the offshore AC voltage, and the control target of the onshore converter station is the DC current.

[0048] The following explains them respectively.

[0049] In the offshore converter station, two fully controlled current source converter valve components use base frequency modulation and control to control the voltage amplitude and frequency on the AC side of the offshore converter station. The control strategy diagram is shown in the figure. Figure 4 As shown, the control method includes:

[0050] The voltage amplitude of the AC side of the offshore converter station With AC voltage amplitude command value The error value generated after the comparison is passed through the PI controller to generate the first trigger angle of the first fully controlled current source converter valve component. ; The trigger angle of the first bridge arm component After the first phase is shifted, a first trigger angle of the second fully controlled current source converter valve assembly is generated. , the first phase can be, for example, 30°. After integration, the offshore AC voltage reference phase is generated And, the first trigger angle and the first firing angle Trigger after limiting. This limit can be set and They are respectively the maximum allowable value and the minimum allowable value of the trigger angle of the fully controlled current source converter valve component.

[0051] In the onshore converter station, PI control is adopted for two fully controlled current source converter valve components. The control strategy diagram is shown in Figure 5 As shown, the method comprises the following steps: converting the DC side current value of the onshore converter station With DC current command value The error value generated is passed through a PI controller to generate a trigger angle of the first fully controlled current source converter valve component and the second fully controlled current source converter valve component. ; The trigger angle Trigger after limiting. This limit can be set and They are respectively the maximum allowable value and the minimum allowable value of the trigger angle of the fully controlled current source converter valve component.

[0052] The modular multi-level integrated compensation component is used to perform harmonic compensation and reactive power compensation on the fully controlled current source converter valve component. The control strategy diagram is shown in Figure 6 As shown, the following steps are included:

[0053] The average voltage of each switch submodule in the modular multi-level integrated compensation component With the submodule voltage command value The error value generated after comparison is passed through the PI controller to generate the fundamental current Axis reference value ; The reactive power of the offshore converter station Reactive power command value The error value generated after comparison is passed through the PI controller to generate the fundamental current Axis reference value The reactive power Q includes the sum of the reactive powers of two fully controlled current source converter valve components and a modular multi-level integrated compensation component; the fundamental current Axis reference value and fundamental current Axis reference value After passing through the fundamental current controller, the current harmonics on the AC side of the offshore converter station are output. The error value generated after comparison with the current harmonic command value (for example, 0) is passed through multiple proportional resonant controllers to perform phase compensation for different harmonics, and the compensation results are superimposed. For example, four proportional resonant controllers can be used to perform phase compensation for the 11th, 13th, 23rd, and 25th harmonics. The output of the fundamental current controller and the superimposed compensation results are passed through After transformation and superposition, the output voltage reference value of the modular multi-level comprehensive compensation component is output In this control process, the proportional integral controller (PI) is used to control the submodule voltage and reactive power in the rotating coordinate system, and the output results are and As current instructions, they are input into the fundamental current controller for phase control. In terms of harmonic compensation, since the current source converter adopts a 12-pulse topology, there are only 12k±1 harmonics in the AC system. Therefore, a proportional resonant controller (PR) is used to perform phase compensation for the 11th, 13th, 23rd, and 25th harmonics. The higher-order harmonics themselves have a small content and are greatly attenuated under system damping, so no compensation is required. Specifically, after the system harmonic current is collected and compared with the reference value, it is calculated through a proportional resonant controller of multiple specific harmonics, and the results are superimposed as the output voltage of each phase.

[0054] In summary, the present invention relates to an offshore wind power flexible direct current transmission system and a control method thereof. The system is composed of a current source converter device composed of two fully controlled current source converter valve components and a modular multi-level integrated compensation component, which respectively constitute an offshore converter station and an onshore converter station, and can reduce the harmonic problem of the current source converter of the base frequency modulation without the help of a filter group; in the control method of the present invention, the fully controlled current source converter valve component of the offshore converter station adopts base frequency modulation; the fully controlled current source converter valve component of the onshore converter station adopts PI control; and the modular multi-level integrated compensation component of the offshore converter station and the onshore converter station is used to perform harmonic compensation and reactive power compensation. The technical solution provided by the present invention can realize the power transmission of the offshore wind power system at a lower switching frequency (for example, 50Hz) with less loss; it has a harmonic suppression function, does not need to configure a large filter group, and has volume and cost advantages; at the same time, it can realize independent smooth regulation of reactive power, and realize the decoupling of active power and reactive power, so as to better maintain the voltage stability of the offshore AC system.

[0055] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, and do not constitute a limitation of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalent forms of such scope and boundaries.

Claims

1. A control method for an offshore wind power flexible direct current transmission system, characterized in that: The system includes an offshore wind farm, an offshore converter station, an onshore converter station and an AC power grid; wherein, The AC side of the onshore converter station is connected to the AC power grid, and the DC side is connected to the DC side of the offshore converter station via a DC submarine cable, so as to maintain power balance in the DC system; The AC side of the offshore converter station is connected to the offshore wind farm via an AC submarine cable, and is used to provide AC system voltage and frequency support for the offshore wind farm; The offshore converter station and the onshore converter station both include a current source converter device, and the current source converter device includes a modular multi-level integrated compensation component, a first fully controlled current source converter valve component, and a second fully controlled current source converter valve component; wherein, The first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly are connected in parallel with the modular multi-level integrated compensation assembly through a Y / Y type converter transformer and a Y / △ type converter transformer respectively; Each of the fully controlled current source converter valve components includes a three-phase H-bridge structure; The modular multi-level integrated compensation component comprises three switch bridge arms, and the three switch bridge arms are connected to each other in a triangle or star manner; The method comprises: The first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly of the offshore converter station and the onshore converter station are subjected to base frequency modulation and PI control; The method of using base frequency modulation and PI control on the first fully controlled current source converter valve assembly and the second fully controlled current source converter valve assembly of the offshore converter station includes: The voltage amplitude U at the AC side of the offshore converter station ac and the AC voltage amplitude command value U ac_ref The error value generated after the comparison is passed through a PI controller to generate a first trigger angle α1 of the first fully controlled current source converter valve assembly; The first trigger angle α1 is shifted by a first phase to generate a second trigger angle α2 of a second fully controlled current source converter valve assembly; The frequency command value f ac_ref After integration, the offshore AC voltage reference phase θ is generated ac ; The modular multi-level integrated compensation components of the offshore converter station and the onshore converter station are used to perform harmonic compensation and reactive power compensation, including: The average voltage U of each switch submodule in the modular multi-level integrated compensation component sm_avg and the submodule voltage command value U sm_avg_ref The error value generated after comparison is passed through the PI controller to generate the fundamental current d-axis reference value i d_ref ; The reactive power Q of the offshore converter station is compared with the reactive power command value Q ref The error value generated after comparison is passed through the PI controller to generate the fundamental current q-axis reference value i q_ref ; The fundamental current d-axis reference value i d_ref and fundamental current q-axis reference value i q_ref Output after passing through fundamental current controller; The current harmonics i on the AC side of the offshore converter station Harm The error value generated after comparison with the current harmonic command value is passed through multiple proportional resonant controllers to perform phase compensation on different harmonics, and the compensation results are superimposed; The output result of the fundamental current controller and the superimposed compensation result are respectively superimposed after dq / abc conversion, and the output voltage reference value U of the modular multi-level comprehensive compensation component is output. abc .

2. The method according to claim 1, characterized in that Also includes: The first trigger angle α1 and the second trigger angle α2 are triggered after being limited.

3. The method according to claim 1, characterized in that The first fully controlled current source converter valve assembly and the second fully controlled current source converter valve assembly of the onshore converter station are subjected to base frequency modulation and PI control, including: The DC side current value i of the onshore converter station dc With the DC current command value i dc_ref The error value generated is passed through a PI controller to generate a trigger angle α of the first fully-controlled current source converter valve assembly and the second fully-controlled current source converter valve assembly; The trigger angle α is limited before triggering.

4. The method according to claim 1, characterized in that: The current harmonic command value is 0.

5. The method according to claim 4, characterized in that The phase-splitting compensation of different harmonics by multiple proportional resonant controllers includes: Four proportional resonant controllers are used to perform phase compensation for the 11th, 13th, 23rd and 25th harmonics.

6. The method according to claim 1, characterized in that In the current source commutation device, each of the switch bridge arms includes a plurality of switch submodules, and the switch submodule includes a full-bridge structure having four fully-controlled switch devices and one capacitor.

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