Flexible DC system modular multilevel converter control method and device

Abstract

The invention provides a flexible DC system modular multilevel converter control method and device, and relates to the technical field of modular multilevel converters. Comprising the following steps: performing constant AC voltage control on a wind power plant modular multilevel converter WFMMC in a flexible DC system so as to keep the AC side voltage of the wind power plant in a stable state; performing constant direct-current voltage control on a grid-connected modular multi-level converter GSMMC so as to keep the direct-current side voltage of the wind power plant in a stable state; an actual voltage vector angle is detected based on a phase-locked loop, and a current source type cascade converter is added to a secondary voltage wave crest of the modular multilevel converter so as to suppress a frequency doubling circulating current component in the modular multilevel converter. Therefore, stable state control of a wind power plant modular multilevel converter WFMMC and a grid-connected modular multilevel converter GSMMC in a flexible DC system can be realized.

Description

Technical Field

[0001] The present disclosure relates to the technical field of modular multi-level converters, and in particular to a control method and device for a modular multi-level converter of a flexible direct current system. Background Art

[0002] The existing technology has some limitations in the control of wind farm modular multilevel converter (WFMMC) in flexible DC system. The traditional control method often uses the traditional PI controller to perform feedback control based on the voltage error. However, this method may not be able to achieve the requirements of fast response and precise control for the AC side voltage control of the wind farm in the flexible DC system. Since the wind farm has a high response requirement to the power grid, the traditional control method may not be able to meet the requirements of maintaining the stable voltage on the AC side of the wind farm.

[0003] Existing DC voltage control methods usually use traditional PID controllers or fuzzy controllers to control according to the DC side voltage error. However, this method has certain limitations in dealing with the dynamic changes and rapid response of the DC side voltage of the wind farm. The stability of the DC side voltage of the wind farm is crucial to the operation and energy management of the system. Existing control methods usually use filters or harmonic suppressors to reduce the double frequency circulating current component. However, these methods often require complex hardware design and additional circuit costs, and cannot completely eliminate the double frequency circulating current component. Summary of the invention

[0004] The present disclosure aims to solve one of the technical problems in the related art at least to some extent.

[0005] The first embodiment of the present disclosure provides a control method for a modular multi-level converter of a flexible direct current system, comprising:

[0006] Perform constant AC voltage control on the wind farm modular multilevel converter WFMMC in the flexible DC system to keep the AC side voltage of the wind farm in a stable state;

[0007] The grid-connected modular multilevel converter GSMMC is controlled by constant DC voltage to keep the DC side voltage of the wind farm in a stable state;

[0008] The actual voltage vector angle is detected based on a phase-locked loop, and a current source type cascade converter is added at the secondary voltage peak of the modular multi-level converter to suppress the double frequency circulating current component in the modular multi-level converter.

[0009] The second aspect of the present disclosure provides a modular multi-level converter control device for a flexible direct current system, comprising:

[0010] The first control module is used to perform constant AC voltage control on the wind farm modular multi-level converter WFMMC in the flexible direct current system to keep the AC side voltage of the wind farm in a stable state;

[0011] The second control module is used to control the grid-connected modular multi-level converter GSMMC by adopting a constant DC voltage control to keep the DC side voltage of the wind farm in a stable state;

[0012] The third control module is used to detect the actual voltage vector angle based on the phase-locked loop, and add a current source type cascade converter at the secondary voltage peak of the modular multi-level converter to suppress the double frequency circulating current component in the modular multi-level converter.

[0013] The third aspect embodiment of the present disclosure proposes an electronic device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the modular multi-level converter control method for a flexible direct current system proposed in the first aspect embodiment of the present disclosure is implemented.

[0014] The fourth aspect of the present disclosure provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the flexible direct current system modular multi-level converter control method provided in the first aspect of the present disclosure is implemented.

[0015] The present disclosure provides a flexible DC system modular multi-level converter control method and device, which has the following features:

[0016] Beneficial effects:

[0017] In the disclosed embodiment, the modular multilevel converter WFMMC of the wind farm in the flexible direct current system is firstly controlled by constant AC voltage to keep the voltage on the AC side of the wind farm in a stable state, and then the modular multilevel converter GSMMC connected to the grid is controlled by constant DC voltage to keep the voltage on the DC side of the wind farm in a stable state, and finally the actual voltage vector angle is detected based on the phase-locked loop, and a current source type cascade converter is added at the secondary voltage peak of the modular multilevel converter to suppress the double frequency circulating current component in the modular multilevel converter. Thus, the stable state control of the modular multilevel converter (WFMMC) of the wind farm and the modular multilevel converter (GSMMC) connected to the grid in the flexible direct current system can be achieved. By controlling the voltage on the AC side of the wind farm with constant AC voltage, the voltage on the grid side can be kept in a stable state. This helps to ensure the normal operation of energy transmission between the wind farm and the grid, and improve the reliability and stability of the grid. The DC side voltage of the grid-connected modular multilevel converter is controlled by constant DC voltage, so that the DC side voltage of the wind farm can be kept in a stable state. This helps to stabilize the storage and management of energy, avoid excessively high or low DC voltages in the power grid, and enhance the controllability and reliability of the system. The actual voltage vector angle is detected by a phase-locked loop, and a current source cascade converter is added to the secondary voltage peak of the modular multi-level converter, which can effectively suppress the double frequency circulating current component. This can reduce power loss and harmonic distortion in the system and improve the efficiency and stability of the entire system. It has beneficially solved the technical problems in the flexible direct current system, including grid-side voltage instability, DC voltage fluctuations and harmonics. By achieving stable control of AC and DC voltages and suppressing double frequency circulating current components, the operating efficiency of wind farms can be improved, energy loss can be reduced, and the safe and reliable operation of the system can be guaranteed.

[0018] Additional aspects and advantages of the present disclosure will be given in part in the following description and in part will be obvious from the following description or learned through practice of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present disclosure will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0020] Figure 1 A flowchart of a modular multi-level converter control method for a flexible DC system provided by an embodiment of the present disclosure;

[0021] Figure 2 A control strategy block diagram of a modular multi-level converter for a flexible DC system provided by an embodiment of the present disclosure;

[0022] Figure 3 The present invention is a flowchart of a modular multi-level converter control device for a flexible DC system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0023] Embodiments of the present disclosure are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present disclosure, and should not be construed as limiting the present disclosure.

[0024] The following describes a method, device, electronic device and storage medium for controlling a modular multi-level converter of a flexible DC system according to an embodiment of the present disclosure with reference to the accompanying drawings.

[0025] Figure 1 A schematic flow chart of a modular multi-level converter control method for a flexible DC system provided by an embodiment of the present disclosure.

[0026] like Figure 1 As shown, the flexible DC system modular multi-level converter control method may include the following steps:

[0027] Step 101, performing constant AC voltage control on the wind farm modular multi-level converter WFMMC in the flexible DC system to keep the AC side voltage of the wind farm in a stable state.

[0028] Among them, the constant AC voltage control strategy (VF control) is a common control method, which is usually used to keep the AC voltage at a specific value. It can control the size of the AC voltage of the wind farm by adjusting the output frequency of the converter. This control strategy enables the wind farm to adapt to the requirements of the power grid and ensure that the voltage operates within a stable range.

[0029] Alternatively, a current control strategy can be used, which can better handle the unique characteristics of multilevel converters in flexible DC systems, such as effectively reducing harmonics and noise. Common current control strategies include DC current control (DCC), current control based on space vector modulation (SVM), model predictive control (MPC), etc. The current control strategy can achieve grid feedback control by adjusting the output current of the converter to keep the voltage on the AC side of the wind farm in a stable state.

[0030] Since the digital controller requires a certain amount of calculation time during the sampling and calculation process, coupled with the rapid control function of the flexible direct current system, it is inevitable that there will be a control delay problem. The control link delay can be equivalent to a delay link e -sT The MMC-HVDC control strategy considering the control link delay is as follows: Figure 2 As shown, Leq represents the equivalent inductance of the upper and lower bridge arms and the output inductor of the MMC, and kp and ki represent the corresponding outer loop and inner loop controller parameters.

[0031] As a possible implementation method, the d-axis reference voltage value and the q-axis reference voltage value of the AC voltage of the wind farm collection system after the dq-axis coordinate transformation can be first obtained, and then the actual d-axis voltage value and the actual q-axis voltage value after the delay link, as well as the actual d-axis current voltage value and the actual q-axis current value are obtained. Then, based on the first formula, according to the d-axis reference voltage value and the q-axis reference voltage value, the actual d-axis voltage value and the actual q-axis voltage value, the first outer loop control parameter, the second outer loop control parameter, the first inner loop control parameter and the second inner loop control parameter, the first reference current value and the second reference current value of the MMC AC valve side are calculated. Then, based on the first reference current value and the second reference current value, the d-axis reference voltage value and the q-axis reference voltage value, the actual d-axis current voltage value and the actual q-axis current value, the first reference voltage value and the second reference voltage value of the MMC AC valve side are determined.

[0032] Among them, the first formula is:

[0033]

[0034] Among them, Uadref_WFMMC is the d-axis reference voltage value, Uaqref_WFMMC is the q-axis reference voltage value, k pwd1 is the first outer loop control parameter, k pwq1 is the second outer loop control parameter, k iwd1 is the first inner loop control parameter, k iwq1 is the second inner loop control parameter, i adref_WFMMC is the first reference current value, i aqref_WFMMC is the second reference current value.

[0035] Optionally, the obtaining of the actual d-axis voltage value and the actual q-axis voltage value after the delay link, and the actual d-axis current voltage value and the actual q-axis current value includes:

[0036] Obtaining a first voltage delay parameter, a second voltage delay parameter, a first current delay parameter, and a second current delay parameter;

[0037] Acquire a first actual current value and a second actual current value of a wind farm output current, and a first actual voltage value and a second actual voltage value of a wind farm output voltage;

[0038] Based on the second formula, the first voltage delay parameter, the second voltage delay parameter, the first actual voltage value and the second actual voltage value, the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link are calculated, wherein the second formula is:

[0039]

[0040] Tud1 and Tuq1 are the first voltage delay parameter and the second voltage delay parameter, respectively; Uad_WFMMC and Uaq_WFMMC represent the first actual voltage value and the second actual voltage value output by the wind farm, respectively; U admWFMMC is the actual value of d-axis voltage,

[0041] U aqmWFMMC is the actual value of q-axis voltage;

[0042] Based on the third formula, the first current delay parameter, the second current delay parameter, the first actual current value and the second actual current value, the d-axis current actual value and the q-axis current actual value after the delay link are calculated, wherein the third formula is:

[0043]

[0044] Among them, T id11 is the first current delay parameter, T iq11 is the second current delay parameter, I ad_WFMMC is the first actual current value, I aq_WFMMC is the second actual current value, I admWFMMC is the actual value of the d-axis current, I aqmWFMMC is the actual value of q-axis current.

[0045] Furthermore, the reference value of the MMC AC valve side voltage can be calculated based on the actual value of the d-axis current and the actual value of the q-axis current, the actual value of the d-axis voltage and the actual value of the q-axis voltage, and the formula is as follows:

[0046]

[0047] Among them, Uadref_WFMMC and Uaqref_WFMMC respectively represent the dq-axis reference voltage values ​​of the AC voltage of the wind farm collection system after the dq-axis coordinate transformation; Uad_WFMMC and Uaq_WFMMC respectively represent the actual voltage values ​​after the change; Uadm_WFMMC and Uaqm_WFMMC respectively represent the actual voltage values ​​after the delay link, Iadm_WFMMC and Iaqm_WFMMC respectively represent the actual current values ​​after the delay link, Tud1, Tuq1 and Tid1, Tiq1 respectively correspond to the delay constants of the control link; Ucdref_WFMMC, Ucqref_WFMMC and Iadref_WFMMC, Iaqref_WFMMC respectively represent the reference values ​​of the voltage and current on the MMC AC valve side, Iad_WFMMC, Iaq_WFMMC respectively represent the actual values ​​of the wind farm output current; Leq represents the equivalent inductance of the upper and lower bridge arms and the export inductance of the MMC; kp and ki represent the corresponding outer loop and inner loop controller parameters.

[0048] Step 102: constant DC voltage control is applied to the grid-connected modular multilevel converter GSMMC to keep the DC side voltage of the wind farm in a stable state.

[0049] Specifically, the mathematical expression that can be used for the constant DC voltage control of the grid-connected modular multilevel converter GSMMC is:

[0050]

[0051]

[0052] Among them, Udcref represents the voltage reference value of the DC bus, Qgref represents the reference value of the reactive power output by GSMMC, Qg represents the reactive power output by GSMMC, Udc represents the actual value of the DC bus voltage, and the physical meaning of the remaining reference values ​​refers to WFMMC.

[0053] Step 103 , detecting the actual voltage vector angle based on a phase-locked loop, and adding a current source type cascade converter at the secondary voltage peak of the modular multi-level converter to suppress the double frequency circulating current component in the modular multi-level converter.

[0054] In the disclosed embodiment, a PLL (phase-locked loop) may be used to detect the actual voltage vector angle and provide it to a controller for coordinate transformation, so as to track and extract the phase information of the grid voltage signal.

[0055] In addition, since MMC has a double frequency circulating current component, this may lead to an increase in losses. In order to suppress this problem, an additional CCSC (Capacitor-Clamped Series Compensator) is required for control. CCSC can reduce the double frequency circulating current component by compensating the voltage waveform, thereby reducing losses.

[0056] The PI controller is a widely used control algorithm that regulates the output of a system to keep it stable with the desired value. Per-unit is a dimensionless representation method that is often used in control systems to simplify calculations and operations. In addition, these controllers control the system in a dq rotating coordinate system. The dq coordinate system is a common transformation method used to convert a three-phase AC system into an equivalent form of a two-axis DC winding. In the dq coordinate system, variables such as voltage, current, and power can be more conveniently processed and system control can be performed.

[0057] In summary, the control system in this paper uses PLL to detect the actual voltage angle and realizes control through coordinate transformation. The system is controlled by PI controller and operates in dq rotating coordinate system. In order to suppress the double frequency circulating current component, CCSC is introduced for compensation. These control strategies are aimed at maintaining the stability and performance of the system.

[0058] Optionally, the control system may also determine the controller coordinate transformation angle after detecting the actual voltage vector angle using a phase-locked loop.

[0059] In a modular multilevel converter, a capacitor clamped multilevel synchronization converter is usually used to adjust the control signal. The capacitor clamped multilevel synchronization converter can achieve synchronous switching of the modular multilevel converter by adjusting the control signal, thereby effectively reducing the power loss in the converter.

[0060] Therefore, the control system uses a phase-locked loop to detect the actual voltage vector angle and determines the coordinate transformation angle based on the angle. At the same time, a capacitor-clamped sub-synchronous converter is used to adjust the control signal to achieve synchronous switching of the modular multi-level converter, further improving the efficiency and stability of the system. Therefore, a variety of strategies can be adopted, including phase-locked loop detection of the actual voltage vector angle, determination of the coordinate transformation angle, and adjustment of the control signal using a capacitor-clamped sub-synchronous converter to achieve stable and efficient operation of the modular multi-level converter.

[0061] In the disclosed embodiment, the modular multilevel converter WFMMC of the wind farm in the flexible direct current system is firstly controlled by constant AC voltage to keep the voltage on the AC side of the wind farm in a stable state, and then the modular multilevel converter GSMMC connected to the grid is controlled by constant DC voltage to keep the voltage on the DC side of the wind farm in a stable state, and finally the actual voltage vector angle is detected based on the phase-locked loop, and a current source type cascade converter is added at the secondary voltage peak of the modular multilevel converter to suppress the double frequency circulating current component in the modular multilevel converter. Thus, the stable state control of the modular multilevel converter (WFMMC) of the wind farm and the modular multilevel converter (GSMMC) connected to the grid in the flexible direct current system can be achieved. By controlling the voltage on the AC side of the wind farm with constant AC voltage, the voltage on the grid side can be kept in a stable state. This helps to ensure the normal operation of energy transmission between the wind farm and the grid, and improve the reliability and stability of the grid. The DC side voltage of the grid-connected modular multilevel converter is controlled by constant DC voltage, so that the DC side voltage of the wind farm can be kept in a stable state. This helps to stabilize the storage and management of energy, avoid excessively high or low DC voltages in the power grid, and enhance the controllability and reliability of the system. The actual voltage vector angle is detected by a phase-locked loop, and a current source cascade converter is added at the secondary voltage peak of the modular multilevel converter, which can effectively suppress the double frequency circulating current component. This can reduce power loss and harmonic distortion in the system and improve the efficiency and stability of the entire system. It has beneficially solved the technical problems in the flexible direct current system, including grid-side voltage instability, DC voltage fluctuations and harmonics. By achieving stable control of AC voltage and DC voltage and suppressing the double frequency circulating current component, the operating efficiency of the wind farm can be improved, energy loss can be reduced, and the safe and reliable operation of the system can be guaranteed. The scheme adopts constant AC voltage control and constant DC voltage control to stabilize the AC side voltage and DC side voltage of the wind farm. At the same time, the actual voltage vector angle is detected by introducing a phase-locked loop, and a current source cascade converter is added at the secondary voltage peak of the modular multilevel converter to suppress the generation of the double frequency circulating current component. These innovative control strategies can overcome the limitations of existing technologies and improve the stability, response speed and efficiency of modular multi-level converters in wind farms in flexible direct current systems.

[0062] Figure 3 This is a structural schematic diagram of a modular multi-level converter control device for a flexible DC system provided in an embodiment of the present disclosure.

[0063] like Figure 3 As shown, the flexible DC system modular multi-level converter control device 300 may include:

[0064] The first control module 310 is used to perform constant AC voltage control on the wind farm modular multi-level converter WFMMC in the flexible DC system to keep the AC side voltage of the wind farm in a stable state;

[0065] The second control module 320 is used to control the grid-connected modular multi-level converter GSMMC by adopting a constant DC voltage control to keep the DC side voltage of the wind farm in a stable state;

[0066] The third control module 330 is used to detect the actual voltage vector angle based on the phase-locked loop, and add a current source type cascade converter at the secondary voltage peak of the modular multi-level converter to suppress the double frequency circulating current component in the modular multi-level converter.

[0067] Optionally, the first control module further includes:

[0068] The first acquisition unit is used to acquire a d-axis reference voltage value and a q-axis reference voltage value of the AC voltage of the wind farm collection system after the dq-axis coordinate transformation;

[0069] A second acquisition unit is used to acquire the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link, as well as the actual value of the d-axis current voltage and the actual value of the q-axis current;

[0070] a calculation unit, configured to calculate a first reference current value and a second reference current value of the MMC AC valve side based on a first formula, according to the d-axis reference voltage value and the q-axis reference voltage value, the d-axis voltage actual value and the q-axis voltage actual value, a first outer loop control parameter, a second outer loop control parameter, a first inner loop control parameter, and a second inner loop control parameter,

[0071] a determining unit, configured to determine a first reference voltage value and a second reference voltage value of an MMC AC valve side based on the first reference current value and the second reference current value, the d-axis reference voltage value and the q-axis reference voltage value, the d-axis current and voltage actual value, and the q-axis current actual value;

[0072] Among them, the first formula is:

[0073]

[0074] Among them, Uadref_WFMMC is the d-axis reference voltage value, Uaqref_WFMMC is the q-axis reference voltage value, k pwd1 is the first outer loop control parameter, k pwq1 is the second outer loop control parameter, k iwd1 is the first inner loop control parameter, k iwq1 is the second inner loop control parameter, i adref_WFMMC is the first reference current value, i aqref_WFMMC is the second reference current value.

[0075] Optionally, the second acquiring unit is specifically configured to:

[0076] Obtaining a first voltage delay parameter, a second voltage delay parameter, a first current delay parameter, and a second current delay parameter;

[0077] Acquire a first actual current value and a second actual current value of a wind farm output current, and a first actual voltage value and a second actual voltage value of a wind farm output voltage;

[0078] Based on the second formula, the first voltage delay parameter, the second voltage delay parameter, the first actual voltage value and the second actual voltage value, the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link are calculated, wherein the second formula is:

[0079]

[0080] Tud1 and Tuq1 are the first voltage delay parameter and the second voltage delay parameter, respectively; Uad_WFMMC and Uaq_WFMMC represent the first actual voltage value and the second actual voltage value output by the wind farm, respectively; U admWFMMC is the actual value of the d-axis voltage, U aqmWFMMC is the actual value of q-axis voltage;

[0081] Based on the third formula, the first current delay parameter, the second current delay parameter, the first actual current value and the second actual current value, the d-axis current actual value and the q-axis current actual value after the delay link are calculated, wherein the third formula is:

[0082]

[0083] Among them, T id11 is the first current delay parameter, T iq11 is the second current delay parameter, I ad_WFMMC is the first actual current value, I aq_WFMMC is the second actual current value, I admWFMMC is the actual value of the d-axis current, I aqmWFMMC is the actual value of q-axis current.

[0084] Optionally, the third control module is further used to:

[0085] Based on the actual voltage vector angle, a controller coordinate transformation angle is determined.

[0086] Optionally, also include:

[0087] The adjustment module is used to adjust the control signal of the modular multi-level converter by using the capacitor clamped sub-synchronous converter.

[0088] In the disclosed embodiment, the modular multilevel converter WFMMC of the wind farm in the flexible direct current system is firstly controlled by constant AC voltage to keep the voltage on the AC side of the wind farm in a stable state, and then the modular multilevel converter GSMMC connected to the grid is controlled by constant DC voltage to keep the voltage on the DC side of the wind farm in a stable state, and finally the actual voltage vector angle is detected based on the phase-locked loop, and a current source type cascade converter is added at the secondary voltage peak of the modular multilevel converter to suppress the double frequency circulating current component in the modular multilevel converter. Thus, the stable state control of the modular multilevel converter (WFMMC) of the wind farm and the modular multilevel converter (GSMMC) connected to the grid in the flexible direct current system can be achieved. By controlling the voltage on the AC side of the wind farm with constant AC voltage, the voltage on the grid side can be kept in a stable state. This helps to ensure the normal operation of energy transmission between the wind farm and the grid, and improve the reliability and stability of the grid. The DC side voltage of the grid-connected modular multilevel converter is controlled by constant DC voltage, so that the DC side voltage of the wind farm can be kept in a stable state. This helps to stabilize the storage and management of energy, avoid excessively high or low DC voltages in the power grid, and enhance the controllability and reliability of the system. The actual voltage vector angle is detected by a phase-locked loop, and a current source cascade converter is added to the secondary voltage peak of the modular multi-level converter, which can effectively suppress the double frequency circulating current component. This can reduce power loss and harmonic distortion in the system and improve the efficiency and stability of the entire system. It has beneficially solved the technical problems in the flexible direct current system, including grid-side voltage instability, DC voltage fluctuations and harmonics. By achieving stable control of AC and DC voltages and suppressing double frequency circulating current components, the operating efficiency of wind farms can be improved, energy loss can be reduced, and the safe and reliable operation of the system can be guaranteed.

[0089] In order to implement the above embodiments, the present disclosure further proposes an electronic device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the modular multi-level converter control method for a flexible direct current system proposed in the above embodiments of the present disclosure is implemented.

[0090] In order to implement the above embodiments, the present disclosure further proposes a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the modular multi-level converter control method of the flexible DC system proposed in the above embodiments of the present disclosure is implemented.

[0091] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0092] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present disclosure, "plurality" means at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0093] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code that includes one or more executable instructions for implementing the steps of a custom logical function or process, and the scope of the preferred embodiments of the present disclosure includes additional implementations in which functions may not be performed in the order shown or discussed, including performing functions in a substantially simultaneous manner or in reverse order depending on the functions involved, which should be understood by technicians in the technical field to which the embodiments of the present disclosure belong.

[0094] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute the instructions), or in combination with these instruction execution systems, devices or apparatuses. For the purpose of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in combination with these instruction execution systems, devices or apparatuses. More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk box (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), a fiber optic device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium and then editing, interpreting or processing in other suitable ways if necessary, and then stored in a computer memory.

[0095] It should be understood that the various parts of the present disclosure can be implemented in hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented in software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used to implement: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0096] A person skilled in the art may understand that all or part of the steps in the method for implementing the above-mentioned embodiment may be completed by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, which, when executed, includes one or a combination of the steps of the method embodiment.

[0097] In addition, each functional unit in each embodiment of the present disclosure may be integrated into a processing module, or each unit may exist physically separately, or two or more units may be integrated into one module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it may also be stored in a computer-readable storage medium.

[0098] The storage medium mentioned above may be a read-only memory, a disk or an optical disk, etc. Although the embodiments of the present disclosure have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations of the present disclosure. A person of ordinary skill in the art may change, modify, replace and modify the above embodiments within the scope of the present disclosure.

Claims

1. A modular multi-level converter control method for a flexible DC system, characterized in that: include: Perform constant AC voltage control on the wind farm modular multilevel converter WFMMC in the flexible DC system to keep the AC side voltage of the wind farm in a stable state; The grid-connected modular multilevel converter GSMMC is controlled by constant DC voltage to keep the DC side voltage of the wind farm in a stable state; The actual voltage vector angle is detected based on a phase-locked loop, and a current source type cascade converter is added at the secondary voltage peak of the modular multi-level converter to suppress the double frequency circulating current component in the modular multi-level converter.

2. The method according to claim 1, characterized in that: The method of controlling the AC voltage of the wind farm modular multi-level converter WFMMC in the flexible direct current system to keep the AC side voltage of the wind farm in a stable state also includes: Obtaining a d-axis reference voltage value and a q-axis reference voltage value of the AC voltage of the wind farm collection system after dq-axis coordinate transformation; Obtain the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link, as well as the actual value of the d-axis current voltage and the actual value of the q-axis current; Based on the first formula, the first reference current value and the second reference current value of the MMC AC valve side are calculated according to the d-axis reference voltage value and the q-axis reference voltage value, the d-axis voltage actual value and the q-axis voltage actual value, the first outer loop control parameter, the second outer loop control parameter, the first inner loop control parameter and the second inner loop control parameter, Determine a first reference voltage value and a second reference voltage value on the MMC AC valve side based on the first reference current value and the second reference current value, the d-axis reference voltage value and the q-axis reference voltage value, the d-axis current and voltage actual value and the q-axis current actual value; Among them, the first formula is: Among them, Uadref_WFMMC is the d-axis reference voltage value, Uaqref_WFMMC is the q-axis reference voltage value, k pwd1 is the first outer loop control parameter, k pwq1 is the second outer loop control parameter, k iwd1 is the first inner loop control parameter, k iwq1 is the second inner loop control parameter, i adref_WFMMC is the first reference current value, i aqref_WFMMC is the second reference current value.

3. The method according to claim 2, characterized in that The obtaining of the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link, as well as the actual value of the d-axis current voltage and the actual value of the q-axis current, includes: Obtaining a first voltage delay parameter, a second voltage delay parameter, a first current delay parameter, and a second current delay parameter; Acquire a first actual current value and a second actual current value of a wind farm output current, and a first actual voltage value and a second actual voltage value of a wind farm output voltage; Based on the second formula, the first voltage delay parameter, the second voltage delay parameter, the first actual voltage value and the second actual voltage value, the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link are calculated, wherein the second formula is: Tud1 and Tuq1 are the first voltage delay parameter and the second voltage delay parameter, respectively; Uad_WFMMC and Uaq_WFMMC represent the first actual voltage value and the second actual voltage value output by the wind farm, respectively; U admWFMMC is the actual value of the d-axis voltage, U aqmWFMMC is the actual value of q-axis voltage; Based on the third formula, the first current delay parameter, the second current delay parameter, the first actual current value and the second actual current value, the d-axis current actual value and the q-axis current actual value after the delay link are calculated, wherein the third formula is: Among them, T id11 is the first current delay parameter, T iq11 is the second current delay parameter, I ad_WFMMC is the first actual current value, I aq_WFMMC is the second actual current value, I admWFMMC is the actual value of the d-axis current, I aqmWFMMC is the actual value of q-axis current.

4. The method according to claim 1, characterized in that: After the actual voltage vector angle is detected based on the phase-locked loop, the method further includes: Based on the actual voltage vector angle, a controller coordinate transformation angle is determined.

5. The method according to claim 1, characterized in that Also includes: A control signal of the modular multi-level converter is adjusted using a capacitor clamped sub-synchronous converter.

6. A modular multi-level converter control device for a flexible DC system, characterized in that: include: The first control module is used to perform constant AC voltage control on the wind farm modular multi-level converter WFMMC in the flexible direct current system to keep the AC side voltage of the wind farm in a stable state; The second control module is used to control the grid-connected modular multi-level converter GSMMC by adopting a constant DC voltage control to keep the DC side voltage of the wind farm in a stable state; The third control module is used to detect the actual voltage vector angle based on the phase-locked loop, and add a current source type cascade converter at the secondary voltage peak of the modular multi-level converter to suppress the double frequency circulating current component in the modular multi-level converter.

7. The device according to claim 6, characterized in that The first control module further includes: The first acquisition unit is used to acquire a d-axis reference voltage value and a q-axis reference voltage value after the AC voltage of the wind farm collection system is transformed by dq-axis coordinates; A second acquisition unit is used to acquire the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link, as well as the actual value of the d-axis current voltage and the actual value of the q-axis current; a calculation unit, configured to calculate a first reference current value and a second reference current value of the MMC AC valve side based on a first formula, according to the d-axis reference voltage value and the q-axis reference voltage value, the d-axis voltage actual value and the q-axis voltage actual value, a first outer loop control parameter, a second outer loop control parameter, a first inner loop control parameter, and a second inner loop control parameter, a determining unit, configured to determine a first reference voltage value and a second reference voltage value of an MMC AC valve side based on the first reference current value and the second reference current value, the d-axis reference voltage value and the q-axis reference voltage value, the d-axis current and voltage actual value, and the q-axis current actual value; Among them, the first formula is: Among them, Uadref_WFMMC is the d-axis reference voltage value, Uaqref_WFMMC is the q-axis reference voltage value, k pwd1 is the first outer loop control parameter, k pwq1 is the second outer loop control parameter, k iwd1 is the first inner loop control parameter, k iwq1 is the second inner loop control parameter, i adref_WFMMC is the first reference current value, i aqref_WFMMC is the second reference current value.

8. The device according to claim 7, characterized in that The second acquisition unit is specifically used to: Obtaining a first voltage delay parameter, a second voltage delay parameter, a first current delay parameter, and a second current delay parameter; Acquire a first actual current value and a second actual current value of a wind farm output current, and a first actual voltage value and a second actual voltage value of a wind farm output voltage; Based on the second formula, the first voltage delay parameter, the second voltage delay parameter, the first actual voltage value and the second actual voltage value, the actual value of the d-axis voltage and the actual value of the q-axis voltage after the delay link are calculated, wherein the second formula is: Tud1 and Tuq1 are the first voltage delay parameter and the second voltage delay parameter, respectively; Uad_WFMMC and Uaq_WFMMC represent the first actual voltage value and the second actual voltage value output by the wind farm, respectively; U admWFMMC is the actual value of the d-axis voltage, U aqmWFMMC is the actual value of q-axis voltage; Based on the third formula, the first current delay parameter, the second current delay parameter, the first actual current value and the second actual current value, the d-axis current actual value and the q-axis current actual value after the delay link are calculated, wherein the third formula is: Among them, T id11 is the first current delay parameter, T iq11 is the second current delay parameter, I ad_WFMMC is the first actual current value, I aq_WFMMC is the second actual current value, I admWFMMC is the actual value of the d-axis current, I aqmWFMMC is the actual value of q-axis current.

9. The device according to claim 6, characterized in that The third control module is further used for: Based on the actual voltage vector angle, a controller coordinate transformation angle is determined.

10. The device according to claim 6, characterized in that Also includes: The adjustment module is used to adjust the control signal of the modular multi-level converter by using the capacitor clamped sub-synchronous converter.

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