A multi-channel offshore wind power DC transmission system and control method thereof

Abstract

The present application discloses a multi-channel offshore wind power DC transmission system and its control method, comprising: first and second channel branches; the AC side of the unidirectional current-type MMC in the first channel branch is connected to the offshore wind farm through a transformer, the DC side is connected to the DC side of the inverter through a DC submarine cable, and the AC side of the inverter is connected to the AC grid through a double-winding transformer; the AC side of the half-bridge MMC in the second channel branch is connected to the offshore wind farm through a transformer, the DC side is connected to the DC side of the inverter through a DC submarine cable, and the AC side of the inverter is connected to the AC grid through a transformer; the first channel branch is connected to the second channel branch through a DC auxiliary communication channel. The present application solves the technical problems in the existing system that large-capacity diode rectifiers occupy large reactive capacity and harmonic compensation resources of the MMC, and that all wind turbines in the offshore wind farm must maintain synchronous operation, and that a failure at any point will affect the operation of the entire system.

Description

Technical Field

[0001] The present application relates to the field of offshore wind power technology, and in particular to a multi-channel offshore wind power direct current transmission system and a control method thereof. Background Art

[0002] Offshore wind energy resources are abundant, wind speeds are stable, and their distance from human settlements allows for larger turbines. Vigorously developing offshore wind power has become a crucial pillar for achieving carbon peak and carbon neutrality. To capture more wind energy, offshore wind power development is gradually moving deeper into the ocean, and ultra-large scale and low cost will be key features of future offshore wind power development.

[0003] Existing offshore wind power DC transmission systems primarily utilize conventional modular multilevel converters (MMCs). However, the low power density of conventional MMC converter valves results in large, heavy, and costly offshore converter platforms, posing significant challenges to their construction and transportation. For example, the offshore converter platform for the Rudong Offshore Wind Power Flexible Direct Current Project (1.1 GW) stands 44 meters tall and weighs 22,000 tons. Consequently, simply replicating several conventional MMC-based DC transmission schemes with capacities of 1-2 GW to achieve a combined transmission capacity of tens of millions of kilowatts would require independent projects, hindering the full utilization of the advantages of ultra-large-scale offshore wind power development and preventing reductions in overall construction costs.

[0004] In view of the fact that offshore wind farms mainly rely on unidirectional power transmission, unidirectional power converters such as diode rectifiers and unidirectional current-type MMCs are used to replace conventional half-bridge MMCs to achieve compactness and lightness of the converter. One type of solution is to use diode rectifiers; the other type of solution is to use Figure 1 The low-cost, compact unidirectional-current MMC (UC-MMC) shown in the figure. However, the power of a unidirectional power converter can only flow in one direction, from the AC side to the DC side, and from the sea to the shore. On the one hand, it is difficult for it to obtain energy from the DC side to achieve a black start for itself and the offshore wind farm. On the other hand, under zero output conditions of the wind farm, the offshore unidirectional current converter also has difficulty absorbing power from the DC side to balance system losses and maintain system operation.

[0005] Regarding the diode rectifier solution, in addition to common issues with unidirectional power converters, such as black start and zero-power operation, the diode rectifier lacks control capabilities and cannot establish grid-connected voltage for offshore AC wind farms. Consequently, the wind farm must utilize a large number of grid-connected wind turbines to jointly support the offshore wind farm's AC system. However, wind turbine grid control is still in the research stage and is still some distance away from large-scale deployment of grid-connected wind turbines. Ultra-large offshore wind farms may contain hundreds of wind turbines, and the performance and coordinated operation of such a large number of wind turbines in the grid require further research. Furthermore, during operation, the diode rectifier can introduce significant voltage harmonics and reactive power issues to offshore AC wind farms. The interaction between the wind farm and the diode rectifier further degrades power quality, potentially causing wind turbines to disconnect from the grid in severe cases.

[0006] like Figure 1 The low-cost, compact UC-MMC shown here features self-commutation capabilities, eliminating the need for grid-connected offshore wind turbines. It can output multiple power levels, making it more grid-friendly. Compared to diode rectification solutions, UC-MMC offers technical advantages. According to existing research, under equivalent capacity conditions, the UC-MMC converter valve can be approximately 40% smaller than conventional MMCs, significantly reducing costs and offering significant economic advantages. However, due to its unidirectional power characteristics, UC-MMCs also face common challenges with unidirectional power converters, such as black start and zero-output operation in offshore wind farms.

[0007] In the context of multiple transmission channels, existing technologies propose to use conventional MMCs and diode rectifiers in different transmission channels, while full-power AC channels are used to connect converter stations. During the startup phase, MMC can provide AC voltage and black start power for the entire wind farm; during normal operation, MMC can also achieve optimal power distribution by controlling AC voltage. Establishing a communication channel between converter stations allows different types of converters to cooperate with each other and effectively utilize the advantages of multi-channel transmission in ultra-large-scale offshore wind farms. However, for the solution of using AC communication between MMC and diode rectifiers, on the one hand, the large-capacity diode rectifier needs to occupy a large amount of reactive capacity and harmonic compensation resources of MMC; on the other hand, under the full-power AC communication mode, all wind turbines in the wind farm must maintain synchronous operation, and a failure at any point will affect the operation of the entire system, bringing certain challenges to system control. Summary of the Invention

[0008] The present application provides a multi-channel offshore wind power DC transmission system and a control method thereof, which are used to solve the technical problems in the existing multi-channel offshore wind power DC transmission system, such as the large-capacity diode rectifier occupying a large amount of reactive capacity and harmonic compensation resources of the MMC, and the fact that all wind turbines in the offshore wind farm must maintain synchronous operation and a failure at any point will affect the operation of the entire system.

[0009] In view of this, the first aspect of the present application provides a multi-channel offshore wind power DC transmission system, comprising: a first channel branch and a second channel branch; the first channel branch includes a first offshore wind farm, a three-winding transformer, a unidirectional current-type MMC, a first inverter provided on shore, a first two-winding transformer, and a first AC power grid;

[0010] The AC side of the unidirectional current-type MMC is connected to the first offshore wind farm through the three-winding transformer, the DC side is connected to the DC side of the first inverter through a DC submarine cable, and the AC side of the first inverter is connected to the first AC power grid through the first two-winding transformer;

[0011] The second channel branch includes a second offshore wind farm, a four-winding transformer, a half-bridge MMC, a second inverter arranged on shore, a second two-winding transformer and a second AC power grid;

[0012] The AC side of the half-bridge MMC is connected to the second offshore wind farm through the four-winding transformer, the DC side is connected to the DC side of the second inverter through a DC submarine cable, and the AC side of the second inverter is connected to the second AC power grid through the second two-winding transformer;

[0013] The first channel branch is connected to the second channel branch via a DC auxiliary communication channel, wherein the DC auxiliary communication channel includes a small-capacity MMC and a small-capacity diode rectifier; the small-capacity MMC and the small-capacity diode rectifier are respectively an MMC and a diode rectifier whose capacities are lower than a preset threshold;

[0014] The AC side of the small-capacity MMC is connected to the three-winding transformer, and the DC side is connected to the DC side of the small-capacity diode rectifier through a medium-voltage DC submarine cable. The AC side of the small-capacity diode rectifier is connected to the four windings.

[0015] Optionally, the number of the first channel branches is one or more; the number of the second channel branches is one or more.

[0016] Optionally, the two large-capacity windings in the four-winding transformer are respectively connected to the AC side of the half-bridge MMC and the second offshore wind farm, and the two small-capacity windings are respectively connected to one of the small-capacity diode rectifiers, wherein the capacity of the large-capacity winding is the same as the capacity of the connected offshore wind farm, and the capacity of the small-capacity winding is 5% to 10% of the capacity of the large-capacity winding.

[0017] Optionally, the two large-capacity windings in the three-winding transformer are respectively connected to the AC side of the unidirectional current-type MMC and the first offshore wind farm, and the small-capacity winding is connected to the small-capacity MMC.

[0018] A second aspect of the present application provides a control method for a multi-channel offshore wind power DC transmission system, which is applied to any of the multi-channel offshore wind power DC transmission systems described in the first aspect. The method includes:

[0019] Control the half-bridge MMC to charge from the DC side;

[0020] Controlling the AC voltage of the half-bridge MMC to be a rated value;

[0021] Control the small-capacity MMC of the DC auxiliary communication channel to charge from the DC side;

[0022] Controlling the AC voltage of the small-capacity MMC to a rated value;

[0023] Control unidirectional current type MMC to charge from the AC side;

[0024] Controlling the unidirectional current-type MMC and the small-capacity MMC to lock;

[0025] Controlling the AC voltage of the unidirectional current-type MMC to a rated value, and controlling the active power through the small-capacity MMC;

[0026] The wind turbine converter charging of the offshore wind farm is started and the system enters a steady-state operation state.

[0027] Optionally, control the half-bridge MMC to charge from the DC side, including:

[0028] In the uncontrolled charging stage, the half-bridge MMC is controlled to be locked, so that the power supply charges the half-bridge MMC through resistor current limiting;

[0029] After the uncontrolled charging stage is completed, the half-bridge MMC is unlocked for controlled charging, and the capacitor voltage of the half-bridge MMC is charged to the rated value.

[0030] Optionally, controlling the AC voltage of the half-bridge MMC to be a rated value includes:

[0031] The three-phase AC rated value of the half-bridge MMC is subtracted from the actual value of the three-phase AC voltage and the result is input into a proportional resonant controller, and the output result of the proportional resonant controller is added to the three-phase AC reference value to obtain an AC intermediate control voltage;

[0032] Performing voltage linear conversion on the AC intermediate control voltage of the half-bridge MMC and the corresponding DC intermediate control voltage to obtain a bridge arm reference voltage; the DC intermediate control voltage of the half-bridge MMC is obtained by subtracting the average value of the capacitor voltage reference value and the actual capacitor voltage value and inputting the result into a proportional-integral controller;

[0033] The bridge arm reference voltage is input to the valve-level controller to generate a switching signal for the IGBT device, thereby controlling the switching of the submodule.

[0034] Optionally, when the system enters a steady-state operation state, the AC side control process of the small-capacity MMC includes:

[0035] Detecting the output of offshore wind farms;

[0036] If the output of the offshore wind farm is greater than the target value, the small-capacity MMC is controlled to be locked;

[0037] If the output of the offshore wind farm is less than the target value, the power of the small-capacity MMC is controlled to be the rated power.

[0038] Optionally, controlling the power of the small-capacity MMC to be rated power includes:

[0039] The difference between the rated power and the actual power of the small-capacity MMC is input into a first proportional-integral controller to obtain a d-axis current reference value;

[0040] The d-axis current reference value is subtracted from the d-axis current actual value and the resultant value is input into a second proportional-integral controller, and the output of the second proportional-integral controller is added to the d-axis voltage and the dq-axis coupling term to obtain a d-axis intermediate control voltage;

[0041] The difference between the q-axis current reference value and the q-axis current actual value is input into a third proportional-integral controller, and the output of the third proportional-integral controller is added to the q-axis voltage and the dq-axis coupling term to obtain the q-axis intermediate control voltage;

[0042] Inputting the d-axis intermediate control voltage and the q-axis intermediate control voltage into a dq to abc conversion module to obtain an AC intermediate control voltage in an abc coordinate system;

[0043] Performing a voltage linear transformation on the AC intermediate control voltage and the corresponding DC intermediate control voltage in the abc coordinate system to obtain a bridge arm reference voltage;

[0044] The bridge arm reference voltage is input to the valve-level controller to generate a switching signal for the IGBT device, thereby controlling the switching of the submodule.

[0045] Optionally, the control acquisition process of the DC intermediate control voltage of the small-capacity MMC includes:

[0046] The difference between the capacitor voltage reference value of the small-capacity MMC and the average value of the capacitor voltage actual value is input into the fourth proportional-integral controller to obtain the DC intermediate control voltage of the small-capacity MMC.

[0047] It can be seen from the above technical solutions that this application has the following advantages:

[0048] The multi-channel offshore wind power DC transmission system provided by this application does not use large-capacity diode rectifiers, thus avoiding the problem of large-capacity diode rectifiers occupying large reactive capacity and harmonic compensation resources of the MMC. In this application, the first channel branch is connected to the second channel branch via a DC auxiliary communication channel, and the small-capacity MMC is connected to the DC side of the small-capacity diode rectifier via a medium-voltage DC submarine cable. This solves the technical problem that all wind turbines in an offshore wind farm must maintain synchronous operation when using AC submarine cables for channel connection, and a failure at any point will affect the operation of the entire system.

[0049] Furthermore, the present application establishes a small-capacity DC auxiliary communication channel between the half-bridge MMC offshore platform and two unidirectional MMC offshore converter platforms, using the bidirectional power capability of the half-bridge MMC to solve the problems of the unidirectional current-type MMC being difficult to black-start from the DC side and difficult to operate reliably when the wind farm is at zero power. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0051] Figure 1 A unidirectional current-mode MMC and two unidirectional current-mode sub-modules (UC-FBSM and UC-CDSM) provided in the embodiments of the present application;

[0052] Figure 2 A schematic structural diagram of a three-channel offshore wind power DC transmission system provided in an embodiment of the present application;

[0053] Figure 3The AC controller of the half-bridge MMC and unidirectional current-type MMC provided in the embodiments of the present application in steady-state operation;

[0054] Figure 4 A DC controller for the half-bridge MMC and unidirectional current-type MMC provided in the embodiments of the present application in a steady-state operation state;

[0055] Figure 5 This is a block diagram of the AC side control of a small-capacity MMC provided in an embodiment of the present application under the condition that the output of an offshore wind farm is less than the target value. DETAILED DESCRIPTION

[0056] In order to help those skilled in the art better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of this application.

[0057] In a first aspect, the present application provides a multi-channel offshore wind power DC transmission system, comprising: a first channel branch and a second channel branch; the first channel branch comprises a first offshore wind farm, a three-winding transformer, a unidirectional current-type MMC, a first inverter disposed on shore, a first two-winding transformer, and a first AC power grid;

[0058] The AC side of the unidirectional current-type MMC is connected to the first offshore wind farm through a three-winding transformer, the DC side is connected to the DC side of the first inverter through a DC submarine cable, and the AC side of the first inverter is connected to the first AC power grid through a first two-winding transformer;

[0059] The second channel branch includes a second offshore wind farm, a four-winding transformer, a half-bridge MMC, a second inverter arranged on shore, a second two-winding transformer and a second AC power grid;

[0060] The AC side of the half-bridge MMC is connected to the second offshore wind farm through a four-winding transformer, the DC side is connected to the DC side of the second inverter through a DC submarine cable, and the AC side of the second inverter is connected to the second AC grid through a second two-winding transformer;

[0061] Each first channel branch is connected to the second channel branch via a DC auxiliary communication channel, wherein the DC auxiliary communication channel includes a small-capacity MMC and a small-capacity diode rectifier; the small-capacity MMC and the small-capacity diode rectifier are respectively an MMC and a diode rectifier whose capacity is lower than a preset threshold;

[0062] The AC side of the small-capacity MMC is connected to a three-winding transformer, and the DC side is connected to the DC side of a small-capacity diode rectifier through a medium-voltage DC submarine cable. The AC side of the small-capacity diode rectifier is connected to the four-winding.

[0063] In an embodiment of the present application, the number of channels of the multi-channel offshore wind power DC transmission system can be 2 or more, that is, the number of first channel branches and second channel branches can be 1 or more respectively, and the number of multiple in the embodiment of the present application is 2 or more.

[0064] Please refer to Figure 2 A three-channel offshore wind power DC transmission system is provided. One offshore channel uses a conventional half-bridge MMC, which offers bidirectional power flow capability and has its AC side connected to an offshore wind farm. The other two offshore channels use unidirectional current-type MMCs, which offer advantages such as compact size, low cost, and high power density. Each AC side is connected to an offshore wind farm. To address the difficulties of unidirectional current-type MMCs in black starting from the DC side and in reliable operation when the wind farm is at zero power, a small-capacity DC auxiliary communication channel is established between the half-bridge MMC offshore platform and the two unidirectional current-type MMC offshore converter platforms. The bidirectional power capability of the half-bridge MMC is used to address the issues of the unidirectional current-type MMC. The capacity of the small-capacity DC auxiliary communication channel is 5% to 10% of that of the half-bridge MMC.

[0065] Since the power of the small-capacity DC auxiliary communication channel can only flow from the half-bridge MMC to the unidirectional current-type MMC, a small-capacity diode rectifier can be directly used on the half-bridge MMC side, while a small-capacity MMC can be used on the unidirectional current-type MMC side. Adding a small-capacity DC channel between stations will not significantly affect system costs.

[0066] The AC side of the half-bridge MMC is connected to a four-winding transformer. Two of the four-winding transformer's windings have large capacities, matching the capacity of the offshore wind farm it connects to. The other two windings have small capacities, each 5% to 10% of the large capacity. The half-bridge MMC connects to one large-capacity winding, while the other large-capacity winding connects to an offshore wind farm. The DC side of the half-bridge MMC is connected to the DC side of an onshore inverter via a DC submarine cable. The AC side of the onshore inverter is connected to the AC grid via a two-winding transformer.

[0067] The AC side of the unidirectional current-type MMC is connected to a three-winding transformer. Two of the three-winding transformer's windings have large capacities, the larger of which matches the capacity of the offshore wind farm it connects to. The other winding has a smaller capacity, which is 5% to 10% of the larger one. The unidirectional current-type MMC is connected to one large-capacity winding, and the other large-capacity winding is connected to an offshore wind farm. The DC side of the unidirectional current-type MMC is connected to the DC side of an onshore inverter via a DC submarine cable. The AC side of the onshore inverter is connected to the AC grid via a two-winding transformer.

[0068] The small-capacity winding in the three-winding transformer connected to the unidirectional current-type MMC is connected to the AC side of a small-capacity MMC. The DC side of the small-capacity MMC is connected to the DC side of a small-capacity diode rectifier through a medium-voltage DC submarine cable. The AC side of the small-capacity diode rectifier is connected to a small-capacity winding of the four-winding transformer connected to the half-bridge MMC.

[0069] The multi-channel offshore wind power DC transmission system in the present application does not use large-capacity diode rectifiers, avoiding the problem of large-capacity diode rectifiers occupying large reactive capacity and harmonic compensation resources of MMC; in the present application, the first channel branch is connected to the second channel branch through a DC auxiliary communication channel, and the small-capacity MMC is connected to the DC side of the small-capacity diode rectifier through a medium-voltage DC submarine cable, solving the technical problem in the prior art of using AC submarine cables for channel connection that all wind turbines in the offshore wind farm must maintain synchronous operation, and a failure at any point will affect the operation of the entire system.

[0070] A second aspect of the present application provides a control method for a multi-channel offshore wind power DC transmission system, which is applied to the aforementioned multi-channel offshore wind power DC transmission system. The control method includes:

[0071] S1 controls the half-bridge MMC to charge from the DC side.

[0072] The charging process is divided into two stages: uncontrolled charging and controlled charging. In the uncontrolled charging stage, the half-bridge MMC is controlled to be locked, and the power supply charges the half-bridge MMC through resistor current limiting.

[0073] After the uncontrolled charging stage is completed, the half-bridge MMC is unlocked for controlled charging, and the capacitor voltage of the half-bridge MMC is charged to the rated value.

[0074] S2 controls the AC voltage of the half-bridge MMC to be the rated value.

[0075] The AC voltage controller is used to control the AC voltage of the half-bridge MMC to the rated value. Figure 3As shown. The three-phase AC rated value of the half-bridge MMC is subtracted from the actual three-phase AC voltage value and then input into the proportional resonant controller. The output of the proportional resonant controller is superimposed on the three-phase AC reference value to obtain the AC intermediate control voltage. The AC intermediate control voltage of the half-bridge MMC or small-capacity MMC and the corresponding DC intermediate control voltage are linearly converted to obtain the bridge arm reference voltage. The bridge arm voltage is input into the valve-level controller to generate the switching signal of the IGBT device, thereby controlling the switching of the submodule. Among them, the DC intermediate control voltage of the half-bridge MMC is obtained by subtracting the average value of the capacitor voltage reference value and the actual capacitor voltage value and then inputting it into the proportional-integral controller.

[0076] S3, controls the small-capacity MMC of the DC auxiliary communication channel to charge from the DC side.

[0077] The DC side charging process of the small-capacity MMC is the same as that of the half-bridge MMC, and will not be described in detail here.

[0078] S4. Control the AC voltage of the small-capacity MMC to the rated value.

[0079] The controller used to control the AC voltage of small capacity MMC to the rated value is Figure 3 same.

[0080] S5. Control the unidirectional current type MMC to charge from the AC side.

[0081] The first stage is uncontrolled charging using the AC side charging resistor. The second stage is to actively bypass some bridge arm submodules to continue charging to the rated value.

[0082] S6, controls the locking of unidirectional current type MMC and small capacity MMC.

[0083] Since the control modes of the unidirectional current type MMC and the small capacity MMC are different from those of the system startup process during normal operation, both the unidirectional current type MMC and the small capacity MMC are locked in this step.

[0084] S7. Control the AC voltage of the unidirectional current type MMC to the rated value and control the active power through the small-capacity MMC.

[0085] The AC voltage of the unidirectional current type MMC is controlled to be the rated value. At the same time, the small-capacity MMC controls the active power to maintain the minimum power requirement of the unidirectional current type MMC.

[0086] S8. Start charging the wind turbine converter of the offshore wind farm, and the system enters a steady-state operation state.

[0087] AC controllers of half-bridge MMC and unidirectional current type MMC in steady state operation are as follows Figure 3When the system enters the steady-state operation state, the AC side control process of the half-bridge MMC or unidirectional current-type MMC includes:

[0088] The three-phase AC reference value u of the half-bridge MMC or unidirectional current type MMC aref , u bref , u cref The actual value of the three-phase AC voltage u a , u b , u c After the difference is made, it is input into the proportional-resonant (PR) controller, and the output result of the proportional resonant controller is superimposed on the three-phase AC reference value u aref , u bref , u cref , get the AC intermediate control voltage e a , e b , e c .

[0089] The DC controller of half-bridge MMC and unidirectional current type MMC in steady state operation is as follows Figure 4 When the system enters the steady-state operation state, the DC side control process of the half-bridge MMC or unidirectional current-type MMC includes:

[0090] The capacitor voltage reference value u of the half-bridge MMC or unidirectional current type MMC capref The average value of the actual value of the capacitor voltage u cap After taking the difference, it is input into the proportional-integral (PI) controller to obtain the DC intermediate control voltage e of the half-bridge MMC or unidirectional current-type MMC. dc .

[0091] After obtaining the AC intermediate control voltage and the DC intermediate control voltage, the six bridge arm reference voltages are obtained through voltage linear transformation:

[0092]

[0093] Where x = a, b, c;

[0094] The six bridge arm reference voltages are input to the valve-level controller to generate switching signals for the IGBT devices, thereby controlling the switching of the submodules.

[0095] The main function of the small-capacity MMC in steady state is to provide the minimum operating power for the unidirectional current type MMC. That is, when the wind farm output is close to 0, the small-capacity MMC provides a certain circulating power to ensure the reliable operation of the unidirectional current type MMC. Specifically, when the system enters the steady-state operation state, the AC side control process of the small-capacity MMC includes:

[0096] Detecting the output of offshore wind farms;

[0097] If the output of the offshore wind farm is greater than the target value, the small-capacity MMC is controlled to be locked and no power needs to be transmitted;

[0098] If the output of the offshore wind farm is less than the target value, the power of the small-capacity MMC is controlled to be the rated power. The specific control block diagram is as follows: Figure 5 As shown, the rated power P of the small capacity MMC auxN With actual power P aux After making the difference, it is input into the first proportional integral controller to obtain the d-axis current reference value i dref ; Set the d-axis current reference value i dref The actual value of the d-axis current i d After the difference is made, it is input to the second proportional integral controller, and the output of the second proportional integral controller is compared with the d-axis voltage u d , dq axis coupling term ωL ac i q Add ( is the system rated angular frequency, L ac is the AC equivalent inductance value, i q is the actual value of the q-axis current), and the d-axis intermediate control voltage e is obtained. d ; For the q-axis, since this MMC does not actively generate reactive power, its q-axis current reference value is 0; compare the q-axis current reference value with the q-axis current actual value i q After the difference is made, it is input to the third proportional integral controller, and the output of the third proportional integral controller is compared with the q-axis voltage u q , dq axis coupling term ωL ac i d Add together to get the q-axis intermediate control voltage e q Input the d-axis intermediate control voltage and the q-axis intermediate control voltage into the dq to abc module to obtain the AC intermediate control voltage in the abc coordinate system;

[0099] The AC intermediate control voltage and the corresponding DC intermediate control voltage in the abc coordinate system are linearly transformed to obtain the six bridge arm reference voltages; the DC intermediate control voltage of the small-capacity MMC is obtained by taking the difference between the capacitor voltage reference value of the small-capacity MMC and the average value of the capacitor voltage actual value and inputting the difference into the fourth proportional-integral controller;

[0100] The six bridge arm reference voltages are input to the valve-level controller to generate switching signals for the IGBT devices, thereby controlling the switching of the submodules.

[0101] The control method of the multi-channel offshore wind power DC transmission system provided in this application can solve the problem that unidirectional current type MMC is difficult to achieve black start by charging from the DC side; the steady-state control method provided can solve the problem that unidirectional current type MMC is difficult to operate reliably under zero power conditions in wind farms.

[0102] In the specification of this application and the above-mentioned drawings, the terms "first," "second," "third," "fourth," etc. (if any) are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. In addition, the terms "including" and "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus that includes a series of steps or elements is not necessarily limited to those steps or elements explicitly listed, but may include other steps or elements not explicitly listed or inherent to such process, method, product, or apparatus.

[0103] It should be understood that in this application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships can exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the previous and next associated objects are in an "or" relationship. "At least one of the following items" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or plural.

[0104] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are merely schematic. For example, the division of the units is merely a logical function division. In actual implementation, there may be other division methods, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0105] The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of these units may be selected to achieve the purpose of this embodiment according to actual needs.

[0106] In addition, the functional units in the various embodiments of the present application may be integrated into a single processing unit, or each unit may exist physically separately, or two or more units may be integrated into a single unit. The aforementioned integrated units may be implemented in the form of hardware or software functional units.

[0107] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the method described in each embodiment of the present application through a computer device (which can be a personal computer, server, or network device, etc.). The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (full name: Read-Only Memory, English abbreviation: ROM), random access memory (full name: Random Access Memory, English abbreviation: RAM), disk or optical disk, and other media that can store program code.

[0108] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A multi-channel offshore wind power DC transmission system, characterized in that: include: A first channel branch and a second channel branch; the first channel branch includes a first offshore wind farm, a three-winding transformer, a unidirectional current-type MMC, a first inverter arranged on shore, a first two-winding transformer and a first AC power grid; The AC side of the unidirectional current-type MMC is connected to the first offshore wind farm through the three-winding transformer, the DC side is connected to the DC side of the first inverter through a DC submarine cable, and the AC side of the first inverter is connected to the first AC power grid through the first two-winding transformer; The second channel branch includes a second offshore wind farm, a four-winding transformer, a half-bridge MMC, a second inverter arranged on shore, a second two-winding transformer and a second AC power grid; The AC side of the half-bridge MMC is connected to the second offshore wind farm through the four-winding transformer, the DC side is connected to the DC side of the second inverter through a DC submarine cable, and the AC side of the second inverter is connected to the second AC power grid through the second two-winding transformer; The first channel branch is connected to the second channel branch via a DC auxiliary communication channel, wherein the DC auxiliary communication channel includes a small-capacity MMC and a small-capacity diode rectifier; the small-capacity MMC and the small-capacity diode rectifier are respectively an MMC and a diode rectifier whose capacities are lower than a preset threshold; The AC side of the small-capacity MMC is connected to the three-winding transformer, and the DC side is connected to the DC side of the small-capacity diode rectifier through a medium-voltage DC submarine cable. The AC side of the small-capacity diode rectifier is connected to the four windings.

2. The multi-channel offshore wind power DC transmission system according to claim 1, characterized in that: The number of the first channel branches is one or more; the number of the second channel branches is one or more.

3. The multi-channel offshore wind power DC transmission system according to claim 1, characterized in that: The two large-capacity windings in the four-winding transformer are respectively connected to the AC side of the half-bridge MMC and the second offshore wind farm, and the two small-capacity windings are respectively connected to one of the small-capacity diode rectifiers. The capacity of the large-capacity winding is the same as the capacity of the connected offshore wind farm, and the capacity of the small-capacity winding is 5% to 10% of the capacity of the large-capacity winding.

4. The multi-channel offshore wind power DC transmission system according to claim 3, characterized in that: The two large-capacity windings in the three-winding transformer are respectively connected to the AC side of the unidirectional current-type MMC and the first offshore wind farm, and the small-capacity winding is connected to the small-capacity MMC.

5. A control method for a multi-channel offshore wind power DC transmission system, characterized in that: The method applied to the multi-channel offshore wind power DC transmission system according to any one of claims 1 to 4 comprises: Control the half-bridge MMC to charge from the DC side; Controlling the AC voltage of the half-bridge MMC to be a rated value; Control the small-capacity MMC of the DC auxiliary communication channel to charge from the DC side; Controlling the AC voltage of the small-capacity MMC to a rated value; Control unidirectional current type MMC to charge from the AC side; Controlling the unidirectional current-type MMC and the small-capacity MMC to lock; Controlling the AC voltage of the unidirectional current-type MMC to a rated value, and controlling the active power through the small-capacity MMC; The wind turbine converter charging of the offshore wind farm is started and the system enters a steady-state operation state.

6. The control method for a multi-channel offshore wind power DC transmission system according to claim 5, characterized in that: Control the half-bridge MMC to charge from the DC side, including: In the uncontrolled charging stage, the half-bridge MMC is controlled to be locked, so that the power supply charges the half-bridge MMC through resistor current limiting; After the uncontrolled charging stage is completed, the half-bridge MMC is unlocked for controlled charging, and the capacitor voltage of the half-bridge MMC is charged to the rated value.

7. The control method for a multi-channel offshore wind power DC transmission system according to claim 5, characterized in that: Controlling the AC voltage of the half-bridge MMC to a rated value includes: The three-phase AC rated value of the half-bridge MMC is subtracted from the actual value of the three-phase AC voltage and the result is input into a proportional resonant controller, and the output result of the proportional resonant controller is added to the three-phase AC reference value to obtain an AC intermediate control voltage; Performing voltage linear conversion on the AC intermediate control voltage of the half-bridge MMC and the corresponding DC intermediate control voltage to obtain a bridge arm reference voltage; the DC intermediate control voltage of the half-bridge MMC is obtained by subtracting the average value of the capacitor voltage reference value and the actual capacitor voltage value and inputting the result into a proportional-integral controller; The bridge arm reference voltage is input to the valve-level controller to generate a switching signal for the IGBT device, thereby controlling the switching of the submodule.

8. The control method for a multi-channel offshore wind power DC transmission system according to claim 5, characterized in that: When the system enters a steady-state operation state, the AC side control process of the small-capacity MMC includes: Detecting the output of offshore wind farms; If the output of the offshore wind farm is greater than the target value, the small-capacity MMC is controlled to be locked; If the output of the offshore wind farm is less than the target value, the power of the small-capacity MMC is controlled to be the rated power.

9. The control method for a multi-channel offshore wind power DC transmission system according to claim 8, characterized in that: Controlling the power of the small-capacity MMC to the rated power includes: The difference between the rated power and the actual power of the small-capacity MMC is input into a first proportional-integral controller to obtain a d-axis current reference value; The d-axis current reference value is subtracted from the d-axis current actual value and the resultant value is input into a second proportional-integral controller, and the output of the second proportional-integral controller is added to the d-axis voltage and the dq-axis coupling term to obtain a d-axis intermediate control voltage; The difference between the q-axis current reference value and the q-axis current actual value is input into a third proportional-integral controller, and the output of the third proportional-integral controller is added to the q-axis voltage and the dq-axis coupling term to obtain the q-axis intermediate control voltage; Inputting the d-axis intermediate control voltage and the q-axis intermediate control voltage into a dq to abc conversion module to obtain an AC intermediate control voltage in an abc coordinate system; Performing a voltage linear transformation on the AC intermediate control voltage and the corresponding DC intermediate control voltage in the abc coordinate system to obtain a bridge arm reference voltage; The bridge arm reference voltage is input to the valve-level controller to generate a switching signal for the IGBT device, thereby controlling the switching of the submodule.

10. The control method for a multi-channel offshore wind power DC transmission system according to claim 9, characterized in that: The control acquisition process of the DC intermediate control voltage of the small-capacity MMC includes: The difference between the capacitor voltage reference value of the small-capacity MMC and the average value of the capacitor voltage actual value is input into the fourth proportional-integral controller to obtain the DC intermediate control voltage of the small-capacity MMC.

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