Multi-channel offshore wind power direct current sending-out system and control method thereof

Abstract

The invention discloses a multi-channel offshore wind power direct current sending-out system and a control method thereof. The multi-channel offshore wind power direct current sending-out system comprises a first channel branch and a second channel branch; the alternating current side of the unidirectional current type MMC in the first channel branch is connected with an offshore wind plant through a transformer, the direct current side is connected with the direct current side of an inverter through a direct current submarine cable, and the alternating current side of the inverter is connected with an alternating current power grid through a double-winding transformer; the alternating-current side of the half-bridge MMC in the second channel branch is connected with the offshore wind plant through a transformer, the direct-current side of the half-bridge MMC is connected with the direct-current side of an inverter through a direct-current submarine cable, and the alternating-current side of the inverter is connected with an alternating-current power grid through a transformer; and the first channel branch is connected with the second channel branch through the direct current auxiliary contact channel. The technical problems that in an existing system, a large-capacity diode rectifier occupies large reactive power capacity and harmonic compensation resources of the MMC, all fans in an offshore wind power plant must keep synchronous operation, and operation of the whole system can be affected when faults occur at any point are solved.

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 good, wind speeds are stable, and they are far from human settlements, allowing for large-scale wind turbines. Vigorously developing offshore wind power has gradually become an important support for achieving carbon peak and carbon neutrality. In order to obtain more wind energy, offshore wind power development is gradually moving towards the deep sea, and ultra-large scale and low cost will be important features of future offshore wind power development.

[0003] The existing offshore wind power DC transmission system mainly uses conventional modular multilevel converters (MMC). However, the conventional MMC converter valve has low power density, resulting in large size and weight of the offshore converter platform, high cost, and great challenges to the construction and transportation of the platform. For example, the offshore converter platform of the Rudong offshore wind power flexible direct current project (1.1GW) has reached a height of 44 meters and weighs 22,000 tons. Therefore, by simply copying several DC transmission schemes based on conventional MMC with a capacity of 1-2GW, the overall transmission capacity reaches tens of millions of kilowatts. Each project is independent of each other, making it difficult to fully utilize the advantages of ultra-large-scale development of offshore wind power and reduce the overall construction cost.

[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 MMCs are used to replace conventional half-bridge MMCs to achieve compactness and lightness of the converter. One solution is to use a diode rectifier; the other is to use Figure 1 The low-cost and compact unidirectional current MMC (UC-MMC) is shown in Figure 1. However, the power of the unidirectional power converter can only flow 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 the black start of itself and the offshore wind farm; on the other hand, under the condition of zero output of the wind farm, it is also difficult for the offshore unidirectional current converter to absorb power from the DC side to balance the system loss and maintain system operation.

[0005] Regarding the diode rectifier solution, in addition to the common problems of unidirectional power converters such as black start and zero power operation, since the diode rectifier has no control capability and cannot establish grid-connected voltage for offshore AC wind farms, wind farms must also use a large number of grid-connected wind turbines to jointly support the AC system of offshore wind farms. However, wind turbine grid control is still in the research stage and is still a long way from the large-scale application of grid-connected wind turbines; ultra-large-scale offshore wind farms may contain hundreds of wind turbines, and the performance and coordinated operation capabilities of such a large number of wind turbines in the grid still need further research. In addition, during operation, the diode rectifier will also bring relatively serious voltage harmonics and reactive power problems to the offshore AC wind farm, and the interaction between the wind farm and the diode rectifier will further deteriorate the power quality, which may cause the wind turbine to be disconnected from the grid in severe cases.

[0006] like Figure 1 The low-cost compact UC-MMC shown has self-commutation capability, and does not require grid-type wind turbines at sea. It can output multiple levels and has more friendly grid-connected characteristics. Compared with the diode rectification solution, UC-MMC has more technical advantages. According to existing research, under the same capacity conditions, the volume of the UC-MMC converter valve can be reduced by about 40% compared with the conventional MMC, and the cost can also be greatly reduced, which has a strong economic advantage. However, due to the unidirectional power characteristics, UC-MMC also has common problems of unidirectional power converters such as black start and zero output operation of offshore wind farms.

[0007] In the context of multiple transmission channels, the existing technology proposes to use conventional MMC and diode rectifiers in different transmission channels, while the converter stations are connected by full-power AC channels. 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, large-capacity diode rectifiers need to occupy a large 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, that is, large-capacity diode rectifiers occupy large reactive capacity and harmonic compensation resources of MMC, and 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 comprises a first offshore wind farm, a three-winding transformer, a unidirectional current-type MMC, a first inverter arranged on shore, a first double-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 double-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 through a DC auxiliary communication channel, and 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;

[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, and 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 1 or more; the number of the second channel branches is 1 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, and 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 be 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 over, the half-bridge MMC is unlocked for controlled charging, and the capacitor voltage of the half-bridge MMC is charged to a rated value.

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

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

[0032] The AC intermediate control voltage of the half-bridge MMC and the corresponding DC intermediate control voltage are subjected to voltage linear transformation to obtain a bridge arm reference voltage; the DC intermediate control voltage of the half-bridge MMC is obtained by inputting the difference between the capacitor voltage reference value and the average value of the capacitor voltage actual value 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 the rated power includes:

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

[0040] The d-axis current reference value and the d-axis current actual value are input into a second proportional-integral controller after the difference is calculated, 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 q-axis current reference value and the q-axis current actual value are input into a third proportional-integral controller after the difference is calculated, 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] Input 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 the present application does not use a large-capacity diode rectifier, thereby avoiding the problem that the large-capacity diode rectifier occupies a large reactive capacity and harmonic compensation resource of the 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, thereby solving the technical problem that all wind turbines in the offshore wind farm must maintain synchronous operation when using AC submarine cables for channel connection in the prior art, 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, and uses 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 drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 creative work.

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

[0052] Figure 2 A schematic diagram of the structure 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 the unidirectional current-type MMC provided in the embodiments of the present application in a steady-state operation state;

[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 The embodiment of the present application provides a control block diagram of the AC side of a small-capacity MMC under the condition that the output of an offshore wind farm is less than the target value. DETAILED DESCRIPTION

[0056] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field 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 arranged on shore, a first double-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 double-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 double-winding transformer;

[0061] Each first channel branch is connected to the second channel branch through a DC auxiliary communication channel, and 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 a 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 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, wherein one offshore channel adopts a conventional half-bridge MMC, which has the ability of bidirectional power flow, and its AC side is connected to an offshore wind farm; the other two offshore channels adopt unidirectional current-type MMC, which has the advantages of small size, low cost, high power density, etc., and its AC side is respectively connected to an offshore wind farm. In order to solve the problems that the unidirectional current-type MMC is difficult to black-start from the DC side and difficult to operate reliably 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, and the bidirectional power capacity of the half-bridge MMC is used to solve the problems of the unidirectional current-type MMC; wherein the capacity of the small-capacity DC auxiliary communication channel is 5% to 10% of the capacity 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 the system cost;

[0066] The AC side of the half-bridge MMC is connected to a four-winding transformer. In the four-winding transformer, two windings have large capacity, and the capacity of the large-capacity winding is the same as the capacity of the offshore wind farm it is connected to; the capacity of the other two windings is small, and the capacity of the small-capacity winding is 5%~10% of the capacity of the large-capacity winding. The half-bridge MMC is connected to a large-capacity winding, and the other large-capacity winding is connected to an offshore wind farm. The DC side of the half-bridge MMC is connected to the DC side of the inverter on the shore through a DC submarine cable, and the AC side of the inverter on the shore is connected to the AC grid through a double-winding transformer.

[0067] The AC side of the unidirectional current type MMC is connected to a three-winding transformer. In the three-winding transformer, two windings have large capacity, and the capacity of the large-capacity winding is the same as the capacity of the offshore wind farm it is connected to; the capacity of the other winding is small, and the capacity of the small-capacity winding is 5%~10% of the capacity of the large-capacity winding. The unidirectional current type MMC is connected to a 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 the inverter on the shore through a DC submarine cable, and the AC side of the inverter on the shore is connected to the AC grid through a double-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, and 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, which solves the technical problem that all wind turbines in the offshore wind farm must maintain synchronous operation when using AC submarine cables for channel connection in the prior art, 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, so that the power supply charges the half-bridge MMC through resistor current limiting.

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

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

[0075] The AC voltage of the half-bridge MMC is controlled to be the rated value by using an AC voltage controller. Figure 3As shown. The three-phase AC rated value of the half-bridge MMC is input into the proportional resonant controller after the difference between the three-phase AC voltage actual value, and the output result 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 the small-capacity MMC and the corresponding DC intermediate control voltage are subjected to voltage linear transformation 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 inputting the difference between the capacitor voltage reference value and the average value of the capacitor voltage actual value 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, controlling the unidirectional current type MMC to charge from the AC side.

[0081] The first stage is to use the AC side charging resistor for uncontrolled charging. The second stage is to actively bypass a part of the bridge arm submodule to achieve continued charging to the rated value.

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

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

[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] The AC controller of half-bridge MMC and unidirectional current type MMC in steady-state operation is 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, 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 working 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 the difference is made, 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 subjected to voltage linear transformation to obtain 6 bridge arm reference voltages; the DC intermediate control voltage of the small-capacity MMC is obtained by inputting the difference between the capacitor voltage reference value of the small-capacity MMC and the average value of the capacitor voltage actual value 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 the present application can solve the problem that the unidirectional current type MMC is difficult to charge from the DC side to achieve black start; the steady-state control method provided can solve the problem that the unidirectional current type MMC is difficult to operate reliably under zero power conditions in the wind farm.

[0102] The terms "first", "second", "third", "fourth", etc. (if any) in the specification of this application and the above-mentioned drawings 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 device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[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 may 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 objects associated before and after are in an "or" relationship. "At least one of the following" 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 multiple.

[0104] In the several embodiments provided in the present 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 only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, 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 separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0106] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of 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 is essentially 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, including a number of 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 in English: Read-Only Memory, English abbreviation: ROM), random access memory (full name in English: Random Access Memory, English abbreviation: RAM), disk or optical disk and other media that can store program codes.

[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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned 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 double-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 double-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 through a DC auxiliary communication channel, and 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; 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, and 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 is 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 is 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, 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.

4. The multi-channel offshore wind power DC transmission system according to claim 3 is 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 be 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 of the multi-channel offshore wind power DC transmission system according to claim 5 is 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 over, the half-bridge MMC is unlocked for controlled charging, and the capacitor voltage of the half-bridge MMC is charged to a rated value.

7. The control method of the 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 rated value of the three-phase AC power of the half-bridge MMC is subtracted from the actual value of the three-phase AC voltage and then inputted into a proportional resonant controller, and the output result of the proportional resonant controller is superimposed on the three-phase AC power reference value to obtain an AC intermediate control voltage; The AC intermediate control voltage of the half-bridge MMC and the corresponding DC intermediate control voltage are subjected to voltage linear transformation to obtain a bridge arm reference voltage; the DC intermediate control voltage of the half-bridge MMC is obtained by inputting the difference between the capacitor voltage reference value and the average value of the capacitor voltage actual value 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 of the 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 of the multi-channel offshore wind power DC transmission system according to claim 8, characterized in that: Controlling the power of the small-capacity MMC to be the rated power includes: The rated power and the actual power of the small-capacity MMC are subtracted and input into a first proportional-integral controller to obtain a d-axis current reference value; The d-axis current reference value and the d-axis current actual value are input into a second proportional-integral controller after the difference is calculated, 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 q-axis current reference value and the q-axis current actual value are input into a third proportional-integral controller after the difference is calculated, and the output of the third proportional-integral controller is added to the q-axis voltage and the dq-axis coupling term to obtain a q-axis intermediate control voltage; Input 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 of the 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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