An offshore wind power hybrid DC transmission system and startup method

Abstract

The present invention provides an offshore wind power hybrid DC transmission system and startup method. The offshore wind power hybrid DC transmission system includes: a control device and a topology circuit, wherein the control device and the topology circuit are connected; wherein the topology circuit includes an offshore wind farm, an offshore converter valve, a DC submarine cable, an onshore converter valve, a three-winding transformer, and an onshore power grid; the offshore converter valve is connected to the offshore wind farm and connected to the onshore converter valve via the DC submarine cable; the onshore converter valve is connected to the three-winding transformer; the three-winding transformer includes a grid-side high-voltage winding, a valve-side high-voltage winding, and a valve-side low-voltage winding; the grid-side high-voltage winding is connected to the onshore power grid via a first circuit breaker, and the valve-side high-voltage winding and the valve-side low-voltage winding are connected to the onshore converter valve via a second circuit breaker and a third circuit breaker, respectively. The system can achieve lightweighting of the offshore wind power DC transmission system, reduce costs, and provide a black start function.

Description

Technical Field

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

[0002] Currently, offshore wind power transmission solutions typically employ flexible DC transmission solutions based on modular multilevel converter valves (MMCs). However, these solutions require large offshore converter valve platforms and high investment costs. To achieve lightweight offshore converter valves and converter station platforms and reduce the cost of offshore wind power transmission systems, diode-based transmission solutions are currently being adopted, including pure diode solutions, parallel solutions with diodes and auxiliary MMCs, and series solutions with diodes and auxiliary MMCs. For these various solutions to be practically implemented in engineering applications, the issue of black starts for offshore wind farms must be addressed. Currently, a black start for an offshore wind farm involves shutting down the entire system, resulting in a complete power outage and a completely "black" state. Power is then transmitted from land to sea to start some of the offshore wind farm's wind turbines. The system's recovery scope is then gradually expanded, ultimately achieving startup and recovery of the entire system.

[0003] Currently, due to the difficulty of achieving a black start using a pure diode solution, the current approach involves adding auxiliary submarine cables for black starts. However, this approach increases costs by laying additional submarine cables. Alternatively, the black start problem can be solved by equipping offshore wind farms with large-capacity diesel engines, but this solution's large size also increases O&M costs. A parallel connection of diodes with auxiliary MMCs can address the black start issue, but because the auxiliary MMCs are connected in parallel on the DC side, they still require a higher rated DC voltage and require a larger number of submodules, which does not effectively reduce size and cost. Summary of the Invention

[0004] Therefore, the technical solution of the present invention mainly solves the defects of large size and high cost of the existing offshore wind power DC transmission system, thereby providing an offshore wind power hybrid DC transmission system and startup method.

[0005] In a first aspect, an embodiment of the present invention provides an offshore wind power hybrid DC transmission system, comprising: a control device and a topology circuit, wherein the control device and the topology circuit are connected; wherein:

[0006] The topology circuit includes an offshore wind farm, an offshore converter valve, a DC submarine cable, an onshore converter valve, a three-winding transformer, and an onshore power grid; the offshore converter valve is connected to the offshore wind farm and is connected to the onshore converter valve via the DC submarine cable; the onshore converter valve is connected to the three-winding transformer; the three-winding transformer includes a grid-side high-voltage winding, a valve-side high-voltage winding, and a valve-side low-voltage winding; the grid-side high-voltage winding is connected to the onshore power grid via a first circuit breaker, and the valve-side high-voltage winding and the valve-side low-voltage winding are connected to the onshore converter valve via a second circuit breaker and a third circuit breaker, respectively;

[0007] the control device is configured to obtain an onshore charging voltage of the onshore converter valve and an offshore charging voltage of the offshore converter valve, and to send an unlocking and power supply instruction to the offshore converter valve based on the onshore charging voltage and the offshore charging voltage; wherein the onshore charging voltage and the offshore charging voltage are generated by charging the onshore converter valve and the offshore converter valve using the first operating voltage output by the onshore power grid through the valve-side low-voltage winding after the first circuit breaker and the third circuit breaker are closed;

[0008] The offshore converter valve is configured to receive the unlocking power supply instruction and supply power to the wind turbines in the offshore wind farm;

[0009] The control device is further configured to, after a preset number of wind turbines have completed startup, control the onshore converter valve so that, when the voltage at the offshore port of the DC submarine cable is higher than the rated voltage, a first disconnect instruction is sent to the third circuit breaker, a second close instruction is sent to the second circuit breaker, and a locking instruction is sent to the onshore converter valve; the third circuit breaker is opened based on the first disconnect instruction, the second circuit breaker is closed based on the second close instruction, and the onshore converter valve is locked based on the locking instruction;

[0010] The onshore power grid is further configured to output a second operating voltage via the valve-side high-voltage winding when the third circuit breaker is disconnected and the second circuit breaker is closed, and the second operating voltage is used to supply power to the onshore converter valve.

[0011] An embodiment of the present invention provides an offshore wind power hybrid DC transmission system and startup method. During the startup phase, the offshore converter valve only puts into use the offshore auxiliary modular multi-level converter valve with a smaller capacity and requires only a smaller DC voltage. Therefore, in order to avoid overmodulation of the onshore converter valve, a three-winding transformer consisting of a valve-side low-voltage winding, a valve-side high-voltage winding and a grid-side high-voltage winding is provided onshore. During the startup phase, the onshore power grid charges the onshore converter valve and the offshore converter valve through the valve-side low-voltage winding, and the control device sends a decoder to the offshore converter valve based on the real-time charging voltage of the onshore converter valve and the offshore converter valve. The onshore power grid receives the lock instruction and then uses the offshore converter valve to provide starting power to the offshore wind farm, realizing the function of the onshore power grid returning the starting power to the offshore wind farm through the onshore converter valve and the offshore converter valve, ensuring the normal startup of the offshore wind farm. Secondly, after the offshore wind farm is started normally, the onshore power grid supplies power to the onshore converter valve through the valve-side high-voltage winding, increases the DC voltage and charges the DC submarine cable until it is slightly higher than the rated voltage. Under the condition of ensuring the reverse shutdown of the diode, the offshore converter valve is put into operation by the diode converter valve, avoiding the impact when the diode converter valve is put into operation, and finally realizing the smooth operation of the offshore wind power transmission system.

[0012] In combination with the first aspect, in one possible implementation, the control device is further used to send at least one control instruction to the first circuit breaker and the third circuit breaker respectively, and the at least one control instruction is used to control the closing of the first circuit breaker and the third circuit breaker, wherein after the third circuit breaker is closed, the onshore power grid outputs the first operating voltage through the valve-side low-voltage winding and then charges the onshore converter valve and the offshore converter valve.

[0013] In combination with the first aspect, in another possible implementation, the control device is further configured to detect in real time the onshore charging voltage of the onshore converter valve and the offshore charging voltage of the offshore converter valve, and send an unlocking power supply instruction to the offshore converter valve when the onshore charging voltage reaches a first voltage threshold and the offshore charging voltage reaches a second voltage threshold.

[0014] In combination with the first aspect, in another possible implementation, the onshore converter valve includes multiple bridge arms, each bridge arm is composed of multiple half-bridge sub-modules; wherein the half-bridge sub-module includes: a capacitor, a first anti-parallel diode, a first fully-controlled device connected in parallel with the first anti-parallel diode, a second anti-parallel diode, and a second fully-controlled device connected in parallel with the second anti-parallel diode.

[0015] In combination with the first aspect, in another possible implementation, the control device is further configured to, when obtaining the closed states of the first circuit breaker and the third circuit breaker, send an uncontrolled charging instruction to the onshore converter valve, obtain the capacitor voltage, and send a controlled charging instruction to the onshore converter valve based on the capacitor voltage.

[0016] In combination with the first aspect, in another possible implementation, the offshore converter valve includes: an offshore auxiliary modular multi-level converter valve, and a first diode converter valve and a second diode converter valve connected in series to a DC side of the offshore auxiliary modular multi-level converter valve;

[0017] The offshore auxiliary modular multi-level converter valve is connected to the offshore wind farm via the fourth circuit breaker, the first connecting transformer and the fifth circuit breaker in sequence; the first diode converter valve is connected to the offshore wind farm via the sixth circuit breaker, the second connecting transformer and the seventh circuit breaker in sequence; the second diode converter valve is connected to the offshore wind farm via the eighth circuit breaker, the third connecting transformer and the ninth circuit breaker in sequence;

[0018] The offshore auxiliary modular multi-level converter valve is configured to supply power to the offshore wind farm when the fourth circuit breaker, the fifth circuit breaker, the seventh circuit breaker, and the ninth circuit breaker are in a closed state; wherein the fourth circuit breaker and the fifth circuit breaker are closed based on the unlocking power supply instruction.

[0019] In combination with the first aspect, in another possible implementation, it further includes: a first bypass switch and a second bypass switch; wherein the first bypass switch is connected in parallel with the DC side of the first diode converter valve, and the second bypass switch is connected in parallel with the DC side of the second diode converter valve.

[0020] In combination with the first aspect, in another possible implementation manner, the device is further used to:

[0021] When detecting that the first circuit breaker and the third circuit breaker are in a closed state, sending a fourth closing instruction to the first bypass switch and the second bypass switch;

[0022] When it is detected that the first bypass switch and the second bypass switch are in a closed state, an uncontrolled charging instruction is sent to the offshore converter valve, and a controlled charging instruction is sent to the offshore converter valve based on the capacitor voltage.

[0023] In combination with the first aspect, in another possible implementation, the control device is further used to send a no-load operation instruction to the offshore auxiliary modular multi-level converter valve when the preset number of wind turbines have completed startup, and to send a second disconnection instruction to the first bypass switch and the second bypass switch respectively.

[0024] In a second aspect, an embodiment of the present invention further provides a method for starting an offshore wind power hybrid DC transmission system, which is applied to an offshore wind power transmission system. The method includes:

[0025] obtaining an onshore charging voltage of the onshore converter valve and an offshore charging voltage of the offshore converter valve, and sending an unlocking power supply instruction to the offshore converter valve based on the onshore charging voltage and the offshore charging voltage to start wind turbines in the offshore wind farm; the onshore charging voltage and the offshore charging voltage are generated by charging the onshore converter valve and the offshore converter valve, respectively, using valve-side low-voltage windings;

[0026] After a preset number of wind turbines have completed startup, the onshore converter valve is controlled so that the voltage at the offshore port of the DC submarine cable is higher than the rated voltage, a first disconnection instruction is sent to the third circuit breaker, a second closing instruction is sent to the second circuit breaker, and a locking instruction is sent to the onshore converter valve, so that power is supplied to the onshore converter valve by utilizing the valve-side high-voltage winding. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 A circuit diagram of an offshore wind power hybrid DC transmission system provided by an embodiment of the present invention;

[0029] Figure 2 A schematic diagram of the internal structure of a half-bridge submodule provided by an embodiment of the present invention;

[0030] Figure 3 A flowchart of a method for starting an offshore wind power hybrid DC transmission system provided by an embodiment of the present invention;

[0031] Figure 4 FIG. 4 is a diagram showing a specific example of an electronic device in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] In the description of the present invention, it should be noted that the terms "first", "second", and "third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance. In addition, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a mechanical connection, or an electrical connection; or it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be internal communication between two components, and it can be a wireless connection or a wired connection. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0034] The embodiment of the present invention provides an offshore wind power hybrid DC transmission system, such as Figure 1 As shown, it includes: a control device 1 and a topology circuit 2, the control device 1 and the topology circuit 2 are connected; wherein,

[0035] The topology circuit 2 includes an offshore wind farm 3, an offshore converter valve 4, a DC submarine cable 5, an onshore converter valve 6, a three-winding transformer 7, and an onshore power grid 8. The offshore converter valve 4 is connected to the offshore wind farm 3 and to the onshore converter valve 6 via the DC submarine cable 5. The onshore converter valve 6 is connected to the three-winding transformer 7. The three-winding transformer 7 includes a grid-side high-voltage winding 9, a valve-side high-voltage winding 10, and a valve-side low-voltage winding 11. The grid-side high-voltage winding 9 is connected to the onshore power grid 8 via a first circuit breaker 12, and the valve-side high-voltage winding 10 and the valve-side low-voltage winding 11 are connected to the onshore converter valve 6 via a second circuit breaker 13 and a third circuit breaker 14, respectively.

[0036] The control device 1 is configured to obtain an onshore charging voltage of the onshore converter valve 6 and an offshore charging voltage of the offshore converter valve 4, and to send an unlocking and power supply instruction to the offshore converter valve 4 based on the above-mentioned charging voltages and the above-mentioned offshore charging voltages. The above-mentioned onshore charging voltages and the above-mentioned offshore charging voltages are generated by charging the onshore converter valve 6 and the above-mentioned offshore converter valve 4 respectively using the first operating voltage output by the onshore power grid 8 through the valve-side low-voltage winding 11 after the first circuit breaker 12 and the third circuit breaker 14 are closed.

[0037] Specifically, the control device 1 is further configured to send at least one control instruction to the first circuit breaker 12 (Brk1) and the third circuit breaker 14 (Brk3), respectively, wherein the at least one control instruction is configured to control the closing of the first circuit breaker 12 (Brk1) and the third circuit breaker 14 (Brk3), wherein after the third circuit breaker 14 (Brk3) is closed, the onshore power grid 8 charges the onshore converter valve 6 and the offshore converter valve 4 via the first operating voltage output by the valve-side low-voltage winding 11.

[0038] Furthermore, the control device 1 is further configured to detect in real time the onshore charging voltage of the onshore converter valve 6 and the offshore charging voltage of the offshore converter valve 4, and to send an unlocking power supply instruction to the offshore converter valve 4 when the onshore charging voltage reaches a first voltage threshold and the offshore charging voltage reaches a second voltage threshold.

[0039] The offshore converter valve 4 is configured to receive the unlocking power supply instruction and supply power to the wind turbines in the offshore wind farm 3 .

[0040] The control device 1 further controls the onshore converter valve 6 after a preset number of wind turbines have completed startup, so that when the voltage at the offshore port of the DC submarine cable 5 is higher than the rated voltage, the control device 1 sends an opening instruction to the third circuit breaker 14, sends a second closing instruction to the second circuit breaker 13, and sends a locking instruction to the onshore converter valve 6; the third circuit breaker 14 opens based on the opening instruction, the second circuit breaker 13 closes based on the second closing instruction, and the onshore converter valve 6 closes based on the locking instruction;

[0041] The onshore power grid 8 is further configured to output a second operating voltage through the valve-side high-voltage winding 10 when the third circuit breaker 14 is disconnected and the second circuit breaker 13 is closed. The second operating voltage is used to supply power to the onshore converter valve 6 .

[0042] Specifically, after a preset number of wind turbines have been started, the control device 1 controls the onshore converter valve so that when the voltage at the offshore port of the DC submarine cable is higher than the rated voltage, the onshore converter valve 6 is locked, the third circuit breaker 14 (Brk3) is disconnected, the second circuit breaker 13 (Brk2) is closed, and the three-winding transformer 7 is switched to the valve-side high-voltage winding 10. After the winding switching is completed, the onshore converter valve 6 is unlocked again to continue charging the submarine cable. At the same time, the sixth circuit breaker 29 (Brk6) and the above-mentioned eighth circuit breaker 32 (Brk8) are closed, and the offshore diode valve is put into operation. Thereafter, the control device 1 controls the DC port voltage to gradually decrease through the onshore converter valve 6 until the diode valve is turned on, and the offshore wind power transmission system enters normal operation.

[0043] This embodiment proposes an offshore wind power hybrid DC transmission system. During the startup phase, the offshore converter valve only uses the smaller-capacity offshore auxiliary modular multi-level converter valve, requiring only a smaller DC voltage. Therefore, to prevent overmodulation of the onshore converter valve, a three-winding transformer consisting of a valve-side low-voltage winding, a valve-side high-voltage winding, and a grid-side high-voltage winding is provided onshore. During the startup phase, the onshore power grid charges the onshore and offshore converter valves via the valve-side low-voltage winding. A control device sends an unlocking command to the offshore converter valve based on the real-time charging voltages of the onshore and offshore converter valves. Then, the offshore converter valve is used to provide starting power to the offshore wind farm, realizing the function of the onshore power grid to reversely supply starting power to the offshore wind farm through the onshore converter valve and the offshore converter valve, ensuring the normal startup of the offshore wind farm; secondly, after the offshore wind farm is normally started, the onshore power grid supplies power to the onshore converter valve through the valve-side high-voltage winding, increases the DC voltage and charges the DC submarine cable until it is slightly higher than the rated voltage. Under the condition of ensuring the reverse shutdown of the diode, the offshore converter valve is put into operation by the diode converter valve, avoiding the impact when the diode converter valve is put into operation, and finally achieving the smooth operation of the offshore wind power transmission system.

[0044] As an optional embodiment of the present invention, Figure 2 As shown, the onshore converter valve 6 includes a plurality of bridge arms, each bridge arm being composed of a plurality of half-bridge sub-modules 15; wherein the half-bridge sub-module 15 includes: a capacitor 16, a first anti-parallel diode 17, a first fully-controllable device 18 connected in parallel with the first anti-parallel diode 17, a second anti-parallel diode 19, and a second fully-controllable device 20 connected in parallel with the second anti-parallel diode 19.

[0045] Specifically, if Figure 2 As shown, the onshore converter valve 6 is provided with three phase units, each phase unit including two upper and lower bridge arms. Figure 2 T1 represents the first fully-controlled device 18, T2 represents the second fully-controlled device 20, wherein the first fully-controlled device 18 and the second fully-controlled device 20 can be insulated gate bipolar transistors (IGBTs) or integrated gate-commutated thyristors (IGCTs), D1 represents the first anti-parallel diode 17, D2 represents the second anti-parallel diode 19, and C represents the capacitor 16.

[0046] As an optional embodiment of the present invention, the control device 1 is further configured to send an uncontrolled charging instruction to the onshore converter valve 6 when obtaining the closed states of the first circuit breaker 12 and the third circuit breaker 14, and to obtain the capacitor voltage, and to send a controlled charging instruction to the onshore converter valve 6 based on the capacitor voltage.

[0047] Furthermore, the onshore power grid 8 performs uncontrolled charging of the capacitor 16 in the half-bridge sub-module 15 through the valve-side low-voltage winding. During the uncontrolled charging phase, the onshore converter valve 6 is closed, and the capacitor 16 is charged through the first anti-parallel diode 17 and the second anti-parallel diode 19 in the half-bridge sub-module 15. Typically, when the capacitor voltage reaches 30% of its rated voltage, the half-bridge sub-module 15 is triggered and controlled. At this time, the control device 1 sends a controlled charging instruction, and the half-bridge sub-module 15 switches from uncontrolled charging to controlled charging.

[0048] Furthermore, after uncontrolled charging is completed, the onshore converter valve 6 is unlocked for controlled charging. A DC voltage is established using constant DC voltage and constant reactive power control. During the startup process, only a relatively small DC voltage is output (e.g., 10% of the rated voltage, at which point the DC voltage command value takes 10% of the rated value, and the reactive power command takes zero). At this time, a relatively small number of half-bridge sub-modules 15 are put into operation. To ensure that the capacitor voltages of all half-bridge sub-modules 15 are balanced during the startup process, the half-bridge sub-modules 15 are put into operation alternately in a rotation manner. The control device 1 monitors the capacitor voltages of all half-bridge sub-modules 15, and alternately puts the half-bridge sub-modules 15 into operation by sorting the capacitor voltages of the half-bridge sub-modules 15. During the process of rotating and alternately putting different half-bridge sub-modules 15 into operation, 10% of the half-bridge sub-modules 15 are always kept in the put-in state.

[0049] As an optional embodiment of the present invention, the offshore converter valve 4 includes: an offshore auxiliary modular multi-level converter valve 21, and a first diode converter valve 22 and a second diode converter valve 23 connected in series to the DC side of the offshore auxiliary modular multi-level converter valve 21;

[0050] The offshore auxiliary modular multilevel converter valve 21 is connected to the offshore wind farm 3 via the fourth circuit breaker 26, the first connecting transformer 27, and the fifth circuit breaker 28 in sequence; the first diode converter valve 22 is connected to the offshore wind farm 3 via the sixth circuit breaker 29, the second connecting transformer 30, and the seventh circuit breaker 31 in sequence; and the second diode converter valve 23 is connected to the offshore wind farm 3 via the eighth circuit breaker 32, the third connecting transformer 33, and the ninth circuit breaker 34 in sequence.

[0051] The above-mentioned offshore auxiliary modular multi-level converter valve 21 is used to supply power to the above-mentioned offshore wind farm 3 when the fourth circuit breaker 26, the above-mentioned fifth circuit breaker 28, the seventh circuit breaker 31 and the ninth circuit breaker 34 are in a closed state; wherein the above-mentioned fourth circuit breaker 26 and the above-mentioned fifth circuit breaker 28 are closed based on the above-mentioned unlocking power supply instruction.

[0052] As an optional embodiment of the present invention, it also includes: a first bypass switch 24 and a second bypass switch 25; wherein, the first bypass switch 24 is connected in parallel with the DC side of the first diode converter valve 22, and the second bypass switch 25 is connected in parallel with the DC side of the second diode converter valve 23.

[0053] As an optional embodiment of the present invention, the control device 1 is further configured to:

[0054] When it is detected that the first circuit breaker 12 and the third circuit breaker 14 are in the closed state, a fourth closing instruction is sent to the first bypass switch 24 and the second bypass switch 25 .

[0055] When it is detected that the first bypass switch and the second bypass switch are in a closed state, an uncontrolled charging instruction is sent to the offshore converter valve, and a controlled charging instruction is sent to the offshore converter valve based on the capacitor voltage.

[0056] Specifically, based on the control instruction sent by the above-mentioned control device 1, the first circuit breaker 12 (Brk1) and the third circuit breaker 14 (Brk3) are closed, and based on the fourth closing instruction, the first bypass switch 24 (BP1) and the second bypass switch 25 (BP2) on both sides of the diode are closed, so that the DC circuit is in an unobstructed state.

[0057] Furthermore, when the charging voltage of the offshore auxiliary modular multilevel converter valve 21 (offshore auxiliary MMC) reaches a preset voltage, the fourth circuit breaker 26 (Brk4), the fifth circuit breaker 28 (Brk5), the seventh circuit breaker 31 (Br7), and the ninth circuit breaker 34 (Brk9) are closed based on the power supply instruction sent by the control device 1, thereby unlocking the offshore auxiliary modular multilevel converter valve 21. Constant AC voltage amplitude and frequency control is adopted to establish the AC side voltage from zero, provide starting power for the offshore wind turbine, and at the same time excite the second connecting transformer 30 and the third connecting transformer 33.

[0058] Furthermore, when the preset number of wind turbines have completed startup, a no-load operation instruction is sent to the offshore auxiliary modular multi-level converter valve 21, and a second disconnection instruction is sent to the first bypass switch 24 and the second bypass switch 25 respectively.

[0059] Furthermore, the offshore wind farm 3 starts a small number of wind turbines (the number of wind turbines can be set according to different operating environments). After the wind turbines are started, the offshore auxiliary modular multi-level converter valve 21 is controlled to operate in a no-load state, and then the first bypass switch 24 (BP1) and the second bypass switch 25 (BP2) are disconnected.

[0060] Furthermore, in order to avoid the DC voltage shock caused by the first diode converter valve 22 and the second diode converter valve 23 when the sixth circuit breaker 29 (Brk6) and the above-mentioned eighth circuit breakers 32 and (Brk8) are closed, the DC voltage is first increased through the onshore converter valve 6 and the DC submarine cable 5 is charged until it is slightly higher than the rated voltage. At this time, the first diode converter valve 22 and the second diode converter valve 23 are reversely closed. At this time, the sixth circuit breaker 29 (Brk6) and the above-mentioned eighth circuit breakers 32 and (Brk8) are closed, and the first diode converter valve 22 and the second diode converter valve 23 are put into operation.

[0061] Furthermore, when the control device 1 detects that a preset number of wind turbines have been completed, the first bypass switch 24 (BP1) and the second bypass switch 25 (BP2) are disconnected, and the onshore converter valve 6 is controlled so that the voltage at the offshore port of the DC submarine cable 5 is higher than the rated voltage, the onshore converter valve 6 is locked, the third circuit breaker 14 (Brk3) is disconnected, and the second circuit breaker 13 (Brk2) is closed. The converter valve 6 is then unlocked again to continue charging the submarine cable. The control device 1 then controls the DC port voltage of the onshore converter valve 6 to gradually decrease until the first diode converter valve 22 and the second diode converter valve 23 are turned on, the DC side between the offshore converter valve 4 and the onshore converter valve is stably connected, and the offshore wind power transmission system enters normal operation. If the offshore wind farm 3 is a grid-connected wind turbine, the energy storage power source or diesel generator at the grid connection point can be orderly withdrawn after the offshore wind power transmission system enters normal operation.

[0062] Furthermore, when it is detected that the above-mentioned preset number of wind turbines have completed startup, the control device 1 obtains the type of wind turbines in the offshore wind farm 3. If the offshore wind farm 3 is a grid-type wind turbine, the grid-type wind turbine operates in island mode; if the offshore wind farm 3 is a grid-following wind turbine, the tenth circuit breaker Brk10 and the eleventh circuit breaker Brk11 are closed, and the energy storage power supply or diesel generator at the grid connection point provides the wind turbine with grid power, or the internal energy consumption device of the wind turbine is used to dissipate the energy of the started wind turbine in a short time before switching to normal operation.

[0063] The above-mentioned offshore converter valve adopts a diode converter valve and an auxiliary MMC converter valve in series, which can effectively reduce the size and cost of the offshore converter station. The diode converter valve is connected in parallel with a bypass switch, and the onshore converter valve adopts a transformer with a valve-side low-voltage winding. The low-voltage starting winding and the bypass switch provide a starting circuit for the black start of the offshore wind farm. While achieving the lightweight of the offshore converter valve, it can ensure the black start function of the system, and can achieve black start by investing fewer sub-modules while keeping the onshore converter valve using conventional half-bridge sub-modules unchanged. During the startup process, the onshore converter valve will not be overmodulated, and the excitation inrush current of the transformer during the startup process can be effectively reduced. Under normal operating conditions after startup, the offshore auxiliary MMC can realize functions such as active filtering and reactive power compensation.

[0064] The following describes the working process of an offshore wind power hybrid DC transmission system through a specific embodiment.

[0065] Example 1:

[0066] First, close the circuit breakers Brk1 and Brk3, and close the bypass switches BP1 and BP2 on both sides of the diode converter valve to make the DC circuit unobstructed. First, the onshore converter valve and the offshore auxiliary MMC converter valve are uncontrolled charged through the low-voltage starting winding.

[0067] After uncontrolled charging is completed, the onshore converter valve is unlocked for controlled charging. Constant DC voltage and constant reactive power control are used to establish the DC voltage. During the startup process, only a small DC voltage is output (for example, 10% of the rated voltage, at which time the DC voltage command value is 10% of the rated value and the reactive power command is zero). At this time, the number of sub-modules put into operation is relatively small. To ensure that the capacitor voltages of all sub-modules are balanced during the startup process, the sub-modules are put into operation alternately in a rotation manner. During the rotation of different sub-modules, 10% of the sub-modules are always kept in the operation state.

[0068] Then close circuit breakers Brk4, Brk5, Brk7, and Brk9, unlock the offshore auxiliary MMC converter valve, use constant voltage amplitude and frequency control, establish the AC side voltage from zero, provide starting power for the offshore wind turbine, and excite the second and third connecting transformers at the same time.

[0069] The offshore wind farm starts a small number of wind turbines, and after the wind turbines are started, the offshore auxiliary MMC converter valves are controlled to operate in a no-load state, and then BP1 and BP2 are disconnected; if the offshore wind farm is a grid-connected wind turbine, the grid-connected wind turbines operate in island mode at this time; if the offshore wind farm is a grid-following wind turbine, when BP1 and BP2 are disconnected, the energy storage power supply or diesel generator at the grid connection point provides the wind turbine with grid power, or the internal energy consumption device of the wind turbine is used to dissipate the energy of the started wind turbine in a short time before switching to normal operation.

[0070] In order to avoid the DC voltage shock caused by the diode converter valve when closing Brk6 and Brk8, the DC voltage is first increased through the onshore converter valve and the DC submarine cable is charged until it is slightly higher than the rated voltage. At this time, the diode valve is reversely closed.

[0071] After that, the onshore converter valve is locked, Brk3 is disconnected, Brk2 is closed, and then the onshore converter valve is unlocked again to continue charging the submarine cable; at the same time, Brk6 and Brk8 are closed, and the offshore diode valve is put into operation.

[0072] Finally, the onshore converter valve is used to control the DC port voltage of the onshore station to gradually decrease until the diode converter valve is turned on, the DC sides of the sea and onshore stations are stably connected, and the system enters normal operation; if the offshore wind farm is a grid-following wind turbine, the energy storage power supply or diesel generator at the grid connection point can be exited in an orderly manner after the system enters normal operation.

[0073] The embodiment of the present invention also discloses a method for starting an offshore wind power hybrid DC transmission system, which is applied to the above-mentioned offshore wind power transmission system. Figure 3 As shown, the above method includes:

[0074] S301. Acquire an onshore charging voltage of an onshore converter valve and an offshore charging voltage of an offshore converter valve, and send an unlocking power supply instruction to the offshore converter valve based on the onshore charging voltage and the offshore charging voltage to start wind turbines in an offshore wind farm; the onshore charging voltage and the offshore charging voltage are generated by charging the onshore converter valve and the offshore converter valve, respectively, using valve-side low-voltage windings.

[0075] Specifically, when the onshore charging voltage reaches a first voltage threshold and the offshore charging voltage reaches a second voltage threshold, an unlocking power supply instruction is sent to the offshore converter valve.

[0076] S302. After a preset number of wind turbines have completed startup, the onshore converter valve is controlled so that the voltage at the offshore port of the DC submarine cable is higher than the rated voltage. A first disconnect instruction is sent to the third circuit breaker, a second close instruction is sent to the second circuit breaker, and a locking instruction is sent to the onshore converter valve to supply power to the onshore converter valve using the valve-side high-voltage winding.

[0077] Specifically, after a preset number of wind turbines have been started, the onshore converter valve is controlled so that the voltage at the offshore port of the DC submarine cable is higher than the rated voltage. The onshore converter valve is locked, the third circuit breaker (Brk3) is disconnected, the second circuit breaker (Brk2) is closed, and the three-winding transformer is switched to the valve-side high-voltage winding. After the winding switching is completed, the onshore converter valve is unlocked again to continue charging the submarine cable. At the same time, the sixth circuit breaker (Brk6) and the above-mentioned eighth circuit breaker (Brk8) are closed, and the offshore diode valve is put into operation. After that, the onshore converter valve is unlocked, and the DC port voltage is gradually reduced by controlling the onshore converter valve until the diode valve is turned on, and the offshore wind power transmission system enters normal operation.

[0078] The present invention provides a startup method for an offshore wind power hybrid DC transmission system. During the startup phase, the offshore converter valve only puts into operation a relatively small-capacity offshore auxiliary modular multi-level converter valve, which requires only a relatively small DC voltage. Therefore, in order to avoid overmodulation of the onshore converter valve, a three-winding transformer consisting of a valve-side low-voltage winding, a valve-side high-voltage winding, and a grid-side high-voltage winding is provided onshore. During the startup phase, the onshore power grid charges the onshore converter valve and the offshore converter valve through the valve-side low-voltage winding. The control device sends an unlocking instruction to the offshore converter valve based on the real-time charging voltage of the onshore converter valve and the offshore converter valve. The offshore converter valve is then used to provide starting power to the offshore wind farm, realizing the function of the onshore power grid feeding back starting power to the offshore wind farm through the onshore converter valve and the offshore converter valve, thereby ensuring the normal starting of the offshore wind farm. Secondly, after the offshore wind farm is started normally, the onshore power grid supplies power to the onshore converter valve through the valve-side high-voltage winding, thereby increasing the DC voltage and charging the DC submarine cable until it is slightly higher than the rated voltage. Under the condition of ensuring the reverse shutdown of the diode, the offshore converter valve is put into operation as the diode converter valve, thereby avoiding the impact when the diode converter valve is put into operation, thereby finally achieving the smooth operation of the offshore wind power transmission system.

[0079] As an optional embodiment of the present invention, it also includes:

[0080] At least one control instruction is sent to the first circuit breaker and the third circuit breaker respectively, and the at least one control instruction is used to control the closing of the first circuit breaker and the third circuit breaker, wherein after the third circuit breaker is closed, the first working voltage output by the onshore power grid passes through the valve-side low-voltage winding to charge the onshore converter valve and the offshore converter valve.

[0081] As an optional embodiment of the present invention, it also includes:

[0082] When the closing state of the above-mentioned first circuit breaker and the above-mentioned third circuit breaker is obtained, an uncontrolled charging instruction is sent to the above-mentioned first fully-controlled device and the above-mentioned second fully-controlled device, and the capacitor voltage is obtained. Based on the above-mentioned capacitor voltage, a third closing instruction is sent to the above-mentioned first fully-controlled device and the above-mentioned second fully-controlled device, and a controlled charging instruction is sent to the above-mentioned first anti-parallel diode and the above-mentioned second anti-parallel diode.

[0083] Specifically, based on the uncontrolled charging instruction, the onshore power grid performs uncontrolled charging of the capacitor in the half-bridge sub-module through the low-voltage starting winding. During the uncontrolled charging stage, the onshore converter valve is locked, and the capacitor is charged through the first anti-parallel diode and the second anti-parallel diode in the half-bridge sub-module. Usually, when the capacitor voltage reaches 30% of its rated voltage, the half-bridge sub-module is triggered and controlled. At this time, based on the controlled charging instruction, the half-bridge sub-module switches from uncontrolled charging to controlled charging.

[0084] Furthermore, after uncontrolled charging is completed, the onshore converter valve is unlocked for controlled charging. A constant DC voltage and constant reactive power control are used to establish a DC voltage. During the startup process, only a relatively small DC voltage is output (such as 10% of the rated voltage, at which time the DC voltage command value is 10% of the rated value, and the reactive power command is zero). At this time, the number of half-bridge sub-modules put into operation is relatively small. In order to ensure that the capacitor voltages of all half-bridge sub-modules are balanced during the startup process, the half-bridge sub-modules are put into operation alternately in a rotation manner, and the capacitor voltages of all half-bridge sub-modules are monitored. The half-bridge sub-modules are put into operation alternately by sorting the capacitor voltages of the half-bridge sub-modules. During the process of rotating and alternating the different half-bridge modules, 10% of the half-bridge sub-modules are always kept in the put-in state.

[0085] As an optional embodiment of the present invention, it also includes:

[0086] When it is detected that the first circuit breaker and the third circuit breaker are in the closed state, a fourth closing instruction is sent to the first bypass switch and the second bypass switch.

[0087] When it is detected that the first bypass switch and the second bypass switch are in a closed state, an uncontrolled charging instruction is sent to the offshore converter valve, and a controlled charging instruction is sent to the offshore converter valve based on the capacitor voltage.

[0088] Specifically, after the first bypass switch and the second bypass switch are closed, a connection is established between the onshore converter valve and the offshore converter valve, and then the offshore converter valve is rotated and charged through the half-bridge submodule in the onshore converter valve.

[0089] Furthermore, when the voltage of the offshore auxiliary modular multi-level converter valve reaches a preset voltage, the fourth circuit breaker (Brk4), the fifth circuit breaker (Brk5), the seventh circuit breaker (Br7), and the ninth circuit breaker (Brk9) are closed based on the power supply instruction, thereby unlocking the offshore auxiliary modular multi-level converter valve. Constant AC voltage amplitude and frequency control is adopted to establish the AC side voltage from zero, provide starting power for the offshore wind turbine, and simultaneously excite the second and third connecting transformers.

[0090] As an optional embodiment of the present invention, it also includes:

[0091] When the preset number of wind turbines have completed startup, a no-load operation instruction is sent to the offshore auxiliary modular multi-level converter valve, and a second disconnection instruction is sent to the first bypass switch and the second bypass switch respectively.

[0092] Specifically, after detecting that the number of wind turbines started reaches a preset number, the onshore converter valve is locked, the third circuit breaker (Brk3) is disconnected, the second circuit breaker (Brk2) is closed, and the onshore converter valve establishes a DC side voltage.

[0093] Furthermore, the offshore wind farm starts a small number of wind turbines (the number of wind turbines can be set according to different operating environments). After the wind turbines are started, the offshore auxiliary modular multi-level converter valve is controlled to operate in a no-load state, and then the first bypass switch (BP1) and the second bypass switch (BP2) are disconnected.

[0094] Furthermore, in order to avoid the DC voltage shock caused by the first diode converter valve and the second diode converter valve when the sixth circuit breaker (Brk6) and the above-mentioned eighth circuit breaker (Brk8) are closed, the DC voltage is first increased through the onshore converter valve and the DC submarine cable is charged until it is slightly higher than the rated voltage. At this time, the first diode converter valve and the second diode converter valve are reversely closed; at this time, the sixth circuit breaker (Brk6) and the above-mentioned eighth circuit breaker (Brk8) are closed, and the first diode converter valve and the second diode converter valve are put into operation; then the DC port voltage of the onshore converter valve is controlled to gradually decrease until the first diode converter valve and the second diode converter valve are turned on, the DC side between the offshore converter valve and the onshore converter valve is stably connected, and the offshore wind power transmission system enters normal operation; wherein, if the offshore wind farm is a grid-following wind turbine, after the offshore wind power transmission system enters normal operation, the energy storage power supply or diesel generator at the grid connection point can be orderly withdrawn.

[0095] Furthermore, when it is detected that the above-mentioned preset number of wind turbines have completed startup, the type of wind turbines in the offshore wind farm is obtained. If the offshore wind farm is a grid-type wind turbine, the grid-type wind turbine operates in island mode; if the offshore wind farm is a grid-following wind turbine, the tenth circuit breaker Brk10 and the eleventh circuit breaker Brk11 are closed, and the energy storage power supply or diesel generator at the grid connection point provides the wind turbine with grid power, or the internal energy consumption device of the wind turbine is used to dissipate the energy of the started wind turbine in a short time before switching to normal operation.

[0096] In addition, an embodiment of the present invention further provides an electronic device, such as Figure 4 As shown, the electronic device may include a processor 110 and a memory 120, wherein the processor 110 and the memory 120 may be connected via a bus or other means. Figure 4 In addition, the electronic device further includes at least one interface 130, which may be a communication interface or other interface, and this embodiment does not limit this.

[0097] The processor 110 may be a central processing unit (CPU). The processor 110 may also be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, or a combination of the above chips.

[0098] Memory 120, as a non-transitory computer-readable storage medium, can be used to store non-transitory software programs, non-transitory computer executable programs, and modules, such as the program instructions / modules corresponding to the video synthesis method in the embodiments of the present invention. Processor 110 executes the non-transitory software programs, instructions, and modules stored in memory 120 to perform various processor functions and data processing, thereby implementing the startup method of an offshore wind power hybrid DC transmission system in the above-mentioned method embodiment.

[0099] The memory 120 may include a program storage area and a data storage area, wherein the program storage area may store an operating system and application programs required for at least one function; the data storage area may store data created by the processor 110, etc. In addition, the memory 120 may include a high-speed random access memory, and may also include a non-volatile memory, such as at least one disk storage device, a flash memory device, or other non-volatile solid-state storage device. In some embodiments, the memory 120 may optionally include a memory remotely located relative to the processor 110, and these remote memories may be connected to the processor 110 via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0100] In addition, at least one interface 130 is used for communication between the electronic device and external devices, such as communication with a server, etc. Optionally, at least one interface 130 can also be used to connect to external input and output devices, such as a keyboard, a display screen, etc.

[0101] The one or more modules are stored in the memory 120 and when executed by the processor 110, perform the following steps: Figures 1 to 3 A method for starting an offshore wind power hybrid DC transmission system in the illustrated embodiment.

[0102] For details of the above electronic equipment, please refer to Figure 1 The corresponding descriptions and effects in the embodiments shown can be understood and will not be repeated here.

[0103] Those skilled in the art will appreciate that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program. The program can be stored in a computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a magnetic disk, an optical disk, a read-only memory (ROM), a random access memory (RAM), a flash memory, a hard disk drive (HDD), or a solid-state drive (SSD). The storage medium can also include a combination of the above-mentioned types of memory.

[0104] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. An offshore wind power hybrid DC transmission system, characterized in that: include: A control device and a topology circuit, wherein the control device and the topology circuit are connected; wherein, The topology circuit includes an offshore wind farm, an offshore converter valve, a DC submarine cable, an onshore converter valve, a three-winding transformer, and an onshore power grid; the offshore converter valve is connected to the offshore wind farm and is connected to the onshore converter valve via the DC submarine cable; the onshore converter valve is connected to the three-winding transformer; the three-winding transformer includes a grid-side high-voltage winding, a valve-side high-voltage winding, and a valve-side low-voltage winding; the grid-side high-voltage winding is connected to the onshore power grid via a first circuit breaker, and the valve-side high-voltage winding and the valve-side low-voltage winding are connected to the onshore converter valve via a second circuit breaker and a third circuit breaker, respectively; the control device is configured to obtain an onshore charging voltage of the onshore converter valve and an offshore charging voltage of the offshore converter valve, and to send an unlocking and power supply instruction to the offshore converter valve based on the onshore charging voltage and the offshore charging voltage; wherein the onshore charging voltage and the offshore charging voltage are generated by charging the onshore converter valve and the offshore converter valve using the first operating voltage output by the onshore power grid through the valve-side low-voltage winding after the first circuit breaker and the third circuit breaker are closed; The offshore converter valve is configured to receive the unlocking power supply instruction and supply power to the wind turbines in the offshore wind farm; The control device is further configured to, after a preset number of wind turbines have completed startup, control the onshore converter valve so that, when the voltage at the offshore port of the DC submarine cable is higher than the rated voltage, a first disconnect instruction is sent to the third circuit breaker, a second close instruction is sent to the second circuit breaker, and a locking instruction is sent to the onshore converter valve; the third circuit breaker is opened based on the first disconnect instruction, the second circuit breaker is closed based on the second close instruction, and the onshore converter valve is locked based on the locking instruction; The onshore power grid is further configured to output a second operating voltage via the valve-side high-voltage winding when the third circuit breaker is disconnected and the second circuit breaker is closed, and the second operating voltage is used to supply power to the onshore converter valve.

2. The offshore wind power hybrid DC transmission system according to claim 1, characterized in that: The control device is further configured to send at least one control instruction to the first circuit breaker and the third circuit breaker respectively, wherein the at least one control instruction is configured to control the closing of the first circuit breaker and the third circuit breaker, wherein after the third circuit breaker is closed, the onshore power grid outputs a first operating voltage through the valve-side low-voltage winding, and then charges the onshore converter valve and the offshore converter valve.

3. The offshore wind power hybrid DC transmission system according to claim 1, characterized in that: The control device is further configured to detect in real time the onshore charging voltage of the onshore converter valve and the offshore charging voltage of the offshore converter valve, and to send an unlocking power supply instruction to the offshore converter valve when the onshore charging voltage reaches a first voltage threshold and the offshore charging voltage reaches a second voltage threshold.

4. The offshore wind power hybrid DC transmission system according to claim 1, characterized in that: The onshore converter valve includes multiple bridge arms, each bridge arm is composed of multiple half-bridge sub-modules; wherein the half-bridge sub-module includes: a capacitor, a first anti-parallel diode, a first fully-controlled device connected in parallel with the first anti-parallel diode, a second anti-parallel diode and a second fully-controlled device connected in parallel with the second anti-parallel diode.

5. The offshore wind power hybrid DC transmission system according to claim 1, characterized in that: The control device is further configured to, when obtaining the closing states of the first circuit breaker and the third circuit breaker, send an uncontrolled charging instruction to the onshore converter valve, obtain a capacitor voltage, and send a controlled charging instruction to the onshore converter valve based on the capacitor voltage.

6. The offshore wind power hybrid DC transmission system according to claim 5, characterized in that: The offshore converter valve comprises: an offshore auxiliary modular multi-level converter valve, and a first diode converter valve and a second diode converter valve connected in series to the DC side of the offshore auxiliary modular multi-level converter valve; The offshore auxiliary modular multi-level converter valve is connected to the offshore wind farm via the fourth circuit breaker, the first connecting transformer and the fifth circuit breaker in sequence; the first diode converter valve is connected to the offshore wind farm via the sixth circuit breaker, the second connecting transformer and the seventh circuit breaker in sequence; the second diode converter valve is connected to the offshore wind farm via the eighth circuit breaker, the third connecting transformer and the ninth circuit breaker in sequence; The offshore auxiliary modular multi-level converter valve is configured to supply power to the offshore wind farm when the fourth circuit breaker, the fifth circuit breaker, the seventh circuit breaker, and the ninth circuit breaker are in a closed state; wherein the fourth circuit breaker and the fifth circuit breaker are closed based on the unlocking power supply instruction.

7. The offshore wind power hybrid DC transmission system according to claim 6, characterized in that: Also includes: A first bypass switch and a second bypass switch; wherein the first bypass switch is connected in parallel with the DC side of the first diode converter valve, and the second bypass switch is connected in parallel with the DC side of the second diode converter valve.

8. The offshore wind power hybrid DC transmission system according to claim 7, characterized in that: The control device is further used for: When detecting that the first circuit breaker and the third circuit breaker are in a closed state, sending a fourth closing instruction to the first bypass switch and the second bypass switch; When it is detected that the first bypass switch and the second bypass switch are in a closed state, an uncontrolled charging instruction is sent to the offshore converter valve, and a controlled charging instruction is sent to the offshore converter valve based on the capacitor voltage.

9. The offshore wind power hybrid DC transmission system according to claim 7, characterized in that: The control device is further configured to send a no-load operation instruction to the offshore auxiliary modular multi-level converter valve when the preset number of wind turbines have completed startup, and to send a second disconnection instruction to the first bypass switch and the second bypass switch respectively.

10. A method for starting an offshore wind power hybrid DC transmission system, characterized in that: Applied to the offshore wind power transmission system according to any one of claims 1 to 9, the method comprises: obtaining an onshore charging voltage of the onshore converter valve and an offshore charging voltage of the offshore converter valve, and sending an unlocking power supply instruction to the offshore converter valve based on the onshore charging voltage and the offshore charging voltage to start wind turbines in the offshore wind farm; the onshore charging voltage and the offshore charging voltage are generated by charging the onshore converter valve and the offshore converter valve, respectively, using valve-side low-voltage windings; After a preset number of wind turbines have completed startup, the onshore converter valve is controlled so that the voltage at the offshore port of the DC submarine cable is higher than the rated voltage, a first disconnection instruction is sent to the third circuit breaker, a second closing instruction is sent to the second circuit breaker, and a locking instruction is sent to the onshore converter valve, so that power is supplied to the onshore converter valve by utilizing the valve-side high-voltage winding.

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