A multi-direct-current interconnected topology and a control method and device thereof

Abstract

The present application relates to the technical field of direct current transmission, and discloses a multi-direct current interconnection topology and a control method and device thereof, and the present application is directed to the core limitations such as the lack of reverse power transmission capacity of DRU and MMC direct current series type converter caused by the unidirectional conduction characteristic, and a multi-loop same-structure direct current channel design is innovatively proposed, and a cooperative operation system is constructed by directly interconnecting the direct current sides of the MMC converters of each channel sending end. The main direct current channel has two flexible switchable operation modes of MMC topology and hybrid conversion topology, can accurately support other channels to complete black start, near-zero power stable operation and the continuous electrification of the sending end bus under zero power working condition, and completely solves the operation bottleneck of the traditional scheme.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, specifically to a multi-DC interconnection topology and its control method and apparatus. Background Technology

[0002] In the field of DC power transmission technology, traditional flexible DC technology based on modular multilevel converters (MMCs) is an important foundation for applications in the scenario of transmitting renewable energy to isolated areas. Hybrid converter technology, which includes diode rectifiers (DRUs) and auxiliary MMCs, has become a key research direction in this field due to the significant economic advantages of its DC-side series-connected topology. However, renewable energy units lack AC grid support, requiring the sending-end converter to have voltage source control functionality. Furthermore, the inherent unidirectional power transmission characteristics of DRUs generally result in these hybrid converters lacking the ability to feed power back from the inverter side to the rectifier side, creating a critical limitation in their operation.

[0003] This limitation manifests itself in two main ways: First, the black start of the sending-end converter station and new energy units is quite difficult, which restricts the promotion and application of diode converter technology in this scenario; Second, when the output of the new energy unit approaches or reaches zero power, the DRU will stop conducting, resulting in the power loss of the sending-end converter bus and the forced shutdown of various power generation, transmission and transformation equipment. Moreover, the equipment restart process is cumbersome, which not only affects the power generation efficiency but also greatly increases the workload of operation and maintenance. These problems are particularly prominent in typical scenarios such as deep-sea wind power.

[0004] To address the aforementioned issues, two main improvement approaches have emerged in the relevant technical fields: One approach uses a traditional MMC flexible DC converter as the main channel, interconnecting it with other uncontrolled rectifier channels via the AC side. This leverages the main channel's reverse power supply capability to provide power support for other channels during startup and operation. However, this approach generally suffers from high costs for the main channel MMC, potentially requiring additional converter investment, and the main channel needs to provide voltage sources for multiple channels, posing certain safety and stability risks. The other approach involves configuring a bypass switch for the DRU and combining it with AC-side interconnected converter equipment to form a reverse power supply system, achieving low-voltage reverse power supply and high-voltage unidirectional power transmission. However, this approach lacks a continuous AC voltage source support at the sending end during normal operation, which can easily lead to system stability issues. Summary of the Invention

[0005] This invention provides a multi-DC interconnect topology and its control method and apparatus to solve the problem that hybrid converters do not have the ability to feed power back from the inverter side to the rectifier side.

[0006] In a first aspect, the present invention provides a multi-DC interconnection topology, comprising: a main DC path and at least one auxiliary DC path, wherein a first AC side of the main DC path is connected to a main sending-end AC bus, a second AC side of the main DC path is connected to a main receiving-end AC bus, the DC side of the main sending-end MMC converter of the main DC path is interconnected with the DC side of the auxiliary sending-end MMC converter of the auxiliary DC path, and the main DC path is used to provide power support during the black start phase, and continuous energization guarantee under near-zero power and zero power conditions for each auxiliary DC path; the first AC side of the auxiliary DC path is connected to an auxiliary sending-end AC bus, the second AC side of the auxiliary DC path is connected to an auxiliary receiving-end AC bus, and the auxiliary DC path is used to cooperate with the main DC path to realize power transmission of the entire system and switching of the main DC path operating topology; the main sending-end AC bus is used to connect a first new energy source, and the auxiliary sending-end AC bus is used to connect a second new energy source; the main receiving-end AC bus and the auxiliary receiving-end AC bus are used to connect to the AC power grid.

[0007] In one optional embodiment, the main DC path includes: a first diode rectifier, a second diode rectifier, a main sending-end MMC converter, a main receiving-end MMC converter, a first bypass switch, a second bypass switch, a first positive switch, and a first negative switch. The AC side of the first diode rectifier is connected to the main sending-end AC bus. The positive DC terminal of the first diode rectifier is connected to the positive DC terminal of the main receiving-end MMC converter via the first positive switch. The negative DC terminal of the first diode rectifier is connected to the positive DC terminal of the main sending-end MMC converter. The negative DC terminal of the converter is also connected to its positive DC terminal through a first bypass switch; the positive DC terminal of the second diode rectifier is connected to the negative DC terminal of the main sending-end MMC converter, and the negative DC terminal of the second diode rectifier is connected to the negative DC terminal of the main receiving-end MMC converter through a first negative switch, and the negative DC terminal of the second diode rectifier is connected to its positive DC terminal through a second bypass switch; the two DC terminals of the main sending-end MMC converter are interconnected with the DC side of the auxiliary sending-end MMC converter of the auxiliary DC path; the AC side of the main receiving-end MMC converter is connected to the main receiving-end AC bus.

[0008] In one optional embodiment, the auxiliary DC path includes: a third diode rectifier, a fourth diode rectifier, an auxiliary sending-end MMC converter, and an auxiliary receiving-end MMC converter. The AC side of the third diode rectifier is connected to the auxiliary sending-end AC bus; the positive DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary receiving-end MMC converter; the negative DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary sending-end MMC converter; the positive DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary sending-end MMC converter; the negative DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary receiving-end MMC converter; the two DC terminals of the auxiliary sending-end MMC converter are interconnected with the DC side of the main sending-end MMC converter in the main DC path; and the AC side of the auxiliary receiving-end MMC converter is connected to the auxiliary receiving-end AC bus.

[0009] In one optional embodiment, the auxiliary DC path includes: a third diode rectifier, a fourth diode rectifier, an auxiliary sending-end MMC converter, an auxiliary receiving-end MMC converter, a third bypass switch, a fourth bypass switch, a second positive switch, and a second negative switch. The AC side of the third diode rectifier is connected to the auxiliary sending-end AC bus. The positive DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary receiving-end MMC converter via the second positive switch. The negative DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary sending-end MMC converter. The negative DC terminal of the rectifier is also connected to its positive DC terminal via a third bypass switch; the positive DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary sending-end MMC converter, and the negative DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary receiving-end MMC converter via a second negative switch, and the negative DC terminal of the fourth diode rectifier is connected to its positive DC terminal via a fourth bypass switch; the two DC terminals of the auxiliary sending-end MMC converter are interconnected with the DC side of the main sending-end MMC converter of the main DC path; the AC side of the auxiliary receiving-end MMC converter is connected to the auxiliary receiving-end AC bus.

[0010] In one optional implementation, the first diode rectifier, the second diode rectifier, and the main sending-end MMC converter are all connected to the main sending-end AC bus via a transformer and a circuit breaker in sequence, and the main receiving-end MMC converter is connected to the main receiving-end AC bus via a transformer and a circuit breaker in sequence; the AC sides of the third diode rectifier, the fourth diode rectifier, and the auxiliary sending-end MMC converter are all connected to the auxiliary sending-end AC bus via a transformer and a circuit breaker in sequence, and the auxiliary receiving-end MMC converter is connected to the auxiliary receiving-end AC bus via a transformer and a circuit breaker in sequence.

[0011] Secondly, the present invention provides a control method for multiple DC interconnect topologies. Based on the above-mentioned multiple DC interconnect topologies, the method includes: controlling the main DC path to start in MMC topology, and enabling the auxiliary DC path to complete black start by injecting power into the auxiliary DC path; when the output of the new energy source is large, the main DC path has insufficient transmission capacity under the MMC topology, and the main DC path is switched to a hybrid converter topology in coordination with the auxiliary DC path; when the output of the new energy source decreases to the point that the diode rectifier of the corresponding DC path is cut off, the main DC path is switched to MMC topology in coordination with the auxiliary DC path.

[0012] In one optional implementation, the black start process includes: controlling the main DC path to switch to MMC topology operation; controlling the main receiving-end MMC converter to unlock in a non-full-voltage state; bypassing the first diode rectifier and the second diode rectifier; controlling the main sending-end MMC converter to unlock in VF mode; controlling the first new energy unit to connect to the grid for power generation; using VF control mode to unlock the auxiliary sending-end MMC converter in a zero-start boost manner; starting the auxiliary load of the second new energy source, while controlling the AC voltage of the second new energy source to be lower than the conduction voltage of the third diode rectifier and the fourth diode rectifier; controlling the second new energy unit to connect to the grid for power generation; adjusting the AC voltage of the auxiliary sending-end MMC converter to the rated voltage; using P-VF control mode to control the auxiliary sending-end MMC converter, and controlling the power of the tie line between the auxiliary sending-end MMC converter and the main DC path within the allowable range of the line.

[0013] In one optional implementation, the process of switching the main DC path to a hybrid converter topology includes: switching the operating mode of the auxiliary sending-end MMC converter of the auxiliary DC path from P-VF control mode to U-VF control mode; switching the operating mode of the main receiving-end MMC converter of the main DC path from constant DC voltage control mode to zero power control mode; disconnecting the first bypass switch and the second bypass switch after the pole current of the main DC path is zero; controlling the operating mode of the main receiving-end MMC converter of the main DC path to full voltage operation; closing the first positive switch and the first negative switch; switching the operating mode of the sending-end MMC converter of the main DC path from VF control mode to P-VF control mode, and controlling the power of the interconnect line between the main DC path and the auxiliary DC path to a preset small value.

[0014] In one optional implementation, the process of switching the main DC path to an MMC topology includes: reducing the AC voltage of the main sending-end MMC converter of the main DC path to below the forward voltage of the first diode rectifier and the second diode rectifier; disconnecting the first positive switch and the first negative switch; after the AC voltage of the main sending-end MMC converter of the main DC path recovers to the rated voltage, switching the receiving-end MMC converter of the main DC path to a non-full-voltage operation state; closing the first bypass switch and the second bypass switch; switching the operating mode of the sending-end MMC converter of the auxiliary DC path from the U-VF control mode to the P-VF control mode; and controlling the power of the tie line between the main DC path and the auxiliary DC path within the allowable range of the line.

[0015] This invention provides a control device for multiple DC interconnect topologies. Based on the above-mentioned control method for multiple DC interconnect topologies, the device includes: a first control module, used to control the main DC path to start in MMC topology and to enable the auxiliary DC path to complete black start by injecting power into the auxiliary DC path; a second control module, used to switch the main DC path to hybrid converter topology when the main DC path has insufficient transmission capacity under MMC topology and with the cooperation of the auxiliary DC path when the output of the new energy source is large; and a third control module, used to switch the main DC path to MMC topology with the cooperation of the auxiliary DC path when the output of the new energy source decreases to the point that the diode rectifier of the corresponding DC path is cut off.

[0016] Beneficial effects: This invention addresses the core limitations of DRU and MMC series DC converters, such as the lack of reverse power supply capability due to their unidirectional conduction characteristics. It innovatively proposes a multi-circuit homogeneous DC channel design, constructing a collaborative operation system through direct interconnection of the DC sides of the MMC converters at the sending end of each channel. The main DC channel innovatively features two flexibly switchable operating modes: MMC topology and hybrid converter topology. This allows for precise support of other channels in achieving black start, near-zero power stable operation, and continuous energization of the sending-end bus under zero-power conditions, completely resolving the operational bottlenecks of traditional solutions.

[0017] This invention only requires the addition of a short-distance DC interconnection line. Under the premise of significantly controlling construction and operation and maintenance costs, it fully taps the control potential of the existing auxiliary MMC. Without the need to add complex hardware such as converters, it can realize the flexible, stable and efficient operation of the entire system under different new energy output conditions, and adapt to the actual needs of isolated island transmission scenarios such as deep-sea wind power. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a multi-DC interconnect topology according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the main control loops of the main sending-end MMC converter and the auxiliary sending-end converter according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the main control loops of the main receiving-end MMC converter and the auxiliary receiving-end converter according to an embodiment of the present invention; Figure 4 This is a flowchart illustrating a control method for multiple DC interconnect topologies according to an embodiment of the present invention; Figure 5 This is a schematic diagram of black start and topology transformation according to an embodiment of the present invention; Figure 6 This is a schematic diagram of black start and topology transformation according to an embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] It is understood that before using the technical solutions disclosed in the various embodiments of the present invention, users should be informed of the types, scope of use, and usage scenarios of the personal information involved in the present invention and their authorization should be obtained in accordance with relevant laws and regulations through appropriate means.

[0022] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0023] This embodiment provides a multi-DC interconnect topology, such as Figure 1As shown, it includes: a main DC path 1 and at least one auxiliary DC path 2, wherein, Figure 1 This example uses a two-loop interconnected topology, but it is only used as an example and is not intended to be a limitation.

[0024] The first AC side of the main DC path 1 is connected to the main sending-end AC bus 3, and the second AC side of the main DC path 1 is connected to the main receiving-end AC bus 4. The DC side of the main sending-end MMC converter of the main DC path 1 is interconnected with the DC side of the auxiliary sending-end MMC converter of the auxiliary DC path 2. The main DC path 1 is used to provide power support for each auxiliary DC path 2 during the black start phase, and continuous energization guarantee in near-zero power and zero power states. The first AC side of the auxiliary DC path 2 is connected to an auxiliary sending-end AC bus 5, and the second AC side of the auxiliary DC path 2 is connected to an auxiliary receiving-end AC bus 6. The auxiliary DC path 2 is used to cooperate with the main DC path 1 to realize the power transmission of the entire system and the switching of the operating topology of the main DC path 1. The main sending-end AC bus 3 is used to connect the first new energy source (i.e., wind farm #A), and the auxiliary sending-end AC bus 5 is used to connect the second new energy source (i.e., wind farm #B). The main receiving-end AC bus 4 and the auxiliary receiving-end AC bus 6 are used to connect to the AC grid.

[0025] The main DC path 1 has two dynamically switchable operating topologies, which can flexibly adjust the working mode according to the changes in the output of new energy sources. Its core function is to provide key power support for each auxiliary DC path 2 during the black start phase, ensuring that the auxiliary DC path 2 can start smoothly from zero power. At the same time, in the scenario of near-zero or zero power operation of new energy sources, the AC bus of the auxiliary DC path 2 is kept energized through continuous power supply, avoiding the cumbersome restart problem caused by power failure of related equipment, and greatly improving the continuity of system operation.

[0026] Auxiliary DC path 2 and main DC path 1 form a collaborative control system. When the main DC path 1 is switching its operating topology, it stabilizes the tie line voltage by regulating its own operating status and temporarily absorbs the new energy power of the main DC path 1, creating a stable operating environment for the topology switching and ensuring that the switching process is shock-free and uninterrupted. At the same time, it undertakes the power transmission task of its corresponding new energy base and works with the main DC path 1 to build an efficient multi-loop power transmission network that adapts to the output fluctuation characteristics of different new energy bases.

[0027] In one alternative implementation, such as Figure 1 As shown, the main DC path 1 includes: a first diode rectifier DRU1, a second diode rectifier DRU2, a main sending end MMC converter 11, a main receiving end MMC converter 12, a first bypass switch BP1, a second bypass switch BP2, a first positive switch BP3, and a first negative switch BP4.

[0028] The AC side of the first diode rectifier DRU1 is connected to the main sending end AC bus 3. The positive DC terminal of the first diode rectifier DRU1 is connected to the positive DC terminal of the main receiving end MMC converter 12 through the first positive switch BP3. The negative DC terminal of the first diode rectifier DRU1 is connected to the positive DC terminal of the main sending end MMC converter 11. The negative DC terminal of the first diode rectifier DRU1 is also connected to its positive DC terminal through the first bypass switch BP1. The positive DC terminal of the second diode rectifier DRU2 is connected to the main sending end AC bus 3. The negative DC terminal of the main receiving end MMC converter 11 is connected, and the negative DC terminal of the second diode rectifier DRU2 is connected to the negative DC terminal of the main receiving end MMC converter 12 through the first negative switch BP4. The negative DC terminal of the second diode rectifier DRU2 is connected to its positive DC terminal through the second bypass switch BP2. The two DC terminals of the main sending end MMC converter 11 are interconnected with the DC side of the auxiliary sending end MMC converter of the auxiliary DC path 2. The AC side of the main receiving end MMC converter 12 is connected to the main receiving end AC bus 4.

[0029] The main sending end MMC converter 11 is the core control and power processing unit at the sending end of the main DC path 1. It has multi-mode control switching capability. It is not only the key to providing a stable AC voltage source and realizing the reverse power supply to start the auxiliary load during the black start stage, but also the core to regulate the tie line power and adjust the AC voltage to control the DRU conduction state. At the same time, it undertakes the important tasks of new energy power access, distribution and coordinated scheduling with auxiliary DC path 2.

[0030] The main and receiving end MMC converter adopts a hybrid structure of full-bridge and half-bridge sub-modules. Its core advantage lies in supporting flexible switching between full-voltage and non-full-voltage operating states. With constant DC voltage, zero power and other control modes, it can achieve stable DC power inversion and reliable transmission to the receiving end AC grid under the hybrid converter topology. It can also reduce pole power to zero during topology switching, creating a safe and shock-free environment for switching operations. At the same time, it can charge relevant energy storage components from the DC side in the early stage of black start, laying the foundation for system startup.

[0031] The first bypass switch BP1 and the second bypass switch BP2 are key control switches for realizing topology mode conversion. Their operation status directly determines the working status of the DRU. When closed, the corresponding DRU can be bypassed, allowing the main DC path 1 to switch to a pure MMC topology, which is suitable for black start, low output or zero output of new energy sources and other scenarios. When open, they create conditions for the DRU to access the circuit, helping the main DC path 1 to switch to a hybrid converter topology to improve transmission capacity.

[0032] The first positive switch BP3 and the first negative switch BP4 are the core components for pole line on / off control. They work together with the bypass switch to accurately realize the opening and closing operations of the pole line of the main DC path 1 during the topology switching process. This effectively isolates the circuit, avoids power surges, and provides reliable circuit on / off protection for the safe access and exit of the DRU and the smooth transition of the operating state of the main DC path 1, ensuring the safety and stability of the entire switching process.

[0033] In one alternative implementation, such as Figure 1 As shown, the auxiliary DC path 2 includes: a third diode rectifier DRU3, a fourth diode rectifier DRU4, an auxiliary sending-end MMC converter 21, and an auxiliary receiving-end MMC converter 22. The AC side of the third diode rectifier DRU3 is connected to the auxiliary sending-end AC bus 5. The positive DC terminal of the third diode rectifier DRU3 is connected to the positive DC terminal of the auxiliary receiving-end MMC converter 22, and the negative DC terminal of the third diode rectifier DRU3 is connected to the positive DC terminal of the auxiliary sending-end MMC converter 21. The positive DC terminal of the fourth diode rectifier DRU4 is connected to the negative DC terminal of the auxiliary sending-end MMC converter 21, and the negative DC terminal of the fourth diode rectifier DRU4 is connected to the negative DC terminal of the auxiliary receiving-end MMC converter 22. The two DC terminals of the auxiliary sending-end MMC converter 21 are interconnected with the DC side of the main sending-end MMC converter 11 of the main DC path 1. The AC side of the auxiliary receiving-end MMC converter 22 is connected to the auxiliary receiving-end AC bus 6.

[0034] In one optional embodiment, the auxiliary DC path 2 includes: a third diode rectifier DRU3, a fourth diode rectifier DRU4, an auxiliary sending-end MMC converter 21, an auxiliary receiving-end MMC converter 22, a third bypass switch, a fourth bypass switch, a second positive switch, and a second negative switch. The AC side of the third diode rectifier DRU3 is connected to the auxiliary sending-end AC bus 5. The positive DC terminal of the third diode rectifier DRU3 is connected to the positive DC terminal of the auxiliary receiving-end MMC converter 22 via the second positive switch. The negative DC terminal of the third diode rectifier DRU3 is connected to the positive DC terminal of the auxiliary sending-end MMC converter 21. The negative DC terminal of the diode rectifier DRU3 is also connected to its positive DC terminal via a third bypass switch; the positive DC terminal of the fourth diode rectifier DRU4 is connected to the negative DC terminal of the auxiliary sending-end MMC converter 21, and the negative DC terminal of the fourth diode rectifier DRU4 is connected to the negative DC terminal of the auxiliary receiving-end MMC converter 22 via a second negative switch, and the negative DC terminal of the fourth diode rectifier DRU4 is connected to its positive DC terminal via a fourth bypass switch; the two DC terminals of the auxiliary sending-end MMC converter 21 are interconnected with the DC side of the main sending-end MMC converter 11 of the main DC path 1; the AC side of the auxiliary receiving-end MMC converter 22 is connected to the auxiliary receiving-end AC bus 6.

[0035] Specifically, the main sending-end MMC converter 11 and the auxiliary sending-end MMC converter 21 adopt a unified voltage level design, and one DC path is selected as the main channel. The sending-end MMC converters of the other DC paths are interconnected with the sending-end MMC converter of the main DC path 1 through DC interconnection lines.

[0036] Specifically, the diode rectifier of the main DC path 1 is equipped with a bypass switch and a pole-side switch, and the receiving-end MMC converter of the main DC path 1 adopts a hybrid topology of full-bridge and half-bridge submodules. The receiving-end MMC converters of the other auxiliary DC paths 2 can adopt a half-bridge submodule topology. The DRU of the auxiliary DC path 2 does not need to be equipped with an additional bypass switch; only a pole-side switch is required to meet the basic operating requirements. If further improvement of system reliability is required, a redundant configuration scheme of dual main channels or multiple main channels can be adopted.

[0037] In one alternative implementation, such as Figure 1 As shown, the first diode rectifier DRU1, the second diode rectifier DRU2, and the main sending-end MMC converter 11 are all connected to the main sending-end AC bus 3 via a transformer and a circuit breaker in sequence. The main receiving-end MMC converter 12 is connected to the main receiving-end AC bus 4 via a transformer and a circuit breaker in sequence. The AC sides of the third diode rectifier DRU3, the fourth diode rectifier DRU4, and the auxiliary sending-end MMC converter 21 are all connected to the auxiliary sending-end AC bus 5 via a transformer and a circuit breaker in sequence. The auxiliary receiving-end MMC converter 22 is connected to the auxiliary receiving-end AC bus 6 via a transformer and a circuit breaker in sequence.

[0038] In one optional implementation, the main control elements of the main sending-end MMC converter 11 and the auxiliary sending-end converter are as follows: Figure 2 As shown, Figure 2 The document presents a general structure compatible with both the main sending-end MMC converter 11 and the auxiliary sending-end converter, allowing selection of control modes based on actual needs. Mode ① is a simple AC voltage source control, i.e., VF mode; Mode ② adds a tie-line power control element outside the AC voltage loop, i.e., P-VF mode; Mode ③ adds a DC voltage control element outside the AC voltage loop, i.e., U-VF mode, which also effectively controls the tie-line voltage.

[0039] In one optional implementation, the main control elements of the primary receiving-end MMC converter 12 and the auxiliary receiving-end converter are as follows: Figure 3 As shown, Figure 3The document presents a general structure compatible with both the main receiving-end MMC converter 12 and the auxiliary receiving-end converter, allowing selection of control modes based on actual needs. Mode ① is a simple AC voltage source control, i.e., VF mode; Mode ② adds a tie-line power control element outside the AC voltage loop, i.e., P-VF mode; Mode ③ adds a DC voltage control element outside the AC voltage loop, i.e., U-VF mode, which also effectively controls the tie-line voltage.

[0040] This embodiment provides a control method for multiple DC interconnect topologies, based on the above-mentioned multiple DC interconnect topologies, such as... Figure 4 As shown, the method includes: Step S1: Control the main DC path 1 to start in MMC topology, and inject power into the auxiliary DC path 2 to enable the auxiliary DC path 2 to complete black start.

[0041] Optionally, the black start process includes: controlling the main DC path 1 to switch to MMC topology operation; controlling the main receiving-end MMC converter 12 to unlock in a non-full voltage state; bypassing the first diode rectifier DRU1 and the second diode rectifier DRU2; controlling the main sending-end MMC converter 11 to unlock in VF mode; controlling the first new energy unit to connect to the grid for power generation; using VF control mode to unlock the auxiliary sending-end MMC converter 21 in a zero-start boost manner; starting the auxiliary load of the second new energy, while controlling the AC voltage of the second new energy to be lower than the conduction voltage of the third diode rectifier DRU3 and the fourth diode rectifier DRU4; controlling the second new energy unit to connect to the grid for power generation; adjusting the AC voltage of the auxiliary sending-end MMC converter 21 to the rated voltage; using P-VF control mode to control the auxiliary sending-end MMC converter 21, and controlling the power of the tie line between the auxiliary sending-end MMC converter 21 and the main DC path 1 within the allowable range of the line.

[0042] Specifically, the black start process includes two key states, as follows: Figure 5 middle dashed line, Figure 6 As shown by the dashed line (DC #A is the main DC path 1 in the diagram), the specific startup steps are as follows: 1) Main DC path 1 operates in MMC topology mode, starting its corresponding new energy units: First, the main receiving end MMC converter 12 of main DC path 1 (DC #A) is unlocked in a non-full-voltage state, charging the submodule capacitors of the sending end MMC converters of DC #A and DC #B from the DC side; then, DRU1 and DRU2 of DC #A are placed in bypass state, and the main sending end MMC converter 11 is unlocked in VF mode, starting the auxiliary load of wind farm #A through reverse power transmission; finally, the units of wind farm #A complete grid connection and power generation, and main DC path 1 (DC #A) achieves forward power transmission. The newly started and energized parts in this state, such as... Figure 5 As indicated by the dashed line.

[0043] 2) Relying on the DC tie line, start the auxiliary sending-end MMC converter 21 of each auxiliary DC path 2 and its corresponding new energy unit: First, the auxiliary sending-end MMC converter 21 of auxiliary DC path 2 (DC #B) adopts VF control mode, unlocks with zero-start voltage boost, and simultaneously starts the auxiliary load of wind farm #B, while controlling its AC voltage to be lower than the conduction voltage threshold of DRU3 and DRU#4; after the wind farm #B unit is connected to the grid for power generation, the auxiliary sending-end MMC converter 21 of DC #B adjusts the AC voltage to the rated value, so that DRU3 and DRU4 enter the normal conduction operation state; finally, the auxiliary sending-end MMC converter 21 of DC #B switches to P-VF control mode, controls the tie line power between it and the main DC path 1 (DC #A) within the preset allowable range, and ensures that even if the new energy is in a zero-output state, DRU3 and DRU4 of DC #B can still be continuously conducted. The newly started and energized parts in this state, such as Figure 6 As indicated by the dashed line.

[0044] Step S2: When the new energy source outputs a large amount of power, the transmission capacity of the main DC path 1 under the MMC topology is insufficient. With the cooperation of the auxiliary DC path 2, the main DC path 1 is switched to the hybrid converter topology.

[0045] Optionally, the process of switching the main DC path 1 to a hybrid converter topology includes: switching the operating mode of the auxiliary sending-end MMC converter 21 of the auxiliary DC path 2 from P-VF control mode to U-VF control mode; switching the operating mode of the main receiving-end MMC converter 12 of the main DC path 1 from constant DC voltage control mode to zero power control mode; after the pole current of the main DC path 1 is zero, disconnecting the first bypass switch BP1 and the second bypass switch BP2; controlling the operating mode of the main receiving-end MMC converter 12 of the main DC path 1 to full voltage operation; closing the first positive switch BP3 and the first negative switch BP4; switching the operating mode of the sending-end MMC converter of the main DC path 1 from VF control mode to P-VF control mode, and controlling the power of the interconnect line between the main DC path 1 and the auxiliary DC path 2 to a preset small value.

[0046] Specifically, when the new energy unit is operating at high output, the power transmission capacity of the main DC path 1 (DC #A) operating in pure MMC topology can no longer meet the demand. It is necessary to switch the main DC path 1 from pure MMC topology to hybrid converter topology with the coordinated cooperation of auxiliary DC path 2 (DC #B). After this switch, newly energized electrical components and lines will be started. Figure 5 The following is a detailed switching process and control logic, marked with a solid line: 1) Zeroing and opening operation of the main DC path 1 pole line power: First, the auxiliary sending-end MMC converter 21 of the auxiliary DC path 2 (DC #B) switches from P-VF composite control mode to U-VF composite control mode, and realizes stable regulation of the tie line voltage between the main and auxiliary channels through DC voltage closed-loop control; at the same time, the main receiving-end MMC converter 12 of the main DC path 1 (DC #A) switches from the conventional DC voltage control mode to the zero power control mode. At this time, all the new energy power of the sending end of the main DC path 1 is transferred to the auxiliary DC path 2, and is inverted by the auxiliary receiving-end MMC converter 22 of the auxiliary DC path 2 (DC #B) and fed into the AC grid; after the pole line current of the main DC path 1 (DC #A) decays to a safe threshold of near zero, BP1 and BP2 are disconnected to create circuit conditions for subsequent topology switching.

[0047] 2) Activation of the hybrid converter topology of main DC path 1 and restoration of pole line power: First, switch the main receiving end MMC converter 12 of main DC path 1 (DC #A) from non-full voltage operation to full voltage operation, and its full bridge submodule is put into operation in the forward direction; Second, close BP3 and BP4 of main DC path 1 (DC #A), at which time the pole line of main DC path 1 is in a standby state with power but close to zero power; Third, switch the sending end MMC of main DC path 1 (DC #A) from VF voltage source control mode to P-VF composite control mode, and control the power of the tie line between the main and auxiliary DC paths within a preset small threshold range to avoid power surge; After the switch is completed, the power generation of each new energy station is mainly transmitted independently through its corresponding DC path, and the whole system enters the high-output high-efficiency transmission condition.

[0048] Step S3: When the output of the new energy source decreases to the point that the diode rectifier of the corresponding DC path is turned off, the main DC path 1 is switched to MMC topology in coordination with the auxiliary DC path 2.

[0049] Optionally, the process of switching the main DC path 1 to an MMC topology includes: reducing the AC voltage of the main sending-end MMC converter 11 of the main DC path 1 to below the conduction voltage of the first diode rectifier DRU1 and the second diode rectifier DRU2; disconnecting the first positive switch BP3 and the first negative switch BP4; after the AC voltage of the main sending-end MMC converter 11 of the main DC path 1 returns to the rated voltage, switching the receiving-end MMC converter of the main DC path 1 to a non-full-voltage operation state; closing the first bypass switch BP1 and the second bypass switch BP2; switching the operating mode of the sending-end MMC converter of the auxiliary DC path 2 from the U-VF control mode to the P-VF control mode; and controlling the power of the tie line between the main DC path 1 and the auxiliary DC path 2 within the allowable range of the line.

[0050] Specifically, when the output of the new energy unit decreases to a critical threshold, which may cause some of the DRUs in auxiliary DC path 2 to be shut down, the main DC path 1 (DC #A) needs to be switched from a hybrid converter topology to a pure MMC topology. After the switch is completed, the system will recover to its previous state. Figure 5 middle dashed line, Figure 6 The specific switching process and control logic for the energized operating status, marked by the dashed line, are as follows: 1) Voltage regulation and pole line open circuit operation of the main DC path 1: The main sending end MMC converter 11 of the main DC path 1 (DC #A) actively regulates the AC voltage to below the conduction voltage threshold of DRU1 and DRU2, forcing DRU1 and DRU2 of DC #A to turn off; at this time, all the renewable energy power of wind farm #A is transferred to the auxiliary DC path 2 (DC #B) for absorption through the DC tie line. After the pole line current of the main DC path 1 decays to a safe zero value, its pole line isolating switch is disconnected; then the main sending end MMC converter 11 of the main DC path 1 (DC #A) smoothly restores the AC voltage to the rated operating voltage, preparing for the subsequent topology switch.

[0051] 2) Main DC Path 1 Pure MMC Topology Activation and Pole Power Restoration: First, switch the main receiving-end MMC converter 12 of the main DC Path 1 (DC #A) from full-voltage operation to non-full-voltage operation, and put its full-bridge submodule into negative operation; Second, close BP1 and BP2 to complete the switch from hybrid converter topology to pure MMC topology; Third, switch the auxiliary sending-end MMC converter 21 of the auxiliary DC Path 2 (DC #B) from U-VF composite control mode to P-VF composite control mode, implement closed-loop control of the tie line power between the main and auxiliary DC paths, and limit it within the preset allowable range; At the same time, ensure that even if the new energy unit is in zero-output operation, DRU3 and DRU4 of DC #B can still be continuously conducted to avoid power loss of the sending-end bus and equipment shutdown.

[0052] This embodiment also provides a control device for multiple DC interconnect topologies, which is used to implement the above embodiments and preferred embodiments; details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that implements a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0053] This embodiment provides a control device for multiple DC interconnect topologies. Based on the above control method for multiple DC interconnect topologies, the device includes: The first control module is used to control the main DC path 1 to start in MMC topology and to enable the auxiliary DC path 2 to complete black start by injecting power into the auxiliary DC path 2. The second control module is used to switch the main DC path 1 to a hybrid converter topology when the main DC path 1 has insufficient transmission capacity under the MMC topology, in coordination with the auxiliary DC path 2, when the output of new energy is large. The third control module is used to switch the main DC path 1 to MMC topology in coordination with the auxiliary DC path 2 when the output of the new energy source decreases to the point that the diode rectifier of the corresponding DC path is turned off.

[0054] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A multi-DC interconnect topology, characterized in that, include: The main DC path and at least one auxiliary DC path, wherein, The first AC side of the main DC path is connected to the main sending end AC bus, and the second AC side of the main DC path is connected to the main receiving end AC bus. The DC side of the main sending end MMC converter of the main DC path is interconnected with the DC side of the auxiliary sending end MMC converter of the auxiliary DC path. The main DC path is used to provide power support for each auxiliary DC path during the black start phase, and continuous energization guarantee under near-zero power and zero power conditions. The first AC side of the auxiliary DC path is connected to an auxiliary sending-end AC bus, and the second AC side of the auxiliary DC path is connected to an auxiliary receiving-end AC bus. The auxiliary DC path is used to cooperate with the main DC path to realize the power transmission of the whole system and the switching of the main DC path operating topology. The main sending end AC bus is used to connect to the first new energy source, and the auxiliary sending end AC bus is used to connect to the second new energy source; The main receiving-end AC bus and the auxiliary receiving-end AC bus are used to connect to the AC power grid.

2. The multi-DC interconnect topology according to claim 1, characterized in that, The main DC path includes: a first diode rectifier, a second diode rectifier, a main sending-end MMC converter, a main receiving-end MMC converter, a first bypass switch, a second bypass switch, a first positive switch, and a first negative switch, wherein... The AC side of the first diode rectifier is connected to the AC bus of the main sending end. The positive DC terminal of the first diode rectifier is connected to the positive DC terminal of the main receiving end MMC converter through the first positive switch. The negative DC terminal of the first diode rectifier is connected to the positive DC terminal of the main sending end MMC converter. The negative DC terminal of the first diode rectifier is also connected to its positive DC terminal through the first bypass switch. The positive DC terminal of the second diode rectifier is connected to the negative DC terminal of the main sending MMC converter. The negative DC terminal of the second diode rectifier is connected to the negative DC terminal of the main receiving MMC converter through the first negative switch. The negative DC terminal of the second diode rectifier is connected to its positive DC terminal through the second bypass switch. The two DC terminals of the main sending-end MMC converter are interconnected with the DC side of the auxiliary sending-end MMC converter of the auxiliary DC path; The AC side of the main receiver MMC converter is connected to the main receiver AC bus.

3. The multi-DC interconnect topology according to claim 2, characterized in that, The auxiliary DC path includes: a third diode rectifier, a fourth diode rectifier, an auxiliary sending-end MMC converter, and an auxiliary receiving-end MMC converter, wherein... The AC side of the third diode rectifier is connected to the auxiliary sending-end AC bus, the positive DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary receiving-end MMC converter, and the negative DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary sending-end MMC converter. The positive DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary sending-end MMC converter, and the negative DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary receiving-end MMC converter. The two DC terminals of the auxiliary sending-end MMC converter are interconnected with the DC side of the main sending-end MMC converter of the main DC path; The AC side of the auxiliary receiving-end MMC converter is connected to the auxiliary receiving-end AC bus.

4. The multi-DC interconnect topology according to claim 2, characterized in that, The auxiliary DC path includes: a third diode rectifier, a fourth diode rectifier, an auxiliary sending-end MMC converter, an auxiliary receiving-end MMC converter, a third bypass switch, a fourth bypass switch, a second positive switch, and a second negative switch, wherein... The AC side of the third diode rectifier is connected to the auxiliary sending-end AC bus. The positive DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary receiving-end MMC converter through the second positive switch. The negative DC terminal of the third diode rectifier is connected to the positive DC terminal of the auxiliary sending-end MMC converter. The negative DC terminal of the third diode rectifier is also connected to its positive DC terminal through the third bypass switch. The positive DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary sending-end MMC converter. The negative DC terminal of the fourth diode rectifier is connected to the negative DC terminal of the auxiliary receiving-end MMC converter through the second negative switch. The negative DC terminal of the fourth diode rectifier is connected to its positive DC terminal through the fourth bypass switch. The two DC terminals of the auxiliary sending-end MMC converter are interconnected with the DC side of the main sending-end MMC converter of the main DC path; The AC side of the auxiliary receiving-end MMC converter is connected to the auxiliary receiving-end AC bus.

5. The multi-DC interconnect topology according to claim 3 or 4, characterized in that, The first diode rectifier, the second diode rectifier, and the main sending-end MMC converter are all connected to the main sending-end AC bus in sequence through a transformer and a circuit breaker. The main receiving-end MMC converter is connected to the main receiving-end AC bus in sequence through a transformer and a circuit breaker. The AC sides of the third diode rectifier, the fourth diode rectifier, and the auxiliary sending-end MMC converter are all connected to the auxiliary sending-end AC bus via a transformer and a circuit breaker, respectively. The auxiliary receiving-end MMC converter is connected to the auxiliary receiving-end AC bus via a transformer and a circuit breaker, respectively.

6. A control method for multiple DC interconnected topologies, characterized in that, Based on the multi-DC interconnect topology according to any one of claims 1-5, the method includes: The main DC path is controlled to start in MMC topology, and power is injected into the auxiliary DC path to enable the auxiliary DC path to complete black start; When the output of new energy sources is large, the transmission capacity of the main DC path under the MMC topology is insufficient. With the cooperation of the auxiliary DC path, the main DC path is switched to the hybrid converter topology. When the output of new energy sources decreases to the point that the diode rectifier in the corresponding DC path is turned off, the main DC path is switched to MMC topology in coordination with the auxiliary DC path.

7. The control method for multiple DC interconnected topologies according to claim 6, characterized in that, The black boot process includes: Control the main DC path to switch to MMC topology operation; The main receiver-side MMC converter is unlocked in a non-full-voltage state. Bypass the first diode rectifier and the second diode rectifier; Control the main sender MMC converter to unlock in VF mode; Control the grid connection and power generation of the first new energy unit; The auxiliary sending-end MMC converter is unlocked using VF control mode with zero-start boost voltage. Start the auxiliary load of the second new energy source, and at the same time control the AC voltage of the second new energy source to be lower than the conduction voltage of the third diode rectifier and the fourth diode rectifier; Control the grid connection and power generation of the second new energy unit; Adjust the AC voltage of the auxiliary MMC converter to the rated voltage. The auxiliary sending-end MMC converter is controlled using P-VF control mode to keep the power of the tie line between the auxiliary sending-end MMC converter and the main DC path within the allowable range of the line.

8. The control method for multiple DC interconnected topologies according to claim 6, characterized in that, The process of switching the main DC path to a hybrid converter topology includes: Switch the operating mode of the auxiliary sending-end MMC converter in the auxiliary DC path from P-VF control mode to U-VF control mode; Switch the operating mode of the main receiver-side MMC converter in the main DC path from constant DC voltage control mode to zero power control mode. After the pole current of the main DC path is zero, disconnect the first bypass switch and the second bypass switch; The operating mode of the main receiver-end MMC converter in the main DC path is switched to full-voltage operation. Close the first positive switch and the first negative switch; Switch the operating mode of the MMC converter at the sending end of the main DC path from VF control mode to P-VF control mode. The power of the connecting line between the main DC path and the auxiliary DC path is controlled at a preset low value.

9. The control method for multiple DC interconnected topologies according to claim 6, characterized in that, The process of switching the main DC path to an MMC topology includes: Reduce the AC voltage of the main DC path's main sending end MMC converter to below the turn-on voltage of the first diode rectifier and the second diode rectifier; Disconnect the first positive switch and the first negative switch; After the AC voltage of the main sending end MMC converter of the main DC path is restored to the rated voltage, the receiving end MMC converter of the main DC path is switched to non-full voltage operation. Close the first bypass switch and the second bypass switch; Switch the operating mode of the MMC converter at the sending end of the auxiliary DC path from U-VF control mode to P-VF control mode; The power of the tie line between the main DC path and the auxiliary DC path shall be controlled within the allowable range of the line.

10. A control device for multiple DC interconnected topologies, characterized in that, The control method based on any one of claims 6-9 for multiple DC interconnect topologies, the apparatus comprising: The first control module is used to control the main DC path to start in MMC topology and to enable the auxiliary DC path to complete black start by injecting power into the auxiliary DC path. The second control module is used to switch the main DC path to a hybrid converter topology when the main DC path has insufficient transmission capacity under the MMC topology, in coordination with the auxiliary DC path, when the output of new energy is large. The third control module is used to switch the main DC path to MMC topology in coordination with the auxiliary DC path when the output of the new energy source decreases to the point that the diode rectifier of the corresponding DC path is turned off.

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