Multi-stage dc voltage boosting aggregation and sending-out system and method based on dc voltage transformer

Abstract

A multi-stage direct current boosting collection and sending-out system and method based on direct current transformers, the system comprising a plurality of new energy power generation units for outputting low-voltage direct current; a first-stage direct current boosting unit having an input end connected with a low-voltage direct current output end of the new energy power generation unit, for boosting the low-voltage direct current to medium-voltage direct current and collecting the medium-voltage direct current through a medium-voltage direct current bus; a second-stage direct current boosting unit having an input end connected with the medium-voltage direct current bus, for boosting the medium-voltage direct current to high-voltage direct current; a third-stage direct current boosting unit having an input end connected with an output end of the second-stage direct current boosting unit, for boosting the high-voltage direct current to extra-high-voltage direct current; and an extra-high-voltage direct current transmission line connected with the output end of the third-stage direct current boosting unit, for sending out the extra-high-voltage direct current; wherein the three-stage direct current boosting units are all direct current-direct current transformers. The application adopts a full direct current form networking, does not need alternating current synchronous support equipment, and can reduce engineering cost.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation and DC power transmission technology, specifically relating to a multi-stage DC boost collection and transmission system and method based on a DC transformer. Background Technology

[0002] The "Shagohuang" region is rich in wind and solar new energy resources, which is conducive to large-scale centralized development. The base as a whole is characterized by photovoltaic power as the main source and wind power as a supplement. Due to the natural environment, the "Shagohuang" region still faces prominent problems such as high uncertainty of power supply, high construction cost of thermal power, low proportion of synchronous generator units, and weak local power grid support capacity. It is urgent to determine a suitable collection and transmission method.

[0003] Currently, the mainstream renewable energy transmission scheme adopts the "AC collection-DC transmission" mode. However, due to the weak grid characteristics on the source side, this scheme faces grid-friendly issues such as voltage instability, transient overvoltage, and wide-frequency oscillations. To ensure stable long-distance collection, a large number of synchronous support devices, such as thermal power units, synchronous condensers, and grid-type energy storage, are required. This not only leads to a significant increase in project costs but also fails to fundamentally solve the AC synchronization stability problem, placing enormous pressure on system construction and operation. Summary of the Invention

[0004] The purpose of this invention is to address the problems in the prior art by providing a multi-stage DC boost collection and transmission system and method based on a DC transformer. This system uses a fully DC network configuration, eliminating the need for AC synchronous support equipment and reducing engineering costs.

[0005] To achieve the above objectives, the present invention provides the following technical solution: Firstly, a multi-stage DC boost collection and transmission system based on a DC transformer is provided, comprising: Multiple new energy power generation units are used to output low-voltage direct current; At least one first-stage DC boost unit, the input terminal of each first-stage DC boost unit is connected to the low-voltage DC output terminal of at least one of the new energy power generation units, for boosting the low-voltage DC power to medium-voltage DC power and collecting it through the medium-voltage DC bus; At least one second-stage DC boost unit, the input of which is connected to the medium-voltage DC bus, is used to boost the medium-voltage DC power to high-voltage DC power. At least one third-stage DC boost unit, the input of which is connected to the output of the second-stage DC boost unit, is used to boost the high-voltage DC to ultra-high-voltage DC. An ultra-high voltage direct current transmission line is connected to the output terminal of the third-stage DC boost unit to transmit the ultra-high voltage direct current. Among them, the first-stage DC-DC boost unit, the second-stage DC-DC boost unit, and the third-stage DC-DC boost unit are all DC-DC converters.

[0006] As a preferred embodiment, the new energy power generation unit includes a photovoltaic power generation unit and / or a wind power generation unit; The photovoltaic power generation unit includes a photovoltaic array and a Boost circuit connected to the photovoltaic array; The wind power generation unit includes a wind turbine and an AC / DC converter connected to the wind turbine.

[0007] As a preferred embodiment, the first-stage DC boost unit adopts a modular, isolated phase-shift control dual active bridge topology with parallel inputs and series outputs.

[0008] As a preferred embodiment, each DC transformer submodule in the first-stage DC boost unit adopts an LLC resonant DC-DC converter topology, with diodes as its high-voltage side switching devices to achieve unidirectional power flow, and each DC transformer submodule has a long-term overload capacity of 1.1 pu.

[0009] As a preferred embodiment, the first-stage DC-DC boost unit comprises multiple parallel basic units, each basic unit consisting of... n The LLC resonant DC-DC converter described above is configured with parallel input and series output to boost low-voltage DC to medium-voltage DC. U z ,in, , U 0 represents the output voltage of a single LLC resonant converter; the number of basic units connected in parallel. N The active power of the new energy power station P and the power capacity of a single LLC resonant DC-DC converter P 0 decisions, in line with .

[0010] As a preferred embodiment, the second-stage DC boost unit adopts a face-to-face MMC topology with an intermediate frequency isolation transformer; The low-voltage side consists of multiple MMCs connected in parallel on the input side and in parallel on the output side, and is connected to the high-voltage side MMC through the intermediate frequency isolation transformer. The operating frequency of the intermediate frequency isolation transformer is 100Hz~200Hz.

[0011] As a preferred embodiment, the third-stage DC boost unit is composed of multiple face-to-face MMCs connected in series and parallel. The DC sides of each MMC on the low-voltage side are connected in parallel, while the MMCs on the high-voltage side are connected in series.

[0012] As a preferred embodiment, for the DC network where the medium-voltage DC bus is located, its positive and negative buses are grounded through a large resistor; For the high-voltage DC line connected to the output of the second-stage DC boost unit, a symmetrical single-pole structure is adopted, and the neutral point of the winding is grounded through a resistor on the AC side of the transformer in the second-stage DC boost unit. For the ultra-high voltage DC transmission line connected to the output end of the third-stage DC boost unit, a true bipolar structure is adopted, and the neutral points of the DC side of the sending end and the receiving end are respectively connected to their respective grounding electrodes through DC grounding electrode leads.

[0013] As a preferred embodiment, the AC connection transformer connected to the isolation transformer in the second-stage DC boost unit or the third-stage DC boost unit adopts a star-grounded-delta YNd connection to isolate the zero-sequence current path between the grid side and the converter side.

[0014] Secondly, a multi-stage DC boost and collection method based on a DC transformer is provided, including the following steps: The low-voltage DC power output from multiple new energy power generation units is boosted to medium-voltage DC power through their respective first-stage DC boost units, and the boosted medium-voltage DC power is collected into the medium-voltage DC bus. The medium-voltage DC power on the medium-voltage DC bus is boosted to high-voltage DC power through the second-stage DC boost unit; The high-voltage DC power is boosted to ultra-high-voltage DC power through a third-stage DC boost unit; The ultra-high voltage direct current is transmitted through an ultra-high voltage direct current transmission line; Among them, the first-stage DC-DC boost unit, the second-stage DC-DC boost unit, and the third-stage DC-DC boost unit are all DC-DC converters.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: To address the issues of poor grid friendliness, such as voltage instability, transient overvoltage, and wideband oscillation, and the high cost due to the need for numerous synchronous support devices when using the "AC aggregation-DC transmission" mode for new energy bases in weak grid areas like the Gobi Desert, this invention proposes a multi-stage DC boost aggregation and transmission system based on a DC transformer. This system employs a multi-stage DC boost architecture, completely eliminating the traditional AC aggregation stage. The first-stage DC boost unit boosts the low-voltage DC output from the new energy power generation unit to medium-voltage DC. The second-stage DC boost unit then boosts the medium-voltage DC to high-voltage DC. Finally, the third-stage DC boost unit boosts the high-voltage DC to ultra-high-voltage DC, which is then transmitted over long distances via ultra-high-voltage DC transmission lines. Since the system operates in DC mode from power generation and collection to transmission, it fundamentally avoids the inherent stability problems of AC systems, such as reactive voltage support, power angle stability, and frequency synchronization. Because the first-stage DC boost unit, the second-stage DC boost unit, and the third-stage DC boost unit are all DC-DC converters, there is no AC link between the new energy power generation unit and the ultra-high voltage DC transmission line. Therefore, there is no need to configure expensive large-scale synchronous support equipment such as thermal power units, synchronous condensers, or grid-type energy storage, which significantly reduces the construction investment and operation and maintenance costs of the new energy base.

[0016] Furthermore, this invention employs an optimized topology in DC transformers at all levels (the medium-voltage stage uses IPOS-LLC, and the high-voltage and ultra-high-voltage stages use face-to-face MMC), which not only meets the transformation requirements of high current on the low-voltage side and high voltage on the high-voltage side, but also achieves flexible expansion of power capacity through modular design, and has significant advantages such as high efficiency, small size and strong adaptability.

[0017] Furthermore, this invention designs differentiated grounding schemes for different voltage levels (medium voltage, high voltage, and ultra-high voltage), which can effectively balance the relationship between the insulation level of DC lines and engineering investment, solve the technical difficulties of multi-point grounding in full DC systems, and is particularly suitable for "sand and barren" areas with complex geological conditions and difficulty in setting up grounding electrodes independently. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 A schematic diagram of the structure of a multi-stage DC boost collection and transmission system based on a DC transformer according to an embodiment of the present invention; Figure 2The first-stage DC boost unit of this invention adopts a modular parallel input and series output isolated phase-shift control dual active bridge topology (DAB-IPOS) structure. Figure 3 A schematic diagram of the topology of a single-phase LLC resonant DC-DC converter according to an embodiment of the present invention; Figure 4 This invention presents a schematic diagram of a structure in which multiple sets of modular, parallel-connected input-side and series-connected output-side isolated phase-shift control dual active bridge topologies (DAB-IPOS) are connected in parallel on the output side. Figure 5 A schematic diagram of the face-to-face MMC topology of the high-voltage DC transformer in this embodiment of the invention; Figure 6 This invention provides a schematic diagram of the topology of an ultra-high voltage DC transformer where each MMC on the low-voltage side is connected in parallel and each MMC on the high-voltage side is connected in series (MMC-IPOS). Figure 7 A schematic diagram of the face-to-face MMC topology of the ultra-high voltage DC transformer in this embodiment of the invention; Figure 8 A schematic diagram of the overall main wiring of a multi-stage DC boost collection and transmission system based on a DC transformer according to an embodiment of the present invention; Figure 9 A schematic diagram of the grounding method of the DC transformer in this embodiment of the invention; Figure 10 Schematic diagram of grounding methods for high-voltage DC transformers and ultra-high-voltage DC transformers according to embodiments of the present invention; Figure 11 Simulation results of active and reactive power at the grid connection point of the 2GW all-DC collection and transmission system of this invention; Figure 12 Simulation results of ultra-high voltage DC pole line voltage and current of the 2GW all-DC collection and transmission system of this invention; Figure 13 Simulation results of high-voltage DC pole line voltage and current for a 2GW all-DC collection and transmission system according to Embodiment 2 of this invention; Figure 14 Simulation results of grid connection point voltage and current for a 2GW all-DC collection and transmission system according to Embodiment 2 of this invention; Figure 15 Simulation results of AC side voltage and current of the ultra-high voltage DC transformer in the 2GW all-DC collection and transmission system of this invention; Figure 16 Simulation results of AC side voltage and current of the high-voltage DC transformer in the 2GW all-DC collection and transmission system of this invention. Detailed Implementation

[0020] 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, and not all embodiments. Based on the embodiments of the present invention, those skilled in the art can obtain other embodiments without creative effort.

[0021] To address the issues of poor grid affinity, such as voltage instability, transient overvoltage, and wideband oscillation, and the high cost due to the need for numerous synchronous support devices when using the "AC aggregation-DC transmission" mode in renewable energy bases in weak grid areas like the "Shagohuang" region, this invention proposes a multi-stage DC boost aggregation and transmission system based on a DC transformer. This system achieves power aggregation and boosting within the field using a fully DC system, eliminating the need for numerous synchronous support devices such as thermal power units, synchronous condensers, and grid-type energy storage. It fundamentally avoids reactive power voltage support and AC power angle stability issues, is not constrained by frequency or phase synchronization, has a wider aggregation range, longer aggregation distance, and lower transmission loss, making it an effective solution for power transmission from renewable energy bases in the "Shagohuang" region.

[0022] Please see Figure 1 The present invention provides a multi-stage DC boost and transmission system based on a DC transformer, comprising: Multiple new energy power generation units are used to output low-voltage direct current; At least one first-stage DC boost unit, the input terminal of each first-stage DC boost unit is connected to the low-voltage DC output terminal of at least one of the new energy power generation units, for boosting the low-voltage DC power to medium-voltage DC power and collecting it through the medium-voltage DC bus; At least one second-stage DC boost unit, the input of which is connected to the medium-voltage DC bus, is used to boost the medium-voltage DC power to high-voltage DC power. At least one third-stage DC boost unit, the input of which is connected to the output of the second-stage DC boost unit, is used to boost the high-voltage DC to ultra-high-voltage DC. An ultra-high voltage direct current transmission line is connected to the output terminal of the third-stage DC boost unit to transmit the ultra-high voltage direct current. Among them, the first-stage DC-DC boost unit, the second-stage DC-DC boost unit, and the third-stage DC-DC boost unit are all DC-DC converters.

[0023] In this embodiment, the first-stage DC boost unit, the second-stage DC boost unit, and the third-stage DC boost unit are respectively a medium-voltage DC transformer, a high-voltage DC transformer, and an ultra-high-voltage DC transformer.

[0024] According to GB / T 35727-2017 and industry standards, in this application, the term "medium-voltage DC" refers to DC power with voltage levels of ±10kV, ±20kV, and ±35kV; the term "high-voltage DC" refers to DC power with voltage levels of ±250kV and ±320kV; and the term "ultra-high-voltage DC" refers to DC power used for long-distance, high-capacity transmission with a voltage level of ±500kV, which is higher than that of "high-voltage DC". It should be noted that the terms "high-voltage DC" and "ultra-high-voltage DC" are specific terms introduced in this application to describe DC boost architecture and are not a redefinition of "high voltage" and "ultra-high voltage" in the standard.

[0025] According to the technical solutions described in the above embodiments, the present invention is based on a multi-voltage level DC system. The system structure has good adaptability and is applicable to both photovoltaic-wind power hybrid new energy power stations and single photovoltaic or wind power power stations.

[0026] In a photovoltaic power station, multiple photovoltaic arrays are connected in parallel to form a power generation unit. Each photovoltaic array is boosted to 1500V by a Boost circuit and then connected to the medium-voltage DC bus through a medium-voltage DC transformer, thereby realizing the power collection of all arrays within the unit. All power generation units within a single power station can share the same medium-voltage DC bus.

[0027] For wind farms, a single wind turbine unit includes a wind turbine generator and an AC / DC converter. All wind turbine units within a single wind farm are also connected to the medium-voltage DC bus via a medium-voltage DC transformer.

[0028] In one possible implementation, the medium-voltage DC voltage level can be ±10kV, ±20kV, or ±35kV. In actual selection, factors such as economy and equipment footprint need to be considered comprehensively. The number of medium-voltage DC transformers to be configured should be evaluated, as well as the technical and economic indicators of key equipment such as cables and DC switches in terms of cost, insulation level, and floor space.

[0029] Medium-voltage DC is stepped up to high-voltage DC by a high-voltage DC transformer and transmitted to an ultra-high-voltage DC booster station via hundreds of kilometers of overhead DC lines. Inside the station, the high-voltage DC is further stepped up by an ultra-high-voltage DC transformer, and finally the combined power of the photovoltaic and wind power stations is transmitted over long distances through the ultra-high-voltage DC lines.

[0030] The selection of high-voltage and ultra-high-voltage DC voltage levels depends on the system's transmission capacity and transmission distance. Based on this, a technical and economic comparison needs to be conducted, taking into account the current-carrying capacity constraints of core components such as converter valves, the investment in converter station construction, the cost of DC lines, and operating losses, to determine the optimal voltage level.

[0031] In summary, taking photovoltaic power plants as an example, regarding the three-stage DC boosting system in a full DC grid connection system for photovoltaic bases, based on the low-voltage DC grid connection of new energy power plants, the first stage is a medium-voltage DC transformer, which boosts the 1.5kV DC voltage to a medium-voltage DC voltage. The second stage is a high-voltage DC transformer, which boosts the medium-voltage DC voltage to a high-voltage DC voltage. The third stage is an ultra-high-voltage DC transformer, which boosts the high-voltage DC voltage to an ultra-high-voltage DC voltage to achieve long-distance power transmission. In one possible implementation, the selection of a medium- or high-voltage, high-capacity DC transformer topology requires comprehensive consideration of the application scenario and performance indicators. Typically, comparisons and selections can be made based on the following three aspects: (1) Based on voltage ratio and voltage level: For high voltage and high ratio scenarios, a face-to-face topology with isolation transformer is recommended; for medium voltage and high ratio scenarios, a parallel input and series output structure can be selected; for high voltage and medium ratio scenarios, a hybrid cascade topology can be considered.

[0032] (2) Based on the transmission power level: single-phase topology is recommended for small and medium capacity scenarios; three-phase or multi-phase topology is recommended for large capacity scenarios, and the capacity level can be further improved by combining port series or bridge arm series / parallel connection technologies.

[0033] (3) Based on system functional requirements: It is necessary to select appropriate bridge arm topology and submodule topology in combination with specific requirements such as power flow reversal, fault isolation and port voltage control. Under the premise of meeting the overall functionality of the converter, the solution with higher safety and better economy should be given priority.

[0034] Based on the above principles, for the all-DC system scheme of the new energy base, the medium voltage link is to adopt the DAB series-parallel combined topology, while the high voltage link will adopt the MMC face-to-face topology with isolation transformer.

[0035] Please see Figure 2 In this embodiment, the first-stage DC-DC boost unit, which is a fully DC-DC converged low-voltage to medium-voltage converter, adopts a modular, isolated phase-shift controlled dual active bridge topology with parallel input and series output. The converters are DC / DC1-DC / DC... n Using the same topology, an LLC resonant DC-DC converter scheme can be adopted. On both sides of the resonant cavity (LLC series resonance), a two-level / three-level converter transforms the low-voltage side DC voltage into a symmetrical square wave as the input to the resonant cavity. The rectifier bridge rectifies and outputs the symmetrical square wave after it has been stepped up by the intermediate isolation transformer. Multiple sub-modules are connected in series on the high-voltage side to form a medium-voltage DC voltage. U z Because wind and solar power transmission is unidirectional and has self-starting capability, unidirectional power flow DC transformers can be used, i.e. Figure 3The LLC resonant converter shown can have its fully controlled high-voltage side devices replaced with diodes, while the converter is required to have a long-term overload capacity of 1.1 pu.

[0036] The medium-voltage DC transformer for all-DC convergence adopts a modular design, and the output voltage of a single LLC resonant converter is... U 0, power capacity is P 0, adopt n A basic unit is formed by connecting two DC transformers in parallel on the input side and in series on the output side, realizing a 1.5kV to... U z The boost converter, at this time, the number of DAB-IPOS modules n for:

[0037] For the power transmission from large-scale renewable energy power plants, given a fixed medium-voltage DC voltage level, the system's power output capacity primarily depends on the current level. To achieve high-capacity power aggregation, multiple DAB-IPOS converter units can be connected in parallel on the output side, such as... Figure 4 As shown, the high current output requirements of the station are met by superimposing currents.

[0038] If the active power of a single photovoltaic or wind farm is P, the power capacity of a single LLC resonant converter is P If 0, then the number of parallel module groups is... N for:

[0039] The high-voltage transformer for DC bundling adopts a face-to-face MMC topology, as shown in the figure. Figure 5 As shown. The intermediate isolation transformer of the face-to-face MMC type transformer can use medium-frequency AC (100Hz~200Hz), which can significantly reduce the size of the isolation transformer while maintaining the same losses and costs. The low-voltage side of the high-voltage DC transformer uses four MMCs connected in parallel on the input and output sides, forming a face-to-face DC transformer through a medium-frequency AC transformer to complete the voltage conversion from medium-voltage DC to high-voltage DC. The rated capacity of the high-voltage side MMC is four times that of the low-voltage side MMC. In the power transmission of large-scale renewable energy power plants, multiple sets of high-voltage DC transformers need to be connected in parallel on the output sides to achieve high-power transmission.

[0040] The topology of an ultra-high voltage DC transformer for DC transmission is as follows: Figure 6 and Figure 7As shown, both the input and output sides are composed of multiple MMCs connected in series and parallel. For high power capacity, multiple sets of face-to-face MMCs are required. The DC sides of each MMC on the low-voltage side are connected in parallel, while the MMCs on the high-voltage side are connected in series (MMC-IPOS structure) to meet voltage and current requirements.

[0041] The overall main wiring method of the multi-stage DC boost collection and transmission system based on DC transformer in this embodiment of the invention is as follows: Figure 8 As shown, the selection of grounding method mainly considers two aspects: the first aspect is the selection of the grounding point, because the selection of the grounding point determines the DC line's voltage to ground, which in turn determines the insulation level of the DC line and has an important impact on the project cost; the second aspect is whether the grounding point will carry operating current during normal operation. If it will carry operating current, then an independent DC grounding electrode must be configured and it cannot share the converter station's own grounding network.

[0042] In wind and solar power plants, the photovoltaic side uses negative grounding to achieve DC voltage output between 0 and 1.5kV; the wind turbine AC side uses neutral grounding, and the voltage is converted into DC power through rectification before being connected to the DC system.

[0043] For medium-voltage DC systems, DC collection is required through a medium-voltage DC transformer, and there is no AC system. Therefore, DC-side grounding can be considered, as it provides a reliable ground potential. A flexible grounding resistor can be configured on the DC bus side to adapt to the actual operating conditions of the system. Figure 9 As shown, the positive and negative busbars of the medium-voltage DC system are grounded through a large resistor, ensuring the consistency of the voltage levels of the positive and negative busbars. The resistance value of the large resistor is selected to meet the design criteria of high-resistance grounding systems, namely, the equivalent zero-sequence resistance of the system is not greater than the distributed capacitive reactance of each phase to ground, in order to limit the single-phase ground fault current. Large resistance or high resistance is a recognized general technical term in the power industry, and the resistance value is usually several hundred to one thousand ohms.

[0044] like Figure 10As shown, due to the complex geological conditions in the "Shagohuang" region, independently configuring grounding electrodes at each DC station presents significant engineering challenges. Therefore, the high-voltage DC system adopts a symmetrical monopole structure. Under this structure, to ensure that the positive and negative voltages remain balanced even in the event of a ground fault, the neutral point of the transformer winding on the AC side of the high-voltage side MMC2 of the medium-voltage DC transformer can be grounded via a resistor. The ultra-high-voltage DC transmission system can adopt a conventional true bipole structure, where the neutral points on the DC sides of both the transmitting and receiving ends (i.e., MMC4 and MMC5) must be connected to their respective grounding electrodes via DC grounding electrode leads. Considering that the zero-sequence current cannot be transmitted to the converter side during an AC grid fault, the connecting transformer must isolate the zero-sequence current path between the grid side and the converter side. Therefore, the AC connecting transformer generally adopts a star-grounded-delta (YNd) connection method.

[0045] Please see Figures 11 to 16 ,based on Figure 8 The embodiment of the present invention illustrates a simulation model of a 2GW full DC collection and transmission system based on the overall main wiring structure of a multi-stage DC boost collection and transmission system using a DC transformer. Figures 11 to 16 The simulation waveforms of steady-state characteristics such as voltage and current at each stage of the system are shown. The simulation waveforms of steady-state characteristics such as voltage and current at each stage of the system show that all operating indicators of the system are normal, which confirms the feasibility and effectiveness of the system structure of the embodiment of the present invention.

[0046] This invention relates to a multi-stage DC boost-and-transmit system based on DC transformers, abandoning the traditional AC boost-and-transmit mode of new energy bases. Addressing the challenges of weak power grids, complex geology, high construction and maintenance costs, and poor adaptability in large-scale new energy bases located in desert and Gobi areas, this invention comprises multiple new energy power generation units, first-stage, second-stage, and third-stage DC boost-and-transmit units, and an ultra-high-voltage DC transmission line. All three boost-and-transmit units are DC-DC converters, constructing a fully DC architecture without AC links, from new energy power generation output, multi-stage boost-and-transmit to ultra-high-voltage DC transmission. Compared to traditional new energy boost-and-transmit systems that rely on AC aggregation and grid connection, this invention eliminates all AC power conversion and transmission links, fundamentally avoiding the inherent stability problems of AC systems, such as insufficient reactive power voltage support, power angle instability, and frequency synchronization difficulties.

[0047] Based on the all-DC boost collection and transmission architecture of this invention, the system does not require expensive large-scale AC synchronous support equipment such as thermal power units, synchronous condensers, and grid-type energy storage. While ensuring the stable collection and transmission of power from new energy bases, it greatly simplifies the system's supporting equipment, effectively reduces the initial construction investment cost of large-scale new energy bases, and reduces the workload and operating expenses of large auxiliary equipment, significantly improving the economic efficiency of the new energy power transmission system.

[0048] This invention features a three-stage progressive DC boost structure: the first-stage DC boost unit boosts the low-voltage DC power from new energy sources to medium-voltage DC power and collects it through a medium-voltage DC bus; the second and third-stage DC boost units sequentially boost the medium-voltage DC power to high-voltage and ultra-high-voltage DC power, forming a multi-stage gradient voltage conversion system of low-voltage-medium-voltage-high-voltage-ultra-high-voltage.

[0049] This invention, based on a multi-level voltage hierarchy, matches differentiated grounding schemes to different voltage levels (medium, high, and ultra-high voltage), precisely adapting to the insulation characteristics and operational requirements of DC lines at each voltage level. This effectively balances the trade-off between DC line insulation configuration and overall project investment. Simultaneously, the differentiated grounding design overcomes the core technical bottleneck of multi-point grounding in all-DC collection systems, ensuring safe and stable system operation without relying on dedicated independent grounding electrodes. This perfectly adapts to the special engineering scenarios of "desert and barren" regions with complex geological conditions, high soil resistivity, and difficulty in independently deploying grounding electrodes, significantly expanding the application scope of large-scale new energy DC transmission systems.

[0050] The three-stage boost units of this invention all employ DC-DC converters. The topology is optimized for the voltage and current characteristics of each stage of the boost unit, forming a hierarchical and adaptable DC conversion system. The first-stage medium-voltage DC boost unit, responsible for low-voltage, high-current boosting and aggregation, adopts an IPOS-LLC topology, perfectly adapting to the low-voltage DC output and high-current aggregation requirements of new energy power generation units, ensuring efficient boosting and aggregation of low-voltage side power. The second and third-stage DC boost units, responsible for high-voltage and ultra-high-voltage boosting and conversion, adopt a face-to-face MMC topology, precisely matching the high-voltage conversion and transmission requirements of the high-voltage and ultra-high-voltage sides. Each stage of the DC-DC converter adopts a modular design, relying on a multi-stage distributed boosting and aggregation architecture, allowing for flexible expansion of power capacity according to the installed capacity of the new energy base, resulting in extremely strong system adaptability. Simultaneously, the optimized hierarchical topology offers advantages such as high operating efficiency, small equipment size, and high integration. Compared to traditional conversion equipment, it further reduces equipment footprint and losses, improving the operating efficiency and engineering practicality of the entire multi-stage DC boosting and aggregation transmission system.

[0051] In summary, this invention solves the problems of poor stability, high investment, difficult construction, and insufficient adaptability of traditional AC power collection and transmission systems in weak power grid areas such as deserts and Gobi. It provides a new, stable, reliable, economical, energy-saving, efficient, and flexible all-DC power collection and transmission solution for large-scale new energy bases in weak power grid areas, and has extremely high engineering promotion value and application prospects.

[0052] Another embodiment of the present invention proposes a multi-stage DC boost and collection method based on a DC transformer, comprising the following steps: The low-voltage DC power output from multiple new energy power generation units is boosted to medium-voltage DC power through their respective first-stage DC boost units, and the boosted medium-voltage DC power is collected into the medium-voltage DC bus. The medium-voltage DC power on the medium-voltage DC bus is boosted to high-voltage DC power through the second-stage DC boost unit; The high-voltage DC power is boosted to ultra-high-voltage DC power through a third-stage DC boost unit; The ultra-high voltage direct current is transmitted through an ultra-high voltage direct current transmission line; Among them, the first-stage DC-DC boost unit, the second-stage DC-DC boost unit, and the third-stage DC-DC boost unit are all DC-DC converters.

[0053] The all-DC collection and transmission system uses DC to achieve on-site power collection and voltage boosting, fundamentally avoiding reactive voltage support and AC power angle stability issues. It is not constrained by frequency and phase synchronization, has a wider collection range, longer collection distance, and lower transmission loss, making it an effective solution for power transmission from the "Shagohuang" new energy base.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A multi-stage DC boost collection and transmission system based on a DC transformer, characterized in that, include: Multiple new energy power generation units are used to output low-voltage direct current; At least one first-stage DC boost unit, the input terminal of each first-stage DC boost unit is connected to the low-voltage DC output terminal of at least one of the new energy power generation units, for boosting the low-voltage DC power to medium-voltage DC power and collecting it through the medium-voltage DC bus; At least one second-stage DC boost unit, whose input terminal is connected to the medium-voltage DC bus, is used to boost the medium-voltage DC power to high-voltage DC power. At least one third-stage DC boost unit, the input of which is connected to the output of the second-stage DC boost unit, is used to boost the high-voltage DC to ultra-high-voltage DC. An ultra-high voltage direct current transmission line is connected to the output terminal of the third-stage DC boost unit to transmit the ultra-high voltage direct current. Among them, the first-stage DC-DC boost unit, the second-stage DC-DC boost unit, and the third-stage DC-DC boost unit are all DC-DC converters.

2. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 1, characterized in that, The new energy power generation unit includes a photovoltaic power generation unit and / or a wind power generation unit; The photovoltaic power generation unit includes a photovoltaic array and a Boost circuit connected to the photovoltaic array; The wind power generation unit includes a wind turbine and an AC / DC converter connected to the wind turbine.

3. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 1, characterized in that, The first-stage DC boost unit adopts a modular, isolated phase-shift control dual active bridge topology with parallel input and series output.

4. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 3, characterized in that, Each DC transformer submodule in the first-stage DC boost unit adopts an LLC resonant DC-DC converter topology, with diodes as its high-voltage side switching devices to achieve unidirectional power flow, and each DC transformer submodule has a long-term overload capacity of 1.1 pu.

5. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 4, characterized in that, The first-stage DC boost unit comprises multiple parallel basic units, each basic unit consisting of... n The LLC resonant DC-DC converter described above is configured with parallel input and series output to boost low-voltage DC to medium-voltage DC. U z ,in, , U 0 represents the output voltage of a single LLC resonant converter; the number of basic units connected in parallel. N The active power of the new energy power station P and the power capacity of a single LLC resonant DC-DC converter P 0 decisions, in line with .

6. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 1, characterized in that, The second-stage DC boost unit adopts a face-to-face MMC topology with an intermediate frequency isolation transformer; The low-voltage side consists of multiple MMCs connected in parallel on the input side and in parallel on the output side, and is connected to the high-voltage side MMC through the intermediate frequency isolation transformer. The operating frequency of the intermediate frequency isolation transformer is 100Hz~200Hz.

7. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 1, characterized in that, The third-stage DC boost unit is composed of multiple face-to-face MMCs connected in series and parallel. The DC sides of each MMC on the low-voltage side are connected in parallel, while the MMCs on the high-voltage side are connected in series.

8. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 1, characterized in that, For the DC network where the medium-voltage DC bus is located, its positive and negative buses are grounded through a large resistor; For the high-voltage DC line connected to the output of the second-stage DC boost unit, a symmetrical single-pole structure is adopted, and the neutral point of the winding is grounded through a resistor on the AC side of the transformer in the second-stage DC boost unit. For the ultra-high voltage DC transmission line connected to the output end of the third-stage DC boost unit, a true bipolar structure is adopted, and the neutral points of the DC side of the sending end and the receiving end are respectively connected to their respective grounding electrodes through DC grounding electrode leads.

9. The multi-stage DC boost collection and transmission system based on a DC transformer according to claim 8, characterized in that, The AC connection transformer connected to the isolation transformer in the second-stage DC boost unit or the third-stage DC boost unit adopts a star-grounded-delta YNd connection to isolate the zero-sequence current path between the grid side and the converter side.

10. A method for multi-stage DC boost collection and transmission based on a DC transformer, characterized in that, Includes the following steps: The low-voltage DC power output from multiple new energy power generation units is boosted to medium-voltage DC power through their respective first-stage DC boost units, and the boosted medium-voltage DC power is collected into the medium-voltage DC bus. The medium-voltage DC power on the medium-voltage DC bus is boosted to high-voltage DC power through the second-stage DC boost unit; The high-voltage DC power is boosted to ultra-high-voltage DC power through a third-stage DC boost unit; The ultra-high voltage direct current is transmitted through an ultra-high voltage direct current transmission line; Among them, the first-stage DC-DC boost unit, the second-stage DC-DC boost unit, and the third-stage DC-DC boost unit are all DC-DC converters.

Images