Hybrid conversion system and control method thereof
Through the coordinated control and modular design of the hybrid converter system, the problem of insufficient reverse power transmission capability of the DRU and MMC DC series topology was solved, realizing efficient black start and stable operation in offshore wind power scenarios, simplifying the operation and maintenance process and reducing costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHINA THREE GORGES CORPORATION
- Filing Date
- 2026-04-21
- Publication Date
- 2026-07-21
AI Technical Summary
In existing DC transmission systems, the DC series topology of DRU and MMC has insufficient reverse power transmission capability, resulting in cumbersome black start operations and frequent equipment outages. This is especially inconvenient for operation and maintenance in offshore wind power scenarios, and existing solutions are costly or significantly increase construction costs.
A hybrid converter system is adopted, including a sending-end converter, a receiving-end converter, and a power-consuming branch. By coordinating the topology switching of the sending-end converter, the output voltage level of the receiving-end converter, and the power consumption of the power-consuming branch, the system achieves black start and power balance. Combined with the modular design of bypassable diode rectifiers and MMC converters, precise switching between bidirectional bypass, hybrid converter, and forward bypass is achieved.
It enables black start without relying on backup power from the new energy side, simplifies the operation process, avoids equipment downtime, improves the system's operational flexibility and stability, adapts to continuous operation under extreme weather conditions, and reduces operation and maintenance risks and construction costs.
Smart Images
Figure CN122437104A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of DC power transmission technology, and more specifically to a hybrid converter system and its control method. Background Technology
[0002] In the field of DC power transmission technology for isolated renewable energy sources, hybrid converter technology combining diode rectifiers (DRUs) and modular multilevel converters (MMCs) has attracted much attention due to its cost advantages, especially the widespread application of topologies where the two are connected in series on the DC side. However, this type of topology has significant limitations: isolated sending-end systems require an AC voltage source to be constructed through the sending-end converter to enable renewable energy units to start up and connect to the grid. The DC series structure of DRUs and MMCs does not have the ability to reverse power transmission from the receiving end to the sending end, causing the system to rely on renewable energy-side auxiliary equipment for black start. When renewable energy sources cannot generate power due to weather or other factors, the DC loop blocking effect of the diodes will force the sending-end converter and related power generation and transmission equipment to shut down. Restarting is cumbersome, causing not only inconvenience in operation and maintenance but also loss of power generation time. This problem is particularly prominent in offshore wind power scenarios.
[0003] To address these issues, various technical solutions have emerged, but they still have significant drawbacks: some solutions achieve black start by adding diesel generators, energy storage, or other self-contained power sources at the sending end, but require starting each generator individually when new energy sources are available, making the operation cumbersome; some solutions use a converter structure that connects diode converters (DUR) and full-bridge MMC converters in series, which can achieve reverse power transmission, but the full-bridge MMC converter equipment is expensive, and the technical difficulty of operating DC submarine cables under negative pressure in offshore wind power scenarios is significant; and some solutions use a bipolar DC transmission structure, utilizing the independence of bipolar operation to achieve reverse power transmission, but the increased number of DC submarine cable connections leads to a significant increase in construction costs, making it difficult to meet the economic requirements of engineering applications. Summary of the Invention
[0004] This invention provides a control method for a hybrid converter system to solve the limitations of reverse power transmission capability caused by the unidirectional conduction characteristics of conventional DRU and MMC DC series converters.
[0005] In a first aspect, the present invention provides a hybrid converter system, comprising: a sending-end converter, a receiving-end converter, and an energy-consuming branch, wherein the AC side of the sending-end converter is connected to the sending-end AC bus, and the DC side of the sending-end converter is connected to the DC side of the receiving-end converter; the sending-end converter is used to switch to a bidirectional bypass topology during black start; to switch to a hybrid converter topology when the output of the renewable energy source is greater than a first preset value; and to switch to a forward bypass topology when the output of the renewable energy source is less than a second preset value; the AC side of the receiving-end converter is connected to the receiving-end AC bus; the energy-consuming branch is connected in parallel to the sending-end AC bus, and the energy-consuming branch is used to consume renewable energy during the switching of the sending-end converter; the sending-end AC bus is used to connect renewable energy sources, and the receiving-end AC bus is used to connect to the power grid; the first preset value is greater than the second preset value.
[0006] In one optional embodiment, the sending-end converter includes: a first bypassable diode rectifier, a second bypassable diode rectifier, and a sending-end MMC converter, wherein the AC side of the first bypassable diode rectifier is connected to the sending-end AC bus, the first DC terminal of the first bypassable diode rectifier is connected to the first DC terminal of the receiving-end converter, and the second DC terminal of the first bypassable diode rectifier is connected to the first DC terminal of the sending-end MMC converter; the AC side of the sending-end MMC converter is connected to the sending-end AC bus, and the second DC terminal of the sending-end MMC converter is connected to the first DC terminal of the second bypassable diode rectifier; the AC side of the second bypassable diode rectifier is connected to the sending-end AC bus, and the second DC terminal of the second bypassable diode rectifier is connected to the second DC terminal of the receiving-end converter.
[0007] In one optional embodiment, both the first bypassable diode rectifier and the second bypassable diode rectifier include: a first diode, a second diode, a diode rectifier bridge, a first switch, a second switch, and a third switch. The AC side of the diode rectifier bridge is the AC side of both the first and second bypassable diode rectifiers. The first DC terminal of the diode rectifier bridge is connected to the cathode of the first diode and the anode of the second diode. The second DC terminal of the diode rectifier bridge is connected to the first terminal of the first switch. The anode of the first diode is connected to the first terminal of the second switch. The cathode of the second diode is connected to the first terminal of the third switch. The second terminal of the first switch is connected to the second terminals of both the second and third switches. The first DC terminal of the diode rectifier bridge is the first DC terminal of both the first and second bypassable diode rectifiers. The second terminal of the first switch is the second DC terminal of both the first and second bypassable diode rectifiers.
[0008] In one alternative implementation, the receiving-end converter is a hybrid MMC converter consisting of a full-bridge and a half-bridge submodule.
[0009] In one optional implementation, the energy-consuming branch includes: an AC energy-consuming resistor, a first converter, and a second converter, wherein the first converter and the second converter are connected back-to-back, and the AC side of the first converter is connected to the sending-end AC bus; the AC side of the second converter is connected to the AC energy-consuming resistor.
[0010] In one optional implementation, the AC side of the sending-end converter and its energy-consuming branch are connected to the sending-end AC bus in sequence via a transformer and a circuit breaker; the AC side of the receiving-end converter is connected to the receiving-end AC bus in sequence via a transformer and a circuit breaker.
[0011] Secondly, the present invention provides a control method for a hybrid converter system. Based on the above-mentioned hybrid converter system, the method includes: achieving system black start by coordinating the control mode of the sending-end converter, the operating topology, the output voltage level of the receiving-end converter, and the energy consumption power of the energy-consuming branch; when the output of the new energy source is greater than a first preset value, switching the sending-end converter to the hybrid converter topology, and achieving power output via the sending-end MMC converter and diode rectifier bridge by coordinating the control mode of the sending-end converter, the output voltage level of the receiving-end converter, and the energy consumption power of the energy-consuming branch; when the output of the new energy source is less than a second preset value, switching the sending-end converter to the forward bypass topology, and achieving power output via the sending-end MMC converter and first diode by coordinating the control mode of the sending-end converter, the output voltage level of the receiving-end converter, and the energy consumption power of the energy-consuming branch.
[0012] In one optional implementation, the process of achieving system black start includes: controlling the first bypassable diode rectifier and the second bypassable diode rectifier to switch to a bidirectional bypass topology; controlling the receiving-end converter to unlock in DC non-full voltage state; controlling the sending-end MMC converter to unlock in VF control mode; and controlling the new energy unit to complete grid connection after the auxiliary load has stabilized.
[0013] In one optional implementation, when the output power of the new energy source exceeds a first preset value, the first converter and the second converter are unlocked sequentially, and the initial AC voltage amplitude of the AC energy-consuming resistor is set to zero. The DC voltage reference value of the receiving-end converter is increased from the DC non-full voltage value to 1.05 times the DC non-full voltage value. The AC voltage of the AC energy-consuming resistor is increased to increase the energy consumption power, so that the system gradually achieves a new power balance. When all the output power of the new energy source is consumed by the energy-consuming resistor, the second switch is disconnected. The DC voltage reference value of the receiving-end converter is increased from 1.05 times the DC non-full voltage value to the DC full voltage value. The first switch is closed. The DC voltage reference value of the second converter is set to be higher than the rated DC voltage value. After the power of the AC energy-consuming resistor drops to zero, the second converter is locked. As the power of the new energy source continues to be injected, the diode rectifier bridge of the first bypassable diode rectifier and the diode rectifier bridge of the second bypassable diode rectifier are turned on.
[0014] In one optional implementation, when the output of the new energy source is less than a second preset value, the second converter is unlocked, and the AC voltage of the sending-end MMC converter is reduced to below the conduction voltage of the first bypassable diode rectifier and the second bypassable diode rectifier, so that the diode rectifier bridge of the first bypassable diode rectifier and the diode rectifier bridge of the second bypassable diode rectifier are cut off; the first switch is disconnected, and the AC voltage of the sending-end MMC converter is restored to the rated voltage; the DC voltage reference value of the receiving-end converter is reduced from the full DC voltage value to 1.05 times the non-full DC voltage value; the second switch is closed, and the DC voltage reference value of the receiving-end converter is reduced to the non-full DC voltage value; the DC voltage reference value of the second converter is set to be higher than the rated DC voltage value; after the power of the energy-consuming resistor drops to zero, the second converter is locked.
[0015] Beneficial effects: This invention utilizes a modular design of the first and second bypassable diode rectifiers in the sending-end converter, combined with the coordinated operation of the internal first / second diodes, diode rectifier bridge, and three sets of switches, to achieve precise switching between three core topologies: bidirectional bypass, hybrid converter, and forward bypass. This perfectly solves the technical limitation of traditional DRU+MMC DC series topologies that cannot reverse power transmission. During the black start phase, the bidirectional bypass topology provides a path for reverse power transmission from the receiving end to the sending end, eliminating the need to rely on self-provided power sources such as diesel generators and energy storage on the renewable energy side, thus completely simplifying the cumbersome operations of traditional black start. When the renewable energy output exceeds a first preset value, the hybrid converter topology achieves efficient forward power transmission through the diode rectifier bridge, matching stable output conditions. When the output is less than a second preset value, the forward bypass topology avoids current discontinuity issues under low power, ensuring continuous energized operation of the system. This completely solves the pain points of forced shutdown and complex restarts of sending-end equipment under extreme weather conditions, making it particularly suitable for isolated island transmission scenarios such as offshore wind power.
[0016] In this invention, the energy-consuming branch adopts a combination structure of AC energy-consuming resistors and back-to-back connected first and second converters. Compared with traditional AC energy-consuming devices, through the outer-layer DC voltage control and open-loop AC voltage control of the first converter and the coordinated regulation of the second converter, the energy-consuming power can be dynamically adjusted to achieve precise matching with the output of new energy sources, and the power compensation accuracy is significantly improved. During topology switching, this energy-consuming branch can quickly consume the surplus power of new energy sources, avoiding voltage fluctuations or equipment impacts, and creating a stable power balance environment for topology conversion of the sending-end converter (such as hybrid converter → forward bypass, forward bypass → hybrid converter). At the same time, its AC-side energy-consuming design has a lower voltage level and higher operational safety compared with DC energy-consuming devices, further reducing system operation and maintenance risks.
[0017] The control method of this invention achieves a standardized process for black start and topology switching by coordinating the control mode of the sending-end converter, the output voltage of the receiving-end converter, and the power of the energy-consuming branch. The black start process does not require waiting for renewable energy to become available; grid connection can be completed through receiving-end MMC non-full-voltage charging and sending-end MMC VF mode reverse power feeding, making the process simple and efficient. The two topology switching processes utilize precise coordination of voltage step adjustment (such as 1.05 times Urec transition) and switch on / off, eliminating the need for complex control algorithms. The operation is convenient and responsive, quickly adapting to dynamic fluctuations in renewable energy output, reducing the workload of maintenance personnel, avoiding the cumbersome process of starting each unit individually and frequent debugging in traditional technologies, and improving system operation and maintenance efficiency. 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 composition diagram of a hybrid converter system according to an embodiment of the present invention; Figure 2 This is a detailed circuit diagram of the first bypassable diode rectifier and the second bypassable diode rectifier according to an embodiment of the present invention; Figure 3 This is a topology diagram of the sending-end MMC converter according to an embodiment of the present invention; Figure 4 This is a control block diagram of the sending-end MMC converter according to an embodiment of the present invention; Figure 5 This is a topology diagram of the receiving-end MMC converter according to an embodiment of the present invention; Figure 6 This is a control block diagram of the receiving-end MMC converter and the first converter according to an embodiment of the present invention; Figure 7 This is a control block diagram of a second converter according to an embodiment of the present invention; Figure 8 This is a flowchart of a control method for a hybrid converter system 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 hybrid converter system, such as Figure 1 As shown, it includes: a sending-end converter 1, a receiving-end converter 2, and an energy-consuming branch 3. The AC side of the sending-end converter 1 is connected to the sending-end AC bus 4, and the DC side of the sending-end converter 1 is connected to the DC side of the receiving-end converter 2. The sending-end converter 1 is used to switch to a bidirectional bypass topology during black start; to switch to a hybrid converter topology when the output of the new energy source is greater than a first preset value; and to switch to a forward bypass topology when the output of the new energy source is less than a second preset value. The AC side of the receiving-end converter 2 is connected to the receiving-end AC bus 5. The energy-consuming branch 3 is connected in parallel to the sending-end AC bus 4 and is used to consume new energy energy when the sending-end converter 1 switches. The sending-end AC bus 4 is used to connect to new energy sources, and the receiving-end AC bus 5 is used to connect to the power grid. The first preset value is greater than the second preset value.
[0024] Specifically, the sending-end converter 1 has multi-topology adaptive switching capability. During the black start phase, it switches to the bidirectional bypass topology to provide basic support for the system's reverse power supply start-up. When the output of the new energy reaches a stable level and is greater than the first preset value, it automatically switches to the hybrid converter topology to complete the forward transmission of new energy power in an efficient mode. When the output of the new energy decreases to less than the second preset value, it switches to the forward bypass topology to avoid the system operation risk under low output conditions and ensure the continuous operation of the equipment under power.
[0025] As the core of power reception and voltage regulation, the receiving-end converter 2 flexibly adapts to the needs of topology switching at the sending end, achieving system voltage matching and stable power reception, and supporting smooth switching of bidirectional transmission modes. The energy-consuming branch 3 plays a core role in power balancing during the topology switching process. During the critical stage of topology conversion at the sending-end converter 1, it precisely consumes the output energy of new energy sources to maintain system power balance, creating stable operating conditions for topology switching and avoiding voltage fluctuations or equipment impacts caused by the change in operating conditions.
[0026] In one alternative implementation, such as Figure 1 As shown, the sending-end converter 1 includes: a first bypassable diode rectifier 11, a second bypassable diode rectifier 12, and a sending-end MMC converter 13. The AC side of the first bypassable diode rectifier 11 is connected to the sending-end AC bus 4, the first DC terminal of the first bypassable diode rectifier 11 is connected to the first DC terminal of the receiving-end converter 2, and the second DC terminal of the first bypassable diode rectifier 11 is connected to the first DC terminal of the sending-end MMC converter 13. The AC side of the sending-end MMC converter 13 is connected to the sending-end AC bus 4, and the second DC terminal of the sending-end MMC converter 13 is connected to the first DC terminal of the second bypassable diode rectifier 12. The AC side of the second bypassable diode rectifier 12 is connected to the sending-end AC bus 4, and the second DC terminal of the second bypassable diode rectifier 12 is connected to the second DC terminal of the receiving-end converter 2.
[0027] The first bypassable diode rectifier 11 and the second bypassable diode rectifier 12 have multi-topology adaptive operation characteristics. They can accurately switch between different working modes such as conventional rectification, bypass, and open circuit according to the power output status of new energy sources and the system operation requirements, providing basic support for system topology transformation. The sending-end MMC converter 13 has both AC voltage construction and energy regulation functions. By flexibly switching control modes, it can adapt to different operating conditions such as reverse power feeding start-up and stable power transmission.
[0028] The three components form a complementary and synergistic whole, which can accurately respond to fluctuations in new energy output, support the smooth switching of DC system between hybrid converter topology and MMC topology, ensure the reliable progress of system black start process, continuous operation under extreme weather conditions, and efficient and stable power transmission of new energy under all operating conditions, effectively improving the system's adaptability and operational reliability to islanded transmission scenarios.
[0029] In one alternative implementation, such as Figure 2As shown, both the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12 include: a first diode D1, a second diode D2, a diode rectifier bridge H1, a first switch BP0, a second switch BP1, and a third switch BP2. The AC side of the diode rectifier bridge H1 is the AC side of both the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12. The first DC terminal of the diode rectifier bridge H1 is connected to the cathode of the first diode D1 and the anode of the second diode D2. The second DC terminal of the diode rectifier bridge H1 is connected to the first DC terminal of the first switch BP0. Terminal connections; the anode of the first diode D1 is connected to the first terminal of the second switch BP1; the cathode of the second diode D2 is connected to the first terminal of the third switch BP2; the second terminal of the first switch BP0 is connected to the second terminal of the second switch BP1 and the second terminal of the third switch BP2; the first DC terminal of the diode rectifier bridge H1 is the first DC terminal of the first bypassable diode rectifier 11 and the first DC terminal of the second bypassable diode rectifier 12; the second terminal of the first switch BP0 is the second DC terminal of the first bypassable diode rectifier 11 and the second DC terminal of the second bypassable diode rectifier 12.
[0030] The first diode D1 and the second diode D2 correspond to the energy conduction paths of forward bypass and reverse bypass, respectively. They can selectively conduct according to the power transmission direction and operating conditions, meeting the operational requirements of special scenarios such as near-zero power transmission and reverse power transmission. The three switches, through differentiated on / off combinations and in conjunction with the natural conduction and turn-off characteristics of the diodes, enable flexible switching between five operating modes: conventional rectification, forward bypass, reverse bypass, bidirectional bypass, and open circuit. This provides crucial assurance for the smooth conversion of the DC system between MMC and hybrid converter topologies. The overall structure, through complementary and synergistic component functions, avoids the high cost of relying on expensive DC circuit breakers for bypassing, while accurately responding to fluctuations in renewable energy output, black start, extreme weather, and other complex operating conditions. This ensures the reliability and operational flexibility of bidirectional power transmission, effectively improving the stability and all-condition adaptability of renewable energy power delivery.
[0031] Optionally, the diode rectifier bridge H1 is a classic 6-pulse or 12-pulse diode rectifier bridge.
[0032] Specifically, the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12 have five flexibly switchable operating topologies, as follows: (1) Conventional rectification operation, corresponding to Figure 2 Bp0 is closed, while Bp1 and Bp2 remain open; (2) Forward bypass operation, corresponding to Figure 2 Bp1 is closed, while Bp0 and Bp2 remain open; (3) Reverse bypass operation, corresponding to Figure 2Bp2 is closed, while Bp0 and Bp1 remain open; (4) Bidirectional bypass operation, corresponding to Figure 2 Bp1 and Bp2 are both closed, while Bp0 remains open; (5) Open-circuit operation, corresponding to Figure 1 Bp0, Bp1, and Bp2 are all kept disconnected.
[0033] Specifically, the DC system corresponds to two core operating topologies: (1) Hybrid commutation topology, in which the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12 operate in the core state of conventional rectifier topology; (2) MMC topology, in which the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12 operate in any one of the bypass topologies of forward bypass, reverse bypass or bidirectional bypass.
[0034] In one alternative implementation, the topology of the sending-end MMC converter 13 is as follows: Figure 3 As shown, HBSM is a half-bridge submodule.
[0035] The main control elements of the sending-end MMC converter 13 are as follows: Figure 4 As shown, it includes an AC voltage loop and a current loop, and has two control modes: one is simple AC voltage source control, namely VF mode (①); the other is adding DC voltage control outside the AC voltage loop, namely U-VF mode (②).
[0036] In one alternative implementation, the topology of the receiving-end MMC converter is as follows: Figure 5 As shown, the receiving-end converter 2 is a hybrid MMC converter with full-bridge and half-bridge submodules, where HBSM is a half-bridge submodule and FBSM is a full-bridge submodule.
[0037] The main control elements of the receiving-end MMC converter are as follows: Figure 6 As shown, it includes DC voltage control, AC current control, and a phase-locked loop.
[0038] In one alternative implementation, such as Figure 1 As shown, the energy-consuming branch 3 includes: an AC energy-consuming resistor 33, a first converter 31 and a second converter 32. The first converter 31 and the second converter 32 are connected back to back. The AC side of the first converter 31 is connected to the sending end AC bus 4. The AC side of the second converter 32 is connected to the AC energy-consuming resistor 33.
[0039] The first converter 31 undertakes the core control function, integrating the outer layer DC voltage control and open-loop AC voltage control logic, and can dynamically adjust the operating parameters to match the system power balance requirements; the second converter 32 and the AC energy dissipation resistor 33 form an energy consumption closed loop to ensure rapid and stable energy absorption; the whole system has a wide range and high precision power regulation capability, and can offset the surplus caused by the power fluctuation of new energy sources in real time during the critical stage of topology switching of the sending-end converter 1, so as to avoid system voltage surges or operational instability.
[0040] The main control elements of the first converter 31 are as follows: Figure 6 As shown, the main control elements of the second converter 32 are as follows: Figure 7 As shown, it includes open-loop AC voltage control and outer-layer DC voltage control, the goal of which is to maintain a constant DC voltage at the sending end MMC through energy-consuming power balancing.
[0041] In one alternative implementation, such as Figure 1 As shown, the AC side of the sending-end converter 1 and the energy-consuming branch 3 are connected to the sending-end AC bus 4 in sequence through a transformer and a circuit breaker; the AC side of the receiving-end converter 2 is connected to the receiving-end AC bus 5 in sequence through a transformer and a circuit breaker.
[0042] This embodiment provides a control method for a hybrid converter system, based on the above-described hybrid converter system, such as... Figure 8 As shown, the method includes: Step S1: Achieve system black start by coordinating the control mode and operating topology of the sending-end converter 1, the output voltage level of the receiving-end converter 2, and the power consumption of the power consumption branch 3.
[0043] Optionally, the process of achieving system black start includes: controlling the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12 to switch to bidirectional bypass topology; controlling the receiving-end converter 2 to unlock in DC non-full voltage state; controlling the sending-end MMC converter 13 to unlock in VF control mode; and controlling the new energy unit to complete grid connection after the auxiliary load has stabilized.
[0044] Specifically, firstly, the first bypassable diode rectifier 11 in the sending-end converter 1 is switched to a bidirectional bypass topology, i.e., BP0 is disconnected, BP1 is closed, and BP2 is closed. At the same time, the receiving-end MMC converter is unlocked in DC non-full voltage (Urec) state and reverse-charges the submodule capacitor of the sending-end MMC converter 13 through the DC pole line. After the capacitor is fully charged, the sending-end MMC converter 13 is switched to VF control mode and unlocked. The auxiliary load of the new energy unit is started through reverse power transmission. After the auxiliary load is running stably, the new energy unit is connected to the grid and the DC system is smoothly switched to the forward power transmission state, realizing the smooth progress of the entire black start process without relying on the auxiliary self-provided power supply on the new energy side, which greatly simplifies the start-up process.
[0045] Step S2: When the output of new energy is greater than the first preset value, switch the sending-end converter 1 to the hybrid converter topology. By coordinating the control mode of the sending-end converter 1, the output voltage level of the receiving-end converter 2 and the energy consumption power of the energy consumption branch 3, the power is sent out through the sending-end MMC converter 13 and the diode rectifier bridge H1.
[0046] Optionally, when the output of the new energy source exceeds the first preset value, the first converter 31 and the second converter 32 are unlocked sequentially, and the initial AC voltage amplitude of the AC energy-consuming resistor 33 is set to zero; the DC voltage reference value of the receiving-end converter 2 is increased from the DC non-full voltage value to 1.05 times the DC non-full voltage value; the AC voltage of the AC energy-consuming resistor 33 is increased to increase the energy consumption power, so that the system gradually achieves a new power balance; when all the output power of the new energy source is consumed by the energy-consuming resistor, the second switch BP1 is disconnected; the DC voltage reference value of the receiving-end converter 2 is increased from 1.05 times the DC non-full voltage value to the DC full voltage value; the first switch BP0 is closed; the DC voltage reference value of the second converter 32 is set to be higher than the rated DC voltage value; after the power of the AC energy-consuming resistor 33 drops to zero, the second converter 32 is locked; as the new energy power continues to be injected, the diode rectifier bridge H1 of the first bypassable diode rectifier 11 and the diode rectifier bridge H1 of the second bypassable diode rectifier 12 are turned on.
[0047] Specifically, when the output of new energy is detected to be greater than the first preset value (corresponding to the stable output condition of new energy), the energy-consuming branch 3 is first started, the first converter 31 is unlocked and its DC capacitor is charged, and then the second converter 32 is unlocked and the AC energy-consuming resistor 33 is connected (the initial AC voltage amplitude is approximately zero); then the DC voltage reference value of the receiving-end MMC converter is increased from Urec to 1.05Urec, causing the DC current to decrease and the sending-end DC voltage to rise. The second converter 32 increases the energy consumption power by increasing the AC voltage of the energy-consuming resistor, so that all the new energy power is consumed by the energy-consuming resistor. After the DC current drops to zero, Bp1 is disconnected; then... The receiving-end MMC converter voltage is boosted to the full DC voltage Uinv and operated under no-load. Bp0 is closed to switch to the conventional rectifier topology. At this time, thanks to the power balancing effect of the energy-consuming system, the DC pole current is approximately zero. Then, the DC voltage reference value of the second converter 32 is set to be slightly higher than the rated value. After the power of the energy-consuming resistor drops to zero, the second converter 32 is locked out. Finally, the sending-end MMC converter 13 switches to the adaptive control mode. As the new energy power is continuously injected, the diode rectifier bridge H1 is turned on, and the power is stably delivered through the sending-end MMC converter 13 and the diode rectifier bridge H1, giving full play to the high-efficiency power transmission advantages of the hybrid converter topology.
[0048] Step S3: When the output of the new energy source is less than the second preset value, the sending-end converter 1 is switched to the forward bypass topology. By coordinating the control mode of the sending-end converter 1, the output voltage level of the receiving-end converter 2, and the energy consumption power of the energy consumption branch 3, the power is sent out through the sending-end MMC converter 13 and the first diode D1.
[0049] Optionally, when the output of the new energy source is less than the second preset value, the second converter 32 is unlocked, and the AC voltage of the sending-end MMC converter 13 is reduced to below the conduction voltage of the first bypassable diode rectifier 11 and the second bypassable diode rectifier 12, so that the diode rectifier bridge H1 of the first bypassable diode rectifier 11 and the diode rectifier bridge H1 of the second bypassable diode rectifier 12 are cut off; the first switch BP0 is opened, and the AC voltage of the sending-end MMC converter 13 is restored to the rated voltage; the DC voltage reference value of the receiving-end converter 2 is reduced from the full DC voltage value to 1.05 times the non-full DC voltage value; the second switch BP1 is closed, and the DC voltage reference value of the receiving-end converter 2 is reduced to the non-full DC voltage value; the DC voltage reference value of the second converter 32 is set to be higher than the rated DC voltage value; after the power of the energy-consuming resistor drops to zero, the second converter 32 is locked.
[0050] Specifically, when it is predicted that the output of new energy is less than the second preset value (corresponding to near-zero output of new energy or extreme weather conditions), the second converter 32 is first unlocked and the AC energy-consuming resistor 33 is connected. The sending-end MMC converter 13 actively reduces the AC voltage to below the conduction voltage of the bypassable diode rectifier, causing the diode rectifier bridge H1 to be cut off. All the power of the new energy is consumed by the energy-consuming resistor. Then, Bp0 is disconnected to switch to an open circuit, and the sending-end AC voltage is restored to the rated voltage. Next, the DC voltage reference value of the receiving-end MMC converter is reduced from Uinv to 1.05Urec and it is run under no-load. Bp1 is closed to switch to the forward bypass topology (at this time, the sending-end DC voltage Urec is lower than the receiving-end voltage 1.05Urec, D1 (Cut off); then the voltage of the receiving-end MMC converter is reduced to Urec to bring D1 close to conduction. At the same time, the DC voltage reference value of the second converter 32 is set to be slightly higher than the rated value. After the power of the energy-consuming resistor drops to zero, the second converter 32 is blocked. Finally, the sending-end MMC converter 13 maintains the adaptive control mode, and the new energy power is continuously injected to make D1 fully conduct. The power is sent out through the sending-end MMC converter 13 and D1 in coordination, which effectively avoids the problem of current discontinuity under low output conditions and ensures continuous operation of the system.
[0051] 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 hybrid converter system, characterized in that, include: Sending-end converter, receiving-end converter, and energy-consuming branch, among which, The AC side of the sending-end converter is connected to the sending-end AC bus, and the DC side of the sending-end converter is connected to the DC side of the receiving-end converter. The sending-end converter is used to switch to a bidirectional bypass topology during black start; to switch to a hybrid converter topology when the output of the new energy source is greater than a first preset value; and to switch to a forward bypass topology when the output of the new energy source is less than a second preset value. The AC side of the receiving-end converter is connected to the receiving-end AC bus; The energy-consuming branch is connected in parallel to the sending-end AC bus, and the energy-consuming branch is used to consume new energy energy when the sending-end converter switches. The sending-end AC bus is used to connect to new energy sources, and the receiving-end AC bus is used to connect to the power grid; The first preset value is greater than the second preset value.
2. The hybrid converter system according to claim 1, characterized in that, The sending-end converter includes: a first bypassable diode rectifier, a second bypassable diode rectifier, and a sending-end MMC converter, wherein... The AC side of the first bypassable diode rectifier is connected to the sending-end AC bus, the first DC terminal of the first bypassable diode rectifier is connected to the first DC terminal of the receiving-end converter, and the second DC terminal of the first bypassable diode rectifier is connected to the first DC terminal of the sending-end MMC converter. The AC side of the sending-end MMC converter is connected to the sending-end AC bus, and the second DC terminal of the sending-end MMC converter is connected to the first DC terminal of the second bypassable diode rectifier. The AC side of the second bypassable diode rectifier is connected to the sending-end AC bus, and the second DC terminal of the second bypassable diode rectifier is connected to the second DC terminal of the receiving-end converter.
3. The hybrid converter system according to claim 2, characterized in that, Both the first bypassable diode rectifier and the second bypassable diode rectifier include: a first diode, a second diode, a diode rectifier bridge, a first switch, a second switch, and a third switch, wherein... The AC side of the diode rectifier bridge is the AC side of the first bypassable diode rectifier and the AC side of the second bypassable diode rectifier. The first DC terminal of the diode rectifier bridge is connected to the cathode of the first diode and the anode of the second diode. The second DC terminal of the diode rectifier bridge is connected to the first terminal of the first switch. The anode of the first diode is connected to the first terminal of the second switch; The cathode of the second diode is connected to the first terminal of the third switch; The second terminal of the first switch is connected to the second terminal of the second switch and the second terminal of the third switch; The first DC terminal of the diode rectifier bridge is the first DC terminal of the first bypassable diode rectifier and the first DC terminal of the second bypassable diode rectifier. The second terminal of the first switch is the second DC terminal of the first bypassable diode rectifier and the second DC terminal of the second bypassable diode rectifier.
4. The hybrid converter system according to claim 3, characterized in that, The receiving-end converter is a hybrid MMC converter consisting of a full-bridge and a half-bridge submodule.
5. The hybrid converter system according to claim 3, characterized in that, The energy-consuming branch includes: an AC energy-consuming resistor, a first converter, and a second converter, wherein the first converter and the second converter are connected back-to-back. The AC side of the first converter is connected to the sending-end AC bus; The AC side of the second converter is connected to the AC power dissipation resistor.
6. The hybrid converter system according to claim 1, characterized in that, The AC side and energy-consuming branch of the sending-end converter are connected to the sending-end AC bus in sequence through a transformer and a circuit breaker. The AC side of the receiving-end converter is connected to the receiving-end AC bus in sequence via a transformer and a circuit breaker.
7. A control method for a hybrid converter system, characterized in that, Based on the hybrid converter system according to any one of claims 1-6, the method comprises: By coordinating the control mode and operating topology of the sending-end converter, the output voltage level of the receiving-end converter, and the power consumption of the power-consuming branch, the system achieves black start. When the output of new energy exceeds the first preset value, the sending-end converter is switched to a hybrid converter topology. Through coordinated control of the control mode of the sending-end converter, the output voltage level of the receiving-end converter, and the energy consumption power of the energy-consuming branch, the power is sent out through the sending-end MMC converter and diode rectifier bridge. When the output of new energy is less than the second preset value, the sending-end converter is switched to the forward bypass topology. Through coordinated control of the control mode of the sending-end converter, the output voltage level of the receiving-end converter and the energy consumption power of the energy consumption branch, the power is sent out through the sending-end MMC converter and the first diode.
8. The control method for the hybrid converter system according to claim 7, characterized in that, The process of achieving a system black boot includes: Control the first bypassable diode rectifier and the second bypassable diode rectifier to switch to a bidirectional bypass topology; The control receiver-end converter is unlocked in a DC non-full voltage state; The sending-end MMC converter is unlocked in VF control mode; After the auxiliary loads have stabilized, control the new energy generating units to complete grid connection.
9. The control method for the hybrid converter system according to claim 7, characterized in that, When the output of new energy sources exceeds the first preset value The first converter and the second converter are unlocked in sequence, and the initial AC voltage amplitude of the control AC energy-consuming resistor is set to zero. The DC voltage reference value of the receiving-end converter is increased from the DC non-full voltage value to 1.05 times the DC non-full voltage value; Increase the AC voltage of the AC power-consuming resistor to increase the power consumption, so that the system can gradually achieve a new power balance; When all the power output from the new energy source is consumed by the energy-consuming resistor, disconnect the second switch; The DC voltage reference value of the receiving-end converter is increased from 1.05 times the DC non-full voltage value to the DC full voltage value; Close the first switch; Set the DC voltage reference value of the second converter to be higher than the rated DC voltage value; Once the power consumption of the AC power-consuming resistor drops to zero, the second converter is locked out. With the continuous injection of new energy power, the diode rectifier bridge of the first bypassable diode rectifier and the diode rectifier bridge of the second bypassable diode rectifier are turned on.
10. The control method for the hybrid converter system according to claim 7, characterized in that, When the output of new energy sources is less than the second preset value Unlock the second converter and control the AC voltage of the sending-end MMC converter to drop below the turn-on voltage of the first bypassable diode rectifier and the second bypassable diode rectifier, so that the diode rectifier bridge of the first bypassable diode rectifier and the diode rectifier bridge of the second bypassable diode rectifier are turned off. Disconnect the first switch to restore the AC voltage of the sending-end MMC converter to the rated voltage. The DC voltage reference value of the receiving-end converter is reduced from the full DC voltage value to 1.05 times the non-full DC voltage value; Close the second switch to reduce the DC voltage reference value of the receiving-end converter to the DC non-full voltage value; Set the DC voltage reference value of the second converter to be higher than the rated DC voltage value; Once the power consumption resistor drops to zero, the second converter is locked out.