A flexible ring closure device

Abstract

The application discloses a flexible loop closing device and method, and relates to the technical field of power electronic equipment of power distribution network, which comprises a first power frequency transformer, a second power frequency transformer and a plurality of power modules; the high-voltage side windings of the first power frequency transformer and the second power frequency transformer are connected with a first medium-voltage power grid and a second medium-voltage power grid respectively. The application transforms the medium-voltage into power frequency low-voltage through the power frequency transformer, so that each power module only bears the power frequency low-voltage, and the voltage resistance requirement of the semiconductor device is reduced; the physical isolation and natural current sharing are realized through the natural electrical isolation between the windings, and the complex voltage sharing control is not needed; when any power module fails, the system automatically exits, and the power supply capacity of the system is smoothly reduced in proportion to the number of the remaining modules. The application is applied to the flexible loop closing operation scene of the power distribution network, and can reduce the system cost, simplify the structure of the control system and improve the long-term operation reliability.

Description

Technical Field

[0001] This invention relates to the field of power electronic equipment for power distribution networks, and in particular to a flexible loop closing device. Background Technology

[0002] Distribution networks typically adopt a "closed-loop design, open-loop operation" mode to avoid excessive short-circuit current. However, this mode has low power supply reliability and low equipment utilization, and cannot meet the dynamic control requirements after the connection of distributed photovoltaic, energy storage, and electric vehicle charging piles, nor can it meet the stringent power quality requirements of modern industrial sensitive loads.

[0003] Existing flexible loop-connected devices mostly employ two modular multilevel AC / DC converters connected back-to-back. However, this approach presents the following technical challenges: Since each power module must withstand the full medium-voltage load, high voltage ratings are required for the semiconductor devices within the power modules, leading to high device costs. Furthermore, high-voltage devices experience significant switching losses, limiting system efficiency. Additionally, because the power modules are connected in series to achieve medium-voltage division, strict requirements are placed on the equalization of capacitor voltages across each module. This necessitates complex modulation and control algorithms, such as carrier phase shifting and voltage equalization loops, to maintain voltage balance among the modules, resulting in a complex control system and a long debugging cycle. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is: addressing the issues of high power module withstand voltage requirements, complex series voltage division and equalization control, and high system cost in existing modular multilevel AC / DC converter back-to-back solutions.

[0005] The above-mentioned technical problems are solved by the following technical solution: The present invention proposes a flexible loop closing device, which includes a first power frequency transformer, the high voltage side winding of which is connected to a first medium voltage power grid; The second power frequency transformer has its high-voltage side winding connected to the second medium-voltage power grid. Multiple power modules, each of which has its two ends electrically connected to the low-voltage side of the first power frequency transformer and the second power frequency transformer, respectively. Each of the power modules receives instructions from the upper level to control the magnitude and direction of the transmitted power in a closed loop. In case of failure, the system automatically exits operation without affecting the normal operation of other power modules, thus achieving online redundancy. The first and second power frequency transformers convert the medium-voltage grid voltage to low-voltage power frequency, so that each power module only bears the low-voltage power frequency, forming physical isolation and natural current sharing.

[0006] In a preferred embodiment of the flexible loop closing device of the present invention: the power module includes a first AC / DC module and a second AC / DC module connected back to back; the AC side of the first AC / DC module and the second AC / DC module are respectively connected to the low-voltage side of the corresponding first power frequency transformer and the second power frequency transformer. The DC sides of the first AC / DC module and the second AC / DC module are connected to form an intermediate DC bus.

[0007] In a preferred embodiment of the flexible loop closing device of the present invention: both the first power frequency transformer and the second power frequency transformer are multi-winding transformers; the low-voltage side of the multi-winding transformer is provided with multiple low-voltage windings, and each of the low-voltage windings is connected to each of the power modules in a one-to-one correspondence.

[0008] In a preferred embodiment of the flexible loop closing device of the present invention: each of the low-voltage windings is electrically isolated, so that there is no direct electrical coupling between each of the power modules, and the voltage division is naturally achieved without the need for voltage equalization control.

[0009] In a preferred embodiment of the flexible loop closing device of the present invention: at least one of the first power frequency transformer or the second power frequency transformer is a double-winding transformer; The dual-winding transformer contains only one primary winding and one secondary winding; One end of each of the power modules is connected in parallel and then connected to the secondary winding.

[0010] In a preferred embodiment of the flexible loop closing device of the present invention: a high-frequency synchronization signal module is provided between each of the power modules to synchronize the switching actions of the first AC / DC module and the second AC / DC module and suppress the high-frequency circulating current between each of the power modules.

[0011] In a preferred embodiment of the flexible loop closing device of the present invention: both the first power frequency transformer and the second power frequency transformer are double-winding transformers; A current sharing control module is also provided between each of the power modules to realize dynamic current sharing among the power modules and solve the current sharing problem during bidirectional power transmission.

[0012] In a preferred embodiment of the flexible loop-closing device of the present invention: the intermediate DC bus of each power module is led out and connected in parallel to form a low-voltage DC port; the voltage of the low-voltage DC port is 750V or 1500V; it is used to construct a DC distribution network and connect photovoltaic, energy storage or charging pile equipment; power can flow bidirectionally.

[0013] In a preferred embodiment of the flexible loop closing device of the present invention: both the first AC / DC module and the second AC / DC module adopt a two-level three-phase bridge circuit. The AC side of both the first AC / DC module and the second AC / DC module is equipped with any one of L-type, LC-type, or LCL-type filters.

[0014] In a preferred embodiment of the flexible loop closing device of the present invention: both the first AC / DC module and the second AC / DC module adopt a three-level diode clamped three-phase bridge circuit. The beneficial effects of this invention are as follows: the first and second power frequency transformers convert the medium voltage to the low power frequency voltage, so that each power module only bears the low power frequency voltage, fundamentally reducing the withstand voltage requirements of the semiconductor devices in the power modules; the power modules are physically isolated from each other through the windings of the power frequency transformers, and the voltage distribution is naturally balanced, without the need for complex voltage equalization control; when any power module fails, it automatically exits operation without affecting other power modules, and the power supply capacity of the system decreases smoothly according to the proportion of the remaining modules, realizing online redundancy of the power modules. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments of the present invention will be briefly described below. Obviously, the drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention. Wherein: Figure 1 This is a structural diagram of the flexible ring-closing device of the present invention; Figure 2 This is a topology diagram of a transformer with two windings at one end, as described in this invention. Figure 3 This is a topology diagram of a transformer with two windings at both ends in this invention; Figure 4 This is a topology diagram of the present invention with a low-voltage DC port; Figure 5 This is a topology diagram of the two-level AC / DC module of the present invention; Figure 6 This is a topology diagram of the three-level AC / DC module of the present invention; Figure 7 This is a simulation model of the flexible ring-closing device of the present invention; Figure 8 The voltage and current simulation waveforms of the flexible loop closing device of the present invention during forward power flow are shown. Figure 9 The voltage and current simulation waveforms of the flexible loop closing device of the present invention when the power flows in reverse are shown. Detailed Implementation

[0016] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0017] The terminology used in this invention is that which is currently widely used in the art in consideration of the function of the invention; however, these terms may vary according to the intent of those skilled in the art, precedent, or new technology in the art. Furthermore, specific terms may be chosen by the applicant, and in such cases, their detailed meanings will be described in the detailed description of the invention. Therefore, the terms used in this specification should not be construed as simple names, but rather based on their meanings and the overall description of the invention.

[0018] Reference Figure 1 and Figure 7 This embodiment provides a flexible loop closing device, including a first power frequency transformer 1, whose high-voltage side winding is connected to a first medium-voltage power grid 4; The second power frequency transformer 2 has its high-voltage side winding connected to the second medium-voltage power grid 5; Multiple power modules 3, with both ends of each power module 3 electrically connected to the low-voltage side of the first power frequency transformer 1 and the second power frequency transformer 2, respectively. Each power module 3 receives instructions from the upper level to control the magnitude and direction of transmitted power in a closed loop; In case of failure, it automatically exits operation without affecting the normal operation of other power modules 3, thus achieving online redundancy; the first power frequency transformer 1 and the second power frequency transformer 2 transform the medium voltage grid voltage into power frequency low voltage, so that each power module 3 only bears the power frequency low voltage, and forms physical isolation and natural current sharing.

[0019] In this embodiment, the high-voltage winding of the first power frequency transformer 1 is connected to the first medium-voltage power grid 4, and the high-voltage winding of the second power frequency transformer 2 is connected to the second medium-voltage power grid 5. One end of each power module 3 is electrically connected to the low-voltage side of the first power frequency transformer 1, and the other end is electrically connected to the low-voltage side of the second power frequency transformer 2; that is, each power module 3 is connected across the first power frequency transformer 1 and the second power frequency transformer 2, forming a flexible closed-loop path between the first medium-voltage power grid 4 and the second medium-voltage power grid 5.

[0020] Each power module 3 receives instructions from the superior control and protection system and controls the magnitude and direction of its transmitted power in a closed loop. When one or more power modules 3 fail, they automatically shut down without affecting the normal operation of other power modules 3, thus achieving online redundancy. The first power frequency transformer 1 and the second power frequency transformer 2 transform the medium-voltage grid voltage to low-voltage power frequency, ensuring that each power module 3 only bears the low-voltage power frequency. Physical isolation and natural current sharing are achieved between the power modules 3 through the first and second power frequency transformers 1 and 2.

[0021] This embodiment employs a combination of two power frequency transformers and multiple power modules, replacing the existing back-to-back connection scheme of two modular multilevel AC / DC converters. Specifically, the medium-voltage voltage is converted to low-voltage power frequency through the first power frequency transformer 1 and the second power frequency transformer 2, transferring the function of medium-voltage voltage division from the power modules to the power frequency transformers. This allows each power module to only bear the low-voltage power frequency, thereby reducing the voltage withstand requirements of the semiconductor devices in the power modules. Simultaneously, the electrical isolation characteristics between the windings of the power frequency transformers enable physical isolation and natural current sharing among the power modules, replacing the complex voltage equalization control in the series voltage division scheme. Furthermore, since each power module is independently connected between the two power frequency transformers, a failure in any power module can automatically shut down without affecting the other power modules, achieving online redundancy of the power modules.

[0022] Reference Figure 1 and Figures 5-6 In one embodiment of the present invention, the power module 3 further includes a first AC / DC module 31 and a second AC / DC module 32 connected back to back; the AC sides of the first AC / DC module 31 and the second AC / DC module 32 are respectively connected to the low-voltage sides of the corresponding first power frequency transformer 1 and the second power frequency transformer 2. The first AC / DC module 31 and the second AC / DC module 32 are connected on their DC sides to form an intermediate DC bus 33.

[0023] In this embodiment, the AC sides of the first AC / DC module 31 and the second AC / DC module 32 are respectively connected to the low-voltage sides of the corresponding first power frequency transformer 1 and second power frequency transformer 2, and the DC sides of the first AC / DC module 31 and the second AC / DC module 32 are connected to form an intermediate DC bus 33. That is, each power module 3 is composed of two AC / DC modules connected back to back through a common DC bus, the AC side is respectively connected to the corresponding low-voltage windings 6 of the two power frequency transformers, and the DC side realizes energy exchange through the intermediate DC bus 33.

[0024] As an option in this embodiment, the first AC / DC module 31 and the second AC / DC module 32 can be adopted as follows: Figure 5 The two-level three-phase bridge circuit shown can also be used Figure 6The three-phase bridge circuit with diode clamping shown can be configured with L-type, LC-type, or LCL-type filters on the AC side as needed. Both the first AC / DC module 31 and the second AC / DC module 32 have bidirectional power flow capability: when power is transferred from the first medium-voltage grid 4 to the second medium-voltage grid 5, the first AC / DC module 31 operates in rectification mode and the second AC / DC module 32 operates in inverter mode; conversely, the first AC / DC module 31 operates in inverter mode and the second AC / DC module 32 operates in rectification mode. The intermediate DC bus 33 serves as an energy buffer for the two AC / DC modules, and its voltage level can be selected according to the system capacity, such as 750V or 1500V.

[0025] The low-voltage side of the multi-winding transformer has multiple low-voltage windings 6, each connected one-to-one to a power module 3. That is, each power module 3 occupies a dedicated low-voltage winding 6, forming a topology of "one power module corresponding to one low-voltage winding". The high-voltage side of the multi-winding transformer consists of a single high-voltage winding connected to the medium-voltage power grid, while the low-voltage side comprises multiple electrically independent low-voltage windings 6. Each low-voltage winding 6 has the same number of turns, voltage level, and power capacity, exhibiting good parameter consistency. The number of windings in the multi-winding transformer can be flexibly configured according to the number of power modules 3, for example, configuring two, three, or more low-voltage windings 6 to accommodate flexible closed-loop devices of different capacities.

[0026] The low-voltage windings 6 of the multi-winding transformer are electrically independent of each other, and the insulating medium between the windings provides natural electrical isolation. Since each power module 3 occupies a low-voltage winding 6, the power modules 3 are physically isolated on the AC side only through the transformer windings, and there is no other direct electrical connection except for the transformer magnetic coupling. Furthermore, since each low-voltage winding 6 is induced by voltage from the same magnetic core, the voltage amplitude is automatically consistent, and the voltage borne by each power module 3 is naturally balanced, without the need for additional voltage equalization control loops, carrier phase shift modulation, or complex voltage equalization algorithms.

[0027] As a synergistic effect of this embodiment, the multi-winding structure of the multi-winding transformer, the back-to-back connection of the AC / DC modules, and the independent control of each power module work together to achieve a safe topology of "medium-voltage closed loop + low-voltage power module": After the first power frequency transformer 1 and the second power frequency transformer 2 transform the medium-voltage voltage to the power frequency low voltage, each power module 3 only bears the power frequency low voltage, which fundamentally reduces the voltage withstand requirements of the semiconductor devices in the power module; each power module 3 independently controls its own power transmission through closed loop; when a power module 3 fails, it automatically shuts down, while the remaining power modules 3 continue to work, and the power supply capacity of the system decreases smoothly according to the proportion of the remaining modules, without causing an overall shutdown; and because the low-voltage windings 6 are isolated from each other, the fault current of the faulty module 3 will not be injected into other power modules 3, and the scope of the fault impact is limited to a single module, which facilitates fault location and on-site maintenance.

[0028] Reference Figure 2 In one embodiment of the present invention, at least one of the first power frequency transformer 1 or the second power frequency transformer 2 is a double-winding transformer 7. The dual-winding transformer 7 contains only one primary winding 71 and one secondary winding 72; Multiple power modules 3 are connected in parallel at one end and then connected to the secondary winding 72.

[0029] Furthermore, a high-frequency synchronization signal module 8 is provided between each power module 3 to synchronize the switching actions of the first AC / DC module 31 and the second AC / DC module 32, and to suppress the high-frequency circulating current between each power module 3.

[0030] In this embodiment, compared to the scheme where both the first power frequency transformer 1 and the second power frequency transformer 2 are multi-winding transformers, one of the power frequency transformers is replaced with a simpler two-winding transformer 7. This replacement simplifies transformer design, reduces transformer size and weight, and lowers transformer costs, while still retaining the core advantage of voltage division of power frequency transformers.

[0031] The primary winding 71 is connected to the corresponding medium-voltage power grid, namely the first medium-voltage power grid 4 or the second medium-voltage power grid 5, and undertakes the medium-voltage level; the secondary winding 72 is connected to one end of the corresponding multiple power modules 3 and undertakes the low-voltage level of the power frequency. The high-voltage side and the low-voltage side of the double-winding transformer 7 achieve energy transfer through magnetic coupling, and the windings are electrically isolated from each other through an insulating medium.

[0032] One end of each of the multiple power modules 3 is connected in parallel to the secondary winding 72. That is, the output terminal of the secondary winding 72 serves as a common connection point, and one end of each of the multiple power modules 3 is connected in parallel at this common connection point to the secondary winding 72. The other end of each power module 3 is still connected to the corresponding low-voltage side of another power frequency transformer, namely the multi-winding transformer in Embodiment 1 or the dual-winding transformer 7 in this embodiment.

[0033] As an extended implementation of this embodiment, at least one of the first power frequency transformer 1 and the second power frequency transformer 2, using a double-winding transformer 7, can be configured in the following ways: Only the first power frequency transformer 1 adopts a double-winding transformer 7, while the second power frequency transformer 2 retains the multi-winding transformer structure; Only the second power frequency transformer 2 adopts a two-winding transformer 7, while the first power frequency transformer 1 retains the multi-winding transformer structure; Both the first power frequency transformer 1 and the second power frequency transformer 2 adopt a double-winding transformer 7. This configuration corresponds to embodiment 3. Both the first power frequency transformer 1 and the second power frequency transformer 2 are multi-winding transformers, i.e., Example 1.

[0034] Preferably, since one end of each power module 3 is connected in parallel on the secondary winding 72, there is direct electrical coupling between the power modules 3, which may generate high-frequency circulating current. That is, when the AC / DC modules within each power module 3 switch on, slight differences in switching timing, device parameters, drive delay, etc., will generate a high-frequency voltage difference at the parallel node, thereby exciting high-frequency circulating current between the power modules 3. This high-frequency circulating current increases the current stress on the devices, reduces system efficiency, and may trigger protection actions in severe cases.

[0035] To suppress high-frequency circulating currents, this embodiment includes a high-frequency synchronization signal module 8 between each power module 3, used to synchronize the switching actions of the first AC / DC module 31 and the second AC / DC module 32. Specifically, the high-frequency synchronization signal module 8 provides a unified switching time base for the controllers of each power module 3: the switching actions of the first AC / DC module 31 and the second AC / DC module 32 remain essentially consistent under the synchronization of the high-frequency synchronization signal module 8, ensuring a high degree of matching in the phase, frequency, and duty cycle of the voltage waveforms generated by each power module 3 at the parallel node, thereby suppressing high-frequency circulating currents between the power modules 3.

[0036] As a preferred embodiment of this implementation, the high-frequency synchronization signal module 8 can be implemented in the following way: High-frequency synchronization buses based on optical fiber communication offer strong resistance to electromagnetic interference and predictable delays, making them suitable for synchronization signal transmission in high-voltage electromagnetic environments. High-frequency synchronization buses based on shielded twisted-pair cables, combined with common-mode inductors, suppress common-mode interference. Wireless synchronization methods, such as UWB (Ultra-Wideband), are suitable for scenarios where cabling is inconvenient. Power line carrier communication methods transmit synchronization signals through the physical connection of the secondary winding 72, eliminating the need for additional cabling.

[0037] As a synergistic effect of this embodiment, the simplified structure of the dual-winding transformer 7, the parallel topology of multiple power modules 3, and the high-frequency synchronization signal module 8 work together to reduce transformer costs while suppressing the high-frequency circulating current problem introduced by parallel connection: compared with multi-winding transformers, the dual-winding transformer 7 has a simpler structure, smaller size, lighter weight, and lower cost; multiple power modules 3 are connected in parallel on the secondary winding 72, expanding the power capacity and redundancy of the system; the high-frequency synchronization signal module 8 suppresses the high-frequency circulating current caused by parallel connection by synchronizing the switching actions of each power module 3, enabling the system to operate stably while retaining cost advantages.

[0038] Reference Figure 3 In one embodiment of the present invention, both the first power frequency transformer 1 and the second power frequency transformer 2 are double-winding transformers 7. A current sharing control module 9 is also provided between each power module 3 to realize dynamic current sharing between each power module 3 and solve the current sharing problem during bidirectional power transmission.

[0039] In this embodiment, both the first power frequency transformer 1 and the second power frequency transformer 2 are double-winding transformers 7. Each of the double-winding transformers 7 at both ends contains only one primary winding 71 and one secondary winding 72: the primary winding 71 is connected to the corresponding first medium-voltage power grid 4 and the second medium-voltage power grid 5 respectively, and undertakes the function of converting the medium-voltage voltage to the power frequency low voltage; the secondary winding 72 is connected in parallel to one end of each of the multiple power modules 3 on their respective sides, so that each power module 3 on each side is electrically connected in parallel on the low-voltage side.

[0040] Since both ends of this embodiment use double-winding transformers 7, and each power module 3 at both ends operates in parallel, in addition to the high-frequency circulating current problem mentioned above, there is also a low-frequency dynamic current sharing problem under bidirectional power transmission conditions: Under the two operating conditions of forward power transmission from the first medium-voltage grid 4 to the second medium-voltage grid 5 and reverse power transmission from the second medium-voltage grid 5 to the first medium-voltage grid 4, the current distribution between the power modules 3 connected in parallel at both ends may be unbalanced due to factors such as differences in switching device parameters, control delay differences, sampling errors, and temperature drift. Long-term imbalance will accelerate the aging of some modules, reduce the overall lifespan of the system, and in severe cases, even trigger module overcurrent protection and cause the device to shut down.

[0041] To address the aforementioned dynamic current sharing issue, this embodiment includes a current sharing control module 9 between each power module 3. The current sharing control module 9 collects the output current signal from each power module 3, generates a current reference value adjustment for each module based on a preset current sharing algorithm, and sends it to each of the first AC / DC modules 31 and the second AC / DC modules 32. This ensures dynamic current sharing for each power module 3 under both forward and reverse power transmission conditions, keeping the load distribution deviation within an acceptable range. The current sharing algorithms employed by the current sharing control module 9 include average current sharing, maximum current automatic master-slave current sharing, and democratic current sharing, which can be flexibly selected or combined according to the device's capacity level, number of modules, and operating conditions.

[0042] The two-end dual-winding transformer scheme in this embodiment and the "at least one end dual-winding" scheme in Embodiment 2 constitute parallel implementations of the present invention in terms of the configuration range of dual-winding transformers. Compared with Embodiment 2, this embodiment, by extending the dual-winding transformer to both ends and adding the current sharing control module 9, is more suitable for closed-loop scenarios where both sides of the medium-voltage power grid 4 and 5 require simplified structures and have large loads.

[0043] Reference Figure 4 In one embodiment of the present invention, the intermediate DC bus 33 of each power module 3 is led out and connected in parallel to form a low-voltage DC port 10; the voltage of the low-voltage DC port 10 is 750V or 1500V; it is used to construct a DC distribution network and connect photovoltaic, energy storage or charging pile equipment; power can flow bidirectionally.

[0044] In this embodiment, the intermediate DC bus 33 of each power module 3 is led out and connected in parallel to form a low-voltage DC port 10. The intermediate DC bus 33 of each power module 3 is led out in parallel via a DC bus to form a unified low-voltage DC port 10. The voltage level of the low-voltage DC port 10 is 750V or 1500V, which can be flexibly selected according to the voltage level of the connected DC equipment: the 750V level is suitable for civil and light industrial and commercial DC distribution networks, and the 1500V level is suitable for medium and high voltage DC equipment such as photovoltaic strings, energy storage battery clusters, and high-power DC charging piles.

[0045] The low-voltage DC port 10 is used to construct a DC distribution network and can connect one or more of the photovoltaic power generation system, energy storage battery system or charging pile equipment. The above-mentioned equipment can achieve DC-side interconnection with this flexible loop device through the low-voltage DC port 10 and share the bidirectional power transmission channel provided by this device.

[0046] In terms of power flow, the power collected at the low-voltage DC port 10 can flow in both directions: when the connected photovoltaic or energy storage equipment feeds power to the distribution network, the power flows from the low-voltage DC port 10 through the intermediate DC bus 33 of each power module 3, the first AC / DC module 31 and the second AC / DC module 32 to the first medium-voltage grid 4 or the second medium-voltage grid 5; when the distribution network supplies power to the connected energy storage or charging pile equipment, the power flow is reversed, flowing from the first medium-voltage grid 4 or the second medium-voltage grid 5 through the power frequency transformer, the first AC / DC module 31 and the second AC / DC module 32, and the intermediate DC bus 33 to the low-voltage DC port 10.

[0047] By setting up a low-voltage DC port 10, this device can not only achieve flexible loop operation between medium-voltage distribution networks 4 and 5, but also provide a unified access point for DC distribution networks, so that DC equipment such as photovoltaic, energy storage, and charging piles can be connected to the distribution network without additional independent inverters or DC / DC converters, thereby reducing the grid connection cost and conversion loss of DC equipment.

[0048] The low-voltage DC port scheme of this embodiment can be arbitrarily combined with any transformer configuration scheme in Embodiments 1 to 3: In the multi-winding transformer scheme, each power module 3 is connected to form an intermediate DC bus 33 in parallel after being connected through an independent low-voltage winding 6; in the dual-winding transformer scheme, each power module 3 is connected to form an intermediate DC bus 33 in parallel after being connected through a secondary winding 72; in the above two cases, the parallel connection of the intermediate DC bus 33 is consistent with the lead-out method of the low-voltage DC port 10, and does not affect the core technical effect of this embodiment.

[0049] Reference Figures 5-6 and Figures 7-9 In one embodiment of the present invention, both the first AC / DC module 31 and the second AC / DC module 32 adopt a two-level three-phase bridge circuit. The AC side of the first AC / DC module 31 and the second AC / DC module 32 is equipped with any one of L-type, LC-type or LCL-type filters.

[0050] Furthermore, both the first AC / DC module 31 and the second AC / DC module 32 adopt a three-level diode clamped three-phase bridge circuit.

[0051] This embodiment provides two circuit topology implementations for the first AC / DC module 31 and the second AC / DC module 32, respectively employing a two-level three-phase bridge circuit or a three-level diode-clamped NPC three-phase bridge circuit, applicable to the AC / DC modules in various embodiments of the present invention.

[0052] In the first implementation of this embodiment, both the first AC / DC module 31 and the second AC / DC module 32 adopt a two-level three-phase bridge circuit. The two-level three-phase bridge circuit consists of six fully controlled switching devices, such as IGBTs, forming three-phase bridge arms. Each phase bridge arm is composed of two switching devices connected in series. The DC side is connected to the positive and negative terminals of the intermediate DC bus 33, and the three-phase AC terminals are connected to the low-voltage winding 6 or the secondary winding 72 of the corresponding power frequency transformer through filters. The two-level three-phase bridge circuit has a simple topology, mature control algorithm, fewer switching devices, and low unit cost, making it suitable for medium- and low-voltage capacity, cost-sensitive applications.

[0053] In the second implementation of this embodiment, both the first AC / DC module 31 and the second AC / DC module 32 adopt a three-level diode clamped NPC three-phase bridge circuit. Based on the two-level three-phase bridge circuit, the three-level NPC three-phase bridge circuit expands the two switching devices of each phase arm into four switching devices connected in series, and adds a pair of clamping diodes in the middle connected to the DC side midpoint, allowing the AC side output voltage of each phase arm to switch between three levels: positive bus, zero-potential DC midpoint, and negative bus. The DC side capacitor consists of two sets of capacitors connected in series, with the midpoint led out as a zero-level reference.

[0054] Compared to a two-level topology, the three-level NPC topology has the following technical advantages: each switching device only bears half of the DC-side voltage in the off state, allowing the use of switching devices with lower withstand voltage ratings to achieve the same medium voltage level, or to increase the voltage level of the device under the same withstand voltage rating; the AC-side output voltage switches between three levels, the voltage waveform is closer to a sine wave, and the current ripple is smaller; the reduced voltage stress on the switching devices leads to a decrease in both switching losses and conduction losses, improving the system conversion efficiency; the reduced effective current value allows for a corresponding reduction in the size and weight of current-conducting devices such as busbars and connecting wires, significantly improving the power density of the entire flexible closed-loop device. As a configurable implementation of this embodiment, the AC sides of the first AC / DC module 31 and the second AC / DC module 32 are configured with any one of L-type, LC-type, or LCL-type filters: the L-type filter has the simplest structure, consisting of only a single inductor, and is suitable for applications with low current ripple requirements and small device capacity; the LC-type filter adds a set of filter capacitors in parallel to the L-type inductor, which can effectively suppress high-order harmonics near the switching frequency, and is suitable for medium-capacity applications with certain power quality requirements; the LCL-type filter adds... A second-order filter composed of a set of capacitors and inductors has the strongest suppression capability for high-order harmonics and is suitable for applications with high current, high power, and strict requirements for grid-connected power quality. For the three-level NPC topology, the switching frequency components are concentrated in a higher frequency band, requiring a smaller filter inductance, and the overall size and cost of the filter can be further reduced. The three types of filters can be flexibly selected according to the capacity level of this flexible loop-connected device, the power grid power quality standards, and the cost budget. As a synergistic effect of this embodiment, the two-level and three-level NPC circuit topologies work together with the "power frequency transformer + multi-power module parallel" architecture of this invention: the two-level topology, with its simple structure, mature control, and low unit cost, provides an economical basic unit for the modular design of this invention; the three-level NPC topology further reduces the device cost and current loss of the power module by reducing the voltage and current stress of the switching devices, and enables this invention to have better power quality performance in sensitive load scenarios and greater deployment flexibility in space-constrained field scenarios such as urban power distribution network renovation; the L / LC / LCL filters can be flexibly configured in the two topologies according to the capacity level, enabling this invention to be flexibly adaptable between low-cost and high-quality scenarios.

[0055] As a verifiable implementation of this embodiment, the flexible ring-closing device of the present invention has been verified by the PLECS simulation platform. Figure 7 In the simulation model shown, two two-level AC / DC converters are connected back-to-back to form a single power module 3. The AC side of the AC / DC converters is connected to two medium-voltage power grids via transformers (represented by equivalent inductance in the simulation). The control strategy is that the first AC / DC converter controls the voltage of the intermediate DC bus 33, and the second AC / DC converter controls the D-axis component of the AC side current. The main simulation parameters are as follows: transformer equivalent inductance 10mH, DC bus capacitance 1mF, AC side line voltage 400V, DC voltage reference value 800V, control period 20μs, controller type PI.

[0056] Simulation of a scenario in a real-world engineering project where two AC ports transfer power to each other: power flows in the forward direction as follows... Figure 8 As shown, the DC-side current is stable at 61A (no fluctuation), the average DC-side voltage is 800V (small fluctuation), and the current amplitude at the second port is 100A; when power flows in reverse, as... Figure 9 As shown, the DC-side current is stable at -61A (no fluctuation), the average DC-side voltage is 800V (small fluctuation), and the current amplitude at the second port is 100A. The simulation results confirm that the two-level three-phase bridge circuit used in Embodiment 5 of this invention can operate normally and stably in a bidirectional power flow scenario, and the AC-side current waveform is smooth and distortion-free, verifying that the flexible loop-closing device of this invention can achieve bidirectional flexible loop closure between medium-voltage distribution networks.

[0057] Finally, it should be noted that the methods and devices described in detail above are merely embodiments, and those skilled in the art can modify these embodiments in different ways as long as they do not depart from the scope of the present invention.

Claims

1. A flexible ring-closing device, characterized in that: include; The first power frequency transformer (1) has its high voltage side winding connected to the first medium voltage power grid (4). The second power frequency transformer (2) has its high-voltage side winding connected to the second medium-voltage power grid (5). Multiple power modules (3), each of which has two ends electrically connected to the low-voltage side of the first power frequency transformer (1) and the second power frequency transformer (2), respectively; Each of the power modules (3) receives instructions from the upper level to control the magnitude and direction of the transmitted power in a closed loop; In case of failure, it will automatically exit operation without affecting the normal operation of other power modules (3), thus achieving online redundancy; the first power frequency transformer (1) and the second power frequency transformer (2) will transform the medium voltage grid voltage into power frequency low voltage, so that each power module (3) only bears the power frequency low voltage, and form physical isolation and natural current sharing.

2. The flexible ring-closing device according to claim 1, characterized in that: The power module (3) includes a first AC / DC module (31) and a second AC / DC module (32) connected back to back; the AC sides of the first AC / DC module (31) and the second AC / DC module (32) are respectively connected to the low-voltage sides of the corresponding first power frequency transformer (1) and second power frequency transformer (2). The first AC / DC module (31) and the second AC / DC module (32) are connected on their DC sides to form an intermediate DC bus (33).

3. The flexible ring-closing device according to claim 1, characterized in that: Both the first power frequency transformer (1) and the second power frequency transformer (2) are multi-winding transformers; the low-voltage side of the multi-winding transformer is provided with multiple low-voltage windings (6), and each low-voltage winding (6) is connected to each power module (3) in a one-to-one correspondence.

4. The flexible ring-closing device according to claim 3, characterized in that: The low-voltage windings (6) are electrically isolated from each other, so that there is no direct electrical coupling between the power modules (3), and the voltage division is naturally achieved without the need for voltage equalization control.

5. The flexible ring-closing device according to claim 2, characterized in that: At least one of the first power frequency transformer (1) or the second power frequency transformer (2) is a double-winding transformer (7); The dual-winding transformer (7) contains only one primary winding (71) and one secondary winding (72). One end of each of the power modules (3) is connected in parallel and then connected to the secondary winding (72).

6. The flexible ring-closing device according to claim 5, characterized in that: A high-frequency synchronization signal module (8) is provided between each of the power modules (3) to synchronize the switching actions of the first AC / DC module (31) and the second AC / DC module (32) and suppress the high-frequency circulating current between each of the power modules (3).

7. The flexible ring-closing device according to claim 1, characterized in that: Both the first power frequency transformer (1) and the second power frequency transformer (2) are double-winding transformers (7); A current sharing control module (9) is also provided between each of the power modules (3) to realize dynamic current sharing between each of the power modules (3) and solve the current sharing problem during bidirectional power transmission.

8. The flexible ring-closing device according to claim 2, characterized in that: The intermediate DC bus (33) of each power module (3) is led out and connected in parallel to form a low-voltage DC port (10); the voltage of the low-voltage DC port (10) is 750V or 1500V; it is used to build a DC distribution network and connect photovoltaic, energy storage or charging pile equipment; power can flow in both directions.

9. The flexible ring-closing device according to claim 2, characterized in that: Both the first AC / DC module (31) and the second AC / DC module (32) adopt a two-level three-phase bridge circuit; The AC side of the first AC / DC module (31) and the second AC / DC module (32) is equipped with any one of L-type, LC-type or LCL-type filters.

10. The flexible ring-closing device according to claim 2, characterized in that: Both the first AC / DC module (31) and the second AC / DC module (32) adopt a three-level diode clamped three-phase bridge circuit.

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