A novel bidirectional dc / dc converter topology

Abstract

The application provides a bidirectional DC / DC converter topology structure, comprising: a converter input end, comprising a plurality of input full-bridge converters connected in series; wherein the number of the input full-bridge converters is 2N; the first input full-bridge converter is connected in series with the second input full-bridge converter; and the converter output end comprises a plurality of output full-bridge converters connected in parallel; the DC positive pole of each output full-bridge converter is connected to the positive pole of a low-voltage bus, and the DC negative pole of each output full-bridge converter is connected to the negative pole of the low-voltage bus; for the full-bridge converter bearing a high absolute voltage, the application can reduce the absolute voltage difference between the primary winding and the secondary winding of the transformer by expanding the number of the transformer, thereby reducing the insulation pressure between the primary winding and the secondary winding of the transformer and reducing the breakdown risk of the equipment.

Description

Technical Field

[0001] This application relates to the field of power equipment, specifically a novel bidirectional DC / DC converter topology. Background Technology

[0002] To meet the high-voltage, high-power transmission requirements of power transmission lines, existing technologies mostly employ an ISOP (Series Input, Parallel Output) topology to construct high-to-low voltage DC-DC converters. This topology consists of multiple isolated power modules. The modules on the input side are connected in series to ensure they can withstand medium to high voltage, while the output side is connected in parallel to the low-voltage bus to output a large current. See [link to ISOP topology] for details. Figure 1 As shown.

[0003] Among them, the isolated DAB converter is a typical example built on the ISOP topology, and it is widely used due to its numerous superior characteristics. The topology of the isolated DAB converter is relatively simple, consisting of a high-frequency inverter on the input side, a high-frequency transformer in the middle, and a high-frequency rectifier on the output side. (See attached diagram for structure.) Figure 2 As shown. Multiple DAB converters can be combined in series and parallel to form an ISOP-DAB DC transformer; see the diagram for its structure. Figure 3 As shown, it has the following technical problems:

[0004] When the input side is a high-voltage DC input and the output side is a low-voltage DC output, Vin is generally the difference between V+ and V-. For example, V+ is the amplitude of a +20kV DC voltage, and V- is the amplitude of a -20kV DC voltage. On several series-connected sides of a high-voltage inverter, the copper wires in the windings of the transformer in the DAB converter, which is closest to the highest amplitude DC voltage, will bear a large absolute voltage. Therefore, the insulation requirements are very high, and the risk of breakdown is relatively large. Summary of the Invention

[0005] To address the problems in the prior art, this application provides a bidirectional DC / DC converter topology that improves upon the traditional bidirectional DC / DC converter topology, thereby preventing the copper wires in the transformer windings from bearing excessively high absolute voltage and reducing the risk of equipment breakdown.

[0006] To solve the above-mentioned technical problems, this application provides the following technical solution:

[0007] This application provides a bidirectional DC / DC converter topology, including:

[0008] The converter input terminal includes multiple stages of input full-bridge converters connected in series; wherein the number of stages of the input full-bridge converter is 2N; the input full-bridge converters from the 1st stage to the 2Nth stage are connected in series; the positive DC terminal of the 1st stage input full-bridge converter is connected to the positive terminal of the high-voltage bus, and the negative DC terminal of the 2Nth stage input full-bridge converter is connected to the negative terminal of the high-voltage bus; the DC terminals of the Nth stage input full-bridge converter and the (N+1)th stage input full-bridge converter are both grounded; N is a positive integer and greater than 2.

[0009] The converter output terminal includes a multi-stage output full-bridge converter connected in parallel; wherein, the number of stages of the output full-bridge converter is: Each stage of the full-bridge output converter has its DC terminal positive terminal connected to the positive terminal of the low-voltage bus, and its DC terminal negative terminal connected to the negative terminal of the low-voltage bus; M is a positive integer and M is less than N.

[0010] Furthermore, the input full-bridge converter is connected to the corresponding output full-bridge converter through a primary transformer.

[0011] Furthermore, the input full-bridge converter is connected to the corresponding output full-bridge converter through cascaded primary and secondary transformers.

[0012] Furthermore, the first transformation ratio of the primary transformer and the second transformation ratio of the secondary transformer are both preset according to the level corresponding to the input full-bridge converter.

[0013] Furthermore, after the positive AC terminal of the input full-bridge converter is connected to the first inductor, it is connected to the negative AC terminal of the input full-bridge converter through the primary side of the corresponding first-stage transformer.

[0014] Furthermore, the input full-bridge converters located at odd-numbered levels and the input full-bridge converters located at even-numbered levels form a pair. In the same pair, the positive secondary terminal of the primary transformer corresponding to the input full-bridge converter located at odd-numbered levels is short-circuited with the positive secondary terminal of the primary transformer corresponding to the input full-bridge converter located at even-numbered levels. In the same pair, the negative secondary terminal of the primary transformer corresponding to the input full-bridge converter located at odd-numbered levels is short-circuited with the negative secondary terminal of the primary transformer corresponding to the input full-bridge converter located at even-numbered levels.

[0015] Furthermore, the positive AC terminal of the output full-bridge converter is connected to the negative AC terminal of the output full-bridge converter through the secondary side of the corresponding secondary transformer.

[0016] Furthermore, the first number of turns of the primary transformer and the second number of turns of the secondary transformer are both preset according to the level corresponding to the input full-bridge converter.

[0017] Furthermore, the primary side of the secondary transformer is connected to a second inductor.

[0018] Furthermore, this application also provides a flexible substation, including the aforementioned bidirectional DC / DC converter topology and a high-voltage DC grid and a low-voltage DC grid.

[0019] To address the problems in the prior art, the bidirectional DC / DC converter topology provided in this application can reduce the absolute voltage difference between the primary and secondary windings of the transformer by increasing the number of transformer stages for full-bridge converters that withstand high absolute voltages. This reduces the insulation pressure between the primary and secondary windings and lowers the risk of equipment breakdown. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the ISOP structure in the prior art;

[0022] Figure 2 This is a schematic diagram of the structure of an isolated DAB converter in the prior art;

[0023] Figure 3 This is a schematic diagram of the structure of the ISOP-DAB DC transformer in the prior art;

[0024] Figure 4 This is a schematic diagram of the bidirectional DC / DC converter topology in an embodiment of this application;

[0025] Figure 5 This is a schematic diagram of the structure of a single full-bridge converter (FBC) in an embodiment of this application;

[0026] Figure 6 This is a simplified structural diagram of the full-bridge converter (FBC) in the embodiments of this application;

[0027] Figure 7 This is a schematic diagram of the DC / DC converter topology in the prior art. Detailed Implementation

[0028] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] As described in the background section, to meet the high-voltage, high-power transmission requirements of power transmission lines, existing technologies mostly employ an ISOP (Series Input, Parallel Output) topology to construct high-to-low voltage DC-DC converters. This topology consists of multiple isolated power modules. The modules on the input side are connected in series to ensure they can withstand medium to high voltages, while the output side is connected in parallel to the low-voltage bus to output a large current. See [link to ISOP topology section] for details. Figure 1 As shown.

[0030] Among them, the isolated DAB converter is a typical example built on the ISOP topology, and it is widely used due to its numerous superior characteristics. The topology of the isolated DAB converter is relatively simple, consisting of a high-frequency inverter on the input side, a high-frequency transformer in the middle, and a high-frequency rectifier on the output side. (See attached diagram for structure.) Figure 2 As shown. Multiple DAB converters can be combined in series and parallel to form an ISOP-DAB DC transformer; see the diagram for its structure. Figure 3 As shown, it has the following technical problems:

[0031] When the input side is a high-voltage DC input and the output side is a low-voltage DC output, Vin is generally the difference between V+ and V-. For example, V+ is the amplitude of a +20kV DC voltage, and V- is the amplitude of a -20kV DC voltage. On several series-connected sides of the high-voltage inverter, the copper wires in the windings of the transformer in the DAB converter closest to the highest amplitude DC voltage will bear a large absolute voltage, thus requiring high insulation and posing a significant risk of breakdown. To address the above issues, in one embodiment, see... Figure 4 In order to improve the traditional bidirectional DC / DC converter topology, avoid the copper wires in the transformer windings from bearing excessively high absolute voltage, and reduce the risk of equipment breakdown, this application provides a novel bidirectional DC / DC converter topology, including: a converter input terminal and a converter output terminal.

[0032] The converter input terminal includes multiple stages of input full-bridge converters connected in series; the number of stages of the input full-bridge converter is 2N; the input full-bridge converters from the 1st stage to the 2Nth stage are connected in series; the positive DC terminal of the 1st stage input full-bridge converter is connected to the positive terminal of the high-voltage bus (UdcL+), and the negative DC terminal of the 2Nth stage input full-bridge converter is connected to the negative terminal of the high-voltage bus (UdcL-); the DC terminals of the Nth stage input full-bridge converter and the (N+1)th stage input full-bridge converter are both grounded; N is a positive integer greater than 2.

[0033] The converter output terminal includes a multi-stage output full-bridge converter connected in parallel; the positive DC terminal of each stage of the output full-bridge converter is connected to the positive terminal of the low-voltage bus (UdcL+), and the negative DC terminal of each stage of the output full-bridge converter is connected to the negative terminal of the low-voltage bus (UdcL-).

[0034] Understandably, for ease of explanation, the FBC on the input side of the bidirectional DC / DC converter is referred to as the input full-bridge converter; and the FBC on the output side of the bidirectional DC / DC converter is referred to as the output full-bridge converter.

[0035] For the topology of converters that convert high-voltage DC to low-voltage DC in the prior art, see [link to relevant documentation]. Figure 7 As shown, a key component of this is the full-bridge converter (hereinafter referred to as FBC). Figure 7 In this diagram, UdcH+ and UdcH- represent the positive and negative terminals of the high-voltage bus, respectively. UdcH is the voltage difference between the positive and negative terminals of the high-voltage bus. UdcL+ and UdcL- represent the positive and negative terminals of the low-voltage bus, respectively. UdcL is the voltage difference between the positive and negative terminals of the low-voltage bus. Ln is the filter inductance, and a:b is the transformer turns ratio. (See also...) Figure 4 a:b can be n1:k, n2:k...m1:1, m2:1...x1:k, x2:k, etc. In specific implementation, it can be reasonably set by those skilled in the art according to the voltage value, and this application is not limited thereto.

[0036] In this traditional topology, because the grounding point is located Figure 7 The middle section shown results in a larger turns ratio n value (including n1, n2, etc.) for the transformer near the FBC connection of UdcH+ and UdcH-, placing high demands on the insulation of the transformer winding copper wires. After the converter has been running for a period of time, high turns ratio winding breakdown due to equipment aging is likely to occur, and the cost of improving the insulation performance of the transformer windings to solve this problem is also high.

[0037] To address the aforementioned technical problems, the novel bidirectional DC / DC converter topology provided in this application consists of several full-bridge converters. See also... Figure 4 The input full-bridge converters, from top to bottom, are: Level 1 input full-bridge converter, Level 2 input full-bridge converter, Level 3 input full-bridge converter... Level 2N-1 input full-bridge converter and Level 2N input full-bridge converter, that is, each input full-bridge converter is divided into 1 to 2N levels. Correspondingly, each output full-bridge converter is also divided into several levels, with the total number of levels of the output full-bridge converters being less than the total number of levels of the input full-bridge converters.

[0038] Specifically, the number of stages of the output full-bridge converter is: Level; M is a positive integer, and M is less than N.

[0039] It should be noted that, see Figure 4 Each input full-bridge converter is cascaded with its corresponding output full-bridge converter via a transformer; there are two specific cascading methods:

[0040] The first type: The transformer includes a primary transformer and a secondary transformer; the input full-bridge converter is connected to the corresponding output full-bridge converter through cascaded primary and secondary transformers. In this case, the primary transformer refers to the transformer directly connected to the AC terminal of the input full-bridge converter; the secondary transformer refers to the transformer directly connected to the AC terminal of the output full-bridge converter.

[0041] The second type: The transformer consists of only one stage transformer; the input full-bridge converter is connected to the corresponding output full-bridge converter through the first-stage transformer. In this case, the primary side of the transformer is connected to the AC terminal of the input full-bridge converter, and the secondary side of the transformer is connected to the AC terminal of the output full-bridge converter.

[0042] It should be noted that for the first cascading method, see [link to relevant documentation]. Figure 4 The input full-bridge converters of two adjacent stages can be grouped together. For example, the input full-bridge converter of stage 1 and stage 2 can be grouped together, the input full-bridge converter of stage 3 and stage 4 can be grouped together, and so on; the input full-bridge converter of stage 2N can be grouped together with the input full-bridge converter of stage 2N-1, the input full-bridge converter of stage 2N-2 can be grouped together with the input full-bridge converter of stage 2N-3, and so on.

[0043] For the same set of input full-bridge converters, the positive sides of the secondary windings of the first-stage transformers corresponding to each stage of the input full-bridge converters are shorted, and the negative sides of the secondary windings of the first-stage transformers corresponding to each stage of the input full-bridge converters are shorted. Thus, for the same set of input full-bridge converters, it is equivalent to sharing the same output full-bridge converter. In other words, the input full-bridge converters at odd levels and the adjacent input full-bridge converters at even levels form a group in pairs. The positive sides of the secondary windings of the first-stage transformers corresponding to the input full-bridge converters at odd levels in the same group are shorted to the positive sides of the secondary windings of the first-stage transformers corresponding to the adjacent input full-bridge converters at even levels, and the negative sides of the secondary windings of the first-stage transformers corresponding to the input full-bridge converters at odd levels in the same group are shorted to the negative sides of the secondary windings of the first-stage transformers corresponding to the adjacent input full-bridge converters at even levels.

[0044] During specific implementation, it can be determined whether to adopt the first cascading method or the second cascading method according to the access voltage levels of each input full-bridge converter. In the embodiments of the present application, when the access voltage of the input full-bridge converter is lower than a certain set threshold, the second cascading method can be adopted. Generally, in Figure 4 , at least two stages of input full-bridge converters close to the grounding point in the figure can be connected to their corresponding output full-bridge converters by adopting the second cascading method.

[0045] It can be understood that the absolute voltages borne by the windings of the first-stage transformers (including at least four stages, such as the four-stage transformers corresponding to the first-stage input full-bridge converter, the second-stage input full-bridge converter, the (2N - 1)-stage input full-bridge converter, and the 2N-stage input full-bridge converter) connected by the FBCs close to UdcH+ and UdcH- are relatively high. Therefore, in the embodiments of the present application, for such FBCs, they can first be transformed through the first-stage transformer w:k (w can be n1, n2,... m1, m2... x1, x2, etc.; 1 < k < n), and then the AC voltage with a relatively high absolute voltage is transformed through the second-stage transformer k:1. For the FBCs (located at the upper and lower ends shown in Figure 4 ) that bear relatively high positive and negative absolute voltages, two-stage transformers can be used for transformation in two steps. Thus, the insulation requirements for the transformer windings are reduced, the occurrence of winding breakdown is decreased, and the manufacturing cost of the winding insulation can also be reduced.

[0046] In summary, if there are many FBCs to be connected in series on the input side, the windings of the transformers corresponding to the input full-bridge converters that bear relatively high positive and negative absolute voltages can be divided into two-stage transformers for transformation; for the transformers corresponding to the input full-bridge converters whose positive and negative absolute voltages are within a preset range, a one-stage transformer can still be adopted for transformation. During specific implementation, a level threshold can be set. When the level corresponding to the input full-bridge converter is less than the preset level threshold, the input full-bridge converter is directly connected to the corresponding output full-bridge converter through the corresponding one-stage transformer. In Figure 4 In the process, the four FBCs connected in series closest to the grounding point are directly transformed using a single-stage transformer.

[0047] It should be noted that after the positive AC terminal of the input full-bridge converter is connected to the first inductor, it is connected to the negative AC terminal of the same input full-bridge converter through the primary side of the corresponding first-stage transformer. The primary side of the second-stage transformer is connected to the second inductor. The function of both the first and second inductors is to filter the high-frequency harmonics generated by the inverter, so that the fundamental frequency ratio passing through the transformer is as high as possible.

[0048] In one embodiment, the turns ratio includes a first turns ratio of the primary transformer and a second turns ratio of the secondary transformer; both the first turns ratio and the second turns ratio are preset according to the level corresponding to the input full-bridge converter. For example, see... Figure 4 ,lie in Figure 4 The first transformer of the first-stage input full-bridge converter at the top has a first turns ratio of n1:k; the second transformer has a second turns ratio of k:1.

[0049] In one embodiment, the positive AC terminal of the output full-bridge converter is connected to the negative AC terminal of the output full-bridge converter through the secondary side of the corresponding secondary transformer.

[0050] In one embodiment, the first number of turns of the primary transformer and the second number of turns of the secondary transformer are both preset according to the level corresponding to the input full-bridge converter. In specific implementation, the input full-bridge converter closer to the high-voltage bus has a relatively larger number of turns, and conversely, the input full-bridge converter farther from the high-voltage bus has a relatively smaller number of turns.

[0051] It should be noted that the topology diagram of a single full-bridge converter can be found in [reference needed]. Figure 5 As shown in the diagram. C is the capacitor, and Q1 to Q4 are all Insulated Gate Bipolar Transistors (IGBTs). Udc+ and Udc- are the positive and negative terminals of the DC side (also called the DC terminal), respectively, with Udc representing the voltage difference between the positive and negative terminals of the DC side. Uac+ and Uac- represent the positive and negative terminals of the AC side, respectively, with Uac representing the voltage difference between the positive and negative terminals of the AC side. In larger-scale power electronic structure diagrams, Figure 5 It can be used Figure 6 To replace it, for simplicity and clarity. Figure 4 The middle is to adopt Figure 6 The simplified symbol used in the language is used to represent FBC.

[0052] Furthermore, the present invention also provides a flexible substation, including the aforementioned bidirectional DC / DC converter topology and a high-voltage DC grid and a low-voltage DC grid; wherein, the positive DC terminal of the first-stage input full-bridge converter in the bidirectional DC / DC converter topology is connected to the positive high-voltage bus of the high-voltage DC grid, and the negative DC terminal of the second-Nth-stage input full-bridge converter in the bidirectional DC / DC converter topology is connected to the negative high-voltage bus of the high-voltage DC grid; the positive DC terminals of each stage of the output full-bridge converter in the bidirectional DC / DC converter topology are all connected to the positive low-voltage bus of the low-voltage DC grid, and the negative DC terminals of each stage of the output full-bridge converter in the bidirectional DC / DC converter topology are all connected to the negative low-voltage bus of the low-voltage DC grid.

[0053] Because of the bidirectional DC / DC converter topology provided in this application, for full-bridge converters that withstand high absolute voltages, the absolute voltage difference between the primary and secondary windings of the transformer can be reduced by increasing the number of transformer stages, thereby alleviating the insulation pressure between the primary and secondary windings and reducing the risk of equipment breakdown. Therefore, applying the bidirectional DC / DC converter topology provided in this application to flexible substations can reduce the failure rate of bidirectional DC / DC converters in flexible substations due to insulation breakdown, reduce the overall insulation manufacturing cost of flexible substations, and thus improve the safety and economy of the converter components in flexible substations.

[0054] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the embodiments of the device implementation method are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0055] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired result. In some embodiments, multitasking and parallel processing are possible or may be advantageous.

[0056] While the embodiments in this specification provide the method operation steps as described in the embodiments or flowcharts, more or fewer operation steps may be included based on conventional or non-inventive means. The order of steps listed in the embodiments is merely one possible order of execution among many steps and does not represent the only possible order of execution. The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, product, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, product, or apparatus. Without further limitations, the presence of other identical or equivalent elements in a process, method, product, or apparatus that includes said elements is not excluded.

[0057] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, system embodiments are basically similar to method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. In the description of this specification, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the embodiments in this specification. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0058] The above description is merely an embodiment of the present specification and is not intended to limit the embodiments of the present specification. For those skilled in the art, various modifications and variations can be made to the embodiments of the present specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of the embodiments of the present specification should be included within the scope of the claims of the embodiments of the present specification.

Claims

1. A bidirectional DC / DC converter topology, characterized in that, include: The converter input terminal includes multiple stages of input full-bridge converters connected in series; wherein the number of stages of the input full-bridge converter is 2N; the input full-bridge converters from the 1st stage to the 2Nth stage are connected in series; the positive DC terminal of the 1st stage input full-bridge converter is connected to the positive terminal of the high-voltage bus, and the negative DC terminal of the 2Nth stage input full-bridge converter is connected to the negative terminal of the high-voltage bus; the negative DC terminal of the Nth stage input full-bridge converter and the positive DC terminal of the (N+1)th stage input full-bridge converter are both grounded; N is a positive integer and greater than 2. The converter output terminal includes a multi-stage output full-bridge converter connected in parallel; wherein, the number of stages of the output full-bridge converter is: Each stage of the full-bridge output converter has its DC terminal positive terminal connected to the positive terminal of the low-voltage bus, and its DC terminal negative terminal connected to the negative terminal of the low-voltage bus; M is a positive integer and M is less than N; Each input full-bridge converter is cascaded with its corresponding output full-bridge converter via a transformer; the cascading methods include two types: The first type: The transformer includes a primary transformer and a secondary transformer; the input full-bridge converter is connected to the corresponding output full-bridge converter through cascaded primary and secondary transformers; wherein, the primary transformer is the transformer directly connected to the AC terminal of the input full-bridge converter; the secondary transformer is the transformer directly connected to the AC terminal of the output full-bridge converter. The second type: The transformer consists of only one stage transformer; the input full-bridge converter is connected to the corresponding output full-bridge converter through the first stage transformer; wherein, the primary side of the transformer is connected to the AC terminal of the input full-bridge converter, and the secondary side of the transformer is connected to the AC terminal of the output full-bridge converter; wherein, 2M is the number of stages of the input full-bridge converter that is connected to the first stage transformer alone.

2. The bidirectional DC / DC converter topology according to claim 1, characterized in that, The first transformation ratio of the primary transformer and the second transformation ratio of the secondary transformer are both preset according to the level corresponding to the input full-bridge converter.

3. The bidirectional DC / DC converter topology according to claim 1, characterized in that, After the positive AC terminal of the input full-bridge converter is connected to the first inductor, it is connected to the negative AC terminal of the input full-bridge converter through the primary side of the corresponding first-stage transformer.

4. The bidirectional DC / DC converter topology according to claim 2, characterized in that, The input full-bridge converters located at odd-numbered levels and those located at even-numbered levels form a pair. In the same pair, the positive secondary terminal of the primary transformer corresponding to the input full-bridge converter at odd-numbered levels is short-circuited with the positive secondary terminal of the primary transformer corresponding to the input full-bridge converter at even-numbered levels. In the same pair, the negative secondary terminal of the primary transformer corresponding to the input full-bridge converter at odd-numbered levels is short-circuited with the negative secondary terminal of the primary transformer corresponding to the input full-bridge converter at even-numbered levels.

5. The bidirectional DC / DC converter topology according to claim 1, characterized in that, The positive AC terminal of one full-bridge output converter is connected to the negative AC terminal of the corresponding secondary transformer; the positive AC terminals of the other 2M full-bridge output converters are connected to the negative AC terminals of the corresponding separately configured primary transformers.

6. The bidirectional DC / DC converter topology according to claim 1, characterized in that, The number of turns of the first stage transformer and the number of turns of the second stage transformer are both preset according to the level corresponding to the input full-bridge converter.

7. The bidirectional DC / DC converter topology according to claim 6, characterized in that, The primary side of the secondary transformer is connected to a second inductor.

8. A flexible substation, characterized in that, The system includes a bidirectional DC / DC converter topology as described in any one of claims 1-7, as well as a high-voltage DC grid and a low-voltage DC grid; wherein, the positive DC terminal of the first-stage input full-bridge converter in the bidirectional DC / DC converter topology is connected to the positive high-voltage bus of the high-voltage DC grid, and the negative DC terminal of the second-Nth-stage input full-bridge converter in the bidirectional DC / DC converter topology is connected to the negative high-voltage bus of the high-voltage DC grid; the positive DC terminals of each stage of the output full-bridge converter in the bidirectional DC / DC converter topology are all connected to the positive low-voltage bus of the low-voltage DC grid, and the negative DC terminals of each stage of the output full-bridge converter in the bidirectional DC / DC converter topology are all connected to the negative low-voltage bus of the low-voltage DC grid.

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