Kun type mixed polarity direct current power distribution system and four-transformer QAB converter

Abstract

The invention provides a kun type mixed polarity direct current power distribution system and a four-transformer QAB converter. A high-voltage side active bridge Hh1 is connected with the positive electrode of a high-voltage side bus, a low-voltage side active bridge Hl1 is connected with the positive electrode of a low-voltage side bus, a high-voltage side active bridge Hh2 is connected with the negative electrode of the high-voltage side bus, and a low-voltage side active bridge Hl2 is connected with the negative electrode of the low-voltage side bus; the high-voltage side active bridge Hh1 and the low-voltage side active bridge Hl1 are interconnected through a high-frequency transformer Tr1, and the high-voltage side active bridge Hh2 and the low-voltage side active bridge Hl2 are interconnected through a high-frequency transformer Tr2; the low-voltage side active bridge Hl1 and the high-voltage side active bridge Hh2 are interconnected through a transformer Te1, and the high-voltage side active bridge Hh1 and the low-voltage side active bridge Hl2 are interconnected through a transformer Te2. The kun type mixed polarity direct current power distribution system has a true bipolar system interconnection function, meanwhile, electrical isolation between bipolar systems is guaranteed, and power supply safety of users is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of a DC power distribution system architecture and a corresponding converter topology, and in particular to a QAB type mixed polarity DC power distribution system and a four-transformer QAB converter. Background Art

[0002] With the development of renewable energy distributed generation and power electronics technology, DC power distribution system has attracted wide attention as an alternative to traditional AC system in certain situations. Compared with traditional AC power distribution, DC system does not have frequency stability and power angle stability problems, and control is simpler; DC sources and loads such as photovoltaic, energy storage system, and electronic load are connected to the DC system without the need for industrial frequency AC and DC power conversion, thereby improving the overall efficiency of the system.

[0003] The DC distribution system architecture can be divided into unipolar structure and bipolar structure. Compared with the unipolar structure, the bipolar structure has advantages in terms of source and load access flexibility, reliability and insulation design. However, the existing bipolar DC distribution system also faces some challenges. A large number of power sources and loads are connected to the DC distribution system through DC-DC converters, which may lead to bipolar voltage and power imbalance. Therefore, additional power electronic devices are often required to maintain the balance of the bipolar system.

[0004] In view of the existing DC power supply system architecture problems, in order to further improve the flexibility and reliability of the DC power distribution system, this application proposes a new DC power distribution system architecture and a corresponding converter topology to implement the architecture.

[0005] Document 1, patent document with publication number CN110783912A discloses an AC / DC hybrid power distribution system and power distribution method through DC collection. The system adds AC / DC converters at the ends of multiple AC distribution feeders, and all AC / DC converters are connected through the DC side to realize DC collection. Through DC collection, multiple AC distribution feeders can be electrically interconnected. The system configures controllers at the incoming switches of each AC distribution feeder and at the DC collection point to realize electrical quantity collection, incoming line protection control and communication with AC / DC converters. The incoming line controller communicates with the DC collection controller wirelessly. However, the patent document still has the defect of requiring additional power electronic devices to maintain the balance of the bipolar system.

[0006] Document 2, Jun-Young Lee, Hyun-Jun Choi, Ju-Young Sim and Jee-Hoon Jung, "Interlink Three-Level Bidirectional DC-DC Converter with Asymmetrical Load Condition," in 2019 10th International Conference on Power Electronics and ECCE Asia (ICPE 2019-ECCE Asia). This document proposes a three-level dual-active bridge DC-DC converter for interconnecting bipolar DC buses. Between the two DC buses, current isolation is provided by a high-frequency transformer of a three-level dual-active bridge. In addition, an asymmetric switch modulation method is proposed to compensate for the asymmetric load condition of the bipolar DC bus system. Compared with this, the topology proposed in this application not only has the ability to interconnect two bipolar DC systems of different voltage levels and realize electrical isolation between two groups of DC systems, but also can balance the bipolar power on the high-voltage side and the low-voltage side.

[0007] Document 3, Jun-Young Lee, Ho-Sung Kim and Jee-Hoon Jung, "Enhanced Dual-Active-Bridge DC-DC Converter for Balancing Bipolar Voltage Level of DC Distribution System," IEEE Transactions on Industrial Electronics (Volume: 67, Issue: 12, December 2020). This document proposes an enhanced DAB converter that can balance bipolar voltage levels without the need for an additional voltage balancer. The bipolar voltage level can be strictly regulated in the case of an unbalanced load, even when one side of the DC pole is unloaded. In addition, its zero-voltage switching capability can be extended to the entire load range including no-load conditions. Compared with this, the topology proposed in this application has the power balancing capability of a bipolar distribution system, and has the ability to operate with high-voltage side short circuits and low-voltage side short circuit fault tolerance. Summary of the invention

[0008] In view of the defects in the prior art, the object of the present invention is to provide a Q type mixed polarity DC power distribution system and a four-transformer QAB converter.

[0009] According to the present invention, a type kun hybrid polarity DC power distribution system is provided, comprising: a high-voltage side bipolar bus, a low-voltage side bipolar bus and a converter part; the high-voltage distribution network and the low-voltage distribution network are connected to each other through the converter part;

[0010] The converter part includes a high-voltage side active bridge H h1 , low voltage side active bridge H l1 , high voltage side active bridge H h2 And the low voltage side active bridge H l2 ;

[0011] The high voltage side active bridge H h1 Connect the positive pole of the high voltage side busbar and the low voltage side active bridge H l1 Connect the positive pole of the low voltage side busbar, the high voltage side active bridge H h2 Connect the negative pole of the high voltage side busbar and the low voltage side active bridge H l2 Connect the negative pole of the low-voltage busbar;

[0012] The high voltage side active bridge H h1 With the low voltage side active bridge H l1 Through the high frequency transformer T r1 interconnected, the high voltage side active bridge H h2 With the low voltage side active bridge H l2 Through the high frequency transformer T r2 Interconnection; the low voltage side active bridge H l1 and the high voltage side active bridge H h2 Through the transformer T e1 interconnected, the high voltage side active bridge H h1 and the low voltage side active bridge H l2 Through the transformer T e2 interconnected.

[0013] Preferably, the high voltage side active bridge H h1 Including MOSFET switch tube S 11 , MOSFET switch tube S 12 , MOSFET switch tube S 13 And MOSFET switch tube S 14 ;

[0014] The low voltage side active bridge H l1 Including MOSFET switch tube S 31 , MOSFET switch tube S 32 , MOSFET switch tube S 33 And MOSFET switch tube S 34 .

[0015] Preferably, the high frequency transformer T r1The turns ratio is 1:1.

[0016] Preferably, the high voltage side active bridge H h2 Including MOSFET switch tube S 21 , MOSFET switch tube S 22 , MOSFET switch tube S 23 And MOSFET switch tube S 24 ;

[0017] The low voltage side active bridge H l2 Including MOSFET switch tube S 41 , MOSFET switch tube S 42 , MOSFET switch tube S 43 And MOSFET switch tube S 44 .

[0018] Preferably, the high frequency transformer T r2 The turns ratio is 1:1.

[0019] Preferably, the transformer T e1 The turns ratio is 1:1.

[0020] Preferably, the transformer T e2 The turns ratio is 1:1.

[0021] Preferably, the high-frequency transformer T on the high-voltage side r1 The transformer T on the high voltage side e2 In series, the high-frequency transformer T on the high-voltage side r2 The transformer T on the high voltage side e1 Series connection.

[0022] Preferably, the high-frequency transformer T on the low-voltage side r1 The transformer T on the low voltage side e1 In parallel, the high frequency transformer T on the low voltage side r2 The transformer T on the low voltage side e2 in parallel.

[0023] The present invention also provides a four-transformer QAB converter, comprising the above-mentioned Kun-type mixed polarity DC power distribution system.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention proposes a full electrical isolation The proposed bipolar DC power distribution system interconnection architecture (Kun) type mixed polarity DC power distribution system has the function of true bipolar system interconnection, while ensuring electrical isolation between bipolar systems to ensure power supply safety for users;

[0026] 2. The present invention is based on the proposed four-transformer QAB circuit. When a short circuit fault occurs in one pole on the low-voltage side, the proposed solution can effectively isolate the low-voltage fault pole to ensure that the fault will not be transmitted to the high-voltage side, thereby improving the power supply reliability of the bipolar DC system;

[0027] 3. The present invention has 4U DC (±2U DC ), 2U DC (2U DC 、-2U DC ±U DC ), U DC (U DC , -U DC ) and other voltage levels, which can provide flexible access for sources and loads;

[0028] 4. The present invention is based on the proposed four-transformer QAB circuit, and both the high-voltage side bipolar DC bus and the low-voltage side bipolar DC bus have the ability to balance positive and negative power;

[0029] 5. The present invention proposes (Kun) type mixed polarity DC distribution system is used to realize isolated interconnection of bipolar DC distribution system and improve the safety of DC system operation;

[0030] 6. The present invention is based on the proposed four-transformer QAB structure, which can achieve power balance control on the high-voltage bipolar and low-voltage bipolar sides; when a pole on the low-voltage side fails, the proposed topology can quickly isolate the faulty pole to ensure continuous operation of the non-faulty pole. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0032] Figure 1 for Structural diagram of the DC power distribution system architecture;

[0033] Figure 2 for Schematic diagram of the structure of the interconnected four-port active bridge in the type power distribution system;

[0034] Figure 3 Schematic diagram of key waveforms for normal operation;

[0035] Figure 4 Schematic diagram of key waveforms for fault-tolerant operation;

[0036] Figure 5 Schematic diagram of the positive bus current on the high voltage side in scenario 1 in Example 3;

[0037] Figure 6 Schematic diagram of the negative bus current on the high voltage side in scenario 1 in Example 3;

[0038] Figure 7 Schematic diagram of the positive bus current on the low voltage side in scenario 1 in Example 3;

[0039] Figure 8 Schematic diagram of the negative bus current on the low voltage side in scenario 1 in Example 3;

[0040] Fig. 9 is a schematic diagram of the leakage inductance L1 current in scenario 1 in embodiment 3;

[0041] Fig.10 Schematic diagram of the leakage inductance L2 current in scenario 1 in embodiment 3;

[0042] Fig.11 Schematic diagram of the negative bus current on the high voltage side in scenario 2 in Example 3;

[0043] Fig.12 Schematic diagram of the low voltage side positive bus current in scenario 2 in embodiment 3;

[0044] Fig.13 Schematic diagram of the negative bus current on the low voltage side in scenario 2 in Example 3;

[0045] Fig.14 Schematic diagram of the leakage inductance L2 current in scenario 2 in embodiment 3;

[0046] Fig.15 Schematic diagram of the positive bus current on the high-voltage side in Scenario 3 in Example 3;

[0047] Fig.16 Schematic diagram of the negative bus current on the high voltage side in scenario 3 in embodiment 3;

[0048] Fig.17 Schematic diagram of the positive bus current on the low voltage side in scenario 3 in embodiment 3;

[0049] Fig.18 Schematic diagram of the negative bus current on the low voltage side in scenario 3 in embodiment 3;

[0050] Fig.19 Schematic diagram of the leakage inductance L1 current in scenario 3 in embodiment 3;

[0051] Fig. 20 Schematic diagram of the leakage inductance L2 current in scenario 3 in embodiment 3;

[0052] Fig.21 Schematic diagram of the low-voltage side positive output voltage US1 in scenario 3 in embodiment 3. DETAILED DESCRIPTION

[0053] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0054] Embodiment 1:

[0055] like Figures 1 to 4 As shown, this embodiment provides a type-K hybrid polarity DC power distribution system, including: a high-voltage side bipolar bus, a low-voltage side bipolar bus, and a converter part; the high-voltage distribution network and the low-voltage distribution network are connected to each other through the converter part, and the converter part includes a high-voltage side active bridge H h1 , low voltage side active bridge H l1 , high voltage side active bridge H h2 And the low voltage side active bridge H l2 , high voltage side active bridge H h1 Connect the positive pole of the high voltage side busbar and the active bridge H on the low voltage side l1 Connect the positive pole of the low voltage side busbar and the high voltage side active bridge H h2 Connect the negative pole of the high voltage side busbar and the active bridge H on the low voltage side l2 Connect the negative pole of the low voltage side busbar and the active bridge H on the high voltage side h1 With the low voltage side active bridge H l1 Through the high frequency transformer T r1 Interconnection, high voltage side active bridge H h2 With the low voltage side active bridge H l2 Through the high frequency transformer T r2 Interconnection; low voltage side active bridge H l1 and the high voltage side active bridge H h2 Through the transformer T e1 Interconnection, high voltage side active bridge H h1 and low voltage side active bridge H l2 Through the transformer T e2 Interconnection. Transformer T e1 The turns ratio of transformer T is 1:1. e2 The turns ratio is 1:1.

[0056] High voltage side active bridge H h2 Including MOSFET switch tube S 21 , MOSFET switch tube S 22 , MOSFET switch tube S 23 And MOSFET switch tube S 24 , low voltage side active bridge H l2Including MOSFET switch tube S 41 , MOSFET switch tube S 42 , MOSFET switch tube S 43 And MOSFET switch tube S 44 . High frequency transformer T r2 The turns ratio is 1:1. The high voltage side active bridge H h1 Including MOSFET switch tube S 11 , MOSFET switch tube S 12 , MOSFET switch tube S 13 And MOSFET switch tube S 14 , low voltage side active bridge H l1 Including MOSFET switch tube S 31 , MOSFET switch tube S 32 , MOSFET switch tube S 33 And MOSFET switch tube S 34 . High frequency transformer T r1 The turns ratio is 1:1.

[0057] High-frequency transformer T on the high-voltage side r1 The transformer T on the high voltage side e2 Series, high-frequency transformer T on the high-voltage side r2 The transformer T on the high voltage side e1 Series. High-frequency transformer T on the low-voltage side r1 The transformer T on the low voltage side e1 Parallel, high-frequency transformer T on the low-voltage side r2 The transformer T on the low voltage side e2 in parallel.

[0058] The present invention also provides a four-transformer QAB converter, comprising the above-mentioned Kun-type mixed polarity DC power distribution system.

[0059] Embodiment 2:

[0060] Those skilled in the art may understand this embodiment as a more specific description of Embodiment 1.

[0061] The embodiment proposed (Kun) type bipolar interconnected DC power distribution system, such as Figure 1 The rated voltage level of the line is the rated voltage amplitude U of the single-pole line on the low-voltage side. DC In unified expression, the single-pole rated voltage of the high-voltage side DC bus is 2U DC .

[0062] The mixed polarity in this embodiment is reflected in that it is divided into three parts: the first is the high-voltage side bipolar busbar, the second is the low-voltage side bipolar busbar, and the third is the four-port converter QT-QAB part that connects the high and low voltage distribution networks. The converter part consists of the high-voltage side active bridge H h1 , low voltage side active bridge H l1 , high voltage side active bridge H h2 And the low voltage side active bridge H l2 , high voltage side active bridge H h1 This part is the core of the mixed polarity distribution system.

[0063] like Figure 2 As shown, the high voltage side active bridge H h1 The four MOSFET switch tubes S 11 , S 12 , S 13 , S 14 Composition, low voltage side active bridge H l2 By the switch tube S 31 , S 32 , S 33 , S 34 The high-voltage side active bridge and the low-voltage side active bridge are connected through a high-frequency transformer T with a turns ratio of 1:1. r1 Connected. High voltage side active bridge H h2 The four MOSFET switch tubes S 21 , S 22 , S 23 , S 24 Composition, low voltage side active bridge H l2 By the switch tube S 41 , S 42 , S 43 , S 44 The high-voltage side active bridge and the low-voltage side active bridge are connected through a high-frequency transformer T with a turns ratio of 1:1. r2 connected.

[0064] High voltage side active bridge H h1 and low voltage side active bridge H l2 Through a transformer T with a turns ratio of 1:1 e2 Interconnection, low voltage side active bridge H l1 and the high voltage side active bridge H h2 Through a transformer T with a turns ratio of 1:1 e1 Interconnection. High voltage side T r1 With T e2 Series, T r2 With T e1 Series. Low voltage side T r1 With T e1 Parallel, T r2 With T e2Parallel connection. Equivalent leakage inductance L 1 For transformer T r1 , T e2 The sum of the leakage inductance, L 2 For transformer T r2 , T e1 The sum of the leakage inductance.

[0065] Basic working principle:

[0066] In the proposed hybrid polarity DC power distribution system, the converter part plays the role of power transmission and voltage conversion between the high and low voltage sides of the system. The positive and negative poles of this system have similar structures, and the rated voltage of the high voltage side bipolar bus is ±2U DC , providing ±2U DC 、4U DC The voltage level of the low voltage side bipolar busbar is rated voltage ±U DC , providing ±U DC , 2U DC The voltage level can meet the needs of flexible access to various types of DC power equipment with different voltage levels.

[0067] Control volume They are respectively the high voltage side active bridge H h2 , low voltage side active bridge H l1 , low voltage side active bridge H l2 Relative high voltage side active bridge H h1 By changing the phase shift angle It can control the power transmission from the positive and negative electrodes on the high voltage side to the positive and negative electrodes on the low voltage side. Take this as an example, the key waveform is as follows Figure 3 In half a cycle, the equivalent leakage inductance L 1 The current is:

[0068]

[0069] In half a cycle, the equivalent leakage inductance L 2 The current is:

[0070]

[0071] Among them, ω=2πf, f is the switching frequency.

[0072] From equations (1) and (2), we can get:

[0073] The positive input power on the high voltage side is:

[0074]

[0075] The negative input power on the high voltage side is:

[0076]

[0077] The positive output power on the low voltage side is:

[0078]

[0079] The negative output power on the low voltage side is:

[0080]

[0081] Fault-tolerant operation. Among them, UP1, UP2, US1, and US2 are Figure 2 Indicated in .

[0082] Assume that one of the output ports (let's take H l2 ) is short-circuited. h1 , H h2 , H l1 Same as the normal mode, H l2 Then let S 41 , S 42 , S 43 , S 44 Always on, that is, H l2 The output power drops to 0, thus isolating the fault and ensuring the normal operation of the converter. Take this as an example, the key waveform is as follows Figure 4 In half a cycle, the equivalent leakage inductance L 1 The current is:

[0083]

[0084] In half a cycle, the equivalent leakage inductance L 2 The current is:

[0085]

[0086] Among them, ω=2πf, f is the switching frequency.

[0087] From equations (7) and (8), we can get

[0088] The positive input power on the high voltage side is:

[0089]

[0090] The negative input power on the high voltage side is:

[0091]

[0092] The positive output power on the low voltage side is:

[0093]

[0094] The negative output power on the low voltage side is:

[0095] P o2 =0 (12).

[0096] Embodiment 3:

[0097] Those skilled in the art may understand this embodiment as a more specific description of Embodiment 1.

[0098] according to Figure 3 Plecs simulation model is built. The positive input voltage V in_high_P =48V, negative input voltage V in_high_N =48V. Low voltage side positive output voltage V in_low_P =24V, negative output voltage V in_low_N =24V. Switching frequency f s =20kHz, leakage inductance L 1 =200uF, leakage inductance L 2 =200uF.

[0099] Scenario 1: Balance the power on the high-voltage side. Figures 5 to 10 As shown, they are the positive and negative busbars on the high and low voltage sides and the leakage inductance L 1 , L 2 The current. At t = 0s to 0.5s, The positive and negative output power on the low voltage side is unbalanced, and the positive and negative input power on the high voltage side is balanced. At t = 0.5s, The positive and negative output powers on the low-voltage side change simultaneously, while the positive and negative input powers on the high-voltage side remain balanced, which proves the effectiveness of the proposed power balancing control on the high-voltage side of the bipolar DC system.

[0100] Scenario 2: Balance the power on the low voltage side. Figures 11 to 14 As shown, they are the positive and negative busbars on the high and low voltage sides and the leakage inductance L 1 , L 2 The current. At t = 0s to 0.5s, The positive and negative input power on the high voltage side is balanced, and the positive and negative output power on the low voltage side is also balanced. At t = 0.5s, The positive input power on the high-voltage side remains unchanged, the negative input power changes, and the positive and negative output powers on the low-voltage side are still balanced, which proves the effectiveness of the proposed low-voltage side power balance control of the bipolar DC system.

[0101] Scenario 3: short-circuit fault-tolerant operation on the low-voltage side. Figures 15 to 21 As shown, they are the positive and negative busbars on the high and low voltage sides and the leakage inductance L 1 , L 2The current. At t = 0s to 0.5s, The positive and negative input power on the high voltage side is balanced. At t = 0.5s, The positive output voltage of the low-voltage side becomes 0, that is, the positive electrode of the low-voltage side is short-circuited. The positive and negative input power of the high-voltage side is still balanced, and the negative electrode of the low-voltage side operates normally. It proves that the proposed bipolar DC system has short-circuit fault-tolerant operation capability.

[0102] The Kun-type mixed polarity DC power distribution system of the present invention has a true bipolar system interconnection function, while ensuring electrical isolation between bipolar systems to ensure power supply safety for users.

[0103] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A type Kun mixed polarity DC power distribution system, characterized in that: include: A high-voltage side bipolar busbar, a low-voltage side bipolar busbar and a converter part; the high-voltage distribution network and the low-voltage distribution network are connected to each other through the converter part; The converter part includes a high-voltage side active bridge H h1 , low voltage side active bridge H l1 , high voltage side active bridge H h2 And the low voltage side active bridge H l2 ; The high voltage side active bridge H h1 Connect the positive pole of the high voltage side busbar and the low voltage side active bridge H l1 Connect the positive pole of the low voltage side busbar, the high voltage side active bridge H h2 Connect the negative pole of the high voltage side busbar and the low voltage side active bridge H l2 Connect the negative pole of the low-voltage busbar; The high voltage side active bridge H h1 With the low voltage side active bridge H l1 Through the high frequency transformer T r1 interconnected, the high voltage side active bridge H h2 With the low voltage side active bridge H l2 Through the high frequency transformer T r2 Interconnection; the low voltage side active bridge H l1 and the high voltage side active bridge H h2 Through the transformer T e1 interconnected, the high voltage side active bridge H h1 and the low voltage side active bridge H l2 Through the transformer T e2 interconnected.

2. The type-U hybrid polarity DC power distribution system according to claim 1, characterized in that: The high voltage side active bridge H h1 Including MOSFET switch tube S 11 , MOSFET switch tube S 12 , MOSFET switch tube S 13 And MOSFET switch tube S 14 ; The low voltage side active bridge H l1 Including MOSFET switch tube S 31 , MOSFET switch tube S 32 , MOSFET switch tube S 33 And MOSFET switch tube S 34 .

3. The type-U hybrid polarity DC power distribution system according to claim 2, characterized in that: The high frequency transformer T r1 The turns ratio is 1:

1.

4. The type mixed polarity DC power distribution system according to claim 3, characterized in that: The high voltage side active bridge H h2 Including MOSFET switch tube S 21 , MOSFET switch tube S 22 , MOSFET switch tube S 23 And MOSFET switch tube S 24 ; The low voltage side active bridge H l2 Including MOSFET switch tube S 41 , MOSFET switch tube S 42 , MOSFET switch tube S 43 And MOSFET switch tube S 44 .

5. The type mixed polarity DC power distribution system according to claim 4, characterized in that: The high frequency transformer T r2 The turns ratio is 1:

1.

6. The type-U hybrid polarity DC power distribution system according to claim 5, characterized in that: The transformer T e1 The turns ratio is 1:

1.

7. The type-U hybrid polarity DC power distribution system according to claim 6, characterized in that: The transformer T e2 The turns ratio is 1:

1.

8. The type mixed polarity DC power distribution system according to claim 7, characterized in that: The high-frequency transformer T on the high-voltage side r1 The transformer T on the high voltage side e2 In series, the high-frequency transformer T on the high-voltage side r2 The transformer T on the high voltage side e1 Series connection.

9. The type mixed polarity DC power distribution system according to claim 8, characterized in that: The high frequency transformer T on the low voltage side r1 The transformer T on the low voltage side e1 In parallel, the high frequency transformer T on the low voltage side r2 The transformer T on the low voltage side e2 in parallel.

10. A four-transformer QAB converter, characterized in that: A mixed polarity DC power distribution system comprising the type Kun described in any one of claims 1 to 9.

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