Kun type hybrid polarity direct current power distribution system and four transformer qab converter

Abstract

The application provides a Kun-type hybrid polarity direct current power distribution system and a four-transformer QAB converter h1 The positive pole of the high-voltage side bus is connected to the high-voltage side active bridge H l1 The positive pole of the low-voltage side bus is connected to the high-voltage side active bridge H h2 The negative pole of the high-voltage side bus is connected to the low-voltage side active bridge H l2 The negative pole of the low-voltage side bus is connected to the high-voltage side active bridge H h1 The high-voltage side active bridge H l1 is connected to the low-voltage side active bridge H r1 through a high-frequency transformer T h2 The high-voltage side active bridge H l2 is connected to the low-voltage side active bridge H r2 through a high-frequency transformer T l1 The low-voltage side active bridge H h2 and the high-voltage side active bridge H e1 are connected through a transformer T h1 The low-voltage side active bridge H l2 and the high-voltage side active bridge H e2 are connected through a transformer T The Kun-type hybrid polarity direct current power distribution system provided by the application has the function of true bipolar system interconnection and guarantees electrical isolation between bipolar systems, thereby ensuring the safety of user power supply.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current power distribution system architecture and corresponding converter topology, in particular, relates to a Kun-type hybrid polarity direct current power distribution system and a four-transformer QAB converter. BACKGROUND

[0002] With the development of renewable energy distributed generation and power electronic technology, direct current power distribution system as a substitute for traditional alternating current system in specific occasions has been widely concerned. Compared with traditional alternating current distribution, there is no frequency stability and power angle stability problem in direct current system, and the control is simpler; photovoltaic, energy storage system, electronic load and other direct current source and load access to direct current system, without power frequency alternating current and direct current power conversion link, so that the overall efficiency of the system can be improved.

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

[0004] In view of the problems of the existing direct current power supply system architecture, in order to further improve the flexibility and reliability of the direct current power distribution system, the present application proposes a new type of direct current power distribution system architecture, and the corresponding converter topology for realizing the architecture.

[0005] Document 1, patent document with publication number CN110783912A, discloses a direct current collection AC-DC hybrid power distribution system and a power distribution method. The system increases AC-DC converters at the end of multiple AC power distribution feeders, all AC-DC converters are connected through the DC side to realize direct current collection, and through direct current collection, the electrical interconnection of multiple AC power distribution feeders can be realized. The system is configured with controllers at the incoming line switch of each AC power distribution feeder and at the direct current collection place to realize electrical quantity acquisition, incoming line protection control and communication with AC-DC converter; the incoming line controller and the direct current collection controller communicate through wireless mode. However, this patent document still has the defect that additional power electronic devices are needed 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 interlinking bipolar DC bus, between two DC buses, current isolation is provided by high frequency transformer of three-level dual active bridge, in addition, a non-symmetrical switching modulation method is proposed to compensate for the non-symmetrical load condition of the bipolar DC bus system. The topology proposed in this application not only has the ability to interlink two different voltage level bipolar DC systems, realize electrical isolation between two sets of DC systems, but also can balance the high voltage side and low voltage side bipolar power.

[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, which can balance the bipolar voltage level without additional voltage balancer. Under the condition of entire load imbalance, even in the case of no-load on one side of the DC pole, the bipolar voltage level can be strictly regulated, in addition, its zero voltage switching capability can be extended to the entire load range including no-load condition. The topology proposed in this application has the power balancing capability of the bipolar distribution system, and has the fault-tolerant operation capability of high-voltage side short circuit and low-voltage side short circuit. SUMMARY

[0008] In view of the defects in the prior art, the purpose of the present application is to provide a Kun-type hybrid polarity DC distribution system and a four-transformer QAB converter.

[0009] The application provides a Kun-type mixed polarity direct current power distribution system, which comprises a high-voltage side bipolar bus, a low-voltage side bipolar bus and a converter part; a high-voltage power distribution network and a low-voltage power distribution network are connected with each other through the converter part.

[0010] The converter part comprises a high-voltage side active bridge H h1 , a low-voltage side active bridge H l1 , a high-voltage side active bridge H h2 and a low-voltage side active bridge H l2 .

[0011] The high-voltage side active bridge H h1 is connected with the positive pole of the high-voltage side bus, the low-voltage side active bridge H l1 is connected with the positive pole of the low-voltage side bus, the high-voltage side active bridge H h2 is connected with the negative pole of the high-voltage side bus, and the low-voltage side active bridge H l2 is connected with the negative pole of the low-voltage side bus.

[0012] The high-voltage side active bridge H h1 is connected with the low-voltage side active bridge H l1 through a high-frequency transformer T r1 , the high-voltage side active bridge H h2 is connected with the low-voltage side active bridge H l2 through a high-frequency transformer T r2 , the low-voltage side active bridge H l1 is connected with the high-voltage side active bridge H h2 through a transformer T e1 , and the high-voltage side active bridge H h1 is connected with the low-voltage side active bridge H l2 through a transformer T e2 .

[0013] Preferably, the high-voltage side active bridge H h1 comprises MOSFET switch tubes S 11 , MOSFET switch tubes S 12 , MOSFET switch tubes S 13 and MOSFET switch tubes S 14 .

[0014] The low-voltage side active bridge H l1 comprises MOSFET switch tubes S 31 , MOSFET switch tubes S 32 , MOSFET switch tubes S 33 and MOSFET switch tubes S 34 .

[0015] Preferably, the high-frequency transformer T r1The number of turns of the high-frequency transformer T

[0016] The high-voltage side active bridge H h2 includes MOSFET switch S 21 , MOSFET switch S 22 , MOSFET switch S 23 and MOSFET switch S 24 ;

[0017] The low-voltage side active bridge H l2 includes MOSFET switch S 41 , MOSFET switch S 42 , MOSFET switch S 43 and MOSFET switch S 44 .

[0018] The number of turns of the high-frequency transformer T r2 is 1:1.

[0019] The number of turns of the transformer T e1 is 1:1.

[0020] The number of turns of the transformer T e2 is 1:1.

[0021] The high-frequency transformer T r1 on the high-voltage side is connected in series with the transformer T e2 on the high-voltage side, the high-frequency transformer T r2 on the high-voltage side is connected in series with the transformer T e1 on the high-voltage side.

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

[0023] The application also provides a four-transformer QAB converter, comprising the above-mentioned Kun-type hybrid polarity direct current power distribution system.

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

[0025] 1. The application provides a full-electrically isolated bipolar direct current power distribution system interconnection architecture, the proposed (Kun) type hybrid polarity direct current power distribution system has true bipolar system interconnection function, while ensuring electrical isolation between bipolar systems, and ensuring user power supply safety;

[0026] 2. Based on the proposed four-transformer QAB circuit, when a short circuit fault occurs on one pole of the low-voltage side, the proposed solution can effectively isolate the low-voltage fault pole, ensuring that the fault will not be transmitted to the high-voltage side, thus improving the power supply reliability of the bipolar DC system.

[0027] 3. This invention has 4U DC (±2U DC ), 2U DC (2U DC -2U DC ±U DC ), U DC (U DC -U DC Multiple voltage levels, including those for power sources and loads, allow for flexible connection.

[0028] 4. Based on the proposed four-transformer QAB circuit, the present invention has the ability to balance the positive and negative power of both the high-voltage side bipolar DC bus and the low-voltage side bipolar DC bus.

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

[0030] 6. Based on the proposed four-transformer QAB structure, this invention can achieve power balance control between the high-voltage bipolar and low-voltage bipolar sides. When a fault occurs on one pole of the low-voltage side, the proposed topology can quickly isolate the faulty pole and ensure the continuous operation of the non-faulty pole. Attached Figure Description

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

[0032] Figure 1 for A schematic diagram of the architecture of a DC power distribution system;

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

[0034] Figure 3 This is a schematic diagram of the key waveforms for normal operation;

[0035] Figure 4 This is a schematic diagram of the key waveforms for fault-tolerant operation;

[0036] Figure 5 This is a schematic diagram of the high-voltage side positive bus current in scenario 1 of Example 3;

[0037] Figure 6 Graph of high voltage side positive bus current for Scenario 1 in Example 3;

[0038] Figure 7 Graph of low voltage side positive bus current for Scenario 1 in Example 3;

[0039] Figure 8 Graph of low voltage side negative bus current for Scenario 1 in Example 3;

[0040] Figure 9 Graph of leakage inductance LI current for Scenario 1 in Example 3;

[0041] Figure 10 Graph of leakage inductance L2 current for Scenario 1 in Example 3;

[0042] Figure 11 Graph of high voltage side negative bus current for Scenario 2 in Example 3;

[0043] Figure 12 Graph of low voltage side positive bus current for Scenario 2 in Example 3;

[0044] Figure 13 Graph of low voltage side negative bus current for Scenario 2 in Example 3;

[0045] Figure 14 Graph of leakage inductance L2 current for Scenario 2 in Example 3;

[0046] Figure 15 Graph of high voltage side positive bus current for Scenario 3 in Example 3;

[0047] Figure 16 Graph of high voltage side negative bus current for Scenario 3 in Example 3;

[0048] Figure 17 Graph of low voltage side positive bus current for Scenario 3 in Example 3;

[0049] Figure 18 Graph of low voltage side negative bus current for Scenario 3 in Example 3;

[0050] Figure 19 Graph of leakage inductance LI current for Scenario 3 in Example 3;

[0051] Figure 20 Graph of leakage inductance L2 current for Scenario 3 in Example 3;

[0052] Figure 21 Graph of low voltage side positive output voltage US1 for Scenario 3 in Example 3. DETAILED DESCRIPTION

[0053] The application will be described in greater detail with reference to specific embodiments. The following embodiments are presented by way of example and are not intended to limit the present application in any manner. It should be noted that, for one of ordinary skill in the art, several changes and modifications can be made without departing from the spirit and scope of the present application. These are all within the scope of the present application.

[0054] Example 1

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

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

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

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

[0059] Example 2

[0060] Those skilled in the art can 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 As shown. The rated voltage level of the line is represented by the rated voltage amplitude U of the single-pole line on the low-voltage side. DC In a unified manner, the rated voltage of a single pole on the high-voltage side DC bus is 2U. DC .

[0062] The mixed polarity in the embodiment is reflected in that it is divided into three parts, one is a high-voltage side bipolar bus, two is a low-voltage side bipolar bus, and three is a four-port converter QT-QAB part connecting the high-voltage and low-voltage distribution networks. The converter part is composed of a high-voltage side active bridge H h1 , a low-voltage side active bridge H l1 , a high-voltage side active bridge H h2 , and a low-voltage side active bridge H l2 , a high-voltage side active bridge H h1 , which is the core of the mixed polarity power distribution system.

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

[0064] The high-voltage side active bridge H h1 and the low-voltage side active bridge H l2 are interconnected through a transformer T e2 with a turns ratio of 1:1, and the low-voltage side active bridge H l1 and the high-voltage side active bridge H h2 are interconnected through a transformer T e1 with a turns ratio of 1:1. The high-voltage side T r1 is connected in series with T e2 , and T r2 is connected in series with T e1 . The low-voltage side T r1 is connected in parallel with T e1 , and T r2 is connected in parallel with T e2Parallel connection. The equivalent leakage inductance L1 is equal to that of transformer T. r1 T e2 The sum of leakage inductance, L2 is the transformer T r2 T e1 The sum of leaks.

[0065] Basic working principle:

[0066] In the proposed hybrid polarity DC power distribution system, the converter section serves as both the high- and low-voltage side power transmission and voltage transformation. 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 Provides ±2U DC 4U DC The voltage level, the rated voltage of the low-voltage side bipolar bus is ±U DC Provide ±U DC 2U DC The voltage level can meet the flexible connection needs of various types of DC electrical equipment with different voltage levels.

[0067] Control quantity These are 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 The phase shift angle. By changing... It can control the power transfer from the positive and negative terminals of the high-voltage side to the positive and negative terminals of the low-voltage side. For example, the key waveform at this time is as follows: Figure 3 As shown. The current flowing through the equivalent leakage inductance L1 during half a cycle is:

[0068]

[0069] The current flowing through the equivalent leakage inductance L2 during half a cycle is:

[0070]

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

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

[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 output power of the low-voltage side positive electrode is:

[0078]

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

[0080]

[0081] Fault-tolerant operation. Wherein, UP1, UP2, US1, US2 are as indicated in Figure 2 .

[0082] Suppose one of the output ports (without loss of generality, take H l2 ) connected to the DC bus short circuit fault. H h1 , H h2 , H l1 and the normal mode of operation state, H l2 then S 41 , S 42 , S 43 , S 44 always on, that is, H l2 output power to 0, so as to isolate the fault while ensuring the normal operation of the converter. At this time, for example, H , the key waveform is shown in Figure 4 . In a half cycle, the current flowing through the equivalent inductance L1 is:

[0083]

[0084] In a half cycle, the current flowing through the equivalent inductance L2 is:

[0085]

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

[0087] From equation (7), (8) can be obtained

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

[0089]

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

[0091]

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

[0093]

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

[0095] P o2=0 (12).

[0096] Example 3

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

[0098] according to Figure 3 Build a Plescs simulation model. High-voltage side 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 terminal output voltage V in_low_N =24V. Switching frequency f s =20kHz, leakage inductance L1=200uF, leakage inductance L2=200uF.

[0099] Scenario 1: Balancing power on the high-voltage side. For example... Figures 5-10 The figures show the currents at the positive and negative busbars on the high and low voltage sides, as well as the leakage inductances L1 and L2. From t = 0s to 0.5s, The output power on the low-voltage side is unbalanced between the positive and negative terminals, while the input power on the high-voltage side is balanced between the positive and negative terminals. At t = 0.5s, The simultaneous change in output power of the positive and negative poles on the low-voltage side, while the input power of the positive and negative poles on the high-voltage side remains balanced, demonstrates the effectiveness of the proposed high-voltage side power balance control for bipolar DC systems.

[0100] Scenario 2: Balancing power on the low-voltage side. For example... Figures 11-14 The figures show the currents at the positive and negative busbars on the high and low voltage sides, as well as the leakage inductances L1 and L2. From t = 0s to 0.5s, The input power to the positive and negative terminals on the high-voltage side is balanced, and the output power to the positive and negative terminals on the low-voltage side is also balanced. At t = 0.5s, With the positive input power on the high-voltage side remaining constant and the negative input power changing, the output power on the low-voltage side remains balanced, demonstrating the effectiveness of the proposed low-voltage side power balance control for bipolar DC systems.

[0101] Scenario 3: Low-voltage side short-circuit fault-tolerant operation. For example... Figures 15-21 The figures show the currents at the positive and negative busbars on the high and low voltage sides, as well as the leakage inductances L1 and L2. From t = 0s to 0.5s, The input power to the positive and negative terminals on the high-voltage side is balanced. At t = 0.5s, The output voltage at the positive terminal of the low-voltage side becomes 0, indicating a short circuit at the positive terminal. The input power at the positive and negative terminals of the high-voltage side remains balanced, and the negative terminal of the low-voltage side operates normally. This demonstrates that the proposed bipolar DC system possesses short-circuit fault-tolerant operation capability.

[0102] The Kun-type hybrid polarity direct current power distribution system has the function of true bipolar system interconnection, ensures electrical isolation between bipolar systems, and guarantees user power supply safety.

[0103] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the specific embodiments described above, and various changes or modifications can be made by those skilled in the art within the scope of the claims, which does not affect the essential content of the present application. The embodiments of the present application and the features in the embodiments can be combined with each other in any way without conflict.

Claims

1. A hybrid Cuk-type mixed polarity DC power distribution system, characterized by, Comprising: a high-voltage side bipolar bus, a low-voltage side bipolar bus and a converter section; the high-voltage power distribution network and the low-voltage power distribution network are connected to each other through the converter section; The converter part comprises a high-voltage side active bridge H h1 , a low-voltage side active bridge H l1 , a high-voltage side active bridge H h2 and a low-voltage side active bridge H l2 ; the high-voltage side active bridge H h1 a positive pole connected to the high-voltage side busbar, the low-voltage side active bridge H l1 a positive pole connected to the low-voltage side busbar, the high-voltage side active bridge H h2 a negative pole connected to the high-voltage side busbar, the low-voltage side active bridge H l2 a negative pole connected to the low-voltage side busbar The high-voltage side active bridge H h1 The low-voltage side active bridge H l1 By means of a high-frequency transformer T r1 The high-voltage side active bridge H h2 The low-voltage side active bridge H l2 By means of a high-frequency transformer T r2 The low-voltage side active bridge H l1 The high-voltage side active bridge H h2 By means of a transformer T e1 The high-voltage side active bridge H h1 The low-voltage side active bridge H l2 By means of a transformer T e2 The low-voltage side active bridge said high frequency transformer on the high voltage side T r1 said transformer on the high voltage side T e2 said high frequency transformer on the high voltage side in series T r2 said transformer on the high voltage side T e1 said transformer on the high voltage side in series said high frequency transformer on the low voltage side T r1 said transformer on the low voltage side T e1 said high frequency transformer on the low voltage side in parallel T r2 said transformer on the low voltage side T e2 in parallel.

2. The Cuk hybrid polarity DC distribution system of claim 1, wherein, The high-voltage side active bridge H h1 Including a MOSFET switch S 11 , a MOSFET switch S 12 , a MOSFET switch S 13 , and a MOSFET switch S 14 ; The low-voltage side active bridge H l1 Including a MOSFET switch S 31 , a MOSFET switch S 32 , a MOSFET switch S 33 , and a MOSFET switch S 34 .

3. The Cuk hybrid polarity DC distribution system of claim 2, wherein, The high-frequency transformer T r1 The turns ratio is 1:

1.

4. The Cuk hybrid polarity DC distribution system of claim 3, wherein, The high-voltage side active bridge H h2 Including a MOSFET switch S 21 , a MOSFET switch S 22 , a MOSFET switch S 23 , and a MOSFET switch S 24 ; The low-voltage side active bridge H l2 Including a MOSFET switch S 41 , a MOSFET switch S 42 , a MOSFET switch S 43 , and a MOSFET switch S 44 .

5. The Cuk hybrid polarity DC distribution system of claim 4, wherein, The high-frequency transformer T r2 The turns ratio is 1:

1.

6. The Cuk hybrid polarity DC distribution system of claim 5, wherein, The transformer T e1 The turns ratio is 1:

1.

7. The Cuk hybrid polarity DC distribution system of claim 6, wherein, The transformer T e2 The turns ratio is 1:

1.

8. A four-transformer QAB converter, characterized by The Kun-type hybrid polarity DC power distribution system comprises the Kun-type hybrid polarity DC power distribution system according to any one of claims 1 to 7.

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