CN112260253B - Push-pull type direct-current autotransformer - Google Patents
Push-pull type direct-current autotransformer Download PDFInfo
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- CN112260253B CN112260253B CN202011172911.4A CN202011172911A CN112260253B CN 112260253 B CN112260253 B CN 112260253B CN 202011172911 A CN202011172911 A CN 202011172911A CN 112260253 B CN112260253 B CN 112260253B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F27/00—Details of transformers or inductances, in general
- H01F27/28—Coils; Windings; Conductive connections
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F30/00—Fixed transformers not covered by group H01F19/00
- H01F30/02—Auto-transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
- H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
- H02M1/0074—Plural converter units whose inputs are connected in series
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract
The invention discloses a push-pull type direct current autotransformer which comprises a first power exchange unit, a second power exchange unit, a third power exchange unit, a positive pole four-winding transformer and a negative pole four-winding transformer, wherein two ports of the positive pole of the first power exchange unit are connected with the positive pole of a second direct current system, two ports of the negative pole of the first power exchange unit are connected with two ports of the positive pole of the second power exchange unit through the positive pole four-winding transformer, two ports of the positive pole of the third power exchange unit are connected with two ports of the negative pole of the second power exchange unit through the negative pole four-winding transformer, the positive poles of a first winding and a second winding, the negative poles of a third winding and a fourth winding and the positive pole of the first direct current system are interconnected together, and the negative poles of the first winding and the second winding, the positive poles of the third winding and the fourth winding and the negative pole of the first direct current system are interconnected together. The converter of the invention adopts the direct current self-coupling technology, thus remarkably reducing the conversion capacity, the running loss and the total cost of the system.
Description
Technical Field
The invention belongs to the technical field of power transmission and distribution of a power system, and particularly relates to a push-pull type direct current autotransformer.
Background
With the rapid development of new energy power generation, High Voltage Direct Current Transmission (HVDC) technology has gained wide attention and research in society, and a plurality of HVDC Transmission lines have been built and operated around the world. Meanwhile, the flexible direct-current transmission technology is mature day by day, and the successful trial operation of a multi-terminal direct-current transmission system interconnects different direct-current transmission systems, so that the technical feasibility and the superiority of constructing a direct-current power grid are more and more obvious. High capacity and multiple voltage levels are one of the development trends of the future direct current power grid, and the direct current transformer (or called as a direct current-direct current converter) has great research significance as indispensable key technical equipment for interconnecting multiple voltage level direct current power transmission systems.
The existing high-voltage high-power direct-current transformer can be mainly divided into two categories according to the implementation mode, the first category adopts a plurality of groups of low-voltage low-power direct-current-direct-current converters to carry out series-parallel combination to realize the overall high-voltage high power, and the second category directly adopts a high-voltage high-power converter topological structure and additional electrical equipment (left-level and the like, ' direct-current-direct-current autotransformer control and direct-current fault isolation ', ' China Motor engineering report, 2016, 36(9): 2398-.
A Dual Active Bridge (DAB) combined dc-dc converter is a typical topology structure of the above-mentioned first type of dc transformer, and its specific implementation is to use a single Dual Active Bridge dc-dc converter as a sub-module to perform serial and parallel operations on multiple Dual Active bridges. The two active bridge ports on the low-voltage direct-current system side are connected in parallel more so as to improve the overall working current, and the two active bridge ports on the high-voltage direct-current system side are connected in series more so as to improve the overall working voltage. Although the double-active-bridge combined DC-DC converter has the advantages of double active bridges, such as soft switching, the problems of voltage and current sharing between different double active bridges, complex system control strategy, high cost and the like are faced.
The facing dc-dc converter is a typical topology structure of the above second type of dc transformer, and its specific implementation manner is to construct two converters, the dc ports of the two converters are respectively connected to the dc terminals of the low voltage first dc system and the high voltage second dc system, and the ac ports of the two converters are connected to each other through an ac circuit such as a reactor or a transformer. Taking the example of power transmission from the first dc system to the second dc system, the operating principle is as follows: the first converter converts the direct current power output by the first direct current system into alternating current power, then transmits the alternating current power to the second converter through an alternating current circuit, and the second converter converts the alternating current power into direct current power and finally transmits the direct current power to the second direct current system. The direct current transformer is characterized in that two direct current systems are not directly electrically connected, and the electrical isolation is realized through an alternating current circuit. The inverter and the additional electrical equipment adopted by the surface type DC-DC converter are mature, so that the control method has the advantages of simple control strategy design, high operation reliability and the like, but has some defects. Firstly, the rated capacities of the two converters are equal to the rated direct current power of the interconnection between the first direct current system and the second direct current system, the rated direct current voltages of the converters are respectively equal to the rated voltages of the connected direct current systems, and the total cost of the converters is high. Secondly, all transmitted direct current power needs to be converted through direct current/alternating current/direct current two-stage power, so that the operation loss is large, and the transmission efficiency is low.
In summary, the existing mainstream high-voltage high-power dc transformers have at least one of the following disadvantages, such as complicated control strategy, high cost, large operation loss, and low transmission efficiency.
Disclosure of Invention
In order to improve the defects of the existing direct current transformer and promote the development of the related technology of a direct current power grid, the invention provides the push-pull type direct current autotransformer which utilizes a direct current autotransformer structure to establish direct electrical connection between two direct current systems to a certain extent and realizes the direct transmission of partial direct current power, namely the partial power does not need to be subjected to direct current/alternating current/direct current two-stage power conversion, so that the power transmission efficiency is improved, and the total current conversion capacity of the system and the equipment cost are reduced.
To achieve the above objects, according to one aspect of the present invention, there is provided a push-pull dc autotransformer for interconnecting two dc systems and controlling power transmission, comprising a first, a second and a third power exchanging units and a positive and a negative pole four-winding transformer, wherein,
the first power exchange unit, the second power exchange unit and the third power exchange unit respectively comprise two identical bridge arms, the positive pole and the negative pole of each power exchange unit are respectively provided with two ports, each bridge arm is formed by connecting a plurality of sub-modules in series, the first power exchange unit comprises a first bridge arm and a second bridge arm, the second power exchange unit comprises a third bridge arm and a fourth bridge arm, and the third power exchange unit comprises a fifth bridge arm and a sixth bridge arm;
the positive pole of the first bridge arm, the positive pole of the second bridge arm and the positive pole of the second direct current system are interconnected together, the negative pole of the first bridge arm is connected with the positive pole of the positive pole four-winding transformer winding III, the negative pole of the second bridge arm is connected with the positive pole of the positive pole four-winding transformer winding IV, the positive pole of the third bridge arm is connected with the negative pole of the positive pole four-winding transformer winding I, the negative pole of the third bridge arm is connected with the positive pole of the negative pole four-winding transformer winding II, the negative pole of the fourth bridge arm is connected with the positive pole of the negative pole four-winding transformer winding II, the positive pole of the fifth bridge arm is connected with the negative pole of the negative pole four-winding transformer winding III, the positive pole of the sixth bridge arm is connected with the negative pole of the negative pole four-winding transformer winding IV, the negative pole of the fifth bridge arm, the negative pole of the sixth bridge arm and the negative pole of the second direct current system are interconnected together, meanwhile, the positive pole of the first winding of the positive pole four-winding transformer, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system are interconnected together, and the negative pole of the first winding of the negative pole four-winding transformer, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system are interconnected together.
According to the scheme, the push-pull type direct current autotransformer controls the power transmitted between the first power unit and the second power unit through the positive pole four-winding transformer by using the alternating current voltage components output by the first bridge arm and the second bridge arm, and controls the power transmitted between the third power unit and the second power unit through the negative pole four-winding transformer by using the alternating current voltage components output by the fifth bridge arm and the sixth bridge arm, and the third bridge arm and the fourth bridge arm through the topological structure, and finally realizes the direct current power transmission control between two direct current systems.
Preferably, the positive pole four-winding transformer and the negative pole four-winding transformer have special properties, and mutual inductance coupling relations among the windings are specially designed, wherein the first winding is strongly coupled with the second winding, the third winding is strongly coupled with the fourth winding, mutual leakage reactance can be ignored, leakage reactance exists among the first winding, the third winding, the first winding, the fourth winding, the second winding and the fourth winding, and the values are the same.
Preferably, the submodules of the bridge arms in the power exchange unit can adopt a half-bridge submodule, a self-resistance submodule, a full-bridge submodule and a clamping submodule, the submodules of the same type are not necessarily adopted in a unified way in a single bridge arm, the submodules of multiple types can also be adopted in a mixed way, and in addition, the submodules in the bridge arms can be formed by connecting a plurality of submodules in parallel so as to realize the improvement of rated working current.
Preferably, the push-pull dc autotransformer has a step-down interconnection capability, that is, a rated dc voltage of the first dc system may be higher than a rated dc voltage of the second dc system, and at this time, sub-modules having a capability of outputting a negative level, such as a full-bridge sub-module, need to be used in bridge arms of the first power exchange unit and the third power exchange unit.
Preferably, the output voltages at the two ends of each bridge arm should include a direct current voltage component and an alternating current voltage component, where the direct current voltage component is used to support a direct current voltage at a port of the power exchange unit, and the alternating current voltage component is used to control power transmission between the first direct current system and the second direct current system.
Preferably, the direct current voltage components of the first bridge arm and the second bridge arm are the same, the direct current voltage components of the fifth bridge arm and the sixth bridge arm are the same and are equal to one half of the rated direct current voltage difference value of the second direct current system and the first direct current system, and the direct current voltage components of the third bridge arm and the fourth bridge arm are the same and are equal to the rated direct current voltage of the first direct current system.
Preferably, the alternating-current voltage components in the bridge arms have a plurality of relation forms, and one preferable control form is as follows: the alternating current voltage component amplitudes of two bridge arms in the first power exchange unit, the second power exchange unit and the third power exchange unit are the same, and the phase difference is 180 degrees. At the moment, the transformer can be combined with a four-winding transformer with a specific topological structure, and in steady-state operation, the positive and negative pole four-winding transformers can be equivalently replaced and analyzed by two identical single-phase double-winding transformers, so that the analysis of power transmission characteristics and the complexity of a control strategy are greatly simplified.
Preferably, when the number of the submodules of the bridge arm in the first power exchange unit, the second power exchange unit and the third power exchange unit is designed, the reserve number of the direct-current submodules, the reserve number of the alternating-current submodules and the reserve number of the redundant submodules need to be considered.
Preferably, the first dc system may be a dc system of an asymmetric unipolar structure, and the second dc system may be a dc system of a symmetric bipolar structure.
Preferably, the first dc system may be a dc system of a symmetrical bipolar structure, and the second dc system may be a dc system of an asymmetrical unipolar structure.
Preferably, the second power exchange unit is further formed by connecting a positive second power exchange unit and a negative second power exchange unit, wherein the positive second power exchange unit includes a positive bridge arm three and a positive bridge arm four, the negative second power exchange unit includes a negative bridge arm three and a negative bridge arm four, a negative pole of the positive bridge arm three, a negative pole of the positive bridge arm four, a positive pole of the negative bridge arm three, and a positive pole of the negative bridge arm four are interconnected, and a connection point is grounded.
Preferably, a common connection point of the negative electrode of the positive electrode bridge arm three, the negative electrode of the positive electrode bridge arm four, the positive electrode of the negative electrode bridge arm three and the positive electrode of the negative electrode bridge arm four is further connected with neutral points of the first direct current system and the second direct current system through a metal return wire, so that the circulation of unbalanced direct current between the two electrodes through a loop formed by the metal return wire in the asymmetric operation is realized, and the corrosion of a pipeline on the direct current line caused by the fact that the unbalanced direct current flows through the ground is avoided.
According to another aspect of the present invention, there is provided a push-pull dc autotransformer for interconnecting and controlling power transmission between a first dc system of positive polarity asymmetric monopole and a second dc system of positive polarity asymmetric monopole, comprising a first power exchanging element, a positive four-winding transformer and a positive second power exchanging element, wherein,
the positive pole of the first bridge arm, the positive pole of the second bridge arm and the positive pole of the second direct current system are interconnected together, the negative pole of the first bridge arm is connected with the positive pole of the positive pole four-winding transformer winding, the negative pole of the second bridge arm is connected with the positive pole of the positive pole four-winding transformer winding, the positive pole of the third bridge arm is connected with the negative pole of the positive pole four-winding transformer winding, the positive pole of the fourth bridge arm is connected with the negative pole of the positive pole four-winding transformer winding, the negative pole of the third bridge arm, the negative pole of the first direct current system and the negative pole of the second direct current system are interconnected together, a connection point is grounded or grounded through a metal return wire, and the positive pole of the first bridge arm, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system are interconnected together;
the alternating current voltage components output by the first bridge arm and the second bridge arm, and the alternating current voltage components output by the third positive bridge arm and the fourth positive bridge arm are utilized to control the power transmitted between the first power unit and the second positive power unit through the four-winding transformer, and finally, the direct current power transmission control between two direct current systems is realized.
According to another aspect of the present invention, there is provided a push-pull dc autotransformer for interconnecting and controlling power transmission between a first dc system of asymmetric unipolar negative polarity and a second dc system of asymmetric unipolar negative polarity, comprising a negative second power exchanging unit, a negative four-winding transformer and a third power exchanging unit, wherein,
the positive pole of the third negative pole bridge arm, the positive pole of the fourth negative pole bridge arm, the positive pole of the first direct current system and the positive pole of the second direct current system are interconnected together, a connection point is grounded or grounded through a metal return wire, the negative pole of the third negative pole bridge arm is connected with the positive pole of the first negative pole four-winding transformer winding, the negative pole of the fourth negative pole bridge arm is connected with the positive pole of the second negative pole four-winding transformer winding, the positive pole of the fifth bridge arm is connected with the negative pole of the third negative pole four-winding transformer winding, the positive pole of the sixth bridge arm is connected with the negative pole of the fourth negative pole four-winding transformer winding, the negative poles of the fifth bridge arm, the negative pole of the sixth bridge arm and the negative pole of the second direct current system are interconnected together, and meanwhile, the negative pole of the first negative pole four-winding transformer winding, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system are interconnected together;
the alternating current voltage components output by the bridge arm five and the bridge arm six, and the alternating current voltage components output by the negative pole bridge arm three and the negative pole bridge arm four are utilized to control the power transmitted between the third power unit and the negative pole second power unit through the negative pole four-winding transformer, and finally, the direct current power transmission control between two direct current systems is realized.
Preferably, the push-pull type direct current autotransformer can be further expanded to realize interconnection and power transmission control among a plurality of direct current systems, the push-pull type direct current autotransformer is provided with a plurality of power exchange units, a four-winding transformer is converted into a coupling type multi-winding transformer, and the total number of windings is equal to twice of the number of the direct current systems.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
(1) according to the scheme, a direct-current self-coupling structure is adopted, direct electrical interconnection is established between two direct-current systems to a certain degree, direct transmission of partial direct-current power is further achieved, the power value and the operation loss of the system after direct-current/alternating-current/direct-current two-stage power conversion are effectively reduced, and when the boost ratio of rated direct-current voltages of the two direct-current systems is closer to 1, the power transmission efficiency of the system is improved more remarkably. Meanwhile, the direct-current voltage of the low-voltage first direct-current system is fully utilized, the direct-current voltage components of bridge arms in the first power exchange unit and the third power exchange unit and the required number of sub-modules are reduced, and the total cost and the manufacturing cost of the system are further reduced.
(2) The bridge arm in the power exchange unit is formed by connecting a plurality of sub-modules in series, and is similar to the bridge arm in the modular multilevel converter, the modulation mode of the bridge arm and the balance control between the capacitor voltages of the sub-modules are mature, the control is simple, and the operation reliability is high.
(3) The scheme of the invention can adopt a four-winding transformer with a specific topological structure, and combines a push-pull type working mode, thereby simplifying the analysis of power transmission characteristics among different bridge arms in steady-state operation, reducing the complexity of a control strategy and enhancing the operation reliability of a system.
(4) The scheme of the invention is easy to be expanded into a multi-port direct current transformer, and realizes the interconnection among three or more direct current systems and the control of power transmission.
Drawings
Fig. 1 is a schematic view showing a typical topology of a dc-dc converter of a dual active bridge combination type in the prior art, which employs a single-phase voltage source type dual active bridge dc-dc converter;
FIG. 2 is a schematic diagram of a typical topology of a prior art face-to-face DC-DC converter that uses a voltage source converter to perform DC/AC (AC/DC) power conversion;
FIG. 3 is a schematic diagram of a push-pull DC autotransformer topology according to one embodiment of the present invention;
FIG. 4 is a schematic diagram of the core winding distribution of a specially designed four-winding transformer of the present invention;
FIG. 5 is a topology of a half-bridge sub-module in the prior art;
FIG. 6 is a topology of a prior art self-blocking sub-module with fault self-clearing capability;
FIG. 7 is a topology of a prior art full bridge sub-module with fault self-clearing and negative level output capability;
FIG. 8 is a topology of a prior art clamped dual submodule with fault self clearing capability;
FIG. 9 is a schematic diagram of a push-pull DC autotransformer topology according to another embodiment of the present invention, wherein the first DC system is a positive polarity asymmetric unipolar DC system and the second DC system is a symmetric bipolar DC system;
FIG. 10 is a schematic diagram of a push-pull DC autotransformer topology according to another embodiment of the present invention, wherein the first DC system is a negative polarity asymmetric unipolar DC system and the second DC system is a symmetric bipolar DC system;
FIG. 11 is a schematic diagram of a push-pull DC autotransformer topology according to another embodiment of the present invention, wherein the first DC system is a symmetric bipolar DC system and the second DC system is a positive polarity asymmetric unipolar DC system;
FIG. 12 is a schematic diagram of a push-pull DC autotransformer topology according to another embodiment of the present invention, wherein the first DC system is a symmetric bipolar DC system and the second DC system is a negative polarity asymmetric unipolar DC system;
fig. 13 is a schematic topology diagram of a push-pull dc autotransformer according to another embodiment of the present invention, in which the second power exchanging unit is formed by connecting a positive second power exchanging unit and a negative second power exchanging unit, and the common connection point is grounded;
FIG. 14 is a schematic diagram of a push-pull DC autotransformer topology according to another embodiment of the present invention, wherein the common connection point of the positive second power exchanging element and the negative second power exchanging element is interconnected with the neutral point of the first DC system and the second DC system by a metallic return line;
fig. 15 is a schematic diagram of a push-pull dc autotransformer topology comprising a first power exchange unit, a positive quad-winding transformer, and a positive second power exchange unit, which may be used to connect two positive-polarity asymmetric unipolar dc systems, according to another embodiment of the present invention;
fig. 16 is a schematic diagram of a push-pull dc autotransformer topology composed of a third power exchanging unit, a negative four-winding transformer and a negative second power exchanging unit, which can be used to connect two negative asymmetric unipolar dc systems according to another embodiment of the present invention;
fig. 17 is a schematic diagram of an extended push-pull dc autotransformer topology capable of interconnecting three dc systems in accordance with another embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The push-pull type direct current autotransformer provided by the embodiment of the invention is mainly used for interconnection of direct current systems with different rated direct current voltage grades and control of power transmission of the direct current systems, and can effectively solve the defects of complex control strategy, high cost, large operation loss, low transmission efficiency and the like in the conventional direct current-direct current converter technology.
Fig. 1 is a schematic diagram showing a typical topology of a dc-dc converter of a dual active bridge combination type in the prior art, which employs a single-phase voltage source type dc-dc converter of a dual active bridge; fig. 2 is a schematic diagram of a typical topology of a prior art facing dc-dc converter that uses a voltage source converter to perform dc/ac (ac/dc) power conversion.
The invention provides a push-pull type direct current autotransformer which is used for realizing interconnection and power transmission control between two direct current systems and is characterized by comprising a first power exchange unit, a second power exchange unit, a third power exchange unit, a positive pole four-winding transformer and a negative pole four-winding transformer, wherein,
the first power exchange unit, the second power exchange unit and the third power exchange unit respectively comprise two identical bridge arms, the positive pole and the negative pole of each power exchange unit are respectively provided with two ports, each bridge arm is formed by connecting a plurality of sub-modules in series, the first power exchange unit comprises a first bridge arm and a second bridge arm, the second power exchange unit comprises a third bridge arm and a fourth bridge arm, and the third power exchange unit comprises a fifth bridge arm and a sixth bridge arm;
the positive pole of the first bridge arm, the positive pole of the second bridge arm and the positive pole of the second direct current system are interconnected together, the negative pole of the first bridge arm is connected with the positive pole of the positive pole four-winding transformer winding III, the negative pole of the second bridge arm is connected with the positive pole of the positive pole four-winding transformer winding IV, the positive pole of the third bridge arm is connected with the negative pole of the positive pole four-winding transformer winding I, the negative pole of the third bridge arm is connected with the positive pole of the negative pole four-winding transformer winding II, the negative pole of the fourth bridge arm is connected with the positive pole of the negative pole four-winding transformer winding II, the positive pole of the fifth bridge arm is connected with the negative pole of the negative pole four-winding transformer winding III, the positive pole of the sixth bridge arm is connected with the negative pole of the negative pole four-winding transformer winding IV, the negative pole of the fifth bridge arm, the negative pole of the sixth bridge arm and the negative pole of the second direct current system are interconnected together, meanwhile, the positive pole of the first winding of the positive pole four-winding transformer, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system are interconnected together, and the negative pole of the first winding of the negative pole four-winding transformer, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system are interconnected together.
On the basis, the invention also provides a push-pull type direct current autotransformer which is used for realizing interconnection between a first direct current system of a positive polarity asymmetric monopole and a second direct current system of the positive polarity asymmetric monopole and controlling power transmission, and comprises a first power exchange unit, a positive pole four-winding transformer and a positive pole second power exchange unit, wherein,
the positive pole of the first bridge arm, the positive pole of the second bridge arm and the positive pole of the second direct current system are interconnected together, the negative pole of the first bridge arm is connected with the positive pole of the positive pole four-winding transformer winding, the negative pole of the second bridge arm is connected with the positive pole of the positive pole four-winding transformer winding, the positive pole of the third bridge arm is connected with the negative pole of the positive pole four-winding transformer winding, the positive pole of the fourth bridge arm is connected with the negative pole of the positive pole four-winding transformer winding, the negative pole of the third bridge arm, the negative pole of the first direct current system and the negative pole of the second direct current system are interconnected together, a connection point is grounded or grounded through a metal return wire, and the positive pole of the first bridge arm, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system are interconnected together;
the alternating current voltage components output by the first bridge arm and the second bridge arm, and the alternating current voltage components output by the third positive bridge arm and the fourth positive bridge arm are utilized to control the power transmitted between the first power unit and the second positive power unit through the four-winding transformer, and finally, the direct current power transmission control between two direct current systems is realized.
The invention also provides a push-pull type direct current autotransformer, which is used for realizing interconnection and power transmission control between a first direct current system with asymmetric monopole with negative polarity and a second direct current system with asymmetric monopole with negative polarity, and comprises a second power exchange unit with negative polarity, a transformer with four windings with negative polarity and a third power exchange unit, wherein,
the positive pole of the third negative pole bridge arm, the positive pole of the fourth negative pole bridge arm, the positive pole of the first direct current system and the positive pole of the second direct current system are interconnected together, a connection point is grounded or grounded through a metal return wire, the negative pole of the third negative pole bridge arm is connected with the positive pole of the first negative pole four-winding transformer winding, the negative pole of the fourth negative pole bridge arm is connected with the positive pole of the second negative pole four-winding transformer winding, the positive pole of the fifth bridge arm is connected with the negative pole of the third negative pole four-winding transformer winding, the positive pole of the sixth bridge arm is connected with the negative pole of the fourth negative pole four-winding transformer winding, the negative poles of the fifth bridge arm, the negative pole of the sixth bridge arm and the negative pole of the second direct current system are interconnected together, and meanwhile, the negative pole of the first negative pole four-winding transformer winding, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system are interconnected together;
the alternating current voltage components output by the bridge arm five and the bridge arm six, and the alternating current voltage components output by the negative pole bridge arm three and the negative pole bridge arm four are utilized to control the power transmitted between the third power unit and the negative pole second power unit through the negative pole four-winding transformer, and finally, the direct current power transmission control between two direct current systems is realized.
The push-pull type direct current autotransformer can be further expanded to realize the interconnection and power transmission control among a plurality of direct current systems, and is provided with a plurality of power exchange units, a four-winding transformer is converted into a coupling type multi-winding transformer, and the total number of windings is equal to twice of the number of the direct current systems.
Fig. 3 shows a schematic topology of a push-pull dc autotransformer 4 according to an embodiment of the present invention, where the push-pull dc autotransformer 4 mainly includes a first power exchanging unit 7, a positive four-winding ac transformer 5, a second power exchanging unit 69, a negative four-winding ac transformer 8, and a third power exchanging unit 10, where,
each power exchange unit comprises two identical bridge arms, so that each power exchange unit is provided with four ports, the positive pole and the negative pole of each power exchange unit are respectively provided with two ports, and each bridge arm is formed by connecting a plurality of sub-modules in series. First power switching unit 7 includes a first bridge arm 71 and a second bridge arm 72, second power switching unit 69 includes a third bridge arm 691 and a fourth bridge arm 692, and third power switching unit 10 includes a fifth bridge arm 101 and a sixth bridge arm 102;
the positive electrode of the first bridge arm 71, the positive electrode of the second bridge arm 72 and the positive electrode of the second direct current system 2 are interconnected together, the negative electrode of the first bridge arm 71 is connected with the positive electrode of the third winding of the positive four-winding transformer 5, the negative electrode of the second bridge arm 72 is connected with the positive electrode of the fourth winding of the positive four-winding transformer 5, the positive electrode of the third bridge arm 691 is connected with the negative electrode of the first winding of the positive four-winding transformer 5, the positive electrode of the fourth bridge arm 692 is connected with the negative electrode of the second winding of the positive four-winding transformer 5, the negative electrode of the third bridge arm 691 is connected with the positive electrode of the first winding of the negative four-winding transformer 8, the negative electrode of the fourth bridge arm 692 is connected with the positive electrode of the second winding of the negative four-winding transformer 8, the positive electrode of the fifth bridge arm 101 is connected with the negative electrode of the third winding of the negative four-winding transformer 8, the positive electrode of the sixth bridge arm 102 is connected with the negative electrode of the fourth winding of the negative winding transformer 8, the negative electrode of the fifth bridge arm 101, The negative pole of the bridge arm six 102 and the negative pole of the second direct current system 2 are interconnected together, meanwhile, the positive pole of the first winding of the positive four-winding transformer 5, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system 1 are interconnected together, and the negative pole of the first winding of the negative four-winding transformer 8, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system 1 are interconnected together.
Fig. 4 shows a schematic diagram of the distribution of the core windings of the positive pole four-winding transformer 5 and the negative pole four-winding transformer 8, which are specifically designed in the present invention, and the structural design can be obtained by setting a center tap on each winding of the single-phase double-winding transformer, wherein the first winding and the second winding are obtained by setting a center tap on the winding on the first core limb 51, the third winding and the fourth winding are obtained by setting a center tap on the winding on the second core limb 52, the homonymous ends between the windings are represented by black dots, and the finally expected mutual inductance coupling relationship between the windings of the four-winding transformer is as follows: the first winding and the second winding are strongly coupled, the third winding and the fourth winding are strongly coupled, mutual leakage reactance can be ignored, leakage reactance exists between the first winding and the third winding, between the first winding and the fourth winding, between the second winding and the third winding, and between the second winding and the fourth winding, and the values are the same.
Fig. 5, 6, 7, and 8 respectively show topologies of a half-bridge submodule, a self-resistance submodule with a fault self-clearing capability, a full-bridge submodule with a fault self-clearing and negative level output capability, and a clamping bimodule with a fault self-clearing capability in the prior art, which can be used as submodules in a bridge arm of a push-pull type direct current autotransformer 4, wherein a single bridge arm adopts a form of a same type of submodule or a mixture of multiple types of submodules, and in addition, the submodules in the bridge arm can also be formed by connecting multiple submodules in parallel, so as to improve rated working current.
If the sub-modules with the capability of outputting negative levels, such as full-bridge sub-modules, are adopted in the arms of the first power exchange unit 7 and the third power exchange unit 10, the output dc voltages of the arm one 71, the arm two 71, the arm five 101, and the arm six 102 can be controlled to be negative, at this time, the push-pull dc autotransformer 4 has the step-down interconnection capability, that is, the rated dc voltage of the first dc system can be higher than the rated dc voltage of the second dc system.
In the embodiment shown in fig. 3, the first dc system 1 and the second dc system 2 are preferably both dc systems with symmetric bipolar structures, and in order to enable the dc systems to operate normally, output voltages at two ends of the first bridge arm 71 and the second bridge arm 72, the fifth bridge arm 101 and the sixth bridge arm 102, and the third bridge arm 691 and the fourth bridge arm 692 all include a dc voltage component and an ac voltage component, where the dc voltage component is used to support a dc voltage at a port of the power exchange unit, and the ac voltage component is used to control power transmission between the first dc system 1 and the second dc system 2. The direct-current voltage components of the first bridge arm 71 and the second bridge arm 72 are the same, the direct-current voltage components of the fifth bridge arm 101 and the sixth bridge arm 102 are the same, the direct-current voltage components are equal to half of the rated direct-current voltage difference value of the second direct-current system 2 and the first direct-current system 1, and the direct-current voltage components of the third bridge arm 691 and the fourth bridge arm 692 are the same and equal to the rated direct-current voltage of the first direct-current system 1. A plurality of control forms are provided between alternating current voltage components output by two bridge arms in the power exchange unit, and one preferable control form is as follows: the amplitudes of the alternating-current voltage components of the two bridge arms in the first power exchange unit 7, the second power exchange unit 69 and the third power exchange unit 10 are the same, and the phase difference is 180 degrees. At this time, if only from the perspective of power transmission characteristics under steady-state operation, a four-winding transformer with a specific topological structure is combined, the positive pole four-winding transformer 5 and the negative pole four-winding transformer 8 can be equivalently replaced by two identical single-phase double-winding transformers for analysis, so that the complexity of analysis and control strategies of the power transmission characteristics of the system is greatly simplified, and the harmonic content on the direct current side is reduced.
When the number of the submodules of the bridge arm in the first power exchange unit 7, the second power exchange unit 69 and the third power exchange unit 10 is designed, the reserve number of the direct current submodules, the reserve number of the alternating current submodules and the reserve number of the redundant submodules need to be considered, the reserve number of the direct current submodules in the bridge arm depends on the direct current voltage component of the bridge arm, the reserve number of the alternating current submodules in the bridge arm has various selection forms, one conventional form is equal to the reserve number of the direct current submodules in the bridge arm, and the reserve number of the redundant submodules in the bridge arm needs to be determined according to actual engineering requirements.
By comparing the present invention with the facing type dc-dc converter of fig. 1, and the dual active bridge combined dc-dc converter of fig. 2, the following differences can be obtained:
(1) the push-pull type dc autotransformer 4 establishes a certain degree of direct electrical interconnection between the first dc system 1 and the second dc system 2, that is, the positive electrode and the negative electrode of the first dc system 1 may be connected to the positive electrode and the negative electrode of the second dc system 2 through the first power exchanging unit 7 and the second power exchanging unit 10, respectively. Has the following advantages: firstly, direct transmission of partial direct current power is realized, namely the partial power does not need direct current/alternating current/direct current two-stage power conversion, and the power transmission efficiency of the system is improved; secondly, the direct-current voltage of the first direct-current system 1 is fully utilized, and the total cost manufacturing cost of the sub-modules in the first power exchange unit 7 and the second power exchange unit 10 is reduced. The traditional facing type direct current-direct current converter and the double-active combined direct current-direct current converter do not establish direct electrical interconnection between the first direct current system 1 and the second direct current system 2, all transmission power needs to be converted through direct current/alternating current/direct current two-stage power, and the operation loss is high. Furthermore, since it does not use the existing dc voltage of the first dc system 1, the total cost is also high.
(2) Compared with a double-active-bridge combined direct current-direct current converter, the modulation mode of the series-connected submodules in the bridge arms of the push-pull direct current autotransformer 4 and the balance control between the capacitor voltages of the submodules are mature, and the problems of voltage equalizing, current equalizing and the like do not exist. Meanwhile, by adopting a four-winding transformer with a specific topological structure and a push-pull type working mode, a mathematical model of the push-pull type direct current autotransformer is fully simplified, a control strategy is simple, and the operation reliability is high.
In one embodiment, a push-pull dc autotransformer 4 is used to interconnect the positive polarity asymmetric unipolar first dc system 1 and the symmetric bipolar second dc system 2, as shown in figure 9.
In one embodiment, a push-pull dc autotransformer 4 is used to interconnect the negative polarity asymmetric unipolar first dc system 1 and the symmetric bipolar second dc system 2, as shown in figure 10.
In one embodiment, a push-pull dc autotransformer 4 is used to interconnect a symmetrical bipolar first dc system 1 and a positive polarity asymmetrical unipolar second dc system 2, as shown in figure 11.
In one embodiment, a push-pull dc autotransformer 4 is used to interconnect a symmetric bipolar first dc system 1 and a negative polarity asymmetric unipolar second dc system 2, as shown in figure 12.
As shown in fig. 13, in an embodiment, the second power exchanging unit 69 may further be formed by connecting a positive second power exchanging unit 6 and a negative second power exchanging unit 9, where the positive second power exchanging unit 6 includes a positive arm three 61 and a positive arm four 62, the negative second power exchanging unit 9 includes a negative arm three 91 and a negative arm four 92, the negative 61 of the positive arm three, the negative 62 of the positive arm four, the positive 91 of the negative arm three, and the positive of the negative arm four 92 are interconnected, and a connection point is grounded. The advantage of this embodiment is that if the first dc system 1 and the second dc system 2 are both symmetrical and bipolar dc systems, when a positive polarity single-pole-to-ground dc short-circuit fault occurs, the first power exchanging unit 7 and the positive second power exchanging unit 6 can be isolated, and the third power exchanging unit 10 and the negative second power exchanging unit 9 are maintained in a normal operating state, so that the push-pull dc autotransformer 469 still has the capability of transmitting half of the rated power. Similarly, when a negative single-pole ground-to-ground direct-current short-circuit fault occurs, the third power exchange unit 10 and the negative second power exchange unit 9 can be isolated, and the first power exchange unit 7 and the positive second power exchange unit 6 are maintained in a normal operation state, so that the push-pull direct-current autotransformer 469 still has the capability of transmitting half of rated power.
As shown in fig. 14, in one embodiment, a common connection point of the negative pole of positive pole arm three 61, the negative pole of positive pole arm four 62, the positive pole of negative pole arm three 91, and the positive pole of negative pole arm four 92 is connected to neutral points of first dc system 1 and second dc system 2 through a metal return line 11. The topology of fig. 14 corresponds substantially to that of fig. 13, which embodiment has the advantage that, when the system is in asymmetrical operation, unbalanced dc currents can circulate through the loop formed by the metallic return 11, no longer flowing to ground, avoiding corrosion of the pipes on the dc line.
As shown in fig. 15, in one embodiment, a push-pull dc autotransformer 411 is used to interconnect the positive polarity asymmetric unipolar first dc system 1 and the positive polarity asymmetric unipolar second dc system 2, which only includes the first power exchanging element 7, the positive four-winding transformer 5, and the positive second power exchanging element 6. The positive pole of the first bridge arm 71, the positive pole of the second bridge arm 72 and the positive pole of the second direct current system 2 are interconnected, the negative pole of the first bridge arm 71 is connected with the three positive poles of the winding of the positive pole four-winding transformer 5, the negative pole of the second bridge arm 72 is connected with the positive pole of the winding four of the positive pole four-winding transformer 5, the positive pole of the winding one of the positive pole four-winding transformer 5 and the positive pole of the winding two, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system 1 are interconnected together, the positive pole of the third positive pole arm 61 is connected with the negative pole of the first positive pole four-winding transformer 5, the positive pole of the fourth positive pole arm 62 is connected with the negative pole of the second positive pole four-winding transformer 5, the negative pole of the third positive pole arm 61 is connected with the negative pole of the fourth positive pole arm 62, and the connection point is directly grounded with the negative pole of the first direct current system 1 and the negative pole of the second direct current system 2 or grounded through a metal loop 11.
As shown in fig. 16, in one embodiment, a push-pull dc autotransformer 412 is used to interconnect the negative polarity asymmetric unipolar first dc system 1 and the negative polarity asymmetric unipolar second dc system 2, which includes only the negative second power exchanging element 9, the negative four-winding transformer 8, and the third power exchanging element 10. The positive pole of the negative pole bridge arm three 91, the positive pole of the negative pole bridge arm four 92, the positive pole of the first direct current system 1 and the positive pole of the second direct current system 2 are interconnected together, and the connection point is grounded or grounded through a metal return wire 11, the cathode of the cathode bridge arm three 91 is connected with the anode of the first winding of the cathode four-winding transformer 8, the cathode of the cathode bridge arm four 92 is connected with the anode of the second winding of the cathode four-winding transformer 8, the anode of the bridge arm five 101 is connected with the cathode of the third winding of the cathode four-winding transformer 8, the anode of the bridge arm six 102 is connected with the cathode of the fourth winding of the cathode four-winding transformer 8, the cathode of the bridge arm five 101, the cathode of the bridge arm six 102 and the cathode of the second direct current system 2 are interconnected together, meanwhile, the negative pole of the first winding of the negative pole four-winding transformer 8, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system 1 are interconnected together;
as shown in fig. 17, in an embodiment, a push-pull dc autotransformer 431 is used to interconnect the first dc system 1, the second dc system 2, and the third dc system 333, and its topology structure can be obtained by topology expansion in fig. 3, and is characterized in that, in addition to the first power exchange unit 7, the second power exchange unit 69, and the third power exchange unit 10 that are already in fig. 3, a fourth power exchange unit 73 (including a bridge arm seven 731 and a bridge arm eight 732) and a fifth power exchange unit 13 (including a bridge arm nine 131 and a bridge arm ten 132) are added, and the positive four-winding transformer 5 and the negative four-winding transformer 8 are respectively changed into a positive six-winding transformer 53 and a negative six-winding transformer 83, wherein,
the positive pole of the bridge arm seven 731, the positive pole of the bridge arm eight 732 and the positive pole of the third direct current system 333 are interconnected together, the negative pole of the bridge arm seven 731 is connected with the positive pole of the winding five of the positive pole six-winding transformer 53, the negative pole of the bridge arm eight 732 is connected with the positive pole of the winding six of the positive pole six-winding transformer 53, the negative pole of the winding five of the positive pole six-winding transformer 53, the negative pole of the winding six of the positive pole six-winding transformer 53, the positive pole of the bridge arm one 71, the positive pole of the bridge arm two 72 and the positive pole of the bridge arm two 72 are interconnected together, the negative pole of the bridge arm one 71 is connected with the positive pole of the winding three of the positive pole six-winding transformer 53, the positive pole of the bridge arm three 691 is connected with the positive pole of the winding one of the positive pole six-winding transformer 53, the positive pole of the bridge arm four is connected with the negative pole of the winding two of the positive pole six-winding transformer 53, the negative pole of the third bridge arm 691 is connected with the positive pole of the first winding of the negative pole six-winding transformer 83, the negative pole of the fourth bridge arm 692 is connected with the positive pole of the second winding of the negative pole six-winding transformer 83, the positive pole of the fifth bridge arm 101 is connected with the negative pole of the third winding of the negative pole six-winding transformer 83, the positive pole of the sixth bridge arm 102 is connected with the negative pole of the fourth winding of the negative pole six-winding transformer 83, the negative pole of the fifth bridge arm 101, the negative pole of the sixth bridge arm 102, the positive pole of the fifth winding of the negative pole six-winding transformer 83, the positive pole of the sixth winding transformer 83 and the negative pole of the second direct current system 2 are interconnected together, the positive pole of the ninth bridge arm 131 is connected with the negative pole of the fifth winding of the negative pole six-winding transformer 83, the positive pole of the tenth arm 132 and the negative pole of the third direct current system 333 are interconnected together, meanwhile, the positive pole of the first winding, the positive pole of the second winding, the negative pole of the third winding and the negative pole of the fourth winding of the positive pole six-winding transformer 53 are interconnected with the positive pole of the first direct current system 1, and the negative pole of the first winding, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system 1 of the negative pole six-winding transformer 83 are interconnected together.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A push-pull DC autotransformer is used for realizing the interconnection between a first DC system and a second DC system and the control of power transmission, and is characterized in that the autotransformer comprises a first power exchange unit, a second power exchange unit, a third power exchange unit, a positive pole four-winding transformer and a negative pole four-winding transformer,
the first power exchange unit, the second power exchange unit and the third power exchange unit respectively comprise two identical bridge arms, the positive pole and the negative pole of each power exchange unit respectively have two ports, each bridge arm is formed by connecting a plurality of sub-modules in series, the first power exchange unit comprises a first bridge arm and a second bridge arm, the second power exchange unit comprises a third bridge arm and a fourth bridge arm, and the third power exchange unit comprises a fifth bridge arm and a sixth bridge arm;
the positive pole of the first bridge arm, the positive pole of the second bridge arm and the positive pole of the second direct current system are interconnected together, the negative pole of the first bridge arm is connected with the positive pole of the positive pole four-winding transformer winding III, the negative pole of the second bridge arm is connected with the positive pole of the positive pole four-winding transformer winding IV, the positive pole of the third bridge arm is connected with the negative pole of the positive pole four-winding transformer winding I, the negative pole of the third bridge arm is connected with the positive pole of the negative pole four-winding transformer winding II, the negative pole of the fourth bridge arm is connected with the positive pole of the negative pole four-winding transformer winding II, the positive pole of the fifth bridge arm is connected with the negative pole of the negative pole four-winding transformer winding III, the positive pole of the sixth bridge arm is connected with the negative pole of the negative pole four-winding transformer winding IV, the negative pole of the fifth bridge arm, the negative pole of the sixth bridge arm and the negative pole of the second direct current system are interconnected together, meanwhile, the positive pole of the first winding of the positive pole four-winding transformer, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system are interconnected together, and the negative pole of the first winding of the negative pole four-winding transformer, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system are interconnected together.
2. The push-pull direct current autotransformer according to claim 1, wherein the positive four-winding transformer has a first winding strongly coupled to a second winding, a third winding strongly coupled to a fourth winding, and leakage reactance between the first winding and the third winding, the first winding and the fourth winding, the second winding and the third winding, and leakage reactance between the second winding and the fourth winding, and the values are the same; the first winding and the second winding and the third winding and the fourth winding of the negative pole four-winding transformer are strongly coupled, mutual leakage reactance is neglected, leakage reactance exists between the first winding and the third winding, between the first winding and the fourth winding, between the second winding and the third winding, between the second winding and the fourth winding, and the values are the same.
3. The push-pull direct-current autotransformer as claimed in claim 1, wherein the submodules of the bridge arms in the first, second and third power exchange units comprise a half-bridge submodule, a self-resistance submodule, a full-bridge submodule and a clamping bimodule, and a single bridge arm is formed by adopting the same type of submodules or a mixed type of submodules.
4. The push-pull type direct current autotransformer according to claim 1, wherein the output voltages at two ends of each bridge arm each include a direct current voltage component and an alternating current voltage component, wherein the direct current voltage component is used for supporting a direct current voltage corresponding to a port of the power exchange unit, and the alternating current voltage component is used for controlling power transmission between the first direct current system and the second direct current system.
5. The push-pull type direct current autotransformer as claimed in claim 4, wherein the direct current voltage components of the first bridge arm and the second bridge arm are the same, the direct current voltage components of the fifth bridge arm and the sixth bridge arm are the same and equal to a half of the rated direct current voltage difference value of the second direct current system and the first direct current system, and the direct current voltage components of the third bridge arm and the fourth bridge arm are the same and equal to the rated direct current voltage of the first direct current system.
6. The push-pull type direct current autotransformer as claimed in claim 1, wherein the second power exchange unit is formed by connecting a positive second power exchange unit and a negative second power exchange unit, the positive second power exchange unit comprises a positive bridge arm three and a positive bridge arm four, the negative second power exchange unit comprises a negative bridge arm three and a negative bridge arm four, a negative pole of the positive bridge arm three, a negative pole of the positive bridge arm four, a positive pole of the negative bridge arm three, and a positive pole of the negative bridge arm four are interconnected, and a connection point is grounded.
7. The push-pull type direct current autotransformer as claimed in claim 6, wherein a common connection point of a negative electrode of the positive bridge arm three, a negative electrode of the positive bridge arm four, a positive electrode of the negative bridge arm three, and a positive electrode of the negative bridge arm four is further connected with neutral points of the first direct current system and the second direct current system through a metal loop, so that circulation of unbalanced direct current between two electrodes through a loop formed by the metal loop is realized when the asymmetric operation is performed.
8. A push-pull type DC autotransformer is used for realizing interconnection between a first DC system with positive polarity and a second DC system with positive polarity and an asymmetric monopole and controlling power transmission, and is characterized by comprising a first power exchange unit, a positive pole four-winding transformer and a positive pole second power exchange unit, wherein,
the first power exchange unit and the positive second power exchange unit both comprise two identical bridge arms, the positive pole and the negative pole of each power exchange unit are respectively provided with two ports, each bridge arm is formed by connecting a plurality of sub-modules in series, the first power exchange unit comprises a first bridge arm and a second bridge arm, and the positive second power exchange unit comprises a third positive bridge arm and a fourth positive bridge arm;
the positive pole of the first bridge arm, the positive pole of the second bridge arm and the positive pole of the second direct current system are interconnected together, the negative pole of the first bridge arm is connected with the positive pole of the positive pole four-winding transformer winding, the negative pole of the second bridge arm is connected with the positive pole of the positive pole four-winding transformer winding, the positive pole of the third bridge arm is connected with the negative pole of the positive pole four-winding transformer winding, the positive pole of the fourth bridge arm is connected with the negative pole of the positive pole four-winding transformer winding, the negative pole of the third bridge arm, the negative pole of the first direct current system and the negative pole of the second direct current system are interconnected together, the connection point is grounded or grounded through a metal return wire, and the positive pole of the first bridge arm, the positive pole of the second winding, the negative pole of the third winding, the negative pole of the fourth winding and the positive pole of the first direct current system are interconnected together.
9. A push-pull type DC autotransformer is used for realizing interconnection between a first DC system with asymmetric monopole with negative polarity and a second DC system with asymmetric monopole with negative polarity and controlling power transmission, and is characterized by comprising a second power exchange unit with negative polarity, a transformer with four windings with negative polarity and a third power exchange unit, wherein,
the negative second power exchange unit and the third power exchange unit respectively comprise two identical bridge arms, the positive pole and the negative pole of each power exchange unit are respectively provided with two ports, each bridge arm is formed by connecting a plurality of sub-modules in series, the negative second power exchange unit comprises a negative bridge arm three and a negative bridge arm four, and the third power exchange unit comprises a bridge arm five and a bridge arm six;
the positive pole of the third negative pole bridge arm, the positive pole of the fourth negative pole bridge arm, the positive pole of the first direct current system and the positive pole of the second direct current system are interconnected together, a connection point is grounded or grounded through a metal return wire, the negative pole of the third negative pole bridge arm is connected with the positive pole of the first negative pole four-winding transformer winding, the negative pole of the fourth negative pole bridge arm is connected with the positive pole of the second positive pole four-winding transformer winding, the positive pole of the fifth bridge arm is connected with the negative pole of the third negative pole four-winding transformer winding, the positive pole of the sixth bridge arm is connected with the negative pole of the fourth negative pole four-winding transformer winding, the negative poles of the fifth bridge arm, the negative pole of the sixth bridge arm and the negative pole of the second direct current system are interconnected together, and the negative pole of the first negative pole four-winding transformer winding, the negative pole of the second winding, the positive pole of the third winding, the positive pole of the fourth winding and the negative pole of the first direct current system are interconnected together.
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