Flexible interconnection device and method for power distribution network based on thyristor AC-AC frequency converter

Abstract

The invention relates to a flexible interconnection device and method for a power distribution network based on thyristor alternating current-alternating current frequency converters, the device is connected between a first alternating current system and a second alternating current system and comprises a current converter, three medium-high frequency transformers and three alternating current-alternating current frequency converters, three input ends of the current converter are respectively connected with three phases of the second alternating current system; the AC input end serves as an AC input end of the flexible interconnection device; three output ends of the converter are connected with terminals of primary windings of the three medium-high frequency transformers; a terminal of a secondary winding of each medium-high frequency transformer is connected with an input end of an AC-AC frequency converter; and two output ends of each AC-AC frequency converter are respectively connected with one phase of the first AC system and one phase of the second AC system. According to the invention, the back-to-back three-phase half-bridge converter is used, a DC bus is reserved, decoupling control of active power and reactive power can be realized, and flexible adjustment of power distribution network energy is realized.

Description

Technical Field

[0001] This invention belongs to the field of flexible interconnection control of power distribution networks, and is particularly applicable to flexible interconnection of high-voltage and high-power power distribution networks. Specifically, it relates to a flexible interconnection device and method for power distribution networks based on a thyristor AC-AC converter. Background Technology

[0002] With the increasing diversification of electricity demand and rising requirements for power quality, coupled with the large-scale integration of distributed renewable energy sources, the power distribution network is shifting from a unidirectional power transmission mode to a multidirectional power flow mode. Traditional switching equipment struggles to adapt to this complex power flow pattern, lacks flexible power regulation, and generates current surges and harmonics during loop-closing operations. Furthermore, existing network structures have poor reconfiguration capabilities, typically relying on adjusting high-frequency transformer taps, node mechanical switches, and capacitor switching for network reconfiguration. However, this method suffers from low accuracy and slow speed, failing to meet reconfiguration requirements. Therefore, a flexible interconnection device is proposed to replace traditional mechanical switches, enabling power flow transfer and rapid fault isolation. This avoids the surge current and harmonic problems associated with traditional mechanical switches, promotes feeder load balancing, and ultimately improves power quality and supply reliability.

[0003] As a type of flexible interconnection device, the dual active bridge converter has strong voltage regulation capability, enabling power conversion and voltage regulation between different voltage levels. It also has good transient response capability for fault isolation when the power grid fails. However, traditional dual active bridge converters use fully controlled IGBTs as switching devices, which have low withstand voltage. In order to ensure that the IGBTs will not break down when the power grid is short-circuited, multiple modules need to be connected in series and parallel to divide the voltage at the AC end. However, this leads to problems such as excessive equipment size and increased switching losses.

[0004] CN118868264A provides a multi-port intensive low-voltage distribution network flexible interconnection device, including: a multilevel converter module and at least one expansion module connected to the module. Both adopt a multilevel converter structure. In the multilevel converter module, each level corresponds to the upper and lower DC side ports of the topology, generating two expansion ports. In the expansion module, each level unit of the multilevel converter structure corresponds to the upper and lower DC side ports of the topology, generating two connection ports. The expansion ports are connected to any one or more connection ports. Corresponding control and adjustment methods, as well as a control and modulation system, are also provided. Compared with this patent, this invention has the following advantages: However, the topology structure of this patent is more complex, and there are more switching devices, resulting in greater overall losses than this invention. Furthermore, each stage in this patent uses capacitors, leading to higher charging and discharging losses. This patent also includes multiple level units and basic unit structures, which are unfavorable for large-scale application due to topology complexity and cost. Moreover, this patent is applicable to low-voltage distribution networks and cannot flexibly adjust the voltage to the required level. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the aforementioned background technology by providing a topology and method for a flexible interconnection device for power distribution networks based on a thyristor AC-AC converter. This improves the applicable voltage level of the equipment, enhances power quality and power supply reliability, and retains the DC bus of the device to form a common DC bus with other flexible interconnection devices for power regulation based on an innovative series-type flexible interconnection device topology and control. This solves the problems of excessive switching losses and high cost in traditional topologies.

[0006] The present invention adopts the following technical solution.

[0007] The first aspect of this invention proposes a flexible interconnection device for power distribution networks based on thyristor AC-AC converters. This device is connected between a first AC system and a second AC system, and includes: a converter, three medium- and high-frequency transformers, and three AC-AC converters. Its characteristic is that: The three-phase AC input terminals of the converter are connected to the three phases of the second AC system, serving as the AC input terminals of the flexible interconnection device. The three-phase AC output terminals of the converter are connected to the primary windings of the corresponding phases of the medium- and high-frequency transformers, respectively; the secondary windings of each phase of the medium- and high-frequency transformers are connected to the input terminals of an AC-AC frequency converter, respectively; and the output terminals of each AC-AC frequency converter are connected in series in the corresponding phases of the first AC system and the second AC system, respectively.

[0008] Preferably, the converter is a back-to-back three-phase half-bridge converter, which consists of two three-phase half-bridge converters connected by a DC bus and converter filter capacitors. The first ends of all upper arms of the two three-phase half-bridge converters are connected to the positive terminal of the DC bus; the second ends of all lower arms of the two three-phase half-bridge converters are connected to the negative terminal of the DC bus; the second ends of the three upper arms of one three-phase half-bridge converter are connected to the first ends of the three lower arms, which are the three input terminals of the back-to-back three-phase half-bridge converter; the second ends of the three upper arms of the other three-phase half-bridge converter are connected to the first ends of the three lower arms, which are the three output terminals of the back-to-back three-phase half-bridge converter. The converter filter capacitor is connected in parallel between the positive and negative terminals of the DC bus; Each upper and lower bridge arm consists of a transistor connected in parallel with an anti-parallel diode.

[0009] Preferably, the back-to-back three-phase half-bridge converter is controlled by SPWM.

[0010] Preferably, the AC-AC inverter is composed of a positive bridge arm, a filter capacitor, and a reverse bridge arm connected in parallel. Both the positive and reverse bridge arms are composed of several bridge arms connected in parallel. Each bridge arm consists of two series-connected switching devices in the same direction. The connection node between the series-connected switching devices of every two bridge arms is the input terminal of a corresponding positive or reverse bridge arm. When the positive and reverse bridge arms are connected in parallel, the parallel connection nodes of the positive and negative terminals of the switching devices in the positive bridge arm are connected to the parallel connection nodes of the negative and positive terminals of the switching devices in the reverse bridge arm, respectively.

[0011] Preferably, the switching devices in the positive and negative bridge arms are thyristors.

[0012] Preferably, the positive and negative bridge arms in the AC-AC inverter are three-phase half-bridges or single-phase half-bridges. When it is a three-phase half-bridge, the AC-AC inverter converts the input three-phase AC voltage into a single-phase AC voltage output; when it is a single-phase half-bridge, the AC-AC inverter converts the input single-phase AC voltage into a single-phase AC voltage output.

[0013] Preferably, when the positive and negative bridge arms are three-phase half-bridges, both the positive and negative bridge arms are composed of three bridge arms connected in parallel, and the corresponding primary and secondary windings of the medium-frequency transformer each have three terminals; when the positive and negative bridge arms are single-phase half-bridges, both the positive and negative bridge arms are composed of two bridge arms connected in parallel, and the corresponding primary and secondary windings of the medium-frequency transformer each have two terminals.

[0014] Preferably, the positive and negative bridge arms in the AC-AC inverter are selected as single-phase half-bridges.

[0015] Preferably, the AC-AC inverter controls the triggering pattern by using the cosine intersection method to form an on / off signal, changing the triggering angle in a sinusoidal manner to modulate the on / off state of the switching devices in the AC-AC inverter.

[0016] A second aspect of the present invention provides a method for flexible interconnection of distribution networks based on thyristor AC-AC converters using the device described in the first aspect of the present invention, comprising: The rectifier side of the back-to-back converter converts the AC power of the second AC system into DC power through the DC bus of the back-to-back converter. The reverse side of the back-to-back converter then converts the DC power back into AC output medium-frequency AC power. The medium-frequency transformer transforms the medium-frequency AC power, and then the frequency is reduced to the power frequency by the AC-AC converter. Finally, it is connected to the phase line of the first AC system and the second AC system to realize the interconnection of the first and second AC systems. When the power grid is under stable control, the rectifier and inverter sides of the back-to-back converter are controlled by a dual-loop voltage and current control. The outer voltage loop of the rectifier side controls the DC bus voltage to stabilize, and the inner current loop of the rectifier side eliminates the harmonic components of the current on the phase line of the first or second AC system. The inverter side controls the power flowing through the phase line of the first or second AC system by changing its output voltage.

[0017] A third aspect of the invention provides an apparatus comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, the processor performing the steps of a flexible interconnection method for a distribution network based on a thyristor AC-AC converter as described in the second aspect of the invention.

[0018] A fourth aspect of the present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, performs the steps of the flexible interconnection method for a distribution network based on a thyristor AC-AC converter as described in the second aspect of the present invention.

[0019] The beneficial effects of this invention are compared with those of the prior art: (1) The present invention provides a topology and method for a flexible interconnection device for a distribution network based on a thyristor AC-AC converter. Compared with the existing series flexible controller topology, this topology uses a back-to-back three-phase half-bridge converter, retains the DC bus, and can achieve decoupled control of active power and reactive power, thereby realizing flexible adjustment of energy in the distribution network.

[0020] (2) The present invention provides a topology and method for a flexible interconnection device for distribution networks based on a thyristor AC-AC converter. This topology replaces the series transformer in the prior art with a medium-high frequency transformer and an AC-AC converter. Compared with the flexible interconnection device topology for distribution networks based on dual active bridge multiplexing, the N series half-bridge converters on the secondary side of the medium-high frequency transformer are replaced with a single AC-AC converter with a lower cost thyristor as the switching transistor. The N series half-bridge converters are changed into a single AC-AC converter, reducing the number of components, manufacturing costs and equipment size. Attached Figure Description

[0021] Figure 1 This is the topology of the AC-AC frequency converter when the positive and negative bridge arms are three-phase half-bridges. Figure 2 This is the topology of the AC-AC frequency converter when the positive and negative bridge arms are single-phase half-bridges. Figure 3 This is a three-phase topology diagram of a flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to the present invention; Figure 4 This is a topology diagram showing the connection between the AC-AC frequency converter and the three-phase half-bridge inverter in the converter when the positive and negative bridge arms are three-phase half-bridges. Figure 5 This is a schematic diagram of the cosine intersection method; Figure 6 The above is a simulation waveform diagram of the AC-AC frequency converter according to an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0023] like Figure 3 As shown, Embodiment 1 of the present invention proposes a flexible interconnection device for power distribution networks based on thyristor AC-AC converters. This device is connected between a first AC system and a second AC system, and includes: a converter, three medium- and high-frequency transformers, and three AC-AC converters. Its characteristic is that: The three-phase AC input terminals of the converter are connected to the three phases of the second AC system, serving as the AC input terminals of the flexible interconnection device. The three-phase AC output terminals of the converter are connected to the primary windings of the corresponding phases of the medium- and high-frequency transformers, respectively; the secondary windings of each phase of the medium- and high-frequency transformers are connected to the input terminals of an AC-AC frequency converter, respectively; and the output terminals of each AC-AC frequency converter are connected in series in the corresponding phases of the first AC system and the second AC system, respectively.

[0024] Preferably, the converter is a back-to-back three-phase half-bridge converter, which consists of two three-phase half-bridge converters connected by a DC bus and converter filter capacitors. The first ends of all upper arms of the two three-phase half-bridge converters are connected to the positive terminal of the DC bus; the second ends of all lower arms of the two three-phase half-bridge converters are connected to the negative terminal of the DC bus; the second ends of the three upper arms of one three-phase half-bridge converter are connected to the first ends of the three lower arms, which are the three input terminals of the back-to-back three-phase half-bridge converter; the second ends of the three upper arms of the other three-phase half-bridge converter are connected to the first ends of the three lower arms, which are the three output terminals of the back-to-back three-phase half-bridge converter. The converter filter capacitor is connected in parallel between the positive and negative terminals of the DC bus; Each upper and lower bridge arm consists of a transistor connected in parallel with an anti-parallel diode.

[0025] Preferably, the back-to-back three-phase half-bridge converter is controlled by SPWM.

[0026] Preferably, the AC-AC inverter is composed of a positive bridge arm, a filter capacitor, and a reverse bridge arm connected in parallel. Both the positive and reverse bridge arms are composed of several bridge arms connected in parallel. Each bridge arm consists of two series-connected switching devices in the same direction. The connection node between the series-connected switching devices of every two bridge arms is the input terminal of a corresponding positive or reverse bridge arm. When the positive and reverse bridge arms are connected in parallel, the parallel connection nodes of the positive and negative terminals of the switching devices in the positive bridge arm are connected to the parallel connection nodes of the negative and positive terminals of the switching devices in the reverse bridge arm, respectively.

[0027] Preferably, the switching devices in the positive and negative bridge arms are thyristors.

[0028] Preferably, the positive and negative bridge arms in the AC-AC inverter are three-phase half-bridges or single-phase half-bridges. When it is a three-phase half-bridge, the AC-AC inverter converts the input three-phase AC voltage into a single-phase AC voltage output; when it is a single-phase half-bridge, the AC-AC inverter converts the input single-phase AC voltage into a single-phase AC voltage output.

[0029] Preferably, such as Figure 1 As shown, when the positive and negative bridge arms are three-phase half-bridges, both the positive and negative bridge arms are composed of three arms connected in parallel, corresponding to three terminals on both the primary and secondary windings of the medium-frequency transformer; for example... Figure 2As shown, when the positive and negative bridge arms are single-phase half-bridges, both the positive and negative bridge arms are composed of two bridge arms connected in parallel, and the corresponding primary and secondary windings of the medium- and high-frequency transformers each have two terminals.

[0030] It should be noted that, taking the positive and negative bridge arms of the AC-AC converter as an example of a three-phase half-bridge, the three output terminals of the converter and the terminals of the primary windings of the three medium- and high-frequency transformers are all connected; the terminals of the secondary windings of each medium- and high-frequency transformer are respectively connected to the input terminal of an AC-AC converter, such as... Figure 4 As shown, the three-phase half-bridge includes: a first switching transistor. S 1 and second switching transistors S The first bridge arm consists of 2 components, and the third switching transistor... S 3 and the fourth switching transistor S The second bridge arm, consisting of 4 components, and the fifth switching transistor. S 5 and the sixth switching transistor S The third bridge arm consists of 6 components, with the positive and negative input terminals of the first bridge arm forming the DC port. V in Medium and high frequency transformers T 1. The branch of the primary winding is connected between the midpoint A of the first arm, the midpoint B of the second arm, and the midpoint C of the third arm of the primary half-bridge; the seventh switch transistor S 7 to 12th switching transistors S 12 The positive converter of the AC-AC frequency converter, the thirteenth switching transistor S 13 Up to the eighteenth switch S 18 The AC-AC converter consists of an inverse converter, where the forward converter and the inverse converter are connected in parallel, and an output filter capacitor. C f The parallel connection between the forward and reverse converters forms an AC port. V out Output filter capacitor C f Used to suppress high-frequency harmonics generated by the switching transistor and smooth the output waveform.

[0031] Preferably, in order to reduce the number of components and lower costs, the positive and negative bridge arms in the AC-AC inverter of this embodiment are selected as single-phase half-bridges.

[0032] Preferably, the AC-AC inverter controls the triggering pattern by using the cosine intersection method to form an on / off signal, changing the triggering angle in a sinusoidal manner to modulate the on / off state of the switching devices in the AC-AC inverter.

[0033] Specifically, trigger angle αThe control should vary sinusoidally between 90° and 0°, so that the instantaneous value of the rectified output voltage is close to the instantaneous value of the desired sinusoidal voltage. The corresponding output waveform during operation is as follows: Figure 5 As shown, the purple, yellow, and green curves represent the three-phase input voltages of the AC-AC converter, while the red curve represents the desired sinusoidal output voltage. The higher the frequency of the three-phase intermediate frequency line voltage connected to the input of the AC-AC converter, the better the quality of the final synthesized single-phase power frequency AC voltage waveform. The positive group control angle starts from... α Starting at 90°, the line voltage output by the AC-AC inverter is... u ab Partial waveforms, positive group thyristors of AC-AC frequency converter S 7 and S 10 When the thyristor is turned on, the other thyristors are turned off. Figure 5 The shaded area represents the portion where the actual voltage applies. α When the angle is 90°, the shaded area and the blank area in the positive half-wave are equal, and the corresponding instantaneous value of the AC output voltage at the power frequency is 0V. Then the control angle... α As the angle decreases from 90° to 0° following a sinusoidal pattern, the area of ​​the shaded region increases. α At 0°, the entire positive half-wave is active, used for the frequency converter output. Therefore, the instantaneous voltage value of the AC output at the power frequency is the highest. As the control angle decreases, the output line voltage of the AC-AC converter... u ab , u ac , u bc , u ba , u ca , u cb The thyristor is connected to the output terminal according to the pattern. S 7 and S 12 , S 9 and S 12 , S 9 and S 7, S 11 and S 7, S 11 and S The combination of 9 is used to conduct the circuit, and then the circuit increases in a sinusoidal manner to... α =90°, the shaded area also decreases until it is equal to the area of ​​the blank area again. The reverse group changes in the same way, and finally forms a voltage waveform with the same frequency as the AC system connected in series after passing through the output filter capacitor.

[0034] Will V dm Let the maximum DC voltage that the AC-AC inverter can output be set to. α 1 indicates the firing angle of the positive group, with α 2 represents the firing angle of the reverse group. The direction of current flowing through the output port distinguishes the operating time intervals of the positive and reverse groups. When the positive group is working, the reverse group is blocked. The voltage at the output terminal can be expressed as: (1) When the reverse group is working, the positive group is blocked, and the voltage at the output terminal can be expressed as: (2) The AC-AC converter operates in a non-circulating current mode, meaning the positive and negative inputs will not conduct simultaneously. The intermediate frequency AC voltage is converted into the industrial frequency AC voltage by the AC-AC converter. v o It can be represented as: (3) Final output voltage amplitude V m The highest DC voltage that an AC-AC inverter can output V dm The ratio, denoted by γ, is called the output voltage ratio. When the positive group is operating, the equation is: v o = v 1. The positive trigger angle can be solved by reverse engineering. α 1. Expression: (4) There is an equation when performing reverse assembly work. v o = v 2. The positive trigger angle can be solved by reverse engineering. α 2. Expression: α 2 = π - α 1.

[0035] Specifically, the Simulink simulation waveform of the three-phase to single-phase AC-AC converter is as follows: Figure 6 As shown, specifically: the input three-phase AC voltage amplitude is 100V and the frequency is 400Hz, and the final output single-phase AC voltage amplitude after passing through the AC-AC converter is 80V and the frequency is 50Hz.

[0036] Embodiment 2 of the present invention proposes a method for flexible interconnection of distribution networks based on thyristor AC-AC frequency converters using the device proposed in Embodiment 1 of the present invention, comprising: The rectifier side of the back-to-back converter converts the AC power of the second AC system into DC power through the DC bus of the back-to-back converter. The reverse side of the back-to-back converter then converts the DC power back into AC output medium-frequency AC power. The medium-frequency transformer transforms the medium-frequency AC power, and then the frequency is reduced to the power frequency by the AC-AC converter. Finally, it is connected to the phase line of the first AC system and the second AC system to realize the interconnection of the first and second AC systems. When the power grid is under stable control, the rectifier and inverter sides of the back-to-back converter are controlled by a dual-loop voltage and current control. The outer voltage loop of the rectifier side controls the DC bus voltage to stabilize, and the inner current loop of the rectifier side eliminates the harmonic components of the current on the phase line of the first or second AC system. The inverter side controls the power flowing through the phase line of the first or second AC system by changing its output voltage.

[0037] Embodiment 3 of the present invention proposes a device including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the steps of the flexible interconnection method for distribution networks based on a thyristor AC-AC converter as described in Embodiment 2 of the present invention.

[0038] Embodiment 4 of the present invention proposes a computer-readable storage medium storing a computer program, wherein the computer program, when executed by a processor, performs the steps of the flexible interconnection method for a distribution network based on a thyristor AC-AC converter as described in Embodiment 2 of the present invention.

[0039] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A flexible interconnection device for a distribution network based on a thyristor AC-AC converter, the device being connected between a first AC system and a second AC system, comprising: The converter, three medium- and high-frequency transformers, and three AC-AC frequency converters are characterized by: The three-phase AC input terminals of the converter are connected to the three phases of the second AC system, serving as the AC input terminals of the flexible interconnection device. The three-phase AC output terminals of the converter are connected to the primary windings of the corresponding phases of the medium- and high-frequency transformers, respectively; the secondary windings of each phase of the medium- and high-frequency transformers are connected to the input terminals of an AC-AC frequency converter, respectively; and the output terminals of each AC-AC frequency converter are connected in series in the corresponding phases of the first AC system and the second AC system, respectively.

2. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 1, characterized in that: The converter is a back-to-back three-phase half-bridge converter, which consists of two three-phase half-bridge converters connected by a DC bus and converter filter capacitors. The first ends of all upper arms of the two three-phase half-bridge converters are connected to the positive terminal of the DC bus; the second ends of all lower arms of the two three-phase half-bridge converters are connected to the negative terminal of the DC bus; the second ends of the three upper arms of one three-phase half-bridge converter are connected to the first ends of the three lower arms, which are the three input terminals of the back-to-back three-phase half-bridge converter; the second ends of the three upper arms of the other three-phase half-bridge converter are connected to the first ends of the three lower arms, which are the three output terminals of the back-to-back three-phase half-bridge converter. The converter filter capacitor is connected in parallel between the positive and negative terminals of the DC bus; Each upper and lower bridge arm consists of a transistor connected in parallel with an anti-parallel diode.

3. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 2, characterized in that: The back-to-back three-phase half-bridge converter is controlled by SPWM.

4. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 1, characterized in that: The AC-AC inverter consists of a positive bridge arm, a filter capacitor, and a reverse bridge arm connected in parallel. Both the positive and reverse bridge arms are composed of several bridge arms connected in parallel. Each bridge arm consists of two series-connected switching devices in the same direction. The connection node between the series-connected switching devices of every two bridge arms is the input terminal of a corresponding positive or reverse bridge arm. When the positive and reverse bridge arms are connected in parallel, the parallel connection nodes of the positive and negative terminals of the switching devices in the positive bridge arm are connected to the parallel connection nodes of the negative and positive terminals of the switching devices in the reverse bridge arm, respectively.

5. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 4, characterized in that: The switching devices in both the positive and negative bridge arms are thyristors.

6. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 4, characterized in that: The positive and negative bridge arms of the AC-AC inverter are either three-phase half-bridges or single-phase half-bridges. When it is a three-phase half-bridge, the AC-AC inverter converts the input three-phase AC voltage into a single-phase AC voltage output; when it is a single-phase half-bridge, the AC-AC inverter converts the input single-phase AC voltage into a single-phase AC voltage output.

7. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 5, characterized in that: When the positive and negative bridge arms are three-phase half-bridges, both the positive and negative bridge arms are composed of three bridge arms connected in parallel, and the corresponding primary and secondary windings of the medium-frequency transformer have three terminals; when the positive and negative bridge arms are single-phase half-bridges, both the positive and negative bridge arms are composed of two bridge arms connected in parallel, and the corresponding primary and secondary windings of the medium-frequency transformer have two terminals.

8. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 6, characterized in that: The positive and negative bridge arms in the AC-AC inverter are selected as single-phase half-bridges.

9. The flexible interconnection device for power distribution networks based on a thyristor AC-AC converter according to claim 8, characterized in that: The AC-AC inverter uses the cosine intersection method to control the triggering law to form on / off signals, changing the trigger angle in a sinusoidal manner to modulate the on / off state of the switching devices in the AC-AC inverter.

10. A method for flexible interconnection of a distribution network based on a thyristor AC-AC frequency converter using the device described in any one of claims 2-9, characterized in that, include: The rectifier side of the back-to-back converter converts the AC power of the second AC system into DC power through the DC bus of the back-to-back converter. The reverse side of the back-to-back converter then converts the DC power back into AC output medium-frequency AC power. The medium-frequency transformer transforms the medium-frequency AC power, and then the frequency is reduced to the power frequency by the AC-AC converter. Finally, it is connected to the phase line of the first AC system and the second AC system to realize the interconnection of the first and second AC systems. When the power grid is under stable control, the rectifier and inverter sides of the back-to-back converter are controlled by a dual-loop voltage and current control. The outer voltage loop of the rectifier side controls the DC bus voltage to stabilize, and the inner current loop of the rectifier side eliminates the harmonic components of the current on the phase line of the first or second AC system. The inverter side controls the power flowing through the phase line of the first or second AC system by changing its output voltage.

11. An apparatus comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, the processor performing the steps of the method for flexible interconnection of a distribution network based on a thyristor AC-AC converter as described in claim 10.

12. A computer-readable storage medium storing a computer program, the computer program being executed by a processor of the steps of the method for flexible interconnection of a distribution network based on a thyristor AC-AC converter as described in claim 10.

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