Single-phase single-stage AC-AC converter capable of reducing voltage

Abstract

The invention provides a step-down single-phase single-stage AC-AC converter, and relates to the technical field of converters. The voltage-reducing single-phase single-stage AC-AC converter comprises a first inductor, an intermediate capacitor, a second inductor, a first bidirectional switch group and a second bidirectional switch tube group, the first end of the first inductor is connected to the first end of an input power supply, and the second end of the first inductor is connected to the second end of the input power supply through the intermediate capacitor and the second inductor; the first end of the first bidirectional switch group is connected to the common end of the first inductor and the intermediate capacitor; the second end is connected to the common end of the first inductor and the input power supply through a load; the first end of the second bidirectional switch group is connected to the common end of the second inductor and the intermediate capacitor; and the second end is connected to the common end of the first bidirectional switch group and the load. According to the invention, one-stage energy AC / AC conversion is realized only by controlling the two switching tube groups which work complementarily, the efficiency is high, the control mode is simple, and the function of the converter can be realized by using fewer peripheral circuits.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a step-down single-phase single-stage AC-AC converter. Background Art

[0002] The power quality of the distribution network substation area will affect the power supply quality. Low-quality power supply will lead to problems such as increased power grid line loss, equipment failure, and power outage accidents. Too high or too low voltage will cause the user's electrical equipment to malfunction.

[0003] To solve the problem of too high supply voltage at the distribution transformer or the end of the line in the substation area, the existing technology generally adjusts the voltage through a UPQC (Unified Power Quality Conditioner) device based on the series-parallel structure of transformers, such as Figure 1 shown. The UPQC device based on the series-parallel structure of transformers first connects a rectifier in parallel at the grid end to generate a DC voltage, and then converts the DC voltage into an alternating current through an inverter. This alternating current is added to the grid through a coupling transformer connected in series in the grid, so that the load-side voltage is superimposed with an alternating voltage generated by the inverter on the basis of the grid voltage, thereby achieving the adjustment of the grid voltage. When the grid voltage is too high, adding a voltage with a phase opposite to the voltage by using the UPQC can achieve voltage reduction.

[0004] However, the UPQC device based on the series-parallel structure of transformers uses two-stage energy conversion of AC / DC + DC / AC, and has low efficiency. Summary of the Invention

[0005] (1) Technical Problems to be Solved

[0006] Aiming at the deficiencies of the existing technology, the present invention provides a step-down single-phase single-stage AC-AC converter, which solves the technical problem of low efficiency of the existing UPQC device.

[0007] (2) Technical Solutions

[0008] To achieve the above object, the present invention is realized through the following technical solutions:

[0009] In a first aspect, the present invention provides a step-down single-phase single-stage AC-AC converter, including a first inductor, an intermediate capacitor, a second inductor, a first bidirectional switch group, and a second bidirectional switch tube group; a first end of the first inductor is connected to a first end of the input power supply, and a second end of the first inductor is connected to a second end of the input power supply via the intermediate capacitor and the second inductor;

[0010] The first end of the first bidirectional switch group is connected to the common end of the first inductor and the intermediate capacitor; the second end is connected to the first end of the load, and the second end of the load is connected to the common end of the first inductor and the input power supply;

[0011] The first end of the second bidirectional switch group is connected to the common end of the second inductor and the intermediate capacitor; the second end is connected to the common end of the first bidirectional switch group and the load;

[0012] Both the first bidirectional switch group and the second bidirectional switch group are connected to a plurality of control terminals, and are adapted to operate in a complementary conduction mode with a fixed duty cycle under the control of a control signal applied to the control terminals, such that the voltage across the load is less than the input voltage.

[0013] Preferably, both the first bidirectional switch group and the second bidirectional switch group are switching devices capable of satisfying a bidirectional current path.

[0014] Preferably, both the first bidirectional switch group and the second bidirectional switch group include a pair of opposite switches.

[0015] Preferably, when the voltage at the second end of the input power supply is positive and the voltage at the first end is negative, the step-down single-phase single-stage AC-AC converter includes two operating modes:

[0016] Operating mode one: The second bidirectional switch group conducts from the first end to the second end, and the first bidirectional switch group is turned off. There are two current flow paths in the circuit, which are respectively: the input power supply current passes through the second inductor and the second bidirectional switch group to supply power to the load; the intermediate capacitor charges the first inductor through the second bidirectional switch group and the load; the voltage at the first end of the load is positive and the voltage at the second end is negative;

[0017] Operating mode two: The second bidirectional switch group is turned off, and the first bidirectional switch group conducts from the first end to the second end. There are two current flow paths in the circuit, which are respectively: the input power supply and the load current charge the intermediate capacitor through the second inductor; the first inductor freewheels through the first bidirectional switch group; the voltage at the first end of the load is positive and the voltage at the second end is negative.

[0018] Preferably, when the voltage at the second end of the input power supply is negative and the voltage at the first end is positive, the step-down single-phase single-stage AC-AC converter includes two operating modes:

[0019] Operating mode three: The second bidirectional switch group conducts from the second end to the first end, and the first bidirectional switch group is turned off. There are two current flow paths in the circuit, which are respectively: the input power supply current passes through the load, the second bidirectional switch group, and the second inductor; the intermediate capacitor current passes through the first inductor, the load, and the second bidirectional switch group; the voltage at the first end of the load is negative and the voltage at the second end is positive;

[0020] Working mode four: The second bidirectional switch group is turned off, and the first bidirectional switch group conducts from the second end to the first end. There are two current flow paths in the circuit, which are respectively: The input power supply passes through the load, charges the intermediate capacitor, and the second inductor to charge the intermediate capacitor; The first inductor freewheels through the first bidirectional switch group; The voltage at the first end of the load is negative, and the voltage at the second end is positive.

[0021] Preferably, when the duty cycle of the first bidirectional switch group is 1 - D and the duty cycle of the second bidirectional switch group is D, the voltage V out across the load and the input voltage V ac have the following quantitative relationship:

[0022] V out = V ac *D.

[0023] In a second aspect, the present invention provides a step - down AC - AC converter applied to a three - phase circuit, which includes three step - down single - phase single - stage AC - AC converters. Each step - down single - phase single - stage AC - AC converter includes a first inductor, an intermediate capacitor, a second inductor, a first bidirectional switch group, and a second bidirectional switch group;

[0024] The second ends of the second inductors of the three step - down single - phase single - stage AC - AC converters are respectively connected to the first ends of the A, B, and C three - phase power supplies, and the second ends of the three - phase power supplies are connected together as the three - phase power common terminal;

[0025] The second end of the first inductor of each step - down single - phase single - stage AC - AC converter is connected to the first end of the second inductor via the intermediate capacitor; The first end of the first bidirectional switch group is connected to the common terminal of the first inductor and the intermediate capacitor; The second end is connected to the first end of the load; The first end of the second bidirectional switch group is connected to the common terminal of the second inductor and the intermediate capacitor; The second end is connected to the common terminal of the first bidirectional switch group and the load; The second end of the load is connected to the first end of the first inductor;

[0026] The second ends of the loads connected to the three step - down single - phase single - stage AC - AC converters are connected together and connected to the three - phase power common terminal;

[0027] The first bidirectional switch group and the second bidirectional switch group in each step - down single - phase single - stage AC - AC converter are each connected to a plurality of control terminals, and are adapted to operate in a complementary conduction mode with a fixed duty cycle under the control of a control signal applied to the control terminals, so that the voltage across the load of each phase is less than the corresponding phase voltage.

[0028] Preferably, the first bidirectional switch group and the second bidirectional switch group are both switch devices capable of satisfying bidirectional current paths;

[0029] Preferably, both the first bidirectional switch group and the second bidirectional switch group include a set of apex-to-apex switches.

[0030] Preferably, in any one-phase circuit, when the duty cycle of the first bidirectional switch group is 1 - D and the duty cycle of the second bidirectional switch group is D, the voltage V across each phase load out and the input voltage V of this phase x have the following quantitative relationship:

[0031] V out = V x *D

[0032] where x = a, b, c, and V x represents the three-phase voltages of V a , V b , V c .

[0033] (III) Advantageous Effects

[0034] The present invention provides a step-down single-phase single-stage AC-AC converter. Compared with the prior art, it has the following advantageous effects:

[0035] The step-down single-phase single-stage AC-AC converter of the present invention can achieve first-level energy-based AC / AC conversion only by controlling two complementary working switch tube groups, has high efficiency, and has a simple control method, and can realize the converter function with fewer peripheral circuits. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0037] Figure 1 FIG. is the architecture diagram of an existing UPQC device based on transformer series-parallel connection;

[0038] Figure 2 FIG. is the architecture diagram of a step-down single-phase single-stage AC-AC converter in Embodiment 1;

[0039] Figure 3 FIG. is Figure 2 the specific circuit diagram of the shown architecture diagram;

[0040] Figure 4 FIG. is Figure 3 the schematic diagram of the first working mode of the shown circuit;

[0041] Figure 5 Schematic diagram of operating mode 2 of the circuit shown Figure 3

[0042] Figure 6 Schematic diagram of operating mode 3 of the circuit shown Figure 3

[0043] Figure 7 Schematic diagram of operating mode 4 of the circuit shown Figure 3

[0044] Figure 8 Circuit diagram of applying the step - down single - phase single - stage AC - AC converter shown Figure 3 in a three - phase circuit Specific implementation manners

[0045] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0046] It should be noted that for the convenience of narration, the switch IGBT is used as a representative of the controllable (conducting and turning - off) switch tube in the embodiments of the present invention, but the switch tube in the present invention is not limited to IGBT. Taking IGBT as an example for illustration, the first end of the IGBT refers to the collector, the second end refers to the emitter, and the control end refers to the gate. A drive control signal is applied to the control end of each switch tube in the embodiments of the present invention. For the sake of simplicity, it will not be elaborated hereinafter. The power switch tubes in the embodiments of the present invention can also be implemented by other controllable switch tube devices other than IGBT, such as MOSFET. At the same time, in the embodiments of the present invention, to ensure the normal operation of each switching device, a free - wheeling diode needs to be connected in parallel with each switching device. The parallel connection direction of the free - wheeling diode is related to the type of the switching device, and those skilled in the art can set it according to the type of the switching device, which is not limited herein. If not specified, the switching device is default to include a free - wheeling diode, and it will be pointed out in this embodiment in special cases.

[0047] By providing a step - down single - phase single - stage AC - AC converter, the embodiments of the present application solve the technical problem of low efficiency of the existing UPQC device, realize one - stage AC / AC conversion, save the number of devices, and improve the operation efficiency.

[0048] The general idea of the technical solutions in the embodiments of the present application to solve the above - mentioned technical problems is as follows:

[0049] ​​​Regarding the problem of excessive power supply voltage at the distribution transformer or the end of the line in the substation area, traditional passive solutions include the on-load tap-changing scheme of the transformer, which adjusts the output voltage by changing the turns ratio of the transformer. However, the adjustment steps are limited, and the flexibility and reliability are poor. The method of switching capacitors uses the method of incorporating capacitors to achieve reactive power support for the line inductance, thereby improving the low voltage. However, it cannot be adjusted seamlessly, and the flexibility and real-time performance are poor. Active solutions are based on power electronics technology. Commonly used ones include bidirectional AC / DC + DC / AC, or AC / AC solutions. All the power in this type of solution passes through the power electronic converter, which has disadvantages such as low efficiency and high cost.

[0050] To overcome the defects of traditional passive and active solutions, the current mainstream technical path is to adopt the UPQC solution based on the series-parallel structure of the transformer. This solution compensates the current through the shunt side, and realizes voltage compensation with the help of the series transformer, and then completes the power electronic conversion of part of the power. However, this solution still uses the two-stage energy conversion mode of AC / DC + DC / AC, and this mode has poor efficiency. In addition, the power frequency transformer used in the solution not only has a large volume, but also leads to low overall efficiency and a significant increase in cost.

[0051] As Figure 1 can be seen, the UPQC device first connects a rectifier in parallel at the grid end to generate a DC voltage, and then inversely converts the DC voltage into an alternating current through an inverter. This alternating current is added to the grid through a coupling transformer connected in series in the grid, so that the voltage on the load side is superimposed with an alternating voltage generated by the inverter on the basis of the grid voltage, thereby achieving the adjustment of the grid voltage. When the grid voltage is too high, adding a voltage with the opposite phase to the voltage by using UPQC can achieve voltage reduction.

[0052] To sum up, due to the two-stage architecture of rectification first and then inversion adopted by UPQC, and the need to connect a coupling transformer in series in the grid, it not only greatly improves the control complexity of the device, but also causes significant problems such as low efficiency, large volume, and high cost.

[0053] To solve the above problems, the embodiment of the present invention proposes a step-down single-phase single-stage AC-AC converter, which can achieve one-stage energy-type AC / AC conversion only by controlling two complementary working switch groups S1 and S2 with a fixed duty cycle. It has high efficiency, simple control method, and can realize the function of the converter with fewer peripheral circuits.

[0054] To better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners.

[0055] Embodiment 1:

[0056] This embodiment proposes a step-down single-phase single-stage AC-AC converter, asFigure 2 As shown, it includes a first inductor, an intermediate capacitor, a second inductor, a first bidirectional switch group, and a second bidirectional switch tube group; the first end of the first inductor is connected to the first end of the input power supply, and the second end of the first inductor is connected to the second end of the input power supply via the intermediate capacitor and the second inductor;

[0057] The first end of the first bidirectional switch group is connected to the common end of the first inductor and the intermediate capacitor; the second end is connected to the first end of the load, and the second end of the load is connected to the common end of the first inductor and the input power supply;

[0058] The first end of the second bidirectional switch group is connected to the common end of the second inductor and the intermediate capacitor; the second end is connected to the common end of the first bidirectional switch group and the load;

[0059] Both the first bidirectional switch group and the second bidirectional switch group are connected to a plurality of control terminals, and are adapted to operate in a complementary conduction mode with a fixed duty cycle under the control of the control signals applied to the control terminals, such that the voltage across the load is less than the input voltage.

[0060] As Figure 3 shown, in this embodiment, both the first bidirectional switch group S1 and the second bidirectional switch group S2 are composed of two opposing IGBTs. It should be noted that in the specific implementation process, other forms of switch tubes such as MOSFETs can also be used to form an opposing switch tube group, or other switch devices with a bidirectional current path. In Figure 3 this, the load is represented by an RC load, but it does not mean that the load can only be an RC load, and it can also be other forms of loads.

[0061] Next, taking the Figure 3 shown circuit topology as an example, the modal analysis of the buck single-phase single-stage AC-AC converter in this embodiment is as follows:

[0062] When the input power supply is positive (i.e., the input voltage is considered positive at the top and negative at the bottom in the reference direction):

[0063] Operating mode one: As Figure 4 shown, the second bidirectional switch group S2 conducts from the first end to the second end (i.e., T3 conducts, and the anti-parallel diode of T4 conducts), and the first bidirectional switch group S1 is turned off (i.e., both T1 and T2 are turned off). There are two current flow paths in the circuit, which are respectively: the input power supply current supplies power to the load through the second inductor L2 and the second bidirectional switch group S2; the intermediate capacitor C charges the first inductor L1 through the second bidirectional switch group S2 and the load. The output voltage is negative at the bottom and positive at the top.

[0064] Operating mode two: As Figure 5As shown, the second bidirectional switch group S2 is turned off (i.e., both T3 and T4 are turned off), and the first bidirectional switch group S1 conducts from the first end to the second end (i.e., T1 conducts, and the anti-parallel diode of T2 conducts). There are two current flow paths in the circuit, which are respectively: the input power supply, and the load current charges the intermediate capacitor C through the second inductor L2; the first inductor L1 freewheels through the first bidirectional switch group S1. The output voltage is negative at the bottom and positive at the top.

[0065] When the input power supply is negative (i.e., the input voltage is considered to be positive at the top and negative at the bottom in the reference direction):

[0066] Operating mode three: As Figure 6 shown, the second bidirectional switch group S2 conducts from the second end to the first end (i.e., T4 conducts, and the anti-parallel diode of T3 conducts), and the first bidirectional switch group S1 is turned off. There are two current flow paths in the circuit, which are respectively: the input power supply current passes through the load, the second bidirectional switch group S2, and the second inductor L2; the current of the intermediate capacitor C passes through the first inductor L1, the load, and the second bidirectional switch group S2. At this time, the output voltage is positive at the bottom and negative at the top.

[0067] Operating mode four: As Figure 7 shown: The second bidirectional switch group S2 is turned off, and the first bidirectional switch group S1 conducts from the second end to the first end (i.e., T2 conducts, and the anti-parallel diode of T1 conducts). There are two current flow paths in the circuit, which are respectively: the input power supply passes through the load, charges the intermediate capacitor C and the second inductor L2 to charge the intermediate capacitor C; the first inductor L1 freewheels through the first bidirectional switch group S1. At this time, the output voltage is positive at the bottom and negative at the top.

[0068] Since the first bidirectional switch group S1 and the second bidirectional switch group S2 conduct complementarily, the duty cycle of the first bidirectional switch group S1 is set to 1 - D, the duty cycle of the second bidirectional switch group S2 is D, and the capacitor voltage is V c , positive on the left and negative on the right, 0 < D < 1.

[0069] Writing the potential balance equations for the inductor and capacitor, ignoring the high-frequency transformation quantity, and approximately considering the voltage to be a constant quantity within adjacent switching cycles. For inductor L1, we get 0 = V out *(1 - D) + (V out - V c )*D. For inductor L2, we get V ac＝ D * V out + (V out + V c )*(1 - D)

[0070] Then the relationship between the output voltage V out and the input voltage V ac is:

[0071] V out = Vac *D

[0072] Therefore, the converter can achieve step-down operation by only controlling the first bidirectional switch group S1 and the second bidirectional switch group S2 that work complementarily with a fixed duty cycle.

[0073] Embodiment 2:

[0074] In this embodiment, the step-down single-phase single-stage AC-AC converter in Embodiment 1 is applied to a three-phase structure, as Figure 8 shown (in this figure, the two bidirectional switch groups in each step-down single-phase single-stage AC-AC converter are taken as opposite switches). The second ends of the second inductors of the three step-down single-phase single-stage AC-AC converters are respectively connected to the first ends of the three-phase power supplies A, B, and C. The second ends of the three-phase power supplies are connected together. The second ends of the loads connected by the three step-down single-phase single-stage AC-AC converters are connected together and connected to the common end of the second ends of the three-phase power supplies. Its working mode is similar to that of Embodiment 1 and will not be elaborated here.

[0075] In summary, compared with the prior art, the following beneficial effects are achieved:

[0076] 1. The step-down single-phase single-stage AC-AC converter according to the embodiment of the present invention can achieve first-level energy-based AC / AC conversion by only controlling two switch tube groups that work complementarily, has high efficiency, and has a simple control method, and can realize the function of the converter with fewer peripheral circuits.

[0077] 2. The output voltage and output current of this circuit are continuous, with few voltage and current harmonics, and are easier to filter.

[0078] 3. By controlling the duty cycles of the first bidirectional switch group and the second bidirectional switch group, the step-down amplitude can be controlled. The control method is simple, and the required voltage can be flexibly output.

[0079] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising a..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the element.

[0080] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A step-down single-phase single-stage AC-AC converter, characterized in that, It includes a first inductor, an intermediate capacitor, a second inductor, a first bidirectional switch group, and a second bidirectional switch group; the first end of the first inductor is connected to the first end of the input power supply, and the second end of the first inductor is connected to the second end of the input power supply via the intermediate capacitor and the second inductor; The first end of the first bidirectional switch group is connected to the common end of the first inductor and the intermediate capacitor; the second end is connected to the first end of the load, and the second end of the load is connected to the common end of the first inductor and the input power supply; The first end of the second bidirectional switch group is connected to the common end of the second inductor and the intermediate capacitor; the second end is connected to the common end of the first bidirectional switch group and the load; The first bidirectional switch group and the second bidirectional switch group are both connected to a plurality of control terminals, and are adapted to work in a complementary conduction mode with a fixed duty cycle under the control of the control signals applied to the control terminals, so that the voltage across the load is less than the input voltage.

2. The step-down single-phase single-stage AC-AC converter according to claim 1, characterized in that, The first bidirectional switch group and the second bidirectional switch group are both switching devices capable of satisfying a bidirectional current path.

3. The step-down single-phase single-stage AC-AC converter according to claim 2, wherein The first bidirectional switch group and the second bidirectional switch group both include a set of opposite switches.

4. The step-down single-phase single-stage AC-AC converter according to any one of claims 1 to 3, characterized in that, When the voltage at the second end of the input power supply is positive and the voltage at the first end is negative, the step-down single-phase single-stage AC-AC converter includes two operating modes: Operating mode one: The second bidirectional switch group conducts from the first end to the second end, and the first bidirectional switch group is turned off. There are two current flow paths in the circuit, which are respectively: the input power supply current supplies power to the load through the second inductor and the second bidirectional switch group; the intermediate capacitor charges the first inductor through the second bidirectional switch group and the load; the voltage at the first end of the load is positive, and the voltage at the second end is negative; Operating mode two: The second bidirectional switch group is turned off, and the first bidirectional switch group conducts from the first end to the second end. There are two current flow paths in the circuit, which are respectively: the input power supply and the load current charge the intermediate capacitor through the second inductor; the first inductor freewheels through the first bidirectional switch group; the voltage at the first end of the load is positive, and the voltage at the second end is negative.

5. The step-down single-phase single-stage AC-AC converter according to any one of claims 1 to 3, characterized in that When the voltage at the second end of the input power supply is negative and the voltage at the first end is positive, the step-down single-phase single-stage AC-AC converter includes two operating modes: Operating mode three: The second bidirectional switch group conducts from the second end to the first end, and the first bidirectional switch group is turned off. There are two current flow paths in the circuit, which are respectively: the input power supply current passes through the load, the second bidirectional switch group, and the second inductor; the intermediate capacitor current passes through the first inductor, the load, and the second bidirectional switch group; the voltage at the first end of the load is negative, and the voltage at the second end is positive; Operating mode four: The second bidirectional switch group is turned off, and the first bidirectional switch group conducts from the second end to the first end. There are two current flow paths in the circuit, which are respectively: the input power supply passes through the load, charges the intermediate capacitor, and the second inductor to charge the intermediate capacitor; the first inductor freewheels through the first bidirectional switch group; the voltage at the first end of the load is negative, and the voltage at the second end is positive.

6. The step-down single-phase single-stage AC-AC converter according to any one of claims 1 to 3, characterized in that, When the duty cycle of the first bidirectional switch group is 1 - D and the duty cycle of the second bidirectional switch group is D, the voltage V out across the load and the input voltage V ac are related by: V out = V ac * D.

7. A step-down AC-AC converter applied in a three-phase circuit, characterized in that, It includes three step-down single-phase single-stage AC-AC converters, and each step-down single-phase single-stage AC-AC converter includes a first inductor, an intermediate capacitor, a second inductor, a first bidirectional switch group, and a second bidirectional switch group; The second ends of the second inductors of the three step-down single-phase single-stage AC-AC converters are respectively connected to the first ends of the three-phase power supplies of A, B, and C, and the second ends of the three-phase power supplies are connected together to serve as the common terminal of the three-phase power supply; The second end of the first inductor of each step-down single-phase single-stage AC-AC converter is connected to the first end of the second inductor via the intermediate capacitor; the first end of the first bidirectional switch group is connected to the common terminal of the first inductor and the intermediate capacitor; the second end is connected to the first end of the load; the first end of the second bidirectional switch group is connected to the common terminal of the second inductor and the intermediate capacitor; the second end is connected to the common terminal of the first bidirectional switch group and the load; the second end of the load is connected to the first end of the first inductor; The second ends of the loads connected to the three step-down single-phase single-stage AC-AC converters are connected together and connected to the common terminal of the three-phase power supply; The first bidirectional switch group and the second bidirectional switch group in each step-down single-phase single-stage AC-AC converter are each connected to a plurality of control terminals, and are adapted to operate in a complementary conduction mode with a fixed duty cycle under the control of a control signal applied to the control terminals, so that the voltage across the load of each phase is less than the corresponding phase voltage.

8. The step-down AC-AC converter applied to a three-phase circuit as claimed in claim 7, wherein The first bidirectional switch group and the second bidirectional switch group are both switching devices capable of satisfying a bidirectional current path.

9. The step-down AC-AC converter applied to a three-phase circuit according to claim 8, characterized in that, The first bidirectional switch group and the second bidirectional switch group each include a set of opposite switches.

10. The step-down AC-AC converter applied to a three-phase circuit according to any one of claims 7 to 9, characterized in that, In any one-phase circuit, when the duty cycle of the first bidirectional switch group is 1 - D and the duty cycle of the second bidirectional switch group is D, the voltage V across each phase load out and the input voltage V x of this phase have the following quantitative relationship: V out = V x * D where x = a, b, c, V x represents V a , V b , V c three-phase voltages.

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