A control method for a single-phase power electronic transformer

Abstract

This invention proposes a control method for a single-phase power electronic transformer, belonging to the field of power electronic transformer technology. The method includes: high-voltage side positive and negative half-bridge units modulating and outputting synchronous 50% duty cycle high-voltage side square wave voltages; and low-voltage side full-bridge units using pulse width modulation to output continuously varying duty cycle low-voltage side square wave voltages. The high-voltage side square wave voltage and the low-voltage side square wave voltage have a phase lag; the voltage difference between the high-voltage side and the low-voltage side square wave voltages acts on a high-frequency transformer, generating a high-frequency current by exciting a resonant network composed of leakage inductance and resonant capacitors of the high-frequency transformer, thereby achieving power transfer between the high-voltage and low-voltage sides. Balanced energy transfer between modules is achieved by adjusting the phase through power deviation adjustments of each power module. In this proposed method, the low-voltage side full-bridge unit's modulation reference voltage does not rely on complex phase-locked loop control, solving the problem of uneven energy transfer between modules caused by inconsistent circuit parameters.

Description

Technical Field

[0001] This invention belongs to the field of power electronic transformer technology, and in particular, relates to a control method for a single-phase power electronic transformer. Background Technology

[0002] As rail transit develops towards higher speeds and greater intelligence, traditional power frequency traction transformers, due to their large size, low power density, and limited harmonic suppression capabilities, have become a key bottleneck restricting the performance improvement of next-generation electric locomotives. Power electronic transformers (PETs), through high-frequency power conversion technology, significantly improve the system's power density and dynamic response capabilities while achieving voltage transformation and electrical isolation, providing a solution for the lightweighting and efficiency enhancement of traction power supply systems.

[0003] Traction power electronic transformers used in rail transit employ a single-phase circuit topology. The mature PET circuit topology consists of multiple power modules cascaded on the high-voltage side and paralleled on the low-voltage DC side, with each power module requiring four conversion stages. However, the numerous power conversion stages result in high power transmission losses and low efficiency, hindering economical system operation. To address this issue, researchers have proposed a three-stage conversion power electronic transformer, comprising a high-voltage side AC / AC half-bridge converter, a resonant capacitor, a high-frequency transformer, and a low-voltage side AC / DC full-bridge converter. The high-voltage side AC / AC half-bridge converter operates in a 50% duty cycle open-loop mode, while the low-voltage side AC / DC full-bridge converter operates in pulse-width modulation (PWM) mode. In PWM mode, the reference voltage requires acquisition of the high-voltage AC port voltage and the use of a phase-locked loop (PLL) control algorithm to obtain the amplitude and phase, leading to complex control and high computational complexity. Furthermore, due to the limitations of manufacturing processes, it is difficult to achieve complete parameter consistency for electrical components, resulting in inconsistent energy transmission between power modules and inconsistent capacitor voltages in the high-voltage side AC / AC half-bridge converter. This can easily lead to overvoltage and excessive single-module losses, affecting operational reliability. Summary of the Invention

[0004] The purpose of this invention is to propose a control method for a single-phase power electronic transformer to solve the problem of uneven energy transmission caused by inconsistent circuit parameters.

[0005] To achieve the above objectives, this invention provides a control method for a single-phase power electronic transformer. The transformer comprises multiple power modules cascaded on the high-voltage side and connected in parallel on the low-voltage side. Each power module consists of a high-voltage side positive half-bridge unit, a high-voltage side negative half-bridge unit, a resonant capacitor, a high-frequency transformer, and a low-voltage side full-bridge unit. The output terminals of the high-voltage side positive and negative half-bridge units of each power module are connected to the primary winding of the high-frequency transformer via the resonant capacitor. The secondary winding of the high-frequency transformer is connected to the low-voltage side full-bridge unit. The control method includes modulating the output of the high-voltage side positive and negative half-bridge units. The high-voltage side square wave voltage is synchronized with a 50% duty cycle, while the low-voltage side full-bridge unit outputs a low-voltage side square wave voltage with a continuously varying duty cycle using pulse width modulation. Both the high-voltage and low-voltage side square wave voltages are high-frequency square wave voltages. There is a phase lag between the high-voltage and low-voltage side square wave voltages. The voltage difference between the high-voltage and low-voltage side square wave voltages acts on the high-frequency transformer, which generates a high-frequency current by exciting a resonant network composed of leakage inductance and resonant capacitor of the high-frequency transformer, thereby realizing power transmission between the high-voltage and low-voltage sides. The phase is adjusted by the power deviation of each power module to achieve balanced energy transmission among the modules.

[0006] The beneficial effects of this invention are as follows:

[0007] (1) In the technical solution of the present invention, the reference voltage signal does not need to obtain the reference voltage phase and amplitude through complex phase-locked loop control. The reference voltage signal can be obtained by only collecting the capacitor voltage and calculating the effective value. The amount of calculation is small and there is no need to collect high voltage AC voltage, which reduces the number of voltage sensors used.

[0008] (2) The present invention adopts a superimposed energy balance control algorithm to collect the effective value of the DC capacitor voltage difference of each power module, and realizes energy transmission balance by balancing the effective value of the voltage. Attached Figure Description

[0009] The accompanying drawings are provided to further illustrate embodiments of the present invention and form part of the specification. They are used together with the following detailed description to explain the embodiments of the present invention, but do not constitute a limitation thereof. In the drawings:

[0010] Figure 1 A schematic diagram of any power module in the half-bridge-full-bridge hybrid single-phase power electronic transformer provided by the present invention;

[0011] Figure 2 This is a schematic diagram of a single-phase power electronic transformer control provided by the present invention. Detailed Implementation

[0012] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above objectives, this invention adopts the following technical solution.

[0013] Figure 1 This is a schematic diagram of any power module in the half-bridge-full-bridge hybrid single-phase power electronic transformer provided by the present invention, as shown below. Figure 1 As shown, the power module consists of a high-voltage side positive half-bridge unit 101, a high-voltage side negative half-bridge unit 102, a resonant capacitor 103, a high-frequency transformer 104, and a low-voltage side full-bridge unit 105. The output terminals of the high-voltage side positive half-bridge unit 101 and the high-voltage side negative half-bridge unit 102 are connected to the primary winding of the high-frequency transformer 104 via the resonant capacitor 103. The secondary winding of the high-frequency transformer 104 is connected to the low-voltage side full-bridge unit 105.

[0014] This invention proposes a novel control method for hybrid single-phase power electronic transformers with half-bridge and full-bridge designs. Figure 2 This is a schematic diagram of a single-phase power electronic transformer control provided by the present invention, which is described below in conjunction with... Figure 1 and Figure 2 The present invention will now be described.

[0015] This invention provides a control method for a single-phase power electronic transformer. The transformer consists of multiple power modules cascaded on the high-voltage side and paralleled on the low-voltage side. Each power module comprises a high-voltage side positive half-bridge unit 101, a high-voltage side negative half-bridge unit 102, a resonant capacitor 103, a high-frequency transformer 104, and a low-voltage side full-bridge unit 105. The output terminals of the high-voltage side positive half-bridge unit 101 and the high-voltage side negative half-bridge unit 102 of each power module are connected to the primary winding of the high-frequency transformer 104 via the resonant capacitor 103. The secondary winding of the high-frequency transformer 104 is connected to the low-voltage side full-bridge unit 105. The control method includes: the high-voltage side positive half-bridge unit 101 and the high-voltage side negative half-bridge unit 102 are connected to the primary winding of the high-frequency transformer 104 via the resonant capacitor 103. The high-voltage side square wave voltage is synchronously output by the modulation unit 102 with a 50% duty cycle. The low-voltage side full-bridge unit 105 outputs a low-voltage side square wave voltage with a continuously varying duty cycle using pulse width modulation. Both the high-voltage side square wave voltage and the low-voltage side square wave voltage are high-frequency square wave voltages. There is a phase lag between the high-voltage side square wave voltage and the low-voltage side square wave voltage. The voltage difference between the high-voltage side square wave voltage and the low-voltage side square wave voltage acts on the high-frequency transformer 104, which generates a high-frequency current by exciting the resonant network composed of the leakage inductance of the high-frequency transformer 104 and the resonant capacitor 103, thereby realizing the power transmission between the high-voltage side and the low-voltage side. The phase is adjusted by the power deviation of each power module to achieve balanced energy transmission of each module.

[0016] This invention addresses the problem of uneven energy transmission among power modules caused by inconsistent parameters of circuit components (such as leakage inductance of high-frequency transformer 104 and DC capacitor). It introduces power deviation adjustment phase in the control loop to achieve balanced energy transmission among the modules.

[0017] In each power module, the high-voltage side positive half-bridge unit 101 includes two fully controlled devices, and the high-voltage side reverse half-bridge unit 102 includes two fully controlled devices. The two fully controlled devices in the high-voltage side positive half-bridge unit 101 and the two fully controlled devices in the high-voltage side reverse half-bridge unit 102 adopt a synchronous 50% duty cycle control mode, and control the phase of the high-frequency square wave voltage of the AC output of the high-voltage side half-bridge unit to be zero. The high-voltage side half-bridge unit includes the high-voltage side positive half-bridge unit 101 and the high-voltage side reverse half-bridge unit 102. The high-frequency square wave voltage of the AC output of the high-voltage side half-bridge unit is the voltage between the first connection point between the two fully controlled devices of the high-voltage side positive half-bridge and the second connection point between the two fully controlled devices of the high-voltage side reverse half-bridge.

[0018] The high-voltage side positive half-bridge unit 101 includes a first DC capacitor and two fully controlled devices connected in series across the first DC capacitor. The high-voltage side negative half-bridge unit 102 includes a second DC capacitor and two fully controlled devices connected in series across the second DC capacitor. The control frequency of the 50% duty cycle control mode ranges from 400Hz to 80kHz. In each control cycle, the fully controlled devices in the high-voltage side positive half-bridge unit 101 connected to the positive terminal of the first DC capacitor and the fully controlled devices in the high-voltage side negative half-bridge unit 102 connected to the positive terminal of the second DC capacitor are turned on during the first 0.5 control cycles. In the next 0.5 control cycles, the fully controlled devices in the high-voltage side positive half-bridge unit 101 connected to the negative terminal of the first DC capacitor and the fully controlled devices in the high-voltage side negative half-bridge unit 102 connected to the negative terminal of the second DC capacitor are turned on.

[0019] The high-voltage side positive half-bridge unit 101 includes a first fully controlled device connected to the positive terminal of the first DC capacitor and a second fully controlled device connected to the negative terminal of the first DC capacitor. The high-voltage side reverse half-bridge unit 102 includes a third fully controlled device connected to the positive terminal of the second DC capacitor and a fourth fully controlled device connected to the negative terminal of the second DC capacitor. In each control cycle, the first and third fully controlled devices are turned on and the second and fourth fully controlled devices are turned off in the first 0.5 control cycles, and the second and fourth fully controlled devices are turned on and the first and third fully controlled devices are turned off in the last 0.5 control cycles. The emitter of the first fully controlled device is connected to the collector of the second fully controlled device, and the emitter of the third fully controlled device is connected to the collector of the fourth fully controlled device.

[0020] The low-voltage side full-bridge unit 105 in each power module includes a third DC capacitor and two series branches connected across the three DC capacitors. Each series branch includes two fully controlled devices. The four fully controlled devices in the low-voltage side full-bridge unit 105 adopt pulse width modulation control and control the output of the low-voltage side square wave voltage with a lagging phase at the AC terminal of the low-voltage side full-bridge unit 105 in each power module. The low-voltage side square wave voltage is the voltage between the third connection point between the two fully controlled devices in the first series branch and the fourth connection point between the two fully controlled devices in the second series branch.

[0021] The output lag phase of the low-voltage side full-bridge unit 105 AC terminal ranges from -60° to 60°.

[0022] The single-phase power electronic transformer control method provided by the present invention further includes: calculating the reference voltage signals of the four fully controlled devices of the low-voltage side full-bridge unit 105 in each power module based on the voltage ratio of the high-frequency transformer 104, the low-voltage DC side voltage control reference value, the DC voltage of the first DC capacitor, and the DC voltage of the second DC capacitor.

[0023] The reference voltage signals for the four fully controlled devices of the low-voltage side full-bridge unit 105 are calculated as follows:

[0024] ;

[0025] Among them, the first reference voltage signal V smi_1a Second reference voltage signal V smi_1b The reference voltage signals are the fifth and sixth fully controlled devices in the first series branch of the low-voltage side full-bridge unit 105 in the i-th power module, and the third reference voltage signal V is... smi_2a and the fourth reference voltage signal V smi_2b k is the reference voltage signal for the seventh and eighth fully controlled devices in the second series branch of the low-voltage side full-bridge unit 105 in the i-th power module. TF For a high-frequency transformer with a voltage ratio of 104, U dc_ref U is the reference value for low-voltage DC side voltage control. smi_C1 U is the DC voltage of the first DC capacitor in the i-th power module. smi_C2 RMS[U] is the DC voltage of the second DC capacitor in the i-th power module. smi_C1 -U smi_C2 [U] represents the DC voltage U of the first DC capacitor in the i-th power module. smi_C1 DC voltage U of the second DC capacitor smi_C2 The effective value of the voltage difference between them, where the emitter of the fifth fully controlled device is connected to the collector of the sixth fully controlled device, the emitter of the seventh fully controlled device is connected to the collector of the eighth fully controlled device, i=1,2,…,M, and M is the number of power modules.

[0026] The single-phase power electronic transformer control method provided by this invention further includes: comparing the reference voltage signals of the four fully controlled devices of the low-voltage side full-bridge unit 105 with the symmetrical triangular carrier voltage signal to generate control commands, wherein the amplitude of the symmetrical triangular carrier voltage signal is 0 to 1, the operating frequency of the symmetrical triangular carrier voltage signal is the same as the control frequency of the high-voltage side positive half-bridge unit 101 and the high-voltage side reverse half-bridge unit 102, and the phase of the symmetrical triangular carrier voltage signal of each power module is the same as the phase of the low-voltage side square wave voltage of each power module. When the first reference voltage signal V smi_1a Less than the symmetrical triangular carrier voltage signal or the second reference voltage signal V smi_1b When the voltage is greater than the symmetrical triangular carrier voltage signal, the fifth fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned on, and the sixth fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned off. When the first reference voltage signal V smi_1a The voltage signal is greater than the symmetrical triangular carrier voltage signal and the second reference voltage signal V smi_1b When the voltage is less than the symmetrical triangular carrier voltage signal, the fifth fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned off, and the sixth fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned on; when the third reference voltage signal V smi_2a Less than the symmetrical triangular carrier voltage signal or the fourth reference voltage signal V smi_2b When the voltage signal is greater than the symmetrical triangular carrier voltage signal, the seventh fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned on, and the eighth fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned off. When the third reference voltage signal V smi_2a Greater than the symmetrical triangular carrier signal and the fourth reference voltage signal V smi_2b When both are less than the symmetrical triangular carrier voltage signal, the seventh fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned off, and the eighth fully controlled device of the low-voltage side full-bridge unit 105 of the i-th power module is turned on.

[0027] The single-phase power electronic transformer control method provided by this invention further includes: in each power module, the phase of the low-voltage side square wave voltage is the same as the phase of the symmetrical triangular carrier voltage signal, the phase of the low-voltage side square wave voltage lags behind the phase of the high-voltage side square wave voltage, the phase of the symmetrical triangular carrier voltage signal lags behind the phase of the high-voltage side square wave voltage, and the symmetrical triangular carrier voltage signal lags behind the phase φ. i (i=1,2,…,M) (i.e., the phase lag of the symmetrical triangular carrier voltage signal of each power module compared to the phase of the high-voltage side square wave voltage of each power module) is obtained through low-voltage DC side voltage single closed-loop control and energy balance control, including:

[0028] Collect the low-voltage side DC port voltage U dc_febThe voltage component containing twice the frequency of the high-voltage AC port voltage is filtered out by a notch filter, and then compared with the low-voltage side DC port voltage reference value U. dc_ref After subtraction, a PI controller is used to generate the common hysteresis phase φ of each power module. com ;

[0029] In the energy balance control stage, the effective values ​​of the voltage differences between the first and second DC capacitors of each power module are summed and averaged. This average is then subtracted from the effective value of the voltage difference between the first and second DC capacitors of the corresponding power module. A PI controller is then used to generate the power deviation adjustment phase φ for each power module. adi (i=1,2,…,M);

[0030] The common hysteresis phase φ of each power module com Subtract the power deviation adjustment phase φ of each power module adi (i=1,2,…,M) then generate the hysteresis phase φ of the symmetrical triangular carrier voltage signal for each power module. i (i=1,2,…,M);

[0031] Among them, by controlling the hysteresis phase φ of the symmetrical triangular carrier voltage signal i This is used to control the transmission power of each power module and the DC voltage on the low-voltage side.

[0032] In this invention, it should be understood that the lag phase of the AC output of the low-voltage side full-bridge unit 105 of each power module is the same as the lag phase of the symmetrical triangular carrier voltage signal of each power module, and both can be represented by φ. i (i=1,2,…,M) indicates that M is the number of power modules.

[0033] The optional embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the embodiments of the present invention are not limited to the specific details in the above embodiments. Within the scope of the technical concept of the embodiments of the present invention, various simple modifications can be made to the technical solutions of the embodiments of the present invention, and these simple modifications all fall within the protection scope of the embodiments of the present invention.

[0034] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the embodiments of the present invention will not describe the various possible combinations separately.

[0035] Furthermore, various different implementations of the present invention can be combined arbitrarily, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed in the present invention.

Claims

1. A control method for a single-phase power electronic transformer, wherein the transformer comprises multiple power modules cascaded on the high-voltage side and connected in parallel on the low-voltage side. Each power module consists of a high-voltage side positive half-bridge unit, a high-voltage side negative half-bridge unit, a resonant capacitor, a high-frequency transformer, and a low-voltage side full-bridge unit. The output terminals of the high-voltage side positive half-bridge unit and the high-voltage side negative half-bridge unit of each power module are connected to the primary winding of the high-frequency transformer via the resonant capacitor. The secondary winding of the high-frequency transformer is connected to the low-voltage side full-bridge unit. The method is characterized in that... The control method includes: The high-voltage side positive half-bridge unit and the high-voltage side negative half-bridge unit modulate and output a high-voltage side square wave voltage with a synchronous 50% duty cycle. The low-voltage side full-bridge unit uses pulse width modulation to output a low-voltage side square wave voltage with a continuously changing duty cycle. Both the high-voltage side square wave voltage and the low-voltage side square wave voltage are high-frequency square wave voltages. There is a phase lag between the high-voltage side square wave voltage and the low-voltage side square wave voltage. The voltage difference between the high-voltage side square wave voltage and the low-voltage side square wave voltage acts on the high-frequency transformer, which generates a high-frequency current by exciting the resonant network composed of the leakage inductance and resonant capacitor of the high-frequency transformer, thereby realizing the power transfer between the high-voltage side and the low-voltage side. Balanced energy transmission among the modules is achieved by adjusting the phase through the power deviation of each module.

2. The single-phase power electronic transformer control method according to claim 1, characterized in that, In each power module, the high-voltage side positive half-bridge unit includes two fully controlled devices, and the high-voltage side reverse half-bridge unit includes two fully controlled devices. The two fully controlled devices in the high-voltage side positive half-bridge unit and the two fully controlled devices in the high-voltage side reverse half-bridge unit adopt a synchronous 50% duty cycle control mode, and control the phase of the high-frequency square wave voltage of the AC output of the high-voltage side half-bridge unit to be zero. The high-voltage side half-bridge unit includes the high-voltage side positive half-bridge unit and the high-voltage side reverse half-bridge unit. The high-frequency square wave voltage of the AC output of the high-voltage side half-bridge unit is the voltage between the first connection point between the two fully controlled devices of the high-voltage side positive half-bridge and the second connection point between the two fully controlled devices of the high-voltage side reverse half-bridge.

3. The single-phase power electronic transformer control method according to claim 2, characterized in that, The high-voltage side positive half-bridge unit includes a first DC capacitor and two fully controlled devices connected in series across the first DC capacitor. The high-voltage side negative half-bridge unit includes a second DC capacitor and two fully controlled devices connected in series across the second DC capacitor. The control frequency of the 50% duty cycle control mode ranges from 400Hz to 80kHz. In each control cycle, the fully controlled devices connected to the positive terminal of the first DC capacitor in the high-voltage side positive half-bridge unit and the fully controlled devices connected to the positive terminal of the second DC capacitor in the high-voltage side negative half-bridge unit are turned on in the first 0.5 control cycles. In the next 0.5 control cycles, the fully controlled devices connected to the negative terminal of the first DC capacitor in the high-voltage side positive half-bridge unit and the fully controlled devices connected to the negative terminal of the second DC capacitor in the high-voltage side negative half-bridge unit are turned on.

4. The single-phase power electronic transformer control method according to claim 3, characterized in that, The high-voltage side positive half-bridge unit includes a first fully controlled device connected to the positive terminal of the first DC capacitor and a second fully controlled device connected to the negative terminal of the first DC capacitor. The high-voltage side reverse half-bridge unit includes a third fully controlled device connected to the positive terminal of the second DC capacitor and a fourth fully controlled device connected to the negative terminal of the second DC capacitor. In each control cycle, the first and third fully controlled devices are turned on and the second and fourth fully controlled devices are turned off for the first 0.5 control cycles, and the second and fourth fully controlled devices are turned on and the first and third fully controlled devices are turned off for the next 0.5 control cycles. The emitter of the first fully controlled device is connected to the collector of the second fully controlled device, and the emitter of the third fully controlled device is connected to the collector of the fourth fully controlled device.

5. The single-phase power electronic transformer control method according to claim 4, characterized in that, The low-voltage side full-bridge unit in each power module includes a third DC capacitor and two series branches connected across the three DC capacitors. Each series branch includes two fully controlled devices. The four fully controlled devices in the low-voltage side full-bridge unit adopt pulse width modulation control and control the output of the low-voltage side square wave voltage with a lagging phase at the AC terminal of the low-voltage side full-bridge unit in each power module. The low-voltage side square wave voltage is the voltage between the third connection point between the two fully controlled devices in the first series branch and the fourth connection point between the two fully controlled devices in the second series branch.

6. The single-phase power electronic transformer control method according to claim 5, characterized in that, The output lag phase of the low-voltage side full-bridge unit AC terminal ranges from -60° to 60°.

7. The single-phase power electronic transformer control method according to claim 6, characterized in that, The method also includes: The reference voltage signals of the four fully controlled devices in the low-voltage side full-bridge unit of each power module are calculated based on the high-frequency transformer voltage ratio, the low-voltage DC side voltage control reference value, the DC voltage of the first DC capacitor, and the DC voltage of the second DC capacitor.

8. The single-phase power electronic transformer control method according to claim 7, characterized in that, The reference voltage signals for the four fully controlled devices of the low-voltage side full-bridge unit are calculated as follows: ； Among them, the first reference voltage signal V smi_1a Second reference voltage signal V smi_1b The reference voltage signals are the fifth and sixth fully controlled devices in the first series branch of the low-voltage side full-bridge unit in the i-th power module, and the third reference voltage signal is V. smi_2a and the fourth reference voltage signal V smi_2b k represents the reference voltage signal for the seventh and eighth fully controlled devices in the second series branch of the low-voltage side full-bridge unit in the i-th power module. TF U is the voltage ratio of a high-frequency transformer. dc_ref U is the reference value for low-voltage DC side voltage control. smi_C1 U is the DC voltage of the first DC capacitor in the i-th power module. smi_C2 RMS[U] is the DC voltage of the second DC capacitor in the i-th power module. smi_C1 -U smi_C2 [U] represents the DC voltage U of the first DC capacitor in the i-th power module. smi_C1 DC voltage U of the second DC capacitor smi_C2 The effective value of the voltage difference between them, where the emitter of the fifth fully controlled device is connected to the collector of the sixth fully controlled device, the emitter of the seventh fully controlled device is connected to the collector of the eighth fully controlled device, i=1,2,…,M, and M is the number of power modules.

9. The single-phase power electronic transformer control method according to claim 8, characterized in that, The method also includes: The reference voltage signals of the four fully controlled devices in the low-voltage side full-bridge unit are compared with the symmetrical triangular carrier voltage signal to generate control commands. The amplitude of the symmetrical triangular carrier voltage signal is 0 to 1, and its operating frequency is the same as the control frequency of the high-voltage side positive half-bridge unit and the high-voltage side negative half-bridge unit. The phase of the symmetrical triangular carrier voltage signal of each power module is the same as the phase of the low-voltage side square wave voltage of each power module. When the first reference voltage signal V... smi_1a Less than the symmetrical triangular carrier voltage signal or the second reference voltage signal V smi_1b When the voltage is greater than the symmetrical triangular carrier voltage signal, the fifth fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned on, and the sixth fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned off. When the first reference voltage signal V... smi_1a The voltage signal is greater than the symmetrical triangular carrier voltage signal and the second reference voltage signal V smi_1b When the voltage is less than the symmetrical triangular carrier voltage signal, the fifth fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned off, and the sixth fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned on; when the third reference voltage signal V... smi_2a Less than the symmetrical triangular carrier voltage signal or the fourth reference voltage signal V smi_2b When the voltage is greater than the symmetrical triangular carrier voltage signal, the seventh fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned on, and the eighth fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned off. When the third reference voltage signal V... smi_2a Greater than the symmetrical triangular carrier signal and the fourth reference voltage signal V smi_2b When both are less than the symmetrical triangular carrier voltage signal, the seventh fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned off, and the eighth fully controlled device of the low-voltage side full-bridge unit of the i-th power module is turned on.

10. The single-phase power electronic transformer control method according to claim 9, characterized in that, The method further includes: in each power module, the phase of the low-voltage side square wave voltage is the same as the phase of the symmetrical triangular carrier voltage signal, the phase of the low-voltage side square wave voltage lags behind the phase of the high-voltage side square wave voltage, the phase of the symmetrical triangular carrier voltage signal lags behind the phase of the high-voltage side square wave voltage, and the symmetrical triangular carrier voltage signal lags behind the phase φ. i This is achieved through single-loop control of the low-voltage DC side voltage and energy balance control, including: Collect the low-voltage side DC port voltage U dc_feb The voltage component containing twice the frequency of the high-voltage AC port voltage is filtered out by a notch filter, and then compared with the low-voltage side DC port voltage reference value U. dc_ref After subtraction, a common hysteresis phase for each power module is generated using a PI controller; In the energy balance control stage, the effective value of the voltage difference between the DC voltage of the first DC capacitor and the DC voltage of the second DC capacitor of each power module is summed and averaged. Then, the difference is taken from the effective value of the voltage difference between the DC voltage of the first DC capacitor and the DC voltage of the second DC capacitor of the corresponding power module. The PI controller is used to generate the power deviation adjustment phase of each power module. The hysteresis phase φ of the symmetrical triangular carrier voltage signal of each power module is generated by subtracting the phase adjustment of the power deviation of each power module from the common hysteresis phase of each power module. i ; Among them, by controlling the lag phase φ of the symmetrical triangular carrier voltage signal i This is used to control the transmission power of each power module and the DC voltage on the low-voltage side.

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