Pet capacitor voltage ripple suppression system and method based on three-port active bridge

Abstract

The application belongs to the technical field of power electronic transformers, and discloses a PET capacitor voltage ripple suppression system and method based on a three-port active bridge. The method uses a three-port active bridge TAB as an isolation stage of an MMDTC-PET, and uses the characteristics that the power AC component amplitudes of the upper and lower bridge arms of an HBSM are equal and the phases are opposite. Under the same phase shift modulation, the two active bridges of the three-port active bridge TAB and the upper and lower bridge arms of the HBSM modular DC chain form a switching capacitor circuit, so that the capacitor ripple current of the two active bridges of the three-port active bridge TAB automatically flows to the next stage. The power AC components are offset at the secondary side of the three-port active bridge TAB. The application does not need to increase the capacitance to suppress the ripple voltage, reduces the cost, and improves the power density.

Description

Technical Field

[0001] This invention belongs to the field of power electronic transformer technology, and particularly relates to a PET capacitor voltage ripple suppression system and method based on a three-port active bridge. Background Technology

[0002] Power electronic transformers (PETs) are a new type of power electronic device that, unlike traditional transformers, improves the flexibility and reliability of AC / DC distribution networks. PETs are typically connected to medium- and high-voltage distribution networks. Due to limitations in the voltage withstand ratings of current power devices, multi-level topologies, such as CHB and MMC, are commonly used for the input stage of PETs. ​​The isolation stage generally employs double-port active bridges (DABs). However, CHB and MMC type PETs have inherent drawbacks such as a large number of devices and complex control. This invention proposes using a Modular Multilevel DC-Link Based T-type Converter (MMDTC) topology as the input stage of the PET.

[0003] DAB-MMDTC-PET topology shortcomings: Similar to CHB-type PET and MMC-type PET, in the MMDTC-PET system with DAB as the isolation stage, AC power is transmitted to the input terminals of each DAB via MMDTC, and then transmitted to the low-voltage DC bus after DAB conversion. Capacitor C acts as a bridge for power transmission between HBSM and DAB. Due to the inherent power fluctuations at the HBSM input, capacitor voltage ripple is unavoidable, and submodule capacitors will generate low-frequency voltage ripple. Typically, large-value capacitors are needed to suppress this voltage ripple, but this significantly increases the device size and cost.

[0004] Existing methods for reducing capacitor voltage ripple include adding hardware circuits, harmonic injection, control strategy optimization, and constructing power channels.

[0005] Additional hardware circuit method: The additional hardware circuit method mainly includes configuring large-capacity capacitors and adding additional filters. Although this method is simple and effective, it will greatly increase the size and cost of PET equipment.

[0006] Harmonic injection method: The harmonic injection method is widely used in CHB and MMC type PET, but MMDTC has no circulating current loop, and the upper and lower bridge arms of MMDTC are fitted to line voltage. Therefore, neither circulating current injection nor zero-sequence voltage injection method is suitable for MMDTC.

[0007] Control strategy optimization method: The control strategy optimization method refers to using optimized control to reduce capacitor voltage ripple, but the control optimization method has limited ability to suppress capacitor voltage ripple.

[0008] The ripple power path construction method refers to reducing or even eliminating voltage ripple by constructing a power path to decouple ripple power from the submodule capacitors and transfer it to the low-voltage side for cancellation. However, this method mostly relies on the mutual coupling control between the three-phase submodules of MMC or CHB to add additional ripple power paths. This increases the driving complexity of the switching devices in the system, reduces the converter control freedom, and is not suitable for MMDTCs with only a single DC link. Summary of the Invention

[0009] To overcome the problems existing in related technologies, the present invention discloses an embodiment of a PET capacitor voltage ripple suppression system and method based on a three-port active bridge.

[0010] The technical solution is as follows: A PET capacitor voltage ripple suppression system based on a three-port active bridge is configured with a TAB-MMDTC-PET topology. The TAB-MMDTC-PET topology utilizes the three-port active bridge TAB as the isolation stage of MMDTC-PET. Taking advantage of the characteristic that the AC power components of the upper and lower arms of the HBSM have equal amplitudes and opposite phases, the two active bridges on the primary side of the three-port active bridge TAB, under the same phase-shift modulation, form a switched capacitor circuit with the upper and lower arms of the HBSM modular DC link, so that the ripple current of the two active bridge capacitors of the three-port active bridge TAB automatically flows to the subsequent stage; the AC power components are mutually canceled on the secondary side of the three-port active bridge TAB, without affecting the low-voltage DC bus LVDC.

[0011] The power transferred from the primary to the secondary side of TAB is controlled by the phase difference between u1, u2, and u3 and the additional inductance L. The power transfer formula is as follows: Phase shift angle β = 180° × D; where V1 and V2 are the voltages on the primary and secondary sides, respectively, D is the phase shift duty cycle, and f s U is the switching frequency, u1 and u2 are the voltages at the two AC input terminals respectively, and u3 is the AC voltage at the output terminal.

[0012] The two active bridges include the upper bridge arm submodule and the lower bridge arm module of the three-port active bridge TAB.

[0013] Furthermore, the three-port active bridge TAB consists of an upper input terminal, a lower input terminal, and an output terminal. The upper input terminal and the lower input terminal are respectively connected to the upper bridge arm sub-module and the lower bridge arm sub-module of the three-port active bridge TAB; the upper bridge arm sub-module and the lower bridge arm sub-module are respectively connected to the upper bridge arm and the lower bridge arm of the HBSM.

[0014] The output terminal is connected to the low-voltage DC bus (LVDC).

[0015] The two input terminals and the output terminal are connected via a capacitor ripple current signal;

[0016] A capacitor C is connected in parallel on each of the upper and lower arms of the HBSM. up With lower capacitor C low ;

[0017] Furthermore, the upper arm sub-module of the three-port active bridge TAB includes: switches S1 and S2 connected in parallel, switch S1 connected in series with switch S3, switch S2 connected in series with switch S4, and switch S3 and switch S4 connected in parallel.

[0018] The current input terminals of the parallel-connected switching transistors S1 and S2 are connected to the upper arm of the HBSM. up The positive electrode is connected; the current output terminals of the parallel-connected switches S3 and S4 are connected to the upper arm of the HBSM. up The negative electrode is connected;

[0019] The common node of the series-connected switches S1 and S3 is connected to the common node of the series-connected switches S2 and S4 through leakage inductance impedance, forming the upper input terminal of the upper arm submodule of the three-port active bridge TAB.

[0020] Furthermore, the three-port active bridge TAB lower bridge arm sub-module includes: switches S5 and S6 connected in parallel, switch S5 connected in series with switch S7, and switch S6 connected in series with switch S8; switch S7 and switch S8 are connected in parallel.

[0021] The current input terminals of the parallel-connected switching transistors S5 and S6 are connected to the lower bridge arm of the HBSM. low The positive electrode is connected; the current output terminals of the parallel-connected switching transistors S7 and S8 are connected to the lower bridge arm of the HBSM. low The negative electrode is connected;

[0022] The common node of the series-connected switches S5 and S6 is connected to the common node of the series-connected switches S6 and S8 through leakage inductance impedance, forming the lower input terminal of the lower bridge arm submodule of the three-port active bridge TAB.

[0023] Furthermore, the low-voltage DC bus (LVDC) includes switching transistors S9 and S1 connected in parallel. 10 Switch S9 is connected in series with switch S 11 Switch S 10 A switching transistor S is connected in series. 12 ; Switching transistor S 11 With the switching transistor S 12Parallel connection;

[0024] The output of the three-port active bridge TAB is connected to a series-connected switching transistor S through one end of the leakage inductance impedance. 10 Switch S 12 The common node of the three-port active bridge TAB is connected to the series-connected switching transistors S9 and S2 through the other end of the leakage inductance impedance. 11 The public nodes.

[0025] Furthermore, the upper bridge arm submodule, lower bridge arm submodule, and low-voltage DC bus LVDC three full bridges of the three-port active bridge TAB all adopt PWM modulation;

[0026] In the primary side of the TAB, the upper bridge arm submodule of the three-port active bridge TAB operates during the first half of the switching cycle T. s1 Internally, the switching transistors S1 / S4 of the full bridge are turned on, and the full bridge outputs a positive level at the upper input terminal. Capacitor C... up The switching transistors S1 / S4 are directly connected in parallel; during the second half of the switching cycle T... s2 Internally, with S2 / S3 of the full-bridge circuit turned on, all input terminals of the full-bridge circuit output a positive level, and capacitor C... up The switching transistors S2 and S3 are connected in parallel directly.

[0027] In the primary side of the TAB, the lower bridge arm submodule of the three-port active bridge TAB operates during the first half of the switching cycle T. s1 Internally, the switching transistors S5 / S8 of the full bridge are turned on, and the entire bridge outputs a positive level at the lower input terminal. Capacitor C... low The switching transistors S5 / S8 are directly connected in parallel; during the second half of the switching cycle T... s2 Internally, with S6 / S7 of the full-bridge circuit turned on, all input terminals of the full-bridge circuit output a positive level, and capacitor C... low The switching transistors S6 and S7 are connected in parallel directly.

[0028] Furthermore, in the secondary phase-shift modulation of TAB, the phase angles of the modulation signals of the two full bridges at the upper and lower input ends are β1 and β2, respectively, with β1 = β2. The phase angle of the full bridge modulation signal at the output end is β3, and the phase shift angle is β = β3 - β1. The power flow of TAB is controlled by controlling the magnitude of the phase shift angle β.

[0029] Another objective of this invention is to provide a PET capacitor voltage ripple suppression method based on a three-port active bridge. This method is implemented through the aforementioned PET capacitor voltage ripple suppression system based on a three-port active bridge. The method utilizes a three-port active bridge (TAB) as the isolation stage of the MMDTC-PET. Taking advantage of the equal amplitude and opposite phase of the AC power components in the upper and lower arms of the HBSM, the two active bridges on the primary side of the three-port active bridge (TAB) form a switched capacitor circuit with the upper and lower arms of the HBSM modular DC link under the same phase-shift modulation. This allows the ripple current of the capacitors in the upper and lower arm submodules of the three-port active bridge (TAB) to automatically flow to the subsequent stage. Mutual cancellation of the AC power components is achieved on the secondary side of the three-port active bridge (TAB).

[0030] Furthermore, the capacitor ripple current of the upper and lower bridge arm submodules of the three-port active bridge TAB automatically flows to the subsequent stage, and the equivalent input impedance of the upper and lower bridge arm submodules is defined as Z. up / low The equivalent input impedance of the upper bridge arm submodule relative to the lower bridge arm submodule is:

[0031] Z up =Z Clow +Z Llow +Z Lup (1)

[0032] In the formula, Z up Z is the equivalent input impedance of the upper bridge arm submodule. Clow Z represents the capacitance impedance of the lower bridge arm submodule. Clow =1 / jωC low Z Lup Z Llow Z is the inductance corresponding to the leakage inductance. Lup / low =jωL up / low ω is the angular frequency, C low For the lower bridge arm submodule capacitor, L up / low For the leakage inductance of the transformer on the primary side of the TAB level connecting the upper and lower bridge arm submodules;

[0033] The equivalent AC source input ripple currents of the capacitors and TAB primary side in the upper and lower bridge arm submodules are i, respectively. Cup / low_sm and i Tup / low i is obtained from the circuit equivalent model. sm_up i Cup_sm i Tup The relationship is:

[0034]

[0035] In the formula, i sm_up For the equivalent AC source of the upper bridge arm submodule, i Tupi represents the ripple current on the primary side of the upper bridge arm TAB. Cup_sm Z represents the ripple current flowing into the submodule from the equivalent AC source of the upper bridge arm. Cup Z represents the capacitance impedance of the upper bridge arm submodule. up The equivalent input impedance of the upper bridge arm submodule;

[0036] From the equivalent alternating current source i Tlow The current flowing into the upper bridge arm is i Cup_Tlow According to Kirchhoff's current law, i Cup_Tlow =i Tlow According to the superposition theorem, and assuming that the capacitors and transformers of each submodule have the same parameters, that is: Z Cup =Z Clow =1 / jωC,Z Lup =Z Llow =jωC, where C and L represent the capacitance and leakage inductance values, respectively, and the current i superimposed on the capacitor of the upper bridge arm submodule is... Cup Represented as:

[0037]

[0038] In the formula, i Cup i is the capacitor current of the upper bridge arm submodule. Cup_Tlow To obtain from the equivalent alternating current source i Tlow The current flowing into the capacitor of the upper bridge arm submodule, Z up The equivalent input impedance of the upper bridge arm submodule;

[0039] Based on the relationship between capacitor voltage and current, we can obtain:

[0040]

[0041] Integrating the equation yields the expression for the capacitor voltage:

[0042]

[0043]

[0044] When θ = 0 Get the valley value U valley ,make The expression is:

[0045]

[0046] From equations (3) and (6), it is known that, based on the low impedance characteristics of the equivalent connection of the switched capacitor circuit, the ripple current i sm_up Completely decoupled from the submodule capacitor, it is transferred to the secondary side of the TAB-level high-frequency transformer without additional ripple power transfer control.

[0047] Furthermore, the PET capacitor voltage ripple suppression method based on a three-port active bridge is applied to the suppression of capacitor voltage ripple in power electronic transformers.

[0048] Combining all the above technical solutions, the beneficial effects of this invention are as follows:

[0049] First, this invention proposes a novel MMDTC-PET structure that utilizes three-port active bridges (TABs) instead of DABs as the isolation stage (MTBs-based MMDTC-PET). When the two full bridges on the TAB primary side employ the same phase-shift modulation, the two sub-modules of the upper and lower bridge arms form a parallel circuit. This method enables the sub-module capacitors to exhibit switched capacitor characteristics, allowing ripple power to flow to the secondary side of the TAB. Simultaneously, by utilizing the characteristic that the AC components of the upper and lower bridge arm sub-modules have equal amplitudes but opposite phases, these AC components cancel each other out on the secondary side of the TAB, reducing the ripple voltage of the sub-module capacitors and having no impact on the low-voltage DC bus.

[0050] Secondly, the TAB-based MMDTC-PET solution reduces capacitor ripple current, lowers submodule capacitor ripple voltage, and reduces submodule capacitor size without requiring additional hardware filtering or complex control. Compared to traditional large-capacitor solutions, it eliminates the need to increase capacitor size to suppress ripple voltage, thereby reducing cost and increasing power density.

[0051] Third, by reducing the capacitor voltage ripple of the MMDTC type power electronic transformer, this invention helps to reduce capacitor size, increase system power density, reduce system construction costs, and decrease system footprint. This invention provides a voltage ripple suppression method for MMDTC type power electronic transformers, solving the problems of large-volume capacitors and complex single-channel control in existing technologies. The purpose of this invention is to overcome the shortcomings of the prior art and provide a capacitor voltage ripple control method and system in MMDTC type power electronic transformers, which effectively controls the capacitor voltage ripple in the MMDTC type power electronic transformer as a whole. Attached Figure Description

[0052] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure;

[0053] Figure 1 This is a topology diagram of TAB-MMDTC-PET provided by the present invention;

[0054] Figure 2 This invention provides Figure 1 Schematic diagram of the connection between the upper and lower bridge arms and the TAB;

[0055] Figure 3 This is the equivalent model diagram of TAB in the capacitor voltage ripple suppression method provided by the present invention;

[0056] Figure 4 This is a schematic diagram of the phase shift modulation of the primary and secondary sides of TAB in the capacitor voltage ripple suppression method provided by the present invention;

[0057] Figure 5 This is a schematic diagram of the connection between HBSM and the primary side of TAB in the first half-cycle TS1 of the capacitor voltage ripple suppression method provided by the present invention.

[0058] Figure 6 This is a schematic diagram of the connection between HBSM and the primary side of TAB in the lower half-cycle TS2 of the capacitor voltage ripple suppression method provided by the present invention;

[0059] Figure 7 This is a schematic diagram of the current flow path in the switched capacitor circuit provided by the present invention;

[0060] Figure 8 This is a schematic diagram of the equivalent impedance model based on a switched capacitor circuit provided by the present invention;

[0061] Figure 9 This is a schematic diagram of the equivalent circuit of the upper bridge arm provided by the present invention;

[0062] Figure 10 This is a schematic diagram of the equivalent circuit of the lower bridge arm provided by the present invention;

[0063] Figure 11 This is a topology diagram of the DAB-MMDTC type PET in the traditional DAB-MMDTC-PET structural simulation model;

[0064] Figure 12 This is a schematic diagram of the connection between HBSM and DAB in a traditional DAB-MMDTC-PET structural simulation model.

[0065] Figure 13 The simulation results show a comparison of the output line voltages of the traditional DAB scheme and the TAB scheme proposed in this invention.

[0066] Figure 14 The simulation results show a comparison of the output current of the traditional DAB scheme and the TAB scheme proposed in this invention.

[0067] Figure 15 The simulation results show the comparison of arm voltages between the traditional DAB scheme and the TAB scheme proposed in this invention.

[0068] Figure 16 The simulation results show the comparison of arm currents between the traditional DAB scheme and the TAB scheme proposed in this invention.

[0069] Figure 17A schematic diagram of the three key measurement points in the traditional DAB scheme;

[0070] Figure 18 The current waveform and FFT analysis diagram of the HBSM output at the key measurement point 1 in the traditional DAB scheme are shown.

[0071] Figure 19 The current waveform and FFT analysis diagram of the capacitor input port at the key measurement point 2 in the traditional DAB scheme are shown.

[0072] Figure 20 The current waveform and FFT analysis diagram of the DAB input at key measurement point 3 in the traditional DAB scheme are shown.

[0073] Figure 21 This is a schematic diagram of three key measurement points of the TAB scheme proposed in this invention;

[0074] Figure 22 The current waveform and FFT analysis diagram of the HBSM output port of the key measurement point 1 of the TAB scheme proposed in this invention are shown.

[0075] Figure 23 The current waveform and FFT analysis diagram of the capacitor input port of the key measurement point 2 of the TAB scheme proposed in this invention are shown.

[0076] Figure 24 The current waveform and FFT analysis diagram of the TAB input port of the key measurement point 3 of the TAB scheme proposed in this invention;

[0077] Figure 25 The diagram shows the HBSM capacitor voltage of each sub-module in the upper and lower arms of the traditional DAB scheme.

[0078] Figure 26 The HBSM capacitor voltage of each sub-module in the upper and lower arms of the TAB scheme proposed in this invention;

[0079] Figure 27 The figure shows the simulation results of the three-phase grid-connected current when the load changes abruptly on the LVDC side;

[0080] Figure 28 The simulation results of the LVDC bus voltage during a sudden load change on the LVDC side are shown in the figure.

[0081] Figure 29 The figure shows the simulation results of the LVDC bus current when the load on the LVDC side changes abruptly.

[0082] Figure 30 The simulation results of the submodule capacitor voltage waveform are shown in the figure when the load on the LVDC side changes abruptly.

[0083] Figure 31 The experimental platform diagram is shown.

[0084] Figure 32 Given Figure 30 Experimental results of current waveforms at three key measurement points;

[0085] Figure 33 The FFT analysis plot of the DC-DC output current;

[0086] Figure 34 FFT analysis plot of capacitive input current;

[0087] Figure 35 FFT analysis plot of the TAB input current;

[0088] Figure 36 The TAB input current and output current waveforms are shown at three measurement points: 3', 4', and 5'.

[0089] Figure 37 The output voltage waveforms of the DC-DC converter and the TAB converter are shown. Detailed Implementation

[0090] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0091] The innovation of this invention lies in the fact that it replaces the traditional two-port active bridge with a three-port active bridge as the isolation stage of the MMDTC type power electronic transformer, so that the ripple power automatically flows to the subsequent stage of the system and is automatically canceled, reducing or even lowering the ripple voltage on the submodule capacitor. This eliminates the need for additional complex control circuits, reduces the capacitance value of the submodule capacitor, lowers the system cost, increases the system power density, and has no impact on the low-voltage DC side of the system, achieving a good ripple voltage suppression effect.

[0092] This invention proposes a TAB-MMDTC-PET voltage ripple suppression technology.

[0093] Example 1: The PET capacitor voltage ripple suppression system based on a three-port active bridge provided in this embodiment of the invention adopts a TAB-MMDTC-PET topology. The TAB-MMDTC-PET topology utilizes a three-port active bridge (TAB) as the isolation stage of MMDTC-PET. Taking advantage of the characteristic that the AC power components of the upper and lower arms of the HBSM have equal amplitudes and opposite phases, the two active bridges on the primary side of the three-port active bridge (TAB) form a switched capacitor circuit with the upper and lower arms of the HBSM modular DC link under the same phase-shift modulation. This allows the ripple current of the two active bridge capacitors of the three-port active bridge (TAB) to automatically flow to the subsequent stage. On the secondary side of the three-port active bridge (TAB), the AC power components cancel each other out, and the AC power components do not affect the low-voltage DC bus (LVDC).

[0094] The power transferred from the primary to the secondary side of TAB is controlled by the phase difference between u1, u2, and u3 and the additional inductance L. The power transfer formula is as follows: Phase shift angle β = 180° × D; where V1 and V2 are the voltages on the primary and secondary sides, respectively, D is the phase shift duty cycle, and f s U is the switching frequency, u1 and u2 are the voltages at the two AC input terminals respectively, and u3 is the AC voltage at the output terminal.

[0095] The two active bridges include the upper bridge arm submodule and the lower bridge arm module of the three-port active bridge TAB;

[0096] For example, a three-port active bridge (TAB) replaces the DAB, utilizing the principle of switched capacitors to reduce input inductive reactance, thereby increasing the transfer ratio ξ to achieve capacitor voltage ripple suppression, such as... Figure 1 This is a diagram of the TAB-MMDTC type PET topology of the present invention, namely the TAB-MMDTC-PET topology. Figure 2 for Figure 1 A schematic diagram of the connection between the upper and lower arms of the HBSM and the TAB in the TAB-MMDTC-PET topology; the high-voltage AC side still uses the MMDTC structure.

[0097] The three-port active bridge TAB consists of an upper input terminal, a lower input terminal, and an output terminal. The upper input terminal and the lower input terminal are respectively connected to the upper bridge arm sub-module and the lower bridge arm module of the three-port active bridge TAB. The upper bridge arm module and the lower bridge arm module are respectively connected to the upper bridge arm and the lower bridge arm of the HBSM.

[0098] The output terminal is connected to the Low Voltage DC (LVDC) bus.

[0099] The two input terminals and the output terminal are connected via a capacitor ripple current signal;

[0100] A capacitor C is connected in parallel on each of the upper and lower arms of the HBSM. up With lower capacitor C low ;

[0101] For example, the upper arm submodule of the three-port active bridge TAB includes: a switch S1 and a switch S2 connected in parallel, a switch S3 connected in series with switch S1, a switch S4 connected in series with switch S2, and a switch S3 and a switch S4 connected in parallel.

[0102] The current input terminals of the parallel-connected switching transistors S1 and S2 are connected to the upper arm of the HBSM. up The positive electrode is connected; the current output terminals of the parallel-connected switches S3 and S4 are connected to the upper arm of the HBSM. up The negative electrode is connected;

[0103] The common node of the series-connected switches S1 and S3 is connected to the common node of the series-connected switches S2 and S4 through leakage inductance impedance, forming the upper input terminal of the upper bridge arm submodule of the three-port active bridge TAB.

[0104] The current flows from the common node of the series-connected switches S1 and S3 to the common node of the series-connected switches S2 and S4 through the leakage inductance impedance.

[0105] For example, the three-port active bridge TAB lower bridge arm submodule includes: switches S5 and S6 connected in parallel, switch S5 connected in series with switch S7, and switch S6 connected in series with switch S8; switch S7 and switch S8 are connected in parallel.

[0106] The current input terminals of the parallel-connected switching transistors S5 and S6 are connected to the lower bridge arm of the HBSM. low The positive electrode is connected; the current output terminals of the parallel-connected switching transistors S7 and S8 are connected to the lower bridge arm of the HBSM. low The negative electrode is connected;

[0107] The common node of the series-connected switches S5 and S6 is connected to the common node of the series-connected switches S6 and S8 through leakage inductance impedance, forming the lower input terminal of the lower bridge arm submodule of the three-port active bridge TAB.

[0108] The current flows from the common node of the series-connected switches S5 and S7 through the leakage inductance impedance to the common node of the series-connected switches S6 and S8.

[0109] The output of the three-port active bridge TAB is connected to the low-voltage DC bus LVDC through leakage inductance impedance.

[0110] The low-voltage DC bus (LVDC) includes switching transistors S9 and S1 connected in parallel. 10 Switch S9 is connected in series with switch S 11 Switch S 10 A switching transistor S is connected in series. 12 ; Switching transistor S 11 With the switching transistor S 12 Forming a parallel connection;

[0111] The output of the three-port active bridge TAB is connected to a series-connected switching transistor S through one end of the leakage inductance impedance. 10 Switch S 12 The common node of the three-port active bridge TAB is connected to the series-connected switching transistors S9 and S2 through the other end of the leakage inductance impedance. 11 public nodes;

[0112] For example, such as Figure 3 The diagram shows the equivalent model of the TAB input in the capacitor voltage ripple suppression method. Both full-bridge inputs of the TAB input employ the exact same phase-shift modulation strategy, such as... Figure 4 The diagram illustrates phase-shift modulation on the primary and secondary sides of the TAB in a capacitor voltage ripple suppression method. Power transfer is achieved through the phase difference between the primary and secondary sides of the TAB via u1, u2, and u3, and an additional inductance L. u1 and u2 represent the voltages at the two AC input terminals, while u3 represents the AC voltage at the output terminal. The phase angles of the modulation signals of the two full-bridge inputs are β1 and β2, with β1 = β2. The phase angle of the modulation signal at the output is β3, and the phase shift angle is β = β3 - β1. Power flow in the TAB is controlled by adjusting the phase shift angle β.

[0113] For example, the upper bridge arm submodule, lower bridge arm module, and low-voltage DC bus LVDC three full bridges of the three-port active bridge TAB all adopt PWM modulation; the switching period is T. s The pulse width is 50%.

[0114] In the primary side of the TAB, the upper bridge arm submodule of the three-port active bridge TAB operates during the first half of the switching cycle T. s1 Internally, the switching transistors S1 / S4 of the full bridge are turned on, and the full bridge outputs a positive level at the upper input terminal. Capacitor C... up The switching transistors S1 and S4 are connected in parallel directly. Similarly, in the second half of the switching cycle T... s2 Internally, with S2 / S3 of the full-bridge circuit turned on, all input terminals of the full-bridge circuit output a positive level, and capacitor C... up The switching transistors S2 and S3 are connected in parallel directly.

[0115] For example, in the primary side of a TAB, the lower bridge arm submodule of a three-port active bridge TAB operates during the first half of the switching cycle T. s1Internally, the switching transistors S5 / S8 of the full bridge are turned on, and the entire bridge outputs a positive level at the lower input terminal. Capacitor C... low The switching transistors S5 / S8 are connected in parallel directly. Similarly, during the second half of the switching cycle T... s2 Internally, with S6 / S7 of the full-bridge circuit turned on, all input terminals of the full-bridge circuit output a positive level, and capacitor C... low Connect directly in parallel via switching transistors S6 / S7;

[0116] Figure 5 In the capacitor voltage ripple suppression method, the upper half-cycle T s1 Schematic diagram of the connection between the inner HBSM and the primary side of TAB. Figure 6 In the capacitor voltage ripple suppression method, the lower half-cycle T s2 Schematic diagram of the connection between the internal HBSM and the primary side of TAB; under high-frequency PWM modulation, capacitor C up With capacitor C low They form a switched capacitor, and the two capacitors charge and discharge each other.

[0117] For example, in the decomposition of AC / DC components of the capacitor voltage of the upper and lower bridge arm submodules of the three-port active bridge (TAB), efforts are made to increase the proportion ξ of the AC component transmitted to the subsequent stage. ξ refers to the ratio of the ripple power flowing to the isolation stage to the ripple power flowing into the submodule. Formulas (1)-(6) analyze the decoupling of the submodule capacitor and ripple power, which can reduce the capacitor voltage ripple and thus reduce the required capacitor value. To this end, this invention proposes to use a three-port active bridge (TAB) instead of a DAB, and to use the switching capacitor principle to reduce the input inductive reactance, thereby increasing the transmission ratio ξ to achieve capacitor voltage ripple suppression.

[0118] Example 2, the PET capacitor voltage ripple suppression based on a three-port active bridge provided in this embodiment of the invention includes:

[0119] Using a three-port active bridge TAB as the isolation stage of MMDTC-PET, and taking advantage of the equal amplitude and opposite phase of the AC power components in the upper and lower arms of the HBSM, the two active bridges on the primary side of the three-port active bridge TAB, under the same phase-shift modulation, form a switched capacitor circuit with the upper and lower arms of the HBSM modular DC link, so that the ripple current of the two active bridge capacitors of the three-port active bridge TAB automatically flows to the subsequent stage; on the secondary side of the three-port active bridge TAB, the AC power components cancel each other out, and the AC power components do not affect the low-voltage DC bus LVDC;

[0120] A specific example is the analysis of the voltage ripple suppression principle.

[0121] Figure 7 This is a schematic diagram of the current flow path in a switched capacitor circuit. Figure 8This is a schematic diagram of the equivalent impedance model based on a switched capacitor circuit. Figures 7-8 As shown, i sm_up i sm_low This represents the equivalent AC current source of the bridge arm submodule; since the impedance of the parallel circuit of the two capacitors is very small, the voltages of the two capacitors are clamped to each other and tend to be consistent.

[0122] In the TAB model proposed in this invention, no external inductor is provided on the primary side of the transformer winding, but the transformer winding itself has leakage inductance. Considering the influence of the transformer leakage impedance, L up L low These are the transformer leakage inductances of the primary side of the TAB level connecting the upper and lower bridge arm submodules, respectively.

[0123] Define the equivalent input impedance of the upper bridge arm submodule and the lower bridge arm submodule as Z. up / low ,like Figure 9 Equivalent circuit of upper bridge arm Figure 10 The equivalent circuit of the lower bridge arm is shown; taking the equivalent circuit of the upper bridge arm as an example, the equivalent input impedance of the upper bridge arm relative to the lower bridge arm is:

[0124] Z up =Z Clow +Z Llow +Z Lup (1)

[0125] In the formula, Z up Z is the equivalent input impedance of the upper bridge arm submodule. Clow Z represents the capacitance impedance of the lower bridge arm submodule. Clow =1 / jωC low Z Lup Z Llow Z is the inductance corresponding to the leakage inductance. Lup / low =jωL up / low ω is the angular frequency, C low For the lower bridge arm submodule capacitor, L up / low For the leakage inductance of the transformer on the primary side of the TAB level connecting the upper and lower bridge arm submodules;

[0126] The equivalent AC source input ripple currents of the capacitors and TAB primary side in the upper and lower bridge arm submodules are i, respectively. Cup / low_sm and i Tup / low i is obtained from the circuit equivalent model. sm_up i Cup_sm i Tup The relationship is:

[0127]

[0128] In the formula, i sm_up For the equivalent AC source of the upper bridge arm submodule, i TupFor the ripple current on the primary side of the TAB input to the equivalent AC source of the upper bridge arm submodule, i Cup_sm Z is the ripple current of the input submodule capacitor as the equivalent AC source of the upper bridge arm submodule. Cup Z represents the capacitance impedance of the upper bridge arm submodule. up The equivalent input impedance of the upper bridge arm submodule;

[0129] It can be understood that the above formula is derived from the innovative topology proposed in this invention, combined with the equivalent model of this structure and circuit knowledge such as KVL, KCL, and superposition theorem.

[0130] exist Figure 7 In the middle, from the equivalent alternating current source i Tlow The current flowing into the upper bridge arm is i Cup_Tlow According to Kirchhoff's current law, i Cup_Tlow =i Tlow According to the superposition theorem, and assuming that the capacitors and transformers of each submodule have the same parameters, that is: Z Cup =Z Clow =1 / jωC,Z Lup =Z Llow =jωC, where C and L represent the capacitance and leakage inductance values, respectively, and the current i superimposed on the capacitor of the upper bridge arm submodule is... Cup Represented as:

[0131]

[0132] In the formula, i Cup i represents the capacitor current of the submodule. Cup_Tlow To obtain from the equivalent alternating current source i Tlow The current flowing into the upper bridge arm, Z up The equivalent input impedance of the upper bridge arm submodule;

[0133] Based on the relationship between capacitor voltage and current, we can obtain:

[0134]

[0135] Integrating the equation yields the expression for the capacitor voltage:

[0136]

[0137] When θ = 0 Get the valley value Uvalley, let The expression is:

[0138]

[0139] From formulas (3) and (6), it is known that due to the presence of leakage inductance L, the ripple current cannot be completely decoupled from the submodule capacitor. However, the leakage inductance can be reduced in the design to minimize its impact. Therefore, the leakage inductance L of the high-frequency transformer in TAB should be designed to be as small as possible to minimize i cup Approaching zero, this means that, based on the low impedance characteristics of the equivalent connection of the switched capacitor circuit, the fluctuating current i sm_up Completely decoupled from the submodule capacitor, it is transferred to the secondary side of the TAB-level high-frequency transformer without additional ripple power transfer control.

[0140] The high-frequency switching signal on the secondary side of TAB has a phase difference with the switching signal on the other side.

[0141] It is understandable that the above formula is derived from the equivalent circuit model, as well as circuit knowledge such as parallel current division and instantaneous power expression.

[0142] For example, the principle of ripple power transfer can therefore be summarized as follows: AC component i sm_up / low Based on the low impedance path formed in the switched capacitor circuit, the power is transmitted to the secondary side of the high-frequency transformer. Then, based on the characteristic that the low-frequency ripple power amplitude of the upper and lower bridge arms is the same but the phase is opposite, the ripple current that originally flowed into the capacitors of the upper and lower bridge arm submodules cancels each other out on the common low-voltage DC bus, thereby eliminating the influence on the submodule capacitors. Only DC power flows into the LVDC bus, while the capacitor voltages of the upper and lower bridge arm submodules are clamped to each other and ripple within a small range.

[0143] To further illustrate the effects of the embodiments of the present invention, the following experiment was conducted: The present invention utilizes TAB as the isolation stage of MMDTC-PET. Taking advantage of the characteristics that the AC power components of the upper and lower bridge arm sub-modules have equal amplitudes and opposite phases, the two active bridges on the primary side of TAB form a switched capacitor circuit with the upper and lower bridge arm sub-modules of the modular DC link under the same phase-shift modulation. This allows the capacitor ripple current of the sub-module to automatically flow to the subsequent stage, achieving mutual cancellation of the AC power components on the secondary side of TAB. The AC power components will not affect the low-voltage DC bus.

[0144] To verify the effectiveness of the TAB suppression submodule capacitor low-frequency voltage ripple proposed in this invention, simulation models of DAB-MMDTC-PET and TAB-MMDTC-PET were built in Simulink, as follows: Figure 11 This is the topology diagram of the DAB-MMDTC type PET in the DAB-MMDTC-PET structural simulation model. Figure 12 This is a schematic diagram of the connection between HBSM and DAB in the DAB-MMDTC-PET structural simulation model;

[0145] TAB-MMDTC-PET simulation model, such as Figures 1-2 As shown;

[0146] The specific simulation parameters are shown in Table 1:

[0147] Table 1. Main Simulation Parameters

[0148] Variable Symbol Value Rated grid voltage Vg 10kV AC filter inductance measurement <![CDATA[L s ]]> 2mH Rated power <![CDATA[P rated ]]> 1MW Number of bridge arm sub-modules N 20 Submodule capacitor voltage <![CDATA[u c ]]> 750V Submodule capacitor value C 0.5mF carrier frequency <![CDATA[f c ]]> 10kHz LVDC voltage <![CDATA[u LVDC ]]> 750V DAB transformer turns ratio <![CDATA[n1:n2]]> 1:1 TAB transformer turns ratio <![CDATA[n1:n2:n3]]> 1:1:1 Switching frequency <![CDATA[f s ]]> 40kHz

[0149] Figure 13 The figure shows the simulation results comparing the output line voltages of the traditional DAB scheme and the TAB scheme proposed in this invention. Figure 14 The figure shows the simulation results comparing the output current of the traditional DAB scheme and the TAB scheme proposed in this invention. Figure 15 The figure shows the simulation results comparing the arm voltages of the traditional DAB scheme and the TAB scheme proposed in this invention. Figure 16 The simulation results comparing the arm currents of the traditional DAB scheme and the TAB scheme proposed in this invention are shown in the figure; the output line voltage, output current, bridge arm voltage, and bridge arm current are displayed. The results show that the basic operating characteristics of the MMDTC remain consistent regardless of the isolation stage configuration.

[0150] For the traditional DAB scheme Figure 17 This is a schematic diagram of the three key measurement points in the traditional DAB scheme. Figure 18 The current waveform and FFT analysis plot of the HBSM output at key measurement point 1 in the traditional DAB scheme are shown. Figure 19 The current waveform and FFT analysis plot of the capacitor input port at key measurement point 2 in the traditional DAB scheme are shown. Figure 20 The current waveform and FFT analysis diagram of the DAB input at key measurement point 3 in the traditional DAB scheme are shown.

[0151] like Figure 18 As shown, the HBSM output current includes a 34A DC component and a 36A tri-frequency component. Figure 19 and Figure 20 The results show that the third harmonic AC component mainly flows into the submodule capacitor, while the DC component is mainly transferred to the DAB input. These observations validate the theoretical analysis presented above.

[0152] Regarding the TAB scheme proposed in this invention, Figure 7 The current waveforms and FFT analysis at the corresponding measurement points are given. Figure 21 A schematic diagram of the three key measurement points of the TAB scheme proposed in this invention. Figure 22 The current waveform and FFT analysis diagram of the HBSM output port of the key measurement point 1 in the TAB scheme proposed in this invention are shown. Figure 23 The image shows the current waveform and FFT analysis diagram of the capacitor input port at key measurement point 2 of the TAB scheme proposed in this invention. Figure 24 The current waveform and FFT analysis diagram of the TAB input port of the key measurement point 3 of the TAB scheme proposed in this invention;

[0153] contrast Figure 19 and Figure 22 The DC and three-frequency AC components at the HBSM output are almost identical. However, when comparing... Figure 20 and Figure 23 At that time, a significant reduction in the three-frequency AC components was observed in the submodule capacitor current. For example... Figure 24 As shown, this demonstrates the successful implementation of the ripple power decoupling mechanism.

[0154] Figures 25-26 The capacitor voltages of each sub-module in the upper and lower arms of the traditional DAB scheme and the TAB scheme proposed in this invention were compared. Figure 25 The diagram shows the HBSM capacitor voltage of each sub-module in the upper and lower arms of the traditional DAB scheme. Figure 26 The HBSM capacitor voltage of each sub-module in the upper and lower arms of the TAB scheme proposed in this invention;

[0155] The comparison results clearly show that, under the same operating conditions, the voltage ripple ratio of the submodule capacitor is significantly reduced from 24% to 2.5%, verifying the excellent ripple suppression capability of the proposed TAB scheme.

[0156] Figure 27 The simulation results show the three-phase grid-connected current under sudden load changes on the LVDC side. Figure 28 The simulation results show the voltage of the LVDC bus when the load on the LVDC side changes abruptly. Figure 29 The simulation results show the current of the LVDC bus when the load on the LVDC side changes abruptly. Figure 30 The simulation results show the capacitor voltage waveform of the submodule when the load on the LVDC side changes abruptly. It can be seen that the system can quickly reach a steady state, has good dynamic characteristics, and can still ensure the stable operation of the TAB-MMDTC-PET system.

[0157] Experimental Analysis: To further verify the correctness and effectiveness of the proposed TAB-based scheme for suppressing low-frequency voltage ripple in the submodule capacitor, an experimental platform based on RT-LAB was built. Due to limited experimental conditions, this patent uses two DC-DC circuits to simulate the third harmonic power output of the upper and lower arms of the MMDTC. The experimental platform structure consists of DC-DC circuits and TAB, as shown in the experimental platform structure below. Figure 31As shown in the figure; the DC-DC circuit generates the switching signal of the DC-DC circuit by comparing the modulated wave containing the third harmonic frequency with the triangular carrier, and then obtains the TAB input current with the third harmonic frequency and opposite phase; in the hardware structure of the experimental platform, RT-LAB is used as the controller, and the main circuit includes two parts: DC-DC and TAB. The TAB consists of a high-frequency winding transformer and three active bridges on both sides, and adopts a modulation method with the same phase on the primary side; the specific experimental parameters are shown in Table 2.

[0158] Table 2 Main experimental parameters

[0159] parameter symbol value Rated power <![CDATA[P rated ]]> 500W DC power input voltage <![CDATA[U1]]> 105V TAB output voltage <![CDATA[U2]]> 100V load R 20Ω Switching frequency <![CDATA[f s ]]> 10kHz Submodule capacitor C 470μF

[0160] Figure 32 Given Figure 31 The experimental results of the current waveforms at the three key measurement points 1', 2', and 3'. Figure 33 The FFT analysis plot of the DC-DC output current is shown below. Figure 34 The FFT analysis plot is for the capacitive input current. Figure 35 FFT analysis plot of the TAB input current;

[0161] The currents at these three measurement points represent the DC-DC output current, the capacitor input current, and the TAB input current, respectively. FFT analysis shows that most of the third harmonic component in the DC-DC output current flows to the TAB, while the capacitor input current contains only higher harmonics.

[0162] Figure 36 The waveforms of the TAB input and output current at three measurement points (3', 4', and 5') are shown. The measured current at points 3' and 4' is the TAB input current, and the measured current at point 5' is the TAB output current. Figure 36 As can be seen, the current input to TAB contains a third-harmonic frequency fluctuation with opposite phase, while the output current does not contain a third-harmonic frequency fluctuation.

[0163] Figure 37 The output voltage waveforms of the DC-DC converter and the TAB converter are presented. It can be observed that the two DC-DC output voltages exhibit third-harmonic ripples with opposite phases, fluctuating by 10V, while the TAB output voltage remains stable at 100V. This indicates that under the same phase modulation of the two full-bridge converters on the primary side, the third-harmonic ripple component in the input current will automatically cancel out on the output side due to the opposite phase characteristic of third-harmonic components. Only the DC component will be transferred to the TAB output side, verifying the effectiveness of the TAB-based scheme for automatically eliminating third-harmonic ripple. Experimental results verify the effectiveness of the voltage ripple suppression based on TAB.

[0164] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A PET capacitor voltage ripple suppression system based on a three-port active bridge, characterized in that, The system is equipped with a TAB-MMDTC-PET topology. The TAB-MMDTC-PET topology uses a three-port active bridge TAB as the isolation stage of MMDTC-PET. Taking advantage of the fact that the upper and lower arms of the HBSM have equal amplitude and opposite phase of AC power components, the two active bridges on the primary side of the three-port active bridge TAB, under the same phase-shift modulation, form a switched capacitor circuit with the upper and lower arms of the HBSM modular DC link. This allows the ripple current of the two active bridge capacitors of the three-port active bridge TAB to automatically flow to the subsequent stage. The AC power components cancel each other out on the secondary side of the three-port active bridge TAB, without affecting the low-voltage DC bus LVDC. TAB primary and secondary sides pass The phase difference between the two sides and the additional inductance L control the power transferred from the primary side to the secondary side. The power transfer formula is as follows: Phase shift angle ;in, These are the voltages on the primary and secondary sides, respectively. For phase shift duty cycle, For switching frequency, These are the voltages at the two AC input terminals, The output AC voltage; The two active bridges include the upper bridge arm submodule and the lower bridge arm module of the three-port active bridge TAB; The upper bridge arm submodule, lower bridge arm module, and low-voltage DC bus LVDC three full bridges of the three-port active bridge TAB all adopt PWM modulation. In the primary side of the TAB, the upper bridge arm submodule of the three-port active bridge TAB operates during the first half of the switching cycle. Inside, the switching transistors of the entire bridge When the circuit is turned on, all bridge inputs at the upper input terminal output a positive level, and the capacitor... Through the switching transistor Direct parallel connection; in the second half of the switching cycle Inside, the entire bridge When the circuit is turned on, all bridge inputs at the upper input terminal output a positive level, and the capacitor... Through the switching transistor Direct parallel connection; In the primary side of the TAB, the lower bridge arm submodule of the three-port active bridge TAB operates during the first half of the switching cycle. Inside, the switching transistors of the entire bridge When the circuit is turned on, all bridge components at the lower input terminal output a positive level, and the capacitor... Through the switching transistor Direct parallel connection; in the second half of the switching cycle Inside, the entire bridge When the circuit is turned on, all bridge components at the lower input terminal output a positive level, and the capacitor... Through the switching transistor Direct parallel connection.

2. The PET capacitor voltage ripple suppression system based on a three-port active bridge according to claim 1, characterized in that, The three-port active bridge TAB consists of an upper input terminal, a lower input terminal, and an output terminal. The upper input terminal and the lower input terminal are respectively connected to the upper bridge arm sub-module and the lower bridge arm module of the three-port active bridge TAB. The upper arm submodule and the lower arm submodule are respectively connected to the upper arm and lower arm of the HBSM; The output terminal is connected to the low-voltage DC bus (LVDC). The two input terminals and the output terminal are connected via a capacitor ripple current signal; A capacitor is connected in parallel on each of the upper and lower arms of the HBSM. With lower capacitor .

3. The PET capacitor voltage ripple suppression system based on a three-port active bridge according to claim 1, characterized in that, The three-port active bridge TAB upper arm submodule includes: interconnected switching transistors. Switching transistor Switching transistor A switching transistor is connected in series. Switching transistor A switching transistor is connected in series. Switching transistor With switching transistor Parallel connection; Parallel-connected switching transistors Switching transistor The current input terminal is connected to the upper arm of the HBSM. The positive electrode is connected; the switching transistors are connected in parallel. With switching transistor Current output terminal and upper arm of HBSM The negative electrode is connected; Series-connected switching transistors Switching transistor The common node is connected to the series switch via leakage inductance impedance. Switching transistor The common node is connected to form the upper input terminal of the upper arm submodule of the three-port active bridge TAB.

4. The PET capacitor voltage ripple suppression system based on a three-port active bridge according to claim 3, characterized in that, The three-port active bridge TAB lower arm submodule includes: interconnected switching transistors. Switching transistor Switching transistor A switching transistor is connected in series. Switching transistor A switching transistor is connected in series. Switching transistor With switching transistor Parallel connection; Parallel-connected switching transistors Switching transistor The current input terminal is connected to the lower bridge arm of the HBSM. The positive electrode is connected; the switching transistors are connected in parallel. With switching transistor Current output terminal and lower bridge arm of HBSM The negative electrode is connected; Series-connected switching transistors Switching transistor The common node is connected to the series switch via leakage inductance impedance. Switching transistor The common node is connected to form the lower input terminal of the lower bridge arm submodule of the three-port active bridge TAB.

5. The PET capacitor voltage ripple suppression system based on a three-port active bridge according to claim 1, characterized in that, The low-voltage direct current (LVDC) bus includes switching transistors connected in parallel. Switching transistor Switching transistor A switching transistor is connected in series. Switching transistor A switching transistor is connected in series. Switching transistor With switching transistor Parallel connection; The output of the three-port active bridge TAB is connected to a series-connected switching transistor through one end of the leakage inductance impedance. Switching transistor The common node of the three-port active bridge TAB is connected to a series-connected switching transistor through the other end of the leakage inductance impedance. Switching transistor The public nodes.

6. The PET capacitor voltage ripple suppression system based on a three-port active bridge according to claim 1, characterized in that, In TAB secondary phase-shift modulation, the phase angles of the modulation signals of the two full-bridge inputs at the upper and lower inputs are: ,and The phase angle of the full-bridge modulated signal at the output is The phase shift angle is By controlling the phase shift angle The size controls the flow of power in the TAB.

7. A method for suppressing PET capacitor voltage ripple based on a three-port active bridge, characterized in that, This method is implemented using the PET capacitor voltage ripple suppression system based on a three-port active bridge as described in any one of claims 1-6. The method utilizes a three-port active bridge (TAB) as the isolation stage of the MMDTC-PET, and leverages the characteristic that the AC power components of the upper and lower arms of the HBSM have equal amplitudes and opposite phases. Under the same phase-shift modulation, the two active bridges on the primary side of the three-port active bridge (TAB) form a switched capacitor circuit with the upper and lower arms of the HBSM modular DC link, enabling the ripple current of the capacitors in the upper and lower arms of the three-port active bridge (TAB) to automatically flow to the subsequent stage. Mutual cancellation of the AC power components is achieved on the secondary side of the three-port active bridge (TAB).

8. The PET capacitor voltage ripple suppression method based on a three-port active bridge according to claim 7, characterized in that, In a three-port active bridge (TAB), the capacitor ripple current of the upper and lower bridge arm submodules automatically flows to the subsequent stage. The equivalent input impedance of the upper and lower bridge arm submodules is defined as follows: The equivalent input impedance of the upper bridge arm submodule relative to the lower bridge arm submodule is: （1）； In the formula, The equivalent input impedance of the upper bridge arm submodule is... The impedance of the lower bridge arm submodule capacitor. ; The inductance is the inductance corresponding to the leakage inductance. ; Angular frequency, For the lower bridge arm submodule capacitor, For the leakage inductance of the transformer on the primary side of the TAB level connecting the upper and lower bridge arm submodules; The ripple currents of the capacitors and TAB primary side in the equivalent AC source input upper and lower bridge arm submodules are respectively and Based on the circuit equivalent model, we obtain The relationship is: （2）； In the formula, For the equivalent AC source of the upper bridge arm submodule, The ripple current on the primary side of the TAB is input to the equivalent AC source of the upper bridge arm. The ripple current of the input submodule capacitor is the equivalent AC source of the upper bridge arm. For the capacitor impedance of the upper bridge arm submodule, The equivalent input impedance of the upper bridge arm submodule; From the equivalent alternating current source The current flowing into the upper bridge arm is According to Kirchhoff's current law, we know According to the superposition theorem, and assuming that the capacitors and transformers of each submodule have the same parameters, that is: , C and L represent the capacitance and leakage inductance values, respectively, and the current superimposed on the capacitor of the upper bridge arm submodule. Represented as: （3）； In the formula, This refers to the current in the capacitor of the upper bridge arm submodule. To obtain from the equivalent alternating current source The current flowing into the upper bridge arm, The equivalent input impedance of the upper bridge arm submodule; Based on the relationship between capacitor voltage and current, we can obtain: （4）； Integrating the equation yields the expression for the capacitor voltage: （5）； ； when hour, Get the valley value ,make The expression is: （6）； From equations (3) and (6), it is known that, based on the low impedance characteristics of the equivalent connection of the switched capacitor circuit, the fluctuating current... Completely decoupled from the submodule capacitor, it is transferred to the secondary side of the TAB-level high-frequency transformer without additional ripple power transfer control.

9. The PET capacitor voltage ripple suppression method based on a three-port active bridge according to claim 7, characterized in that, The proposed PET capacitor voltage ripple suppression method based on a three-port active bridge is applied to the suppression of capacitor voltage ripple in power electronic transformers.

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