A power electronic transformer system and a control method thereof

Abstract

The application provides a power electronic transformer system and a control method thereof, and relates to the technical field of power electronic transformers, and comprises an AC-DC power conversion submodule, a network-side filter reactor, an AC soft starting circuit, a resonance capacitor, a resonance inductor, a matrix transformer, a plurality of rectifiers and a plurality of bus capacitors. The application omits the long power frequency DC intermediate link in the conventional topology, has fewer power conversion links, and greatly improves the electric energy conversion efficiency of the system. Since one end of the upper and lower bridge arm groups respectively collects positive and negative poles of a high-frequency loop, after the resonance capacitor and the primary branch of the matrix transformer are connected in series, the whole is connected between the positive and negative poles of the high-frequency loop on the DC side, rather than being directly connected to the network-side AC end, so that the resonance capacitor does not directly bear a great power frequency line voltage bias voltage stress, the damage of the reactive element due to overvoltage is effectively avoided, and the operation reliability of the whole power electronic transformer system is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of power electronic transformer technology, and in particular to a power electronic transformer system and its control method. Background Technology

[0002] Solid-state transformers, as core components of next-generation data center power supply architectures, are widely used in medium-voltage direct-connection and high-voltage DC power supply. However, traditional solid-state transformer topologies often employ multi-stage power conversion circuits, typically requiring a four-stage conversion process: from power frequency AC to DC, then to high-frequency AC, and finally back to DC. This conventional topology has lengthy power conversion stages, with energy accumulating at each stage of the transmission link, severely limiting further improvements in the overall operating efficiency of solid-state transformer systems.

[0003] To shorten the conversion path and improve system efficiency, existing technologies have proposed solid-state transformers based on isolation matrix converters. This solution directly converts the three-phase power frequency AC input to high-frequency AC using a matrix converter composed of bidirectional switches, and the energy is transferred through a resonant circuit formed by the inductor and capacitor connected in series on the primary side of the transformer. This solution eliminates the intermediate power frequency DC link, reducing one stage of power conversion and improving the overall energy conversion efficiency of the system to a certain extent.

[0004] However, in this topology that directly converts power frequency to high frequency, the resonant capacitor directly bears extremely high power frequency bias voltage stress, which can easily lead to accelerated aging of components and reduce the operational reliability of the transformer system. Therefore, in order to shorten the power conversion link and improve system efficiency while reducing the power frequency bias voltage stress borne by the resonant capacitor, how to provide a power electronic transformer system that balances high efficiency and high reliability has become an urgent problem to be solved in this field. Summary of the Invention

[0005] This invention provides a power electronic transformer system and its control method to solve the technical problem of how to provide a power electronic transformer system that balances high efficiency and high reliability.

[0006] This invention provides a power electronic transformer system, including an AC / DC power conversion submodule, a grid-side filter reactor, an AC soft start circuit, a resonant capacitor, a resonant inductor, a matrix transformer, multiple rectifiers, and multiple bus capacitors.

[0007] The AC ports of several AC / DC power conversion submodules are connected in series to form a power bridge arm. The six power bridge arms are divided into an upper bridge arm group and a lower bridge arm group. Both the upper bridge arm group and the lower bridge arm group include three power bridge arms.

[0008] One end of each of the three power arms of the upper bridge arm group is connected to form the positive terminal of the high-frequency circuit, and one end of each of the three power arms of the lower bridge arm group is connected to form the negative terminal of the high-frequency circuit.

[0009] The other ends of the three power arms of the upper bridge arm group are respectively connected to one end of the three-phase grid-side filter reactor through the corresponding resonant inductor;

[0010] The other ends of the three power arms of the lower arm group are respectively connected to one end of the three-phase grid-side filter reactor through the corresponding resonant inductor, and the other end of the three-phase grid-side filter reactor is connected to the external three-phase AC power grid through the AC soft start circuit.

[0011] The resonant capacitor and the primary branch of the matrix transformer are connected in series and then connected between the positive terminal and the negative terminal of the high-frequency circuit.

[0012] The AC ports of the multiple rectifiers are respectively connected to the multiple secondary AC ports of the matrix transformer, and the DC ports of the rectifiers are respectively connected to the corresponding bus capacitors. The positive and negative terminals of each bus capacitor are combined and connected to the low-voltage DC bus to form the DC port of the power electronic transformer system.

[0013] According to a power electronic transformer system provided by the present invention, the AC / DC power conversion submodule consists of a full-bridge circuit composed of four power devices and a filter capacitor.

[0014] According to the present invention, a power electronic transformer system is provided, wherein the matrix transformer is composed of multiple discrete high-frequency transformers;

[0015] The primary sides of multiple high-frequency transformers are connected in series to form a high-voltage AC port, and the secondary sides of multiple high-frequency transformers form multiple secondary AC ports.

[0016] According to a power electronic transformer system provided by the present invention, the AC / DC power conversion submodule is an energy storage submodule, which includes an energy storage unit, a filter inductor, a filter capacitor, a DC soft start circuit, multiple power devices, and a bypass switch;

[0017] The four power devices and the filter capacitor form a full-bridge circuit, and the bypass switch is connected in parallel to the AC output port of the full-bridge circuit.

[0018] The energy storage unit, the filter inductor, and the DC soft start circuit are connected in series to form an energy storage branch, which is connected in parallel with the branch formed by the filter capacitor.

[0019] The present invention also provides a control method for a power electronic transformer system, comprising:

[0020] Based on the grid connection current and power command, the low-frequency modulation voltage of the six power arms in the power electronic transformer system is calculated.

[0021] The high-frequency modulation voltage of the six power bridge arms is calculated based on the low-voltage DC bus voltage.

[0022] The final modulation voltage is obtained by superimposing the low-frequency modulation voltage and the high-frequency modulation voltage.

[0023] The switching on and off of each switch in the six power bridge arms is controlled based on the final modulation voltage, the feedback grid current, the power command, and the low-voltage DC bus voltage.

[0024] According to the control method for a power electronic transformer system provided by the present invention, the low-frequency modulation voltage of the six power arms in the power electronic transformer system is calculated based on the grid-connected current and power command, including:

[0025] The active current command is calculated by summing the power command with the low-voltage DC output power of the power electronic transformer system.

[0026] Closed-loop control is performed based on the active current command and the sampled grid-connected current to generate a reference low-frequency modulation voltage for the three-phase power bridge arm;

[0027] Based on the three-phase power bridge arm and the reference low-frequency modulation voltage, the low-frequency modulation voltage of the six power bridge arms is generated, and the low-frequency modulation voltage, the grid-connected current, the active current command, the power command, and the low-voltage DC output power are output to the control loop for processing.

[0028] According to the control method of the power electronic transformer system provided by the present invention, the AC / DC power conversion submodule is an energy storage submodule;

[0029] The control method further includes:

[0030] Obtain the current state of charge of each of the energy storage submodules;

[0031] Based on the current state of charge of each bridge arm, the corresponding inter-arm equalization voltage, inter-phase equalization voltage, and intra-phase equalization voltage are calculated.

[0032] The inter-arm equalization voltage, the inter-phase equalization voltage, and the intra-phase equalization voltage are superimposed on the low-frequency modulation voltage to update the low-frequency modulation voltage corresponding to each energy storage submodule, and the current state of charge is used as the feedback quantity for closed-loop control for modulation control.

[0033] According to the control method of the power electronic transformer system provided by the present invention, the corresponding inter-arm equalization voltage is calculated based on the current state of charge of each arm, including:

[0034] The amplitude of the circulating current in the upper and lower bridge arms is calculated based on the difference in the current average state of charge of the corresponding phases of the energy storage bridge arms in the upper and lower bridge arm groups.

[0035] The phase of the circulating current in the upper and lower bridge arms is calculated based on the power frequency angular frequency and phase angle, and a circulating current command is generated based on the amplitude and the phase.

[0036] The deviation between the circulating current command and the actual circulating current of each energy storage arm obtained by sampling is input to the closed-loop controller for control, generating the equalization voltage between the arms, and dynamically tracking the difference between the current average state of charge based on the phase angle and the power frequency during the control process.

[0037] According to the control method of the power electronic transformer system provided by the present invention, the amplitude of the circulating current of the upper and lower bridge arms is calculated based on the difference in the current average state of charge of the energy storage bridge arms of corresponding phases in the upper and lower bridge arm groups, including:

[0038] The difference between the current average state of charge of the lower arm and the current average state of charge of the upper arm in the same phase is used to obtain the state of charge deviation.

[0039] The state of charge deviation is subjected to proportional-integral control to calculate the amplitude of the circulating current of the upper and lower bridge arms, and the circulating current modulation control is performed based on the amplitude, wherein the state of charge deviation is used as the input feedback quantity in the proportional-integral control.

[0040] According to the control method of the power electronic transformer system provided by the present invention, the corresponding phase-to-phase equalization voltage is calculated based on each of the current states of charge, including:

[0041] Based on the current state of charge of the three-phase energy storage bridge arm, coordinate transformation is performed to obtain the active and reactive components of the current state of charge.

[0042] The preset capacitor voltage reference value is subtracted from the active component and the reactive component respectively to obtain the component difference value;

[0043] Using the proportional-integral control, the coordinate transformation and inverse coordinate transformation, a closed-loop operation is performed on the component difference to obtain the rotation component. The phase-to-phase equalization voltage is then generated based on the rotation component, and the phase-to-phase equalization voltage and the preset capacitor voltage reference value are used together as feedback quantities to act on the three-phase energy storage bridge arm.

[0044] The power electronic transformer system and its control method provided by this invention connect sub-modules in series to form power arms, and the upper and lower arms are directly connected at one end to form high-frequency positive and negative poles. A resonant capacitor is connected in series with the primary branch of a matrix transformer between the high-frequency positive and negative poles, realizing the direct conversion of power frequency AC voltage to high-frequency AC voltage. After being stepped down by the matrix transformer and rectified, a low-voltage DC output is produced. This eliminates the lengthy power frequency DC intermediate links in conventional topologies, reducing the power conversion links and greatly improving the system's power conversion efficiency. Because the upper and lower arms respectively connect to form the positive and negative poles of the high-frequency circuit, the resonant capacitor, after being connected in series with the primary branch of the matrix transformer, is connected between the positive and negative poles of the high-frequency circuit on the DC side, rather than being directly connected to the AC terminal on the grid side. This DC-side coupled resonance topology prevents the resonant capacitor from directly bearing the extremely high power frequency line voltage bias stress, effectively avoiding accelerated aging and damage of reactive components due to overvoltage, thereby significantly improving the overall operational reliability of the power electronic transformer system. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0046] Figure 1 This is one of the structural schematic diagrams of the power electronic transformer system provided by the present invention.

[0047] Figure 2 This is the second structural schematic diagram of the power electronic transformer system provided by the present invention.

[0048] Figure 3 This is a flowchart of the control method for the power electronic transformer system provided by the present invention. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0050] With the explosive growth of AI computing power, the power of a single AIDC rack for training has exceeded 100kW, placing unprecedented demands on the capacity, efficiency, and reliability of power supply systems. Solid-state transformers (SSTs), with their high-frequency, multi-port, and intelligent controllability characteristics, are considered core equipment for next-generation data center power supply architectures. They can achieve direct medium-voltage connection and directly output high-voltage DC to power AI servers.

[0051] Currently, most solid-state transformer topologies employ multi-stage power conversion circuits. First, the power frequency AC is converted to DC through cascaded H-bridges or modular multilevel circuits. Then, the DC is inverted to high-frequency AC. After isolation by a high-frequency transformer, the high-frequency AC is converted back to DC. This involves four stages of power conversion, resulting in a long link and room for further efficiency improvement.

[0052] Existing power supply systems based on SST technology include two independently operating power supply systems: one is the power grid supplying power to the load through a solid-state transformer, and the other is the energy storage battery supplying power to the load through a DC-DC converter circuit. The two independent subsystems supply power to the load respectively, and the hardware cost for supplying power to the load is relatively high.

[0053] In the prior art, the following topologies and control methods for solid-state transformers have been proposed:

[0054] The method discloses an isolation matrix converter and its control method. The input terminal of the isolation matrix converter is connected to a three-phase AC power supply. Each phase of the isolation matrix converter includes a bidirectional switch, that is, two series-connected anti-parallel diodes with opposite directions, allowing current to flow in both vertical and horizontal directions. The primary winding of the transformer is connected in series with an inductor and a capacitor to form an LLC resonant circuit. The matrix converter realizes the direct conversion of power frequency AC to high-frequency AC, reducing the number of power conversion stages. However, in this topology, the resonant capacitor bears a large power frequency bias voltage stress.

[0055] Scheme 2 discloses a solid-state transformer and its control method and power supply system. It integrates the energy storage battery onto the two arms of the low-voltage side H-bridge via two inductors, allowing the circuit supplying power to the load via AC power and the circuit supplying power to the load via the energy storage battery to share the low-voltage side H-bridge, reducing the hardware cost of powering the load. However, reusing the low-voltage side H-bridge causes the energy storage charging / discharging control and the solid-state transformer power supply processes to be coupled during circuit control and modulation, resulting in complex control and difficulty in further optimizing efficiency.

[0056] Scheme 3 discloses a solid-state transformer topology, mixing modulation, and soft-switching control method based on MMC. Through a three-stage topology (input stage, intermediate stage, and output stage), it addresses the technical problems of existing DAB-type MMC-SST topologies, which have numerous switching devices, require frequency selection networks for decoupling low-frequency and high-frequency ports, and lack a suitable and simple method to control the switching devices on the primary and secondary sides of the transformer to achieve soft-switching effects. However, this method relies on common-mode and differential-mode modulation and specific sub-module topologies, resulting in strong coupling between soft-switching and decoupling control. Furthermore, the single-stage mixing and highly coupled sub-module functions complicate design and equalization.

[0057] Scheme 4 discloses a solid-state transformer topology and modulation method based on MMC submodule bridge arm multiplexing. By using a three-phase circuit topology with medium-voltage AC, medium-voltage DC, low-voltage AC, and low-voltage DC terminals, and a modulation method that incorporates a high-frequency cancellation submodule corresponding to the high-frequency submodule in the single-phase submodule, the number of switching transistors can be significantly reduced. Decoupling of high and low frequency modulation signals in the submodules can be achieved without a frequency selection network, avoiding mutual interference between high-frequency and low-frequency switching signals. However, multi-stage power conversion leads to a gradual accumulation of energy loss, limiting power density improvement. Furthermore, to achieve high-low frequency decoupling, this patent not only increases the number of submodules but also requires strict control synchronization between paired submodules, limiting the system's modular expansion capabilities.

[0058] Scheme 5 discloses an energy storage-type MMC topology and its control method that avoids battery micro-circulation. It consists of six arms in a three-phase configuration, using the common point of the three-phase arms as the DC port and the midpoint of the upper and lower arms as the AC port for connection to DC and AC systems. Each arm comprises an arm inductor and N identical energy storage sub-modules connected in series. Each sub-module includes a half-bridge power module, a battery energy storage module, and an interface device between them. This patent can determine the operating mode of the energy storage battery based on system operating parameters and switch the battery on demand, avoiding battery micro-circulation problems, reducing battery life loss, and lowering the total life-cycle cost of the energy storage system. However, this patent uses a power conversion link of AC-DC-AC-DC, requiring multiple conversions in the energy transmission link. This lengthy conversion process limits system efficiency, significantly increases the total system loss after cascading, and makes it difficult to improve power density.

[0059] The following is combined Figures 1 to 3 The present invention describes a power electronic transformer system and its control method.

[0060] like Figure 1 and Figure 2As shown, the power electronic transformer system provided by this invention includes an AC / DC power conversion submodule, a grid-side filter reactor, an AC soft start circuit, a resonant capacitor, a resonant inductor, a matrix transformer, multiple rectifiers, and multiple bus capacitors.

[0061] Several AC / DC power conversion submodules are connected in series to form a power bridge arm. The six power bridge arms are divided into an upper bridge arm group and a lower bridge arm group. Both the upper bridge arm group and the lower bridge arm group include three power bridge arms.

[0062] One end of each of the three power arms of the upper arm group is connected to form the positive terminal of the high-frequency circuit, and one end of each of the three power arms of the lower arm group is connected to form the negative terminal of the high-frequency circuit.

[0063] The other ends of the three power arms of the upper bridge arm group are respectively connected to one end of the three-phase grid-side filter reactor through the corresponding resonant inductor;

[0064] The other ends of the three power arms of the lower arm group are respectively connected to one end of the three-phase grid-side filter reactor through the corresponding resonant inductor. The other end of the three-phase grid-side filter reactor is connected to the external three-phase AC power grid through the AC soft start circuit.

[0065] The resonant capacitor and the primary branch of the matrix transformer are connected in series and then connected between the positive and negative terminals of the high-frequency circuit.

[0066] The AC ports of multiple rectifiers are connected to the AC ports of multiple secondary sides of the matrix transformer, and the DC ports of the rectifiers are connected to the corresponding bus capacitors. The positive and negative terminals of each bus capacitor are combined and connected to the low-voltage DC bus to form the DC port of the power electronic transformer system.

[0067] The AC / DC power conversion submodule, as the basic power conversion unit of the system, consists of multiple submodules connected in series to form a power bridge arm. The grid-side filter reactor is connected between the AC side of the system and the power grid to filter out high-frequency switching harmonics. The AC soft-start circuit limits the transient inrush current generated when the grid side charges the submodule capacitors during the initial power-on phase. The resonant capacitor is the core reactive power compensation element of the high-frequency resonant cavity, used to isolate the power frequency voltage component and participate in high-frequency resonance. Each power bridge arm has a corresponding resonant inductor connected in series at its end, participating in high-frequency sinusoidal resonance. The matrix transformer provides safe electrical isolation between the high-voltage and low-voltage sides and achieves voltage matching and power transfer. The rectifier converts the high-frequency AC power from the secondary side of the matrix transformer into DC power. The bus capacitor is connected in parallel to the DC output side of the rectifier to smooth the DC voltage ripple.

[0068] Several AC / DC power conversion submodules have their AC ports connected in series to form a power arm. The power electronic transformer system contains six power arms, divided into an upper arm group and a lower arm group. The upper arm group includes three power arms, corresponding to phases A, B, and C respectively; the lower arm group also includes three power arms, corresponding to phases A, B, and C respectively.

[0069] The three power arms of the upper arm group are physically connected at one end to form the positive terminal of the high-frequency circuit; similarly, the three power arms of the lower arm group are physically connected at one end to form the negative terminal of the high-frequency circuit.

[0070] The other ends of the three power arms of the upper bridge arm group are each connected to one end of the three-phase grid-side filter reactor through a corresponding resonant inductor; the other ends of the three power arms of the lower bridge arm group are each connected to one end of the three-phase grid-side filter reactor through a corresponding resonant inductor.

[0071] The other end of the three-phase grid-side filter reactor is connected to the external three-phase medium-high voltage AC power grid via an AC soft start circuit.

[0072] The resonant capacitor and the primary branch of the matrix transformer are connected in series and then connected as a whole between the positive and negative terminals of the high-frequency circuit to form a high-frequency series resonant circuit.

[0073] The AC ports of multiple rectifiers are connected to the secondary AC ports of the matrix transformer. The DC ports of the rectifiers are connected in parallel to their corresponding bus capacitors. The positive and negative terminals of all the bus capacitors are then connected to the low-voltage DC positive and negative buses, thus forming the DC ports of the power electronic transformer system.

[0074] The operation process of a power electronic transformer system includes:

[0075] When the system starts up, the electrical energy from the external three-phase medium-high voltage AC grid first passes through the AC soft starter circuit. At this time, the soft starter resistor is connected with current limiting, and the electrical energy is slowly charged to the internal capacitors of each AC / DC power conversion submodule in the six power bridge arms through the grid-side filter reactor to prevent damage to the devices from the initial inrush current. When the submodule capacitor voltage rises to the preset operating value, the switch inside the AC soft starter circuit closes the bypass current limiting resistor.

[0076] Each AC / DC power conversion submodule in the six power arms synthesizes a power frequency AC voltage with stepped waveform characteristics through complementary switching of internal switching transistors. This voltage, in conjunction with the grid-side filter reactor, enables controlled bidirectional flow of active and reactive power between the system and the external three-phase medium-high voltage AC power grid.

[0077] The controller generates a high-frequency sinusoidal voltage between the positive and negative terminals of the high-frequency circuit by simultaneously superimposing high-frequency common-mode modulation waves of the same phase and amplitude onto the six power bridge arms.

[0078] The high-frequency sinusoidal voltage drives a series resonant circuit composed of a resonant capacitor, a resonant inductor, and the equivalent leakage inductance of the primary side of the matrix transformer, which makes the high-frequency current sinusoidal and couples the high-voltage side energy to the secondary side of the matrix transformer with extremely high efficiency near the resonance point.

[0079] The secondary side of the matrix transformer outputs low-voltage, high-frequency AC power, which is fed into the corresponding rectifiers for rectification. After the high-frequency ripple is filtered out by the corresponding bus capacitors, the AC power is finally collected at the low-voltage DC positive and negative buses to output stable low-voltage DC power.

[0080] As described above, this invention connects sub-modules in series to form power arms, and the upper and lower arms are directly connected at one end to form high-frequency positive and negative terminals. A resonant capacitor is connected in series with the primary branch of the matrix transformer between these high-frequency positive and negative terminals, enabling direct conversion of power frequency AC voltage to high-frequency AC voltage. After being stepped down by the matrix transformer and rectified, a low-voltage DC output is produced. This eliminates the lengthy power frequency DC intermediate links found in conventional topologies, reducing the power conversion path and significantly improving the system's energy conversion efficiency. Because the upper and lower arms respectively connect to form the positive and negative terminals of the high-frequency circuit, the resonant capacitor, after being connected in series with the primary branch of the matrix transformer, is connected between the positive and negative terminals of the high-frequency circuit on the DC side, rather than directly to the AC terminal on the grid side. This DC-side coupled resonance topology prevents the resonant capacitor from directly bearing the extremely high power frequency line voltage bias stress, effectively avoiding accelerated aging and damage of reactive components due to overvoltage, thus significantly improving the overall operational reliability of the power electronic transformer system.

[0081] In one embodiment, such as Figure 1 As shown, the AC / DC power conversion submodule of the present invention can be composed of four power devices and a filter capacitor to form a full-bridge circuit.

[0082] Four power devices form two half-bridge arms, which are connected in parallel across the filter capacitor. The midpoint of the two half-bridge arms is led out as the AC port of the AC / DC power conversion submodule.

[0083] The power devices can be insulated-gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs). The controller sends complementary pulse-width modulation (PWM) signals with dead time to the four power devices. By controlling their on and off states, the submodule can output three different voltage levels at its AC port.

[0084] During grid-connected operation, each submodule cycles through positive, negative, and zero levels, thereby synthesizing a power frequency multi-level AC voltage that closely approximates a sine wave on the series power bridge arm.

[0085] Meanwhile, during the high-frequency modulation cycle, the submodule alternately switches the power devices on and off according to the high-frequency carrier cycle, superimposing high-frequency voltage components on the bridge arm to drive the high-frequency circuit for power exchange.

[0086] This invention employs four power devices and a filter capacitor to form a full-bridge circuit, enabling the AC / DC power conversion submodule to switch arbitrarily between positive, negative, and zero voltage levels. Compared to a half-bridge submodule that can only output positive and zero voltage, the full-bridge submodule provides stronger AC / DC bidirectional power regulation capability and waveform synthesis margin when directly connected to the medium-voltage grid, significantly improving the system's synthesis accuracy for AC / DC multi-frequency hybrid modulation waveforms.

[0087] In one embodiment, such as Figure 1 and Figure 2 As shown, the matrix transformer of the present invention can be composed of multiple discrete high-frequency transformers;

[0088] The primary sides of multiple high-frequency transformers are connected in series to form a high-voltage AC port, and the secondary sides of multiple high-frequency transformers form multiple secondary AC ports.

[0089] A matrix transformer is composed of multiple physically discrete high-frequency transformers.

[0090] The primary windings of multiple discrete high-frequency transformers are connected in series, end to end, to form a single primary high-voltage AC port, which is connected between the positive and negative terminals of the high-frequency circuit. The secondary windings of the multiple discrete high-frequency transformers are independent and not connected to each other, each forming an independent secondary AC port, and each secondary AC port is physically connected to an independent rectifier.

[0091] A high-frequency sinusoidal voltage is applied to the primary winding port of the series circuit. Based on the principle of impedance and inductance sharing, this high-frequency voltage is evenly distributed across the primary windings of each discrete high-frequency transformer. Each high-frequency transformer, through alternating electromagnetic coupling, reduces the high-frequency energy distributed to its primary winding and transmits it to the corresponding secondary winding, thus achieving power shunting and balanced output to multiple low-voltage sides.

[0092] The DC-side high-frequency circuit generates high-voltage high-frequency alternating current. This high-frequency current flows through the primary windings of the series-connected discrete high-frequency transformers, establishing alternating magnetic flux in each transformer and achieving proportional voltage division. Each independent secondary winding, driven by the magnetic flux, induces low-voltage high-frequency alternating current, which is then sent to its corresponding independently connected rectifier.

[0093] The matrix transformer of this invention employs a structure in which multiple discrete high-frequency transformers are connected in series on their primary sides and their secondary sides form multiple secondary AC ports. This effectively overcomes the insulation process bottlenecks and manufacturing difficulties of a single large-capacity transformer operating under high voltage and high power conditions. The series voltage divider structure on the primary side significantly reduces the insulation withstand voltage level that a single discrete transformer's primary side needs to withstand, and the multiple outputs on the secondary side facilitate modular and uniform power distribution and heat dissipation on the low-voltage side, improving the system's combination flexibility and scalability.

[0094] In one embodiment, such as Figure 2 As shown, the AC / DC power conversion submodule of the present invention can be an energy storage submodule, which includes an energy storage unit, a filter inductor, a filter capacitor, a DC soft start circuit, multiple power devices, and a bypass switch.

[0095] Four power devices and a filter capacitor form a full-bridge circuit, and a bypass switch is connected in parallel to the AC output port of the full-bridge circuit.

[0096] The energy storage unit, the filter inductor, and the DC soft start circuit are connected in series to form the energy storage branch, which is then connected in parallel with the branch formed by the filter capacitor.

[0097] Four power devices and a filter capacitor are connected together to form a full-bridge circuit. A bypass switch is connected in parallel to the AC output port of the full-bridge circuit. The energy storage unit, filter inductor, and DC soft-start circuit are connected in series to form an independent energy storage branch. This energy storage branch, as a whole, is connected in parallel across the capacitor branch formed by the filter capacitor.

[0098] The energy storage unit can be an energy storage carrier such as a lithium-ion battery, sodium-ion battery, supercapacitor, or lithium supercapacitor. The filter inductor smooths the DC current during energy storage charging and discharging, and suppresses harmonic current injection into the energy storage unit caused by high-frequency switching. The DC soft-start circuit limits current when the energy storage unit is first connected or the system is woken up, preventing destructive overcurrent between the energy storage unit and the filter capacitor due to transient high voltage differences. The bypass switch can be a fast thyristor, vacuum contactor, or electromagnetic switch, used to quickly close the circuit when a fault is detected in this module, short-circuiting the AC terminal of the submodule and ensuring continuous operation of the entire power bridge arm series circuit.

[0099] When the energy storage system is powered on, the soft-start resistor of the DC soft-start circuit is connected in series with current limiting, and the energy storage unit slowly charges the filter capacitor until the voltage is consistent. Then, the DC soft-start circuit closes its internal switch to bypass the soft-start resistor. In grid charging mode, the system controls the power devices to chop the grid-side AC energy, converting it into DC and storing it in the capacitor. The current flows through the filter inductor to charge the energy storage unit. In discharging mode, the energy storage unit releases DC, which, after being inverted by the full-bridge power devices, can either supply energy to the grid-side AC grid or, through a high-frequency common-mode modulation wave, transfer energy to the low-voltage DC side load via the matrix transformer T. If a fault occurs inside a submodule, such as power device breakdown or overheating of the energy storage, the bypass switch closes instantaneously, short-circuiting the AC terminal of the faulty module. Energy flows through the faulty module via the bypass switch, ensuring that other normal submodules in the bridge arm operate in series.

[0100] This invention connects the energy storage branch, consisting of an energy storage unit, a filter inductor, and a DC soft-start circuit connected in series, directly in parallel across the filter capacitor of the full-bridge circuit. This achieves in-situ deep integration of the high-voltage side cascaded submodules with the energy storage system, eliminating the need for an externally mounted high-voltage independent bidirectional DC / DC converter and high-voltage power distribution equipment. This significantly reduces system hardware costs and footprint, shortens the physical stages of energy transfer during charging and discharging, and substantially improves energy dispatch efficiency. Furthermore, the parallel bypass switch ensures rapid and uninterrupted fault self-healing in the event of partial module failure, enhancing equipment reliability.

[0101] like Figure 3 As shown, the control method for the power electronic transformer system provided by the present invention includes, but is not limited to, steps S1, S2, S3 and S4.

[0102] Step S1: Calculate the low-frequency modulation voltage of the six power arms in the power electronic transformer system based on the grid-connected current and power command.

[0103] The grid-connected current is the actual power frequency sinusoidal current flowing between the power arm and the three-phase AC grid. The power command is the expected active and reactive power reference for the interaction between the system and the grid, issued by the upper-level energy dispatch system. The low-frequency modulated voltage is the power frequency modulated phase voltage reference generated on the six power arms by the system to control the grid-connected current in response to the power command.

[0104] The active and reactive current closed-loop decoupling control in a rotating orthogonal coordinate system (i.e., the dq coordinate system) can be used, or the proportional resonance (PR) control in a two-phase stationary coordinate system (i.e., the αβ coordinate system) can be used to obtain the low-frequency modulated voltage.

[0105] A reference low-frequency modulated voltage can be generated by closed-loop control of the grid-connected current in a rotating orthogonal dq coordinate system. The control formulas for its direct axis (d-axis) and quadrature axis (q-axis) are as follows:

[0106] ;

[0107] in, These are the target low-frequency modulation voltage components along the direct and quadrature axes, respectively; v gd v gq , i, and ω represent the direct-axis and quadrature-axis components of the grid voltage, respectively; w is the grid power frequency angular frequency; L is the inductance of the equivalent grid-side filter reactor; i d i q These are the direct-axis and quadrature-axis feedback quantities of the actual grid-connected current obtained from sampling, respectively; These are the direct-axis and quadrature-axis reference command values ​​of the expected grid-connected current calculated based on the input power command; K p K is the proportional gain coefficient. i is the integral gain coefficient; s is the complex frequency domain Laplace operator.

[0108] The above rotating coordinate system After inverse Parker transformation back to the three-phase stationary coordinate system, the three-phase low-frequency reference modulation voltage is obtained. Therefore, the low-frequency modulation voltages of the six power bridge arms (upper bridge arm group p, lower bridge arm group n, each containing three phases a, b, and c) are obtained:

[0109] ;

[0110] in: These are the low-frequency modulation voltages for the three phases of the upper bridge arm, A, B, and C, respectively. These are the low-frequency modulation voltages for the three phases of the lower bridge arm, A, B, and C, respectively.

[0111] Step S2: Calculate the high-frequency modulation voltage of the six power bridge arms based on the low-voltage DC bus voltage.

[0112] The low-voltage DC bus voltage is the real-time terminal voltage value at both ends of the DC side bus of the system; the high-frequency modulated voltage is a high-frequency common-mode modulated voltage signal with a frequency consistent with the resonant frequency of the resonant cavity that is superimposed on the bridge arm to control the low-voltage DC bus voltage.

[0113] The actual low-voltage DC bus voltage U can be obtained by sampling. dc With the preset target reference voltage U dcref Perform proportional-integral (PI) closed-loop control to generate the amplitude U of a high-frequency common-mode modulated wave. amp :

[0114] ;

[0115] Based on the amplitude U obtained from the closed loop amp And the resonant frequency of the high-frequency circuit, generating the instantaneous value of the common-mode high-frequency modulated voltage. .

[0116] Step S3: The low-frequency modulation voltage and the high-frequency modulation voltage are superimposed to obtain the final modulation voltage.

[0117] The final modulation voltage is the waveform of the integrated control signal for each bridge arm of the pulse width modulation generator, which is obtained by algebraically summing the low-frequency modulation voltage and the high-frequency common-mode modulation wave.

[0118] The generated low-frequency modulation voltage and high-frequency modulation voltage can be algebraically superimposed:

[0119] ;

[0120] in, These are the final modulation voltages corresponding to the upper bridge arms of phases A, B, and C, respectively. These are the final modulation voltages corresponding to the lower bridge arms of phases A, B, and C, respectively.

[0121] Step S4: Based on the final modulation voltage, the feedback grid current, the power command, and the low-voltage DC bus voltage, control the on and off of each switch in the six power bridge arms.

[0122] Based on the calculated final modulation voltage, feedback grid current, power command, and DC bus voltage, the modulation generator generates PWM drive pulses to directly control the on and off states of each switching device in the power bridge arm.

[0123] As described above, the control method of the present invention controls the switching transistor by calculating the low-frequency modulation voltage for controlling the grid-connected current and power, and the high-frequency modulation voltage for controlling the DC-side voltage, and then algebraically summing the two to generate the final modulation voltage. This achieves complete decoupled independent control of multiple voltage frequency components (power frequency and high frequency) within a single bridge arm, significantly simplifying the complexity of the system's multivariable control algorithm. It can simultaneously ensure that the grid-connected current has excellent low harmonic distortion rate and that the low-voltage DC-side bus voltage has a fast and robust closed-loop regulation response speed.

[0124] In one embodiment, step S1 may include:

[0125] The active current command is calculated by summing the power command with the low-voltage DC output power of the power electronic transformer system.

[0126] Closed-loop control is performed based on the active current command and the sampled grid current to generate the reference low-frequency modulation voltage for the three-phase power bridge arm.

[0127] Based on the three-phase power bridge arms and the reference low-frequency modulation voltage, the low-frequency modulation voltage of the six power bridge arms is generated, and the low-frequency modulation voltage, grid current, active current command, power command and low-voltage DC output power are output to the control loop for processing.

[0128] The low-voltage DC output power is the instantaneous total power consumed by the load on the low-voltage DC bus side; the active current command is the active direct axis (d-axis) reference current value used for closed-loop tracking and feedback.

[0129] The system can acquire the output voltage and load current of the low-voltage DC bus in real time and calculate its actual low-voltage DC output power. The controller directly algebraically sums the charging and discharging power commands from the upper-level energy storage battery with the DC output power to calculate the total target active power required by the system. Then, based on the current grid voltage direct-axis amplitude, it calculates the active current command for the grid-connected current. This active current command is used as a reference input, and the difference between it and the real-time grid-connected active current is sent to the PI controller to generate the power frequency reference low-frequency modulation voltage for the three-phase power bridge arm. Finally, low-frequency modulated voltages for the six bridge arms are generated, and these state variables are output to the control loop for processing.

[0130] This invention introduces low-voltage DC output power as a feedforward component, which is algebraically summed with the energy storage power command to convert into the grid-connected active current command, thus constructing a direct power feedforward response control mechanism. When the DC-side load undergoes large-scale sudden changes, the system can skip the time delay of the conventional feedback regulation loop and instantly adjust the grid-side current command for active power compensation. This fundamentally suppresses the overshoot or drop in low-voltage DC-side bus voltage caused by drastic load changes, greatly enhancing the system's transient stability and load disturbance rejection capability.

[0131] In one embodiment, the AC / DC power conversion submodule is an energy storage submodule;

[0132] The control method of the present invention may further include:

[0133] Obtain the current state of charge of each energy storage submodule;

[0134] Based on the current state of charge, the corresponding inter-arm equalization voltage, inter-phase equalization voltage, and intra-phase equalization voltage are calculated.

[0135] The equalization voltage between bridge arms, the equalization voltage between phases, and the equalization voltage within a phase are superimposed on the low-frequency modulation voltage to update the low-frequency modulation voltage corresponding to each energy storage submodule, and the current state of charge is used as the feedback quantity for closed-loop control for modulation control.

[0136] correspond Figure 2The current state of charge (SOC) is the percentage of remaining energy in each individual energy storage submodule; the inter-arm equalization voltage is the regulating voltage injected to eliminate the average SOC deviation between upper and lower arms in the same phase due to the difference in battery impedance; the inter-phase equalization voltage is the zero-sequence regulating voltage injected to eliminate the overall uneven energy storage between phases A, B, and C; the intra-phase equalization voltage (also known as inter-module voltage compensation) is the voltage compensation command calculated for a single submodule to eliminate the SOC divergence between series submodules within the same arm.

[0137] It can obtain the current actual state of charge (SOC) of each submodule. pjk (representing the current SOC of the k-th submodule of the j-th phase upper arm, where j∈{a,b,c}), and calculated in three spatial dimensions respectively:

[0138] Inter-arm balancing: The current average state of charge deviation between the upper and lower arms is used to calculate and generate a balancing circulating current command. The control system tracks this circulating current in a closed loop to generate an inter-arm balancing voltage.

[0139] Phase-to-phase equalization: The phase-to-phase equalization voltage is calculated by performing coordinate transformation on the current average state of charge of the three-phase energy storage and injecting the fundamental zero-sequence voltage.

[0140] Intra-phase (inter-submodule) balancing: Taking the kth submodule of the upper arm as an example, based on its current state of charge (SOC)... pjk The average SOC of the bridge arm of this phase pj The difference between the current i and the actual current i flowing through the current bridge arm pj The product is used to calculate and output the intra-phase equalization compensation voltage Δu corresponding to each submodule. pjk :

[0141] ;

[0142] Where: K is the intra-phase equilibrium adjustment coefficient; SOC pj The average actual state of charge (SNP) of all energy storage submodules in the upper arm of phase j; i pj This is the current actual AC power frequency current feedback quantity of this bridge arm.

[0143] Finally, the calculated three-level equalization voltages between bridge arms, between phases, and within phases are directly linearly superimposed onto the low-frequency modulation voltage of the corresponding energy storage submodule to achieve fine-tuning of the final drive waveform.

[0144] This invention acquires the state of charge (SOC) of each series-connected energy storage submodule and calculates and modulates the three-level equalization voltage between bridge arms, between phases, and within phases. This achieves highly dynamic, multi-dimensional distributed energy storage SOC fully closed-loop automatic balance regulation, which can eliminate the long-term SOC inconsistency caused by differences in cell manufacturing and ambient operating temperature in cascaded energy storage systems. It also avoids overcharging or over-discharging of individual cells, greatly extending the lifespan of the energy storage system while ensuring the safe and continuous operation of the energy storage transformer.

[0145] In one embodiment, calculating the corresponding inter-arm equalization voltage based on each current state of charge may include:

[0146] The amplitude of the circulating current in the upper and lower bridge arms is calculated based on the difference in the current average state of charge of the corresponding phases of the energy storage bridge arms in the upper and lower bridge arm groups.

[0147] The phase of the circulating current in the upper and lower bridge arms is calculated based on the power frequency angular frequency and phase angle, and a circulating current command is generated based on the amplitude and phase.

[0148] The deviation between the circulating current command and the actual circulating current of each energy storage arm obtained from sampling is input to the closed-loop controller for control, generating an equal voltage between the arms, and dynamically tracking the difference in the current average state of charge based on the phase angle and power frequency during the control process.

[0149] The common-mode circulating current of the upper and lower bridge arms is the internal common-mode AC current component flowing between the upper and lower bridge arms of the same phase. It is not manifested in the external circuit and on the grid-connected side and does not interfere with the power grid. The circulating current command is the target common-mode current waveform that the system expects to establish within the phase to achieve energy distribution between the upper and lower arms.

[0150] The current average state of charge (SOC) of the energy storage in the lower arm of phase j can be used as a reference. nj Compared with the current average state of charge (SOC) of the upper arm energy storage pj The deviation is used to calculate and generate a reference command for the amplitude of the common-mode fundamental circulation through a proportional-integral (PI) closed-loop controller. :

[0151] ;

[0152] In the formula: K p K is the proportionality coefficient. i is the integral coefficient; s is the Laplace operator.

[0153] Based on the calculated amplitude command The system's operating angular frequency w and phase angle φ cirj (Obtained through dynamic conversion of the power grid phase-locked angle), the instantaneous target command value for generating the fundamental frequency circulating current. :

[0154] ;

[0155] The target command is used to perform closed-loop control adjustment with the actual internal circulating current difference between the upper and lower bridge arms in the same phase, which is obtained by actual sampling and calculation from the AC transformer. The corresponding control voltage component is generated as the equalization voltage between the bridge arms, and the SOC deviation is dynamically tracked and leveled during modulation.

[0156] This invention calculates the circulating current amplitude based on the difference in average state of charge between the upper and lower bridge arms, and generates instantaneous circulating current commands based on angular frequency and phase angle to perform closed-loop control and generate balanced voltage between bridge arms. This enables flexible transfer of active power between the upper and lower bridge arms within a phase without changing or deteriorating the grid-connected power quality, effectively mitigating the power imbalance between the upper and lower half of the bridge arms.

[0157] In one embodiment, calculating the amplitude of the circulating current in the upper and lower bridge arms based on the difference in the current average state of charge of the corresponding phases of the energy storage bridge arms in the upper and lower bridge arm groups may include:

[0158] The difference between the current average state of charge of the lower arm and the current average state of charge of the upper arm in the same phase is used to obtain the state of charge deviation.

[0159] The state of charge deviation is subjected to proportional-integral control to calculate the amplitude of the circulating current in the upper and lower bridge arms, and the circulating current modulation control is performed based on the amplitude. In the proportional-integral control, the state of charge deviation is used as the input feedback quantity.

[0160] State of charge deviation is the deviation between the current average SOC of energy storage in the lower arm of the same phase and the current average SOC of energy storage in the upper arm.

[0161] Circulation amplitude command The closed-loop control law formula is:

[0162] ;

[0163] In each algorithm loop timer interrupt, the controller directly calculates the state-of-charge (SOC) deviation of the j-th phase. nj -SOC pj The deviation is used as the sole closed-loop feedback input to the proportional-integral (PI) controller. After proportional amplification and integral accumulation, the amplitude reference command of the common-mode circulating current of that phase is directly output. And based on this parameter, precise chopping modulation of the circulating current is carried out.

[0164] This invention inputs the state-of-charge (SOC) deviation of the upper and lower arms of the same physical phase into a proportional-integral (PI) controller for closed-loop generation of circulating current amplitude, achieving automatic SOC energy alignment of the upper and lower arms without steady-state error. The introduction of the integral stage automatically eliminates systematic steady-state errors caused by battery internal resistance estimation errors, temperature drift of sampling elements, and uneven parasitic impedance of the arms, greatly improving the disturbance rejection robustness and control accuracy of the energy transfer control loop.

[0165] In one embodiment, calculating the corresponding interphase equalization voltage based on each current state of charge may include:

[0166] Based on the current state of charge of the three-phase energy storage bridge arm, coordinate transformation is performed to obtain the active and reactive components of the current state of charge.

[0167] The preset capacitor voltage reference value is subtracted from the active component and the reactive component respectively to obtain the component difference value;

[0168] By utilizing the operational relationships of proportional-integral control, coordinate transformation, and inverse coordinate transformation, a closed-loop operation is performed on the component difference to obtain the rotating component. Based on the rotating component, an interphase equalization voltage is generated, which, together with a preset capacitor voltage reference value, is used as a feedback quantity to act on the three-phase energy storage bridge arm.

[0169] Coordinate transformation is the process of transforming the average state of charge (SOC) in a three-phase stationary coordinate system. a SOC b SOC c The mathematical mapping process of transforming to the synchronous rotating dual-axis orthogonal dq coordinate system (including Clarke transformation and Parker transformation); the rotation component is the DC deviation compensation voltage component between the direct axis and the quadrature axis generated after closed-loop operation adjustment in the dq rotating coordinate system.

[0170] First, calculate the average state of charge (SOC) of each of the three-phase energy storage arms. a SOC b SOC c Then, the Parker transformation matrix is ​​used to project it onto the rotating dq coordinate system to obtain the direct-axis component SOC of the interphase charge state in the rotating coordinate system. phd and cross-axis component SOC phq The preset capacitor voltage / SOC balance reference value (SOC) is used. ref Compare with the obtained direct-axis component SOC respectively phd and cross-axis component SOC phq The component differences are calculated. A proportional-integral (PI) controller is used to perform closed-loop calculations on the component differences, and the rotation component is output. and :

[0171] ;

[0172] The obtained rotational components are subjected to an inverse Parker transformation to generate a zero-sequence phase angle reference for power frequency AC in a stationary coordinate system. This signal is the phase-to-phase equalization voltage, which is directly superimposed on the low-frequency modulation wave of the three-phase bridge arm and used in conjunction with the capacitor voltage reference value as feedback to dynamically adjust the three-phase active power distribution.

[0173] This invention transforms the three-phase energy storage state of charge (SOC) to a rotating dq-axis system. In this DC-domain coordinate system, a zero-sequence voltage (phase-to-phase equalization voltage) is generated through closed-loop calculation and inverse transformation using a proportional-integral (PI) controller. This transforms the multivariable coupled phase-to-phase energy balance problem into a single-variable independent decoupled control on the DC / AC rotating axes. By utilizing the fundamental zero-sequence voltage injection mechanism, it can achieve precise redistribution of the active power of three-phase energy storage and high-precision leveling of the phase-to-phase SOC without changing the grid-side line voltage and grid-connected current quality.

[0174] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A power electronic transformer system, characterized in that, Includes AC / DC power conversion submodules, grid-side filter reactors, AC soft start circuits, resonant capacitors, resonant inductors, matrix transformers, multiple rectifiers, and multiple bus capacitors: The AC ports of several AC / DC power conversion submodules are connected in series to form a power bridge arm. The six power bridge arms are divided into an upper bridge arm group and a lower bridge arm group. Both the upper bridge arm group and the lower bridge arm group include three power bridge arms. One end of each of the three power arms of the upper bridge arm group is connected to form the positive terminal of the high-frequency circuit, and one end of each of the three power arms of the lower bridge arm group is connected to form the negative terminal of the high-frequency circuit. The other ends of the three power arms of the upper bridge arm group are respectively connected to one end of the three-phase grid-side filter reactor through the corresponding resonant inductor; The other ends of the three power arms of the lower arm group are respectively connected to one end of the three-phase grid-side filter reactor through the corresponding resonant inductor, and the other end of the three-phase grid-side filter reactor is connected to the external three-phase AC power grid through the AC soft start circuit. The resonant capacitor and the primary branch of the matrix transformer are connected in series and then connected between the positive terminal and the negative terminal of the high-frequency circuit. The AC ports of the multiple rectifiers are respectively connected to the multiple secondary AC ports of the matrix transformer, and the DC ports of the rectifiers are respectively connected to the corresponding bus capacitors. The positive and negative terminals of each bus capacitor are combined and connected to the low-voltage DC bus to form the DC port of the power electronic transformer system.

2. The power electronic transformer system according to claim 1, characterized in that, The AC / DC power conversion submodule consists of a full-bridge circuit composed of four power devices and a filter capacitor.

3. The power electronic transformer system according to claim 1, characterized in that, The matrix transformer is composed of multiple discrete high-frequency transformers; The primary sides of multiple high-frequency transformers are connected in series to form a high-voltage AC port, and the secondary sides of multiple high-frequency transformers form multiple secondary AC ports.

4. The power electronic transformer system according to claim 1, characterized in that, The AC / DC power conversion submodule is an energy storage submodule, which includes an energy storage unit, a filter inductor, a filter capacitor, a DC soft start circuit, multiple power devices, and a bypass switch. The four power devices and the filter capacitor form a full-bridge circuit, and the bypass switch is connected in parallel to the AC output port of the full-bridge circuit. The energy storage unit, the filter inductor, and the DC soft start circuit are connected in series to form an energy storage branch, which is connected in parallel with the branch formed by the filter capacitor.

5. A control method for a power electronic transformer system according to any one of claims 1-4, characterized in that, include: Based on the grid connection current and power command, the low-frequency modulation voltage of the six power arms in the power electronic transformer system is calculated. The high-frequency modulation voltage of the six power bridge arms is calculated based on the low-voltage DC bus voltage. The final modulation voltage is obtained by superimposing the low-frequency modulation voltage and the high-frequency modulation voltage. The switching on and off of each switch in the six power bridge arms is controlled based on the final modulation voltage, the feedback grid current, the power command, and the low-voltage DC bus voltage.

6. The control method according to claim 5, characterized in that, Based on the grid-connected current and power command, the low-frequency modulation voltages of the six power arms in the power electronic transformer system are calculated, including: The active current command is calculated by summing the power command with the low-voltage DC output power of the power electronic transformer system. Closed-loop control is performed based on the active current command and the sampled grid-connected current to generate a reference low-frequency modulation voltage for the three-phase power bridge arm; Based on the three-phase power bridge arm and the reference low-frequency modulation voltage, the low-frequency modulation voltage of the six power bridge arms is generated, and the low-frequency modulation voltage, the grid-connected current, the active current command, the power command, and the low-voltage DC output power are output to the control loop for processing.

7. The control method according to claim 5, characterized in that, The AC / DC power conversion submodule is an energy storage submodule; The control method further includes: Obtain the current state of charge of each of the energy storage submodules; Based on the current state of charge of each bridge arm, the corresponding inter-arm equalization voltage, inter-phase equalization voltage, and intra-phase equalization voltage are calculated. The inter-arm equalization voltage, the inter-phase equalization voltage, and the intra-phase equalization voltage are superimposed on the low-frequency modulation voltage to update the low-frequency modulation voltage corresponding to each energy storage submodule, and the current state of charge is used as the feedback quantity for closed-loop control for modulation control.

8. The control method according to claim 7, characterized in that, Based on the current state of charge of each arm, the corresponding inter-arm equalization voltage is calculated, including: The amplitude of the circulating current in the upper and lower bridge arms is calculated based on the difference in the current average state of charge of the corresponding phases of the energy storage bridge arms in the upper and lower bridge arm groups. The phase of the circulating current in the upper and lower bridge arms is calculated based on the power frequency angular frequency and phase angle, and a circulating current command is generated based on the amplitude and the phase. The deviation between the circulating current command and the actual circulating current of each energy storage arm obtained by sampling is input to the closed-loop controller for control, generating the equalization voltage between the arms, and dynamically tracking the difference between the current average state of charge based on the phase angle and the power frequency during the control process.

9. The control method according to claim 8, characterized in that, The amplitude of the circulating current in the upper and lower bridge arms is calculated based on the difference in the current average state of charge of the corresponding phases of the energy storage bridge arms in the upper and lower bridge arm groups, including: The difference between the current average state of charge of the lower arm and the current average state of charge of the upper arm in the same phase is used to obtain the state of charge deviation. The state of charge deviation is subjected to proportional-integral control to calculate the amplitude of the circulating current of the upper and lower bridge arms, and the circulating current modulation control is performed based on the amplitude, wherein the state of charge deviation is used as the input feedback quantity in the proportional-integral control.

10. The control method according to claim 7, characterized in that, Based on the current state of charge of each phase, the corresponding phase-to-phase equalization voltage is calculated, including: Based on the current state of charge of the three-phase energy storage bridge arm, coordinate transformation is performed to obtain the active and reactive components of the current state of charge. The preset capacitor voltage reference value is subtracted from the active component and the reactive component respectively to obtain the component difference value; Using the proportional-integral control, the coordinate transformation and inverse coordinate transformation, a closed-loop operation is performed on the component difference to obtain the rotation component. The phase-to-phase equalization voltage is then generated based on the rotation component, and the phase-to-phase equalization voltage and the preset capacitor voltage reference value are used together as feedback quantities to act on the three-phase energy storage bridge arm.

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