Solid state transformer and load side self-current-sharing control method and system thereof

Abstract

The application provides a solid-state transformer and a load-side self-current-sharing control method and system thereof, and relates to the field of power electronic converters.The method steps are as follows: step S1 signal acquisition, step S2 average value calculation, step S3 voltage outer loop control, step S4 current inner loop control, step S5 each module feedforward compensation and control quantity generation, and step S6 dynamic self-current-sharing adjustment.Through a three-layer composite control architecture of "voltage outer loop for output voltage, current inner loop for total current, and DC voltage feedforward for current sharing", the DC voltage inside each module is used as a current sharing proxy variable, and under the premise of no high-speed communication between modules, high-precision, fast-response, and strong-robust self-current-sharing control of the solid-state transformer output side multi-module parallel connection is realized, the system reliability is significantly improved, the hardware cost is reduced, and the method is widely applicable to key scenes such as AI data center DC power supply, intelligent microgrid, and new energy grid connection.

Description

Technical Field

[0001] This invention relates to the field of power electronic converters, and more specifically, to solid-state transformers and their load-side self-current sharing control methods and systems. Background Technology

[0002] Solid-state transformers (SSTs) can rapidly adjust power flow and amplitude through power electronic conversion technology, smoothing power generation fluctuations and significantly improving the grid's ability to absorb new energy sources. Simultaneously, SSTs can also be used to build smart microgrids. In industrial parks, remote areas, and other scenarios, SSTs can achieve interconnection between multiple microgrids and seamless switching between microgrids and the main grid, effectively ensuring power supply reliability. An 800V DC power supply system using SSTs as key conversion equipment effectively reduces power transmission losses and copper usage by minimizing AC / DC conversion stages and increasing voltage levels, improving the overall end-to-end operating efficiency of the system and significantly saving space. Its technical economy and voltage adaptability are well demonstrated. Currently, to enable SSTs to directly draw power from 10kV or 35kV grids, isolated SSTs commonly employ three design approaches: first, using series-connected semiconductor devices; second, using a modular multilevel converter structure; and third, using an input series and output parallel or input series and output series structure.

[0003] However, existing isolated solid-state transformers generally rely on the design approach of series-connected semiconductor devices, which is severely limited by the technological development of the semiconductor devices themselves. Although 10kV wide-bandgap semiconductor devices are available on the market, they are expensive and difficult to widely apply. In existing solid-state transformer designs based on MMC and ISOP, the voltage equalization of each DC-link capacitor is a critical issue due to the use of multi-chain series connection on the high-voltage side of the input. Simultaneously, how to achieve output current sharing when the modules are connected in parallel is another key issue for reliable system operation. Existing technologies mostly focus on solving the input-side voltage equalization problem, while lacking effective solutions for the current sharing problem of multiple modules connected in parallel on the output side. When the modules on the output side are directly connected in parallel, the differences in parameters, losses, and control delays of the DC / DC converters of each module will lead to an imbalance in the output current of each module. Excessive output current in some modules may cause overcurrent damage to switching devices; insufficient output current in some modules will prevent full utilization of their power capacity, reducing the overall system efficiency. Summary of the Invention

[0004] The present invention aims to solve the problem of uneven output current of parallel modules on the output side of existing solid-state transformers.

[0005] To address the aforementioned problems, this invention provides a self-current sharing control method for the load side of a solid-state transformer, applied to the output side of the solid-state transformer. The solid-state transformer comprises n power modules, each power module having an AC / DC rectifier stage, a DC / DC isolation stage, and a DC link capacitor connected between the AC / DC rectifier stage and the DC / DC isolation stage. The DC output terminals of the n power modules are directly connected in parallel to serve as the DC bus on the load side. The method steps are as follows: Step S1: Signal acquisition. Acquire the voltage across the DC link capacitor inside each power module, denoted as Vdc1, Vdc2, ..., Vdcn; acquire the total current signal Iout and voltage signal Vout at the DC output terminal on the load side. Step S2: Average value calculation. Sum Vdc1 to Vdcn and divide by n to obtain the average voltage Vavg of the DC link. Step S3: Voltage outer loop control. Set the target value Vref of the DC output voltage on the load side, and subtract Vref from Vout to obtain the voltage error value. V, will After V is regulated by the first PI regulator, a current reference signal Iref is generated for the inner current loop. Step S4: Inner current loop control, subtract Iref from Iout to obtain the current error value. I will I is adjusted by the second PI controller to obtain the basic adjustment value φref; Step S5: Feedforward compensation and control quantity generation for each module. For the i-th power module (i=1,...,n), calculate the DC voltage error value of that module. Vdci = Vavg - Vdci After Vdci is regulated by the third PI regulator, the voltage compensation amount Ucom_i is obtained. Then, the final control amount φi=φref-Ucom_i or φi=φref+Ucom_i is calculated, and φi is used to generate the control signal to drive the DC / DC isolation stage switch of the module. Step S6: Dynamic self-current equalization adjustment. Through the closed-loop control of steps S1 to S5, the output current of each power module is dynamically balanced.

[0006] The massage mechanism provided by this invention has, but is not limited to, the following beneficial effects compared to existing technologies: This invention indirectly reflects the degree of imbalance in output current by detecting the deviation of the DC voltage Vdci inside each module, and automatically adjusts the phase shift angle or switching frequency of the DC / DC converter of each module so that the output current of each module dynamically converges to a balanced state.

[0007] This invention achieves decoupled control with the dual objectives of stable output voltage and balanced output current. The outer voltage loop ensures that the output voltage Vout strictly tracks the given value Vref with no steady-state error. The inner current loop ensures that the total output current meets the load power requirements. The current sharing compensation loop, based on macroscopic power distribution, performs microscopic corrections to each module, achieving proportional current sharing among modules while meeting the total power requirements. The three loops have separated bandwidths and decoupled functions, preventing interference between them and facilitating engineering tuning.

[0008] This invention eliminates the need for high-speed communication between modules, reducing system complexity and cost. It also eliminates the need for high-speed communication interfaces and protocol stacks, thus lowering hardware costs. Furthermore, it eliminates the impact of communication latency on dynamic current sharing performance. A single module's communication failure will not propagate to other modules, significantly improving system robustness.

[0009] This invention utilizes an internal DC voltage as a proxy variable for current sharing, achieving indirect yet precise current sharing control. It eliminates the need for current sensors at the output of each module, significantly reducing the number of sensors required. Existing DC-link voltage sensors are reused, resulting in zero incremental hardware cost. The physical relationship is clear, the control polarity is inherently correct, and positive feedback oscillations are avoided.

[0010] This invention features fast dynamic response, high steady-state accuracy, fully analog / digital local control, no communication delay, and a response speed that is 1-2 orders of magnitude faster.

[0011] This invention utilizes a three-layer composite control architecture of "external environmental protection output voltage, internal environmental protection total current, and DC voltage feedforward to ensure current sharing". By using the internal DC voltage of each module as a proxy variable for current sharing, it achieves high-precision, fast-response, and robust self-current sharing control of multiple modules in parallel on the output side of the solid-state transformer without the need for high-speed communication between modules. This significantly improves system reliability, reduces hardware costs, and is widely applicable to key scenarios such as DC power supply for AI data centers, smart microgrids, and grid connection of new energy sources.

[0012] Furthermore, step S5 specifically involves: Step S51: Calculate the DC voltage error value of the module. Vdci, i.e. Vdci = Vavg - Vdci; Step S52, to Vdci is fed into the third PI regulator, that is, the independent PI regulator of each module is adjusted to obtain the voltage compensation amount Ucom_i of the module. Step S53: Calculate the final control quantity φi of the module, i.e., φi = φref - Ucom_i, or according to the polarity design, φi = φref + Ucom_i; Step S54: Send the final control quantity φi to the pulse width modulation (PWM) generator or phase shift modulator of the module to generate the control signal driving the DC / DC isolation stage switch in the module; for dual active bridge (DAB) topology, φi is the phase shift angle; for LLC resonant converter topology, φi is the switching frequency or duty cycle.

[0013] Furthermore, when the output current Iouti of the i-th power module is too small in step S5, the specific adjustment process is as follows: Energy will accumulate on the DC / DC input side of this power module, resulting in a higher internal DC voltage Vdci for this power module. A higher Vdci results in a slightly higher average DC link voltage Vavg, but the difference between Vavg and Vdci is... Vdci decreases, meaning a positive value decreases or becomes negative; After Vdci is regulated by the third PI regulator, the output voltage compensation amount Ucom_i decreases accordingly or even reverses. According to the relationship φi=φref-Ucom_i, a decrease in Ucom_i leads to an increase in the final control quantity φi; Increasing φi increases the phase shift angle of the DC / DC converter or changes the switching frequency, thereby increasing the output power of the module. The output current Iouti of the module is increased, while Vdci decreases until equilibrium is restored.

[0014] Furthermore, when the output current Iouti of the i-th power module is too large in step S5, the specific adjustment process is as follows: The power module will lack power on the DC / DC input side, resulting in a low internal DC voltage Vdci for the power module. A low Vdci results in a slightly lower average DC link voltage Vavg, but the difference between Vavg and Vdci is... Vdci increases; After Vdci is regulated by the third PI regulator, the output voltage compensation amount Ucom_i increases accordingly; According to the relationship φi=φref-Ucom_i, an increase in Ucom_i leads to a decrease in the final control quantity φi; Decreasing φi reduces the phase shift angle of the DC / DC converter or changes the switching frequency, thereby reducing the output power of the module. The output current Iouti of the module is lowered, while Vdci rises until equilibrium is restored.

[0015] Furthermore, the first PI controller, the second PI controller, and the third PI controller all adopt a discrete incremental PID algorithm or a positional PID algorithm, and the proportional coefficient and integral coefficient of each PI controller are independently tuned according to the system's dynamic response requirements and stability requirements.

[0016] Furthermore, in step S5, the DC / DC isolation stage switch adopts a dual active bridge DAB topology. The final control quantity φi of the dual active bridge DAB topology is the phase shift angle between the primary and secondary sides of the DAB converter. The magnitude and direction of the transmitted power are controlled by adjusting the phase shift angle.

[0017] Furthermore, in step S5, the DC / DC isolation stage switch adopts an LLC resonant converter topology. The final control quantity φi of the LLC resonant converter topology is the switching frequency or duty cycle, and the output power is controlled by adjusting the switching frequency or duty cycle.

[0018] Furthermore, in step S1, the DC current sampling sensor installed at the DC output terminal on the load side is a Hall effect current sensor or a shunt resistor current sensor; the DC voltage sampling sensor installed at the DC output terminal on the load side in step S1 is a resistor voltage divider voltage sensor or an isolated voltage sensor.

[0019] A solid-state transformer system based on load-side self-current sharing control function includes: The main power circuit of the solid-state transformer comprises n power modules, each with an AC / DC rectifier stage, a DC / DC isolation stage, and a DC link capacitor connecting the two. The DC output terminals of the n power modules are directly connected in parallel to form a load-side DC bus. A DC current sampling sensor CT group is installed at the load-side DC output terminal to collect the total current signal Iout. A DC voltage sampling sensor PT group is installed at the load-side DC output terminal to collect the voltage signal Vout. A voltage sensor is installed inside each power module to collect its respective DC link voltage Vdc1, Vdc2, ..., Vdcn. A digital controller executes the above-described solid-state transformer method based on load-side self-current sharing control function to generate control signals for the DC / DC isolation stages of each power module.

[0020] A solid-state transformer includes a main power circuit and a three-level control system; the input terminal of the main power circuit is connected to a medium- and high-voltage power grid, and the main power circuit is connected to the three-level control system. The main power circuit includes power module valve groups; the power module valve groups include A-phase power module valve group stacks, B-phase power module valve group stacks and C-phase power module valve group stacks, which are connected in parallel, and each phase power module valve group stack contains multiple power modules. Each power module is equipped with three H-bridge circuits, one of which is an AC / DC rectifier module, and the other two are DC / DC chopper modules. The two DC / DC chopper modules are respectively located on the primary side and the secondary side of the isolation transformer. The three-level control system includes a main controller, three phase controllers, and multiple module controllers. The main power circuit is signal-connected to the main controller. The three phase controllers are connected one-to-one with the valve groups of the A-phase power module, B-phase power module, and C-phase power module. The phase controllers are bidirectionally connected to the main controller via a pair of fiber optic interfaces. The phase controllers are bidirectionally connected to the phase controllers of the other two phases via two pairs of fiber optic interfaces. The phase controllers are connected to multiple module controllers of their respective phases via multiple pairs of fiber optic interfaces. The module controllers and power modules are signal-connected one-to-one. The isolation transformer adopts the solid-state transformer system based on the load-side self-current sharing control function described above. The DC output terminals of each power module are directly connected in parallel, and the automatic balancing of the output current is achieved through the aforementioned method. Attached Figure Description

[0021] Figure 1 This is a logic control diagram of the solid-state transformer load-side self-current sharing control method according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the main power circuit of the solid-state transformer according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the main controller interface of the solid-state transformer according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the phase controller interface of the solid-state transformer according to an embodiment of the present invention; Figure 5 This is a schematic diagram showing the module controller interface, drive adapter circuit, and module power circuit connection of the solid-state transformer according to an embodiment of the present invention. Figure 6 This is a schematic diagram showing the numbering of the DAB circuit and the H-bridge SiCMOSFET device in Embodiment 1 of the present invention; Figure 7 This is a timing diagram of the single-phase shift control of the DAB circuit in Embodiment 1 of the present invention; Figure 8 This is a graph showing the relationship between transmission power and phase shift angle under single-phase-shift control of the DAB circuit in Embodiment 1 of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this application clearer, specific embodiments of this application are described clearly and completely below with reference to the accompanying drawings. It should be understood that the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments described in this application without creative effort will fall within the scope of protection of this application.

[0023] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in the specification of this application is for the purpose of describing specific embodiments only and is not intended to limit this application; the terms "comprising," "including," "having," "containing," "comprise," etc., in the specification, claims, and accompanying drawings of this application are open-ended terms, indicating that a method comprises one or more steps, or an apparatus comprises one or more elements, but do not exclude the inclusion of other steps or elements. The terms "first," "second," etc., in the specification, claims, or accompanying drawings of this application are used to distinguish different objects, not to describe a specific order or primary / secondary relationship. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0024] In the description of this application, it should be understood that the terms "upper", "lower", "left", "right", "front", "rear", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0025] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "attachment" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0026] In this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this application, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0027] See Figures 1-5 This invention discloses a self-current sharing control method for the load side of a solid-state transformer, applied to the output side of the solid-state transformer. The solid-state transformer comprises n power modules, each power module having an AC / DC rectifier stage, a DC / DC isolation stage, and a DC link capacitor connected between the AC / DC rectifier stage and the DC / DC isolation stage; the DC output terminals of the n power modules are directly connected in parallel to serve as the DC bus on the load side; the method steps are as follows: Step S1: Signal acquisition. Acquire the voltage across the DC link capacitor inside each power module, denoted as Vdc1, Vdc2, ..., Vdcn; acquire the total current signal Iout and voltage signal Vout at the DC output terminal on the load side. Step S2: Average value calculation. Sum Vdc1 to Vdcn and divide by n to obtain the average voltage Vavg of the DC link. Step S3: Voltage outer loop control. Set the target value Vref of the DC output voltage on the load side, and subtract Vref from Vout to obtain the voltage error value. V, will After V is regulated by the first PI regulator, a current reference signal Iref is generated for the inner current loop. Step S4: Inner current loop control, subtract Iref from Iout to obtain the current error value. I will I is adjusted by the second PI controller to obtain the basic adjustment value φref; Step S5: Feedforward compensation and control quantity generation for each module. For the i-th power module (i=1,...,n), calculate the DC voltage error value of that module. Vdci = Vavg - Vdci After Vdci is regulated by the third PI regulator, the voltage compensation amount Ucom_i is obtained. Then, the final control amount φi=φref-Ucom_i or φi=φref+Ucom_i is calculated, and φi is used to generate the control signal to drive the DC / DC isolation stage switch of the module. Step S6: Dynamic self-current equalization adjustment. Through the closed-loop control of steps S1 to S5, the output current of each power module is dynamically balanced.

[0028] In step S1 of this invention, a voltage sensor is set inside each power module to collect the voltage across each DC link capacitor in real time, denoted as the internal DC voltage Vdc1, Vdc2, ..., Vdcn; a DC current sampling sensor CT group is set at the DC output terminal on the load side to collect the total current signal Iout at the DC output terminal on the load side; and a DC voltage sampling sensor PT group is set at the DC output terminal on the load side to collect the voltage signal Vout at the DC output terminal on the load side. Step S2: Sum the n internal DC voltages collected and divide by n to obtain the average voltage Vavg of the DC link, i.e.: Vavg=(Vdc1+Vdc2+...+Vdcn) / n; Step S3 sets the target value Vref of the DC output voltage on the load side. This target value is set according to the requirements of the downstream load, such as 800V, 400V, etc. The target value Vref is subtracted from the actual value Vout to obtain the voltage error value. V, that is: V = Vref - Vout; This represents the voltage error value. V is fed into the first PI regulator for adjustment, and the voltage outer loop regulation output is obtained. This output is used as the current reference signal Iref of the current inner loop. Step S4: Subtract the current reference signal Iref from the acquired total DC output current signal Iout from the load side to obtain the current error value. I, that is: I = Iref - Iout; This sets the current error value. I is fed into the second PI controller for adjustment, and the basic adjustment amount φref is obtained; Step S5: For the i-th power module, i=1,...,n, calculate the DC voltage error value of that module. Vdci = Vavg - Vdci After Vdci is regulated by the third PI regulator, the voltage compensation amount Ucom_i is obtained. Then, the final control amount φi=φref-Ucom_i or φi=φref+Ucom_i is calculated, and φi is used to generate the control signal to drive the DC / DC isolation stage switch of the module. Step S6, through the closed-loop control of steps S1 to S5 above, when the output current of a certain power module deviates from the balance value, the internal DC voltage Vdci of the module will change accordingly. Then, the control logic automatically adjusts the power transmission capability of the DC / DC converter of the module, so that the module with a small output current increases the output power and the module with a large output current decreases the output power, thereby achieving dynamic balance of the output current of all power modules.

[0029] This invention indirectly reflects the degree of imbalance in output current by detecting the deviation of the DC voltage Vdci inside each module, and automatically adjusts the phase shift angle or switching frequency of the DC / DC converter of each module so that the output current of each module dynamically converges to a balanced state.

[0030] This invention achieves decoupled control with the dual objectives of stable output voltage and balanced output current. The outer voltage loop ensures that the output voltage Vout strictly tracks the given value Vref with no steady-state error. The inner current loop ensures that the total output current meets the load power requirements. The current sharing compensation loop, based on macroscopic power distribution, performs microscopic corrections to each module, achieving proportional current sharing among modules while meeting the total power requirements. The three loops have separated bandwidths and decoupled functions, preventing interference between them and facilitating engineering tuning.

[0031] This invention eliminates the need for high-speed communication between modules, reducing system complexity and cost. It also eliminates the need for high-speed communication interfaces and protocol stacks, thus lowering hardware costs. Furthermore, it eliminates the impact of communication latency on dynamic current sharing performance. A single module's communication failure will not propagate to other modules, significantly improving system robustness.

[0032] This invention utilizes an internal DC voltage as a proxy variable for current sharing, achieving indirect yet precise current sharing control. It eliminates the need for current sensors at the output of each module, significantly reducing the number of sensors required. Existing DC-link voltage sensors are reused, resulting in zero incremental hardware cost. The physical relationship is clear, the control polarity is inherently correct, and positive feedback oscillations are avoided.

[0033] This invention features fast dynamic response, high steady-state accuracy, fully analog / digital local control, no communication delay, and a response speed that is 1-2 orders of magnitude faster.

[0034] This invention utilizes a three-layer composite control architecture of "external environmental protection output voltage, internal environmental protection total current, and DC voltage feedforward to ensure current sharing". By using the internal DC voltage of each module as a proxy variable for current sharing, it achieves high-precision, fast-response, and robust self-current sharing control of multiple modules in parallel on the output side of the solid-state transformer without the need for high-speed communication between modules. This significantly improves system reliability, reduces hardware costs, and is widely applicable to key scenarios such as DC power supply for AI data centers, smart microgrids, and grid connection of new energy sources.

[0035] Furthermore, step S5 specifically involves: Step S51: Calculate the DC voltage error value of the module. Vdci, that is: Vdci = Vavg - Vdci; Step S52, to Vdci is fed into the third PI regulator, that is, the independent PI regulator of each module is adjusted to obtain the voltage compensation amount Ucom_i of the module. Step S53: Calculate the final control quantity φi of the module, i.e.: φi=φref-Ucom_i, or according to the polarity design, φi=φref+Ucom_i; Step S54: Send the final control quantity φi to the pulse width modulation (PWM) generator or phase shift modulator of the module to generate the control signal driving the DC / DC isolation stage switch in the module; for dual active bridge (DAB) topology, φi is the phase shift angle; for LLC resonant converter topology, φi is the switching frequency or duty cycle.

[0036] Steps S51-S54 of this invention, through a closed-loop link of "Vdci deviation detection → PI compensation → superposition with φref → topology mapping", achieve high-precision, zero steady-state error, and fast-response self-current sharing control of multiple DC / DC modules in parallel on the output side of the solid-state transformer without the need for high-speed communication between modules and additional current sensors. It is also compatible with various mainstream DC / DC topologies such as DAB, LLC, and PSFB. This is the core innovative execution link that distinguishes this invention from existing current sharing technologies.

[0037] Furthermore, when the output current Iouti of the i-th power module is too small in step S5, the specific adjustment process is as follows: Energy will accumulate on the DC / DC input side of this power module, resulting in a higher internal DC voltage Vdci for this power module. A higher Vdci results in a slightly higher average DC link voltage Vavg, but the difference between Vavg and Vdci is... Vdci decreases, meaning a positive value decreases or becomes negative; After Vdci is regulated by the third PI regulator, the output voltage compensation amount Ucom_i decreases accordingly or even reverses. According to the relationship φi=φref-Ucom_i, a decrease in Ucom_i leads to an increase in the final control quantity φi; Increasing φi increases the phase shift angle of the DC / DC converter or changes the switching frequency, thereby increasing the output power of the module. The output current Iouti of the module is increased, while Vdci decreases until equilibrium is restored.

[0038] This invention fully reveals the relationship between "low output current → high Vdci →" The negative feedback regulation chain of "Vdci decreases / becomes negative → Ucom_i decreases / reverses → φi increases → output power increases → current recovers" proves that the present invention can indirectly detect current deviation using existing DC voltage sensors and automatically, quickly, and without steady-state error complete current sharing regulation without the need for inter-module communication. At the same time, this regulation process has multiple advantages such as automatic polarity identification, single-module fault isolation, and symmetrical response to positive and negative deviations. This is the core innovative mechanism that distinguishes the present invention from traditional current sharing technology.

[0039] Furthermore, when the output current Iouti of the i-th power module is too large in step S5, the specific adjustment process is as follows: The power module's DC / DC input side will lack power, resulting in a deficiency in the corresponding internal DC power of the power module. A low Vdci results in a slightly lower average DC link voltage Vavg, but the difference between Vavg and Vdci is... Vdci increases; After Vdci is regulated by the third PI regulator, the output voltage compensation amount Ucom_i increases accordingly; According to the relationship φi=φref-Ucom_i, an increase in Ucom_i leads to a decrease in the final control quantity φi; Decreasing φi reduces the phase shift angle of the DC / DC converter or changes the switching frequency, thereby reducing the output power of the module. The output current Iouti of the module is lowered, while Vdci rises until equilibrium is restored.

[0040] Together with the previous paragraph, this invention constitutes the "complete regulation profile" of the current sharing control of this invention: when the output current is too high, it works through "energy deficiency → Vdci decrease → The negative feedback chain of "Vdci increases → Ucom_i increases → φi decreases → output power decreases → current drops" automatically, quickly, and without steady-state error completes current sharing regulation. When the deviation is small, it forms a strict symmetrical regulation logic, which proves that the present invention has a complete regulation capability with bidirectional, symmetrical, and automatic polarity response to positive and negative deviations. This is the core innovative advantage of the present invention that distinguishes it from traditional unidirectional droop control or complex communication current sharing schemes.

[0041] Furthermore, the first PI controller, the second PI controller, and the third PI controller all adopt a discrete incremental PID algorithm or a positional PID algorithm, and the proportional coefficient and integral coefficient of each PI controller are independently tuned according to the system's dynamic response requirements and stability requirements.

[0042] This invention clarifies that the three PI regulators can employ incremental or positional PID algorithms, and emphasizes that the proportional and integral coefficients must be independently tuned according to the dynamic response and stability requirements of each loop. This achieves functional decoupling and bandwidth separation of the voltage outer loop, current inner loop, and current sharing compensation loop, enabling each loop to obtain optimal dynamic performance while ensuring system stability. It also provides a clear technical path for phased commissioning, parameter standardization, and on-site maintenance.

[0043] Furthermore, in step S5, the DC / DC isolation stage switch adopts a dual active bridge DAB topology. The final control quantity φi of the dual active bridge DAB topology is the phase shift angle between the primary and secondary sides of the DAB converter. The magnitude and direction of the transmitted power are controlled by adjusting the phase shift angle.

[0044] This invention establishes a direct mapping relationship between the control algorithm and the power topology by explicitly adopting a dual active bridge (DAB) topology for the DC / DC isolation stage and defining the final control quantity φi as the phase shift angle between the primary and secondary sides: increasing φi → increasing phase shift angle → increasing transmitted power, decreasing φi → decreasing phase shift angle → decreasing transmitted power. This scheme not only makes the control polarity clear, the response direct, and the power precisely and continuously adjustable, but also naturally supports bidirectional power transmission and ZVS soft switching. It is deeply coupled with the current sharing control method of this invention, achieving a high-efficiency and high-reliability system design while ensuring current sharing accuracy.

[0045] Furthermore, in step S5, the DC / DC isolation stage switch adopts an LLC resonant converter topology. The final control quantity φi of the LLC resonant converter topology is the switching frequency or duty cycle, and the output power is controlled by adjusting the switching frequency or duty cycle.

[0046] Furthermore, in step S1, the DC current sampling sensor installed at the DC output terminal on the load side is a Hall effect current sensor or a shunt resistor current sensor; the DC voltage sampling sensor installed at the DC output terminal on the load side in step S1 is a resistor voltage divider voltage sensor or an isolated voltage sensor.

[0047] This invention clarifies that the DC / DC isolation stage can adopt an LLC resonant converter topology and defines the final control quantity φi as the switching frequency or duty cycle, establishing a mapping relationship between the control algorithm and the LLC topology: in frequency conversion mode, increasing the frequency leads to decreasing output power, and in pulse width modulation mode, increasing the duty cycle leads to increasing output power. This scheme fully utilizes the inherent advantages of the LLC topology, such as full-range soft switching, high efficiency, low EMI, and high power density, and is deeply coupled with the current sharing control method of this invention. While achieving high-precision current sharing with zero steady-state error, it maintains the excellent efficiency and power density characteristics of the LLC converter.

[0048] A solid-state transformer system based on load-side self-current sharing control function includes: The main power circuit of the solid-state transformer comprises n power modules, each with an AC / DC rectifier stage, a DC / DC isolation stage, and a DC link capacitor connecting the two. The DC output terminals of the n power modules are directly connected in parallel to form a load-side DC bus. A DC current sampling sensor CT group is installed at the load-side DC output terminal to collect the total current signal Iout. A DC voltage sampling sensor PT group is installed at the load-side DC output terminal to collect the voltage signal Vout. A voltage sensor is installed inside each power module to collect its respective DC link voltage Vdc1, Vdc2, ..., Vdcn. A digital controller executes the above-described solid-state transformer method based on load-side self-current sharing control function to generate control signals for the DC / DC isolation stages of each power module.

[0049] This invention constructs a solid-state transformer output-side self-current sharing control system based on a defined hardware system architecture—including a main power circuit with n parallel modules, distributed sensors (n DC voltage sensors + 1 total current transformer + 1 total voltage transformer) and a centralized digital controller—encompassing "distributed sensing, centralized computing, and modular execution." This system utilizes existing DC link voltage sensors for current sharing control, achieving zero incremental hardware cost and reducing the number of sensors by approximately 50% compared to traditional solutions. It also supports flexible expansion, redundant design, and hot-swappable maintenance of power modules and translates the aforementioned control method into an engineering-featured hardware platform, thus forming dual patent protection for both the "method" and the "system."

[0050] A solid-state transformer includes a main power circuit and a three-level control system; the input terminal of the main power circuit is connected to a medium- and high-voltage power grid, and the main power circuit is connected to the three-level control system. The main power circuit includes power module valve groups; the power module valve groups include A-phase power module valve group stacks, B-phase power module valve group stacks and C-phase power module valve group stacks, which are connected in parallel, and each phase power module valve group stack contains multiple power modules. Each power module is equipped with three H-bridge circuits, one of which is an AC / DC rectifier module, and the other two are DC / DC chopper modules. The two DC / DC chopper modules are respectively located on the primary side and the secondary side of the isolation transformer. The three-level control system includes a main controller, three phase controllers, and multiple module controllers. The main power circuit is signal-connected to the main controller. The three phase controllers are connected one-to-one with the valve groups of the A-phase power module, B-phase power module, and C-phase power module. The phase controllers are bidirectionally connected to the main controller via a pair of fiber optic interfaces. The phase controllers are bidirectionally connected to the phase controllers of the other two phases via two pairs of fiber optic interfaces. The phase controllers are connected to multiple module controllers of their respective phases via multiple pairs of fiber optic interfaces. The module controllers and power modules are signal-connected one-to-one. The isolation transformer adopts the solid-state transformer system based on the load-side self-current sharing control function described above. The DC output terminals of each power module are directly connected in parallel, and the automatic balancing of the output current is achieved through the aforementioned method.

[0051] This invention constructs a complete power electronic conversion system from 10kV / 35kV grid direct connection to low-voltage DC output by defining a complete solid-state transformer architecture—including a main power circuit directly connected to medium and high voltage (A / B / C three-phase valve stack, multiple power modules per phase), a three-level control system (main controller + phase controller + module controller, fiber optic interconnection), and a three-H bridge topology (AC / DC rectification + dual DC / DC chopper) within each module. This system seamlessly integrates the aforementioned "solid-state transformer system based on load-side self-current sharing control function" into the three-level control architecture, utilizes fiber optic communication to achieve electrical isolation and high-speed data transmission between the high-voltage and low-voltage sides, supports independent three-phase control and inter-phase coordination, and forms a three-level patent protection system of "control method → ​​control system → solid-state transformer whole machine".

[0052] like Figure 6-8 The following is an embodiment of the present invention. The DC / DC converter in the power module adopts a DAB circuit topology and uses single phase shift control (SPS). The outputs of the H-bridges on both sides of the isolation transformer in the DC / DC converter are two-level voltages with a duty cycle of 50%. The power transmission is controlled by changing the phase shift angle between the two-level voltages.

[0053] k is the turns ratio of the isolation transformer, L is the series inductance, f is the control frequency, Ths is the time of half a working cycle (Ths = 1 / 2 * f), D is the shift ratio of half a working cycle, and φ = D * Ths.

[0054] When 0≤D≤1, power flows from the DC terminal Vdci inside the power module to the DC output terminal Vout on the load side, that is, from the primary side of the isolation transformer in the DC converter to the secondary side; when -1≤D<0, power flows from the DC output terminal Vout on the load side of the power module to the internal DC terminal Vdci, that is, from the secondary side of the isolation transformer in the DC converter to the primary side; the direction of power transmission can be controlled by changing D. This paper only considers the positive transmission of power from the primary side to the secondary side.

[0055] For the DAB circuit and H-bridge SiCMOSFET device numbers, see [link to relevant documentation]. Figure 6 As shown, the timing principle of single-phase shift control is as follows: Figure 7 As shown, U1 is the output voltage of the primary H-bridge, U2 is the voltage of the secondary H-bridge output voltage referred to the primary side by the transformer, UL is the voltage across the equivalent inductor, and iL is the inductor current.

[0056] Transmission power can be expressed as:

[0057] As can be seen from the above formula, the transmission power is not only related to circuit parameters such as equivalent inductance, switching frequency, and transformer turns ratio, but also to operating conditions such as input and output voltage and shift ratio.

[0058] The transmission power is normalized, and a base value is taken as:

[0059] This reference value represents the maximum transmission power of the converter when D=0.5.

[0060] The normalized transmission power is expressed as:

[0061] Based on the above formula, the power transfer characteristic curve is plotted as follows: Figure 7 As can be seen, with the increase of the shift ratio, the transmission power first increases and then decreases, and the characteristic curve is symmetrical about D=0.5, where it reaches its maximum value. In actual control, in order to stabilize the control system, the range of D is generally controlled in the interval [0, 0.5].

[0062] When the output current Iouti of the i-th power module is relatively small, energy will accumulate on the DC / DC input side of that power module, causing the corresponding internal DC voltage Vdci to be relatively high, resulting in a slight increase in Vavg. However, the difference between Vavg and Vdci is not significant. As Vdci decreases, the difference is subtracted from φref after PI adjustment, resulting in a larger difference φi. φi = Di * Ths serves as the phase shift angle of the DAB circuit, increasing the output power of the power module. The output current Iouti of the module is then increased until the internal DC voltage Vdci of the power module returns to its normal level, thus achieving consistent output current across all power modules.

[0063] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A self-current sharing control method for the load side of a solid-state transformer, characterized in that, This is applied to the output side of a solid-state transformer, which contains n power modules. Each power module has an AC / DC rectifier stage, a DC / DC isolation stage, and a DC link capacitor connected between the AC / DC rectifier stage and the DC / DC isolation stage. The DC output terminals of the n power modules are directly connected in parallel to serve as the DC bus on the load side. The method and steps are as follows: Step S1: Signal acquisition. Acquire the voltage across the DC link capacitor inside each power module, denoted as Vdc1, Vdc2, ..., Vdcn; acquire the total current signal Iout and voltage signal Vout at the DC output terminal on the load side. Step S2: Average value calculation. Sum Vdc1 to Vdcn and divide by n to obtain the average voltage Vavg of the DC link. Step S3: Voltage outer loop control. Set the target value Vref of the DC output voltage on the load side, and subtract Vref from Vout to obtain the voltage error value. V, will After V is regulated by the first PI regulator, a current reference signal Iref is generated for the inner current loop. Step S4: Inner current loop control, subtract Iref from Iout to obtain the current error value. I will I is adjusted by the second PI controller to obtain the basic adjustment value φref; Step S5: Feedforward compensation and control quantity generation for each module. For the i-th power module (i=1,...,n), calculate the DC voltage error value of that module. Vdci = Vavg - Vdci After Vdci is regulated by the third PI regulator, the voltage compensation amount Ucom_i is obtained. Then, the final control amount φi=φref-Ucom_i or φi=φref+Ucom_i is calculated, and φi is used to generate the control signal to drive the DC / DC isolation stage switch of the module. Step S6: Dynamic self-current equalization adjustment. Through the closed-loop control of steps S1 to S5, the output current of each power module is dynamically balanced.

2. The solid-state transformer load-side self-current sharing control method according to claim 1, characterized in that, Step S5 specifically involves: Step S51: Calculate the DC voltage error value of the module. Vdci, i.e. Vdci = Vavg - Vdci; Step S52, to Vdci is fed into the third PI regulator, that is, the independent PI regulator of each module is adjusted to obtain the voltage compensation amount Ucom_i of the module. Step S53: Calculate the final control quantity φi of the module, i.e., φi = φref - Ucom_i, or according to the polarity design, φi = φref + Ucom_i; Step S54: Send the final control quantity φi to the pulse width modulation (PWM) generator or phase shift modulator of the module to generate the control signal driving the DC / DC isolation stage switch in the module; for dual active bridge (DAB) topology, φi is the phase shift angle; for LLC resonant converter topology, φi is the switching frequency or duty cycle.

3. The solid-state transformer load-side self-current sharing control method according to claim 2, characterized in that, When the output current Iouti of the i-th power module is too small in step S5, the specific adjustment process is as follows: Energy will accumulate on the DC / DC input side of this power module, resulting in a higher internal DC voltage Vdci for this power module. A higher Vdci results in a slightly higher average DC link voltage Vavg, but the difference between Vavg and Vdci is... Vdci decreases, meaning a positive value decreases or becomes negative; After Vdci is regulated by the third PI regulator, the output voltage compensation amount Ucom_i decreases accordingly or even reverses. According to the relationship φi=φref-Ucom_i, a decrease in Ucom_i leads to an increase in the final control quantity φi; Increasing φi increases the phase shift angle of the DC / DC converter or changes the switching frequency, thereby increasing the output power of the module. The output current Iouti of the module is increased, while Vdci decreases until equilibrium is restored.

4. The solid-state transformer load-side self-current sharing control method according to claim 3, characterized in that, When the output current Iouti of the i-th power module is too large in step S5, the specific adjustment process is as follows: The power module will lack power on the DC / DC input side, resulting in a low internal DC voltage Vdci for the power module. A low Vdci results in a slightly lower average DC link voltage Vavg, but the difference between Vavg and Vdci is... Vdci increases; After Vdci is regulated by the third PI regulator, the output voltage compensation amount Ucom_i increases accordingly; According to the relationship φi=φref-Ucom_i, an increase in Ucom_i leads to a decrease in the final control quantity φi; Decreasing φi reduces the phase shift angle of the DC / DC converter or changes the switching frequency, thereby reducing the output power of the module. The output current Iouti of the module is lowered, while Vdci rises until equilibrium is restored.

5. The solid-state transformer load-side self-current sharing control method according to claim 3, characterized in that, The first, second, and third PI controllers all employ discrete incremental PID algorithms or positional PID algorithms, and the proportional and integral coefficients of each PI controller are independently tuned according to the system's dynamic response and stability requirements.

6. The solid-state transformer load-side self-current sharing control method according to claim 4, characterized in that, In step S5, the DC / DC isolation stage switch adopts a dual active bridge DAB topology. The final control quantity φi of the dual active bridge DAB topology is the phase shift angle between the primary and secondary sides of the DAB converter. The magnitude and direction of the transmitted power are controlled by adjusting the phase shift angle.

7. The solid-state transformer load-side self-current sharing control method according to claim 4, characterized in that, In step S5, the DC / DC isolation stage switch adopts an LLC resonant converter topology. The final control quantity φi of the LLC resonant converter topology is the switching frequency or duty cycle. The output power is controlled by adjusting the switching frequency or duty cycle.

8. The solid-state transformer load-side self-current sharing control method according to claim 7, characterized in that, In step S1, the DC current sampling sensor set at the DC output terminal on the load side is a Hall effect current sensor or a shunt resistor current sensor; the DC voltage sampling sensor set at the DC output terminal on the load side in step S1 is a resistor voltage divider voltage sensor or an isolated voltage sensor.

9. A solid-state transformer system based on load-side self-current sharing control function, characterized in that, include: The main power circuit of the solid-state transformer comprises n power modules, each with an AC / DC rectifier stage, a DC / DC isolation stage, and a DC link capacitor connecting the two. The DC output terminals of the n power modules are directly connected in parallel to form a load-side DC bus. A DC current sampling sensor CT group is installed at the load-side DC output terminal to collect the total current signal Iout. A DC voltage sampling sensor PT group is installed at the load-side DC output terminal to collect the voltage signal Vout. A voltage sensor is installed inside each power module to collect its respective DC link voltage Vdc1, Vdc2, ..., Vdcn. A digital controller that performs the method according to any one of claims 1 to 7, generating control signals for the DC / DC isolation stages of each power module.

10. A solid-state transformer, characterized in that, It includes a main power circuit and a three-level control system; the input terminal of the main power circuit is connected to the medium- and high-voltage power grid, and the main power circuit is connected to the three-level control system. The main power circuit includes power module valve groups; the power module valve groups include A-phase power module valve group stacks, B-phase power module valve group stacks and C-phase power module valve group stacks, which are connected in parallel, and each phase power module valve group stack contains multiple power modules. Each power module is equipped with three H-bridge circuits, one of which is an AC / DC rectifier module, and the other two are DC / DC chopper modules. The two DC / DC chopper modules are respectively located on the primary side and the secondary side of the isolation transformer. The three-level control system includes a main controller, three phase controllers, and multiple module controllers; the main power circuit is signal-connected to the main controller; the three phase controllers are connected one-to-one with the valve groups of the A-phase power module, the B-phase power module, and the C-phase power module; the phase controllers are bidirectionally connected to the main controller via a pair of fiber optic interfaces; the phase controllers are bidirectionally connected to the phase controllers of the other two phases via two pairs of fiber optic interfaces; the phase controllers are connected to multiple module controllers of their respective phases via multiple pairs of fiber optic interfaces; the module controllers and power modules are signal-connected one-to-one; wherein, the isolation transformer adopts the solid-state transformer system based on the load-side self-current sharing control function as described in claim 9, the DC output terminals of each power module are directly connected in parallel, and the automatic balancing of the output current is achieved through the solid-state transformer load-side self-current sharing control method described in any one of claims 1 to 7.

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