Three-level cascaded solid state transformer and control method

Abstract

This disclosure relates to the field of power conversion technology, and discloses a three-level cascaded solid-state transformer and its control method. The solid-state transformer includes: three phases, each phase consisting of multiple modular power units connected in series, each modular power unit consisting of a three-level full-bridge input unit, an isolation conversion unit, and a DC output unit connected in parallel, wherein the three-level full-bridge input unit is a diode-neutral-clamped three-level full-bridge; and five terminal units, each terminal unit consisting of a switch, a soft-start circuit, and an inductor connected in series. Three terminal units are connected in series with the input terminals of each of the three phases, and the output terminals of each phase are connected in parallel to form two total phase output terminals. The two terminal units are then connected in series with each total phase output terminal to form the three-level cascaded solid-state transformer. Through the three-level full-bridge input unit, the maximum voltage that the switch can withstand is half of the DC bus voltage, reducing the voltage stress on the devices.

Description

Technical Field

[0001] This disclosure relates to the field of power conversion technology, specifically to a three-level cascaded solid-state transformer and its control method. Background Technology

[0002] In the field of power conversion technology, solid-state transformers, also known as power electronic transformers, are a new type of power conversion device based on power electronics technology and the principle of high-frequency electromagnetic induction. In existing technologies, the high-voltage side power conversion topologies of solid-state transformers mainly include schemes based on modular multilevel converters and schemes based on cascaded H-bridges. While the modular multilevel converter-based scheme offers excellent harmonic performance, its system complexity results in high timing control complexity and high cost. The solid-state transformer composed of a two-level cascaded H-bridge scheme presents a trade-off between high device voltage stress and high cost. To save costs, the two-level cascaded H-bridge scheme results in high device voltage stress and high switching losses. Reducing device voltage stress further increases costs. Therefore, existing solid-state transformers suffer from high device voltage stress, high cost, and high timing control complexity. Summary of the Invention

[0003] This disclosure addresses the problems existing in the prior art by providing a three-level cascaded solid-state transformer and a control method.

[0004] To achieve the above objectives, the technical solution adopted in this disclosure is as follows: The first aspect of this disclosure discloses a three-level cascaded solid-state transformer, comprising: three phases, each phase consisting of multiple modular power units connected in series, each modular power unit consisting of a three-level full-bridge input unit, an isolation conversion unit, and a DC output unit connected in parallel, wherein the three-level full-bridge input unit is a diode-neutral-clamped three-level full-bridge used to output zero level, half of the DC bus voltage, and negative half of the DC bus voltage; five terminal units, each terminal unit consisting of a switch, a soft-start circuit, and an inductor connected in series; three of the terminal units are connected in series with the input terminals of each of the three phases, and the output terminals of each of the three phases are connected in parallel to form two total phase output terminals, and two of the terminal units are connected in series with each of the total phase output terminals, thereby forming the three-level cascaded solid-state transformer.

[0005] In some embodiments of this disclosure, the three-level full-bridge input unit includes: a capacitor module consisting of two capacitors connected in series and two bridge arms, wherein the capacitor module and the two bridge arms are connected in parallel; each bridge arm consists of four switches and two clamping diodes, wherein the connection point of the first switch and the second switch is connected to the cathode of the first clamping diode, the anode of the first clamping diode is connected to the midpoint of the capacitor connection of the two capacitors, the connection point of the third switch and the fourth switch is connected to the anode of the second clamping diode, and the cathode of the second clamping diode is connected to the midpoint of the capacitor connection.

[0006] In some embodiments of this disclosure, the isolation transformation unit includes: a primary side consisting of two parallel bridge arms, a secondary side consisting of a two-level full bridge, and the primary side and the secondary side are coupled by a high-frequency isolation transformer.

[0007] In some embodiments of this disclosure, the DC output unit includes: three bidirectional half-bridge converters connected in parallel with phase-interleaved configurations, each bidirectional half-bridge converter consisting of two switches connected in series; three inductors, each inductor being connected to each bidirectional half-bridge converter in a one-to-one correspondence, with the first end of each inductor connected to the converter midpoint of the corresponding bidirectional half-bridge converter; and two capacitors, with the second end of each inductor connected to the first end of a first capacitor, the first end of each bidirectional half-bridge converter being connected to the second end of the first capacitor and the first end of the second capacitor, respectively, and the second end of each bidirectional half-bridge converter being connected to the second end of the second capacitor.

[0008] A second aspect of this disclosure discloses a control method for controlling a three-level cascaded solid-state transformer, comprising: a system-level controller acquiring in real time the module output voltage and module output current of each modular power unit of the three-level cascaded solid-state transformer; calculating the real-time total system power of the solid-state transformer based on the module output voltage and module output current; acquiring the operating status of each modular power unit in the solid-state transformer, and determining the effective capacity coefficient of each modular power unit based on the operating status; determining a module power adjustment reference value for each modular power unit based on the effective capacity coefficient, the real-time total system power, and a preset system target power, wherein the module power adjustment reference value is used to represent the power value that the modular power unit should adjust at the next moment from the current moment; and adaptively adjusting the output power of each modular power unit in the solid-state transformer based on the module power adjustment reference value to achieve system-level control of the output power of each modular power unit.

[0009] In some embodiments of this disclosure, the method further includes: acquiring, in real time, the rectified output voltage of the three-level full-bridge input unit in each modular power unit; determining an active current reference value based on the rectified output voltage and a preset target rectified output voltage; acquiring, in real time, the real-time grid-side current and real-time grid-side voltage; determining, based on the real-time grid-side current, the active current reference value, a preset reactive current reference value, and the real-time grid-side voltage, each first switch drive signal of the three-level full-bridge input unit, wherein the first switch drive signal is used to indicate the opening or closing of the corresponding switch in the three-level full-bridge input unit at the next time step; acquiring, in real time, the secondary voltage and secondary current of the isolation converter unit of the modular power unit; and based on the module power adjustment reference value, the... The secondary-side voltage and the secondary-side current generate the second switch drive signals of the isolation conversion unit, wherein the second switch drive signals are used to indicate whether the corresponding switch in the isolation conversion unit is open or closed at the next moment of the current moment; the actual inductor current of the DC output unit of the modular power unit is acquired in real time; an inductor current reference value is generated based on the module output voltage and the preset module target output voltage; a set of three-phase interleaved switch drive signals is generated based on the inductor current reference value and the actual inductor current, wherein the switch drive signal set includes three third switch drive signals, the phase difference of each third switch drive signal is 120°, and the third switch drive signals are used to indicate whether the corresponding switch in the DC output unit is open or closed at the next moment of the current moment, so as to achieve the target power of the output module.

[0010] In some embodiments of this disclosure, the step of generating each second switch drive signal of the isolation converter unit based on the module power adjustment reference value, the secondary voltage, and the secondary current includes: calculating the secondary voltage and the secondary current using a power calculation formula to obtain the actual transmission power of the isolation converter unit; obtaining the historical actual transmission power of the isolation converter unit at the previous time in the current time; calculating the sum of the module power adjustment reference value and the historical actual transmission power, and calculating the difference between the sum and the actual transmission power; processing the difference using a PI regulator to obtain the optimal phase shift angle combination corresponding to each operating mode in the preset dual operating mode, and selecting a target phase shift angle combination that is conducive to achieving the preset optimization target from each of the optimal phase shift angle combinations in combination with a preset optimization target; and generating each second switch drive signal of the isolation converter unit based on the target phase shift angle combination.

[0011] In some embodiments of this disclosure, determining the module power adjustment reference value of each modular power unit based on each of the effective capability coefficients, the real-time total system power, and the preset system target power includes: determining the total effective capability coefficient in the solid-state transformer based on each of the effective capability coefficients; obtaining the rated power of the modular power unit and using the product of the rated power and the total effective capability coefficient as a safe operating power threshold; determining the total power adjustment reference value based on the real-time total system power and the preset system target power; and determining the module power adjustment reference value of each modular power unit based on the total power adjustment reference value and the effective capability coefficient of each modular power unit when the sum of the total power adjustment reference value and the real-time total system power is not greater than the safe operating power threshold.

[0012] In some embodiments of this disclosure, the method further includes: real-time detection of the real-time capacitor voltage values ​​of the capacitors on both sides of the capacitor connection midpoint in each of the modular power units; and calculation of a zero-sequence voltage compensation amount based on the two real-time capacitor voltage values, so as to adjust the timing of the current injected into the capacitor connection midpoint by each bridge arm in the modular power unit based on the zero-sequence voltage compensation amount, thereby realizing a symmetrical three-level output of the three-level full-bridge input unit.

[0013] In some embodiments of this disclosure, the method further includes: real-time detection of the operating status of the bidirectional half-bridge converter in each of the modular power units; when there is a bidirectional half-bridge converter with abnormal operation, generating a set of two-phase interleaved switch drive signals based on the inductor current reference value and the actual inductor current, or generating a third switch drive signal, wherein the two-phase interleaved switch drive signal set includes two third switch drive signals with a phase difference of 180°.

[0014] Compared with the prior art, this disclosure has the following beneficial effects: By using a three-level full-bridge input unit, three output levels are achieved, with the maximum level being half of the DC bus voltage. This is significantly better than existing solid-state transformers that can only output DC bus voltage and negative DC bus voltage. In this solution, the voltage stress borne by each switching device in the solid-state transformer is half that of existing solid-state transformers. This greatly reduces the voltage stress on the devices with only a small increase in cost (i.e., a small increase in electronic components, such as switching devices), achieving a balance between cost and device voltage stress.

[0015] Furthermore, the control method proposed in this scheme coordinates the power allocation of each modular power unit through a system-level controller. Each modular power unit, based on the power allocated by the system-level controller, performs internal state awareness and local closed-loop control to achieve autonomous adjustment across the entire link. This significantly simplifies the module control logic and reduces the complexity of timing control. It avoids the problem of high timing control complexity inherent in centralized control of solid-state transformers, which involves opening and closing individual switches. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a three-level cascaded solid-state transformer according to an embodiment of this disclosure; Figure 2 This is a schematic diagram of a three-level full-bridge input unit provided according to an embodiment of the present disclosure; Figure 3 This is a schematic diagram of an isolation transformation unit provided according to an embodiment of the present disclosure; Figure 4 This is a schematic diagram of a DC output unit provided according to an embodiment of the present disclosure; Figure 5 This is a flowchart illustrating a control method provided according to an embodiment of the present disclosure; Figure 6 This is a schematic diagram of an output current waveform provided according to an embodiment of the present disclosure. Detailed Implementation

[0017] The present disclosure will now be further described with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present disclosure and should not be construed as limiting the scope of protection of the present disclosure. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application.

[0018] The acquisition, transmission, storage, use, and processing of data in this disclosed technical solution comply with relevant national laws and regulations. In the embodiments of this disclosure, certain existing industry solutions such as software, components, and models may be mentioned. These should be considered exemplary, intended only to illustrate the feasibility of implementing the technical solution of this disclosure, and do not imply that the applicant has already used or necessarily used such solutions.

[0019] All terms used in this disclosure have the same meaning as understood by one of ordinary skill in the art to which this disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in general dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant art, and not as idealized or highly formalized, unless expressly defined herein.

[0020] Example 1; This embodiment proposes a three-level cascaded solid-state transformer, the structure of which can be shown in the figure below. Figure 1 As shown, it includes: The three phases, each phase X is composed of multiple modular power units 10 connected in series. Each modular power unit 10 is composed of a three-level full-bridge input unit 110, an isolation conversion unit 120, and a DC output unit 130 connected in parallel. The three-level full-bridge input unit is a diode midpoint clamped three-level full-bridge, used to output zero level, half of the DC bus voltage, and negative half of the DC bus voltage.

[0021] Five terminal units 20, each of which consists of a switch 201, a soft-start circuit 202 and an inductor 203 connected in series.

[0022] The three terminal units are connected in series with the input terminals 1301 of each of the three phases, and the output terminals of each phase are connected in parallel to form two total phase output terminals 1302. The two terminal units are connected in series with each total phase output terminal 1302 to form the three-level cascaded solid-state transformer.

[0023] The three-level full-bridge input unit converts high-voltage AC power into stable DC power; the isolation converter converts the DC power output from the three-level full-bridge input unit into stable low-voltage DC power that meets the requirements; and the DC output unit modulates the stable low-voltage DC power output from the isolation converter to improve power quality. The number of modular power units comprising each phase can be set according to actual needs, for example, 2, 3, 4, or other numbers; no specific limitation is imposed here. A phase refers to a physically independent power channel, with no direct electrical connection between phases, forming three independent and symmetrical circuits, each capable of outputting DC power. The output terminal of the three-level full-bridge input unit can output 0, ... Three levels, This refers to the DC bus voltage. This refers to half the DC bus voltage; 0 refers to zero level. This refers to the negative half of the DC bus voltage. A diode-clamped three-level full-bridge is a type of bridge that limits the voltage level to a specific value using clamping diodes. The topology of the circuit is as follows. The soft-start circuit consists of a pre-charge resistor and a relay connected in parallel, used to achieve overcurrent limiting, fault isolation, and startup protection.

[0024] Specifically, such as Figure 2As shown, the three-level full-bridge input unit 110 includes: a capacitor module 01 consisting of two capacitors connected in series and two bridge arms 02, wherein the capacitor module and the two bridge arms are connected in parallel; each bridge arm 02 consists of four switches and two clamping diodes, wherein the connection point A of the first switch S11 and the second switch S12 is connected to the cathode of the first clamping diode D1, the anode of the first clamping diode D1 is connected to the midpoint O of the capacitor connection of the two capacitors, the connection point B of the third switch S13 and the fourth switch S14 is connected to the anode of the second clamping diode D2, and the cathode of the second clamping diode D2 is connected to the midpoint O of the capacitor connection.

[0025] Therefore, the three-level full-bridge input unit, through capacitor voltage division and clamping diode structure, ensures that each switching device only bears half of the DC bus voltage. This means that when a switching device is turned on or off, it only needs to withstand a maximum of half the DC bus voltage, whereas the switching devices of a solid-state transformer based on a cascaded H-bridge scheme need to withstand the entire DC bus voltage when turned on or off. Consequently, the switching losses of the three-level full-bridge input unit in this scheme are significantly reduced. Furthermore, due to its lower voltage transition and zero-level midpoint, the three-level full-bridge input unit provides greater controllability of the inductor current of the isolation switching unit. It can maintain sufficient commutation current under various operating conditions to achieve natural charging and discharging of the MOSFET parasitic capacitance, thereby broadening the range of soft switching (ZVS) and providing superior soft switching implementation conditions, which is beneficial for further reducing switching losses.

[0026] Specifically, the isolation conversion unit 120 includes: a primary side consisting of two parallel bridge arms, a secondary side consisting of a two-level full-bridge, and the primary side and the secondary side are coupled through a high-frequency isolation transformer. The structure of the bridge arms is the same as that of the bridge arms in the three-level full-bridge input unit 110, and the clamping diodes in each bridge arm of the isolation conversion unit 120 are connected to the capacitor connection midpoint O in accordance with the connection method of the clamping diodes and capacitors in each bridge arm of the aforementioned three-level full-bridge input unit 110.

[0027] Furthermore, regarding the structure of the isolation transformation unit 120, as follows: Figure 3 As shown, the primary side 300 consists of two parallel bridge arms. The midpoint N1 of the first bridge arm 03 is connected to one end of the front stage of the high-frequency isolation transformer 310, and the midpoint N2 of the second bridge arm 04 is connected to the other end of the front stage of the high-frequency isolation transformer 310. In the secondary side 320, the midpoint N3 of the third bridge arm 05 is connected to one end of the rear stage of the high-frequency isolation transformer 310, and the midpoint N4 of the fourth bridge arm 06 is connected to the other end of the rear stage of the high-frequency isolation transformer 310.

[0028] Therefore, the primary side of the isolation converter unit adopts the same bridge arm structure as the three-level full-bridge input unit, allowing the primary side to reuse the capacitor modules in the three-level full-bridge input unit without the need for additional independent support capacitors or voltage divider circuits. This not only significantly reduces the number of electronic components and the size of the solid-state transformer, lowering hardware costs, but also ensures high compatibility between the isolation converter unit and the three-level full-bridge input unit in terms of level generation capability, voltage stress distribution, and switching timing logic. This facilitates the implementation of a unified switching modulation strategy, simplifies timing control, and thus improves the integration, reliability, and control consistency of the entire modular power unit.

[0029] Specifically, such as Figure 4 As shown, the DC output unit 130 includes: three bidirectional half-bridge converters 400 connected in parallel with phase-interleaved configurations, each bidirectional half-bridge converter 400 consisting of two switches connected in series; three inductors, each inductor L connected to each bidirectional half-bridge converter 400 in a one-to-one correspondence, the first end of each inductor connected to the converter midpoint QO of the corresponding bidirectional half-bridge converter; and two capacitors, the second end of each inductor L connected to the first end of a first capacitor 4001, the first end of each bidirectional half-bridge converter 400 connected to the second end of the first capacitor 4001 and the first end of the second capacitor 4002 respectively, and the second end of each bidirectional half-bridge converter 400 connected to the second end of the second capacitor.

[0030] In summary, the proposed three-level cascaded solid-state transformer includes: three phases, each phase consisting of multiple modular power units connected in series. Each modular power unit comprises a three-level full-bridge input unit, an isolation conversion unit, and a DC output unit connected in parallel. The three-level full-bridge input unit is a diode-clamped three-level full-bridge, used to output zero level, half of the DC bus voltage, and half of the negative DC bus voltage; five terminal units, each terminal unit consisting of a switch, a soft-start circuit, and an inductor connected in series. Three of the terminal units are connected in series with the input terminals of each of the three phases, and the output terminals of each phase are connected in parallel to form two total phase output terminals. Two of the terminal units are then connected in series with each of the total phase output terminals, thus forming the three-level cascaded solid-state transformer. Through the three-level full-bridge input unit, three output levels are achieved, with the maximum level being half of the DC bus voltage. This is significant compared to existing solid-state transformers that can only output DC bus voltage and negative DC bus voltage. In this solution, the voltage stress borne by each switching device in the solid-state transformer is half that of the switching devices in existing solid-state transformers. This significantly reduces device voltage stress with only a small increase in cost (i.e., a small increase in electronic components, such as switching devices), achieving a balance between cost and device voltage stress. Furthermore, the three phases of the solid-state transformer in this solution are not directly electrically connected to each other, forming three independent and structurally symmetrical input links, giving the solid-state transformer excellent scalability and ease of maintenance. All modular power units have consistent structures, identical control methods, and operate independently, further enhancing the solid-state transformer's excellent scalability and ease of maintenance.

[0031] Example 2: This example provides a control method for controlling a three-level cascaded solid-state transformer. The specific process is as follows: Figure 5 As shown, it includes: Step 510: The system-level controller collects the module output voltage and module output current of each modular power unit of the three-level cascaded solid-state transformer in real time.

[0032] The system-level controller is used to adjust the output power of each modular power unit in each of the three phases based on the difference between the target output power and the actual output power of the entire solid-state transformer composed of three phases, so as to realize the control of the output power of each modular power unit at the system level of the solid-state transformer. The three-level cascaded solid-state transformer is the solid-state transformer proposed in Embodiment 1 of this scheme.

[0033] Step 520: Calculate the real-time total system power of the solid-state transformer based on the output voltage and output current of each module.

[0034] The real-time total system power refers to the power actually output by the solid-state transformer at the current moment.

[0035] Specifically, in step 520 above, calculating the real-time total system power of the solid-state transformer based on the output voltage and output current of each module includes: for each modular power unit in the solid-state transformer, calculating the product of the module output voltage and the module output current of the modular power unit to obtain the module output power of the modular power unit; and adding the output power of each module to obtain the real-time total system power of the solid-state transformer.

[0036] Step 530: Obtain the operating status of each modular power unit in the solid-state transformer, and determine the effective capacity coefficient of each modular power unit based on the operating status.

[0037] The operating states include normal operation, power-limited operation, standby operation, and fault operation. Normal operation means the modular power unit is in use and can output rated power according to instructions. Power-limited operation means the modular power unit is in use, but its output power is limited (i.e., it cannot output rated power, and the maximum output power is lower than its rated power). Standby operation means the modular power unit is unused and can be switched to normal operation at any time. Fault operation means the modular power unit is unusable. The effective capacity coefficient refers to the actual output power capability of the solid-state transformer, with reference to the rated power output of the modular power unit. Each operating state corresponds one-to-one with the effective capacity coefficient. The effective capacity coefficient for normal operation is 1. The effective capacity coefficient for standby operation is 0. The effective capacity coefficient for fault operation is 0. The effective capacity coefficient for power-limited operation is greater than 0 and less than 1. The effective capacity coefficient for power-limited operation can be preset; for example, the effective capacity coefficient for the modular power unit in power-limited operation can be set to 0.5. The effective capacity coefficient corresponding to the power-limited operation state can also be determined based on the ratio of the maximum output power of the modular power unit to its rated power under the power-limited operation state.

[0038] Step 540: Based on the effective capacity coefficients, the real-time total system power, and the preset system target power, determine the module power adjustment reference value for each modular power unit, wherein the module power adjustment reference value is used to indicate the power value that the modular power unit should adjust in the next moment from the current moment.

[0039] The preset system target power refers to the power that the solid-state transformer needs to output.

[0040] Specifically, in step 540 above, determining the module power adjustment reference value of each modular power unit based on each of the effective capacity coefficients, the real-time total system power, and the preset system target power includes: determining the total effective capacity coefficient in the solid-state transformer based on each of the effective capacity coefficients; obtaining the rated power of the modular power unit and using the product of the rated power and the total effective capacity coefficient as a safe operating power threshold; determining the total power adjustment reference value based on the real-time total system power and the preset system target power; and determining the module power adjustment reference value of each modular power unit based on the total power adjustment reference value and the effective capacity coefficient of each modular power unit when the sum of the total power adjustment reference value and the real-time total system power is not greater than the safe operating power threshold.

[0041] The total effective capacity factor refers to the actual output power capability of a solid-state transformer (SSE) with reference to its rated output power. The safe operating power threshold refers to the maximum power that a SSE can output under safe and stable operating conditions. Rated power refers to the rated power that a single modular power unit can output. The total power adjustment reference value indicates the power value that the SSE should adjust at the next moment from the current moment.

[0042] Further, determining the total effective capacity coefficient of the solid-state transformer based on each of the effective capacity coefficients includes: adding the effective capacity coefficients together to obtain the total effective capacity coefficient of the solid-state transformer.

[0043] Further, determining the total power adjustment reference value based on the real-time system total power and the preset system target power includes: calculating the difference between the preset system target power and the real-time system total power to obtain the system power deviation; and using a PI controller to calculate and process the system power deviation to obtain the total power adjustment reference value.

[0044] The PI controller is a linear feedback control algorithm used to determine the system adjustment reference value (i.e., the total power adjustment reference value) based on the difference between the system target value (i.e., the preset system target power in this scheme) and the actual system output value (i.e., the real-time total system power) (i.e., the system power deviation), so that the system can actually output the system target value.

[0045] For example, the system power deviation is calculated using a PI controller to obtain a total power adjustment reference value, as shown in the following formula:

[0046] in These are the proportional control parameters for the PI controller. These are the integral control parameters for the PI controller. This is the reference value for total power adjustment. This represents the system power deviation.

[0047] Further, determining the module power adjustment reference value for each modular power unit based on the total power adjustment reference value and the effective capability coefficient of each modular power unit includes: for each phase, calculating the sum of the effective capability coefficients of each modular power unit in the phase to obtain the phase effective capability coefficient; calculating a first ratio of the phase effective capability coefficient to the total effective capability coefficient, and calculating the product of the first ratio and the total power adjustment reference value to obtain the phase power adjustment reference value; for each modular power unit in each phase, calculating a second ratio of the effective capability coefficient of the modular power unit to the phase effective capability coefficient, and calculating the product of the phase power adjustment reference value and the second ratio to obtain the module power adjustment reference value.

[0048] The phase effective capacity coefficient refers to the actual output power capability of a phase, with the phase's rated output power as a reference. The phase power adjustment reference value indicates the power value that the phase should adjust to in the next moment from the current moment.

[0049] For example, the phase power adjustment reference value is obtained by multiplying the first ratio by the total power adjustment reference value, as shown in the following formula:

[0050] in, This is the reference value for total power adjustment; The first ratio; Let m be the phase effective capacity coefficient of phase m; This represents the overall effective capacity coefficient. This is the reference value for phase power adjustment of phase m.

[0051] For example, the module power adjustment reference value is obtained by calculating the product of the phase power adjustment reference value and the second ratio, as shown in the following formula:

[0052] in, This is the reference value for phase power adjustment of phase m. This is the second ratio; Let m be the phase effective capacity coefficient of phase m; For phase m Effective capability coefficient of modular power unit. For phase m Reference value for module power adjustment of modular power unit.

[0053] In some examples, when the sum of the total power adjustment reference value and the real-time system total power is greater than the safe operating power threshold, the difference between the safe operating power threshold and the real-time system total power is updated to the total power adjustment reference value, and the module power adjustment reference value of each modular power unit is determined based on the total power adjustment reference value and the effective capability coefficient of each modular power unit.

[0054] Therefore, by introducing a safe operating power threshold to dynamically limit the total power adjustment reference value, when the sum of the total power adjustment reference value and the real-time system total power exceeds the safe operating power threshold, it is automatically corrected to a feasible value that does not exceed the safe operating power threshold. While ensuring that the solid-state transformer always operates within the safe boundary, it aims to output the preset system target power as much as possible, effectively avoiding problems such as excessive device stress, thermal runaway, or protection shutdown caused by power overload. This significantly improves the operational safety, scheduling robustness, and long-term reliability of the solid-state transformer.

[0055] Step 550: Based on the power adjustment reference value of each module, the output power of each modular power unit in the solid-state transformer is adaptively adjusted to achieve system-level control of the output power of each modular power unit.

[0056] Therefore, by acquiring the output voltage and current of each modular power unit in real time through a system-level controller, the real-time total system power of the solid-state transformer is accurately calculated. Combined with the operating status of each module, its effective capacity coefficient is dynamically evaluated. Then, based on the preset system target power and the current load level (real-time total system power), the power adjustment reference values ​​for each modular power unit are adaptively allocated. This achieves refined, differentiated, and coordinated control of the power of multiple modules (modular power units), ensuring that each module participates in power output efficiently and evenly within its actual available capacity, avoiding overload or idleness. It also significantly improves the power distribution fairness, operating efficiency, and dynamic response capability of the entire solid-state transformer, while enhancing the fault tolerance and stability of the solid-state transformer under conditions of partial module derating or failure.

[0057] Specifically, in step 550 above, the output power of each modular power unit in the solid-state transformer is adaptively adjusted based on the power adjustment reference value of each module, including: the system-level controller sends the power adjustment reference value of each module to the module-level controller corresponding to each modular power unit, and the module-level controller adaptively adjusts the corresponding modular power unit based on the module power adjustment reference value, so that the adjusted modular power unit outputs the module target power, and further makes the adjusted solid-state transformer output the preset system target power.

[0058] The module-level controller regulates the modular power units based on their corresponding module power adjustment reference values, ensuring that each regulated modular power unit outputs its target module power, and further enabling the regulated solid-state transformer to output its target output power. The module-level controller regulates the output power of the modular power units at the module level. The target module power refers to the power that the modular power unit should output. In some examples, the method further includes: real-time detection of the real-time capacitor voltage values ​​of the capacitors on both sides of the capacitor connection midpoint in each of the modular power units; and calculation of a zero-sequence voltage compensation amount based on the two real-time capacitor voltage values, so as to adjust the timing of the current injected into the capacitor connection midpoint by each bridge arm in the modular power unit based on the zero-sequence voltage compensation amount, thereby realizing a symmetrical three-level output of the three-level full-bridge input unit.

[0059] The capacitor connection midpoint refers to the capacitor connection midpoint in the three-level full-bridge input unit of the modular power unit. Specifically, it is the capacitor connection midpoint of the capacitor module in the three-level full-bridge input unit, and the capacitors on both sides of the capacitor connection midpoint are the two capacitors that make up the capacitor module. The real-time capacitor voltage value refers to the voltage of the capacitor in the capacitor module that is currently being collected. The zero-sequence voltage compensation amount refers to the compensation applied to the modular power unit to balance the voltage at the capacitor connection midpoint of the capacitor module in the modular power unit, further confirming that the three output levels of the three-level full-bridge are symmetrically stable with zero level as the symmetrical point.

[0060] Specifically, the zero-sequence voltage compensation amount is calculated based on the two real-time capacitor voltage values, including: calculating the voltage difference between the two real-time capacitor voltage values, and using a PI controller to process the voltage difference to obtain the zero-sequence voltage compensation amount.

[0061] Specifically, adjusting the timing of the current injection at the midpoint of the capacitor connection of each bridge arm in the modular power unit based on the zero-sequence voltage compensation amount to achieve symmetrical three-level output of the three-level full-bridge input unit includes: superimposing the zero-sequence voltage compensation amount with the modulation wave of the modular power unit so that the bridge arm in the modular power unit changes to a zero-level output during the original non-zero-level output period, thereby achieving symmetrical three-level output of the three-level full-bridge input unit.

[0062] Therefore, by real-time monitoring of the capacitor voltages on both sides of the midpoint of the capacitor connection in the modular power unit, the zero-sequence voltage compensation is accurately calculated, and the timing of current injection into the midpoint by each bridge arm is dynamically adjusted accordingly, effectively suppressing the capacitor voltage imbalance problem. Furthermore, without adding additional hardware voltage equalization circuitry, the symmetry of the output voltage waveform of the three-level full-bridge input unit is achieved, reducing common-mode voltage and low-frequency voltage fluctuations, and improving the operational stability of the solid-state transformer.

[0063] In some examples, the method further includes: acquiring, in real time, the rectified output voltage of the three-level full-bridge input unit in each modular power unit; determining an active current reference value based on the rectified output voltage and a preset target rectified output voltage; acquiring, in real time, the real-time grid-side current and real-time grid-side voltage; determining, based on the real-time grid-side current, the active current reference value, a preset reactive current reference value, and the real-time grid-side voltage, each first switch drive signal of the three-level full-bridge input unit, wherein the first switch drive signal is used to indicate whether the corresponding switch in the three-level full-bridge input unit is open or closed at the next time step; acquiring, in real time, the secondary-side voltage and secondary-side current of the isolation converter unit of the modular power unit; and determining, based on the module power adjustment reference value and the secondary-side voltage... The voltage and the secondary current generate the second switch drive signals of the isolation conversion unit, wherein the second switch drive signals are used to indicate whether the corresponding switch in the isolation conversion unit is open or closed at the next moment of the current moment; the actual inductor current of the DC output unit of the modular power unit is acquired in real time, and an inductor current reference value is generated based on the module output voltage and the preset module target output voltage; a set of three-phase interleaved switch drive signal groups is generated based on the inductor current reference value and the actual inductor current, the switch drive signal group including three third switch drive signals, the phase difference of each third switch drive signal is 120°, and the third switch drive signals are used to indicate whether the corresponding switch in the DC output unit is open or closed at the next moment of the current moment, so as to achieve the target power of the output module.

[0064] The preset target rectified output voltage refers to the voltage that the three-level full-bridge input unit needs to output. The active current reference value indicates the active current to be absorbed from the grid; it is the output of the voltage outer loop and used for current inner loop tracking. Specifically, the active current reference value is generated by the voltage regulator based on the difference between the rectified output voltage and the preset target rectified output voltage. Grid side refers to the input terminal of the solid-state transformer connected to the AC grid. Real-time grid-side current refers to the grid-side current at the current moment, and real-time grid-side voltage refers to the grid-side voltage at the current moment. The preset reactive current reference value refers to the preset reactive current value that the solid-state transformer should absorb from the grid. The first switch drive information has a one-to-one correspondence with the switches in the three-level full-bridge input unit. The secondary voltage refers to the output voltage of the isolation converter unit at the current moment, and the secondary current refers to the output current of the isolation converter unit at the current moment. The second switch drive signal has a one-to-one correspondence with the switches in the isolation converter unit. The preset module target output voltage refers to the voltage value that the modular power unit needs to output. The inductor current reference value refers to the inductor current that each inductor in the DC output unit should output. The third switch drive signal has a one-to-one correspondence with the switch in the DC output unit.

[0065] Among them, the switch drive signal group is used to ensure that the drive signals of each bidirectional half-bridge converter in the DC output unit are staggered in phase at equal intervals under the same switching frequency, such as... Figure 6 The actual inductor current of each inductor corresponding to each bidirectional half-bridge converter ( The three phase currents exhibit the characteristics of having the same amplitude but staggered phases. They are connected in parallel and superimposed at the output terminal to form the module's output current. Due to phase interleaving, the ripple valleys and peak values ​​of the current output by each bidirectional half-bridge converter appear out of time, thus canceling each other out when superimposed, effectively suppressing the ripple of the module output current and improving the quality of the module output current.

[0066] Therefore, by constructing a three-level collaborative closed-loop control architecture within the modular power unit—the input stage dynamically generates a first switching drive signal based on the deviation between the rectified output voltage of the current three-level full-bridge input unit and the preset target rectified output voltage to regulate the active and reactive currents on the grid side at the next moment; the isolation stage calculates and generates a second switching drive signal in real time based on the current secondary-side power and the module power adjustment reference value issued by the system level to accurately regulate high-frequency isolated energy transmission; and the output stage generates a three-phase interleaved third switching drive signal with a 120° phase difference based on the error between the module output voltage and the preset target module output voltage, achieving low-ripple, high-precision DC regulated output. Through the three-level adaptive collaborative control of the input stage, isolation stage, and output stage, each module can complete the entire link autonomous adjustment by utilizing internal state perception and local closed loop, under the premise of only receiving a single power command (i.e., the module power adjustment reference value) issued by the system level controller, significantly simplifying the module control logic and the complexity of timing control. Furthermore, the system-level controller coordinates the power allocation of each modular power unit in a unified manner, and the module-level controller performs refined adaptive execution on this basis. This not only ensures the flexibility and security of global power scheduling, but also improves the response speed and robustness of individual modules, realizing a highly efficient collaborative control architecture of "centralized scheduling and distributed autonomy". This provides key technical support for the high reliability and high efficiency operation of large-scale modular solid-state transformers.

[0067] Specifically, based on the real-time grid-side current, the active current reference value, the preset reactive current reference value, and the real-time grid-side voltage, the first switch drive signals of the three-level full-bridge input unit are determined, including: transforming the real-time grid-side current into real-time grid-side d-axis current and real-time grid-side q-axis current in the dq rotating coordinate system; using a current PI regulator to calculate the difference between the active current reference value and the real-time grid-side d-axis current to obtain the d-axis voltage reference value; using a current PI regulator to calculate the difference between the preset reactive current reference value and the real-time grid-side q-axis current to obtain the q-axis voltage reference value; and transforming the real-time grid-side current into a d-axis current and a q-axis current in the dq rotating coordinate system. The voltage is transformed into real-time grid-side d-axis voltage and real-time grid-side q-axis voltage in the dq rotating coordinate system; the d-axis voltage difference between the reference value of the d-axis voltage and the real-time grid-side d-axis voltage is obtained; the q-axis voltage difference between the reference value of the q-axis voltage and the real-time grid-side q-axis voltage is obtained; the d-axis voltage difference and the q-axis voltage difference are subjected to inverse Park transformation to obtain α voltage modulation signal and β voltage modulation signal in the two-phase (α-β) stationary coordinate system; the α voltage modulation signal and the β voltage modulation signal are subjected to inverse Clark transformation to generate a set of three-phase modulation waves; the set of three-phase modulation waves are subjected to carrier phase shift modulation to generate the first switch drive signals of each of the three-level full-bridge input units.

[0068] In the dq rotating coordinate system, the d-axis corresponds to the active component, and the q-axis corresponds to the reactive component. A set of three-phase modulation waves refers to three continuous-time signals used for PWM modulation. Carrier phase-shift modulation refers to a modulation method in which multiple modulation waves have staggered phases.

[0069] Specifically, the step of generating the second switch drive signals of the isolation converter unit based on the module power adjustment reference value, the secondary voltage, and the secondary current includes: calculating the secondary voltage and the secondary current using a power calculation formula to obtain the actual transmission power of the isolation converter unit; obtaining the historical actual transmission power of the isolation converter unit at the previous moment; calculating the sum of the module power adjustment reference value and the historical actual transmission power, and calculating the difference between the sum and the actual transmission power; processing the difference using a PI regulator to obtain the optimal phase shift angle combination corresponding to each operating mode in the preset dual operating mode, and selecting a target phase shift angle combination that is conducive to achieving the preset optimization target from each optimal phase shift angle combination in combination with a preset optimization target; and generating the second switch drive signals of the isolation converter unit based on the target phase shift angle combination.

[0070] Here, actual transmission power refers to the actual power transferred from the primary side to the secondary side by the isolation transformation unit at the current moment. Preset dual operating modes refer to two preset operating modes, namely Mode A (0≤...). ≤ ≤1) and Pattern B (0≤ ≤ ≤1), , To isolate the phase shift angle on the primary side of the converter unit, This refers to the phase shift angle on the secondary side of the isolation converter unit. The optimal phase shift angle combination refers to the combination of phase shift angles that, under a given operating mode, enables the isolation converter unit to achieve the module power adjustment reference value, allowing it to output the required transmission power while facilitating the achievement of preset optimization goals. A set of phase shift angle combinations is formed. The preset optimization target refers to a pre-set performance optimization direction used to guide the operation strategy of the isolation converter unit; for example, when the preset optimization target is to improve efficiency, then, under the premise of meeting transmission power requirements, the set of phase shift angles that maximizes system efficiency is selected from the feasible phase shift angle combinations corresponding to the operating mode as the optimal phase shift angle combination. The target phase shift angle combination refers to the set of phase shift angle combinations that is most conducive to achieving the preset optimization target among all optimal phase shift angle combinations.

[0071] For example, the actual transmission power of the isolation converter unit can be obtained by calculating the secondary voltage and the secondary current using a power calculation formula, as shown in the following formula:

[0072] in, For actual transmission power, This is the secondary voltage. This is the secondary current.

[0073] Specifically, the difference is processed using a PI controller to obtain the optimal phase angle combination corresponding to each working mode in the preset dual working mode, including: processing the difference using a PI controller to obtain the phase angle adjustment amount; and calculating the phase angle adjustment amount using the DPS phase shift calculation formula based on each working mode in the preset dual working mode to obtain the optimal phase angle combination corresponding to each working mode.

[0074] The phase shift adjustment refers to the equivalent phase correction value required to achieve the power output by the isolation converter unit. The DPS phase shift calculation formula refers to the functional relationship between transmission power and phase shift angle.

[0075] Furthermore, to achieve ZVS turn-off, the method further includes: acquiring the primary current, primary voltage, secondary voltage, and equivalent transmission inductance of the isolation converter in real time, and calculating the predicted primary current of the isolation converter at the next moment based on the primary current, primary voltage, secondary voltage, and equivalent transmission inductance; determining the zero-crossing time of the primary current based on the predicted primary current, and adjusting the driving signals of each second switch of the isolation converter based on the zero-crossing time to complete the opening or closing of each switch in the isolation converter at or near the zero-crossing time.

[0076] ZVS shutdown refers to opening or closing a switch under zero voltage or zero current conditions.

[0077] For example, if the predicted primary current is 0, then the next time step after the current time step is the zero-crossing time of the primary current.

[0078] Therefore, by real-time acquisition of the primary-side current, primary-side voltage, secondary-side voltage, and equivalent transmission inductance of the isolation converter, the predicted primary-side current at the next moment can be estimated, and the zero-crossing moment of the primary-side current can be accurately deduced accordingly. Furthermore, based on this zero-crossing moment, the timing of the second switching drive signal is dynamically adjusted to ensure that the power switching devices complete the turn-on or turn-off operation near the natural current zero-crossing point. This effectively achieves zero-voltage switching (ZVS) across the entire load range, significantly reducing switching losses and electromagnetic interference, and enhancing the soft-switching robustness and operational reliability of the solid-state transformer.

[0079] Specifically, generating an inductor current reference value based on the module output voltage and the preset target output voltage of the module includes: calculating the voltage difference between the preset target output voltage of the module and the module output voltage, and using a voltage PI regulator to process the voltage difference to obtain the inductor current reference value.

[0080] Specifically, the method further includes: real-time detection of the operating status of the bidirectional half-bridge converter in each of the modular power units; when there is a bidirectional half-bridge converter with abnormal operation, generating a set of two-phase interleaved switch drive signals based on the inductor current reference value and the actual inductor current, or generating a third switch drive signal, wherein the two-phase interleaved switch drive signal set includes two third switch drive signals with a phase difference of 180°.

[0081] Therefore, by monitoring the operating status of the bidirectional half-bridge converters in each modular power unit in real time, the output mode of the modular power unit can be quickly reconstructed when an abnormal bidirectional half-bridge converter is detected: seamlessly switching from the normal three-phase interleaved output mode to a two-phase interleaved output mode or degrading to a single-phase output mode. Fault-tolerant operation is achieved without interrupting the system power supply. Although this comes at the cost of slightly sacrificing the output current ripple performance of the modular power unit, it effectively ensures the continuity and stability of the output current, significantly improving the reliability, availability, and fault resilience of the modular power unit. It is particularly suitable for applications with stringent requirements for power supply continuity, such as data center power systems, electric vehicle on-board charging devices, and energy storage converter systems.

[0082] It should be noted that the terms "first," "second," and similar terms used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different parts. Terms such as "including" or "contains" mean that the element preceding the word covers the element listed after the word, and do not exclude the possibility of covering other elements as well.

[0083] Although operations are described in a specific order in the accompanying drawings in this disclosure, it should not be construed as requiring these operations to be performed in the specific order or serial order shown, or requiring all of the shown operations to obtain the desired result. In certain environments, multitasking and parallel processing may be advantageous.

[0084] Finally, it should be noted that the above content is only used to illustrate the technical solution of this disclosure, and is not intended to limit the scope of protection of this disclosure. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of this disclosure do not depart from the substance and scope of the technical solution of this disclosure.

Claims

1. A three-level cascaded solid-state transformer, characterized in that, include: At least three phases, each phase is composed of multiple modular power units connected in series, the multiple modular power units in each phase have the same circuit structure, each modular power unit is composed of a three-level full-bridge input unit, an isolation conversion unit and a DC output unit connected in parallel, the three-level full-bridge input unit is a diode midpoint clamped three-level full-bridge, used to output zero level, half of the DC bus voltage and negative half of the DC bus voltage; Three input protection units, each of which consists of a switch, a soft-start circuit and an inductor connected in series, are connected in series with the input terminal of each of the three phases to protect the circuit in that phase. Two output protection units are provided. Each output unit consists of a switch, a soft-start circuit, and an inductor connected in series. Each output protection unit is connected in series with each of the two total phase outputs. The total phase outputs are formed by connecting the outputs of the same electrodes of the three phases in parallel.

2. The three-level cascaded solid-state transformer according to claim 1, characterized in that, The three-level full-bridge input unit includes: A capacitor module consisting of two capacitors connected in series and two bridge arms, wherein the capacitor module and the two bridge arms are connected in parallel; Each of the bridge arms consists of four switches and two clamping diodes. The connection point of the first switch and the second switch is connected to the cathode of the first clamping diode. The anode of the first clamping diode is connected to the midpoint of the capacitor connection of the two capacitors. The connection point of the third switch and the fourth switch is connected to the anode of the second clamping diode. The cathode of the second clamping diode is connected to the midpoint of the capacitor connection.

3. The three-level cascaded solid-state transformer according to claim 2, characterized in that, The isolation transformation unit includes: The primary side consists of two parallel bridge arms, the secondary side consists of a two-level full bridge, and the primary side and the secondary side are coupled by a high-frequency isolation transformer.

4. The three-level cascaded solid-state transformer according to claim 1, characterized in that, The DC output unit includes: Three bidirectional half-bridge converters connected in parallel with phases interleaved, each of the bidirectional half-bridge converters consisting of two switches connected in series; Three inductors are connected one-to-one with each of the bidirectional half-bridge converters, and the first end of each inductor is connected to the converter midpoint of the corresponding bidirectional half-bridge converter. Two capacitors are provided, with the second end of each inductor connected to the first end of the first capacitor. The first end of each bidirectional half-bridge converter is connected to the second end of the first capacitor and the first end of the second capacitor, respectively. The second end of each bidirectional half-bridge converter is connected to the second end of the second capacitor.

5. A control method, characterized in that, Solid-state transformers used to control three-level cascaded circuits include: The system-level controller collects the module output voltage and module output current of each modular power unit of the three-level cascaded solid-state transformer in real time. The real-time total system power of the solid-state transformer is calculated based on the output voltage and output current of each module. The operating status of each modular power unit in the solid-state transformer is obtained, and the effective capacity coefficient of each modular power unit is determined based on the operating status. Based on the effective capacity coefficients, the real-time total system power, and the preset system target power, the module power adjustment reference value of each modular power unit is determined, wherein the module power adjustment reference value is used to indicate the power value that the modular power unit should adjust in the next moment of the current moment. The output power of each modular power unit in the solid-state transformer is adaptively adjusted based on the power adjustment reference value of each module, so as to achieve system-level control of the output power of each modular power unit.

6. The control method according to claim 5, characterized in that, The method further includes: For each of the modular power units, the rectified output voltage of the three-level full-bridge input unit in the modular power unit is acquired in real time; The active current reference value is determined based on the rectified output voltage and the preset target rectified output voltage; Real-time acquisition of grid-side current and grid-side voltage; Based on the real-time grid-side current, the active current reference value, the preset reactive current reference value, and the real-time grid-side voltage, the first switch drive signals of the three-level full-bridge input unit are determined, wherein the first switch drive signal is used to indicate whether the corresponding switch in the three-level full-bridge input unit is turned on or off at the next time of the current time. Real-time acquisition of the secondary voltage and secondary current of the isolation converter unit of the modular power unit; Based on the module power adjustment reference value, the secondary voltage, and the secondary current, the second switch drive signal of the isolation conversion unit is generated, wherein the second switch drive signal is used to indicate whether the corresponding switch in the isolation conversion unit is turned on or off at the next time of the current time. The actual inductor current of the DC output unit of the modular power unit is obtained in real time. An inductor current reference value is generated based on the module's output voltage and the preset target output voltage of the module. Based on the inductor current reference value and the actual inductor current, a set of three-phase interleaved switch drive signal groups is generated. The switch drive signal group includes three third switch drive signals, and the phase difference between each third switch drive signal is 120°. The third switch drive signals are used to indicate the opening or closing of the corresponding switch in the DC output unit at the next moment of the current moment, so as to achieve the target power of the output module.

7. The control method according to claim 6, characterized in that, The process of generating the second switch drive signals for the isolation converter unit based on the module power adjustment reference value, the secondary voltage, and the secondary current includes: The actual transmission power of the isolation converter unit is obtained by calculating the secondary voltage and the secondary current using the power calculation formula. Obtain the historical true transmission power of the isolation transformation unit at the previous time step at the current time step; Calculate the sum of the module power adjustment reference value and the historical actual transmission power, and calculate the difference between the sum and the actual transmission power; The difference is processed using a PI controller to obtain the optimal phase shift angle combination corresponding to each working mode in the preset dual working mode. Then, a target phase shift angle combination that is conducive to achieving the preset optimization target is selected from each of the optimal phase shift angle combinations in combination with the preset optimization target. The second switch drive signals of the isolation conversion unit are generated based on the target phase shift angle combination.

8. The control method according to claim 5, characterized in that, The determination of the module power adjustment reference value for each modular power unit based on the effective capacity coefficients, the real-time total system power, and the preset system target power includes: The overall effective capacity coefficient of the solid-state transformer is determined based on each of the aforementioned effective capacity coefficients. Obtain the rated power of the modular power unit, and use the product of the rated power and the total effective capacity coefficient as the safe operating power threshold; A total power adjustment reference value is determined based on the real-time total system power and the preset system target power; When the sum of the total power adjustment reference value and the real-time system total power is not greater than the safe operating power threshold, the module power adjustment reference value of each modular power unit is determined based on the total power adjustment reference value and the effective capability coefficient of each modular power unit.

9. The control method according to claim 5, characterized in that, The method further includes: Real-time detection of the real-time capacitor voltage values ​​of the capacitors on both sides of the midpoint of the capacitor connection in each modular power unit; The zero-sequence voltage compensation is calculated based on the two real-time capacitor voltage values. The timing of the current injection at the midpoint of the capacitor connection of each bridge arm in the modular power unit is adjusted based on the zero-sequence voltage compensation, so as to realize the symmetrical three-level output of the three-level full-bridge input unit.

10. The control method according to claim 6, characterized in that, The method further includes: The operating status of the bidirectional half-bridge converter in each modular power unit is monitored in real time. When there is a bidirectional half-bridge converter with abnormal operation, a set of two-phase interleaved switch drive signal groups is generated based on the inductor current reference value and the actual inductor current, or a third switch drive signal is generated. The two-phase interleaved switch drive signal group includes two third switch drive signals with a phase difference of 180°.

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