High-power dc transformer based on four-level topology and balance control method thereof

Abstract

The present application relates to the technical field of transformer, in particular to a high-power DC transformer based on four-level topology and a balance control method thereof, comprising a high-power DC transformer primary side for converting DC into AC and a high-power DC transformer secondary side for converting AC into DC; the high-power DC transformer primary side comprises a hybrid multi-level converter and a transformer primary winding; the hybrid multi-level converter comprises a DC side and one or two phases, each phase comprises eight switching devices and a flying capacitor; the DC side comprises three DC bus capacitors connected in series, a first capacitor and a second capacitor; the balance control of the high-power DC transformer is used to maintain the voltage stability of each capacitor under high-frequency output, and is suitable for general DC transformer output power control.

Description

Technical Field

[0001] This invention relates to the field of transformer technology, specifically to a high-power DC transformer based on a four-level topology and its balance control method. Background Technology

[0002] With the continuous development and transformation of energy technologies, the importance of energy storage systems and related fields of new energy power is becoming increasingly prominent. Solid-state transformers, as a novel type of power conversion device, are gradually becoming a research hotspot and key development direction in this field due to their numerous significant advantages. Solid-state transformers, with their small size, light weight, bidirectional energy flow, and low noise, are expected to become key components in the construction of energy storage systems. Furthermore, solid-state transformers can also be widely used in new energy power systems, DC transmission systems, electric vehicle charging, and other fields.

[0003] However, existing solid-state transformers have certain limitations in terms of topology. Most solid-state transformers adopt a modular multilevel topology, which, while having its own advantages to some extent, suffers from a large number of components. Too many components not only increase the complexity and cost of the system, but more importantly, it hinders the improvement of the system's power density. Furthermore, there is a lack of a hybrid multilevel topology control method at high frequencies, making it impossible to achieve stable voltage across individual capacitors in a hybrid multilevel topology at high-frequency output.

[0004] Therefore, there is an urgent need for a high-power DC transformer based on a four-level topology and its balance control method, which can solve the problems of the large number of devices corresponding to existing fixed transformers using modular multilevel topology and the lack of hybrid multilevel topology control methods at high frequencies, so as to maintain the stability of the voltage of each capacitor under high-frequency output. Summary of the Invention

[0005] One of the objectives of this invention is to provide a high-power DC transformer based on a four-level topology and its balance control method, which can solve the problems of numerous components in existing fixed transformers using modular multilevel topologies and the lack of hybrid multilevel topology control methods at high frequencies, thereby achieving stable voltage of each capacitor at high-frequency output.

[0006] To achieve the above objectives, a high-power DC transformer based on a four-level topology is provided, including a primary side of a high-power DC transformer for converting DC to AC and a secondary side of a high-power DC transformer for converting AC to DC. The primary side of the high-power DC transformer includes a hybrid multilevel converter and the transformer primary winding. The hybrid multilevel converter includes a DC side and one or two phases, each phase including eight switching devices and a flying capacitor. ; ; The DC side includes three DC bus capacitors connected in series. First capacitor Second capacitor The DC side is connected to an external DC power supply. The three DC bus capacitors The first capacitor is connected in series with a DC power supply. Second capacitor Series connection with 3 DC bus capacitors Parallel connection; the first capacitor Second capacitor A first neutral point is set between them; The eight switching devices are divided into four groups, namely: , , , ; The The first terminal is connected to the DC bus capacitor. The positive terminal is connected to the first terminal, and the second terminal is connected to the second terminal. The first end and The second end; the The first end is also with The first end is connected to the second end, and the second end is connected to the DC bus capacitor. The negative electrode; the Second end connection The first end; the The first end is connected to the DC bus capacitor. The positive terminal, the second terminal connected The first end and The first end; the The second end is connected to the DC bus capacitor. The negative electrode; the Second end connection The first terminal; when the multilevel converter contains two phases, the The second end and the The connection point corresponding to the second end is connected to both ends of the primary side of the transformer as the AC output terminal of the primary side of the high-power DC transformer; when the multilevel converter contains only one phase, the... The second end and the The connection point corresponding to the second end is connected to one end of the primary side of the transformer, and the first capacitor Second capacitor The first neutral point between them is connected to the other end of the primary side of the transformer; The The second end and The first end is connected to a flying capacitor. The positive electrode, the The second end and The first end is connected to a flying capacitor. The negative electrode.

[0007] Technical principles and effects of this solution: In this solution, the hybrid multilevel converter includes a DC side, which consists of three DC bus capacitors connected in series. and the first capacitor in series Second capacitor Composition: 3 DC bus capacitors Connected in series with a DC power supply, it is used to divide the input DC voltage. First capacitor. Second capacitor After being connected in series, it is connected in parallel with three DC bus capacitors to further stabilize the voltage, and in the first capacitor Second capacitor A first neutral point is set between the points, which can be used as a reference potential point. An external DC power supply is connected to the DC side.

[0008] The output of the DC power supply is input to the hybrid multilevel converter, which also includes one or two phases; each phase has eight switching devices, divided into four groups of switching devices. , , , Different output levels are achieved by controlling the on and off states of these switching devices. The output level of the hybrid multilevel converter is controlled by controlling the on and off states of each group of switching devices.

[0009] Among them, the flying capacitor is connected in the middle of the bridge arm. During the switching process of the switching devices, it can absorb and release energy to ensure that the voltage stress at both ends of each switching device is within a reasonable range.

[0010] A high-power DC transformer uses a hybrid multilevel converter to transform DC voltage into AC voltage of different levels on its primary side. This AC voltage is input to the primary side of the transformer, where energy is transferred to the secondary side through electromagnetic induction. The secondary side of the transformer then converts the received AC voltage back into DC voltage, thus realizing the entire transformation process from DC input to DC output.

[0011] The presence of flying capacitors solves the problems of voltage stress imbalance that may be caused by series connection of switching devices, which can lead to damage to switching devices and degradation of system performance, and further improves the stability and reliability of the system.

[0012] Hybrid multilevel converters consist of only eight switching devices and one additional flying capacitor per phase. Compared to modular multilevel converters and cascaded H-bridge converters with the same number of levels, hybrid multilevel converters use fewer power devices. Furthermore, their redundant switching states provide more degrees of freedom for control and more options for capacitor voltage control. Flexible control strategies: Due to its topology, various control strategies can be employed to achieve precise control of output voltage and current, meeting the needs of different application scenarios. For example, voltage regulation and power transfer can be achieved by controlling the conduction of switching devices. By controlling each switching device, the charging and discharging process of the capacitor can be adjusted, allowing the capacitor voltage to gradually approach equilibrium. In other words, by precisely selecting appropriate switching combinations, the charging and discharging process of the capacitor can be effectively intervened, enabling the capacitor voltage to be quickly controlled to near its rated value within a short time. This rapid response mechanism effectively avoids system performance degradation caused by long-term deviations of the capacitor voltage from its rated value, greatly improving the system's ability to cope with voltage fluctuations. This solves the problems of numerous components associated with existing fixed transformers using modular multilevel topologies and the lack of hybrid multilevel topology control methods at high frequencies, enabling the maintenance of stable capacitor voltages at high-frequency output (primary side output frequency greater than 1kHz).

[0013] It can achieve four-level output, which, compared to traditional two-level or three-level topologies, results in more output voltage levels, a waveform closer to a sine wave, and lower harmonic content. This helps reduce the impact of harmonics in the output voltage on the transformer, thus extending the transformer's service life.

[0014] Furthermore, the secondary side of the high-power DC transformer includes four switching devices, an output capacitor, and a transformer secondary winding, which are respectively... The and Forward series connection, the and Forward series connection; both series branches are connected in parallel with the output capacitor; The and The series connection point, and the and The series connection point is connected to the secondary side of the transformer as the AC input terminal of the secondary side of the high-power DC transformer.

[0015] Beneficial effects: In this scheme, the AC power output from the secondary side of the transformer is input to a circuit consisting of four switching devices. In the rectifier circuit, by properly controlling the on and off sequence of these four switching devices, alternating current can be converted into direct current. This results in a relatively stable DC output voltage, providing a reliable power supply to the load.

[0016] The primary side uses a hybrid multilevel converter to convert high-power DC to AC, while the secondary side uses a rectifier circuit composed of four switching devices to convert the AC back to DC. The primary and secondary sides are electrically isolated and energy transferred via a transformer, enabling the entire system to handle high-power power conversion. It is suitable for high-power DC-DC conversion applications such as high-voltage DC transmission and large-scale industrial power supplies, meeting the demands of these fields for high-power, high-efficiency DC-DC conversion.

[0017] Furthermore, the hybrid multilevel converter also includes a phase-shifting inductor. The The second end and the The second terminal is connected to the phase-shifting inductor The first end is connected to the phase-shifting inductor. The second end is connected to the primary side of the transformer.

[0018] Beneficial effects: Phase-shifting inductors can change the rate of change of current during the turn-on and turn-off processes of switching devices, and are an important design parameter for the maximum output power of solid-state transformers.

[0019] Furthermore, it also includes a feature for acquiring the capacitance of three DC bus terminals. In addition, a voltage sensor for the capacitor voltage corresponding to the flying capacitor and a sensor for collecting the output current corresponding to the AC output terminal of the primary side of the high-power DC transformer. Current sensor.

[0020] Beneficial effects: By monitoring the voltage of these capacitors, the control system can adjust the on and off strategies of the switching devices in the hybrid multilevel converter based on the voltage feedback information, ensuring the stability and balance of the voltage of each capacitor, avoiding damage to the switching devices due to excessive voltage stress caused by capacitor voltage imbalance, and improving the reliability and stability of the system.

[0021] This invention also provides a balance control method for a high-power DC transformer, using the aforementioned high-power DC transformer based on a four-level topology, comprising the following steps: S1. Acquire the capacitor voltage data and output current data collected by the voltage sensor and current sensor; S2. Based on the collected capacitor voltage data and output current data, and using the preset capacitor deviation calculation strategy, calculate the capacitor voltage deviation value corresponding to each capacitor. S3. Determine whether the absolute value of the capacitor voltage deviation value corresponding to each capacitor is less than the preset first deviation threshold. S4. If the judgment result is yes, then the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer. S5. If the judgment result is negative, then at the moment when the output level of the hybrid multilevel converter switches or the current reverses, based on the output value corresponding to the output level at the next moment, and based on a preset selection strategy, the switching state of each switching device that can prevent the capacitor voltage deviation value of the capacitor with the largest deviation value from the maximum is selected and each switching device is controlled until the absolute value of the capacitor voltage deviation value corresponding to all capacitors is less than the preset second deviation threshold. Then, at the moment when the output level of the hybrid multilevel converter switches or the current reverses, the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer; the preset first deviation threshold is greater than the preset second deviation threshold.

[0022] The technical principle and effects of this solution: In this solution, voltage and current sensors collect real-time voltage data of the capacitors in a high-power DC transformer based on a four-level topology, as well as output current data from the primary side AC output terminal. This data forms the basis for subsequent control decisions and reflects the current operating status of the system.

[0023] Based on a pre-defined capacitor deviation calculation strategy, the collected capacitor voltage data and output current data are used to calculate the capacitor voltage deviation value for each capacitor. This deviation value represents the difference between the current capacitor voltage and the expected voltage, and is used to determine whether the capacitor voltage is in a balanced state.

[0024] The absolute value of the voltage deviation of each capacitor is compared with a preset first deviation threshold. The preset first deviation threshold is a pre-set standard value used to determine whether the capacitor voltage deviation is within an acceptable range.

[0025] If the absolute value of the capacitor voltage deviation is less than the preset first deviation threshold, it indicates that the capacitor voltage is in a relatively balanced state. At this time, the preset switching state rotation control strategy is invoked. The purpose of this strategy is to keep the switching frequency consistent with the output frequency by alternately switching the on and off states of the switching devices, thereby reducing the losses caused by the switching process and improving system efficiency while maintaining stable system operation.

[0026] If the absolute value of the capacitor voltage deviation is greater than or equal to a preset first deviation threshold, it indicates a significant imbalance in the capacitor voltage. At the moment of output level switching or current commutation of the hybrid multilevel converter, based on the output value of the next moment's output level, a preset selection strategy is used to select the switching state of the switching device that prevents the capacitor voltage corresponding to the capacitor with the largest voltage deviation from further deviation. In this way, the charging and discharging process of the capacitors is adjusted, gradually bringing the capacitor voltages towards balance. When the absolute value of the capacitor voltage deviation of all capacitors is less than a preset second deviation threshold (the preset first deviation threshold is greater than the preset second deviation threshold), it indicates that the capacitor voltages have basically reached a balanced state. At this point, at the next moment of output level switching or current commutation of the hybrid multilevel converter, a preset switching state rotation control strategy is adopted to continue maintaining stable system operation.

[0027] When the capacitor voltage deviates significantly, this solution cleverly utilizes the redundant switching states of the hybrid multilevel converter to quickly adjust the circuit's operating mode. By precisely selecting appropriate switching combinations, it effectively intervenes in the capacitor's charging and discharging process, rapidly controlling the capacitor voltage to near its rated value within a short time. This rapid response mechanism effectively avoids system performance degradation caused by long-term capacitor voltage deviations from the rated value, greatly improving the system's ability to cope with voltage fluctuations.

[0028] When the capacitor voltage deviation is low, a switching state rotation strategy is adopted. Under this strategy, the switching frequency of the power switching devices is consistent with the output voltage frequency, which theoretically can maintain the voltage balance of each capacitor. The switching state rotation strategy ensures that the switching frequency of the power switching devices matches the output voltage frequency, avoiding unnecessary high-frequency switching operations. This significantly reduces energy loss during the switching process, as switching losses are closely related to the switching frequency. Lower switching losses mean that the system can convert more input energy into useful output energy, thereby improving the overall system efficiency. By organically combining a fast correction strategy for large voltage deviations with a switching state rotation strategy for low voltage deviations, full-process capacitor voltage balance can be achieved with an increase of less than 2% in the switching frequency. This means that throughout the entire system operation, regardless of the capacitor voltage state, effective control and regulation can be achieved, ensuring that the system always operates in a stable and efficient state.

[0029] A reasonable capacitor voltage control strategy and switching state rotation strategy effectively reduce the stress and losses experienced by key equipment such as switching devices and capacitors during operation. Switching devices are prevented from premature aging and damage caused by frequent high-voltage switching and prolonged high-load operation, and the stable capacitor voltage results in lower harmonics in the primary-side AC output.

[0030] Furthermore, the preset capacitance deviation calculation strategy is as follows: Based on the collected capacitor voltage and output current data, and using a preset current calculation formula, the three DC bus capacitors corresponding to the hybrid multilevel converter are calculated. The current corresponding to the connection point between them, and the flyover current flowing through each phase. ; The preset current calculation formula is:

[0031]

[0032]

[0033]

[0034]

[0035] In the formula, , Three DC bus capacitors The current corresponding to the connection point between them; A and B are phase A and phase B on the primary side of the hybrid multilevel converter, respectively. In phase A The current corresponding to the component, In phase B The current corresponding to the component, In phase x The current corresponding to the component; In phase A The current corresponding to the component, In phase B The current corresponding to the component, In phase x The current corresponding to the component, , as well as These represent the switch states corresponding to phase x, with 1 indicating the switch is on and 0 indicating the switch is off. Let x be the output current corresponding to phase x; Based on the three DC bus capacitors corresponding to the hybrid multilevel converter The current corresponding to the connection point between them, and the flyover current flowing through each phase. Based on the preset formula for calculating capacitor voltage deviation, the capacitor voltage deviation of each capacitor in the hybrid multilevel converter is calculated.

[0036]

[0037]

[0038]

[0039] In the formula, , , , These represent the upper DC bus capacitor, the middle DC bus capacitor, and the lower DC bus capacitor, respectively. x The capacitance value of the phase-flying capacitor. This is the capacitor voltage deviation value corresponding to the upper DC bus capacitor.

[0040] Beneficial effects: By utilizing a preset formula for calculating capacitor voltage deviation, the system further calculates the voltage deviation of each capacitor. This deviation accurately reflects the difference between the current and expected voltage of the capacitor, providing a clear picture of its actual state. Based on the real-time capacitor voltage deviation, the system can take appropriate measures to adjust the voltage at an early stage, preventing further deviation. This helps improve the system's response speed and stability, and reduces system failures caused by abnormal capacitor voltage. Attached Figure Description

[0041] Figure 1 This is a flowchart of the balance control method based on a four-level topology in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the structure of a high-power DC transformer based on a four-level topology in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram showing the relationship between the primary side output and the output of the two-phase hybrid multilevel converter in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram showing the switching state ordering and output of phase A and phase B during switching state rotation in Embodiment 1 of the present invention. Figure 5 This is a simulation waveform diagram of capacitor voltage balance in Embodiment 1 of the present invention. Detailed Implementation

[0042] The following detailed description illustrates the specific implementation method: Example 1 High-power DC transformers based on four-level topology are basically as follows: Figure 1 and Figure 2 As shown, it includes the primary side of a high-power DC transformer for converting DC to AC and the secondary side of a high-power DC transformer for converting AC to DC. The primary side of the high-power DC transformer includes a hybrid multilevel converter and the transformer primary winding. The hybrid multilevel converter includes a DC side and one or two phases, each phase including eight switching devices and a flying capacitor. In this embodiment, the hybrid multilevel converter can be phase-selected. When it is a single phase, it can be either phase A or phase B; when it is a two-phase converter, it can be phase A or phase B. In the two-phase configuration, phase A has eight switching devices. , , , and 1 flying capacitor The B phase also has 8 switching devices. , , , and 1 flying capacitor The structure consists of flying capacitors, all of which have a capacitance of 150μF.

[0043] The DC side includes three DC bus capacitors connected in series. First capacitor Second capacitor The DC side is connected to an external DC power supply.

[0044] The three DC bus capacitors The first capacitor is connected in series with a DC power supply. Second capacitor Series connection with 3 DC bus capacitors Parallel connection; the first capacitor Second capacitor A first neutral point is set between them; The eight switching devices are divided into four groups of switching devices, namely: , , , ; The The first terminal is connected to the DC bus capacitor. The positive terminal is connected to the first terminal, and the second terminal is connected to the second terminal. The first end and The second end; the The first end is also with The first end is connected to the second end, and the second end is connected to the DC bus capacitor. The negative electrode; the Second end connection The first end; the The first end is connected to the DC bus capacitor. The positive terminal, the second terminal connected The first end and The first end; the The second end is connected to the DC bus capacitor. The negative electrode; the Second end connection The first terminal; when the multilevel converter contains two phases, the The second end and the The connection point corresponding to the second end is connected to both ends of the primary side of the transformer as the AC output terminal of the primary side of the high-power DC transformer; when the multilevel converter contains only one phase, the... The second end and the The connection point corresponding to the second end is connected to one end of the primary side of the transformer, and the first capacitor Second capacitor The first neutral point between them is connected to the other end of the primary side of the transformer; The The second end and The first end is connected to a flying capacitor. The positive electrode, the The second end and The first end is connected to a flying capacitor. The negative terminal. In this embodiment, the eight switching devices in each phase of the hybrid multilevel converter are divided into... , , , These four groups, x = A or B; the switching states of the two switching devices in each group are complementary, and and , The switch states are the same.

[0045] When controlling the switching state of switching devices, the eight switching state names are respectively named as follows: These 8 switch states , ， The on / off states are sequentially 000, 001, 010, 100, 011, 101, 110, 111 (0 indicates off, 1 indicates on). In this embodiment, the single-phase output of the hybrid multilevel converter is a 4-level voltage (0, E, 2E, 3E respectively). By controlling the switching time of the output level of the two-phase hybrid converter, different waveforms such as square wave, triangle wave, and sine wave can be output on the primary side for different modulation methods such as square wave modulation, triangle wave modulation, and sine wave modulation. Figure 3 The diagram shows the relationship between the primary output and the output of the two-phase hybrid multilevel converter.

[0046] The secondary side of the high-power DC transformer includes four switching devices, an output capacitor, and a transformer secondary winding, which are respectively... The and Forward series connection, the and Forward series connection; both series branches are connected in parallel with the output capacitor; The and The series connection point, and the and The series connection point is connected to the secondary side of the transformer as the AC input terminal of the secondary side of the high-power DC transformer.

[0047] The hybrid multilevel converter also includes a phase-shifting inductor. The The second end and the The second terminal is connected to the phase-shifting inductor The first end is connected to the phase-shifting inductor. The second end is connected to the primary side of the transformer.

[0048] It also includes a method for acquiring the capacitance of three DC bus terminals. In addition, a voltage sensor for the capacitor voltage corresponding to the flying capacitor and a sensor for collecting the output current corresponding to the AC output terminal of the primary side of the high-power DC transformer. Current sensor.

[0049] This embodiment also discloses a balance control method for a high-power DC transformer, using the aforementioned high-power DC transformer based on a four-level topology, including the following steps: S1. Acquire the capacitor voltage data and output current data collected by the voltage sensor and current sensor; S2. Based on the collected capacitor voltage data and output current data, and using the preset capacitor deviation calculation strategy, calculate the capacitor voltage deviation value corresponding to each capacitor. S3. Determine whether the absolute value of the capacitor voltage deviation value corresponding to each capacitor is less than the preset first deviation threshold. S4. If the judgment result is yes, then the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer. In this embodiment, the preset switching state rotation control strategy corresponds to the following rotation sequence: a sequence consisting of 3 voltage cycles forming one cycle. V1 and V8 Each of the three states appears 3 times, and other switch states appear 2 times each. For example, The relationship between the switching state of a two-phase hybrid multilevel converter and the primary output voltage and phase-shifting inductor current is as follows: Figure 4 As shown.

[0050] S5. If the judgment result is negative, then at the moment when the output level of the hybrid multilevel converter switches or the current reverses, based on the output value corresponding to the output level at the next moment, and according to a preset selection strategy, the switching state of each switching device that can prevent the capacitor voltage deviation of the largest capacitor voltage deviation value from deviating the largest is selected, and each switching device is controlled until the absolute value of the capacitor voltage deviation value corresponding to all capacitors is less than a preset second deviation threshold. Then, at the next moment when the output level of the hybrid multilevel converter switches or the current reverses, a preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer; the preset first deviation threshold is greater than the preset second deviation threshold. For example, if the output level is 0E, then the following can be selected: If the output level is E, then it can be selected. The output level is 2E, which can be selected. The output level 3E can be selected. ).

[0051] The preset capacitance deviation calculation strategy is as follows: Based on the collected capacitor voltage and output current data, and using a preset current calculation formula, the three DC bus capacitors corresponding to the hybrid multilevel converter are calculated. The current corresponding to the connection point between them, and the flyover current flowing through each phase. ; The preset current calculation formula is:

[0052]

[0053]

[0054]

[0055]

[0056] In the formula, , Three DC bus capacitors The current corresponding to the connection point between them; A and B are phase A and phase B on the primary side of the hybrid multilevel converter, respectively. In phase A The current corresponding to the component, In phase B The current corresponding to the component, In phase x The current corresponding to the component; In phase A The current corresponding to the component, In phase B The current corresponding to the component, In phase x The current corresponding to the component, , as well as These represent the switch states corresponding to phase x, with 1 indicating the switch is on and 0 indicating the switch is off. Let x be the output current corresponding to phase x; Based on the three DC bus capacitors corresponding to the hybrid multilevel converter The current corresponding to the connection point between them, and the flyover current flowing through each phase. Based on the preset formula for calculating capacitor voltage deviation, the capacitor voltage deviation of each capacitor in the hybrid multilevel converter is calculated.

[0057]

[0058]

[0059]

[0060] In the formula, , , , These represent the upper DC bus capacitor, the middle DC bus capacitor, and the lower DC bus capacitor, respectively. x The capacitance value of the phase-flying capacitor. This represents the capacitor voltage deviation value corresponding to the upper DC bus capacitor. In this embodiment, the initial voltages of the upper DC bus capacitor, the middle DC bus capacitor, and the lower DC bus capacitor are set to 1100V, 950V, and 950V, respectively, and the initial voltages of the A-phase flying capacitor and the B-phase flying capacitor are set to 1100V and 900V, respectively.

[0061] For example, at t=0s, if the absolute value of the capacitor voltage deviation from the rated value is greater than the set threshold... (In this example, the threshold is set to 30V). The program proceeds to step S5, selects the switching state according to the rules to balance the voltage of each capacitor, and after 3ms, the absolute value of the capacitor voltage deviation is less than another set threshold. In this embodiment, the threshold is set to 10V. The program enters step S4, adopting a switching control method. Then, based on the capacitor voltage deviation value, the control method switches between steps S4 and S5. During the simulation, the various capacitor voltage values ​​are as follows: Figure 5 As shown.

[0062] The above descriptions are merely embodiments of the present invention. Commonly known structures and characteristics are not described in detail here. Those skilled in the art are aware of all common technical knowledge in the field prior to the application date or priority date, are aware of all existing technologies in that field, and have the ability to apply conventional experimental methods prior to that date. Those skilled in the art can, based on the guidance provided in this application, improve and implement this solution in combination with their own capabilities. Some typical known structures or methods should not be obstacles for those skilled in the art to implement this application. It should be noted that those skilled in the art can make several modifications and improvements without departing from the structure of the present invention. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A high-power DC transformer based on a four-level topology, characterized in that: This includes the primary side of a high-power DC transformer for converting direct current to alternating current and the secondary side of a high-power DC transformer for converting alternating current to direct current; The primary side of the high-power DC transformer includes a hybrid multilevel converter and the transformer primary winding. The hybrid multilevel converter includes a DC side and one or two phases, each phase including eight switching devices and a flying capacitor. ; A and B are phase A and phase B on the primary side of the hybrid multilevel converter, respectively. The DC side includes three DC bus capacitors connected in series. First capacitor Second capacitor The DC side is connected to an external DC power supply. The three DC bus capacitors The first capacitor is connected in series with a DC power supply. Second capacitor Series connection with 3 DC bus capacitors Parallel connection; the first capacitor Second capacitor A first neutral point is set between them; The eight switching devices are divided into four groups, namely: , , , ; The The first terminal is connected to the DC bus capacitor. The positive terminal is connected to the first terminal, and the second terminal is connected to the second terminal. The first end and The second end; the The first end is also with The first end is connected to the second end, and the second end is connected to the DC bus capacitor. The negative electrode; the Second end connection The first end; the The first end is connected to the DC bus capacitor. The positive terminal, the second terminal connected The first end and The first end; the The second end is connected to the DC bus capacitor. The negative electrode; the Second end connection The first terminal; when the multilevel converter contains two phases, the The second end and the The connection point corresponding to the second end is connected to both ends of the primary side of the transformer as the AC output terminal of the primary side of the high-power DC transformer; when the multilevel converter contains only one phase, the... The second end and the The connection point corresponding to the second end is connected to one end of the primary side of the transformer, and the first capacitor Second capacitor The first neutral point between them is connected to the other end of the primary side of the transformer; The The second end and The first end is connected to a flying capacitor. The positive electrode, the The second end and The first end is connected to a flying capacitor. The negative electrode; It also includes a control system for balancing high-power DC transformers. The control system acquires capacitor voltage data and output current data collected by voltage and current sensors. Based on the acquired capacitor voltage data and output current data, and using a preset capacitor deviation calculation strategy, it calculates the capacitor voltage deviation value corresponding to each capacitor. Determine whether the absolute value of the capacitor voltage deviation value corresponding to each capacitor is less than a preset first deviation threshold; If the judgment result is yes, then the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer. If the judgment result is negative, then at the moment when the output level of the hybrid multilevel converter switches or the current reverses, based on the output value corresponding to the output level at the next moment, and according to a preset selection strategy, the switching state of each switching device that can prevent the capacitor voltage deviation value of the largest capacitor voltage deviation value from deviating the largest is selected and each switching device is controlled until the absolute value of the capacitor voltage deviation value corresponding to all capacitors is less than the preset second deviation threshold. Then, at the moment when the output level of the hybrid multilevel converter switches or the current reverses, the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer; the preset first deviation threshold is greater than the preset second deviation threshold.

2. The high-power DC transformer based on a four-level topology according to claim 1, characterized in that: The secondary side of the high-power DC transformer includes four switching devices, an output capacitor, and a transformer secondary winding, which are respectively... The and Forward series connection, the and Forward series connection; both series branches are connected in parallel with the output capacitor; The and The series connection point, and the and The series connection point is connected to the secondary side of the transformer as the AC input terminal of the secondary side of the high-power DC transformer.

3. The high-power DC transformer based on a four-level topology according to claim 2, characterized in that: The hybrid multilevel converter also includes a phase-shifting inductor. The The second end and the The second terminal is connected to the phase-shifting inductor The first end is connected to the phase-shifting inductor. The second end is connected to the primary side of the transformer.

4. The high-power DC transformer based on a four-level topology according to claim 3, characterized in that: It also includes a method for acquiring the capacitance of three DC bus terminals. In addition, a voltage sensor for the capacitor voltage corresponding to the flying capacitor and a sensor for collecting the output current corresponding to the AC output terminal of the primary side of the high-power DC transformer. Current sensor.

5. A balance control method for a high-power DC transformer, using a high-power DC transformer based on a four-level topology according to any one of claims 1 to 4, characterized in that: Includes the following steps: S1. Acquire the capacitor voltage data and output current data collected by the voltage sensor and current sensor; S2. Based on the collected capacitor voltage data and output current data, and using the preset capacitor deviation calculation strategy, calculate the capacitor voltage deviation value corresponding to each capacitor. S3. Determine whether the absolute value of the capacitor voltage deviation value corresponding to each capacitor is less than the preset first deviation threshold. S4. If the judgment result is yes, then the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer. S5. If the judgment result is negative, then at the moment when the output level of the hybrid multilevel converter switches or the current reverses, based on the output value corresponding to the output level at the next moment, and according to the preset selection strategy, the switching state of each switching device that can prevent the capacitor voltage deviation value of the capacitor with the largest deviation value from the maximum is selected and each switching device is controlled until the absolute value of the capacitor voltage deviation value corresponding to all capacitors is less than the preset second deviation threshold. Then, at the moment when the output level of the hybrid multilevel converter switches or the current reverses, the preset switching state rotation control strategy is invoked to control the switching state of each switching device on the primary side of the high-power DC transformer; the preset first deviation threshold is greater than the preset second deviation threshold.

6. The balance control method for a high-power DC transformer according to claim 5, characterized in that: The preset capacitance deviation calculation strategy is as follows: Based on the collected capacitor voltage and output current data, and using a preset current calculation formula, the three DC bus capacitors corresponding to the hybrid multilevel converter are calculated. The current corresponding to the connection point between them, and the flyover current flowing through each phase. ; The preset current calculation formula is: In the formula, , Three DC bus capacitors The current corresponding to the connection point between them; A and B are phase A and phase B on the primary side of the hybrid multilevel converter, respectively. In phase A The current corresponding to the component, In phase B The current corresponding to the component, In phase x The current corresponding to the component; In phase A The current corresponding to the component, In phase B The current corresponding to the component, In phase x The current corresponding to the component, , as well as These represent the switch states corresponding to phase x, with 1 indicating the switch is on and 0 indicating the switch is off. Let x be the output current corresponding to phase x; Based on the three DC bus capacitors corresponding to the hybrid multilevel converter The current corresponding to the connection point between them, and the flyover current flowing through each phase. Based on the preset formula for calculating capacitor voltage deviation, the capacitor voltage deviation of each capacitor in the hybrid multilevel converter is calculated. In the formula, , , , These represent the upper DC bus capacitor, the middle DC bus capacitor, and the lower DC bus capacitor, respectively. x The capacitance value of the phase-flying capacitor. This is the capacitor voltage deviation value corresponding to the upper DC bus capacitor.

Images