A modular series step-up high-voltage autotransformer and its control method

Abstract

This invention relates to a modular series-connected step-up high-voltage autotransformer and its control method, belonging to the field of power electronics technology, and solves the problems of high cost and complex control of existing DC transformers. The active DC transformer is composed of N cascaded energy buffer bridge arm modules; each energy buffer bridge arm module includes a first energy buffer bridge arm and a second energy buffer bridge arm, a control switch group, and a DC capacitor C; by controlling different configurations of the control switch group, the first and second energy buffer bridge arms selectively form charging or discharging circuits with the DC capacitor C. The above technical solution can achieve reliable turn-off of soft switching and thyristors, improving performance and reliability; and through precise control of the bridge arm voltage, the bridge arm current waveform can be flexibly controlled, thereby accurately adjusting the transmission power and maintaining the energy balance of the bridge arm itself.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a modular series step-up high-voltage autotransformer and its control method. Background Technology

[0002] With the large-scale grid connection of new energy sources such as wind and solar power, DC transformers, with their efficient and economical power collection capabilities, are becoming key equipment in building integrated DC transmission and distribution networks, and are crucial for promoting the modernization of the power system. To adapt to this development trend, the research and development of new types of DC transformers urgently needs to be advanced.

[0003] Currently, isolated high-voltage solutions rely on bulky power frequency AC transformers, resulting in high costs and manufacturing difficulties. Meanwhile, non-isolated low-voltage solutions using IGBT devices face the technical challenge of dynamic voltage equalization when operating in high-voltage series. To address these issues, some researchers have proposed DC-DC converter solutions based on modular multilevel converters (MFMCs). However, while the traditional FTF-MMC structure avoids direct series connection of devices, its two-stage conversion architecture leads to decreased system efficiency and requires an additional 50% of sub-modules. Traditional autotransformers replace bulky AC transformers with sub-module reuse, but require injecting AC circulating current to achieve power equalization, which undoubtedly increases control complexity.

[0004] Therefore, how to design a new topology that combines economy and reliability to meet the actual needs of DC transformers for new energy power transmission and distribution has become a core technical problem that urgently needs to be solved in the current power field. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a modular series step-up high-voltage autotransformer and its control method to solve the problems of high cost and complex control of existing DC transformers.

[0006] On one hand, this invention provides a modular series-connected step-up high-voltage autotransformer, wherein the active DC transformer is composed of N cascaded energy buffer bridge arm modules; wherein, Each of the energy buffer bridge arm modules includes a first energy buffer bridge arm and a second energy buffer bridge arm, a control switch group, and a DC capacitor C; by controlling different configurations of the control switch group, the first energy buffer bridge arm and the second energy buffer bridge arm can selectively form a charging or discharging circuit with the DC capacitor C.

[0007] Based on the above solution, the present invention also makes the following improvements: Furthermore, the control switch group includes thyristor valves T1 and T2 and diode valves D1, D2, and D3; The high-voltage end of the first energy buffer bridge arm Arm1 is connected to the anode of diode valve D1 and one end of DC capacitor C, respectively. The cathode of diode valve D1 is connected to the high-voltage end of the second energy buffer bridge arm Arm2 and the anode of diode valve D3, respectively. The other end of DC capacitor C is connected to the cathode of diode valve D3. The low-voltage end of the first energy buffer bridge arm Arm1 is connected to the anode of diode valve D2 and the anode of thyristor valve T1, respectively. The low-voltage end of the second energy buffer bridge arm Arm2 is connected to the cathode of diode valve D2 and the anode of thyristor valve T2, respectively. The cathodes of thyristor valves T1 and T2 are both grounded. The high-voltage end of the first energy buffer bridge arm Arm1 and the cathode of the diode valve D3 serve as the input and output ends of the energy buffer bridge arm module, respectively.

[0008] Furthermore, both the first and second energy buffer bridge arms include multiple cascaded sub-modules and a bridge arm inductor L; wherein, The input terminal of the first submodule serves as one end of the first or second energy buffer bridge arm, and the output terminal of the last submodule is connected to one end of the bridge arm inductor L. The other end of the bridge arm inductor L serves as the other end of the first or second energy buffer bridge arm.

[0009] Furthermore, the submodules of the first energy buffer bridge arm are either full-bridge submodules or T-type submodules; the submodules of the second energy buffer bridge arm are either full-bridge submodules or half-bridge submodules.

[0010] Furthermore, thyristor valve T1 and thyristor valve T2 are used to control the charging of the first energy buffer bridge arm and the second energy buffer bridge arm, respectively. Diode valve D1 is used to control the separation of the charging circuits of the first and second energy buffer bridge arms from the high-voltage side; Diode valves D2 and D3 are used together to control the discharge of the first energy buffer bridge arm and the second energy buffer bridge arm.

[0011] On the other hand, the present invention also provides a control method for a modular series step-up high-voltage autotransformer, the control method comprising: Within each control cycle, the synchronous control of each energy buffer bridge arm module in the active DC transformer sequentially executes the following four operating modes: Working Mode 1: In t 0- t During the three time periods, control thyristor valves T1 and T2 to be turned on simultaneously, and control the first energy buffer bridge arm and the second energy buffer bridge arm to absorb energy from the preceding voltage until thyristor valves T1 and T2 are turned off. Working Mode 2: In t 3- tDuring the 4-period period, the voltage of the first and second energy buffer bridge arms is greater than the voltage of the preceding stage, and the duration exceeds the reverse recovery time of thyristor valves T1 and T2. Working Mode 3: In t 4- t During the 7-period period, the first and second energy buffer bridge arms are controlled to release energy to the subsequent voltage until diode valves D2 and D3 are turned off; Working Mode 4: In t 7- t During the 8-period period, the sum of the voltages of the first and second energy buffer bridge arms is less than the voltage of the subsequent stage, and the duration exceeds the reverse recovery time of diode valves D2 and D3.

[0012] Based on the above solution, the present invention also makes the following improvements: Furthermore, in the first working mode, the following is performed: exist t = t At time 0, control thyristor valves T1 and T2 to be turned on simultaneously, and control the voltage of the first energy buffer bridge arm and the second energy buffer bridge arm to be less than the voltage of the preceding stage. When the current in the first energy buffer bridge arm rises to its maximum value, the voltage of the first energy buffer bridge arm is controlled to equal the voltage of the preceding stage. This is recorded as... t = t At time 1; when the arm current on the second energy buffer bridge arm rises to its maximum value, the arm voltage of the second energy buffer bridge arm is controlled to equal the voltage of the preceding stage, and this time is recorded as... t = t 1' moment; exist t = t At time 2, the voltage of both the first and second energy buffer bridge arms is greater than the voltage of the preceding stage, until the thyristor valves T1 and T2 are turned off. in, t 1 and t 1' is less than t 2.

[0013] Furthermore, in the third working mode, the following is performed: exist t 4- t During the 5-period interval, the voltage of the first energy buffer bridge arm is controlled to be negative, and the voltage of the second energy buffer bridge arm is controlled to be positive. exist t 5- t During the 6-period interval, the first and second energy buffer bridge arms release the energy absorbed in working mode one; exist t 6- t During the 7-period interval, the direction of the arm voltage of the first energy buffer bridge arm and the second energy buffer bridge arm remains unchanged, while the absolute value of the corresponding bridge arm voltage is reduced, so that the sum of the arm voltages of the first energy buffer bridge arm and the second energy buffer bridge arm is less than the voltage of the subsequent stage, until the diode valves D2 and D3 are turned off.

[0014] Furthermore, the preceding voltage refers to the input voltage of the current energy buffer bridge arm module; the following voltage refers to the output voltage of the current energy buffer bridge arm module.

[0015] Furthermore, in the first working mode, energy balance control is performed on the submodule. Output voltage control is performed in the third operating mode. In all four operating modes, bridge arm inductor current control, submodule capacitor voltage balance control, and carrier phase shift modulation control are performed.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: This invention provides a modular series-connected step-up high-voltage autotransformer. Based on a modular multilevel converter architecture and device series technology, this transformer possesses both high-voltage withstand capability and, through the use of semi-controlled devices, effectively avoids complex series drive and voltage equalization issues, while also enabling soft switching. This topology employs sub-module-based active bridge arms for energy transfer and uses thyristors as control switches. Its normal operation hinges on precise control of the bridge arm energy and reliable thyristor turn-off.

[0017] The control method for a modular series-connected step-up high-voltage autotransformer provided by this invention fully utilizes the active characteristics of the bridge arms. On the one hand, it achieves soft switching and reliable thyristor turn-off, thereby improving performance and reliability. On the other hand, through precise control of the bridge arm voltage, the bridge arm current waveform becomes flexible and controllable, thus accurately adjusting the transmission power and maintaining the energy balance of the bridge arms themselves, ultimately achieving the comprehensive goals of cost reduction and optimized control. Simultaneously, to consider the scalability of the equipment, an energy buffer bridge arm module is introduced. A direct electrical circuit is constructed between the low-voltage and high-voltage sides using a DC capacitor as a medium; thyristors and diode converter valves are used to reduce the cost of topology switching devices, reduce switching losses, and improve operating efficiency; two energy buffer bridge arms form a modular unit, utilizing different switching states of the sub-modules to achieve parallel charging and series discharging, thereby completing the step-up function. This invention uses an active energy buffer bridge arm as the energy transfer medium, which can meet the application requirements of high voltage and large capacity. It adopts a modular design and has good scalability. It uses a semi-controlled converter valve to improve the efficiency and economy of the topology. In addition, the parallel charging and series discharging control method for the energy buffer bridge arm can achieve high transformation ratio boost.

[0018] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0019] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. Figure 1 This is a topology diagram of the modular series step-up high-voltage autotransformer provided in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the energy buffer bridge arm module provided in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the structure of the full-bridge submodule provided in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the waveform changes of the energy buffer bridge arm module corresponding to the control method of the modular series step-up high-voltage autotransformer provided in Embodiment 2 of the present invention. Figure 5 This is a schematic diagram of the modular series step-up high-voltage autotransformer provided in Embodiment 2 of the present invention in operating mode one; Figure 6 This is a schematic diagram of the modular series step-up high-voltage autotransformer provided in Embodiment 2 of the present invention in operating mode two; Figure 7 This is a schematic diagram of the modular series step-up high-voltage autotransformer provided in Embodiment 2 of the present invention in operating mode three. Figure 8 This is a block diagram illustrating the principle of energy balance control for submodule strings provided in Embodiment 2 of the present invention. Figure 9 This is a block diagram illustrating the principle of submodule capacitor voltage balance control provided in Embodiment 2 of the present invention. Figure 10 This is a principle block diagram of phase-shift carrier pulse width modulation provided in Embodiment 2 of the present invention; Figure 11 This is a simulation curve of the output voltage of the first energy buffer bridge arm module in the simulation verification of this invention; Figure 12 The diagram shows the overall current waveform of the two energy buffer bridge arm branches in the first energy buffer bridge arm module during the simulation verification of this invention. Figure 13 This is a diagram showing the capacitor voltage fluctuation of the sub-module of the energy buffer bridge arm branch of the first energy buffer bridge arm group module in the simulation verification of this invention. Detailed Implementation

[0020] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0021] Specific embodiment 1 of the present invention discloses a modular series step-up high-voltage autotransformer, the topology of which is shown in the figure below. Figure 1 As shown. The active DC transformer provided in this embodiment is composed of N cascaded energy buffer bridge arm modules; each energy buffer bridge arm module includes a first energy buffer bridge arm and a second energy buffer bridge arm, a control switch group and a DC capacitor C; by controlling different configurations of the control switch group, the first energy buffer bridge arm and the second energy buffer bridge arm can selectively form a charging or discharging circuit with the DC capacitor C.

[0022] Preferably, in this embodiment, the control switch group includes thyristor valves T1 and T2 and diode valves D1, D2, and D3; wherein, the high-voltage end of the first energy buffer bridge arm Arm1 is connected to the anode of diode valve D1 and one end of DC capacitor C, respectively; the cathode of diode valve D1 is connected to the high-voltage end of the second energy buffer bridge arm Arm2 and the anode of diode valve D3, respectively; the other end of DC capacitor C is connected to the cathode of diode valve D3; the low-voltage end of the first energy buffer bridge arm Arm1 is connected to the anode of diode valve D2 and the anode of thyristor valve T1, respectively; the low-voltage end of the second energy buffer bridge arm Arm2 is connected to the cathode of diode valve D2 and the anode of thyristor valve T2, respectively; the cathodes of thyristor valves T1 and T2 are both grounded. The high-voltage end of the first energy buffer bridge arm Arm1 and the cathode of diode valve D3 serve as the input and output ends of the energy buffer bridge arm group module, respectively. A schematic diagram of the energy buffer bridge arm group module is shown below. Figure 2 As shown.

[0023] In the energy buffer bridge arm module provided in this embodiment, thyristor valves T1 and T2 are used to control the charging of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2, respectively; diode valve D1 is used to control the separation of the charging circuit of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 from the high-voltage side; diode valves D2 and D3 are used together to control the discharging of the first energy buffer bridge arm and the second energy buffer bridge arm. In the above-mentioned active DC transformer, the input terminal of the first energy buffer bridge arm module receives the low-voltage side voltage. V dc1 The output terminal of the last energy buffer bridge arm module outputs the high-voltage side voltage. V dc2 .

[0024] In this embodiment, both the first and second energy buffer bridge arms include multiple cascaded sub-modules (SMs) and a bridge arm inductor L. The input terminal of the first sub-module serves as one end of either the first or second energy buffer bridge arm, and the output terminal of the last sub-module is connected to one end of the bridge arm inductor L. The other end of the bridge arm inductor L serves as the other end of either the first or second energy buffer bridge arm. Preferably, the sub-modules of the first energy buffer bridge arm must meet the requirements of outputting two polarities of voltage and unidirectional current; specifically, full-bridge sub-modules, T-type sub-modules, etc., can be used. The sub-modules of the second energy buffer bridge arm must meet the requirements of outputting unipolar voltage and bidirectional current; specifically, half-bridge sub-modules, full-bridge sub-modules, etc., can be used.

[0025] Embodiment 2 of the present invention provides a control method for a modular series step-up high-voltage autotransformer. This control method includes: synchronously controlling each energy buffer bridge arm module in the active DC transformer to sequentially execute the following four operating modes within each control cycle. Therefore, the operating modes of all energy buffer bridge arm modules remain consistent, meaning that the conduction status of the converter valves in all energy buffer bridge arm modules is the same.

[0026] The following description focuses on a specific energy buffer bridge arm module. Thyristor valves T1 and T2 operate according to the switching cycle T. s Synchronous conduction, considering the reverse recovery process after the thyristor valve is turned off, the energy buffer bridge arm module includes four operating modes, as detailed below. A schematic diagram of the waveform changes of the energy buffer bridge arm module corresponding to this control method is shown below. Figure 4 As shown. Schematic diagrams for working modes 1, 2, and 3 are respectively shown below. Figure 5 , Figure 6 , Figure 7 As shown.

[0027] (1) Working Mode 1 exist t 0- t During the three time periods, thyristor valves T1 and T2 are simultaneously turned on, and the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 are controlled to receive voltage from the preceding stage. V pre It absorbs energy until the thyristor valves T1 and T2 are turned off.

[0028] In this embodiment, the front-end voltage V pre This refers to the input voltage of the current energy buffer bridge arm module. For the first energy buffer bridge arm module, the preceding voltage... V pre This refers to the low-voltage side voltage. V dc1 For the second and subsequent energy buffer bridge arm modules, the front-end voltage... V pre This refers to the output voltage of the previous energy buffer bridge arm module of the current energy buffer bridge arm module.

[0029] 1) In t = t At time 0, thyristor valves T1 and T2 are simultaneously turned on, controlling the arm voltages of the first energy buffer arm Arm1 and the second energy buffer arm Arm2. v arm1 , v arm2 All are less than the voltage of the preceding stage. V pre .

[0030] Therefore, diode valve D1 is turned on, and the inductor on the first energy buffer bridge arm Arm1 withstands the voltage from the preceding stage. V pre and v arm1 The voltage difference between them, the inductor on the second energy buffer bridge arm Arm2 withstands the voltage from the previous stage. V pre and v arm2 The voltage difference between them. At this time, the bridge arm currents on the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 both increase linearly.

[0031] It should be noted that, in this embodiment, the bridge arm voltage of the energy buffer bridge arm refers to the voltage across all sub-modules cascaded on the energy buffer bridge arm, and the bridge arm current on the energy buffer bridge arm refers to the current flowing through the energy buffer bridge arm.

[0032] 2) When the arm current on the first energy buffer arm Arm1 rises to the maximum value of the arm current on the first energy buffer arm Arm1. I arm1_cha At that time, the voltage of the first energy buffer arm Arm1 is equal to the voltage of the preceding stage. V pre When the arm current on the second energy buffer arm Arm2 rises to the maximum value of the arm current on the second energy buffer arm Arm2. I arm2_cha At that time, the voltage of the second energy buffer arm Arm2 is equal to the voltage of the preceding stage. V pre .

[0033] When the arm current on the first energy buffer arm Arm1 rises to the maximum value of the arm current of the first energy buffer arm Arm1 I arm1_cha At that time, the voltage of the first energy buffer arm Arm1 is equal to the voltage of the preceding stage. V pre To maintain the bridge arm current on the first energy buffer bridge arm Arm1, let's denote this time... t = t 1 moment.

[0034] When the arm current on the second energy buffer arm Arm2 rises to the maximum value of the arm current of the second energy buffer arm Arm2 I arm2_cha At that time, the voltage of the second energy buffer arm Arm2 is equal to the voltage of the preceding stage. V pre To maintain the bridge arm current on the second energy buffer bridge arm Arm2, let's denote this time... t = t1' moment.

[0035] It is worth noting that, since the parameters of the submodules of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 may differ, t 1 and t 1' may not be the same if and only if the device parameters and control parameters of the two energy buffer bridge arms are the same. t 1= t 1'.

[0036] 3) In t = t At time 2, control the arm voltages of the first energy buffer arm Arm1 and the second energy buffer arm Arm2. v arm1 , v arm2 All are greater than the preceding voltage. V pre until both thyristor valves T1 and T2 are turned off.

[0037] Specifically, in t = t At time 2, control the arm voltage of the first energy buffer arm Arm1. v arm1 Greater than the preceding voltage V pre This causes the arm current on the first energy buffer arm Arm1 to decrease linearly until... t At time 3, the current drops to zero. During this period, the capacitor in the submodule SM on the first energy buffer bridge arm Arm1 is charged. At the same time, the current in thyristor valve T1 crosses zero and is subjected to reverse voltage, thus beginning to turn off.

[0038] At the same time, t = t At time 2, the arm voltage of the second energy buffer arm Arm2 is also controlled. v arm2 Greater than the preceding voltage V pre This causes the arm current on the second energy buffer arm Arm2 to decrease linearly until... t At time 3', the current drops to zero. During this period, the capacitor in submodule SM on the second energy buffer bridge arm Arm2 is charged. Simultaneously, during this process, the current of thyristor valve T2 crosses zero and experiences reverse voltage, initiating its shutdown.

[0039] It is worth noting that the parameters of the submodules in the two energy buffer bridge arms may differ. t 3 and t 3' may not be the same if and only if the device parameters and control parameters in both energy buffer arms are identical.t 3= t 3'. At the same time, during this process, the current of thyristor valve T2 crosses zero and is subjected to reverse pressure, thus starting to shut off.

[0040] Furthermore, it should be noted that in operating mode one, the front-end voltage... V pre With DC capacitor C Discharge to the next stage in series, such as Figure 5 As shown.

[0041] (2) Working Mode Two exist t 3- t During the 4 time periods, control the arm voltages of the first energy buffer arm Arm1 and the second energy buffer arm Arm2. v arm1 , v arm2 All are greater than the preceding voltage. V pre The duration exceeds the reverse recovery time of thyristor valves T1 and T2. t rr .

[0042] Specifically, assuming t 3- > t 3', then in t 3- t During the 4th time period, since the current of thyristor valves T1 and T2 is already zero in operating mode one, thyristor valves T1 and T2 enter the turn-off process. At this time, the arm voltages controlling the first energy buffer arm Arm1 and the second energy buffer arm Arm2 remain greater than the preceding stage voltage. V pre This ensures reliable shut-off of thyristor valves T1 and T2. At this time, thyristor valves T1-T2 and diode valves D1-D2 are both in a reverse blocking state. Furthermore, it should be noted that because the thyristor has a reverse recovery current, the duration of operating mode two must exceed the reverse recovery time of the thyristor valves. t rr .

[0043] Furthermore, it should be noted that in operating mode two, the front-end voltage... V pre With DC capacitor C Discharge to the next stage in series, such as Figure 6 As shown.

[0044] (3) Working Mode 3: In t 4- t During the 7-period interval, control the voltage of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 to the next stage.V sub Energy is released until diode valves D2-D3 shut off automatically.

[0045] In this embodiment, the subsequent voltage V sub This refers to the output voltage of the current energy buffer bridge arm module. For the last energy buffer bridge arm module, the subsequent voltage... V sub This refers to the high-voltage side voltage. V dc2 For the residual energy buffer bridge arm module, the subsequent voltage... V sub This refers to the output voltage of the current energy buffer bridge arm module, which is the input voltage of the next energy buffer bridge arm module.

[0046] 1) In t 4- t During the 5-period time period, control the arm voltage of the first energy buffer arm Arm1. v arm1 It is a negative value, and controls the arm voltage of the second energy buffer arm Arm2. v arm2 The value is positive. At this time, diode valve D1 experiences a negative voltage drop and is in the off state.

[0047] During this period, the current control strategy is similar to that of Mode 1. Specifically, the arm voltages of the first energy buffer arm and the second energy buffer arm... v arm1 , v arm2 With the preceding voltage V pre The sum of the three is greater than the voltage of the subsequent stage. V sub Therefore, diode valves D2 and D3 are turned on because they both bear a positive voltage, and the inductors on the two energy buffer bridge arms share the load. v arm1 , v arm2 With the preceding voltage V pre The sum of the three and V sub The voltage difference between them. At this time, the bridge arm currents on both the first and second energy buffer bridge arms increase linearly.

[0048] At this point, the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 are connected in series. When the bridge arm current on the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 reaches its maximum value, the low-voltage side voltage, the high-voltage side voltage, and the voltage of the two bridge arms should satisfy the following relationship: (1) 2) In t 5- t During the 6-period interval, the first energy buffer arm Arm1 and the second energy buffer arm Arm2 release the energy they absorbed in working mode one.

[0049] Of the released energy, a portion is from the DC capacitor. C The charge is applied, and the remaining portion flows to the high-voltage side, such as... Figure 7 As shown. Similar to operating mode one, the arm currents of the two energy buffer bridge arms exhibit trapezoidal waves, and the energy released by the two energy buffer bridge arms varies from the maximum amplitude of the current. I arm_rel Decide.

[0050] 3) In t 6- t During the 7-period control, the direction of the voltage of the first energy buffer arm Arm1 and the second energy buffer arm Arm2 remains unchanged, while the absolute value of the corresponding arm voltage is reduced, so that the sum of the voltages of the first energy buffer arm Arm1 and the second energy buffer arm Arm2 is less than the voltage of the subsequent stage. V sub This causes the bridge arm currents on the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 to decrease linearly until... t At time 7, the current drops to zero. Diode valves D2 and D3 enter the off state due to the current crossing zero and the reverse voltage. During this period, the capacitor in submodule SM is discharged.

[0051] Working Mode 4: In t 7- t During the 8-period time period, the sum of the voltages of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 is less than the voltage of the subsequent stage. V sub The duration exceeds the reverse recovery time of diode valves D2 and D3.

[0052] Specifically, in t 7- t During the 8-period time interval, diode valves D1 and D2 have entered the off state. After the bridge arm current in operating mode three drops to zero, the sum of the bridge arm voltages of the first and second energy buffer bridge arms is maintained below the subsequent stage voltage. V sub This is to ensure that the diode valve can be reliably shut off.

[0053] In addition, it should be noted that the voltage of the preceding stage at this time... V pre With DC capacitor C Discharge to the next stage in series, such as Figure 5 As shown.

[0054] In the specific implementation of the control method in this embodiment, to ensure the control effect, submodule string energy balance control is performed in operating mode one; output voltage control is performed in operating mode three; and bridge arm inductor current control, submodule capacitor voltage balance control, and carrier phase-shift modulation control are also performed in all four operating modes. That is, the control method for the modular series step-up high-voltage autotransformer provided in this embodiment includes five parts: submodule string energy balance control, output voltage control, bridge arm inductor current control, submodule capacitor voltage balance control, and carrier phase-shift modulation. Among them, submodule string energy balance control is mainly responsible for realizing the operation of operating mode one; output voltage control is mainly responsible for realizing the operation of operating mode three; and bridge arm inductor current control, submodule capacitor voltage balance control, and carrier phase-shift modulation all play a control role in all four operating modes. In specific implementation, the energy exchanged between the energy buffer bridge arms during commutation is related to the maximum value of the energy buffer bridge arm current. By controlling the maximum current of the energy buffer bridge arm inductor, the energy exchanged between the energy buffer bridge arms is controlled, thereby controlling the output voltage on the high-voltage side.

[0055] It is important to emphasize that in this embodiment, the arm voltages of the first energy buffer arm Arm1 and the second energy buffer arm Arm2 are controlled by respectively controlling the number of sub-modules deployed within them. Specifically, carrier phase-shift modulation control can be applied to each sub-module. The output of the carrier phase-shift modulation is the trigger signal for the corresponding IGBT of each sub-module, which can control the deployment and bypassing of sub-modules, thereby changing the total number of deployed sub-modules in the entire arm and thus controlling the arm voltage of the entire energy buffer arm. However, the carrier phase-shift modulation control is constrained by sub-module string energy balance control, output voltage control, arm inductor current control, and sub-module capacitor voltage balance control. Therefore, through the cooperation of these control methods, the arm voltage of the energy buffer arm that matches the aforementioned operating mode is ultimately controlled.

[0056] Specifically, in this embodiment, the submodule string energy balance control includes: using a first proportional-integral controller to control the maximum value of the current absorbed by the two bridge arms to achieve energy buffer bridge arm energy balance control; the output voltage control includes: using a second proportional-integral controller to control the power absorbed by the DC capacitor to achieve output voltage stability; the bridge arm inductor current control includes: multiplying the bridge arm absorption current and discharge current by a waveform control signal function respectively, adding them together to obtain a reference value for the bridge arm current, and using a third proportional-integral controller for control. The submodule capacitor voltage balance control includes: adding a correction signal to adjust the capacitor voltage signal of each submodule, and using a proportional controller for control; adding the submodule string reference voltage signal and the equalization capacitor voltage difference of the submodule to obtain the modulation signal of each submodule, and using carrier phase-shift modulation to obtain the control signal of each submodule.

[0057] The principle block diagram of submodule string energy balance control is as follows: Figure 8 As shown. It should be noted that, in the operating mode, the sub-module string energy balance control of the energy buffer bridge arm is executed, specifically including: using a first proportional-integral controller to control the maximum current absorbed by the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2. I arm1_cha , I arm2_cha The method achieves energy balance control of the two energy buffer bridge arms by adding a correction signal to adjust the capacitor voltage signal of each submodule, and using the proportional controller in the first proportional-integral controller for control; the submodule is connected in series with the reference voltage signal. V SM_ref The voltage difference between the equalizing capacitors of the submodules is added to obtain the modulation signal for each submodule. This modulation signal is then phase-shift modulated by a carrier wave to obtain the control signal for each submodule. In the submodule string energy balance control, the mathematical equation for the first proportional-integral controller is: (2) In the formula, , These are the reference values ​​for the current absorbed by the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2, respectively. , These are the proportional gain coefficients of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 of the first proportional-integral controller, respectively. , These are the integral gain coefficients of the first energy buffer bridge arm Arm1 and the second energy buffer bridge arm Arm2 of the first proportional-integral controller, respectively. This refers to the rated capacitor voltage value of a single submodule. This represents the average capacitor voltage of the N sub-modules in the first energy buffer bridge arm. This represents the average capacitor voltage of the N sub-modules in the second energy buffer bridge arm.

[0058] In operating mode three, output voltage control is required, which includes stabilizing the output voltage by using a second proportional-integral controller to control the power absorption of the DC capacitor. The mathematical equation for the second proportional-integral controller in output voltage control is: (3) In the formula, DC capacitor C Reference value for voltage. DC capacitor C Voltage, This is a reference value for the discharge current of the first or second energy buffer bridge arm (the two bridge arms are in series and the current is the same). k p_rel This is the gain coefficient of the proportional element in the second proportional-integral controller. This is the gain coefficient of the integral element of the second proportional-integral controller.

[0059] Furthermore, in the control of the bridge arm inductor current, the mathematical equation of the third proportional-integral controller is: (4) (5) In the formula, This is a reference value for the capacitor voltage of the first energy buffer bridge arm. This is a reference value for the capacitor voltage of the second energy buffer bridge arm; This is the current reference value for the first energy buffer bridge arm. This is the current reference value for the second energy buffer bridge arm. and These are the control signal functions for the first and second waveforms, respectively. and These are the gain coefficients of the proportional elements of the first and second energy buffer bridge arms of the third proportional-integral controller, respectively. and These are the gain coefficients of the first and second energy buffer bridge arm integral links of the third proportional-integral controller, respectively.

[0060] The principle block diagram of the capacitor voltage balance control of the submodule is as follows: Figure 9 As shown. In the capacitor voltage balance control of the submodule, the mathematical equation of the proportional controller is: (6) In the formula, The first energy buffer bridge arm i Individual module capacitor voltage, For the second energy buffer bridge arm i Individual module capacitor voltage; The first energy buffer bridge arm i The voltage difference of the equalizing capacitors in each submodule For the second energy buffer bridge arm i The equalization capacitor voltage difference of each submodule; and These are the gain coefficients of the proportional element of the first and second energy buffer bridge arms, respectively. This represents the sign function of the first energy buffer arm current. Indicates the sign function of the second energy buffer bridge arm current; N The number of submodules in the first or second energy buffer bridge arm; This indicates the current in the first energy buffer bridge arm; This indicates the current in the second energy buffer arm.

[0061] The principle block diagram of phase-shift carrier pulse width modulation is as follows: Figure 10 As shown. Bridge arm voltage reference value. v arm_ref With each submodule's correction item Δ v sm_i This approach combines system-level power control and internal voltage equalization control. The synthesized reference signal is then fed into a phase-shifted carrier pulse-width modulation (PSC-PWM) unit. In this scheme, each submodule is assigned a triangular carrier with a specific phase shift, thereby uniformly distributing switching actions within a switching cycle. This method increases the equivalent switching frequency, improving output waveform quality while reducing switching losses.

[0062] To verify the beneficial effects of this invention, scientific demonstration was conducted through comparative experiments. Based on Figure 1 The DC transformer structure shown is built using an existing software platform. The simulation verification of this topology is performed, and the simulation parameters are shown in Table 1 below.

[0063] Table 1 Simulation parameters of DC transformer

[0064] Under the operating conditions shown in the table above, with a given input voltage of 20kV, an output voltage of 50kV, and a rated transmission power of 10MW, and to ensure reliable thyristor turn-off, a switching frequency of 150Hz is set. The simulation results are as follows. Figures 11-13 As shown. Figure 11 The waveform of the output voltage during the simulation verification of this invention shows that the voltage remained stable at around 50kV throughout the process. Figure 12This is a simulation verification diagram of the overall current waveforms of the two energy buffer bridge arm branches in the first group of energy buffer bridge arm modules. It can be seen that the current waveforms of the two energy buffer bridge arm branches change in the form of trapezoidal waves, consistent with the theoretical analysis. Specifically, the current direction of the first energy buffer bridge arm does not change with the operating mode, while the current direction of the second energy buffer bridge arm differs under different operating modes. Figure 13 This is a voltage fluctuation diagram of the submodules in the energy buffer bridge arm group module of the first group of energy buffer bridge arm modules in the simulation verification of this invention. Taking the capacitor voltage of one submodule as an example, the voltage remains stable at around 2kV throughout the process. The above simulation verification proves the effectiveness of the control method of this invention.

[0065] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0066] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A modular series voltage boosting high voltage self-coupled active DC transformer, characterized by, The active DC transformer is composed of N cascaded energy buffer bridge arm modules; wherein... Each of the energy buffer bridge arm modules includes a first energy buffer bridge arm and a second energy buffer bridge arm, a control switch group, and a DC capacitor C; by controlling different configurations of the control switch group, the first energy buffer bridge arm and the second energy buffer bridge arm can selectively form a charging or discharging circuit with the DC capacitor C.

2. The modular series-compounded high voltage autotransformer active DC transformer according to claim 1, characterized in that, The control switch group includes thyristor valves T1 and T2 and diode valves D1, D2, and D3; The high-voltage end of the first energy buffer bridge arm Arm1 is connected to the anode of diode valve D1 and one end of DC capacitor C, respectively. The cathode of diode valve D1 is connected to the high-voltage end of the second energy buffer bridge arm Arm2 and the anode of diode valve D3, respectively. The other end of DC capacitor C is connected to the cathode of diode valve D3. The low-voltage end of the first energy buffer bridge arm Arm1 is connected to the anode of diode valve D2 and the anode of thyristor valve T1, respectively. The low-voltage end of the second energy buffer bridge arm Arm2 is connected to the cathode of diode valve D2 and the anode of thyristor valve T2, respectively. The cathodes of thyristor valves T1 and T2 are both grounded. The high-voltage end of the first energy buffer bridge arm Arm1 and the cathode of the diode valve D3 serve as the input and output ends of the energy buffer bridge arm module, respectively.

3. The modular series-compounded high voltage autotransformer active DC-DC converter according to claim 2, characterized in that, Both the first and second energy buffer bridge arms include multiple cascaded sub-modules and a bridge arm inductor L; among which, The input terminal of the first submodule serves as one end of the first or second energy buffer bridge arm, and the output terminal of the last submodule is connected to one end of the bridge arm inductor L. The other end of the bridge arm inductor L serves as the other end of the first or second energy buffer bridge arm.

4. The modular series-compounded high voltage autotransformer active DC-DC converter according to claim 3, characterized in that, The submodules of the first energy buffer bridge arm are either full-bridge submodules or T-type submodules; the submodules of the second energy buffer bridge arm are either full-bridge submodules or half-bridge submodules.

5. The modular series step-up high-voltage autotransformer according to any one of claims 2-4, characterized in that, Thyristor valve T1 and thyristor valve T2 are used to control the charging of the first energy buffer bridge arm and the second energy buffer bridge arm, respectively. Diode valve D1 is used to control the separation of the charging circuits of the first and second energy buffer bridge arms from the high-voltage side; Diode valves D2 and D3 are used together to control the discharge of the first energy buffer bridge arm and the second energy buffer bridge arm.

6. A control method of a modular series voltage boosting high voltage self-coupled active DC transformer, characterized in that, The control method includes: Within each control cycle, the synchronous control of each energy buffer bridge arm module in the active DC transformer sequentially executes the following four operating modes: Working mode one: in t 0- t In the 3 period, the thyristor valves T1 and T2 are controlled to be turned on at the same time, and the first energy buffer bridge arm and the second energy buffer bridge arm are controlled to absorb energy from the front-stage voltage until the thyristor valves T1 and T2 are turned off; Working mode two: in t 3- t 4 period, the bridge arm voltage of the first energy buffer bridge arm and the second energy buffer bridge arm are both greater than the front stage voltage, and the duration exceeds the reverse recovery time of the thyristor valves T1 and T2; Working Mode 3: In t 4- t During the 7-period period, the first and second energy buffer bridge arms are controlled to release energy to the subsequent voltage until diode valves D2 and D3 are turned off; Working Mode 4: In t 7- t During the 8-period period, the sum of the voltages of the first and second energy buffer bridge arms is less than the voltage of the subsequent stage, and the duration exceeds the reverse recovery time of diode valves D2 and D3.

7. The control method for the modular series step-up type high-voltage autotransformer according to claim 6, characterized in that, In the first working mode, the following is executed: exist t = t At time 0, control thyristor valves T1 and T2 to conduct simultaneously, and control the voltage of the first energy buffer bridge arm and the second energy buffer bridge arm to be less than the voltage of the preceding stage. When the current in the first energy buffer bridge arm rises to its maximum value, the voltage of the first energy buffer bridge arm is controlled to equal the voltage of the preceding stage. This is recorded as... t = t At time 1; when the arm current on the second energy buffer bridge arm rises to its maximum value, the arm voltage of the second energy buffer bridge arm is controlled to equal the voltage of the preceding stage, and this time is recorded as... t = t 1' moment; exist t = t At time 2, the voltage of both the first and second energy buffer bridge arms is greater than the voltage of the preceding stage, until the thyristor valves T1 and T2 are turned off. in, t 1 and t 1' is less than t 2.

8. The control method for the modular series step-up type high-voltage autotransformer according to claim 7, characterized in that, In the third working mode, the following is executed: exist t 4- t During the 5-period interval, the voltage of the first energy buffer bridge arm is controlled to be negative, and the voltage of the second energy buffer bridge arm is controlled to be positive. exist t 5- t During the 6-period interval, the first and second energy buffer bridge arms release the energy absorbed in working mode one; exist t 6- t During the 7-period interval, the direction of the arm voltage of the first energy buffer bridge arm and the second energy buffer bridge arm remains unchanged, while the absolute value of the corresponding bridge arm voltage is reduced, so that the sum of the arm voltages of the first energy buffer bridge arm and the second energy buffer bridge arm is less than the voltage of the subsequent stage, until the diode valves D2 and D3 are turned off.

9. The control method for the modular series step-up type high-voltage autotransformer according to claim 8, characterized in that, The preceding voltage refers to the input voltage of the current energy buffer bridge arm module; the following voltage refers to the output voltage of the current energy buffer bridge arm module.

10. The control method for a modular series step-up high-voltage autotransformer according to any one of claims 6-9, characterized in that, In the operating mode, the energy balance control of the execution submodule is performed. Output voltage control is performed in the third operating mode. In all four operating modes, bridge arm inductor current control, submodule capacitor voltage balance control, and carrier phase shift modulation control are performed.

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