A method for minimizing dc capacitor of high-frequency transformer-based converter
By using magnetic circuit coupling in the high-frequency transformer to offset fluctuating power and reduce the capacitance of the DC-side capacitor, the problem of excessive capacitor size and weight in traditional converters is solved, and the power density is improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- XI AN JIAOTONG UNIV
- Filing Date
- 2022-05-07
- Publication Date
- 2026-05-19
AI Technical Summary
Traditional isolated converters transfer power fluctuations to the DC side when handling AC power, requiring large electrolytic capacitors, which increases the size and weight of the equipment and has a short lifespan, affecting power density.
By using magnetic circuit coupling in the high-frequency transformer to offset fluctuating power and reduce the capacitance of the DC-side capacitor, a high-frequency converter and a buffer branch structure are adopted to control power distribution and reduce the demand for DC-side capacitors.
Within the same voltage fluctuation range, the capacitance value of the DC-side capacitor is significantly reduced, which increases the power density of the device, reduces the size and weight of the capacitor, and avoids the addition of additional components.
Smart Images
Figure CN114726237B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of power electronics and power conversion technology, and relates to a method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer. Background Technology
[0002] With the increasing severity of global energy shortages and environmental pollution, the development and utilization of new energy sources have received growing attention, while also posing new challenges to power equipment. In the power conversion process, traditional power frequency transformers suffer from large size, making high power density technology a research hotspot and a challenge. On the other hand, due to the different voltage levels, power supply types, and load requirements of distributed power sources, multi-port power electronic transformers are being used more and more widely. Therefore, research on power equipment with high power density, multi-port coordination, and electrical isolation capabilities is both necessary and meaningful.
[0003] The proposed converter topologies based on high-frequency transformers have solved the technical challenges of multi-port operation and magnetic isolation. As the core component of such converters, the multi-winding high-frequency transformer greatly facilitates the expansion of the number of ports, enabling multi-port functionality. Simultaneously, the presence of the high-frequency transformer achieves magnetic isolation between the ports, improving the converter's reliability.
[0004] However, this converter still faces a technical challenge: the DC-side capacitor value needs further optimization to improve power density. Traditional isolated converter units transfer power fluctuations to the DC side when handling AC power, requiring large electrolytic capacitors on the DC side to buffer these fluctuations. These large capacitors result in significant equipment size and weight, increasing costs, and also suffer from serious drawbacks such as short lifespan. Therefore, further optimization of the capacitor value is urgently needed to improve power density. Summary of the Invention
[0005] To address the aforementioned issues, this patent proposes a method for reducing the capacitance value of the DC-side capacitor in a converter based on a high-frequency transformer. Without adding additional components, this method reduces the fluctuating power buffered at the capacitor by utilizing the principle that fluctuating power at the AC port is canceled out through the magnetic circuit. This minimizes the capacitance value of the DC-side capacitor, thereby reducing its size and weight and addressing the technical requirements for power density.
[0006] To achieve the above objectives, the present invention adopts the following technical solution.
[0007] A method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer includes:
[0008] Using the input converter as the reference direction, the total ripple power is obtained by summing the instantaneous power at all ports. This power is then distributed to each DC-side capacitor to calculate the power buffer required for each DC-side capacitor. ,in ;
[0009] Detect the input power of each port. Control the power transmitted from each high-frequency converter to the high-frequency transformer, so that the average value of this power during the high-frequency cycle is... equal to port input power DC-side capacitor buffer power The difference, where, .
[0010] As a further improvement of the present invention, the total fluctuation power When allocating power to each DC-side capacitor, the power should be distributed either equally or in any proportion, specifically including:
[0011] Total fluctuation power The power is evenly distributed among the DC-side capacitors, ensuring that each DC-side capacitor buffers an equal amount of power. ;
[0012] Total fluctuation power The power is allocated to each DC-side capacitor in any other proportion, and the power buffered by each DC-side capacitor is... ,in For the first The power buffered by each DC-side capacitor and the total ripple power The ratio, and satisfy .
[0013] As a further improvement of the present invention, the high-frequency transformer includes an m+n winding high-frequency transformer, m+n buffer branches, m+n high-frequency converters, m+n DC-side capacitors, and m interface converters; the terminals of each winding of the m+n winding high-frequency transformer are connected in series with a buffer branch, the other end of each buffer branch is connected to the AC port of the high-frequency converter, and the DC port of each high-frequency converter is connected in parallel with a DC-side capacitor; wherein, each of the m DC-side capacitors is cascaded with an interface converter to lead out m AC ports, and the n DC-side capacitors directly lead out n DC ports from both ends.
[0014] As a further improvement of the present invention, the frequency of the high-frequency transformer is any frequency in the range of several hundred hertz to several hundred kilohertz; the voltage and current frequencies of each AC port are power frequency and low frequency.
[0015] As a further improvement of the present invention, the number of AC ports is one, two to m; the number of DC ports is zero, one to n; and the type of switching transistor is IGBT, MOSFET or other arbitrary type of semiconductor switching transistor device.
[0016] As a further improvement of the present invention, the buffer branch between the high-frequency converter and the transformer winding adopts an LC series resonant branch structure. The high-frequency converter adopts a single-phase full-bridge / half-bridge converter. The switching frequency of the high-frequency converter is close to but not equal to the LC resonant frequency. The power output from the high-frequency converter to the high-frequency transformer is controlled by a phase-shifting control strategy. The total fluctuating power is evenly distributed or distributed to each DC-side capacitor in any proportion to calculate the power buffered by each DC-side capacitor.
[0017] As a further improvement of the present invention, the buffer branch between the high-frequency converter and the transformer winding adopts a single inductor branch structure, and the high-frequency converter adopts a single-phase full-bridge converter. The power output from the high-frequency converter to the transformer is precisely controlled by the phase-shifting control strategy. At this time, the total fluctuating power can be evenly distributed or distributed to each DC-side capacitor in any proportion to calculate the power buffered by each DC-side capacitor.
[0018] As a further improvement of the present invention, the buffer branch between the high-frequency converter and the transformer winding adopts an LC series resonant branch structure. The high-frequency converter adopts a single-phase full-bridge / half-bridge converter. The switching frequency of the high-frequency converter is equal to the LC resonant frequency. Each high-frequency converter adopts a square wave signal with the same phase and a duty cycle of 50%. The total fluctuation power is evenly distributed to each DC-side capacitor to calculate the power buffered by each DC-side capacitor.
[0019] The advantages of this invention are as follows:
[0020] This invention improves the power density of power equipment by optimizing capacitor values, solving the problem of large size and weight of DC-side capacitors in converters based on high-frequency transformers. Within the same allowable voltage fluctuation range (e.g., within ±5% of the steady-state value), smaller capacitor values can be used, resulting in higher power density. Compared to the traditional series H-bridge structure, this invention reduces the DC-side capacitor value by more than 66% under the worst operating conditions; and by more than 90% under conditions of perfectly symmetrical three-phase AC ports. Furthermore, this invention does not require additional sensors or devices, making it simple and reliable to implement. Attached Figure Description
[0021] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the invention in any way. Furthermore, the shapes and proportions of the components in the drawings are merely schematic to aid in understanding the invention and do not specifically limit the shapes and proportions of the components. In the drawings:
[0022] Figure 1 The diagram shows a converter structure based on a high-frequency transformer.
[0023] Figure 2The proposed method is illustrated using a three-phase AC-DC converter as an example.
[0024] Figure 3 The figure shows the simulated waveforms of the three-phase AC ports on the input side of the high-frequency transformer. Detailed Implementation
[0025] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.
[0026] It should be noted that when an element is referred to as being "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.
[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.
[0028] Traditional isolated medium-voltage high-power converter units transfer low-frequency power fluctuations from the AC port input to the DC side, where they are buffered by DC-side capacitors. The large-capacity capacitor array increases the device's size and weight, hindering power density improvement. To address the significant power density issues caused by large-capacity capacitor arrays, this invention utilizes magnetic coupling to cancel out power fluctuations and reduce capacitor value. In practical implementation, the topology of the high-frequency converter at the AC port, the form of the buffer branch, the switching frequency, and the type of switching transistor are not limited.
[0029] This invention proposes a method for reducing the capacitance value of the DC-side capacitor in a converter based on a high-frequency transformer. Without adding additional components, it reduces the fluctuating power buffered at the capacitor by reducing the fluctuating power at the AC port through the magnetic circuit, thereby minimizing the capacitance value of the DC-side capacitor, thus reducing the size and weight of the capacitor and addressing the technical requirements for power density.
[0030] To explain how this method works, Figure 1 The following example illustrates a converter based on an m+n winding high-frequency transformer. This converter consists of an m+n winding high-frequency transformer, m+n buffer branches, m+n high-frequency converters, m+n DC-side support capacitors, and m interface converters. Each winding of the m+n winding high-frequency transformer has a buffer branch connected in series at its terminals. The other end of each buffer branch is connected to the AC port of the high-frequency converter. Each high-frequency converter is connected to a DC-side capacitor. Each DC-side capacitor is cascaded with an interface converter to either lead out an AC port or directly lead out to form a DC port.
[0031] The frequency of the high-frequency transformer of the present invention is any frequency in the range of several hundred hertz to several hundred kilohertz.
[0032] Where m and n are both positive integers.
[0033] The main function of a high-frequency converter is to provide square wave voltage to a high-frequency transformer. Its structure adopts, for example... Figure 1 (b) and Figure 1 (c) shows a single-phase half-bridge or single-phase full-bridge circuit. The buffer branch uses, for example... Figure 1 (d) and Figure 1 (e) shows an LC series resonant branch or a single-inductor branch. The interface converter adopts a single-phase full-bridge converter structure. The converter has at least one AC port, i.e., the number of AC ports is one, two, or up to m. The number of DC ports can be zero, one, or up to n. The type of switching transistor is not limited and can be IGBT, MOSFET, or other switching transistors.
[0034] With the input converter as the reference direction, the instantaneous power input to all ports is summed to obtain the total ripple power. .
[0035] In this configuration, the DC components of the input power at each port sum to zero, while the fluctuating power components at each AC port cancel each other out to some extent. Therefore, the total fluctuating power is zero. This is a relatively small value. Each winding of the high-frequency transformer is connected to a high-frequency converter, which is equipped with a capacitor on the DC side to reduce the total fluctuating power. The power is distributed evenly or in any other arbitrary proportion to each winding, thereby calculating the power buffered by the DC-side capacitor of each winding. ) ,in , Let be the i-th DC-side capacitor. Detect the input power at each port ( ), controlling the power output from each winding high-frequency converter to the transformer, so that the average value of this power within the high-frequency cycle ( ) equals the winding input power ( ) and the power buffered by the DC-side capacitor ( The difference in power, i.e. According to the law of conservation of power, it satisfies This allows the low-frequency fluctuation power input from the AC port to be transferred to the high-frequency transformer to cancel each other out, significantly reducing the amount of fluctuation power that the DC-side capacitor needs to buffer, thereby reducing the capacitor value and increasing the system power density.
[0036] When allocating the power required for buffering the DC-side capacitors of each winding, different power allocation methods are needed for different combinations of high-frequency converters and buffer branches:
[0037] (1) When the high-frequency converter adopts a single-phase full-bridge or single-phase half-bridge converter, the buffer branch is an LC resonant branch, and the switching frequency of the high-frequency converter is close to but not equal to the LC resonant frequency, the total fluctuation power can be distributed to the DC side capacitors of each winding in an average or other arbitrary proportion, and the power output of the high-frequency converter to the high-frequency transformer can be precisely controlled by the phase shift control strategy.
[0038] (2) When the high-frequency converter adopts a single-phase full-bridge or single-phase half-bridge converter, the buffer branch is an LC resonant branch, and the switching frequency of the high-frequency converter is equal to the LC resonant frequency, the total fluctuation power can only be distributed equally to the DC side capacitors of each winding. Each high-frequency converter uses a square wave switching signal with the same phase and a duty cycle of 50%, and the buffer power of the DC side capacitors of each port is equal.
[0039] (3) When the high-frequency converter adopts a single-phase full-bridge converter and the buffer branch is an LC resonant branch, the total fluctuating power can be distributed to the DC side capacitors of each winding in any other proportion. The power output of the high-frequency converter to the high-frequency transformer can be precisely controlled by the phase shift control strategy.
[0040] The following will provide a detailed explanation with specific examples.
[0041] by Figure 2 The proposed method is illustrated using a three-phase AC-DC converter as an example. This circuit is a three-phase AC-DC converter with four windings: three primary windings and one secondary winding. Both the interface converter and the high-frequency converter employ a full-bridge circuit, and the buffer branch uses an LC resonant branch. The switching frequency of the high-frequency converter is equal to the LC resonant frequency. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.
[0042] The voltage and power rating parameters of this circuit are summarized in Table 1. Based on this, the Matlab / Simulink three-phase AC-DC converter simulation model fully achieved the expected design goals and implemented the capacitor reduction function. Simulation waveforms are summarized in... Figure 3 middle.
[0043] Table 1 Main parameters of the simulation model
[0044]
[0045] Figure 3 The diagram shows the simulated waveforms of the three-phase AC ports on the input side of the high-frequency transformer. Sub-figure (a) shows the three-phase AC voltage waveform; sub-figure (b) shows the three-phase AC current waveform; and sub-figure (c) shows the input power at the A-phase port after multiplying the A-phase AC voltage and A-phase AC current. Sub-diagram (d) shows the average power output of the A-phase high-frequency converter after multiplying the square wave voltage from the winding resonant current and removing the switching subharmonics. Sub-diagram (e) shows the input power at phase A port. With the output power of the high-frequency converter The power that the DC-side capacitor needs to handle after subtraction Sub-diagram (f) shows the DC-side capacitor voltage at phase A port. .
[0046] At the start of the simulation, the system operates under no-load conditions. At t=0.1 seconds, the load power absorption jumps from 0W to the rated power of 200kW. At t=0.2 seconds, a single-phase ground fault occurs on the input side (phase C), and the load operates at a reduced rate. Throughout the simulation, the fluctuation components of the three-phase input power are canceled out by the transformer magnetic circuit. After subtracting the output power of the high-frequency converter from the input power of phase A, the fluctuation power that the DC-side capacitor needs to bear is very small. Even with a very small DC-side capacitor value, the voltage fluctuation amplitude is still controlled within 5%. When the capacitor value is... Even under the worst operating conditions, the capacitor voltage ripple is within 100V, which is about 5% of the voltage setting value (2000V). Compared with the traditional series H-bridge structure, the converter in this invention reduces the capacitor value by more than 66%, proving its good ability to reduce the capacitor value.
[0047] Therefore, under the same ripple performance requirements, the method proposed in this invention to cancel the fluctuating power of the AC port by using a coupled magnetic circuit significantly reduces the power buffered by the DC-side capacitor, thereby minimizing the capacitance value and verifying the effectiveness of the proposed method.
[0048] Many embodiments and applications beyond the examples provided will be apparent to those skilled in the art upon reading the foregoing description. Therefore, the scope of this teaching should not be determined by reference to the foregoing description, but rather by reference to the foregoing claims and the full scope of their equivalents. For purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended as a waiver of that subject matter, nor should it be construed as an indication that the applicant has not considered that subject matter as part of the disclosed inventive subject matter.
[0049] The above content provides a further detailed description of the present invention. It should not be construed that the specific embodiments of the present invention are limited to this. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection of the present invention as defined by the submitted claims.
Claims
1. A method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer, characterized in that, include: Using the input converter as the reference direction, the total ripple power is obtained by summing the instantaneous power at all ports. Total fluctuation power Distribute the power to each DC-side capacitor to calculate the required buffer power for each DC-side capacitor. ,in ; Let m be the number of AC ports and n be the number of DC ports; Detect the input power of each port. The power transmitted from each high-frequency converter to the high-frequency transformer is controlled so that the average power transmitted to the high-frequency transformer is within the high-frequency cycle. equal to port input power DC-side capacitor buffer power The difference, where, .
2. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 1, characterized in that, Total fluctuation power When allocating power to each DC-side capacitor, the power should be distributed either equally or in any proportion, specifically including: Total fluctuation power The power is evenly distributed among the DC-side capacitors, ensuring that each DC-side capacitor buffers an equal amount of power. ; Total fluctuation power The power is allocated to each DC-side capacitor in any other proportion, and the power buffered by each DC-side capacitor is... ,in For the first The power buffered by each DC-side capacitor and the total ripple power The ratio, and satisfy .
3. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 1, characterized in that, The high-frequency transformer includes an m+n winding high-frequency transformer, m+n buffer branches, m+n high-frequency converters, m+n DC-side capacitors, and m interface converters. Each winding terminal of the m+n winding high-frequency transformer is connected in series with a buffer branch, and the other end of each buffer branch is connected to the AC port of the high-frequency converter. Each DC port of the high-frequency converter is connected in parallel with a DC-side capacitor. Among them, each of the m DC-side capacitors is cascaded with an interface converter to lead out m AC ports, and the n DC-side capacitors directly lead out n DC ports from both ends.
4. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 3, characterized in that, The frequency of the high-frequency transformer is any frequency in the range of several hundred hertz to several hundred kilohertz; the voltage frequency of each AC port is the power frequency or low frequency, and the current frequency is the power frequency or low frequency.
5. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 3, characterized in that, The number of AC ports is one, two, or m; the number of DC ports is zero, one, or n.
6. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 3, characterized in that, In high-frequency converters, the switching transistors are either IGBTs or MOSFETs.
7. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 3, characterized in that, The buffer branch between the high-frequency converter and the transformer winding adopts an LC series resonant branch structure. The high-frequency converter adopts a single-phase full-bridge / half-bridge converter. The switching frequency of the high-frequency converter is close to but not equal to the LC resonant frequency. The power output from the high-frequency converter to the high-frequency transformer is controlled by a phase-shifting control strategy. The total fluctuating power is evenly distributed or distributed to each DC-side capacitor in any proportion to calculate the power buffered by each DC-side capacitor.
8. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 3, characterized in that, The buffer branch between the high-frequency converter and the transformer winding adopts a single inductor branch structure. The high-frequency converter adopts a single-phase full-bridge converter. The power output from the high-frequency converter to the transformer is precisely controlled by the phase-shifting control strategy. At this time, the total fluctuating power can be evenly distributed or distributed to each DC-side capacitor in any proportion to calculate the power buffered by each DC-side capacitor.
9. The method for minimizing the DC-side capacitance of a converter based on a high-frequency transformer according to claim 3, characterized in that, The buffer branch between the high-frequency converter and the transformer winding adopts an LC series resonant branch structure. The high-frequency converter adopts a single-phase full-bridge / half-bridge converter. The switching frequency of the high-frequency converter is equal to the LC resonant frequency. Each high-frequency converter adopts a square wave signal with the same phase and a duty cycle of 50%. The total fluctuation power is evenly distributed to each DC-side capacitor to calculate the power buffered by each DC-side capacitor.