A method of asymmetric waveform operation of a dc-dc converter

Abstract

This invention discloses an asymmetric waveform operation method for a DC-DC converter, comprising the following steps: Step 1, designing an asymmetric AC voltage waveform; Step 2, designing an asymmetric AC current waveform; Step 3, designing an optimal asymmetric AC current waveform; and Step 4, designing a control method for asymmetric waveform operation. This invention adjusts the distribution of the number of modules within the DC-DC converter across different branches by designing the asymmetric AC voltage waveform, ensuring a constant total number of modules. By designing the asymmetric AC current waveform, it reduces the AC current amplitude on the DC current component side of the devices, thereby reducing the peak current of the devices and increasing power capacity. This invention can significantly improve the transmission power capacity of the DC-DC converter without changing its circuit structure and the number of devices, which is of great significance for the application of existing DC-DC converter topologies in DC power grids.

Description

Technical Field

[0001] This invention belongs to the field of power electronics and high-voltage direct current transmission, and relates to an asymmetric waveform operation method for a DC converter. Background Technology

[0002] With the development of DC power grid technology, DC converters, as the core hub of DC power grids, have received increasing attention. However, large capacity has always been a key technical challenge for DC converters, and existing DC converters are limited by the peak current of the devices and cannot achieve large capacity.

[0003] CN105576982B discloses a non-isolated DC transformer that utilizes a modular multilevel converter with three phases directly connected in parallel at the output to increase the power transmission capacity. However, because it requires the generation of AC voltage and current internally to balance the capacitor energy, a large-capacity filter is needed on the DC side to filter out the internal AC voltage and current, resulting in significant size and weight. CN117318474A discloses a DC transformer for DC grid interconnection and its control and protection method. It adopts an active filtering method, using cascaded sub-module bridge arms to replace passive filters. However, because the bridge arms contain both DC and AC current components, and the AC current and AC voltage are symmetrical waveforms with positive and negative amplitudes, and the sinusoidal AC voltage and current have high amplitudes, the peak current of the devices within the bridge arms becomes very large after the superposition of the DC current component amplitude and the AC current component amplitude, making it difficult for the DC converter to achieve a large capacity.

[0004] Therefore, in order to improve the power capacity of DC converters in DC power grids, it is urgent to invent a new operating method to reduce the peak current of DC converters. Summary of the Invention

[0005] This invention provides an asymmetric waveform operation method for DC-DC converters. By designing an asymmetric AC voltage waveform within the DC-DC converter, the distribution of the number of modules within the converter across different branches is adjusted, ensuring a constant total number of modules. Furthermore, by designing an asymmetric AC current waveform, the amplitude of the AC current on the DC current component side of the devices is reduced, thereby lowering the device current peak value and increasing power capacity. This invention proposes optimal constraints for AC current design, ensuring that the designed asymmetric operating waveform is optimal in terms of both current peak value and RMS current, significantly reducing the device current peak value. This invention also proposes a control method for asymmetric operation, achieving stability of DC voltage, branch current, and converter energy. This invention can significantly improve the transmission power capacity of the DC-DC converter without changing its circuit structure and the number of devices, which is of great significance for the application of existing DC-DC converter topologies in DC power grids.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] An asymmetric waveform operation method for a DC-DC converter includes the following steps:

[0008] Step 1: Design the asymmetrical AC voltage waveform:

[0009] For each phase in the DC-DC converter, the sum of the voltage of the first branch and the voltage of the intermediate branch equals the first DC voltage U1, and the sum of the voltage of the intermediate branch and the voltage of the second branch equals the second DC voltage U2. The voltages of the first branch, the intermediate branch, and the second branch all contain both DC and AC voltage components. The AC voltages in the first and intermediate branches cancel each other out of phase, and the AC voltages in the intermediate and second branches cancel each other out of phase. The specific steps are as follows:

[0010] For each phase in the DC-DC converter, the positive amplitude of the AC voltage in the intermediate branch is U. p The duration is T p The negative amplitude of the AC voltage in the intermediate branch is U. n The duration is T n , then U p T p equal to U n T n To satisfy the AC voltage cancellation relationship, the positive amplitude of the AC voltage in the first branch is U. n The duration is T n The negative amplitude is U p The duration is T p The positive amplitude of the AC voltage in the second branch is U. p The duration is T p The negative amplitude is U n The duration is T n Under this design method, the AC voltages in the first branch and the middle branch cancel each other out in phase, and the AC voltages in the middle branch and the second branch cancel each other out in phase, so the AC voltages will not affect the DC side; by designing U p Greater than U n Reduce the number of submodules in the first branch, increase the number of submodules in the middle and second branches, and maintain a balance in the total number of submodules.

[0011] Step 2: Design the asymmetrical AC current waveform:

[0012] For each phase of the DC-DC converter, the AC current in the intermediate branch is the sum of the AC current in the first branch and the AC current in the second DC-DC converter. The AC currents in the first branch of the three phases are staggered by 120 degrees and cancel each other out. The AC currents in the second branch of the three phases are staggered by 120 degrees and cancel each other out, thus preventing AC current from appearing on the DC side. The specific steps are as follows:

[0013] For each phase in the DC-DC converter, the positive amplitude of the AC current in the intermediate branch is I. p The duration is T p The negative amplitude of the AC current in the intermediate branch is I. n The duration is T n , then I p T p equals I n T n The positive amplitude of the AC current in the first branch is I. p 1. The negative amplitude of the AC current in the first branch is I. n 1. The positive amplitude of the AC current in the second branch is I. p 2. The negative amplitude of the AC current in the second branch is I. n 2. Then the positive amplitude of the AC current in the first branch is I. p The positive amplitude of the AC current in the first and second branches is I. p The sum of 2 equals the positive amplitude I of the AC current in the intermediate branch. p Similarly, the negative amplitude I of the AC current in the first branch n 1 and the negative amplitude of the AC current in the second branch I n The sum of 2 equals I n When the DC current component of the intermediate branch is negative, the peak current of the intermediate branch is reduced by designing Ip to be greater than In; when the DC current component of the intermediate branch is positive, the peak current of the intermediate branch is reduced by designing Ip to be less than In.

[0014] Step 3: Design the optimal asymmetrical AC current waveform:

[0015] For the asymmetrical AC circuit waveform of a DC-DC converter, the optimal asymmetrical AC current waveform is designed from two perspectives: minimizing the peak value of the AC current and minimizing the effective value of the AC current. The specific steps are as follows:

[0016] Within a period [0, T], to ensure the minimum peak value of the alternating current and to keep the alternating current constant within the time interval [T / 3, 2T / 3], the amplitude is I. p To ensure the minimum effective value of the alternating current, the alternating current remains constant in both the [0, T / 3] and [2T / 3, T] time periods, with an amplitude of I. p / 2=I n ;

[0017] Step 4: Design a control method for asymmetric waveform operation:

[0018] Based on the DC voltage reference command Uref, the DC voltage is controlled using proportional-integral (PI) control to obtain the DC current reference command Iref for the first or second branch. Multiplying this command by the asymmetrical AC current waveform designed in steps two and three yields the internal AC current reference. Further, based on the DC current reference command Iref, the DC current of the first or second branch is controlled using PI control to obtain the voltage reference command Us_ref for the first or second branch. Multiplying this command by the asymmetrical AC voltage waveform designed in step one yields the internal AC voltage reference, thus obtaining the branch voltage and achieving voltage and current stability for the converter. Based on the DC converter submodule capacitor reference command UC_ref, the average voltage UC_avg of the submodule capacitors in each branch of the DC converter is controlled using PI control to obtain the AC voltage amplitude reference command Up_ref. This further adjusts the AC voltage amplitude in each phase of the DC converter, regulating the energy absorption or release of each branch submodule capacitor, achieving energy balance in the DC converter.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1. Compared with the sinusoidal operation mode of existing DC converters, this invention proposes an asymmetric operation method to transfer power with constant voltage and current amplitude, which can improve the utilization rate of voltage and current and reduce the peak current.

[0021] 2. Compared with the symmetrical operation mode of existing DC converters, the asymmetrical operation method proposed in this invention can reduce the amplitude of AC current on the DC current component side and reduce the peak current.

[0022] 3. The optimal asymmetrical AC current waveform in this invention can achieve the lowest effective value and peak value of AC current when operating under asymmetrical operation method.

[0023] 4. The control method for asymmetrical operation proposed in this invention can achieve energy balance, stable DC voltage control, and stable branch current control of each branch of the DC converter. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the asymmetric waveform operation method of a DC-DC converter.

[0025] Figure 2 This is a schematic diagram of a three-phase asymmetrical AC voltage waveform.

[0026] Figure 3 This is a waveform diagram of a three-phase asymmetrical alternating current.

[0027] Figure 4 This is the voltage waveform of the first branch of the three-phase circuit;

[0028] Figure 5 This is the waveform of the first DC current in the three phases;

[0029] Figure 6 This is the voltage waveform of the three-phase intermediate branch;

[0030] Figure 7 This is the waveform of the three-phase intermediate branch current;

[0031] Figure 8 This is the voltage waveform of the second branch of the three-phase circuit;

[0032] Figure 9 This is the current waveform of the second branch of the three-phase circuit;

[0033] Figure 10 This is a control block diagram for the asymmetric operation method of a DC-DC converter. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0035] This invention provides an asymmetric waveform operation method for a DC-DC converter, such as... Figure 1 As shown, the DC-DC converter consists of three identical phase circuits. Each phase circuit includes a first branch, a middle branch, and a second branch. The first and middle branches are constructed by cascading half-bridge submodules, and the second branch is constructed by cascading full-bridge submodules. U1 and U2 are DC-side voltages, and I1 and I2 are DC-side currents. In this embodiment, U1 is greater than U2, and I1 is less than I2. p U is the positive amplitude of the AC voltage in the intermediate branch. n I represents the negative amplitude of the AC voltage in the intermediate branch. p I is the positive amplitude of the AC current in the intermediate branch. n This represents the negative amplitude of the AC current in the intermediate branch.

[0036] The trapezoidal wave operation mode of a T-type DC transformer will be explained in detail below:

[0037] I. Designing an asymmetrical AC voltage waveform:

[0038] like Figure 4 , Figure 6 , Figure 8The voltage waveforms of each branch shown indicate that for each phase of the DC-DC converter, the sum of the voltage of the first branch and the voltage of the intermediate branch equals the first DC voltage U1, and the sum of the voltage of the intermediate branch and the voltage of the second branch equals the second DC voltage U2. The voltages of the first branch, the intermediate branch, and the second branch all contain both DC and AC voltage components. Therefore, the AC voltages in the first and intermediate branches cancel each other out, and the AC voltages in the intermediate and second branches also cancel each other out.

[0039] Specifically, taking one phase as an example, the positive amplitude of the AC voltage in the intermediate branch is U. p The duration is T / 3, and the negative amplitude of the AC voltage in the intermediate branch is U. n The duration is 2T / 3, therefore U p Equal to 2U n To satisfy the AC voltage cancellation relationship, the positive amplitude of the AC voltage in the first branch is U. n The duration is 2T / 3, and the negative amplitude is U. p The duration is T / 3. The positive amplitude of the AC voltage in the second branch is U. p The duration is T / 3, and the negative amplitude is U. n The duration is 2T / 3. Under this design method, the AC voltages in the first branch and the middle branch cancel each other out, and the AC voltages in the middle branch and the second branch cancel each other out, so the AC voltages do not affect the DC side. Design U p =2U n Compared to the traditional symmetrical operation mode, it reduces the number of sub-modules in the first branch, increases the number of sub-modules in the middle and second branches, and maintains a balance in the total number of sub-modules.

[0040] II. Designing asymmetrical AC current waveforms:

[0041] like Figure 5 , Figure 7 , Figure 9 The current waveforms for each branch are shown. For each phase of the DC-DC converter, the AC current in the middle branch is the sum of the AC current in the first branch and the AC current in the second DC-AC branch. The AC currents in the first branch of the three phases are staggered by 120 degrees and cancel each other out, preventing AC current from appearing on the DC side. The second branch of the three phases contains no AC current, only DC current.

[0042] Specifically, taking one phase as an example, the positive amplitude of the AC current in the intermediate branch is I. p The duration is T / 3, and the negative amplitude of the AC current in the intermediate branch is I. n The duration is 2T / 3, therefore I p Equal to 2I n The positive amplitude of the AC current in the first branch is I. p The negative amplitude of the AC current in the first branch is I.n The AC current in the second branch is 0. The sum of the AC currents in the first and second branches equals the AC current in the middle branch. The DC current component in the middle branch is (I1-I2) / 3 < 0. Design I... p =2I n This reduces the peak current in the intermediate branch. Therefore, the asymmetrical AC current waveform reduces the peak current of the DC-DC converter, which can improve power capacity.

[0043] III. Designing the optimal asymmetrical AC current waveform:

[0044] For the asymmetrical AC circuit waveform of a DC-DC converter, the optimal asymmetrical AC current waveform is designed from two perspectives: minimizing the peak value of the AC current and minimizing the effective value of the AC current. The AC current waveform of a three-phase DC-DC converter satisfies the periodicity characteristic, and one cycle [0, T] is used as an example for illustration.

[0045] First, the maximum current peak of the DC converter occurs within the time period [T / 3, 2T / 3]. When the current is constant during this period, the AC current peak is the minimum; otherwise, the current peak will increase for any other shape.

[0046] Secondly, according to the formula for calculating the effective value of alternating current:

[0047]

[0048] To ensure the minimum effective value of the alternating current, the alternating current remains constant in both the [0,T / 3] and [2T / 3,T] ​​time periods, with an amplitude of I. p / 2=I n Among them, I RMS Let T be the effective value of the alternating current, T be the period of the alternating current, and i(t) be the alternating current.

[0049] Finally, considering the line inductance, the alternating current changes with a certain slope, exhibiting a trapezoidal wave. This yields the final optimal three-phase alternating current waveform, as shown below. Figure 3 As shown.

[0050] The three-phase optimal AC current waveform designed in this invention combines the advantages of minimum AC current stress and minimum AC current effective value. Without changing the circuit structure and number of components of the DC-DC converter, it reduces the peak current of the components compared with the traditional symmetrical operation mode, thereby increasing the power capacity.

[0051] IV. Design of control methods for asymmetric waveform operation:

[0052] like Figure 10As shown, under the asymmetrical waveform operation method, the DC voltage is controlled using proportional-integral (PI) control based on the DC voltage reference command Uref, resulting in the DC current reference command Iref for the first or second branch. Multiplying this command by the asymmetrical AC current waveform designed in steps two and three yields the internal AC current reference. Further, based on the DC current reference command Iref, the DC current of the first or second branch is controlled using PI control, resulting in the voltage reference command Us_ref for the first or second branch. Multiplying this command by the asymmetrical AC voltage waveform designed in step one yields the internal AC voltage reference, thus obtaining the branch voltage and achieving voltage and current stability for the converter. Based on the DC converter submodule capacitor reference command UC_ref, the average voltage UC_avg of the submodule capacitors in each branch of the DC converter is controlled using PI control, resulting in the AC voltage amplitude reference command Up_ref. This further adjusts the AC voltage amplitude in each phase of the DC converter, regulating the energy absorption or release of each branch submodule capacitor, achieving energy balance in the DC converter.

Claims

1. An asymmetric waveform operation method for a DC-DC converter, characterized in that... The method includes the following steps: Step 1: Design the asymmetrical AC voltage waveform: For each phase in the DC-DC converter, the sum of the voltage of the first branch and the voltage of the intermediate branch is equal to the first DC voltage U1, and the sum of the voltage of the intermediate branch and the voltage of the second branch is equal to the second DC voltage U2. The voltage of the first branch, the voltage of the intermediate branch, and the voltage of the second branch all contain DC voltage components and AC voltage components. The AC voltages in the first branch and the intermediate branch cancel each other out of phase, and the AC voltages in the intermediate branch and the second branch cancel each other out of phase. Step 2: Design the asymmetrical AC current waveform: For each phase of the DC converter, the AC current in the intermediate branch is the sum of the AC current in the first branch and the AC current in the second branch. The AC currents in the first branch of the three phases are staggered by 120 degrees and cancel each other out. The AC currents in the second branch of the three phases are staggered by 120 degrees and cancel each other out, so as to avoid AC current appearing on the DC side. Step 3: Design the optimal asymmetrical AC current waveform: To address the asymmetrical AC current waveform of a DC-DC converter, the optimal asymmetrical AC current waveform is designed from two perspectives: minimizing the peak value of the AC current and minimizing the effective value of the AC current. Step 4: Design a control method for asymmetric waveform operation: Based on the DC voltage reference command Uref, the DC voltage is controlled using proportional-integral (PI) control to obtain the DC current reference command Iref for the first or second branch. Multiplying this command by the asymmetrical AC current waveform designed in steps two and three yields the internal AC current reference. Further, based on the DC current reference command Iref, the DC current of the first or second branch is controlled using PI control to obtain the voltage reference command Us_ref for the first or second branch. Multiplying this command by the asymmetrical AC voltage waveform designed in step one yields the internal AC voltage reference, thus obtaining the branch voltage and achieving voltage and current stability for the converter. Based on the DC converter submodule capacitor reference command UC_ref, the average voltage UC_avg of the submodule capacitors in each branch of the DC converter is controlled using PI control to obtain the AC voltage amplitude reference command Up_ref. This further adjusts the AC voltage amplitude in each phase of the DC converter, regulating the energy absorption or release of each branch submodule capacitor, achieving energy balance in the DC converter.

2. The asymmetric waveform operation method for a DC-DC converter according to claim 1, characterized in that... The specific steps of step one are as follows: For each phase in the DC-DC converter, the positive amplitude of the AC voltage in the intermediate branch is U. p The duration is T p The negative amplitude of the AC voltage in the intermediate branch is U. n The duration is T n , then U p T p equal to U n T n To satisfy the AC voltage cancellation relationship, the positive amplitude of the AC voltage in the first branch is U. n The duration is T n The negative amplitude is U p The duration is T p The positive amplitude of the AC voltage in the second branch is U. p The duration is T p The negative amplitude is U n The duration is T n Under this design method, the AC voltages in the first branch and the middle branch cancel each other out in phase, and the AC voltages in the middle branch and the second branch cancel each other out in phase, so the AC voltages will not affect the DC side; by designing U p Greater than U n Reduce the number of submodules in the first branch, increase the number of submodules in the middle and second branches, and maintain a balance in the total number of submodules.

3. The asymmetric waveform operation method for a DC-DC converter according to claim 1, characterized in that... The specific steps of step two are as follows: For each phase in the DC-DC converter, the positive amplitude of the AC current in the intermediate branch is I. p The duration is T p The negative amplitude of the AC current in the intermediate branch is I. n The duration is T n , then I p T p equals I n T n The positive amplitude of the AC current in the first branch is I. p 1. The negative amplitude of the AC current in the first branch is I. n 1. The positive amplitude of the AC current in the second branch is I. p 2. The negative amplitude of the AC current in the second branch is I. n 2. Then the positive amplitude of the AC current in the first branch is I. p The positive amplitude of the AC current in the first and second branches is I. p The sum of 2 equals the positive amplitude I of the AC current in the intermediate branch. p Similarly, the negative amplitude I of the AC current in the first branch n 1 and the negative amplitude of the AC current in the second branch I n The sum of 2 equals I n When the DC current component of the intermediate branch is negative, by designing I p Greater than I n This reduces the peak current in the intermediate branch; when the DC current component of the intermediate branch is positive, it is achieved by designing I... p Less than I n This reduces the peak current in the intermediate branches.

4. The asymmetric waveform operation method of the DC-DC converter according to claim 1, characterized in that... The specific steps of step three are as follows: Within a period [0, T], to ensure the minimum peak value of the alternating current, the alternating current is kept constant within the time interval [T / 3, 2T / 3], with an amplitude of I. p To ensure the minimum effective value of the alternating current, the alternating current remains constant in both the [0, T / 3] and [2T / 3, T] time periods, with an amplitude of I. p / 2=I n .

5. The asymmetric waveform operation method of the DC-DC converter according to claim 1, 2 or 3, characterized in that... The first branch and the intermediate branch are formed by cascading half-bridge sub-modules, and the second branch is formed by cascading full-bridge sub-modules.

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