Capacitor voltage ripple control method and system in MMC type power electronic transformer

Abstract

This invention discloses a capacitor voltage ripple control method and system in an MMC type power electronic transformer, comprising: constructing a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system; generating a capacitor energy reference value based on an optimal operating strategy for partial ripple power transfer; and inputting the capacitor energy reference value into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value, thereby completing the capacitor voltage ripple control in the MMC type power electronic transformer. This method and system effectively control the capacitor voltage ripple in the MMC type power electronic transformer as a whole.

Description

Technical Field

[0001] This invention belongs to the field of AC / DC hybrid power distribution network technology, and relates to a capacitor voltage ripple control method and system in an MMC type power electronic transformer. Background Technology

[0002] With the large-scale integration of distributed power sources, energy storage, and electric vehicles into the power grid, traditional AC distribution networks face severe challenges. Hybrid AC / DC distribution networks, due to their advantages in integrating renewable energy and reducing power conversion stages and transmission losses, have become the preferred solution for future power distribution systems. Power electronic transformers, also known as solid-state transformers or power routers, can connect AC and DC buses of various voltage levels, improving the flexibility of power conversion and are the core equipment of hybrid AC / DC distribution networks. The power electronic transformer topology based on cascaded H-bridge converters has been a research hotspot in recent years. However, it cannot provide a medium-voltage DC bus, which limits its application in hybrid AC / DC distribution networks. Modular multilevel converters (MMCs) have received widespread academic attention in the construction of power electronic transformers due to their excellent modularity, high reliability, flexible scalability, and superior harmonic performance.

[0003] However, in almost all MMC-based topologies, large capacitors are required to suppress low-frequency voltage ripple, which limits the improvement of power density and thus restricts their application. In traditional MMC topologies, there are three main methods for suppressing voltage ripple: high-frequency injection, improved operating modes, and ripple power transfer. High-frequency injection injects common-mode voltage and circulating current into the arms, generating power components to cancel out the ripple power within the arms. While these methods effectively suppress voltage ripple, they inevitably increase the current stress on power devices, limiting the AC voltage range. Improved operating modes can suppress voltage ripple to some extent, but the added hardware increases size and cost, and the control scheme becomes more complex. Ripple power transfer utilizes the symmetry of ripple power. It has been proven that all ripple power components are three-phase symmetrical, regardless of whether they are odd or even orders. Therefore, power paths can be constructed using external circuitry to cancel out the ripple power of different arms. However, this method has the disadvantage of increased hardware cost and is rarely used in practical engineering. Fortunately, in MMC-type power electronic transformers, the inherent DAB (Direct Adjacent Block) can serve as a power path, transferring ripple power from the MMC stage to the low-voltage side. Due to symmetry, the ripple power of different arms cancels each other out on the low-voltage DC side, without introducing significant low-frequency voltage ripple. Therefore, this topology can suppress voltage ripple by transferring ripple power without increasing hardware complexity. Experiments have demonstrated that this method of suppressing voltage ripple can significantly reduce the required capacitor size and increase the power density of MMC-type power electronic transformers.

[0004] However, most articles to date treat this topology as two independent converters, MMC and DAB, and control them separately without addressing the control of ripple power, thus failing to fully leverage the power density advantages of this topology. The paper "Capacitor Optimization Method for MMC-type Solid State Transformer Submodules Based on Fluctuating Power Transfer" (Zhou Jianqiao et al., Capacitor Optimization Method for MMC-type Solid State Transformer Submodules Based on Fluctuating Power Transfer [J]. Proceedings of the CSEE, 2020, 40(12): 3990-4003) employs a voltage ripple suppression strategy based on online estimation of the phase shift angle. The total phase shift angle of DAB is obtained by superimposing the AC component required for ripple power transfer and the DC component required for low-voltage DC side voltage regulation. This open-loop control structure is highly sensitive to circuit parameters and non-ideal factors, thus exhibiting a non-negligible steady-state error. Furthermore, if a traditional closed-loop control structure with voltage ripple as feedback signal and phase shift angle as controller output is adopted (Tu Chunming, Xiao Fan, Yuan Jingbing. DC voltage secondary ripple suppression strategy for cascaded power electronic transformers [J]. Journal of Electrical Engineering, 2019, 34(14): 2990-3003), there is strong coupling between voltage ripple control, low-voltage DC side voltage control, and MMC-level control. The coupling between different control loops makes controller parameter design difficult and limits the improvement of dynamic performance. In addition, when the DAB is used as the ripple power channel, the power loss of power devices and transformers will increase to a certain extent, resulting in reduced efficiency. In practice, it is not necessary to transmit 100% of the ripple power; only a portion needs to be transmitted to limit the voltage ripple within a certain range. However, it is not possible to control the capacitor voltage ripple as a whole. To date, existing literature has not studied this problem. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and system for controlling capacitor voltage ripple in an MMC type power electronic transformer. This method and system effectively control the capacitor voltage ripple in the MMC type power electronic transformer as a whole.

[0006] To achieve the above objectives, the capacitor voltage ripple control method in the MMC-type power electronic transformer of the present invention includes:

[0007] Construct a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system;

[0008] A capacitor energy reference value is generated based on the optimal operating strategy for partial ripple power transfer.

[0009] The capacitor energy reference value is input into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value and complete the capacitor voltage ripple control in the MMC type power electronic transformer.

[0010] The specific process of inputting the capacitor energy reference value into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value is as follows:

[0011] In the capacitor voltage ripple control loop, the difference between the capacitor energy reference value and the submodule capacitor energy is input to the PI regulator, and in the PI regulator, it is superimposed with each component of the MMC level ripple power to obtain each axis component of the DAB ripple power command.

[0012] In the low-voltage DC side voltage control loop, the difference between the reference value and the actual value of the capacitor energy on the low-voltage DC side is input to the PI regulator to obtain the DAB DC power command.

[0013] The DAB ripple power command generated by the capacitor voltage ripple control loop is superimposed with the DAB DC power command generated by the low-voltage DC side voltage control loop after Park inverse transformation to obtain the total DAB power command.

[0014] The phase shift angle of the DAB is calculated based on the preset modulation strategy and the total power command of the DAB, and the MMC type power electronic transformer is controlled according to the phase shift angle of the DAB.

[0015] The process of generating the capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer is as follows:

[0016] Based on the optimal operating strategy of partial ripple power transfer, the voltage ripple factor is used as the feedback signal and the ripple power transfer ratio is used as the controller output to generate the capacitor energy reference value.

[0017] The specific operation for generating the capacitor energy reference value based on the optimal operation strategy of partial ripple power transfer is as follows:

[0018] The difference between the maximum allowable voltage ripple factor and the actual voltage ripple factor is passed through a PI regulator and a limiting circuit to generate the ripple power transfer ratio.

[0019] Based on the ripple power transfer ratio, the components of the capacitor energy reference value in the dual rotating coordinate system are calculated to obtain the capacitor energy reference value in the dual rotating coordinate system.

[0020] The capacitor voltage ripple control system in the MMC-type power electronic transformer of the present invention includes:

[0021] The module is used to construct a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system;

[0022] The generation module is used to generate a capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer.

[0023] The tracking module is used to input the capacitor energy reference value into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value and complete the capacitor voltage ripple control in the MMC type power electronic transformer.

[0024] The tracking module includes:

[0025] The first control module is used to input the difference between the capacitor energy reference value and the capacitor energy of the submodule into the PI regulator in the capacitor voltage ripple control loop, and then superimpose it with each component of the MMC level ripple power in the PI regulator to obtain each axis component of the DAB ripple power command.

[0026] The second control module is used to input the difference between the reference value and the actual value of the capacitor energy on the low-voltage DC side into the PI regulator in the low-voltage DC side voltage control loop to obtain the DAB DC power command.

[0027] The third control module is used to superimpose the DAB ripple power command generated by the capacitor voltage ripple control loop, after Park inverse transformation, with the DAB DC power command generated by the low-voltage DC side voltage control loop to obtain the total DAB power command.

[0028] The fourth control module is used to calculate the phase shift angle of the DAB according to the preset modulation strategy and the total power command of the DAB, and control the MMC type power electronic transformer according to the phase shift angle of the DAB.

[0029] The process of generating the capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer is as follows:

[0030] Based on the optimal operating strategy of partial ripple power transfer, the voltage ripple factor is used as the feedback signal and the ripple power transfer ratio is used as the controller output to generate the capacitor energy reference value.

[0031] The generation module includes:

[0032] The fifth control module is used to generate the ripple power transfer ratio by passing the difference between the maximum allowable voltage ripple factor and the actual voltage ripple factor through a PI regulator and a limiting circuit.

[0033] The sixth control module is used to calculate the components of the capacitor energy reference value in the dual rotating coordinate system based on the ripple power transfer ratio, and obtain the capacitor energy reference value in the dual rotating coordinate system.

[0034] The present invention has the following beneficial effects:

[0035] The capacitor voltage ripple control method in the MMC type power electronic transformer described in this invention generates a capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer to improve the efficiency of DAB. It uses a voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value, avoids the coupling between different control loops which makes controller parameter design difficult, improves the dynamic performance of the transformer, and achieves the purpose of overall control of capacitor voltage ripple in the MMC type power electronic transformer. Attached Figure Description

[0036] Figure 1 This is a topology diagram of the MMC type power electronic transformer in this invention;

[0037] Figure 2a This is a schematic diagram of the internal structure of an MMC type power electronic transformer;

[0038] Figure 2b This is a schematic diagram of the main circuit variables of an MMC type power electronic transformer;

[0039] Figure 3 This is a diagram of the voltage ripple decoupling closed-loop control structure in the dual rotating coordinate system of this invention.

[0040] Figure 4 This is a schematic diagram of the optimal operation strategy based on partial ripple power transfer in this invention.

[0041] Figure 5 The steady-state waveform diagram is shown when the present invention is used;

[0042] Figure 6 This is a transient waveform diagram before and after enabling the present invention;

[0043] Figure 7 This is a steady-state waveform diagram of the voltage ripple dual closed-loop control scheme implemented in this invention;

[0044] Figure 8 This is a graph showing the DAB efficiency measured in the experiment. Detailed Implementation

[0045] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, not all embodiments, and are not intended to limit the scope of the present invention. Furthermore, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion regarding the concepts disclosed in the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0046] The accompanying drawings show structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not drawn to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0047] refer to Figure 1 , Figure 2a and Figure 2b The capacitor voltage ripple overall control method of the present invention includes the following steps:

[0048] 1) Construct a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system, generate the phase shift angle of DAB according to the voltage ripple decoupling closed-loop control structure, and control the MMC type power electronic transformer according to the phase shift angle of DAB.

[0049] 2) Generate the reference value of capacitor energy required for the decoupled closed-loop control structure based on the optimal operating strategy of partial ripple power transfer.

[0050] In step 1), refer to Figure 3 The voltage ripple decoupling closed-loop control structure in the dual rotating coordinate system uses the capacitor energy of the submodule in the dual rotating coordinate system as the feedback quantity, the ripple power command of DAB as the controller output, and introduces MMC-level ripple power feedforward to achieve decoupling between capacitor voltage ripple control, low-voltage DC side voltage control and MMC-level control.

[0051] The specific implementation steps include:

[0052] 11) In the capacitor voltage ripple control loop, the difference between the capacitor energy reference value and the submodule capacitor energy is input to the PI regulator, and in the PI regulator, it is superimposed with each component of the MMC level ripple power to obtain each axis component of the DAB ripple power command.

[0053] 12) In the low-voltage DC side voltage control loop, the reference value of the capacitor energy on the low-voltage DC side is subtracted from the actual value, and the result of the subtraction is processed by the PI regulator to obtain the DC power command of DAB.

[0054] 13) The ripple power command generated by the capacitor voltage ripple control loop is superimposed with the DC power command generated by the low-voltage DC side voltage control loop after Park inverse transformation to obtain the total power command of DAB.

[0055] 14) Calculate the phase shift angle of DAB based on the preset modulation strategy and the total power command generated in step 13).

[0056] In step 2), based on the optimal operating strategy of partial ripple power transfer, using the voltage ripple factor as the feedback signal and the ripple power transfer ratio as the controller output, a capacitor energy reference for the voltage ripple decoupling closed-loop control structure is generated. Specific steps include:

[0057] 21) The difference between the maximum allowable voltage ripple factor and the actual voltage ripple factor is calculated, and the difference is then passed through a PI regulator and a limiting circuit to generate the ripple power transfer ratio.

[0058] 22) Based on the ripple power transfer ratio, calculate each component of the capacitor energy reference value in the dual rotating coordinate system to obtain the capacitor energy reference value in the dual rotating coordinate system.

[0059] 23) The obtained capacitor energy reference value in the dual rotating coordinate system is input into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value and realize dual closed-loop control of voltage ripple.

[0060] Based on the above method, this invention discloses a capacitor voltage ripple control system in an MMC-type power electronic transformer, comprising:

[0061] The module is used to construct a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system;

[0062] The generation module is used to generate a capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer.

[0063] The tracking module is used to input the capacitor energy reference value into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value and complete the capacitor voltage ripple control in the MMC type power electronic transformer.

[0064] The tracking module described in this embodiment includes:

[0065] The first control module is used to input the difference between the capacitor energy reference value and the capacitor energy of the submodule into the PI regulator in the capacitor voltage ripple control loop, and then superimpose it with each component of the MMC level ripple power in the PI regulator to obtain each axis component of the DAB ripple power command.

[0066] The second control module is used to input the difference between the reference value and the actual value of the capacitor energy on the low-voltage DC side into the PI regulator in the low-voltage DC side voltage control loop to obtain the DAB DC power command.

[0067] The third control module is used to superimpose the DAB ripple power command generated by the capacitor voltage ripple control loop, after Park inverse transformation, with the DAB DC power command generated by the low-voltage DC side voltage control loop to obtain the total DAB power command.

[0068] The fourth control module is used to calculate the phase shift angle of the DAB according to the preset modulation strategy and the total power command of the DAB, and control the MMC type power electronic transformer according to the phase shift angle of the DAB.

[0069] The generation module described in this embodiment includes:

[0070] The fifth control module is used to generate the ripple power transfer ratio by passing the difference between the maximum allowable voltage ripple factor and the actual voltage ripple factor through a PI regulator and a limiting circuit.

[0071] The sixth control module is used to calculate the components of the capacitor energy reference value in the dual rotating coordinate system based on the ripple power transfer ratio, and obtain the capacitor energy reference value in the dual rotating coordinate system.

[0072] Confirmatory Experiment

[0073] To verify the present invention, a scaled-down version of the MMC-type power electronic transformer experimental prototype was built, and the main parameters are shown in Table 1.

[0074] Table 1

[0075]

[0076]

[0077] The steady-state waveform when using this invention is as follows: Figure 5 As shown, all energy reference values ​​were set to zero to facilitate observation of steady-state error. The capacitor voltage exhibits only a small switching ripple besides the DC component, without any significant low-frequency components. The stable medium-voltage AC side current and low-voltage DC side voltage demonstrate that the voltage ripple control structure described in this invention has high steady-state accuracy. This is due to the conversion of the harmonic components of the voltage ripple into DC components in a dual rotating coordinate system, and the use of a PI controller with infinite gain at 0Hz.

[0078] The transient waveforms before and after enabling this invention are as follows: Figure 6 As shown, the capacitor voltage ripple rapidly decreases to zero, while the changes in the medium-voltage AC side current, low-voltage DC side voltage, and even the DC component of the submodule capacitor voltage are not significant. This means that the control at the MMC level and the control of the low-voltage DC side voltage are unaffected by voltage ripple control. This is due to: a) the elimination of coupling between voltage ripple control and low-voltage DC side voltage control by utilizing the first-order characteristics of DAB through improvements to the closed-loop structure; and b) the elimination of coupling between voltage ripple control and MMC level control by feedforward control. Therefore, the controller parameters for different loops can be designed independently, simplifying the adjustment of controller parameters and improving dynamic performance.

[0079] Figure 7 The waveform diagrams before and after enabling this invention show that the maximum allowable voltage ripple factor is set to 10%, the peak voltage of the submodule capacitor is initially 8.5V, and quickly drops to 5V. The medium-voltage AC side current and the low-voltage DC side voltage remain unchanged, verifying that... Figure 4 The feasibility of the optimal capacitor energy reference closed-loop generation method is shown.

[0080] Based on experimental data, Figure 8 The diagram shows the efficiency curves of the DAB under three ripple power transfer methods. When there is no ripple power transfer, the efficiency can reach over 96%. As the power on the medium-voltage AC side increases, the DAB needs to transfer more ripple power to completely suppress voltage ripple, which will lead to a significant decrease in the DAB's efficiency. For the proposed optimized operation strategy based on partial ripple power transfer, when the medium-voltage AC side power is below 200W, the efficiency can remain unchanged because the voltage ripple coefficient requirement can be met even without ripple power transfer. Therefore, in... Figure 4 Under the adjustment of the external control loop shown, the ripple power transfer ratio tends to saturate to zero, and the voltage ripple control loop no longer functions. When the medium voltage AC side power exceeds 200W, the ripple power transferred by the DAB will gradually increase, limiting the voltage ripple within the allowable range, but the efficiency can still be maintained above 95%.

[0081] It should be noted that the present invention has the following characteristics:

[0082] The harmonic components of the voltage ripple are converted into DC components in a dual rotating coordinate system, and a sufficiently high steady-state accuracy is achieved using a PI controller with infinite gain at 0 Hz.

[0083] Meanwhile, by improving the closed-loop structure, the coupling between voltage ripple control and low-voltage DC side voltage control is eliminated by utilizing the first-order characteristics of DAB, and feedforward control eliminates the coupling between voltage ripple control and MMC-level control. Therefore, the controller parameters for different control loops can be designed independently, simplifying the adjustment of controller parameters and improving dynamic performance.

[0084] In the optimal operating strategy based on partial ripple power transfer, an external control loop is introduced, with the voltage ripple factor as the feedback signal and the ripple power transfer ratio as the controller output. This loop adjusts the magnitude of the transferred ripple power in real time according to the operating conditions. Experimental results show that, compared with the traditional method of full ripple power transfer, the proposed optimized operating strategy can significantly improve the efficiency of the DAB (Distributed Amplifier Block).

[0085] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for controlling capacitor voltage ripple in an MMC-type power electronic transformer, characterized in that, include: Construct a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system; A capacitor energy reference value is generated based on the optimal operating strategy for partial ripple power transfer. The capacitor energy reference value is input into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value and complete the capacitor voltage ripple control in the MMC type power electronic transformer. The specific process of inputting the capacitor energy reference value into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value is as follows: In the capacitor voltage ripple control loop, the difference between the capacitor energy reference value and the submodule capacitor energy is input to the PI regulator, and in the PI regulator, it is superimposed with each component of the MMC level ripple power to obtain each axis component of the DAB ripple power command. In the low-voltage DC side voltage control loop, the difference between the reference value and the actual value of the capacitor energy on the low-voltage DC side is input to the PI regulator to obtain the DAB DC power command. The DAB ripple power command generated by the capacitor voltage ripple control loop is superimposed with the DAB DC power command generated by the low-voltage DC side voltage control loop after Park inverse transformation to obtain the total DAB power command. The phase shift angle of the DAB is calculated based on the preset modulation strategy and the total power command of the DAB, and the MMC type power electronic transformer is controlled based on the phase shift angle of the DAB. The specific operation for generating the capacitor energy reference value based on the optimal operation strategy of partial ripple power transfer is as follows: The difference between the maximum allowable voltage ripple factor and the actual voltage ripple factor is passed through a PI regulator and a limiting circuit to generate the ripple power transfer ratio. Based on the ripple power transfer ratio, the components of the capacitor energy reference value in the dual rotating coordinate system are calculated to obtain the capacitor energy reference value in the dual rotating coordinate system.

2. The capacitor voltage ripple control method in the MMC type power electronic transformer according to claim 1, characterized in that, The process of generating the capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer is as follows: Based on the optimal operating strategy of partial ripple power transfer, the voltage ripple factor is used as the feedback signal and the ripple power transfer ratio is used as the controller output to generate the capacitor energy reference value.

3. A capacitor voltage ripple control system for an MMC-type power electronic transformer, characterized in that, include: The module is used to construct a voltage ripple decoupling closed-loop control structure in a dual rotating coordinate system; The generation module is used to generate a capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer. The tracking module is used to input the capacitor energy reference value into the voltage ripple decoupling closed-loop control structure to track the capacitor energy reference value and complete the capacitor voltage ripple control in the MMC type power electronic transformer. The tracking module includes: The first control module is used to input the difference between the capacitor energy reference value and the capacitor energy of the submodule into the PI regulator in the capacitor voltage ripple control loop, and then superimpose it with each component of the MMC level ripple power in the PI regulator to obtain each axis component of the DAB ripple power command. The second control module is used to input the difference between the reference value and the actual value of the capacitor energy on the low-voltage DC side into the PI regulator in the low-voltage DC side voltage control loop to obtain the DAB DC power command. The third control module is used to superimpose the DAB ripple power command generated by the capacitor voltage ripple control loop, after Park inverse transformation, with the DAB DC power command generated by the low-voltage DC side voltage control loop to obtain the total DAB power command. The fourth control module is used to calculate the phase shift angle of the DAB according to the preset modulation strategy and the total power command of the DAB, and control the MMC type power electronic transformer according to the phase shift angle of the DAB. The generation module includes: The fifth control module is used to generate the ripple power transfer ratio by passing the difference between the maximum allowable voltage ripple factor and the actual voltage ripple factor through a PI regulator and a limiting circuit. The sixth control module is used to calculate the components of the capacitor energy reference value in the dual rotating coordinate system based on the ripple power transfer ratio, and obtain the capacitor energy reference value in the dual rotating coordinate system.

4. The capacitor voltage ripple control system in the MMC type power electronic transformer according to claim 3, characterized in that, The process of generating the capacitor energy reference value based on the optimal operating strategy of partial ripple power transfer is as follows: Based on the optimal operating strategy of partial ripple power transfer, the voltage ripple factor is used as the feedback signal and the ripple power transfer ratio is used as the controller output to generate the capacitor energy reference value.

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