Circulating current suppression method and device for modular multi-level direct-current transformer

Abstract

The invention discloses a modular multi-level direct-current transformer circulating current suppression method and device, and belongs to the field of distributed energy, and the method comprises the steps: determining a first KVL equation of a first bridge arm unit and a second KVL equation of a second bridge arm unit according to an equivalent model of a topological structure of a modular multi-level direct-current transformer; according to the first KVL equation, the second KVL equation and the direct current and double-frequency component decomposition formula, determining a transfer function equation of double-frequency harmonic control, and according to the transfer function equation, controlling the double-frequency component of the full-bridge sub-module capacitor through a PR controller; and according to the direct current and double frequency component decomposition formula and the equivalent model, determining a transfer function equation of capacitor direct current voltage control, and according to the transfer function equation, controlling the direct current voltage of the full-bridge sub-module capacitor through a PI controller. By implementing the application, the problem that the modular multi-level direct-current transformer in the prior art has circulating current can be solved.

Description

Technical Field

[0001] This invention relates to the field of distributed energy, and in particular to a method and apparatus for suppressing circulating current in a modular multilevel DC transformer. Background Technology

[0002] With the large-scale penetration of new distributed energy sources (such as photovoltaics and energy storage), inspired by modular multilevel DC converters (MMCs), the primary side voltage level can be effectively improved by replacing the full primary bridge of the dual active bridge (DAB) in the modular multilevel DC transformer (MMDCT) with two phase bridge arms of the MMC, thus adapting to medium-voltage scenarios.

[0003] However, while inheriting the high-voltage advantage of MMC, modular multilevel DC transformers introduce circulating current problems. The circulating current in the medium-voltage side bridge arm is superimposed on the current in the upper and lower bridge arms, which leads to an increase in the rated capacity requirement of power devices, an increase in system losses, and an increase in cost. In severe cases, it may even threaten the safe operation of the device. Summary of the Invention

[0004] This invention provides a method and apparatus for suppressing circulating current in modular multilevel DC transformers. The method can solve the technical problem of circulating current in existing modular multilevel DC transformers.

[0005] To address the aforementioned technical problems, one embodiment of the present invention provides a method for suppressing circulating current in a modular multilevel DC transformer, comprising: Obtain an equivalent model of the topology of a modular multilevel DC transformer; wherein the equivalent model includes a DC voltage source, a full-bridge submodule, a first bridge arm unit, and a second bridge arm unit; the full-bridge submodule includes a full-bridge submodule capacitor; Based on the equivalent model, determine the first KVL equation of the first bridge arm element and the second KVL equation of the second bridge arm element; Based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas, the transfer function equation for second harmonic control is calculated. Based on the DC and second harmonic component decomposition formulas and the equivalent model, the transfer function equation for capacitor DC voltage control is calculated. Based on the transfer function equation for the second harmonic control, the second harmonic component of the full-bridge submodule capacitor is controlled by the PR controller; Based on the transfer function equation for the DC voltage control of the capacitor, the DC voltage of the capacitor in the full-bridge submodule is controlled by a PI controller.

[0006] Further, the calculation of the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition and the equivalent model includes: Based on the equivalent model, construct the voltage and current dynamic equations for the capacitor of the full-bridge submodule; Based on the aforementioned DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitors are converted into a first equation in the form of DC and second harmonic components; The first equation is transformed into a second equation that only concerns the DC component, and the second equation is subjected to a Laplace transform to obtain the transfer function equation for the DC voltage control of the capacitor.

[0007] Further, the step of calculating the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas includes: Add the first KVL equation and the second KVL equation to obtain the summed equation, which is used as the third equation; The third equation is simplified to obtain the simplified third equation. The DC and second harmonic component decomposition equations are then substituted into the simplified third equation to obtain the fourth equation. Extract the second harmonic component from the fourth equation to obtain the second harmonic component equation; The Laplace transform of the second harmonic component equation yields the transfer function equation for the second harmonic control.

[0008] Furthermore, the input terminal of the full-bridge submodule is connected to the output terminal of the DC voltage source, and the output terminal of the full-bridge submodule is connected to the input terminals of the first bridge arm unit and the second bridge arm unit, respectively; the output terminal of the first bridge arm unit is connected to the input terminal of the DC voltage source, and the output terminal of the second bridge arm unit is connected to the input terminal of the DC voltage source.

[0009] Furthermore, the full-bridge submodule also includes a first switching device, a second switching device, a third switching device, and a fourth switching device; The emitter of the first switching device is connected to the collector of the second switching device; The emitter of the third switching device is connected to the collector of the fourth switching device; One end of the full-bridge submodule capacitor is connected to the collector of the first switching device and the collector of the third switching device, respectively, and the other end of the full-bridge submodule capacitor is connected to the emitter of the second switching device and the emitter of the fourth switching device, respectively. The connection point between the emitter of the first switching device and the collector of the second switching device serves as the input terminal of the full-bridge submodule, and the connection point between the emitter of the third switching device and the collector of the fourth switching device serves as the output terminal of the full-bridge submodule.

[0010] An embodiment of the present invention also provides a modular multilevel DC transformer circulating current suppression device, comprising: The data acquisition module is used to acquire an equivalent model of the topology of a modular multilevel DC transformer; wherein, the equivalent model includes a DC voltage source, a full-bridge submodule, a first bridge arm unit, and a second bridge arm unit; the full-bridge submodule includes a full-bridge submodule capacitor; The bridge arm analysis module is used to determine the first KVL equation of the first bridge arm element and the second KVL equation of the second bridge arm element based on the equivalent model. The first calculation module is used to calculate the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas. The second calculation module is used to calculate the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition formula and the equivalent model. The first control module is used to control the second harmonic component of the full-bridge submodule capacitor through a PR controller based on the transfer function equation of the second harmonic control. The second control module is used to control the DC voltage of the capacitor in the full-bridge submodule via a PI controller based on the transfer function equation for the DC voltage control of the capacitor.

[0011] Further, the calculation of the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition and the equivalent model includes: Based on the equivalent model, construct the voltage and current dynamic equations for the capacitor of the full-bridge submodule; Based on the aforementioned DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitors are converted into a first equation in the form of DC and second harmonic components; The first equation is transformed into a second equation that only concerns the DC component, and the second equation is subjected to a Laplace transform to obtain the transfer function equation for the DC voltage control of the capacitor.

[0012] Further, the step of calculating the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas includes: Add the first KVL equation and the second KVL equation to obtain the summed equation, which is used as the third equation; The third equation is simplified to obtain the simplified third equation. The DC and second harmonic component decomposition equations are then substituted into the simplified third equation to obtain the fourth equation. Extract the second harmonic component from the fourth equation to obtain the second harmonic component equation; The Laplace transform of the second harmonic component equation yields the transfer function equation for the second harmonic control.

[0013] Furthermore, the input terminal of the full-bridge submodule is connected to the output terminal of the DC voltage source, and the output terminal of the full-bridge submodule is connected to the input terminals of the first bridge arm unit and the second bridge arm unit, respectively; the output terminal of the first bridge arm unit is connected to the input terminal of the DC voltage source, and the output terminal of the second bridge arm unit is connected to the input terminal of the DC voltage source.

[0014] Furthermore, the full-bridge submodule also includes a first switching device, a second switching device, a third switching device, and a fourth switching device; The emitter of the first switching device is connected to the collector of the second switching device; The emitter of the third switching device is connected to the collector of the fourth switching device; One end of the full-bridge submodule capacitor is connected to the collector of the first switching device and the collector of the third switching device, respectively, and the other end of the full-bridge submodule capacitor is connected to the emitter of the second switching device and the emitter of the fourth switching device, respectively. The connection point between the emitter of the first switching device and the collector of the second switching device serves as the input terminal of the full-bridge submodule, and the connection point between the emitter of the third switching device and the collector of the fourth switching device serves as the output terminal of the full-bridge submodule.

[0015] The following benefits can be obtained by implementing the present invention: This invention provides a method and apparatus for suppressing circulating current in a modular multilevel DC transformer. The method is based on an equivalent model, writing KVL equations for the first and second bridge arm units respectively to describe the dynamic relationship between the output voltage of the first bridge arm unit and the full-bridge submodule. Since the modular multilevel DC transformer exhibits DC components and second harmonics during operation, the variables in the KVL equations are decomposed into DC and second harmonic components using a preset decomposition formula, achieving separate control. For the second harmonic component, its transfer function is derived using the KVL equation, and a PR controller is used to handle the second harmonic. The PR controller has infinite gain at the second harmonic resonance, enabling harmonic suppression without steady-state error and eliminating the influence of bridge arm power fluctuations on the capacitor voltage. For the capacitor DC voltage, its transfer function is derived, and a PI controller is used to stabilize the capacitor DC voltage. The PI controller can quickly eliminate the steady-state error of the DC component, ensuring the energy balance of the submodule, thereby effectively suppressing the circulating current in the modular multilevel DC transformer. Attached Figure Description

[0016] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a flowchart illustrating a modular multilevel DC transformer circulating current suppression method provided in a certain embodiment of this application; Figure 2 This is a topology of a modular multilevel DC transformer provided in one embodiment of this application; Figure 3 This is an equivalent model of the topology of a modular multilevel DC transformer provided in a certain embodiment of this application; Figure 4 This is a flowchart of a PR controller controlling a second harmonic component according to a certain embodiment of this application; Figure 5 This is a flowchart of a PI controller controlling DC voltage according to a certain embodiment of this application; Figure 6 This is a Fourier analysis diagram of the arm current of a modular multilevel DC transformer provided in a certain embodiment of this application; Figure 7 This is a waveform and Fourier decomposition diagram of the bridge arm current after circulating current suppression of a modular multilevel DC transformer provided in a certain embodiment of this application; Figure 8 This is a schematic diagram of the structure of a modular multilevel DC transformer circulating current suppression device provided in a certain embodiment of this application; The reference numerals for the accompanying drawings in the specification are as follows: DC voltage source 1, full-bridge submodule 2, first bridge arm unit 3, second bridge arm unit 4, full-bridge submodule capacitor C, first switching device Second switching device Third switching device Fourth switching device . Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] 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 application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0020] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0021] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0022] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0023] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0024] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0025] See Figure 1To address the technical problem of circulating current in existing modular multilevel DC transformers, an embodiment of the present invention provides a method for suppressing circulating current in modular multilevel DC transformers, comprising: S1. Obtain the equivalent model of the topology of the modular multilevel DC transformer; wherein, the equivalent model includes a DC voltage source 1, a full-bridge submodule 2, a first bridge arm unit 3, and a second bridge arm unit 4; the full-bridge submodule 2 includes a full-bridge submodule capacitor C; Specifically, in order to suppress the second-harmonic circulation current in the MMDCT bridge arm, such as Figure 2 As shown, the topology of the modular multilevel DC transformer in this embodiment introduces a full-bridge submodule 2. After the introduction of the full-bridge submodule 2, the switching state of the submodule can be flexibly controlled on the bridge arm of the modular multilevel DC transformer to output a compensation voltage that is opposite to the second harmonic circulating current, thereby canceling or weakening the second harmonic circulating current in the bridge arm. Specifically, Figure 2 The topology can be simplified to Figure 3 The equivalent model shown.

[0026] See Figure 3 In a preferred embodiment, the input terminal of the full-bridge submodule 2 is connected to the output terminal of the DC voltage source 1, and the output terminal of the full-bridge submodule 2 is connected to the input terminal of the first bridge arm unit 3 and the input terminal of the second bridge arm unit 4, respectively; the output terminal of the first bridge arm unit 3 is connected to the input terminal of the DC voltage source 1, and the output terminal of the second bridge arm unit 4 is connected to the input terminal of the DC voltage source 1. Specifically, see Figure 3 The full-bridge submodule 2, as the core conversion unit, has its input directly connected to the output of the DC voltage source 1. It receives DC power and converts voltage by switching internal switching devices. The output of the full-bridge submodule 2 forms a common connection node, which is connected to the input of the first bridge arm unit 3 and the input of the second bridge arm unit 4, respectively, and transmits the converted voltage signal synchronously to the two bridge arm units. The output of the first bridge arm unit 3 is connected to the input of the DC voltage source 1, and the output of the second bridge arm unit 4 is also connected to the input of the DC voltage source 1, forming a closed loop. After buffering and regulating the energy, the two bridge arm units jointly feed the power back to the input of the DC voltage source 1, completing the energy circulation and dynamic balance.

[0027] In a preferred embodiment, the full-bridge submodule 2 further includes a first switching device. Second switching device Third switching device and the fourth switching device ; The first switching device The emitter and the second switching device collector connection; The third switching device The emitter and the fourth switching device collector connection; One end of the capacitor C in the full-bridge submodule is connected to the first switching device. The collector and the third switching device The collector of the capacitor is connected to the capacitor, and the other end of the capacitor C in the full-bridge submodule is connected to the second switching device. The emitter and the fourth switching device emitter connection; The first switching device The emitter and the second switching device The collector connection point serves as the input terminal of the full-bridge submodule 2, and the third switching device The emitter and the fourth switching device The collector connection point serves as the output terminal of the full-bridge submodule 2; Specifically, the full-bridge submodule 2 of the modular multilevel DC transformer topology has a negative voltage output capability that is key to suppressing circulating current: after detecting the second harmonic circulating current in the bridge arm, the output of the full-bridge submodule 2 can be adjusted to a compensation voltage that is opposite to the circulating current, thereby forming a reverse potential to cancel the circulating current, reducing the harmonic components in the bridge arm current, and reducing device losses and stress. The full-bridge submodule 2 adapts to the voltage requirements of medium and high voltage scenarios through its three major characteristics: multilevel output, bidirectional energy flow, and active circulating current compensation.

[0028] S2. Based on the equivalent model, determine the first KVL equation of the first bridge arm unit 3 and the second KVL equation of the second bridge arm unit 4; Specifically, according to Figure 3 Based on the equivalent model, the first KVL equation of the first bridge arm element 3 is written as follows: (1) In the formula, This refers to the voltage on the medium-voltage side of the MMDCT. For the switching function of full-bridge submodule 2; The voltage at capacitor C of the full-bridge submodule; For the upper arm switching function of the first bridge arm unit 3; This is the switching function for the lower bridge arm of the first bridge arm unit 3; The voltage value of the bridge arm submodule string corresponding to the upper bridge arm switching function of the first bridge arm unit 3; The voltage value of the bridge arm submodule string corresponding to the upper bridge arm switching function of the second bridge arm unit 4; For bridge arm inductance; This refers to the common-mode current component of the first bridge arm unit 3; Specifically, according to Figure 3 The equivalent model is used to write the second KVL equations for the second bridge arm element 4, and the specific expressions are as follows: (2) In the formula, For the upper arm switching function of the second bridge arm unit 4; This is the lower bridge arm switching function for the second bridge arm unit 4; The voltage value of the bridge arm submodule string corresponding to the lower bridge arm switching function of the first bridge arm unit 3; The voltage value of the bridge arm submodule string corresponding to the lower bridge arm switching function of the second bridge arm unit 4; This is the common-mode current component of the second bridge arm unit 4.

[0029] S3. Based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas, calculate the transfer function equation for the second harmonic control. In a preferred embodiment, the step of calculating the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and a preset decomposition formula for DC and second harmonic components includes: Add the first KVL equation and the second KVL equation to obtain the summed equation, which is used as the third equation; The third equation is simplified to obtain the simplified third equation. The DC and second harmonic component decomposition equations are then substituted into the simplified third equation to obtain the fourth equation. Extract the second harmonic component from the fourth equation to obtain the second harmonic component equation; The Laplace transform of the second harmonic component equation yields the transfer function equation for the second harmonic control. Specifically, the first KVL equation and the second KVL equation are added together to obtain the equation after addition, which is used as the third equation. The specific expression is as follows: (3) The third-party program is simplified to obtain the simplified third-party program, and its specific expression is as follows: (4) In the formula, This represents the DC component of the voltage across capacitor C in the full-bridge submodule. This is the second harmonic component of the voltage across capacitor C in the full-bridge submodule; It should be noted that since the circulating component consists of the second harmonic, the value in equation (3) is... , , , , , , , Write it in DC + double frequency form Simplify; The circulating current in the MMDCT bridge arm is mainly a second harmonic, therefore the switching function s of the full-bridge submodule capacitor C, and the current flowing through the full-bridge submodule capacitor C... It can be written in DC and second harmonic form, that is, the decomposition formula of DC and second harmonic components, and the specific expression is as follows: (5) Substituting the DC and second harmonic component decompositions into the simplified third equation, we obtain the fourth equation, the specific expression of which is as follows: (6) Extracting the second harmonic component from the fourth equation yields the second harmonic component equation, the specific expression of which is as follows: (7) Performing a Laplace transform on the second harmonic component equation yields the transfer function equation for the second harmonic control, the specific expression of which is as follows: (8) S4. Based on the DC and second harmonic component decomposition formulas and the equivalent model, the transfer function equation for capacitor DC voltage control is calculated. In a preferred embodiment, the step of calculating the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition and the equivalent model includes: Based on the equivalent model, construct the voltage and current dynamic equations for the capacitor C of the full-bridge submodule; Based on the DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitor C are converted into the first equation in the form of DC and second harmonic components; The first equation is transformed into a second equation that only concerns the DC component, and the second equation is subjected to a Laplace transform to obtain the transfer function equation for the DC voltage control of the capacitor. Specifically, based on the structure of the full-bridge submodule 2 in the equivalent model, the voltage and current dynamic equations of the capacitor C of the full-bridge submodule are constructed, and the specific expressions are as follows: (9) In the formula, The voltage across capacitor C of the full-bridge submodule; Specifically, based on the aforementioned DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitor C are converted into a first equation in the form of DC and second harmonic components, as shown in the following specific expression: (10) When controlling the DC component of capacitor C in the full-bridge submodule, let the second harmonic current component... The first equation is transformed into a second equation that concerns only the DC component, as shown in the following expression: (11) Performing a Laplace transform on the second equation yields the transfer function equation for the DC voltage control of the capacitor, the specific expression of which is as follows: (12) S5. Based on the transfer function equation of the second harmonic control, the second harmonic component of the full-bridge submodule capacitor C is controlled by the PR controller; Specifically, based on the transfer function equation of the second harmonic control, i.e., equation (8), a PR controller is introduced to control the second harmonic component of the full-bridge submodule capacitor C. The control block diagram is as follows: Figure 4 As shown, its transfer function is as follows: (13) In the formula, This represents the proportional gain, used to adjust the system's response speed and stability. Represents the resonant gain, the resonant component in a control system; Indicates the resonant frequency; This represents the reference value for the second harmonic circulating current. like Figure 4 As shown, a PR controller is used to track the second harmonic circulating current, thereby generating an opposite second harmonic current in the introduced full-bridge submodule capacitor C, which in turn cancels the second harmonic circulating current on the MMDCT bridge arm.

[0030] S6. Based on the transfer function equation for the DC voltage control of the capacitor, the DC voltage of the capacitor C in the full-bridge submodule is controlled by a PI controller; Specifically, based on the transfer function equation for the DC voltage control of the capacitor, i.e., equation (12), a PI controller is introduced to control the DC voltage of the capacitor C in the full-bridge submodule. The control block diagram is as follows: Figure 5 As shown, its transfer function is as follows: (14) In the formula, This represents the proportional gain of the PI controller; This represents the integral gain of the PI controller; This indicates the reference value for the DC voltage of the capacitor; The error is controlled by a PI controller. Output switching function According to equation (12), the output voltage Function and feedback, through control of DC voltage Stability ensures that the voltage of capacitor C in the full-bridge submodule remains stable.

[0031] To better illustrate this application, the modular multilevel DC transformer of this application was simulated and verified using MATLAB / Simulink software. Figure 6 Fourier analysis of the arm current of the modular multilevel DC transformer (MMDCT) in this application shows that the circulating current in the arm is mainly second harmonic, consistent with the simulation and theory. The circulating current suppression method for the MMDCT proposed in this invention uses a PR controller to track the second harmonic circulating current, thereby generating an opposite second harmonic current in the introduced full-bridge capacitor, thus canceling the second harmonic circulating current on the MMDCT arm. Under otherwise unchanged conditions, such as... Figure 7 As shown, the second harmonic circulating current of the bridge arm decreased from 18.8% to 0.08% at this time; simulation proves that the modular multilevel DC transformer circulating current suppression method proposed in this invention has a significant effect on suppressing circulating current.

[0032] See Figure 8 This invention provides a modular multilevel DC transformer circulating current suppression device, comprising: The data acquisition module is used to acquire an equivalent model of the topology of the modular multilevel DC transformer; wherein, the equivalent model includes a DC voltage source 1, a full-bridge submodule 2, a first bridge arm unit 3, and a second bridge arm unit 4; the full-bridge submodule 2 includes a full-bridge submodule capacitor C; The bridge arm analysis module is used to determine the first KVL equation of the first bridge arm unit 3 and the second KVL equation of the second bridge arm unit 4 based on the equivalent model. The first calculation module is used to calculate the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas. The second calculation module is used to calculate the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition formula and the equivalent model. The first control module is used to control the second harmonic component of the capacitor C of the full-bridge submodule through the PR controller based on the transfer function equation of the second harmonic control. The second control module is used to control the DC voltage of the capacitor C in the full-bridge submodule through a PI controller based on the transfer function equation of the capacitor DC voltage control.

[0033] In a preferred embodiment, the step of calculating the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition and the equivalent model includes: Based on the equivalent model, construct the voltage and current dynamic equations for the capacitor C of the full-bridge submodule; Based on the DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitor C are converted into the first equation in the form of DC and second harmonic components; The first equation is transformed into a second equation that only concerns the DC component, and the second equation is subjected to a Laplace transform to obtain the transfer function equation for the DC voltage control of the capacitor.

[0034] In a preferred embodiment, the step of calculating the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and a preset decomposition formula for DC and second harmonic components includes: Add the first KVL equation and the second KVL equation to obtain the summed equation, which is used as the third equation; The third equation is simplified to obtain the simplified third equation. The DC and second harmonic component decomposition equations are then substituted into the simplified third equation to obtain the fourth equation. Extract the second harmonic component from the fourth equation to obtain the second harmonic component equation; The Laplace transform of the second harmonic component equation yields the transfer function equation for the second harmonic control.

[0035] In a preferred embodiment, the input terminal of the full-bridge submodule 2 is connected to the output terminal of the DC voltage source 1, and the output terminal of the full-bridge submodule 2 is connected to the input terminal of the first bridge arm unit 3 and the input terminal of the second bridge arm unit 4, respectively; the output terminal of the first bridge arm unit 3 is connected to the input terminal of the DC voltage source 1, and the output terminal of the second bridge arm unit 4 is connected to the input terminal of the DC voltage source 1.

[0036] In a preferred embodiment, the full-bridge submodule 2 further includes a first switching device. Second switching device Third switching device and the fourth switching device ; The first switching device The emitter and the second switching device collector connection; The third switching device The emitter and the fourth switching device collector connection; One end of the capacitor C in the full-bridge submodule is connected to the first switching device. The collector and the third switching device The collector of the capacitor is connected to the capacitor, and the other end of the capacitor C in the full-bridge submodule is connected to the second switching device. The emitter and the fourth switching device emitter connection; The first switching device The emitter and the second switching device The collector connection point serves as the input terminal of the full-bridge submodule 2, and the third switching device The emitter and the fourth switching device The connection point of the collector is used as the output terminal of the full-bridge submodule 2.

[0037] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for suppressing circulating current in a modular multilevel DC transformer, characterized in that, include: Obtain an equivalent model of the topology of a modular multilevel DC transformer; wherein the equivalent model includes a DC voltage source, a full-bridge submodule, a first bridge arm unit, and a second bridge arm unit; the full-bridge submodule includes a full-bridge submodule capacitor; Based on the equivalent model, determine the first KVL equation of the first bridge arm element and the second KVL equation of the second bridge arm element; Based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas, the transfer function equation for second harmonic control is calculated. Based on the DC and second harmonic component decomposition formulas and the equivalent model, the transfer function equation for capacitor DC voltage control is calculated. Based on the transfer function equation for the second harmonic control, the second harmonic component of the full-bridge submodule capacitor is controlled by the PR controller; Based on the transfer function equation for the DC voltage control of the capacitor, the DC voltage of the capacitor in the full-bridge submodule is controlled by a PI controller.

2. The modular multilevel DC transformer circulating current suppression method as described in claim 1, characterized in that, The transfer function equation for capacitor DC voltage control, calculated based on the DC and second harmonic component decomposition and the equivalent model, includes: Based on the equivalent model, construct the voltage and current dynamic equations for the capacitor of the full-bridge submodule; Based on the aforementioned DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitors are converted into a first equation in the form of DC and second harmonic components; The first equation is transformed into a second equation that only concerns the DC component, and the second equation is subjected to a Laplace transform to obtain the transfer function equation for the DC voltage control of the capacitor.

3. The modular multilevel DC transformer circulating current suppression method as described in claim 1, characterized in that, The step of calculating the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas includes: Add the first KVL equation and the second KVL equation to obtain the summed equation, which is used as the third equation; The third equation is simplified to obtain the simplified third equation. The DC and second harmonic component decomposition equations are then substituted into the simplified third equation to obtain the fourth equation. Extract the second harmonic component from the fourth equation to obtain the second harmonic component equation; The Laplace transform of the second harmonic component equation yields the transfer function equation for the second harmonic control.

4. The modular multilevel DC transformer circulating current suppression method as described in claim 1, characterized in that, The input terminal of the full-bridge submodule is connected to the output terminal of the DC voltage source, and the output terminal of the full-bridge submodule is connected to the input terminal of the first bridge arm unit and the input terminal of the second bridge arm unit respectively; the output terminal of the first bridge arm unit is connected to the input terminal of the DC voltage source, and the output terminal of the second bridge arm unit is connected to the input terminal of the DC voltage source.

5. The modular multilevel DC transformer circulating current suppression method as described in claim 4, characterized in that, The full-bridge submodule also includes a first switching device, a second switching device, a third switching device, and a fourth switching device; The emitter of the first switching device is connected to the collector of the second switching device; The emitter of the third switching device is connected to the collector of the fourth switching device; One end of the full-bridge submodule capacitor is connected to the collector of the first switching device and the collector of the third switching device, respectively, and the other end of the full-bridge submodule capacitor is connected to the emitter of the second switching device and the emitter of the fourth switching device, respectively. The connection point between the emitter of the first switching device and the collector of the second switching device serves as the input terminal of the full-bridge submodule, and the connection point between the emitter of the third switching device and the collector of the fourth switching device serves as the output terminal of the full-bridge submodule.

6. A modular multilevel DC transformer circulating current suppression device, characterized in that, include: The data acquisition module is used to acquire an equivalent model of the topology of a modular multilevel DC transformer; wherein, the equivalent model includes a DC voltage source, a full-bridge submodule, a first bridge arm unit, and a second bridge arm unit; the full-bridge submodule includes a full-bridge submodule capacitor; The bridge arm analysis module is used to determine the first KVL equation of the first bridge arm element and the second KVL equation of the second bridge arm element based on the equivalent model. The first calculation module is used to calculate the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas. The second calculation module is used to calculate the transfer function equation for capacitor DC voltage control based on the DC and second harmonic component decomposition formula and the equivalent model. The first control module is used to control the second harmonic component of the full-bridge submodule capacitor through a PR controller based on the transfer function equation of the second harmonic control. The second control module is used to control the DC voltage of the capacitor in the full-bridge submodule via a PI controller based on the transfer function equation for the DC voltage control of the capacitor.

7. The modular multilevel DC transformer circulating current suppression device as described in claim 6, characterized in that, The transfer function equation for capacitor DC voltage control, calculated based on the DC and second harmonic component decomposition and the equivalent model, includes: Based on the equivalent model, construct the voltage and current dynamic equations for the capacitor of the full-bridge submodule; Based on the aforementioned DC and second harmonic component decomposition formula, the voltage and current dynamic equations of the full-bridge submodule capacitors are converted into a first equation in the form of DC and second harmonic components; The first equation is transformed into a second equation that only concerns the DC component, and the second equation is subjected to a Laplace transform to obtain the transfer function equation for the DC voltage control of the capacitor.

8. The modular multilevel DC transformer circulating current suppression device as described in claim 6, characterized in that, The step of calculating the transfer function equation for second harmonic control based on the first KVL equation, the second KVL equation, and the preset DC and second harmonic component decomposition formulas includes: Add the first KVL equation and the second KVL equation to obtain the summed equation, which is used as the third equation; The third equation is simplified to obtain the simplified third equation. The DC and second harmonic component decomposition equations are then substituted into the simplified third equation to obtain the fourth equation. Extract the second harmonic component from the fourth equation to obtain the second harmonic component equation; The Laplace transform of the second harmonic component equation yields the transfer function equation for the second harmonic control.

9. The modular multilevel DC transformer circulating current suppression device as described in claim 6, characterized in that, The input terminal of the full-bridge submodule is connected to the output terminal of the DC voltage source, and the output terminal of the full-bridge submodule is connected to the input terminal of the first bridge arm unit and the input terminal of the second bridge arm unit respectively; the output terminal of the first bridge arm unit is connected to the input terminal of the DC voltage source, and the output terminal of the second bridge arm unit is connected to the input terminal of the DC voltage source.

10. The modular multilevel DC transformer circulating current suppression device as described in claim 9, characterized in that, The full-bridge submodule also includes a first switching device, a second switching device, a third switching device, and a fourth switching device; The emitter of the first switching device is connected to the collector of the second switching device; The emitter of the third switching device is connected to the collector of the fourth switching device; One end of the full-bridge submodule capacitor is connected to the collector of the first switching device and the collector of the third switching device, respectively, and the other end of the full-bridge submodule capacitor is connected to the emitter of the second switching device and the emitter of the fourth switching device, respectively. The connection point between the emitter of the first switching device and the collector of the second switching device serves as the input terminal of the full-bridge submodule, and the connection point between the emitter of the third switching device and the collector of the fourth switching device serves as the output terminal of the full-bridge submodule.

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