A method and system for suppressing capacitor voltage fluctuations in SiC MMC submodules

Abstract

This invention discloses a method and system for suppressing capacitor voltage fluctuations in SiC MMC submodules, belonging to the field of electronic power technology. Based on the analysis results of the optimal broadband harmonic circulating current injection boundary and order for SiC MMC submodules, an optimized model for submodule capacitor voltage fluctuations is determined that simultaneously injects 2nd, 4th, and 8th harmonic circulating currents and 3rd harmonic voltages, which can fully utilize the higher control bandwidth characteristic of SiC MMC. By solving the optimized model for submodule capacitor voltage fluctuations, the amplitude and phase of the reference values ​​of the 2nd, 4th, and 8th harmonic circulating currents to be injected, as well as the amplitude and phase of the corresponding 3rd harmonic voltage, are determined. Then, by injecting 2nd, 4th, and 8th harmonic circulating currents and jointly injecting the 3rd harmonic voltage, broadband harmonic joint injection is achieved, reducing the capacitor voltage fluctuations of the submodule and realizing a method for suppressing capacitor voltage fluctuations in SiC MMC submodules with high suppression effect.

Description

Technical Field

[0001] This invention belongs to the field of electronic power technology, and more specifically, relates to a method and system for suppressing voltage fluctuations in SiC MMC submodule capacitors. Background Technology

[0002] MMCs boast high operating efficiency, a wide power range, high output power quality, and strong scalability. However, current MMC converter stations are bulky and complex in structure, necessitating lightweight design. MMC submodules mainly consist of switching devices and capacitors. Currently, Si devices are primarily used as switching devices, but they suffer from high losses and low switching frequencies. Furthermore, their utilization in existing engineering projects has reached its theoretical limit, making it difficult to reduce the weight and size of MMCs. In contrast, SiC devices offer high switching frequencies and low switching losses. Therefore, replacing traditional Si MMCs with SiC MMCs to achieve lightweight design is a crucial future research direction.

[0003] During MMC operation, the charging and discharging effect of the bridge arm current on the submodule capacitors causes all submodule capacitor voltages to fluctuate, requiring suppression. Directly using larger capacitors to suppress submodule voltage fluctuations would undoubtedly increase the MMC system size; therefore, existing MMC submodule voltage fluctuation suppression technologies typically employ additional control and topology improvements. Topology improvements mainly involve submodule modifications and adding power channels. These solutions usually require additional circuitry, increasing hardware costs. Additional control methods primarily include submodule capacitor voltage reduction control, high-frequency circulating current combined with high-frequency common-mode voltage injection, and circulating harmonic injection. The first two methods are prone to overshoot in high-voltage direct current transmission due to the high modulation ratio of the MMC. Harmonic injection, however, requires no additional circuitry and can effectively avoid overshoot through proper control; therefore, circulating harmonic injection is often used to suppress MMC submodule voltage fluctuations.

[0004] However, existing circulating harmonic injection methods are designed for current MMC submodules based on Si IGBT devices. Limited by the switching frequency and power loss of the Si IGBTs themselves, these methods cannot fully utilize the advantages of SiC MMC to maximize the suppression effect when directly used to suppress capacitor voltage fluctuations in SiC MMC submodules. Therefore, there is an urgent need to provide a capacitor voltage fluctuation suppression method for SiC MMC submodules to fully leverage the advantages of SiC devices and further achieve lightweight MMC design. Summary of the Invention

[0005] In view of the above-mentioned defects or improvement needs of the prior art, the present invention provides a method and system for suppressing capacitor voltage fluctuations in SiC MMC submodules. The purpose is to provide a method for suppressing capacitor voltage fluctuations in SiC MMC submodules to achieve a higher suppression effect.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a method for suppressing voltage fluctuations in SiC MMC submodule capacitors; wherein the SiC MMC includes: three phases and six bridge arms; each phase includes upper and lower bridge arms, and each bridge arm includes: a bridge arm inductor and multiple cascaded submodules; the submodules include: SiC MOSFETs and capacitors;

[0007] The above-mentioned fluctuation suppression methods include:

[0008] S1. Input the circuit parameters of SiC MMC into the submodule capacitor voltage fluctuation optimization model for calculation to obtain the amplitude and phase of the reference values ​​of the second, fourth and eighth harmonic circulating currents to be injected, as well as the amplitude and phase of the corresponding third harmonic voltage.

[0009] S2. Based on the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​and the amplitude and phase of the corresponding 3rd harmonic voltage, generate the harmonic injection reference wave for each phase.

[0010] S3. A harmonic injection controller is used to control the circulating current of each phase of the SiC MMC to track its corresponding harmonic injection reference wave, thereby suppressing the fluctuation of the capacitor voltage of the SiC MMC submodule.

[0011] The submodule capacitor voltage fluctuation optimization model is constructed in the following way:

[0012] Simultaneously injecting 2nd, 4th, and 8th harmonic circulating current reference values ​​into the j-phase circulating current of SiC MMC yields the corresponding circulating current expressions for the phase; j represents a, b, or c.

[0013] Substituting the circulating current expression into the bridge arm current expression of the j-phase r-arm, and multiplying it by the average switching function of the corresponding bridge arm injected with the third harmonic voltage, we obtain the average current expression flowing through the submodule capacitor. Then, based on the relationship between the current and voltage across the capacitor, we obtain the submodule capacitor voltage fluctuation expression; the r-arm is either the upper or lower bridge arm.

[0014] An optimization model for submodule capacitor voltage fluctuation is constructed with the objective of minimizing the difference between the maximum and minimum values ​​of the submodule capacitor voltage fluctuation obtained from the expression for submodule capacitor voltage fluctuation.

[0015] More preferably, the circulation expression for phase j is:

[0016]

[0017] Among them, i cirj For the j-phase circulating current of SiC MMC; I dc For SiC MMC DC side current; I km ω is the amplitude of the reference value for the kth harmonic circulating current to be injected; ω is the grid angular frequency; t is the system running time; when j is a, n is 0; when j is b, n is -1; when j is c, n is 1. The phase of the reference value for the kth harmonic circulating current to be injected.

[0018] More preferably, the constraints of the submodule capacitor voltage fluctuation optimization model include:

[0019]

[0020] Where, δ s δ c γ s γ c μ1 and μ2 are all preset ratios; I m The amplitude of the output current of the SiC MMC; I km The amplitude of the reference value for the kth harmonic circulating current to be injected; The phase of the reference value for the kth harmonic circulating current to be injected; k = 2, 4, 8; U m The amplitude of the SiCMMC output voltage; m 3s =u 3s / U dc ;m 3c =u 3c / U dc ;u 3s =U 3m sin(β3); u 3c =U 3m cos(β3); U 3m β3 represents the amplitude of the third harmonic voltage to be injected; β3 represents the phase of the third harmonic voltage to be injected; U dc Indicates the DC side voltage of SiCMMC; u jr (t) represents the voltage of the j-phase r-arm at time t; T represents one grid cycle.

[0021] More preferably, δ s δ c γ s and γ c All are preset ratios that are greater than 0 and less than 1;

[0022] When the SiC MMC is a half-bridge MMC, μ1 and μ2 are both preset ratios that are greater than 0 and less than 1;

[0023] When the SiC MMC is a hybrid MMC or a full-bridge MMC, μ1 and μ2 are both preset ratios that are greater than -1 and less than 1.

[0024] More preferably, S3 includes:

[0025] S31. Using a harmonic injection controller, obtain the differential mode voltage of each phase corresponding to the 3rd harmonic voltage to be injected and the common mode voltage of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents. After superimposing the differential mode voltage and the common mode voltage of each phase onto the bridge arm reference wave of the corresponding phase, obtain the circulating current of each phase of the current SiC MMC.

[0026] S32. Determine whether the absolute value of the difference between the per-unit value of each phase circulating current of the current SiC MMC and the corresponding harmonic injection reference wave is less than or equal to the preset threshold. If so, complete the fluctuation suppression operation of the capacitor voltage of the SiC MMC submodule; otherwise, adjust the parameters of the harmonic injection controller and proceed to step S31.

[0027] More preferably, the above-mentioned harmonic injection controller includes: a voltage harmonic injection controller and a circulating harmonic injection controller;

[0028] The voltage harmonic injection controller is used to obtain the differential mode voltage of each phase corresponding to the third harmonic voltage to be injected;

[0029] The circulating harmonic injection controller is used to obtain the common-mode voltage of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents to be injected.

[0030] More preferably, the above-mentioned circulating harmonic injection controller is a PIR controller.

[0031] In a second aspect, the present invention provides a SiC MMC submodule capacitor voltage fluctuation suppression system, comprising: a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to perform the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of the present invention.

[0032] Thirdly, the present invention provides a modular multilevel converter system, including: SiC MMC and MMC controller;

[0033] The MMC controller is used to execute the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of the present invention.

[0034] Fourthly, the present invention also provides a computer-readable storage medium comprising a stored computer program, wherein the computer program, when executed by a processor, controls the device where the storage medium is located to execute the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of the present invention.

[0035] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0036] 1. This invention provides a method for suppressing capacitor voltage fluctuations in SiC MMC submodules. By analyzing and determining the optimal boundary and number of broadband harmonic circulating current injections for SiC MMC submodules, an optimized model for capacitor voltage fluctuations in submodules is determined, which simultaneously injects 2nd, 4th, and 8th harmonic circulating currents and 3rd harmonic voltages. This fully utilizes the higher control bandwidth characteristic of SiC MMC. By solving the optimized model for capacitor voltage fluctuations in submodules, the amplitude and phase of the reference values ​​of the 2nd, 4th, and 8th harmonic circulating currents to be injected, as well as the amplitude and phase of the corresponding 3rd harmonic voltage, are determined. Then, by injecting 2nd, 4th, and 8th harmonic circulating currents and combining them with the injection of 3rd harmonic voltage, broadband harmonic joint injection is achieved, reducing capacitor voltage fluctuations in submodules. This method for suppressing capacitor voltage fluctuations in SiC MMC submodules has a high suppression effect.

[0037] 2. Furthermore, the SiC MMC submodule capacitor voltage fluctuation suppression method provided by the present invention, by supplementing the upper and lower bounds of the bridge arm reference voltage as constraints, makes the method applicable to both half-bridge MMCs that cannot output negative levels and hybrid MMCs or full-bridge MMCs that have negative level output capabilities, thus having a wide range of applications.

[0038] 3. Furthermore, the SiC MMC submodule capacitor voltage fluctuation suppression method provided by the present invention uses a PIR controller to avoid the coupling problem of traditional PI control during multiple harmonic injections, enabling it to complete the closed-loop control and injection of MMC circulating current according to the optimal solution of harmonic injection amplitude and phase, thereby further improving the capacitor voltage fluctuation suppression effect.

[0039] 4. This invention provides a modular multilevel converter system, including a SiC MMC and an MMC controller; wherein, the MMC controller is used to execute the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of this invention, which fully utilizes the higher control bandwidth of SiC MMC to achieve a higher suppression effect. Compared with existing modular multilevel converter systems, it can further reduce the volume of the submodule capacitor under the same capacitor voltage fluctuation requirements, and realize the lightweight design of the modular multilevel converter system. Attached Figure Description

[0040] Figure 1 A flowchart of a method for suppressing capacitor voltage fluctuations in a SiC MMC submodule provided in this embodiment of the invention;

[0041] Figure 2 A typical SiC MMC topology diagram provided for embodiments of the present invention;

[0042] Figure 3 The SiC MMC equivalent model provided in the embodiments of the present invention; wherein, (a) is the SiC MMC differential mode equivalent circuit; (b) is the SiC MMC common mode equivalent circuit;

[0043] Figure 4 A schematic diagram of a broadband harmonic injection control strategy provided in an embodiment of the present invention is shown; wherein, (a) is a schematic diagram of the injected harmonic reference value generation module; and (b) is a schematic diagram of the PIR controller.

[0044] Figure 5 A flowchart illustrating an implementation of a preferred scheme for suppressing capacitor voltage fluctuations in a SiC MMC submodule, provided in an embodiment of the present invention;

[0045] Figure 6 The circulating current fluctuation component and its reference wave of the MMC system during harmonic order switching provided in the embodiments of the present invention;

[0046] Figure 7 The submodule voltage and bridge arm current waveforms during harmonic suppression and injection are provided in the embodiments of the present invention; wherein, (a) to (f) are the submodule capacitor voltage waveforms during circulating current suppression, typical 2nd circulating current injection, 2+3rd harmonic injection, 2+3+4th harmonic injection, 2+3+4+8th, and 2+3+4+8+10th harmonic injection; (g) to (l) are the bridge arm current waveforms during circulating current suppression, typical 2nd circulating current injection, 2+3rd harmonic injection, 2+3+4th harmonic injection, 2+3+4+8th, and 2+3+4+8+10th harmonic injection. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0048] To achieve the above objectives, in a first aspect, the present invention provides a method for suppressing voltage fluctuations in SiC MMC submodule capacitors; wherein the SiC MMC includes: three phases and six bridge arms; each phase includes upper and lower bridge arms, and each bridge arm includes: a bridge arm inductor and multiple cascaded submodules; the submodules include: SiC MOSFETs and capacitors;

[0049] like Figure 1 As shown, the above-mentioned fluctuation suppression method includes:

[0050] S1. Input the circuit parameters of SiC MMC into the submodule capacitor voltage fluctuation optimization model for calculation to obtain the amplitude and phase of the reference values ​​of the second, fourth and eighth harmonic circulating currents to be injected, as well as the amplitude and phase of the corresponding third harmonic voltage.

[0051] S2. Based on the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​and the amplitude and phase of the corresponding 3rd harmonic voltage, generate the harmonic injection reference wave for each phase.

[0052] S3. A harmonic injection controller is used to control the circulating current of each phase of the SiC MMC to track its corresponding harmonic injection reference wave, thereby suppressing the fluctuation of the capacitor voltage of the SiC MMC submodule.

[0053] The submodule capacitor voltage fluctuation optimization model is constructed in the following way:

[0054] a. Simultaneously inject 2nd, 4th, and 8th harmonic circulating current reference values ​​into the j-phase circulating current of SiC MMC to obtain the corresponding circulating current expression; j is a, b, or c;

[0055] In one alternative implementation, the circulation expression for phase j is:

[0056]

[0057] Among them, i cirj For the j-phase circulating current of SiC MMC; I dc For SiC MMC DC side current; I km ω is the amplitude of the reference value for the kth harmonic circulating current to be injected; ω is the grid angular frequency; t is the system running time; when j is a, n is 0; when j is b, n is -1; when j is c, n is 1. The phase of the reference value for the kth harmonic circulating current to be injected.

[0058] b. Substitute the circulating current expression into the bridge arm current expression of the j-phase r-arm, and multiply it with the average switching function of the corresponding bridge arm injected with the third harmonic voltage to obtain the average current expression flowing through the submodule capacitor. Then, based on the relationship between the current and voltage across the capacitor, obtain the submodule capacitor voltage fluctuation expression; where r is p or n, representing the upper or lower bridge arm.

[0059] c. To minimize the difference between the maximum and minimum values ​​of the submodule capacitor voltage fluctuation obtained from the submodule capacitor voltage fluctuation expression, construct an optimization model for the submodule capacitor voltage fluctuation.

[0060] In one optional implementation, the constraints of the submodule capacitor voltage fluctuation optimization model include:

[0061]

[0062] Where, δ s δ c γ s γ c μ1 and μ2 are preset ratios; I m The amplitude of the output current of the SiC MMC; I km The amplitude of the reference value for the kth harmonic circulating current to be injected; The phase of the reference value for the kth harmonic circulating current to be injected; k = 2, 4, 8; U m The amplitude of the SiC MMC output voltage; m 3s =u 3s / U dc ;m 3c =u 3c / U dc ;u 3s =U 3m sin(β3); u 3c =U 3m cos(β3); U 3m β3 represents the amplitude of the third harmonic voltage to be injected; β3 represents the phase of the third harmonic voltage to be injected; U dc Indicates the DC side voltage of SiCMMC; u jr (t) represents the voltage of the j-phase r-arm at time t; T represents one grid cycle.

[0063] It should be noted that δ s δ c γ s and γ c All are preset ratios that are greater than 0 and less than 1;

[0064] When the SiC MMC is a half-bridge MMC, μ1 and μ2 are both preset ratios that are greater than 0 and less than 1;

[0065] When the SiC MMC is a hybrid MMC or a full-bridge MMC, μ1 and μ2 are both preset ratios that are greater than -1 and less than 1.

[0066] In one optional implementation, taking SiC MMC as a half-bridge MMC as an example, considering a certain margin, the above preset ratio is taken as follows: δ s =0.5, δ c =0.5, γ s =0.5, γ c =0.5, μ1=0.02, μ2=0.98.

[0067] In one alternative implementation, S3 includes:

[0068] S31. Using a harmonic injection controller, obtain the differential mode voltage of each phase corresponding to the 3rd harmonic voltage to be injected and the common mode voltage of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents. After superimposing the differential mode voltage and the common mode voltage of each phase onto the bridge arm reference wave of the corresponding phase, obtain the circulating current of each phase of the current SiC MMC.

[0069] S32. Determine whether the absolute value of the difference between the per-unit value of each phase circulating current of the current SiC MMC and the corresponding harmonic injection reference wave is less than or equal to the preset threshold. If so, complete the fluctuation suppression operation of the capacitor voltage of the SiC MMC submodule; otherwise, adjust the parameters of the harmonic injection controller and proceed to step S31.

[0070] Furthermore, S3 also includes: when the absolute value of the difference between the per-unit value of each phase circulating current of the current SiC MMC and the corresponding harmonic injection reference wave is less than or equal to a preset threshold, before completing the fluctuation suppression operation of the SiC MMC submodule capacitor voltage, it is further determined whether the bridge arm reference value is overshooting; if so, the preset ratio δ is readjusted. s δ c γ s γ c 1, μ1, μ2, and proceed to step S1.

[0071] In one alternative implementation, the preset threshold is 0.03.

[0072] In one alternative implementation, the harmonic injection controller includes: a voltage harmonic injection controller and a circulating harmonic injection controller;

[0073] The voltage harmonic injection controller is used to obtain the differential mode voltage of each phase corresponding to the third harmonic voltage to be injected;

[0074] The circulating harmonic injection controller is used to obtain the common-mode voltage of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents to be injected.

[0075] It should be noted that the circulating current harmonic injection controller can be a PIR controller, a PI controller, etc. Preferably, the circulating current harmonic injection controller is a PIR controller. Using a PIR controller can avoid the coupling problem of traditional PI control during multiple harmonic injections, enabling it to complete the closed-loop control and injection of the MMC circulating current based on the optimal solution of the harmonic injection amplitude and phase, further improving the capacitor voltage fluctuation suppression effect.

[0076] It should be noted that the boundary and order analysis process for the optimal broadband harmonic circulation current injection of the SiC MMC submodule is as follows:

[0077] 1) To ensure the stable operation of MMC, when using the harmonic injection method to suppress the voltage fluctuation of the submodule capacitor, the number of the injected circulating harmonics can only be an even number and cannot be a multiple of 3;

[0078] 2) Every kth harmonic injection will inevitably generate a smaller amplitude of the kth or higher oscillation component, which will then need to be injected to suppress it. In the end, although the harmonic component of the bridge arm power oscillation increases, the harmonic amplitude decreases, thereby reducing the power oscillation evenly distributed to each submodule and reducing the submodule capacitance oscillation.

[0079] 3) Taking the simultaneous injection of 2nd, 4th, 8th and 10th harmonic circulating current reference values ​​into the j-phase circulating current of SiC MMC as an example (here, 10th means 10th and above), the circulating current expression of the j-phase is obtained;

[0080]

[0081] Where j is a, b, or c;

[0082] Substituting the circulating current expression of phase j into the bridge arm current expression of phase j's r-arm, and multiplying it by the average switching function of the corresponding bridge arm injected with the 3rd harmonic voltage, we obtain the average current expression flowing through the submodule capacitor. Then, based on the relationship between the current and voltage across the capacitor, we obtain the voltage fluctuation expressions for the submodule capacitor when simultaneously injected with 2nd, 4th, 8th, and 10th harmonic circulating currents and the 3rd harmonic voltage. The 2nd, 3rd, 4th, 8th, and 10th harmonics are represented by their sine and cosine components, respectively, as i. 2ms i 2mc i 4ms i 4mc i 8ms i 8mc i 10ms i 10mc u 3s u 3c :

[0083] Where k is the injected harmonic order, taking values ​​of 2, 4, 8, and 10; m k The modulation ratio of the injected k-th harmonic voltage satisfies

[0084]

[0085] Where m and m3 are the modulation ratios of the fundamental frequency and the injected third harmonic voltage; C is the capacitance value of the submodule. The power factor angle; m ks The sinusoidal component of the modulation ratio of the corresponding harmonic voltage or the third harmonic voltage caused by the k-th circulating harmonic; m kc The cosine component of the modulation ratio of the corresponding harmonic voltage or the third harmonic voltage caused by the k-th circulating harmonic; β k The phase of the corresponding harmonic voltage or the third harmonic voltage caused by the kth circulating harmonic.

[0086] Among them are:

[0087]

[0088] Taking the upper bridge arm of phase a as an example, the expression for the voltage fluctuation of the submodule capacitor of this bridge arm is as follows:

[0089]

[0090] The l-th component of the submodule capacitor voltage fluctuation is:

[0091]

[0092]

[0093]

[0094]

[0095] 4) Based on the expression for the voltage fluctuation of the submodule capacitor at this time, the amplitude coefficient of the 1st harmonic is at least 1 / (2l) of the fundamental component. Therefore, the amplitude coefficients of harmonic components above the 10th order are less than 1 / 20 of the fundamental component and can be ignored. Simultaneously, when the 10th harmonic is injected, the 1st, 3rd, 4th, and 5th harmonics of the submodule capacitor voltage fluctuation do not contain terms related to the 10th harmonic. Furthermore, only 8 terms in the 2nd harmonic voltage fluctuation are related to the injected 10th harmonic, all of which are product terms with the smaller amplitude 8th harmonic. Therefore, the injected 10th harmonic has little effect on the voltage fluctuation of the submodule capacitor, especially the dominant low-frequency component. The above conclusions can also be derived for the injection of higher harmonics.

[0096] Therefore, in order to effectively suppress the voltage fluctuation of the submodule, the 2nd, 4th and 8th harmonic currents can be injected in sequence and the 3rd harmonic voltage can be injected in conjunction to effectively reduce the voltage fluctuation of the SiC MMC submodule capacitor.

[0097] In summary, this invention analyzes and determines the optimal boundaries and order of broadband harmonic circulating current injection for SiC MMC submodules, thereby establishing optimized models for submodule capacitor voltage fluctuations that can inject 2nd, 4th, and 8th harmonics. This invention fully utilizes the higher control bandwidth of SiC MMCs, and by solving the optimized submodule capacitor voltage fluctuation model, determines the amplitude and phase of the reference values ​​for the 2nd, 4th, and 8th harmonic circulating currents to be injected, as well as the amplitude and phase of the corresponding 3rd harmonic voltage. Furthermore, by injecting 2nd, 4th, and 8th harmonic circulating currents, combined with the injection of the 3rd harmonic voltage, broadband harmonic joint injection is achieved, reducing submodule capacitor voltage fluctuations and realizing a capacitor voltage fluctuation suppression method for SiC MMC submodules with high suppression effectiveness.

[0098] To further illustrate the SiC MMC submodule capacitor voltage fluctuation suppression method provided by the present invention, a specific embodiment is described below:

[0099] This embodiment is for a three-phase 23-level SiC MMC system, wherein the topology of the SiC MMC is as follows: Figure 2 As shown. Where, u j i j Let a, b, c represent the j-phase voltage and j-phase current on the AC side of the SiC MMC, respectively; u rj Indicates the voltage of the j-phase r-arm bridge; i rj This represents the current in the j-phase r-arm bridge; r = p, n; p represents the upper arm bridge, n represents the lower arm bridge; i pj Indicates the current in the upper arm of phase j; i nj Represents the lower arm current of phase j; u pj Represents the voltage of the upper arm of phase j; u nj This represents the voltage of the lower arm of phase j. U dc I dc This represents the DC-side voltage and current of the SiC MMC, u sm This represents the steady-state submodule voltage. The bridge arm inductance is L0, the bridge arm parasitic resistance is R0, and the submodule capacitance is C0. The specific implementation steps are as follows:

[0100] Based on the MMC mathematical model, taking the upper bridge arm of phase a as an example, the expressions for the submodule capacitor voltage fluctuation u under the injection of the 2nd, 3rd, 4th, and 8th harmonics are established. cpap The specific process is as follows:

[0101] By simultaneously injecting reference values ​​of the 2nd, 4th, and 8th harmonic circulating currents into the a-phase circulating current of the MMC, the circulating current of the upper arm of the a-phase is obtained as follows:

[0102]

[0103] Among them, I dc I is the DC side current of the MMC. km ω is the amplitude of the reference value for the kth harmonic circulating current to be injected; ω is the grid angular frequency; t is the system running time. The phase of the reference value for the kth harmonic circulating current to be injected.

[0104] Figure 3 The diagram shows the equivalent circuit of MMC, where u diffa For differential mode voltage, u coma This is the differential mode voltage. The current i in the upper arm of phase a is... pa It can be represented as:

[0105]

[0106] Ignoring the influence of circuit impedance in the differential mode circuit, the voltage u of the upper bridge arm of phase a is... pa for: Therefore, S p Let i be the average switching function of the upper bridge arm, and the current i flowing through the submodule. cpa for:

[0107]

[0108] Based on the relationship between the current and voltage across the capacitor, the calculation method for the capacitor voltage fluctuation of the MMC submodule can be obtained as follows:

[0109]

[0110] In the formula, U c This represents the DC component of the submodule capacitor voltage; the integral part is the capacitor voltage fluctuation value.

[0111] The second, third, fourth, and eighth harmonics are represented by their sine and cosine components, respectively, as i. 2ms i 2mc i 4ms i 4mc i 8ms i 8mc u 3s u 3c :

[0112]

[0113] Then the expression for the voltage fluctuation of the capacitor in the established submodule can be obtained. cpap :

[0114]

[0115] Where m and m3 are the modulation ratios of the fundamental frequency and the injected third harmonic voltage; C is the capacitance value of the submodule. β3 is the power factor angle; β3 is the phase of the third harmonic voltage to be injected; m k β is the modulation coefficient for the injected k-th harmonic voltage; k The phase of the corresponding harmonic voltage or the third harmonic voltage caused by the kth circulating harmonic; k = 2, 4, 8.

[0116] Among them are:

[0117]

[0118] Therefore, the optimization model for the voltage fluctuation of the submodule capacitor is established as follows:

[0119] obj.min f(i 2ms i 2mc i 4ms i 4mc i 8ms i 8mc ,u 3s ,u 3c )

[0120] =max(u cpap )-min(u cpap )

[0121] (k = 2, 4, 8)

[0122] Meanwhile, to ensure the normal operation of the MMC system, the input harmonic voltage is constrained:

[0123]

[0124] In the formula, δ s δ c γ s γ c μ1 and μ2 are preset ratios that are both greater than 0 and less than 1; u ap (t) represents the phase a voltage of the upper bridge arm at time t; in this embodiment, δs and δc are both taken as 0.5; γs and γ c The values ​​are all 0.5; μ1 is 0.02; and μ2 is 0.98.

[0125] Therefore, the upper bridge arm voltage u of phase a is obtained. ap (t) is:

[0126]

[0127] An intelligent algorithm is used to solve the capacitor voltage fluctuation optimization model of the above sub-module. In this embodiment, the specific algorithm used is the `fmin` function from the MATLAB toolbox. After inputting system parameters, the objective function, and constraints, the optimal amplitude and phase of each harmonic injection are obtained. The proposed optimal amplitude U for each harmonic injection is... 3m I km and phase β3, Based on solving the above submodule capacitor voltage fluctuation optimization model, we can obtain:

[0128]

[0129] The parameters of the 3-phase 23-level SiC MMC system are shown in Table 1.

[0130] Table 1. Main circuit parameters of the SiC MMC simulation model

[0131]

[0132] After obtaining the optimal amplitude and phase of each harmonic injection, the corresponding harmonics are injected using the appropriate harmonic injection control method to suppress voltage fluctuations in the SiC MMC submodule capacitor.

[0133] The control method for harmonic injection is as follows: Figure 4 As shown, it includes two control modules: Figure (a) is the injected harmonic reference value generation module, which is used to generate the harmonic injection reference wave; Figure (b) is the PIR controller, which is used to enable the real-time circulating current measurement value to track the generated harmonic injection reference wave through closed-loop control.

[0134] The injected harmonic reference value generation module obtains the harmonic phase angle by multiplying the positive sequence phase angle by its power k and adding the calculated optimal initial phase angle. This positive sequence phase angle is obtained through a phase-locked loop (PLL). When solving for phase a as an example, according to the phase shift relationship of the three phases, the phase coefficients of phases a, b, and c are 0, -1, and 1, respectively. Due to the presence of line parameters such as bridge arm inductance, to avoid unnecessary phase angle deviations, this positive sequence phase angle is obtained by phase-locking the fundamental reference wave.

[0135] The PIR controller subtracts the original circulating current signal from its DC signal after passing through a low-pass filter (LPF) to obtain its ripple component. This harmonic component is then controlled by the PIR controller to track the reference value, thus achieving the purpose of injecting circulating current as needed.

[0136] The above-mentioned SiC MMC submodule capacitor voltage fluctuation suppression method is as follows: Figure 5As shown, after determining the harmonic injection order and boundaries, optimization models for submodule capacitor voltage fluctuations under simultaneous injection of 2nd, 4th, and 8th harmonic circulating currents and 3rd harmonic voltages were established. SiC MMC circuit parameters were input, and constraint conditions were set to a preset ratio. An optimization model for submodule capacitor voltage fluctuations under multiple harmonic co-injection was established, simultaneously solving for the optimal amplitude and phase reference values. These values ​​were then injected into the MMC system via a PIR controller. If the preset conditions were not met, the adjustment parameters were returned; if they were met, the fluctuation suppression of SiC MMC submodule capacitor voltages could be achieved.

[0137] Example simulation results are as follows Figure 6 As shown, the circulating harmonic component i cir It tracked the reference harmonic i quite well. cirref This verifies the effectiveness of the invention in suppressing capacitor voltage fluctuations in SiC MMC submodules through broadband harmonic injection.

[0138] also, Figure 7 The submodule voltage and bridge arm current waveforms during harmonic suppression and injection were recorded. The following conclusions can be drawn:

[0139] 1) The harmonic injection method proposed in this invention, which injects harmonics of the 2nd, 3rd, 4th, and 8th orders, achieves the best suppression effect on capacitor voltage fluctuations. After injecting harmonics of the 10th order or higher, the suppression effect on capacitor voltage fluctuations becomes insignificant, and may even lead to an increase in capacitor voltage fluctuations.

[0140] 2) The broadband harmonic injection method disclosed in this invention can effectively reduce the voltage fluctuation of the submodule capacitor and reduce the bridge arm current. After adopting the broadband harmonic injection proposed in this invention, compared with the circulating current suppression and typical second-order circulating current injection schemes, the capacitor voltage fluctuation is reduced by 71.12% and 54.17% respectively, and the effective value of the bridge arm current is close to that of the typical injection scheme, without increasing the bridge arm current and generating additional losses.

[0141] In a second aspect, the present invention provides an MMC submodule capacitor voltage fluctuation suppression system, comprising: a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to perform the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of the present invention.

[0142] The related technical solutions are the same as the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of this invention, and will not be described in detail here.

[0143] Thirdly, the present invention provides a modular multilevel converter system, including: SiC MMC and MMC controller;

[0144] The MMC controller is used to execute the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of the present invention.

[0145] The related technical solutions are the same as the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of this invention, and will not be described in detail here.

[0146] Fourthly, the present invention also provides a computer-readable storage medium comprising a stored computer program, wherein the computer program, when executed by a processor, controls the device where the storage medium is located to execute the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of the present invention.

[0147] The related technical solutions are the same as the SiC MMC submodule capacitor voltage fluctuation suppression method provided in the first aspect of this invention, and will not be described in detail here.

[0148] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for suppressing capacitor voltage fluctuations in a SiC MMC submodule, characterized in that, The SiC MMC includes: three phases and six bridge arms; each phase includes upper and lower bridge arms, and each bridge arm includes: bridge arm inductor and multiple cascaded sub-modules; the sub-modules include: SiC MOSFETs and capacitors; The fluctuation suppression method includes: S1. Input the circuit parameters of the SiC MMC into the submodule capacitor voltage fluctuation optimization model for calculation to obtain the amplitude and phase of the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents to be injected, as well as the amplitude and phase of the corresponding 3rd harmonic voltage. S2. Based on the amplitude and phase of the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents to be injected and the amplitude and phase of the corresponding 3rd harmonic voltage, generate the harmonic injection reference wave for each phase. S3. A harmonic injection controller is used to control the circulating current of each phase of the SiC MMC to track its corresponding harmonic injection reference wave, thereby suppressing the fluctuation of the capacitor voltage of the SiC MMC submodule. The submodule capacitor voltage fluctuation optimization model is constructed in the following way: Simultaneously injecting 2nd, 4th, and 8th harmonic circulating current reference values ​​into the j-phase circulating current of SiC MMC yields the corresponding circulating current expressions for the phase; j represents a, b, or c. Substituting the circulating current expression into the bridge arm current expression of the j-phase r-arm, and multiplying it by the average switching function of the corresponding bridge arm injected with the third harmonic voltage, we obtain the average current expression flowing through the submodule capacitor. Then, based on the relationship between the current and voltage across the capacitor, we obtain the submodule capacitor voltage fluctuation expression; the r-arm is either the upper or lower bridge arm. An optimization model for the submodule capacitor voltage fluctuation is constructed with the objective of minimizing the difference between the maximum and minimum values ​​of the submodule capacitor voltage fluctuation obtained based on the submodule capacitor voltage fluctuation expression.

2. The method for suppressing capacitor voltage fluctuations in SiC MMC submodules according to claim 1, characterized in that, The circulation expression for phase j is: Among them, i cirj For the j-phase circulating current of SiC MMC; I dc For SiC MMC DC side current; I km ω is the amplitude of the reference value for the kth harmonic circulating current to be injected; ω is the grid angular frequency; t is the system running time; when j is a, n is 0; when j is b, n is -1; when j is c, n is 1. The phase of the reference value for the kth harmonic circulating current to be injected.

3. The method for suppressing capacitor voltage fluctuations in SiC MMC submodules according to claim 1, characterized in that, The constraints of the submodule capacitor voltage fluctuation optimization model include: Where, δ s δ c γ s γ c μ1 and μ2 are all preset ratios; I m The amplitude of the output current of the SiC MMC; I km The amplitude of the reference value for the kth harmonic circulating current to be injected; The phase of the reference value for the kth harmonic circulating current to be injected; k = 2, 4, 8; U m The amplitude of the SiCMMC output voltage; m 3s =u 3s / U dc m 3c =u 3c / U dc ;u 3s =U 3m sin(β3); u 3c =U 3m cos(β3); U 3m β3 represents the amplitude of the third harmonic voltage to be injected; β3 represents the phase of the third harmonic voltage to be injected; Udc represents the DC-side voltage of the SiCMMC; u jr (t) represents the voltage of the j-phase r-arm at time t; T represents one grid cycle.

4. The method for suppressing capacitor voltage fluctuations in a SiCMMC submodule according to claim 3, characterized in that, δ s δ c γ s and γ c All are preset ratios that are greater than 0 and less than 1; When SiCMMC is a half-bridge MMC, μ1 and μ2 are both preset ratios that are greater than 0 and less than 1; When the SiC MMC is a hybrid MMC or a full-bridge MMC, μ1 and μ2 are both preset ratios that are greater than -1 and less than 1.

5. The method for suppressing capacitor voltage fluctuations in SiC MMC submodules according to any one of claims 1-4, characterized in that, S3 includes: S31. Using a harmonic injection controller, obtain the differential mode voltage of each phase corresponding to the 3rd harmonic voltage to be injected and the common mode voltage of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents. After superimposing the differential mode voltage and the common mode voltage of each phase onto the bridge arm reference wave of the corresponding phase, obtain the circulating current of each phase of the current SiC MMC. S32. Determine whether the absolute value of the difference between the per-unit value of each phase circulating current of the current SiC MMC and the corresponding harmonic injection reference wave is less than or equal to a preset threshold. If so, complete the fluctuation suppression operation of the capacitor voltage of the SiC MMC submodule; otherwise, adjust the parameters of the harmonic injection controller and proceed to S31.

6. The method for suppressing capacitor voltage fluctuations in a SiC MMC submodule according to claim 5, characterized in that, The harmonic injection controller includes: a voltage harmonic injection controller and a circulating harmonic injection controller; The voltage harmonic injection controller is used to obtain the differential mode voltages of each phase corresponding to the third harmonic voltage to be injected. The circulating harmonic injection controller is used to obtain the common-mode voltage of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents to be injected.

7. The method for suppressing capacitor voltage fluctuations in a SiC MMC submodule according to claim 6, characterized in that, The circulating harmonic injection controller is a PIR controller.

8. A SiC MMC submodule capacitor voltage fluctuation suppression system, characterized in that, include: A memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to perform the SiC MMC submodule capacitor voltage fluctuation suppression method according to any one of claims 1-7.

9. A modular multilevel converter system, characterized in that, include: SiC MMC and MMC controllers; The MMC controller is used to execute the SiC MMC submodule capacitor voltage fluctuation suppression method according to any one of claims 1-7.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored computer program, wherein, when the computer program is run by a processor, it controls the device where the storage medium is located to perform the SiC MMC submodule capacitor voltage fluctuation suppression method according to any one of claims 1-7.

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