SiC MMC sub-module capacitor voltage fluctuation suppression method and system

Abstract

The invention discloses a SiC MMC sub-module capacitor voltage fluctuation suppression method and system, and belongs to the technical field of electronic power. According to the analysis result of the optimal broadband harmonic circulation injection boundary and number of times of the SiC MMC sub-module, a sub-module capacitor voltage fluctuation optimization model for simultaneously injecting second, fourth and eighth harmonic circulation and third harmonic voltage is determined, and the characteristic of higher control bandwidth of the SiC MMC can be fully utilized; by solving a submodule capacitor voltage fluctuation optimization model, the amplitudes and phases of reference values of second, fourth and eighth harmonic circulation needing to be injected and the amplitude and phase of corresponding third harmonic voltage are determined, then the second, fourth and eighth harmonic circulation is injected, and the third harmonic voltage injection is combined to realize broadband harmonic combined injection. The capacitance voltage fluctuation of the sub-module is reduced, the capacitance voltage fluctuation suppression method for the SiC MMC sub-module is realized, and the suppression effect is relatively high.

Description

Technical Field

[0001] The present invention belongs to the field of electronic power technology, and more specifically, relates to a method and system for suppressing capacitor voltage fluctuation of a SiC MMC submodule. Background Art

[0002] MMC has high operating efficiency, large power range, high output power quality and strong scalability, but the current MMC converter station is large in size and complex in structure, so it is urgent to realize the lightweight design of MMC. MMC submodules are mainly composed of switching devices and capacitors. Among them, the switching devices are currently mainly Si devices, but Si devices have large losses and low switching frequencies, and the utilization effect of them in existing projects has reached its theoretical limit, resulting in the difficulty in reducing the weight and volume of MMC. SiC devices have high switching frequencies and low switching losses. Therefore, using SiC MMC to replace traditional Si MMC to achieve lightweight MMC is an important research direction in the future.

[0003] During the operation of MMC, the charging and discharging effect of the bridge arm current on the submodule capacitor causes the voltage of all submodule capacitors to fluctuate, which needs to be suppressed. If a larger capacitor is directly used to suppress the submodule voltage fluctuation, it will undoubtedly increase the volume of the MMC system. Therefore, the existing MMC submodule voltage fluctuation suppression technology usually adopts additional control and improved topology methods. The improved topology scheme is mainly divided into improved submodules, increased power channels and other schemes. Such schemes usually require the addition of additional circuits, which increases additional hardware costs. The additional control methods mainly include submodule capacitor voltage reduction control method, high-frequency circulating current combined with high-frequency common-mode voltage injection method and circulating current harmonic injection method. The first two methods are prone to overshoot in the field of high-voltage direct current transmission due to the relatively high MMC modulation. The harmonic injection method does not require the addition of additional circuits, and can effectively avoid overshoot through reasonable control. Therefore, the circulating current harmonic injection method is often used to suppress the voltage fluctuation of MMC submodules.

[0004] However, the existing circulating harmonic injection method is designed for the current MMC submodule based on Si IGBT devices. It is limited by the switching frequency and power loss of the Si IGBT itself. When it is directly used to suppress the fluctuation of the capacitor voltage of the SiC MMC submodule, it cannot fully utilize the advantages of SiC MMC to maximize the suppression effect. Therefore, it is urgent to provide a method for suppressing the capacitor voltage fluctuation for SiC MMC submodules, so as to give full play to the advantages of SiC devices and further realize the lightweight design of MMC. Summary of the invention

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

[0006] In order to achieve the above-mentioned object, in the first aspect, the present invention provides a method for suppressing capacitor voltage fluctuation of SiC MMC submodule; wherein, SiC MMC comprises: three-phase six bridge arms; each phase comprises upper and lower two bridge arms, each bridge arm comprises: a bridge arm inductor and a plurality of cascaded submodules; the submodule comprises: SiC MOSFET and capacitor;

[0007] The above-mentioned fluctuation suppression method includes:

[0008] S1. Input the circuit parameters of SiC MMC into the submodule capacitor voltage fluctuation optimization model for calculation, and obtain 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;

[0009] S2. Generate a harmonic injection reference wave for each phase according to the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​injected as needed and the amplitude and phase of the corresponding 3rd harmonic voltage;

[0010] S3, using a harmonic injection controller 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] Among them, the submodule capacitor voltage fluctuation optimization model is constructed in the following way:

[0012] The reference values ​​of the 2nd, 4th and 8th harmonic circulating currents are injected into the j-phase circulating current of SiC MMC at the same time to obtain the circulating current expression of the corresponding phase; j is a, b or c;

[0013] Substitute the circulating current expression into the bridge arm current expression of the j-phase r bridge arm, and multiply it by 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, and then obtain the submodule capacitor voltage fluctuation expression based on the relationship between the current and voltage across the capacitor; the r bridge arm is the upper bridge arm or the lower bridge arm;

[0014] With the goal 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, a submodule capacitor voltage fluctuation optimization model is constructed.

[0015] Further preferably, the circulation expression of phase j is:

[0016]

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

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

[0019]

[0020] Among them, δ s ,δ c , γ s , γ c , μ 1 , μ 2 All are preset ratios; I m is the amplitude of the SiC MMC output current; I km is the amplitude of the kth harmonic circulating current reference value that needs to be injected; is the phase of the kth harmonic circulating current reference value to be injected; k = 2, 4, 8; U m is the amplitude of the SiCMMC output voltage; m 3s =u 3s / U dc ;m 3c =u 3c / U dc ;u 3s =U 3m sin(β 3 );u 3c =U 3m cos(β 3 );U 3m is the amplitude of the third harmonic voltage to be injected; β 3 is the phase of the third harmonic voltage to be injected; U dc Indicates the DC side voltage of SiCMMC; u jr (t) is the voltage of the j-phase r bridge arm at time t; T is a power grid cycle.

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

[0022] When the SiC MMC is a half-bridge MMC, μ 1 , μ 2 They are all preset ratios greater than 0 and less than 1;

[0023] When the SiC MMC is a hybrid MMC or a full-bridge MMC, μ 1 , μ 2 They are all preset ratios greater than -1 and less than 1.

[0024] Further preferably, the above S3 includes:

[0025] S31, using a harmonic injection controller to 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 value of the 2nd, 4th and 8th harmonic circulating current, and after superimposing the differential mode voltage of each phase and the common mode voltage of each phase on the bridge arm reference wave of the corresponding phase, obtain the circulating current of each phase of the current SiC MMC;

[0026] S32, judging whether the absolute values ​​of the differences between the current circulating currents of each phase of the SiC MMC and the per-unit values ​​of the corresponding harmonic injection reference waves are all less than or equal to the preset threshold value, if so, completing the fluctuation suppression operation of the SiC MMC sub-module capacitor voltage; otherwise, adjusting the parameters of the harmonic injection controller and going to step S31.

[0027] Further preferably, the above harmonic injection controller includes: a voltage harmonic injection controller and a circulating current 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 current harmonic injection controller is used to obtain the common mode voltages of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents that need to be injected.

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

[0031] In a second aspect, the present invention provides a SiC MMC sub-module capacitor voltage fluctuation suppression system, comprising: a memory and a processor, the memory storing a computer program, and the processor executing the SiC MMC sub-module capacitor voltage fluctuation suppression method provided by the first aspect of the present invention when executing the computer program.

[0032] In a third aspect, the present invention provides a modular multilevel converter system, comprising: a SiC MMC and an MMC controller;

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

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

[0035] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0036] 1. The present invention provides a method for suppressing capacitor voltage fluctuation of a SiC MMC submodule. The boundary and number of optimal broadband harmonic circulating current injection for the SiC MMC submodule are determined by analysis, thereby determining a submodule capacitor voltage fluctuation optimization model for simultaneously injecting 2nd, 4th and 8th harmonic circulating currents and 3rd harmonic voltages, which can fully utilize the characteristic of SiC MMC with a higher control bandwidth; by solving the submodule capacitor voltage fluctuation optimization model, the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​to be injected and the amplitude and phase of the corresponding 3rd harmonic voltage are determined, and then by injecting 2nd, 4th and 8th harmonic circulating currents and combining with 3rd harmonic voltage injection to achieve broadband harmonic joint injection, the submodule capacitor voltage fluctuation is reduced, and the capacitor voltage fluctuation suppression method for the SiC MMC submodule is realized, which has a high suppression effect.

[0037] 2. Furthermore, the SiC MMC sub-module 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 with negative level output capabilities, and has a wide range of applications.

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

[0039] 4. The present invention provides a modular multilevel converter system, comprising a SiC MMC and an MMC controller; wherein the MMC controller is used to execute the SiC MMC sub-module capacitor voltage fluctuation suppression method provided in the first aspect of the present invention, making full use of the characteristic of SiC MMC having a higher control bandwidth, achieving a higher suppression effect, and compared with the existing modular multilevel converter system, it can further reduce the volume of the sub-module capacitor under the same capacitor voltage fluctuation requirements, thereby realizing the lightweight design of the modular multilevel converter system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A flow chart of a method for suppressing capacitor voltage fluctuation of a SiC MMC submodule provided by an embodiment of the present invention;

[0041] Figure 2 A topological structure diagram of a typical SiC MMC provided in an embodiment of the present invention;

[0042] Figure 3 The SiC MMC equivalent model provided by the embodiment 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 Schematic diagram of a broadband harmonic injection control strategy provided by an embodiment of the present invention; wherein (a) is a schematic diagram of the structure of an injected harmonic reference value generation module; (b) is a schematic diagram of the structure of a PIR controller;

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

[0045] Figure 6 The circulating current fluctuation component and the reference wave of the MMC system when the harmonic injection order is switched by the harmonic injection controller provided by the embodiment of the present invention;

[0046] Figure 7 Submodule voltage and bridge arm current waveforms during harmonic suppression and injection provided in an embodiment of the present invention; wherein, (a) to (f) are 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, 2+3+4+8+10th harmonic injection; (g) to (l) are 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, 2+3+4+8+10th harmonic injection. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0048] In order to achieve the above-mentioned object, in the first aspect, the present invention provides a method for suppressing capacitor voltage fluctuation of SiC MMC submodule; wherein, SiC MMC comprises: three-phase six bridge arms; each phase comprises upper and lower two bridge arms, each bridge arm comprises: a bridge arm inductor and a plurality of cascaded submodules; the submodule comprises: SiC MOSFET and capacitor;

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

[0050] S1. Input the circuit parameters of SiC MMC into the submodule capacitor voltage fluctuation optimization model for calculation, and obtain 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;

[0051] S2. Generate a harmonic injection reference wave for each phase according to the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​injected as needed and the amplitude and phase of the corresponding 3rd harmonic voltage;

[0052] S3, using a harmonic injection controller 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] Among them, the submodule capacitor voltage fluctuation optimization model is constructed in the following way:

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

[0055] In an optional implementation manner, the circulation expression of phase j is:

[0056]

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

[0058] b. Substitute the circulating current expression into the bridge arm current expression of the j-phase r bridge arm, and multiply it by 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, and then obtain the submodule capacitor voltage fluctuation expression based on the relationship between the current and voltage across the capacitor; where r is p or n, indicating the upper bridge arm or the lower bridge arm;

[0059] c. With the goal of minimizing the difference between the maximum and minimum values ​​of the submodule capacitor voltage fluctuations obtained based on the submodule capacitor voltage fluctuation expression, a submodule capacitor voltage fluctuation optimization model is constructed.

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

[0061]

[0062] Among them, δ s , δ c , γ s , γ c , μ 1 , μ 2 is the preset ratio; I m is the amplitude of the SiC MMC output current; I km is the amplitude of the kth harmonic circulating current reference value that needs to be injected; is the phase of the kth harmonic circulating current reference value to be injected; k = 2, 4, 8; U m is 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(β 3 );u 3c =U 3m cos(β 3 );U 3m is the amplitude of the third harmonic voltage to be injected; β 3 is the phase of the third harmonic voltage to be injected; U dc Indicates the DC side voltage of SiCMMC; u jr (t) is the voltage of the j-phase r bridge arm at time t; T is a power grid cycle.

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

[0064] When the SiC MMC is a half-bridge MMC, μ 1 , μ 2 They are all preset ratios greater than 0 and less than 1;

[0065] When the SiC MMC is a hybrid MMC or a full-bridge MMC, μ 1 , μ 2 They are all preset ratios greater than -1 and less than 1.

[0066] In an 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, μ 1 =0.02, μ 2 =0.98.

[0067] In an optional implementation manner, the above S3 includes:

[0068] S31, using a harmonic injection controller to 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 value of the 2nd, 4th and 8th harmonic circulating current, and after superimposing the differential mode voltage of each phase and the common mode voltage of each phase on the bridge arm reference wave of the corresponding phase, obtain the circulating current of each phase of the current SiC MMC;

[0069] S32, judging whether the absolute values ​​of the differences between the current circulating currents of each phase of the SiC MMC and the per-unit values ​​of the corresponding harmonic injection reference waves are all less than or equal to the preset threshold value, if so, completing the fluctuation suppression operation of the SiC MMC sub-module capacitor voltage; otherwise, adjusting the parameters of the harmonic injection controller and going to step S31.

[0070] Furthermore, the above S3 also includes: when the absolute value of the difference between the current phase circulating current of the SiC MMC and the corresponding per-unit value of the harmonic injection reference wave is less than or equal to the preset threshold, before completing the fluctuation suppression operation of the SiC MMC sub-module capacitor voltage, it is further determined whether the bridge arm reference value is over-adjusted, and if so, the preset ratio δ is readjusted. s , δ c , γ s , γ c , μ 1 , μ 2 , and go to step S1.

[0071] In an optional implementation manner, the preset threshold is 0.03.

[0072] In an optional implementation manner, the above-mentioned harmonic injection controller includes: a voltage harmonic injection controller and a circulating current 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 current harmonic injection controller is used to obtain the common mode voltages of each phase corresponding to the reference values ​​of the 2nd, 4th and 8th harmonic circulating currents that need to be injected.

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

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

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

[0078] 2) Each injection of the kth harmonic will inevitably generate a fluctuation component with a smaller amplitude above the kth order, and then it is necessary to continue to inject the corresponding harmonic for suppression. Although the harmonic component of the bridge arm power fluctuation is increased in the end, the harmonic amplitude is reduced, thereby reducing the power fluctuation distributed to each sub-module and reducing the sub-module capacitance fluctuation;

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

[0080]

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

[0082] After substituting the j-phase circulating current expression into the bridge arm current expression of the j-phase r bridge arm, multiply it by the corresponding bridge arm average switching function injected with the 3rd 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, the submodule capacitor voltage fluctuation expression when the 2nd, 4th, 8th and 10th harmonic circulating current and the 3rd harmonic voltage are injected simultaneously is obtained. The 2nd, 3rd, 4th, 8th and 10th harmonics are represented by their sine components and cosine components as i 2ms 、i2mc 、i 4ms 、i 4mc 、i 8ms 、i 8mc 、i 10ms 、i 10mc 、u 3s 、u 3c :

[0083] Where k is the number of injected harmonics, which can be 2, 4, 8, or 10; m k is the modulation ratio of the injected kth harmonic voltage, satisfying

[0084]

[0085] Among them, m, m 3 is the modulation ratio of the fundamental wave and the injected third harmonic voltage; C is the capacitance value of the submodule; is the power factor angle; m ks m is the sinusoidal component of the corresponding harmonic voltage or the modulation ratio of the third harmonic voltage caused by the kth harmonic of the circulating current; kc is the cosine component of the corresponding harmonic voltage or the 3rd harmonic voltage modulation ratio caused by the kth harmonic of circulating current; β k It is the phase of the corresponding harmonic voltage or the third harmonic voltage caused by the kth harmonic of circulating current.

[0086] Among them, there are:

[0087]

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

[0089]

[0090] Among them, the l-th component of the submodule capacitor voltage fluctuation is:

[0091]

[0092]

[0093]

[0094]

[0095] 4) According to the expression of the submodule capacitor voltage fluctuation at this time, the amplitude coefficient of the lth harmonic is at least 1 / (2l) of the fundamental component. Therefore, the amplitude coefficient of the harmonic components above the 10th order is less than 1 / 20 of the fundamental component and can be ignored. At the same time, when the 10th harmonic is injected, the 1st, 3rd, 4th, and 5th capacitor voltage fluctuations in the low-order harmonics of the submodule capacitor voltage do not contain items related to the 10th harmonic, and there are only 8 items related to the injected 10th harmonic in the 2nd harmonic of the voltage fluctuation, all of which are product items with the 8th harmonic with a smaller amplitude. Therefore, the injected 10th harmonic has no obvious effect on the submodule capacitor voltage fluctuation, especially the dominant low-frequency component of the voltage fluctuation. The above conclusions can also be derived to the case of injecting higher-order 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 injection can effectively reduce the capacitor voltage fluctuation of the SiC MMC submodule.

[0097] In summary, the present invention determines the boundary and number of optimal broadband harmonic circulating current injection for SiC MMC sub-modules through analysis, thereby determining the sub-module capacitor voltage fluctuation optimization model that can inject 2nd, 4th and 8th harmonics; the present invention can make full use of the characteristic of SiC MMC with higher control bandwidth, and by solving the sub-module capacitor voltage fluctuation optimization model, the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​that need to be injected and the amplitude and phase of the corresponding 3rd harmonic voltage are determined, and then by injecting 2nd, 4th and 8th harmonic circulating currents and combining with 3rd harmonic voltage injection to achieve broadband harmonic joint injection, the sub-module capacitor voltage fluctuation is reduced, and the capacitor voltage fluctuation suppression method for SiC MMC sub-module is realized, which has a high suppression effect.

[0098] In order 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 directed to a three-phase 23-level SiC MMC system, where the topology of the SiC MMC is as follows: Figure 2 As shown. Among them, u j 、i j They represent the j-phase voltage and j-phase current on the AC side of SiC MMC, j = a, b, c; u rj represents the voltage of the bridge arm of phase j; i rj represents the current of the bridge arm of phase j; r = p, n; p represents the upper bridge arm, n represents the lower bridge arm; i pj represents the upper bridge arm current of phase j; i nj represents the lower bridge arm current of phase j; u pj represents the voltage of the upper bridge arm of phase j; u njIndicates the voltage of the lower bridge arm of phase j. dc ,I dc Represents the voltage and current at the DC side of SiC MMC, u sm is the steady-state submodule voltage. The bridge arm inductance is L 0 , the parasitic resistance of the bridge arm is R 0 , the submodule capacitance is C 0 The following are the specific implementation steps:

[0100] According to the MMC mathematical model, taking the upper bridge arm of phase a as an example, the submodule capacitor voltage fluctuation expression u is established when the 2nd, 3rd, 4th and 8th harmonics are injected: cpap , the specific process is as follows:

[0101] The 2nd, 4th and 8th harmonic circulating current reference values ​​are injected into the a-phase circulating current of the MMC at the same time, and the circulating current of the upper bridge arm of the a-phase is obtained as follows:

[0102]

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

[0104] Figure 3 The figure shows the MMC equivalent circuit, where u diffa is the differential mode voltage, u coma is the differential voltage. pa It can be expressed as:

[0105]

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

[0107]

[0108] According to 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] Where U c is the DC component of the submodule capacitor voltage, and the integral part is the capacitor voltage fluctuation value.

[0111] The 2nd, 3rd, 4th and 8th 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 established submodule capacitance voltage fluctuation expression u can be obtained cpap :

[0114]

[0115] Among them, m, m 3 is the modulation ratio of the fundamental wave and the injected third harmonic voltage; C is the capacitance value of the submodule; is the power factor angle; β 3 is the phase of the third harmonic voltage to be injected; m k is the modulation coefficient of the injected kth harmonic voltage; β k It is the phase of the corresponding harmonic voltage or the third harmonic voltage caused by the kth harmonic of circulating current; k=2,4,8.

[0116] Among them, there are:

[0117]

[0118] The submodule capacitor voltage fluctuation optimization model 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] At the same time, in order 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 , μ 1 , μ 2 are preset ratios that are both greater than 0 and less than 1; u ap (t) is the voltage of phase a of the upper bridge arm at time t; in this embodiment, δs and δc are both 0.5; γs and γ c The average value is 0.5; μ 1 The value is 0.02; μ 2 The value is 0.98.

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

[0126]

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

[0128]

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

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

[0131]

[0132] After solving the optimal amplitude and phase of each harmonic injection, the corresponding harmonic injection control method is used to inject the corresponding harmonic to achieve the suppression of capacitor voltage fluctuation of SiC MMC sub-module.

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

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

[0135] Among them, the PIR controller subtracts the original circulating current signal from the DC signal after it passes through the low-pass filter (LPF) to obtain its fluctuation component, and controls this part of the harmonic component through the PIR to track the reference value, so as to achieve the purpose of injecting the circulating current as required.

[0136] The above SiC MMC submodule capacitor voltage fluctuation suppression method is as follows Figure 5 As shown in the figure, after determining the number and boundary of harmonic injection, the sub-module capacitor voltage fluctuation optimization model under the simultaneous injection of 2nd, 4th, 8th harmonic circulating current and 3rd harmonic voltage is determined, the SiC MMC circuit parameters are input and the preset proportion of constraint conditions is set, and the sub-module capacitor voltage fluctuation optimization model under multiple harmonic coordinated injection is established. The optimal amplitude and phase reference values ​​are solved at the same time, and injected into the MMC system through the PIR controller. If the preset conditions are not met, the adjustment parameters are returned; if they are met, the fluctuation suppression of the SiC MMC sub-module capacitor voltage can be achieved.

[0137] The simulation results are as follows: Figure 6 As shown, the circulating current harmonic component i cir Better tracking of the reference harmonic i cirref , which verifies the effectiveness of the SiC MMC sub-module capacitor voltage fluctuation suppression through broadband harmonic injection provided by the present invention.

[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 by the present invention with the injection number of 2+3+4+8 harmonics can achieve the best capacitor voltage fluctuation suppression effect. After injecting 10 harmonics or more, the suppression effect on capacitor voltage fluctuation is no longer significant, and may even lead to an increase in capacitor voltage fluctuation.

[0140] 2) The broadband harmonic injection method disclosed in the present invention can effectively reduce the voltage fluctuation of the submodule capacitor and reduce the bridge arm current. After adopting the broadband harmonic injection proposed by the present invention, compared with the circulating current suppression and the typical secondary circulating current injection scheme, 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 the typical injection scheme, and the bridge arm current is not increased to generate additional losses.

[0141] In a second aspect, the present invention provides an MMC sub-module capacitor voltage fluctuation suppression system, comprising: a memory and a processor, the memory storing a computer program, and the processor executing the SiC MMC sub-module capacitor voltage fluctuation suppression method provided in the first aspect of the present invention when executing the computer program.

[0142] The related technical solution is the same as the SiC MMC sub-module capacitor voltage fluctuation suppression method provided in the first aspect of the present invention, and will not be described in detail here.

[0143] In a third aspect, the present invention provides a modular multilevel converter system, comprising: a SiC MMC and an MMC controller;

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

[0145] The related technical solution is the same as the SiC MMC sub-module capacitor voltage fluctuation suppression method provided in the first aspect of the present invention, and will not be described in detail here.

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

[0147] The related technical solution is the same as the SiC MMC sub-module capacitor voltage fluctuation suppression method provided in the first aspect of the present invention, and will not be described in detail here.

[0148] It will be easily understood by those skilled in the art that the above description is only 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 in the protection scope of the present invention.

Claims

1. A method for suppressing capacitor voltage fluctuation of SiC MMC submodule, characterized in that: The SiC MMC includes: three phases and six bridge arms; each phase includes two upper and lower bridge arms, each bridge arm includes: a bridge arm inductor and a plurality of cascaded sub-modules; the sub-modules include: SiC MOSFET and capacitor; The fluctuation suppression method comprises: S1. Input the circuit parameters of the SiC MMC into the submodule capacitance voltage fluctuation optimization model for calculation, and obtain the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​to be injected and the amplitude and phase of the corresponding 3rd harmonic voltage; S2. Generate a harmonic injection reference wave for each phase according to the amplitude and phase of the 2nd, 4th and 8th harmonic circulating current reference values ​​to be injected and the amplitude and phase of the corresponding 3rd harmonic voltage; S3, using a harmonic injection controller 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: The reference values ​​of the 2nd, 4th and 8th harmonic circulating currents are injected into the j-phase circulating current of SiC MMC at the same time to obtain the circulating current expression of the corresponding phase; j is a, b or c; Substitute the circulating current expression into the bridge arm current expression of the j-phase r bridge arm, and multiply it by 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, and then obtain the submodule capacitor voltage fluctuation expression according to the relationship between the current and voltage across the capacitor; the r bridge arm is the upper bridge arm or the lower bridge arm; The submodule capacitor voltage fluctuation optimization model is constructed with the goal of minimizing the difference between the maximum value and the minimum value of the submodule capacitor voltage fluctuation obtained based on the submodule capacitor voltage fluctuation expression.

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

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

4. The method for suppressing capacitor voltage fluctuation of SiCMMC submodule according to claim 3, characterized in that: δ s , δ c , γ s and γ c They are all preset ratios greater than 0 and less than 1; When SiCMMC is a half-bridge MMC, μ1 and μ2 are both preset ratios 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 greater than -1 and less than 1.

5. The method for suppressing capacitor voltage fluctuation of SiC MMC submodule according to any one of claims 1 to 4, characterized in that: The S3 includes: S31, using a harmonic injection controller to 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 value of the 2nd, 4th and 8th harmonic circulating current, and after superimposing the differential mode voltage of each phase and the common mode voltage of each phase on the bridge arm reference wave of the corresponding phase, obtain the circulating current of each phase of the current SiC MMC; S32, judging whether the absolute values ​​of the differences between the current circulating currents of each phase of the SiC MMC and the per-unit values ​​of the corresponding harmonic injection reference waves are all less than or equal to the preset threshold value, and if so, completing the fluctuation suppression operation of the SiC MMC sub-module capacitor voltage; otherwise, adjusting the parameters of the harmonic injection controller and going to S31.

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

7. The method for suppressing capacitor voltage fluctuation of SiC MMC submodule according to claim 6, characterized in that: The circulating current 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 when the processor executes the computer program, the method for suppressing capacitor voltage fluctuation of a SiC MMC submodule according to any one of claims 1 to 7 is executed.

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 sub-module capacitor voltage fluctuation suppression method according to any one of claims 1 to 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 executed by a processor, the device where the storage medium is located is controlled to execute the SiC MMC sub-module capacitor voltage fluctuation suppression method according to any one of claims 1 to 7.

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