A high-modulation-ratio hybrid MMC and a control method thereof

Abstract

This invention relates to a high modulation ratio hybrid MMC, employing a three-phase six-arm structure, wherein each phase includes an upper arm and a lower arm, which are connected via an intermediate submodule. The two ends of a DC bus are respectively connected to the upper and lower arms of the three-phase structure, and the voltage of the DC bus is V. dc The three intermediate sub-modules correspond to the output voltage and current, respectively, and the three-phase output voltage and current are connected in series to inductor L. ac The AC power supply is grounded in all three cases. To address the issue of high power loss in power electronic converters, this invention proposes a high modulation ratio hybrid MMC. Each bridge arm submodule is composed of a combination of one SiCMOSFET device and multiple SiIGBT devices. Utilizing the low switching loss characteristics of SiCMOSFETs, high-frequency components are concentrated on the SiCMOSFET submodules, thus reducing the power loss of the MMC.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter technology, specifically to a high modulation ratio hybrid MMC and its control method. Background Technology

[0002] Power electronic converters (MECs) are a crucial type of equipment widely used in power distribution networks, and their performance and economic indicators directly determine the quality of output power. Compared to other converters, modular multilevel converters (MMCs) are highly modular, have low output harmonics, and are easily expandable, leading to their widespread application in flexible DC transmission, variable frequency speed control, and wind farms. Currently, Si IGBTs are commonly used power semiconductor devices in medium and high voltage MMCs. These devices have low switching frequencies and low power densities, and the performance of MMCs is affected by these characteristics, thus impacting transmission efficiency and increasing power losses. Furthermore, improving the modulation ratio of MMCs is a critical issue. A higher modulation ratio improves DC bus utilization, while lower utilization leads to energy waste and reduced conversion efficiency. Existing technologies often inject high-order harmonic components to improve the modulation ratio, resulting in a large amount of high-order harmonics in the MMC's output voltage, necessitating further optimization of the output voltage. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a high modulation ratio hybrid MMC and control method to solve the problems of high power loss and low modulation ratio in the prior art of power electronic converters.

[0004] To solve the above technical problems, the technical solution of this invention is as follows: A high modulation ratio hybrid MMC is provided, the innovation of which lies in: adopting a three-phase six-arm structure, wherein each phase includes an upper arm and a lower arm, the upper arm and the lower arm are connected through an intermediate submodule, and the two ends of the DC bus are respectively connected to the upper arm and the lower arm of the three-phase structure, the voltage of the DC bus being V. dc The three intermediate sub-modules correspond to the output voltage and current, respectively, and the three-phase output voltage and current are connected in series to inductor L. ac There are three AC power supplies, all of which are grounded.

[0005] Furthermore, both the upper and lower bridge arms are formed by connecting N bridge arm sub-modules in series, and the N bridge arm sub-modules are SM1-SM2. N Each of the intermediate submodules is connected at both ends to the upper bridge arm submodule SM via a coupling inductor L. N And the lower bridge arm submodule SM1.

[0006] Furthermore, the number N of bridge arm sub-modules corresponding to the upper or lower bridge arm is determined by the input DC bus voltage and the withstand voltage rating of the switching devices used in the bridge arm sub-module.

[0007] Furthermore, each phase includes a silicon carbide metal-oxide-semiconductor field-effect transistor (SiC MOSFET) and a silicon insulated-gate bipolar transistor (Si IGBT), wherein each phase has an upper bridge arm submodule SM1 and a lower bridge arm submodule SM N SiC MOSFET devices are used, while the remaining bridge arm sub-modules and intermediate sub-modules all use Si IGBT devices.

[0008] Furthermore, each of the aforementioned bridge arm submodules and intermediate submodules is connected in parallel with a capacitor, and the bridge arm submodule SM1 of each upper bridge arm and the bridge arm submodule SM of each lower bridge arm are connected in parallel. N A full-bridge structure is adopted, while the remaining bridge arm sub-modules and intermediate sub-modules all adopt a half-bridge structure.

[0009] To address the aforementioned technical problems, this invention also provides a control method based on a high modulation ratio hybrid MMC, the innovation of which lies in: specifically divided into current inner-loop control and modulation strategy, and specifically including the following steps:

[0010] S1: Inner loop current control:

[0011] (1) Set the current reference value i vd * i vq * The voltage of the DC bus is V dc Collect the three-phase current value i on the AC side oa i ob i oc With the three-phase voltage source voltage value u sa u sb u sc ;

[0012] (2) The three-phase voltage source voltage value on the AC side obtained in step (1) is passed through a phase-locked loop device to obtain the phase θ required for the Parker transformation;

[0013] (3) Using the Park transformation, the sinusoidal AC quantity in the three-phase stationary coordinate system is transformed into the DC component in the two-axis synchronous rotating coordinate system d and q, that is, the AC three-phase current value i collected in step (1) is transformed into the DC component. oa i ob i oc Transformed into output variable i through Park transformation vd i vq The AC three-phase voltage source voltage value u collected in step (1) is used to... sa u sbu sc The variable is transformed into a perturbation variable u through the Park transform. sd u sq ;

[0014] (4) The output variable i obtained in step (3) vd i vq The current reference value i set in step (1) vd * i vq * After subtraction, the outputs are processed by PI controllers and the two outputs are introduced into the disturbance variable u obtained in step (3). sd u sq and voltage feedforward quantity ωLi vq ,ωLi vd To eliminate the coupling between the d and q axes, the reference value i of the control variable is obtained. diffd * i diffq * Finally, the required three-phase voltage reference value u is obtained through the d-q inverse converter. refa u refb u refc ;

[0015] (5) Set the upper bridge arm voltage reference value to u. refuj The lower bridge arm voltage reference value is u. refwj j = a,b,c, representing the three phases a, b, and c, according to the formula and The voltage reference values ​​of the three-phase upper and lower bridge arms can be obtained separately, and the voltage reference values ​​of the three-phase upper and lower bridge arms are used as modulation signals and input into the modulation module.

[0016] S2: Modulation strategy:

[0017] (1) Formulate modulation strategy: Each phase's upper and lower bridge arms are composed of N bridge arm sub-modules. Let the capacitor voltage of each bridge arm sub-module be V. c Then V c =V dc / N, setting the reference voltage for each bridge arm to V. ref The SUPWM modulation strategy only includes the upper bridge arm submodule SM1 and the lower bridge arm submodule SM. N Using SiC MOSFETs, the bridge arm submodules employing SiC MOSFETs no longer participate in the sorting and selection of capacitor voltages from other bridge arm submodules. Instead, they are fixed to use PWM modulation, with the triangular carrier voltage set to u. carrier According to the triangular carrier voltage u carrierIn comparison, the bridge arm submodule SM1 that generates the upper bridge arm or the bridge arm submodule SM that generates the lower bridge arm N The driving signal generates a voltage of u. PWM The remaining bridge arm submodules are sorted according to their capacitor voltages, and then sorted in ascending order using a sorting and selection algorithm. Let k1 and k2 represent the number of upper or lower bridge arm submodules in the non-SiC MOSFET bridge arm submodules. The values ​​of k1 and k2 are determined by the direction of the bridge arm current in the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule. The number n of bridge arm submodules in the upper or lower bridge arm of each phase is... arm The number of bridge arm sub-modules that do not use SiC MOSFETs, k1 or k2, and the upper bridge arm sub-module SM1 or the lower bridge arm sub-module SM can be selected. N The cutting states are added together to obtain the result, where k1 and k2 are obtained by rounding down using the floor and ceil functions, respectively.

[0018] The intermediate submodule can be deployed to either the upper or lower bridge arm, depending on the needs of the upper or lower bridge arm. If the upper bridge arm requires N bridge arm submodules and its bridge arm submodule SM1 is currently under negative pressure, then the intermediate submodule is deployed to the upper bridge arm; if the lower bridge arm requires N bridge arm submodules and its bridge arm submodule SM1 is currently under negative pressure, then the intermediate submodule is deployed to the upper bridge arm. N If a negative voltage is output, the intermediate submodule is engaged in the lower bridge arm. If neither the upper nor lower bridge arm requires the engagement of the intermediate submodule, the intermediate submodule determines its switching state according to the capacitor voltage balance principle.

[0019] (2) According to the modulation strategy, based on the relationship between the bridge arm voltage of the upper or lower bridge arm and the engaged bridge arm submodule, the following relationship can be obtained, where j = u and w, representing the upper and lower bridge arms respectively:

[0020] kV c <u refj <(k+1)V c

[0021] (3) According to the modulation strategy, the number of bridge arm submodules to be put into operation, k1 or k2, is determined using the floor or ceil function. The selection principle is determined by the bridge arm current of the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule. Based on the relationship between the voltage of the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule, the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM2 of the lower bridge arm can be obtained. N bridge arm voltage u PWM The calculation formula is as follows:

[0022]

[0023]

[0024]

[0025] (4) According to the modulation strategy, if the bridge arm current of the upper or lower bridge arm is greater than 0, and the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is... N If the capacitor voltage is less than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 generated by the floor function is selected. If the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is selected, then the number of bridge arm submodules k1 is selected. N If the capacitor voltage is greater than the reference voltage of each bridge arm, then select the number of bridge arm submodules k2 generated by the rounding function Ceil; if the bridge arm current of the upper or lower bridge arm is less than 0, and the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm... N If the capacitor voltage is greater than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 generated by the floor function is selected. If the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is greater than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 is selected. N If the capacitor voltage is less than the reference voltage of each bridge arm, then select the number of bridge arm submodules k2 generated by the floor function Ceil;

[0026] The upper arm submodule SM1 or the lower arm submodule SM1 is calculated based on step (3). N bridge arm voltage u PWM That is, the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm. N The modulation signal, within the same bridge arm, is processed by a delay module to obtain a new carrier signal from the triangular carrier. This carrier signal is compared with the modulation signal to generate the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm. N The driving signal;

[0027] (5) According to the modulation strategy, the activation status of the intermediate submodule is determined based on the needs of the upper and lower bridge arms. Let S represent the switching state of the intermediate submodule. If neither the upper nor lower bridge arms need to activate it, its switching state is determined by its own capacitor voltage balance principle. If the capacitor voltage of the intermediate submodule is greater than the average capacitor voltage of the bridge arm submodule and the output current is greater than 0, the intermediate submodule is in the "on" state, defined as S=1 mode. If the output current is less than 0, the intermediate submodule is in the "off" state, defined as S=0 mode. If the capacitor voltage of the intermediate submodule is less than the average capacitor voltage of the bridge arm submodule and the output current is less than 0, the intermediate submodule is in the "on" state. If the output current is greater than 0, the intermediate submodule is in the "off" state.

[0028] Compared with existing technologies, the advantages of this invention are as follows:

[0029] (1) In view of the problem of high power loss of power electronic converter, the present invention proposes a high modulation ratio hybrid MMC, in which each bridge arm sub-module is composed of a SiC MOSFET device and multiple Si IGBT devices. By utilizing the low switching loss characteristics of SiC MOSFET, the high frequency components are concentrated on the sub-module using SiC MOSFET, which can reduce the power loss of MMC.

[0030] (2) In view of the problem of low DC bus utilization rate of power electronic converter, this invention proposes a high modulation ratio hybrid MMC, which adds an intermediate sub-module that can be flexibly deployed between the upper and lower bridge arms, which is beneficial to improve the output voltage amplitude and improve the DC bus utilization rate.

[0031] (3) The high modulation ratio hybrid MMC proposed in this invention adds three intermediate sub-modules between the upper and lower bridge arms of the three phases, which improves the modulation ratio and reduces the number of three sub-modules for the three-phase MMC topology, thus saving costs. Attached Figure Description

[0032] Figure 1 This is a structural diagram of a high modulation ratio hybrid MMC proposed in this invention.

[0033] Figure 2 This is a block diagram of the current inner loop control of the high modulation ratio hybrid MMC in this invention.

[0034] Figure 3 This is the circuit diagram of the single-phase MMC and the schematic diagram of SUPWM modulation in this invention.

[0035] Figure 4 This is a diagram illustrating the working principle of the intermediate submodule and the capacitor voltage balance principle in this invention.

[0036] Figure 5 This example compares the power loss of the MMC when using a hybrid structure and an all-Si IGBT structure, respectively.

[0037] Figure 6 This is the AC three-phase voltage output waveform of the high modulation ratio hybrid MMC in the embodiment.

[0038] Figure 7 This is the AC side three-phase line voltage output waveform of the high modulation ratio hybrid MMC in the embodiment.

[0039] Figure 8 This is the AC three-phase current output waveform of the high modulation ratio hybrid MMC in the embodiment.

[0040] Figure 9 The waveforms are the capacitor voltage waveforms of each submodule of the upper bridge arm of phase A in the embodiment.

[0041] Figure 10 The waveforms of the capacitor voltages of each submodule of the lower bridge arm in phase A of the embodiment are shown.

[0042] Figure 11 This is an example that derives the relationship between the modulation ratio m and the number of submodules N in each bridge arm. Detailed Implementation

[0043] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0044] This invention provides a high modulation ratio hybrid MMC, the specific structure of which is as follows: Figure 1 As shown, a three-phase six-arm bridge structure is adopted, where each phase includes an upper arm and a lower arm. The upper and lower arms are connected through an intermediate submodule. The two ends of the DC bus are connected to the upper and lower arms of the three-phase structure, respectively. The voltage of the DC bus is V. dc The three intermediate submodules correspond to the output voltage and current, respectively, and the three-phase output voltage and current are connected to inductor L in sequence. ac There are three AC power supplies, all of which are grounded.

[0045] The upper and lower bridge arms of this invention are each formed by N bridge arm sub-modules connected in series, and the N bridge arm sub-modules are SM1-SM2. N Each intermediate submodule is connected to the upper bridge arm submodule SM via a coupling inductor L at both ends. N And the lower bridge arm submodule SM1.

[0046] The number N of bridge arm sub-modules corresponding to the upper or lower bridge arm of the present invention is determined by the input DC bus voltage and the withstand voltage rating of the switching devices used in the bridge arm sub-modules. N can be obtained by dividing the DC bus voltage by the withstand voltage value of the switching devices.

[0047] Each phase includes a silicon carbide metal-oxide-semiconductor field-effect transistor (SiC MOSFET) and a silicon insulated-gate bipolar transistor (Si IGBT), wherein each phase has an upper bridge arm submodule SM1 and a lower bridge arm submodule SM N SiC MOSFET devices are used, while the remaining bridge arm sub-modules and intermediate sub-modules all use Si IGBT devices.

[0048] Each bridge arm submodule and intermediate submodule is connected in parallel with a capacitor. The bridge arm submodule SM1 of the upper bridge arm and the bridge arm submodule SM of the lower bridge arm of each phase... N A full-bridge structure is adopted, while the remaining bridge arm sub-modules and intermediate sub-modules all adopt a half-bridge structure.

[0049] To address the aforementioned technical problems, this invention also provides a control method based on a high modulation ratio hybrid MMC, specifically comprising current inner-loop control and a modulation strategy, wherein the current inner-loop control is as follows: Figure 2 As shown, the modulation strategy is as follows Figure 3 As shown, the specific steps include:

[0050] S1: Inner loop current control:

[0051] (1) Set the current reference value i vd * i vq * The voltage of the DC bus is V dc Collect the three-phase current value i on the AC side oa i ob i oc With the three-phase voltage source voltage value u sa u sb u sc ;

[0052] (2) The three-phase voltage source voltage value on the AC side obtained in step (1) is passed through a phase-locked loop device to obtain the phase θ required for the Parker transformation;

[0053] (3) Using the Park transformation, the sinusoidal AC quantity in the three-phase stationary coordinate system is transformed into the DC component in the two-axis synchronous rotating coordinate system d and q, that is, the AC three-phase current value i collected in step (1) is transformed into the DC component. oa i ob i oc Transformed into output variable i through Park transformation vd i vq The AC three-phase voltage source voltage value u collected in step (1) is used to... sa u sb u sc The variable is transformed into a perturbation variable u through the Park transform. sd u sq ;

[0054] (4) The output variable i obtained in step (3) vd i vq The current reference value i set in step (1) vd * i vq * After subtraction, the outputs are processed by PI controllers and the two outputs are introduced into the disturbance variable u obtained in step (3). sd u sq and voltage feedforward quantity ωLi vq ,ωLi vd To eliminate the coupling between the d and q axes, the reference value i of the control variable is obtained. diffd* i diffq * Finally, the required three-phase voltage reference value u is obtained through the d-q inverse converter. refa u refb u refc ;

[0055] (5) Set the upper bridge arm voltage reference value to u. refuj The lower bridge arm voltage reference value is u. refwj j = a,b,c, representing the three phases a, b, and c, according to the formula and The voltage reference values ​​of the three-phase upper and lower bridge arms can be obtained separately, and the voltage reference values ​​of the three-phase upper and lower bridge arms are used as modulation signals and input into the modulation module.

[0056] S2: Modulation strategy:

[0057] (1) Formulate modulation strategy: Each phase's upper and lower bridge arms are composed of N bridge arm sub-modules. Let the capacitor voltage of each bridge arm sub-module be V. c Then V c =V dc / N, setting the reference voltage for each bridge arm to V. ref The modulation strategy of this invention is SUPWM, and its modulation principle is as follows: Figure 3 As shown, Figure 3 (a) is a single-phase MMC circuit diagram. Figure 3 (b) is the output voltage waveform of the upper bridge arm. Figure 3 (c) is the output voltage waveform of the lower bridge arm. Figure 3 (d) are the enable signals for the upper and lower bridge arms. The SUPWM modulation strategy only includes the upper bridge arm submodule SM1 and the lower bridge arm submodule SM. N Using SiC MOSFETs, the bridge arm submodules employing SiC MOSFETs no longer participate in the sorting and selection of capacitor voltages from other bridge arm submodules. Instead, they are fixed to use PWM modulation, with the triangular carrier voltage set to u. carrier According to the triangular carrier voltage u carrier In comparison, the bridge arm submodule SM1 that generates the upper bridge arm or the bridge arm submodule SM that generates the lower bridge arm N The driving signal generates a voltage of u. PWM The remaining bridge arm submodules are sorted according to their capacitor voltages, and then sorted in ascending order using a sorting and selection algorithm. Let k1 and k2 represent the number of upper or lower bridge arm submodules in the non-SiC MOSFET bridge arm submodules. The values ​​of k1 and k2 are determined by the direction of the bridge arm current in the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule. The number n of bridge arm submodules in the upper or lower bridge arm of each phase is...arm The number of bridge arm sub-modules that do not use SiC MOSFETs, k1 or k2, and the upper bridge arm sub-module SM1 or the lower bridge arm sub-module SM can be selected. N The cutting states are added together to obtain the result, where k1 and k2 are obtained by rounding down using the floor and ceil functions, respectively.

[0058] (2) The intermediate submodule can be deployed in either the upper or lower bridge arm. Deployment depends on the needs of the upper and lower bridge arms. The operating principle and capacitor voltage balance principle of the intermediate submodule are as follows: Figure 4 As shown, Figure 4 (a) is the working principle. Figure 4 (b) is based on the capacitor voltage balance principle, according to the required number of bridge arm sub-modules k1 or k2, the number of bridge arm sub-modules N, and the voltage u of the upper bridge arm sub-module SM1 or the lower bridge arm sub-module SMN. PWM The enable signals E for the upper and lower bridge arms can be obtained. nP E nN ,according to Figure 4 Based on the working principle and capacitor voltage balancing principle, the intermediate submodule can be flexibly connected to the upper and lower bridge arms. If the upper bridge arm needs to be connected with N bridge arm submodules, and the bridge arm submodule SM1 of the upper bridge arm is at a negative voltage, then the intermediate submodule is connected to the upper bridge arm; if the lower bridge arm needs to be connected with N bridge arm submodules, and the bridge arm submodule SM1 of the lower bridge arm is at a negative voltage, then the intermediate submodule is connected to the upper bridge arm. N If a negative voltage is output, the intermediate submodule is engaged in the lower bridge arm. If neither the upper nor lower bridge arm requires the engagement of the intermediate submodule, the intermediate submodule determines its switching state according to the capacitor voltage balance principle.

[0059] (2) According to the modulation strategy, based on the relationship between the bridge arm voltage of the upper or lower bridge arm and the engaged bridge arm submodule, the following relationship can be obtained, where j = u and w, representing the upper and lower bridge arms respectively:

[0060] kV c <u refj <(k+1)V c

[0061] (3) According to the modulation strategy, the number of bridge arm submodules to be put into operation, k1 or k2, is determined using the floor or ceil function. The selection principle is determined by the bridge arm current of the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule. Based on the relationship between the voltage of the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule, the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM2 of the lower bridge arm can be obtained. N bridge arm voltage u PWM The calculation formula is as follows:

[0062]

[0063]

[0064]

[0065] (4) According to the modulation strategy, if the bridge arm current of the upper or lower bridge arm is greater than 0, and the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is... N If the capacitor voltage is less than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 generated by the floor function is selected. If the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is selected, then the number of bridge arm submodules k1 is selected. N If the capacitor voltage is greater than the reference voltage of each bridge arm, then select the number of bridge arm submodules k2 generated by the rounding function Ceil; if the bridge arm current of the upper or lower bridge arm is less than 0, and the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm... N If the capacitor voltage is greater than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 generated by the floor function is selected. If the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is greater than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 is selected. N If the capacitor voltage is less than the reference voltage of each bridge arm, then select the number of bridge arm submodules k2 generated by the floor function Ceil;

[0066] The upper arm submodule SM1 or the lower arm submodule SM1 is calculated based on step (3). N bridge arm voltage u PWM That is, the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm. N The modulation signal, within the same bridge arm, is processed by a delay module to obtain a new carrier signal from the triangular carrier. This carrier signal is compared with the modulation signal to generate the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm. N The driving signal;

[0067] (5) According to the modulation strategy, the activation status of the intermediate submodule is determined based on the needs of the upper and lower bridge arms. Let S represent the switching state of the intermediate submodule. If neither the upper nor lower bridge arms require its activation, its switching state is determined by its own capacitor voltage balance principle. If the capacitor voltage u of the intermediate submodule is... cm Greater than the average capacitor voltage u of the bridge arm submodule cp,avg or u cn,avg And the output current i oj If the value is greater than 0, the intermediate submodule is in the "on" state, defined as S=1 mode; if the output current i oj If the value is less than 0, the intermediate submodule is in the "off" state, defined as S=0 mode. If the capacitor voltage u of the intermediate submodule is less than 0 at this time... cm Less than the average capacitor voltage u of the bridge arm submodulecp,avg or u cn,avg And the output current i oj If the value is less than 0, the intermediate submodule is in the "on" state. If the output current i oj If the value is greater than 0, the intermediate submodule is in the "off" state.

[0068] To illustrate the adjustable modulation ratio proposed in this invention, the relationship between the modulation index m of the MMC and the N bridge arm submodules of each bridge arm is explained under different hybrid schemes. In a three-phase six-bridge arm MMC, the amplitudes of the DC bus voltage and the AC side phase voltage must satisfy the following:

[0069]

[0070] Among them, V dc V is the DC bus voltage. oj Let m be the phase voltage amplitude of phase j on the AC side, and m be the modulation ratio (0 ≤ m ≤ 0). <m<1)。

[0071] In this invention, assuming all bridge arm submodules are equal, the maximum output AC voltage amplitude is V'. o And since the voltage drop across the bridge arm inductor is negligible, the capacitor voltage of each bridge arm submodule and the bridge arm voltages of the upper and lower bridge arms can be obtained as follows:

[0072]

[0073]

[0074]

[0075] In this context, the subscript uj represents the upper arm of phase j, the subscript wj represents the lower arm of phase j, the subscript on_uj represents the number of submodules deployed in the upper arm of phase j, and the subscript on_wj represents the number of submodules deployed in the upper arm of phase j.

[0076] In this invention, the maximum value of the AC voltage amplitude can be obtained according to the above formula:

[0077]

[0078]

[0079] At any given moment in this invention, the output voltage amplitude should satisfy the following relationship:

[0080] V o ≤V' o

[0081] Based on the aforementioned output voltage amplitude relationship, we can obtain:

[0082]

[0083] After simplification, we can obtain

[0084]

[0085] To further describe the high modulation ratio hybrid MMC and its control method, the present invention is described below with reference to specific embodiments:

[0086] This invention constructs a three-phase six-arm MMC. The DC side voltage of the simulation model is 7.5kV and the frequency is 50Hz.

[0087] Simulations of different MMC schemes were performed to verify the study that the high modulation ratio hybrid MMC in this embodiment can reduce power loss. This embodiment uses a CAS300M17BM2 SiC MOSFET device and a 5SNG0300Q170300 Si IGBT device. Figure 5 The figure shows a comparative analysis of conduction and switching losses generated by power semiconductor devices in MMCs when using high modulation ratio hybrid MMCs, traditional hybrid MMCs, and all-Si IGBT structures at the same power level. Figure 5 It can be seen that the conduction loss of the high modulation ratio hybrid structure is not much different from that of the all-Si IGBT structure, but the switching loss of the two is quite different. The power loss of the high modulation ratio hybrid MMC is reduced by 59%, and compared with the traditional hybrid structure, the power loss of the high modulation ratio hybrid MMC is reduced by 27.8%. Therefore, the power loss of the power semiconductor device in the high modulation ratio hybrid structure is much smaller than that in the all-Si IGBT structure.

[0088] SiC MOSFETs have low switching losses, which can effectively reduce power loss and improve transmission efficiency when used in converters. However, considering the high manufacturing cost of SiC MOSFETs, this invention fully utilizes their characteristics by using only the upper bridge arm submodule SM1 and the lower bridge arm submodule SM2. N SiC MOSFETs are used, while the remaining bridge arm sub-modules all use SiIGBTs. Figure 6-10 The waveforms of each port of the high modulation ratio hybrid MMC are given to verify the effectiveness of the proposed MMC.

[0089] The relationship between the modulation index m of MMC and the N submodules of each bridge arm under different hybrid schemes is as follows: Figure 11 As shown. Compared to traditional hybrid MMCs, the high modulation ratio hybrid MMC presented in this paper can maintain a high modulation index m regardless of the number of bridge arm submodules. Therefore, a larger output voltage amplitude can be obtained, and the utilization rate of the DC bus can be greatly improved.

[0090] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A control method based on a high modulation ratio hybrid MMC, wherein the high modulation ratio hybrid MMC adopts a three-phase six-arm structure, each phase including an upper arm and a lower arm, the upper arm and the lower arm being connected through an intermediate submodule, and the two ends of a DC bus being connected to the upper arm and the lower arm of the three-phase structure respectively, and the voltage of the DC bus being V. dc The three intermediate submodules correspond to the output voltage and current, respectively, and the three-phase output voltage and current are connected to inductor L in sequence. ac AC power supply, all three AC power supplies are grounded; Both the upper and lower bridge arms are formed by connecting N bridge arm sub-modules in series, and the N bridge arm sub-modules are SM1-SM2. N Each of the intermediate submodules is connected at both ends to the upper bridge arm submodule SM via a coupling inductor L. N and the lower bridge arm submodule SM1; The number N of bridge arm sub-modules corresponding to the upper or lower bridge arm is determined by the input DC bus voltage and the withstand voltage rating of the switching devices used in the bridge arm sub-module. Each phase includes a silicon carbide metal-oxide-semiconductor field-effect transistor (SiC MOSFET) and a silicon insulated-gate bipolar transistor (Si IGBT), wherein each phase has an upper bridge arm submodule SM1 and a lower bridge arm submodule SM N SiCMOSFET devices are used, while the remaining bridge arm sub-modules and intermediate sub-modules all use Si IGBT devices; Each of the bridge arm submodules and intermediate submodules is connected in parallel with a capacitor, and the bridge arm submodule SM1 of the upper bridge arm and the bridge arm submodule SM of the lower bridge arm of each phase are connected in parallel. N The bridge adopts a full-bridge structure, while the remaining bridge arm sub-modules and intermediate sub-modules all adopt a half-bridge structure. Its characteristics are: the control process is divided into current inner loop control and modulation strategy, specifically including the following steps: S1: Inner loop current control: (1) Set the current reference value i vd * i vq * The voltage of the DC bus is V dc Collect the three-phase current value i on the AC side oa i ob i oc With the three-phase voltage source voltage value u sa u sb u sc ; (2) The three-phase voltage source voltage value obtained in step (1) is passed through a phase-locked loop device to obtain the phase θ required for the Parker transformation; (3) Using the Park transformation, the sinusoidal AC quantity in the three-phase stationary coordinate system is transformed into the DC component in the two-axis synchronous rotating coordinate system d and q, that is, the AC three-phase current value i collected in step (1) is transformed into the DC component. oa i ob i oc Transformed into output variable i through Park transformation vd i vq The AC three-phase voltage source voltage value u collected in step (1) is used to... sa u sb u sc The variable is transformed into a perturbation variable u through the Park transform. sd u sq ; (4) The output variable i obtained in step (3) vd i vq The current reference value i set in step (1) vd * i vq * After subtraction, the outputs are processed by PI controllers and the two outputs are introduced into the disturbance variable u obtained in step (3). sd u sq and voltage feedforward quantity ωLi vq ,ωLi vd To eliminate the coupling between the d and q axes, the reference value i of the control variable is obtained. diffd * i diffq * Finally, the required three-phase voltage reference value u is obtained through the d-q inverse converter. refa u refb u refc ; (5) Set the upper bridge arm voltage reference value to u refuj The lower bridge arm voltage reference value is u. refwj j=a,b,c, representing three-phase a,b,c, according to the formula and The voltage reference values ​​of the three-phase upper and lower bridge arms are obtained respectively, and the voltage reference values ​​of the three-phase upper and lower bridge arms are used as modulation signals and entered into the modulation module. S2: Modulation strategy: (1) Formulate modulation strategy: Each phase's upper and lower bridge arms are composed of N bridge arm sub-modules. Let the capacitor voltage of each bridge arm sub-module be V. c Then V c =V dc / N, setting the reference voltage for each bridge arm to V. ref The SUPWM modulation strategy only has the upper bridge arm submodule SM1 and the lower bridge arm submodule SM. N Using SiC MOSFETs, the bridge arm submodules employing SiC MOSFETs no longer participate in the sorting and selection of capacitor voltages from other bridge arm submodules. Instead, they are fixed to use PWM modulation, with the triangular carrier voltage set to u. carrier According to the triangular carrier voltage u carrier In comparison, the bridge arm submodule SM1 that generates the upper bridge arm or the bridge arm submodule SM that generates the lower bridge arm N The driving signal generates a voltage of u. PWM For bridge arm submodules that do not use SiC MOSFETs, they are sorted according to their capacitor voltages and then sorted in ascending order using a sorting and selection algorithm. Let k1 and k2 represent the number of upper or lower bridge arm submodules in the non-SiC MOSFET submodules, where the values ​​of k1 and k2 are determined by the direction of the bridge arm current in the upper or lower bridge arm and the capacitor voltage of the bridge arm submodule. The number n of bridge arm submodules in the upper or lower bridge arm of each phase is... arm The number of bridge arm sub-modules that do not use SiC MOSFETs, k1 or k2, and the upper bridge arm sub-module SM1 or the lower bridge arm sub-module SM N The cutting states are summed to obtain k1 and k2, which are obtained by rounding by the floor function Floor and Ceil, respectively. The intermediate submodule can be deployed to either the upper or lower bridge arm, depending on the needs of the upper or lower bridge arm. If the upper bridge arm requires N bridge arm submodules and its bridge arm submodule SM1 is currently under negative pressure, then the intermediate submodule is deployed to the upper bridge arm; if the lower bridge arm requires N bridge arm submodules and its bridge arm submodule SM1 is currently under negative pressure, then the intermediate submodule is deployed to the upper bridge arm. N If a negative voltage is output, the intermediate submodule is engaged in the lower bridge arm. If neither the upper nor lower bridge arm requires the engagement of the intermediate submodule, the intermediate submodule determines its switching state according to the capacitor voltage balance principle. (2) According to the modulation strategy, based on the relationship between the bridge arm voltage of the upper or lower bridge arm and the input bridge arm submodule, the following formula is obtained. , where j=u and w represent the upper arm and lower arm respectively; (3) According to the modulation strategy, the number of bridge arm sub-modules to be put into operation, k1 or k2, is determined by the floor or ceil function. The selection principle is determined by the bridge arm current of the upper or lower bridge arm and the capacitor voltage of the bridge arm sub-module. Based on the relationship between the voltage of the upper or lower bridge arm and the capacitor voltage of the bridge arm sub-module, the bridge arm sub-module SM1 of the upper bridge arm or the bridge arm sub-module SM2 of the lower bridge arm can be obtained. N bridge arm voltage u PWM The calculation formula is as follows: (4) According to the modulation strategy, if the bridge arm current of the upper or lower bridge arm is greater than 0, and the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is SM N If the capacitor voltage is less than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 generated by the floor function is selected. If the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is selected, then the number of bridge arm submodules k1 is selected. N If the capacitor voltage is greater than the reference voltage of each bridge arm, then select the number of bridge arm submodules k2 generated by the rounding function Ceil; if the bridge arm current of the upper or lower bridge arm is less than 0, and the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm... N If the capacitor voltage is greater than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 generated by the floor function is selected. If the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm is greater than the reference voltage of each bridge arm, then the number of bridge arm submodules k1 is selected. N If the capacitor voltage is less than the reference voltage of each bridge arm, then select the number of bridge arm submodules k2 generated by the floor function Ceil; The upper arm submodule SM1 or the lower arm submodule SM1 is calculated based on step (3). N bridge arm voltage u PWM That is, the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm. N The modulation signal, within the same bridge arm, is processed by a delay module to obtain a new carrier signal from the triangular carrier. This carrier signal is compared with the modulation signal to generate the bridge arm submodule SM1 of the upper bridge arm or the bridge arm submodule SM of the lower bridge arm. N The driving signal; (5) According to the modulation strategy, the activation status of the intermediate submodule is determined based on the needs of the upper and lower bridge arms. Let S represent the switching state of the intermediate submodule. If neither the upper nor lower bridge arms need to activate it, its switching state is determined by its own capacitor voltage balance principle. If the capacitor voltage of the intermediate submodule is greater than the average capacitor voltage of the bridge arm submodule and the output current is greater than 0, the intermediate submodule is in the "on" state, defined as S=1 mode. If the output current is less than 0, the intermediate submodule is in the "off" state, defined as S=0 mode. If the capacitor voltage of the intermediate submodule is less than the average capacitor voltage of the bridge arm submodule and the output current is less than 0, the intermediate submodule is in the "on" state. If the output current is greater than 0, the intermediate submodule is in the "off" state.

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