A modular multi-level converter AC side impedance reshaping method and system

Abstract

The present invention discloses a method and system for reshaping the impedance on the AC side of a modular multilevel converter, comprising obtaining the DC side voltage of the modular multilevel converter, the common mode voltage reference value of any phase during circulating current control, and the differential mode voltage reference value of the current phase during output voltage control; using a compensation algorithm to calculate the compensation item of the differential mode voltage reference; based on the common mode voltage reference value, the differential mode voltage reference value, the compensation item of the differential mode voltage reference, and the DC side voltage, respectively calculating the modulation coefficient of the compensated upper bridge arm and the modulation coefficient of the compensated lower bridge arm; based on the modulation coefficient of the compensated upper bridge arm and the modulation coefficient of the compensated lower bridge arm, controlling the number of sub-modules put into the upper bridge arm and the number of sub-modules put into the lower bridge arm, so as to realize the impedance reshaping of the AC side port of the modular multilevel converter; the method is easy to realize, has a simple structure, can be calculated online, improves system stability, is easy to apply in engineering, and has good control effects under various working conditions.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a method and system for reshaping the alternating current side impedance of a modular multi-level converter. Background Art

[0002] The modular multilevel converter was proposed by German professor R. Marquardt in 2001. It is a type of converter with excellent performance among cascade converters. Compared with traditional low-level converters, it has many advantages such as modularity, good output waveform quality, and few harmonics. It has attracted widespread attention from researchers at home and abroad and has become the most promising multilevel converter topology in the field of high-voltage direct current transmission applications.

[0003] However, due to the cyclic coupling of steady-state capacitor voltage fluctuations with bridge arm voltage and bridge arm current, the internal dynamic characteristics of the modular multilevel converter are complex, and will further affect the characteristics of the modular multilevel converter AC side port, thereby bringing about stability problems in the operation of the modular multilevel converter system; and when performing harmonic interaction and stability analysis on the modular multilevel converter system, very complex methods are often used to model the internal dynamics, such as the harmonic state space modeling method, which makes stability analysis difficult and cannot use a simple average model to provide design guidance for the controller parameters like a two-level system, and is difficult to apply in actual engineering; therefore, the reshaping of the AC side impedance characteristics of the modular multilevel converter is an urgent problem to be solved, which can not only improve the stability of the modular multilevel converter system, but also further simplify the design process of the controller parameters.

[0004] Existing methods for reshaping the AC side impedance of modular multilevel converters mainly include: a method for reshaping the AC side impedance of modular multilevel converters using a proportional-integral resonant controller and a method for reshaping the AC side impedance of modular multilevel converters using energy damping control. Although the above methods can improve the stability of the modular multilevel converter system, they all have the problem that the design of controller parameters in the system is relatively difficult. Moreover, in the method for reshaping the AC side impedance of the modular multilevel converter using energy damping control, a new energy damping control is added to the control system, resulting in a more complicated control structure.

[0005] Based on the above background, there is an urgent need for an AC side impedance reshaping method for a modular multilevel converter that is easy to implement, has a simple structure, can be calculated online, improves system stability, is easy to apply in engineering, and can perform impedance reshaping under various working conditions. Summary of the invention

[0006] In view of the above problems, an object of the present invention is to provide a method for reshaping the impedance on the AC side of a modular multilevel converter. The method reshapes the impedance on the AC side of each phase of the modular multilevel converter based on capacitor voltage feedforward. The method is simple to implement, so that the impedance characteristics of the AC side port of the modular multilevel converter are the same as the impedance characteristics of the traditional two-level voltage source converter port, thereby simplifying the low-frequency stability analysis process of the modular multilevel converter interconnection system and a simple average model can be used to provide design guidance for the parameters of the controller; the method is easy to implement, simple in structure, can be calculated online, improves system stability, is easy to apply in engineering, and can perform impedance reshaping under various working conditions and has a good control effect.

[0007] A second object of the present invention is to provide a modular multi-level converter AC side impedance reshaping system.

[0008] The first technical solution adopted by the present invention is: a method for reshaping the AC side impedance of a modular multi-level converter, comprising the following steps:

[0009] S100: Obtaining a DC side voltage of a modular multilevel converter, a common mode voltage reference value of any phase during circulating current control, and a differential mode voltage reference value of a current phase during output voltage control;

[0010] S200: Calculate a compensation item of a differential mode voltage reference using a compensation algorithm;

[0011] S300: Calculate the modulation coefficient of the upper bridge arm after compensation and the modulation coefficient of the lower bridge arm after compensation respectively based on the common mode voltage reference value, the differential mode voltage reference value, the compensation term of the differential mode voltage reference and the DC side voltage;

[0012] S400: Based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, the number of submodules put into the upper bridge arm and the number of submodules put into the lower bridge arm are controlled to achieve impedance reshaping of the AC side port of the modular multilevel converter.

[0013] Preferably, the common mode voltage reference value in step S100 is obtained by the following formula:

[0014]

[0015] In the formula, v cm * is the common mode voltage reference value; V dc is the DC side voltage; K Pic , K Ric are the proportional coefficient and resonance coefficient of the proportional resonant regulator respectively; ω0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; i cir *is the circulation reference value; i cir is the actual value of circulation.

[0016] Preferably, the differential mode voltage reference value is obtained by the following formula:

[0017]

[0018] In the formula, v s * is the differential mode voltage reference value; v a is the actual value of the AC side output voltage; K Pis , K Ris are the proportional coefficient and resonance coefficient of the PR regulator respectively; ω0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; i s * is the reference value of the output current on the AC side; i s It is the actual value of the output current on the AC side.

[0019] Preferably, the AC side output current reference value is obtained by the following formula:

[0020]

[0021] In the formula, i s * K is the reference value of the AC side output current; Pvd , K Rvd are the proportional coefficient and resonance coefficient of the PR regulator respectively; ω0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; v a * is the reference value of the AC side output voltage; v a is the actual value of the AC side output voltage.

[0022] Preferably, the compensation term of the differential mode voltage reference in step S200 is obtained by the following formula:

[0023]

[0024] In the formula, △v s is the compensation term of the differential mode voltage reference; V dc is the DC side voltage; v cm * is the common mode voltage reference value; v s * is the differential mode voltage reference value; v cu Σ is the total capacitance voltage of the upper bridge arm; v cl Σ is the total capacitor voltage of the lower bridge arm.

[0025] Preferably, the modulation coefficient after upper bridge arm compensation in step S300 is obtained by the following formula:

[0026]

[0027] Where n u c is the modulation coefficient after upper bridge arm compensation; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term for the differential mode voltage reference.

[0028] Preferably, the modulation coefficient after the lower bridge arm compensation in step S300 is obtained by the following formula:

[0029]

[0030] Where n l c is the modulation coefficient after compensation of the lower bridge arm; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term for the differential mode voltage reference.

[0031] Preferably, the common mode modulation coefficient is obtained by the following formula:

[0032]

[0033] Where n cm is the common mode modulation coefficient; v cm * is the common mode voltage reference value; V dc is the DC side voltage;

[0034] The compensated differential mode modulation coefficient is obtained by the following formula:

[0035]

[0036] In the formula, is the differential mode modulation coefficient after compensation; V dc is the DC side voltage; vs * is the differential mode voltage reference value; △v s is the compensation term for the differential mode voltage reference.

[0037] Preferably, the S400 is specifically:

[0038] Based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, the nearest level approximation modulation method is used to obtain the number of sub-modules that need to be put into use on the bridge arm at each moment; a sorting algorithm is used to generate a trigger pulse to put the modular multilevel converter sub-module into and out, so as to achieve impedance reshaping of the AC side port of the modular multilevel converter.

[0039] The second technical solution adopted by the present invention is: a modular multi-level converter AC side impedance reshaping system, comprising an acquisition module, a first calculation module, a second calculation module and a control module;

[0040] The acquisition module is used to acquire the DC side voltage of the modular multilevel converter, the common mode voltage reference value of any phase during circulating current control, and the differential mode voltage reference value of the current phase during output voltage control;

[0041] The first calculation module is used to calculate the compensation term of the differential mode voltage reference using a compensation algorithm;

[0042] The second calculation module is used to calculate the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation respectively based on the common mode voltage reference value, the differential mode voltage reference value, the compensation term of the differential mode voltage reference and the DC side voltage;

[0043] The control module is used to control the number of sub-modules put into the upper bridge arm and the number of sub-modules put into the lower bridge arm based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, so as to achieve impedance reshaping of the AC side port of the modular multilevel converter.

[0044] The above technical solution has the following beneficial effects:

[0045] (1) The AC side impedance reshaping method of a modular multilevel converter disclosed in the present invention can take each phase of a three-phase modular multilevel converter as a control object. First, the compensation term of the differential mode voltage reference required to achieve the AC side impedance reshaping is derived, and the differential mode modulation coefficient obtained by the compensation term is superimposed on the traditional differential mode modulation coefficient to form a new differential mode modulation coefficient. Together with the common mode modulation coefficient, the number of sub-modules put into the upper and lower bridge arms is controlled to achieve the impedance reshaping of the AC side port of the modular multilevel converter, so that the impedance characteristics of the AC side port of the modular multilevel converter are the same as the impedance characteristics of the traditional two-level voltage source converter port, simplifying the low-frequency stability analysis process of the modular multilevel converter interconnection system and providing design guidance for controller parameters through a simple average model.

[0046] (2) Compared with the AC side impedance reshaping method in the prior art, the modular multi-level converter AC side impedance reshaping method disclosed in the present invention is easy to implement, has a simple structure, can be calculated online, improves system stability, is easy to apply in engineering practice, and can perform impedance reshaping under various working conditions and has a good control effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 A flow chart of a method for reshaping the AC side impedance of a modular multi-level converter provided by one embodiment of the present invention;

[0048] Figure 2 A schematic block diagram of an AC side impedance reshaping method for a modular multi-level converter provided in an embodiment of the present invention;

[0049] Figure 3 A schematic diagram of the principle of a method for reshaping the AC side impedance of a modular multi-level converter provided by an embodiment of the present invention;

[0050] Figure 4 A structural diagram of a single-phase modular multilevel converter, an internal structural diagram of a half-bridge submodule, an average model of the modular multilevel converter, and a differential common mode equivalent circuit diagram provided by an embodiment of the present invention;

[0051] Figure 5 A Bode diagram of the output impedance of the AC side port of the modular multilevel converter during AC voltage open-loop control before applying the AC side impedance reshaping method provided in an embodiment of the present invention;

[0052] Figure 6 A Bode diagram of the output impedance of the AC side port of the modular multilevel converter during AC voltage closed-loop control before applying the AC side impedance reshaping method provided in an embodiment of the present invention;

[0053] Figure 7A Bode diagram of the output impedance of the AC side port of the modular multilevel converter during AC voltage open-loop control after applying the AC side impedance reshaping method provided in an embodiment of the present invention;

[0054] Figure 8 A Bode diagram of the output impedance of the AC side port of the modular multilevel converter during AC voltage closed-loop control after applying the AC side impedance reshaping method provided in an embodiment of the present invention;

[0055] Fig. 9 A schematic structural diagram of an AC side impedance reshaping system of a modular multilevel converter provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0056] The following detailed description of the embodiments of the present invention is further described in detail in conjunction with the accompanying drawings and examples. The detailed description of the following embodiments and the accompanying drawings are used to exemplarily illustrate the principles of the present invention, but cannot be used to limit the scope of the present invention, that is, the present invention is not limited to the preferred embodiments described, and the scope of the present invention is defined by the claims.

[0057] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0058] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0059] Embodiment 1

[0060] like Figures 1 to 3 As shown, the present invention provides a method for reshaping the AC side impedance of a modular multilevel converter, comprising the following steps:

[0061] S100: Calculate and obtain a common-mode voltage reference value of any phase of the modular multilevel converter during circulating current control and a differential-mode voltage reference value of the current phase during output voltage control;

[0062] (1) The AC side impedance reshaping method of the modular multilevel converter in this embodiment takes each phase of the modular multilevel converter as a control object, calculates the common mode voltage reference value of any phase of the modular multilevel converter during circulating current control, and specifically includes the following sub-steps:

[0063] S111: Input circulating current reference value i cir* , get the actual value of circulation i cir , get the DC side voltage V dc ;

[0064] S112: Using a proportional resonant regulator (PR regulator) to adjust the circulating current reference value i cir * The actual value of the circulation cir The error is adjusted to obtain the adjusted error result;

[0065] S113: Add the adjusted error result to the feedforward DC side voltage component V dc / 2, obtain the common mode voltage reference value v cm * ; The specific formula is as follows:

[0066]

[0067] In the formula, v cm * is the common mode voltage reference value; V dc is the DC side voltage; K Pic , K Ric are the proportional coefficient and resonance coefficient of the PR regulator respectively; ω0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; i cir * is the circulation reference value; i cir is the actual value of circulation.

[0068] (2) The differential mode voltage reference value is obtained through the output voltage control loop, which includes an output voltage control outer loop and an output current control inner loop. Based on the AC side output current reference value, the AC side output current actual value and the AC side output voltage actual value, the differential mode voltage reference value v of the current phase of the modular multilevel converter during output voltage control is calculated. s * , specifically including the following sub-steps:

[0069] S121: Input an AC side output voltage reference value to obtain an AC side output voltage actual value and an AC side output current actual value;

[0070] S122: Based on the AC side output voltage reference value and the AC side output voltage actual value, the AC side output current reference value is obtained using the PR regulator, and the formula is as follows:

[0071]

[0072] In the formula, i s * K is the reference value of the AC side output current; Pvd , KRvd are the proportional coefficient and resonance coefficient of the PR regulator respectively; ω0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; v a * is the reference value of the AC side output voltage; v a is the actual value of the AC side output voltage.

[0073] S123: Based on the AC side output current reference value, the AC side output current actual value and the AC side output voltage actual value, the PR regulator is used to output the differential mode voltage reference value, and the formula is as follows:

[0074]

[0075] In the formula, v s * is the differential mode voltage reference value; v a is the actual value of the AC side output voltage; K Pis , K Ris are the proportional coefficient and resonance coefficient of the PR regulator respectively; ω0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; i s * is the reference value of the output current on the AC side; i s It is the actual value of the output current on the AC side.

[0076] S200: Calculate a compensation item of a differential mode voltage reference using a compensation algorithm;

[0077] The compensation term of the differential mode voltage reference is calculated using the compensation algorithm, which specifically includes the following sub-steps:

[0078] S210: Add all capacitor voltages of the upper bridge arm to obtain a total capacitor voltage of the upper bridge arm, and add all capacitor voltages of the lower bridge arm to obtain a total capacitor voltage of the lower bridge arm;

[0079] S220: Based on the total capacitor voltage of the upper bridge arm, the total capacitor voltage of the lower bridge arm, the common mode voltage reference value, the differential mode voltage reference value and the DC side voltage, a compensation item of the differential mode voltage reference is calculated. The specific formula is as follows:

[0080]

[0081] In the formula, △v s is the compensation term of the differential mode voltage reference; V dc is the DC side voltage; v cm * is the common mode voltage reference value; v s * is the differential mode voltage reference value; v cu Σ is the total capacitance voltage of the upper bridge arm; v clΣ is the total capacitor voltage of the lower bridge arm.

[0082] S300: Based on the common mode voltage reference value v cm * , differential mode voltage reference value v s * , compensation term △v of differential mode voltage reference s and DC side voltage V dc Calculate the modulation coefficient n after upper bridge arm compensation respectively u c And the modulation coefficient n after the lower bridge arm compensation l c ;

[0083] Modulation coefficient n after upper arm compensation u c Obtained by the following formula:

[0084]

[0085] Where n u c is the modulation coefficient after upper bridge arm compensation; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term for the differential mode voltage reference.

[0086] Modulation coefficient n after lower arm compensation l c Obtained by the following formula:

[0087]

[0088] Where n l c is the modulation coefficient after compensation of the lower bridge arm; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term for the differential mode voltage reference.

[0089] Among them, the common mode modulation coefficient n cmObtained by the following formula:

[0090]

[0091] Compensated differential mode modulation coefficient Obtained by the following formula:

[0092]

[0093] Where V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term of the differential mode voltage reference; v cm * is the common mode voltage reference value.

[0094] S400: Based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, the number of sub-modules put into the upper bridge arm and the number of sub-modules put into the lower bridge arm are controlled to achieve impedance reshaping of the AC side port of the modular multilevel converter.

[0095] According to the modulation coefficient after compensation of the upper and lower bridge arms, the nearest level approximation modulation method is used to obtain the number of sub-modules that need to be put into use on the bridge arm at each moment; the sorting algorithm is combined to generate trigger pulses, so that the sub-modules of the modular multilevel converter can be reasonably put into and cut out, so as to achieve the impedance reshaping of the AC side port of the modular multilevel converter.

[0096] The practical effect of the present invention is analyzed below in conjunction with specific examples:

[0097] The AC side impedance reshaping method of the present invention is phase-controlled, so the AC side impedance reshaping method is applicable to both single-phase and multi-phase systems. For the convenience of description, a single-phase system is constructed below, and the single-phase system is taken as an example to illustrate the modular multi-level converter AC side impedance reshaping method based on capacitor voltage feedforward.

[0098] Figure 4 A structural diagram of a single-phase modular multilevel converter, an internal structural diagram of a half-bridge submodule, an average model of the modular multilevel converter, and a differential common mode equivalent circuit diagram are provided in an embodiment of the present invention; wherein the single-phase system comprises: an upper bridge arm, a lower bridge arm, an upper bridge arm inductor, an upper bridge arm equivalent resistor, a lower bridge arm inductor, and a lower bridge arm equivalent resistor; and the upper bridge arm and the lower bridge arm in the single-phase system are both formed by N half-bridge submodules SM connected in series.

[0099] In the main circuit of the single-phase modular multilevel converter, the upper and lower bridge arms of each phase include N half-bridge sub-modules SM connected in series, an equivalent bridge arm resistor R arm and bridge arm inductance L arm ;vu and i u are the bridge arm voltage and bridge arm current of the upper bridge arm respectively (due to the symmetry of the upper and lower bridge arms of each phase, only the upper bridge arm is used as an example for explanation); wherein the half-bridge submodule includes two power switching devices T1 and T2, two diodes and an energy storage capacitor C; v cuj is the voltage on the jth submodule capacitor of the upper bridge arm (j=1, 2, 3, ..., N); v SM is the output voltage of the submodule; L k is the equivalent leakage inductance of the AC line and transformer; v a is the actual value of the AC side output voltage; R L and L L are load resistance and load inductance respectively; n u is the upper bridge arm modulation coefficient, v cu Σ is the sum of the capacitor voltages of the upper bridge arm submodules; L dm and R dm are differential mode equivalent inductance and differential mode equivalent resistance respectively, and L dm =L arm / 2+L k , R dm =R arm / 2; v s and i s are the actual values ​​of differential mode voltage and AC side output current respectively; L cm and R cm are the common-mode equivalent inductance and common-mode equivalent resistance respectively, and L cm =L arm , R cm =R arm ;v cm and i cm are the common mode voltage and common mode current respectively; V dc is the DC side voltage.

[0100] Figure 5 The Bode diagram comparison of the theoretical calculation and simulated frequency sweep of the AC side output impedance of the modular multilevel converter when the AC voltage is open-loop controlled before the AC side impedance reshaping method is adopted in the system; Figure 6 The Bode diagram comparison of the theoretical calculation and simulated frequency sweep of the AC side output impedance of the modular multilevel converter before the system adopts the AC side impedance reshaping method and when the AC voltage is closed-loop controlled; Figure 7 The Bode diagram comparison between the theoretical calculation and simulated frequency sweep of the AC side output impedance of the modular multilevel converter when the AC voltage is open-loop controlled after the system adopts the AC side impedance reshaping method; Figure 8 The Bode diagram comparison between the theoretical calculation and simulated frequency sweep of the AC side output impedance of the modular multilevel converter when the AC voltage is closed-loop controlled after the system adopts the AC side impedance reshaping method;

[0101] from Figures 5 to 8 It can be concluded that the theoretical calculation and simulation frequency sweep are consistent, which illustrates the accuracy of the impedance modeling of the AC side port of the modular multilevel converter before and after the AC side impedance reshaping method is adopted; and, before the AC side impedance reshaping method is adopted, the output impedance of the AC side port of the modular multilevel converter is approximately capacitive in the low-frequency region, and after the AC side impedance reshaping method is adopted, the output impedance of the AC side port of the modular multilevel converter is approximately inductive in the low-frequency region; in addition, through the AC side impedance reshaping, the impedance characteristics of the AC side port of the modular multilevel converter are consistent with the impedance characteristics of the AC side port of the traditional two-level voltage source converter, thereby simplifying the low-frequency stability analysis process of the modular multilevel converter interconnection system and providing design guidance for the controller parameters through a simple average model.

[0102] The AC side impedance reshaping method of a modular multilevel converter disclosed in the present invention can take each phase of a three-phase modular multilevel converter as a control object, first derive the compensation item of the differential mode voltage reference required to achieve the AC side impedance reshaping, and superimpose the differential mode modulation coefficient obtained by the compensation item on the traditional differential mode modulation coefficient to form a new differential mode modulation coefficient, which is used together with the common mode modulation coefficient to control the number of sub-modules put into the upper and lower bridge arms to achieve the impedance reshaping of the AC side port of the modular multilevel converter, so that the impedance characteristics of the AC side port of the modular multilevel converter are the same as the impedance characteristics of the traditional two-level voltage source converter port, simplify the low-frequency stability analysis process of the modular multilevel converter interconnection system, and provide design guidance for controller parameters through a simple average model.

[0103] Compared with the AC side impedance reshaping method in the prior art, the modular multi-level converter AC side impedance reshaping method disclosed in the present invention is easy to implement, has a simple structure, can be calculated online, improves system stability, is easy to apply in engineering practice, and can perform impedance reshaping under various working conditions and has a good control effect.

[0104] Embodiment 2

[0105] like Fig. 9 As shown, the present invention provides an AC side impedance reshaping system for a modular multi-level converter, comprising an acquisition module, a first calculation module, a second calculation module and a control module;

[0106] The acquisition module is used to obtain the DC side voltage of the modular multilevel converter, the common mode voltage reference value of any phase during circulating current control, and the differential mode voltage reference value of the current phase during output voltage control;

[0107] The first calculation module is used to calculate the compensation item of the differential mode voltage reference by using a compensation algorithm;

[0108] The second calculation module is used to calculate the modulation coefficient of the upper bridge arm after compensation and the modulation coefficient of the lower bridge arm after compensation respectively based on the common mode voltage reference value, the differential mode voltage reference value, the compensation term of the differential mode voltage reference and the DC side voltage;

[0109] The control module is used to control the number of sub-modules put into the upper bridge arm and the number of sub-modules put into the lower bridge arm based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, so as to achieve impedance reshaping of the AC side port of the modular multilevel converter.

[0110] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed in the embodiments of the present invention can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0111] In the embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0112] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0113] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit.

[0114] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: various media that can store program codes, such as USB flash drives, mobile hard disks, ROM, RAM, magnetic disks, or optical disks.

[0115] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A modular multilevel converter AC side impedance reshaping method, It is characterized in that The following steps are involved: S100: Obtaining a DC side voltage of a modular multilevel converter, a common mode voltage reference value of any phase during circulating current control, and a differential mode voltage reference value of a current phase during output voltage control; S200: Calculate a compensation item of a differential mode voltage reference using a compensation algorithm; S300: Calculate the modulation coefficient of the upper bridge arm after compensation and the modulation coefficient of the lower bridge arm after compensation respectively based on the common mode voltage reference value, the differential mode voltage reference value, the compensation term of the differential mode voltage reference and the DC side voltage; S400: Based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, the number of submodules put into the upper bridge arm and the number of submodules put into the lower bridge arm are controlled to achieve impedance reshaping of the AC side port of the modular multilevel converter; The modulation coefficient after upper bridge arm compensation in step S300 is obtained by the following formula: Where n u c is the modulation coefficient after upper bridge arm compensation; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term of the differential mode voltage reference; The modulation coefficient after the lower bridge arm compensation is obtained by the following formula: Where n l c is the modulation coefficient after compensation of the lower bridge arm; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term of the differential mode voltage reference; The common mode modulation coefficient is obtained by the following formula: Where n cm is the common mode modulation coefficient; v cm * is the common mode voltage reference value; V dc is the DC side voltage; The compensated differential mode modulation coefficient is obtained by the following formula: In the formula, is the differential mode modulation coefficient after compensation; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term for the differential mode voltage reference.

2. The AC side impedance reshaping method according to claim 1, It is characterized in that The common mode voltage reference value in step S100 is obtained by the following formula: In the formula, v cm * is the common mode voltage reference value; V dc is the DC side voltage; K Pic , K Ric are the proportional coefficient and resonance coefficient of the first PR regulator respectively; ω 0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; i cir * is the circulation reference value; i cir is the actual value of circulation.

3. The AC side impedance reshaping method according to claim 1, It is characterized in that The differential mode voltage reference value is obtained by the following formula: In the formula, v s * is the differential mode voltage reference value; v a is the actual value of the AC side output voltage; K Pis , K Ris are the proportional coefficient and resonance coefficient of the second PR regulator respectively; ω 0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; i s * is the reference value of the output current on the AC side; i s It is the actual value of the output current on the AC side.

4. The AC side impedance reshaping method according to claim 3, It is characterized in that The AC side output current reference value is obtained by the following formula: In the formula, i s * K is the reference value of the AC side output current; Pvd , K Rvd are the proportional coefficient and resonance coefficient of the third PR regulator respectively; ω 0 is the fundamental angular frequency; ω is the cutoff angular frequency; s is the Laplace operator; v a * is the reference value of the AC side output voltage; v a is the actual value of the AC side output voltage.

5. The AC side impedance reshaping method according to claim 1, It is characterized in that The compensation term of the differential mode voltage reference in step S200 is obtained by the following formula: In the formula, △v s is the compensation term of the differential mode voltage reference; V dc is the DC side voltage; v cm * is the common mode voltage reference value; v s * is the differential mode voltage reference value; v cu Σ is the total capacitance voltage of the upper bridge arm; v cl Σ is the total capacitor voltage of the lower bridge arm.

6. The AC side impedance reshaping method according to claim 1, It is characterized in that The S400 is specifically: Based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, the nearest level approximation modulation method is used to obtain the number of sub-modules that need to be put into use on the bridge arm at each moment; A sorting algorithm is used to generate trigger pulses to switch the submodules of the modular multilevel converter on and off, so as to achieve impedance reshaping of the AC side ports of the modular multilevel converter.

7. A modular multi-level converter AC side impedance reshaping system, It is characterized in that It includes an acquisition module, a first calculation module, a second calculation module and a control module; The acquisition module is used to acquire the DC side voltage of the modular multilevel converter, the common mode voltage reference value of any phase during circulating current control, and the differential mode voltage reference value of the current phase during output voltage control; The first calculation module is used to calculate the compensation term of the differential mode voltage reference using a compensation algorithm; The second calculation module is used to calculate the modulation coefficient after upper bridge arm compensation and the modulation coefficient after lower bridge arm compensation respectively based on the common mode voltage reference value, the differential mode voltage reference value, the compensation term of the differential mode voltage reference and the DC side voltage; wherein the modulation coefficient after upper bridge arm compensation is obtained by the following formula: Where n u c is the modulation coefficient after upper bridge arm compensation; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term of the differential mode voltage reference; The modulation coefficient after the lower bridge arm compensation is obtained by the following formula: Where n l c is the modulation coefficient after compensation of the lower bridge arm; n cm is the common mode modulation coefficient; is the differential mode modulation coefficient after compensation; v cm * is the common mode voltage reference value; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term of the differential mode voltage reference; The common mode modulation coefficient is obtained by the following formula: Where n cm is the common mode modulation coefficient; v cm * is the common mode voltage reference value; V dc is the DC side voltage; The compensated differential mode modulation coefficient is obtained by the following formula: In the formula, is the differential mode modulation coefficient after compensation; V dc is the DC side voltage; v s * is the differential mode voltage reference value; △v s is the compensation term of the differential mode voltage reference; The control module is used to control the number of sub-modules put into the upper bridge arm and the number of sub-modules put into the lower bridge arm based on the modulation coefficient after the upper bridge arm compensation and the modulation coefficient after the lower bridge arm compensation, so as to achieve impedance reshaping of the AC side port of the modular multilevel converter.

Images