A method and device for harmonic suppression of a modular multilevel converter and a converter

Abstract

The application relates to the technical field of power control, and discloses a harmonic suppression method and device of a modular multilevel converter and the converter. The grid-connected point current of an acquired target power grid is subjected to delay processing and positive and negative sequence separation, and the obtained twice-frequency harmonic positive sequence current component and twice-frequency harmonic negative sequence current component are subjected to proportional integral control respectively to obtain a twice-frequency harmonic positive sequence voltage component and a twice-frequency harmonic negative sequence voltage component. A positive sequence modulation signal is obtained according to the first amplitude of the twice-frequency harmonic positive sequence current component and the twice-frequency harmonic positive sequence voltage component, and a negative sequence modulation signal is obtained according to the second amplitude of the twice-frequency harmonic negative sequence current component and the twice-frequency harmonic negative sequence voltage component. The positive sequence modulation signal, the negative sequence modulation signal and a first modulation signal output by the modular multilevel converter are superposed to obtain a second modulation signal, so as to suppress the twice-frequency harmonic current of the target power grid. The method effectively suppresses the harmonic current of the modular multilevel converter.

Description

Technical Field

[0001] This invention relates to the field of power control technology, and in particular to a harmonic suppression method, device and converter for a modular multilevel converter. Background Technology

[0002] Flexible DC transmission technology is a high-voltage DC transmission technology based on modular multilevel converters (MMC). It has received widespread attention and application in many fields and has achieved full development and engineering application in scenarios such as wind power transmission, grid interconnection, and large-capacity long-distance power transmission.

[0003] However, in practical applications, when AC and DC transmission lines operate in parallel, a 50Hz harmonic component is coupled into the DC line. This harmonic component, after being modulated by the modular multilevel converter (MMC), generates a DC voltage on the AC side. The presence of this DC voltage causes transformer saturation in the MMC converter station, resulting in a large number of second harmonics, which seriously affects the safe and stable operation of the entire power system.

[0004] Therefore, how to suppress harmonics in modular multilevel converters has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] This invention provides a harmonic suppression method, device, and converter for a modular multilevel converter, to solve the technical problem of how to suppress harmonics in a modular multilevel converter, thereby effectively suppressing harmonics in the modular multilevel converter and enhancing the stability and power supply reliability of the flexible DC transmission system.

[0006] In a first aspect, the present invention provides a harmonic suppression method for a modular multilevel converter, the method comprising:

[0007] The grid connection point current of the target power grid is obtained, and the grid connection point current is sequentially delayed and separated into positive and negative sequences to obtain the positive sequence current component of the second harmonic and the negative sequence current component of the second harmonic.

[0008] The positive-sequence current component of the second harmonic is subjected to proportional-integral control to obtain the positive-sequence voltage component of the second harmonic, and the negative-sequence current component of the second harmonic is subjected to proportional-integral control to obtain the negative-sequence voltage component of the second harmonic.

[0009] According to the first amplitude of the second harmonic positive sequence current component and the second harmonic positive sequence voltage component, a positive sequence modulation signal is obtained, according to the second amplitude of the second harmonic negative sequence current component and the second harmonic negative sequence voltage component, a negative sequence modulation signal is obtained, and the positive sequence modulation signal and the negative sequence modulation signal are superimposed to obtain a harmonic suppression signal;

[0010] The harmonic suppression signal and a first modulation signal output by a modular multilevel converter installed in the target power grid loop are superimposed to obtain a second modulation signal, and the second modulation signal is sent to the modular multilevel converter to suppress the second harmonic current of the target power grid.

[0011] Preferably, the grid-connected point current is sequentially subjected to delay processing and positive and negative sequence separation to obtain a second harmonic positive sequence current component and a second harmonic negative sequence current component, comprising:

[0012] The grid-connected point current is subjected to Clarke transformation to obtain a current component in a two-phase static coordinate system;

[0013] The current component is sequentially subjected to delay processing and positive and negative sequence separation to obtain a second harmonic positive sequence current and a second harmonic negative sequence current in the two-phase static coordinate system;

[0014] The second harmonic positive sequence current and the second harmonic negative sequence current are respectively subjected to Park transformation to correspondingly obtain a second harmonic positive sequence current component and a second harmonic negative sequence current component in a synchronous rotating coordinate system.

[0015] Preferably, the second harmonic positive sequence current component is subjected to proportional integral control to obtain a second harmonic positive sequence voltage component, and the second harmonic negative sequence current component is subjected to proportional integral control to obtain a second harmonic negative sequence voltage component, comprising:

[0016] The second harmonic positive sequence current component is subtracted from zero and subjected to proportional integral control to obtain the second harmonic positive sequence voltage component in the synchronous rotating coordinate system;

[0017] The second harmonic negative sequence current component is subtracted from zero and subjected to proportional integral control to obtain the second harmonic negative sequence voltage component in the synchronous rotating coordinate system.

[0018] Preferably, the positive sequence modulation signal is obtained according to the product of the first amplitude of the second harmonic positive sequence current component and the second harmonic positive sequence voltage component, and the negative sequence modulation signal is obtained according to the product of the second amplitude of the second harmonic negative sequence current component and the second harmonic negative sequence voltage component, comprising:

[0019] The amplitudes of the positive-sequence current component and the negative-sequence current component of the second harmonic are calculated respectively to obtain the first amplitude of the positive-sequence current component and the second amplitude of the negative-sequence current component of the second harmonic.

[0020] The positive sequence voltage component of the second harmonic is converted from the synchronous rotating coordinate system to the three-phase coordinate system and then multiplied with the first amplitude to obtain the positive sequence modulation signal;

[0021] The negative sequence current component of the second harmonic is converted from the synchronous rotating coordinate system to the three-phase coordinate system and then multiplied with the second amplitude to obtain the negative sequence modulation signal.

[0022] Preferably, the first amplitude is the second harmonic positive sequence current. The square of the axial component and the second harmonic positive sequence current The square root of the sum of the squares of the axial components;

[0023] The second amplitude is the second harmonic negative sequence current. The square of the axial component and the second harmonic negative sequence current The square root of the sum of the squares of the axial components.

[0024] Preferably, the first modulation signal includes a 50Hz modulation signal output from the current loop of the modular multilevel converter and a 100Hz modulation signal output from the circulating current suppression loop of the modular multilevel converter.

[0025] Preferably, the step of superimposing the harmonic suppression signal and the first modulation signal output by the modular multilevel converter to obtain the second modulation signal includes:

[0026] The first weight of the harmonic suppression signal, the second weight of the 50Hz modulation signal, and the third weight of the 100Hz modulation signal are obtained through simulation modeling.

[0027] The harmonic suppression signal, the 50Hz modulation signal, and the 100Hz modulation signal are weighted and superimposed according to the first weight, the second weight, and the third weight to obtain the second modulation signal.

[0028] Preferably, suppressing the second harmonic current of the modular multilevel converter according to the second modulation signal includes:

[0029] The second modulation signal is compared with the triangular carrier wave of each sub-module in the modular multilevel converter to obtain the comparison result;

[0030] Based on each comparison result, the switching state of the insulated gate bipolar transistor in each submodule is determined.

[0031] In a second aspect, the present application further provides a harmonic suppression device of a modular multilevel converter, which implements the harmonic suppression method of the modular multilevel converter as described above, and the device comprises a positive and negative sequence separation module, a positive and negative sequence proportional integral control module, a harmonic suppression signal generation module and a signal superposition module.

[0032] The positive and negative sequence separation module is configured to obtain a grid-connected point current of a target power grid, and sequentially perform delay processing and positive and negative sequence separation on the grid-connected point current to obtain a double-frequency harmonic positive sequence current component and a double-frequency harmonic negative sequence current component.

[0033] The positive and negative sequence proportional integral control module is configured to perform proportional integral control on the double-frequency harmonic positive sequence current component to obtain a double-frequency harmonic positive sequence voltage component, and perform proportional integral control on the double-frequency harmonic negative sequence current component to obtain a double-frequency harmonic negative sequence voltage component.

[0034] The harmonic suppression signal generation module is configured to obtain a positive sequence modulation signal according to a first amplitude of the double-frequency harmonic positive sequence current component and the double-frequency harmonic positive sequence voltage component, obtain a negative sequence modulation signal according to a second amplitude of the double-frequency harmonic negative sequence current component and the double-frequency harmonic negative sequence voltage component, and superimpose the positive sequence modulation signal and the negative sequence modulation signal to obtain a harmonic suppression signal.

[0035] The signal superposition module is configured to superimpose the harmonic suppression signal and a first modulation signal output by the modular multilevel converter installed in a loop of the target power grid to obtain a second modulation signal, and send the second modulation signal to the modular multilevel converter to suppress the double-frequency harmonic current of the target power grid.

[0036] In a third aspect, the present application further provides a modular multilevel converter, which adopts the harmonic suppression device of the modular multilevel converter as described above to suppress the double-frequency harmonic current.

[0037] The present application provides a harmonic suppression method, device and converter of a modular multilevel converter, and the beneficial effects of the present application are as follows compared with the prior art.

[0038] The harmonic suppression method of the modular multilevel converter disclosed in the present application can be adaptively controlled according to the double-frequency harmonic current content in the target power grid, and when the target power grid is normally operated, the original stability of the system will not be affected. The output harmonic suppression signal can be superimposed with the output signal of the original control link of the modular multilevel converter, so as to realize the double-frequency harmonic suppression of the target power grid, and additional physical filters do not need to be added from the outside, thereby reducing the procurement and installation costs of hardware devices. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a schematic diagram of a harmonic suppression method of a modular multilevel converter according to an embodiment of the present application;

[0040] Figure 2 is a schematic diagram of a flexible HVDC system topology according to an embodiment of the present application;

[0041] Figure 3 is a schematic diagram of a positive and negative sequence separation control topology according to an embodiment of the present application;

[0042] Figure 4 is a schematic diagram of a harmonic suppression signal generation control topology according to an embodiment of the present application;

[0043] Figure 5 is a schematic diagram of a harmonic suppression system of a modular multilevel converter according to an embodiment of the present application;

[0044] Figure 6 is a current simulation waveform and FFT analysis result according to an embodiment of the present application;

[0045] Reference Signs:

[0046] 1 - positive and negative sequence separation module, 2 - positive and negative sequence proportional integral control module, 3 - harmonic suppression signal generation module, 4 - signal superposition module. DETAILED DESCRIPTION

[0047] Embodiments of the present application will be described in detail below with reference to the drawings, which are given by way of illustration only and thus are not restrictive of the present application. The present application is not limited in its application to the details set forth in the description below and / or in the drawings. The present application is capable of other embodiments and of being practiced or carried out in various ways. Examples are given for illustrative purposes only and are not intended to limit the scope of the present application. The drawings are not necessarily to scale; instead, emphasis is placed on the principles of the present application. The drawings are merely schematic representations, not intended to portray specific structural details of the application. In the drawings, like numbers refer to like objects throughout the several views, unless otherwise indicated. The use of the alternative (e.g., "or") is meant to include both "and" and "or" unless otherwise indicated. The use of "an" is meant to include both "an" and "a" unless otherwise indicated. The use of "comprise", "comprises", "comprising", "include", "includes", "including" and "contain", "contains", "containing" are meant to be open-ended, non-limiting identifiers that allow for the inclusion of more than what is specifically recited. The use of "first" and "second" is meant to indicate a difference between the two elements, but does not mean a limitation on the number of elements. The use of "one" is meant to indicate a singular element, but does not mean a limitation on the number of elements. The use of "another" is meant to indicate a different element than the one previously recited, but does not mean a limitation on the number of elements.

[0048] In the description of the present application, it should be noted that, unless otherwise explicitly defined and limited, the terms "mounting", "connection", "connection" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected, it can be mechanically connected, or it can be electrically connected, it can be directly connected, or indirectly connected through an intermediate medium, it can be the communication inside two elements. The term "and / or" used herein includes any and all combinations of one or more related listed items. For those skilled in the art, the specific meaning of the above terms in the present application can be understood in specific cases.

[0049] In the description of the present application, it should be noted that, unless otherwise defined, all technical and scientific terms used in the present application are the same as those commonly understood by those skilled in the art. The terms used in the specification of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. For those skilled in the art, the specific meaning of the above terms in the present application can be understood in specific cases.

[0050] Please refer to Figure 1 The step diagram of a harmonic suppression method of a modular multilevel converter is shown. In the embodiments of the present application, a harmonic suppression method of a modular multilevel converter is provided, which comprises:

[0051] S1, obtaining grid-connected point current, separating positive and negative sequences of the grid-connected point current to obtain two-frequency harmonic positive sequence current component and two-frequency harmonic negative sequence current component in synchronous rotating coordinate system; In the preferred embodiments of the present application, the modular multilevel converter (Module Multilevel Converter, MMC) is applied to the flexible DC power transmission system, so that Figure 2 The flexible DC power transmission system topology structure diagram is shown for illustration. The flexible DC power transmission system comprises an AC network, a YgY transformer and a MMC converter station with PQ control. The MMC converter station is a DC power transmission converter station based on modular multilevel converter technology, and in Figure 2 , For AC voltage, and are grid voltage and grid current respectively, and are grid voltage and grid current respectively, is the YgY transformer ratio, , are MMC converter station AC side voltage and MMC converter station AC side current respectively, and are positive DC voltage and negative DC voltage respectively, both are half. DC voltage This refers to the circulating current inside the MMC; PLL stands for phase-locked loop. The phase angle is the output phase angle of the phase-locked loop. and These are the inner current loops. Shaft command current and current inner loop Shaft command current, and These are the first modulation signals output from the modular multilevel converter, where, This is the 50Hz modulated signal output from the current loop. The 100Hz modulated signal output by the circulating current suppression loop. Represents the positive sequence current of the second harmonic. Axial components, Represents the positive sequence current component of the second harmonic. Axial components, Represents the second harmonic negative sequence current. Axial components, Represents the second harmonic negative sequence current. Axial components, Represents the positive sequence voltage of the second harmonic. Axial components, Represents the positive sequence voltage of the second harmonic. Axial components, Represents the negative sequence voltage of the second harmonic. Axial components, Represents the negative sequence voltage of the second harmonic. Axial components, Indicates harmonic suppression signal, This indicates the second modulation signal.

[0052] In a preferred embodiment of this application, the grid connection point current is obtained. At this time, the grid connection point current is the three-phase current in a three-phase coordinate system, such as... Figure 3 The diagram shows a positive and negative sequence separation control topology. The three-phase currents at the grid connection point are respectively... , and , Tg is the power frequency cycle delay, Tg / 4 is the quarter power frequency cycle delay, the grid-connected point current is subjected to Clarke transformation to obtain the current component in the two-phase static coordinate system, at this time the fundamental wave of the grid-connected point current is a 50Hz sine wave and the double-frequency harmonic wave is a 100Hz sine wave. The positive sequence component and the negative sequence component are separated by using the phase difference characteristics after delay, the current component is sequentially subjected to delay processing and positive and negative sequence separation by Tg / 4, for the positive sequence double-frequency harmonic wave, the phase is offset by 90° after delay by Tg / 4, for the negative sequence double-frequency harmonic wave, the phase is offset by -90° after delay by Tg / 4, then the double-frequency harmonic wave positive sequence current and the double-frequency harmonic wave negative sequence current in the two-phase static coordinate system are obtained by addition and subtraction operation. Further, the double-frequency harmonic wave positive sequence current and the double-frequency harmonic wave negative sequence current are subjected to Park transformation respectively to obtain the double-frequency harmonic wave positive sequence current component and the double-frequency harmonic wave negative sequence current component in the synchronous rotating coordinate system.

[0053] S2, the double-frequency harmonic wave positive sequence current component is subjected to proportional integral control to obtain a double-frequency harmonic wave positive sequence voltage component, and the double-frequency harmonic wave negative sequence current component is subjected to proportional integral control to obtain a double-frequency harmonic wave negative sequence voltage component; in the preferred embodiment of the application, the double-frequency harmonic wave positive sequence current component and the double-frequency harmonic wave negative sequence current component are respectively subjected to proportional integral control by a PI loop. Specifically, referring to the harmonic suppression signal generation control topological structure diagram shown in Figure 4 , the double-frequency harmonic wave positive sequence current component is subtracted from zero and input into the PI loop for proportional integral control to obtain the double-frequency harmonic wave positive sequence voltage component in the synchronous rotating coordinate system; the double-frequency harmonic wave negative sequence current component is subtracted from zero and input into the PI loop for proportional integral control to obtain the double-frequency harmonic wave negative sequence voltage component in the synchronous rotating coordinate system.

[0054] S3, a positive sequence modulation signal is obtained according to the first amplitude of the double-frequency harmonic wave positive sequence current component and the double-frequency harmonic wave positive sequence voltage component, and a negative sequence modulation signal is obtained according to the second amplitude of the double-frequency harmonic wave negative sequence current component and the double-frequency harmonic wave negative sequence voltage component, the positive sequence modulation signal and the negative sequence modulation signal are superimposed to obtain a harmonic suppression signal; in a preferred embodiment of the application, the double-frequency harmonic wave positive sequence current component and the double-frequency harmonic wave negative sequence current component are respectively subjected to amplitude calculation to obtain the first amplitude of the double-frequency harmonic wave positive sequence current component and the second amplitude of the double-frequency harmonic wave negative sequence current component, the calculation formula of the first amplitude is:

[0055]

[0056] wherein, the first amplitude is represented by I1, the double-frequency harmonic wave positive sequence current is represented by I1d, the axis component is represented by I1d0, and Represents the positive sequence current of the second harmonic. Axial components.

[0057] The formula for calculating the second amplitude is:

[0058]

[0059] in, Indicates the first value. Represents the second harmonic negative sequence current. Axial components, Represents the second harmonic negative sequence current. Axial components.

[0060] The first amplitude is used as the feedback coefficient for the positive-sequence voltage component of the second harmonic, and the second amplitude is used as the feedback coefficient for the negative-sequence voltage component of the second harmonic. Further, the positive-sequence voltage component of the second harmonic is transformed from a synchronous rotating coordinate system to a three-phase coordinate system and multiplied by the first amplitude to obtain the positive-sequence modulation signal. The negative-sequence current component of the second harmonic is transformed from the synchronous rotating coordinate system to the three-phase coordinate system and then multiplied with the second amplitude to obtain the negative-sequence modulation signal. The positive-sequence modulation signal and the negative-sequence modulation signal are superimposed to obtain the harmonic suppression signal. The first and second amplitudes reflect the magnitude of the second harmonic in the grid connection point current. During normal operation of the flexible DC transmission system, there is no second harmonic current at the grid connection point, so the first and second amplitudes are zero, thus not affecting the stability of the flexible DC transmission system during normal operation. When a 50Hz harmonic component is coupled out of the DC line, the grid connection point current contains a second harmonic current. The harmonic suppression method of the modular multilevel converter in this application yields a positive-sequence modulation signal and a negative-sequence modulation signal. These two signals are then superimposed to obtain the harmonic suppression signal. .

[0061] S4. The harmonic suppression signal is superimposed with the first modulation signal output by the modular multilevel converter installed in the target power grid circuit to obtain a second modulation signal. The second modulation signal is sent to the modular multilevel converter to suppress the second harmonic current of the target power grid. In a preferred embodiment of this application, the harmonic suppression signal is superimposed with the first modulation signal output by the modular multilevel converter itself installed in the target power grid circuit to generate a second modulation signal. The second modulation signal is input into the modular multilevel converter to quickly suppress the second harmonic current generated by transformer saturation. The modular multilevel converter of this application has a PQ control loop, such as... Figure 2As shown, the generation process of the first modulation signal includes the following steps: the AC side voltage of the MMC converter station is processed by a phase-locked loop to output the phase angle of the AC side voltage of the MMC converter station, the internal circulating current of the MMC is suppressed according to the phase angle to output a 100Hz modulation signal; meanwhile, the AC side voltage and the AC side current of the MMC converter station are calculated to obtain active power and reactive power, the active power and the reactive power are controlled in the inner power loop to generate the axis instruction current and the current inner loop of the axis instruction current, the current inner loop of the axis instruction current and the current inner loop of the axis instruction current is controlled in the current inner loop to generate a 50Hz modulation signal. The 50Hz modulation signal and the 100Hz modulation signal are superimposed and then superimposed with the harmonic suppression signal to obtain a second modulation signal. In the preferred embodiment of the present application, the weights of the superimposition of the harmonic suppression signal, the 50Hz modulation signal and the 100Hz modulation signal need to be comprehensively considered according to the requirements of the flexible DC power transmission system on harmonic suppression, power control and circulating current suppression, and the first weight of the harmonic suppression signal, the second weight of the 50Hz modulation signal and the third weight of the 100Hz modulation signal are obtained by simulation modeling. Further, the first weight is taken as the weight value of the harmonic suppression signal, the second weight is taken as the weight value of the 50Hz modulation signal, and the third weight is taken as the weight value of the 100Hz modulation signal. The harmonic suppression signal, the 50Hz modulation signal and the 100Hz modulation signal are superimposed by weighting to obtain the second modulation signal, which can effectively suppress the double-frequency harmonic current and flexibly adjust the weights of the superimposition of the harmonic suppression signal, the 50Hz modulation signal and the 100Hz modulation signal according to the requirements of the flexible DC power transmission system on harmonic suppression, power control and circulating current suppression, and adapt to the requirements of the flexible DC power transmission system.

[0062] The second modulation signal is input into the modular multilevel converter to quickly suppress the double-frequency harmonic current generated due to transformer saturation. The modular multilevel converter includes a plurality of sub-modules, each of which includes an insulated gate bipolar transistor (IGBT), and each sub-module uses triangular carriers of the same frequency and amplitude, but there is a certain phase shift between the triangular carriers of adjacent sub-modules. The second modulation signal is compared with the triangular carriers of each sub-module in the modular multilevel converter to obtain a comparison result, and the switching state of the insulated gate bipolar transistor in each sub-module is determined according to the comparison result. For example, when the second modulation signal is greater than the triangular carrier of the sub-module, the upper tube of the corresponding sub-module is turned on and the lower tube is turned off; when the second modulation signal is less than or equal to the triangular carrier of the sub-module, the lower tube of the corresponding sub-module is turned on and the upper tube is turned off. With the increase of the number of sub-modules, the equivalent switching frequency is improved, and the output waveform is closer to a sine wave, thereby effectively suppressing the double-frequency harmonic current.

[0063] In the preferred embodiment of the present application, the grid point current of the target power grid is obtained, the grid point current is sequentially subjected to delay processing and positive and negative sequence separation to obtain a double-frequency harmonic positive sequence current component and a double-frequency harmonic negative sequence current component; proportional integral control is performed on the double-frequency harmonic positive sequence current component and the double-frequency harmonic negative sequence current component respectively to correspondingly obtain a double-frequency harmonic positive sequence voltage component and a double-frequency harmonic negative sequence voltage component; a positive sequence modulation signal is obtained according to a first amplitude of the double-frequency harmonic positive sequence current component and the double-frequency harmonic positive sequence voltage component, a negative sequence modulation signal is obtained according to a second amplitude of the double-frequency harmonic negative sequence current component and the double-frequency harmonic negative sequence voltage component, and the positive sequence modulation signal and the negative sequence modulation signal are superimposed to obtain a harmonic suppression signal; the harmonic suppression signal and a first modulation signal output by the modular multilevel converter are superimposed to obtain a second modulation signal, and the second modulation signal is sent to the modular multilevel converter to suppress the double-frequency harmonic current of the target power grid. The harmonic suppression method of the modular multilevel converter disclosed in the present application can be adaptively controlled according to the double-frequency harmonic current content in the target power grid, and when the target power grid is normally operated, the original stability of the system will not be affected. The output harmonic suppression signal can be superimposed with the output signal of the original control link of the modular multilevel converter, so that the double-frequency harmonic suppression of the target power grid is realized, and there is no need to increase an additional physical filter from the outside, thereby reducing the procurement and installation costs of hardware devices.

[0064] Correspondingly, as shown in the structure schematic diagram of the harmonic suppression system of the modular multilevel converter, Figure 5 the embodiment of the present application also provides a harmonic suppression device of the modular multilevel converter, which realizes the harmonic suppression method of the modular multilevel converter disclosed in the embodiment of the present application, and the device comprises: a positive and negative sequence separation module 1, a positive and negative sequence proportional integral control module 2, a harmonic suppression signal generation module 3 and a signal superimposition module 4;

[0065] The positive and negative sequence separation module 1 is used to obtain the grid point current of the target power grid, sequentially perform delay processing and positive and negative sequence separation on the grid point current to obtain a double-frequency harmonic positive sequence current component and a double-frequency harmonic negative sequence current component;

[0066] The positive and negative sequence proportional integral control module 2 is used to perform proportional integral control on the double-frequency harmonic positive sequence current component to obtain a double-frequency harmonic positive sequence voltage component, and perform proportional integral control on the double-frequency harmonic negative sequence current component to obtain a double-frequency harmonic negative sequence voltage component;

[0067] The harmonic suppression signal generation module 3 is configured to obtain a positive sequence modulation signal according to the first amplitude of the second harmonic positive sequence current component and the second harmonic positive sequence voltage component, obtain a negative sequence modulation signal according to the second amplitude of the second harmonic negative sequence current component and the second harmonic negative sequence voltage component, and superimpose the positive sequence modulation signal and the negative sequence modulation signal to obtain a harmonic suppression signal.

[0068] The signal superimposition module 4 is configured to superimpose the harmonic suppression signal and a first modulation signal output by a modular multilevel converter installed in the target power grid loop to obtain a second modulation signal, and send the second modulation signal to the modular multilevel converter to suppress the second harmonic current of the target power grid.

[0069] The specific limitations of the harmonic suppression device of the modular multilevel converter can refer to the limitations of the harmonic suppression method of the modular multilevel converter described above, which will not be repeated here. Those skilled in the art can realize that the various modules and steps described in combination with the embodiments disclosed in the present application can be realized in hardware, software or both. Whether the functions are realized in hardware or software depends on the specific application and design constraints of the technical solution. Professionals can use different methods for each specific application to realize the described functions, but such implementation should not be considered beyond the scope of the present application.

[0070] Correspondingly, the embodiments of the present application provide a modular multilevel converter, which uses the harmonic suppression device of the modular multilevel converter disclosed in the embodiments of the present application to suppress the second harmonic, so as to improve the grid power quality.

[0071] The harmonic suppression method of the modular multilevel converter of the present application is simulated and verified. A 50Hz harmonic source is added at the DC side to simulate the 50Hz current coupled in the DC line due to the parallel operation of the AC-DC transmission line in the real engineering. As shown in Figure 6 The current simulation waveform and FFT analysis result are shown in the figure. The 50Hz harmonic source at the DC side is added at t1=2s, and the harmonic suppression device of the modular multilevel converter of the present application is put into use at t2=6s.

[0072] As can be seen from Figure 6 When the harmonic suppression device of the modular multilevel converter of the present application is not in use, the content of the second harmonic at the AC side is as high as 12.87%, which seriously affects the power supply quality of the modular multilevel converter. When the harmonic suppression device of the modular multilevel converter of the present application is put into use, the content of the second harmonic in the modular multilevel converter is significantly reduced to 1.21%, which fully verifies the effectiveness of the harmonic suppression method and device of the modular multilevel converter of the present application.

[0073] In conclusion, the embodiment of the present application provides a harmonic suppression method and device of a modular multilevel converter and the converter, which solves the technical problem of how to improve the integrity and intelligent management level of new energy industry data. The method comprises the following steps: obtaining grid-connected point current of a target power grid, sequentially performing delay processing and positive and negative sequence separation on the grid-connected point current to obtain a two-frequency harmonic positive sequence current component and a two-frequency harmonic negative sequence current component; performing proportional integral control on the two-frequency harmonic positive sequence current component and the two-frequency harmonic negative sequence current component respectively to correspondingly obtain a two-frequency harmonic positive sequence voltage component and a two-frequency harmonic negative sequence voltage component; obtaining a positive sequence modulation signal according to a first amplitude of the two-frequency harmonic positive sequence current component and the two-frequency harmonic positive sequence voltage component, obtaining a negative sequence modulation signal according to a second amplitude of the two-frequency harmonic negative sequence current component and the two-frequency harmonic negative sequence voltage component, and superimposing the positive sequence modulation signal and the negative sequence modulation signal to obtain a harmonic suppression signal; superimposing the harmonic suppression signal and a first modulation signal output by the modular multilevel converter to obtain a second modulation signal, and sending the second modulation signal to the modular multilevel converter to suppress the two-frequency harmonic current of the target power grid. The harmonic suppression method of the modular multilevel converter disclosed in the present application can be adaptively controlled according to the two-frequency harmonic current content in the target power grid. When the target power grid is normally operated, the original stability of the system will not be affected. The output harmonic suppression signal can be superimposed with the output signal of the original control link of the modular multilevel converter, so that the two-frequency harmonic suppression of the target power grid is realized. No additional physical filter needs to be added from the outside, and the procurement and installation costs of hardware devices are reduced.

[0074] Each of the embodiments in the specification is described in a progressive manner, and the same or similar parts of each of the embodiments can be referred to each other. Each of the embodiments focuses on the difference from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, it is described more simply, and the related parts can be referred to the part of the description of the method embodiment. It should be noted that, each of the technical features of the above-mentioned embodiments can be combined arbitrarily, in order to make the description simple, not all possible combinations of the technical features of the above-mentioned embodiments are described, however, as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the description.

[0075] The above-described embodiments only express several preferred implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the patent scope of the application. It should be noted that, for ordinary skilled persons in the technical field, some improvements and replacements can be made without departing from the technical principles of the present application, and these improvements and replacements should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the protection scope of the claims.

Claims

1. A harmonic suppression method for a modular multilevel converter, characterized in that, The method includes: The grid connection point current of the target power grid is obtained, and the grid connection point current is sequentially delayed and separated into positive and negative sequences to obtain the positive sequence current component of the second harmonic and the negative sequence current component of the second harmonic. The positive-sequence current component of the second harmonic is subjected to proportional-integral control to obtain the positive-sequence voltage component of the second harmonic, and the negative-sequence current component of the second harmonic is subjected to proportional-integral control to obtain the negative-sequence voltage component of the second harmonic. A positive-sequence modulation signal is obtained based on the first amplitude of the positive-sequence current component of the second harmonic and the positive-sequence voltage component of the second harmonic. A negative-sequence modulation signal is obtained based on the second amplitude of the negative-sequence current component of the second harmonic and the negative-sequence voltage component of the second harmonic. The positive-sequence modulation signal and the negative-sequence modulation signal are superimposed to obtain a harmonic suppression signal. The harmonic suppression signal is superimposed with the first modulation signal output by the modular multilevel converter installed in the target power grid circuit to obtain a second modulation signal. The second modulation signal is then sent to the modular multilevel converter to suppress the second harmonic current of the target power grid.

2. The harmonic suppression method for a modular multilevel converter as described in claim 1, characterized in that, The process of sequentially delaying and separating the positive and negative sequence of the grid connection point current to obtain the second harmonic positive sequence current component and the second harmonic negative sequence current component includes: The current at the grid connection point is subjected to Clarke transformation to obtain the current components in the two-phase stationary coordinate system. The current components are sequentially delayed and separated into positive and negative sequences to obtain the second harmonic positive sequence current and the second harmonic negative sequence current in the two-phase stationary coordinate system. The positive-sequence current of the second harmonic and the negative-sequence current of the second harmonic are subjected to Park transformation respectively to obtain the positive-sequence current component of the second harmonic and the negative-sequence current component of the second harmonic in the synchronous rotating coordinate system.

3. The harmonic suppression method for a modular multilevel converter as described in claim 2, characterized in that, The step of performing proportional-integral control on the positive-sequence current component of the second harmonic to obtain the positive-sequence voltage component of the second harmonic, and performing proportional-integral control on the negative-sequence current component of the second harmonic to obtain the negative-sequence voltage component of the second harmonic, includes: The second harmonic positive sequence current component is subtracted from zero and then subjected to proportional-integral control to obtain the second harmonic positive sequence voltage component in the synchronous rotating coordinate system. The negative-sequence second harmonic current component is subtracted from zero and then subjected to proportional-integral control to obtain the negative-sequence second harmonic voltage component in the synchronous rotating coordinate system.

4. The harmonic suppression method for a modular multilevel converter as described in claim 3, characterized in that, The step of obtaining a positive-sequence modulation signal based on the product of the first amplitude of the positive-sequence current component of the second harmonic and the positive-sequence voltage component of the second harmonic, and obtaining a negative-sequence modulation signal based on the product of the second amplitude of the negative-sequence current component of the second harmonic and the negative-sequence voltage component of the second harmonic, includes: The amplitudes of the positive-sequence current component and the negative-sequence current component of the second harmonic are calculated respectively to obtain the first amplitude of the positive-sequence current component and the second amplitude of the negative-sequence current component of the second harmonic. The positive sequence voltage component of the second harmonic is converted from the synchronous rotating coordinate system to the three-phase coordinate system and then multiplied with the first amplitude to obtain the positive sequence modulation signal; The negative sequence voltage component of the second harmonic is converted from the synchronous rotating coordinate system to the three-phase coordinate system and then multiplied with the second amplitude to obtain the negative sequence modulation signal.

5. The harmonic suppression method for a modular multilevel converter as described in claim 4, characterized in that, The first amplitude is the second harmonic positive sequence current. The square of the axial component and the second harmonic positive sequence current The square root of the sum of the squares of the axial components; The second amplitude is the second harmonic negative sequence current. The square of the axial component and the second harmonic negative sequence current The square root of the sum of the squares of the axial components.

6. The harmonic suppression method for a modular multilevel converter as described in claim 1, characterized in that, The first modulation signal includes a 50Hz modulation signal output from the current loop of the modular multilevel converter and a 100Hz modulation signal output from the circulating current suppression loop of the modular multilevel converter.

7. The harmonic suppression method for a modular multilevel converter as described in claim 6, characterized in that, The step of superimposing the harmonic suppression signal and the first modulation signal output from the modular multilevel converter to obtain the second modulation signal includes: The first weight of the harmonic suppression signal, the second weight of the 50Hz modulation signal, and the third weight of the 100Hz modulation signal are obtained through simulation modeling. The harmonic suppression signal, the 50Hz modulation signal, and the 100Hz modulation signal are weighted and superimposed according to the first weight, the second weight, and the third weight to obtain the second modulation signal.

8. The harmonic suppression method for a modular multilevel converter as described in claim 1, characterized in that, The step of suppressing the second harmonic current of the modular multilevel converter according to the second modulation signal includes: The second modulation signal is compared with the triangular carrier wave of each sub-module in the modular multilevel converter to obtain the comparison result; Based on each comparison result, the switching state of the insulated gate bipolar transistor in each submodule is determined.

9. A harmonic suppression device for a modular multilevel converter, used to implement the harmonic suppression method of the modular multilevel converter according to any one of claims 1-8, characterized in that, The device includes: a positive and negative sequence separation module, a positive and negative sequence proportional-integral control module, a harmonic suppression signal generation module, and a signal superposition module; The positive and negative sequence separation module is used to obtain the grid connection point current of the target power grid, and to perform delay processing and positive and negative sequence separation on the grid connection point current in sequence to obtain the second harmonic positive sequence current component and the second harmonic negative sequence current component. The positive and negative sequence proportional-integral control module is used to perform proportional-integral control on the positive sequence current component of the second harmonic to obtain the positive sequence voltage component of the second harmonic, and to perform proportional-integral control on the negative sequence current component of the second harmonic to obtain the negative sequence voltage component of the second harmonic. The harmonic suppression signal generation module is used to obtain a positive-sequence modulation signal based on the first amplitude of the positive-sequence current component of the second harmonic and the positive-sequence voltage component of the second harmonic, to obtain a negative-sequence modulation signal based on the second amplitude of the negative-sequence current component of the second harmonic and the negative-sequence voltage component of the second harmonic, and to superimpose the positive-sequence modulation signal and the negative-sequence modulation signal to obtain a harmonic suppression signal. The signal superposition module is used to superimpose the harmonic suppression signal and the first modulation signal output by the modular multilevel converter installed in the target power grid circuit to obtain a second modulation signal, and send the second modulation signal to the modular multilevel converter to suppress the second harmonic current of the target power grid.

10. A modular multilevel converter, characterized in that, The harmonic suppression device of the modular multilevel converter described in claim 9 is used to suppress the second harmonic current.

Images