MMC carrier phase-shift modulation high-frequency harmonic evaluation and elimination method

Abstract

The invention discloses an MMC carrier phase shift modulation high-frequency harmonic wave evaluation and elimination method, and the evaluation method comprises the following steps: calculating a valve side AC voltage; calculating a bridge arm reference wave; calculating a multiplication bridge arm reference wave; generating a continuous carrier wave; generating a synchronous discrete carrier wave; calculating discrete output voltages of upper and lower bridge arms; performing broadband fast Fourier transform to obtain high-frequency harmonic distribution; the elimination method specifically comprises the following steps: obtaining a triangular wave of a lower bridge arm based on an evaluation method; a quantization unit is added on the triangular wave of the lower bridge arm; and the elimination is completed by quantizing the triangular wave of the unit lifting lower bridge arm. According to the evaluation method, factors such as different frequencies, ranges and quantization units are considered, and the frequency band and content of the direct-current high-frequency harmonic waves caused by triangular wave discretization are rapidly evaluated. According to the method for eliminating the high-frequency harmonic waves, the direct-current high-frequency harmonic waves caused by triangular wave discretization can be simply and conveniently eliminated.

Description

Technical Field

[0001] The invention belongs to the field of power transmission, and in particular relates to a method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation. Background Art

[0002] Modular Multilevel Converter (MMC) has a modular design, a large number of output levels, good harmonic characteristics, and can easily achieve flexible voltage level design through series connection of sub-modules. At present, MMC has played an important role in the field of high-voltage flexible DC transmission. At the same time, its influence in medium and low voltage fields such as DC distribution networks, new energy stations, and wind turbine converters is also gradually increasing.

[0003] In the medium and low voltage field, the number of bridge arm submodules is usually small. In order to reduce low-frequency voltage harmonics, the nearest level approximation modulation is usually not used, but the carrier phase shift modulation method with better low-frequency harmonic characteristics is used: a triangular wave equal to the number of bridge arm submodules is generated, the modulated wave is compared with the triangular wave, and the control signal of the submodule switch device is generated to control the input and bypass of the bridge arm submodule. In theory, since the DC component of the upper and lower bridge arm modulation waves is always 1 / 2, the input and bypass of the upper and lower bridge arm submodules (the upper bridge arm submodule is input and the lower bridge arm submodule is bypassed; or the upper bridge arm submodule is bypassed and the lower bridge arm submodule is input) occurs simultaneously, thereby maintaining the sum of the upper and lower bridge arm input submodules equal at any time, and there is no harmonic on the DC side.

[0004] However, the actual controller is a digital controller, and the triangular wave is generated by a digital chip. Therefore, the triangular wave generated in the digital controller approximates a continuous value by discrete data. Figure 1 Taking the upper and lower bridge arms as an example, each with a submodule, the switching of the upper and lower bridge arm submodules in a triangle wave cycle is plotted. Figure 1 As shown in the figure, the triangular wave naturally generated by the digital circuit is always lower than the ideal triangular wave, which will cause the switching error of "delayed bypass and early switching", so that the sum of the upper and lower bridge arm sub-modules does not remain constant, but a voltage spike appears near the switching moment. At the same time, the actual MMC bridge arm may have several to dozens of sub-modules, and each sub-module may have two voltage spikes in one triangular wave cycle, resulting in high-frequency harmonics of the DC voltage.

[0005] In existing research, there is no evaluation method for DC high-frequency harmonics caused by the discretization of triangular waves in MMC carrier phase-shift modulation, which makes it difficult to effectively evaluate such high-frequency harmonics from a theoretical level; at the same time, there is no elimination method from a hardware level, which makes it difficult to effectively eliminate such high-frequency harmonics. Summary of the invention

[0006] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation.

[0007] The technical solution of the present invention is: a method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation, wherein the evaluation method comprises the following steps:

[0008] A. Calculate the AC voltage on the valve side;

[0009] B. Calculate the bridge arm reference wave;

[0010] C. Calculate the multiplied bridge arm reference wave;

[0011] D. Generate continuous carrier;

[0012] E. Generate synchronous discrete carrier;

[0013] F. Calculate the discrete output voltages of the upper and lower bridge arms;

[0014] G. Perform broadband fast Fourier transform to obtain high-frequency harmonic distribution.

[0015] Furthermore, the elimination method and the specific process are as follows:

[0016] First, the triangle wave of the lower bridge arm is obtained based on the evaluation method;

[0017] Then, add a quantization unit to the triangle wave of the lower bridge arm;

[0018] Finally, the elimination is completed by raising the triangle wave of the lower bridge arm through the quantization unit.

[0019] Furthermore, step A calculates the valve side AC voltage, and the specific process is as follows:

[0020] First, the rated AC voltage, AC current, and AC connection reactor data are obtained;

[0021] Then, the time domain expression of the valve-side AC voltage of the MMC is calculated;

[0022] Finally, the valve side AC voltage is obtained.

[0023] Furthermore, step B calculates the bridge arm reference wave, and the specific process is as follows:

[0024] First, obtain the valve side AC voltage and DC rated voltage;

[0025] Then, based on the above parameters, the bridge arm reference wave expression is established and the bridge arm reference wave is calculated.

[0026] Furthermore, step C calculates the multiplied bridge arm reference wave, and the specific process is as follows:

[0027] First, assume that the digital circuit generates a triangle wave that varies from 0 to k;

[0028] Then, the bridge arm voltage is amplified to 0~k by multiplication.

[0029] Furthermore, step D generates a continuous carrier wave, and the specific process is as follows:

[0030] First, set the sampling rate to no less than 50MHz;

[0031] Then, a nearly continuous carrier wave is generated;

[0032] Finally, the corresponding carrier expression of the jth submodule of the upper and lower bridge arms is obtained.

[0033] Furthermore, step E generates a synchronous discrete carrier wave, and the specific process is as follows:

[0034] First, obtain the continuous carrier generated in step D;

[0035] Then, based on the continuous carrier, the synchronized discrete carrier is calculated.

[0036] Furthermore, step F calculates the discrete output voltages of the upper and lower bridge arms, and the specific process is as follows:

[0037] First, build the expression of the comparator;

[0038] Then, the output voltage of the submodules at the same position of the upper and lower bridge arms is obtained through a comparator;

[0039] Finally, calculate the output voltage of the upper and lower bridge arms.

[0040] Furthermore, step G performs broadband fast Fourier transform to obtain high-frequency harmonic distribution. The specific process is as follows:

[0041] Perform fast Fourier transform on the DC voltage to obtain the high-frequency harmonic distribution.

[0042] The beneficial effects of the present invention are as follows:

[0043] The evaluation method of the present invention takes into account factors such as different frequencies, ranges, and quantization units, and quickly evaluates the frequency band and content of direct current high-frequency harmonics caused by the discretization of triangular waves.

[0044] The high-frequency harmonic elimination method of the present invention can simply and efficiently eliminate direct current high-frequency harmonics caused by the discretization of triangular waves. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a schematic diagram of the asynchronism of the switching of the upper and lower bridge arms caused by the triangular wave generated by the digital controller in the background technology of the present invention;

[0046] Figure 2 It is a flow chart of the DC high frequency harmonic theoretical evaluation method in the present invention. DETAILED DESCRIPTION

[0047] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and embodiments:

[0048] like Figure 1 to Figure 2 As shown, a method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation, the evaluation method includes the following steps:

[0049] A. Calculate the AC voltage on the valve side;

[0050] B. Calculate the bridge arm reference wave;

[0051] C. Calculate the multiplied bridge arm reference wave;

[0052] D. Generate continuous carrier;

[0053] E. Generate synchronous discrete carrier;

[0054] F. Calculate the discrete output voltages of the upper and lower bridge arms;

[0055] G. Perform broadband fast Fourier transform to obtain high-frequency harmonic distribution.

[0056] The specific process of its elimination is as follows:

[0057] First, the triangle wave of the lower bridge arm is obtained based on the evaluation method;

[0058] Then, add a quantization unit to the triangle wave of the lower bridge arm;

[0059] Finally, the elimination is completed by raising the triangle wave of the lower bridge arm through the quantization unit.

[0060] Step A calculates the valve side AC voltage. The specific process is as follows:

[0061] First, the rated AC voltage, AC current, and AC connection reactor data are obtained;

[0062] Then, the time domain expression of the valve-side AC voltage of the MMC is calculated;

[0063] Finally, the valve side AC voltage is obtained.

[0064] Step B calculates the bridge arm reference wave, and the specific process is as follows:

[0065] First, obtain the valve side AC voltage and DC rated voltage;

[0066] Then, based on the above parameters, the bridge arm reference wave expression is established and the bridge arm reference wave is calculated.

[0067] Step C calculates the multiplied bridge arm reference wave, and the specific process is as follows:

[0068] First, assume that the digital circuit generates a triangle wave that varies from 0 to k;

[0069] Then, the bridge arm voltage is amplified to 0~k by multiplication.

[0070] Step D generates a continuous carrier wave, and the specific process is as follows:

[0071] First, set the sampling rate to no less than 50MHz;

[0072] Then, a nearly continuous carrier wave is generated;

[0073] Finally, the corresponding carrier expression of the jth submodule of the upper and lower bridge arms is obtained.

[0074] Step E generates a synchronous discrete carrier wave, and the specific process is as follows:

[0075] First, obtain the continuous carrier generated in step D;

[0076] Then, based on the continuous carrier, the synchronized discrete carrier is calculated.

[0077] Step F calculates the discrete output voltages of the upper and lower bridge arms. The specific process is as follows:

[0078] First, build the expression of the comparator;

[0079] Then, the output voltage of the submodules at the same position of the upper and lower bridge arms is obtained through a comparator;

[0080] Finally, calculate the output voltage of the upper and lower bridge arms.

[0081] Step G performs broadband fast Fourier transform to obtain high-frequency harmonic distribution. The specific process is as follows:

[0082] Perform fast Fourier transform on the DC voltage to obtain the high-frequency harmonic distribution.

[0083] The evaluation method of the present invention aims to quickly evaluate the frequency range and specific content of high-frequency harmonics caused by different situations under different frequencies, ranges, quantitative units and other factors in a computer.

[0084] The elimination method of the present invention aims to eliminate the error caused by the triangular wave quantization process in an actual digital controller.

[0085] Embodiment 1

[0086] A method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation, the evaluation method comprising the following steps:

[0087] A. Calculate the AC voltage on the valve side

[0088] According to the rated AC voltage, AC current, AC connection reactor and other parameters, the time domain expression of the valve side AC voltage of the MMC is calculated as follows:

[0089]

[0090] Among them, u x-valv Indicates the valve side AC voltage of phase x;

[0091] u x represents the AC voltage on the grid side of phase x, where x = a, b, c;

[0092] L ac Indicates the equivalent connection inductance on the AC side;

[0093] i x It represents the alternating current of phase x, t is the time, and d is the differential sign.

[0094] u x and i x The expression is

[0095]

[0096] B. Calculate the bridge arm reference wave

[0097] According to the valve side AC voltage and DC rated voltage, calculate the bridge arm reference wave according to the following formula:

[0098]

[0099] C. Calculate the multiplied bridge arm reference wave

[0100] Assume that the digital circuit generates a triangle wave that changes from 0 to k, and amplifies the bridge arm voltage to 0~k by multiplication, that is:

[0101]

[0102] D. Generate continuous carrier

[0103] In the computer, the sampling rate is not less than 50MHz, and an approximately continuous carrier is generated in the computer, wherein the corresponding carrier relationship of the j-th submodule of the upper and lower bridge arms is:

[0104] carr nj =k-carr pj (6)

[0105] Among them, carr nj is the continuous carrier corresponding to the jth submodule of the lower bridge arm;

[0106] carr pjis the continuous carrier corresponding to the jth submodule of the upper bridge arm, and the carrier variation range of the upper and lower bridge arms is between 0 and k.

[0107] E. Generate synchronous discrete carrier

[0108] Based on the continuous carrier in step D, the synchronous discrete carrier is calculated as follows:

[0109]

[0110] Among them, carr' pj and carr' nj They are the discretized triangle waves respectively;

[0111] floor(·) is the floor function.

[0112] F. Calculate the discrete output voltage of the upper and lower bridge arms

[0113] First, the output voltage of the upper and lower bridge arms at the same position of the sub-module can be obtained by the comparator, and the calculation

[0114] The method is:

[0115]

[0116] Then, calculate the output voltage of the upper and lower bridge arms using the following method:

[0117]

[0118] G. Broadband Fast Fourier Transform.

[0119] Perform fast Fourier transform on the DC voltage to obtain the high-frequency harmonic distribution.

[0120] A method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation, the elimination method comprising the following steps:

[0121] In order to eliminate the DC harmonics generated by the quantization process, a method is proposed to add a quantization unit to the triangle wave of the lower bridge arm, that is:

[0122]

[0123] Thereby raising the triangle wave of the lower bridge arm.

[0124] The evaluation method of the present invention takes into account factors such as different frequencies, ranges, and quantization units, and quickly evaluates the frequency band and content of direct current high-frequency harmonics caused by the discretization of triangular waves.

[0125] The high-frequency harmonic elimination method of the present invention can simply and efficiently eliminate direct current high-frequency harmonics caused by the discretization of triangular waves.

Claims

1. A method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation, characterized in that: The evaluation method includes the following steps: A. Calculate the AC voltage on the valve side; B. Calculate the bridge arm reference wave; C. Calculate the multiplied bridge arm reference wave; D. Generate continuous carrier; E. Generate synchronous discrete carrier; F. Calculate the discrete output voltages of the upper and lower bridge arms; G. Perform broadband fast Fourier transform to obtain high-frequency harmonic distribution.

2. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: The specific process of its elimination is as follows: First, the triangle wave of the lower bridge arm is obtained based on the evaluation method; Then, add a quantization unit to the triangle wave of the lower bridge arm; Finally, the elimination is completed by raising the triangle wave of the lower bridge arm through the quantization unit.

3. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step A calculates the valve side AC voltage. The specific process is as follows: First, the rated AC voltage, AC current, and AC connection reactor data are obtained; Then, the time domain expression of the valve-side AC voltage of the MMC is calculated; Finally, the valve side AC voltage is obtained.

4. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step B calculates the bridge arm reference wave, and the specific process is as follows: First, obtain the valve side AC voltage and DC rated voltage; Then, based on the above parameters, the bridge arm reference wave expression is established and the bridge arm reference wave is calculated.

5. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step C calculates the multiplied bridge arm reference wave, and the specific process is as follows: First, suppose the digital circuit generates k Varying triangle waves; Then, the bridge arm voltage is amplified to 0~ by multiplication. k .

6. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step D generates a continuous carrier wave, and the specific process is as follows: First, set the sampling rate to no less than 50MHz; Then, a nearly continuous carrier wave is generated; Finally, we get the upper and lower bridge arms j The corresponding carrier expression of each submodule.

7. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step E generates a synchronous discrete carrier wave, and the specific process is as follows: First, obtain the continuous carrier generated in step D; Then, based on the continuous carrier, the synchronized discrete carrier is calculated.

8. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step F calculates the discrete output voltages of the upper and lower bridge arms. The specific process is as follows: First, build the expression of the comparator; Then, the output voltage of the submodules at the same position of the upper and lower bridge arms is obtained through a comparator; Finally, calculate the output voltage of the upper and lower bridge arms.

9. The method for evaluating and eliminating high-frequency harmonics of MMC carrier phase-shift modulation according to claim 1, characterized in that: Step G performs broadband fast Fourier transform to obtain high-frequency harmonic distribution. The specific process is as follows: Perform fast Fourier transform on the DC voltage to obtain the high-frequency harmonic distribution.

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