Optimized PWM (Pulse Width Modulation) method capable of completely eliminating zero-sequence circulating current of parallel converter

Abstract

The invention relates to the field of power electronic parallel converters, in particular to an optimized PWM (Pulse Width Modulation) method capable of completely eliminating zero-sequence circulating current of a parallel converter, and solves the problem that the optimized PWM method cannot completely solve the zero-sequence circulating current in the prior art. The method comprises the following steps: S1, discretizing to represent an optimized PWM waveform; s2, carrying out parallel converter zero-sequence circulating current modeling; s3, carrying out harmonic modeling on the parallel converter; and S4, establishing an optimization model and solving. According to the method, PWM waveform discretization expression is optimized, and the output level value is taken as a control variable, so that PWM output can be directly controlled and optimized conveniently, direct control over the common-mode voltage difference can be achieved conveniently, and then control over the zero-sequence circulating current is achieved; while eliminating low-order harmonics, the zero-sequence circulating current of the parallel converter is completely eliminated, and the operation efficiency of the parallel converter is remarkably improved.

Description

Technical Field

[0001] The invention relates to the field of power electronic parallel converters, in particular to an optimized PWM method capable of completely eliminating zero-sequence circulating current of parallel converters. Background Art

[0002] Parallel converters, which are technologies that connect two or more power electronic converters in parallel, can significantly increase the power level of the system at the same voltage level, thereby meeting the needs of high-power application scenarios. In recent years, with the growing demand for high power density and high efficiency in fields such as new energy power generation, electric vehicles, and industrial drives, parallel converter technology has been widely used. However, in actual operation, due to differences in sub-unit parameters, control delays, and inconsistent device characteristics, the output waveforms of different sub-units of the parallel converter may be inconsistent, resulting in zero-sequence circulating currents between sub-units.

[0003] Zero-sequence circulating current is a common-mode current flowing between the subunits of the parallel converter. It not only increases the conduction loss and switching loss of the switching device and reduces the overall efficiency of the system, but may also cause problems such as device overheating and electromagnetic interference. In severe cases, it may even affect the stability and reliability of the system. Therefore, it is necessary to comprehensively consider the characteristics of zero-sequence circulating current in the design of the modulation method to achieve efficient and stable operation of the parallel converter.

[0004] Optimized PWM is an offline pulse width modulation method with the advantages of strong harmonic suppression capability, low switching loss, and high system stability. It is especially suitable for scenarios with high requirements for efficiency and reliability. However, although the existing optimized PWM method has achieved certain results in suppressing zero-sequence circulating current, it still has limitations. Since the zero-sequence circulating current is mainly caused by triple frequency harmonics (such as 3rd, 9th, 15th, etc.), and the number of these harmonics is infinite, the optimized PWM method can only suppress some low-order harmonics through a limited switching angle α, and cannot completely eliminate all triple frequency harmonics, so it is impossible to completely eliminate zero-sequence circulating current.

[0005] In order to solve the above problems, a new optimized PWM method that can eliminate zero-sequence circulating current is urgently needed. Summary of the invention

[0006] The present invention proposes an optimized PWM method which can completely eliminate the zero-sequence circulating current of the parallel converter, thereby solving the problem that the optimized PWM in the prior art cannot completely solve the zero-sequence circulating current.

[0007] The technical solution of the present invention is implemented as follows: an optimized PWM method that can completely eliminate the zero-sequence circulating current of a parallel converter comprises the following steps: S1 discretization representation optimized PWM waveform: based on a specific sampling frequency, the continuous optimized PWM waveform is sampled, and the level value of each sampling point forms a discretized optimized PWM waveform, and is modeled, with the output level value as the variable to be optimized of the optimized PWM waveform; S2 parallel converter zero-sequence circulating current modeling: on the basis of S1, the difference in common-mode voltage of the parallel converter is modeled using the discrete output level value as a controllable variable, thereby achieving accurate modeling of the zero-sequence circulating current; S3 parallel converter harmonic modeling: based on the discrete optimization representation optimized PWM waveform method in step S1, the harmonic modeling is completed, that is, the fundamental and harmonic components of the output voltage are represented by the discrete output level value as a variable; S4 establishes and solves the optimized PWM model: based on the above modeling, an optimization model with the discrete output level value as the optimization variable is established and solved, so that the output waveform with the output level value as a variable meets the requirements of zero-sequence circulating current elimination and harmonic suppression.

[0008] As a specific technical solution, the output level value of the discrete sampling in step S1 is l i As a variable; the discretized optimized PWM waveform is used n Vector of dimensional variables l = [ l 1 , l 2 , …, l n ]express.

[0009] In a further technical solution, the common mode voltage difference can be expressed as: (1) in, V A1 to V C1 is the three-phase PWM output voltage of the first converter, V A2 to V C2 is the three-phase PWM output voltage of the second converter; based on the discretization representation of the optimized PWM waveform in step S1, the available variables l A1 to l C1 , l A2 to l C2 replace V A1 to VC1 , V A2 to V C2 ,Right now: (2) According to the working characteristics of the parallel converter, its zero-sequence circulating current can be expressed by discrete output levels: (3) in, L is the total inductance of the bridge arms of the parallel converter; l 1 , l 2 They are the actual level values ​​output by the first inverter and the second inverter respectively.

[0010] As a further technical solution, the optimized PWM model that can achieve zero-sequence circulating current and harmonic elimination in step S4 is: (4) Among them, the optimization goal is to minimize the output harmonics of the converter; k is the fundamental wave and target harmonic order; s 1 and s k They are the fundamental wave, each harmonic ( k Fourier coefficients of subharmonics); A is the fundamental amplitude; i ZSCC is the model of zero-sequence circulating current; x The maximum actual output level value of each converter; Constraint 1 is: constrain the output fundamental waves of the two converters to specific values; Constraint 2 is: to achieve zero-sequence circulating current elimination, where the optional values ​​of the two converter output level value variables are related to the number of levels of the parallel converters, where each converter is N Level converter, formula (4) x That is ( N -1) / 2.

[0011] The present invention discloses an optimized PWM method capable of completely eliminating zero-sequence circulating current of parallel converters, which has the following beneficial effects: 1. By optimizing the discrete representation of the PWM waveform and taking the output level value as the control variable, it is convenient to directly control the output of the optimized PWM, so as to realize the direct control of the common mode voltage difference, and then realize the control of the zero-sequence circulating current; 2. Optimized PWM waveform based on discretization representation Accurately model the zero-sequence circulating current and harmonics; 3. Propose an optimized PWM method that can eliminate the zero-sequence circulating current of parallel converters. The optimized PWM method can completely eliminate the zero-sequence circulating current of parallel converters while eliminating low-order harmonics, greatly improving the operating efficiency of parallel converters; BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0013] Figure 1 : Schematic diagram of the discretization representation of the optimized PWM waveform in the present invention; Figure 2 : Schematic diagram of simulation model results; DETAILED DESCRIPTION

[0014] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Specific implementation method 1 The optimized PWM method in this embodiment includes the following steps: S1 Discrete representation of optimized PWM waveform: based on a specific sampling frequency, the continuous optimized PWM waveform is sampled, and the level value of each sampling point forms a discrete optimized PWM waveform, and is modeled, and the traditional switch angle α is transformed as a variable to be optimized to an output level value as a variable to be optimized for the optimized PWM waveform; S2 Parallel converter zero-sequence circulating current modeling: on the basis of S1, the difference in common-mode voltage of the parallel converter is modeled using the discrete output level value as a controllable variable, thereby achieving accurate modeling of the zero-sequence circulating current; S3 Parallel converter harmonic modeling: based on the discrete optimization representation optimized PWM waveform method in step S1, the harmonic modeling is completed, that is, the fundamental and harmonic components of the output voltage are represented by the discrete output level value as a variable; S4 Establishment and solution of the optimized PWM model: based on the above modeling, an optimization model with the discrete output level value as the optimization variable is established and solved, so that the output waveform with the output level value as a variable meets the requirements of zero-sequence circulating current elimination and harmonic suppression.

[0016] In this embodiment, the output level value is used as a control variable through conversion variables to achieve control of the output level value, thereby achieving accurate modeling of the zero-sequence circulating current; on this basis, the zero-sequence circulating current modeling of the parallel converter and the harmonic modeling of the parallel converter are carried out to significantly improve their modeling accuracy; by establishing and solving an optimized PWM model, low-order harmonics are eliminated while completely eliminating the zero-sequence circulating current.

[0017] In the specific operation process, the optimized PWM waveform is discretized by taking the output level value as a variable. The optimization goal is to minimize the output harmonics of the converter. The constraints are set as follows: the output fundamental waves of the two converters are constrained to a specific value determined according to the actual engineering requirements, and the zero-sequence circulating current is eliminated; thereby achieving complete elimination of low-order harmonics and zero-sequence circulating current. Specific implementation method 2 Based on the specific implementation mode 1, Figure 1 As shown in the schematic diagram of the optimized PWM waveform discretization representation in the present invention, the output level value of the discrete sampling in step S1 is l i As a variable; the discrete sampling is sampled at a specific sampling frequency, that is, the angle between adjacent sampling points, or a fundamental wave period divided by the number of sampling points in the period; the optimized PWM waveform after discrete sampling is sampled with a n Vector of dimensional variables l = [ l 1 , l 2 , …, l n ] indicates that the optimized variable conversion of the optimized PWM waveform can be realized. The common-mode voltage difference of the parallel converter is further accurately modeled. The common-mode voltage difference refers to one-third of the sum of the three-phase PWM voltages. For the parallel converter, the zero-sequence circulating current depends on the common-mode voltage difference, that is: , accurately modeling the common-mode voltage difference is conducive to accurately modeling the zero-sequence circulating current; a common-mode voltage difference representation is disclosed: (1) in, V A1 to V C1 is the three-phase PWM output voltage of the first converter, V A2 to V C2 is the three-phase PWM output voltage of the second converter; based on the discretization representation of the optimized PWM waveform in step S1, the available variables l A1 to lC1 , l A2 to l C2 replace V A1 to V C1 , V A2 to V C2 ,Right now: (2) According to the working characteristics of the parallel converter, its zero-sequence circulating current can be expressed by discrete output levels: (3) in, L is the total inductance of the bridge arms of the parallel converter; l 1 , l 2 They are the actual level values ​​output by the first inverter and the second inverter respectively.

[0019] An optimized PWM model that can achieve zero-sequence circulating current and harmonic elimination is disclosed as: (4) Among them, the optimization goal is to minimize the output harmonics of the converter; k is the fundamental wave and target harmonic order; s 1 and s k They are the fundamental wave, each harmonic ( k Fourier coefficients of subharmonics); A is the fundamental amplitude; i ZSCC is the model of zero-sequence circulating current; x is the maximum actual output level value of each converter; Constraint 1 is: constrain the output fundamental waves of the two converters to a specific value; Constraint 2 is: achieve zero-sequence circulating current elimination, where the optional values ​​of the output level value variables of the two converters are related to the number of levels of the parallel converters, where each converter is N Level converter, formula (4) x That is ( N -1) / 2.

[0020] This embodiment specifically optimizes the discrete representation of PWM waveform, and discloses a finite common-mode voltage difference representation method and a PWM optimization model, establishes specific conditions for target optimization and constraints, and can use mature optimization solution algorithm packages such as YALMIP, Gurobi, etc. to solve the model, obtain the optimized PWM discrete output level value, and then achieve the elimination of zero-sequence circulating current and harmonics.

[0021] This embodiment can completely eliminate the zero-sequence circulating current while eliminating low-order harmonics, thereby significantly reducing the impact of the zero-sequence circulating current on the parallel converter, reducing the switching loss and conduction loss caused by the zero-sequence circulating current, and not only improving the operating efficiency of the parallel converter, but also improving the operating life and device reliability of the parallel converter. Figure 2 The result diagram of the simulation model shows that the zero-sequence circulating current in the parallel converter has been completely eliminated and the output current has a satisfactory waveform quality.

[0022] Of course, without departing from the spirit and essence of the invention, technicians familiar with the field should be able to make various corresponding changes and modifications based on the invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the invention.

Claims

1. An optimized PWM method for completely eliminating zero-sequence circulating current of parallel converters, characterized in that: The following steps are involved: S1 Discretization represents the optimized PWM waveform: based on a specific sampling frequency, the continuous optimized PWM waveform is sampled, and the level value of each sampling point forms a discrete optimized PWM waveform, which is modeled, and the output level value is used as the variable to be optimized for the optimized PWM waveform; S2 Parallel converter zero-sequence circulating current modeling: Based on S1, the difference in common-mode voltage of parallel converters is modeled using discrete output level values ​​as controllable variables, thereby achieving accurate modeling of zero-sequence circulating current; S3 parallel converter harmonic modeling: based on the discrete representation optimization PWM waveform method in step S1, the harmonic modeling is completed, that is, the fundamental and harmonic components of the output voltage are represented by using discrete output level values ​​as variables; S4 Establishing and solving an optimized PWM model: Based on the above modeling, an optimization model with discrete output level values ​​as optimization variables is established and solved, so that the output waveform with the output level value as a variable meets the requirements of zero-sequence circulating current elimination and harmonic suppression.

2. The optimized PWM method for completely eliminating zero-sequence circulating current of parallel converters according to claim 1 is characterized in that: The output level value of the discrete sampling in step S1 l i As a variable; the discretized optimized PWM waveform is used n Vector of dimensional variables l = [ l 1, l 2, …, l n ] indicates that the conversion of variables to be optimized for the optimized PWM waveform can be realized.

3. The optimized PWM method for completely eliminating zero-sequence circulating current of parallel converters according to claim 2 is characterized in that: The difference in common mode voltage can be expressed as: (1) in, V A1 to V C1 is the three-phase PWM output voltage of the first converter, V A2 to V C2 is the three-phase PWM output voltage of the second converter; based on the discretization representation of the optimized PWM waveform in step S1, the available variables l A1 to l C1 , l A2 to l C2 replace V A1 to V C1 , V A2 to V C2 ,Right now: (2) According to the working characteristics of the parallel converter, its zero-sequence circulating current can be expressed by discrete output levels: (3) in, L is the total inductance of the bridge arms of the parallel converter; l 1. l 2 are the actual level values ​​output by the first inverter and the second inverter respectively.

4. The optimized PWM method for completely eliminating zero-sequence circulating current of parallel converters according to claim 3 is characterized in that: The optimized PWM model that can achieve zero-sequence circulating current and harmonic elimination in step S4 is: (4) Among them, the optimization goal is to minimize the output harmonics of the converter; k is the fundamental wave and target harmonic order; s 1 and s k They are the fundamental wave, each harmonic ( k Fourier coefficients of subharmonics); A is the fundamental amplitude; i ZSCC is the model of zero-sequence circulating current; x The maximum actual output level value of each converter; Constraint 1 is: constrain the output fundamental waves of the two converters to specific values; Constraint 2 is: to achieve zero-sequence circulating current elimination, where the optional values ​​of the two converter output level value variables are related to the number of levels of the parallel converters, where each converter is N Level converter, formula (4) x That is ( N -1) / 2.

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