Accurate evaluation method for harmonic coupling degree of output current of LCL grid-connected converter

Abstract

The invention provides a precise evaluation method for the harmonic coupling degree of the output current of an LCL grid-connected converter, and the method comprises the steps: deducing a double Fourier series decomposition expression of the voltage of an output port of the converter, and obtaining an amplitude expression of the voltage harmonic of the output port; substituting the direct current side voltage and the typical modulation degree into an amplitude expression to calculate a fundamental wave amplitude; substituting the fundamental wave amplitude into a preset inequality to obtain a corresponding target frequency when the output port voltage sideband harmonic amplitude is attenuated to a smaller value; and obtaining a harmonic coupling degree evaluation result of the converter according to the target frequency. According to the method, the double Fourier series decomposition theory is combined, the influence of the modulation mode, the direct-current side voltage and other factors on the harmonic coupling degree is considered, accurate evaluation of the harmonic coupling degree of the LCL grid-connected converter is achieved, an effective basis is provided for selection of design modes of system parameters of converters such as a controller and a filter, and the method is suitable for large-scale popularization and application. The method is of great significance in improving the quality of current output by the converter.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter. Background Art

[0002] The switching frequency of high-power LCL grid-connected converters used for photovoltaic and wind power is often between 1kHz and 5kHz. Although the span of this switching frequency is not large, there are obvious differences in the degree of harmonic coupling of LCL grid-connected converters at different switching frequencies. First, the situation where there is no harmonic coupling is that the sideband harmonics generated by modulation, the resonance peak of the LCL filter and the bandwidth of the current control loop occupy independent frequency zones, namely high, medium and low frequency zones, and there is no overlap. At this time, even if the controller, filter, etc. are designed independently, the converter can obtain good performance, and the design range of the filter resonance frequency is wide. Secondly, the situation where there is mild coupling is that the sideband harmonics generated by modulation only overlap with the resonance peak. At this time, the controller and filter can also be designed independently, but the design range of the filter resonance frequency is narrow and should be as low as possible under the condition of being higher than the bandwidth of the current control loop. Finally, the situation of severe coupling is that the sideband harmonics overlap with both the resonance peak and the control loop. At this time, the design range of the filter resonance frequency is narrower, and the controller and filter must be designed at the same time to enable the converter to obtain better performance. In short, the degree of harmonic coupling of LCL grid-connected converters is different, and the design method of the converter system is also different. Therefore, when the system power level, modulation method and switching frequency are determined, accurately evaluating the harmonic coupling degree of the grid-connected converter is the prerequisite for determining the optimal design method of the grid-connected converter system parameters.

[0003] The existing method roughly judges the output current harmonic coupling degree of the grid-connected converter based on experience only according to the system switching frequency, so the accuracy of the existing LCL grid-connected converter harmonic coupling degree evaluation method is not high. Summary of the invention

[0004] In order to solve the above problems existing in the prior art, the present invention provides a method for accurately evaluating the degree of harmonic coupling of the output current of an LCL grid-connected converter. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0005] An accurate evaluation method for the degree of harmonic coupling of output current of an LCL grid-connected converter includes:

[0006] S1, determining the topology of the power circuit of the converter and the modulation mode of the converter in the LCL grid-connected converter system;

[0007] S2, using the topological structure, derive the double Fourier series decomposition expression of the output port voltage of the converter under the modulation mode, and obtain the amplitude expression of the harmonic of the output port voltage of the converter;

[0008] S3, setting the current control loop bandwidth according to actual requirements, and determining the typical modulation system of the LCL grid-connected converter system according to different modulation systems and operation requirements of the LCL grid-connected converter system;

[0009] S4, substituting the obtained DC side voltage and typical modulation index of the LCL grid-connected converter system into the amplitude expression in S2, and calculating the fundamental wave amplitude of the output port of the AC converter;

[0010] S5, substituting the fundamental wave amplitude into a predetermined inequality to obtain a target frequency corresponding to when the voltage sideband harmonic amplitude of the output port of the converter is attenuated to a smaller value;

[0011] S6, obtaining a harmonic coupling degree evaluation result of the converter according to a mathematical relationship between the target frequency, the current control loop bandwidth, and the acquired switching frequency.

[0012] Optionally, the topology is a three-phase NPC three-level topology, and the modulation method is in-phase stacked carrier pulse width modulation.

[0013] Optionally, S2 includes:

[0014] S2-1, using the topological structure and according to the double Fourier series decomposition theory, derive the Fourier series expansion of the converter output line voltage under the modulation mode;

[0015] S2-2, using the Fourier series expansion of the converter output line voltage, derive the phase voltage u output from the converter output port n The amplitude of the high-frequency harmonic component h|u n_h |.

[0016] Optionally, the Fourier series expansion of the converter output line voltage in the modulation mode in S2-1 is expressed as:

[0017]

[0018] Where U dc is the DC side voltage of the LCL grid-connected converter system, M r is the modulation index, ω0 is the fundamental angular frequency, m is the carrier index variable, n is the fundamental index variable, ω sw is the switching angular frequency, J n is the first kind Bessel function, and its expression is:

[0019]

[0020] Where x is the order of the Bessel function;

[0021] Phase voltage u in S2-2n The amplitude of the high-frequency harmonic component h|u n_h |, which is represented by:

[0022]

[0023] In the formula, |u ab_h | for u ab The amplitude of the high-frequency harmonic component h.

[0024] Optionally, S3 includes:

[0025] S3-1, according to the different actual requirements of the LCL grid-connected converter system for rapidity, set the current control loop bandwidth f of the LCL grid-connected converter system b ;

[0026] S3-2, according to the operation requirements of the LCL grid-connected converter system and different modulation systems, determine the modulation range M of the LCL grid-connected converter system. r_1 ;

[0027] S3-3, select the middle value of the modulation range as the typical modulation.

[0028] Optional, current control loop bandwidth f b Different values ​​correspond to different actual requirements for speed.

[0029] Optionally, the modulation range has different modulation ranges under different modulation systems;

[0030] If the LCL grid-connected converter system works under low modulation, the modulation range M r_1 =[0.1,0.6), typical modulation index M r_t Take 0.35; if the LCL grid-connected converter system works under the conventional modulation index, the modulation index range is M r_1 =[0.6,1), typical modulation index M r_t Take 0.8; if the LCL grid-connected converter system works under over-modulation, the modulation range is M r_1 =[1,1.5), typical modulation index M r_t Take 1.25.

[0031] Optionally, S4 includes:

[0032] S4-1, obtain the DC side voltage U of the LCL grid-connected converter system dc and the switching frequency f sw ;

[0033] S4-2, the DC side voltage U dc and typical modulation M r_tSubstitute the amplitude expression in S2 to calculate the harmonic amplitude u at the output port of the AC converter. n_ht ;

[0034] S4-3, according to the output port harmonic amplitude u n_ht , calculate the fundamental amplitude u of the output port voltage of the AC converter n_1 .

[0035] Optionally, S5 includes:

[0036] The fundamental amplitude u n_1 Substitute the predetermined inequality |u n_ht |≤0.2%u n_1 The target frequency f corresponding to the voltage sideband harmonic amplitude of the converter output port attenuates to a smaller value is obtained. a .

[0037] Optionally, S6 includes:

[0038] S6-1, determine the target frequency f a Is it less than (1 / 2)f sw , if not, outputting a first evaluation result that there is no harmonic coupling in the LCL grid-connected converter system;

[0039] S6-2, if the target frequency f a Less than (1 / 2)f sw , then determine the target frequency f a Is it less than the current control loop bandwidth f b , if not, outputting a second evaluation result that the LCL grid-connected converter system has mild harmonic coupling;

[0040] S6-3, if the target frequency f a Smaller than the current control loop bandwidth f b , then the third evaluation result that the LCL grid-connected converter system has severe harmonic coupling is output.

[0041] Beneficial effects:

[0042] The present invention provides a precise evaluation method for the harmonic coupling degree of the output current of an LCL grid-connected converter. The method derives a double Fourier series decomposition expression of the output port voltage of the converter under a modulation mode to obtain an amplitude expression of the voltage harmonics of the output port of the converter; then, the current control loop bandwidth is set according to actual needs, and the typical modulation degree of the LCL grid-connected converter system is determined according to different modulation systems and operation requirements of the LCL grid-connected converter system; the DC side voltage and the typical modulation degree of the LCL grid-connected converter system obtained are substituted into the amplitude expression to calculate the fundamental amplitude of the output port of the AC converter; the fundamental amplitude is substituted into a predetermined inequality to obtain a target frequency corresponding to when the amplitude of the voltage sideband harmonics of the output port of the converter is attenuated to a smaller value; and the harmonic coupling degree evaluation result of the converter is obtained according to the mathematical relationship between the target frequency, the current control loop bandwidth, and the obtained switching frequency. The present invention combines the double Fourier series decomposition theory and takes into account the influence of factors such as modulation mode and DC side voltage on the degree of harmonic coupling. It thus achieves an accurate evaluation of the degree of harmonic coupling of the LCL grid-connected converter, thereby providing an effective basis for the design selection of converter system parameters such as controllers and filters, which is of great significance to the improvement of the output current quality of the converter.

[0043] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of a method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter provided by the present invention;

[0045] Figure 2 It is a detailed schematic diagram of a method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter provided by the present invention;

[0046] Figure 3 It is a schematic diagram of a three-phase NPC three-level topology structure provided by the present invention;

[0047] Figure 4 It is a schematic diagram of the in-phase stacked carrier pulse width modulation of the NPC three-level converter provided by the present invention;

[0048] Figure 5 is a schematic diagram of the distribution and percentage of harmonics of the output current of the converter under the first parameter condition provided by the present invention;

[0049] Figure 6 It is a schematic diagram of the distribution and percentage of harmonics of the output current of the converter under the second parameter condition provided by the present invention. DETAILED DESCRIPTION

[0050] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0051] The technical concept of the present invention is as follows:

[0052] In a current-controlled LCL grid-connected converter, hereinafter referred to as an AC converter, the current control loop bandwidth f is usually set to b The area below is considered the low frequency region; the resonant frequency of LCL filters is often between f b and f sw / 2, this is the intermediate frequency region, where f sw is the switching frequency; and f sw / 2 and above is occupied by the sideband harmonics generated by the modulation, which is the high frequency area. When the switching frequency is high, the control loop, LCL filter resonance peak and sideband harmonics basically do not affect each other and occupy independent frequency bands. This situation is ideal, and the design of controller, filter and other parameters is very easy. However, when the switching frequency drops to a certain level, the above ideal state becomes increasingly difficult to achieve. This is because the sideband harmonics generated by the modulation that are lower than the switching frequency enter the LCL resonance peak area and even the control loop area, resulting in harmonic coupling. It can be seen that the sideband harmonics in the output voltage of the converter that are lower than the switching frequency can be effectively coupled as long as they enter the intermediate frequency area (f sw / 2), then it can be said that the converter system will not have harmonic coupling; otherwise, the system is very likely to have mild harmonic coupling in the intermediate frequency region; but if the sideband harmonics in the converter output voltage that are lower than the switching frequency enter the current control loop bandwidth f b If the sideband harmonics of the output voltage of the converter have not been attenuated to a very small amplitude before the frequency drops to f, then the converter is very likely to have severe coupling in the medium frequency and low frequency regions. sw / 2 and f b The previously attenuated smaller amplitude is used as a basis for judgment, thereby estimating the output current harmonic coupling degree of the LCL grid-connected converter.

[0053] Combination Figure 1 and Figure 2 The present invention provides a method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter, including:

[0054] S1, determining the topology of the power circuit of the converter and the modulation mode of the converter in the LCL grid-connected converter system;

[0055] The topological structure that can be adopted in the practice of the present invention is a three-phase NPC three-level topological structure, and the modulation method is an in-phase stacked carrier pulse width modulation.

[0056] S2, using the topological structure, derive the double Fourier series decomposition expression of the output port voltage of the converter under the modulation mode, and obtain the amplitude expression of the harmonic of the output port voltage of the converter;

[0057] S3, setting the current control loop bandwidth according to actual requirements, and determining the typical modulation system of the LCL grid-connected converter system according to different modulation systems and operation requirements of the LCL grid-connected converter system;

[0058] Where, the current control loop bandwidth f b Different values ​​correspond to different actual requirements for rapidity. Usually, the current control loop bandwidth f b Often set to (1 / 10~1 / 5)f sw If the actual demand has a higher requirement on the converter speed, then f b Set to (1 / 5)f sw ; If the actual demand does not place too high a requirement on the speed of the converter, then f b Set to (1 / 8)f sw .

[0059] S5: According to the converter modulation mode and operation requirements, determine the modulation range of the converter system, and then take the middle value as the typical modulation M. r_t ;

[0060] Modulation range There are different modulation ranges under different modulation systems; among them, if the LCL grid-connected converter system works under a low modulation system, the modulation range M r_1 =[0.1,0.6), typical modulation index M r_t Take 0.35; if the LCL grid-connected converter system works under the conventional modulation index, the modulation index range is M r_1 =[0.6,1), typical modulation index M r_t Take 0.8; if the LCL grid-connected converter system works under over-modulation, the modulation range is M r_1 =[1,1.5), typical modulation index M r_t Take 1.25.

[0061] S4, substituting the obtained DC side voltage and typical modulation index of the LCL grid-connected converter system into the amplitude expression in S2, and calculating the fundamental wave amplitude of the output port of the AC converter;

[0062] S5, substituting the fundamental wave amplitude into a predetermined inequality to obtain a target frequency corresponding to when the voltage sideband harmonic amplitude of the output port of the converter is attenuated to a smaller value;

[0063] In this step, the fundamental amplitude u n_1 Substitute the predetermined inequality |u n_ht |≤0.2%un_1 The target frequency f corresponding to the voltage sideband harmonic amplitude of the converter output port attenuates to a smaller value is obtained. a .

[0064] S6, according to the mathematical relationship between the target frequency, the current control loop bandwidth and the switching frequency, the harmonic coupling degree evaluation result of the converter is obtained.

[0065] Since the existing methods lack consideration of factors such as modulation mode and DC side voltage, and lack accurate and reasonable calculation methods. The present invention provides an accurate evaluation method for the harmonic coupling degree of the output current of an LCL grid-connected converter, which derives the double Fourier series decomposition expression of the output port voltage of the converter under the modulation mode, and obtains the amplitude expression of the output port voltage harmonic of the converter; then the current control loop bandwidth is set according to actual needs, and the typical modulation degree of the LCL grid-connected converter system is determined according to different modulation systems and operation requirements of the LCL grid-connected converter system; the DC side voltage and typical modulation degree of the LCL grid-connected converter system are substituted into the amplitude expression to calculate the fundamental amplitude of the output port of the AC converter; the fundamental amplitude is substituted into the predetermined inequality to obtain the target frequency corresponding to the attenuation of the harmonic amplitude of the output port voltage sideband of the converter to a smaller value; according to the mathematical relationship between the target frequency, the current control loop bandwidth, and the obtained switching frequency, the harmonic coupling degree evaluation result of the converter is obtained. The present invention combines the double Fourier series decomposition theory and takes into account the influence of factors such as modulation mode and DC side voltage on the degree of harmonic coupling. It thus achieves an accurate evaluation of the degree of harmonic coupling of the LCL grid-connected converter, thereby providing an effective basis for the design selection of converter system parameters such as controllers and filters, which is of great significance to the improvement of the output current quality of the converter.

[0066] In an optional embodiment of the present invention, S2 includes:

[0067] S2-1, using the topological structure and according to the double Fourier series decomposition theory, the Fourier series expansion of the converter output line voltage under the modulation mode is derived, which is expressed as:

[0068]

[0069] Where U is the DC side voltage of the LCL grid-connected converter system, M r is the modulation index, ω0 is the fundamental angular frequency, m is the carrier index variable, n is the fundamental index variable, ω sw is the switching angular frequency, J n is the first kind Bessel function, and its expression is:

[0070]

[0071] Wherein, x is the order of the Bessel function, which can be any real number. In the present invention, x is taken as 20.

[0072] It is worth noting that the double Fourier series decomposition theory is described in document 1 (Mouton T, Putzeys B. Understanding the PWM nonlinearity: single-sided modulation [J]. IEEE Transactions on Power Electronics, 2012, 27 (4): 2116-2128.) The first kind of Bessel function is described in document 2 (Gross F B. New approximations to J / sub 0 / and J / sub 1 / bessel functions [J]. IEEE Transactions on Antennas and Propagation, 1995, 43 (8): 904-907).

[0073] S2-2, using the Fourier series expansion of the converter output line voltage, derive the phase voltage u output from the converter output port n The amplitude of the high-frequency harmonic component h|u n_h |, which is represented by:

[0074]

[0075] In the formula, |u ab_h | for u ab The amplitude of the high-frequency harmonic component h.

[0076] In an optional embodiment of the present invention, S3 includes:

[0077] S3-1, according to the different actual requirements of the LCL grid-connected converter system for rapidity, set the current control loop bandwidth f of the LCL grid-connected converter system b ;

[0078] S3-2, according to the operation requirements of the LCL grid-connected converter system and different modulation systems, determine the modulation range M of the LCL grid-connected converter system. r_1 ;

[0079] S3-3, select the middle value of the modulation range as the typical modulation.

[0080] In an optional embodiment of the present invention, S4 includes:

[0081] S4-1, obtain the DC side voltage U of the LCL grid-connected converter system dc and the switching frequency fsw ;

[0082] S4-2, the DC side voltage U dc and typical modulation M r_t Substitute the amplitude expression in S2 to calculate the harmonic amplitude u at the output port of the AC converter. n_ht ;

[0083] S4-3, according to the output port harmonic amplitude u n_ht , calculate the fundamental amplitude u of the output port voltage of the AC converter n_1 .

[0084] In an optional embodiment of the present invention, S6 includes:

[0085] S6-1, determine the target frequency f a Is it less than (1 / 2)f sw , if not, outputting a first evaluation result that there is no harmonic coupling in the LCL grid-connected converter system;

[0086] In addition to outputting the first evaluation result, this step also outputs the optimal design method of system parameters such as the controller and the filter: the system parameters such as the controller and the filter can be designed independently, and the design range of the LCL filter resonant frequency is relatively large.

[0087] S6-2, if the target frequency f a Less than (1 / 2)f sw , then determine the target frequency f a Is it less than the current control loop bandwidth f b , if not, outputting a second evaluation result that the LCL grid-connected converter system has mild harmonic coupling;

[0088] In addition to outputting the second evaluation result, this step also outputs the optimal design method for system parameters such as the controller and filter: system parameters such as the controller and filter can be designed independently, but the design range of the LCL filter resonant frequency is narrow and should be as low as possible while being higher than the control bandwidth.

[0089] S6-3, if the target frequency f a Smaller than the current control loop bandwidth f b , then the third evaluation result that the LCL grid-connected converter system has severe harmonic coupling is output.

[0090] In addition to outputting the third evaluation result, this step also outputs the optimal design method for system parameters such as the controller and filter: the filter resonant frequency can be designed in a narrower range, and the controller and filter must be designed at the same time.

[0091] Here, an example is given to illustrate the accuracy of the method proposed in the present invention. Taking a three-phase NPC three-level LCL grid-connected converter using in-phase stacked carrier pulse width modulation as an example, Figure 3 The three-phase NPC three-level topology is shown, where Udc = 700V and the switching frequency f sw =2.2kHz, requiring the control system to have a faster adjustment speed and the converter to work under conventional modulation. Figure 4 As shown. Figure 1 The steps describe in detail the evaluation process of the harmonic coupling of the converter output current in this embodiment.

[0092] First, determine that the power circuit topology of the LCL grid-connected converter is a three-phase NPC three-level topology, and the modulation method is in-phase stacked carrier pulse width modulation; derive the double Fourier series decomposition expression of the converter output port voltage and the port voltage amplitude expression |u n_h |; According to the double Fourier series decomposition theory, the output line voltage u of the converter under this modulation mode can be derived ab The Fourier series expansion of is shown in equation (1). Determine the DC side voltage U of the LCL grid-connected converter system dc =700V, switching frequency f sw =2.2kHz. Since the control system is required to have a faster adjustment speed, f b Set to (1 / 5)f sw , that is, f b =440Hz. Since the converter works under the conventional modulation index, the modulation index range is M r_n =[0.6,1), so the typical modulation index M r_t is 0.8. dc =700V and M r =M r_t =0.8 is substituted into the expression of the voltage harmonic amplitude of the converter output port derived from S2, and recorded as u n_ht , and calculate the fundamental amplitude u of the converter port voltage in this embodiment n_1 =U dc M r_t 2=280V. Solve the inequality |u n_ht |≤0.2%u n_1 , that is, solving the inequality |u n_ht |≤0.56. Since the sideband harmonics that may be coupled with the filter resonance peak and the control loop are the sideband harmonics at 1 times the switching frequency, the carrier index variable m=1, so n≥26 can be solved. This shows that the output phase voltage sideband harmonics of the converter below the switching frequency are at (f sw-nf0) are all less than 0.56V (where n≥26), that is, the output phase voltage sideband harmonics of the converter are from (f sw -nf0) means that it has decayed to a smaller value since 900Hz, that is, f a =900Hz. Since f a Less than (1 / 2)f sw , that is, f a Less than 1100Hz, so we judge f a Is it greater than f b , due to f a Greater than f b , so the output converter harmonic coupling degree evaluation results are: LCL grid-connected converter has slight coupling, and the best design method of output controller, filter and other system parameters: system parameters such as controller and filter can be designed independently, but the design range of LCL filter resonant frequency is narrow, and should be as low as possible while being higher than the control bandwidth.

[0093] The following is a simulation verification of this embodiment.

[0094] First, build in Matlab / Simulink Figure 3 The topology shown in FIG. 1 and the parameters in this embodiment are used to obtain the harmonic distribution and amplitude of the converter output line voltage uab, which is divided by The harmonic distribution and amplitude of the converter output phase voltage can be obtained as follows: Figure 5 As shown. Observing the magnitude of the harmonic amplitude, it can be seen that at 900Hz, the sideband harmonic is already less than 0.56V, which means that the f a The solution calculation is correct.

[0095] Secondly, the LCL grid-connected converter system in this embodiment is built in Matlab / Simulink, using proportional-integral + active damping control, where the proportional coefficient is 0.9, the integral coefficient is 900, and the active damping coefficient is 0.9; and the effective value of the grid line voltage is set to 315V, and the output power is 120kW; the LCL filter parameters are 0.6mH on the converter side, 0.3mH on the grid side, and 160CF on the capacitor, so the filter resonant frequency is 889Hz. The converter output current harmonic distribution and percentage are as follows Figure 6 As shown in the figure, the blue arrow points to the frequency corresponding to the sideband harmonics generated by the modulation, the orange arrow points to the filter resonant frequency, and the dotted line frames the coupled harmonics of the sideband harmonics and the resonant peak. Figure 6 It can be seen that the sideband harmonics generated by the modulation are coupled with the filter resonance peak at 800Hz~950Hz (i.e. 16~19th), causing the 16th and 18th sideband harmonics to be amplified; however, the sideband harmonics generated by the modulation are not coupled with the control loop. Therefore, Figure 6 This shows that the system in this embodiment has slight coupling. This simulation result is consistent with the result obtained by the evaluation method proposed in the present invention, which shows that the evaluation method for the harmonic coupling degree of the converter output current proposed in the present invention is accurate. Figure 6 It can be seen that this mild harmonic coupling significantly increases the harmonic amplitude near the resonant frequency, affecting the quality of the converter output current. The LCL filter resonant frequency should be as low as possible while being higher than the control bandwidth to improve the converter output current quality and performance, thereby achieving the optimal converter design.

[0096] Although the present application is described herein in conjunction with various embodiments, in the process of implementing the claimed application, those skilled in the art may understand and implement other variations of the disclosed embodiments by reviewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude a plurality of components or steps.

[0097] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter, characterized in that: include: S1, determining a topological structure of a power circuit of a converter in an LCL grid-connected converter system and a modulation mode of the converter; S2, using the topological structure, deriving a double Fourier series decomposition expression of the output port voltage of the converter under the modulation mode, and obtaining an amplitude expression of the output port voltage harmonic of the converter; S3, setting the current control loop bandwidth according to actual requirements, and determining the typical modulation degree of the LCL grid-connected converter system according to different modulation degrees and operation requirements of the LCL grid-connected converter system; S4, substituting the obtained DC side voltage of the LCL grid-connected converter system and the typical modulation index into the amplitude expression in S2 to calculate the fundamental wave amplitude of the output port of the AC converter; S5, substituting the fundamental wave amplitude into a predetermined inequality to obtain a target frequency corresponding to when the voltage sideband harmonic amplitude of the output port of the converter is attenuated to a smaller value; S6, obtaining a harmonic coupling degree evaluation result of the converter according to a mathematical relationship between the target frequency, the current control loop bandwidth, and the acquired switching frequency.

2. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 1 is characterized in that: The topology structure is a three-phase NPC three-level topology structure, and the modulation method is an in-phase stacked carrier pulse width modulation.

3. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 1, characterized in that S2 include: S2-1, using the topological structure and according to the double Fourier series decomposition theory, deriving the Fourier series expansion of the output line voltage of the converter under the modulation mode; S2-2, using the Fourier series expansion of the output line voltage of the converter, derive the phase voltage u output from the output port of the converter n The amplitude of the high-frequency harmonic component h|u n_h |.

4. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 3 is characterized in that: The Fourier series expansion of the output line voltage of the converter under the modulation mode in S2-1 is expressed as: Where U dc is the DC side voltage of the LCL grid-connected converter system, M r is the modulation index, ω0 is the fundamental angular frequency, m is the carrier index variable, n is the fundamental index variable, ω sw is the switching angular frequency, J n is the first kind Bessel function, and its expression is: Where x is the order of the Bessel function; Phase voltage u in S2-2 n The amplitude of the high-frequency harmonic component h|u n_h |, which is represented by: In the formula, |u ab_h | for u ab The amplitude of the high-frequency harmonic component h.

5. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 1, characterized in that S3 include: S3-1, according to the different actual requirements for the rapidity of the LCL grid-connected converter system, set the current control loop bandwidth f of the LCL grid-connected converter system b ; S3-2, according to the operation requirements of the LCL grid-connected converter system and different modulation systems, determine the modulation range M of the LCL grid-connected converter system r_1 ; S3-3, selecting the middle value of the modulation range as the typical modulation.

6. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 5, characterized in that: The current control loop bandwidth f b Different values ​​correspond to different actual requirements for speed.

7. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 5, characterized in that: The modulation range has different modulation ranges under different modulation systems; If the LCL grid-connected converter system operates under a low modulation regime, the modulation regime range M r_1 =[0.1,0.6), the typical modulation index M r_t Take 0.35; if the LCL grid-connected converter system operates under conventional modulation, the modulation range is M r_1 =[0.6,1), the typical modulation index M r_t Take 0.8; if the LCL grid-connected converter system works under over-modulation, the modulation range is M r_1 =[1,1.5), the typical modulation index M r_t Take 1.

25.

8. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 1, characterized in that S4 include: S4-1, obtain the DC side voltage U of the LCL grid-connected converter system dc and the switching frequency f sw ; S4-2, the DC side voltage U dc and typical modulation M r_t Substitute the amplitude expression in S2 to calculate the harmonic amplitude u at the output port of the AC converter. n_ht ; S4-3, according to the output port harmonic amplitude u n_ht , calculate the fundamental amplitude u of the output port voltage of the AC converter n_1 .

9. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 1, characterized in that S5 include: The fundamental amplitude u n_1 Substitute the predetermined inequality |u n_ht |≤0.2%u n_1 The target frequency f corresponding to the voltage sideband harmonic amplitude of the output port of the converter is obtained when it is attenuated to a smaller value. a .

10. The method for accurately evaluating the degree of harmonic coupling of output current of an LCL grid-connected converter according to claim 1, characterized in that S6 include: S6-1, determine the target frequency f a Is it less than (1 / 2)f sw , if not, outputting a first evaluation result that the LCL grid-connected converter system does not have harmonic coupling; S6-2, if the target frequency f a Less than (1 / 2)f sw , then determine the target frequency f a Is it less than the current control loop bandwidth f b , if not, outputting a second evaluation result that the LCL grid-connected converter system has mild harmonic coupling; S6-3, if the target frequency f a Smaller than the current control loop bandwidth f b , then a third evaluation result indicating that the LCL grid-connected converter system has severe harmonic coupling is output.

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