A harmonic current injection based offshore wind farm harmonic suppression method

Abstract

The application provides a harmonic suppression method for offshore wind farms based on harmonic current injection, comprising the following steps: S1, applying a harmonic signal to a d / q axis current control link of a wind turbine generator set grid-side converter control system; S2, determining the amplitude, phase and frequency of the injected harmonic current component in the d / q axis and the amplitude, phase and frequency of the corresponding generated harmonic current component in the phase coordinate system at the wind turbine port; S3, determining the transfer function model between the injected harmonic current component at the wind turbine port and the harmonic current component to be suppressed at the grid connection point, i.e. a plurality of groups of harmonic with different amplitudes and phases of the actively injected harmonic current component, calculating the observed harmonic current component amplitude and phase at the harmonic observation point, and then calculating the transfer function model; and S4, according to the amplitude and phase of the harmonic current component to be suppressed and the determined transfer function model, finally calculating the amplitude and phase of the harmonic current component to be injected. The application avoids increasing the equipment and affecting the layout of the onshore converter station equipment when the equipment is applied for suppression.

Description

TECHNICAL FIELD

[0001] The present application relates to the offshore wind power generation technical field, specifically relates to a kind of offshore wind farm harmonic suppression method based on harmonic current injection. BACKGROUND

[0002] Due to the characteristic of sea cable to ground capacitance, sea cable and system form resonance and amplify the tiny harmonic current component generated by wind turbine converter, which leads to the harmonic current amplitude exceeding at grid-connected point, and high-amplitude harmonic component can cause harm to equipment and affect the safe and stable operation of system. Active and passive devices can alleviate or even suppress the problem of harmonic amplitude exceeding in system to some extent, but they all need to apply primary equipment and the capacity of the device needed to suppress amplified harmonic component is large, which affects the equipment layout of onshore station of wind farm. Therefore, it is of great significance to use the suppression ability of wind turbine converter itself to reduce the amplitude of harmonic current component in system without applying additional primary equipment. SUMMARY

[0003] In view of the deficiencies in the prior art, the present application provides a kind of offshore wind farm harmonic suppression method based on harmonic current injection. The present application considers using the ability of wind turbine converter itself, by applying harmonic current component in secondary side d / q axis control system, to generate harmonic current component in system and neutralize the original existing harmonic current in system, thereby avoiding the increase of large-capacity equipment when suppressing by applying primary equipment and affecting the equipment layout of onshore converter station.

[0004] To solve the above technical problems, the present application realizes by the following technical scheme:

[0005] A kind of offshore wind farm harmonic suppression method based on harmonic current injection, characterized in that: comprising the following steps:

[0006] S1, harmonic signal is applied to the d / q axis current control link of wind turbine grid-side converter control system;

[0007] S2, determine the amplitude, phase and frequency of harmonic current component injected under d / q axis, and the amplitude, phase and frequency of corresponding generated harmonic current component under phase coordinate system at wind turbine port;

[0008] S3, determine the transfer function model between injected harmonic current component at wind turbine port and harmonic current component that needs to be suppressed at grid-connected point, i.e. multiple groups of harmonics with different amplitude and phase of actively injected harmonic current component, corresponding calculated observed harmonic current component amplitude and phase at harmonic observation point, and then calculate the transfer function model;

[0009] S4, according to the amplitude and phase of the harmonic current component to be suppressed and the determined transfer function model, the amplitude and phase of the harmonic current component to be injected are finally calculated.

[0010] Further, in step S1, the harmonic signal only needs to be added to the d-axis current reference value link of the wind turbine grid-side converter control system. Since the ω s +ω0 component in the d / q coordinate system will generate harmonic components with frequencies of ω s and ω s +2ω0 in the system, and the ω s -ω0 component will generate harmonic components with frequencies of ω s and ω s -2ω0 in the system, that is, the ω s ±ω0 components in the d / q coordinate system will both generate ω s components in the system, but the phase sequences are different. For the case of abc three-phase harmonic current imbalance in the system, ω s +ω0 and ω s -ω0 components are selected to be injected at the same time.

[0011] The mathematical model of the harmonic injection signal is as follows:

[0012]

[0013] where I d11 , I d12 , and I are the amplitudes and phases of the ω s1 +ω0 and ω s1 -ω0 components, respectively; I d21 , I d22 , and I are the amplitudes and phases of the ω s2 +ω0 and ω s2 -ω0 components, respectively; I dn1 , I dn2 , and I are the amplitudes and phases of the ω sn +ω0 and ω sn -ω0 components, respectively.

[0014] The amplitudes and phases of the above-mentioned injected harmonic current components can be adjusted.

[0015] Further, in step S2, harmonic current components with a certain amplitude and frequencies of ω s ±ω0 are injected into the d-axis current reference value link, and through coordinate transformation, the calculation expression of the three-phase current in the phase coordinate system is as follows:

[0016]

[0017] In the above formula, I as_ref , I bs_ref and I cs_ref are harmonic current components in three-phase coordinate system; ω0and are the angular frequency and phase of the power frequency component of the phase-locked loop; I d1 , I d2 , and are the amplitude and phase of the ω s ±ω0component; according to the above formula (2), the harmonic current expressions of phase a, phase b and phase c in the phase coordinate system are respectively shown in formula (3), formula (4) and formula (5):

[0018]

[0019]

[0020]

[0021] Only the harmonic components with the frequency of ω s in the above formula (3), formula (4) and formula (5) are concerned, and they are written in the following vector form:

[0022]

[0023]

[0024]

[0025] According to the above expressions (6), (7) and (8), I and I are the ω s +ω0component which generates the ω s harmonic component of negative sequence in the system; and I are the ω s -ω0component which generates the ω s harmonic component of positive sequence in the system.

[0026] Further, in step S3, the harmonic current injected by the wind turbine at the grid-connected point C is related as follows:

[0027]

[0028] In the above formula, I is the harmonic current of phase a at the grid-connected point; and I These are the negative and positive order components, respectively; T 11 (jω s ) and T 12 (jω s )for Quantity about and Transfer function of harmonic components; T 21 (jω s ) and T 22 (jω s )for Quantity about and The transfer function of harmonic components; the aforementioned transfer function T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s It can be obtained through measurement based on active perturbation testing.

[0029] Furthermore: The transfer function T is measured using a method based on active perturbation testing. 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s The specific calculation method is as follows: do not inject any harmonic components into the system, that is, I d1 and I d2 Set the amplitude to 0 and calculate the harmonic components present in the system at this time. and positive / negative order components Will I d2 Set the amplitude to 0 and record the amplitude I of the injected harmonic current. d1 and the corresponding phase Record the harmonic components in the system at this time and positive / negative order components Will I d1 Set the amplitude to 0 and record the amplitude I of the injected harmonic current. d2 and the corresponding phase Record the harmonic components in the system at this time and positive / negative order components Combining equation (9), we can see that:

[0030]

[0031] Where T11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s Let be the transfer function to be determined, and then we can know that:

[0032]

[0033]

[0034]

[0035]

[0036] When high measurement accuracy is required, it is advisable to inject multiple sets of different harmonic currents. and And calculate the corresponding harmonic currents for different operating conditions, and finally calculate T for the n operating conditions. 11-n (jω s ), T 12-n (jω s ), T 21-n (jω s ) and T 22-n (jω s The final T is obtained by averaging the values. 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s This can improve the accuracy of calculations.

[0037] Furthermore: In step S4, after obtaining the transfer function T... 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s Based on this, the amplitude and phase of the harmonic current to be injected are further calculated; for the frequency ω at the grid connection point... s The harmonic components, according to power quality standards, can have their current harmonic limits found to be [value missing]. The three-phase harmonic components at the grid-connected point are mainly positive and negative harmonic components, and the amplitude of the zero sequence component is relatively small, so the harmonic amplitude can be reduced to the required value of the harmonic amplitude at the grid-connected point by suppressing the two parts of harmonic components;

[0038] Without any suppression measures, the harmonic components of each phase at the grid-connected point are and By vector transformation, the positive and negative sequence harmonic components can be calculated respectively:

[0039]

[0040]

[0041] In order to suppress the above components, the negative and positive sequence harmonic components with opposite amplitudes that need to be injected at the grid-connected point are and Combining formula (6) and formula (9), the relationship between the injected harmonic components and the harmonic components existing in the system is shown in the following formula:

[0042]

[0043] Further, the harmonic current that needs to be injected on the d-axis is finally calculated:

[0044]

[0045] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0046] The suppression method and the harmonic signal of the present application only need to be added in the current reference value link of the variable flow control of the wind turbine generator, and then the ability of the variable flow device itself can be utilized without adding a primary device. At the same time, compared with the voltage-based harmonic signal, the current-based harmonic signal is easier to adjust. The suppression method and the harmonic signal based on the current-based harmonic injection proposed in the present application are added to the secondary side variable flow control system of the wind turbine generator, and the proposed suppression method can reduce the harmonic current amplitude at the grid-connected point of the offshore wind farm. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The position of the offshore wind turbine generator variable flow controller and the harmonic signal is applied;

[0048] Figure 2 The I in the embodiment dh The internal structure topology of the module;

[0049] Figure 3 The offshore wind farm power collection and transmission submarine cable circuit topology. DETAILED DESCRIPTION

[0050] In order for those skilled in the art to better understand the technical solutions of the present application, the preferred embodiments of the present application are described below in combination with specific examples, but it should be understood that the drawings are only used for illustrative description and cannot be understood as a limitation on the present application; in order to better illustrate the present embodiment, some components in the drawings may be omitted, enlarged or reduced, and do not represent the actual product size; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted. The positional relationship described in the drawings is only used for illustrative description and cannot be understood as a limitation on the present application.

[0051] The present application is further described below in combination with the drawings and examples, but is not used as a basis for limiting the present application.

[0052] As shown in Figures 1 to 3 , a harmonic suppression method for offshore wind farm based on harmonic current injection, comprising the following steps:

[0053] S1, applying a harmonic signal to the d / q axis current control link of the wind turbine grid-side converter control system;

[0054] S2, determining the amplitude, phase and frequency of the injected harmonic current component in the d / q axis and the amplitude, phase and frequency of the corresponding generated harmonic current component in the phase coordinate system at the wind turbine port;

[0055] S3, determining the transfer function model between the injected harmonic current component at the wind turbine port and the harmonic current component to be suppressed at the grid connection point, i.e. multiple groups of harmonics with different harmonic current component amplitudes and phases are actively injected, and the observed harmonic current component amplitudes and phases are calculated at the harmonic observation point, and then the transfer function model is calculated;

[0056] S4, according to the amplitude and phase of the harmonic current component to be suppressed and the determined transfer function model, finally calculating the amplitude and phase of the harmonic current component to be injected.

[0057] The harmonic signal generates a harmonic current, and the harmonic signal is I dh module injection, I dh The injection position and internal structure of the module are respectively shown in Figure 1 and dh , and Figure 2 , in the step S1, the harmonic signal only needs to be attached to the d-axis current reference value link of the wind turbine grid-side converter control system, because the ω s +ω0 component in the d / q coordinate system will generate harmonic components with frequencies of ω s and ω s +2ω0 in the system, and the ω s -ω0 component will generate harmonic components with frequencies of ωs with ω s -2ω0harmonic components, i.e. ω s ±ω0components will generate ω s components in the system, but the phase sequence is different, for the case of abc three-phase harmonic current imbalance in the system, thus select to inject ω s +ω0and ω s -ω0components at the same time;

[0058] The mathematical model of the harmonic injection signal is as follows:

[0059]

[0060] Where I d11 , I d12 , and are the amplitudes and phases of ω s1 +ω0and ω s1 -ω0components respectively; I d21 , I d22 , and are the amplitudes and phases of ω s2 +ω0and ω s2 -ω0components respectively; I dn1 , I dn2 , and are the amplitudes and phases of ω sn +ω0and ω sn -ω0components respectively;

[0061] The above injected harmonic current components, amplitudes and phases can be adjusted.

[0062] In the step S2, taking the suppression of the harmonic current component with the frequency of ω s in the system as an example, harmonic current components with the frequency of ω s ±ω0and a certain amplitude are injected into the d-axis current reference value link, and through coordinate transformation, the calculation expression of three-phase current in the phase coordinate system is as follows:

[0063]

[0064] In the above formula, I as_ref , I bs_ref and I cs_ref are harmonic current components in the three-phase coordinate system; ω0and are the angular frequency and phase of the phase-locked loop link power frequency component respectively; I d1 , I d2 , and ω s The amplitude and phase of the ±ω0 component; according to the above equation (2), the harmonic current expressions of phase a, phase b and phase c in the phase coordinate system are calculated as shown in equation (3), equation (4) and equation (5) respectively:

[0065]

[0066]

[0067]

[0068] Focusing only on the frequencies of equations (3), (4), and (5) above, ω s The harmonic components are expressed in the following vector form:

[0069]

[0070]

[0071]

[0072] According to the above expressions (6), (7), and (8), it can be seen that... and For ω s The +ω0 component produces a negative-order ω in the system. s Harmonic components; and For ω s The -ω0 component produces positive-order ω in the system. s Harmonic components.

[0073] In step S3, the harmonic current relationship generated at grid connection point C by the harmonic current injected by the wind turbine is as follows:

[0074]

[0075] In the above formula, This refers to the harmonic current of phase a at the grid connection point; and These are the negative and positive order components, respectively; T 11 (jω s ) and T 12 (jω s )for Quantity about and Transfer function of harmonic components; T 21 (jω s ) and T 22 (jω s )for Quantity about and The transfer function of harmonic components; the aforementioned transfer function T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s It can be obtained through measurement using a method based on active perturbation testing.

[0076] The transfer function T is measured using a method based on active perturbation testing. 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s The specific calculation method is as follows: do not inject any harmonic components into the system, that is, I d1 and I d2 Set the amplitude to 0 and calculate the harmonic components present in the system at this time. and positive / negative order components Will I d2 Set the amplitude to 0 and record the amplitude I of the injected harmonic current. d1 and the corresponding phase Record the harmonic components in the system at this time and positive / negative order components Will I d1 Set the amplitude to 0 and record the amplitude I of the injected harmonic current. d2 and the corresponding phase Record the harmonic components in the system at this time and positive / negative order components Combining equation (9), we can see that:

[0077]

[0078] Where T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s Let be the transfer function to be determined, and then we can know that:

[0079]

[0080]

[0081]

[0082]

[0083] When the accuracy of the measurement is required, it can be considered to inject different harmonic currents in multiple groups and corresponding to the calculation of different harmonic currents, and finally the T 11-n (jω s ), T 12-n (jω s ), T 21-n (jω s ) and T 22-n (jω s ) of n working conditions calculated are averaged to obtain the final T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s ), so as to improve the accuracy of the calculation.

[0084] In step S4, on the basis of obtaining the transfer functions T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s ), the amplitude and phase of the harmonic current to be injected are further calculated; for the harmonic component at the grid-connected point with a frequency of ω s , according to the standard of power quality, the limit value of the current harmonic can be obtained The three-phase harmonic components at the grid-connected point are mainly positive and negative sequence harmonic components, and the amplitude of the zero sequence component is relatively small, so suppressing these two parts of the harmonic components can reduce the harmonic amplitude to the required value of the harmonic amplitude at the grid-connected point.

[0085] Without any suppression measures, the harmonic components of each phase at the grid-connected point are respectively and Through vector transformation, the positive and negative sequence harmonic components can be calculated respectively:

[0086]

[0087]

[0088] To suppress the aforementioned components, the negative-sequence and positive-sequence harmonic components with reverse amplitudes that need to be injected at the grid connection point are respectively... and Combining formulas (6) and (9), the relationship between the injected harmonic components and the harmonic components existing in the system can be seen as follows:

[0089]

[0090] Furthermore, the harmonic current that needs to be injected along the d-axis is finally calculated:

[0091]

[0092] Example 1:

[0093] The design process of harmonic signals is illustrated using the example of 11th harmonic amplitude exceeding the standard under a certain operating condition of the system.

[0094] (1) Injection component analysis

[0095] When the system is operating under a certain condition, there is an 11th harmonic (ω). s =11) In cases where the standard is exceeded, the harmonics of phases A, B, and C at the grid connection point ( as well as The values ​​are 107∠145, 77∠-65, and 55∠8.5, respectively.

[0096] According to equation (15), the corresponding positive sequence harmonic components in the system are calculated. and negative sequence harmonic components The values ​​are 32.35∠130.98 and 75.86∠150.77, respectively. Therefore, the equivalent harmonic components injected at the grid connection point are 32.35∠310.98 and 75.86∠330.77.

[0097] (2) Solving the transfer function

[0098] The transfer function parameters are solved using active perturbation testing. Taking a specific set of examples, I... d2 Component set to 0, injected I d1 and When set to 0.02 and 135° respectively, the corresponding negative and positive order components are 19.08∠290.91 and 35.89∠259.17 respectively; T is calculated according to equations (11) and (12). 11 (j550) and T 12 (j550) are 954∠155.9 and 1794.5∠124.17 respectively. I d1 Component set to 0, injected I d2 and respectively, the corresponding generated negative and positive sequence components are 17.63∠347.6° and 35∠328.9° respectively; T 21 (j550) and T 22 (j550) are 881.5∠(152.6°) and 1750∠(133.9°) respectively.

[0099] (3) d-axis injected current solving

[0100] According to the harmonic amplitude value required to be injected at the grid-connected point and the transfer function, I d1 , I d2 , and The calculation results are shown in the following table:

[0101]

[0102] Before and after the active harmonic current injection method is adopted, the three-phase harmonic current components in the system are respectively:

[0103] a phase b phase c phase no injection 107∠145 77∠-65 55∠8.5 with injection 7.1∠126 4.2∠-123 7∠-18

[0104] According to the description and drawings of the present application, a person skilled in the art can easily manufacture or use the offshore wind farm harmonic suppression method based on harmonic current injection, and can produce the positive effects described in the present application.

[0105] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Any simple modification or equivalent change to the above embodiment according to the technical essence of the present application falls within the protection scope of the present application.

Claims

1. A harmonic current injection based offshore wind farm harmonic mitigation method, characterized in that: It comprises the following steps: S1, applying a harmonic signal to the d / q axis current control link of the wind turbine grid-side converter control system; S2, determining the amplitude, phase and frequency of the injected harmonic current component in the d / q axis and the corresponding generated harmonic current component in the phase coordinate system at the wind turbine port; S3, determining the transfer function model between the injected harmonic current component at the wind turbine port and the harmonic current component to be suppressed at the grid connection point, i.e. multiple groups of harmonics with different harmonic current component amplitudes and phases are actively injected, and the observed harmonic current component amplitudes and phases are calculated at the harmonic observation point, and then the transfer function model is calculated; S4, according to the amplitude and phase of the harmonic current component to be suppressed and the determined transfer function model, the amplitude and phase of the harmonic current component to be injected are finally calculated.

2. The harmonic suppression method for offshore wind farm based on harmonic current injection according to claim 1, characterized in that: In the step S1, the harmonic signal only needs to be added to the d-axis current reference value link of the wind turbine grid-side converter control system. Since the ω s +ω0 component in the d / q coordinate system will generate harmonic components with frequencies of ω s and ω s +2ω0 in the system, the ω s -ω0 component will generate harmonic components with frequencies of ω s and ω s -2ω0 in the system, that is, the ω s ±ω0 components in the d / q coordinate system will both generate ω s components in the system, but the phase sequence is different. For the case of abc three-phase harmonic current imbalance in the system, ω s +ω0 and ω s -ω0 components are selected to be injected simultaneously. The mathematical model of the harmonic injection signal is as follows: where I d11 , I d12 , and I are the amplitudes and phases of the ω s1 + ω0and ω s1 - ω0components, respectively; I d21 , I d22 , and I are the amplitudes and phases of the ω s2 + ω0and ω s2 - ω0components, respectively; I dn1 , I dn2 , and I are the amplitudes and phases of the ω sn + ω0and ω sn - ω0components, respectively. The amplitude and phase of the injected harmonic current component can be adjusted.

3. The harmonic suppression method for offshore wind farm based on harmonic current injection according to claim 2, characterized in that: In the step S2, harmonic current components with a certain amplitude and a frequency of ω s ±ω0are injected into the d-axis current reference value link, respectively. Through coordinate transformation, the calculation expression of three-phase current in the phase coordinate system is calculated as follows: In the above formula, I as_ref , I bs_ref , and I cs_ref are harmonic current components in a three-phase coordinate system, respectively. ω0 and These represent the angular frequency and phase of the power frequency component of the phase-locked loop; I d1 I d2 , and ω s The amplitude and phase of the ±ω0 component; according to the above equation (2), the harmonic current expressions of phase a, phase b and phase c in the phase coordinate system are calculated as shown in equation (3), equation (4) and equation (5) respectively: Only the frequency components of the above equation of formula (3), formula (4) and formula (5) are ω s The harmonic components are written in the following vector form: From the above expressions (6), (7), (8), and is the ω s -ω0component generates a positive sequence of ω s harmonic component in the system. and is the ω s +ω0component generates a negative sequence of ω s harmonic component in the system.

4. The harmonic suppression method for offshore wind farm based on harmonic current injection of claim 3, wherein: In step S3, the relationship between the harmonic current injected by the wind turbine and the harmonic current generated at the grid connection point C is as follows: in the above formula, is the a-phase harmonic current at the point of common coupling; and are the negative and positive sequence components, respectively; 11 (jω s ) and T 12 (jω s ) are the transfer functions of the harmonic components with respect to and ; T 21 (jω s ) and T 22 (jω s ) are the transfer functions of the harmonic components with respect to and ; the above transfer functions T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s ) can be measured by a method based on active disturbance test.

5. The harmonic current injection based offshore wind farm harmonic mitigation method of claim 4, wherein: The transfer functions T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s ) are measured by the method based on active disturbance test. The specific calculation method is as follows: no harmonic component is injected into the system, i.e. the amplitudes of I d1 and I d2 are set to 0, the harmonic components and the positive / negative sequence components existing in the system at this time are calculated. The amplitude of I d2 is set to 0, the injected harmonic current amplitude I d1 and the corresponding phase are recorded. The harmonic components and the positive / negative sequence components existing in the system at this time are recorded. The amplitude of I d1 is set to 0, the injected harmonic current amplitude I d2 and the corresponding phase are recorded. The harmonic components and the positive / negative sequence components existing in the system at this time are recorded. It can be known from formula (9) that: Where T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s Let be the transfer function to be determined, and then we can know that: When the accuracy of the measurement is required, it can be considered to inject different harmonic currents in different groups and corresponding calculations of different harmonic currents, and finally T 11-n (jω s ), T 12-n (jω s ), T 21-n (jω s ) and T 22-n (jω s ) of n working conditions calculated are taken average to obtain the final T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s ), thereby improving the accuracy of the calculation.

6. The harmonic current injection based offshore wind farm harmonic mitigation method of claim 5, wherein: In step S4, on the basis of the transfer functions T 11 (jω s ), T 12 (jω s ), T 21 (jω s ) and T 22 (jω s ), the amplitude and phase of the harmonic current to be injected are further calculated; for the harmonic component at the grid point with a frequency of ω s , according to the standard of power quality, the limit value of the harmonic current can be obtained as The three-phase harmonic components at the grid point are mainly the positive and negative sequence harmonic components, and the amplitude of the zero sequence component is relatively small, so suppressing these two parts of the harmonic components can reduce the harmonic amplitude to the required value of the harmonic amplitude at the grid point; Without any suppression measures applied, the harmonic components of each phase at the point of interconnection are respectively and By vector transformation, the positive and negative sequence harmonic components can be calculated respectively: In order to suppress the above components, the amplitude of the negative sequence and positive sequence harmonic components injected at the grid-connected point in the opposite direction are respectively and Combining formula (6) with formula (9), the relationship between the injected harmonic components and the harmonic components existing in the system is shown in the following formula: Further, the harmonic current to be injected on the d-axis is finally calculated as follows: 。

Images