A photovoltaic inverter harmonic suppression system and method

Abstract

The present application provides a kind of photovoltaic inverter harmonic suppression system and method, obtains the target current error of current period, and the branch period delay of last period is generated according to target current error and its corresponding gain coefficient in repetitive control branch, and is summarized as total delay in current period;Current period total delay is input into PI controller, to make it generate target voltage instruction.Both retain the tracking ability of PI controller to fundamental current, do not affect the current inner loop design of general inverter, and enhance the suppression effect of periodic harmonic through repeater controller.At the same time, through optimizing repeater controller design, it becomes the composite repeater controller including at least two repeater control branches, widens its effective bandwidth, so that it can adapt to the fluctuation of power grid frequency, thereby maintaining excellent grid-connected current harmonic suppression performance in the wide frequency variation range.

Description

Technical Field

[0001] This invention relates to the field of photovoltaics, and more specifically, to a harmonic suppression system and method for photovoltaic inverters. Background Technology

[0002] Global photovoltaic (PV) installed capacity has grown rapidly for several consecutive years, and PV power plants are transitioning from supplementary energy to a primary energy source. This means that their impact on the power grid is no longer negligible, but rather crucial. PV inverters are the key interface connecting PV arrays to the power grid, and their performance directly determines power quality and system stability.

[0003] Traditional control strategies such as proportional-integral (PI) control have excellent regulation capabilities for DC signals in a synchronous rotating coordinate system, but their ability to suppress periodic AC harmonics is limited, especially when the frequency fluctuates, where the suppression effect decreases. Power grid standards impose strict limits on the total harmonic distortion rate and the content of each harmonic in the grid-connected current. Substandard power quality can lead to problems such as transformer and line overheating, relay protection malfunctions, and damage to precision equipment. Summary of the Invention

[0004] The purpose of this invention is to provide a photovoltaic inverter harmonic suppression system and method to improve the above-mentioned problems.

[0005] To achieve the above objectives, the technical solutions adopted in the embodiments of the present invention are as follows:

[0006] In a first aspect, embodiments of the present invention provide a photovoltaic inverter harmonic suppression system, the photovoltaic inverter harmonic suppression system comprising: a first arithmetic unit, a composite repetitive controller and a PI controller, the composite repetitive controller comprising at least two repetitive control branches and a second arithmetic unit;

[0007] The first arithmetic unit is used to obtain the target current error of the current cycle, wherein the target current error is the d-phase current error or the q-phase current error of the inverter;

[0008] The repetitive control branch is used to generate the branch cycle delay of the current cycle based on the target current error and its corresponding gain coefficient and the branch cycle delay of the previous cycle.

[0009] The second arithmetic unit is used to sum up the branch cycle delay of each repetitive control branch in the current cycle into the total delay of the current cycle;

[0010] The PI controller is used to generate a target voltage command based on the total delay of the current cycle; wherein, when the target current error is a d-phase current error, the target voltage command is a d-phase voltage command, and when the target current error is a q-phase current error, the target voltage command is a q-phase voltage command.

[0011] Secondly, embodiments of the present invention provide a photovoltaic inverter harmonic suppression method, applied to the aforementioned photovoltaic inverter harmonic suppression system, the method comprising:

[0012] Obtain the target current error for the current cycle, where the target current error is the d-phase current error or q-phase current error of the inverter.

[0013] The target current error is input into each repetitive control branch in the composite repetitive controller. The repetitive control branch, combined with its corresponding gain coefficient and the branch period delay of the previous cycle, generates the branch period delay of the current cycle. The composite repetitive controller includes at least two repetitive control branches.

[0014] The branch cycle delay of each repetitive control branch in the current cycle is summed into the total delay of the current cycle;

[0015] The total delay of the current cycle is input into the PI controller to generate the target voltage command;

[0016] Wherein, when the target current error is a d-phase current error, the target voltage command is a d-phase voltage command; when the target current error is a q-phase current error, the target voltage command is a q-phase voltage command.

[0017] Compared to existing technologies, the photovoltaic inverter harmonic suppression system and method provided in this invention obtains the target current error for the current cycle. The target current error is either the d-phase current error or the q-phase current error of the inverter. The target current error is input into each repetitive control branch in the composite repetitive controller. The repetitive control branch, combined with its corresponding gain coefficient and the branch cycle delay of the previous cycle, generates the branch cycle delay for the current cycle. The branch cycle delays of each repetitive control branch in the current cycle are summed into the total delay for the current cycle. The total delay for the current cycle is input into the PI controller to generate the target voltage command. This approach retains the PI controller's ability to track the fundamental current, without affecting the current inner loop design of a typical inverter, while enhancing the suppression effect on periodic harmonics through the repetitive controller. Furthermore, by optimizing the repetitive controller design to create a composite repetitive controller with at least two repetitive control branches, its effective bandwidth is broadened, enabling it to adapt to grid frequency fluctuations and maintain excellent grid-connected current harmonic suppression performance over a wide frequency variation range.

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is one of the structural schematic diagrams of a photovoltaic inverter harmonic suppression system provided in an embodiment of the present invention.

[0021] Figure 2 This is the second schematic diagram of the structure of the photovoltaic inverter harmonic suppression system provided in the embodiment of the present invention.

[0022] Figure 3 This is one of the flowcharts illustrating the photovoltaic inverter harmonic suppression method provided in this embodiment of the invention.

[0023] Figure 4 This is the second schematic flowchart of the photovoltaic inverter harmonic suppression method provided in the embodiments of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0026] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this invention, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0027] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0028] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of this invention is usually placed when in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0029] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set" and "connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0030] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0031] This invention provides a photovoltaic inverter harmonic suppression system. Please refer to [link / reference]. Figure 1 , Figure 1 This is one of the structural schematic diagrams of a photovoltaic inverter harmonic suppression system provided in an embodiment of the present invention. The photovoltaic inverter harmonic suppression system includes: a first operational unit, a composite repetitive controller, and a PI controller. The composite repetitive controller includes at least two repetitive control branches and a second operational unit. Figure 1 The example shown uses three repetitive control branches. It should be noted that a composite repetitive controller can also include more or fewer repetitive control branches, such as two or four.

[0032] The first arithmetic unit is used to obtain the target current error for the current cycle. The target current error is the d-phase current error or q-phase current error of the inverter.

[0033] Optionally, the d-phase current error = the d-phase current reference value of the inverter (i dref - Actual value of the d-phase current of the inverter (i d ); q-phase current error = inverter q-phase current reference value (i qref - Actual value of the q-phase current of the inverter (i q The inverter in this embodiment of the invention may be, but is not limited to, a photovoltaic inverter.

[0034] The repetitive control branch is used to generate the branch cycle delay of the current cycle based on the target current error and its corresponding gain coefficient and the branch cycle delay of the previous cycle.

[0035] The second arithmetic unit is used to sum up the branch cycle delay of each repetitive control branch in the current cycle into the total delay of the current cycle.

[0036] It should be understood that the summation here can refer to the summation of the branch cycle delays of each repetitive control branch in the current cycle, thereby obtaining the total delay of the current cycle.

[0037] The PI controller is used to generate a target voltage command based on the total delay of the current cycle.

[0038] Wherein, when the target current error is the d-phase current error, the target voltage command is the d-phase voltage command (v d When the target current error is the q-phase current error, the target voltage command is the q-phase voltage command (v). q ).

[0039] The target voltage command is transmitted through a PWM modulator to generate a switching signal that drives the three-level bridge arm (which may be, but is not limited to, the NPC-I type three-level bridge arm), thereby achieving high-precision and dynamic control of the grid-connected current.

[0040] In the photovoltaic inverter harmonic suppression system provided in this embodiment of the invention, the tracking capability of the PI controller for the fundamental current is retained, without affecting the current inner loop design of a typical inverter. Furthermore, the suppression effect on periodic harmonics is enhanced through a repetitive controller. Simultaneously, by optimizing the repetitive controller design to make it a composite repetitive controller including at least two repetitive control branches, its effective bandwidth is widened, enabling it to adapt to grid frequency fluctuations and thus maintain excellent grid-connected current harmonic suppression performance over a wide frequency variation range.

[0041] Please continue to refer to this. Figure 1 The repetitive control branch includes a gain unit, a third arithmetic unit, and a filtering unit.

[0042] The gain unit is used to multiply the target current error by its corresponding gain coefficient and provide the product to the third arithmetic unit.

[0043] The filtering unit is used to process the branch cycle delay of the previous cycle using a filtering function, and provides the obtained delay error to the third arithmetic unit.

[0044] The third arithmetic unit is used to add the obtained product to the delay error to obtain the branch cycle delay of the current cycle.

[0045] Please continue to refer to this. Figure 1 The picture k i This represents the gain coefficient corresponding to the i-th repetitive control branch. Let represent the filter function corresponding to the i-th repetitive control branch. This represents the branch period delay corresponding to the i-th repetitive control branch. Figure 1 The value of i can be 1, 2, or 3.

[0046] Please refer to Figure 2 , Figure 2 This is a second schematic diagram of the structure of a photovoltaic inverter harmonic suppression system provided in an embodiment of the present invention. The photovoltaic inverter harmonic suppression system also includes a comb filter. In an optional embodiment, the comb filter can also be integrated with a composite repetitive controller.

[0047] The comb filter is used to process the total delay of the current cycle output of the second arithmetic unit and transmit the comb filter result to the PI controller.

[0048] The PI controller is used to generate target voltage commands based on the results of comb filtering.

[0049] Introducing a comb filter into the output stage of the composite repetitive controller serves to address the inherent resonance peak problem of LCL filters. By precisely aligning the notch frequency of the comb filter with the system's resonant point, the risk of oscillation at that frequency can be effectively suppressed. The greatest advantage of this approach lies in its frequency selectivity; the filter has virtually no effect on frequency bands outside the resonant point. Not only does it maintain a constant amplitude-frequency response, but its zero-phase-off characteristic eliminates the need for complex phase compensation design, contributing to maintaining system stability and dynamic response performance.

[0050] Alternatively, the comb filter can be represented as:

[0051]

[0052] in, This represents the result of the comb filtering process. This indicates the total delay of the current period.

[0053] Comb filters provide a notch characteristic targeting the resonant frequency, but their attenuation range is limited and they lack the necessary suppression capability for high-frequency disturbances. Therefore, in the photovoltaic inverter harmonic suppression system provided in this embodiment of the invention, a composite repetitive controller, a comb filter, and a PI controller are connected in series. This combination leverages the advantages of their respective frequency characteristics: the comb filter accurately suppresses the resonant peak, while the subsequent PI controller naturally provides wide-bandwidth high-frequency attenuation, thus jointly ensuring the overall stability of the system.

[0054] Regarding how to determine the gain coefficient corresponding to the repetitive control branch, this embodiment of the invention also provides an optional implementation method, please refer to the following.

[0055] The gain coefficient of each repetitive control branch is matched with the design frequency and the stable frequency of the power grid corresponding to the repetitive control branch.

[0056] The formula for the gain coefficient of the repetitive control branch is:

[0057]

[0058]

[0059] in, This represents the gain coefficient corresponding to the i-th repetitive control branch. This represents the triangle membership degree corresponding to the i-th repeated control branch. This represents the triangle membership degree corresponding to the j-th repeated control branch. Indicates the stable frequency of the power grid. Let i represent the design frequency corresponding to the i-th repetitive control branch, and N be the total number of repetitive control branches in the composite repetitive controller, where 1 ≤ i ≤ N.

[0060] Please continue to refer to this. Figure 1 In one optional implementation, the triangular membership functions corresponding to the three repetitive control branches are represented as follows:

[0061]

[0062] in, .

[0063] Considering the frequency adaptability requirements of the inverter, the photovoltaic inverter does not require the grid frequency to operate for a long time when it is less than 48.5Hz or greater than 51Hz. Therefore, 49Hz is selected as the design frequency of the first repetitive control branch, 50Hz is selected as the design frequency of the second repetitive control branch, and 51Hz is selected as the design frequency of the third repetitive control branch.

[0064] Set sampling frequency If the frequency is 16kHz, then the delay parameters for each branch are derived from the corresponding frequencies:

[0065]

[0066] in, This represents the delay parameter corresponding to the i-th repetitive control branch. This represents the design frequency corresponding to the i-th repetitive control branch. Indicates the sampling frequency.

[0067] Here I am Under the given conditions, we can obtain:

[0068] .

[0069] For the gain of the repeating controller in each branch, the weighting coefficients are obtained by detecting the current frequency of the power grid.

[0070] In one alternative implementation, the process of obtaining the grid stable frequency includes: the photovoltaic inverter extracts the frequency of the grid positive sequence voltage through a (SOGI) phase-locked loop and filters it appropriately to obtain the grid stable frequency.

[0071] The specific process is as follows: Obtain the three-phase voltage v output by the inverter in the three-phase stationary coordinate system. abc Then, it is transformed to obtain the α-axis voltage component v in the two-dimensional stationary coordinate system. α and β-axis voltage component v β ;

[0072] The α-axis voltage component v α and β-axis voltage component v β Input (SOGI) phase-locked loop to enable the phase-locked loop to combine the angle information of the previous cycle and extract the α-axis positive sequence voltage and β-axis positive sequence voltage corresponding to the power grid;

[0073] Combining the angle information from the previous cycle, the α-axis positive sequence voltage and β-axis positive sequence voltage in the two-dimensional stationary coordinate system are transformed into the rotating coordinate system (dq coordinate system) to obtain the d-phase voltage component and the q-phase voltage component.

[0074] The q-phase voltage component is low-pass filtered and then added to the fundamental angular frequency to obtain the intermediate sum;

[0075] Based on this intermediate sum, an integral operation (1 / s) is performed to obtain the angle information (θ) of the current cycle. + );

[0076] Multiply the intermediate sum by (1 / 2π) and perform an average operation to obtain the stable frequency of the power grid.

[0077] Using the composite repetitive + PI controller of this invention, single repetitive control + PI controller and non-repetitive controller were tested at different grid frequencies. The output current harmonics of the inverter were observed, and it can be seen that the composite repetitive + PI series controller provides a better grid-connected current harmonic suppression effect.

[0078] This invention also provides a method for suppressing harmonics in a photovoltaic inverter, applicable to the photovoltaic inverter harmonic suppression system described above. Please refer to [link / reference]. Figure 3 Harmonic suppression methods for photovoltaic inverters include:

[0079] S11, obtain the target current error for the current cycle.

[0080] The target current error is the d-phase current error or q-phase current error of the inverter;

[0081] S12, the target current error is input into each repetitive control branch in the composite repetitive controller. The repetitive control branch combines its corresponding gain coefficient and the branch period delay of the previous cycle to generate the branch period delay of the current cycle.

[0082] The composite repetitive controller includes at least two repetitive control branches.

[0083] S13, sum up the branch cycle delay of each repetitive control branch in the current cycle into the total delay of the current cycle;

[0084] S14: Input the total delay of the current cycle into the PI controller so that it can generate the target voltage command;

[0085] Specifically, when the target current error is the d-phase current error, the target voltage command is the d-phase voltage command; when the target current error is the q-phase current error, the target voltage command is the q-phase voltage command.

[0086] Please refer to Figure 4 In an optional implementation, after summing the branch cycle delays of each repetitive control branch in the current cycle into the total delay of the current cycle, the photovoltaic inverter harmonic suppression method further includes:

[0087] S15, use a comb filter to process the total delay of the current cycle, and transmit the comb filter result to the PI controller;

[0088] S16, the PI controller generates the target voltage command based on the comb filtering result.

[0089] In summary, the photovoltaic inverter harmonic suppression system and method provided by this invention obtains the target current error for the current cycle. The target current error is either the d-phase current error or the q-phase current error of the inverter. The target current error is input into each repetitive control branch in the composite repetitive controller. The repetitive control branch, combined with its corresponding gain coefficient and the branch cycle delay of the previous cycle, generates the branch cycle delay for the current cycle. The branch cycle delays of each repetitive control branch in the current cycle are summed into the total delay for the current cycle. The total delay for the current cycle is input into the PI controller to generate the target voltage command. This system retains the PI controller's ability to track the fundamental current, without affecting the current inner loop design of a typical inverter, while enhancing the suppression effect on periodic harmonics through the repetitive controller. Furthermore, by optimizing the repetitive controller design to make it a composite repetitive controller with at least two repetitive control branches, its effective bandwidth is broadened, enabling it to adapt to grid frequency fluctuations and maintain excellent grid-connected current harmonic suppression performance over a wide frequency variation range.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0091] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A photovoltaic inverter harmonic suppression system, characterized in that, The photovoltaic inverter harmonic suppression system includes: a first computing unit, a composite repetitive controller, and a PI controller. The composite repetitive controller includes at least two repetitive control branches and a second computing unit. The first arithmetic unit is used to obtain the target current error of the current cycle, wherein the target current error is the d-phase current error or the q-phase current error of the inverter; The repetitive control branch is used to generate the branch cycle delay of the current cycle based on the target current error and its corresponding gain coefficient and the branch cycle delay of the previous cycle. The second arithmetic unit is used to sum up the branch cycle delay of each repetitive control branch in the current cycle into the total delay of the current cycle; The PI controller is used to generate a target voltage command based on the total delay of the current cycle; wherein, when the target current error is a d-phase current error, the target voltage command is a d-phase voltage command, and when the target current error is a q-phase current error, the target voltage command is a q-phase voltage command. The gain coefficient of each repetitive control branch is matched with the design frequency and the stable grid frequency of the repetitive control branch. The formula for the gain coefficient of the repetitive control branch is: in, This represents the gain coefficient corresponding to the i-th repetitive control branch. This represents the triangle membership degree corresponding to the i-th repeated control branch. This represents the triangle membership degree corresponding to the j-th repeated control branch. Indicates the stable frequency of the power grid. Let i represent the design frequency corresponding to the i-th repetitive control branch, and N be the total number of repetitive control branches in the composite repetitive controller, where 1 ≤ i ≤ N.

2. The photovoltaic inverter harmonic suppression system as described in claim 1, characterized in that, The repetitive control branch includes a gain unit, a third arithmetic unit, and a filtering unit; The gain unit is used to multiply the target current error by its corresponding gain coefficient and provide the product to the third arithmetic unit; The filtering unit is used to process the branch cycle delay of the previous cycle using a filtering function, and provides the obtained delay error to the third calculation unit. The third arithmetic unit is used to add the obtained product to the delay error to obtain the branch cycle delay of the current cycle.

3. The photovoltaic inverter harmonic suppression system as described in claim 1, characterized in that, The photovoltaic inverter harmonic suppression system also includes a comb filter; The comb filter is used to process the total delay of the current cycle output by the second arithmetic unit and transmit the comb filter result to the PI controller; The PI controller is used to generate a target voltage command based on the comb filtering result.

4. The photovoltaic inverter harmonic suppression system as described in claim 3, characterized in that, The comb filter is represented as follows: in, This represents the result of the comb filtering process. This indicates the total delay of the current period.

5. The photovoltaic inverter harmonic suppression system as described in claim 1, characterized in that, The process of obtaining the stable grid frequency includes: the photovoltaic inverter extracts the frequency of the positive sequence voltage of the grid through a phase-locked loop and filters it appropriately to obtain the stable grid frequency.

6. The photovoltaic inverter harmonic suppression system as described in claim 1, characterized in that, d-phase current error = Inverter d-phase current reference value - Inverter d-phase current actual value; Q-phase current error = Inverter's q-phase current reference value - Inverter's q-phase current actual value.

7. A method for harmonic suppression in a photovoltaic inverter, characterized in that, The method, applied to the photovoltaic inverter harmonic suppression system according to any one of claims 1-6, comprises: Obtain the target current error for the current cycle, where the target current error is the d-phase current error or q-phase current error of the inverter. The target current error is input into each repetitive control branch in the composite repetitive controller. The repetitive control branch, combined with its corresponding gain coefficient and the branch period delay of the previous cycle, generates the branch period delay of the current cycle. The composite repetitive controller includes at least two repetitive control branches. The branch cycle delay of each repetitive control branch in the current cycle is summed into the total delay of the current cycle; The total delay of the current cycle is input into the PI controller to generate the target voltage command; Wherein, when the target current error is a d-phase current error, the target voltage command is a d-phase voltage command; when the target current error is a q-phase current error, the target voltage command is a q-phase voltage command.

8. The photovoltaic inverter harmonic suppression method as described in claim 7, characterized in that, After summing the branch cycle delays of each repetitive control branch in the current cycle into the total delay of the current cycle, the method further includes: The total delay of the current cycle is processed using a comb filter, and the comb filter result is transmitted to the PI controller. The PI controller generates a target voltage command based on the comb filtering result.

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