A photovoltaic inverter power conversion control method

Abstract

The present application relates to photovoltaic inverter electric energy conversion control technical field, specifically a kind of photovoltaic inverter electric energy conversion control method.The present application carries out positive and negative sequence separation to grid-connected point voltage, obtains positive sequence voltage amplitude, negative sequence voltage amplitude and positive and negative sequence voltage phase angle difference cosine value, with the product of three as benefit estimation value, the reduction degree of the double-frequency power oscillation amplitude of grid-connected point when each unit negative sequence current is injected by inverter is characterized;With negative sequence unbalance degree dynamic calculation benefit threshold, benefit threshold decreases with the increase of negative sequence unbalance degree;When benefit estimation value is not lower than benefit threshold, distribution value of three components of positive sequence reactive current, negative sequence current and positive sequence active current is determined in turn;When benefit estimation value is lower than benefit threshold, negative sequence current injection is smoothly exited in control cycle by time within not more than one fundamental period, and the current margin released is supplemented to positive sequence reactive current distribution value and positive sequence active current distribution value in turn.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic inverter power conversion control technology, specifically a photovoltaic inverter power conversion control method. Background Technology

[0002] In grid-connected photovoltaic (PV) power generation systems, grid voltage imbalance can cause second-harmonic oscillations in the inverter's output power, affecting its stable operation. Injecting negative-sequence current into the grid connection point can suppress these second-harmonic power oscillations, but this injection consumes the inverter's limited current margin. Therefore, rationally allocating the current margin between negative-sequence current injection and positive-sequence reactive and active currents is a crucial issue in the power conversion control of PV inverters.

[0003] In existing technologies, photovoltaic inverters typically employ fixed-ratio or fixed-priority current allocation strategies under grid voltage imbalance conditions, lacking a quantitative assessment of the actual benefits of injecting negative-sequence current. On one hand, when the amplitudes of positive and negative-sequence voltages at the grid connection point are small or the phase angle difference between them is large, the effect of injecting negative-sequence current on suppressing the amplitude of second-harmonic power oscillations is limited. Continuing to inject negative-sequence current unnecessarily occupies current margin, squeezing the allocation space for positive-sequence reactive and active currents, resulting in insufficient reactive power support and limited active power output of the inverter. On the other hand, existing technologies often use a step-like approach when exiting negative-sequence current injection. Sudden changes in current allocation values ​​cause instantaneous fluctuations in the inverter's output current, adversely affecting grid stability. Furthermore, existing technologies do not consider the dynamic impact of voltage imbalance on the benefit assessment threshold, nor do they modify the negative-sequence current exit conditions under voltage dips, making it difficult to achieve refined management of the inverter's current margin under complex grid conditions. Summary of the Invention

[0004] This invention provides a photovoltaic inverter power conversion control method to achieve dynamic evaluation of the benefits of negative sequence current injection and adaptive allocation of current components under unbalanced power grid conditions.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a photovoltaic inverter power conversion control method, comprising: S1: Separate the positive and negative sequence voltages at the grid connection point to obtain the positive sequence voltage amplitude, negative sequence voltage amplitude, and phase angle difference between the positive and negative sequence voltages; S2: The product of the positive sequence voltage amplitude, the negative sequence voltage amplitude, and the cosine of the phase angle difference between the positive and negative sequence voltages is used as the benefit estimate. The benefit estimate characterizes the degree of reduction in the second harmonic power oscillation amplitude at the grid connection point when the inverter injects a unit negative sequence current. S3: The ratio of negative sequence voltage amplitude to positive sequence voltage amplitude is the negative sequence imbalance. The benefit threshold is dynamically calculated based on the negative sequence imbalance. The benefit threshold decreases as the negative sequence imbalance increases. S4: When the estimated benefit value is not lower than the benefit threshold, the inverter output current is decomposed into three components: positive sequence reactive current, negative sequence current and positive sequence active current. The allocation value of each component is determined in turn, with the constraint that the sum of the amplitudes of the three components does not exceed the upper limit of the inverter current. S5: When the benefit estimate is lower than the benefit threshold, the negative sequence current allocation value is reduced in each control cycle within a time period not exceeding one fundamental cycle. The reduction amount in each control cycle is positively correlated with the product of the benefit estimate and the current negative sequence current allocation value. The current margin released by the reduction of the negative sequence current allocation value in each control cycle is successively supplemented to the positive sequence reactive current allocation value and the positive sequence active current allocation value until the negative sequence current allocation value is reduced to zero.

[0006] As a preferred embodiment of the present invention, in S1, the step of separating the positive and negative sequences includes: A second-order generalized integrator is applied to the grid connection point voltage signal to output a component that is in phase and frequency with the grid connection point voltage signal, as well as a component that is orthogonal to the same frequency. The positive-sequence voltage component and the negative-sequence voltage component are reconstructed by linear combinations of in-phase and quadrature components of the same frequency, respectively. The amplitude and phase angle of the positive-sequence voltage component and the negative-sequence voltage component are extracted respectively, and the difference between the positive-sequence voltage phase angle and the negative-sequence voltage phase angle is taken as the positive-negative-sequence voltage phase angle difference.

[0007] As a preferred embodiment of the present invention, in S2, the step of updating the benefit estimate includes: In each control cycle, the positive-sequence voltage amplitude, negative-sequence voltage amplitude, and positive-negative-sequence voltage phase angle difference are resampled, and the benefit estimate is recalculated. The absolute value of the difference between the estimated benefit value of the current control cycle and the estimated benefit value of the previous control cycle is used to obtain the change in the estimated benefit value. When the change in the estimated benefit value exceeds the preset change threshold, the current estimated benefit value is replaced by the value calculated for the current control period. When the change in the estimated benefit value is not greater than the preset change threshold, the estimated benefit value of the previous control period remains unchanged.

[0008] As a preferred embodiment of the present invention, in S3, the dynamic calculation step of the benefit threshold includes: Using negative order imbalance as the independent variable, the benefit threshold is calculated as a linear function. The higher the negative order imbalance, the lower the benefit threshold. The lower limit of the benefit threshold is the estimated benefit value corresponding to the difference between the upper limit of the inverter current and the minimum positive sequence reactive current specified by the power grid standard. When the benefit threshold calculated by the linear function is lower than the lower limit of the benefit threshold, the lower limit of the benefit threshold shall be used as the benefit threshold for this control period. When the benefit threshold calculated by the linear function is not lower than the lower limit of the benefit threshold, the benefit threshold calculated by the linear function shall be used as the benefit threshold for this control cycle.

[0009] As a preferred embodiment of the present invention, in S3, the benefit threshold is further corrected based on the positive sequence voltage amplitude, and the correction step includes: Compare the positive sequence voltage amplitude with the rated voltage amplitude at the grid connection point; When the positive sequence voltage amplitude is lower than half of the rated voltage amplitude at the grid connection point, the difference between half of the rated voltage amplitude at the grid connection point and the positive sequence voltage amplitude is multiplied by a preset correction coefficient to obtain the correction amount. The correction amount is then added to the benefit threshold, and the result of the addition is used as the actual benefit threshold for this control cycle. When the positive sequence voltage amplitude is not less than half of the rated voltage amplitude at the grid connection point, no correction is applied to the benefit threshold.

[0010] As a preferred embodiment of the present invention, in S4, the step of sequentially determining the allocation value of each component includes: The positive sequence reactive current allocation value is determined by multiplying the difference between the rated voltage amplitude at the grid connection point and the positive sequence voltage amplitude by the rated current. When the positive sequence reactive current allocation value exceeds the inverter current limit, the inverter current limit is used as the positive sequence reactive current allocation value. The difference between the inverter current upper limit and the positive sequence reactive current allocation value is used as the upper limit of the negative sequence current allocation value, and the negative sequence current allocation value is determined within the range of the upper limit of the negative sequence current allocation value. The positive sequence active current allocation value is obtained by subtracting the positive sequence reactive current allocation value from the inverter current upper limit and then subtracting the negative sequence current allocation value.

[0011] As a preferred embodiment of the present invention, in S4, the constraint determination step of ensuring that the sum of the amplitudes of the three components does not exceed the upper limit of the inverter current includes: The positive sequence reactive current distribution value, negative sequence current distribution value and positive sequence active current distribution value are directly added together, and the sum is used as the upper limit of the instantaneous peak value of the three-phase current. The constraint condition is that the upper limit of the instantaneous peak value of the three-phase current does not exceed the upper limit of the inverter current. When the sum of the positive-sequence reactive current allocation value and the negative-sequence current allocation value exceeds the inverter current limit, the difference between the inverter current limit and the positive-sequence reactive current allocation value is used as the negative-sequence current allocation value, and the positive-sequence active current allocation value is set to zero.

[0012] As a preferred embodiment of the present invention, in S5, the step of decreasing the negative sequence current allocation value cycle by cycle includes: The remaining number of control cycles is determined by the difference between the total number of control cycles corresponding to one fundamental cycle and the number of control cycles that have been executed and exited. The upper limit of the reduction in negative sequence current is obtained by dividing the negative sequence current allocation value of this control cycle by the number of remaining control cycles. The target reduction in negative sequence current is determined by multiplying the estimated benefit value by the negative sequence current allocation value for this control cycle. When the target reduction of negative sequence current does not exceed the upper limit of the reduction of negative sequence current, the target reduction of negative sequence current shall be used as the actual reduction of negative sequence current in this control cycle. When the target reduction of negative sequence current exceeds the upper limit of negative sequence current reduction, the upper limit of negative sequence current reduction shall be used as the actual reduction of negative sequence current in this control cycle. The negative sequence current allocation value minus the actual decrease in negative sequence current is used as the negative sequence current allocation value for the next control cycle. The process ends when the negative sequence current allocation value of the next control cycle decreases to zero.

[0013] As a preferred embodiment of the present invention, in S5, the step of sequentially supplementing the current margin to the positive sequence reactive current allocation value and the positive sequence active current allocation value includes: The actual decrease in negative sequence current during this control cycle is taken as the current margin to be released. When the sum of the positive sequence reactive current allocation value and the released current margin does not exceed the minimum positive sequence reactive current value specified by the power grid standard, the released current margin will be fully added to the positive sequence reactive current allocation value. When the sum of the positive sequence reactive current allocation value and the released current margin exceeds the minimum positive sequence reactive current value specified by the power grid standard, the positive sequence reactive current allocation value will be supplemented to the minimum positive sequence reactive current value specified by the power grid standard, and the portion of the released current margin that exceeds the supplementary amount will be added to the positive sequence active current allocation value.

[0014] As a preferred embodiment of the present invention, it further includes step S6, determining the relationship between the estimated benefit value and the benefit threshold in each control cycle: When the estimated benefit value changes from not lower than the benefit threshold to lower than the benefit threshold, the positive sequence reactive current allocation value, negative sequence current allocation value and positive sequence active current allocation value determined in this control cycle remain unchanged, and the negative sequence current allocation value is reduced cycle by cycle according to S5 starting from the next control cycle. When the estimated benefit value changes from being below the benefit threshold to being above the benefit threshold, the S5 process is terminated, the negative sequence current allocation value of this control cycle remains unchanged, and the allocation value of each component is redetermined according to S4 from the next control cycle.

[0015] The beneficial effects of this invention are: 1. This invention uses the product of the positive-sequence voltage amplitude, the negative-sequence voltage amplitude, and the cosine of the phase angle difference between the positive and negative-sequence voltages as the benefit estimate, thereby achieving a quantitative characterization of the actual benefit of injecting negative-sequence current into the inverter. Compared with the existing technology that uses a fixed ratio or fixed priority to allocate current margin and lacks benefit assessment, this invention can promptly stop unnecessary negative-sequence current injection when the benefit is insufficient, thus improving the utilization efficiency of the inverter's current margin.

[0016] 2. This invention dynamically calculates the benefit threshold based on the negative sequence imbalance and sets a lower limit for the benefit threshold. At the same time, it corrects the benefit threshold based on the positive sequence voltage amplitude. Compared with the shortcomings of the prior art, which does not consider the dynamic impact of the voltage imbalance degree on the benefit threshold and does not make corrections for voltage drop conditions, this invention enables the inverter to adaptively adjust the negative sequence current injection strategy under different voltage imbalance degrees and voltage drop conditions, thereby enhancing the adaptability of the control method to complex grid conditions.

[0017] 3. When the estimated benefit value is lower than the benefit threshold, this invention reduces the negative sequence current allocation value in a controlled cycle within a time period not exceeding one fundamental cycle. The released current margin is preferentially replenished to the positive sequence reactive current allocation value and then to the positive sequence active current allocation value. Compared with the defects of the current allocation value caused by the step exit of the prior art, this invention achieves the smooth exit of the negative sequence current and the orderly redistribution of the current margin, ensuring the stable operation of the inverter under unbalanced grid conditions. Attached Figure Description

[0018] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a flowchart illustrating a photovoltaic inverter power conversion control method according to the present invention. Figure 2 This is a schematic diagram illustrating the dynamic calculation of the benefit threshold of this invention; Figure 3 This is a schematic diagram of the dynamic change curve of current distribution in this invention. Detailed Implementation

[0019] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0020] Example 1: As Figure 1 As shown, the present invention provides a photovoltaic inverter power conversion control method, comprising: S1: Separate the positive and negative sequence voltages at the grid connection point to obtain the positive sequence voltage amplitude, negative sequence voltage amplitude, and phase angle difference between the positive and negative sequence voltages; Further, in S1, the step of separating the positive and negative orders includes: A second-order generalized integrator is applied to the grid connection point voltage signal to output a component that is in phase and frequency with the grid connection point voltage signal, as well as a component that is orthogonal to the same frequency. The positive-sequence voltage component and the negative-sequence voltage component are reconstructed by linear combinations of in-phase and quadrature components of the same frequency, respectively. The amplitude and phase angle of the positive-sequence voltage component and the negative-sequence voltage component are extracted respectively, and the difference between the positive-sequence voltage phase angle and the negative-sequence voltage phase angle is taken as the positive-negative-sequence voltage phase angle difference.

[0021] Specifically, the instantaneous values ​​of the three-phase voltages at the grid connection point are obtained after sampling. A Clarke transform is then performed on these instantaneous values ​​to obtain the two-phase voltage components in the stationary coordinate system, denoted as follows: and ,in and They are mutually orthogonal.

[0022] right and A second-order generalized integrator is applied, with its resonant frequency set to the fundamental angular frequency of the power grid. , Provided in real time by a phase-locked loop. The second-order generalized integrator... Output two signals: and In-phase and in-frequency components and with Orthogonal components of the same frequency ;right Similarly, the output and In-phase and in-frequency components and with Orthogonal components of the same frequency .

[0023] Using the four signals mentioned above, the positive-sequence voltage component and the negative-sequence voltage component are reconstructed using the following formula: ; ; in For the positive sequence voltage component in the stationary coordinate system Components on the axis, For the positive sequence voltage component in the stationary coordinate system Components on the axis, For second-order generalized integrators The output components that are in phase and at the same frequency. For second-order generalized integrators The output components with the same frequency and orthogonality, For second-order generalized integrators The output components that are in phase and at the same frequency. For second-order generalized integrators The output consists of orthogonal components of the same frequency.

[0024] ; ; in For the negative sequence voltage component in the stationary coordinate system Components on the axis, For the negative sequence voltage component in the stationary coordinate system Components on the axis, The meaning is the same as in the above formula.

[0025] Extracting the amplitude and phase angle of the positive sequence voltage component, positive sequence voltage amplitude for: ; in For the positive sequence voltage component in the stationary coordinate system Components on the axis, For the positive sequence voltage component in the stationary coordinate system Components on the axis.

[0026] Positive sequence voltage phase angle for: ; in For the positive sequence voltage component in the stationary coordinate system Components on the axis, For the positive sequence voltage component in the stationary coordinate system Components on the axis.

[0027] Similarly, the negative sequence voltage component is obtained. Phase angle of negative sequence voltage ,in Through the negative sequence voltage component in the stationary coordinate system Components on the axis and Components on the axis We get the square root of the sum of the squares. pass and The ratio of the arctangent to the tangent is obtained.

[0028] Positive and negative sequence voltage phase angle difference The difference between the positive-sequence voltage phase angle and the negative-sequence voltage phase angle: ; in The positive sequence voltage phase angle, This is the negative sequence voltage phase angle.

[0029] This yields the positive sequence voltage amplitude required for subsequent steps. Negative sequence voltage amplitude and the phase angle difference between positive and negative sequence voltages .

[0030] S2: The product of the positive sequence voltage amplitude, the negative sequence voltage amplitude, and the cosine of the phase angle difference between the positive and negative sequence voltages is used as the benefit estimate. The benefit estimate characterizes the degree of reduction in the second harmonic power oscillation amplitude at the grid connection point when the inverter injects a unit negative sequence current. Specifically, within each control cycle, the positive sequence voltage amplitude obtained from S1 is... Negative sequence voltage amplitude Phase angle difference between positive and negative sequence voltages Calculate the estimated benefit value : ; in This is the positive sequence voltage amplitude. The magnitude of the negative sequence voltage. The phase angle difference between positive and negative sequence voltages. This is the cosine of the phase angle difference between the positive and negative sequence voltages. In actual control implementation, B is calculated using the rated voltage amplitude at the grid connection point. As a benchmark and After normalization, B is a dimensionless number.

[0031] Due to normalization and All are not greater than 1. , The theoretical upper limit is 1. In actual unbalanced operating conditions, the amplitude of the negative sequence component is much smaller than that of the positive sequence component. It is usually much less than 1.

[0032] Furthermore, the update step of the benefit estimate specifically includes: Resampling within each control cycle , and The benefit estimate is then recalculated using the above formula, and the benefit estimate obtained for this control cycle is recorded as follows: The estimated benefit value for the previous control period is denoted as... The change in the estimated benefit value is obtained by taking the absolute value of the difference between the two. : ; in This is the estimated benefit value calculated for this control period. This is the estimated benefit value from the previous control period. This represents the change in the estimated benefit value.

[0033] Change in the estimated benefit value Compared with the preset change threshold Comparison, This is a fixed value preset based on the system noise level. When Greater than When this occurs, it indicates a significant change in the voltage state at the grid connection point, based on the calculated value for this control cycle. Replace the current benefit estimate, that is, let ;when Not greater than When this occurs, it indicates that the voltage state at the grid connection point has not changed significantly, and the estimated benefit value from the previous control cycle remains unchanged, i.e., let... .

[0034] This gives us the estimated benefit value for the current control cycle. This is for use in subsequent steps.

[0035] S3: The ratio of negative sequence voltage amplitude to positive sequence voltage amplitude is the negative sequence imbalance. The benefit threshold is dynamically calculated based on the negative sequence imbalance. The benefit threshold decreases as the negative sequence imbalance increases. Specifically, the negative sequence voltage amplitude obtained from S1 With positive sequence voltage amplitude The ratio defines the negative order imbalance. : ; in The magnitude of the negative sequence voltage. This is the positive sequence voltage amplitude. It represents the negative order imbalance.

[0036] negative order imbalance Using [variable name] as the independent variable, calculate the benefit threshold using a linear function. : ; in The intercept of the linear function. Let be the slope of the linear function. and All values ​​are positive, ensuring negative order imbalance. The higher the threshold, the greater the benefit. The lower. and It is pre-tuned according to specific system parameters.

[0037] Benefit threshold A lower limit is set , This is a fixed threshold pre-tuned according to system parameters. Its physical meaning is: in the extreme case where all the remaining current margin of the inverter is allocated to the negative sequence current, the minimum benefit level required to ensure the priority of the positive sequence reactive current is guaranteed; if it is lower than this value, the benefit of injecting the negative sequence current is insufficient to support the current margin it occupies. The specific value is determined by engineering debugging and is not uniquely expressed by analytical formulas.

[0038] The benefit threshold obtained from the linear function calculation With benefit threshold lower limit Comparison. When Below At that time, with As the benefit threshold for this control cycle; when Not less than At that time, with This serves as the benefit threshold for this control cycle.

[0039] Furthermore, the benefit threshold is also based on the positive sequence voltage amplitude. Make corrections. Adjust the positive sequence voltage amplitude. With the rated voltage amplitude at the grid connection point Comparison. When Below When it is halfway, one-half and Difference divided by Then multiply by the preset correction factor Obtain the correction amount : ; in Preset correction coefficients for dimensionless quantities. The rated voltage amplitude at the grid connection point. This is the positive sequence voltage amplitude. This is the correction amount, compared to the estimated benefit value. All quantities are dimensionless and have the same dimensions. The correction amount will be... The results are superimposed to the benefit threshold, and the superimposed result is used as the actual benefit threshold for this control period. : ; in To set the benefit threshold before the adjustment is applied. For correction amount, This is the actual benefit threshold for this control cycle. When... Not less than When the effective benefit threshold is half, no adjustment is applied to the actual benefit threshold. equal .

[0040] This yields the actual benefit threshold for this control cycle. This is for use in subsequent steps.

[0041] S4: When the estimated benefit value is not lower than the benefit threshold, the inverter output current is decomposed into three components: positive sequence reactive current, negative sequence current and positive sequence active current. The allocation value of each component is determined in turn, with the constraint that the sum of the amplitudes of the three components does not exceed the upper limit of the inverter current. Specifically, when the estimated benefit value obtained by S2 Not lower than the actual benefit threshold obtained from S3 When the time is right, proceed to this step. Add the positive-sequence reactive current allocation value, negative-sequence current allocation value, and positive-sequence active current allocation value directly. Use the sum as the upper limit of the instantaneous peak value of the three-phase current, ensuring that the upper limit of the instantaneous peak value of the three-phase current does not exceed the inverter current limit. As a constraint, the allocation values ​​of the three components are determined sequentially according to the following steps.

[0042] First, determine the positive sequence reactive current distribution value. Based on the rated voltage amplitude at the grid connection point. With positive sequence voltage amplitude The difference multiplied by the rated current Obtain the positive sequence reactive current distribution value : ; in The rated voltage amplitude at the grid connection point. This is the positive sequence voltage amplitude. Rated current, This represents the positive-sequence reactive current distribution value. When the above formula yields... Exceeding the inverter current limit At that time, with As the positive sequence reactive current allocation value.

[0043] Then the negative sequence current allocation value was determined. Based on the inverter current upper limit Distribution value of positive sequence reactive current The difference is used as the upper limit for the negative sequence current allocation value, and the negative sequence current allocation value is determined within this upper limit range. .when and The sum exceeds At that time, with and The difference is used as the negative sequence current allocation value, while the positive sequence active current allocation value is set to zero.

[0044] Finally, the positive sequence active current distribution values ​​were determined. Based on the inverter current upper limit Subtract the positive sequence reactive current distribution value Subtract the negative sequence current distribution value. The result is used as the positive sequence active current distribution value. ,in This is the upper limit of the inverter current. This represents the positive sequence reactive current distribution value. Assigning values ​​to negative sequence currents. This represents the positive sequence active current allocation value.

[0045] It should be noted that, if Calculated value exceeds ,but Cut off to At this time, the negative sequence current distribution value Zero, positive sequence active current distribution value Since the current margin is also zero, the inverter's entire current margin is used for positive-sequence reactive power support, and there is no issue of negative-sequence current needing to exit. Therefore, it will not enter the S5 process. Entering S5 requires the existence of a non-zero negative-sequence current allocation value, which implies... Not cut off to The positive sequence reactive current distribution value has not yet reached the minimum positive sequence reactive current value specified by the power grid standard. In this case, the two current margin compensation scenarios described above completely cover all possibilities during S5 operation. Thus, the positive sequence reactive current distribution value for this control cycle is obtained. Negative sequence current distribution value Distribution value of positive sequence active current This is for use in subsequent steps.

[0046] S5: When the benefit estimate is lower than the benefit threshold, the negative sequence current allocation value is reduced in each control cycle within a time period not exceeding one fundamental cycle. The reduction amount in each control cycle is positively correlated with the product of the benefit estimate and the current negative sequence current allocation value. The current margin released by the reduction of the negative sequence current allocation value in each control cycle is successively supplemented to the positive sequence reactive current allocation value and the positive sequence active current allocation value until the negative sequence current allocation value is reduced to zero.

[0047] Specifically, when the estimated benefit value obtained by S2 Below the actual benefit threshold of S3 When this step begins, the negative sequence current distribution value is reduced cycle by cycle.

[0048] The total number of control cycles corresponding to one fundamental frequency period is denoted as The number of exit control cycles that have been executed is recorded as The difference between the two is the number of remaining control cycles. ; Furthermore, the step of decreasing the negative sequence current allocation value cycle by cycle includes: The negative sequence current allocation value for this control cycle Divide by the number of remaining control cycles The upper limit of the decrease in negative sequence current is obtained. : ; in This is the negative sequence current allocation value for this control cycle. This represents the number of remaining control cycles. This represents the upper limit of the reduction in negative sequence current.

[0049] Based on benefit estimates Negative sequence current allocation value in this control cycle The product determines the target reduction amount of the negative sequence current. : ; in This is an estimated benefit value. This is the negative sequence current allocation value for this control cycle. This is the amount by which the negative sequence current is reduced.

[0050] Reduce the target amount of negative sequence current Upper limit of negative sequence current reduction Comparison. When No more than At that time, with As the actual reduction of negative sequence current in this control cycle ;when Exceed At that time, with As the actual reduction of negative sequence current in this control cycle The negative sequence current allocation value for this control cycle. Subtract the actual reduction in negative sequence current The result is used as the negative sequence current allocation value for the next control cycle, and the number of control cycles exited is [number missing]. Increment by one, proceed to the next control cycle. The process ends when the negative sequence current allocation value in the next control cycle decreases to zero. The remaining control cycles... When the value decreases to 1, the negative sequence current allocation value is directly set to zero, and the negative sequence current allocation value for this control cycle is used. As the actual reduction of negative sequence current The released residual current margin is sequentially replenished to the positive sequence reactive current allocation value and the positive sequence active current allocation value, and the exit process ends.

[0051] Furthermore, the step of sequentially supplementing the current margin to the positive-sequence reactive current allocation value and the positive-sequence active current allocation value includes: It should be noted that when entering S5, the positive sequence reactive current allocation value has not yet reached the minimum positive sequence reactive current value specified by the power grid standard. The following two situations fully cover all possibilities of current margin supplementation.

[0052] The actual reduction in negative sequence current during this control cycle As a released current margin, it is sequentially replenished to the positive-sequence reactive current distribution value and the positive-sequence active current distribution value. When the positive-sequence reactive current distribution value... With the released current margin The sum shall not exceed the minimum positive sequence reactive current specified in the power grid standard. At that time, the current margin will be released. All added to the positive sequence reactive current distribution value ,in This is the minimum positive-sequence reactive current specified by the power grid standard. When and The sum exceeds At that time, the positive sequence reactive current distribution value will be... Supplement to With the released current margin The portion exceeding the compensation amount is added to the positive sequence active current distribution value. ,in This represents the positive sequence active current allocation value.

[0053] This completes the negative sequence current exit process for this control cycle, and the updated positive sequence reactive current distribution values ​​are now available. Distribution value of positive sequence active current Used in the inverter current control circuit.

[0054] The present invention further includes step S6, which determines the relationship between the estimated benefit value and the benefit threshold in each control cycle: Specifically, within each control period, the estimated benefit value obtained by S2 is... The threshold for actual benefits obtained from S3 The results are compared and contrasted with the comparison results from the previous control cycle to determine whether the magnitude relationship between the two has changed.

[0055] When the benefit estimate From a threshold no lower than the actual benefit level Becoming below the actual benefit threshold When this occurs, it indicates that the benefit of injecting negative-sequence current is insufficient to support the current margin it occupies, and negative-sequence current injection needs to be stopped. Maintain the positive-sequence reactive current allocation value determined according to S4 for this control cycle. Negative sequence current distribution value Distribution value of positive sequence active current The negative sequence current allocation value remains unchanged. Starting from the next control cycle, the negative sequence current allocation value will be reduced cycle by cycle according to S5. The number of control cycles that have been executed to exit control cycle is as follows. Counting starts from zero.

[0056] When the benefit estimate From below the actual benefit threshold Change to no less than the actual benefit threshold When this occurs, it indicates that the injected negative sequence current has regained its benefit, the S5 process is terminated, and the negative sequence current allocation value of this control cycle is maintained. The positive sequence reactive current distribution value remains unchanged, and starting from the next control cycle, it will be redetermined according to S4. Negative sequence current distribution value Distribution value of positive sequence active current The calculation result of S4 directly overwrites the intermediate state during the exit process of S5.

[0057] This completes the state switching judgment of the control process, ensuring that when the relationship between the benefit estimate and the benefit threshold changes, the switching between S4 and S5 is completed within one control cycle.

[0058] Example 2: Example 2 takes a distribution network in a certain area with uneven three-phase load distribution as the background. The grid connection point of this distribution network has long suffered from voltage imbalance and prominent double-frequency power oscillation. The stable operation of photovoltaic inverters connected to this distribution network is affected. Therefore, the photovoltaic inverter power conversion control method of the present invention is used to dynamically manage the inverter output current.

[0059] During a certain operating period, the three-phase voltage imbalance at the grid connection point worsens. After separation of positive and negative sequences in S1, the positive-sequence voltage amplitude is 280V, the negative-sequence voltage amplitude is 42V, and the phase angle difference between the positive and negative sequences is 60°. In S2, the positive and negative sequence voltage amplitudes are normalized using the grid connection point's rated voltage amplitude of 311V as a benchmark and then substituted into the formula, yielding an estimated benefit value of 0.061. In S3, the negative-sequence imbalance is 0.15, and the benefit threshold is dynamically calculated using a linear function, resulting in a value of 0.11. Figure 2 The relationship between the benefit threshold and the negative sequence imbalance is shown, as well as the impact of the correction on the benefit threshold when the positive sequence voltage amplitude is less than half of the rated voltage amplitude at the grid connection point. Figure 2 It can be seen that the benefit threshold decreases linearly with the increase of negative sequence imbalance. After the negative sequence imbalance exceeds a certain level, it is cut off by the lower limit and maintained at the lower limit of 0.04 without further decreasing. When the positive sequence voltage amplitude is lower than half of the rated voltage amplitude at the grid connection point, the correction will raise the benefit threshold as a whole, making the inverter more inclined to withdraw from negative sequence current injection under voltage drop conditions, and giving priority to ensuring positive sequence reactive power support capability.

[0060] During this operating phase, the positive sequence voltage amplitude is 280V, which is 155.5V, half of the rated voltage amplitude of 311V at the grid connection point. No correction is applied, and the actual benefit threshold is 0.11. The estimated benefit value of 0.061 is lower than the actual benefit threshold of 0.11, indicating that the reduction in the second harmonic power oscillation amplitude that can be achieved by continuing to inject negative sequence current into the inverter is limited and insufficient to support the current margin occupied by the negative sequence current. Therefore, the process enters S5, initiating the negative sequence current smooth exit process.

[0061] In this embodiment, the inverter control frequency is 10kHz, and one fundamental frequency cycle (20ms) corresponds to 200 control cycles, i.e., N=200. Figure 3 The curves showing the changes in negative-sequence current allocation, positive-sequence reactive current allocation, and positive-sequence active current allocation as a function of the number of control cycles exiting during the S5 exit process are illustrated. Within a timeframe not exceeding one fundamental cycle (20ms, corresponding to 200 control cycles), the negative-sequence current allocation value decreases from 30A each control cycle. Initially, it decreases linearly due to the upper limit of the negative-sequence current reduction, but later it exhibits an exponential decrease dominated by proportional decay, finally decreasing steadily to zero within the 200th control cycle. The current margin released in each control cycle sequentially replenishes the positive-sequence reactive current allocation and positive-sequence active current allocation values. Figure 3 It is evident that the positive sequence reactive current allocation value is increased from 75A first, and after reaching the minimum value of 80A specified by the grid standard, it is no longer increased. The remaining release margin is used to supplement the positive sequence active current allocation value, which is gradually increased from 5A to 30A. The inverter restores active power output under the premise of meeting the reactive power support requirements of the grid standard.

[0062] Compared to the situation without the method of this invention, if the inverter continuously injects 30A of negative-sequence current without benefit assessment, the 30A current margin cannot effectively reduce the second harmonic power oscillation amplitude when the estimated benefit value is already below the benefit threshold. Simultaneously, the positive-sequence reactive current allocation value is only 75A, lower than the minimum value of 80A specified by the grid standard, and the positive-sequence active current allocation value is only 5A, severely limiting active power output. This invention, through dynamic comparison of the estimated benefit value and the actual benefit threshold, promptly initiates a smooth exit process when the benefit from injected negative-sequence current is insufficient, completing the switchover within 20ms, avoiding sudden changes in current allocation, and ensuring stable operation of the inverter under unbalanced grid conditions.

[0063] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A photovoltaic inverter power conversion control method, characterized in that, include: S1: Separate the positive and negative sequence voltages at the grid connection point to obtain the positive sequence voltage amplitude, negative sequence voltage amplitude, and phase angle difference between the positive and negative sequence voltages; S2: Based on the rated voltage amplitude at the grid connection point, normalize the positive-sequence voltage amplitude and the negative-sequence voltage amplitude respectively; use the product of the normalized positive-sequence voltage amplitude, the normalized negative-sequence voltage amplitude, and the cosine of the phase angle difference between the positive and negative-sequence voltages as the benefit estimate, which characterizes the degree of reduction in the second harmonic power oscillation amplitude at the grid connection point when the inverter injects a unit negative-sequence current; S3: The ratio of negative sequence voltage amplitude to positive sequence voltage amplitude is the negative sequence imbalance. The benefit threshold is dynamically calculated based on the negative sequence imbalance. The benefit threshold decreases as the negative sequence imbalance increases. The dynamic calculation steps for the benefit threshold include: Using negative order imbalance as the independent variable, the benefit threshold is calculated as a linear function. The higher the negative order imbalance, the lower the benefit threshold. A fixed threshold predetermined by the engineering process is used as the lower limit of the benefit threshold; When the benefit threshold calculated by the linear function is lower than the lower limit of the benefit threshold, the lower limit of the benefit threshold shall be used as the benefit threshold for this control period. When the benefit threshold calculated by the linear function is not lower than the lower limit of the benefit threshold, the benefit threshold calculated by the linear function shall be used as the benefit threshold for this control cycle. The benefit threshold is further corrected based on the positive sequence voltage amplitude, and the correction steps include: Compare the positive sequence voltage amplitude with the rated voltage amplitude at the grid connection point; When the positive sequence voltage amplitude is lower than half of the rated voltage amplitude at the grid connection point, the difference between half of the rated voltage amplitude at the grid connection point and the positive sequence voltage amplitude is divided by the rated voltage amplitude at the grid connection point and then multiplied by a preset correction coefficient to obtain the correction amount. The correction amount is then added to the benefit threshold, and the result of the addition is used as the actual benefit threshold for this control cycle. When the positive sequence voltage amplitude is not less than half of the rated voltage amplitude at the grid connection point, no correction is applied to the benefit threshold, and the benefit threshold is used as the actual benefit threshold for this control cycle. S4: When the estimated benefit value is not lower than the actual benefit threshold, the inverter output current is decomposed into three components: positive sequence reactive current, negative sequence current and positive sequence active current. The allocation value of each component is determined in turn, with the constraint that the sum of the amplitudes of the three components does not exceed the upper limit of the inverter current. S5: When the estimated benefit value is lower than the actual benefit threshold, the negative sequence current allocation value is reduced in each control cycle within a time period not exceeding one fundamental cycle. The reduction amount in each control cycle is positively correlated with the product of the estimated benefit value and the current negative sequence current allocation value. The current margin released by the reduced negative sequence current allocation value in each control cycle is successively supplemented to the positive sequence reactive current allocation value and the positive sequence active current allocation value until the negative sequence current allocation value is reduced to zero.

2. The photovoltaic inverter power conversion control method according to claim 1, characterized in that, In S1, the step of separating the positive and negative orders includes: A second-order generalized integrator is applied to the grid connection point voltage signal to output a component that is in phase and frequency with the grid connection point voltage signal, as well as a component that is orthogonal to the same frequency. The positive-sequence voltage component and the negative-sequence voltage component are reconstructed by linear combinations of in-phase and quadrature components of the same frequency, respectively. The amplitude and phase angle of the positive-sequence voltage component and the negative-sequence voltage component are extracted respectively, and the difference between the positive-sequence voltage phase angle and the negative-sequence voltage phase angle is taken as the positive-negative-sequence voltage phase angle difference.

3. The photovoltaic inverter power conversion control method according to claim 1, characterized in that, In S2, the update step of the benefit estimate includes: In each control cycle, the positive-sequence voltage amplitude, negative-sequence voltage amplitude, and positive-negative-sequence voltage phase angle difference are resampled, and the benefit estimate is recalculated. The absolute value of the difference between the estimated benefit value of the current control cycle and the estimated benefit value of the previous control cycle is used to obtain the change in the estimated benefit value. When the change in the estimated benefit value exceeds the preset change threshold, the current estimated benefit value is replaced by the value calculated for the current control period. When the change in the estimated benefit value is not greater than the preset change threshold, the estimated benefit value of the previous control period remains unchanged.

4. The photovoltaic inverter power conversion control method according to claim 1, characterized in that, In S4, the constraint determination step of ensuring that the sum of the three component amplitudes does not exceed the upper limit of the inverter current includes: The positive sequence reactive current distribution value, negative sequence current distribution value and positive sequence active current distribution value are directly added together, and the sum is used as the upper limit of the instantaneous peak value of the three-phase current. The constraint condition is that the upper limit of the instantaneous peak value of the three-phase current does not exceed the upper limit of the inverter current. When the sum of the positive-sequence reactive current allocation value and the negative-sequence current allocation value exceeds the inverter current limit, the difference between the inverter current limit and the positive-sequence reactive current allocation value is used as the negative-sequence current allocation value, and the positive-sequence active current allocation value is set to zero.

5. The photovoltaic inverter power conversion control method according to claim 1, characterized in that, In S5, the step of decreasing the negative sequence current allocation value cycle by cycle includes: The remaining number of control cycles is determined by the difference between the total number of control cycles corresponding to one fundamental cycle and the number of control cycles that have been executed and exited. When the number of remaining control cycles is 1, the negative sequence current allocation value is directly set to zero, and the negative sequence current allocation value of this control cycle is used as the actual reduction of the negative sequence current, and the process ends. When the number of remaining control cycles is greater than 1, the following steps are performed: the upper limit of the reduction of the negative sequence current is obtained by dividing the negative sequence current allocation value of this control cycle by the number of remaining control cycles. The target reduction in negative sequence current is determined by multiplying the estimated benefit value by the negative sequence current allocation value for this control cycle. When the target reduction of negative sequence current does not exceed the upper limit of the reduction of negative sequence current, the target reduction of negative sequence current shall be used as the actual reduction of negative sequence current in this control cycle. When the target reduction of negative sequence current exceeds the upper limit of negative sequence current reduction, the upper limit of negative sequence current reduction shall be used as the actual reduction of negative sequence current in this control cycle. The negative sequence current allocation value minus the actual decrease in negative sequence current is used as the negative sequence current allocation value for the next control cycle. The process ends when the negative sequence current allocation value of the next control cycle decreases to zero.

6. The photovoltaic inverter power conversion control method according to claim 1, characterized in that, In S5, the step of sequentially supplementing the current margin to the positive-sequence reactive current allocation value and the positive-sequence active current allocation value includes: The actual decrease in negative sequence current during this control cycle is taken as the current margin to be released. When the sum of the positive sequence reactive current allocation value and the released current margin does not exceed the minimum positive sequence reactive current value specified by the power grid standard, the released current margin will be fully added to the positive sequence reactive current allocation value. When the sum of the positive sequence reactive current allocation value and the released current margin exceeds the minimum positive sequence reactive current value specified by the power grid standard, the positive sequence reactive current allocation value will be supplemented to the minimum positive sequence reactive current value specified by the power grid standard, and the portion of the released current margin that exceeds the supplementary amount will be added to the positive sequence active current allocation value.

7. The photovoltaic inverter power conversion control method according to claim 1, characterized in that, It also includes step S6, which determines the relationship between the estimated benefit value and the actual benefit threshold in each control cycle: When the estimated benefit value changes from not lower than the actual benefit threshold to lower than the actual benefit threshold, the positive sequence reactive current allocation value, negative sequence current allocation value and positive sequence active current allocation value determined in this control cycle remain unchanged, and the negative sequence current allocation value is reduced cycle by cycle according to S5 starting from the next control cycle. When the estimated benefit value changes from being lower than the actual benefit threshold to being no lower than the actual benefit threshold, the S5 process is terminated, the negative sequence current allocation value of this control cycle remains unchanged, and the allocation value of each component is re-determined according to S4 from the next control cycle.

Images