Electric energy quality optimal compensation control method and device based on photovoltaic converter

Abstract

The invention discloses an electric energy quality optimal compensation control method and device based on a photovoltaic converter, and the method and device achieve the optimal compensation control of the electric energy quality of the photovoltaic converter by exploring the self-regulation potential of the photovoltaic converter and carrying out the autonomous adjustment of the electric energy quality while having the renewable energy consumption capability. According to the basic principle of the control method, reactive, negative sequence and harmonic current components are detected from a compensation object, and a compensation current reference value of the photovoltaic converter is generated by combining the output condition of the current photovoltaic converter and through instruction optimization of electric energy quality treatment current; therefore, the purpose of maximally utilizing the capacity of the photovoltaic converter to treat the electric energy quality is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power quality compensation and distributed photovoltaic converter control, and relates to a power quality optimal compensation control method and device based on a photovoltaic converter. Background Art

[0002] Photovoltaic converter systems generally use a voltage-type inverter main circuit, which is consistent with the main circuit structure of the APF. Traditional photovoltaic grid-connected systems and active filters have many similarities:

[0003] ① Both are controlled current sources, and both generate the required grid-connected current according to the command current signal;

[0004] ② The converter topologies of the two are similar, both including a DC-AC converter and DC bus capacitors;

[0005] ③ The key control technologies of both include DC voltage control, current tracking control and filtering, and phase-locked technology.

[0006] This suggests that the remaining capacity of the photovoltaic inverter device can be used to achieve power quality management. Without changing the original device, the dual functions of photovoltaic grid connection and power quality management can be realized. This can not only improve the power quality of the power system, but also increase the utilization rate of the photovoltaic grid-connected system device, saving additional equipment investment.

[0007] One of the key technical challenges is how to effectively utilize the remaining capacity of PV converters to improve busbar power quality while ensuring their safe operation. To address these issues, many studies have proposed control and optimization schemes. However, existing methods focus on the comprehensive evaluation of post-treatment power quality indicators and simply consider the compensation capacity of PV converters as the sum of the squares of reactive, unbalanced, and harmonic currents. They lack consideration of three-phase current limiting under the interaction of positive and negative sequence components. Consequently, existing methods are unable to ensure that the three-phase currents do not exceed the limit or maximize the utilization of the remaining capacity of the PV converter. Furthermore, existing methods focus on optimizing solutions using Lagrangian and heuristic algorithms to generate compensation current reference values for PV converters, making them difficult to implement in real time within DSPs.

[0008] Therefore, how to maximize the use of the remaining capacity of the photovoltaic converter to manage power quality while meeting the safe operation constraints of the converter with three-phase current limitation needs further research, and how to enable the designed control strategy to be quickly solved or calculated in real time in DSP is a problem that needs to be solved. Summary of the Invention

[0009] To address the limitations of existing photovoltaic converter power quality management solutions, which ignore the phasor relationship between positive and negative sequence components and lack consideration for real-time control, the present invention provides a photovoltaic converter-based power quality optimal compensation control method and device. By exploring the photovoltaic converter's own regulation potential, while maintaining the ability to absorb renewable energy, the method independently adjusts power quality and achieves optimal power quality compensation control for the photovoltaic converter. The basic principle of the control method is to detect reactive, negative sequence, and harmonic current components from the compensation object, combine them with the current output of the photovoltaic converter, and generate a compensation current reference value for the photovoltaic converter through power quality management current command optimization, thereby achieving the purpose of maximizing the utilization of the photovoltaic converter capacity to manage power quality.

[0010] The present invention is implemented by adopting the following technical solutions: First, the present invention provides an optimal compensation control method for power quality based on a photovoltaic converter. The method detects reactive, negative sequence and harmonic current components from the compensation object, combines the current output of the photovoltaic converter, optimizes the power quality control current instruction, and generates a compensation current reference value for the photovoltaic converter, thereby achieving the purpose of maximizing the utilization of the photovoltaic converter capacity to control power quality.

[0011] Furthermore, without changing the original device of the existing two-stage photovoltaic converter, the dual functions of photovoltaic grid connection and power quality management are achieved. The specific control architecture is as follows:

[0012] The photovoltaic converter involved in power quality management consists of a DC-DC converter and a DC-AC converter. The converter uses an LCL filter to suppress harmonics in the switching frequency band, and uses a capacitor branch with a series resistor to suppress the resonant peak of the LCL filter.

[0013] The DC-AC part includes filtering, phase-locked loop unit, DC bus voltage control and current inner loop control, as well as compensation current generation and current tracking control; by adjusting the grid-connected positive sequence active current command signal of the DC-AC converter to maintain the DC bus voltage stable, the power output of the photovoltaic array to the grid is realized; at the same time, according to the power quality management requirements, the positive sequence reactive current command, harmonic current command and negative sequence current command that need to be compensated are obtained, combined with the real-time positive sequence active current of the photovoltaic converter , through the power quality management current instruction optimization, the compensation current reference value of the photovoltaic converter is obtained, and then through current tracking control, the converter outputs the corresponding compensation current according to the compensation current reference value;

[0014] In the DC-DC part, photovoltaics use MPPT control to enable the photovoltaic array to output the maximum available power. , or use the perturbation observation method P&O power control method to make the photovoltaic output a given power , and in Output power .

[0015] Furthermore, reactive, negative sequence and harmonic current components are detected from other reactive, unbalanced and harmonic load compensation objects on the same bus, specifically including:

[0016] Additional current sensors are installed to sample the three-phase current flowing through the compensation object. The measured current direction is from the load to the busbar. The three-phase photovoltaic converter is a three-phase three-wire system, so zero-sequence current compensation is not considered.

[0017] After Clark transformation, we get Current below the shaft , where the subscript L represents the load, and the fundamental positive sequence component of the current is obtained by the second-order generalized integrator SOGI and the 90-degree lag link , and the fundamental negative sequence component ;

[0018] Will minus and ,get Harmonic current components under the shaft , and the harmonic components of the three-phase current ;

[0019] After Park transformation and 100Hz sliding average filtering, the positive sequence current component under the dq axis is obtained, that is, the positive sequence active current and positive sequence reactive current ;

[0020] After Park transformation and 100Hz sliding average filtering, the negative sequence current component under the dq axis is obtained and .

[0021] Furthermore, the process of optimizing the power quality management current instructions is to generate the compensation current reference value of the photovoltaic converter based on the reactive, negative sequence and harmonic current components detected from the compensation object; and to execute the optimal control strategy or the local control strategy according to the local computing power of the photovoltaic converter.

[0022] Furthermore, the optimal control strategy is suitable for situations where a local computer is deployed on the PV converter with sufficient computing power. After sampling the reactive, negative sequence, and harmonic current components, the PV converter uploads the program to the local computer for optimization calculation, generates the compensation current reference value for the PV converter, and then sends it to the PV converter for execution.

[0023] In this control strategy, the input of the power quality management current instruction optimization module is the harmonic component of the three-phase current , positive sequence reactive current , negative sequence current component and , and the real-time positive sequence active current of the photovoltaic converter ;

[0024] After optimization calculation, the output of the power quality management current instruction optimization process is the harmonic current reference value ,in is the harmonic current compensation coefficient and , indicating that the compensated harmonic current is proportional to the harmonic current flowing through the compensation object, and the reactive current instruction , negative sequence current instruction and , 、 、 and That is, the variables that need to be solved for the optimization problem;

[0025] Specifically, during power quality compensation, the PV converter must ensure that the effective value of the three-phase current does not exceed the limit to prevent excessive heating of components. Based on the vector relationship between the positive-sequence and negative-sequence currents, and the conversion rules between the sequence components and the three-phase currents, the three-phase current limit constraint is expressed as:

[0026]

[0027]

[0028]

[0029]

[0030] in is the maximum allowable value of the three-phase current of the converter, is the maximum value of the three-phase harmonic current effective value;

[0031] The control objective is to maximize the compensation of reactive, negative-sequence, and harmonic current components detected using the remaining capacity of the photovoltaic converter in addition to active power generation. That is, the sum of the uncompensated reactive, negative-sequence, and harmonic current components is minimized. Therefore, the control objective is expressed as:

[0032]

[0033] in is the objective function, is the uncompensated reactive current part, is the uncompensated harmonic current part, The uncompensated unbalanced current part, the superscript w represents the uncompensated part, and the subscript un represents the three-phase unbalanced part, that is, the negative sequence part. is the coefficient corresponding to different power quality pollution;

[0034] The uncompensated reactive, negative sequence and harmonic current components are expressed as:

[0035]

[0036]

[0037]

[0038] The optimization problem consists of quadratic constraints. The Lagrange multiplier method is used to directly solve the optimization problem. 、 、 and This is the current compensation instruction of the photovoltaic converter.

[0039] Furthermore, the local control strategy is suitable for situations where the local computing power of the photovoltaic converter is insufficient. After sampling the reactive, negative-sequence, and harmonic current components, the photovoltaic converter directly calculates them in the DSP of the photovoltaic converter controller to generate and execute the compensation current reference value of the photovoltaic converter. Compared with the optimal control strategy, although the local control strategy cannot meet the control objective of minimizing the sum of the uncompensated reactive, negative-sequence, and harmonic current components, it is executed in real time in the DSP and meets the constraint that the effective value of the three-phase current does not exceed the limit.

[0040] In this control strategy, the input of the power quality management current instruction optimization module is the harmonic component of the three-phase current , positive sequence reactive current , negative sequence current component and , and the real-time positive sequence active current of the photovoltaic converter ;

[0041] After optimization calculation, the output of the power quality management current instruction optimization module is the harmonic current reference value , reactive current command , negative sequence current instruction and ,in is the harmonic current compensation coefficient, is the reactive current compensation coefficient, is the negative sequence current compensation coefficient, and , , , indicating that the compensated reactive, negative sequence and harmonic currents are proportional to the current flowing through the compensation object. 、 、 The size between them is proportional to the coefficient corresponding to the power quality pollution , that is, the variables required to solve the optimization problem;

[0042] Specifically, during power quality compensation, the PV converter should ensure that the effective value of the three-phase current does not exceed the limit. Based on the vector relationship between the positive-sequence and negative-sequence currents, and the conversion rules between the sequence components and the three-phase currents, the three-phase current limit constraint is expressed as:

[0043]

[0044]

[0045]

[0046]

[0047] in is the maximum allowable value of the three-phase current of the converter, is the maximum effective value of the three-phase harmonic current, is the negative sequence current amplitude flowing through the load, that is , is the phase angle of the negative sequence current flowing through the load in the negative sequence dq coordinate system, expressed as:

[0048]

[0049] For the three-phase current limit constraint, if The constraint is simplified to 、 、 A quadratic equation with one of the three compensation coefficients as a variable is directly solved in DSP to obtain the value of the compensation coefficient;

[0050] right The three equations are solved for compensation coefficients in DSP respectively, and the smallest compensation coefficient is selected as the final output solution. 、 、 and This is the current compensation instruction of the photovoltaic converter.

[0051] Furthermore, the process of current tracking control for causing the converter to output a corresponding compensation current according to the compensation current reference value includes:

[0052] Since the ideal PR controller has component errors when it is digitally implemented, a quasi-PR controller is used to track the fundamental and harmonic currents. The expression of the PR controller is:

[0053]

[0054] in is the proportional gain, is the cutoff frequency, is the resonant gain, is the resonant frequency;

[0055] Considering that most of the harmonics in the system are odd harmonics, and 6k+1 is mostly positive sequence, 6k-1 is mostly negative sequence, and 3k is zero sequence, which are absorbed by the delta-connected transformer, the starting point of the coordinate transformation in the PR controller solution is changed from The coordinate system is moved to the fundamental wave rotation coordinate system, and the 6k+1 and 6k-1 harmonics will both be converted into the 6k harmonic, and the amount of calculation required for control will be reduced by half;

[0056] In addition, due to the sampling and control delays in the discretized system, the influence of the delay compensation on the system stability needs to be analyzed in the control. For the 6k±1 harmonic, the corresponding relationship between the compensated phase angle and the system characteristic root distribution is established according to the control block diagram of the system, and the delay compensation phase angle of the PR controller with different frequencies is determined. .

[0057] Furthermore, the process of discretizing the PR controller with delay compensation includes:

[0058] In the PR controller, a pre-corrected bilinear transformation is used in the process of discretizing the controller;

[0059] And when the system frequency shifts, a variable-frequency high-order harmonic delay compensation is designed. If the system transmission frequency changes, the coefficient of the PR controller will be updated in real time.

[0060] In a second aspect, the present invention also provides an optimal compensation control device for power quality based on a photovoltaic converter, comprising a memory and one or more processors, wherein the memory stores executable code, and when the processor executes the executable code, it implements the optimal compensation control method for power quality based on a photovoltaic converter.

[0061] In a third aspect, the present invention further provides a computer-readable storage medium having a program stored thereon, which, when executed by a processor, implements the aforementioned method for optimal compensation control of power quality based on a photovoltaic converter.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] (1) The present invention provides a method for optimal power quality compensation control based on a photovoltaic converter. Based on the reactive, negative sequence and harmonic current components detected from the compensation object, combined with the current output of the photovoltaic converter, the method generates a compensation current reference value for the photovoltaic converter through power quality control current instruction optimization, thereby maximizing the use of the photovoltaic converter capacity to control power quality.

[0064] (2) The present invention takes into account the phasor relationship between the positive and negative sequence components when calculating the compensation current reference value of the photovoltaic converter, thereby controlling the current of each phase below the maximum allowable current of the photovoltaic converter, thereby ensuring the safe operation of the converter;

[0065] (3) The present invention designs two control methods for optimizing the calculation of the power quality management current instructions based on the computing power of the photovoltaic converter location. The optimal control strategy is executed in the local computer to minimize the sum of the uncompensated reactive power, negative sequence and harmonic current components. The local control strategy is executed in the DSP. It is simple to implement and can be executed in real time in the DSP, and it meets the constraint that the effective value of the three-phase current does not exceed the limit. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0067] Figure 1 Schematic diagram of the unified control architecture of photovoltaic converters for achieving photovoltaic grid connection and power quality management.

[0068] Figure 2 Schematic diagram of filtering, sequence component extraction, and phase-locked link to obtain the positive and negative sequence components of the voltage and current fundamental waves.

[0069] Figure 3 The phasor relationship diagram of different current components of the photovoltaic converter involved in power quality compensation.

[0070] Figure 4 A schematic diagram of optimizing the reactive, negative sequence and harmonic current components and the current instructions for power quality management is provided for detecting the reactive, negative sequence and harmonic current components from the compensation object.

[0071] Figure 5 Schematic diagram of current inner loop control - multi-PR control based on fundamental wave rotating coordinate system.

[0072] Figure 6 Schematic diagram of high-order harmonic delay compensation stability analysis and characteristic root distribution.

[0073] Figure 7 The waveforms of the photovoltaic converter and grid current when the optimal control strategy is executed in the application example.

[0074] Figure 8 This is the FFT analysis diagram of the grid current when the optimal control strategy is executed in the application example.

[0075] Figure 9 The waveforms of the photovoltaic converter and grid current when the local control strategy is implemented in the application example.

[0076] Figure 10 This is the FFT analysis diagram of the grid current when the local control strategy is executed in the application example.

[0077] Figure 11 This is a structural diagram of a power quality optimal compensation control device based on a photovoltaic converter according to the present invention. DETAILED DESCRIPTION

[0078] The following is an explanation and description of the technical solutions of the embodiments of the present invention in conjunction with the drawings of the embodiments of the present invention, but the following embodiments are only preferred embodiments of the present invention and are not exhaustive. Based on the embodiments in the implementation mode, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0079] Embodiment: The present invention is a method for optimal power quality compensation control based on a photovoltaic converter, which comprises the following steps:

[0080] S1, without changing the original device of the existing two-stage photovoltaic converter, realizes the dual functions of photovoltaic grid connection and power quality management, and builds a unified control architecture for photovoltaic converter active power generation and power quality management. The specific process includes:

[0081] like Figure 1 As shown in (a), a unified control architecture for photovoltaic converters is implemented to achieve both photovoltaic grid connection and power quality management. The photovoltaic converter involved in power quality management consists of a DC-DC converter and a DC-AC converter. The converter uses an LCL filter to suppress harmonics in the switching frequency band, and a capacitor branch with a series resistor is used to suppress the resonant peak of the LCL filter.

[0082] For the DC-AC part, in addition to the traditional filtering, phase-locked loop unit, DC bus voltage control and current inner loop control, it also includes the generation of compensation current and current tracking control. By adjusting the grid-connected positive sequence active current command signal of the DC-AC converter to maintain the DC bus voltage stable, the power output of the photovoltaic array to the grid is achieved; at the same time, according to the power quality management requirements, the positive sequence reactive current command, harmonic current command, and negative sequence current command that need to be compensated are obtained, combined with the real-time positive sequence active current of the photovoltaic converter. , through the power quality management current instruction optimization, the compensation current reference value of the photovoltaic converter is obtained, and then through appropriate current tracking control, the converter outputs the corresponding compensation current according to the compensation current reference value;

[0083] In the DC-DC part, photovoltaics use MPPT control to enable the photovoltaic array to output the maximum available power. , or use the perturbation observation method P&O power control method to make the photovoltaic output a given power , and in Output power , its specific control strategy is as follows Figure 1 As shown in (b);

[0084] S2 detects reactive, negative sequence, and harmonic current components from the compensation target (i.e., other reactive, unbalanced, and harmonic loads on the same bus). The specific process includes:

[0085] Additional current sensors are installed to sample the three-phase current flowing through the compensation object. The measured current direction is from the load to the busbar. Common three-phase photovoltaic converters use a three-phase three-wire system, so zero-sequence current compensation is not considered.

[0086] Figure 2 The filtering, sequence component extraction, and phase-locked links of the converter are demonstrated. Through these links, the positive and negative sequence components of the voltage and current fundamentals, as well as the phase of the grid-connected point voltage, can be obtained. Specifically, the Clark transform is used to obtain Current below the shaft , where the subscript L represents the load, and the fundamental positive sequence component of the current is further obtained through the second-order generalized integrator (SOGI) and the 90-degree lag link , and the fundamental negative sequence component ; The phase of the positive sequence voltage at the grid connection point can be obtained by Park transformation and controlling the q-axis voltage to 0. , Can be used for Park transformation of positive sequence voltage and current;

[0087] Figure 3 The phasor relationship diagram of different current components of the photovoltaic converter involved in power quality compensation is shown. Figure 3 It can be seen that the phase currents determined by active, reactive, unbalanced, and harmonic currents are not in a square sum relationship. The vector relationship of these currents must be considered when generating the compensation current reference value for the PV converter to ensure that the three-phase currents do not exceed the limit.

[0088] Furthermore, the control scheme of reactive power, negative sequence and harmonic current components detected from the compensation object is as follows Figure 4 As shown, for harmonic current, minus and ,get Harmonic current components under the shaft , and the harmonic components of the three-phase current ;

[0089] For reactive current, After Park transformation and 100Hz sliding average filtering, the positive sequence current component under the dq axis is obtained, that is, the positive sequence active current , and positive sequence reactive current ;

[0090] For negative sequence current, After Park transformation and 100Hz sliding average filtering, the negative sequence current component under the dq axis is obtained and ;

[0091] Compared with the damping coefficient Taking a second-order Butterworth filter with a value of 0.707 (bandwidth 30 Hz), the bandwidth of a 100 Hz sliding average filter is 44.3 Hz. The sliding average filter has a higher bandwidth, a faster response, and better attenuation of negative sequence and characteristic subharmonics.

[0092] S3, based on the reactive power, negative sequence and harmonic current components detected from the compensation object and the compensation current reference value of the photovoltaic converter is generated, that is, the power quality management current instruction is optimized and calculated. According to the local computing power of the photovoltaic converter, it can be divided into two different control strategies, namely (1) optimal control strategy or (2) local control strategy, which are executed in the local computer or DSP respectively. The corresponding power quality management current instruction optimization diagram is shown in FIG. Figure 4 As shown, the specific process includes:

[0093] (1) Optimal control strategy

[0094] This control strategy is suitable for situations where a local computer is deployed on the PV converter with sufficient computing power. After sampling the reactive, negative sequence, and harmonic current components, the PV converter uploads the program to the local computer for optimization calculation, generates the compensation current reference value for the PV converter, and then sends it to the PV converter for execution.

[0095] In this control strategy, the input of the power quality management current instruction optimization module is the harmonic component of the three-phase current (The maximum value of the three-phase harmonic current effective value is recorded as ), positive sequence reactive current , negative sequence current component and , and the real-time positive sequence active current of the photovoltaic converter ;

[0096] After optimization calculation, the output of the power quality management current instruction optimization module is the harmonic current reference value (in is the harmonic current compensation coefficient and , indicating that the compensated harmonic current is proportional to the harmonic current flowing through the compensation object), reactive current instruction , negative sequence current instruction and , 、 、 and That is, the variables that need to be solved for the optimization problem;

[0097] Specifically, during power quality compensation, the PV converter must ensure that the effective value of the three-phase current does not exceed the limit to prevent excessive heating of components. Based on the vector relationship between the positive-sequence and negative-sequence currents, and the conversion rules between the sequence components and the three-phase currents, the three-phase current limit constraint can be expressed as:

[0098]

[0099]

[0100]

[0101]

[0102] in is the maximum allowable value of the three-phase current of the converter;

[0103] The control objective is to maximize the compensation of reactive, negative-sequence, and harmonic current components detected using the remaining capacity of the PV converter in addition to active power generation. That is, the sum of the uncompensated reactive, negative-sequence, and harmonic current components is minimized. Therefore, the control objective can be expressed as:

[0104]

[0105] in is the objective function, is the uncompensated reactive current part, is the uncompensated harmonic current part, The uncompensated unbalanced current part, the superscript w represents the uncompensated part, and the subscript un represents the three-phase unbalanced part, that is, the negative sequence part. is the coefficient corresponding to different power quality pollution;

[0106] The uncompensated reactive, negative sequence and harmonic current components can be expressed as:

[0107]

[0108]

[0109]

[0110] The optimization problem consists of quadratic constraints and can be solved directly on a local computer using the Lagrange multiplier method (the solution time is less than 0.01 seconds and the solution interval can be set to ten to one hundred milliseconds). 、 、 and That is the current compensation instruction of the photovoltaic converter;

[0111] (2) On-site control strategy

[0112] This control strategy is suitable for situations where the local computing power of the photovoltaic converter is insufficient. After sampling the reactive, negative-sequence, and harmonic current components, the photovoltaic converter directly calculates them in the DSP of the photovoltaic converter controller to generate and execute the compensation current reference value of the photovoltaic converter. Compared with the optimal control strategy, although the local control strategy cannot meet the control goal of minimizing the sum of the uncompensated reactive, negative-sequence, and harmonic current components, it is simple to implement, executes in real time in the DSP, and meets the constraint that the effective value of the three-phase current does not exceed the limit.

[0113] In this control strategy, the input of the power quality management current instruction optimization module is the harmonic component of the three-phase current (The maximum value of the three-phase harmonic current effective value is recorded as ), positive sequence reactive current , negative sequence current component and , and the real-time positive sequence active current of the photovoltaic converter ;

[0114] After optimization calculation, the input of the power quality management current instruction optimization module is the harmonic current reference value , reactive current command , negative sequence current instruction and (in is the harmonic current compensation coefficient, is the reactive current compensation coefficient, is the negative sequence current compensation coefficient, and , , , indicating that the compensated reactive, negative sequence and harmonic currents are proportional to the current flowing through the compensation object. 、 、 The size is proportional to ), which is the variable that needs to be solved for the optimization problem;

[0115] Specifically, during power quality compensation, the PV converter should ensure that the effective value of the three-phase current does not exceed the limit. Based on the vector relationship between the positive-sequence and negative-sequence currents, and the transformation rules between the sequence components and the three-phase currents, the three-phase current limit constraint can be expressed as:

[0116]

[0117]

[0118]

[0119]

[0120] in is the maximum allowable value of the three-phase current of the converter, is the negative sequence current amplitude flowing through the load, that is , is the phase angle of the negative sequence current flowing through the load in the negative sequence dq coordinate system, which can be expressed as:

[0121]

[0122] For the three-phase current limit constraint, if The constraint can be simplified to 、 、 A quadratic equation with one of the three compensation coefficients as a variable (if If a certain compensation coefficient is calculated to be greater than 1, then set it equal to 1 and calculate the remaining two compensation coefficients until all three compensation coefficients are less than or equal to 1). Therefore, the size of the compensation coefficient can be directly solved in DSP;

[0123] right The three equations are solved for compensation coefficients in DSP respectively, and the smallest compensation coefficient is selected as the final output solution. 、 、 and That is the current compensation instruction of the photovoltaic converter;

[0124] S4, performs current tracking control, so that the converter outputs the corresponding compensation current according to the compensation current reference value, and discretizes the controller. The specific process includes:

[0125] Since the ideal PR controller has component errors when it is digitally implemented, a quasi-PR controller is used to track the fundamental and harmonic currents. The expression of the PR controller is:

[0126]

[0127] in is the proportional gain, is the cutoff frequency, is the resonant gain, is the resonant frequency;

[0128] The national standard GB 17625.1-2022 "Electromagnetic compatibility limits Part 1: Harmonic current emission limits (equipment input current per phase ≤ 16A)" stipulates the harmonic emission limits within the 25th order of the equipment. For the compensation of harmonics up to the 25th order, In the coordinate system, since a PR controller can only correspond to a harmonic of a specific frequency, the compensation of harmonic currents in steps is relatively cumbersome and requires large computing resources;

[0129] Considering that most of the harmonics in the system are odd harmonics, and 6k+1 is mostly positive sequence, 6k-1 is mostly negative sequence, and 3k is zero sequence (which can be absorbed by the delta-connected transformer), the starting point of the coordinate transformation in the PR controller solution is changed from The coordinate system is moved to the fundamental wave rotation coordinate system, and the 6k+1 and 6k-1 harmonics will be converted into 6k harmonics. The amount of calculation required for control will be reduced by half. The specific current inner loop control diagram is shown in the figure. Figure 5 As shown;

[0130] In addition, due to the sampling and control delays in the discretized system, the compensation effect of high-order harmonic currents is poor. Time delay compensation must be added when performing high-order harmonic current compensation control. In the control, it is necessary to analyze the impact of the time delay compensation on the system stability. For the 6k±1 harmonics, the corresponding relationship between the compensated phase angle and the system characteristic root distribution is established according to the system control block diagram, and the time delay compensation phase angle of the PR controller with different frequencies is determined. ;

[0131] According to the mathematical model of photovoltaic converter, The control block diagram of the harmonic current loop in the coordinate system can be expressed as follows Figure 6 The structure shown in the figure, where L is the inductance value of the AC side, R is the equivalent resistance of the inductance of the AC side, is the period corresponding to the switching frequency. And the modulation strategy adopted by the system is an asymmetric regular sampling modulation strategy, so the time constant of the small inertia link of the current loop signal sampling delay is .

[0132] In order to analyze the impact of time delay compensation on system stability, taking the 25th harmonic as an example, the corresponding relationship between the number of compensated switching cycles and the distribution of system characteristic roots is established according to the control block diagram of the system, where k is the number of compensated PWM switching cycles and the corresponding compensation angle is ,like Figure 6 As shown in the figure, it can be seen that when the number of compensated switching cycles, k, varies, the system is stable only when the number of compensated switching cycles, k, is between 1 and 4. In practical engineering applications, for the time delay compensation of each harmonic, if the time delay compensation is close to 90° but less than 90° (at 90°, the integral effect will be zero, which will be detrimental to system control), the conditions for system stability can be met. For higher-frequency harmonics, the time delay compensation can be selected closer to 90°, while for lower-frequency harmonics, the time delay compensation can be selected to be smaller.

[0133] In addition, in the PR controller, the accuracy of the resonant frequency is very high. Due to the existence of , there may be problems caused by pole mismatch during DSP operation, so a pre-corrected bilinear transformation is used in the process of discretizing the controller;

[0134] Moreover, when the system frequency shifts, the controller with fixed parameters will cause the harmonic frequency, especially the higher harmonics, to be compensated by the resonant frequency shift of the sinusoidal signal integrator. Therefore, a variable-frequency higher-order harmonic delay compensation is designed. When the above-mentioned PR controller is implemented in engineering using DSP code, if the system transmission frequency changes, the coefficient of the PR controller will be updated in real time.

[0135] Finally, the z-domain expression of the quasi-PR controller with phase compensation is obtained as follows:

[0136]

[0137] Application Example: To verify the effectiveness of the proposed control strategy, a system consisting of a residential PV inverter and reactive, unbalanced, and harmonic current loads located at the same grid connection point was selected. The system voltage level was 230 / 400V. The harmonic load was a 13.5kW three-phase six-pulse rectifier. The three-phase RL load totaled 3kW / 1kVar, the single-phase RL load on phase a totaled 4kW / 1kVar, and the single-phase RL load on phase b totaled 2kW / 1kVar. The PV inverter involved in power quality compensation was a common residential three-phase PV inverter with a rated power of 10kW, a switching frequency of 20kHz, and LCL filter parameters of 2mH / 3uF / 0.2mH. The maximum current allowed to flow through each phase was 1.2 times the rated current corresponding to the maximum PV power (effective value 18A). The coefficients corresponding to different power quality pollution levels were also calculated. Both are equal to 1.

[0138] The application example was simulated and tested using MATLAB / Similink-2023a. The system initial state does not contain any harmonic control measures. After the power output of the photovoltaic converter stabilizes, the proposed optimal power quality compensation control method based on the photovoltaic converter is enabled in the first second to perform power quality control. In different simulations, two control methods are used to optimize the calculation of the power quality control current instructions, namely (1) the optimal control strategy or (2) the local control strategy. The current output of the photovoltaic after compensation and the grid current at the grid connection point are observed to evaluate the compensation effect under different control strategies. For the local control strategy, in order to verify its ability to respond to changes in the grid operation state in real time, the illumination is continuously reduced by a total of 50W / m in the first 1.3s-1.8s. 2 .

[0139] Table 1 Power quality compensation effects after using different control methods

[0140]

[0141] Table 1 summarizes the power quality compensation effects after different control methods are enabled. The current to be compensated flowing through the load is = -6.818A, =8.66A, = -2.76A, =10.185;

[0142] For the optimal control strategy, the Lagrange method is used to solve the current compensation instruction. After 0.003 seconds of solution on the local computer, the obtained 、 、 and This is the current compensation instruction of the photovoltaic converter. After the optimal control strategy is adopted in the first second, the uncompensated reactive, negative sequence and harmonic current components 、 、 As shown in Table 1, the sum of the uncompensated reactive, negative sequence, and harmonic current components is f=37.94, and the power quality of the grid-connected current is significantly improved.

[0143] For local control strategies, The three equations are solved for compensation coefficients in DSP (i.e., solving quadratic equations), and the smallest compensation coefficient is selected as the final output solution. 、 、 and This is the current compensation command for the photovoltaic converter. After the local control strategy is adopted in the first second, the sum of the uncompensated reactive, negative sequence, and harmonic current components, f, is 68.47. The power quality of the grid-connected current is improved. Although the compensation effect is not as good as the optimal control strategy, this control strategy is executed in real time in the DSP and is easy to implement.

[0144] Figures 7 and 8 The simulation waveform diagram after adopting the optimal control strategy in the application example of the present invention is as follows: Figure 7 As shown in (a), the corresponding three-phase current effective value is as follows Figure 7 As shown in (b), Figure 7 (c) in the figure shows the change of grid current before and after compensation. Figure 7 It can be seen that after adopting the proposed optimal power quality compensation control based on photovoltaic converters, the injection current of the photovoltaic converter and the grid current at the grid connection point change accordingly after the control strategy is implemented, and they can transition to a steady state in a short time. It can be seen that after adopting the proposed control strategy, the photovoltaic converter participating in power quality compensation can quickly respond to control commands and the three-phase current is maintained within the maximum allowable current. Figure 8 The steady-state current waveform and FFT analysis results of phase a before and after power quality compensation are shown. It can be seen that after adopting the proposed control strategy, the grid-connected point phase a current changes from 46.92A (THD=19.62%) to 43.39A (THD=12.55%), and the grid-connected point harmonic current up to the 25th order is suppressed to a certain extent.

[0145] Figures 9 and 10 This is a simulation waveform diagram after adopting the local control strategy in the application example of the present invention. The three-phase current waveforms of the photovoltaic converter before and after power quality compensation are as follows: Figure 9 As shown in (a) in the figure, the change of the effective value of the three-phase current is as follows: Figure 9As shown in (b), Figure 9 (c) shows the change of grid current before and after compensation. Figure 9 (d) is the uncompensated reactive power, negative sequence and harmonic current amplitude in the grid current at the grid connection point, which is given by Figure 9 It can be seen that after adopting the proposed control strategy, the photovoltaic converter involved in power quality compensation can quickly respond to control commands and maintain the three-phase current within the maximum allowable current. Moreover, when the operating conditions of the photovoltaic converter change, the compensation current output by the photovoltaic converter can be adjusted in real time according to the output of the photovoltaic converter, avoiding overcurrent while maximizing the use of the remaining capacity of the photovoltaic converter. Figure 10 The steady-state current waveform and FFT analysis results of phase a before and after illumination changes after power quality compensation are shown. It can be seen that after adopting the proposed control strategy, the grid-connected phase a current changes from 46.92 A (THD = 19.62%) to 42.52 A (THD = 15.58%). After the illumination change, it further changes to 42.66 A (THD = 14.69%).

[0146] Corresponding to the aforementioned embodiment of the power quality optimal compensation control method based on a photovoltaic converter, the present invention further provides an embodiment of a power quality optimal compensation control device based on a photovoltaic converter.

[0147] See also Figure 11 An embodiment of the present invention provides an optimal compensation control device for power quality based on a photovoltaic converter, comprising a memory and one or more processors. The memory stores executable code. When the processor executes the executable code, it is used to implement an optimal compensation control method for power quality based on a photovoltaic converter in the above embodiment.

[0148] The embodiment of the power quality optimal compensation control device based on photovoltaic converter provided by the present invention can be applied to any device with data processing capability, and the device with data processing capability can be a device or apparatus such as a computer. The device embodiment can be implemented through software, or through hardware or a combination of software and hardware. Taking software implementation as an example, as a device in a logical sense, it is formed by the processor of any device with data processing capability in which it is located reading the corresponding computer program instructions in the non-volatile memory into the memory for execution. From the hardware level, if Figure 11 As shown in the figure, it is a hardware structure diagram of any device with data processing capability where the power quality optimal compensation control device based on photovoltaic converter provided by the present invention is located. Figure 11In addition to the processor, memory, network interface, and non-volatile memory shown, any device with data processing capabilities in which the apparatus in the embodiment is located may also include other hardware, generally based on the actual functions of the device with data processing capabilities, which will not be described in detail.

[0149] The implementation process of the functions and effects of each unit in the above-mentioned device is specifically described in the implementation process of the corresponding steps in the above-mentioned method, and will not be repeated here.

[0150] For the device embodiments, since they basically correspond to the method embodiments, the relevant parts can be referred to the partial description of the method embodiments. The device embodiments described above are merely illustrative, wherein the units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the present invention. A person of ordinary skill in the art can understand and implement the present invention without inventive work.

[0151] An embodiment of the present invention further provides a computer-readable storage medium having a program stored thereon. When the program is executed by a processor, the method for optimal power quality compensation control based on a photovoltaic converter in the above embodiment is implemented.

[0152] The computer-readable storage medium may be an internal storage unit of any device with data processing capabilities described in any of the aforementioned embodiments, such as a hard disk or memory. The computer-readable storage medium may also be an external storage device of any device with data processing capabilities, such as a plug-in hard disk, a smart media card (SMC), an SD card, a flash card, etc. equipped on the device. Furthermore, the computer-readable storage medium may also include both an internal storage unit and an external storage device of any device with data processing capabilities. The computer-readable storage medium is used to store the computer program and other programs and data required by any device with data processing capabilities, and may also be used to temporarily store data that has been output or is to be output.

[0153] The present invention also provides a computer program product, comprising a computer program, which, when executed by a processor, implements the method for optimal compensation control of power quality based on a photovoltaic converter.

[0154] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for optimal power quality compensation control based on photovoltaic converters, characterized in that: This method detects reactive, negative sequence and harmonic current components from the compensation object, combines the current output of the current PV converter, and optimizes the power quality management current instruction to generate a compensation current reference value for the PV converter, thereby achieving the purpose of maximizing the use of the PV converter capacity to manage power quality.

2. The method for optimal power quality compensation control based on photovoltaic converter according to claim 1, characterized in that: Without changing the original device of the existing two-stage photovoltaic converter, the dual functions of photovoltaic grid connection and power quality management are achieved. The specific control architecture is as follows: The photovoltaic converter involved in power quality management consists of a DC-DC converter and a DC-AC converter. The converter uses an LCL filter to suppress harmonics in the switching frequency band, and uses a capacitor branch with a series resistor to suppress the resonant peak of the LCL filter. The DC-AC part includes filtering, phase-locked loop unit, DC bus voltage control and current inner loop control, as well as compensation current generation and current tracking control; by adjusting the grid-connected positive sequence active current command signal of the DC-AC converter to maintain the DC bus voltage stable, the power output of the photovoltaic array to the grid is realized; at the same time, according to the power quality management requirements, the positive sequence reactive current command, harmonic current command and negative sequence current command that need to be compensated are obtained, combined with the real-time positive sequence active current of the photovoltaic converter , through the power quality management current instruction optimization, the compensation current reference value of the photovoltaic converter is obtained, and then through current tracking control, the converter outputs the corresponding compensation current according to the compensation current reference value; In the DC-DC part, photovoltaics use MPPT control to enable the photovoltaic array to output the maximum available power. , or use the perturbation observation method P&O power control method to make the photovoltaic output a given power , and in Output power .

3. The method for optimal power quality compensation control based on photovoltaic converter according to claim 1, characterized in that: The reactive, negative sequence and harmonic current components are detected from the compensation objects of other reactive, unbalanced and harmonic loads on the same bus, including: Additional current sensors are installed to sample the three-phase current flowing through the compensation object. The measured current direction is from the load to the busbar. The three-phase photovoltaic converter is a three-phase three-wire system, so zero-sequence current compensation is not considered. After Clark transformation, we get Current below the shaft , where the subscript L represents the load, and the fundamental positive sequence component of the current is obtained by the second-order generalized integrator SOGI and the 90-degree lag link , and the fundamental negative sequence component ; Will minus and ,get Harmonic current components under the shaft , and the harmonic components of the three-phase current ; After Park transformation and 100Hz sliding average filtering, the positive sequence current component under the dq axis is obtained, that is, the positive sequence active current and positive sequence reactive current ; After Park transformation and 100Hz sliding average filtering, the negative sequence current component under the dq axis is obtained and .

4. The method for optimal power quality compensation control based on photovoltaic converter according to claim 1, characterized in that: The process of optimizing the power quality management current instructions, that is, generating the compensation current reference value of the photovoltaic converter based on the reactive, negative sequence and harmonic current components detected from the compensation object; and executing the optimal control strategy or local control strategy according to the local computing power of the photovoltaic converter.

5. The method for optimal power quality compensation control based on photovoltaic converter according to claim 4, characterized in that: The optimal control strategy is suitable for situations where a local computer with sufficient computing power is deployed on the PV converter. After sampling the reactive, negative sequence, and harmonic current components, the PV converter uploads the program to the local computer for optimization calculations. The generated compensation current reference value for the PV converter is then sent to the PV converter for execution. In this control strategy, the input of the power quality management current instruction optimization module is the harmonic component of the three-phase current , positive sequence reactive current , negative sequence current component and , and the real-time positive sequence active current of the photovoltaic converter ; After optimization calculation, the output of the power quality management current instruction optimization process is the harmonic current reference value ,in is the harmonic current compensation coefficient and , indicating that the compensated harmonic current is proportional to the harmonic current flowing through the compensation object, and the reactive current instruction , negative sequence current instruction and , 、 、 and That is, the variables required to solve the optimization problem; Specifically, during power quality compensation, the PV converter must ensure that the effective value of the three-phase current does not exceed the limit to prevent excessive heating of components. Based on the vector relationship between the positive-sequence and negative-sequence currents, and the conversion rules between the sequence components and the three-phase currents, the three-phase current limit constraint is expressed as: in is the maximum allowable value of the three-phase current of the converter, is the maximum value of the three-phase harmonic current effective value; The control objective is to maximize the compensation of reactive, negative-sequence, and harmonic current components detected using the remaining capacity of the photovoltaic converter in addition to active power generation. That is, the sum of the uncompensated reactive, negative-sequence, and harmonic current components is minimized. Therefore, the control objective is expressed as: in is the objective function, is the uncompensated reactive current part, is the uncompensated harmonic current part, The uncompensated unbalanced current part, the superscript w represents the uncompensated part, and the subscript un represents the three-phase unbalanced part, that is, the negative sequence part. is the coefficient corresponding to different power quality pollution; The uncompensated reactive, negative sequence and harmonic current components are expressed as: The optimization problem consists of quadratic constraints. The Lagrange multiplier method is used to directly solve the optimization problem. 、 、 and This is the current compensation instruction of the photovoltaic converter.

6. The method for optimal power quality compensation control based on photovoltaic converter according to claim 4, characterized in that: The local control strategy is suitable for situations where the local computing power of the photovoltaic converter is insufficient. After sampling the reactive, negative-sequence, and harmonic current components, the photovoltaic converter directly calculates them in the DSP of the photovoltaic converter controller to generate and execute the compensation current reference value of the photovoltaic converter. Compared with the optimal control strategy, although the local control strategy cannot meet the control objective of minimizing the sum of the uncompensated reactive, negative-sequence, and harmonic current components, it is executed in real time in the DSP and meets the constraint that the effective value of the three-phase current does not exceed the limit. In this control strategy, the input of the power quality management current instruction optimization module is the harmonic component of the three-phase current , positive sequence reactive current , negative sequence current component and , and the real-time positive sequence active current of the photovoltaic converter ; After optimization calculation, the output of the power quality management current instruction optimization module is the harmonic current reference value , reactive current command , negative sequence current instruction and ,in is the harmonic current compensation coefficient, is the reactive current compensation coefficient, is the negative sequence current compensation coefficient, and , , , indicating that the compensated reactive, negative sequence and harmonic currents are proportional to the current flowing through the compensation object, 、 、 The size between them is proportional to the coefficient corresponding to the power quality pollution , that is, the variables required to solve the optimization problem; Specifically, during power quality compensation, the PV converter should ensure that the effective value of the three-phase current does not exceed the limit. Based on the vector relationship between the positive-sequence and negative-sequence currents, and the conversion rules between the sequence components and the three-phase currents, the three-phase current limit constraint is expressed as: in is the maximum allowable value of the three-phase current of the converter, is the maximum effective value of the three-phase harmonic current, is the negative sequence current amplitude flowing through the load, that is , is the phase angle of the negative sequence current flowing through the load in the negative sequence dq coordinate system, expressed as: For the three-phase current limit constraint, if The constraint is simplified to 、 、 A quadratic equation with one of the three compensation coefficients as a variable is directly solved in DSP to obtain the value of the compensation coefficient; right The three equations are solved for compensation coefficients in DSP respectively, and the smallest compensation coefficient is selected as the final output solution. 、 、 and This is the current compensation instruction of the photovoltaic converter.

7. The method for optimal power quality compensation control based on photovoltaic converter according to claim 1, characterized in that: The process of current tracking control for causing the converter to output a corresponding compensation current according to a compensation current reference value includes: Since the ideal PR controller has component errors when it is digitally implemented, a quasi-PR controller is used to track the fundamental and harmonic currents. The expression of the PR controller is: in is the proportional gain, is the cutoff frequency, is the resonant gain, is the resonant frequency; Considering that most of the harmonics in the system are odd harmonics, and 6k+1 is mostly positive sequence, 6k-1 is mostly negative sequence, and 3k is zero sequence, which are absorbed by the delta-connected transformer, the starting point of the coordinate transformation in the PR controller solution is changed from The coordinate system is moved to the fundamental wave rotation coordinate system, and the 6k+1 and 6k-1 harmonics will both be converted into the 6k harmonic, and the amount of calculation required for control will be reduced by half; In addition, due to the sampling and control delays in the discretized system, the influence of the delay compensation on the system stability needs to be analyzed in the control. For the 6k±1 harmonic, the corresponding relationship between the compensated phase angle and the system characteristic root distribution is established according to the control block diagram of the system, and the delay compensation phase angle of the PR controller with different frequencies is determined. .

8. The method for optimal power quality compensation control based on photovoltaic converter according to claim 5, characterized in that: The process of discretizing the PR controller with delay compensation includes: In the PR controller, a pre-corrected bilinear transformation is used in the process of discretizing the controller; And when the system frequency shifts, a variable-frequency high-order harmonic delay compensation is designed. If the system transmission frequency changes, the coefficient of the PR controller will be updated in real time.

9. A power quality optimal compensation control device based on a photovoltaic converter, comprising a memory and one or more processors, wherein the memory stores executable code, characterized in that: When the processor executes the executable code, the method for optimal power quality compensation control based on a photovoltaic converter according to any one of claims 1 to 8 is implemented.

10. A computer-readable storage medium having a program stored thereon, characterized in that: When the program is executed by a processor, the method for optimal power quality compensation control based on a photovoltaic converter according to any one of claims 1 to 8 is implemented.

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