Electric energy quality treatment method and system for adaptive capacity allocation of photovoltaic grid-connected system

Abstract

The invention discloses a photovoltaic grid-connected system self-adaptive capacity allocation electric energy quality treatment method and system, and the method comprises the steps: detecting whether a photovoltaic inverter is in a power generation state or not, and switching to an electric energy quality treatment mode if the photovoltaic inverter is not in the power generation state; if the photovoltaic inverter is in a power generation state and the electric energy quality of the grid-connected point exceeds the grid-connected requirement, and the residual capacity SR of the photovoltaic inverter is greater than the required compensation capacity SF, enabling the compensation capacity to be minimum while the electric energy quality is optimal through an optimal compensation model of the grid-connected point; otherwise, performing active reduction on the photovoltaic inverter, calculating the active reduction quantity, and enabling the compensation capacity to be minimum while the electric energy quality is optimal through an active reduction grid-connected point optimal compensation model. According to the invention, the photovoltaic multifunctional grid-connected inverter can be used to realize the electric energy quality management function, and the compensation capacity can be minimized when the electric energy quality of the grid-connected point is optimal.

Description

Technical Field

[0001] The present invention relates to a method and system for controlling power quality of a photovoltaic grid-connected system with adaptive capacity allocation, and belongs to the field of utilization of power quality control technology. Background Art

[0002] In order to achieve the strategic goals of "carbon peak" and "carbon neutrality", it has become an inevitable trend for new power generation such as solar energy and wind to gradually replace traditional power generation modes, and the installed capacity has increased significantly. By the end of 2022, the cumulative installed capacity of distributed photovoltaics is 157.62GW, and the capacity of 35kv, 20 / 10kv, and 380 / 220V voltage levels accounts for approximately 3%, 30%, and 67%; distributed photovoltaic grid connection and local consumption of electricity have become a gradual trend. The high proportion of power electronic equipment in distributed photovoltaic power generation systems and the access of more nonlinear, reactive and unbalanced local loads have led to power quality problems at the common coupling point (Point of Common Coupling) between the two, posing a serious threat to power supply security and grid stability.

[0003] In response to the power quality problems caused by distributed photovoltaic grid connection and nonlinear loads, traditional power quality management measures include installing active power filters (APF), static var compensators (SVC), static var generators (SVG), dynamic voltage regulators and other devices. Distributed photovoltaic installed capacity is small, the number is large, and the distribution is scattered. If each photovoltaic inverter grid connection point is equipped with active power quality management devices, it will require huge investment costs, the power grid system will become more complicated, and new problems such as power loss will arise. The photovoltaic inverter is consistent with the topology of the active power quality management device in terms of topology. By changing the control strategy, the photovoltaic inverter has harmonic suppression and reactive power compensation functions, thereby improving the power quality of the grid connection point. Photovoltaic power generation is intermittent and fluctuating. The photovoltaic inverter does not always work at the rated state, and there is a certain amount of surplus capacity. Therefore, its surplus capacity can be used to achieve power quality management.

[0004] The existing research on using photovoltaic inverters for power quality management is relatively mature, and can achieve harmonic suppression, reactive power compensation, three-phase imbalance, voltage deviation and other functions while connecting active power to the grid. However, most of them ignore the fact that photovoltaic inverters have different output states in different environments and there are large differences in residual capacity. How to allocate residual capacity for power quality management in different states affects the operating status and economic benefits of the power grid. Summary of the invention

[0005] Purpose of the invention: In order to solve the problem that photovoltaic inverters have different output states and large differences in residual capacity under different environments, the present invention provides a power quality management method for adaptive capacity allocation of photovoltaic grid-connected systems, which completes active grid connection and power quality management, realizes adaptive adjustment of the allocation of residual capacity of photovoltaic inverters under different states, and achieves optimal power quality management.

[0006] Technical solution: To achieve the above purpose, the technical solution adopted by the present invention is: A method for controlling power quality of a photovoltaic grid-connected system with adaptive capacity allocation. The distributed photovoltaic grid-connected system mainly consists of a photovoltaic array, a boost circuit, and a DC side capacitor. C PV , three-phase full-bridge inverter, LCL filter, functional outer loop module, current inner loop tracking control module, SVPWM control module, power grid, nonlinear load, etc. The solar energy is converted into electrical energy through the photovoltaic array. After passing through the boost circuit, the DC power is boosted to the voltage required for inverter, and then converted into sinusoidal AC power through the three-phase inverter. After being filtered by the filter inductor LCL, it is connected to the power grid. The photovoltaic inverter adopts a dual-loop control strategy, namely functional outer loop control and current inner loop control. The functional outer loop control module includes a reduction module, a power quality detection module and a power control module. The power quality detection module is based on the input load current i L Calculating compensation current for power quality management and i L ; The grid-connected fundamental current command calculated by the power control module i pref , i qref ;The current inner loop tracks the current command calculated by the outer loop to obtain the reference signal of SVPWM, which controls the on and off of the inverter switch. In the power quality detection module, the remaining capacity of the photovoltaic inverter is calculated according to the active output state of the photovoltaic inverter, and it is determined whether the photovoltaic inverter needs to reduce its active power to release more capacity for power quality management. The present invention can adaptively adjust the remaining capacity of the photovoltaic inverter according to the output state of the photovoltaic inverter and the power quality of the grid-connected power, and realize the power quality management of the grid-connected point, which specifically includes the following steps: Step 1: Check whether the photovoltaic inverter is in power generation state. If the photovoltaic inverter is not in power generation state, the photovoltaic inverter becomes a power quality management device, switches to power quality management mode, and enters step 2. If the photovoltaic inverter is in power generation state, check the power quality of the grid connection point and enter step 3.

[0007] Step 2: When the power quality of the grid connection point meets the requirements, no work is performed. If the power quality of the grid connection point does not meet the requirements, the photovoltaic inverter uses the remaining capacity to manage the power quality. While the power quality of the grid connection point meets the grid connection requirements, the optimal compensation model of the grid connection point is used to ensure that the comprehensive power quality indicators are optimal while the compensation capacity is minimized.

[0008] Step 3: If the PV inverter is in power generation state and the power quality at the grid connection point meets the requirements, power quality management is not performed and the PV inverter works in normal state.

[0009] If the photovoltaic inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirements, first calculate the compensation capacity required to meet the grid connection requirements. S F , and then calculate the remaining capacity of the PV inverter S R .

[0010] Step 4: If the PV inverter has remaining capacity S R Larger than the required compensation capacity S F , through the optimal compensation model of the grid-connected point, the compensation capacity is minimized while the power quality is optimal.

[0011] If the PV inverter has remaining capacity S R Less than the required compensation capacity S F , the active power of the photovoltaic inverter is reduced, the remaining capacity of the photovoltaic inverter is further released, and the active power reduction is calculated after compensation to ensure that the photovoltaic inverter meets the grid-connected requirements. The optimal compensation model of the grid-connected point with active power reduction is used to obtain the minimum compensation capacity while achieving the best power quality.

[0012] Optimal compensation model for grid connection point: The objective function is: ; The constraints are: ; ; ; ; in, F Represents the comprehensive index of power quality, w 1 is the harmonic weight factor, w 2 is the reactive weight factor, Indicates the harmonic current coefficient of the power grid, Indicates the reactive power coefficient of the grid connection point, S T is the required compensation capacity, is the margin factor, α 1 is the harmonic compensation coefficient, α 2 is the reactive power compensation coefficient, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, S R is the remaining capacity of the photovoltaic grid-connected inverter, P is the grid-connected active power, Indicates the grid-connected point power factor before compensation, To compensate for the harmonic coefficient of the previous grid connection point, Indicates the active power reduction of the PV inverter.

[0013] Optimum: The optimal compensation model for active power reduction is: The objective function is: ; The constraints are: ; <5%; >0.98; ; ; in, The compensation capacity required after the active power reduction is: represents the margin factor, Indicates the grid voltage, α 1 is the compensation coefficient 1, α 2 is the compensation coefficient 2, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, It indicates the comprehensive index of power quality after active power reduction and compensation. is the compensation margin coefficient, w 1 is the harmonic weight factor, w 2 is the reactive weight factor, Indicates the harmonic coefficient after active power reduction, Indicates the reactive power coefficient after active power reduction, Indicates the power factor of the grid-connected point after compensation, S T is the required compensation capacity, S R is the remaining capacity of the photovoltaic grid-connected inverter, Indicates the power factor of the grid-connected point before compensation, Represents the harmonic coefficient before governance.

[0014] Preferred: In step 2, when evaluating the power quality of distributed photovoltaic grid-connected power, the comprehensive power quality index F is the target layer, and the criterion layer includes the grid current harmonic distortion rate, grid point reactive power coefficient x q , build a comprehensive power quality model; The comprehensive model of power quality is: ; ; in, F Represents the comprehensive index of power quality, w 1 is the harmonic weight factor, w 2 is the reactive weight factor, Indicates the harmonic current coefficient of the power grid, Indicates the reactive power coefficient of the grid connection point, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, I 1 is the effective value of the fundamental grid-connected current.

[0015] Optimum: Active power reduction amount is: .

[0016] in, Indicates the amount of active power reduction, Indicates meritorious contribution. It indicates the active contribution after reduction; Remaining capacity of inverter after active power reduction S r for: ; in, S R In order to reduce the remaining capacity of the inverter after active power generation, Indicates the rated capacity of the inverter.

[0017] After active power reduction, the grid-connected fundamental wave active power is: ; ; ; ; ; ; ; ; in, Indicates the grid-connected fundamental wave active power after active power reduction. P is the grid-connected active power, Indicates the active power reduction of the photovoltaic inverter. Indicates the grid voltage, Indicates the grid-connected fundamental active current after active power reduction. Represents the grid current, α 1 is the harmonic compensation coefficient, α 2 is the reactive power compensation coefficient, I , I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, Indicates the effective value of the reactive component of the grid-connected fundamental current before compensation. Indicates the output current of the photovoltaic inverter, express, Indicates the load fundamental active current, represents the harmonic coefficient after compensation, Indicates the effective value of harmonic current before compensation, Represents the reactive power coefficient after compensation, Indicates the grid-side power coefficient after compensation, represents the apparent power after compensation, S R is the remaining capacity of the photovoltaic grid-connected inverter, Indicates the compensation capacity after active power reduction.

[0018] Preferably, the PR controller of the photovoltaic grid-connected system satisfies the following formula: ; in, represents the PR control closed-loop transfer function, K p is the proportionality coefficient, K r is the resonance coefficient, ω c is the bandwidth factor, yes hThe frequency of the subharmonic signal.

[0019] Optimum: Harmonic capacity to be compensated before and after compensation: ; ; Power factor of grid connection point before and after compensation: ; ; ; ; Power factor of grid-connected point after compensation PF satisfy: in, Indicates the harmonic capacity to be compensated before compensation. Indicates the capacity of harmonics to be compensated after compensation. Indicates the grid voltage, represents the harmonic coefficient, Indicates the effective value of the fundamental grid-connected current, Indicates the power factor, Indicates the grid-connected active power, Indicates the grid-connected fundamental reactive power before compensation. It represents the grid-connected fundamental reactive power after compensation. Indicates the power factor of the grid-connected point after compensation, Indicates the compensation of reactive power factor, α 1 is the harmonic compensation coefficient, α 2 is the reactive power compensation coefficient.

[0020] Another object of the present invention is to provide a power quality management device for adaptive capacity allocation of a photovoltaic grid-connected system, which is used to implement the power quality management method for adaptive capacity allocation of the photovoltaic grid-connected system, including a photovoltaic inverter power generation state detection unit, a normal power generation mode unit, a power quality management mode unit, a maximum power point tracking control unit, and an active power reduction control unit, wherein: The photovoltaic inverter power generation state detection unit is used to detect whether the photovoltaic inverter is in the power generation state. If the photovoltaic inverter is not in the power generation state, the photovoltaic inverter becomes a power quality management device and switches to the power quality management mode unit. If the photovoltaic inverter is in the power generation state, the power quality of the grid connection point is detected and the process goes to step 3.

[0021] The power quality management mode unit is used to stop working when the power quality of the grid connection point meets the requirements. If the power quality of the grid connection point does not meet the requirements, the photovoltaic inverter uses the remaining capacity to manage the power quality, and while the power quality of the grid connection point meets the grid connection requirements, the optimal compensation model of the grid connection point is used to ensure that the comprehensive power quality indicators are optimal while the compensation capacity is minimal.

[0022] The normal power generation mode unit is used for not performing power quality management if the photovoltaic inverter is in power generation state and the power quality of the grid connection point meets the requirements, and the photovoltaic inverter works in a normal state.

[0023] The maximum power point tracking control unit first calculates the compensation capacity required to meet the grid connection requirements if the photovoltaic inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirements. S F , and then calculate the remaining capacity of the PV inverter S R If the remaining capacity of the photovoltaic inverter S R Larger than the required compensation capacity S F , through the optimal compensation model of the grid-connected point, the compensation capacity is minimized while the power quality is optimal.

[0024] The active power reduction control unit is used to S R Less than the required compensation capacity S F , the active power of the photovoltaic inverter is reduced, the remaining capacity of the photovoltaic inverter is further released, and the active power reduction is calculated after compensation to ensure that the photovoltaic inverter meets the grid-connected requirements. The optimal compensation model of the grid-connected point with active power reduction is used to obtain the minimum compensation capacity while achieving the best power quality.

[0025] Another object of the present invention is to provide a computer system, comprising a memory and a processor, wherein the memory is used to store computer programs / instructions. The processor is used to execute the computer programs / instructions to implement the power quality management method for adaptive capacity allocation of photovoltaic grid-connected systems.

[0026] Compared with the prior art, the present invention has the following beneficial effects: While realizing power quality management by using a photovoltaic multifunctional grid-connected inverter, the present invention adaptively adjusts the remaining capacity of the photovoltaic inverter involved in power quality management according to the output state of the photovoltaic multifunctional inverter and the power quality of the grid-connected point, and selectively and flexibly compensates for the grid-connected current harmonics and reactive components through a multi-objective optimal control strategy, thereby minimizing the compensation capacity when the power quality of the grid-connected point is optimal. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a main circuit topology diagram of the distributed photovoltaic grid-connected system in Example 1; Figure 2 Calculation method of reference current command for photovoltaic multifunctional inverter; Figure 3 FBD harmonic current detection calculation block diagram; Figure 4 Comprehensive evaluation model of power quality; Figure 5 PV inverter remaining capacity adaptive capacity allocation flow chart; Figure 6 It is the MATLAB / Simulink simulation topology diagram of the system in Example 1; Figure 7 Multi-objective optimization Parato optimization boundary; Figure 8 The grid current waveform of each compensation section; Fig. 9 A phase grid voltage uga and grid current iga waveform; Fig.10 Harmonic distortion rate of grid-connected point current in each period; Fig.11 Active power at the grid connection point; Fig.12 Reactive power at the grid connection point; Fig.13 Pareto frontier after merit reduction; Fig.14 Grid-side voltage and current waveforms after load changes; Fig.15 Harmonic distortion rate of grid current after active power reduction optimization compensation; Fig.16 The harmonic distortion rate of the grid current after optimizing compensation for active power reduction. DETAILED DESCRIPTION

[0028] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0029] In order to better describe a method of using the remaining capacity of distributed photovoltaic inverters to manage power quality, a distributed photovoltaic grid-connected system is selected, the grid-connected active power is 20kW, the photovoltaic inverter is rated at 22kVA, the switching frequency is 10kHz, and the grid voltage is 220V. Figure 1-5 As shown, the following steps are included: Step 1: Design system structure: The distributed photovoltaic grid-connected system mainly consists of photovoltaic array, boost circuit, DC side capacitor C PV , three-phase full-bridge inverter, LCL filter, power calculation module, current inner loop tracking control module, SVPWM control module, power grid, nonlinear load, etc. Figure 1 shown.

[0030] in U PV , I PV are the DC side voltage and current respectively. L 0 is the DC side inductance, which is equal to the DC side capacitance C PV Composition boost voltage. S 0 is the DC side inverter switch, S 1— S 6 Three-phase AC circuit switch, L 1 is the inverter side filter inductor, C is the filter capacitor, R is the damping resistor, L 2 is the grid-side filter inductor. u abc , i abc is the three-phase grid voltage and current. i gabc is the inverter power side current. The inverter circuit includes 6 fully controlled IGBT devices, whose gate level on and off are controlled by the SPVWM control circuit. The photovoltaic multifunctional inverter system model is established, and the front-stage DC side power supply can use a constant DC voltage source U dc Equivalent.

[0031] Step 2: Calculation of reference current of photovoltaic multi-function inverter: The reference current of the photovoltaic multifunctional inverter consists of two parts, including the grid-connected fundamental wave command current and the compensation current used for grid-connected point power quality management. This patent adopts a current detection method based on the FBD (Fryze-Buchholz-Döppenbrock) power theory. Figure 1 The grid voltage and load current are used to calculate the instantaneous active and reactive power of the load, which are then divided by the square of the effective value of the grid voltage to obtain the total conductance and total susceptance. After filtering with a low-pass filter, the fundamental conductance and fundamental susceptance are obtained. The fundamental conductance and susceptance are multiplied by the grid voltage to obtain the fundamental active current and fundamental reactive current. The harmonic current component is obtained by subtracting the fundamental active and fundamental reactive currents from the load current.

[0032] (1) Calculation of grid-connected current command The calculation formula for active and reactive power is as follows in, U , I They correspond to the effective values ​​of phase voltage and phase current in a three-phase symmetrical system. P , Q Corresponding to the active and reactive grid-connected power reference values ​​respectively. By solving the above equation, we can get the value based on the two-phase rotating coordinate system. dq Grid-connected power command current under the axis: (2) Compensation current command calculation The three-phase voltage of the power grid can be expressed as: In the formula U m is the three-phase voltage amplitude. φ u is the voltage phase angle. ω is the fundamental angular frequency. The three-phase equivalent load current vector can be expressed as the sum of positive sequence, negative sequence, and zero sequence: In the formula, I pk , I nk , I 0k They are k The amplitude of the positive, negative and zero sequence current components, φ pk , φ nk , φ 0k They are k The phase angles corresponding to the positive-sequence, negative-sequence, and zero-sequence current components.

[0033] Load equivalent conductance G , Electron B Generally, it can be calculated by the instantaneous active power of the load p and reactive power q The calculation results are: After low-pass filtering, the fundamental wave conductance is obtained G 1. Electrical Susceptance B 1, and then multiplying it with the grid voltage can get the fundamental current component i 1p and i 1q : In order to achieve adaptive allocation of the remaining capacity of the photovoltaic inverter, the harmonic and reactive compensation components can be multiplied by the compensation coefficients α 1, α 2. Selectively compensate for the harmonics and reactive current at the grid connection point to achieve the best comprehensive power quality indicators when the compensation capacity of the distributed photovoltaic inverter is limited.

[0034] Step 3: Comprehensive evaluation of power quality at the grid connection point Considering the limited compensation capacity of distributed photovoltaic inverters, it mainly compensates for the reactive power and harmonic current of the electrical downstream and achieves the optimal management of the power quality at the grid connection point under the premise of limited capacity, which can organically combine multiple power quality indicators into a comprehensive power quality indicator. This method adopts the power quality assessment model as shown in the figure. The criterion layer includes the harmonic coefficient x h , reactive power coefficient x q .

[0035] in: I h is the effective value of the grid-connected harmonic current. I q It is the effective value of the reactive component of the grid-connected fundamental current. I 1 is the effective value of the fundamental grid-connected current.

[0036] The target layer is the comprehensive index of power quality F , w 1, w 2 are the harmonic coefficient and reactive power coefficient index weights respectively.

[0037] The harmonic coefficient is calculated by the hierarchical analysis method, and the reactive coefficient weight is: The comprehensive power quality index is obtained by multiplying the value of each single index by the weight: in, x h and x q satisfy: Step 4: Calculate the compensation capacity for power quality management The rated capacity of the photovoltaic inverter when connected to the grid is S N , the active output in this period is P ref, the grid-connected reactive power instruction is Q ref =0, then the remaining capacity S R It can be expressed as: Photovoltaic inverters are used to manage power quality issues that occur during grid connection, and compensation margin coefficients are set for harmonic compensation components and reactive compensation components. =1.5, the required compensation capacity is S T , which can be expressed as: Where: S h It is the harmonic component compensation capacity. S q is the reactive component compensation capacity. The effective values ​​of harmonic currents before and after compensation are I h0 and I h , before and after compensation current THD Expressed as x h0 and x h . U g is the grid voltage. Harmonic capacity to be compensated before and after compensation S h0 and S h It can be expressed as: Assume that the required reactive capacity of the current before and after compensation is S q0 and S q , the corresponding power factor PF 0 and PF It can be expressed as: Where: Q 0 and Q is the grid-connected fundamental reactive power before and after compensation, P is the grid-connected active power, φ 0 and φ is the power factor angle of the grid-connected current before and after compensation. According to the reactive power compensation analysis, the capacity of the multifunctional grid-connected inverter used to compensate reactive power can be obtained. ΔQ, as shown in formula (28).

[0038] Power factor of grid-connected point after compensation PF satisfy: Step 5: Adaptive adjustment strategy of the remaining capacity of the photovoltaic inverter Under different weather conditions, the output state of the photovoltaic inverter is different, and the change in active power leads to uncertainty in the remaining capacity of the photovoltaic inverter. The present invention adjusts the control mode of the photovoltaic inverter according to the different photovoltaic output states, fully utilizes the remaining capacity of the photovoltaic inverter, and reduces the investment and operation costs of the power quality management device at the grid connection point. Figure 5 The figure is the switching process of photovoltaic inverter adaptive mode. The photovoltaic inverter can work in four working modes: 1. normal power generation mode, 2. power quality management mode, 3. maximum power point tracking control, and 4. active power reduction control.

[0039] 1 When the weather conditions are bad or at night, the photovoltaic inverter is not in working state and switches to the power quality management mode. When there is a power quality problem at the grid connection point, the optimal compensation control is adopted. The economic benefits of achieving a thorough management of the power quality at the grid connection point are not enough to offset the economic cost of the investment. In this mode, the power quality index at the PCC is maximized with the minimum compensation capacity. Based on the above analysis, NSGA-Ⅱ is used for multi-objective optimization to solve the compensation coefficient α 1, α 2.

[0040] According to input capacity S T Optimized compensation model can be selected F 1. Comprehensive index value of power quality F 2 The objective function obtained is: Next, we further establish the constraints of the optimization model: (1) Each compensation coefficient should be between 0 and 1, that is: (2) The compensation capacity should be less than the remaining capacity of the PV inverter (3) After compensation, all power quality indicators should meet the grid connection requirements, and the power quality at the grid connection point should be improved, that is: 2 When the weather conditions are good and the photovoltaic inverter is in working condition, if the power quality meets the requirements, power quality management will not be performed. The photovoltaic inverter works in MPPT mode to maintain the maximum power output of active power.

[0041] 3 If the photovoltaic inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirements, first calculate the compensation capacity required for power quality management to meet the grid connection requirements. S T , and then calculate the remaining capacity of the photovoltaic inverter. If the remaining capacity is greater than the compensation capacity required to meet the grid connection requirements, then S T > S p , also adopts a multi-objective optimization approach to achieve the best power quality management with the least compensation capacity. The objective function in this mode is: The constraints are: Harmonic distortion x h <5%.

[0042] Power Factor PF >0.98.

[0043] Each compensation coefficient should be between 0 and 1, that is: The compensation capacity should be less than the remaining capacity of the PV inverter: In the solution process, NSGA-Ⅱ is used for multi-objective optimization to solve the compensation coefficient as in mode 1. α 1, α 2.

[0044] 4 When the PV inverter is in the power generation state, the power quality at the grid connection point exceeds the grid connection requirements, and the power quality at the grid connection point is poor, the PV inverter can be considered to perform active power reduction to further increase the compensation capacity of the PV inverter for power quality management. When active power reduction is not performed, the remaining capacity of the PV inverter is S R , the compensation capacity required when the grid connection point meets the grid connection requirements is S P ,like S P > S R When the PV inverter performs active power reduction, the active power reduction required to meet the grid connection requirements isP cut , the calculation formula is as follows: The reduction of active power reduction will reduce the income of distributed photovoltaic grid-connected electricity prices, and it can also release a certain degree of surplus capacity to reduce the cost of power quality management. S r : After the photovoltaic reduction, the inverter grid-connected current command will change. The inverter grid-connected command current is defined as I g , after active power reduction, the grid-connected fundamental active current is , the harmonic coefficient is , the reactive power coefficient is .

[0045] After active power reduction, the grid-connected fundamental wave active power is: In this mode, the requirements that need to be met when performing power quality management are: (1) Each compensation coefficient should be between 0 and 1, that is: (2) Harmonic distortion rate <5%.

[0046] (3) Power factor >0.98.

[0047] (4) The compensation capacity should be less than the remaining capacity of the PV inverter (5) After compensation, all power quality indicators should meet the grid connection requirements, and the power quality at the grid connection point should be improved, that is: In the solution process, as in mode 1, NSGA-Ⅱ is used for multi-objective optimization to solve the compensation coefficient. α 1,α 2.

[0048] Therefore, Figure 5 The photovoltaic inverter adaptive mode switching process shown is: (1) First, check whether the photovoltaic inverter is in the power generation state. If the weather is bad or at night, the photovoltaic inverter is not in the power generation state, then the photovoltaic inverter becomes a power quality management equipment to achieve the management of the power quality of the grid connection point. When the power quality of the grid connection point meets the requirements, it does not work. If the power quality of the grid connection point does not meet the requirements, the photovoltaic inverter uses the remaining capacity to manage the power quality. While making the power quality of the grid connection point meet the grid connection requirements, the multi-objective genetic algorithm NSGA-Ⅱ is used to calculate the minimum compensation capacity while meeting the optimal comprehensive power quality indicators.

[0049] (2) If the PV inverter is in power generation state and the power quality at the grid connection point meets the requirements, power quality management is not performed and the PV inverter operates in normal state.

[0050] (3) If the PV inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirements, first calculate the compensation capacity required to meet the grid connection requirements. S F , and then calculate the remaining capacity of the PV inverter S R The multi-objective genetic algorithm NSGA-Ⅱ is used to calculate the optimal power quality while minimizing the compensation capacity.

[0051] (4) When the photovoltaic inverter is in the power generation state, the power quality at the grid connection point exceeds the grid connection requirements, and the power quality at the grid connection point is poor, the photovoltaic inverter can be considered to reduce active power to further release the remaining capacity of the photovoltaic inverter. Although the active power reduction of the photovoltaic inverter will save the cost of power quality management, it will also reduce the grid connection income of the photovoltaic inverter. In this case, after compensation is performed to ensure that the photovoltaic inverter meets the grid connection requirements, the multi-objective genetic algorithm NSGA-Ⅱ is used to calculate the active power reduction amount while ensuring the optimal power quality and the minimum compensation capacity.

[0052] The calculation method of comprehensive power quality index is: When evaluating the power quality of distributed photovoltaic grid-connected power, the comprehensive power quality index F is the target layer, and the criterion layer includes the grid current harmonic distortion rate (THD) x h , Reactive power coefficient of grid connection point x q , construct a comprehensive power quality model. The comprehensive power quality index can be expressed as: in, w 1, w 2 are harmonic coefficient and reactive power coefficient index weights, which are calculated by analytic hierarchy process. w 1, w 2. I h is the harmonic current component of the power grid, I q is the reactive current component of the power grid, I 1 is the fundamental current of the power grid.

[0053] The calculation method of compensation capacity is: (1) Use FBD current detection method to separate the harmonic current components in the grid-connected current I h Reactive power compensation component I q .

[0054] (2) The remaining capacity of the photovoltaic grid-connected inverter fluctuates with the state of the photovoltaic inverter. In order to achieve the optimal comprehensive power quality indicators of the grid-connected point with the minimum compensation capacity while meeting the grid-connected requirements, a selective and flexible compensation strategy is implemented. The compensation coefficient is multiplied by the harmonic component and the reactive power compensation component. α 1, α 2. Construct a multi-objective optimization model based on the NSGA-Ⅱ algorithm to calculate the corresponding compensation coefficient, realize the flexible allocation of photovoltaic inverters under different output states, and achieve optimal compensation for the grid-connected point.

[0055] The objective function is: in, S R is the remaining capacity of the photovoltaic grid-connected inverter, S N is the rated capacity of the photovoltaic grid-connected inverter, P is the rated active power, P cut is the active power reduction of the photovoltaic inverter, for.

[0056] The constraints are: (1) Each compensation coefficient should be between 0 and 1, that is: (2) The compensation capacity should be less than the remaining capacity of the PV inverter (3) After compensation, all power quality indicators should meet the grid connection requirements, and the power quality at the grid connection point should be improved, that is: (4) If the PV inverter needs to reduce active power, the amount of active power reduction cannot exceed the rated active power of the inverter. The compensation coefficient is calculated through the established multi-objective optimization model. α 1, α 2 optimal solution, thus obtaining the required compensation capacity.

[0057] The control strategy for photovoltaic inverters is the current tracking link: The power quality management and control of photovoltaic inverters mainly focuses on the generation of command current and current tracking control. The current generated by the nonlinear load is separated into harmonic components and reactive components through the current detection link, and then the polarity is reversed with the grid-connected fundamental command current as the reference current of the current inner loop. i ref , reference current i ref and inverter output current i abc The difference is then output through the current tracking control link of the grid-connected inverter to generate a control signal. The control signal is modulated to obtain a pulse signal to drive the IGBT, and finally the command current is tracked.

[0058] The current controller is: Photovoltaic inverters need to have power quality management functions. The command current contains not only fundamental grid-connected current, but also high-order harmonic current. Traditional PI control cannot accurately track the AC component. The PR control method can provide gain to the AC signal, which can effectively reduce the grid-connected current stability error. The ideal PR controller must meet the requirements that the frequency of the controlled AC signal and the harmonic frequency are consistent to ensure control accuracy. The resonant frequency bandwidth coefficient is added to the ideal PR controller to deal with the frequency deviation problem. Quasi-PR control satisfies the formula: in K p is the proportionality coefficient, K r is the resonance coefficient, ω c The bandwidth factor can control the bandwidth of the resonant frequency and improve the phenomenon of rapid gain drop caused by frequency deviation. ω h yes h The frequency of the subharmonic signal.

[0059] In another embodiment, a power quality management device for adaptive capacity allocation of a photovoltaic grid-connected system is provided, which is used to implement the power quality management method for adaptive capacity allocation of the photovoltaic grid-connected system, including a photovoltaic inverter power generation state detection unit, a normal power generation mode unit, a power quality management mode unit, a maximum power point tracking control unit, and an active power reduction control unit, wherein: The photovoltaic inverter power generation state detection unit is used to detect whether the photovoltaic inverter is in the power generation state. If the photovoltaic inverter is not in the power generation state, the photovoltaic inverter becomes a power quality management device and switches to the power quality management mode unit. If the photovoltaic inverter is in the power generation state, the power quality of the grid connection point is detected and the process goes to step 3.

[0060] The power quality management mode unit is used to stop working when the power quality of the grid connection point meets the requirements. If the power quality of the grid connection point does not meet the requirements, the photovoltaic inverter uses the remaining capacity to manage the power quality, and while the power quality of the grid connection point meets the grid connection requirements, the optimal compensation model of the grid connection point is used to ensure that the comprehensive power quality indicators are optimal while the compensation capacity is minimal.

[0061] The normal power generation mode unit is used for not performing power quality management if the photovoltaic inverter is in power generation state and the power quality of the grid connection point meets the requirements, and the photovoltaic inverter works in a normal state.

[0062] The maximum power point tracking control unit first calculates the compensation capacity required to meet the grid connection requirements if the photovoltaic inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirements. S F , and then calculate the remaining capacity of the PV inverter S R If the remaining capacity of the photovoltaic inverter S R Larger than the required compensation capacity S F , through the optimal compensation model of the grid-connected point, the compensation capacity is minimized while the power quality is optimal.

[0063] The active power reduction control unit is used to S R Less than the required compensation capacity S F , the active power of the photovoltaic inverter is reduced, the remaining capacity of the photovoltaic inverter is further released, and the active power reduction is calculated after compensation to ensure that the photovoltaic inverter meets the grid-connected requirements. The optimal compensation model of the grid-connected point with active power reduction is used to obtain the minimum compensation capacity while achieving the best power quality.

[0064] In another embodiment, a computer system is provided, including a memory and a processor, wherein the memory is used to store computer programs / instructions, and the processor is used to execute the computer programs / instructions to implement the power quality management method for adaptive capacity allocation of photovoltaic grid-connected systems.

[0065] Figure 6 This is the Matlab / Simulink simulation topology diagram of the distributed photovoltaic grid-connected system in Example 1. During the implementation process, the front-stage DC side power supply of the distributed photovoltaic grid-connected system adopts a constant DC voltage source U dc Equivalent. The simulation parameters are shown in Table 1: Table 1 System simulation parameters The grid current waveforms of each compensation section are as follows: Figure 8 As shown, taking the photovoltaic output state as an example, when no load is connected, the change of the three-phase current through the grid is as follows Fig.10 As shown, it can be seen that the three-phase grid current is sinusoidal and stable, and the grid A phase voltage and current changes are as follows Fig. 9 As shown, the grid voltage and current are in phase, and the harmonic distortion rate of the grid current is 0.63%. It can be seen from the grid-connected active power curve that the active power of the photovoltaic inverter is fully connected to the grid, which can reflect the grid-connected function of the photovoltaic inverter. The load is connected at 0.1. After the load is connected, the grid current is severely distorted, the current harmonic distortion rate is 24.79%, and the power factor is low, which is 0.965. The active power consumed by the load is 32.62kW, provided by the photovoltaic inverter and the grid, and the reactive power consumed is 3.47kVar, provided by the grid.

[0066] Since the photovoltaic inverter provides active power and is in output state, the remaining capacity allocation mode is mode 3. The power quality optimization compensation of the grid-connected point is performed. First, the remaining capacity of the photovoltaic inverter is calculated. The grid-connected active power instruction is 20kW, and the rated capacity of the photovoltaic inverter is 22kW. The remaining capacity of the photovoltaic grid-connected inverter is calculated. S N =9.16kVA, if the power quality of the grid connection point is thoroughly treated, the required compensation capacity is 7.50kVA. After treatment, the required compensation capacity to meet the grid connection point requirements is 4.56kVA, and the comprehensive power quality index of the grid connection point is 0.1136. Therefore, the photovoltaic inverter does not need to reduce active power to release more residual capacity for power quality compensation. After using the NSGA-2 algorithm for multi-objective optimization, the Pareto frontier is obtained as follows Figure 7 shown.

[0067] Before determining the optimal compensation coefficient, the Pareto frontier obtained through optimization shows that when the comprehensive power quality F2 tends to be smaller, the required power quality compensation capacity F1 is more. Since NSGA-2 will calculate the congestion of the obtained non-dominated solution during the calculation process, combined with the comprehensive power quality index to compensate the capacity, the compensation coefficient is selected in this method. α 1=0.98, α 2=0.57 is the optimal solution.

[0068] After 0.2 seconds, the optimized compensation is carried out, and after 0.3 seconds, the full compensation is carried out. Compared with the full compensation, after the optimal compensation, the grid current waveform is almost the same as the full compensation, and the grid current phase can also track the grid voltage phase well. The harmonic distortion rate is 1.01%, and the power factor is 0.99, which meets the grid connection requirements. The compensation capacity required for the optimal compensation is 5.94kVA, and the comprehensive power quality index is 0.052, which can save 20% capacity compared with the full compensation.

[0069] After 0.3 seconds, full compensation coefficient is used α 1=1, α 2=1. After compensation, the harmonic distortion rate at the grid connection point is 1.0%, which is much lower than the grid connection requirement. The grid current and voltage are in phase, the load reactive power is all provided by the photovoltaic inverter, and the grid connection point power factor is 1.

[0070] When the nonlinear load changes, the harmonic distortion rate at the grid connection point is 26.06%, and the power factor is 0.9. The I h0 =7.54A,I q0 =12.03A, the required compensation capacity is 10.02KVA, which exceeds the remaining capacity of the photovoltaic inverter. The photovoltaic inverter works in mode 4. According to the photovoltaic inverter, the remaining capacity of the inverter is increased by reducing the active power to improve the power quality.

[0071] Fig.11 The figure shows the active power at the grid connection point; Fig.12 The figure shows the reactive power at the grid connection point; Fig.13 The Pareto frontier after active power reduction is shown; Fig.14 The waveform diagram of grid-side voltage and current after load changes is shown; Fig.15 The figure shows the harmonic distortion rate of the grid current after the active power reduction optimization compensation. Fig.16 The figure shows the harmonic distortion rate of the grid current after the active power reduction optimization compensation.

[0072] The solution to the first non-dominated sorting problem is solved by using a multi-objective optimization algorithm: α 1=0.967, α 2=0.583, output the Pareto optimal solution set Fig.13As shown in the figure, the calculated active power reduction required by the photovoltaic inverter is 600W. While slightly reducing the active power output, the optimal compensation for power quality can be achieved. After compensation, it can be seen that the harmonic distortion rate at the grid connection point is 1.17%, the power factor is 0.981, the grid current has good sinusoidal properties, and there is no obvious difference between the current phase and the voltage phase, which meets the grid connection requirements.

[0073] Through the above analysis, it can be seen that this method can realize adaptive allocation of the remaining capacity of the photovoltaic inverter according to the state of the distributed photovoltaic system and the changes in nonlinear loads, so as to achieve optimal management of the power quality at the grid connection point.

[0074] 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 principle 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 controlling power quality of a photovoltaic grid-connected system with adaptive capacity adjustment, characterized in that: The following steps are involved: Step 1: Detect whether the photovoltaic inverter is in a power generation state. If the photovoltaic inverter is not in a power generation state, the photovoltaic inverter becomes a power quality management device, switches to a power quality management mode, and enters step 2; If the PV inverter is in power generation state, check the power quality of the grid connection point and proceed to step 3; Step 2: When the power quality of the grid-connected point meets the requirements, no work is performed; if the power quality of the grid-connected point does not meet the requirements, the photovoltaic inverter uses the remaining capacity to manage the power quality, and while the power quality of the grid-connected point meets the grid-connected requirements, the optimal compensation model of the grid-connected point is used to ensure that the comprehensive power quality indicators are optimal while the compensation capacity is minimized; Step 3: If the photovoltaic inverter is in power generation state and the power quality of the grid connection point meets the requirements, power quality management is not performed and the photovoltaic inverter works in a normal state; If the photovoltaic inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirements, first calculate the compensation capacity S required to meet the grid connection requirements. F , and then calculate the remaining capacity S of the photovoltaic inverter R ; Step 4: If the remaining capacity of the PV inverter is S R Greater than the required compensation capacity S F , through the optimal compensation model of the grid connection point, the power quality is optimized while the compensation capacity is minimized; If the remaining capacity of the PV inverter is S R Smaller than the required compensation capacity S F , the active power of the photovoltaic inverter is reduced, the remaining capacity of the photovoltaic inverter is further released, and the active power reduction is calculated after compensation to ensure that the photovoltaic inverter meets the grid-connected requirements. The optimal compensation model of the grid-connected point with active power reduction is used to obtain the minimum compensation capacity while achieving the best power quality.

2. The power quality management method for adaptive capacity adjustment of photovoltaic grid-connected system according to claim 1 is characterized by: Optimal compensation model for grid connection points: The objective function is: ； The constraints are: ； ； ； ； Among them, F represents the comprehensive index of power quality, w1 is the harmonic weight factor, w2 is the reactive power weight factor, Indicates the harmonic current coefficient of the power grid, Represents the reactive power coefficient of the grid connection point, S T is the required compensation capacity, is the margin coefficient, α1 is the harmonic compensation coefficient, α2 is the reactive power compensation coefficient, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, S R is the remaining capacity of the photovoltaic grid-connected inverter, P is the grid-connected active power, Indicates the grid-connected point power factor before compensation, To compensate for the harmonic coefficient of the previous grid connection point, Indicates the active power reduction of the PV inverter.

3. The power quality management method for adaptive capacity adjustment of photovoltaic grid-connected system according to claim 2 is characterized by: The optimal compensation model for active power curtailment at the grid connection point is: The objective function is: ； The constraints are: ； <5%； >0.98； ； ； in, The compensation capacity required after the active power reduction is: represents the margin factor, represents the grid voltage, α1 is the compensation coefficient 1, α2 is the compensation coefficient 2, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, It indicates the comprehensive index of power quality after active power reduction and compensation. is the compensation margin factor, w1 is the harmonic weight factor, w2 is the reactive power weight factor, Indicates the harmonic coefficient after active power reduction, Indicates the reactive power coefficient after active power reduction, Indicates the power factor of the grid-connected point after compensation, S T is the required compensation capacity, S R is the remaining capacity of the photovoltaic grid-connected inverter, Indicates the grid-connected point power factor before compensation, Represents the harmonic coefficient before governance.

4. The power quality management method for adaptive capacity adjustment of photovoltaic grid-connected system according to claim 3 is characterized by: In step 2, when evaluating the power quality of distributed photovoltaic grid-connected power, the comprehensive power quality index F is used as the target layer, and the criterion layer includes the grid-connected current harmonic distortion rate, the grid-connected point reactive power coefficient x q , build a comprehensive power quality model; The comprehensive model of power quality is: ； ； Among them, F represents the comprehensive index of power quality, w1 is the harmonic weight factor, w2 is the reactive power weight factor, Indicates the harmonic current coefficient of the power grid, Indicates the reactive power coefficient of the grid connection point, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the fundamental current of the grid, and I1 is the effective value of the fundamental current of the grid.

5. The power quality management method for adaptive capacity adjustment of photovoltaic grid-connected system according to claim 4 is characterized by: The active power reduction is: ； in, Indicates the amount of active power reduction, Indicates meritorious contribution. It indicates the effective output after reduction; After reducing the active power, the remaining capacity of the inverter S R for: ； Among them, S R In order to reduce the remaining capacity of the inverter after active power generation, Indicates the rated capacity of the inverter; After active power reduction, the grid-connected fundamental wave active power is: ； ； ； ； ； ； ； ； in, It indicates the grid-connected fundamental wave active power after active power reduction, P is the grid-connected active power, Indicates the active power reduction of the photovoltaic inverter. Indicates the grid voltage, Indicates the grid-connected fundamental active current after active power reduction. represents the grid current, α1 is the harmonic compensation coefficient, α2 is the reactive power compensation coefficient, I, I h is the effective value of the grid-connected harmonic current, I q is the effective value of the reactive component of the grid-connected fundamental current, Indicates the effective value of the reactive component of the grid-connected fundamental current before compensation. Indicates the output current of the photovoltaic inverter, express, Indicates the load fundamental active current, represents the harmonic coefficient after compensation, Indicates the effective value of harmonic current before compensation, Represents the reactive power coefficient after compensation, Indicates the grid-side power coefficient after compensation, Represents the apparent power after compensation, S R is the remaining capacity of the photovoltaic grid-connected inverter, Indicates the compensation capacity after active power reduction.

6. The power quality management method for adaptive capacity adjustment of photovoltaic grid-connected system according to claim 5 is characterized by: The PR controller of the photovoltaic grid-connected system satisfies the following equation: ； in, represents the PR control closed-loop transfer function, K p is the proportionality coefficient, K r is the resonance coefficient, ω c is the bandwidth factor, is the frequency of the hth harmonic signal.

7. The method for controlling power quality of photovoltaic grid-connected system with adaptive capacity adjustment according to claim 1, characterized in that: Harmonic capacity to be compensated before and after compensation: ； ； Power factor of grid connection point before and after compensation: ； ； ； ； After compensation, the grid-connected point power factor PF satisfies: in, Indicates the harmonic capacity to be compensated before compensation. Indicates the capacity of harmonics to be compensated after compensation. Indicates the grid voltage, represents the harmonic coefficient, Indicates the effective value of the fundamental grid-connected current, Indicates the power factor, Indicates the grid-connected active power, Indicates the grid-connected fundamental reactive power before compensation. It represents the grid-connected fundamental reactive power after compensation. Indicates the power factor of the grid-connected point after compensation, It represents the reactive power factor compensation, α1 is the harmonic compensation coefficient, and α2 is the reactive power compensation coefficient.

8. A power quality management device for adaptive capacity adjustment of photovoltaic grid-connected systems, characterized by: A power quality management method for realizing the adaptive capacity allocation of a photovoltaic grid-connected system as described in any one of claim 7, comprising a photovoltaic inverter power generation state detection unit, a normal power generation mode unit, a power quality management mode unit, a maximum power point tracking control unit, and an active power reduction control unit, wherein: The photovoltaic inverter power generation state detection unit is used to detect whether the photovoltaic inverter is in a power generation state. If the photovoltaic inverter is not in a power generation state, the photovoltaic inverter becomes a power quality management device and switches to a power quality management mode unit; if the photovoltaic inverter is in a power generation state, the power quality of the grid connection point is detected and the process proceeds to step 3; The power quality management mode unit is used to not work when the power quality of the grid connection point meets the requirements; if the power quality of the grid connection point does not meet the requirements, the photovoltaic inverter uses the remaining capacity to manage the power quality, and while the power quality of the grid connection point meets the grid connection requirements, the optimal compensation model of the grid connection point is used to ensure that the comprehensive power quality index is optimal while the compensation capacity is minimal; The normal power generation mode unit is used for not performing power quality management if the photovoltaic inverter is in power generation state and the power quality of the grid connection point meets the requirements, and the photovoltaic inverter works in a normal state; If the photovoltaic inverter is in the power generation state and the power quality at the grid connection point exceeds the grid connection requirement, the maximum power point tracking control unit first calculates the compensation capacity S required to meet the grid connection requirement. F , and then calculate the remaining capacity S of the photovoltaic inverter R ; If the remaining capacity of the photovoltaic inverter is S R Greater than the required compensation capacity S F , through the optimal compensation model of the grid connection point, the power quality is optimized while the compensation capacity is minimized; The active power reduction control unit is used to reduce the remaining capacity S of the photovoltaic inverter. R Smaller than the required compensation capacity S F , the active power of the photovoltaic inverter is reduced, the remaining capacity of the photovoltaic inverter is further released, and the active power reduction is calculated after compensation to ensure that the photovoltaic inverter meets the grid-connected requirements. The optimal compensation model of the grid-connected point with active power reduction is used to obtain the minimum compensation capacity while achieving the best power quality.

9. A computer system, characterized in that: It includes a memory and a processor, the memory is used to store computer programs / instructions; the processor is used to execute the computer programs / instructions to implement the power quality management method for adaptive capacity allocation of photovoltaic grid-connected systems as described in any one of claims 1-8.

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