Photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking

Abstract

The invention discloses a photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking. The photovoltaic inverter control method comprises the following steps of 1, obtaining photovoltaic voltage, output voltage and control current through a hardware sampling circuit; step 2, obtaining a photovoltaic sampling voltage Uspv, an output voltage Usbat and a control current Isbat after AC / DC filtering, and calculating an output power Psout; step 3, detecting whether the inverter is in a protection state or has no photovoltaic energy demand and is not in the protection state and has the photovoltaic demand, and entering step 4; 4, according to the demand of the inverter at the present stage, giving the maximum output voltage limit Uset and the maximum control current limit Iset, and calculating the maximum power limit Pset; and step 5, a PI algorithm with output voltage as an outer loop and control current as an inner loop is used as a control algorithm. Solar energy can be utilized to the maximum extent and efficiently under the safe and reliable conditions.

Description

Technical Field

[0001] The present invention relates to the field of electricity, and in particular to a photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking. Background Art

[0002] As an important part of human energy consumption, electricity has always played a vital role. However, current electricity production mainly relies on the burning of fossil fuels, which not only leads to the depletion of natural resources, but also produces a large amount of greenhouse gas emissions. In order to solve these problems, many countries and regions are vigorously developing renewable energy, especially solar energy. In this context, photovoltaic inverters came into being, which can effectively convert solar energy into electricity that can be used by humans, providing a new solution to solve current energy problems.

[0003] Photovoltaic inverter is a key device that can convert direct current into alternating current. It plays an important role in solar power generation system. Compared with traditional generators, photovoltaic inverter has higher energy conversion efficiency and can maximize power generation. In recent years, with the development of technology, the performance of photovoltaic inverter has also been significantly improved, including higher conversion efficiency, stronger reliability and lower cost.

[0004] Although photovoltaic inverters have shown great potential in many aspects, there are still some shortcomings and challenges. First, the initial investment cost of photovoltaic inverters is high, which constitutes a barrier for many potential users. Second, the working stability of photovoltaic inverters is greatly affected by external environmental factors (such as temperature, humidity and sunlight intensity), which may lead to reduced power generation efficiency.

[0005] In the face of the above challenges, photovoltaic inverters need to optimize the utilization of sunlight energy by integrating sunlight energy, load demand, and energy management. When energy is needed, they can stably and quickly track the maximum power point. When the battery capacity is saturated, constant voltage and current limiting are used to control the battery to keep it fully charged and active in a safe operating range.

[0006] Explanation of the name:

[0007] MPPT: Maximum Power Point Tracking, which is based on the PU characteristic curve of photovoltaics. Through continuous adjustment, the photovoltaic equipment tracks the maximum power in the PU characteristic curve, so as to maximize the utilization of photovoltaic energy.

[0008] DC component filtering algorithm: a processing method used to filter the DC component interference caused by the CPU itself and the sampling circuit

[0009] AC component filtering algorithm: a method for filtering the AC component interference coupled from the inverter AC output

[0010] PI control method: It is a classic feedback control method widely used in automatic control systems. Its core idea is to adjust the output of the controlled object by adding the proportional and integral links to achieve precise control of its state.

[0011] Conductance increment method: It is a method used to determine the current position of the photovoltaic PU characteristic curve. When the conductance increment tends to 0, it is currently at the maximum power point. When the conductance increment is a negative value, it is currently on the right side of the maximum power point in the PU characteristic curve. When the conductance increment is a positive value, it is currently on the left side of the maximum power point in the PU characteristic curve. Summary of the invention

[0012] In order to solve the above technical problems, the present invention proposes a photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking.

[0013] The purpose of the present invention is achieved through the following technical solutions:

[0014] A photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking comprises the following steps:

[0015] Step 1: Obtain photovoltaic voltage, output voltage, and control current;

[0016] Step 2: In order to filter the zero drift interference of the DC component on the AD sampling of the CPU, before the control function is run, the zero drift Zero of the photovoltaic voltage, output voltage and control current is calculated first, and then the photovoltaic voltage, output voltage and control current are filtered through the DC component filtering algorithm to obtain the photovoltaic voltage Uspv-d, output voltage Usbat-d and control current Isbat after DC filtering; the photovoltaic voltage Uspv-d and output voltage Usbat-d after DC filtering are filtered through the AC component filtering algorithm to filter the 60Hz AC interference generated by the inverter output to obtain the final photovoltaic voltage Uspv and output voltage Usbat, and calculate the output power Psout, Psout = Usbat*Isbat;

[0017] Step 3: Check whether the inverter is in protection state or has no photovoltaic energy demand. If it is not in protection state and has photovoltaic demand, go to step 4; if it is in protection state or has no photovoltaic demand, exit without output;

[0018] Step 4: According to the current requirements of the inverter, the maximum output voltage limit Uset and the maximum control current limit Iset are given, and the maximum power limit Pset is calculated, Pset = Uset*Iset;

[0019] Step 5. The control algorithm uses the PI algorithm with the output voltage as the outer loop and the control current as the inner loop; when the output voltage and the control current are both less than the maximum output voltage limit Uset and the maximum control current limit Iset of the inverter, the proportional coefficient and the integral coefficient are automatically adjusted to enable the control algorithm to perform MPPT maximum power point tracking; if the control current is greater than the maximum control current limit Iset, the current limiting control is performed by the PI algorithm of the current inner loop; when the output voltage is greater than the maximum output voltage limit Uset of the inverter, the constant voltage control is performed by the PI algorithm of the voltage outer loop.

[0020] As a further improvement, in step one, the photovoltaic voltage, output voltage and control current are obtained through a hardware sampling circuit.

[0021] As a further improvement, the DC component filtering algorithm is as follows:

[0022]

[0023] f(x)=P(x)–Zero

[0024] f(x) is the DC filter function, P(x) is the photovoltaic voltage / output voltage / control current collected by the CPU through the sampling circuit at the current moment; P(i) is the photovoltaic voltage / output voltage / control current sampling value at the i moment in i=1~L; Zero is the photovoltaic voltage / output voltage / control current AD sampling zero drift corresponding to the photovoltaic output before the photovoltaic output; L is the total number of sampling points within the preset time.

[0025] As a further improvement, the AC component filtering algorithm is as follows:

[0026]

[0027] f(x) is the AC filter function, D(r) is the photovoltaic voltage Uspv-d / output voltage Usbat-d at time r, D(A) is the photovoltaic voltage Uspv-d / output voltage Usbat-d at time A, It is the integral sum of the photovoltaic voltage Uspv-d / output voltage Usbat-d from 1 to A-1; Fsample is the sampling frequency of 10000Hz, and Ffilter is the filtering frequency of 60Hz; INT() means taking the integer part, and Round() means taking the decimal part; therefore A=166, B=0.66667.

[0028] Further improvement, in step 5, the control algorithm is as follows:

[0029] PI algorithm of voltage outer loop:

[0030] eu(t)=Usbat(t)–Uset;

[0031] U(t)=Kpu*eu(t)+Kiu*∫eu(t);

[0032] PI algorithm of the current inner loop:

[0033] ei(t)=i(t)–U(t);

[0034] If Kii*∫ei(t)>Iset,Intg(t)=Iset; otherwise Intg(t)=Kii*∫ei(t);

[0035] I(t)=Kpi*ei(t)+Intg(t);

[0036] t represents the current moment, Usbat(t) represents the output voltage after filtering at the current moment, Uset represents the maximum output voltage limit, eu(t) represents the difference between the output voltage and the maximum output voltage limit Uset; U(t) represents the voltage loop output value at the current moment, Kpu represents the proportional coefficient of the voltage loop, Kiu represents the integral coefficient of the voltage loop, ∫eu(t) represents the cumulative sum from eu(t) from time 0 to time t, i(t) represents the control current Isbat at the current moment, ei(t) represents the difference between the voltage loop output value and the control current, Intg(t) represents the current loop integral link, Kii represents the integral coefficient of the current loop, Kpi is the proportional coefficient of the current loop, ∫ei(t) represents the cumulative sum from ei(t) from time 0 to time t; I(t) is the output of the current loop, which is also the duty cycle of the switch tube controlled by the control algorithm;

[0037] If the output voltage Usbat(t) after filtering is greater than the maximum output voltage limit Uset, U(t) increases, ei(t) decreases, the I(t) control duty cycle tends to decrease, and the output energy decreases; if the output voltage Usbat(t) after filtering is less than the maximum output voltage limit Uset, U(t) decreases, ei(t) increases, the I(t) control duty cycle tends to increase, and the output energy increases; the duty cycle is adjusted through algorithm control to make the output voltage approach the maximum output voltage limit Uset and maintain constant voltage, that is, the PI algorithm of the voltage outer loop performs constant voltage control; when the control current is greater than the maximum control current limit value Iset, Intg(t) = Iset, which limits the integral link, so that the I(t) control duty cycle is limited to the maximum value, the output energy decreases, and the control current is limited to the maximum control current limit value Iset, and the current is maintained, that is, the PI algorithm of the current inner loop performs current limiting control; when the voltage is less than the maximum output voltage limit Uset, and the control current is less than the maximum control current limit value Iset, MPPT maximum power point tracking is performed.

[0038] Further improvement, in step 5, the steps of automatically adjusting the proportional coefficient and the integral coefficient so that the control algorithm can track the MPPT maximum power point are as follows:

[0039] With the X-axis as the photovoltaic voltage and the Y-axis as the photovoltaic output power, the Vspv-Psout curve is drawn. The duty cycle is inversely proportional to the photovoltaic voltage, that is, the smaller the duty cycle, the greater the photovoltaic voltage; five reference points P1, P2, P3, P4, and P5 are set on the Vspv-Psout curve, and P1-P5 are the photovoltaic voltage and output power corresponding to the duty cycle from large to small respectively; among them, p3 is the maximum power point, and the left side of p3 is set as area A, and the right side of p3 is set as area B, then P1 and P2 are in area A, and P4 and P5 are in area B; in area A, the output duty cycle decreases, the photovoltaic voltage increases, the output power increases, the photovoltaic voltage and the output power are in the same direction, and the conductance increment is positive. In area B, the output duty cycle increases, the photovoltaic voltage decreases, the output power increases, the photovoltaic voltage and the output power are in opposite directions, and the conductance increment is negative;

[0040] When performing MPPT maximum power point tracking, if the operation is in the direction from P5 to P3, the conductance increment is used to identify that it is in area B, and the control algorithm is used to adjust the duty cycle to increase, so that the photovoltaic voltage decreases and the output power increases; when the MPPT is running in the direction from P5 to P1 across P3, the conductance increment is used to identify that it is in area A. If the direction of the control algorithm is not adjusted, the output duty cycle will continue to increase, and the output power will decrease, deviating from the goal of maximum power point tracking; at this time, the proportional coefficient Kpu and integral coefficient Kiu of the voltage loop and the proportional coefficient Kpi and integral coefficient Kii of the current loop are reversed in direction, that is, multiplied by -1 to take a negative value, so that the output duty cycle decreases, the photovoltaic voltage increases, the output power increases, and it approaches p3;

[0041] When performing MPPT maximum power point tracking, if the operation is from P1 to P3 to P5, then make the opposite adjustment from P5 to P3 to P1, so that the proportional coefficient Kpu and integral coefficient Kiu in the PI algorithm of the voltage outer loop and the proportional coefficient Kpi and integral coefficient Kii of the current loop are adjusted back to positive values ​​to ensure that the output duty cycle increases, the output power increases, and approaches p3; at this time, the MPPT will oscillate between the three points of p2-p3-p4. In order to reduce the oscillation and stabilize the output power at the maximum power point P3, it is detected that there are N AB interval cross-zone oscillations in the M duty cycle output, and the voltage loop integral coefficient Kiu and the current loop integral coefficient Kii in the PI algorithm of the voltage outer loop are automatically adjusted to make the voltage loop integral coefficient Kiu and the current loop integral coefficient Kii = the original integral coefficient Through the above-mentioned integral coefficients Kiu and Kii, the integral response speed is reduced to lock the MPPT at the highest point; if the change in photovoltaic energy causes a significant change in the Vspv-Psout curve, causing the MPPT to jump from P3 to P1 or P5, and it is detected that there is no AB cross-zone oscillation within the preset time, the voltage loop integral coefficient Kiu and the current loop integral coefficient Kii are amplified back to the initial value, and the integral response speed is increased to quickly return to the MPPT maximum power point P3.

[0042] The beneficial effects of the present invention are:

[0043] 1. It can adaptively adjust the step size and direction according to the deviation feedback. Compared with the fixed step size adjustment of the disturbance observation method, it has faster and more accurate adjustment capabilities. Compared with the fixed unidirectional PI algorithm, it has the advantages of bidirectional adjustment and reducing the maximum power point oscillation to improve efficiency.

[0044] 2. It can filter out the inherent DC sampling error of the CPU and the AC component interference caused by the AC output of the inverter

[0045] 3. It can automatically integrate multiple factors such as photovoltaic intensity, load intensity, battery status and user needs to achieve constant voltage, current limiting and MPPT functions, and maximize and efficiently utilize solar energy in a safe and reliable manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The present invention is further described with reference to the accompanying drawings, but the contents in the accompanying drawings do not constitute any limitation to the present invention.

[0047] Figure 1 It is a schematic diagram of the control algorithm operation (including photovoltaic VP characteristic curve);

[0048] Figure 2 This is a schematic diagram of photovoltaic inverter application;

[0049] Figure 3 This is a schematic diagram of the photovoltaic inverter system architecture;

[0050] Figure 4 This is a schematic diagram of the integrated management and operation of photovoltaic inverters;

[0051] Figure 5 This is a schematic diagram of the photovoltaic inverter control algorithm;

[0052] Figure 6 Schematic diagram of automatic parameter adjustment of photovoltaic inverter algorithm. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solutions and advantages of the invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples.

[0054] Example 1

[0055] The technical means and functions of the present invention are further described below in conjunction with the accompanying drawings and examples. The inverter photovoltaic control method integrating constant voltage, current limiting and maximum power point tracking proposed by the present invention is described in detail as follows:

[0056] like Figure 2 As shown, the main functions of the photovoltaic inverter are AC inverter output for load supply, photovoltaic charging or photovoltaic load supply, and battery management. The present invention is mainly responsible for the photovoltaic-photovoltaic inverter-battery aspect.

[0057] like Figure 3 As shown, the ADC sampling module enters the comprehensive management module through the filtering module.

[0058] The filtering is two-layer filtering. The first layer is DC component filtering to eliminate the inherent DC component of AD sampling. For example, if the ADC can sample about 8 without adding quantity, then remember the average value, and the DC filtering output is the ADC sampling minus the average value.

[0059] Since there is AC interference when the photovoltaic inverter is output, the AC filter is entered after the DC filter is completed. The AC component filtering algorithm makes the positive and negative AC interference components in one cycle cancel each other out to restore the original AD sampling. For example, the control frequency is 10kHz and the AC interference is 60Hz. The integer part A = INT (10000 / 60) = 166, and the decimal part B = ROUND (10000 / 60, 5) = 0.66667. P(r) is the DC filtering value at time r, and f(x) is the filtering value input to the integrated management module after AC filtering.

[0060] Energy management determines whether the photovoltaic inverter needs solar energy and how much solar energy is needed, which is then entered into the integrated management module. The battery has BMS data which also enters the integrated management module.

[0061] When there is a BMS, the integrated management module gives priority to using the battery voltage data of the BMS. If not, the sampling and filtering data will be used to make the control without line loss and directly linked to the real-time data inside the battery.

[0062] The real-time self-check protection of the photovoltaic inverter is also input into the integrated management module to prevent photovoltaic output from continuing when the hardware is damaged, causing more serious hardware damage and safety hazards.

[0063] like Figure 4As shown in the figure, the integrated management module combines sampling filtering, battery state data, photovoltaic inverter protection status, and energy demand to obtain the maximum output voltage limit Uset, the maximum control current limit Iset, and the maximum power setting Pset = Uset * Iset provided to the control algorithm. For example, when the output voltage charges the battery to near Uset and is close to full charge, the control algorithm feeds back and modulates to gradually decrease the output, so that the output voltage stabilizes near Uset. For example, when the battery needs to be activated with a continuous small current, control Iset = small current, and the control algorithm will limit the control current to the specified current. The control algorithm performs algorithm modulation around the elements output by the above integrated management module.

[0064] The basic control algorithm of the present invention is as Figure 5 shown, which is a dynamic PI control algorithm with a voltage outer loop and a current inner loop. Taking the output voltage Usbat of the integrated management module as the input quantity, the maximum output voltage outer loop reference voltage output by the integrated management module. The voltage deviation eu(t) = Usbat(t) – Uset. U(t) = Kpu * eu(t) + Kiu * ∫eu(t). When Usbat(t) < Uset, U(t) decreases, I(t) increases, and the photovoltaic energy increases the output to raise the output voltage. When Usbat(t) > Uset, U(t) increases, and I(t) gradually decreases to lower the output voltage to Uset. As the output voltage approaches Uset, eu(t) ≈ 0, and I(t) gradually decreases to equal the load current, so that the output voltage of the photovoltaic inverter is held constant at Uset. The above is the constant voltage mode of the control algorithm. If the battery is in a battery activation state that will be protected when receiving large current charging, or the user needs to set a specified charging current, these scenarios require the photovoltaic output of the photovoltaic inverter to be constant (sufficient energy) or not greater than (insufficient energy) a current. The limit control of the current inner loop will limit the current output to Iset or below Iset to achieve the effect of current limiting.

[0065] (If Kii * ∫ei(t) > Iset, Intg(t) = Iset; otherwise Intg(t) = Kii * ∫ei(t); I(t) = Kpi * ei(t) + Intg(t)) When the photovoltaic energy is greater than the demand, triggering the above condition makes Intg(t) = Iset, so that the finally output control current is limited to the value of Iset. For example, if the maximum control current of the photovoltaic energy can reach 50A, the charging current is set to 20A, and the load current is 10A, then Iset = 30A, and the control algorithm will lock the control current to Iset = 30A. The above-mentioned current limiting part is the current limiting mode of the control algorithm

[0066] The control algorithm of the present invention automatically adjusts parameters as Figure 6As shown in the figure, Kpu_init is the initial value of the voltage loop proportional coefficient, Kiu_init is the initial value of the voltage loop integral coefficient, Kpi_init is the initial value of the current loop proportional coefficient, and Kii_init is the initial value of the current loop integral coefficient. Figure 1 In area B, the output duty cycle increases, the photovoltaic voltage decreases, the output power increases, the photovoltaic voltage and the output power are in opposite directions, and the conductance increment is negative; in area A, the output duty cycle increases, the photovoltaic voltage decreases, the output power decreases, the photovoltaic voltage and the output power are in the same direction, and the conductance increment is positive. Therefore, for the unidirectional PI control algorithm, it is necessary to adjust the proportional coefficient and the integral coefficient to achieve bidirectional matching of the photovoltaic VP curve. The current position on the photovoltaic curve is determined by the conductance increment method. When the conductance increment is greater than 0, on the right side of the maximum power point, the output duty cycle increases, and the output power increases. At this time, the proportional coefficient and integral coefficient of the voltage loop and the current loop are automatically set to positive values, that is, kpu=Kpu_init, kiu=Kiu_init, kpi=Kpi_init, kii=Kii_init, so that the duty cycle and output power are synchronized. When the conductance increment is less than 0, on the left side of the maximum power point, the duty cycle increases and the output power decreases. At this time, the proportional coefficient and integral coefficient of the voltage loop and the current loop are automatically set to negative values, that is, kpu=-Kpu_init, kiu=-Kiu_init, kpi=-Kpi_init, kii=-Kii_init. After adjustment, the control algorithm reduces the output duty cycle and increases the output power, forcing the output point to approach the maximum power point from right to left. This is done by modulating the proportional coefficient and the integral coefficient within an algorithm to make the photovoltaic inverter output oscillate near the maximum power point. If constant voltage or current limiting causes power to drop, the PI modulation will modulate to the right of the maximum power point to stabilize it at one place. The control algorithm provides a direction for the coefficient of PI modulation.

[0067] The automatic parameter adjustment of the control algorithm of the present invention to improve efficiency near the maximum power point is as follows: Figure 6 As shown. It is known that automatic parameter adjustment will lock the photovoltaic output of the photovoltaic inverter to oscillate near the maximum power point. However, if the oscillation amplitude is too large, the MPPT efficiency will be reduced, and if the oscillation amplitude is too small, the output modulation response speed will be too slow. Therefore, the present invention provides a solution to the problem. The number of oscillations is recorded at a certain output number, and then the integral coefficient of the PI control algorithm is enlarged or reduced. Before each coefficient adjustment, the values ​​of the integral coefficients Kiu and Kii will be restored to the original values ​​Kiu_init or Kii_init, but the positive and negative signs of the integral coefficients will be retained. After the photovoltaic inverter modulates the output M times, it is detected that the output power switches and oscillates in the AB interval N times, and the value of the voltage loop integral coefficient Kiu is adjusted to The value of the integral coefficient Kii of the current loop is adjusted to When the number of oscillations is high near the maximum power point, the coefficient is adjusted to a smaller value, so that the duty cycle of each modulation becomes smaller and the modulation amplitude becomes smaller. For example, if the modulation output is 100 times and 99 AB interval switching oscillations are detected near the maximum power point, then the integral coefficient That is, the integral coefficient is reduced by 1%, and then this coefficient is maintained continuously, and the efficiency is improved. If the number of oscillations detected is very small, it means that it is far away from the maximum power point. The PI integral coefficient is gradually increased or restored. For example, if 100 outputs are detected and 0 AB interval switching oscillations are detected, the integral coefficient The integral coefficient, that is, the value of the integral coefficient is restored to the initial value, so that it is accelerated to be re-modulated to the vicinity of the maximum power point.

[0068] After the above control algorithm is executed, the modulated coefficient is fed back to the PI control algorithm, the duty cycle is output by the PI control algorithm, and the photovoltaic inverter performs output modulation.

[0069] After completing the above steps, return to step 1 and wait for the next operation.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking, characterized in that: The steps include: Step 1: Obtain photovoltaic voltage, output voltage, and control current; Step 2: In order to filter the zero drift interference of the DC component on the AD sampling of the CPU, before the control function is run, the zero drift Zero of the photovoltaic voltage, output voltage and control current is calculated first, and then the photovoltaic voltage, output voltage and control current are filtered through the DC component filtering algorithm to obtain the photovoltaic voltage Uspv-d, output voltage Usbat-d and control current Isbat after DC filtering; the photovoltaic voltage Uspv-d and output voltage Usbat-d after DC filtering are filtered through the AC component filtering algorithm to filter the 60Hz AC interference generated by the inverter output to obtain the final photovoltaic voltage Uspv and output voltage Usbat, and calculate the output power Psout, Psout = Usbat*Isbat; Step 3: Check whether the inverter is in protection state or has no photovoltaic energy demand. If it is not in protection state and has photovoltaic demand, go to step 4; if it is in protection state or has no photovoltaic demand, exit without output; Step 4: According to the current requirements of the inverter, the maximum output voltage limit Uset and the maximum control current limit Iset are given, and the maximum power limit Pset is calculated, Pset = Uset*Iset; Step 5. The control algorithm uses the PI algorithm with the output voltage as the outer loop and the control current as the inner loop; when the output voltage and the control current are both less than the maximum output voltage limit Uset and the maximum control current limit Iset of the inverter, the proportional coefficient and the integral coefficient are automatically adjusted to enable the control algorithm to perform MPPT maximum power point tracking; if the control current is greater than the maximum control current limit Iset, the current limiting control is performed by the PI algorithm of the current inner loop; when the output voltage is greater than the maximum output voltage limit Uset of the inverter, the constant voltage control is performed by the PI algorithm of the voltage outer loop.

2. The photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking as claimed in claim 1, characterized in that: In the step 1, the photovoltaic voltage, output voltage and control current are obtained through a hardware sampling circuit.

3. The photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking as claimed in claim 2, characterized in that: The DC component filtering algorithm is as follows: f(x)=P(x)–Zero f(x) is the DC filter function, P(x) is the photovoltaic voltage / output voltage / control current collected by the CPU through the sampling circuit at the current moment; P(i) is the photovoltaic voltage / output voltage / control current sampling value at the i moment in i=1~L; Zero is the photovoltaic voltage / output voltage / control current AD sampling zero drift corresponding to the photovoltaic output before the photovoltaic output; L is the total number of sampling points within the preset time.

4. The photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking as claimed in claim 2, characterized in that: The AC component filtering algorithm is as follows: f(x) is the AC filter function, D(r) is the photovoltaic voltage Uspv-d / output voltage Usbat-d at time r, D(A) is the photovoltaic voltage Uspv-d / output voltage Usbat-d at time A, It is the integral sum of the photovoltaic voltage Uspv-d / output voltage Usbat-d from 1 to A-1; Fsample is the sampling frequency of 10000Hz, and Ffilter is the filtering frequency of 60Hz; INT() means taking the integer part, and Round() means taking the decimal part; therefore A=166, B=0.66667.

5. The photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking according to claim 1, characterized in that: In step 5, the control algorithm is as follows: PI algorithm of voltage outer loop: eu(t)=Usbat(t)–Uset; U(t)=Kpu*eu(t)+Kiu*∫eu(t); PI algorithm of the current inner loop: ei(t)=i(t)–U(t); If Kii*∫ei(t)>Iset,Intg(t)=Iset; otherwise Intg(t)=Kii*∫ei(t); I(t)=Kpi*ei(t)+Intg(t); t represents the current moment, Usbat(t) represents the output voltage after filtering at the current moment, Uset represents the maximum output voltage limit, eu(t) represents the difference between the output voltage and the maximum output voltage limit Uset; U(t) represents the voltage loop output value at the current moment, Kpu represents the proportional coefficient of the voltage loop, Kiu represents the integral coefficient of the voltage loop, ∫eu(t) represents the cumulative sum from eu(t) from time 0 to time t, i(t) represents the control current Isbat at the current moment, ei(t) represents the difference between the voltage loop output value and the control current, Intg(t) represents the current loop integral link, Kii represents the integral coefficient of the current loop, Kpi is the proportional coefficient of the current loop, ∫ei(t) represents the cumulative sum from ei(t) from time 0 to time t; I(t) is the output of the current loop, which is also the duty cycle of the switch tube controlled by the control algorithm; If the output voltage Usbat(t) after filtering is greater than the maximum output voltage limit Uset, U(t) increases, ei(t) decreases, the I(t) control duty cycle tends to decrease, and the output energy decreases; if the output voltage Usbat(t) after filtering is less than the maximum output voltage limit Uset, U(t) decreases, ei(t) increases, the I(t) control duty cycle tends to increase, and the output energy increases; The duty cycle is adjusted through algorithm control to make the output voltage approach the maximum output voltage limit Uset and maintain constant voltage, that is, the PI algorithm of the voltage outer loop performs constant voltage control; when the control current is greater than the maximum control current limit value Iset, Intg(t) = Iset, which limits the integral link, so that the I(t) control duty cycle is limited to the maximum value, the output energy is reduced, and the control current is limited to the maximum control current limit value Iset to maintain current limiting, that is, the PI algorithm of the current inner loop performs current limiting control; when the voltage is less than the maximum output voltage limit Uset, and the control current is less than the maximum control current limit value Iset, MPPT maximum power point tracking is performed.

6. The photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking as claimed in claim 5, characterized in that: In step 5, the steps of automatically adjusting the proportional coefficient and the integral coefficient so that the control algorithm can track the MPPT maximum power point are as follows: With the X-axis as the photovoltaic voltage and the Y-axis as the photovoltaic output power, the Vspv-Psout curve is drawn. The duty cycle is inversely proportional to the photovoltaic voltage, that is, the smaller the duty cycle, the greater the photovoltaic voltage; five reference points P1, P2, P3, P4, and P5 are set on the Vspv-Psout curve, and P1-P5 are the photovoltaic voltage and output power corresponding to the duty cycle from large to small respectively; among them, p3 is the maximum power point, and the left side of p3 is set as area A, and the right side of p3 is set as area B, then P1 and P2 are in area A, and P4 and P5 are in area B; in area A, the output duty cycle decreases, the photovoltaic voltage increases, the output power increases, the photovoltaic voltage and the output power are in the same direction, and the conductance increment is positive. In area B, the output duty cycle increases, the photovoltaic voltage decreases, the output power increases, the photovoltaic voltage and the output power are in opposite directions, and the conductance increment is negative; When performing MPPT maximum power point tracking, if the operation is in the direction from P5 to P3, the conductance increment is used to identify that it is in area B, and the control algorithm is used to adjust the duty cycle to increase, so that the photovoltaic voltage decreases and the output power increases; when the MPPT is running in the direction from P5 to P1 across P3, the conductance increment is used to identify that it is in area A. If the direction of the control algorithm is not adjusted, the output duty cycle will continue to increase, and the output power will decrease, deviating from the goal of maximum power point tracking; at this time, the proportional coefficient Kpu and integral coefficient Kiu of the voltage loop and the proportional coefficient Kpi and integral coefficient Kii of the current loop are reversed in direction, that is, multiplied by -1 to take a negative value, so that the output duty cycle decreases, the photovoltaic voltage increases, the output power increases, and it approaches p3; When performing MPPT maximum power point tracking, if the operation is from P1 to P3 to P5, then make the opposite adjustment from P5 to P3 to P1, so that the proportional coefficient Kpu and integral coefficient Kiu in the PI algorithm of the voltage outer loop and the proportional coefficient Kpi and integral coefficient Kii of the current loop are adjusted back to positive values ​​to ensure that the output duty cycle increases, the output power increases, and approaches p3; at this time, the MPPT will oscillate between the three points of p2-p3-p4. In order to reduce the oscillation and stabilize the output power at the maximum power point P3, it is detected that there are N cross-region AB interval oscillations in the M duty cycle output, and the voltage loop integral coefficient Kiu and the current loop integral coefficient Kii in the PI algorithm of the voltage outer loop are automatically adjusted to make the voltage loop integral coefficient Kiu and the current loop integral coefficient Kii Through the above-mentioned integral coefficients Kiu and Kii, the integral response speed is reduced to lock the MPPT at the highest point; if the change in photovoltaic energy causes a significant change in the Vspv-Psout curve, causing the MPPT to jump from P3 to P1 or P5, and it is detected that there is no AB cross-zone oscillation within the preset time, the voltage loop integral coefficient Kiu and the current loop integral coefficient Kii are amplified back to the initial value, and the integral response speed is increased to quickly return to the MPPT maximum power point P3.

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