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

Abstract

The present invention discloses a photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking, comprising the following steps: step 1, obtaining photovoltaic voltage, output voltage and control current through a hardware sampling circuit; step 2, obtaining photovoltaic sampling voltage Uspv, output voltage Usbat and control current Isbat after AC / DC filtering, and calculating output power Psout; step 3, detecting whether the inverter is in a protection state or has no photovoltaic energy demand; if it is not in a protection state and has photovoltaic demand, proceeding to step 4; step 4, providing a maximum output voltage limit Uset and a maximum control current limit Iset according to the current demand of the inverter, and calculating a maximum power limit Pset; step 5, using a PI algorithm with output voltage as the outer loop and control current as the inner loop in the control algorithm. The present invention can maximize and efficiently utilize solar energy in a safe and reliable manner.

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 a crucial component of human energy consumption, electricity has always played a vital role. However, current electricity production relies primarily on the combustion of fossil fuels, which not only depletes natural resources but also generates significant greenhouse gas emissions. To address these issues, many countries and regions are vigorously developing renewable energy, particularly solar energy. In this context, photovoltaic inverters have emerged. They can efficiently convert solar energy into usable electricity, providing a new solution to current energy challenges.

[0003] A photovoltaic inverter is a key device that converts direct current (DC) to alternating current (AC), playing a crucial role in solar power generation systems. Compared to traditional generators, a photovoltaic inverter offers higher energy conversion efficiency, maximizing power generation. In recent years, technological advancements have significantly improved the performance of photovoltaic inverters, resulting in higher conversion efficiency, greater reliability, and lower costs.

[0004] While photovoltaic inverters demonstrate significant potential in many areas, they still face several challenges. First, the high initial investment cost of photovoltaic inverters presents a barrier to many potential users. Second, the operational stability of photovoltaic inverters is significantly affected by environmental factors (such as temperature, humidity, and sunlight intensity), which can lead to reduced power generation efficiency.

[0005] To address these challenges, photovoltaic inverters must optimize solar energy utilization by integrating sunlight energy, load demand, and energy management. They must be able to stably and quickly track the maximum power point when energy is needed. When battery capacity is saturated, constant voltage and current limiting are used to maintain battery charge and maintain a safe operating range.

[0006] Name explanation:

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

[0008] DC component filtering algorithm: a method for filtering 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] Conductivity increment method: This method is used to determine the current position on the photovoltaic power generation (PU) characteristic curve. When the conductance increment approaches 0, the current position is at the maximum power point (MPP). When the conductance increment is negative, the current position is to the right of the MPP on the PU characteristic curve. When the conductance increment is positive, the current position is to the left of the MPP on the PU characteristic curve. Summary of the Invention

[0012] To solve the above technical problems, the present invention proposes a photovoltaic inverter control method that integrates 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 includes the following steps:

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

[0016] Step 2: In order to filter the DC component zero drift interference on the CPU AD sampling, 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 by 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 by 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 there is photovoltaic demand, proceed to step 4; if it is in protection state or has no photovoltaic demand, exit and do not 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 1, 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 time i 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 time 1 to A-1; Fsample is the sampling frequency of 10000 Hz, and Ffilter is the filtering frequency of 60 Hz; INT() means taking the integer part, and Round() means taking the decimal part; therefore, A=166, B=0.66667.

[0028] For 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 filtered output voltage 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 represents 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 reduced 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] As a 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. 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 output power have the same direction, and the conductance increment 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 increases. is negative;

[0040] When performing MPPT maximum power point tracking, if the operation is in the direction from P5 to P3, the conductance increment identifies that it is in area B, and the control algorithm adjusts the duty cycle to increase, so that the photovoltaic voltage decreases and the output power increases. If the MPPT is running in the direction from P5 to P1 across P3 and the conductance increment identifies that it is in area A, if the direction of the control algorithm is not adjusted, the output duty cycle will continue to increase, the output power will decrease, and it will deviate 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, that is, multiplied by -1 to obtain 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 and then to P5, then the opposite adjustment is made 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 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 and the MPPT is locked at the highest point. If the change in photovoltaic energy causes the Vspv-Psout curve to change significantly, 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 the efficient use of 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 drawings do not constitute any limitation to the present invention.

[0047] Figure 1 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 Schematic diagram of 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 clear, 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 with reference to the accompanying drawings and examples. The present invention proposes an inverter photovoltaic control method that integrates constant voltage, current limiting, and maximum power point tracking, as described in detail below:

[0056] like Figure 2 As shown, the main functions of the photovoltaic inverter are AC inversion output for load, photovoltaic charging or photovoltaic load supply, and battery management. The present invention mainly focuses on 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 a two-layer process. The first layer is a DC filter to eliminate the inherent DC component of the ADC sampling. For example, if the ADC can sample about 8 without adding any more data, the average value is recorded. The DC filter output is the ADC sampling minus the average value.

[0059] Because the PV inverter output contains AC interference, the AC filter enters the circuit after the DC filter. The AC component filtering algorithm cancels out the positive and negative AC interference components within a cycle, restoring the original AD sampling. For example, if 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 filtered value input to the integrated management module after AC filtering.

[0060] Energy management determines whether the PV inverter requires solar power and how much solar power it needs, which is then fed into the integrated management module. Battery BMS data is also fed into 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, it uses the sampled and filtered data, so that the control has no line of sight loss and is 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 even when the hardware is damaged, causing more serious hardware damage and safety hazards.

[0063] like Figure 4As shown, 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 and 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, making the output voltage of the photovoltaic inverter 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, the above condition is triggered to make 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, 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 output power have the same direction, and the conductance increment is positive. Therefore, for a unidirectional PI control algorithm, bidirectional matching of the PV VP curve requires adjustment of the proportional and integral coefficients. The current position on the PV curve is determined by the conductance increment method. When the conductance increment is greater than 0, to the right of the maximum power point, the output duty cycle increases, and the output power increases. In this case, the proportional and integral coefficients of the voltage and current loops are automatically set to positive values: kpu = Kpu_init, kiu = Kiu_init, kpi = Kpi_init, and kii = Kii_init, synchronizing the duty cycle and output power. When the conductance increment is less than 0, to the left of the maximum power point, the duty cycle increases, and the output power decreases. In this case, the proportional and integral coefficients of the voltage and current loops are automatically set to negative values: kpu = -Kpu_init, kiu = -Kiu_init, kpi = -Kpi_init, and kii = -Kii_init. This adjustment reduces the output duty cycle and increases the output power, forcing the output point to move from right to left toward the maximum power point. This algorithm modulates the proportional and integral coefficients within a single system to keep the PV inverter output oscillating near its maximum power point. If constant voltage or current limiting causes power to drop, PI modulation will adjust the output to the right of the maximum power point to stabilize it. The control algorithm provides a direction for the PI modulation coefficients.

[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, recording the number of oscillations at a certain output time, and then amplifying or reducing the integral coefficient of the PI control algorithm. 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, then 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 oscillation frequency is high near the maximum power point, the coefficient is adjusted to a smaller value so that the duty cycle of each modulation is smaller and the modulation amplitude is 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 is 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 there are 0 AB interval switching oscillations, the integral coefficient is 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, and 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 solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions 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 DC component zero drift interference on the CPU AD sampling, 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 by 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 by 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 there is photovoltaic demand, proceed to step 4; if it is in protection state or has no photovoltaic demand, exit and do not 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 a PI algorithm with the output voltage as the outer loop and the control current as the inner loop. When both the output voltage and the control current are 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 PI algorithm of the current inner loop performs current limiting control. When the output voltage is greater than the maximum output voltage limit Uset of the inverter, the PI algorithm of the voltage outer loop performs constant voltage control. 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 filtered output voltage 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 represents 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, maintaining 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, maintaining the current limit, 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; 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. 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 output power have the same direction, and the conductance increment Positive, in area B, the output duty cycle increases, the photovoltaic voltage decreases, the output power increases, the photovoltaic voltage and 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 identifies that it is in area B, and the control algorithm adjusts the duty cycle to increase, so that the photovoltaic voltage decreases and the output power increases. If the MPPT is running in the direction from P5 to P1 across P3 and the conductance increment identifies that it is in area A, if the direction of the control algorithm is not adjusted, the output duty cycle will continue to increase, the output power will decrease, and it will deviate 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, that is, multiplied by -1 to obtain 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 and then to P5, then the opposite adjustment is made from P5 to P3 and then to P1, so that the proportional coefficient Kpu, integral coefficient Kiu in the PI algorithm of the voltage outer loop and the proportional coefficient Kpi, integral coefficient Kii of the current loop are adjusted back to positive values, ensuring that the output duty cycle increases, the output power increases, and approaches p3; at this time, the MPPT will oscillate between the three points 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-region 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 above-mentioned integral coefficients Kiu and Kii are used to reduce the integral response speed and lock the MPPT at the highest point. If the change in photovoltaic energy causes the Vspv-Psout curve to change significantly, causing the MPPT to jump from P3 to P1 or P5, and no cross-zone AB oscillation is detected 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. In the step 1, the photovoltaic voltage, output voltage, and control current are obtained through a hardware sampling circuit; 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 time i 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.

2. The photovoltaic inverter control method integrating constant voltage, current limiting and maximum power point tracking according to claim 1, 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, and D(A) is the photovoltaic voltage Uspv-d / output voltage at time A. It is the integral sum of the photovoltaic voltage Uspv-d / output voltage Usbat-d from time 1 to A-1; Fsample is the sampling frequency of 10000 Hz, and Ffilter is the filtering frequency of 60 Hz; INT() means taking the integer part, and Round() means taking the decimal part; therefore, A=166, B=0.66667.

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