Photovoltaic system maximum power point tracking control method under greenhouse load condition change

Abstract

The present application relates to a greenhouse load operating condition change photovoltaic system maximum power point tracking control method, including photovoltaic components, two-phase interleaved parallel boost converter, proportion integration (PI) and advance correction combined controller. The method proposed by the present application is to use two-phase interleaved parallel boost converter as a boost converter, add PI and advance correction module, and use improved variable step power prediction method as maximum power point optimization control method. Under the conditions of irradiance fluctuation and load operating condition change, the control method can reduce the oscillation in the power optimization process of the photovoltaic system, shorten the optimal power optimization time, improve the maximum power point tracking efficiency of the independent photovoltaic power generation system, solve the mismatch between the load power and photovoltaic output of the independent photovoltaic greenhouse power generation system, and improve the independent greenhouse system photovoltaic power generation consumption rate.

Description

Technical Field

[0001] This invention relates to the field of renewable energy utilization and photovoltaic power generation, and in particular to a maximum power point tracking control method for photovoltaic greenhouse systems considering changes in greenhouse load conditions. Background Technology

[0002] When greenhouse loads regulate light, temperature, and humidity within the greenhouse, changes in load conditions cause the photovoltaic system's output power to deviate from its maximum power point. This leads to output power oscillations during the search for the maximum power point, increasing the power optimization time and reducing the efficiency of Dynamic Maximum Power Point Tracking (MPPT). Extensive research has been conducted both domestically and internationally on the oscillation problem around the maximum power point, primarily focusing on algorithm optimization. However, the optimized models often struggle to balance speed and stability. With the rapid development of power electronics technology and increasing demands on converter power ratings and circuit device reliability, a single converter is no longer sufficient for system requirements. Furthermore, using a single converter necessitates larger inductors and filter capacitors to eliminate higher ripple in the output voltage and current. Therefore, the interleaved parallel connection technique of multiple converters to improve system speed and stability is currently a research hotspot. Introducing a high-performance controller into a photovoltaic greenhouse system is another way to solve the problem of oscillation around the maximum power point. The advantage of a PI controller is that it is a type I system with a transfer function, which can achieve the output voltage tracking the given reference value without static error. The disadvantage is that it cannot provide a positive angle and cannot achieve a high crossover frequency. The advantage of a lead compensator is that it can provide a positive phase margin and achieve a high crossover frequency. The disadvantage is that there is a static error between the reference voltage and the output voltage. Currently, there is little research on controllers that combine the two.

[0003] The combination of photovoltaic (PV) power generation and greenhouses, as a novel agricultural production method, enables agricultural production in special environments. By regulating the greenhouse environment, it meets the needs of crops at different growth stages. The PV power generation system can meet the electricity demand of the greenhouse load. However, due to the mismatch between the load power and PV output of an independent PV greenhouse power generation system, the actual power generation of the PV system is not fully utilized. Coordinating the balance between PV output and greenhouse load regulation is currently a research hotspot in the context of "smart grids." However, traditional MPPT control methods are prone to misjudgment when there are sudden changes in light intensity and temperature. Taking the perturbation-observation method, a commonly used MPPT control method in PV systems, as an example, this algorithm cannot determine whether the change in system power is caused by a change in light intensity or by the perturbation itself. When the light intensity changes abruptly, the voltage regulation direction is opposite to the actual voltage change direction, causing the power regulation to enter the unstable region, thereby increasing the power optimization time. If the light intensity changes drastically, this algorithm may even cause the optimal power optimization to fail, reducing the dynamic MPPT efficiency and leading to system instability. Therefore, based on the misjudgment phenomenon of the perturbation-observation method, some scholars have proposed a power prediction method, assuming that the power point at the current time kT is P. (k) ,exist At any given moment, the power of an artificially added sampling point is... Then, at time kT, the power at time (k+1)T is estimated. Predicted value P ( ' k) Then compare with the actual working point P at the next moment (k+1) In comparison, this algorithm can avoid misjudging the optimization direction of the maximum power point and improve the maximum power point tracking speed compared to the perturbation and observation method. However, this is on the premise that the light intensity is constant within a sampling period. If the solar irradiance and load conditions change simultaneously within a sampling period, the algorithm will still produce a large deviation. Therefore, the MPPT control method for independent photovoltaic greenhouse systems under load-controlled greenhouse conditions needs further optimization. Summary of the Invention

[0004] The present invention aims to solve the above-mentioned problems and defects by providing a method for tracking and controlling the maximum power point of a photovoltaic system under changing greenhouse load conditions.

[0005] A method for tracking the maximum power point of a photovoltaic system under varying greenhouse load conditions is disclosed. To meet the high power requirements of photovoltaic systems, this invention typically employs multiple converters operating in parallel, and utilizes interleaved parallel technology within this parallel operation. Therefore, this invention employs a two-phase interleaved parallel Boost converter as the boost converter.

[0006] The tracking control method includes using a two-phase interleaved parallel Boost converter as a boost converter; the specific circuit is: power supply Vi A circuit is formed by resistor R1, inductor L1, diode D1, resistor R, and resistor R2 connected in series; inductor L2 and diode D2 are connected in series and then in parallel with inductor L1 and diode D1; one end of switch S1 is connected between resistor R and resistor R2, and the other end of switch S1 is connected between inductor L1 and diode D1; one end of switch S2 is connected between resistor R and resistor R2, and the other end of switch S2 is connected between inductor L2 and diode D2; filter capacitor C3 is connected in parallel to power supply V. i Two ends; one end of the filter capacitor C1 is connected between the resistor R1 and the inductor L1, and the other end of the filter capacitor C1 is connected to the power supply V. i Between resistor R1 and resistor R2; one end of filter capacitor C2 is connected between diode D1 and resistor R, and the other end of filter capacitor C2 is connected between resistor R and resistor R2; filter capacitor C4 is connected in parallel across resistor R.

[0007] A two-phase interleaved parallel Boost converter is selected as the boost converter. The two branches operate in parallel. The control signals are two PWM signals with the same frequency and a phase difference of 180°, which make the two branches interleave. The circuit of the two-phase interleaved parallel Boost converter operates in the inductor current continuous mode. The switching transistors S1 and S2 operate with equal duty cycles and a phase difference of 180 degrees. The inductors L1 and L2 are equal.

[0008] The diodes D1 and D2 are freewheeling diodes; R1 and R2 are the input current sensing resistor and the output current sensing resistor, respectively.

[0009] Given the input and output voltages, the circuit requires that the inductor current ripple be less than 20% of its average current and the voltage ripple be less than 5% of its average voltage. The inductance, capacitance, and current hysteresis widths are calculated accordingly.

[0010] The current in the circuit flows out from the positive terminal, one branch passes through the filter capacitor C3 and returns to the negative terminal, and the other branch passes through the input current sensing resistor R1. Based on the conduction status of the switching transistors S1 and S2, the operating state of the circuit within one switching cycle can be divided into four stages:

[0011] Stage 1: Switches S1 and S2 are turned on. At this time, in branch 1, switch S1 and inductor L1 are connected in series, and in branch 2, switch S2 and inductor L2 are connected in series. Then, branch 1 and branch 2 are connected in parallel and pass through the output current sensing resistor R2. Branch 3 is equipped with a filter capacitor C1. The three branches are connected in parallel and then merge. During this process, inductors L1 and L2 store energy.

[0012] Phase 2: Switch S1 is turned off, and S2 continues to conduct. At this time, inductor L1 forms a freewheeling circuit through diode D1 and releases energy, while inductor L2 continues to store energy.

[0013] Stage 3: Switch S1 is turned on and S2 is turned off. Inductor L1 stores energy, and inductor L2 forms a freewheeling circuit through diode D2 to release energy.

[0014] Phase 4: Switches S1 and S2 are both turned off. At this time, inductors L1 and L2 form freewheeling circuits through diodes D1 and D2 respectively and release energy.

[0015] For the two-phase interleaved parallel Boost converter selected in this invention, a control method combining two controllers is used to improve the system performance. Specifically, the output voltage of the photovoltaic string is input to a controller combining PI and lead compensation to adjust the duty cycle. The hardware specifically uses a TI C2000 series DSP chip. The DSP chip's port is connected to a universal asynchronous transceiver (UART) bus; the DSP chip's serial peripheral interface is connected to the external environment; and the acquired solar panel output voltage V... in Solar panel output current I in The output voltage V of the two-phase interleaved parallel Boost converter bus The output current I of the two-phase interleaved parallel Boost converter bus The DSP is connected via an analog-to-digital converter; the pulse width modulation module in the DSP chip outputs four PWM signals to drive the half-bridge circuit, with two signals forming a complementary PWM signal group, which makes the two branches with switching transistors in the two-phase interleaved parallel Boost converter interleaved; the solar panel is connected to the two-phase interleaved parallel Boost converter and the auxiliary power supply respectively. The output voltage of the solar panel is boosted by the two-phase interleaved parallel Boost converter and then enters the inverter for DC-AC conversion. After passing through the auxiliary power supply, the DSP chip is powered by a two-stage buck mode of DC-DC converter and low dropout linear regulator.

[0016] The specific implementation method includes the following steps:

[0017] Step 1: Obtain the transfer function of the controller combining PI and lead compensation;

[0018] The transfer function of the PI controller is The transfer function of the lead compensation controller is: By aligning the zeros of the two controllers, the transfer function of the controller combining PI and lead compensation is obtained as follows:

[0019] In the formula, S is the independent variable representing the complex frequency, and K p K represents the proportional adjustment coefficient. i K represents the integral adjustment coefficient. c T1 and T2 represent the parameters of a controller combining PI and lead compensation. Assuming T1 > T2, S = 2πf (where f represents the switching frequency), f = f cAt 1000Hz, the controller can provide a phase margin of 45°;

[0020] Step 2: Solve for the unknown parameters in the transfer function;

[0021] Using Mathematics, the controller parameter K can be expressed by listing the following three equations. c Configure T1 and T2 as follows:

[0022]

[0023]

[0024]

[0025] In the formula, f c The crossover frequency is indicated; amp[f] and phase[f] represent the amplitude-frequency characteristic and phase-frequency characteristic of the controlled object, i.e., the two-phase interleaved parallel Boost converter, respectively, when f = 1000 Hz.

[0026] These represent the amplitude-frequency and phase-frequency characteristics of the controller combining PI and lead compensation at f = 1000 Hz, respectively.

[0027] Finally, the unknown parameter K of the controller combining PI and lead compensation was obtained. c T1, T2.

[0028] The tracking control method described in this invention also includes using an improved variable step size power prediction method as a control method for finding the maximum power point. By introducing load impedance change as a correction coefficient to optimize the control algorithm, the load in the photovoltaic greenhouse can be adjusted based on crop demand.

[0029] The improved variable step size power prediction method of this invention includes the following steps:

[0030] Assuming a sufficiently high sampling frequency, the rate of change of light intensity is constant within one sampling period. Using Newton's interpolation method, a three-point sampling function model y = f(x) is established. Within one sampling period kT to (k+1)T, the number of sampling modules increases... and sampling at time and Then calculate and exist The system is constantly disturbed by P. (k) , and A function model is established using Newton's interpolation method, with a mean difference of...

[0031]

[0032]

[0033]

[0034] The function expression for the three-point Newton interpolation method is:

[0035]

[0036] Substituting x = k + 1 into the above equation, we can obtain the predicted power at time (k + 1). The expression for the changing step size is V ref(k+1) =V ref(k) +DX|dP|; where: D is the direction of adjustment; X is the variable step size parameter; |dP| is the absolute value of the power change. The value of D is the direction of the disturbance, when P... (k+1) >P ( ' k) AndU (k+1) >U (k) When P = 1, D = 1; when P = 1, D = 1. (k+1) >P ( ' k) AndU (k+1) (k) When D = -1, the value of X is determined according to the following principles:

[0037] (1) The value of X is determined by the load type and operating conditions. In this invention, the load operating conditions at the summer solstice and winter solstice are used as the boundary for the value of X.

[0038] (2) When the absolute value of dP is greater than or equal to e1, X = a, and a larger step size is used to make the system track to MPPT quickly and reduce tracking loss.

[0039] (3) When the absolute value of dP is less than or equal to e1 and greater than or equal to e2, X = b, the system switches to a smaller step size for slow and precise optimization, minimizing oscillation loss to the greatest extent.

[0040] (4) When the absolute value of dP is less than e2, X = c, where c is the perturbation threshold.

[0041] ​The specific parameters e1, e2, a, b, and c are related to the greenhouse load. Tests were conducted on the winter solstice and summer solstice. The greenhouse load was adjusted according to the time to control the temperature inside the greenhouse, thereby determining the values ​​of e1, e2, a, b, and c. The test period started at 9:00 and ended at 18:00. e1, e2, a, b, and c are variable parameters set in the flowchart and have no practical significance. With the goal of finding a suitable internal environment for greenhouse crop growth, the distribution changes of greenhouse light, temperature, and humidity on two typical days (i.e., the winter solstice and summer solstice) were analyzed by adjusting the greenhouse load to obtain the optimal values ​​of e1, e2, a, b, and c.

[0042] The beneficial effects of this invention are as follows:

[0043] 1) Because the two-phase interleaved parallel Boost converter adopts interleaved parallel technology, the power capacity and reliability of the converter are improved, the current stress of the switching tube is further reduced, thereby saving the cost of the photovoltaic greenhouse system and meeting the requirements of the higher power level of the photovoltaic system, and improving the power density of the converter.

[0044] 2) Since the output filter capacitor has been expanded in the circuit topology of the two-phase interleaved parallel Boost converter, the electrical energy stored in the capacitor as a power device can smooth out fluctuations when oscillations occur during the optimization process, and at the same time, it can reduce the output ripple.

[0045] 3) By introducing a robust, high-gain, and highly anti-interference PI controller combined with a lead-compensation controller into the two-phase interleaved parallel Boost converter, the tracking rate is further improved from a hardware perspective. This not only enables the system to respond quickly but also eliminates static errors, thus enhancing the overall circuit performance. Simultaneously, it improves response speed and system stability, thereby increasing the dynamic MPPT efficiency of the independent photovoltaic greenhouse system.

[0046] 4) Since the maximum power point tracking control method of the two-phase interleaved parallel Boost converter adopts the improved variable step size power prediction method, it solves the problem of mismatch between photovoltaic output and load power. At the same time, by introducing an impedance change coefficient into the variable step size factor, the load condition can be adjusted according to the needs of the crop. Overall, it improves system efficiency, reduces system loss, smooths out optimization fluctuations, and shortens optimization time.

[0047] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the two-phase interleaved parallel Boost converter circuit topology of the present invention;

[0049] Figure 2This is a hardware block diagram of the method for tracking and controlling the maximum power point of a photovoltaic system under varying greenhouse load conditions according to the present invention.

[0050] Figure 3 The flowchart of the improved variable step size power prediction method MPPT of this invention is shown below;

[0051] Figure 4 This is a block diagram illustrating the control principle of the two-phase interleaved parallel Boost converter used in the photovoltaic greenhouse system of this invention. Detailed Implementation

[0052] The invention will be further described in detail below with reference to the accompanying drawings:

[0053] To address the oscillation problem during the maximum power point optimization process, this invention employs two methods: optimizing the circuit topology and introducing a controller combining PI control and lead compensation. (See attached diagram) Figure 1 This is a two-phase interleaved parallel Boost converter circuit topology, which will be discussed below in conjunction with the attached diagram. Figure 1 This invention describes the technical solution for optimizing circuit topology:

[0054] The photovoltaic greenhouse system uses a two-phase interleaved parallel Boost converter as the boost converter. The two branches operate in parallel, controlled by two PWM signals of the same frequency but 180° out of phase, allowing the two branches to operate alternately. The circuit of the two-phase interleaved parallel Boost converter operates in Continuous Conduction Mode (CCM). Switches S1 and S2 operate with equal duty cycles and a 180° phase difference. Inductors L1 and L2 are equal, and diodes D1 and D2 are freewheeling diodes. In the circuit diagram, R1 and R2 are the input and output current sensing resistors, respectively. Given the input and output voltages, the inductor current ripple is required to be less than 20% of its average current, and the voltage ripple is required to be less than 5% of its average voltage. The inductance, capacitance, and current hysteresis width are calculated accordingly. The MOSFET selected is STB60NF06T4; the diode selected is STPSC4H065B-TR. The output filter capacitor C4 was expanded during the design.

[0055] Analyze the operation of this circuit and make the following assumptions:

[0056] (1) All switching components in the circuit are ideal components;

[0057] (2) The inductors and capacitors in the circuit are all ideal devices;

[0058] After the current flows out from the positive terminal, one branch passes through the filter capacitor C3 and returns to the negative terminal, while the other branch passes through the input current sensing resistor R1. Based on the conduction status of the switching transistors S1 and S2, the circuit's operating state within one switching cycle can be divided into four stages:

[0059] Stage 1: Switches S1 and S2 are turned on. At this time, in branch 1, switch S1 and inductor L1 are connected in series, and in branch 2, switch S2 and inductor L2 are connected in series. Then, branch 1 and branch 2 are connected in parallel and pass through the output current sensing resistor R2. Branch 3 is equipped with a filter capacitor C1. The three branches are connected in parallel and then merge. During this process, inductors L1 and L2 store energy.

[0060] Phase 2: Switch S1 is turned off, and S2 continues to conduct. At this time, inductor L1 forms a freewheeling circuit through diode D1 and releases energy, while inductor L2 continues to store energy.

[0061] Stage 3: Switch S1 is turned on and S2 is turned off. Inductor L1 stores energy, and inductor L2 forms a freewheeling circuit through diode D2 to release energy.

[0062] Phase 4: Switches S1 and S2 are both turned off. At this time, inductors L1 and L2 form freewheeling circuits through diodes D1 and D2 respectively and release energy.

[0063] Appendix Figure 2 The hardware block diagram for the maximum power point tracking control method of a photovoltaic system under varying greenhouse load conditions is shown below. (See attached diagram.) Figure 2 This invention describes the technical solution of introducing a controller that combines PI and lead compensation:

[0064] The photovoltaic greenhouse system hardware uses TI C2000 series DSP chips. The DSP chip's Universal Asynchronous Receiver / Transmitter (UART) port connects to a Universal Serial Bus (USB); the DSP chip's Serial Peripheral Interface (SPI) connects to the external environment; and the acquired solar panel output voltage V... in Solar panel output current I in The output voltage V of the two-phase interleaved parallel Boost converter bus The output current I of the two-phase interleaved parallel Boost converter busThe DSP is connected via an analog-to-digital converter (ADC). The DSP chip's pulse-width modulation (PWM) module outputs four PWM signals to drive a half-bridge circuit. Two signals form a complementary PWM signal group, causing the two branches with switching transistors in the two-phase interleaved parallel boost converter to be interleaved. Solar panels are connected to both the two-phase interleaved parallel boost converter and the auxiliary power supply (AUX power). The solar panel output voltage is boosted by the two-phase interleaved parallel boost converter and then enters the inverter for DC-AC conversion. After passing through the auxiliary power supply, it powers the DSP chip via a two-stage buck converter (DC-DC converter + low dropout regulator, LDO). A controller combining PI and lead compensation is introduced into the two-phase interleaved parallel boost converter. The specific implementation method is as follows:

[0065] Step 1: Obtain the transfer function of the controller combining PI and lead compensation;

[0066] The transfer function of the PI controller is The transfer function of the lead compensation controller is: By aligning the zeros of the two controllers, the transfer function of the controller combining PI and lead compensation is obtained as follows:

[0067] In the formula, S is the independent variable representing the complex frequency, and K p K represents the proportional adjustment coefficient. i K represents the integral adjustment coefficient. c T1 and T2 represent the parameters of a controller combining PI and lead compensation. Assuming T1 > T2, S = 2πf (where f represents the switching frequency), f = f c At 1000Hz, the controller can provide a phase margin of 45°;

[0068] Step 2: Solve for the unknown parameters in the transfer function;

[0069] Using Mathematics, the controller parameter K can be expressed by listing the following three equations. c Configure T1 and T2 as follows:

[0070]

[0071]

[0072]

[0073] In the formula, f cThe crossover frequency is indicated; amp[f] and phase[f] represent the amplitude-frequency characteristic and phase-frequency characteristic of the controlled object, i.e., the two-phase interleaved parallel Boost converter, respectively, when f = 1000 Hz.

[0074] These represent the amplitude-frequency and phase-frequency characteristics of the controller combining PI and lead compensation at f = 1000 Hz, respectively.

[0075] Finally, the unknown parameter K of the controller combining PI and lead compensation was obtained. c T1, T2.

[0076] To address the mismatch between photovoltaic power output and greenhouse load power, as well as the problem that traditional MPPT control methods are prone to misjudgment when there are sudden changes in light and temperature, this invention adopts an improved variable step size power prediction method to solve the problem. Figure 3 The flowchart of the improved variable step size power prediction method MPPT will be presented below. Figure 3 This invention describes the technical solution of the improved variable step size power prediction method:

[0077] Assuming a sufficiently high sampling frequency, the rate of change of light intensity is constant within one sampling period. Using Newton's interpolation method, a three-point sampling function model y = f(x) is established. Within one sampling period kT to (k+1)T, the number of sampling modules increases... and sampling at time and Then calculate and exist The system is constantly disturbed by P. (k) , and A function model is established using Newton's interpolation method, with a mean difference of...

[0078]

[0079]

[0080]

[0081] The function expression for the three-point Newton interpolation method is:

[0082]

[0083] Substituting x = k + 1 into the above equation, we can obtain the predicted power at time (k + 1). The expression for the changing step size is V ref(k+1) =V ref(k)+DX|dP|; where: D is the direction of adjustment; X is the variable step size parameter; |dP| is the absolute value of the power change. The value of D is the disturbance direction, when P... (k+1) >P ( ' k) AndU (k+1) >U (k) When P = 1, D = 1; when P = 1, D = 1. (k+1) >P ( ' k) AndU (k+1) (k) When D = -1, the value of X is determined according to the following principles:

[0084] (1) The value of X is determined by the load type and operating conditions. In this invention, the load operating conditions at the summer solstice and winter solstice are used as the boundary for the value of X.

[0085] (2) When the absolute value of dP is greater than or equal to e1, X = a, and a larger step size is used to make the system track to MPPT quickly and reduce tracking loss.

[0086] (3) When the absolute value of dP is less than or equal to e1 and greater than or equal to e2, X = b, the system switches to a smaller step size for slow and precise optimization, minimizing oscillation loss to the greatest extent.

[0087] (4) When the absolute value of dP is less than e2, X = c, where c is the perturbation threshold.

[0088] The specific parameters e1, e2, a, b, and c are related to the greenhouse load. This invention selects the winter solstice and summer solstice for testing, and adjusts the greenhouse load according to time to control the temperature inside the greenhouse, thereby determining the values ​​of e1, e2, a, b, and c. The test time starts at 9:00 and ends at 18:00. In order to better adjust the operating state of the load in the time domain, it is proposed to set operating constraints on the greenhouse system. Within the constraints, the operating time of the load is adjusted according to the cooling and heating capacity of the load inside the greenhouse. e1, e2, a, b, and c are variable parameters set in the flowchart and have no practical meaning. With the goal of seeking a suitable internal environment for greenhouse crop growth, the optimal values ​​of e1, e2, a, b, and c are obtained by adjusting the greenhouse load and analyzing the changes in the distribution of greenhouse light, temperature, and humidity on two typical days (i.e., the winter solstice and summer solstice).

[0089] An improved variable step-size power prediction method is adopted to reduce misjudgments, and the traditional variable step-size factor V is used. ref(k) By introducing load impedance variations, the load inside the photovoltaic greenhouse can be adjusted according to the needs of the crops.

[0090] Appendix Figure 4 ​This document presents the control principle block diagram of a two-phase interleaved parallel Boost converter used in a photovoltaic greenhouse system. A dual closed-loop control system for voltage and current is established for the two-phase interleaved parallel Boost converter. The output voltage is detected in real time by the ADC module in the DSP, and the output voltage value is compared with the reference voltage value U calculated by the MPPT algorithm. ref The voltage outer loop control is used for comparison; the desired current value is obtained through the outer loop control, and this value is compared with the output current as the current inner loop control. Finally, the control signals are generated through two dual closed-loop control systems to realize the turn-on and turn-off of the switching transistors of the two-phase interleaved parallel Boost converter, thereby making the output current and output voltage tend to the steady-state value.

[0091] The above descriptions are merely some specific embodiments of the present invention. Commonly known details or common knowledge in the solutions are not described in detail here (including but not limited to abbreviations, acronyms, and units conventionally used in the art). It should be noted that the above embodiments do not limit the present invention in any way. For those skilled in the art, any technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for tracking the maximum power point of a photovoltaic system under varying load conditions in a greenhouse, characterized in that, The tracking control method comprises using a two-phase interleaved parallel Boost converter as a boost converter; a specific circuit is as follows: a loop is formed by connecting a power supply V i , a resistor R1, an inductor L1, a diode D1, a resistor R and a resistor R2 in series; the inductor L2 and the diode D2 are connected in series and then connected in parallel with the inductor L1 and the diode D1; one end of a switch tube S1 is connected between the resistor R and the resistor R2, and the other end of the switch tube S1 is connected between the inductor L1 and the diode D1; one end of a switch tube S2 is connected between the resistor R and the resistor R2, and the other end of the switch tube S2 is connected between the inductor L2 and the diode D2; a filter capacitor C3 is connected in parallel between the power supply V i and the resistor R2; one end of a filter capacitor C1 is connected between the resistor R1 and the inductor L1, and the other end of the filter capacitor C1 is connected between the power supply V i and the resistor R2; one end of a filter capacitor C2 is connected between the diode D1 and the resistor R, and the other end of the filter capacitor C2 is connected between the resistor R and the resistor R2; and a filter capacitor C4 is connected in parallel between the resistor R. The two-phase interleaved parallel Boost converter is selected as the boost converter, two branches are in parallel operation, the control signals are two PWM signals with the same frequency and a phase difference of 180 degrees, so that the two branches are in phase communication, the two-phase interleaved parallel Boost converter works in the inductor current continuous mode, the switch S1 and the switch S2 work, the duty cycles are equal and the phases are different by 180 degrees, the inductor L1 and the inductor L2 are equal; The diode D1 and the diode D2 are freewheeling diodes, and the resistor R1 and the resistor R2 are input and output current detection resistors respectively; The current of the circuit flows out from the positive electrode, one branch returns to the negative electrode through the filtering capacitor C3, and the other branch passes through the input current detection resistor R1 and can be adjusted according to the conduction state of the switch S1 and the switch S2, so that the working state of the circuit in a switching cycle can be divided into four stages: Stage 1: the switch S1 and the switch S2 are turned on, at this time, the switch S1 and the inductor L1 in branch 1 are in series, the switch S2 and the inductor L2 in branch 2 are in series, then branch 1 and branch 2 are in parallel and pass through the output current detection resistor R2, branch 3 is provided with the filtering capacitor C1, and the three branches are in parallel and then converge, wherein the inductor L1 and the inductor L2 store energy; Stage 2: the switch S1 is turned off and the switch S2 continues to be turned on, at this time, the inductor L1 forms a freewheeling circuit through the diode D1 and releases energy, and the inductor L2 continues to store energy; Stage 3: the switch S1 is turned on and the switch S2 is turned off, the inductor L1 stores energy, and the inductor L2 forms a freewheeling circuit through the diode D2 and releases energy; Stage 4: the switch S1 and the switch S2 are both turned off, at this time, the inductor L1 and the inductor L2 form a freewheeling circuit through the diode D1 and the diode D2 respectively and release energy.

2. The tracking control method according to claim 1, characterized by, The tracking control method further comprises a control method combining two controllers to improve the performance of the system, namely: inputting the photovoltaic string output voltage into the PI and lead correction combined controller to adjust the duty cycle; The specific hardware is that a TI C2000 series DSP chip is adopted, a universal asynchronous receiver transmitter of a port of the DSP chip is connected with a universal serial bus, a serial peripheral interface of the DSP chip is connected with an external environment, a solar panel output voltage V in , a solar panel output current I in , an output voltage V bus of a two-phase interleaved parallel Boost converter, an output current I bus of the two-phase interleaved parallel Boost converter, and the like are collected; the collected signals are connected with a DSP through an analog-digital converter; a pulse width modulation module in the DSP chip outputs 4 paths of PWM to drive a half-bridge circuit, two paths are a group of complementary PWM signals, so that two branches with switching tubes in the two-phase interleaved parallel Boost converter are staggered; a solar cell panel is connected with the two-phase interleaved parallel Boost converter and an auxiliary power supply respectively, the solar panel output voltage is boosted through the two-phase interleaved parallel Boost converter and then enters an inverter to perform DC-AC conversion, and after passing through the auxiliary power supply, the voltage is supplied to the DSP chip through two-stage voltage reduction modes of a DC-DC converter and a low-dropout linear regulator.

3. The tracking control method according to claim 2, characterized by, The tracking control method further comprises a control method combining two controllers to improve the performance of the system, namely: inputting the photovoltaic string output voltage into the PI and lead correction combined controller to adjust the duty cycle; the specific implementation method comprises the following steps: step one: obtaining the transfer function of the PI and lead correction combined controller; The transfer function of the PI controller is The transfer function of the lead correction controller is The transfer function of the combined PI and lead correction controller is where S is a variable representing complex frequency, K p represents a proportional regulation coefficient, K i represents an integral regulation coefficient, K c , T1, T2 all represent parameters of the PI and lead correction combined controller, it is assumed that T1>T2, S=2πf, wherein f represents a switching frequency, f=f c provides a phase angle margin of 45° when f=1000HZ. Step two: solving the unknown parameters in the transfer function; The controller parameters K, T1 and T2 are set by listing the following 3 equations using Mathematic: c K = 1 / (T1 + T2) In the formula, f c represents the crossover frequency; amp[f], phase[f] respectively represent the amplitude-frequency characteristic and the phase-frequency characteristic of the controlled object, i.e. the two-phase interleaved Boost converter, at f=1000HZ; respectively represent the amplitude and phase frequency characteristics of the controller with PI and lead correction combined at f = 1000 Hz; The unknown parameters K of the controller combining PI and lead correction are finally obtained c T1 and T2.

4. The tracking control method according to claim 1, characterized by, The tracking control method further comprises an improved variable step power prediction method as the control method of maximum power point optimization, and the control algorithm is optimized by introducing the load impedance change as a correction coefficient to realize the adjustment of the load in the photovoltaic greenhouse based on the crop demand.

5. The tracking control method according to claim 4, characterized by, The improved variable step power prediction method comprises the following steps: Assuming that the sampling frequency is high enough, the rate of change of light intensity is constant in a sampling period, and the three-point sampling function model y = f(x) is established by using Newton interpolation method. In a sampling period kT to (k+1)T, the sampling module increases and sampling at time and Then the and At time, the system is disturbed, and P (k) , and The function model is established by using Newton interpolation method, and the equal difference is The function expression of the three-point Newton interpolation method is: Substituting x = k + 1 into the above equation, the predicted power at (k + 1)th time can be obtained The expression of the variable step is V ref(k+1) = V ref(k) + DX|dP|; where D is the disturbance direction, X is the variable step parameter, and |dP| is the absolute value of the power change.

6. The tracking control method according to claim 5, characterized by, D is the direction of disturbance, when P (k+1) > P' (k) and U (k+1) > U (k) then D = 1; when P (k+1) > P' (k) and U (k+1) < U (k) then D = -1; the determination of the value of X follows the following principles: (1) the value of X is determined by the load type and working condition, and the summer solstice and winter solstice load operating conditions are taken as the X value boundary; (2) when the absolute value of dP is greater than or equal to e1, X=a; (3) when the absolute value of dP is less than e1 and greater than or equal to e2, X=b; (4) when the absolute value of dP is less than e2, X=c, and c is a perturbation threshold; The e1, e2, a, b, c are related to the greenhouse load, and are selected by testing on the winter solstice day and the summer solstice day, the greenhouse load is controlled according to the time, the temperature in the greenhouse is controlled, and then the values of e1, e2, a, b and c are determined, and the test time starts from 9:00 and ends at 18:00.

Images