MPPT control method for centralized photovoltaic DC-DC converters

Abstract

The present invention provides an MPPT control method suitable for a centralized photovoltaic DC-DC converter, and belongs to the technical field of photovoltaic power generation system control. The control method is based on the CVT method and the P&O method, combined with the dual closed-loop control strategy of the centralized photovoltaic DC-DC converter, and accurately approximates the maximum power point by sampling three points with variable step length for comparison. The present invention enables the centralized DC-DC converter to effectively track the maximum power point of the photovoltaic array under the condition of changes in light intensity and temperature, and has the following advantages: (1) High tracking accuracy: Compared with the traditional P&O method, the present invention can avoid misjudgment of the disturbance direction, and can significantly improve the oscillation at the MPP by changing the disturbance step length; (2) Fast tracking rate: Although the sampling time for the present invention to complete a comparison is twice that of the P&O method, it can be significantly improved by combining with CVT and adding variable step length disturbance; (3) Simple control method: The present invention only requires simple logical judgment, and does not require high processor computing power.

Description

Technical Field

[0001] The invention relates to the technical field of photovoltaic power generation system control, and is an MPPT control method suitable for a centralized photovoltaic DC-DC converter. Background Art

[0002] Faced with the increasing depletion of fossil energy and environmental pollution, low-carbon, clean and renewable energy represented by solar energy has been vigorously developed. With the continuous innovation of power electronic devices, DC boost grid-connected technology has become increasingly mature, making it possible for large-scale photovoltaic power stations to adopt DC boost collection access systems. Photovoltaic cells and DC-DC converters are key devices for energy conversion and DC boost. It is of great research significance to ensure the efficient and stable operation of the system through the cooperation of the two.

[0003] The maximum power of photovoltaic cells changes with the changes in light intensity and temperature. In conventional photovoltaic AC grid-connected systems, photovoltaic modules mostly adopt a string structure, connecting several strings to multiple inverters, or connecting to one inverter through multiple power optimizers. Regardless of the method used, MPPT is achieved by relying on the Boost circuit in the front stage of the power optimizer or inverter. Since the Boost circuit has low power, simple structure, and easy control, most MPPT algorithms can be used directly. Compared with the Boost circuit, the high-power centralized DC-DC converter is more complex, and it is necessary to study the adaptability of the MPPT algorithm in combination with its own operating characteristics.

[0004] The transformation ratio and duty cycle of a centralized DC-DC converter are nonlinearly related. When the grid-connected current becomes smaller, the inductor current will be intermittent. At this time, the sensitivity of the converter transformation ratio to the duty cycle change will change. In many MPPT algorithms, directly setting the disturbance object as the duty cycle is a common method to speed up the tracking rate. However, for the Boost full-bridge isolated converter (BFBIC) converter based on the input parallel output series (IPOS) structure, the complexity of the circuit makes it difficult to determine the duty cycle disturbance step. Directly disturbing the duty cycle can easily cause the system to lose control due to misjudgment or oscillation. The adaptability of traditional MPPT algorithms in centralized DC-DC converters needs to be further verified. Summary of the invention

[0005] The present invention provides an MPPT control method suitable for a centralized photovoltaic DC-DC converter, which overcomes the shortcomings of the above-mentioned prior art and enables the centralized DC-DC converter to effectively track the maximum power point of the photovoltaic array, avoid misjudgment, and improve the operating efficiency of large-scale photovoltaic DC boost grid-connected.

[0006] The technical solution of the present invention is achieved by the following measures: A MPPT control method suitable for a centralized photovoltaic DC-DC converter comprises the following steps:

[0007] Step 1: At the initial start-up of the centralized photovoltaic DC boost transmission system, constant voltage control is used to set the input voltage reference value of the DC-DC converter double closed-loop control. V ref It is between 70% and 80% of the open circuit voltage of the photovoltaic array. Make the output voltage of the photovoltaic array close to the maximum power point voltage;

[0008] Step 2: When the output voltage of the photovoltaic array in step 1 reaches the voltage reference value given by the constant voltage controller, the voltage and current values ​​output by the photovoltaic array at this time are taken as the first set of data and set as V C , I C , and calculate the power P C ; By setting the perturbation step size Δ V , change the input voltage reference value of the dual closed-loop control V ref , which in turn becomes V C +Δ V and V C -Δ V , and sample the second and third groups of data of the photovoltaic array output voltage and current in sequence, set as V R , I R and V L , I L , and calculate the corresponding power P R and P L , and at the same time P 1=| P L - P C |, P 2=| P R - P C |, will P C , P R , P L , P 1 and P 2 as the basis for three-point comparison;

[0009] Step three, the sampling data obtained in step two is compared by three points to obtain the next disturbance direction, and the next disturbance step length is calculated by a variable step length calculation method.

[0010] The three comparison methods are as follows:

[0011] When satisfied P L ≤ P C < P R or P L > P C = P R When it is judged as a positive disturbance, the double closed-loop control is set V ref = V C +Δ V When satisfied P L ≤ P C = P R or P L = P C > P R or P L > P C < P R or P L < P C > P R or P L > P C > P R and P 1≥ P 2 is determined as a central disturbance and the double closed-loop control is set V ref = V C When satisfied P L > P C > P R and P 1< P2 is judged as reverse disturbance, and the double closed-loop control is set V ref = V C -Δ V .

[0012] The variable step length calculation method is as follows:

[0013] Set the perturbation step size Δ V is the step size factor Δ P With coefficient λ When the disturbance is positive, Δ P =| P R - P C |; When the reverse disturbance occurs, Δ P =| P L - P C |; When the center is disturbed, Δ P =|| P L - P C |-| P R - P C ||; λ is the maximum perturbation step length near the MPP under standard conditions γ Corresponding to Δ P The standard condition is irradiation intensity 1000W / m 2 , temperature 25℃; maximum perturbation step length near MPP γ It refers to the maximum voltage change that the system can withstand to ensure stable operation. V The value range is limited by the limiting link, that is, ɛ ≤Δ V < γ , ɛ It is the minimum voltage change that the DC-DC converter can recognize.

[0014] Step 4: V ref The dual closed-loop controller outputs to the centralized DC-DC converter, and the duty cycle of the output converter is controlled by the dual closed-loop D , and then control the opening and closing of IGBT in the centralized DC-DC converter through PWM drive signal. Among them, double closed-loop control is a control method to make the centralized DC-DC converter run stably, which consists of the input voltage (photovoltaic array output voltage) outer loop and the output current (grid-connected current) inner loop.

[0015] After completing a three-point comparison to determine the disturbance step size and disturbance direction, return to sampling and start a new round of discrimination, thereby gradually approaching the maximum power point of the photovoltaic array.

[0016] The MPPT control method for a centralized photovoltaic DC-DC converter described in the present invention can enable the centralized DC-DC converter to effectively track the maximum power point of the photovoltaic array under the condition of changes in light intensity and temperature, and has the following advantages: (1) High tracking accuracy: Compared with the traditional P&O method, the present invention can avoid misjudgment of the disturbance direction and significantly improve the oscillation at the MPP by changing the disturbance step size; (2) Fast tracking rate: Although the sampling time for completing a comparison of the present invention is twice that of the P&O method, it can be significantly improved by combining with CVT and adding variable step size disturbance; (3) Simple control method: The present invention only requires simple logical judgment and does not require high processor computing power. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Attached Figure 1 It is a flow chart of the MPPT control method applicable to the centralized photovoltaic DC-DC converter described in the present invention.

[0018] Attached Figure 2 It is a topological diagram of a centralized photovoltaic DC boost transmission system in an embodiment of the present invention.

[0019] Attached Figure 3 It is a control schematic diagram of a centralized DC-DC converter in an embodiment of the present invention.

[0020] Attached Figure 4 This is the new algorithm tracking process when the light intensity and temperature are constant in the embodiment of the present invention.

[0021] Attached Figure 5 This is the new algorithm tracking process when the light intensity or temperature changes in the embodiment of the present invention.

[0022] Attached Figure 6 This is a comparison of the tracking effects between the variable step-size three-point comparison MPPT control method and the P&O method in an embodiment of the present invention. DETAILED DESCRIPTION

[0023] The present invention is not limited by the following embodiments, and specific implementation methods can be determined based on the technical solution of the present invention and actual conditions.

[0024] In the present invention, unless otherwise specified, the algorithms described are the existing publicly known algorithms.

[0025] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments:

[0026] Example: As attached Figure 1As shown, the MPPT control method applicable to a centralized photovoltaic DC-DC converter comprises the following steps:

[0027] Step 1: At the initial start-up of the centralized photovoltaic DC boost transmission system, constant voltage control (CVT) is used to keep the photovoltaic array output voltage near the maximum power point voltage;

[0028] Step 2: When the output voltage of the photovoltaic array reaches the voltage reference value given by the constant voltage controller in step 1, the voltage and current values ​​output by the photovoltaic array at this time are taken as the center point data of the three-point comparison, and then the disturbance step size is changed twice continuously, and the voltage and current values ​​output by the photovoltaic array are sampled in turn as the right point and left point data of the three-point comparison;

[0029] Step 3: After processing the sampled data obtained in step 2 as required, a three-point comparison tracking method is used to determine the next disturbance direction, and a variable step length calculation method is used to obtain the next disturbance step length;

[0030] Step 4: Combine the center point voltage obtained in steps 2 and 3 with the disturbance step size, set it as the input voltage given value of the dual closed-loop control of the DC-DC converter, output the converter duty cycle through the dual closed-loop control, and then control the switching of the IGBT in the DC-DC converter through the PWM signal.

[0031] In step 1, the method of using the constant voltage method is as follows:

[0032] At the initial stage of system startup, the input voltage setpoint of the DC-DC converter double closed-loop control is set between 70% and 80% of the photovoltaic array open-circuit voltage.

[0033] The sampling processing method of the three groups of sampling data in step 2 is as follows:

[0034] The output voltage and current of the photovoltaic array after constant voltage control are taken as the first set of data, which is set as V C , I C , and calculate the power P C ; By setting the perturbation step size Δ V , changing the input voltage reference value of the dual closed-loop control, which becomes V C +Δ V and V C -Δ V , and sample the second and third groups of data of the photovoltaic array output voltage and current in sequence, set as V R , I R andV L , I L , and calculate the corresponding power P R and P L , and at the same time P 1=| P L - P C |, P 2=| P R - P C |, will P C , P R , P L , P 1 and P 2 as the basis for the three-point comparison.

[0035] The three-point comparison tracking method in step 3 is as follows:

[0036] When satisfied P L ≤ P C < P R or P L > P C = P R When it is judged as a positive disturbance, the double closed-loop control is set V ref = V C +Δ V When satisfied P L ≤ P C = P R or P L = P C > P R or P L > P C < P R or P L < P C >P R or P L > P C > P R and P 1≥ P 2 is determined as a central disturbance and the double closed-loop control is set V ref = V C When satisfied P L > P C > P R and P 1< P 2 is judged as reverse disturbance, and the double closed-loop control is set V ref = V C -Δ V .

[0037] The variable step size method in step three is as follows:

[0038] Set the perturbation step size Δ V is the step size factor Δ P With coefficient λ When the disturbance is positive, Δ P =| P R - P C |; When the reverse disturbance occurs, Δ P =| P L - P C |; When the center is disturbed, Δ P =|| P L - P C |-| P R - P C ||; λ is the maximum perturbation step length near the MPP under standard conditions γ Corresponding to Δ P ratio.

[0039] in λ The standard condition for taking the value is the irradiation intensity of 1000W / m 2 , temperature 25℃. Maximum perturbation step length γIt refers to the maximum voltage change that the system can withstand to ensure stable operation.

[0040] Perturbation step length Δ V The value range is limited by the limiting link, that is, ɛ ≤Δ V < γ , ɛ It is the minimum voltage change that the DC-DC converter can recognize.

[0041] The dual closed-loop control in step 4 refers to the control method of the DC-DC converter, which consists of an outer loop of input voltage (PV array output voltage) and an inner loop of output current (grid-connected current).

[0042] The specific control process is Figure 2 The centralized photovoltaic DC boost grid-connected topology shown in the figure is that the photovoltaic array is centrally boosted by the DC-DC converter and then directly connected to the medium-voltage DC grid through the bipolar DC transmission line. The relationship between MPPT control, dual closed-loop control and DC-DC converter is shown in the attached figure. Figure 3 As shown, the output voltage of the photovoltaic array is sampled V PV and current I PV , output voltage given value through MPPT controller V PV *, that is, the voltage reference value of the double closed-loop control V ref , through the input voltage outer loop and the grid-connected current inner loop controller, the output converter duty cycle D , and then control the opening and closing of each IGBT through the PWM drive signal. A ±10 kV / 360 kW centralized photovoltaic DC grid-connected system model was built using PSIM software to simulate and verify the present invention.

[0043] The coefficient k of CVT control in step 1 is set to 0.75, the simulation step size is set to 5e-6, the upper limit γ of the disturbance step size is set to 10 V, the lower limit ɛ is set to 1 V, and the lower limit is assigned only when the disturbance step size tends to 0, the disturbance coefficient λ is set to 0.003, the sampling frequency is set to 5 kHz, and the calculation is performed every 0.02 s. Different light intensity and temperature environments are set for the photovoltaic array, and the system response is simulated and analyzed.

[0044] Attached Figure 4 This is the tracking process after the system is started when the light intensity and temperature are constant as standard conditions. The system was under CVT control before 0.06 s. It can be seen that the disturbance step size is always 0 during CVT control, and the photovoltaic output voltage can be quickly tracked to 0.75 V OC, the rest of the electrical quantities are stabilized near the MPP following the photovoltaic output voltage. The three-point comparison principle is introduced after 0.06 s, and the initial disturbance step is the minimum value ɛ. After the first judgment, the disturbance starts from the maximum disturbance step γ. When approaching the MPP, the disturbance step begins to decrease and gradually approaches the MPP. It can be stabilized at the MPP within 0.58 s.

[0045] Attached Figure 5 It is the response of the system when the light intensity or temperature changes. The light intensity is set to decrease to 800 W / m2 at 0.4 s, and the temperature is set to increase to 40 ℃ at 0.72 s. It can be seen from the photovoltaic output voltage and current curves that when the light intensity suddenly changes, the current mutation degree is greater than the temperature mutation, while the voltage corresponding to the MPP is the opposite. After the mutation point, the new perturbation step size follows the result of the previous comparison, and no misjudgment occurs. When the temperature changes, the MPP voltage is far away from the mutation point, and the perturbation step size rapidly approaches from large to small. It can be seen from the photovoltaic output power curve that when the light intensity or temperature suddenly changes, it can quickly transition to the new MPP, whether in the tracking process or in stable operation. From the above analysis, it can be seen that the system response is consistent with the theory of the present invention, and the tracking results are accurate and effective.

[0046] Attached Figure 6 This is a comparative verification of the present invention and the conventional perturbation observation method (P&O). It can be seen that the perturbation step size of the P&O method is constant during the perturbation process. In order to avoid excessive ripple during stabilization, the perturbation step size is set to 5 V. In order to ensure the tracking speed, the perturbation is usually reversed from the open circuit voltage to the MPP. Both algorithms start the system under standard conditions. The present invention can stabilize at the MPP slightly faster than the P&O method through the CVT stage and variable step size perturbation. It can be seen from the local enlarged diagram from 0.54 s to 0.78 s that the P&O method has overshoot and obvious oscillation after stabilization, while the present invention has obvious improvement.

[0047] In summary, the MPPT control method for centralized photovoltaic DC-DC converters described in the present invention can enable centralized DC-DC converters to effectively track the maximum power point of photovoltaic arrays under the condition of light intensity and temperature changes, and has the following advantages: (1) High tracking accuracy: Compared with the traditional P&O method, the present invention can avoid misjudgment of the disturbance direction, and can significantly improve the oscillation at the MPP by changing the disturbance step size; (2) Fast tracking rate: Although the sampling time for completing a comparison of the present invention is twice that of the P&O method, it can be significantly improved by combining with CVT and adding variable step size disturbance; (3) Simple control method: The present invention only requires simple logical judgment and does not require high processor computing power. The above technical features constitute the embodiments of the present invention, which have strong adaptability and implementation effect, and can increase or decrease non-essential technical features according to actual needs to meet the needs of different situations.

Claims

1. A MPPT control method for a centralized photovoltaic DC-DC converter, characterized in that It includes the following steps: Step 1, at the initial stage of starting the centralized photovoltaic DC boost and transmission system, adopt constant voltage control to make the output voltage of the photovoltaic array near the maximum power point voltage; Step 2, when the output voltage of the photovoltaic array in Step 1 reaches the voltage reference value given by the constant voltage controller, take the voltage and current values output by the photovoltaic array at this time as the central point data for three-point comparison, and then continuously change the perturbation step size twice, and sample the voltage and current values output by the photovoltaic array in turn as the right point data and left point data for three-point comparison; Among them, the sampling processing method is as follows: First, take the voltage and current output by the photovoltaic array after constant voltage control as the first group of data, set as VC and IC, and calculate the power PC; by setting the perturbation step size ΔV, change the input voltage reference value of the double closed-loop control to VC + ΔV and VC - ΔV in turn, and sample the voltage and current output by the photovoltaic array in turn to obtain the second group and third group of data, set as VR, IR and VL, IL, and calculate the corresponding power PR and PL. At the same time, let P1 = |PL - PC|, P2 = |PR - PC|, and use PC, PR, PL, P1 and P2 as the basis for three-point comparison; Step 3, after processing the sampling data obtained in Step 2 as required, adopt the three-point comparison tracking method to determine the next perturbation direction, and obtain the next perturbation step size through the variable step size calculation method; Among them, the three-point comparison tracking method is as follows: When PL ≤ PC < PR or PL > PC = PR is satisfied, it is determined as a positive disturbance, and Vref of the double closed-loop control is set to VC + ΔV; when PL ≤ PC = PR or PL = PC > PR or PL > PC < PR or PL <pc>When PR or PL > PC > PR and P1 ≥ P2, it is determined as central perturbation, and set Vref of the double closed-loop control to VC; when PL > PC > PR and P1 < P2 are satisfied, it is determined as reverse perturbation, and set Vref of the double closed-loop control to VC - ΔV; < / pc> Among them, the variable step size method is as follows: Set the perturbation step size ΔV as the product of the step size factor ΔP and the coefficient λ. When it is a forward perturbation, ΔP = |PR - PC|; when it is a reverse perturbation, ΔP = |PL - PC|; when it is a central perturbation, ΔP = ||PL - PC| - |PR - PC||; λ is the ratio of the maximum perturbation step size γ near the MPP under standard conditions to the corresponding ΔP; Step 4, combine the central point voltage and the perturbation step size obtained in Steps 2 and 3, set it as the input voltage given value of the double closed-loop control of the DC-DC converter, output the duty ratio of the converter through the double closed-loop control, and then control the on and off of the IGBT in the DC-DC converter through the PWM signal.

2. The MPPT control method for a centralized photovoltaic DC-DC converter according to claim 1, characterized in that In the above Step 1, the usage method of the constant voltage method is as follows: At the initial stage of the system startup, set the input voltage given value of the double closed-loop control of the DC-DC converter to be between 70% and 80% of the open-circuit voltage of the photovoltaic array.

3. The MPPT control method for a centralized photovoltaic DC-DC converter according to claim 1, characterized in that: λ The standard condition for taking the value is the irradiation intensity of 1000W / m 2 , temperature 25℃.

4. The MPPT control method for a centralized photovoltaic DC-DC converter according to claim 1, characterized in that: Maximum perturbation step length near the MPP γ It refers to the maximum voltage change that the system can withstand to ensure stable operation.

5. The MPPT control method for a centralized photovoltaic DC-DC converter according to claim 1 or 2, characterized in that: Perturbation step length Δ V The value range is limited by the limiting link, that is, ɛ ≤Δ V < γ , ɛ It is the minimum voltage change that the DC-DC converter can recognize.

6. The MPPT control method for a centralized photovoltaic DC-DC converter according to claim 1, characterized in that The double closed-loop control in Step 4 refers to the control mode of the DC-DC converter, which is composed of an input voltage outer loop and an output current inner loop.

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