阅读说明：本技术 一种基于llc拓扑光伏发电系统的mppt控制方法 (MPPT control method based on LLC topology photovoltaic power generation system ) 是由 任景 周鑫 张小东 程松 褚云龙 马晓伟 董丹 王鹏 李俊 薛斌 李宁 于 2021-08-26 设计创作，主要内容包括：本发明公开了一种基于LLC拓扑光伏发电系统的MPPT控制方法,包括对光伏发电系统进行软起动,采集光伏板的输出电流I-(pv),判断I-(pv)是否大于阈值η,若大于,根据LLC电路的最小开关频率f-(min)和最大开关频率f-(max)确定f-(1)和f-(2),f-(1＝)f-(min),f-(2＝)f-(max),通过f-(1)和f-(2)计算出[f-(1),f-(2)]内的f-(3)、f-(4)和f-(5),计算出当前f-(3),f-(4),f-(5)对应的光伏板最大输出功率P-(max)和最小输出功率P-(min),判断P-(max)与P-(min)的差值是否小于误差值ξ,若小于,得出最终的最大功率P-(max)＝P(f-(3)),将f-(3)转化为脉冲信号,驱动光伏发电系统中开关管Q-(1)-Q-(4),即将光伏发电系统控制在最大功率点上。(The invention discloses an MPPT control method based on an LLC (logical Link control) topological photovoltaic power generation system, which comprises the steps of carrying out soft start on the photovoltaic power generation system and collecting the output current I of a photovoltaic panel pv Judgment of I pv Whether it is greater than threshold eta, if so, according to minimum switching frequency f of LLC circuit min And a maximum switching frequency f max Determination of f 1 And f 2 ，f 1＝ f min ，f 2＝ f max Through f 1 And f 2 Calculate [ f ] 1 ，f 2 ]Inner f 3 、f 4 And f 5 Calculating the current f 3 ，f 4 ，f 5 Corresponding maximum output power P of photovoltaic panel max And minimum output power P min Judgment of P max And P min If the difference is less than the error value xi, the final maximum power P is obtained max ＝P(f 3 ) A 1 is to f 3 Converted into pulse signals to drive a switching tube Q in the photovoltaic power generation system 1 ‑Q 4 Namely, the photovoltaic power generation system is controlled at the maximum power point.)

1. An MPPT control method based on an LLC topological photovoltaic power generation system is characterized by comprising the following steps:

step 1, carrying out soft start on a photovoltaic power generation system;

step 2, collecting the output current I of the photovoltaic panel after soft start_pvJudging whether the current value is greater than a threshold eta, if so, entering a step 3, otherwise, entering a step 1;

step 3, according to the minimum switching frequency f of the LLC circuit_minAnd a maximum switching frequency f_maxDetermining a first switching frequency f₁And a second switching frequency f₂，f₁＝f_min，f₂＝f_maxThrough f₁And f₂Calculate [ f ]₁，f₂]Inner third switching frequency f₃Fourth switching frequency f₄And a fifth switching frequency f₅Wherein

Wherein a and b are both constants, and a + b is 2;

step 4, calculating the current three switching frequencies f₃，f₄，f₅Corresponding maximum output power P of photovoltaic panel_maxAnd minimum output power P of photovoltaic panel_min；

Step 5, judging P_maxAnd P_minIf the difference value is smaller than the preset convergence error value xi, entering a step 7, otherwise entering a step 6;

step 6, according to P_maxUpdating f₁，f₂，f₃According to the updated frequency f₁、f₂、f₃And the update frequency f of equations (2) and (3)₄And f₅Then entering step 4;

step 7, obtaining the final maximum power P_max＝P_pv(f₃) Namely, tracking of the maximum power point of the photovoltaic power generation system based on LLC topology is completed, and the frequency f corresponding to the maximum power is output₃Will frequency f₃Converted into pulse signals to drive a switching tube Q in the photovoltaic power generation system₁、Q₂、Q₃And Q₄Namely, the photovoltaic power generation system is controlled at the maximum power point.

2. The MPPT control method based on the LLC topology photovoltaic power generation system according to claim 1, characterized in that said step 3 further comprises the following steps:

step 3.1, draw P_pv-f_sGraph P_pvFor photovoltaic panel output power, f_sIs switching frequency, s ═ {1,2, … …, n };

step 3.2, adding P_pv-f_sThe graph is divided into three parts, respectively And

step 3.3, judgeIf the sign of (b) is negative, the maximum power point position is atIf the value of a is more than 1.1 and the value of b is less than 0.9, entering the step 3.4;

step 3.4, judgeIf positive, the maximum power point position is atTaking the value of a to be less than 0.9 and the value of b to be more than 1.1; if the maximum power point is negative, the maximum power point is positionedTaking the values of a and b as [0.9, 1.1 ]]And (4) the following steps.

3. The MPPT control method based on the LLC topology photovoltaic power generation system according to claim 2, characterized in that in the step 3.1, according to the photovoltaic panel output power P_pvAnd the switching frequency f_sIs plotted against a functional relationship of P_pv-f_sGraph P_pvAnd f_sThe functional relationship of (a) is as follows:

the voltage gain expression of the LLC resonant circuit is

Wherein G represents a voltage gain, F_xRepresenting the normalized frequency, f_rRepresenting the resonant frequency, Q representing the quality factor,L_nrepresenting the ratio of the resonant inductance to the excitation inductance,lr is resonance inductance, Lm is excitation inductance of transformer, C_rRepresented by the resonant capacitance, R_acIs the equivalent resistance of the LLC circuit;

assuming no losses, according to the law of conservation of energy:

in the formula, V_pvRepresenting the output voltage of the photovoltaic panel, I_pvRepresenting the output current of the photovoltaic panel, R_LRepresenting the load resistance of the converter, V_oRepresenting the converter load R_LVoltage of_oRepresenting the flow through the converter load R_LThe current of (a);

the converter load absorbed power P is related to the voltage gain G by

Photovoltaic panel output power P_pvAnd the output voltage V of the photovoltaic panel_pvHas the relation of

I_scAnd V_ocRespectively the short-circuit current and the open-circuit voltage of the photovoltaic panel, wherein:

in the formula, C₁Represents a first coefficient, C₂Represents a second coefficient, V_mAnd I_mRespectively corresponding output voltage and output current when the photovoltaic panel outputs the maximum power;

deducing the output power P of the photovoltaic panel according to a formula (4) to a formula (10)_pvAnd the switching frequency f_sThe implicit function of (c) is as follows:

P_pv＝P(f_s) (11)。

4. the MPPT control method based on the LLC topology photovoltaic power generation system according to claim 3, characterized in that in the step 4, according to the photovoltaic panel output power P_pvCalculating the switching frequency f by an implicit function of the switching frequency fs₃，f₄，f₅Corresponding photovoltaic panel output power, wherein

P_max＝max{P_pv(f₃)，P_pv(f₄)，P_pv(f₅)} (12)

P_min＝min{P_pv(f₃)，P_pv(f₄)，P_pv(f₅)} (13)

In the formula, P_pv(f₃) For a third switching frequency f₃Corresponding photovoltaic panel output power, P_pv(f₄) Is the fourth switching frequency f₄Corresponding photovoltaic panel output power, P_pv(f₅) Is a fifth switching frequency f₅And outputting power by the corresponding photovoltaic panel.

5. The MPPT control method based on the LLC topology photovoltaic power generation system according to claim 4, characterized in that in the step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P_pv(f₃) Then f is₁＝f₄，f₂＝f₅，f₃＝f₃。

6. According to claimThe MPPT control method based on the LLC topology photovoltaic power generation system according to claim 4, characterized in that in the step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P_pv(f₄) Then f is₁＝f₁，f₂＝f₃，f₃＝f₄。

7. The MPPT control method based on the LLC topology photovoltaic power generation system according to claim 4, characterized in that in the step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P_pv(f₅) Then f is₁＝f₃，f₂＝f₂，f₃＝f₅。

Technical Field

The invention belongs to the technical field of new energy power generation, and relates to an MPPT control method based on an LLC (logical link control) topological photovoltaic power generation system.

Background

In recent years, along with huge energy consumption while the economy of China is rapidly developed, the increasing energy demand and supply are double challenges that China must face. Coal consumption accounts for nearly 70% of energy requirements in China and will continue to play a key role in promoting economic development in China. However, excessive dependence on fossil energy not only leads to gradual shortage of energy in China but also causes serious pollution to the environment in China. The effective utilization of renewable energy can well improve the energy supply structure of China and avoid the environmental deterioration. Among various renewable energy sources, solar energy is a completely renewable, pollution-free and easily available energy resource, and solar photovoltaic power generation is rapidly developed and has huge potential, so that the solar photovoltaic power generation technology is a hot spot technology for disputed research in various countries including China.

Photovoltaic cells (Photovoltaic cells) are one of the key elements of Photovoltaic power generation, which deeply affect the benefits of solar Photovoltaic power generation, and the characteristics and efficiency of the solar cells are the key problems in Photovoltaic Cell research. However, the output characteristic of the photovoltaic cell has nonlinearity, and the output characteristic thereof is influenced by the environment such as the intensity of light, the ambient temperature, the ambient humidity, the region, and the load. Under different working environments, the photovoltaic cell panel shows different output characteristic curves. On the same output characteristic curve, different output load conditions can output different power values. When the output voltage is equal to a corresponding value, the output Power reaches a Maximum value, and the Power output Point at the time is called a Maximum Power Point (MPP) of the photovoltaic cell. Therefore, it is necessary to ensure that the maximum power point is tracked at all times so that the photovoltaic panel is output at the maximum power value. Maximum Power Point Tracking (MPPT) is thus one of the key technologies to improve the performance of photovoltaic power generation systems.

The LLC resonant converter has the following advantages as a DCDC topology of a photovoltaic power generation system:

1) high switching frequency and high efficiency;

2) a wide load range, a high gain range and a wide input voltage range;

3) the leakage inductance of the transformer is used as a resonance element, so that the total number of parts is reduced;

4) the series resonant capacitor may provide an advantageous dc blocking for the transformer.

Most of the existing mainstream MPPT methods adopt a disturbance observation method (P & O), a conductance incremental method (INC) and some improvement methods (such as variable step length and the like), but the two methods cannot simultaneously consider disturbance speed and precision in an LLC resonant converter, and have some defects of energy loss and the like due to oscillation at a steady-state working point.

Disclosure of Invention

The invention aims to provide an MPPT control method based on an LLC (logical link control) topology photovoltaic power generation system, and solves the problem that the conventional MPPT method cannot give consideration to both disturbance speed and precision when the topology is an LLC resonant converter.

The invention adopts the technical scheme that an MPPT control method based on an LLC topological photovoltaic power generation system comprises the following steps:

step 1, carrying out soft start on a photovoltaic power generation system;

step 2, collecting the output current I of the photovoltaic panel after soft start_pvJudging whether the current value is greater than a threshold eta, if so, entering a step 3, otherwise, entering a step 1;

step 3, according to the minimum switching frequency f of the LLC circuit_minAnd a maximum switching frequency f_maxDetermining a first switching frequency f₁And a second switching frequency f₂，f₁＝f_min，f₂＝f_maxThrough f₁And f₂Calculate [ f ]₁，f₂]Inner third switching frequency f₃Fourth switching frequency f₄And a fifth switching frequency f₅Wherein

Wherein a and b are both constants, and a + b is 2;

step 4, calculating the current three switching frequencies f₃，f₄，f₅Corresponding maximum output power P of photovoltaic panel_maxAnd minimum output power P of photovoltaic panel_min；

Step 5, judging P_maxAnd P_minIf the difference value is smaller than the preset convergence error value xi, entering a step 7, otherwise entering a step 6;

step 6, according to P_maxUpdating f₁，f₂，f₃According to the updated frequency f₁、f₂、f₃And the update frequency f of equations (2) and (3)₄And f₅Then entering step 4;

step 7, obtaining the final maximumPower P_max＝P_pv(f₃) Namely, tracking of the maximum power point of the photovoltaic power generation system based on LLC topology is completed, and the frequency f corresponding to the maximum power is output₃Will frequency f₃Converted into pulse signals to drive a switching tube Q in the photovoltaic power generation system₁、Q₂、Q₃And Q₄Namely, the photovoltaic power generation system is controlled at the maximum power point.

Wherein, step 3 also includes the following steps:

step 3.1, draw P_pv-f_sGraph P_pvFor photovoltaic panel output power, f_sIs switching frequency, s ═ {1,2, … …, n };

step 3.2, adding P_pv-f_sThe graph is divided into three parts, which are respectively [ f₁， And

step 3.3, judgeIf the sign of (b) is negative, the maximum power point position is atIf the value of a is more than 1.1 and the value of b is less than 0.9, entering the step 3.4;

step 3.4, judgeIf positive, the maximum power point position is atTaking the value of a to be less than 0.9 and the value of b to be more than 1.1; if it is negativeThe maximum power point position is atTaking the values of a and b as [0.9, 1.1 ]]And (4) the following steps.

In step 3.1, according to the output power P of the photovoltaic panel_pvAnd the switching frequency f_sIs plotted against a functional relationship of P_pv-f_sGraph P_pvAnd f_sThe functional relationship of (a) is as follows:

the voltage gain expression of the LLC resonant circuit is

In the above formula, G represents a voltage gain, F_xRepresenting the normalized frequency, f_sRepresents a switching frequency, s is 1,2, 3, 4, 5, f_rRepresenting the resonant frequency, Q representing the quality factor,L_nrepresenting the ratio of the resonant inductance to the excitation inductance,lr is resonance inductance, Lm is excitation inductance of transformer, C_rRepresented by the resonant capacitance, R_acIs the equivalent resistance of the LLC circuit.

Assuming no losses, according to the law of conservation of energy: ,

in the formula, V_pvRepresenting the output voltage of the photovoltaic panel, I_pvRepresenting the output current of the photovoltaic panel, R_LRepresenting the load resistance of the converter, V_oRepresenting the converter load R_LVoltage of_oRepresenting the flow through the converter load R_LThe current of (a);

the converter load absorbed power P is related to the voltage gain G by

Photovoltaic panel output power P_pvAnd the output voltage V of the photovoltaic panel_pvHas the relation of

I_scAnd V_ocRespectively the short-circuit current and the open-circuit voltage of the photovoltaic panel, wherein:

in the formula, C₁Represents a first coefficient, C₂Represents a second coefficient, V_mAnd I_mThe output voltage and the output current are respectively corresponding to the maximum output power of the photovoltaic panel.

Deducing the output power P of the photovoltaic panel according to a formula (4) to a formula (10)_pvAnd the switching frequency f_sThe implicit function of (c) is as follows:

P_pv＝P(f_s) (11)

in step 4, according to the output power P of the photovoltaic panel_pvCalculating the switching frequency f by an implicit function of the switching frequency fs₃，f₄，f₅Corresponding photovoltaic panel output power, wherein

P_max＝max{P_pv(f₃)，P_pv(f₄)，P_pv(f₅)} (12)

P_min＝min{P_pv(f₃)，P_pv(f₄)，P_pv(f₅)} (13)

In the formula, P_pv(f₃) For a third switching frequency f₃Corresponding photovoltaic panel output power, P_pv(f₄) Is the fourth switching frequency f₄Corresponding photovoltaic panel output power, P_pv(f₅) Is a fifth switching frequency f₅And outputting power by the corresponding photovoltaic panel.

In step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P_pv(f₃) Then f is₁＝f₄，f₂＝f₅，f₃＝f₃。

In step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P_pv(f₄) Then f is₁＝f₁，f₂＝f₃，f₃＝f₄。

In step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P_pv(f₅) Then f is₁＝f₃，f₂＝f₂，f₃＝f₅。

The invention has the advantages that the defects of the traditional P & O and conductance increment method in the photovoltaic power generation MPPT technology based on the LLC circuit are solved, the dynamic effect and the steady-state effect in the traditional method cannot be considered at the same time, for example, if the dynamic effect is rapid, the steady-state effect can vibrate, the circuit efficiency is lost, if the steady-state vibration is small, the dynamic effect is slow, the time reaching the maximum power point is prolonged, compared with the traditional median frequency conversion MPPT method, the dynamic tracking effect can be improved on curves of different photovoltaic panel powers and switching frequencies through the design of constants a and b, and the steady-state vibration is gradually reduced when the dynamic effect is rapid, so that the loss is reduced, and the precision is improved.

Drawings

FIG. 1 is a topological diagram of a solar photovoltaic power generation system in an embodiment of the present invention;

FIG. 2 is a schematic flow chart of an MPPT control method based on an LLC topology photovoltaic power generation system;

FIG. 3 is a graph of absorbed power of a load with different switching frequencies for a fixed output load according to an embodiment of the present invention;

FIG. 4 shows an embodiment of the present invention P_pv-f_sGraph is shown.

Detailed Description

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

The invention relates to an MPPT control method based on an LLC (logical link control) topological photovoltaic power generation system, wherein a topological diagram is shown in figure 1, Lr is resonant inductance, Lm is excitation inductance of a transformer, Cr is resonant capacitance, and Q is₁、Q₂、Q₃And Q₄Are all switch tubes.

The invention relates to an MPPT control method based on an LLC (logical link control) topological photovoltaic power generation system, which comprises the following steps of:

step 1, detecting a photovoltaic voltage U through a voltage sensor and a current sensor_pvAnd current I_pvAnd calculating the power P_pv＝U_pv·I_pvCarrying out soft start on the photovoltaic power generation system;

step 2, collecting the output current I of the photovoltaic panel after soft start_pvJudgment of I_pvJudging whether the current value is greater than a threshold eta, wherein the threshold eta is smaller than a threshold eta, and if the current value is greater than the threshold eta, the step 3 is performed, otherwise, the step 1 is performed;

step 3, according to the minimum switching frequency f of the LLC circuit_minAnd a maximum switching frequency f_maxDetermining a first switching frequency f₁And a second switching frequency f₂，f_min＝0.5f_r，f_max＝2f_r，f_rIs the resonant frequency, f₁＝f_min，f₂＝f_maxThrough f₁And f₂Calculate [ f ]₁，f₂]Inner third switching frequency f₃Fourth switching frequency f₄And a fifth switching frequency f₅Wherein

Wherein a and b are constants, and a + b is 2;

step 3 also comprises the following steps:

step 3.1, according to the output power P of the photovoltaic panel_pvAnd the switching frequency f_sIs plotted against a functional relationship of P_pv-f_sGraph, s ═ {1,2, … …, n }, P_pvAnd f_sThe functional relationship of (a) is obtained by the following method:

the voltage gain expression of the LLC resonant circuit is

In the above formula, G represents a voltage gain, F_xRepresenting the normalized frequency, f_sRepresents a switching frequency, s is 1,2, 3, 4, 5, f_rRepresenting the resonant frequency, Q representing the quality factor,L_nrepresenting the ratio of the resonant inductance to the excitation inductance,lr is resonance inductance, Lm is excitation inductance of transformer, C_rRepresented by the resonant capacitance, R_acIs the equivalent resistance of the LLC circuit.

Assuming no losses, according to the law of conservation of energy:

in the formula, V_pvRepresenting the output voltage of the photovoltaic panel, I_pvRepresenting the output current of the photovoltaic panel, R_LRepresenting the load resistance of the converter, V_oRepresenting the converter load R_LVoltage of_oRepresenting the flow through the converter load R_LThe current of (a);

the converter load absorbed power P is related to the voltage gain G by

Photovoltaic panel output power P_pvAnd the output voltage V of the photovoltaic panel_pvHas the relation of

I_scAnd V_ocRespectively the short-circuit current and the open-circuit voltage of the photovoltaic panel, wherein:

in the formula, C₁Represents a first coefficient, C₂Represents a second coefficient, V_mAnd I_mThe output voltage and the output current are respectively corresponding to the maximum output power of the photovoltaic panel.

Deducing the output power P of the photovoltaic panel according to a formula (4) to a formula (10)_pvAnd the switching frequency f_sImplicit functions of, i.e. P_pvAnd f_sIs as follows:

P_pv＝P(f_s) (11)

from the equations (7) and (8), it can be derived that the switching frequency f is changed with the output load fixed_sTime, photovoltaic output power P_pvCurve of absorbed power P of converter load, resonant frequency f_r130kHz, as shown in FIG. 3, it can be seen from FIG. 3 that the switching frequency f is different_sCorresponding photovoltaic cell output power P_pvDifferent, the maximum power point can be found by changing the switching frequency;

p plotted in this example_pv-f_sThe graph is shown in FIG. 4, in which the abscissa is the switching frequency f_sThe ordinate is the output power P of the photovoltaic panel_pv。

Step 3.2, adding P_pv-f_sThe graph is divided into three parts, respectively And

step 3.3, judgeIf the sign of (b) is negative, the maximum power point position is atIf the value of a is more than 1.1 and the value of b is less than 0.9, entering the step 3.4;

step 3.4, judgeIf positive, the maximum power point position is atTaking the value of a to be less than 0.9 and the value of b to be more than 1.1; if the maximum power point is negative, the maximum power point is positionedTaking the values of a and b as [0.9, 1.1 ]]And (4) the following steps.

Step 4, according to the output power P of the photovoltaic panel_pvAnd the switching frequency f_sThe implicit function of (2) calculates the current three switching frequencies f₃，f₄，f₅Finding out the maximum output power P of the photovoltaic panel corresponding to the output power of the photovoltaic panel_maxAnd minimum output power P of photovoltaic panel_minWherein

P_max＝max{P_pv(f₃)，P_pv(f₄)，P_pv(f₅)} (12)

P_min＝min{P_pv(f₃)，P_pv(f₄)，P_pv(f₅)} (13)

In the formula, P_pv(f₃) For a third switching frequency f₃Corresponding photovoltaic panel output power, P_pv(f₄) Is the fourth switching frequency f₄Corresponding photovoltaic panel output power, P_pv(f₅) Is a fifth switching frequency f₅And outputting power by the corresponding photovoltaic panel.

Step 5, judging P_maxAnd P_minIf yes, go to step 7, otherwise go to step 6;

step 6, according to P_maxUpdating f₁，f₂，f₃If P is a value of_max＝P(f₃) Then f is₁＝f₄，f₂＝f₅，f₃＝f₃If P is_max＝P(f₄) Then f is₁＝f₁，f₂＝f₃，f₃＝f₄If P is_max＝P(f₅) Then f is₁＝f₃，f₂＝f₂，f₃＝f₅Then according to the updated frequency f₁、f₂、f₃And the update frequency f of equations (2) and (3)₄And f₅Finally, entering step 4;

step 7, obtaining the final maximum power P_max＝P_pv(f₃) Namely, tracking of the maximum power point of the photovoltaic power generation system based on LLC topology is completed, and the frequency f corresponding to the maximum power is output₃Will frequency f₃Converted into pulse signals to drive a switching tube Q in the photovoltaic power generation system₁、Q₂、Q₃And Q₄Namely, the photovoltaic power generation system is controlled at the maximum power point.

The switching frequency f corresponding to the maximum output power point of the photovoltaic panel can be reversely deduced by comparing the output power of the photovoltaic panel corresponding to each switching frequency_mppComparison of P (f)₃)，P(f₄)，P(f₅) Power value if P (f)₄)≤P(f₃)、P(f₃)≥P(f₅) Interval of switching frequency [ f ]₄，f₅]As the next quarter interval; if P (f)₄)≤P(f₃)、P(f₄)≥P(f₅) Interval of switching frequency [ f ]₁，f₃]As the next quarter interval; if P (f)₃)≤P(f₅)、P(f₅)≥P(f₄) Interval of switching frequency [ f ]₃，f₂]As the next quarter partition.

From the above analysis, the output power of the photovoltaic panel corresponding to the three median switching frequencies is compared, and once P (f) is determined₃)、P(f₄)、P(f₅) And determining the equal partition interval again to carry out maximum power point search, and gradually reducing the switching frequency search step until the preset convergence error is met.