CN103019294A - Maximum power point tracking (MPPT) method of self-adaption disturbance frequency and step - Google Patents

Abstract

The invention discloses a maximum power point tracking (MPPT) method of self-adaption disturbance frequency and step, and particularly relates to a maximum power point tracking method of a solar photovoltaic cell and belongs to the technical field of new energy. The MPPT method comprises the following steps of: after each disturbance (namely changing an output voltage reference value of the photovoltaic cell), calculating the difference of the power before and after disturbance when the output voltage of the photovoltaic cell is in a steady state, and ensuring the judging accuracy of power changing direction; accumulating the power difference to accurately obtain the changing direction of the power after each disturbance; and regulating the disturbance step in real time according to the time of the reference values accumulated by the power difference, and regulating the reference values of the power accumulated values in real time according to the output power of the photovoltaic cell. Therefore, the maximum power point can be rapidly and accurately tracked, the disturbance frequency and the disturbance step near the maximum power point are minimum, the power loss caused by disturbance can be effectively avoided, and the maximization of the output power of the photovoltaic cell is realized.

Description

The maximum power point tracing method of a kind of self-adaptation forcing frequency and step-length

Technical field

The invention belongs to technical field of new energies, what relate to is a kind of maximum power point tracing method of solar-energy photo-voltaic cell, be particularly related to the maximum power point tracing method of a kind of self-adaptation forcing frequency and step-length, i.e. MPPT (Maximum Power Point Tracking) method.

Background technology

Worldwide energy crisis and environmental problem, various countries pay close attention to and the focus of research so that solar photovoltaic technology becomes.

Solar-energy photo-voltaic cell is core and the critical component of photovoltaic generating system, the solar-energy photo-voltaic cell output voltage u under particular light and temperature conditions _PVWith output power p _PVThe typical relation curve as shown in Figure 1.1 as can be known with reference to the accompanying drawings, under the identical external environment condition, the output power from photovoltaic cells and output voltage have corresponding relation, in order at utmost to bring into play the effectiveness of photovoltaic cell, at utmost improve the generated energy of photovoltaic cell, increase the production capacity of photovoltaic cell, need to adjust the photovoltaic cell output end voltage, make the output power of photovoltaic cell constantly remain on maximum power point (the M point in the accompanying drawing 1), namely need to adopt MPPT maximum power point tracking (Maximum Power Point Tracking, MPPT) technology.

Several MPPT methods commonly used mainly contain at present: constant voltage process, conductance increment method, disturbance observation etc.The disturbance observation is the output voltage that whenever changes at regular intervals photovoltaic array, 2 output power value was to change the direction of regulation voltage before and after Real Time Observation relatively changed, finally be stabilized near the maximum power point, because disturbance observation algorithm is simple, realization is convenient and widespread use.When the disturbance observation realizes MPPT, choosing of disturbance step-length and forcing frequency is very crucial, conventional disturbance observation adopts fixing disturbance step-length and forcing frequency, if step-length is too small, then making photovoltaic cell be trapped in for a long time low-power output goes, if step-length is excessive, can make again the system oscillation aggravation, the power loss in the time of particularly stable state can being increased.This is that step-length is larger, departs from real maximum power point during disturbance far away because the output voltage of photovoltaic cell will near always disturbance maximum power point, and the power loss that causes also can be larger.In order to address this problem, the research worker improves precision and the rapidity that MPPT follows the tracks of from the angle that changes the disturbance step-length usually, but seldom proposes this solution of problem countermeasure from the angle that changes forcing frequency.

Notice, in the MPPT method based on the disturbance observation, the most critical issue is the Obtaining Accurate that applies power change direction after the disturbance, and applying of next time disturbance need to be carried out after this disturbance reaches stable state more simultaneously, otherwise it is disconnected to produce the erroneous judgement of perturbation direction.For the output voltage that guarantees photovoltaic cell applies next time disturbance after having reached stable state again, common way is to choose the relatively long MPPT cycle, guarantees that output voltage has reached stable state when a MPPT end cycle.Yet corresponding different disturbance step-length, output voltage reach stable time and different, and therefore, fixedly the mode in MPPT cycle can't make and become disturbance step-length MPPT tracking effect and reach optimum.

Summary of the invention

The present invention is directed to the existing existing deficiency of MPPT method, the maximum power point tracing method of a kind of self-adaptation forcing frequency and step-length is proposed, can overcome the shortcoming of conventional disturbance observation, variable step disturbance observation, make photovoltaic cell quick, stable trace into maximum power point.

In order to achieve the above object and effect, the maximum power point tracing method of self-adaptation forcing frequency provided by the present invention and step-length is taked following steps:

(1) the output voltage u of detection solar cell _PV, current i _PV

(2) judge input voltage count value N _UKWhether greater than input voltage count value benchmark N _{UK_R}

If N _UK≤ N _{UK_R}, then change step (3) over to;

If N _UK＞N _{UK_R}, then change step (4) over to;

(3) judge solar cell output voltage u _PVWith the Error Absolute Value of the photovoltaic cell output voltage reference value u (k) in current MPPT cycle whether less than the voltage error reference value Δ u that sets _R

If | u _PV-u (k) |＜Δ u _R, N then _UKIncrease 1, finish;

If | u _PV-u (k) | 〉=Δ u _R, N then _UKBe set to 0, finish;

(4) according to the output voltage u of photovoltaic cell _PV, current i _PVCalculate the output power p of photovoltaic cell _PV:

p _PV＝u _PVi _PV

Make simultaneously power calculation count value N _PIncrease 1, then calculate the present output power p of photovoltaic cell _PVWith the difference of a upper MPPT cycle photovoltaic cell average output power P (k-1), and this power difference added power difference accumulated value Δ p:

Δp＝Δp+p _PV-P(k-1)

(5) judge the absolute value of power difference accumulated value | whether Δ p| is greater than power difference reference value Δ P _R:

If | Δ p|＞Δ P _R, then change step (6) over to;

If | Δ p|≤Δ P _R, then withdraw from;

(6) symbol of judgement power difference accumulated value Δ p:

If Δ p＞0, then continued the relation of the photovoltaic cell voltage reference value u (k-1) in the current photovoltaic cell output voltage reference value u of judgement (k) and a upper MPPT cycle: if u (k)＞u (k-1), then change step (7) over to, otherwise change step (8) over to;

If Δ p≤0, then continued the relation of the photovoltaic cell voltage reference value u (k-1) in the current photovoltaic cell output voltage reference value u of judgement (k) and a upper MPPT cycle: if u (k)＞u (k-1), then change step (8) over to, otherwise change step (7) over to;

(7) increase photovoltaic cell output voltage reference value, namely so that interior photovoltaic cell output voltage reference value u (k+1) of next MPPT cycle=u (k)+Δ u, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then changes step (9) over to;

(8) reduce photovoltaic cell output voltage reference value, namely so that interior photovoltaic cell output voltage reference value u (k+1) of next MPPT cycle=u (k)-Δ u, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then changes step (9) over to;

(9) change power difference reference value Δ P according to following formula _R, again with input voltage count value N _UKBe set to 0, power difference accumulated value Δ p is set to 0, power calculation count value N _PBe set to 0:

${\mathrm{\ΔP}}_R=k_P\frac{N_P}{N_{\mathrm{PR}}}P\left(k\right)$

Wherein, N _PRBe the reference value of N, k _PFor power difference reference value is calculated scale-up factor and k _P＞0, current MPPT end cycle withdraws from.

In above-mentioned steps (7) and step (8), photovoltaic cell output voltage disturbance step delta u calculates according to following formula:

$\mathrm{\Δu}=\frac{k_u}{N_P}\·{\mathrm{\ΔU}}_R$

Wherein: k _uBe photovoltaic cell output voltage disturbance step size computation scale-up factor and k _u＞0, Δ U _RBe output voltage disturbance step-length reference value.

Compared with prior art the present invention has the following advantages:

(1) maximum power point tracing method of the present invention can be adjusted forcing frequency and disturbance step-length automatically, so that the output power from photovoltaic cells point departs from maximum power point when far away, forcing frequency and disturbance step-length are larger, guarantees the rapidity of tracking velocity;

(2) maximum power point tracing method of the present invention can be adjusted forcing frequency and disturbance step-length automatically, so that the output power from photovoltaic cells point is when reaching maximum power point, forcing frequency and disturbance step-length are minimum, guarantee near the stability of tracking when maximum power point is, reduce because the power loss that disturbance causes;

(3) maximum power point tracing method shown in the present is being judged the judgement that begins again the change direction of power after input terminal voltage reaches stable state, can guarantee validity and the stability of disturbance, avoids undisturbed generation;

(4) change suddenly when causing the output power from photovoltaic cells to be undergone mutation when external environment condition, forcing frequency and step-length can be adjusted automatically, realize following the tracks of fast.

Description of drawings

Further specify the present invention below in conjunction with the drawings and specific embodiments.

Fig. 1 is solar-energy photo-voltaic cell output voltage and characteristics of output power curve;

Fig. 2 is the process flow diagram of the maximum power point tracing method of self-adaptation forcing frequency of the present invention and step-length;

Fig. 3 is that embodiment one corresponding photovoltaic generating system consists of schematic diagram;

Symbol description among the figure: u _PV, i _PV, p _PVBe respectively photovoltaic cell output voltage, output current and output power; In the accompanying drawing 3, C ₁, C ₂Be electric capacity, S ₁～S ₄Be switching tube, L is inductance.

Embodiment

For technological means, creation characteristic that the present invention is realized, reach purpose and effect is easy to understand, below in conjunction with concrete diagram, further set forth the present invention.

In order to narrate conveniently, below in conjunction with accompanying drawing 1 and accompanying drawing 2 explanations technical scheme of the present invention.

The photovoltaic cell output voltage u shown in 1 with reference to the accompanying drawings _PVWith output power p _PVCurve as can be known, the output power from photovoltaic cells from maximum power point (M point in the accompanying drawing 1) more away from, output power is less, simultaneously slope of a curve is larger, and near maximum power point, the output power from photovoltaic cells maximum but the efficient of curve is minimum.Therefore in order to make the output power from photovoltaic cells arrive fast maximum power point, photovoltaic cell stable output power when making maximum power point simultaneously, forcing frequency and disturbance step-length are reduced with the increase of the output power from photovoltaic cells, and it is the fundamental starting point of the maximum power point tracing method of self-adaptation forcing frequency of the present invention and step-length.

Based on above-mentioned principle, the maximum power point tracing method process flow diagram of self-adaptation forcing frequency of the present invention and step-length is taked following steps as shown in Figure 2 during implementation:

(1) the output voltage u of detection solar cell _PV, current i _PV

(2) judge input voltage count value N _UKWhether greater than input voltage count value benchmark N _{UK_R}

If N _UK≤ N _{UK_R}, then change step (3) over to;

If N _UK＞N _{UK_R}, then change step (4) over to;

(3) judge solar cell output voltage u _PVWith the Error Absolute Value of the photovoltaic cell output voltage reference value u (k) in current MPPT cycle whether less than the voltage error reference value Δ u that sets _R

If | u _PV-u (k) |＜Δ u _R, N then _UKIncrease 1, finish;

If | u _PV-u (k) | 〉=Δ u _R, N then _UKBe set to 0, finish;

(4) according to photovoltaic cell voltage u _PV, current i _PVCalculate photovoltaic cell power p _PV:

p _PV＝u _PVi _PV

Make simultaneously power calculation count value N _PIncrease 1, then calculate the present output power p of photovoltaic cell _PVWith the difference of a upper MPPT cycle photovoltaic cell average output power P (k-1), and this power difference added power difference accumulated value Δ p:

Δp＝Δp+p _PV-P(k-1)

(5) judge the absolute value of power difference accumulated value | whether Δ p| is greater than power difference reference value Δ P _R:

If | Δ p|＞Δ P _R, then change step (6) over to;

If | Δ p|≤Δ P _R, then withdraw from;

(6) symbol of judgement power difference accumulated value Δ p:

If Δ p＞0, then continued the relation of the photovoltaic cell voltage reference value u (k-1) in the current photovoltaic cell output voltage reference value u of judgement (k) and a upper MPPT cycle: if u (k)＞u (k-1), then change step (7) over to, otherwise change step (8) over to;

If Δ p≤0, then continued the relation of the photovoltaic cell voltage reference value u (k-1) in the current photovoltaic cell output voltage reference value u of judgement (k) and a upper MPPT cycle: if u (k)＞u (k-1), then change step (8) over to, otherwise change step (7) over to;

(7) increase photovoltaic cell output voltage reference value, namely so that interior photovoltaic cell output voltage reference value u (k+1) of next MPPT cycle=u (k)+Δ u, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then changes step (9) over to;

(8) reduce photovoltaic cell output voltage reference value, namely so that interior photovoltaic cell output voltage reference value u (k+1) of next MPPT cycle=u (k)-Δ u, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then changes step (9) over to;

(9) change power difference reference value Δ P according to following formula _R, again with input voltage count value N _UKBe set to 0, power difference accumulated value Δ p is set to 0, and power calculation count value NP is set to 0, as the starting point in next cycle:

${\mathrm{\ΔP}}_R=k_P\frac{N_P}{N_{\mathrm{PR}}}P\left(k\right)$

Wherein, N _PRBe the reference value of N, k _PFor power difference reference value is calculated scale-up factor and k _P＞0, current MPPT end cycle withdraws from;

In above-mentioned steps (7) and step (8), photovoltaic cell output voltage disturbance step delta u calculates according to following formula:

$\mathrm{\Δu}=\frac{k_u}{N_P}\·{\mathrm{\ΔU}}_R$

Wherein: k _uBe photovoltaic cell output voltage disturbance step size computation scale-up factor and k _u＞0, Δ U _RBe output voltage disturbance step-length reference value.

According to above-mentioned implementation step as can be known:

(1) to the judgement of the output power from photovoltaic cells after the photovoltaic cell output end voltage reaches stable state, after namely each MPPT cycle applies disturbance, only have when the photovoltaic cell output end voltage stable (with the deviation of given voltage reference less than the error amount Δ u that sets _R) after just begin to judge the change direction of power, after this can guarantee to apply disturbance at every turn, the output end voltage of photovoltaic cell has all reached setting value, avoids the too fast and photovoltaic cell voltage follow of disturbance velocity not go up mistake disturbance or the disconnected problem of erroneous judgement that disturbance velocity is brought.

(2) directly the output power from photovoltaic cells after the disturbance and the difference power in a upper MPPT cycle (before applying disturbance) were added up, can obtain accurately the power change direction, owing to having set the cumulative reference value Δ P of difference power _R, for same reference value Δ P _R, when disturbance later power changed greatly, it is shorter that difference power is added to the time of reference value, shows that the MPPT forcing frequency is higher, and therefore, when power was larger with the slope of change in voltage, the corresponding MPPT cycle was shorter, forcing frequency is higher.The output power from photovoltaic cells shown in 1 and voltage curve relation are as can be known with reference to the accompanying drawings, the output power from photovoltaic cells point from maximum power point more away from, corresponding power is larger with the slope of change in voltage, so maximum power point tracing method of the present invention can be accelerated the speed that MPPT follows the tracks of by automatically adjusting forcing frequency.

(3) the cumulative reference value Δ P of power _RBe proportional to the output power from photovoltaic cells, the output power from photovoltaic cells is larger, the cumulative reference value of power is larger, the output power from photovoltaic cells is less, the cumulative reference value of power is less, show the output power from photovoltaic cells point from maximum power point more close to, the cumulative reference value of power is larger, corresponding MPPT forcing frequency is lower, the output power from photovoltaic cells point from maximum power point more away from, the cumulative reference value of power is less, and corresponding MPPT forcing frequency is higher, can further improve the speed of tracking and the degree of stability that the maximum power point place follows the tracks of.

(4) photovoltaic cell output voltage disturbance step delta u is proportional to the MPPT disturbance cycle, namely be inversely proportional to the MPPT forcing frequency, show the output power from photovoltaic cells point from maximum power point more away from, u is larger for the disturbance step delta, maximum power point is nearer in the output power from photovoltaic cells point, u is less for the disturbance step delta, also can further improve the speed of tracking and the degree of stability that the maximum power point place follows the tracks of.

(5) be in maximum power point when the output power from photovoltaic cells point, if change suddenly causes output power to be undergone mutation to external environment condition this moment (illumination, block etc.), then corresponding power difference in change Δ p also can follow and become large, and the cumulative reference value Δ P of power this moment _RNot yet change, therefore difference power time of being added to reference value also can shorten automatically, corresponding MPPT forcing frequency improves automatically, therefore maximum power point tracing method of the present invention can be adjusted MPPT forcing frequency and step-length automatically when illumination condition changes suddenly, improves the rapidity of following the tracks of this moment.

In one embodiment of the invention, maximum power point tracing method of the present invention (MPPT) is based on Freescale MC56F8245 DSP, adopt the C Programming with Pascal Language to realize, be used to the control of DC-DC transducer (four switch buck-boost converters), be used for realizing the MPPT maximum power point tracking of photovoltaic cell.

The photovoltaic generating system of this embodiment consists of schematic diagram as shown in Figure 3.Whole system comprises that solar-energy photo-voltaic cell, electric current and voltage adopt module, driving circuit, DSP, DC-DC transducer and load.Wherein solar-energy photo-voltaic cell connects load by the DC-DC transducer, and DSP adopts module to connect solar-energy photo-voltaic cell by electric current and voltage, and by driving circuit control DC-DC transducer.The DC-DC transducer is by capacitor C ₁, C, switching tube S ₁～S ₄And inductance L connects to form.

This system is by sampling photovoltaic cell output voltage and output current, control DC-DC converter switches pipe turns on and off after the DSP computing, realize the maximal power tracing of photovoltaic cell, the output of DC-DC transducer can load can be accumulator, combining inverter or other load.In the present embodiment, the input voltage range of DC-DC transducer is 7～60V, and output voltage range is 0～60V, rated power 300W, switching tube switching frequency 100kHz in the DC-DC transducer.

Above demonstration and described ultimate principle of the present invention, principal character and advantage of the present invention.The technician of the industry should understand; the present invention is not restricted to the described embodiments; that describes in above-described embodiment and the instructions just illustrates principle of the present invention; without departing from the spirit and scope of the present invention; the present invention also has various changes and modifications, and these changes and improvements all fall in the claimed scope of the invention.The claimed scope of the present invention is defined by appending claims and equivalent thereof.

Claims (2)

1. the maximum power point tracing method of a self-adaptation forcing frequency and step-length is characterized in that, said method comprising the steps of:

(1) the output voltage u of detection solar cell _PV, current i _PV

(2) judge input voltage count value N _UKWhether greater than input voltage count value benchmark N _{UK_R}

If N _UK≤ N _{UK_R}, then change step (3) over to;

If N _UK＞N _{UK_R}, then change step (4) over to;

(3) judge solar cell output voltage u _PVWith the Error Absolute Value of the photovoltaic cell output voltage reference value u (k) in current MPPT cycle whether less than the voltage error reference value Δ u that sets _R

If | u _PV-u (k) |＜Δ u _R, N then _UKIncrease 1, finish;

If | u _PV-u (k) | 〉=Δ u _R, N then _UKBe set to 0, finish;

(4) according to the output voltage u of photovoltaic cell _PV, current i _PVCalculate the output power p of photovoltaic cell _PV:

p _PV＝u _PVi _PV

Make simultaneously power calculation count value N _PIncrease 1, then calculate the present output power p of photovoltaic cell _PVWith the difference of a upper MPPT cycle photovoltaic cell average output power P (k-1), and this power difference added power difference accumulated value Δ p:

Δp＝Δp+p _PV-P(k-1)

(5) judge the absolute value of power difference accumulated value | whether Δ p| is greater than power difference reference value Δ P _R:

If | Δ p|＞Δ P _R, then change step (6) over to;

If | Δ p|≤Δ P _R, then withdraw from;

(6) symbol of judgement power difference accumulated value Δ p:

If Δ p＞0, then continued the relation of the photovoltaic cell voltage reference value u (k-1) in the current photovoltaic cell output voltage reference value u of judgement (k) and a upper MPPT cycle: if u (k)＞u (k-1), then change step (7) over to, otherwise change step (8) over to;

If Δ p≤0, then continued the relation of the photovoltaic cell voltage reference value u (k-1) in the current photovoltaic cell output voltage reference value u of judgement (k) and a upper MPPT cycle: if u (k)＞u (k-1), then change step (8) over to, otherwise change step (7) over to;

(7) increase photovoltaic cell output voltage reference value, namely so that interior photovoltaic cell output voltage reference value u (k+1) of next MPPT cycle=u (k)+Δ u, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then changes step (9) over to;

(8) reduce photovoltaic cell output voltage reference value, namely so that interior photovoltaic cell output voltage reference value u (k+1) of next MPPT cycle=u (k)-Δ u, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then changes step (9) over to;

(9) change power difference reference value Δ P according to following formula _R, and then with input voltage count value N _UKBe set to 0, power difference accumulated value Δ p is set to 0, power calculation count value N _PBe set to 0:

${\mathrm{\ΔP}}_R=k_P\frac{N_P}{N_{\mathrm{PR}}}P\left(k\right)$

Wherein, N _PRBe the reference value of N, k _PFor power difference reference value is calculated scale-up factor and k _P＞0, current MPPT end cycle withdraws from.

2. the maximum power point tracing method of a kind of self-adaptation forcing frequency according to claim 1 and step-length is characterized in that, in described step (7) and the step (8), photovoltaic cell output voltage disturbance step delta u calculates according to following formula:

$\mathrm{\Δu}=\frac{k_u}{N_P}\·{\mathrm{\ΔU}}_R$

Wherein: k _uBe photovoltaic cell output voltage disturbance step size computation scale-up factor and k _u＞0, Δ U _RBe output voltage disturbance step-length reference value.

Images