CN103019294B - 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, in particular 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, make solar photovoltaic technology become the focus of various countries' concern and research.

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 _pVtypical relation curve as shown in Figure 1.1 is known with reference to the accompanying drawings, under identical external environment condition, the output power from photovoltaic cells and output voltage have corresponding relation, in order at utmost play the effectiveness of photovoltaic cell, at utmost improve the generated energy of photovoltaic cell, increase the production capacity of photovoltaic cell, need adjustment photovoltaic cell output end voltage, the output power moment of photovoltaic cell is made to remain on maximum power point (the M point in accompanying drawing 1), namely need to adopt MPPT maximum power point tracking (Maximum Power Point Tracking, MPPT) technology.

Several MPPT methods conventional at present mainly contain: constant voltage process, conductance increment method, disturbance observation etc.Disturbance observation is the output voltage often changing photovoltaic array at regular intervals, Real Time Observation compares the output power value of before and after change 2 to change the direction of regulation voltage, finally be stabilized near maximum power point, because disturbance observation algorithm is simple, it is convenient to realize and widespread use.When 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 make photovoltaic cell be trapped in low-power output for a long time to go, if step-length is excessive, system oscillation can be made again to aggravate, particularly can increase power loss during stable state.This is that step-length is larger, departs from real maximum power point far away during disturbance, and the power loss caused also can be larger because the output voltage of photovoltaic cell will disturbance near maximum power point always.In order to solve this problem, research worker improves precision and the rapidity of MPPT tracking usually from the angle changing disturbance step-length, but seldom proposes the solve scheme of this problem from the angle changing forcing frequency.

Notice, based in the MPPT method of disturbance observation, the most critical issue is the Obtaining Accurate in changed power direction after applying disturbance, and the applying of disturbance next time simultaneously needs to carry out after this disturbance reaches stable state again, otherwise can produce the erroneous judgement of perturbation direction.In order to ensure that the output voltage of photovoltaic cell applies disturbance next time after having reached stable state again, common way chooses the relatively long MPPT cycle, ensures that output voltage reaches stable state when a MPPT end cycle.But corresponding different disturbance step-length, output voltage reaches the stable time and different, and therefore, the mode in fixing MPPT cycle cannot make change disturbance step-length MPPT tracking effect reach optimum.

Summary of the invention

The present invention is directed to the deficiency existing for existing MPPT method, the maximum power point tracing method of a kind of self-adaptation forcing frequency and step-length is proposed, the shortcoming of conventional disturbance observation, variable step disturbance observation can be overcome, 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 takes following steps:

(1) the output voltage u of solar cell is detected _pV, current i _pV;

(2) input voltage count value N is judged _uKwhether be greater than input voltage count value benchmark N _{uK_R};

If N _uK≤ N _{uK_R}, then step (3) is proceeded to;

If N _uK>N _{uK_R}, then step (4) is proceeded to;

(3) solar cell output voltage u is judged _pVthe voltage error reference value Δ u of setting whether is less than with the Error Absolute Value of photovoltaic cell output voltage reference value u (k) in current MPPT cycle _r;

If | u _pV-u (k) | < Δ u _r, then N _uKincrease 1, terminate;

If | u _pV-u (k) |>=Δ u _r, then N _uKbe set to 0, terminate;

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

Δp＝Δp+p _PV-P(k-1)

(5) absolute value of power difference accumulated value is judged | whether Δ p| is greater than power difference reference value Δ P _r:

If | Δ p|> Δ P _r, then step (6) is proceeded to;

If | Δ p|≤Δ P _r, then exit;

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

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

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

(7) photovoltaic cell output voltage reference value is increased, namely photovoltaic cell output voltage reference value u (k+1)=u (k)+Δ u in the next MPPT cycle is made, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then proceeds to step (9);

(8) photovoltaic cell output voltage reference value is reduced, namely photovoltaic cell output voltage reference value u (k+1)=u (k)-Δ u in the next MPPT cycle is made, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then proceeds to step (9);

(9) power difference reference value Δ P is changed according to the following formula _r, then by 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:

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

Wherein, N _pRfor the reference value of N, k _pfor power difference reference value calculates scale-up factor and k _p>0, current MPPT end cycle, exits.

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

$\mathrm{\&Delta;u}=\frac{k_u}{N_P}\&CenterDot;\mathrm{\&Delta;}{U}_R$

Wherein: k _ufor photovoltaic cell output voltage disturbance step size computation scale-up factor and k _u>0, Δ U _rfor 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 adjust forcing frequency and disturbance step-length automatically, make the output power from photovoltaic cells point depart from maximum power point far away time, forcing frequency and disturbance step-length larger, ensure the rapidity of tracking velocity;

(2) maximum power point tracing method of the present invention can adjust forcing frequency and disturbance step-length automatically, when making the output power from photovoltaic cells point reach maximum power point, forcing frequency and disturbance step-length minimum, the stability of following the tracks of when ensureing near maximum power point, reduces the power loss because disturbance causes;

(3) illustrated maximum power point tracing method, judging the judgement starting the change direction of power after input terminal voltage reaches stable state again, can ensure validity and the stability of disturbance, avoiding undisturbed generation;

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

Accompanying drawing explanation

The present invention is further illustrated 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;

The photovoltaic generating system of Fig. 3 corresponding to embodiment one forms schematic diagram;

Symbol description in figure: u _pV, i _pV, p _pVbe respectively photovoltaic cell output voltage, output current and output power; In accompanying drawing 3, C ₁, C ₂for electric capacity, S ₁~ S ₄for switching tube, L is inductance.

Embodiment

The technological means realized to make the present invention, creation characteristic, reaching object and effect is easy to understand, below in conjunction with concrete diagram, setting forth the present invention further.

Convenient in order to describe, below in conjunction with accompanying drawing 1 and accompanying drawing 2, technical scheme of the present invention is described.

Photovoltaic cell output voltage u with reference to the accompanying drawings shown in 1 _pVwith output power p _pVcurve is known, the output power from photovoltaic cells from maximum power point (in accompanying drawing 1 M point) more away from, output power is less, and simultaneously slope of a curve is larger, and near maximum power point, the output power from photovoltaic cells is maximum but slope of a curve is minimum.Therefore in order to make the output power from photovoltaic cells arrive maximum power point fast, when making maximum power point, the output power from photovoltaic cells is stablized simultaneously, forcing frequency and disturbance step-length should be made to reduce 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 as shown in Figure 2, specifically takes following steps when implementing:

(1) the output voltage u of solar cell is detected _pV, current i _pV;

(2) input voltage count value N is judged _uKwhether be greater than input voltage count value benchmark N _{uK_R};

If N _uK≤ N _{uK_R}, then step (3) is proceeded to;

If N _uK>N _{uK_R}, then step (4) is proceeded to;

(3) solar cell output voltage u is judged _pVthe voltage error reference value Δ u of setting whether is less than with the Error Absolute Value of photovoltaic cell output voltage reference value u (k) in current MPPT cycle _r;

If | u _pV-u (k) | < Δ u _r, then N _uKincrease 1, terminate;

If | u _pV-u (k) |>=Δ u _r, then N _uKbe set to 0, terminate;

(4) according to photovoltaic cell voltage u _pV, current i _pVcalculate photovoltaic power p _pV:

p _PV＝u _PVi _PV

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

Δp＝Δp+p _PV-P(k-1)

(5) absolute value of power difference accumulated value is judged | whether Δ p| is greater than power difference reference value Δ P _r:

If | Δ p|> Δ P _r, then step (6) is proceeded to;

If | Δ p|≤Δ P _r, then exit;

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

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

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

(7) photovoltaic cell output voltage reference value is increased, namely photovoltaic cell output voltage reference value u (k+1)=u (k)+Δ u in the next MPPT cycle is made, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then proceeds to step (9);

(8) photovoltaic cell output voltage reference value is reduced, namely photovoltaic cell output voltage reference value u (k+1)=u (k)-Δ u in the next MPPT cycle is made, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then proceeds to step (9);

(9) power difference reference value Δ P is changed according to the following formula _r, then by 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, the starting point as the next cycle:

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

Wherein, N _pRfor the reference value of N, k _pfor power difference reference value calculates scale-up factor and k _p>0, current MPPT end cycle, exits;

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

$\mathrm{\&Delta;u}=\frac{k_u}{N_P}\&CenterDot;\mathrm{\&Delta;}{U}_R$

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

According to above-mentioned implementation step:

(1) to the judgement of the output power from photovoltaic cells after photovoltaic cell output end voltage reaches stable state, namely, after each MPPT cycle applies disturbance, only have to stablize when photovoltaic cell output end voltage and (be less than the error amount Δ u of setting with the deviation of given voltage reference _r) after just start the change direction judging power, after this can ensure to apply disturbance at every turn, the output end voltage of photovoltaic cell all reaches setting value, avoids disturbance velocity too fast and photovoltaic cell voltage follow does not go up the mistake disturbance or erroneous judgement problem that disturbance velocity brings.

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

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

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

(5) when the output power from photovoltaic cells point is in maximum power point, if now change suddenly causes output power to be undergone mutation to external environment condition (illumination, block), then corresponding changed power difference Δ p also can follow and become large, and now power adds up reference value Δ P _rnot yet change, therefore the time that difference power is 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 adjust MPPT forcing frequency and step-length automatically when illumination condition changes suddenly, improves the rapidity of now following the tracks of.

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

The photovoltaic generating system of this embodiment forms schematic diagram as shown in Figure 3.Whole system comprises solar-energy photo-voltaic cell, electric current and voltage adopts module, driving circuit, DSP, DC-DC converter and load.Wherein solar-energy photo-voltaic cell connects load by DC-DC converter, and DSP adopts model calling solar-energy photo-voltaic cell by electric current and voltage, and controls DC-DC converter by driving circuit.DC-DC converter is by electric capacity 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, turning on and off of DC-DC converter switching tube is controlled after DSP computing, realize the maximal power tracing of photovoltaic cell, the output of DC-DC converter can load can be accumulator, combining inverter or other load.In the present embodiment, the input voltage range of DC-DC converter is 7 ~ 60V, and output voltage range is 0 ~ 60V, rated power 300W, DC-DC converter breaker in middle pipe switching frequency 100kHz.

More than show and describe 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; what describe in above-described embodiment and 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.Application claims protection domain is defined by appending claims and equivalent thereof.

Claims (1)

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

(1) the output voltage u of solar cell is detected _pV, current i _pV;

(2) input voltage count value N is judged _uKwhether be greater than input voltage count value benchmark N _{uK_R};

If N _uK≤ N _{uK_R}, then step (3) is proceeded to;

If N _uK>N _{uK_R}, then step (4) is proceeded to;

(3) solar cell output voltage u is judged _pVthe voltage error reference value Δ u of setting whether is less than with the Error Absolute Value of photovoltaic cell output voltage reference value u (k) in current MPPT cycle _r;

If | u _pV-u (k) | < Δ u _r, then N _uKincrease 1, terminate;

If | u _pV-u (k) |>=Δ u _r, then N _uKbe set to 0, terminate;

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

Δp＝Δp+p _PV-P(k-1)

(5) absolute value of power difference accumulated value is judged | whether Δ p| is greater than power difference reference value Δ P _r:

If | Δ p|> Δ P _r, then step (6) is proceeded to;

If | Δ p|≤Δ P _r, then exit;

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

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

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

(7) photovoltaic cell output voltage reference value is increased, namely photovoltaic cell output voltage reference value u (k+1)=u (k)+Δ u in the next MPPT cycle is made, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then proceeds to step (9);

(8) photovoltaic cell output voltage reference value is reduced, namely photovoltaic cell output voltage reference value u (k+1)=u (k)-Δ u in the next MPPT cycle is made, wherein Δ u is photovoltaic cell output voltage disturbance step-length, then proceeds to step (9);

(9) power difference reference value Δ P is changed according to the following formula _r, and then by 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:

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

Wherein, N _pRfor the reference value of N, k _pfor power difference reference value calculates scale-up factor and k _p>0, current MPPT end cycle, exits;

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

$\mathrm{\&Delta;u}=\frac{k_u}{N_P}\&CenterDot;\mathrm{\&Delta;}{U}_R$

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