CN104615189A - Maximum power tracing method suitable for single-stage grid-connected photovoltaic inverter system - Google Patents

Abstract

The invention discloses a maximum power tracing method suitable for a single-stage grid-connected photovoltaic inverter system. The maximum power tracing method is characterized by periodically executing the following steps that 1 photovoltaic array output voltage is sampled, whether the photovoltaic array output voltage is lowered to a value below a set critical value is judged, if yes, the steps 2, 3 and 4 are skipped, the step 5 is executed, and otherwise, the step 2 is executed; 2 whether the photovoltaic array output voltage is increased is judged, if yes, the step 3 is executed, and otherwise, the step 3 is skipped and the step 4 is executed; 3 present photovoltaic array voltage and a grid-connected current instruction are recorded; 4 grid-connected current instructions are increased, amplitude limiting is conducted, and the amplitude limiting value is a recorded value in the step 5; 5 present grid-connected current instructions are recorded and decreased; 6 after the grid-connected current instructions run through a current loop, a PWM control signal is obtained, motion of a switching tube is controlled, and maximum power tracing is achieved. According to the method, the sampling quantity is reduced, and the global maximum power point can be traced under the multi-extreme-point working condition.

Description

A kind of maximum power tracking method being applicable to single-stage photovoltaic grid-connected inversion system

Technical field

The invention belongs to technical field of new energies, more specifically, relate to a kind of maximum power tracking method being applicable to single-stage photovoltaic grid-connected inversion system.

Background technology

Grid-connected photovoltaic system is subject to the extensive concern of researchist because of its advantage such as pollution-free.Because of photovoltaic array output power with Changes in weather wide variation, thus need to utilize maximal power tracing technology to make photovoltaic array utilization factor maintain higher level.

Stage type system is formed by DC-DC circuit and DC-AC circuit connected in series, usually realizes maximal power tracing by DC-DC transform part, and DC-AC link generates grid-connected current instruction according to photovoltaic array outer voltage, maintains photovoltaic array voltage constant.Different from stage type system, single stage type system can cancel capacitance voltage control loop, and directly generate final grid-connected current instruction by maximum power tracking method, this avoid adding of photovoltaic array Voltage loop, Controller gain variations is simpler.

In single-stage photovoltaic grid-connected system, conventional maximum power tracking method thankss for your hospitality dynamic observation and incremental conductance method etc.According to the change direction of power and voltage, this kind of algorithm judges that present operating point is positioned on the left of maximum power point or right side, realize maximal power tracing.Usual needs are sampled light photovoltaic array voltage and photovoltaic array output current simultaneously, and needs to consider that MPPT algorithm is on the impact of system stability, increases anti-unstability algorithm simultaneously.

In addition, tradition maximum power tracking method can not distinguish local maximum power point and global maximum power point usually, thus need to use extra algorithm, such as based on the distribution MPPT algorithm of particle cluster algorithm to adapt to Multi-maximum point operating mode, this algorithm adds implementation complexity to a great extent.

Summary of the invention

For above defect or the Improvement requirement of prior art, the invention provides a kind of maximum power tracking method being applicable to the univoltage sampling of single-stage photovoltaic grid-connected system, decrease the sampling quantity that maximal power tracing needs, the natural ability possessing tracing global maximum power point of algorithm, avoids the possibility of photovoltaic array collapse of voltage simultaneously.

For achieving the above object, the invention provides the maximum power tracking method of a kind of univoltage sampling, comprise and periodically perform following steps: the output voltage of (1) sampled light photovoltaic array, judge whether photovoltaic array output voltage is reduced to below the critical value of setting, skip step (2) (3) (4) and perform step (5), otherwise perform step (2); (2) judge whether photovoltaic array voltage increases according to double photovoltaic array sampled value, if then perform step (3), otherwise skip step (3) execution step (4); (3) current photovoltaic array voltage and grid-connected current instruction is recorded; (4) increase grid-connected current instruction, but maximal value is less than the grid-connected current instruction that step (5) records; (5) record the instruction of current time grid-connected current, and reduce grid-connected current instruction; (6) grid-connected current instruction obtains pwm control signal after electric current loop, controls the switch motion of inverter circuit switching tube, realizes maximal power tracing.

Preferably, in described step (1), the Critical Light photovoltaic array voltage of setting is Δ V in formula _lmaxfor maximum pressure drop on filtering circuit; To single-phase full bridge system, U _gridfor line voltage effective value, to three-phase system, U _gridfor phase voltage effective value, according to SPWM modulation, modulate according to SVPWM k _cfor safety factor, for being greater than the constant of 1.

Preferably, in described step (3), current-order reduces amplitude and should be chosen for: u in formula _xcorresponding photovoltaic array voltage is increased, I for making photovoltaic array voltage for the last time _xfor grid-connected current instruction now, U _nfor the photovoltaic array voltage that present sample arrives, k is safety factor, be greater than 1 constant.

Owing to only whether occurring that collapsibility declines or is reduced to below rated voltage according to DC voltage, maximum power tracking method that the present invention carries, without the need to direct current input current of sampling, decreases sampling quantity, reduces the hardware cost that maximum power tracking method realizes.Secondly, described maximum power tracking method utilizes output power in single stage type system to be greater than photovoltaic array energy global maximum power to there will be photovoltaic array collapse of voltage characteristic and realize maximal power tracing, can't produce collapse of voltage phenomenon in Local Extremum, thus natural possess the function of following the tracks of global maximum power point.Finally, under this methodology, the impact of maximal power tracing on system stability is directly processed, and thus can not cause system unstability because of maximum power tracking method erroneous judgement.

Accompanying drawing explanation

Fig. 1 is the realization flow figure of photovoltaic system maximum power tracking method provided by the invention;

Fig. 2 is single-stage photovoltaic grid-connected system provided by the invention;

Fig. 3 is experiment test figure of the present invention; Wherein Fig. 3 (a) is system tracks unimodal curve experimental waveform; The bimodal curve that Fig. 3 (b) simulates for utilizing photovoltaic dummy source; Fig. 3 (c) is system tracks bimodal curve experimental waveform; Fig. 3 (a) is the zero point of oscillograph respective channel with left side numeral in Fig. 3 (c).

Embodiment

In order to make object of the present invention, technical scheme and advantage clearly understand, below in conjunction with drawings and Examples, the present invention is further elaborated.Should be appreciated that specific embodiment described herein only in order to explain the present invention, be not intended to limit the present invention.

Fig. 1 is maximum power tracking method of the present invention, see Fig. 1, and 101, according to sampled value, Gather and input photovoltaic array voltage, i.e. photovoltaic array output voltage, judge whether photovoltaic array voltage is less than if so, then redirect 105; Otherwise redirect 102;

102: according to adjacent twice photovoltaic array voltage sample value, judge whether photovoltaic array voltage increases, if then perform 103, otherwise skip 103 execution 104;

103: record current photovoltaic array voltage and grid-connected current instruction, for the power reduction after Voltage Drop provides data foundation;

104: increase grid-connected current instruction, grid-connected current instruction is larger, and maximal power tracing speed is faster, but fluctuates during stable state; Otherwise then tracking velocity is slower, stable fluctuation is little; In real system, current increment can be taken as 0.05% ~ 0.2% of rated power, but the instruction of increase after-current need be less than the current-order of record in 105, avoids photovoltaic array voltage again to drop into below critical value;

105: record current grid-connected current instruction, and reduce grid-connected current instruction, instruction reduces step-length and is not less than Δ I _d; Δ I _dcalculating formula be

106: grid-connected current instruction obtains exporting pwm control signal after electric current loop, controls the switch motion of inverter circuit switching tube, realizes maximal power tracing.

Be applicable to the maximum power tracking method of single-stage photovoltaic grid-connected inversion system in order to further description is provided by the invention, details are as follows for existing composition graphs 2 and instantiation:

Fig. 2 shows structural drawing and the control strategy of single-stage photovoltaic grid-connected electricity generation system, and single-stage photovoltaic grid-connected electricity generation system comprises the photovoltaic array, inverter circuit and the filtering circuit that are connected on electrical network incoming end successively.Its control method is: as shown in Figure 1, maximum power tracking method exports grid-connected active current instruction to maximum power tracking method, and reactive power instruction is arranged by host computer, is defaulted as 0; Grid-connected current draws and exports meritorious, reactive current size after abc/dq coordinate transform, above-mentioned instruction and actual current value are subtracted each other, and under controller (conventional proportional-plus-integral controller) forms d, q axle steering order, under d, q axle, steering order forms pwm control signal after dq/abc coordinate transform and SVPWM algorithm, the action of gauge tap pipe, realizes maximal power tracing.

Embodiment is further illustrated for a 630kW three-phase photovoltaic grid-connected system.In system, dc-link capacitance is 25.2mF, line voltage is 181.8V (phase effective value), and adopt SVPWM modulation, peak power algorithm disturbance step-length is 1A (peak value), every 100ms that circulates as described in Figure 1 performs once, and when injecting meritorious, maximum induction pressure drop is 54.4V.Obtain according to above-mentioned formulae discovery:

$U_c=k_{c}n\sqrt{U_{\mathrm{grid}}^2+\mathrm{\Δ}{V}_{L\max }^2}=1.05\×\sqrt{6}\×\sqrt{{181.8}^2+54.4^2}=488V,$ Get 490V.Namely photovoltaic array charge threshold level is chosen for 490V.

Current-order decrease is by formula calculate.

After arranging above-mentioned variable, system cloud gray model oscillogram as shown in Figure 3.Fig. 3 (a) is system tracks unimodal curve experimental waveform.In figure, DC bus-bar voltage occurs fall fast and drop into below critical value, and grid-connected current instruction reduces to maintain system stability; Observe from run time long, after adopting this algorithm, system does not occur that DC bus-bar voltage declines fast and causes the phenomenon of system shakiness.The bimodal curve that Fig. 3 (b) simulates for utilizing photovoltaic dummy source, Fig. 3 (c) is system tracks bimodal curve experimental waveform, in figure there is first peak value of the corresponding bimodal curve of decline fast in DC bus-bar voltage first time, owing to not reaching global maximum power point, busbar voltage is gone up after of short duration decline, and average anode current continues to rise after voltage recovery, system moves towards global maximum power point, in figure, second time voltage falls correspondence fast and reaches maximum power point, its characteristic and Fig. 3 (a) similar.Test result shows, described maximum power tracking method natural adaptation multi-peak point curve, can effectively trace into global maximum power electricity.

Those skilled in the art will readily understand; the foregoing is only preferred embodiment of the present invention; not in order to limit the present invention, all any amendments done within the spirit and principles in the present invention, equivalent replacement and improvement etc., all should be included within protection scope of the present invention.

Claims (3)

1. be applicable to a maximum power tracking method for single stage type combining inverter, it is characterized in that, periodically perform following steps:

(1) obtain photovoltaic array voltage sample value, judge whether photovoltaic array output voltage is reduced to the critical value U of setting _cbelow, if so, then proceed to step (5), if not, then enter step (2);

(2) judge whether photovoltaic array voltage increases according to double photovoltaic array sampled value, if so, then enter step (3), if not, then proceed to step (4);

(3) photovoltaic array voltage and the grid-connected current instruction of current time is recorded;

(4) increase described grid-connected current instruction, and proceed to step (6);

Wherein, current increment is 0.05% ~ 0.2% of rated power, and the instruction of increase after-current need be less than the current-order of record in last step (5);

(5) record the grid-connected current instruction of current time, and reduce described grid-connected current instruction;

The reduction step-length of wherein said grid-connected current instruction is not less than Δ I _d;

(6) grid-connected current instruction obtains exporting pwm control signal after electric current loop, controls the switch motion of inverter circuit switching tube, realizes maximal power tracing.

2. maximum power tracking method as claimed in claim 1, the critical value U of setting _cfor inverter inversion minimum threshold voltage, $U_c=k_{c}n\sqrt{U_{\mathrm{grid}}^2+{\mathrm{\ΔV}}_{L\max }^2},$

Wherein, Δ V _lmaxfor maximum pressure drop on filtering circuit; To single-phase full bridge system, U _gridfor line voltage effective value, to three-phase system, U _gridfor phase voltage effective value, according to SPWM modulation, modulate according to SVPWM k _cfor safety factor, for being greater than the constant of 1.

3. maximum power tracking method as claimed in claim 1, it is characterized in that, in described step (5), the photovoltaic array voltage that photovoltaic array voltage, grid-connected current and present sample when increasing according to last photovoltaic array voltage arrive calculates minimum current decrease Δ I _d, its computing formula is: ${\mathrm{\ΔI}}_D=k\frac{U_x-U_n}{U_x}\×I_x;$

Wherein, U _x, I _xfor the value of record in step (3), i.e. U _xphotovoltaic array voltage is made to increase corresponding photovoltaic array voltage, I for the last time _xfor the grid-connected current instruction in corresponding moment, U _nfor the photovoltaic array voltage that present sample arrives, k is safety factor, be greater than 1 constant.