CN103618471A - Single-phase photovoltaic grid-connected inverter - Google Patents

Abstract

The invention discloses a single-phase photovoltaic grid-connected inverter which is composed of an inverter unit, a secondary power decoupling circuit and an output filter. The output filter is composed of two inductors and one capacitor. The secondary power decoupling circuit is composed of one energy storage inductor and two bidirectional power switches, wherein a bridge arm composed of the two bidirectional power switches is directly connected with the direct-current capacitor in parallel. The energy storage inductor stretches across between the bridge arm composed of the two directional power switches and a bridge arm composed of two blocking type bidirectional power switches. According to the single-phase photovoltaic grid-connected inverter, due to the fact that the secondary power decoupling circuit and the output filter are arranged, secondary ripple wave components in the voltage of the direct-current side can be effectively restrained, the service life of a photovoltaic array is prolonged, the quality of grid-connected currents is improved, and in addition, the needed direct-current filter capacitor is small. Thus, the size of a whole system is reduced, and cost is lowered.

Description

A kind of single-phase photovoltaic grid-connected inverter

Technical field

The invention belongs to parallel network power generation technical field, relate to a kind of single-phase photovoltaic grid-connected inverter.

Background technology

Grid-connected photovoltaic system is comprised of photovoltaic array and inverter two parts conventionally, and wherein photovoltaic array converts solar energy into electrical energy, and inverter injects AC network by electric energy.Single-phase single-grade photovoltaic combining inverter generally adopts single-stage energy conversion link, and that this inverter has is simple in structure, cost is lower and system effectiveness advantages of higher.Therefore, in residents photovoltaic generating system, there is good application and research prospect.Traditional single-phase single-grade photovoltaic combining inverter, controlled in grid-connected current is and the same same phase time frequently of line voltage in power output, to contain secondary ripple component, i.e. secondary power disturbance.This disturbance meeting causes containing in DC voltage a large amount of secondary ripple components, and the secondary ripple component in DC voltage can cause on the one hand and in grid-connected current, contain three times a large amount of ripple currents, thereby reduces grid-connected current quality; Can affect on the other hand the accuracy of photovoltaic array MPPT output reference voltage and the useful life of photovoltaic array.

Deficiency for single-phase single-grade photovoltaic parallel in system, now widely used solution is to suppress secondary power disturbance by designing suitable power decoupled electric capacity, the DC filter capacitor that the method needs is very large, makes like this system bulk and cost greatly increase.

Traditional single-phase single-grade photovoltaic combining inverter topology, is comprised of photovoltaic array, DC filter capacitor C, single-phase electricity die mould inverter and filter inductance L conventionally, and two groups of brachium pontis that wherein single-phase electricity die mould inverter is comprised of 4 two-way power switch T1～T4 form.Now by the operation principle of traditional single-phase single-grade photovoltaic combining inverter, and the problem existing details are as follows.

If v _sfor the voltage of line voltage after isolating transformer, make voltage v _sexpression formula be:

v _s(t)=V _scos(ωt)

V in formula _sfor voltage peak, ω is voltage angle frequency.Control grid-connected current and the same phase time of line voltage, i.e. grid-connected current i under unity power factor _sfor:

i _s(t)=I _scos(ωt)

I in formula _sfor grid-connected current peak value.

Ignore the loss of inverter, the power output P of photovoltaic array _pvfor:

$P_{\mathrm{pv}}=v_s{i}_s=\frac{I_s{V}_s}{2}(1+\cos \left(2\mathrm{\ωt}\right))$

From above formula, photovoltaic array power output contains secondary ripple power

p _pv% is produced by DC filter capacitor C.The charge-discharge energy equation of C is:

$\frac{1}{2}C(u_{\mathrm{pv}}{\left(t\right)}^2-{\stackrel{\‾}{U}}_{\mathrm{pv}}^2)=\underset{0}{\overset{t}{\∫}}\frac{I_s{V}_s}{2}\cos \left(2\mathrm{\ωt}\right)\mathrm{dt}$

In formula

average voltage for filter capacitor C two ends.

Can be obtained fom the above equation, the expression formula of direct voltage is:

$u_{\mathrm{pv}}\left(t\right)=\sqrt{{\stackrel{\‾}{U}}_{\mathrm{pv}}^2+\frac{I_s{V}_s}{2\mathrm{\ωC}}\sin \left(2\mathrm{\ωt}\right)}$

place, carries out Taylor series expansion to above formula:

From above formula, the ripple power P in capacitor C _pvduring % < 0, electric capacity discharges; Ripple power P in capacitor C _pvduring % > 0, electric capacity charges.By charge or discharge process, can obtain following relational expression:

$\frac{I_s{V}_s}{\mathrm{\&omega;}}=C(U_{\max }^2-U_{\min }^2)$

U in formula _maxand U _minbe respectively u _pv(t) maximum and minimum value, due to

the ripple voltage peak-to-peak value △ U=U of capacitor C _max-U _min, can obtain:

$\mathrm{\&Delta;U}=\frac{I_s{V}_s}{2\mathrm{\&omega;C}{\stackrel{\&OverBar;}{U}}_{\mathrm{pv}}}$

Select suitable capacitor C can effectively reduce △ U, but can not eliminate.In MPPT process, MPPT algorithm utilizes the power output of photovoltaic array to calculate DC reference voltage

and then change photovoltaic array operating point, therefore, the fluctuation of photovoltaic array output voltage and electric current should be as far as possible little, to reduce the fluctuation of photovoltaic array power output.Yet the MPPT algorithm that system adopts is the extremum search algorithm based on direct voltage secondary ripple, ripple voltage and current signal are enough large again, being detected by transducer exactly.Comprehensive above analysis, generally getting △ U is 2% left and right of rated direct voltage, can make system have good output characteristic.For example, if the rated power of a system is 1000W, DC rated voltage is 175V, can obtain the value of capacitor C up to 10400 μ F.So large capacitance has not only increased system cost and volume, and can have a strong impact on the response speed of system, therefore, only utilizes capacitor C to reduce the method for direct voltage ripple inadvisable.

Summary of the invention

For overcoming the deficiency of prior art, the object of the present invention is to provide a kind of topological circuit of New single-phase single-stage photovoltaic grid-connected inverter, to suppress the secondary ripple component in DC voltage, improve the quality of grid-connected current, extend the useful life of photovoltaic array, and the DC filter capacitor in topological circuit is reduced greatly.

For achieving the above object, the present invention takes following technical scheme to realize single-phase photovoltaic grid-connected inverter design: this single-phase photovoltaic grid-connected inverter is comprised of inversion unit, secondary power decoupling zero circuit and 3 parts of output filter, described inversion unit is comprised of 1 DC capacitor and 4 blocking-up type two-way power switch, and described 4 two-way power switch form single-phase single-grade full bridge inverter; Described output filter is comprised of 2 inductance and 1 electric capacity, described DC capacitor docks with photovoltaic cell, inductance and the capacitor terminal of described output filter get access to grid, and described inductance is connected with 2 brachium pontis that blocking-up type two-way power switch forms respectively with capacitor terminal; Described secondary power decoupling zero circuit is comprised of an energy storage inductor and 2 two-way power switch, wherein the brachium pontis of 2 two-way power switch compositions is directly in parallel with described DC capacitor, and energy storage inductor is across between the brachium pontis forming at 2 two-way power switch and the brachium pontis of 2 blocking-up type two-way power switch compositions.

Compared with prior art, single-phase photovoltaic grid-connected inverter of the present invention is by the setting of secondary power decoupling zero circuit and output filter, not only can effectively suppress the secondary ripple component in DC voltage, extend the useful life of photovoltaic array, improve the quality of grid-connected current, and required DC filter capacitor is less.Like this, system bulk is reduced, cost.

Accompanying drawing explanation

Below in conjunction with accompanying drawing, the present invention will be further described.

Fig. 1 is the topological circuit structure chart of single-phase photovoltaic grid-connected inverter of the present invention.

Embodiment

As shown in Figure 1, single-phase photovoltaic grid-connected inverter of the present invention is comprised of inversion unit 2, secondary power decoupling zero circuit 3 and 4 three parts of output filter, and described inversion unit 2 is by 1 DC capacitor C6 and 4 blocking-up type two-way power switch T ₁, T ₂, T ₃and T ₄form described 4 two-way power switch T ₁, T ₂, T ₃and T ₄form single-phase single-grade full bridge inverter; Described output filter 4 is by 2 inductance L ₁, L ₃with 1 capacitor C ₁form, described DC capacitor C6 docks with Photovoltaic array 1, the L of described output filter 4 ₁and capacitor C ₁terminal gets access to grid 5, inductance L ₁, L ₃and capacitor C ₁terminal respectively with T ₃and T ₄the brachium pontis a point and the T that form ₁and T ₂the brachium pontis b point forming connects; Described secondary power decoupling zero circuit 3 is by an energy storage inductor L ₂7 and 2 two-way power switch T ₅, T ₆form 2 two-way power switch T ₅and T ₆the brachium pontis forming is directly in parallel with described DC capacitor C6, energy storage inductor L ₂7 across at 2 two-way power switch T ₅and T ₆between the brachium pontis that the brachium pontis forming and blocking-up type two-way power switch T1 and T2 form.

Single-phase photovoltaic grid-connected inverter topological structure of the present invention and principle:

The operation principle of single-phase photovoltaic grid-connected inverter topological circuit of the present invention is as follows:

Make energy storage inductor L ₂current expression be:

i _L=I _Lcos(ωt-α) (1)

I in formula _lfor inductive current peak, α is that inductive current is with respect to the angle of retard of line voltage.

Energy storage inductor L ₂eNERGY E _l2and power P _l2can be expressed as:

$E_{L2}=\frac{L_2{i}_L^2\left(t\right)}{2}---\left(2\right)$

$P_{L2}=-\frac{1}{2}\mathrm{\&omega;}{L}_2{I}_L^2\sin (2\mathrm{\&omega;t}-2\mathrm{\&alpha;})---\left(3\right)$

For grid-connected LCL filter, generally in grid-connected current, do not contain high order harmonic component, therefore the effect of capacitor C 1 is little.Therefore, to simplify the analysis, ignore the impact of capacitor C 1, and make equivalent inductance L=L ₁+ L ₃.For equivalent inductance, L is analyzed as follows:

The power P of grid-connected inductance L _lfor:

$P_L=-\frac{1}{2}\mathrm{\&omega;L}{I}_s^2\sin \left(2\mathrm{\&omega;t}\right)---\left(4\right)$

Ignore inverter losses, can obtain following relational expression:

$\frac{I_s{V}_s}{2}+\frac{I_s{V}_s}{2}\cos \left(2\mathrm{\&omega;t}\right)-\frac{\mathrm{\&omega;L}{I}_s^2\sin \left(2\mathrm{\&omega;t}\right)}{2}=P_{\mathrm{pv}}\left(t\right)-\frac{\mathrm{\&omega;}{L}_2{I}_L^2\sin (2\mathrm{\&omega;t}-2\mathrm{\&alpha;})}{2}---\left(5\right)$

If met p _pv %+P _l=P _l2in photovoltaic array power output, secondary ripple component is inhibited, and has realized the decoupling zero of alternating current-direct current component in power, and in direct voltage, secondary ripple component also can be reduced largely so, has:

In formula

Inductance L can be obtained fom the above equation ₂electric current I _lrelational expression with angle of retard α:

$I_L=\sqrt{\sqrt{{\left(I_s{V}_s\right)}^2+{\left(\mathrm{\&omega;L}{I}_s^2\right)}^2}/\mathrm{\&omega;}{L}_2}---\left(7\right)$

By Substitution method, can obtain energy storage inductor L ₂current i _lfor:

In formula

From above formula, after system power and grid-connected inductance L are determined, i _lpeak value with be inversely proportional to, and L ₂value is larger, can cause system bulk to increase, therefore, and inductance L ₂value to consider from above two aspects.

List this switching mode that improves topological inverter as shown in table 1,

Table 1

In table, Sa, Sb and Sm are respectively the switch function of three brachium pontis: Sk=1, brachium pontis switch conduction in expression, and lower brachium pontis switch turn-offs; Sk=0, represents lower brachium pontis switch conduction, and upper brachium pontis switch turn-offs (k=a, b, m).Inductive drop and the grid-connected voltage relationship of main circuit switch pattern and correspondence thereof are as shown in table 1.By table 1, can obtain u _aband u _lrelational expression be:

$\left\{\begin{array}{c}{u}_{\mathrm{ab}}=(S_a-S_b)U_{\mathrm{pv}}\\ u_L=(S_m-S_b)U_{\mathrm{pv}}\end{array}\right.---\left(10\right)$

Get T ₁, T ₃and T ₅duty ratio be respectively db, da and dm, and upper and lower bridge arm switch opens pattern complementation, above formula can be converted into:

$\left\{\begin{array}{c}{u}_{\mathrm{ab}}^{*}=(d_a-d_b)\\ U_L^{*}=(d_m-d_b)\end{array}\right.---\left(11\right)$

In formula

consider that duty cycle of switching must meet 0<d<1, can be obtained fom the above equation:

$\left\{\begin{array}{c}0<d_m<1\\ \mathrm{\&Delta;u}<d_m<1+\mathrm{\&Delta;u}\end{array}\right.---\left(12\right)$

In formula

the duty ratio dm value of T5 can be summarized as: max (0, △ u) <d _m<min (1,1+ △ u), based on mean value theorem, can obtain dm and be:

d _m=0.5(max(0,△u)+min(1,1+△u)) (13)

Db and da can be drawn by following formula:

$\left\{\begin{array}{c}{d}_a=d_m-\mathrm{\&Delta;u}\\ d_b=d_m-u_L/U_{\mathrm{pv}}\end{array}\right.---\left(14\right)$

Da, db and dm respectively with carrier wave ratio, can draw the pwm control signal of 6 power switchs.

Claims (1)

1. a single-phase photovoltaic grid-connected inverter, it is characterized in that: it is comprised of inversion unit, secondary power decoupling zero circuit and 3 parts of output filter, described inversion unit is comprised of 1 DC capacitor and 4 blocking-up type two-way power switch, and described 4 two-way power switch form single-phase single-grade full bridge inverter; Described output filter is comprised of 2 inductance and 1 electric capacity, described DC capacitor docks with photovoltaic cell, inductance and the capacitor terminal of described output filter get access to grid, and described inductance is connected with 2 brachium pontis that blocking-up type two-way power switch forms respectively with capacitor terminal; Described secondary power decoupling zero circuit is comprised of an energy storage inductor and 2 two-way power switch, wherein the brachium pontis of 2 two-way power switch compositions is directly in parallel with described DC capacitor, and energy storage inductor is across between the brachium pontis forming at 2 two-way power switch and the brachium pontis of 2 blocking-up type two-way power switch compositions.