CN106208782A - Cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control - Google Patents

Abstract

The present invention discloses a kind of cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control, and described method includes: writes out all 256 kinds of on off states of four H bridge cascaded inverters and calculates photovoltaic cell parasitic capacitor voltage sum V of correspondence_cValue；Select on off state and form Switch State Combination in Power Systems；Calculate the output voltage V of inverter ac side_k；The voltage V of on-line checking k moment electrical network_g(k) and power network current i (k), and calculate power network current value i (k+1) in the k+1 moment；The line voltage V that on-line checking is obtained_gK () is phase-locked, obtain phase theta, θ calculate given value of current value i in k+1 moment^*(k+1) valuation functions h is substituted into, and together with Current calculation value i (k+1) in k+1 moment；Select the Switch State Combination in Power Systems that minimum valuation functions h is corresponding, and drive switching tube as output by drive circuit.The method can not only suppression system leakage current effectively, and extend conveniently, and be applicable to the single-phase non-isolated cascaded H-bridges photovoltaic combining inverter of even number H bridge.

Description

Cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control

Technical field

The present invention relates to a kind of cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control；It is fitted For the grid-connected field of non-isolation type.

Background technology

Compared with conventional inverter, cascaded H-bridges multi-electrical level inverter has that grid current harmonic is little, switching frequency is low, filter Ripple body amasss little and is prone to the advantages such as modularity, has therefore obtained the concern of more and more scholar.

Additionally, the DC side of each module of cascaded H-bridges multi-electrical level inverter can be independently-powered by one piece of photovoltaic panel so that it is solely Vertical MPPT controls to be possibly realized, and therefore cascaded H-bridges many level topological structure is particularly suited for photovoltaic combining inverter.

Due to cascaded H-bridges inverter module structure, grid-connected required voltage can be reached by cascading certain quantity, Therefore can save the transformator of raising voltage and buffer action, reduce cost further and improve power density.

But, owing to lacking transformer isolation, between photovoltaic panel and electrical network, there is directly electrical connection, can be in photovoltaic panel And in the parasitic capacitance between the earth, producing leakage current, leakage current can affect the efficiency of system, reduce system reliability, threat people The safety of body and generation electromagnetic interference etc., be therefore highly desirable to suppress leakage current.

At present, traditional drain current suppressing method can be largely classified into following three kinds: 1) use the topological structure improved, as The topologys such as H5, H6；2) passive filter is used, such as common mode inductance, electromagnetic interface filter etc.；3) suitable modulation strategy is sought.

But, unlike single module inverter topology, the composition of cascaded H-bridges topology leakage current not only with this module Output is relevant, and the also output with other modules of cascade is associated.Therefore, the suppressing method of single H bridge leakage current can not be straight Connect the suppression applying to cascaded H-bridges topology leakage current, cause method and the level of existing single module inverter suppression leakage current A little mismatch is there is between connection H bridge inverter drain current suppressing method.

To this end, scholars have done a lot of effort and trial in terms of the drain current suppressing of cascaded H-bridges inverter, such as 2016 IEEE document " Single Phase Cascaded H5Inverter with Leakage Current Elimination For Transformerless Photovoltaic System " (" non-isolation type Cascade H 5 photovoltaic DC-to-AC converter common mode current is special Property analyze " 2016 years IEEE energy sciences can plenary session collection of thesis) and propose a kind of about Cascade H 5 topology suppression electric leakage The modulation strategy of stream, although inhibit leakage current to a certain extent, but be as number of modules and increase modulation strategy and can answer very much Miscellaneous, it is unfavorable for system extension and modularized design.Additionally, Cascade H 5 topology proposed compares H4 topology, cost and loss all Can increase.

IEEE document " Analysis and Suppression of Leakage Current in 2014 Cascaded-Multilevel–Inverter-Based PV Systems,”Y.Zhou and H.Li,《IEEE Trans.Power Electron. ", 2014,29 (10), 5,265 5277 (" cascade connection multi-level photovoltaic inverter leakage current analyses With suppression ", " IEEE journal-power electronics periodical ", volume 29 the 10th phase page 5,265 5277 in 2014) propose at DC side Add common-mode filter suppression leakage current with AC respectively, but its switching frequency is set as 10kHz, this and use cascaded topology The original intention reducing switching frequency does not meets.IEEE document " A Modulation Strategy for Single-in 2013 phase HB-CMI to Reduce Leakage Ground Current in Transformer-less PV Appl Ications " (" cascade connection multi-level photovoltaic inverter leakage current is analyzed and suppression " IEEE energy science in 2013 can all meetings Words collected works) modulation strategy that proposes makes parasitic capacitor voltage change by power frequency staircase waveform, but this modulation strategy is the most multiple Miscellaneous and be not easy to system extension.Additionally, Chinese invention patent application prospectus CN 105450059 A is in December, 2015 " suppress the modulator approach of two H bridge cascaded inverter leakage currents " disclosed in 22 days, traditional carrier wave stacking modulation improved, Propose a kind of new modulation strategy, the electric leakage of the single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter containing two H bridges can be suppressed Stream, but the method is only used for the situation of two H bridges, it is impossible to spread over multiple H bridge, have certain limitation, be not easy to Practical implementation.

In sum, for single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter, existing drain current suppressing method is mainly deposited In following problem:

(1) prior art is concentrated mainly on the leakage current of the single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter containing two H bridges Suppression, this largely limits the expanded application of single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter more multimode, fails fully Play the advantage of cascaded H-bridges photovoltaic DC-to-AC converter；

(2) the drain current suppressing method of the existing single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter containing multiple H bridges mainly changes Become existing topology or use new topological realization.As added common-mode filter respectively at DC side and AC, use level Connection H5 or H6 topology, this can increase cost and the loss of system undoubtedly, reduce the power density of inverter.

Summary of the invention

The technical problem to be solved in the present invention is exactly the limitation overcoming above-mentioned various scheme, inverse for non-isolation type photovoltaic Become device produces leakage current owing to lacking transformer isolation problem in parasitic capacitance between photovoltaic panel and the earth, it is proposed that A kind of based on Model Predictive Control drain current suppressing method, it is convenient that the method has extension, calculates the advantages such as simple and low cost.

For solving the technical problem of the present invention, the technical scheme key step used is as follows:

1, a kind of cascaded H-bridges photovoltaic combining inverter drain current suppressing method based on Model Predictive Control, including selecting Switch State Combination in Power Systems and the voltage and the electric current that detect electrical network, key step is as follows:

Step 1, if four module DC voltages of four H bridge cascaded inverters are identical, and is designated as V_dc, and calculate with following formula Corresponding photovoltaic cell parasitic capacitor voltage sum V_cValue,

$V_c=\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\Σ}}\[4-2(i-1)\]S_{ia}-\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\Σ}}(4-2i)S_{ib}-2V_g$

Wherein, V_gFor the voltage of electrical network, S_1aIt is the switch function of pipe, S on first left brachium pontis of H bridge_1bIt is that first H bridge is right The switch function of pipe, S on brachium pontis_2aIt is the switch function of pipe, S on second left brachium pontis of H bridge_2bIt is to manage on second right brachium pontis of H bridge Switch function, S_3aIt is the switch function of pipe, S on the 3rd the left brachium pontis of H bridge_3bIt it is the switch letter of pipe on the 3rd the right brachium pontis of H bridge Number, S_4aIt is the switch function of pipe, S on the 4th the left brachium pontis of H bridge_4bIt is the switch function of pipe on the 4th the right brachium pontis of H bridge, and full Foot:

Switch function is carried out permutation and combination and obtains Switch State Combination in Power Systems S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bValue, Obtain four H bridge cascaded inverter S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bAll 256 kinds of on off states；

Step 2, according to all 256 kinds of on off states of four H bridge cascaded inverters that step 1 is write out, according to maintaining light Volt battery parasitic capacitance voltage sum V_cFor power frequency sinusoidal quantity andRequirement, Select the on off state meeting requirement, form following Switch State Combination in Power Systems:

10101010-10100010-10110010-10011110-11110110-01100001-01001101- 01011101-01010101, this switches set amounts to comprise 9 kinds of on off states, every kind of corresponding output level of on off state；

Step 3, the 9 kinds of on off states selected according to step 2, calculate the output level V of inverter ac side_kValue, Wherein,Its value is [-4 ,-3 ,-2 ,-1,0,1,2,3,4] V_dc, have 9 kinds of level；

Step 4, on-line checking single-phase non-isolated cascaded H-bridges inverter is at the voltage V of k moment electrical network_g(k) and power network current I (k), and by the output voltage V of calculated for step 3 inverter ac side_kSubstitute into the discrete model letter of grid-connected current in the lump Number, it was predicted that go out power network current value i (k+1) in the k+1 moment, wherein, discrete model function meets following formula:

$i(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i\left(k\right)+\frac{T_s}{L}(V_k-V_g(k\left)\right)$

In formula, T_sBeing the sampling period of line voltage, L and R is respectively described single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter Inductance value and the resistance value of correspondence, V_kIt it is k moment inverter ac side output voltage；

Step 5, the line voltage V that step 4 on-line checking is obtained_gK () carries out phase-locked, obtain phase theta, θ calculate The sinusoidal quantity of one frequently homophase same with line voltage is as given value of current value i in kth+1 moment^*(k+1), and with step 4 obtain The power network current value of calculation i (k+1) in the k+1 moment substitute into valuation functions h together, valuation functions h that selects to send as an envoy to minimum one Individual output level；Wherein, valuation functions h=| i^*(k+1)-i(k+1)|；

Step 6, the output level selected according to step 5, from the Switch State Combination in Power Systems that step 2 has been selected, select correspondence On off state, and drive switching tube as output by drive circuit.

The present invention relative to the advantage of prior art is:

1, the suitability is strong, can not only suppression system leakage current effectively, and extend conveniently, it is adaptable to four H bridge modules The photovoltaic DC-to-AC converter of cascade.

2, need not by changing existing topology or using new topology to realize, the switch control proposed by the present invention System strategy just can realize the drain current suppressing of the single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter to four H bridges, is possible not only to fall The cost of low system and switching loss, and the power density of inverter can be improved.

Accompanying drawing explanation

Fig. 1 is the single-phase cascaded H-bridges photovoltaic DC-to-AC converter topological structure containing four H bridges.

Fig. 2 is the equivalent circuit of the single-phase cascaded H-bridges photovoltaic DC-to-AC converter containing four H bridges.

Fig. 3 is control block diagram based on Model Predictive Control suppression leakage current method.

Detailed description of the invention

In order to make the purpose of the present invention, technical scheme and advantage clearer, below in conjunction with the accompanying drawings and embodiment, right The present invention makees the most clearly and completely to describe.

Cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control disclosed by the invention is applicable to The leakage current of the tandem photovoltaic inverter of four H bridges of suppression.

Fig. 1 is the single-phase non-isolated cascaded H-bridges photovoltaic combining inverter topological structure described in this patent, comprises four H bridges, Each H bridge DC side is independently-powered by one piece or polylith photovoltaic battery panel, and each H bridge AC is serially connected, by difference Mould inductance L₁And L₂Being connected with electrical network, the feature of this cascaded H-bridges topology maximum is each H bridge AC can be exported superposition To form many level.Wherein, parameter C_pvk1And C_pvk2(k=1,2,3,4) is photovoltaic panel parasitic capacitance over the ground, this capacitance size Relevant with external factor such as the area of photovoltaic panel and weather, L₁And L₂For net side filter inductance；R₁And R₂For net side filter inductance Dead resistance；V_gFor AC point of common coupling voltage.

Fig. 2 is single-phase four module-cascade H bridge equivalent circuits, wherein C shown in Fig. 1_pvk=C_pvk1//C_pvk2.With power network current It is analyzed as a example by positive half period, and remembers net side filter inductance L₁=L₂, R₁=R₂。

Fig. 3 is the control block diagram based on Model Predictive Control that patent of the present invention proposes, and rate-determining steps includes: on-line checking Single-phase non-isolated cascaded H-bridges inverter is at the voltage V of k moment electrical network_g(k) and power network current i (k), and substitute into grid-connected current Discrete model function, it was predicted that go out power network current value i (k+1) in the k+1 moment；The line voltage obtaining on-line checking is phase-locked, Obtain phase theta, by θ calculate one with line voltage with frequency homophase and sinusoidal quantity that amplitude is A as the electric current in kth+1 moment Set-point i^*(k+1)；By given value of current value i in kth+1 moment^*(k+1) with power network current at the value of calculation i (k+1) in k+1 moment Substitute into valuation functions together, the output level that the valuation functions that selects to send as an envoy to is minimum；According to the output level selected, enter one Step selects the on off state of correspondence, and as the output switching tube by drive circuit cascaded H bridge photovoltaic DC-to-AC converter.

For the single-phase cascaded H-bridges photovoltaic DC-to-AC converter of shown in Fig. 1 four H bridges, disclosed by the invention based on model prediction control The basic step of the cascaded H-bridges photovoltaic inverter leakage current suppressing method of system is as follows:

See Fig. 1, Fig. 2, Fig. 3.

Step 1, if four module DC voltages of four H bridge cascaded inverters are identical, and is designated as V_dc, calculate four H bridges All 256 kinds of on off state (S of cascaded inverter_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4b) and the photovoltaic cell parasitism of correspondence Capacitance voltage sum V_cValue,

Wherein, V_gFor the voltage of electrical network, S_1aIt is the switch function of pipe, S on first left brachium pontis of H bridge_1bIt is that first H bridge is right The switch function of pipe, S on brachium pontis_2aIt is the switch function of pipe, S on second left brachium pontis of H bridge_2bIt is to manage on second right brachium pontis of H bridge Switch function, S_3aIt is the switch function of pipe, S on the 3rd the left brachium pontis of H bridge_3bIt it is the switch letter of pipe on the 3rd the right brachium pontis of H bridge Number, S_4aIt is the switch function of pipe, S on the 4th the left brachium pontis of H bridge_4bIt is the switch function of pipe on the 4th the right brachium pontis of H bridge, and full Foot:

Each switch function S_1a、S_1b、S_2a、S_2b、S_3a、S_3b、S_4a、S_4bValue be 0 or 1, the value of switch function is arranged Row group is incorporated as on off state S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bValue, totally 2⁸Kind, these are 2 years old⁸Just plant the value of on off state It is four all of 256 kinds of on off state (S of H bridge cascaded inverter_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4b)；

According to Fig. 2, can be able to be obtained by Kirchhoff's second law, parasitic capacitor voltage V_ckMeet following formula:

$\left\{\begin{array}{c}-V_{1a}+V_L+V_R+V_g+V_{c1}=0\\ -V_{4b}-V_L-V_R+V_{cn}=0\\ -V_{1b}+V_{2a}-V_{c2}+V_{c1}=0\\ -V_{2b}+V_{3a}-V_{c3}+V_{c2}=0\\ -V_{3b}+V_{4a}-V_{c4}+V_{c3}=0\end{array}\right.$

After arrangement, calculate parasitic capacitor voltage V_ckMeet following formula:

$V_{\mathrm{ck}}=\underset{i=1}{\overset{k-1}{\mathrm{\Σ}}}\frac{V_{\mathrm{DMi}}}{2}+V_{\mathrm{CMk}}-\underset{i=k+1}{\overset{4}{\mathrm{\Σ}}}\frac{V_{\mathrm{DMi}}}{2}-\frac{V_g}{2}$

Wherein, V_CMkAnd V_DMkRepresent common-mode voltage and the differential mode voltage of kth (k=1,2,3,4) individual module respectively；

According to the most defined switch function S_1a、S_1b、S_2a、S_2b、S_3a、S_3b、S_4a、S_4b, calculate four each brachium pontis of module Output terminals A₁、B₁、A₂、B₂、A₃、B₃、A₄And B₄Voltage V_1a、V_1b、V_2a、V_2b、V_3a、V_3b、V_4aAnd V_4bValue,

V_1a=S_1aV_dc,

V_1b=S_1bV_dc,

V_2a=S_2aV_dc,

V_2b=S_2bV_dc,

V_3a=S_3aV_dc,

V_3b=S_3bV_dc,

V_4a=S_4aV_dc,

V_4b=S_4bV_dc；

Finally, according to system parasitic capacitance voltage V obtained above_ck, system leakage current i_leakAnd four each bridges of module The output terminals A of arm₁、B₁、A₂、B₂、A₃、B₃、A₄And B₄Voltage V_1a、V_1b、V_2a、V_2b、V_3a、V_3b、V_4aAnd V_4bValue, calculate four Voltage sum V of individual module H bridge cascaded inverter parasitic capacitance_cValue,

$\begin{array}{c}{V}_c=\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\Σ}}\[4-2(i-1)\]S_{ia}-\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\Σ}}(4-2i)S_{ib}-2V_g\\ ={\mathrm{mV}}_{dc}-2V_g\end{array}$

Wherein, m meets following formula:

$m=\underset{i=1}{\overset{4}{\Σ}}\frac{\[4-2(i-1)\]}{2}{S}_{ia}-\underset{i=1}{\overset{4}{\Σ}}\frac{(4-2i)}{2}{S}_{ib}$

By the 256 kinds of on off state (S write out_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4b) it is calculated 256 kinds of photovoltaic electrics Pond parasitic capacitor voltage sum V_c；

Step 2, according to all 256 kinds of on off states of four H bridge cascaded inverters that step 1 is write out, according to maintaining light Volt battery parasitic capacitance voltage sum V_cFor power frequency sinusoidal quantity andRequirement, choose The on off state that the foot that is full requires, according to above-mentioned requirements, finally selects the 9 kinds of on off states meeting requirement, and composition is following to be opened Off status combines:

10101010-10100010-10110010-10011110-11110110-01100001-01001101- 01011101-01010101, every kind of corresponding output level of on off state；

9 kinds of on off states of four the H bridge cascaded inverters selected and voltage sum V of corresponding parasitic capacitance_cAnd m Value is as shown in table 1:

The on off state of 1 four modules of table and m value

Output voltage	S1a/S1b/S2a/S2b/S3a/S3b/S4a/S4b	m
			-4Vdc	0101 0101	2
-3Vdc	0101 1101	2
			-2Vdc	0100 1101	2
-Vdc	0110 0001	2
			0	1111 0110	2
Vdc	1001 1110	2
			2Vdc	1011 0010	2
3Vdc	1010 0010	2
			4Vdc	1010 1010	2

As shown in Table 1, the m value that these 9 kinds of on off states of selecting are corresponding is 2, this make parasitic capacitance voltage it And V_cPower frequency sinusoidal quantity can be kept constant, and meetTherefore, selection this 9 Plant on off state and can form Switch State Combination in Power Systems；

Step 3, the 9 kinds of on off states selected according to step 2, calculate the output voltage V of inverter ac side_k, wherein,Its value is [-4 ,-3 ,-2 ,-1,0,1,2,3,4] V_dc, have 9 kinds of level；

Step 4, on-line checking single-phase non-isolated cascaded H-bridges inverter is at the voltage V of k moment electrical network_g(k) and power network current I (k), and by the output voltage V of calculated for step 3 inverter ac side_kSubstitute into the discrete model letter of grid-connected current in the lump Number, it was predicted that go out power network current value i (k+1) in the k+1 moment, wherein, discrete model function meets following formula:

$i(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i\left(k\right)+\frac{T_s}{L}(V_k-V_g(k\left)\right)$

In formula, T_sBeing the sampling period of line voltage, L and R is respectively described single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter Inductance value and the resistance value of correspondence, and R=R₁+R₂, L=L₁+L₂, the value of sampling instant k along with controller operation and It is continuously increased, k=1,2,3 ...；

Step 5, the line voltage obtaining on-line checking is phase-locked, obtains phase theta, θ calculate one and line voltage With frequency homophase and sinusoidal quantity that amplitude is A as given value of current value i in kth+1 moment^*(k+1) electrical network, and with step 4 obtained The electric current value of calculation i (k+1) in the k+1 moment substitutes into valuation functions h together, selects one group of switch shape that valuation functions h is minimum State combines.Wherein, valuation functions h=| i^*(k+1)-i(k+1)|.Because only that make valuation functions minimum, guarantee electrical network electricity The tracking effect of stream is best, meets the elementary object of cutting-in control；

Step 6, the output level selected according to step 5, from the Switch State Combination in Power Systems that step 2 has been selected, select correspondence On off state, and drive switching tube as output by drive circuit.

Unlike existing modulation technique, the cascaded H-bridges photovoltaic inversion based on Model Predictive Control that the present invention proposes Device drain current suppressing method need not carrier wave and participates in modulation, but selected the method replacement of on off state by Model Predictive Control Carrier modulation.According to above-mentioned steps, can maintain photovoltaic cell parasitic capacitor voltage sum is power frequency sinusoidal quantity, thus realizes cascade H bridge photovoltaic inverter leakage current suppresses.

The method of cascaded H-bridges photovoltaic inverter leakage current based on Model Predictive Control disclosed by the invention is applied to four The tandem photovoltaic grid-connected system of H bridge.Based on embodiments of the invention, those skilled in the art is not making creative work On the premise of obtain other embodiments, the protection domain of this patent all should be belonged to.

Claims (1)

1. a cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control, including selecting on off state Combine and detect voltage and the electric current of electrical network, it is characterised in that key step is as follows:

Step 1, if four module DC voltages of four H bridge cascaded inverters are identical, and is designated as V_dc, and calculate correspondence with following formula Photovoltaic cell parasitic capacitor voltage sum V_cValue,

$V_c=\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\mathrm{\Σ}}}\[4-2\left(i-1\right)\]S_{ia}-\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\mathrm{\Σ}}}\left(4-2i\right)S_{ib}-2V_g$

Wherein, V_gFor the voltage of electrical network, S_1aIt is the switch function of pipe, S on first left brachium pontis of H bridge_1bIt is first right brachium pontis of H bridge The switch function of upper pipe, S_2aIt is the switch function of pipe, S on second left brachium pontis of H bridge_2bIt is the opening of pipe on second right brachium pontis of H bridge Close function, S_3aIt is the switch function of pipe, S on the 3rd the left brachium pontis of H bridge_3bIt is the switch function of pipe on the 3rd the right brachium pontis of H bridge, S_4aIt is the switch function of pipe, S on the 4th the left brachium pontis of H bridge_4bIt is the switch function of pipe on the 4th the right brachium pontis of H bridge, and meets:

Switch function is carried out permutation and combination and obtains Switch State Combination in Power Systems S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bValue, obtain Four H bridge cascaded inverter S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bAll 256 kinds of on off states；

Step 2, according to all 256 kinds of on off states of four H bridge cascaded inverters that step 1 is write out, according to maintaining photovoltaic electric Pond parasitic capacitor voltage sum V_cFor power frequency sinusoidal quantity andRequirement, select Meet the on off state required, form following Switch State Combination in Power Systems:

10101010-10100010-10110010-10011110-11110110-01100001-01001101-01011101- 01010101, this Switch State Combination in Power Systems comprises 9 kinds of on off states, every kind of corresponding output level of on off state altogether；

Step 3, the 9 kinds of on off states selected according to step 2, calculate the output level V of inverter ac side_kValue, wherein,Its value is [-4 ,-3 ,-2 ,-1,0,1,2,3,4] V_dc, have 9 kinds of level；

Step 4, on-line checking single-phase non-isolated cascaded H-bridges inverter is at the voltage V of k moment electrical network_g(k) and power network current i (k), And by the output voltage V of calculated for step 3 inverter ac side_kSubstitute into the discrete model function of grid-connected current in the lump, in advance Measuring power network current value i (k+1) in the k+1 moment, wherein, discrete model function meets following formula:

$i(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i\left(k\right)+\frac{T_s}{L}(V_k-V_g(k\left)\right)$

In formula, T_sBeing the sampling period of line voltage, L and R is respectively the electricity of described single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter Inductance value and the resistance value of correspondence, V_kIt it is k moment inverter ac side output voltage；

Step 5, the line voltage V that step 4 on-line checking is obtained_gK () carries out phase-locked, obtain phase theta, by θ calculate one with The sinusoidal quantity of line voltage same frequency homophase is as given value of current value i in kth+1 moment^*(k+1) electrical network, and with step 4 obtained The electric current value of calculation i (k+1) in the k+1 moment substitutes into valuation functions h together, the output that valuation functions h that selects to send as an envoy to is minimum Level；Wherein, valuation functions h=| i^*(k+1)-i(k+1)|；

Step 6, the output level selected according to step 5, from the Switch State Combination in Power Systems that step 2 has been selected, select corresponding opening Off status, and drive switching tube as output by drive circuit.