Modulation wave hybrid control strategy and system of cascaded grid-connected inverter

Abstract

The application discloses a modulation wave mixing control strategy and system of a cascade grid-connected inverter, belongs to the electrical engineering field, and is used for expanding the stable operation range of the cascade grid-connected inverter. The application aims at solving the problem of over-modulation of some units caused by uneven power of photovoltaic components of each H-bridge unit of the single-phase cascade grid-connected inverter. The steps include: controlling the direct-current voltage of all the H-bridge units according to the selected power; controlling the grid-connected current; when some H-bridge units are over-modulated, the corresponding modulation waves of each H-bridge unit are calculated according to the range of the maximum modulation degree. Compared with the prior art, the application expands the stable operation range of the cascade grid-connected inverter and improves the ability of the cascade grid-connected inverter to cope with the uneven power of photovoltaic power.

Description

TECHNICAL FIELD

[0001] The application belongs to the photovoltaic power generation technology in the field of electrical engineering, and particularly relates to a modulation wave hybrid control strategy and system of a cascaded grid-connected inverter. BACKGROUND

[0002] In order to realize large-scale photovoltaic grid connection, the capacity of grid-connected converters is gradually expanded, which puts forward higher requirements on photovoltaic utilization rate and grid-connected waveform quality. The single-phase cascaded grid-connected inverter has a modular structure and has the following advantages: easy to flexibly expand to adapt to different power and voltage level requirements; independent DC source of each module, which can realize multi-path MPPT, even component-level MPPT, and high photovoltaic utilization rate; low harmonic content of output multi-level ladder wave, simple and low-cost filter design. Therefore, it becomes a new scheme for photovoltaic power generation grid connection. However, the photovoltaic panels on the independent DC side of the single-phase cascaded grid-connected inverter may be blocked by clouds and fallen leaves, and the power output characteristics change. There is a power difference between the modules, and the module with relatively high power is difficult to completely transmit the DC side power, which is prone to over-modulation, affecting the quality of grid-connected current. There are related researches on this problem. Literature“A.Eskandari, V. Javadian, H. Iman-Eini and M. Yadollahi, "Stable operation of grid connected Cascaded H-Bridge inverter under unbalanced insolation conditions," 2013 3rd International Conference on Electric Power and Energy Conversion Systems, Istanbul, Turkey, 2013, pp. 1-6, doi: 10.1109 / EPECS.2013.6713006.”(A.Eskandari, V. Javadian, H. Iman Eini and M. Yadollahi, "Stable operation of grid connected Cascaded H-Bridge inverter under unbalanced insolation conditions," 2013 3rd International Conference on Electric Power and Energy Conversion Systems, Istanbul, Turkey, 2013, pp. 1-6, doi: 10.1109 / EPECS.2013.6713006) proposes an improved MPPT algorithm, which makes the over-modulation module exit the MPPT operation, reduces the power that needs to be transmitted from the source, and avoids over-modulation. However, this method loses the power generation of the system, the photovoltaic utilization rate is not high, and the adjustment speed is slow, which is limited by the MPPT step length.

[0003] The document“Zhao T, Zhang X, Mao W, Xu J, Gu YL, Zhao DY, Jiang C. Power imbalance control strategy for cascaded H-bridge photovoltaic inverter based on reactive power compensation[J]. Proceedings of the CSEE, 2017, 37(17): 5076-5085+5227” proposes a reactive power compensation control strategy, which reduces the total modulation voltage amplitude by compensating the reactive angle, thereby avoiding HB over-modulation to some extent. However, this method injects reactive power into the grid and cannot work at unity power factor, which does not meet the grid connection requirements.

[0004] The document“M. Moosavi, G. Farivar, H. Iman-Eini, and S. M. Shekarabi, “A voltage balancing strategy with extended operating region for cascaded H-bridge converters,” IEEE Trans. Power Electron., vol. 29, no. 9, pp. 5044-5053, Sep. 2014” proposes a hybrid modulation strategy that can ensure unity power factor grid connection conditions, match photovoltaic power and module transmission power, reduce the risk of over-modulation, and expand the linear modulation range of single-phase cascaded grid-connected inverters to 1.273. However, unlike the traditional carrier phase-shifted modulation which uses a PI regulator to accurately control the DC side voltage, the hybrid modulation strategy determines the charge and discharge state of the DC side capacitor according to the operating requirements, which makes it difficult to accurately control the DC side voltage of each module, is not conducive to the implementation of MPPT, and will lose photovoltaic power generation as the input power difference further expands.

[0005] In summary, the prior art still has the following problems:

[0006] 1) The improved MPPT method is not conducive to improving photovoltaic utilization;

[0007] 2) The power balance control strategy based on reactive power compensation cannot achieve unity power factor grid connection;

[0008] 3) The hybrid modulation strategy is difficult to accurately control the DC side voltage of each module, which is not conducive to the efficiency of MPPT, and the linear modulation range of single-phase cascaded grid-connected inverters is limited to 1.273. SUMMARY

[0009] To overcome the limitations of the above scheme, the application provides a modulation wave hybrid control strategy and system for expanding the stable operation range of a cascaded grid-connected inverter, which can accurately control the DC side voltage of each module when the photovoltaic power is unbalanced, further improving the ability of a single-phase cascaded grid-connected inverter to cope with the degree of unbalanced photovoltaic power.

[0010] To solve the technical problems of the application, the application provides a modulation wave hybrid control strategy for a cascaded grid-connected inverter, the grid-connected inverter being a single-phase inverter, containing N identical H-bridge units, any one of the N H-bridge units being denoted as H-bridge unit HB j , j = 1, 2,..., N, N being a positive integer greater than 1; a capacitor C j and a photovoltaic module PV j are connected in parallel on the DC side of each H-bridge unit HB j ; the AC side of the N H-bridge units HB j is output in series, then connected to the grid through a filter inductor L s .

[0011] The control strategy is characterized in that the steps are as follows:

[0012] S1, obtaining the DC voltage V dcj and output current I PVj of the N photovoltaic modules PV j ; taking a DC voltage reference value

[0013] S2, controlling the DC voltage V dcj and the DC voltage reference value through a voltage regulator to obtain the voltage regulator output I j , multiplying the DC voltage V dcj and the voltage regulator output I j to obtain the control output power P j of the H-bridge unit HB cj , and denoted as selected power 1;

[0014] S3, obtaining the grid voltage amplitude V gM and grid voltage phase angle θ according to the sampled grid voltage v g ; obtaining the grid current d-axis component I d and grid current q-axis component I q according to the sampled grid current i g ;

[0015] S4, obtaining the grid current d-axis component reference value I drefand grid current d-axis component I d , to obtain the d-axis modulation voltage U d of the inverter; and according to the grid current q-axis component reference value I qref and grid current q-axis component I q , to obtain the q-axis modulation voltage U q of the inverter;

[0016] S5, according to the d-axis modulation voltage U d of the inverter and the q-axis modulation voltage U q of the inverter, to obtain the total modulation voltage amplitude V r of the inverter, the total modulation voltage amplitude V r of the inverter, and the angle δ between the total modulation voltage amplitude V j of the inverter and the grid voltage;

[0017] S6, to obtain the reference power P bj of the H-bridge unit HB j , and to record it as the selected power 2; according to the size relationship between the selected power 1 of the H-bridge unit HB j and the selected power 2 of the H-bridge unit HB j , to generate the selected power P j of the H-bridge unit HB dcj ;

[0018] S7, according to the DC voltage V r , the total modulation voltage amplitude V j of the inverter, and the selected power P j of the H-bridge unit HB j , to obtain the modulation degree M j of the H-bridge unit HB j ;

[0019] S8, to obtain the over-modulation unit and the non-over-modulation unit in the N H-bridge units, to perform modulation wave calculation and generation:

[0020] For the over-modulation unit, the modulation degree M j of the H-bridge unit HB j , the angle δ, and the grid voltage phase angle θ are used to calculate the modulation voltage m Rj of the H-bridge unit HB j , and then according to the modulation voltage m Rj of the H-bridge unit HB j , the modulation degree M j of the H-bridge unit HB j , the angle δ, and the grid voltage phase angle θ, to calculate the compensation harmonic hf j of the H-bridge unit HB hojmaxThe compensation is calculated for the H-bridge unit HB in the non-overmodulation unit. j The harmonics, and denoted as the reverse compensated harmonics ho j And calculate the H-bridge unit HB in the non-overmodulation unit. i modulation voltage m Rj .

[0021] Preferably, the DC voltage reference value in S1 The implementation method is as follows:

[0022] DC voltage V dcj and output current I PVj Multiplying yields the H-bridge element HB. j Photovoltaic power generation P PVj Regarding the photovoltaic power generation P Pvj Maximum power point tracking control is performed to obtain the H-bridge unit HB. j The maximum power point voltage of the H-bridge unit HB j The maximum power point voltage is used as the H-bridge unit HB j The reference value of the DC voltage is denoted as the DC voltage reference value.

[0023] Preferably, the voltage regulator output I in S2 j The expression is as follows:

[0024]

[0025] Where, k vP k is the proportional coefficient of the voltage regulator. vI is the integral coefficient of the voltage regulator, and s is the Laplace operator.

[0026] Preferably, in S3, the sampled grid voltage v g The grid voltage amplitude V is obtained through a phase-locked loop (PLL) circuit. gM and the grid voltage phase angle θ; the grid-connected current i g The α-axis component i of the grid-connected current is obtained by performing SOGI calculation. α The grid-connected current β-axis component iβ is obtained by performing Park transformation to obtain the grid-connected current d-axis component I. d and the q-axis component of the grid-connected current I q The transformation formulas for the Park transformation are as follows:

[0027]

[0028] Wherein, sinθ represents the sine value of the grid voltage phase angle θ, and cosθ represents the cosine value of the grid voltage phase angle θ.

[0029] Preferably, the grid-connected current q-axis component reference value I qref =0, the grid-connected current d-axis component reference value I dref is calculated by delaying one beat, and the calculation formula of the grid-connected current d-axis component reference value I dref is as follows:

[0030]

[0031] wherein P j is the selected power of the H-bridge unit HB j in the last control period;

[0032] The grid-connected current d-axis component reference value I dref and the grid-connected current d-axis component I d are controlled by a current regulator, and the grid voltage amplitude V gM is fed back, so as to obtain the d-axis modulation voltage U d of the inverter.

[0033] The grid-connected current q-axis component reference value I qref and the grid-connected current q-axis component I q are controlled by a current regulator, so as to obtain the q-axis modulation voltage U q of the inverter, and the calculation formulas of the d-axis modulation voltage U d and the q-axis modulation voltage U q are as follows:

[0034]

[0035] wherein k iP is the proportional coefficient of the current regulator, k iI is the integral coefficient of the current regulator, and s is the Laplace operator.

[0036] Preferably, the calculation formulas of the total modulation voltage amplitude V r and the included angle δ of the inverter are as follows:

[0037]

[0038] wherein arctan(U q / U d ) represents the inverse tangent value of U q / U d .

[0039] Preferably, the selected power 2, i.e., the reference power P bj , is calculated according to the following formula:

[0040]

[0041] P j is the selected power of the H-bridge unit HB j in the last control cycle;

[0042] Preferably, S6 calculates the reference power P bj as follows:

[0043]

[0044] P j is the selected power of the H-bridge unit HB j in the last control cycle;

[0045] The selected power P j of the H-bridge unit HB j is selected as follows:

[0046]

[0047] Preferably, S7 calculates the modulation degree M j of the H-bridge unit HB j as follows:

[0048]

[0049] Preferably, in S8, the H-bridge unit HB j with M i > 1 is an over-modulation unit, and the H-bridge unit HB j with M i ≤ 1 is a non-over-modulation unit.

[0050] The specific process of the modulation wave calculation of the over-modulation unit is as follows:

[0051] First, judge the range of the maximum value max(M j ) of the modulation degree M j of the N H-bridge units HB j , and according to the modulation degree M j , the included angle δ and the grid voltage phase angle θ of the H-bridge unit HB j , calculate the modulation voltage m i of the H-bridge unit HB Rj in the over-modulation unit:

[0052] If max(M j ) ≤ 1.155,

[0053]

[0054] If max(M j ) > 1.155,

[0055]

[0056] wherein, is the trigger angle of the H-bridge unit HB j , which is obtained by inverse solution using the following equation:

[0057]

[0058] The compensation harmonic hf j of the H-bridge unit HB j is calculated again, and the calculation formula is as follows:

[0059] hf j = m Rj - M j cos(θ+δ)

[0060] Let the compensation harmonic hf j of the H-bridge unit HB j of all non-overmodulation units be 0, and calculate the total harmonic hf

[0061]

[0062] The specific process of calculating the modulation wave of the non-overmodulation unit is as follows:

[0063] The margin V j of the H-bridge unit HB hojmax in the non-overmodulation unit for injecting reverse harmonic is calculated, and the calculation formula is as follows:

[0064] V hojmax = 1-M j

[0065] Let the margin V j of the H-bridge unit HB hojmax of all overmodulation units for injecting reverse harmonic be 0, calculate the harmonic compensated to the H-bridge unit HB j in the non-overmodulation unit, and mark it as reverse compensation harmonic ho j , and the calculation formula is as follows:

[0066]

[0067] The modulation voltage m i of the H-bridge unit HB Rj in the non-overmodulation unit is calculated, and the calculation formula is as follows:

[0068] m Rj = M j cos(θ+δ)+ho j .

[0069] The application also provides a modulation wave hybrid control system of a cascade grid-connected inverter, comprising:

[0070] N same H-bridge units, each H-bridge unit HB j is parallel with a capacitor C j and a photovoltaic component PVj ， N H-bridge units HB j are serially connected on the AC side, and then output through a filter inductor L s and are connected into a power grid;

[0071] a sampling module for sampling a direct current V dcj and an output current I PVj ;

[0072] a control module for controlling the direct current V dcj and a direct current reference value and obtaining a voltage regulator output I j ;

[0073] a calculation module for obtaining a grid voltage amplitude V g and a grid voltage phase angle θ according to the sampled grid voltage v gM ; a calculation module for obtaining a grid current d-axis component I g and a grid current q-axis component I d according to the sampled grid current i q ;

[0074] a calculation module for obtaining an inverter d-axis modulation voltage U dref according to a grid current d-axis component reference value I d and the grid current d-axis component I d ; a calculation module for obtaining an inverter q-axis modulation voltage U qref according to a grid current q-axis component reference value I q and the grid current q-axis component I q ;

[0075] a calculation module for obtaining a total modulation voltage amplitude V r of the inverter, and an angle δ between the total modulation voltage amplitude V r of the inverter and the grid voltage;

[0076] a calculation selection module for generating a selected power P j of the H-bridge unit HB j ;

[0077] a calculation module for obtaining a modulation degree M j of the H-bridge unit HB j ;

[0078] a processing module for modulation wave calculation and generation of the over-modulation unit and the non-over-modulation unit in each H-bridge unit;

[0079] and a microprocessor and a memory, each of the modules, the microprocessor being programmed or configured to perform the steps of the modulation wave hybrid control strategy of the cascaded grid-connected inverter according to any one of claims 1-9.

[0080] The application also provides a computer readable storage medium having stored therein a computer program programmed or configured to perform the modulation wave hybrid control strategy of the cascaded grid-connected inverter according to any one of claims 1-9.

[0081] The application has the following advantages over the prior art:

[0082] 1. When the photovoltaic power of the H-bridge unit is unbalanced, the system can still operate at a unit power factor without reducing the photovoltaic power generation, and directly control the DC side voltage, thereby ensuring the MPPT efficiency.

[0083] 2. The application further improves the ability of the single-phase cascaded grid-connected inverter to cope with the unbalanced degree of photovoltaic power. BRIEF DESCRIPTION OF DRAWINGS

[0084] Figure 1 is the main circuit topology structure of the single-phase cascaded grid-connected inverter in the embodiment of the application.

[0085] Figure 2 is the control block diagram of the single-phase cascaded grid-connected inverter in the embodiment of the application.

[0086] Figure 3 is the hybrid modulation wave calculation sub-module in the control block diagram of the single-phase cascaded grid-connected inverter in the embodiment of the application.

[0087] Figure 4 is the grid voltage v g and the grid current i g simulation waveform when the power of each H-bridge unit is unbalanced and the conventional control strategy is used.

[0088] Figure 5 is the grid voltage v g and the grid current i g simulation waveform when the power of each H-bridge unit is unbalanced and the modulation wave hybrid control strategy of the cascaded grid-connected inverter is used. DETAILED DESCRIPTION

[0089] The application will be described in detail below with reference to the accompanying drawings and embodiments.

[0090] Figure 1is a single-phase cascaded grid-connected inverter main circuit topology in the embodiment of the present application. As can be seen from the figure, the single-phase cascaded grid-connected inverter comprises N identical H-bridge units, and any one of the N H-bridge units is denoted as H-bridge unit HB j , j = 1, 2, …, N, N is a positive integer greater than 1; one capacitor C j and one photovoltaic module PV j are connected in parallel at the DC side of each H-bridge unit HB j ; the AC side of the N H-bridge units HB j is connected in series, and then connected to the grid through a filter inductor L s .

[0091] Specifically, each H-bridge unit HB j comprises four switching tubes with reverse-parallel diodes, denoted as switching tube S j1 , switching tube S j2 , switching tube S j3 and switching tube S j4 , wherein the emitter of switching tube S j1 and the collector of switching tube S j2 are connected in series to form an a bridge arm of the H-bridge unit HB j , and the junction of the emitter of switching tube S j1 and the collector of switching tube S j2 is denoted as an a bridge arm midpoint ja; the emitter of switching tube S j3 and the collector of switching tube S j4 are connected in series to form a b bridge arm of the H-bridge unit HB j , and the junction of the emitter of switching tube S j3 and the collector of switching tube S j4 is denoted as a b bridge arm midpoint jb. The a bridge arm midpoint 1a of the H-bridge unit HB1 is connected in series with the filter inductor L s and then connected to the single-phase grid; the a bridge arm midpoint ja of the H-bridge unit HB j is connected in series with the b bridge arm midpoint (j-1)b of the H-bridge unit HB j-1 , the b bridge arm midpoint jb of the H-bridge unit HB j is connected in series with the a bridge arm midpoint (j+1)a of the H-bridge unit HB j+1 , j = 2, 3, …, N-1; and the b bridge arm midpoint Nb of the H-bridge unit HB N is connected to the grid ground.

[0092] Figure 2 is a control block diagram of the single-phase cascaded grid-connected inverter in the embodiment of the present application. Figure 3 is a hybrid modulation wave calculation submodule in the control block diagram of the single-phase cascaded grid-connected inverter in the embodiment of the present application. As can be seen from the figure, Figure 2 , Figure 3It can be seen that the modulation wave hybrid control strategy of the cascade grid-connected inverter of the application is used to expand the stable operation range of the cascade grid-connected inverter, and comprises DC bus voltage control, grid-connected current control, reference power selection and modulation wave generation, and the steps are as follows:

[0093] I. DC bus voltage control

[0094] Step 1, the output voltage of N photovoltaic components PV j is sampled, and the sampling value is recorded as DC voltage V dcj , the output current of N photovoltaic components PV j is sampled, and the sampling value is recorded as output current I PVj , DC voltage V dcj is multiplied by output current I PVj to obtain the photovoltaic power P j of H-bridge unit HB PVj , I PVj = V dcj I PVj .

[0095] Step 2, the photovoltaic power P j of H-bridge unit HB PVj is subjected to maximum power point tracking control to obtain the maximum power point voltage of H-bridge unit HB j , the maximum power point voltage of the H-bridge unit HB j is taken as the reference value of the DC voltage of the H-bridge unit HB j , and is recorded as DC voltage reference value

[0096] In the attached Figure 2 , the maximum power point tracking control is recorded as MPPT control.

[0097] Or in the range of variation of the DC voltage V j of the photovoltaic component PV dcj , a constant is set as the DC voltage reference value, recorded as The setting mode of the DC voltage reference value is not drawn in the attached Figure 2 .

[0098] Step 3, the DC voltage V dcj and the DC voltage reference value are controlled through a voltage regulator to obtain voltage regulator output I j , the DC voltage V dcj and the voltage regulator output I j are multiplied to obtain the control output power P j of H-bridge unit HB cj , Pcj = V dcj I j , and is recorded as the selected power 1.

[0099] In the embodiment, the voltage regulator outputs I j , and the expression is as follows:

[0100]

[0101] wherein k vP is the proportional coefficient of the voltage regulator, k vI is the integral coefficient of the voltage regulator, and s is the Laplace operator. In the embodiment, k vP = 5.

[0102] II. Grid-connected current control

[0103] Step 4, the grid voltage v g is sampled, and the grid voltage v g is passed through a phase-locked loop (PLL) link to obtain the grid voltage amplitude V gM and the grid voltage phase angle θ; the grid-connected current i g is sampled, and the grid-connected current i g is subjected to SOGI operation to obtain the grid-connected current α-axis component i α and the grid-connected current β-axis component i β , and then park transformation is performed to obtain the grid-connected current d-axis component I d and the grid-connected current q-axis component I q .

[0104] In the embodiment, the transformation formula of the park transformation is as follows:

[0105]

[0106] wherein sinθ represents the sine value of the grid voltage phase angle θ, and cosθ represents the cosine value of the grid voltage phase angle θ.

[0107] Step 5, the grid-connected current q-axis component reference value I qref = 0, and the grid-connected current d-axis component reference value I dred is calculated by using the method of delaying one beat;

[0108] In the embodiment, the calculation formula of the grid-connected current d-axis component reference value I dref is as follows:

[0109]

[0110] wherein P j is the H-bridge unit HB jSelect power.

[0111] Step 6, set the reference value I of the d-axis component of the grid-connected current. dred and the d-axis component of the grid-connected current I d Controlled by a current regulator, the grid voltage amplitude V gM After feedforward, the d-axis modulation voltage U of the inverter is obtained. d ; The reference value I of the q-axis component of the grid-connected current qred and the q-axis component of the grid-connected current I q The q-axis modulation voltage U of the inverter is obtained through current regulation. q ;

[0112] In this embodiment, the d-axis modulation voltage U d and q-axis modulation voltage U q The specific calculations are as follows:

[0113]

[0114] Where, k iP k is the proportional coefficient of the current regulator. iI Let be the integral coefficient of the current regulator, and s be the Laplace operator. In this embodiment, k is taken as... iP =1,k iI =50.

[0115] Step 7, based on the inverter's d-axis modulation voltage U d and the q-axis modulation voltage U of the inverter q The total modulation voltage amplitude V of the inverter is calculated. r The total modulation voltage amplitude V of the inverter r The angle δ between the voltage and the grid voltage is denoted as angle δ.

[0116] In this embodiment, the total modulation voltage amplitude V of the inverter r The formulas for calculating the included angle δ are as follows:

[0117]

[0118] Among them, arctan(U q / U d ) represents U q / U d The arctangent value.

[0119] III. Reference Power Selection

[0120] Step 8: Calculate the H-bridge element HB using the one-time delay method. j The reference power P bj And as the selected power 2; according to the H-bridge unit HBj Control output power P cj With H-bridge unit HB j The reference power P bj The size relationship between them generates the H-bridge element HB. j Select power P j .

[0121] In this embodiment, the reference power P bj The calculation formula is as follows:

[0122]

[0123] Among them, P j For the H-bridge unit HB of the previous control cycle j Select power;

[0124] The H-bridge unit HB j Select power P j The options are as follows:

[0125]

[0126] Step 9, based on the DC voltage V dcj The total modulation voltage amplitude V of the inverter r and H-bridge unit HB j Select power P j The H-bridge element HB was calculated. j The adjustment system M j .

[0127] In this embodiment, the H-bridge unit HB j The adjustment system M j The calculation formula is as follows:

[0128]

[0129] IV. Modulation Wave Generation

[0130] Step 10, in N H-bridge units, M j H-bridge element HB > 1 i For overmodulation units, all overmodulation units proceed to step 11, M j H-bridge element HB ≤1 i For non-overmodulation units, all non-overmodulation units proceed to step 12; the H-bridge unit HB... i The modulation voltage is denoted as m Rj .

[0131] Step 11, Calculation of modulated wave through modulation unit

[0132] Determine the HB values ​​of N H-bridge units jThe adjustment system M j The maximum value in the middle is max(M) j The range of ) and based on the H-bridge unit HB j The adjustment system M j The H-bridge element HB is obtained by calculating the included angle δ and the grid voltage phase angle θ. j modulation voltage m Rj Then, based on the H-bridge unit HB... j modulation voltage m Rj H-bridge unit HB j The adjustment system M j The calculation of the included angle δ and the grid voltage phase angle θ yields the H-bridge unit HB in the overmodulation unit. j Compensation harmonics hf j ;

[0133] Let all H-bridge units HB that did not enter step 11 j Compensation harmonics hf j All values ​​are 0, and the total harmonic hf compensated by all overmodulation units is calculated.

[0134] In this embodiment, the specific process of calculating the modulated wave by the overmodulation unit is as follows:

[0135] First, determine the N H-bridge units HB. j The adjustment system M j The maximum value in the middle is max(M) j The range of ) and the H-bridge unit HB in the modulation unit were calculated. i modulation voltage m Rj :

[0136] If max(M) j )≤1.155,

[0137]

[0138] If max(M) j () > 1.155,

[0139]

[0140] in, H-bridge element HB j The trigger angle can be obtained by inverse solving the following formula:

[0141]

[0142] Then calculate the H-bridge element HB. j Compensation harmonics hf j The calculation formula is as follows:

[0143] hf j =mRj -M j cos(θ+δ)

[0144] Let all H-bridge units HB j that have not entered step 11 have their compensation harmonics hf j equal to 0, and calculate the total harmonics hf compensated by all over-modulation units:

[0145]

[0146] Step 12, non-over-modulation unit modulation wave calculation

[0147] Let the margin of the injected reverse harmonics of H-bridge units HB j in the non-over-modulation unit be margin V hojmax , V hojmax =1-M j , let all H-bridge units HB j that have not entered step 12 have their margin V hojmax =0, and calculate the harmonics compensated to H-bridge units HB j in the non-over-modulation unit according to the total harmonics hf compensated by all over-modulation units and margin V hojmax , and record them as reverse compensation harmonics ho j , and calculate the modulation voltage m Rj of H-bridge units HB i in the non-over-modulation unit.

[0148] In this embodiment, the specific process of the non-over-modulation unit modulation wave calculation is as follows:

[0149] Calculate the margin V hojmax of the injected reverse harmonics of H-bridge units HB j in the non-over-modulation unit, and its calculation formula is as follows:

[0150] V hojmax =1-M j

[0151] Let all H-bridge units HB j that have not entered step 12 have their margin V hojmax of the injected reverse harmonics equal to 0, and calculate the harmonics compensated to H-bridge units HB j in the non-over-modulation unit, and record them as reverse compensation harmonics ho i , and its calculation formula is as follows:

[0152]

[0153] Calculate the modulation voltage m Rj of H-bridge units HB i in the non-over-modulation unit, and its calculation formula is as follows:

[0154] m Rj =M j cos(θ+δ)+ho j 。

[0155] To prove the effect of the application, the application is simulated, and Figure 4 and Figure 5 are obtained.

[0156] Figure 4 is the grid voltage v g and the grid current i g waveform when the conventional control strategy is used under the condition of power imbalance of each H-bridge unit. Before the simulation time 1.2s, the light intensity received by the DC side photovoltaic cell panel of the five H-bridge units is 1000W / m 2 , 1000W / m 2 , 600W / m 2 , 500W / m 2 and 450W / m 2 , and the temperature is 25℃. After the simulation time 1.2s, the light intensity received by the DC side photovoltaic cell panel of the five H-bridge units is 1000W / m 2 , 1000W / m 2 , 400W / m 2 , 350W / m 2 and 300W / m 2 , and the temperature is 25℃. In this embodiment, the photovoltaic module outputs the maximum power when the light intensity is 1000W / m 2 and the temperature is 25℃, and the maximum power is 150W, and the maximum power point voltage is 33.2V. Obviously, when the light intensity of the photovoltaic cell panel of each H-bridge unit is imbalanced, power imbalance will occur, over-modulation will easily occur, and grid current waveform distortion or even oscillation overcurrent will be caused.

[0157] Figure 5 and Figure 4 the simulation waveforms of the grid voltage v g and the grid current i g when the modulation wave hybrid control strategy of the cascaded grid-connected inverter is used under the same configuration condition of the photovoltaic cell panel and the same change setting of the light intensity. It can be seen that the quality of the grid current waveform is good.

[0158] The application further provides a modulation wave hybrid control system of a cascaded grid-connected inverter, which comprises:

[0159] N identical H-bridge units, and each H-bridge unit HB j is parallel connected with a capacitor C j and a photovoltaic module PV jN H-bridge units HB j The AC side output is connected in series, and the filter inductor L s is connected to the grid; a module for obtaining the DC voltage V dcj and the output current I PVj ; a sampling module for obtaining the DC voltage V dcj and the output current I PVj ; a control module for controlling the DC voltage V dcj and the DC voltage reference value and obtaining the voltage regulator output I j ; a calculation module for obtaining the grid voltage amplitude V g and the grid voltage phase angle θ according to the sampled grid voltage v gM ; a calculation module for obtaining the grid current d-axis component I g and the grid current q-axis component I d according to the sampled grid current i q ; a calculation module for obtaining the inverter d-axis modulation voltage U dref according to the grid current d-axis component reference value I d and the grid current d-axis component I d ; a calculation module for obtaining the inverter q-axis modulation voltage U qref according to the grid current q-axis component reference value I q and the grid current q-axis component I q ; a calculation module for calculating the total modulation voltage amplitude V r of the inverter, the angle δ between the total modulation voltage amplitude V r of the inverter and the grid voltage; a calculation selection module for generating the selection power P j of the H-bridge unit HB j ; a calculation module for obtaining the modulation degree M j of the H-bridge unit HB j ; a processing module for modulation wave calculation and generation of the over-modulation unit and the non-over-modulation unit in each H-bridge unit; and a microprocessor and a memory, each of the modules, the microprocessor being programmed or configured to perform the steps of the modulation wave hybrid control strategy of the cascaded grid-connected inverter according to any one of claims 1-9.

[0160] The application also provides a computer readable storage medium, wherein the computer readable storage medium stores a computer program which is programmed or configured to perform the modulation wave hybrid control strategy of the cascaded grid-connected inverter according to any one of claims 1-9.

[0161] The circuit topology, control strategy and method of the present application can be considered as a hardware embodiment of the circuit topology alone, a software embodiment of the control strategy and method alone, or a software and hardware combined embodiment based on the circuit topology and the control strategy and method. Moreover, the control strategy and method of the present application can be implemented on one or more computer usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer usable program codes, in the form of a computer program product; and can be implemented in various computer languages, such as object-oriented programming language Java and direct interpretation script language JavaScript, etc.

[0162] Furthermore, embodiments of the present application are described in connection with flow diagrams and / or block diagrams, which can be understood as each respective flow and / or block in the flow diagrams and / or block diagrams of the present application, and the combination of the flows and / or blocks in the flow diagrams and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general purpose computer, a special purpose computer, an embedded processor, or other programmable data processing apparatus to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing apparatus produce an apparatus that implements the functions specified in the flow Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks. These computer program instructions can also be stored in a computer readable memory that can direct the computer or other programmable data processing apparatus to work in a specific manner, so that the instructions stored in the computer readable memory produce a manufactured product including instruction apparatus that implements the functions specified in the flow Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks. These computer program instructions can also be loaded into a computer or other programmable data processing apparatus, so that a series of operation steps are performed on the computer or other programmable data processing apparatus to produce a computer implemented process, so that the instructions executed on the computer or other programmable data processing apparatus provide a process for implementing the functions specified in the flow Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks. These computer program instructions can also be loaded into a computer or other programmable data processing apparatus, so that a series of operation steps are performed on the computer or other programmable data processing apparatus to produce a computer implemented process, so that the instructions executed on the computer or other programmable data processing apparatus provide a process for implementing the functions specified in the flow

[0163] Therefore, the above is only the preferred embodiment of the present application, and does not limit the present application. The modifications and changes made by those skilled in the art based on the specific embodiments of the present application and the above description should be considered as equivalent solutions of the present application, and should fall within the protection scope of the present application.

Claims

1. A modulation wave hybrid control strategy of a cascaded grid-connected inverter, the grid-connected inverter being a single-phase inverter, comprising N identical H-bridge units, any one of the N H-bridge units being denoted as H-bridge unit HB j , , N being a positive integer greater than 1; each H-bridge unit HB j has a capacitor C j and a photovoltaic module PV j connected in parallel at the DC side of the H-bridge unit HB j ; the AC sides of the N H-bridge units HB s are connected in series, and the series connection is connected to a grid through a filter inductor L. characterized in that The control strategy steps are as follows: S1, obtain N photovoltaic modules PV j DC voltage and output current Take a DC voltage reference value. ; S2, DC voltage and DC voltage reference value Controlled by a voltage regulator, the output of the voltage regulator is obtained. DC voltage and voltage regulator output Multiplying yields the H-bridge element HB. j Control output power And denoted as Select Power 1; S3, the grid voltage obtained from the sampling , the grid voltage amplitude and the grid voltage phase angle ; According to the sampled grid-connected current , a grid-connected current d-axis component and a grid-connected current q-axis component are obtained; S4, the grid-connected current d-axis component reference value and the grid-connected current d-axis component to obtain the d-axis modulation voltage of the inverter ; and according to the grid-connected current q-axis component reference value and the grid-connected current q-axis component to obtain the q-axis modulation voltage of the inverter ; S5, calculating total modulation voltage amplitude of the inverter and q-axis modulation voltage of the inverter , calculating total modulation voltage amplitude of the inverter total modulation voltage amplitude of the inverter angle with grid voltage ; S6, obtaining a reference power of the H-bridge unit HB j of the H-bridge unit HB , and denoted as selected power 2; according to a size relationship between the selected power 1 of the H-bridge unit HB j and the selected power 2 of the H-bridge unit HB j , generating a selected power of the H-bridge unit HB j ;​ S7, the direct voltage is selected according to the direct voltage , the total modulation voltage amplitude of the inverter and the H-bridge unit HB j , the selected power , the modulation degree of the H-bridge unit HB j is calculated ; S8, obtaining over-modulation units and non-over-modulation units in the N H-bridge units, and performing modulation wave calculation and generation: For overmodulation units, H-bridge unit HB j The adjustment system Angle Phase angle with grid voltage The H-bridge element HB was calculated. j modulation voltage Then, based on the H-bridge element HB j modulation voltage H-bridge unit HB j The adjustment system Angle Phase angle with grid voltage The calculation yields the H-bridge unit HB in the overmodulation unit. j Compensation harmonics ; And the total harmonic compensated by the over-modulation unit for the non-over-modulation unit And the margin The harmonic of the H-bridge unit HB j in the non-over-modulation unit is calculated and recorded as the reverse compensation harmonic The modulation voltage of the H-bridge unit HB i in the non-over-modulation unit is calculated ; Specifically, the H-bridge unit HB in S8 i is an over-modulation unit, the H-bridge unit HB i is a non-over-modulation unit; The specific process of modulation wave calculation of the over-modulation units is as follows: First, determine the N H-bridge units HB. j The adjustment system maximum value The range, and according to the H-bridge unit HB j The adjustment system Angle Phase angle with grid voltage The H-bridge unit HB in the modulation unit has been calculated. i modulation voltage : If , If > 1.155, wherein, H-bridge unit HB j trigger angle of the H-bridge unit HB.

2. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, characterized in that, The direct current voltage reference value S1 The implementation is as follows: DC voltage and output current Multiplying yields the H-bridge element HB. j Photovoltaic power generation Regarding photovoltaic power generation Maximum power point tracking control is performed to obtain the H-bridge unit HB. j The maximum power point voltage of the H-bridge unit HB j The maximum power point voltage is used as the H-bridge unit HB j The reference value of the DC voltage is denoted as the DC voltage reference value. .

3. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, S2 the voltage regulator output The expression is as follows: wherein is a proportional coefficient of the voltage regulator, is an integral coefficient of the voltage regulator, is a Laplacian operator.

4. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, The sampled grid voltage in S3 The grid voltage amplitude obtained through the phase-locked loop (PLL) link And the grid voltage phase angle The grid current The grid current obtained through SOGI operation The d-axis component And the grid current The q-axis component The d-axis component of the grid current obtained through park transformation And the q-axis component of the grid current The transformation formula of the park transformation is as follows: wherein denotes the sine value of the grid voltage phase angle , denotes the cosine value of the grid voltage phase angle .

5. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, The grid-connected current q-axis component reference value described in S4 The grid-connected current d-axis component reference value is calculated by a one-beat delay method The calculation formula of the grid-connected current d-axis component reference value is as follows: wherein, H-bridge unit HB of the previous control cycle j selected power; the grid current d-axis component reference value and the grid current d-axis component controlled by the current regulator, and the grid voltage magnitude after the feedforward, the d-axis modulation voltage of the inverter is obtained ; a grid-connected current q-axis component reference value and a grid-connected current q-axis component a q-axis modulation voltage of the inverter is obtained by current regulator control , the d-axis modulation voltage and the q-axis modulation voltage the calculation formulae of which are as follows: wherein is a proportional coefficient of the current regulator, is an integral coefficient of the current regulator, is a Laplacian operator.

6. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, S5 total modulation voltage amplitude of the inverter and the included angle The calculation formula is as follows: wherein represents the arctangent value of 7. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, S6 selects the power 2, i.e. the reference power The calculation formula is as follows: wherein, H-bridge unit HB of the previous control cycle j selected power; The H-bridge unit HB j of the selected power is selected as follows: 。 8. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, The H-bridge unit HB of S7 j of the modulation degree The calculation formula is as follows: 。 9. The modulation wave hybrid control strategy of a cascaded grid-connected inverter according to claim 1, wherein, The H-bridge unit HB described in S8 j of the triggering angle The inverse solution is obtained using the following formula: Recalculating the compensation harmonics of the H-bridge unit HB j The calculation formula is as follows:​ Let all the non-overmodulation unit H-bridge unit HB j of the compensation harmonic is 0, calculate all the overmodulation unit compensation total harmonic : ； The specific process of modulation wave calculation of the non-over-modulation units is as follows: The H-bridge unit HB in the non-overmodulation unit j Margin of injection of reverse harmonics The calculation formula is as follows: Let the margin of injection of reverse harmonics of all over-modulation units H-bridge units HB j be , the harmonic of compensation to H-bridge units HB j in non-over-modulation units is calculated and recorded as reverse compensation harmonic , the calculation formula is as follows: The modulation voltage of the H-bridge unit HB in the non-overmodulation unit is calculated i The calculation formula is as follows:​ 。 10. A modulation wave mixing control system of a cascaded grid-connected inverter, characterized by, comprise: N identical H-bridge units, each H-bridge unit HB j has a DC side connected in parallel with a capacitor C j and a photovoltaic module PV j , N H-bridge units HB j have AC sides connected in series to each other, pass through a filter inductor L s and enter the power grid; Sampling module for obtaining a direct voltage and an output current ​ A control module for controlling a DC voltage reference value and a DC voltage reference value and obtaining a voltage regulator output ​ A method for determining a grid voltage value from a grid voltage sample A calculation module for determining a grid voltage amplitude and a grid voltage phase angle from the grid voltage sample A calculation module for determining a grid current d-axis component and a grid current q-axis component from the grid current sample Used to reference the d-axis component of the grid-connected current. and the d-axis component of the grid-connected current The d-axis modulation voltage of the inverter is obtained. The calculation module is used to calculate the q-axis component reference value of the grid-connected current. and the q-axis component of the grid-connected current Obtain the q-axis modulation voltage of the inverter The calculation module; for calculating the total modulation voltage amplitude of an inverter , the total modulation voltage amplitude of an inverter the angle to the grid voltage calculation module For generating an H-bridge unit HB j of the selected power of the computing selection module for obtaining a h-bridge unit HB j of a modulation degree of a calculation module a processing module for performing modulation wave calculation and generation on the over-modulation units and the non-over-modulation units in each H-bridge unit; and a microprocessor and a memory, each of the modules, the microprocessor being programmed or configured to perform the steps of the modulation wave hybrid control strategy of the cascaded grid-connected inverter according to any one of claims 1-9.

11. A computer readable storage medium characterized in that, The computer readable storage medium stores a computer program programmed or configured to perform the modulation wave hybrid control strategy of the cascaded grid-connected inverter according to any one of claims 1-9.

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