CN112510761B - Power self-adaptive harmonic compensation strategy for cascaded H-bridge photovoltaic inverter - Google Patents

Power self-adaptive harmonic compensation strategy for cascaded H-bridge photovoltaic inverter Download PDF

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CN112510761B
CN112510761B CN202011425081.1A CN202011425081A CN112510761B CN 112510761 B CN112510761 B CN 112510761B CN 202011425081 A CN202011425081 A CN 202011425081A CN 112510761 B CN112510761 B CN 112510761B
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CN112510761A (en
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张兴
王明达
吴孟泽
郭梓暄
王付胜
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Hefei University of Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/493—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode the static converters being arranged for operation in parallel
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/50—Arrangements for eliminating or reducing asymmetry in polyphase networks

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Abstract

The invention discloses a power self-adaptive harmonic compensation strategy for a cascaded H-bridge photovoltaic inverter, belongs to the field of photovoltaic power generation, and aims to solve various problems caused by unbalanced transmission power between modules of the cascaded H-bridge inverter. The method comprises the following steps: controlling the direct-current side capacitor voltage of all H-bridge converters to enable the photovoltaic component corresponding to each H-bridge converter to work at the maximum power point; selecting the reference power of each H-bridge converter according to the modulation degree of each H-bridge converter, and obtaining an active current instruction value after the selected reference power is operated; controlling the grid-connected current; and calculating the modulation degrees of all the H-bridge converters, judging the working mode of the system, and calculating the modulation wave of each H-bridge converter by adopting different modulation wave calculation methods. Compared with the prior art, the linear modulation range of the cascade H-bridge photovoltaic inverter is further expanded, and the capacity of the cascade H-bridge photovoltaic inverter for coping with power imbalance is improved.

Description

Power self-adaptive harmonic compensation strategy for cascaded H-bridge photovoltaic inverter
Technical Field
The invention belongs to the field of photovoltaic power generation in the field of electrical engineering, and particularly relates to a power self-adaptive harmonic compensation strategy for a cascade H-bridge photovoltaic inverter.
Background
Compared with the traditional inverter, the cascade H-bridge photovoltaic inverter has the advantages of module-level Maximum Power Point Tracking (MPPT-Maximum Power Point Tracking), low grid-connected current harmonic content, small filter volume, easiness in modularization and the like, and has wide development prospect and market potential.
Each H-bridge converter in the cascaded H-bridge photovoltaic inverter is connected with one photovoltaic module, and the photovoltaic modules can work at the maximum power point by independently adjusting the voltage of the capacitor at the direct current side. However, if part of the photovoltaic modules are blocked or damaged, a large difference occurs in the active power transmitted between the H-bridge converters, and since the current flowing through each H-bridge converter is equal, the H-bridge converter with a large transmission power is likely to have overmodulation, which further causes the grid-connected current performance of the whole system to be poor, and even causes the system to be incapable of operating normally.
The documents "m.miranbeigi and h.iman-eini.hybrid modulation technique for grid-connected cascaded photovoltaic grid-connected systems.ieee trans.ind.electron., vol.63, No.12, pp.7843-7853, dec.2016." (m.miranbeigi, h.iman-Eini, hybrid modulation technique of cascaded pv grid-connected systems, IEEE industrial electronics journal, 2016, 12 th vol.63, 12 th p.7843 to 7853, 12 th p.12, 2016) propose a hybrid modulation strategy, which combines low-frequency square waves and high-frequency spwaves, which can extend the linear modulation range of H-bridge converters to 4/pi, thereby avoiding overmodulation of H-bridge converters under certain power imbalance conditions. However, the method can cause large voltage fluctuation of a capacitor on the direct current side of the H-bridge converter, so that the MPPT efficiency is influenced, and the power generation amount of a system is reduced.
Document "zhao, zhanxing, mao wang, xujun, both, zhaobraong, jiangcai.cascade H-bridge photovoltaic inverter power imbalance control strategy based on reactive compensation. 5076 and 5085. "(Chinese Motor engineering journal, 37, vol.37, 17, 5076 and 5085 pages) propose a reactive power compensation method which utilizes a power factor as a degree of freedom to stabilize the operation of an inverter, and the method can normally operate under the condition that the power of an H-bridge converter is seriously unbalanced, but can reduce the power factor of a system.
Documents "Tao Zhao, Xing Zhang, Wang Mao, Fusheng Wang, Jun Xu, Yilei Gu, and Xinyu wang.an optimized third harmonic compensation strategy for single-phase cascaded H-bridge photovoltaic inverter, IEEE trans.ind.electron, vol.65, No.11, pp.8635-8645, nov.2018." (Tao Zhao, Xing Zhang, Wang Mao, Fusheng wa, Jun Xu, Yilei Gu, and Xinyu Wang), an optimized third harmonic compensation strategy for a single-phase cascaded H-bridge photovoltaic inverter, IEEE industrial electronics, journal 11, vol.65, page 8635 to page 8645) ensure that the converter will not fluctuate to a linear harmonic current factor H155 by compensating for a linear harmonic in a linear current converter system, which is not subject to fluctuations in a linear current factor H-converter system, and which is not subject to fluctuations in a linear current factor H-1. However, it has a weak ability to cope with power imbalance, and cannot cope with some cases of severe power imbalance.
The invention of Chinese patent CN201710948192.2 discloses a method for controlling harmonic Compensation of a cascade H-Bridge type photovoltaic grid-connected inverter, and "Tao Zhao, Xing Zhang, Wang Mao, Mingda Wang, Fusheng Wang, Xinyu Wang, and Jun Xu. harmonic Compensation method for expanding the Operating Range of the photovoltaic grid-connected inverter for expanding the same, and" the Power Compensation method for expanding the Operating Range of the cascade H-Bridge PV inverse of the cascade H-Bridge PV, IEEE Journal of electromagnetic and Selected Topics in Po Electronics, vol.8, No.2, pp.1341-1350 ", June 2020" (Tao Zhao, Xing Zhang, Wang Mao, Mingda Wang, Fusheng Wang, Xinyu Wang, Jun Wang, Ju, expanding the Operating Range of the H-Bridge, expanding the harmonic Compensation method for the cascade H-Bridge type photovoltaic grid-connected inverter, and the harmonic Compensation method for expanding the Operating Range of the H-Bridge type photovoltaic grid-connected inverter, and the harmonic Compensation method for expanding the harmonic Compensation method for the harmonic Range of the cascade H-Bridge type photovoltaic grid-Bridge, and the cascade H-Bridge, wherein the harmonic Compensation method for expanding the harmonic Range of the harmonic Compensation method for expanding the harmonic of the cascade H-Bridge, and the harmonic Compensation method for expanding the harmonic of the cascade H-Bridge, JP, wherein the harmonic Compensation method for expanding the harmonic of the cascade H-Bridge, Tao, JP 8, JP 2, JP 8, JP 1, JP 8, JP 2, JP 8, JP 1, JP 2, JP 1, JP 2, JP 1, JP 2, the capacity of the partial H-bridge converter to cope with power imbalance is obviously higher than that of a third harmonic compensation strategy, but when the modulation degree of the partial H-bridge converter is more than 4/pi, the method also fails.
In summary, the existing method for expanding the operation range of the cascaded H-bridge photovoltaic inverter has the following disadvantages:
1) the linear modulation range of the H-bridge converter can be expanded to 4/pi through hybrid modulation, but the voltage fluctuation of a direct current side capacitor of the H-bridge converter is increased, and the MPPT efficiency is influenced.
2) Reactive compensation strategies can cope with severe power imbalance situations, but this approach can reduce the system power factor.
3) The third harmonic compensation strategy does not aggravate voltage fluctuation of a direct current side capacitor of the H-bridge converter and can ensure unit power factor operation of the system, but the capacity of coping with power unbalance is weak.
4) The harmonic compensation strategy retains the advantages of third harmonic compensation and further expands the linear modulation range of the H-bridge converter, but the harmonic compensation strategy also fails in the case of more severe power imbalance.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the limitation of the scheme and provide a power adaptive harmonic compensation strategy for the cascaded H-bridge photovoltaic inverter, and when the cascaded H-bridge photovoltaic inverter is in serious power imbalance, the system can still be kept to normally operate. Compared with the existing control strategy, the method can further expand the system operation range.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows: a power self-adaptive harmonic compensation strategy for a cascaded H-bridge photovoltaic inverter is characterized in that the cascaded H-bridge photovoltaic grid-connected inverter is a single-phase inverter and comprises N identical H-bridge converters, wherein N is an integer larger than 1, a capacitor is connected in parallel to the direct current side of each H-bridge converter, and a photovoltaic component is connected in parallel to each capacitor;
the adaptive harmonic compensation strategy comprises the steps of H-bridge converter direct-current side capacitor voltage control, reference power selection, grid-connected current control and H-bridge converter modulation wave calculation, and the adaptive harmonic compensation strategy comprises the following steps:
step 1, controlling the DC side capacitor voltage of the H-bridge converter
Step 1.1, respectively sampling the DC side capacitor voltages of N H-bridge converters and the output currents of N photovoltaic modules to obtain N DC side capacitor voltage sampling values V dci And N photovoltaic module output current sampling values I dci ，i＝1，2，...，N；
Step 1.2, the voltage sampling values V of the capacitors at the direct current side of the N H-bridge converters obtained in the step 1.1 are compared dci And N photovoltaic module output current sampling values I dci Carrying out maximum power point tracking to obtain maximum power point voltages of N H-bridge converters
Figure BDA0002820245570000031
i＝1，2，...，N；
Step 1.3, frequency of use 2f g Hz wave trap to the DC side of the N H-bridge converters obtained in step 1.1Capacitor voltage sampling value V dci Filtering to obtain N filtered voltage sampling values V of capacitors at the direct current side of the H-bridge converter dcAi ，i＝1，2，...，N；f g Is the grid voltage frequency;
step 1.4, obtaining the maximum power point voltage of the N H-bridge converters obtained in the step 1.2
Figure BDA0002820245570000041
As the DC side capacitor voltage reference value of the H-bridge converter, N voltage regulators are used for respectively sampling N filtered DC side capacitor voltage sampling values V of the H-bridge converter dcAi Controlled by a current signal I output from the voltage regulator i 1, 2, N, calculated as:
Figure BDA0002820245570000042
wherein, K vP Is the proportionality coefficient of the voltage regulator, K vI Is the integral coefficient of the voltage regulator, s is the laplace operator;
step 1.5, sampling values V of N filtered DC side capacitor voltage of the H-bridge converter dcAi And current signals I output by N voltage regulators i Multiplying to obtain the output power P of N H bridges i 1, 2, N, calculated as:
P i ＝V dcAi I i
step 2, selecting reference power
Obtaining reference power P of N H-bridge converters by delaying one beat refi 1, 2, N, calculated as:
Figure BDA0002820245570000043
wherein,
Figure BDA0002820245570000044
ith H-bridge calculated for last control cycleModulation degree of the converter; v HAB_o Outputting voltage fundamental wave amplitude P for a control period on a cascaded H-bridge photovoltaic inverter T_o Outputting total power for a control period of the cascaded H-bridge photovoltaic inverter, wherein the calculation formula is shown as follows;
Figure BDA0002820245570000045
in the formula, P refi_o The reference power obtained in the last control period;
step 3, grid-connected current control
Step 3.1, respectively sampling the power grid voltage and the grid-connected current to obtain a power grid voltage sampling value v g And grid-connected current sampling value i g ；
Step 3.2, grid-connected current sampling value i g Delaying 90 degrees to obtain a grid-connected current sampling value i g Orthogonal signal i Q I is to g And i Q Converting the two-phase static coordinate system into a two-phase rotating coordinate system to obtain an active current feedback value i d And a reactive current feedback value i q ；
Step 3.3, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 3.1 g Phase locking is carried out to obtain a power grid voltage angular frequency signal omega t and a power grid voltage amplitude signal V g ；
Step 3.4, reference value of reactive current
Figure BDA0002820245570000051
Set to 0, calculate the active current reference value
Figure BDA0002820245570000052
The calculation formula is as follows:
Figure BDA0002820245570000053
wherein, P T Outputting total power for the control period of the cascaded H-bridge photovoltaic inverter, wherein the calculation formula is shown as follows;
Figure BDA0002820245570000054
step 3.5, respectively passing through the active current regulator and the reactive current regulator pair i d And i q Control to obtain active modulation voltage v d And a reactive modulation voltage v q The calculation formula is:
Figure BDA0002820245570000055
wherein, K iP Is the proportionality coefficient, K, of active and reactive current regulators iI The integral coefficients of the active current regulator and the reactive current regulator are shown;
step 3.6, calculating the fundamental wave amplitude V of the output voltage of the cascaded H-bridge photovoltaic inverter HAB The included angle alpha of the output voltage and the power grid voltage is calculated as follows:
Figure BDA0002820245570000056
step 3.7, calculating the modulation degree M of the N H-bridge converters i 1, 2, N, calculated as:
Figure BDA0002820245570000057
step 4, calculating the modulation wave of the H-bridge converter
Step 4.1, the modulation degree M of the N H-bridge converters obtained in the step 3.7 i Judging the working mode of the system:
if the modulation degree M of all H-bridge converters in the N H-bridge converters 1 If not, the system works in a mode 1 and executes a step 4.2;
if the modulation degree M of all H-bridge converters in the N H-bridge converters i Are not more than 4/pi, and the modulation degree of the partial H-bridge converter is between 1 and 4Pi, the system works in the mode 2, and the step 4.3 is executed;
if the modulation degree of a part of H-bridge converters in the N H-bridge converters is not more than 1, the modulation degree of the part of H-bridge converters is between 1 and 4/pi, and the modulation degree of the part of H-bridge converters is not less than 4/pi, the system works in a mode 3, and the step 4.4 is executed;
step 4, 2, the system works in the mode 1, and the modulation wave m of each H-bridge converter is directly calculated i 1, 2, N, and the calculation formula is;
m i ＝M i cos(ωt+α)i＝1，2，...，N
4.3, the system works in a mode 2, firstly, setting the modulation degree of the 1st, 2 nd, 2 th, x-bridge converters to be not more than 1, setting the modulation degree of the N H-bridge converters to be between 1 and 4/pi, wherein x is a positive integer and x is less than N;
step 4.3.1, calculating the modulation waves m of the x +1 th to N H-bridge converters i N, calculated as:
Figure BDA0002820245570000061
Figure BDA0002820245570000062
wherein,
Figure BDA0002820245570000063
for calculating intermediate variables in the process, π is the circumferential rate, arcsin (π M) i /4) denotes π M i An anti-sine value of/4;
step 4.3.2, calculating the total harmonic voltage v compensated by the x +1 th to N H-bridge converters hf The calculation formula is:
Figure BDA0002820245570000064
step 4.3.3, calculate the 1st to x H bridgeConverter compensated total harmonic voltage v ho The calculation formula is:
Figure BDA0002820245570000065
step 4.3.4, calculating the maximum harmonic voltage amplitude V that the 1st to x th H-bridge converters can compensate himax The calculation formula is: v himax ＝(1-M i )V dci i＝1，2，...，x
Step 4.3.5, distributing the total harmonic voltage v compensated by the 1st to the x th H-bridge converters ho For the 1st to the x th H-bridge converters, the 1st to the x th H-bridge converters compensate the harmonic ho i The calculation formula of (c) is:
Figure BDA0002820245570000071
step 4.3.6, calculating the modulation wave m of the 1st to x th H-bridge converters i X, calculated as:
m i ＝M i cos(ωt+α)+ho i i＝1，2，...，x
step 4.4, the system works in a mode 3, in which the modulation degrees of the 1st, 2 nd, 9 th H-bridge converters are set to be not more than 1, the modulation degrees of the y th +1 th, y +2 th, z th H-bridge converters are set to be between 1 and 4/pi, the modulation degrees of the z th +1 th, z +2 th, 6 th, N H-bridge converters are set to be not less than 4/pi, y and z are positive integers, and y < z < N; secondly, because the modulation degree of part of the H-bridge converters is not less than 4/pi and exceeds the linear modulation range of the H-bridge converters, the modulation degrees of N H-bridge converters need to be recalculated, and the new modulation degree of N H-bridge converters is marked as S i 1, 2, N, calculated as:
Figure BDA0002820245570000072
step 4.4.1, calculating the (y + 1) th to the N (N) th H-bridge converter modulation waves m i I +1, y +2,.., N, calculated as:
Figure BDA0002820245570000073
Figure BDA0002820245570000074
wherein, tau is an intermediate variable in the calculation process, pi is a circumferential rate, arcsin (pi S) i /4) represents π S i An inverse sine value of/4;
step 4.4.2, calculating the total harmonic voltage v compensated by the (y + 1) th-N H-bridge converters hf_o The calculation formula is:
Figure BDA0002820245570000081
step 4.4.3, calculating the total harmonic voltage v compensated by the 1st to y th H-bridge converters ho_o The calculation formula is:
Figure BDA0002820245570000082
step 4.4.4, calculating the maximum harmonic voltage amplitude V which can be compensated by the 1st to y H-bridge converters himax_o The calculation formula is:
V himax_o ＝(1-S i )V dci i＝1，2，...，y
step 4.4.5, distributing the total harmonic voltage v compensated by the 1st to y H-bridge converters ho_o For 1st to y H-bridge converters, 1st to y H-bridge converters compensate harmonic ho o_i The calculation formula of (c) is:
Figure BDA0002820245570000083
step 4.4.6, calculating the modulation wave m of the 1st to the y th H-bridge converters i ，i＝1，2，..Y, the calculation formula is:
m i ＝S i cos(ωt+α)+ho o_i i＝1，2，...，y。
compared with the prior art, the invention has the beneficial effects that:
1. compared with a harmonic compensation strategy proposed by the existing literature, the method can further expand the linear modulation range of the cascaded H-bridge inverter, so that the cascaded H-bridge inverter can cope with a more serious power imbalance condition;
2. the method can ensure the stable operation of the cascaded H-bridge inverter when the power is unbalanced, does not influence the power factor of the inverter, and does not aggravate the voltage fluctuation of the direct current side of the H-bridge.
Drawings
Fig. 1 is a main circuit topology of a single-phase cascaded H-bridge photovoltaic inverter embodying the present invention.
Fig. 2 is a control block diagram of a cascaded H-bridge photovoltaic inverter embodying the present invention.
Fig. 3 is a flow chart of the calculation of the modulated wave of the H-bridge converter according to the embodiment of the present invention.
FIG. 4 shows sampled values v of the grid voltage in case of severe power imbalance, using a harmonic compensation strategy g And grid-connected current sampling value i g The waveform of (2).
FIG. 5 shows a modulation wave m of a first H-bridge converter in case of severe power imbalance and adopting a power adaptive harmonic compensation strategy 1 Grid voltage sampling value v g And grid-connected current sampling value i g The waveform of (2).
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more clearly understood, the present invention is further clearly and completely described below with reference to the accompanying drawings and embodiments.
Fig. 1 is a main circuit topology of a single-phase cascade H-bridge photovoltaic inverter implemented by the present invention, as can be seen from fig. 1, comprising N identical H-bridge converters, N being an integer greater than 1, each H-bridge converter consisting of 4 fully-controlled switching devices. The direct current side of each H-bridge converter is connected with a capacitor in parallel, and each capacitor is connected with a photovoltaic module in parallel. Inverter outputThe terminals are connected to the network via a filter inductor L. In the figure, C 1 ，C 2 ，...，C N Capacitors connected in parallel to the direct current sides of the 1st, 2 nd, N H-bridge converters, respectively; PV (photovoltaic) 1 ，PV 2 ，...，PV N 1, 2, a photovoltaic module to which N H-bridge converters are connected, I dci And V dci The ith H-bridge converter corresponds to the output current of the photovoltaic module and the sampling value of the dc-side capacitor voltage, i is 1, 2. v. of g For sampled values of grid voltage and grid current, i g Is the sampling value of the grid-connected current.
Fig. 2 is a control block diagram of a cascaded H-bridge photovoltaic inverter implemented by the present invention, which includes a dc-side capacitor voltage control module of an H-bridge converter, a reference power selection module, a grid-connected current control module, and a modulation wave calculation module of the H-bridge converter.
FIG. 3 is a flow chart of the calculation of the modulation wave of the H-bridge converter according to the present invention, first determining the working mode corresponding to the system according to the modulation degree of each H-bridge, and referring to the calculation method of the modulation wave of the H-bridge converter under different modes to obtain the modulation wave m of each H-bridge i And finally, calculating the switch driving signal of each H-bridge converter by carrier phase shift modulation.
Referring to fig. 1, 2 and 3, the adaptive harmonic compensation strategy according to the present invention includes the following steps:
step 1, controlling the DC side capacitor voltage of the H-bridge converter
Step 1.1, respectively sampling N direct current side capacitor voltages of the H-bridge converter and N photovoltaic module output currents to obtain N direct current side capacitor voltage sampling values V dci And N photovoltaic module output current sampling values I dci ，i＝1，2，...，N。
Step 1.2, the voltage sampling values V of the capacitors at the DC side of the N H-bridge converters obtained in the step 1.1 dci And N photovoltaic module output current sampling values I dci Performing maximum power point tracking to obtain maximum power point voltages of N H-bridge converters
Figure BDA0002820245570000091
，i＝1，2，...，N。
Step 1.3, the frequency of use is 2f g Hz wave trap to the N DC side capacitor voltage sampling values V of the H-bridge converter obtained in step 1.1 dci Filtering to obtain N filtered voltage sampling values V of capacitors at the direct current side of the H-bridge converter dcAi ，i＝1，2，...，N；f g Is the grid voltage frequency.
Step 1.4, obtaining the maximum power point voltage of the N H-bridge converters obtained in the step 1.2
Figure BDA0002820245570000105
As the DC side capacitor voltage reference value of the H-bridge converter, N voltage regulators are used for respectively sampling N filtered DC side capacitor voltage sampling values V of the H-bridge converter dcAi Controlled by a current signal I output from the voltage regulator i 1, 2, N, calculated as:
Figure BDA0002820245570000101
wherein, K vP Is the proportionality coefficient of the voltage regulator, K vI S is the laplacian, which is the integral coefficient of the voltage regulator. In this embodiment, K vP ＝8，K vI ＝150。
Step 1.5, sampling values V of N filtered DC side capacitor voltage of the H-bridge converter dcAi And current signals I output by N voltage regulators i Multiplying to obtain the output power P of N H bridges i 1, 2, N, calculated as:
P i ＝V dcAi I i
step 2, selecting reference power
Method for obtaining reference power P of N H-bridge converters by delaying one beat refi 1, 2, N, calculated as:
Figure BDA0002820245570000102
wherein,
Figure BDA0002820245570000103
the modulation degree of the ith H-bridge converter calculated for the previous control period; v HAB_o Outputting voltage fundamental wave amplitude P for a control period on a cascaded H-bridge photovoltaic inverter T_o Outputting total power for a control period of the cascaded H-bridge photovoltaic inverter, wherein the calculation formula is as follows;
Figure BDA0002820245570000104
in the formula, P refi_o The reference power obtained in the last control period.
Step 3, grid-connected current control
Step 3.1, respectively sampling the power grid voltage and the grid-connected current to obtain a power grid voltage sampling value v g And grid-connected current sampling value i g 。
Step 3.2, grid-connected current sampling value i g Delaying 90 degrees to obtain a grid-connected current sampling value i g Orthogonal signal i Q I is to g And i Q Converting the two-phase static coordinate system into a two-phase rotating coordinate system to obtain an active current feedback value i d And a reactive current feedback value i q 。
Step 3.3, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 3.1 g Performing phase locking to obtain a power grid voltage angular frequency signal omega t and a power grid voltage amplitude signal V g 。
Step 3.4, reference value of reactive current
Figure BDA0002820245570000111
Set to 0, calculating the active current reference value
Figure BDA0002820245570000112
The calculation formula is:
Figure BDA0002820245570000113
wherein, P T Outputting total power for the control period of the cascaded H-bridge photovoltaic inverter, wherein the calculation formula is shown as follows;
Figure BDA0002820245570000114
step 3.5, respectively passing through the active current regulator and the reactive current regulator pair i d And i q Control to obtain active modulation voltage v d And a reactive modulation voltage v q The calculation formula is:
Figure BDA0002820245570000115
wherein, K iP Is the proportionality coefficient, K, of active and reactive current regulators iI The integral coefficient of the active current regulator and the reactive current regulator. In this example, K iP ＝1.5，K iI ＝50。
Step 3.6, calculating the fundamental wave amplitude V of the output voltage of the cascaded H-bridge photovoltaic inverter HAB The included angle alpha of the output voltage and the power grid voltage is calculated as follows:
Figure BDA0002820245570000116
step 3.7, calculating the modulation degree M of the N H-bridge converters i 1, 2, N, calculated as:
Figure BDA0002820245570000117
step 4, calculating the modulation wave of the H-bridge converter
Step 4.1, obtained according to step 3.7Modulation degree M of N H-bridge converters i Judging the working mode of the system:
if the modulation degree M of all H-bridge converters in the N H-bridge converters i If not, the system works in a mode 1 and executes a step 4.2;
if the modulation degree M of all H-bridge converters in the N H-bridge converters i The modulation degree of the partial H-bridge converter is between 1 and 4/pi, the system works in a mode 2, and a step 4.3 is executed;
and if the modulation degree of part of the H-bridge converters in the N H-bridge converters is not more than 1, the modulation degree of part of the H-bridge converters is between 1 and 4/pi, and the modulation degree of part of the H-bridge converters is not less than 4/pi, the system works in a mode 3, and the step 4.4 is executed.
Step 4, 2, the system works in the mode 1, and the modulation wave m of each H-bridge converter is directly calculated i 1, 2, N, calculated as;
m i ＝M i cos(ωt+α)i＝1，2，...，N
and 4.3, operating the system in a mode 2, firstly setting the modulation degree of the 1st, 2 nd, 2 th, x-bridge converters to be not more than 1, setting the modulation degree of the N H-bridge converters to be between 1 and 4/pi, wherein x is a positive integer and x is less than N.
Step 4.3.1, calculating the modulation waves m of the x +1 th to N H-bridge converters i I ═ x +1, x + 2.
Figure BDA0002820245570000121
Figure BDA0002820245570000122
Wherein,
Figure BDA0002820245570000123
for calculating intermediate variables in the process,. pi.is the circumference ratio, arcsin (π M) i /4) denotes π M i Arcsine of/4The value is obtained.
Step 4.3.2, calculating the total harmonic voltage v compensated by the x +1 th to N H-bridge converters hf The calculation formula is:
Figure BDA0002820245570000124
step 4.3.3, calculating the total harmonic voltage v compensated by the 1st to x th H-bridge converters ho The calculation formula is:
Figure BDA0002820245570000131
step 4.3.4, calculating the maximum harmonic voltage amplitude V that the 1st to x th H-bridge converters can compensate himax The calculation formula is:
V himax ＝(1-M i )V dci i＝1，2，...，x
step 4.3.5, distributing the total harmonic voltage v compensated by the 1st to x th H-bridge converters ho The 1st to the x th H-bridge converters and the 1st to the x th H-bridge converters compensate the harmonic ho i The calculation formula of (A) is as follows:
Figure BDA0002820245570000132
step 4.3.6, calculating the modulation wave m of the 1st to x th H-bridge converters i X, calculated as:
m i ＝M i cos(ωt+α)+ho i i＝1，2，...，x
step 4.4, the system works in a mode 3, in which the modulation degrees of the 1st, 2 nd, 9 th H-bridge converters are set to be not more than 1, the modulation degrees of the y th +1 th, y +2 th, z th H-bridge converters are set to be between 1 and 4/pi, the modulation degrees of the z th +1 th, z +2 th, 6 th, N H-bridge converters are set to be not less than 4/pi, y and z are positive integers, and y < z < N; secondly, the modulation degree of the partial H-bridge converter is not less than 4/pi, and the linear modulation range of the H-bridge converter is exceededRecalculating the modulation degrees of the N H-bridge converters, and recording the new modulation degrees of the N H-bridge converters as S i 1, 2, N, calculated as:
Figure BDA0002820245570000133
step 4.4.1, calculating the (y + 1) th to the N (N) th H-bridge converter modulation waves m i I +1, y +2,.., N, calculated as:
Figure BDA0002820245570000134
Figure BDA0002820245570000135
wherein, tau is an intermediate variable in the calculation process, pi is a circumferential rate, arcsin (pi S) i /4) represents π S i An anti-sine value of/4;
step 4.4.2, calculating the total harmonic voltage v compensated by the (y + 1) th to the N (N) th H-bridge converters hf_o The calculation formula is:
Figure BDA0002820245570000141
step 4.4.3, calculating the total harmonic voltage v compensated by the 1st to y th H-bridge converters ho_o The calculation formula is:
Figure BDA0002820245570000142
step 4.4.4, calculating the maximum harmonic voltage amplitude V which can be compensated by the 1st to y H-bridge converters himax_o The calculation formula is:
V himax_o ＝(1-S i )V dci i＝1，2，...，y
step 4.4.5, distributing the total compensated by the 1st to the y H-bridge convertersHarmonic voltage v ho_o For the 1st to the y th H-bridge converters, the 1st to the y H-bridge converters compensate the harmonic bo o_i The calculation formula of (c) is:
Figure BDA0002820245570000143
step 4.4.6, calculating the modulation wave m of the 1st to the y th H-bridge converters i 1, 2, y, calculated as:
m i ＝S i cos(ωt+α)+ho o_i i＝1，2，...，y。
FIG. 4 shows sampled values v of the grid voltage in case of severe power imbalance, using a harmonic compensation strategy g And grid-connected current sampling value i g The waveform of (2). Wherein, the illumination intensity of the front-stage photovoltaic module of the five H-bridge converters is 1000W/m respectively 2 、1000W/m 2 、300W/m 2 、250W/m 2 And 200W/m 2 The temperatures were all 25 ℃. In the embodiment, the photovoltaic module is 1Soltech1STH-225-P, and the illumination intensity is 1000W/m 2 At 25 ℃, the maximum output power is 225W, and the maximum power point voltage is 29.6V. Obviously, there is a serious imbalance in the output power of the photovoltaic module, and overmodulation occurs due to the large output power of the first H-bridge converter and the second H-bridge converter. However, the harmonic content in the grid-connected current waveform is high due to the limited adjustment range of the harmonic compensation strategy.
FIG. 5 shows a modulation wave m of a first H-bridge converter in case of severe power imbalance and adopting a power adaptive harmonic compensation strategy 1 Grid voltage sampling value v g And grid-connected current sampling value i g The waveform of (2). As can be seen from fig. 5, the modulation wave of the first H-bridge converter is compensated to be a square wave and has an amplitude smaller than 1; the inverter can maintain unit power factor operation, and grid-connected current waveform quality is better.

Claims (1)

1. A power self-adaptive harmonic compensation strategy for a cascaded H-bridge photovoltaic inverter is a single-phase inverter and comprises N identical H-bridge converters, wherein N is an integer larger than 1, a capacitor is connected in parallel to the direct current side of each H-bridge converter, and a photovoltaic module is connected in parallel to each capacitor;
the method is characterized in that the self-adaptive harmonic compensation strategy comprises the steps of DC side capacitor voltage control of the H-bridge converter, reference power selection, grid-connected current control and H-bridge converter modulation wave calculation, and the method comprises the following steps:
step 1, controlling the DC side capacitor voltage of the H-bridge converter
Step 1.1, respectively sampling N direct current side capacitor voltages of the H-bridge converter and N photovoltaic module output currents to obtain N direct current side capacitor voltage sampling values V dci And N photovoltaic module output current sampling values I dci ，i＝1，2，...，N；
Step 1.2, the voltage sampling values V of the capacitors at the direct current side of the N H-bridge converters obtained in the step 1.1 are compared dci And N photovoltaic module output current sampling values I dci Performing maximum power point tracking to obtain maximum power point voltages of N H-bridge converters
Figure FDA0003681868270000011
Figure FDA0003681868270000012
Step 1.3, the frequency of use is 2f g Hz wave trap to the N DC side capacitor voltage sampling values V of the H-bridge converter obtained in step 1.1 dci Filtering to obtain N filtered voltage sampling values V of capacitors at the DC side of the H-bridge converter dcAi ，i＝1，2，...，N；f g Is the grid voltage frequency;
step 1.4, obtaining the maximum power point voltage of the N H-bridge converters obtained in the step 1.2
Figure FDA0003681868270000013
As the DC side capacitor voltage reference value of the H-bridge converter, N voltage regulators are used for respectively sampling N filtered DC side capacitor voltage sampling values V of the H-bridge converter dcAi Control the output of the voltage regulator to be currentSignal I i 1, 2, N, calculated as:
Figure FDA0003681868270000014
wherein, K vP Is the proportionality coefficient of the voltage regulator, K vI Is the integral coefficient of the voltage regulator, s is the laplace operator;
step 1.5, sampling values V of N filtered DC side capacitor voltage of the H-bridge converter dcAi And current signals I output by N voltage regulators i Multiplying to obtain the output power P of N H bridges i 1, 2, N, calculated as:
P i ＝V dcAi I i
step 2, selecting reference power
Method for obtaining reference power P of N H-bridge converters by delaying one beat refi 1, 2, N, calculated as:
Figure FDA0003681868270000021
wherein,
Figure FDA0003681868270000022
the modulation degree of the ith H-bridge converter calculated for the previous control period; v HAB_o Outputting voltage fundamental wave amplitude P for a control period on a cascaded H-bridge photovoltaic inverter T_o Outputting total power for a control period of the cascaded H-bridge photovoltaic inverter, wherein the calculation formula is as follows;
Figure FDA0003681868270000023
in the formula, P refi_o The reference power obtained in the last control period;
step 3, grid-connected current control
Step 3.1, respectively sampling the power grid voltage and the grid-connected current to obtain a power grid voltage sampling value v g And grid-connected current sampling value i g ；
Step 3.2, grid-connected current sampling value i g Delaying 90 degrees to obtain a grid-connected current sampling value i g Orthogonal signal i Q A is to i g And i Q Converting the two-phase static coordinate system into a two-phase rotating coordinate system to obtain an active current feedback value i d And a reactive current feedback value i q ；
Step 3.3, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 3.1 g Phase locking is carried out to obtain a power grid voltage angular frequency signal omega t and a power grid voltage amplitude signal V g ；
Step 3.4, reference value of reactive current
Figure FDA0003681868270000024
Set to 0, calculating the active current reference value
Figure FDA0003681868270000025
The calculation formula is as follows:
Figure FDA0003681868270000026
wherein, P T Outputting total power for the control period of the cascaded H-bridge photovoltaic inverter, wherein the calculation formula is shown as follows;
Figure FDA0003681868270000027
step 3.5, i through active current regulator and reactive current regulator pair respectively d And i q Control to obtain active modulation voltage v d And a reactive modulation voltage v q The calculation formula is:
Figure FDA0003681868270000031
wherein, K iP Is the proportionality coefficient, K, of active and reactive current regulators iI The integral coefficients of the active current regulator and the reactive current regulator are shown;
step 3.6, calculating the fundamental wave amplitude V of the output voltage of the cascaded H-bridge photovoltaic inverter HAB The included angle alpha of the output voltage and the power grid voltage is calculated as follows:
Figure FDA0003681868270000032
step 3.7, calculating the modulation degree M of the N H-bridge converters i 1, 2, N, calculated as:
Figure FDA0003681868270000033
step 4, calculating the modulation wave of the H-bridge converter
Step 4.1, the modulation degree M of the N H-bridge converters obtained in step 3.7 i Judging the working mode of the system:
if the modulation degree M of all H-bridge converters in the N H-bridge converters i If not, the system works in a mode 1 and executes a step 4.2;
if the modulation degree M of all H-bridge converters in the N H-bridge converters i The modulation degree of the partial H-bridge converter is between 1 and 4/pi, the system works in a mode 2, and a step 4.3 is executed;
if the modulation degree of part of the H-bridge converters in the N H-bridge converters is not more than 1, the modulation degree of part of the H-bridge converters is between 1 and 4/pi, and the modulation degree of part of the H-bridge converters is not less than 4/pi, the system works in a mode 3, and the step 4.4 is executed;
step 4, 2, the system works in the mode 1, and the modulation wave m of each H-bridge converter is directly calculated i 1, 2, N, and the calculation formula is;
m i ＝M i cos(ωt+α)i＝1，2，...，N
step 4.3, the system works in a mode 2, firstly, the modulation degree of the 1st, 2 nd, right, x H-bridge converters is set to be not more than 1, the modulation degree of the x +1 th, x +2 th, right, N H-bridge converters is set to be between 1 and 4/pi, x is a positive integer, and x is less than N;
step 4.3.1, calculating the modulation waves m of the x +1 th to N H-bridge converters i I ═ x +1, x + 2.
Figure FDA0003681868270000041
Figure FDA0003681868270000042
Wherein,
Figure FDA0003681868270000043
for calculating intermediate variables in the process, π is the circumferential rate, arcsin (π M) i /4) denotes π M i An inverse sine value of/4;
step 4.3.2, calculating the total harmonic voltage v compensated by the x +1 th to N H-bridge converters hf The calculation formula is:
Figure FDA0003681868270000044
step 4.3.3, calculating the total harmonic voltage v compensated by the 1st to x th H-bridge converters ho The calculation formula is:
Figure FDA0003681868270000045
step 4.3.4, calculating the maximum harmonic voltage amplitude V that the 1st to x th H-bridge converters can compensate himax The calculation formula is:
V himax ＝(1-M i )V dci i＝1，2，...，x
step 4.3.5, distributing the total harmonic voltage v compensated by the 1st to the x th H-bridge converters ho For the 1st to the x th H-bridge converters, the 1st to the x th H-bridge converters compensate the harmonic ho i The calculation formula of (A) is as follows:
Figure FDA0003681868270000046
step 4.3.6, calculating the modulation wave m of the 1st to x th H-bridge converters i 1, 2.., x, calculated as:
m i ＝M i cos(ωt+α)+ho i i＝1，2，...，x
step 4.4, the system works in a mode 3, in which the modulation degrees of the 1st, 2 nd, 9 th H-bridge converters are set to be not more than 1, the modulation degrees of the y th +1 th, y +2 th, z th H-bridge converters are set to be between 1 and 4/pi, the modulation degrees of the z th +1 th, z +2 th, 6 th, N H-bridge converters are set to be not less than 4/pi, y and z are positive integers, and y < z < N; secondly, because the modulation degree of part of the H-bridge converters is not less than 4/pi and exceeds the linear modulation range of the H-bridge converters, the modulation degrees of N H-bridge converters are needed to be recalculated, and the new modulation degree of the N H-bridge converters is marked as S i 1, 2, N, calculated as:
Figure FDA0003681868270000051
step 4.4.1, calculating the (y + 1) th to the N (N) th H-bridge converter modulation waves m i Y +1, y +2, N, calculated as:
Figure FDA0003681868270000052
Figure FDA0003681868270000053
wherein, tau is an intermediate variable in the calculation process, pi is a circumferential rate, arcsin (pi S) i /4) represents π S i An anti-sine value of/4;
step 4.4.2, calculating the total harmonic voltage v compensated by the (y + 1) th to the N (N) th H-bridge converters hf_o The calculation formula is:
Figure FDA0003681868270000054
step 4.4.3, calculating the total harmonic voltage v compensated by the 1st to y th H-bridge converters ho_o The calculation formula is:
Figure FDA0003681868270000055
step 4.4.4, calculating the maximum harmonic voltage amplitude V which can be compensated by the 1st to y H-bridge converters himax_o The calculation formula is:
V himax_o ＝(1-S i )V dci i＝1，2，...，y
step 4.4.5, distributing the total harmonic voltage v compensated by the 1st to y H-bridge converters ho_o For 1st to y H-bridge converters, 1st to y H-bridge converters compensate harmonic ho o_i The calculation formula of (A) is as follows:
Figure FDA0003681868270000061
step 4.4.6, calculating the modulation wave m of the 1st to the y th H-bridge converters i 1, 2, y, calculated as:
m i ＝S i cos(ωt+α)+ho o_i i＝1，2，...，y。
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