CN110311406B - Control method for expanding operation range of cascaded H-bridge photovoltaic inverter - Google Patents

Control method for expanding operation range of cascaded H-bridge photovoltaic inverter Download PDF

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CN110311406B
CN110311406B CN201910492341.8A CN201910492341A CN110311406B CN 110311406 B CN110311406 B CN 110311406B CN 201910492341 A CN201910492341 A CN 201910492341A CN 110311406 B CN110311406 B CN 110311406B
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grid
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CN110311406A (en
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张兴
赵涛
王明达
毛旺
刘芳
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
    • H02J3/01—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
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
    • H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
    • H02J3/1857—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters wherein such bridge converter is a multilevel converter
    • H02J3/385—
    • 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
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers
    • 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
    • Y02E40/20—Active power filtering [APF]
    • 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
    • Y02E40/30—Reactive power compensation
    • 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

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Abstract

The invention discloses a control method for expanding the operation range of a cascade H-bridge photovoltaic inverter, and belongs to the field of photovoltaic power generation. The method mainly comprises the following steps: (1) controlling the direct-current side voltages of all H-bridge units to obtain an active current instruction; (2) controlling fundamental wave of the power grid current, controlling third harmonic of the power grid current and obtaining reverse-phase third harmonic voltage to be compensated; (3) calculating the modulation degree of the H bridge unit, and compensating third harmonic for the overmodulation H bridge unit to obtain a modulation wave of the overmodulation H bridge unit; (4) according to the proposed control method, the inverse third harmonic voltage is distributed to the non-overmodulating H-bridge cells, resulting in a modulated wave of the non-overmodulating H-bridge cells. Compared with the existing published documents, the method can ensure that the unit power factor of the cascaded H-bridge type photovoltaic grid-connected inverter normally operates, the voltage fluctuation of the direct-current side capacitor is small, and the third harmonic content of the power grid current is low when the input power of the cascaded H-bridge type photovoltaic grid-connected inverter is seriously unbalanced.

Description

Control method for expanding operation range of cascaded H-bridge photovoltaic inverter
Technical Field
The invention belongs to the photovoltaic power generation technology in the field of electrical engineering, and particularly relates to a control method for expanding the operation range of a single-phase cascade H-bridge photovoltaic inverter.
Background
Compared with the traditional inverter, the cascade H-bridge type multi-level inverter has the advantages of low harmonic content of the power grid current, low switching frequency, small filter volume, easiness in modularization and the like, so that the cascade H-bridge type multi-level inverter is paid attention to by a plurality of scholars. In addition, the direct current side of each H-bridge unit of the cascaded H-bridge multi-level inverter can be independently supplied with Power by one photovoltaic cell, so that the independent Maximum Power Point Tracking (MPPT-Maximum Power Point Tracking) control of the cascaded H-bridge multi-level inverter becomes possible, and therefore, the cascaded H-bridge multi-level topology structure is particularly suitable for a photovoltaic grid-connected inverter.
Although each H-bridge unit of the cascaded H-bridge type photovoltaic grid-connected inverter can improve the power generation capacity of the system through independent MPPT control, if the photovoltaic cell is influenced by factors such as shielding or damage, the output power of part of the photovoltaic cells can be seriously reduced, and the H-bridge unit corresponding to the photovoltaic module with normal output power can be overmodulating due to the fact that the currents flowing through each H-bridge unit are equal and the transmitted power difference is large, so that the output current performance is poor and even the system is unstable.
At present, how to enlarge the operation range of the cascaded H-bridge type inverter has become a hot spot of research on the cascaded H-bridge type photovoltaic inverter. Documents "l.liming, l.hui, x.yaosuo and l.wenxin, Reactive power compensation and optimization strategy for grid-interactive captured photovoltaic systems IEEE trans.power electron, vol.30, No.1, pp.188-202, jan.2015." (l.liming, l.hui, x.yaosuo and l.wenxin, Reactive power compensation and optimization strategy for cascade grid-connected photovoltaic systems, IEEE power electronics, journal 2015.vol.30, page 188 to page 202) can ensure that all H-bridge modules do not overmodulation when the output power of each H-bridge module is severely unbalanced. However, this approach may reduce the power factor of the inverter.
Documents "m.mirambeigi, and h.iman-Eini, Hybrid modulation technique for grid-connected modulated photovoltaic system.ieee trans.ind.electron., vol.63, No.12, pp.7843-7853, dec.2016." (m.mirambeigi, and h.iman-Eini, mixed modulation strategy of cascade type photovoltaic grid-connected power generation system, IEEE industrial electronics magazine, volume 63 in 12 of 12 months in 2016, pages 7843 to 7853) propose an equalization control strategy combining low-frequency square wave modulation and high-frequency sine wave pulse width modulation, and improve the utilization rate of the voltage on the dc side of the H bridge by using the characteristic that the maximum square wave modulation degree is 4/pi. However, this method allocates each H-bridge module to charge or discharge according to the system operating state, and does not precisely control the dc-side capacitor voltage, which causes large fluctuation of the dc-side capacitor voltage. The fluctuation of the voltage on the direct current side enables the photovoltaic assembly to deviate from the maximum power point to operate, and the average power generation amount of the photovoltaic assembly is reduced.
Documents "y.ko, m.andresen, g.butichi, and m.liserre, Power Routing for cascaded H-bridge converters.ieee trans.power electronics, Early Access, 2017" (y.ko, m.andresen, g.butichi, and m.liserre, Power path for cascaded H-bridge converters, IEEE electronics journal, published in advance in 2017) propose a third harmonic compensation strategy that can extend the modulation of H-bridge elements to 1.155, avoiding H-bridge element modulation within certain limits. Meanwhile, the method can also ensure that the system operates under the unit power factor and the voltage fluctuation of the direct-current side capacitor is small. Compared with a hybrid modulation strategy and a reactive power compensation scheme, the third harmonic compensation strategy has better comprehensive performance. However, the inverse third harmonic compensation strategy proposed in this document is an open-loop calculation method, and it cannot be guaranteed that the grid current does not contain a third harmonic component. Secondly, the document does not mention the problem of the distribution of the inverted third harmonic in the non-overmodulation block, which has certain application limitations.
In summary, the existing method for expanding the operation range of the cascaded H-bridge type photovoltaic grid-connected inverter has the following disadvantages:
1) and when the output power of each H-bridge module is seriously unbalanced, the reactive compensation control strategy can ensure that all the H-bridge modules are not modulated, but the power factor of the system is low, and the grid-connected standard cannot be met.
2) Although the hybrid modulation strategy can expand the operation range of the system to a certain extent, the voltage fluctuation of the direct-current side capacitor is large, and the power generation amount of the system is reduced.
3) Although the third harmonic compensation strategy can enable the system to operate under a unit power factor and the voltage fluctuation of the direct-current side capacitor is small, the compensation of the reversed-phase third harmonic is open-loop compensation, and the fact that the current of the power grid does not contain a third harmonic component cannot be guaranteed. Furthermore, the problem of the distribution of the inverted third harmonic between non-overmodulation modules is not mentioned.
Disclosure of Invention
The invention aims to solve the problems that the limitation of various schemes is overcome, and the control method for expanding the operation range of the cascade H-bridge photovoltaic inverter is provided. In addition, an inverse third harmonic compensation strategy based on a quasi-resonant controller is provided, and third harmonic components in the power grid current can be effectively reduced.
In order to solve the technical problem of the invention, the adopted technical scheme is as follows:
a control method for expanding the operation range of a cascade H-bridge photovoltaic inverter belongs to a single-phase photovoltaic inverter and comprises N identical H-bridge units, wherein N is a positive integer, each H-bridge unit consists of four fully-controlled switching devices, the front end of each H-bridge unit is respectively connected with an electrolytic capacitor in parallel, and each electrolytic capacitor is respectively connected with a photovoltaic module in parallel.
Step 1, controlling the DC side capacitor voltage of the H-bridge unit
Step 1.1, respectively sampling the direct current side capacitor voltage of the N H bridge units and the output current of the corresponding photovoltaic module to obtain direct current side capacitor voltage sampling values of the N H bridge units and output current sampling values of the corresponding photovoltaic module of the N H bridge units, and respectively recording the direct current side capacitor voltage sampling values and the output current sampling values as VdciAnd IPVi，i＝1,2,...,N；
Step 1.2, obtaining the DC side capacitor voltage sampling values V of the N H-bridge units according to the step 1.1dciAnd output current sampling value I of photovoltaic module of N H-bridge unitsPViRespectively tracking the maximum power point of the photovoltaic assembly connected with the N H-bridge units to obtain the maximum power point voltage of the photovoltaic assembly connected with the N H-bridge units
Figure BDA0002087442960000032
Then the maximum power point voltage
Figure BDA0002087442960000033
As a command value of the dc-side capacitor voltage of the H-bridge unit, i is 1, 2., N;
step 1.3, respectively carrying out comparison on the DC side capacitance voltage sampling values V of the N H-bridge units obtained in the step 1.1 by using a 100Hz wave trapdciFiltering, and recording the voltage sampling value of the DC side capacitor of the N H-bridge units as VPVi，i＝1,2,...,N；
Step 1.4, respectively calculating output power P of N H-bridge units by using N same voltage regulatorsiAnd summing the output power of all H-bridge units to obtain the total power P transmitted from the direct current side of the H-bridge to the alternating current sideTThe calculation formula is respectively:
Figure BDA0002087442960000031
Figure BDA0002087442960000041
wherein, KVPIs the proportionality coefficient of the voltage regulator, KVIIs the integral coefficient of the voltage regulator, s is the Laplace operator;
step 2, grid-connected current control
Step 2.1, respectively sampling the power grid voltage and the grid-connected current to obtain a power grid voltage sampling value vgAnd grid-connected current sampling value ig；
Step 2.2, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 2.1gPhase locking is carried out to obtain a phase angle theta and an angular frequency omega of the power grid voltage0And the grid voltage amplitude VM；
Step 2.3, the grid-connected current sampling value i obtained in the step 2.1 is processedgDelaying 90 degrees to obtain a grid-connected current sampling value igOrthogonal signal iQHandle igAnd iQConverting the two-phase static vertical coordinate system to a synchronous rotating coordinate system to obtain an active current feedback value IdAnd a reactive current feedback value IqThe calculation formula is as follows:
Figure BDA0002087442960000042
wherein cos (theta) represents a cosine value of the grid voltage phase angle theta, and sin (theta) represents a sine value of the grid voltage phase angle theta;
step 2.4, setting the reactive current instruction value of the inverter
Figure BDA0002087442960000046
Given as 0, the active current command value
Figure BDA0002087442960000043
Is calculated as follows:
Figure BDA0002087442960000044
step 2.5, calculating the amplitude U of the active modulation voltage of the inverter through the active current regulator and the reactive current regulator respectivelydAnd the amplitude U of the reactive modulation voltageqThe calculation formula is respectively:
Figure BDA0002087442960000045
wherein, KiPIs the proportionality coefficient of the current regulator, KiIIs the integral coefficient of the current regulator;
step 2.6, a quasi-resonance controller of triple grid angular frequency is used, and grid-connected current i is converted into grid-connected currentgThe output of the quasi-resonant controller controlled to be 0, tripling the angular frequency of the power grid is the third harmonic voltage v which needs to be compensated by the systemPR3The calculation formula is as follows:
Figure BDA0002087442960000051
wherein, ω iscCut-off frequency, k, of quasi-resonant controllers for tripling the angular frequency of the networkrThe proportionality coefficient of the quasi-resonance controller is three times of the angular frequency of the power grid;
step 3, calculating modulation degree and modulation wave of H-bridge unit
The N H-bridge units are divided into the following two classes: setting the modulation degree of the 1 st, 2 nd, … th and x H-bridge units between 1-1.155, and calling the H-bridge units as overmodulation H-bridge units; setting the modulation degree of the (x + 1) (…) () N H-bridge units to be less than 1, and calling the H-bridge units as non-overmodulation H-bridge units, wherein x is a positive integer and is less than N;
step 3.1, calculating the total modulation voltage amplitude V of the inverterrThe included angle theta between the total modulation voltage and the grid voltagerModulation degree S of N H-bridge units i1,2, N, the calculation formula is as follows:
Figure BDA0002087442960000052
wherein, arctan (U)q/Ud) Represents Uq/UdThe arctan value of;
and 3.2, compensating third harmonic waves for the modulation waves of the overmodulation H bridge units, specifically, calculating to obtain modulation waves m of x overmodulation H bridge units Ai1,2, …, x, calculated as follows:
Figure BDA0002087442960000053
step 3.3, the third harmonic voltage v which needs to be compensated by the systemPR3Distributing to non-overmodulation H bridge units, and specifically calculating distribution coefficients q of K non-overmodulation H bridge unitsiI ═ x +1, x + 2.., N, k ═ N-x, which is calculated as follows:
Figure BDA0002087442960000054
step 3.4, calculating to obtain modulation waves m of k non-overmodulation H bridge unitsBiI ═ x +1, x + 2.., N, calculated as follows:
Figure BDA0002087442960000055
compared with the prior art, the invention has the beneficial effects that:
1. when the input power of the H-bridge unit is unbalanced, the system can normally operate in a unit power factor mode, and the voltage fluctuation of the direct-current side capacitor is not large.
2. And a closed-loop reverse third harmonic compensation strategy is adopted, so that the third harmonic component of the power grid current can be effectively reduced.
Drawings
Fig. 1 shows a main circuit topology structure of a single-phase cascaded H-bridge type photovoltaic grid-connected inverter implemented by the invention.
Fig. 2 is a control block diagram of a single-phase cascaded H-bridge type photovoltaic grid-connected inverter implemented by the invention.
Fig. 3 is a flow chart of the control strategy of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more clearly and clearly understood, the present invention will be further clearly and completely described below with reference to the accompanying drawings and embodiments.
Fig. 1 is a main circuit topology structure of a single-phase cascaded H-bridge type photovoltaic grid-connected inverter, which is implemented by the invention and comprises N identical H-bridge units, wherein each H-bridge unit consists of four fully-controlled switching devices. The front end of each H bridge is respectively connected with an electrolytic capacitor C in paralleli1,2, N, each electrolytic capacitor being associated with a respective one of the photovoltaic cells PViLink, i ═ 1, 2. After the alternating current side outputs of all H bridges are mutually connected in series, the alternating current side outputs pass through a filter inductor L1And L2Connected to the power grid, wherein R1And R2Are respectively a filter inductance L1And L2The equivalent resistance of (c). In the figure VdciAnd IPViRespectively representing a direct-current side capacitor voltage sampling value of an ith H-bridge unit and a corresponding photovoltaic module output current sampling value, wherein i is 1, 2. v. ofgAnd igAnd respectively representing a grid voltage sampling value and a grid-connected current sampling value. In the present embodiment, the front stage of each H-bridge unit is connected in parallel with a capacitor CiAre each 27.2mF, i 1, 2.., N, filter inductance L1＝L20.75mH, its equivalent resistance R1＝R2The amplitude and frequency of the grid voltage is 100V and 50Hz, respectively, 0.005 omega.
Fig. 2 is a control block diagram of a single-phase cascaded H-bridge type photovoltaic grid-connected inverter implemented by the present invention, which is composed of a main controller and N H-bridge controllers. The main controller realizes grid-connected current control and calculates the modulation wave of each H-bridge module. The H-bridge controller realizes Maximum Power Point Tracking (MPPT-Maximum Power Point Tracking) control of the photovoltaic module, direct-current side capacitor voltage control of the H-bridge unit and generation of driving signals of four fully-controlled switching devices corresponding to the H-bridge unit according to modulation waves of the H-bridge unit calculated by the main controller.
FIG. 3 is a flow chart of a control strategy implemented by the present invention. Firstly, the direct-current side voltages of all H-bridge units are controlled to obtain an active current instruction. Secondly, the fundamental wave of the power grid current is controlled, the third harmonic wave of the power grid current is controlled, and the inverse third harmonic wave voltage v required to be compensated is obtainedPR3. Then, the modulation degree of the H-bridge unit is calculated, and the third harmonic is compensated for the overmodulation H-bridge unit, so that the modulation wave of the overmodulation H-bridge unit is obtained. Finally, according to the proposed control method, the inverse third harmonic voltage v is appliedPR3And distributing the modulated wave to the non-overmodulation H-bridge unit to obtain the modulated wave of the non-overmodulation H-bridge unit.
Referring to fig. 1,2 and 3, the implementation of the present invention is as follows:
step 1, controlling the DC side capacitor voltage of the H-bridge unit
Step 1.1, respectively sampling the direct current side capacitor voltage of the N H bridge units and the output current of the corresponding photovoltaic module to obtain direct current side capacitor voltage sampling values of the N H bridge units and output current sampling values of the corresponding photovoltaic module of the N H bridge units, and respectively recording the direct current side capacitor voltage sampling values and the output current sampling values as VdciAnd IPVi，i＝1,2,...,N；
Step 1.2, obtaining the DC side capacitor voltage sampling values V of the N H-bridge units according to the step 1.1dciAnd output current sampling value I of photovoltaic module of N H-bridge unitsPViRespectively carrying out the photovoltaic module connection of N H-bridge unitsTracking the maximum power point to obtain the maximum power point voltage of the photovoltaic module connected with the N H-bridge units
Figure BDA0002087442960000071
Then the maximum power point voltage
Figure BDA0002087442960000072
As a command value of the dc-side capacitor voltage of the H-bridge unit, i is 1, 2., N;
step 1.3, respectively carrying out comparison on the DC side capacitance voltage sampling values V of the N H-bridge units obtained in the step 1.1 by using a 100Hz wave trapdciFiltering, and recording the voltage sampling value of the DC side capacitor of the N H-bridge units as VPVi，i＝1,2,...,N；
Step 1.4, respectively calculating output power P of N H-bridge units by using N same voltage regulatorsiAnd summing the output power of all H-bridge units to obtain the total power P transmitted from the direct current side of the H-bridge to the alternating current sideTThe calculation formula is respectively:
Figure BDA0002087442960000073
Figure BDA0002087442960000074
wherein, KVPIs the proportionality coefficient of the voltage regulator, KVIS is the laplacian operator, which is the integral coefficient of the voltage regulator. Voltage regulator proportionality coefficient KVPAnd the voltage regulator integral coefficient KVIDesigned according to a conventional grid-connected inverter, in the implementation, KVP＝8，KVI＝150。
Step 2, grid-connected current control
Step 2.1, respectively sampling the power grid voltage and the grid-connected current to obtain a power grid voltage sampling value vgAnd grid-connected current sampling value ig；
Step 2.2, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 2.1gPhase locking is carried out to obtain a phase angle theta and an angular frequency omega of the power grid voltage0And the grid voltage amplitude VM；
Step 2.3, the grid-connected current sampling value i obtained in the step 2.1 is processedgDelaying 90 degrees to obtain a grid-connected current sampling value igOrthogonal signal iQHandle igAnd iQConverting the two-phase static vertical coordinate system to a synchronous rotating coordinate system to obtain an active current feedback value IdAnd a reactive current feedback value IqThe calculation formula is as follows:
Figure BDA0002087442960000081
wherein cos (theta) represents a cosine value of the grid voltage phase angle theta, and sin (theta) represents a sine value of the grid voltage phase angle theta;
step 2.4, setting the reactive current instruction value of the inverter
Figure BDA0002087442960000082
Given as 0, the active current command value
Figure BDA0002087442960000083
Is calculated as follows:
Figure BDA0002087442960000084
step 2.5, calculating the amplitude U of the active modulation voltage of the inverter through the active current regulator and the reactive current regulator respectivelydAnd the amplitude U of the reactive modulation voltageqThe calculation formula is respectively:
Figure BDA0002087442960000085
wherein, KiPAs a proportion of current regulatorsCoefficient, KiIIs the integral coefficient of the current regulator and s is the laplacian operator. Current regulator proportionality coefficient KiPIntegral coefficient K of sum current regulatoriIDesigned according to a conventional grid-connected inverter, in the implementation, KiP＝1.5，KiI＝50。
Step 2.6, a quasi-resonance controller of triple grid angular frequency is used, and grid-connected current i is converted into grid-connected currentgThe output of the quasi-resonant controller controlled to be 0, tripling the angular frequency of the power grid is the third harmonic voltage v which needs to be compensated by the systemPR3The calculation formula is as follows:
Figure BDA0002087442960000086
wherein, ω iscCut-off frequency, k, of quasi-resonant controllers for tripling the angular frequency of the networkrThe proportionality coefficient of the quasi-resonance controller is three times of the angular frequency of the power grid; omegacAnd krThe design is carried out according to the design method of a conventional standard resonant controller, in the embodiment, omegac＝3.14，kr＝80。
Step 3, calculating modulation degree and modulation wave of H-bridge unit
The N H-bridge units are divided into the following two classes: setting the modulation degree of the 1 st, 2 nd, … th and x H-bridge units between 1-1.155, and calling the H-bridge units as overmodulation H-bridge units; setting the modulation degree of the (x + 1) (…) () N H-bridge units to be less than 1, and calling the H-bridge units as non-overmodulation H-bridge units, wherein x is a positive integer and is less than N;
step 3.1, calculating the total modulation voltage amplitude V of the inverterrThe included angle theta between the total modulation voltage and the grid voltagerModulation degree S of N H-bridge units i1,2, N, the calculation formula is as follows:
Figure BDA0002087442960000091
wherein, arctan (U)q/Ud) Represents Uq/UdThe arctan value of;
and 3.2, compensating third harmonic waves for the modulation waves of the overmodulation H bridge units, specifically, calculating to obtain modulation waves m of x overmodulation H bridge units Ai1,2, …, x, calculated as follows:
Figure BDA0002087442960000092
step 3.3, the third harmonic voltage v which needs to be compensated by the systemPR3Distributing to non-overmodulation H bridge units, and specifically calculating distribution coefficients q of K non-overmodulation H bridge unitsiI ═ x +1, x + 2.., N, k ═ N-x, which is calculated as follows:
Figure BDA0002087442960000093
step 3.4, calculating to obtain modulation waves m of k non-overmodulation H bridge unitsBiI ═ x +1, x + 2.., N, calculated as follows:
Figure BDA0002087442960000094
after the modulation waves of all H-bridge units are calculated by adopting the steps, the switch driving signals of all H-bridge units can be obtained by adopting a carrier phase-shifting sine wave pulse width modulation strategy. The carrier phase-shifted sine wave pulse width modulation strategy refers to a modulation strategy commonly applied by a cascaded H-bridge converter, and is a more and mature technology used in the cascaded H-bridge converter. The pulse width modulation of the carrier phase-shifted sine wave is described in detail in the literature, for example, pages 84 to 88 of the monograph "high performance cascaded multilevel converter principle and application" published by mechanical industry publishers in kyoto and chen asia 2013.

Claims (1)

1. A control method for expanding the operation range of a cascade H-bridge photovoltaic inverter belongs to a single-phase photovoltaic inverter and comprises N identical H-bridge units, wherein N is a positive integer, each H-bridge unit consists of four fully-controlled switching devices, the front end of each H-bridge unit is respectively connected with an electrolytic capacitor in parallel, and each electrolytic capacitor is respectively connected with a photovoltaic module in parallel.
Step 1, controlling the DC side capacitor voltage of the H-bridge unit
Step 1.1, respectively sampling the direct current side capacitor voltage of the N H bridge units and the output current of the corresponding photovoltaic module to obtain direct current side capacitor voltage sampling values of the N H bridge units and output current sampling values of the corresponding photovoltaic module of the N H bridge units, and respectively recording the direct current side capacitor voltage sampling values and the output current sampling values as VdciAnd IPVi，i＝1,2,...,N；
Step 1.2, obtaining the DC side capacitor voltage sampling values V of the N H-bridge units according to the step 1.1dciAnd output current sampling value I of photovoltaic module of N H-bridge unitsPViRespectively tracking the maximum power point of the photovoltaic assembly connected with the N H-bridge units to obtain the maximum power point voltage of the photovoltaic assembly connected with the N H-bridge units
Figure FDA0002803334930000012
Then the maximum power point voltage
Figure FDA0002803334930000013
As a command value of the dc-side capacitor voltage of the H-bridge unit, i is 1, 2., N;
step 1.3, respectively carrying out comparison on the DC side capacitance voltage sampling values V of the N H-bridge units obtained in the step 1.1 by using a 100Hz wave trapdciFiltering, and recording the voltage sampling value of the DC side capacitor of the N H-bridge units as VPVi，i＝1,2,...,N；
Step 1.4, respectively calculating output power P of N H-bridge units by using N same voltage regulatorsiAnd summing the output power of all H-bridge units to obtain the total transmission from the direct current side of the H-bridge to the alternating current sidePower PTThe calculation formula is respectively:
Figure FDA0002803334930000011
Figure FDA0002803334930000014
wherein, KVPIs the proportionality coefficient of the voltage regulator, KVIIs the integral coefficient of the voltage regulator, s is the Laplace operator;
step 2, grid-connected current control
Step 2.1, respectively sampling the power grid voltage and the grid-connected current to obtain a power grid voltage sampling value vgAnd grid-connected current sampling value ig；
Step 2.2, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 2.1gPhase locking is carried out to obtain a phase angle theta and an angular frequency omega of the power grid voltage0And the grid voltage amplitude VM；
Step 2.3, the grid-connected current sampling value i obtained in the step 2.1 is processedgDelaying 90 degrees to obtain a grid-connected current sampling value igOrthogonal signal iQHandle igAnd iQConverting the two-phase static vertical coordinate system to a synchronous rotating coordinate system to obtain an active current feedback value IdAnd a reactive current feedback value IqThe calculation formula is as follows:
Figure FDA0002803334930000021
step 2.4, setting the reactive current instruction value of the inverter
Figure FDA0002803334930000022
Given as 0, the active current command value
Figure FDA0002803334930000023
Is calculated as follows:
Figure FDA0002803334930000024
step 2.5, calculating the amplitude U of the active modulation voltage of the inverter through the active current regulator and the reactive current regulator respectivelydAnd the amplitude U of the reactive modulation voltageqThe calculation formula is respectively:
Figure FDA0002803334930000025
wherein, KiPIs the proportionality coefficient of the current regulator, KiIIs the integral coefficient of the current regulator;
step 2.6, a quasi-resonance controller of triple grid angular frequency is used, and grid-connected current sampling values i are obtainedgThe output of the quasi-resonant controller controlled to be 0, tripling the angular frequency of the power grid is the third harmonic voltage v which needs to be compensated by the systemPR3The calculation formula is as follows:
Figure FDA0002803334930000026
wherein, ω iscCut-off frequency, k, of quasi-resonant controllers for tripling the angular frequency of the networkrThe proportionality coefficient of the quasi-resonance controller is three times of the angular frequency of the power grid;
step 3, calculating modulation degree and modulation wave of H-bridge unit
The N H-bridge units are divided into the following two classes: setting the modulation degree of the 1 st, 2 nd, x H-bridge units between 1-1.155, and calling the modulation degree as an overmodulation H-bridge unit; setting the modulation degree of the (x + 1) ·, N H bridge units to be less than 1, and calling the modulation degree as a non-overmodulation H bridge unit, wherein x is a positive integer and is less than N;
step 3.1, calculating the total modulation voltage of the inverterAmplitude VrThe included angle theta between the total modulation voltage and the grid voltagerModulation degree S of N H-bridge unitsi1,2, N, the calculation formula is as follows:
Figure FDA0002803334930000031
wherein, arctan (U)q/Ud) Represents Uq/UdThe arctan value of;
and 3.2, compensating third harmonic waves for the modulation waves of the overmodulation H bridge units, specifically, calculating to obtain modulation waves m of x overmodulation H bridge unitsAi1, 2.. times, x, calculated as follows:
Figure FDA0002803334930000032
step 3.3, the third harmonic voltage v which needs to be compensated by the systemPR3Distributing to non-overmodulation H bridge units, and specifically calculating distribution coefficients q of K non-overmodulation H bridge unitsiI ═ x +1, x + 2.., N, k ═ N-x, which is calculated as follows:
Figure FDA0002803334930000033
step 3.4, calculating to obtain modulation waves m of k non-overmodulation H bridge unitsBiI ═ x +1, x + 2.., N, calculated as follows:
Figure FDA0002803334930000034
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