CN112564172B - Control method of single-phase cascade photovoltaic grid-connected inverter - Google Patents
Control method of single-phase cascade photovoltaic grid-connected inverter Download PDFInfo
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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
- H02J3/381—Dispersed generators
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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
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- 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
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
- H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
- H02M3/1582—Buck-boost converters
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- 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
- H02M7/53—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 using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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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
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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- 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
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- 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 of a single-phase cascade type photovoltaic grid-connected inverter, and belongs to the field of photovoltaic power generation. The method is used for dealing with the working scene of serious unbalance of the output power of the photovoltaic module, and compared with the existing single-phase cascade H-bridge photovoltaic grid-connected inverter, the method can enlarge the operation range of the system to a greater extent. The method comprises the following steps: controlling the average value of the DC bus capacitor voltage of the H-bridge converter; controlling the grid-connected current; using modified nearest level approximation modulation; the method comprises the steps of controlling a single-phase active filter and controlling the input direct-current bus voltage of a four-switch tube Buck-Boost converter. By adopting the method, even in a working scene with seriously unbalanced output power of the photovoltaic components, all the photovoltaic components can still work at the self maximum power point, and the power generation capacity of the system is maximized.
Description
Technical Field
The invention belongs to the photovoltaic power generation technology in the field of electrical engineering, and particularly relates to a control method of a single-phase cascade type photovoltaic grid-connected inverter.
Background
In 2017, new standards are set by the national electrical code of the united states, and the photovoltaic facilities on all buildings are required to have the capability of realizing the rapid turn-off of the component level in an emergency, which inevitably promotes the rapid development and wide application of the component level power electronic technology. Because each Power unit of the single-phase Cascaded H-bridge (CHB) photovoltaic grid-connected inverter can be independently supplied with Power by one photovoltaic component, maximum Power Point Tracking (MPPT) of a component level, monitoring of the component level and turn-off of the component are easy to realize, and the single-phase CHB photovoltaic grid-connected inverter is particularly suitable for low-voltage low-Power household photovoltaic Power generation occasions. However, under the influence of factors such as partial shielding or damage, the output power of some photovoltaic modules is seriously reduced, and since the currents flowing through each H-bridge are equal and the transmission power difference is large, the power unit corresponding to the photovoltaic module with normal output power is overmodulatied, which leads to the performance deterioration of the output current and even the unstable operation of the system.
At present, how to enable a single-phase CHB photovoltaic grid-connected inverter to normally operate under a power imbalance condition has become a research hotspot of the single-phase CHB photovoltaic grid-connected inverter. Documents "Zhao Tao, zhang Xing, mao Wang, xu Jun, gu Yilei, zhao Deyong, jiang Cai. Cascade H-bridge photovoltaic inverter power imbalance control strategy based on reactive compensation. 5076-5085 ("proceedings of the Chinese Motor engineering, 2017, volume 37, phase 17, pages 5076-5085) proposes a centralized control strategy with reactive compensation function, which follows the principles of active proportional distribution and reactive demand distribution to ensure that all H-bridge converters cannot be overmodulating when the power is seriously unbalanced. However, this method may reduce the power factor of the system, limiting its practical application.
Documents "Tao Zhao, xing Zhang, wang Mao, fusheng Wang, jun Xu, and Yilei gu.a modified hybrid modulation strategy for applying dc voltage modulation of modulated H-bridge photovoltaic inverter" "(Tao Zhao, xing Zhang, wang Mao, fusheng Wang, jun Xu, and Yilei gu.a modified hybrid modulation strategy, vol.65, no.5, pp.3932-3941, may.2018." (Tao Zhao, xing Zhang, wang Mao, fusheng Wang, jun Xu, and Yilei Gu), an improved hybrid modulation strategy for cascaded H-bridge photovoltaic inverters, IEEE industrial electronics journal, 5 th volume 65 of 2018, pages 3932 to 3941) propose an improved hybrid modulation strategy that uses a combination of square wave modulation and high frequency modulation strategy, which can avoid the occurrence of unbalanced H-bridge pulse width modulation under certain conditions of H-bridge over-voltage, and which can avoid the occurrence of unbalanced H-bridge over-voltage modulation under certain conditions. However, when this method is adopted, the dc bus capacitor voltage of the H-bridge converter still fluctuates greatly and there is a static difference in control, which reduces the power generation amount and MPPT efficiency.
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 inverters", a strategy for optimized third harmonic compensation of single-phase cascaded H-bridge photovoltaic inverters (Tao Zhao, xing Zhang, wang Mao, fusheng wao, jun Xu, yilei Gu, and Xinyu Wang), a strategy for optimized third harmonic compensation of single-phase cascaded H-bridge photovoltaic inverters, IEEE industrial electronics journal compensation, 65 th volume 11, 8635 th page 8645 of 2018 transformer, and a linear harmonic compensation to a third harmonic compensation system, which does not enlarge the power conversion range of the H-bridge system by a linear harmonic compensation of 1H-phase modulated dc voltage. However, this method has a weak capability to cope with power imbalance, and is not suitable for a scenario in which power is severely imbalanced.
Chinese invention patent CN201710947222.8, which is published and authorized in 8/2 in 2019, a square wave compensation control method for expanding the operation range of a cascaded H-bridge type photovoltaic inverter, and Chinese invention patent CN201710948192.2, which is published and authorized in 8/27 in 2019, a harmonic compensation control method for a cascaded H-bridge type photovoltaic grid-connected inverter, respectively propose different harmonic compensation strategies, the advantages of a third harmonic compensation strategy are retained by the two methods, the linear modulation range of the H-bridge converter can be expanded to 4/pi, and the capacity of the two methods to power unbalance is obviously higher than that of the third harmonic compensation strategy. However, with further imbalance of the output power of the photovoltaic module, the modulation degree of the partial H-bridge converter will be larger than 4/pi, and such a method will also fail.
The literature "Abbas Eskandari, vahid java dian, hossei im-Eini, and broad yadolaihi. Stable operation of grid connection cascade H-bridge inverter under unbalanced photovoltaic operation conditions, 20133rd international conference on electric power and energy conversion system, istanbul, turnkey, october2-4, 2013." (Abbas Eskandari, vahid java dian, hossei an im-Eini, and Milad yadolaihi, stable operation of a grid-connected cascade H-bridge inverter under unbalanced lighting conditions, third year international conference on electric and energy conversion systems in 2013, turkish, eistanula, 2013, day 2 to 10), and improved balance output of a photovoltaic module is reduced by an improved MPPT control module. However, this method may reduce the power generation of the system.
In summary, the existing single-phase cascaded photovoltaic grid-connected inverter and the control strategy thereof applied to the low-voltage household photovoltaic power generation occasions have the following disadvantages:
(1) The reactive power compensation strategy can deal with serious power imbalance conditions, but the method can reduce the power factor of the system, and the practical application can be limited.
(2) The linear modulation range of the H-bridge converter can be expanded from 1 to 4/pi by the improved hybrid modulation strategy, but the voltage fluctuation of a direct-current bus capacitor of the H-bridge converter is large, static control exists, and the power generation and MPPT efficiency of a system can be reduced.
(3) The third harmonic compensation strategy can not only aggravate the fluctuation of the DC bus capacitor voltage of the H-bridge converter, but also ensure the unit power factor operation of the system, but the method has weak capability of coping with power imbalance.
(4) The harmonic compensation strategy retains the advantages of the third harmonic compensation strategy and further expands the linear modulation range of the H-bridge converter, but the harmonic compensation strategy also fails in more severe power imbalance situations (e.g., some H-bridge converters have a modulation greater than 4/pi).
(5) Although the improved MPPT control strategy can deal with a severe power imbalance scene, the method reduces the power generation amount of the system.
Disclosure of Invention
The invention aims to solve the problem of overcoming the limitations of various schemes and provides a control method of a single-phase cascade type photovoltaic grid-connected inverter. The control method of the single-phase cascade type photovoltaic grid-connected inverter can ensure that the system can still normally operate under the condition of serious unbalanced illumination intensity, is suitable for household photovoltaic grid-connected power generation occasions, and has better comprehensive performance. In order to solve the technical problem of the invention, the adopted technical scheme is as follows:
a control method of a single-phase cascade type photovoltaic grid-connected inverter comprises N modules with the same structure, wherein N is a positive integer larger than 1, each module is formed by connecting a four-switch tube Buck-Boost converter in series with an H-bridge converter, direct current input ends of all the modules are respectively connected with a photovoltaic component in parallel, alternating current output ends of all the modules are connected in series, one end of each module is connected with a power grid through a filter inductor after being connected in series, the other end of each module is connected with the power grid through a single-phase active filter, the control method comprises average value control of direct current bus capacitor voltage of the H-bridge converter, grid-connected current control, improved nearest level approximation modulation, single-phase active filter control and four-switch tube Buck-Boost converter control, and the specific steps are as follows:
Step 1.1, respectively sampling the DC bus capacitor voltage of N H-bridge converters to obtain DC bus capacitor voltage sampling values V of the N H-bridge converters Hi ，i＝1，2，...，N；
Step 1.2, calculating to obtain an average value V of voltage sampling values of direct current bus capacitors of N H-bridge converters HAver The calculation formula is as follows:
step 1.3, using an H-bridge voltage regulator to obtain an average value V of voltage sampling values of direct current bus capacitors of N H-bridge converters HAver Controlled to a reference voltage V ref The output of the H-bridge voltage regulator is the amplitude I of the grid-connected current instruction value of the single-phase cascade photovoltaic grid-connected inverter M The calculation formula is as follows:
wherein, K HVP Is the proportionality coefficient of the H-bridge voltage regulator, K HVI Is the integral coefficient of the H-bridge voltage regulator, and s is a Laplace operator;
Step 2.1, 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 2.2, useThe digital phase-locked loop is used for comparing the power grid voltage sampling value v obtained in the step 2.1 g Phase locking is carried out to obtain a phase angle theta of the power grid voltage and an amplitude value V of the power grid voltage g ；
Step 2.3, grid-connected current sampling value i g The phase angle is delayed by pi/2 while the amplitude value is kept unchanged, and a grid-connected current sampling value i is obtained 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 The calculation formula is as follows:
wherein cos (theta) is a cosine value of a power grid voltage phase angle theta, and sin (theta) is a sine value of the power grid voltage phase angle theta;
step 2.4, setting a reactive current reference value for enabling the single-phase cascade type photovoltaic grid-connected inverter to operate at a unit power factor
Is 0, active current reference value>
The amplitude I of the grid-connected current instruction value of the single-phase cascade type photovoltaic grid-connected inverter calculated in the step 1.3 M Using an active current regulator to feed back an active current value i d Is controlled as an active current reference value->
Feeding back a reactive current value i using a reactive current regulator q Control as a reactive current reference value->
Amplitude v directed to the active modulation voltage d And the amplitude v of the reactive modulation voltage q The calculation formula is as follows: />
Wherein, K IP1 As the proportionality coefficient of the active current regulator, K II1 Is the integral coefficient of the active current regulator; k is IP2 Is the proportionality coefficient of the reactive current regulator, K II2 Is the integral coefficient of the reactive current regulator;
step 2.5, calculating the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter r And amplitude V of modulation wave of single-phase cascade type photovoltaic grid-connected inverter RM The calculation formula is as follows:
wherein, arctan (v) q /v d ) Denotes v q /v d The arctan value of;
step 3, improved type nearest level approximation modulation
Step 3.1, dividing the modulation wave vr of the single-phase cascade type photovoltaic grid-connected inverter calculated in the step 2.5 into four intervals in one period, and referring the four intervals to be an interval I, an interval II, an interval III and an interval IV, and defining as follows:
interval I: v. of r V is not less than 0 r Monotonically increasing
Interval II: v. of r V is not less than 0 r Monotonically decreasing
Interval III: v. of r < 0 and v r Monotonically decreasing
Interval IV: v. of r < 0 and v r Monotonically increasing
Step 3.2, obtaining the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter through calculation according to the interval I, the interval II, the interval III and the interval IV which are divided in the step 3.1 and the step 2.5 r Calculating the actual modulation wave v of the single-phase cascade photovoltaic grid-connected inverter Real The calculation formula is as follows:
step 3.3, V Hj J =1,2, j, N, representing the voltage sampling value of the direct-current bus capacitor of the jth H-bridge converter, and calculating the actual modulation wave v of the single-phase cascade photovoltaic grid-connected inverter according to the step 3.2 Real And judging the operation modes of the N H-bridge converters:
(1) When v is r When located in the interval I
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =1; otherwise, the operation mode H of the ith H-bridge converter Mi ＝0，i＝1，2，...，N；
(2) When v is r When located in interval II
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =0; otherwise, the operation mode H of the ith H-bridge converter Mi ＝1，i＝1，2，...，N；/>
(3) When v is r When located in interval III
If expression
If yes, the operation mode H of the ith H-bridge converter Mi -1; otherwise, the operation mode H of the ith H-bridge converter Mi ＝0，i＝1，2，...，N；
(4) When v is r When located in the interval IV
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =0; otherwise, the operation mode H of the ith H-bridge converter Mi ＝-1，i＝1，2，...，N；
Wherein, | v Real L is v Real Absolute value of (d); h Mi =1 ac output voltage v of ith H-bridge converter HOi ＝V Hi ；H Mi =0 ac output voltage v of ith H-bridge converter HOi ＝0；H Mi -1 represents the alternating output voltage v of the ith H-bridge converter HOi -V Hi ；
Step 4.1, calculating by using recursive discrete Fourier transform to obtain grid-connected current sampling value i g Fundamental component i of gF Then calculating a grid-connected current sampling value i g The harmonic component i contained gH The calculation formula is as follows:
i gH ＝i g -i gF
step 4.2, harmonic component i contained in grid-connected current sampling value is subjected to proportional control gH Control is carried out, and the output of the controller is a harmonic voltage signal v gH The calculation formula is as follows:
v gH ＝K FP i gH
wherein, K FP Is the proportionality coefficient of the proportional controller;
step 4.3, sampling the direct current bus capacitor voltage of the single-phase active filter to obtain a direct current bus capacitor voltage sampling value V of the single-phase active filter F ；
Step 4.4, using an active filtering voltage regulator to sample the voltage value V of the direct current bus capacitor of the single-phase active filter F Controlled to a reference voltage V ref The output of the active filter voltage regulator is the amplitude V of the voltage command value C The calculation formula is as follows:
wherein, K FVP Is the scale factor, K, of an active filter voltage regulator FVI Is the integral coefficient of the active filter voltage regulator;
step 4.5, calculating the amplitude V of the voltage command value output by the active filter voltage regulator according to the phase angle theta of the power grid voltage obtained in the step 2.2 and the amplitude V of the voltage command value output by the active filter voltage regulator obtained in the step 4.4 C Calculating to obtain fundamental voltage signal v CF The calculation formula is as follows:
v CF ＝V C cos(θ)
step 4.6, obtaining the harmonic voltage signal v by calculation according to the step 4.2 gH And step 4.5, calculating to obtain a fundamental voltage signal v CF Adding to obtain the total modulation voltage v of the single-phase active filter gF Then divided by the voltage sampling value V of the DC bus capacitor of the single-phase active filter F Then, obtaining the modulation wave m of the single-phase active filter gF The calculation formula is as follows:
m gF ＝(v gH +v CF )/V F
step 5, four-switch tube Buck-Boost converter control
Step 5.1, respectively sampling the direct-current bus capacitor voltage of the N four-switch tube Buck-Boost converters and the output current of the N photovoltaic modules to obtain direct-current bus capacitor voltage sampling values V of the N four-switch tube Buck-Boost converters PVi And output current sampling values I of N photovoltaic modules PVi ，i＝1，2，...，N；
Step 5.2, obtaining voltage sampling values V of the direct current bus capacitor of the N four-switch tube Buck-Boost converter according to the step 5.1 PVi And output current sampling values I of N photovoltaic modules PVi Respectively carrying out maximum power point tracking control on the N photovoltaic assemblies to obtain maximum power point voltages of the N photovoltaic assemblies
i＝1，2，...，N；
Step 5.3, obtaining the maximum power point voltage of the N photovoltaic modules obtained in the step 5.2
Direct-current bus capacitor electricity serving as four-switch tube Buck-Boost converterVoltage reference value, using N identical DC/DC voltage regulators to respectively sample voltage values V of DC bus capacitor of N four-switch tube Buck-Boost converters PVi Controlling the N DC/DC voltage regulators to output a current inner ring instruction value (or greater than or equal to the value) of the N four-switch tube Buck-Boost converters>
i =1,2,.., N, calculated as:
wherein, K DVP Is the proportionality coefficient of the DC/DC voltage regulator, K DVI Is the integral coefficient of the DC/DC voltage regulator;
and 5.4, respectively sampling the inductive currents of the N four-switch tube Buck-Boost converters to obtain inductive current sampling values i of the N four-switch tube Buck-Boost converters LDi ，i＝1，2，...，N；
Step 5.5, using N same DC/DC current regulators to obtain inductance current sampling values i of N four-switch tube Buck-Boost converters LDi Control is the current inner loop instruction value of four-switch tube Buck-Boost converter
The outputs of the N DC/DC current regulators are respectively the duty ratios d of N four-switch tube Buck-Boost converters i I =1,2,.., N, calculated as:
wherein, K DIP Is the proportionality coefficient of the DC/DC current regulator, K DII Is the integral coefficient of the DC/DC current regulator.
Compared with the prior art, the invention has the beneficial effects that:
(1) The single-phase cascade type photovoltaic grid-connected inverter and the control method thereof can deal with the working scene of serious unbalance of the output power of the photovoltaic component, and can enlarge the operation range of the system to a greater degree compared with the traditional single-phase cascade H-bridge photovoltaic grid-connected inverter.
(2) Even in a working scene with seriously unbalanced output power of the photovoltaic components, all the photovoltaic components can still work at the self maximum power point, and the generated energy of the system is maximized.
(3) By adopting the scheme of connecting the single-phase active filter in series, not only can the low-order harmonic of the output voltage of the inverter be filtered, but also the harmonic component of the voltage of the power grid can be filtered, and the better performance of the current of the power grid is ensured.
(4) Compared with the existing nearest level approximation modulation strategy, the improved nearest level approximation modulation strategy can ensure that the pulse widths output by all H-bridge converters have smaller difference, and further ensure that the consistency of the power transmission capability of the H-bridge converters is better.
Drawings
Fig. 1 shows a main circuit topology of a single-phase cascade type photovoltaic grid-connected inverter implemented by the present invention.
Fig. 2 is a main circuit topology of a single phase active filter embodying the present invention.
Fig. 3 is a control block diagram of a single-phase cascade-type photovoltaic grid-connected inverter implemented by the present invention.
Fig. 4 is a flowchart of a control method of a single-phase cascade-type photovoltaic grid-connected inverter implemented by the present invention.
FIG. 5 shows a modulation wave v of a single-phase cascaded grid-connected photovoltaic inverter according to an embodiment of the present invention r Interval division schematic diagram and actual modulation wave v of single-phase cascade type photovoltaic grid-connected inverter Real Schematic diagram of the waveform of (1).
FIG. 6 illustrates the AC output voltages v of four H-bridge converters in a four-module cascade system, using the improved recent level modulation strategy implemented in the present invention HO1 、v HO2 、v HO3 、v HO4 And the ac total output voltage v of the inverter H 。
FIG. 7 is a diagram of a system in which four modules are cascaded, as an example, using a conventional methodAC output voltage v of four H-bridge converters in recent level modulation strategy HO1 、v HO2 、v HO3 、v HO4 And the ac total output voltage v of the inverter H 。
FIG. 8 is an ith H-bridge converter operating mode H implemented in accordance with the present invention Mi ＝1、H Mi =0 and H Mi And (4) a method for generating a switch driving signal of the ith H-bridge converter when the signal is-1.
FIG. 9 shows the total AC output voltage v of a single-phase cascaded grid-connected PV inverter embodying the present invention H AC output voltage v of single-phase active filter F And a waveform diagram of the grid-connected point voltage vT.
FIG. 10 is a duty cycle d of an ith four-switch Buck-Boost converter in accordance with an embodiment of the invention i A method of generating corresponding four switching tube driving signals.
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 cascade type photovoltaic grid-connected inverter implemented by the invention, which includes N modules with the same structure, wherein N is a positive integer greater than 1, and each module is composed of a four-switch tube Buck-Boost converter connected in series with an H-bridge converter. Wherein, the ith (i =1,2,.., N) four-switch tube Buck-Boost converter is composed of a direct current bus capacitor C PVi A filter inductor L Di And four fully-controlled switching devices S with anti-parallel diodes Dij (j =1,2,3,4); the ith H-bridge converter is composed of a DC bus capacitor C Hi And four fully-controlled switching devices S with anti-parallel diodes Hij And (4) forming. Direct-current bus capacitor C of four-switch tube Buck-Boost converter PVi Each of which is connected in parallel with a photovoltaic module PV i For supplying power to the entire system. In addition, the alternating current output ends of all the modules are connected in series, and one end of the alternating current output ends passes through the filter inductor L after the alternating current output ends are connected in series g Is connected with the power grid, and the other end is connected with the power grid through a single-phase active filterAnd (6) connecting. In FIG. 1, v g And i g Respectively representing a sampling value of the power grid voltage and a sampling value of the grid-connected current; v PVi And I PVi Respectively representing a direct-current bus capacitor voltage sampling value of an ith four-switch tube Buck-Boost converter and an output current sampling value of a photovoltaic module connected with the direct-current bus capacitor voltage sampling value; i all right angle LDi The method comprises the steps of representing an inductive current sampling value of an ith four-switch tube Buck-Boost converter; v Hi Representing a voltage sampling value of a direct current bus capacitor of the ith H-bridge converter; v. of HOi Represents the ac output voltage of the H-bridge converter of the ith H-bridge converter; v. of H Representing the AC total output voltage, v, of a single-phase cascaded grid-connected photovoltaic inverter T And the grid-connected point voltage of the single-phase cascade type photovoltaic grid-connected inverter is represented.
FIG. 2 shows the main circuit topology of a single-phase active filter implemented by the present invention, which consists of a DC bus capacitor C F1 Four full-control type switching devices S with anti-parallel diodes Fj (j =1,2,3,4), filter inductance L of single-phase active filter F And a filter capacitor C of the single-phase active filter F2 And (4) forming. Wherein, V F Representing the voltage sample value of the DC bus capacitor of a single-phase active filter, v F Representing the ac output voltage of a single-phase active filter.
FIG. 3 is a control block diagram of a single-phase cascaded grid-connected photovoltaic inverter embodying the present invention, including average control of all H-bridge converter DC bus capacitor voltages, using a digital phase-locked loop to grid voltage v g Phase locking and grid current i g Switching device S for performing control, improved nearest level approximation modulation strategy and generating N H-bridge converters Hij Drive signal Q of Hij (i =1,2., N, j =1,2,3,4), control of dc bus capacitance voltage of N four-switch tube Buck-Boost converter, and switching device S generating N four-switch tube Buck-Boost converter Dij Drive signal Q of Dij And control and generation switching device S of a single-phase active filter Fj Drive signal Q of Fj 。
Fig. 4 is a flowchart of a control method of a single-phase cascade-type photovoltaic grid-connected inverter implemented by the present invention. As can be seen from fig. 3 and 4, the control method of the invention includes average value control of the dc bus capacitor voltage of the H-bridge converter, grid-connected current control, improved nearest level approximation modulation, single-phase active filter control and four-switch tube Buck-Boost converter control, and includes the following steps:
Step 1.1, respectively sampling the DC bus capacitor voltage of N H-bridge converters to obtain DC bus capacitor voltage sampling values V of the N H-bridge converters Hi ，i＝1，2，...，N。
Step 1.2, calculating to obtain an average value V of voltage sampling values of direct current bus capacitors of N H-bridge converters HAver The calculation formula is as follows:
step 1.3, using an H-bridge voltage regulator to obtain an average value V of voltage sampling values of direct current bus capacitors of N H-bridge converters HAver Controlled to a reference voltage V ref The output of the H-bridge voltage regulator is the amplitude I of the grid-connected current instruction value of the single-phase cascade type photovoltaic grid-connected inverter M The calculation formula is as follows:
wherein, K HVP Is the proportionality coefficient of the H-bridge voltage regulator, K HVI Is the integral coefficient of the H-bridge voltage regulator and s is the laplacian operator. In this embodiment, K HVP ＝10，K HVI ＝350。
Step 2.1, 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 2.2, obtaining by using a digital phase-locked loop in step 2.1Of the network voltage sampling value v g Phase locking is carried out to obtain a phase angle theta of the power grid voltage and an amplitude value V of the power grid voltage g 。
Step 2.3, grid-connected current sampling value i g The phase angle is delayed by pi/2 while the amplitude value is kept unchanged, and a grid-connected current sampling value i is obtained 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 The calculation formula is as follows:
wherein cos (theta) is a cosine value of a power grid voltage phase angle theta, and sin (theta) is a sine value of the power grid voltage phase angle theta.
In the embodiment, the grid current slave i is realized by using a second-order generalized integrator g Conversion to a signal i of equal amplitude but phase-delayed by pi/2 Q An all-pass filter may also be used to achieve this function.
Step 2.4, setting a reactive current reference value for enabling the single-phase cascade type photovoltaic grid-connected inverter to operate at a unit power factor
Is 0 and the active current reference value->
The amplitude I of the grid-connected current instruction value of the single-phase cascade type photovoltaic grid-connected inverter calculated in the step 1.3 M Using an active current regulator to feed back an active current value i d Is controlled as an active current reference value->
Feeding back a reactive current value i using a reactive current regulator q Control as a reactive current reference value->
Obtaining the amplitude v of the active modulation voltage d And the amplitude v of the reactive modulation voltage q The calculation formula is as follows:
wherein, K IP1 As the proportionality coefficient of the active current regulator, K II1 Is the integral coefficient of the active current regulator; k IP2 Is the proportionality coefficient of the reactive current regulator, K II2 Is the integral coefficient of the reactive current regulator; in this embodiment, K IP1 ＝1.5，K II1 ＝50，K IP2 ＝1.5，K II2 ＝50。
Step 2.5, calculating the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter r And amplitude V of modulation wave of single-phase cascade type photovoltaic grid-connected inverter RM The calculation formula is as follows:
wherein, arctan (v) q /v d ) Denotes v q /v d The arctan value of (c).
Step 3, improved type nearest level approximation modulation
Step 3.1, the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter obtained by the calculation in the step 2.5 r The four intervals are divided into four intervals in one cycle, and the four intervals are called an interval I, an interval II, an interval III, and an interval IV, and are defined as follows:
interval I: v. of r V is not less than 0 r Monotonically increasing
Interval II: v. of r V is not less than 0 r Monotonically decreasing
Interval III: v. of r < 0 and v r Monotonically decreasing in magnitude
Interval IV: v. of r < 0 and v r Monotonically increasing
Step 3.2, partitioning according to step 3.1The interval I, the interval II, the interval III and the interval IV, and the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter calculated in the step 2.5 r Calculating the actual modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter Real The calculation formula is as follows:
FIG. 5 shows a modulation wave v of a single-phase cascaded grid-connected photovoltaic inverter according to an embodiment of the present invention r Interval division schematic diagram and actual modulation wave v of single-phase cascade type photovoltaic grid-connected inverter Real Schematic diagram of the waveform of (1).
Step 3.3, V Hj J =1,2, j, N, representing the voltage sampling value of the direct-current bus capacitor of the jth H-bridge converter, and calculating the actual modulation wave v of the single-phase cascade photovoltaic grid-connected inverter according to the step 3.2 Real And judging the operation modes of the N H-bridge converters:
(1) When v is r When located in the interval I
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =1; otherwise, the working mode H of the ith H-bridge converter Mi ＝0，i＝1，2，...，N；/>
(2) When v is r When located in interval II
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =0; otherwise, the operation mode H of the ith H-bridge converter Mi ＝1，i＝1，2，...，N；
(3) When v is r When located in interval III
If expression
If yes, the operation mode H of the ith H-bridge converter Mi -1; otherwise, the operation mode H of the ith H-bridge converter Mi ＝0，i＝1，2，...，N；
(4) When v is r When located in the interval IV
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =0; otherwise, the operation mode H of the ith H-bridge converter Mi ＝-1，i＝1，2，...，N；
Wherein, | v Real L is v Real Absolute value of (d); h Mi =1 ac output voltage v of ith H-bridge converter HOi ＝V Hi ；H Mi =0 ac output voltage v of ith H-bridge converter HOi ＝0；H Mi = -1 ac output voltage v of ith H-bridge converter HOi ＝-V Hi ；
FIG. 6 illustrates the AC output voltages v of four H-bridge converters in a four-module cascade system, using the improved recent level modulation strategy implemented in the present invention HO1 、v HO2 、v HO3 、v HO4 And the ac total output voltage v of the inverter H . FIG. 7 illustrates an example of an AC output voltage v of four H-bridge converters in a four-module cascade system, using a conventional recent level modulation strategy HO1 、v HO2 、v HO3 、v HO4 And the ac total output voltage v of the inverter H . It can be seen that, although the traditional nearest level approximation modulation strategy and the improved nearest level approximation modulation strategy provided by the invention are adopted, the total AC output voltage v of the inverter H Is identical, but when the traditional recent level approximation debugging strategy is adopted, the alternating current output voltages v of the four H-bridge converters HO1 、v HO2 、v HO3 And v HO4 The pulse width of the converter is greatly different, which means that the power transmitted by the four H-bridge converters is greatly different; when the improved nearest level approach debugging strategy provided by the invention is adopted, four H bridgesAC output voltage v of converter HO1 、v HO2 、v HO3 And v HO4 The pulse width difference of the H-bridge converter is small, and the power difference transmitted by the H-bridge converter is also small.
Working modes H of all H-bridge converters are obtained through calculation according to the step 3.3 Mi Thereafter, the switching drive signals of the N H-bridge converters are generated according to the method for generating the switching drive signals of the ith H-bridge converter implemented by the present invention shown in fig. 8. Taking the ith H-bridge converter as an example: if the working mode H Mi =1, switching tube S Hi1 Drive signal Q of Hi1 Is at a high level, S Hi1 Conducting, switching tube S Hi2 Drive signal Q of Hi2 Is at a low level, S Hi2 Turn-off, switch tube S Hi3 Drive signal Q of Hi3 Is at a low level, S Hi3 Turn-off, switch tube S Hi4 Drive signal Q of Hi4 Is at a high level, S Hi4 Conducting; if the working mode H Mi = -1, switching tube S Hi1 Drive signal Q of Hi1 At a low level, S Hi1 Turn-off, switch tube S Hi2 Drive signal Q of Hi2 Is at a high level, S Hi2 Conducting, switching tube S Hi3 Drive signal Q of Hi3 Is at a high level, S Hi3 Conducting, switching tube S Hi4 Drive signal Q of Hi4 Is at a low level, S Hi4 And (6) turning off. If the working mode H Mi =0, there are two methods for generating the driving signal of the switching tube: method 1, switching tube S Hi1 Drive signal Q of Hi1 Is at a high level, S Hi1 Conducting, switching tube S Hi2 Drive signal Q of Hi2 Is at a low level, S Hi2 Turn off and switch tube S Hi3 Drive signal Q of Hi3 Is at a high level, S Hi3 Conducting, switching tube S Hi4 Drive signal Q of Hi4 Is at a low level, S Hi4 Turning off; method 2, switching tube S Hi1 Drive signal Q of Hi1 Is at a low level, S Hi1 Turn-off, switch tube S Hi2 Drive signal Q of Hi2 Is at a high level, S Hi2 Conducting, switching tube S Hi3 Drive signal Q of Hi3 Is at a low level, S Hi3 Turn-off, switch tube S Hi4 Drive signal Q of Hi4 Is at a high level, S Hi4 And conducting.
Step 4.1, calculating by using recursive discrete Fourier transform to obtain grid-connected current sampling value i g Fundamental component i of gF Then calculating a grid-connected current sampling value i g The harmonic component i contained gH The calculation formula is as follows:
i gH ＝i g -i gF
the recursive discrete fourier transform used herein refers to a method commonly used in harmonic analysis and extraction, and the method has been reported in a large number of scientific documents, such as documents "Chen Qiao, zhang Xing, li Ming, guo Zixuan, liu Xiaoxi, zhao is. 123-129. "(" power system automation "volume 44, no. 7, pages 123-129) in 2020) and documents" "" Chen Qiao, zhang Xing, li Ming, liu Xiaoxi, guo Zixuan. Short circuit ratio measurement method based on impedance identification. Electrical engineering newspaper, 2019, 14 (2): 7-11. "(journal of Electrical engineering 2020, vol.14, no. 2, pages 7-11).
Step 4.2, harmonic component i contained in grid-connected current sampling value is subjected to proportional control gH Control is carried out, and the output of the controller is a harmonic voltage signal v gH The calculation formula is as follows:
v gH ＝K FP i gH
wherein, K FP In this embodiment, K is the scaling factor of the proportional controller FP ＝10。
Step 4.3, sampling the direct current bus capacitor voltage of the single-phase active filter to obtain a direct current bus capacitor voltage sampling value V of the single-phase active filter F 。
Step 4.4, using an active filtering voltage regulator to sample the voltage value V of the direct current bus capacitor of the single-phase active filter F Controlled to a reference voltage V ref The output of the active filter voltage regulator is the amplitude V of the voltage command value C Which isThe calculation formula is as follows:
wherein, K FVP Is the scale factor, K, of an active filter voltage regulator FVI Is the integral coefficient of the active filtering voltage regulator. K FVP ＝2.4，K FVI ＝120。
Step 4.5, calculating the amplitude V of the voltage command value output by the active filter voltage regulator according to the phase angle theta of the power grid voltage obtained in the step 2.2 and the amplitude V of the voltage command value output by the active filter voltage regulator obtained in the step 4.4 C Calculating to obtain fundamental voltage signal v CF The calculation formula is as follows:
v CF ＝V C cos(θ)
step 4.6, obtaining the harmonic voltage signal v by calculation according to the step 4.2 gH And step 4.5, calculating to obtain a fundamental voltage signal v CF Adding to obtain the total modulation voltage v of the single-phase active filter gF Then divided by the voltage sampling value V of the DC bus capacitor of the single-phase active filter F Then, obtaining the modulation wave m of the single-phase active filter gF The calculation formula is as follows:
m gF ＝(v gH +v CF )/V F
calculating to obtain a modulation wave m of the single-phase active filter gF Then, four switching devices S are obtained according to Pulse Width Modulation (PWM) commonly used in power electronics technology Fj (j =1,2,3,4) switching drive signal Q Fj 。
FIG. 9 shows the total AC output voltage v of a single-phase cascade-type grid-connected PV inverter implemented according to the present invention H AC output voltage v of single-phase active filter F And the voltage v of the grid-connected point T Schematic diagram of the waveform of (1). The single-phase active filter is mainly used for filtering out the AC total output voltage v of the single-phase cascade type photovoltaic grid-connected inverter H The low-order harmonic component ensures that the performance of the current of the power grid is better.
Step 5, four-switch tube Buck-Boost converter control
Step 5.1, respectively sampling the direct-current bus capacitor voltage of the N four-switch tube Buck-Boost converters and the output current of the N photovoltaic modules to obtain direct-current bus capacitor voltage sampling values V of the N four-switch tube Buck-Boost converters PVi And output current sampling values I of N photovoltaic modules PVi ，i＝1，2，...，N。
Step 5.2, obtaining voltage sampling values V of the direct current bus capacitor of the N four-switch tube Buck-Boost converter according to the step 5.1 PVi And output current sampling values I of N photovoltaic modules PVi Respectively carrying out maximum power point tracking control on the N photovoltaic assemblies to obtain maximum power point voltages of the N photovoltaic assemblies
i＝1，2，...，N。
Step 5.3, obtaining the maximum power point voltage of the N photovoltaic assemblies obtained in the step 5.2
As a reference value of the direct-current bus capacitor voltage of the four-switch tube Buck-Boost converter, N same DC/DC voltage regulators are used for respectively carrying out sampling value V on the direct-current bus capacitor voltage of the N four-switch tube Buck-Boost converters PVi Controlling the N DC/DC voltage regulators to output a current inner ring instruction value (or greater than or equal to the value) of the N four-switch tube Buck-Boost converters>
i =1,2,.., N, calculated as:
wherein, K DVP Is the proportionality coefficient of the DC/DC voltage regulator, K DVI Is the integral coefficient of the DC/DC voltage regulator. In this embodiment, K DVP ＝5，K DVI ＝20。
Step 5.4, respectively aligning NThe inductive current of the four-switch tube Buck-Boost converter is sampled to obtain inductive current sampling values i of N four-switch tube Buck-Boost converters LDi ，i＝1，2，...，N。
And (5) in the step (5.5), N same DC/DC current regulators are used for sampling inductive current i of N four-switch tube Buck-Boost converters LDi Control is the current inner loop instruction value of four-switch tube Buck-Boost converter
The outputs of the N DC/DC current regulators are respectively the duty ratio d of the N four-switch tube Buck-Boost converters i I =1,2,.., N, calculated as:
wherein, K DIP Is the proportionality coefficient of the DC/DC current regulator, K DII Is the integral coefficient of the DC/DC current regulator. In this embodiment, K DIP ＝5，K DII ＝20。
FIG. 10 is a duty cycle d of an ith four-switch Buck-Boost converter in accordance with an embodiment of the invention i A method of generating corresponding four switching tube driving signals. When the duty ratio is 0 < d i When the duty ratio is less than or equal to 0.5, the four-switch Buck-Boost converter works in a Buck mode, and the equivalent duty ratio at the moment can be expressed as d i1 ＝2d i Switching device S Di1 Drive signal Q of Di1 Can be controlled by duty cycle d i1 And carrier v cD1 Obtained by comparison when d i1 ＞v cD1 When is, Q Di1 At a high level, the switching device S Di1 Conducting; when d is i1 ≤v cD1 When is, Q Di1 At a low level, the switching device S Di1 And (6) turning off. Switching device S Di2 Drive signal Q of Di2 Is and Q Di1 Complementary signals, i.e. Q Di1 At high level, Q Di2 Is low level, Q Di1 At low level, Q Di2 Is high. In this operating mode, the switching device S Di3 Is driven by letterNumber Q Di3 Always at high level, i.e. switching devices S Di3 Conducting all the time; switching device S Di4 Drive signal Q of Di4 Always at low level, i.e. switching devices S Di4 Is always turned off. When the duty ratio is 0.5 < d i When the duty ratio is less than or equal to 1, the four-switch Buck-Boost converter works in a Boost mode, and the equivalent duty ratio at the moment can be expressed as d i2 ＝2d i -1. In this operating mode, the switching device S Di1 Drive signal Q of Di1 Always at high level, i.e. switching devices S Di1 Conducting all the time; switching device S Di2 Drive signal Q of Di2 Always at low level, i.e. switching devices S Di2 Is always turned off. Switching device S Di4 Drive signal Q of Di4 Can be controlled by duty cycle d i2 And carrier v cD2 Obtained by comparison when d i2 ＞v cD2 When is, Q Di4 At a high level, the switching device S Di4 Conducting; when d is i2 ＞v cD2 When is, Q Di4 At a low level, the switching device S Di4 And (6) turning off. Switching device S Di3 Drive signal Q of Di3 Is and Q Di4 Complementary signals, i.e. Q Di4 At high level, Q Di3 Is low level, Q Di4 At low level, Q Di3 Is high.
Claims (1)
1. A control method of a single-phase cascade type photovoltaic grid-connected inverter comprises N modules with the same structure, wherein N is a positive integer larger than 1, each module is formed by connecting a four-switch tube Buck-Boost converter in series with an H-bridge converter, direct current input ends of all the modules are respectively connected with a photovoltaic component in parallel, alternating current output ends of all the modules are connected in series, one end of each module is connected with a power grid through a filter inductor after being connected in series, and the other end of each module is connected with the power grid through a single-phase active filter.
Step 1,H average control of DC bus capacitor voltage of bridge converter
Step 1.1, respectively sampling the DC bus capacitor voltage of N H-bridge converters to obtain DC bus capacitor voltage sampling values V of the N H-bridge converters Hi ，i＝1，2，...，N；
Step 1.2, calculating to obtain an average value V of voltage sampling values of direct current bus capacitors of N H-bridge converters HAver The calculation formula is as follows:
step 1.3, using an H-bridge voltage regulator to obtain an average value V of voltage sampling values of direct current bus capacitors of N H-bridge converters HAver Controlled to a reference voltage V ref The output of the H-bridge voltage regulator is the amplitude I of the grid-connected current instruction value of the single-phase cascade type photovoltaic grid-connected inverter M The calculation formula is as follows:
wherein, K HVP Is the proportionality coefficient of the H-bridge voltage regulator, K HVI Is the integral coefficient of the H-bridge voltage regulator, and s is a 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 v g And grid-connected current sampling value i g ；
Step 2.2, using a digital phase-locked loop to compare the grid voltage sampling value v obtained in the step 2.1 g Performing phase locking to obtain a phase angle theta of the power grid voltage and an amplitude V of the power grid voltage g ；
Step 2.3, grid-connected current sampling value i g The phase angle is delayed by pi/2 while the amplitude value is kept unchanged, and a grid-connected current sampling value i is obtained 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 The calculation formula is as follows:
wherein cos (theta) is a cosine value of a power grid voltage phase angle theta, and sin (theta) is a sine value of the power grid voltage phase angle theta;
step 2.4, setting a reactive current reference value for enabling the single-phase cascade type photovoltaic grid-connected inverter to operate at a unit power factor
Is 0 and the active current reference value->
The amplitude I of the grid-connected current instruction value of the single-phase cascade type photovoltaic grid-connected inverter calculated in the step 1.3 M Using an active current regulator to feed back an active current value i d Is controlled as an active current reference value->
Feeding back a reactive current value i using a reactive current regulator q Control as a reactive current reference value->
Obtaining the amplitude v of the active modulation voltage d And the amplitude v of the reactive modulation voltage q The calculation formula is as follows: />
Wherein, K IP1 As the proportionality coefficient of the active current regulator, K II1 For active current regulationThe integral coefficient of the device; k IP2 Is the proportionality coefficient of the reactive current regulator, K II2 Is the integral coefficient of the reactive current regulator;
step 2.5, calculating the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter r And amplitude V of modulation wave of single-phase cascade type photovoltaic grid-connected inverter RM The calculation formula is as follows:
wherein, arctan (v) q /v d ) Denotes v q /ν d The arctan value of;
step 3, improved type nearest level approximation modulation
Step 3.1, the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter obtained by the calculation in the step 2.5 r The four intervals are divided into four intervals in one cycle, and the four intervals are called an interval I, an interval II, an interval III, and an interval IV, and are defined as follows:
interval I: v. of r V is not less than 0 r Monotonically increasing
Interval II: v. of r V is not less than 0 r Monotonically decreasing
Interval III: v. of r < 0 and v r Monotonically decreasing
Interval IV: v. of r < 0 and v r Monotonically increasing
Step 3.2, obtaining the modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter through calculation according to the interval I, the interval II, the interval III and the interval IV which are divided in the step 3.1 and the step 2.5 r Calculating the actual modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter Real The calculation formula is as follows:
step 3.3, V Hj J =1,2,.., N, root, representing dc bus capacitor voltage sample value of jth H-bridge converterThe actual modulation wave v of the single-phase cascade type photovoltaic grid-connected inverter calculated according to the step 3.2 Real And judging the operation modes of the N H-bridge converters:
(1) When v is r When located in the interval I
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =1; otherwise, the operation mode H of the ith H-bridge converter Mi ＝0，i＝1，2，...，N；
(2) When v is r When located in interval II
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =0; otherwise, the operation mode H of the ith H-bridge converter Mi ＝1，i＝1，2，...，N；
(3) When v is r When located in interval III
If expression
If yes, the operation mode H of the ith H-bridge converter Mi -1; otherwise, the operation mode H of the ith H-bridge converter Mi ＝0，i＝1，2，...，N；
(4) When v is r When located in the interval IV
If expression
If yes, the operation mode H of the ith H-bridge converter Mi =0; otherwise, the operation mode H of the ith H-bridge converter Mi ＝-1，i＝1，2，...，N；
Wherein, | v Real L is v Real Absolute value of (d); h Mi =1 ac output voltage v of ith H-bridge converter HOi ＝V Hi ；H Mi =0 for ith H bridgeAC output voltage v of the converter HOi ＝0；H Mi -1 represents the alternating output voltage v of the ith H-bridge converter HOi -V Hi ；
Step 4, single-phase active filter control
Step 4.1, calculating by using recursive discrete Fourier transform to obtain grid-connected current sampling value i g Fundamental component i of gF Then calculating a grid-connected current sampling value i g The harmonic component i contained gH The calculation formula is as follows:
i gH ＝i g -i gF
step 4.2, harmonic component i contained in grid-connected current sampling value is subjected to proportional control gH Control is carried out, and the output of the controller is a harmonic voltage signal v gH The calculation formula is as follows:
v gH ＝K FP i gH
wherein, K FP Is the proportionality coefficient of the proportional controller;
step 4.3, sampling the direct current bus capacitor voltage of the single-phase active filter to obtain a direct current bus capacitor voltage sampling value V of the single-phase active filter F ；
Step 4.4, using an active filtering voltage regulator to sample the voltage value V of the direct current bus capacitor of the single-phase active filter F Controlled to a reference voltage V ref The output of the active filter voltage regulator is the amplitude V of the voltage command value C The calculation formula is as follows:
wherein, K FVP Is the scale factor, K, of an active filter voltage regulator FVI Is the integral coefficient of the active filter voltage regulator;
step 4.5, calculating the amplitude V of the voltage command value output by the active filter voltage regulator according to the phase angle theta of the power grid voltage obtained in the step 2.2 and the amplitude V of the voltage command value output by the active filter voltage regulator obtained in the step 4.4 C Calculating to obtain fundamental voltage signal v CF The calculation formula is as follows:
v CF ＝V C cos(θ)
step 4.6, obtaining the harmonic voltage signal v by calculation according to the step 4.2 gH And step 4.5, calculating to obtain a fundamental voltage signal v CF Adding to obtain the total modulation voltage v of the single-phase active filter gF Then divided by the voltage sampling value V of the DC bus capacitor of the single-phase active filter F Then, obtaining the modulation wave m of the single-phase active filter gF The calculation formula is as follows:
m gF ＝(v gH +v CF )/V F
step 5, four-switch tube Buck-Boost converter control
Step 5.1, respectively sampling the direct-current bus capacitor voltage of the N four-switch tube Buck-Boost converters and the output current of the N photovoltaic modules to obtain direct-current bus capacitor voltage sampling values V of the N four-switch tube Buck-Boost converters PVi And output current sampling values I of N photovoltaic modules PVi ，i＝1，2，...，N；
Step 5.2, obtaining voltage sampling values V of the direct current bus capacitor of the N four-switch tube Buck-Boost converter according to the step 5.1 PVi And output current sampling values I of N photovoltaic modules PVi Respectively carrying out maximum power point tracking control on the N photovoltaic assemblies to obtain maximum power point voltages of the N photovoltaic assemblies
i＝1，2，...，N；/>
Step 5.3, obtaining the maximum power point voltage of the N photovoltaic modules obtained in the step 5.2
As a reference value of the direct-current bus capacitor voltage of the four-switch tube Buck-Boost converter, N same DC/DC voltage regulators are used for respectively carrying out sampling value V on the direct-current bus capacitor voltage of the N four-switch tube Buck-Boost converters PVi The outputs of the N DC/DC voltage regulators are N four switching tubes BuCurrent inner loop instruction value of ck-Boost converter>
The calculation formula is as follows:
wherein, K DVP Is the proportionality coefficient of the DC/DC voltage regulator, K DVI Is the integral coefficient of the DC/DC voltage regulator;
and 5.4, respectively sampling the inductive currents of the N four-switch tube Buck-Boost converters to obtain inductive current sampling values i of the N four-switch tube Buck-Boost converters LDi ，i＝1，2，...，N；
Step 5.5, using N same DC/DC current regulators to sample the inductive current i of N four-switch tube Buck-Boost converters LDi Control is the current inner loop instruction value of four-switch tube Buck-Boost converter
The outputs of the N DC/DC current regulators are respectively the duty ratio d of the N four-switch tube Buck-Boost converters i I =1,2,.., N, calculated as:
wherein, K DIP Is the proportionality coefficient of the DC/DC current regulator, K DII Is the integral coefficient of the DC/DC current regulator.
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