CN110611451B - A photovoltaic inverter based on gallium nitride device and control method thereof - Google Patents
A photovoltaic inverter based on gallium nitride device and control method thereof Download PDFInfo
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- 229910002601 GaN Inorganic materials 0.000 title claims abstract description 26
- JMASRVWKEDWRBT-UHFFFAOYSA-N Gallium nitride Chemical compound [Ga]#N JMASRVWKEDWRBT-UHFFFAOYSA-N 0.000 title claims abstract description 25
- 238000000034 method Methods 0.000 title claims abstract description 21
- 239000003990 capacitor Substances 0.000 claims abstract description 47
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 8
- 229910052710 silicon Inorganic materials 0.000 claims description 8
- 239000010703 silicon Substances 0.000 claims description 8
- 239000010409 thin film Substances 0.000 claims description 6
- 238000004804 winding Methods 0.000 claims description 6
- 230000008901 benefit Effects 0.000 abstract description 4
- 238000010586 diagram Methods 0.000 description 6
- 238000005516 engineering process Methods 0.000 description 6
- 230000000694 effects Effects 0.000 description 5
- 239000004065 semiconductor Substances 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 3
- 230000002035 prolonged effect Effects 0.000 description 3
- 230000007547 defect Effects 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 230000010354 integration Effects 0.000 description 2
- 238000010248 power generation Methods 0.000 description 2
- 238000012827 research and development Methods 0.000 description 2
- 239000002210 silicon-based material Substances 0.000 description 2
- 238000013459 approach Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 230000005284 excitation Effects 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 238000002955 isolation Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
Classifications
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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
- H02M3/22—Conversion of DC power input into DC power output with intermediate conversion into AC
- H02M3/24—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters
- H02M3/28—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC
- H02M3/325—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal
- H02M3/335—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/33569—Conversion of DC power input into DC power output with intermediate conversion into AC by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate AC using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
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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
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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
- H02M7/539—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 with automatic control of output wave form or frequency
- H02M7/5395—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 with automatic control of output wave form or frequency by pulse-width modulation
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0048—Circuits or arrangements for reducing losses
- H02M1/0054—Transistor switching losses
- H02M1/0058—Transistor switching losses by employing soft switching techniques, i.e. commutation of transistors when applied voltage is zero or when current flow is zero
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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
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
- Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
-
- 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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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Inverter Devices (AREA)
- Dc-Dc Converters (AREA)
Abstract
本发明公开了一种基于氮化镓器件的光伏逆变器及其控制方法，包括用于将光伏电池产生的低压直流电转变为高压直流电的第一级变换器，以及用于将所述高压直流电转换成正弦波的第二级变换器；包括一个防反二极管、低压直流滤波电容、低压全桥逆变电路、谐振电路、高频变压器、倍压整流电路、高压直流滤波电容、高压全桥逆变电路和输出滤波器；整机的各个部分经优化设计后具有高电压增益、低损耗、高功率密度的优点。
The present invention discloses a photovoltaic inverter based on gallium nitride devices and a control method thereof, comprising a first-stage converter for converting low-voltage direct current generated by a photovoltaic cell into high-voltage direct current, and a second-stage converter for converting the high-voltage direct current into a sine wave; comprising an anti-reverse diode, a low-voltage direct current filter capacitor, a low-voltage full-bridge inverter circuit, a resonant circuit, a high-frequency transformer, a voltage doubler rectifier circuit, a high-voltage direct current filter capacitor, a high-voltage full-bridge inverter circuit and an output filter; each part of the whole machine has the advantages of high voltage gain, low loss and high power density after being optimized.
Description
Technical Field
The invention relates to the technical field of photovoltaic power generation, in particular to a photovoltaic inverter based on a gallium nitride device and a control method thereof.
Background
The photovoltaic power generation technology is a new energy technology for converting light energy into electric energy. The photovoltaic inverter converts direct current output by the photovoltaic cell panel into alternating current and sends the electric energy into an alternating current power grid. The photovoltaic inverter can be classified into a centralized photovoltaic inverter, a string type photovoltaic inverter, a micro photovoltaic inverter, and the like according to the capacity. Among them, the micro inverter is attracting attention because of its advantages in terms of maximum power tracking efficiency, flexibility, reliability, and the like. The research and development of the miniature grid-connected photovoltaic inverter with high efficiency and high power density has huge market value and good development prospect. The technical staff can use smaller-sized elements such as inductors, transformers and the like in the power electronic converter due to the improvement of the working frequency of the circuit, so that the whole machine size is reduced, and the power density is improved; there is a trend to employ faster semiconductor switching devices.
At present, a silicon-based semiconductor device is mainly adopted in the micro photovoltaic inverter, however, the performance of the silicon-based semiconductor device gradually approaches the theoretical limit of a silicon material, the updating speed is continuously slowed down, and the performance of the inverter is difficult to further improve. How to provide an inverter to improve the performance of the whole machine, optimize the circuit topology, improve the efficiency and reduce the cost.
The basic requirement of the photovoltaic inverter is long-time stable grid-connected operation, and the service life of the micro photovoltaic inverter is generally required to reach 20-25 years. The electrolytic capacitor in the main loop is a bottleneck of the service life of all power electronic converters, and the long-life design must reduce or avoid the use of the electrolytic capacitor, so that research and development of power conversion technology without the electrolytic capacitor are required.
Disclosure of Invention
The invention aims to provide a photovoltaic inverter based on a gallium nitride device and a control method thereof, which are used for solving one of the defects or the defects caused by the prior art.
In order to achieve the above purpose, the invention is realized by adopting the following technical scheme:
A photovoltaic inverter based on gallium nitride devices, comprising a first-stage converter for converting low-voltage direct current generated by a photovoltaic cell into high-voltage direct current, and a second-stage converter for converting the high-voltage direct current into a sine wave;
The first-stage converter comprises a low-voltage direct-current filter capacitor C 1, a low-voltage full-bridge inverter circuit, a resonant circuit, a high-frequency transformer T 1 and a voltage doubling rectifying circuit, wherein the low-voltage direct-current filter capacitor C 1 is connected to two ends of a photovoltaic cell; the direct current side of the low-voltage full-bridge inverter circuit is connected with the low-voltage direct current filter capacitor, the alternating current side of the low-voltage full-bridge inverter circuit is connected with the resonant circuit and the primary winding of the high-frequency transformer T 1, and the secondary winding combination of the high-frequency transformer T 1 is connected with the input end of the voltage doubling rectifying circuit;
The second-stage converter comprises a high-voltage filter capacitor C 4 and a high-voltage full-bridge inverter circuit, the output end of the voltage doubling rectifying circuit is connected with the high-voltage direct-current filter capacitor C 4 and the direct-current side of the high-voltage full-bridge inverter circuit, and the alternating-current side of the high-voltage full-bridge inverter circuit is connected with a power grid.
Further, the photovoltaic cell further comprises an anti-reflection diode, wherein the anode of the anti-reflection diode is connected with the positive electrode end of the photovoltaic cell, and the cathode of the anti-reflection diode and the negative electrode end of the photovoltaic cell are respectively connected to the two ends of the low-voltage direct-current filter capacitor C 1.
Further, the resonant circuit includes a resonant capacitor C r and a resonant inductance L r.
Further, an output filter is further connected between the alternating current side of the full-bridge inverter circuit and the power grid.
Further, the high frequency transformer T 1 is a step-up transformer.
Further, the low-voltage full-bridge inverter circuit comprises four gallium nitride switches.
Further, the high-voltage filter capacitor C 4 is a thin film capacitor.
Further, the full-bridge inverter circuit comprises a low-frequency bridge arm and a high-frequency bridge arm, wherein the low-frequency bridge arm comprises two silicon switches, and the high-frequency bridge arm comprises two gallium nitride switches.
The invention also provides a control method of the photovoltaic inverter based on the gallium nitride device, which comprises the following steps:
acquiring a predicted value of the voltage of the high-voltage direct-current bus in the next period according to the acquired instantaneous value of the voltage of the high-voltage direct-current bus in the current period;
according to the predicted value of the high-voltage direct-current bus of the next period, obtaining the voltage gain ratio K of the next period of the first-stage converter;
obtaining an equivalent load resistance R of a first-stage converter;
Acquiring the switching frequency f 1 of the next period of the first-stage converter according to the voltage gain ratio K and the equivalent load resistance R of the next period;
And acquiring a driving signal of a switch of the low-voltage full-bridge inverter circuit according to the switching frequency f 1.
Further, the method further comprises:
Obtaining a reference value U dcref of a low-voltage direct-current bus voltage instantaneous value;
According to the reference value U dcref, acquiring the amplitude I acref of the reference value of the output current of the photovoltaic inverter;
Acquiring a phase factor sin theta of the power grid voltage according to the power grid voltage instantaneous value u ac;
acquiring a reference value I acref of the output current of the photovoltaic inverter according to the phase factor sin theta of the power grid voltage and the amplitude I acref of the reference value of the output current of the photovoltaic inverter;
Obtaining a modulation ratio d of the output voltage of the photovoltaic inverter according to the reference value i acref of the output current;
According to the modulation ratio d, driving signals of two switches in the high-voltage full-bridge inverter circuit are obtained;
And obtaining driving signals of the other two switches of the high-voltage full-bridge inverter circuit according to the power grid voltage instantaneous value u ac.
According to the technical scheme, the embodiment of the invention has at least the following effects:
1. The LLC resonant soft switching circuit commonly used for the voltage reduction circuit is designed into a voltage boosting mode and is combined with the voltage doubling rectifying circuit, so that the effect of further improving the voltage boosting ratio is achieved, and the LLC resonant soft switching circuit is particularly suitable for application occasions in which the photovoltaic cell terminal voltage in the miniature photovoltaic inverter is low and grid-connected inversion requires high direct current bus voltage; meanwhile, due to the adoption of the novel gallium nitride switching device, the circuit can work at high frequency, and LLC resonance realizes zero voltage switching-on (ZVS), so that the circuit response is quick, ripple waves of voltage and current are reduced, the volumes of main elements of the circuit such as an inductor, a capacitor, a transformer and the like can be reduced, and therefore the power density is greatly improved, and the loss is reduced.
2. According to the invention, the two bridge arms of the full-bridge inverter circuit respectively use the silicon switch controlled by the power frequency and the gallium nitride switch modulated by the high frequency, so that the advantages of the two switching devices are fully exerted, the requirement on the waveform quality of the grid-connected current is met, and the loss and the cost are reduced as much as possible.
3. According to the power decoupling control method adopting power feedforward and voltage feedback control, the fluctuation of the voltage of the high-voltage direct-current bus is detected by introducing the power feedforward, and the working frequency is adjusted according to the principle of an LLC resonant soft switching circuit so as to control the voltage gain K of the first-stage DC-DC converter to fluctuate along with alternating current power; the output voltage of the first-stage DC-DC converter actively adapts to voltage fluctuation caused by power fluctuation on the high-voltage direct-current bus, so that the voltage of the low-voltage direct-current bus can be kept basically stable. As a result, on the one hand, the voltage ripple of the low-voltage direct current bus is small, which is favorable for the photovoltaic cell to maintain the maximum generated power, and meanwhile, the capacity of the low-voltage filter capacitor C 1 is reduced; on the other hand, the control method allows the high-voltage direct current bus to fluctuate in a larger amplitude, and the high-voltage capacitor with larger capacity but shorter service life is not needed to control the voltage ripple, so that the thin film capacitor with small capacity and high reliability can be used, and the service life of the whole machine is greatly prolonged.
Drawings
Fig. 1 is a circuit configuration diagram of an inverter according to an embodiment of the present invention;
FIG. 2 is a schematic diagram illustrating analysis of power and voltage fluctuations throughout an inverter according to an embodiment of the present invention;
FIG. 3 is a flow chart of a method of controlling a first stage DC-DC circuit in accordance with an embodiment of the present invention;
FIG. 4 is a block diagram of a second stage DC-AC circuit control method in accordance with an embodiment of the present invention;
FIG. 5 is a flow chart of a disturbance observation method employed in embodiments of the present invention;
FIG. 6 is a schematic diagram of K-F x relationship in an embodiment of the present invention.
Detailed Description
The invention is further described in connection with the following detailed description, in order to make the technical means, the creation characteristics, the achievement of the purpose and the effect of the invention easy to understand.
A two-stage soft-start Guan Weixing photovoltaic inverter with high efficiency, high power density and high reliability, which is formed by gallium nitride devices; the invention utilizes the advantage of fast switching speed of the gallium nitride device, optimally designs the first-stage soft switching DC-DC converter working at high switching frequency, reduces the volume and improves the efficiency of the whole machine. In the second-stage DC-AC converter, a single-phase full-bridge inverter circuit with a low-frequency bridge arm and a high-frequency bridge arm mixed is adopted, wherein the low-frequency bridge arm adopts a conventional silicon-based semiconductor switching device, and the high-frequency bridge arm adopts a gallium nitride switching device, so that the performance index is met, and meanwhile, the cost is reduced; finally, high-efficiency power conversion from the photovoltaic cell panel to the single-phase alternating current power grid is realized.
As shown in FIG. 1, the photovoltaic inverter based on the gallium nitride device provided by the invention comprises an anti-reflection diode, a low-voltage direct-current filter capacitor C 1, a low-voltage full-bridge inverter circuit, a resonant circuit, a high-frequency transformer T 1, a voltage doubling rectifying circuit, a high-voltage direct-current filter capacitor C 4, a high-voltage full-bridge inverter circuit and an output filter; The anode of the anti-reflection diode is connected with the positive terminal of the photovoltaic cell, and the cathode of the anti-reflection diode and the negative terminal of the photovoltaic cell are respectively connected to the two ends of the low-voltage direct-current filter capacitor C 1; the four switches of the low-voltage full-bridge inverter circuit adopt gallium nitride switches Q 1～Q4, the direct current side of the gallium nitride switches Q 1～Q4 is connected with a low-voltage direct current filter capacitor C 1, and the alternating current side of the gallium nitride switches Q 1～Q4 is connected with the resonant circuit and the primary winding of a high-frequency transformer T 1; The resonant circuit comprises a resonant capacitor C r and a resonant inductor L r; two terminals of a secondary winding of the high-frequency transformer T 1 are respectively connected between two rectifying diodes D 1、D2 of the voltage doubling rectifying circuit and between two capacitors C 2、C3; The output end of the voltage doubling rectifying circuit is connected with the direct current side of the high-voltage direct current filter capacitor C 4 and the high-voltage full-bridge inverter circuit; The output filter comprises an inductor L f and a capacitor C f, wherein the inductor L f is connected in series between the alternating-current side of the high-voltage full-bridge inverter circuit and the power grid, and the capacitor C f is connected in parallel to two terminals of the single-phase power grid.
The first-stage soft switching DC-DC converter in the photovoltaic inverter is composed of a low-voltage direct-current filter capacitor C 1, a low-voltage full-bridge inverter circuit, a resonant circuit, a high-frequency transformer T 1 and a voltage doubling rectifying circuit. The low-voltage direct current output by the photovoltaic cell is converted into high-voltage direct current, and the input and output of the whole machine are electrically isolated. The low-voltage full-bridge inverter circuit has small voltage and current stress, adopts a low-voltage gallium nitride switch, and is divided into two groups Q 1、Q4 and Q 2、Q3, and two paths of high-frequency square wave control signals with opposite phases and proper dead time are respectively applied; zero voltage turn-on (ZVS) of Q 1～Q4 is achieved through the resonant circuit, effectively reducing switching losses. The primary side leakage inductance of the high-frequency transformer T 1 is used for replacing the resonant inductance L r by a magnetic integration technology, so that the whole volume is reduced; the turns ratio of the primary side and the secondary side of the high-frequency transformer T 1 is 1:n, and the voltage doubling rectifying circuit can increase the rectified direct current voltage by one time, so that the turns of the secondary side of the transformer T 1 can be properly reduced, and the transformer core with smaller volume is beneficial to use.
The second-stage soft switching DC-AC converter in the photovoltaic inverter is composed of a high-voltage direct-current filter capacitor C 4, a high-voltage full-bridge inverter circuit and an output filter. The high-voltage direct current output by the first-stage soft switching DC-DC converter is changed into sine wave through SPWM modulation control and is output to a single-phase alternating current power grid. Because the current of the high-voltage direct current bus is smaller, the required capacitance is smaller, the high-voltage direct current filter capacitor C 4 adopts a thin film capacitor with small capacitance, and then the double frequency power fluctuation caused by single-phase inversion can be absorbed by combining with a proper control technology, so that the fluctuation is prevented from being transmitted to the low-voltage direct current bus of the first-stage DC-DC converter, the adoption of a high-capacity electrolytic capacitor (usually thousands to tens of thousands of uF) at the low-voltage side is avoided, and the service life of the whole machine is effectively prolonged. The high-voltage full-bridge inverter circuit adopts a switch with higher withstand voltage, wherein two switches Q 5、Q6 of a left bridge arm are silicon switches, power frequency control is adopted, the effect of reducing the switching loss of the inverter circuit is achieved, two switches Q 7、Q8 of a right bridge arm are GaN switches, and the equivalent switching frequency can be improved by adopting high-frequency modulation, so that the volume of an output filter can be reduced.
Specifically, the terminal voltage of the photovoltaic cell connected with the common micro photovoltaic inverter is about 30-45V, so that the terminal voltage of the low-voltage direct-current capacitor C 1 is very low, four switches of the low-voltage full-bridge inverter circuit can adopt 100V gallium nitride MOSFET switches, Q 1、Q4 and Q 2、Q3 are divided into two groups, two paths of high-frequency square wave control signals with opposite phases and dead time of 0.1-0.2 microsecond are respectively applied, the control signal frequency can reach more than 1MHz, and the control signal frequency is far higher than the switching frequency of a silicon switch; zero voltage turn-on (ZVS) of Q 1～Q4 is realized through the resonant circuit, effectively reducing switching loss; the primary side leakage inductance of the high-frequency transformer T 1 is used for replacing the resonant inductance L r by a magnetic integration technology, so that the whole volume is reduced.
If the rated voltage of the single-phase power grid connected with the micro photovoltaic inverter is 220V, the voltage of the high-voltage direct-current bus needs to reach 320-380V. Because the single-phase inversion causes large high-voltage direct-current voltage fluctuation, the high-voltage direct-current filter capacitor C 4 can select a high-voltage thin film capacitor with the withstand voltage of 650V. The high-voltage full-bridge inverter circuit adopts a voltage-resistant 600V or 650V switch, wherein two switches Q 5、Q6 of a left bridge arm are silicon material MOSFET switches, power frequency control is adopted to achieve the effect of reducing the switching loss of the inverter circuit, two switches Q 7、Q8 of a right bridge arm are gallium nitride material MOSFET switches, and high-frequency SPWM (sinusoidal pulse width modulation) with a carrier frequency of thousands of hertz is adopted to improve the equivalent switching frequency, so that the volume of an output filter can be reduced.
Because the direct current voltage difference between the low voltage side and the high voltage side is large, the high frequency transformer T 1 is designed as a step-up transformer, and the turn ratio of the primary side and the secondary side is 1:n. Further preferably, a voltage doubling rectifying circuit capable of doubling the voltage is used at the output end of the transformer, so that the number of turns of the secondary side of the transformer T 1 can be properly reduced, and the transformer core with smaller size is facilitated to be used. The high-frequency transformer T 1 also plays a role in realizing the input/output electrical isolation of the whole machine.
When the power is fixed, the current of the high-voltage direct-current bus is smaller, and the required capacitance is smaller, so that the high-voltage direct-current filter capacitor C 4 adopts a thin film capacitor with small capacity, and the double frequency power fluctuation caused by single-phase inversion can be absorbed at the high-voltage side by combining the power decoupling control method adopting the power feedforward and the voltage feedback control, so that the fluctuation is prevented from being transmitted to the low-voltage direct-current bus, and the service life of the whole machine is effectively prolonged by avoiding adopting a high-capacity electrolytic capacitor (usually thousands to tens of thousands of uF) at the low-voltage side.
The invention also discloses a control method of the photovoltaic inverter based on the gallium nitride device, which adopts a power decoupling control method of power feedforward and voltage feedback control and comprises the steps of generating a driving signal of a switch of a low-voltage full-bridge inverter circuit in a first-stage DC-DC circuit; and generating a drive signal for a switch of a high voltage full bridge inverter circuit in the second stage DC-AC circuit.
The first stage DC-DC circuit control algorithm flow is shown in FIG. 3, and comprises the following steps:
Step 1: the method comprises the steps of collecting a grid voltage instantaneous value u ac and a photovoltaic inverter output current instantaneous value i ac, and calculating a photovoltaic inverter output alternating current instantaneous power p ac＝uac×iac. Collecting a low-voltage direct-current bus voltage instantaneous value U dc1 and a photovoltaic cell output current instantaneous value I dc1, respectively calculating average values U dc1 and I dc1 of the low-voltage direct-current bus voltage instantaneous value U dc1 and the photovoltaic cell output current instantaneous value I dc1 with a period of 10ms as a period, and calculating photovoltaic cell output power P pv＝Udc1×Idc1. Collecting a high-voltage direct-current bus voltage instantaneous value u dc2, and calculating the variation delta u dc2 of u dc2 in the next switching period according to the following formula, wherein T 0 is the current switching period of the first-stage DC-DC converter:
adding the obtained u dc2 and Deltau dc2 to obtain a predicted value u' dc2＝udc2+Δudc2 of the voltage of the high-voltage direct-current bus of the next switching period, and then calculating the voltage gain ratio K of the first-stage DC-DC converter of the next switching period according to the following formula:
Wherein n is the transformation ratio of the high-frequency transformer.
Step 2: collecting an input current instantaneous value I dc2 at the direct current side of the high-voltage full-bridge inverter circuit, respectively calculating average values U dc2 and I dc2 of U dc2 and I dc2 obtained in the step1 with a period of 10ms, and calculating an equivalent load resistance R of the first-stage DC-DC converter according to the following formula:
let quality factor Inductance ratio parameterLLC resonant frequencyNormalized switching frequencyWherein L r is the resonant inductance of LLC, C r is the resonant capacitance of LLC, L m is the primary excitation inductance of the high-frequency transformer T 1, and f 1 is the switching frequency of the next period of the first-stage DC-DC converter to be solved. According to the principles of an LLC soft-switching resonant converter, the following formula will be given:
FIG. 6 is an example of a K-F x relationship. According to the offline calculated K-F x relation curve, the solution of F x meeting the following conditions is obtained by table look-up:
Then according to the relation between F x and F 1, F 1＝Fx×fr is obtained and is used as the switching frequency of the next period of the low-voltage full-bridge inverter circuit; and t 1＝1/f1 of the next cycle.
Step 3: and (3) performing voltage-frequency conversion on the switching frequency obtained in the step (2) to obtain a square wave signal with the duty ratio of 50%, inverting the square wave signal, and adding proper dead time to serve as driving signals of two groups of switches Q 1、Q4 and Q 2、Q3.
The second stage DC-AC circuit control has the steps of:
Step 1: according to the low-voltage direct-current bus voltage instantaneous value U dc1 and the photovoltaic cell output current instantaneous value i dc1, a disturbance observation method is adopted as an algorithm for tracking the maximum power of the photovoltaic cell to calculate a reference value U dcref of U dc1 with a period of 100 ms. The algorithm block diagram is shown in fig. 5.
Step 2: calculating the error between the reference value U dcref and the instantaneous value U dc1 obtained in the step 1, and obtaining the amplitude I acref of the reference value of the output current of the photovoltaic inverter by using a proportional integral controller (PI); the first-stage DC-DC circuit controls the power grid voltage instantaneous value u ac obtained in the step 1, and the phase factor sin theta of the power grid voltage is obtained through the phase-locked loop module, so that the reference value i acref＝Iacref multiplied by sin theta of the output current is obtained; calculating an error between a reference value i acref of the output current and the instantaneous value i ac obtained in the step 1 of the first-stage DC-DC circuit control, and obtaining a modulation ratio d of the output voltage of the photovoltaic inverter by using a proportional controller (P); the modulation ratio d is subtracted from the power frequency square wave with the amplitude of 1, the obtained result and the high-frequency triangular carrier are subjected to SPWM (sinusoidal pulse width modulation) modulation, so that switching signals of two switches Q 7、Q8 of a right bridge arm of the high-voltage full-bridge inverter circuit are obtained, and switching signals of two switches Q 5、Q6 of a left bridge arm are power frequency square waves with the duty ratio of 50%. The algorithm block diagram is shown in fig. 4.
It will be appreciated by those skilled in the art that the present invention can be carried out in other embodiments without departing from the spirit or essential characteristics thereof. Accordingly, the above disclosed embodiments are illustrative in all respects, and not exclusive. All changes that come within the scope of the invention or equivalents thereto are intended to be embraced therein.
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