CN107134937B - Three-level multi-pulse output transformerless inverter circuit - Google Patents

Three-level multi-pulse output transformerless inverter circuit Download PDF

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CN107134937B
CN107134937B CN201710424860.1A CN201710424860A CN107134937B CN 107134937 B CN107134937 B CN 107134937B CN 201710424860 A CN201710424860 A CN 201710424860A CN 107134937 B CN107134937 B CN 107134937B
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frequency link
switching
power
frequency
capacitor
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CN107134937A (en
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张玉林
胡伟
朱国忠
徐锡军
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Shanghai Chint Power Systems Co ltd
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Shanghai Chint Power Systems Co ltd
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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
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H02M1/126—Arrangements for reducing harmonics from ac input or output using passive filters
    • 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

Abstract

The invention provides a three-level multi-pulse output transformerless inverter circuit, which comprises: a power frequency link, a high frequency link and a filtering link; the power frequency link comprises two direct-current capacitors and four switching tubes; the high-frequency link is composed of a plurality of high-frequency link units; each high-frequency link unit comprises two switching tubes; the filtering link comprises a plurality of filtering inductors and a filtering capacitor, and each high-frequency link unit corresponds to one filtering inductor. Aiming at the transformer-free grid-connected photovoltaic inverter, the invention provides a new circuit for converting direct current into alternating current, and can realize that each phase of inversion voltage is output as multi-path staggered three-level pulses, thereby reducing output filter inductance and capacitance; meanwhile, the utilization rate of the low-frequency power switching device is improved, and electromagnetic interference and switching loss of the power switching device are reduced; higher conversion efficiency, power density and lower cost are realized, and the system power capacity is easier to expand.

Description

Three-level multi-pulse output transformerless inverter circuit
Technical Field
The invention belongs to the technical field of new energy grid-connected power generation, relates to a circuit for converting direct-current voltage into alternating-current voltage, and particularly relates to a three-level multi-pulse output transformerless inverter circuit.
Background
Solar power generation is an important component of new energy and is increasingly developed. In a solar power generation system, an inverter circuit for converting direct current generated by a solar panel into alternating current plays a very important role as a core of the solar power generation system.
Solar energy systems can be generally classified into independent type and grid-connected type, and the latter is a main application type of photovoltaic systems at present because energy emitted from a solar panel can be utilized to the greatest extent. Inverter topologies can be generally divided into the type with transformer isolation and transformerless. The isolation transformer can realize the functions of isolation and boosting, but the power frequency transformer has the advantages of heavy volume, low efficiency, high cost and inconvenient installation; the high-frequency transformer has the advantages that the volume and the weight are reduced, but the system is complex, the output current has a direct current component, and the efficiency is also low; the transformerless inverter is a mainstream of grid-connected photovoltaic power generation because of the rapid development of the system with simple structure, high efficiency, small volume and low cost.
At present, the three-level inverter circuit topology is widely applied to a three-phase transformer-free grid-connected inverter. Compared with a two-level topology, the high-frequency common-mode voltage circuit has lower power switch voltage and lower high-frequency common-mode voltage, so that the switching loss and the filter inductance are reduced, and the conversion efficiency and the power density are improved. However, the power switch devices are all operated in a high-frequency switch state, the performance of the power switch devices is still relatively difficult to optimize, the utilization rate is low, and the further improvement of the conversion efficiency and the power density of the inverter is limited.
The active neutral point clamped three-level inverter circuit topology can switch part of power switching tubes in a low-frequency mode, so that the conduction characteristic and the switching characteristic of a power switching device can be optimized and selected respectively, and the conversion efficiency and the power density are further improved. However, the on-time utilization rate of the low-frequency power switch tube is only about 50%, and the parallel current sharing of the high-frequency power switch tube is relatively difficult, so that the maximum power capacity of the high-frequency power switch tube is relatively low compared with that of the low-frequency power switch tube, and the maximum power capacity of the system is limited.
Compared with a single inverter circuit, the staggered parallel technology can reduce filter inductance and filter capacitance, and meanwhile, the system capacity is easier to improve. Based on the staggered parallel technology, the magnetic coupling inductor is adopted, so that the volume and weight of the filter inductor can be further reduced, and the performance of the inverter can be optimized. But cannot improve the operation performance and the utilization rate of the power switching device.
The three-level inverter circuit is widely applied to a three-phase transformer-free grid-connected inverter. Compared with a two-level inverter circuit, the inductor has smaller volume and higher conversion efficiency and power density. At present, an I-type three-level inverter circuit topology and a T-type three-level inverter circuit topology are mostly adopted by a three-phase transformerless grid-connected inverter. Each power switch tube in the classical I-type three-level inverter circuit topology and the T-type three-level inverter circuit topology works in a high-frequency switch state in a half power frequency period, and works in a continuous conduction state in the other half power frequency period. The design and the selection of the power switch tube need to simultaneously consider the switching characteristic and the conduction characteristic, and the optimization of the switching and the conduction loss cannot be realized.
In the active neutral point clamped three-level inverter circuit topology, four low-frequency power switching tubes are used for power frequency switching and two high-frequency switching tubes are used for high-frequency switching, so that the conduction characteristic and the switching characteristic can be optimized respectively, and the conversion efficiency and the power density are further improved. However, the on-time utilization rate of the four low-frequency power switching devices is only 50%, and the parallel current sharing of the high-frequency power switching tubes is relatively difficult, so that the maximum power capacity of the high-frequency power switching tubes is relatively low compared with that of the low-frequency power switching tubes, the maximum power capacity of the system is limited, and the further improvement of the conversion efficiency and the power density is hindered.
The traditional three-phase inverter circuit topology only outputs one column of pulse voltage per phase. By adopting the staggered parallel technology, each phase uses a plurality of inverter circuits to stagger and parallel output a plurality of parallel pulse sequences, the utilization rate of a filtering element can be improved, the filtering inductance and the filtering capacitance can be reduced, and the power capacity of the system can be improved relatively easily. However, the utilization rate of the power switch device is not improved, and the improvement of the power density is hindered.
Disclosure of Invention
The invention aims to solve the technical problems of reducing the output filter inductance and the capacitance of a circuit for converting direct-current voltage into alternating-current voltage, improving the utilization rate of a low-frequency power switch device, reducing the switching loss of the power switch device and realizing higher conversion efficiency, power density and lower cost.
In order to solve the technical problems, the technical scheme of the invention is to provide a three-level multi-pulse output transformer-free inverter circuit, which is characterized in that: comprising: a power frequency link, a high frequency link and a filtering link;
the power frequency link comprises two direct current capacitors C1-C2 and four switching tubes T1-T4; the high-frequency link consists of n high-frequency link units, wherein n is a positive integer not less than 2; each high-frequency link unit comprises two switching tubes Tx1 and Tx2, x is the number of the high-frequency link unit, and x=1 and 2 … … n; the filtering link comprises n filtering inductors Lf1, lf2 … and Lfn and a filtering capacitor Cf, and each high-frequency link unit corresponds to one filtering inductor;
in the power frequency link, one end of a capacitor C1 is connected with the positive end of a direct-current voltage source and one end of a switching tube T1, and is marked as '1'; the other end of the capacitor C1 is connected to one end of the capacitor C2, one end of the switching tube T2 and one end of the switching tube T3, and is marked as '0'; the other end of the capacitor C2 is connected to the negative end of the direct-current voltage source and one end of the switching tube T4, and is marked as '2'; the serial ends of the switching tube T1 and the switching tube T2 are marked as '3', and the serial ends of the switching tube T3 and the switching tube T4 are marked as '4';
in the high-frequency link, n high-frequency link units are included, in each high-frequency link unit, a switching tube Tx1 and a switching tube Tx2 are connected in series, and the serial end is marked as 'x 5'; the other end of the switching tube Tx1 is connected to the serial ends '3' of the power frequency link switching tubes T1 and T2, and the other end of the switching tube Tx2 is connected to the serial ends '4' of the power frequency link switching tubes T3 and T4;
in the filtering link, the filter comprises n filter inductors Lf 1-Lfn and a filter capacitor Cf; one end of the filter inductor Lfx is connected to the serial ends 'x 5' of the switching tubes Tx1 and Tx2 of the high-frequency link unit x, the other end of the filter inductor Lfx is connected to one end of the filter capacitor Cf, and the phase line Lg of the power grid is marked as '6'; one end of the filter capacitor Cf is connected with one ends of the inductors Lf 1-Lfn and the phase line Lg '6' of the power grid, and the other end of the filter capacitor Cf is connected with the serial end '0' of the direct-current capacitors C1 and C2.
Preferably, the switching transistors T1 to T4, the switching transistor Tx1 and the switching transistor Tx2 are composed of switching transistors with inverse diodes inside.
Preferably, the switching transistors T1 to T4, the switching transistor Tx1, and the switching transistor Tx2 are formed by antiparallel switching transistors without antiparallel diodes and independent diodes.
Preferably, the switching transistors T1 to T4, the switching transistor Tx1, and the switching transistor Tx2 are formed by connecting the same type or different types of switching transistors in parallel. The switching tube may also be a parallel combination of various switching devices. Such as an IGBT in parallel with a MOSFET; a plurality of IGBTs are connected in parallel; a plurality of MOSFETs are connected in parallel; IGBTs are connected in parallel with MOSFETs, diodes, etc.
Preferably, the switching tubes T1 to T4, the switching tube Tx1, and the switching tube Tx2 are IGBT tubes or MOSFET tubes.
Preferably, the switching tubes T1 to T4, the switching tube Tx1 and the switching tube Tx2 are formed by connecting IGBT tubes, MOSFET tubes and diodes in parallel. Such as: the IGBT is connected with the MOSFET in parallel; or IGBT, MOSFET and diode are connected in parallel, and are two common parallel combinations.
Preferably, the filter capacitor Cf is a single capacitor, or a combination of multiple capacitors connected in parallel.
Preferably, the filter inductors Lf1 to Lfn are independent inductors or inductors with magnetic coupling between them by adopting a magnetic integration technology.
Preferably, each capacitor C1, C2 in the power frequency link is a single electrolytic capacitor or a thin film capacitor, or a combination of multiple electrolytic capacitors or thin film capacitors connected in parallel, or a combination of electrolytic capacitors and thin film capacitors connected in parallel.
Preferably, the power frequency link switching tubes T1 to T4 are switched according to the power grid voltage phase power frequency, wherein T1 and T3 are synchronously switched, T2 and T4 are synchronously switched, and T1 and T3 are complementarily switched with T2 and T4;
the high-frequency link switching tubes Tx1 and Tx2 are switched according to the pulse width high-frequency switch modulated by the sine wave of the power grid according to the requirements of the voltage and the power factor of the power grid; wherein a pair of switching tubes Tx1, tx2 of each high frequency link unit are complementarily switched; the switching tubes in each high-frequency link have no special time sequence requirement, but the switching moments are staggered, so that the volume and the weight of the filter are minimum.
Preferably, in the positive half period of the power grid, the power frequency link switching tubes T1 and T3 are turned on, and the power frequency link switching tubes T2 and T4 are turned off; half of the output voltage Vdc of the direct-current voltage source is 0.5Vdc and is directly applied to the high-frequency link, and the conversion from direct current to alternating current voltage is realized through the high-frequency switching action of the switching tube of the high-frequency link.
Preferably, in the negative half period of the power grid, the power frequency link switching tubes T1 and T3 are turned off, and the power frequency link switching tubes T2 and T4 are turned on; half-0.5 Vdc of the output voltage Vdc of the direct-current voltage source is directly applied to the high-frequency link, and the conversion from direct current to alternating current voltage is realized through the high-frequency switching action of the switching tube of the high-frequency link.
Aiming at the transformer-free grid-connected photovoltaic inverter, the invention provides a new circuit for converting direct current into alternating current, and can realize that each phase of inversion voltage is output as multi-path staggered three-level pulses, thereby reducing output filter inductance and capacitance; meanwhile, the utilization rate of the low-frequency power switching device is improved, and electromagnetic interference and switching loss of the power switching device are reduced; higher conversion efficiency, power density and lower cost are realized, and the system power capacity is easier to expand.
Drawings
FIG. 1 is a schematic diagram of a three-level multi-pulse output transformerless inverter circuit;
FIG. 2 is a schematic diagram of a three-phase inverter using a three-level multi-pulse output transformerless inverter circuit;
FIG. 3 is a schematic diagram of driving signals of each switching tube in the conversion circuit;
FIG. 4a is a schematic diagram of a positive half cycle active circuit;
FIG. 4b is a schematic diagram of a positive half cycle reactive freewheel loop;
FIG. 5a is a schematic diagram of a positive half cycle active freewheel circuit;
FIG. 5b is a schematic diagram of a positive half cycle reactive circuit;
FIG. 6a is a schematic diagram of a negative half cycle active circuit;
FIG. 6b is a schematic diagram of a negative half-cycle reactive freewheel loop;
FIG. 7a is a schematic diagram of a negative half cycle active freewheel loop;
FIG. 7b is a schematic diagram of a negative half cycle reactive circuit;
FIG. 8 is a schematic diagram of an inverted output multi-path three-level voltage;
FIG. 9 is a schematic diagram of a parallel combination of different types of switching transistors;
FIG. 10 is a schematic diagram of a power frequency link using an IGBT with low large capacity conduction loss as a switching device and a high frequency link using a MOSFET with low switching loss but low capacity as a switching device;
fig. 11 is a schematic diagram of a magnetic coupling inductor of the filter inductors Lf1 and Lf2 … Lfn;
fig. 12 is a schematic diagram of the filter inductors Lf1 and Lf2 as one magnetic coupling inductor in the same direction, and Lf3 and Lf4 as one magnetic coupling inductor in the same direction;
fig. 13 is a schematic diagram of the filter inductors Lf1 and Lf2 as one reverse magnetic coupling inductor, and Lf3 and Lf4 as one reverse magnetic coupling inductor.
Detailed Description
The invention will be further illustrated with reference to specific examples.
The invention provides a circuit for converting direct current into alternating current, which can be used for solar grid-connected power generation. As shown in fig. 1, the input end of the circuit is a solar generator SG, and the output end is an alternating current power grid Ug. The device comprises three links, namely a power frequency link, a high-frequency link and a filtering link.
The high-frequency loop circuit is composed of 2-n high-frequency link units, n is a set value, and the actual number can be determined according to design requirements. The power frequency link comprises two direct current capacitors C1-C2 and four switching tubes T1-T4. Each high frequency link unit comprises two switching tubes Tx1, tx2, where x is the high frequency link unit number, x=1, 2 … …. The filtering link comprises a plurality of filtering inductors Lf1, lf2 … and Lfn and a filtering capacitor Cf, wherein each high-frequency link unit corresponds to one filtering inductor.
In the power frequency link, the capacitors C1-C2 are sequentially connected in series, and the switching tubes T1-T4 are sequentially connected in series. One end of the capacitor C1 is connected to the positive end of the solar generator SG and one end of the switching tube T1, and is marked as '1'; the other end of the capacitor C1 is connected to one end of the capacitor C2, one end of the switching tube T2 and one end of the switching tube T3, and the other end is marked as '0'. One end of the capacitor C2 is connected to one end of the C1, one end of the switch tube T2 and one end of the switch tube T3, namely '0'; the other end of the capacitor C2 is connected to the negative end of the solar generator SG and one end of the switching tube T4, denoted as "2". The serial ends of the switching tube T1 and the switching tube T2 are marked as '3', the serial ends of the switching tube T3 and the switching tube T4 are marked as '4', the other end of the switching tube T1 is connected with the positive end of the solar generator SG and one end '1' of the capacitor C1, the other end of the switching tube T4 is connected with the negative end of the solar generator SG and one end '2' of the capacitor C2, and the serial ends of the switching tubes T2 and T3 are connected with the serial ends '0' of the capacitors C1 and C2.
The two capacitors C1 and C2 are connected in series and connected in parallel with the solar generator SG, and when the output voltage of the solar generator SG is Vdc, the voltages on the capacitors C1 and C2 are both 0.5Vdc. A reference potential can be set, and the series terminal "0" is marked as "0V", so that voltages of positive terminal "1" and negative terminal "2" of the capacitor C1 and C2 relative to the 0V point are +0.5vdc and-0.5vdc, respectively.
In the high frequency link, 2-n high frequency link units are included. In each high-frequency link unit, the switching transistors Tx1 and Tx2 are connected in series, the series end is denoted as "x5", the other end of Tx1 is connected to the series end "3" of the power frequency link switching transistors T1, T2, and the other end of Tx2 is connected to the series end "4" of the power frequency link switching transistors T3, T4.
In the filtering link, the filter comprises 2 to n filter inductors Lf1, lf2 to Lfn and a filter capacitor Cf. One end of the filter inductor Lfx is connected to the series connection end "x5" of the switching tubes Tx1, tx2 of the high-frequency link unit x, and the other end of Lfx is connected to one end of the filter capacitor Cf and the phase line Lg of the power grid, which is denoted as "6". One end of the filter capacitor Cf is connected with one ends of the inductors Lf 1-Lfn and the phase line Lg 6 of the power grid, and the other end is connected with the serial end 0 of the direct-current capacitors C1 and C2.
Fig. 2 is a schematic diagram of a three-phase inverter formed by using the proposed three-level multi-pulse output transformerless inverter, where the power frequency link has two basic operating states and the high frequency link has two basic operating states according to different voltage phases and power factor settings of the power grid in positive and negative grid periods.
For simplicity of description, the working state of the circuit is described in detail by taking two high-frequency link units as examples, but in practical application, multiple high-frequency link units can be used according to design requirements.
In one grid period, the timing sequence of the driving pulse of each switching tube is shown in fig. 3, wherein T1 to T4, T11, T12, T21, T22 respectively represent the driving pulse of the switching tubes T1 to T4, T11, T12, T21, T22. And setting the switching period of the high-frequency link switching tube as Ts. The power frequency link switching tubes T1-T4 are switched according to the power grid voltage phase power frequency, wherein T1 and T3 are synchronously switched, T2 and T4 are synchronously switched, and T1 and T3 are complementarily switched with T2 and T4. The high-frequency link switching tubes T11, T12, T21 and T22 are switched according to the pulse width high-frequency switch modulated by the sine wave of the power grid according to the requirements of the voltage and the power factor of the power grid. Wherein the pair of switching tubes Tx1, tx2 of each high frequency unit are complementary switches, e.g. T11, T12 complementary switches, T21, T22 complementary switches. No special time sequence requirement exists among the switching tubes of each high-frequency link, but the switching moments are staggered by Ts/N, so that staggered complementary switching can be realized, and the utilization rate of the filter circuit is improved.
(1) Operating state of positive half cycle
During the positive half-cycle of the grid, as shown in fig. 3. The power frequency link switching tubes T1 and T3 are turned on, and the power frequency link switching tubes T2 and T4 are turned off. The +0.5vdc is directly applied to the high frequency link 1 and the high frequency link 2 at half of the output voltage of the solar generator SG. The high-frequency switching actions (such as 30 kHz) of the high-frequency link 1 switching tubes T11 and T12 and the high-frequency link 2 switching tubes T21 and T22 realize the conversion from direct current to alternating current voltage.
State 1:
when the high-frequency link switching tube Tx1 is turned on and the high-frequency link switching tube Tx2 is turned off, current flows out from the end 1 of the capacitor C1, sequentially passes through the power frequency switching tube T1, the high-frequency switching tube Tx1, the inductor Lfx, the power grid Ug and the filter capacitor Cf, and then flows into the end 0 of the capacitor C1, and at the moment, the inversion output voltage Vinv (namely, the voltage between the high-frequency link output end x5 and the reference point 0) is 0.5Vdc. The current flow direction when the active current is output is shown in fig. 4a, and the current flow direction when the reactive current is output is shown in fig. 4b.
State 2:
when the high-frequency link switching tube Tx1 is turned off and Tx2 is turned on, because the inductance current cannot be suddenly changed, the current flows out from the 0 end of the capacitor C1, sequentially passes through the power frequency switching tube T3, the high-frequency switching tube Tx2, the inductor Lfx, the power grid Ug and the filter capacitor Cf, and then flows into the 0 end of the capacitor C1, at the moment, the inversion output voltage Vinv (namely, the voltage between the high-frequency link output end 'x 5' and the reference point 0) is 0Vdc, the current flow is shown in fig. 5a when the active current is output, and the current flow is shown in fig. 5b when the reactive current is output.
High frequency ringEach high-frequency link unit in the node has independent working state, so the maximum existence of the circuit can be 2 according to the number of the high-frequency link units n An operating state.
The switching of the multipath output inversion voltage pulse between 0V and 0.5Vdc is realized through the two working states of the high-frequency link unit in the positive half period, and the switching time of each high-frequency link unit can be staggered by Ts/N.
(2) Operating state of negative half cycle
During the negative half-cycle of the grid, as shown in fig. 3. The power frequency link switching tubes T1 and T3 are turned off, and the power frequency link switching tubes T2 and T4 are turned on. Half of the output voltage of the solar generator SG, -0.5Vdc is directly applied to the high frequency link 1, high frequency link 2. The high-frequency switching actions (such as 30 kHz) of the high-frequency link 1 switching tubes T11 and T12 and the high-frequency link 2 switching tubes T21 and T22 realize the conversion from direct current to alternating current voltage.
State 1:
when the high-frequency link switching tube Tx1 is turned off and the high-frequency link switching tube Tx2 is turned on, current flows out of the end 2 of the capacitor C2, sequentially passes through the power frequency switching tube T4, the high-frequency switching tube Tx2, the inductor Lfx, the power grid Ug and the filter capacitor Cf, and then flows into the end 0 of the capacitor C2, and at the moment, the inversion output voltage Vinv (namely, the voltage between the high-frequency link output end x5 and the reference point 0) is minus 0.5Vdc. The current flow when active current is output is shown in fig. 6a, and the current flow when reactive current is output is shown in fig. 6b.
State 2:
when the high-frequency link switching tube Tx1 is turned on and Tx2 is turned off, because the inductance current cannot be suddenly changed, the current flows out from the 0 end of the capacitor C2, sequentially passes through the power frequency switching tube T2, the high-frequency switching tube Tx1, the inductor Lfx, the power grid Ug and the filter capacitor Cf, and then flows into the 0 end of the capacitor C2, at the moment, the inversion output voltage Vinv (namely, the voltage between the high-frequency link output end 'x 5' and the reference point 0) is 0Vdc, the current flow direction is shown in fig. 7a when the active current is output, and the current flow direction is shown in fig. 7b when the reactive current is output.
Each high-frequency link unit in the high-frequency links has independent working state, so that 2 can exist at maximum according to the number of the high-frequency link units n An operating state.
The switching of the multipath output inversion voltage pulse between 0V and-0.5 Vdc is realized through two working states of the high-frequency link unit in the negative half period, and the switching time of each high-frequency link unit can be staggered by Ts/N.
Therefore, in one power grid period, each high-frequency link unit inverter output voltage comprises three voltage levels of-0.5 Vdc, 0V and 0.5Vdc, and the waveform of the inverter voltage is shown in fig. 8. In the circuit, the waveform of the inversion output voltage in the high-frequency link is three levels of multipath staggered parallel connection, the harmonic content is low, the required filter inductance is small, and the system cost and the power loss are reduced.
As described in the above state, the power frequency link switching tubes T1, T2, T3, and T4 are power frequency switches, so that the conduction performance of the switching tubes can be optimized, and the switching devices with better conduction characteristics can be selected preferentially. The working states of the high-frequency link units are independent, capacity expansion can be achieved by connecting more high-frequency links in parallel, so that the requirements on the conduction performance of the switching devices of the high-frequency link units are relatively low, and the high-frequency link switches Tx1 and Tx2 are always switched at high frequency, so that the switching performance of the high-frequency link switching tubes can be optimized, and the switching devices with better switching characteristics can be selected preferentially. All the currents of the high-frequency link units flow through the power frequency switching tube of the power frequency link, so that the utilization rate of the power frequency switching tube is high, and higher power density can be realized.
It should be understood that the above-described embodiments are merely illustrative of the present invention and are not intended to limit the scope of the present invention. Further, it is understood that various changes and modifications may be made by those skilled in the art after reading the teachings of the present invention, and such equivalents are intended to fall within the scope of the claims appended hereto. The following modifications were made:
1. the power switch tube can be composed of switch transistors with anti-parallel diodes inside, or composed of switch transistors without anti-parallel diodes inside and independent diodes after anti-parallel connection, or composed of switch transistors of the same type or different types in parallel connection. The switching transistor may be IGBT (Insulation Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field-effect transistor) or other switching device. For example, the power switch tube may be a MOSFET as shown in fig. 9a, an IGBT as shown in fig. 9b and an independent diode in anti-parallel, an IGBT as shown in fig. 9c and a MOSFET in parallel, or an IGBT as shown in fig. 9d and an independent diode in anti-parallel and then in parallel.
The topology is the same as that of an active neutral point clamped three-level inverter circuit, high-frequency link switching devices Tx1 and Tx2 in the circuit are optimal by using high-speed switching devices such as MOSFET and SiC MOSFET, and power frequency link switching devices T1, T2, T3 and T4 are better by using high-capacity low-speed switching devices such as IGBT. Fig. 10 shows a preferred scheme, in which an IGBT with small on loss is used as a switching device in the power frequency link, and a MOSFET with small on loss but small capacity is used as a switching device in the high frequency link.
2. The filter link circuit can be composed of filter inductors Lf1 and Lf2 … Lfn and filter capacitor Cf, and can also be composed of filter inductors Lf1 and Lf2 … Lfn. The filter capacitor Cf may be a single capacitor or a combination of a plurality of capacitors connected in parallel. The filter inductors Lf1 to Lfn may be independent inductors or inductors that are magnetically coupled to each other by a magnetic integration technique.
The following list of 3 possible inductance schemes is given in detail, and alternatives with different numbers of inductances and inductances can be easily deduced.
Fig. 11 shows a scheme in which the filter inductors Lf1 and Lf2 … Lfn are magnetic coupling inductors.
Fig. 12 shows a scheme in which the filter inductors Lf1 and Lf2 are one magnetic coupling inductor in the same direction, and the filter inductors Lf3 and Lf4 are one magnetic coupling inductor in the same direction. It is also possible to use more magnetic coupling inductances, each of which can be coupled by more inductances.
Fig. 13 shows a scheme in which the filter inductors Lf1 and Lf2 are one reverse magnetic coupling inductor, and Lf3 and Lf4 are one reverse magnetic coupling inductor. It is also possible to use more magnetic coupling inductances, each of which can be coupled by more inductances.
3. The filter capacitor Cf can be connected to one end of 0 or 1 or 2, and has the same filtering effect. Under some application conditions, the filter capacitor Cf can also be connected to an alternating current neutral line, the ground or connected with other phases to float.
4. Each capacitor C1 and C2 in the power frequency link can be a single electrolytic capacitor or a thin film capacitor, can be a combination of a plurality of electrolytic capacitors or thin film capacitors connected in parallel, and can also be a combination of electrolytic capacitors and thin film capacitors connected in parallel.
5. The high-frequency link switching tubes in the circuit are mutually staggered with the Ts/N switch, so that the purposes of reducing the filter inductance and the filter capacitance are realized, but the high-frequency link switching tubes can be synchronously switched or mutually staggered with other arbitrary delay switches.
6. The input of the proposed circuit may be a solar cell or other dc voltage source.
7. The output "6" of the proposed circuit may be a ground network or other load.

Claims (10)

1. A three-level multi-pulse output transformerless inverter circuit, comprising: a power frequency link, a high frequency link and a filtering link;
the power frequency link comprises two direct current capacitors C1-C2 and four switching tubes T1-T4; the high-frequency link consists of n high-frequency link units, wherein n is a positive integer not less than 2; each high-frequency link unit comprises two switching tubes Tx1 and Tx2, x is the number of the high-frequency link unit, and x=1 and 2 … … n; the filtering link comprises n filtering inductors Lf1, lf2 … and Lfn and a filtering capacitor Cf, and each high-frequency link unit corresponds to one filtering inductor;
in the power frequency link, one end of a capacitor C1 is connected with the positive end of a direct-current voltage source and one end of a switching tube T1, and is marked as '1'; the other end of the capacitor C1 is connected to one end of the capacitor C2, one end of the switching tube T2 and one end of the switching tube T3, and is marked as '0'; the other end of the capacitor C2 is connected to the negative end of the direct-current voltage source and one end of the switching tube T4, and is marked as '2'; the serial ends of the switching tube T1 and the switching tube T2 are marked as '3', and the serial ends of the switching tube T3 and the switching tube T4 are marked as '4';
in the high-frequency link, n high-frequency link units are included, in each high-frequency link unit, a switching tube Tx1 and a switching tube Tx2 are connected in series, and the serial end is marked as 'x 5'; the other end of the switching tube Tx1 is connected to the serial ends '3' of the power frequency link switching tubes T1 and T2, and the other end of the switching tube Tx2 is connected to the serial ends '4' of the power frequency link switching tubes T3 and T4;
in the filtering link, the filter comprises n filter inductors Lf 1-Lfn and a filter capacitor Cf; one end of the filter inductor Lfx is connected to the serial ends 'x 5' of the switching tubes Tx1 and Tx2 of the high-frequency link unit x, the other end of the filter inductor Lfx is connected to one end of the filter capacitor Cf, and the phase line Lg of the power grid is marked as '6'; one end of the filter capacitor Cf is connected with one ends of the inductors Lf 1-Lfn and the phase line Lg '6' of the power grid, and the other end of the filter capacitor Cf is connected with the serial end '0' of the direct-current capacitors C1 and C2.
2. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: the switching transistors T1 to T4, the switching transistor Tx1 and the switching transistor Tx2 are composed of switching transistors with inverse diodes.
3. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: the switching transistors T1 to T4, the switching transistor Tx1 and the switching transistor Tx2 are formed by antiparallel switching transistors without antiparallel diodes and independent diodes.
4. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: the switching tubes T1 to T4, the switching tube Tx1, and the switching tube Tx2 are IGBT tubes or MOSFET tubes.
5. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: the filter capacitor Cf is a single capacitor or a combination of multiple capacitors connected in parallel.
6. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: the filter inductors Lf 1-Lfn are independent inductors or inductors with magnetic coupling with each other by adopting a magnetic integration technology.
7. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: each capacitor C1 and C2 in the power frequency link is a single electrolytic capacitor or a thin film capacitor, or a combination of a plurality of electrolytic capacitors or thin film capacitors connected in parallel, or a combination of the electrolytic capacitors and the thin film capacitors connected in parallel.
8. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: when the power frequency switching tube is used, the power frequency link switching tubes T1-T4 are switched according to the power grid voltage phase power frequency, wherein the T1 is synchronously switched with the T3, the T2 is synchronously switched with the T4, and the T1, the T3 are complementarily switched with the T2 and the T4;
the high-frequency link switching tubes Tx1 and Tx2 are switched according to the pulse width high-frequency switch modulated by the sine wave of the power grid according to the requirements of the voltage and the power factor of the power grid; wherein a pair of switching tubes Tx1, tx2 of each high frequency link unit are complementarily switched; no special time sequence requirement exists between the switching tubes of each high-frequency link.
9. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: in the positive half period of the power grid, the power frequency link switching tubes T1 and T3 are turned on, and the power frequency link switching tubes T2 and T4 are turned off; half of the output voltage Vdc of the direct-current voltage source is 0.5Vdc and is directly applied to the high-frequency link, and the conversion from direct current to alternating current voltage is realized through the high-frequency switching action of the switching tube of the high-frequency link.
10. The three-level multi-pulse output transformerless inverter circuit of claim 1, wherein: in the negative half period of the power grid, the power frequency link switching tubes T1 and T3 are turned off, and the power frequency link switching tubes T2 and T4 are turned on; half-0.5 Vdc of the output voltage Vdc of the direct-current voltage source is directly applied to the high-frequency link, and the conversion from direct current to alternating current voltage is realized through the high-frequency switching action of the switching tube of the high-frequency link.
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