Based on wide gain and low leakage current unipolar inverter

Abstract

The present invention provides a unipolar inverter based on wide gain and low leakage current, and relates to the technical field of inverters. The unipolar inverter based on wide gain and low leakage current of the present invention comprises a DC / DC conversion module with a dual structure, a bus capacitor, a DC / AC inverter and a filter circuit; wherein the DC / DC conversion module comprises two DC / DC conversion units, the structures and performance parameters of the two DC / DC conversion units are the same, and the two DC / DC conversion units are connected in a symmetrical manner; the front stage of the inverter adopts a symmetrical structure, which can reduce the voltage stress of the switch tube when it is turned off, realize multi-level output, and at the same time, greatly reduce the change frequency of the common mode voltage, and the leakage current suppression effect is good.

Description

Technical Field

[0001] The invention relates to the technical field of inverters, and in particular to a unipolar inverter based on wide gain and low leakage current. Background Art

[0002] Non-isolated single-phase full-bridge inverter has gained wide attention in photovoltaic power generation systems due to its advantages such as small size, low cost and high efficiency.

[0003] When the PV array outputs a low DC voltage, the main structure of the traditional non-isolated PV inverter is a two-stage converter structure combining DC / DC and DC / AC. The schematic diagram is shown in the figure. Figure 1 The DC low voltage is boosted and then inverted before being connected to the grid. Through two-stage energy conversion, a sine wave that meets the grid connection requirements is obtained.

[0004] In the actual operation of non-isolated grid-connected systems, due to the existence of parasitic capacitance between the photovoltaic array and the earth, and the formation of a common-mode loop between the inverter bridge arm, output filter, and power grid, the changing common-mode voltage will generate a corresponding common-mode current, i.e. leakage current, on this common-mode loop. The leakage current will not only affect other operating equipment in the photovoltaic system, causing electromagnetic interference, but also cause problems such as grid-connected current distortion. Summary of the invention

[0005] 1. Technical issues to be resolved

[0006] In view of the deficiencies of the prior art, the present invention provides a unipolar inverter based on wide gain and low leakage current, which solves the technical problem that leakage current is generated in the traditional non-isolated photovoltaic inverter during the operation of the grid-connected system.

[0007] (II) Technical solution

[0008] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0009] The present invention provides a wide-gain low-leakage current unipolar inverter, comprising a dual-structure DC / DC conversion module, a bus capacitor, a DC / AC inverter and a filter circuit;

[0010] The DC / DC conversion module includes two DC / DC conversion units, the structures and performance parameters of the two DC / DC conversion units are the same, and the two DC / DC conversion units are connected in a symmetrical manner; the two DC / DC conversion units each include an input end and two output ends; the input ends of the two DC / DC conversion units are respectively connected to the positive and negative poles of the DC power supply; the first output end of the first DC / DC conversion unit is connected to the first input end of the DC / AC inverter, and the second output end of the first DC / DC conversion unit is connected to the second output end of the second DC / DC conversion unit; the first output end of the second DC / DC conversion unit is connected to the second input end of the DC / AC inverter; a bus capacitor is connected between the first output end of the first DC / DC conversion unit and the first output end of the second DC / DC conversion unit, and the common end of the first DC / DC conversion unit and the second DC / DC conversion unit is connected to the midpoint of the bus capacitor;

[0011] The two output ends of the DC / AC inverter are connected to the grid after passing through a filter circuit.

[0012] Preferably, the dual-structure DC / DC conversion module includes a plurality of switch tubes, and the switch tubes include control terminals, and the disconnection and conduction of the switch tubes are controlled by control signals, and the DC / DC conversion module outputs two levels, three levels, or five levels;

[0013] and / or,

[0014] The working modes of the wide-gain low-leakage current unipolar inverter include a DC mode and a current conversion mode;

[0015] Among them, the DC mode means that the DC / DC conversion module outputs a DC voltage, and the DC / AC inverter operates in a high-frequency switching state;

[0016] The current conversion mode includes two states: State 1: the DC / DC conversion module outputs a DC voltage, and the DC / AC inverter works in a high-frequency switching state; State 2: the DC / DC conversion module outputs a partially sinusoidal AC voltage, and the DC / AC inverter works in a direct-on or off state.

[0017] Preferably, the input terminals of the two DC / DC conversion units are respectively set to point p and point n, the first output terminal of the second DC / DC conversion unit is point N, and the two output terminals of the DC / AC inverter are respectively point A and point B; the voltage between point p and point N is , the voltage between point n and point N is , the voltage between point A and point N is , the voltage between point B and point N is ; The common mode voltage of the wide gain low leakage current unipolar inverter The calculation formula is as follows:

[0018]

[0019] The wide-gain low-leakage current unipolar inverter keeps the common-mode voltage unchanged in different working modes.

[0020] Preferably, an inductor group is respectively arranged between the input ends of the two DC / DC conversion units and the positive and negative electrodes of the DC power supply.

[0021] Preferably, the first DC / DC conversion unit includes a first diode, a second diode, a first switch tube, a second switch tube and a first flying capacitor;

[0022] The cathode of the first diode is the first output end of the first DC / DC conversion unit, the anode of the first diode is connected to the cathode of the second diode, and the cathode of the second diode is connected to the first end of the second switch tube via the first switch tube; the second end of the second switch tube is the second output end of the first DC / DC conversion unit;

[0023] Two ends of the first flying capacitor are respectively connected to the common end of the first diode and the second diode, and to the common end of the first switch tube and the second switch tube;

[0024] The common end of the second diode and the first switch tube is the input end of the first DC / DC conversion unit;

[0025] The second DC / DC conversion unit includes a third diode, a fourth diode, a third switch tube, a fourth switch tube and a second flying capacitor;

[0026] The cathode of the fourth diode is the first output end of the first DC / DC conversion unit, the anode of the fourth diode is connected to the cathode of the third diode, and the cathode of the third diode is connected to the first end of the third switch tube via the fourth switch tube; the second end of the third switch tube is the second output end of the second DC / DC conversion unit;

[0027] Two ends of the second flying capacitor are respectively connected to the common end of the fourth diode and the third diode, and the common end of the fourth switch tube and the third switch tube;

[0028] The common end of the third diode and the fourth switch tube is the input end of the DC / DC conversion unit.

[0029] Preferably, the DC / AC inverter comprises an H4 full bridge circuit or an H5 bridge or an H6 bridge or a HERIC circuit or a Heric circuit.

[0030] Preferably, when the DC / AC inverter is an H4 full-bridge circuit, the H4 full-bridge circuit includes a fifth switch tube, a sixth switch tube, a seventh switch tube and an eighth switch tube;

[0031] Wherein, the first end of the fifth switch tube is connected to the first end of the seventh switch tube, and the common end thereof is the first input end of the DC / AC inverter;

[0032] The second end of the fifth switch tube is connected to the first end of the sixth switch tube, and the common end thereof is the first output end of the DC / AC inverter;

[0033] The second end of the seventh switch tube is connected to the first end of the eighth switch tube, and the common end thereof is the second output end of the DC / AC inverter;

[0034] The second end of the sixth switch tube is connected to the second end of the eighth switch tube, and the common end thereof is the second input end of the DC / AC inverter.

[0035] Preferably, the common end of the second diode and the first switch tube is set as point p, the common end of the fourth switch tube and the third diode is set as point n, the common end of the second flying capacitor and the fourth diode is set as point N; the common end of the fifth switch tube and the sixth switch tube is set as point A, and the common end of the seventh switch tube and the eighth switch tube is set as point B; the voltage between point p and point N is , the voltage between point n and point N is , the voltage between point A and point N is , the voltage between point B and point N is ; The common mode voltage of the wide gain low leakage current unipolar inverter The calculation formula is as follows:

[0036]

[0037] The wide-gain low-leakage current unipolar inverter keeps the common-mode voltage unchanged in different working modes.

[0038] Preferably, when the DC / DC conversion module outputs a DC voltage and the DC / AC inverter operates in a high-frequency operating state, the positive half-cycle operating state based on the wide-gain low-leakage current unipolar inverter includes four operating modes, specifically including:

[0039] In working mode 1, the first switch tube and the fourth switch tube of the front stage are turned off, the second switch tube and the third switch tube are turned on, the fifth switch tube and the eighth switch tube of the rear stage are turned on, and the sixth switch tube and the seventh switch tube are turned off; in working mode 1, the first flying capacitor and the second flying capacitor store energy, and the common mode voltage ;in is the voltage across the DC bus;

[0040] In working mode 2, the first switch tube and the fourth switch tube of the front stage are turned on, the second switch tube and the third switch tube are turned off, the fifth switch tube and the eighth switch tube of the rear stage are turned on, and the sixth switch tube and the seventh switch tube are turned off; in working mode 2, the first flying capacitor and the second flying capacitor release energy, and the common mode voltage ;

[0041] Working mode three, the first switch tube and the fourth switch tube of the front stage are turned off, the second switch tube and the third switch tube are turned on, the fifth switch tube and the eighth switch tube of the rear stage are turned off, and the sixth switch tube and the seventh switch tube are turned on; in working mode three, the first flying capacitor and the second flying capacitor store energy, and the common mode voltage ;

[0042] Working mode four, the first switch tube and the fourth switch tube of the front stage are turned on, the second switch tube and the third switch tube are turned off, the fifth switch tube and the eighth switch tube of the rear stage are turned off, and the sixth switch tube and the seventh switch tube are turned on; in working mode four, the first flying capacitor and the second flying capacitor release energy, and the common mode voltage .

[0043] Preferably, when the DC / DC conversion module outputs a partially sinusoidal AC voltage and the DC / AC inverter operates in a direct-on or off state, the negative half-cycle operating state based on the wide-gain low-leakage current unipolar inverter includes three operating modes:

[0044] Working mode 1, all the switches of the front-stage DC / DC conversion module are turned on, the fifth and eighth switches of the back-stage H4 are turned off, and the sixth and seventh switches are turned on; the energy of the first flying capacitor and the second flying capacitor remains unchanged, and the common mode voltage of the front-stage DC / DC conversion module is ; Equivalent common mode voltage of the subsequent H4 bridge inverter , the common-mode voltage of the entire wide-gain low-leakage unipolar inverter ;

[0045] Working mode 2, the first switch tube and the fourth switch tube of the front stage are turned on, the second switch tube and the third switch tube are turned off, the fifth switch tube and the eighth switch tube of the rear stage are turned off, the sixth switch tube and the seventh switch tube are turned on, the first flying capacitor and the second flying capacitor store energy, and the common mode voltage of the entire wide gain low leakage current unipolar inverter is ;

[0046] Working mode three, the first switch tube and the fourth switch tube of the front stage are turned off, the second switch tube and the third switch tube are turned on, the fifth switch tube and the eighth switch tube of the rear stage are turned off, the sixth switch tube and the seventh switch tube are turned on, the first flying capacitor and the second flying capacitor release energy, and the common mode voltage of the entire wide gain low leakage current unipolar inverter is .

[0047] (III) Beneficial effects

[0048] The present invention provides a unipolar inverter based on wide gain and low leakage current. Compared with the prior art, it has the following beneficial effects:

[0049] The present invention provides a wide-gain low-leakage current unipolar inverter, which comprises a dual-structure DC / DC conversion module, a bus capacitor, a DC / AC inverter and a filter circuit; wherein the DC / DC conversion module comprises two DC / DC conversion units, the structures and performance parameters of the two DC / DC conversion units are the same, and the two DC / DC conversion units are connected in a symmetrical manner; the front stage of the inverter adopts a symmetrical structure, which can reduce the voltage stress borne by the switch tube when it is turned off, realize multi-level output, and at the same time, greatly reduce the change frequency of the common mode voltage, and the leakage current suppression effect is good. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] Figure 1 This is a schematic diagram of the DC / DC+DC / AC structure of a traditional non-isolated photovoltaic inverter;

[0052] Figure 2 This is a schematic diagram of the traditional DC / DC+DC / AC topology;

[0053] Figure 3~Figure 6 This is a specific circuit for improving the topology of the rear-stage H4 inverter bridge in the traditional DC / DC+DC / AC topology in the traditional solution, in which: Figure 3 For H5 bridge, Figure 4 For H6 bridge, Figure 5 For the Heric circuit, Figure 6 It is a ZVR-Hreic circuit;

[0054] Figure 7 for Figure 2 Schematic diagram of unipolar sinusoidal pulse width modulation drive signal of the rear-stage H4 full-bridge inverter of the unified DC / DC+DC / AC topology shown;

[0055] Figure 8 for Figure 2 Schematic diagram of bipolar sinusoidal pulse width modulation drive signal of the rear-stage H4 full-bridge inverter of the unified DC / DC+DC / AC topology shown;

[0056] Fig. 9 for Figure 2 The schematic diagram of the unipolar double frequency sinusoidal pulse width modulation driving signal of the rear stage H4 full bridge inverter of the unified DC / DC+DC / AC topology shown;

[0057] Fig.10 It is a schematic diagram of a dual DC / DC and DC / AC two-stage structure based on a wide-gain, low-leakage current unipolar inverter in Example 1;

[0058] Fig.11 for Fig.10 The topology described above is a schematic diagram of photovoltaic output, bus capacitor, inverter output and output voltage waveform under the control of bipolar SPWM modulation strategy;

[0059] Fig.12 for Fig.10 The topology described above is a schematic diagram of the photovoltaic output, bus capacitor, inverter output and output voltage waveform under the control of the unipolar SPWM modulation strategy;

[0060] Fig.13 The front stage adopts a dual Boost circuit, the rear stage adopts an H4 inverter bridge, and a partial first-stage energy conversion control strategy is adopted. The front stage outputs three levels as a case to analyze the working mode of the converter structure proposed in Example 1. Under this working topology and modulation strategy, the output waveforms of each level of the system structure are;

[0061] Fig.14 for Fig.10 A specific circuit diagram of the structure shown, the circuit is a dual three-level dual Boost + H4 topology;

[0062] Fig.15 for Fig.14 The topology shown is a schematic diagram of the switch tube drive signal in DC mode;

[0063] Figure 16~Figure 19 for Fig.15 Driven by the switch tube drive signal shown, Fig.14 The four operating modes of the positive half cycle of the topology shown, among which, Fig.16 is working mode 1, Fig.17 For working mode 2, Fig.18 For working mode three, Fig.19 It is working mode four;

[0064] Fig. 20 for Fig.15 for Fig.14 The schematic diagram of the topology of the switch drive signal is shown in the figure, where ① refers to the DC mode, the front-stage DC / DC outputs a DC voltage, and the rear-stage inverter works in a high-frequency switching state. ② refers to the current conversion mode, the front-stage DC / DC outputs a partial sinusoidal voltage, and the rear-stage inverter works in a direct-on or off state;

[0065] Figure 21~Figure 23 for Fig. 20 Driven by the switch tube drive signal shown, Fig.14 The three operating modes of the negative half cycle of the topology shown, among which, Fig.21 is working mode 1, Fig. 22 For working mode 2, Fig.23 This is working mode three. DETAILED DESCRIPTION

[0066] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0067] It should be noted that, for the convenience of description, the switch IGBT is used to represent the controllable (on and off) switch tube in the embodiment of the present invention, but the switch tube in the present invention is not limited to IGBT. Take IGBT as an example for explanation. The first end of the IGBT refers to the collector, the second end refers to the emitter, and the control end refers to the gate. A driving control signal is applied to the control end of each switch tube in the embodiment of the present invention. For the sake of brevity, it will not be repeated later. The power switch tube in the embodiment of the present invention can also be implemented by other controllable switch tube devices other than IGBT, such as MOSFET. At the same time, in order to ensure the normal operation of each switch device in the embodiment of the present invention, a freewheeling diode needs to be connected in parallel to each switch device. The parallel connection direction of the freewheeling diode is related to the type of the switch device. The technician can set it according to the type of the switch device, which is not limited here. If not specified, the switch device defaults to including a freewheeling diode, which will be pointed out in this embodiment in special cases.

[0068] The embodiment of the present application solves the technical problem that a traditional non-isolated photovoltaic inverter will generate leakage current during the operation of a grid-connected system by providing a wide-gain, low-leakage current unipolar inverter, thereby achieving leakage current suppression.

[0069] The technical solution in the embodiment of the present application is to solve the above technical problems, and the overall idea is as follows:

[0070] In order to solve the technical problem that traditional non-isolated photovoltaic inverters will generate leakage current during the operation of the grid-connected system, the embodiments of the present invention mainly study the two-stage converter topology and control strategy, and propose a new two-stage topology combining DC / DC and DC / AC with leakage current suppression effect, and a control strategy combining partial one-stage energy conversion DC / DC with high efficiency and low switching stress and DC / AC.

[0071] For single-phase photovoltaic systems that combine DC / DC and DC / AC, the front-stage DC / DC module uses a traditional Boost converter. Due to its high efficiency and stability, the full-bridge non-isolated inverter is widely used in photovoltaic power stations. When the rear-stage inverter is a full-bridge structure, the traditional topology diagram is as follows: Figure 2 shown.

[0072] In terms of topology improvement, considering the suppression of leakage current, the traditional solution is to improve the topology of the subsequent H4 inverter bridge. Specific technical solutions include H5 bridge, H6 bridge, Heric circuit and ZVR-Hreic circuit.

[0073] The topology diagram of the H5 bridge inverter technology solution is as follows: Figure 3 As shown in the figure, the topology connects a switch tube in series in the main loop of the full-bridge circuit. , in the working process, cooperate with the unipolar modulation strategy to decouple the photovoltaic panel from the main circuit in the freewheeling stage, so as to achieve the purpose of suppressing leakage current. Compared with the traditional single-phase full-bridge, this topology only adds one switch tube, which has the advantages of simple structure and high efficiency. However, due to the asymmetric topology, there is a problem of local heat dissipation, which affects the service life. Figure 4 As shown, it adopts unipolar modulation, and its common mode voltage can be kept constant at Vdc / 2. The switching voltage of all switches is half of the input voltage. Compared with the single-phase full-bridge inverter, the switching loss is reduced. Figure 5 As shown, two switching tubes are added to the AC filter side of the single-phase full bridge. and , through reverse series connection to form a bidirectional freewheeling loop, cut off the connection between the grid and the solar panel during the freewheeling stage, realize the decoupling of the AC side and the DC side, and achieve the purpose of suppressing leakage current. ZVR-Heric topology is an improved topology based on Heric topology, such as Figure 6Compared with the traditional Heric topology, the ZVR-Heric topology replaces the original freewheeling circuit with four diodes and a switch tube. Although the working principle is similar to the Heric topology, there are three power tubes turned on during the freewheeling process, resulting in relatively large power loss in the ZVR-Heric topology under the same working conditions. This increased loss directly leads to a decrease in the overall efficiency of the ZVR-Heric system.

[0074] In terms of control strategy, the traditional control strategy adopts fixed duty cycle modulation for the front-stage Boost circuit, which requires the switch tube to work in a high duty cycle state for a long time. The control strategy of the rear-stage H4 full-bridge inverter includes unipolar sinusoidal pulse width modulation, bipolar sinusoidal pulse width modulation and unipolar doubled frequency sinusoidal pulse width modulation.

[0075] Among them, in the unipolar sinusoidal pulse width modulation technology solution, the inverter AC side output pulse has a unipolar characteristic. Within the power frequency range, the inverter only outputs positive pulses during the positive half cycle and only outputs negative pulses during the negative half cycle. Figure 2 Take the H4 full-bridge inverter circuit as an example, in the positive half cycle, the switch and Working in power frequency working state, and Working in high frequency working state, the driving signal is as follows Figure 7 shown.

[0076] In the bipolar sinusoidal pulse width modulation technology solution, the inverter output pulse has bipolar characteristics in both the positive and negative half cycles. The switch tube drive signal is generated by carrier modulation, with the desired output sinusoidal waveform as the modulation signal and a bipolar triangle wave as the carrier signal. The drive pulse signal is obtained by the carrier comparison method. Figure 2 Taking the H4 inverter bridge as an example, the switch tubes all work in a high-frequency working state. and The driving signal is the same. and The driving signals are the same and the two sets of driving signals are complementary, such as Figure 8 shown.

[0077] In the unipolar double frequency sine pulse width modulation technology solution, the modulation frequency of the inverter output pulse is twice the carrier frequency, and the output pulse is unipolar. If the carrier frequency is the same as the carrier frequency during unipolar SPWM control, then the modulation frequency of the H4 inverter bridge output pulse under unipolar double frequency SPWM control is twice the output pulse frequency under unipolar SPWM control, which helps to reduce the distortion rate of the output waveform. Figure 2 Taking the H4 inverter bridge as an example, when unipolar double frequency sinusoidal pulse width modulation is adopted, and Complementary conduction, and Complementary conduction, the driving signal diagram is as follows Fig. 9 shown.

[0078] Through the above topology combination analysis and control strategy analysis, it can be seen that the improved topology structure based on full-bridge realizes the decoupling of the DC side and the AC side, but there are problems such as a large number of semiconductor devices in the energy transfer or freewheeling stage, which leads to increased system losses, or a large number of semiconductor devices in the topology itself but low utilization.

[0079] In the two-stage conversion structure combining DC / DC and DC / AC, when the topology of the rear-stage full-bridge inverter circuit is improved, the voltage stress of the front-stage boost circuit switch tube when it is turned off is the voltage across the DC bus capacitor. Since the DC bus capacitor voltage is stable at a higher voltage value, long-term operation is prone to damage components. At the same time, under high duty cycle conditions, the energy conversion efficiency in the circuit may be affected. Excessive duty cycle may cause part of the energy to be lost during the conversion process and cannot be effectively output to the load end, and the circuit may be more sensitive to fluctuations in the input voltage. Once the input voltage changes significantly, the stability of the output voltage may be seriously affected.

[0080] In SPWM modulation, the switch tube of the rear inverter frequently switches at the carrier frequency. This high-frequency switching operation leads to large switching losses. The increase in switching losses not only reduces the efficiency of the inverter, but may also cause the switch tube to overheat, thereby affecting the long-term stable operation of the inverter. At the same time, since the switch tube works at a high frequency, a large number of harmonic components will be generated. These harmonic components will be superimposed on the output voltage, resulting in a decrease in the waveform quality of the output voltage.

[0081] In order to solve the above problems, the embodiments of the present invention propose a new type of topology with leakage current suppression effect, and propose a low voltage stress and high efficiency partial primary energy conversion control strategy for this type of new topology.

[0082] It should be noted that in the field of inverter technology, the overall control strategy is divided into two working modes - DC mode and conversion mode, in which the front-stage rectification and the back-stage inverter switch tube of the DC mode both work in a high-frequency switching state, which is a two-stage energy conversion. In the conversion mode, there are two states, among which, state one: the DC / DC conversion module outputs a DC voltage, and the DC / AC inverter works in a high-frequency switching state; state two: the DC / DC conversion module outputs a partially sinusoidal AC voltage, and the DC / AC inverter works in a direct-on or off state, which is equivalent to only undergoing a first-stage energy conversion of the front-stage rectification, and most of the time in a cycle is a first-stage energy conversion, so it is called a partial first-stage energy conversion.

[0083] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0084] Embodiment 1:

[0085] The embodiment of the present invention proposes a unipolar inverter based on wide gain and low leakage current. The inverter is a two-stage topology architecture of DC / DC and DC / AC. Its front stage is a DC / DC conversion module with a dual structure. The DC / DC conversion module includes two DC / DC conversion units that are independent of each other but can work together. They optimize space utilization and efficiency by sharing certain components (such as input power supply, output filter capacitor, etc.).

[0086] like Fig.10 As shown, the wide-gain low-leakage current unipolar inverter according to the embodiment of the present invention includes a dual-structure DC / DC conversion module, a bus capacitor, a DC / AC inverter and a filter circuit;

[0087] The DC / DC conversion module includes two DC / DC conversion units, the structures and performance parameters of the two DC / DC conversion units are the same, and the two DC / DC conversion units are connected in a symmetrical manner; the two DC / DC conversion units each include an input end and two output ends; the input ends of the two DC / DC conversion units are respectively connected to the positive and negative poles of a DC power supply (in this embodiment, a photovoltaic power supply); the first output end of the first DC / DC conversion unit is connected to the first input end of the DC / AC inverter, and the second output end of the first DC / DC conversion unit is connected to the second output end of the second DC / DC conversion unit; the first output end of the second DC / DC conversion unit is connected to the second input end of the DC / AC inverter; a bus capacitor is connected between the first output end of the first DC / DC conversion unit and the first output end of the second DC / DC conversion unit, and the common end of the first DC / DC conversion unit and the second DC / DC conversion unit is connected to the midpoint of the bus capacitor;

[0088] The two output ends of the DC / AC inverter are connected to the grid after passing through a filter circuit.

[0089] It should be noted that, in the specific implementation process, the input ends of the first DC / DC conversion unit and the second DC / DC conversion unit are respectively connected to the positive and negative electrodes of the DC power supply via an inductor group. The functions of these two groups of inductors mainly include: when the DC voltage output by the photovoltaic array is low, the inductor group is the front-stage inductor of the boost voltage circuit, and has the functions of energy storage and filtering during the operation of the circuit. If the input DC voltage is high and needs to be stepped down and rectified, the two groups of inductors can be moved back between the DC / DC unit and the bus capacitor to play a freewheeling role.

[0090] In the specific implementation process, the control strategy of the dual structure DC / DC conversion module of the front stage and the DC / AC circuit of the back stage is as follows:

[0091] The dual-structure DC / DC conversion module of the previous stage can output different levels under different control strategies:

[0092] Two-level output: Under the basic control strategy, the DC / DC unit works in a traditional way to adjust the output voltage and achieve a basic two-level output. In this mode, the circuit is simple and suitable for applications that do not require high output waveform quality.

[0093] Three-level output: By adjusting the control strategy, the output voltage of the DC / DC unit is superimposed or interleaved to form a three-level output. This output mode helps to reduce the harmonic content of the output voltage and improve the power quality.

[0094] Five-level output: Further adjustment of the control strategy can introduce more intermediate level states to obtain five-level output. If additional switches are added or the circuit topology is adjusted, even more levels of output can be achieved. Multi-level output can suppress harmonics more effectively and is suitable for occasions with extremely high requirements for power quality.

[0095] The DC / AC circuit is highly flexible and supports multiple inverter topologies such as H4 full-bridge circuit, H5 bridge, H6 bridge, HERIC circuit, etc. These different topological modes have their own advantages and are suitable for different application scenarios. When the DC / AC circuit of the subsequent stage adopts different topologies, different modulation methods can be used to achieve unipolar or bipolar output.

[0096] For the front-stage DC / DC, the control strategy can adopt a fixed duty cycle mode to keep the voltage at both ends of the DC bus constant. At this time, the modulation mode of the rear-stage inverter can adopt unipolar SPWM, bipolar SPWM and unipolar frequency-doubled SPWM modulation according to the different topological structures. The front-stage DC / DC can also adopt a control strategy of partial primary energy conversion, by controlling the duty cycle of the DC / DC unit to control the voltage on the DC bus to a waveform that combines DC and partial sine. When the front-stage DC / DC outputs DC, the rear-stage inverter bridge works in the same way as the fixed duty cycle mode. When the DC / DC outputs a partial sine waveform, the rear-stage inverter circuit works in a direct-on or off state, reducing the number of energy conversions and improving efficiency.

[0097] When using a partial primary energy conversion control strategy, unipolar or bipolar SPWM modulation can be used according to the different inverter topologies of the subsequent stage. Under different modulation strategies, the schematic diagrams of the photovoltaic output, bus capacitor, inverter output and output voltage waveforms are shown in the figure below. Fig.11 , 12 shown.

[0098] From the above description, it can be seen that the topological architecture proposed in this embodiment realizes flexible switching of output voltage levels and diversification of inverter circuit output characteristics under different control strategies through the dual structure of the front-stage DC / DC circuit and the diverse topological modes of the rear-stage DC / AC circuit, providing new ideas and methods for the design and optimization of power electronic systems.

[0099] Embodiment 2:

[0100] This embodiment adopts a dual Boost circuit in the front stage, an H4 inverter bridge in the back stage, and a partial first-stage energy conversion control strategy. The front stage outputs three levels as an example to analyze the working mode of the converter structure proposed in the embodiment of the present invention. Under this working topology and modulation strategy, the output waveforms of each stage of the system are as follows: Fig.13 As shown, the dual three-level dual Boost + H4 topology is as follows Fig.14 As shown, the switch tube and Using the same drive signal, and Using the same drive signal and the signal lagging behind and Half a cycle (Note: The front stage of the topology adopts Boost circuit, but it does not mean that only boost module can be used; the rear inverter circuit can also modify the H4 inverter bridge to obtain different single and bipolar output characteristics; the front stage DC / DC can also be controlled by fixed duty cycle, and the front stage outputs DC waveform; the circuit diagram uses IGBT symbol to represent the switch tube, but it does not mean that only IGBT can be used for the switch tube; all diodes in the topology can be replaced by fully controlled devices that meet the modal flow direction; the load is replaced by AC grid-connected, but it does not mean that it can only be AC ​​grid-connected, and the converter can operate off-grid. The front stage DC / DC switch tube can also adopt an asymmetric control method, and each switch tube can be controlled separately to achieve output multi-level switching and bus capacitor voltage adjustment).

[0101] In this converter topology, and For flying capacitors, the two capacitors have the same capacitance. and The two capacitors have the same capacitance value, and the voltage across the capacitors is half of the DC bus voltage. In this circuit, the input terminals of the two DC / DC conversion units are points p and n, the first output terminal of the second DC / DC conversion unit is point N, and the two output terminals of the DC / AC inverter are points A and B. When the common-mode equivalent model is established for the topology, when the common-mode voltage of the entire topology is When the voltage remains unchanged, the converter has the minimum leakage current during operation.

[0102] In a DC converter, taking a Boost converter as an example, when the input voltage remains unchanged, the output voltage is determined by the duty cycle of the converter. When the photovoltaic output DC voltage is low, the DC / DC conversion unit needs to work in a high duty cycle state for a long time, which puts a lot of pressure on the converter. When the control strategy of partial primary energy conversion is adopted, the output waveform can be changed by controlling the change of the duty cycle of the Boost converter. When the duty cycle changes according to the sine law, the converter works in the variable current mode. When the control strategy of partial primary energy conversion is adopted, the output voltage of the photovoltaic array, the voltage across the DC bus capacitor and the output voltage waveform of the AC side are as follows: Fig.13 shown.

[0103] In DC mode, the rear H4 inverter bridge works in high-frequency working state. In variable current mode, the H4 inverter has two states, one is working in high-frequency switching state, and the other is working in direct-on or off state. This control strategy is adopted in the dual three-level dual Boost + H4 topology converter topology, and the topology working state is analyzed.

[0104] In the DC mode, the working mode of the topology is analyzed by taking the positive half-cycle working state as an example. The switch tube driving signal is as follows: Fig.15 In this mode, there are four working modes in the positive half cycle, and the mode diagram is as follows Figures 16 to 20 shown.

[0105] Working mode 1, front stage switch tube and Shutdown, and Conducting, the next stage switch tube and Conductivity, and Shutdown, working mode diagram as shown Fig.16 As shown, the flying capacitor and Stored energy, common mode voltage .in is the voltage across the DC bus. During the topology operation, Remains unchanged, that is, the topological common-mode voltage remains unchanged in this mode. constant.

[0106] Working mode 2, front stage switch tube and Conductivity, and Turn off, the subsequent switch tube and Conductivity, and Shutdown, working mode diagram as shown Fig.17 As shown, the flying capacitor and Release energy, and the front switch tube and The voltage stress is , the common mode voltage of the entire topology .

[0107] Working mode three, front stage switch tube and Shutdown, and Conducting, the next stage switch tube and Shutdown, and Conducting, the working mode diagram is as follows Fig.18 As shown, the flying capacitor and Energy is stored, and the current of the subsequent inverter flows through and Anti-parallel diode freewheeling, common mode voltage .

[0108] Working mode four, front stage switch tube and Conductivity, and Turn off, the subsequent switch tube and Shutdown, and Conducting, the working mode diagram is as follows Fig.19 As shown, the flying capacitor and Release energy, and the front switch tube and The voltage stress is , the subsequent inverter current flows through and The anti-parallel diode freewheels, and the common-mode voltage of the entire topology .

[0109] That is, in the four working modes of the entire DC mode positive half-cycle, the common-mode voltage of this topology remains unchanged at twice the DC bus voltage, which can effectively suppress the generation of leakage current. At the same time, the voltage stress borne by the front-stage Boost circuit switch when it is turned off is one-fourth of the DC bus voltage, which is lower than the traditional solution, and the negative half-cycle is similar.

[0110] In the current conversion mode, the negative half-cycle working state is taken as an example to analyze the working mode of the topology. The switch tube driving signal is as follows: Fig. 20In this mode, the DC / DC conversion module outputs a partially sinusoidal AC voltage, the DC / AC inverter works in the on or off state, and the topology has three working modes. The mode diagram is shown in Figures 21 to 23 shown.

[0111] Working mode 1: All switches in the front-stage Boost circuit are turned on, and the back-stage H4 switch is turned on. and Shutdown, and conduction, its working mode is as follows Fig.21 Flying capacitor and The energy remains constant, the inductor and The current increases and energy is stored. The potentials of points p and n in the front-stage Boost circuit are the same, which is the midpoint voltage of the two electrolytic capacitors on the DC bus side. Then the common mode voltage .

[0112] In the subsequent H4 inverter bridge, due to the DC bus electrolytic capacitor and The existence of the switch and Keep the switch on, then the potential difference between point A and point N is , the potential difference between the midpoints B and N of the latter bridge arm is At this time, the equivalent common mode voltage of the H4 bridge inverter is , then the common-mode voltage of the entire topology ;

[0113] Working mode 2, pre-stage Boost circuit Pass, Turn off, the next stage H4 switch tube and Shutdown, and conduction, its working mode is as follows Fig. 22 As shown. At this time, the flying capacitor and Energy storage, inductance and Current decreases, energy is released, and topological common-mode voltage ;

[0114] Working mode three, pre-stage boost circuit Shutdown, Open, the H4 switch tube of the rear stage and Shutdown, and Open, its working mode is as follows Fig.23 As shown. At this time, the flying capacitor and Release energy, inductance and Current decreases, energy is released, and topological common-mode voltage ;

[0115] In the current conversion mode, the output voltage of the front stage changes according to the partial sine law at the power frequency, and the common mode voltage also changes according to the sine law of the power frequency. Compared with the SPWM modulation of high-frequency switching, this scheme can greatly reduce the frequency of change of the common mode voltage, has the effect of suppressing leakage current, and at this time, the H4 inverter bridge of the rear stage does not need to perform high-frequency switching action, but is directly turned on, which can reduce the harmonic components caused by the high-frequency action of the switch tube and reduce switching losses.

[0116] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0117] 1. The front stage adopts a symmetrical structure, which can reduce the voltage stress of the switch tube when it is turned off, realize multi-level output, and at the same time, greatly reduce the frequency of change of common mode voltage, and have a good leakage current suppression effect.

[0118] 2. Avoid the front-stage DC / DC converter from continuously operating at a high duty cycle when using a two-stage structure, reduce the voltage stress on the switch tube, and improve the stability of the topology. Extend the service life of the equipment, have the advantages of frequency doubling characteristics and small demand for magnetic components, and can also be used for string photovoltaic inverters.

[0119] 3. When the corresponding bridge arm of the rear-stage full-bridge inverter is directly connected, the energy is only converted by the front-stage DC converter. The energy loss during the flow process is reduced. At the same time, the number of switching actions of the switch tube is greatly reduced, which can reduce the switching loss, suppress the harmonic components generated by the high-frequency action of the switch tube, and improve the power density of the converter and the efficiency.

[0120] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0121] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A wide-gain low-leakage current unipolar inverter, characterized in that: It includes a dual-structure DC / DC conversion module, a bus capacitor, a DC / AC inverter and a filter circuit; The DC / DC conversion module includes two DC / DC conversion units, the structures and performance parameters of the two DC / DC conversion units are the same, and the two DC / DC conversion units are connected in a symmetrical manner; the two DC / DC conversion units each include an input end and two output ends; the input ends of the two DC / DC conversion units are respectively connected to the positive and negative poles of the DC power supply; the first output end of the first DC / DC conversion unit is connected to the first input end of the DC / AC inverter, and the second output end of the first DC / DC conversion unit is connected to the second output end of the second DC / DC conversion unit; the first output end of the second DC / DC conversion unit is connected to the second input end of the DC / AC inverter; a bus capacitor is connected between the first output end of the first DC / DC conversion unit and the first output end of the second DC / DC conversion unit, and the common end of the first DC / DC conversion unit and the second DC / DC conversion unit is connected to the midpoint of the bus capacitor; The two output ends of the DC / AC inverter are connected to the grid after passing through a filter circuit; The first DC / DC conversion unit includes a first diode, a second diode, a first switch tube, a second switch tube and a first flying capacitor; The cathode of the first diode is the first output end of the first DC / DC conversion unit, the anode of the first diode is connected to the cathode of the second diode, and the cathode of the second diode is connected to the first end of the second switch tube via the first switch tube; the second end of the second switch tube is the second output end of the first DC / DC conversion unit; Two ends of the first flying capacitor are respectively connected to the common end of the first diode and the second diode, and to the common end of the first switch tube and the second switch tube; The common end of the second diode and the first switch tube is the input end of the first DC / DC conversion unit; The second DC / DC conversion unit includes a third diode, a fourth diode, a third switch tube, a fourth switch tube and a second flying capacitor; The cathode of the fourth diode is the first output end of the first DC / DC conversion unit, the anode of the fourth diode is connected to the cathode of the third diode, and the cathode of the third diode is connected to the first end of the third switch tube via the fourth switch tube; the second end of the third switch tube is the second output end of the second DC / DC conversion unit; Two ends of the second flying capacitor are respectively connected to the common end of the fourth diode and the third diode, and the common end of the fourth switch tube and the third switch tube; The common end of the third diode and the fourth switch tube is the input end of the DC / DC conversion unit.

2. The wide-gain low-leakage current unipolar inverter according to claim 1, characterized in that: The dual-structure DC / DC conversion module includes a plurality of switch tubes, each of which includes a control terminal, and the disconnection and conduction of the switch tube are controlled by a control signal. The DC / DC conversion module outputs two levels, three levels, or five levels. and / or, The working modes of the wide-gain low-leakage current unipolar inverter include a DC mode and a current conversion mode; Among them, the DC mode means that the DC / DC conversion module outputs a DC voltage, and the DC / AC inverter operates in a high-frequency switching state; The current conversion mode includes two states: State 1: the DC / DC conversion module outputs a DC voltage, and the DC / AC inverter works in a high-frequency switching state; State 2: the DC / DC conversion module outputs a partially sinusoidal AC voltage, and the DC / AC inverter works in a direct-on or off state.

3. The wide-gain low-leakage current unipolar inverter according to any one of claims 1 to 2, characterized in that: The input terminals of the two DC / DC conversion units are set to point p and point n, the first output terminal of the second DC / DC conversion unit is point N, and the two output terminals of the DC / AC inverter are point A and point B; the voltage between point p and point N is , the voltage between point n and point N is , the voltage between point A and point N is , the voltage between point B and point N is ; The common mode voltage of the wide gain low leakage current unipolar inverter The calculation formula is as follows: The wide-gain low-leakage current unipolar inverter keeps the common-mode voltage unchanged in different working modes.

4. The wide-gain low-leakage current unipolar inverter according to any one of claims 1 to 2, characterized in that: An inductor group is respectively arranged between the input ends of the two DC / DC conversion units and the positive and negative electrodes of the DC power supply.

5. The wide-gain low-leakage current unipolar inverter according to claim 1, characterized in that: The DC / AC inverter includes an H4 full bridge circuit, an H5 bridge, an H6 bridge, a HERIC circuit, or a Heric circuit.

6. The wide-gain low-leakage current unipolar inverter according to claim 1, characterized in that: When the DC / AC inverter is an H4 full-bridge circuit, the H4 full-bridge circuit includes a fifth switch tube, a sixth switch tube, a seventh switch tube and an eighth switch tube; Wherein, the first end of the fifth switch tube is connected to the first end of the seventh switch tube, and the common end thereof is the first input end of the DC / AC inverter; The second end of the fifth switch tube is connected to the first end of the sixth switch tube, and the common end thereof is the first output end of the DC / AC inverter; The second end of the seventh switch tube is connected to the first end of the eighth switch tube, and the common end thereof is the second output end of the DC / AC inverter; The second end of the sixth switch tube is connected to the second end of the eighth switch tube, and the common end thereof is the second input end of the DC / AC inverter.

7. The wide-gain low-leakage current unipolar inverter according to claim 6, characterized in that: Set the common end of the second diode and the first switch tube as point p, the common end of the fourth switch tube and the third diode as point n, the common end of the second flying capacitor and the fourth diode as point N; the common end of the fifth switch tube and the sixth switch tube as point A, the common end of the seventh switch tube and the eighth switch tube as point B; the voltage between point p and point N is , the voltage between point n and point N is , the voltage between point A and point N is , the voltage between point B and point N is ; The common mode voltage of the wide gain low leakage current unipolar inverter The calculation formula is as follows: The wide-gain low-leakage current unipolar inverter keeps the common-mode voltage unchanged in different working modes.

8. The wide-gain low-leakage current unipolar inverter according to claim 7, characterized in that: When the DC / DC conversion module outputs a DC voltage and the DC / AC inverter operates in a high-frequency operating state, the positive half-cycle operating state based on the wide-gain low-leakage current unipolar inverter includes four operating modes, specifically including: In working mode 1, the first switch tube and the fourth switch tube of the front stage are turned off, the second switch tube and the third switch tube are turned on, the fifth switch tube and the eighth switch tube of the rear stage are turned on, and the sixth switch tube and the seventh switch tube are turned off; in working mode 1, the first flying capacitor and the second flying capacitor store energy, and the common mode voltage ;in is the voltage across the DC bus; In working mode 2, the first switch tube and the fourth switch tube of the front stage are turned on, the second switch tube and the third switch tube are turned off, the fifth switch tube and the eighth switch tube of the rear stage are turned on, and the sixth switch tube and the seventh switch tube are turned off; in working mode 2, the first flying capacitor and the second flying capacitor release energy, and the common mode voltage ; Working mode three, the first switch tube and the fourth switch tube of the front stage are turned off, the second switch tube and the third switch tube are turned on, the fifth switch tube and the eighth switch tube of the rear stage are turned off, and the sixth switch tube and the seventh switch tube are turned on; in working mode three, the first flying capacitor and the second flying capacitor store energy, and the common mode voltage ; Working mode four, the first switch tube and the fourth switch tube of the front stage are turned on, the second switch tube and the third switch tube are turned off, the fifth switch tube and the eighth switch tube of the rear stage are turned off, and the sixth switch tube and the seventh switch tube are turned on; in working mode four, the first flying capacitor and the second flying capacitor release energy, and the common mode voltage .

9. The wide-gain low-leakage current unipolar inverter according to claim 7, characterized in that: When the DC / DC conversion module outputs a partially sinusoidal AC voltage and the DC / AC inverter operates in a direct-on or off state, the negative half-cycle operating state based on the wide-gain low-leakage current unipolar inverter includes three operating modes: Working mode 1, all the switches of the front-stage DC / DC conversion module are turned on, the fifth and eighth switches of the back-stage H4 are turned off, and the sixth and seventh switches are turned on; the energy of the first flying capacitor and the second flying capacitor remains unchanged, and the common mode voltage of the front-stage DC / DC conversion module is ; Equivalent common mode voltage of the subsequent H4 bridge inverter , the common-mode voltage of the entire wide-gain low-leakage unipolar inverter ; Working mode 2, the first switch tube and the fourth switch tube of the front stage are turned on, the second switch tube and the third switch tube are turned off, the fifth switch tube and the eighth switch tube of the rear stage are turned off, the sixth switch tube and the seventh switch tube are turned on, the first flying capacitor and the second flying capacitor store energy, and the common mode voltage of the entire wide gain low leakage current unipolar inverter is ; Working mode three, the first switch tube and the fourth switch tube of the front stage are turned off, the second switch tube and the third switch tube are turned on, the fifth switch tube and the eighth switch tube of the rear stage are turned off, the sixth switch tube and the seventh switch tube are turned on, the first flying capacitor and the second flying capacitor release energy, and the common mode voltage of the entire wide gain low leakage current unipolar inverter is .

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