A three-level inverter structure, inverter and photovoltaic power supply system

Abstract

The present invention discloses a three-level inverter structure, an inverter, and a photovoltaic power supply system, belonging to the field of inverter circuit topology. The three-level circuit requires five power switch tubes, wherein the switching timing of power switches S1 and S4, and S2 and S3 are the same, thereby effectively reducing the number of drive circuits. The three-level circuit has only three operating modes. In the 0-level mode, a DC power supply charges the first capacitor, thereby ensuring that the charging and discharging of the first capacitor are carried out at the switching frequency scale, maintaining the voltage across the first capacitor stable; on the other hand, under non-unity power factor, the charging circuit of the first capacitor of the circuit is separated from the freewheeling circuit of the power grid, and the excess electric energy of the first capacitor can be fed back to the DC power supply, so that the voltage of the first capacitor of the circuit is stable within the full power factor range, and the circuit has excellent stable working performance within the full power factor range.

Description

Technical Field

[0001] The present invention relates to the field of inverter circuit topology, and in particular to a three-level inverter structure, an inverter and a photovoltaic power supply system. Background Art

[0002] Currently, there are three types of non-isolated photovoltaic grid-connected inverter circuits with leakage current suppression capabilities: a two-level full-bridge inverter circuit with bipolar modulation, a half-bridge circuit, and a common ground circuit. Since the two-level output voltage of the two-level full-bridge inverter circuit with bipolar modulation is two-level, it is necessary to increase the switching frequency or filter capacity to reduce the distortion rate of the grid-connected current. Therefore, this method is not conducive to improving the power density of the grid-connected inverter device. The half-bridge circuit has a three-level output voltage level, thus reducing the harmonic distortion rate of the grid-connected current without increasing the switching frequency or filter capacity. However, the peak value of the inverter output is only half of the input DC voltage, which increases the design difficulty of the inverter's pre-stage boost circuit. The common ground type grid-connected inverter realizes the natural suppression of leakage current, and the peak value of the output voltage is consistent with the input DC voltage, and the voltage utilization rate is 100%. However, the current common ground type grid-connected inverter still has two problems. On the one hand, the current common ground type inverter does not have the expansion capability. In order to further improve the output power quality and the output voltage gain, it is still necessary to increase the filter capacity and increase the boost capability of the front-stage boost circuit; on the other hand, the current common ground type inverter often shows the problem of insufficient non-unity power factor processing capability when connected to the grid. That is, when the non-power factor angle is large, the terminal voltage of the capacitor fluctuates greatly, which is not conducive to the stable operation of the grid-connected system. Summary of the Invention

[0003] In view of the deficiencies in the prior art, the present invention proposes a three-level inverter structure, an inverter and a photovoltaic power supply system.

[0004] The purpose of the present invention can be achieved through the following technical solutions:

[0005] A three-level inverter structure includes: a first power switch tube, a second power switch tube, a third power switch tube, a fourth power switch tube, a fifth power switch tube, a first capacitor and an AC filter inductor;

[0006] The drain of the first power switch tube is connected to the positive electrode of the DC power supply; the source of the first power switch tube is connected to the source of the third power switch tube and the drain of the fifth power switch tube; the drain of the third power switch tube is connected to the positive electrode of the first capacitor; the source of the fifth power switch tube is connected to the drain of the fourth power switch tube and the drain of the second power switch tube; the negative electrode of the first capacitor is connected to the source of the fourth power switch tube and one end of the AC filter inductor; the other end of the AC filter inductor is connected to one end of the AC power distribution network; the other end of the AC power distribution network is a common ground end; the common ground end is connected to the source of the second power switch tube and the negative electrode of the DC power supply.

[0007] Furthermore, the first power switch tube, the second power switch tube, the third power switch tube, the fourth power switch tube and the fifth power switch tube are at least one of a metal-oxide semiconductor field effect transistor, an insulated gate bipolar transistor and a silicon carbide field effect transistor.

[0008] Furthermore, the driving signals of the first power switch tube, the second power switch tube, the third power switch tube, the fourth power switch tube and the fifth power switch tube are generated by modulation of a modulation wave and a high-frequency triangular carrier; the modulation wave is an industrial frequency of 50 Hz, and the frequency of the high-frequency triangular carrier is 50 kHz.

[0009] Furthermore, the first power switch tube and the fourth power switch tube have the same switching timing and share a driving circuit; the second power switch tube and the third power switch tube have the same switching timing and share a driving circuit.

[0010] Furthermore, the three-level inverter structure further includes an expansion unit; the expansion unit includes the following structure:

[0011] The positive electrode of the n-1th capacitor is connected to the drain of the 5n-4th power switch tube;

[0012] The source of the 5n-4th power switch tube is connected to the source of the 5n-2th power switch tube and the drain of the 5nth power switch tube;

[0013] The drain of the 5n-2th power switch tube is connected to the positive electrode of the nth capacitor;

[0014] The source of the 5nth power switch tube is connected to the drain of the 5n-1th power switch tube and the drain of the 5n-3th power switch tube;

[0015] The source electrode of the 5n-3th power switch tube is connected to the negative electrode of the n-1th capacitor;

[0016] The negative electrode of the nth capacitor is connected to the source of the 5n-1th power switch tube and one end of the AC filter inductor; the negative electrode of the nth capacitor is an output port of the extension unit, and the other output port of the extension unit is connected to the source of the second power switch.

[0017] Furthermore, the three-level inverter structure includes three operating modes:

[0018] Mode 1: The first, fourth, and fifth power switches are turned on, and the DC power supply provides a forward voltage to the AC distribution network. At this point, the current flowing through the loop formed by the first, fourth, and fifth power switches, the DC power supply, the AC filter inductor, and the AC distribution network is equal to the current of the AC distribution network.

[0019] Mode 2: The first, second, third, and fourth power switches are turned on; the freewheeling branch formed by the second and fourth power switches equalizes the potentials at points a and N; the freewheeling circuit of the AC power distribution network is separated from the circuit for charging the first capacitor by the DC power supply; when the voltage across the first capacitor reaches the DC power supply, the potential is maintained.

[0020] Mode 3: The second, third, and fifth power switches are turned on, so that the first capacitor is connected in series to the AC distribution network for discharge. At this time, the loop current formed by the second, third, and fifth power switches, the first capacitor, the AC filter inductor, and the AC distribution network is the current of the AC distribution network.

[0021] In a second aspect, the present invention further provides an inverter comprising the three-level inverter structure as described in any one of the above items.

[0022] In a third aspect, the present invention further provides a photovoltaic power supply system, comprising:

[0023] a photoelectric device, the photoelectric device serving as a DC power supply for outputting a DC voltage;

[0024] AC distribution network;

[0025] As described above, the input end of the inverter is connected to the photovoltaic device, and the output end of the inverter is connected to the AC power distribution network. The inverter is used to convert the DC voltage into AC voltage and output it to the AC power distribution network.

[0026] Beneficial effects of the present invention:

[0027] The inverter circuit of the present invention uses only a small number of components to directly connect the neutral line of the AC distribution network to the negative electrode of the photovoltaic panel, forming a common ground structure to eliminate leakage current. The three-level circuit requires five power switches, among which the switching timing of power switches S1 and S4, and S2 and S3 are the same, reducing the number of drive circuits and lowering costs.

[0028] The charging and discharging of the first capacitor in this three-level circuit occurs only during the negative half-cycle of the AC power distribution network and is performed at the switching frequency scale, achieving voltage stability across the first capacitor. At non-unity power factors, the circuit separates the charging circuit for the first capacitor from the freewheeling circuit of the AC power distribution network, and excess energy from the first capacitor can be fed back to the DC power supply, ensuring that the voltage of the first capacitor remains stable across the entire power factor range. This inverter circuit has good scalability. When the number of expansion units increases to n, the number of inverter output voltage levels increases to (2n+1), effectively improving the power quality of the inverter output. The peak value of the output voltage is up to n times the DC input, reducing the pressure on the boost circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below with reference to the accompanying drawings.

[0030] Figure 1 Schematic diagram of the structure of the expandable three-level inverter circuit of the present invention;

[0031] Figure 2 This is a schematic diagram of the power switch tube driving signal of the present invention;

[0032] Figure 3 This is the energy transfer working mode diagram of the positive half-cycle of the AC distribution network voltage;

[0033] Figure 4 This is the freewheeling working mode diagram of the AC distribution network;

[0034] Figure 5 This is the working mode diagram of energy transfer in the negative half cycle of AC distribution network voltage;

[0035] Figure 6 This is the operating waveform diagram under unity power factor;

[0036] Figure 7 This is the operating waveform diagram when the current leads the voltage of the AC distribution network by 90°;

[0037] Figure 8 This is the operating waveform diagram when the current lags the voltage by 90° in the AC distribution network;

[0038] Figure 9 is a waveform diagram of the voltage of the first capacitor at unity power factor;

[0039] Figure 10 This is the operating waveform diagram when the current leads the voltage of the AC distribution network by 90°;

[0040] Figure 11 This is the operating waveform diagram when the current lags the voltage by 90° in the AC distribution network;

[0041] Figure 12 This is the expanded circuit diagram of the circuit. DETAILED DESCRIPTION

[0042] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only 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 making any creative efforts shall fall within the scope of protection of the present invention.

[0043] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0044] See attached Figure 1 The present invention provides a three-level inverter circuit comprising: a first capacitor C1, a first power switch tube S1, a second power switch tube S2, a third power switch tube S3, a fourth power switch tube S4, a fifth power switch tube S5, an AC filter inductor L1, a DC power supply V pv , single-phase AC distribution network u g .

[0045] DC power supply V pv The positive electrode of the first power switch tube S1 is connected to the drain of the first power switch tube S1; the source of the first power switch tube S1 is connected to the source of the third power switch tube S3 and the drain of the fifth power switch tube S5; the drain of the third power switch tube S3 is connected to the positive electrode of the first capacitor C1; the source of the fifth power switch tube S5 is connected to the drain of the fourth power switch tube S4 and the drain of the second power switch tube S2; the negative electrode of the first capacitor C1 is connected to the output port a of the inverter circuit, the source of the fourth power switch tube S4 and one end of the AC filter inductor L1; the other end of the AC filter inductor L1 is connected to the AC distribution network u g One end is connected to the AC distribution network u g The other end is the common ground terminal N of the inverter circuit and the source of the second power switch tube S2 and the DC power supply V pvThe negative pole is connected.

[0046] Figure 2 The figure shows the schematic diagram of the driving signal of the power switch tube of the three-level inverter circuit, where the modulation wave v M is the power frequency (50Hz), v tri It is a 50kHz high frequency triangular carrier. M With v tri Modulation generates drive signals for S1, S2, S3, S4, and S5. S1 and S4 have the same switching timing and share a drive circuit, and S2 and S3 have the same switching timing and share a drive circuit, effectively reducing the number of drivers, simplifying control, and lowering costs.

[0047] There are three working modes in this embodiment:

[0048] Mode 1: If Figure 3 As shown, it is the positive half-cycle energy transfer mode of the AC distribution network, which is provided by the DC power supply V pv Provides a positive voltage to the AC power distribution network. The first capacitor C1 does not participate in the work. In this state, the switches S1, S4 and S5 are turned on. At this time, the switches S1, S4, S5 and the DC power supply V pv The current flowing through the loop formed by the AC filter inductor and the AC distribution network is equal to the AC distribution network current.

[0049] Mode 2: If Figure 4 As shown, it is the freewheeling mode. Turn on the power switches S1, S2, S3 and S4. At this time, the freewheeling branch formed by S2 and S4 makes the potential of points a and N equal. The freewheeling loop of the AC distribution network is connected to the DC power supply V pv The charging circuit of the first capacitor C1 is separated, and the two will not affect each other. When the voltage across the first capacitor C1 is charged to V pv The potential is maintained.

[0050] Mode 3: If Figure 5 As shown, this is the energy transfer mode of the negative half-cycle of the AC distribution network. The power switches S2, S3 and S5 are turned on, so that the first capacitor C1 is connected in series to the AC distribution network for discharge. At this time, the loop current formed by the switches S2, S3, S5, C1, the AC filter inductor and the AC distribution network is the AC distribution network current.

[0051] The inverter circuit works in the non-unity power factor mode and the unity power factor mode exactly the same, such as Figure 3 、 Figure 4 and Figure 5 As shown in Figure 2. Under reactive power transmission, when the AC distribution network voltage is positive and the incoming current is negative, the reactive energy is transferred between the load and the DC power supply V pv The switch mode is as follows: Figure 3As shown; when entering Figure 4 In the freewheeling mode 2 shown, the AC distribution network and the first capacitor C1 do not form a loop, and the first capacitor C1 is directly connected in parallel with the DC power supply. When the voltage across the first capacitor C1 is less than the DC power supply, the DC power supply V pv The first capacitor C1 is charged. When the voltage across the first capacitor C1 is greater than the DC power supply V pv When the voltage across both ends is pv In the process, the voltage across the first capacitor C1 is kept stable; when the AC distribution network voltage is negative and the incoming current is positive, as shown in FIG. Figure 5 The reactive energy shown is exchanged between the load and the first capacitor C1. At this time, the voltage across the first capacitor C1 will rise. However, in the next freewheeling mode, the excess electric energy of the first capacitor C1 will be fed back to the DC power supply, thereby always maintaining the voltage across the first capacitor C1 stable.

[0052] Figure 6 、 Figure 7 and Figure 8 : The operating waveforms of the inverter circuit of this embodiment under unity power factor and non-unity power factor when it is in three-level output, wherein: Figure 6 is the inverter output u at unity power factor aN 、u g 、i g The operating waveform, Figure 7 When the incoming current leads the AC distribution network voltage by 90°, u aN 、u g 、i g The operating waveform of Figure 8 u is when the incoming current lags behind the AC distribution network voltage by 90° aN 、u g 、i g The operating waveform of aN The peak voltage and DC power supply voltage V pv The voltage is consistent with 400V, u g The peak voltage is 311V, and the grid current i g The peak current is 6.43A and the power frequency is 50Hz. It can be seen that the inverter output can stably output good power quality under both unity power factor and extreme non-unity power factor.

[0053] Figure 9 、 Figure 10 and Figure 11 is the waveform of the voltage of the first capacitor C1 of the inverter circuit in this embodiment, wherein, Figure 9 is the waveform of the first capacitor voltage at unity power factor, Figure 10 This is the operating waveform when the incoming current leads the AC distribution network voltage by 90°. Figure 11 The operating waveform is shown when the grid current lags the AC distribution network voltage by 90°. It can be seen that when the capacitance of first capacitor C1 is 0.1mF and the switching frequency is 50kHz, the voltage of first capacitor C1 is stabilized at 400V at both unity power factor and extreme non-unity power factor, outputting a stable, high-quality grid current. Therefore, the inverter circuit has an excellent ability to transmit reactive power to the AC distribution network.

[0054] Figure 12 This is an extended circuit of the three-level inverter circuit of this embodiment. The capacitor of the extended inverter circuit operates at the switching frequency scale, so the capacitor voltage can remain stable. It is worth noting that as the number of levels increases, the same level may have multiple switching modes, which is beneficial for achieving the doubling of the differential mode operating frequency. Figure 12 The extended circuit shown in the figure satisfies formula (1). For every additional extended unit, that is, every additional five switches and one capacitor, two levels can be added and the output voltage gain can be doubled. N represents the number of levels. C Indicates the number of capacitors, N S Indicates the number of switches, Ng indicates the number of gate drivers, G indicates the output voltage gain, f DM Indicates the equivalent inverter differential mode operating frequency, f S Indicates the switching frequency;

[0055] The positive electrode of the n-1th capacitor is connected to the drain of the 5n-4th power switch tube; the source of the 5n-4th power switch tube is connected to the source of the 5n-2th power switch tube and the drain of the 5nth power switch tube; the drain of the 5n-2th power switch tube is connected to the positive electrode of the n-th capacitor; the source of the 5nth power switch tube is connected to the drain of the 5n-1th power switch tube and the drain of the 5n-3th power switch tube; the source of the 5n-3th power switch tube is connected to the negative electrode of the n-1th capacitor;

[0056] The negative electrode of the nth capacitor is connected to the source of the 5n-1th power switch tube and one end of the AC filter inductor; the negative electrode of the nth capacitor is an output port of the expansion unit, and the other output port of the expansion unit is connected to the source of the second power switch.

[0057]

[0058] All power switches described in this embodiment are metal-oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors, or silicon carbide field effect transistors.

[0059] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the invention as claimed.

Claims

1. A three-level inverter structure, characterized in that: include: A first power switch tube, a second power switch tube, a third power switch tube, a fourth power switch tube, a fifth power switch tube, a first capacitor and an AC filter inductor; The drain of the first power switch tube is connected to the positive electrode of the DC power supply; the source of the first power switch tube is connected to the source of the third power switch tube and the drain of the fifth power switch tube; the drain of the third power switch tube is connected to the positive electrode of the first capacitor; the source of the fifth power switch tube is connected to the drain of the fourth power switch tube and the drain of the second power switch tube; the negative electrode of the first capacitor is connected to the source of the fourth power switch tube and one end of the AC filter inductor; the other end of the AC filter inductor is connected to one end of the AC power distribution network; the other end of the AC power distribution network is a common ground terminal; the common ground terminal is connected to the source of the second power switch tube and the negative electrode of the DC power supply; The first power switch tube, the second power switch tube, the third power switch tube, the fourth power switch tube and the fifth power switch tube are at least one of a metal-oxide semiconductor field-effect transistor, an insulated gate bipolar transistor and a silicon carbide field-effect transistor; The driving signals of the first power switch tube, the second power switch tube, the third power switch tube, the fourth power switch tube and the fifth power switch tube are generated by modulation of a modulation wave and a high-frequency triangular carrier; the modulation wave is an industrial frequency of 50 Hz, and the frequency of the high-frequency triangular carrier is 50 kHz.

2. The three-level inverter structure according to claim 1, characterized in that: The first power switch tube and the fourth power switch tube have the same switching timing and share a driving circuit; the second power switch tube and the third power switch tube have the same switching timing and share a driving circuit.

3. The three-level inverter structure according to claim 1, characterized in that: The three-level inverter structure further includes an expansion unit; the expansion unit includes the following structure: The positive electrode of the n-1th capacitor is connected to the drain of the 5n-4th power switch tube; The source of the 5n-4th power switch tube is connected to the source of the 5n-2th power switch tube and the drain of the 5nth power switch tube; The drain of the 5n-2th power switch tube is connected to the positive electrode of the nth capacitor; The source of the 5nth power switch tube, the drain of the 5n-1th power switch tube, and the drain of the 5n-3th power switch tube; The source electrode of the 5n-3th power switch tube is connected to the negative electrode of the n-1th capacitor; The negative electrode of the nth capacitor is connected to the source electrode of the 5n-1th power switch tube and one end of the AC filter inductor; the negative electrode of the nth capacitor is an output port of the extension unit, the negative electrode of the n-1th capacitor is another output port of the extension unit, and the other output port of the extension unit is connected to the source electrode of the fourth power switch tube.

4. The three-level inverter structure according to claim 1, characterized in that: The three-level inverter structure includes three working modes: Mode 1: The first, fourth, and fifth power switches are turned on, and the DC power supply provides a forward voltage to the AC distribution network. At this point, the current flowing through the loop formed by the first, fourth, and fifth power switches, the DC power supply, the AC filter inductor, and the AC distribution network is equal to the current of the AC distribution network. Mode 2: The first, second, third, and fourth power switches are turned on; the freewheeling branch formed by the second and fourth power switches equalizes the potentials at points a and N; the freewheeling circuit of the AC power distribution network is separated from the circuit for charging the first capacitor by the DC power supply; when the voltage across the first capacitor reaches the DC power supply, the potential is maintained. Mode 3: The second, third, and fifth power switches are turned on, so that the first capacitor is connected in series to the AC distribution network for discharge. At this time, the loop current formed by the second, third, and fifth power switches, the first capacitor, the AC filter inductor, and the AC distribution network is the current of the AC distribution network.

5. An inverter, characterized in that: It comprises the three-level inverter structure according to any one of claims 1 to 4.

6. A photovoltaic power supply system, characterized in that: include: a photoelectric device, the photoelectric device serving as a DC power supply for outputting a DC voltage; AC distribution network; The inverter according to claim 5, wherein the input end of the inverter is connected to the photovoltaic device, the output end of the inverter is connected to the AC distribution network, and the inverter is used to convert the DC voltage into an AC voltage and output it to the AC distribution network.

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