Common ground type single phase three-level inverter circuit

Abstract

The application discloses a common ground type three-level inverter circuit and belongs to the technical field of power electronics, comprising a power switch network, a switch capacitor, an independent diode and an AC filter inductor. One AC output end of the inverter circuit is directly connected with a negative end of an input DC voltage, thereby forming a common ground structure and keeping the common mode voltage constant. The output voltage of the inverter circuit is three-level, and the differential mode voltage has low harmonic content. The switch capacitor works in a switch frequency charging and discharging mode, and the charging loop of the switch capacitor is separated from the freewheeling loop of the power grid, thereby effectively inhibiting the rising amplitude of the switch capacitor voltage under non-unit factor operation. The application is suitable for application of small and medium power non-isolated photovoltaic grid-connected inverter systems under unit and non-unit power factors.

Description

Technical Field

[0001] This invention relates to a common-ground type single-phase three-level inverter circuit technology, belonging to the field of power electronics technology. Background Technology

[0002] Non-isolated grid-connected photovoltaic (PV) inverters, lacking a transformer, offer significant advantages in terms of size and efficiency compared to isolated PV inverters. However, the absence of a transformer in a non-isolated PV inverter results in a lack of electrical isolation, creating parasitic capacitance between the PV array and ground, and a common-mode loop between the PV inverter and ground, leading to leakage current. This leakage current causes additional losses, damages equipment in the leakage current flow path, and can even endanger personnel safety.

[0003] Currently, single-phase non-isolated photovoltaic grid-connected inverters generally employ bipolar modulation full-bridge inverters, half-bridge inverters, and ground-type inverters. However, bipolar modulation full-bridge inverters output two levels, exhibiting poor differential characteristics. Half-bridge inverters require their DC-side input voltage to be twice the inverter's maximum output voltage, which is unfavorable for small-to-medium power photovoltaic grid-connected boost applications. Ground-type inverters directly connect the grid neutral point to the negative terminal of the photovoltaic panel, eliminating leakage current. However, ground-type inverters require, on the one hand, to ensure that the switching capacitors operate at the switching frequency scale to maintain capacitor voltage balance; on the other hand, if grid current flows through the switching capacitors during grid cycles when the switching capacitors are not discharging, the switching capacitor voltage will rise significantly, which is detrimental to providing high-quality power to the grid. Summary of the Invention

[0004] The purpose of this invention is to provide a common-ground single-phase three-level inverter circuit and its modulation method that are simple in structure and control, and can effectively suppress leakage current in non-isolated photovoltaic grid-connected systems. It also features the ability to separate the charging circuit of the switched capacitor from the grid freewheeling circuit during the grid freewheeling phase, achieve voltage balance of the switched capacitor at unity power factor, and suppress the voltage rise of the switched capacitor at non-unity power factor, thus maintaining stable voltage balance. This makes it suitable for applications in small-to-medium power non-isolated photovoltaic grid-connected inverter systems.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a common-ground type single-phase three-level inverter circuit, and a photovoltaic DC power supply ( ), first capacitor ( ), first power switch ( ), second power switch ( ), third power switch ( ), fourth power switch ( ), the fifth power switch ( ), first power diode ( ) and AC filter inductor ( );

[0006] Photovoltaic DC power supply ( The positive terminal of ) is connected to the first power switch ( The drain of the first power switch is connected to the first power switch. The source of the second power switch () The drain of the first power diode and the first power diode. The anode of the second power switch is connected; The source of the third power switch () The source of the first capacitor, the negative terminal of the first capacitor, and the output port a of the inverter circuit are connected; the third power switch ( The drain of the fourth power switch () The drains of the electrodes are connected to form a back-to-back structure;

[0007] Fourth power switch ( The source of ) and the fifth power switch ( The source of the photovoltaic DC power supply. The negative terminal of the fifth power switch is connected to the neutral wire at the center point of the power grid; The drain of ) is respectively connected to the first power diode ( The cathode of the inverter circuit is connected to the positive terminal of the first capacitor, and the output port a of the inverter circuit of this invention is connected to the AC filter inductor ( One end of the AC filter inductor is connected to the other end; The other end of the network is connected to the AC distribution network. One end is connected to the AC distribution network; The other end is connected to the inverter output port, i.e., to the DC-side photovoltaic DC power supply. The negative terminal of ) is connected.

[0008] As an optional embodiment of the present invention, the power switch is a metal-oxide-semiconductor field-effect transistor, an insulated-gate bipolar transistor, or a silicon carbide field-effect transistor.

[0009] As an optional embodiment of the present invention, the power diode... It can be a Schottky diode or a silicon power switching diode.

[0010] As an optional embodiment of the present invention, the AC filter circuit is an inductive filter. ), or a capacitive filter or an inductor-capacitor combination filter.

[0011] As an optional embodiment of the present invention, the load is an AC power grid ( ), or purely resistive load, inductive load, or capacitive load.

[0012] The beneficial effects of this invention are: by directly connecting the neutral point of the power grid to the negative terminal of the photovoltaic panel, a common ground structure is formed, which naturally eliminates the leakage current caused by the parasitic capacitance of the photovoltaic panel; the control is simple, wherein... and , and The two pairs of switches are complementary. Switching status and The switching states are consistent; during the grid freewheeling phase, the charging circuit of the switched capacitor is separated from the grid freewheeling circuit, achieving voltage balance of the switched capacitor under unity power factor, and suppressing the voltage rise of the switched capacitor under non-unity power factor, maintaining the voltage balance and stability of the switched capacitor, so that the inverter, as a small and medium power grid-connected inverter, has excellent reactive power compensation capability and optimizes the power quality of the input grid. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the common-ground type single-phase three-level inverter circuit in this embodiment.

[0014] Figure 2 This is a schematic diagram of the power switch drive signal of the common-ground single-phase three-level inverter circuit in this embodiment;

[0015] Figure 3(a) shows the working mode 1 of the power transfer during the positive half-cycle of the grid voltage in this embodiment;

[0016] Figure 3(b) shows the freewheeling operating mode 2 of the positive and negative half-cycles of the grid voltage in this embodiment;

[0017] Figure 3(c) shows the power transfer mode 3 during the negative half-cycle of the grid voltage in this embodiment;

[0018] Figure 4(a) shows the working mode 4 of the grid voltage positive half-cycle energy transfer in this embodiment;

[0019] Figure 4(b) shows the freewheeling operating mode 5 of the positive and negative half-cycles of the grid voltage in this embodiment;

[0020] Figure 4(c) shows the power transfer mode 6 during the negative half-cycle of the grid voltage in this embodiment;

[0021] Figure 5 The output three-level and mains voltage waveforms in this embodiment;

[0022] Figure 6(a) shows the operating waveform under unity power factor in this embodiment;

[0023] Figure 6(b) shows the operating waveform when the grid current leads the voltage by 30° in this embodiment;

[0024] Figure 6(c) shows the operating waveform when the grid current lags the voltage by 30° in this embodiment;

[0025] Figure 7(a) shows the waveform of the first capacitor voltage under unity power factor in this embodiment;

[0026] Figure 7(b) shows the operating waveform when the grid current leads the voltage by 30° in this embodiment;

[0027] Figure 7(c) shows the operating waveform when the grid current lags the voltage by 30° in this embodiment. Detailed Implementation

[0028] The present invention will now be described in detail with reference to the accompanying drawings and embodiments thereof.

[0029] See appendix Figure 1 The common-ground type single-phase three-level inverter circuit of the present invention includes: a DC power supply. First capacitor First power switching transistor Second power switching transistor Third power switching transistor Fourth power switching transistor Fifth power switching transistor First power diode AC filter inductor and single-phase AC distribution network .

[0030] The photovoltaic DC power supply The positive terminal and the first power switch The drains are connected; the first power switch transistor The source and the second power switch drain and first power diode The anode is connected; the second power switch transistor The source and the third power switch The source, the first capacitor The negative terminal is connected to the output port a of the inverter circuit of this invention; the third power switch transistor The drain of the fourth power switch The drains are connected, forming a back-to-back structure; the fourth power switch transistor The source and the fifth power switch Source, photovoltaic DC power supply The negative terminal is connected to the neutral wire at the center point of the power grid; the fifth power switch transistor The drain of the first power diode cathode and first capacitor The positive terminals are connected.

[0031] The output port a of the inverter circuit of this invention and the AC filter inductor Connect the left end; AC filter inductor The right end is connected to one end of the AC distribution network; the other end of the AC distribution network is connected to the inverter output port, i.e., to the DC-side photovoltaic DC power supply. The negative terminal is connected.

[0032] Figure 2 The diagram shows the power switch drive signal of a common-ground single-phase three-level inverter circuit, where the modulation wave... It is the power frequency (50Hz). It is a 50kHz high-frequency triangular carrier signal. and Modulation generation ~ The drive signal. Among them, the power switch. and The drive signals are the same and are high-frequency signals during the negative half-cycle of the power grid; power switches... With power switch Complementary. Power switch and They are complementary and are high-frequency signals of the positive half-cycle of the power grid.

[0033] The common-ground type single-phase three-level inverter of the present invention has three operating modes.

[0034] Mode 1, as shown in Figure 3(a), is the energy transfer mode during the positive half-cycle of the power grid, with the first power switch turned on. Second power switching transistor and the third power switch Fourth power switch Fifth power switching transistor When disconnected, current does not flow through the third switching transistor, and the DC power supply... When directly connected in series to the power grid, the output voltage of a common-ground type single-phase three-level inverter circuit is equal to the DC input voltage.

[0035] Mode 2, as shown in Figure 3(b), represents the grid freewheeling current and its relationship to the first capacitor. The charging mode turns on the first power switch. Third power switching transistor Fourth power switching transistor Second power switch Fifth power switching transistor Disconnect, in this mode, the switching transistor and switching transistor After being turned on, the output ports a and N of the inverter circuit of this invention have the same potential, the output voltage is 0V, and the freewheeling current of the power grid flows through the switching transistor. Switching transistor and AC filter inductor Then return to single-phase AC distribution network Meanwhile, in the switching transistor After being turned on, the current flowing from the DC power supply passes through the switching transistor. ,diode Switching transistor and switching transistor The resulting conducting circuit is a capacitor. Charging is then performed. It is important to note that the charging circuit and the freewheeling circuit are separate; the freewheeling current from the grid does not pass through the switched capacitor. When the capacitor When the voltage exceeds the DC power supply voltage, the diode The cathode potential will be higher than the anode potential, thus cutting off charging and stopping the capacitor. The voltage is maintained below the DC power supply voltage.

[0036] Mode 3, as shown in Figure 3(c), represents the energy transfer and mode during the negative half-cycle of the power grid, with the fourth power switch turned on. Fifth power switching transistor First power switch Second power switching transistor Third power switching transistor Disconnected, by the first capacitor Provides reverse voltage to the power grid, so that current does not pass through the switching transistor. Thus, the output voltage of the common-ground single-phase three-level inverter circuit is equal to the negative value of the first capacitor voltage. Simultaneously, in freewheeling mode 2, the first capacitor is charged by the DC power supply. Therefore, the first capacitor operates at the switching frequency scale of the negative half-cycle, maintaining the first capacitor voltage at a stable DC voltage value.

[0037] Mode 4 is shown in Figure 4(a). The grid voltage is positive and the grid current is negative. The switching state of the power switch is the same as in Mode 1, and the reverse current does not pass through the switch. The current generated by the power grid passes through the switching transistor. and The current flows to the DC power supply, and the output voltage of the common-ground single-phase three-level inverter circuit is equal to the voltage of the DC power supply.

[0038] Mode 5 is shown in Figure 4(b). This mode has the same switching state as Mode 2 under unity power factor. In this freewheeling mode, the grid current flows through the switching transistor. and switching transistor The resulting branch returns to the power grid, therefore the power grid will not be used for the first capacitor. Charging, while the first capacitor is powered by a DC power supply. Charging is performed if the capacitor When the voltage exceeds the DC power supply voltage, the diode The cathode potential will be higher than the anode potential, thus cutting off charging.

[0039] Mode 6 is shown in Figure 4(c). The grid voltage is negative and the grid current is positive. The switching state of the power switch is the same as in Mode 3. In this mode, the grid current flows through the first capacitor. And charging it will cause capacitor The voltage rise, and the magnitude of the rise, are related to the angle of non-power factor:

[0040] In mode 6 with a non-unity power factor, the capacitance can be calculated based on the lead-lag angle Ψ between the output current and voltage when the grid voltage is negative and the input current is positive. The increase in amplitude. At this time, the current flowing through the capacitor... The current is the same as the grid current, according to the capacitor. characteristic formula Equation (1) can be obtained, and by combining it with equation (2), the capacitance can be obtained. The increase in value. Among them, The peak current of the power grid is T, the power frequency period is M, and the modulation index is M. , It is the peak value of the output sinusoidal voltage. The peak current of the power grid is T, and the power frequency period is T.

[0041] (1)

[0042] (2)

[0043] (3)

[0044] Figure 5 The present invention provides a common-ground type single-phase three-level inverter circuit with three-level output. and grid voltage Waveform, in which, The peak voltage is the same as the photovoltaic DC power supply voltage Vdc, which is 400V. The peak voltage is 311V and the frequency is 50Hz (power frequency).

[0045] Figure 6 shows the operating waveforms of a common-ground type single-phase three-level inverter circuit provided by the present invention under unity power factor and non-unity power factor conditions when in three-level output. Figure 6(a) shows the operating waveform under unity power factor conditions, where the grid voltage is... With grid current In phase, grid-connected current The peak value is 6.43A; Figure 6(b) shows the operating waveform when the grid current leads the voltage by 30°, and Figure 6(c) shows the operating waveform when the grid current lags the voltage by 30°. It can be seen that, under unity power factor or non-unity power factor operation, the grid-connected current of this invention... The waveforms are relatively smooth, which can provide high power quality input to the power grid. It also means that when there is reactive power in the power grid, the circuit of this invention can still operate normally and stably.

[0046] Figure 7 shows the first capacitor of a common-ground type single-phase three-level inverter circuit provided by the present invention. The voltage waveforms are shown in Figure 7(a), where Figure 7(b) shows the waveform of the first capacitor voltage under unity power factor; Figure 7(c) shows the operating waveform when the grid current leads the voltage by 30°; and Figure 7(d) shows the operating waveform when the grid current lags the voltage by 30°. It can be seen that when the second capacitor... With a capacitance of 0.1mF, at a switching frequency of 50kHz, and with unity power factor, the first capacitor... The voltage is stabilized at 400V. When the grid current leads the voltage by 30° and lags the voltage by 30°, the voltage of the first capacitor will increase slightly during the negative half-cycle of the grid, but will still remain relatively stable. Therefore, the common-ground single-phase three-level inverter circuit has a good ability to transmit reactive power to the grid.

[0047] The power switch described in this embodiment - The power diode described in this embodiment employs a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated-gate bipolar transistor, or a silicon carbide field-effect transistor. It can be a Schottky diode or a silicon power switching diode, etc.

[0048] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A common-ground type single-phase three-level inverter circuit, characterized in that: Photovoltaic DC power supply ( ), first capacitor ( ), first power switch ( ), second power switch ( ), third power switch ( ), fourth power switch ( ), the fifth power switch ( ), first power diode ( ) and AC filter inductor ( ); Photovoltaic DC power supply ( The positive terminal of ) is connected to the first power switch ( The drain of the first power switch is connected to the first power switch. The source of the second power switch () The drain of the first power diode and the first power diode. The anode of the second power switch is connected; The source of the third power switch () The source of the inverter, the negative terminal of the first capacitor, and the output port a of the inverter circuit are connected. Third power switch ( The drain of the fourth power switch () The drains of the electrodes are connected to form a back-to-back structure; Fourth power switch ( The source of ) and the fifth power switch ( The source of the photovoltaic DC power supply. The negative terminal of the fifth power switch is connected to the neutral wire at the center point of the power grid; The drain of ) is respectively connected to the first power diode ( The cathode of the inverter circuit is connected to the positive terminal of the first capacitor, and the output port a of the inverter circuit is connected to the AC filter inductor ( One end of the AC filter inductor is connected to the other end; The other end of the network is connected to the AC distribution network. One end is connected to the AC distribution network; The other end is connected to the inverter output port, i.e., to the DC-side photovoltaic DC power supply. The negative terminal of the ) is connected; The power switch described herein employs a metal-oxide-semiconductor field-effect transistor, an insulated-gate bipolar transistor, or a silicon carbide field-effect transistor; The first power diode ( () is a Schottky diode or a silicon power switching diode.

2. The common-ground type single-phase three-level inverter circuit according to claim 1, characterized in that: The AC filter inductor ( It can be an inductive filter, a capacitive filter, or an inductive-capacitive combination filter.

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