A single-phase NPC non-isolated photovoltaic grid-connected inverter circuit and modulation method

Abstract

The present invention discloses a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit and a modulation method thereof, wherein the collector of the first switch tube S1 is respectively connected to the positive electrode of the DC source and the positive electrode of the first voltage-dividing capacitor Cdc1, the emitter of the first switch tube S1 is respectively connected to the cathode of the first freewheeling diode D1 and one end of the first filter inductor L1, the emitter of the third switch tube S3 is respectively connected to the anode of the first freewheeling diode D1, and the collector of the third switch tube S3 is respectively connected to the negative electrode of the first voltage-dividing capacitor Cdc1, the positive electrode of the second voltage-dividing capacitor Cdc2, the emitter of the fourth switch tube S4 and the negative end of the power grid. The present invention discloses a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit and a modulation method thereof, which can effectively solve the leakage current problem.

Description

Technical Field

[0001] The invention relates to a non-isolated photovoltaic grid-connected inverter circuit based on a single-phase NPC and a control method thereof, belonging to the technical field of power electronic photovoltaic grid-connected power generation. Background Art

[0002] In recent years, with the increase in energy demand and the introduction of environmental protection policies, renewable energy has received more and more attention. Among various renewable energy sources, solar energy has developed rapidly due to its safety, cheapness and reliability. In traditional isolated photovoltaic grid-connected systems, there is a transformer to increase the voltage and isolate the grid from the photovoltaic side. The transformer has the disadvantages of large size, high cost and high loss. Therefore, non-isolated photovoltaic grid-connected inverters are more widely used due to their high efficiency, small size and low weight.

[0003] However, in non-isolated photovoltaic grid-connected inverter systems, since there is no electrical isolation between the photovoltaic cells and the grid, leakage current may generate an electrical loop through the parasitic capacitance of the photovoltaic array and the filter capacitors and filter inductors in the circuit. The leakage current is caused by the high-frequency pulsation of the common-mode voltage, which can cause serious leakage current in the common-mode circuit, thereby endangering the safety of the entire system and distorting the output current. Summary of the invention

[0004] The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit and a modulation method, which can effectively solve the leakage current problem.

[0005] The object of the present invention is achieved in that:

[0006] A single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, characterized in that: the collector of the first switch tube S1 is respectively connected to the positive electrode of the DC source and the positive electrode of the first voltage-dividing capacitor Cdc1, the emitter of the first switch tube S1 is respectively connected to the cathode of the first freewheeling diode D1 and one end of the first filter inductor L1, the emitter of the third switch tube S3 is connected to the anode of the first freewheeling diode D1, the collector of the third switch tube S3 is respectively connected to the negative electrode of the first voltage-dividing capacitor Cdc1, the positive electrode of the second voltage-dividing capacitor Cdc2, the emitter of the fourth switch tube S4 and the negative end of the power grid, the collector of the fourth switch tube S4 is connected to the cathode of the second freewheeling diode D2, the collector of the second switch tube S2 is respectively connected to the anode of the second freewheeling diode D2 and one end of the second filter inductor L2, the emitter of the second switch tube S2 is respectively connected to the negative electrode of the DC source and the negative electrode of the second voltage-dividing capacitor Cdc2, and the other end of the first filter inductor L1 is respectively connected to the other end of the second filter inductor L2 and the positive end of the power grid.

[0007] The present invention discloses a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, wherein the first switch tube S1, the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are insulated gate bipolar transistors IGBT or power field effect transistors MOSFET.

[0008] The present invention discloses a modulation method for a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, and the modulation method includes four working modes:

[0009] Working mode 1: In the power transmission mode of the positive half cycle of the grid voltage, the first switch tube S1 is turned on, and the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are all turned off; at this time, the common mode voltage Ucm=Udc / 2;

[0010] Working mode 2: In the freewheeling mode of the positive half cycle of the grid voltage, the current on the first filter inductor L1 is freewheeling, the third switch tube S3 is turned on, the first switch tube S1 is turned on, and the second switch tube S2 and the fourth switch tube S4 are both turned off; at this time, the common mode voltage Ucm=Udc / 2;

[0011] Working mode three: In the power transmission mode of the negative half cycle of the grid voltage, the second switch tube S2 is turned on, and the first switch tube S1, the third switch tube S3 and the fourth switch tube S4 are all turned off; at this time, the common mode voltage Ucm=Udc / 2;

[0012] Working mode four: In the freewheeling mode of the negative half cycle of the grid voltage, the current on the second filter inductor L2 continues to flow, the fourth switch tube S4 is turned on, and the first switch tube S1, the second switch tube S2 and the third switch tube S3 are all turned off; at this time, the common mode voltage Ucm=Udc / 2.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The single-phase NPC non-isolated photovoltaic grid-connected inverter circuit of the present invention can keep the common-mode voltage of the inverter always unchanged at Udc / 2, eliminate the high-frequency pulsation of the common-mode voltage, and thus effectively suppress the ground leakage current; by constructing a new freewheeling loop to change the freewheeling current path, the freewheeling path does not pass through the body diode with large conduction loss, completely eliminating the reverse recovery loss of the diode, which is beneficial to improving efficiency; without adding additional dead time, the bridge arm direct-through problem is solved by inductors L1 and L2, so that there is no need to design additional dead time, which means that the output grid-connected current distortion caused by the dead time is eliminated. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A schematic diagram of the topological structure of a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit of the present invention;

[0016] Figure 2A schematic diagram of a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit in the positive half-cycle of the grid voltage power transmission mode of the present invention;

[0017] Figure 3 A schematic diagram of a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit in the positive half-cycle freewheeling mode of the grid voltage according to the present invention;

[0018] Figure 4 A schematic diagram of a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit in the negative half cycle of grid voltage power transmission mode of the present invention;

[0019] Figure 5 A schematic diagram of a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit in the negative half-cycle freewheeling mode of the grid voltage according to the present invention;

[0020] in:

[0021] A first voltage-dividing capacitor Cdc1 and a second voltage-dividing capacitor Cdc2;

[0022] A first freewheeling diode D1 and a second freewheeling diode D2;

[0023] A first switch tube S1, a second switch tube S2, a third switch tube S3, and a fourth switch tube S4;

[0024] The first filter inductor L1 and the second filter inductor L2. DETAILED DESCRIPTION

[0025] See also Figures 1 to 5 The present invention relates to a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, wherein the collector of the first switch tube S1 is respectively connected to the positive electrode of the DC source and the positive electrode of the first voltage-dividing capacitor Cdc1, the emitter of the first switch tube S1 is respectively connected to the cathode of the first freewheeling diode D1 and one end of the first filter inductor L1, the emitter of the third switch tube S3 is respectively connected to the anode of the first freewheeling diode D1, the collector of the third switch tube S3 is respectively connected to the negative electrode of the first voltage-dividing capacitor Cdc1, the positive electrode of the second voltage-dividing capacitor Cdc2, the emitter of the fourth switch tube S4 and the negative end of the power grid, the collector of the fourth switch tube S4 is connected to the cathode of the second freewheeling diode D2, the collector of the second switch tube S2 is respectively connected to the anode of the second freewheeling diode D2 and one end of the second filter inductor L2, the emitter of the second switch tube S2 is respectively connected to the negative electrode of the DC source and the negative electrode of the second voltage-dividing capacitor Cdc2, and the other end of the first filter inductor L1 is respectively connected to the other end of the second filter inductor L2 and the positive end of the power grid.

[0026] Furthermore, the first switch tube S1, the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are insulated gate bipolar transistors IGBTs, or power field effect transistors MOSFETs.

[0027] A modulation method for a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, the modulation method comprising four working modes:

[0028] Working mode 1: In the power transmission mode of the positive half-cycle of the grid voltage, the photovoltaic cell as a DC source emits power, the first switch tube S1 is turned on, and the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are all turned off; at this time, the output voltage input to the grid is Udc / 2; therefore, the common mode voltage is:

[0029]

[0030] Working mode 2: In the freewheeling mode of the positive half cycle of the grid voltage, the current on the first filter inductor L1 is freewheeling, the third switch tube S3 is turned on, the first switch tube S1 is turned on, and the second switch tube S2 and the fourth switch tube S4 are both turned off; at this time, the output voltage input to the grid is 0; therefore, the common mode voltage is

[0031]

[0032] Working mode three: In the power transmission mode of the negative half-cycle of the grid voltage, the photovoltaic cell as a DC source emits power, the second switch tube S2 is turned on, and the first switch tube S1, the third switch tube S3 and the fourth switch tube S4 are all turned off; at this time, the output voltage input to the grid is -UDC / 2, so the common mode voltage is:

[0033]

[0034] Working mode 4: In the freewheeling mode of the negative half cycle of the grid voltage, the current on the second filter inductor L2 continues to flow, the fourth switch tube S4 is turned on, and the first switch tube S1, the second switch tube S2 and the third switch tube S3 are all turned off; at this time, the output voltage input to the grid is 0, so the common mode voltage is:

[0035] ;

[0036] In summary, the common mode voltage is always kept at Udc / 2, which can eliminate the high-frequency pulsation of the common mode voltage and effectively suppress the ground leakage current;

[0037] In addition: It should be noted that the above specific implementation is only an optimization scheme of this patent. Any changes or improvements made by technicians in this field based on the above concept are within the scope of protection of this patent.

Claims

1. A modulation method for a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, characterized in that: The modulation method is based on a single-phase NPC non-isolated photovoltaic grid-connected inverter circuit, wherein the collector of the first switch tube S1 is respectively connected to the positive electrode of the DC source and the positive electrode of the first voltage-dividing capacitor Cdc1, the emitter of the first switch tube S1 is respectively connected to the cathode of the first freewheeling diode D1 and one end of the first filter inductor L1, the emitter of the third switch tube S3 is connected to the anode of the first freewheeling diode D1, the collector of the third switch tube S3 is respectively connected to the negative electrode of the first voltage-dividing capacitor Cdc1, the positive electrode of the second voltage-dividing capacitor Cdc2, the emitter of the fourth switch tube S4 and the negative end of the power grid, and the fourth switch tube S The collector of the second switch tube S4 is connected to the cathode of the second freewheeling diode D2, the collector of the second switch tube S2 is respectively connected to the anode of the second freewheeling diode D2 and one end of the second filter inductor L2, the emitter of the second switch tube S2 is respectively connected to the negative electrode of the DC source and the negative electrode of the second voltage-dividing capacitor Cdc2, and the other end of the first filter inductor L1 is respectively connected to the other end of the second filter inductor L2 and the positive end of the power grid; the first switch tube S1, the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are insulated gate bipolar transistors IGBT, or power field effect transistors MOSFET; The modulation method includes four working modes: Working mode 1: In the power transmission mode of the positive half cycle of the grid voltage, the first switch tube S1 is turned on, and the second switch tube S2, the third switch tube S3 and the fourth switch tube S4 are all turned off; at this time, the common mode voltage Ucm=Udc / 2; Working mode 2: In the freewheeling mode of the positive half cycle of the grid voltage, the current on the first filter inductor L1 is freewheeling, the third switch tube S3 is turned on, the first switch tube S1 is turned on, and the second switch tube S2 and the fourth switch tube S4 are both turned off; at this time, the common mode voltage Ucm=Udc / 2; Working mode three: In the power transmission mode of the negative half cycle of the grid voltage, the second switch tube S2 is turned on, and the first switch tube S1, the third switch tube S3 and the fourth switch tube S4 are all turned off; at this time, the common mode voltage Ucm=Udc / 2; Working mode four: In the freewheeling mode of the negative half cycle of the grid voltage, the current on the second filter inductor L2 continues to flow, the fourth switch tube S4 is turned on, and the first switch tube S1, the second switch tube S2 and the third switch tube S3 are all turned off; at this time, the common mode voltage Ucm=Udc / 2.

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