CN207218562U - A low leakage current grid-connected inverter circuit - Google Patents

Abstract

The utility model discloses a kind of low-leakage current grid-connected inverter circuit.The utility model adds two switching tubes and the full-bridge being made up of four diodes on the basis of single-phase full bridge topology.One end of two switching tubes is connected with the DC voltage side midpoint of single-phase full bridge inverter circuit, and the full-bridge that the other end forms with diode connects, and the other end of the full-bridge of diode composition is electrically connected by two and network interface inductance with single phase power supply power network.Therefore reversals freewheeling period common-mode voltage and the power output stage common-mode voltage be to maintain it is constant, i.e. relative to the earth for be constant voltage source, because constant pressure will not produce electric current for electric capacity and then inhibit common mode current, that is, reduce leakage current.

Description

A low leakage current grid-connected inverter circuit

technical field

The utility model relates to the field of electrical engineering, in particular to an application of power electronic equipment in electrical engineering, which is a grid-connected inverter circuit with low leakage current.

Background technique

An inverter is a device that converts DC power into AC power. Through the control of the switching tube in the inverter, the DC power can be converted into civilian AC 220V power, and it can also be converted into AC power with different frequencies to control the motor speed. With the continuous maturity of new energy technology development, it has become a method to improve the reliability of power supply by merging the electric energy generated by solar energy into large-scale power supply network. The main equipment to realize electric energy grid connection is the inverter. In the process of electric energy conversion, the inverter needs high-frequency on-off of high-power switching tubes, and high-frequency switching of power devices will generate high-frequency common-mode voltage. As we all know, photovoltaic power generation is mostly composed of a large number of photovoltaic panels. The photovoltaic array composed of parallel connection has a large parasitic capacitance to the ground, so under the action of high-frequency common-mode voltage, a common-mode current, that is, a leakage current, is generated. The high-frequency leakage current makes the conduction and radiation interference of the photovoltaic power generation system serious, increases the harmonics of the grid-connected current and system loss, and even threatens personal safety in severe cases. Suppressing the leakage current in the photovoltaic power generation system is one of the key issues in solving the grid-connected technology of non-isolated photovoltaic inverters. The most common way to suppress the leakage current is to change the topological structure of the inverter circuit of the inverter and construct a new freewheeling circuit. In the freewheeling phase of the inverter switching cycle, the output terminal of the photovoltaic cell is decoupled from the grid side to prevent the formation of a common mode loop. The most representative topologies are H5, H6, and Heric topologies. These topological structures change the transmission path of the freewheeling circuit by increasing the number of switching devices, thereby achieving the purpose of suppressing the leakage current. The H5 topology is to add a switching tube to the H4 bridge. The disadvantage is that the current must flow through the added switching tube during the grid-connected phase, resulting in an increase in the conduction loss of the switching tube. However, the H5 bridge has an increased number of switches compared to other topologies. At least, the control method is simple and the cost is low. There are many researches and improvements on the topology of the H6 bridge. By adding two switch tubes, a freewheeling circuit is formed to keep the photovoltaic panel disconnected from the grid. The topological structure of H6 can be varied, but it is necessary to optimize the turn-on sequence of the switch tubes to achieve the best efficiency. Further improvement of efficiency is limited by the performance of MOSFETs and IGBTs. A single-phase non-isolated photovoltaic grid-connected inverter and its control method with application number 201610246182.X is an inverter circuit with H6 topology.

Therefore, it is an urgent requirement to design a topological structure that reduces the leakage current and apply it in the inverter.

Contents of the invention

The technical problem to be solved by the utility model is to provide a grid-connected inverter circuit with low leakage current, which has the function of effectively suppressing the leakage current.

The technical solution of the technical problem to be solved by the utility model is: a low-leakage current grid-connected inverter circuit, including a DC power supply, the first and second voltage dividing capacitors, the first, second, third, fourth, fifth and sixth switching tubes , The first, second, third, and fourth diodes, the first and second grid-connected inductors, the first, second, third, fourth, fifth, and sixth freewheeling diodes and controllers. The first and second voltage dividing capacitors are connected in series to the positive and negative poles of the DC power supply. The collectors of the first and third switching tubes are electrically connected to the positive pole of the DC power supply after being connected in parallel, the emitters of the second and fourth switching tubes are connected in parallel and are electrically connected to the negative pole of the DC power supply, and the emitters of the first switching tube and the The collector of the second switch tube is electrically connected, and the emitter of the third switch tube is electrically connected with the collector of the fourth switch tube. A first grid-connected inductance is connected in series between the emitter of the first switching tube and the live line of the single-phase power supply grid, and a second grid-connecting inductance is connected in series between the emitter of the third switching tube and the neutral line of the single-phase power supply grid. The cathodes of the first and third diodes are connected in parallel and electrically connected to the collector of the fifth switching tube, and the emitter of the fifth switching tube is electrically connected to the connection point of the first and second voltage dividing capacitors. After the anodes of the second and fourth diodes are connected in parallel, they are electrically connected to the emitter of the sixth switching tube, and the collector of the sixth switching tube is electrically connected to the connection point of the first and second voltage dividing capacitors. After the anode of the first diode is connected to the cathode of the second diode, it is electrically connected to the emitter of the first switch tube, and after the anode of the third diode is connected to the cathode of the fourth diode, it is connected to the third switch tube. The emitter electrical connection. The first, second, third, fourth, fifth and sixth freewheeling diodes are respectively connected with opposite polarities of the first, second, third, fourth, fifth and sixth switching tubes. The gates of the first, second, third, fourth, fifth and sixth switch tubes are electrically connected to the controller for controlling the switching on and off of the switch tubes.

More preferably, the DC power supply is a solar panel array.

More preferably, the controller adopts a single-chip microcomputer as a micro-control chip.

More preferably, the first, second, third, fourth, fifth and sixth switch tubes use IGBT modules.

The beneficial effects of the utility model are:

1. The utility model adds two switching tubes and a full bridge composed of diodes on the basis of the single-phase full-bridge inverter circuit, which has the beneficial effects of simple circuit structure and low cost;

2. The utility model keeps the voltage at the midpoint of the bridge arm at half of the output voltage of the DC power supply in the freewheeling stage when the output power is zero, so that the common-mode voltage tends to be constant, so it has the beneficial effect of greatly reducing the leakage current;

3. The utility model has the beneficial effect that the switch tube control method is simple.

Description of drawings

Fig. 1 is a circuit structure diagram of the present utility model,

Fig. 2 is the equivalent circuit diagram of the ground parasitic capacitance that the utility model produces in the actual operation process,

Fig. 3 is a non-isolated single-phase full-bridge grid-connected inverter circuit structure in the prior art,

Fig. 4 is a simplified common-mode model circuit of a non-isolated single-phase full-bridge grid-connected inverter circuit structure in the prior art,

Fig. 5 is the timing diagram of the drive control signal of the utility model,

Fig. 6 (a) is the electric current flow diagram of working state 1 of the present utility model,

Fig. 6 (b) is the current flow diagram of the utility model working state 2,

Fig. 6 (c) is the electric current flow figure of working state 3 of the present utility model,

Fig. 6(d) is a current flow diagram of working state 4 of the utility model.

Detailed ways

In order to make the technical solutions and beneficial effects of the present utility model clearer, the implementation of the present utility model will be further explained in detail below.

As shown in Figure 1, a low-leakage current grid-connected inverter circuit includes a DC power supply, a main circuit composed of a diode, a switch tube, a capacitor, and an inductor, and a controller that controls the switch tube to be turned on and off.

The utility model is improved on the basis of a single-phase full-bridge inverter circuit, so the main structure of the circuit is relatively simple. In order to keep the voltage stable, the first and second divider capacitors C1 and C2 connected in series are connected in parallel at both ends of the DC power supply. Preferably, the DC power supply here is DC power generated by photovoltaic power generation.

The collector of the first switching tube S1 is electrically connected to the positive pole of the DC power supply, the emitter of the first switching tube S1 is electrically connected to the collector of the second switching tube S2, and the emitter of the second switching tube S2 is electrically connected to the negative pole of the DC power supply . The collector of the third switching tube S3 is electrically connected to the positive pole of the DC power supply, the emitter of the third switching tube S3 is electrically connected to the collector of the fourth switching tube S4, and the emitter of the fourth switching tube S4 is electrically connected to the negative pole of the DC power supply .

A first grid-connected inductance L1 is connected in series between the emitter of the first switching tube S1 and the live line of the single-phase power supply grid, and a second grid-connecting inductor L1 is connected in series between the emitter of the third switching tube S3 and the neutral line of the single-phase power supply grid Inductor L2.

The cathodes of the first and third diodes D1 and D3 are connected in parallel and electrically connected to the collector of the fifth switching tube S5, and the emitter of the fifth switching tube S5 is electrically connected to the connection point of the first and second voltage dividing capacitors C1 and C2. The anodes of the second and fourth diodes D2 and D4 are connected in parallel and electrically connected to the emitter of the sixth switching tube S6, and the collector of the sixth switching tube S6 is electrically connected to the connection point of the first and second voltage dividing capacitors C1 and C2.

The anode of the first diode D1 and the cathode of the second diode D2 are electrically connected to the emitter of the first switching transistor S1. The anode of the third diode D3 and the cathode of the fourth diode D4 are electrically connected to the emitter of the third switching transistor S3.

Since the inverter circuit is improved on the basis of the single-phase full-bridge inverter circuit, the original freewheeling diodes are retained, that is, each switch tube is connected to a diode in parallel, and the anode of the diode is connected to the emission of the switch tube. pole connection, the cathode of the diode is connected to the collector of the switch tube. Therefore, the first, second, third, fourth, fifth, and sixth freewheeling diodes DX1, DX2, DX3, DX4, DX5, and DX6 are also provided with the first, second, third, fourth, fifth, and sixth switches respectively in the circuit. Pipes S1, S2, S3, S4, S5, S6 are connected in parallel.

In the actual operation process of photovoltaic power plants connected to the grid, there are ground parasitic capacitance Cpv and ground inductance Lg, and its equivalent circuit is shown in Figure 2. Between the negative pole of the DC power supply and the ground, and define the connection point between the ground parasitic capacitance Cpv and the negative pole of the DC power supply as n points. The neutral terminal of the single-phase power supply network is electrically connected to the earth. Among them, the ground parasitic capacitance Cpv and the ground inductance Lg are the inductive capacitance and inductive inductance generated due to the relatively large change in the voltage of the inverter circuit at point n during operation.

In order to control the turn-on and turn-off of the six switch tubes, the gates of the switch tubes are electrically connected to the controller, and the peripheral circuits related to the controller are used to realize the normal operation of the controller.

Figure 3 is a topology of a non-isolated single-phase full-bridge grid-connected inverter circuit, and Figure 4 is a simplified common-mode model of the structure. In the figure, the connection point between the ground parasitic capacitance and the topology of the grid-connected inverter circuit is defined as n points, where Ucm_ab is the common mode voltage, Udm_ab is the differential mode voltage, and Utcm in the formula is the system common mode voltage. It can be seen from the circuit diagram that

The total mode voltage of the system is:

As shown in formula (3), when La≠Lb, Udm_ab will directly affect the size of the total mode voltage Utcm of the system. But in general, the values of grid-connected inductance La and Lb are the same, then formula (3) can be simplified as:

The calculation formula of the common mode current is shown in formula (5),

When the value of Utcm is constant, Utcm can be regarded as a constant-voltage voltage source. According to formula (5), it can be seen that the common mode current of the system is zero. It can be concluded that when the voltage value of the common-mode voltage is stable, the common-mode current can be eliminated, that is, the leakage current can be eliminated.

The utility model can greatly eliminate the leakage current according to the circuit structure and the control method. The above is the circuit structure of the utility model and the principle of eliminating the leakage current, and the control method of the grid-connected inverter circuit with low leakage current will be described below.

The simplest control method is to switch and conduct the first and fourth switching tubes S1 and S4 and the second and third switching tubes S2 and S3 to realize the generation of AC power at the output terminal. Of course, there are other different control modes such as PWM control mode, SPWM control mode, etc. for different topology structures of inverter circuits, and these existing technologies will not be described in detail here.

The periodic change of current with time is called alternating current, and a cycle of alternating current is generally divided into a positive half cycle and a negative half cycle. In order to convert DC to AC, it is necessary to output positive electricity in the positive half cycle and output negative electricity in the negative half cycle. As shown in Figure 5, its control method is:

In the positive half cycle, the controller drives the first and fourth switching tubes S1 and S4 to conduct at high frequency, and the second and third switching tubes S2 and S3 are always turned off. During this period, the fifth and sixth switching tubes S5 and S6 The driving signal of is complementary to the driving signals of the first and fourth switching transistors S1 and S4. Specifically:

When the first and fourth switching tubes S1 and S4 are turned on, the fifth and sixth switching tubes S5 and S6 are turned off. At this time, it is the power transmission stage, and the current flow direction is: the positive pole of the DC power supply → the first switching tube S1 → the first Grid-connected inductor L1→single-phase power supply grid→second grid-connected inductor L2→fourth switching tube S4→negative pole of DC power supply;

When the first and fourth switching tubes S1 and S4 are turned off, the fifth and sixth switching tubes S5 and S6 are turned on. At this time, it is a freewheeling phase, and the current flow direction is, single-phase power supply grid→second grid-connected inductor L2→second Three diodes D3→fifth switching tube S5→sixth switching tube S6→second diode D2→first grid-connected inductor L1→single-phase power supply grid.

In the negative half cycle, the controller drives the second and third switching tubes S2 and S3 to conduct at high frequency, and the first and fourth switching tubes S1 and S4 are always turned off. During this period, the fifth and sixth switching tubes S5 and S6 The driving signal is complementary to the driving signals of the second and third switching transistors S2 and S3. Specifically:

When the second and third switching tubes S2 and S3 are turned on, the fifth and sixth switching tubes S5 and S6 are turned off. At this time, it is the power transmission stage, and the current flow direction is: the positive pole of the DC power supply → the third switching tube S3 → the second Grid-connected inductor L2→single-phase power supply grid→first grid-connected inductor L1→second switching tube S2→negative pole of DC power supply;

When the second and third switching tubes S2 and S3 are turned off, the fifth and sixth switching tubes S5 and S6 are turned on. At this time, it is the freewheeling phase, and the current flow direction is, single-phase power supply grid → first grid-connected inductor L1 → second A diode D1→fifth switching tube S5→sixth switching tube S6→fourth diode D4→second grid-connected inductor L2→single-phase power supply grid.

This control method produces the following four working states in one cycle:

First, define the connection point between the ground parasitic capacitance Cpv and the grid-connected inverter circuit as n points in Fig. 6 .

Working state 1, output positive voltage in the positive half cycle, power output stage. In this working state, the first and fourth switching tubes S1 and S4 are turned on, and the second, third, fifth and sixth switching tubes S2, S3, S5 and S6 are off. At this time, the DC power supply transmits power to the single-phase power supply grid. The flow direction of the current is: the positive pole of the DC power supply→the first switch tube S1→the first grid-connected inductor L1→single-phase power supply network→the second grid-connected inductor L2→the fourth switch tube S4→the negative pole of the DC power supply. It can be seen from Fig. 6(a) that Uan=Upv, Ubn=0, and according to the formula (4), it can be known that the common mode voltage is 0.5Upv.

Working state 2, output zero voltage in the positive half cycle, freewheeling stage. In this working state, the first, second, third and fourth switching tubes S1, S2, S3, and S4 are turned off, and the fifth and sixth switching tubes S5 and S6 are turned on. At this time, the fifth and sixth switching tubes S5, S6 and The second and third diodes D2 and D3 provide a loop for the grid-connected inductor current, and the direction of the grid-connected inductor current is the same as that in working state 1. The flow direction of the current is: single-phase power supply network→second grid-connected inductor L2→third diode D3→fifth switch tube S5→sixth switch tube S6→second diode D2→first grid-connected inductor L1→ Single-phase power grid. As shown in Figure 6(b), since the fifth and sixth switching tubes are connected to the midpoint of the two voltage-dividing capacitors of the DC link, that is, the first and second voltage-dividing capacitors C1 and C2, the voltage at the midpoint of the bridge arm is the photovoltaic output voltage One-half of Upv, ie Uan=Ubn=0.5Upv. Therefore, it can be seen from formula (4) that the common mode voltage is 0.5Upv.

Working state 3, output negative voltage in the negative half cycle, power output stage. In this working state, the second and third switching tubes S2 and S3 are turned on, the first, fourth, fifth and sixth switching tubes S1, S4, S5 and S6 are turned off, and the DC current source transmits power to the single-phase power supply grid. The flow direction of the current is: the positive pole of the DC power supply→the third switch tube S3→the second grid-connected inductor L2→single-phase power supply network→the first grid-connected inductor L1→the second switch tube S2→the negative pole of the DC power supply. As can be seen from Fig. 6(c), Uan=0, Ubn=Upv, Uab=-Upv, according to the formula (4), the common mode voltage is 0.5Upv.

Working state 4, output zero voltage in the negative half cycle, freewheeling stage. In this working state, the first, second, third and fourth switching tubes S1 , S2 , S3 and S4 are turned off, the fifth and sixth switching tubes S5 and S6 are turned on, and the direction of the inductor current is the same as working state 3 . The flow direction of the current is: single-phase power supply network→first grid-connected inductor L1→first diode D1→fifth switch tube S5→sixth switch tube S6→fourth diode D4→second grid-connected inductor L2→ As shown in Figure 6(d) of the single-phase power supply network, Uan=Ubn=0.5Upv, so it can be known from formula (4) that the common-mode voltage is 0.5Upv.

Through the analysis, it can be seen that in the four working states, the value of the common-mode voltage is always maintained at 0.5 Upv. According to the formula (5), it can be known that the common-mode current is zero when the common-mode voltage is constant. Therefore, it can be concluded from theoretical analysis that the inverter circuit has the beneficial effect of effectively reducing the leakage current.

In order to verify the correctness of the theory, the simulation analysis and data comparison of the utility model and heric inverter circuit topology are carried out through MATLAB/Simulink. Among them, the input voltage Upv=350V, the first and second divider capacitors C1 and C2 are 250uF respectively, the first and second grid-connected inductors L1 and L2 are both 1.8mH, and the parasitic capacitance Cpv to ground is 100uF. The value of grounding inductance Lg is very small and is neglected in the calculation. Through simulation, it is found that the leakage current of the heric inverter topology is about 0.15A. Although the leakage current of the utility model is not completely eliminated, the amplitude is relatively small, about 0.04A. The above is enough to prove that the utility model can greatly reduce the leakage current and realize the goal of energy saving and high efficiency.

In summary, it is only a preferred embodiment of the utility model, and is not used to limit the scope of the utility model. Through the above description, relevant staff can completely , perform various changes and modifications. The technical scope of the present utility model is not limited to the content on the description, and all the so-called equal changes and modifications of the shape, structure, characteristics and spirit described in the scope of the utility model shall be included in the claims of the utility model. within range.

Claims (4)

1. A low leakage current grid-connected inverter circuit, characterized in that: Including DC power supply, first and second voltage divider capacitors (C1, C2), first, second, third, fourth, fifth, and sixth switch tubes (S1, S2, S3, S4, S5, S6), first, second, Three and four diodes (D1, D2, D3, D4), the first and second grid-connected inductors (L1, L2), the first, second, third, fourth, fifth and sixth freewheeling diodes (DX1, DX2, DX3, DX4 , DX5, DX6) and the controller, The first and second voltage dividing capacitors (C1, C2) are connected in series to the positive and negative poles of the DC power supply, The collectors of the first and third switch tubes (S1, S3) are connected in parallel to the positive pole of the DC power supply, and the emitters of the second and fourth switch tubes (S2, S4) are connected in parallel to the negative pole of the DC power supply. connection, the emitter of the first switching tube (S1) is electrically connected to the collector of the second switching tube (S2), the emitter of the third switching tube (S3) is electrically connected to the collector of the fourth switching tube (S4), A first grid-connected inductance (L1) is connected in series between the emitter of the first switching tube (S1) and the live line of the single-phase power supply grid, and between the emitter of the third switching tube (S3) and the neutral line of the single-phase power supply grid There is a second grid-connected inductor (L2) in series, The cathodes of the first and third diodes (D1, D3) are connected in parallel to the collector of the fifth switching tube (S5), and the emitter of the fifth switching tube (S5) is connected to the first and second voltage dividing capacitors ( The connection points of C1, C2) are electrically connected, The anodes of the second and fourth diodes (D2, D4) are connected in parallel to the emitter of the sixth switch tube (S6), and the collector of the sixth switch tube (S6) is connected to the first and second voltage dividing capacitors ( The connection points of C1, C2) are electrically connected, The anode of the first diode (D1) is electrically connected to the emitter of the first switching tube (S1) after being connected to the cathode of the second diode (D2), and the anode of the third diode (D3) is connected to the cathode of the fourth diode (D3). After the cathode of the diode (D4) is connected, it is electrically connected with the emitter of the third switching tube (S3), The first, second, third, fourth, fifth, and sixth freewheeling diodes (DX1, DX2, DX3, DX4, DX5, DX6) are respectively connected with the first, second, third, fourth, fifth, and sixth switching tubes (S1, S2, S3 , S4, S5, S6) polarities are reversed and connected, The gates of the first, second, third, fourth, fifth and sixth switch tubes (S1, S2, S3, S4, S5, S6) are electrically connected to the controller to control the switching on and off of the switch tubes. 2. A low leakage current grid-connected inverter circuit according to claim 1, characterized in that: The DC power source is a solar panel array. 3. A low-leakage current grid-connected inverter circuit according to claim 1, characterized in that: The controller adopts a single-chip microcomputer as a micro-control chip. 4. A low leakage current grid-connected inverter circuit according to claim 1, characterized in that: The first, second, third, fourth, fifth and sixth switch tubes (S1, S2, S3, S4, S5, S6) use IGBT modules.