CN107834888A - A kind of Transformer-free single-phase photovoltaic inverter of voltage hybrid clamp - Google Patents

Abstract

The invention discloses a voltage hybrid clamping transformerless single-phase photovoltaic inverter, which comprises an energy storage voltage dividing unit, a power inverter unit, a hybrid clamping unit and a filter unit. When the inverter of the present invention is working, five power switches with anti-parallel diodes act in coordination, supplemented by six diodes, so that when the inverter outputs zero level, the AC side continues to flow, and its common-mode voltage is clamped To one-half of the DC input voltage, so as to ensure a constant common-mode voltage throughout the cycle, thereby realizing the complete elimination of common-mode current, and the required bus voltage is only half of that of the half-bridge circuit. At the same time, the invention adopts unipolar pulse width modulation, the output current ripple is small, the volume and quality of the filter are reduced, and the loss on the magnetic element is reduced at the same time; the number of switching actions in the switching cycle is small, and the switching loss is reduced, so The inverter of the invention has high output efficiency and can obtain an inverter efficiency as high as 98%.

Description

A Transformerless Single-Phase Photovoltaic Inverter with Voltage Hybrid Clamping

technical field

The invention belongs to the technical field of power electronics, and in particular relates to a transformerless single-phase photovoltaic inverter with voltage hybrid clamping.

Background technique

Nowadays, the problems of energy depletion and environmental pollution are becoming more and more serious, and the development and utilization of various new energy sources are getting more and more attention. Solar energy is currently one of the cleanest and most promising renewable energy sources for large-scale development and utilization, and its photovoltaic utilization has attracted widespread attention from all over the world. Solar photovoltaic power generation is the main development trend of solar photovoltaic utilization, and it will develop more and more rapidly in the future.

As the last or only energy conversion device in the photovoltaic power generation system, the inverter's efficiency and safety performance will directly affect the performance and investment of the entire system. According to the configuration of the transformer in the inverter, the existing inverters can be divided into inverters with power frequency transformers, inverters with high frequency transformers and transformerless inverters. Inverters with power frequency transformers or high frequency transformers can realize the functions of boosting and isolation, but inverters with power frequency transformers have problems of increased volume and weight, high price and inconvenient installation; Although the size and weight of the type inverter are greatly reduced, the multi-level structure leads to a complex system structure and a decrease in overall efficiency. The transformerless inverter has been paid more and more attention due to its simple system structure, high efficiency, small size and low cost.

In the transformerless photovoltaic grid-connected system, due to the loss of the electrical isolation of the transformer, the parasitic capacitance between the photovoltaic cell array and the ground, the photovoltaic grid-connected inverter and the ground will form a common mode loop as shown in Figure 1 . According to the data, there is a parasitic capacitance between the flat plate structure of the crystalline silicon photovoltaic cell and the ground, which is about 50-150nF/kWp, and its capacitance is much larger than the parasitic capacitance of the power device to the ground. Therefore, if there are high-frequency ripples in the common-mode voltage of the system, a large common-mode leakage current will be generated in the common-mode loop. The common-mode leakage current will not only cause serious EMI problems, but also reduce the quality of grid-connected current and bring hidden dangers to the personal safety of photovoltaic cell maintenance personnel. Therefore, in the transformerless type grid-connected inverter, the problem of high-frequency common-mode current must be solved.

In the existing disclosed technology, the half-bridge inverter circuit and the midpoint clamping circuit directly clamp one end of the power grid to the midpoint of the DC bus voltage, so that the voltage at both ends of the parasitic capacitance of the photovoltaic cell is constant, thereby suppressing the common Mode current generation. However, the DC bus voltage required by the above two schemes is twice the bus voltage required by the ordinary full bridge. Therefore, in the case of low DC input voltage, these two schemes must boost the bus voltage by boosting the voltage of the booster circuit. Voltage. The use of the pre-stage boost circuit not only increases the cost of the system, but also reduces the overall conversion efficiency of the inverter.

European Patent Publication No. EP2086102A2 discloses a high-efficiency common-mode current-mode topology (HERIC), which requires half the input voltage of the half-bridge inverter circuit, so in many cases, no additional step-up The voltage circuit boosts the busbar. Based on the common full-bridge topology, this solution adds two switching devices on the AC side. In the stage of energy transmission from the DC side to the AC side, the work of this circuit is the same as that of the full-bridge unipolar circuit, and the common-mode voltage of the system is half of the input voltage; It ensures that the system outputs zero level and at the same time realizes the decoupling of the DC side and the AC side. At this time, the common mode voltage of the system is kept at half of the input voltage under ideal conditions. Therefore, there is no high-frequency disturbance in the common-mode voltage of the circuit, thereby suppressing the common-mode leakage current of the system. However, in actual working conditions, when the DC side and the AC side circuit are decoupled, the voltage on the AC side is in a suspended state relative to the DC side, and is not constant at half of the DC input voltage. Considering the parasitic parameters in the circuit: such as the switching tube Output junction capacitance, distribution parameters such as lead inductance, etc., the above parameters and the inductance and capacitance in the common mode circuit have high frequency resonance, causing high frequency common mode current in the common mode circuit of the system. Therefore, this technique cannot achieve complete cancellation of high-frequency common-mode currents.

The European Patent Publication No. EP1626494A2 discloses another H5 topology with leakage current suppression capability. This structure adds a power switch on the DC side of the full bridge circuit to ensure that the DC side and the AC side of the grid-connected inductance freewheeling stage decoupling. This circuit, like a HERIC circuit, cannot completely eliminate common-mode leakage. In addition, its circuit structure is asymmetrical, and the working hours of the five switching transistors are not equal, resulting in unbalanced heat generation of the switching devices, and high requirements for device heat dissipation design; in the energy transfer stage, three power devices are in the on state (2 in the HERIC topology). ), reducing the overall efficiency of the inverter.

Contents of the invention

Aiming at the above-mentioned technical problems existing in the prior art, the present invention provides a transformerless single-phase photovoltaic inverter with voltage hybrid clamping, which can effectively eliminate high-frequency common-mode current and has high conversion efficiency.

A transformerless single-phase photovoltaic inverter with voltage hybrid clamping, comprising:

An energy storage voltage division unit, used for energy storage and voltage division of the input photovoltaic DC voltage;

A power inverter unit, configured to convert the photovoltaic DC voltage into a three-level DC voltage;

The hybrid clamping unit is used to carry out freewheeling on the AC side of the inverter when the power inverter unit outputs zero level, and clamp the midpoint voltage of the freewheeling to half of the photovoltaic DC voltage;

The filter unit is used for low-pass filtering the three-level DC voltage, so as to output a sinusoidal AC voltage.

The energy storage and voltage dividing unit includes two input capacitors C _dc1 -C _dc2 ; wherein, the positive pole of the input capacitor C _dc1 is connected to the positive pole of the photovoltaic DC source, the negative pole of the input capacitor C _dc1 is connected to the positive pole of the input capacitor C _dc2 , The negative pole of the input capacitor C _dc2 is connected to the negative pole of the photovoltaic DC source.

The input capacitors C _dc1 and C _dc2 are composed of one electrolytic capacitor or multiple electrolytic capacitors connected in series and parallel.

The power inverter unit adopts a single-phase full-bridge inverter structure, which includes four power switch tubes S1-S4 with anti-parallel diodes; wherein, one end of the power switch tube S1 is connected in parallel with one end of the power switch tube S3 The positive pole of the photovoltaic DC source, one end of the power switch tube S2 is connected to one end of the power switch tube S4 and connected to the negative pole of the photovoltaic DC source, and the other end of the power switch tube S1 is connected to the other end of the power switch tube S2 as a power inverter unit The first voltage output terminal, the other end of the power switch tube S3 is connected to the other end of the power switch tube S4 as the second voltage output terminal of the power inverter unit; the four power switch tubes S1-S4 all receive switch control provided by external equipment Signal.

The hybrid clamping unit includes a power switch tube S5 and six diodes D1-D6; wherein, one end of the power switch tube S5 is connected to the cathodes of the diodes D2, D4 and D6, and the other end of the power switch tube is connected to the diodes D1, The anodes of D3 and D5 are connected, the cathode of diode D1 is connected to the anode of diode D2 and connected to the first voltage output end of the power inverter unit, the cathode of diode D3 is connected to the anode of diode D4 and connected to the second voltage of the power inverter unit At the output end, the cathode of the diode D5 is connected to the anode of the diode D6 and the energy storage voltage dividing unit to extract half of the photovoltaic DC voltage; the power switch tube S5 receives the switch control signal provided by the external device.

The power switch tubes all use IGBT (Insulated Gate Bipolar Transistor).

The filtering unit adopts a symmetrical inductance filter or a symmetrical LCL (inductance-capacitance-inductance) filter.

The symmetrical inductance filter includes two filter inductors L ₁ ~ L ₂ ; wherein, one end of the filter inductor L ₁ is connected to the first voltage output terminal of the power inverter unit, and one end of the filter inductor L ₂ is connected to the power inverter The second voltage output terminals of the unit are connected, and the other terminals of the filter inductors L ₁ and L ₂ output the sinusoidal AC voltage.

The symmetrical LCL filter includes two filter inductors L ₁ ~ L ₂ and a filter capacitor C; wherein, one end of the filter inductor L ₁ is connected to the first voltage output terminal of the power inverter unit, and the filter inductor L ₂ One end is connected to the second voltage output end of the power inverter unit, and the other ends of the filter inductors L ₁ and L ₂ are respectively connected to both ends of the filter capacitor C and output the sinusoidal AC voltage.

The modulation method of the single-phase photovoltaic inverter of the present invention adopts unipolar pulse width modulation. In the positive half cycle of the power frequency, the power switch tubes S1 and S4 operate synchronously at high frequency, the power switch tubes S2 and S3 are kept disconnected, and the power switch tubes S5 and S1 complement high-frequency action. In the negative half cycle of the power frequency, the power switch tubes S2 and S3 operate synchronously at high frequency, the power switch tubes S1 and S4 remain disconnected, and the power switch tubes S5 and S2 complement each other at high frequency.

When the single-phase photovoltaic inverter of the present invention is working, five power switches with anti-parallel diodes act in coordination, supplemented by six diodes, so that when the inverter outputs zero level, the AC side carries out freewheeling, and through hybrid clamping Clamps its common-mode voltage to one-half of the DC input voltage to ensure a constant common-mode voltage throughout the cycle, thereby achieving complete elimination of common-mode current, and requires only half the bus voltage of a half-bridge circuit . At the same time, the invention adopts unipolar pulse width modulation, the output current ripple is small, the volume and quality of the filter are reduced, and the loss on the magnetic element is reduced at the same time; the number of switching actions in the switching cycle is small, and the switching loss is reduced, so The inverter of the invention has high output efficiency and can obtain an inverter efficiency as high as 98%.

Description of drawings

Figure 1 is a schematic diagram of a common-mode circuit in a transformerless photovoltaic system;

Fig. 2 is a schematic diagram of the topological structure of the single-phase photovoltaic inverter of the present invention;

Fig. 3 is the waveform diagram of each switch control signal of the present invention adopting the unipolar pulse width modulation mode;

Figure 4(a)-(d) are respectively schematic diagrams of the four working modes of the single-phase photovoltaic inverter of the present invention;

Fig. 5 is a comparison waveform diagram of the common mode current between the single-phase photovoltaic inverter of the present invention and the HERIC inverter.

Detailed ways

In order to describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Fig. 2, a transformerless single-phase photovoltaic inverter with voltage hybrid clamping includes: an energy storage voltage dividing unit, a power inverter unit, a hybrid clamping unit and a filter unit. in:

The energy storage and voltage dividing unit is used to store energy and divide the input photovoltaic direct current to extract the midpoint voltage of the photovoltaic direct current output; in this embodiment, it includes a first input capacitor C _dc1 and a second input capacitor C _dc2 . Wherein, the positive pole of the first input capacitor C _dc1 is connected to the positive pole of the input DC terminal, the negative pole of the second input capacitor C _dc2 is connected to the negative pole of the input DC terminal, and the negative pole of the first input capacitor C _dc1 is connected to the positive pole of the second input capacitor C _dc2 Form the voltage midpoint of the energy storage voltage division unit.

The power inverter unit is used to convert photovoltaic direct current into three-level direct voltage; in this embodiment, it includes a first power switch S ₁ , a second power switch S ₂ , a third power switch S ₃ , and a fourth Power switch tube S ₄ . Wherein, the drain of the _first power switch S1 and the drain of the _third power switch S3 are connected to the anode of the input DC terminal; the source of the _second power switch S2 and the source of the _fourth power switch S4 The pole is connected with the negative pole of the input DC terminal; the source of the _first power switch S1 is connected with the drain of the _second power switch S2 to form the first output voltage terminal of the power inverter unit; the _third power switch S3 The source is connected to the drain of the _fourth power switching transistor S4 to form a second output voltage terminal of the power inverter unit. The control poles of the four power switch tubes S ₁ -S ₄ all receive switch control signals provided by external devices.

The hybrid clamping unit is used to carry out freewheeling on the AC side when the power inverter unit outputs zero level, and clamp the freewheeling midpoint voltage to the midpoint voltage of the photovoltaic DC output; in this embodiment, it includes a power The switch tube S5 and six diodes D1-D6, the cathodes of the diodes D2, D4 and D6 are all connected to one end of the power switch tube S5, the anodes of the diodes D1, D3 and D5 are all connected to the other end of the power switch tube, and the diode D1 The cathode is connected to the anode of the diode D2 and connected to the first voltage output end of the power inverter unit, the cathode of the diode D3 is connected to the anode of the diode D4 and connected to the second voltage output end of the power inverter unit, the cathode of the diode D5 is connected to the diode D6 The anode is connected to the mid-point voltage port of the energy storage voltage divider unit; the power switch tube S5 receives the switch control signal provided by the external device.

The filtering unit is used for performing low-pass filtering on the three-level DC voltage, so as to output a sinusoidal AC voltage; in this embodiment, an AC output filter F is used. Wherein, the first input terminal of the filter F is connected to the first output voltage terminal of the power inverter unit, the second input terminal of the filter F is connected to the second output voltage terminal of the power inverter unit, and the first input terminal of the filter F , the second output terminal is connected to both ends of the AC power grid.

In this embodiment, the _first power switch S1 is formed by parallel connection of the first switching transistor _T1 and the _first anti-parallel diode DT1, and the second power switch S2 is formed by the _second switching transistor _T2 and the _second anti-parallel diode DT2 The _third power switch S3 is formed by parallel connection of the third switching transistor T3 and the _third anti-parallel diode D _T3 , and the fourth power switch S4 is formed by the _fourth switching transistor T4 and the _fourth anti-parallel diode D _T4 The _fifth power switch S5 is formed by parallel connection of the fifth switching transistor T5 and the _fifth anti-parallel diode D _T5 ; the parallel connection mode of the switching transistor and the anti-parallel diode is: the drain or collector of the switching transistor and the The cathode of the parallel diode is connected to form the drain of the power switch, and the source or emitter of the switching transistor is connected to the anode of the anti-parallel diode to form the source of the power switch.

In this embodiment, the input capacitors are all electrolytic capacitors, the power switch tubes are IGBT tubes, the filter is symmetrical inductors L ₁ and L ₂ , and the modulation method is unipolar pulse width modulation.

FIG. 3 is a schematic diagram of waveforms in the present invention using unipolar pulse width modulation. In the positive half cycle of the power frequency, the first switching transistor T ₁ and the fourth switching transistor T ₄ operate synchronously at high frequency, and the second switching transistor T ₂ , the third switching transistor T ₃ , the fifth switching transistor T ₅ and the first switching transistor T ₁ complementary high frequency action. In the negative half cycle of power frequency, the second switching transistor T ₂ and the third switching transistor T ₃ operate synchronously at high frequency, the first switching transistor T ₁ and the fourth switching transistor T ₄ remain off, and the fifth switching transistor T ₅ and the third switching transistor T 5 The _two switching transistors T2 are complementary to high-frequency action.

During the whole working process of the inverter in this embodiment, there are mainly four working modes as shown in Fig. 4(a)-(d). In working mode 1, the current flows through the first switching transistor T ₁ , the filter inductor L ₁ , the power grid, the filter inductor L ₂ , and the fourth switch transistor T ₄ in sequence, and the inverter outputs a positive voltage. In working mode 2, the current flows sequentially through the filter inductor L ₁ , the power grid, the filter inductor L ₂ , the diode D4, the fifth switching transistor T ₅ , and the diode D1, and the inverter outputs zero voltage. In the working mode 3, the current flows through the third switching transistor T ₃ , the filter inductor L ₂ , the power grid, the filter inductor L ₁ , and the second switch transistor T ₂ in sequence, and the inverter outputs a negative voltage. In working mode 4, the current flows sequentially through the filter inductor L ₂ , the power grid, the filter inductor L ₁ , the diode D2, the fifth switching transistor T ₅ , and the diode D3, and the inverter outputs zero voltage. In working modes 1 and 3, V _CM =(V _AN +V _BN )/2=V _DC /2. In working modes 2 and 4, the first switching transistor T ₁ and the first anti-parallel diode D ₁ , the second switching transistor T ₂ and the second anti-parallel diode D ₂ , the third switching transistor T ₃ and the third anti-parallel diode The diode D ₃ , the fourth switching transistor T ₄ and the fourth anti-parallel diode D ₄ are all turned off. Both V _AN and V _BN are clamped to the midpoint voltage of the capacitor by diode D5 or D6, so (V _AN +V _BN )/2=V _DC /2. Therefore, the common-mode voltage is a constant value throughout the process, thereby ensuring effective suppression of the common-mode current.

The single-phase inverter in this embodiment is tested and verified on a power platform with an input of 400V and 2kW. The comparative waveforms of the leakage current experiment between the inverter of this embodiment and the HERIC inverter are shown in Figure 5. The experimental data shows that the leakage current suppression effect of the voltage hybrid clamp type transformerless single-phase inverter of the present invention is better than that of HERIC inverter, and the experimental efficiency of the single-phase inverter of the present invention is as high as 98%.

The above experimental results show that the inverter of the present invention has reliable common-mode current suppression capability and extremely high energy conversion efficiency, and is very suitable for transformerless single-phase photovoltaic grid-connected inverter systems.

Finally, it should be noted that the combination of the above embodiments only illustrates the technical solution of the present invention rather than limiting it. Those of ordinary skill in the art should understand that those skilled in the art can modify or equivalently replace the specific embodiments of the present invention, but these modifications or changes are within the protection scope of the pending claims.

Claims (9)

1. A transformerless single-phase photovoltaic inverter with voltage hybrid clamping, characterized in that it comprises: An energy storage voltage division unit, used for energy storage and voltage division of the input photovoltaic DC voltage; A power inverter unit, configured to convert the photovoltaic DC voltage into a three-level DC voltage; The hybrid clamping unit is used to carry out freewheeling on the AC side of the inverter when the power inverter unit outputs zero level, and clamp the midpoint voltage of the freewheeling to half of the photovoltaic DC voltage; The filter unit is used for low-pass filtering the three-level DC voltage, so as to output a sinusoidal AC voltage. 2. The transformerless single-phase photovoltaic inverter according to claim 1, characterized in that: the energy storage voltage division unit includes two input capacitors C _dc1 ~ C _dc2 ; wherein, the positive pole of the input capacitor C _dc1 It is connected to the positive pole of the photovoltaic DC source, the negative pole of the input capacitor C _dc1 is connected to the positive pole of the input capacitor C _dc2 , and the negative pole of the input capacitor C _dc2 is connected to the negative pole of the photovoltaic DC source. 3 . The transformerless single-phase photovoltaic inverter according to claim 2 , wherein the input capacitors C _dc1 and C _dc2 are composed of one electrolytic capacitor or multiple electrolytic capacitors connected in series and parallel. 4. The transformerless single-phase photovoltaic inverter according to claim 1, characterized in that: the power inverter unit adopts a single-phase full-bridge inverter structure, which includes four power switches with anti-parallel diodes Tubes S1-S4; where one end of the power switch tube S1 is connected to one end of the power switch tube S3 and connected to the positive pole of the photovoltaic DC source, one end of the power switch tube S2 is connected to one end of the power switch tube S4 and connected to the negative pole of the photovoltaic DC source , the other end of the power switch tube S1 is connected to the other end of the power switch tube S2 as the first voltage output end of the power inverter unit, and the other end of the power switch tube S3 is connected to the other end of the power switch tube S4 as the power inverter unit The second voltage output terminal; the four power switch tubes S1-S4 all receive switch control signals provided by external equipment. 5. The transformerless single-phase photovoltaic inverter according to claim 1, wherein the hybrid clamping unit includes a power switch tube S5 and six diodes D1-D6; wherein, the power switch tube S5 One end of the power switch tube is connected to the cathodes of diodes D2, D4 and D6, the other end of the power switch tube is connected to the anodes of diodes D1, D3 and D5, and the cathode of diode D1 is connected to the anode of diode D2 and connected to the first voltage of the power inverter unit At the output end, the cathode of the diode D3 is connected to the anode of the diode D4 and connected to the second voltage output end of the power inverter unit, and the cathode of the diode D5 is connected to the anode of the diode D6 and the energy storage voltage dividing unit to extract half of the photovoltaic power DC voltage; the power switch tube S5 receives a switch control signal provided by an external device. 6. The transformerless single-phase photovoltaic inverter according to claim 4 or 5, characterized in that: the power switch tubes are all IGBTs. 7. The transformerless single-phase photovoltaic inverter according to claim 1, characterized in that: the filter unit adopts a symmetrical inductor filter or a symmetrical LCL filter. 8. The transformerless single-phase photovoltaic inverter according to claim 7, characterized in that: the symmetrical inductance filter includes two filter inductors L ₁ -L ₂ ; wherein, one end of the filter inductor L ₁ Connected to the first voltage output end of the power inverter unit, _one end of the filter inductor L2 is connected to the _second voltage output end of the power inverter unit, and the other ends of the filter inductors L1 and _L2 output the sinusoidal AC voltage. 9. The transformerless single-phase photovoltaic inverter according to claim 7, characterized in that: the symmetrical LCL filter includes two filter inductors L ₁ ~ L ₂ and a filter capacitor C; wherein, the filter One end of the inductor L1 is connected to the _first voltage output end of the power inverter unit, _one end of the filter inductor L2 is connected to the _second voltage output end of the power inverter unit, and the other ends of the filter inductors L1 and _L2 are respectively connected to the filter Both ends of the capacitor C are connected and output the sinusoidal AC voltage.