CN111682793B - Low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter - Google Patents
Low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
- H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
- H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
- H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
- H02M7/53873—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with digital control
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/12—Arrangements for reducing harmonics from AC input or output
- H02M1/126—Arrangements for reducing harmonics from AC input or output using passive filters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0003—Details of control, feedback or regulation circuits
- H02M1/0038—Circuits or arrangements for suppressing, e.g. by masking incorrect turn-on or turn-off signals, e.g. due to current spikes in current mode control
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/12—Arrangements for reducing harmonics from AC input or output
- H02M1/123—Suppression of common mode voltage or current
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Abstract
本申请公开一种低漏电流改进H8型非隔离三相并网逆变器，包括：直流电源udc、三相逆变桥、滤波电路、直流侧电容Cdc、交流电网和直流侧电路，udc的正负极与Cdc的正负极连接；直流侧电路包括第一功率开关管S1、第二功率开关管S2、与S1反并联的二极管D1以及与S2反并联的二极管D2，S1的漏极与Cdc的正极连接，S2的源极与Cdc的负极连接；直流侧电路还包括二极管D11、二极管D12、二极管D21和二极管D22，D11和D12的正极分别连接在S1的漏极，D11和D12的负极分别连接第一功率开关管S1的源极；D21和D22的正极分别连接在S2的源极，D21和D22的负极连接在S2的漏极。显然，本申请在直流侧电路的S1和S2分别反并联两个二极管，降低并网电路共模电压高频成分，有效抑制系统漏电流。
The present application discloses an improved H8 type non-isolated three-phase grid-connected inverter with low leakage current, comprising: a DC power supply u dc , a three-phase inverter bridge, a filter circuit, a DC side capacitor C dc, an AC power grid and a DC side circuit, The positive and negative poles of u dc are connected to the positive and negative poles of C dc ; the DC side circuit includes a first power switch tube S1, a second power switch tube S2, a diode D1 in anti-parallel with S1, and diodes D2 and S1 in anti-parallel with S2 The drain of S2 is connected to the anode of C dc , and the source of S2 is connected to the cathode of C dc ; the DC side circuit also includes diode D11, diode D12, diode D21 and diode D22, and the anodes of D11 and D12 are connected to the drain of S1 respectively. , the negative poles of D11 and D12 are respectively connected to the source of the first power switch S1; the positive poles of D21 and D22 are respectively connected to the source of S2, and the negative poles of D21 and D22 are connected to the drain of S2. Obviously, in the present application, two diodes are respectively connected in anti-parallel to S1 and S2 of the DC side circuit, so as to reduce the high frequency component of the common mode voltage of the grid-connected circuit and effectively suppress the leakage current of the system.
Description
Technical Field
The embodiment of the application relates to the technical field of photovoltaic non-isolated three-phase photovoltaic grid-connected inverters, in particular to a low-leakage current improved H8 type non-isolated three-phase grid-connected inverter.
Background
With the development of a distributed photovoltaic power generation system, a non-isolated three-phase photovoltaic grid-connected inverter is more prone to application scenes such as a micro-grid. The non-isolated photovoltaic non-isolated three-phase photovoltaic grid-connected inverter has the advantages of small volume, light weight, low cost and the like, and is widely concerned at home and abroad.
In practical engineering, because the non-isolated photovoltaic non-isolated three-phase photovoltaic grid-connected inverter is free of transformer for electrical isolation, the common-mode voltage with high frequency variation generated by the high-frequency switch acts on a stray capacitor between a PV plate and the ground (equipment shell), so that the problem of leakage current can be caused, the leakage current can cause grid-connected current distortion and Total Harmonic Distortion (THD) increase, the problem of electromagnetic interference can be caused, and even the safety of personnel and equipment is endangered.
Compared with a diode clamping type, a cascade type, a flying capacitor type and other three-phase inverters, a voltage source three-phase full-bridge inverter (B6-type inverter, B6) is widely applied to a three-phase photovoltaic system due to the advantages of high efficiency, small volume, few devices and the like. However, the application of the B6 inverter in a transformerless photovoltaic grid-connected system is limited due to the large leakage current, and the scholars at home and abroad make intensive research for improving the leakage current characteristic of the B6 inverter.
In the prior art, two symmetrical switching tubes are added on the DC side of B6 to form an H8 structure, when a system works at V1-V6, S7-S8 are conducted, and when the system works at a zero vector, S7-S8 are turned off and S1-S6 are all conducted, so that AC-DC side isolation is realized, and common mode voltage is reduced. However, due to the irregular switching of the switching tube, grid-connected current contains more harmonic waves, and the leakage current effect is still not ideal.
Disclosure of Invention
The embodiment of the application provides a low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter which is used for connecting two diodes in anti-parallel in a power switch tube of a direct-current side circuit, so that the common-mode voltage high-frequency component of a grid-connected circuit is reduced and the system leakage current is effectively inhibited by not increasing the system loss and the control difficulty.
The first aspect of the embodiments of the present application provides a low leakage current improved H8 type non-isolated three-phase grid-connected inverter, including:
DC power supply udcThree-phase inverter bridge, filter circuit and direct-current side capacitor CdcAn AC power grid and a DC side circuit, the DC power source udcPositive and negative poles of the capacitor C are respectively connected with the direct current side capacitor CdcThe positive electrode and the negative electrode are connected;
the DC side circuit comprises a first power switch tube S1A second power switch tube S2, a diode D1 connected in anti-parallel with the first power switch tube S1, and a diode D2 connected in anti-parallel with the second power switch tube S2, wherein the drain of the first power switch tube S1 and the DC side capacitor CdcThe source of the second power switch tube S2 is connected to the dc side capacitor CdcThe negative electrode of (1) is connected;
the direct current side circuit further comprises a diode D11, a diode D12, a diode D21 and a diode D22, anodes of the diode D11 and the diode D12 are respectively connected to the drain of the first power switch tube S1, and cathodes of the diode D11 and the diode D12 are respectively connected to the source of the first power switch tube S1; the anodes of the diode D21 and the diode D22 are respectively connected to the source of the second power switch S2, and the cathodes of the diode D21 and the diode D22 are respectively connected to the drain of the second power switch S2.
Optionally, the three-phase inverter bridge includes a third power switch tube S3, a fourth power switch tube S4, a fifth power switch tube S5, a sixth power switch tube S6, a seventh power switch tube S7, an eighth power switch tube S8, and a diode D3, a diode D4, a diode D5, a diode D6, a diode D7, and a diode D8, which are connected in anti-parallel to the third power switch tube S3, the fourth power switch tube S4, the fifth power switch tube S5, the sixth power switch tube S6, the seventh power switch tube S7, and the eighth power switch tube S8, respectively;
a source of the third power switch tube S3 is connected to a drain of the fourth power switch tube S4, a source of the fifth power switch tube S5 is connected to a drain of the sixth power switch tube S6, and a source of the seventh power switch tube S7 is connected to a drain of the eighth power switch tube S8.
Optionally, the filter circuit includes an a-phase filter inductor La, a B-phase filter inductor Lb, a C-phase filter inductor Lc, an a-phase filter resistor Ra, a B-phase filter resistor Rb, and a C-phase filter resistor Rc;
the phase A filter inductor La is connected with the phase A filter resistor Ra and then connected with an alternating current power grid Ea, the phase B filter inductor Lb is connected with the phase B filter resistor Rb and then connected with an alternating current power grid Eb, and the phase C filter inductor Lc is connected with the phase C filter resistor Rc and then connected with an alternating current power grid Ec.
Optionally, the diode D1, the diode D11, the diode D12, the diode D2, the diode D21, the diode D22, the diode D3, the diode D4, the diode D5, the diode D6, the diode D7, the diode D71, the diode D72, and the diode D8 are all fast recovery diodes.
Optionally, the first power switch tube S1, the second power switch tube S2, the third power switch tube S3, the fourth power switch tube S4, the fifth power switch tube S5, the sixth power switch tube S6, the seventh power switch tube S7, and the eighth power switch tube S8 are all mosfets.
Optionally, inductance values corresponding to the phase a filter inductance La, the phase B filter inductance Lb, and the phase C filter inductance Lc are equal.
Optionally, the resistances of the a-phase filter resistor Ra, the B-phase filter resistor Rb, and the C-phase filter resistor Rc are equal to each other.
Optionally, the low leakage current improved H8 type non-isolated three-phase grid-connected inverter further comprises a stray capacitor CPV，The stray capacitance CPV is respectively connected with the direct current power supply udcIs connected with the connection point of the alternating current power grid Ea, the alternating current power grid Eb and the alternating current power grid Ec.
Optionally, the diode D11, the diode D12, the diode D21 and the diode D22 have the same parameters respectively.
Compared with the prior art, the embodiment of the application has the following advantages:
the low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter comprises a diode D11, a diode D12, a diode D21 and a diode D22, wherein anodes of the diode D11 and the diode D12 are respectively connected to a drain electrode of a first power switch tube S1, and cathodes of the diode D11 and a diode D12 are respectively connected to a source electrode of a seventh power switch tube S1; the anodes of the diode D81 and the diode D82 are respectively connected to the source of the second power switch tube S2, and the cathodes of the diode D21 and the diode D22 are respectively connected to the drain of the second power switch tube S2, so that when the switching sequences of the fourth power switch tube S4 and the sixth power switch tube S6 are 000 or 111, the electric output voltage in three phases can be stabilized at half of the dc bus voltage, thereby reducing the variation amplitude of the common mode voltage of the system, lowering the switching frequency, and achieving the purpose of suppressing the leakage current; and the added four diodes belong to uncontrollable devices, so that the control and loss of the system are not influenced.
Drawings
Fig. 1 is a schematic diagram of a low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided in an embodiment of the present application;
fig. 2 is a schematic diagram of a modulation strategy of each power switching tube of the low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided by the embodiment of the present application
Fig. 3 is a schematic circuit diagram of a connection between a low leakage current improved H8 type non-isolated three-phase grid-connected inverter and a control circuit provided in an embodiment of the present application;
fig. 4 is a schematic diagram of a conventional H8 inverter mode provided in an embodiment of the present application;
fig. 5 is a diagram illustrating an analysis of an inverter mode according to an embodiment of the present application;
fig. 6 is a waveform diagram of three-phase grid-connected voltage and current provided by the embodiment of the present application through a simulation model;
fig. 7 is a schematic diagram of a leakage current model of a low leakage current improved H8 type non-isolated three-phase grid-connected inverter according to an embodiment of the present application;
FIG. 8 is a simulation diagram of the common mode voltage, the voltages at two ends of the stray capacitor, and the leakage current of the conventional H8 according to the embodiment of the present invention;
fig. 9 is a simulation diagram of a common-mode voltage, a voltage across a stray capacitor, and a leakage current of the low leakage current improved H8 type non-isolated three-phase grid-connected inverter according to the embodiment of the present invention.
Detailed Description
Embodiments of the present application will now be described with reference to the accompanying drawings, and it is to be understood that the described embodiments are merely illustrative of some, but not all, embodiments of the present application. As can be appreciated by those skilled in the art, with the advent of new technologies, the technical solutions provided in the embodiments of the present application are also applicable to similar technical problems.
The terms "first," "second," "third," "fourth," and the like in the description and in the claims of the embodiments of the application and in the drawings described above, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the application described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "comprising" and "having," as well as any variations thereof, are intended to cover non-exclusive inclusions.
The embodiment of the application provides a low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter which is used for connecting two diodes in anti-parallel in a power switch tube of a direct-current side circuit, so that the common-mode voltage high-frequency component of a grid-connected circuit is reduced and the system leakage current is effectively inhibited by not increasing the system loss and the control difficulty.
It should be noted that the low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided in the embodiment of the present application may be an improved H8 type non-isolated three-phase photovoltaic grid-connected circuit based on a bypass strategy, specifically, on the basis of an original H8 type non-isolated three-phase photovoltaic grid-connected circuit, a high-performance diode is added to a bypass power switching tube to reduce a common-mode voltage high-frequency component of the grid-connected circuit, and the present application is described in this embodiment and subsequent embodiments with a non-isolated three-phase photovoltaic grid-connected circuit.
The non-isolated photovoltaic grid-connected circuit provided by the embodiment of the application is described below.
Referring to fig. 1, fig. 1 is a schematic diagram of an improved H8 type non-isolated three-phase grid-connected inverter with low leakage current according to an embodiment of the present disclosure; as shown in FIG. 1, the improved H8 type non-isolated three-phase grid-connected inverter with low leakage current provided by the embodiment of the application comprises a direct-current power supply udcThree-phase inverter bridge, filter circuit and direct-current side capacitor CdcAn AC power network andand a DC side circuit.
Wherein, the DC power supply u of the present embodimentdcPositive electrode of and DC side capacitor CdcIs connected with the negative electrode of the direct current side capacitor CdcThe negative electrode of (1) is connected; the alternating current power grid comprises an alternating current power grid Ea connection, an alternating current power grid Eb and an alternating current power grid Ec, the alternating current power grid Ea connection, the alternating current power grid Eb and the alternating current power grid Ec are respectively connected with a filter circuit, and a three-phase inverter bridge is connected with the filter circuit.
As shown in fig. 1, the dc-side circuit provided in the embodiment of the present application includes a first power switch transistor S1, a second power switch transistor S2, a diode D1 connected in anti-parallel with the first power switch transistor S1, and a diode D2 connected in anti-parallel with the second power switch transistor S2;
specifically, the anode of the diode D1 is connected to the drain of the first power switch S1, and the cathode of the diode D1 is connected to the source of the first power switch S1; the anode of the diode D2 is connected to the drain of the second power switch tube S1, and the cathode of the diode D2 is connected to the source of the second power switch tube S2; the drain of the first power switch tube S1 and the DC side capacitor CdcIs connected with the positive pole of the second power switch tube S2, the source electrode of the second power switch tube S2 and the DC side capacitor CdcIs connected to the negative electrode of (1).
It should be noted that the dc-side circuit provided in this embodiment further includes a diode D11, a diode D12, a diode D21, and a diode D22, wherein an anode of the diode D11 is connected to the drain of the first power switch S1, and a cathode of the diode D11 is connected to the source of the first power switch S1; the anode of the diode D12 is connected to the drain of the first power switch S1, and the cathode of the diode D12 is connected to the source of the first power switch S1.
The anode of the diode D21 is connected to the source of the second power switch tube S2, and the cathode of the diode D21 is connected to the drain of the second power switch tube S2; the anode of the diode D22 is connected to the source of the second power switch S2, and the cathode of the diode D21 is connected to the drain of the second power switch S2.
Alternatively, the parameters of the diode D11, the diode D12, the diode D21 and the diode D22 may be the same. Of course, this embodiment is only an embodiment, and in other implementation manners, the number of diodes connected in anti-parallel to the first power switch S1 and the second power switch S2 may be other, and is not limited herein.
In a possible implementation manner, as shown in fig. 1, the three-phase inverter bridge includes a third power switch tube S3, a fourth power switch tube S4, a fifth power switch tube S5, a sixth power switch tube S6, a seventh power switch tube S7, an eighth power switch tube S8, and a diode D3, a diode D4, a diode D5, a diode D6, a diode D7, and a diode D8, which are connected in anti-parallel to the third power switch tube S3, the fourth power switch tube S4, the fifth power switch tube S5, the sixth power switch tube S6, the seventh power switch tube S7, and the eighth power switch tube S8, respectively.
Specifically, the anode of the diode D3 is connected to the drain of the third power switch tube S3, and the cathode of the diode D3 is connected to the source of the third power switch tube S3; the anode of the diode D4 is connected to the drain of the fourth power switch tube S4, and the cathode of the diode D4 is connected to the source of the fourth power switch tube S4; the anode of the diode D5 is connected to the drain of the fifth power switch tube S5, and the cathode of the diode D5 is connected to the source of the fifth power switch tube S5; the anode of the diode D6 is connected to the drain of the sixth power switch tube S6, and the cathode of the diode D6 is connected to the source of the sixth power switch tube S6; the anode of the diode D7 is connected to the drain of the seventh power switch tube S7, and the cathode of the diode D7 is connected to the source of the seventh power switch tube S7; the anode of the diode D8 is connected to the drain of the eighth power switch S8, and the cathode of the diode D8 is connected to the source of the eighth power switch S8.
It can be understood that the driving signals of the first power switch tube S1, the second power switch tube S2, the third power switch tube S3, the fourth power switch tube S4, the fifth power switch tube S5, the fourth power switch tube S6, the fourth power switch tube S7, and the eighth power switch tube S8 provided in this embodiment are G1, G2, G3, G4, G5, G6, G7, and G8, which can be specifically obtained by:
monitoring a power grid voltage signal in real time through a voltage sensor, and transmitting the signal into a three-phase-locked loop to obtain a real-time phase theta of the power grid voltage; detecting three-phase grid-connected current through a current sensor, completing conversion from three-phase rotation to two-phase static conversion on the detected three-phase grid-connected current through Clark, and completing conversion from two-phase static rotation to two-phase rotation through Park to obtain dq-axis current; after dq axis error operation, sending the data to a PI regulator capable of realizing no-static-error control, and obtaining modulation voltage signals VA, VB and VC through inverse Park and inverse Clark conversion; VA and carrier are sent to a comparator to obtain a G3 signal, and a G3 signal is converted into a G4 signal through an inverter; VB and the carrier are sent to a comparator to obtain a G5 signal, and a G5 signal is converted into a G6 signal through an inverter; VC and carrier are sent to a comparator to obtain a G7 signal, and a G7 signal is converted into a G8 signal through an inverter; the carrier wave is a high-frequency triangular wave or a sawtooth wave; g4, G5, G6, G7, G8 and G9 are high-frequency on-off signals; the nand operation of G3, G5, and G7 yields G1, and the nand operation of G4, G6, and G8 yields G2.
In a possible implementation manner, as shown in fig. 1, the filter circuit includes an a-phase filter inductor La, a B-phase filter inductor Lb, a C-phase filter inductor Lc, an a-phase filter resistor Ra, a B-phase filter resistor Rb, and a C-phase filter resistor Rc.
The A-phase filter inductor La is connected with the A-phase filter resistor Ra and then connected with an alternating current power grid Ea, the B-phase filter inductor Lb is connected with the B-phase filter resistor Rb and then connected with an alternating current power grid Eb, and the C-phase filter inductor Lc is connected with the C-phase filter resistor Rc and then connected with an alternating current power grid Ec.
It is understood that the a-phase filter inductor La is also connected to the connection point a of the third power switching tube S3 and the fourth power switching tube S4; the B-phase filter inductor Lb is also connected with a connection point B of the fifth power switch tube S5 and the sixth power switch tube S6; the C-phase filter resistor Rc is further connected to a connection point C of the seventh power switching tube S7 and the eighth power switching tube S8.
In a possible implementation manner, the low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided by the embodiment further comprises a stray capacitor CPVStray capacitance CPVOne end of (1) and a DC power supply udcIs connected to the negative electrode of the capacitor CPVAnd the other end of the second connection line is connected to a connection point n of the alternating current grid Ea, the alternating current grid Eb and the alternating current grid Ec.
In a possible implementation manner, the diode D1, the diode D11, the diode D12, the diode D2, the diode D21, the diode D22, the diode D3, the diode D4, the diode D5, the diode D6, the diode D7, and the diode D8 are all fast recovery diodes, but may be other types of diodes.
In a possible implementation manner, the first power switch tube S1, the second power switch tube S2, the third power switch tube S3, the fourth power switch tube S4, the fifth power switch tube S5, the sixth power switch tube S6, the seventh power switch tube S7, and the eighth power switch tube S8 are all mosfet transistors, but may be other types of transistors.
In one possible implementation manner, the a-phase filter resistor Ra, the B-phase filter resistor Rb, and the C-phase filter resistor Rc have equal corresponding resistance values.
The low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter comprises a diode D11, a diode D12, a diode D21 and a diode D22, wherein anodes of the diode D11 and the diode D12 are respectively connected to a drain electrode of a first power switch tube S1, and cathodes of the diode D11 and a diode D12 are respectively connected to a source electrode of a seventh power switch tube S1; the anodes of the diode D81 and the diode D82 are respectively connected to the source of the second power switch tube S2, and the cathodes of the diode D21 and the diode D22 are respectively connected to the drain of the second power switch tube S2, so that when the switching sequences of the fourth power switch tube S4 and the sixth power switch tube S6 are 000 or 111, the electric output voltage in three phases can be stabilized at half of the dc bus voltage, thereby reducing the variation amplitude of the common mode voltage of the system, lowering the switching frequency, and achieving the purpose of suppressing the leakage current; and the added four diodes belong to uncontrollable devices, so that the control and loss of the system are not influenced.
The low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided by the embodiment of the present application is described above, the working principle of the low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided by the embodiment of the present application is described below,
the improved H8 type non-isolated three-phase grid-connected inverter with low leakage current comprises a photovoltaic component and a three-phase alternating current power grid, wherein the photovoltaic component is adopted as a direct current input source.
The CMV is an average value of a Voltage difference between an inverter output and a DC source negative electrode, which is defined by Common-Mode Voltage (CMV)
Fig. 2 is a schematic diagram of a modulation strategy of each power switching tube of the low leakage current improved H8 type non-isolated three-phase grid-connected inverter, where G1 to G8 are driving signals corresponding to the power switching tubes S1 to S8, and VA, VB, and VC are modulation voltage signals. The driving signals of G3, G5 and G7 are obtained by a sine modulation wave and a carrier wave through a comparator, G3, G5 and G7 are sent to an inverter to obtain G4, G6 and G8 respectively, G3, G5 and G7 are subjected to NAND operation to obtain G1, and G4, G6 and G8 are subjected to NAND operation to obtain G2.
Fig. 3 is a schematic circuit diagram of a connection between the low leakage current improved H8 type non-isolated three-phase grid-connected inverter and a control circuit, where the control circuit is used to drive power switching tubes S1-S8, and the control strategy is specifically as follows:
the control circuit adopts a current loop control strategy and injects three-phase current with the same frequency and phase as the voltage of the power grid into the power grid. Monitoring a power grid voltage signal in real time through a voltage sensor, and transmitting the signal into a three-phase-locked loop (PLL) to obtain a real-time phase theta of the power grid voltage; detecting three-phase grid-connected current through a current sensor, completing conversion from three-phase rotation to two-phase static conversion on the detected three-phase grid-connected current through Clark, and completing conversion from two-phase static rotation to two-phase rotation through Park to obtain dq-axis current; sending the data to a PI regulator which can realize no static error control after dq axis error operation, and obtaining modulation voltage signals VA, VB and VC through inverse Park and inverse Clark conversion; VA and carrier are sent to a comparator to obtain a G3 signal, and a G3 signal is converted into a G4 signal through an inverter; VB and the carrier are sent to a comparator to obtain a G5 signal, and a G5 signal is converted into a G6 signal through an inverter; VC and carrier are sent to a comparator to obtain a G7 signal, and a G7 signal is converted into a G8 signal through an inverter; the carrier wave is a high-frequency triangular wave or a sawtooth wave; g3, G4, G5, G6, G7 and G8 are high frequency on-off signals; the nand operation of G3, G5, and G7 yields G1, and the nand operation of G4, G6, and G8 yields G2. And d, error operation of the dq axis is that the id and the iq are respectively subtracted from the id and the iq, active and reactive instantaneous quantities of the inverter are related to the id and the iq under the dq coordinate, and in order to ensure that the power factor of the non-isolated three-phase photovoltaic grid-connected inverter is close to 1, the id is 0, and the iq can be set according to the actual power requirement.
As can be seen from fig. 2 and 3, the low leakage current improved H8 type non-isolated three-phase grid-connected inverter system has eight switching sequences, which are divided into 4 modes according to the conduction conditions of the upper arm third power switching tube S3, the fifth power switching tube S5 and the seventh power switching tube S7, and the switching sequences are respectively: one-phase conduction, two-phase conduction, three-phase conduction and non-conduction are carried out, and four working modes are analyzed in detail by combining a conduction signal and a control strategy.
Working mode 1: in the working mode 1, one phase of the upper bridge arm is conducted, G1 is 1, G2 is 1, S1 and S2 are conducted and are equivalent to ideal conducting wires, the switching sequences of the third power switch tube S3, the fifth power switch tube S5 and the seventh power switch tube S7 are (100,101,001), which can be obtained from the formula (1), ucm is 1/3 udc;
and (3) working mode 2: in the working mode 2, the upper bridge arm is conducted, G1 is 1, G2 is 1, S1 and S2 are conducted and are equivalent to ideal wires, the switching sequences of the third power switch tube S3, the fifth power switch tube S5 and the seventh power switch tube S7 are (101, 011 and 110), which can be obtained from the formula (1), ucm is 2/3 udc;
working mode 3: in the operating mode 3, the upper arm is three-phase connected, as can be seen from fig. 2, G1 is 0, G2 is 1, S1 is connected, which is equivalent to an ideal conducting wire, and the switching sequences of the third power switching tube S3, the fifth power switching tube S5, and the seventh power switching tube S7 are 111. In a traditional H8 topology, D4, D6 and D8 are connected in parallel and then connected in series with D1, and the inverter output CMV is ucm-1/4 udc, as shown in FIG. 4; in the study topology, D4, D6, D8 are connected in parallel, D1, D11, and D12 are connected in parallel, two parallel branches are connected in series, and the inverter output CMV is ucm ═ 1/2udc, as shown in fig. 5;
the working mode 4 is as follows: in the operating mode 4, the upper arm three phases are not conductive, as can be seen from fig. 2, G1 is equal to 1, G2 is equal to 0, S2 is conductive, which is equivalent to an ideal conducting wire, and the switching sequences of the third power switching tube S3, the fifth power switching tube S5, and the seventh power switching tube S7 are 000. In a traditional H8 topology, D3, D5 and D7 are connected in parallel and then connected in series with D2, and the inverter output CMV is ucm-1/4 udc, as shown in FIG. 4; in the topology of the research, D3, D5, D7 are connected in parallel, D2, D21, and D22 are connected in parallel, the two parallel branches are connected in series, and the inverter output CMV is ucm ═ 1/2udc, as shown in fig. 5.
In order to verify the correctness of theoretical analysis, a simulation model is built, and grid-connected voltage and current waveforms are obtained as shown in fig. 6, wherein fig. 6 is a three-phase grid-connected voltage and current waveform diagram provided by the embodiment of the application and passing through the simulation model; the invention can convert the direct current into the sine alternating current with the same phase and frequency as the power grid voltage through the inverter.
The following explains the principle that the present application can effectively reduce the amplitude variation of the common mode voltage of the system, reduce the switching frequency, and effectively suppress the leakage current:
fig. 7 is a schematic view of a leakage current model of a low leakage current improved H8 type non-isolated three-phase grid-connected inverter provided in an embodiment of the present application, where ielagage is affected by three high-frequency pulse direct current sources and three alternating current sources that are different from each other by 120 °, and a leakage component is complex. As can be seen from the above implementation, La ═ Lb ═ Lc, Ra ═ Rb ═ Rc, and the ac power grid is an ideal three-phase symmetric power grid. And analyzing the leakage current simplified model by adopting a superposition theorem, and when the action of a certain excitation source is analyzed, the rest excitation sources are regarded as ideal wires. The voltages of the high-frequency direct current source and the alternating current source acting on the stray capacitor can be obtained respectively.
Wherein, ω isERepresenting the grid angular frequency and omega the switching angular frequency.
The frequency of a power grid in China is 50Hz, and compared with 20-40 kHz of switching frequency, leakage current generated by the power grid can be ignored.
Leakage current is generated by common mode voltage acting on stray capacitance between a PV plate and the ground (equipment shell), and when the leakage current flows greatly, the grid-connected current is distorted, total harmonic distortion of the grid-connected current is increased, electromagnetic interference is caused, and even the equipment and personnel safety is endangered. The relationship between the leakage current and the common mode voltage is:
the combination of modal analysis and common-mode voltage results in the common-mode voltage values of the conventional H8 inverter and the improved H8 inverter of the present invention under different modes and switching sequences as shown in table 1:
TABLE 1 common mode Voltage contrast in different modes
As can be seen from the comparison in table 1, the common-mode voltage variation range of the H8 inverter is 1/4 udc-3/4 udc, and is switched between 4 states; the common-mode voltage change range of the H8 inverter after improvement is 1/3 udc-2/3 udc, and the common-mode voltage change range is switched among 3 states. As can be seen from equation (4), the reduction of the variation amplitude of the common mode voltage and the reduction of the switching frequency are beneficial to suppressing the leakage current.
In order to verify the effectiveness of the improved topology, a simulation model is built under the MATLAB/Simulink environment, and the improved H8 and the traditional H8 inverter are subjected to contrast study in the simulation. Stray capacitance between the monocrystalline silicon photovoltaic panel and the ground is about 60-110 nF/kWp, the amplitude of simulation model grid-connected current is 10A, power is about 1.5kW, and CPV is taken as 100 nF; f is the grid frequency, fs is the switching frequency, and E is the single-phase voltage amplitude. The simulation parameters are shown in table 2.
TABLE 2 simulation Circuit parameters
| Parameter(s) | Numerical value | Parameter(s) | Numerical value |
| udc/V | 700 | L1/mH | 5 |
| E/V | 311 | R/Ω | 1 |
| f/Hz | 50 | CPV/nF | 100 |
| fs/kHz | 20 | Cdc/uF | 340 |
Fig. 8-9 show simulation waveforms of a conventional H8 non-isolated three-phase pv grid-connected inverter and a non-isolated three-phase pv grid-connected inverter after the improvement of the present invention, where (a) is a common mode voltage, fig. (b) is a voltage across a stray capacitor, and fig. (c) is a leakage current ieleakage waveform.
As shown in fig. 8(a), CMV of the H8 inverter is switched among four states of 175V, 233.3V, 466.6V, and 525V. The UCPV has more high-frequency components, larger waveform burrs and a leakage current root mean square value of about 150mA, meets the provisions of VDE 0126-1-1-2006, has grid-connected current THD of only 0.43 percent and meets the IEEE std.929-2000 standard.
The improved H8 type non-isolated three-phase grid-connected inverter with low leakage current is further improved on the basis of an H8 inverter, so that the problem of leakage current is optimized. As shown in fig. 9(a), the CMV of the H8 inverter of the present invention is switched among three states, namely 233.3V, 350V and 466.6V, and the amplitude is further reduced, which is beneficial to further reduce the high frequency component of the UCPV and suppress the leakage current. As shown in FIG. 9(c), the root mean square value of the leakage current of the improved H8 inverter is about 99.6mA, the leakage current is reduced by 1/3 compared with that of the H8 inverter, and the standard of VDE 0126-1-1-2006 is met. Compared with an H8 inverter, the grid-connected current THD is further reduced by only 0.32%, and the standard of IEEE Std.929-2000 is met. Compared with an H8 inverter, the improved H8 inverter can work in a higher-power non-isolated grid-connected occasion within the range that the device can bear.
The invention discloses a low-leakage-current improved H8 type non-isolated three-phase grid-connected inverter, which adopts a diode bypass strategy, and by connecting two diodes with the same parameters in parallel in reverse directions beside two direct-current side switching tubes of the inverter, when the switching sequence of the inverter is all 0 and all 1, the output voltage of a three-phase bridge arm is stabilized at half of the direct-current bus voltage, so that the common-mode voltage amplitude of the inverter is changed from (1/4-3/4) to (1/3-2/3) the direct-current side bus voltage, and the switching state is reduced from four states to three states, thereby reducing the high-frequency component of the common-mode voltage and inhibiting the common-mode leakage current of a system. Meanwhile, after the device is improved, only 4 diodes are added to the device, and system loss and control difficulty are not affected. The parallel diodes are connected in parallel, so that the common-mode voltage transformation amplitude of the system is reduced, the switching frequency is reduced, and the common-mode leakage current of the system is effectively inhibited.
It is clear to those skilled in the art that, for convenience and brevity of description, the specific working processes of the above-described systems, apparatuses and units may refer to the corresponding processes in the foregoing method embodiments, and are not described herein again.
The above embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions in the embodiments of the present application.
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| CN101673945A (en) * | 2008-09-11 | 2010-03-17 | 通用电气公司 | Micro-electromechanical switch protection in series parallel topology |
| CN104038094A (en) * | 2014-04-01 | 2014-09-10 | 燕山大学 | Non-isolated three-phase photovoltaic grid-connected inverter and control method |
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