CN111864794A - Double-frequency transformer-free single-phase photovoltaic grid-connected inverter - Google Patents
Double-frequency transformer-free single-phase photovoltaic grid-connected inverter Download PDFInfo
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- H—ELECTRICITY
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
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- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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Abstract
The invention relates to the technical field of inverters, in particular to a double-frequency transformer-free single-phase photovoltaic grid-connected inverter which comprises a shell, wherein a circuit board is arranged in the shell and comprises a control unit and a main loop unit; the main loop unit is divided into a power unit and a harmonic elimination unit, and the harmonic elimination unit and the power unit are connected in parallel to two sides of a single-phase power grid; the bus cathode of the resonance elimination unit is connected with the bus cathode of the power unit through a connecting capacitor; the LA and LB terminals of the power unit are connected to L, N two terminals of a power grid through power filters; the HA and HB terminals of the harmonic elimination unit are connected to L, N two terminals of a power grid through a harmonic elimination filter to form a main loop unit; by means of a novel topological structure and a novel control algorithm, the earth leakage current of the transformer-free grid-connected inverter is effectively restrained, and meanwhile the overall efficiency and the power density of the inverter are improved.
Description
[ technical field ] A method for producing a semiconductor device
The invention relates to the technical field of inverters, in particular to a double-frequency transformer-free single-phase photovoltaic grid-connected inverter.
[ background of the invention ]
The development of renewable energy is an important means for solving the environmental problem, a photovoltaic grid-connected power generation system is an important form of renewable energy, a photovoltaic grid-connected inverter is an important component of the renewable energy, and how to further improve the efficiency of the grid-connected inverter and the quality of grid-connected current and reduce the volume and the cost are also widely concerned. The transformer-less grid-connected inverter does not need a single-phase transformer between the inverter and the grid, so that the transformer-less grid-connected inverter has advantages in the aspects of cost, reliability, structural simplicity, efficiency and the like, and is widely applied to a distributed photovoltaic power generation system within the power range of 2.5 kW-8 kW. However, since there is no isolation transformer in the system, a current loop through the parasitic capacitance between the photovoltaic panel and ground is generated, thereby generating a current leakage to ground.
In order to eliminate the leakage current of the transformer-free grid-connected inverter, various topological structures are proposed, and the H5 structure and the HERIC structure are most widely applied. The H5 structure is on the basis of the full-bridge inverter, set up a power device between positive pole of the direct current bus and inverter bridge, through turning on and turning off of the control power device, guarantee to realize the isolation of direct current bus (photovoltaic cell panel) and alternating current electric wire netting at the stage of afterflow, have reached the purpose to reduce the leakage current. The HERIC structure is characterized in that a bidirectional switch consisting of two series-connected power devices (each device is connected with a freewheeling diode in an anti-parallel mode) is connected between the output end of the full-bridge structure and the filter in parallel, and the isolation of a direct-current bus (a photovoltaic cell panel) and an alternating-current power grid in a freewheeling stage is realized by controlling the on and off of the two power devices, so that the leakage current is restrained.
At present, a transformer-free single-phase photovoltaic grid-connected inverter mainly has the problems of large leakage current, further improvement of efficiency, complex structure and control and the like.
In the aspect of leakage current, the positive bus and the negative bus of the H5 type and the HERIC type transformerless single-phase grid-connected inverters are in a disconnected state with a power grid in a follow current stage under unipolar modulation, so that the voltage of a photovoltaic cell panel (the positive electrode and the negative electrode of a direct current bus) to ground at the stage cannot be guaranteed to change along with the voltage of the power grid, the voltage at two ends of a parasitic capacitor changes rapidly in the next power transmission stage, and then large earth leakage current is generated, and the safety performance of the system is influenced.
For the aspect of efficiency improvement, the existing various transformer-free grid-connected inverters do not solve the contradiction between the switching frequency and the efficiency and the size. To improve the efficiency of the inverter, it is necessary to use a lower switching frequency, which in turn increases the volume of the passive filter; conversely, in order to reduce the size of the passive filter, the switching frequency needs to be increased, which in turn reduces the efficiency of the inverter and increases the design difficulty of the cooling system. Meanwhile, for the H5 structure and the HERIC structure, the switching frequency is reduced, and the amplitude of the high-frequency component of the negative electrode (positive electrode) voltage of the direct-current bus is increased, so that the amplitude of the earth leakage current is increased.
In terms of structural and control algorithm complexity, various improvements are proposed in terms of modulation strategies, increasing the number of switches to improve the topology, neutral point voltage clamping, blocking leakage current loops and the like in order to further degrade the leakage current level, but compared with the topologies of H5 type and HERIC type grid-connected inverters with basically no voltage transformation, the difficulties of the topology or (and) control strategies are increased to different degrees, and the cost of the system is increased to a certain degree.
It can be seen that although the transformer-less grid-connected inverter has been widely used, further improvement is required in terms of magnitude of leakage current, efficiency, and implementation complexity.
[ summary of the invention ]
The invention aims to provide a dual-frequency transformer-free single-phase photovoltaic grid-connected inverter aiming at the defects and shortcomings of the prior art, effectively inhibits the earth leakage current of the transformer-free grid-connected inverter through a novel topological structure and a control algorithm, and simultaneously improves the overall efficiency and power density of the inverter.
The invention relates to a double-frequency transformerless single-phase photovoltaic grid-connected inverter, which comprises a shell, wherein a circuit board is arranged in the shell, and the circuit board comprises a control unit and a main circuit unit; the main loop unit is divided into a power unit and a harmonic elimination unit, and the harmonic elimination unit and the power unit are connected in parallel to two sides of a single-phase power grid; the bus cathode of the resonance elimination unit is connected with the bus cathode of the power unit through a connecting capacitor;
The LA and LB terminals of the power unit are connected to L, N two terminals of a power grid through power filters; the HA and HB terminals of the harmonic elimination unit are connected to L, N two terminals of a power grid through a harmonic elimination filter to form a main loop unit; the control unit generates power unit driving signals P1, P2, P3, P4, P5, P6 and harmonic elimination unit driving signals P7, P8, P9, P10 according to the feedback signal and the given signal.
Further, the power unit comprises a power converter device and a power filter, wherein the power converter device is connected with the power filter in series; the power converter comprises six fully-controlled power devices S1-S6, six freewheeling diodes D1-D6 and a filter capacitor CL; the S1 is connected with the S2 in series, and the S3 is connected with the S4 in series and are connected with the two transmission conductors LLP and LLN in parallel; s5 and S6 are connected in series and are connected in parallel at a LA point and a LB point; the S1 is connected with a D1 in an antiparallel manner, the S2 is connected with a D2 in an antiparallel manner, the S3 is connected with a D3 in an antiparallel manner, the S4 is connected with a D4 in an antiparallel manner, the S5 is connected with a D5 in an antiparallel manner, and the S6 is connected with a D6 in an antiparallel manner; the filter capacitor CL is connected in parallel with the two transmission conductors LLP and LLN to form a power converter sub-circuit; the power converter subcircuits are connected with a power filter through terminals LA and LB; the power filter comprises an inductor L1 and an inductor L2, and forms a power filter branch circuit;
Furthermore, the harmonic elimination unit comprises an H-bridge converter and a harmonic elimination filter, wherein the H-bridge converter is connected with the harmonic elimination filter in series; the H-bridge current transformation device comprises four fully-controlled power devices S7-S10, four freewheeling diodes D7-D10 and a filter capacitor CH; the S7 is connected with the S8 in series, and the S9 is connected with the S10 in series and are connected to the two power transmission conductors LHP and LHN in parallel; the S7 is connected with a D7 in an antiparallel manner, the S8 is connected with a D8 in an antiparallel manner, the S9 is connected with a D9 in an antiparallel manner, and the S10 is connected with a D10 in an antiparallel manner; the filter capacitor CH is connected in parallel to the two power transmission conductors LHP and LHN to form an H-bridge current transformation device sub-circuit; the harmonic elimination filter comprises an inductor L3 and an inductor L4, and forms a harmonic elimination filter branch circuit. The L-terminal of the power cell is connected to the terminal H-of the detuning unit via a connecting capacitor CC.
Further, the control unit comprises a detuning control loop and a power control loop; the harmonic elimination control loop comprises a current regulator ACR2, one end of the current regulator ACR2 is connected with the voltage regulator AVR, and the other end of the current regulator ACR2 is connected with the PWM module 2; the power control loop comprises a current regulator ACR1, and one end of the current regulator ACR1 is connected with the PWM module 1.
Further, the output u of the current regulator ACR2 ACR2With a feedforward compensation amount uCOMAdding the modulated signals u as detuning unitsref2. Amount of feedforward compensationu COMIs used for generating the current i by using a feedforward control method1Harmonic-eliminating current u with opposite medium rippleCOMIs calculated by：
Wherein L is3And L4For the harmonic-elimination filter inductance, L1And L2As power filter inductance, uGIs the network voltage, VHFor detuning the cell bus voltage, i* GFor grid-connected current set-point, uin1Is the output voltage of the power unit current transformer.
Furthermore, the harmonic elimination unit adopts a bipolar PWM modulation method, the negative pole of a bus of the harmonic elimination unit is connected with the negative pole of the bus of the power unit through a capacitor, and the voltage of the negative pole of the bus of the power unit is clamped at u by the harmonic elimination unitGL-＝(VL-uG) And/2, eliminating high-frequency components in the voltage of the parasitic capacitance to the ground and realizing the suppression of leakage current.
The invention has the beneficial effects that: the invention relates to a double-frequency transformer-free single-phase photovoltaic grid-connected inverter, which reduces the high-frequency component in the voltage of a direct-current bus connected with a photovoltaic cell panel to the ground, reduces the switching frequency of a power device bearing the fundamental frequency current component of grid-connected current, and achieves the purposes of reducing leakage current, improving efficiency and simplifying a topological structure by utilizing a harmonic current cancellation principle.
[ description of the drawings ]
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, and are not to be considered limiting of the invention, in which:
FIG. 1 is a schematic of the topology of the present invention;
FIG. 2 is a schematic diagram of the circuit of the present invention;
FIG. 3 is a schematic diagram of the control system circuit of the present invention;
fig. 4 is a schematic diagram of a leakage current equivalent circuit in the present invention.
[ detailed description ] embodiments
The present invention will now be described in detail with reference to the drawings and specific embodiments, wherein the exemplary embodiments and descriptions are provided only for the purpose of illustrating the present invention and are not to be construed as limiting the present invention.
As shown in fig. 1 to 4, the dual-frequency transformer-free single-phase photovoltaic grid-connected inverter according to the present embodiment includes a housing, a circuit board is disposed in the housing, and the circuit board includes a control unit and a main circuit unit; the main loop unit is divided into a power unit and a harmonic elimination unit, and the harmonic elimination unit and the power unit are connected in parallel to two sides of a single-phase power grid; the bus cathode of the resonance elimination unit is connected with the bus cathode of the power unit through a connecting capacitor; the LA and LB terminals of the power unit are connected to L, N two terminals of a power grid through power filters; the HA and HB terminals of the harmonic elimination unit are connected to L, N two terminals of a power grid through a harmonic elimination filter to form a main loop unit;
The power unit converts direct current electric energy into alternating current electric energy, the power device adopts a full-control type power device with large capacity and low cost, the switching frequency is low, and the switching loss is reduced; harmonic elimination unit generation and power unit current i1Current i with opposite ripple2Suppressing the harmonic wave of the grid-connected current, and adopting a power device with high switching frequency, small capacity, high switching frequency and small on-resistance;
the control unit generates power unit driving signals P1, P2, P3, P4, P5, P6 and harmonic elimination unit driving signals P7, P8, P9, P10 through a control algorithm taking feed-forward control as a core according to the feedback signal and the given signal.
Further, the power unit comprises a power converter device and a power filter, wherein the power converter device is connected with the power filter in series; the power converter comprises six fully-controlled power devices S1-S6, six freewheeling diodes D1-D6 and a filter capacitor CL; the S1 is connected with the S2 in series, and the S3 is connected with the S4 in series and are connected with the two transmission conductors LLP and LLN in parallel; s5 and S6 are connected in series and are connected in parallel at a LA point and a LB point; the S1 is connected with a D1 in an antiparallel manner, the S2 is connected with a D2 in an antiparallel manner, the S3 is connected with a D3 in an antiparallel manner, the S4 is connected with a D4 in an antiparallel manner, the S5 is connected with a D5 in an antiparallel manner, and the S6 is connected with a D6 in an antiparallel manner; the filter capacitor CL is connected in parallel with the two transmission conductors LLP and LLN to form a power converter sub-circuit; the power converter subcircuits are connected with a power filter through terminals LA and LB; the power filter comprises an inductor L1 and an inductor L2, and forms a power filter branch circuit;
Furthermore, the harmonic elimination unit comprises an H-bridge converter and a harmonic elimination filter, wherein the H-bridge converter is connected with the harmonic elimination filter in series; the H-bridge current transformation device comprises four fully-controlled power devices S7-S10, four freewheeling diodes D7-D10 and a filter capacitor CH; the S7 is connected with the S8 in series, and the S9 is connected with the S10 in series and are connected to the two power transmission conductors LHP and LHN in parallel; the S7 is connected with a D7 in an antiparallel manner, the S8 is connected with a D8 in an antiparallel manner, the S9 is connected with a D9 in an antiparallel manner, and the S10 is connected with a D10 in an antiparallel manner; the filter capacitor CH is connected in parallel to the two power transmission conductors LHP and LHN to form an H-bridge current transformation device sub-circuit; the harmonic elimination filter comprises an inductor L3 and an inductor L4, and forms a harmonic elimination filter branch circuit. The L-terminal of the power cell is connected to the terminal H-of the detuning unit via a connecting capacitor CC.
Further, the control unit comprises a detuning control loop and a power control loop; the harmonic elimination control loop comprises a current regulator ACR2, one end of the current regulator ACR2 is connected with the voltage regulator AVR, and the other end of the current regulator ACR2 is connected with the PWM module 2; the power control loop comprises a current regulator ACR1, and one end of the current regulator ACR1 is connected with the PWM module 1.
Further, the output u of the current regulator ACR2 ACR2With a feedforward compensation amount uCOMAdding the modulated signals u as detuning unitsref2. Amount of feedforward compensationu COMIs used for generating the current i by using a feedforward control method1Harmonic-eliminating current u with opposite medium rippleCOMThe calculation method comprises the following steps:
wherein L is3And L4For the harmonic-elimination filter inductance, L1And L2As power filter inductance, uGIs the network voltage, VHFor detuning the cell bus voltage, i* GFor grid-connected current set-point, uin1Is the output of a power cell inverterA voltage.
Furthermore, the harmonic elimination unit adopts a bipolar PWM modulation method, the negative pole of a bus of the harmonic elimination unit is connected with the negative pole of the bus of the power unit through a capacitor, and the voltage of the negative pole of the bus of the power unit is clamped at u by the harmonic elimination unitGL-＝(VL-uG) And/2, eliminating high-frequency components in the voltage of the parasitic capacitance to the ground and realizing the suppression of leakage current.
The working principle of the invention is as follows:
1. the description of the various functions in the present invention is as follows:
(1) the function of the power unit is to supply a current i1Providing a channel, current i1The fundamental frequency component of the grid-connected current is equal to the given value I of the amplitude of the grid-connected current* mSince the power unit uses a low switching frequency, the current i1The grid-connected inverter has large switching ripples, but can reduce the switching loss of the whole grid-connected inverter. The power device adopts a low-cost and large-capacity fully-controlled power device, such as an IGBT.
(2) Capacitance C in power unitLThe function of the power unit is to restrain the voltage ripple on the direct current bus of the power unit and maintain the voltage V of the direct current bus of the power unitLAnd (4) stabilizing.
(3) Inductor L1Inductor L2Has the effect of initially suppressing the current i1Since the high-frequency harmonic elimination unit is included in the present topology, an inductor with a small inductance value can be selected as the filter inductor of the power unit.
(4) The harmonic elimination unit is used for generating the harmonic current i1Harmonic elimination current i with opposite switching ripples2And a device with low loss can be adopted, and because the unit bears low current, the increased loss of the unit is less than the reduced loss caused by the reduction of the switching frequency of the power unit, so that the overall efficiency of the grid-connected inverter can be improved.
(5) Capacitance C in a detuning unitHThe high-frequency harmonic elimination unit is used for providing direct-current voltage support for the high-frequency harmonic elimination unit, and the harmonic elimination unit works in a rectification mode when working, so that the harmonic elimination unit does not need an independent power supply for supplying power.
(6) Inductor L3、L4Has the function of inhibiting the current i2Medium high frequency ripple, small inductance due to high switching frequency of the unit, and low inductance due to current i2And the size of the inductor is small.
(7) Connecting capacitor CCHas the function of isolating the power unit direct current negative bus L LNDC negative bus L of harmonic elimination unitHND.c. voltage, capacitor C in steady stateCThe voltage on is the dc voltage difference between the two negative buses.
2. The control system of the design has the following control targets:
(1) stabilizing the bus voltage V of the resonance elimination unitHIs made equal to a given value V* H；
(2) Method for eliminating grid-connected current i by utilizing feedforward control method1The switching current ripple in (1);
(3) controlling power cell current i1Are made equal to respective given values I* mAnd thetap+ θG，θpAnd thetaGRespectively a power factor angle and a grid voltage phase angle.
As shown in fig. 3, the control system of the present invention consists of a detuning control loop and a power control loop;
(1) the resonance elimination control loop adopts a double closed loop structure, the outer loop is a voltage loop, V* HFor the given voltage of the DC bus voltage of the harmonic elimination unit, the voltage regulator AVR is used to make the bus voltage V of the harmonic elimination unitHEqual to its given value, output I of AVR* VMultiplication by sin θGObtaining a given value component i of the current* v. Given value I of grid-connected current amplitude* mMultiplied by sin (theta)G+θp) Obtaining given value i of grid-connected current* G，i*GAnd grid-connected current value iGSubtract i from the subtraction* vAs input to the current regulator ACR 2.
The control method has the advantages that: (1) on the basis of the power control unit, the grid-connected current i can be further controlled GMake it equal to i* GInhibition of iGCurrent harmonics in (1); (2) harmonic elimination current i due to steady state2Is related to the current i1The current with opposite ripple, and in this control method, i is not directly utilized2As a feedback quantity of the current loop, i can be eliminated2The effect of the sharp fluctuations on the control effect.
(2) Output u of current regulator ACR2ACR2With a feedforward compensation amount uCOMAdding the modulated signals u as detuning unitsref2. Amount of feedforward compensationu COMIs used for generating the current i by using a feedforward control method1Harmonic-eliminating current u with opposite medium rippleCOMThe calculation method comprises the following steps:
u in the formulain1The output voltage of the power unit current transformation device; l is3And L4For the harmonic-elimination filter inductance, L1And L2As power filter inductance, uGIs the network voltage, VHFor detuning the cell bus voltage, i* GFor grid-connected current set-point, uin1Is the output voltage of the power unit current transformer.
The PWM module 2 is based on the modulation signal uref2Generating S by bipolar modulation7、S8、S9、S10Drive signal P of7、P8、P9、P10Realize the power device S7、S8、S9、S10And (4) controlling.
The power control loop in the invention is used for realizing the control of the power unit so as to lead the current i of the power unit1The power factor is adjustable and can be changed along with the change of the given value. Instantaneous set current value i GFor the setting of the signal of the current loop, the current regulator ACR1 is a PR regulator, the output value u of which is the output value uacr1And a feedforward compensation amount uGAdd to obtain a modulated signal uref1. PWM module 1 according to signal uref1Generating the driving signal P by using a unipolar modulation method1、P2、P3、P4、P5、P6To control the power devices S of the power cells respectively1、S2、S3、 S4、S5、S6。
3. In the present invention, the principle of active detuning to suppress harmonic currents is as follows:
power cell current i1Can be decomposed into fundamental frequency current iBAnd harmonic current iHIs a sum of
i1＝iB+iH(1)
The voltage across the power filter inductor is
uL12＝uin1-uG(2)
Wherein: u. ofin1Is the output voltage of the power converter, uGIs the grid voltage.
At steady state, iHCorresponds to uin1Harmonic voltage u inHAnd under the action of the power control unit has iB＝i* G，i*GGiven value of grid-connected current, voltage uHCan be expressed as
Current iHCan be expressed as
Since the harmonic elimination unit adopts high switching frequency, if the voltage harmonic can be ignored, the output voltage u isin2Can be expressed as
uin2＝VH(uACR2+uCOM) (5)
Wherein: vHFor the DC bus voltage of the detuning unit uACR2For the output of the regulator ACR2, uCOMFor the feedforward compensation amount, the calculation method is
Wherein: l is3+L4The inductance value of the detuning filter.
Inductor L3+L4A voltage across is
At steady state uACR20, then the harmonic-eliminating unit current i2Is composed of
Grid-connected current i G＝i1+i2According to formulae (8), (4) and (1) have
iG＝iB(9)
It can be seen that the grid connection i is eliminated by the above methodGThe medium and high frequency harmonic waves achieve the purpose of improving the current quality of the power grid.
4. In the invention, the principle of inhibiting the floor drain current is as follows:
the harmonic elimination unit adopts an H-bridge structure and utilizes a bipolar modulation method to carry out modulation, so that the harmonic elimination unit and the switching device S are connected7～S10The potential difference between the H-point and the G point of the negative pole of the direct current bus of the harmonic elimination unit is constant
Therefore, a series equivalent inductor L can be usedEHThe voltage source of (3) replaces the detuning unit, resulting in an equivalent circuit diagram as shown in fig. 4.
As can be seen from FIG. 4, when the circuit is stable, the capacitor C is connectedCSo that the capacitance value of the capacitor C is larger than that of the capacitor C under the power grid frequency of 50HzCHas sufficiently small impedance, neglects AC voltage drop, connects the capacitor voltage uCCIs of a size of
For the circuit 1 of FIG. 4, the voltage at the L-point to the G-point is known to be
Wherein VLIs the power cell bus voltage.
From equation (12), it is found that the potential of the negative electrode of the power cell dc bus is clamped to (V)L-uG) At/2 and VLIs DC voltage and will not be in parasitic capacitance CPVIn which a leakage current is generated, uGIs 50Hz AC voltage, and has a capacitor CPVThe capacitor has large capacitive reactance to the alternating voltage of 50Hz, so that the scheme can realize the suppression of leakage current.
The invention has the following advantages:
(1) the method has the advantages of simple topological structure and easy control.
(2) Through a novel topological structure and a control algorithm, the harmonic elimination unit and the connection capacitor are utilized, the high-frequency component in the voltage to ground of the power unit direct-current bus connected with the photovoltaic cell panel is reduced, and the current to ground leakage is reduced.
(3) The efficiency of the grid-connected inverter is improved by reducing the switching frequency of the power unit bearing the fundamental frequency component of the grid-connected current.
(4) By utilizing the harmonic current cancellation principle, the total product and the cost of the power filter are reduced.
The invention has the beneficial effects that: the invention relates to a double-frequency transformer-free single-phase photovoltaic grid-connected inverter, which reduces the high-frequency component in the voltage of a direct-current bus connected with a photovoltaic cell panel to the ground, reduces the switching frequency of a power device bearing the fundamental frequency current component of grid-connected current, and achieves the purposes of reducing leakage current, improving efficiency and simplifying a topological structure by utilizing a harmonic current cancellation principle.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention, and all equivalent changes and modifications made based on the features and principles described in the claims of the present invention are included in the scope of the present invention.
Claims (6)
1. A double-frequency transformer-free single-phase photovoltaic grid-connected inverter comprises a shell, wherein a circuit board is arranged in the shell, and the circuit board comprises a control unit and a main loop unit; the main loop unit is divided into a power unit and a harmonic elimination unit, and the harmonic elimination unit and the power unit are connected in parallel to two sides of a single-phase power grid; the bus cathode of the resonance elimination unit is connected with the bus cathode of the power unit through a connecting capacitor;
the LA and LB terminals of the power unit are connected to L, N two terminals of a power grid through power filters; the HA and HB terminals of the harmonic elimination unit are connected to L, N two terminals of a power grid through a harmonic elimination filter to form a main loop unit; the control unit generates power unit driving signals P1, P2, P3, P4, P5, P6 and harmonic elimination unit driving signals P7, P8, P9, P10 according to the feedback signal and the given signal.
2. The dual-frequency transformerless single-phase photovoltaic grid-connected inverter as claimed in claim 1, wherein: the power unit comprises a power converter and a power filter, and the power converter is connected with the power filter in series; the power converter comprises six fully-controlled power devices S1-S6, six freewheeling diodes D1-D6 and a filter capacitor CL; the S1 is connected with the S2 in series, and the S3 is connected with the S4 in series and are connected with the two transmission conductors LLP and LLN in parallel; s5 and S6 are connected in series and are connected in parallel at a LA point and a LB point; the S1 is connected with a D1 in an antiparallel manner, the S2 is connected with a D2 in an antiparallel manner, the S3 is connected with a D3 in an antiparallel manner, the S4 is connected with a D4 in an antiparallel manner, the S5 is connected with a D5 in an antiparallel manner, and the S6 is connected with a D6 in an antiparallel manner; the filter capacitor CL is connected in parallel with the two transmission conductors LLP and LLN to form a power converter sub-circuit; the power converter subcircuits are connected with a power filter through terminals LA and LB; the power filter comprises an inductor L1 and an inductor L2, and forms a power filter subcircuit.
3. The dual-frequency transformerless single-phase photovoltaic grid-connected inverter as claimed in claim 1, wherein: the harmonic elimination unit comprises an H-bridge converter and a harmonic elimination filter, and the H-bridge converter is connected with the harmonic elimination filter in series; the H-bridge current transformation device comprises four fully-controlled power devices S7-S10, four freewheeling diodes D7-D10 and a filter capacitor CH; the S7 is connected with the S8 in series, and the S9 is connected with the S10 in series and are connected to the two power transmission conductors LHP and LHN in parallel; the S7 is connected with a D7 in an antiparallel manner, the S8 is connected with a D8 in an antiparallel manner, the S9 is connected with a D9 in an antiparallel manner, and the S10 is connected with a D10 in an antiparallel manner; the filter capacitor CH is connected in parallel to the two power transmission conductors LHP and LHN to form an H-bridge current transformation device sub-circuit; the harmonic elimination filter comprises an inductor L3 and an inductor L4, and forms a harmonic elimination filter branch circuit. The L-terminal of the power cell is connected to the terminal H-of the detuning unit via a connecting capacitor CC.
4. The dual-frequency transformerless single-phase photovoltaic grid-connected inverter as claimed in claim 1, wherein: the control unit comprises a harmonic elimination control loop and a power control loop; the harmonic elimination control loop comprises a current regulator ACR2, one end of the current regulator ACR2 is connected with the voltage regulator AVR, and the other end of the current regulator ACR2 is connected with the PWM module 2; the power control loop comprises a current regulator ACR1, and one end of the current regulator ACR1 is connected with the PWM module 1.
5. The dual-frequency transformerless single-phase photovoltaic grid-connected inverter as claimed in claim 4, wherein: output u of said current regulator ACR2ACR2With a feedforward compensation amount uCOMAdding the modulated signals u as detuning unitsref2Feed forward compensation amountu COMIs used for generating the current i by using a feedforward control method1Harmonic-eliminating current u with opposite medium rippleCOMThe calculation method comprises the following steps:
wherein L is3And L4For the harmonic-elimination filter inductance, L1And L2As power filter inductance, uGIs the network voltage, VHFor detuning the cell bus voltage, i* GFor grid-connected current set-point, uin1Is the output voltage of the power unit current transformer.
6. The dual-frequency transformerless single-phase photovoltaic grid-connected inverter as claimed in claim 1, wherein: the harmonic elimination unit adopts a bipolar PWM modulation method, the negative pole of a bus of the harmonic elimination unit is connected with the negative pole of the bus of the power unit through a capacitor, and the harmonic elimination unit clamps the voltage of the negative pole of the bus of the power unit at uGL-＝(VL-uG) And/2, eliminating high-frequency components in the voltage of the parasitic capacitance to the ground and realizing the suppression of leakage current.
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