CN112003493A - Low-common-mode-voltage non-isolated bidirectional DC/AC converter and control method thereof - Google Patents

Low-common-mode-voltage non-isolated bidirectional DC/AC converter and control method thereof Download PDF

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CN112003493A
CN112003493A CN202010828005.9A CN202010828005A CN112003493A CN 112003493 A CN112003493 A CN 112003493A CN 202010828005 A CN202010828005 A CN 202010828005A CN 112003493 A CN112003493 A CN 112003493A
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current
grid
voltage
control method
bus
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尹强
黄军伟
熊泽成
甘江华
陈天锦
方支剑
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Xuji Group Co Ltd
XJ Electric Co Ltd
Xuji Power Co Ltd
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Xuji Group Co Ltd
XJ Electric Co Ltd
Xuji Power Co Ltd
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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
    • H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
    • H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
    • H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with 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
    • H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with 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
    • 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/28—Arrangements for balancing of the load in a network by storage of energy
    • H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
    • 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
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • 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
    • H02J5/00—Circuit arrangements for transfer of electric power between ac networks and dc networks
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/22—The renewable source being solar energy
    • H02J2300/24—The renewable source being solar energy of photovoltaic origin
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E70/00—Other energy conversion or management systems reducing GHG emissions
    • Y02E70/30—Systems combining energy storage with energy generation of non-fossil origin

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

The invention relates to a low common mode voltage non-isolated bidirectional DC/AC converter and a control method thereof, wherein the non-isolated bidirectional DC/AC converter is composed of a direct current bus, a bridge type conversion circuit and an LC filter circuit, and the converter is controlled by a combined control strategy composed of a rectification constant voltage control method and a grid-connected constant current control method, so that bidirectional energy flow between a single-phase power grid and the bus is realized, and the low common mode voltage and energy bidirectional flow control method has the advantages of reducing leakage current existing in non-isolated access of distributed energy such as photovoltaic power generation and the like, and realizing multi-mode control operation of the power grid.

Description

Low-common-mode-voltage non-isolated bidirectional DC/AC converter and control method thereof
Technical Field
The invention relates to the field related to power circuits, in particular to a low-common-mode-voltage non-isolated bidirectional DC/AC converter and a control method thereof.
Background
The non-isolated photovoltaic inverter topology omits a transformer used for isolation, reduces the manufacturing cost, has the advantages of high efficiency, light weight, small volume and the like, and is very suitable for a low-power photovoltaic inverter system. However, the omission of the isolation element also brings about a serious problem that a large earth leakage current may occur in the system. The problem of leakage current can be solved through a direct current bypass topology and a topology evolved on the basis of the direct current bypass topology.
The energy storage technology is introduced into the household photovoltaic power generation system, the problem of unbalanced power supply in the household photovoltaic power generation system can be solved, the requirement of normal work of a load is met, the reliable operation of the whole household photovoltaic power generation system can be ensured, and the method is also an effective method for solving the problem of simultaneous interruption of instantaneous power supply of the household photovoltaic battery and an alternating current power grid.
The introduction of the stored energy enables the battery to provide energy for the power grid, and the power grid can also charge the battery, so that the requirement that the inversion topology in the traditional photovoltaic power generation system cannot reverse the energy flow needs to be broken. Therefore, the need for a low common mode voltage non-isolated bidirectional DC/AC converter and a control method thereof should arise.
Disclosure of Invention
Based on the above situation in the prior art, the purpose of the present invention is to solve the problem that the energy of a photovoltaic power generation grid-connected system can only flow from a bus to a power grid in a single-phase manner in the prior art by providing a low common-mode voltage non-isolated bidirectional DC/AC converter and a control method thereof, and enable bidirectional flow between the bus and the power grid through a driving waveform signal timing sequence of SPWM, thereby expanding the application range and obtaining better application in a photovoltaic energy storage system.
To achieve the above object, according to one aspect of the present invention, there is provided a low common mode voltage non-isolated bidirectional DC/AC converter, comprising:
the direct current bus, the bridge type conversion circuit and the LC filter circuit;
the direct current bus, the bridge type conversion circuit and the LC filter circuit are connected in this way, and the output end of the LC filter circuit is connected to a power grid;
the bridge type conversion circuit comprises a first bridge arm and a second bridge arm, wherein the first bridge arm and the second bridge arm respectively comprise three switching tubes;
the LC filter circuit comprises an inductor and a capacitor.
Further, the first bridge arm comprises switching tubes Q1, Q5 and Q3 respectively provided with anti-parallel diodes, and the second bridge arm comprises switching tubes Q2, Q6 and Q4 respectively provided with anti-parallel diodes.
Further, the switching tubes Q1, Q5 and Q3 are sequentially connected in series, the collector of the switching tube Q1 is connected with the positive electrode of the direct current bus, and the emitter of the switching tube Q3 is connected with the negative electrode of the direct current bus; the switching tubes Q2, Q6 and Q4 are sequentially connected in series, the collector of the switching tube Q2 is connected with the positive electrode of the direct current bus, and the emitter of the switching tube Q4 is connected with the negative electrode of the direct current bus
Further, the converter further comprises diodes D1 and D2, a connection point of an emitter of the switching tube Q5 and a collector of the switching tube Q3 is a point a, a connection point of an emitter of the switching tube Q6 and a collector of the switching tube Q4 is a point B, an anode of the diode D1 is connected with the point a, and a cathode of the diode D1 is connected with the emitter of the switching tube Q2 and the collector of the switching tube Q6; the anode of the diode D2 is connected to the point B, and the cathode of the diode D2 is connected to the emitter of the switching tube Q1 and the collector of the switching tube Q5.
Further, the LC filter circuit comprises inductors L1 and L2, and a capacitor C2, one end of the inductor L1 is connected to the point a, one end of the inductor L2 is connected to the point B, and the other ends of the inductors L1 and L2 are connected to two ends of the capacitor C2, respectively, and are used as output ends of the LC filter circuit.
According to another aspect of the invention, a control method for the low common-mode voltage non-isolated bidirectional DC/AC converter is provided, the control method comprises a combined control strategy consisting of a rectification constant voltage control method and a grid-connected constant current control method, and the corresponding control method is implemented according to the working condition of the converter.
Further, the rectification constant voltage control method includes:
obtaining direct current bus voltage feedback signal UdcGrid voltageUgridGrid current igrid；
Feeding back the DC bus voltage to a signal UdcAnd a voltage given signal UdcrefComparing, inputting the compared error signal into a PI + QPR control voltage loop, and taking the output of the control voltage loop as a given signal i of the power grid currentgref；
Will the network voltage UgridObtaining a phase locking angle theta through orthogonal virtual voltage and alpha beta/dq coordinate transformation, wherein the grid current igridAfter alpha beta/dq coordinate transformation and a phase locking angle theta, a feedback current i is formedg；
The feedback current igAnd a current given signal igrefComparing, and inputting the compared error signal into a PI + QPR control current loop;
the output of the current loop forms a carrier wave through dq/alpha beta and a phase locking angle theta, and the carrier wave is compared with a modulation wave to form an SPWM control signal;
and driving a switching tube in the bridge type conversion circuit after the SPWM signal is isolated and amplified.
Further, the grid-connected constant current control method comprises the following steps:
obtaining a grid voltage UgridGrid current igrid；
Will the network voltage UgridObtaining a phase locking angle theta through orthogonal virtual voltage and alpha beta/dq coordinate transformation, wherein the grid current igridAfter alpha beta/dq coordinate transformation and a phase locking angle theta, a feedback current i is formedg；
The feedback current igAnd a current given signal igrefComparing, and inputting the compared error signal into a PI + QPR control current loop;
the output of the current loop forms a carrier wave through dq/alpha beta and a phase locking angle theta, and the carrier wave is compared with a modulation wave to form an SPWM control signal;
and driving a switching tube in the bridge type conversion circuit after the SPWM signal is isolated and amplified.
Further, in the rectification constant-voltage control and the grid-connected constant-current control, the current given signal is a positive value or a negative value, when the current given signal is the positive value, energy is transmitted from the direct-current bus to the power grid, and when the current given signal is the negative value, energy is transmitted from the power grid to the direct-current bus.
In summary, the invention provides a low common mode voltage non-isolated bidirectional DC/AC converter and a control method thereof, the non-isolated bidirectional DC/AC converter is composed of a direct current bus, a bridge type conversion circuit and an LC filter circuit, and the converter is controlled by a combined control strategy composed of a rectification constant voltage control method and a grid-connected constant current control method, so that bidirectional energy flow between a single-phase power grid and the bus is realized, and the invention has the advantages of reducing leakage current existing in non-isolated access of distributed energy such as photovoltaic power generation, and realizing multi-mode control operation of the power grid, and low common mode voltage and bidirectional energy flow.
Drawings
FIG. 1 is an overall structural diagram and a control method schematic diagram of a low common mode voltage non-isolated bidirectional DC/AC converter according to the present invention;
FIG. 2 is a circuit diagram of a bridge converter circuit in a low common mode voltage non-isolated bi-directional DC/AC converter according to the present invention;
FIG. 3 is a timing diagram of SPWM driving waveform signals of a bridge conversion circuit in a low common mode voltage non-isolated bidirectional DC/AC converter according to the present invention;
FIG. 4 is a schematic diagram of a vector transformation of a single phase locking technique;
FIG. 5 is a block diagram of the phase-locked loop closed-loop control in the method for controlling the low common mode voltage non-isolated bidirectional DC/AC converter according to the present invention
FIG. 6 is a block diagram of the current inner loop control for the rectified constant voltage control in the control method of the low common mode voltage non-isolated bidirectional DC/AC converter of the present invention;
FIG. 7 is a voltage outer loop control block diagram of the rectified constant voltage control in the low common mode voltage non-isolated bidirectional DC/AC converter control method of the present invention;
FIG. 8 is a block diagram of grid-connected constant current control in the low common mode voltage non-isolated bidirectional DC/AC converter control method of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings in conjunction with the following detailed description. It should be understood that the description is intended to be exemplary only, and is not intended to limit the scope of the present invention. Moreover, in the following description, descriptions of well-known structures and techniques are omitted so as to not unnecessarily obscure the concepts of the present invention.
The technical solution of the present invention will be described in detail below with reference to the accompanying drawings. According to an embodiment of the present invention, there is provided a low common mode voltage non-isolated bidirectional DC/AC converter, fig. 1 is an overall structural diagram and a control method schematic diagram of the low common mode voltage non-isolated bidirectional DC/AC converter of the present invention, as shown in fig. 1, the converter includes: the direct current bus, the bridge type conversion circuit and the LC filter circuit; the direct current bus, the bridge type conversion circuit and the LC filter circuit are connected in this way, and the output end of the LC filter circuit is connected to a power grid.
Fig. 2 shows a circuit structure of a bridge type converting circuit in a non-isolated bidirectional DC/AC converter with low common mode voltage, and as shown in fig. 2, the bridge type converting circuit comprises a first bridge arm composed of switching tubes Q1, Q5 and Q3 respectively having anti-parallel diodes, and a second bridge arm composed of switching tubes Q2, Q6 and Q4 respectively having anti-parallel diodes. The switch tube can be a power switch tube such as MOEFET and IGBT which are common in the field. Further, the switching tubes Q1, Q5 and Q3 are sequentially connected in series, the collector of the switching tube Q1 is connected with the positive electrode of the direct current bus, and the emitter of the switching tube Q3 is connected with the negative electrode of the direct current bus; the switching tubes Q2, Q6 and Q4 are connected in series in sequence, the collector of the switching tube Q2 is connected with the positive electrode of the direct current bus, and the emitter of the switching tube Q4 is connected with the negative electrode of the direct current bus. The converter further comprises diodes D1 and D2, the connection point of the emitter of the switching tube Q5 and the collector of the switching tube Q3 is point A, the connection point of the emitter of the switching tube Q6 and the collector of the switching tube Q4 is point B, the anode of the diode D1 is connected with point A, and the cathode of the diode D1 is connected with the emitter of the switching tube Q2 and the collector of the switching tube Q6; the anode of the diode D2 is connected to the point B, and the cathode of the diode D2 is connected to the emitter of the switching tube Q1 and the collector of the switching tube Q5. The LC filter circuit comprises inductors L1 and L2 and a capacitor C2, one end of the inductor L1 is connected with a point A, one end of the inductor L2 is connected with a point B, and the other ends of the inductors L1 and L2 are respectively connected with two ends of the capacitor C2 and serve as output ends of the LC filter circuit. The point A is connected with the L of the power grid through an inductor L1; point B is connected to N of the grid via an inductor L2.
Fig. 3 shows a timing diagram of SPWM driving waveform signals of a bridge converter circuit in a low common mode voltage non-isolated bidirectional DC/AC converter according to the present invention, as shown in fig. 3, in the SPWM driving signals of the bridge converter circuit, the driving signals of the switching tubes Q1 and Q4 are the same, and the driving signals of the switching tubes Q6 and Q1 are complementary; the driving signals of the switching tube Q2 and the switching tube Q3 are the same, and the driving signals of the switching tube Q5 and the switching tube Q2 are complementary. Carrier ucarrierGreater than the modulating wave ucontrolWhen the voltage is high, the driving signal of the switching tube Q1 is high, so that in the positive half period of the modulation wave, the driving signal of the switching tube Q1 is from small to large and then from large to small, and in the negative half period of the modulation wave, the driving signal of the switching tube Q1 is low; carrier ucarrierGreater than the modulating wave ucontrolWhen the driving signal of the switching tube Q2 is high,therefore, in the positive half period of the modulation wave, the driving signal of the switching tube Q2 is from small to large and then from large to small, and in the negative half period of the modulation wave, the driving signal of the switching tube Q2 is low.
The operating mode of the bridge converter circuit is further explained below.
In the positive half period of the voltage of the alternating-current side, the switching tube Q5 is in a direct-current state, the switching tubes Q2 and Q3 are normally off, the high-frequency switches of the switching tubes Q1 and Q4 are driven by the same driving signal, the switching tube Q6 is in a high-frequency switch, and the switching signals of the switching tubes Q1 and Q4 are complementary with the switching signal of the switching tube Q6 (the dead zone is ignored). In the negative half period of the alternating-current side voltage, the switching tube Q6 is directly conducted, the switching tubes Q1 and Q4 are normally off, the high-frequency switches of the switching tubes Q2 and Q3 are driven by the same signal, the switching tube Q5 is switched at a high frequency, and the switching signals of the switching tubes Q2 and Q3 are complementary with the switching signal of the switching tube Q5 (the dead zone is ignored).
(1) Working state 1:
as shown in fig. 3, in the positive half cycle of the grid voltage, the switching tubes Q2, Q3, Q6 are in the open circuit state, at which the switching tube Q5 keeps the closed state, and the switching tubes Q1, Q4 are in the high frequency switching state, and the pulse width thereof varies sinusoidally according to the grid voltage. In this state, the dc input power U is supplied while the switching tubes Q1 and Q4 are ondcThrough switching tubes Q1, Q4, filter inductors L1, L2 and an alternating current power grid UgForm a loop to supply power to the power grid. The bridge arm output voltage is:
Figure BDA0002636939710000061
(2) and 2, working state:
as shown in fig. 3, the circuit is in the off state of the high-frequency switches Q1 and Q4, at this time, the inductor current flows continuously, the inductor current flows through the switch Q5, the filter inductors L1 and L2, the alternating current network, and the diode D2 form a continuous current loop, and the grid-connected current is maintained in the original direction. At this time, the output voltage of the bridge arm is
Figure BDA0002636939710000071
(3) And 3, working state:
as shown in fig. 3, in the negative half cycle of the grid voltage, the switching tubes Q1, Q4, Q5 are in the open circuit state, at which the switching tube Q6 keeps the closed state, and the switching tubes Q2, Q3 are in the high frequency switching state, and the pulse width of the switching tubes varies sinusoidally according to the grid voltage. In this state, the dc input power U is supplied while the switching tubes Q2 and Q3 are ondcThrough switching tubes Q2, Q3, filter inductors L1, L2 and an alternating current power grid UgForm a loop to supply power to the power grid. At this time, the output voltage of the bridge arm is
Figure BDA0002636939710000072
(4) And the working state 4:
as shown in fig. 3, the circuit is in the off state of the high-frequency switches Q2 and Q3, at this time, the inductor current flows continuously, the inductor current flows through the switch Q6, the filter inductors L1 and L2, the alternating current network, and the diode D2 form a continuous current loop, and the grid-connected current is maintained in the original direction. At this time, the output voltage of the bridge arm is
Figure BDA0002636939710000073
In summary, the output effect of the converter provided by the present invention is the same as the modulation mode of the unipolar SPWM, and the voltage of the output bridge arm of the converter appears as 1, 0 in the positive half cycle and 0, -1 in the negative half cycle of the grid voltage.
The effect of suppressing the common mode current of the low common mode voltage non-isolated bidirectional DC/AC converter of the present invention is further described as follows:
when the grid voltage works in a positive half cycle and the switching tubes Q1 and Q4 are in the conducting state of the high-frequency switch, the voltage of the point A of the bridge arm to the point N of the direct current ground is equal to the input voltage Udc(ii) a And under the condition of not considering the conduction voltage drop of the switching tube, the voltage of the bridge arm B point to the direct current ground N point is equal to 0V. Common mode voltage U at this stagecmIs composed of
Ucm＝0.5(Udc+0)＝0.5Udc (5)
When the high-frequency switch is in the off state of the switching tubes Q1 and Q4, the freewheeling circuit exists at the time. The voltage of the bridge arm A point to the direct current ground N is half of the input voltage; the voltage of the bridge arm B point to the direct current ground N is also half of the input voltage. Common mode voltage U at this stagecmIs composed of
Ucm＝0.5(0.5Udc+0.5Udc)＝0.5Udc (6)
As can be seen from equations (5) and (6), if the input voltage can be kept constant, the common mode voltage U is constantcmIf the common mode current can be always kept constant, the common mode current is 0 as shown by equation (7). Likewise, the same effect can be obtained when the grid voltage is operating at negative half cycles.
Figure BDA0002636939710000081
According to another embodiment of the invention, a control method for the low common-mode voltage non-isolated bidirectional DC/AC converter is provided, and since the converter is a bidirectional converter, the control method includes a combined control strategy consisting of a rectification constant voltage control method and a grid-connected constant current control method, and the corresponding control method is implemented according to the working condition of the converter. In the grid-connected state, the output does not need to stabilize the grid voltage but follows the voltage and frequency of the grid because the grid voltage exists and varies within a certain range, generally around 230VAC/115 VAC. The rectification constant voltage control of a bridge type conversion circuit in the converter adopts a double closed loop control mode of a bus voltage outer loop and a power grid current inner loop, the inner loop is used for controlling the sine of grid-connected current, and the outer loop is used for controlling the constant voltage of a system direct current bus; the grid-connected constant current control adopts the current loop control of a power grid, and the flowing direction of the energy between the bus voltage and the power grid is controlled according to the direction of the current. The rectification constant voltage control method comprises the following steps:
obtaining direct current bus voltage feedback signal UdcGrid voltageUgridGrid current igrid；
Feeding back the DC bus voltage to a signal UdcAnd a voltage given signal UdcrefComparing, inputting the compared error signal into a PI + QPR control voltage loop, and taking the output of the control voltage loop as a given signal i of the power grid currentgref；
Will the network voltage UgridObtaining a phase locking angle theta through orthogonal virtual voltage and alpha beta/dq coordinate transformation, wherein the grid current igridAfter alpha beta/dq coordinate transformation and a phase locking angle theta, a feedback current i is formedg；
The feedback current igAnd a current given signal igrefComparing, and inputting the compared error signal into a PI + QPR control current loop;
the output of the current loop forms a carrier wave through dq/alpha beta and a phase locking angle theta, and the carrier wave is compared with a modulation wave to form an SPWM control signal;
and driving a switching tube in the bridge type conversion circuit after the SPWM signal is isolated and amplified.
The grid-connected constant current control method comprises the following steps:
obtaining a grid voltage UgridGrid current igrid；
Will the network voltage UgridObtaining a phase locking angle theta through orthogonal virtual voltage and alpha beta/dq coordinate transformation, wherein the grid current igridAfter alpha beta/dq coordinate transformation and a phase locking angle theta, a feedback current i is formedg；
The feedback current igAnd a current given signal igrefComparing, and inputting the compared error signal into a PI + QPR control current loop;
the output of the current loop forms a carrier wave through dq/alpha beta and a phase locking angle theta, and the carrier wave is compared with a modulation wave to form an SPWM control signal;
and driving a switching tube in the bridge type conversion circuit after the SPWM signal is isolated and amplified.
Further, in the rectification constant-voltage control and the grid-connected constant-current control, the current given signal is a positive value or a negative value, when the current given signal is the positive value, energy is transmitted from the direct-current bus to the power grid, and when the current given signal is the negative value, energy is transmitted from the power grid to the direct-current bus.
This example will be described in detail below. Alpha beta-dq conversion of the DC/AC converter refers to Park conversion in three-phase inverter control so as to achieve the purpose of converting alternating current into direct current. Applying the coordinate transformation requires at least two independent phases, such as two-phase sinusoids with 90 ° phase difference required in the Park transformation. In order to obtain two independent phases, an alternating current output variable which is different from an actual alternating current output variable of the single-phase inverter by 90 degrees is virtualized according to the concept of an orthogonal virtual circuit, and two static coordinate systems similar to a coordinate system alpha beta are formed. Fig. 4 shows a vector transformation diagram of the single-phase-locking technique. In FIG. 4, VgridIs the network voltage, VPLLIs the voltage obtained using a phase locked loop. When the two are not coincident, a new vector V can be sent out because of the angle differenceaddThis parasitic vector can cause overshoot of the system. Therefore, the purpose of the single-phase-locked loop control technology is to ensure that V isPLLCompletely coincident with V, i.e. theta1θ. Wherein, single-phase voltage satisfies the following relational expression:
Figure BDA0002636939710000101
Figure BDA0002636939710000102
the phase locked loop closed loop control block of the present invention is shown in fig. 5. By applying the network voltage VgridAnd performing Park transformation on the vector and the virtual orthogonal vector to obtain VdAnd Vq. Wherein VdFor the d-axis component, V, of the output voltage of the phase-locked loopqIs the q-axis component of the output voltage. To make VPLLCompletely coincident with V, provided that V is guaranteedPLLThe q-axis component in the synchronous rotating coordinate system is always 0, i.e. V q0. When V isqAn output through a PI regulator of0, finally only the grid angular frequency omegaffAnd outputting a phase-locking angle theta under the action of the integrator.
The embodiment of the control method provided by the invention comprises rectification constant voltage control and grid-connected constant current control, wherein the rectification constant voltage control further comprises current inner loop control and voltage outer loop control. The control method described above will be described in detail below.
(1) Rectification constant voltage control
1) Current inner loop control
According to an embodiment of the present invention, a block diagram of the current inner loop control is shown in fig. 6, and as can be seen from fig. 6, only one PI + QPR regulator is included in the entire controller, and the parameter setting is relatively simple. Given value i of grid-connected currentg_refAnd network side current feedback value ig_fbObtaining a modulation wave signal v through a PI + QPR regulatorcThe resulting signal is compared with a triangular wave to obtain a driving duty D of SPWM modulation, 1/V in FIG. 6mIs the transfer function of the triangular wave. After the duty ratio D acts on the DC/AC converter, the output voltage V of the middle point of the bridge arm is obtainedNFrom VNMinus the net side voltage vgridObtaining the voltage V at two ends of the AC output inductorN-vgridThe current into the grid or the current out of the grid can be obtained by ohm's law. Wherein the DC/AC converter link can be equivalent to Vdc. The current inner loop control is used for ensuring the quality of output electric energy. Here, it is only necessary to convert both the given signal and the feedback signal from the α β coordinate system to the dq coordinate system, and to convert the given signal and the feedback signal from the dq coordinate system to the α β coordinate system when outputting the drive signal.
2) Voltage outer loop control
Under the grid-connected state, the bus needs to be kept stable and cannot fluctuate greatly, so that the grid-connected power is adjusted through voltage outer ring control, when the grid-connected power is smaller than the output power of distributed energy sources such as a photovoltaic array and the like, the bus capacitor is charged, and the voltage on the bus is increased; when the grid-connected power is larger than the output power of distributed energy sources such as a photovoltaic array and the like, the bus capacitor discharges, and the voltage on the bus drops and falls back, so that the voltage on the bus is constant. The voltage outer loop control block diagram of the bus voltage is as followsFIG. 7 shows the bus voltage feedback value Vdc_fbAnd a bus voltage reference value Vdc_refThe difference value is calculated to obtain a grid-connected current reference value i through a PI + QPR regulator and a limit valueg_ref，Gc(s) is a current inner loop regulator model, grid-connected current reference value ig_refObtaining a network side current i through the current inner loopg. Here, it is only necessary to convert both the given signal and the feedback signal from the α β coordinate system to the dq coordinate system, and to convert the given signal and the feedback signal from the dq coordinate system to the α β coordinate system when outputting the drive signal.
Let the effective value of the network voltage be vgridEffective value of grid-connected current is igThe DC bus voltage is VdcThe current output by the bus capacitor is idcFrom the power balance relationship, one can derive:
vgrid×ig＝vdc×idc (10)
let kd＝vgrid/vdcThen, there are:
idc＝kd×vgrid (11)
through the link, the current output by the bus capacitor is obtained according to the charge-discharge relation of the capacitor on the direct current bus, and can also be obtained,
Figure BDA0002636939710000111
converting this formula into the frequency domain, there are:
Figure BDA0002636939710000112
the resulting dc bus voltage is thus obtained.
(2) Grid-connected constant current control
A block diagram of the grid-connected constant current control is shown in fig. 8. The whole controller is only provided with one PI + QPR regulator, and parameter setting is simple. Given value i of grid-connected currentg_refAnd network side current feedback value ig_fbIs obtained by a PI + QPR regulatorModulated wave signal vcWhich is compared to the triangular wave to obtain the driving duty D of the SPWM modulation, 1/V in FIG. 8mIs the transfer function of the triangular wave. After the duty ratio D acts on the DC/AC converter, the output voltage V of the middle point of the bridge arm is obtainedN，VNMinus the net side voltage vgridAnd obtaining the voltage at two ends of the alternating current output inductor, and obtaining the current entering the power grid or the current flowing out of the power grid through ohm's law. Wherein the DC/AC converter link can be equivalent to Vdc. Here, it is only necessary to convert both the given signal and the feedback signal from the α β coordinate system to the dq coordinate system, and to convert the given signal and the feedback signal from the dq coordinate system to the α β coordinate system when outputting the drive signal. The bus voltage at this time is stabilized by the output of distributed energy such as a photovoltaic array or the like or the conversion output of an energy storage battery.
In summary, the invention relates to a low common mode voltage non-isolated bidirectional DC/AC converter and a control method thereof, the non-isolated bidirectional DC/AC converter is composed of a DC bus, a bridge conversion circuit and an LC filter circuit, and the converter is controlled by a combined control strategy composed of a rectification constant voltage control method and a grid-connected constant current control method, so that different controls can be realized according to the actual working conditions of the converter. The converter realizes bidirectional energy flow between a single-phase power grid and a bus, and has the advantages of reducing leakage current existing in non-isolated access of distributed energy such as photovoltaic power generation and the like, realizing multi-mode control operation of the power grid and low common-mode voltage; and the bus and the power grid can flow in two directions through the driving waveform signal time sequence of the SPWM, so that the application range of the bus and the power grid is expanded, and the bus and the power grid can be better applied to a photovoltaic energy storage system.
It is to be understood that the above-described embodiments of the present invention are merely illustrative of or explaining the principles of the invention and are not to be construed as limiting the invention. Therefore, any modification, equivalent replacement, improvement and the like made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. Further, it is intended that the appended claims cover all such variations and modifications as fall within the scope and boundaries of the appended claims or the equivalents of such scope and boundaries.

Claims (9)

1. A low common mode voltage non-isolated bidirectional DC/AC converter, comprising:
the direct current bus, the bridge type conversion circuit and the LC filter circuit;
the direct-current bus, the bridge type conversion circuit and the LC filter circuit are sequentially connected, and the output end of the LC filter circuit is connected to a power grid;
the bridge type conversion circuit comprises a first bridge arm and a second bridge arm, wherein the first bridge arm and the second bridge arm respectively comprise three switching tubes;
the LC filter circuit comprises an inductor and a capacitor.
2. The converter according to claim 1, wherein the first leg comprises switching tubes Q1, Q5 and Q3 having anti-parallel diodes, respectively, and the second leg comprises switching tubes Q2, Q6 and Q4 having anti-parallel diodes, respectively.
3. The converter according to claim 2, wherein the switching tubes Q1, Q5 and Q3 are connected in series in sequence, and the collector of the switching tube Q1 is connected with the positive pole of the dc bus, and the emitter of the switching tube Q3 is connected with the negative pole of the dc bus; the switching tubes Q2, Q6 and Q4 are connected in series in sequence, the collector of the switching tube Q2 is connected with the positive electrode of the direct current bus, and the emitter of the switching tube Q4 is connected with the negative electrode of the direct current bus.
4. The converter according to claim 3, further comprising diodes D1 and D2, wherein the connection point of the emitter of the switch tube Q5 and the collector of the switch tube Q3 is point A, the connection point of the emitter of the switch tube Q6 and the collector of the switch tube Q4 is point B, the anode of the diode D1 is connected with point A, and the cathode of the diode D1 is connected with the emitter of the switch tube Q2 and the collector of the switch tube Q6; the anode of the diode D2 is connected to the point B, and the cathode of the diode D2 is connected to the emitter of the switching tube Q1 and the collector of the switching tube Q5.
5. The current transformer of claim 4, wherein the LC filter circuit comprises inductors L1 and L2 and a capacitor C2, one end of the inductor L1 is connected to the point A, one end of the inductor L2 is connected to the point B, and the other ends of the inductors L1 and L2 are connected to two ends of the capacitor C2 respectively and serve as output ends of the LC filter circuit.
6. A control method of a low common mode voltage non-isolated bidirectional DC/AC converter according to any one of claims 1 to 5, characterized in that the control method comprises a combined control strategy consisting of a rectification constant voltage control method and a grid-connected constant current control method, and the corresponding control method is implemented according to the working condition of the converter.
7. The control method according to claim 6, wherein the rectifying constant-voltage control method includes:
obtaining direct current bus voltage feedback signal UdcGrid voltageUgridGrid current igrid；
Feeding back the DC bus voltage to a signal UdcAnd a voltage given signal UdcrefComparing, inputting the compared error signal into a PI + QPR control voltage loop, and taking the output of the control voltage loop as a given signal i of the power grid currentgref；
Will the network voltage UgridObtaining a phase locking angle theta through orthogonal virtual voltage and alpha beta/dq coordinate transformation, wherein the grid current igridAfter alpha beta/dq coordinate transformation and a phase locking angle theta, a feedback current i is formedg；
The feedback current igAnd a current given signal igrefComparing, and inputting the compared error signal into a PI + QPR control current loop;
the output of the current loop forms a carrier wave through dq/alpha beta and a phase locking angle theta, and the carrier wave is compared with a modulation wave to form an SPWM control signal;
and driving a switching tube in the bridge type conversion circuit after the SPWM signal is isolated and amplified.
8. The control method according to claim 6, wherein the grid-connected constant current control method comprises:
obtaining grid voltageUgridGrid current igrid；
Will the network voltage UgridObtaining a phase locking angle theta through orthogonal virtual voltage and alpha beta/dq coordinate transformation, wherein the grid current igridAfter alpha beta/dq coordinate transformation and a phase locking angle theta, a feedback current i is formedg；
The feedback current igAnd a current given signal igrefComparing, and inputting the compared error signal into a PI + QPR control current loop;
the output of the current loop forms a carrier wave through dq/alpha beta and a phase locking angle theta, and the carrier wave is compared with a modulation wave to form an SPWM control signal;
and driving a switching tube in the bridge type conversion circuit after the SPWM signal is isolated and amplified.
9. The control method according to claim 7 or 8, wherein in the rectification constant voltage control and the grid-connected constant current control, the current given signal has a positive value or a negative value, and when the current given signal has a positive value, energy is transferred from the direct current bus to a grid, and when the current given signal has a negative value, energy is transferred from the grid to the direct current bus.
CN202010828005.9A 2020-08-17 2020-08-17 Low-common-mode-voltage non-isolated bidirectional DC/AC converter and control method thereof Pending CN112003493A (en)

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