CN112701725B - Grid-connected inverter with mixed conduction mode - Google Patents

Grid-connected inverter with mixed conduction mode Download PDF

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CN112701725B
CN112701725B CN202011532606.1A CN202011532606A CN112701725B CN 112701725 B CN112701725 B CN 112701725B CN 202011532606 A CN202011532606 A CN 202011532606A CN 112701725 B CN112701725 B CN 112701725B
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grid
current
machine side
module
conduction mode
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CN112701725A (en
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张宇
赵普
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Huazhong University of Science and Technology
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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
    • 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
    • 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
    • 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
    • H02M7/539—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 with automatic control of output wave form or frequency
    • H02M7/5395—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 with automatic control of output wave form or frequency by pulse-width modulation
    • 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
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

The invention discloses a grid-connected inverter with a mixed conduction mode, which belongs to the field of grid-connected inverter control and optimization and comprises the following components: the system comprises a bridge type inversion module, an LCL filtering module and a closed-loop control module; in the current switching period, when the average current of a machine side inductor in the LCL filtering module is consistent with the reference direction of capacitor voltage, if the electric charge quantity flowing through the machine side inductor is small, the bridge type inversion module is controlled to be in a P-O intermittent conduction mode, and if the electric charge quantity is large, the bridge type inversion module is controlled to be in a P-O-N critical conduction mode; when the average current of the machine side inductor in the LCL filtering module is not consistent with the reference direction of the capacitor voltage, if the electric charge quantity flowing through the machine side inductor is small, the bridge type inversion module is controlled to be in an O-N intermittent conduction mode, and if the electric charge quantity is large, the bridge type inversion module is controlled to be in a P-O-N critical conduction mode. The soft switching of the power switching device is realized without an additional resonant circuit, the current stress and the switching loss of a switching tube are reduced, the size of the radiator is reduced, the inverter can select a filter inductor with smaller inductance, the size of the inductor is reduced, and the efficiency and the power density are improved.

Description

Grid-connected inverter with mixed conduction mode
Technical Field
The invention belongs to the field of grid-connected inverter control and optimization, and particularly relates to a grid-connected inverter in a hybrid conduction mode.
Background
The micro LCL grid-connected inverter can directly invert and connect the electric energy of each photovoltaic polar plate to the grid. High efficiency, high power density, simple and easy control algorithm and high-quality grid-connected current control effect (low total harmonic distortion) are still the performance targets of the micro grid-connected inverter. These several performance indicators are typically interrelated, tightly coupled. The efficiency improvement means that various losses occurring in the grid-connected system are reduced as much as possible, and mainly include losses of magnetic elements such as switching devices and inductances in the inverter bridge, and losses of stray inductances and equivalent resistances of the line. The losses of the switching device generally mainly include turn-on turn-off losses, on-state losses of the switching tube, and free-wheeling losses and reverse recovery losses of the anti-parallel diode. Turn-on turn-off losses are caused by voltage and current overlap of the switching devices during turn-on and turn-off; the on-state loss of the switching device is determined by the on-state resistance of the switching tube and the flowing current; the current flowing through the switching tube per switching cycle can be reduced by increasing the switching frequency of the device.
Under the condition of medium and high power, the traditional LCL grid-connected inverter works in a Continuous Conduction Mode (CCM) in consideration of the ripple amplitude requirement of machine side inductance current, and in this case, switching devices are all hard switches, so that the switching loss of the devices is inevitably increased. In a low-power situation, the soft switching technology is sometimes realized by adding a resonant circuit, and can be divided into a resonant DC link, a resonant pole, an auxiliary resonant buffer, a main and auxiliary switching circuit, carrier control and the like according to the working principle and the circuit position. When the resonant circuit is used to realize soft switching, the resonant process may generate high voltage stress and current stress on the switching device. In addition, the resonant circuit needs to be added with an auxiliary capacitor, an inductor, a switching tube and other elements, which makes the control strategy of the inverter power supply very complex and affects the stable operation of the inverter power supply. Therefore, in the field of micro grid-connected inverter, how to realize soft switching without increasing a resonant circuit is a concern for those skilled in the art.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a grid-connected inverter in a mixed conduction mode, and aims to realize the soft switching of a power switching device under the condition of not adding a resonant circuit, reduce the current stress and the switching loss of a switching tube, further reduce the volume of a radiator, select a filter inductor with smaller inductance for the inverter, reduce the volume of the inductor and improve the efficiency and the power density.
To achieve the above object, according to an aspect of the present invention, there is provided a hybrid conduction mode grid-connected inverter including: the system comprises a bridge type inversion module, an LCL filtering module and a closed-loop control module; the input side of the LCL filtering module is connected with the alternating current side of the bridge type inversion module, and the output side of the LCL filtering module is connected with a power grid; in the current switching period, when the average current of a machine side inductor in the LCL filtering module is consistent with the reference direction of capacitor voltage, if the charge amount flowing through the machine side inductor is smaller than a first critical charge amount, the closed-loop control module is used for controlling the bridge type inverter module to be in a P-O intermittent conduction mode, and if the charge amount flowing through the machine side inductor is not smaller than the first critical charge amount and not larger than a second critical charge amount, the closed-loop control module is used for controlling the bridge type inverter module to be in a P-O-N critical conduction mode; in the current switching period, when the average current of a machine side inductor in the LCL filter module is not consistent with the reference direction of a capacitor voltage, if the amount of electric charge flowing through the machine side inductor is less than a third critical electric charge amount, the closed-loop control module is configured to control the bridge inverter module to be in an O-N discontinuous conduction mode, and if the amount of electric charge flowing through the machine side inductor is not less than the third critical electric charge amount, the closed-loop control module is configured to control the bridge inverter module to be in the P-O-N critical conduction mode.
Furthermore, the controlled model of the LCL filtering module is a direct average current model which takes the average current of the machine side inductor as an input quantity, takes the grid-connected current as an output quantity, takes the grid-connected voltage as a disturbance quantity and is provided with grid-connected voltage feedforward, and the direct average current model is a second-order model; the closed-loop control module takes grid-connected current as a direct control object, carries out PID closed-loop control on the LCL filtering module, control parameters are determined by the direct average current model, an expected system damping ratio and natural oscillation frequency, and the expression of the direct average current model is as follows:
Figure GDA0003516635670000031
wherein, Ig(s) is the grid-connected current, L2The inductance value of the network side inductor, C is the capacitance value of the capacitor in the LCL filter module, RgIs the equivalent resistance of power frequency at network side, s is the physical quantity under continuous domain, IL1_avg(s) is the average current of the machine side inductance, ug(s) is the grid-connected voltage.
Furthermore, the bridge type inverter module is a T-type inverter bridge, a capacitor C1 and a capacitor C2 are arranged between the positive input side and the negative input side of the T-type inverter bridge in series, a switch tube S1 and a switch tube S2 are further sequentially connected between the positive input side and the negative input side of the T-type inverter bridge, and a switch tube S3 and a switch tube S4 which are reversely connected in series are sequentially connected between the connection point of the capacitor C1 and the capacitor C2 and the connection point of the switch tube S1 and the switch tube S2.
Furthermore, in the P-O discontinuous conduction mode, the switch tube S2 and the switch tube S4 are turned off, the switch tube S3 is kept normally on during the whole switching period, and the duty ratio D of the switch tube S11Comprises the following steps:
Figure GDA0003516635670000032
wherein L is1The inductance value of the machine side inductor ucAnd QL1_avrThe capacitor voltage and the charge quantity, T, flowing through the machine side inductor in the current switching periodsAnd E is the direct current voltage at the input side of the bridge type inversion module in a switching period.
Furthermore, in the P-O-N critical conduction mode, the switch tube S2 and the switch tube S4 are turned off, and the duty ratio D of the switch tube S11And duty cycle D of switching tube S33Respectively as follows:
Figure GDA0003516635670000033
Figure GDA0003516635670000034
A0＝0.5E
B0＝-(0.5E+uc)
Figure GDA0003516635670000041
wherein A is0Is a coefficient of a quadratic term, B0Is a coefficient of a first order term, C0Is a zero-order coefficient, L1The inductance value of the machine side inductor ucAnd QL1_avrThe capacitor voltage and the charge quantity, T, flowing through the machine side inductor in the current switching periodsAnd E is the direct current voltage at the input side of the bridge type inversion module in a switching period.
Furthermore, in the O-N discontinuous conduction mode, the switch tube S1, the switch tube S2 and the switch tube S4 are turned off, and the duty ratio D of the switch tube S33Comprises the following steps:
Figure GDA0003516635670000042
wherein L is1The inductance value of the machine side inductor ucAnd QL1_avrThe capacitor voltage and the charge quantity, T, flowing through the machine side inductor in the current switching periodsAnd E is the direct current voltage at the input side of the bridge type inversion module in a switching period.
Furthermore, the system also comprises a sampling module used for sampling the grid-connected voltage, the grid-connected current and the capacitor voltage; the closed-loop control module comprises a differential phase-locked loop, a PID controller and a mode discrimination and calculation unit; the differential phase-locked loop is used for obtaining phase angle information according to a grid-connected voltage sampling value of the previous switching period, multiplying the phase angle information by a grid-connected current set value and outputting a multiplication result to the PID controller; the PID controller is used for carrying out PID adjustment on a difference value between the multiplication result and the grid-connected current sampling value of the previous switching period and outputting the average current of the side inductor in the current switching period; the mode distinguishing and calculating unit is used for calculating the charge quantity of the machine side inductor according to the average current of the machine side inductor in the current switching period and distinguishing the working mode of the bridge type inverter module according to the charge quantity of the machine side inductor.
Furthermore, the device also comprises an input voltage-sharing control module, wherein the input voltage-sharing control module is used for sampling the difference value between the voltages of the capacitor C1 and the capacitor C2 and outputting a corresponding correction coefficient after PI control; and if the given value of the grid-connected current in the previous switching period is zero, the closed-loop control module is also used for correcting the average current of the machine side inductor in the current switching period by using the correction coefficient, and the mode distinguishing and calculating unit calculates the charge amount of the machine side inductor according to the corrected average current of the machine side inductor in the current switching period.
Furthermore, the closed-loop control module further comprises a PWM unit for outputting a driving signal to a switching tube in the T-type inverter bridge according to the determination result of the mode determination and calculation unit.
Furthermore, the mode discrimination and calculation unit is further configured to determine the current switching period according to the capacitor voltage ucInductance L of machine side inductor1、TsAnd E calculating the first, second and third critical charge amounts.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) on the occasion of low power, the current is zero at the starting moment of the switching period in the intermittent conduction mode and the critical conduction mode, so that zero current switching-on of the switching device can be realized; meanwhile, in an intermittent conduction mode, a resonance process of a parasitic capacitor and a filter inductor of a switching tube at a zero current exists, and reverse recovery loss does not exist when the follow current of the anti-parallel diode is finished; in addition, in the high-frequency low-power micro grid-connected inverter, the rising amplitude of the inductive current in a single switching period is limited, so that the improvement of the current stress of the switching tube caused by the two modes can be ignored; the soft switching of the power switching device is realized without an additional resonant circuit, and the current stress and the switching loss of a switching tube are reduced; the loss of magnetic elements such as inductors mainly comprises magnetic core loss and coil loss, for a grid-connected inverter with a certain power level, the power density depends on the volume of the device, a discontinuous conduction mode is adopted, the efficiency can be effectively improved, the volume of a radiator is simplified, and the machine side filter inductor is miniaturized due to the intermittent characteristic of current, so that the power density is also effectively improved;
(2) a mathematical model is established for a grid-connected inverter in a hybrid conduction mode, an LCL filter is equivalent to a second-order model with the input of the average current of a machine side inductor and the output of the average current of the grid-connected inverter as the grid-connected current, order reduction processing is realized compared with a traditional controlled model, the model does not have a resonance peak and needs damping, PID design can be directly carried out through a three-degree-of-freedom zero pole configuration method, and design of a controller is greatly simplified.
Drawings
Fig. 1A is a circuit diagram of a three-phase T-type three-level LCL grid-connected inverter;
fig. 1B is a circuit diagram of a single-phase T-type three-level LCL grid-connected inverter;
fig. 2 is a schematic structural diagram of a grid-connected inverter with a hybrid conduction mode according to the present embodiment;
fig. 3A is a circuit diagram of an LCL filter module of a grid-connected controlled object in the present embodiment;
fig. 3B is a transfer function block diagram of the grid-connected controlled system in this embodiment;
FIG. 4 is a comparison bode diagram of the conventional controlled system and the controlled system in the present embodiment;
FIG. 5 is a block diagram of a PID closed loop control system with grid-connected voltage feedforward;
6A-6C are circuit diagrams of three levels of P, O, N output by the T-shaped inverter bridge at the middle point of the bridge arm in the positive half power frequency period respectively;
FIG. 7 is a schematic diagram of voltage waveforms of machine side inductor average current and capacitor in a power frequency period;
FIG. 8 is a schematic diagram of average current, instantaneous current and mode selection of machine side inductors within a power frequency cycle;
FIGS. 9A-9C are schematic diagrams of (P-O) DCM, (P-O-N) BCM, (O-N) DCM switching timing and machine side inductor average current waveforms, respectively;
FIGS. 10A-10C are schematic diagrams of waveforms of (P-O) DCM, (P-O-N) BCM, (O-N) DCM switching timing and the side inductor average current with the criterion threshold, respectively;
fig. 11 is a waveform diagram of grid-connected current setting, grid-connected current, machine side inductance average current, machine side inductance instantaneous current and mode flag in five power frequency cycles under 500W of the grid-connected inverter in the embodiment;
fig. 12 is a waveform diagram of the machine side inductance average current, the machine side inductance instantaneous current, the controller output equivalent charge absolute value, and the 2 mode discrimination critical charge magnitude in one power frequency cycle of the grid-connected inverter 500W according to the present embodiment;
FIGS. 13A-13B are simulated waveforms of machine side inductor currents under (O-N) DCM and (P-O) DCM, respectively;
FIGS. 13C-13D are a simulated waveform diagram and a simulated amplified waveform diagram, respectively, of the machine side inductor current under the (P-O-N) BCM.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
the system comprises a bridge type inversion module 1, an LCL filtering module 2, a closed-loop control module 3, a sampling module 4 and an input voltage-sharing control module 5.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
In the present application, the terms "first," "second," and the like (if any) in the description and the drawings are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order.
Fig. 2 is a schematic structural diagram of the grid-connected inverter with the hybrid conduction mode according to the present embodiment. Referring to fig. 2, a grid-connected inverter with a hybrid conduction mode according to the present embodiment will be described in detail with reference to fig. 1A to 13D.
The grid-connected inverter in the hybrid conduction mode comprises a bridge type inversion module 1, an LCL filtering module 2 and a closed-loop control module 3. The input side of the LCL filter module 2 is connected to the ac side of the bridge inverter module 1, and the output side is connected to the grid, as shown in fig. 1A and 1B.
In the current switching period, when the average current of the machine side inductor in the LCL filter module 2 is consistent with the reference direction of the capacitor voltage, if the amount of electric charge flowing through the machine side inductor is smaller than the first critical amount of electric charge, the closed-loop control module 3 controls the bridge type inverter module 1 to be in the P-O discontinuous conduction mode, and if the amount of electric charge flowing through the machine side inductor is not smaller than the first critical amount of electric charge and not larger than the second critical amount of electric charge, the closed-loop control module 3 controls the bridge type inverter module 1 to be in the P-O-N critical conduction mode. When the average current of the machine side inductor in the LCL filtering module 2 is not consistent with the reference direction of the capacitor voltage, if the charge amount flowing through the machine side inductor is smaller than a third critical charge amount, the closed-loop control module 3 controls the bridge type inverter module 1 to be in an O-N intermittent conduction mode, and if the charge amount flowing through the machine side inductor is not smaller than the third critical charge amount, the closed-loop control module 3 controls the bridge type inverter module 1 to be in a P-O-N critical conduction mode.
In this embodiment, the grid-connected inverter in the hybrid conduction mode will be described by taking the bridge inverter module 1 as a single-phase T-type inverter bridge shown in fig. 1B as an example. In the LCL filter module 2, an inductor L1Is machine side inductance, inductance L2Is a network side inductor, a resistor rgThe capacitor C is a filter capacitor, and the capacitor voltage in this embodiment refers to the voltage of the filter capacitor C.
For the continuous conduction mode of the traditional grid-connected inverter, due to the linear relation between the output voltage and the duty ratio of the bridge arm, a linear state space average model can be established and directly modulated by adopting an SPWM (sinusoidal pulse width modulation) strategy, and a controlled object is a transfer function G of the output voltage-grid-connected current of the bridge arm1(s)：
Figure GDA0003516635670000081
Wherein, Uinv(s) is bridge arm output voltage, Ig(s) is the grid-connected current, L1Is a machine side inductance, RL1Is equivalent AC resistance of machine side inductor, L2Is a network side inductor, RgThe equivalent resistance is the power frequency of the network side, and C is a filter capacitor.
The model is a three-order controlled model, and an inherent resonance peak exists in a middle and high frequency band, as shown in fig. 4. The design of the controller needs active damping processing, the parameter configuration of the controller is complex, and the design difficulty of the controller is increased. When the inductive current is in a discontinuous conduction mode, because the working state of the inductive current after returning to zero is added in the embodiment of the invention, a mathematical model in the continuous mode is not applicable any more, and becomes highly nonlinear, and the design of the controller is more difficult.
In view of this, an embodiment of the invention provides a model for controlling the average current of a machine-side inductor, as shown in fig. 3A. According to the state space average model modeling method, the LCL filter module 2 is equivalent to a second-order model with the input of the machine side inductance average current, the output of the machine side inductance average current and the output of the disturbance quantity of the grid-connected current as grid-connected voltage, and PID closed-loop control with grid-connected voltage feedforward takes the grid-connected current as a control target, wherein a transfer function block diagram of the second-order model is shown in FIG. 3B, and the expression is as follows:
Figure GDA0003516635670000091
wherein, Ig(s) is the grid-connected current, L2The inductance of the network side inductor, C the capacitance of the filter capacitor, RgIs the equivalent resistance of power frequency at network side, s is the physical quantity under continuous domain, IL1_avg(s) is the average current of the machine side inductance, ugAnd(s) is grid-connected voltage.
The bode diagrams of the controlled object and the conventional controlled object in the present embodiment are shown in fig. 4. Referring to fig. 4, it can be seen that the machine side inductor average current control model in this embodiment does not have a resonance peak requiring damping, is a second-order controlled system, and can directly perform pole configuration and design of the PID controller by a three-degree-of-freedom pole-zero configuration method, and a transfer function block diagram of the closed-loop control system is shown in fig. 5.
In this embodiment, the bridge inverter module 1 is a T-type inverter bridge, a capacitor C1 and a capacitor C2 are arranged between the positive input side and the negative input side of the T-type inverter bridge in series, a switch tube S1 and a switch tube S2 are further sequentially connected between the positive input side and the negative input side of the T-type inverter bridge, and a switch tube S3 and a switch tube S4 which are connected in series in reverse are sequentially connected between the connection point of the capacitor C1 and the capacitor C2 and the connection point of the switch tube S1 and the switch tube S2.
Taking a positive half power frequency period as an example, the on-states of the circuit switching tubes when the midpoint output P, O, N of the topology bridge arm of the T-type inverter bridge is at three levels are shown in fig. 6A, 6B, and 6C, respectively, black indicates that the corresponding switching device is driven to be on, and light gray indicates that the corresponding switching device is in an off-state.
With the change of the average current output by the controller, in order to ensure that each switching period outputs the corresponding charge amount (the product of the average current and the switching period) on the premise of the minimum current stress, the embodiment of the invention provides the grid-connected inverter in the mixed conduction mode. Referring to fig. 7 and 8, in a power frequency period, when the average current of the machine side inductor is consistent with the reference direction of the capacitor voltage, the modulation method is in a P-O discontinuous conduction mode under a low charge amount and in a P-O-N critical conduction mode under a high charge amount. Because the voltage of the filter capacitor lags behind the average current by a small phase angle, the average current of the inductor and the voltage of the filter capacitor can generate a non-in-phase working condition in a plurality of switching periods near the zero crossing of the average current of the inductor at the machine side, and at the moment, if the previous criterion circuit is adopted, the circuit can enter a P-O-N critical conduction mode to cause the conditions of current waveform instability and sudden increase of current stress of a switching tube, so that a new conduction mode needs to be introduced into the non-in-phase region. Specifically, when the average current of the machine side inductor is inconsistent with the reference direction of the capacitor voltage, the O-N discontinuous conduction mode is adopted when the charge quantity is small, and the P-O-N critical conduction mode is adopted when the charge quantity is large.
Fig. 9A, 9B, and 9C are schematic diagrams of waveforms of the machine side inductor current in one switching cycle in the P-O discontinuous conduction mode, the P-O-N critical conduction mode, and the O-N discontinuous conduction mode, respectively, and schematic diagrams of the driving signals G1-G4 applied to the switching tubes S1-S4, respectively, where the shaded areas indicate the amount of charge flowing through the machine side inductor in one switching cycle, and are equal in value to the product of the average inductor current and the switching cycle in each switching cycle. No matter which topology is adopted by the bridge type inversion module 1, the machine side inductor current works in which mode, and the average current value flowing through the machine side inductor in the current switching period and the duty ratio required by the switching tube can be in one-to-one correspondence through corresponding conversion.
The mode discrimination and calculation unit is also used for judging the current capacitor voltage u in the current switching periodcInductance L of machine side inductor1Switching period TsAnd bridge type inversion moduleThe direct-current voltage E on the input side of the block 1 calculates a first critical charge amount, a second critical charge amount, and a third critical charge amount. In FIG. 10A, the area surrounded by the dashed line (i) (-) is the first critical charge amount; in fig. 10B, the area surrounded by the intersection of the first dotted line and the second dotted line is the first critical charge amount, and the area surrounded by the intersection of the first dotted line and the second dotted line is the second critical charge amount; in fig. 10C, the area surrounded by the dotted line (C) is the third critical charge amount. A first critical charge amount in a current switching period
Figure GDA0003516635670000101
Second critical amount of charge
Figure GDA0003516635670000102
And a third critical amount of charge
Figure GDA0003516635670000103
Respectively as follows:
Figure GDA0003516635670000104
Figure GDA0003516635670000105
Figure GDA0003516635670000106
wherein, TsFor the switching period, E is the DC voltage at the input side of the bridge inverter module 1, L1The inductance value of the machine side inductor ucThe capacitor voltage in the current switching period.
In this embodiment, the grid-connected inverter in the hybrid conduction mode further includes a sampling module 4 and an input voltage-sharing control module 5. The sampling module 4 is used for sampling grid-connected voltage, grid-connected current and capacitance voltage. The input voltage-sharing control module 5 is configured to sample a difference between voltages of the capacitor C1 and the capacitor C2, and output a corresponding correction coefficient after PI control. Referring to fig. 2, the operation process of the grid-connected inverter in the hybrid conduction mode is as follows:
it should be noted that, the k-th switching period may be equivalent to the previous switching period, and the k + 1-th switching period may be equivalent to the current switching period. Firstly, at the beginning of the kth switching period, the sampling module 4 samples the grid-connected voltage ug(k) Grid-connected current ig(k) And the capacitor voltage uc(k) (ii) a Will be connected to the grid voltage ug(k) The phase angle information sin theta (k) of the kth switching period is obtained by inputting the phase angle information sin theta (k) into a differential phase-locked loop DU-PLL (digital phase-locked Loop), and the differential phase-locked loop enables the phase angle information sin theta (k) and a grid-connected current given value I to be givengm *Multiplying to obtain the grid-connected current given value I of the kth switching periodgm *sinθ(k)。
Secondly, grid-connected current given value I of kth switching periodgm *sin theta (k) and sampled grid-connected current ig(k) Subtracting to obtain an offset value e (k), and outputting the average current i which does not consider the problem of input voltage imbalance and flows through the machine side inductor in the (k +1) th switching period through a designed PID (proportion integration differentiation) controllerL1_aver(k+1)。
Then, in order to correct the problem of input voltage-sharing imbalance caused by incomplete symmetry of upper and lower half-waves of machine side inductive current under the complete power frequency positive and negative half-wave working period, input side voltage difference ring PI control is introduced. Specifically, if the grid-connected current given value in the kth switching period is just zero, the input voltage-sharing control module 5 samples to obtain the voltage difference u of the input-side voltage-stabilizing capacitorcdiff(i) Outputting a correction coefficient 1+ k in the current power frequency period through a PI controllercorec(i) I from the main control loop outputL1_averMultiplying (k +1) to obtain corrected average current given i of machine side inductorL1_ref(k + 1); therefore, the closed-loop control module 3 outputs the average current i which flows through the machine side inductor in the (k +1) th switching period after the input voltage unbalance is consideredL1_ref(k+1)＝[1+kcorec(i)]iL1_aver(k+1)。
Finally, the mode judging and calculating unit obtains the critical value for judging the conduction mode of the grid-connected inverter according to the capacitance voltage and other circuit parameters obtained by sampling in the current switching periodA criterion value; the mode discrimination and calculation unit does not need correction according to the average current i of the machine side inductor in the k +1 th switching period before correctionL1_aver(k +1) the amount of charge of the computer side inductance; the mode discrimination and calculation unit judges and calculates the average current i of the machine side inductor in the k +1 th switching period after correctionL1_ref(k +1) the amount of charge of the computer side inductance; comparing the machine side inductance charge quantity obtained by calculation with the critical criterion value to determine a conduction mode, and outputting duty ratio information corresponding to each switching tube according to the average current-duty ratio conversion relation in the corresponding mode. Specifically, the method comprises the following steps:
in P-O discontinuous conduction mode, i.e. the average current of the machine side inductor is consistent with the reference direction of the capacitor voltage and
Figure GDA0003516635670000121
when the switch tube S2 is disconnected from the switch tube S4, the switch tube S3 is kept normally on in the whole switching period, and the duty ratio D of the switch tube S11Comprises the following steps:
Figure GDA0003516635670000122
P-O-N critical conduction mode, i.e. the average current of the machine side inductor is aligned with the reference direction of the capacitor voltage and
Figure GDA0003516635670000123
when the average current of the side inductor is not consistent with the reference direction of the capacitor voltage
Figure GDA0003516635670000124
When the current is detected, the switch tube S2 and the switch tube S4 are disconnected, and the duty ratio D of the switch tube S1 is1And duty cycle D of switching tube S33Respectively as follows:
Figure GDA0003516635670000125
Figure GDA0003516635670000126
A0＝0.5E
B0＝-(0.5E+uc)
Figure GDA0003516635670000127
wherein A is0Is a coefficient of a quadratic term, B0Is a coefficient of a first order term, C0Is a zero-order term coefficient.
O-N discontinuous conduction mode, i.e. the average current of the machine side inductor is not in accordance with the reference direction of the capacitor voltage and
Figure GDA0003516635670000128
when the switch tube S1, the switch tube S2 and the switch tube S4 are disconnected, the duty ratio D of the switch tube S3 is3Comprises the following steps:
Figure GDA0003516635670000129
the closed-loop control module 3 further comprises a PWM unit for outputting a driving signal to a switching tube in the T-type inverter bridge according to the result of the mode discrimination and calculation unit to complete the modulation of the current switching period, and repeating the above operations until the next switching period.
The T-type three-level inverter bridge works in an inductive current mixed conduction mode by adopting the filter inductor with small inductance, and the direct average current control method is adopted, and the controller which takes the inductive average current as the control output quantity is used for uniformly controlling a plurality of conduction modes, so that the inverter can realize standard sine wave in-phase grid connection in a wider power range. Firstly, the invention enables the inverter to select the filter inductor with smaller inductance, thereby reducing the inductor volume and reducing or eliminating the magnetic core loss; secondly, under the condition of not needing an additional resonant circuit, soft switching of the power switching device is realized through the design of several conduction modes, and on the basis, the current stress of a switching tube is reduced to the greatest extent, the switching loss is greatly reduced, and the volume of a radiator is reduced; finally, a control scheme different from the active damping control of the traditional LCL grid-connected inverter is provided, the order reduction of a controlled object is realized, the design of a controller is facilitated, and the grid-connected inverter can realize safe grid connection and reliable operation under the condition of a non-ideal power grid through a feedforward link. Therefore, the efficiency and the power density of the whole inverter are greatly improved, and the closed-loop control is simplified. The method is suitable for the low-power single-phase and three-phase T-shaped LCL grid-connected inverter with high power density and high performance.
In order to verify the practicability of the invention, an MATLAB/Simulink simulation model of a direct average current control method of the mixed conduction mode grid-connected inverter is established based on the topological structure of the single-phase T-type three-level grid-connected inverter shown in FIG. 1B, a corresponding control algorithm is realized by utilizing S-function, and simulation verification under the average power of 500W is completed. When the grid-connected inverter device uses the scheme of the invention, the waveform diagrams of grid-connected current given, grid-connected current, machine side inductance average current, machine side inductance instantaneous current and mode marks in five power frequency periods under 500W are shown in figure 11. When the grid-connected inverter device uses the scheme of the invention, the waveform of the average current of the machine side inductor and the instantaneous current of the machine side inductor under 500W, the absolute value of the equivalent charge quantity output by the controller and the waveform of the critical charge quantity value judged by 2 modes in a power frequency period are shown in figure 12. Fig. 13A, 13B, 13C, and 13D show graphs of inductor current amplification simulation waveforms in the respective modes. Simulation results show that under any working condition of 500W or below, the grid-connected inverter can keep grid-connected current to be sine wave in a mixed conduction mode. The grid-connected inverter control method can quickly and accurately track the input command grid-connected current and effectively improve the efficiency of the device.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (10)

1. A grid-connected inverter with a hybrid conduction mode is characterized by comprising: the system comprises a bridge type inversion module (1), an LCL filtering module (2) and a closed-loop control module (3); the input side of the LCL filter module (2) is connected with the alternating current side of the bridge type inverter module (1), and the output side of the LCL filter module is connected with a power grid;
in the current switching period, when the average current of a machine side inductor in the LCL filtering module (2) is consistent with the reference direction of a capacitor voltage, if the charge amount flowing through the machine side inductor is smaller than a first critical charge amount, the closed-loop control module (3) is used for controlling the bridge type inversion module (1) to be in a P-O intermittent conduction mode, and if the charge amount flowing through the machine side inductor is not smaller than the first critical charge amount and not larger than a second critical charge amount, the closed-loop control module (3) is used for controlling the bridge type inversion module (1) to be in a P-O-N critical conduction mode;
in the current switching period, when the average current of a machine side inductor in the LCL filtering module (2) is not consistent with the reference direction of a capacitor voltage, if the charge amount flowing through the machine side inductor is smaller than a third critical charge amount, the closed-loop control module (3) is used for controlling the bridge type inversion module (1) to be in an O-N intermittent conduction mode, and if the charge amount flowing through the machine side inductor is not smaller than the third critical charge amount, the closed-loop control module (3) is used for controlling the bridge type inversion module (1) to be in a P-O-N critical conduction mode;
in the current switching cycle, the first critical charge amount, the second critical charge amount, and the third critical charge amount are respectively:
Figure FDA0003516635660000011
Figure FDA0003516635660000012
Figure FDA0003516635660000013
wherein Q isL1_BCM1、QL1_BCM2、QL1_BCM3Respectively, a first critical charge amount, a second critical charge amount, a third critical charge amount, TsFor the switching period, E is the DC voltage at the input side of the bridge inverter module (1), L1The inductance value of the machine side inductor ucThe capacitor voltage in the current switching period.
2. The grid-connected inverter with hybrid conduction mode according to claim 1, wherein the controlled model of the LCL filter module (2) is a direct average current model which takes the average current of the machine side inductor as an input quantity, takes the grid-connected current as an output quantity, takes the grid-connected voltage as a disturbance quantity, and is provided with grid-connected voltage feedforward, and the direct average current model is a second-order model; the closed-loop control module (3) takes grid-connected current as a direct control object, performs PID closed-loop control on the LCL filtering module (2), control parameters are determined by the direct average current model, an expected system damping ratio and natural oscillation frequency, and the expression of the direct average current model is as follows:
Figure FDA0003516635660000021
wherein, Ig(s) is the grid-connected current, L2The inductance value of the network side inductor, C is the capacitance value of the capacitor in the LCL filter module (2), RgIs the equivalent resistance of power frequency at network side, s is the physical quantity under continuous domain, IL1_avg(s) is the average current of the machine side inductance, ug(s) is the grid-connected voltage.
3. The grid-connected inverter with hybrid conduction mode according to claim 1, wherein the bridge inverter module (1) is a T-type inverter bridge, a capacitor C1 and a capacitor C2 are arranged between the positive input side and the negative input side of the T-type inverter bridge in series, a switch tube S1 and a switch tube S2 are further connected between the positive input side and the negative input side of the T-type inverter bridge in sequence, and a switch tube S3 and a switch tube S4 which are connected in series in reverse direction are sequentially connected between the connection point of the capacitor C1 and the capacitor C2 and the connection point of the switch tube S1 and the switch tube S2.
4. The grid-connected inverter with hybrid conduction mode as claimed in claim 3, wherein in the P-O discontinuous conduction mode, the switch tube S2 and the switch tube S4 are turned off, the switch tube S3 is kept normally on for the whole switching period, and the duty ratio D of the switch tube S11Comprises the following steps:
Figure FDA0003516635660000022
wherein L is1The inductance value of the machine side inductor ucAnd QL1_avrThe capacitor voltage and the charge quantity, T, flowing through the machine side inductor in the current switching periodsAnd E is the direct current voltage at the input side of the bridge type inversion module (1) in a switching period.
5. The grid-connected inverter with hybrid conduction mode as claimed in claim 3, wherein in the P-O-N critical conduction mode, the switch tube S2 and the switch tube S4 are turned off, and the duty ratio D of the switch tube S11And duty cycle D of switching tube S33Respectively as follows:
Figure FDA0003516635660000031
Figure FDA0003516635660000032
A0＝0.5E
B0＝-(0.5E+uc)
Figure FDA0003516635660000033
wherein A is0Is a coefficient of a quadratic term, B0Is a coefficient of a first order term, C0Is a zero-order coefficient, L1The inductance value of the machine side inductor ucAnd QL1_avrThe capacitor voltage and the charge quantity, T, flowing through the machine side inductor in the current switching periodsAnd E is the direct current voltage at the input side of the bridge type inversion module (1) in a switching period.
6. The grid-connected inverter with hybrid conduction mode as claimed in claim 3, wherein in the O-N discontinuous conduction mode, the switch tube S1, the switch tube S2 and the switch tube S4 are turned off, and the duty ratio D of the switch tube S3 is3Comprises the following steps:
Figure FDA0003516635660000034
wherein L is1The inductance value of the machine side inductor ucAnd QL1_avrThe capacitor voltage and the charge quantity, T, flowing through the machine side inductor in the current switching periodsAnd E is the direct current voltage at the input side of the bridge type inversion module (1) in a switching period.
7. The grid-connected inverter with hybrid conduction mode according to any one of claims 3 to 6, further comprising a sampling module (4) for sampling the grid-connected voltage, the grid-connected current and the capacitor voltage;
the closed-loop control module (3) comprises a differential phase-locked loop, a PID controller and a mode discrimination and calculation unit;
the differential phase-locked loop is used for obtaining phase angle information according to a grid-connected voltage sampling value of the previous switching period, multiplying the phase angle information by a grid-connected current set value and outputting a multiplication result to the PID controller; the PID controller is used for carrying out PID adjustment on a difference value between the multiplication result and the grid-connected current sampling value of the previous switching period and outputting the average current of the side inductor in the current switching period; the mode distinguishing and calculating unit is used for calculating the charge quantity of the machine side inductor according to the average current of the machine side inductor in the current switching period and distinguishing the working mode of the bridge type inversion module (1) according to the charge quantity of the machine side inductor.
8. The grid-connected inverter with hybrid conduction mode according to claim 7, further comprising an input voltage equalizing control module (5) for sampling a difference between the voltages of the capacitor C1 and the capacitor C2, and outputting a corresponding correction coefficient after PI control;
if the grid-connected current set value in the previous switching period is zero, the closed-loop control module (3) is also used for correcting the average current of the machine side inductor in the current switching period by using the correction coefficient, and the mode distinguishing and calculating unit calculates the charge quantity of the machine side inductor according to the corrected average current of the machine side inductor in the current switching period.
9. The grid-connected inverter with hybrid conduction mode according to claim 7, wherein the closed-loop control module (3) further comprises a PWM unit for outputting a driving signal to a switching tube in the T-shaped inverter bridge according to the determination result of the mode determination and calculation unit.
10. The grid-connected inverter with hybrid conduction mode according to claim 7, wherein the mode discrimination and calculation unit is further configured to determine the current switching period according to the capacitor voltage ucInductance L of machine side inductor1、TsAnd E calculating the first, second and third critical charge amounts.
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