CN109302094A - Three level parallel inverter of non-isolation type modularization, control method and system - Google Patents
Three level parallel inverter of non-isolation type modularization, control method and system Download PDFInfo
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- CN109302094A CN109302094A CN201811217717.6A CN201811217717A CN109302094A CN 109302094 A CN109302094 A CN 109302094A CN 201811217717 A CN201811217717 A CN 201811217717A CN 109302094 A CN109302094 A CN 109302094A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/539—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
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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Abstract
The invention discloses three level parallel inverter of non-isolation type modularization, control method and systems, realize the inhibition of common mode resonance electric current, circulation and leakage current.Wherein, a kind of three level parallel inverter of non-isolation type modularization, main circuit are I font three-level inverter modular parallel, and filter is modified LCL filter; modified LCL filter changes common mode loop structure, realizes the inhibition of high frequency circulating currents, leakage current.In order to inhibit common mode resonance caused by modified LCL filter, the present invention increases the inhibition that active controller realizes common mode resonance electric current in common mode circuit.The invention also achieves low frequency loop current suppressions, current tracking and the resonance inhibition of the balance control of DC side mid-point voltage and differential mode circuit simultaneously.
Description
Technical Field
The invention belongs to the field of power electronic control, and relates to a non-isolated modular three-level parallel inverter, a control method and a system.
Background
In order to solve the problems of energy crisis and environmental pollution, new energy power generation is more and more widely concerned. Photovoltaic power generation in new energy power generation has the advantages of low carbon, environmental protection and low cost, and is one of the only energy sources in the future world, so that the photovoltaic power generation is vigorously developed. As the photovoltaic power generation power level increases, the I-shaped three-level inverter modular parallel system is widely used by supplying high-quality large current. However, modular parallel connection of inverters causes circulating currents. Meanwhile, in a non-isolated photovoltaic system, leakage current is inevitably generated due to the existence of parasitic capacitance. The circulation and the leakage current increase the system loss, cause the distortion of the grid-connected current, increase the electromagnetic interference and even bring huge potential safety hazards to equipment and operators.
By analyzing and finding that the circulating current includes a high frequency component and a low frequency component, the leakage current contains only the high frequency component. The control method for the high-frequency component in the circulating current is generally realized by changing the carrier wave. The method needs real-time communication between parallel system stages, has large calculation amount and is limited by communication speed and communication system stability. For the problem of suppression of leakage current, a method of changing modulation and changing topology is generally adopted. The essence of both methods is that the suppression of leakage current is achieved by varying the common mode voltage. The modulation method is changed, and the amplitude of the common-mode voltage is suppressed by removing the vector with large common-mode voltage, so that the suppression of the leakage current is realized. However, this method may cause an increase in the Total Harmonic Distortion (THD) of the output waveform, which may deteriorate the system performance. The method for changing the topology realizes the suppression of the common mode voltage by adding an auxiliary switching device or an auxiliary diode to change the structure of a common mode loop. The use of auxiliary devices, however, increases the losses and volume of the system, reducing the efficiency of the system.
Disclosure of Invention
In order to solve the defects of the prior art, a first object of the present invention is to provide a non-isolated three-level parallel inverter, in which an improved LCL filter changes a circulating current loop of a modular three-level parallel inverter by changing a structure of a common mode loop.
The invention discloses a non-isolated three-level parallel inverter, which comprises a main circuit and an LCL filter, and is characterized in that the main circuit is an I-shaped three-level inverter modular parallel circuit; the non-isolated three-level parallel inverters are connected in parallel with an alternating current-direct current bus, and the midpoints of the direct current sides of the modules are connected together;
the capacitance neutral point of the LCL filter is connected with the neutral point of the direct current side to form an improved LCL filter; the improved LCL filter changes the structure of a common-mode loop and realizes the suppression of high-frequency circulation and leakage current.
The invention adopts an improved LCL filter to reduce the high-frequency components of leakage current and circulating current. Compared with the traditional LCL filter, the middle point of the filter capacitor of the improved LCL filter is connected with the middle point of the straight-side capacitor through a lead.
The circulation loop based on the traditional LCL filter is a first-order system, and the circulation loop based on the improved LCL filter is a third-order system. Compared with the traditional LCL filter, the improved LCL filter has better capability of restraining the high-frequency components of the circulating current. Also, the improved LCL filter is more resistant to leakage current than conventional LCL filters.
Because the improved LCL filter changes the structure of the common-mode loop, the common-mode voltage excites common-mode current resonance on the common-mode loop. The common mode current resonance increases leakage current and circulation current, and destroys the output waveform quality and the system efficiency of the modular parallel system.
The improved LCL filter effectively inhibits the high-frequency component and the leakage current of the circulating current by changing the structure of the common-mode loop. The method realizes effective suppression of leakage current and circulation high-frequency components on the premise of hardly increasing cost and not changing control and modulation.
The second purpose of the invention is to provide a control method of a non-isolated three-level parallel inverter.
The control method of the non-isolated three-level parallel inverter only controls the midpoint voltage of the direct current side of the first non-isolated three-level inverter module, and the other control parts of other non-isolated three-level inverter modules in the non-isolated three-level parallel inverter are the same as the first non-isolated three-level inverter module.
Further, in the control process of the first non-isolated three-level inverter module:
proportional control is adopted to realize the balance control of the midpoint voltage at the direct current side;
the suppression of the differential mode loop resonant current is realized by adopting capacitor voltage feedforward control;
the given value of the common mode loop current is 0, and the given value and the common mode loop current are subjected to PI control to realize resonance suppression of the common mode current;
and in a dq coordinate system, the tracking control of the current is realized by adopting PI control, and further the control of the transmission power of the system is realized.
Furthermore, the seven-segment SVPWM modulation is adopted, the action time of the positive and negative small vectors is adjusted in a closed loop mode, and the suppression of common mode resonance current and low frequency circulation current and the control of the midpoint voltage of the direct current side are achieved.
The improved LCL filter has the same characteristics as the traditional LCL filter in a differential mode loop, and has an inherent resonance point, so that the resonance of grid-connected current is caused. The invention adopts capacitor voltage feedforward control to realize the inhibition of the resonant current of the differential mode loop.
In the differential mode loop, the invention adopts capacitor voltage feedforward control to solve the problem of differential mode current resonance. On the other hand, in the differential mode loop, in order to realize tracking control of the current, control of the transmission power is realized by using a PI controller in a dq coordinate system.
The third purpose of the invention is to provide a controller of a non-isolated three-level parallel inverter.
The controller of the non-isolated three-level parallel inverter of the present invention is configured to:
and only the midpoint voltage of the direct current side of the first non-isolated three-level inverter module is controlled, and the other control parts of other non-isolated three-level inverter modules in the non-isolated three-level parallel inverter are the same as the first non-isolated three-level inverter module.
Further, the controller is configured to:
proportional control is adopted to realize the balance control of the midpoint voltage at the direct current side;
the suppression of the differential mode loop resonant current is realized by adopting capacitor voltage feedforward control;
the given value of the common mode loop current is 0, and the given value and the common mode loop current are subjected to PI control to realize resonance suppression of the common mode current;
and in a dq coordinate system, the tracking control of the current is realized by adopting PI control, and further the control of the transmission power of the system is realized.
Further, the controller is configured to:
seven-segment SVPWM modulation is adopted, and the common mode resonance current, the low-frequency circulation current and the control of the midpoint voltage on the direct current side are realized by closed-loop regulation of the action time of the positive and negative small vectors.
The fourth purpose of the invention is to provide a control system of a non-isolated three-level parallel inverter.
The following provides two technical schemes of a control system of a non-isolated three-level parallel inverter:
the first control system of the non-isolated three-level parallel inverter comprises the controller.
A control system for a non-isolated three-level parallel inverter, comprising:
a first controller that realizes balance control of a midpoint voltage on a direct current side by using proportional control;
a second controller that implements suppression of a differential mode loop resonant current using capacitance voltage feedforward control;
the given value of the common-mode loop current of the third controller is 0, and the difference between the given value and the common-mode loop current is subjected to PI control so as to realize resonance suppression of the common-mode current;
the fourth controller adopts PI control to realize current tracking control under the dq coordinate system so as to realize control of system transmission power;
and the first controller, the second controller, the third controller and the fourth controller are all connected with the PWM modulator.
Further, the PWM modulator is configured to:
seven-segment SVPWM modulation is adopted, and the common mode resonance current, the low-frequency circulation current and the control of the midpoint voltage on the direct current side are realized by closed-loop regulation of the action time of the positive and negative small vectors.
Compared with the prior art, the invention has the beneficial effects that:
(1) in order to reduce high-frequency components of leakage current and circulating current, the invention adopts an improved LCL filter;
the circulation loop based on the traditional LCL filter is a first-order system, and the circulation loop based on the improved LCL filter is a third-order system. Compared with the traditional LCL filter, the improved LCL filter has better inhibition capability on the high-frequency component of the circulating current;
also, the improved LCL filter is more resistant to leakage current than conventional LCL filters.
(2) The invention provides a PI controller for realizing resonance suppression of common mode current.
(3) In order to solve the problem of differential mode current resonance, the invention adopts capacitor voltage feedforward control.
(4) In the differential mode loop, in order to realize the tracking control of the current, a PI controller is adopted to realize the control of the transmission power in a dq coordinate system.
(5) The invention can also realize low-frequency circulation suppression, balance control of the midpoint voltage at the direct current side and current tracking and resonance suppression of the differential mode loop;
the control algorithm provided by the invention realizes the suppression of resonance on the premise of not requiring passive damping, can save cost, improves the system conversion efficiency and safety and stability, and has very high practical value.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the application and, together with the description, serve to explain the application and are not intended to limit the application.
FIG. 1 is a single I-shaped three-level non-isolated photovoltaic inverter topology based on an improved LCL filter;
FIG. 2 is a main circuit diagram of a non-isolation type modularized three-level I-shaped parallel inverter based on an improved LCL filter;
FIG. 3 is a block diagram of midpoint voltage control on the DC side of a three-level I-shaped parallel inverter;
FIG. 4 is a control block diagram of a differential mode loop of a three-level inverter;
FIG. 5 is a three-level inverter space vector diagram;
fig. 6 is a control block diagram of a first inverter in a non-isolated modular three-level I-shaped parallel inverter system.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The invention relates to a common-mode resonance current, system circulation current and system leakage current suppression method for a non-isolated module three-level conversion parallel inverter system.
Fig. 1 shows a single I-shaped three-level non-isolated photovoltaic inverter topology based on an improved LCL filter. The topology has 12 switching tubes and 6 clamping diodes, wherein each phase bridge arm has 4 switching tubes and 2 clamping diodes. Two capacitors are connected in series on the direct current side, a neutral point O is formed between the two capacitors, and the middle of two clamping diodes of each phase of bridge arm is connected with the neutral point O, so that the neutral point of the alternating current bridge arm generates zero potential. Obviously, in this circuit topology, the conduction states of the four tubes of each phase are different, and three levels of + Udc/2, -Udc/2 and 0 can be generated. Wherein Udc is a dc-side input voltage value.
Fig. 2 is a main circuit diagram of a non-isolated modular three-level I-shaped parallel inverter based on an improved LCL filter. In the system, the alternating current output end of each three-level module is connected with a power grid, the direct current input ends of the three-level modules are connected together, and the positive direct current bus, the negative direct current bus and the neutral line of each three-level module are connected together.
The improved LCL filter circuit is characterized in that a capacitor common end is connected to a neutral point O of a filter capacitor on the direct current side through a lead, a low-impedance capacitor branch is added in a common-mode loop, the circuit structure of the common-mode loop is changed, the high-frequency component of the circulating current and the leakage current flow back to the inverter through the low-impedance capacitor branch, and the high-frequency component of the circulating current and the leakage current are greatly reduced.
Since the improved LCL filter changes the structure of the common-mode loop, the common-mode voltage excites common-mode current resonance on the common-mode loop. The invention adopts the PI controller to realize the resonance suppression of the common mode current. In particular to collect the three-phase network side current i of each inverter moduleajibjicjAnd bridge arm side current iAjiBjiCj. Obtaining a common-mode loop current iojComprises the following steps:
ioj＝(iAj+iBj+iCj)-(iaj+ibj+icj)
to suppress resonance of the common mode current, the given value of the common mode loop current is 0. And the given value and the common-mode loop current are subjected to difference through a PI controller, so that resonance suppression of the common-mode current is realized. The output of the PI controller may be dzComprises the following steps:
wherein,is a given value of common mode loop current;
kp1and ki1Proportional and integral coefficients of the PI controller, respectively, that suppress resonance of the common mode current.
In order to suppress the low-frequency component of the circulating current, the PI controller realizes suppression of the low-frequency circulating current as a circulating current controller. The circulating current of a modular three-level parallel inverter is defined as:
izj＝iaj+ibj+icj
the output of the PI controller controlling the low frequency component of the circulating current is:
wherein,a given value for controlling the circulating low-frequency current;
kp2and ki2Respectively, a proportional coefficient and an integral coefficient of a PI controller for controlling the low-frequency component of the circulating current.
The balanced control of the midpoint voltage on the dc side is a problem that has to be considered in three-level systems. Since the non-isolated modular three-level parallel inverter system shares the direct-current bus and the midpoint connection, only one of the parallel systems is needed to realize the control of the direct-current bus, and the control of the midpoint voltage balance of the direct-current bus is realized in the first control system. Capacitor voltage U on the DC sidepAnd lower capacitor voltage UnComprises the following steps:
wherein ipAnd inCurrent through the upper and lower capacitors, respectively, assuming C1＝C2C, so the difference between the upper and lower capacitance voltages is:
iothe present invention controls the current flowing through the neutral line using a proportional controller for the purpose of achieving a balanced control of the midpoint voltage by controlling the current flowing through the neutral line. The control block diagram is shown in fig. 3.
Wherein the output of the proportional controller is:
yz＝kp3[(Up-Un)*-(Up-Un)]
wherein (U)p-Un)*The given value of the voltage difference of the upper and lower capacitors;
kp3is the scaling factor of the proportional controller.
The open-loop transfer function of the DC side midpoint voltage control system is as follows:
Tdfor sampling delay time, TsFor a switching period, KPWMIs the gain of the inverter module. Due to TdAnd TsBoth of these stiffness constants are small and can be combined and satisfy the following relationship:
the open loop transfer function can be simplified to:
the above formula shows that the system is a type I system, and the controller is designed according to a typical type I system, so that the system has quick dynamic response performance and steady-state performance. According to a typical type I system design, the parameters of the proportional controller are:
in order to control the transmission power of the system, the current inner loop employs a PI controller in dq coordinate system. In addition, the inherent resonance of the LCL in the differential mode loop can also make the system unstable due to the improved LCL controller, which is also a problem to be solved. The invention adopts an active damping control strategy based on capacitor voltage feedforward to solve the resonance problem of a differential mode loop. The control current of the invention is the output current of the bridge arm side of the inverter, taking the phase A of the inverter as an example, the output current I of the bridge arm sideAOutput voltage U to bridge arm sideANThe transfer function of (a) is:
therefore, the resonant frequency is:
in order to suppress the resonance of the differential mode loop, a capacitor voltage feed forward control is employed. In order to realize the control of the transmission power of the system, the current inner loop adopts a PI controller, so the control block diagram of the differential mode loop is shown in FIG. 4.
From fig. 4, the open loop transfer function of the differential mode loop can be obtained as:
wherein M(s) is 1/KPWMThe open-loop transfer function of the simplified differential mode control system is as follows:
open loop crossover frequency f to suppress adverse effects of high frequency harmonicscAnd the turning frequency of the PI controller is as follows:
wherein k isp4And ki4The proportional system and the integral coefficient of a PI controller in a differential mode control system.
In order to ensure the stability of the system and simultaneously meet the requirements of dynamic performance, the relation satisfied by the turning frequency, the resonance frequency and the cut-off frequency is fL＜fc＜fr. Meanwhile, at the turning frequency, the amplitude of the open-loop transfer function of the differential mode system satisfiesThus, the parameters of the PI controller in the differential mode control system are obtained as follows:
the closed loop transfer function of the differential mode loop is thus obtained as:
obtaining a control bandwidth f of the closed loop systemB2.38kHz, and meets the control requirement.
The on and off of the switch tube is controlled by the modulation module, the input of the modulation module is a three-phase modulation wave signal, and the output is the on and off signal of the switch tube. The invention adopts seven-segment SVPWM modulation. The three-phase three-level inverter has 27 space vectors as shown in fig. 5. According to the difference of the amplitudes of the space voltage vectors, the 27 space voltage vectors are divided into 6 large vectors, 6 medium vectors, 6 pairs of small vectors and 3 zero vectors. The paired small vectors do not change the output voltage, but can be used for the implementation of other control objectives. The control of the midpoint voltage, the common mode resonant current and the low-frequency circulating current on the direct current side is realized by changing the action time of the positive and negative small vectors.
Fig. 6 is a control block diagram of a first inverter in a non-isolated modular three-level I-shaped parallel inverter system. The control block diagram comprises control of a differential mode loop, control of a common mode loop, control of circulating current and control of midpoint voltage on a direct current side. It should be noted that only the first inverter controls the dc-side midpoint voltage, and other inverters in the parallel system do not need to control the dc-side midpoint voltage, and the rest of the control parts are the same as those of the first inverter.
The invention realizes the suppression of common mode resonant current, circulation current and leakage current of the non-isolated modular three-level parallel inverter on the premise of hardly increasing the cost and the control and modulation difficulty. Meanwhile, the invention realizes the resonance suppression of the differential mode loop and the balance control of the midpoint voltage at the direct current side. The invention improves the quality of the output waveform of the system and improves the safety and the stability of the system.
The invention provides a controller of a non-isolated three-level parallel inverter, which is configured to:
and only the midpoint voltage of the direct current side of the first non-isolated three-level inverter module is controlled, and the other control parts of other non-isolated three-level inverter modules in the non-isolated three-level parallel inverter are the same as the first non-isolated three-level inverter module.
In particular, the controller is configured to:
proportional control is adopted to realize the balance control of the midpoint voltage at the direct current side;
the suppression of the differential mode loop resonant current is realized by adopting capacitor voltage feedforward control;
the given value of the common mode loop current is 0, and the given value and the common mode loop current are subjected to PI control to realize resonance suppression of the common mode current;
and in a dq coordinate system, the tracking control of the current is realized by adopting PI control, and further the control of the transmission power of the system is realized.
In particular, the controller is configured to:
seven-segment SVPWM modulation is adopted, and the common mode resonance current, the low-frequency circulation current and the control of the midpoint voltage on the direct current side are realized by closed-loop regulation of the action time of the positive and negative small vectors.
The invention provides two technical schemes of a control system of a non-isolated three-level parallel inverter, which comprise the following steps:
the first control system of the non-isolated three-level parallel inverter comprises the controller.
A control system for a non-isolated three-level parallel inverter, comprising:
a first controller that realizes balance control of a midpoint voltage on a direct current side by using proportional control;
a second controller that implements suppression of a differential mode loop resonant current using capacitance voltage feedforward control;
the given value of the common-mode loop current of the third controller is 0, and the difference between the given value and the common-mode loop current is subjected to PI control so as to realize resonance suppression of the common-mode current;
the fourth controller adopts PI control to realize current tracking control under the dq coordinate system so as to realize control of system transmission power;
and the first controller, the second controller, the third controller and the fourth controller are all connected with the PWM modulator.
Specifically, the PWM modulator is configured to:
seven-segment SVPWM modulation is adopted, and the common mode resonance current, the low-frequency circulation current and the control of the midpoint voltage on the direct current side are realized by closed-loop regulation of the action time of the positive and negative small vectors.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive efforts by those skilled in the art based on the technical solution of the present invention.
Claims (10)
1. A non-isolation type three-level parallel inverter comprises a main circuit and an LCL filter, and is characterized in that the main circuit is an I-shaped three-level inverter modularized parallel circuit; the non-isolated three-level parallel inverters are connected in parallel with an alternating current-direct current bus, and the midpoints of the direct current sides of the modules are connected together;
the capacitance neutral point of the LCL filter is connected with the neutral point of the direct current side to form an improved LCL filter; the improved LCL filter changes the structure of a common-mode loop and realizes the suppression of high-frequency circulation and leakage current.
2. The control method of the non-isolated three-level parallel inverter according to claim 1, wherein only a midpoint voltage on a direct current side of a first non-isolated three-level inverter module is controlled, and the remaining control portions of other non-isolated three-level inverter modules in the non-isolated three-level parallel inverter are the same as the first non-isolated three-level inverter module.
3. The method for controlling the non-isolated three-level parallel inverter according to claim 2, wherein in the process of controlling the first non-isolated three-level inverter module:
proportional control is adopted to realize the balance control of the midpoint voltage at the direct current side;
the suppression of the differential mode loop resonant current is realized by adopting capacitor voltage feedforward control;
the given value of the common mode loop current is 0, and the given value and the common mode loop current are subjected to PI control to realize resonance suppression of the common mode current;
and in a dq coordinate system, the tracking control of the current is realized by adopting PI control, and further the control of the transmission power of the system is realized.
4. The method for controlling the non-isolated three-level parallel inverter according to claim 3, wherein a seven-segment SVPWM modulation is adopted, and the common-mode resonant current, the low-frequency ring current suppression and the direct-current side midpoint voltage control are realized by adjusting the action time of positive and negative small vectors in a closed loop.
5. A controller based on the non-isolated three-level parallel inverter of claim 1, wherein the controller is configured to:
and only the midpoint voltage of the direct current side of the first non-isolated three-level inverter module is controlled, and the other control parts of other non-isolated three-level inverter modules in the non-isolated three-level parallel inverter are the same as the first non-isolated three-level inverter module.
6. The controller of a non-isolated three-level parallel inverter of claim 5, wherein the controller is configured to:
proportional control is adopted to realize the balance control of the midpoint voltage at the direct current side;
the suppression of the differential mode loop resonant current is realized by adopting capacitor voltage feedforward control;
the given value of the common mode loop current is 0, and the given value and the common mode loop current are subjected to PI control to realize resonance suppression of the common mode current;
and in a dq coordinate system, the tracking control of the current is realized by adopting PI control, and further the control of the transmission power of the system is realized.
7. The controller of the non-isolated three-level parallel inverter of claim 6, wherein the controller is configured to:
seven-segment SVPWM modulation is adopted, and the common mode resonance current, the low-frequency circulation current and the control of the midpoint voltage on the direct current side are realized by closed-loop regulation of the action time of the positive and negative small vectors.
8. A control system for a non-isolated three-level parallel inverter, comprising a controller according to any one of claims 5-7.
9. A control system of a non-isolated three-level parallel inverter is characterized by comprising:
a first controller that realizes balance control of a midpoint voltage on a direct current side by using proportional control;
a second controller that implements suppression of a differential mode loop resonant current using capacitance voltage feedforward control;
the given value of the common-mode loop current of the third controller is 0, and the difference between the given value and the common-mode loop current is subjected to PI control so as to realize resonance suppression of the common-mode current;
the fourth controller adopts PI control to realize current tracking control under the dq coordinate system so as to realize control of system transmission power;
and the first controller, the second controller, the third controller and the fourth controller are all connected with the PWM modulator.
10. The control system of a non-isolated three-level parallel inverter of claim 9, wherein the PWM modulator is configured to:
seven-segment SVPWM modulation is adopted, and the common mode resonance current, the low-frequency circulation current and the control of the midpoint voltage on the direct current side are realized by closed-loop regulation of the action time of the positive and negative small vectors.
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