CN110649649A - Low-cost voltage-free sensor carrier phase shifting method used under island microgrid - Google Patents

Low-cost voltage-free sensor carrier phase shifting method used under island microgrid Download PDF

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CN110649649A
CN110649649A CN201910809801.5A CN201910809801A CN110649649A CN 110649649 A CN110649649 A CN 110649649A CN 201910809801 A CN201910809801 A CN 201910809801A CN 110649649 A CN110649649 A CN 110649649A
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CN110649649B (en
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何晋伟
董哲
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Tianjin University
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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
    • 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
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • 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
    • H02M7/493—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 the static converters being arranged for operation in parallel
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

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Abstract

The invention discloses a low-cost voltage-free sensor carrier phase shifting method used under an island microgrid. The method realizes power sharing through the parallel droop system, and utilizes the characteristic that the voltage phase angles of the parallel units are the same to formulate a global carrier synchronization signal so as to replace a central controller and a carrier synchronization communication cable, thereby improving the reliability of the parallel operation of the inverters and avoiding stopping working due to the damage of the communication cable. And because the voltage reference amount generated by droop control is approximately equal to the output voltage of the inverter, a voltage sensor for collecting the output voltage of the inverter does not need to be additionally arranged when the inverter is designed, and the equipment cost is saved. According to the invention, on the fundamental frequency, the voltage drop of a circuit is suppressed through the fundamental virtual impedance, the voltage of the parallel connection point of the inverter is tracked without difference, on the harmonic frequency, the specific subharmonic of the circuit is suppressed through the harmonic virtual impedance, and the influence of the nonlinear load on the electric energy quality of the whole system is weakened.

Description

Low-cost voltage-free sensor carrier phase shifting method used under island microgrid
Technical Field
The invention relates to the technical field of low-cost voltage-free sensor carrier phase shifting under an island microgrid, in particular to a method for realizing the discrete control of an inverter and the global synchronization of an inverter carrier in a droop control mode, utilizing virtual impedance to inhibit low-order harmonic waves of a circuit, and inhibiting switching frequency subharmonic waves by a carrier phase shifting technology to improve the electric energy quality of a system.
Background
Micro-grids have been widely accepted as an efficient way to integrate a large number of adjacent distributed power supplies and loads into a main grid. In contrast to the control of a single distributed generation unit (DG), the coordinated operation of the DG units may provide sustained power to critical loads within the grid in the event of a failure of the main grid. For islanding operation of a microgrid, an important task is to achieve proper power sharing among parallel DG units in order to obtain good energy management and avoid underloading or overloading of DG units. In previous studies, real power frequency droop control and reactive power voltage amplitude droop control and variables have been extensively studied. The main advantage of these approaches is that communication between parallel DG units is not required for power sharing. Therefore, the island system can have good robustness and resist long-distance communication faults. However, there are some problems with the conventional droop method, including stability and reactive power sharing problems. On the other hand, multi-party applications regarding droop control have been proposed so far, including virtual impedance control, virtual voltage/frequency control and hierarchical control of low bandwidth communication. However, as far as the authors know, they typically address the specific problem of islanding microgrid power sharing under given operating conditions. Furthermore, implementing these methods typically requires additional information on system parameters or additional measurements.
For each inverter, it is usually equipped with an LC filter with a lower cut-off frequency to sufficiently absorb the switching ripple of the inverter branches. In addition, the above-described power sharing method of virtual impedance assisted droop control is achieved by accurate closed loop tracking of the filter capacitor voltage. However, complex closed loop voltage tracking and virtual impedance control may be associated with more computational load and measurement. In addition, the LC filter for each inverter is expensive, so an L-type filter with a small inductance is chosen for each inverter, as well as a compact interleaved topology and a common capacitor filter for all parallel inverters have been widely studied for high power inverter applications. However, this requires a centralized processor to implement carrier phase shifting to reduce the effects of switching ripple.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provides a low-cost voltage-sensor-free carrier phase shifting method for an island microgrid. The high and low order harmonics in the system are suppressed by virtual impedance compensation and a carrier phase shifting technology, and the quality of electric energy is improved.
The purpose of the invention is realized by the following technical scheme:
the low-cost voltage-free sensor carrier phase shifting method for the island microgrid comprises the following steps of:
(1) the power sharing under the multi-inverter parallel system is realized by using a droop control mode, and the reference voltage V generated by utilizing droop controlrefApproximately equal to the inverter output voltage VoutputBecause the reference voltage is approximately equal to the output voltage of the inverter, a voltage transformer for collecting the output voltage of the inverter is not needed to be installed when the inverter is designed;
(2) obtaining droop voltage generated by droop control approximately equal to the voltage of the common connection point of the inverter according to the virtual impedance characteristic, thereby realizing the global synchronization of the carrier wave of the inverter; the virtual harmonic impedance is utilized to suppress the low-order harmonic of the line, and the influence of the nonlinear load on the line is reduced;
(3) the real-time carrier phase shifting technology is utilized, the global synchronization of the carrier is realized by means of droop control, the switch subharmonic output by the inverter is suppressed, and the power quality is improved.
Further, the step (1) specifically comprises the following steps:
according to the fundamental component I of the inverter output current obtained by samplingL,fAnd a reference voltage VrefCalculating to obtain the active power P output by the systemDGAnd reactive power QDGThe formula is as follows:
PDG＝1/2(Vref·IL,f+Vref_delay·IL,f_delay) (1-1)
QDG＝1/2(Vref·IL,f_delay-Vref_delay·IL,f) (1-2)
wherein IL,f_delayAnd Vref_delayIs IL,fAnd VrefThe quadrature component of (a); then generating a reference voltage V output by the inverter according to the droop controlref(ii) a The formula is as follows:
ωref＝ω*-DP·PDG (1-3)
Eref＝E*-DQ·QDG (1-4)
Figure BDA0002184759020000021
wherein ω is*Is the rated angular frequency, omega, of the inverterrefIs a reference angular frequency, D, generated by inverter controlPIs the active sag coefficient, PDGAnd QDGIs the active power and the reactive power output by the inverter; e*Is the rated output voltage amplitude of the inverter, ErefIs the inverter output voltage reference value, DQIs the reactive droop coefficient;
Figure BDA0002184759020000022
is the voltage reference of the inverter output.
Due to virtual impedance RV,hAnd XV,hOf the inverter, reference voltage VrefAnd
Figure BDA0002184759020000023
is as follows
Figure BDA0002184759020000024
Wherein Δ VVIA voltage drop generated for the virtual impedance; the calculation formula is as follows
Figure BDA0002184759020000031
Wherein R isV,hAnd XV,hIs a virtual impedance of harmonic order, RV,fAnd XV,fIs a virtual impedance of fundamental order, IL,fIs the fundamental sub-current, IL,f_delayIs the quadrature magnitude of the fundamental current; i isL,hIs a harmonic sub-current, IL,h_delayIs the quadrature magnitude of the harmonic current.
Further, the step (2) comprises the following steps:
based on known line parameters, droop control is generatedAnd the output voltage V of the inverteroutput,fThe relationship of (a) to (b) is as follows:
Figure BDA0002184759020000033
at this time, the actual line impedance R is taken into accountline,fAnd Xline,fOf the inverter output voltage Voutput,fVoltage V at point of common connection with inverterPCC,fThe relationship between them is as follows:
VPCC,f＝Voutput,f+ΔVline,f＝Voutput,f+IL,fRline,f-IL,f_delayXline,f (1-9)
because the virtual impedance and the actual physical impedance have RV,f≈-Rline,fAnd XV,f≈-Xline,fThe voltage generated by the inverter droop controlVoltage V at point of common connection with inverterPCC,fApproximately in agreement, therefore
Figure BDA0002184759020000035
Simulating common node voltage VPCC,fAnd the carrier synchronization among the inverters is realized.
Further, the step (3) comprises the following steps: based on the principle of carrier phase shift, on the basis of parallel connection of a plurality of inverters, the carrier phase of each inverter is set by using a PI controller with a dead zone, so that the switching ripples of the inverters interact with each other at the common connection point of the inverters under the condition of keeping the phase of the fundamental wave of the system unchanged, and the suppression effect of the switching subharmonic is effectively realized.
Compared with the prior art, the technical scheme of the invention has the following beneficial effects:
1. the invention realizes power sharing through the parallel droop system, and utilizes the characteristic that the voltage phase angles of the parallel units are the same to formulate a global carrier synchronization signal so as to replace a central controller and a carrier synchronization communication cable, thereby improving the reliability of the parallel operation of the inverters and avoiding stopping working due to the damage of the communication cable. And because the voltage reference amount generated by droop control is approximately equal to the output voltage of the inverter, a voltage sensor for collecting the output voltage of the inverter does not need to be additionally arranged when the inverter is designed, and the equipment cost is saved.
2. Because the nonlinear load has negative influence on the power quality of the system in an island state, the invention inhibits the voltage drop of the line through the fundamental wave virtual impedance on the fundamental wave frequency, tracks the voltage of the parallel connection point of the inverter without difference, and inhibits the specific subharmonic wave of the line through the harmonic wave virtual impedance on the harmonic wave frequency to weaken the influence of the nonlinear load on the power quality of the whole system. The dynamic carrier phase shifting technology is used for inhibiting the system switch subharmonic, because of the characteristic of carrier phase shifting, the influence of inverter switch ripples in a system is reduced, the design difficulty of a passive filter of a high-power low-switching-frequency inverter is greatly reduced, the problem that the difference between the resonant frequency of the filter and the inverter switching frequency is too small is not considered, therefore, the resonant frequency of the filter is allowed to be close to the switching frequency, the method can reduce 70% of the filter capacity, the economy of the whole system is improved, the capacity of a capacitor is reduced, and the reliability of system operation can be improved.
Drawings
Fig. 1 is a block diagram of a multi-inverter parallel control architecture.
Fig. 2 is a control structure diagram of an inverter internal controller.
Fig. 3 is a graph of the interaction of virtual impedance with actual physical resistance at fundamental and harmonic frequencies.
FIG. 4 shows V under the influence of a virtual impedanceoutput,f，
Figure BDA0002184759020000041
And VPCC,fA vector diagram of the relationship of (2).
Fig. 5 is a diagram of adaptive carrier timing control and PWM modulation.
Fig. 6 shows the output voltage and output current of the inverter in the conventional control stage (stage one), the virtual impedance action stage (stage two), and the carrier phase shift stage (stage three).
Fig. 7 shows the total distortion rate variation of the voltage of the inverter in three stages.
Detailed Description
The following describes a specific embodiment of the present invention for a low-cost voltage-free sensor carrier phase shifting technique under an island microgrid with reference to the attached drawings, so as to facilitate those skilled in the art to better understand the present invention.
Based on droop control, the inverter realizes power sharing on the basis of no communication line and depends on a droop control parameter VrefAnd the output voltage V of the inverteroutputThe relationship of (a) replaces a physical voltage sensor; based on
Figure BDA0002184759020000042
And VPCC,fThe carrier synchronization among inverters is realized, low-order harmonic waves in the system are suppressed by utilizing the virtual impedance, and the switching-order harmonic waves of the system are suppressed by utilizing a carrier phase-shifting technology. The distributed control of the multi-inverter with low cost and high power quality is realized.
The invention is used for the low-cost non-voltage sensor carrier phase shifting technology under the island micro-network and comprises the following basic steps:
step 1: in the conventional application of inverters, each inverter is provided with a passive filter to suppress inverter switching harmonics, and the embodiment proposes that the inverters are connected in parallel as shown in fig. 1, wherein the outlet of each inverter is only connected with a filter inductorThe inverter is controlled by a local controller, and a filter capacitor is arranged at a common connection Point (PCC) of the inverter so as to filter out system higher harmonics. Based on droop control, the inverter realizes power sharing on the basis of no communication line and depends on a droop control parameter VrefAnd the output voltage V of the inverteroutputThe relationship of (a) replaces a physical voltage sensor; based onAnd VPCC,fThe relationship (2) realizes carrier synchronization between inverters. According to the fundamental component I of the inverter output current obtained by samplingL,fAnd a reference voltage VrefCalculating to obtain the active power P output by the systemDGAnd reactive power QDGThe formula is as follows:
PDG＝1/2(Vref·IL,f+Vref_delay·IL,f_delay) (1-1)
QDG＝1/2(Vref·IL,f_delay-Vref_delay·IL,f) (1-2)
wherein IL,f_delayAnd Vref_delayIs IL,fAnd VrefThe quadrature component of (a). Then generating a reference voltage V output by the inverter according to the droop controlref. The formula is as follows:
ωref＝ω*-DP·PDG (1-3)
Eref＝E*-DQ·QDG (1-4)
Figure BDA0002184759020000051
wherein ω is*Is the rated angular frequency, omega, of the inverterrefIs a reference angular frequency, D, generated by inverter controlPIs the active sag coefficient, PDGAnd QDGThe active power and the reactive power output by the inverter. E*Is the rated output voltage amplitude of the inverter, ErefIs the inverter output voltage reference value, DQIs reactive sag factor。
Figure BDA0002184759020000052
Is the voltage reference of the inverter output.
Because of the virtual impedance RV,hAnd XV,hOf the inverter, reference voltage VrefAnd
Figure BDA0002184759020000053
is as follows
Figure BDA0002184759020000054
Wherein Δ VVIThe voltage drop generated for the virtual impedance. The calculation formula is as follows
Figure BDA0002184759020000055
Wherein R isV,hAnd XV,hIs a virtual impedance of harmonic order, RV,fAnd XV,fIs a virtual impedance of fundamental order, IL,fIs the fundamental sub-current, IL,f_delayIs the quadrature magnitude of the fundamental current. I isL,hIs a harmonic sub-current, IL,h_delayIs the quadrature magnitude of the harmonic current.
And VrefCan also replace V collected by a voltage transformeroutAnd participates in droop control as a feedback amount. As shown in fig. 2.
Step 2: the virtual impedance suppresses system low order harmonics and achieves inverter carrier global synchronization.
Based on known line parameters, droop control is generatedAnd the output voltage V of the inverteroutput,fThe relationship of (a) to (b) is as follows:
at this time, the actual line resistance is consideredanti-Rline,fAnd Xline,fFig. 3 shows an equivalent circuit of the actual line impedance and the virtual impedance, where the virtual impedance is equivalent to the internal resistance of the controller, and the actual line impedance is cancelled by a negative control coefficient. The voltage variation caused by the virtual impedance and the actual line impedance is shown in FIG. 4, and the inverter output voltage Voutput,fVoltage V at point of common connection with inverterPCC,fThe relationship between them is as follows:
VPCC,f＝Voutput,f+ΔVline,f＝Voutput,f+IL,fRline,f-IL,f_delayXline,f (1-9)
because the virtual impedance has R with the actual physical impedanceV,h≈-Rline,fAnd XV,h≈-Xline,fCan find the voltage generated by the droop control of the inverter
Figure BDA0002184759020000061
Voltage V at point of common connection with inverterPCC,fApproximately coincide, so can use
Figure BDA0002184759020000062
Simulating common node voltage VPCC,fAnd the carrier synchronization among the inverters is realized. Under the effect of the virtual impedance, 3 rd, 5 th and 7 th harmonics of the system are well suppressed as shown in fig. 7.
Step 3, dynamic carrier phase shifting is carried out to inhibit switch subharmonic;
as already mentioned above, droop control occurs after virtual impedance compensationVoltage V to PCC grid-connected pointPCC,fIt is possible to implement a non-biased tracking so that each inverter has local parametersOn-line correction of the carrier phase of inverters, particularly control, for reference, enabling carrier synchronization between inverters without any communication cableThe details are shown in fig. 5. The whole controller carries out data acquisition and calculation on the electric signals of the inverter at the sampling frequency of 24kHz to obtain IDG(k),Vdc(k),PDG(k),QDG(k),Vref(k) And
Figure BDA0002184759020000065
in one aspect, the controller generates droop control
Figure BDA0002184759020000066
Line impedance compensation is performed as shown in part a of FIG. 5, the physical impedance on the line that is not compensated by the virtual impedance is further compensated to estimate the voltage phase angle θ at PCCPCC(k) At this time, the line impedance (R)DGAnd XDG) Upper induced phase angle change Δ θDG(k) The calculation process is as follows:
Figure BDA0002184759020000067
wherein, VrefFor the inverter output voltage, PDGAnd QDGIs the output power of the inverter. Therefore, the voltage phase theta at the PCC can be estimatedPCC(k)：
Figure BDA0002184759020000068
Obtaining the PCC node voltage phase thetaPCC(k) Thereafter, the controller monitors θ using a zero crossing detection modulePCC(k) Zero crossing state, which is important to point out, under discrete controller, the zero crossing point is not the actual zero point due to sampling precision problem, and zero crossing threshold value [ -tau, tau ] is set]When theta isPCC(k) When the inverter carrier falls into a zero-crossing region, the adaptive carrier control system is triggered, and the specific details are shown in a part b in the figure, the inverter carrier is generated by a counter which counts in an Up-down mode (Up-down mode), and the working frequency of the counter is 75 MHz.
Considering that the whole control system is a discrete system, in order to accurately obtain the carrier corresponding to the voltage phase zero-crossing pointPhase position
Figure BDA0002184759020000071
It is also necessary to perform voltage phase zero crossing carrier phase
Figure BDA0002184759020000072
Accurate prediction of (1), wherein
Figure BDA0002184759020000073
The calculation formula is as follows:
Figure BDA0002184759020000074
the controller is based on the preset reference carrier phase angle
Figure BDA0002184759020000075
Carrying out PI control on the carrier frequency, wherein the PI controller comprises a control dead zone, and the carrier frequency can obtain:
Figure BDA0002184759020000076
its output carrier frequencyThe gradual phase shift of the carrier will be realized as the carrier frequency of the next fundamental period. The control details are shown at the bottom of fig. 5 b.
Therefore, the carrier phase angle controls the change of the digital carrier frequency through the PI controller, the adjusting frequency of the carrier frequency is 50Hz, and the dynamic carrier phase shift is indirectly realized. In the next fundamental wave period, the carrier controller will continuously change the carrier frequency and slowly adjust the carrier phase to the reference carrier phase
Figure BDA0002184759020000078
On the other hand, the controller obtains the direct current bus V according to the samplingdc(k) To Vref(k) Performing per unit to generate modulated wave
Figure BDA0002184759020000079
Further, the adaptive carrier modulation generated by the above control generates a gate signal to control the inverter to operate as shown in part c of fig. 5. As shown in fig. 6, it can be found that the inverter output voltage is low and the subharmonic is well suppressed in the second stage, and the switching harmonic is also well suppressed in the output voltage in the third stage. The total distortion rate of the voltage at each stage is also described in detail in fig. 7.
In conclusion, the method can effectively inhibit the current harmonic of the grid-connected point of the inverter on the premise of removing the voltage transformer of the inverter, removing the communication cable between the inverters and reducing the inductance of the filter. The control strategy provided by the invention reduces the production cost, improves the reliability of the parallel operation of the multiple inverters, improves the electric energy quality under an island system, and effectively inhibits low-order harmonic waves and switch-order harmonic waves in a circuit.
The present invention is not limited to the above-described embodiments. The foregoing description of the specific embodiments is intended to describe and illustrate the technical solutions of the present invention, and the above specific embodiments are merely illustrative and not restrictive. Those skilled in the art can make many changes and modifications to the invention without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (4)

1. The low-cost voltage-free sensor carrier phase shifting method used under the island microgrid is characterized by comprising the following steps of:
(1) the power sharing under the multi-inverter parallel system is realized by using a droop control mode, and the reference voltage V generated by utilizing droop controlrefApproximately equal to the inverter output voltage VoutputBecause the reference voltage is approximately equal to the output voltage of the inverter, a voltage transformer for collecting the output voltage of the inverter is not needed to be installed when the inverter is designed;
(2) obtaining droop voltage generated by droop control approximately equal to the voltage of the common connection point of the inverter according to the virtual impedance characteristic, thereby realizing the global synchronization of the carrier wave of the inverter; the virtual harmonic impedance is utilized to suppress the low-order harmonic of the line, and the influence of the nonlinear load on the line is reduced;
(3) the real-time carrier phase shifting technology is utilized, the global synchronization of the carrier is realized by means of droop control, the switch subharmonic output by the inverter is suppressed, and the power quality is improved.
2. The low-cost voltage-free sensor carrier phase shifting method for the island microgrid according to claim 1, characterized in that the step (1) specifically comprises the following steps:
according to the fundamental component I of the inverter output current obtained by samplingL，fAnd a reference voltage VrefCalculating to obtain the active power P output by the systemDGAnd reactive power QDGThe formula is as follows:
PDG＝1/2(Vref·IL，f+Vref_delay·IL，fdelay) (1-1)
QDG＝1/2(Vref·IL，f_delay-Vref_delay·IL，f) (1-2)
wherein IL，f_delayAnd Vref_delayIs IL，fAnd VrefThe quadrature component of (a); then generating a reference voltage V output by the inverter according to the droop controlref(ii) a The formula is as follows:
ωref＝ω*-DP·PDG (1-3)
Eref＝E*-DQ·QDG (1-4)
Figure FDA0002184759010000011
wherein ω is*Is the rated angular frequency, omega, of the inverterrefIs a reference angular frequency, D, generated by inverter controlPIs the active sag coefficient, PDGAnd QDGIs active work output by the inverterRate and reactive power; e*Is the rated output voltage amplitude of the inverter, ErefIs the inverter output voltage reference value, DQIs the reactive droop coefficient;
Figure FDA0002184759010000012
is the voltage reference of the inverter output.
Due to virtual impedance RV，hAnd XV，hOf the inverter, reference voltage VrefAndis as follows
Figure FDA0002184759010000014
Wherein Δ VVIA voltage drop generated for the virtual impedance; the calculation formula is as follows
Figure FDA0002184759010000021
Wherein R isV，hAnd XV，hIs a virtual impedance of harmonic order, RV，fAnd XV，fIs a virtual impedance of fundamental order, IL，fIs the fundamental sub-current, IL，f_delayIs the quadrature magnitude of the fundamental current; i isL，hIs a harmonic sub-current, IL，h_delayIs the quadrature magnitude of the harmonic current.
3. The low-cost voltage-free sensor carrier phase shifting method for the island microgrid according to claim 1, characterized in that the step (2) comprises the following steps:
based on known line parameters, droop control is generatedAnd the output voltage V of the inverteroutput，fThe relationship of (a) to (b) is as follows:
Figure FDA0002184759010000023
at this time, the actual line impedance R is taken into accountline，fAnd Xline，fOf the inverter output voltage Voutput，fVoltage V at point of common connection with inverterPCC，fThe relationship between them is as follows:
VPCC，f＝Voutput，f+ΔVline，f＝Voutput，f+IL，fRline，f-IL，f_delayXline，f (1-9)
because the virtual impedance and the actual physical impedance have Rv，f≈-Rline，fAnd XV，f≈-Xline，fThe voltage generated by the inverter droop control
Figure FDA0002184759010000024
Voltage V at point of common connection with inverterPCC，fApproximately in agreement, therefore
Figure FDA0002184759010000025
Simulating common node voltage VPCC，fAnd the carrier synchronization among the inverters is realized.
4. The low-cost voltage-free sensor carrier phase shifting method for the island microgrid according to claim 1, characterized in that the step (3) comprises the following steps: based on the principle of carrier phase shift, on the basis of parallel connection of a plurality of inverters, the carrier phase of each inverter is set by using a PI controller with a dead zone, so that the switching ripples of the inverters interact with each other at the common connection point of the inverters under the condition of keeping the phase of the fundamental wave of the system unchanged, and the suppression effect of the switching subharmonic is effectively realized.
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