CN113013921A - Virtual oscillator improvement method applied to three-phase grid-connected inverter - Google Patents
Virtual oscillator improvement method applied to three-phase grid-connected inverter Download PDFInfo
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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
- H02J3/381—Dispersed generators
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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
- H02J3/01—Arrangements for reducing harmonics or ripples
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
- H02J3/241—The oscillation concerning frequency
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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Abstract
The invention discloses a virtual oscillator improvement method applied to a three-phase grid-connected inverter, wherein the virtual oscillator comprises an oscillating capacitor, an inductor, a resistor, a nonlinear current source and a controlled current source, wherein the inductor, the resistor, the nonlinear current source and the controlled current source are connected with the oscillating capacitor in parallel, and the virtual oscillator improvement method mainly comprises the following steps: step 1: determining an oscillating capacitor C, an inductor L, a resistor R and a current gain K of a virtual oscillator according to the grid-connected requirement of the inverteriAnd a voltage gain Kv(ii) a Step 2: the non-linear current source of the virtual oscillator is designed to reduce the third harmonic in the output voltage of the virtual oscillator. Based on the technical scheme, the virtual oscillator improvement method applied to the three-phase grid-connected inverter can effectively solve the problem of abundant third harmonic in the traditional virtual oscillator, and can also solve the problem of abundant third harmonic in the traditional virtual oscillatorThe droop characteristic of the grid-connected inverter is not influenced, the quality of the grid-connected current is improved, and the operation effect of the grid-connected inverter is optimized.
Description
The technical field is as follows:
the invention belongs to the field of micro-grid control, and particularly relates to an improvement method of a virtual oscillator applied to a three-phase grid-connected inverter.
Background art:
in recent years, distributed power generation and renewable energy sources such as wind energy and solar energy have attracted much attention. Distributed power generation devices play an important role in improving the quality of electric energy and reducing the transmission loss of electric power systems. Most distributed generation installations require a DC/AC inverter (voltage source inverter) which is connected to the grid by a Point of Common Coupling (PCC). In grid-tie control, the control target of the voltage source inverter is the output voltage of the inverter, not the current injected into the grid. A microgrid typically contains a plurality of DC/AC inverters connected in parallel. Therefore, the entire DC/AC inverter system must provide and maintain the voltage and frequency of the microgrid.
Droop control is a classical control method applicable to voltage source grid-connected inverters in alternating current micro-grids. The goal of droop control is to simulate the droop out characteristics of a virtual synchronous machine by allowing deviations from nominal voltage and frequency to achieve power balance. In resistive networks, active power is closely related to voltage amplitude, while reactive power is related to phase angle.
Virtual oscillator control is an emerging voltage source inverter control technique that can simulate the dynamics of limit cycle oscillators, such as Van der Pol oscillators. The nonlinear dynamics of the virtual oscillator inherently embed the droop characteristics. Meanwhile, the virtual oscillator reacts to the instantaneous current without other filters or power calculation, and has a fast transient response characteristic. Another significant feature of the virtual oscillator is that, unlike droop control, the virtual oscillator does not require additional voltage and current tracking loops, thereby simplifying the controller structure.
Although the existing virtual oscillator shows very good dynamic response, the output voltage of the virtual oscillator always contains rich third harmonic, and a considerable third harmonic current is generated in the grid-connected current in a grid-connected mode, so that the grid-connected operation effect of the inverter is seriously affected, and the application of the existing virtual oscillator is still limited.
The invention content is as follows:
in view of this, the invention aims to solve the problem that the output voltage of a traditional virtual oscillator in a three-phase grid-connected inverter has abundant third harmonic waves, improve the quality of grid-connected current and optimize the grid-connected operation effect of the inverter.
The idea of the invention is as follows: firstly, parameters of a virtual oscillator are determined according to grid connection requirements of an inverter, then reasons of existence of third harmonic in output voltage of the traditional virtual oscillator are analyzed, and current sources of the virtual oscillator are improved according to the requirements of harmonic and response speed of the inverter.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a virtual oscillator improvement method applied to a three-phase grid-connected inverter comprises an oscillation capacitor, an inductor, a resistor, a nonlinear current source and a controlled current source, wherein the inductor, the resistor, the nonlinear current source and the controlled current source are connected with the oscillation capacitor in parallel, and the virtual oscillator improvement method mainly comprises the following steps:
step 1: determining an oscillating capacitor C, an inductor L, a resistor R and a current gain K of a virtual oscillator according to the grid-connected requirement of the inverteriAnd a voltage gain Kv；
The oscillation capacitor C, the inductor L and the current gain KiAnd a voltage gain KvThe following formula is satisfied:
wherein, ω is*Is the grid voltage frequency, VocIs the maximum value of the allowed voltage, V, of the inverterminIs the minimum value of the inverter let-through voltage, PrateIs the inverter output rated power;
step 2: designing a nonlinear current source of the virtual oscillator to reduce third harmonic in the output voltage of the virtual oscillator;
the current i on the side of the power gridL2abcI after abc-alpha beta coordinate changeα(t) as input to a controlled current source, said controlled current sourceCurrent source is Ki*iα；
The dynamic response of the virtual oscillator is determined by the following equation:
the non-linear current source in a conventional virtual oscillator is determined by the following equation:
wherein cos3The term is the main cause of the presence of the third harmonic in the output voltage of a conventional virtual oscillator, which satisfies the following formula:
therefore, the nonlinear current source determined by equation (5) can be simplified and improved, and the nonlinear current source of the improved virtual oscillator is determined by the following equation:
the virtual oscillator output voltage v (t) is determined by the formula:
solving the joint formulas (1) and (2) to obtain a voltage amplitude VRMSAnd frequency ω is as follows:
preferably, in order to meet the performance requirement of the inverter, the selection range of the oscillator capacitance C is 0.1759F ≦ C ≦ 0.2031F.
Based on the technical scheme, the virtual oscillator improvement method applied to the three-phase grid-connected inverter can effectively solve the problem of abundant third harmonic waves in the traditional virtual oscillator, does not affect the droop characteristic of the traditional virtual oscillator, improves the quality of grid-connected current, and optimizes the operation effect of the grid-connected inverter.
Drawings
FIG. 1 is a circuit topology diagram of a three-phase grid-connected inverter controlled by a virtual oscillator according to the present invention;
FIG. 2 is a schematic diagram of a conventional virtual oscillator circuit;
FIG. 3 is a schematic diagram of a virtual oscillator circuit of the present invention;
FIG. 4(a) is a graph of the FFT spectrum of a conventional virtual oscillator output voltage;
FIG. 4(b) is a graph of FFT spectrum of the network current of a conventional virtual oscillator;
FIG. 5(a) is a graph of the FFT spectrum of the output voltage of the virtual oscillator of the present invention;
FIG. 5(b) is a FFT spectrogram of the net current of the virtual oscillator of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The embodiments of the present invention, and all other embodiments obtained by a person of ordinary skill in the art without any inventive work, belong to the scope of protection of the present invention.
Referring to fig. 1-3, fig. 1 is a circuit topology diagram of a three-phase grid-connected inverter controlled by a virtual oscillator according to the present invention, fig. 2 is a schematic diagram of a conventional virtual oscillator circuit, and fig. 3 is a schematic diagram of a virtual oscillator circuit according to the present invention.
A virtual oscillator improvement method applied to a three-phase grid-connected inverter comprises an oscillation capacitor, an inductor, a resistor, a nonlinear current source and a controlled current source, wherein the inductor, the resistor, the nonlinear current source and the controlled current source are connected with the oscillation capacitor in parallel, and the virtual oscillator improvement method mainly comprises the following steps:
step 1: according to the grid connection and performance requirements of the inverter, an appropriate oscillating capacitor C, an inductor L, a resistor R and a current gain K of the virtual oscillator are selectediAnd a voltage gain Kv；
According to the requirement of the ministry of China's electric power industry, the fundamental frequency of the voltage of the power grid is 50Hz, the specific formula of the capacitor C and the inductor L of the oscillator is as follows:
wherein, ω is*Is the grid voltage frequency.
In order to meet the performance requirement of the inverter, the selection range of the oscillator capacitor is as follows: 0.1759F is less than or equal to C is less than or equal to 0.2031F.
Current gain KiAnd a voltage gain KvShould be selected to satisfy the following conditions:
the inverter allows the deviation of the permitted voltage not to exceed 5% of the nominal voltage, i.e. V, in response to the requirements of the ministry of the China's electric industryoc＝(1+5％)Vrate，Vmin＝(1-5％)Vrate。
Wherein, PrateIs the rated power, V, output by the inverterocIs the maximum value of the allowed voltage, V, of the inverterminIs the minimum value of the inverter allowed voltage;
step 2: analyzing the reason of the third harmonic in the output voltage of the traditional virtual oscillator, and improving the current source of the virtual oscillator according to the harmonic and response speed requirements of the inverter so as to reduce the third harmonic in the output voltage of the virtual oscillator;
the current i on the side of the power gridL2abcI after abc-alpha beta coordinate changeα(t) as input to a controlled current source, said controlled current source being Ki*iα；
The dynamic response of the virtual oscillator is determined according to the following equation:
thus, the output voltage v (t) of the virtual oscillator and the non-linear current source is(t) correlating.
The non-linear current source in the conventional virtual oscillator is determined by the following formula:
by simplifying the trigonometric function, cos is found3The term is the main cause of the presence of third harmonics in the oscillator output voltage:
therefore, in the improved non-linear current source, cos is added3(ω t + θ (t)) is replaced with 3/4cos (ω t + θ (t)). The improved non-linear current source is determined by the following formula:
substituting equation (17) into equations (13), (14) yields:
the individual harmonic content of the oscillator output voltage can be approximated by multi-scale and perturbation analysis methods. Convert equation (18) to
In time coordinates, then equation (18) can be rewritten as:
where ε → 0.
The approximate solution of the above equation is decomposed into the sum of the components on two time scales:
v(τ)＝v(τ0,ε)≈v0(τ0,τ1)+εv1(τ0,τ1) (20)
wherein tau is0Is the original time scale, and1＝ετ0is a slower time scale.
Substituting the formula (20) into the formula (19) to obtain
In order for the equation to hold, the parts in parentheses must be 0.
As can be seen from (22) and (23), the virtual oscillator of the present invention does not have a third harmonic term on both time scales. Conventional virtual oscillator low time scale component v1As shown in the following equation:
it can be seen from equations (23) and (24) that the lower time scale v in the virtual oscillator of the present invention is achieved by replacing the non-linear current source of the oscillator1The approximate solution above does not contain the third harmonic term.
The virtual oscillator can be applied to the grid-connected inverter because the nonlinear dynamic characteristic of the virtual oscillator can simulate the drooping-like behavior, thereby realizing the power distribution of the load. The virtual oscillator of the present invention must therefore exhibit droop-like characteristics.
Substituting equation (8) into equation (3) yields equation (25):
from equation (25), equation (26) can be derived,
substituting equation (26) into equations (13) and (14) can obtain voltage VRMS(t) and the derivative equation of the phase θ (t):
solving equations (27) and (28) can obtainTo a voltage amplitude VRMSAnd frequency ω:
when P is 0, VRMSThere is a maximum value
Voltage V according to equation (29)RMSThe relationship with power P can be further written in the form:
that is, equation (29) can be written as a droop-like characteristic:
VRMS(t)*Kv＝Voc+mPP(t) (32)
similarly, equation (30) can also be written as a similar droop characteristic:
ω＝ω*+mQQ(t) (33)
wherein m isQIs the reactive droop coefficient:
in a simulation experiment, the superiority of the control method provided by the invention is highlighted by comparing control strategies, and the invention respectively adopts the following two methods for comparison, namely:
the method comprises the following steps: an LCL grid-connected inverter controlled based on a traditional virtual oscillator;
the second method comprises the following steps: the invention provides a control method.
The experiment verifies the effectiveness and superiority of the method provided by the invention through a three-phase 3kW LCL grid-connected inverter system and a simulation experiment by comparing control methods;
referring to fig. 1-3, fig. 1 is a schematic circuit diagram of a three-phase grid-connected inverter controlled by a virtual oscillator, fig. 2 is a schematic circuit diagram of a conventional virtual oscillator, and fig. 3 is a schematic circuit diagram of a virtual oscillator according to the present invention.
Wherein L is1,L2And C is the LCL filter parameter, LgIs the grid inductance. v (t) is the virtual oscillator output voltage, vPCC1Is the phase voltage at the point of common coupling of the inverter system. v. ofgIs an equivalent power grid, the effective value of the voltage of the power grid is 120V, and the fundamental wave frequency f of the power grid0Is 50Hz, and the DC side voltage U of the inverterdc350V, inverter switching frequency fsIs 10 kHz. The system adopts SVPWM (voltage space vector PWM) modulation and grid current feedback control. The parameters of the system are shown in tables 1 and 2.
TABLE 1
Virtual oscillator parameters
TABLE 2
Inverter parameters
When the method one is adopted, the FFT spectrum of the output voltage of the conventional virtual oscillator is shown in fig. 4 (a). It can be seen that the output voltage of a conventional virtual oscillator has an undesirable third harmonic component. In the grid-connected mode, the third harmonic voltage in the oscillator output voltage generates a larger third harmonic current in the grid-connected side current. The FFT spectrum of the grid-connected current is shown in fig. 4(b), and the third harmonic in the grid-connected current is close to 4%, which seriously reduces the power quality of the power grid.
When the second method is adopted, the inverter is controlled by the virtual oscillator provided by the invention, and the FFT spectrum of the output voltage of the virtual oscillator provided by the invention is shown in fig. 5(a), so that the third harmonic component in the output voltage of the virtual oscillator provided by the invention is greatly reduced. In the grid-connected operation, the FFT spectrum of the grid-connected current is shown in fig. 5 (b). By using the virtual oscillator of the invention, the third harmonic component in the network-access current is reduced from 4% to 0.2%, which greatly improves the waveform of the network-injection current and is beneficial for connecting the DC/AC inverter to the network.
According to the comparison of the two methods, the virtual oscillator improvement method applied to the three-phase grid-connected inverter can effectively solve the problem that abundant third harmonic waves exist in the output voltage of the traditional virtual oscillator, improve the quality of grid-connected current and optimize the grid-connected operation effect of the inverter.
Claims (2)
1. The virtual oscillator improvement method applied to the three-phase grid-connected inverter is characterized by mainly comprising the following steps of:
step 1: determining an oscillating capacitor C, an inductor L, a resistor R and a current gain K of a virtual oscillator according to the grid-connected requirement of the inverteriAnd a voltage gain Kv；
The oscillation capacitor C, the inductor L and the current gain KiAnd a voltage gain KvThe following formula is satisfied:
wherein, ω is*Is the grid voltage frequency, VocIs the maximum value of the allowed voltage, V, of the inverterminIs the minimum value of the voltage allowed to pass by the inverter,Prateis the inverter output rated power;
step 2: determining a nonlinear current source of the virtual oscillator to reduce third harmonics in the output voltage of the virtual oscillator;
the current i on the side of the power gridL2abcI after abc-alpha beta coordinate changeα(t) as input to a controlled current source, said controlled current source being Ki*iα；
The dynamic response of the virtual oscillator is determined by the following equation:
the non-linear current source in a conventional virtual oscillator is determined by the following equation:
wherein cos3The term is the main cause of the presence of the third harmonic in the output voltage of a conventional virtual oscillator, which satisfies the following formula:
therefore, the determination formula of the nonlinear current source in the traditional virtual oscillator can be simplified and improved, and the nonlinear current source of the improved virtual oscillator is determined by the following formula:
the virtual oscillator output voltage v (t) is determined by the formula:
joint formula
Solving to obtain a voltage amplitude VRMSAnd frequency ω is as follows:
2. the method of claim 1, wherein the oscillator capacitance C is selected from the range of 0.1759F C0.2031F.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN113452040A (en) * | 2021-07-16 | 2021-09-28 | 中国科学院电工研究所 | Nonlinear virtual oscillator control method of three-phase grid-connected converter |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| US20170316135A1 (en) * | 2016-04-29 | 2017-11-02 | Alliance For Sustainable Energy, Llc | Virtual oscillator control |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| US20170316135A1 (en) * | 2016-04-29 | 2017-11-02 | Alliance For Sustainable Energy, Llc | Virtual oscillator control |
Non-Patent Citations (2)
| Title |
|---|
| BRAIN B. JOHNSON 等: ""Synthesizing Virtual Oscillators to Control Islanded Inverters"", 《IEEE TRANSACTIONS ON POWER ELECTRONICS 》, vol. 31, no. 8, 3 November 2015 (2015-11-03), pages 6002 - 6015 * |
| SIYI LUO 等: ""A Novel Third-Harmonic Elimination Method for VOC-Based Three-Phase DC/AC Inverter"", 《 IECON 2020 THE 46TH ANNUAL CONFERENCE OF THE IEEE INDUSTRIAL ELECTRONICS SOCIETY》, 18 November 2020 (2020-11-18), pages 3170 - 3175 * |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113452040A (en) * | 2021-07-16 | 2021-09-28 | 中国科学院电工研究所 | Nonlinear virtual oscillator control method of three-phase grid-connected converter |
| CN113452040B (en) * | 2021-07-16 | 2022-09-13 | 中国科学院电工研究所 | Nonlinear virtual oscillator control method of three-phase grid-connected converter |
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