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.
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, V
RMSThere 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.