A transient-free control method for a VOC-based three-phase grid-connected inverter

Abstract

This invention discloses a transient-free control method for a three-phase grid-connected inverter based on Virtual Oscillator Control (VOC). The method includes sampling the three-phase grid-connected current of the inverter and performing a three-phase / two-phase stationary coordinate transformation. The two-phase grid-connected current command values ​​are calculated based on the active power command and reactive power command values. A second-order inductor-capacitor oscillator circuit is used to obtain the inverter control modulation wave. The steady-state operating point is approximated based on the power command value, filter inductance, and equivalent resistance. The inverter voltage output signal for the next cycle is calculated. An oscillator operating at the ideal steady-state point is used to couple and synchronize the VOC output modulation wave. A superimposed voltage is calculated. The inverter voltage output signal for the next cycle and the superimposed voltage are added to obtain the voltage command signal, which is then modulated by PWM to generate a switching signal to drive the switching transistors. This invention effectively reduces transient processes after changes in operating conditions, improves transient current waveforms, and has strong versatility.

Description

Technical Field

[0001] This invention belongs to the field of three-phase grid-connected inverter control technology, specifically a transient-free control method for three-phase grid-connected inverters based on VOC. Background Technology

[0002] Traditional grid stability and inverter synchronization rely on synchronous generators. Inverters are controlled in a "grid-following" manner, adjusting the current injected into the grid to track the power command value. However, in a pure inverter grid without synchronous generators, this control strategy cannot maintain stability and global synchronization. Virtual Oscillation Control (VOC) is a newly emerging control method for inverters in recent years. Inverters under this control can not only achieve power dispatching but also function as a controllable voltage source, suitable for both grid-connected and islanded modes. VOC incorporates the characteristics of traditional droop control without requiring a phase-locked loop (PLL), needing only grid current information, enabling completely communication-free control and showing broad application prospects in the inverter field.

[0003] However, traditional VOC control struggles to balance the contradiction between transient processes and steady-state performance, making it difficult to simultaneously achieve fast power command tracking speed and small steady-state frequency error. Summary of the Invention

[0004] The purpose of this invention is to provide a transient-free control method for a three-phase grid-connected inverter based on VOC.

[0005] A transient-free control method for a three-phase grid-connected inverter based on VOC, provided by the present invention, includes:

[0006] The grid-connected three-phase current of the inverter is sampled and transformed by abc / αβ to obtain the grid-connected three-phase current;

[0007] Calculate the grid-connected current command value based on the active power command value and the reactive power command value;

[0008] The virtual oscillator is controlled according to the grid-connected current command value. The capacitor voltage and inductor current of the virtual oscillator are collected and amplified as the inverter grid-connected voltage.

[0009] Calculate the phase angle difference between the inverter output voltage and the grid voltage based on the active power command value, reactive power command value and three-phase grid-connected inverter parameters;

[0010] Based on the calculated steady-state data, an oscillator operating at the ideal steady-state point is virtually generated, which couples to the VOC output modulation wave, thereby enabling rapid switching of the controller output state.

[0011] The zero-crossing point of the current after the change in operating conditions is selected as the update time of the voltage signal, and the coupling effect is applied from the beginning.

[0012] Select the superimposed voltage amplitude based on the DC power supply margin, and calculate the superimposed voltage application time;

[0013] The voltage command signal is obtained by adding the inverter voltage output signal of the next cycle and the superimposed voltage. After PWM modulation, the switching transistor on / off signal is generated to drive the switching transistor.

[0014] Preferably, the inverter grid-connected three-phase current i abc The coordinate transformation matrix for sampling and performing the abc / αβ transformation is:

[0015]

[0016] Preferably, the grid-connected current command value is calculated based on the active power command value and the reactive power command value, using the following formula:

[0017]

[0018] Among them, v α and v β Indicates the inverter's grid connection voltage. P * Q is the active power command value. * This is the reactive power command value.

[0019] Preferably, the virtual oscillator includes an inductor L, a capacitor C, and a controlled voltage source v. m Controlled voltage source v u Controlled current source i m Controlled current source i u .

[0020] An inductor L is connected in series with a controlled voltage source v m and a controlled voltage source v u Then, a capacitor C and a controlled current source i are connected in parallel. m A controlled current source i u .

[0021] Preferably, the method for controlling the virtual oscillator according to the grid-connected current command value, and for collecting and amplifying the capacitor voltage and inductor current of the virtual oscillator as the inverter grid-connected voltage is as follows:

[0022] The grid-connected three-phase current i α i β With grid-connected current command value and The error is amplified by K after being processed by the rotation matrix. i We obtain u1 and u2, where u1 serves as the controlled current source i. u The current u2, after passing through the coefficient ε, becomes the controlled voltage source v.u The voltage;

[0023] Collect the voltage u of capacitor C of the second-order LC oscillator C and inductor current i L As state variables x1 and x2, the controlled voltage source v is calculated based on the state variables. m and controlled current source i m The signal, a state variable, is amplified to a fixed amplitude V. n As the inverter voltage command v α v β .

[0024] Preferably, the coefficient ε and the controlled voltage source v m and controlled current source i m Specifically:

[0025]

[0026] Where, k i X represents the amplification factor, ω is the power grid angular frequency, x1 and x2 are state variables, and X... n Let |x|| be the nominal value of the state variable, |x|| be the modulus of the state variable, and ξ be a constant.

[0027] Preferably, the active power command value P is used. * and reactive power command value Q * The phase angle difference between the inverter output voltage and the grid voltage is obtained by approximating the circuit parameters. The specific formula is as follows:

[0028]

[0029] Among them, V g R is the grid voltage amplitude, R is the circuit equivalent resistance, and L is the voltage level. eq It is the sum of the filter inductance and the grid-side inductance.

[0030] Preferably, the inverter voltage output signal v′ for the next cycle is calculated. αβ The corresponding formula is:

[0031]

[0032] v' αβ =V n sin(wt+δ * );

[0033] Where, δ * P represents the next steady-state phase angle difference, δ0 represents the current steady-state phase angle difference, and P represents the next steady-state phase angle difference. * P0 represents the next steady-state power command, while P1 represents the current steady-state power command.

[0034] Preferably, the feedback applied to the output modulated wave by the ideal steady-state oscillator is calculated based on the Kuramoto model, and the corresponding formula is:

[0035]

[0036] Where θ1 is the phase of the oscillator in AHO, and θ2 is the phase of the oscillator operating in the desired state. K represents the coupling coefficient, Wnom is the rated frequency, and ΔV' α and ΔV' β These represent the coupling effect exerted by the ideal oscillator on the output modulated wave.

[0037] Preferably, the duration Δt of the superimposed voltage is calculated based on the instantaneous current value at the time of voltage signal update in each cycle, and the corresponding formula is:

[0038]

[0039] Among them, u add To superimpose the voltage amplitude, I0 is the instantaneous current value at the time of voltage signal update, and L... f For the filter inductor, R f This is the filter resistor.

[0040] Compared with the prior art, the present invention has the following significant advantages: by calculating the ideal steady-state value, the present invention uses coupling to quickly make the modulated wave approach the ideal steady state, and then by accurately controlling the time of the change of the modulated wave and applying an appropriate intensity and time superposition voltage, the transient process can be effectively reduced, the current response speed can be increased, and the invention has strong versatility.

[0041] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0042] Figure 1 This invention relates to a three-phase grid-connected inverter topology based on VOC.

[0043] Figure 2 This is a control block diagram for VOCs.

[0044] Figure 3 This is a schematic diagram of the coupling effect of an ideal oscillator.

[0045] Figure 4 This is a control block diagram for non-transient VOCs.

[0046] Figure 5 This is a comparison chart of active power waveforms with and without the transient-free control algorithm.

[0047] Figure 6 This is a comparison chart of current waveforms with and without the transient control algorithm. Detailed Implementation

[0048] The invention will now be described in detail with reference to specific examples.

[0049] To more clearly describe the objectives, technical solutions, and advantages of the present invention, specific embodiments are illustrated through examples and accompanying drawings. Obviously, the described embodiments are only used to explain the present invention and are not limited to it.

[0050] A transient-free control method for a three-phase grid-connected inverter based on VOC, the method specifically includes:

[0051] Step 1: Sample the three-phase current of the inverter connected to the grid and perform an abc / αβ transformation, i.e., obtain the three-phase current signal i. abc After αβ transformation, the grid-connected three-phase current value i is obtained. α and i β , ;

[0052] In a further embodiment, coordinate transformation: for the inverter grid-connected three-phase current i abc Sampling is performed, followed by an abc / αβ transformation. The coordinate transformation matrix is ​​as follows:

[0053]

[0054] Step 2: Calculate the grid-connected current command value based on the active power command value and the reactive power command value;

[0055] According to the active power command value P * and reactive power command value Q * The formula for calculating the grid-connected current command value is as follows:

[0056] Among them, v α and v β Indicates the inverter's grid connection voltage.

[0057] Step 3: Control the virtual oscillator according to the grid-connected current command value, collect the capacitor voltage and inductor current of the virtual oscillator and amplify them as the inverter grid-connected voltage;

[0058] The virtual oscillator includes an inductor L, a capacitor C, and a controlled voltage source v. m Controlled voltage source v u Controlled current source i m Controlled current source i u .

[0059] An inductor L is connected in series with a controlled voltage source v m and a controlled voltage source v uThen, a capacitor C and a controlled current source i are connected in parallel. m A controlled current source i u .

[0060] Control of the virtual oscillator: The grid-connected three-phase current i α i β With grid-connected current command value and The error is amplified by K after being processed by the rotation matrix. i We obtain u1 and u2, where u1 serves as the controlled current source i. u The current u2, after passing through the coefficient ε, becomes the controlled voltage source v. u The voltage; the voltage u of capacitor C of the second-order LC oscillator is collected. C and inductor current i L As state variables x1 and x2, the controlled voltage source v is calculated based on the state variables. m and controlled current source i m The signal, a state variable, is amplified to a fixed amplitude V. n As the inverter voltage command v α v β .

[0061] In a further embodiment, the coefficient ε and the controlled voltage source v m and controlled current source i m It is given by the following formula:

[0062]

[0063] Where k i X represents the amplification factor, ω is the power grid angular frequency, x1 and x2 are state variables, and X... n Let |x|| be the nominal value of the state variable, |x|| be the modulus of the state variable, and ξ be a constant.

[0064] Step 4: Calculation of approximate steady-state value of the system: via power command P * Q * The phase angle difference between the inverter output voltage and the grid voltage is obtained by approximating the circuit parameters, and the corresponding formula is:

[0065]

[0066] Where V g R is the grid voltage amplitude, R is the circuit equivalent resistance, and L is the voltage level. eq It is the sum of the filter inductance and the grid-side inductance.

[0067] Step 5: Select the same zero-crossing point of the current in each cycle as the update time of the voltage signal, and compare the actual active power P with the active power command P. * The error is compared and then processed through the proportional element K.p Then, the voltage phase angle θ for the next cycle is obtained. Based on the voltage phase angle θ for the next cycle, the inverter voltage output signal v' for the next cycle is calculated. αβ The specific formula is as follows:

[0068] v′ αβ =V n sin(wt+δ * );

[0069] In a further embodiment, the proportional element is determined by the approximate steady-state value of the phase angle difference, specifically by the following formula:

[0070]

[0071] Where δ * P represents the next steady-state phase angle difference, δ0 represents the current steady-state phase angle difference, and P represents the next steady-state phase angle difference. * P0 represents the next steady-state power command, while P1 represents the current steady-state power command.

[0072] Step 6: Apply coupling feedback to the output modulated wave using an oscillator operating in an ideal steady state. The corresponding formula is:

[0073]

[0074] Where θ1 is the phase of the oscillator in AHO, and θ2 is the phase of the oscillator operating in the desired state. K represents the coupling coefficient, Wnom is the rated frequency, and ΔV' α and ΔV' β These represent the coupling effect exerted by the ideal oscillator on the output modulated wave.

[0075] Step 7: Superimposed Voltage Calculation: Select the superimposed voltage amplitude U based on the DC power supply margin. add The duration Δt of the superimposed voltage is calculated based on the instantaneous current value at the time of voltage signal update in each cycle, and the corresponding formula is:

[0076]

[0077] Among them, u add To superimpose the voltage amplitude, I0 is the instantaneous current value at the time of voltage signal update, and L... f For the filter inductor, R f This is the filter resistor.

[0078] Step 8: PWM modulation module: Convert the inverter voltage output signal v' for the next cycle αβ The voltage command signal is obtained by adding the superimposed voltage and then generating the switching signal of the switching transistor after PWM modulation.

[0079] Example

[0080] like Figure 1 The diagram shown illustrates the hardware circuit portion of a VOC-based transient-free control method for a three-phase grid-connected inverter, as provided in this invention example. It includes a DC power supply, a three-phase IGBT full-bridge, and a filter inductor L. f Circuit equivalent resistance R, grid-side inductance L g Three-phase AC power grid, current sensor, control motherboard and drive module.

[0081] like Figure 2 The diagram shown illustrates a VOC control method according to an example of the present invention. The control part includes: calculating the command current, and converting the sampled current i... abc After coordinate transformation and comparison with the command current, the error is fed into the virtual oscillation controller to finally obtain the voltage signal v. αβ .

[0082] like Figure 3 The diagram shown illustrates the coupling effect of an ideal oscillator according to an example of the present invention. The coupling application process includes: detecting whether the power command value changes; maintaining normal VOC operation when it does not change; initiating the coupling effect after a change; and then applying coupling and synchronization to the controller through the ideal oscillator.

[0083] like Figure 4 The diagram shown illustrates the control section of a VOC-based transient-free control method for a three-phase grid-connected inverter, as provided in this invention. The main process includes: In the normal VOC section, the ideal steady-state phase angle difference is calculated at the moment the power command value changes; transient-free control is performed at the current zero-crossing point; the control signal is calculated and coupled using the formula from step 6; a superimposed voltage signal is calculated and applied; and after being added to the inverter voltage, it is modulated by PWM to obtain the switching transistor on / off signal.

[0084] To demonstrate the correctness and effectiveness of the control strategy proposed in this invention, according to Figure 1 Figure 2 Figure 4 A simulation model was established, in which the active power command changed from 500W to 2000W at 0.5s.

[0085] Figure 5 A comparison of active power waveforms between traditional Virtual Oscillation Control (AHO) and the Transient-Free VOC (NT-VOC) proposed in this invention is presented. Simulation results show that under traditional AHO, a new steady state is reached 0.2 seconds after the active power command, resulting in a slow dynamic response. The method proposed in this invention, however, achieves the active power command in approximately 0.02 seconds, enabling a rapid attainment of a new steady state. This demonstrates that the proposed control method can quickly track the power command and effectively reduce the transient process between two steady states.

[0086] Figure 6A comparison of the grid-side current waveforms of traditional virtual oscillation control and the transient-free control proposed in this invention shows that the transient-free control proposed in this invention has a fast current response and no obvious transients, which is a significant advantage over traditional virtual oscillator control.

Claims

1. A transient-free control method for a three-phase grid-connected inverter based on VOC, characterized in that, include: The grid-connected three-phase current of the inverter is sampled and transformed by abc / αβ to obtain the grid-connected three-phase current; Calculate the grid-connected current command value based on the active power command value and the reactive power command value; The virtual oscillator is controlled according to the grid-connected current command value. The capacitor voltage and inductor current of the virtual oscillator are collected and amplified as the inverter grid-connected voltage. Calculate the phase angle difference between the inverter output voltage and the grid voltage under steady-state conditions based on the active power command value, the reactive power command value and the parameters of the three-phase grid-connected inverter. Based on the calculated steady-state data, an oscillator is virtually generated that operates at a set steady-state point. At a set time, the oscillator is coupled and synchronized with the VOC output modulation wave to achieve rapid switching of the controller output state. The zero-crossing point of the current after the change in operating conditions is selected as the update time of the voltage signal, and the coupling effect is applied from the beginning. According to the Kuramoto model, using an oscillator operating in an ideal steady state to apply coupling feedback to the output modulated wave, the corresponding formula is: Where θ1 is the phase of the oscillator in AHO, θ2 is the phase of the oscillator operating in the desired state, K represents the coupling coefficient, and W... nom For the rated frequency, ΔV' α and ΔV' β These represent the coupling effect exerted by the ideal oscillator on the output modulated wave; The amplitude of the superimposed voltage is selected based on the DC power supply margin. The application time of the superimposed voltage is calculated. The application time Δt is calculated based on the instantaneous current value at the voltage signal update time of each cycle. The corresponding formula is: Among them, u add To superimpose the voltage amplitude, I0 is the instantaneous current value at the time of voltage signal update, and L... f For the filter inductor, R f For filtering resistors; The voltage command signal is obtained by adding the inverter voltage output signal of the next cycle and the superimposed voltage. After PWM modulation, the switching transistor on / off signal is generated to drive the switching transistor.

2. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 1, characterized in that: The three-phase current i of the inverter grid connection abc The coordinate transformation matrix for sampling and performing the abc / αβ transformation is: 。 3. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 1, characterized in that, The grid-connected current command value is calculated based on the active power command value and the reactive power command value. The specific formula is as follows: Among them, v α and v β Indicates the inverter's grid connection voltage. P * Q is the active power command value. * This is the reactive power command value.

4. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 1, characterized in that, The virtual oscillator includes an inductor L, a capacitor C, and a controlled voltage source v. m Controlled voltage source v u Controlled current source i m Controlled current source i u An inductor L is connected in series with a controlled voltage source v. m and a controlled voltage source v u Then, a capacitor C and a controlled current source i are connected in parallel. m A controlled current source i u .

5. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 1, characterized in that, The specific method for controlling the virtual oscillator based on the grid-connected current command value, and for acquiring and amplifying the capacitor voltage and inductor current of the virtual oscillator as the inverter grid-connected voltage is as follows: The grid-connected three-phase current i α i β With grid-connected current command value and The error is amplified by K after being processed by the rotation matrix. i We obtain u1 and u2, where u1 serves as the controlled current source i. u The current u2, after passing through the coefficient ε, becomes the controlled voltage source v. u The voltage; Collect the voltage u across capacitor C of the LC second-order oscillator C and inductor current i L As state variables x1 and x2, the controlled voltage source v is calculated based on the state variables. m and controlled current source i m The signal, a state variable, is amplified to a fixed amplitude V. n As the inverter voltage command v α v β .

6. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 5, characterized in that, coefficient ε, controlled voltage source v m and controlled current source i m Specifically: Where, k i X represents the amplification factor, ω is the power grid angular frequency, x1 and x2 are state variables, and X... n Let |x|| be the nominal value of the state variable, |x|| be the modulus of the state variable, and ξ be a constant.

7. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 1, characterized in that, Through the active power command value P * and reactive power command value Q * The phase angle difference between the inverter output voltage and the grid voltage is obtained by approximating the circuit parameters. The specific formula is as follows: Among them, V g R is the grid voltage amplitude, R is the circuit equivalent resistance, and L is the voltage level. eq It is the sum of the filter inductance and the grid-side inductance.

8. The transient-free control method for a three-phase grid-connected inverter based on VOC according to claim 1, characterized in that, The inverter voltage output signal v' for the next cycle is calculated. αβ The corresponding formula is: v' αβ NV n sin(wt+δ * )4 Where, δ * P represents the next steady-state phase angle difference, δ0 represents the current steady-state phase angle difference, and P represents the next steady-state phase angle difference. * P0 represents the next steady-state power command, while P1 represents the current steady-state power command.

Images