An inverter synchronization control method based on a current source type virtual resonator

Abstract

The application discloses an inverter synchronization control method based on a current source type virtual resonator. The method constructs two-phase orthogonal resonant current states with an equivalent LC oscillation structure, and takes the resonant current vector as the inverter current inner loop reference, so that the inverter has the self-synchronization ability of the virtual oscillator while maintaining the controlled current source external characteristic. The power factor angle is calculated by collecting the AC terminal voltage and output current, and the power factor angle error is fed back along the direction rotated by π / 2 with respect to the resonant current vector, so that the phase adjustment only changes the resonant current angular velocity without directly disturbing the nonlinear amplitude adjustment, and the phase and current amplitude are orthogonally decoupled. In the grid-connected mode, the current amplitude instruction and the target power factor angle are generated according to the active and reactive power instructions and the terminal voltage amplitude; when the current is not saturated at the rated frequency, the active and reactive power zero steady-state error tracking can be realized, and the overcurrent can be inhibited by power scaling or current limiting when the voltage drops.

Description

Technical Field

[0001] This invention relates to the field of grid-connected control of power electronic converters and microgrid control technology, specifically to a synchronous control method for inverters based on a current-source type virtual resonator, and more particularly to a grid-following virtual oscillator control method that uses the resonant current vector as the autonomous oscillation state, achieves synchronization through power factor angle feedback, and also has grid-connected power regulation and current constraint capabilities. Background Technology

[0002] With the large-scale integration of new energy power generation, energy storage converters, and power electronic loads, inverters need to maintain stable operation under various operating conditions, including strong grids, weak grids, grid voltage dips, and isolated microgrids. Existing inverter control can generally be divided into grid-following control, which focuses on output current regulation, and grid-connected control, which focuses on output voltage establishment. Grid-following control facilitates precise power regulation and current limiting, making it suitable for strong grid connection, but it typically relies on phase-locked loops or external grid voltage as a synchronization reference, limiting its voltage support and islanding capabilities.

[0003] Virtual oscillator control, by constructing nonlinear oscillatory dynamics within the controller, can achieve autonomous synchronization without the need for traditional phase-locked loops and exhibits a fast time-domain dynamic response. Most existing virtual oscillator controls directly use the oscillator state to generate the inverter voltage reference, making the inverter behave as a controlled voltage source. This structure has advantages in weak grids and islanded operation scenarios, but in strong grids or when multiple voltage-source inverters are directly coupled through very small line impedances, even small voltage amplitude or phase deviations can cause significant circulating currents, typically requiring additional virtual impedance, damping, or impedance shaping components.

[0004] Therefore, a new virtual oscillator control structure is needed to enable the inverter to maintain the characteristics of a controlled current source and current constraint when connected to the grid, while retaining the autonomous synchronization mechanism of the virtual oscillator. Furthermore, the control structure also needs to be able to maintain synchronization and limit current when the grid voltage drops, and achieve AC voltage support through current regulation in islanded mode, so as to be suitable for multi-inverter parallel scenarios with low line impedance or even near-zero line impedance. Summary of the Invention

[0005] The purpose of this invention is to provide an inverter synchronization control method based on a current-source virtual resonator to solve problems such as circulating current in existing voltage-source virtual oscillators in parallel systems with strong grids and near-zero line impedance, reliance on external synchronization links for traditional grid-following control and insufficient island voltage support capability, and contradictions between power command and allowable current when grid voltage drops.

[0006] To achieve the above objectives, this invention modifies the virtual oscillator from a "voltage reference generator" to a "current reference generator." An equivalent LC oscillation structure is established internally by the controller, and two orthogonal current quantities are selected as oscillation states. This oscillation state directly generates the reference value for the inverter's inner current loop, maintaining the grid-connected controlled current source characteristics from a control perspective. Simultaneously, the power factor angle is calculated using the AC terminal voltage and output current, and the power factor angle error is fed back along the orthogonal direction of the resonant current vector to change the angular velocity of the resonant current vector, thereby achieving synchronization.

[0007] The present invention further employs A power factor angle feedback vector is constructed by rotation, making it orthogonal to the resonant current vector. Since the inner product of the orthogonal vector and the original vector is zero, the phase adjustment feedback does not directly affect the dynamics of the resonant current amplitude; therefore, the phase synchronization and nonlinear amplitude adjustment of the resonant current can be decoupled at the vector level.

[0008] During grid-connected operation, the current amplitude command and target power factor angle are generated based on active and reactive power references and terminal voltage amplitude analysis. Under the conditions of rated frequency and current non-saturation, zero steady-state error tracking of active and reactive power can be achieved. When the voltage drops, the apparent power can be transmitted and the power command can be scaled based on the allowable current calculation, or the current amplitude command can be directly limited, thereby maintaining the operation of the controlled current source and avoiding overcurrent.

[0009] In islanded operation, the current amplitude command is generated by the voltage support loop, allowing the inverter to still regulate its output as a current source. However, the output current amplitude will vary with the bus voltage deviation, thus creating voltage support. For multiple inverters in parallel, under low or near-zero line impedance conditions, each inverter shares the common bus voltage. By configuring the voltage-active power droop factor, active power can be distributed proportionally to the rated capacity, and corresponding reactive power distribution can be achieved under common power factor angle conditions.

[0010] Beneficial effects:

[0011] 1. This invention modifies the traditional voltage source type virtual oscillator into a current source type implementation, directly generating the inverter current reference with the resonant current state, thereby introducing the autonomous synchronization capability of the virtual oscillator while maintaining the inverter current regulation and current limiting characteristics.

[0012] 2. By rotating the power factor angle error along the resonant current vector Feedback is provided in the orthogonal direction so that phase adjustment only affects the angular velocity of the resonant current and does not directly affect its amplitude dynamics, thus achieving structural decoupling between phase adjustment and current amplitude adjustment;

[0013] 3. In grid-connected operation mode, this invention generates current amplitude commands and target power factor angles based on active and reactive power commands and terminal voltage amplitudes, and can achieve zero steady-state error tracking of active and reactive power under rated frequency and current unsaturation conditions.

[0014] 4. When the grid voltage drops, this invention can suppress overcurrent by current limiting or power derating while maintaining the original resonant synchronization mechanism, thereby improving the current controllability during the fault period.

[0015] 5. In islanded operation mode, this invention generates current amplitude commands based on terminal voltage deviation, enabling the current source type virtual resonator to have AC voltage support capability. It is applicable to the parallel operation and power distribution of multiple inverters under low line impedance or even near-zero line impedance conditions, thereby reducing the circulating current risk caused by voltage amplitude or phase deviation when multiple rigid voltage sources are directly connected in parallel in the traditional way. Attached Figure Description

[0016] The following accompanying drawings are provided to further illustrate embodiments of the present invention and form part of the specification. They are used together with the embodiments of the present invention to explain the invention and do not constitute a limitation thereof. In the drawings, the same reference numerals generally represent the same components or steps.

[0017] Figure 1 A comparison diagram of the control structures of a conventional voltage source type grid VOC and the current source type grid VOC of the present invention is shown;

[0018] Figure 2 The experimental waveforms at the active power reference step are shown.

[0019] Figure 3 The waveforms of the low-voltage fault ride-through experiment during a grid voltage drop are shown.

[0020] Figure 4 The experimental waveforms of the grid-type VOC and the grid-type VOC of the present invention under the condition of varying short-circuit ratio are shown.

[0021] Figure 5 A flowchart of the inverter control method based on a voltage-controlled current source virtual resonator in this invention is shown. Detailed Implementation

[0022] The specific embodiments of the present invention will be described below in conjunction with the control principles of the present invention. These embodiments are intended to explain the present invention and not to limit its scope of protection. Those skilled in the art can make equivalent substitutions for the coordinate transformation form, power coefficient, current inner loop structure, modulation method, filter type, or current limiting priority without altering the core concept of the present invention; these substitutions can all be considered implementations of the technical concept of the present invention.

[0023] like Figure 5 As shown, this embodiment provides a synchronous control method for an inverter based on a current-source virtual resonator, the steps of which are as follows:

[0024] S1. Collect the voltage vector and output current vector at the AC terminal of the inverter, obtain the power factor angle based on the voltage vector and output current vector, and form the power factor angle error between the power factor angle and the target power factor angle.

[0025] S2. Construct a virtual resonator with two-phase orthogonal current components as state variables, and form a nonlinear amplitude adjustment term along the radial direction of the resonant current vector based on the current amplitude command to adjust the amplitude of the resonant current vector.

[0026] S3. A phase adjustment feedback term is formed based on the power factor angular error, and the phase adjustment feedback term is made orthogonal to the resonant current vector to adjust the angular velocity of the resonant current vector without directly changing its amplitude.

[0027] S4. Combine the nonlinear amplitude adjustment term, the phase adjustment feedback term, and the rated oscillation term of the virtual harmonic oscillator to form the resonant current state equation, and output the resonant current reference vector.

[0028] S5. Input the resonant current reference vector into the inverter current inner loop, generate a modulation voltage command from the current inner loop, and drive the inverter through pulse width modulation.

[0029] I. System Structure and Signal Definition;

[0030] This embodiment focuses on a voltage source inverter with a DC-side bus, bridge power devices, an AC filter network, and a digital controller. The controller acquires the inverter's AC terminal voltage and output current, and obtains the voltage vector through a stationary coordinate transformation. and current vector For three-phase amplitude invariant type Transformation, power coefficient 3 / 2 is acceptable; if other normalization methods are used, then... Determined according to the corresponding power definition.

[0031] ;

[0032] in, and These are the instantaneous active power and reactive power output by the inverter, respectively.

[0033] II. Construction of a voltage-controlled current source virtual resonator;

[0034] Construct an equivalent LC oscillating structure with a rated angular frequency. and impedance scale parameters They are respectively:

[0035] ;

[0036] Based on the polarity of the equivalent LC structure and the signal scaling relationship, the inductor current is... and capacitor voltage scaled by the scaling factor μ Choose two orthogonal current states:

[0037] ;

[0038] Resonant current vector The current is directly fed into the inner current loop as a reference. The inner current loop is based on... The error between the actual output current and the actual output current generates α-axis and β-axis modulated voltage commands, which drive the power devices via PWM or other equivalent modulation methods. Therefore, the virtual resonator does not directly generate the inverter port voltage reference, but instead generates a current reference with oscillating characteristics.

[0039] To achieve nonlinear current amplitude regulation, a radial adjustment term along the ir direction is set in the virtual resonator. Let... This is the amplitude adjustment coefficient. If the command is for current amplitude, then the following can be used:

[0040] .

[0041] III. Power factor angle feedback and π / 2 orthogonal decoupling;

[0042] Calculate the voltage phase angle from the AC terminal voltage and output current. and current phase angle And define the power factor angle φ as:

[0043] ;

[0044] Let the target power factor angle be... Then the power factor angle error is Rotate the resonant current vector Then, as feedback direction:

[0045] ;

[0046] in, This is the phase feedback gain. By superimposing the rated oscillation term, the nonlinear amplitude adjustment term, and the power factor angle feedback term, the core resonant current control law of this invention can be obtained:

[0047] ;

[0048] In equation (7), the first term determines the rated rotation, the power factor angular error corrects the angular velocity through the Jir direction, and the second term adjusts the current amplitude along the ir direction.

[0049] To explain why π / 2 rotation achieves decoupling of amplitude and phase, let .Depend on We can obtain:

[0050] ;

[0051] Therefore, for any non-zero initial resonant current, the steady state satisfies The power factor angle feedback term does not appear directly in equation (8), indicating that it mainly changes the phase evolution of the resonant current vector, while the current amplitude is changed by... and The decision was made. This orthogonal structure provides the basis for independent mapping of grid-connected power commands.

[0052] IV. Power command mapping in grid-connected mode;

[0053] During grid-connected operation, the AC terminal voltage is primarily established by the external power grid. This invention regulates active and reactive power through current amplitude and power factor angle. The AC terminal voltage amplitude is defined as follows: Given active power command and reactive power command The apparent power command is:

[0054] ;

[0055] Since the steady-state amplitude of the resonant current vector is Therefore, the settings are as follows:

[0056] ;

[0057] From equation (7), the dynamic phase angle of the resonant current can be obtained:

[0058] ;

[0059] V. Grid voltage dips and current limiting;

[0060] When the grid voltage amplitude drops to If the original state is maintained... , The required current will be according to Increase. To avoid overcurrent, set... For the allowable current amplitude, calculate the maximum apparent power that can be transferred based on the dropout voltage:

[0061] ;

[0062] When using proportional power derating, the definition is:

[0063] ;

[0064] Then, P̄* and Q̄* are used to replace the original power command for recalculation. and If the current limit is not reached, then If the current limit is reached, the current is kept within the allowable range by scaling the power command. According to the grid connection specifications, current limiting strategies such as reactive current priority can also be adopted without changing the resonant synchronization control law described in equation (7). As long as the AC terminal voltage vector has not completely collapsed to zero, the power factor angle can still be defined, and the phase synchronization mechanism can continue to work.

[0065] IX. Experimental Verification;

[0066] To verify the operating characteristics of the control method, experimental waveforms were used to illustrate three operating conditions: changes in active power command, grid voltage dips, and changes in grid short-circuit ratio.

[0067] like Figure 2 As shown, at t=0.55 s, the active power reference jumps from 300 W to 400 W. After the jump, the output briefly transitions into a new steady state, indicating that the control method can adjust the resonant current amplitude and power factor angle according to the power command.

[0068] like Figure 3 As shown, the AC terminal voltage drops from 1 pu to 0.2 pu at t=0.55 s and recovers to 1 pu at t=1.175 s. During the voltage drop, the inverter maintains controlled current output. After the voltage recovers, the system re-enters normal steady state, indicating that the resonant synchronization structure can work with the current limiting strategy to achieve low-voltage fault ride-through.

[0069] like Figure 4 As shown, the system short-circuit ratio (SCR) changes from 13.3 to 15.2 at t=0.22 s. The waveform comparison shows that the proposed current-source grid-connected VOC remains stable under this operating condition, while the voltage-source grid-connected VOC exhibits significant oscillations, demonstrating the adaptability of the proposed current-source control structure to strong grid conditions.

[0070] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above embodiments. Those skilled in the art, without departing from the core technical concept of realizing virtual oscillation with resonant current state, adjusting phase with power factor angle feedback orthogonal to the resonant current vector, and generating current amplitude commands according to the operating mode, can make equivalent changes or substitutions to the coordinate transformation method, controller parameters, current inner loop structure, power command mapping, current limiting strategy, voltage support loop, and modulation method; the corresponding changes or substitutions should be within the scope of protection defined in the claims.

Claims

1. A method for synchronous control of an inverter based on a current-source type virtual resonator, characterized in that, Includes the following steps: S1. Collect the voltage vector and output current vector at the AC terminal of the inverter, obtain the power factor angle based on the voltage vector and output current vector, and form the power factor angle error between the power factor angle and the target power factor angle. S2. Construct a virtual resonator with two-phase orthogonal current components as state variables, and form a nonlinear amplitude adjustment term along the radial direction of the resonant current vector based on the current amplitude command to adjust the amplitude of the resonant current vector. S3. A phase adjustment feedback term is formed based on the power factor angular error, and the phase adjustment feedback term is made orthogonal to the resonant current vector to adjust the angular velocity of the resonant current vector without directly changing its amplitude. S4. Combine the nonlinear amplitude adjustment term, the phase adjustment feedback term, and the rated oscillation term of the virtual harmonic oscillator to form the resonant current state equation, and output the resonant current reference vector. S5. Input the resonant current reference vector into the inverter current inner loop, generate a modulation voltage command from the current inner loop, and drive the inverter through pulse width modulation.

2. The inverter synchronization control method based on a current-source virtual resonator as described in claim 1, characterized in that, The virtual resonator is composed of an equivalent LC oscillation structure, and its rated angular frequency is... and impedance scale parameters satisfy: ； and the equivalent inductance current i L The equivalent capacitance voltage μv scaled by the scaling factor μ C As two orthogonal current states respectively, the resonant current vector is made... ; Among them, i α * i α * These are the current components in the αβ coordinate system, respectively.

3. The inverter synchronization control method based on a current-source virtual resonator as described in claim 2, characterized in that, Define rotation matrix The power factor angle Power factor angle error And the phase adjustment feedback satisfies: ; in, The phase angle of the voltage vector. The phase angle of the output current vector. The target power factor angle is η, and the phase feedback gain is η. This corresponds to the orthogonal vector obtained by rotating the resonant current vector by π / 2.

4. The inverter synchronization control method based on a current-source virtual resonator as described in claim 3, characterized in that, The state equation for the resonant current is: ； Where ξ>0 is the amplitude adjustment coefficient. This is a current amplitude command; define ρ = ||i r When ||², the voltage amplitude dynamically satisfies , so that under non-zero initial value ||i r Convergence to .

5. The inverter synchronization control method based on a current-source virtual resonator as described in claim 4, characterized in that, In grid-connected mode, according to the active power command Reactive power command and AC terminal voltage amplitude Generate the current amplitude command Angle with target power factor ,in: ； in, The power coefficient is determined by the coordinate transformation and power definition used; when the grid angular frequency is equal to Furthermore, when the inner current loop accurately tracks and current saturation is not triggered, the steady-state active power and reactive power converge to [the specified values]. and .

6. The inverter synchronization control method based on a current-source virtual resonator as described in claim 5, characterized in that, When the AC terminal voltage drops, according to the allowable current Limit active and reactive power, or current amplitude commands; when using power commands for proportional scaling, and according to... , and voltage amplitude after drop Regenerate and This is to ensure that the output current does not exceed the allowable range.

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