Method for circulating current suppression and grid forming control of modular grid-connected converter based on virtual oscillator

Abstract

This invention discloses a method for circulating current suppression and grid-connected control of modular grid-connected converters based on virtual oscillators, belonging to the field of electrical engineering. The control method of this invention includes an Andronov-Hopf oscillator grid-connected control method with dual voltage and current closed loops, and a dead-zone oscillator circulating current suppression method. Addressing the problem in existing technologies where modular grid-connected converter grid-connected control and high-frequency zero-sequence circulating current suppression technologies are independent and lack coordinated design, failing to simultaneously achieve grid-connected performance and circulating current suppression effectiveness, this invention provides a modular grid-connected converter circulating current suppression and grid-connected control technology based on virtual oscillators. This technology achieves integrated control of grid-connected capability and circulating current suppression function without requiring communication between individual grid-connected converter modules.

Description

Technical Field

[0001] This invention relates to the field of electrical engineering, and in particular to a modular grid-connected converter circulating current suppression and grid control technology based on a virtual oscillator. Background Technology

[0002] With the rapid development of new energy power generation technologies, modular grid-connected converters are increasingly widely used in distributed generation systems due to their advantages of flexible capacity expansion and strong redundancy. The main component is the grid-connected converter module, with multiple modules connected in parallel to achieve capacity expansion. However, during parallel operation, the asynchronous carrier waves of each unit can easily induce high-frequency zero-sequence circulating currents, leading to decreased system efficiency and increased device losses. At the same time, new energy power grids place higher demands on the grid-connected capabilities of converters, requiring them to possess voltage and frequency regulation capabilities similar to synchronous generators to ensure grid-connected operation stability.

[0003] In existing technologies, Virtual Oscillator Control (VOC) provides a new solution to the aforementioned challenges of grid construction and circulation suppression. The Andronov-Hopf oscillator is the core component of grid construction control, its fundamental theory originating from Hopf's theory of bifurcation in dynamical systems proposed in 1942. Hopf bifurcation is a dynamic phenomenon in autonomous systems based on nonlinear ordinary differential equations, referring to the phenomenon where, with continuous change of a key parameter, a pair of conjugate complex eigenvalues ​​of the corresponding Jacobian matrix near the system's equilibrium point shifts from the left half-plane, perpendicularly across the imaginary axis, to the right half-plane. This instantaneous shift alters the stability of the equilibrium point, leading to a new periodic solution. This lays a solid theoretical foundation for the subsequent engineering application of this oscillator in converter grid construction control.

[0004] The paper "A Grid-compatible Virtual Oscillator Controller: Analysis and Design," published by Minghui Lu, Soham Dutta, Victor Purba, and others at the IEEE Energy Conversion Congress and Exposition (ECCE) in 2019, was the first to formally introduce the Andronov-Hopf oscillator into the field of converter grid control and propose a complete implementation scheme. Based on the Andronov-Hopf nonlinear dynamic model, this paper designs a controller suitable for both grid-connected and islanded operation. By constructing an architecture containing a virtual LC resonant circuit and a nonlinear state-dependent source, a perfect circular limit loop is generated to ensure voltage and current quality. Simultaneously, orthogonal signal characteristics are embedded to adapt to three-phase systems, overcoming the shortcomings of traditional VOC controllers, such as severe harmonics and incompatibility with three-phase operating conditions. This represents a pioneering research achievement in this field.

[0005] Building upon the aforementioned fundamental research, researchers have gradually promoted the practical application of Andronov-Hopf oscillators in converter grid control. Among them, the paper "Dispatchable Virtual-oscillator-controlled Inverters with Current-limiting and MPPT Capabilities," published at the ECCE conference in 2021 by Minghui Lu, Rahul Mallik, Brian Johnson, and others, integrates voltage and current dual closed-loop control with Andronov-Hopf oscillator grid control, specifically addressing the insufficient power point tracking performance of pure Andronov-Hopf oscillator grid control algorithms.

[0006] To address the synchronization problem between Andronov-Hopf oscillators and the power grid, MA Awal, Md Rifat KaisarRachi, Md Rashed Hassan Bipu, and others published a paper at the ECCE conference in 2021 entitled "Adaptive Pre-Synchronization Strategy for Hopf Oscillator-Based Grid-Forming Inverters." This paper discloses an adaptive pre-synchronization strategy that can stably achieve precise matching between the oscillator output and the power grid voltage even when the grid voltage, amplitude, and frequency deviate from their rated values.

[0007] In addition, Jian Hu and Hao Ma published a paper titled "Synchronization of the Carrier Wave of Parallel Three-Phase Inverters With Virtual Oscillator Control" in the journal IEEE Transactions on Power Electronics in 2017, which proposed a circulating current suppression technique based on dead-zone oscillators to suppress high-frequency zero-sequence circulating current in multi-machine parallel systems through carrier synchronization.

[0008] In summary, in existing technologies, the Andronov-Hopf oscillator network control algorithm with dual voltage and current closed loops and the dead-zone oscillator circulating current suppression technology are independent of each other and lack collaborative design. Specifically, they often exist as independent solutions, failing to achieve integrated optimized control. Therefore, how to integrate the two, achieve functional complementarity and collaborative operation, has become an urgent technical challenge to be solved. Summary of the Invention

[0009] The technical problem this invention aims to solve is that in existing technologies, the grid-connected converter network control and high-frequency zero-sequence circulating current suppression technology are independent and lack coordinated design, making it impossible to simultaneously achieve both grid-connected performance and circulating current suppression effectiveness. To address this problem, this invention provides a modular grid-connected converter circulating current suppression and grid-connected control technology based on a virtual oscillator, achieving integrated control of grid-connected capability and circulating current suppression function without requiring communication between individual grid-connected converter modules.

[0010] The technical solution of the present invention is as follows.

[0011] A modular grid-connected converter circulating current suppression and grid-connected control method based on a virtual oscillator is applied to a grid-connected converter parallel system. A grid-connected converter module is formed by connecting in parallel with a common DC bus and a common AC bus. Any one of these grid-connected converter modules is denoted as a grid-connected converter. , The grid-connected converter The system comprises a DC bus capacitor, a main inverter, and an LCL filter connected in series. The LCL filter includes a machine-side inductor, a grid-side inductor, and a filter capacitor. The system includes the following steps: Step 1, collect data from the grid-connected converter. The three-phase voltage of the filter capacitor of the LCL filter , , The three-phase current of the machine-side inductor , , The three-phase current of the grid-side inductor , , ; Step 2: Obtain the grid-connected converter using the Andronov-Hopf oscillator grid control method with voltage and current dual closed loops. Three-phase modulated wave signal , , Generate grid-connected converter The three-phase bridge arm PWM control signal; Step 3: During the grid control process in Step 2, when a high-frequency zero-sequence circulating current occurs, it is suppressed using a dead-zone oscillator circulating current suppression method. Specifically, this is based on the grid-connected converter... Three-phase current of the grid-side inductor of the LCL filter , , Zero-sequence circulating current calculation is performed to obtain the grid-connected converter. Zero-order circulation ; Zero-sequence circulation After filtering with a bandpass filter, the grid-connected converter is obtained. zero-sequence current at switching frequency And obtain the grid-connected converter Input current of dead-zone oscillator ; grid-connected converter Input current of dead-zone oscillator Input to grid-connected converter In a dead-zone oscillator, the output voltage And at the terminal voltage When the signal changes from negative to positive, a carrier reset signal is sent to control the grid-connected converter. The triangular carrier is reset to phase 0 to suppress the high-frequency zero-sequence circulating current.

[0012] Preferably, step 2 is implemented as follows: Step 2.1, convert the three-phase current of the grid-side inductor , , By using the constant amplitude Clark transform, we obtain Components of the grid-side inductor current in the coordinate system , ; Step 2.2, according to Components of the grid-side inductor current in the coordinate system , Grid-connected converter Active power command and grid-connected converter reactive power command Calculate the output voltage of the Andronov-Hopf oscillator; specifically, first perform... Grid-connected converter in coordinate system The grid-side current command is calculated to obtain the grid-side current command at the current sampling time. , Based on this, grid-connected converters are developed. The Andronov-Hopf oscillator was calculated to obtain the output voltage of the Andronov-Hopf oscillator at the current sampling time. , ; The grid-side current command at the current sampling time , The calculation formula is as follows:

[0013] in, , These are the output voltages of the Andronov-Hopf oscillator at the previous sampling time. ; The output voltage of the Andronov-Hopf oscillator at the current sampling time , It is given by the following differential equation:

[0014] in, , They are , The derivative with respect to time, i.e. , ; For grid-connected converters The convergence rate constant of the Andronov-Hopf oscillator For grid-connected converters The voltage amplification factor of the Andronov-Hopf oscillator For grid-connected converters The current amplification factor of the Andronov-Hopf oscillator This refers to the rated voltage amplitude of the Andronov-Hopf oscillator. For grid-connected converters The rated angular frequency of the Andronov-Hopf oscillator, For grid-connected converters The current error rotation angle of the Andronov-Hopf oscillator For grid-connected converters The virtual capacitance parameters of the Andronov-Hopf oscillator ; Step 2.3, according to the calculation formula Obtaining electrical angle ; and according to the electrical angle The three-phase current of the grid-side inductor , , Perform a constant-amplitude rotating coordinate transformation to obtain the current component of the grid-side inductance of grid-connected converter i in the da coordinate system; and perform a three-phase voltage transformation on the filter capacitor. , , The grid-connected converter is obtained by performing a constant amplitude rotation coordinate transformation. Voltage components in the dq coordinate system , Three-phase current of the machine-side inductor , , The grid-connected converter is obtained by performing a constant amplitude rotation coordinate transformation. Current components of the machine-side inductor in the dq coordinate system , For grid-connected converters The output voltage of the Andronov-Hopf oscillator at the current sampling time , The voltage components of the Andronov-Hopf oscillator in the dq coordinate system are obtained by performing an equal-amplitude rotational coordinate transformation. , ; Step 2.4, based on the voltage components in the dq coordinate system of the Andronov-Hopf oscillator. , and electrical angle A dual closed-loop control of voltage and current is implemented; specifically, the grid-connected converter is obtained through the outer loop control equation of voltage in the dq coordinate system. The given value of the inner current loop in the dq coordinate system , Then, the grid-connected converter is obtained through the inner current control equation. Modulated wave signal in dq coordinate system , Subsequently, the grid-connected converter is obtained through an inverse coordinate transformation with equal amplitude rotation. Three-phase modulated wave signal , , It is then compared with a triangular carrier wave to generate a grid-connected converter. The PWM control signal for the three-phase bridge arm.

[0015] Preferably, the voltage outer loop control equation in step 2.4 is:

[0016] in, For grid-connected converters The value of the filter capacitor for each phase in the LCL filter, For grid-connected converters The voltage outer loop proportionality coefficient, For grid-connected converters The voltage outer loop integral coefficient, For the Laplace operator; The formula for calculating the current inner loop control equation is as follows:

[0017] in, For grid-connected converters The inductance value per camera side of the LCL filter, For grid-connected converters The current inner loop proportionality coefficient, For grid-connected converters The integral coefficient of the inner loop of the current.

[0018] Preferably, the implementation process of step 3 is as follows: Step 3.1, based on the grid-connected converter Three-phase current of the grid-side inductor of the LCL filter , , Perform zero-sequence circulating current calculations to obtain the grid-connected converter. Zero-order circulation , ; Step 3.2, connect the grid-connected converter zero-sequence current After filtering with a bandpass filter, the grid-connected converter is obtained. zero-sequence current at switching frequency ; based on the zero-sequence current of the switching frequency Calculate the grid-connected converter Input current of dead-zone oscillator , .in For grid-connected converters The zero-sequence current scaling factor of the switching frequency; Step 3.3, connect the grid-connected converter Input current of dead-zone oscillator Input grid-connected converter In a dead-zone oscillator, the dead-zone oscillator calculates its own output voltage. ; Step 3.4, for the grid-connected converter The terminal voltage of the dead zone oscillator Perform positive zero-crossing monitoring and carrier reset, specifically at the terminal voltage. When the signal changes from negative to positive, a carrier reset signal is sent to control the grid-connected converter. The triangular carrier is reset to phase 0.

[0019] Preferably, the dead-zone oscillator is described by the following differential equation:

[0020] in, For grid-connected converters The inductance value of the dead-zone oscillator, For grid-connected converters The capacitance value of the dead-zone oscillator, For grid-connected converters The resistance value of the dead-zone oscillator; For grid-connected converters The expression for the piecewise linear voltage-controlled current source of the dead-zone oscillator is as follows:

[0021] in, For grid-connected converters The boundary point of the piecewise linear voltage-controlled current source of the dead-zone oscillator. For grid-connected converters The voltage slope of the piecewise linear voltage-controlled current source of the dead-zone oscillator.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Innovative integration of Andronov-Hopf oscillator grid control with dual voltage and current closed loops and dead-zone oscillator circulating current suppression technology, achieving integrated optimization of grid construction and circulating current suppression, filling the gap in existing technology; 2. This method does not require communication between the grid-connected converter modules, thus eliminating the need to deploy a communication network in each grid-connected converter module. This reduces costs, lowers system complexity, improves reliability, and facilitates modular expansion. 3. This method is effective for parallel systems composed of any number of grid-connected converter modules, and there is no limit to the number of grid-connected converter modules, making it easy to scale up applications for high-power systems. Attached Figure Description

[0023] Figure 1 This is a block diagram of the overall structure of the grid-connected converter parallel system of the present invention.

[0024] Figure 2 This is a diagram illustrating the circulating current suppression and grid control of a modular grid-connected converter based on a virtual oscillator, as presented in this invention.

[0025] Figure 3 The graph shows the total grid-connected current waveform of the grid-connected converter parallel system of the present invention.

[0026] Figure 4 The image shows the dead-zone oscillator waveforms of two grid-connected converter modules in the grid-connected converter parallel system of the present invention.

[0027] Figure 5 The diagram shows the triangular carrier waveform of two grid-connected converter modules in the grid-connected converter parallel system of the present invention.

[0028] Figure 6 The diagram shows the zero-sequence circulating current waveform of the grid-connected converter parallel system of the present invention.

[0029] Figure 7This is a flowchart of the modular grid-connected converter circulating current suppression and grid control method based on a virtual oscillator according to the present invention. Detailed Implementation

[0030] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0031] Figure 1 This is a block diagram of the overall structure of the grid-connected converter parallel system of the present invention. Figure 1 It can be seen that the grid-connected converter parallel system applying this control method consists of A grid-connected converter module is formed by connecting in parallel with a common DC bus and a common AC bus. Any one of these grid-connected converter modules is denoted as a grid-connected converter. , The grid-connected converter It includes a DC bus capacitor, a main inverter, and an LCL filter connected in series. The LCL filter includes a machine-side inductor, a grid-side inductor, and a filter capacitor.

[0032] Figure 1 middle, This is the DC bus voltage. For grid-connected converters The capacitance value of the DC bus capacitor. L f_i For grid-connected converters The inductance value of the grid-side inductor of the LCL filter. For grid-connected converters The inductance value of the grid-side inductor of the LCL filter. For grid-connected converters The resistance value of the line resistor of the LCL filter, Grid-connected converter The resistance value of the grid-side line resistance of the LCL filter. For grid-connected converters The resistance value of the damping resistor of the LCL filter. , , This is the voltage of the three-phase power grid.

[0033] In this embodiment, select This means that the grid-connected converter parallel system contains two grid-connected converter modules. For any grid-connected converter , The parameters of the LCL filter are: , , , , , The effective value of the phase voltage of a three-phase power grid is 220V.

[0034] Figure 2 This is a structural diagram of the modular grid-connected converter circulating current suppression and grid control method based on a virtual oscillator according to the present invention. Figure 7 This is a flowchart of the modular grid-connected converter circulating current suppression and grid control method based on a virtual oscillator according to the present invention. Figure 2 and Figure 7 As can be seen, the present invention includes the following steps: Step 1, collect data from the grid-connected converter. The three-phase voltage of the filter capacitor of the LCL filter , , The three-phase current of the machine-side inductor , , The three-phase current of the grid-side inductor , , .

[0035] Step 2: Obtain the grid-connected converter using the Andronov-Hopf oscillator grid control method with voltage and current dual closed loops. Three-phase modulated wave signal , , Generate grid-connected converter The PWM control signal for the three-phase bridge arm.

[0036] Step 3: During the grid control process in Step 2, when a high-frequency zero-sequence circulating current occurs, it is suppressed using a dead-zone oscillator circulating current suppression method. Specifically, this is based on the grid-connected converter... Three-phase current of the grid-side inductor of the LCL filter , , Zero-sequence circulating current calculation is performed to obtain the grid-connected converter. Zero-order circulation ; Zero-sequence circulation After filtering with a bandpass filter, the grid-connected converter is obtained. zero-sequence current at switching frequency And obtain the grid-connected converter Input current of dead-zone oscillator ; grid-connected converter Input current of dead-zone oscillator Input to grid-connected converter In a dead-zone oscillator, the output voltage And at the terminal voltage When the signal changes from negative to positive, a carrier reset signal is sent to control the grid-connected converter. The triangular carrier is reset to phase 0 to suppress the high-frequency zero-sequence circulating current.

[0037] In this embodiment, step 2 is implemented as follows: Step 2.1, convert the three-phase current of the grid-side inductor , , By using the constant amplitude Clark transform, we obtain Components of the grid-side inductor current in the coordinate system , ; Step 2.2, according to Components of the grid-side inductor current in the coordinate system , Grid-connected converter Active power command and grid-connected converter reactive power command Calculate the output voltage of the Andronov-Hopf oscillator; specifically, first perform... Grid-connected converter in coordinate system The grid-side current command is calculated to obtain the grid-side current command at the current sampling time. , Based on this, grid-connected converters are developed. The Andronov-Hopf oscillator was calculated to obtain the output voltage of the Andronov-Hopf oscillator at the current sampling time. , ; The grid-side current command at the current sampling time , The calculation formula is as follows:

[0038] in, , These are the output voltages of the Andronov-Hopf oscillator at the previous sampling time. ; The output voltage of the Andronov-Hopf oscillator at the current sampling time , It is given by the following differential equation:

[0039] in, , They are , The derivative with respect to time, i.e. , ; For grid-connected converters The convergence rate constant of the Andronov-Hopf oscillator For grid-connected converters The voltage amplification factor of the Andronov-Hopf oscillator For grid-connected converters The current amplification factor of the Andronov-Hopf oscillator This refers to the rated voltage amplitude of the Andronov-Hopf oscillator. For grid-connected converters The rated angular frequency of the Andronov-Hopf oscillator, For grid-connected converters The current error rotation angle of the Andronov-Hopf oscillator For grid-connected converters The virtual capacitance parameters of the Andronov-Hopf oscillator .

[0040] Step 2.3, according to the calculation formula Obtaining electrical angle ; and according to the electrical angle The three-phase current of the grid-side inductor , , Perform a constant-amplitude rotation coordinate transformation to obtain the grid-connected converter. Current components of the grid-side inductor in the dq coordinate system , ; Three-phase voltage of the filter capacitor , , The grid-connected converter is obtained by performing a constant amplitude rotation coordinate transformation. Voltage components in the dq coordinate system , Three-phase current of the machine-side inductor , , The grid-connected converter is obtained by performing a constant amplitude rotation coordinate transformation. Current components of the machine-side inductor in the dq coordinate system , For grid-connected converters The output voltage of the Andronov-Hopf oscillator at the current sampling time , The voltage components of the Andronov-Hopf oscillator in the dq coordinate system are obtained by performing an equal-amplitude rotational coordinate transformation. , .

[0041] Step 2.4, based on the voltage components in the dq coordinate system of the Andronov-Hopf oscillator. , and electrical angle A dual closed-loop control of voltage and current is implemented; specifically, the grid-connected converter is obtained through the outer loop control equation of voltage in the dq coordinate system. The given value of the inner current loop in the dq coordinate system , Then, the grid-connected converter is obtained through the inner current control equation. Modulated wave signal in dq coordinate system , Subsequently, the grid-connected converter is obtained through an inverse coordinate transformation with equal amplitude rotation. Three-phase modulated wave signal , , It is then compared with a triangular carrier wave to generate a grid-connected converter. The PWM control signal for the three-phase bridge arm.

[0042] The voltage outer loop control equation is as follows:

[0043] in, For grid-connected converters The value of the filter capacitor for each phase in the LCL filter, For grid-connected converters The voltage outer loop proportionality coefficient, For grid-connected converters The voltage outer loop integral coefficient, For the Laplace operator; The formula for calculating the current inner loop control equation is as follows:

[0044] in, For grid-connected converters The inductance value per camera side of the LCL filter, For grid-connected converters The current inner loop proportionality coefficient, For grid-connected converters The integral coefficient of the inner loop of the current.

[0045] In this embodiment, step 3 is implemented as follows: Step 3.1, based on the grid-connected converter Three-phase current of the grid-side inductor of the LCL filter , , Perform zero-sequence circulating current calculations to obtain the grid-connected converter. Zero-order circulation , ; Step 3.2, connect the grid-connected converter zero-sequence current After filtering with a bandpass filter, the grid-connected converter is obtained. zero-sequence current at switching frequency ; based on the zero-sequence current of the switching frequency Calculate the grid-connected converter Input current of dead-zone oscillator , .in For grid-connected converters The zero-sequence current scaling factor of the switching frequency; Step 3.3, connect the grid-connected converter Input current of dead-zone oscillator Input grid-connected converter In a dead-zone oscillator, the dead-zone oscillator calculates its own output voltage. ; Step 3.4, for the grid-connected converter The terminal voltage of the dead zone oscillator Perform positive zero-crossing monitoring and carrier reset, specifically at the terminal voltage. When the signal changes from negative to positive, a carrier reset signal is sent to control the grid-connected converter. The triangular carrier is reset to phase 0.

[0046] The dead-zone oscillator is described by the following differential equation:

[0047] in, For grid-connected converters The inductance value of the dead-zone oscillator, For grid-connected converters The capacitance value of the dead-zone oscillator, For grid-connected converters The resistance value of the dead-zone oscillator; For grid-connected converters The expression for the piecewise linear voltage-controlled current source of the dead-zone oscillator is as follows:

[0048] in, For grid-connected converters The boundary point of the piecewise linear voltage-controlled current source of the dead-zone oscillator. For grid-connected converters The voltage slope of the piecewise linear voltage-controlled current source of the dead-zone oscillator.

[0049] In this embodiment, for any grid-connected converter ,set up , The parameters of its Andronov-Hopf oscillator, which contains both voltage and current closed-loop circuits, are set as follows: , , , , , , .

[0050] , , , . . , , ; , .

[0051] To demonstrate the technical effects of this invention, the following is an example of using... Figure 2 The control method of the grid-connected converter parallel system was simulated and verified. At simulation time 0s, grid-connected converter 1 began pre-synchronization. After pre-synchronization, grid-connected converter 1 automatically connected to the grid. After grid connection, grid-connected converter 1 began to execute Andronov-Hopf oscillator grid-connection control with voltage and current dual closed loops. At simulation time 0.1s, grid-connected converter 2 began pre-synchronization. After pre-synchronization, grid-connected converter 2 automatically connected to the grid. After grid connection, grid-connected converter 2 began to execute Andronov-Hopf oscillator grid-connection control with voltage and current dual closed loops. At simulation time 0.35s, grid-connected converter 1 and grid-connected converter 2 simultaneously executed dead-time oscillator circulating current suppression.

[0052] Figure 3 The diagram shows the total grid-connected current waveform of the parallel grid-connected converter system of the present invention. It can be seen that grid-connected converter 1 and grid-connected converter 2 are connected to the grid sequentially, followed by dead-zone oscillator circulating current suppression. After a period of adjustment, once circulating current suppression is successfully completed, the parallel grid-connected converter system returns to stable operation.

[0053] Figure 4The diagram shows the waveforms of the terminal voltage of grid-connected converter 1 and the terminal voltage of the dead-zone oscillator of grid-connected converter 2 in the parallel grid-connected converter system of the present invention. When dead-zone oscillator circulating current suppression is not performed, the terminal voltages of the dead-zone oscillator of grid-connected converter 1 and the dead-zone oscillator of grid-connected converter 2 are out of phase. After dead-zone oscillator circulating current suppression is performed, both grid-connected converter 1 and grid-connected converter 2 begin to adjust the terminal voltages of their respective dead-zone oscillators, and ultimately the terminal voltages of the dead-zone oscillator of grid-connected converter 1 and the dead-zone oscillator of grid-connected converter 2 are in phase.

[0054] Figure 5 The diagram shows the triangular carrier waveforms of two grid-connected converter modules in the parallel grid-connected converter system of the present invention. As can be seen from the diagram, before dead-time oscillator circulating current suppression is implemented, the triangular carriers of grid-connected converter 1 and grid-connected converter 2 are out of phase. After dead-time oscillator circulating current suppression is implemented, both grid-connected converter 1 and grid-connected converter 2 begin to adjust their respective triangular carrier phases. Ultimately, the triangular carrier phases of grid-connected converter 1 and grid-connected converter 2 are completely synchronized, thus completing the suppression of zero-sequence circulating current.

[0055] Figure 6 This is a waveform diagram of the zero-sequence circulating current in the grid-connected converter parallel system of the present invention. As can be seen from the figure, after grid-connected converter 2 is connected to the grid, a zero-sequence circulating current appears in the grid-connected converter parallel system. After the dead-zone oscillator circulating current suppression is started, the zero-sequence circulating current is successfully suppressed after a period of adjustment.

Claims

1. A modular grid-connected converter circulating current suppression and grid-connected control method based on a virtual oscillator. The grid-connected converter parallel system applying this control method consists of… A grid-connected converter module is formed by connecting in parallel with a common DC bus and a common AC bus. Any one of these grid-connected converter modules is denoted as a grid-connected converter. , The grid-connected converter The system comprises a DC bus capacitor, a main inverter, and an LCL filter connected in series. The LCL filter includes a machine-side inductor, a grid-side inductor, and a filter capacitor. Its characteristic is that... Includes the following steps: Step 1, collect data from the grid-connected converter. The three-phase voltage of the filter capacitor of the LCL filter , , The three-phase current of the machine-side inductor , , The three-phase current of the grid-side inductor , , ; Step 2: Obtain the grid-connected converter using the Andronov-Hopf oscillator grid control method with voltage and current dual closed loops. Three-phase modulated wave signal , , Generate grid-connected converter The three-phase bridge arm PWM control signal; Step 3: During the grid control process in Step 2, when a high-frequency zero-sequence circulating current occurs, it is suppressed using a dead-zone oscillator circulating current suppression method. Specifically, this is based on the grid-connected converter... Three-phase current of the grid-side inductor of the LCL filter , , Zero-sequence circulating current calculation is performed to obtain the grid-connected converter. Zero-order circulation ; Zero-sequence circulation After filtering with a bandpass filter, the grid-connected converter is obtained. Zero-sequence current at switching frequency And obtain the grid-connected converter Input current of dead-zone oscillator ; grid-connected converter Input current of dead-zone oscillator Input to grid-connected converter In a dead-zone oscillator, the output voltage And at the terminal voltage When the signal changes from negative to positive, a carrier reset signal is sent to control the grid-connected converter. The triangular carrier is reset to phase 0 to suppress the high-frequency zero-sequence circulating current.

2. The modular grid-connected converter circulating current suppression and grid control method based on a virtual oscillator according to claim 1, characterized in that, The implementation process of step 2 is as follows: Step 2.1, convert the three-phase current of the grid-side inductor , , By using the constant amplitude Clark transform, we obtain Components of the grid-side inductor current in the coordinate system , ; Step 2.2, according to Components of the grid-side inductor current in the coordinate system , Grid-connected converter Active power command and grid-connected converter reactive power command Calculate the output voltage of the Andronov-Hopf oscillator; specifically, first perform... Grid-connected converter in coordinate system The grid-side current command is calculated to obtain the grid-side current command at the current sampling time. , ; Grid-connected converter Andronov-Hopf oscillator calculation, the output voltage of the Andronov-Hopf oscillator at the current sampling moment is obtained , ; the grid-side current instruction of the current sampling moment , The calculation formula is as follows: wherein , are the output voltages of the Andronov-Hopf oscillator at the previous sampling instant, ; The output voltage of the Andronov-Hopf oscillator at the current sampling instant , is given by the following differential equation: in, , They are , The derivative with respect to time, i.e. , ; For grid-connected converters The convergence rate constant of the Andronov-Hopf oscillator For grid-connected converters The voltage amplification factor of the Andronov-Hopf oscillator For grid-connected converters The current amplification factor of the Andronov-Hopf oscillator This refers to the rated voltage amplitude of the Andronov-Hopf oscillator. For grid-connected converters The rated angular frequency of the Andronov-Hopf oscillator For grid-connected converters The current error rotation angle of the Andronov-Hopf oscillator For grid-connected converters The virtual capacitance parameters of the Andronov-Hopf oscillator ; Step 2.3, according to the calculation formula Obtaining electrical angle ; and according to the electrical angle The three-phase current of the grid-side inductor , , Perform a constant-amplitude rotation coordinate transformation to obtain the grid-connected converter. Current components of the grid-side inductor in the dq coordinate system , ; Three-phase voltage of the filter capacitor , , The grid-connected converter is obtained by performing a constant amplitude rotation coordinate transformation. Voltage components in the dq coordinate system , Three-phase current of the machine-side inductor , , The grid-connected converter is obtained by performing a constant amplitude rotation coordinate transformation. Current components of the machine-side inductor in the dq coordinate system , For grid-connected converters The output voltage of the Andronov-Hopf oscillator at the current sampling time , The voltage components of the Andronov-Hopf oscillator in the dq coordinate system are obtained by performing an equal-amplitude rotating coordinate transformation. , ; Step 2.4, based on the voltage components in the dq coordinate system of the Andronov-Hopf oscillator. , and electrical angle A dual closed-loop control of voltage and current is implemented; specifically, the grid-connected converter is obtained through the outer loop control equation of voltage in the dq coordinate system. The given value of the inner current loop in the dq coordinate system , Then, the grid-connected converter is obtained through the inner current control equation. Modulated wave signal in dq coordinate system , Subsequently, the grid-connected converter is obtained through an inverse coordinate transformation with equal amplitude rotation. Three-phase modulated wave signal , , It is then compared with a triangular carrier wave to generate a grid-connected converter. The PWM control signal for the three-phase bridge arm.

3. The virtual oscillator based modular grid-connected converter circulating current suppression and grid-forming control method of claim 2, wherein, The voltage outer loop control equation described in step 2.4 is as follows: in, For grid-connected converters The value of the filter capacitor for each phase in the LCL filter, For grid-connected converters The voltage outer loop proportionality coefficient, For grid-connected converters The voltage outer loop integral coefficient, For the Laplace operator; The formula for calculating the inner loop control equation of the current is: in, For grid-connected converters The inductance value per camera side of the LCL filter, For grid-connected converters The current inner loop proportionality coefficient, For grid-connected converters The integral coefficient of the inner loop of the current.

4. The virtual oscillator based modular grid-connected converter circulating current mitigation and grid-forming control method of claim 1, wherein, The implementation process of step 3 is as follows: Step 3.1, based on the grid-connected converter Three-phase current of the grid-side inductor of the LCL filter , , Perform zero-sequence circulating current calculations to obtain the grid-connected converter. Zero-order circulation , ; Step 3.2, connect the grid-connected converter zero-sequence current After filtering with a bandpass filter, the grid-connected converter is obtained. Zero-sequence current at switching frequency ; based on the zero-sequence current of the switching frequency Calculate the grid-connected converter Input current of dead-zone oscillator , .in For grid-connected converters The zero-sequence current scaling factor of the switching frequency; Step 3.3, connect the grid-connected converter Input current of dead-zone oscillator Input grid-connected converter In a dead-zone oscillator, the dead-zone oscillator calculates its own output voltage. ; Step 3.4, for the grid-connected converter The terminal voltage of the dead zone oscillator Perform positive zero-crossing monitoring and carrier reset, specifically at the terminal voltage. When the signal changes from negative to positive, a carrier reset signal is sent to control the grid-connected converter. The triangular carrier is reset to phase 0.

5. The virtual oscillator based modular grid-connected converter circulating current mitigation and grid-forming control method of claim 4, wherein, The dead-zone oscillator is described by the following differential equation: in, For grid-connected converters The inductance value of the dead-zone oscillator, For grid-connected converters The capacitance value of the dead-zone oscillator, For grid-connected converters The resistance value of the dead-zone oscillator; For grid-connected converters The expression for the piecewise linear voltage-controlled current source of the dead-zone oscillator is as follows: in, For grid-connected converters The boundary point of the piecewise linear voltage-controlled current source of the dead-zone oscillator. For grid-connected converters The voltage slope of the piecewise linear voltage-controlled current source of the dead-zone oscillator.

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