Method for seamless switching between grid-connected inverter and constructed network based on adaptive virtual impedance and feedforward technology

Abstract

The invention provides a grid-connected inverter and grid-following seamless switching method based on self-adaptive virtual impedance and a feedforward technology. The method comprises the following steps: 1, judging whether the switching type of a grid-connected inverter is switching from a grid-constructing type control mode to a grid-following type control mode or not; 2, if yes, latching and switching the d-axis and q-axis inductive current values of the grid-connected inverter in the net-building mode at the moment, and taking the d-axis and q-axis inductive current values as reference values of a current loop in the net-building mode, so as to generate a control signal; 3, if not, latching the capacitance voltage amplitude and the phase angle in the following network mode, and in the initial stage of switching to the network construction mode, respectively taking the capacitance voltage amplitude and the phase angle as initial values of voltage and phase angle references so as to generate a network construction control signal; and 4, adding linearly decreasing self-adaptive virtual impedance into the voltage loop under network-forming control. According to the invention, smooth switching between dual modes can be effectively realized when the power grid intensity changes and low-frequency oscillation occurs, so that current impact and oscillation in the switching process can be remarkably reduced, and the system stability is improved.

Description

Technical Field

[0001] This invention relates to the field of new energy utilization technology, specifically to a method for seamless grid-connected inverter switching based on adaptive virtual impedance and feedforward technology. Background Technology

[0002] Battery formation and capacity testing are core processes in the power battery production chain, directly impacting battery performance. To facilitate large-scale production, many manufacturers have adopted a novel Battery Formation and Capacity Testing System (BFGTS) to simultaneously perform formation and capacity testing on thousands of batteries. A key advantage of this system is that energy recovered during the discharge of some batteries can be used to charge others, significantly reducing manufacturers' energy costs. Under normal operating conditions, the grid-connected inverter within the BFGTS typically employs a grid-based control mode, providing a stable DC voltage to the DC / DC converter, which then regulates battery charging and discharging. However, the BFGTS must also support islanded operation during grid failures or achieve energy self-circulation according to specific operating mode requirements. In such cases, the system needs to introduce a grid-based control mode inverter to establish and maintain a stable AC voltage reference, ensuring that the grid-connected inverter and the entire system can continue to provide stable and reliable power to the batteries. Therefore, in practical applications, a hybrid BFGTS structure combining network-based and follow-network-based control modes is a promising solution, effectively improving system reliability and operational efficiency. However, this solution still faces a key challenge in practical applications: how to achieve seamless switching between network-based and follow-network-based control modes.

[0003] Extensive research has been conducted on methods for seamless switching between network-based and follow-network-based control modes. One approach is to use the same... qThe system can switch from grid-following control mode to grid-connecting control mode with minimal adjustments to the voltage loop, current loop, and sensor group. Secondly, a smooth dual-mode switching strategy for grid-connecting and grid-connecting control modes suitable for different grid strengths is proposed. Through smooth phase angle switching and dq-axis reference current switching, fluctuations in phase angle and dq-axis reference current are effectively suppressed, thus ensuring the system's operational stability under SCR changes. Furthermore, under grid voltage imbalance conditions, a dual-sequence control strategy further achieves seamless switching between grid-connecting and grid-connecting control modes. The positive-sequence controller ensures the continuity of power transmission, maintaining consistent phase angle, current reference, and voltage amplitude; the negative-sequence controller actively suppresses unbalanced current. Most existing transition methods rely on certain shared control loops, especially the common current inner loop. While ensuring the continuity of key variables such as voltage, current, and angle, this prevents either mode from operating at its theoretical optimal state, sacrificing some performance. For example, in grid-connecting control mode, using a PI regulator in the current loop to achieve error-free tracking of the inductor current is a common method. In grid-connected control mode, when a proportional regulator is used in the current loop, the inverter functions well as a voltage source. However, in this case, although the same current reference is applied to the current loop after mode switching, current surges cannot be suppressed within permissible limits, especially when switching from grid-connected to grid-connected control mode. Therefore, a novel mode switching method is urgently needed to improve the stability and control performance of the switching process, considering the different current loop structures that may exist in the system. Summary of the Invention

[0004] To address the aforementioned issues, this invention aims to propose a seamless grid-connected inverter switching method based on adaptive virtual impedance and feedforward technology. This method is designed to enable smooth switching between dual modes even when grid strength changes or low-frequency oscillations occur, thereby significantly reducing current surges and oscillations during the switching process and improving system stability.

[0005] To achieve the above objectives, the technical solution of the present invention is implemented as follows: The present invention provides a method for seamless grid-connected inverter switching based on adaptive virtual impedance and feedforward technology, characterized by the following steps: Step 1: Determine whether the switching type of the grid-connected inverter is switching from grid-connected control mode to grid-following control mode. If yes, execute Step 2 at the moment of switching. Otherwise, it means that the switching type is switching from grid-following control mode to grid-connected control mode, and execute Step 3 and Step 4 at the moment of switching. Step 2: Latch the d-axis inductor current value and q-axis inductor current value output by the grid-connected inverter in the grid-connected control mode, and use them as reference values ​​for the current loop in the grid-connected control mode, thereby obtaining the control signal of the grid-connected inverter in the grid-connected control mode, and ending the process; Step 3: Latch the capacitor voltage amplitude and phase angle of the grid-connected inverter output in the grid-connected control mode, and in the initial stage of switching to the grid-connected control mode, use the latched capacitor voltage amplitude and phase angle as the initial reference values ​​of the capacitor voltage amplitude and phase angle of the grid-connected inverter in the grid-connected control mode, respectively, so as to obtain the control signals of the capacitor voltage amplitude and phase angle of the grid-connected inverter in the grid-connected control mode; Step 4: Add a linearly decreasing adaptive virtual impedance to the voltage loop in the grid-connected control mode to suppress the current surge generated by the grid-connected inverter during mode switching.

[0006] The present invention provides a method for seamless grid-connected inverter switching based on adaptive virtual impedance and feedforward technology. The method is characterized in that, in step 2, the d-axis control signal of the grid-connected inverter in grid-connected control mode is obtained using equation (1). and q-axis control signal : (1) In equation (1), A PI controller for the current loop; These are the decoupling coefficients; , To switch the momentarily latched d-axis and q-axis inductor current values ​​of the grid-connected inverter output; , These are the actual values ​​of the d-axis inductor current and the q-axis inductor current output by the grid-connected inverter. s This represents the Laplace operator.

[0007] Furthermore, in step 3, the control signal for the capacitor voltage amplitude of the grid-connected inverter in the grid-connected control mode is obtained using equation (2). Control signal with phase angle : (2) In equation (2), and To switch the momentarily latched capacitor voltage amplitude and phase angle of the grid-connected inverter output; and These are the reference values ​​for the active power and reactive power of the grid-connected inverter. and This refers to the actual active power and actual reactive power of the grid-connected inverter. This is virtual inertia; The damping coefficient; This is the reactive power regulation coefficient. s This represents the Laplace operator.

[0008] Furthermore, in step 4, the adaptive virtual impedance is designed using equation (3): (3) In equation (3), and These are adaptive virtual resistance and adaptive virtual reactance. and These are the initial values ​​of the adaptive virtual resistance and the adaptive virtual reactance. and Let be the slope of the resistance and reactance.

[0009] The present invention provides an electronic device, including a memory and a processor, characterized in that the memory is used to store a program supporting the processor in performing the method described therein, and the processor is configured to execute the program stored in the memory.

[0010] The present invention discloses a computer-readable storage medium storing a computer program, characterized in that the computer program is executed by a processor to perform the steps of the method described thereon.

[0011] Compared with existing methods, the beneficial effects of the present invention are: 1. This invention achieves smooth and seamless mode switching, effectively suppressing switching current surges. By combining a feedforward-based reference value transfer method with a linearly decreasing adaptive virtual impedance, it can significantly reduce current surges and oscillations during mode switching, achieving a near-seamless smooth transition, even when the current loop structures and parameters differ between the grid-type and follow-grid control modes.

[0012] 2. This invention balances dynamic suppression and steady-state performance without affecting normal system operation. The adaptive virtual impedance used only takes effect during mode switching, and its impedance value automatically decays to zero over time. This effectively suppresses transient current during switching while ensuring that the original impedance characteristics and output current of the inverter in grid-connected control mode are not affected during stable operation, thus solving the problem of steady-state performance degradation that may be caused by traditional fixed virtual impedance. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the dual-mode control structure of the grid-connected inverter in this invention; Figure 2a This is a schematic diagram of the inductor current locking circuit in the network control of this invention; Figure 2b This is a schematic diagram of the current loop reference value switching process in the grid-type control of this invention; Figure 3a This is a schematic diagram of the process of locking the phase and amplitude of the output capacitor voltage in the grid-controlled phase-locked loop of the present invention; Figure 3b This is a schematic diagram of the capacitor voltage phase and amplitude feedforward process in the VSG algorithm of the network control in this invention; Figure 4 This is a schematic diagram of the adaptive virtual impedance element in this invention; Figure 5a This is the current waveform diagram of the sixth inverter when only voltage reference feedforward control is used in this invention; Figure 5b This is the current waveform diagram of the entire system when only voltage reference feedforward control is used in this invention; Figure 6a This is the current waveform diagram of the sixth inverter when adaptive virtual impedance and feedforward technology are used in this invention; Figure 6b This is the current waveform diagram of the entire system when adaptive virtual impedance and feedforward technology are used in this invention. Detailed Implementation

[0014] The specific embodiments and working principles of the present invention will be further described in detail below with reference to the accompanying drawings.

[0015] In this embodiment, the main circuit and dual-mode control structure block diagram of the BFGTS inverter under study are as follows: Figure 1 As shown. The grid-connected control mode operates under grid-connected conditions and mainly includes a DC voltage loop, a phase-locked loop, and a current loop, controlling the electrical energy absorbed or generated from the grid based on the magnitude of the DC bus voltage. The network-connected control mode mainly includes a virtual synchronous generator algorithm and a voltage and current dual closed loop, which can operate under both off-grid and grid-connected conditions to suppress broadband oscillations in weak grids.

[0016] The main circuit section of a grid-connected inverter is the same as that of a conventional grid-connected inverter. V dc This is the DC-side input voltage. L f , L g These are the inverter-side filter inductor and the grid inductor, respectively. C f For filtering capacitors, R c This refers to the parasitic resistance of the filter capacitor. u cabc and u cdq They are respectively abccoordinate system and dq The terminal voltage of the inverter filter inductor in the coordinate system. i Labc and i Ldq They are respectively abc coordinate system and dq The current in the filter inductor in the coordinate system. i gabc This represents the grid-side inductor current.

[0017] Based on the overall control structure of grid-connected inverters with both grid-connected and grid-following control modes, a seamless grid-connected inverter switching method based on adaptive virtual impedance and feedforward technology can not only quickly suppress the current surge during mode switching, but also solve the mode switching problem caused by the different current loop structures and parameters resulting from the dynamic performance requirements of grid-connected and grid-following control modes. Specifically, this method is implemented as follows: Step 1: Determine whether the switching type of the grid-connected inverter is switching from grid-connected control mode to grid-following control mode. If yes, execute Step 2 at the moment of switching. Otherwise, it means that the switching type is switching from grid-following control mode to grid-connected control mode, and execute Step 3 and Step 4 at the moment of switching. The switching type of the inverter is determined based on conditions such as grid connection / off-grid status and weak grid oscillation switching. When the switch signal is set to 1, the grid-connected control mode is effective; when the switch signal is 0, the grid-connected control mode is effective.

[0018] Step 2: Latch the d-axis inductor current value and q-axis inductor current value output by the grid-connected inverter in the grid-connected control mode, and use them as reference values ​​for the current loop in the grid-connected control mode, thereby obtaining the control signal of the grid-connected inverter in the grid-connected control mode, and ending the process; When the switch signal changes from 0 to 1, through Figure 2a The latch latches the d-axis and q-axis values ​​of the actual inductor current in the grid-type control mode at the moment of switching. It is worth noting that in the grid-type control mode, the phase-locked loop in the grid-type control mode always remains effective to synchronize with the phase of the output voltage.

[0019] After switching to mesh control mode, current loop control is as follows: Figure 2b As shown, the latched inductor current , As a reference value for the current loop in the grid-following control mode, it ensures that the inductor current can track the actual inductor current in the grid-following control mode, rather than the reference current value of the controller, at the moment of switching. This ensures that the current can be switched almost seamlessly even when there are significant differences in the current loop structure and parameters between the two modes.

[0020] The d-axis control signal of the grid-connected inverter in the grid-connected control mode is obtained using equation (1). and q-axis control signal : (1) In equation (1), A PI controller for the current loop; These are the decoupling coefficients; , To switch the momentarily latched d-axis and q-axis inductor current values ​​of the grid-connected inverter output; , These are the actual values ​​of the d-axis inductor current and the q-axis inductor current output by the grid-connected inverter. s This represents the Laplace operator.

[0021] Step 3: Latch the capacitor voltage amplitude and phase angle of the grid-connected inverter output in the grid-connected control mode, and in the initial stage of switching to the grid-connected control mode, use the latched capacitor voltage amplitude and phase angle as the initial reference values ​​of the capacitor voltage amplitude and phase angle of the grid-connected inverter in the grid-connected control mode, respectively, so as to obtain the control signals of the capacitor voltage amplitude and phase angle of the grid-connected inverter in the grid-connected control mode; When the switching signal changes from 1 to 0, the inverter in grid-connected control mode needs to exhibit voltage source characteristics. Therefore, to ensure that the capacitor voltage is the same when switching to grid-connected control mode as in grid-connected control mode, such as... Figure 3a In the middle, the latch latches the measured value of the capacitor voltage in the grid control mode at the moment of switching. d Axis width Phase angle of the phase-locked loop output That is, the phase angle and amplitude of the output capacitor voltage; after switching to the network control mode, such as Figure 3b As shown, the latched phase and voltage amplitude are fed forward to the active-phase link and reactive-voltage link respectively, serving as reference values ​​for the capacitor voltage loop in the grid-type control mode, thus avoiding the phenomenon of long switching process and large oscillation amplitude caused by phase jump.

[0022] The control signal for the capacitor voltage amplitude of the grid-connected inverter in the grid-connected control mode is obtained using equation (2). Control signal with phase angle : (2) In equation (2), and To switch the momentarily latched capacitor voltage amplitude and phase angle of the grid-connected inverter output; and These are the reference values ​​for the active power and reactive power of the grid-connected inverter. and This refers to the actual active power and actual reactive power of the grid-connected inverter. This is virtual inertia; The damping coefficient; This is the reactive power regulation coefficient. s This represents the Laplace operator.

[0023] Step 4: Add a linearly decreasing adaptive virtual impedance to the voltage loop in the grid-connected control mode to suppress the current surge generated by the grid-connected inverter during mode switching.

[0024] However, the switching method in step three is based on the principle of keeping the capacitor voltage constant. Under system oscillation, there may be a phenomenon where the inductor current cannot be suppressed quickly at certain switching moments. In order to suppress the large current surge that may occur during mode switching under oscillation, this patent adds... Figure 4 The adaptive virtual impedance shown is used to suppress the current during switching. To avoid altering the active and reactive currents of the network output due to the addition of virtual impedance, a linearly decreasing adaptive virtual impedance is designed, with the specific expression as follows: (3) In equation (3), and These are adaptive virtual resistance and adaptive virtual reactance. and These are the initial values ​​of the adaptive virtual resistance and the adaptive virtual reactance. and Let be the slope of the resistance and reactance.

[0025] The virtual impedance gradually decays to 0 over time, thus ensuring that the virtual impedance does not change the impedance characteristics and output current of the grid-connected inverter during stable operation, and only suppresses the current surge generated during inverter mode switching.

[0026] In this embodiment, an electronic device includes a memory and a processor. The memory stores a program that supports the processor in executing the above-described method, and the processor is configured to execute the program stored in the memory.

[0027] In this embodiment, a computer-readable storage medium stores a computer program, which is executed by a processor to perform the steps of the above method.

[0028] Example: To verify the effectiveness of the proposed switching mode method and its effectiveness in improving stability under different grid strengths, the following scenario was designed: Initially, six grid-connected units operated with an SCR of 3.528. At 0.14 seconds, the SCR switched to 1.764, and low-frequency oscillations occurred. At 0.2 seconds, the sixth inverter was commanded to switch to grid-connected control mode, and the low-frequency oscillations should have been eliminated. At 0.6 seconds, the SCR increased to 1.764, and the low-frequency oscillations occurred again. At 0.7 seconds, the inverter in grid-connected control mode switched back to grid-connected control mode to stabilize the system. Furthermore, it is worth noting that the regulators and parameters of the current loop are different in grid-connected and grid-connected control modes.

[0029] Therefore, the system will perform two mode switches: a 0.2s switch from network-following control mode to network-building control mode, and a 0.7s switch from network-building control mode to network-following control mode. Two schemes were designed for comparison. Option 1: Use the traditional parameter feedforward method to switch control methods. Option 2: Employing the novel parameter feedforward control method with adaptive virtual impedance proposed in this patent. The results of switching control methods using traditional parameter feedforward methods are as follows: Figure 5a and Figure 5b As shown, the novel control method using parameter feedforward and adaptive virtual impedance proposed in this patent is as follows: Figure 6a and Figure 6b As shown.

[0030] Depend on Figure 5a It can be seen that when the traditional parameter feedforward method is used to switch the control method, at 0.2s, the grid-based control mode switches to the network-connected control mode, and the maximum current of the sixth inverter increases to over 1000A. Simultaneously, the entire system current oscillates at a low frequency before decaying to... Figure 5b The steady state is shown. Clearly, this significant current surge indicates that suppressing the current increase solely through output voltage reference feedforward is limited. At 0.7s, the grid-based control mode switches to grid-following control mode, and the current of the sixth inverter and the entire system smoothly transitions from grid-based to grid-following control mode. In contrast, at 0.2s, using adaptive virtual impedance and feedforward technology, as... Figure 6a As shown, at 0.2s, the current waveform of the sixth inverter is... Figure 5a The waveforms in the video are much smoother. Figure 6b The current waveform of the entire system shown exhibits no sudden current changes during switching. Simultaneously, when the inverter switches from grid-connected control mode to grid-following control mode, i.e., at 0.7 seconds, as... Figure 6a , 6bThe current waveform can remain stable and smooth. Therefore, when the current loop parameters of the grid-based control mode and the follow-grid control mode are different, the seamless switching method of the follow-grid based on adaptive virtual impedance and feedforward technology is effective and can effectively suppress current fluctuations at the switching moment.

Claims

1. A method for seamless grid-connected inverter switching based on adaptive virtual impedance and feedforward technology, characterized in that, Includes the following steps: Step 1: Determine whether the switching type of the grid-connected inverter is switching from grid-connected control mode to grid-following control mode. If yes, execute Step 2 at the moment of switching. Otherwise, it means that the switching type is switching from grid-following control mode to grid-connected control mode, and execute Step 3 and Step 4 at the moment of switching. Step 2: Latch the d-axis inductor current value and q-axis inductor current value output by the grid-connected inverter in the grid-connected control mode, and use them as reference values ​​for the current loop in the grid-connected control mode, thereby obtaining the control signal of the grid-connected inverter in the grid-connected control mode, and ending the process; Step 3: Latch the capacitor voltage amplitude and phase angle of the grid-connected inverter output in the grid-connected control mode, and in the initial stage of switching to the grid-connected control mode, use the latched capacitor voltage amplitude and phase angle as the initial reference values ​​of the capacitor voltage amplitude and phase angle of the grid-connected inverter in the grid-connected control mode, respectively, so as to obtain the control signals of the capacitor voltage amplitude and phase angle of the grid-connected inverter in the grid-connected control mode; Step 4: Add a linearly decreasing adaptive virtual impedance to the voltage loop in the grid-connected control mode to suppress the current surge generated by the grid-connected inverter during mode switching.

2. The seamless grid-connected inverter and grid-connected switching method based on adaptive virtual impedance and feedforward technology according to claim 1, characterized in that, In step 2, the d-axis control signal of the grid-connected inverter in the grid-connected control mode is obtained using equation (1). and q-axis control signal : (1) In equation (1), A PI controller for the current loop; These are the decoupling coefficients; , To switch the momentarily latched d-axis and q-axis inductor current values ​​of the grid-connected inverter output; , These are the actual values ​​of the d-axis inductor current and the q-axis inductor current output by the grid-connected inverter. s This represents the Laplace operator.

3. The seamless grid-connected inverter and grid-connected switching method based on adaptive virtual impedance and feedforward technology according to claim 1, characterized in that, In step 3, the control signal for the capacitor voltage amplitude of the grid-connected inverter in the grid-connected control mode is obtained using equation (2). Control signal with phase angle : (2) In equation (2), and To switch the momentarily latched capacitor voltage amplitude and phase angle of the grid-connected inverter output; and These are the reference values ​​for the active power and reactive power of the grid-connected inverter. and This refers to the actual active power and actual reactive power of the grid-connected inverter. This is virtual inertia; The damping coefficient; This is the reactive power regulation coefficient. s This represents the Laplace operator.

4. The seamless grid-connected inverter and grid-connected switching method based on adaptive virtual impedance and feedforward technology according to claim 1, characterized in that, In step 4, the adaptive virtual impedance is designed using equation (3): (3) In equation (3), and These are adaptive virtual resistance and adaptive virtual reactance. and These are the initial values ​​of the adaptive virtual resistance and the adaptive virtual reactance. and Let be the slope of the resistance and reactance.

5. An electronic device, comprising a memory and a processor, characterized in that, The memory is used to store a program that supports a processor in executing the method of any one of claims 1-4, the processor being configured to execute the program stored in the memory.

6. A computer-readable storage medium storing a computer program thereon, characterized in that, The computer program is executed by the processor to perform the steps of the method according to any one of claims 1-4.

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