A control method and system for multi-vsg inverter circulating current suppression and fault handling

Abstract

This invention provides a control method and system for suppressing circulating current and handling faults in multiple VSG inverters, comprising: during grid-connection switching from off-grid to grid-connected operation: for each VSG inverter with its power parameters set, a pre-synchronization control compensation method is used to compensate each VSG inverter until the grid connection requirements are met; during steady-state operation after grid connection: if an asymmetrical load fault occurs, a current-source inverter is connected in parallel with the VSG inverters with their power parameters set and connected to the grid to compensate for the unbalanced current; wherein, the power parameters of each VSG inverter are set based on its rated capacity. The multi-VSG inverter control method proposed in this invention not only effectively ensures that power distribution can be achieved through parameter settings for multiple VSG inverters, but also ensures synchronization with the grid during the pre-synchronization process, effectively suppressing grid-connection inrush current, improving power quality, and achieving seamless switching between operating modes. Furthermore, it eliminates the circulating current problem between multiple VSG inverters.

Description

Technical Field

[0001] This invention belongs to the field of smart grid grid-connected power generation technology, specifically relating to a control method and system for suppressing circulating current and handling faults in multi-VSG inverters. Background Technology

[0002] In microgrids, grid-connected and off-grid (islanded) operating modes are key to demonstrating the technological and economic advantages of microgrids. Therefore, seamless and smooth switching technology is crucial to ensuring a smooth transition between these two operating modes. Virtual Synchronous Generator (VSG) technology, by simulating the operating mechanism of a synchronous generator, can freely transition from islanded to grid-connected operation, thus eliminating the cumbersome V / F to P / Q conversion. Furthermore, its inertial mechanism facilitates the integration of distributed power sources and improves power system stability.

[0003] Although VSG inverters offer some amplitude and phase compensation capabilities compared to conventional inverters, their main circuitry is still composed of fragile power electronic components, resulting in weak overvoltage and overcurrent resistance. If VSG inverters still employ the traditional method of synchronous generator-driven grid connection, or if emergency situations such as load asymmetry occur after grid connection or disconnection, the inrush current generated during grid connection could severely damage the inverter's switching devices and magnetic components. Furthermore, traditional grid-connected inverters require two coordinate transformations and five PI stages in the pre-synchronization phase to ensure successful pre-synchronization, a cumbersome and time-consuming process.

[0004] During the off-grid to grid-connected switching process of multiple inverters, especially at the moment of switching from off-grid to grid-connected operation, or in emergency situations such as sudden load changes, there will be a large inrush current, which will cause distortion of grid-connected current and PCC point voltage, reduce power quality, and cause serious damage to electronic components. Therefore, in addition to considering the pre-synchronization problem during switching, when multiple VSGs are connected to form a microgrid, there is also a serious circulating current problem. Therefore, the pre-synchronization grid-connected control of multiple VSG inverters, the pre-synchronization grid-connected control under off-grid load faults, power distribution and circulating current suppression are particularly important. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention proposes a control method for circulating current suppression and fault handling in multi-VSG inverters, the improvement of which includes:

[0006] When switching from off-grid to grid-connected: For each VSG inverter that has completed the power parameter setting, the pre-synchronization control compensation method is used to compensate each VSG inverter separately until the grid connection requirements are met;

[0007] When the grid is connected and in steady state: if an unbalanced load fault occurs, the current-source inverter and the VSG inverter after the power parameters have been set will be connected in parallel to the grid to compensate for the unbalanced current.

[0008] The power parameters of each VSG inverter are set based on its rated capacity.

[0009] The first preferred technical solution provided by the present invention is improved in that, the step of setting the power parameters of each VSG inverter based on the rated capacity of each VSG inverter includes:

[0010] Obtain the rated capacity of each VSG inverter;

[0011] Set the power parameters for each VSG inverter according to the ratio of the rated capacity of each VSG inverter;

[0012] The power parameters include virtual reactance and reactive voltage droop coefficient, which are inversely proportional to the rated capacity, and active frequency droop coefficient, virtual moment of inertia, and virtual damping coefficient, which are proportional to the rated capacity.

[0013] The second preferred technical solution provided by the present invention is improved in that, for each VSG inverter that has completed power parameter setting, a pre-synchronization control compensation method is used to compensate each VSG inverter separately until the grid connection requirements are met, including:

[0014] For each VSG inverter with completed power parameter settings, pre-synchronization control is performed to compensate for the angular frequency and voltage of the VSG inverter;

[0015] Determine whether each VSG inverter meets the grid connection requirements after pre-synchronization control: if so, connect the VSG inverter that meets the grid connection conditions to the grid; otherwise, continue pre-synchronization control until the grid connection requirements are met.

[0016] The third preferred technical solution provided by the present invention is improved in that the pre-synchronization control compensation of the angular frequency and voltage of the VSG inverter includes:

[0017] Phase-locked loops are used to obtain the voltage phase angle of the power grid;

[0018] The output voltage of the VSG inverter is transformed by dq based on the phase angle to obtain the d-axis component and the q-axis component.

[0019] The d-axis component is input into the PI controller to obtain the angular frequency adjustment amount, and the deviation between the q-axis component and the three-phase grid voltage amplitude is input into the PI controller to obtain the amplitude adjustment amount.

[0020] The reference angular frequency of the islanding mode is added to the angular frequency of the angular frequency adjustment to obtain the reference angular frequency of the pre-synchronization stage. The reference voltage amplitude of the pre-synchronization stage is added to the amplitude of the amplitude adjustment and the reference voltage amplitude of the islanding mode.

[0021] The fourth preferred technical solution provided by the present invention is improved in that determining whether the VSG inverter meets the grid connection requirements includes:

[0022] The output voltage of the VSG inverter is converted to dq to obtain the d-axis output voltage and the q-axis output voltage.

[0023] Determine whether the absolute value of the q-axis output voltage is less than the q-axis threshold, and determine whether the error between the peak value of the d-axis output voltage and the grid phase voltage is less than the d-axis threshold.

[0024] When both judgment results are yes, the VSG inverter meets the grid connection requirements; otherwise, it does not meet the grid connection requirements.

[0025] The fifth preferred technical solution provided by the present invention is improved in that, after the grid connection requirements are met, it further includes:

[0026] Set the angular frequency and voltage compensation of the VSG inverter after it is connected to the grid to zero.

[0027] The sixth preferred technical solution provided by the present invention is improved in that the step of connecting the current-source inverter and the VSG inverter after the power parameters have been set in parallel and connecting them to the power grid to compensate for unbalanced current includes:

[0028] The current-source inverter is connected in parallel with the VSG inverter after the power parameters have been set and then connected to the power grid.

[0029] The load current is decomposed into balanced active current, balanced reactive current, unbalanced current and no-load current components using conservative power theory.

[0030] The unbalanced current is compensated by controlling the output current of the current-source inverter to be equal to the unbalanced current and the no-load current component in the load current.

[0031] The seventh preferred technical solution provided by the present invention is improved in that it further includes:

[0032] When operating off-grid and experiencing a load asymmetry fault: For each VSG inverter with completed power parameter settings, a negative sequence control method is used to control each VSG inverter separately, reducing the output voltage imbalance during off-grid load asymmetry faults.

[0033] The eighth preferred technical solution provided by the present invention is improved in that the step of using a negative sequence control method to control the VSG inverter includes:

[0034] The three-phase voltage and three-phase current of the VSG inverter in the three-phase stationary coordinate system are converted into two-phase voltage and current in the two-phase stationary coordinate system.

[0035] A second-order generalized integrator is used to separate the positive and negative sequences of the two-phase voltage and current to obtain the positive and negative sequence components of the two-phase voltage and current.

[0036] The positive-sequence and negative-sequence components of the two-phase voltage and current are transformed from the two-phase stationary coordinate system to the two-phase rotating coordinate system to obtain the positive-sequence dq-axis voltage and current and the negative-sequence dq-axis voltage and current, respectively.

[0037] The positive-sequence dq-axis voltage and current are controlled, and the negative-sequence dq-axis voltage is set to zero and controlled, so that the inverter output voltage contains only the positive-sequence component.

[0038] Based on the same inventive concept, the present invention also provides a control system for circulating current suppression and fault handling of multiple VSG inverters, the improvement of which is that it includes: a grid-connected switching module and a grid-connected fault handling module;

[0039] The grid-connected fault handling module is used to, when the grid is connected to steady state, if an asymmetrical load fault occurs, connect the current-source inverter and the VSG inverter (after power parameter setting) in parallel to the grid to compensate for the unbalanced current.

[0040] The power parameters of each VSG inverter are set based on its rated capacity.

[0041] The ninth preferred technical solution provided by the present invention is improved in that it further includes a parameter setting module for setting the power parameters of each VSG inverter, the parameter setting module including: a capacity acquisition unit and a parameter setting unit;

[0042] The capacity acquisition unit is used to acquire the rated capacity of each VSG inverter;

[0043] The parameter setting unit is used to set power parameters for each VSG inverter according to the ratio of the rated capacity of each VSG inverter.

[0044] The power parameters include virtual reactance and reactive voltage droop coefficient, which are inversely proportional to the rated capacity, and active frequency droop coefficient, virtual moment of inertia, and virtual damping coefficient, which are proportional to the rated capacity.

[0045] The tenth preferred technical solution provided by the present invention is improved in that the grid connection switching module includes: a pre-synchronization control unit and a grid connection judgment unit;

[0046] The pre-synchronization control unit is used to perform pre-synchronization control compensation of the angular frequency and voltage of each VSG inverter that has completed the power parameter setting.

[0047] The grid connection judgment unit is used to determine whether each VSG inverter has met the grid connection requirements after pre-synchronization control. If so, the VSG inverter that has met the grid connection conditions will be connected to the grid. Otherwise, pre-synchronization control will continue until the grid connection requirements are met.

[0048] The eleventh preferred technical solution provided by the present invention is improved in that the grid connection fault handling module includes: a current-source inverter grid connection unit, a current decomposition unit and a current compensation unit.

[0049] The current-source inverter grid connection unit is used to connect the current-source inverter and the VSG inverter (after power parameter setting) in parallel to the power grid.

[0050] The current decomposition unit is used to decompose the load current into a balanced active current, a balanced reactive current, an unbalanced current and a no-load current component using conservative power theory.

[0051] The current compensation unit is used to compensate for the unbalanced current by controlling the output current of the current-source inverter to be equal to the unbalanced current and the no-load current component in the load current.

[0052] Compared with the closest existing technology, the present invention has the following beneficial effects:

[0053] This invention provides a control method and system for suppressing circulating current and handling faults in multiple VSG inverters, comprising: setting the power parameters of each VSG inverter based on its rated capacity; during grid-connection switching from off-grid to on-grid operation: for each VSG inverter with its power parameters set, a pre-synchronization control compensation method is used to compensate each VSG inverter until the grid connection requirements are met; during steady-state operation after grid connection: if an asymmetrical load fault occurs, a current-source inverter is connected in parallel with the VSG inverters with their power parameters set and connected to the grid to compensate for the unbalanced current. The multi-VSG inverter control method proposed in this invention not only effectively ensures that power distribution can be achieved through parameter settings for multiple VSG inverters, but also ensures synchronization with the grid during the pre-synchronization process, effectively suppressing grid-connection inrush current, improving power quality, and achieving seamless switching between operating modes. Furthermore, it eliminates the circulating current problem between multiple VSG inverters.

[0054] This invention requires only one phase-locked loop and two PI circuits in its calculation process, and directly connects the frequency compensation circuit to the VSG rotor, greatly reducing the computational load and making the grid connection algorithm simple, fast, and effective. It not only reduces current surges and achieves power distribution but also effectively suppresses circulating currents and handles faults during grid connection.

[0055] This invention not only enables correct power distribution, but also effectively suppresses problems such as seamless switching between off-grid and on-grid operations and the suppression of circulating currents and harmonics during grid connection in emergency situations. Attached Figure Description

[0056] Figure 1 This is a schematic flowchart of a control method for suppressing circulating current and handling faults in a multi-VSG inverter provided by the present invention.

[0057] Figure 2 This is a schematic diagram of the pre-synchronization process in a control method for suppressing circulating current and handling faults in a multi-VSG inverter provided by the present invention.

[0058] Figure 3 Diagram of the main circuit structure for two VSG inverters operating in parallel after incorporating the pre-synchronization strategy;

[0059] Figure 4 This invention provides a pre-synchronization control block diagram for a control method for suppressing circulating current and handling faults in multi-VSG inverters.

[0060] Figure 5 An improved VSG control block diagram with the introduction of a negative sequence control element provided by the present invention;

[0061] Figure 6 A schematic diagram of the phase current waveforms of the busbar of a microgrid with two VSGs connected in parallel without the pre-synchronization control strategy.

[0062] Figure 7 A schematic diagram of the active / reactive power waveforms output by VSG1 without the pre-synchronization control strategy;

[0063] Figure 8 A schematic diagram of the active / reactive power waveforms output by VSG2 without the pre-synchronization control strategy;

[0064] Figure 9 A schematic diagram of the phase current waveform of the busbar of the VSG1 parallel microgrid with pre-synchronization;

[0065] Figure 10 A schematic diagram of the phase current waveform of the busbar of the VSG2 parallel microgrid with pre-synchronization;

[0066] Figure 11 A schematic diagram of the active / reactive power waveforms of the VSG1 output with pre-synchronization added;

[0067] Figure 12 A schematic diagram of the active / reactive power waveforms of the VSG2 output with pre-synchronization added;

[0068] Figure 13 A schematic diagram of the circulating current waveform between two inverters before and after grid connection without the addition of pre-synchronization control;

[0069] Figure 14 A schematic diagram of the circulating current waveform between the two inverters before and after grid connection with pre-synchronization control;

[0070] Figure 15 This is a schematic diagram of the grid voltage and current waveforms before and after applying the grid current compensation scheme under asymmetrical load.

[0071] Figure 16 This invention provides a schematic diagram of the basic structure of a control system for suppressing circulating current and handling faults in a multi-VSG inverter.

[0072] Figure 17 This invention provides a detailed structural diagram of a control system for suppressing circulating current and handling faults in a multi-VSG inverter. Detailed Implementation

[0073] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0074] Example 1:

[0075] A schematic flowchart of a control method for circulating current suppression and fault handling of multi-VSG inverters provided by this invention is shown below. Figure 1 As shown, it includes:

[0076] Step 1: When switching from off-grid to grid-connected: For each VSG inverter that has completed the power parameter setting, the pre-synchronization control compensation method is used to compensate each VSG inverter until the grid connection requirements are met.

[0077] Step 2: When the grid is connected and in steady state: If an unbalanced load fault occurs, connect the current-source inverter and the VSG inverter (after power parameter setting) in parallel to the grid to compensate for the unbalanced current.

[0078] The power parameters of each VSG inverter are set based on its rated capacity.

[0079] This invention addresses the challenges of large inrush currents, circulating currents, or faults that occur when multiple inverters of different capacities are switched from off-grid to grid connection in emergency situations. It aims to quickly and accurately synchronize with the grid voltage and achieve power distribution, circulating current suppression, and fault handling.

[0080] The embodiments of the present invention will be described in detail below.

[0081] This invention targets a microgrid system composed of multiple VSG grid-connected inverters. First, power distribution is achieved by configuring suitable VSG power parameters. Then, phase-locked loop (PLL) technology is used to extract the grid voltage phase and await the pre-synchronization enable signal for grid connection. Upon arrival of this signal, the voltage at the microgrid bus is transformed using the grid voltage phase as a reference. The error between the q-axis output value and zero, and the error between the d-axis output value and the grid voltage amplitude, of each VSG inverter when disconnected from the grid are continuously monitored. If both errors are less than a pre-set threshold, it indicates that the voltage deviation on both sides of the PCC point is sufficiently small. This allows for rapid determination of the required compensation frequency and amplitude, closing the grid connection switch, and achieving grid connection. Considering the impact of asymmetrical load conditions before and after grid connection, a current-source inverter operating in parallel with the VSG inverters is added to compensate for the unbalanced current component of the load current, effectively suppressing grid current imbalance and PCC point voltage distortion.

[0082] The control strategy adopted requires fewer phase-locked loops and fewer PI control links than the traditional inverter pre-synchronization strategy. Furthermore, the frequency compensation link is directly compensated into the rotor equation of the VSG. Even under load asymmetry faults, it can quickly, accurately, and easily achieve seamless switching of multiple VSG inverters from off-grid to on-grid. Moreover, the circulating current in the microgrid is well suppressed at the moment of pre-synchronization, thus improving power quality.

[0083] The specific process of the control method for circulating current suppression and fault handling of multi-VSG inverters is as follows:

[0084] Step 101: Establishment of the Virtual Synchronous Generator Model

[0085] The virtual synchronous generator model includes the electronic and electromagnetic equations, torque and frequency equations, motion equations, and virtual excitation equations of the VSG. The forms of these equations are well-known and will not be described in this application.

[0086] Step 102: Analysis of VSG Off-Grid / On-Grid Switching Process

[0087] During the analysis, it was found that when switching from grid-connected to off-grid, the VSG inverter does not exhibit a significant transient process during the mode switching process. However, during the process of switching from off-grid to grid-connected, a large inrush current is generated, which can even lead to grid connection failure. The maximum instantaneous deviation can reach twice the grid voltage amplitude, i.e., 2U. This also leads to voltage distortion and deterioration of power quality.

[0088] Step 103: Selection of pre-synchronization control strategy for a single VSG inverter

[0089] Based on step 102, a PLL-based VSG inverter pre-synchronization control strategy was designed. By extracting the grid voltage phase using the PLL method, the grid connection switch is closed only when the output voltage amplitude, frequency and phase of the VSG inverter are consistent with the grid during the pre-synchronization stage. This control strategy avoids the drawbacks of direct grid connection.

[0090] Step 104: Generation of grid connection signal for a single VSG grid-connected inverter

[0091] After step 103 is completed, i.e. after the pre-synchronization enable signal for grid connection arrives, the frequency compensation circuit Δω and the amplitude compensation circuit ΔU begin operation. Figure 2 The following is the specific process:

[0092] The grid voltage phase is obtained through a phase-locked loop. When the pre-synchronization enable signal arrives, the VSG algorithm runs with the algorithm after adding pre-synchronization control and continuously checks whether the grid connection requirements are met. If not, the pre-synchronization adjustment continues; if it has been met, the grid connection switch is closed and the compensation amount is set to zero.

[0093] By continuously monitoring the q-axis output voltage U of the inverter after dq conversion q Is the absolute value less than the q-axis threshold u? qerms d-axis output voltage U d Is the error between the peak value of the phase voltage and the grid voltage less than the d-axis threshold u? derms If both of the above conditions are met, it indicates that the voltage deviation on both sides of the PCC point is small enough, and it is considered that the synchronization between the output voltage of the VSG inverter and the grid voltage has been basically achieved, allowing grid connection. At this time, the grid connection switch is closed, ultimately achieving a seamless switch from off-grid to grid connection for a single VSG inverter.

[0094] Step 105: Power Distribution and Parameter Configuration for Multiple VSG Inverters

[0095] For VSG inverters of different capacities operating in parallel within a microgrid, power distribution is crucial for stable system operation, regardless of whether it's in off-grid or grid-connected mode. Therefore, the first step is to configure power distribution and its parameters, followed by the design of a pre-synchronization control strategy. This is illustrated using two VSG inverters as an example. Figure 3 It can show its specific implementation method.

[0096] Figure 3The diagram shows the main circuit structure of two VSG inverters operating in parallel after incorporating a pre-synchronization control strategy. The diagram includes the main circuit topology and control circuit. The main circuit topology uses a typical LC three-phase bridge circuit, which can operate in both off-grid and grid-connected modes. The connection between the external power system and the power supply network is achieved through the Point of Common Coupling (PCC). When the PCC is disconnected from the main grid, the system automatically operates in off-grid mode, ensuring uninterrupted power supply to the loads within the microgrid. The control circuit includes a power calculation module, a VSG algorithm module, and a voltage and current dual closed-loop control module. The power calculation module samples the active and reactive power output of the inverter in real time; the VSG algorithm module makes the inverter's output characteristics similar to those of a synchronous generator; and the voltage and current dual closed-loop control further improves system stability. The frequency compensation value Δω (within the dashed box) is added during the pre-synchronization process. i (i=1,2) and voltage amplitude compensation value ΔU i (i=1,2) can achieve pre-synchronization of the two inverters respectively.

[0097] Step 105-a: Ignoring line impedance, the active power and reactive power output at the generator terminals of each VSG inverter are as follows: Among them, U g E represents the voltage amplitude of the power grid. i Let δ be the amplitude of the output voltage of the i-th VSG inverter; i Z represents the phase angle of the output voltage of the i-th VSG inverter. i Let be the equivalent impedance between the i-th VSG inverter and the power grid.

[0098] Step 105-b: Ensure that during the speed regulation process, the electromagnetic power and mechanical power are equal in a steady state. Therefore, the electromagnetic power P of the i-th VSG inverter... i and excitation electromotive force E i It can be represented as: P i =P refi +k ωi (ω ref -ω mi E i =E ref +k qi (Q refi -Q i ), where k ωi k represents the reactive voltage droop factor of the i-th VSG inverter. qi P represents the active frequency droop factor of the i-th VSG inverter. refi ω represents the reference value of active power given by the i-th VSG inverter. refThis represents the reference angular frequency value given by the VSG inverter, ω. mi E represents the true angular frequency of the i-th VSG inverter. ref Q represents the reference value of the voltage amplitude given at the grid connection point of the VSG inverter. refi This represents the reference value of the reactive power given by the i-th VSG inverter.

[0099] Step 105-c: Set the rated capacity S of the two VSG inverters. i * If i = 1, 2, and the ratio is N, then when the two excitation electromotive forces are equal during stable operation, the angles of the two grid-connected inverters are also equal, i.e., E1 = E2, δ1 = δ2. Then the active power ratio can be obtained as: Z2 = NZ1.

[0100] Step 105-d: Next, consider the distribution of reactive power. When the reactive power satisfies k... q2 =k q1 N allows the VSG inverters to be allocated according to their rated capacity ratio.

[0101] Step 105-e: To ensure good dynamic response characteristics for VSG inverters operating in parallel, considering the changes in active power of the two inverters as ΔP1 and ΔP2 respectively, we can obtain: When VSG inverters of different capacities are operating in parallel, the parameters should be set so that the virtual reactance X of each inverter is... i reactive voltage droop coefficient k ωi Inversely proportional to capacity; active frequency droop factor k qi Virtual rotational inertia J i Virtual damping coefficient D i With capacity S i * This relationship is proportional to the sum of its parts, enabling power distribution and providing good dynamic characteristics during disturbances. The specific relationship is as follows:

[0102]

[0103] Step 106: Control strategy for pre-synchronization, circulating current suppression, and fault handling of multi-VSG inverters in emergency situations.

[0104] Based on the power distribution achieved in step 105 for the parallel operation of multiple VSGs, the system operation control is handled in the following three operating conditions:

[0105] Scenario 1: Under normal circumstances, when multiple VSG inverters are operating in parallel and switching between grid and off-grid operation, the frequency compensation Δω and amplitude compensation ΔU of each VSG inverter are determined by the output voltage setpoint when it is off-grid. Figure 3 The specific calculation methods for Δω and ΔU are shown within the dashed box in the middle. Figure 4As shown, by ensuring that the voltage at the common connection point (PCC) does not deviate significantly from the grid voltage due to the instantaneous amplitude and phase differences in the output voltage of each VSG inverter, inrush current is effectively suppressed, achieving seamless switching from off-grid to grid-connected operation of multiple inverters. Furthermore, the proposed control strategy effectively suppresses circulating current issues among multiple inverters.

[0106] Scenario 2: When multiple VSG inverters are connected in parallel and off-grid, an emergency situation of load asymmetry occurs. This solution introduces a negative sequence control loop into the traditional VSG inverter control loop. The improved VSG control structure diagram is shown below. Figure 5 As shown. Inverter output voltage U in three-phase stationary coordinate system. abc and output current i abc U is transformed into a two-phase stationary coordinate system by coordinate transformation. αβ and i αβ After separating the positive and negative sequences using a second-order generalized integrator (SOGI), the positive and negative sequence components of the voltage and current are transformed from a two-phase stationary coordinate system to a two-phase rotating coordinate system. At this point, the positive and negative sequence voltage and current are controlled independently. The d-axis and q-axis negative sequence voltage setpoints in the negative sequence control loop are both set to 0, ensuring that the output voltage of the VSG inverter contains only the positive sequence component. When the VSG inverter system is in a stable operating state after the introduction of the negative sequence control loop, the pre-synchronization enable signal arrives. As described in Case 1, the output voltage setpoint of each VSG inverter when it is off-grid is compensated with its own frequency Δω and amplitude ΔU, which can also effectively suppress inrush current and achieve seamless switching from off-grid to grid-connected operation of multiple inverters.

[0107] Scenario 3: Building upon Scenario 1's successful implementation of multiple VSG inverters operating in parallel with seamless off-grid switching, the system is already in a stable operating state. In the event of an emergency load asymmetry, this solution adds a current-source inverter operating in parallel with the VSG inverters. Using conservative power theory (CPT), the load current is decomposed into balanced active current, balanced reactive current, unbalanced current, and no-load current components. The output current of this current-source inverter is controlled to equalize the unbalanced current and no-load current components in the load current, thereby compensating for these components. After reaching steady state, the load's external characteristics will exhibit a three-phase balanced load, as shown in the structural diagram below. Figure 3 As shown, this effectively suppresses grid current imbalance and PCC point voltage distortion.

[0108] Figure 4 This is a pre-synchronization control block diagram, which provides specific details. Figure 3 The calculation method for medium frequency and voltage amplitude compensation. The designed pre-synchronization control strategy mainly includes a frequency compensation element Δω and an amplitude compensation element ΔU. Three-phase grid voltage u gabc The phase angle of the output grid voltage after phase-locked loop is θg VSG inverter output voltage u abc With θ g Performing a dq transformation on the reference, we obtain the d-axis components U. d and q-axis component U q . Will U q The reference value is set to 0, U q The deviation from 0 is fed into a PI controller, and the output adjustment Δω is related to the islanding mode reference angular frequency ω. * Together, they constitute the reference angular frequency ω of the pre-synchronization element. m And the d-axis component U d The reference value is the three-phase grid voltage amplitude U. gm U d with U gm The deviation is fed into the PI controller, and the output adjustment ΔU is superimposed on the islanded mode reference voltage amplitude E to obtain the reference voltage amplitude U. ref Finally, together with the reference phase, they synthesize a three-phase reference voltage, thereby achieving pre-synchronization control.

[0109] Figure 5 This is an improved VSG control block diagram that introduces a negative sequence control element. The inverter output voltage U in the three-phase stationary coordinate system is shown in the diagram. abc and output current i abc U is transformed into a two-phase stationary coordinate system by coordinate transformation. αβ and i αβ After the positive and negative sequence components of the voltage and current are separated by a second-order generalized integrator (SOGI), they are transformed from a two-phase stationary coordinate system to a two-phase rotating coordinate system. At this time, the positive sequence voltage and current and the negative sequence voltage and current are controlled independently. The d-axis and q-axis negative sequence voltage setpoints in the negative sequence control loop are both set to 0. The control realizes that the output voltage of the VSG inverter contains only the positive sequence component, thereby reducing the imbalance of the output voltage when the off-grid load has an asymmetrical fault.

[0110] Example 2:

[0111] The following is a specific application example of a control method for circulating current suppression and fault handling in multi-VSG inverters. This control method is used to perform simulation analysis and verification when two VSGs of different capacities are operated in parallel. VSG1 has a capacity of 20kW + 10kvar, and VSG2 has a capacity of 10kW + 5kvar, with a capacity ratio of 2:1. Their common local load is 18kW + 15kvar. Figures 6-8The figures show the phase current waveform of the microgrid bus and the active / reactive power waveforms of VSG1 and VSG2, respectively. The simulation results show that in a microgrid system with multiple VSGs connected in parallel, grid connection can be achieved without any pre-synchronization control strategy. However, the microgrid bus current experiences a significant surge, approximately eight times the normal grid connection current. Simultaneously, the output power of the two inverters also generates substantial spikes and oscillations, reducing power quality.

[0112] The pre-synchronization control strategy proposed in this patent is introduced into the above-mentioned multi-machine parallel system. Figures 9-12 The simulation results show the phase current waveforms of the microgrid bus and the active / reactive power waveforms of VSG1 and VSG2 after the addition of pre-synchronization control. The simulation results demonstrate that VSG1 and VSG2 can allocate power according to the inverter capacity ratio based on local load demand in off-grid operation mode. In grid-connected operation mode, they provide energy to the grid and load according to the power reference value, proving the correctness of the power allocation control strategy. With the addition of the pre-synchronization control strategy, the inrush current of the microgrid bus is effectively suppressed, and power spikes and oscillations are significantly reduced, enabling seamless switching between off-grid and grid-connected operation of multiple VSG inverters with different capacities. The simulation results also show the phase current waveforms of the microgrid bus and the output power waveforms of VSG1 and VSG2 after the addition of pre-synchronization control. Figures 9-12 The waveform without the pre-synchronization control strategy Figures 6-8 The comparison shows that the inrush current was reduced from about 440A to about 80A, and the power spikes and oscillations were suppressed, demonstrating the correctness and effectiveness of the above control strategy.

[0113] The effect of adding a compensation stage on circulation suppression, for example Figures 13-14 As shown, the circulating current decreased from 80A to 12A, indicating that the control strategy can also effectively suppress the circulating current generated between inverters at the moment of grid connection.

[0114] When an asymmetrical load fault occurs, a solution is to add an additional current-source inverter to compensate for the nonlinear current, such as... Figure 15 As shown, a load asymmetry fault occurs at 0.2s, at which point the current and voltage become distorted. A current compensation scheme is added at 0.7s. It can be seen that this scheme effectively suppresses the distortion and imbalance of the grid-connected voltage and current. The total harmonic distortion (THD) of the grid-connected voltage decreases from 6.85% to 3.26%, and the THD of the grid-connected current decreases from 4.57% to 0.66%, significantly improving the grid-connected power quality. Simulation results show that this method can effectively suppress the distortion of the grid-connected voltage and current, significantly reduce the imbalance of the three-phase grid-connected current, effectively reduce the THD of the grid-connected voltage and current, and improve the grid-connected power quality.

[0115] Example 3:

[0116] Based on the same inventive concept, this invention also provides a control system for circulating current suppression and fault handling of multi-VSG inverters. Since the principle of these devices in solving technical problems is similar to the control method for circulating current suppression and fault handling of multi-VSG inverters, the repetition will not be repeated.

[0117] The basic structure of the system is as follows Figure 16 As shown, it includes: a grid connection switching module and a grid connection fault handling module;

[0118] in,

[0119] The grid-connected switching module is used to compensate each VSG inverter that has completed the power parameter setting by using a pre-synchronization control compensation method until the grid connection requirements are met when switching from off-grid to grid-connected.

[0120] The grid connection fault handling module is used to compensate for unbalanced current when the load is unbalanced during steady-state operation after grid connection if an unbalanced fault occurs. This is done by connecting the current-source inverter and the VSG inverter (with power parameters set) in parallel to the grid.

[0121] The power parameters of each VSG inverter are set based on its rated capacity.

[0122] The detailed structure of a control system for circulating current suppression and fault handling of multi-VSG inverters is as follows: Figure 17 As shown.

[0123] The system also includes a parameter setting module for setting the power parameters of each VSG inverter. The parameter setting module includes a capacity acquisition unit and a parameter setting unit.

[0124] The capacity acquisition unit is used to acquire the rated capacity of each VSG inverter;

[0125] The parameter setting unit is used to set the power parameters for each VSG inverter according to the ratio of the rated capacity of each VSG inverter;

[0126] The power parameters include virtual reactance and reactive voltage droop coefficient, which are inversely proportional to the rated capacity, and active frequency droop coefficient, virtual moment of inertia, and virtual damping coefficient, which are proportional to the rated capacity.

[0127] The grid connection switching module includes: a pre-synchronization control unit and a grid connection judgment unit;

[0128] The pre-synchronization control unit is used to perform pre-synchronization control compensation of the angular frequency and voltage of each VSG inverter after the power parameters have been set.

[0129] The grid connection judgment unit is used to determine whether each VSG inverter has met the grid connection requirements after pre-synchronization control. If so, the VSG inverter that has met the grid connection conditions will be connected to the grid. Otherwise, pre-synchronization control will continue until the grid connection requirements are met.

[0130] The pre-synchronization control unit includes: a phase angle acquisition subunit, a first dq transformation subunit, an adjustment quantum unit, and a reference value subunit;

[0131] The phase angle acquisition subunit is used to acquire the voltage phase angle of the power grid using a phase-locked loop;

[0132] The first dq conversion subunit is used to perform dq conversion on the output voltage of the VSG inverter based on the phase angle to obtain the d-axis component and the q-axis component.

[0133] The quantum control unit is used to input the d-axis component into the PI controller to obtain the angular frequency adjustment amount, and to input the deviation between the q-axis component and the three-phase grid voltage amplitude into the PI controller to obtain the amplitude adjustment amount;

[0134] The reference value sub-unit is used to add the angular frequency adjustment amount to the islanded mode reference angular frequency to obtain the reference angular frequency of the pre-synchronization link, and to add the amplitude adjustment amount to the islanded mode reference voltage amplitude to obtain the reference voltage amplitude of the pre-synchronization link.

[0135] The grid connection judgment unit includes: a second dq transformation subunit, a dq judgment subunit, and a grid connection judgment subunit;

[0136] The second dq conversion subunit is used to perform dq conversion on the output voltage of the VSG inverter to obtain the d-axis output voltage and the q-axis output voltage.

[0137] The dq judgment subunit is used to determine whether the absolute value of the q-axis output voltage is less than the q-axis threshold and whether the error between the peak value of the d-axis output voltage and the grid phase voltage is less than the d-axis threshold.

[0138] The grid connection judgment subunit is used to determine whether the VSG inverter meets the grid connection requirements when both judgment results are yes; otherwise, it does not meet the grid connection requirements.

[0139] The grid connection switching module also includes a zeroing unit;

[0140] The zeroing unit is used to reset the angular frequency and voltage compensation of the VSG inverter after it is connected to the power grid to zero.

[0141] The grid connection fault handling module includes: a current-source inverter grid connection unit, a current decomposition unit, and a current compensation unit.

[0142] The current-source inverter grid connection unit is used to connect the current-source inverter and the VSG inverter after the power parameters have been set to the grid.

[0143] The current decomposition unit is used to decompose the load current into balanced active current, balanced reactive current, unbalanced current and no-load current components using conservative power theory.

[0144] The current compensation unit is used to compensate for unbalanced current by controlling the output current of the current-source inverter to be equal to the unbalanced current and the no-load current component in the load current.

[0145] The control system for multi-VSG inverter circulating current suppression and fault handling also includes an off-grid fault handling module;

[0146] The off-grid fault handling module is used when an off-grid operation occurs and an asymmetrical load fault occurs: for each VSG inverter with completed power parameter settings, a negative sequence control method is used to control each VSG inverter separately to reduce the output voltage imbalance when an off-grid load asymmetrical fault occurs.

[0147] The off-grid fault handling module includes: a first conversion unit, a positive-negative separation unit, a second conversion unit, and a positive-negative sequence control unit;

[0148] The first conversion unit is used to convert the three-phase voltage and three-phase current of the VSG inverter in the three-phase stationary coordinate system into the two-phase voltage and current in the two-phase stationary coordinate system.

[0149] The positive and negative separation unit is used to separate the positive and negative sequences of two-phase voltage and current using a second-order generalized integrator to obtain the positive and negative sequence components of the two-phase voltage and current.

[0150] The second conversion unit is used to transform the positive-sequence and negative-sequence components of the two-phase voltage and current from the two-phase stationary coordinate system to the two-phase rotating coordinate system, so as to obtain the positive-sequence dq-axis voltage and current and the negative-sequence dq-axis voltage and current, respectively.

[0151] The positive and negative sequence control unit is used to control the positive sequence dq axis voltage and current, and to set the negative sequence dq axis voltage to zero and control it, so that the inverter output voltage contains only the positive sequence component.

[0152] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0153] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0154] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0155] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and not to limit its protection scope. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading this application, they can still make various changes, modifications or equivalent substitutions to the specific implementation of the application, but these changes, modifications or equivalent substitutions are all within the protection scope of the claims pending approval.

Claims

1. A control method for suppressing circulating current and handling faults in multi-VSG inverters, characterized in that, include: When switching from off-grid to grid-connected: For each VSG inverter that has completed the power parameter setting, the pre-synchronization control compensation method is used to compensate each VSG inverter separately until the grid connection requirements are met; When the grid is connected and in steady state: if an unbalanced load fault occurs, the current-source inverter and the VSG inverter after the power parameters have been set will be connected in parallel to the grid to compensate for the unbalanced current. The power parameters of each VSG inverter are preset based on the rated capacity of each VSG inverter. The power parameters of each VSG inverter are set based on its rated capacity, including: Obtain the rated capacity of each VSG inverter; Set the power parameters for each VSG inverter according to the ratio of the rated capacity of each VSG inverter; The power parameters include virtual reactance and reactive voltage droop coefficient, which are inversely proportional to the rated capacity, and active frequency droop coefficient, virtual moment of inertia and virtual damping coefficient, which are proportional to the rated capacity. The step of connecting the current-source inverter in parallel with the VSG inverter after power parameter setting and connecting it to the power grid to compensate for unbalanced current includes: The current-source inverter is connected in parallel with the VSG inverter after the power parameters have been set and then connected to the power grid. The load current is decomposed into balanced active current, balanced reactive current, unbalanced current and no-load current components using conservative power theory. The unbalanced current is compensated by controlling the output current of the current-source inverter to be equal to the unbalanced current and the no-load current component in the load current. For each VSG inverter with completed power parameter settings, a pre-synchronization control compensation method is used to compensate each VSG inverter individually until the grid connection requirements are met, including: For each VSG inverter with completed power parameter settings, pre-synchronization control is performed to compensate for the angular frequency and voltage of the VSG inverter; Determine whether each VSG inverter meets the grid connection requirements after pre-synchronization control: if yes, connect the VSG inverter that meets the grid connection conditions to the grid; otherwise, continue pre-synchronization control until the grid connection requirements are met. The method further includes: When operating off-grid and experiencing a load asymmetry fault: For each VSG inverter with completed power parameter settings, a negative sequence control method is used to control each VSG inverter separately, reducing the output voltage imbalance during off-grid load asymmetry faults. The pre-synchronization control compensation of the VSG inverter's angular frequency and voltage includes: Phase-locked loops are used to obtain the voltage phase angle of the power grid; The output voltage of the VSG inverter is transformed by dq based on the phase angle to obtain the d-axis component and the q-axis component. The d-axis component is input into the PI controller to obtain the angular frequency adjustment amount, and the deviation between the q-axis component and the three-phase grid voltage amplitude is input into the PI controller to obtain the amplitude adjustment amount. The angular frequency adjustment is added to the island mode reference angular frequency to obtain the reference angular frequency of the pre-synchronization link, and the amplitude adjustment is added to the island mode reference voltage amplitude to obtain the reference voltage amplitude of the pre-synchronization link. The VSG inverter is controlled using a negative sequence control method, including: The three-phase voltage and three-phase current of the VSG inverter in the three-phase stationary coordinate system are converted into two-phase voltage and current in the two-phase stationary coordinate system. A second-order generalized integrator is used to separate the positive and negative sequences of the two-phase voltage and current to obtain the positive and negative sequence components of the two-phase voltage and current. The positive-sequence and negative-sequence components of the two-phase voltage and current are transformed from the two-phase stationary coordinate system to the two-phase rotating coordinate system to obtain the positive-sequence dq-axis voltage and current and the negative-sequence dq-axis voltage and current, respectively. The positive-sequence dq-axis voltage and current are controlled, and the negative-sequence dq-axis voltage is set to zero and controlled, so that the inverter output voltage contains only the positive-sequence component.

2. The method as described in claim 1, characterized in that, Determining whether the VSG inverter meets the grid connection requirements includes: The output voltage of the VSG inverter is converted to dq to obtain the d-axis output voltage and the q-axis output voltage. Determine whether the absolute value of the q-axis output voltage is less than the q-axis threshold, and determine whether the error between the peak value of the d-axis output voltage and the grid phase voltage is less than the d-axis threshold. When both judgment results are yes, the VSG inverter meets the grid connection requirements; otherwise, it does not meet the grid connection requirements.

3. The method as described in claim 1, characterized in that, After the grid connection requirements are met, the following are also included: Set the angular frequency and voltage compensation of the VSG inverter after it is connected to the grid to zero.

4. A control system for suppressing circulating current and handling faults in multi-VSG inverters, characterized in that, include: Grid connection switching module and grid connection fault handling module; The grid-connected switching module is used to compensate each VSG inverter with a pre-synchronization control compensation method when switching from off-grid to grid-connected: for each VSG inverter that has completed the power parameter setting, until the grid-connected requirements are met. The grid-connected fault handling module is used to, when the grid is connected to steady state, if an asymmetrical load fault occurs, connect the current-source inverter and the VSG inverter (after power parameter setting) in parallel to the grid to compensate for the unbalanced current. The power parameters of each VSG inverter are set based on the rated capacity of each VSG inverter. It also includes a parameter setting module for setting the power parameters of each VSG inverter, the parameter setting module comprising: a capacity acquisition unit and a parameter setting unit; The capacity acquisition unit is used to acquire the rated capacity of each VSG inverter; The parameter setting unit is used to set power parameters for each VSG inverter according to the ratio of the rated capacity of each VSG inverter. The power parameters include virtual reactance and reactive voltage droop coefficient, which are inversely proportional to the rated capacity, and active frequency droop coefficient, virtual moment of inertia and virtual damping coefficient, which are proportional to the rated capacity. The grid connection fault handling module includes: a current-source inverter grid connection unit, a current decomposition unit, and a current compensation unit. The current-source inverter grid connection unit is used to connect the current-source inverter and the VSG inverter (after power parameter setting) in parallel to the power grid. The current decomposition unit is used to decompose the load current into a balanced active current, a balanced reactive current, an unbalanced current and a no-load current component using conservative power theory. The current compensation unit is used to compensate for unbalanced current by controlling the output current of the current-source inverter to be equal to the unbalanced current and the no-load current component in the load current. The grid connection switching module includes: a pre-synchronization control unit and a grid connection judgment unit; The pre-synchronization control unit is used to perform pre-synchronization control compensation of the angular frequency and voltage of each VSG inverter that has completed the power parameter setting. The grid connection judgment unit is used to determine whether each VSG inverter has met the grid connection requirements after pre-synchronization control: if so, the VSG inverter that has met the grid connection conditions will be connected to the grid; otherwise, pre-synchronization control will continue until the grid connection requirements are met. The control system for multi-VSG inverter circulating current suppression and fault handling also includes an off-grid fault handling module; The off-grid fault handling module is used when an off-grid operation occurs and an asymmetrical load fault occurs: for each VSG inverter with completed power parameter settings, a negative sequence control method is used to control each VSG inverter separately to reduce the output voltage imbalance when an off-grid load asymmetrical fault occurs. The pre-synchronization control unit includes: a phase angle acquisition subunit, a first dq transformation subunit, an adjustment quantum unit, and a reference value subunit; The phase angle acquisition subunit is used to acquire the voltage phase angle of the power grid using a phase-locked loop; The first dq conversion subunit is used to perform dq conversion on the output voltage of the VSG inverter based on the phase angle to obtain the d-axis component and the q-axis component. The quantum control unit is used to input the d-axis component into the PI controller to obtain the angular frequency adjustment amount, and to input the deviation between the q-axis component and the three-phase grid voltage amplitude into the PI controller to obtain the amplitude adjustment amount; The reference value subunit is used to add the angular frequency adjustment amount to the islanded mode reference angular frequency to obtain the reference angular frequency of the pre-synchronization link, and to add the amplitude adjustment amount to the islanded mode reference voltage amplitude to obtain the reference voltage amplitude of the pre-synchronization link. The off-grid fault handling module includes: a first conversion unit, a positive-negative separation unit, a second conversion unit, and a positive-negative sequence control unit; The first conversion unit is used to convert the three-phase voltage and three-phase current of the VSG inverter in the three-phase stationary coordinate system into the two-phase voltage and current in the two-phase stationary coordinate system. The positive and negative separation unit is used to separate the positive and negative sequences of two-phase voltage and current using a second-order generalized integrator to obtain the positive and negative sequence components of the two-phase voltage and current. The second conversion unit is used to transform the positive-sequence and negative-sequence components of the two-phase voltage and current from the two-phase stationary coordinate system to the two-phase rotating coordinate system, so as to obtain the positive-sequence dq-axis voltage and current and the negative-sequence dq-axis voltage and current, respectively. The positive and negative sequence control unit is used to control the positive sequence dq axis voltage and current, and to set the negative sequence dq axis voltage to zero and control it, so that the inverter output voltage contains only the positive sequence component.

Images