A circulating current transient suppression method based on output voltage reconstruction

Abstract

The application discloses a loop current transient suppression method based on output voltage reconstruction and relates to the technical field of flexible control of a distributed power generation system, and solves the problems of coupling of voltage control and loop current suppression, poor adaptability of transient mode and difficulty in considering multiple types of loop current suppression in the existing multi-converter parallel connection. Specifically, the application can effectively suppress multiple types of loop current problems caused by control parameter deviation, phase asynchronization or circuit parameter mismatch without directly modifying the output voltage reference instruction of the converter, so that the loop current suppression process and the output voltage control process are decoupled, the system design is simplified, the overall operation efficiency is improved, the safe and reliable operation of the multi-converter parallel connection system under various working modes is ensured, and the application has high practical value and wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of flexible control technology for distributed generation systems, specifically a method for suppressing circulating current transients based on output voltage reconstruction. Background Technology

[0002] Distributed generation technology improves energy efficiency and enhances the grid's resilience by utilizing renewable energy sources locally. In distributed generation systems, converters are crucial for effective and reliable power regulation. Typically, multiple converters are connected in parallel to adapt to high-voltage, high-capacity distributed generation applications. Each converter handles the same power output, maintaining a balance between power supply and demand at the generation and load ends, and ensuring stable AC bus voltage.

[0003] To flexibly adapt to energy regulation needs under different operating scenarios, multiple parallel converters can switch their control strategies and structures according to instructions from the upper-level controller. For example, in grid-connected mode, the converters precisely control output voltage and frequency to ensure synchronous operation with the grid and achieve efficient energy transmission; in off-grid mode, the converters simulate synchronous generator operation, using droop control to undertake load power supply tasks and maintain stable system operation. Furthermore, when the system is in a transient transition process, such as switching from off-grid to grid or vice versa, the converters can quickly adjust their output characteristics to ensure a smooth and seamless switching process. However, when the converters use a direct voltage control structure, circulating currents are prone to occur in multi-converter parallel systems due to factors such as voltage deviations and inconsistent impedance characteristics on the bridge arms of each converter. This not only reduces available active power and disrupts the supply-demand balance, but excessive circulating currents may also falsely trigger system protection mechanisms, affecting the operational stability of the distributed generation system. This has become a common problem in current multi-converter parallel distributed generation systems. At present, the main strategy adopted is to combine droop control with impedance reshaping. By adjusting the output voltage reference command of each converter, the equivalent output impedance is indirectly changed, so that the output voltage of each converter LC port tends to be consistent, thereby suppressing the circulating current of the multi-converter parallel system.

[0004] However, the droop control strategy is a control strategy that simulates the operating characteristics of a synchronous generator. It achieves average distribution of the output process to suppress circulating current by establishing the relationship between the active and reactive components of the output power and the frequency and amplitude of the voltage. However, it is heavily dependent on the accuracy of the line impedance parameters. Once the line impedance changes, the droop control equation will change fundamentally. The virtual impedance strategy corrects the voltage control reference command through the feedback of the output current of a single converter. It uses software algorithms to simulate a sufficiently large series impedance at the output end to compensate for the inconsistency of the output characteristics of each converter. Although it can suppress circulating current between converters to a certain extent, the excessively large series impedance will affect the voltage quality at the output end and affect the AC bus voltage. On the other hand, when the parallel converter system enters a transient transition process such as forced disconnection or synchronous grid connection, and needs to further adjust the amplitude or phase of the output voltage to achieve the requirement of being in phase and frequency with the grid voltage, the traditional strategy cannot guarantee that circulating current suppression and smooth switching of operating modes can be carried out simultaneously. This can easily lead to the failure of the temporary circulating current suppression effect or synchronization failure of the multi-converter system, which seriously affects the transient operating stability of the system.

[0005] Furthermore, existing methods mostly target the suppression of fundamental frequency circulating current, lacking control strategies to effectively suppress non-fundamental frequency circulating currents such as harmonics and switching frequencies simultaneously. Therefore, there is an urgent need for an effective method that can decouple circulating current suppression from voltage control in a multi-converter parallel system under direct voltage control mode, adapt to transient modes, and achieve unified suppression for different types of circulating currents. To address this, we propose a circulating current transient suppression method based on output voltage reconstruction. Summary of the Invention

[0006] The purpose of this invention is to provide a circulating current transient suppression method based on output voltage reconstruction, which aims to solve the problems of voltage control and circulating current suppression coupling, poor transient mode adaptability, and difficulty in simultaneously suppressing multiple types of circulating current in existing multi-converter parallel systems. This invention provides a unified suppression method for multiple types of circulating current based on output voltage reconstruction, which can effectively suppress circulating current while ensuring the stable operation of multi-converter parallel systems in both steady-state and transient modes.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a circulating current transient suppression method based on output voltage reconstruction, comprising the following steps:

[0008] Step 1: The circulating current suppression of a multi-converter parallel system is equivalent to the current sharing process of each converter output. Based on the current system load or total output demand, and combined with the number of parallel converters, the target output current of each converter at the current moment is dynamically generated.

[0009] Step 2: Real-time acquisition of the output current of each converter The deviation between the output current of a single converter and the average output current of the system is calculated, and this deviation is defined as the disturbance current component. As a basis for control of circulation suppression;

[0010] Step 3: Based on the rated equivalent output characteristics of each converter group, design the circulating current suppression feedforward compensation admittance Y. dis (s) introduces the circulating current disturbance component, and generates a voltage disturbance compensation vector based on the disturbance current. Used to counteract the excitation effect of disturbances on the circulation;

[0011] Step 4: By reconstructing the converter output voltage vector, the reconstructed converter output voltage is controlled by the fundamental component of the voltage closed-loop control. and circulation suppression component It is formed by superposition, in which the basic voltage component Used to maintain the rated voltage of the AC bus, circulating current suppression component This is used to offset the deviation between the output current of a single converter and the average output current of the system.

[0012] Preferably, the circulating current is defined as follows: when circulating current exists in a system containing multiple sets of converters connected in parallel, the output currents of at least two sets of converters are not equal, and the circulating currents formed by any two pairs of converters do not affect each other, and are superimposed to form the total circulating current in the parallel system.

[0013] When the parallel system has M groups of converters, the circulating current between any two groups of converters is calculated using the formula for the difference in their output currents:

[0014]

[0015] in Let M represent the circulating current between the j-th converter and the k-th converter, 0≤j,k≤M, and j≠k, with the direction of the flow from the j-th converter to the k-th converter being positive;

[0016] For the output current of the j-th converter, Let be the output current of the k-th converter.

[0017] Preferably, the target output current in step one The average system output current is obtained by dividing the total system output current by the number of parallel converters M, as shown in the following formula:

[0018]

[0019] in Let M be the real-time output current of the m-th converter, where M is the total number of parallel converters.

[0020] Preferably, the formula for calculating the disturbance current component in step two is:

[0021]

[0022] in This refers to the disturbance current component caused by the presence of circulating current in the m-th converter group.

[0023] Preferably, the transfer function formula for calculating the circulating current suppression feedforward compensation admittance Ydis(s) in step three is as follows:

[0024]

[0025] Where Z0 is the equivalent output load in the voltage control mode of the multi-parallel converter system, L is the LC filter inductance, C is the LC filter capacitance, and Rd is the LC filter damping resistor.

[0026] Preferably, the basic components in step four Generated by the converter's own voltage control closed loop, circulating current suppression component It is generated by circulating current disturbance calculation and does not participate in or affect the converter's own voltage command generation and closed-loop control process. It can achieve dual decoupling of circulating current suppression and voltage control, and the circulating current suppression component... It is generated by multiplying equation (3) and equation (4);

[0027] The formula for calculating the circulating current suppression component of the m-th converter in a parallel system is:

[0028]

[0029] Preferably, the method introduces a flexible control strategy, which can flexibly adapt to various complex operating scenarios without changing the original system control architecture.

[0030] Preferably, the method introduces an adaptive adjustment mechanism, which can dynamically adjust the calculation parameters of the circulating current suppression component according to the system operating status and the output current change trend;

[0031] When there is a sudden change in load or a grid fault, the controller can quickly update the transfer function of the feedforward compensation admittance Ydis(s) through online identification technology, so that the circulating current suppression strategy always matches the current operating conditions.

[0032] A multi-converter parallel system for large-capacity distributed generation is characterized in that: the system adopts a circulating current transient suppression method based on output voltage reconstruction to achieve circulating current suppression between converters;

[0033] The system includes multiple sets of converters, output filters, loads, and control modules connected in parallel to the same AC bus. The control module is used to perform voltage vector reconstruction and circulating current suppression algorithms.

[0034] An application of a circulating current transient suppression method based on output voltage reconstruction is presented for distributed generation systems, which include power generation systems with grid-connected and off-grid operation. When multiple converters are connected in parallel for high-capacity demand, each converter generates a set of voltage regulation vectors and superimposes them on its own output voltage, thereby suppressing the circulating current in the multi-converter parallel system.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] 1. This invention treats circulating current as a disturbance component in the converter control system. It can effectively suppress various types of circulating current problems caused by control parameter deviation, phase asynchrony, or circuit parameter mismatch without directly modifying the converter output voltage reference command. In addition, in this way, the circulating current suppression process is decoupled from the output voltage control process, which simplifies the system design, improves the overall operating efficiency, and ensures the safe and reliable operation of the multi-converter parallel system in various working modes. It has high practical value and broad application prospects.

[0037] 2. This invention decouples the circulating current suppression from the output voltage control process by reconstructing the output voltage, avoiding the mutual interference between the two in traditional methods. This ensures both stable output voltage and effective suppression of circulating current.

[0038] 3. This invention has strong transient adaptability and is suitable for transient modes such as off-grid switching and synchronous grid connection of microgrids, solving the problem of performance degradation of droop control during transient processes;

[0039] 4. This invention can suppress multiple types of circulating currents, including DC, fundamental frequency, harmonics, and switching frequency circulating currents, without the need to design special strategies for a single type.

[0040] 5. This invention has high robustness and can maintain good performance under scenarios such as load changes, parameter mismatch, and carrier asynchrony, thereby improving the stability and reliability of system operation. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the three-phase equivalent structure of the parallel system of multiple converters based on the embodiments of the present invention;

[0042] Figure 2 This is a schematic diagram of the three-phase equivalent circuit based on two sets of converters connected in parallel, which is the basis of the embodiments of the present invention.

[0043] Figure 3This is an equivalent schematic diagram of the circulating current path based on two sets of converters connected in parallel, which is the basis of the embodiments of the present invention;

[0044] Figure 4 This is a block diagram of the control structure for implementing a unified suppression strategy for circulating current in a multi-converter parallel system based on output voltage reconstruction, as used in an embodiment of the present invention.

[0045] Figure 5 This is a software flowchart of the unified suppression strategy for circulating current in a multi-converter parallel system based on output voltage reconstruction, as used in an embodiment of the present invention.

[0046] Figure 6 The waveform diagram of the simulation experiment results is not used when the amplitude deviation of the modulation wave of the two sets of converters is 100% in this embodiment of the invention, compared with the simulation experiment results using the scheme of the present invention.

[0047] Figure 7 The waveform diagrams of the simulation experiment results of the present invention are shown in the embodiment of the present invention when the phase deviation of the modulation waves of the two sets of converters is 180°, without using the simulation experiment results of the present invention and with the present invention.

[0048] Figure 8 The waveform diagram of the simulation experiment results without using the scheme of the present invention is shown in the embodiment of the present invention when the carrier phase deviation of the two sets of converter PWM modulation processes is 180°.

[0049] Figure 9 The waveform diagram of the simulation experiment results is not used when the inductance values ​​of the LC filter circuits on the AC side of the two sets of converters deviate by 80% in this embodiment of the invention.

[0050] Figure 10 In this embodiment of the invention, when the capacitor values ​​of the LC filter circuits on the AC side of the two sets of converters deviate by 80%, the waveform diagram of the simulation experiment results using the solution of this invention is not adopted.

[0051] Figure 11 The following are waveform diagrams showing the simulation results of the traditional feeddown control and the scheme of the present invention in two sets of converter transient operation modes according to an embodiment of the present invention.

[0052] Figure 12 The waveform diagrams shown are from experimental results of a prototype using the present invention under linear load conditions, without employing the method described in the present invention.

[0053] Figure 13 The waveform diagrams shown are from the prototype experiments conducted under nonlinear load conditions in this embodiment of the invention, without using the waveforms of the prototype with the solution of this invention.

[0054] Figure 14 The waveform diagrams show the prototype test results of the traditional feeddown control and the scheme of the present invention in two sets of converter transient operation modes according to an embodiment of the present invention.

[0055] In the diagram: 1.1 Three-phase AC bus; 1.2 Grid-connected / off-grid switching switch; 1.3 Converter inlet circuit breaker; 1.4 Three-phase two-stage converter circuit; 1.4.1 DC / AC conversion stage; 1.4.2 DC / DC conversion stage; 1.4.3 LC filter circuit; 1.5 Renewable energy power generation device; 1.6 Three-phase AC load. Detailed Implementation

[0056] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0057] Please see Figures 1 to 14 The present invention provides an embodiment of a circulating current transient suppression method based on output voltage reconstruction, comprising the following steps:

[0058] Step 1: The circulating current suppression of a multi-converter parallel system is actually equivalent to the current sharing process of each converter output. Based on the current system load or total output demand, and combined with the number of parallel converters, the target output current of each converter at the current moment is dynamically generated.

[0059] Step 2: Real-time acquisition of the output current of each converter The deviation between the output current of a single converter and the average output current of the system is calculated, and this deviation is defined as the disturbance current component. As a basis for control of circulation suppression;

[0060] Step 3: Based on the rated equivalent output characteristics of each converter group, design the circulating current suppression feedforward compensation admittance Y. dis (s) introduces the circulating current disturbance component, and generates a voltage disturbance compensation vector based on the disturbance current. Used to counteract the excitation effect of disturbances on the circulation;

[0061] Step 4: By reconstructing the converter output voltage vector, the reconstructed converter output voltage is controlled by the fundamental component of the voltage closed-loop control. and circulation suppression component It is formed by superposition, in which the basic voltage component Used to maintain the rated voltage of the AC bus, circulating current suppression component This is used to offset the deviation between the output current of a single converter and the average output current of the system.

[0062] The aforementioned circulating current refers to the situation where, in a system containing multiple sets of converters connected in parallel, at least two sets of converters will have unequal output currents. Furthermore, the circulating currents formed by any two pairs of converters are independent of each other and are superimposed to form the total circulating current within the parallel system. Specifically:

[0063] In a multi-converter parallel system containing M converters, the grid-side output current matrix of each converter is expressed as:

[0064]

[0065] Among them, the circulating current between any two sets of converters (1≤j,k≤M) is represented as:

[0066]

[0067] When the parallel system has M groups of converters, the circulating current between any two groups of converters is calculated using the formula for the difference in their output currents:

[0068]

[0069] in Let M represent the circulating current between the j-th converter and the k-th converter, 0≤j,k≤M, and j≠k, with the direction of the flow from the j-th converter to the k-th converter being positive;

[0070] For the output current of the j-th converter, Let be the output current of the k-th converter. The magnitude of the circulating current between any two converters in a parallel converter system is related to the output voltage deviation (including amplitude, phase, and harmonic components) and the deviation of the output circuit characteristic parameters.

[0071] Target output current in step one The average system output current is obtained by dividing the total system output current by the number of parallel converters M, as shown in the following formula:

[0072]

[0073] in Let be the real-time output current of the m-th converter, and M be the total number of parallel converters.

[0074] The formula for calculating the disturbance current component in step two is:

[0075]

[0076] in This refers to the disturbance current component caused by the circulating current in the m-th converter group, and its magnitude is related to the output current of that converter group itself.

[0077] In step three, the converter closed-loop control system introduces a circulating current disturbance component. The relationship between the output current, the converter output voltage, and the circulating current compensation voltage is as follows:

[0078]

[0079] Therefore, the circulating current suppression feedforward compensation admittance Y can be calculated. dis The formula for calculating the transfer function of (s) is:

[0080]

[0081] Where Z0 is the equivalent output load of the multi-parallel converter system under voltage control mode, L is the LC filter inductance, C is the LC filter capacitance, and R... d This is the damping resistor for the LC filter.

[0082] The basic components in step four Generated by the converter's own voltage control closed loop, circulating current suppression component It is generated by circulating current disturbance calculation and does not participate in or affect the converter's own voltage command generation and closed-loop control process. It can achieve dual decoupling of circulating current suppression and voltage control, and the circulating current suppression component... It is generated by multiplying equation (3) and equation (4);

[0083] The formula for calculating the circulating current suppression component of the m-th converter in a parallel system is:

[0084]

[0085] The calculated converter output voltage is decomposed into a fundamental component and a circulating current suppression component, where the fundamental component... The circulating current suppression component is generated by the converter's own voltage control closed loop. This is generated by circulating current disturbance calculation and does not participate in or affect the converter's own voltage command generation and closed-loop control process. In the transient operating mode of a multi-converter parallel system, the fundamental component... Used to achieve a smooth transition of operating modes, circulation suppression component This is used to achieve unified suppression of circulating current, thereby realizing the dual decoupling of circulating current suppression and voltage control;

[0086] The generation process of the circulating current suppression voltage component comprehensively considers the deviation between the actual value of the converter output current and the target current value, and dynamically adjusts it in conjunction with the system operating status to ensure effective circulating current suppression under different operating conditions. Furthermore, by introducing a flexible control strategy, this method can flexibly adapt to various complex operating scenarios without changing the original system control architecture, thereby improving the system's compatibility and robustness.

[0087] This method introduces an adaptive adjustment mechanism, which can dynamically adjust the calculation parameters of the circulating current suppression component according to the system operating status and the output current change trend;

[0088] Under extreme conditions such as sudden load changes or grid failures, the controller can quickly update the feedforward compensation admittance Y through online identification technology. dis The transfer function of (s) ensures that the circulation suppression strategy always matches the current operating conditions, thereby effectively avoiding the performance degradation problem caused by fixed parameters in traditional methods.

[0089] A multi-converter parallel system for large-capacity distributed generation applications is disclosed. The system employs the aforementioned method to achieve circulating current suppression between converters. The system includes multiple converters connected in parallel to the same AC bus, an output filter, a load, and a control module. The control module is used to execute voltage vector reconstruction and circulating current suppression algorithms.

[0090] An application of a circulating current transient suppression method based on output voltage reconstruction can be applied to distributed generation systems with multiple functions such as grid-connected and off-grid operation, especially in high-capacity demand scenarios involving multiple sets of converters connected in parallel. Each converter generates a set of voltage regulation vectors and superimposes them into its own output voltage, thereby suppressing the circulating current in the multi-converter parallel system.

[0091] This invention is simulated and experimentally verified in a three-phase three-wire AC power grid system containing two converters connected in parallel. The specific parameters are as follows:

[0092] l Three-phase AC bus rated voltage <![CDATA[U p ]]> 220 V 2 Three-phase AC bus rated frequency f 50 Hz 3 Number of converters in parallel M 2 - 4 DC side voltage of converter <![CDATA[U dc ]]> 300 V 5 Three-phase load rated power <![CDATA[P R ]]> 20 kW 6 LC filter inductance value L 0.5 mH 7 LC filter capacitance value C 80 μF 8 LC filter damping resistor value <![CDATA[R d ]]> 2 Ω 9 Converter PWM modulation carrier frequency <![CDATA[f c ]]> 2 kHz 10 Sampling frequency (calculation frequency) of the converter control system <![CDATA[f sam ]]> 10 kHz

[0093] See attached document Figure 6 (a) and Figure 6 (b) represents the extreme operating condition where the amplitude deviation of the modulation waves of the two sets of converters reaches 100%, that is, the modulation ratio of converter 1 is 0.8 and the modulation ratio of converter 2 is 0.4, and the two phases are strictly synchronized. The waveforms were not compared with the simulation experimental results using the scheme of this invention.

[0094] When the present invention is not used, the output current I of the two sets of converters is... g1 with I g2 The amplitude difference is significant, and the system circulating current I h The peak value exceeds 400A;

[0095] When the solution of this invention is adopted, the disturbance current I is calculated in real time. dis Generate the inverse voltage compensation vector U based on feedforward compensation admittance. dis This value is then superimposed on the converter base voltage reference value to decouple circulating current suppression from voltage control. The converter output currents essentially overlap, and the system circulating current I... h The peak current was reduced to 5A, and the voltage distortion rate at the grid connection point was 1.27%. This verifies the efficient suppression capability of the proposed solution for circulating currents with modulation amplitude deviation.

[0096] See attached document Figure 7(a) and Figure 7 (b) represents the extreme operating conditions where the amplitude of the modulation waves of the two sets of converters are completely equal but the phase deviation is 180°, that is, when the phase of the modulation wave of converter 2 lags the phase of the modulation wave of converter 1 by 180°, the waveforms of the simulation experiments using the scheme of the present invention are not compared.

[0097] When the present invention is not used, the output current I of the two sets of converters is... g1 with I g2 The amplitudes not only differed significantly, but the phase deviation was also nearly 90°, indicating a system circulation I. h The peak value exceeds 1000A;

[0098] When the solution of this invention is adopted, the disturbance current I is calculated in real time. dis Generate the inverse voltage compensation vector U based on feedforward compensation admittance. dis This value is then superimposed on the converter base voltage reference value to decouple circulating current suppression from voltage control. The converter output currents essentially overlap, and the system circulating current I... h The peak current was reduced to 12A, and the voltage distortion rate at the grid connection point was 2.49%. This verifies that the present invention can effectively cancel phase deviation and significantly suppress circulating current under extreme interference with reversed phase of the modulated wave.

[0099] See attached document Figure 8 (a) and Figure 8 (b) represents the extreme working condition where the PWM carrier phase deviation of the two sets of converters reaches 180°, that is, the modulation amplitude of converter 1 and converter 2 are completely equal and the phase is completely consistent. However, when the carrier phase deviation is 180°, the waveform is not compared with the simulation experimental results using the scheme of this invention.

[0100] When the present invention is not used, the output current I of the two sets of converters is... g1 with I g2 The amplitude and phase of the system exhibit significant deviations and distortion, and the system circulation I... h The peak value exceeds 300A;

[0101] When the solution of this invention is adopted, the disturbance current I is calculated in real time. dis Generate the inverse voltage compensation vector U based on feedforward compensation admittance. dis The current is then superimposed onto the converter base voltage reference value, achieving decoupling of circulating current suppression and voltage control. The amplitudes and phases of the output currents Ig1 and Ig2 of the two sets of converters are basically consistent, the circulating current amplitude is reduced to below 10A, and the voltage distortion rate at the grid connection point is only 1.46%. This verifies that the proposed solution can accurately offset the driving effect caused by the carrier phase difference, force multiple converters to synchronize their outputs, efficiently suppress circulating current, and strongly support the stable operation of the system under abnormal carrier synchronization scenarios.

[0102] See attached document Figure 9 (a) and Figure 9 (b) represents the extreme operating condition where the inductance values ​​of the LC filters on the output side of the two sets of parallel converters deviate by 100%, i.e., when the inductance value of the output filter of converter 1 is 0.5mH and the inductance value of the output filter of converter 2 is 0.25mH. The waveforms of the simulation experiment results using the present invention were not compared.

[0103] When the present invention is not used, the output current I of the two sets of converters is... g1 with I g2 Due to inductance mismatch, there is a significant amplitude deviation and waveform distortion. The system circulating current amplitude exceeds 100A and contains a large number of high-frequency components.

[0104] When the solution of this invention is adopted, the output current I of the two sets of converters g1 with I g2 The waveform converges rapidly, the amplitude becomes more uniform, the high-frequency components in the circulating current are significantly reduced, and the voltage distortion rate at the grid connection point is 4.89%. This verifies that the present invention can effectively suppress the high-frequency circulating current and output voltage waveform distortion caused by differences in filter inductor parameters when facing the situation of filter inductor deviation in a multi-group parallel converter system.

[0105] See attached document Figure 10 (a) and Figure 10 (b) represents the extreme operating condition where the capacitance value of the LC filter on the output side of the two sets of parallel converters deviates by 80%, i.e., when the capacitance value of the output filter of converter 1 is 80μF and the capacitance value of the output filter of converter 2 is 40μF, and the waveform of the simulation experiment results using the present invention is not compared.

[0106] Similar to the inductance mismatch condition (Example 4), when the solution of this invention is not adopted, the output current I of the two sets of converters is... g1 with I g2 Due to capacitor mismatch, there is a significant amplitude deviation and waveform distortion. The system circulating current amplitude exceeds 50A and also contains a large number of high-frequency components.

[0107] When the solution of this invention is adopted, the output current I of the two sets of converters g1 with I g2 The waveform converges rapidly, the amplitude becomes more uniform, the high-frequency components in the circulating current are significantly reduced, and the voltage distortion rate at the grid connection point is 2.62%. This verifies that the present invention can effectively suppress the high-frequency circulating current and output voltage waveform distortion caused by differences in filter capacitor parameters when facing the situation of filter capacitor deviation in a multi-group parallel converter system.

[0108] See attached document Figure 11 (a) and Figure 11(b) shows the comparison waveforms of simulation results of two sets of parallel converters operating in transient mode without and with the present invention. This embodiment takes the synchronous grid connection process of the converter as an example, and the grid connection / off-grid switching switch is closed at time t0 in the figure.

[0109] When the solution of this invention is not used, the amplitude of the circulating current continues to increase throughout the synchronization process;

[0110] When the solution of this invention is adopted, the circulating current of the system is well suppressed throughout the entire stage, ensuring the stability of the converter parallel system under transient operating mode.

[0111] See attached document Figure 12 With appendix Figure 13 The waveforms are compared between prototype experiments under linear load and nonlinear load conditions, showing the results of parallel connection of two sets of converters without and with the present invention.

[0112] in, Figure 12 (a) and Figure 13 (a) Output voltage and current under linear and nonlinear load conditions, respectively, without any circulating current suppression strategy. It can be observed that the output current I of the two sets of converters... g1 with I g2 The waveform has obvious deviations, which will inevitably lead to the existence of circulating current. According to the results analysis, the two sets of converters connected in parallel in the prototype system are inconsistent in the amplitude of the modulation command, and the parameters of the inductor-capacitor filter are different.

[0113] When the solution of this invention is adopted, the waveforms of the prototype under two types of load conditions are referenced. Figure 12 (b) and Figure 13 (b) At this time, the output current I of the two sets of converters g1 with I g2 The waveforms are essentially identical in amplitude and phase, indicating that the output currents of the two sets of converters are basically the same, and the circulating current in the system is well suppressed. This verifies that the proposed solution can still effectively suppress the circulating current in the system under different types of load conditions.

[0114] See attached document Figure 14 (a) and Figure 14 (b) shows the comparative waveforms of the experimental results of the two sets of parallel converters operating in transient mode without and with the present invention. Corresponding to Embodiment Six, as a preferred embodiment, this embodiment takes the synchronous grid connection process of the converter as an example, and the grid connection / off-grid switching switch is closed at time t1 in the figure;

[0115] When the conventional strategy of droop control and virtual impedance is used instead of the solution of this invention, the amplitude of the circulating current continues to increase throughout the synchronization process, and the waveform quality of the converter output voltage and current is significantly distorted.

[0116] When the solution of this invention is adopted, the circulating current of the system is well suppressed throughout the entire stage. When the output voltage reference command starts to track the amplitude and phase of the grid voltage by adjusting its own parameters, the proposed strategy can still significantly maintain the sinusoidal nature of the output waveform and the circulating current suppression effect, thus ensuring the stability of the converter parallel system in transient operating mode.

[0117] In summary, through simulation and experimental verification of multiple embodiments, this invention demonstrates its excellent performance under various extreme conditions. Whether it is modulation amplitude deviation, phase deviation, carrier phase deviation, or LC filter circuit parameter mismatch, the solution of this invention can effectively suppress circulating current while ensuring the stability of the system output voltage and the low distortion rate of the grid connection point voltage.

[0118] Specifically, under extreme conditions where the modulation wave amplitude deviation reaches 100%, the proposed solution reduces the system circulating current peak from over 400A to below 5A and significantly improves the voltage quality at the grid connection point. Even with a modulation wave phase deviation of 180°, the proposed solution also achieves converter output current synchronization, reducing the circulating current peak from over 1000A to below 12A, further demonstrating its adaptability to phase-reversal interference. Furthermore, in complex scenarios such as carrier phase deviation and LC filter circuit parameter mismatch, the proposed solution exhibits excellent robustness, successfully solving technical challenges that traditional control methods struggle to address. Therefore, the proposed method not only possesses high-precision circulating current suppression capabilities but also maintains system stability under various transient modes, providing reliable technical support for the practical application of multi-converter parallel systems.

[0119] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A circulating current transient suppression method based on output voltage reconstruction, characterized in that: Includes the following steps: Step 1: The circulating current suppression of a multi-converter parallel system is equivalent to the current sharing process of each converter output. Based on the current system load or total output demand, and combined with the number of parallel converters, the target output current of each converter at the current moment is dynamically generated. Step 2: Real-time acquisition of the output current of each converter The deviation between the output current of a single converter and the average output current of the system is calculated, and this deviation is defined as the disturbance current component. This serves as the basis for controlling circulation suppression; The formula for calculating the disturbance current component is: （3） in This refers to the disturbance current component caused by the presence of circulating current in the m-th converter group, the magnitude of which is related to the output current of that converter group itself. Step 3: Based on the rated equivalent output characteristics of each converter group, design the circulating current suppression feedforward compensation admittance Y. dis (s) introduces the circulating current disturbance component, and generates a voltage disturbance compensation vector based on the disturbance current. This is used to counteract the excitation effect of disturbances on the circulation; Circulation suppression feedforward compensation admittance Y dis The formula for calculating the transfer function of (s) is: （4） in For the equivalent output load in the voltage control mode of the multi-parallel converter system, L is the inductance of the LC filter, C is the capacitance of the LC filter, and Rd is the damping resistor of the LC filter. Step 4: Reconstruct the converter output voltage vector. The reconstructed converter output voltage is controlled by the fundamental component of the voltage closed-loop control. and circulation suppression component It is formed by superposition, in which the basic voltage component Used to maintain the rated voltage of the AC bus, circulating current suppression component This is used to offset the deviation between the output current of a single converter and the average output current of the system; Basic components Generated by the converter's own voltage control closed loop, circulating current suppression component It is generated by circulating current disturbance calculation and does not participate in or affect the converter's own voltage command generation and closed-loop control process. It can achieve dual decoupling of circulating current suppression and voltage control, and the circulating current suppression component... It is generated by multiplying equation (3) and equation (4); The formula for calculating the circulating current suppression component of the m-th converter in a parallel system is: （5）。 2. The circulating current transient suppression method based on output voltage reconstruction according to claim 1, characterized in that: The circulating current is defined as follows: when a system containing multiple sets of converters connected in parallel has circulating current, at least two sets of converters have different output currents, and the circulating currents formed by any two sets of converters do not affect each other, and are superimposed to form the total circulating current in the parallel system. When the parallel system has M groups of converters, the circulating current between any two groups of converters is calculated using the formula for the difference between their output currents: （1） in Let represent the circulating current between the j-th converter and the k-th converter, 0 ≤ j, k ≤ M, and j ≠ k, with the direction of the flow from the j-th converter to the k-th converter being positive; For the output current of the j-th converter, Let be the output current of the k-th converter.

3. The circulating current transient suppression method based on output voltage reconstruction according to claim 2, characterized in that: The target output current in step one The average system output current is obtained by dividing the total system output current by the number of parallel converters M, which is the formula for the average system output current: （2） in, Let be the real-time output current of the m-th converter, and M be the total number of parallel converters.

4. The circulating current transient suppression method based on output voltage reconstruction according to claim 3, characterized in that: The proposed method introduces a flexible control strategy, which can flexibly adapt to various complex operating scenarios without changing the original system control architecture.

5. The circulating current transient suppression method based on output voltage reconstruction according to claim 4, characterized in that: The method introduces an adaptive adjustment mechanism, which can dynamically adjust the calculation parameters of the circulating current suppression component according to the system operating status and the output current change trend. When there are sudden load changes or grid faults, the controller can quickly update the feedforward compensation admittance Y through online identification technology. dis The transfer function of (s) ensures that the circulation suppression strategy always matches the current operating conditions.

6. A multi-converter parallel system for large-capacity distributed generation applications, characterized in that: The system employs the method described in any one of claims 1-5 to achieve circulating current suppression between converters; The system includes multiple sets of converters, output filters, loads, and control modules connected in parallel to the same AC bus. The control module is used to perform voltage vector reconstruction and circulating current suppression algorithms.

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