Circulating current transient suppression method based on output voltage reconstruction

Abstract

The invention discloses a circulating current transient suppression method based on output voltage reconstruction, relates to the technical field of flexible control of a distributed power generation system, and solves the problems of voltage control and circulating current suppression coupling, poor transient mode adaptability, difficulty in considering multi-type circulating current suppression and the like in the existing parallel connection of multiple groups of converters. Specifically, on the premise that an output voltage reference instruction of the converter is not directly modified, various types of circulating current problems caused by control parameter deviation, phase asynchronization or circuit parameter mismatch can be effectively suppressed, so that a circulating current suppression process and an output voltage control process are decoupled, the system design is simplified, and the system reliability is improved. And the overall operation efficiency is improved, safe and reliable operation of the multi-converter parallel system in various working modes is ensured, and the method has high practical value and wide application prospects.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of flexible control of distributed generation system, and particularly relates to a circulating current transient suppression method based on output voltage reconstruction. BACKGROUND

[0002] Distributed generation technology can improve energy utilization efficiency and enhance the anti-disturbance ability of power grid by using renewable energy for power generation nearby. In a distributed generation system, a converter is an important part for effective and reliable regulation of power energy. Usually, multiple converters are connected in parallel to adapt to high-voltage and large-capacity distributed generation occasions. Each converter bears the same power output to maintain the balance between power supply and demand at the power generation end and the load end and the stability of the AC bus voltage.

[0003] In order to flexibly adapt to the energy regulation requirements in different working scenarios, the multiple parallel converters can switch their control strategies and control structures according to the instructions of the upper controller. For example, in the grid-connected mode, the converter controls the output voltage and frequency accurately to ensure synchronous operation with the power grid to realize efficient energy transmission. In the off-grid mode, the converter simulates the operation of a synchronous generator and adopts droop control to bear the load power supply task and maintain the stable operation of the system. In addition, when the system is in a transient transition process, such as switching from off-grid to grid-connected or vice versa, the converter can adjust the output characteristics quickly to ensure the smoothness and seamless connection of the switching process. When the direct voltage control structure is used inside the converter, due to the voltage deviation of each converter bridge arm side, the inconsistent impedance characteristics and other factors, circulating current is easily generated in the multiple parallel converter system, which not only reduces the available active power and destroys the balance between supply and demand, but also may trigger the system protection mechanism due to excessive circulating current, affecting the operation stability of the distributed generation system, which is a common problem in the current multiple parallel converter distributed generation system. At present, the strategy of combining droop control and impedance remodeling is mainly adopted, that is, by adjusting the output voltage reference instruction 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 multiple parallel converter system.

[0004] However, the droop control strategy is a control strategy for simulating the operation characteristics of the synchronous generator, which realizes the average distribution of the output process to suppress the circulating current by establishing the relationship between the active and reactive components in the output power and the frequency and amplitude of the voltage respectively, but it is seriously dependent on the accuracy of the line impedance parameter, and once the line impedance changes, the droop control equation will change essentially. The virtual impedance strategy is to correct the voltage control reference command through the feedback of the output current of a single converter, and to simulate a large enough series impedance at the output end by using a software algorithm to offset the inconsistency of the output characteristics of each converter, which can suppress the circulating current between the converters to a certain extent, but the excessive series impedance will affect the voltage quality of the output end voltage and the AC bus voltage. On the other hand, when the parallel converter system enters the forced splitting or synchronous grid connection transient transition process, the amplitude or phase of the output voltage needs to be further adjusted to realize the same frequency and phase requirements with the grid voltage, and the traditional strategy cannot guarantee the simultaneous implementation of circulating current suppression and smooth switching of the operation mode, which easily leads to the invalidation of the short-term circulating current suppression of the multi-converter system or the failure of synchronization, and seriously affects the stability of the system transient operation;

[0005] And the existing method is mainly for the suppression of fundamental frequency circulating current, and lacks a control strategy for effectively suppressing non-fundamental frequency circulating current such as harmonic and switching frequency, therefore, an effective method for realizing the decoupling of circulating current suppression and voltage control of a multi-converter parallel system in a direct voltage control mode, adapting to transient mode, and uniformly suppressing different types of circulating current is urgently needed, and for this purpose, the present application provides a circulating current transient suppression method based on output voltage reconstruction. SUMMARY

[0006] The present application aims to provide a circulating current transient suppression method based on output voltage reconstruction, and aims to solve the problems of coupling between voltage control and circulating current suppression, poor transient mode adaptability, and difficulty in considering multiple types of circulating current suppression in the existing multi-converter parallel system, and to provide a multi-type circulating current uniform suppression method based on output voltage reconstruction, which realizes effective circulating current suppression while ensuring the stable operation of the multi-converter parallel system in steady state and transient mode.

[0007] To achieve the above-mentioned purpose, the present application provides the following technical scheme: a circulating current transient suppression method based on output voltage reconstruction, comprising the following steps:

[0008] Step one: the circulating current suppression of the multi-converter parallel system and the output current sharing process of each converter are equivalent, and according to the current system load or total output demand, the number of parallel converters is combined to dynamically generate the unified target output current of each converter at the current time;

[0009] Step two: real-time acquisition of the output current of each converter The deviation of the output current of each group of converters from the average output current of the system is calculated, and the deviation is defined as the disturbance current component as the basis for control of circulating current suppression;

[0010] Step three: Based on the rated equivalent output characteristics of each group of converters, a circulating current suppression feedforward compensation admittance Y is designed dis (s), a circulating current disturbance component is introduced, and a voltage disturbance compensation vector is generated according to the disturbance current to counteract the stimulating effect of the disturbance on the circulating current;

[0011] Step four: The output voltage vector of the converter is reconstructed, and the reconstructed output voltage of the converter is composed of a basic component controlled by a voltage loop and a circulating current suppression component , wherein the basic voltage component is used to maintain the rated voltage of the AC bus, and the circulating current suppression component is used to counteract the deviation of the output current of each converter from the average output current of the system.

[0012] Preferably, the circulating current is the output current of at least two groups of converters when circulating current exists in a system containing multiple groups of parallel converters, and the circulating currents formed by any two converters do not affect each other and are superimposed to become the total circulating current in the parallel system.

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

[0014]

[0015] wherein and represent the circulating current between the jth converter and the kth converter, 0≤j, k≤M, and j≠k, and the direction from the jth converter to the kth converter is positive.

[0016] is the output current of the jth converter, and is the output current of the kth converter.

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

[0018]

[0019] wherein is the real-time output current of the mth converter, and M is the total number of parallel converters.

[0020] Preferably, the calculation formula of the disturbance current component in step two is:

[0021]

[0022] Wherein is the disturbance current component in the mth group of converters caused by the existence of the circulating current.

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

[0024]

[0025] Wherein Z0 is 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 resistance of the LC filter.

[0026] Preferably, the basic component in step four is generated by the voltage control closed loop of the converter itself, and the circulating current suppression component is generated by the circulating current disturbance calculation, does not participate in and does not affect the generation of the voltage instruction of the converter itself and the closed loop control process, can realize the double decoupling of the circulating current suppression and the voltage control, and the circulating current suppression component is calculated and generated by the multiplication of formula (3) and formula (4);

[0027] The calculation formula of the circulating current suppression component of the mth group of converters in the 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 state and the output current change trend;

[0031] When the load is suddenly changed or the power grid is faulty, 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 condition requirements.

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

[0033] And the system comprises multiple groups of converters, output filters, loads and control modules connected in parallel to the same AC bus, and the control modules are used for executing voltage vector reconstruction and circulating current suppression algorithms.

[0034] The application of a circulating current transient suppression method based on output voltage reconstruction is used for a distributed power generation system, and the distributed power generation system comprises a power generation system with grid-connected and off-grid operation.

[0035] Compared with the prior art, the application has the beneficial effects that:

[0036] 1. The application regards the circulating current as a disturbance component in the converter control system, can effectively suppress various types of circulating current problems caused by control parameter deviation, phase desynchronization or circuit parameter mismatch without directly modifying the converter output voltage reference instruction, in addition, through this way, the circulating current suppression process and the output voltage control process are decoupled, which simplifies the system design, improves the overall operation efficiency, ensures the safe and reliable operation of the multi-converter parallel system in various working modes, has high practical value and wide application prospect;

[0037] 2. The application decouples the circulating current suppression and the output voltage control process through output voltage reconstruction, avoids the mutual interference of the two in the traditional method, can guarantee the stability of the output voltage and effectively suppress the circulating current;

[0038] 3. The application has strong transient adaptability and is suitable for off-grid switching, synchronous grid connection and other transient modes of the microgrid, solves the problem of performance degradation of the droop control in the transient process;

[0039] 4. The application can suppress multiple types of circulating currents, can simultaneously suppress DC, fundamental frequency, harmonic and switching frequency and other types of circulating currents, and does not need to design a special strategy for a single type;

[0040] 5. The application has high robustness, can maintain good performance in the scenes of load mutation, parameter mismatch and carrier desynchronization, improves the stability and reliability of the system operation. DETAILED DESCRIPTION

[0041] Figure 1 A three-phase equivalent structure schematic diagram of a multi-converter parallel system relied on by the embodiment of the application;

[0042] Figure 2 A three-phase equivalent circuit schematic diagram based on two-converter parallel relied on by the embodiment of the application;

[0043] Figure 3An equivalent schematic diagram of a circulating current path based on parallel connection of two groups of converters for an embodiment of the present application;

[0044] Figure 4 A control structure block diagram for implementing a circulating current unified suppression strategy of a multi-converter parallel system based on output voltage reconstruction for an embodiment of the present application;

[0045] Figure 5 A software flow chart for implementing a circulating current unified suppression strategy of a multi-converter parallel system based on output voltage reconstruction for an embodiment of the present application;

[0046] Figure 6 A simulation experimental result waveform diagram of an embodiment of the present application without and with the present application scheme when the modulation wave amplitude deviation of two groups of converters is 100%;

[0047] Figure 7 A simulation experimental result waveform diagram of an embodiment of the present application without and with the present application scheme when the modulation wave phase deviation of two groups of converters is 180°;

[0048] Figure 8 A simulation experimental result waveform diagram of an embodiment of the present application without and with the present application scheme when the carrier wave phase deviation in the PWM modulation process of two groups of converters is 180°;

[0049] Figure 9 A simulation experimental result waveform diagram of an embodiment of the present application without and with the present application scheme when the inductance value deviation of an AC side LC filter circuit of two groups of converters is 80%;

[0050] Figure 10 A simulation experimental result waveform diagram of an embodiment of the present application without and with the present application scheme when the capacitance value deviation of an AC side LC filter circuit of two groups of converters is 80%;

[0051] Figure 11 A simulation experimental result waveform diagram of an embodiment of the present application without and with the present application scheme in a transient operation mode of two groups of converters, using a traditional downfeed control;

[0052] Figure 12 A prototype experimental result waveform diagram of an embodiment of the present application without and with the present application scheme in a linear load case;

[0053] Figure 13 A prototype experimental result waveform diagram of an embodiment of the present application without and with the present application scheme in a nonlinear load case;

[0054] Figure 14 A prototype experimental result waveform diagram of an embodiment of the present application without and with the present application scheme in a transient operation mode of two groups of converters, using a traditional downfeed control.

[0055] In the figure: 1.1, three-phase AC bus; 1.2, on / off grid switch; 1.3, converter inlet circuit breaker; 1.4, three-phase two-stage conversion circuit; 1.4.1, DC / AC conversion link; 1.4.2, DC / DC conversion link; 1.4.3, LC filter circuit; 1.5, renewable energy power generation device; 1.6, three-phase AC load. DETAILED DESCRIPTION

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

[0057] Please refer to Figures 1 to 14 An embodiment provided by the present application is a loop current transient suppression method based on output voltage reconstruction, comprising the following steps:

[0058] Step one: loop current suppression of a multi-converter parallel system is actually equivalent to output current sharing process of each converter, according to current system load or total output demand, in combination with the number of parallel converters, dynamically generating the unified target output current of each converter at the current time;

[0059] Step two: real-time acquisition of output current of each converter Calculation of deviation of output current of a single group of converters from system average output current, the deviation is defined as disturbance current component as the control basis for loop current suppression;

[0060] Step three: based on rated equivalent output characteristics of each group of converters, designing loop current suppression feedforward compensation admittance Y dis (s), introducing loop current disturbance component, generating voltage disturbance compensation vector according to disturbance current for offsetting the excitation effect of disturbance on loop current;

[0061] Step four: through reconstruction of converter output voltage vector, the reconstructed converter output voltage is composed of voltage closed-loop control basic component and loop current suppression component , wherein the basic voltage component is used to maintain the rated voltage of the AC bus, and the loop current suppression component is used to offset the deviation of output current of a single converter from the system average output current.

[0062] The loop current described above is that when there is loop current in the system containing multiple groups of parallel converters, the output currents of at least two groups of converters are necessarily not equal, and the loop currents formed by any two converters do not affect each other and are superimposed to become the total loop current in the parallel system, specifically:

[0063] A multi-inverter parallel system comprising M groups of inverters, the output current matrix of each group of inverters is represented as:

[0064]

[0065] The circulating current between any two groups of inverters is represented as: (1≤j,k≤M) is represented as:

[0066]

[0067] When the parallel system comprises M groups of inverters, the circulating current between any two groups of inverters is the difference between the output currents of the two groups of inverters, and the calculation formula is:

[0068]

[0069] wherein and represent the circulating current between the jth inverter and the kth inverter, 0≤j,k≤M, and j≠k, and the direction from the jth inverter to the kth inverter is positive;

[0070] is the output current of the jth inverter, is the output current of the kth inverter, and the size of the circulating current between any two inverters in the parallel inverter system is related to the output voltage deviation (including amplitude, phase, harmonic component) and the output circuit characteristic parameter deviation.

[0071] The target output current in step one is The system output average current, which is obtained by dividing the system output total current by the number of parallel inverters M, is as follows:

[0072]

[0073] wherein is the real-time output current of the mth inverter, and M is the total number of parallel inverters.

[0074] The calculation formula of the disturbance current component in step two is:

[0075]

[0076] wherein is the disturbance current component caused by the circulating current in the mth group of inverters, and the size is related to the output current of the group of inverters itself

[0077] In step three, the circulating current disturbance component is introduced into the closed-loop control system of the inverter, and the relationship between the output current and the inverter output voltage and the circulating current compensation voltage is:

[0078]

[0079] Thus, the circulating current suppression feedforward compensation admittance Y dis The calculation transfer function formula of (s) is:

[0080]

[0081] Wherein, Z0 is 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, R d is the damping resistance of the LC filter.

[0082] The basic component in step four is generated by the voltage control closed loop of the converter itself, and the circulating current suppression component is generated by the circulating current disturbance calculation, does not participate in and does not affect the generation of the voltage instruction of the converter itself and the closed loop control process, and can realize the double decoupling of the circulating current suppression and the voltage control, and the circulating current suppression component is generated by the multiplication calculation of formula (3) and formula (4);

[0083] The circulating current suppression component calculation formula of the mth group of converters in the parallel system is:

[0084]

[0085] The calculated converter output voltage is decomposed into a basic component and a circulating current suppression component, wherein the basic component is generated by the voltage control closed loop of the converter itself, and the circulating current suppression component is generated by the circulating current disturbance calculation, does not participate in and does not affect the generation of the voltage instruction of the converter itself and the closed loop control process. In the transient working mode of the multi-converter parallel system, the basic component is used to complete the smooth transition of the working mode, and the circulating current suppression component is used to complete the unified suppression of the circulating current, so as to realize the double decoupling of the circulating current suppression and the voltage control;

[0086] The generation process of the circulating current suppression voltage component comprehensively considers the deviation between the actual value and the target current value of the converter output current, and dynamically adjusts in combination with the system running state, so as to ensure that effective circulating current suppression effect can be realized under different working conditions. In addition, the method can flexibly adapt to various complex running scenes on the premise of not changing the original system control architecture, so as to improve the compatibility and robustness of the system.

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

[0088] When extreme conditions such as load mutation or power grid failure occur, the controller can quickly update the feedforward compensation admittance Y dis (s) through online identification technology, so that the circulating current suppression strategy always matches the current working condition requirements, thereby effectively avoiding the performance degradation problem caused by fixed parameters in the traditional method.

[0089] A multi-group converter parallel system applied to large-capacity distributed power generation occasions, the system adopts the above method to realize circulating current suppression between converters; and the system comprises a plurality of converters connected in parallel to the same AC bus, an output filter, a load and a control module, and the control module is used to execute voltage vector reconstruction and circulating current suppression algorithm.

[0090] A circulating current transient suppression method based on output voltage reconstruction is applied to a distributed power generation system with functions such as grid-connected and off-grid operation, especially large-capacity occasions with multiple converter parallel connection, each converter generates a set of voltage regulating vectors and superimposes them into the output voltage of itself, thereby realizing circulating current suppression of multi-converter parallel system.

[0091] The simulation and experimental verification are carried out in a three-phase three-wire AC power grid with two converter parallel systems, and the specific parameters are as follows:

[0092] Serial number Parameter Symbol Value Unit l Three-phase AC bus rated voltage U p ]]> 220 V 2 Three-phase AC bus rated frequency f 50 Hz 3 Number of parallel connected converters M 2 - 4 Converter DC side voltage U dc ]]> 300 V 5 Three-phase load rated power P R ]] 20 kW 6 LC filter inductance value L 0.5 mH 7 LC filter capacitance value C 80 pF 8 LC filter damping resistance value [R d ]] 2 Ohm 9 Converter PWM modulation carrier frequency f c ]]> 2 kHz 10 Converter control system sampling frequency (calculation frequency) f sam ]]> 10 kHz

[0093] Referring to the accompanying Figure 6 (a) and Figure 6 (b) are respectively the extreme working conditions in which the amplitude deviation of the modulation wave of the two groups of converters reaches 100%, that is, the modulation ratio of converter 1 is 0.8, the modulation ratio of converter 2 is 0.4, and the phases of the two are strictly synchronized, and the simulation results before and after the application of the scheme of the present application are compared;

[0094] When the scheme of the present application is not used, the output currents I g1 and I g2 of the two groups of converters have a significant amplitude difference, and the peak value of the circulating current I h of the system exceeds 400A;

[0095] When the scheme of the present application is used, the disturbance current I dis is calculated in real time, the anti-phase voltage compensation vector U dis is generated based on the feedforward compensation admittance, and is superimposed on the basic voltage reference value of the converter, thereby realizing the decoupling of circulating current suppression and voltage control, the output currents of the converters are basically coincident, the peak value of the circulating current I h of the system is reduced to 5A, and the voltage distortion rate at the grid connection point is 1.27%. The high-efficiency suppression ability of the scheme of the present application to the modulation wave amplitude deviation type circulating current is verified.

[0096] Referring to the accompanying Figure 7(a) with Figure 7 (b), respectively, are the simulation results of the extreme condition that the modulation wave amplitude of the two groups of converters is completely equal but the phase deviation is 180°, that is, the modulation wave phase of converter 2 lags behind the modulation wave phase of converter 1 by 180°, and the comparison waveforms of the simulation results without adopting the scheme of the application and the simulation results with adopting the scheme of the application;

[0097] When the scheme of the application is not adopted, the output currents Ig1 and Ig2 of the two groups of converters are obviously different in amplitude and phase, and there is obvious distortion, and the peak value of the system circulating current Ic is over 300 A; g1 and Ig2 are obviously different in amplitude and phase, and there is obvious distortion, and the peak value of the system circulating current Ic is over 300 A; g2 The amplitude difference is significant, and the phase deviation is nearly 90°, the system circulating current Ic is over 1000 A; h The peak value of the system circulating current Ic is over 1000 A;

[0098] When the scheme of the application is adopted, the disturbance current Ic is calculated in real time, the anti-phase voltage compensation vector Uc is generated based on the feedforward compensation admittance, and is superimposed on the converter basic voltage reference value, so as to realize the decoupling of circulating current suppression and voltage control, the output currents Ig1 and Ig2 of the two groups of converters are basically coincident, the peak value of the system circulating current Ic is reduced to 12 A, and the voltage distortion rate of the grid connection point is 2.49%. It is verified that the scheme of the application can efficiently offset the phase deviation under the extreme interference that the modulation wave phases are opposite. dis dis The peak value of the system circulating current Ic is reduced to 12 A, and the voltage distortion rate of the grid connection point is 2.49%. It is verified that the scheme of the application can efficiently offset the phase deviation under the extreme interference that the modulation wave phases are opposite.

[0099] Referring to the accompanying drawings Figure 8 (a) with Figure 8 (b), respectively, are the simulation results of the extreme condition that the modulation wave amplitude of the two groups of converters is completely equal but the phase deviation is 180°, that is, the modulation wave phase of converter 2 lags behind the modulation wave phase of converter 1 by 180°, and the comparison waveforms of the simulation results without adopting the scheme of the application and the simulation results with adopting the scheme of the application;

[0100] When the scheme of the application is not adopted, the output currents Ig1 and Ig2 of the two groups of converters are obviously different in amplitude and phase, and there is obvious distortion, and the peak value of the system circulating current Ic is over 300 A; g1 and Ig2 are obviously different in amplitude and phase, and there is obvious distortion, and the peak value of the system circulating current Ic is over 300 A; g2 The amplitude difference is significant, and the phase deviation is nearly 90°, the system circulating current Ic is over 1000 A; h The peak value of the system circulating current Ic is over 300 A;

[0101] When the scheme of the application is adopted, the disturbance current Ic is calculated in real time, the anti-phase voltage compensation vector Uc is generated based on the feedforward compensation admittance, and is superimposed on the converter basic voltage reference value, so as to realize the decoupling of circulating current suppression and voltage control, the output currents Ig1 and Ig2 of the two groups of converters are basically coincident, the peak value of the system circulating current Ic is reduced to 12 A, and the voltage distortion rate of the grid connection point is 2.49%. It is verified that the scheme of the application can efficiently offset the phase deviation under the extreme interference that the modulation wave phases are opposite. dis dis The amplitude difference is significant, and the phase deviation is nearly 90°, the system circulating current Ic is over 1000 A; g1 The peak value of the system circulating current Ic is reduced to 12 A, and the voltage distortion rate of the grid connection point is 2.49%. It is verified that the scheme of the application can efficiently offset the phase deviation under the extreme interference that the modulation wave phases are opposite. ​​

[0102] Refer to the accompanying drawings Figure 9 (a) and Figure 9 (b) are respectively the extreme condition of 100% deviation of inductance value in LC filter on output side of two groups of parallel connected converters, that is, the inductance value of output filter of converter 1 is 0.5 mH, and the inductance value of output filter of converter 2 is 0.25 mH, the simulation experimental results of not using and using the scheme of the application are compared in waveform;

[0103] When not using the scheme of the application, the output currents I g1 and I g2 present significant amplitude deviation and waveform distortion due to inductance mismatch, the amplitude of system circulation current is over 100 A, and a large amount of high frequency components are contained;

[0104] When using the scheme of the application, the output currents I g1 and I g2 present fast convergence of waveform and consistent amplitude, the high frequency components in circulation current are significantly reduced, and the voltage distortion rate at grid connection point is 4.89%. It is verified that the scheme of the application can effectively suppress the problems of high frequency circulation current and output voltage waveform distortion caused by parameter difference of filter inductance when facing the inductance deviation of filter in parallel system of multiple groups of converter circuits.

[0105] Refer to the accompanying drawings Figure 10 (a) and Figure 10 (b) are respectively the extreme condition of 80% deviation of capacitance value in LC filter on output side of two groups of parallel connected converters, that is, the capacitance value of output filter of converter 1 is 80 μF, and the capacitance value of output filter of converter 2 is 40 μF, the simulation experimental results of not using and using the scheme of the application are compared in waveform;

[0106] Similarly to the inductance value mismatch condition (Example Four), when not using the scheme of the application, the output currents I g1 and I g2 present significant amplitude deviation and waveform distortion due to capacitance mismatch, the amplitude of system circulation current is over 50 A, and a large amount of high frequency components are contained;

[0107] When using the scheme of the application, the output currents I g1 and I g2 present fast convergence of waveform and consistent amplitude, the high frequency components in circulation current are significantly reduced, and the voltage distortion rate at grid connection point is 2.62%. It is verified that the scheme of the application can effectively suppress the problems of high frequency circulation current and output voltage waveform distortion caused by parameter difference of filter capacitance when facing the capacitance deviation of filter in parallel system of multiple groups of converter circuits.

[0108] Refer to the accompanying drawings Figure 11 (a) and Figure 11(b), respectively, are simulation experimental results of two groups of parallel converters running in transient operation mode without and with the scheme of the application, in this embodiment, the synchronization parallel grid process of the converter is taken as an example, and the on / off grid switching switch is closed at time t0 in the figure;

[0109] When the scheme of the application is not used, the amplitude of the circulating current continuously increases in the whole synchronization process;

[0110] When the scheme of the application is used, the system circulating current is well inhibited in the whole stage, and the stability of the parallel converter system in the transient operation mode is ensured.

[0111] Referring to the accompanying drawings Figure 12 and the accompanying drawings Figure 13 , respectively, are sample experimental results of two groups of parallel converters without and with the scheme of the application under linear load and nonlinear load conditions;

[0112] Among them, Figure 12 (a) and Figure 13 (a) are output voltages and currents under linear load and nonlinear load conditions without any circulating current suppression strategy. It can be found that the output currents I g1 and I g2 of the two groups of parallel converters have obvious deviations, which will inevitably lead to the existence of circulating current. According to the result analysis, the two groups of parallel converters in the sample system have inconsistent modulation command amplitudes, and the parameters of the inductance-capacitance filter are different;

[0113] When the scheme of the application is used, the sample result waveforms under two types of load conditions are as follows Figure 12 (b) and Figure 13 (b), at this time, the output currents I g1 and I g2 of the two groups of parallel converters are basically coincident in amplitude and phase, indicating that the output currents of the two groups of parallel converters are basically consistent, and the system circulating current is well inhibited. It is verified that the scheme of the application can still effectively suppress the system circulating current under different types of load conditions.

[0114] Referring to the accompanying drawings Figure 14 (a) and Figure 14 (b), respectively, are sample experimental results of two groups of parallel converters running in transient operation mode without and with the scheme of the application. Corresponding to embodiment six, as preferred, this embodiment takes the synchronization parallel grid process of the converter as an example, and the on / off grid switching switch is closed at time t1 in the figure;

[0115] When the conventional strategy of droop control and virtual impedance is adopted without the scheme of the application, the amplitude of circulating current continuously increases throughout the synchronization process, and the waveform quality of converter output voltage and current is obviously distorted;

[0116] When the scheme of the application is adopted, the system circulating current is well suppressed throughout the stage, and 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 degree of the output waveform and the circulating current suppression effect, ensuring the stability of the converter parallel system in the transient operation mode.

[0117] In summary, through simulation and experimental verification of multiple embodiments, the application demonstrates its excellent performance in different extreme working conditions. Whether it is modulation wave amplitude deviation, phase deviation, carrier phase deviation, or LC filter circuit parameter mismatch, the scheme of the application 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, in the extreme working condition of modulation wave amplitude deviation reaching 100%, the scheme of the application reduces the system circulating current peak from more than 400A to below 5A, and significantly improves the grid connection point voltage quality. In the case of modulation wave phase deviation of 180°, the application also achieves synchronization of the converter output current, and the circulating current peak is reduced from more than 1000A to within 12A, further demonstrating its adaptability to phase reversal interference. In addition, in complex scenarios such as carrier phase deviation and LC filter circuit parameter mismatch, the scheme of the application shows good robustness, successfully solving the technical problems that traditional control methods cannot handle. It can be seen that the method proposed in the application not only has high-precision circulating current suppression capability, but also can maintain system stability in multiple transient modes, providing reliable technical support for the practical application of multi-converter parallel system.

[0119] It is obvious to those skilled in the art that the application is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or essential characteristics of the application. Therefore, the embodiments should be regarded as exemplary and non-limiting, and the scope of the application is defined by the appended claims rather than the above description, and it is intended to encompass all changes falling within the meaning and scope of the equivalent elements of the claims. Any reference signs in the claims should not be considered as limiting the claims involved.

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. As a basis for control of circulation suppression; 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; 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.

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: 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; 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, as shown in the following formula: 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 formula for calculating the disturbance current component in step two is as follows: in This refers to the disturbance current component caused by the presence of circulating current in the m-th converter group, and its magnitude is related to the output current of that converter group itself.

5. The circulating current transient suppression method based on output voltage reconstruction according to claim 4, characterized in that: In step three, the circulating current suppression feedforward compensation admittance Y dis The formula for calculating the transfer function of (s) is: 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.

6. The circulating current transient suppression method based on output voltage reconstruction according to claim 5, characterized in that: 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); The formula for calculating the circulating current suppression component of the m-th converter in a parallel system is:

7. The circulating current transient suppression method based on output voltage reconstruction according to claim 6, 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.

8. The circulating current transient suppression method based on output voltage reconstruction according to claim 7, 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.

9. 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 1-8 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.

10. An application of a circulating current transient suppression method based on output voltage reconstruction, characterized in that: Used in distributed generation systems, which include power generation systems with grid-connected and off-grid operation, when there is a large capacity demand due to multiple converters connected in parallel, each converter generates a set of voltage regulation vectors and adds them to its own output voltage, thereby suppressing the circulating current in the multi-converter parallel system.

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