Method for circulating current suppression of multi-inverter parallel system based on extended state observer

Abstract

The application discloses a circulating current suppression method of a multi-inverter parallel system based on an extended state observer, and belongs to the technical field of power electronics. The circulating current is calculated through the output current of each inverter collected by an inverter parallel operation equivalent circuit, and the circulating current is differentiated. The compensation voltage is obtained through the means of supplementing the voltage difference. Then, the unknown part of the system is estimated by using a linear extended state observer, and the output voltage reference value of the inverter is obtained. After that, the power balance and the circulating current suppression effect of the system under the no circulating current suppression strategy and the circulating current suppression strategy based on the ESO under the VSG control are verified. The circulating current calculation model of the multi-inverter parallel system is combined with the extended state observer, so that the inverter voltage compensation reference value required for minimizing the circulating current can be accurately calculated without knowing the line impedance parameters. The circulating current suppression strategy can be directly realized without detailed micro-grid line parameters, and the applicability of the circulating current suppression strategy is enhanced.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a circulating current suppression method for a multi-inverter parallel system based on an extended state observer. Background Technology

[0002] Currently, there are still some problems with the circulating current suppression technology in existing multi-inverter parallel systems, as follows:

[0003] Reference [1] uses the same virtual impedance at the output port of each inverter, with the magnitude of the virtual impedance significantly exceeding the line impedance. This method suppresses uneven power distribution caused by differences in line impedance. While this method is simple and practical, there is a trade-off between power sharing accuracy and bus voltage distortion. Increasing the virtual impedance can improve power sharing efficiency, but it can also amplify imbalances and introduce harmonics.

[0004] Reference [2] proposes an adaptive adjustment of the output virtual impedance, which solves the problem of inaccurate power distribution caused by line impedance mismatch. However, this method requires the use of line impedance parameters, and accurately detecting line impedance is a challenge in practical applications. In addition, changes in the microgrid structure may cause this method to fail. Summary of the Invention

[0005] The purpose of this invention is to provide a circulating current suppression method for a multi-inverter parallel system based on an extended state observer. By calculating the difference between the inverter output voltage and the voltage at the common PCC point, this deviation is used as a compensation amount to increase the reference voltage output of the virtual synchronous machine (VSG) control loop, forming a reference value for the inverter output voltage. Furthermore, even without knowing the line impedance parameters, the inverter voltage compensation reference value required to minimize the circulating current can be accurately calculated.

[0006] To achieve the above objectives, this invention provides a circulating current suppression method for a multi-inverter parallel system based on an extended state observer, comprising the following steps:

[0007] S1. Construct a simulation model of two inverters connected in parallel in an isolated microgrid, collect data from each part of the two inverters, and calculate the circulating current based on the output current of each inverter collected from the equivalent circuit of the inverters operating in parallel.

[0008] S2. Perform differential calculation on the circulating current obtained in step S1, and obtain the compensation voltage by supplementing the voltage difference.

[0009] S3. Use the linear extended state observer to estimate the unknown part of the system, express the LESO expression of the system, and discretize it using LESO to obtain the output voltage reference value of the inverter.

[0010] S4. The power equalization and circulating current suppression effects of the system under VSG control without circulating current suppression strategy and ESO-based circulating current suppression strategy are compared through experiments to verify the circulating current suppression effect.

[0011] Preferably, in step S1, the calculation of the circulating current is as follows:

[0012] Based on the collected output currents I1 and I2 of each inverter, the circulating current is calculated as shown in the following formula (1):

[0013]

[0014] Where I1 and I2 are the output currents of the two inverters. Average current;

[0015] Based on the equivalent circuit of two inverters operating in parallel, the output current of the inverter can also be expressed as:

[0016]

[0017] Substituting formula (2) into (1), the circulating current is rewritten as:

[0018]

[0019] Where Z1 and Z2 are the output impedances of the two inverters, Z load1 Z load2 The line impedance is E1∠θ1, E1∠θ2, and the inverter's output voltage is U. L ∠δ0、Z L These are the load voltage and the load impedance, respectively.

[0020] Therefore, assuming E1∠θ1=E2∠θ2=E∠θ, the circulating current is:

[0021]

[0022] Preferably, in step S2, the circulating current I in step S1 is... cir The differential equation is as follows:

[0023]

[0024] Where Z and L represent the equivalent impedance and inductance of the circulating current in the circuit, respectively, and ΔE cir It is the voltage difference between the parallel inverters that causes the circulating current;

[0025] By adding an appropriate compensation ΔE to the inverter's reference output voltage cir Reduce circulation;

[0026] Compensation voltage The calculation method is as follows:

[0027]

[0028] in This is the reference value for the circulation, set to 0.

[0029] Preferably, in step S3, the LESO expression of the system is as follows:

[0030]

[0031] Substituting formula (5) into formula (7), we get:

[0032]

[0033] in,

[0034] Discretizing formula (8) yields formula (10):

[0035]

[0036] make Calculated using formula (10)

[0037] The inverter's output voltage reference value can be expressed as:

[0038]

[0039] Among them, U vsg It is the reference voltage output by the VSG control loop of the virtual synchronous machine.

[0040] Preferably, in step S4, the system data under both the VSG-controlled non-circulating current suppression strategy and the ESO-based circulating current suppression strategy are collected by the same device. The two inverters have the same rated capacity. The active power, reactive power, inverter output current, and system circulating current of the two inverters are collected respectively. The power balance and circulating current of the system under the VSG-controlled non-circulating current suppression strategy and the ESO-based circulating current suppression strategy are compared to verify the circulating current suppression effect.

[0041] Therefore, the circulating current suppression method for a multi-inverter parallel system based on an extended state observer, using the above-described structure, has the following beneficial effects:

[0042] This invention combines a circulating current calculation model for a multi-inverter parallel system with an extended state observer, enabling accurate calculation of the inverter voltage compensation reference value required to minimize circulating current even without knowing the line impedance parameters. This control method can be implemented directly without detailed microgrid line parameters, enhancing the applicability of circulating current suppression strategies.

[0043] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the equivalent circuit of a parallel inverter in a method for suppressing circulating current in a multi-inverter parallel system based on an extended state observer, according to the present invention.

[0045] Figure 2 This is a schematic diagram of the output waveform of the test results of the parallel inverters under the resistive line without any circulating current suppression strategy in the circulating current suppression method of the multi-inverter parallel system based on the extended state observer of the present invention.

[0046] Figure 3 This is a schematic diagram of the output waveform of the test results of the circulating current suppression control strategy based on ESO for parallel inverters under resistive lines in the circulating current suppression method of a multi-inverter parallel system based on an extended state observer according to the present invention. Detailed Implementation

[0047] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0048] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0049] Example

[0050] This invention provides a circulating current suppression method for a multi-inverter parallel system based on an extended state observer, comprising the following steps:

[0051] Step S1: Figure 1 This shows the equivalent circuit of two inverters connected in parallel in an islanded microgrid. Z1 and Z2 are the output impedances of the two inverters. load1 Z load2 The line impedance is E1∠θ1, E2∠θ2, and the inverter's output voltage is U. L ∠δ0、Z L These are the load voltage and load impedance, respectively. I1 and I2 are the output currents of the two inverters, and I is the load current. Based on the collected output currents I1 and I2 of each inverter, the circulating current can be calculated as shown in the following formula (1):

[0052]

[0053] in This represents the average current.

[0054] According to such Figure 1 The equivalent circuit shown, with two inverters operating in parallel, can also be represented by the following expression for the inverter's output current:

[0055]

[0056] Substituting formula (2) into (1), the circulating current can be rewritten as:

[0057]

[0058] In practical systems, ensuring that different inverters have the same output voltage is simple, but ensuring that inverters in different locations have exactly the same output impedance and line impedance is difficult. Therefore, assuming E1∠θ1=E2∠θ2=E∠θ, the circulating current can be derived as:

[0059]

[0060] Step S2: Circulating current I cir The differential equation is as follows:

[0061]

[0062] Where Z and L represent the equivalent impedance and inductance of the circulating current through the circuit, respectively. ΔE cir The circulating current is caused by the voltage difference between the parallel inverters. This can be compensated by adding an appropriate compensation ΔE to the inverter's reference output voltage. cir This can reduce circulation.

[0063] Compensation voltage The calculation method is as follows:

[0064]

[0065] in This is the reference value for the circulation, set to 0.

[0066] Step 3: The Linear Extended State Observer (LESO) only requires the relative degrees of the system, and therefore can be used without an exact model. Therefore, this paper uses LESO to estimate the unknown parts of the system. Based on the controller's design requirements, the LESO of the system can be expressed as follows:

[0067]

[0068] Substituting formula (5) into formula (7), we get:

[0069]

[0070] in,

[0071] Discretizing formula (8) yields formula (10).

[0072]

[0073] make It can be calculated using formula (10)

[0074] The inverter's output voltage reference value can be expressed as:

[0075]

[0076] Among them, U vsg It is the reference voltage output by the Virtual Synchronizer (VSG) control loop.

[0077] Step S4: To verify the effectiveness of the proposed strategy, a simulation model of two inverters operating in parallel under islanded mode was designed and established based on the fast control prototype MT1050 and the hardware-in-the-loop NIPXI-1082. The experiment compared the power equalization and circulating current suppression effects of the system under VSG control without circulating current suppression and under ESO-based circulating current suppression strategies. All data were acquired using an oscilloscope (YOKOGAWADL950). Inverter #1 and Inverter #2 have the same rated capacity, i.e., S* = 10kVA. The DC bus voltage is 750V. The rated voltage is 380V. The rated frequency is 50Hz. The control cycle is 100μs.

[0078] Power grid transmission lines are configured as resistive lines. The power distribution and circulating current suppression effects under different control strategies are as follows: Figure 3As shown. The waveforms from top to bottom represent the active power, reactive power, inverter output current, and system circulating current of the two inverters, respectively. The initial system load is P = 13000W and Q = 6200var. After stable operation, the system is connected to a load, increasing the load to P = 7000W and Q = 3500var. Comparing the graphs, it can be seen that under the condition of line impedance mismatch, Figure 2 The results shown are from a test conducted without any circulating current suppression strategy. It can be seen that power cannot be accurately allocated, and the circulating current is large, with a maximum value of 4A. The error in active power allocation will also increase as the load increases. Figure 3 The test results using the ESO-based circulating current suppression control strategy are shown, demonstrating that the active and reactive power outputs of the two inverters are consistent. The circulating current suppression effect is significant, with a maximum circulating current of 0.8A.

[0079] Ultimately, by adjusting the output voltage according to the voltage reference value, the circulating current suppression between parallel inverters can be achieved.

[0080] Therefore, this invention employs the aforementioned circulating current suppression method for multi-inverter parallel systems based on an extended state observer. By combining the circulating current calculation model of the multi-inverter parallel system with the extended state observer, it can accurately calculate the inverter voltage compensation reference value required to minimize the circulating current even without knowing the line impedance parameters. This control method can be implemented directly without requiring detailed microgrid line parameters, thus enhancing the applicability of the circulating current suppression strategy.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A circulating current suppression method for a multi-inverter parallel system based on an extended state observer, characterized in that, Includes the following steps: S1. Construct a simulation model of two inverters connected in parallel in an isolated microgrid, collect data from each part of the two inverters, and calculate the circulating current based on the output current of each inverter collected from the equivalent circuit of the inverters operating in parallel. In step S1, the steps for calculating the circulating current are as follows: Based on the collected output current of each inverter , The circulating current is calculated as shown in the following formula (1): （1）； in, , It is the output current of the two inverters. Average current; Based on the equivalent circuit of two inverters operating in parallel, the output current of the inverter can also be expressed as: （2）； Substituting formula (2) into (1), the circulating current is rewritten as: （3）； in, , It refers to the output impedance of the two inverters. , It is the line impedance. , The inverter's output voltage, This is the load voltage; Therefore, the assumption is... The circulating current is then obtained as follows: （4）； S2. Perform differential calculation on the circulating current obtained in step S1, and obtain the compensation voltage by supplementing the voltage difference. In step S2, the circulating current from step S1 is... The differential equation is as follows: （5）； in and These represent the equivalent impedance and inductance of the circulating current through the circuit, respectively. It is the voltage difference between the parallel inverters that causes the circulating current; By adding appropriate compensation to the inverter's reference output voltage Reduce circulation; Compensation voltage The calculation method is as follows: (6)； in This is the reference value for the circulation, set to 0; S3. Use the linear extended state observer to estimate the unknown part of the system, express the LESO expression of the system, and discretize it using LESO to obtain the output voltage reference value of the inverter. In step S3, the LESO expression of the system is as follows: （7）； Substituting formula (5) into formula (7), we get: （8）； in, (9); Discretizing formula (8) yields formula (10): （10）； make Calculated using formula (10) ; S4. The power equalization and circulating current suppression effects of the system under VSG control without circulating current suppression strategy and under LESO-based circulating current suppression strategy are compared by experiment to verify the circulating current suppression effect. In step S4, the system data under both the VSG-controlled non-circulating current suppression strategy and the LESO-based circulating current suppression strategy are collected by the same device. The two inverters have the same rated capacity. The active power, reactive power, inverter output current, and system circulating current of the two inverters are collected respectively. The power balancing and circulating current suppression effects of the systems under the VSG-controlled non-circulating current suppression strategy and the LESO-based circulating current suppression strategy are compared to verify the circulating current suppression effect.

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