Adaptive virtual impedance remodeling harmonic resonance suppression method for parallel inverter microgrid

Abstract

Aiming at the stability problem caused by harmonic resonance of multiple parallel photovoltaic inverters, the invention provides a parallel inverter micro-grid-oriented adaptive virtual impedance remodeling harmonic resonance suppression method, which comprises a virtual impedance remodeling unit of a grid-connected inverter, a parameter adaptive adjustment unit and a power grid impedance online detection unit. The virtual impedance remodeling unit can improve the output impedance characteristic of the grid-connected inverter and inject a certain damping component into the micro-grid; and the parameter adaptive adjustment unit adaptively adjusts control parameters in the virtual impedance remodeling unit and the current adjustment controller according to the impedance parameters of the power grid so as to maintain the stability of a system control loop. The power grid impedance online detection unit provides power grid equivalent impedance as the input of the parameter adaptive adjustment unit. The three units work cooperatively to form a self-adaptive virtual impedance remodeling multi-inverter micro-grid harmonic resonance suppression strategy, so that the characteristics of a current loop are improved, the inverters can be kept stable under various grid impedance conditions, and the reliability of a multi-inverter micro-grid system is improved.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics and new energy power generation, and particularly relates to an adaptive virtual impedance reshaping harmonic resonance suppression method for a parallel-inverter microgrid. Background Art

[0002] With the continuous aggravation of the global energy crisis and environmental problems, it has stimulated the rapid development of grid-connected technologies for renewable energy sources (such as solar energy, wind energy, biomass energy, etc.). A microgrid is a unified whole composed of local multiple distributed power sources, energy storage devices, and power consumption units, and is one of the key technologies for the future energy chain. As a key device connecting a distributed generation system and the public grid, the performance and control method of a grid-connected inverter directly determine the quality of the grid-connected electric energy.

[0003] The harmonic resonance problem that appears in an LCL-type grid-connected inverter will seriously threaten the stable operation of the microgrid system. At present, many studies mainly focus on the harmonic resonance suppression of a single grid-connected inverter. However, the harmonic interaction and resonance problems in a microgrid are particularly complex in a multi-machine grid-connected system with continuously expanding capacity. Suppressing the harmonic resonance existing in multiple grid-connected inverters is a problem with great research value. In the field of microgrids, with the wide adoption of the multi-inverter parallel grid-connected structure, establishing a grid-connected model that more conforms to the actual system, through an adaptive virtual impedance reshaping control strategy, enabling the grid-connected inverter to inject different damping amounts into the microgrid according to different situations of the grid impedance, and combining with a current control strategy to achieve the purpose of suppressing harmonic resonance is the problem that the present invention is committed to solving. Summary of the Invention

[0004] Since the LCL-filter grid-connected inverter with grid-side current feedback cannot achieve single-loop control stability, other feedback control quantities need to be introduced in the grid-side current feedback control. The present invention adopts a control strategy of inverter machine-side current feedback (as Figure 1 shown). This algorithm is simple. Under the conditions of appropriate controller design and appropriate control parameter selection, the inverter machine-side current control can ensure the stability of the system and avoid the occurrence of inverter resonance phenomena. The purpose of the present invention is to propose an adaptive virtual impedance reshaping harmonic resonance suppression method for a parallel-inverter microgrid aiming at the problems existing in the existing multi-inverter microgrid system, such as complex control system, fixed control parameters, and low adjustment process efficiency. This method can not only effectively suppress the series-parallel harmonic resonance caused by multiple inverters in parallel, but also adaptively adjust the control parameters in the virtual impedance reshaping unit and the current regulation controller according to the situation of the microgrid system.

[0005] The above technical problems of the present invention are mainly solved by the following technical solutions:

[0006] An adaptive virtual impedance reshaping harmonic resonance suppression method for a parallel-inverter microgrid, based on inverter machine-side current feedback control, is characterized in that: a virtual impedance reshaping unit, a parameter adaptive adjustment unit, and a grid impedance online detection unit are implemented as digital programs in the controller. Among them, the virtual impedance reshaping unit can improve the output impedance characteristics of the grid-connected inverter and inject a certain damping component into the microgrid; the parameter adaptive adjustment unit adaptively adjusts the control parameters in the virtual impedance reshaping unit and the current regulation controller according to the grid impedance parameters to maintain the stability of the system control loop; the grid impedance online detection unit provides the grid equivalent impedance as the input of the parameter adaptive adjustment unit.

[0007] An adaptive virtual impedance reshaping harmonic resonance suppression method for a parallel-inverter microgrid is characterized in that: in the grid-connected inverter control system, the inverter machine-side inductor current and the filter circuit capacitor voltage, etc. are used as state feedback quantities to change the grid-connected inverter equivalent output impedance, so as to achieve the same harmonic resonance suppression effect as the actual impedance in series and parallel of the inverter machine-side inductor and the filter capacitor. This can not only improve the output impedance of the grid-connected inverter, but also improve the network impedance of the distributed microgrid when multiple inverters are connected in parallel. However, while the virtual impedance in the control link suppresses high-order harmonic resonance, it will also have some influence on the fundamental frequency current component of the power frequency, thereby affecting the tracking accuracy of the grid-connected power. In particular, the series impedance of the filter inductor will reduce the low-frequency gain of the LCL filter. In order to only reshape other frequency components in the network except the power frequency, it is necessary to first obtain the harmonic output voltage and current of the grid-connected inverter. In the present invention, a fundamental frequency notch filter is introduced into the virtual impedance reshaping unit to eliminate the unnecessary fundamental frequency components in this unit. The transfer function of this fundamental frequency notch filter is:

[0008]

[0009] where k is the damping coefficient and uo is the grid fundamental angular frequency. The characteristic of Equation 1 is that the voltage (or current) component of the fundamental frequency will be greatly attenuated, while the components of other frequencies can pass through this fundamental frequency notch filter without loss.

[0010] The control equivalent block diagram of a single grid-connected inverter based on virtual impedance reshaping is as Figure 2 shown. Among them, the control parameters of the virtual impedance reshaping unit mainly include the virtual impedance RSC and RPC, and the control parameters of the current regulation controller mainly include the proportional coefficient kp and the resonance coefficient ky. Figure 2 The open-loop transfer function of the shown control system is:

[0011]

[0012] Among them, \(H_i\) is the current gain coefficient; \(H_{UI}\) is the feedback sampling coefficient of the inductor current on the machine side of the inverter; \(G_i(s)\) is the QPR current regulation controller; is the inverter gain; \(m\) refers to the number of grid-connected inverters, and \(m\omega L_g\) is obtained by the on-line detection technology of the grid impedance.

[0013] The grid impedance is not only a factor affecting the stability of a single grid-connected inverter, but also a factor affecting the stability of a system composed of multiple parallel grid-connected inverters. It can be analyzed that when multiple grid-connected inverters are connected in parallel, the equivalent grid impedance of any one of the grid-connected inverters at the Point of Common Coupling (PCC) is equal to the product of the grid impedance value and the number of grid-connected inverters connected in parallel at the PCC. Therefore, the change in the number of parallel grid-connected inverters will cause the equivalent grid impedance of a single inverter to change, and the grid-connected inverters will generate series and parallel harmonic resonances through the grid impedance. At this time, the control parameters of the current controller are not the most appropriate, and the stable operation of the system of the present invention depends on the selection of the control parameters of the current controller. The present invention proposes an adaptive virtual impedance reshaping method for the above problems. This method can adaptively adjust the control parameters in the virtual impedance reshaping unit and the current regulation controller, so that the grid-connected inverter can maintain stable control characteristics under various grid impedance conditions. For the proportional coefficient \(k_p\), the resonance coefficient \(k_r\) in the current regulation controller, and the virtual impedance \(R_{SC}\), \(R_{PC}\) in the virtual impedance reshaping unit, the design process is as follows:

[0014] For the proportional coefficient \(k_p\), the cut-off frequency is generally designed to be 10 times the fundamental frequency and less than 1 / 5 of the switching frequency. The present invention designs the cut-off frequency of the system to be 1 kHz. When considering the frequency domain characteristics before the cut-off frequency, the capacitive branch can be ignored. At this time, the LCL filter can be simplified to a single L filter. At the same time, at the crossover frequency \(f_c\), the QPR current regulator is mainly affected by \(k_p\). At this time, the open-loop transfer function of the control system can be simplified as:

[0015]

[0016] According to the definition of the cut-off frequency \(f_c\), we can get:

[0017]

[0018] After back-calculation, the adaptive adjustment calculation formula for the proportional coefficient \(k_p\) is obtained as:

[0019]

[0020] Resonant coefficient \(k_y\). To achieve effective tracking of the grid current with respect to the grid voltage, the low-frequency gain \(T\) of the system at 50 Hz is designed to be 60 dB. At the reference frequency \(f_0\), since \(f_0 < f\), the system can still be simplified to a single-L filter. At this time, the QPR current regulator is mainly affected by \(K_y\), and the open-loop transfer function of the control system can be simplified as follows:

[0021]

[0022] From the definition of the fundamental frequency gain \(T\), we get:

[0023]

[0024] Rearranging Equation (7), the binary linear equation about the resonant coefficient \(k\) is obtained as:

[0025] \(a_1k + b_1h + G_1 = 0\) Equation (8)

[0026] where:

[0027]

[0028] It can be seen that the adaptive adjustment calculation formula for the resonant coefficient \(k\) is:

[0029]

[0030] In the present invention, the damping coefficient of the control strategy is a key parameter of the resonant system. If its value is too large, the stability margin of the system may be reduced; if it is too small, harmonic resonance cannot be effectively suppressed. Generally, in engineering, it is taken between 0.5 and 1. To ensure that the system still has a certain stability margin under various grid impedance conditions, the system damping coefficient \(r_p\) is designed to be 0.707. Here, considering the filtering effect of the LCL filter and the efficiency of the system, since the change in the grid impedance has little effect on the harmonic resonance suppression ability of the virtual series impedance RSC, within a large range of grid impedance changes, good suppression of each harmonic can be ensured. Therefore, the value of RSC is set in advance in the virtual impedance reshaping link, and only the value of the virtual impedance RPC is adjusted. According to Equation (2), the closed-loop transfer function of the control system can be obtained as follows:

[0031]

[0032] The solution idea of RPC in the present invention is to analyze the denominator of \(G_U\) on the principle of ensuring the invariance of the system damping coefficient, define the damping coefficient \(r_p\) and the resonant frequency \(\omega_{rp}\), solve the functional relationship of \(r_p\) with respect to RPC, and obtain the adaptive adjustment function calculation formula of the virtual impedance RPC with respect to \(m\omega L_g\) through backstepping. In the present invention, RPC is adjusted accordingly.

[0033] It can be calculated according to the grid impedance parameters by using the corresponding adaptive adjustment calculation formula, and the control parameters in the adaptive adjustment virtual impedance reshaping unit and the current regulation controller are adjusted adaptively to maintain the cut-off frequency, fundamental frequency gain and damping of the control system unchanged, so as to maintain the system stability.

[0034] The on-line grid impedance detection unit detects the grid impedance condition. The present invention adopts the mature small-signal injection method. A high-frequency measurement current im with a certain amplitude is injected into the reference current iref of the current regulator, causing the grid-connected voltage lpcc and the grid-connected current ig to generate responses at the measurement frequency fm. The fast Fourier transform (FFT) analysis is performed on the grid-connected voltage and the grid-connected current, and the amplitude and phase information at the measurement frequency fm are obtained respectively, then the grid impedance can be calculated as:

[0035]

[0036] According to Equation XIII, the grid inductive reactance Lg and the grid impedance R can be obtained as:

[0037]

[0038] Due to the application of the above technical solutions, the present invention has the following characteristics:

[0039] 1. The present invention adopts the virtual impedance reshaping technology to improve the output impedance characteristics of the grid-connected inverter. When multiple grid-connected inverters are connected in parallel to the microgrid, adopting this control strategy can effectively change the network impedance of the microgrid, especially enhance the series-parallel resistive components of the network, suppress the series-parallel resonance of high-order harmonics in the network, and to a certain extent improve the current loop characteristics of the system and improve the stability of the system;

[0040] 2. The parameter adaptive adjustment unit of the present invention can adaptively adjust the control parameters in the virtual impedance reshaping unit and the current regulation controller according to the grid impedance parameters, which can not only ensure the stability of the system under the ideal grid condition, but also ensure the stability of the system under the weak grid condition;

[0041] 3. The present invention adopts the on-line grid impedance detection technology based on the small-signal injection method, which can obtain the grid impedance in real time and automatically send this parameter to the parameter adaptive adjustment unit in real time, avoiding manual continuous correction of the controller parameters, saving manpower and material resources, and improving the intelligent control of the entire grid-connected control system. Description of the Drawings

[0042] Figure 1 It is the control system block diagram of the three-phase LCL type grid-connected inverter without resonance suppression link adopted in the present invention;

[0043] Figure 2It is the equivalent block diagram of the control of a single grid-connected inverter based on virtual impedance reshaping in the present invention;

[0044] Figure 3 It is the control block diagram of the grid-connected inverter with the adaptive virtual impedance reshaping function in the present invention;

[0045] Figure 4 It is the flowchart of an adaptive virtual impedance reshaping harmonic resonance suppression method for a parallel-inverter microgrid in the present invention;

[0046] Figure 5 It is the simulation diagram of the harmonic resonance suppression effect of the present invention. Detailed implementation manners

[0047] The technical solutions will be clearly and completely described below in conjunction with the preferred embodiments and drawings of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention, rather than limiting the protection scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0048] The present invention provides an adaptive virtual impedance reshaping harmonic resonance suppression method for a parallel-inverter microgrid. The inverter machine-side current feedback control strategy is adopted to prevent the grid-connected inverter from resonating. Once it is detected that resonance occurs at the PCC of the microgrid system, the grid impedance parameters are detected, and the parameter adaptive adjustment unit is started to adjust the control parameters in the virtual impedance reshaping unit and the current regulation controller, so that the system reaches a stable state in a short time, improves the current loop characteristics of the system, enables the inverter to maintain stability under various grid impedance conditions, and improves the reliability of the multi-inverter microgrid system.

[0049] The structural schematic diagram of an embodiment is as Figure 3 shown, and the flowchart is as Figure 4 shown. The main contents of this embodiment include:

[0050] The adaptive control scheme includes: a virtual impedance reshaping unit, a parameter adaptive adjustment unit, and a grid impedance online detection unit. The grid impedance online detection technology is used to obtain the equivalent grid impedance of each grid-connected inverter at the PCC. This technology superimposes harmonic currents with a certain frequency and amplitude on the reference current, causing the response of the grid-connected voltage and grid-connected current at this frequency. The FFT algorithm is used to analyze the grid-connected voltage and current at the PCC to obtain the amplitude and phase information of the grid-connected voltage lpcc and grid-connected current ig at the measured frequency, so as to obtain the real-time equivalent grid impedance value. Based on this value, the grid inductance Lg is calculated. According to the topology parameters of the grid-connected inverter and the grid impedance parameters, the control parameters in the virtual impedance reshaping unit and the current regulation controller are calculated and corrected to ensure that the grid-connected inverter can always operate stably and efficiently under various grid impedance and inverter parameter conditions.

[0051] Taking the parallel system of two grid-connected inverters adopting Figure 3 the control scheme as an example, the harmonic resonance suppression effect of the microgrid is analyzed, and the effect is as Figure 5 shown. Some parameters of the grid-connected inverter, the grid, and the control link are shown in Table 1. Through the inherent resonance characteristics of the inverter, it is found that there is a resonance point at about 4.16 kHz in the system.

[0052] Table 1 shows some parameters of the grid-connected inverter, the grid, and the control link

[0053]

[0054] According to Figure 3 a simulation system is built in Matlab. According to Table 1, the preset control parameters are set and the simulation starts to run. First, a single inverter is connected to the grid, and its output current waveform is as Figure 5 shown by ig1 in. At t = 0.06 s, 5% of the 4.16 kHz harmonic is added to the grid voltage. At t = 0.12 s, the inverter machine-side current feedback is added for resonance suppression. At t = 0.18 s, the virtual impedance reshaping unit is added for harmonic suppression. The control block diagram is as Figure 2 shown. It can be Figure 5 seen that after adding the inverter machine-side current feedback and the virtual impedance reshaping unit, the harmonic resonance of the single grid-connected inverter is effectively suppressed.

[0055] After the grid-connected inverter with a virtual impedance reshaping unit operates stably, at t = 0.24 s, the second LCL-type grid-connected inverter is connected to the PCC. Its filter parameters and control parameters are exactly the same as those of the first one. At this time, the equivalent grid impedance mLg of a single grid-connected inverter at the PCC changes, and harmonic resonance phenomena occur in the PCC voltage upcc and the current ig. At t = 0.30 s, the parameter adaptive adjustment unit and the on-line grid impedance detection unit are put into operation, and the control parameters in the QPR current regulation controller and the virtual impedance reshaping unit are corrected successively.

[0056] The process of parameter adaptive adjustment is as follows:

[0057] Step S110: The grid-connected inverter with a single virtual impedance reshaping unit operates stably, and step S120 is executed;

[0058] Step S120: After multiple LCL-type grid-connected inverters are connected in parallel, it is detected whether grid-connected inverter microgrid resonance occurs. If it occurs, step S130 is executed; otherwise, step S110 is executed;

[0059] Step S130: The on-line grid impedance detection unit is put into operation, and mLg is obtained through this unit. The calculation formula is:

[0060]

[0061] Step S140 is executed;

[0062] Step S140: The mLg value in step S130 is input into the parameter adaptive adjustment unit, and the control parameters kp, k r , RPC are calculated in the DSP controller. The calculation formulas are:

[0063] k r Among them,

[0064]

[0065] According to the functional relationship of rp with respect to RPC, the adaptive adjustment functional relationship of the virtual impedance RPC with respect to mLg is obtained; the new control parameters kp, k r , RPC are obtained, and step S150 is executed;

[0066] Step S150: The control parameter RPC in the virtual impedance reshaping unit and the control kp, k in the current regulation controller are corrected, and step S160 is executed;

[0067] Step S160: After completing the correction of the main control parameters of the virtual impedance reshaping unit and the current regulation controller, execute Step S120 to determine whether the harmonic resonance phenomenon in the grid-connected inverter microgrid system is effectively suppressed. If it is effectively suppressed, the parameter adaptive adjustment unit and the grid impedance on-line detection unit can be disconnected to release the system controller resources and improve the working efficiency of the controller.

[0068] From Figure 5 The simulation waveforms show that after adding the parameter adaptive adjustment unit, without manually correcting the controller parameters, the waveforms of the grid-connected voltage, grid-connected current, grid voltage, and current output by the inverter have small distortions, and the system has a high stability margin. It can be seen that the adaptive virtual impedance reshaping technology in the present invention can effectively suppress the harmonic resonance phenomenon of the multi-grid-connected inverter microgrid adaptively.

[0069] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly. It cannot be used to limit the protection scope of the present invention. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein. Any equivalent changes or modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims

1. An adaptive virtual impedance reshaping harmonic resonance suppression method for parallel inverter microgrid, characterized in that: The method includes: when a single LCL-type grid-connected inverter is connected to the grid or multiple LCL-type grid-connected inverters are connected to the grid in parallel, a virtual impedance reshaping strategy is adopted to improve the output impedance of the grid-connected inverter and inject a certain damping component into the microgrid. If each photovoltaic grid-connected inverter in the microgrid adopts the virtual impedance reshaping strategy, the network impedance of the microgrid can be effectively changed, especially the series-parallel resistance component of the network can be enhanced, and the series-parallel resonance of the higher harmonics in the network can be suppressed; The parameter adaptive adjustment unit is combined with the grid impedance online detection technology to adaptively adjust the control parameters in the virtual impedance reshaping unit and the current regulation controller according to the grid impedance parameters, which can ensure the stability of the system under ideal grid conditions and weak grid conditions. The grid impedance online detection unit adopts a small signal injection method to obtain the grid impedance in real time, and automatically sends the parameter to the parameter adaptive adjustment unit in real time, avoiding manual correction of controller parameters, saving manpower and material resources, and improving the intelligent control of the entire grid-connected control system; The method and device can solve the problems existing in the existing multi-inverter microgrid system: complex control system, fixed control parameters, low efficiency of the regulation process, etc.

2. The method according to claim 1, characterized in that An adaptive virtual impedance reshaping harmonic resonance suppression method for parallel inverter microgrids includes: taking the inductor current on the machine side of the grid-connected inverter as a state variable, using the state variable as a negative feedback link of the feedback quantity, and achieving the purpose of suppressing harmonic resonance with the same actual impedance of the inductor series on the machine side of the grid-connected inverter through a fundamental frequency notch filter and a virtual impedance coefficient; The voltage of the grid-connected inverter filter capacitor is taken as the state variable, and the negative feedback link with the state variable as the feedback quantity is used. Through the fundamental frequency notch filter and the virtual impedance coefficient, the purpose of the harmonic resonance suppression effect of the same actual impedance in parallel at both ends of the inverter filter capacitor is achieved.

3. The method according to claim 1, characterized in that The process of adaptive parameter adjustment includes: After a single machine is running or multiple machines are connected in parallel, if harmonic resonance is detected, the grid impedance online detection unit and the parameter adaptive adjustment unit are put into operation. The grid impedance online detection unit is used to obtain the equivalent grid impedance value of each grid-connected inverter at the PCC. The equivalent grid impedance is used as the input of the parameter adaptive adjustment unit, and the unit obtains the control parameters in the new virtual impedance reshaping unit and the current regulation controller through the adaptive adjustment calculation formula, including: virtual impedance RPC, current regulation controller proportional coefficient kp, resonance coefficient kr; The control parameters RPC, kp, k in the virtual impedance reshaping unit and the current regulation controller are r Correction is made to achieve adaptive adjustment of control parameters.

4. The method according to claim 2 or 3, characterized in that The process of virtual impedance reshaping: by creating virtual series and parallel impedance on a single inverter, the output impedance of the grid-connected inverter is changed, thereby changing the network impedance of the microgrid, so as to achieve the purpose of suppressing the series and parallel resonance of high-order harmonics in the network; The working principle of the parameter adaptive adjustment unit is: starting from the system cutoff frequency, fundamental frequency gain and damping that meet the stability of the control system, through calculation, the functional relationship between the equivalent grid impedance and the control parameters in the virtual impedance reshaping unit and the current regulation controller is obtained, and the updated control parameters are calculated based on this relationship.

5. The method for suppressing harmonic resonance in a multi-inverter microgrid according to claim 1, characterized in that: When the number of grid-connected inverters in parallel at the PCC changes, the equivalent grid impedance of any grid-connected inverter at the PCC is equal to the product of the grid impedance value and the number of grid-connected inverters in parallel at the PCC. At this time, the control parameters of the original control link of a single grid-connected inverter may not be appropriate. The parameter adaptive adjustment unit combined with the grid impedance online detection technology can change the control parameters in the virtual impedance reshaping unit and the current regulation controller in real time according to the equivalent grid impedance of a single grid-connected inverter at the PCC, so that the grid-connected inverter can maintain stable control characteristics under various grid impedance conditions, thereby improving the stability of parallel operation of multiple LCL-type grid-connected inverters.

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