LCLLC filter-based grid-connected inverter hybrid damping control method

Abstract

The invention discloses a grid-connected inverter hybrid damping control method based on an LCLLC filter, passive damping adopts a mode that a filter capacitor is connected with a resistor in parallel, active damping adopts a state variable feedback mode, and a feedback variable is a filter capacitor current; a core model is a single-phase main circuit topological structure based on an LCLLC filter, and active damping parameters and current QPR controller parameters are designed through gain steady-state errors, gain margins and phase margins, so that the stability and control precision of a system in a full working condition range are ensured.

Description

Technical Field

[0001] This invention mainly relates to the field of filters, and in particular to a hybrid damping control method for grid-connected inverters based on LCC filters. Background Technology

[0002] With the rapid development and large-scale application of new energy power generation technologies such as photovoltaics and wind power, grid-connected inverters, as core power electronic devices for achieving efficient and safe grid connection of new energy sources, directly affect the power quality and operational stability of the grid. In inverter systems, filters are a critical component, and their topology selection and parameter design have a crucial impact on the inverter's output characteristics, efficiency, and reliability. Conventional inverters, due to their high-frequency switching modulation, typically contain a large number of harmonics in their output current, especially high-amplitude harmonic components at the switching frequency and its harmonics. If these are not suppressed, they will severely affect the power quality of the grid. Therefore, configuring high-performance filters at the inverter output to effectively filter out harmonic components has become an indispensable part of achieving safe grid connection of new energy systems.

[0003] Inverters use sinusoidal pulse width modulation (SPWM) technology to control the switching transistors. However, this control process generates high-frequency harmonics, which, if transmitted to the grid, degrade the grid current quality. Therefore, a filter needs to be added after the inverter full-bridge structure to attenuate high-frequency harmonics, thereby ensuring a low total harmonic distortion (THD) of the grid-connected current to meet grid connection requirements. For grid-connected converter systems, standards such as IEEE 519-1992 and IEEE 1547-2008 require that each current harmonic above the 35th order should be limited to less than 0.3% of the rated fundamental current. If traditional L-type or LCL-type filters are used to filter out PWM current harmonics to meet the aforementioned requirements, the cost of the filters will increase significantly. L-type filters are first-order filters with the simplest structure, but their filtering effect depends heavily on the inductor size. They are poor at high frequencies, with a harmonic attenuation rate of only -20dB / dec. Therefore, to achieve better filtering, the inductor needs to be increased. However, this not only increases the filter's size but also increases core losses, leading to decreased efficiency and affecting the system's dynamic response speed. LC-type filters are mainly used in off-grid inverters. By adding a filter capacitor, they become second-order filters, enhancing their high-frequency filtering capability. Their attenuation rate at high frequencies is -40dB / dec. However, in grid-connected environments, the grid voltage alters the voltage across the filter capacitor, resulting in a deterioration in filtering performance. LCL-type filters, compared to LC filters, add an inductor, forming a third-order filter with better high-frequency filtering. Compared to L-type and LC filters, they have lower current harmonics and a smaller total inductance, while reducing system size, cost, and losses. Their attenuation rate at high frequencies is -60dB / dec, demonstrating significantly better harmonic attenuation capability. However, LCL filters often only focus on the total harmonic distortion (THD) of the grid-connected current in applications. To ensure that the switching frequency and its integer multiples of the current harmonics meet grid standards, filter parameters need to be increased, raising inverter costs. LLCL filters are a new type of high-order filter with an LC series resonant branch that can bypass the first harmonic current at the switching frequency, significantly reducing switching frequency harmonics in the grid-connected current. Compared to traditional LCL filters, LLCL filters have negligible cost due to the small increase in resonant inductance that does not carry power current, while offering significantly improved filtering performance. However, the harmonic attenuation rate of LLCL filters at high frequencies is only -20dB / dec, resulting in insufficient attenuation of current harmonics at frequencies two times or higher than the switching frequency, making it difficult to meet grid standards.The LCLLC filter has a more complex structure, adding an LC series resonant branch to the LCL filter. It retains not only the high-frequency harmonic attenuation rate of the LCL filter but also the switching frequency harmonics of the LC series resonant branch. However, this type of filter has two resonant spikes, requiring necessary measures to suppress them.

[0004] Second-order and higher-order filters exhibit resonant spikes (at the resonant frequency, where impedance is minimum) caused by filter element coupling, requiring damping techniques to suppress them. Damping techniques mainly include passive and active damping. Passive damping involves connecting resistors in series or parallel in actual filter circuits; to achieve better absorption, the resistance value often needs to be increased, however, excessively large resistance values ​​increase losses. Active damping includes filter-based active damping control and state-variable feedback-based active damping control. Filter-based active damping control works by designing a notch filter matched to the filter's resonant frequency to generate an inverse resonant spike, thus canceling the inherent resonant spike of the original filter. Furthermore, the gain at other frequencies is close to zero, thus achieving ideal damping effects without affecting the original filter's low-frequency gain and high-frequency attenuation performance. However, designing a notch filter requires precise calculation of the original filter's resonant frequency using mathematical modeling to suppress resonant spikes. This process demands high precision and is complex. In practical engineering applications, electronic components such as inductors and capacitors can experience frequency shifts due to aging and other factors, weakening the notch filter's damping effect. Excessive frequency shift can lead to damping failure and system instability. Active damping control based on state variable feedback simulates a virtual resistor in the control process by feeding back appropriate state variables, thereby absorbing resonant spikes. Compared to filter-based active damping technology, this approach is not only simpler to design but also effectively addresses the impact of parameter variations.

[0005] Depending on the connection method of the damping resistor, there are two ways: series-parallel connection of inductor and damping resistor, and series-parallel connection of capacitor and damping resistor. Among them, the series-parallel connection of capacitor and damping resistor is the best in terms of both absorption effect of resonant peaks and increase of circuit loss, and is more commonly used in practical applications. With capacitors in series, at high frequencies, the capacitive reactance of the capacitor is small, while the series resistance increases the impedance, resulting in a worse ability to suppress high-frequency harmonics. At low frequencies, the capacitive reactance of the capacitor is much greater than the impedance of the series resistance, which is negligible and has no effect on low-frequency signals. Although the damping resistor can absorb resonant peaks, it reduces the system's ability to suppress high-frequency harmonics. With capacitors in parallel, at high frequencies, the capacitive reactance of the capacitor is much smaller than the impedance of the parallel resistance, and has no effect on the ability to suppress high-frequency harmonics. At low frequencies, the inductive reactance of the inductor is much smaller than the impedance of the parallel resistance, and has no effect on the gain capability of low-frequency signals. Therefore, the capacitor in parallel with the resistor does not affect the system's ability to suppress both high and low frequencies.

[0006] This invention combines the advantages of passive damping and active damping technologies to propose a hybrid control strategy. Passive damping employs a filter capacitor connected in parallel with a resistor, while active damping uses state variable feedback, with the filter capacitor current as the feedback variable. This invention effectively overcomes the problems of purely passive damping schemes, which easily increase system losses and affect operational stability, and purely active damping, which has limited effectiveness in suppressing high-frequency resonant peaks. The proposed method not only effectively suppresses two resonant peaks in the system but also maintains the resonant valley characteristics and preserves the system's gain performance in the low-frequency range and its high-frequency harmonic suppression capability. This scheme possesses comprehensive advantages such as low damping resistor loss and strong adaptability. The active damping parameters and current QPR controller parameters are designed using gain steady-state error, gain margin, and phase margin, thereby ensuring the system's stability and control accuracy across the entire operating range. Summary of the Invention

[0007] Therefore, it is necessary to provide a hybrid damping control method for grid-connected inverters based on LCC filters to address the existing problems.

[0008] In a first aspect, embodiments of this application provide a hybrid damping control method for grid-connected inverters based on LCC filters, characterized in that it includes:

[0009] A hybrid damping control method for grid-connected inverters based on LCC filters, characterized by comprising:

[0010] Passive damping uses a parallel resistor to the filter capacitor, while active damping uses state variable feedback, with the filter capacitor current as the feedback variable. Its core model is a single-phase main circuit topology based on an LCClC filter, where Udc is the DC bus voltage; L1 is the filter inverter-side inductance; Cd is the filter capacitor; Rd is the parallel damping resistor; Id is the filter capacitor current; Lf and Cf are the series resonant branch inductance and capacitance; L2 is the grid-side inductance; Ig is the grid-connected current; Ug is the grid-connected voltage; the dashed box represents the control algorithm; PLL is the phase-locked loop; Iref is the reference current; QPR is the quasi-proportional resonant current controller; K is the capacitor current feedback coefficient, i.e., the active damping coefficient; and PWM is the pulse width modulation module.

[0011] Preferably, the transfer function includes:

[0012] (1)

[0013] in, ; ; ; ; ; This is the capacitor current feedback coefficient, also known as the active damping coefficient.

[0014] Preferably, the active damping parameters and current controller parameters are designed based on the system's phase margin, gain margin, and gain steady-state error; the current controller is a quasi-proportional resonant QPR controller, and its transfer function is as follows:

[0015] (2)

[0016] in, This is the proportionality coefficient; The resonant coefficient; The fundamental angular frequency; Here is the cutoff angular frequency of the current controller; the system loop gain is as follows:

[0017] (3)

[0018] The grid current Ig is decomposed into the form obtained by multiplying the currents of the preceding and following stages.

[0019] (4)

[0020] During normal system operation, the switching frequency Needs to be met;

[0021] (5)

[0022] in, The cutoff frequency; This is the resonant frequency of the low-frequency resonant peak; when determining the frequency band of the cutoff frequency, since the cutoff frequency... Much smaller than the switching frequency Therefore, the effect of the filter capacitor can be ignored, and the system loop gain function can be simplified to:

[0023] (6)

[0024] Make the system at the cutoff frequency The amplitude gain at that point is 1, at the fundamental frequency. The amplitude gain at that point is much greater than 1, and Then the grid current at the fundamental frequency The expression is:

[0025] (7)

[0026] The steady-state error of a grid-connected inverter is divided into gain error and phase angle error. Due to the role of the phase-locked loop in the control system, The phase lags behind The lag angle is ,and The phase is the same as the grid voltage; in addition, the QPR controller can eliminate dynamic errors at the base frequency and minimize phase angle errors, so the phase angle errors can be ignored.

[0027] Preferably, gain error The definition is as follows:

[0028] (8)

[0029] At the base frequency, the expression for the QPR controller is as follows:

[0030] (9)

[0031] When the system frequency is greater than the cutoff frequency, there is And from what has been said above, we can obtain Then the proportional coefficient of the controller can be derived. The expression is as follows:

[0032] (10)

[0033] The LCLLC filter at the second resonant peak Phase will cross In order to increase the gain margin A positive value indicates the resonant frequency in the high-frequency range. Requires a frequency less than 0Hz; for stable system operation, phase margin is required. It needs to be greater than 0dB, and The influence of the filter capacitor current at that location cannot be ignored.

[0034] Preferably, gain margin The definition is as follows:

[0035] (11)

[0036] Phase margin The definition is as follows:

[0037] (12)

[0038] The resonance coefficient can be derived from the above formula. The expression is as follows:

[0039] (13)

[0040] make , As phase margin and gain error The constraints determined are shown in the following two equations:

[0041] (14)

[0042] (15)

[0043] The active damping coefficient under constraint conditions can be obtained. The expression is shown in the following formula:

[0044] (16).

[0045] Preferably, when the phase margin Gain margin Gain error The system exhibits good dynamic performance when the following three constraints are met:

[0046] ; ; ;

[0047] Based on the derived relational expressions and constraints, the active damping coefficient can be obtained. proportionality coefficient resonance coefficient .

[0048] Preferably, the expressions for pure passive damping and pure active damping in the control block diagram are different; the numerator of pure passive damping has an additional differential term. In the hybrid damping strategy, the passive damping resistance... The value of follows the following two principles: (1) The value of cannot exceed the value in the pure passive damping strategy, otherwise it will be impossible to effectively reduce the excessive loss caused by the damping resistor; (2) The value of should be determined based on the filter's ability to attenuate high-frequency harmonics. In a hybrid damping control strategy, The larger the value, the better the absorption effect of the resonant peak, but the weaker the attenuation ability of high-frequency harmonics, and the more the system loss will increase.

[0049] Preferably, the active damping coefficient in the hybrid damping control strategy and passive damping resistor The value of needs to ensure system stability; the stability of the system is verified by the Routh-Hurwitz stability criterion, and the active damping coefficient is obtained by using the characteristic equation of the system's closed-loop transfer function. and passive damping resistor The range of values ​​for ; under this criterion, the necessary and sufficient condition for system stability is that all coefficients of the closed-loop system are positive, that is, all elements in the first column of the Routh table are positive, so the hybrid damping parameter needs to satisfy this condition.

[0050] Preferably, the LCCL filter belongs to a fifth-order system. Setting the denominator of the transfer function to 0, the characteristic equation of the system is obtained as follows:

[0051] (17)

[0052] in, ; ; ; ; ; .

[0053] Compared with existing technologies, this invention has the following advantages: it effectively overcomes the problems of pure passive damping schemes easily increasing system losses and affecting operational stability, as well as the limited suppression effect of pure active damping on high-frequency resonant peaks. The proposed method can not only effectively suppress the two resonant peaks in the system, but also maintain the resonant valley characteristics unchanged, and maintain the system's gain performance in the low-frequency range and high-frequency harmonic suppression capability. This scheme has comprehensive advantages such as low damping resistor loss and strong adaptability. The active damping parameters and current QPR controller parameters are designed by using gain steady-state error, gain margin, and phase margin, thereby ensuring the stability and control accuracy of the system across the entire operating range. Attached Figure Description

[0054] Exemplary embodiments of the present invention can be more fully understood by referring to the accompanying drawings. The drawings are provided to further illustrate the embodiments of this application and form part of the specification. They are used together with the embodiments of this application to explain the present invention and do not constitute a limitation thereof. In the drawings, the same reference numerals generally represent the same components or steps.

[0055] Figure 1 This refers to the main circuit topology of the inverter.

[0056] Figure 2 This is a block diagram of the hybrid damping control for an LCC-type filter. Detailed Implementation

[0057] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0058] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0059] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0060] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0061] This application provides a hybrid damping control method for grid-connected inverters based on LCC filters, which will be described below with reference to the accompanying drawings.

[0062] Grid-connected inverter model based on LCC filter: Single-phase main circuit topology based on LCC filter, such as... Figure 1 As shown in the diagram. Where Udc is the DC bus voltage; L1 is the inverter-side inductance of the filter; Cd is the filter capacitor; Rd is the parallel damping resistor; Id is the filter capacitor current; Lf and Cf are the series resonant branch inductance and capacitance; L2 is the grid-side inductance; Ig is the grid-connected current; Ug is the grid-connected voltage; the dashed box represents the control algorithm; PLL is the phase-locked loop; Iref is the reference current; QPR is the quasi-proportional resonant current controller; K is the capacitor current feedback coefficient, i.e., the active damping coefficient; and PWM is the pulse width modulation module.

[0063] Design of Hybrid Damping Control Strategy for LCLLC Filter Grid-Connected Inverter: This invention proposes a hybrid damping strategy for LCLLC filter grid-connected inverters that combines passive and active damping technologies. This strategy effectively suppresses two resonant spikes in the system while maintaining the resonant valley characteristics and preserving the system's gain performance in the low-frequency band and harmonic suppression capability in the high-frequency band. The overall control block diagram is as follows: Figure 2 As shown, the transfer function includes:

[0064] (1)

[0065] in, ; ; ; ; ; This is the capacitor current feedback coefficient, also known as the active damping coefficient.

[0066] Preferably, the active damping parameters and current controller parameters are designed based on the system's phase margin, gain margin, and gain steady-state error; the current controller is a quasi-proportional resonant QPR controller, and its transfer function is as follows:

[0067] (2)

[0068] in, This is the proportionality coefficient; The resonant coefficient; The fundamental angular frequency; Here is the cutoff angular frequency of the current controller; the system loop gain is as follows:

[0069] (3)

[0070] The grid current Ig is decomposed into the form obtained by multiplying the currents of the preceding and following stages.

[0071] (4)

[0072] During normal system operation, the switching frequency Needs to be met;

[0073] (5)

[0074] in, The cutoff frequency; This is the resonant frequency of the low-frequency resonant peak; when determining the frequency band of the cutoff frequency, since the cutoff frequency... Much smaller than the switching frequency Therefore, the effect of the filter capacitor can be ignored, and the system loop gain function can be simplified to:

[0075] (6)

[0076] Make the system at the cutoff frequency The amplitude gain at that point is 1, at the fundamental frequency. The amplitude gain at that point is much greater than 1, and Then the grid current at the fundamental frequency The expression is:

[0077] (7)

[0078] The steady-state error of a grid-connected inverter is divided into gain error and phase angle error. Due to the role of the phase-locked loop in the control system, The phase lags behind The lag angle is ,and The phase is the same as the grid voltage; in addition, the QPR controller can eliminate dynamic errors at the base frequency and minimize phase angle errors, so the phase angle errors can be ignored.

[0079] Preferably, gain error The definition is as follows:

[0080] (8)

[0081] At the base frequency, the expression for the QPR controller is as follows:

[0082] (9)

[0083] When the system frequency is greater than the cutoff frequency, there is And from what has been said above, we can obtain Then the proportional coefficient of the controller can be derived. The expression is as follows:

[0084] (10)

[0085] The LCLLC filter at the second resonant peak Phase will cross In order to increase the gain margin A positive value indicates the resonant frequency in the high-frequency range. Requires a frequency less than 0Hz; for stable system operation, phase margin is required. It needs to be greater than 0dB, and The influence of the filter capacitor current at that location cannot be ignored.

[0086] Preferably, gain margin The definition is as follows:

[0087] (11)

[0088] Phase margin The definition is as follows:

[0089] (12)

[0090] The resonance coefficient can be derived from the above formula. The expression is as follows:

[0091] (13)

[0092] make , As phase margin and gain error The constraints determined are shown in the following two equations:

[0093] (14)

[0094] (15)

[0095] The active damping coefficient under constraint conditions can be obtained. The expression is shown in the following formula:

[0096] (16).

[0097] Preferably, when the phase margin Gain margin Gain error The system exhibits good dynamic performance when the following three constraints are met:

[0098] ; ; ;

[0099] Based on the derived relational expressions and constraints, the active damping coefficient can be obtained. proportionality coefficient resonance coefficient .

[0100] Preferably, the expressions for pure passive damping and pure active damping in the control block diagram are different; the numerator of pure passive damping has an additional differential term. In the hybrid damping strategy, the passive damping resistance... The value of follows the following two principles: (1) The value of cannot exceed the value in the pure passive damping strategy, otherwise it will be impossible to effectively reduce the excessive loss caused by the damping resistor; (2) The value of should be determined based on the filter's ability to attenuate high-frequency harmonics. In a hybrid damping control strategy, The larger the value, the better the absorption effect of the resonant peak, but the weaker the attenuation ability of high-frequency harmonics, and the more the system loss will increase.

[0101] Preferably, the active damping coefficient in the hybrid damping control strategy and passive damping resistor The value of needs to ensure system stability; the stability of the system is verified by the Routh-Hurwitz stability criterion, and the active damping coefficient is obtained by using the characteristic equation of the system's closed-loop transfer function. and passive damping resistor The range of values ​​for ; under this criterion, the necessary and sufficient condition for system stability is that all coefficients of the closed-loop system are positive, that is, all elements in the first column of the Routh table are positive, so the hybrid damping parameter needs to satisfy this condition.

[0102] Preferably, the LCCL filter belongs to a fifth-order system. Setting the denominator of the transfer function to 0, the characteristic equation of the system is obtained as follows:

[0103] (17)

[0104] in, ; ; ; ; ; .

[0105] The Rolls table is shown in Table 1.

[0106]

[0107] In Table 1, ; ; ; ; ; .

[0108] It should be noted that the flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of this application. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of code containing one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions marked in the blocks may occur in a different order than those marked in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, can be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions.

[0109] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0110] In the several embodiments provided in this application, it should be understood that the disclosed apparatus and methods can be implemented in other ways. The apparatus embodiments described above are merely illustrative. For example, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. Furthermore, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Additionally, the displayed or discussed mutual couplings, direct couplings, or communication connections may be through some communication interfaces; indirect couplings or communication connections between devices or units may be electrical, mechanical, or other forms.

[0111] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0112] In addition, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0113] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application.

Claims

1. A hybrid damping control method for grid-connected inverters based on LCC filters, characterized in that, include: Passive damping uses a parallel resistor to the filter capacitor, while active damping uses state variable feedback, with the filter capacitor current as the feedback variable. Its core model is a single-phase main circuit topology based on an LCClC filter, where Udc is the DC bus voltage; L1 is the filter inverter-side inductance; Cd is the filter capacitor; Rd is the parallel damping resistor; Id is the filter capacitor current; Lf and Cf are the series resonant branch inductance and capacitance; L2 is the grid-side inductance; Ig is the grid-connected current; Ug is the grid-connected voltage; the dashed box represents the control algorithm; PLL is the phase-locked loop; Iref is the reference current; QPR is the quasi-proportional resonant current controller; and K is the capacitor current feedback coefficient, i.e., the active damping coefficient. PWM stands for Pulse Width Modulation.

2. The method according to claim 1, characterized in that, Transfer functions include: (1) in, ; ; ; ; ; This is the capacitor current feedback coefficient, also known as the active damping coefficient.

3. The method according to claim 2, characterized in that, The active damping parameters and current controller parameters are designed based on the system's phase margin, gain margin, and steady-state gain error. The current controller is a quasi-proportional resonant QPR controller, and its transfer function is as follows: (2) in, This is the proportionality coefficient; The resonant coefficient; The fundamental angular frequency; Here is the cutoff angular frequency of the current controller; the system loop gain is as follows: (3) The grid current Ig is decomposed into the form obtained by multiplying the currents of the preceding and following stages. (4) During normal system operation, the switching frequency Needs to be met; (5) in, The cutoff frequency; This is the resonant frequency of the low-frequency resonant peak; when determining the frequency band of the cutoff frequency, since the cutoff frequency... Much smaller than the switching frequency Therefore, the effect of the filter capacitor can be ignored, and the system loop gain function can be simplified to: (6) Make the system at the cutoff frequency The amplitude gain at that point is 1, at the fundamental frequency. The amplitude gain at that point is much greater than 1, and Then the grid current at the fundamental frequency The expression is: (7) The steady-state error of a grid-connected inverter is divided into gain error and phase angle error. Due to the role of the phase-locked loop in the control system, The phase lags behind The lag angle is ,and The phase is the same as the grid voltage; in addition, the QPR controller can eliminate dynamic errors at the base frequency and minimize phase angle errors, so the phase angle errors can be ignored.

4. The method according to claim 3, characterized in that, Gain error The definition is as follows: (8) At the base frequency, the expression for the QPR controller is as follows: (9) When the system frequency is greater than the cutoff frequency, there is And from what has been said above, we can obtain Then the proportional coefficient of the controller can be derived. The expression is as follows: (10) The LCLLC filter at the second resonant peak Phase will cross In order to increase the gain margin A positive value indicates the resonant frequency in the high-frequency range. Requires a frequency less than 0Hz; for stable system operation, phase margin is required. It needs to be greater than 0dB, and The influence of the filter capacitor current at that location cannot be ignored.

5. The method according to claim 4, characterized in that, Gain margin The definition is as follows: (11) Phase margin The definition is as follows: (12) The resonance coefficient can be derived from the above formula. The expression is as follows: (13) make , As phase margin and gain error The constraints determined are shown in the following two equations: (14) (15) The active damping coefficient under constraint conditions can be obtained. The expression is shown in the following formula: (16)。 6. The method according to claim 5, characterized in that, When phase margin Gain margin Gain error The system exhibits good dynamic performance when the following three constraints are met: ； ； ； Based on the derived relational expressions and constraints, the active damping coefficient can be obtained. proportionality coefficient resonance coefficient .

7. The method according to claim 6, characterized in that, The expressions for pure passive damping and pure active damping in the control block diagram are different; pure passive damping has an additional differential term in the numerator. In a hybrid damping strategy, the passive damping resistance... The value of follows the following two principles: (1) The value of cannot exceed the value in the pure passive damping strategy, otherwise it will be impossible to effectively reduce the excessive loss caused by the damping resistor; (2) The value of should be determined based on the filter's ability to attenuate high-frequency harmonics. In a hybrid damping control strategy, The larger the value, the better the absorption effect of the resonant peak, but the weaker the attenuation ability of high-frequency harmonics, and the more the system loss will increase.

8. The method according to claim 7, characterized in that, Active damping coefficient in hybrid damping control strategy and passive damping resistor The value of needs to ensure system stability; the stability of the system is verified by the Routh-Hurwitz stability criterion, and the active damping coefficient is obtained by using the characteristic equation of the system's closed-loop transfer function. and passive damping resistor The range of values ​​for ; under this criterion, the necessary and sufficient condition for system stability is that all coefficients of the closed-loop system are positive, that is, all elements in the first column of the Routh table are positive, so the hybrid damping parameter needs to satisfy this condition.

9. The method according to claim 8, characterized in that, The LCLLC filter is a fifth-order system. Setting the denominator of the transfer function to 0, the characteristic equation of the system is as follows: (17) in, ; ; ; ; ; .

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