Method and device for improving stability of grid-connected inverter based on state feedback loop

Abstract

The invention discloses a grid-connected inverter stability improving method and device based on a state feedback loop. The method comprises the following steps: constructing a grid-connected inverter small signal model comprising a phase-locked loop, a voltage loop and a current loop; a state feedback loop is introduced into the control link of the phase-locked loop to increase the damping of the grid-connected inverter; meanwhile, a delay link is introduced into a control link of the phase-locked loop to limit a state feedback loop coefficient; a secondary integral loop is introduced into a control link of the phase-locked loop, and an error between an output phase and an actual phase of the phase-locked loop is eliminated through output of the secondary integral loop, so that zero-static-error tracking is realized. According to the method, the stability of the grid-connected inverter in an extremely weak power grid environment can be effectively improved, the state feedback method is simple and easy to implement in practical application, and a certain stability margin can be reserved for the system.

Description

Technical Field

[0001] The present invention relates to the technical field of stability control of new energy power systems, and in particular to a method and device for improving the stability of a grid-connected inverter based on a state feedback loop. Background Art

[0002] New power systems with new energy as the theme are being vigorously promoted. Due to the lack of support from conventional generators, new energy power stations in remote areas often exhibit weak grid characteristics with low short-circuit ratios, that is, they contain non-negligible grid impedance. The grid-connected inverter is the interface unit for distributed new energy to access the grid. In a weak grid environment, the existence of grid impedance will seriously affect the control performance of the grid-connected inverter and even cause instability. Therefore, it is of great research value to improve the stability of the grid-connected inverter in a weak grid environment.

[0003] At present, the conventional method to improve stability is to optimize the inverter control parameters reasonably. However, the interaction between different control loops of grid-connected inverters in weak power grids intensifies, and the parameters of one control loop will affect the dynamics of other control loops, which makes the optimization design of control parameters difficult, and the optimized control parameters may still not achieve the desired goal. The intelligent optimization algorithm is used to coordinately optimize the control parameters of all control loops, while reducing the integral parameters of the control loops, so that the oscillation modes of different frequency bands can meet the stability requirements at the same time. However, this method will cause some frequency bands to be in a critical stability state. The stable operation of the system requires a certain stability margin to avoid instability due to the uncertainty of certain parameters in the controlled object. Summary of the invention

[0004] The purpose of the present invention is to overcome the above-mentioned defects and problems existing in the prior art, and to provide a method and device for improving the stability of a grid-connected inverter based on a state feedback loop, which can effectively improve the stability of the grid-connected inverter in an extremely weak power grid environment. The state feedback method is simple and easy to implement in practical applications, and can leave a certain stability margin for the system.

[0005] To achieve the above objectives, the technical solution of the present invention is: a method for improving the stability of a grid-connected inverter based on a state feedback loop, comprising:

[0006] Construct a small signal model of the grid-connected inverter including a phase-locked loop, a voltage loop, and a current loop;

[0007] A state feedback loop is introduced into the control link of the phase-locked loop to increase the damping of the grid-connected inverter; at the same time, a delay link is introduced into the control link of the phase-locked loop to limit the coefficient of the state feedback loop;

[0008] A secondary integral loop is introduced into the control link of the phase-locked loop, and the error between the output phase of the phase-locked loop and the actual phase is eliminated through the output of the secondary integral loop, thereby achieving zero-static-error tracking.

[0009] The small signal model of the grid-connected inverter is:

[0010]

[0011] In the formula, Δi dref is the d-axis component of the current loop reference current; N 1 、N 2 、N 3 、N 4 is the intermediate variable, N 1 =3 / (2U dc0 C dc ), N 2 =U g cosδ 0 , N 3 =U g i d0 sinδ 0 -U g i q0 cosδ 0 , N 4 =U g sinδ 0 , U dc0 is the steady-state value of the DC side voltage, C dc is the DC side capacitance, U g is the amplitude of the grid voltage, δ 0 is the stable value of the phase difference between the grid voltage phase and the phase-locked loop output phase; Δi d is the d-axis component of the actual current in the current loop; Δi q is the q-axis component of the actual current in the current loop; Δθ is the phase angle of the grid-connected voltage; L g is the equivalent reactance of the power grid; i d0 is the stable value of the d-axis component of the actual current in the current loop; s is the complex frequency variable of the Laplace transform; i q0 is the stable value of the q-axis component of the actual current in the current loop; k vp k is the DC voltage loop proportional coefficient; vi is the DC voltage loop integral coefficient; Δi qref is the q-axis component of the current loop reference current; X g is the equivalent inductive reactance of the power grid, X g =ω 0 L g ,ω 0 is the stable value of angular frequency; k tp k is the proportional coefficient of the AC voltage loop; tik is the AC voltage loop integral coefficient; pp k is the proportional link coefficient of the phase-locked loop; pi k is the integral coefficient of the phase-locked loop; ip k is the current loop proportional coefficient; ii is the current loop integral coefficient; L is the AC filter inductance; Δi qref is the q-axis component of the current loop reference current.

[0012] The AC voltage loop of the grid-connected inverter is shielded, and in steady state there is i q0 =i qref =0, the small signal model of the grid-connected inverter is simplified to:

[0013]

[0014] The dynamic equation of the dq axis current loop is rewritten as:

[0015]

[0016] The closed-loop transfer function of the current loop is:

[0017]

[0018] N at unity power factor 3 =i d0 N 4 , substitute the dynamic equation of the dq axis current loop and the closed-loop transfer function of the current loop into the dynamic equation of the DC voltage loop and the phase-locked loop in the small signal model to obtain:

[0019]

[0020] The small signal model of the phase-locked loop with the state feedback loop and delay link is:

[0021]

[0022] Where Δω is the phase-locked loop output; Δu tq is the q-axis component of the grid-connected point voltage; K 2 is the state feedback loop coefficient.

[0023] The selection range of the state feedback loop coefficient is obtained by the following method:

[0024] The approximate result is obtained by making a first-order pade approximation on the delay link in the small signal model of the phase-locked loop:

[0025]

[0026] Substituting the approximate result into the small signal model of the phase-locked loop, the closed-loop input-output relationship of the phase-locked loop is obtained as follows:

[0027]

[0028] According to Routh criterion, the stability condition of the phase-locked loop is:

[0029]

[0030] Introducing the state feedback loop and ignoring the delay link, the open-loop input-output relationship of the phase-locked loop is:

[0031]

[0032] A secondary integral loop is introduced into the control link of the phase-locked loop. The output of the secondary integral loop is dynamically adjusted according to the output Δω of the phase-locked loop without considering the delay link, and the error between the output phase of the phase-locked loop and the actual phase is eliminated through the output of the secondary integral loop, thereby achieving zero-static-error tracking.

[0033] A device for improving the stability of a grid-connected inverter based on a state feedback loop, the device is applied to the above-mentioned method, and the device comprises:

[0034] A small signal model building module is used to build a small signal model of a grid-connected inverter including a phase-locked loop, a voltage loop, and a current loop;

[0035] The state feedback loop introduction module is used to introduce a state feedback loop into the control link of the phase-locked loop to increase the damping of the grid-connected inverter; at the same time, a delay link is introduced into the control link of the phase-locked loop to limit the coefficient of the state feedback loop;

[0036] The secondary integral loop introduction module is used to introduce the secondary integral loop into the control link of the phase-locked loop, and eliminate the error between the output phase and the actual phase of the phase-locked loop through the output of the secondary integral loop, thereby realizing zero-static-error tracking.

[0037] A device for improving the stability of a grid-connected inverter based on a state feedback loop, comprising a memory and a processor;

[0038] The memory is used to store computer program code and transmit the computer program code to the processor;

[0039] The processor is used to execute the method according to the instructions in the computer program code.

[0040] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method described above is implemented.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] In the method and device for improving the stability of a grid-connected inverter based on a state feedback loop of the present invention, a negative feedback loop is introduced in a phase-locked loop to improve the system damping, and the greater the feedback coefficient, the better the stability of the system. At the same time, the control delay of the control loop is considered, the value range of the feedback coefficient is calculated, and a secondary integral loop is introduced to eliminate the steady-state error caused by the introduced feedback loop, thereby achieving zero-static error tracking. Compared with the traditional control parameter optimization method, the present invention can effectively improve the adaptability of the inverter under extremely weak power grids. The state feedback method is simple and easy to implement in practical applications, and can leave a certain stability margin for the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 The present invention is a flow chart of a method for improving the stability of a grid-connected inverter based on a state feedback loop.

[0044] Figure 2 Schematic diagram of the topological structure and control framework of the grid-connected inverter in the embodiment of the present invention.

[0045] Figure 3 It is a schematic diagram of a phase-locked loop based on a synchronous reference coordinate system in an embodiment of the present invention.

[0046] Figure 4 Schematic diagram of the small signal model structure in an embodiment of the present invention.

[0047] Figure 5 It is a schematic diagram of a phase-locked loop including a state feedback loop and a secondary integration loop in an embodiment of the present invention.

[0048] Figure 6 It is a small signal model of a phase-locked loop taking into account a delay link in an embodiment of the present invention.

[0049] Figure 7 It is a schematic diagram of disturbance simulation waveforms before and after the state feedback loop is introduced in an embodiment of the present invention.

[0050] Figure 8 The present invention is a structural block diagram of a device for improving the stability of a grid-connected inverter based on a state feedback loop.

[0051] Fig. 9 It is a structural block diagram of a grid-connected inverter stability improvement device based on a state feedback loop of the present invention. DETAILED DESCRIPTION

[0052] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0053] See also Figure 1 , a method for improving the stability of a grid-connected inverter based on a state feedback loop, comprising:

[0054] S1. Construct a small signal model of the grid-connected inverter including a phase-locked loop, a voltage loop and a current loop;

[0055] S2. Introduce a state feedback loop in the control link of the phase-locked loop to increase the damping of the grid-connected inverter; at the same time, introduce a delay link in the control link of the phase-locked loop to limit the state feedback loop coefficient;

[0056] S3. A secondary integral loop is introduced into the control link of the phase-locked loop, and the error between the output phase of the phase-locked loop and the actual phase is eliminated through the output of the secondary integral loop, thereby achieving zero-static-error tracking.

[0057] The method for improving the stability of a grid-connected inverter based on a state feedback loop proposed in the present invention effectively improves the stability of the grid-connected inverter in a weak power grid environment.

[0058] New energy grid-connected inverter topology and control framework Figure 2 As shown, it includes a phase-locked loop, a DC voltage loop, an AC voltage loop and a current loop. The phase-locked loop in the present invention adopts a traditional phase-locked loop based on a synchronous reference coordinate system, and its structure is as follows: Figure 3 As shown. Among them, H PLL (s), H DVC (s), H AVC (s) and H CC (s) represent the transfer functions of the phase-locked loop, DC voltage loop, AC voltage loop and current loop PI controller respectively, and H PLL (s) = k pp +k pi / s,R DVC (s) = k vp +k vi / s,H AVC (s) = k tp +k ti / s and H CC (s) = k ip +k ii / s; among them, U g and θ g are the amplitude and phase angle of the grid voltage respectively; U t and θ t are the amplitude and phase angle of the grid-connected point voltage respectively; θ is the phase angle of the phase-locked loop output, ω is the phase-locked loop output angular frequency, s is the complex frequency variable of Laplace transform; U dc and C are the DC side voltage and DC side capacitance respectively; P in and P e are the active power input and output on the DC side respectively; L is the AC filter inductance; Z g is the equivalent impedance of the power grid, and Zg =L g s+R g , where L g is the grid reactance value, R g is the grid resistance value; the subscripts d and q represent the d-axis and q-axis components in the rotating coordinate system respectively; the subscript ref represents the reference value of the control variable.

[0059] The grid-connected inverter control system is established in the dq synchronous rotating coordinate system based on the phase of the phase-locked loop output, while the voltage and current equations in the circuit are established in the xy synchronous rotating coordinate system based on the grid-connected point voltage phase. In order to avoid multiple conversions between coordinate systems during the modeling process, the grid voltage is converted to the dq coordinate system:

[0060]

[0061] In the formula, u gd is the d-axis component of the grid voltage; u gq is the q-axis component of the grid voltage; δ is the phase difference between the grid voltage phase and the phase-locked loop output phase.

[0062] The DC voltage loop is used to control the DC side capacitor voltage to a constant value to ensure the active power balance of the AC / DC side. Its output is used as the reference value of the d-axis current loop. Its linearization model is as follows:

[0063]

[0064] In the formula, ΔP in is the active power input on the DC side; ΔP e is the DC side output active power; U dc0 is the steady-state value of the DC side voltage; C dc is the DC side capacitance; ΔU dc is the DC side voltage; u td0 is the steady-state value of the d-axis component of the grid-connected point voltage; u tq0 is the steady-state value of the q-axis component of the grid-connected point voltage; Δu td is the d-axis component of the grid-connected point voltage; Δu tq is the q-axis component of the grid-connected point voltage; Δi dref is the d-axis component of the current loop reference value; ΔU dcref It is the reference voltage of DC voltage loop.

[0065] The AC voltage loop is used to maintain the voltage amplitude of the grid connection point within a given range. Its output is used as the reference value of the q-axis current loop. Its linearization model is as follows:

[0066]

[0067] In the formula, ΔU tis the voltage amplitude at the grid connection point; ΔU tref It is the reference voltage of AC voltage loop.

[0068] The phase-locked loop is used to track the phase of the inverter grid-connected point voltage in real time and provide a phase reference for the control of the inverter. The present invention adopts a traditional phase-locked loop based on a synchronous reference coordinate system, and its linearization model is as follows:

[0069]

[0070] The current loop and AC filter are used to realize the power control of the voltage outer loop. At the same time, the dq axis decoupling term of the output filter inductor is considered. The linearization model is as follows:

[0071]

[0072] The network impedance ignores the resistance component. Since the inductive reactance component in the high-voltage transmission network is much larger than the impedance component, this simplification is reasonable. The linearization model is as follows:

[0073]

[0074] In the formula, δ 0 is the stable value of the phase difference between the grid voltage phase and the phase-locked loop output phase; X g =ω 0 L g ,ω 0 is the stable value of the angular frequency.

[0075] Under the constraint of maximum power tracking control, the active power input to the DC side of the inverter is approximately constant in a short period of time, so the disturbance of the inverter input power is zero, that is, ΔP in = 0. At the same time, ΔU dcref =0, ΔU tref =0, then the small-size model of the inverter can be organized as:

[0076]

[0077] In the formula, Δi dref is the d-axis component of the current loop reference current; N 1 、N 2 、N 3 、N 4 is the intermediate variable, N 1 =3 / (2U dc0 C dc ), N 2 =U g cosδ 0 , N 3 =U g i d0 sinδ0 -U g i q0 cosδ 0 , N 4 =U g sinδ 0 ; Δi d is the d-axis component of the actual current in the current loop; Δi q is the q-axis component of the actual current in the current loop; Δθ is the phase angle of the grid-connected voltage; L g is the equivalent reactance of the power grid; i d0 is the stable value of the d-axis component of the actual current in the current loop; s is the complex frequency variable of the Laplace transform; i q0 is the stable value of the q-axis component of the actual current in the current loop; k vp k is the DC voltage loop proportional coefficient; vi is the DC voltage loop integral coefficient; Δi qref is the q-axis component of the current loop reference current; X g is the equivalent inductive reactance of the power grid; k tp k is the proportional coefficient of the AC voltage loop; ti k is the integral coefficient of the AC voltage loop; pp k is the proportional link coefficient of the phase-locked loop; pi k is the integral coefficient of the phase-locked loop; ip k is the current loop proportional coefficient; ii is the current loop integral coefficient; L is the AC filter inductance; Δi qref is the q-axis component of the current loop reference current.

[0078] The complete small signal model of the grid-connected inverter is relatively complex. The small signal model will be simplified according to the rules in the future. For details, see Figure 4 .

[0079] The photovoltaic power grid connection standard points out that the power factor of photovoltaic power stations connected to the grid at a voltage level of 10kV to 35kV should not be less than 0.98, and new energy stations are usually equipped with special reactive power compensation devices. In this case, the AC voltage loop of the grid-connected inverter should be shielded, and there should be i in steady state. q0 =i qref =0, the small signal model of the grid-connected inverter is simplified to:

[0080]

[0081] In order to obtain a simple and clear small signal model framework, the dynamic equation of the dq axis current loop is rewritten as:

[0082]

[0083] The closed-loop transfer function of the current loop is:

[0084]

[0085] N at unity power factor 3 =i d0 N 4 , substitute the dynamic equation of the dq axis current loop and the closed-loop transfer function of the current loop into the dynamic equation of the DC voltage loop and the phase-locked loop in the small signal model to obtain:

[0086]

[0087] The feedback loop is artificially added to the phase-locked loop to increase the system damping. It can be seen from the simplified grid-connected inverter small signal model that the differential term L g i d0 s represents the differential dynamics of the network inductance, which is equivalent to introducing an additional positive feedback loop in the phase-locked loop control loop. Using the parameter root locus method, this term is regarded as an open-loop gain, and the root locus diagram of the system is drawn and analyzed. When the differential term L g i d0 As s increases, the low-frequency pole moves significantly to the right of the imaginary axis, indicating that the positive feedback loop introduced by the differential dynamic equivalent of the network inductance will increase with the differential term L g i d0 The increase of s value reduces the damping of the phase-locked loop, thereby reducing the stability of the system.

[0088] Inspired by the above conclusions, if a proportional negative feedback loop is artificially added to the phase-locked loop control loop, the stability of the system can be effectively increased. This method increases the damping of the inverter's low-frequency oscillation mode while preventing the medium- and high-frequency oscillation modes from crossing the imaginary axis and entering an unstable state. Compared with the method of modifying the control parameters, it can better improve the stability and stability margin of the system.

[0089] The larger the feedback coefficient of the introduced proportional negative feedback loop, the greater the increase in the damping of the system. However, the actual control system is discrete and there is a certain control delay, which will limit the feedback coefficient. Once the feedback coefficient exceeds the constraint, the system will experience high-frequency oscillation. Figure 5 and Figure 6 , the small signal model of the phase-locked loop with the state feedback loop and delay link is:

[0090]

[0091] Where Δω is the phase-locked loop output; Δu tq is the q-axis component of the grid-connected point voltage; K2 is the state feedback loop coefficient.

[0092] The approximate result is obtained by making a first-order pade approximation on the delay link in the small signal model of the phase-locked loop:

[0093]

[0094] Substituting the approximate result into the small signal model of the phase-locked loop, the closed-loop input-output relationship of the phase-locked loop is obtained as follows:

[0095]

[0096] According to Routh criterion, the stability condition of the phase-locked loop is:

[0097]

[0098] The above is the selection range of the state feedback loop coefficient.

[0099] For a phase-locked loop, its integral parameter is usually much smaller than the control frequency of the controller. Therefore, the second inequality in the above phase-locked loop stability condition is generally valid. Based on this, the range of the state feedback coefficient can be obtained as follows:

[0100]

[0101] That is, the state feedback loop coefficient needs to be smaller than the inverse of the controller proportional parameter.

[0102] After the phase-locked loop introduces the state feedback loop, the structure of the control system changes. When the operating point of the grid-connected inverter deviates from the rated operating point, the state variable will have a steady-state error. When the state feedback loop is introduced and the delay link is not considered, the open-loop input-output relationship of the phase-locked loop is:

[0103]

[0104] The original PI control link is a Type I system, while the control link with the state feedback loop is a Type 0 system. This system cannot achieve zero-error tracking of the step signal, that is, there is a steady-state error between the output phase of the phase-locked loop and the actual phase, and the grid phase value cannot be accurately output.

[0105] To solve this problem, a secondary integral loop is introduced in the control link. The basic idea is to dynamically adjust the rated value of the state variable according to the output of the controller. Due to the existence of the integral link, Δω=0, then Δu tq =0, thus achieving zero-static-error tracking.

[0106] In order to verify the gain effect of the present invention on the stability of the grid-connected inverter system, a simulation model was built in Matlab / Simulink for verification. The simulation step size was 0.1ms, the proportional coefficient in the state feedback loop was set to 3, the integral coefficient a on the secondary integral loop of the control loop used to eliminate the steady-state error was set to 10, and the system short-circuit ratio was set to 1.6, which belongs to an extremely weak power grid environment. A 2% power increment disturbance was applied to the DC input terminal at 2s, and the stability of the system was judged by observing the fluctuation amplitude and response time of the DC side voltage. According to Figure 7 It can be seen that after the introduction of state feedback, the time for the inverter to recover stability after disturbance is shortened and the fluctuation amplitude becomes smaller, which confirms the effectiveness of the present invention in improving system stability.

[0107] See also Figure 8 The present invention also provides a device for improving the stability of a grid-connected inverter based on a state feedback loop, which is applied to the above-mentioned method for improving the stability of a grid-connected inverter based on a state feedback loop, and the device comprises:

[0108] A small signal model building module is used to build a small signal model of a grid-connected inverter including a phase-locked loop, a voltage loop, and a current loop;

[0109] The state feedback loop introduction module is used to introduce a state feedback loop into the control link of the phase-locked loop to increase the damping of the grid-connected inverter; at the same time, a delay link is introduced into the control link of the phase-locked loop to limit the coefficient of the state feedback loop;

[0110] The secondary integral loop introduction module is used to introduce the secondary integral loop into the control link of the phase-locked loop, and eliminate the error between the output phase and the actual phase of the phase-locked loop through the output of the secondary integral loop, thereby realizing zero-static-error tracking.

[0111] See also Fig. 9 , the present invention also provides a device for improving the stability of a grid-connected inverter based on a state feedback loop, comprising a memory and a processor;

[0112] The memory is used to store computer program code and transmit the computer program code to the processor;

[0113] The processor is used to execute the above-mentioned method for improving the stability of a grid-connected inverter based on a state feedback loop according to the instructions in the computer program code.

[0114] The present invention also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the above-mentioned method for improving the stability of a grid-connected inverter based on a state feedback loop is implemented.

[0115] Generally speaking, the computer instructions for implementing the method of the present invention may be carried in any combination of one or more computer-readable storage media. Non-transitory computer-readable storage media may include any computer-readable media, except for the signal itself that is temporarily propagating.

[0116] The computer-readable storage medium may be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, device or device, or any combination thereof. More specific examples of computer-readable storage media (a non-exhaustive list) include: an electrical connection with one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EKROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof. In the present invention, a computer-readable storage medium may be any tangible medium containing or storing a program that may be used by or in conjunction with an instruction execution system, device, or device.

[0117] Computer program code for performing the operation of the present invention can be written in one or more programming languages ​​or a combination thereof, including object-oriented programming languages ​​such as Java, Smalltalk, C++, and conventional procedural programming languages ​​such as "C" or similar programming languages, in particular, Python suitable for neural network computing and platform frameworks based on TensorFlow, PyTorch, etc. can be used. The program code can be executed entirely on the user's computer, partially on the user's computer, as an independent software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer can be connected to the user's computer or to an external computer (for example, using an Internet service provider to connect via the Internet) through any type of network, including a local area network (LAN) or a wide area network (WAN).

[0118] The above-mentioned device and non-temporary computer-readable storage medium can refer to the specific description of a method for improving the stability of a grid-connected inverter based on a state feedback loop and its beneficial effects, which will not be repeated here.

[0119] Although the embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for improving the stability of a grid-connected inverter based on a state feedback loop, characterized in that: include: Construct a small signal model of the grid-connected inverter including a phase-locked loop, a voltage loop, and a current loop; A state feedback loop is introduced into the control link of the phase-locked loop to increase the damping of the grid-connected inverter; at the same time, a delay link is introduced into the control link of the phase-locked loop to limit the coefficient of the state feedback loop; A secondary integral loop is introduced into the control link of the phase-locked loop, and the error between the output phase of the phase-locked loop and the actual phase is eliminated through the output of the secondary integral loop, thereby achieving zero-static-error tracking.

2. A method for improving the stability of a grid-connected inverter based on a state feedback loop according to claim 1, characterized in that: The small signal model of the grid-connected inverter is: In the formula, Δi dref is the d-axis component of the current loop reference current; N1, N2, N3, N4 are intermediate variables, N1 = 3 / (2U dc0 C dc ), N2=U g cosδ0,N3=U g i d0 sinδ0-U g i q0 cosδ0,N4=U g sin δ0,U dc0 is the steady-state value of the DC side voltage, C dc is the DC side capacitance, U g is the amplitude of the grid voltage, δ0 is the stable value of the phase difference between the grid voltage phase and the phase-locked loop output phase; Δi d is the d-axis component of the actual current in the current loop; Δi q is the q-axis component of the actual current in the current loop; Δθ is the phase angle of the grid-connected voltage; L g is the equivalent reactance of the power grid; i d0 is the stable value of the d-axis component of the actual current in the current loop; S is the complex frequency variable of Laplace transform; i q0 is the stable value of the q-axis component of the actual current in the current loop; k vp k is the DC voltage loop proportional coefficient; vi is the DC voltage loop integral coefficient; Δi qref is the q-axis component of the current loop reference current; X g is the equivalent inductive reactance of the power grid, X g =ω0L g , ω0 is the stable value of angular frequency; k tp k is the proportional coefficient of the AC voltage loop; ti k is the AC voltage loop integral coefficient; pp k is the proportional link coefficient of the phase-locked loop; pi k is the integral coefficient of the phase-locked loop; ip k is the current loop proportional coefficient; ii is the current loop integral coefficient; L is the AC filter inductance; Δi qref is the q-axis component of the current loop reference current.

3. A method for improving the stability of a grid-connected inverter based on a state feedback loop according to claim 2, characterized in that: The AC voltage loop of the grid-connected inverter is shielded, and in steady state there is i q0 =i qref =0, the small signal model of the grid-connected inverter is simplified to:

4. The method for improving the stability of a grid-connected inverter based on a state feedback loop according to claim 3, characterized in that: The dynamic equation of the dq axis current loop is rewritten as: The closed-loop transfer function of the current loop is: N3=i at unity power factor d0 N4, substitute the dynamic equation of the dq axis current loop and the closed-loop transfer function of the current loop into the dynamic equation of the DC voltage loop and the phase-locked loop in the small signal model to obtain:

5. A method for improving the stability of a grid-connected inverter based on a state feedback loop according to claim 4, characterized in that: The small signal model of the phase-locked loop with the state feedback loop and delay link is: Where Δω is the phase-locked loop output; Δu tq is the q-axis component of the grid-connected point voltage; K2 is the state feedback loop coefficient.

6. A method for improving the stability of a grid-connected inverter based on a state feedback loop according to claim 5, characterized in that: The selection range of the state feedback loop coefficient is obtained by the following method: The approximate result is obtained by making a first-order pade approximation on the delay link in the small signal model of the phase-locked loop: Substituting the approximate result into the small signal model of the phase-locked loop, the closed-loop input-output relationship of the phase-locked loop is obtained as follows: According to Routh criterion, the stability condition of the phase-locked loop is:

7. A method for improving the stability of a grid-connected inverter based on a state feedback loop according to claim 6, characterized in that: Introducing the state feedback loop and ignoring the delay link, the open-loop input-output relationship of the phase-locked loop is: A secondary integral loop is introduced into the control link of the phase-locked loop. The output of the secondary integral loop is dynamically adjusted according to the output Δω of the phase-locked loop without considering the delay link, and the error between the output phase of the phase-locked loop and the actual phase is eliminated through the output of the secondary integral loop, thereby achieving zero-static-error tracking.

8. A device for improving the stability of a grid-connected inverter based on a state feedback loop, characterized in that: The device is applied to the method described in any one of claims 1 to 7, and the device comprises: A small signal model building module is used to build a small signal model of a grid-connected inverter including a phase-locked loop, a voltage loop, and a current loop; The state feedback loop introduction module is used to introduce a state feedback loop into the control link of the phase-locked loop to increase the damping of the grid-connected inverter; at the same time, a delay link is introduced into the control link of the phase-locked loop to limit the coefficient of the state feedback loop; The secondary integral loop introduction module is used to introduce the secondary integral loop into the control link of the phase-locked loop, and eliminate the error between the output phase and the actual phase of the phase-locked loop through the output of the secondary integral loop, thereby realizing zero-static-error tracking.

9. A device for improving the stability of a grid-connected inverter based on a state feedback loop, characterized in that: including memory and processor; The memory is used to store computer program code and transmit the computer program code to the processor; The processor is configured to execute the method according to any one of claims 1 to 7 according to instructions in the computer program code.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method according to any one of claims 1 to 7 is implemented.

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