An online non-intrusive grid impedance estimation method based on grid-connected inverters

Abstract

The application discloses an online non-intrusive power grid impedance estimation method of a grid-connected inverter, a grid-connected inverter grid-connected system, and an inverter connected to an alternating current power grid through grid-connected control. The inverter outputs a voltage with continuously changing amplitude and phase in a transient process. The inverter output voltage and the power grid current are collected, and the power grid impedance is estimated based on the following estimation algorithm: the online non-intrusive power grid impedance estimation method based on the grid-connected inverter has good power grid impedance estimation effect, and can estimate the changing power grid impedance in real time.

Description

Technical Field

[0001] This invention relates to the field of power analysis technology, and in particular to an online non-intrusive grid impedance estimation method based on a grid-connected inverter. Background Technology

[0002] The equivalent grid outside the grid-connected inverter port can be regarded as a series connection of a voltage source and an impedance. This impedance parameter directly affects the transient stability of the grid-connected inverter.

[0003] Currently, the addition of grid-connected inverters alters grid characteristics, including the equivalent grid impedance seen by each inverter at its point of common coupling (PCC). The PCC impedance is a time-varying parameter, changing occasionally but generally considered constant over a certain period. Grid impedance significantly impacts the control and stability of grid-connected inverters. It is a key parameter for inverter control, directly affecting its overall performance. For grid-connected inverters acting as current sources, grid impedance can cause coupling with the phase-locked loop (PLL), affecting the system's small-signal stability. Understanding the grid impedance beforehand helps decouple from the PLL, eliminating its adverse effects on system stability. For grid-connected inverters acting as voltage sources, grid impedance determines the power control method. For purely inductive grids, the frequency and amplitude of the AC voltage are regulated by active and reactive power, respectively. However, the resistor-inductor gate impedance causes coupling between active and reactive power, leading to coupling in active and reactive power control, as well as some transient stability issues. Meanwhile, some methods to improve converter performance, such as virtual impedance, reduced-order control, or low-voltage ride-through (LVRT), require prior knowledge of grid impedance. Therefore, estimating grid impedance is crucial for the control and stability analysis of grid-connected inverters. Measuring grid impedance with satisfactory accuracy and dynamics can help design the controller of a grid-connected inverter, thereby improving its performance.

[0004] Online grid impedance estimation methods can be broadly categorized into invasive and non-invasive methods. Invasive methods estimate grid impedance by measuring the system's response to disturbances. However, the additional interference injected into the grid can lead to power quality problems. Furthermore, data processing is complex, and impedance measurements are inaccurate due to time delays and frequency offset interference. Non-invasive methods, on the other hand, utilize information present during normal system operation to estimate grid impedance without additional interference. The Extended Kalman Filter (EKF) has proven to be an effective tool for estimating grid impedance. This method can estimate gate impedance under high distortion voltages. However, adjusting the noise covariance matrix is ​​very difficult, affecting the estimation performance of the EKF algorithm. Other non-invasive methods utilize the steady-state information of the converter to estimate grid impedance, thus requiring consideration of converter stability. Inherent harmonic signals introduced by nonlinear or unbalanced loads can also be used for impedance estimation. Obviously, this method is only effective when there is significant harmonic distortion in the output voltage, which can pollute the grid. Some researchers have proposed a non-invasive grid impedance estimation method using grid-connected inverter network control. By using network control, the phase difference and amplitude difference between the converter output voltage and the grid voltage can be obtained. The grid impedance can be calculated using a simple circuit equation. However, this method requires prior knowledge of the grid voltage and does not allow for real-time impedance estimation. It can be seen that passive methods applicable to grid-connected inverters all utilize their steady-state information, and the estimation results are not ideal. Therefore, it is necessary to design a novel grid impedance estimation method for grid-connected inverters. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing an online, non-intrusive grid impedance estimation method based on grid-connected inverters.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: An online non-intrusive grid impedance estimation method based on a grid-connected inverter is designed. The inverter is connected to the AC grid through grid-connection control. The grid impedance estimation algorithm is implemented based on the sampled output voltage and current after grid connection, as shown in the following formula:

[0007] ,

[0008] ,

[0009] ,

[0010] In the formula: V cd (k),V cq (k) represents the d-axis and q-axis components of the converter output voltage at the k-th sampling point in the synchronous coordinate system (dq reference frame); i gd (k),i gq(k) represent the d-axis and q-axis components of the grid current sampled at the kth time in the synchronous coordinate system (dq reference frame); θ=[Lf g R g v g ] T This is a parameter vector representing the grid inductance, grid resistance, and grid voltage amplitude. Given the estimated grid impedance parameter vector, the grid impedance can be estimated using the recursive least squares algorithm. Forgetting factor, usually To ensure that the parameter estimates converge to the true values ​​quickly, take... .

[0011] Preferably, the transient process of the grid-connected inverter is used to estimate the grid impedance.

[0012] The control law for the grid-connected inverter; the power outer loop control law is as follows:

[0013] ,

[0014] The voltage and current loop transfer functions are as follows:

[0015] ,

[0016] ,

[0017] In the formula, ω ref With V ref These are the frequency reference and voltage amplitude reference of the converter, respectively; ω0 and V0 are the grid voltage angular frequency and grid voltage amplitude, respectively; P0 and P LPF These are the active power reference of the converter and the output active power after passing through the low-pass filter (LPF), respectively; Q0 ​​and Q... LPF These are the reactive power reference of the converter and the output reactive power after passing through the LPF, respectively; K P K Qp and K Qi All are controller parameters; k vp and k vi These are the proportional and integral coefficients of the voltage loop, respectively; k ip and k ii These are the proportional and integral coefficients of the current loop, respectively.

[0018] Preferably, the specific steps are as follows: 1. The grid-connected inverter synchronizes with the grid through a phase-locked loop (PLL). After grid connection, the PLL is disabled, the inverter's output power is 0, and voltage and current sensors sample the output voltage and grid current data; 2. The inverter's output power tracks the reference power, and the inverter's output voltage will change accordingly; 3. The sampled voltage and current data are transformed using a dq transformation to obtain V. cd (k),Vcq (k),i gd (k),igq(k),Import impedance estimation algorithm, this recursive algorithm can estimate the grid impedance value before the converter enters steady state;4.Since the algorithm is only effective when the grid-connected inverter is in transient state, it needs to be stopped after the system enters steady state;when the grid impedance changes and the system enters transient state again, the estimation algorithm will estimate the new grid impedance value in the new transient state.

[0019] This invention proposes an online, non-intrusive grid impedance estimation method based on grid-connected inverters. The advantages are as follows: Unlike most impedance estimation methods based on converter steady-state information, this method utilizes the constantly changing amplitude and phase of the output voltage during the inverter's transient process to estimate the grid impedance, without requiring grid voltage information. Furthermore, this method is passive and does not cause any interference to the grid. The grid impedance can be estimated before the converter enters steady state, and the estimation algorithm is updated synchronously with grid-connected control, resulting in excellent real-time estimation performance. Attached Figure Description

[0020] Figure 1 Topology diagram of a grid-connected inverter system;

[0021] Figure 2 This is a grid connection control block diagram;

[0022] Figure 3 Here is a flowchart of the estimation algorithm;

[0023] Figure 4 A graph showing the active power output of the converter;

[0024] Figure 5 The output reactive power curve of the converter;

[0025] Figure 6 A graph showing the estimated inductance of the power grid;

[0026] Figure 7 This is the curve for estimating the grid resistance. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0028] Example 1:

[0029] Grid-connected control of grid-connected inverters:

[0030] like Figure 1-2As shown; the converter is connected to the AC grid via a grid-connected control and an LCL filter. The grid voltage phase is the reference phase, and the converter's output voltage is... If the phase of the grid voltage is taken as the reference phase, then the grid voltage is V. g e j0 The grid current is The grid impedance is Z g =R g +jX g =R g +jω0(L g +L f ), then we have .

[0031] The active power P and reactive power Q injected into the grid by the grid-connected inverter are derived from the complex product of the voltage and conjugate current phasors, as shown below:

[0032] ;

[0033] ;

[0034] The converter's output power P and Q can also be expressed as:

[0035] ,

[0036] In the formula, V cd V cq ,I gd ,I gq These are the dq components of the converter output voltage and the grid current, respectively.

[0037] Voltage and current loop control:

[0038] The system uses the dq synchronous coordinate system, in which the steady-state output voltage of the converter and the grid current are both DC. Therefore, a proportional-integral (PI) controller can be used to ensure rapid tracking of the reference voltage.

[0039] The voltage and current loops are designed as follows:

[0040] ,

[0041] ,

[0042] ω ref With V ref These are the frequency reference and voltage amplitude reference of the converter, respectively; ω0 and V0 are the grid voltage angular frequency and grid voltage amplitude, respectively.

[0043] Modeling of power grid impedance parameters:

[0044] The system satisfies the following model during the transient process:

[0045] ,

[0046] ,

[0047] In the formula L fg =(L g +L f Discretizing it yields the following linear model:

[0048] ,

[0049] .

[0050] Grid impedance estimation:

[0051] refer to Figure 3 After obtaining the linear discrete model of the grid impedance parameters, the grid impedance parameters can be estimated using the recursive least squares algorithm. The estimation algorithm can be expressed as:

[0052] ,

[0053] Forgetting factor in the formula We set it to 0.99. The algorithm can quickly estimate the grid impedance. The algorithm's initialization is achieved by the following formula:

[0054] ,

[0055] ,

[0056] In the formula It is a very small positive real number. It is an identity matrix.

[0057] After the converter is connected to the grid, its output voltage will track the reference voltage generated by the power loop under the action of the voltage-current loop until the converter's output power is the reference power. When the grid impedance changes, the converter's output power will change accordingly.

[0058] Simulation analysis:

[0059] To verify the feasibility and theoretical correctness of the grid impedance estimation method based on the transient state of the grid-connected inverter, the proposed grid impedance estimation method was simulated and verified using the Matlab / Simulink simulation platform. The simulation parameter settings are shown in Table 1:

[0060] Table 1: Simulation parameters of grid-connected inverter system

[0061] ,

[0062] ;

[0063] The simulation parameters of the grid-connected inverter system are shown in Table 1. The grid impedance estimation method is based on the transient state of the grid-connected inverter.

[0064] Actual results are as follows Figure 4-7 As shown, it can be observed that the estimation algorithm has already estimated the parameter values ​​of the grid impedance while the output power of the converter is still changing during the transient process.

[0065] As can be seen from the above, the grid impedance estimation method based on the transient state of the grid-connected inverter of this invention differs from most impedance estimation methods based on the steady-state information of the converter. It utilizes the continuously changing amplitude and phase of the output voltage during the transient process of the grid-connected inverter to estimate the grid impedance, without requiring grid voltage information. Furthermore, this method is passive and does not cause any interference to the grid. The grid impedance can be estimated before the converter enters steady state, and the estimation algorithm is updated synchronously with grid-connected control, resulting in excellent real-time estimation performance.

[0066] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for online non-intrusive grid impedance estimation based on a grid-connected inverter, characterized in that, The converter is connected to the AC power grid via grid-connection control. The grid impedance estimation algorithm is implemented based on the sampled output voltage and current after grid connection, as shown in the following formula: ， In the formula: V cd (k),V cq (k) represents the d-axis and q-axis components of the converter output voltage at the k-th sampling point in the synchronous coordinate system (dq reference frame); i gd (k),i gq (k) represent the d-axis and q-axis components of the grid current sampled at the kth time in the synchronous coordinate system (dq reference frame); θ=[Lf g R g v g ] T This is a parameter vector representing the grid inductance, grid resistance, and grid voltage amplitude. Given the estimated grid impedance parameter vector, the grid impedance can be estimated using the recursive least squares algorithm; λ is the forgetting factor, 0.95≤λ≤1; to ensure that the parameter estimates converge to the true values ​​quickly, λ=0.99 is chosen. The specific steps are as follows:

1. The grid-connected inverter is synchronized with the grid through a phase-locked loop. After grid connection, the phase-locked loop is disabled, the inverter's output power is 0, and voltage and current sensors sample the output voltage and grid current data.

2. The converter output power tracks the reference power, and the converter output voltage will change accordingly; 3. Perform dq transformation on the sampled voltage and current data to obtain V. cd (k),V cq (k),i gd (k),igq(k),Import impedance estimation algorithm, the impedance estimation algorithm can estimate the grid impedance value before the converter enters steady state; 4. Since the impedance estimation algorithm is only effective during the transient state of the grid-connected inverter, it needs to be stopped from being updated after the system enters steady state. When the grid impedance changes and the system re-enters the transient state, the impedance estimation algorithm will estimate the new grid impedance value in the new transient state.

2. The online non-intrusive grid impedance estimation method based on grid- connected inverter according to claim 1, characterized in that, Grid impedance estimation is performed using the transient process of a grid-connected inverter.

3. The online non-intrusive grid impedance estimation method based on grid- connected inverter according to claim 2, characterized in that, The control law for the grid-connected inverter; the power outer loop control law is as follows: oh ref =ω0+K P ·(P0-P LPF ), V ref = V0+ (K Qp + K Qi / s) · (Q0- Q LPF ), The voltage and current loop transfer functions are as follows: ， ， In the formula, ω ref With V ref These are the frequency reference and voltage amplitude reference of the converter, respectively; ω0 and V0 are the grid voltage angular frequency and grid voltage amplitude, respectively; P0 and P LPF These are the active power reference of the converter and the output active power after passing through the low-pass filter (LPF), respectively; Q0 ​​and Q... LPF These are the reactive power reference of the converter and the output reactive power after passing through the LPF, respectively; K P K Qp and K Qi All are controller parameters; k vp and k vi These are the proportional and integral coefficients of the voltage loop, respectively; k ip and k ii These are the proportional and integral coefficients of the current loop, respectively.

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