A grid-connected inverter stability analysis method and device

Abstract

The application discloses a grid-connected inverter stability analysis method and device, the method comprises the following steps: according to the circuit structure of the grid-connected inverter system in the dq domain, a small signal model of the grid-connected inverter system is established in the controller dq coordinate system; the latch voltage of the control loop of the grid-connected inverter system in the controller dq coordinate system is calculated; according to the conversion relationship between the latch voltage of the phase-locked loop control loop, the controller dq coordinate system and the grid dq coordinate system, the grid synchronization small signal model of the grid-connected inverter system in the control dq coordinate system is obtained; according to the grid synchronization small signal model, the loop gain is calculated and the synchronization stability of the grid-connected inverter system is obtained. By adopting the application, the stability margin of the system can be identified through the constructed single-input single-output grid synchronization small signal model, and the stability of the grid-connected inverter can be improved.

Description

Technical Field

[0001] This invention relates to the field of inverter analysis, and more particularly to a method and apparatus for analyzing the stability of grid-connected inverters. Background Technology

[0002] With the massive consumption of traditional fossil fuels and their adverse environmental impacts, distributed generation technology has received widespread attention and application. Inverters, as the interface device between distributed generation and the power grid, play a crucial role in transmitting renewable energy to the grid. However, the uneven distribution of renewable energy means that its generation units are often deployed in remote areas. Simultaneously, the high impedance of long-distance transmission lines leads to the grid exhibiting weak grid characteristics. For distributed generation systems, a grid can be defined as weak when the short-circuit ratio (SCR) is ≤10. Under weak grid conditions, the influence of grid impedance causes stability problems such as wide-frequency oscillations after the inverter is connected to the grid, seriously threatening the safe and reliable operation of grid-connected inverters. Therefore, it is urgent to conduct research on model construction and stability analysis of grid-connected inverters under weak grid conditions.

[0003] Impedance analysis is a relatively good method for analyzing the interactions between converters and the power grid, and between converters themselves. Essentially, this method treats the grid-connected converter and the power grid as two independent subsystems, and uses Norton's or Thevenin's theorem to represent them as parallel impedances of current sources and series impedances of voltage sources, thus analyzing the stability of the entire grid-connected system. The advantages of impedance analysis are significant: it allows for the establishment of separate impedance models for each subsystem based on their respective control structures and parameter characteristics. Changes in the control structure or parameters of any subsystem will not affect the impedance model of the other subsystem, thus eliminating the need to rebuild impedance models and greatly reducing the difficulty of system analysis.

[0004] Impedance analysis was first applied to DC systems to analyze the stability of DC-DC converters with loads. For three-phase AC systems, a synchronous rotating coordinate transformation (dq-axis) is typically used to convert the three-phase AC to DC along the dq axes before impedance analysis is applied to analyze the stability of the grid-connected inverter system. However, during coordinate transformation, coupling exists between the dq axes, resulting in a multi-input multi-output (MIMO) second-order matrix form for the impedance model of the grid-connected inverter system. This necessitates the use of the more complex generalized Nyquist criterion to analyze the stability of the grid-connected inverter, making the analysis process relatively complex. Furthermore, since the generalized Nyquist criterion analyzes system stability by determining whether the Nyquist curves of the eigenvalues ​​enclose the point (-1, j0), it does not directly demonstrate the system's stability margin. Moreover, the Nyquist curves of the eigenvalues ​​do not provide amplitude-frequency and phase-frequency characteristic curves, making the design of control methods that optimize the impedance amplitude-phase-frequency characteristic curves extremely difficult. Summary of the Invention

[0005] This invention provides a method and apparatus for analyzing the stability of grid-connected inverters. By constructing a single-input single-output grid synchronization small-signal model, the stability margin of the system can be identified, which facilitates the improvement of the stability of the grid-connected inverter.

[0006] To achieve the above objectives, a first aspect of this application provides a method for analyzing the stability of a grid-connected inverter, comprising:

[0007] Based on the circuit structure of the grid-connected inverter system in the dq domain, a small-signal model of the grid-connected inverter system is established in the controller dq coordinate system.

[0008] Based on the response of the delay element, the inverter modulation wave response, and the positive feedback element in the small-signal model, calculate the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system.

[0009] Based on the latching voltage of the phase-locked loop control circuit, the transformation relationship between the controller dq coordinate system and the grid dq coordinate system, the small-signal model of grid synchronization of the grid-connected inverter system in the control dq coordinate system is obtained.

[0010] The loop gain is calculated based on the small-signal model of grid synchronization to obtain the synchronization stability of the grid-connected inverter system; the loop gain refers to the ratio of the main circuit response to the phase-locked loop control circuit response.

[0011] In one possible implementation of the first aspect, the step of calculating the loop gain based on the grid synchronization small-signal model of the grid-connected inverter and obtaining the synchronization stability of the grid-connected inverter system specifically includes:

[0012] Based on the grid synchronization small-signal model of the grid-connected inverter, the amplitude-frequency response curve of the main circuit and the amplitude-frequency response curve of the phase-locked loop control circuit are calculated.

[0013] The crossover frequency is obtained based on the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit; the crossover frequency is the frequency corresponding to the intersection point of the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit.

[0014] Calculate the phase margin of the grid-connected inverter system at each switching frequency;

[0015] If the phase margin at all switching frequencies is greater than 0, the grid-connected inverter system is in a stable state.

[0016] In one possible implementation of the first aspect, calculating the phase margin of the grid-connected inverter system at each switching frequency specifically includes:

[0017] Obtain the phase angle of the main circuit response and the phase angle of the phase-locked loop control circuit response at the switching frequency;

[0018] The complementary angle between the phase angle of the phase-locked loop control circuit response and the phase angle of the main circuit response is taken as the phase margin of the grid-connected inverter system.

[0019] In one possible implementation of the first aspect, calculating the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system based on the response of the delay element, the inverter modulation wave response, and the positive feedback element response in the small-signal model specifically includes:

[0020] Based on the response of the delay element, the inverter modulation wave response, and the positive feedback element in the small-signal model, the equivalent output impedance of the grid-connected inverter system in the controller dq coordinate system and the common access point voltage in the controller dq coordinate system are obtained.

[0021] By selecting the common access point voltage in the controller's dq coordinate system as the orientation, the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller's dq coordinate system is obtained.

[0022] In one possible implementation of the first aspect, the grid-connected inverter system, in the control dq coordinate system, specifically includes a grid synchronization small-signal model of the grid.

[0023] The main circuit response, the phase-locked loop control circuit response, the latching voltage of the phase-locked loop control circuit, and the phase angle of the common access point voltage in the grid dq coordinate system.

[0024] A second aspect of this application provides a grid-connected inverter stability analysis device, comprising:

[0025] The signal model building module establishes a small-signal model of the grid-connected inverter system in the controller dq coordinate system based on the circuit structure of the grid-connected inverter system in the dq domain.

[0026] The latch voltage calculation module is used to calculate the latch voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system based on the response of the delay element, the response of the inverter modulation wave, and the response of the positive feedback element in the small-signal model.

[0027] The synchronization model establishment module obtains the grid synchronization small-signal model of the grid-connected inverter system in the control dq coordinate system based on the latching voltage of the phase-locked loop control loop, the transformation relationship between the controller dq coordinate system and the grid dq coordinate system;

[0028] The analysis module is used to calculate the loop gain based on the grid synchronization small-signal model and obtain the synchronization stability of the grid-connected inverter system; the loop gain refers to the ratio of the main circuit response to the phase-locked loop control circuit response.

[0029] In one possible implementation of the second aspect, the analysis module is specifically used for:

[0030] Based on the grid synchronization small-signal model of the grid-connected inverter, the amplitude-frequency response curve of the main circuit and the amplitude-frequency response curve of the phase-locked loop control circuit are calculated.

[0031] The crossover frequency is obtained based on the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit; the crossover frequency is the frequency corresponding to the intersection point of the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit.

[0032] Calculate the phase margin of the grid-connected inverter system at each switching frequency;

[0033] If the phase margin at all switching frequencies is greater than 0, the grid-connected inverter system is in a stable state.

[0034] In one possible implementation of the second aspect, calculating the phase margin of the grid-connected inverter system at each switching frequency specifically includes:

[0035] Obtain the phase angle of the main circuit response and the phase angle of the phase-locked loop control circuit response at the switching frequency;

[0036] The complementary angle between the phase angle of the phase-locked loop control circuit response and the phase angle of the main circuit response is taken as the phase margin of the grid-connected inverter system.

[0037] In one possible implementation of the second aspect, the latch voltage calculation module is specifically used for:

[0038] Based on the response of the delay element, the inverter modulation wave response, and the positive feedback element in the small-signal model, the equivalent output impedance of the grid-connected inverter system in the controller dq coordinate system and the common access point voltage in the controller dq coordinate system are obtained.

[0039] By selecting the common access point voltage in the controller's dq coordinate system as the orientation, the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller's dq coordinate system is obtained.

[0040] In one possible implementation of the second aspect, the grid-connected inverter system's small-signal grid synchronization model in the control dq coordinate system specifically includes:

[0041] The main circuit response, the phase-locked loop control circuit response, the latching voltage of the phase-locked loop control circuit, and the phase angle of the common access point voltage in the grid dq coordinate system.

[0042] Compared with existing technologies, the grid-connected inverter stability analysis method and apparatus provided in this embodiment of the invention establishes a grid synchronization small-signal model of the grid-connected inverter system, converting the multi-input multi-output model of the grid-connected inverter system in the dq domain into a single-input single-output form. Since it only needs to obtain the phase angle of the voltage at the system's common access point, the stability analysis process of the grid-connected inverter is simplified. At the same time, it considers the influence of active and reactive power on the stability of the grid-connected inverter, overcoming the technical limitations of the modeling method in the polar coordinate system.

[0043] In addition, during the establishment of the small-signal model for power grid synchronization, a positive feedback loop was introduced at the modulation signal, making the established single-input single-output model more complete. Attached Figure Description

[0044] Figure 1 This is a flowchart illustrating a grid-connected inverter stability analysis method according to an embodiment of the present invention;

[0045] Figure 2 This is a control structure diagram of a grid-connected inverter system in the dq domain according to an embodiment of the present invention;

[0046] Figure 3 This is a structural diagram of a phase-locked loop control circuit in a grid-connected inverter system provided by an embodiment of the present invention;

[0047] Figure 4 This is a diagram showing the relationship between the controller dq coordinate system and the power grid dq coordinate system according to an embodiment of the present invention;

[0048] Figure 5 This is a schematic diagram of a small-signal model of a grid-connected inverter system in the controller dq coordinate system according to an embodiment of the invention;

[0049] Figure 6 This is a schematic diagram of a small-signal grid synchronization model of a grid-connected inverter system in the controller dq coordinate system according to an embodiment of the invention;

[0050] Figure 7 This is a stability analysis result diagram of a grid-connected inverter system provided by an embodiment of the invention;

[0051] Figure 8 This is a stability analysis result diagram of another grid-connected inverter system provided by one embodiment of the invention. Detailed Implementation

[0052] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] Please see Figure 1 An embodiment of the present invention provides a method for stability analysis of a grid-connected inverter, comprising:

[0054] S10. Based on the circuit structure of the grid-connected inverter system in the dq domain, establish a small-signal model of the grid-connected inverter system in the controller dq coordinate system.

[0055] S11. Based on the response of the delay element, the inverter modulation wave response, and the positive feedback element in the small-signal model, calculate the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system.

[0056] S12. Based on the latching voltage of the phase-locked loop control circuit, the transformation relationship between the controller dq coordinate system and the grid dq coordinate system, the small-signal model of the grid-connected inverter system in the control dq coordinate system is obtained.

[0057] S13. Calculate the loop gain based on the grid synchronization small-signal model and obtain the synchronization stability of the grid-connected inverter system; the loop gain refers to the ratio of the main circuit response to the phase-locked loop control circuit response.

[0058] This invention provides a method for modeling a small-signal grid synchronization model of a grid-connected inverter that considers the influence of modulation signals. This modeling method can simultaneously consider the impact of active and reactive power on the stability of the grid-connected inverter, simplifying the MIMO model to a SISO model while overcoming the limitations of traditional polar coordinate impedance models. Furthermore, the constructed single-input single-output synchronization model not only identifies the system's stability margin but also facilitates optimized design, improving the stability of the grid-connected inverter.

[0059] For ease of explanation, the stability analysis process will be illustrated below using a 14kW three-phase LCL grid-connected inverter under unity power factor grid-connected current control as an example. It should be noted that this solution is applicable to all three-phase inverter AC systems and is not limited to a 14kW three-phase LCL grid-connected inverter.

[0060] Figure 2 This describes the main circuit and control circuit of a 14kW three-phase LCL grid-connected inverter under unity power factor grid-connected current control. Figure 2 in, udc DC bus voltage; inverter-side filter inductor L1, filter capacitor C f The grid-side filter inductor L2 forms an LCL filter; R d For damping resistor; u inv i is the inverter bridge arm output voltage; i2 is the grid-connected current; u pcc The voltage at the grid connection point; u g Z is the grid voltage; g Let G be the grid impedance. The inner current loop uses a PI controller with a transfer function G. i (s)=k ip +k ii / s. I 2dref I 2qref These are the reference values ​​for the grid-connected current on the d-axis and q-axis, respectively. Md v Mq These are the modulation signals output by the current loop PI controller on the d-axis and q-axis, respectively.

[0061] Figure 3 This is a PLL (synchronous reference frame PLL, SRF-PLL) structure in a synchronous reference coordinate system, used to achieve phase tracking between the inverter and the grid. H... pi (s)=k ppll +k ipll / s represents the PI controller inside the phase-locked loop, k ppll k is the proportional gain of the PI controller. ipll ω is the integral coefficient of the PI controller. ref ω is the rated angular frequency of the power grid. o This is the output angular frequency of the PLL.

[0062] Under normal operating conditions, the phase angle of the phase-locked loop (PLL) output is synchronized with the power grid. However, when considering the influence of power grid impedance, the PLL output phase angle is affected by small-signal disturbances, causing the controller's dq coordinate system to no longer be consistent with the power grid's dq coordinate system, resulting in a phase angle difference between the two. Figure 4 As shown. This means that the entire grid-connected system has two coordinate systems: one is the controller dq coordinate system, denoted by the superscript "c"; the other is the system dq coordinate system, denoted by the superscript "g". Furthermore, the following transformation relationship exists between the two coordinate systems.

[0063]

[0064]

[0065] In the above formula, the variable x can represent voltage, current, duty cycle, etc.

[0066] For example, S11 specifically includes:

[0067] S110. Based on the delay element response, inverter modulation wave response, and positive feedback element response in the small-signal model, the equivalent output impedance of the grid-connected inverter system in the controller dq coordinate system and the common access point voltage in the controller dq coordinate system are obtained.

[0068] S111. Select the common access point voltage in the controller dq coordinate system as the orientation to obtain the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system.

[0069] To analyze the synchronous stability of a grid-connected inverter system under weak grid conditions, the control circuit and main circuit models of the grid-connected inverter system need to be constructed in the controller's dq coordinate system. However, when establishing the main circuit model in the controller's dq coordinate system, since the modulation signal needs to undergo coordinate transformation, the influence of small perturbations in the PLL at the modulation signal must be considered. The specific process can be expressed as follows:

[0070]

[0071] Based on the above analysis, we can Figure 2 The equivalent control structure of the grid-connected inverter system in the dq domain is obtained, resulting in the small-signal control block diagram of the grid-connected inverter system in the controller dq coordinate system, as follows: Figure 5 As shown.

[0072] In this context, bold letters represent matrix forms. Figure 5 In the middle, G i (s) is the current inner loop controller, G de (s) is a delay element, K PWM For the equivalent element of the inverter modulation wave, G PF (s) represents the positive feedback loop introduced at the modulation signal by the PCC point voltage. According to Figure 5 From the small-signal model of the inverter system, the equivalent output impedance of the grid-connected inverter in the control dq coordinate system can be derived as follows:

[0073]

[0074] Meanwhile, based on the relationship between the PCC point voltage and the grid voltage, and choosing the PCC point voltage in the controller's dq coordinate system as the orientation, we can obtain:

[0075]

[0076]

[0077] Where, Δδ=Δθ g -Δθ.

[0078] Combining equations (5) and (6), we can obtain the following relationship:

[0079]

[0080] Based on the above analysis, the expression for the PLL latching q-axis voltage can be derived as follows:

[0081]

[0082] For example, the grid-connected inverter system's small-signal model for grid synchronization in the control dq coordinate system specifically includes:

[0083] The main circuit response, the phase-locked loop control circuit response, the latching voltage of the phase-locked loop control circuit, and the phase angle of the common access point voltage in the grid dq coordinate system.

[0084] Combining equations (2) and (8), the small-signal control block diagram for grid synchronization stability can be obtained as follows: Figure 6 As shown. In Figure 6 In the PLL control loop, there is a T(s) and a PLL controller. T(s) includes the grid impedance and the LCL filtering, current control, and PWM components of the grid-connected inverter. Therefore, based on the obtained small-signal model, the function of other parts of the PLL grid-connected inverter can be analyzed, thereby analyzing the stability of the grid-connected inverter system.

[0085] For example, S13 specifically includes:

[0086] S130. Based on the grid synchronization small-signal model of the grid-connected inverter, the amplitude-frequency characteristic curve of the main circuit response and the amplitude-frequency characteristic curve of the phase-locked loop control circuit response are calculated.

[0087] S131. Obtain the crossover frequency based on the amplitude-frequency response curve of the main circuit and the amplitude-frequency response curve of the phase-locked loop control circuit; the crossover frequency is the frequency corresponding to the intersection point of the amplitude-frequency response curve of the main circuit and the amplitude-frequency response curve of the phase-locked loop control circuit.

[0088] S132. Calculate the phase margin of the grid-connected inverter system at each switching frequency.

[0089] S132. If the phase margin at all switching frequencies is greater than 0, the grid-connected inverter system is in a stable state.

[0090] For example, calculating the phase margin of the grid-connected inverter system at each switching frequency specifically includes:

[0091] Obtain the phase angle of the main circuit response and the phase angle of the phase-locked loop control circuit response at the switching frequency;

[0092] The complementary angle between the phase angle of the phase-locked loop control circuit response and the phase angle of the main circuit response is taken as the phase margin of the grid-connected inverter system.

[0093] The synchronous stability of a grid-connected inverter can be determined by the loop gain of the grid synchronization small-signal model, which can be judged by the ratio of T(s) to 1 / (PI(s) / s).

[0094]

[0095] However, it is worth noting that when using the ratio of T(s) to 1 / (PI(s) / s) in equation (9) to judge the synchronous stability of the grid-connected inverter, there are two situations.

[0096] 1) There is a crossover frequency ω between T(s) and 1 / (PI(s) / s). cr Only when PM>0 can the grid-connected inverter system maintain synchronous stability.

[0097] PM=180°-(∠PLL(jω cr )-∠T(jω cr (10)

[0098] 2) T(s) and 1 / (PI(s) / s) have multiple intersection frequencies ω cr1 ω cr2 …ω crn When, and only when PM1, PM2...PM n Only when all values ​​are greater than 0 can the grid-connected inverter system maintain synchronous stability.

[0099]

[0100] To verify the correctness of the above-mentioned single-input single-output modeling method for grid-connected inverters that considers the influence of modulation signals, Figure 7 Stability analysis results are presented using a grid-connected inverter grid synchronization model when Lg = 11mH and PLL bandwidth fPLL = 50Hz. Figure 8 It can be seen that there are three intersection points between the amplitude-frequency curves of PLL(s) and T(s) at this time, and the phase margin of the grid-connected inverter system at the three intersection frequencies is greater than 0, which means that the grid-connected inverter is in a stable state.

[0101] Figure 8 Given when L g =12mH, PLL bandwidth f PLL Stability analysis results obtained using a grid synchronization model of a grid-connected inverter at 50Hz. Figure 7It can be seen that the amplitude-frequency curves of PLL(s) and T(s) intersect at three points. However, the phases at one of these intersection points at the cutoff frequency are 97.09° and -87.62°, respectively. Therefore, the phase margin of the grid-connected inverter system at this point is PM = 180° - (97.09° + 87.62°) = -4.71° < 0, which does not meet the stability criterion, meaning that the grid-connected inverter is in an unstable state.

[0102] This demonstrates that the single-input single-output modeling method for grid-connected inverters, which considers the influence of modulation signals, proposed in this patent can accurately and effectively identify the stability of grid-connected inverter systems even under non-unity power factor conditions. Furthermore, the proposed modeling method can intuitively display the stability margin of grid-connected inverters under different grid strengths, enabling the use of methods such as impedance reshaping to improve the stability of grid-connected inverters.

[0103] Compared with existing technologies, the grid-connected inverter stability analysis method and apparatus provided in this embodiment of the invention establishes a grid synchronization small-signal model of the grid-connected inverter system, converting the multi-input multi-output model of the grid-connected inverter system in the dq domain into a single-input single-output form. Since it only needs to obtain the phase angle of the voltage at the system's common access point, the stability analysis process of the grid-connected inverter is simplified. At the same time, it considers the influence of active and reactive power on the stability of the grid-connected inverter, overcoming the technical limitations of the modeling method in the polar coordinate system.

[0104] In addition, during the establishment of the small-signal model for power grid synchronization, a positive feedback loop was introduced at the modulation signal, making the established single-input single-output model more complete.

[0105] One embodiment of this application provides a grid-connected inverter stability analysis device, including: a signal model establishment module, a latch voltage calculation module, a synchronization model establishment module, and an analysis module.

[0106] The signal model building module establishes a small-signal model of the grid-connected inverter system in the controller dq coordinate system based on the circuit structure of the grid-connected inverter system in the dq domain.

[0107] The latch voltage calculation module is used to calculate the latch voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system based on the response of the delay element, the response of the inverter modulation wave, and the response of the positive feedback element in the small-signal model.

[0108] The synchronization model establishment module obtains the grid synchronization small-signal model of the grid-connected inverter system in the control dq coordinate system based on the latching voltage of the phase-locked loop control loop, the transformation relationship between the controller dq coordinate system and the grid dq coordinate system;

[0109] The analysis module is used to calculate the loop gain based on the grid synchronization small-signal model and obtain the synchronization stability of the grid-connected inverter system; the loop gain refers to the ratio of the main circuit response to the phase-locked loop control circuit response.

[0110] For example, the analysis module is specifically used for:

[0111] Based on the grid synchronization small-signal model of the grid-connected inverter, the amplitude-frequency response curve of the main circuit and the amplitude-frequency response curve of the phase-locked loop control circuit are calculated.

[0112] The crossover frequency is obtained based on the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit; the crossover frequency is the frequency corresponding to the intersection point of the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit.

[0113] Calculate the phase margin of the grid-connected inverter system at each switching frequency;

[0114] If the phase margin at all switching frequencies is greater than 0, the grid-connected inverter system is in a stable state.

[0115] For example, calculating the phase margin of the grid-connected inverter system at each switching frequency specifically includes:

[0116] Obtain the phase angle of the main circuit response and the phase angle of the phase-locked loop control circuit response at the switching frequency;

[0117] The complementary angle between the phase angle of the phase-locked loop control circuit response and the phase angle of the main circuit response is taken as the phase margin of the grid-connected inverter system.

[0118] For example, the latch voltage calculation module is specifically used for:

[0119] Based on the response of the delay element, the inverter modulation wave response, and the positive feedback element in the small-signal model, the equivalent output impedance of the grid-connected inverter system in the controller dq coordinate system and the common access point voltage in the controller dq coordinate system are obtained.

[0120] By selecting the common access point voltage in the controller's dq coordinate system as the orientation, the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller's dq coordinate system is obtained.

[0121] For example, the grid-connected inverter system's small-signal model for grid synchronization in the control dq coordinate system specifically includes:

[0122] The main circuit response, the phase-locked loop control circuit response, the latching voltage of the phase-locked loop control circuit, and the phase angle of the common access point voltage in the grid dq coordinate system.

[0123] Compared to existing technologies, the grid-connected inverter stability analysis device provided in this embodiment of the invention establishes a grid synchronization small-signal model of the grid-connected inverter system, converting the multi-input multi-output model of the grid-connected inverter system in the dq domain into a single-input single-output form. Since it only needs to obtain the phase angle of the voltage at the system's common access point, it simplifies the stability analysis process of the grid-connected inverter. At the same time, it considers the influence of active and reactive power on the stability of the grid-connected inverter, overcoming the technical limitations of the modeling method in the polar coordinate system.

[0124] In addition, during the establishment of the small-signal model for power grid synchronization, a positive feedback loop was introduced at the modulation signal, making the established single-input single-output model more complete.

[0125] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process of the device described above can be referred to the corresponding process in the foregoing method embodiments, and will not be elaborated further here.

[0126] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for stability analysis of grid-connected inverters, characterized in that, include: Based on the circuit structure of the grid-connected inverter system in the dq coordinate system, a small-signal model of the grid-connected inverter system is established in the controller's dq coordinate system; wherein, the circuit structure of the grid-connected inverter system includes a main circuit and a control circuit; the control circuit includes a phase-locked loop control circuit; The delay response, inverter modulation wave response, and positive feedback response are obtained from the small-signal model. Based on these responses, the disturbance propagation relationship in the small-signal model is established. The delay response refers to the time delay from the generation of the modulation signal generated by the control loop to its application to the main loop. The inverter modulation wave response refers to the proportional gain generated when the modulation signal is converted into a voltage output. The positive feedback response refers to the positive feedback effect generated by the disturbance to the modulation signal caused by the phase-locked loop control loop. Based on the disturbance propagation relationship, the equivalent output impedance matrix of the grid-connected inverter system in the controller dq coordinate system is derived; The equivalent output impedance matrix is ​​combined with the pre-established grid impedance model, and the latching voltage of the phase-locked loop control circuit of the grid-connected inverter system in the controller dq coordinate system is obtained by using the preset common access point voltage as the orientation; wherein, the latching voltage represents the error input of the phase-locked loop control circuit. Based on the latching voltage of the phase-locked loop control circuit, the transformation relationship between the controller dq coordinate system and the grid dq coordinate system, the small-signal model of grid synchronization of the grid-connected inverter system in the grid dq coordinate system is obtained. The loop gain is calculated based on the small-signal model of grid synchronization to obtain the synchronization stability of the grid-connected inverter system; the loop gain refers to the ratio of the main circuit response to the phase-locked loop control circuit response.

2. The grid-connected inverter stability analysis method as described in claim 1, characterized in that, The loop gain is calculated based on the aforementioned small-signal grid synchronization model to obtain the synchronization stability of the grid-connected inverter system, specifically including: Based on the aforementioned small-signal model of power grid synchronization, the amplitude-frequency response curves of the main circuit and the phase-locked loop control circuit are calculated. The crossover frequency is obtained based on the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit; the crossover frequency is the frequency corresponding to the intersection point of the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit. Calculate the phase margin of the grid-connected inverter system at each switching frequency; wherein the specific formula for calculating the phase margin is as follows: In the formula, PM represents the phase margin. The phase angle of the phase-locked loop control loop at the crossover frequency. The phase angle of the main loop response at the junction frequency; If the phase margin at all switching frequencies is greater than 0, the grid-connected inverter system is in a stable state.

3. The grid-connected inverter stability analysis method as described in claim 1, characterized in that, The grid-connected inverter system's grid synchronization small-signal model in the grid dq coordinate system specifically includes: The main circuit response, the control circuit response, the latching voltage of the phase-locked loop control circuit, and the phase angle of the common access point voltage in the grid dq coordinate system.

4. A grid-connected inverter stability analysis device, characterized in that, include: The signal model establishment module establishes a small-signal model of the grid-connected inverter system in the controller's dq coordinate system based on the circuit structure of the grid-connected inverter system in the dq coordinate system; wherein, the circuit structure of the grid-connected inverter system includes a main circuit and a control circuit; the control circuit includes a phase-locked loop control circuit; The latch voltage calculation module is used to obtain the delay response, inverter modulation wave response, and positive feedback response from the small-signal model; establish the disturbance propagation relationship in the small-signal model based on the delay response, inverter modulation wave response, and positive feedback response; wherein, the delay response refers to the time delay from the generation of the modulation signal generated by the control loop to its effect on the main loop; the inverter modulation wave response refers to the proportional gain generated when the modulation signal is converted into voltage output; the positive feedback response refers to the positive feedback effect generated by the disturbance of the modulation signal by the phase-locked loop control loop; based on the disturbance propagation relationship, derive the equivalent output impedance matrix of the grid-connected inverter system in the controller dq coordinate system; combine the equivalent output impedance matrix with the pre-established grid impedance model, and solve for the latch voltage of the phase-locked loop control loop in the controller dq coordinate system with the preset common access point voltage in the controller dq coordinate system as the orientation; wherein, the latch voltage represents the error input of the phase-locked loop control loop; The synchronization model establishment module obtains the grid synchronization small-signal model of the grid-connected inverter system in the grid dq coordinate system based on the latching voltage of the phase-locked loop control loop, the transformation relationship between the controller dq coordinate system and the grid dq coordinate system; The analysis module is used to calculate the loop gain based on the grid synchronization small-signal model and obtain the synchronization stability of the grid-connected inverter system; the loop gain refers to the ratio of the main circuit response to the phase-locked loop control circuit response.

5. The grid-connected inverter stability analysis device as described in claim 4, characterized in that, The analysis module is specifically used for: Based on the aforementioned small-signal model of power grid synchronization, the amplitude-frequency response curves of the main circuit and the phase-locked loop control circuit are calculated. The crossover frequency is obtained based on the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit; the crossover frequency is the frequency corresponding to the intersection point of the amplitude-frequency response characteristic curve of the main circuit and the amplitude-frequency response characteristic curve of the phase-locked loop control circuit. Calculate the phase margin of the grid-connected inverter system at each switching frequency; wherein the specific formula for calculating the phase margin is as follows: In the formula, PM represents the phase margin. The phase angle of the phase-locked loop control loop at the crossover frequency. The phase angle of the main loop response at the junction frequency; If the phase margin at all switching frequencies is greater than 0, the grid-connected inverter system is in a stable state.

6. The grid-connected inverter stability analysis device as described in claim 4, characterized in that, The grid-connected inverter system's grid synchronization small-signal model in the grid dq coordinate system specifically includes: The main circuit response, the phase-locked loop control circuit response, the latching voltage of the phase-locked loop control circuit, and the phase angle of the common access point voltage in the grid dq coordinate system.

Images