Strong series compensation grid-connected stability regulation and control method and related device of grid-forming type energy storage converter

Abstract

The invention provides a strong series compensation grid-connected stability regulation and control method of a grid-forming type energy storage converter and a related device, and belongs to the technical field of grid connection of a power grid. According to the method, a three-phase grid-connected inverter complete sequence impedance model controlled by the virtual synchronous generator is established by using a harmonic linearization method, and reasons for causing subsynchronous oscillation by using a double-closed-loop control strategy are analyzed. In order to improve the grid-connected stability of a strong series compensation power grid of a grid-forming type energy storage converter, a PI-LADRC control strategy is provided, a sequence impedance model under the control mode is established, and grid-connected stability margin information under the PI-LADRC control strategy is obtained by comparing output impedance characteristics of a grid-connected inverter under a double closed-loop control mode and a PI-LADRC control mode. According to the invention, the PI-LADRC control strategy is adopted, so that the grid-connected stability can be effectively regulated and controlled.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid connection of power grids, and particularly relates to a method and related device for regulating and controlling the grid connection stability of a network-forming energy storage converter with strong series compensation. Background Art

[0002] With the intensification of the environmental and energy crises and the progress of power electronics technology, the development of new energy sources such as wind energy and solar energy in distributed microgrids has been promoted. The application of power electronic devices has improved the control flexibility of microgrids, but also reduced the system inertia and damping, affecting the stability. Therefore, researchers have developed the virtual synchronous generator (VSG) control method to simulate the functions of synchronous generators, provide inertia support, and enhance system stability. The VSG control strategy enables the energy storage converter to play a key role in power systems with a high proportion of new energy access, improving system stability, and thus is widely used in distributed power grids and power systems with a high proportion of renewable energy access.

[0003] Currently, the most commonly used control method for the virtual synchronous generator VSG is the voltage-current double closed-loop control based on a PI regulator, which has a simple structure and wide application. To improve the transmission efficiency of new energy power systems and reduce power and voltage losses, reactive power compensation devices, including series capacitor compensation, are installed and operated in the system. However, the VSG with PI-PI control is prone to sub-synchronous oscillation when connected to a series compensation circuit, resulting in system instability, and even equipment failure or unit disconnection from the grid. Oscillation accidents caused by series compensation have occurred in systems at home and abroad, so it is of great significance to study this problem.

[0004] Sub-synchronous oscillation is mainly caused by the interaction between the output impedance of the virtual synchronous generator VSG and the low-frequency impedance of the series-compensated power grid. The solution is to adjust the low-frequency output impedance of the VSG. Currently, by introducing virtual impedance to change the output characteristics of the inverter, the stability of the grid-connected system is enhanced. However, virtual impedance will increase the control complexity and may cause voltage drop. The network-forming energy storage converter is very important in VSG control. It optimizes the operation of the power grid through energy management, improving the stability and reliability of power systems with a high proportion of new energy access. Through the above analysis, the problems and defects existing in the prior art are as follows:

[0005] The network-forming energy storage converter has good small-signal stability under a relatively weak series-compensated power grid, but its small-signal stability will decrease as the strength of the series-compensated power grid increases, and there may even be a risk of sub-synchronous oscillation causing instability. Moreover, the grid connection stability of the existing network-forming energy storage converter connected to a strong series-compensated power grid is poor. Summary of the Invention

[0006] To overcome the problems existing in the related art, the present invention provides a method and related device for regulating and controlling the grid connection stability of a network-forming energy storage converter with strong series compensation.

[0007] To achieve the above object, the technical solution provided by the present invention is as follows:

[0008] In a first aspect, the present invention provides a method for regulating the grid connection stability of a network-forming energy storage converter with strong series compensation, including the following steps:

[0009] S1. Use the harmonic linearization method to establish a complete sequence impedance model of a three-phase grid-connected inverter controlled by a virtual synchronous generator, and obtain the information on the instability of sub / super-synchronous oscillations caused by small signals under the increase of the grid series compensation strength in the double-loop PI control strategy;

[0010] S2. Based on the hybrid control strategy of proportional integral PI and linear active disturbance rejection control LADRC, that is, the PI-LADRC control strategy, establish a sequence impedance model under the PI-LADRC control strategy;

[0011] S3. Based on the information on the instability of sub / super-synchronous oscillations, compare the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the differences in grid connection characteristics under the double-loop PI control strategy and the PI-LADRC control strategy, and obtain the information on the grid connection stability margin of the network-forming energy storage converter with strong series compensation under the PI-LADRC control strategy.

[0012] Further, in step S1, using the harmonic linearization method to establish a complete sequence impedance model of a three-phase grid-connected inverter controlled by a virtual synchronous generator includes:

[0013] Model the power loop through the harmonic linearization method and perform double-loop modeling of voltage and current. Combine the power loop model with the double-loop model to obtain the positive sequence output impedance of the inverter;

[0014] Calculate the negative sequence impedance based on the positive sequence output impedance.

[0015] Further, the information on the instability of sub / super-synchronous oscillations is determined according to the impedance stability criterion, and the impedance stability criterion is:

[0016] The phase margin at the intersection of the output impedance of the virtual synchronous generator VSG and the grid impedance should be greater than 0°; the expression of the phase margin at the intersection is:

[0017]

[0018] In the formula, is the phase margin, and are the positive sequence impedances of the grid impedance and the inverter output under PI-PI control respectively.

[0019] Further, establishing a sequence impedance model under the PI-LADRC control strategy includes:

[0020] Construct the mathematical model of the virtual synchronous generator;

[0021] Design the LADRC control strategy based on the mathematical model of the virtual synchronous generator, and combine it with the proportional-integral (PI) controller to obtain the PI-LADRC control strategy;

[0022] Derive the positive-sequence and negative-sequence impedance models based on the mathematical model and the PI-LADRC control strategy.

[0023] Furthermore, constructing the mathematical model of the virtual synchronous generator includes:

[0024] Select the filter capacitor voltage and the output current as state variables, and obtain the state-space equations of the virtual synchronous generator (VSG) inverter in the dq coordinate system through Clark and Park transformations based on Kirchhoff's current law and Kirchhoff's voltage law;

[0025] Left-multiply the state-space equations of the VSG inverter by the Park matrix to obtain the space equations in the dq coordinate system, which are used as the mathematical model of the virtual synchronous generator.

[0026] Furthermore, designing the LADRC control strategy includes designing the equivalent form of the first-order LADRC as follows:

[0027]

[0028] where is the control signal, , , represents the given signal, y represents the output of the controlled object, represents the control gain of the system input, s is the complex frequency domain variable, is the bandwidth of the first-order extended state observer, is the bandwidth of the PD controller.

[0029] Furthermore, in the comparison of the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the grid connection characteristics under the double closed-loop control strategy and the PI-LADRC control strategy, the analysis of the output impedance characteristics of the grid-connected inverter under the PI-LADRC control strategy includes:

[0030] In the PI-LADRC control, determine the small-signal components of the reference values of the three-phase inductor currents in the dq coordinate system and the expressions of the small-signal components of the three-phase inverter voltages in the dq coordinate system through the equivalent form of the first-order LADRC; where, is used as the feedforward compensation transfer function, is used as the feedback compensation transfer function;

[0031] Based on the determined small-signal component expression and combined with circuit parameters, the positive-sequence output impedance expression and negative-sequence impedance expression of PI-LADRC control are obtained for analyzing the output impedance characteristics of the grid-connected inverter under the PI-LADRC control strategy.

[0032] In a second aspect, the present invention provides a strong series compensation grid connection stability regulation device for a grid-forming energy storage converter, including:

[0033] A three-phase grid-connected inverter complete sequence impedance module, which is used to establish a complete sequence impedance model of the three-phase grid-connected inverter controlled by a virtual synchronous generator by using the harmonic linearization method, and obtain the sub / super-synchronous oscillation instability information caused by small signals under the increase of the grid series compensation intensity under the double-loop PI control strategy;

[0034] A PI-LADRC control strategy module, which is used to establish a sequence impedance model under the PI-LADRC control strategy based on the hybrid control strategy of proportional integral PI and linear active disturbance rejection control LADRC, that is, the PI-LADRC control strategy;

[0035] A comparison and verification module, which is used to compare the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the differences in grid connection characteristics under the double-loop PI control strategy and the PI-LADRC control strategy based on the sub / super-synchronous oscillation instability information, and obtain the strong series compensation grid connection stability margin information of the grid-forming energy storage converter under the PI-LADRC control strategy.

[0036] In a third aspect, the present invention provides a computer device, which includes a processor and a memory:

[0037] The memory is used to store a computer program and send the instructions of the computer program to the processor;

[0038] The processor executes a strong series compensation grid connection stability regulation method for a grid-forming energy storage converter as described in the first aspect according to the instructions of the computer program.

[0039] In a fourth aspect, the present invention provides a computer-readable storage medium, on which a computer program is stored, and when the computer program is executed by a processor, it implements a strong series compensation grid connection stability regulation method for a grid-forming energy storage converter as described in the first aspect.

[0040] In summary, the present invention provides a method and related device for regulating the grid connection stability of a network-forming energy storage converter. By using the harmonic linearization method, a complete sequence impedance model of a three-phase grid-connected inverter with virtual synchronous generator control is established, and the reason for the sub-synchronous oscillation caused by the double closed-loop control strategy is analyzed. To improve the grid connection stability of the network-forming energy storage converter with strong series compensation, a PI-LADRC control strategy is proposed, and the sequence impedance model under this control mode is established. By comparing the output impedance characteristics of the grid-connected inverter under the double closed-loop control and the PI-LADRC control modes, the grid connection stability margin information under the PI-LADRC control strategy is obtained. The present invention proposes that the PI-LADRC control strategy can effectively regulate the grid connection stability. Description of the Drawings

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0042] Figure 1 Schematic diagram of the PI-LADRC hybrid control strategy for the network-forming energy storage converter provided by the embodiment of the present invention;

[0043] Figure 2 is f provided by the embodiment of the present invention p Small-signal equivalent circuit diagram of frequency;

[0044] Figure 3 is f provided by the embodiment of the present invention p -2f 0 Small-signal equivalent circuit diagram of coupling frequency;

[0045] Figure 4 Positive and negative sequence impedance models and scanning curve diagrams of the virtual synchronous generator VSG with traditional double closed-loop control provided by the embodiment of the present invention;

[0046] Figure 5 Circuit model of the series-compensated power grid provided by the embodiment of the present invention;

[0047] Figure 6 Curve diagram of the impedance characteristics of the inverter and the power grid with traditional double closed-loop control under the power grid conditions of SCR = 4 and SCL = 0.3 provided by the embodiment of the present invention;

[0048] Figure 7 Curve diagram of the impedance characteristics of the inverter and the power grid with traditional double closed-loop control under the power grid conditions of SCR = 2 and SCL = 0.4 provided by the embodiment of the present invention;

[0049] Figure 8 is the first-order LADRC control framework provided by the embodiments of the present invention;

[0050] Figure 9 is the equivalent form of the first-order LADRC provided by the embodiments of the present invention;

[0051] Figure 10 is the curve graph of the inverter and grid impedance characteristics under the grid conditions of SCL = 0.3 and SCR = 4, 5 with PI-LADRC control provided by the embodiments of the present invention;

[0052] Figure 11 is the curve graph of the inverter and grid impedance characteristics under the grid conditions of SCR = 2 and SCL = 0.4, 0.5 with PI-LADRC control provided by the embodiments of the present invention;

[0053] Figure 12 is the output current of the inverter with traditional double closed-loop control under the grid conditions of SCR = 2, 3, 4 and SCL = 0.3 provided by the embodiments of the present invention;

[0054] Figure 13 is the output current of the inverter with traditional double closed-loop control under the grid conditions of SCR = 2 and SCL = 0.2, 0.3, 0.4 provided by the embodiments of the present invention;

[0055] Figure 14 is the output current of the inverter with PI-LADRC control under the grid conditions of SCR = 3, 4, 5 and SCL = 0.3 provided by the embodiments of the present invention;

[0056] Figure 15 is the output current of the inverter with PI-LADRC control under the grid conditions of SCR = 2 and SCL = 0.3, 0.4, 0.5 provided by the embodiments of the present invention. Detailed implementation manners

[0057] To make the objectives, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0058] The embodiments of the present invention provide a method for regulating the grid-connected stability of a grid-forming energy storage converter with strong series compensation, including the following steps:

[0059] S1. Use the harmonic linearization method to establish a complete sequence impedance model of a three-phase grid-connected inverter with virtual synchronous generator control, and obtain the instability information of sub- / supra-synchronous oscillation caused by small signals under the condition of increasing the grid series compensation strength for the double-loop PI control strategy.

[0060] It should be noted that in the power system, the three-phase grid-connected inverter with virtual synchronous generator (VSG) control is a key part of the network-forming energy storage converter. The harmonic linearization method is a mathematical means that can approximate the complex nonlinear characteristics of the inverter as a linear relationship under certain conditions for easy analysis. The complete sequence impedance model established by this method covers positive sequence, negative sequence, and zero sequence impedances, comprehensively describing the electrical characteristics of the inverter at different frequencies.

[0061] The double-loop PI control strategy is relatively common in the control of power electronic devices. It consists of a current inner loop and a voltage outer loop and is used to regulate the output of the inverter. When the grid series compensation strength increases, the electrical parameters of the system change, and small-signal disturbances may trigger sub- / supra-synchronous oscillations. In this step, by analyzing and calculating the established model above, the relevant information on the instability of sub- / supra-synchronous oscillations caused by the double-loop PI control strategy in this case is obtained.

[0062] S2. Based on the hybrid control strategy of proportional-integral (PI) and linear active disturbance rejection control (LADRC), namely the PI-LADRC control strategy, establish a sequence impedance model under the PI-LADRC control strategy.

[0063] It should be noted that proportional-integral (PI) control can adjust the steady-state error of the system, while linear active disturbance rejection control (LADRC) has strong anti-interference ability and the ability to compensate for system uncertainties. Combining the two to form the PI-LADRC hybrid control strategy aims to integrate the advantages of both to improve system performance.

[0064] Based on the PI-LADRC control strategy, using relevant knowledge of circuit theory and control theory, establish a sequence impedance model under this control strategy. This model reflects the characteristics of the inverter output impedance under different sequence components under the new control strategy, providing a model basis for subsequent comparative analysis of the influence of different control strategies on the system.

[0065] S3. Based on the above-mentioned sub- / supra-synchronous oscillation instability information, compare the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the differences in grid connection characteristics under the double-loop PI control strategy and the PI-LADRC control strategy, and obtain the grid connection stability margin information of the network-forming energy storage converter under the PI-LADRC control strategy in a strongly series-compensated grid.

[0066] It should be noted that, with the sub- / supra-synchronous oscillation instability information obtained in step S1 as a reference, the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance under the double-loop PI control strategy and the PI-LADRC control strategy is analyzed respectively. For example, the changes in parameters such as the phase difference and amplitude ratio of the impedances of the two under different control strategies, as well as the differences in grid connection characteristics such as power transmission stability and voltage stability, are studied. These differences can intuitively reflect the influence of different control strategies on system stability.

[0067] Through the above comparative analysis, the stability margin information of the grid-forming energy storage converter when connected to the grid in a strongly series-compensated grid under the PI-LADRC control strategy is evaluated. The stability margin is an index to measure the ability of the system to maintain stable operation after being disturbed. Obtaining this information helps to judge whether the new control strategy can effectively improve the stability of the system under complex grid conditions and provides an important basis for practical engineering applications.

[0068] This embodiment provides a hybrid control strategy for a grid-forming energy storage converter based on proportional-integral (PI) and linear active disturbance rejection control (LADRC), where the voltage outer loop uses proportional-integral (PI) control and the current inner loop uses linear active disturbance rejection control. This strategy combines the strong anti-interference ability of LADRC and the fast tracking characteristics of PI control to solve the instability problem caused by sub-synchronous oscillation of the traditional double-loop controlled grid-forming converter under strongly series-compensated grid conditions. By comparing the output impedance characteristics of the grid-connected inverter and its interaction with the grid impedance under the traditional double-loop control and the proposed control strategy, the grid connection performance differences between the two strategies are analyzed. The proposed strategy significantly enhances the stability margin of the grid-forming converter under strongly series-compensated grid conditions. With the access of renewable energy, the energy storage converter needs to remain stable in a strongly series-compensated grid environment. The proposed control strategy can effectively improve the grid connection stability and current waveform of the energy storage converter and enhance its adaptability in the grid.

[0069] The following combines Figures 1-15 to introduce some other embodiments of the present invention.

[0070] As Figure 1 shown, in the control of the virtual synchronous generator. Distributed energy sources such as batteries and photovoltaic panels are equivalently represented as a DC power supply, denoted as . and are the filter inductor and capacitor respectively. and are the series equivalent resistances of the filter inductor and capacitor respectively. The currents flowing through the three-phase inductors are represented as , and , while , and represent the three-phase currents at the output terminals of the inverter, , and represent the three-phase voltages on the inverter side, , and represent the three-phase voltages at the output terminals of the inverter, and represent the grid-side filter inductance and equivalent impedance respectively. The virtual synchronous generator control loop includes a power outer loop, a voltage loop, and a current inner loop. The power outer loop collects the voltage and current outputs of the inverter, calculates the active power and reactive power output by the inverter, and uses them as input variables for the virtual synchronous generator control loop. Through virtual synchronous generator control, the reference voltage amplitude E and phase angle can be obtained, and then the reference voltage for voltage loop control is formed. The voltage loop in this study adopts a proportional-integral control algorithm to ensure that the output voltage (x = d, q) can closely track the reference voltage (x = d, q), and outputs the reference value (x = d, q) of the current loop. The current loop uses a linear active disturbance rejection control algorithm to ensure that the inductor current (x = d, q) closely tracks the reference current (x = d, q), and outputs the inverter modulation voltage (x = d, q).

[0071] In one embodiment, a method for establishing a complete sequence impedance model of a double-loop PI control for a three-phase grid-connected inverter with virtual synchronous generator control using the harmonic linearization method is proposed as follows:

[0072] The traditional double closed-loop control strategy adopts double-loop PI control, and the control expressions are as follows:

[0073] (1)

[0074] The sequence impedance modeling of the virtual synchronous generator VSG grid-connected inverter needs to consider the double mirror frequency coupling effect. When the inverter injects a positive sequence disturbance at a frequency of f p , positive and negative sequence responses will be generated at frequencies of f p and f p - 2f 0 respectively. Similarly, when a negative sequence disturbance is injected at a frequency of f p , positive and negative sequence responses will be generated at frequencies of f p and f p + 2f 0Negative and positive sequence responses are generated respectively. Taking phase A as an example, when a positive sequence harmonic disturbance of the injection frequency is applied to the grid side, the equivalent small-signal model of the main circuit is as Figure 2 the f of p the small-signal equivalent circuit diagram of the frequency Figure 3 is the coupling frequency f p -2f 0 the small-signal equivalent circuit diagram.

[0075] Among them, , , and are , , ,. For the positive and negative sequence responses, is the positive sequence voltage harmonic disturbance with a frequency of injected into the grid side. According to Figure 2 , Figure 3 shown, when a positive sequence voltage small-signal disturbance is injected into the grid side, due to the existence of mirror frequency coupling, the time-domain expressions of the small-signal components of the output voltage, grid-connected current, and output current of phase A are:

[0076] (2)

[0077] Among them , and are the disturbance angular frequency and the negative sequence coupling angular frequency respectively; , ; , and , are the amplitudes of the output voltage, output current, and inductor current at the positive sequence disturbance frequency and the negative sequence coupling frequency respectively; , ; , and , are the initial phase angles of the output voltage, output current, and inductor current at the positive sequence disturbance frequency and the negative sequence coupling frequency respectively.

[0078] After the output voltage, output current, and inductor current undergo Park transformation, the steady-state component of the output voltage, , and the small-signal component , , the steady-state component , of the output current, , , and the steady-state component of the grid-connected current , and the small-signal component , . Their expressions are as follows:

[0079]

[0080] (3)

[0081]

[0082] (4)

[0083]

[0084] (5)

[0085] Among them, and are the initial phase angle and the small-signal perturbation of the phase angle, respectively.

[0086] The small-signal components of the active / reactive power output by the virtual synchronous generator VSG converter are shown in Equation (6):

[0087] (6)

[0088] Among them, is the transfer function of the low-pass filter, is the cut-off frequency of the low-pass filter.

[0089] The core of the active power-frequency control of the virtual synchronous generator VSG grid-connected inverter is to utilize droop control to simulate the frequency response characteristics of the synchronous machine, and at the same time introduce the rotor motion equation to simulate the inertia and damping characteristics of the synchronous machine rotor. The reactive power-voltage control is adjusted through reactive power droop control. The specific expression is shown in Equation (7):

[0090] (7)

[0091] In Equation (7), and are the moment of inertia and the damping coefficient, respectively, represents the reactive-voltage regulation coefficient, is the angular frequency output by the virtual synchronous generator, and are the reference values of the small-signal components of the output voltage in the coordinate system. Due to the existence of voltage and current perturbations, phase perturbations will occur in the system. Therefore, the output phase angle of the active power loop can be expressed as , where and respectively represent the steady-state component and the small-signal perturbation component of the fundamental phase angle.

[0092] The small-signal output of the power outer loop will enter the voltage and current inner loops. Therefore, the small-signal output after loop control is:

[0093] (8)

[0094] where and are the voltage-current double closed-loop PI controllers, and are the small-signal components of the output voltage of the double PI control loop on the dq axes.

[0095] Taking phase A as an example, the small-signal output of the loop is obtained :

[0096] (9)

[0097] contains small-signal components of two frequencies. Combining equations (3) to (9), the expressions of and are as follows:

[0098] (10)

[0099] where X 1 (s) - X 8 (s) is expressed as shown in A1:

[0100] (A1)

[0101] In the formula:

[0102]

[0103]

[0104]

[0105]

[0106]

[0107] From Figure 3 and equation (10), the mirror-coupled current is as shown in equation (11):

[0108] (11)

[0109] Among them, the variables , and are shown as in A(2):

[0110] (A2)

[0111] Substitute the image frequency current obtained in Equation (16) into Equation (14), and combine with Figure 3 the result of

[0112] (12)

[0113] Among them, the variables , and are given in A(3).

[0114] (A3)

[0115] The derivation process of the negative sequence impedance of the inverter is similar to that of the positive sequence impedance. Using the conversion relationship between the positive sequence impedance and the negative sequence impedance of the inverter output, the formula for the negative sequence impedance is obtained as follows: .

[0116] The theoretical values of the positive and negative sequence output impedances are as shown by Figure 4 the solid line. In order to verify the accuracy of the impedance modeling, the sweep frequency method is used to measure the output impedance. It can be seen from Figure 4 that the theoretical values are basically in agreement with the sweep frequency points, verifying the accuracy of the impedance modeling.

[0117] In one embodiment, Figure 5 the equivalent circuit of the series-compensated power grid and its equivalent impedance model are shown as follows:

[0118] (13)

[0119] The compensation ability of the series-compensated power grid is usually measured by the series compensation level (SCL). Usually, the maximum SCL of the series-compensated power grid is about 50% to 60%. At this time, the power grid belongs to a strongly series-compensated power grid. The expression of SCL is:

[0120] (14)

[0121] The short-circuit ratio (SCR) of the power grid may change due to changes in power grid operation and power transmission. As the SCR changes, the series compensation level will be adjusted accordingly. Therefore, it is crucial to study the grid connection stability of the power grid after adding series compensation capacitors under different SCR conditions. The calculation formula of SCR is as follows:

[0122] (15)

[0123] Among them, represents the short-circuit capacity of the power grid, represents the rated power, is the effective value of the rated phase voltage of the power grid, represents the equivalent impedance of the power grid.

[0124] Series compensation devices are usually used in weak power grids with a low short-circuit ratio (SCR). Therefore, research is carried out on weak power grids with an SCR below 5. Figure 4 It shows that the virtual synchronous generator VSG inverter equipped with a dual-PI loop control system exhibits capacitive positive-sequence output impedance and inductive negative-sequence output impedance characteristics in the sub-synchronous frequency range below the fundamental frequency. The capacitive characteristic of the positive-sequence output impedance of the virtual synchronous generator VSG will interact with the inductive impedance of the power grid.

[0125] According to the stability requirements, the phase margin at the intersection of the output impedance of the virtual synchronous generator VSG and the power grid impedance should be greater than 0°. The expression of the phase margin at the intersection is:

[0126] (16)

[0127] When the phase margin at the intersection of the inverter output impedance and the power grid impedance is insufficient, it is easy to cause system oscillation instability; as Figure 6 and 7 shown in the impedance characteristic curves of the inverter and the power grid under different series compensation power grid conditions, when the power grid impedance is SCR = 4, SCL = 0.3 and SCR = 2, SCL = 0.4, the power grid impedance interacts with the impedance of the inverter using double closed-loop control in the sub-synchronous frequency band, and the impedance amplitude characteristic curves intersect at 31.3 Hz and 33 Hz respectively. According to formula (16), the phase margins at the intersections are and respectively, and the phase margins are both negative values, not meeting the robust stability conditions.

[0128] In one embodiment, a method for establishing an impedance model under the PI-LADRC control strategy is provided. The mathematical model of the inverter based on the virtual synchronous generator VSG is:

[0129] (17)

[0130] In Equation (17), t is time.

[0131] Left-multiplying Equation (17) by the Park matrix gives:

[0132] (18)

[0133] Rearranging (18) gives:

[0134] (19)

[0135] The d-axis of Equation (19) is a first-order mathematical model in the time domain. Therefore, a first-order LADRC controller needs to be designed. For a first-order controlled object, its control equation is:

[0136] (20)

[0137] Where represents the control gain of the system input, represents the output of the controller, y represents the output of the controlled object, and f represents the total disturbance.

[0138] Let , and the first-order extended state space equation is:

[0139] (21)

[0140] Where: .

[0141] The first-order extended state observer is expressed as follows:

[0142] (22)

[0143] Where , is the gain of the extended state observer, , are the estimated state variables respectively. By selecting appropriate , the LESO can achieve , for , real-time tracking with a small steady-state error.

[0144] (23)

[0145] Based on further derivation of Equation (23), the characteristic equation of the LESO can be expressed as:

[0146] (24)

[0147] Among them , is the bandwidth of LESO.

[0148] The PD controller is designed as:

[0149] (25)

[0150] Among them, represents the given signal, is the controller gain, is the bandwidth of the controller.

[0151] (26)

[0152] According to equations (23)-(25), the control block diagram of the first-order LADRC is as shown in Figure 8 , and the equivalent form of the first-order LADRC is as shown in Figure 9 , where P(s) is the controlled object.

[0153] In one embodiment, designing the LADRC control strategy includes designing the equivalent form of the first-order LADRC.

[0154] According to (22)-(25), the equivalent form of the first-order LADRC is as follows:

[0155] (27)

[0156] In equation (27): , .

[0157] When designing the LADRC control strategy in this embodiment, by transforming the LADRC control strategy into the equivalent form as above, it provides a standardized framework for analyzing the relationship between the subsequent control signal , the given signal and the output y, enabling the quantitative analysis of the system dynamic characteristics under PI-LADRC control.

[0158] This equivalent form can provide a theoretical bridge for deriving the sequence impedance model under the PI-LADRC control mode. By integrating the equivalent form of the control strategy into small-signal analysis (such as the small-signal component expressions of three-phase inductor current and inverter voltage), the sequence impedance model can accurately reflect the influence of the control strategy on the impedance characteristics, and further reveal the interaction between the output impedance of the grid-connected inverter and the grid impedance.

[0159] In a further embodiment, in comparing the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the differences in grid-connection characteristics under the double closed-loop control strategy and the PI-LADRC control strategy, the analysis of the output impedance characteristics of the grid-connected inverter under the PI-LADRC control strategy includes:

[0160] In PI-LADRC control, the small-signal components of the reference values of the three-phase inductor currents in the dq coordinate system and the expressions of the small-signal components of the three-phase inverter voltages in the dq coordinate system are determined through the first-order LADRC equivalent form; where As the feedforward compensation transfer function, As the feedback compensation transfer function;

[0161] Based on the determined small-signal component expressions and combined with the circuit parameters, the positive-sequence output impedance expression and the negative-sequence impedance expression of the PI-LADRC control are obtained for analyzing the output impedance characteristics of the grid-connected inverter under the PI-LADRC control strategy.

[0162] For PI-LADRC control, and The expressions of the small-signal components are as follows:

[0163] (28)

[0164] The positive-sequence output impedance expression of the PI-LADRC control is obtained as follows:

[0165] (29)

[0166] In the formula, , , , , , The expression of

[0167] A (4)

[0168] In the formula, the variable - The expression of is as follows:

[0169] A (5)

[0170] Where:

[0171]

[0172]

[0173] Based on the conversion relationship between the positive-sequence and negative-sequence impedances of the inverter output, the formula for the negative-sequence impedance can be derived. .

[0174] In this embodiment, the equivalent forms and , as the core part of the LADRC control strategy, directly participate in the construction of the sequence impedance model under the PI-LADRC control strategy. In the positive-sequence output impedance , directly participate in the parameter operations of the numerator and denominator. Through coupling with other intermediate variables ( , , etc.) and system parameters ( , , etc.), they determine the amplitude and phase characteristics of the positive-sequence output impedance, reflecting the regulation effect of the control strategy on the stability of the positive-sequence impedance.

[0175] Since the negative-sequence impedance is determined based on the positive-sequence output impedance, and 's conjugate characteristics indirectly affect the calculation of the negative-sequence impedance through the substitution of the complex frequency domain variable s. Its essence is to map the control effect of the positive-sequence impedance model to the negative-sequence scenario through conjugate operation, ensuring that both the positive-sequence and negative-sequence impedance models can reflect the consistent regulation of the impedance characteristics by the LADRC control strategy, providing a complete basis for analyzing the stability of the grid-connected system under different sequence components.

[0176] The positive-sequence impedance of the virtual synchronous generator VSG with PI-LADRC hybrid control and the grid impedance are as shown in Figure 10 , the impedance characteristic curves of the inverter and the grid under different series-compensated power grid conditions.

[0177] Comparing Figure 6 and Figure 10 the inverter output impedance characteristic curves, it can be seen that the phase margin of the positive-sequence impedance of the virtual synchronous generator VSG inverter with PI-LADRC control is improved in the subsynchronous frequency band near the fundamental frequency, and the anti-interference ability and robustness are enhanced. As shown in Figure 10 the impedance characteristic curves of the inverter and the grid, when SCL = 0.3, SCR = 4, 5, the impedance amplitude characteristic curves intersect at 39 Hz and 41 Hz respectively. According to formula (21), the phase margins at the intersection points are 6° and 5° respectively, and the phase margins are both greater than 0. The grid-connected system is stable. Comparing Figure 7 and Figure 11 the inverter output impedance characteristic curves, it can be seen that the phase margin of the positive-sequence impedance of the virtual synchronous generator VSG inverter with PI-LADRC control is improved in the subsynchronous frequency band near the fundamental frequency, and the anti-interference ability and robustness are enhanced. As shown in Figure 11As shown in the impedance characteristic curves of the inverter and the grid, when SCR = 2, SCR = 0.4, and 0.5, the impedance amplitude characteristic curves intersect at 37 Hz and 40 Hz respectively. According to formula (21), the phase margin at the intersection point is 6°, and the phase margin is greater than 0. The grid-connected system is stable. It can be seen that the grid-forming energy storage converter using PI-LADRC control can improve the grid-connected stability under a strong series compensation grid.

[0178] Based on the same inventive concept, an embodiment of the present application also provides a device for regulating the grid-connected stability of a strong series compensation of a grid-forming energy storage converter for implementing the method for regulating the grid-connected stability of a strong series compensation of a grid-forming energy storage converter involved above. The solution provided by this device to solve the problem is similar to the solution described in the above method. Therefore, the specific limitations in the embodiment of the device for regulating the grid-connected stability of a strong series compensation of a grid-forming energy storage converter provided below can refer to the limitations on the method for regulating the grid-connected stability of a strong series compensation of a grid-forming energy storage converter in the above text, and will not be repeated here.

[0179] An embodiment of the present invention also provides a device for regulating the grid-connected stability of a strong series compensation of a grid-forming energy storage converter, including:

[0180] A three-phase grid-connected inverter complete sequence impedance module, which is used to establish a complete sequence impedance model of a three-phase grid-connected inverter with virtual synchronous generator control by using the harmonic linearization method, and obtain the sub / super-synchronous oscillation instability information caused by small signals under the increase of the grid series compensation intensity by the double-loop PI control strategy;

[0181] A PI-LADRC control strategy module, which is used to establish a sequence impedance model under the PI-LADRC control strategy based on the hybrid control strategy of proportional integral PI and linear active disturbance rejection control LADRC, that is, the PI-LADRC control strategy;

[0182] A comparison and verification module, which is used to compare the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the differences in grid-connected characteristics under the double-loop PI control strategy and the PI-LADRC control strategy based on the sub / super-synchronous oscillation instability information, and obtain the grid-connected stability margin information of the grid-forming energy storage converter with strong series compensation grid under the PI-LADRC control strategy.

[0183] To verify the superiority of the PI-LADRC control method, a three-phase virtual synchronous generator VSG inverter grid-connected system is built using an RT-LAB semi-physical experimental device. The model of the real-time digital simulator is 0P5600, and the model of the upper computer processor is 3.7GHz Intel Corei3.

[0184] Verify the adaptability of the virtual synchronous generator (VSG) with traditional dual-PI control to the change of the strength of the series-compensated power grid. The process is as follows:

[0185] Under the grid conditions of SCL = 0.3, ACR = 2 - 4 and SCR = 2, SCL = 0.2 - 0.4, the waveforms of the output current of the virtual synchronous generator (VSG) with dual-PI closed-loop control are as shown in Figure 12 , Figure 13. It can be seen from the figure that under the condition of fixed series compensation (SCL = 0.3), when SCR = 2, 3, the current is stable. As SCR increases to 4, the current begins to be unstable. When the short-circuit ratio is fixed (SCR = 2) and SCL increases from 0.2 to 0.4, the current is stable when SCL = 0.2, 0.3, but the current cannot be stabilized when SCL = 0.4, and the grid-connected system loses stability at this time. It can be seen from this that the virtual synchronous generator (VSG) with traditional dual-PI control cannot adapt to the scenario of a strongly series-compensated power grid.

[0186] Verify the adaptability of the virtual synchronous generator (VSG) with PI-LADRC control to the change of the strength of the series-compensated power grid. The process is as follows:

[0187] Under the grid conditions of SCL = 0.3, ACR = 3 - 5 and SCR = 2, SCL = 0.3 - 0.5, the waveforms of the output current of the virtual synchronous generator (VSG) with dual-PI closed-loop control are as shown in Figure 14 , Figure 15. It can be seen from the figure that under the condition of fixed series compensation (SCL = 0.3), the current remains stable as SCR increases from 3 to 5. When the short-circuit ratio is fixed (SCR = 2) and SCL increases from 0.3 to 0.5, the current remains stable, and the grid-connected system is stable at this time. It can be seen from this that the virtual synchronous generator (VSG) with PI-LADRC control retains its grid-connected stability under a weakly series-compensated power grid while improving the grid-connected stability of a strongly series-compensated power grid, proving that PI-LADRC has better adaptability in the scenario of a series-compensated power grid and can significantly improve the grid-connected stability of the series-compensated power grid.

[0188] Those skilled in the art can clearly understand that, for the convenience and brevity of description, only the division of the above-mentioned functional units and modules is used as an example. In actual applications, the above-mentioned functions can be allocated to different functional units and modules as needed, that is, the internal structure of the system is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated in a processing unit, or each unit can exist physically alone, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of a software functional unit. In addition, the specific names of the functional units and modules are only for the convenience of mutual distinction and do not limit the protection scope of this application. The specific working process of the units and modules in the above system can refer to the corresponding process in the foregoing method embodiment and will not be elaborated here.

[0189] An embodiment of the present invention further provides a computer device, including: a memory, a processor, and a computer program stored on the memory. When the computer program is executed on the processor, it implements a method for regulating the grid connection stability of a grid-forming energy storage converter with strong series compensation as described in any one of the above methods.

[0190] The computer device can be a computing device such as a desktop computer, a notebook, a palm computer, and a cloud server. The computer device may include, but is not limited to, a processor and a memory.

[0191] The so-called processor may be a central processing unit (CPU), and the processor may also be other general-purpose processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0192] In some embodiments, the memory may be an internal storage unit of the computer device, such as the hard disk or memory of the computer device. In other embodiments, the memory may also be an external storage device of the computer device, such as a plug-in hard disk, a Smart Media Card (SMC), a Secure Digital (SD) card, a Flash Card, etc., equipped on the computer device. Further, the memory may also include both the internal storage unit and the external storage device of the computer device. The memory is used to store an operating system, application programs, a BootLoader, data, and other programs, such as the program code of the computer program. The memory may also be used to temporarily store data that has been output or will be output.

[0193] An embodiment of the present invention further provides a computer-readable storage medium, on which a computer program is stored. When the computer program is run by a processor, it implements a method for regulating the grid connection stability of a grid-forming energy storage converter in series compensation as described in any one of the above methods.

[0194] In this embodiment, if the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, to implement all or part of the processes in the above embodiment methods of the present application, a computer program can be used to instruct relevant hardware to complete. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by a processor, it can implement the steps of the above various method embodiments. Among them, the computer program includes computer program code, and the computer program code can be in the form of source code, object code, an executable file, or some intermediate form, etc. The computer-readable medium can at least include: any entity or device that can carry the computer program code to the photographing device / terminal device, a recording medium, a computer memory, a Read-Only Memory (ROM), a Random Access Memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium. For example, a USB flash drive, a mobile hard disk, a magnetic disk, or an optical disc, etc. In some jurisdictions, according to legislation and patent practice, the computer-readable medium cannot be an electrical carrier signal and a telecommunication signal.

[0195] In the above embodiments, the descriptions of the various embodiments have their own focuses. For parts not detailed or recorded in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0196] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed herein can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of this application.

[0197] In the embodiments disclosed in this application, it should be understood that the disclosed devices / terminal devices and methods can be implemented in other ways. For example, the device / terminal device embodiments described above are merely illustrative. For example, the division of the modules or units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed couplings or direct couplings or communication connections to each other can be through some interfaces. The indirect couplings or communication connections of the devices or units can be electrical, mechanical or other forms.

[0198] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. And these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling the grid-connected stability of a strong series compensation grid-connected energy storage converter, characterized in that: The steps include: S1, using the harmonic linearization method to establish a complete sequence impedance model of a three-phase grid-connected inverter controlled by a virtual synchronous generator, and obtain the information on the sub / supersynchronous oscillation instability caused by small signals under the double closed-loop PI control strategy with the increase of the grid series compensation intensity; S2, based on the hybrid control strategy of proportional integral PI and linear active disturbance rejection rate (LADRC), namely PI-LADRC control strategy, the sequence impedance model under PI-LADRC control strategy is established; S3, based on the sub / super synchronous oscillation instability information, compare the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the difference in grid-connected characteristics under the dual closed-loop PI control strategy and the PI-LADRC control strategy, and obtain the grid-connected stability margin information of the strong series compensation grid of the grid-connected energy storage converter under the PI-LADRC control strategy.

2. The strong series compensation grid-connected stability control method of the grid-connected energy storage converter according to claim 1 is characterized in that: In step S1, a complete sequence impedance model of a three-phase grid-connected inverter controlled by a virtual synchronous generator is established by using a harmonic linearization method, including: The power loop is modeled by the harmonic linearization method and the voltage and current double closed-loop model is performed. The power loop model is combined with the double closed-loop model to obtain the inverter positive sequence output impedance. The negative sequence impedance of the inverter is determined based on the positive sequence output impedance of the inverter.

3. The method for controlling the strong series compensation grid-connected stability of a grid-connected energy storage converter according to claim 1, characterized in that: The sub-synchronous / super-synchronous oscillation instability information is determined according to an impedance stability criterion, and the impedance stability criterion is: The phase margin at the intersection of the output impedance of the virtual synchronous generator VSG and the grid impedance should be greater than 0°; the phase margin expression at the intersection is: In the formula, is the phase margin, and are the grid impedance and the positive sequence impedance of the inverter output under PI-PI control respectively.

4. The strong series compensation grid-connected stability control method of the grid-connected energy storage converter according to claim 1 is characterized in that: Establish the sequence impedance model under the PI-LADRC control strategy, including: Construct a mathematical model of a virtual synchronous generator; The LADRC control strategy is designed based on the mathematical model of the virtual synchronous generator, and combined with the proportional integral PI to obtain the PI-LADRC control strategy; Positive-sequence and negative-sequence impedance models are derived based on the mathematical model and the PI-LADRC control strategy.

5. The method for controlling the strong series compensation grid-connected stability of a grid-connected energy storage converter according to claim 4 is characterized in that: The mathematical model of the virtual synchronous generator is constructed by: The filter capacitor voltage and output current are selected as state variables, and the state space equation of the virtual synchronous generator VSG inverter in the dq coordinate system is obtained by using Kirchhoff's current law and Kirchhoff's voltage law through Clark and Park transformation; The state space equation of the virtual synchronous generator VSG inverter is multiplied on the left by the Park matrix to obtain a space equation in a dq coordinate system, and the space equation is used as the mathematical model of the virtual synchronous generator.

6. The method for controlling the strong series compensation grid-connected stability of a grid-connected energy storage converter according to claim 4, characterized in that: Designing the LADRC control strategy includes designing the equivalent form of the first-order LADRC as follows: in is the control signal, , , represents the given signal, y represents the output of the controlled object, represents the control gain of the system input, s is a complex frequency domain variable, is the bandwidth of the first-order extended state observer, is the bandwidth of the PD controller.

7. The method for controlling the strong series compensation grid-connected stability of a grid-connected energy storage converter according to claim 6, characterized in that: In comparing the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the difference in grid-connected characteristics under the dual closed-loop control strategy and the PI-LADRC control strategy, the output impedance characteristics analysis of the grid-connected inverter under the PI-LADRC control strategy includes: In PI-LADRC control, the small signal component of the reference value of the three-phase inductor current in the dq coordinate system and the expression of the small signal component of the inverter three-phase voltage in the dq coordinate system are determined by the first-order LADRC equivalent form; among them, As the feedforward compensation transfer function, As feedback compensation transfer function; Based on the determined small signal component expression and combined with the circuit parameters, the positive sequence output impedance expression and negative sequence impedance expression of PI-LADRC control are obtained to analyze the output impedance characteristics of the grid-connected inverter under the PI-LADRC control strategy.

8. A strong series compensation grid-connected stability control device for a grid-connected energy storage converter, characterized in that: include: The three-phase grid-connected inverter complete sequence impedance module is used to establish a complete sequence impedance model of the three-phase grid-connected inverter controlled by a virtual synchronous generator using the harmonic linearization method, and obtain the sub / supersynchronous oscillation instability information caused by small signals under the double closed-loop PI control strategy with increased grid series compensation intensity; PI-LADRC control strategy module, which is used to establish the sequence impedance model under the PI-LADRC control strategy based on the hybrid control strategy of proportional integral PI and linear active disturbance rejection LADRC; A comparison and verification module is used to compare the interaction between the output impedance characteristics of the grid-connected inverter and the grid impedance and the difference in grid-connected characteristics under the dual closed-loop PI control strategy and the PI-LADRC control strategy based on the sub / super synchronous oscillation instability information, and obtain the grid-connected stability margin information of the strong series compensation grid of the grid-connected energy storage converter under the PI-LADRC control strategy.

9. A computer device, characterized in that: The device comprises a processor and a memory: The memory is used to store a computer program and send instructions of the computer program to the processor; The processor executes a strong series compensation grid-connected stability control method for a grid-connected energy storage converter according to any one of claims 1 to 7 according to the instructions of the computer program.

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 the processor, it implements a strong series compensation grid-connected stability control method for a grid-connected energy storage converter according to any one of claims 1 to 7.

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