Low-voltage ride-through control method for network construction type converter

Abstract

The invention is suitable for the technical field of power electronics and power transmission, and provides a low voltage ride-through control method and system for a network construction type converter, and the method comprises the steps: carrying out the conventional VSG control according to a VSG control strategy, and continuously monitoring the difference between the voltage of a power grid and a set reference value, so as to judge whether a fault occurs or not; when it is detected that the power grid breaks down, homodyne control is carried out on an active power control ring and a reactive power control ring of the grid-forming type converter so as to fix the phase and the amplitude of the output voltage of the converter; calculating and injecting a positive sequence compensation voltage and a negative sequence compensation voltage according to the drop degree of the power grid voltage; and according to the predicted impact current value, dynamically calculating and inputting adaptive virtual impedance. The power angle and the voltage in the fault period can be ensured to be stable, transient overcurrent is effectively restrained, and large power fluctuation is not caused.

Description

Technical Field

[0001] This application belongs to the field of power electronics and electric drive technology, and in particular relates to a low voltage ride-through control method for grid-type converters. Background Technology

[0002] Faced with the increasingly severe energy crisis and climate change, the world is actively promoting a low-carbon transition. Renewable energy sources such as photovoltaics and wind power are accelerating the replacement of traditional thermal power, a shift that brings new challenges to the stable operation of power systems. Among these, grid-connected converters, with their active voltage and frequency regulation capabilities, can be widely used in weak grids and will be an important guarantee for the safe and stable operation of "high-voltage and high-efficiency" power systems. Grid-connected converters based on virtual synchronous generator (VSG) control can simulate the inertia and damping characteristics of synchronous generators and have typical voltage source characteristics. However, when a grid fault occurs, the sudden changes in voltage amplitude and phase angle of the faulted phase will lead to an imbalance in the system's power supply and demand, potentially causing instability or even divergence in the power angle of new energy units. At the same time, grid faults can also cause a sudden increase in converter output current, which can easily damage the power electronic switching devices in the converter. Therefore, to ensure the stable operation of grid-connected converters during low-voltage ride-through, a reliable control strategy must be adopted to enhance system stability and effectively suppress overcurrent. Summary of the Invention

[0003] This application provides a low-voltage ride-through control method for grid-connected converters, which can solve the problems of instability and overcurrent faced by grid-connected systems based on VSG control during grid faults.

[0004] This application provides a low-voltage ride-through control method for a grid-connected converter, including: performing conventional VSG control according to a VSG control strategy, and continuously monitoring the difference between the grid voltage and a set reference value to determine whether a fault has occurred;

[0005] When a grid fault is detected, zero-difference control is implemented on the active power control loop and reactive power control loop of the grid-type converter to fix the phase and amplitude of the converter output voltage.

[0006] Based on the degree of voltage drop in the power grid, calculate and inject positive-sequence compensation voltage and negative-sequence compensation voltage;

[0007] Based on the predicted inrush current value, an adaptive virtual impedance is dynamically calculated and applied.

[0008] Specifically, the implementation of zero-difference control for the active power control loop and the reactive power control loop includes:

[0009] Set the power deviation of the active control loop to zero so that the converter continues to operate at the voltage phase before the fault.

[0010] Set the power deviation of the reactive power control loop to zero, so that the converter continues to operate at the voltage amplitude before the fault.

[0011] Specifically, the calculation of the positive-sequence compensation voltage and the negative-sequence compensation voltage includes:

[0012] Based on the degree of voltage drop in the power grid, determine the reference values ​​for positive-sequence reactive current and negative-sequence reactive current;

[0013] With the goal of maximizing the positive sequence active current output, the negative sequence active current is set to 0, and the reference value of the positive sequence active current is calculated.

[0014] By combining the reference values ​​of positive-sequence reactive current, negative-sequence reactive current, negative-sequence active current, and positive-sequence active current, the dq-axis components of the positive- and negative-sequence compensation voltage are obtained through positive- and negative-sequence vector calculation.

[0015] Among them, the maximum peak value of the converter output current is predicted when a power grid fault occurs and after the fault is recovered;

[0016] The predicted maximum peak value is compared with the current withstand threshold of the converter. If it exceeds the current withstand threshold, the required virtual impedance coefficient is calculated based on the proportion of current exceeding the threshold.

[0017] During the fault occurrence phase, a virtual impedance value is generated based on the virtual impedance coefficient, and the virtual impedance value is withdrawn after being used for a period of time.

[0018] During the fault recovery phase, a virtual impedance value is generated based on the virtual impedance coefficient, and after being put into operation for a period of time, it is smoothly withdrawn in a linear decay manner.

[0019] This application provides a low-voltage ride-through control device for a grid-type converter, comprising:

[0020] The fault detection module is used to perform routine VSG control according to the VSG control strategy and continuously monitor the difference between the grid voltage and the set reference value to determine whether a fault has occurred.

[0021] The power zero-difference control module is used to implement zero-difference control on the active power control loop and reactive power control loop of the grid-type converter when a grid fault is detected, so as to fix the phase and amplitude of the converter output voltage.

[0022] The voltage compensation module is used to calculate and inject positive-sequence compensation voltage and negative-sequence compensation voltage based on the degree of voltage drop in the power grid.

[0023] The adaptive virtual impedance module is used to dynamically calculate and apply an adaptive virtual impedance based on the predicted inrush current value.

[0024] Specifically, the voltage compensation module is configured as follows:

[0025] The positive and negative sequence compensation voltage components are calculated independently using the positive and negative sequence decomposition unit.

[0026] The positive and negative sequence compensation voltage components are independently controlled and injected through a positive and negative dual-sequence voltage and current loop.

[0027] The above-mentioned solution in this application has the following beneficial effects:

[0028] In the embodiments of this application, firstly, zero-difference control is implemented in the power control loop to facilitate system power angle and voltage stability; then, positive and negative sequence compensation voltages are calculated according to national standards to ensure that the VSG maintains voltage source characteristics while avoiding steady-state overcurrent, and to improve the system voltage support capability and reduce three-phase current imbalance; secondly, adaptive virtual impedance is designed to suppress transient overcurrent, ensuring that the virtual impedance value has a small impact on power fluctuations within a reasonable range. Furthermore, a system state equation is established for the proposed control strategy, and its stability during fault periods is proven. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a grid-connected system diagram of a grid-type converter based on VSG control according to an embodiment of this application;

[0031] Figure 2 This is an equivalent circuit diagram of a single-phase sequence component under VSG control during a power grid fault, according to an embodiment of this application.

[0032] Figure 3 This is an improved low-penetration penetration control block diagram according to an embodiment of this application;

[0033] Figure 4 This is a vector diagram of steady-state voltage and current during a fault, according to an embodiment of this application.

[0034] Figure 5 This is a topology diagram of a positive and negative dual-sequence voltage-current loop according to an embodiment of this application;

[0035] Figure 6 This is an equivalent circuit diagram of the improved single-phase sequence component control during a power grid fault according to an embodiment of this application;

[0036] Figure 7 This is a flowchart of the low-pass control of a grid-type converter according to an embodiment of this application;

[0037] Figure 8 This is a waveform diagram of the converter output current under a single-phase fault in a conventional VSG control according to an embodiment of this application.

[0038] Figure 9 The improved low-throughput control waveform of the converter under single-phase fault is an embodiment of this application.

[0039] Figure 10 (a) is a waveform diagram of the active power output of the converter under a single-phase fault according to a conventional VSG control according to an embodiment of this application;

[0040] Figure 10 (b) is a waveform diagram of the converter output active power under a single-phase fault, which is an embodiment of the present application of the improved low-voltage control.

[0041] Figure 11 (a) is a waveform diagram of the reactive power output of the converter under a single-phase fault according to a conventional VSG control according to an embodiment of this application;

[0042] Figure 11 (b) is a waveform diagram of the reactive power output of the converter under a single-phase fault, which is an embodiment of the present application of the improved low-voltage control.

[0043] Figure 12 The output current waveforms of the converter under a three-phase fault are shown in the traditional VSG control and improved low-voltage control according to an embodiment of this application.

[0044] Figure 13 The waveform diagram of the converter output active power under a three-phase fault is shown for conventional VSG control and improved low-voltage control according to an embodiment of this application.

[0045] Figure 14 The waveform diagram of the reactive power output of the converter under a three-phase fault is shown in the traditional VSG control and the improved low-voltage control according to an embodiment of this application. Detailed Implementation

[0046] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0047] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0048] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0049] As used in this application specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if detected [the described condition or event]" may be interpreted, depending on the context, as meaning "once determined," "in response to determination," "once detected [the described condition or event]," or "in response to detection [the described condition or event]."

[0050] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0051] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0052] To address the stability issues of grid-connected converters during low-voltage ride-through (LVRT), this application provides a LVRT control method for grid-connected converters. This method first implements zero-difference control in the power control loop to facilitate system power angle and voltage stability. Then, it calculates positive and negative sequence compensation voltages according to national standards to ensure the VSG maintains its voltage source characteristics while avoiding steady-state overcurrent, improving system voltage support capability, and reducing three-phase current imbalance. Secondly, it suppresses transient overcurrent by designing an adaptive virtual impedance, ensuring that the virtual impedance value, within a reasonable range, has minimal impact on power fluctuations. Furthermore, it establishes system state equations for the proposed control strategy and proves its stability during fault periods.

[0053] The low voltage ride-through control method for grid converters provided in this application will be illustrated below with reference to specific embodiments.

[0054] This application provides a low-voltage ride-through control method for a grid-connected converter, including: performing conventional VSG control according to a VSG control strategy, and continuously monitoring the difference between the grid voltage and a set reference value to determine whether a fault has occurred;

[0055] When a grid fault is detected, zero-difference control is implemented on the active power control loop and reactive power control loop of the grid-type converter to fix the phase and amplitude of the converter output voltage.

[0056] Based on the degree of voltage drop in the power grid, calculate and inject positive-sequence compensation voltage and negative-sequence compensation voltage;

[0057] Based on the predicted inrush current value, an adaptive virtual impedance is dynamically calculated and applied.

[0058] Specifically, the implementation of zero-difference control for the active power control loop and the reactive power control loop includes:

[0059] Set the power deviation of the active control loop to zero so that the converter continues to operate at the voltage phase before the fault.

[0060] Set the power deviation of the reactive power control loop to zero, so that the converter continues to operate at the voltage amplitude before the fault.

[0061] Specifically, the calculation of the positive-sequence compensation voltage and the negative-sequence compensation voltage includes:

[0062] Based on the degree of voltage drop in the power grid, determine the reference values ​​for positive-sequence reactive current and negative-sequence reactive current;

[0063] With the goal of maximizing the positive sequence active current output, the negative sequence active current is set to 0, and the reference value of the positive sequence active current is calculated.

[0064] By combining the reference values ​​of positive-sequence reactive current, negative-sequence reactive current, negative-sequence active current, and positive-sequence active current, the dq-axis components of the positive- and negative-sequence compensation voltage are obtained through positive- and negative-sequence vector calculation.

[0065] Among them, the maximum peak value of the converter output current is predicted when a power grid fault occurs and after the fault is recovered;

[0066] The predicted maximum peak value is compared with the current withstand threshold of the converter. If it exceeds the current withstand threshold, the required virtual impedance coefficient is calculated based on the proportion of current exceeding the threshold.

[0067] During the fault occurrence phase, a virtual impedance value is generated based on the virtual impedance coefficient, and the virtual impedance value is withdrawn after being used for a period of time.

[0068] During the fault recovery phase, a virtual impedance value is generated based on the virtual impedance coefficient, and after being put into operation for a period of time, it is smoothly withdrawn in a linear decay manner.

[0069] This application provides a low-voltage ride-through control device for a grid-type converter, comprising:

[0070] The fault detection module is used to perform routine VSG control according to the VSG control strategy and continuously monitor the difference between the grid voltage and the set reference value to determine whether a fault has occurred.

[0071] The power zero-difference control module is used to implement zero-difference control on the active power control loop and reactive power control loop of the grid-type converter when a grid fault is detected, so as to fix the phase and amplitude of the converter output voltage.

[0072] The voltage compensation module is used to calculate and inject positive-sequence compensation voltage and negative-sequence compensation voltage based on the degree of voltage drop in the power grid.

[0073] The adaptive virtual impedance module is used to dynamically calculate and apply an adaptive virtual impedance based on the predicted inrush current value.

[0074] Specifically, the voltage compensation module is configured as follows:

[0075] The positive and negative sequence compensation voltage components are calculated independently using the positive and negative sequence decomposition unit.

[0076] The positive and negative sequence compensation voltage components are independently controlled and injected through a positive and negative dual-sequence voltage and current loop.

[0077] In the embodiments of this application, firstly, zero-difference control is implemented in the power control loop to facilitate system power angle and voltage stability; then, positive and negative sequence compensation voltages are calculated according to national standards to ensure that the VSG maintains voltage source characteristics while avoiding steady-state overcurrent, and to improve the system voltage support capability and reduce three-phase current imbalance; secondly, adaptive virtual impedance is designed to suppress transient overcurrent, ensuring that the virtual impedance value has a small impact on power fluctuations within a reasonable range. Furthermore, a system state equation is established for the proposed control strategy, and its stability during fault periods is proven.

[0078] 1. Analysis of VSG low-voltage transmission characteristics considering phase transition

[0079] like Figure 1 The diagram shows a grid-connected converter system based on VSG control. Among them, , These are the output voltage and current after LC filtering, respectively. , These are the AC line resistance and inductance, respectively. This is the grid voltage.

[0080] The VSG control strategy mainly consists of two power controllers. The active frequency controller shown in Equation (1) is used to simulate the rotor motion characteristics of a traditional synchronous generator, providing damping and inertial support for the power grid, and realizing coordinated regulation of active power and system frequency. The reactive voltage controller shown in Equation (2) is used to simulate the excitation voltage regulation device of a traditional synchronous generator, realizing coordinated regulation of reactive power and output voltage.

[0081] (1)

[0082] (2)

[0083] in: , These are the rated angular frequency and the actual angular frequency; , For rotational inertia and damping; , These are the reference and actual values ​​for active power. , These are the reference and actual values ​​for the internal potential amplitude. , These are the reactive power control coefficient and the reactive power integral coefficient; , These are the reference and actual values ​​for reactive power.

[0084] When an asymmetrical fault occurs in the power grid, the converter outputs active power. and reactive power The expression is:

[0085] (3)

[0086] (4)

[0087] in: , These are the average components of the active and reactive power outputs of the inverter, respectively. , , , These represent the amplitudes of active and reactive power fluctuations according to a sine (cosine) distribution.

[0088] The active average component This can be further expressed as:

[0089] (5)

[0090] in: , , , They are respectively The positive and negative sequence dq-axis voltage components, , , , They are respectively The positive and negative sequence dq axis current components.

[0091] When the resistive component in the circuit is ignored, the negative sequence voltage component , All of this is applied to the line inductance, from which we can deduce Furthermore, the more severe the power grid fault, the stronger the positive sequence voltage component. , The smaller the value, the more likely it is to result in an average active power component. It also decreases accordingly.

[0092] Substituting equation (3) into equation (1), we obtain the following equation:

[0093] (6)

[0094] in: , .

[0095] This formula is about The first-order nonhomogeneous differential equation. When a minor fault occurs in the power grid, the VSG regulates the positive-sequence voltage to make the average active power component... At this time, angular frequency Will surround The system oscillates periodically at twice the frequency, resulting in power angle oscillations. However, during severe grid faults, the positive-sequence voltage drops excessively, causing the average active power component to oscillate. It will always be less than the active power reference value. (Right now ), at this time angular frequency Will always be greater than This state causes the power angle of the VSG system to continuously increase, eventually leading to system instability.

[0096] In reactive power controllers, the presence of an integral element makes... Substituting equation (4) into equation (2), we can derive the following equation:

[0097] (7)

[0098] in , C is the integration constant.

[0099] From equation (7), we can see that the internal potential Also surrounding The power grid oscillates periodically at twice the frequency. In summary, asymmetrical faults in the power grid will cause twice-frequency fluctuations in the output power of the VSG. This periodic power disturbance will lead to significant power angle and voltage oscillations in the system.

[0100] like Figure 2 As shown, when an asymmetrical fault occurs in the power grid, it can be equivalent to the superposition of two symmetrical circuits, one with positive sequence and one with negative sequence. Based on symmetry, only the single-phase circuit needs to be analyzed. At this time, the total current of the single-phase VSG... The output expression is:

[0101] (8)

[0102] in: To output full current in positive sequence, It outputs the full current in negative sequence.

[0103] Based on the transient analysis theory of power systems, the positive-sequence output total current can be written. The expression is:

[0104] (9)

[0105] in: , These are the steady-state periodic components of the positive-sequence output current after a fault and their initial values, respectively. This refers to the instantaneous positive sequence current before the fault. The DC component current attenuation coefficient is... .

[0106] Instantaneous positive sequence current before a power grid fault for:

[0107] (10)

[0108] in, The phase of the converter output voltage during a fault; , These are the voltage amplitude and voltage angle before the fault, respectively; The system impedance angle, .

[0109] Steady-state periodic component of positive sequence output current during power grid faults for:

[0110] (11)

[0111] in: , These represent the positive sequence voltage amplitude and voltage angle during the fault period, respectively.

[0112] when At that time, the initial value of the steady-state periodic component of the output current after a power grid fault can be obtained from equation (11). for:

[0113] (12)

[0114] Since the VSG negative sequence output voltage and instantaneous negative sequence current were close to 0 before the fault, the total negative sequence output current can be calculated. The expression is:

[0115] (13)

[0116] in: , These represent the magnitude and phase of the negative sequence voltage during the fault, respectively.

[0117] In summary, when an asymmetrical fault occurs in the power grid, it faces the dual threat of steady-state cyclic overcurrent and transient impulse overcurrent caused by free components. Among these, the negative sequence component not only causes imbalance in the steady-state cyclic current but may also exacerbate transient impulse current. Furthermore, the more severe the power grid fault and the lower the system impedance, the more serious the overcurrent problem will be.

[0118] The improved low-altitude penetration control strategy proposed in this application is as follows: Figure 3 As shown, the improvement mainly includes three aspects. The zero-difference power circuit ensures power angle and voltage stability during faults. The voltage compensation circuit achieves the combined goals of avoiding steady-state overcurrent, improving voltage support capability, and enhancing current balance while maintaining the characteristics of the VSG voltage source. The adaptive virtual impedance circuit suppresses transient overcurrent and minimizes power fluctuations during the shutdown process.

[0119] In the event of grid asymmetric faults, voltage dips and phase jumps directly affect the active power output capability of inverters. However, traditional VSG control strategies still attempt to adjust the voltage angle... To maintain the original active power output. This approach can cause system frequency and power angle oscillations under minor faults, and under severe faults, it can lead to power angle divergence and system instability. Furthermore, due to virtual inertia, the active power control adjustment process is not instantaneous; it may not be complete even after the grid fault ends, making it difficult to stabilize the VSG power angle during voltage dips. To avoid frequency and power angle oscillations at the source and to prevent the power angle from being continuously adjusted during faults due to slow active power loop adjustment, this application implements zero-difference control (i.e., zero-difference control) in the active power control loop during grid faults. Figure 3 (The active power deviation in the active power control loop is set to 0), so that VSG operates at the voltage angle before the fault. Continuous operation maintains system frequency stability and avoids power angle divergence.

[0120] Furthermore, if reactive power droop control is maintained during asymmetrical grid faults, the second-harmonic frequency fluctuations of reactive power will cause voltage amplitude to fluctuate through negative feedback. This produces synchronous oscillation. Voltage amplitude The oscillations will further exacerbate the reactive power oscillations of the system, thus adversely affecting the stability of the VSG control system. This application implements zero-difference control in the reactive power controller stage during grid faults (…). Figure 3 The reactive power deviation in the reactive power control loop is set to 0 to ensure the VSG voltage amplitude. The output value remains constant until the fault occurred.

[0121] According to national standards, when a grid fault occurs, the magnitude of the positive-sequence reactive current injected into the grid and the negative-sequence current absorbed from the grid should be determined based on the grid voltage drop. The reference value for the generated positive-sequence reactive current is specified below. and negative sequence reactive current reference value for:

[0122] (14)

[0123] in: , These represent the positive and negative sequence reactive current output values ​​before the power grid fault. , These are the proportional coefficients of the dynamic positive and negative sequence reactive currents of the VSG system. This is the rated current of the converter.

[0124] To maximize positive-sequence active current output while ensuring reactive current support capability, the following settings can be configured: This leads to the derivation of the positive sequence active current. for:

[0125] (15)

[0126] Figure 4 The diagram shows the steady-state voltage and current during a fault. Under the limiting conditions of each current component, the compensation voltage can be calculated. The positive and negative order dq axis components are:

[0127] (16)

[0128] in: , , , They are respectively Positive and negative sequence dq axis voltages This represents the amplitude of the VSG output voltage at the instant of the fault.

[0129] Figure 5 A dual-sequence voltage-current loop topology is demonstrated. Compared with traditional voltage-current loops, this structure effectively solves the steady-state error problem caused by the negative sequence component by independently controlling the positive and negative sequence components, achieving precise tracking of the converter output voltage. The voltage-compensated grid system maintains the characteristics of the VSG voltage source and avoids steady-state overcurrent, while its generated reactive current fully complies with national standards. Specifically, the positive sequence reactive current enhances the system's voltage support capability, while the negative sequence reactive current component effectively improves the three-phase current imbalance.

[0130] Networked systems may experience transient current surges during both fault occurrence and recovery. Virtual impedance can limit transient currents, but a value that is too small can easily lead to current exceeding limits, while a value that is too large will reduce the output current capability. Therefore, this application designs an adaptive virtual impedance to optimize its effect on suppressing transient currents.

[0131] like Figure 6 The diagram shows the improved single-phase sequence component equivalent circuit for power grid fault control. Among them, , , , These improvements are aimed at enhancing the amplitude and angle of the positive and negative sequence voltages output by the control converter.

[0132] Expressions for positive and negative sequence total current during power grid faults , for:

[0133] (17)

[0134] in: , , , These represent the positive and negative sequence steady-state output current amplitude and current angle during the fault period, respectively. , These represent the positive and negative sequence current amplitudes before the fault, respectively. , These represent the positive and negative sequence current phases at the moment of the fault.

[0135] Because the degree of voltage drop and phase jump in the fault phase are random, the phase angle of the positive and negative sequence currents varies. , It is subject to uncertainty. Furthermore, the timing of the fault is unpredictable, and the initial phase of the instantaneous positive and negative sequence currents during the fault is also uncertain. and Similarly, it is difficult to determine. Furthermore, according to the conclusions of traditional power system short-circuit current analysis, the larger the initial value of the transient current component, the larger the peak value of the inrush current.

[32] Therefore, when , At this time, a single phase of the inverter will suffer the maximum current surge, and the expression at this time is... for:

[0136] (18)

[0137] Theoretically, the moment when the derivative of equation (18) is 0 should be the moment when its amplitude is at its maximum. However, setting the derivative to 0 will construct a transcendental equation, making it impossible to accurately determine its maximum value and the corresponding moment. From the analysis of short-circuit current in power systems, it can be seen that the peak value of the inrush current after a voltage drop is approximately at the peak position of its steady-state component. Therefore, the maximum value of the total current when a fault occurs can be approximately expressed as:

[0138] (19)

[0139] in: This is the maximum peak current that a single phase of the converter needs to withstand when a fault occurs.

[0140] Also note that the maximum peak inrush current that the converter can withstand is ,in This is the inverter's peak current rating. If... If this occurs, the system is at risk of transient overcurrent surge, and a virtual impedance should be introduced for compensation. This is because the introduction of the virtual impedance will not change the pre-fault current amplitude. and Therefore, the virtual impedance coefficient at the time of the fault for:

[0141] (20)

[0142] Considering that the transient current component exhibits an exponential decay characteristic, in After a certain period of time, the impedance essentially decays to zero, therefore the virtual impedance can be removed at that point. The formula for calculating the virtual impedance at the time of the fault can be obtained as follows:

[0143] (twenty one)

[0144] Similarly, the maximum peak value of the single-phase current after fault recovery can be derived using the above method. for:

[0145] (twenty two)

[0146] in: This refers to the maximum peak current that a single phase of the converter needs to handle after the fault is repaired. , These represent the positive and negative sequence current amplitudes before fault recovery, respectively.

[0147] when At the same time, there is also a risk of transient surge overcurrent. The virtual impedance coefficient after fault recovery can be calculated. for:

[0148] (twenty three)

[0149] Considering that changes in virtual impedance will cause power fluctuations, and the active power regulation timescale of the VSG system is on the order of seconds, the virtual impedance is designed to be between 0 and... The time period remained constant, and then... It linearly decays to zero within a 1-second time interval. Therefore, the virtual impedance calculation formula is:

[0150] (twenty four)

[0151] The low-drive-through control process for a grid-type converter considering the phase jump of a faulty phase, as proposed in this application, is as follows: Figure 7 As shown. Based on conventional VSG control, fault detection is achieved by continuously monitoring the difference between the grid voltage and the set reference value. When a grid fault is detected, the zero-difference power control mode is first activated, and the required positive and negative sequence voltage compensation is calculated based on equation (16), and the virtual impedance value to be applied is determined by equation (21). After the grid fault is restored, the system exits the zero-difference power control mode and shuts down the voltage compensation link. Then, the virtual impedance value is calculated and applied based on equation (24), and finally, the virtual impedance is smoothly reduced to zero within 1 second.

[0152] During a grid fault, zero-delay control is applied to the power loop to maintain a constant dq-axis output voltage in the VSG control loop. If the dynamic effects of the voltage and current loops are ignored, modeling the grid connection lines yields the following expression:

[0153] (25)

[0154] After adopting the voltage compensation and adaptive virtual impedance strategy proposed in this application, the converter output voltage characteristics can be described as follows:

[0155] (26)

[0156] Among them, the voltage generated by the virtual impedance This can be further expressed as:

[0157] (27)

[0158] Combining equations (21), (25), (26), and (27), the simplified dynamic equation of the system during the fault period can be derived as follows:

[0159] (28)

[0160] in: .

[0161] System state matrix and constant matrix The mathematical expressions are as follows:

[0162] ,

[0163] Error states can be defined ,in If the system is at equilibrium, then the dynamic equation becomes:

[0164] (29)

[0165] At this point, the system is a strictly linear time-invariant system, and its stability is entirely determined by the state matrix. The decision is made. By solving the characteristic equation of the state matrix, the system's eigenvalues ​​can be obtained as follows:

[0166] (30)

[0167] Grid-connected systems are typically resistive-inductive circuits, which makes it easy to conclude... , When the system needs to be supplemented with virtual impedance, it can be seen from equation (20) that... When the system does not require the addition of virtual impedance, then there is... Therefore, it can be seen that all characteristic roots of the system are distributed in the left half-plane of the complex plane, proving that the improved control strategy can remain stable during faults. It can also be concluded that the voltage compensation element does not affect the system's stability, but it does change its steady-state operating point. While the introduction of virtual impedance can improve the system's stability margin, it will make the system's oscillation frequency under disturbances more pronounced.

[0168] To verify the correctness and effectiveness of the proposed control strategy, this application follows... Figure 1 The topology shown is used to build a grid-connected system model on a host computer. The VSG grid-connected controller hardware is built and tested using an in-loop test system, which mainly consists of a host computer, an RT-LAB real-time emulator, a DSP controller, and an oscilloscope.

[0169] The system parameters are shown in Table 1. The effective values ​​of the steady-state periodic current and the effective values ​​of the instantaneous inrush current that the converter can withstand are 28.2 A and 36.7 A, respectively. To verify the effectiveness of the proposed low-voltage surge strategy under different types of faults, experiments were conducted under single-phase and three-phase faults. The experiment duration was 2 seconds: the grid was in rated operation for the first 0.4 seconds; a fault occurred at 0.4 seconds; and the grid switched back to rated operation at 0.8 seconds until the end of the experiment. This application sets up experiments under both traditional VSG control strategy and improved control strategy, comparing waveforms and analyzing the results.

[0170] Table 1 System Parameters

[0171] This section sets the grid phase a voltage to drop to 40% of its rated value (0.4U0) under fault conditions, accompanied by a 20° phase lag jump. After the fault is cleared, the phase a voltage amplitude returns to its rated value, and the phase jumps back to the pre-fault state.

[0172] Figure 8 The waveform of the converter output current under traditional VSG control during a single-phase fault is shown. During the fault occurrence and recovery phases, the peak inrush currents are 82.0A and 46.4A, respectively, reaching 2.23 and 1.26 times the withstand value. The peak steady-state currents during the fault are: phase a 41.1A, phase b 6.2A, and phase c 47.0A. Both phase a and phase c currents exceed the steady-state current limit, and the three-phase current imbalance reaches 80.3%. This demonstrates that traditional control systems pose an overcurrent risk during both the fault occurrence and recovery phases under single-phase fault conditions. Figure 9 The improved low-throughput control waveform of the converter output current under single-phase fault is shown. During the fault occurrence and recovery phases, through the coordinated control of voltage compensation and adaptive virtual impedance, the peak inrush currents are 29.4A and 20.5A, respectively, with instantaneous inrush current values ​​being 0.80 and 0.56 times the withstand value, respectively. During the fault, the peak steady-state currents are: phase a 23.6A, phase b 27.9A, and phase c 20.7A, at which point the system current imbalance is 15.2%. This demonstrates that the improved control under single-phase faults can not only prevent fault current from exceeding limits but also improve three-phase current imbalance.

[0173] Figure 10 (a) shows the active power waveform of the converter output under a single-phase fault using conventional VSG control. During the fault occurrence phase, the peak active power surge reaches 26.82 kW. During the fault, the active power exhibits second-harmonic oscillation characteristics with an amplitude of 8.25 kW. Figure 10(b) illustrates the converter output active power waveform under a single-phase fault due to the improved low-throughput control. During the fault occurrence and recovery phases, the improved control effectively suppresses active power surges near the setpoint. During the fault, the active power fluctuation amplitude is reduced to 3.06 kW. This demonstrates that the improved control not only effectively addresses active power surges under single-phase faults but also improves active power fluctuations during the fault period.

[0174] Figure 11 (a) illustrates the reactive power waveform of the converter output under conventional VSG control during a single-phase fault. During the fault occurrence phase, the peak reactive power surge reaches 21.62 kVar. During the steady-state period, the reactive power fluctuation amplitude reaches 8.52 kVar, with an average reactive power output of 0 kVar. During the fault recovery phase, the system experiences a reverse reactive power surge with an amplitude of 13.78 kVar. Figure 11 (b) illustrates the reactive power waveform of the converter output under a single-phase fault, as shown by the improved low-voltage control. During both the fault occurrence and recovery phases, the improved control effectively suppresses reactive power surges. In steady state, the reactive power fluctuation amplitude is reduced to 2.42 kVar, and the average reactive power output is 1.05 kVar. This demonstrates that under single-phase fault conditions, the improved control not only suppresses reactive power surges and improves reactive power fluctuations but also provides a certain degree of voltage support capability.

[0175] This section sets the three-phase voltage of the power grid to drop to 40% of its rated value (0.4U0) under fault conditions, accompanied by a 20° phase lag jump. After the fault is cleared, the three-phase voltage amplitude returns to its rated value, and the phase jumps back to the pre-fault state.

[0176] Figure 12 The converter output current waveforms under three-phase faults are shown for both conventional VSG control and improved low-voltage surge control. During the fault occurrence and recovery phases, the peak inrush current of the conventional control was 102.0A and 103.0A, respectively, reaching 2.78 and 2.80 times the tolerance value. Under conventional control, the peak inrush current was 31.2A and 30.9A, respectively, reaching 0.85 and 0.84 times the tolerance value. During the fault, the output current under conventional control continuously increased, reaching a peak of 36.6A before the fault was cleared, significantly exceeding the steady-state effective current value. The improved control achieved precise control of the steady-state peak current of 28.1A, reaching the maximum allowable output while ensuring the current does not exceed the limit. Therefore, the improved control reliably ensures that the output current does not exceed the safety threshold even under the most severe three-phase faults.

[0177] Figure 13The active power waveforms of the converter output under three-phase faults are shown for both conventional VSG control and improved low-voltage surge control. During the fault occurrence and recovery phases, the peak active power surges of the conventional control were 28.54 kW and 17.94 kW, respectively, while the active power output of the improved control remained stable below 11.93 kW throughout the fault. During the fault, the conventional control failed to reach a steady state, while the improved control stabilized the output at 4.60 kW within 0.1 s. This demonstrates that under three-phase faults, the improved control can effectively suppress active power surges and maintain stable active power output during the fault period.

[0178] Figure 14 The reactive power waveforms of the converter output under three-phase faults are shown for both conventional VSG control and improved low-voltage surge control. During the fault occurrence phase, the peak active power surge of the conventional control is 30.22 kVar, while that of the improved control is only 7.08 kVar. During the fault, the reactive power output of the conventional control is close to 0 kVar, while the improved control outputs a stable 6.62 kVar. During the fault recovery phase, the conventional control exhibits a reverse reactive power surge with a peak value of 17.52 kVar, while the improved control reduces it to 8.41 kVar. This demonstrates that under three-phase faults, the improved control not only effectively suppresses reactive power surges but also provides stronger voltage support capabilities.

[0179] This application addresses the instability and overcurrent problems faced by grid-connected systems based on VSG control during grid faults. It proposes a low-voltage ride-through control strategy combining voltage compensation and adaptive virtual impedance. This strategy specifically considers the phase jump phenomenon of the fault phase under asymmetrical faults, effectively improving the reliability and stability of the system under fault conditions. The conclusions are as follows:

[0180] (1) The voltage compensation method proposed in this application, while taking into account the characteristics of VSG voltage source and the suppression of steady-state overcurrent, achieves the improvement of system voltage support capability and the improvement of three-phase current balance.

[0181] (2) The adaptive virtual impedance designed in this application can effectively suppress transient overcurrent and does not cause large power fluctuations during the virtual impedance exit process.

[0182] (3) The stability of the proposed control strategy during faults was proved by establishing the system state equations. Experimental results show that the strategy can achieve good low-voltage ride-through control under different types of faults.

Claims

1. A low-voltage ride-through control method for a grid-connected converter, characterized in that, include: Perform routine VSG control according to the VSG control strategy, and continuously monitor the difference between the grid voltage and the set reference value to determine whether a fault has occurred. When a grid fault is detected, zero-difference control is implemented on the active power control loop and reactive power control loop of the grid-type converter to fix the phase and amplitude of the converter output voltage. Based on the degree of voltage drop in the power grid, calculate and inject positive-sequence compensation voltage and negative-sequence compensation voltage; Based on the predicted inrush current value, an adaptive virtual impedance is dynamically calculated and applied.

2. The low-voltage ride-through control method for a grid-type converter according to claim 1, characterized in that, The implementation of zero-difference control for the active power control loop and the reactive power control loop specifically includes: Set the power deviation of the active control loop to zero so that the converter continues to operate at the voltage phase before the fault. Set the power deviation of the reactive power control loop to zero, so that the converter continues to operate at the voltage amplitude before the fault.

3. The low-voltage ride-through control method for a grid-type converter according to claim 1, characterized in that, The calculation of the positive-sequence compensation voltage and the negative-sequence compensation voltage specifically includes: Based on the degree of voltage drop in the power grid, determine the reference values ​​for positive-sequence reactive current and negative-sequence reactive current; With the goal of maximizing the positive sequence active current output, the negative sequence active current is set to 0, and the reference value of the positive sequence active current is calculated. By combining the reference values ​​of positive-sequence reactive current, negative-sequence reactive current, negative-sequence active current, and positive-sequence active current, the dq-axis components of the positive- and negative-sequence compensation voltage are obtained through positive- and negative-sequence vector calculation.

4. The low-voltage ride-through control method for a grid-type converter according to claim 1, characterized in that, Predict the maximum peak value of the converter output current when a power grid fault occurs and after the fault is recovered; The predicted maximum peak value is compared with the current withstand threshold of the converter. If it exceeds the current withstand threshold, the required virtual impedance coefficient is calculated based on the proportion of current exceeding the threshold. During the fault occurrence phase, a virtual impedance value is generated based on the virtual impedance coefficient, and the virtual impedance value is withdrawn after being used for a period of time. During the fault recovery phase, a virtual impedance value is generated based on the virtual impedance coefficient, and after being put into operation for a period of time, it is smoothly withdrawn in a linear decay manner.

5. A low-voltage ride-through control device for a grid-connected converter, characterized in that, include: The fault detection module is used to perform routine VSG control according to the VSG control strategy and continuously monitor the difference between the grid voltage and the set reference value to determine whether a fault has occurred. The power zero-difference control module is used to implement zero-difference control on the active power control loop and reactive power control loop of the grid-type converter when a grid fault is detected, so as to fix the phase and amplitude of the converter output voltage. The voltage compensation module is used to calculate and inject positive-sequence compensation voltage and negative-sequence compensation voltage based on the degree of voltage drop in the power grid. The adaptive virtual impedance module is used to dynamically calculate and apply an adaptive virtual impedance based on the predicted inrush current value.

6. The low-voltage ride-through control device for a grid-type converter according to claim 5, characterized in that, The voltage compensation module is specifically configured as follows: The positive and negative sequence compensation voltage components are calculated independently using the positive and negative sequence decomposition unit. The positive and negative sequence compensation voltage components are independently controlled and injected through a positive and negative dual-sequence voltage and current loop.

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