A low-frequency oscillation suppression system and control method for a meshed network type converter

Abstract

The application provides a grid-forming converter low-frequency oscillation suppression system and a control method, comprising: an inner loop control module and an additional damping control module; the additional damping control module is connected with the inner loop control module, and the additional damping control module suppresses the low-frequency oscillation of the grid-forming converter; the inner loop control module comprises an alternating current voltage loop controller and an alternating current current loop controller; the additional damping control link processes the q-axis voltage component input into the alternating current voltage loop controller, applies the processed q-axis voltage component to the d-axis current reference value of the alternating current current loop controller, simultaneously applies the processed q-axis voltage component to the q-axis current reference value of the alternating current current loop controller, and dynamically adjusts the current reference value of the dq axis. The application enhances the low-frequency oscillation stability of the grid-forming converter under the grid condition by adding the additional damping control link, can effectively suppress the low-frequency oscillation, and has good dynamic performance.

Description

Technical Field

[0001] This application relates to the field of new energy power generation technology, specifically to a low-frequency oscillation suppression system and control method for a grid-type converter. Background Technology

[0002] The increasing proportion of new energy power generation, such as wind and solar power, is driving the power system towards a "dual-high" model of high-proportion renewable energy and high-proportion power electronic equipment. Grid-type converters, with their autonomous voltage establishment and active grid support characteristics, have become core equipment for building new power systems.

[0003] However, with the large-scale deployment of grid-connected converters in high-voltage power grid scenarios, low-frequency oscillations have gradually become apparent. Research indicates that the negative damping effect caused by interactions between control loops is the core cause of oscillations. Existing technologies for addressing low-frequency oscillations in grid-connected converters have key shortcomings: First, the impact of the current feedforward element in the AC voltage loop on the small-signal stability of the system has long been neglected. While this element can improve the suppression of grid disturbances, it may significantly alter the system's damping characteristics, thereby affecting system stability. Second, existing suppression strategies are mostly developed based on simplified single-loop control architectures. When applied to grid-connected converter systems with current feedforward in the AC voltage loop, their stability assurance mechanisms lack universality.

[0004] In the prior art, application publication number CN119010068A discloses an additional damping control method to improve the oscillation suppression capability of grid-connected equipment. It proposes to address the low-frequency oscillation problem of grid-connected converters (or equipment) under strong power grid / high proportion power electronic systems by adopting additional damping control. The system damping is improved without changing the main control structure. Although an additional damping control loop is added, the transfer functions of the power oscillation damping controller and the virtual power system stabilizer are more complex, containing multiple DC blocking links and lead-lag links, making parameter tuning relatively more complicated.

[0005] Therefore, it is urgent to propose targeted low-frequency oscillation suppression schemes based on the stability mechanism analysis of grid-type converters considering the influence of current feedforward, so as to improve the stable operation capability of grid-type converters. Summary of the Invention

[0006] In view of one of the defects in the prior art, the purpose of this application is to provide a low-frequency oscillation suppression system and control method for a grid-type converter.

[0007] A first aspect of this application provides a low-frequency oscillation suppression system for a grid-type converter, comprising: an inner loop control module and an additional damping control module; The additional damping control module is connected to the inner loop control module, and the additional damping control module is used to suppress low-frequency oscillations of the grid-type converter; The inner loop control module includes an AC voltage loop controller and an AC current loop controller. The additional damping control module processes the q-axis voltage component input to the AC voltage loop controller and applies the processed q-axis voltage component to the d-axis current reference value of the AC current loop controller. At the same time, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller, thereby dynamically adjusting the dq-axis current reference value.

[0008] Optionally, the additional damping control module includes a high-pass filter, a gain circuit, and a limiting circuit connected in sequence. The q-axis voltage component of the AC voltage loop controller is processed sequentially through the high-pass filter, the gain filter, and the limiting filter, and then superimposed onto the d-axis current reference value and the q-axis current reference value of the AC current loop controller, thereby dynamically adjusting the dq-axis current reference value.

[0009] Optionally, the adjustment method for the d-axis current reference value is as follows: The original d-axis current reference value is superimposed with the first adjustment amount to obtain the adjusted d-axis current reference value. The first adjustment amount is the signal obtained after the q-axis voltage component is processed by the high-pass filter, the gain filter and the limiting filter in sequence. The adjusted d-axis current reference value is expressed as follows: ; In the formula, i cdref_old This is the original d-axis current reference value. i cdref_new This is the adjusted d-axis current reference value; This is the reference value for the q-axis voltage. V sq This is the measured value of the q-axis voltage; T d The time constant of the high-pass filter. K d This refers to the gain coefficient of the gain element in the additional damping control circuit. s These are complex variables in the transfer function.

[0010] Optionally, the q-axis current reference value can be adjusted as follows: The original q-axis current reference value is superimposed with the second adjustment amount to obtain the adjusted q-axis current reference value. The second adjustment amount is the signal obtained after the q-axis voltage component is processed by the high-pass filter stage, the gain stage, and the limiting stage in sequence. The adjusted q-axis current reference value is expressed as follows: ; In the formula, i cqref_old This is the original q-axis current reference value. i cqref_new This is the adjusted q-axis current reference value. This is the reference value for the q-axis voltage. V sq This is the measured value of the q-axis voltage; T d The time constant of the high-pass filter. K d This refers to the gain coefficient of the gain element in the additional damping control circuit. s These are complex variables in the transfer function.

[0011] Optionally, the AC voltage loop controller H v ( s )for, H v ( s )= k p-v + k i-v / s ; The high-pass filter H d ( s )for, H d ( s )= T d · s / ( T d · s +1); In the formula, k p-v and k i-v These are the proportional and integral coefficients of the PI control in the AC voltage loop controller; T d The time constant of the high-pass filter; s These are complex variables in the transfer function.

[0012] Optionally, the high-pass filter is used to filter out constant DC quantities in the input signal; Wherein, the time constant of the high-pass filter T d It is determined by the frequency range of the low-frequency oscillation and satisfies the following conditions; T d ≥1 / (2π· fc_min ); In the formula, f c_min This is the minimum value of the low-frequency oscillation frequency generated by the grid-type converter under strong power grid conditions.

[0013] Optionally, the gain coefficient of the gain circuit is determined based on the actual compensated system phase margin, and the range of the gain coefficient is 10 to 50. The limiting value in the limiting circuit is 20% of the rated voltage.

[0014] Optionally, the inner loop control module further includes an AC current loop controller, an AC voltage loop controller, a voltage feedforward link, and a current feedforward link; The AC current loop control includes a voltage feedforward element, and the AC voltage loop control includes a current feedforward element. The AC current loop controller, the AC voltage loop controller, the voltage feedforward link, and the current feedforward link form a voltage and current dual closed-loop structure. The AC current loop controller and the voltage feedforward link constitute AC current closed-loop control; the AC voltage loop controller and the current feedforward link constitute AC voltage closed-loop control.

[0015] Optionally, it also includes an outer loop control module, which is connected to the AC voltage loop control and is used to control the active and reactive power of the grid-type converter and generate a synchronous phase angle; The outer loop control module includes a synchronization link for VSG control and a reactive power-voltage control link. The d-axis voltage reference value and the q-axis voltage reference value are generated by the reactive power-voltage control loop.

[0016] A second aspect of this application provides a control method for a low-frequency oscillation suppression system of a grid converter, comprising: Obtain the q-axis voltage component of the AC voltage loop controller; The q-axis voltage component is processed by an additional damping control module to obtain the processed q-axis voltage component. The processed q-axis voltage component is applied to the d-axis current reference value of the AC current loop controller to obtain the adjusted d-axis current reference value; at the same time, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller to obtain the adjusted q-axis current reference value. Based on the adjusted d-axis current reference value and the adjusted q-axis current reference value, the current reference values ​​of the d and q axes are dynamically adjusted to suppress the low-frequency oscillation of the grid converter.

[0017] This application provides a low-frequency oscillation suppression system for grid-connected converters. It employs an additional damping control element added to the AC voltage loop controller, effectively enhancing the low-frequency oscillation stability of grid-connected converters under strong grid conditions. While maintaining low-frequency oscillation suppression capabilities, this application offers faster dynamic response and quicker oscillation convergence. The application requires minimal modification to the original control structure, resulting in a simple control structure that not only effectively suppresses low-frequency oscillations but also possesses excellent dynamic performance. It can be used in grid-connected power electronic equipment such as photovoltaic power generation units, direct-drive wind turbines, and energy storage systems.

[0018] Other technical effects resulting from the additional features will be further illustrated in the corresponding embodiments. Attached Figure Description

[0019] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a structural diagram illustrating a low-frequency oscillation suppression system for a grid-type converter according to an exemplary embodiment.

[0020] Figure 2 The diagram shows the output active power waveform of a system before and after implementing the proposed low-frequency oscillation suppression method under a strong power grid, according to an exemplary embodiment.

[0021] Figure 3 The present invention is a flowchart illustrating a method for suppressing low-frequency oscillations in a grid-type converter based on additional damping control, according to an exemplary embodiment. Detailed Implementation

[0022] The present application will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present application, and these all fall within the protection scope of the present application. Parts not described in detail in the following embodiments can be implemented using existing technology.

[0023] In the description of the embodiments of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature.

[0025] In the description of the embodiments in this application, "multiple" means two or more, unless otherwise explicitly specified. In this application, unless otherwise explicitly specified and limited, the terms "installed," "connected," "linked," "fixed," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0026] The terms "comprising" and "having," and any variations thereof, in the embodiments of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such processes, methods, products, or devices.

[0027] In existing technologies, the impact of the current feedforward element of the AC voltage loop on the small-signal stability of the system has long been neglected. Although this element can improve the grid disturbance suppression capability, it may significantly change the system damping characteristics, thereby affecting system stability. Based on the above problems, this application provides a low-frequency oscillation suppression system for a grid-connected converter to solve the aforementioned issues.

[0028] Reference Figure 1 As shown in one embodiment of this application, a low-frequency oscillation suppression system for a grid-type converter includes an inner loop control module and an additional damping control module. The additional damping control module is connected to the inner loop control module and is used to suppress low-frequency oscillations in the grid-type converter.

[0029] The inner loop control module includes an AC voltage loop controller and an AC current loop controller. The additional damping control module processes the q-axis voltage component input to the AC voltage loop controller and applies the processed q-axis voltage component to the d-axis current reference value of the AC current loop controller. Simultaneously, the processed... q The q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller to dynamically adjust the dq-axis current reference value.

[0030] Specifically, in the low-frequency oscillation suppression system of the grid-type converter based on additional damping control, the inner loop control module is used to control the AC side voltage and current of the grid-type converter and generate the modulation voltage. The AC voltage loop control provides the additional damping control link in the additional damping control module. First, the q-axis voltage component of the input AC voltage loop controller is processed, and then the processed q-axis voltage component is applied to the d-axis current reference value of the AC current loop controller. At the same time, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller, thus completing the processing of the voltage component and the adjustment of the current reference value.

[0031] The embodiments described above in this application, by setting an additional damping control loop, process the q-axis voltage component on the AC voltage loop controller and apply it to the corresponding current reference value. The processed q-axis component is then simultaneously superimposed onto both the q-axis and d-axis current reference values. This effectively suppresses low-frequency oscillations in the grid-connected converter and alters the dynamic characteristics of the AC voltage loop controller. While maintaining low-frequency oscillation suppression capabilities, this application offers a faster dynamic response and facilitates faster oscillation convergence. Furthermore, this application involves minimal modifications to the original control structure, resulting in a simpler control structure. It not only effectively suppresses low-frequency oscillations but also possesses excellent dynamic performance, thereby enhancing the system's ability to suppress low-frequency oscillations, improving the stability and reliability of the grid-connected converter, and ensuring the safe and stable operation of the power system.

[0032] In some specific embodiments of this application, the additional damping control module includes a high-pass filter, a gain circuit, and a limiting circuit connected in sequence.

[0033] The q-axis voltage component of the input AC voltage loop controller is processed by a high-pass filter, a gain filter, and a limiting filter, and then applied to the d-axis current reference value and the q-axis current reference value of the AC current loop controller, thereby realizing the dynamic adjustment of the d-axis and q-axis current reference values.

[0034] Specifically, refer to Figure 1 As shown, the input AC voltage loop controller H v ( s The q-axis voltage component is passed through a high-pass filter (to extract high-frequency components), a gain stage (to adjust the amplitude), and a limiting stage (to limit the output range) before being applied to the d-axis current reference value of the AC current loop. The adjusted d-axis current reference value is obtained by summing the results. Simultaneously, this adjusted value is input to the AC voltage loop controller. H v ( s The q-axis voltage component is passed through a high-pass filter. H d ( s Gain coefficient K dThe limiting circuit is applied to the q-axis current reference value of the AC current loop, and the summation yields the adjusted q-axis current reference value.

[0035] The high-pass filter allows only high-frequency signals to pass through while attenuating low-frequency components. By introducing the high-frequency components of the voltage into the current reference value, the current output is quickly adjusted to suppress disturbances during dynamic processes (when the frequency changes). In steady state (low-frequency or DC components), the output of the high-pass filter is close to zero, avoiding static influence on the reference current, thereby achieving low-frequency suppression.

[0036] In the embodiments described above, the high-frequency dynamic components of the voltage are extracted by a high-pass filter, and the current reference value is dynamically corrected after gain and limiting. This effectively suppresses the impact of low-frequency disturbances on system stability, while ensuring control accuracy in steady state. The high-pass filter isolates low-frequency signals and avoids static errors, while the gain and limiting circuits improve dynamic response speed and safety.

[0037] It should be noted that the high-pass filter, gain circuit, and limiting circuit connected in sequence form the additional damping control circuit in the additional damping control module.

[0038] In some specific embodiments of this application, the d-axis current reference value is adjusted by superimposing the original d-axis current reference value with a first adjustment amount to obtain the adjusted d-axis current reference value. The first adjustment amount is the signal obtained after the q-axis voltage component is processed by a high-pass filter, a gain filter, and a limiting filter.

[0039] The adjusted d-axis current reference value is expressed as follows: ; In the formula, i cdref_old This is the original d-axis current reference value. i cdref_new This is the adjusted d-axis current reference value; This is the reference value for the q-axis voltage. V sq This is the measured value of the q-axis voltage; T d The time constant of the high-pass filter. K d This refers to the gain coefficient of the gain element in the additional damping control circuit. s These are complex variables in the transfer function.

[0040] Specifically, the difference between the q-axis voltage reference value and the q-axis voltage measurement value of the AC voltage loop controller is processed through an additional damping control loop to obtain the processed q-axis voltage component.

[0041] In the embodiments described above, by superimposing the signals (first adjustment amount) obtained after the original d-axis current reference value is processed sequentially through a high-pass filter, a gain filter, and a limiting filter, the dynamic error of the q-axis voltage component is introduced into the d-axis (active) current command, establishing a coupling channel between voltage oscillation and active current regulation, which can effectively suppress low-frequency oscillation energy; the high-pass filter ensures that the additional signal is zero in steady state, without affecting the steady-state current tracking accuracy of the system; the gain coefficient and the limiting filter provide flexible damping strength adjustment and safety protection, and the superimposed structure requires little modification to the original control system and is easy to implement.

[0042] In some specific embodiments of this application, the q-axis current reference value is adjusted by superimposing the original q-axis current reference value with a second adjustment amount to obtain the adjusted q-axis current reference value. The second adjustment amount is the signal obtained after the q-axis voltage component is processed by a high-pass filter, a gain filter, and a limiting filter in sequence.

[0043] The adjusted q-axis current reference value is expressed as follows: ; In the formula, i cqref_old This is the original q-axis current reference value. i cqref_new This is the adjusted q-axis current reference value. This is the reference value for the q-axis voltage. V sq This is the measured value of the q-axis voltage; T d The time constant of the high-pass filter. K d This refers to the gain coefficient of the gain element in the additional damping control circuit. s These are complex variables in the transfer function.

[0044] The embodiments described above in this application achieve symmetrical and joint correction of active and reactive currents by synchronously introducing the same q-axis voltage dynamic error into the q-axis (reactive / torque) current command. This enables more comprehensive suppression of low-frequency oscillations (because oscillations are often reflected in both active and reactive power). By dynamically correcting the q-axis current command, voltage oscillations can be responded to quickly, and additional electrical damping can be provided to accelerate the dissipation of oscillation energy. The d-axis and q-axis use the exact same control structure and parameters, simplifying controller design, avoiding new coupling problems introduced by parameter asymmetry, and maintaining the original control framework unchanged, thus improving adaptability.

[0045] In some specific embodiments of this application, the AC voltage loop controller H v ( s )for, H v ( s )= k p-v + k i-v / s ; High-pass filter H d ( s )for, H d ( s )= T d · s / ( T d · s +1); In the formula, k p-v and k i-v These are the proportional and integral coefficients of the PI control in the AC voltage loop controller; T d This represents the time constant of the high-pass filter; s These are complex variables in the transfer function.

[0046] In some specific embodiments of this application, a high-pass filter is used to filter out constant DC quantities in the input signal; wherein, the time constant of the high-pass filter is... T d It is determined by the frequency range of the low-frequency oscillation and satisfies the following conditions; T d ≥1 / (2π· f c_min ); In the formula, f c_min This is the minimum value of the low-frequency oscillation frequency generated by the grid-type converter under strong power grid conditions.

[0047] It should be noted that the minimum value of the low-frequency oscillation frequency... f c_min It is generally around 2Hz.

[0048] For example, refer to Figure 2 As shown, increasing the short-circuit ratio from 5 to 11 results in a system low-frequency oscillation frequency of 3.05 Hz. The time constant of the high-pass filter is set to 0.0589 s.

[0049] Specifically, a high-pass filter is used to filter out constant DC input signals, and its time constant is... T dThe tuning should be based on the frequency range of low-frequency oscillations generated by the grid-type converter under strong power grid conditions, and specifically needs to meet the following requirements. T d ≥1 / (2π· f c_min By correlating the time constant of the high-pass filter with the lower limit of the oscillation frequency, the filter can effectively pass through and extract high-frequency dynamic components related to low-frequency oscillations. At the same time, it can fully attenuate DC and extremely low-frequency signals that do not help suppress oscillations, so that the additional damping control can be precisely applied to the oscillation frequency band, thereby improving the targeting and effectiveness of the additional damping control and enhancing the low-frequency oscillation suppression capability of the grid-type converter under strong power grid conditions.

[0050] In some specific embodiments of this application, the gain coefficient of the gain circuit is determined based on the actual compensated system phase margin, and the range of the gain coefficient is 10 to 50; the limiting value in the limiting circuit is 20% of the rated voltage, which is used to avoid the limiting value being too small, resulting in insufficient damping and reduced oscillation suppression effect; the limiting value being too large may cause voltage instability or transient instability.

[0051] In some specific embodiments of this application, the inner loop control module further includes an AC current loop controller, an AC voltage loop controller, a voltage feedforward link, and a current feedforward link. The AC current loop controller includes a voltage feedforward link, and the AC voltage loop controller includes a current feedforward link; the AC current loop controller, AC voltage loop controller, voltage feedforward link, and current feedforward link form a voltage and current dual closed-loop structure. The AC current loop controller and voltage feedforward link constitute AC current closed-loop control; the AC voltage loop controller and current feedforward link constitute AC voltage closed-loop control.

[0052] Specifically, in AC current loop control, the AC current loop controller monitors and regulates the AC current in real time. Simultaneously, a voltage feedforward loop is introduced to detect voltage changes in advance and quickly feed them back to the current control loop. This assists the AC current loop controller in adjusting its control strategy promptly to respond quickly to the impact of voltage changes on the current, thus forming AC current closed-loop control. In AC voltage loop control, the AC voltage loop controller monitors and regulates the AC voltage, while the current feedforward loop acquires current change information in real time and feeds it back to the voltage control loop. This helps the AC voltage loop controller quickly respond to the impact of current changes on the voltage, thus forming AC voltage closed-loop control. These two loops work together to form a dual closed-loop structure of voltage and current, achieving precise control of the system voltage and current.

[0053] Among them, such as Figure 1 As shown, the voltage loop control and current loop control are cascaded, controlling the voltage and current feedback signals of the main circuit, respectively. Specifically, in the voltage loop control, the voltage feedback signal... V sd qThe difference between the input and the reference value is used as the input to the voltage loop controller; in the current loop control, the current feedback signal... i cd q The difference between the voltage loop and the reference value is used as the input to the current loop controller. Therefore, both the voltage loop and the current loop are "closed-loop" controls.

[0054] The current feedforward element is a component of the voltage loop control, and the voltage feedforward element is a component of the current loop control.

[0055] It should be noted that the inner loop control module is used to control the AC side voltage and current of the grid-connected converter and generate the modulation voltage; the additional damping control module is used to increase the system damping to suppress the low-frequency oscillations of the grid-connected converter under strong grid conditions and to improve the stability of the converter under strong grid conditions.

[0056] Secondly, the input to the additional damping control loop in this application is the current signal output by the "voltage loop controller," which refers to a PI controller. H v ( s The current signal output by the "voltage loop control" is the input reference signal for the "current loop control".

[0057] The embodiments described above in this application achieve independent and coordinated control of AC current and AC voltage by setting up an AC current loop controller, an AC voltage loop controller, a voltage feedforward link, and a current feedforward link to form a voltage and current dual closed-loop structure. The voltage feedforward link in the AC current loop control and the current feedforward link in the AC voltage loop control can improve the system's response speed and anti-interference capability, enabling rapid and accurate output adjustment in the face of external interference and load changes, ensuring the stability of AC current and voltage, improving the overall system's control accuracy and stability, and enhancing the system's adaptability to different operating conditions.

[0058] In some specific embodiments of this application, an outer loop control module is also included. The outer loop control module is connected to the AC voltage loop controller and is used to control the active and reactive power of the grid-type converter and generate the synchronous phase angle; the outer loop control module includes a VSG control synchronization link and a reactive-voltage control link; The d-axis voltage reference value and the q-axis voltage reference value are generated through the VSG reactive-voltage control loop.

[0059] It should be noted that the outer loop control module is used to simulate the inertial characteristics, damping characteristics, and primary frequency and voltage regulation characteristics of a synchronous generator, control the active and reactive power of the grid-type converter, and generate the synchronous phase angle. Specifically, refer to Figure 1 As shown, in the synchronization loop, the active power reference value... P refWith actual power P Do the difference, after 1 / ω The change obtained after zero is equivalent to the torque change. This change is then processed through a virtual moment of inertia and damping circuit to obtain the angular frequency, which is then integrated to obtain the output phase angle. In the reactive power-voltage control circuit, the reactive power reference value... Q ref With feedback value Q The deviation, and then compared with the reference value of the output voltage amplitude. V tref and measured values V sd After subtraction, a reactive voltage droop factor is applied. D q The difference between the output and the input is given to a gain of 1 / K The integrator is used to obtain the d-axis voltage reference value. The inner loop control module includes AC voltage loop control and AC current loop control. The AC voltage loop control includes a current feedforward component; the AC current loop control includes a voltage feedforward component. In the AC voltage loop control, a reference value for the q-axis AC voltage loop control is set. =0, so that the voltage measurement value at the grid connection point is 0. V sd and V sq Follow each and The output signal of the AC voltage loop control also serves as the reference value for the AC current loop control. i cdref and i cqref These are respectively related to the current at the converter output. i cd and i cq After subtraction, the d and q components of the output modulation voltage are obtained through the PI control loop. Based on the frequency, phase, and voltage amplitude output by the outer loop control module, a three-phase modulation wave can be formed. After PWM modulation, the switching signals of the converter power devices are obtained. Additional damping control module: This module is used to improve system damping to suppress low-frequency oscillations of the grid-type converter under strong grid conditions and to improve the stability of the converter under strong grid conditions.

[0060] Reference Figure 3 As shown, based on the same inventive concept, another embodiment of this application provides a method for suppressing low-frequency oscillations in a grid-type converter, which can be implemented using the low-frequency oscillation suppression system for the grid-type converter in the aforementioned embodiment, specifically including: S100: Obtain the q-axis voltage component of the AC voltage loop controller; S200. The q-axis voltage component is processed by the additional damping control module to obtain the processed q-axis voltage component. S300. The processed q-axis voltage component is applied to the d-axis current reference value of the AC current loop controller to obtain the adjusted d-axis current reference value; at the same time, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller to obtain the adjusted q-axis current reference value. S400. Based on the adjusted d-axis current reference value and the adjusted q-axis current reference value, the current reference values ​​of the d and q axes are dynamically adjusted to suppress low-frequency oscillations that occur in grid-type converters under strong power grid conditions.

[0061] Specifically, the q-axis voltage component is obtained by measuring the difference between the q-axis voltage reference value and the q-axis voltage measurement value of the AC voltage loop controller. Then, the q-axis voltage component is fed into the additional damping control loop for processing. After processing, the processed q-axis voltage component is applied to the d-axis current reference value of the AC current loop controller to obtain the adjusted d-axis current reference value. Simultaneously, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller to obtain the adjusted q-axis current reference value. Finally, based on the adjusted d-axis and q-axis current reference values, the dq-axis current reference values ​​are dynamically adjusted to suppress low-frequency oscillations in the grid-connected converter under strong grid conditions.

[0062] The embodiments described above in this application obtain relevant voltage and current reference values ​​from the AC voltage loop controller, process the q-axis voltage component using an additional damping control loop, and generate adjusted d-axis current reference values ​​and adjusted q-axis current reference values ​​based on the processing results. This enables the suppression of low-frequency oscillations in grid-connected converters under strong grid conditions, effectively improving the operational stability of grid-connected converters under strong grid conditions, reducing the impact of low-frequency oscillations on system performance, improving the power supply quality and reliability of the power system, and ensuring the safe and stable operation of the power system.

[0063] The specific implementation techniques of each step in the above-mentioned low-frequency oscillation suppression method for grid-type converters correspond to the implementation techniques of each module in the aforementioned low-frequency oscillation suppression system for grid-type converters, and can be referred to each other. They will not be repeated here.

[0064] The preferred features in the above embodiments can be used individually in any embodiment, or in any combination thereof, provided they do not conflict with each other. Furthermore, parts not described in detail in the embodiments can be implemented using existing technologies.

[0065] The following examples and comparative examples will be used to further illustrate this application in order to better understand the above-mentioned technical solutions. It should be understood that the following are only some examples and are not intended to limit this application.

[0066] Application Example 1: When the system in the example has been running stably for 3 seconds, the grid short-circuit ratio is increased from 5 to 11. The grid-connected current of the grid-connected converter system gradually exhibits constant-amplitude oscillations. This low-frequency oscillation frequency is 3.05Hz. The circuit parameters and control parameters of the system in the example remain unchanged. The time constant of the high-pass filter in the additional damping control module is set. T d The gain coefficient is 0.0589s. K d The value is 35. In the example, when the system runs for 6 seconds, the low-frequency oscillation suppression method is activated, which causes the grid-connected current of the system to gradually converge, thus suppressing the low-frequency oscillation that occurs in the grid-type converter under strong grid conditions.

[0067] Table 1 Parameters of Network Converter

[0068] The foregoing has described some specific embodiments of this application. It should be understood that this application is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the substantive content of this application. The above-described preferred features can be used in any combination without conflict.

Claims

1. A low-frequency oscillation suppression system for a grid-type converter, characterized in that, include: Inner loop control module and additional damping control module; The additional damping control module is connected to the inner loop control module, and the additional damping control module is used to suppress low-frequency oscillations of the grid-type converter; The inner loop control module includes an AC voltage loop controller and an AC current loop controller. The additional damping control module processes the q-axis voltage component input to the AC voltage loop controller and applies the processed q-axis voltage component to the d-axis current reference value of the AC current loop controller. At the same time, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller, thereby dynamically adjusting the dq-axis current reference value. The additional damping control module includes a high-pass filter, a gain circuit, and a limiting circuit connected in sequence. The q-axis voltage component of the AC voltage loop controller is processed by the high-pass filter, the gain circuit, and the limiting circuit, and then applied to the d-axis current reference value and the q-axis current reference value of the AC current loop controller, thereby dynamically adjusting the d-axis and q-axis current reference values. The q-axis voltage component is the difference between the q-axis voltage reference value and the q-axis voltage measurement value.

2. The low-frequency oscillation suppression system for a grid-type converter according to claim 1, characterized in that, The adjustment method for the d-axis current reference value is as follows: The original d-axis current reference value is superimposed with the first adjustment amount to obtain the adjusted d-axis current reference value. The first adjustment amount is the signal obtained after the q-axis voltage component is processed by the high-pass filter, the gain filter and the limiting filter in sequence. The adjusted d-axis current reference value is expressed as follows: ； In the formula, i cdref_old This is the original d-axis current reference value. i cdref_new This is the adjusted d-axis current reference value; V q This is the reference value for the q-axis voltage. V sq This is the measured value of the q-axis voltage; T d The time constant of the high-pass filter. K d This refers to the gain coefficient of the gain element in the additional damping control circuit. s These are complex variables in the transfer function.

3. The low-frequency oscillation suppression system for a grid-type converter according to claim 1, characterized in that, The q-axis current reference value is adjusted as follows: The original q-axis current reference value is superimposed with the second adjustment amount to obtain the adjusted q-axis current reference value. The second adjustment amount is the signal obtained after the q-axis voltage component is processed by the high-pass filter stage, the gain stage, and the limiting stage in sequence. The adjusted q-axis current reference value is expressed as follows: ； In the formula, i cqref_old This is the original q-axis current reference value. i cqref_new This is the adjusted q-axis current reference value; V q This is the reference value for the q-axis voltage. V sq This is the measured value of the q-axis voltage; T d The time constant of the high-pass filter. K d This refers to the gain coefficient of the gain element in the additional damping control circuit. s These are complex variables in the transfer function.

4. The low-frequency oscillation suppression system for a grid-type converter according to claim 1, characterized in that, The AC voltage loop controller H v ( s )for: H v ( s )= k p-v + k i-v / s ； The high-pass filter H d ( s )for: H d ( s )= T d · s / ( T d · s +1)； In the formula, k p-v and k i-v These are the proportional and integral coefficients of the PI control in the AC voltage loop controller; T d The time constant of the high-pass filter; s These are complex variables in the transfer function.

5. A low-frequency oscillation suppression system for a grid-type converter according to claim 4, characterized in that, The high-pass filter is used to filter out constant DC quantities in the input signal; Wherein, the time constant of the high-pass filter T d The frequency range of the low-frequency oscillation is determined, and the following conditions are met: T d ≥ 1 / (2π· f c_min )； In the formula, f c_min This is the minimum value of the low-frequency oscillation frequency generated by the grid-type converter under strong power grid conditions.

6. A low-frequency oscillation suppression system for a grid-type converter according to claim 4, characterized in that, The gain coefficient of the gain circuit is determined based on the actual compensated system phase margin, and the range of the gain coefficient is 10 to 50. The limiting value of the limiting element is 20% of the rated voltage.

7. The low-frequency oscillation suppression system for a grid-type converter according to claim 1, characterized in that, The inner loop control module also includes a voltage feedforward circuit and a current feedforward circuit; The AC current loop controller is provided with the voltage feedforward link, and the AC voltage loop control is provided with the current feedforward link; The AC current loop controller, the AC voltage loop controller, the voltage feedforward link, and the current feedforward link form a voltage and current dual closed-loop structure; The AC current loop controller and the voltage feedforward link constitute AC current closed-loop control; the AC voltage loop controller and the current feedforward link constitute AC voltage closed-loop control.

8. The low-frequency oscillation suppression system for a grid-type converter according to claim 1, characterized in that, It also includes an outer loop control module, which is connected to the AC voltage loop control and is used to control the active and reactive power of the grid-type converter and generate the synchronous phase angle; The outer loop control module includes a synchronization link for VSG control and a reactive power-voltage control link. The d-axis voltage reference value and the q-axis voltage reference value are generated through the reactive power-voltage control loop.

9. A method for suppressing low-frequency oscillations in a grid-type converter, characterized in that, A low-frequency oscillation suppression system for a grid-type converter according to any one of claims 1-8, comprising: Obtain the q-axis voltage component of the AC voltage loop controller; The q-axis voltage component is processed by an additional damping control module to obtain the processed q-axis voltage component. The processed q-axis voltage component is applied to the d-axis current reference value of the AC current loop controller to obtain the adjusted d-axis current reference value; at the same time, the processed q-axis voltage component is applied to the q-axis current reference value of the AC current loop controller to obtain the adjusted q-axis current reference value. Based on the adjusted d-axis current reference value and the adjusted q-axis current reference value, the current reference values ​​of the d and q axes are dynamically adjusted to suppress the low-frequency oscillation of the grid converter. The additional damping control module includes a high-pass filter, a gain circuit, and a limiting circuit connected in sequence. The q-axis voltage component of the AC voltage loop controller is processed by the high-pass filter, the gain circuit, and the limiting circuit, and then applied to the d-axis current reference value and the q-axis current reference value of the AC current loop controller, thereby dynamically adjusting the d-axis and q-axis current reference values. The q-axis voltage component is the difference between the q-axis voltage reference value and the q-axis voltage measurement value.

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