Full-band cooperative oscillation suppression method for heterogeneous inverter series-parallel system

Abstract

The invention discloses a full-band cooperative oscillation suppression method for a heterogeneous inverter series-parallel system. Establishing an equivalent impedance model without introducing a cooperative oscillation suppression strategy, constructing a following network type medium-high frequency stability enhancement controller, setting parameters of the following network type medium-high frequency stability enhancement controller, constructing a construction network type damping dynamic coordination controller, selecting parameters of the construction network type damping dynamic coordination controller, and performing dynamic coordination control on the construction network type damping dynamic coordination controller. And establishing an equivalent impedance model to which the collaborative oscillation suppression strategy is introduced, comparing the Bode diagram and the Nyquist diagram of the impedance model to which the collaborative oscillation suppression strategy is not introduced and the impedance model to which the collaborative oscillation suppression strategy is introduced, and verifying the correctness of theoretical analysis and the effectiveness of the proposed suppression strategy. The risk of oscillation of the system in the full frequency band is reduced, and the stability of the system in the full frequency band is enhanced.

Description

Technical Field

[0001] The present application relates to the technical field of control and stability analysis of power electronic converters in new energy stations, and in particular to a full-band collaborative oscillation suppression method for a heterogeneous inverter hybrid system. Background Art

[0002] In recent years, the system stability problems caused by the interaction between new energy units and the power grid have become increasingly prominent. Large-scale new energy unit off-grid accidents and system stability problems have occurred frequently, posing a severe challenge to the safe operation of the new power system. Wideband oscillations occur from time to time and have become a major problem threatening the stable operation of the power system.

[0003] Currently, most grid-connected converters use a phase-locked loop (PLL)-based grid-following control strategy, with their external characteristics behaving as a current source. However, grid-following converters are prone to oscillation when the grid strength is weak. To improve the operational performance of large-scale renewable energy stations, a common approach currently used in engineering projects is to incorporate grid-forming devices into grid-following stations. Compared to grid-following control, grid-forming control has the ability to independently establish voltage and frequency, and its external characteristics behave as a voltage source, eliminating the need for a phase-locked loop (PLL) to follow the grid phase. Even when connected to a weak grid, grid-forming converters maintain high stability and are less prone to oscillation.

[0004] There are many methods for analyzing the broadband oscillation problem caused by the grid connection of power electronic equipment. The system stability can be analyzed and judged from the impedance perspective. The main methods include characteristic value analysis, sequence impedance analysis and time domain simulation analysis.

[0005] The sequential impedance analysis method determines system stability by determining whether the phase of an interacting system exhibits negative damping characteristics at frequencies with equal impedance amplitudes. This method intuitively displays the frequency characteristics of impedance from the perspective of RLC circuits, treating power electronic equipment and the power grid as independent systems. System stability can be quantitatively analyzed by measuring only the characteristics outside the ports. This method offers greater model adaptability and scalability, and is applicable to both white-box and black-box models.

[0006] The grid configuration with fixed damping has many disadvantages, such as poor dynamic adaptability, limited oscillation suppression capability and insufficient robustness to changes in grid parameters.

[0007] Fixed damping is difficult to cope with the oscillation characteristics of different scenarios (such as low-frequency oscillations and subsynchronous oscillations). In scenarios with weak power grids or a high proportion of renewable energy access, the system damping requirements are complex, and fixed damping may not be able to effectively suppress oscillations in specific frequency bands.

[0008] Fixed damping cannot be adaptively adjusted when parameters such as grid strength (such as short-circuit ratio) and line impedance change. This can result in insufficient damping in weak grids and excessive damping in strong grids, leading to decreased dynamic performance.

[0009] To suppress the problem of medium and high frequency oscillations, the current existing technology mainly introduces two methods of damping into the system: passive damping and active damping. Passive damping is to use series or parallel damping in the filter to consume oscillation energy. It has the advantages of simple structure and no need for control algorithm coordination. However, its disadvantages are that it introduces additional losses, reducing system efficiency, and the resistance value needs to be accurately calculated. Otherwise, too large a resistance value will affect the filtering effect, while too small a resistance value will result in insufficient damping. Active damping does not require additional hardware. It collects current or voltage signals through sensors and injects virtual damping into the control algorithm, such as capacitor current feedback and grid voltage feedback. The damping effect is flexible and adjustable, making it suitable for high frequency bands. Therefore, active damping is adopted as a technical solution to suppress high-frequency oscillations of the converter.

[0010] Existing active damping technology for suppressing broadband oscillations has the following problems: Active damping control is difficult to be compatible with other control loops. Designing a new energy converter that is compatible with multiple supporting functions such as voltage and frequency is the technical basis for realizing the transformation of new energy equipment from passive adaptation to active support. However, the implementation of each supporting function is only designed for a specific scenario, ignoring the coupling effect between the various control loops. On the one hand, the introduction of voltage and frequency control links may change the broadband dynamic response characteristics of new energy equipment, causing new oscillation modes to be excited by disturbances in new energy equipment, and the active damping control designed for new energy equipment may not be applicable. On the other hand, there may be interactions between the control loops of different functions in the converter, and the introduction of active damping control branches may affect the stable operation of new energy equipment under other working conditions (such as low voltage ride-through).

[0011] The active damping function is difficult to cover the entire broadband frequency band. The multi-modal drift characteristics of broadband oscillations highlight the higher requirements for the scenario adaptability of the active damping control of the grid-connected system of new energy equipment. The existing source-side converter control parameter optimization and compensation branch design only consider the damping requirements near a certain resonant frequency. Although the source-side additional damping controller has a certain robustness in multiple scenarios, the control structure is based on an offline design of a fixed new energy equipment grid-connected model. During the oscillation process, the operating point of the grid-connected system of new energy equipment changes continuously with the changes in the system operating status and network topology. The offline broadband oscillation model is difficult to match the actual oscillation scenario, and the control strategy is difficult to meet the damping requirements of multiple oscillation modes and multiple oscillation scenarios.

[0012] The oscillation diffusion path is difficult to track. Tracking the oscillation diffusion path is the basis for effectively blocking the development of broadband oscillations. Revealing the development path of oscillations can provide early warning of oscillation risks and effectively block the further development of broadband oscillations by adding damping control at weak nodes in the system. However, the coupling between power electronic equipment at different time scales is complex, making it difficult to reveal the mechanism of oscillation occurrence and development. At the same time, as oscillations spread regionally, changes in network topology and wind turbine operation mode may cause the control mode of converter equipment to continuously switch, exciting multiple oscillation modes and changing the key equipment that dominates the oscillation mode. Summary of the Invention

[0013] The embodiments of the present application provide a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system to solve the above-mentioned technical problems.

[0014] In view of this, the present application provides a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system, which is characterized by comprising the following steps: S1. Establish an equivalent impedance model without introducing cooperative oscillation suppression strategy; S2. Construct a grid-following medium and high frequency stability enhancement controller; S3. Adjusting the parameters of the constructed grid-following medium and high frequency stability enhancement controller; S4, constructing a network-type damping dynamic coordination controller; S5, selecting parameters of the constructed network-type damping dynamic coordination controller; S6. Based on the grid-following medium- and high-frequency stability enhancement controller constructed and parameterized in steps S2 to S5 and the grid-forming damping dynamic coordination controller constructed and parameterized, an equivalent impedance model that introduces a cooperative oscillation suppression strategy is established; S7. Compare the Bode plot and Nyquist plot of the equivalent impedance model without the cooperative oscillation suppression strategy and the equivalent impedance model with the cooperative oscillation suppression strategy, and verify the correctness of the theoretical analysis and the effectiveness of the proposed suppression strategy based on the comparison results.

[0015] Optionally, in step S1, the equivalent impedance model established without introducing the cooperative oscillation suppression strategy includes positive and negative sequence impedance models of a grid-forming and grid-following inverter hybrid system, and the expressions of the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid system are:

[0016]

[0017] Where n is the number of inverters, , , , , , , is the filter capacitor, is the filter resistor, , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle; is the terminal voltage output value; and are the fundamental amplitudes of voltage and current respectively; is the filter inductor; , is the power angle of network control; ; is the initial phase of the current fundamental wave.

[0018] Optionally, in step S2, the grid-following medium and high frequency stability enhancement controller is constructed, and the frequency domain expression of the medium and high frequency stability enhancement controller is: , in, , is the cutoff frequency, .

[0019] Optionally, in step S3, the tuning of the parameters of the constructed grid-following medium and high frequency stability enhancement controller includes selecting a cutoff frequency To improve the impedance characteristics in the mid- and high-frequency bands.

[0020] Optionally, in step S4, the mathematical expression for constructing the network-type damping dynamic coordination controller is: , in, is the initial damping coefficient of the grid-type inverter; is the coordination coefficient; is the short-circuit ratio of the grid-side line impedance.

[0021] Optionally, in step S5, the selection of parameters of the constructed network damping dynamic coordination controller includes selecting the coordination coefficient To improve the system impedance characteristics in the low frequency band.

[0022] Optionally, in step S6, the equivalent impedance model for introducing the cooperative oscillation suppression strategy established by the harmonic linearization method includes positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems, and the mathematical expressions of the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems are:

[0023] , Where n is the number of inverters, , , , , , , , , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle; is the terminal voltage output value; and are the fundamental amplitudes of voltage and current respectively; is the filter inductor; , is the power angle of network control; ; is the initial phase of the current fundamental wave.

[0024] Optionally, the physical characteristics of the high-frequency stability enhancement controller in the grid-following inverter are manifested as second-order low-pass filter characteristics.

[0025] Optionally, the upper limit of the cutoff frequency of the medium and high frequency stability enhancement controller is set to 100 Hz.

[0026] Optionally, in step S7, the Bode diagram and Nyquist diagram of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are compared, and the correctness of the theoretical analysis and the effectiveness of the proposed suppression strategy are verified based on the comparison results, specifically including: drawing a Bode diagram of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy, and obtaining the Bode diagram amplitude-frequency and phase-frequency curves of the two models in the target frequency band by the sweep frequency method. The phase margin of the system can be expressed as the phase and phase of the equivalent impedance of the grid-type and grid-type hybrid systems. The distance between them, the phase margin is less than or greater than When the system is in a negative damping state, the improvement of the phase margin of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are compared; the equivalent impedance of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are respectively compared with the equivalent impedance of the power grid to draw the Nyquist curve, and observe whether the system can still surround the (-1, j0) point after the introduction of the cooperative oscillation suppression strategy, so as to judge the stability of the system.

[0027] It can be seen from the above technical solutions that the embodiments of the present application have the following advantages: The advantage of adaptive damping is that it can dynamically adjust the damping coefficient according to the real-time operating conditions of the power grid (such as frequency fluctuations, power disturbances, impedance changes, etc.), significantly improving the system's dynamic response speed and stability, effectively suppressing multi-band oscillations (such as low-frequency, subsynchronous oscillations), and enhancing adaptability to complex scenarios such as weak power grids and high proportions of new energy access; at the same time, by optimizing damping distribution, it reduces equipment losses and extends its life, and suppresses impact currents and accelerates recovery during faults. Combined with intelligent algorithms and collaborative control technologies, it takes into account the flexibility, robustness and efficiency of the power system.

[0028] The present invention adopts a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system, changes the amplitude intersection point of the hybrid system's equivalent impedance and the grid impedance, and improves the system phase margin after the introduction of the cooperative suppression method. At the same time, the ratio of the system's equivalent impedance to the grid impedance after the introduction of the cooperative suppression method meets the Nyquist stability criterion, reducing the risk of system oscillation in the full frequency band and enhancing the system's stability in the full frequency band.

[0029] Compared to existing techniques that generally rely on small-signal models to analyze broadband oscillation stability, employ local Bode plot phase margin criteria, and analyze the impedance of individual converters in isolation, this invention achieves optimization through equivalent impedance modeling and source-grid collaborative analysis. By introducing the Nyquist stability criterion to globally evaluate the closed curve of the impedance ratio, this approach mitigates the "pseudo-stability" risk of local phase margin criteria, providing a theoretically rigorous and engineering-feasible solution for broadband oscillation suppression in systems with a high proportion of renewable energy grid-connected systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly express the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 This is a step diagram of a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application; Figure 2 This is a typical control structure diagram of the network-building type and the network-following type of a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application; FIG3 (a) and FIG3 (b) are control loop diagrams of a network-forming type and a network-following type of a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application without introducing a cooperative oscillation suppression strategy; FIG4 (a) and FIG4 (b) are Bode diagrams and Nyquist diagrams of the system equivalent impedance of a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application without introducing a cooperative oscillation suppression strategy; FIG5( a ) and FIG5 ( b ) are control loop diagrams of a network-forming type and a network-following type of a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application, which introduce a cooperative oscillation suppression strategy; 6( a ) and 6 ( b ) are Bode plots at different cutoff frequencies and damping coefficients for a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application; 7( a ) and 7 ( b ) are Bode diagrams and Nyquist diagrams of the system equivalent impedance of a full-band cooperative oscillation suppression method for a heterogeneous inverter hybrid system provided in an embodiment of the present application that introduces a cooperative oscillation suppression strategy. DETAILED DESCRIPTION

[0032] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. The details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other unless they conflict.

[0033] It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention. Therefore, the drawings only show the layers related to the present invention and are not drawn according to the number, shape and size ratio of the layers in actual implementation. In actual implementation, the type and number of each layer can be changed at will, and the layer layout may also be more complicated.

[0034] In the following description, numerous details are set forth to provide a more thorough explanation of the embodiments of the present invention; however, it is apparent to one skilled in the art that the embodiments of the present invention may be practiced without these specific details.

[0035] like Figures 1 to 7(b) This embodiment provides a method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system, including the following steps: S1. Establish an equivalent impedance model without introducing cooperative oscillation suppression strategy; S2. Construct a grid-following medium and high frequency stability enhancement controller; S3. Adjusting the parameters of the constructed grid-following medium and high frequency stability enhancement controller; S4, constructing a network-type damping dynamic coordination controller; S5, selecting parameters of the constructed network-type damping dynamic coordination controller; S6. Based on the grid-following medium- and high-frequency stability enhancement controller constructed and parameterized in steps S2 to S5 and the grid-forming damping dynamic coordination controller constructed and parameterized, an equivalent impedance model that introduces a cooperative oscillation suppression strategy is established; S7. Compare the Bode plot and Nyquist plot of the equivalent impedance model without the cooperative oscillation suppression strategy and the equivalent impedance model with the cooperative oscillation suppression strategy, and verify the correctness of the theoretical analysis and the effectiveness of the proposed suppression strategy based on the comparison results.

[0036] The amplitude intersection point of the hybrid system's equivalent impedance and the grid impedance is changed, and the system phase margin is also improved after the introduction of the collaborative suppression method. At the same time, the ratio of the system's equivalent impedance to the grid impedance after the introduction of the collaborative suppression method meets the Nyquist stability criterion, reducing the risk of system oscillation in the full frequency band and enhancing the system's stability in the full frequency band.

[0037] Compared to existing techniques that generally rely on small-signal models to analyze broadband oscillation stability, employ local Bode plot phase margin criteria, and analyze the impedance of individual converters in isolation, this invention achieves optimization through equivalent impedance modeling and source-grid collaborative analysis. By introducing the Nyquist stability criterion to globally evaluate the closed curve of the impedance ratio, this approach mitigates the "pseudo-stability" risk of local phase margin criteria, providing a theoretically rigorous and engineering-feasible solution for broadband oscillation suppression in systems with a high proportion of renewable energy grid-connected systems.

[0038] Furthermore, in step S1, the equivalent impedance model established without introducing the cooperative oscillation suppression strategy includes positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems. The expressions of the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems are:

[0039]

[0040] Where n is the number of inverters, , , , , , , is the filter capacitor, is the filter resistor, , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle; is the terminal voltage output value; and are the fundamental amplitudes of voltage and current respectively; is the filter inductor; , is the power angle of network control; ; is the initial phase of the current fundamental wave.

[0041] Specifically, step S1 is based on Figure 2 , the circuit topology and network following and network building control block diagrams of Figure 3(a) and Figure 3(b), and the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy is established by the harmonic linearization method. 、 、 are the three-phase voltages at the grid connection point; 、 、 They are three-phase output currents respectively; 、 、 is a three-phase modulated signal; 、 are the dq axis voltages at the grid connection point respectively; 、 are dq axis output current respectively; is the output voltage reference value; 、 The output current reference value of the dq axis; 、 They are PLL and second-order VSG control output phase angle respectively.

[0042] According to the grid-type control block diagram in Figure 3(a), the active power controller of the voltage-type VSG simulates the inertia and primary frequency modulation characteristics of the synchronous generator. The mathematical equations of the active power controller and reactive power controller of the voltage-type VSG are as follows:

[0043]

[0044]

[0045]

[0046] , Where, is the virtual moment of inertia; and are the output angular frequency of the voltage-type VSG and the rated angular frequency of the grid respectively; and They are torque given and electromagnetic torque respectively; and are the active damping coefficient and the reactive damping coefficient respectively; and The active power and reactive power are given respectively; and are the instantaneous output active power and reactive power respectively; is the phase of the potential inside the voltage-type VSG; is the reactive inertia coefficient; and They are the rated voltage RMS and output voltage RMS respectively; is the effective value of the internal potential of the voltage type VSG.

[0047] Output active power of voltage type VSG and reactive power It can be calculated based on instantaneous power theory, and the calculation formula is:

[0048] Where, and for Output current of voltage-type VSG in the coordinate system; and for The output voltage of the voltage type VSG in the coordinate system.

[0049] The modulation wave of the voltage-controlled VSG is determined by the output of the active power controller and the reactive power controller, and its expression is:

[0050] Where, 、 and is the modulation wave of VSG.

[0051] The expressions for the internal potential, output voltage, and output current of a voltage-type VSG are:

[0052] When the system is three-phase symmetrical, we can get The frequency domain expressions of the output voltage and output current of the voltage-type VSG in the coordinate system are:

[0053]

[0054] Using the frequency domain convolution theorem, the expression of active power in the frequency domain can be obtained as follows:

[0055] Ignoring the small signal quadratic term, we can get The expression in the frequency domain is shown as follows:

[0056] The phase angle of the three-phase modulation wave Phase angle perturbation is introduced ,Right now . is the phase angle disturbance caused by the harmonic small disturbance signal, is the phase angle of the fundamental component in the three-phase modulated wave. The expression in the frequency domain is:

[0057] Positive and negative sequence disturbance voltage and current The influence of the positive sequence disturbance voltage and current, negative sequence disturbance voltage and current and relationship. yes Due to the small disturbance amount generated by the harmonic disturbance signal, it can be obtained:

[0058] Positive and negative sequence disturbance current and The relationship is: , Where, , is the power angle of voltage type VSG.

[0059] The output voltage fluctuation of voltage type VSG is small, so it can be considered as is a constant. Considering the effects of voltage and current signal sampling delay, PWM delay, and low-pass filter, the frequency domain positive and negative sequence impedance of the voltage-type VSG can be obtained as:

[0060]

[0061] Where, , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle.

[0062] In addition to the filter inductor, the inverter based on voltage-type VSG control also includes filter capacitors and damping resistors. Therefore, the positive and negative sequence impedance models of the inverter based on VSG control are:

[0063]

[0064] According to the grid-following control block diagram in Figure 3(b), similar to the above derivation, the positive and negative sequence impedance expressions of the traditional grid-following inverter can be obtained as follows:

[0065]

[0066] In addition to the filter inductor, the inverter based on grid-following control also includes filter capacitors and damping resistors. Therefore, the positive and negative sequence impedance models of the inverter based on grid-following control are:

[0067]

[0068] According to the national requirement that the proportion of grid-connected equipment in the capacity of new energy power generation equipment should be no less than 20%, the positive and negative sequence impedance model expression of the grid-connected and grid-following inverter hybrid system in the new energy station is:

[0069]

[0070] The grid impedance is generally equalized by connecting inductance and resistance in series, and the reactance value of the connection is often used to represent the strength of the grid.

[0071] The grid side impedance equivalent model is:

[0072] According to Figure 4(a) and Figure 4(b), the intersection frequencies of the system equivalent impedance and the grid impedance are 44 Hz and 56 Hz in the low frequency band, and 167 Hz and 320 Hz in the medium and high frequency bands. The phases corresponding to the intersection frequencies in the medium and high frequency bands are all in the negative damping range. At this time, the system is unstable as judged by the Bode plot, and the curve of the ratio of the system equivalent impedance to the grid impedance surrounds the point (-1, j0), which does not meet the Nyquist stability criterion.

[0073] Furthermore, in the step S2, the medium-high frequency stability enhancement controller of the grid-following type is constructed, and the frequency domain expression of the medium-high frequency stability enhancement controller is: , The control block diagram after the control structure is changed is shown in Figure 5(b), where: , is the cutoff frequency, .

[0074] Furthermore, in step S3, the parameters of the constructed grid-following type medium and high frequency stability enhancement controller are adjusted, including selecting the cutoff frequency In order to improve the impedance characteristics of the medium and high frequency bands, according to Figure 6(a), when the cutoff frequency increases, the station equivalent impedance does not have an intersection frequency with the grid impedance in the medium and high frequency bands, and the dangerous range where oscillation may occur in the low frequency band also increases. By observing the intersection frequency and corresponding phase margin of the system equivalent impedance and the grid equivalent impedance of the Bode diagram, the appropriate cutoff frequency is selected.

[0075] Furthermore, in step S4, the mathematical expression for constructing the network-type damping dynamic coordination controller is: , The control block diagram after the control structure is changed is shown in Figure 5(a), where: is the initial damping coefficient of the grid-type inverter; is the coordination coefficient; is the short-circuit ratio of the grid-side line impedance.

[0076] Furthermore, in step S5, the selection of parameters of the constructed network damping dynamic coordination controller includes selecting the coordination coefficient To improve the system impedance characteristics in the low frequency band.

[0077] Specifically, the parameters of the network damping dynamic coordination controller are selected, including the selection of the coordination coefficient This improves the system impedance characteristics in the low-frequency band. Figure 6(b) shows that as the damping coefficient increases, the dangerous range where oscillations may occur in the low-frequency band gradually decreases. However, when the damping coefficient increases to a certain level, the reduction in the dangerous range of low-frequency oscillations is minimal and the effect is insignificant. Furthermore, an excessively large damping coefficient can inhibit the system's dynamic response speed, causing delays in the VSG's regulation of frequency fluctuations or power changes. In the event of sudden changes in grid frequency or rapid load changes, the VSG may be unable to adjust its output power in a timely manner, affecting the system's dynamic stability. Excessive damping can also alter the system's small-signal stability characteristics, potentially causing certain oscillation modes to enter unstable regions.

[0078] Select the appropriate coordination coefficient by observing the changing trend of the system equivalent impedance in the dangerous range where oscillation may occur in the low frequency band of the Bode diagram .

[0079] Furthermore, in step S6, the equivalent impedance model for introducing the cooperative oscillation suppression strategy established by the harmonic linearization method includes positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems. The mathematical expressions of the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems are:

[0080] , Where n is the number of inverters, , , , , , , , , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle; is the terminal voltage output value; and are the fundamental amplitudes of voltage and current respectively; is the filter inductor; , is the power angle of network control; ; is the initial phase of the current fundamental wave.

[0081] Specifically, step S6 uses the harmonic linearization method to establish an equivalent impedance model that incorporates a cooperative oscillation suppression strategy based on the grid-following medium- and high-frequency stability enhancement controller constructed and tuned in steps S2 to S5 and the grid-forming damping dynamic coordinated controller constructed and selected. The mathematical expression of the equivalent impedance of the grid-type inverter is:

[0082]

[0083] Where, .

[0084] In addition to the filter inductor, the inverter based on grid control also includes filter capacitors and damping resistors. Therefore, the positive and negative sequence impedance models of the grid-based inverter are:

[0085]

[0086] The mathematical expression of the equivalent impedance of the grid-following inverter is:

[0087]

[0088] The positive and negative sequence impedance model expressions of the grid-connected and grid-following inverter hybrid systems in the new energy station after the introduction of the cooperative oscillation suppression strategy are as follows:

[0089]

[0090] Furthermore, the physical characteristics of the high-frequency stability enhancement controller in the grid-following inverter are manifested as second-order low-pass filter characteristics.

[0091] Furthermore, the upper limit of the cutoff frequency of the medium and high frequency stability enhancement controller is set to 100 Hz.

[0092] Furthermore, in step S7, the Bode diagram and Nyquist diagram of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are compared, and the correctness of the theoretical analysis and the effectiveness of the proposed suppression strategy are verified based on the comparison results, specifically including: drawing the Bode diagram of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy, and obtaining the Bode diagram amplitude-frequency and phase-frequency curves of the two models in the target frequency band by the sweep frequency method. The phase margin of the system can be expressed as the phase and phase of the equivalent impedance of the grid-type and grid-type hybrid systems. The distance between them, the phase margin is less than or greater than When the system is in a negative damping state, the improvement of the phase margin of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are compared; the equivalent impedance of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are compared with the equivalent impedance of the power grid to draw the Nyquist curve, and observe whether the system can still surround the (-1, j0) point after the introduction of the cooperative oscillation suppression strategy, so as to judge the stability of the system. Moreover, the impedance model with the introduction of the cooperative oscillation suppression strategy not only narrows the dangerous range where oscillations may occur in the low-frequency band, but also avoids intersection with the grid impedance amplitude in the medium and high frequency bands. At the same time, the ratio of the system equivalent impedance to the grid equivalent impedance meets the Nyquist stability criterion, which greatly reduces the possibility of broadband oscillations, realizes the reshaping of the system impedance characteristics, and has a strong anti-interference ability. The terms "first," "second," "third," "fourth," and the like (if any) in the specification of the present application and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, so that the embodiments of the present application described herein can, for example, be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or apparatus.

[0093] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system, characterized in that: The following steps are involved: S1. Establish an equivalent impedance model without introducing cooperative oscillation suppression strategy; S2. Construct a grid-following medium and high frequency stability enhancement controller; S3. Adjusting the parameters of the constructed grid-following medium and high frequency stability enhancement controller; S4, constructing a network-type damping dynamic coordination controller; S5, selecting parameters of the constructed network-type damping dynamic coordination controller; S6. Based on the grid-following medium- and high-frequency stability enhancement controller constructed and parameterized in steps S2 to S5 and the grid-forming damping dynamic coordination controller constructed and parameterized, an equivalent impedance model that introduces a cooperative oscillation suppression strategy is established; S7. Compare the Bode plot and Nyquist plot of the equivalent impedance model without the cooperative oscillation suppression strategy and the equivalent impedance model with the cooperative oscillation suppression strategy, and verify the correctness of the theoretical analysis and the effectiveness of the proposed suppression strategy based on the comparison results.

2. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 1, characterized in that: In step S1, the equivalent impedance model established without introducing the cooperative oscillation suppression strategy includes the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems. The expressions of the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems are: Where n is the number of inverters, , , , , , , is the filter capacitor, is the filter resistor, , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle; is the terminal voltage output value; and are the fundamental amplitudes of voltage and current respectively; is the filter inductor; , is the power angle of network control; ; is the initial phase of the current fundamental wave.

3. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 1, characterized in that: In the step S2, the grid-following medium and high frequency stability enhancement controller is constructed, and the frequency domain expression of the medium and high frequency stability enhancement controller is: ， in, , is the cutoff frequency, .

4. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 3, characterized in that: In step S3, the parameters of the constructed grid-following medium and high frequency stability enhancement controller are adjusted, including selecting the cutoff frequency To improve the impedance characteristics in the mid- and high-frequency bands.

5. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 1, characterized in that: In step S4, the mathematical expression for constructing the network-type damping dynamic coordination controller is: ， in, is the initial damping coefficient of the grid-type inverter; is the coordination coefficient; is the short-circuit ratio of the grid-side line impedance.

6. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 5, characterized in that: In step S5, the parameters of the constructed network damping dynamic coordination controller are selected including selecting the coordination coefficient To improve the system impedance characteristics in the low frequency band.

7. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 1, characterized in that: In step S6, the equivalent impedance model for introducing the cooperative oscillation suppression strategy established by the harmonic linearization method includes positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems. The mathematical expressions of the positive and negative sequence impedance models of the grid-forming and grid-following inverter hybrid systems are: ， Where n is the number of inverters, , , , , , , , , and is the cutoff frequency of the low-pass filter for voltage and current signals; is the switching cycle; is the terminal voltage output value; and are the fundamental amplitudes of voltage and current respectively; is the filter inductor; , is the power angle of network control; ; is the initial phase of the current fundamental wave.

8. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 3, characterized in that: The physical characteristics of the high-frequency stability enhancement controller in the grid-following inverter are manifested as second-order low-pass filter characteristics.

9. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 3, characterized in that: The upper limit of the cutoff frequency of the medium and high frequency stability enhancement controller is set to 100 Hz.

10. The method for suppressing full-band cooperative oscillation of a heterogeneous inverter hybrid system according to claim 1, characterized in that: In step S7, the Bode plot and Nyquist plot of the equivalent impedance model without the cooperative oscillation suppression strategy and the equivalent impedance model with the cooperative oscillation suppression strategy are compared, and the correctness of the theoretical analysis and the effectiveness of the proposed suppression strategy are verified based on the comparison results, specifically including: The equivalent impedance model without the cooperative oscillation suppression strategy and the equivalent impedance model with the cooperative oscillation suppression strategy are plotted as Bode diagrams. The amplitude-frequency and phase-frequency curves of the Bode diagrams of the two models in the target frequency band are obtained by the frequency sweep method. The phase margin of the system can be expressed as the phase and the phase of the equivalent impedance of the grid-type and grid-type hybrid systems. The distance between them, the phase margin is less than or greater than When the system is in a negative damping state, the improvement of the phase margin of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are compared; the equivalent impedance of the equivalent impedance model without the introduction of the cooperative oscillation suppression strategy and the equivalent impedance model with the introduction of the cooperative oscillation suppression strategy are respectively compared with the equivalent impedance of the power grid to draw the Nyquist curve, and observe whether the system can still surround the (-1, j0) point after the introduction of the cooperative oscillation suppression strategy, so as to judge the stability of the system.

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