Method, device and equipment for suppressing oscillation of grid-connected hybrid photovoltaic system

Abstract

The application provides an oscillation suppression method, device and equipment for a grid-connected hybrid photovoltaic system, and relates to the technical field of power system stability control. The method comprises the following steps: constructing a sequence impedance analytical model based on the system topology of an alternating current power grid to be suppressed; determining the working condition of the alternating current power grid to be suppressed according to the sequence impedance analytical model; determining the impedance stability criterion of the equivalent impedance of a photovoltaic side and the impedance of a power grid based on an equivalent impedance model of the hybrid photovoltaic system; determining the boundary of the control parameter and the lower limit of the grid-connected photovoltaic proportion in the working condition according to the impedance stability criterion, with the lowest oscillation risk in the working condition as the optimization target and the phase margin as the constraint condition; and adjusting the grid-connected hybrid photovoltaic system based on the boundary of the control parameter and the lower limit of the grid-connected photovoltaic proportion. The application can effectively improve the stability and oscillation suppression capability of the system.

Description

Technical Field

[0001] This invention relates to the field of power system stability control technology, and in particular to an oscillation suppression method, apparatus and equipment for a grid-connected hybrid photovoltaic system. Background Technology

[0002] As the proportion of new energy units such as wind and solar power in the power system continues to increase, solar energy is gradually evolving from a supplementary energy source to a substitute energy source. Due to the continuous connection of a high proportion of new energy sources to the grid, weak grid oscillations are easily triggered. Under weak grid conditions, photovoltaic inverters using traditional grid-connected control are prone to interacting with grid impedance, causing subsynchronous / supersynchronous oscillations, which poses new challenges to the stable operation of grid-connected photovoltaic units.

[0003] To improve the absorption rate and stability of new energy sources, grid-connected photovoltaic (PV) systems are considered one of the key technologies for solving grid-connected oscillation problems under weak grid conditions. Under weak grid conditions, grid-connected PV systems do not require phase-locked loops (PLLs) and can achieve autonomous synchronization by simulating synchronous generator characteristics through strategies such as matched control, exhibiting good adaptability and stability under weak grid conditions. Therefore, converting some grid-connected PV power generation units into grid-connected systems, forming a hybrid grid / connected PV system, is expected to become an effective means of suppressing subsynchronous / supersynchronous oscillations.

[0004] Currently, research on oscillations in grid-connected / grid-connected hybrid photovoltaic (PV) systems mainly employs impedance analysis and eigenvalue analysis. However, the inventors have found that these two methods cannot provide direct criteria for determining the stability of hybrid systems, and it is difficult to quantify the required grid-connected PV ratio and the stability boundaries of key control parameters under different grid strengths, thus failing to improve the operational stability of hybrid PV systems. Summary of the Invention

[0005] This invention provides a method, apparatus, and device for suppressing oscillations in a grid-connected hybrid photovoltaic system, in order to solve the problem that current methods cannot improve the operational stability of hybrid photovoltaic systems.

[0006] In a first aspect, embodiments of the present invention provide an oscillation suppression method for a grid-connected hybrid photovoltaic system, comprising: An analytical model of sequence impedance is constructed based on the system topology of the AC power grid to be suppressed; wherein, the power grid to be suppressed includes grid-connected photovoltaic and grid-connected photovoltaic. Based on the sequence impedance analytical model, the operating conditions of the AC power grid to be suppressed are determined; Based on the equivalent impedance model of a hybrid photovoltaic system, the impedance stability criteria for the equivalent impedance of the photovoltaic side and the grid impedance are determined. Based on the impedance stability criterion, with the lowest oscillation risk under this operating condition as the optimization objective and the phase margin as the constraint, the boundaries of the control parameters and the lower limit of the proportion of grid-type photovoltaics under this operating condition are determined. The grid-connected hybrid photovoltaic system is adjusted based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics.

[0007] In one possible implementation, constructing an analytical model of sequence impedance based on the system topology of the AC power grid to be suppressed includes: Construct an electromagnetic transient simulation model of the aforementioned grid-connected hybrid photovoltaic system; Based on the electromagnetic transient simulation model, the frequency domain steady-state operating points of root-grid photovoltaic and grid-type photovoltaic are determined respectively. Based on the steady-state operating point in the frequency domain, a small-signal AC voltage disturbance of a set frequency is applied to determine the small-signal frequency domain relationships of multiple set electrical quantities; The sequence impedance analytical model is determined based on the small-signal frequency domain relationships of multiple set electrical quantities.

[0008] In one possible implementation, the AC voltage small signal of the set frequency is a positive-sequence voltage small signal with the set frequency. The sequence impedance analytical model is constructed based on the resistance and inductance on the high-voltage side and the impedance on the low-voltage side.

[0009] In one possible implementation, determining the operating condition of the AC power grid to be suppressed based on the sequence impedance analytical model includes: The short-circuit ratio is determined based on the ratio of the magnitude of the grid rated voltage to the grid sequence impedance and the rated transmission power on the DC side. Based on the short-circuit ratio and the set judgment criteria, the operating conditions of the AC power grid to be suppressed are determined; wherein, the operating conditions include strong power grid, weak power grid and extremely weak power grid.

[0010] In one possible implementation, the impedance stability criterion for determining the equivalent impedance of the photovoltaic side and the grid impedance based on the equivalent impedance model of the hybrid photovoltaic system includes: Based on the equivalent impedance model, the amplitude-frequency characteristic curve and phase-frequency characteristic curve of the photovoltaic equivalent impedance and the grid impedance are determined. Based on the amplitude-frequency response curve and the phase-frequency response curve, the impedance stability criterion is determined.

[0011] In one possible implementation, determining the impedance stability criterion based on the amplitude-frequency response curve and the phase-frequency response curve includes: Based on the intersection of the amplitude frequency response curve and the phase frequency response curve, the phase angle difference at the intersection point is determined; If the phase angle difference is less than a set value, then the hybrid photovoltaic system is determined to be stable; If the phase angle difference is greater than or less than the set value, the hybrid photovoltaic system is determined to be unstable, and the frequency at the intersection point is the subsynchronous oscillation frequency or the supersynchronous oscillation frequency.

[0012] In one possible implementation, before adjusting the grid-connected hybrid photovoltaic system based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics, the method further includes: Based on the intersection of the amplitude frequency response curve and the phase frequency response curve, the phase margin at the intersection point is determined; Based on the phase margin at the intersection point and the preset safety threshold, it is determined whether the risk of oscillation in other frequency bands is introduced while suppressing synchronous or supersynchronous oscillation.

[0013] In one possible implementation, the control parameters include the phase-locked loop proportional coefficient and the inner current loop integral parameter.

[0014] Secondly, embodiments of the present invention provide an oscillation suppression device for a grid-connected hybrid photovoltaic system, comprising: A model building module is used to construct an analytical model of sequence impedance based on the system topology of the AC power grid to be suppressed; wherein, the power grid to be suppressed includes grid-connected photovoltaic and grid-connected photovoltaic. The first determining module is used to determine the operating conditions of the AC power grid to be suppressed based on the sequence impedance analytical model. The second determination module is used to determine the impedance stability criteria of the photovoltaic side equivalent impedance and the grid impedance based on the equivalent impedance model of the hybrid photovoltaic system. The third determining module is used to determine the boundaries of the control parameters and the lower limit of the proportion of grid-type photovoltaics under the operating condition based on the impedance stability criterion, with the lowest oscillation risk under the operating condition as the optimization objective and the phase margin as the constraint condition. The adjustment module is used to adjust the grid-connected hybrid photovoltaic system based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics.

[0015] Thirdly, embodiments of the present invention provide an electronic device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the method described in the first aspect or any possible implementation thereof.

[0016] In this embodiment of the invention, to improve the stability and oscillation suppression capability of the hybrid photovoltaic system, a sequence impedance analytical model is constructed based on the system topology of the AC grid to be suppressed. This model allows the determination of the current operating condition of the AC grid. Then, to determine the stability criterion of the hybrid photovoltaic system, the impedance stability criterion between the equivalent impedance of the photovoltaic side and the grid impedance can be determined based on the equivalent impedance model of the hybrid photovoltaic system. Based on this criterion, it can be determined that the grid-connected photovoltaic system is equivalent to connecting a voltage source branch with positive damping characteristics in parallel with the grid-connected photovoltaic system. Therefore, guided by the obtained impedance stability criterion, the boundaries of control parameters and the lower limit of the proportion of grid-connected photovoltaic systems under different operating conditions can be quantitatively determined, providing a basis for subsequent parameter configuration. This effectively improves the stability and oscillation suppression capability of the hybrid photovoltaic system. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the system topology of the AC power grid to be suppressed according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating the implementation of the oscillation suppression method for a grid-connected hybrid photovoltaic system provided in this embodiment of the invention. Figure 3 This is a frequency sweep verification diagram of the positive sequence impedance analytical model of a grid-type photovoltaic system provided in an embodiment of the present invention; Figure 4 This is a frequency sweep verification diagram of the grid-connected photovoltaic positive sequence impedance analytical model provided in an embodiment of the present invention; Figure 5 This is an equivalent impedance model diagram of a hybrid photovoltaic system provided in an embodiment of the present invention; Figure 6 This is the time-domain waveform of the active power P output by the grid-connected photovoltaic system when the grid-connected proportion is switched from 5% to 15% to meet the stability boundary under a weak grid with SCR=2.57, as provided in this embodiment of the invention. Figure 7 This invention provides a time-domain waveform of the active power P output by a grid-connected photovoltaic system, obtained by reducing the phase-locked loop proportional gain from 1.0 pu to 0.13 pu under an extremely weak grid with SCR=1.93. Figure 8 The SCR provided in this embodiment of the invention is changed from 3.84 to 1.93 to excite oscillation, and the phase-locked loop proportional coefficient is reduced from 1.0 pu to the stable boundary of 0.13 pu, so as to obtain the time-domain waveform of the active power P output by the grid-connected photovoltaic system. Figure 9 This is a schematic diagram of the oscillation suppression device for a grid-connected hybrid photovoltaic system provided in an embodiment of the present invention; Figure 10 This is a schematic diagram of an electronic device provided in an embodiment of the present invention. Detailed Implementation

[0018] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0019] As introduced in the background section, impedance analysis and eigenvalue analysis are currently the main methods used to study the oscillation problem of hybrid photovoltaic systems. The core of impedance analysis is to establish a small-signal frequency domain impedance model of the power electronic device, and then use the Nyquist criterion or its extended form to determine the system stability. Eigenvalue analysis first linearizes the system at a specific operating point, and then obtains key information such as the mode's damping characteristics and oscillation frequency by solving for the eigenvalues ​​and corresponding eigenvectors of the linearized system model, thus determining the system stability at that operating point. However, both methods still have the following limitations: First, there is a lack of impedance criteria that can directly determine the stability of hybrid systems, making it difficult to quantify the influence boundaries of key factors on system stability. Second, it cannot clearly reveal the intrinsic mechanism by which grid-connected photovoltaic systems improve the oscillation characteristics of grid-connected photovoltaic systems, and it is also difficult to quantify the required proportion of grid-connected photovoltaic systems and the stability boundaries of key control parameters under different grid strengths.

[0020] To address the aforementioned technical problems, this invention provides a method, apparatus, and device for suppressing oscillations in a grid-connected hybrid photovoltaic system.

[0021] Figure 1 This is a schematic diagram of the system topology of the AC power grid to be suppressed according to an embodiment of the present invention. Figure 2 A flowchart illustrating the implementation of the oscillation suppression method for a grid-connected hybrid photovoltaic system provided in this embodiment of the invention is shown below in detail: S210. Based on the system topology of the AC power grid to be suppressed, construct an analytical model of sequence impedance.

[0022] Among them, the grid to be suppressed includes grid-connected photovoltaic and grid-connected photovoltaic.

[0023] In some embodiments, the harmonic linearization method can be used to establish the sequence impedance analytical model of grid-connected photovoltaic, grid-connected photovoltaic and the power grid.

[0024] In this embodiment, an electromagnetic transient simulation model of the root-grid hybrid photovoltaic system is first constructed. Then, based on the electromagnetic transient simulation model, the frequency domain steady-state operating points of the root-grid and grid-connected photovoltaic systems are determined respectively. Next, based on the frequency domain steady-state operating points, a small-signal AC voltage disturbance of a set frequency is applied, and the small-signal frequency domain relationships of several set electrical quantities are determined. Finally, based on the small-signal frequency domain relationships of the several set electrical quantities, an analytical model of the sequence impedance is determined.

[0025] Specifically, an electromagnetic transient simulation model of a hybrid photovoltaic (PV) system (grid-mounted / parallel-mounted) can be built in simulation software. The steady-state operating points in the frequency domain are established for both grid-mounted and parallel-mounted PV systems. Then, a small-signal AC voltage disturbance of a specific frequency is applied to these steady-state operating points. By deriving the small-signal frequency vectors of each electrical quantity and eliminating intermediate variables through simultaneous equations, analytical models of the positive-sequence and negative-sequence impedances at the AC ports are obtained. Finally, impedance scanning is performed on the constructed simulation model using a frequency sweep method. The sweep results are compared with the calculation results of the analytical models to verify the accuracy of the obtained sequence impedance models.

[0026] In this application, the AC voltage small signal with a set frequency is a positive sequence voltage small signal with a set frequency. The sequence impedance analytical model includes the positive sequence impedance model of grid-connected photovoltaics, the positive sequence impedance model of grid-connected photovoltaics, and the grid sequence impedance model; the grid sequence impedance analytical model is constructed based on the resistance and inductance on the high-voltage side and the impedance on the low-voltage side.

[0027] For example, MATLAB / Simulink can be used as simulation software to determine the steady-state operating point in the frequency domain for each electrical quantity. Taking the A-phase AC voltage as an example, its steady-state vector can be expressed as: ; in, = V g1 / 2, V g1 This represents the amplitude of the fundamental frequency phase voltage of phase A. V g1 Indicates the frequency of AC voltage. f The steady-state component of 1; Indicates conjugate.

[0028] Based on the steady-state AC voltage of phase A, a frequency of... f p If the positive-sequence voltage small-signal disturbance is a given, then the small-signal frequency sequence is: ; Therefore, the small-signal vector of the phase A AC voltage is: ; in, The frequency of phase A AC voltage is f p -2 f The small signal component of 1; The frequency of phase A AC voltage is f p The small signal component.

[0029] The single-input single-output positive-sequence impedance analytical model and negative-sequence impedance analytical model of GFL-PV based on matched control are presented. The model fully includes DC bus dynamics, reactive power outer loop, virtual impedance control, current inner loop, and matched control loop. GFL-PV exhibits capacitive negative damping characteristics in the 40~49 Hz and 62~83 Hz frequency bands under weak grid conditions, providing an impedance-level basis for explaining its susceptibility to subsynchronous / supersynchronous oscillations. The small-signal frequency domain relationships of the main circuit, phase-locked loop, power outer loop, current inner loop, matched control loop, and virtual impedance control are derived respectively, thus obtaining the AC port positive-sequence impedance analytical model.

[0030] The positive sequence impedance formulas for grid-connected photovoltaic (PV) and grid-linked PV are as follows: ; in, Z GFMP ( s This is the positive sequence impedance model for the AC port of a grid-type photovoltaic system. Z GFLP ( s This is the positive sequence impedance model for the AC port of a grid-connected photovoltaic system. The frequency of the AC voltage of phase A in a grid-type photovoltaic system is... f p The small signal component of the current, , The AC voltage and current frequency of phase A of the grid-connected photovoltaic system are... f p The small-signal current component.

[0031] After obtaining the positive sequence impedance formulas for grid-connected and grid-linked photovoltaic systems, it is necessary to verify the correctness of the obtained impedance formulas using the frequency scanning method. For example... Figure 3 and 4 As shown, Figure 3 This is a frequency sweep verification diagram for the positive sequence impedance analytical model of a grid-type photovoltaic system. Figure 4 Frequency sweep verification diagram for the positive sequence impedance analytical model of grid-connected photovoltaic system.

[0032] The power grid sequence impedance model is as follows: ; in, R g , L g The resistor and inductor are on the 110 kV side; Z g This is the impedance referred to the 35 kV side.

[0033] S220. Based on the sequence impedance analytical model, determine the operating conditions of the AC power grid to be suppressed.

[0034] In some embodiments, the short circuit ratio can be determined first according to the ratio of the modulus of the grid sequence impedance of the AC power grid to the rated transmission power at the DC side, with the grid rated voltage as a reference. Then, the operating condition of the AC power grid to be suppressed is determined according to the short circuit ratio and a set judgment criterion; wherein the operating conditions include strong power grids, weak power grids and extremely weak power grids.

[0035] In this embodiment, the strength of the AC power grid can be distinguished by the short circuit ratio (SCR), which is defined as: ; wherein, U N is the rated voltage of the power grid; Z g = R g +j100π L g , |Zg| represents Z g modulus value of. P N is the rated transmission power at the DC side.

[0036] For example, when SCR>3, it is a strong power grid; when 2<SCR<3, it is a weak power grid; when SCR<2, it is an extremely weak power grid.

[0037] S230: Based on the equivalent impedance model of the hybrid photovoltaic system, determine the impedance stability criterion for the photovoltaic-side equivalent impedance and the grid impedance.

[0038] In some embodiments, first, the amplitude-frequency characteristic curves and phase-frequency characteristic curves of the photovoltaic-side equivalent impedance and the grid impedance can be determined according to the equivalent impedance model. Then, the impedance stability criterion is determined according to the amplitude-frequency characteristic curves and the phase-frequency characteristic curves.

[0039] In this embodiment, as shown in Figure 5 the equivalent impedance model diagram of the hybrid photovoltaic system, the transfer function of the total current I g can be listed. Let the photovoltaic-side equivalent impedance be Z eq =( Z M + Z GFMPBL ) / / ( Z L + Z GFLPBL ), wherein Z M and Z L are line impedances, Z GFMPBL and ZGFLPBL These are the equivalent positive sequence impedances of multiple grid-connected and grid-connected photovoltaic units, respectively. The expression for the total grid-connected current can be derived as follows: ; in, TF 1. TF 2. TF 3 is: ; Under ideal power grid conditions, If both grid-connected photovoltaic (GFM-PV) and grid-connected photovoltaic (GFL-PV) systems can operate stably, then for GFM-PV, V s 1 / Z GFMPBL The poles are all in the left half-plane. For GFL-PV, I s 1 / Z GFLPBL The poles of all are located in the left half-plane. Therefore... I g The stability is due to TF 1. TF 2. TF 3. Decision.

[0040] As can be seen from the above, a hybrid photovoltaic system with a grid connection can achieve this if and only if it meets the following four conditions. I g is stable: First, under ideal power grid conditions, both GFL-PV and GFM-PV can operate stably; second... Z M / Z GFMPBL satisfies the Nyquist stability criterion; third, Z L / Z GFLPBL satisfies the Nyquist stability criterion; fourth, Z g / (( Z M+ Z GFMPBL) / / ( Z L+ Z GFLPBL)) satisfies the Nyquist stability criterion.

[0041] In this embodiment, the phase angle difference at the intersection of the amplitude-frequency response curve and the phase-frequency response curve can be determined. If the phase angle difference is less than a set value, the hybrid photovoltaic system is determined to be stable. If the phase angle difference is greater than or less than the set value, the hybrid photovoltaic system is determined to be unstable, and the frequency at the intersection is the subsynchronous oscillation frequency or the supersynchronous oscillation frequency.

[0042] Specifically, the phase margin at the intersection of the amplitude-frequency response curve and the phase-frequency response curve can be determined. Then, based on the phase margin at the intersection and a preset safety threshold, it can be determined whether to introduce oscillation risks in other frequency bands while suppressing synchronous or supersynchronous oscillations.

[0043] Based on the above analysis, it can be seen that the stability of a hybrid photovoltaic system depends on... Z eq With grid impedance Z g Interaction.

[0044] For example, draw respectively Z eq and Z g Find the intersection point of the amplitude-frequency response curve and the phase-frequency response curve, and read the value at that intersection point. Z eq and Z g The phase angle difference Δφ. Therefore, the necessary and sufficient condition for system stability is Δφ < 180°, that is, phase margin PM = 180° - Δφ > 0°. If the phase margin PM > 0°, the system is stable; otherwise, the system is unstable, and the frequency corresponding to the intersection point is the subsynchronous / supersynchronous oscillation frequency.

[0045] As can be seen from the above, by analyzing the impedance stability criterion, the correction effect of grid-connected photovoltaic (PV) on the equivalent impedance phase characteristics of the PV side is revealed, and the suppression mechanism is disclosed. After grid-connected PV, it is equivalent to connecting a voltage source branch Z with positive damping characteristics in parallel with the grid-connected PV side. M +Z GFMPBL This branch changes the equivalent impedance on the photovoltaic side. Z eq The phase characteristics reduce the capacitive negative damping of the photovoltaic subsystem, thereby improving the phase margin and effectively suppressing oscillations.

[0046] S240. Based on the impedance stability criterion, with the lowest oscillation risk under this operating condition as the optimization objective and the phase margin as the constraint, determine the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics under this operating condition.

[0047] The control parameters include the phase-locked loop proportional coefficient and the inner current loop integral parameter.

[0048] With the goal of minimizing the oscillation risk of the system under weak and extremely weak grid conditions, and with a phase margin greater than 0° as a constraint, the lower limit of the stability of the proportion of grid-connected photovoltaics under different grid strengths is determined based on impedance stability criteria, and the ratio coefficient of the phase-locked loop for grid-connected photovoltaics is optimized. K pllp and current inner loop integral parametersK ii2 .

[0049] For example, based on the impedance criterion, the lower stability limit of the proportion of grid-forming photovoltaic under different grid strengths is quantitatively given: the lower limit is 0 for strong grids, ≥15% for weak grids, and ≥20% for extremely weak grids. In addition, the upper stability limit of the proportional coefficient of the phase-locked loop of grid-following photovoltaic is 0.13 pu, and the integral parameter of the inner current loop K ii2 has a lower stability limit of 2.86 pu. These quantitative boundaries can be directly used for engineering design and parameter tuning, overcoming the shortcomings of traditional methods that rely on trial-and-error and lack quantitative basis.

[0050] For example, increasing K ii2 can significantly reduce the capacitive negative damping of the equivalent impedance on the photovoltaic side, increase the phase margin, and accelerate oscillation convergence. When the proportion of GFM-PV is increased from 5% to 15% under a weak grid, the active power oscillation gradually converges. When K pllp is reduced from 1.0 pu to 0.13 p.u., or K ii2 is increased from 1.0 pu to 2.86 pu, the oscillation amplitude decays rapidly and the voltage recovery time is shortened, which proves that the method has clear engineering application value.

[0051] For example, reference values of the lower stability limit of GFM-PV proportion under different grid strengths: when the strong grid has SCR > 3, the lower stability limit is 0; when the weak grid has 2 < SCR < 3, the lower stability limit is ≥ 15%; when the extremely weak grid has SCR < 2, the lower stability limit is ≥ 20%, which clarifies the quantitative relationship that the weaker the grid, the higher the required proportion of grid-forming power generation.

[0052] In some embodiments, to ensure that no oscillation risk in other frequency bands is introduced while suppressing sub-synchronous / super-synchronous oscillations, a stability check for non-sub-synchronous / non-super-synchronous oscillations is also added.

[0053] In this embodiment, the phase margin at the intersection can be determined according to the intersection of the amplitude-frequency characteristic curve and the phase-frequency characteristic curve. Then, according to the phase margin at the intersection and a preset safety threshold, it is determined whether an oscillation risk in other frequency bands is introduced while suppressing sub-synchronous or super-synchronous oscillations.

[0054] For example, by checking the equivalent impedance on the photovoltaic side Z eq and grid impedance Z g to check whether the phase margin at intersections of other frequencies is greater than the preset safety threshold, so as to ensure that no oscillation risk in other frequency bands is introduced while suppressing sub-synchronous / super-synchronous oscillations.

[0055] S250 adjusts the grid-connected hybrid photovoltaic system based on the boundaries of control parameters and the lower limit of the proportion of grid-connected photovoltaics.

[0056] Once the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics are obtained, the optimized parameters can be configured into the control loop of the hybrid photovoltaic system.

[0057] To verify the effectiveness of the oscillation suppression method provided by this invention, the grid configuration ratio satisfying the stability boundary and the optimized control parameters were configured into a hybrid photovoltaic system (grid-connected / joint-connected). Time-domain simulation was performed based on the electromagnetic transient model in simulation software. Under a weak grid with SCR=2.57, the total number of photovoltaic units was 100, and the initial grid configuration ratio was 5%, at which point the system was unstable. At t=3s, the grid configuration ratio was switched from 5% to 15% satisfying the stability boundary, and the time-domain waveform of the active power P output by the grid-connected photovoltaic system was obtained, as shown below. Figure 6 As shown in the figure, it can be seen that the active power P oscillated continuously before the switch; after the switch, the oscillation amplitude of the active power P gradually decreased, the oscillation converged, and the system entered a steady state. This indicates that when the proportion of the grid configuration reaches 15%, the system phase margin is greater than 0°, and increasing the proportion of the grid configuration can effectively suppress subsynchronous / supersynchronous oscillations, proving the effectiveness of the method of the present invention.

[0058] Under an extremely weak grid with SCR=1.93, the proportion of grid-connected photovoltaic power is fixed at 10%, and the initial phase-locked loop (PLL) proportional gain is 1.0 pu. At this point, the system oscillates. At t=2s, the PLL proportional gain is reduced from 1.0 pu to 0.13 pu, yielding the time-domain waveform of the active power P output by the grid-connected photovoltaic system, as shown below. Figure 7 As shown in the figure, before parameter adjustment, the active power P oscillated continuously; after adjustment, the oscillation amplitude gradually decreased and tended to converge, and the system returned to stability. This indicates that reducing the proportional gain of the phase-locked loop can effectively suppress subsynchronous / supersynchronous oscillations, proving the effectiveness of the method of this invention.

[0059] Under an extremely weak grid with an SCR of 1.93, the grid-connected photovoltaic system has a fixed proportion of 10%, and the initial inner-loop integral coefficient is 1.0 pu. At t=1s, the SCR changes from 3.84 to 1.93, triggering oscillation. After 0.5s, Kii2 increases to 2.86 (pu) and 14.3 (pu) respectively. At t=1.5s, the phase-locked loop proportional coefficient is reduced from 1.0 pu to the stability boundary of 0.13 pu, yielding the time-domain waveform of the active power P output by the grid-connected photovoltaic system, as shown below. Figure 10As shown, before parameter adjustment, the active power P oscillated continuously; after adjustment, the oscillation amplitude decayed rapidly, and the system tended to stabilize. Further increasing the integral parameter to 14.3 pu further accelerated the oscillation convergence speed, and the system quickly entered a steady state. This indicates that increasing the integral parameter control of the inner current loop can effectively increase the positive damping of the system and significantly suppress subsynchronous / supersynchronous oscillations, proving the effectiveness of the method of this invention.

[0060] This invention reveals, from an impedance perspective, the suppression mechanism by which parallel positive damping branches of grid-connected photovoltaic systems improve system phase characteristics and reduce capacitive negative damping by using a stability condition where the phase margin at the intersection of the amplitude-frequency characteristics of the photovoltaic equivalent impedance and the grid impedance is greater than 0°. Based on this criterion, optimization directions are given for increasing the proportion of grid-connected photovoltaic systems, decreasing the proportional coefficient of the grid-connected photovoltaic phase-locked loop, and increasing the integral parameter of the inner current loop under weak grid conditions. This effectively improves the accuracy and engineering practicality of parameter optimization and avoids the blindness of traditional trial-and-error methods.

[0061] In this embodiment of the invention, to improve the stability and oscillation suppression capability of the hybrid photovoltaic system, a sequence impedance analytical model is constructed based on the system topology of the AC grid to be suppressed. This model allows the determination of the current operating condition of the AC grid. Then, to determine the stability criterion of the hybrid photovoltaic system, the impedance stability criterion between the equivalent impedance of the photovoltaic side and the grid impedance can be determined based on the equivalent impedance model of the hybrid photovoltaic system. Based on this criterion, it can be determined that the grid-connected photovoltaic system is equivalent to connecting a voltage source branch with positive damping characteristics in parallel with the grid-connected photovoltaic system. Therefore, guided by the obtained impedance stability criterion, the boundaries of control parameters and the lower limit of the proportion of grid-connected photovoltaic systems under different operating conditions can be quantitatively determined, providing a basis for subsequent parameter configuration. This effectively improves the stability and oscillation suppression capability of the hybrid photovoltaic system.

[0062] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0063] The following are device embodiments of the present invention. For details not described in detail, please refer to the corresponding method embodiments described above.

[0064] Figure 9 A schematic diagram of the oscillation suppression device for a grid-connected hybrid photovoltaic system provided in an embodiment of the present invention is shown. For ease of explanation, only the parts relevant to the embodiment of the present invention are shown, and are described in detail below: like Figure 9 As shown, the oscillation suppression device for a grid-connected hybrid photovoltaic system includes: Model module 910 is used to construct an analytical model of sequence impedance based on the system topology of the AC power grid to be suppressed; wherein the power grid to be suppressed includes grid-connected photovoltaic and grid-connected photovoltaic. The first determining module 920 is used to determine the operating conditions of the AC power grid to be suppressed based on the sequence impedance analytical model. The second determining module 930 is used to determine the impedance stability criteria of the photovoltaic side equivalent impedance and the grid impedance based on the equivalent impedance model of the hybrid photovoltaic system. The third determination module 940 is used to determine the boundaries of the control parameters and the lower limit of the proportion of grid-type photovoltaics under the operating condition based on the impedance stability criterion, with the lowest oscillation risk under the operating condition as the optimization objective and the phase margin as the constraint condition. The adjustment module 950 is used to adjust the grid-connected hybrid photovoltaic system based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaic.

[0065] In one possible implementation, a model module 910 is constructed to construct an electromagnetic transient simulation model of a grid-connected hybrid photovoltaic system. Based on the electromagnetic transient simulation model, the frequency domain steady-state operating points of root-grid photovoltaic and grid-type photovoltaic are determined respectively. Based on the steady-state operating point in the frequency domain, a small-signal AC voltage disturbance of a set frequency is applied to determine the small-signal frequency domain relationship of multiple set electrical quantities; Based on the small-signal frequency domain relationships of multiple set electrical quantities, a sequence impedance analytical model is determined.

[0066] In one possible implementation, the AC voltage small signal with a set frequency is a positive-sequence voltage small signal with a set frequency; The sequence impedance analytical model includes the positive sequence impedance model of grid-connected photovoltaics, the positive sequence impedance model of grid-connected photovoltaics, and the grid sequence impedance model; the grid sequence impedance analytical model is constructed based on the resistance and inductance on the high-voltage side and the impedance on the low-voltage side.

[0067] In one possible implementation, the first determining module 920 is used to determine the short-circuit ratio based on the ratio of the magnitude of the grid rated voltage to the grid sequence impedance and the DC side rated transmission power. Based on the short-circuit ratio and the set judgment criteria, the operating conditions of the AC power grid to be suppressed are determined; among them, the operating conditions include strong power grid, weak power grid and extremely weak power grid.

[0068] Figure 10 This is a schematic diagram of an electronic device provided in an embodiment of the present invention. For example... Figure 10As shown, the electronic device 10 of this embodiment includes a processor 100 and a memory 101. The memory 101 stores a computer program 102. When the processor 100 executes the computer program 102, it implements the steps in the various method embodiments described above. Alternatively, when the processor 100 executes the computer program 102, it implements the functions of each module / unit in the various device embodiments described above.

[0069] For example, computer program 102 may be divided into one or more modules / units, which are stored in memory 101 and executed by processor 100 to complete the present invention. The one or more modules / units may be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of computer program 102 in electronic device 10.

[0070] Electronic device 10 may include, but is not limited to, processor 100 and memory 101. Those skilled in the art will understand that... Figure 10 This is merely an example of electronic device 10 and does not constitute a limitation on electronic device 10. It may include more or fewer components than shown, or combine certain components, or different components. For example, electronic device 10 may also include input / output devices, network access devices, buses, etc.

[0071] For the sake of simplicity and clarity, only the above-described functional modules / units are used as examples. In practical applications, the functions described above can be assigned to different functional modules / units as needed. These modules / units can be implemented in hardware, software, or a combination of both.

[0072] In the above embodiments, the descriptions of each embodiment have their own emphasis. Parts not detailed or described in a particular embodiment can be referred to in the relevant descriptions of other embodiments. Unless otherwise specified or in conflict with logic, the terminology and / or descriptions between different embodiments are consistent and can be referenced interchangeably. Technical features in different embodiments can be combined to form new embodiments based on their inherent logical relationships.

[0073] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for suppressing oscillations in a grid-connected hybrid photovoltaic system, characterized in that, include: An analytical model of sequence impedance is constructed based on the system topology of the AC power grid to be suppressed; wherein, the power grid to be suppressed includes grid-connected photovoltaic and grid-connected photovoltaic. Based on the sequence impedance analytical model, the operating conditions of the AC power grid to be suppressed are determined; Based on the equivalent impedance model of a hybrid photovoltaic system, the impedance stability criteria for the equivalent impedance of the photovoltaic side and the grid impedance are determined. Based on the impedance stability criterion, with the lowest oscillation risk under this operating condition as the optimization objective and the phase margin as the constraint, the boundaries of the control parameters and the lower limit of the proportion of grid-type photovoltaics under this operating condition are determined. The grid-connected hybrid photovoltaic system is adjusted based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics.

2. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to claim 1, characterized in that, The system topology based on the AC power grid to be suppressed is used to construct an analytical model of sequence impedance, including: Construct an electromagnetic transient simulation model of the aforementioned grid-connected hybrid photovoltaic system; Based on the electromagnetic transient simulation model, the frequency domain steady-state operating points of root-grid photovoltaic and grid-type photovoltaic are determined respectively. Based on the steady-state operating point in the frequency domain, a small-signal AC voltage disturbance of a set frequency is applied to determine the small-signal frequency domain relationships of multiple set electrical quantities; The sequence impedance analytical model is determined based on the small-signal frequency domain relationships of multiple set electrical quantities.

3. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to claim 2, characterized in that, The AC voltage small signal at the set frequency is a positive-sequence voltage small signal with the set frequency. The sequence impedance analytical model includes the positive sequence impedance model of grid-connected photovoltaics, the positive sequence impedance model of grid-connected photovoltaics, and the grid sequence impedance model; the grid sequence impedance model is constructed based on the resistance and inductance on the high-voltage side and the impedance on the low-voltage side.

4. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to claim 1, characterized in that, The step of determining the operating conditions of the AC power grid to be suppressed based on the sequence impedance analytical model includes: The short-circuit ratio is determined based on the ratio of the magnitude of the grid rated voltage to the grid sequence impedance and the rated transmission power on the DC side. Based on the short-circuit ratio and the set judgment criteria, the operating conditions of the AC power grid to be suppressed are determined; wherein, the operating conditions include strong power grid, weak power grid and extremely weak power grid.

5. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to claim 1, characterized in that, The equivalent impedance model based on the hybrid photovoltaic system determines the impedance stability criteria for the equivalent impedance on the photovoltaic side and the grid impedance, including: Based on the equivalent impedance model, the amplitude-frequency characteristic curve and phase-frequency characteristic curve of the photovoltaic equivalent impedance and the grid impedance are determined. Based on the amplitude-frequency response curve and the phase-frequency response curve, the impedance stability criterion is determined.

6. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to claim 5, characterized in that, The determination of the impedance stability criterion based on the amplitude-frequency response curve and the phase-frequency response curve includes: Based on the intersection of the amplitude frequency response curve and the phase frequency response curve, the phase angle difference at the intersection point is determined; If the phase angle difference is less than a set value, then the hybrid photovoltaic system is determined to be stable; If the phase angle difference is greater than or less than the set value, the hybrid photovoltaic system is determined to be unstable, and the frequency at the intersection point is the subsynchronous oscillation frequency or the supersynchronous oscillation frequency.

7. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to claim 6, characterized in that, Before adjusting the hybrid photovoltaic system based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics, the method further includes: Based on the intersection of the amplitude frequency response curve and the phase frequency response curve, the phase margin at the intersection point is determined; Based on the phase margin at the intersection point and the preset safety threshold, it is determined whether the risk of oscillation in other frequency bands is introduced while suppressing synchronous or supersynchronous oscillation.

8. The oscillation suppression method for a grid-connected hybrid photovoltaic system according to any one of claims 1-7, characterized in that, The control parameters include the phase-locked loop proportional coefficient and the current inner loop integral parameter.

9. An oscillation suppression device for a grid-connected hybrid photovoltaic system, characterized in that, include: A model building module is used to construct an analytical model of sequence impedance based on the system topology of the AC power grid to be suppressed; wherein, the power grid to be suppressed includes grid-connected photovoltaic and grid-connected photovoltaic. The first determining module is used to determine the operating conditions of the AC power grid to be suppressed based on the sequence impedance analytical model. The second determination module is used to determine the impedance stability criteria of the photovoltaic side equivalent impedance and the grid impedance based on the equivalent impedance model of the hybrid photovoltaic system. The third determining module is used to determine the boundaries of the control parameters and the lower limit of the proportion of grid-type photovoltaics under the operating condition based on the impedance stability criterion, with the lowest oscillation risk under the operating condition as the optimization objective and the phase margin as the constraint condition. The adjustment module is used to adjust the grid-connected hybrid photovoltaic system based on the boundaries of the control parameters and the lower limit of the proportion of grid-connected photovoltaics.

10. An electronic device, characterized in that, It includes a memory and a processor, the memory storing a computer program, and the processor executing the computer program to implement the method as described in any one of claims 1 to 8.

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