Wide-frequency domain parameter dynamic adjustment method for flexible low-frequency power transmission photovoltaic inverter

Abstract

The invention discloses a wide frequency domain parameter dynamic adjustment method for a flexible low-frequency power transmission photovoltaic inverter, and relates to the technical field of power electronics and flexible power transmission. According to the method, based on filter parameters and control parameters, an output impedance frequency domain model is established at a low-frequency band of 10-50 Hz; in combination with power grid impedance, a low-frequency stability index is defined according to a Nyquist criterion, and the index is a weighted sum of an amplitude margin and a phase margin which can be adaptively adjusted; constructing a multi-target optimization model with a maximum stability index and a harmonic distortion rate as targets; solving an optimal parameter vector by adopting an intelligent optimization algorithm; the system frequency is monitored in real time, and when a low-frequency mode is switched or the frequency change exceeds a threshold value, parameters are updated online, and filter topology and controller parameters are dynamically adjusted. According to the invention, flexible switching and stable operation of the photovoltaic inverter between the power frequency mode and the low frequency mode are realized, and the reliability and the electric energy quality of the flexible low-frequency power transmission system are effectively improved.

Description

Technical Field

[0001] This invention relates to the fields of power electronics and flexible power transmission technology, specifically to a method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission. Background Technology

[0002] Flexible low-frequency power transmission technology is a novel power transmission method that reduces line impedance, increases transmission capacity, and reduces losses by lowering the transmission frequency (e.g., 10Hz–50Hz). It is particularly suitable for long-distance offshore wind power and power transmission from photovoltaic power plants. However, traditional photovoltaic inverters are designed for operation at the power frequency (50Hz / 60Hz), and their control parameters, filter topologies, and phase-locked loop mechanisms are optimized for power frequency. Directly applying these to low-frequency power transmission faces the following problems: 1. Risk of instability: The impedance characteristics of the system change greatly at low frequencies. Traditional fixed-parameter control strategies are prone to causing a decrease in stability or even oscillation.

[0003] 2. Poor output power quality: When operating at low frequencies, the original design of the LCL filter may cause resonance shift, resulting in increased output waveform distortion.

[0004] 3. Insufficient dynamic response: The bandwidth of the current loop and phase-locked loop needs to be readjusted at low frequencies, otherwise the dynamic performance will degrade.

[0005] 4. Lack of mode switching capability: Existing inverters do not support smooth and stable switching between power frequency grid connection and low frequency power transmission modes.

[0006] Therefore, there is an urgent need for a parameter adjustment method for photovoltaic inverters that can adapt to the demand for flexible low-frequency power transmission and achieve stable operation in a wide frequency range. Summary of the Invention

[0007] The technical problem to be solved by this invention is how to solve the problem of parameter mismatch and stability decline caused by changes in operating frequency in flexible low-frequency power transmission technology.

[0008] This invention solves the above-mentioned technical problems through the following technical means: a method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission, comprising: S1. Based on the filter parameters and control parameters of the photovoltaic inverter, establish the frequency domain expression of the output impedance in the 10Hz~50Hz frequency band in the rotating coordinate system corresponding to low-frequency operation. ,in For the Laplace operator; S2, Combining the aforementioned output impedance frequency domain expression Equivalent grid impedance of flexible low-frequency transmission system Based on the Nyquist stability criterion, the low-frequency stability criterion index is determined. ,in, For the magnitude margin, For phase margin, The weighting coefficient for the gain margin. The weighting coefficients for phase margin are: and It can adaptively adjust to changes in low-frequency power transmission frequency; S3. Define the filter parameters and control parameters as a parameter vector to be optimized. To maximize With the goal of considering the power quality of low-frequency output, an optimization problem model was established based on the constraints. S4. The optimal parameter vector corresponding to the target low-frequency frequency is obtained by iteratively solving using an intelligent optimization algorithm. ; S5. Monitor the operating frequency of the power transmission system in real time. When the system switches to flexible low-frequency transmission mode or the low-frequency operating frequency changes beyond the preset threshold, take the target low-frequency frequency as input, repeat steps S1 to S4, and update the optimal parameter vector online. It also dynamically adjusts the filter topology and control parameters of the photovoltaic inverter.

[0009] Furthermore, the filter mentioned in step S1 is a reconfigurable LCL filter, and the filter parameters include the inverter-side inductance. , grid-side inductor and filter capacitor The control parameters include the current loop proportional coefficient. Integral coefficient Phase-locked loop proportional coefficient Integral coefficient Capacitor current feedback coefficient and low-frequency voltage feedforward coefficient The parameter vector to be optimized .

[0010] Furthermore, the reconfigurable LCL filter described in step S1 has a low-frequency capacitor switching module, which increases the value of the filter capacitor or connects an auxiliary capacitor in parallel when switching to the low-frequency transmission mode to adapt to the filtering requirements under low-frequency output.

[0011] Furthermore, the phase-locked loop corresponding to the control parameters in step S1 adopts a dual-mode phase-locked loop structure, including a power frequency phase-locked loop and a low-frequency phase-locked loop. The bandwidth and center frequency of the low-frequency phase-locked loop are adjustable, and the two are switched according to system instructions.

[0012] Furthermore, the magnitude margin mentioned in step S2 and phase margin The ratio of inverter output impedance to grid impedance. The margin of the Nyquist curve relative to the point (-1, j0), where j0 is the coordinate point with the imaginary axis at 0, and the calculation interval corresponds to the frequency range of 10Hz to 50Hz.

[0013] Furthermore, the weighting coefficients for the magnitude margin and phase margin... and Dynamic adjustments are made based on the current low-frequency transmission frequency and system stability status, specifically including: Obtain the current low-frequency transmission frequency and system operating status parameters; Calculate the frequency impact factor: ,in, The power frequency reference value, This is the lowest permissible low-frequency operating frequency for the system. Calculate the stability requirement factor: ,in, This is the estimated phase margin of the current system. This is the phase margin threshold. This is the adjustment coefficient; Calculate the weighting coefficients: , ,in Basic weight value, and These are the weighting coefficients of the frequency factor and the stability factor, respectively. and The value range is [0,1].

[0014] Furthermore, the objective expression of the optimization problem model described in step S3 is:

[0015] in, , This represents the harmonic distortion rate of the inverter at low-frequency output.

[0016] Furthermore, the constraint conditions described in step S3 are as follows: Parameter boundary constraints: ,in, This represents the minimum value within the parameter's range. This represents the maximum value within the parameter's range. Low-frequency dynamic performance constraints: ,in, This is the cutoff frequency of the current loop in the low-frequency range. This is the lower limit of the cutoff frequency. This is the upper limit of the cutoff frequency; Low-frequency voltage accuracy constraints: ,in For low-frequency output voltage deviation, The upper limit of the allowable deviation; Low-frequency power smoothness constraint: ,in, For low-frequency output power fluctuations, This represents the upper limit of allowed fluctuations.

[0017] Furthermore, the rotating coordinate system is either the dq coordinate system or the αβ coordinate system.

[0018] Furthermore, the intelligent optimization algorithm described in step S4 is any one of particle swarm optimization, genetic algorithm, or differential evolution algorithm.

[0019] The advantages of this invention are: This invention solves the problem of model mismatch and control parameter mismatch caused by frequency changes in flexible low-frequency power transmission by constructing a wide-frequency impedance model in the 10Hz-50Hz low-frequency range and using an adaptively adjustable weighted sum of amplitude margin and phase margin as a stability criterion. It also addresses the challenges of traditional methods in flexible low-frequency power transmission by constructing a wide-frequency impedance model in the 10Hz-50Hz low-frequency range and using an adaptively adjustable weighted sum of amplitude margin and phase margin as a stability criterion. Furthermore, it achieves rapid tuning of globally optimal parameters by using an intelligent optimization algorithm to perform multi-objective collaborative optimization of filter and control parameters while considering low-frequency harmonic distortion. By monitoring the operating frequency in real time and dynamically updating the optimal parameter vector online, the inverter can flexibly switch between power frequency and low-frequency modes, significantly improving the system's stability margin and power quality over a wide frequency range and avoiding the oscillation risk during low-frequency operation. Attached Figure Description

[0020] Figure 1 This is a flowchart of a method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission, according to Embodiment 1 of the present invention. Figure 2 This is a comparison chart of the stability margins of the fixed parameters in Embodiment 1 of the present invention and the dynamically adjusted parameters of the present invention; Figure 3 This is a waveform diagram of the output voltage during mode switching in Embodiment 1 of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1 like Figure 1As shown, a method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission includes: S1. Based on the filter parameters and control parameters of the photovoltaic inverter, establish the frequency domain expression of the output impedance in the 10Hz~50Hz frequency band in the rotating coordinate system corresponding to low-frequency operation. ,in For the Laplace operator.

[0023] Specifically, the rotating coordinate system corresponding to low-frequency operation is the dq coordinate system or the αβ coordinate system.

[0024] The filter is a reconfigurable LCL filter, and its parameters include the inverter-side inductance. , grid-side inductor and filter capacitor Control parameters include the current loop proportional coefficient. Integral coefficient Phase-locked loop proportional coefficient Integral coefficient Capacitor current feedback coefficient and low-frequency voltage feedforward coefficient ; Parameter vector to be optimized .

[0025] The reconfigurable LCL filter features a low-frequency capacitor switching module. When switching to low-frequency transmission mode, it increases the filter capacitor value or connects an auxiliary capacitor in parallel to adapt to the filtering requirements under low-frequency output. For example... Figure 3 As shown.

[0026] The phase-locked loop corresponding to the control parameters adopts a dual-mode phase-locked structure, including a power frequency phase-locked loop and a low-frequency phase-locked loop. The bandwidth and center frequency of the low-frequency phase-locked loop are adjustable, and the two are switched according to system commands.

[0027] S2, Combining the frequency domain expression of output impedance Equivalent grid impedance of flexible low-frequency transmission system Based on the Nyquist stability criterion, the low-frequency stability criterion index is determined. ,in, For the magnitude margin, For phase margin, The weighting coefficient for the gain margin. The weighting coefficients for phase margin are: and It can adaptively adjust to changes in low-frequency power transmission frequency.

[0028] Specifically, this embodiment takes a photovoltaic inverter in a flexible low-frequency power transmission system as the application object, adapting to the wide frequency range operation requirements of 10Hz to 50Hz. By collecting real-time operating parameters of the system, the frequency influence factor and stability requirement factor are calculated, and finally the weighting coefficient is realized. , The dynamic adjustment is implemented as follows: 1. Core parameter configuration To achieve adaptive weight calculation, the core parameters of the flexible low-frequency transmission system and photovoltaic inverter need to be pre-configured. The values, physical meanings, and configuration basis of each parameter are as follows. All parameters are determined based on engineering practice and system stability requirements: Adaptive weight calculation requires pre-configuration of the following core parameters: Power frequency reference value 50.0Hz, the lowest permissible low-frequency operating frequency 10.0Hz, phase margin threshold 30.0°, adjustment coefficient 0.1, base weight value 0.5, frequency factor weighting coefficient Stability factor: 0.2 : 0.3.

[0029] 2. Adaptive weight calculation In this embodiment, the adaptive weighting coefficients α (amplitude margin weight) and β (phase margin weight, β=1) are used. The calculation of α is accomplished through the following steps: (1) Obtain current running status parameters Real-time acquisition of current operating parameters of the low-frequency power transmission system, including: Current low-frequency operating frequency The unit is Hz, and the current phase margin estimate is... The unit is degrees (°), and it calls the preset core parameters.

[0030] (2) Calculate the frequency influence factor

[0031] Frequency Influence Factor The degree to which the current operating frequency deviates from the power frequency reference value is characterized, and its calculation follows a piecewise linear saturation function: when At this time, the frequency influence factor This indicates that the system is in an extremely low frequency band, at which point the amplitude margin weight should reach its maximum. when At this time, the frequency influence factor This indicates that the system is operating close to the power frequency, and the frequency has a negligible impact on the weights. when (When the current frequency is in the low-frequency operating range) Calculate using the following formula:

[0032] The lower the frequency, The larger the value, the greater the weight of the amplitude margin in the overall stability margin assessment, in order to compensate for the control uncertainty caused by the increased phase information delay in the low frequency band.

[0033] (3) Calculate the stability demand factor

[0034] Stability demand factor Based on the deviation between the current phase margin and the threshold, the Sigmoid function is used to achieve smooth nonlinear mapping, and its calculation formula is as follows:

[0035] Where k is the adjustment coefficient, controlling The steepness of the transition near the threshold. When the phase margin is below the threshold. This indicates that the system is in an unstable state, and the magnitude margin weight needs to be increased to enhance robustness; when the phase margin is higher than the threshold, This indicates that the system has sufficient stability margin, and the amplitude weight can be appropriately reduced to optimize the dynamic response.

[0036] (4) Calculate the weighting coefficients Based on the basic weights, frequency influence factor, and stability requirement factor, the initial weighting coefficients of the gain margin are calculated using a linear superposition method. The calculation formula is as follows: Calculate the weighting coefficients: , ,in Basic weight value, and These are the weighting coefficients for the frequency factor and the stability factor, and and The value range is [0,1].

[0037] Through weighting coefficients To ensure the stability of the computational benchmark, by and This allows for independent adjustment of frequency factors and stability requirements, preventing weight calculations from falling into local extrema.

[0038] To verify the effectiveness of this embodiment, the verification results of the following three typical operating scenarios are provided: Scenario 1: Low-frequency operation and insufficient stability margin Operating conditions: , (Below the threshold of 30°).

[0039] Calculation results: , , , .

[0040] The system is in a double disadvantageous state of low frequency and insufficient phase margin. The algorithm adjusts the magnitude margin weight to 0.763 and the phase margin weight to 0.237, so that the controller relies more on magnitude information when evaluating the system stability margin, effectively avoiding misjudgment caused by phase delay.

[0041] Scenario 2: Low-frequency operation but with sufficient stability margin Operating conditions: , (30° above the threshold).

[0042] Calculation results: , , , .

[0043] Although the frequency is still low, the phase margin is sufficient. Accordingly, the algorithm adjusts the amplitude margin weight to 0.650 and the phase weight to 0.350, so as to ensure robustness while taking into account dynamic response performance.

[0044] Scenario 3: Operating near power frequency Operating conditions: , .

[0045] Calculation results: , , , .

[0046] With the frequency close to the power frequency, the frequency influence factor λ_f approaches 0, and the phase margin slightly exceeds the threshold, the algorithm automatically adjusts the weighting coefficients to near neutral. ≈ (≈0.5), allowing the stability margin assessment to revert to the traditional power frequency mode. This scenario verifies the algorithm's ability to smoothly transition over a wide frequency range.

[0047] S3. Define the filter parameters and control parameters as the parameter vector to be optimized. To maximize With the goal of considering the quality of low-frequency output power, an optimization problem model was established based on the constraints.

[0048] Specifically, the objective expression for the optimization problem model is:

[0049] in, and These are the weighting coefficients, , This represents the harmonic distortion rate of the inverter at low-frequency output.

[0050] The constraints are: Parameter boundary constraints: ,in, This represents the minimum value within the parameter's range. This represents the maximum value within the parameter's range.

[0051] Low-frequency dynamic performance constraints: ,in, This is the cutoff frequency of the current loop in the low-frequency range. This is the lower limit of the cutoff frequency. This is the upper limit of the cutoff frequency.

[0052] Low-frequency voltage accuracy constraints: ,in For low-frequency output voltage deviation, This represents the upper limit of the allowable deviation.

[0053] Low-frequency power smoothness constraint: ,in, For low-frequency output power fluctuations, This represents the upper limit of allowed fluctuations.

[0054] S4. The optimal parameter vector corresponding to the target low-frequency frequency is obtained by iteratively solving using an intelligent optimization algorithm. .

[0055] Specifically, the intelligent optimization algorithm can be any one of particle swarm optimization, genetic algorithm, or differential evolution algorithm. The specific steps are as follows: Initialize the particle swarm, where the position of each particle represents a parameter vector X to be optimized, and the velocity is randomly generated; Calculate the fitness value of each particle, i.e., the stability criterion index. ; Update the individual optimal position and the global optimal position; The state of the particles is iteratively updated based on the velocity and position update formulas of the particle swarm optimization algorithm. When the maximum number of iterations is reached or the fitness value converges to a stable value, the globally optimal position is output as X_opt.

[0056] S5. Monitor the operating frequency of the power transmission system in real time. When the system switches to flexible low-frequency transmission mode or the low-frequency operating frequency changes beyond the preset threshold, take the target low-frequency frequency as input, repeat steps S1 to S4, and update the optimal parameter vector online. The system dynamically adjusts the filter topology of the photovoltaic inverter (e.g., adding / removing some capacitors / inductors) and control parameters to ensure the inverter outputs stable low-frequency voltage and current that meet requirements. Figure 2 As shown, comparing the stability margins of fixed parameters and dynamically adjusted parameters, the stability phase margin of dynamically adjusted parameters is improved by 87°. The above 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.

Claims

1. A method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission, characterized in that, include: S1. Based on the filter parameters and control parameters of the photovoltaic inverter, establish the frequency domain expression of the output impedance in the 10Hz~50Hz frequency band in the rotating coordinate system corresponding to low-frequency operation. ,in For the Laplace operator; S2, Combining the aforementioned output impedance frequency domain expression Equivalent grid impedance of flexible low-frequency transmission system Based on the Nyquist stability criterion, the low-frequency stability criterion index is determined. ,in, For the magnitude margin, For phase margin, The weighting coefficient for the gain margin. The weighting coefficients for phase margin are: and It can adaptively adjust to changes in low-frequency power transmission frequency; S3. Define the filter parameters and control parameters as a parameter vector to be optimized. To maximize With the goal of considering the power quality of low-frequency output, an optimization problem model was established based on the constraints. S4. The optimal parameter vector corresponding to the target low-frequency frequency is obtained by iteratively solving using an intelligent optimization algorithm. ; S5. Monitor the operating frequency of the power transmission system in real time. When the system switches to flexible low-frequency transmission mode or the low-frequency operating frequency changes beyond the preset threshold, take the target low-frequency frequency as input, repeat steps S1 to S4, and update the optimal parameter vector online. It also dynamically adjusts the filter topology and control parameters of the photovoltaic inverter.

2. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The filter mentioned in step S1 is a reconfigurable LCL filter, and the filter parameters include the inverter-side inductance. , grid-side inductor and filter capacitor The control parameters include the current loop proportional coefficient. Integral coefficient Phase-locked loop proportional coefficient Integral coefficient Capacitor current feedback coefficient and low-frequency voltage feedforward coefficient The parameter vector to be optimized .

3. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 2, characterized in that, The reconfigurable LCL filter described in step S1 has a low-frequency capacitor switching module. When switching to low-frequency power transmission mode, the filter capacitor value is increased or an auxiliary capacitor is connected in parallel to adapt to the filtering requirements under low-frequency output.

4. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 2, characterized in that, The phase-locked loop corresponding to the control parameters mentioned in step S1 adopts a dual-mode phase-locked loop structure, including a power frequency phase-locked loop and a low-frequency phase-locked loop. The bandwidth and center frequency of the low-frequency phase-locked loop are adjustable, and the two are switched according to system instructions.

5. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The gain margin mentioned in step S2 and phase margin The ratio of inverter output impedance to grid impedance. The margin of the Nyquist curve relative to the point (-1, j0), where j0 is the coordinate point with the imaginary axis at 0, and the calculation interval corresponds to the frequency range of 10Hz to 50Hz.

6. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The weighting coefficients for the magnitude margin and phase margin and Dynamic adjustments are made based on the current low-frequency transmission frequency and system stability status, specifically including: Obtain the current low-frequency transmission frequency and system operating status parameters; Calculate the frequency impact factor: ,in, The power frequency reference value, This is the lowest permissible low-frequency operating frequency for the system. Calculate the stability requirement factor: ,in, This is the estimated phase margin of the current system. This is the phase margin threshold. This is the adjustment coefficient; Calculate the weighting coefficients: , ,in Basic weight value, and These are the weighting coefficients of the frequency factor and the stability factor, respectively. and The value range is [0,1].

7. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The objective expression of the optimization problem model described in step S3 is: in, , This represents the harmonic distortion rate of the inverter at low-frequency output.

8. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The constraint conditions mentioned in step S3 are: Parameter boundary constraints: ,in, This represents the minimum value within the parameter's range. This represents the maximum value within the parameter's range. Low-frequency dynamic performance constraints: ,in, This is the cutoff frequency of the current loop in the low-frequency range. This is the lower limit of the cutoff frequency. This is the upper limit of the cutoff frequency; Low-frequency voltage accuracy constraints: ,in For low-frequency output voltage deviation, The upper limit of the allowable deviation; Low-frequency power smoothness constraint: ,in, For low-frequency output power fluctuations, This represents the upper limit of allowed fluctuations.

9. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The rotating coordinate system is either the dq coordinate system or the αβ coordinate system.

10. The method for dynamic adjustment of wide-frequency domain parameters of a photovoltaic inverter for flexible low-frequency power transmission according to claim 1, characterized in that, The intelligent optimization algorithm mentioned in step S4 is any one of particle swarm optimization, genetic algorithm, or differential evolution algorithm.

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