A grid-connected inverter hybrid damping adaptive control method considering short-circuit ratio

Abstract

This invention discloses a hybrid damping adaptive control method for grid-connected inverters that considers the short-circuit ratio, belonging to the field of power conversion technology. The steps include: constructing a three-phase grid-connected inverter main circuit system; obtaining an error signal by subtracting the inverter grid-connected current from a reference current, and using the error signal as the input to a quasi-resonant controller; and obtaining a feedforward signal u from the capacitor current through an improved hybrid damping control loop. hr An adaptive control equation consisting of the short-circuit ratio λ, passive weighting coefficient K1, and active weighting coefficient K2 is introduced. Optimal hybrid damping control parameters K1 and K2 are generated based on the real-time detected value of the short-circuit ratio λ. The output signal u of the quasi-resonant controller is then... pr Subtract the feedforward signal u hr The modulation signal is obtained; the modulation signal is compared with the carrier control signal to control the switching on and off of the three-phase inverter power transistors. This invention, applied to LCL-type grid-connected inverter systems, can adapt to a wide range of grid strength variations, maintain sufficient phase margin in the system, and improve system stability.

Description

Technical Field

[0001] This invention relates to the field of power conversion technology, and in particular to a hybrid damping adaptive control method for grid-connected inverters that takes into account the short-circuit ratio. Background Technology

[0002] Renewable energy sources are connected to the AC grid via grid-connected inverters, forming a dynamic interconnected system with a common coupling point exhibiting resonant interaction. As the installed capacity of renewable energy increases, the penetration rate of new energy installations has exceeded 50% in some regions. At this point, the electricity transmitted to the grid system is primarily supplied by renewable energy generation, with the surplus being supplemented by thermal power plants. Currently, grid-connected inverters mostly use LCL filters for grid connection. There are generally two solutions to the LCL filter resonance problem: passive damping and active damping control. Passive damping mainly increases damping by connecting an impedance in series with the LCL; active damping mainly increases damping by simulating damping in the control loop.

[0003] Passive damping introduces additional losses and lacks flexibility; active damping offers greater flexibility, but it is detrimental to system stability when grid impedance changes. Currently, researchers have proposed a hybrid damping control method that combines the advantages of both methods, avoiding additional losses and improving system stability. However, in grid-connected systems with high penetration rates, the volatility of renewable energy generation leads to wide variations in grid strength, making traditional hybrid damping control methods unable to maintain consistent system stability.

[0004] To address the aforementioned issues, a hybrid damping adaptive control method needs to be proposed to maintain system stability and provide technical support for the construction of new power systems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a hybrid damping adaptive control method for grid-connected inverters that takes into account the short-circuit ratio. When applied to LCL-type grid-connected inverter systems, this method can not only adapt to a wide range of grid intensity variations, but also maintain sufficient phase margin in the system and improve system stability.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A hybrid damping adaptive control method for grid-connected inverters that takes into account the short-circuit ratio includes the following steps:

[0008] S1. Construct a three-phase grid-connected inverter main circuit system;

[0009] S2. The error signal is obtained by subtracting the inverter grid current from the reference current, and the error signal is used as the input of the quasi-resonant controller.

[0010] S3. The capacitor current is passed through an improved hybrid damping control circuit to obtain the feedforward signal u. hr ;

[0011] S4. In order to ensure that the system can remain stable under a wide range of grid strength, an adaptive control equation composed of short-circuit ratio λ, passive weighting coefficient K1 and active weighting coefficient K2 is introduced. The optimal hybrid damping control parameters K1 and K2 are generated based on the real-time detection value of short-circuit ratio λ.

[0012] S5, the output signal u of the quasi-resonant controller pr Subtract the feedforward signal u obtained in S3 hr The modulated signal is obtained;

[0013] S6. The modulation signal is compared with the carrier control signal to control the power transistors of the three-phase inverter to turn on and off.

[0014] A further improvement of the technical solution of the present invention is that: in S1, the main circuit structure of the three-phase grid-connected inverter includes six power switching devices, an LCL filter and a grid-side inductor; wherein, the six power switching devices form a three-phase voltage-type bridge inverter structure, the LCL filter is used to filter out the high-order voltage harmonics output by the power switching devices, and the grid-side inductor is used to simulate the line impedance; the capacitor branch of the LCL filter is connected in series with a damping resistor R.

[0015] A further improvement to the technical solution of this invention lies in: in S2, the transfer function of the quasi-resonant controller is:

[0016]

[0017] In the formula, K p As a proportion, K r ω is the integral coefficient. c ω is the bandwidth coefficient. o denoted as the fundamental angular frequency, and s as the Laplace operator.

[0018] A further improvement of the technical solution of the present invention is that: in S3, the improved hybrid damping control link is: the control strategy of the grid-connected inverter simultaneously adopts passive damping and active damping, and two control parameters are added, namely the passive weighting coefficient K1 and the active weighting coefficient K2.

[0019] Adding the passive weighting coefficient K1 to the resistor R of the passive damping control loop yields a new passive weighted control loop with the following transfer function:

[0020]

[0021] The active weighting coefficient K2 is added to the active damping control element H. i The above yields a new active weighted control loop, whose transfer function is:

[0022] G Hi (s)=K2H i (8).

[0023] A further improvement to the technical solution of this invention lies in: In S4, the adaptive control equation considering the short-circuit ratio is:

[0024]

[0025] In the formula, ζ is the optimal damping ratio, and PM is the phase margin.

[0026] A further improvement of the technical solution of the present invention is that: through the analysis of impedance stability criteria and the influence of grid strength on the stability of the grid-connected system, the formula is derived and the adaptive equation is designed according to the optimal damping ratio ζ = 0.707 and the phase margin PM = 60°;

[0027] The specific governing equations are as follows:

[0028]

[0029] In the formula, K pwm L1 represents the inverter gain, which is related to the carrier signal amplitude; L1, L2, and C are the filter parameter values, f c The cutoff frequency is related to the inverter output impedance and the impedance of the power grid system; U g I is the effective value of the phase voltage of the power grid. N H represents the rated phase current value output by the inverter. i R and R are the active damping proportional coefficient and passive damping value in the initial state, respectively.

[0030] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows:

[0031] 1. By setting hybrid damping, this invention can effectively suppress the resonance problem caused by LCL filter, and make full use of the advantages of both passive damping and active damping to enhance the stability of the system.

[0032] 2. The hybrid damping control method that takes into account the short-circuit ratio in this invention can not only suppress grid-connected resonance in a strong grid system with constant grid strength, but also adapt to a wide range of grid strength variations, solving the instability problem caused by grid strength variations due to the volatility of new energy power generation in grid-connected systems with high penetration rates.

[0033] 3. This invention uses an adaptive equation to adjust the inverter output impedance based on real-time monitoring of the short-circuit ratio, thereby avoiding interactive resonance with the power grid system. Attached Figure Description

[0034] Figure 1This is a system topology diagram of the present invention applied to a high-penetration grid-connected system;

[0035] Figure 2 This is a block diagram of the SOGI-PLL control.

[0036] Figure 3 Diagram of traditional hybrid damping control strategy for three-phase LCL grid-connected inverter;

[0037] Figure 4 Bode plot of inverter system impedance under varying grid strength;

[0038] Figure 5 To simplify the control block diagram for hybrid damping adaptive control of grid-connected inverters that takes into account the short-circuit ratio;

[0039] Figure 6 Bode plot of inverter system impedance under varying passive and active weighting coefficients;

[0040] Figure 7 Simulated waveforms of grid-connected current for inverters using traditional hybrid damping control when grid strength changes;

[0041] Figure 8 The simulation waveform of the inverter grid-connected current using the control scheme of this invention is shown. Detailed Implementation

[0042] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0043] like Figure 1 As shown, this invention can be applied to grid-connected systems with high penetration rates. The stability of such systems is greatly affected by the volatility of renewable energy generation, and the grid strength is prone to changes. The system may transition from a strong grid to a weak grid, or vice versa, with a wide range of variations. This invention does not limit the inverter structure; it is applicable to both single-phase and three-phase systems. A three-phase inverter is selected for detailed explanation in the specific implementation method.

[0044] A hybrid damping adaptive control method for grid-connected inverters that takes into account the short-circuit ratio includes the following steps:

[0045] S1. Construct a three-phase grid-connected inverter main circuit system and transform it to obtain the main circuit model in the αβ axis coordinate system;

[0046] S1.1 Construct the main circuit structure of a three-phase grid-connected inverter;

[0047] like Figure 1As shown, the main circuit structure of the three-phase grid-connected inverter includes six power switching devices, an LCL filter, and a grid-side inductor. The six power switching devices form a three-phase voltage-source bridge inverter structure. The LCL filter is used to filter out high-order voltage harmonics from the output of the power switching devices. The grid-side inductor is used to simulate line impedance. The capacitor branch of the LCL filter is connected in series with a damping resistor R.

[0048] S1.2 The phase angle of the three-phase voltage at the common coupling point is obtained using SOGI-PLL, which is used for coordinate transformation and reference current generation;

[0049] like Figure 2 As shown, using the specific structure of SOGI-PLL, the phase angle θ is obtained for generating the reference current and coordinate transformation in subsequent steps;

[0050]

[0051] In the formula, and These are the reference currents for the α and β axes, respectively; and These are the reference current values ​​for the d and q axes, respectively; θ is the phase angle generated by the phase-locked loop.

[0052] S1.3 The grid-connected current, capacitor current and common coupling point voltage are transformed by coordinates to obtain the main circuit model in the αβ axis coordinate system;

[0053] This invention relates to grid-connected power; therefore, abc-αβ employs equal power conversion, resulting in:

[0054]

[0055] In the formula, i α / g and i β / g These are the components of the output grid-connected current in the αβ coordinate system; i a / g i b / g and i c / g These are the components of the output grid-connected current in the abc coordinate system;

[0056]

[0057] In the formula, i α / c and i β / c These are the components of the capacitor current in the αβ coordinate system; i a / c i b / c and i c / c These are the components of the capacitor current in the abc coordinate system;

[0058]

[0059] In the formula, u α / pccand i β / pcc These are the components of the voltage at the common coupling point in the αβ coordinate system; i a / pcc i b / pcc and i c / pcc These are the components of the voltage at the common coupling point in the abc coordinate system;

[0060] The coordinate transformation from abc to αβ axes does not involve coupling between variables. Taking one axis, such as the α axis, as an example, and selecting inductor current and capacitor voltage as state variables, we can obtain:

[0061]

[0062] In the formula, all variables are components along the α-axis: u d The inverter output voltage; u c This is the capacitor voltage; i is the inverter-side inductor current; g For grid-side inductor current; u pcc The voltage at the common coupling point;

[0063] The main circuit structure in the αβ axis coordinate system can be obtained based on the relationship between the variables in equation (5).

[0064] S2. The error signal is obtained by subtracting the inverter grid-connected current and the reference current in the main circuit model under the αβ axis coordinate system. The error signal is used as the input of the quasi-resonant controller (Quasi Proportional Resonance, hereinafter referred to as QPR).

[0065] The selected current controller is QPR, and its transfer function is:

[0066]

[0067] In the formula, K p As a proportion, K r ω is the integral coefficient. c ω is the bandwidth coefficient. o denoted as the fundamental angular frequency, and s as the Laplace operator.

[0068] S3. The capacitor current is passed through an improved hybrid damping control circuit to obtain the feedforward signal u. hr ;

[0069] The improved hybrid damping control loop is as follows: the control strategy of the grid-connected inverter adopts both passive damping and active damping, and two control parameters are added, namely the passive weighting coefficient K1 and the active weighting coefficient K2.

[0070] Adding the passive weighting coefficient K1 to the resistor R of the passive damping control loop yields a new passive weighted control loop with the following transfer function:

[0071]

[0072] The active weighting coefficient K2 is added to the active damping control element H. i The above yields a new active weighted control loop, whose transfer function is:

[0073]

[0074] Figure 3 The diagram shows the traditional hybrid damping control strategy for a three-phase LCL grid-connected inverter. This control structure is used in traditional grid-connected inverter systems that employ hybrid damping control. This invention applies it to a high-penetration grid-connected system scenario, thereby introducing the impact of grid strength changes on system stability.

[0075] Figure 4 The Bode plot shows the inverter output impedance when the grid strength changes. Under traditional hybrid damping control, changes in grid strength will cause changes in inverter output impedance. According to the impedance stability criterion, when the inverter system and the grid system are interconnected, any change in the impedance of either side will affect the stability of the interconnected system. Therefore, the impact of grid strength changes on system stability must be considered.

[0076] S4. To ensure system stability under a wide range of grid strength conditions, an adaptive control equation consisting of the short-circuit ratio λ, passive weighting coefficient K1, and active weighting coefficient K2 is introduced. Optimal hybrid damping control parameters K1 and K2 are generated based on the real-time detected value of the short-circuit ratio λ. The specific control block diagram is shown below. Figure 5 As shown;

[0077] The adaptive control equation taking into account the short-circuit ratio is:

[0078]

[0079] In the formula, the formula is derived by analyzing the impedance stability criterion and the influence of grid strength on the stability of the grid-connected system, and the adaptive equation is designed according to the optimal damping ratio ζ = 0.707 and the phase margin PM = 60°.

[0080] The specific governing equations are as follows:

[0081]

[0082] In the formula, K pwm L1 represents the inverter gain, which is related to the carrier signal amplitude; L1, L2, and C are the filter parameter values, f c The cutoff frequency is related to the inverter output impedance and the impedance of the power grid system; U g I is the effective value of the phase voltage of the power grid. NH represents the rated phase current value output by the inverter. i R and R are the active damping proportional coefficient and passive damping value in the initial state, respectively;

[0083] Figure 6 The Bode plots of inverter system impedance are shown for the changes in passive and active weighting coefficients. As can be seen from the plots, the hybrid damping adaptive control, which takes into account the short-circuit ratio, can adjust the inverter output impedance to maintain system stability.

[0084] S5, convert the output signal u of QPR pr Subtract the feedforward signal u obtained in S3 hr This yields two modulation signals on the αβ axis;

[0085] The expression for the modulated signal is:

[0086]

[0087] In the formula, e α and e β These are two modulation signals in the αβ axis coordinate system.

[0088] S6, two modulation signals e α and e β The coordinate transformation yields a three-phase modulation signal, which is compared with the three-phase carrier control signal to control the switching on and off of the three-phase inverter power transistors.

[0089] The effectiveness of this invention was verified as follows:

[0090] System parameters: DC side voltage of grid-connected inverter is 700V; peak phase voltage of grid is 311V; inverter-side inductance is 4mH, grid-side inductance is 1mH, capacitance is 6μF, inverter gain is 700, cutoff frequency is 1500Hz, active power of a single three-phase inverter is 3kW, and rated reactive power is 0.

[0091] Simulation results are as follows Figure 7 and Figure 8 As shown, Figure 7 The graph shows the grid-connected current waveform when the grid strength changes by 0.15s during grid-connected operation using traditional hybrid damping control (i.e., control parameters without adaptive control). As can be seen from the graph, the current harmonic content is large. Figure 8 For the grid-connected current waveform using the control scheme of this invention under the same operating conditions, when the grid intensity changes by 0.15s, the weighting coefficients K1 and K2 are adaptively adjusted in real time. Figure 8 Compared to reducing grid-connected current harmonics, waveform quality is improved, resonance suppression is significantly enhanced, which is beneficial for stable system operation.

[0092] In summary, when applied to LCL-type grid-connected inverter systems, this invention can not only adapt to a wide range of grid intensity variations, but also maintain sufficient phase margin and improve system stability.

Claims

1. A hybrid damping adaptive control method for grid-connected inverters considering the short-circuit ratio, characterized in that: Includes the following steps: S1. Construct a three-phase grid-connected inverter main circuit system. The three-phase grid-connected inverter main circuit structure includes six power switching devices, an LCL filter, and a grid-side inductor. Among them, the six power switching devices form a three-phase voltage-source bridge inverter structure, the LCL filter is used to filter out high-order voltage harmonics output by the power switching devices, and the grid-side inductor is used to simulate line impedance. The capacitor branch of the LCL filter is connected in series with a damping resistor R. S2. The error signal is obtained by subtracting the inverter grid current from the reference current, and the error signal is used as the input of the quasi-resonant controller. S3. The capacitor current is processed through an improved hybrid damping control circuit to obtain a feedforward signal. The improved hybrid damping control mechanism employs both passive and active damping in the grid-connected inverter's control strategy, with two control parameters added: a passive weighting coefficient and an active damping coefficient. and active weighting coefficients The passive weighting coefficients Adding this to the resistor R in the passive damping control loop results in a new passive weighted control loop with the following transfer function: （7） Active weighting coefficients Added to the active damping control stage The above yields a new active weighted control loop, whose transfer function is: （8）； S4. To ensure system stability under a wide range of grid strength conditions, a short-circuit ratio is introduced. Passive weighting coefficients and active weighting coefficients The adaptive control equations are composed based on the short-circuit ratio. The real-time detection values ​​generate the optimal hybrid damping control parameters. and The adaptive control equation taking into account the short-circuit ratio is: （9） In the formula, The optimal damping ratio is given by PM, which represents the phase margin. Based on the impedance stability criterion and the analysis of the impact of grid strength on the stability of the grid-connected system, the formula is derived and applied according to the optimal damping ratio. and phase margin To design the adaptive equations, the specific control equations are as follows: （10） In the formula, The gain of the inverter is related to the amplitude of the carrier signal. , C represents the filter parameter values. The cutoff frequency is related to the inverter output impedance and the impedance of the power grid system. This represents the effective value of the phase voltage of the power grid. This represents the rated phase current value output by the inverter. R and R are the active damping proportional coefficient and passive damping value in the initial state, respectively; S5, the output signal of the quasi-resonant controller Subtract the feedforward signal obtained in S3 The modulated signal is obtained; S6. The modulation signal is compared with the carrier control signal to control the power transistors of the three-phase inverter to turn on and off.

2. The hybrid damping adaptive control method for grid-connected inverters considering short-circuit ratio according to claim 1, characterized in that: In S2, the transfer function of the quasi-resonant controller is: （6） In the formula, As a proportion, The integral coefficient is... This is the bandwidth factor. denoted as the fundamental angular frequency, and s as the Laplace operator.

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