Feedforward control method to enhance system stability and filter out high-order background harmonics in power grid

Abstract

The present invention discloses a feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid, comprising an LCL topology and a control loop, wherein a decoupling term and a capacitor current feedback active damping are connected in the control loop; the feedforward control method comprises the following steps: collecting a grid-connected current, a common coupling point voltage, and a capacitor current; obtaining a voltage phase angle of the common coupling point voltage through a phase-locked loop, and transforming the grid-connected current, the common coupling point voltage, and the capacitor current into dq axes through the voltage phase angle; indicating an angle difference between the dq axes of the system and the dq axes of the control loop; adopting a full feedforward strategy for the grid d-axis voltage feedforward and a full feedforward strategy for the grid q-axis voltage feedforward, deriving a duty cycle signal of the control system based on a transfer function of the d-axis and q-axis voltage feedforwards, and further controlling the on and off of a bridge arm through SVPWM.

Description

Technical Field

[0001] The present invention relates to the fields of electronic power technology and new energy power generation, and in particular to a feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid. Background Art

[0002] In today's society, renewable energy generation has become a focus of worldwide attention because the use of traditional energy sources such as oil, coal, and natural gas as fuels in the power generation process will cause environmental pollution (such as large-scale emissions of carbon dioxide) and also face the problem of depletion.

[0003] Renewable energy sources like solar and wind are widely distributed and difficult to collect and utilize in a unified manner. Therefore, distributed generation systems have become one of the most effective means of solving the current energy crisis. In recent years, my country's grid-connected renewable energy capacity has continued to increase rapidly, and it is expected that by 2050, renewable energy will account for approximately 60% of total power generation.

[0004] In existing technologies, distributed power generation systems mostly use long-distance transmission lines and multiple transformers to interconnect the systems and connect them to the public power grid. High-penetration distributed power generation systems exhibit weak grid characteristics with low short circuit ratio (SCR), that is, the grid side exhibits non-negligible impedance.

[0005] To synchronize the frequency and phase of the output current with the grid voltage, a phase-locked loop (PLL) is widely used in grid-connected inverters. However, the PLL introduces negative resistance into the inverter output impedance. As the PLL bandwidth increases, the negative resistance range of the output impedance also increases. The addition of grid voltage feedforward further increases the negative resistance range of the output impedance, compromising system stability.

[0006] In order to improve the stability of grid-connected inverters in weak grid conditions, some scholars have proposed a method to reduce the bandwidth of the phase-locked loop.

[0007] In the prior art, a Chinese patent application with publication number CN113162117A discloses a method for designing the bandwidth of a grid-connected inverter controller under a weak power grid. The method includes sampling, first determining the open-loop transfer function of the grid-connected inverter current control loop, then calculating the amplitude of the open-loop transfer function of the current control loop, and finally, combining the Nyquist stability criterion to determine the constraint relationship between the current control loop bandwidth and the phase-locked loop bandwidth under the weak power grid, thereby completing the parameter design of the current control loop and phase-locked loop controller. However, reducing the bandwidth of the phase-locked loop will impair the dynamic performance of the phase-locked loop.

[0008] In addition, some scholars have proposed to reconstruct the inverter output impedance through grid voltage feedforward, but this method does not take into account the impact of active damping strategy on impedance and the filtering of high-order background harmonics of the grid.

[0009] To address the above issues, further research is needed to consider the impact of the phase-locked loop, grid voltage feedforward, and capacitor current feedback on the inverter output impedance, as well as the control strategy for filtering out the high-order background harmonics of the grid. Summary of the Invention

[0010] In order to solve the problems existing in the prior art, the present invention provides a feedforward control method for enhancing system stability and filtering out high-order background harmonics of the power grid.

[0011] A feedforward control method for enhancing system stability and filtering out high-order background harmonics of the power grid includes an LCL topology and a control loop.

[0012] A decoupling term and capacitive current feedback active damping are connected in the control loop;

[0013] The feedforward control method comprises the following steps:

[0014] S1 collects grid current, common coupling point voltage and capacitor current;

[0015] S2 obtains the voltage phase angle of the common coupling point voltage through a phase-locked loop, and transforms the grid current, common coupling point voltage, and capacitor current into the dq axis through the voltage phase angle;

[0016] Due to the dynamics of the phase-locked loop, there is an angle difference Δθ between the dq axis of the system and the dq axis of the control loop, which is expressed as:

[0017]

[0018] Among them, G PLL is the transfer function between the q-axis voltage disturbance at the system common coupling point and the angle difference Δθ, is the q-axis voltage disturbance at the common coupling point;

[0019] The S4 grid d-axis voltage feedforward adopts a full feedforward strategy, and the transfer function is:

[0020]

[0021] Among them, K c is the capacitor current feedback coefficient, is a constant, Cs is the filter capacitor, K PWM is the PWM gain, L1 is the inverter side inductor, and s is the differential operator;

[0022] The transfer function of the grid q-axis voltage feedforward is:

[0023]

[0024] in, is the d-axis capacitive current, is the d-axis system output current, is the duty cycle of the d-axis, is the common coupling point voltage on the d-axis; G id (s) is the current PI controller;

[0025] S5 according to G f , G h The duty cycle signal of the control topology is obtained, and then the on and off of the bridge arm is controlled by SVPWM.

[0026] The control loop is connected to a decoupling term G dec The expression is:

[0027]

[0028] Among them, L1 and L2 are the inverter side inductance and grid side inductance respectively, ω0 is the grid base frequency, V dc is the DC side voltage.

[0029] The transfer function G between the q-axis voltage disturbance at the system common coupling point and the angle difference Δθ PLL The expression is:

[0030]

[0031] Among them, k ppll is the proportional term of the controller in the phase-locked loop, k ipll is the integral term of the controller in the phase-locked loop, and s is the differential operator.

[0032] Specifically, the G h The derivation process is as follows:

[0033] According to the system structure, the closed-loop output impedance matrix expression of the system is:

[0034]

[0035] Among them, Z out is the system open-loop output impedance matrix, B and C are the filter capacitor admittance matrix and the grid-side filter inductor impedance matrix respectively, G il is the transfer function matrix from topology to output current, These are the disturbances caused by the dynamic phase-locked loop to the output current, capacitor current, PCC point voltage, and system duty cycle;

[0036] Z out The expression of Z is: out =(I+AB) -1 (C+A+ABC)

[0037] Where I is the unit matrix, A is the inverter side filter inductor impedance matrix;

[0038] G il The expression is: G il =-V dc (C+A+ABC) -1

[0039] The expressions of A, B, and C are:

[0040]

[0041] Where R1 is the parasitic resistance of the inverter-side inductor, ω is the grid fundamental frequency, and R2 is the parasitic resistance of the grid-side inductor;

[0042] The expressions are as follows:

[0043]

[0044]

[0045]

[0046]

[0047] in, is the system output current, is the system capacitance current, D s is the system duty cycle, subscript d represents the d-axis signal, and subscript q represents the q-axis signal;

[0048] S4-2 Substituting each expression into the closed-loop output impedance expression yields an expression where the sum of the disturbance terms containing the phase-locked loop dynamics equals zero on the q-axis:

[0049]

[0050] The solution is:

[0051]

[0052] The d-axis voltage feedforward adopts the full feedforward strategy G f Known, combined with G f , G h The feedforward matrix G of the grid voltage is obtained, and the expression of G is as follows:

[0053]

[0054] The matrix G is used to obtain the duty cycle signal of the control system, and then the on and off of the bridge arm is controlled by SVPWM.

[0055] Preferably, the current controller is a PI controller, which is used to adjust the current in the control loop to be consistent with a given value.

[0056] Preferably, the LCL topology structure further includes a line impedance connected in series with the grid-side inductor.

[0057] Compared with the prior art, the present invention is beneficial in that:

[0058] Aiming at the adverse effects of PLL, grid voltage feedforward, and capacitor current feedback on system stability in weak grid conditions, as well as the interference of grid background harmonics on grid-connected current, an improved grid voltage feedforward scheme is proposed. In this scheme, the d-axis of the grid voltage feedforward adopts a full feedforward method, which can filter out most of the grid background harmonics. The q-axis of the grid voltage feedforward is used to reduce the negative resistance bandwidth of the q-axis introduced by PLL, grid voltage feedforward, and capacitor current feedback, thereby increasing the phase margin of the system, improving the system's ability to operate stably in weak grid conditions, and better maintaining the performance of PLL. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 This is a schematic diagram of the feedforward control method provided by the present invention for enhancing system stability and filtering out high-order background harmonics of the power grid.

[0060] Figure 2 A small signal model of the feedforward control method for enhancing system stability and filtering out high-order background harmonics of the power grid provided by the present invention.

[0061] Figure 3 The q-axis output impedance Bode diagram of the feedforward control method for enhancing system stability and filtering out high-order background harmonics of the power grid provided by the present invention and the traditional voltage feedforward method.

[0062] Figure 4 The simulation results of the output current waveform of the feedforward control method for enhancing system stability and filtering out high-order background harmonics of the power grid provided by the present invention and the traditional voltage feedforward method under weak power grid are presented.

[0063] Figure 5 The present invention provides a feedforward control method for enhancing system stability and filtering out higher-order background harmonics of a power grid, and a traditional voltage feedforward method for outputting current waveform simulation results when the power grid contains background harmonics. DETAILED DESCRIPTION

[0064] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0065] like Figure 1 As shown, a feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid includes an LCL topology and a control loop. The LCL topology also includes a line impedance connected in series with a grid-side inductor.

[0066] A decoupling term and capacitive current feedback active damping are connected in the control loop;

[0067] The feedforward control method comprises the following steps:

[0068] S1 collects the grid current i ga 、i gb 、i gc , voltage at the point of common coupling u pcca 、u pccb 、u pccc , and the capacitor current i Ca 、i Cb 、i Cc ;

[0069] S2 obtains the voltage phase angle of the common coupling point voltage through a phase-locked loop, and transforms the grid current, common coupling point voltage, and capacitor current into the dq axis through the voltage phase angle;

[0070] Due to the dynamics of the phase-locked loop, there is an angle difference Δθ between the dq axis of the system and the dq axis of the control loop, which is expressed as:

[0071]

[0072] Among them, G PLL is the transfer function between the q-axis voltage disturbance at the system common coupling point and the angle difference Δθ, is the q-axis voltage disturbance at the common coupling point;

[0073] The S4 grid d-axis voltage feedforward adopts a full feedforward strategy, and the transfer function is:

[0074]

[0075] Among them, K c is the capacitor current feedback coefficient, is a constant, Cs is the filter capacitor, K PWM is the PWM gain, the inverter side inductance, and s is the differential operator;

[0076] The transfer function of the grid q-axis voltage feedforward is:

[0077]

[0078] in, is the d-axis capacitive current, is the duty cycle of the d-axis, is the common coupling point voltage on the d-axis; G id (s) is the current PI controller;

[0079] The current controller is a PI controller, which is used to adjust the current in the control loop to be consistent with a given value;

[0080] S5 according to G f, G h The duty cycle signal of the control topology is obtained, and then the on and off of the bridge arm is controlled by SVPWM.

[0081] The control loop is connected to a decoupling term G dec The expression is:

[0082]

[0083] Among them, L1 and L2 are the inverter side inductance and grid side inductance respectively, ω0 is the grid base frequency, V dc is the DC side voltage.

[0084] The transfer function G between the q-axis voltage disturbance at the system common coupling point and the angle difference Δθ PLL The expression is:

[0085]

[0086] Among them, k ppll is the proportional term of the controller in the phase-locked loop, k ipll is the integral term of the controller in the phase-locked loop, and s is the differential operator.

[0087] Specifically, the G h The derivation process is as follows:

[0088] according to Figure 2 The system structure shown in the figure, the closed-loop output impedance matrix expression of the system is:

[0089]

[0090] Among them, Z out is the system open-loop output impedance matrix, B and C are the filter capacitor admittance matrix and the grid-side filter inductor impedance matrix respectively, G il is the transfer function matrix from topology to output current, These are the disturbances caused by the dynamic phase-locked loop to the output current, capacitor current, PCC point voltage, and system duty cycle;

[0091] Z out The expression of Z is: out =(I+AB) -1 (C+A+ABC)

[0092] Where I is the unit matrix, A is the inverter side filter inductor impedance matrix;

[0093] G il The expression is: G il =-V dc (C+A+ABC) -1

[0094] The expressions of A, B, and C are:

[0095]

[0096] Where R1 is the parasitic resistance of the inverter-side inductor, ω is the grid fundamental frequency, and R2 is the parasitic resistance of the grid-side inductor;

[0097] The expressions are as follows:

[0098]

[0099]

[0100]

[0101]

[0102] in, is the system output current, is the system capacitance current, D s is the system duty cycle, subscript d represents the d-axis signal, and subscript q represents the q-axis signal;

[0103] S4-2 Substituting each expression into the closed-loop output impedance expression yields an expression where the sum of the disturbance terms containing the phase-locked loop dynamics equals zero on the q-axis:

[0104]

[0105] The solution is:

[0106]

[0107] The d-axis voltage feedforward adopts the full feedforward strategy G f Known, combined with G f , G h The feedforward matrix G of the grid voltage is obtained, and the expression of G is as follows:

[0108]

[0109] The matrix G is used to obtain the duty cycle signal of the control system, and then the on and off of the bridge arm is controlled by SVPWM.

[0110] like Figure 3 As shown, the feedforward control method provided by this embodiment can eliminate the negative resistance brought by the dynamic phase-locked loop to the q-axis output impedance of the inverter, thereby enhancing the stability of the system under weak power grid conditions.

[0111] like Figure 4As shown, when the traditional grid voltage feedforward control strategy is adopted, the output current of the system is unstable, while when the feedforward control strategy provided by this embodiment is adopted, the three-phase output current of the system is stable.

[0112] like Figure 5 As shown, when the grid contains background harmonics, the waveform of the output current has obvious fluctuations when the grid voltage feedforward strategy is not adopted. However, when the feedforward control strategy provided by this embodiment is adopted, the waveform of the output current is significantly improved.

Claims

1. A feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid, comprising an LCL topology and a control loop, characterized in that: A decoupling term and capacitive current feedback active damping are connected in the control loop; The feedforward control method comprises the following steps: S1 collects grid current, common coupling point voltage and capacitor current; S2 obtains the voltage phase angle of the common coupling point voltage through a phase-locked loop, and transforms the grid current, common coupling point voltage, and capacitor current into the dq axis through the voltage phase angle; Due to the dynamics of the phase-locked loop, there is an angle difference Δθ between the dq axis of the system and the dq axis of the control loop, which is expressed as: Among them, G PLL is the transfer function between the q-axis voltage disturbance at the system common coupling point and the angle difference Δθ, is the q-axis voltage disturbance at the common coupling point; The S4 grid d-axis voltage feedforward adopts a full feedforward strategy, and the transfer function is: Among them, K c is the capacitor current feedback coefficient, that is, the active damping is a constant, C is the filter capacitor, K PWM is the PWM gain, L1 is the inverter side inductor, and s is the differential operator; The transfer function of the grid q-axis voltage feedforward is calculated as: in, is the d-axis capacitive current, is the d-axis system output current, is the duty cycle of the d-axis, is the common coupling point voltage on the d-axis; G id is the current PI controller; S5 according to G f , G h The duty cycle signal of the control system is obtained, and then the on and off of the bridge arm is controlled by SVPWM.

2. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 1, characterized in that: The control loop is connected to a decoupling term G dec The expression is: Among them, L1 and L2 are the inverter side inductance and grid side inductance respectively, ω0 is the grid base frequency, V dc is the DC side voltage.

3. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 2, characterized in that: The transfer function G between the q-axis voltage disturbance at the system common coupling point and the angle difference Δθ PLL The expression is: Among them, k ppll is the proportional term of the controller in the phase-locked loop, k ipll is the integral term of the controller in the phase-locked loop, and s is the differential operator.

4. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 3, characterized in that: In step S4, the transfer function G h The specific calculation steps are as follows: S4-1 According to the system structure, the closed-loop output impedance matrix expression of the system is: Among them, Z out is the system open-loop output impedance matrix, B and C are the filter capacitor admittance matrix and the grid-side filter inductor impedance matrix respectively, G il is the transfer function matrix from topology to output current, These are the disturbances caused by the dynamic phase-locked loop to the output current, capacitor current, PCC point voltage, and system duty cycle; Z out The expression of Z is: out =(I+AB) -1 (C+A+ABC) Where I is the unit matrix, A is the inverter side filter inductor impedance matrix; G il The expression is: G il =-V dc (C+A+ABC) -1 The expressions of A, B, and C are: Where R1 is the parasitic resistance of the inverter-side inductor, ω is the grid fundamental frequency, and R2 is the parasitic resistance of the grid-side inductor; The expressions are as follows: in is the system output current, is the system capacitance current, D s is the system duty cycle, subscript d represents the d-axis signal, and subscript q represents the q-axis signal; S4-2 Substitute each expression into the closed-loop output impedance expression to obtain an expression in which the sum of the disturbance terms containing the dynamics of the phase-locked loop is equal to 0 on the q-axis. Solve this expression to obtain G h .

5. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 4, characterized in that: The expression in step 4-2 is as follows: Solve the equation to get the transfer function G of the grid q-axis voltage feedforward h .

6. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 5, characterized in that: The d-axis voltage feedforward adopts the full feedforward strategy G f Known, combined with G f , G h The feedforward matrix G of the grid voltage is obtained, and the expression of G is as follows: The matrix G is used to obtain the duty cycle signal of the control system, and then the on and off of the bridge arm is controlled by SVPWM.

7. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 1, characterized in that: The current controller is a PI controller, which is used to adjust the current in the control loop to be consistent with a given value.

8. The feedforward control method for enhancing system stability and filtering out high-order background harmonics of a power grid according to claim 1, characterized in that: The LCL topology also includes a line impedance connected in series with the grid-side inductor.

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