Robust current control method of LCL type SAPF based on adaptive IIR filter

Abstract

The present invention relates to an LCL-type SAPF robust current control method based on an adaptive IIR filter link, comprising: an inverter-side current control loop, a grid current control loop, and a common coupling point voltage feedforward branch. The beneficial effects of the present invention are as follows: the present invention can fully compensate for the adverse effects of digital control delay, and expand the active damping range of the inverter-side current feedback to a maximum extent close to the Nyquist frequency, so that the initial resonant frequency of the LCL is always within its active damping region, thereby effectively improving the robustness of the system to the grid impedance. In addition, the stability margin of the system under the condition of large grid impedance can be significantly increased, and the high-frequency resonance unit of the grid current controller can be prevented from having multiple ‑180° crossings, thereby effectively improving the stability of the system under the condition of large grid impedance.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected inverter, and in particular relates to an LCL-type SAPF robust current control method based on an adaptive IIR filter link. Background Art

[0002] The rapid growth of large-scale renewable energy power generation systems, power equipment and power storage devices has led to the continuous advancement of the power electronics level of the power grid, and power quality issues such as harmonic pollution have become increasingly prominent. At the same time, modern production and service equipment has continuously improved the requirements for power supply quality. The LCL-type shunt active power filter (SAPF) is a flexible conversion device that comprehensively compensates for grid harmonics, reactive power and asymmetric components. It can accurately and quickly achieve grid harmonic control and improve power supply quality, and has therefore received widespread attention.

[0003] As the grid interface of SAPF, LCL filter can achieve the same switching ripple filtering effect as L filter with smaller inductance, and has obvious cost advantage, but it has resonance phenomenon, which can easily cause system instability. The LCL-type SAPF dual-loop current control method based on grid current and inverter current sampling inherently has active damping of inverter current proportional feedback. Without adding additional sensors or active damping loops, the resonance suppression of LCL filter can be achieved to ensure the stable operation of the system. However, in practical applications, SAPF is often implemented in digital control mode. At this time, the control loop will contain a digital control delay of 1.5 beats, so that the active damping area of ​​inverter current proportional feedback only reaches 1 / 6 sampling frequency, which is difficult to cover the possible resonant frequency of LCL filter, resulting in poor robustness of the system to grid impedance.

[0004] In addition, SAPF usually introduces a common coupling point voltage unit feedforward in the control loop to avoid excessive impact current during the startup process, reduce the modulation burden of the current controller and suppress grid voltage disturbances. However, the common coupling point voltage feedforward will introduce a grid-connected current positive feedback loop and provide another action channel for the grid impedance, thereby significantly reducing the stability margin of the system under large grid impedance conditions. Considering that SAPF often uses high-frequency resonant units to adjust the harmonic compensation current, the reduction in stability margin can easily cause the high-frequency resonant unit to cross -180° multiple times, thereby causing system stability problems. Summary of the invention

[0005] The object of the present invention is to overcome the deficiencies in the prior art and provide a robust current control method for an LCL-type SAPF based on an adaptive IIR filter link.

[0006] The LCL-type SAPF robust current control method based on the adaptive IIR filter link comprises: an inverter side current control loop, a grid current control loop and a common coupling point voltage feedforward branch.

[0007] The inverter side current control loop adopts the LCL initial resonant frequency f r0 Offline adaptive IIR delay compensation link G IIR1 (z) to adjust the inverter side current i inv , so that it can accurately follow the inverter side current reference value i inv * .

[0008] The grid current control loop adopts a grid current controller G ch (z) to accurately adjust the grid current i s The harmonic components of the grid are adjusted to follow the grid harmonic current reference value i sh * =0; the grid current controller is formed by cascading a proportional unit and a high-frequency resonance unit, and the expression is:

[0009]

[0010] Among them, K pf is the harmonic proportional gain; K rn ,φ n ,ω n are the resonance gain, compensation phase angle and resonance angular frequency of each high-frequency resonance unit respectively; ω n =nω1, n=6k±1 is the harmonic order, k∈[1,2,3,4]; T s is the sampling period.

[0011] The common coupling point voltage feedforward branch adopts an IIR filter feedforward link G based on online adaptation of harmonic content. IIR2 (z) and the voltage at the common coupling point u g multiplied together and its output added to the other current controller outputs.

[0012] As a preference, the common coupling point voltage feedforward branch adopts an IIR filter feedforward link G based on online adaptation of harmonic content. IIR2 (z) and the voltage at the common coupling point u g multiplied together and its output added to the other current controller outputs.

[0013] Preferably, in step 1, the initial resonant frequency of the LCL is r0 The expression is

[0014]

[0015] Among them, L1, L2, C f They are the inverter side inductor, grid side inductor and filter capacitor of the LCL filter respectively;

[0016] The expression of the IIR delay compensation link is:

[0017]

[0018] Among them, a1 and a2 are delay compensation coefficients; K piinv is the gain coefficient, and its upper limit of stable value can be determined by the inverter side current loop gain T iinv The closed-loop root locus of (z) is obtained; the expression of the inverter side current loop gain is

[0019]

[0020] Wherein, b1 and b2 are order coefficients, and their values ​​are determined by the offline adaptive law of the LCL initial resonant frequency; the offline adaptive law is

[0021]

[0022] The active damping area of ​​the inverter side current feedback is composed of the inverter side current feedback equivalent impedance Z eq The resistance part of (ω) is determined, and the expression of the equivalent impedance is:

[0023]

[0024] Among them, R eq (ω) is Z eq The resistance part of (ω), R eq (ω) is Z eq (ω) is the inductive reactance part, where ω is the angular frequency.

[0025] As a priority, in step 2, the expression of the IIR filter feedforward link is:

[0026]

[0027] Among them, T s is the sampling period; ω Lp1 is the first low-pass cutoff frequency, which is selected as the highest harmonic compensation angular frequency of SAPF; ω Lp2 is the second low-pass cutoff frequency, which is selected to be 200~400Hz;

[0028] The harmonic content of the common coupling point voltage is calculated by discrete Fourier transform and is expressed as

[0029]

[0030] Among them, the i-th harmonic voltage amplitude u gi The expression is

[0031]

[0032] Where m is an integer; f i is the i-th harmonic voltage frequency, f i =i·f1, f1 is the fundamental voltage frequency; N = f s / f1,f s is the sampling frequency.

[0033] Preferably, m1 and m2 are order coefficients, and their values ​​are determined by the online adaptive law of harmonic content, which can be divided into the startup online adaptive law and the grid-connected online adaptive law.

[0034] Preferably, starting the online adaptive law comprises the following steps:

[0035] Step 1, enable the signal sampling function, phase-locked loop function and harmonic content calculation function of SAPF;

[0036] Step 2: Set m1=1, m2=1, and after the output of the phase-locked loop and the IIR filter feedforward link is stable, perform the SAPF grid-connected operation, gradually increase the output limit of the grid current controller from 0 to the steady-state upper limit, and measure the voltage u at the common coupling point at this time. g Harmonic content, recorded as THD ref ;

[0037] Step 3: Set m1=1, m2=0 and measure the voltage u at the common coupling point. g Harmonic content, recorded as THD su1 If THD su1 ≥110%THD ref , determine that m1=1, m2=1 is the grid-connected steady-state value and restore; otherwise, update THD ref =THD su1 , proceed to step 4;

[0038] Step 4: Set m1 = 0, m2 = 0, and measure the harmonic content of the common coupling point voltage ug, recorded as THD su2 If THD su2 ≥110%THD ref , determine that m1=1, m2=0 is the grid-connected steady-state value and restore; otherwise, update THD ref =THD su2 , determine m1=0, m2=0 as the grid-connected steady-state values.

[0039] As a preferred embodiment, the grid-connected online adaptive law comprises the following steps:

[0040] Step 1: Periodically measure the voltage u at the common coupling point g Harmonic content, recorded as THD temp If THD temp ≤110%THD ref , keep the grid-connected steady-state values ​​of m1 and m2 unchanged, and update THD ref =THD temp ; Otherwise, proceed to step 2.

[0041] Step 2: Detect the grid-connected steady-state values ​​of m1 and m2. If m1 = 1 and m2 = 1, keep the grid-connected steady-state values ​​of m1 and m2 unchanged; if m1 = 1 and m2 = 0, proceed to step 3; if m1 = 0 and m2 = 0, set m1 = 1 and m2 = 0, and measure the harmonic content of the common coupling point voltage ug, recorded as THD gc1 If THD gc1 ≤110%THD ref , Update THD ref =THD gc1 , and determine that m1=1, m2=0 are the grid-connected steady-state values; otherwise, go to step 3.

[0042] Step 3: Set m1=1, m2=1, and measure the voltage u at the common coupling point. g Harmonic content, recorded as THD gc2 , Update THD ref =THD gc2 , and determine that m1=1 and m2=1 are the grid-connected steady-state values.

[0043] Preferably, the delay compensation coefficients a1 and a2 are selected from 0.8 <a1≤1、0.8<a2≤1。

[0044] Preferably, the gain coefficient K piinv It is 1 / 2 of the upper limit of the stable value.

[0045] The beneficial effects of the present invention are as follows: the present invention adopts an IIR delay compensation link based on offline adaptation of the LCL initial resonant frequency to replace the proportional unit of the inverter side current controller, and is used to constitute the inverter side current IIR delay compensation feedback, so as to fully compensate for the adverse effects of digital control delay, and expand the active damping range of the inverter side current feedback to a maximum extent close to the Nyquist frequency, so that the LCL initial resonant frequency is always located in its active damping region, thereby effectively improving the system's robustness to grid impedance.

[0046] In addition, the present invention also adopts an IIR filter unit based on online adaptation of harmonic content to replace the unit controller of the common coupling point voltage feedforward branch, which is used to constitute the common coupling point voltage IIR filter feedforward, which can significantly increase the stability margin of the system under the large grid impedance condition, and avoid multiple -180° crossings in the high-frequency resonance unit of the grid current controller, thereby effectively improving the stability of the system under the large grid impedance condition. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a schematic diagram of the dual-loop current control method of the LCL type SAPF;

[0048] Figure 2 A schematic diagram of a robust current control method for an LCL-type SAPF provided by the present invention;

[0049] Figure 3 A flowchart of starting the online adaptive law provided by the present invention;

[0050] Figure 4 A flow chart of the grid-connected online adaptive law provided by the present invention;

[0051] Figure 5 The R corresponding to the inverter side current proportional feedback and IIR delay compensation feedback provided by the present invention eq (ω) Schematic diagram of amplitude-frequency curve;

[0052] Figure 6 is the large grid impedance L in the embodiment g =1.53mH working condition, the open-loop Bode diagram of the grid current control loop corresponding to the common coupling point voltage unit feedforward and IIR filter feedforward;

[0053] Figure 7 In the embodiment, when the grid impedance L g Schematic diagram of SAPF compensation waveform corresponding to inverter side current proportional feedback when switching from 0 to 280μH;

[0054] Figure 8 In the embodiment, when the grid impedance L g Schematic diagram of SAPF compensation waveform corresponding to the inverter-side current IIR delay compensation feedback when switching from 0 to 280μH;

[0055] Fig. 9 In the embodiment, when the grid impedance L g Schematic diagram of SAPF compensation waveform corresponding to the common coupling point voltage unit feedforward when switching from 0 to 1.53mH;

[0056] Fig.10 In the embodiment, when the grid impedance L gSchematic diagram of SAPF compensation waveform corresponding to the common coupling point voltage IIR filter feedforward when switching from 0 to 1.53mH. DETAILED DESCRIPTION

[0057] The present invention is further described below in conjunction with embodiments. The description of the following embodiments is only used to help understand the present invention. It should be noted that for ordinary persons in the art, without departing from the principle of the present invention, the present invention can also be modified in some ways, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0058] Embodiment 1:

[0059] Aiming at the adverse effects of digital control delay and common coupling point voltage unit feedforward on the LCL-type SAPF dual-loop current control method, a robust current control method is determined.

[0060] The dual-loop current control method of LCL type SAPF is as follows Figure 1 As shown, it includes an inverter side current control loop, a grid current control loop and a common coupling point voltage feedforward branch.

[0061] The inverter side current control loop adopts the inverter side current controller G cf (z) to adjust the inverter side current i inv , so that it can accurately follow the inverter side current reference value i inv * , thereby maintaining the DC bus voltage constant. The inverter side current controller consists of a proportional resonant control unit, and the expression is

[0062]

[0063] Among them, K pf is the proportional unit gain, which inherently introduces the proportional feedback of the inverter side current; K r1 , φ1, ω1 are the resonance gain, compensation phase angle and resonance angular frequency of the fundamental resonance unit respectively; T s is the sampling period.

[0064] The grid current control loop adopts the grid current controller G ch (z) to accurately adjust the grid current i s The harmonic components of the grid are adjusted to follow the grid harmonic current reference value i sh * = 0, used to achieve harmonic compensation of grid current. The grid current controller is composed of a proportional unit and a high-frequency resonance unit in cascade, and the expression is:

[0065]

[0066] Among them, K pfis the harmonic proportional gain; K rn ,φ n ,ω n are the resonance gain, compensation phase angle and resonance angular frequency of each high-frequency resonance unit respectively; ω n =nω1, n=6k±1 is the harmonic order, k∈[1,2,3,4]; T s is the sampling period.

[0067] The common coupling point voltage feedforward branch uses a unit controller F(z) and the common coupling point voltage u g Multiply them and add their output to the output of other current controllers to avoid large inrush currents during startup, reduce the regulation burden of the inverter-side current controller, and suppress the impact of grid voltage disturbances. The expression of this unit controller is

[0068]

[0069] Among them, K pwm is the pulse width modulation gain, which is approximately 1.

[0070] It should be noted that the adverse effects of digital control delay and common coupling point voltage unit feedforward on the dual-loop current control method include:

[0071] (1) The active damping area of ​​the inverter current proportional feedback due to digital control delay only reaches 1 / 6 of the sampling frequency (f s / 6), and the design range of the LCL filter resonant frequency can be close to the Nyquist frequency (f s / 2). When the initial resonance frequency of LCL is f r0 Outside the active damping region of the inverter current proportional feedback, the actual resonant frequency of the system may be lower than f due to grid impedance fluctuations. s / 6, the control parameters originally designed for the system no longer meet the Nyquist stability criterion, which results in poor robustness of the system to grid impedance;

[0072] (2) The voltage unit feedforward at the common coupling point reduces the stability margin of the system under large grid impedance conditions, making it easy for the high-frequency resonant unit of the grid current controller to experience multiple -180° crossings, which in turn causes poor system stability.

[0073] In this regard, the present invention provides a robust current control method for an LCL-type SAPF based on an adaptive IIR filter link, such as Figure 2As shown, on the basis of the dual-loop current control method, the inverter side current IIR delay compensation feedback based on the offline adaptation of the LCL initial resonant frequency and the common coupling point voltage IIR filter feedforward based on the online adaptation of the harmonic content are adopted, specifically including: an inverter side current control loop, a grid current control loop and a common coupling point voltage feedforward branch.

[0074] Among them, the inverter side current control loop adopts the LCL initial resonant frequency f r0 Offline adaptive IIR delay compensation link G IIR1 (z) to adjust the inverter side current i inv , so that it can accurately follow the inverter side current reference value i inv * .

[0075] The voltage feed-forward branch of the common coupling point adopts an IIR filter feed-forward link G based on online adaptation of harmonic content. IIR2 (z) and the voltage at the common coupling point u g multiplied together and its output added to the other current controller outputs.

[0076] For the explanation of the grid current control loop, please refer to the explanation of the grid current control loop in the above-mentioned LCL-type SAPF double-loop current control method, which will not be repeated in the present invention.

[0077] In the inverter side current control loop, the above-mentioned IIR delay compensation link is used to replace the proportional unit of the inverter side current controller in the LCL type SAPF dual-loop current control method, and constitutes the inverter side current IIR delay compensation feedback, which is used to expand the active damping area of ​​the inverter side current feedback to cover the possible resonant frequency of the LCL filter. The active damping area of ​​the inverter side current feedback is expanded to cover the possible resonant frequency of the LCL filter.

[0078] Moreover, in the IIR delay compensation link based on offline adaptation of the LCL initial resonant frequency, the LCL initial resonant frequency f r0 The expression is

[0079]

[0080] Among them, L1, L2, C f They are the inverter side inductor, grid side inductor and filter capacitor of the LCL filter respectively.

[0081] The expression of IIR delay compensation link is:

[0082]

[0083] Among them, a1 and a2 are delay compensation coefficients, usually 0.8 <a1≤1、0.8<a2≤1。Kpiinv is the gain coefficient, and its upper limit of stable value can be determined by the inverter side current loop gain T iinv The closed-loop root locus of (z) is obtained, and K is usually selected piinv is 1 / 2 of the upper limit of the stable value. The expression of the inverter side current loop gain is

[0084]

[0085] b1 and b2 are order coefficients, whose values ​​are determined by the offline adaptive law of the LCL initial resonant frequency. The offline adaptive law is

[0086]

[0087] The active damping area of ​​the inverter side current feedback is determined by the resistance part of the inverter side current feedback equivalent impedance Zeq(ω), which is expressed as

[0088]

[0089] Among them, R eq (ω) is Z eq The resistance part of (ω), R eq (ω) is Z eq (ω) is the inductive reactance part, where ω is the angular frequency. eq (ω) amplitude-frequency curve, the active damping area of ​​the inverter side current feedback can be obtained.

[0090] In the common coupling point voltage feedforward branch, the present invention adopts an IIR filter feedforward link based on online adaptation of harmonic content to replace the unit controller of the common coupling point voltage feedforward branch in the LCL type SAPF double-loop current control method, and constitutes a common coupling point voltage IIR filter feedforward, which is used to improve the stability margin of the system under large grid impedance conditions.

[0091] The expression of the IIR filter feedforward link in the IIR filter link based on online adaptation of harmonic content is:

[0092]

[0093] Among them, T s is the sampling period; ω Lp1 is the first low-pass cutoff frequency, which can be selected as the highest harmonic compensation angular frequency of the SAPF; ω Lp2 is the second low-pass cutoff frequency, which can be selected as 200~400Hz; m1 and m2 are order coefficients.

[0094] The harmonic content of the common coupling point voltage is calculated by discrete Fourier transform and is expressed as

[0095]

[0096] Among them, the i-th harmonic voltage amplitude u gi The expression is

[0097]

[0098] Where m is an integer; f i is the i-th harmonic voltage frequency, fi=i·f1; N=f s / f1.

[0099] Considering that the even-order harmonic content of the common coupling point voltage is low and the highest harmonic compensation frequency of the SAPF usually does not exceed the 25th harmonic, in practical applications, the harmonic content expression of the common coupling point voltage ug can be simplified to

[0100]

[0101] The values ​​of m1 and m2 are determined by the harmonic content online adaptive law, which can be divided into a startup online adaptive law and a grid-connected online adaptive law.

[0102] Start the online adaptive law process as follows Figure 3 As shown, specifically including:

[0103] Step 301: First, enable the signal sampling function, phase-locked loop function and harmonic content calculation function of the SAPF.

[0104] Step 302: Set m1=1, m2=1, and after the output of the phase-locked loop and the IIR filter feedforward link is stable, perform the SAPF grid-connected operation, gradually increase the output limit of the grid current controller from 0 to the steady-state upper limit, and measure the voltage u at the common coupling point at this time. g Harmonic content, recorded as THD ref .

[0105] Step 303: Set m1=1, m2=0, and measure the voltage u at the common coupling point. g Harmonic content, recorded as THD su1 If THD su1 ≥110%THD ref , determine that m1=1, m2=1 is the grid-connected steady-state value and restore; otherwise, update THD ref =THD su1 , proceed to step 304.

[0106] Step 304: Set m1=0, m2=0, and measure the harmonic content of the common coupling point voltage ug, recorded as THD su2 If THD su2 ≥110%THD ref, determine that m1=1, m2=0 is the grid-connected steady-state value and restore; otherwise, update THD ref =THD su2 , determine m1=0, m2=0 as the grid-connected steady-state values.

[0107] The grid-connected online adaptive law process is as follows Figure 4 As shown, specifically

[0108] Step 401: Periodically measure the voltage u at the common coupling point g Harmonic content, recorded as THD temp If THD temp ≤110%THD ref , keep the grid-connected steady-state values ​​of m1 and m2 unchanged, and update THD ref =THD temp ; Otherwise, proceed to step 402.

[0109] Step 402: Detect the grid-connected steady-state values ​​of m1 and m2. If m1=1, m2=1, keep the grid-connected steady-state values ​​of m1 and m2 unchanged; if m1=1, m2=0, proceed to step 403; if m1=0, m2=0, set m1=1, m2=0, and measure the harmonic content of the common coupling point voltage ug, recorded as THD gc1 If THD gc1 ≤110%THD ref , Update THD ref =THD gc1 , and determine that m1=1, m2=0 are grid-connected steady-state values; otherwise, go to step 403.

[0110] Step 403: Set m1=1, m2=1, and measure the voltage u at the common coupling point. g Harmonic content, recorded as THD gc2 , Update THD ref =THD gc2 , and determine that m1=1 and m2=1 are the grid-connected steady-state values.

[0111] After completing the above steps, the staff can evaluate the system's improvement effect on grid impedance robustness and stability under large grid impedance conditions, including:

[0112] (1) Improvement effect of the system on grid impedance robustness

[0113] For example, Figure 5 The R corresponding to the inverter side current proportional feedback and IIR delay compensation feedback when using the system and control parameters shown in Table 1 and Table 2 is shown. eq(ω) Amplitude-frequency curve. As can be seen from the figure, compared with the inverter-side current proportional feedback, the inverter-side current IIR delay compensation feedback based on the LCL initial resonant frequency offline adaptation can fully compensate for the digital control delay, and expand the active damping range of the inverter-side current feedback to a maximum level close to the Nyquist frequency, so that the LCL initial resonant frequency is always within the active damping region, thereby effectively improving the system's robustness to grid impedance.

[0114] (2) Improvement effect of system stability under large grid impedance conditions

[0115] Figure 6 It shows that when the system and control parameters shown in Table 1 and Table 2 are used, the large grid impedance L g =1.53mH, the open-loop Bode diagram of the grid current control loop corresponding to the common coupling point voltage unit feedforward and IIR filter feedforward. It can be seen from the figure that compared with the common coupling point voltage unit feedforward, the common coupling point voltage IIR filter feedforward based on online adaptive harmonic content can significantly increase the stability margin of the system under large grid impedance conditions, avoid multiple -180° crossings in the high-frequency resonance unit of the grid current controller, and effectively improve the stability of the system under large grid impedance conditions.

[0116] Table 1 System parameters of LCL-type SAPF

[0117]

[0118] Table 2 Control parameters of LCL type SAPF

[0119]

[0120] Embodiment 2:

[0121] A three-phase LCL SAPF simulation model was built in Matlab / Simulink to verify the effectiveness of the above-mentioned robust current control method. The system and control parameters of the simulation model are consistent with those in Tables 1 and 2.

[0122] Figure 7 , Figure 8 It shows that when the grid impedance L g When switching from 0 to 280μH, the SAPF compensation waveform corresponding to the inverter side current proportional feedback and IIR delay compensation feedback. At this time, the system uses a common coupling point voltage low-pass feedforward with a bandwidth of 150Hz, so that the common coupling point voltage feedforward effect is not introduced at other frequencies outside the bandwidth, so as to observe the effect of only changing the inverter side current feedback. Figure 7 , Figure 8 , and found that when L gWhen switching from 0 to 280μH, the output current of the SAPF using the inverter-side current proportional feedback shows obvious oscillation and gradually diverges, while the SAPF using the inverter-side current IIR delay compensation feedback maintains a good harmonic compensation effect and the grid current always remains sinusoidal.

[0123] Fig. 9 , Fig.10 It shows that when the grid impedance L g When switching from 0 to 1.53mH, the SAPF compensation waveform corresponding to the common coupling point voltage unit feedforward and IIR filter feedforward. At this time, the system uses the inverter side current IIR delay compensation feedback based on the offline adaptation of the LCL initial resonant frequency to ensure that the resonant frequency of the LCL filter is always within the active damping area of ​​the inverter side current feedback. Fig. 9 , Fig.10 , and found that when L g When switching from 0 to 1.53mH, the output current of the SAPF using common coupling point voltage unit feedforward oscillates and the grid current waveform is obviously distorted, while the SAPF using common coupling point voltage IIR filter feedforward maintains a good harmonic compensation effect and the grid current always remains sinusoidal.

[0124] In summary, the use of robust current control method can ensure that the active damping area of ​​the inverter side current feedback covers the possible resonant frequency of the LCL filter, and significantly increase the stability margin of the system under large grid impedance conditions, thereby improving the system's robustness to grid impedance and stability under large grid impedance conditions, and effectively solving the adverse effects of digital control delay and common coupling point unit feedforward.

Claims

1. The LCL-type SAPF robust current control method based on adaptive IIR filtering is characterized by: The method comprises: an inverter side current control loop, a grid current control loop and a common coupling point voltage feedforward branch; The inverter side current control loop adopts the LCL initial resonant frequency f r0 Offline adaptive IIR delay compensation link G IIR1 (z) to adjust the inverter side current i inv , so that it can accurately follow the inverter side current reference value i inv * ; The grid current control loop adopts a grid current controller G ch (z) to accurately adjust the grid current i s The harmonic components of the grid are adjusted to follow the grid harmonic current reference value i sh * =0; the grid current controller is formed by cascading a proportional unit and a high-frequency resonance unit, and the expression is: Among them, K pf is the harmonic proportional gain; K rn ,φ n ,ω n are the resonance gain, compensation phase angle and resonance angular frequency of each high-frequency resonance unit respectively; ω n =nω1, n=6k±1 is the harmonic order, k∈[1,2,3,4]; T s is the sampling period; The common coupling point voltage feedforward branch adopts an IIR filter feedforward link G based on online adaptive harmonic content. IIR2 (z) and the voltage at the common coupling point u g multiplied together and its output added to the other current controller outputs.

2. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 1 is characterized in that: The LCL initial resonant frequency f r0 The expression is Among them, L1, L2, C f They are the inverter side inductor, grid side inductor and filter capacitor of the LCL filter respectively; The IIR delay compensation link G IIR1 The expression of (z) is Among them, a1 and a2 are delay compensation coefficients; K piinv is the gain coefficient, and its upper limit of stable value can be determined by the inverter side current loop gain T iinv The closed-loop root locus of (z) is obtained; the expression of the inverter side current loop gain is Wherein, b1 and b2 are order coefficients, and their values ​​are determined by the offline adaptive law of the LCL initial resonant frequency; the offline adaptive law is The active damping area of ​​the inverter side current feedback is composed of the inverter side current feedback equivalent impedance Z eq The resistance part of (ω) is determined, and the expression of the equivalent impedance is: Where Req(ω) is the resistance part of Zeq(ω), X eq (ω) is the inductive reactance part of Zeq(ω), and ω is the angular frequency.

3. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 1 is characterized in that: The IIR filter feedforward link G IIR2 The expression of (z) is Among them, T s is the sampling period; ω Lp1 is the first low-pass cutoff frequency, which is selected as the highest harmonic compensation angular frequency of SAPF; ω Lp2 is the second low-pass cutoff frequency, which is selected to be 200-400Hz; m1 and m2 are order coefficients; The harmonic content of the common coupling point voltage is calculated by discrete Fourier transform and is expressed as Among them, the i-th harmonic voltage amplitude u gi The expression is Where m is an integer; f i is the i-th harmonic voltage frequency, f i =i·f1, f1 is the fundamental voltage frequency; N = f s / f1,f s is the sampling frequency.

4. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 3 is characterized in that: The values ​​of m1 and m2 are determined by an online adaptive law for harmonic content, and the online adaptive law for harmonic content can be divided into a startup online adaptive law and a grid-connected online adaptive law.

5. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 4 is characterized in that: The starting of the online adaptive law comprises the following steps: Step 1, enable the signal sampling function, phase-locked loop function and harmonic content calculation function of SAPF; Step 2: Set m1=1, m2=1, and after the output of the phase-locked loop and the IIR filter feedforward link is stable, perform the SAPF grid-connected operation, gradually increase the output limit of the grid current controller from 0 to the steady-state upper limit, and measure the voltage u at the common coupling point at this time. g Harmonic content, recorded as THD ref ; Step 3: Set m1=1, m2=0 and measure the voltage u at the common coupling point. g Harmonic content, recorded as THD su1 ; If THD su1 ≥110%THD ref , determine that m1=1, m2=1 is the grid-connected steady-state value and restore; otherwise, update THD ref =THD su1 , proceed to step 4; Step 4: Set m1 = 0, m2 = 0, and measure the harmonic content of the common coupling point voltage ug, recorded as THD su2 ; If THD su2 ≥110%THD ref , determine that m1=1, m2=0 is the grid-connected steady-state value and restore; otherwise, update THD ref =THD su2 , determine m1=0, m2=0 as the grid-connected steady-state values.

6. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 4 is characterized in that: The grid-connected online adaptive law comprises the following steps: Step 1: Periodically measure the voltage u at the common coupling point g Harmonic content, recorded as THD temp ; If THD temp ≤110%THD ref , keep the grid-connected steady-state values ​​of m1 and m2 unchanged, and update THD ref =THD temp ; Otherwise, proceed to step 2; Step 2: Detect the grid-connected steady-state values ​​of m1 and m2; if m1=1, m2=1, keep the grid-connected steady-state values ​​of m1 and m2 unchanged; if m1=1, m2=0, proceed to step 3; if m1=0, m2=0, set m1=1, m2=0, and measure the harmonic content of the common coupling point voltage ug, recorded as THD gc1 ; If THD gc1 ≤110%THD ref , Update THD ref =THD gc1 , and determine that m1=1, m2=0 are the grid-connected steady-state values; otherwise, go to step 3; Step 3: Set m1=1, m2=1, and measure the voltage u at the common coupling point. g Harmonic content, recorded as THD gc2 , Update THD ref =THD gc2 , and determine that m1=1 and m2=1 are the grid-connected steady-state values.

7. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 2 is characterized in that: The delay compensation coefficients a1 and a2 are selected to be 0.8 <a1≤1、0.8<a2≤1。 8. The LCL-type SAPF robust current control method based on the adaptive IIR filter link according to claim 2 is characterized in that: The gain factor K piinv It is 1 / 2 of the upper limit of the stable value.

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