A method for harmonic adaptive harmonic current suppression of a photovoltaic inverter

Abstract

The application discloses a harmonic adaptive harmonic current suppression method of a photovoltaic inverter, belongs to the technical field of power system power quality control, and adopts an adaptive harmonic component compensation control strategy of the photovoltaic inverter. The strategy adopts a "detection-reverse injection" framework, takes the difference between the remaining apparent capacity of the inverter after power supply of a fundamental wave and the harmonic power borne by the inverter as an adaptive quantity, and reduces the total harmonic distortion rate of a grid connection point to the maximum extent under the premise of no overload. Firstly, a real-time harmonic detector based on a cascaded generalized integrator is constructed. Secondly, an inverter impedance model is established from the perspective of impedance stability, and control parameter stability boundaries are determined. Then, the adaptive harmonic control strategy is introduced into a single-inverter grid-connected system, and a reasonable parameter setting method is proposed on the basis of considering stability limitation. Subsequently, a capacity-matched harmonic power distribution algorithm is proposed for a multi-inverter grid-connected system, and the efficiency and operation stability of the photovoltaic grid-connected power generation system are improved.

Description

Technical Field

[0001] This invention relates to the field of power system power quality control technology, specifically a method for harmonic adaptive harmonic current suppression in photovoltaic inverters. Background Technology

[0002] With the integration of distributed power sources and the widespread application of power electronic equipment, the characteristics of power electronics in modern power distribution networks are becoming increasingly apparent, and power grid harmonic pollution is becoming more and more serious.

[0003] To improve power quality at the grid connection point of a distribution network, traditional methods for mitigating harmonic currents typically involve installing passive or active filters. Some literature introduces active power quality controllers at the common coupling point between the microgrid and the distribution network to improve power quality through harmonic current compensation. However, this method has high operating costs and significantly increases the difficulty of system maintenance. Existing grid-connected inverters for distributed generation systems are highly similar to active power quality control devices such as filters in terms of topology and control strategies. They can achieve renewable energy grid connection while simultaneously addressing power quality at the grid connection point, avoiding the need for additional power quality devices in the distribution network.

[0004] However, the power quality management function of grid-connected inverters has limited compensation capacity. When using grid-connected inverters to achieve harmonic current compensation, the inverter must bear the harmonic power generated by the harmonic current in addition to outputting the fundamental power. Therefore, in scenarios where the load harmonic content is high and the inverter's fundamental output power is large, the inverter is very likely to be affected by harmonic power overload and thus affect normal operation. In addition, due to the differences in electrical distance and line impedance between each inverter and the nonlinear load, the inverter closer to the harmonic source often bears more harmonic current, while the inverter with smaller capacity is more prone to overload.

[0005] Therefore, how to reasonably allocate harmonic power under the condition of multiple inverters operating in parallel is also an urgent problem to be solved in suppressing harmonics. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes an adaptive harmonic parameter control strategy based on local inverter information. This strategy employs a "detection-reverse injection" framework, using the difference between the inverter's remaining apparent capacity after fundamental power supply and the harmonic power it bears as the adaptive value. This strategy minimizes the total harmonic distortion rate at the grid connection point without overload.

[0007] The objective of this invention can be achieved through the following technical solution: a method for adaptive harmonic current suppression in a photovoltaic inverter, comprising the following steps:

[0008] The three-phase current at the grid connection point is collected and input into the cascaded generalized integrator CGI network. The nth target harmonic current component is extracted synchronously through the parallel TOGI and SOGI modules.

[0009] The extracted harmonic current is transformed to the dq coordinate system using Clarke-Park transformation to obtain the current component i of the harmonic current along the dq axis. nd and i nq The controller then sends i nd and i nq Multiply by the harmonic compensation factor K h Then, the dq-axis fundamental current reference value i is compared with the fundamental active and reactive power generated by MPPT. gdref and i gqref Perform vector superposition to generate a comprehensive current command i gdref1 and i gqref1 ;

[0010] The comprehensive current command i gdref1 and i gqref1 The input cross-coupled PI dual closed loop, the inner current loop introduces ω0L feedforward compensation to cancel inductive coupling, and the output modulated signal is sent to the SVPWM module to drive the inverter.

[0011] Based on the inverter's rated capacity S N To constrain this, the remaining capacity S is calculated periodically. R =S N -(S1+S H ), where S1 is the fundamental apparent power, S H Harmonic power;

[0012] If S R >S H Increase K according to the set slope h ;

[0013] If S R Approaching 0, K decays proportionally. h Until S1+S H ≤S N ;

[0014] Each inverter is based on its local remaining capacity S R With harmonic power S H Dynamic calculation of harmonic distribution coefficient k vi Automatically adjusts harmonic compensation, increasing the k value for units with sufficient capacity. vi Units with tight capacity reduce k vi When a global overload is detected, a reduced-order compensation strategy is executed, retaining the specified order harmonic compensation while reducing the harmonic compensation coefficient K. h And freeze its growth.

[0015] Preferably, the transfer function of the CGI network includes:

[0016] TOGI's transfer function:

[0017]

[0018] The transfer function of the CGI harmonic detection method obtained by paralleling TOGI and SOGI:

[0019]

[0020] In the formula, G1(s), G2(s), and G3(s) are the three closed-loop transfer functions of TOGI; Gd1(s) and Gq1(s) are the closed-loop transfer functions of the CGI model used; ω is the angular frequency for extracting harmonics; Y(s) is the Laplace transform of the input signal to be detected y(t); Y1(s), Y2(s), and Y3(s) are the Laplace transforms of the input signals y1(t), y2(t), and y3(t); y(s) is the input signal of CGI; yh(s) and yqh(s) are the AC output signals of CGI; and K is the system damping factor, with a value range of (0,1).

[0021] Preferably, the cross-coupled PI dual closed loop further includes an adaptive harmonic compensation control strategy:

[0022] Detecting harmonic current i at the grid connection point h,pcc Generate a harmonic compensation reference current i with the same amplitude but opposite phase. h.ref After the reference quantity and the harmonic component of the filter inductor current are finely adjusted in amplitude and initially corrected in phase by a PI regulator, the phase compensation stage compensates for the lag in the control loop, and the inverter outputs a reverse harmonic current i. inv Vector cancellation is achieved at the grid connection point; the harmonic current extracted by CGI is transformed into the dq-axis component i through synchronous rotating coordinate system transformation. nd and i nq， Then, through the harmonic compensation coefficient K h Adjustment, with fundamental current reference command i gdref and i gqref The signals are superimposed to form a current reference signal i that includes harmonic compensation information. gdref1 and i gqref1 The specific expression is:

[0023]

[0024] In the formula, i gdref1 i gqref1 To introduce new reference values ​​for the d and q axes after harmonic compensation; i gdref and i gqref Reference command for fundamental current; id i q i represents the d-axis and q-axis components of the inverter output current. nd i nq K represents the d-axis and q-axis components of the nth harmonic current. h The harmonic compensation coefficient; u d u q For inverter output voltage command; k p k i ω0 represents the proportional gain and integral gain of the inner-loop PI controller; ω0 represents the fundamental angular frequency; L represents the inverter-side filter inductance value; i0 represents the current inner-loop PI controller. gd i gq The d-axis and q-axis components of the grid-connected current; u gd u gq These are the d-axis and q-axis components of the grid voltage.

[0025] Preferably, it also includes a harmonic compensation coefficient K. h Stability constraint mechanism:

[0026] An impedance model based on the inverter and power grid is established and analyzed using the Nyquist criterion, and the harmonic compensation coefficient K is evaluated. h The stability boundary is quantitatively derived as follows:

[0027]

[0028] In the formula, I g For the grid-connected current expression; I s It treats the grid-connected inverter as a current source, Z inv Z is the filter impedance; grd U is the power grid impedance. g It is an ideal voltage source;

[0029]

[0030] In the formula, i a, i b, i c The three-phase current generated by the inverter; u a, u b, u c The three-phase voltage at the grid connection point; m a, m b, m c K represents the PWM modulation ratio. m For modulation gain; V dc d is the DC bus voltage; L is the filter inductance; d / dt is the rate of change of current over time.

[0031]

[0032] In the formula, V1 is the fundamental frequency voltage; I1 and φ i1 These represent the amplitude and phase of the fundamental frequency current, respectively; V p and φ vp V represents the amplitude and phase of the voltage during the positive-sequence disturbance. n and φ vn For the magnitude and phase of the voltage of the negative sequence disturbance; I p and φ ip I represents the amplitude and phase of the positive-sequence disturbance current response; n and φ in The magnitude and phase of the negative-sequence disturbance current response;

[0033]

[0034] In the formula, H i The transfer functions for the d-axis and q-axis current controllers; k d m is the decoupling coefficient; d m q The modulated signal component in the rotating coordinate system; Ф i The fundamental current phase is represented by Δθ, which is the frequency deviation of the PLL. dr I is the reference component of the d-axis current. qr This is the q-axis current reference component; I0 is the positive sequence current amplitude under small-signal disturbance; I1 is the fundamental current amplitude; I0 is the positive sequence current amplitude under small-signal disturbance. n The amplitude of the nth harmonic current;

[0035]

[0036] In the formula, m d ,m q The modulation signal is in a rotating coordinate system; ω p The frequency ω0 of the injected disturbance voltage is the reference angular frequency 2πf1; Δθ is the frequency deviation of the PLL;

[0037]

[0038] In the formula, Z P (s) represents the positive sequence impedance of the inverter; m d m q The modulated signal component in the rotating coordinate system; This represents the amplitude of the small-signal positive-sequence voltage disturbance. I0 is the amplitude of the small-signal positive-sequence current disturbance; I1 is the amplitude of the fundamental current; I0 is the amplitude of the small-signal positive-sequence current disturbance; I0 is the amplitude of the fundamental current. n The amplitude of the nth harmonic current; Ф n The phase of the nth harmonic current; K h V is the harmonic compensation coefficient; dc H is the inverter DC bus voltage; i (ωp -ω) is the current inner loop controller at (ω) p Transfer function at frequency -ω); k d Ф is the decoupling coefficient; L is the filter inductance value; Ф i The fundamental current phase is represented by S; the Laplace operator is represented by V1; the fundamental voltage amplitude is represented by G. PLL (ω p -ω) represents the phase-locked loop at (ω) p Transfer function at a frequency of -ω).

[0039] Preferably, each inverter is based on its local remaining capacity S R With harmonic power S H Dynamic calculation of harmonic distribution coefficient k vi The formula is:

[0040]

[0041] In the formula, P and Q are the measured active power and reactive power of the inverter, respectively; S N Indicates the rated capacity of the inverter; S R Indicates the remaining capacity of the inverter; S H Represents harmonic power; U 1,rms and I h,rms These represent the effective values ​​of the fundamental voltage and harmonic current, respectively.

[0042]

[0043] In the formula, P and Q are the measured active power and reactive power of the inverter, respectively; S N Indicates the rated capacity of the inverter; S R,j k represents the remaining capacity of the j-th inverter in the distribution network; vi Indicates the harmonic distribution coefficient; i pcc,dq i represents the dq-axis harmonic current obtained after harmonic detection of the sampled grid connection point current. * pcc,dq The harmonic current is the compensation component on the dq axis after adopting the adaptive compensation strategy.

[0044] Preferably, the process of automatically adjusting the harmonic compensation amount is as follows:

[0045] When k vi When the value is greater than 0.7, the inverter is deemed to have sufficient capacity margin, and the harmonic compensation coefficient is automatically increased to accelerate harmonic elimination.

[0046] When k vi When the value is less than 0.3, the inverter is deemed to have limited capacity. Priority is given to ensuring the fundamental power output and the harmonic compensation coefficient is reduced accordingly.

[0047] Preferably, the order reduction compensation strategy is: when the S of all inverters R H If this continues for more than two control cycles, retain compensation for the 5th, 7th, 11th, and 13th harmonics, disable compensation for higher harmonics, and set K... h Reduce the gain to 0.8x in one go and stop increasing the gain until S. R >S H .

[0048] Compared with the prior art, the beneficial effects of the present invention are:

[0049] This invention employs an adaptive harmonic component compensation control strategy for photovoltaic inverters. This strategy uses a "detection-reverse injection" framework, employing the difference between the inverter's remaining apparent capacity after fundamental wave power supply and the harmonic power it bears as an adaptive quantity. This minimizes the total harmonic distortion rate at the grid connection point without overload. First, a real-time harmonic detector based on a cascaded generalized integrator is constructed. Second, an inverter impedance model is established from the perspective of impedance stability, and the stability boundary of the control parameters is determined. Then, an adaptive harmonic control strategy is introduced into a single-inverter grid-connected system, and a reasonable parameter tuning method is proposed considering stability constraints. Subsequently, a capacity-matching harmonic power allocation algorithm is proposed for multi-inverter parallel systems, achieving dynamic coordination of harmonic suppression capabilities, effectively reducing the impact of harmonics on the power grid, and improving the efficiency and operational stability of the photovoltaic grid-connected power generation system. Attached Figure Description

[0050] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0051] Figure 1 An adaptive control flowchart provided for an embodiment of the present invention;

[0052] Figure 2 A control structure block diagram of a single TOGI and SOGI provided in an embodiment of the present invention;

[0053] Figure 3 A schematic diagram of the cascaded CGI harmonic detection structure provided in an embodiment of the present invention;

[0054] Figure 4 The closed-loop transfer function G provided in the embodiments of the present invention d1 (s), G q1 Bode plot of (s);

[0055] Figure 5 ​This is a block diagram of the adaptive harmonic control strategy provided in an embodiment of the present invention;

[0056] Figure 6 Bode plot of output impedance of grid-connected inverter provided in an embodiment of the present invention. Detailed Implementation

[0057] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0058] See Figures 1 to 6 The adaptive harmonic component compensation control method for multi-photovoltaic inverters in this embodiment includes the following steps:

[0059] This embodiment proposes a harmonic extraction method based on a CGI harmonic detector, aiming to improve the power quality control capability of grid-connected inverters in complex harmonic environments. The method involves collecting three-phase current at the grid connection point and inputting it into a cascaded generalized integrator (CGI) network. The nth target harmonic current component is extracted synchronously through parallel TOGI and SOGI modules. By bandpass amplification and deep attenuation of the fundamental component, pure harmonic current is obtained in real time.

[0060] The extracted harmonic current is transformed to the dq coordinate system using Clarke-Park transformation to obtain the current component i of the harmonic current along the dq axis. nd and i nq The controller then sends i nd and i nq Multiply by the harmonic compensation factor K h Then, the dq-axis fundamental current reference value i is compared with the fundamental active and reactive power generated by MPPT. gdref and i gqref Perform vector superposition to generate a comprehensive current command i gdref1 and i gqref1 Before superposition, multiply by the harmonic compensation coefficient K. h This allows the system to adjust the degree of harmonic suppression in real time.

[0061] The comprehensive current command i gdref1 and i gqref1 The input cross-coupled PI dual closed loop, the inner current loop introduces ω0L feedforward compensation to cancel inductive coupling, and the output modulated signal is sent to the SVPWM module to drive the inverter.

[0062] The PI controller is responsible for quickly eliminating instantaneous errors; the regulated voltage command is decomposed into the duty cycles of eight basic vectors via space vector SVPWM, reducing switching losses.

[0063] To avoid harmonic power overload in single-unit scenarios with multiple inverters, the system introduces an adaptive harmonic compensation mechanism based on remaining apparent capacity. In a multi-inverter parallel system, the units do not need to communicate with each other, but only utilize local S... R With S H The allocation coefficient k is automatically calculated using the capacity matching formula. vi :

[0064] Each inverter is based on its local remaining capacity S R With harmonic power S H Dynamic calculation of harmonic distribution coefficient k vi Automatically adjust the harmonic compensation amount, that is:

[0065]

[0066] In the formula, P and Q are the measured active power and reactive power of the inverter, respectively; S N Indicates the rated capacity of the inverter; S R Indicates the remaining capacity of the inverter; S H Represents harmonic power; U 1,rms and I h,rms These represent the effective values ​​of the fundamental voltage and harmonic current, respectively.

[0067]

[0068] In the formula, P and Q are the measured active power and reactive power of the inverter, respectively; S N Indicates the rated capacity of the inverter; S R,j k represents the remaining capacity of the j-th inverter in the distribution network; vi Indicates the harmonic distribution coefficient; i pcc,dq i represents the dq-axis harmonic current obtained after harmonic detection of the sampled grid connection point current. * pcc，dq The harmonic current in the dq axis after adopting the adaptive compensation strategy;

[0069] Ample capacity units increase k vi Units with tight capacity reduce k vi When a global overload is detected, a reduced-order compensation strategy is executed, retaining the specified order harmonic compensation while reducing the harmonic compensation coefficient K. h And freeze its growth;

[0070] The process of automatically adjusting the harmonic compensation amount is as follows:

[0071] When kvi When the value is greater than 0.7, the inverter is deemed to have sufficient capacity margin, and the harmonic compensation coefficient is automatically increased to accelerate harmonic elimination.

[0072] When k vi When the value is less than 0.3, the inverter is deemed to have limited capacity. Priority is given to ensuring the fundamental power output and the harmonic compensation coefficient is reduced accordingly.

[0073] By proportionally allocating power to each inverter in the distribution network using only the allocation factor, units with sufficient capacity automatically increase k. vi Units with limited capacity that handle more harmonic power will automatically reduce k. vi Prioritize ensuring fundamental frequency output; when the remaining capacity S of all inverters... R Less than the current harmonic power S H When this condition persists for more than two control cycles, compensation for the 5th, 7th, 11th, and 13th harmonics is retained, while compensation for higher harmonics is temporarily disabled, and the harmonic compensation coefficient K is adjusted. h The gain is reduced in one go by 0.8, and then stopped increasing in subsequent cycles until S. R >S H .

[0074] To avoid harmonic resonance caused by high gain, K h The upper limit is obtained through offline scanning using the inverter-grid small-signal impedance model and the Nyquist criterion, with approximately one-third of the phase margin reserved as the online adjustable range; when running K h When approaching the upper limit, the controller automatically freezes the gain to prevent the system from crossing the stability boundary.

[0075] This dynamic flow ensures that no single unit is overloaded while minimizing harmonic distortion at the common coupling point. Overall, this method fully utilizes the margins of existing grid-connected photovoltaic inverters, achieving adaptive harmonic mitigation across the entire process, frequency band, and load conditions without additional filtering hardware. This provides a highly efficient and flexible power quality improvement method for the distribution network. By calculating the margin of each inverter after handling the fundamental active power in real time and dynamically adjusting its corresponding K value based on the current harmonic injection situation, this method achieves this. h This method ensures that each inverter can collaboratively bear harmonic power without exceeding its own capacity constraints. It effectively alleviates the overload phenomenon of inverters near harmonic sources, realizes the self-balancing allocation of harmonic compensation tasks among multiple inverters, and improves the system's safety and continuous compensation capability.

[0076] The design and implementation method of the adaptive harmonic component compensation control strategy for multi-photovoltaic inverters provided by the present invention will be described in detail below with reference to the accompanying drawings.

[0077] like Figure 2The diagram shown is a schematic of a three-phase photovoltaic power generation grid-connected system provided in an embodiment of the present invention. It consists of a photovoltaic array, a Boost converter based on a DC / DC structure, a photovoltaic grid-connected inverter, an AC LC filter, and a line impedance Z. line Grid impedance Z g And nonlinear loads.

[0078] Combination Figure 2 and Figure 3 The adaptive harmonic component compensation control strategy for multi-photovoltaic inverters of this invention employs a cascaded generalized integrator harmonic current detection method, the specific implementation steps of which are as follows:

[0079] Regarding the extraction of harmonic compensation components, this paper employs a cascaded generalized integrator, whose multi-stage parallel structure can simultaneously separate common harmonic components such as 5th, 7th, 11th, and 13th harmonics in a single operation. The input signal y is fed into a CGI quadrature signal generator to generate quadrature signal pairs. Next, the obtained signals are fed into another CGI filter to further purify the target harmonic frequency components, ultimately achieving accurate extraction and isolation of the target frequency harmonics.

[0080] First, the transfer function for TOGI is given as follows:

[0081]

[0082] The transfer function of the CGI harmonic detection method obtained by connecting TOGI and SOGI in parallel is given:

[0083]

[0084] In the formula, G1(s), G2(s), and G3(s) are the three closed-loop transfer functions of TOGI, respectively; G d1 (s), G q1 Y(s) is the closed-loop transfer function of the CGI model used; ω is the angular frequency for extracting harmonics; Y(s) is the Laplace transform of the input signal to be detected y(t); Y1(s), Y2(s), and Y3(s) are the Laplace transforms of the input signals y1(t), y2(t), and y3(t); y(s) is the input signal of the CGI; y h (s),y qh (s) is the AC output signal of CGI; K is the system damping factor, with a value range of (0,1).

[0085] See Figure 4 The closed-loop transfer function G is given. d1 (s), G q1The Bode plot of (s) shows that CGI responds significantly to specific harmonic frequencies in the input signal, accurately filtering out the target harmonic frequency components and demonstrating excellent frequency selectivity. Furthermore, different values ​​of the parameter k significantly affect the system's detection performance. Smaller k values ​​result in a wider bandwidth for harmonic detection but lower selectivity; conversely, increasing k narrows the detection bandwidth and enhances frequency selectivity, thereby improving the accuracy of harmonic detection.

[0086] Specifically, such as Figure 5 As shown, in the current dual closed-loop control framework of the inverter provided by this embodiment of the invention, a harmonic suppression strategy of "detection-reverse injection" is proposed. First, the harmonic current i at the grid connection point is detected. h,pcc Subsequently, a harmonic compensation reference current i with the same amplitude but opposite phase is generated. h.ref Then, the reference value and the harmonic component of the filter inductor current are processed by a PI regulator for amplitude fine-tuning and preliminary phase correction. Finally, the phase compensation stage compensates for the lag in the control loop, ultimately causing the inverter to output a reverse harmonic current i. inv Vector cancellation is achieved at the grid connection point, thereby significantly reducing the grid connection point current distortion rate. At the same time, it can adapt to the dynamic changes of load harmonics without the need for additional power hardware, effectively suppressing grid connection point harmonics.

[0087] To address the need for precise control of harmonic components, a generalized cascaded integrator (CGI) is employed to extract the harmonic current at the grid connection point in real time. The harmonic current extracted by the CGI is then transformed using a synchronously rotating coordinate system to obtain the dq-axis component (i...). nd and i nq Then, through the harmonic compensation coefficient K h Adjustment, with fundamental current reference command (i gdref and i gqref The signals are superimposed to form a current reference signal (i) that contains harmonic compensation information. gdref1 and i gqref1 The specific expression is as follows:

[0088]

[0089] In the formula, i gdref1 i gqref1 To introduce new reference values ​​for the d and q axes after harmonic compensation; i gdref and i gqref Reference command for fundamental current; i d i q i represents the d-axis and q-axis components of the inverter output current. nd i nq K represents the d-axis and q-axis components of the nth harmonic current. h The harmonic compensation coefficient; u d uq For inverter output voltage command; k p k i ω0 represents the proportional gain and integral gain of the inner-loop PI controller; ω0 represents the fundamental angular frequency; L represents the inverter-side filter inductance value; i0 represents the current inner-loop PI controller. gd i gq The d-axis and q-axis components of the grid-connected current; u gd u gq These are the d-axis and q-axis components of the grid voltage.

[0090] like Figure 6 The figure shows the Bode plot of the output impedance of the grid-connected inverter provided by this invention. To verify the obtained mathematical model, a Simulink simulation model was established using the same parameters. This paper uses the seventh harmonic as the analysis object. The yellow solid line represents the obtained theoretical model of the output impedance, and the blue dashed line represents the actual output impedance characteristics obtained in the time-domain simulation using the harmonic small-signal method. Figure 1 As can be seen, the theoretical model and the actual model of the grid-connected inverter are consistent, verifying the correctness of the impedance mathematical model.

[0091] like Figure 1 As shown in the flowchart of the adaptive control proposed in this invention, the process first utilizes a cascaded generalized integrator to separate multiple harmonic currents at the grid connection point in real time; then, it verifies the system stability based on the Nyquist criterion and adaptively tunes the harmonic compensation coefficient K accordingly. h ; after obtaining a new K h Then, the algorithm calculates the remaining apparent capacity S of the inverter. R With the harmonic power to be compensated S H If S R >S H Directly generate harmonic current reference values; otherwise, appropriately reduce K. h The inverter is then recalibrated until the overload constraint is met. Finally, the inverter injects reverse harmonic current according to the reference value, achieving stable and efficient harmonic suppression under capacity-limited conditions.

[0092] An impedance model based on the inverter and power grid is established and analyzed using the Nyquist criterion, and the harmonic compensation coefficient K is evaluated. h The quantitative derivation process of the stability boundary is as follows:

[0093] Based on impedance stability analysis, the dynamic coupling between the grid-connected inverter impedance and the grid impedance directly determines the stability of the grid-connected system. Therefore, the grid-connected inverter should first be modeled with refined impedance to lay the theoretical foundation for subsequent harmonic suppression and adaptive compensation coefficients. The harmonic linearization method is used for sequence impedance modeling, mainly by adding harmonic compensation coefficients in the inner current loop modeling. If the ratio of grid impedance to inverter output impedance satisfies the Nyquist criterion, the grid-connected inverter can operate stably under this condition; otherwise, it cannot.

[0094] This formula represents the inverter output stability when the grid impedance is zero. The stability of the output current depends on the second term on the right-hand side of the following equation. This expression is similar to the closed-loop transfer function of a system with negative feedback control, where the positive gain is 1 and the feedback gain is Z. grd (s) / Z inv (s); At this point, if the ratio of the grid impedance to the inverter output impedance satisfies the Nyquist criterion, the grid-connected inverter can operate stably under these grid conditions, and the stability margin of the grid-connected system can also be determined by Z. grd (s) / Z inv The Nyquist curve of (s) is used to characterize it;

[0095]

[0096] In the formula, I g For the grid-connected current expression; I s It treats the grid-connected inverter as a current source, Z inv Z is the filter impedance; grd U is the power grid impedance. g It is an ideal voltage source;

[0097] This formula is used to establish the relationship between voltage, current, and control elements;

[0098]

[0099] In the formula, i a, i b, i c The three-phase current generated by the inverter; u a, u b, u c The three-phase voltage at the grid connection point; m a, m b, m c K represents the PWM modulation ratio. m For modulation gain; V dc d is the DC bus voltage; L is the filter inductance; d / dt is the rate of change of current over time.

[0100] Assuming the disturbance voltage at the grid connection point is a positive-sequence component, we obtain the time-domain expressions for the voltage and current of phase a at the grid connection point at this time, which can be used to construct the control loop later.

[0101]

[0102] In the formula, V1 is the fundamental frequency voltage; I1 and φ i1 These represent the amplitude and phase of the fundamental frequency current, respectively; V p and φ vp V represents the amplitude and phase of the voltage during the positive-sequence disturbance. n and φ vn For the magnitude and phase of the voltage of the negative sequence disturbance; I p and φ ip I represents the amplitude and phase of the positive-sequence disturbance current response; n and φ in The magnitude and phase of the negative-sequence disturbance current response;

[0103] According to the appendix Figure 5 The adaptive control structure diagram shown, along with the input d- and q-axis currents and their command values, allows us to derive the expressions for the true sequence disturbance components of the d- and q-axis modulation signals:

[0104]

[0105] In the formula, H i The transfer functions for the d-axis and q-axis current controllers; k d m is the decoupling coefficient; d m q The modulated signal component in the rotating coordinate system; Ф i The fundamental current phase is represented by Δθ, which is the frequency deviation of the PLL. dr I is the reference component of the d-axis current. qr This is the q-axis current reference component; I0 is the positive sequence current amplitude under small-signal disturbance; I1 is the fundamental current amplitude; I0 is the positive sequence current amplitude under small-signal disturbance. n The amplitude of the nth harmonic current;

[0106] This formula is the modulation signal expression of phase a in the stationary coordinate system obtained by inverse Park transformation of the above formula. It is used to substitute the relationship between the voltage and current loop and the control loop to obtain the impedance expression.

[0107]

[0108] In the formula, m d ,m q The modulation signal is in a rotating coordinate system; ω p The frequency ω0 of the injected disturbance voltage is the reference angular frequency 2πf1; Δθ is the frequency deviation of the PLL.

[0109] Substituting the above equations, we obtain the positive sequence impedance of the grid-connected inverter:

[0110]

[0111] In the formula, Z P (s) represents the positive sequence impedance of the inverter; m d m q The modulated signal component in the rotating coordinate system; This represents the amplitude of the small-signal positive-sequence voltage disturbance. I0 is the amplitude of the small-signal positive-sequence current disturbance; I1 is the amplitude of the fundamental current; I0 is the amplitude of the small-signal positive-sequence current disturbance; I0 is the amplitude of the fundamental current. n The amplitude of the nth harmonic current; Ф n The phase of the nth harmonic current; K h V is the harmonic compensation coefficient; dc H is the inverter DC bus voltage; i (ω p -ω) is the current inner loop controller at (ω) p Transfer function at frequency -ω); k d Ф is the decoupling coefficient; L is the filter inductance value; Ф i The fundamental current phase is represented by S; the Laplace operator is represented by V1; the fundamental voltage amplitude is represented by G. PLL (ω p -ω) represents the phase-locked loop at (ω) p Transfer function at a frequency of -ω).

[0112] Through the above embodiments, this invention utilizes an adaptive harmonic component compensation control strategy for photovoltaic inverters. This strategy employs a "detection-reverse injection" framework, using the difference between the inverter's remaining apparent capacity after fundamental wave power supply and the harmonic power it bears as an adaptive quantity. This minimizes the total harmonic distortion rate at the grid connection point without overload. First, a real-time harmonic detector based on a cascaded generalized integrator is constructed. Second, an inverter impedance model is established from the perspective of impedance stability, and the stability boundary of control parameters is determined. Then, an adaptive harmonic control strategy is introduced into a single-inverter grid-connected system, and a reasonable parameter tuning method is proposed considering stability constraints. Subsequently, a capacity-matching harmonic power allocation algorithm is proposed for multi-inverter parallel systems, achieving dynamic coordination of harmonic suppression capabilities, effectively reducing the impact of harmonics on the power grid, and improving the efficiency and operational stability of the photovoltaic grid-connected power generation system.

[0113] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0114] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for adaptive harmonic current suppression in a photovoltaic inverter, characterized in that, Includes the following steps: The three-phase current at the grid connection point is collected and input into the cascaded generalized integrator CGI network. The nth target harmonic current component is extracted synchronously through the parallel TOGI and SOGI modules. The extracted harmonic current is transformed to the dq coordinate system using Clarke-Park transformation to obtain the current components of the harmonic current along the dq axis. i nd and i nq The controller will then... i nd and i nq Multiply by harmonic compensation factor K h Subsequently, the dq-axis fundamental current reference value is compared with the fundamental active and reactive power generated by MPPT. i gdref and i gqref Perform vector superposition to generate a comprehensive current command. i gdref1 and i gqref1 ; The comprehensive current command i gdref1 and i gqref1 Input cross-coupled PI dual closed loop, current inner loop introduced ω 0L feedforward compensation cancels inductive coupling, where ω 0 is the fundamental angular frequency, L is the value of the inverter-side filter inductance, and the output modulation signal is sent to the SVPWM module to drive the inverter; Based on the inverter's rated capacity S N To constrain this, the remaining capacity is calculated periodically. S R = S N -( S 1+ S H ),in S 1 represents the fundamental apparent power. S H Harmonic power; like S R > S H Increase the slope according to the set value K h ; like S R Approaching 0, decaying proportionally K h Until S 1+ S H ≤ S N ; Each inverter is based on its local remaining capacity S R With harmonic power S H Dynamic calculation of harmonic distribution coefficient k vi Automatically adjusts harmonic compensation, improving the efficiency of units with sufficient capacity. k vi Units with tight capacity reduce k vi When a global overload is detected, a reduced-order compensation strategy is executed, retaining the specified subharmonic compensation while reducing the harmonic compensation coefficient. K h And freeze its growth; Each inverter is based on its local remaining capacity. S R With harmonic power S H Dynamic calculation of harmonic distribution coefficient k vi The formula is: In the formula, P , Q These are the measured active power and reactive power of the inverter, respectively. S N Indicates the rated capacity of the inverter; S R Indicates the remaining capacity of the inverter; S H Represents harmonic power; U 1,rms and I h,rms These represent the effective values ​​of the fundamental voltage and harmonic current, respectively. In the formula, P , Q These are the measured active power and reactive power of the inverter, respectively. S N Indicates the rated capacity of the inverter; S R,j This represents the remaining capacity of the j-th inverter in the distribution network; k vi Indicates the harmonic distribution coefficient; i pcc,dq The sampled grid connection point current is obtained after harmonic detection. dq Shaft harmonic current, The harmonic current is the compensation component on the dq axis after adopting the adaptive compensation strategy.

2. The method for adaptive harmonic current suppression in a photovoltaic inverter according to claim 1, characterized in that, The transfer function of the CGI network includes: TOGI's transfer function: The transfer function of the CGI harmonic detection method obtained by paralleling TOGI and SOGI: In the formula, G1(s), G2(s), and G3(s) are the three closed-loop transfer functions of TOGI, respectively; G d1 (s) and G q1 (s) is the closed-loop transfer function of the CGI model used; ω To extract the angular frequency of harmonics; Y (s) represents the input signal to be detected. y Laplace transform of (t); Y 1(s), Y 2(s), Y 3(s) is the input signal y 1(t), y 2(t), y Laplace transform of 3(t); y (s) is the input signal for CGI; y dh (s) and y qh (s) is the AC output signal of CGI; K The system damping factor has a value range of (0, 1).

3. The method for adaptive harmonic current suppression in a photovoltaic inverter according to claim 1, characterized in that, The cross-coupled PI dual closed loop also includes an adaptive harmonic compensation control strategy: Detecting harmonic current at grid connection point i h,pcc Generate harmonic compensation reference currents with the same amplitude but opposite phase. i h,ref The reference value and the harmonic component of the filter inductor current are processed by a PI regulator for amplitude fine-tuning and preliminary phase correction. Then, the phase compensation stage compensates for the lag in the control loop, and the inverter outputs a reverse harmonic current. i inv Vector cancellation is achieved at the grid connection point; the harmonic current extracted by CGI is obtained through a synchronous rotating coordinate system transformation. dq Axial components i nd and i nq， Then through harmonic compensation coefficient K h Adjustment, relative to fundamental current reference value i gdref and i gqref The data are superimposed to form a comprehensive current command that includes harmonic compensation information. i gdref1 and i gqref1 The specific expression is: In the formula, i gdref1 , i gqref1 For comprehensive current commands; i gdref and i gqref This is the reference value for the fundamental current. i d , i q For the inverter output current d , q Axial components; i nd , i nq nth harmonic current d , q Axial components; K h This refers to the harmonic compensation coefficient; u d , u q This is the inverter output voltage command; k p , k i For the proportional gain and integral gain of the current inner-loop PI controller; ω 0 represents the fundamental angular frequency; L represents the value of the inverter-side filter inductance. i gd , i gq For grid-connected current d , q Axial components; u gd , u gq For grid voltage d , q Axial components.

4. The method for adaptive harmonic current suppression in a photovoltaic inverter according to claim 1, characterized in that, It also includes harmonic compensation coefficients. K h Stability constraint mechanism: An impedance model based on the inverter and power grid is established and analyzed using the Nyquist criterion to determine the harmonic compensation coefficient. K h The stability boundary is quantitatively derived as follows: In the formula, I g Here is the expression for the grid-connected current; I s It treats the grid-connected inverter as a current source. Z inv The filter impedance; Z grd For grid impedance; U g It is an ideal voltage source; In the formula, i a, i b, i c The three-phase current generated by the inverter; u a, u b, u c The three-phase voltage at the grid connection point; m a, m b, m c This is the PWM modulation ratio; K m Modulation gain; V dc d is the DC bus voltage; L is the filter inductance; d / dt is the rate of change of current over time. In the formula, V 1 represents the fundamental frequency voltage; I 1 and φ i1 These represent the amplitude and phase of the fundamental frequency current, respectively. V p and φ vp The magnitude and phase of the voltage for the positive-sequence disturbance; V n and φ vn The magnitude and phase of the voltage for the negative sequence disturbance; I p and φ ip I represents the amplitude and phase of the positive-sequence disturbance current response; n and φ in The magnitude and phase of the negative-sequence disturbance current response; In the formula, H i for d shaft and q Transfer function of shaft current controller; k d These are the decoupling coefficients; m d , m q The modulated signal component in the rotating coordinate system; Ф i The phase of the fundamental current; Δ θ This refers to the frequency deviation of the PLL. I dr This is the reference component for the d-axis current. I qr This is the q-axis current reference component; This represents the positive sequence current amplitude under small-signal disturbance. I 1 represents the amplitude of the fundamental current; I n The amplitude of the nth harmonic current; In the formula, ω p Frequency of injected disturbance voltage ω 0 is the reference angular frequency 2π f 1; Δ θ This refers to the frequency deviation of the PLL. In the formula, Z P (s) represents the positive sequence impedance of the inverter; m d , m q The modulated signal component in the rotating coordinate system; This represents the amplitude of the small-signal positive-sequence voltage disturbance. The amplitude of the small-signal positive-sequence current disturbance; I 1 represents the amplitude of the fundamental current; I n The amplitude of the nth harmonic current; Ф n The phase of the nth harmonic current; K h This refers to the harmonic compensation coefficient; V dc This refers to the DC bus voltage of the inverter. H i ( ω p - ω ) is the current inner loop controller in ( ω p - ω Transfer function at frequency 100000; k d L is the decoupling coefficient; L is the value of the inverter-side filter inductance. Ф i The phase of the fundamental current; S For the Laplace operator; V 1 represents the fundamental voltage amplitude; G PLL ( ω p - ω ) is a phase-locked loop in ( ω p - ω The transfer function at a given frequency.

5. The method for adaptive harmonic current suppression in a photovoltaic inverter according to claim 1, characterized in that, The process of automatically adjusting the harmonic compensation amount is as follows: when k vi When the value is greater than 0.7, the inverter is deemed to have sufficient capacity margin, and the harmonic compensation coefficient is automatically increased to accelerate harmonic elimination. when k vi When the value is less than 0.3, the inverter is deemed to have limited capacity. Priority is given to ensuring the fundamental power output and the harmonic compensation coefficient is reduced accordingly.

6. The method for adaptive harmonic current suppression in a photovoltaic inverter according to claim 1, characterized in that, The order reduction compensation strategy is as follows: when all inverters S R < S H If this continues for more than two control cycles, retain compensation for the 5th, 7th, 11th, and 13th harmonics, and disable compensation for higher harmonics. K h Reduce the gain to 0.8x at once and stop increasing the gain until... S R > S H .

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