A control method and system for an LCL type grid-connected inverter

Abstract

The present invention discloses a control method and system for an LCL-type grid-connected inverter, which relates to the field of electric power transmission technology. The method comprises: obtaining predicted values ​​of the grid current and capacitor voltage of the LCL-type grid-connected inverter at time k+1; extracting the high-frequency resonance component of the capacitor voltage of the LCL-type grid-connected inverter at a set time k, obtaining the predicted value of the high-frequency resonance component at time k+1, constructing a tracking value function J for suppressing the resonance of the LCL-type grid-connected inverter system; obtaining the minimum value of the tracking value function J, and obtaining the optimal voltage reference value u of the inverter in a stationary α-β coordinate system. α * and u β * This generates a control pulse signal, which is then applied to the switching transistors of the LCL-type grid-connected inverter to achieve predictive control of the LCL-type grid-connected inverter. This invention eliminates the need for multiple weighting factors, avoids the problem of unstable switching frequency in single-vector finite-value control, and improves the steady-state control performance of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of electric drive, and in particular to a control method and system for an LCL type grid-connected inverter. Background Art

[0002] Photovoltaic energy, with its numerous advantages such as cleanliness, renewable energy, flexibility, and reliability, is a crucial component of distributed energy systems and is experiencing increasing penetration in my country's renewable energy sector. Grid-connected photovoltaic power systems transmit generated electricity to the grid. As the key interface between distributed photovoltaic power sources and the grid, the performance of three-phase grid-connected inverters directly impacts grid connection quality and grid stability. Because grid-connected inverters utilize power electronic components, their switching frequency is high during normal operation, injecting high-frequency harmonics into the grid, impacting power quality and operational stability. To achieve satisfactory high-order harmonic suppression, LCL filters are typically installed between the three-phase grid-connected inverter and the grid. However, the introduction of LCL filters not only increases the system's control order but also causes grid current resonance, seriously threatening the stability and reliability of the photovoltaic grid-connected system. Therefore, a novel LCL-type three-phase grid-connected inverter control method with intuitive concept, simple implementation, and strong resonance suppression capabilities is needed.

[0003] In the prior art, a multivariable finite set single vector predictive control strategy is typically used for predictive control to suppress LCL resonance. The specific process involves selecting a multivariable model predictive control algorithm suitable for LCL-type grid-connected inverters, confirming that the control targets are the inverter output current, filter capacitor voltage, and grid current. A precise mathematical model of the LCL-type grid-connected inverter is established, and a multivariable predictive controller is designed. This controller considers the control of multiple variables, including the inverter output current, filter capacitor voltage, and grid current. The switching state corresponding to the basic voltage vector that minimizes the cost function is applied to the inverter.

[0004] The defects of the above-mentioned existing technologies are: the multivariable model predictive control algorithm is based on a single vector finite control set, and a large number of weight factors need to be set, which easily leads to unstable switching frequency and poor system steady-state performance. Summary of the Invention

[0005] Based on this, it is necessary to provide a control method and system for an LCL type grid-connected inverter to address the above technical problems.

[0006] An embodiment of the present invention provides a control method for an LCL type grid-connected inverter, comprising:

[0007] Obtain the state space model of the LCL grid-connected inverter in the dq synchronous rotating coordinate system and set the time k Discretize the state space model to obtain a discrete state space model;

[0008] Obtain the grid current and capacitor voltage of the LCL type grid-connected inverter in the discrete state space model k The predicted value at time +1; based on the capacitor voltage at k +1 moment prediction value, extract the LCL type grid-connected inverter at the set time k The high-frequency resonant component of the capacitor voltage is obtained, and the high-frequency resonant component is obtained at k +1 predicted value;

[0009] According to the high frequency resonant components of the grid current and capacitor voltage k The predicted value at time +1 is used to construct a tracking value function for suppressing the resonance of the LCL type grid-connected inverter system J ;

[0010] According to the tracking value function J The optimal voltage reference value of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained by the minimum value of the optimal voltage reference value. u d * and u q * ; For the optimal voltage reference expression u d * and u q * Perform Park inverse transformation to obtain the optimal voltage reference value of the LCL type grid-connected inverter in the stationary α-β coordinate system u α * and u β * ;

[0011] Optimal voltage reference value based on stationary α-β coordinate system u α * and u β * A control pulse signal including a plurality of basic voltage vectors is generated, and the control pulse signal is applied to a switch tube of an LCL type grid-connected inverter to realize control of the LCL type grid-connected inverter.

[0012] In addition, the state space model of the three-phase LCL type grid-connected inverter in the continuous domain in the dq synchronous rotating coordinate system is:

[0013]

[0014] Where: is the state variable, is the control input, i fd 、 i fq is the inverter side current, u cd 、 u cq is the capacitor voltage, i gd 、 i gq is the grid current, u d 、 u q is the inverter output voltage, u gd 、 u gq is the grid voltage;

[0015] Coefficient matrix A and B They are:

[0016] ,

[0017] in, L f is the filter inductor on the output side of the inverter, R f is the parasitic resistance on the inverter output side, L g is the filter inductance on the grid side, R g is the parasitic resistance on the grid side, C f is the filter capacitor, ω e is the fundamental angular frequency of the grid voltage.

[0018] In addition, the discrete state space model is obtained, which specifically includes:

[0019] Set the system sampling period to T s ,exist k The state space model in the continuous domain is discretized based on the zero-order holder at all times, and the discrete state space model is obtained as follows:

[0020]

[0021] In the formula, k "and" k +1" respectively k and k +1 sampling moment, xk =[ i fd,k i fq,k u cd,k u cq,k i gd,k i gq,k ] T for k System state variables collected at all times, x k+1 =[ i fd,k+1 i fq,k+1 u cd,k+1 u cq,k+1 i gd,k+1 i gq,k+1 ] T The inverter side current, capacitor voltage and grid current state variables are k +1 time prediction value, u k =[ u d,k u q,k u gd,k u gq,k ] T for k The inverter voltage and grid voltage at the moment, A d 、 B d is the system discretization model coefficient matrix calculated using the zero-order holder method:

[0022]

[0023] in, e is the base of natural logarithms, I 6 is the sixth-order identity matrix.

[0024] In addition, the extraction LCL type grid-connected inverter is set at the time k The high-frequency resonant component of the capacitor voltage is filtered using a high-pass filter, which specifically includes:

[0025]

[0026] in, u c,k+1 =[u cd,k+1 u cq,k+1 ]for k The predicted value of the capacitor voltage in the dq synchronous rotating coordinate system at time +1, u c,k =[ u cd,k u cq,k ]for k The capacitor voltage in the dq synchronous rotating coordinate system at the moment, =[ ]for k The high-frequency resonant component of the capacitor voltage extracted in the dq synchronous rotating coordinate system at time t, =[ ]for k The predicted value of the high-frequency resonant component of the capacitor voltage extracted in the dq synchronous rotating coordinate system at time +1; ɑ The high-pass filter frequency f c The filter coefficient is determined by the formula:

[0027]

[0028] in, T s is the system sampling period, f c is the high-pass filter frequency.

[0029] In addition, the tracking value function constructed to suppress the resonance of the grid-connected inverter system J , which specifically include:

[0030] While tracking the reference grid current according to the grid current prediction value, the high-frequency resonance component prediction value of the capacitor voltage is also tracked to its reference value 0, and the resonance tracking value function for suppressing the grid-connected inverter system is obtained. J for:

[0031]

[0032] in, i g * =[ i gd * i gq * ] is the grid current reference in the dq synchronous rotating coordinate system, i g,k+1 =[ igd,k+1 i gq,k+1 ]for k The predicted value of the grid current at time +1, =[ ]for k The predicted value of the high-frequency resonance component of the capacitor voltage extracted at time +1, λ is the weight factor used to balance the two tracking items.

[0033] In addition, the tracking value function J The optimal voltage reference value of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained by the minimum value of the optimal voltage reference value. u d * and u q * :

[0034] Let the tracking value function J The partial derivative of is 0 and the optimal solution for achieving the minimum value is calculated as follows:

[0035]

[0036] in, is the predicted value of the high-frequency resonant component of the capacitor voltage extracted at time k+1, λ is the weight factor used to balance the two tracking items, i g * is the grid current reference in the dq synchronous rotating coordinate system, i g,k+1 for k +1 time grid current forecast value;

[0037] The optimal voltage reference expression of the inverter is obtained u d * and u q * for:

[0038]

[0039] in, u * =[ u d * u q * ] is the optimal voltage reference expression of the inverter in the dq synchronous rotating coordinate system, , , , , , , , C 1. C 2. C 3. C 4. C 5. C 6. C 7 is the coefficient matrix;

[0040] After the inverse Park transformation matrix , perform the Park inverse transform:

[0041]

[0042] Get the optimal voltage reference value of the inverter in the stationary α-β coordinate system u αβ * =[ u α * u β * ].

[0043] In addition, the optimal voltage reference value based on the stationary α-β coordinate system u α * and u β * Generate a control pulse signal containing multiple basic voltage vectors, which specifically includes:

[0044] Obtaining the optimal voltage reference value of the inverter through the inverse tangent function u αβ * The angle is:

[0045]

[0046] According to the voltage reference u αβ * Angle δ Determine the sector where the voltage reference vector is located, and select two adjacent non-zero vectors corresponding to the sector from the inverter basic voltage vector u αβ,x = u α,x + ju β,x 、 u αβ,y = uα,y + ju β,y and the zero vector u αβ,0 =0 as the three optimal voltage vectors for synthesizing the optimal voltage reference value of the inverter, where j is an imaginary unit;

[0047] The tracking error between the three optimal voltage vectors and the inverter optimal voltage reference value is defined as:

[0048]

[0049] Where, e α,i and e β,i is the tracking error, subscript i = x , y ,0 correspond to three optimal voltage vectors respectively;

[0050] According to the principle that the synthetic vector of the three optimal voltage vectors according to their duty ratios can track the optimal voltage reference value of the inverter within one sampling period, we have:

[0051]

[0052] The specific expression for the optimal duty cycle of the three optimal voltage vectors is:

[0053]

[0054] The switch states corresponding to the three optimal vectors are sequentially acted according to the optimal duty cycle, and a control pulse signal containing multiple basic voltage vectors is generated according to the symmetrical 7-segment mode as follows:

[0055]

[0056] in, d x , d y and d 0 are the duty cycles of the three optimal voltage vectors.

[0057] In addition, a control system for an LCL type grid-connected inverter includes:

[0058] Model building module, used to obtain the state space model of LCL type grid-connected inverter in dq synchronous rotating coordinate system, and set the time k Discretize the state space model to obtain a discrete state space model;

[0059] The prediction module is used to obtain the grid current and capacitor voltage of the LCL type grid-connected inverter in the discrete state space model. k The predicted value at time +1; based on the capacitor voltage at k +1 moment prediction value, extract the LCL type grid-connected inverter at the set time k The high-frequency resonant component of the capacitor voltage is obtained, and the high-frequency resonant component is obtained at k +1 predicted value;

[0060] Function building block for generating the high frequency resonant components of grid current and capacitor voltage in k The predicted value at time +1 is used to construct a tracking value function for suppressing the resonance of the LCL type grid-connected inverter system J ;

[0061] Optimization module, used to track the value function J The optimal voltage reference value of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained by the minimum value of the optimal voltage reference value. u d * and u q * ; For the optimal voltage reference expression u d * and u q * Perform Park inverse transformation to obtain the optimal voltage reference value of the LCL type grid-connected inverter in the stationary α-β coordinate system u α * and u β * ;

[0062] Pulse generation module for optimal voltage reference based on stationary α-β coordinate system u α * and u β * A control pulse signal including a plurality of basic voltage vectors is generated, and the control pulse signal is applied to a switch tube of an LCL type grid-connected inverter to realize control of the LCL type grid-connected inverter.

[0063] The control method of the LCL type grid-connected inverter provided by the embodiment of the present invention has the following beneficial effects compared with the prior art:

[0064] In the prior art, a multi-variable finite set single vector predictive control strategy is usually used to perform predictive control to suppress LCL resonance, which requires the introduction of multiple weight factors, resulting in an unstable switching frequency and poor steady-state performance of the LCL type grid-connected inverter.

[0065] The present invention uses the high frequency resonance component of the grid current and capacitor voltage to k +1 moment prediction value builds tracking value function J , it is possible to realize grid current reference tracking without introducing multiple weight factors; then the tracking value function J The minimum value representing the optimal voltage reference value is processed to generate a control pulse signal containing multiple basic voltage vectors. Control is performed through multiple basic voltage vectors to avoid the problem of unstable switching frequency in single-vector finite control, thereby improving the steady-state control performance of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 A schematic structural diagram of a control method for an LCL-type grid-connected inverter provided in one embodiment;

[0067] Figure 2 A schematic diagram of a grid-connected current steady-state waveform and harmonic spectrum when no resonance suppression item is added in a control method for an LCL-type grid-connected inverter provided in one embodiment;

[0068] Figure 3 A schematic diagram of a grid-connected current steady-state waveform and a harmonic spectrum when a resonance suppression item is added to a control method of an LCL-type grid-connected inverter provided in one embodiment;

[0069] Figure 4 The figure is a grid-connected current dynamic response waveform diagram of a control method for an LCL type grid-connected inverter provided in one embodiment. DETAILED DESCRIPTION

[0070] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present 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 only used to explain the present invention and are not intended to limit the present invention.

[0071] Figure 1 The schematic diagram of the structure corresponding to the method of the present invention is shown. The DC bus voltage source is converted into an AC voltage square wave signal by a three-phase voltage source inverter, and then connected to the grid through an output LCL filter. The three-phase filter inductor current, three-phase filter capacitor voltage, three-phase grid current, and three-phase grid voltage are sampled sequentially.

[0072] In the figure, V dc is the DC bus voltage;R f is the equivalent series resistance inside the filter inductor; L f and R f is the filter inductance and parasitic resistance on the inverter output side, L g and R g is the grid-side filter inductance and parasitic resistance, C f For the filter capacitor. i f,abc is the three-phase filter inductor current; u c,abc is the three-phase filter capacitor voltage; i g,abc is the grid current; u g,abc is the grid voltage; i g * is the grid reference current; i g,k 、 u g,k 、 u c,k and i f,k They are k The grid current, grid voltage, filter capacitor voltage and filter inductor current in the stationary coordinate system at time α-β; the sampled grid voltage is sent to the phase-locked loop to obtain the grid voltage phase θ ; u αβ * is the optimal voltage reference of the inverter in the stationary αβ coordinate system; S abc It is the switching state of the inverter upper arm switch tube.

[0073] In one embodiment, a control method for an LCL-type grid-connected inverter is provided, the method comprising:

[0074] Step 1: Obtain the state space model of the LCL grid-connected inverter in the dq synchronous rotating coordinate system and set the time k The state space model is discretized to obtain a discrete state space model.

[0075] The state space model of the three-phase LCL grid-connected inverter in the continuous domain in the dq synchronous rotating coordinate system is:

[0076]

[0077] Where: ,

[0078] ,

[0079] in, i fd 、 i fq 、 u cd 、 u cq 、 i gd 、 i gq 、 u d 、 u q 、 u gd 、 u gq are the inverter side current, capacitor voltage, grid current, inverter output voltage and grid voltage in the dq synchronous rotating coordinate system, respectively. L f and R f Inverter output side filter inductor and parasitic resistance, L g and R g is the grid-side filter inductance and parasitic resistance, C f is the filter capacitor, ω e is the fundamental angular frequency of the grid voltage.

[0080] Set the system sampling period to T s ,exist k The state space model in the continuous domain is discretized based on the zero-order holder to obtain a discrete state space model. The state variables of the inverter side current, capacitor voltage and grid current are k The predicted value at time +1 is:

[0081]

[0082] In the formula, the subscript “ k "and" k +1" respectively k and k +1 sampling moment, x k =[ i fd,k i fq,k ucd,k u cq,k i gd,k i gq,k ] T for k The system state variable matrix collected at every moment, x k+1 =[ i fd,k+1 i fq,k+1 u cd,k+1 u cq,k+1 i gd,k+1 i gq,k+1 ] T The inverter side current, capacitor voltage and grid current state variables are k +1 time prediction value, u k =[ u d,k u q,k u gd,k u gq,k ] T for k The inverter voltage and grid voltage vector at the moment, A d 、 B d is the system discretization model coefficient matrix calculated using the zero-order holder method: ,in e is the base of natural logarithms, I 6 is the sixth-order identity matrix.

[0083] Step 2: Obtain the grid current and capacitor voltage of the LCL type grid-connected inverter in the discrete state space model. k The predicted value at time +1; based on the capacitor voltage at k +1 moment prediction value, extract the LCL type grid-connected inverter at the set time k The high-frequency resonant component of the capacitor voltage is obtained, and the high-frequency resonant component is obtained at k The predicted value at time +1 is expressed by the following difference equation:

[0084]

[0085] Where u c,k+1 =[u cd,k+1 u cq,k+1 ]fork +1 moment dq synchronous rotating coordinate system capacitor voltage prediction value, u c,k =[u cd,k u cq,k ]for k The capacitor voltage in the dq synchronous rotating coordinate system at the moment, =[ ]for k The high-frequency resonant component of the capacitor voltage extracted in the dq synchronous rotating coordinate system at time t, =[ ]for k The predicted value of the high-frequency resonant component of the capacitor voltage extracted in the dq synchronous rotating coordinate system at time +1. ɑ The high-pass filter frequency f c The filter coefficient is determined by the formula:

[0086]

[0087] in, T s is the system sampling period, f c is the high-pass filter frequency.

[0088] Step 3: According to the high frequency resonant components of the grid current and capacitor voltage k The predicted value at time +1 is used to construct a tracking value function for suppressing the resonance of the LCL type grid-connected inverter system J .

[0089] In order to track and control the grid current while effectively suppressing the resonance of the grid-connected inverter system, that is, while tracking the reference grid current according to the grid current prediction value, the high-frequency resonance component prediction value of the capacitor voltage is also tracked to its reference value 0, and a tracking value function for suppressing the resonance of the grid-connected inverter system can be constructed. J as follows:

[0090]

[0091] Where, i g * =[ i gd * i gq * ] is the grid current reference in the dq synchronous rotating coordinate system. i g,k+1 =[ i gd,k+1i gq,k+1 ]for k The predicted value of the grid current at time +1. =[ ]for k Predicted value of the high-frequency resonant component of the capacitor voltage extracted at time +1. λ is the weight factor used to balance the two tracking items.

[0092] Step 4: Based on the tracking value function J The optimal voltage reference value of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained by the minimum value of the optimal voltage reference value. u d * and u q * ; For the optimal voltage reference expression u d * and u q * Perform Park inverse transformation to obtain the optimal voltage reference value of the LCL type grid-connected inverter in the stationary α-β coordinate system u α * and u β * .

[0093] Specifically, let the tracking value function J The partial derivative is 0, and the equation for solving the optimal solution can be constructed as follows:

[0094]

[0095] By inversely solving the above equation u , we can get the expression of the optimal voltage reference of the inverter as:

[0096]

[0097] Where, u * =[ u d * u q * ] is the optimal voltage reference matrix of the inverter in the dq synchronous rotating coordinate system. C 1 to C 7 is the coefficient matrix:

[0098] ,

[0099] , , , ,

[0100] After the inverse Park transformation matrix , the optimal voltage reference matrix of the inverter in the stationary α-β coordinate system is obtained u αβ * =[ u α * u β * ], as shown in the following formula:

[0101]

[0102] Step 5: Optimal voltage reference value based on the stationary α-β coordinate system u α * and u β * Generates a control pulse signal containing multiple basic voltage vectors.

[0103] Specifically, the inverter voltage reference vector is first obtained by the inverse tangent function u αβ * The angles are as follows:

[0104]

[0105] Since the six sectors divided by the two-level inverter are spaced at intervals of π / 3, the inverter voltage reference vector u αβ * Angle δ The sector where the optimal voltage reference vector is located can be quickly determined, that is: when δ When the following conditions are met, the inverter optimal voltage reference u αβ * Falling in l Sector:

[0106]

[0107] By judgment u αβ *After the sector is located, the two adjacent basic voltage vectors corresponding to the sector can be directly selected as the optimal voltage reference value for synthesizing the inverter u αβ * The two optimal non-zero voltage vectors are u αβ,x = u α,x + ju β,x 、 u αβ,y = u α,y + ju β,y The zero vector is selected based on the principle of minimum switching state transition. u αβ,0 = 0. When the voltage vector u αβ * In sector I, the adjacent voltage vectors u αβ,1 and u αβ,2 Then it is directly output as the optimal voltage vector. Similarly, the voltage vector u αβ * In sector II, the adjacent voltage vectors are u αβ,2 and u αβ,3 ; Voltage vector u αβ * In sector III, the adjacent voltage vectors are u αβ,3 and u αβ,4 ; Voltage vector u αβ * In sector IV, the adjacent voltage vectors are u αβ,4 and u αβ,5 ; Voltage vector u αβ * In sector V, the adjacent voltage vectors are u αβ,5 and u αβ,6 ; Voltage vector u αβ * In sector VI, the adjacent voltage vectors are u αβ,6 and u αβ,1 .

[0108] After determining the three optimal voltage vectors suitable for synthesizing the optimal voltage reference value of the inverter, the system expects the final output synthesized voltage vector to track the voltage reference value within one control cycle to reduce current fluctuations. Therefore, it is necessary to accurately calculate the duty cycle of the three voltage vectors and reasonably distribute them.

[0109] First, the tracking error between the three optimal voltage vectors and the inverter optimal voltage reference value is defined as:

[0110]

[0111] Where, e α,i and e β,i is the tracking error, subscript i = x , y ,0 correspond to the three optimal voltage vectors respectively.

[0112] Then, based on the principle that the optimal voltage reference value of the inverter is tracked within one sampling period, the synthetic vector of the three optimal voltage vectors according to their duty ratio is used. u * = u k+1 , calculate the duty cycle of the voltage vector:

[0113]

[0114] Where, d x , d y and d 0 are the duty cycles of the three optimal voltage vectors. From the above formula, the specific expression of the optimal duty cycle is:

[0115]

[0116] To achieve satisfactory steady-state performance at a fixed switching frequency, the switching states corresponding to the three optimal vectors are sequentially applied at a specified duty cycle, and a symmetrical 7-segment pattern is used to generate control signal pulses containing multiple basic voltage vectors. The specific implementation is as follows:

[0117]

[0118] Step 6: Apply the control pulse signal to the switch tube of the LCL type grid-connected inverter to realize the control of the LCL type grid-connected inverter.

[0119] In one embodiment, a control system for an LCL-type grid-connected inverter is provided, the system comprising:

[0120] Model building module, used to obtain the state space model of LCL type grid-connected inverter in dq synchronous rotating coordinate system, and set the time k The state space model is discretized to obtain a discrete state space model.

[0121] The prediction module is used to obtain the grid current and capacitor voltage of the LCL type grid-connected inverter in the discrete state space model. k The predicted value at time +1; based on the capacitor voltage at k +1 moment prediction value, extract the LCL type grid-connected inverter at the set time k The high-frequency resonant component of the capacitor voltage is obtained, and the high-frequency resonant component is obtained at k +1 predicted value at time.

[0122] Function building block for generating the high frequency resonant components of grid current and capacitor voltage in k The predicted value at time +1 is used to construct a tracking value function for suppressing the resonance of the LCL type grid-connected inverter system J .

[0123] Optimization module, used to track the value function J The optimal voltage reference value of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained by the minimum value of the optimal voltage reference value. u d * and u q * ; For the optimal voltage reference expression u d * and u q * Perform Park inverse transformation to obtain the optimal voltage reference value of the LCL type grid-connected inverter in the stationary α-β coordinate system u α * and u β * .

[0124] Pulse generation module for optimal voltage reference based on stationary α-β coordinate system u α * and u β * A control pulse signal including a plurality of basic voltage vectors is generated, and the control pulse signal is applied to a switch tube of an LCL type grid-connected inverter to realize control of the LCL type grid-connected inverter.

[0125] Example 1

[0126] The method provided by the present invention is applied to a grid-connected inverter system with an LCL type, and the system parameters are given in Table 1.

[0127] Table 1

[0128]

[0129] Figure 2 This is a schematic diagram of the steady-state waveform of the grid-connected current and the harmonic spectrum when the resonance suppression embedded predictive control method of the present invention does not add the resonance suppression term. Figure 3 The diagram of the grid-connected current steady-state waveform and harmonic spectrum when the resonance suppression term is added to the resonance suppression embedded predictive control method of the present invention. Figure 2 and Figure 3 By comparison, the method provided by the present invention can effectively suppress system resonance in steady state, ensuring stable operation of the power grid. Comparison with the harmonic spectrum diagram shows that the method provided by the present invention can achieve lower grid current harmonic distortion and a fixed switching frequency compared to traditional methods.

[0130] Figure 4 This is the grid-connected current waveform diagram when the reference current suddenly changes from 10A to 20A at 0.2s using the control method proposed by the present invention. Figure 4 It can be seen that under the control method provided by the present invention, the system dynamic process can also effectively suppress resonance when the grid current suddenly changes, ensuring the stability of the system under various conditions. Therefore, the method provided by the present invention can suppress system resonance and effectively improve the operational stability of the LCL type grid-connected inverter system.

[0131] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A control method for an LCL type grid-connected inverter, characterized in that: include: Obtain a state space model of the LCL type grid-connected inverter in a dq synchronous rotating coordinate system, and discretize the state space model at a set time k to obtain a discrete state space model; Obtain predicted values ​​of the grid current and capacitor voltage of the LCL-type grid-connected inverter at time k+1 in the discrete state space model; extract the high-frequency resonant component of the capacitor voltage of the LCL-type grid-connected inverter at a set time k based on the predicted value of the capacitor voltage at time k+1, and obtain a predicted value of the high-frequency resonant component at time k+1; According to the predicted values ​​of the high-frequency resonant components of the grid current and capacitor voltage at time k+1, a tracking value function J is constructed to suppress the resonance of the LCL type grid-connected inverter system; The tracking value function J for suppressing the resonance of the grid-connected inverter system is constructed, which specifically includes: While tracking the reference grid current according to the predicted grid current value, the predicted value of the high-frequency resonance component of the capacitor voltage is also tracked to its reference value 0, and the tracking value function J for suppressing the resonance of the grid-connected inverter system is obtained as follows: Among them, i g * =[i gd * i gq * ] is the grid current reference in the dq synchronous rotating coordinate system, i g,k+1 =[i gd,k+1 i gq,k+1 ] is the predicted value of the grid current at time k+1, is the predicted value of the high-frequency resonant component of the capacitor voltage extracted at time k+1, and λ is the weight factor used to balance the two tracking items; According to the minimum value of the optimal voltage reference value characterized by the tracking value function J, the optimal voltage reference value u of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained. d * and u q * ; The optimal voltage reference value u of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system is obtained according to the minimum value of the optimal voltage reference value characterized by the tracking value function J. d * and u q * , which specifically include: Let the partial derivative of the tracking value function J be 0 to calculate the optimal solution to achieve its minimum value. The formula is as follows: in, is the predicted value of the high-frequency resonant component of the capacitor voltage extracted at time k+1, λ is the weight factor used to balance the two tracking items, i g * is the grid current reference in the dq synchronous rotating coordinate system, i g,k+1 is the predicted value of the grid current at time k+1; The optimal voltage reference expression u of the inverter is obtained d * and u q * for: Among them, u * =[u d * u q * ] is the optimal voltage reference expression of the inverter in the dq synchronous rotating coordinate system, C1, C2, C3, C4, C5, C6, and C7 are coefficient matrices; After the inverse Park transformation matrix Perform inverse Park transform: Get the optimal voltage reference value u of the inverter in the stationary α-β coordinate system αβ * =[u α * u β * ]; For the optimal voltage reference expression u d * and u q * Perform Park inverse transformation to obtain the optimal voltage reference value u of the LCL type grid-connected inverter in the stationary α-β coordinate system α * and u β * ; Optimal voltage reference value u based on the stationary α-β coordinate system α * and u β * A control pulse signal including a plurality of basic voltage vectors is generated, and the control pulse signal is applied to a switch tube of an LCL type grid-connected inverter to realize control of the LCL type grid-connected inverter.

2. The control method of an LCL type grid-connected inverter according to claim 1, characterized in that: The state space model of the three-phase LCL grid-connected inverter in the continuous domain in the dq synchronous rotating coordinate system is: Where: x=[i fd i fq u cd u cq i gd i gq ] T is the state variable, u=[u d u q u gd u gq ] T is the control input, i fd 、i fq is the inverter side current, u cd 、u cq is the capacitor voltage, i gd 、i gq is the grid current, u d 、u q is the inverter output voltage, u gd 、u gq is the grid voltage; The coefficient matrices A and B are: Among them, L f is the filter inductor on the output side of the inverter, R f is the parasitic resistance on the inverter output side, L g is the filter inductor on the grid side, R g is the parasitic resistance on the grid side, C f is the filter capacitor, ω e is the fundamental angular frequency of the grid voltage.

3. The control method of the LCL type grid-connected inverter according to claim 2, characterized in that: The discrete state space model is obtained, which specifically includes: Set the system sampling period to T s , based on the zero-order holder at time k, the state space model in the continuous domain is discretized, and the discrete state space model is obtained as follows: x k+1 =A d x k +B d u k Where "k" and "k+1" refer to the k and k+1 sampling moments, and x k =[i fd,k i fq,k u cd,k u cq,k i gd,k i gq,k ] T is the system state variable collected at time k, x k+1 =[i fd,k+1 i fq,k+1 u cd,k+1 u cq,k+1 i gd,k+1 i gq,k+1 ] T are the predicted values ​​of the inverter side current, capacitor voltage and grid current state variables at time k+1, u k =[u d,k u q,k u gd,k u gq,k ] T is the inverter voltage and grid voltage at time k, A d 、B d is the system discretization model coefficient matrix calculated using the zero-order holder method: Where e is the base of natural logarithm and I6 is the sixth-order identity matrix.

4. The control method of the LCL type grid-connected inverter according to claim 1, wherein: The method of extracting the high-frequency resonance component of the capacitor voltage of the LCL type grid-connected inverter at the set time k adopts a high-pass filter, which specifically includes: Among them, u c,k+1 =[u cd,k+1 u cq,k+1 ] is the predicted value of the capacitor voltage in the dq synchronous rotating coordinate system at time k+1, u c,k =[u cd,k u cq,k ] is the capacitor voltage in the dq synchronous rotating coordinate system at time k, is the high-frequency resonant component of the capacitor voltage extracted in the dq synchronous rotating coordinate system at time k, is the predicted value of the high-frequency resonance component of the capacitor voltage extracted in the dq synchronous rotating coordinate system at time k+1; ɑ is the high-pass filter frequency f c The filter coefficient is determined by the formula: Among them, T s is the system sampling period, f c is the high-pass filter frequency.

5. The control method of the LCL type grid-connected inverter according to claim 1, characterized in that: The optimal voltage reference value u based on the stationary α-β coordinate system α * and u β * Generate a control pulse signal containing multiple basic voltage vectors, which specifically includes: Obtain the optimal voltage reference value u of the inverter through the inverse tangent function αβ * The angle is: According to the voltage reference value u αβ * The angle δ determines the sector where the voltage reference vector is located, and selects two adjacent non-zero vectors u corresponding to the sector from the inverter basic voltage vector αβ,x =u α,x +ju β,x 、u αβ,y =u α,y +ju β,y and the zero vector u αβ,0 =0 as three optimal voltage vectors for synthesizing the optimal voltage reference value of the inverter, where j is an imaginary unit; The tracking error between the three optimal voltage vectors and the inverter optimal voltage reference value is defined as: Among them, e α,i and e β,i is the tracking error, and the subscripts i = x, y, and 0 correspond to the three optimal voltage vectors respectively; According to the principle that the synthetic vector of the three optimal voltage vectors according to their duty ratios can track the optimal voltage reference value of the inverter within one sampling period, we have: The specific expression for the optimal duty cycle of the three optimal voltage vectors is: The switch states corresponding to the three optimal vectors are sequentially acted according to the optimal duty cycle, and a control pulse signal containing multiple basic voltage vectors is generated according to the symmetrical 7-segment mode as follows: Among them, d x ,d y and d0 are the duty cycles of the three optimal voltage vectors respectively.

6. A control system for an LCL type grid-connected inverter based on the control method for an LCL type grid-connected inverter according to any one of claims 1 to 5, characterized in that: include: A model building module is used to obtain a state space model of the LCL type grid-connected inverter in a dq synchronous rotating coordinate system and discretize the state space model at a set time k to obtain a discrete state space model; A prediction module is used to obtain predicted values ​​of the grid current and capacitor voltage of the LCL type grid-connected inverter at time k+1 in a discrete state space model; based on the predicted value of the capacitor voltage at time k+1, extract the high-frequency resonant component of the capacitor voltage of the LCL type grid-connected inverter at a set time k, and obtain the predicted value of the high-frequency resonant component at time k+1; A function construction module is used to construct a tracking value function J for suppressing the resonance of the LCL type grid-connected inverter system according to the predicted values ​​of the high-frequency resonance components of the grid current and the capacitor voltage at time k+1; The optimization module is used to obtain the optimal voltage reference value u of the LCL type grid-connected inverter in the dq synchronous rotating coordinate system according to the minimum value of the optimal voltage reference value characterized by the tracking value function J d * and u q * ; For the optimal voltage reference expression u d * and u q * Perform Park inverse transformation to obtain the optimal voltage reference value u of the LCL type grid-connected inverter in the stationary α-β coordinate system α * and u β * ; Pulse generation module for optimal voltage reference value u based on the stationary α-β coordinate system α * and u β * A control pulse signal including a plurality of basic voltage vectors is generated, and the control pulse signal is applied to a switch tube of an LCL type grid-connected inverter to realize control of the LCL type grid-connected inverter.

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