Grid-connected inverter control method, device and system

Abstract

The invention discloses a grid-connected inverter control method, device and system, and relates to the technical field of inverter control, and the grid-connected inverter control method comprises the following steps: S10, obtaining a power grid side three-phase current, and carrying out the dq conversion of the power grid side three-phase current to obtain a d-axis current component and a q-axis current component of the power grid side three-phase current; s20, calculating the d-axis current component and the q-axis current component of the three-phase current at the power grid side according to a preset fractional order self-coupling PI lambda control algorithm to obtain a control output signal; and S30, performing dq inverse transformation and pulse width modulation processing on the control output signal, and inputting the control output signal to a grid-connected inverter. The fractional order self-coupling PI lambda control algorithm is adopted to control the grid-connected inverter, on one hand, the characteristic that existing self-coupling PI control is high in disturbance rejection capacity is inherited, on the other hand, the control precision of the existing self-coupling PI control is further improved by adding a fractional order integration link, and the electric energy quality of an input power grid of the grid-connected inverter is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and particularly relates to a grid-connected inverter control method, device and system. Background Art

[0002] LCL-type grid-connected inverters are widely used as the interface for new energy distributed systems to access the power grid system. New energy distributed systems such as wind power generation and photovoltaic power generation have complex working environments and often face problems such as power mutations and severe distortion of grid-connected currents, making it impossible for LCL-type grid-connected inverters to filter out grid-connected current harmonics, seriously affecting the power quality of the grid-connected inverter output.

[0003] In order to improve the power quality of the grid-connected inverter output, many scholars have tried to make improvements in the control strategy direction. Among them, the autotransformer PI control is an improved PID control method, which has the characteristics of strong anti-disturbance ability and high control accuracy. However, in practical applications, when the autotransformer PI control is applied to the LCL-type grid-connected inverter, its control accuracy is still slightly insufficient. Therefore, it is necessary to further improve the existing autotransformer PI control to meet the control requirements of the LCL-type grid-connected inverter. Summary of the Invention

[0004] Embodiments of the present invention provide a grid-connected inverter control method, device and system to solve the technical problem of low control accuracy in the related art where the existing grid-connected inverter uses autotransformer PI control.

[0005] In a first aspect, a grid-connected inverter control method is provided, including the following steps:

[0006] Obtain the three-phase grid-side current, and perform dq transformation on the three-phase grid-side current to obtain the d-axis current component and q-axis current component of the three-phase grid-side current;

[0007] Perform operations on the d-axis current component and q-axis current component of the three-phase grid-side current according to a preset fractional-order autotransformer PI λ control algorithm to obtain a control output signal;

[0008] Perform dq inverse transformation and pulse width modulation processing on the control output signal and then input it to the grid-connected inverter.

[0009] In some embodiments, the step of performing operations on the d-axis current component and q-axis current component of the three-phase grid-side current according to a preset fractional-order autotransformer PI λ control algorithm to obtain a control output signal includes:

[0010] Construct a preset fractional-order autotransformer PI λ control algorithm function based on the mathematical model of the grid-connected inverter as follows:

[0011]

[0012] Among them, u x is the control output signal, and x is d or q;

[0013] b is the control gain constant, b = K SVPWM / (L1 + L2), where K SVPWM is the inverter gain constant, L1 is the inductor on the inverter side, and L2 is the inductor on the grid side;

[0014] z c is the speed factor;

[0015] e is the system error is the d-axis current component or q-axis current component of the given current, and i gx is the d-axis current component or q-axis current component of the three-phase current on the grid side;

[0016] is the fractional-order error integral, is the fractional calculus, and λ is the fractional order.

[0017] In some embodiments, the fractional calculus is represented as follows:

[0018]

[0019] Among them, t and a are the preset upper and lower limits of the operator respectively.

[0020] In some embodiments, the step of calculating the control output signal by operating on the d-axis current component and q-axis current component of the three-phase current on the grid side according to a preset fractional-order auto-coupled PI λ control algorithm includes:

[0021] Construct a multi-objective optimization function related to the system error and the speed factor;

[0022] Use the dandelion optimization algorithm to optimize the system error and the speed factor of the multi-objective optimization function.

[0023] In some embodiments, the multi-objective optimization function is as follows:

[0024]

[0025] Among them, h1 and h2 are the optimization weights of e and respectively.

[0026] In some embodiments, the optimization weights satisfy h1 + h2 = 1 and h1 = kh2, where k > 90.

[0027] In a second aspect, a grid-connected inverter control device is provided, including:

[0028] A converter for obtaining three-phase grid-side currents, performing dq transformation on the three-phase grid-side currents to obtain the d-axis current component and q-axis current component of the three-phase grid-side currents;

[0029] A controller for operating on the d-axis current component and q-axis current component of the three-phase grid-side currents according to a preset fractional-order self-coupled PI λ control algorithm to obtain a control output signal;

[0030] A modulator for performing dq inverse transformation and pulse-width modulation processing on the control output signal and then inputting it into the grid-connected inverter.

[0031] In some embodiments, the step of the controller for operating on the d-axis current component and q-axis current component of the three-phase grid-side currents according to a preset fractional-order self-coupled PI λ algorithm to obtain a control output signal includes:

[0032] Constructing a preset fractional-order self-coupled PI λ control algorithm function based on the mathematical model of the grid-connected inverter as follows:

[0033]

[0034] where u x is the control output signal, x is d or q;

[0035] b is the control gain constant, b = K SVPWM / (L1 + L2), K SVPWM is the inverter gain constant, L1 is the inductor on the inverter side, and L2 is the inductor on the grid side;

[0036] z c is the speed factor;

[0037] e is the system error is the d-axis current component or q-axis current component of the given current, i gx is the d-axis current component or q-axis current component of the three-phase grid-side currents;

[0038] is the fractional-order error integral, is the fractional-order calculus, and λ is the fractional-order order.

[0039] In some embodiments, the representation form of the fractional-order calculus is as follows:

[0040]

[0041] Among them, t and a are respectively the preset upper and lower limits of the operator.

[0042] In a third aspect, a grid-connected inverter control system is provided, including the aforementioned grid-connected inverter control device.

[0043] The beneficial effects brought by the technical solution provided by the present invention include:

[0044] The embodiment of the present invention provides a grid-connected inverter control method, device and system. The grid-connected inverter control method first obtains the three-phase current on the grid side, performs dq transformation on the three-phase current on the grid side to obtain the d-axis current component and q-axis current component of the three-phase current on the grid side, and then performs operation on the d-axis current component and q-axis current component of the three-phase current on the grid side according to a preset fractional-order autotransformer PI λ control algorithm to obtain a control output signal, and finally performs dq inverse transformation and pulse width modulation processing on the control output signal and then inputs it to the grid-connected inverter. The present invention adopts a fractional-order autotransformer PI λ control algorithm to control the grid-connected inverter. On the one hand, it inherits the characteristics of strong anti-disturbance ability of the existing autotransformer PI control. On the other hand, by adding a fractional-order integral link, it further improves the control accuracy of the existing autotransformer PI control, effectively improving the power quality of the grid-connected inverter input into the grid and meeting the control requirements of the grid-connected inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0046] Figure 1 It is a flowchart of a grid-connected inverter control method provided by an embodiment of the present invention;

[0047] Figure 2 It is a topology diagram of an LCL three-level neutral point clamped grid-connected inverter provided by an embodiment of the present invention;

[0048] Figure 3 It is a control block diagram of a fractional-order autotransformer PI λ control algorithm function provided by an embodiment of the present invention;

[0049] Figure 4 It is a Bode diagram of a fractional-order autotransformer PI λ control algorithm function provided by an embodiment of the present invention;

[0050] Figure 5For the grid-side incoming current and grid voltage waveform diagram (normal condition) of the grid-connected inverter control method provided by the embodiments of the present invention; Figure 1 The grid-side incoming current and grid voltage waveform diagram (normal condition) of the grid-connected inverter control method provided by the embodiments of the present invention;

[0051] Figure 6 For the grid-side incoming current and grid voltage waveform diagram (normal condition) of the grid-connected inverter control method provided by the embodiments of the present invention; Figure 1 The FFT analysis diagram of the grid-side incoming current of the grid-connected inverter control method provided by the embodiments of the present invention (normal condition);

[0052] Figure 7 For the grid-side incoming current and grid voltage waveform diagram (power mutation) of the grid-connected inverter control method provided by the embodiments of the present invention; Figure 1 The grid-side incoming current and grid voltage waveform diagram (power mutation) of the grid-connected inverter control method provided by the embodiments of the present invention;

[0053] Figure 8 For the grid-side incoming current and grid voltage waveform diagram (power mutation) of the grid-connected inverter control method provided by the embodiments of the present invention; Figure 1 The FFT analysis diagram of the grid-side incoming current of the grid-connected inverter control method provided by the embodiments of the present invention (power mutation). Specific embodiments

[0054] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0055] The embodiments of the present invention provide a grid-connected inverter control method, which can solve the technical problem that the existing grid-connected inverter uses an autotransformer PI control and the control accuracy is still not high.

[0056] See Figure 1 As shown, the embodiments of the present invention provide a grid-connected inverter control method, including the following steps:

[0057] Step S10, obtain the three-phase current on the grid side, and perform dq transformation on the three-phase current on the grid side to obtain the d-axis current component and q-axis current component of the three-phase current on the grid side.

[0058] Specifically, the grid-connected inverter in the embodiments of the present invention is described by taking the LCL three-level neutral-point clamped grid-connected inverter as an example. Figure 2 is the topology diagram of the LCL three-level neutral-point clamped grid-connected inverter, including a three-level neutral-point clamped main circuit and an LCL filter. The embodiments of the present invention obtain the three-phase current i ga 、i gb 、i gc input from the LCL three-level neutral-point clamped grid-connected inverter to the grid, and perform dq transformation on the three-phase current i ga, i gb , i gc Perform dq transformation to obtain the d-axis current component i gd and the q-axis current component i gq of the three-phase grid current.

[0059] Step S20: Perform operations on the d-axis current component and the q-axis current component of the three-phase grid current according to a preset fractional-order auto-coupled PI λ control algorithm to obtain control output signals u d and u q .

[0060] Specifically, the step of performing operations on the d-axis current component and the q-axis current component of the three-phase grid current according to a preset fractional-order auto-coupled PI λ control algorithm to obtain control output signals includes:

[0061] Construct a preset fractional-order auto-coupled PI λ control algorithm function based on the mathematical model of the grid-connected inverter as follows:

[0062]

[0063] where u x is the control output signal, x is d or q;

[0064] b is the control gain constant, b = K SVPWM / (L1 + L2), K SVPWM is the inverter gain constant, L1 is the inductor on the inverter side, and L2 is the inductor on the grid side;

[0065] z c is the speed factor;

[0066] e is the system error is the d-axis current component or q-axis current component of the given current, i gx is the d-axis current component or q-axis current component of the three-phase grid current;

[0067] is the fractional-order error integral, is the fractional-order calculus, and λ is the fractional-order order.

[0068] Figure 3 This is the control block diagram of the fractional-order auto-coupled PI λ control algorithm function of the embodiment of the present invention. Figure 3 In it, f a is the total disturbance, i gx = u x + f a . The embodiment of the present invention adds a fractional-order integral link Added to the autotransformer PI control, it realizes the unified control of two physical links with different properties of proportional and integral, and the control accuracy is improved.

[0069] Among them, the fractional calculus has the following representation:

[0070]

[0071] Among them, t and a are respectively the upper and lower limits of the preset operator. When Re(λ)>0, represents the fractional-order differential on the interval [0, t]; when Re(λ)=0, is the constant 1; when Re(λ)<0, represents the fractional-order integral on the interval [a, t]. Figure 4 is the Bode diagram of the fractional-order autotransformer PI λ control algorithm function under different λ value conditions.

[0072] Step S30: Perform dq inverse transformation and pulse width modulation processing on the control output signal and then input it to the grid-connected inverter.

[0073] Specifically, as shown in Figure 2 , for the control output signals u d and u q of step S20, after performing dq inverse transformation and pulse width modulation processing and inputting them to each arm (controllable switch) of the three-level neutral-point clamped main circuit of the grid-connected inverter, the inverter-side currents i ia 、i ib 、i ic can be output. The inverter-side current is filtered by the LCL filter and then changed into the grid-side current and input into the grid, thus forming a closed-loop control loop.

[0074] In the grid-connected inverter control method in the embodiment of the present invention, the fractional-order autotransformer PI λ control algorithm is used to control the grid-connected inverter. On the one hand, it inherits the strong anti-disturbance ability of the existing autotransformer PI control. On the other hand, by adding a fractional-order integral link, the control accuracy of the existing autotransformer PI control is further improved, effectively improving the power quality of the grid-connected inverter input into the grid and meeting the control requirements of the grid-connected inverter.

[0075] As an optional implementation manner, in an embodiment of the invention, the step of performing operations on the d-axis current component and q-axis current component of the three-phase grid current according to the preset fractional-order autotransformer PI λ control algorithm to obtain the control output signal includes:

[0076] Construct a multi-objective optimization function related to the system error and speed factor;

[0077] The dandelion optimization algorithm is used to optimize the system error and speed factor of the multi-objective optimization function.

[0078] Generally, the smaller the system adjustment time is, the faster the response speed is. However, the system error also needs to be considered to ensure the control accuracy. Therefore, the embodiments of the present invention construct the following multi-objective optimization function:

[0079]

[0080] where h1 and h2 are the optimization weights of e and respectively. z c_min and z c_max are the given minimum and maximum values.

[0081] In the fractional-order autotransformer PI λ control algorithm, to effectively obtain the control parameters, the system error and speed factor are optimized. The dandelion optimization algorithm is used to optimize the multi-objective optimization function, effectively ensuring the control accuracy and response speed. Among them, the system adjustment time t s in the optimization process satisfies That is, the smaller the system adjustment time is, the faster the response speed is.

[0082] In addition, the dandelion optimization algorithm simulates the diffusion process of dandelion seeds, which includes three stages: rising, falling, and finally landing at a certain position. First, the algorithm needs to be initialized to determine the maximum number of iterations, the number of dandelion populations, and the control parameter range (i.e., the above constraints). Then, it enters the rising stage of the algorithm, simulating the random diffusion process of dandelion seeds under the action of wind. When a certain number of seeds diffuse to the set parameter margin, it enters the falling stage. In the falling stage, the algorithm calculates the multi-objective function calculation results of each dandelion seed, selects a batch of dandelion seeds with the smallest calculation results as elite dandelion seeds, delimits a region where the optimal solution may exist for each elite dandelion seed, and ends the falling stage and enters the landing stage. In the landing stage, all ordinary dandelion seeds approach the surrounding area of the nearest elite dandelion seed and randomly land, and the landing coordinates of each seed represent a solution. The dandelion optimization algorithm will calculate the multi-objective optimization function results of all solutions and output a minimum value, thus completing the iterative operation.

[0083] Optionally, considering that the system error and speed factor are not in the same order of magnitude, the embodiments of the present invention set the optimization weights to satisfy h1 + h2 = 1 and h1 = kh2, k > 90, to improve the optimization efficiency and accuracy. For example, h1 = 0.99 and h2 = 0.01.

[0084] Based on Figure 2For the shown topological graph, a simulation model is built in the simulation software, and the parameters of the simulation model are shown in Table 1 below.

[0085] Table 1

[0086]

[0087] Simulation Case 1: For the grid-connected inverter control method according to the embodiment of the present invention, when the LCL-type grid-connected inverter is operating normally, the grid-connected current i g and the grid voltage u g are as shown in Figure 5 . The FFT analysis of the grid current i g is as shown in Figure 6 . It can be seen that the adjustment time of the grid-connected inverter control method according to the embodiment of the present invention is 1 ms, and the total harmonic distortion (THD) is 0.14% (the total harmonic distortion of the corresponding existing autotransformer PI control is generally about 3%). That is, when the LCL-type grid-connected inverter is operating normally, the embodiment of the present invention realizes a short system adjustment time and higher control accuracy, verifies the superiority of the system adjustment time and the control accuracy of the grid-connected inverter control method according to the embodiment of the present invention, and the output power quality is higher.

[0088] Simulation Case 2: Assume that the value of the given current steps from the initial 20 A to 30 A at 0.2 s to simulate the power mutation state. When a power mutation occurs, for the grid-connected inverter control method according to the embodiment of the present invention, the grid-connected current i and the grid voltage u g are as shown in g Figure 7 . The FFT analysis of the grid current i g is as shown in Figure 8 . After the power mutation occurs, it can be seen that the adjustment time of the grid-connected inverter control method according to the embodiment of the present invention is 1 ms, and the total harmonic distortion (THD) is 0.59% (the total harmonic distortion of the corresponding existing autotransformer PI control is generally about 6%). That is, when the LCL-type grid-connected inverter responds to a power mutation, the embodiment of the present invention realizes a short system adjustment time and higher control accuracy, verifying that the grid-connected inverter control method according to the embodiment of the present invention has significant advantages in anti-disturbance.

[0089] The embodiment of the present invention also provides a grid-connected inverter control device, including: a converter, a controller, and a modulator.

[0090] The converter is used to obtain the three-phase grid currents, and perform dq transformation on the three-phase grid currents to obtain the d-axis current component and the q-axis current component of the three-phase grid currents.

[0091] ​The controller is used to calculate the d-axis current component and q-axis current component of the three-phase grid-side current according to a preset fractional-order auto-coupled PI λ control algorithm to obtain a control output signal.

[0092] The modulator is used to perform dq inverse transformation and pulse-width modulation processing on the control output signal and then input it to the control switch of the grid-connected inverter.

[0093] As an optional implementation manner, in an embodiment of the invention, the step that the controller is used to calculate the d-axis current component and q-axis current component of the three-phase grid-side current according to a preset fractional-order auto-coupled PI λ control algorithm to obtain a control output signal includes:

[0094] Construct a preset fractional-order auto-coupled PI λ control algorithm function based on the mathematical model of the grid-connected inverter as follows:

[0095]

[0096] wherein, u x is the control output signal, x is d or q;

[0097] b is the control gain constant, b = K SVPWM / (L1 + L2), K SVPWM is the inverter gain constant, L1 is the inductor on the inverter side, and L2 is the inductor on the grid side;

[0098] z c is the speed factor;

[0099] e is the system error is the d-axis current component or q-axis current component of the given current, i gx is the d-axis current component or q-axis current component of the three-phase grid-side current;

[0100] is the fractional-order error integral, is the fractional-order calculus, and λ is the fractional-order order.

[0101] Furthermore, the representation form of the fractional-order calculus is as follows:

[0102]

[0103] wherein, t and a are respectively the preset operator upper and lower limits.

[0104] An embodiment of the invention further provides a grid-connected inverter control system, including the aforementioned grid-connected inverter control device.

[0105] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by terms such as "upper" and "lower" is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention. Unless otherwise clearly specified and defined, the terms "installed", "connected" and "coupled" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0106] It should be noted that in the present invention, relative terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variation thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising one..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the said element.

[0107] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features of the invention herein.

Claims

1. A grid-connected inverter control method, characterized in that, It includes the following steps: Obtain the three-phase current on the grid side, and perform dq transformation on the three-phase current on the grid side to obtain the d-axis current component and q-axis current component of the three-phase current on the grid side; The d-axis current component and q-axis current component of the three-phase current on the grid side are calculated according to a preset fractional-order auto-coupled PI λ control algorithm to obtain a control output signal; Perform dq inverse transformation and pulse width modulation processing on the control output signal and then input it to the grid-connected inverter.

2. The grid-connected inverter control method according to claim 1, characterized in that The step of calculating a control output signal by performing operations on the d-axis current component and the q-axis current component of the three-phase current on the grid side according to a preset fractional-order auto-coupled PI λ control algorithm includes: Construct a preset fractional-order auto-coupled PI based on the mathematical model of the grid-connected inverter λ The control algorithm function is as follows: where u x is the control output signal, and x is d or q; b is the control gain constant, b = K SVPWM / (L1 + L2), where K SVPWM is the inverter gain constant, L1 is the inductor on the inverter side, and L2 is the inductor on the grid side; z c is the speed factor; e is the systematic error is the d-axis current component or q-axis current component of the given current, i gx is the d-axis current component or q-axis current component of the three-phase grid-side current; is the fractional error integral, is the fractional calculus, and λ is the fractional order.

3. The grid-connected inverter control method according to claim 2, wherein The fractional calculus is expressed as follows: Wherein, t and a are respectively the upper and lower limits of the preset operator.

4. The grid-connected inverter control method according to claim 3, characterized in that The step of calculating a control output signal by operating on the d-axis current component and the q-axis current component of the three-phase current on the grid side according to a preset fractional-order auto-coupled PI λ control algorithm includes: Construct a multi-objective optimization function related to the system error and speed factor; Use the dandelion optimization algorithm to optimize the system error and speed factor of the multi-objective optimization function.

5. The grid-connected inverter control method according to claim 4, wherein, The multi-objective optimization function is as follows: where h1 and h2 are the optimized weights of e and respectively.

6. The grid-connected inverter control method according to claim 5, characterized in that: The optimization weights satisfy h1 + h2 = 1 and h1 = kh2, where k > 90.

7. A grid-connected inverter control device, characterized in that, It includes: A converter, which is used to obtain the three-phase current on the grid side and perform dq transformation on the three-phase current on the grid side to obtain the d-axis current component and q-axis current component of the three-phase current on the grid side; A controller, which is used to calculate a control output signal by performing operations on the d-axis current component and the q-axis current component of the three-phase current on the grid side according to a preset fractional-order auto-coupled PI λ control algorithm; A modulator, which is used to perform dq inverse transformation and pulse width modulation processing on the control output signal and then input it to the grid-connected inverter.

8. The grid-connected inverter control device according to claim 7, wherein, The controller is used for performing operations on the d-axis current component and the q-axis current component of the three-phase current on the grid side according to a preset fractional-order auto-coupled PI λ control algorithm to obtain a control output signal, including: Construct a preset fractional-order auto-coupled PI based on the mathematical model of the grid-connected inverter λ The control algorithm function is as follows: where u x is the control output signal, and x is d or q; b is the control gain constant, b = K SVPWM / (L1 + L2), where K SVPWM is the inverter gain constant, L1 is the inductor on the inverter side, and L2 is the inductor on the grid side; z c is the speed factor; $e$ is the systematic error is the d-axis current component or q-axis current component of the given current, $i$ gx is the d-axis current component or q-axis current component of the three-phase grid-side current; is the fractional - order error integral, is the fractional - order calculus, and λ is the fractional - order order.

9. The grid-connected inverter control device according to claim 8, characterized in that, The fractional calculus is represented as follows: Wherein, t and a are respectively the upper and lower limits of the preset operator.

10. A grid-connected inverter control system, characterized in that, It includes the grid-connected inverter control device described in claim 7.

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