A Current Loop Control Method and System Suitable for Grid Connection of Photovoltaic Inverters

Abstract

The present invention discloses a current loop control method and system applicable to grid connection of a photovoltaic inverter, which relates to the technical field of photovoltaic control, and includes: acquiring three-phase currents at the grid connection point, inputting the three-phase currents into a pre-constructed Clark coordinate converter for coordinate transformation to obtain current components on the α-axis and β-axis; acquiring the grid connection point voltage, inputting the grid connection point voltage into a pre-constructed voltage outer-loop PI controller, and after coordinate transformation, entering a phase-locked loop to obtain a voltage deviation value, and obtaining the reference current components on the α-axis and β-axis through PI control of the voltage deviation value; inputting the current components on the α-axis and β-axis and the reference current components on the α-axis and β-axis into a pre-constructed nF-DPCI controller, outputting a PWM modulation signal, inputting the PWM modulation signal into a PWM generator for modulation to obtain a switching tube drive signal, and realizing the control of the current of the grid-connected inverter based on the switching tube drive signal.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic control, and specifically to a current loop control method and system applicable to grid connection of a photovoltaic inverter. Background Art

[0002] With the rapid expansion of the construction scale of global solar photovoltaic power generation systems, the control of photovoltaic systems has become the focus of research. Photovoltaic power generation systems are often connected to the power grid through grid-connected inverters. Grid-connected inverters play the role of power grid power transmission, and grid-connected current control is the core issue of precise control of photovoltaic systems. Currently, commonly used grid-connected control currents include constant frequency hysteresis control, predictive control, repetitive control, single-cycle control, sliding mode control, feedback linearization control, deadbeat control, proportional-integral control, etc. Among them, proportional-integral control is the most commonly used control method, and its principle and design are relatively simple and easy to implement.

[0003] The current inner loop control using proportional-integral current control has an infinite gain only for direct current. And positive-sequence alternating current signals appear as direct current signals in the dq coordinate system. Therefore, precise control of positive-sequence alternating current signals can be achieved through a PI controller. However, the coupling effect between the d-axis and q-axis currents in the dq coordinate system will affect the dynamic characteristics of the current loop. Summary of the Invention

[0004] To solve the deficiencies mentioned in the above background art, the purpose of the present invention is to provide a current loop control method and system applicable to grid connection of a photovoltaic inverter.

[0005] In a first aspect, the purpose of the present invention can be achieved through the following technical solutions: A current loop control method applicable to grid connection of a photovoltaic inverter, the method comprising the following steps:

[0006] Obtain the three-phase currents at the grid connection point, input the three-phase currents into a pre-constructed Clark coordinate converter for coordinate conversion to obtain the current components on the α-axis and β-axis;

[0007] Obtain the grid connection point voltage, input the grid connection point voltage into a pre-constructed voltage outer loop PI controller, enter a phase-locked loop through coordinate conversion to obtain a voltage deviation value, and obtain the reference current components on the α-axis and β-axis through PI control of the voltage deviation value;

[0008] Input the current components on the α-axis and β-axis and the reference current components on the α-axis and β-axis into a pre-constructed nF-DPCI controller, output a PWM modulation signal, input the PWM modulation signal into a PWM generator for modulation to obtain a switch tube drive signal, and realize the control of the grid-connected inverter current based on the switch tube drive signal.

[0009] In combination with the first aspect, in some implementations of the first aspect, the method further includes: The voltage outer-loop PI controller is as follows:

[0010]

[0011] In the formula, K p is the proportionality coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional-order complex frequency domain.

[0012] In combination with the first aspect, in some implementations of the first aspect, the method further includes: The pre-constructed nF-DPCI controller is constructed by introducing a complex proportional-integral link s - jω based on the voltage outer-loop PI controller.

[0013] In combination with the first aspect, in some implementations of the first aspect, the method further includes: The control transfer function of the nF-DPCI is:

[0014]

[0015] where K p is the proportionality coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional-order complex frequency domain, j is the imaginary unit, ω is the control frequency, and the grid frequency is f.

[0016] In combination with the first aspect, in some implementations of the first aspect, the method further includes: The ω N = 2πf, K p = k·L, K i = k·R L where L represents the filter inductor, R L is the equivalent resistance, and k is the controller gain.

[0017] In combination with the first aspect, in some implementations of the first aspect, the method further includes: The mathematical model of the grid-connected converter in the two-phase stationary α, β coordinate system:

[0018]

[0019] In the formula, u α (t), u β (t) are the components of the converter output voltage on the α and β axes respectively; i α (t), i β (t) are the components of the converter output current on the α and β axes respectively.

[0020] Second aspect, to achieve the above object, the present invention discloses a current-loop control system applicable to grid connection of a photovoltaic inverter, including:

[0021] The first conversion module is configured to obtain the three-phase current at the grid connection point, input the three-phase current into a pre-constructed Clark coordinate converter for coordinate conversion, and obtain the current components on the α-axis and β-axis.

[0022] The second conversion module is configured to obtain the grid connection point voltage, input the grid connection point voltage into a pre-constructed voltage outer loop PI controller, perform coordinate conversion and enter a phase-locked loop to obtain a voltage deviation value, and obtain the reference current components on the α-axis and β-axis through PI control of the voltage deviation value.

[0023] The signal output module is configured to input the current components on the α-axis and β-axis and the reference current components on the α-axis and β-axis into a pre-constructed nF-DPCI controller, output a PWM modulation signal, input the PWM modulation signal into a PWM generator for modulation to obtain a switching tube drive signal, and implement control of the grid-connected inverter current based on the switching tube drive signal.

[0024] Combined with the second aspect, in some implementation manners of the second aspect, the system further includes: the voltage outer loop PI controller of the second conversion module is as follows:

[0025]

[0026] In the formula, K p is the proportionality coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional-order complex frequency domain;

[0027] The pre-constructed nF-DPCI controller of the signal output module is constructed based on introducing a complex proportional integral link s - jω into the voltage outer loop PI controller;

[0028] The control transfer function of nF-DPCI is:

[0029]

[0030] where K p is the proportionality coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional-order complex frequency domain, j is the imaginary unit, ω is the control frequency, and the grid frequency is f;

[0031] ω N = 2πf, K p = k·L, K i = k·R L where L represents the filter inductor, R L is the equivalent resistance, and k is the controller gain;

[0032] Mathematical model of the converter connected to the grid by the first conversion module in the two-phase stationary α, β coordinate system:

[0033]

[0034] In the formula, u α (t), u β (t) are the components of the converter output voltage on the α and β axes respectively; i α (t), i β (t) are the components of the converter output current on the α and β axes respectively.

[0035] In another aspect of the present invention, in order to achieve the above object, a terminal device is disclosed, including a memory, a processor, and a computer program stored in the memory and capable of running on the processor. The memory stores a computer program capable of running on the processor. When the processor loads and executes the computer program, a current loop control method applicable to grid connection of a photovoltaic inverter as described above is adopted.

[0036] In another aspect of the present invention, in order to achieve the above object, a computer-readable storage medium is disclosed. The computer-readable storage medium stores a computer program. When the computer program is loaded and executed by a processor, a current loop control method applicable to grid connection of a photovoltaic inverter as described above is adopted.

[0037] The present invention can directly process three-phase AC signals, avoiding the need to convert signals to the dq coordinate system, and has good dynamic response and steady-state performance. The advantages include: eliminating the phase-locked loop (PLL), reducing errors caused by synchronization problems, avoiding loss of synchronization due to frequency or phase changes in microgrids, aerospace applications or remote networks, reducing control calculation complexity, being applicable to smaller and cost-sensitive systems (such as photovoltaic grid-connected converters), having better response to unbalanced grids and imperfect system inductance imbalance, and fractional-order control being able to estimate and online compensate for disturbances, significantly improving the transient response ability and tracking performance of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings;

[0039] Figure 1 is a schematic diagram of the method flow of the present invention;

[0040] Figure 2 is a schematic diagram of the photovoltaic grid-connected inverter of the present invention;

[0041] Figure 3 It is a schematic diagram of the inverter control model based on current feedback decoupling of the present invention;

[0042] Figure 4 It is a schematic diagram of the nF-DPCI control principle of the present invention;

[0043] Figure 5 It is a schematic diagram of the three-phase grid-connected current of the present invention;

[0044] Figure 6 It is the grid-connected phase current I a and the phase voltage U a schematic diagram;

[0045] Figure 7 It is a schematic diagram of the change of phase A current after perturbation of the present invention;

[0046] Figure 8 It is a schematic diagram of the system structure of the present invention. Specific embodiments

[0047] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

[0048] Embodiment 1:

[0049] Next, the relevant terms involved in the embodiments of the present application will be introduced:

[0050] Photovoltaic inverter: A photovoltaic inverter (PV inverter or solar inverter) can convert the variable DC voltage generated by a photovoltaic (PV) solar panel into AC power at the commercial power frequency, which can be fed back to the commercial power transmission system or used for an off-grid power grid. A photovoltaic inverter is one of the important balance of systems (BOS) in a photovoltaic array system and can be used with equipment powered by general AC power. A solar inverter has special functions to cooperate with a photovoltaic array, such as maximum power point tracking and anti-islanding protection functions.

[0051] As Figure 1 shown, a current loop control method applicable to grid connection of a photovoltaic inverter includes the following steps:

[0052] S101: Obtain the three-phase current at the grid connection point, input the three-phase current into a pre-constructed Clark coordinate converter for coordinate conversion, and obtain the current components on the α-axis and β-axis;

[0053] Mathematical model of grid-connected converter in two-phase stationary α, β coordinate system:

[0054]

[0055] Where u α (t), u β (t) are the components of the converter output voltage on the α and β axes respectively; i α (t), i β (t) are the components of the converter output current on the α and β axes respectively.

[0056] At this time, the complex vector f αβ = f α + jf β is defined, so the voltage and current can be expressed in the form of complex vectors:

[0057]

[0058] Rewrite the formula into vector form

[0059]

[0060] According to the transfer function of the converter in the two-phase coordinate system, it can be written as:

[0061]

[0062] The transfer function of the nF-DPCI current inner loop control is:

[0063]

[0064] After simplification, it can be obtained:

[0065] From the above formula, the closed-loop transfer function of the current loop can be derived:

[0066]

[0067] S102: Obtain the grid connection point voltage, input the grid connection point voltage into the pre-constructed voltage outer loop PI controller, enter the phase-locked loop through coordinate transformation to obtain the voltage deviation value, and obtain the reference current components on the α and β axes through PI control of the voltage deviation value;

[0068] The voltage outer loop PI controller is as follows:

[0069]

[0070] Where, K p is the proportional coefficient of the controller, K i is the integral coefficient of the controller, sλ is the fractional - order complex frequency domain.

[0071] S103: Input the current components of the α - axis and β - axis and the reference current components of the α - axis and β - axis into the pre - constructed nF - DPCI controller, output the PWM modulation signal, input the PWM modulation signal into the PWM generator for modulation to obtain the switching - tube drive signal, and realize the control of the grid - connected inverter current based on the switching - tube drive signal.

[0072] The pre - constructed nF - DPCI controller is constructed by introducing a complex proportional - integral link s - jω based on a voltage - outer - loop PI controller.

[0073] The control transfer function of the nF - DPCI is:

[0074]

[0075] where K p is the proportional coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional - order complex frequency domain, j is the imaginary unit, ω is the control frequency, and the grid frequency is f.

[0076] The ω N = 2πf, K p = k·L, K i = k·R L where L represents the filter inductor, R L is the equivalent resistance, and k is the controller gain.

[0077] Specifically, the solution of the present invention will be further elaborated through the following embodiments:

[0078] According to Figure 4 the nF - DPCI mathematical model, compared with the traditional controller, the nF - DPCI control is itself a complex controller, and it is difficult to realize the controller principle. However, by using the relationship of m α = jm β in the two - phase stationary coordinate system, the complex number j can be realized, that is, the amplitude remains unchanged and the phase rotates forward by 90°.

[0079] According to Figure 5 , Figure 6 the simulation results verify the dynamic performance of the nF - DPCI. Figure 5 is the three - phase grid - connected current waveform output by the photovoltaic grid - connected inverter. It can be seen that after closing the switch at 0.02 s, the current reaches the steady - state value after about one cycle. At the same time, according to Figure 6 the A - phase current and voltage taken in Figure 7At 0.6 seconds, a 0.1-second perturbation is applied to mimic the power change of the photovoltaic due to the external environment change. It can be seen that after adopting the nF-DPCI current controller, the phase A current quickly recovers to the stable value, and the system has good dynamic stability and fast tracking performance.

[0080] Embodiment 2: Second aspect, as Figure 8 shown, to achieve the above object, the present invention discloses a current loop control system applicable to the grid connection of a photovoltaic inverter, including:

[0081] The first conversion module 11 is used to obtain the three-phase current at the grid connection point, input the three-phase current into a pre-constructed Clark coordinate converter for coordinate conversion, and obtain the current components on the α-axis and β-axis;

[0082] The second conversion module 12 is used to obtain the grid connection point voltage, input the grid connection point voltage into a pre-constructed voltage outer loop PI controller, enter the phase-locked loop through coordinate conversion to obtain the voltage deviation value, and obtain the reference current components on the α-axis and β-axis through PI control of the voltage deviation value;

[0083] The signal output module 13 is used to input the current components on the α-axis and β-axis and the reference current components on the α-axis and β-axis into a pre-constructed nF-DPCI controller, output a PWM modulation signal, input the PWM modulation signal into a PWM generator for modulation to obtain a switch tube drive signal, and realize the control of the grid-connected inverter current based on the switch tube drive signal.

[0084] Combined with the second aspect, in some implementation manners of the second aspect, the system further includes: The voltage outer loop PI controller of the second conversion module 12 is as follows:

[0085]

[0086] In the formula, K p is the proportional coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional-order complex frequency domain;

[0087] The pre-constructed nF-DPCI controller of the signal output module 13 is constructed based on the voltage outer loop PI controller by introducing a complex proportional integral link s - jω;

[0088] The control transfer function of nF-DPCI is:

[0089]

[0090] Where K p is the proportional coefficient of the controller, K i is the integral coefficient of the controller, s λThe complex frequency domain is fractional-order, j is the imaginary unit, ω is the control frequency, and the power grid frequency is f;

[0091] ω N = 2πf, K p = k·L, K i = k·R L where L represents the filter inductor, R L is the equivalent resistance, and k is the controller gain;

[0092] The mathematical model of the converter connected to the grid by the first conversion module 11 in the two-phase stationary α, β coordinate system:

[0093]

[0094] In the formula, u α (t), u β (t) are the components of the converter output voltage on the α and β axes respectively; i α (t), i β (t) are the components of the converter output current on the α and β axes respectively.

[0095] Based on the same inventive concept, the present invention also provides a computer device, which includes: one or more processors, and a memory for storing one or more computer programs; the program includes program instructions, and the processor is used to execute the program instructions stored in the memory. The processor may be a Central Processing Unit (CPU), or may also be other general-purpose processors, Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, and is used to implement one or more instructions, specifically used to load and execute one or more instructions in the computer storage medium to implement the above method.

[0096] It should be further noted that, based on the same inventive concept, the present invention also provides a computer storage medium, on which a computer program is stored, and when the computer program is run by a processor, it executes the above method. The storage medium may adopt any combination of one or more computer-readable media. The computer-readable media may be a computer-readable signal medium or a computer-readable storage medium. The computer-readable storage medium may be, for example, but not limited to, an electrical, magnetic, optical, electro-magnetic, infrared, or semiconductor system, apparatus, or device, or any combination of the above. More specific examples (non-exhaustive list) of the computer-readable storage medium include: an electrical connection having one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the above. In the present invention, the computer-readable storage medium may be any tangible medium that contains or stores a program, and the program may be used by or in combination with an instruction execution system, apparatus, or device.

[0097] In the description of this specification, the description with reference to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples.

[0098] The above shows and describes the basic principles, main features, and advantages of the present disclosure. Those skilled in the art of this industry should understand that the present disclosure is not limited by the above embodiments, and the above embodiments and the descriptions in the specification only illustrate the principles of the present disclosure. Without departing from the spirit and scope of the present disclosure, the present disclosure will have various changes and improvements, and these changes and improvements all fall within the scope of the present disclosure claimed.

Claims

1. A current loop control method applicable to grid connection of a photovoltaic inverter, characterized in that, The method includes the following steps: Obtain the three-phase current at the grid connection point, input the three-phase current into a pre-constructed Clark coordinate converter for coordinate transformation to obtain the current components on the α-axis and β-axis; Obtain the grid connection point voltage, input the grid connection point voltage into a pre-constructed outer voltage loop PI controller, and through coordinate transformation enter the phase-locked loop to obtain the voltage deviation value, and obtain the reference current components on the α-axis and β-axis through PI control of the voltage deviation value; Input the current components on the α-axis and β-axis and the reference current components on the α-axis and β-axis into a pre-constructed nF-DPCI controller, output a PWM modulation signal, input the PWM modulation signal into a PWM generator for modulation to obtain a switching tube drive signal, and realize the control of the grid-connected inverter current based on the switching tube drive signal; The control transfer function of the nF-DPCI is: Where K p is the proportionality coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional-order complex frequency domain, j is the imaginary unit, ω is the control frequency, and the grid frequency is f; The ω N = 2πf, K p = k·L, K i = k·R L where L represents the filter inductor, R L is the equivalent resistance, and k is the controller gain.

2. The current loop control method applicable to grid connection of a photovoltaic inverter according to claim 1, wherein, The outer voltage loop PI controller is as follows: Where K p is the proportional coefficient of the controller, and K i is the integral coefficient of the controller, and s λ is the fractional-order complex frequency domain.

3. A current loop control method applicable to grid connection of a photovoltaic inverter, characterized in that, The pre-constructed nF-DPCI controller is constructed based on the outer voltage loop PI controller by introducing a complex proportional integral link s - jω.

4. A current loop control method applicable to grid connection of a photovoltaic inverter, characterized in that, The mathematical model of the grid-connected converter in the two-phase stationary α, β coordinate system: where u α (t), u β (t) are the components of the converter output voltage on the α and β axes respectively; i α (t), i β (t) are the components of the converter output current on the α and β axes respectively.

5. A current loop control system applicable to the grid connection of a photovoltaic inverter, characterized in that, Includes: A first conversion module, which is used to obtain the three-phase current at the grid connection point, input the three-phase current into a pre-constructed Clark coordinate converter for coordinate transformation to obtain the current components on the α-axis and β-axis; A second conversion module, which is used to obtain the grid connection point voltage, input the grid connection point voltage into a pre-constructed outer voltage loop PI controller, and through coordinate transformation enter the phase-locked loop to obtain the voltage deviation value, and obtain the reference current components on the α-axis and β-axis through PI control of the voltage deviation value; A signal output module, which is used to input the current components on the α-axis and β-axis and the reference current components on the α-axis and β-axis into a pre-constructed nF-DPCI controller, output a PWM modulation signal, input the PWM modulation signal into a PWM generator for modulation to obtain a switching tube drive signal, and realize the control of the grid-connected inverter current based on the switching tube drive signal; The control transfer function of the nF-DPCI is: Where K p is the proportional coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional - order complex frequency domain, j is the imaginary unit, ω is the control frequency, and the grid frequency is f; ω N = 2πf, K p = k·L, K i = k·R L where L represents the filter inductor, R L is the equivalent resistance, and k is the controller gain.

6. The current loop control system applicable to grid connection of a photovoltaic inverter according to claim 5, wherein, The outer voltage loop PI controller of the second conversion module is as follows: where K p is the proportional coefficient of the controller, K i is the integral coefficient of the controller, s λ is the fractional - order complex frequency domain; The pre-constructed nF-DPCI controller of the signal output module is constructed based on the outer voltage loop PI controller by introducing a complex proportional integral link s - jω; The mathematical model of the grid-connected converter of the first conversion module in the two-phase stationary α, β coordinate system: where \(u\) α (t), \(u\) β (t) are the components of the converter output voltage on the α and β axes respectively; \(i\) α (t), \(i\) β (t) are the components of the converter output current on the α and β axes respectively.

7. A terminal device, comprising a memory, a processor, and a computer program stored in the memory and capable of running on the processor, characterized in that, The memory stores a computer program that can run on a processor. When the processor loads and executes the computer program, it adopts a current loop control method for grid connection of a photovoltaic inverter as described in any one of claims 1 to 4.

8. A computer-readable storage medium storing a computer program therein, characterized in that, When the computer program is loaded and executed by the processor, it adopts a current loop control method for grid connection of a photovoltaic inverter as described in any one of claims 1 to 4.

Images