Distributed multi-source power generation input quasi-Z-source inverter control system

Abstract

The invention provides a distributed multi-source power generation input quasi-Z-source inverter control system, and belongs to the technical field of power grid control. The control system is connected to a grid-connected inverter comprising a quasi-Z-source impedance network, and comprises a control module and a grid-connected management module. Wherein the control module adopts a sliding mode control strategy on a direct current side and adopts a repetitive control strategy on an alternating current side. According to the sliding mode control, a straight-through duty ratio D is calculated by formulating a sliding mode surface and an equivalent control law; in the repetitive control, a modulation wave signal is generated by establishing a state space model of a standard repetitive controller; and inputting the shoot-through duty ratio D and the modulation wave signal into an SPWM modulator based on a constant switching frequency boost modulation strategy so as to implement dynamic control on the grid-connected inverter. And the grid-connected management module is responsible for connecting the alternating current output by the inverter into a power grid, and comprises a phase-locked loop technology and an island protection function, so that the safety and the stability of a grid-connected process are ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid control, and in particular, relates to a quasi-Z-source inverter control system for distributed multi-source power generation input. Background Art

[0002] Inverters play a vital role in power generation equipment, especially those used in renewable energy related power generation equipment. First of all, the core function of the inverter is to convert direct current into alternating current. In many power generation equipment, such as solar panels and certain types of wind turbines, the generated electricity is initially in the form of direct current. However, most modern power systems and household appliances require alternating current to operate properly. Inverters enable these power generation equipment to effectively feed electricity into the grid or directly supply it to AC loads by converting direct current into alternating current, thereby achieving efficient utilization and distribution of electrical energy.

[0003] Secondly, the inverter also has the function of adjusting the output voltage and frequency to ensure that the power quality meets the requirements of the grid or load. In a distributed power generation system, multiple power generation equipment may supply power to the grid at the same time, and the output characteristics of these equipment may vary due to environmental conditions or load changes. The inverter can monitor and adjust the output parameters in real time to ensure that the output AC power has a stable voltage and frequency, thereby ensuring the stability and reliability of the power system. In addition, the inverter can also provide power factor correction, optimize power quality, and reduce harmonic pollution to the grid, further improving the performance of power generation equipment and the operating efficiency of the grid.

[0004] In order to meet the actual needs of new energy power conversion technology, a new type of grid-connected inverter, the Z-source inverter, was introduced. The Z-source inverter places an impedance network composed of capacitors and inductors between the inverter bridge and the input power supply, and uses the switch tube of the inverter bridge to conduct up and down at the same time to achieve the buck-boost function. This design does not require the addition of a traditional chopper circuit, thereby significantly reducing the system size and reducing costs. In view of the performance improvement capabilities of the Z-source inverter, the industry continues to propose the technical direction of the quasi-Z-source inverter, which not only solves the limitations of the Z-source inverter, but also reduces the capacitor voltage pressure.

[0005] According to the related public technologies, the technical solution with the publication number CN107968590A proposes a phase angle compensation control method for a three-phase LCL grid-connected inverter under a weak power grid, introduces common coupling point voltage feedforward, reduces the phase angle of the output admittance of the grid-connected inverter at the system cutoff frequency, solves the system instability problem caused by the large bandwidth of the phase-locked loop, and ensures the dynamic response speed of the system. The publication number US20150115722A1 proposes a control method for controlling multiple inverters connected to a public power grid connection point, by setting detection points in the inverters of multiple power generation equipment, and generating the working parameters of each inverter after the detection data is collected, so as to implement simultaneous control of a large number of inverters. The technical solution with the publication number WO2016113838A1 proposes an inverter control device, which changes the inverter working mode of the photovoltaic system by detecting whether the DC input voltage of the photovoltaic power generation is less than the lower limit value.

[0006] The above technical solutions all propose several control systems for inverters that enable new energy power generation equipment to access the power grid. However, in order to realize the precise control solution of inverters for the current complex distributed multi-source power generation input, a more efficient and accurate control method is still needed.

[0007] The foregoing discussion of the background art is intended only to facilitate an understanding of the present invention. This discussion does not acknowledge or admit that any of the material referred to is part of the common general knowledge. Summary of the invention

[0008] The purpose of the present invention is to provide a quasi-Z source inverter control system for distributed multi-source power generation input, which belongs to the field of power grid control technology. The control system is connected to a grid-connected inverter including a quasi-Z source impedance network, and includes a control module and a grid-connected management module. Among them, the control module includes a sliding mode control strategy on the DC side and a repetitive control strategy on the AC side. The sliding mode control calculates the direct duty cycle D by formulating the sliding mode surface and the equivalent control law; the repetitive control generates a modulation wave signal by establishing a state space model of a standard repetitive controller; the direct duty cycle D and the modulation wave signal are input into an SPWM modulator based on a constant switching frequency boost modulation strategy to implement dynamic control of the grid-connected inverter. The grid-connected management module is responsible for connecting the AC power output by the inverter to the power grid, including phase-locked loop technology and island protection functions, to ensure the safety and stability of the grid-connected process.

[0009] The present invention adopts the following technical solution: a quasi-Z-source inverter control system for distributed multi-source power generation input, the control system is used to control the operation of a grid-connected inverter between one or more power generation devices and a public power grid in a distributed power supply network; wherein the front end of the grid-connected inverter is configured with a quasi-Z-source impedance network; In addition, the control system adopts pre-stage control and post-stage control for the DC input side and AC output side of the grid-connected inverter respectively; the pre-stage control adopts a sliding mode control strategy with capacitor voltage error, inductor current error and the integral of capacitor voltage error as state variables to achieve stable regulation of the DC input side voltage. During the control process, the DC side input voltage U dc , capacitor voltage u C1 and the inductor current i L1 Input to the sliding mode controller to calculate the direct duty cycle D; The latter stage controls the measured three-phase output voltage U of the alternating current abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq , the reference current I under the rotating coordinate dq_ref and I dq Input a repetitive controller to generate a modulated wave signal through a repetitive control algorithm; Finally, the direct duty cycle D and the modulation wave signal are input into the modulator, and the switch tube of the inverter is controlled through the SPWM modulation technology to realize the operation control of the inverter.

[0010] Preferably, the control system is configured with a sliding mode controller on the input side of the grid-connected inverter, for implementing the sliding mode control strategy and calculating the DC duty cycle; the control system is configured with a repetitive controller on the output side of the grid-connected inverter, for implementing the repetitive control algorithm and generating a modulated wave signal.

[0011] Preferably, the modulator adopts constant switching frequency boost as a modulation strategy.

[0012] Preferably, in the quasi-Z source impedance network, the capacitance values ​​C of the two configured capacitors are required to be: , Among them, I L is the DC component of the input, D is the direct duty cycle, δ is the voltage ripple coefficient of the capacitor, U dc is the input voltage on the DC side, f s is the switching frequency of the inverter.

[0013] Preferably, in the quasi-Z source impedance network, the inductance values ​​L of the two configured inductors are required to be: , Where, σ is the inductor current ripple rate, and P is the rated output power of the inverter.

[0014] Preferably, when the three-phase output voltage U abc and the filter inductor current Iabc Before Park transformation, an LC filter is set to filter the AC output signal. The inductance value L of the inductor used in the LC filter is f for: , Among them, ω1 is the fundamental angular frequency; I0 is the effective value of the fundamental current; U0 is the effective value of the fundamental voltage; ω0 is the cut-off angular frequency, which can be expressed as: ; And, based on the following relationship, calculate the capacitance value C of the capacitor used in the LC filter f : , Among them, T S is the sampling period of the grid-connected inverter.

[0015] Furthermore, a quasi-Z-source inverter control method for distributed multi-source power generation input is proposed, and the control method is applied to the quasi-Z-source inverter control system for the distributed multi-source power generation input; the control method includes the following control stages: Pre-control stage: The sliding mode control strategy with capacitor voltage error, inductor current error and the integral of capacitor voltage error as state variables is used to achieve stable regulation of DC input voltage. During the control process, the DC input voltage U dc , capacitor voltage u C1 and the inductor current i L1 Input to the sliding mode controller to calculate the direct duty cycle D; Post-stage control stage: The post-stage control measures the three-phase output voltage U of the AC power abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq , input the reference current I under the rotating coordinate dq_ref , and generate a modulated wave signal through a repetitive control algorithm; Finally, the direct duty cycle D and the modulation wave signal are input into the modulator, and the switch tube of the inverter is controlled through the SPWM modulation technology to realize the operation control of the grid-connected inverter.

[0016] The beneficial effects achieved by the present invention are: The control system of the present technical solution adopts a sliding mode control strategy to perform pre-stage control on the DC input side of the grid-connected inverter in the pre-stage control stage on the DC input side, and can accurately calculate the direct duty cycle D to achieve stable regulation of the DC input side voltage. This control method is highly robust to parameter changes and external disturbances, and can effectively suppress fluctuations in the DC side voltage, ensuring stable operation of the inverter under different working conditions, and improving system reliability and power quality.

[0017] In the control system of this technical solution, a repetitive control algorithm is used in the post-control stage of the AC output side. By using a repetitive controller, an accurate modulation wave signal can be generated. This control method can effectively suppress the harmonics in the grid-connected current, achieve high-precision tracking of the grid-connected current, improve the quality of power, meet the strict requirements of the power grid on the grid-connected current, and enhance the stability and reliability of the system.

[0018] The control system of the present technical solution can be applied to various forms of power generation equipment in a distributed multi-source power generation network. By adjusting the values ​​of capacitors, inductors and other components in the control system, the control system can have higher application applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention can be further understood from the following description in conjunction with the accompanying drawings. The components in the figures are not necessarily drawn to scale, but the emphasis is placed on illustrating the principles of the embodiments. In different views, the same reference numerals designate corresponding parts.

[0020] Description of the accompanying drawings: 1-power generation equipment; 2-grid-connected inverter; 3-public power grid; 10-quasi-Z source impedance network; 20-sliding mode controller; 30-repetitive controller; 40-microprocessor for Park transformation; 50-microprocessor for inverse Park transformation; 60-modulator; Figure 1 A schematic diagram of the connection architecture between the power generation equipment, the grid-connected inverter and the public power grid; Figure 2 is a schematic diagram of the architecture of the control system described in an embodiment of the present invention; Figure 3 Schematic diagram of a DC side equivalent circuit diagram of a quasi-Z-source inverter in an embodiment of the present invention; Figure 4 Schematic diagram of a circuit of a quasi-Z source impedance network in a through mode according to an embodiment of the present invention; Figure 5 Schematic diagram of a circuit of a quasi-Z source impedance network in a non-through mode according to an embodiment of the present invention; Figure 6 Schematic diagram of a repeated control topology structure of a repeated controller in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with its embodiments; it should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. For those skilled in the art, after reviewing the following detailed description, other systems, methods and / or features of the present embodiment will become apparent. It is intended that all such additional systems, methods, features and advantages are included in this specification. Included within the scope of the present invention and protected by the appended claims. Additional features of the disclosed embodiments are described in the following detailed description, and these features will be apparent from the following detailed description.

[0022] The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right" and the like indicating the orientation or positional relationship, they are based on the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or component referred to must have a specific orientation. The invention is constructed and operated in a specific orientation, so the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as a limitation on the present application. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to the specific circumstances.

[0023] Embodiment 1: Exemplarily, a quasi-Z source inverter control system for distributed multi-source power generation input is proposed, wherein the control system is applied to control the operation of a grid-connected inverter between one or more power generation devices and a public power grid in a distributed power supply network; wherein a quasi-Z source impedance network is configured at the front end of the grid-connected inverter; In addition, the control system adopts pre-stage control and post-stage control for the DC input side and AC output side of the grid-connected inverter respectively; the pre-stage control adopts a sliding mode control strategy with capacitor voltage error, inductor current error and the integral of capacitor voltage error as state variables to achieve stable regulation of the DC input side voltage. During the control process, the DC side input voltage U dc , capacitor voltage u C1 and the inductor current i L1 Input to the sliding mode controller to calculate the direct duty cycle D; The latter stage controls the measured three-phase output voltage U of the alternating current abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq , input the reference current I under the rotating coordinate dq_ref , and generate a modulated wave signal through a repetitive control algorithm; Finally, the direct duty cycle D and the modulation wave signal are input into the modulator, and the switch tube of the inverter is controlled through the SPWM modulation technology to realize the operation control of the inverter.

[0024] Preferably, the control system is configured with a sliding mode controller on the input side of the grid-connected inverter, for implementing the sliding mode control strategy and calculating the DC duty cycle; the control system is configured with a repetitive controller on the output side of the grid-connected inverter, for implementing the repetitive control algorithm and generating a modulated wave signal.

[0025] Preferably, the modulator adopts constant switching frequency boost as a modulation strategy.

[0026] Preferably, in the quasi-Z source impedance network, the capacitance values ​​C of the two configured capacitors are required to be: , Among them, I L is the DC component of the input, D is the direct duty cycle, δ is the voltage ripple coefficient of the capacitor, U dc is the input voltage on the DC side, f s is the switching frequency of the inverter.

[0027] Preferably, in the quasi-Z source impedance network, the inductance values ​​L of the two configured inductors are required to be: , Where, σ is the inductor current ripple rate, and P is the rated output power of the inverter.

[0028] Preferably, when the three-phase output voltage U abc and the filter inductor current I abc Before Park transformation, an LC filter is set to filter the AC output signal. The inductance value L of the inductor used in the LC filter is f for: , Among them, ω1 is the fundamental angular frequency; I0 is the effective value of the fundamental current; U0 is the effective value of the fundamental voltage; ω0 is the cut-off angular frequency, which can be expressed as: ; And, based on the following relationship, calculate the capacitance value C of the capacitor used in the LC filter f : , Among them, T S is the sampling period of the grid-connected inverter.

[0029] At the same time, a quasi-Z-source inverter control method for distributed multi-source power generation input is proposed, and the control method is applied to the quasi-Z-source inverter control system for the distributed multi-source power generation input; the control method includes the following control stages: Pre-control stage: The sliding mode control strategy with capacitor voltage error, inductor current error and the integral of capacitor voltage error as state variables is used to achieve stable regulation of DC input voltage. During the control process, the DC input voltage U dc , capacitor voltage u C1 and the inductor current i L1 Input to the sliding mode controller to calculate the direct duty cycle D.

[0030] Post-stage control stage: The post-stage control measures the three-phase output voltage U of the AC power abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq , input the reference current I under the rotating coordinate dq_ref , and generate a modulated wave signal through a repetitive control algorithm; Finally, the direct duty cycle D and the modulation wave signal are input into the modulator, and the switch tube of the inverter is controlled through the SPWM modulation technology to realize the operation control of the grid-connected inverter.

[0031] As attached Figure 1 The block diagram shown is the connection architecture between the power generation equipment and the power grid.

[0032] In a distributed multi-source power generation network, power generation equipment 1 (such as solar panels, wind turbines) converts natural energy into direct current. The grid-connected inverter 2 is configured to convert direct current into alternating current, ensuring that its voltage and frequency match those of the public power grid. The public power grid 3 is responsible for transmitting the power to the power users. The above architecture works together to realize the entire process from power generation to power transmission.

[0033] Further, as attached Figure 2 As shown, it is a specific architecture block diagram of the control system.

[0034] The DC input of the power generation device 1 is connected to the quasi-Z source impedance network 10; the quasi-Z source impedance network 10 converts the input voltage U dc , capacitor voltage u C1 and the inductor current i L1 The input is sent to the sliding mode controller 20 to implement the pre-control stage; thereafter, the sliding mode controller 20 calculates the through duty cycle signal D and sends the through duty cycle signal D to the modulator 60 .

[0035] The DC input enters the grid-connected inverter 2, which converts it into AC power and outputs it to the AC side. On the AC side, an LC filter is set up on the AC side to filter the AC output. After that, the control system implements the post-control stage, firstly converting the measured output voltage U abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq Based on the indirect current control strategy, the reference current I dq_ref and I dq The repetitive controller 30 is input to generate a modulated wave signal through a repetitive control algorithm.

[0036] Specifically, as attached Figure 3 The equivalent circuit diagram of the DC side of the quasi-Z source inverter is shown in FIG. Figure 3 The figure shows the topology of the quasi-Z source inverter, which is composed of inductors L1 and L2, capacitors C1 and C2, and diodes. in and i in are the voltage and current values ​​of the DC link respectively.

[0037] When the quasi-Z source inverter is working, there are two states: direct-on and non-direct-on. Figure 4 As shown in the figure, when the same bridge arm of the quasi-Z source inverter is turned on at the same time, the grid-connected inverter works in the direct-through mode. At this time, the diode is turned off due to the reverse bias voltage, the inductors L1 and L2 begin to store energy, and the capacitors C1 and C2 release the stored energy. Figure 5 As shown, when the grid-connected inverter works in the non-through mode, the diode is turned on due to the positive bias voltage, and the voltage it withstands is very small, which can be regarded as a short circuit. At this time, capacitors C1 and C2 take on the role of energy storage.

[0038] The voltages on inductors L1 and L2 are set to u L1 and u L2 The voltages on capacitors C1 and C2 are u C1 and u C2 , the voltage relationship in the through state can be derived: , formula 1; Furthermore, the voltage relationship in the non-shoot-through state can be derived: , formula 2; In steady-state mode, the average voltage of inductors L1 and L2 in one switching cycle is zero, so: , formula 3; In the above formula, D is the duty cycle; M is the boost ratio, that is, the ratio between the output voltage and the input voltage of the inverter; i L1 、i L2 is the current value passing through inductors L1 and L2.

[0039] In a preferred embodiment, the quasi-Z source impedance network is mathematically modeled using the state-space averaging method. The state-space averaging method can accurately describe the steady-state characteristics and low-frequency small-signal dynamic behavior of the PWM inverter, and is widely used in engineering practice. The core of the state-space averaging method is that it allows the specific switching action of the switching element to be ignored when analyzing the behavior of the PWM inverter in the current continuous mode, and approximates the nonlinear dynamic characteristics of the inverter by using a linear equivalent model. Starting from a circuit model containing linear resistors, inductors, capacitors and switching elements, appropriate state variables and output variables are selected to fully describe the state of the circuit, and a state-space model is established accordingly. By weighted averaging the state-space equations in different working stages, an equivalent state equation applicable throughout the entire switching cycle can be obtained.

[0040] Preferably, the implementation steps of the state space averaging method include: S100: According to different working states of the grid-connected inverter, the working modes of the inverter circuit are divided, and an equivalent circuit in each working mode is determined.

[0041] S200: Select appropriate state variables for each working mode, and write corresponding state equations for each mode.

[0042] S300: Based on the time proportion when each working mode exists, a weighted average state equation is obtained by using a weighted average method.

[0043] S400: Based on the fact that the average value of the inductor current and the average value of the capacitor voltage are both zero under steady-state conditions, the weighted average state equation is solved using this characteristic, thereby obtaining a circuit equation for the system in steady-state operation.

[0044] S500: After the system reaches equilibrium, a small low-frequency disturbance is introduced and incorporated into the steady-state equation. By separating the steady-state part and the disturbance part and ignoring the nonlinear effects caused by the disturbance, a linear equation system can be obtained, which is the small signal equation that describes the dynamic behavior of the system under small disturbances.

[0045] S600: Perform Laplace transform on the small signal equation and apply the superposition principle of linear systems to derive the transfer function of the system.

[0046] Preferably, when the state space averaging method is used to model the voltage-type quasi-Z-source inverter, the following assumptions are made to simplify the calculation: (1) The quasi-Z-source inverter operates in the current continuous mode; (2) Set C1 and C2 in the circuit to be equal, denoted as C, and L1 and L2 to be equal, denoted as L; set the series resistance R of capacitor C and the parasitic resistance r of inductor L; (3) All switching devices are assumed to be in ideal state without considering their on and off resistances.

[0047] First, the state space equation is introduced. The state space equation is a mathematical model that describes the dynamic behavior of a system. It expresses the relationship between the state variables of the system through a set of first-order differential equations. The standard form of the state space equation is: , formula 4; In Equation 4, x(t) is the state vector of the system, which represents the state of the system at time t; u(t) is the system input vector, which represents the external input acting on the system; It represents the derivative of the system state vector x(t) with respect to time t, that is, the rate of change of the state vector x. A is the system matrix, which describes the internal dynamic relationship between the system states. B is the input matrix, which describes the impact of the input on the system state.

[0048] Compare with the attached Figure 4 , the state space equation of the quasi-Z-source inverter in the direct-flow state is: , Formula 5; On the other hand, compared with the attached Figure 5 , the state space equation of the quasi-Z-source inverter in the non-shoot-through state is: , formula 6.

[0049] The state space equation of the quasi-Z source inverter is expressed as , based on equations 5 and 6, the state equations of the direct mode and the non-direct mode are established respectively, the direct duty cycle D and the non-direct duty cycle (1-D) are introduced, and the state space averaging method is introduced for analysis and optimization: , formula 7; We can get: , formula 8; Finally, the state space equation of the quasi-Z source inverter can be solved as The form can be expressed as: , formula 9; Furthermore, by performing Laplace transform on the small signal equation and eliminating the disturbance terms that have little impact on the control performance, the transfer function G(s) of the system can be obtained as follows: , formula 10; Where s is the complex frequency variable of the Laplace transform, , I in ,I L1 ,I L2 , U C1 , U C2 They are respectively the current value of the DC link when the grid-connected inverter is in steady state, the current value through the inductors L1 and L2, and the voltage value of the capacitors C1 and C2.

[0050] In a preferred embodiment, in order to keep the DC link voltage in the quasi-Z source network stable, the DC input side of the control system uses a sliding mode controller to implement sliding mode control. Sliding mode control is a nonlinear control technology with excellent robustness and anti-interference ability to achieve precise control of the DC link voltage of the quasi-Z source inverter. The core feature of sliding mode control is that the system structure is dynamically adjusted according to the real-time state during operation, so as to ensure that the system can operate according to the preset "sliding mode" trajectory.

[0051] Preferably, when designing a sliding mode controller, it is necessary to first set a suitable switching surface (sliding mode surface) in the state space, then guide the system trajectory to the predetermined sliding mode surface according to the system's discontinuous control strategy, and then transition to the steady-state point according to the preset control law to ensure that the system stably converges to the equilibrium point. Therefore, when designing a sliding mode control, the main steps are as follows: (1) Design a sliding surface hs(x) suitable for this control system; (2) Design an appropriate control law to achieve discontinuous switching. The control law aims to force the system state trajectory to converge quickly to the sliding surface hs(x) before it touches the sliding surface.

[0052] First, the formal function of the state space representation of the control system is established as: ,among, ; The function in the above formula is a multidimensional function, and there exists a sliding surface hs(x)=hs(x1,x2……,x n )=0, which separates the spatial state into two parts: hs(x)>0 and hs(x)<0.

[0053] Furthermore, the control law is set to cu(x), and cu(x) satisfies: , formula 11; In the formula, , and meets the following requirements: the sliding model state exists; the control algorithm can meet the reachability conditions of the system, and any state point other than hs(x)=0 can reach the vicinity of the sliding surface in a finite time; and ensure the stability and robustness of the sliding motion.

[0054] Specifically, on the DC side, according to Formula 3, the relationship between the capacitor voltage and the DC side voltage is: , formula 12; In the quasi-Z source impedance network, the capacitor voltage is easier to obtain and adjust as a continuous signal. As shown in Formula 12, the capacitor voltage is related to the DC side output voltage of the inverter. Therefore, preferably, the control system controls the capacitor voltage u in the quasi-Z source network. C1 To indirectly control the DC side output voltage, thereby ensuring the stable operation of the grid-connected system of the distributed multi-source power generation system.

[0055] Furthermore, the control law is derived; Specifically, through formula 12, the reference value of the DC side voltage is Converted to capacitor voltage reference value : , formula 13; Ignoring the series resistance R of capacitor C and the parasitic resistance r of inductor L, we can get the following equation (9): , formula 14; Among them, ST indicates the straight-through state, and `S`T indicates the non-straight-through state.

[0056] The capacitor voltage error, inductor current error and the integral of the capacitor voltage error are selected as state variables, namely: , in, is the reference value of the inductor current.

[0057] According to the mathematical model established above, based on the formal function of state space representation, the standard form of the state space equation can be obtained: , formula 15; Furthermore, the sliding surface equation hs(x) is assumed to be: , formula 16; in, is the sliding mode coefficient vector. When the state variables of the system touch the sliding mode interface (i.e., hs=0), the state variables will begin to follow the reference trajectory.

[0058] The quasi-Z-source inverter has two working modes: direct and non-direct operation. Therefore, the switching function can be defined according to its switching characteristics: , make , substituting into formula 15, we can get: , formula 17; The equivalent control law can be obtained from formula 17: , formula 18; In fact, the equivalent control law cu eq Equivalent to the direct duty cycle D of the sliding mode controller, in the quasi-Z source inverter, The optimal values ​​of a1, a2, and a3 can be selected through simulation experiments.

[0059] Moreover, the existence, reachability and stability of the above sliding mode controller can be proved, which shows that the sliding mode controller can adapt to different input and load conditions.

[0060] Embodiment 2: This embodiment should be understood to include at least all the features of any of the above embodiments, and further improve upon them; In a preferred embodiment, a repetitive control strategy is selected on the output side of the grid-connected inverter, ie, the grid-connected side, to achieve accurate tracking of the periodic signal, have good dynamic response characteristics, and effectively improve the quality of the grid-connected current.

[0061] In the inverter, the control signal often has periodic harmonic characteristics; in a preferred embodiment, an internal model is used to describe the control signal in the control system. The internal model refers to embedding the dynamic model of the external input signal (such as a periodic signal, a step signal, etc.) into the controller so that the entire feedback control system can track or suppress these signals without error. The internal model can significantly improve the tracking accuracy and disturbance suppression ability of the control system for specific signals. For example, for periodic signals, the internal model can achieve zero-static tracking, thereby effectively reducing the steady-state error of the system.

[0062] Preferably, the internal model function is a specific mathematical representation of the internal model. The internal model function is as follows: ; Among them, Q(s) is a compensation function, usually a low-pass filter or a constant close to 1. Its function is to adjust the gain of the internal model to ensure that the gain of the system in the high frequency band gradually decreases, thereby improving the stability and dynamic performance of the system.

[0063] Z -N is a delay function, and the delay time is one period T (including N sampling points). It is used to simulate the periodic delay characteristics of periodic signals.

[0064] Furthermore, the form of the compensation function Q(s) can be various, such as proportional compensation, integral compensation, etc. In a preferred embodiment, due to the delay link in the repetitive control system, when the system faces external disturbances or command changes, it will use the control signal of the previous cycle in the current cycle, which may cause control errors. To accelerate the dynamic response of the system and improve the overall performance, a compensation function can be connected in series in the controller. After the compensation function is corrected, the following form is selected: , Equation 21; In Equation 21, Q´(s) is the corrected compensation function, and K represents the control gain. Its preferred range is 0 < K < 1 to assist the system in amplitude compensation. When K takes a relatively large value, although the convergence speed of the system can be accelerated and the error can be reduced, the stability of the system will be sacrificed; on the contrary, if K takes a relatively small value, although the stability of the system can be enhanced, the convergence speed may be slowed down.

[0065] Z w is the lead phase compensation, that is, the aforementioned delay function Z -N . In the system, the controlled object and the filter may cause phase lag. By introducing Z w it can be ensured that no additional phase shift will be introduced during the compensation process in the medium and low frequency ranges. Q(s) is the filter part, usually a low-pass filter or a more complex filter form, which is used to adjust the frequency response of the system, reduce or eliminate the influence of resonance peaks, and improve the stability of the system.

[0066] Furthermore, in a preferred implementation manner, as shown in the attached Figure 6 figure, it is the repetitive control topology structure of the repetitive controller. Among them, Q(s) is the transfer function in continuous time of a first-order low-pass filter, τ is the delay time, and e -sτ is the pure delay link, where e is the base of the natural logarithm, which is used to simulate the delay generated during the signal transmission process.

[0067] The input-output relationship formula of the repetitive controller after adding the low-pass filter is as follows: , Equation 22; Among them, y rc (s) and e(s) are the Laplace transforms of the output y rc (t) and the error e(t) respectively. And, based on the transfer function of the first-order low-pass filter and combined with Equation 22, the state space equation of the repetitive control topology structure in the attached Figure 3 figure can be obtained as: , Equation 23; In Equation 23, x rc (t) represents the state variable of the low-pass filter.

[0068] By setting the cutoff frequency ω of transient and steady-state response c , and then according to the inverter parameters, the LMI toolbox of MATLAB is used to perform verification calculations to determine the gain parameters K1 and K2 of the repetitive controller. And through conversion, the gain of K1 can be equivalently decomposed into K11 and K12, corresponding to the current signal gain and voltage signal gain on the AC side.

[0069] Furthermore, in a preferred embodiment, the current control method on the grid-connected side adopts an indirect current control strategy to control the grid-connected side inductor current and combines a control method based on grid voltage to ensure the stability of the grid-connected current and improve the current waveform.

[0070] Furthermore, in the system using the quasi-Z source impedance network, in order to control the ripple factor within a specific range and make the quasi-Z source impedance network have good anti-interference performance, a larger capacitor capacity is usually required, but at the same time, it is also necessary to balance the cost, energy loss and dynamic response speed of the system. In a single switching cycle, the quasi-Z source network will alternate between two working modes: one is to charge the capacitor in the non-direct state, at this time, the two capacitors are connected in parallel to jointly bear the voltage fluctuation of the current, so as to maintain the stability of the DC link voltage, which is crucial to the output AC voltage sinusoidal waveform; the capacitor is discharged during the direct state. Therefore, the capacitance C of the required capacitor can be designed by the following formula: ; At the same time, since the fundamental wave period is much longer than the direct-flow time, choosing a sufficiently large inductor can make the current of the circuit in the direct-flow stage approximately constant. Therefore, the analysis can be simplified: ; When a constant switching frequency boost modulation strategy is adopted, the capacitor voltage undergoes six fluctuations in a single switching cycle, that is, the working time of each direct-on state is: ; Preferably, the switching frequency f of the inverter s is 10kHz.

[0071] In the quasi-Z source impedance network, if the capacitor voltage ripple coefficient is set to a, the capacitor voltage fluctuation du C ≤daC, thus the capacitance calculation formula is derived as follows: ; In addition, in the quasi-Z source impedance network, c1 ≥u c2, if two capacitors of equal value are selected, it can be obtained that when the specification of capacitor C1 meets the predetermined ripple requirement, capacitor C2 will also meet the requirement. Therefore, the selection standard of the capacitor in the quasi-Z source impedance network can be a single formula, which is expressed as: .

[0072] Embodiment 3: This embodiment should be understood to include at least all the features of any of the above embodiments, and further improve upon them; In a preferred embodiment, the components in the control system are set according to the following values: Quasi-Z source network inductance L1, L2: 2mH; Quasi-Z source network capacitors C1, C2: 500μF; Filter inductor L f : 3mH; Filter capacitor C f :10μF; Furthermore, each gain coefficient is set according to the following values: a1: 0.005; a2: 50500; a3: 81; K11: -167.69; K12: -150.44; K2: 5240; Furthermore, the public grid voltage is 220V, and the DC bus voltage U in 700V; The performance of the DC side grid-connected inverter using sliding mode control strategy and PI control strategy is compared. dc and the quasi-Z source DC link voltage u in When the set value is disturbed, the capacitor voltage u is obtained respectively C and DC link voltage u in Performance statistics.

[0073] In the first case, when the DC bus voltage is set to 700V, the input voltage drops from 600V to 500V in 0.2 seconds, and then suddenly rises by 50V in 0.4 seconds. Under both control modes, the DC link voltage can be accurately maintained at the set value of 700V. However, when the input voltage suddenly changes, under the PI control strategy, the system adjustment time is 55 milliseconds, the time to return to a stable state is long, and the overshoot reaches 15.61%; under the sliding mode control strategy, the system adjustment time is shortened to 23 milliseconds, and the overshoot is reduced to 8.21%, which shows that sliding mode control can provide faster dynamic response and better system stability.

[0074] Assuming the input voltage is 600V, the DC bus voltage reference value increases rapidly from 700V to 800V in 0.2 seconds, and then drops back to 700V in 0.4 seconds. Under these two different control methods, the time it takes for the system to reach a new stable state is 45 milliseconds under the PI control strategy; in contrast, this time is shortened to 20 milliseconds when the sliding mode control is applied. This once again verifies the advantages of the sliding mode control strategy designed in this paper in terms of dynamic response speed and anti-interference.

[0075] Although the present invention has been described above with reference to various embodiments, it should be understood that many changes and modifications may be made without departing from the scope of the present invention. That is, the methods, systems and devices discussed above are examples. Various configurations may appropriately omit, replace or add various processes or components. For example, in alternative configurations, the method may be performed in an order different from the order described, and / or various components may be added, omitted and / or combined. Moreover, the features described with respect to certain configurations may be combined in various other configurations, such as different aspects and elements of the configurations may be combined in a similar manner. In addition, the elements therein may be updated as the technology develops, i.e., many elements are examples and do not limit the scope of the present disclosure or claims.

[0076] Specific details are given in the specification to provide a thorough understanding of the exemplary configurations including implementations. However, the configurations may be practiced without these specific details. For example, well-known circuits, processes, algorithms, structures, and techniques have been shown without unnecessary details to avoid obscuring the configurations. This description provides only example configurations and does not limit the scope, applicability, or configurations of the claims. On the contrary, the foregoing description of the configurations will provide those skilled in the art with an enabling description for implementing the described techniques. Various changes may be made to the functions and arrangements of the elements without departing from the spirit or scope of the present disclosure.

[0077] In summary, it is intended that the above detailed description is considered to be illustrative rather than restrictive, and it should be understood that the above embodiments should be understood to be only used to illustrate the present invention and not to limit the scope of protection of the present invention. After reading the contents of the present invention, the technician can make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A quasi-Z-source inverter control system for distributed multi-source power generation input, characterized in that: The control system is used to control the operation of a grid-connected inverter between one or more power generation devices and a public power grid in a distributed power supply network; wherein a quasi-Z source impedance network is configured at the front end of the grid-connected inverter; In addition, the control system adopts pre-stage control and post-stage control for the DC input side and AC output side of the grid-connected inverter respectively; the pre-stage control adopts a sliding mode control strategy with capacitor voltage error, inductor current error and the integral of capacitor voltage error as state variables to achieve stable regulation of the DC input side voltage. During the control process, the DC side input voltage U dc , capacitor voltage u C1 and the inductor current i L1 Input to the sliding mode controller to calculate the direct duty cycle D; The latter stage controls the measured three-phase output voltage U of the alternating current abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq , the reference current I under the rotating coordinate dq_ref and I dq Input a repetitive controller to generate a modulated wave signal through a repetitive control algorithm; Finally, the direct duty cycle D and the modulation wave signal are input into the modulator, and the switch tube of the inverter is controlled through the SPWM modulation technology to realize the operation control of the inverter.

2. The control system according to claim 1, characterized in that: The control system is configured with a sliding mode controller on the input side of the grid-connected inverter, which is used to implement the sliding mode control strategy and calculate the DC duty cycle; the control system is configured with a repetitive controller on the output side of the grid-connected inverter, which is used to implement the repetitive control algorithm and generate a modulated wave signal.

3. The control system according to claim 2, characterized in that: The modulator adopts constant switching frequency boost as the modulation strategy.

4. The control system according to claim 3, characterized in that: In the quasi-Z source impedance network, the capacitance values ​​C of the two configured capacitors are required to be: ， Among them, I L is the DC component of the input, D is the direct duty cycle, δ is the voltage ripple coefficient of the capacitor, U dc is the input voltage on the DC side, f s is the switching frequency of the inverter.

5. The control system according to claim 4, characterized in that: In the quasi-Z source impedance network, the inductance value L of the two configured inductors is required to be: ， Where, σ is the inductor current ripple rate, and P is the rated output power of the inverter.

6. The control system according to claim 5, characterized in that: When the three-phase output voltage U abc and the filter inductor current I abc Before Park transformation, an LC filter is set to filter and sort the AC output signal; The inductance value L of the inductor used in the LC filter f for: ， Among them, ω1 is the fundamental angular frequency; I0 is the effective value of the fundamental current; U0 is the effective value of the fundamental voltage; ω0 is the cut-off angular frequency, which can be expressed as: ； And, based on the following relationship, calculate the capacitance value C of the capacitor used in the LC filter f : ， Among them, T S is the sampling period of the grid-connected inverter.

7. A quasi-Z-source inverter control method for distributed multi-source power generation input, the control method is applied to a quasi-Z-source inverter control system for distributed multi-source power generation input as claimed in claim 6; the control method comprises the following control stages: Pre-control stage: The sliding mode control strategy with capacitor voltage error, inductor current error and the integral of capacitor voltage error as state variables is used to achieve stable regulation of DC input voltage. During the control process, the DC input voltage U dc , capacitor voltage u C1 and the inductor current i L1 Input to the sliding mode controller to calculate the direct duty cycle D; Post-stage control stage: The post-stage control measures the three-phase output voltage U of the AC power abc and the filter inductor current I abc Perform Park transformation to obtain the voltage U under the two-phase rotating coordinates dq and current I dq , input the reference current I under the rotating coordinate dq_ref , and generate a modulated wave signal through a repetitive control algorithm; Finally, the direct duty cycle D and the modulation wave signal are input into the modulator, and the switch tube of the inverter is controlled through the SPWM modulation technology to realize the operation control of the grid-connected inverter.

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