Impedance source photovoltaic inverter with additional controllable input port and control method

Abstract

The invention discloses an impedance source photovoltaic inverter with an additional controllable input port and a control method. The impedance source photovoltaic inverter comprises a photovoltaic array, an impedance source network and a three-phase two-level inverter, the input end of the three-phase two-level inverter is connected with the impedance source network, and the function of controlling the output voltage of the photovoltaic array is achieved through the impedance source network. The output end of the three-phase two-level inverter is connected with a power grid side, so that the function of converting direct current output by the impedance source network into alternating current is realized; the power electronic conversion technology based on the impedance source network is adopted, more-degree-of-freedom control over the DC side control port of the photovoltaic impedance source inverter is achieved, the inverter can be controlled more accurately in the operation process, and more stable and efficient electric energy conversion between photovoltaic power generation and a power grid can be achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic converter control, and particularly relates to an impedance source photovoltaic inverter with an additional controllable input port and a control method therefor. Background Art

[0002] In recent years, the Z-source inverter (ZSI) has received extensive attention in the field of renewable energy. Its main advantages are that it can achieve buck and boost functions in a single-stage circuit and has high reliability due to supporting the short-through (ST) operating mode. The traditional ZSI adopts an X-shaped network structure, which was initially proposed by Professor Peng. Although this topology solves some problems of traditional voltage source inverters, as a basic topology, there are still some criticisms, mainly from different technical perspectives.

[0003] The quasi-Z-source inverter (Quasi ZSI), series Z-source inverter (Series ZSI), multi-level series Z-source inverter (Multiple Series ZSI), switched-inductor quasi-Z-source inverter (Switched-Inductor Quasi ZSI), and extended boost Z-source inverter (Extended Boost ZSI) are all full-bridge topologies that optimize the performance of the traditional ZSI. At the same time, a topology design for half-bridge inverters has also been proposed, which uses fewer switching elements to achieve DC-AC conversion, but its boost factor is equivalent to that of the traditional ZSI. To increase the boost factor, Reference [1] proposed an isolated topology based on the combination of full-bridge and half-bridge structures and a transformer, which led to an increase in the volume and size of the inverter.

[0004] Reference [1] Ke Jin and Xinbo Ruan, "Hybrid Full-Bridge Three-Level LLC Resonant Converter - A Novel DC-DC Converter Suitable for Fuel Cell Power System," 2005 IEEE 36th Power Electronics Specialists Conference, Dresden, Germany, 2005, pp. 361-367, doi: 10.1109 / PESC.2005.1581649; In addition, when the ZSI is used as a power conversion interface between a photovoltaic panel and the grid / load, the control input is at the DC side , and the modulation index MIt is used for AC - side control. Therefore, this may violate the constraint relationship between them. This problem makes the closed - loop control of the ZSI more complex, and it is necessary to provide a control port with more control degrees of freedom on the DC side. Therefore, it is necessary to design an impedance - source photovoltaic inverter and a control method with an additional controllable input port to solve the above problems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an impedance - source photovoltaic inverter and a control method with an additional controllable input port, aiming to solve the problems that the closed - loop control of the Z - source inverter in the prior art is complex and cannot take into account both the improvement of the boost factor and the small volume. By setting multiple control inputs on the DC side, more control degrees of freedom are provided on the DC side.

[0006] To solve the above - mentioned technical problems, the technical solution adopted by the present invention is as follows: An impedance - source photovoltaic inverter with an additional controllable input port includes an impedance - source network, and the impedance - source network is connected between a photovoltaic array and a three - phase two - level inverter; the impedance - source network includes a diode D, an inductor an inductor a switch a switch a diode a diode a capacitor and a capacitor connected in sequence; the cathode of the diode D is connected to the inductor and the switch , the inductor is connected to the anode of the diode , the diode is connected to the capacitor , the capacitor is connected to the inductor and the cathode of the diode , the anode of the diode is connected to the capacitor , the switch is connected between the inductor and the anode of the diode , the capacitor serves as the negative input terminal and the negative output terminal of the impedance - source network, and the negative - pole node of the capacitor serves as the positive output terminal of the impedance - source network.

[0007] Preferably, the output - side voltage of the photovoltaic array is input between the positive input terminal and the negative input terminal of the impedance - source network; the DC voltage is output between the positive output terminal and the negative output terminal of the impedance - source network to the DC - bus side of the three - phase two - level inverter.

[0008] Preferably, when the input voltage of the three-phase two-level inverter is constant, by adjusting the direct-through duty ratio Adjust the output voltage of the photovoltaic array , by Control to find the maximum power point, according to Determine the direct-through signal; On the AC side, by using the modulation index M And the phase of the inverter reference voltage θ To stabilize the input voltage of the inverter and synchronize the output current with the grid voltage.

[0009] Preferably, the control method adopted on the AC side is the sinusoidal pulse width modulation SPWM technology.

[0010] Furthermore, in this way, when performing the maximum power point tracking MPPT, using and taking As a constant can eliminate the possibility of overlap between the ST state and the active state.

[0011] Preferably, the switching factor N Is the control input on the DC side: ; Where Is a constant; Represents the average voltage of the capacitor in the steady state. By adjusting the switching factor N Control the working mode of the MPPT module, and at the same time eliminate the overlap between the ST state and the active state.

[0012] Preferably, through feedforward control to accelerate the convergence process, the value of the switching factor N is determined by the sum of the output of the PI controller and the output of the feedforward controller.

[0013] Preferably, in the sinusoidal pulse width modulation SPWM technology, the reference waveform , , and Represents a sinusoidal waveform with a frequency of 50Hz, respectively having phases , , and , as well as the triangular carrier waveform ; To generate different working modes, four straight lines are used: the lines symmetric about the time axis And Are used to create the ST mode, the And Symmetric about the time axis are used to control And Switch, generate A-nST and nA-nST modes; Compare the reference signal of each phase with the triangular carrier signal, and is compared with a triangular carrier waveform to generate a gating signal for the ST mode to generate a gating signal for the inverter switch; The and are compared with a carrier waveform to generate and gating signals; The value of the parameter is calculated by the following formula: ; ; The switching factor N is used as a control input, is a constant, and the overlap between the ST state and the active state is eliminated.

[0014] Preferably, N is a control input on the DC side for performing maximum power point tracking MPPT; in the MPPT module, the reference voltage of the photovoltaic PV is calculated by the perturbation and observation method P&O, and the photovoltaic output voltage tracks the calculated reference value through a PI controller.

[0015] Preferably, when extracting the dynamic model of the proposed inverter, it is assumed that the equivalent series resistance ESR of the capacitor, switch, and diode is zero, and the calculation is as follows: ; ; ; ; ; The variables in the formula include the voltage across the capacitor , the inverter input voltage , the current through the inductor , the switching function N, and the current through the DC link in n the ST operating mode ; By applying the Kirchhoff's current law KCL equation to the capacitor and the Kirchhoff's voltage law KVL equation to the inductor in different operating modes, averaging these equations and neglecting second-order small quantities, the small-signal model of the proposed inverter is obtained as follows: ; ; ; In the above equations, r and are the equivalent series resistance of the inductor and the capacitance value of the input capacitance, respectively; L represents the inductance parameter; C represents the capacitance parameter.

[0016] Preferably, from to the transfer function expression is as follows: ; Preferably, in the formula, represents the transfer function from the capacitor voltage to the control input; represents the complex frequency variable; a and b represent the constant terms composed of system parameters; k1 and k2 represent the feedback gain coefficients in the control loop.

[0017] The beneficial effects of the present invention are as follows: Compared with the prior art, the impedance source photovoltaic inverter of the present invention with an additional controllable input port has two independent control inputs on the DC side, which enables the inverter to perform more precise control during operation and achieve more stable and efficient power conversion between photovoltaic power generation and the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a structural diagram of an impedance source photovoltaic inverter with an additional controllable input port; Figure 2 is a circuit diagram of an impedance source photovoltaic inverter with an additional controllable input port; Figure 3 is a circuit diagram of a three-phase two-level inverter Figure 4 is a block diagram of the control system; Figure 5 is a schematic diagram of the SPWM implementation method; Figure 6 is the photovoltaic voltage (PV voltage), photovoltaic current (PV current), photovoltaic power (PV power), and maximum power point (MPP) values corresponding to a solar irradiance level of 1000 W / m² in the time interval t = [0, 0.3]; Figure 7 is the photovoltaic voltage, photovoltaic current, photovoltaic power, and maximum power point values corresponding to a solar irradiance level of 700 W / m² in the time interval t = [0.3, 0.6]; Figure 8 is the photovoltaic voltage, photovoltaic current, photovoltaic power, and maximum power point values corresponding to a solar irradiance level of 500 W / m² in the time interval t = [0.6, 0.9]. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Embodiment 1: As shown Figure 1 in the figure, an impedance-source photovoltaic inverter with an additional controllable input port includes an impedance-source network, which is connected between a photovoltaic array and a three-phase two-level inverter; the impedance-source network includes a diode D, an inductor , an inductor , a switching tube , a switching tube , a diode , a diode , a capacitor and a capacitor connected in sequence; the cathode of the diode D is connected to the inductor and the switching tube , the inductor is connected to the anode of the diode , the diode is connected to the capacitor , the capacitor is connected to the inductor and the cathode of the diode , the anode of the diode and the capacitor are connected, the switching tube is connected between the inductor and the anode of the diode , the capacitor serves as the negative input terminal and the negative output terminal of the impedance-source network, and the negative node of the capacitor serves as the positive output terminal of the impedance-source network.

[0020] Preferably, the output-side voltage of the photovoltaic array is input between the positive input terminal and the negative input terminal of the impedance-source network; the DC voltage is output between the positive output terminal and the negative output terminal of the impedance-source network to the DC bus side of the three-phase two-level inverter.

[0021] Preferably, when the input voltage of the three-phase two-level inverter is constant, the output voltage of the photovoltaic array is adjusted by adjusting the direct-conduction duty ratio , the maximum power point is searched by controlling , and the direct-conduction signal is determined according to ; On the AC side, the input voltage of the inverter is stabilized and the output current is synchronized with the grid voltage by using the modulation index M and the phase of the inverter reference voltage θ .

[0022] Preferably, the control method adopted on the AC side is the sine pulse width modulation SPWM technology.

[0023] Furthermore, in this way, when performing the maximum power point tracking MPPT, use and will As a constant, it can eliminate the possibility of overlap between the ST state and the active state.

[0024] Preferably, the switching factor N is the control input on the DC side: ; where is a constant; represents the average voltage of the capacitor in the steady state. By adjusting the switching factor N the working mode of the MPPT module is controlled, and at the same time, the overlap between the ST state and the active state is eliminated.

[0025] Preferably, to accelerate the convergence process through feedforward control, the value of the switching factor N is determined by the sum of the output of the PI controller and the output of the feedforward controller.

[0026] Preferably, in the sinusoidal pulse width modulation SPWM technology, the reference waveforms , , and represent sinusoidal waveforms with a frequency of 50 Hz, having phases , , and , respectively, and the triangular carrier waveform ; To generate different working modes, four straight lines are used: the lines and symmetric about the time axis are used to create the ST mode, and the and symmetric about the time axis are used to control the and switches to generate the A-nST and nA-nST modes; The reference signal of each phase is compared with the triangular carrier signal, and are compared with the triangular carrier waveform to generate the gating signal for the ST mode to generate the gating signal for the inverter switch; The and are compared with the carrier waveform to generate the and gating signals; The value of the parameter is calculated by the following formula: ; ; The switching factor N is used as the control input, is a constant, and the overlap between the ST state and the active state is eliminated.

[0027] Preferably, N is the control input on the DC side for performing maximum power point tracking (MPPT); in the MPPT module, the reference voltage of the photovoltaic (PV) is calculated by the perturbation and observation (P&O) method, and the PV output voltage tracks the calculated reference value through a PI controller.

[0028] Preferably, when extracting the dynamic model of the proposed inverter, it is assumed that the equivalent series resistance (ESR) of the capacitor, switch, and diode is zero, and the calculation is as follows: ; ; ; ; ; The variables in the formula include the voltage across the capacitor , the inverter input voltage , the current through the inductor , the switch function N, and the current through the DC link in the n ST operating mode ; By applying Kirchhoff's current law (KCL) equations to the capacitor and Kirchhoff's voltage law (KVL) equations to the inductor in different operating modes, averaging these equations and neglecting second-order small quantities, the small-signal model of the proposed inverter is obtained as follows: ; ; ; In the above equations, r and are the equivalent series resistance of the inductor and the capacitance value of the input capacitor, respectively; L represents the inductor parameter; C represents the capacitor parameter.

[0029] Preferably, the transfer function expression from to is as follows: ; Preferably, in the formula, represents the transfer function from the capacitor voltage to the control input; represents the complex frequency variable; a and b represent constant terms composed of system parameters; k1 and k2 represent the feedback gain coefficients in the control loop.

[0030] Embodiment 2: In the Z-source inverter, the ST mode should be placed in the zero state of the inverter; therefore, and M are not completely independent; choosing a value for one variable imposes a constraint on the other variable. On the other hand, when these inverters are used as the interface between photovoltaic (PV) and the grid, it is necessary to use the variables M and as control inputs. Generally, these inputs independently control the DC side and the AC side respectively. Therefore, the closed-loop control may cause the ST mode to overlap with the effective state of the inverter and generate distortion in the output current waveform, and when the ZSI is used as the interface between photovoltaic (PV) and the grid / load, the control input for the DC side is , while the modulation index M is used for the AC side control. Therefore, this may violate the constraint relationship between them. This problem makes the closed-loop control of the ZSI more complex.

[0031] To address the above problems, this embodiment provides an impedance source photovoltaic inverter with an additional controllable input port, as Figure 2 shown: An impedance source network is added between the photovoltaic array and the three-phase two-level inverter. The impedance source network includes a diode D, an inductor , an inductor , a switching tube , a switching tube , a diode , a diode , a capacitor , a capacitor connected in sequence; the cathode of the diode D is connected to the inductor and the switching tube , the inductor is connected to the anode of the diode , the diode is connected to the capacitor , the capacitor is connected to the cathode of the inductor and the diode , the anode of the diode and the capacitor are connected, the switching tube is connected between the inductor and the anode of the diode , the capacitor serves as the negative input terminal and the negative output terminal of the impedance source network; the negative node of the capacitor serves as the positive output terminal of the impedance source network.

[0032] The output side voltage of the photovoltaic array is input between the positive input terminal and the negative input terminal of the impedance source network; the DC voltage is output between the positive output terminal and the negative output terminal of the impedance source network to the DC bus side of the three-phase two-level inverter.

[0033] As Figure 3 shown, the three-phase two-level inverter includes switching tubes , switching tubes , switching tubes , switching tubes , switching tubes , switching tubes .

[0034] Embodiment 3: Regarding a control method for an impedance source photovoltaic inverter with an additional controllable input port, this embodiment provides a control method for an impedance source photovoltaic inverter with an additional controllable input port, which uses the impedance source inverter with an additional control input as described in Embodiment 1, including: When the input voltage of the three-phase two-level inverter is constant, the output voltage of the photovoltaic array is adjusted by adjusting the direct-through duty cycle . By controlling , the maximum power point is found. According to , the direct-through signal is determined. On the AC side, by using the modulation index and the phase of the inverter reference voltage M , the input voltage of the inverter is stabilized and the output current is synchronized with the grid voltage. And the control method adopted on the AC side is the sinusoidal pulse width modulation SPWM technology; in this way, when performing the maximum power point tracking MPPT, using and taking θ as a constant can eliminate the possibility of overlap between the ST state and the active state. As

[0035] shown, regarding the sinusoidal pulse width modulation SPWM technology, the reference waveforms Figure 5 , , , and represent sinusoidal waveforms with a frequency of 50 Hz, having phases , , and , respectively, and the triangular carrier waveform . To generate different working modes, four straight lines are used. The lines and symmetric about the time axis are used to create the ST mode. The and symmetric about the time axis are used to control and The switch generates A-nST and nA-nST modes.

[0036] Similar to the traditional three-phase SPWM method, to generate the gating signals of the inverter switches, the reference signal of each phase is compared with the triangular carrier signal. and is compared with the triangular carrier waveform to generate the gating signal of the ST mode. To generate and the gating signals of, and are compared with the carrier waveform. The values of these parameters are calculated by the following formulas: ; ; The switching factor ( N ) can be used as a control input. Therefore, can be regarded as a constant. In this way, the overlap between the ST state and the active state is eliminated. The complete block diagram of the proposed control method is as Figure 4 shown. According to this block diagram, N is the control input on the DC side for performing maximum power point tracking (MPPT).

[0037] In the MPPT module, the reference voltage of the photovoltaic (PV) is calculated by the traditional perturbation and observation method (P&O). Through a PI controller, the PV output voltage tracks the calculated reference value. When extracting the dynamic model of the proposed inverter, as shown in the figure, it is assumed that the equivalent series resistance (ESR) of the capacitor, switch, and diode is zero. The following variables are considered in the analysis: the voltage across the capacitor ( ), the inverter input voltage ( ), the current through the inductor ( ), the switching function (N), and the current through the DC link in the n ST operating mode ( ) ; ; ; ; ; By applying the Kirchhoff's current law (KCL) equation to the capacitor and the Kirchhoff's voltage law (KVL) equation to the inductor in different operating modes, averaging these equations, substituting the above formulas, and neglecting the second-order small quantities, the small-signal model of the proposed inverter is obtained as follows: ; ; ; In the above equation, r and are the equivalent series resistance of the inductor and the capacitance value of the input capacitor, respectively; Figure 6 are the PV voltage, PV current, PV power, and maximum power point (MPP) values corresponding to a solar irradiance level of 1000 W / m² in the time interval t = [0, 0.3]; Figure 7 are the PV voltage, PV current, PV power, and maximum power point values corresponding to a solar irradiance level of 700 W / m² in the time interval t = [0.3, 0.6]; Figure 8 are the PV voltage, PV current, PV power, and maximum power point values corresponding to a solar irradiance level of 500 W / m² in the time interval t = [0.6, 0.9].

[0038] From to the transfer function expression is as follows: ; Thus, the design of the PI controller is completed as Figure 4 . The system also adds a feedforward control to accelerate the convergence process. The sum of the PI controller output and the feedforward controller output determines the value of N.

[0039] ; The method for finding the maximum power point is one of the perturbation and observation method and the incremental derivative method. Perturbation and observation method: By perturbing (adjusting) the operating voltage or current of the photovoltaic cell and observing the change in power. If the power increases, continue to adjust in the current direction; if the power decreases, adjust in the reverse direction. By continuously perturbing and observing, the maximum power point is finally found.

Claims

1. An impedance source photovoltaic inverter with an additional controllable input port, characterized in that, It includes an impedance source network, which is connected between a photovoltaic array and a three-phase two-level inverter; the impedance source network includes a diode D, an inductor , an inductor , a switching tube , a switching tube , a diode , a diode , a capacitor and a capacitor ; the cathode of the diode D is connected to the inductor and the switching tube , the inductor is connected to the anode of the diode , the diode is connected to the capacitor , the capacitor is connected to the inductor and the cathode of the diode , the anode of the diode and the capacitor are connected, the switching tube is connected between the inductor and the anode of the diode , the capacitor serves as the negative input terminal and negative output terminal of the impedance source network, and the negative node of the capacitor serves as the positive output terminal of the impedance source network.

2. The impedance source photovoltaic inverter with an additional controllable input port according to claim 1, wherein The output side voltage of the photovoltaic array is input between the positive input terminal and the negative input terminal of the impedance source network; the DC voltage is output between the positive output terminal and the negative output terminal of the impedance source network to the DC bus side of the three-phase two-level inverter.

3. The control method of an impedance-source photovoltaic inverter with an additional controllable input port according to claim 2, characterized in that, When the input voltage of a three-phase two-level inverter is constant, by adjusting the direct-conduction duty cycle Adjust the output voltage of the photovoltaic array , by Control to find the maximum power point, and determine the direct-conduction signal according to ; On the AC side, by using the modulation index M and the phase of the inverter reference voltage θ to stabilize the input voltage of the inverter and synchronize the output current with the grid voltage.

4. A control method for an impedance-source photovoltaic inverter with an additional controllable input port according to claim 3, characterized in that, The control method adopted on the AC side is the sinusoidal pulse width modulation SPWM technology.

5. A control method for an impedance-source photovoltaic inverter with an additional controllable input port according to claim 3, characterized in that, Switching factor N is the control input on the DC side: ； wherein is a constant; represents the average voltage of the capacitor at steady state, and controls the operation mode of the MPPT module by adjusting the switching factor N to eliminate the overlap between the ST state and the active state.

6. A control method for an impedance-source photovoltaic inverter with an additional controllable input port according to claim 3, characterized in that, The feedforward control is adopted to accelerate the convergence process, and the value of the switching factor N is determined by the sum of the output of the PI controller and the output of the feedforward controller.

7. A control method for an impedance source photovoltaic inverter with an additional controllable input port according to claim 4, characterized in that In the sinusoidal pulse width modulation (SPWM) technique, the reference waveforms , , and represent sinusoidal waveforms with a frequency of 50 Hz, having phases , , and , respectively, and the triangular carrier waveform ; To generate different operating modes, four straight lines are used: lines symmetric about the time axis and are used to create the ST mode, and lines symmetric about the time axis and are used to control the switches of and to generate the A-nST and nA-nST modes; Compare the reference signal of each phase with the triangular carrier signal, and compare with the triangular carrier waveform to generate a gating signal in ST mode to generate a gating signal for the inverter switch; Compare and with the carrier waveform to generate and gating signals; The values of the parameters are calculated by the following formula: ； ； Switching factor N Used as a control input, being a constant, the overlap of the ST state and the active state is eliminated.

8. A control method for an impedance-source photovoltaic inverter with an additional controllable input port according to claim 7, characterized in that N is the control input on the DC side and is used to perform maximum power point tracking (MPPT); in the MPPT module, the reference voltage of the photovoltaic (PV) is calculated by the perturbation and observation (P&O) method, and the PV output voltage tracks the calculated reference value through a PI controller.

9. A control method for an impedance-source photovoltaic inverter with an additional controllable input port according to claim 8, characterized in that, When extracting the dynamic model of the proposed inverter, it is assumed that the equivalent series resistance ESR of the capacitor, switch and diode is zero, and the calculation is as follows: ； ； ； ； ； The variables in the formula include the voltage across the capacitor , the input voltage of the inverter , the current through the inductor , the switching function N, and the current through the DC link in n the ST operating mode ; By applying the Kirchhoff's current law KCL equation to the capacitor and the Kirchhoff's voltage law KVL equation to the inductor under different operating modes, averaging these equations and neglecting the second-order small quantities, the small-signal model of the proposed inverter is obtained as follows: ； ； ； In the above equation, r and are the equivalent series resistance of the inductor and the capacitance value of the input capacitance, respectively; L represents the inductance parameter; C represents the capacitance parameter.

10. A control method for an impedance source photovoltaic inverter with an additional controllable input port according to claim 9, characterized in that, From to the transfer function expression is as follows: ； wherein, represents the transfer function from the capacitor voltage to the control input; represents the complex frequency variable; a and b represent constant terms composed of system parameters; k1 and k2 represent feedback gain coefficients in the control loop.

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