Single-phase inverter and control method and control system thereof

Abstract

The application discloses a single-phase inverter and a control method and system thereof, and relates to the technical field of inverters. The single-phase inverter comprises a Semi-Z source inverter, an input source connected to the input side of the Semi-Z source inverter, a voltage boosting module connected to the output side of the Semi-Z source inverter, and a filter module connected to the voltage boosting module. The application proposes an improved topology and control for the Semi-Z source inverter topology, and the novel double-end common-ground single-phase inverter is combined with the Semi-Z source inverter, thereby breaking the limitation of a gain of 1, realizing high gain, that is, a positive gain is infinite and a negative gain is also infinite.

Description

TECHNICAL FIELD

[0001] The present application relates to a non-isolated inverter system, in particular to a single-phase inverter and a control method and system thereof. BACKGROUND

[0002] In photovoltaic grid-connected system, photovoltaic inverter plays an important role in the process of power conversion. As an important interface between photovoltaic cells and power grid, photovoltaic grid-connected inverter can be divided into isolated inverter and non-isolated inverter according to whether the inverter system contains an isolation transformer. In the isolated inverter system, a high-frequency transformer is usually installed on the DC side or a low-frequency transformer is installed on the low-frequency side. The transformer not only boosts the DC voltage of the photovoltaic cell, but also isolates the DC side and the AC side, so that there is no DC loop between them. However, the presence of the transformer increases the loss of the entire system, greatly reduces the efficiency, and the transformer is relatively large in size and high in cost. The non-isolated inverter overcomes the shortcomings of the isolated inverter system, but due to the lack of electrical isolation, the non-isolated system has many new problems such as reliability and leakage current.

[0003] Inverters are divided into voltage source inverters and current source inverters, and most of the traditional voltage source inverters are step-down circuits, that is, the output AC voltage is lower than the input DC voltage. Therefore, many applications currently add a boost circuit (such as a Boost circuit) before the inverter circuit. In this way, the inverter becomes a two-stage structure, which increases the volume and reduces the system stability. Therefore, it is essential to study single-stage high-gain inverters. Therefore, some scholars have proposed Z-source inverters, which are impedance networks composed of two inductors and two capacitors, and can realize the function of voltage boosting, so they have been widely studied.

[0004] There are two main ideas in the prior art to solve the leakage current problem of the non-isolated inverter system. One idea is to improve the topology and modulation. Domestic and foreign scholars have proposed many improved topological structures, which can be mainly divided into single-inductor structures and symmetric inductor structures. The symmetric inductor structure can be further divided into DC side bypass and AC side bypass structures. Typical structures include H5, H6, improved H6, hybrid H6, and HERIC topological structures. Although these improved topologies and controls can reduce the leakage current to some extent, they can only suppress it and cannot solve the leakage current problem from the root. The other idea is to use a topology structure with input and output common ground. The leakage current is generated because there is a parasitic capacitor between the photovoltaic array and the ground. At the same time, due to the lack of isolation of the transformer, current flows through the parasitic capacitor to form a loop in the circuit, thereby generating leakage current. If a topology with input and output common ground is constructed, the parasitic capacitor can be bypassed, thereby solving the leakage current problem from the root.

[0005] D. Cao, S. Jiang, X. Yu and F. Z. Peng, "Low-Cost Semi-Z-source Inverter for Single-Phase Photovoltaic Systems," in IEEE Transactions on Power Electronics , vol. 26, no. 12, pp. 3514-3523, Dec. 2011 derived two kinds of inverter circuit on the basis of Z-source inverter, one named Semi-Z-source inverter, as shown in FIG. 1 , another named Semi-quasi-Z-source inverter, as shown in FIG. 3 Compared with the traditional Z-source inverter, only two switching tubes are used, at the same time, the impedance network of Z-source is retained, but the shoot-through state of Z-source is not used. More importantly, the input and output are common, and the leakage current problem is completely solved. Semi-Z-source inverter topology and Semi-quasi-Z-source inverter topology realize inversion with only two switching tubes, and realize double-end common ground. However, the circuit has a big drawback, that is, the maximum forward gain of the two topologies can only reach 1, and the negative gain can reach infinity, so the inverter can only achieve 1 times gain.

[0006] The invention patent CN112019076B optimizes Semi-Z-source inverter, constructs Boost circuit on the basis of Semi-Z-source inverter, widens the gain range of original Semi-Z-source inverter, and realizes high gain output. The topology proposed in the patent improves the gain of the original inverter, but introduces too many switching devices, obviously increases the cost, and is not conducive to industrial low-cost application. In addition, the control is complex, needs to judge the voltage for control switching, and is not conducive to the stability of the system. In addition, the boost circuit is fitted by the boost part waveform, and the output waveform has no inductance filter, so the waveform quality is relatively poor. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a single-phase inverter and a control method and system thereof, which breaks the limitation of gain being 1 without introducing too many switching devices.

[0008] To solve the above technical problems, the technical scheme adopted by the present application is: a single-phase inverter, comprising a double-inductor boost converter; the double-inductor boost converter comprises two parallel branches, a first branch being a Semi-Z source inverter, and a second branch comprising a first switch tube; the input ends of the two branches are connected with an input source, and the output ends of the two branches are connected with a boost module; the boost module is connected with a filter module.

[0009] The present application combines a Semi-Z source inverter with a double-inductor boost converter, so that the generated high-gain inverter has a simple topology structure, and only three switch tubes are needed to achieve high-gain inversion, greatly reducing the cost. In addition, the generated high-gain inverter topology contains the advantages of the Semi-Z source inverter, and the negative end of the input side is directly connected with the negative end of the output side, so that the parasitic capacitor can be bypassed, and the leakage current problem can be solved. Further, the generated high-gain inverter topology has the double-inductor characteristics of the double-inductor boost converter, and due to the existence of the double inductors on the input and output sides, the input current and the output current are continuous, and the output filtering effect is good and the waveform quality is high.

[0010] The Semi-Z source inverter comprises a second switch tube and a third switch tube; the positive electrode of the third switch tube is connected with the positive electrode of the input source through a first inductor and a first capacitor in sequence; the connection point of the first inductor and the first capacitor is connected with the positive electrode of the second switch tube; the negative electrode of the second switch tube is connected with the connection point of a second capacitor and a second inductor; the positive electrode of the third switch tube is connected with the negative electrode of the second capacitor, and the positive electrode of the second inductor is connected with the positive electrode of the input source. The advantage of this structure is that only two switch tubes are needed to achieve inversion, and the control is relatively simple. In addition, the negative end of the input is directly connected with the negative end of the output, so that the parasitic capacitor can be bypassed, and the leakage current problem can be solved. In addition, since there are only three switch tubes, their work is to combine the working state of Semi-Z and the working state of double-inductor boost, and they work together to improve the gain, so the control method of this topology is also simple and easy to implement.

[0011] The boost module comprises a third capacitor and a third inductor connected in series with the third capacitor; the negative electrode of the first switch tube is connected with the connection midpoint of the third capacitor and the third inductor; and the negative electrode of the third switch tube is connected with the positive electrode of the third capacitor. This structure has one inductor on the input and output sides, so that the input current and the output current are continuous, and has a good filtering effect. In addition, the control of this structure is also simple, and only a fixed duty ratio PWM wave is needed to achieve voltage boosting.

[0012] The filter module comprises a filter inductor and a filter capacitor; the filter capacitor is connected in parallel with the boost module; and the filter inductor is connected between the positive terminal of the boost module and the positive pole of the filter capacitor.

[0013] The calculation formula of the gain G of the single-phase inverter is:

[0014] ;

[0015] wherein, is the angular frequency of the output sine wave, and t is the working time of the single-phase inverter, , k is the maximum gain coefficient. , A is the ratio of the output peak voltage of the single-phase inverter to the input peak voltage. is the input source voltage value. is the load voltage value connected to the single-phase inverter. According to the above expression, it can be seen that the gain expression of the single-phase high-gain inverter is relatively simple, and only the expression of the modulation wave needs to be calculated according to the desired gain. The gain expression is a simple sinusoidal function and a one-variable first-order function, which can be easily solved by substituting the variable, and is also easy to realize in the control system.

[0016] The application further provides a control method of the single-phase inverter, and the method comprises the following steps:

[0017] comparing the first duty cycle signal D1 with a carrier signal (the carrier is a sawtooth wave, for example, the time required for counting once is a, and the set carrier period is Ts, so that Ts / a times of counting can obtain a rising edge waveform, and when the time is Ts, 0 is directly output, so that the sawtooth wave signal can be obtained), to generate a first driving signal to drive a first switch tube;

[0018] comparing a sine modulation wave (a sine table containing multiple values is used, and a PWM wave with a certain duty cycle is generated every period through a timer module, and the sine modulation wave can be obtained by filtering the output) with a carrier signal, to generate a second driving signal to drive a second switch tube and a third switch tube of the Semi-Z source inverter;

[0019] wherein, , k is the maximum gain coefficient.

[0020] It can be seen from the above control expression that the expression is simple, the k value can be defined by itself, the greater the k value, the greater the duty ratio of D1, and the greater the gain realized, which needs to be formulated according to the implementation requirement. Secondly, if the gain A of the topology to be realized is required, such as 100V input, 200V AC output, let A=2 and substitute into the expression, the modulation wave expression can be solved. Therefore, the implementation method of the present application is simple and convenient to set. At the same time, the form of the expression is simple, there is no complex expression, the calculation is simple, it is easy to realize in control, the stability of the control system is improved, and the rapid response ability of the control system is improved.

[0021] The single-phase inverter comprises three working modes:

[0022] Working mode 1, the first switch tube and the third switch tube are turned on, and the second switch tube is turned off;

[0023] Working mode 2, the first switch tube and the second switch tube are turned on, and the third switch tube is turned off;

[0024] Working mode 3, the second switch tube and the third switch tube are turned on, and the first switch tube is turned off;

[0025] The working time of working mode 1 is Ts(1- D 1);The working time of working mode 2 is Ts( D 1+ D 2-1);The working time of working mode 3 is Ts(1- D 1);Ts is the working time of a period; , is the angular frequency of the output sine wave, A is the ratio of the peak value of the single-phase inverter output voltage to the peak value of the input voltage.

[0026] As an inventive concept, the present application also provides a control system of the single-phase inverter, comprising a computer device; the computer device is configured or programmed to realize the steps of the control method of the present application.

[0027] Compared with the prior art, the present application has the beneficial effects of:

[0028] 1. The present application proposes an improved topology and control for Semi-Z source inverter topology. The new double-ended common ground single-phase inverter is a combination of Semi-Z source inverter, which breaks the limitation of gain of 1 and realizes high gain, that is, the positive gain is infinite and the negative gain is also infinite.

[0029] 2、 The inverter of the present application has the feature of double-ended common ground, and can bypass parasitic capacitance, thus fundamentally solving the problem of leakage current. In addition, inductance exists at the input and output sides of the inverter, so that the input and output currents are continuous;

[0030] 3、 Few switching devices, low cost;

[0031] 4、 Simple control, easy to implement. BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. 1 is a Semi-Z source inverter schematic diagram;

[0033] FIG. 2 is a double-inductor boost converter circuit schematic diagram;

[0034] FIG. 3 is a Semi-quasi-Z source inverter schematic diagram;

[0035] FIG. 4 is a schematic diagram of the embodiment 1 of the present application;

[0036] FIG. 5 is a schematic diagram of the embodiment 1 of the present application in working mode 1;

[0037] FIG. 6 is a schematic diagram of the embodiment 1 of the present application in working mode 2;

[0038] FIG. 7 is a schematic diagram of the embodiment 1 of the present application in working mode 3;

[0039] FIG. 8 is a driving timing diagram of the embodiment 1 of the present application;

[0040] FIG. 9 is a schematic diagram of the control method of the embodiment 2 of the present application. DETAILED DESCRIPTION

[0041] The topology structure of the new high-gain double-ended common ground single-phase inverter based on Semi-Z source inverter proposed by the embodiment 1 of the present application is shown in FIG. 4 The input source V in , inductance L 1, inductance L 2, inductance L 3, inductance L 4, capacitor C 1, capacitor C 2, capacitor C o , switching tube S 1, switching tube S 2 and switching tube S 3, etc. Among them, capacitorC 1, capacitor C 2, inductor L 1, inductor L 2, switch S 2 and switch S 3 constitute a Semi-Z source inverter. In the inverter part, according to the Semi-Z source inverter, switches S 2 and switch S 3 cannot be on at the same time, similarly, for the boost circuit, the series branch of switches S 1 and switches S 2 and switch S 3 cannot be on at the same time. Therefore, only two switches can be on at a time, therefore, the proposed high gain dual half-bridge inverter has three working modes.

[0042] Working mode 1: as shown in the figure. At this time, switches FIG. 5 1 and switch S 3 are on, and switch S 2 is off. According to the principle that the inductor current direction does not change, the circuit is as shown in the figure. In order to simplify the calculation, it is assumed that S 1 = FIG. 5 2 = C , C 1 = C 2 = L . According to Kirchhoff's voltage law, we have: L L

[0043]

[0044] Working mode 2: as shown in the figure. At this time, switches FIG. 6 1 and switch S 2 are on, and switch S 3 is off. According to the principle that the inductor current direction does not change, the circuit is as shown in the figure. According to Kirchhoff's voltage law, we have: S FIG. 6

[0045]

[0046] Working mode 3: as shown in the figure. At this time, switches FIG. 7 2 and switch S 3 are on, and switch S 1 is off. According to the principle that the inductor current direction does not change, the circuit is as shown in the figure. According to Kirchhoff's voltage law, we have: S FIG. 7

[0047]

[0048] ​​​​​​​​​​The driving timing sequence of the novel high-gain double-ended common-ground single-phase inverter based on Semi-Z source inverter is shown in the figure. FIG. 8 The driving of switch tube S 1 is PWM, and the conduction time is fixed time, which is used for boosting. The duty cycle of switch tube S 2 and switch tube S 3 varies with sine, which is used for generating sine output. Ts(1- D 1) is the action time of mode 1, at this time, switch tube S 1 and switch tube S 3 are turned on, and switch tube S 2 is turned off. Ts( D 1+ D 2-1) is the action time of mode 2, at this time, switch tube S 1 and switch tube S 2 are turned on, and switch tube S 3 is turned off. Ts(1- D 1) is the action time of mode 3, at this time, switch tube S 2 and switch tube S 3 are turned on, and switch tube S 1 is turned off. Wherein Ts is the working time of a period.

[0049] According to the expressions of the three states, the following expression can be obtained from the volt-second balance principle of inductor:

[0050]

[0051] From the above expression, the gain of the inverter proposed at present can be obtained as:

[0052]

[0053] Wherein, Vout / Vin is the ratio of the peak value of inverter output voltage to the peak value of input voltage. A Here, the expression of

[0054] 1 is defined as: D

[0055]

[0056] Wherein, Kmax is the maximum gain coefficient, and its value is determined by the DC boosting part in the proposed inverter. k Then the expression of

[0057] 2 can be obtained as: D

[0058]

[0059] Switch tube​​S 3 can be driven by the switch tube S 1 of the switch tube S 2 is obtained by driving through an XOR gate.

[0060] The specific implementation process of the control method of embodiment 2 of the present application is shown in the figure. FIG. 9 The driving signal of switch tube S1 is generated by comparing the expression of D1 with the carrier signal, which is used to realize voltage boosting. The driving signals of switch tube S2 and switch tube S3 are generated by comparing the sine modulation wave (a sine table containing multiple values is used, and after passing through a timer module, a PWM wave with a certain duty ratio is generated every period, and the output is filtered to obtain the sine modulation wave) with the carrier signal, which is used to realize inversion. Through the cooperation of the three driving signals, high gain and inversion are realized at the same time.

[0061] Working mode 1: At this time, switch tubes S1 and S3 are turned on, and switch tube S2 is turned off. Inductors L1 and L2 are discharged, capacitors C1 and C2 are charged, and inductors L3 and L4 are charged.

[0062] Working mode 2: At this time, switch tubes S1 and S2 are turned on, and switch tube S3 is turned off. Capacitors C1 and C2 discharge to inductors L2 and L1. Inductors L3 and L4 are in the same state as working mode 1.

[0063] Working mode 3: At this time, switch tubes S2 and S3 are turned on, and switch tube S1 is turned off. Inductors L1 and L2 continue to maintain the state of working mode 2. Inductors L3 and L4 start to discharge.

[0064] Through the above three working modes, a high-gain sine wave is formed on the passive devices in the circuit and output to the load.

Claims

1. A single-phase inverter, characterized in that, It includes a dual-inductor boost converter; the dual-inductor boost converter includes two parallel branches, the first branch is a Semi-Z source inverter, and the second branch includes a first switching transistor; the input terminals of both branches are connected to the input source, and the output terminals of both branches are connected to the boost module; the boost module is connected to the filter module. The formula for calculating the gain G of the single-phase inverter is as follows: ； in, To output the angular frequency of a sine wave, , k This is the maximum gain coefficient; , A This is the ratio of the peak output voltage to the peak input voltage of a single-phase inverter. This is the input source voltage value; This is the load voltage value connected to the single-phase inverter; The Semi-Z source inverter includes a second switch and a third switch; the positive terminal of the third switch is connected to the positive terminal of the input source in sequence through a first inductor and a first capacitor; the connection point of the first inductor and the first capacitor is connected to the positive terminal of the second switch; the negative terminal of the second switch is connected to the connection point of the second capacitor and the second inductor; the negative terminal of the second capacitor is connected to the positive terminal of the third switch, and the positive terminal of the second inductor is connected to the positive terminal of the input source; The boost module includes a third capacitor and a third inductor connected in series with the third capacitor; the negative terminal of the first switch is connected to the midpoint of the connection between the third capacitor and the third inductor; the negative terminal of the third switch is connected to the positive terminal of the third capacitor.

2. The single-phase inverter according to claim 1, characterized in that, The filtering module includes a filtering inductor and a filtering capacitor; the filtering capacitor is connected in parallel with the boost module; the filtering inductor is connected between the positive terminal of the boost module and the positive terminal of the filtering capacitor.

3. A control method for a single-phase inverter as described in claim 1 or 2, characterized in that, The method includes: Compare the first duty cycle signal D1 with the carrier signal to generate a first drive signal, which drives the first switching transistor. By comparing the sinusoidal modulation wave with the carrier signal, a second drive signal is generated to drive the second and third switches of the Semi-Z source inverter.

4. The method according to claim 3, characterized in that, The single-phase inverter includes three operating modes: Operating mode 1: The first and third switching transistors are turned on, and the second switching transistor is turned off. Operating mode 2: the first and second switching transistors are turned on, and the third switching transistor is turned off. Operating mode 3: the second and third switches are turned on, and the first switch is turned off. The duration of working mode 1 is Ts(1- D 1); The duration of operation mode 2 is Ts( D 1+ D 2-1); The duration of operation mode 3 is Ts(1- D 1); Ts is the working time of one cycle; , To output the angular frequency of a sine wave, A This is the ratio of the peak output voltage to the peak input voltage of a single-phase inverter.

5. A control system for the single-phase inverter according to claim 1 or 2, characterized in that, Includes a computer device; said computer device is configured or programmed to implement the steps of the method of claim 3 or 4.

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