A grid-connected inverter

Abstract

The application discloses a grid-connected inverter which is composed of a direct-current power supply, active switching tubes, diodes, capacitors and inductors. The application completely eliminates the leakage current on the parasitic capacitance of the photovoltaic cell panel by grounding the cathodes of the input side and the output side. The transformer or the front-stage step-up / down voltage circuit is not needed to realize the wide-range output voltage regulation, so the application can be applied to occasions with different power requirements. The intermediate capacitor in the circuit has low voltage resistance requirement, so the non-electrolytic capacitor can be used to prolong the circuit life and reduce the maintenance cost. The zeta and buck-boost converters are used as the working loops in the positive and negative half cycles respectively, the active devices used in the circuit are less, most of which are in the power frequency working state, the passive devices in the positive and negative half cycles are multiplexed, the circuit structure is greatly simplified, the system loss and the volume can be effectively reduced, and the circuit output is the current flow, so the grid-connected operation is easy.

Description

TECHNICAL FIELD

[0001] The application relates to the field of power electronics, in particular to a grid-connected inverter. BACKGROUND

[0002] Since entering the new century, the rapid development of China's economy and society cannot be separated from the consumption of various types of energy, among which fossil energy such as coal and oil accounts for the majority. However, the use of fossil energy often generates various types of pollution sources, which is contrary to the strengthening of ecological civilization construction in China, and fossil energy is a non-renewable energy, so finding a new clean energy to replace fossil energy has become a major focus in China's energy field. Solar energy, as a new type of clean energy, has attracted widespread attention since its discovery. Its universal, harmless, huge and long-lasting advantages make it stand out among various new energy sources and become one of the energy sources that scholars focus on. Photovoltaic power generation is a way to effectively use solar energy, and a photovoltaic inverter is an important part of a photovoltaic power generation system. Using different photovoltaic inverters will change the energy conversion efficiency and application occasions of the photovoltaic system, so in order to cope with different occasions and improve the energy conversion efficiency, photovoltaic inverters with different structures and control strategies are constantly being proposed. The input of the photovoltaic system will be disturbed by environmental factors such as sunlight intensity and humidity, in order to keep the output stable, the photovoltaic inverter needs to have the ability to step up and step down. Transformers are used in some traditional circuits to obtain step-up and step-down capability, which are divided into power frequency transformers and high frequency transformers, but both have problems such as large system size, low inverter efficiency and slow response speed. If a front-stage DC-DC step-up and step-down and rear-stage DC-AC inverter mode is used, the various problems caused by transformers can be solved, but the two-stage structure makes the efficiency and safety of the system cannot be guaranteed. In view of the shortcomings of traditional circuits, single-stage non-isolated inverters emerge as the times require, which have the advantages of high inverter efficiency, small system size and simple control method, but because there is no electrical isolation between the photovoltaic panel and the output side, and there is a parasitic capacitor between the photovoltaic panel and the ground, it is easy to cause leakage current, thereby affecting the safe operation of the photovoltaic system. SUMMARY

[0003] In order to solve the problems of the existing circuit, the application provides a new inverter which can completely eliminate the ground leakage current of the photovoltaic panel, easily operate in parallel, adapt to the application scenarios of small power input side disturbance through its step-up and step-down function, and has no electrolytic capacitor.

[0004] To solve the above technical problems, the technical scheme adopted by the application is:

[0005] A grid-connected inverter comprises a direct current power supply, four switching tubes, two diodes, two capacitors and three inductors; the direct current power supply P end is connected with the first inductor L1, the first diode D1 cathode end and the first end of the fourth switching tube S4 through the first switching tube S1; the other end of the fourth switching tube S4 is connected with the first capacitor C1, the first capacitor C1 other end is connected with the second diode D2 cathode and the first end of the second inductor L2; the first diode D1 anode is connected with the other end of the second inductor L2 through the third switching tube S3, and is connected with the load or the grid through the filter inductor Lf;

[0006] The direct current power supply N end is connected with the other end of the first inductor L1 and the other end of the second diode D2 through the end point of the second switching tube S2, the first end of the second inductor L2 through the end point of the filter capacitor Cf and the other end of the load or the grid.

[0007] The further improvement of the technical scheme of the present application is that the direct current power supply N end is directly connected with the grid common end B, that is, the input and output are common ground, and there will be no inter-electrode current in the parasitic capacitance in the photovoltaic cell panel, so that the influence of the leakage current can be completely eliminated.

[0008] The further improvement of the technical scheme of the present application is that the inverter works in zeta mode in the positive half cycle and works in buck-boost mode in the negative half cycle, and the forward output characteristic of the zeta converter and the reverse output characteristic of the buck-boost converter are utilized to realize the output alternating current signal.

[0009] The further improvement of the technical scheme of the present application is that the output alternating current voltage can be adjusted within a certain range before and after the direct current power supply input voltage E.

[0010] The further improvement of the technical scheme of the present application is that neither of the two capacitors in the inverter uses electrolytic capacitor, so that electrolytic capacitorization is realized.

[0011] The further improvement of the technical scheme of the present application is that the first capacitor C1 and the filter capacitor Cf f Both use polypropylene film capacitors.

[0012] The further improvement of the technical scheme of the present application is that the output is a current signal, which is easy to operate in parallel with the grid.

[0013] The further improvement of the technical scheme of the present application is that for the positive half cycle, the circuit works in zeta mode:

[0014]

[0015]

[0016]

[0017] The current ripple coefficient is

[0018]

[0019] The voltage ripple coefficient is

[0020]

[0021] In the above formula, D is the duty ratio of the first switch S1, P is the output power, U o is the output voltage effective value, f s is the switching frequency of the first switch S1.

[0022] Wherein, the filter inductance L f = 3mH, the filter capacitance C f = 4.7uF.

[0023] The circuit works in the buck-boost mode:

[0024]

[0025] Due to the adoption of the above technical scheme, the technical progress achieved by the application is:

[0026] 1. The input side photovoltaic cell panel N end is short-circuited with the output side load end or the power grid common end B, the parasitic capacitance of the photovoltaic cell panel to the ground is short-circuited, there is no inter-electrode current, and thus the leakage current is completely eliminated.

[0027] 2. Without using a transformer or a front-end circuit structure, the boost-buck inverter can be realized. Figure 10 、 Figure 11 、 Figure 12 、 Figure 14 、 Figure 15 、 Figure 16 The inductance current continuous working mode, the inductance current discontinuous working mode, the boost-buck inverting capability of the inverter are respectively demonstrated.

[0028] 3. The value and voltage resistance requirement of the first capacitor C1 in the circuit are relatively low compared with other inverter circuits, the non-electrolytic capacitor can realize the basic function of the circuit, and the electrolytic capacitor of the system is removed. At the same time, the values of the first inductance L1 and the second inductance L2 are relatively low, and the inverter circuit can be lightened.

[0029] 4. Only four switch tubes are used in the entire inverter circuit, and the driving signal waveforms of the switch tubes are shown in the specification drawings. Figure 2 Among them, three switch tubes are in the power frequency working state, which can effectively reduce the system loss and reduce the operation cost.

[0030] 5. The circuit output is current, which is easy to operate in parallel.

[0031] 6、Control structure is simple, the specification is attached Figure 18 The control structure of the circuit is shown, and only one single voltage / current loop is used to have good tracking performance and response speed. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is the schematic diagram of the circuit of the present application;

[0033] Figure 2 It is the schematic diagram of the modulation signal and the switch tube driving signal of the circuit of the present application;

[0034] Figure 3 It is the schematic diagram of the working mode I of the circuit of the present application;

[0035] Figure 4 It is the schematic diagram of the working mode II of the circuit of the present application;

[0036] Figure 5 It is the schematic diagram of the working mode III of the circuit of the present application;

[0037] Figure 6 It is the schematic diagram of the working mode IV of the circuit of the present application;

[0038] Figure 7 It is the schematic diagram of the working mode V of the circuit of the present application;

[0039] Figure 8 It is the schematic diagram of the working mode VI of the circuit of the present application;

[0040] Figure 9 It is the waveform diagram of the first inductance current, the first capacitance voltage and the second inductance current of the circuit of the present application in the inductance current continuous mode;

[0041] Figure 10 It is the waveform diagram of the DC input voltage and the AC output voltage of the circuit of the present application in the inductance current continuous mode for step-up inversion;

[0042] Figure 11 It is the waveform diagram of the DC input voltage and the AC output voltage of the circuit of the present application in the inductance current continuous mode for flat inversion;

[0043] Figure 12 It is the waveform diagram of the DC input voltage and the AC output voltage of the circuit of the present application in the inductance current continuous mode for step-down inversion;

[0044] Figure 13 It is the waveform diagram of the first inductance current, the first capacitance voltage and the second inductance current of the circuit of the present application in the inductance current discontinuous mode;

[0045] Figure 14is the waveform diagram of DC input voltage and AC output voltage when the circuit of the present application is in boost inverter mode under discontinuous inductor current mode;

[0046] Figure 15 is the waveform diagram of DC input voltage and AC output voltage when the circuit of the present application is in flat inverter mode under discontinuous inductor current mode;

[0047] Figure 16 is the waveform diagram of DC input voltage and AC output voltage when the circuit of the present application is in boost inverter mode under discontinuous inductor current mode;

[0048] Figure 17 is the waveform diagram of output voltage and output current of the present application;

[0049] Figure 18 is the control circuit structure diagram of the circuit of the present application. DETAILED DESCRIPTION

[0050] The present application will be further described in detail below in combination with embodiments:

[0051] Traditional photovoltaic inverters usually use improved modulation strategies or improved circuit structures to suppress leakage current. If the method of using improved modulation strategies is used, the requirements for the control system will be higher than other circuits, and at the same time the reliability of the system will be reduced. If the method of using improved circuit structure is used, the symmetric design is often used to eliminate the influence of differential mode voltage, which leads to the increase of the number of devices used, the increase of the volume, the increase of the loss, and the reduction of the system inverter efficiency. At the same time, the above two methods can only suppress leakage current to a certain extent. When the external environment of the system is extreme, the parasitic capacitance value of the photovoltaic panel deviates greatly from the theoretical design value, at this time the reliability of the system cannot be guaranteed. The present application uses input and output common ground design, and the ground parasitic capacitance branch of the photovoltaic panel is short-circuited, so no matter what changes the external environment causes to the parasitic capacitance value, the system will not have leakage current.

[0052] Traditional photovoltaic inverters can usually only realize step-down inversion, and even some improved circuits can only realize step-up and step-down within a narrow range. For example, Roopa Viswadev Damodaran et al. proposed a common ground type inverter based on an improved buck converter in the article A Common Ground Four Quadrant Buck Converter for DC-AC Conversion published in IEEE Access in 2022. The circuit structure of the inverter is simple, and the common ground design can completely eliminate the leakage current. However, the voltage gain range of the inverter is narrow, and the minimum operating voltage of the circuit is high, so the circuit is more likely to stop in harsh weather conditions. The new inverter proposed in the present application has a wide range of step-up and step-down capability, can adapt to input side disturbances in extreme environments, and can be applied to more occasions with large input disturbances.

[0053] In most existing common ground type inverter schemes, in order to pursue symmetry of working principle and input-output common ground design, the working loop of the positive and negative half cycles of the circuit is usually the same type of converter. In this way, in order to ensure that the basic characteristics of the circuit remain unchanged, and to prevent parasitic loops from occurring in the circuit, more switching tube devices and energy storage devices are used, which greatly increases the complexity of the circuit structure and increases the circuit loss. For example, Sze Sing Lee et al. proposed a common ground type inverter based on a buck-boost converter in the article A Common-Ground-Type Single-Stage Buck-Boost Inverter with Sinusoidal Output Voltage published in ECCE-Asia. The circuit uses a total of eight switching tubes, which has a complex structure and high circuit loss. The present application bypasses the inherent thinking of pursuing symmetry, and organically combines the zeta and buck-boost converters, which have different but common points. This greatly simplifies the structure while ensuring the step-up and step-down capability of the circuit and the input-output common ground. Only four switching tubes are used, and three of them are in the power frequency operating state, so the circuit loss is low.

[0054] The electrolytic capacitor has the disadvantages of short service life, high temperature performance, and sensitivity to mechanical stress, which greatly affects the overall service life of the circuit and increases the operation and maintenance cost of the system. The new inverter proposed in the present application realizes electrolytic capacitor-free operation, and all capacitors in the circuit are non-electrolytic capacitors, thereby prolonging the service life of the circuit and reducing the operation and maintenance cost.

[0055] Embodiment 1

[0056] The application provides a grid-connected inverter.

[0057] The DC bus P end is connected with the first inductor L1, the one end of the fourth switch tube S4 and the cathode end of the first diode D1 through the first switch tube S1.

[0058] The DC bus N end is connected with the other end of the first inductor L1 and the other end of the second diode D2 through the second switch tube S2, and the one end of the second inductor L2 is connected with the other end of the load or the grid through the end point of the filter capacitor Cf.

[0059] The switch tube driving signal of the circuit of the application is shown in Figure 2 When the output voltage is the positive half cycle, the first switch tube S1 is in the high-frequency switching state, the second switch tube S2 and the fourth switch tube S4 are in the keeping-on state, and the third switch tube S3 is in the keeping-off state.

[0060] According to the driving signal waveform shown in Figure 2 and the value of the first inductor, the circuit of the application can be divided into two working modes of inductor current continuous and inductor current discontinuous.

[0061] If the inverter works in the inductor current continuous working mode, in order to linearize the input and output of the inverter, a nonlinear modulation technology is adopted; if the inverter works in the inductor current discontinuous working mode, the modulation method adopted is the SPWM modulation. Figure 2 As shown in

[0062] The inductor current continuous working mode can be divided into four working modes, wherein the working modes of the output voltage being the positive half cycle are mode I and mode II, and the working modes of the output voltage being the negative half cycle are mode III and mode IV, and the specific modes are as follows:

[0063] Mode I

[0064] The working mode of the circuit in the mode is as shown in Figure 3As shown in the figure. The first switch S1, the second switch S2, the fourth switch S4 are in the on state, and the third switch S3 is in the off state. The power supply E charges the first inductor L1 through the first switch S1, and at the same time supplies power to the load side through the first switch S1, the fourth switch S4, the first capacitor C1, the second inductor L2 and the filter inductor Lf. The first capacitor C1 supplies power to the load side through the second inductor L2 and the filter inductor Lf.

[0065] Working mode II

[0066] In this mode, the working form of the circuit is as shown in the figure. Figure 4 The second switch S2 and the third switch S4 are in the on state, and the first switch S1 and the third switch S3 are in the off state. The first inductor L1 charges the first capacitor C1 through the second switch S2, the fourth switch S4 and the second diode D2. The second inductor L2 freewheels through the filter inductor Lf, the second switch S2 and the second diode D2.

[0067] Working mode III

[0068] In this mode, the working form of the circuit is as shown in the figure. Figure 5 The first switch S1 and the third switch S3 are in the on state, and the second switch S2 and the fourth switch S4 are in the off state. The power supply E charges the first inductor L1 through the first switch S1. The filter inductor Lf reverse freewheels through the filter capacitor Cf to the load side.

[0069] Working mode IV

[0070] In this mode, the working form of the circuit is as shown in the figure. Figure 6 The third switch S3 is in the on state, and the first switch S1, the second switch S2 and the fourth switch S4 are in the off state. The first inductor L1 supplies reverse power to the load side through the filter inductor Lf, the third switch S3 and the first diode D2.

[0071] The above four working modes can be represented by Table 1.

[0072] Table 1: Switching tube combination state of inverter in continuous mode

[0073] Working mode UAB S1 S2 S3 S4 Corresponding figures Ⅰ >0 1 1 0 1 Figure 3 Ⅱ >0 0 1 0 1 Figure 4 Ⅲ <0 1 0 1 0 Figure 5

[0074] For the discontinuous inductor current mode, there are six working modes, of which the output voltage is positive half cycle working mode I, II, V, and the output voltage is negative half cycle working mode III, IV, VI, and working mode I, II is the same as working mode I, II in continuous mode, and working mode III, IV is the same as working mode III, IV in continuous mode. Working mode V, VI is as follows:

[0075] Working mode V

[0076] The working form of the circuit in this mode is shown in Figure 7 The second switch S2 and the third switch S4 are in the on state, and the first switch S1 and the third switch S3 are in the off state. The energy on the first inductor L1 is released, and the first capacitor C1 supplies power to the load side through the second inductor L2, the filter inductor Lf, and the fourth switch S4, while charging the first inductor L1.

[0077] Working mode VI

[0078] The working form of the circuit in this mode is shown in Figure 8 The third switch S3 is in the on state, and the first switch S1, the second switch S2, and the fourth switch S4 are in the off state. The energy on the first inductor L1 is released, and the filter capacitor Cf supplies current to the load side through the filter inductor Lf.

[0079] The above six working modes can be represented by Table 2.

[0080] Table 2: Switching tube combination state of inverter in discontinuous mode

[0081] Working mode UAB S1 S2 S3 S4 Corresponding figures Ⅰ >0 1 1 0 1 Figure 3 Ⅱ >0 0 1 0 1 Figure 4 Ⅴ >0 0 1 0 1 Figure 7 Ⅲ <0 1 0 1 0 Figure 5 Ⅳ <0 0 0 1 0 Figure 6 Ⅵ <0 0 0 1 0 Figure 8

[0082] The circuit proposed in the present application makes the following assumptions for simplicity of analysis:

[0083] (1) All components in the circuit are ideal components; (2) The circuit works in a stable state.

[0084] Since the inverter of the present application is composed of a zeta circuit and a buck-boost circuit, the two circuits can be analyzed separately. The parameters of each passive component in the circuit can be calculated using the calculation method of the zeta and buck-boost circuits.

[0085] For the positive half cycle, the circuit works in zeta mode, and has:

[0086]

[0087]

[0088]

[0089] The current ripple coefficient is defined as

[0090]

[0091] The voltage ripple coefficient is defined as

[0092]

[0093] In the above formula, D is the duty ratio of the first switch S1, P is the output power, U o is the effective value of the output voltage, f s is the switching frequency of the first switch S1.

[0094] In the above formula, L f = 3mH, C f = 4.7mF.

[0095] Since the first capacitor C1 and the second inductor L2 are only enabled in the positive half-cycle zeta operating mode, their values can be directly determined by formula (2) and formula (3), respectively. The first inductor L1 is also in working state in the negative half-cycle buck-boost operating mode, so the value of the first inductor L1 needs to be analyzed in combination with the calculation formula of the buck-boost.

[0096] As can be seen from formula (3), in the case of small and medium power, the value of the first capacitor C1 is very small, and the voltage across the first capacitor C1 is the output voltage, so the required voltage resistance is also small, and therefore an electrolytic capacitor is not needed.

[0097] For the negative half-cycle, the circuit works in the buck-boost mode, and has:

[0098]

[0099] The value of the first inductor L1 can be obtained by formula (1) and formula (6).

[0100] In the above formula, L f is the filter inductor, and C f is the filter capacitor. The derivation process is as follows:

[0101] The transfer function of the CL filter is:

[0102]

[0103] The CL filter is a typical second-order system, and as can be seen from formula (7), the natural oscillation angular frequency of the filter and the damping coefficient are:

[0104]

[0105] The working of the LC filter is affected by the damping ratio and the corner frequency f n , and the optimal damping ratio is taken as 0.707. In the design, the natural oscillation frequency needs to be much smaller than the switching frequency f s of the inverter, so that the harmonics in the frequency band around the switching frequency and its integer multiples can be effectively filtered out. At the same time, the natural oscillation frequency needs to be much larger than the fundamental frequency f e , that is:

[0106] 10f e <f n <0.1f s (9)

[0107] According to the above relationship, the filter inductance L f = 3 mH, and the filter capacitance C f = 4.7 μF are finally determined.

Claims

1. A grid-connected inverter comprising a direct current power source, four switching tubes, two diodes, two capacitors and three inductors; characterized in that: The direct current power supply P is connected with the first inductor L1, the one end of the fourth switch tube S4 and the cathode end of the first diode D1 through the first switch tube S1; the other end of the fourth switch tube S4 is connected with the one end of the first capacitor C1, and the other end of the first capacitor C1 is connected with the cathode of the second diode D2 and the one end of the second inductor L2; the anode of the first diode D1 is connected with the other end of the second inductor L2 through the third switch tube S3, and is connected with the load or the power grid through the filter inductor L f and the power grid. The N terminal of the direct current power source is connected with the other terminal of the first inductor L1, the other terminal of the second diode D2, the terminal of the second switch S2, the one terminal of the second inductor L2, the terminal of the filter capacitor C and the other terminal of the load or the power grid, and the N terminal of the direct current power source is directly connected with the B terminal of the power grid. f The positive half cycle of the inverter works in zeta mode and the negative half cycle works in buck-boost mode, and the forward output characteristic of the zeta converter and the reverse output characteristic of the buck-boost converter are utilized to realize the output alternating current signal; for the positive half cycle, the circuit works in zeta mode: （1） （2） （3） The current ripple coefficient is （4） The voltage ripple coefficient is （5） In the above formula, D is the duty ratio of the first switch S1, P is the output power, U o is the effective value of the output voltage, f s is the switching frequency of the first switch S1; Wherein, the filter inductance L f = 3 mH, the filter capacitance C f = 4.7 μF; The circuit works in buck-boost mode: （6）。 2. The grid-connected inverter of claim 1, wherein: The output AC voltage can be adjusted within a certain range before and after the DC power input voltage E.

3. The grid-connected inverter of claim 1, wherein: Both capacitors in the inverter do not use electrolytic capacitors.

4. The grid-connected inverter of claim 3, wherein: The first capacitor C1 and the filter capacitor C f Both use polypropylene film capacitors.

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