Back-to-back cascaded photovoltaic system and method of use

Abstract

This invention belongs to the field of photovoltaic system power control technology, and discloses a back-to-back cascaded photovoltaic system and its usage method. Each power module includes a cascaded H-bridge inverter, a photovoltaic module, an isolated DC-DC converter, and a cascaded H-bridge rectifier connected in series. The AC side of the H-bridge module of the cascaded H-bridge inverter connected in series at both ends of the power module is connected to the positive and negative terminals of the power grid, respectively. The AC side of the H-bridge module of the cascaded H-bridge rectifier connected in series at both ends of the power module is connected to the positive and negative terminals of the power grid, respectively. The DC side of the H-bridge module in the cascaded H-bridge inverter is connected to the photovoltaic module. The output terminal of the isolated DC-DC converter is connected to the DC side of the H-bridge module in the cascaded H-bridge rectifier. This invention can expand the operating range of photovoltaic grid-connected power generation systems based on cascaded H-bridge converters while avoiding increasing system costs and control complexity.

Description

Technical Field

[0001] This invention belongs to the field of photovoltaic system power control technology, specifically relating to a back-to-back cascaded photovoltaic system and its usage method. Background Technology

[0002] Cascaded H-bridge converter-based photovoltaic grid-connected power generation systems offer numerous advantages over traditional photovoltaic grid-connected systems, including: integrating multiple independent photovoltaic modules and achieving maximum power point tracking; eliminating the need for a step-up transformer to improve system efficiency; and reducing switching frequency for better electromagnetic compatibility. Therefore, they have broad application prospects. However, because the AC sides of each H-bridge module in a cascaded H-bridge converter are connected in series, the current flowing through each module is the same. Combined with differences in the characteristics of each photovoltaic module and their environmental conditions, this results in uneven power generation. The H-bridge module connected to a photovoltaic module with higher power generation also has a higher AC output voltage. When the DC voltage amplitude required for AC output voltage modulation exceeds the photovoltaic module voltage, overmodulation occurs, causing distortion of the H-bridge module output voltage, mismatch between the power transmitted by the H-bridge module and the power generated by the photovoltaic module, and distortion of the grid-connected current.

[0003] Existing methods consider three aspects. First, improving the modulation strategy of the H-bridge module increases the utilization rate of the DC bus voltage; however, this method can only extend the modulation factor operating range of the H-bridge module to 1.27. Second, connecting an energy storage H-bridge module in series with the photovoltaic H-bridge module increases the grid-connected current amplitude and reduces the modulation factor of the photovoltaic H-bridge module. Third, connecting an energy storage H-bridge module in parallel with the photovoltaic H-bridge module increases the grid-connected current amplitude while reducing the output power of the H-bridge module, thus reducing the modulation factor. While these methods can solve the overmodulation problem caused by uneven power generation from the H-bridge module, they each have drawbacks, including limited expansion of the H-bridge module's operating range, increased system cost from adding energy storage H-bridge modules, and increased system control complexity due to the need for energy storage module state-of-charge balancing.

[0004] Chinese patent publication number CN111277159A, entitled "A Modular Three-Phase Photovoltaic Inverter and Its Topology System," includes: a front-end multi-port isolated DC / DC converter and three independent H-bridge inverters in the back-end. The front-end DC / DC converter uses a multi-winding high-frequency transformer for isolation and has three rectified output terminals. The three H-bridge inverters in the back-end are respectively connected to the three rectified output terminals of the DC / DC converter, forming three output H-bridges. The three output H-bridges of multiple modular units are cascaded to a three-phase power grid to form a complete photovoltaic power generation system. This patent application has high system cost and high control complexity. Summary of the Invention

[0005] To overcome the problems existing in the prior art, the present invention aims to provide a back-to-back cascaded photovoltaic system and its usage method. By connecting n power modules in series, each power module includes a cascaded H-bridge inverter, a photovoltaic module, an isolated DC-DC converter, and a cascaded H-bridge rectifier connected in series, thereby further expanding the operating range of the photovoltaic grid-connected power generation system based on the cascaded H-bridge converter while avoiding increasing system cost and control complexity.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a back-to-back cascaded photovoltaic system, comprising n power modules connected in series; each power module includes a cascaded H-bridge inverter, a photovoltaic module, an isolated DC-DC converter, and a cascaded H-bridge rectifier connected in series; both the cascaded H-bridge inverter and the cascaded H-bridge rectifier have H-bridge modules; wherein, the AC side of the H-bridge module of the cascaded H-bridge inverter of the power modules connected in series at both ends is connected to the positive and negative terminals of the power grid, respectively; the AC side of the H-bridge module of the cascaded H-bridge rectifier of the power modules connected in series at both ends is connected to the positive and negative terminals of the power grid, respectively; the DC side of the H-bridge module in the cascaded H-bridge inverter is connected to the photovoltaic module, and the other end of the photovoltaic module is connected to the input terminal of the isolated DC-DC converter; the output terminal of the isolated DC-DC converter is connected to the DC side of the H-bridge module in the cascaded H-bridge rectifier.

[0007] Optionally, the AC side of the H-bridge module of the cascaded H-bridge inverter of the power module is connected to the positive terminal of the power grid through a first grid-connected inductor Lga; the AC side of the H-bridge module of the cascaded H-bridge rectifier of the power module is connected to the positive terminal of the power grid through a second grid-connected inductor Lgb.

[0008] Optionally, the isolated DC-DC converter is a dual active bridge DC-DC converter.

[0009] Optionally, each isolated DC-DC converter has the same transmission power.

[0010] Secondly, the present invention provides a method for using a back-to-back cascaded photovoltaic system, comprising the following steps: When the cascaded H-bridge rectifier and isolated DC-DC converter are out of service, what happens in the cascaded H-bridge inverter? n Modulation factor of each H-bridge module M 1-ai ; like n Modulation factor of each H-bridge module M 1-ai ≤4 / π, control the cascaded H-bridge inverter to start system operation; control the cascaded H-bridge rectifier and isolated DC-DC converter to stop system operation; like n Modulation factor of each H-bridge module M 1-ai >4 / π, controls the cascaded H-bridge inverter, cascaded H-bridge rectifier, and isolated DC-DC converter to be put into system operation.

[0011] Optionally, if n Modulation factor of each H-bridge module M 1-ai >4 / π, by calculating the reference power of the H-bridge module in the cascaded H-bridge inverter, the input power of the H-bridge module in the cascaded H-bridge rectifier, and the transmission power of the isolated DC-DC converter, power closed-loop control is adopted to control the power of the H-bridge module in the cascaded H-bridge inverter, the H-bridge module in the cascaded H-bridge rectifier, and the isolated DC-DC converter respectively.

[0012] Optionally, the modulation factor of the H-bridge module in the cascaded H-bridge inverter M 1-ai (1≤ i ≤ n The formula for calculating ) is:

[0013] in, P * PVi For the first i Reference power of each photovoltaic module P * PV For all n The sum of the reference power of each photovoltaic module, V * Ha This is the reference amplitude for the output voltage of the cascaded H-bridge inverter. V PVi For the first i DC side voltage of each photovoltaic module.

[0014] Optionally, when the H-bridge modules in the cascaded H-bridge rectifiers are put into operation, the input reference power of the H-bridge modules in each cascaded H-bridge rectifier is controlled to be the same.

[0015] Optionally, in a cascaded H-bridge rectifier n Input reference power of each H-bridge module P * Hbi (1≤ i ≤ n The calculation formula is:

[0016] in, P *PVmax This represents the reference power of the photovoltaic module with the highest power generation capacity. M 1-amax This represents the modulation factor of the H-bridge module with the largest power output in the cascaded H-bridge inverter when the input power of the cascaded H-bridge rectifier is zero.

[0017] Optionally, the photovoltaic modules operate in maximum power point tracking mode.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a back-to-back cascaded photovoltaic system and its usage method. Based on a cascaded H-bridge photovoltaic grid-connected inverter, a cascaded H-bridge rectifier and an isolated DC-DC converter are introduced. By introducing energy from the grid, the output power of each H-bridge module in the cascaded H-bridge photovoltaic grid-connected inverter is changed, reducing the power output difference between each H-bridge module and lowering the modulation factor of the H-bridge module connected to the photovoltaic module with high power generation. This effectively solves the problems of power generation loss and voltage and current distortion caused by overmodulation. Attached Figure Description

[0019] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the invention in any way. Furthermore, the shapes and proportions of the components in the drawings are merely schematic to aid in understanding the invention and do not specifically limit the shapes and proportions of the components. In the drawings: Figure 1 This is a topology diagram of a back-to-back cascaded photovoltaic system according to an embodiment of the present invention; Figure 2 This is a preferred back-to-back cascaded photovoltaic system topology diagram according to an embodiment of the present invention; Figure 3 This is a flowchart of a power control method for a back-to-back cascaded photovoltaic system according to an embodiment of the present invention; Figure 4 This is the equivalent circuit diagram of mode I of the back-to-back cascaded photovoltaic system according to an embodiment of the present invention; Figure 5 This is the equivalent circuit diagram of Mode II of the back-to-back cascaded photovoltaic system according to an embodiment of the present invention; Among them, 1, first power module; 2, second power module; n-2, n-2nd power module; n-1, n-1st power module; n, nth power module; 11, first cascaded H-bridge inverter; 12, first photovoltaic module; 13, first isolated DC-DC converter; 14, first cascaded H-bridge rectifier. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0021] Therefore, the following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention.

[0022] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper", "lower", "horizontal", "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of the invention is usually placed during use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0023] When an element is referred to as being "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments. The use of the term "horizontal" does not imply that the component is required to be absolutely horizontal, but rather that it may be slightly tilted. "Horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but rather that it may be slightly tilted.

[0024] It should be noted that similar reference numerals and letters in the following figures denote similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or collections thereof.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the specification and appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0026] The present invention will now be described in detail with reference to the accompanying drawings.

[0027] This invention discloses a back-to-back cascaded photovoltaic system, comprising n power modules connected in series; each power module includes a cascaded H-bridge inverter, a photovoltaic module, an isolated DC-DC converter, and a cascaded H-bridge rectifier connected in series; both the cascaded H-bridge inverter and the cascaded H-bridge rectifier have H-bridge modules; wherein, the AC side of the H-bridge module of the cascaded H-bridge inverter of the power modules connected in series at both ends is connected to the positive and negative terminals of the power grid, respectively; the AC side of the H-bridge module of the cascaded H-bridge rectifier of the power modules connected in series at both ends is connected to the positive and negative terminals of the power grid, respectively; the DC side of the H-bridge module in the cascaded H-bridge inverter is connected to the photovoltaic module, and the other end of the photovoltaic module is connected to the input terminal of the isolated DC-DC converter; the output terminal of the isolated DC-DC converter is connected to the DC side of the H-bridge module in the cascaded H-bridge rectifier.

[0028] This invention, based on a cascaded H-bridge photovoltaic grid-connected inverter, introduces a cascaded H-bridge rectifier and an isolated DC-DC converter. By introducing energy from the grid, it changes the output power of each H-bridge module in the cascaded H-bridge photovoltaic grid-connected inverter, reduces the power output difference between each H-bridge module, and lowers the modulation factor of the H-bridge module connected to the photovoltaic module with high power generation capacity. This effectively solves the problems of power generation loss and voltage and current distortion caused by overmodulation.

[0029] Example 1 A back-to-back cascaded photovoltaic system includes n parallel power modules. Each power module includes a cascaded H-bridge inverter, a photovoltaic module, an isolated DC-DC converter, and a cascaded H-bridge rectifier. Both the cascaded H-bridge inverter and the cascaded H-bridge rectifier have H-bridge modules. The AC side of the H-bridge modules in the cascaded H-bridge inverter is connected in series with an AC inductor and then connected to the power grid. The AC side of the H-bridge modules in the cascaded H-bridge rectifier is connected in series with an AC inductor and then connected to the power grid. The DC side of the H-bridge modules in the cascaded H-bridge inverter is connected to the photovoltaic module, and the other end of the photovoltaic module is connected to the input terminal of the isolated DC-DC converter. The output terminal of the isolated DC-DC converter is connected to the DC side of the H-bridge modules in the cascaded H-bridge rectifier.

[0030] Specifically, such as Figure 2 As shown, the back-to-back cascaded photovoltaic system includes n power modules connected in parallel, namely, first power module 1, second power module 2, ..., nth power module n.

[0031] Each power module has a first positive terminal va1+ and a first negative terminal va1- on one end, and a second positive terminal vb1+ and a second negative terminal vb1- on the other end.

[0032] Wherein: the first positive terminal va1+ of the first power module 1 is connected to the positive terminal AC+ of the power grid; the second positive terminal vb1+ of the first power module 1 is connected to the positive terminal AC+ of the power grid; the first negative terminal va1- of the first power module 1 is connected to the first positive terminal va2+ of the second power module 2; the second negative terminal vb1- of the first power module 1 is connected to the second positive terminal vb2+ of the second power module 2.

[0033] Similarly, the first positive terminal va1+ of the (n-1)th power module n-1 is connected to the first negative terminal van-2- of the (n-2)th power module n-2.

[0034] The second positive terminal vbn-1+ of the (n-1)th power module n-1 is connected to the second negative terminal vbn-2- of the (n-2)th power module n-2.

[0035] The first negative terminal van-1 of the (n-1)th power module is connected to the first positive terminal van+ of the nth power module n.

[0036] The second negative terminal vbn-1- of the (n-1)th power module n-1 is connected to the second positive terminal vbn+ of the nth power module n.

[0037] The first negative terminal van- of the nth power module n is connected to the negative terminal AC- of the power grid.

[0038] The second negative terminal vbn- of the nth power module n is connected to the negative terminal AC- of the power grid.

[0039] Optionally, the first positive terminal va1+ of the first power module 1 is connected to the positive terminal AC+ of the power grid through the first grid-connected inductor Lga. The second positive terminal vb1+ of the first power module 1 is connected to the positive terminal AC+ of the power grid through the second grid-connected inductor Lgb.

[0040] Each power module includes a cascaded H-bridge inverter, photovoltaic modules, an isolated DC-DC converter, and a cascaded H-bridge rectifier.

[0041] Specifically, the first power module 1 includes a first cascaded H-bridge inverter 11, a first photovoltaic module 12, a first isolated DC-DC converter 13, and a first cascaded H-bridge rectifier 14.

[0042] The first cascaded H-bridge inverter 11 includes a first power transistor S11, a second power transistor S12, a third power transistor S13, and a fourth power transistor S14.

[0043] The source of the first power transistor S11 is connected to the first node P1.

[0044] The first node P1 is connected to the first positive terminal va1+ of the first power module 1; and the first node P1 is connected to the drain of the second power transistor S12.

[0045] The drain of the first power transistor S11 is connected to the drain of the third power transistor S13, the first terminal of the first capacitor C1, and the first terminal of the first photovoltaic module 12.

[0046] The source of the second power transistor S12 is connected to the source of the fourth power transistor S14, the second terminal of the first capacitor C1, and the second terminal of the first photovoltaic module 12.

[0047] The source of the third power transistor S13 is connected to the second node p2.

[0048] The second node p2 is connected to the drain of the fourth power transistor S14 and the first positive terminal va2+ of the second power module 2.

[0049] The first end of the first photovoltaic module 12 is connected to the first input end of the first isolated DC-DC converter 13, and the second end of the first photovoltaic module 12 is connected to the second input end of the first isolated DC-DC converter 13.

[0050] The first cascaded H-bridge rectifier 14 includes: a first power transistor T11, a second power transistor T12, a third power transistor T13, a fourth power transistor T14, and a second capacitor C2.

[0051] The first terminal of the second capacitor C2 is connected to the first output terminal of the first isolated DC-DC converter 13, the drain of the third power transistor T13 of the rectifier, and the drain of the first power transistor T11 of the rectifier. The second terminal of the second capacitor C2 is connected to the second output terminal of the first isolated DC-DC converter 13, the source of the fourth power transistor T14 of the rectifier, and the source of the second power transistor T12 of the rectifier.

[0052] The source of the first power transistor T11 in the rectifier is connected to a third node P3.

[0053] The third node P3 is connected to the drain of the second power transistor T12 of the rectifier and the second positive terminal vb1+ of the first power module 1.

[0054] The source of the third power transistor T13 in the rectifier is connected to the fourth node P4.

[0055] The fourth node P4 is connected to the drain of the fourth power transistor T14 of the rectifier and the second positive terminal vb2+ of the second power module 2.

[0056] Each power module has the same circuit structure as the first power module 1.

[0057] Optionally, such as Figure 2 As shown, in this embodiment, the first isolated DC-DC converter 13 adopts a dual active bridge DC-DC converter, including: a primary-side first power transistor S15, a primary-side second power transistor S16, a primary-side third power transistor S17, and a primary-side fourth power transistor S18; a secondary-side first power transistor T15, a secondary-side second power transistor T16, a secondary-side third power transistor T17, and a secondary-side fourth power transistor T18; an inductor Lr; and a transformer. The transformer includes a primary-side inductor Lm and a secondary-side inductor.

[0058] The drain of the first power transistor S15 on the primary side is connected to the first output terminal of the first photovoltaic module 12, the drain of the third power transistor S17 on the primary side, the drain of the third power transistor T17 on the secondary side, the drain of the first power transistor T15 on the secondary side, and the first terminal of the second capacitor C2 of the first cascaded H-bridge rectifier 14.

[0059] The source of the first power transistor S15 on the primary side is connected to the first terminal of the inductor Lr and the drain of the second power transistor S16 on the primary side. The second terminal of the inductor Lr is connected to the first terminal of the primary side inductor Lm of the transformer. The second terminal of the primary side inductor Lm of the transformer is connected to the drain of the fourth power transistor S18 on the primary side and the source of the third power transistor S17 on the primary side.

[0060] The source of the second power transistor S16 on the primary side is connected to the second output terminal of the first photovoltaic module 12, the source of the fourth power transistor S18 on the primary side, the source of the fourth power transistor T18 on the secondary side, the source of the second power transistor T16 on the secondary side, and the second terminal of the second capacitor C2 of the first cascaded H-bridge rectifier 14.

[0061] The source of the third power transistor T17 on the secondary side is connected to the drain of the fourth power transistor T18 on the secondary side and the second terminal of the transformer's secondary inductor.

[0062] The source of the first power transistor T15 on the secondary side is connected to the first terminal of the inductor on the secondary side of the transformer and the drain of the second power transistor T16 on the secondary side.

[0063] Optionally, the inductance of inductor Lr is the sum of the transformer leakage inductance and the external additional inductance.

[0064] Example 2 like Figure 3 As shown, this embodiment provides a method for using a back-to-back cascaded photovoltaic system based on Embodiment 1, including the following steps: S1: Calculate the performance of the cascaded H-bridge inverter when the cascaded H-bridge rectifier and isolated DC-DC converter are out of service. n Modulation factor of each H-bridge module M 1-ai ; Modulation factor of H-bridge module in cascaded H-bridge inverter M 1-ai (1≤ i ≤ n The formula for calculating ) is: Formula 1 in, P * PVi Indicates the first i Reference power of each photovoltaic module P * PV Indicates all n The sum of the reference power of each photovoltaic module, V * Ha This indicates the reference amplitude of the output voltage of the cascaded H-bridge inverter. V PVi Indicates the first i DC side voltage of each photovoltaic module.

[0065] S2: Judgment n Modulation factor of each H-bridge module M 1-ai Based on the relationship between the magnitude of 4 / π and the above judgment results, the back-to-back cascaded photovoltaic systems operate according to Mode I and Mode II respectively: S2.1: If M 1-ai For systems with a voltage of ≤4 / π, back-to-back cascaded photovoltaic systems operate in Mode I. Mode I consists of: controlling the cascaded H-bridge inverter to start operating in the system; controlling the cascaded H-bridge rectifier to stop operating in the system; and controlling the isolated DC-DC converter to stop operating in the system.

[0066] S2.2: If M 1-ai >4 / π, the back-to-back cascaded photovoltaic system operates in Mode II.

[0067] Mode II is as follows: Controlling the cascaded H-bridge inverter to operate in the system; controlling the cascaded H-bridge rectifier to operate in the system, causing the cascaded H-bridge rectifier... n Each H-bridge module has the same input power, and its input reference power is... P * Hbi (1≤ i ≤ n The calculation formula is: Formula 2 in, P * PVmax This represents the reference power of the photovoltaic module with the highest power generation capacity. M 1-amax This represents the modulation factor of the H-bridge module with the largest power output in the cascaded H-bridge inverter when the input power of the cascaded H-bridge rectifier is zero.

[0068] Control the isolated DC-DC converter to put it into system operation, so that n The power transfer from the cascaded H-bridge rectifier side to the cascaded H-bridge inverter side is the same for each isolated DC-DC converter, and its power transfer reference is the same. P * Di (1≤ i ≤ n The calculation formula is: Formula 3 S3: Calculate the following when the cascaded H-bridge rectifier and isolated DC-DC converter are put into system operation: In the cascaded H-bridge inverter... n Reference power of each H-bridge module P * Hai (1≤ i ≤ n The calculation formula is: Formula 4 Step S4: According to P * Hai (1≤ i ≤ n ), P * Hbi (1≤ i ≤ n )and P * Di (1≤ i ≤ n Power closed-loop control is adopted for the cascaded H-bridge inverters. n In individual H-bridge modules and cascaded H-bridge rectifiers n H-bridge module and n An isolated DC-DC converter is used for power control.

[0069] When the cascaded H-bridge rectifier and isolated DC-DC converter in steps S1 and S2.1 are taken out of the system, P * Hbi (1≤i ≤ n )and P * Di (1≤ i ≤ n The formula for calculating ) is: Formula 5 In both steps S1 and S2, the photovoltaic modules operate in maximum power point tracking mode.

[0070] Figure 4 This is the equivalent circuit diagram of Mode I of a back-to-back cascaded photovoltaic system. The cascaded H-bridge rectifier and the isolated DC-DC converter are out of operation. The power of the cascaded H-bridge rectifier and the isolated DC-DC converter is zero. The power of each H-bridge module in the cascaded H-bridge inverter is the power of the photovoltaic module.

[0071] Figure 5 This is the equivalent circuit diagram of a back-to-back cascaded photovoltaic system in Mode II. The cascaded H-bridge rectifier and the isolated DC-DC converter are put into operation. The power of the cascaded H-bridge rectifier and the isolated DC-DC converter is not zero. The power of each H-bridge module of the cascaded H-bridge inverter is the sum of the power of the photovoltaic module and the power of the isolated DC-DC converter.

[0072] Unless otherwise specified, the equipment components involved in the above embodiments are all conventional equipment components, and the structural settings, working methods or control methods involved are all conventional settings, working methods or control methods in the art unless otherwise specified.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A back-to-back cascaded photovoltaic system, characterized in that, The system comprises n power modules connected in series. Each power module includes a cascaded H-bridge inverter, a photovoltaic module, an isolated DC-DC converter, and a cascaded H-bridge rectifier, all connected in series. Both the cascaded H-bridge inverter and the cascaded H-bridge rectifier have H-bridge modules. The AC side of the H-bridge module in the cascaded H-bridge inverter connected in series at both ends is connected to the positive and negative terminals of the power grid, respectively. The AC side of the H-bridge module in the cascaded H-bridge rectifier connected in series at both ends is connected to the positive and negative terminals of the power grid, respectively. The DC side of the H-bridge module in the cascaded H-bridge inverter is connected to the photovoltaic module, and the other end of the photovoltaic module is connected to the input terminal of the isolated DC-DC converter. The output terminal of the isolated DC-DC converter is connected to the DC side of the H-bridge module in the cascaded H-bridge rectifier.

2. The back-to-back cascaded photovoltaic system according to claim 1, characterized in that, The AC side of the H-bridge module of the cascaded H-bridge inverter of the power module is connected to the positive terminal of the power grid through the first grid-connected inductor Lga; the AC side of the H-bridge module of the cascaded H-bridge rectifier of the power module is connected to the positive terminal of the power grid through the second grid-connected inductor Lgb.

3. The back-to-back cascaded photovoltaic system according to claim 1, characterized in that, The isolated DC-DC converter adopts a dual active bridge DC-DC converter.

4. The back-to-back cascaded photovoltaic system according to claim 1, characterized in that, Each isolated DC-DC converter has the same transmission power.

5. A method of using a back-to-back cascaded photovoltaic system as described in any one of claims 1 to 4, characterized in that, Includes the following steps: When the cascaded H-bridge rectifier and isolated DC-DC converter are out of service, what happens in the cascaded H-bridge inverter? n Modulation factor of each H-bridge module M 1-ai ; like n Modulation factor of each H-bridge module M 1-ai ≤4 / π, control the cascaded H-bridge inverter to start system operation; control the cascaded H-bridge rectifier and isolated DC-DC converter to stop system operation; like n Modulation factor of each H-bridge module M 1-ai >4 / π, controls the cascaded H-bridge inverter, cascaded H-bridge rectifier, and isolated DC-DC converter to be put into system operation.

6. The method of using a back-to-back cascaded photovoltaic system according to claim 5, characterized in that, like n Modulation factor of each H-bridge module M 1-ai >4 / π, by calculating the reference power of the H-bridge module in the cascaded H-bridge inverter, the input power of the H-bridge module in the cascaded H-bridge rectifier, and the transmission power of the isolated DC-DC converter, power closed-loop control is adopted to control the power of the H-bridge module in the cascaded H-bridge inverter, the H-bridge module in the cascaded H-bridge rectifier, and the isolated DC-DC converter respectively.

7. The method of using a back-to-back cascaded photovoltaic system according to claim 5, characterized in that, Modulation factor of H-bridge module in cascaded H-bridge inverter M 1-ai (1≤ i ≤ n The formula for calculating ) is: in, P * PVi For the first i Reference power of each photovoltaic module P * PV For all n The sum of the reference power of each photovoltaic module, V * Ha This is the reference amplitude for the output voltage of the cascaded H-bridge inverter. V PVi For the first i DC side voltage of each photovoltaic module.

8. The method of using a back-to-back cascaded photovoltaic system according to claim 5, characterized in that, When the H-bridge modules in a cascaded H-bridge rectifier are put into operation, the input reference power of each H-bridge module in the cascaded H-bridge rectifier is the same.

9. A method of using a back-to-back cascaded photovoltaic system according to claim 8, characterized in that, In cascaded H-bridge rectifiers n Input reference power of each H-bridge module P * Hbi (1≤ i ≤ n The calculation formula is: in, P * PVmax This represents the reference power of the photovoltaic module with the highest power generation capacity. M 1-amax This represents the modulation factor of the H-bridge module with the largest power output in the cascaded H-bridge inverter when the input power of the cascaded H-bridge rectifier is zero.

10. The method of using a back-to-back cascaded photovoltaic system according to claim 5, characterized in that, The photovoltaic modules operate in maximum power point tracking mode.

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