Large-scale offshore floating photovoltaic full DC collection and transmission system and construction method

Abstract

The present invention relates to a large-scale offshore floating photovoltaic full DC collection and transmission system and a construction method, in which photovoltaic strings are formed by photovoltaic modules on a floating body, and the photovoltaic strings, junction boxes, primary boost DC converters, secondary boost DC converters and onshore inverter stations are sequentially connected by cables. The present invention aims to alleviate the problems of high line loss, low efficiency, small capacity, short distance and other problems faced by traditional AC collection and transmission systems under the trend of deep-sea construction and large-scale development of floating photovoltaic power stations. At the same time, the present invention provides a photovoltaic array arrangement and low-voltage DC cable wiring method suitable for different floating bodies in the construction method of the full DC collection and transmission system. The combination of the two can realize the safe, stable, economical and efficient collection and transmission of large-scale deep-sea offshore floating photovoltaic power.

Description

Technical Field

[0001] The present invention belongs to the field of offshore electrical and electrical technology, and in particular to a large-scale offshore floating photovoltaic full DC collection and transmission system and a construction method thereof. Background Art

[0002] The growing energy demand of countries around the world and the worsening climate problems such as the global greenhouse effect have led to unprecedented attention to renewable energy represented by wind and solar. Among them, solar energy has become one of the most popular new energy sources in the world due to its easy availability and abundance on the earth's surface. However, due to the large-scale construction of centralized photovoltaic power stations in the early stage, there are few land resources available for use. Therefore, people have turned their attention to the vast ocean. Ocean floating photovoltaics can not only alleviate the tension of land resources, but also due to the cooling and cleaning effect of a large amount of seawater, the power generation efficiency of the components will be higher than that of land photovoltaics and building photovoltaics, and the attenuation and degradation rate will also be reduced, which has good development prospects.

[0003] Traditional photovoltaic power stations all use the technical solution of AC collection and AC transmission. Although the relevant technology is relatively mature, problems such as system stability under the support of weak power grids and losses caused by the skin effect of AC cables still exist, which limits the further improvement of collection efficiency, transmission distance and installed capacity. In the engineering planning and design of large-scale floating photovoltaic power generation systems in the deep sea, the economic problem cannot be ignored. With the development of high-power density power electronic device manufacturing technology, the research and development and manufacturing level of high-efficiency, wide-gain DC transformers and large-capacity DC circuit breakers has been continuously improved, and DC transmission and distribution technology has been put into practical application in a large number of projects. Therefore, the use of full DC collection and grid-connected technology, which is different from the traditional AC collection and transmission, is an effective way to solve the problems existing in the above-mentioned AC system and promote the design, planning, development and construction of large-scale floating photovoltaic power stations. However, there is currently no system that uses full DC collection and grid-connected technology. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a large-scale offshore floating photovoltaic all-DC collection and transmission system and a construction method, which can effectively improve the problems of low efficiency and poor operating stability of traditional AC collection and transmission systems under the background of long offshore distance and large installed capacity, and provide an effective technical solution for deep-sea development and large-scale construction of floating photovoltaic power stations.

[0005] The present invention solves the technical problem by adopting the following technical solutions:

[0006] A large-scale floating offshore photovoltaic full DC collection and transmission system includes a floating body, photovoltaic modules, a junction box, a cable, a primary boost DC converter, a secondary boost DC converter and an onshore inverter station; wherein the cables include: a low-voltage DC cable, a medium-voltage DC dynamic cable and a high-voltage DC cable; photovoltaic modules and a junction box are installed on the floating body; a primary boost DC converter is installed above the central platform of the floating body; photovoltaic modules are connected in series through low-voltage DC cables to form photovoltaic strings; photovoltaic strings are connected in parallel to the junction boxes on the corresponding floating body; multiple junction boxes are connected in parallel to the primary boost DC converter for boosting conversion from low voltage to medium voltage; multiple primary boost DC converters are connected in parallel to the secondary boost DC converter through medium-voltage DC dynamic cables for boosting conversion from medium voltage to high voltage; the secondary boost DC converter is connected to the onshore inverter station through high-voltage DC cables for transmitting electric energy to the onshore inverter station; the onshore inverter station is used to convert electric energy from DC to AC for grid connection.

[0007] Furthermore, the low voltage DC cable is protected by a special protective casing.

[0008] Moreover, the one-stage boost DC converter is used to increase DC power from low voltage to medium voltage, and at the same time provide multi-channel MPPT for photovoltaic strings to achieve maximum power tracking. The control target of the one-stage boost DC converter is the low-voltage side DC voltage, and the voltage reference value is tracked through unit feedback control.

[0009] Moreover, the medium voltage DC dynamic cable floats in the seawater, and a linear structure is achieved through floating blocks and gravity blocks.

[0010] Furthermore, the two-stage DC boost converter is installed on an independent pile-type platform.

[0011] Furthermore, the primary boost DC converter and the secondary boost DC converter use high-frequency switching power semiconductor devices to achieve miniaturization and lightness of the DC converter.

[0012] Furthermore, the onshore inverter station adopts a voltage source inverter to control the DC bus voltage stability of the high voltage DC transmission system.

[0013] A method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system comprises the following steps:

[0014] Step 1: Connect the external electrical equipment of the floating body by connecting the combiner box, the primary boost DC converter and the secondary boost DC converter through corresponding cables according to the topological structure of the full DC collection and transmission system;

[0015] Step 2: Arrangement and connection of photovoltaic modules inside the floating body. Determine whether partitioning is required based on the shape of the floating body. If partitioning is required, proceed to step 3; otherwise, proceed to step 4.

[0016] Step 3: After partitioning, the array is designed in triangles. According to the geometric characteristics of the float and the components, the number of components arranged in each row of the square array is calculated through multiple iterations;

[0017] Step 4: If partitioning is not required, the array design is performed in rectangular units, and the result is calculated in one step according to the geometric characteristics of the floating body and the components;

[0018] Step 5: Calculate the number of strings based on the electrical parameters of the primary boost DC converter and components and arrange low-voltage DC cables in the full DC collection and transmission system.

[0019] The advantages and positive effects of the present invention are:

[0020] 1. The present invention forms a photovoltaic string through photovoltaic modules on a floating body, and constructs a large-scale offshore floating photovoltaic full DC collection and transmission system through photovoltaic strings, junction boxes, primary boost DC converters, secondary boost DC converters and onshore inverter stations connected in sequence by cables. The present invention realizes the primary boost collection and secondary boost transmission of DC voltage through a large-capacity DC transformer. The DC collection and transmission scheme does not have problems such as synchronous stability and low-order harmonic pollution, and the control strategy is concise and clear. Moreover, under the same insulation level, the DC cable has stronger transmission capacity and lower line loss, which can improve the operating efficiency of the internal collection system and the transmission system of the station. The DC scheme can effectively alleviate the limitations of the traditional AC scheme on the collection efficiency, transmission distance and transmission capacity of large-scale floating photovoltaic power stations in the deep sea.

[0021] 2. In the all-DC collection and transmission system of the present invention, the boost DC converters at each level are interconnected by dynamic DC cables. Through the special design of the armor layer and the optimization of the cable configuration, the problems of cable damage and breakage caused by excessive tension and insufficient curvature caused by the movement of the floating body can be effectively alleviated, and its service life can be significantly extended.

[0022] 3. The construction process of the all-DC collection and transmission system of the present invention involves the photovoltaic array arrangement design and low-voltage DC cable wiring method, which can realize the automatic array arrangement design according to the geometric characteristics of different types of floating bodies such as rectangles, triangles, hexagons, etc. combined with the specification parameters of the components, and then generate a wiring plan in combination with the input electrical parameters of the DC converter. The construction method of the present invention can not only reduce the error of manual calculation, maximize the use of floating body space, and reduce the economic losses caused by it; and when the installed capacity is large, due to wave breaking, anchoring and mooring, etc., there may be a variety of floating bodies with different shapes and sizes. At this time, automatic arrangement and wiring can greatly improve the design efficiency in the early stage of scheme planning. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1This is a schematic diagram of the structure of a large-scale offshore floating photovoltaic full DC collection and transmission system of the present invention;

[0024] Figure 2 This is a flow chart of the photovoltaic array arrangement and low-voltage DC cable wiring method of the present invention;

[0025] Figure 3 It is a schematic diagram of the application effect of the photovoltaic array arrangement and low-voltage DC cable wiring method of the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below with reference to the accompanying drawings.

[0027] Terminology explanation:

[0028] DC converter: refers to a power-level power electronic device (DC-DC) that directly realizes the voltage level conversion of DC electric energy. In the present invention, it is used to boost the DC electric energy twice.

[0029] Primary boost: In the present invention, it refers to increasing low voltage DC (below 10kV) to medium voltage DC (10kV~100kV), that is, the voltage level of the photovoltaic string is increased to the voltage level of the collection system.

[0030] Secondary boost: In the present invention, it refers to increasing medium voltage DC (10kV~100kV) to high voltage DC (100kV~500kV), that is, increasing the voltage level of the collection system to the voltage level of the transmission system.

[0031] Aggregation: refers to the collection and concentration of electric energy within the station. In the present invention, it refers to the collection of DC electric energy output by photovoltaic modules and the first-stage boost DC converter, that is, all links between the module output end and the second-stage boost converter input end.

[0032] Transmission: refers to the transmission of electric energy from the station to the grid connection point. In the present invention, it refers to the process of transmitting the output electric energy of the secondary boost DC converter to the onshore inverter station through a high-voltage DC cable, that is, the part after the secondary boost converter.

[0033] Large-scale floating photovoltaic full DC collection and transmission system, such as Figure 1As shown, it includes a floating body, photovoltaic modules, a junction box, a cable, a primary boost DC converter, a secondary boost DC converter and an onshore inverter station; wherein the cables include: a low-voltage DC cable, a medium-voltage DC dynamic cable and a high-voltage DC cable. The photovoltaic modules and the junction box are installed on the floating body. The photovoltaic modules on the floating body are connected in series through low-voltage DC cables to form a photovoltaic string, and then connected in parallel to the junction box on the floating body. The junction boxes on multiple floating bodies are then connected in parallel for a second time to the primary boost DC converter. The primary boost DC converter is point-to-point connected to the secondary boost DC converter through the medium-voltage DC dynamic cable to realize star networking. The secondary boost DC converter is connected to the onshore inverter station through a high-voltage DC cable to send the electric energy to the land and convert it into AC to realize grid connection.

[0034] Among them, all low-voltage DC cables are protected by special cable protection sleeves (such as CPVC pipes, etc.) and laid above the buoy. The protection sleeves should have excellent waterproof, UV protection, and corrosion resistance. At the same time, considering the high wind level on the sea surface, the sleeves must also have a certain rigidity and be reliably fixed to avoid frequent pulling and shaking of the cable, which may lead to breakage. Enhance the protection ability of low-voltage DC cables under environmental factors such as marine salt spray corrosion and strong ultraviolet radiation, and the fatigue resistance under complex wind conditions.

[0035] The combiner box is installed in the middle of each floating body to combine the DC power of multiple strings on the floating body into one channel and output it to the nearest primary step-up DC converter. The use of combiner boxes can greatly reduce the number of DC cables across the floating body and improve reliability. Combiner boxes are usually equipped with surge protectors, leakage protectors, disconnectors, fuses, etc., and have fault identification and circuit protection functions. They can quickly cut off and isolate strings that have overvoltage, overcurrent, short circuit, lightning strike, grounding, reverse connection and other faults. They are of great significance in marine environments with frequent lightning and rainstorms.

[0036] Due to limitations of structural stability and anchoring and mooring, the installed capacity of the photovoltaic array on a single floating body is approximately 200kW to 500kW. Therefore, considering the economic feasibility of the project, a first-stage boost DC converter will not be installed on each floating body. Instead, the power of multiple floating bodies will be collected through a junction box and then connected in a unified manner for boosting. The number of floating bodies connected to each first-stage boost DC converter needs to be further determined based on the system scale and equipment capacity.

[0037] The first-stage boost DC converter directly increases the DC power from low voltage to medium voltage, and provides multiple MPPTs for the photovoltaic strings on multiple floating bodies to achieve maximum power tracking. Its control target is the DC voltage on the low voltage side (i.e., the photovoltaic string), and the voltage reference value is tracked through unit feedback control. It is installed in the center of the floating body to keep the center of gravity of the structure stable and reduce the difficulty of construction. At the same time, an additional outer shell cabin is added to improve the protection level, ensuring good dust and water resistance and heat dissipation performance.

[0038] All first-stage boost DC converter outlets are connected to the low-voltage side inlet of the second-stage boost DC converter through medium-voltage DC dynamic cables, realizing a point-to-point double-ended star topology, maximizing operational reliability, avoiding the impact of upstream equipment failures on downstream equipment in traditional radial topology, and preventing the expansion of the fault area.

[0039] In the present invention, the medium voltage DC dynamic cable floats in the seawater, and it is necessary to configure bend limiters at both ends to avoid excessive bending, install buoys and counterweights in the middle to achieve the configuration, and install mooring and anchoring devices on the seabed to prevent excessive displacement. Single wave or multi-wave line type should be selected according to the hydrological environment such as the depth of the water area and the degree of waves at the project site, effectively alleviating the fatigue damage caused by the frequent movement of the cable due to the stress of waves and ocean currents.

[0040] In this embodiment, there is only one secondary DC boost converter in the station, and its total capacity is about 100MW to 500MW. It can be packaged by combining multiple small-capacity devices in series and parallel, avoiding the difficulty and high cost of making large-capacity devices. The secondary DC boost converter controls the DC voltage stability on its low-voltage side (i.e., the collection system). Because its operating status is directly related to the power delivery of the entire station, its reliability priority is relatively high, and it needs to be installed on an independent pile-type fixed platform, so it only needs to be connected to the onshore inverter station through an ordinary submarine static high-voltage DC cable.

[0041] The onshore inverter station uses a voltage source inverter (which can be a modular multilevel converter with a half-bridge or full-bridge structure) to convert DC power into three-phase industrial frequency AC power and feed it into the onshore main grid. It has two control degrees of freedom, which are used to control the DC side voltage and the AC side voltage or reactive power.

[0042] A method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system, such as Figure 2 As shown, the following steps are included:

[0043] Step 1: Connect the external electrical equipment of the floating body. According to the topological structure of the full DC collection and transmission system, connect the combiner box, the first-stage boost DC converter and the second-stage boost DC converter through corresponding cables.

[0044] Step 2: Arrange and connect the photovoltaic modules inside the floating body. Determine whether partitioning is required based on the shape of the floating body. If partitioning is required, proceed to step 3; otherwise, proceed to step 4.

[0045] Common shapes of floating bodies used for offshore floating photovoltaics are rectangles, triangles, and regular polygons represented by regular hexagons. The first two are basic structures and do not require further partitioning, while polygons such as regular hexagons can be divided into multiple regular triangles before the next step of calculation.

[0046] Step 3: After partitioning, the array is designed in triangles. According to the geometric characteristics of the float and components, the number of components arranged in each row of the array is calculated through multiple iterations.

[0047] According to the judgment result of step 2, when the floating body is a regular hexagon, it is necessary to first divide it into six triangular areas, and then use this as the basic unit to iterate and calculate the array arrangement plan through formulas (1) to (3). Formula (1) determines whether the i-th row can arrange components, formula (2) calculates the number of components in the i-th row, and formula (3) calculates the length of the base of the equivalent triangle corresponding to the i-th row. The calculation is repeated iteratively:

[0048]

[0049]

[0050]

[0051] Among them, C i-1 is the length of the base of the triangle corresponding to the i-th row, n i is the number of components installed in the i-th row, h i-1 is the height of the equivalent triangle corresponding to the i-th row, such as Figure 3 shown.

[0052] Step 4: If partitioning is not required, the array design is performed in rectangular units, and the result is calculated in one step according to the geometric characteristics of the float and components.

[0053] According to the judgment result of step 2, when the floating body is a rectangle, it can be calculated in one step by formula (4):

[0054]

[0055] Wherein, m and n are the number of components in the horizontal and vertical directions respectively, floor is rounded down, A and B are the length and width of the rectangular float respectively, A' and B' are the lengths reserved for reasons such as maintenance channels and equipment installation, and a and b are the length and width of the component taking into account the installation space.

[0056] Step 5: Calculate the number of strings based on the electrical parameters of the converter and components and lay out DC cables in the full DC collection and transmission system.

[0057] The number of modules in the PV string is calculated based on the open-circuit voltage and operating voltage of the module, as well as the maximum allowable DC voltage and MPPT voltage at the input end of the first-stage boost DC converter. The number is calculated using formulas (5) to (6). According to the number of modules in series, the modules are connected starting from the first row and then to the center of the floating body. The outgoing wires at both ends of each string are led to the nearest truss and fixed, and then connected to the combiner box at the center of the floating body.

[0058]

[0059]

[0060] Where N is the number of components under the maximum input voltage constraint, N min and N max K is the maximum and minimum number of components under MPPT voltage constraints, v is the open circuit voltage temperature coefficient of the photovoltaic module; K ’ v is the working voltage temperature coefficient of the photovoltaic module; t is the extreme low temperature under the working conditions of the photovoltaic module (℃); t' is the extreme high temperature under the working conditions of the photovoltaic module (℃); V dcmax V is the maximum DC input voltage allowed by the inverter (V); MPPTmax and V MPPTmin are the maximum and minimum values ​​of the inverter MPPT voltage (V); V oc is the open circuit voltage of the photovoltaic module (V); V pm is the operating voltage of the PV module (V).

[0061] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific implementation manner. Any other implementation manners derived by those skilled in the art based on the technical solution of the present invention also fall within the scope of protection of the present invention.

Claims

1. A method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system, characterized in that: The construction method is implemented based on a large-scale offshore floating photovoltaic full DC collection and transmission system, and includes the following steps: Step 1: Connect the external electrical equipment of the floating body by connecting the combiner box, the primary boost DC converter and the secondary boost DC converter through corresponding cables according to the topological structure of the full DC collection and transmission system; Step 2: Arrangement and connection of photovoltaic modules inside the floating body. Determine whether partitioning is required based on the shape of the floating body. If partitioning is required, proceed to step 3; otherwise, proceed to step 4. Step 3: After partitioning, the array is designed in triangles. According to the geometric characteristics of the float and the components, the number of components arranged in each row of the square array is calculated through multiple iterations; According to the judgment result of step 2, when the floating body is a regular hexagon, it is necessary to first divide it into six triangular areas, and then use these as basic units to iteratively calculate the array arrangement plan through formulas (1) to (3); formula (1) determines whether the i-th row can arrange components, formula (2) calculates the number of components in the i-th row, and formula (3) calculates the length of the base of the equivalent triangle corresponding to the i-th row, and the calculation is repeated iteratively: Among them, C i-1 is the length of the base of the triangle corresponding to the i-th row, n i is the number of components installed in the i-th row, h i-1 is the height of the equivalent triangle corresponding to the i-th row; Step 4: If partitioning is not required, the array design is performed in rectangular units, and the result is calculated in one step according to the geometric characteristics of the floating body and the components; According to the judgment result of step 2, when the floating body is a rectangle, it can be calculated in one step by formula (4): Wherein, m and n are the number of components in the horizontal and vertical directions, respectively, floor is rounded down, A and B are the length and width of the rectangular float, respectively, A' and B' are the lengths reserved for maintenance access and equipment installation, respectively, and a and b are the length and width of the component taking into account the installation space; Step 5: Calculate the number of strings based on the electrical parameters of the converter and components and arrange DC cables in the full DC collection and transmission system; The large-scale offshore floating photovoltaic full DC collection and transmission system comprises a floating body, photovoltaic modules, a junction box, a cable, a primary boost DC converter, a secondary boost DC converter and an onshore inverter station; wherein the cables comprise: a low voltage DC cable, a medium voltage DC dynamic cable and a high voltage DC cable; photovoltaic modules and a junction box are installed on the floating body; a primary boost DC converter is installed above the central platform of the floating body; photovoltaic modules are connected in series through low voltage DC cables to form photovoltaic strings; photovoltaic strings are connected in parallel to the junction boxes on the corresponding floating body; a plurality of junction boxes are connected in parallel to the primary boost DC converter for performing a boost conversion from low voltage to medium voltage; a plurality of primary boost DC converters are connected in parallel to the secondary boost DC converter through medium voltage DC dynamic cables for performing a boost conversion from medium voltage to high voltage; a secondary boost DC converter is connected to the onshore inverter station through a high voltage DC cable for transmitting electric energy to the onshore inverter station; the onshore inverter station is used to convert electric energy from DC to AC for grid connection.

2. The method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system according to claim 1 is characterized in that: The low voltage DC cable is protected by a special protective casing.

3. The method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system according to claim 1 is characterized in that: The first-stage boost DC converter is used to increase DC power from low voltage to medium voltage, and at the same time provide multi-channel MPPT for photovoltaic strings to achieve maximum power tracking. The control target of the first-stage boost DC converter is the low-voltage side DC voltage, and the voltage reference value is tracked through unit feedback control.

4. The method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system according to claim 1 is characterized in that: The medium voltage DC dynamic cable floats in the seawater, and a linear structure is achieved through floating blocks and gravity blocks.

5. The method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system according to claim 1 is characterized in that: The two-stage boost DC converter is installed on an independent pile-type platform.

6. The method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system according to claim 1, characterized in that: The first-stage boost DC converter and the second-stage boost DC converter use power semiconductor devices with high-frequency switches to achieve miniaturization and lightness of the DC converter.

7. The method for constructing a large-scale offshore floating photovoltaic full DC collection and transmission system according to claim 1, characterized in that: The onshore inverter station adopts a voltage source inverter to control the DC bus voltage stability of the high-voltage DC transmission system.

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