Standardization proposal and product design of offshore floating photovoltaic system cables

The cable product for sea surface floating photovoltaic system is a waterproof cable with a highly comprehensive performance developed from the wiring application of the land photovoltaic system, which is mainly used for the electrical connection between modules of sea surface floating photovoltaic system, as well as the electrical connection of each assembly to the combiner box and combiner box to the inverter. This work introduced the main product types of photovoltaic cables in domestic and internationally, focusing on environmental adaptability, mechanical performance, and special waterproof performance analysis. On this basis, the standardization proposal for cables used in floating photovoltaic systems on the sea was proposed to facilitate the generalization, serialization, and standardization of such products, and further introduce the corresponding product design scheme, material selection, and process control, so as to provide reference for the cable manufacturing enterprises.


INTRODUCTION
Inhibiting climate warming is a major problem facing mankind in the 21st century, and vigorously developing clean and renewable energy is one of the important measures.Over the past decade, photovoltaic power generation have grown rapidly, becoming a major form of global renewable energy development [1] .With the continuous growth of onshore PV installed capacity, land resources and relevant policy restrictions are becoming the key elements in restricting the large-scale development of PV.Therefore, the construction of photovoltaic power stations has shifted to the wider water space, and offshore floating photovoltaic has come into view at a clip.At the same time, due to the wide sea area, sufficient and unobstructed sunshine on the sea, and the natural environmental advantages of photovoltaic layout, it has become a globally recognized application form.On the one hand, China has a nearly 18,000 kilometers long coastline, so it is feasible to make full use of the resources along the line to develop offshore human centralized photovoltaic power generation, which can greatly save the increasingly precious taxi resources on land.On the other hand, the eastern coastal areas of China have developed economy and population density.This provides an ideal solution for the development of offshore photovoltaic power generation for the growing power demand of load centers [2] .The offshore floating photovoltaic project built by Singapore solar company Sunseap in Johor Strait has been completed, which is one of the largest offshore floating photovoltaic systems in the world.
Ocean Sun and Korean energy company EN Technologies developed the pilot project of a floating solar energy station near the Saemangeum tidal flat on the coast of the Yellow Sea; The first batch of the 2022 offshore floating photovoltaic key technology project of China Energy Investment Group Corporation is in the sea near Putuo Baisha Island, with a Giant floater composed of hundreds of photovoltaic panels floating on the sea.After the completion of the project, it will become China's first offshore floating photovoltaic power generation project; The maritime floating scheme of Norwegian Ocean Sun company is currently used by Chinese companies in the waters of Yantai, and construction has started with an installed capacity of 1 MW.OceanSun's floating photovoltaic system components are installed on the blue plastic film supported by the buoyancy ring.The plastic mold can fluctuate up and down with the waves, which can greatly reduce the wind resistance and cushion the wave so that the photovoltaic system can more easily operate stably in the wind, wave, and current environment or resist stronger typhoons.
Cable products for offshore floating photovoltaic systems (Figure1) are waterproof cables with a high comprehensive performance developed in the wiring application of the onshore photovoltaic system, which is mainly used for the electrical connection between strings of offshore floating photovoltaic system and between strings, as well as the collection of strings to the combiner box.In addition to the influence of mechanical load, the electrical performance and high temperature performance parameters of dynamic Marine cable insulation materials also need to meet the safe operation requirements of Marine cable [2] .On the premise of meeting the requirements of high mechanical strength, UV resistance, long life, and other comprehensive properties of photovoltaic cables, the special waterproof performance of cables for offshore floating photovoltaic systems has been greatly improved.For this reason, compared with the traditional photovoltaic cables, it is more suitable for laying and application in the sea surface environment.In order to promote the development of cable products for offshore floating photovoltaic systems towards generalization, serialization, and standardization, this paper has carried out research on relevant standardization and product design.

Environmental adaptability
Photovoltaic cable with photovoltaic power stations fixed in remote, harsh climate areas, and change with the seasons to extreme temperature erosion, so its application environment is complex and circulation experience, but generally can be summarized as high temperature, low temperature, ozone resistance, ultraviolet resistance, acid, alkaline and other changes of random combination.These combinations and changes affect the characteristics of the photovoltaic cable, so the environmental performance of the photovoltaic cable is very important.In general, high temperature, low temperature, ozone resistance, UV resistance, acid, alkaline, and other conditions are the general factors in photovoltaic cables, so we can directly refer to the existing EU EN 50618 "wire and cable for photovoltaic system" standard [4] .

Electrical properties
The photovoltaic cable includes not only the electric unit of power transmission but also the control unit with signal transmission function.Therefore, the electrical performance requirements, such as DC resistance, insulation resistance, dielectric strength, and other requirements, need to be included in the category of product standardization.Additionally, the impact of environmental changes on the electrical performance of products, such as the requirements of insulation resistance and dielectric strength when high-temperature and low-temperature changes, should be paid attention to it.

2.3Mechanical property
In the process of installation, laying, application, transportation, storage, maintenance, and replacement, the photovoltaic cable may be subjected to bending and torsion for a long time.The mechanical properties of the product directly affect the working life, so the requirement for the cable to bear mechanical stress is higher than before.The cable should adopt a tinned copper conductor for the 5-class soft cable according to EN 60228, and the stranded mode should be suitable for small spaces and extremely bending wiring conditions and shall be used with polyolefin insulation and sheath.The strength of the cable shall exceed 8 MPa, and the elongation rate exceeds 125% to ensure that the cable can withstand a certain stress load when bent and fixed at small angles.Because the photovoltaic cable between the photovoltaic system group string and the group string is in a bare laying state, it should also have the power-cutting ability to prevent the cable from suffering from impact damage.

Special waterproof performance
Photovoltaic Cable for Shoal (CEEIA263) of China Electrical Appliance Industry Association proposed to conduct a saltwater resistance test for some cables if it is necessary in combination with the use environment of the shoal.The cable used in the sea surface floating photovoltaic system is in the wet salt fog environment for a long time in the whole life cycle.The cable insulation and sheath materials shall also pass the long-term DC stability test, water absorption capacitance increment test, seawater immersion resistance test, subsequent insulation resistance test, and other special waterproof performance verification.We ensure that the electrical performance does not decrease significantly to improve the reliability of long-term operation.

STANDARDIZATION RECOMMENDATIONS
High-quality photovoltaic cable products are not only achieved overnight by virtue of advanced single technical indicators or multiple environmental adaptability points but also reflected in the ability of product performance to meet the requirements of the actual application environment requirements, which is closely related to the accuracy and comprehensiveness of product quality and reliability verified by simulated application environment [5] .The development of a new type of Marine cable system should firstly make the cable structure design and material selection according to the parameters of its application engineering [6] .On the basis of the performance analysis of the cables used in the traditional onshore photovoltaic power stations, the corresponding standardization suggestions are put forward for the cables used for the floating photovoltaic systems on the sea surface, and the test points of the cables are further specified, as shown in Table 1 for details.
Table 1.Inspection items of cable used for sea surface floating photovoltaic systems

Test item points Test conditions and requirements (a) Electrical test Conductor resistance
The test sample length is 1.5 m and the ambient temperature is 20℃; the conductor resistance is required.

Voltage withstand test
Test sample length is 20 m, water temperature is 20℃, soaking time is 1 h, after 6.5 kV(a.c.) or 15 kV (d.c.), 5 min Voltage test; No breakdown is required.

Capacitance change
The length of the test sample was 3 m, the water temperature was 90℃, the soaking time is 14 d, and the maximum capacitor value is 10% for 1-14 d, and 4% for 7-14 d.

Sheath surface resistance
The number of sampling tests is 3, the length of each test sample is 250 mm, the distance between two electrodes is 100 mm, the water temperature is 20 ℃, the immersion time is 24 h, and the test voltage is DC100-500 V, and the surface resistance is greater than or equal to 1.0×10 9 Ω.

Insulation resistance at 20℃
The test sample is 5 m long, the water bath temperature is 20℃, the water bath immersion time is 2 h, and the test voltage is DC500 V; The insulation resistance is required to be greater than or equal to the specified value.

Insulation resistance at 90℃
The test sample length is 5 m, the water bath temperature is 90℃, the water bath immersion time is 2 h, and the test voltage is DC 500 V; The required insulation resistance is greater than or equal to the specified value.

The DC stability test
The test sample length is 5 m, the water temperature (containing 1% sodium chloride in water) is 90℃, the test time is 2016 h, and the test voltage is DC 3.6 kV; no breakdown or damage is required.

Insulation thickness
The number of test samples is 3, and each sample is measured 6 times; the required average thickness is greater than or equal to the specified nominal thickness, and the minimum insulation thickness is greater than or equal to 90% nominal insulation thickness of 0.1 mm.

Cover thickness
The number of test samples is 3, and each sample is measured 6 times; the required average thickness is greater than or equal to the specified nominal thickness; the minimum sheath thickness is greater than or equal to 85% nominal sheath thickness of 0.1 mm.

Cable ellipticity
Make two measurements on the same section of the finished cable, and the difference between any two measured outer diameter values shall not exceed 15%.

Outside diameter of the cable
The number of test samples is 3; the outer diameter of the finished product is less than or equal to the specified value.

Tensile testing both before and after aging in the air aging box
The number of test samples before and after aging is 5, the temperature is (150 ± 2)℃, and the aging time is 168 h; the tensile strength is greater than 8 MPa, the fracture elongation is greater than or equal to 125%, the change rate of tensile strength should be less than or equal to -30%, and the change rate of fracture elongation is less than or equal to -30%.

Hot extension
The number of test samples is 2, the sample length is 100 mm, the spacing between the two identifiers is 20 mm, the air oven temperature is 250℃, the applied load time is 15 min, and the mechanical stress is 20 N/cm 2 ; Requirements for elongation less than 100% under load and permanent deformation less than or equal to 25% after cooling

Insulation water absorption
The water temperature is (85 ± 2)℃, and the immersion time is 336 h; the required maximum weight increase is less than or equal to 1 mg/cm 2 .

Cover water absorption
The water temperature is (85 ± 2)℃, and the immersion time is 336 h; the required maximum weight increase is less than or equal to 5 mg/cm 2 .

Ozone proof
The test method is GB / T 2951.21, the temperature is (25 ± 2)℃, the ozone concentration is (250-300) 10-4% (volume fraction), and the test time is 24 h; no crack is required.

Acid and alkali resistant
The number of test samples is 5, standard oxalic acid (0.5 mol/L) and standard sodium hydroxide solution (1 mol/L), the temperature is 23℃, the soaking time in the solution is 168 h; the change rate of tensile strength of the sheath is less than or equal to ± 30%, and the break elongation is greater than or equal to 100%.

Jacket thermal contraction test
Place a section of 300 mm specimen with sheath in a (120 ± 2)℃ oven for 1 h, requiring a shrinkage rate of less than or equal to 2%.

Sheath weather resistance / anti-UV test
The number of test samples is 10.Take 5 test pieces and expose them to the xenon arc light source of 60 W/m 2 ± 15% intensity for 720 h.Test the tensile strength and elongation at break of 5 exposed and 5 non-exposed test pieces respectively.The test values of the tensile strength and elongation at the break of the exposed test pieces are required to be at least 70% of the non-exposed test pieces

Halogen test
The number of test samples is 1, the sample mass is 1000 mg, the gas flow rate is (0.0157D2) I•h-1, D=38 mm, and the combustion time is 30 min.The flow rate of chlorine and bromine content measured by HCl is (0.0157D2)I • h-1, D=38 mm.The fluorine content test conditions The number of test samples: 1, the test method: ion selective electrode method, combustion support: 2-3 drops of dodecanol, the absorption solution: 5 ml 0.5 M sodium hydroxide solution, buffer reagent: 5 ml TISAB, the atomic mass of fluorine: 19.0 mol /L.The pH value is required to be greater than or equal to 4.3, and the conductivity is required to be less than or equal to 10 μ S /mm, chlorine and bromine content (calculated by HCl) is less than or equal to 0.5%, fluorine content is less than or equal to 0.1%

Thermal life test
The test procedure is as per IEC 60216, requiring a temperature index of 120℃ at 20000 h, and a fracture elongation retention rate of 50%.

Finished cable test Low-temperature bending test
The test temperature is (40 ± 2)℃; the sheath shall not crack.

Low-temperature stretching test
The test temperature is (40 ± 2)℃, the low-temperature treatment time is 16 h, the separation rate is 25 mm/min; the sheath extension rate is greater than or equal to 30%.

Low-temperature impact test
The test temperature is (40 ± 2)℃, the number of samples is 3, the time is 16 h, the drop hammer weight meets the EN 50618 Appendix C, and the sheath does not crack.

Power cut through
The number of test samples is 1, the measurement is 4 times, the temperature is 20℃, the outer diameter of steel wire is 0.45 mm, and the loading speed is 1 N/s; the load is greater than or equal to Fmin =150√dL.

Compatibility test
Aging condition of finished cable: number of test samples is 3, the test temperature is (135 ± 2)℃, the test time is 168 h; the required maximum variation rate of tensile strength of insulation and sheath is ± 30%, the maximum variation rate of fracture elongation is ± 30%.

Smoke density
The test length of the sample is 1 m, the storage time is 16 h at 20℃, the distance between the lower surface of the sample and the bottom of the alcohol disk is 150 mm, the pretreatment temperature of the test box is 23℃, and the test time is 40 min; the light transmittance is 60% within 40 min after the flame is extinguished

Single vertical combustion
The number of test samples is 1, the sample length is 600 mm, and the combustion time is 60 s; 50 mm from the upper edge and 540 mm from the lower edge.(e) Special environmental resistance performance test:

Salt spray test
Put the sample into the salt spray box, spray salt spray (produced by 5% sodium chloride solution) at 50 ℃ for 336 h, and test the change rate of tensile strength and elongation at break of cable sheath is less than or equal to -30%; No breakdown is required after the voltage test of 6.5 kV (a.c.) or 15 kV (d.c.) for 5 min; Insulation resistance value at 20℃ and 90℃ is greater than or equal to the specified value.

Seawater immersion corrosion test
Put the sample into the sodium chloride solution with temperature 90℃ and 5% mass fraction, immersion depth 300 mm, after 720 h, test the change rate of tensile strength and fracture elongation of sheath -30%; 6.5 kV(a.c.) or 15 kV(d.c.), 5 min voltage test; no insulation resistance of 20℃ and 90℃.

Damp heat test
After 2000 h in the damp heat test box with a temperature of 90℃ and 85% of relative humidity, test the change rate of tensile strength and fracture elongation of the cable sheath by -30%; 6.5 kV(a.c.) or 15 kV(d.c.), 5 min voltage test, no breakdown; the specified value of insulation resistance of 20℃ and 90℃.

Mould proof test
The temperature is 30℃, the relative humidity is not less than 90%, and it is placed in the mold test box for 28 d, and the mildew-proof grade is required to be grade 0.

Structural design of offshore photovoltaic cable
With the rapid development of new energy, a large number of conventional units are replaced, resulting in the reduction of the disturbance resistance capacity of the system and the insufficient adjustment capacity of the power grid, which brings challenges to the safe operation of the power grid [7] .According to the use requirements of the sea surface photovoltaic system, in addition to the special waterproof performance requirements, the electrical performance and mechanical performance of the sea surface photovoltaic cable are basically consistent with the traditional onshore photovoltaic cable.Therefore, the sea surface photovoltaic cable should follow the terrestrial photovoltaic cables' conductor, insulation, and sheath structure.The conductor should be a class 5 annealed tin-plated stranded copper conductor that meets the IEC 60228:2005 standard to ensure the overall softness and current carrying capacity of the cable; The insulation and sheath should be low smoke halogen-free flame retardant material with remarkable waterproof performance.The overall structure of the DC 1.5 kV 4 mm 2 specification offshore photovoltaic cable with the largest amount in the current photovoltaic power station is shown in Figure 2.

Research on sea surface photovoltaic cable materials
Under the constraint of a large framework consistent with low smoke halogen-free characteristics, the preferred three types of materials are silicone rubber, irradiated crosslinked polyolefin, and silane natural crosslinked polyolefin materials, and all of them have certain waterproof properties.The natural crosslinked silane polyolefin material with PE as the matrix material and mixed with EVA as well as related compatible agents.It can effectively improve the compatibility between the PE matrix and the inorganic filler, and make the composite material more dense, not easy to absorb water.Through the test and verification of these three types of cables, it is further confirmed that the silane naturally crosslinked polyolefin material has the best comprehensive performance in all aspects, which best meets the actual needs of long-term application of photovoltaic cables on the sea.The resistance and mechanical properties of the three materials are shown in Figure 3 and Figure 4 respectively, and other performances are shown in Table 2.
Figure 3 The resistance of the three materials at 20°C Figure 4 The mechanical properties of the three materials

4.3Sea-surface photovoltaic cable process control
According to the structural design and material selection of DC1.5 kV 4 mm 2 offshore photovoltaic cable, the production process includes conductor winding, insulation extrusion, sheath extrusion, and natural cross-linking of the insulation sheath.The conductor production process of offshore photovoltaic cable is generally the same as that of traditional terrestrial photovoltaic cable.Through trial production verification, the conductor of DC1.5 kV 4 mm 2 offshore photovoltaic cable can put to use 48 pieces of 0.300 mm monofilament stranded structure, and control the outer diameter and roundness of the stranded conductor according to the layering method of 3+9+15+21, to ensure that the outer diameter of the stranded conductor is less than or equal to 2.8 mm, the DC resistance of the conductor at 20℃ is less than or equal to 5.09 Ω/km, the conductor roundness is greater than or equal to 92%, and the conductor surface is smooth.Silane naturally crosslinked polyolefin materials are used for the insulation and sheath.Because a large number of inorganic flame retardants are added to this type of material, the material fluidity during extrusion is poor.Once the friction heat of materials in the screw melting section and homogenizing section is too high, the material will be decomposed or pre-crosslinked, which will easily lead to the particles on the extrusion surface.Therefore, it is necessary to keep the materials dry during mixing to avoid the introduction of moisture.The retention time of materials in the machine head shall be further strictly controlled to be less than or equal to 10 min to ensure a smooth extruded surface of insulation and sheath.In the meantime, in view of the problem of insulation shrinkage due to the tensile crystallization characteristics, the conductor should be preheated to more than 60℃ during extrusion to reduce the shrinkage stress generated by the contact between the insulation material and the conductor, and after extrusion, progressive water cooling should be adopted to reduce the shrinkage stress in the material and avoid the insulation retraction.

CONCLUSION AND PROSPECT
After nearly 10 years of development, research, technical breakthrough and use verification of corresponding application environment of cables for photovoltaic systems, the technology of cable products for offshore floating photovoltaic systems has become gradually mature, and the corresponding product technical assessment requirements have been gradually clear.We conduct standardization research and through the form of product standard curing technical requirements, for development, production and use all parties to provide a unified standard technical assessment basis, to the products to the direction of generalization, serialization, and standardization, and further introduce the corresponding product design scheme, material selection, and process control, to provide a reference for the cable manufacturing enterprises.

Figure 1
Figure 1 Sea surface floating photovoltaic system Meet the requirements of class 5 tinned copper conductors specified in IEC 60228

Figure 2
Figure 2 DC 1.5 kV 4 mm 2 specification sea surface photovoltaic cable structure diagram

Table 2 .
Key Performance Test of Samples