Photovoltaic cables are frequently exposed to sunlight, and solar energy systems are often used in harsh environments, such as high temperatures and ultraviolet radiation. In Europe, sunny days can cause on-site temperatures for solar energy systems to reach 100°C. Currently, various materials are available, including PVC, rubber, TPE, and high-quality cross-linking materials. However, unfortunately, rubber cables with a rated temperature of 90°C, and even PVC cables with a rated temperature of 70°C, are often used outdoors. With the frequent launch of the national Golden Sun Project, many contractors, in order to save costs, do not choose dedicated solar energy system cables but instead choose ordinary PVC cables as substitutes for photovoltaic cables. Obviously, this will greatly affect the lifespan of the system.
The characteristics of photovoltaic cables are determined by their specialized insulation and sheath materials, known as cross-linked polyethylene (PE). Irradiation by an accelerator alters the molecular structure of the cable material, thus improving its various properties. Regarding mechanical load resistance, during installation and maintenance, cables may be laid along the sharp edges of roof structures, and must withstand pressure, bending, tension, cross-tension loads, and strong impacts. If the cable sheath strength is insufficient, the cable insulation will be severely damaged, affecting the overall lifespan of the cable or leading to problems such as short circuits, fires, and personal injury hazards.
Performance of photovoltaic cables
Electrical performance
1. DC resistance
The DC resistance of the conductive core of the finished cable at 20℃ is no greater than 5.09Ω/km.
2 Immersion Voltage Test
The finished cable (20m) does not break down after being immersed in water at (20±5)℃ for 1 hour and then subjected to a 5-minute voltage test (AC 6.5kV or DC 15kV).
3. Long-term withstand DC voltage
The sample is 5m long and is placed in distilled water containing 3% sodium chloride (NaCl) at (85±2)℃ for (240±2)h, with both ends 30cm above the water surface. A DC voltage of 0.9kV is applied between the conductor and the water (conducting conductor connected to the positive terminal, water connected to the negative terminal). After removing the sample, an immersion voltage test is performed at an AC voltage of 1kV, requiring no breakdown.
4. Insulation resistance
The insulation resistance of the finished cable at 20℃ shall not be less than 10¹⁴ Ω·cm, and the insulation resistance of the finished cable at 90℃ shall not be less than 10¹¹ Ω·cm.
5. Surface resistance of the sheath
The surface resistance of the finished cable sheath should be no less than 10⁹ Ω.
Other performance
1. High temperature pressure test (GB/T 2951.31-2008): Temperature (140±3)℃, time 240min, k=0.6, indentation depth not exceeding 50% of the total thickness of insulation and sheath. An AC 6.5kV, 5min voltage test shall also be performed, requiring no breakdown.
2. Damp heat test
The sample was placed in an environment of 90℃ and 85% relative humidity for 1000 hours. After cooling to room temperature, the change rate of tensile strength and the change rate of elongation at break were ≤-30% compared with before the test.
3. Acid and alkali solution resistance test (GB/T 2951.21-2008)
The two groups of samples were immersed in oxalic acid solution with a concentration of 45 g/L and sodium hydroxide solution with a concentration of 40 g/L, respectively, at a temperature of 23℃ for 168 h. Compared with before immersion, the tensile strength change rate was ≤ ±30% and the elongation at break was ≥100%.
4. Compatibility test
After the cable is aged for 7×24h at (135±2)℃, the change rate of tensile strength of the insulation before and after aging is ≤±30%, and the change rate of elongation at break is ≤±30%; the change rate of tensile strength of the sheath before and after aging is ≤-30%, and the change rate of elongation at break is ≤±30%.
5. Low-temperature impact test (8.5 in GB/T 2951.14-2008)
Cooling temperature -40℃, time 16h, drop hammer mass 1000g, impact block mass 200g, drop height 100mm, and there should be no visible cracks on the surface.
6. Low-temperature bending test (8.2 in GB/T 2951.14-2008)
Cooling temperature (-40±2)℃, time 16h, test rod diameter is 4 to 5 times the outer diameter of the cable, wrapped 3 to 4 times, after the test there should be no visible cracks on the surface of the sheath.
7. Ozone resistance test
The sample length is 20 cm, and it is placed in a desiccator for 16 hours. The diameter of the test rod used for the bending test is (2±0.1) times the outer diameter of the cable. The test chamber settings are: temperature (40±2)℃, relative humidity (55±5)%, ozone concentration (200±50)×10-6%, and air flow rate: 0.2～0.5 times the test chamber volume/min. The sample is placed in the test chamber for 72 hours. After the test, there should be no visible cracks on the surface of the sheath.
8 Weather resistance/UV test
Each cycle consists of: 18 minutes of water spraying, 102 minutes of xenon lamp drying, at a temperature of (65±3)℃, relative humidity of 65%, and a minimum power of (60±2) W/m² under wavelength conditions of 300–400 nm. After 720 hours, a bending test is performed at room temperature. The test rod diameter is 4–5 times the cable's outer diameter, and there should be no visible cracks on the sheath surface after the test.
9. Dynamic Penetration Test
At room temperature, the cutting speed was 1 N/s, and the number of cutting tests was 4. Each time, the sample had to be moved forward 25 mm and rotated 90° clockwise before proceeding with the test. The penetration force F at the instant the spring steel needle contacted the copper wire was recorded, and the average value was ≥150·Dn1/2 N (4 mm² cross-section Dn = 2.5 mm).
10 Dent Resistance
Take three sample sections, and make four indentations on each section at 25mm intervals and rotated 90°. The indentations are 0.05mm deep and perpendicular to the copper conductor. Place the three sample sections in test chambers at -15℃, room temperature, and +85℃ respectively for 3 hours. Then, wind them onto a mandrel with a diameter of (3±0.3) times the minimum outer diameter of the cable in their respective test chambers. At least one indentation on the outer side of each sample is required. Perform an AC 0.3kV immersion voltage test without breakdown.
11. Sheath heat shrinkage test (11 in GB/T 2951.13-2008)
The sample was cut to a length L1=300mm, placed in an oven at 120℃ for 1 hour, and then cooled to room temperature. This hot and cold cycle was repeated 5 times. Finally, the sample was cooled to room temperature. The thermal shrinkage rate of the sample was required to be ≤2%.
12 Vertical Burning Test
After the finished cable is placed at (60±2)℃ for 4 hours, a vertical burning test as specified in GB/T 18380.12-2008 is conducted.
13 Halogen content test
pH and conductivity
Sample placement: 16h, temperature (21～25)℃, humidity (45～55)%. Two samples, each (1000±5)mg, crushed into particles less than 0.1mg. Air flow rate (0.0157·D2)l·h⁻¹±10%, distance between the combustion boat and the edge of the effective heating zone of the furnace ≥300mm, temperature at the combustion boat ≥935℃, temperature ≥900℃ at 300m away from the combustion boat (in the direction of air flow).
The gas generated by the test sample was collected through a gas washing bottle containing 450 ml of distilled water (pH 6.5 ± 1.0; conductivity ≤ 0.5 μS/mm). Test cycle: 30 min. Requirements: pH ≥ 4.3; conductivity ≤ 10 μS/mm.
Cl and Br content
Sample placement: 16h, temperature (21～25)℃, humidity (45～55)%. Two samples, each (500～1000)mg, crushed to 0.1mg.
The air flow rate was (0.0157·D2)l·h-1±10%, and the sample was uniformly heated to (800±10)℃ for 40min and held for 20min.
The gas generated by the test sample was drawn through a gas wash bottle containing 220 ml of 0.1 M sodium hydroxide solution. The liquid from the two gas wash bottles was poured into a volumetric flask. At the same time, the gas wash bottles and their accessories were rinsed with distilled water and poured into the volumetric flask to a volumetric volume. After cooling to room temperature, 200 ml of the test solution was dropped into the volumetric flask with a pipette. 4 ml of concentrated nitric acid, 20 ml of 0.1 M silver nitrate, and 3 ml of nitrobenzene were added, and then the mixture was stirred until a white flocculent precipitate was formed. 40% ammonium sulfate aqueous solution and a few drops of nitric acid solution were added and mixed completely. The mixture was stirred with a magnetic stirrer, and then ammonium thiocyanate titration solution was added.
Requirements: The mean values of the two sample test values should be: HCl ≤ 0.5%; HBr ≤ 0.5%.
The test value of each sample is ≤ the mean of the test values of the two samples ± 10%.
F content
Place 25–30 mg of sample material into a 1 L oxygen container, add 2–3 drops of alkanol, and then add 5 ml of 0.5 M sodium hydroxide solution. Burn the sample completely, and then gently rinse the residue into a 50 ml measuring cup.
Mix 5 ml of buffer solution with the sample solution and rinsing solution until the calibration line is reached. Plot a calibration curve to determine the fluoride concentration in the sample solution, and calculate the percentage of fluoride in the sample.
Requirement: ≤0.1%.
14 Mechanical properties of insulation and sheathing materials
Before aging, the insulation tensile strength is ≥6.5 N/mm2 and the elongation at break is ≥125%, while the sheath tensile strength is ≥8.0 N/mm2 and the elongation at break is ≥125%.
After aging at (150±2)℃ for 7×24h, the change rate of tensile strength of insulation and sheath before and after aging is ≤-30%, and the change rate of elongation at break of insulation and sheath before and after aging is ≤-30%.
15. Thermal stretching test
Under a load of 20 N/cm2, after a thermal extension test at (200±3)℃ for 15 min, the median value of the elongation of the insulation and sheath should not exceed 100%, and the median value of the increase in the distance between the marked lines after the specimen is taken out of the oven and cooled should not exceed 25% of the distance before the specimen was put into the oven.
16 Thermal life
The test was conducted according to the Allene curves of EN 60216-1 and EN 60216-2, with a temperature index of 120°C and a duration of 5000 hours. The insulation and sheath elongation at break retention rate was ≥50%. A bending test was then performed at room temperature. The test bar diameter was twice the cable outer diameter, and there should be no visible cracks on the sheath surface after the test. Required lifespan: 25 years.
