Talking about the performance requirements and test methods of low-temperature flexible cables

Release date: 2021/5/18 17:33:31 | Publisher: Shenzhen Cptele Electronics Co., Ltd

 

In general, wind farms are located in harsh environments with special climatic conditions, such as strong winds, strong ultraviolet rays, and highly salinized air. Because of this, cable performance in wind power applications is undoubtedly higher than in other applications. The moving parts within the wind turbine further increase the importance of the correct cable selection. "

Both the maintenance of existing wind farms and the development of new large-scale wind farms require consideration of high-grade power cables, data and control cables, and communication cables, which determine the interconnection quality of the grid and communication systems. The number of cables required for a single wind turbine is greater than one might think. For example, a 90-meter-tall 1.25MW wind turbine requires about 1km of power cables. In this way, a wind farm with an installed capacity of 50MW will need 40km of cables.

Wind turbines operate in harsh environments with a wide temperature range (about -40°C to 50°C) and exposure to extremely strong ultraviolet light. Therefore, to achieve the expected service life, the special cables used need to be able to withstand temperatures as low as -40°C and be resistant to UV radiation. For the moving parts in the wind turbine, the cable should have excellent torsional and bending flexibility and have a small bend radius. Cables also need to be resistant to fuels, freezers, oils, corrosive chemicals, and abrasion. If the wind farm is on land close to the coast or offshore, the cables must also be resistant to high salt water erosion. For safety reasons, the cable is required to be flame retardant in addition to the above requirements. In some cases, other features such as low-smoke, zero-halogen (LSZH) materials, and EMI protection are also required.

In summary, cables used in wind power applications should generally meet the following requirements:

(1) Wire
To maximize flexibility, it is recommended that design engineers use only annealed soft copper wire with multiple strands. In bending and winding applications, short concentric stranded wire construction is used; In torsional winding applications, long concentric stranded wires are used. Conductors with an area greater than 6 mm2 (10 AWG) require a composite stranded wire construction.

(2) Insulation
To increase the flexibility of low temperatures, thermoplastic rubber (TPE), ethylene propylene diene monomer (EPR, one EPM or EPDM), or silicone rubber (SiR) are often selected as insulating materials to resist ozone corrosion and heat-induced aging. PVC/nylon insulation is also widely used due to its high dielectric strength.

(3) Sheathing
Cable sheaths can be thermosetting compounds such as polyvinyl chloride (CPE), polychloroprene (neoprene), chlorosulfonated polyethylene (CSPE) synthetic rubber, etc.; It can also be thermoplastic compounds such as TPE, TPE-PVC alloys and polyurethane. These materials are resistant to oils, fuels, solvents, etc., and have excellent flexibility at low temperatures. This property makes it an ideal sheath material for wind power cables.

It should be noted that the cable structure is also a decisive factor in the flexibility of the cable. Symmetrical wire designs with balanced construction are typically highly flexible.

Even if these general rules are followed when the cable is manufactured, it is highly recommended that a thorough test be carried out to simulate the "real-world" application.

Cable Test Methods and Procedures

Depending on the wind direction, the angle of the fan needs to be adjusted by the yaw drive. Power, control, and communication cables either bend along a horizontal axis or rotate along a vertical axis. This makes the torsional flex requirements more stringent and requires more attention. While there are currently no standards or regulations for torsional flexure, end users are still typically looking for cables that can pass some form of testing before they are put into service.

The following are the general test methods adopted by end users in the cable industry.

(1) Torsional stress test of a single cable at low temperature (-40°C):
A 10 m long vertically suspended cable sample is fixed at the top and the bottom end is bound to a rotating device. First, twist the cable clockwise 4 times (+1440o) and then turn it counterclockwise 4 times to return to its original position. Then twist the cable counterclockwise 4 times (-1440o) and then turn it 4 times clockwise to return to its original position. Repeat the entire process 5,000 times to simulate 20 years of usage. If after 5 minutes at 2.5U0, the cable is not broken down and the sheath is not cracked, then the cable has passed the test.
Note: Depending on the voltage level of the cable, U0 can be 600, 1000 or 2000V.

(2) Torsional stress test of a bundle of cables
The test procedure is the same as (1), except that the cable harness is replaced.
Wind power cable standards

There is currently no standard specific to the use of cables in wind power applications. Many cable manufacturers follow the IEC 60228 Class 5 or 6 standard (similar to DIN VDE 0295 Class 5 or 6, HD 383, GB/T 3956 Class 5 or 6) and use smooth or metallized annealed stranded copper wire as wind power cable conductors to obtain the required flexibility. Interestingly, IEC 60228 only specifies the nominal cross-sectional area of the conductor and the number and size of wires in the conductor for power cables, which gives cable manufacturers a great deal of freedom. As a result, even if the cable meets the requirements of IEC 60288 Class 5 or 6, the cable performance is often unsatisfactory. UL 62 (which involves several ASTM standards) specifies not only the size and number of wires per strand in a conductor, but also the conductor structure (such as twisted, composite and ensembled) which are key to the flexibility of cables. As for insulation and sheaths, many manufacturers follow DIN VDE 0207-20 and DIN VDE 0207-21. HD 22.1, HD 22.4, UL 44 and UL 62 have also become common standards for cable production.

Other standards such as UL 758, UL 1581, UL 1277, UL 2277, IEC 60332, etc., are also frequently used to support additional characteristics such as the Wind Turbine Rack Cable (WTTC) specification and flammability rating requirements.

Since European countries developed cables for the wind energy market earlier than North American countries, cable manufacturers are now adopting more European standards. Nonetheless, similar U.S. UL standards serve the same purpose, and in some cases, UL standards have more stringent requirements for wind energy applications.

 

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