Which Steel Strand Is Both Economical and Safe for Flexible PV Brackets? (Issue 5)

Steel Strand Is the Backbone of Flexible PV Brackets

Steel strand is the principal material of PV flexible brackets, accounting for more than 60% of the bracket's material content. Depending on the structural form, the steel share varies; in some specially designed flexible brackets the strand content can reach as high as 80%. Correct strand selection is therefore a decisive factor in both cost control and quality assurance.

Two Main Categories of Steel Strand

1. Power-fittings strand. Used mainly in power transmission and transformation line projects. In the early stage of flexible bracket development, most projects adopted this type of power strand. It is mostly made of 355B grade material, has relatively high elongation, is usually suitable for low-load equipment, and is inexpensive. Manufacturers of this product are concentrated in Renqiu, Hebei.

2. Prestressed strand. The base material is generally 82B, with common strength grades of 1770 MPa, 1860 MPa and 1960 MPa. The typical wire structure consists of seven round-section wires — one centre wire with six wires helically stranded around it. Thanks to its material properties, it offers high strength, good toughness and low relaxation. Common diameters used in flexible brackets are 12.7 mm, 15.2 mm, 17.8 mm and 21.6 mm. Prestressed strand is produced in many regions, with a strong concentration of manufacturers in the Tianjin area. The two most widely used diameters are 12.7 mm and 15.2 mm; the unsaturated annual production capacity for these two sizes in the Tianjin region alone is about 4.5 million tonnes. With an average strand consumption of about 7 tonnes per MW of flexible bracket, even half of that capacity would be sufficient for 300 GW of flexible bracket demand. Tianjin's strand industry chain is complete, with good quality, competitive prices and short delivery cycles.

How to Choose the Right Strand by Local Climate

As infrastructure demand has changed, many strand variants have emerged: galvanized strand, zinc-aluminium alloy strand, PE unbonded strand (galvanized or non-galvanized), filled epoxy strand, and filled epoxy PE strand. These are widely used in bridges, civil buildings, wind power and PV flexible brackets. Because of the rapid growth of PV flexible brackets, strand demand has risen sharply. Yet there are many opinions on the best strand choice, because people's understanding of strand performance differs. The scientific approach is to select the strand according to the actual conditions of the project site — above all, its climate. The following selection suggestions for flexible bracket strand are offered for reference:

• Annual rainfall below 300 mm: non-galvanized unbonded strand — the lower-cost option.
• Annual rainfall 300–500 mm: non-galvanized unbonded strand or galvanized strand.
• Annual rainfall 500–700 mm: zinc-aluminium alloy strand.
• Annual rainfall 700–1200 mm: galvanized unbonded strand.
• Annual rainfall above 1200 mm (including areas within 30 km of the coast, beaches and offshore flexible brackets): filled epoxy strand with an additional PE sheath. This type has excellent sealing performance — highly concentrated corrosive salt mist cannot penetrate inward — which effectively prevents corrosion and rusting and avoids shortening the service life. Filled epoxy strand offshore typically lasts more than 30 years, making it the ideal strand for offshore wind and offshore PV projects.

Key Takeaways

• Strand is the largest material cost item of a flexible bracket — choose it by local climate, not habit.
• 12.7 mm and 15.2 mm prestressed strand dominate the flexible bracket market today.
• For high-rainfall and offshore sites, filled epoxy strand with a PE sheath is the reliable, long-life solution.