VOLTAGE STRESSES VARY INTERNATIONALLY
The U.S., Europe and Japan have different approaches to insulation stress limits.
- Published in Southwire's Power Cable Update newsletter in August 1997
- Reprint permission granted
How much insulation stress is too much? It depends on who you ask.
"In the U.S., voltage stress guidelines are more conservative than in Europe or Japan," says Steve Campbell, technical manager for solid-dielectric, high-voltage products at Southwire's Heflin facility. "In some cases especially at higher voltages-the differences can be significant."
Where do the differences come from? Varying approaches to overcoming practical limits.
Size Limits Insulation Thickness
"You can only put so much insulation on a conductor before the cable gets too big and inflexible to handle well in the field," says Campbell. "As the cable gets less flexible, the reels get too big to transport easily."
These field considerations put a practical limit of about 30mm (1,200 mil) on insulation thickness. Above 35kV, designers try to hold down overall cable diameter by letting the stress level in the insulation climb.
"High-quality XLP cables will take about 4OkV/mm (1000V/mil) before failure in an AC test. These cables typically operate in the U.S. at a maximum stress of less than 4kV/mm (100V/mil)," says Campbell. "The question is how much safety factor you build in. In Europe and Japan, the compromises are a little more aggressive than in the U.S."
Guidelines Make a Difference
"One difference is in the way guidelines are set," says Nick Ware, manager of Southwire's D.B. Cofer Technology Center. "In the U.S., the Association of Edison Illuminating Companies (AEIC) sets user-driven standards with specific insulation thickness guidelines. There's room for variation but historically, most manufacturers have followed the guidelines."
Ware adds, "In Europe and Japan, guidelines are less specific. Designers and users are more willing to accept higher stresses. Typical designs reflect that."
Campbell concludes, "When cables operate above 66kV, the insulation is usually protected from moisture. You'll normally find a moisture barrier in these applications, no matter where the cable is built." |
